National Renewable Energy Laboratory 

Innovation for Our Energy Future 

A national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

NREL is operated by Midwest Research Institute ● Battelle Contract No. DE-AC36-99-GO10337 

Lessons Learned from 

Case Studies of Six 

High-Performance Buildings 

P. Torcellini, S. Pless, M. Deru, B. Griffith, 

N. Long, and R. Judkoff 

Technical Report 

NREL/TP-550-37542 

June 2006 
Lessons Learned from 

Case Studies of Six 

High-Performance Buildings 

P. Torcellini, S. Pless, M. Deru, B. Griffith, 

N. Long, and R. Judkoff 

Prepared under Task No(s). BEC31001, BEC61012 

Technical Report 

NREL/TP-550-37542 

June 2006 

National Renewable Energy Laboratory 

1617 Cole Boulevard, Golden, Colorado 80401-3393 

303-275-3000 • www.nrel.gov 

Operated for the U.S. Department of Energy 

Office of Energy Efficiency and Renewable Energy 

by Midwest Research Institute • Battelle 

Contract No. DE-AC36-99-GO10337
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CONTENTS

Acknowledgments......................................................................................................................................vi

Executive Summary ..................................................................................................................................vii

Lessons Learned Summary ..............................................................................................................viii

Best Practices for High-Performance Buildings................................................................................ix

Acronyms...................................................................................................................................................xii

1 Introduction.......................................................................................................................................1

1.1 Problem Definition.................................................................................................................1

1.2 Methodology..........................................................................................................................2

1.3 Goals and Objectives .............................................................................................................3

1.4 Report Organization...............................................................................................................3

2 Case Studies.......................................................................................................................................4

2.1 Adam Joseph Lewis Center for Environmental Studies, Oberlin College.............................5

2.2 Zion Visitor Center ................................................................................................................5

2.3 Cambria Office Building........................................................................................................6

2.4 Philip Merrill Environmental Center, Chesapeake Bay Foundation......................................7

2.5 Thermal Test Facility.............................................................................................................7

2.6 BigHorn Home Improvement Center.....................................................................................8

2.7 Energy Performance Summary ..............................................................................................9

3 Lessons Learned..............................................................................................................................13

3.1 Applying a Whole-Building Design Process .......................................................................13

3.1.1 Owners provide the main motivation for low-energy buildings..............................15

3.1.2 Setting measurable goals is crucial to achieve low-energy buildings....................15

3.1.3 Cost justification of integrated design ....................................................................19

3.1.4 Using energy modeling throughout process ...........................................................20

3.1.5 Common roadblocks to successful integrated design process implementation ......23

3.1.6 Measured versus predicted performance................................................................24

3.1.7 Monitoring leads to better management and improved performance.....................25

3.2 Lighting and Daylighting Systems.......................................................................................26

3.2.1 Daylighting designs ................................................................................................27

3.2.2 Daylighting and lighting controls...........................................................................31

3.2.3 Lighting energy savings..........................................................................................33

3.2.4 Six elements of successful daylighting systems.......................................................37

3.3 Integrating the Envelope and Mechanical Systems .............................................................43

3.3.1 HVAC system descriptions......................................................................................44

3.3.2 Using the architectural design and envelope to create low-energy buildings........45

3.3.3 Passive solar heating ..............................................................................................49

3.3.4 Natural ventilation..................................................................................................52

3.3.5 Evaporative cooling and cooltowers.......................................................................57

3.3.6 Ground-source heat pumps.....................................................................................63

iii
3.3.7 Energy recovery ventilators....................................................................................65

3.3.8 Electric resistance heating......................................................................................69

3.3.9 HVAC controls........................................................................................................69

3.4 Photovoltaic Systems...........................................................................................................70

3.4.1 Design grid-connected PV systems to have no parasitic standby loads.................73

3.4.2 Consider an automatic monitoring system for PV system operation......................73

3.4.3 Locate PV panels where they will not be shaded....................................................74

3.4.4 Consider specifications for how a PV-based UPS system transitions to and from 

utility power............................................................................................................75

3.4.5 Use maximum power-point tracking controllers ....................................................76

3.4.6 Grid-connected PV systems and inverters should be carefully designed as an 

integrated system ....................................................................................................76

3.4.7 Integrate PV systems with demand-responsive controls and on-site thermal 

storage to maximize energy cost savings and payback...........................................77

3.5 Peak Demand and Demand Management ............................................................................78

3.5.1 Low-energy buildings peak demand profiles..........................................................81

3.5.2 PV systems have had limited success reducing peak demands...............................82

3.5.3 Demand-responsive controls can save energy costs and increase load factors.....83

3.5.4 Low-energy buildings can have low load factors ...................................................86

3.6 Plug and Equipment Loads..................................................................................................87

3.6.1 Parasitic loads should be minimized ......................................................................88

3.6.2 Domestic hot water loads are small........................................................................92

3.7 Postoccupancy Evaluation Techniques................................................................................94

3.7.1 Determining whole-building energy savings ..........................................................94

3.7.2 Measuring energy use ...........................................................................................101

3.7.3 Daylighting evaluations........................................................................................104

3.7.4 PV systems evaluations.........................................................................................105

3.7.5 Performance metrics.............................................................................................106

4 Conclusions....................................................................................................................................109

4.1 Lessons Learned.................................................................................................................109

4.1.1 Applying a whole-building energy design process................................................110

4.1.2 Lighting and daylighting systems..........................................................................112

4.1.3 Integrating the envelope and mechanical systems................................................113

4.1.4 Photovoltaic systems.............................................................................................114

4.1.5 Peak demand and demand management...............................................................115

4.1.6 Plug and equipment loads.....................................................................................116

4.1.7 Postoccupancy evaluation techniques ..................................................................116

4.2 Best Practices for High-Performance Buildings................................................................118

4.3 Recommended Future Research.........................................................................................121

5 References......................................................................................................................................124

Appendix A: Zero-Energy Buildings: Definitions and Analysis........................................................127

A.1 Boundary Definitions and Energy Flows...........................................................................127

A.1.1 Grid connection is allowed and necessary for energy balances...........................127

A.1.2 Prioritize supply-side technologies to those available on site and within the 

footprint ................................................................................................................127

iv
A.2 Definitions..........................................................................................................................129

A.3 Low- and Zero-Energy Buildings: Examples ...................................................................130

A.4 How Definition Determines Design...................................................................................130

A.4.1 Net zero site energy building ................................................................................130

A.4.2 Net zero source energy building ...........................................................................131

A.4.3 Net zero energy cost building ...............................................................................132

A.4.4 Net zero energy emissions building ......................................................................133

A.4.5 ZEB definitions applied to a sample of current generation low-energy buildings134

A.4.6 The ZEB definition selected can have an impact on future ZEB designs..............134

Appendix B: National Daylighting Potential.......................................................................................136

v
ACKNOWLEDGMENTS 

The authors would like to thank the U.S. Department of Energy’s (DOE) Office of Building 

Technologies, which managed and funded this research. 

The authors would also like to thank those who helped to edit and review the document: Peter Ellis, 

Stefanie Woodward, and Lauren Poole (NREL), Dru Crawley (DOE), Nadav Malin (BuildingGreen), 

Tom Wood (Montana State University), and Keith Emerson (United Power). 

vi
EXECUTIVE SUMMARY 

Commercial buildings have a significant impact on energy use and the environment. They account for 

approximately 18% (17.9 quads) of the total primary energy consumption in the United States (DOE 

2005). The energy used by the building sector continues to increase, primarily because new buildings are 

added to the national building stock faster than old buildings are retired. Energy consumption by 

commercial buildings will continue to increase until buildings can be designed to produce more energy 

than they consume. As a result, the U.S. Department of Energy’s (DOE) Building Technologies Program 

has established a goal to create the technology and knowledgebase for marketable zero-energy 

commercial buildings (ZEBs) by 2025. 

To help DOE reach its ZEB goal, the Buildings and Thermal Systems Center at the National Renewable 

Energy Laboratory (NREL) studied six buildings in detail over the past four years to understand the 

issues related to the design, construction, operation, and evaluation of the current generation of lowenergy commercial buildings. These buildings and the lessons learned from them help inform a set of 

best practices—beneficial design elements, technologies, and techniques that should be encouraged in 

future buildings, as well as pitfalls to be avoided. The 

lessons learned from these six buildings are also used 

to guide future research on commercial buildings to 

meet DOE’s goal for facilitating marketable ZEBs by 

2025. The six buildings are: 

High-Performance Buildings Case Study 

Reports 

• Energy Performance Evaluation of an 

Educational Facility: The Adam Joseph 

Lewis Center for Environmental Studies, 

Oberlin College, Oberlin, Ohio

• Evaluation of the Low-Energy Design and 

Energy Performance of the Zion National 

Park Visitor Center

• Analysis of the Design and Energy 

Performance of the Pennsylvania 

Department of Environmental Protection 

Cambria Office Building

• Analysis of the Energy Performance of the 

Chesapeake Bay Foundation’s Philip Merrill 

Environmental Center

• Evaluation of the Energy Performance and 

Design Process of the Thermal Test Facility 

at the National Renewable Energy 

Laboratory 

 

• Energy Design and Performance Analysis of

the BigHorn Home Improvement Center

• “Oberlin”—The Adam Joseph Lewis Center 

for Environmental Studies, Oberlin College, 

Ohio 

• “Zion”—The Visitor Center at Zion National 

Park, Springdale, Utah 

• “Cambria”—The Cambria Department of 

Environmental Protection Office Building, 

Ebensburg, Pennsylvania

• “CBF”—The Philip Merrill Environmental 

Center, Chesapeake Bay Foundation, 

Annapolis, Maryland 

• “TTF”—The Thermal Test Facility, National 

Renewable Energy Laboratory, Golden, 

Colorado 

• “BigHorn”—The BigHorn Home 

Improvement Center, Silverthorne, Colorado 

Each of the six commercial buildings we studied has a unique purpose and function, but all have 

commonalities. Each building must provide visual, acoustic, and thermal comfort for the occupants. All 

must stand up to climatic conditions, and all must meet or exceed the programmatic requirements for their 

spaces. The six buildings in this study are successful in these respects, and are all good energy 

performers. All had owners who pushed low-energy or sustainability goals and considered energy 

efficiency as part of the decision-making process. The architects and engineers then created a design to 

implement the vision, which required a whole-building design process to achieve the goals. The wholebuilding design process requires that the team responsible for the building design, including the architect, 

engineers (lighting, electrical, and mechanical), energy and other consultants, and the building’s owner 

and occupants—work together to set and understand the energy performance goals. The purpose of the 

vii
whole-building design approach is to enable the full design team to interact throughout the design process 

to fully understand the building systems’ interdependencies. A systematic analysis of these 

interdependencies can help ensure that a much more efficient and cost-effective building is produced. 

Various incarnations of the whole-building design process were used to produce the buildings 

documented in the case studies. For Zion, BigHorn, and TTF, we were directly involved in the design 

process, providing energy analysis and facilitating the design process as it applied to energy performance. 

We monitored all six buildings after they were built for at least one year and analyzed the data to 

determine their energy performance with respect to design goals. 

All the buildings use innovative combinations of “state-of-the-shelf” energy technologies to reduce their 

energy use and minimize their environmental impacts. The buildings all have thermal envelopes that 

exceed current energy codes. They also use daylighting, radiant heating, natural ventilation, evaporative 

cooling, ground-source heat pumps, photovoltaics (PV), and passive solar strategies. All use much less 

energy than comparable energy code-compliant buildings. We found that the buildings’ energy 

consumption was 25% to 70% lower than code. Although each building is a good energy performer, 

additional energy efficiency is required for these buildings to reach DOE’s ZEB goal. We learned a great 

deal from these case studies about the whole-building design process and about which technologies work 

best under given circumstances. 

Lessons Learned Summary 

Below is a summary list of lessons learned that applied to all six case study buildings: 

• Owners provide the main motivation for low-energy buildings. The owner was the driving 

force in each case. Each owner set the goals and made decisions to keep the project on track. 

The architects and engineers strived to meet the goals of the building owners, which resulted in 

the need for the whole-building design process. 

• Setting measurable energy saving goals at the outset of the project is crucial to realizing 

low-energy buildings. In the case studies, all the owners and design teams set aggressive energy 

saving goals at the outset. The goals ranged from 40% better than code to net-zero energy 

performance. In general, the teams that set the strongest energy performance goals and used 

energy simulation to understand the energy impacts of design decisions had the best energy 

performance. 

• Many decisions are not motivated by cost. Building owners make decisions based on values. 

Quite often owners will pay for features they really want in a building—this is especially true of 

architectural features. Conversely, if an owner does not want a feature, cost is often used as the 

reason to eliminate it. 

• Today’s technologies can substantially change how buildings perform. Properly applied offthe-shelf or state-of-the-shelf technologies are available to achieve low-energy buildings. 

However, these strategies must be applied together and properly integrated in the design, 

installation, and operation to realize energy savings. There is no single efficiency measure or 

checklist of measures to achieve low-energy buildings. 

• A whole-building design approach is a good way to lower energy use and cost. An integrated 

whole-building approach begins with a design team that is committed to the energy goals. The 

building must be engineered as a system if the technologies are to be integrated in design and 

operation. This included using computer simulations to help guide the design process—these 

simulations can perform trade-off analysis to examine energy impacts of architecture choices and 

HVAC&L (heating, ventilation, air-conditioning, and lighting) designs. 

• Low-energy buildings do not always operate as they were designed. The design community 

rarely goes back to see how their buildings perform after they have been constructed. 

viii
Measurements in all six buildings showed that they used more energy and produced less energy 

than predicted in the design/simulation stage. Several reasons were documented: 

3⁄4 There was often a lack of control software or appropriate control logic to allow the 

technologies to work well together. 

3⁄4 Design teams were too optimistic about the behavior of the occupants and their acceptance of 

systems. 

3⁄4 Energy savings from daylighting were substantial, but were generally less than expected. 

3⁄4 Plug loads were often greater than design predictions. 

3⁄4 Effective insulation values are often inflated when comparing the actual building to the asdesigned building. 

3⁄4 PV systems experienced a range of operational performance degradations. Common 

degradation sources included snow, inverter faults, shading, and parasitic standby losses. 

• Information leads to better management and improved performance. Setting and following 

design goals or traditional commissioning does not guarantee that the goals will be satisfied in 

actual operations. The whole-building energy performance must be tracked and verified. 

Monitoring provides valuable feedback that can help maintain the efficient performance of 

systems to ensure design goals are met. 

• This set of current generation low-energy buildings shows progress toward achieving a 

ZEB goal in actual buildings. Each of these buildings saved energy, with energy use 25% to 

70% lower than code. Although each building is a good energy performer, additional energy 

efficiency and on-site generation is required for these buildings to reach DOE’s ZEB goal. At 

current levels of performance, the one-story buildings—Zion, BigHorn, and TTF—could 

accomplish the ZEB goal with PV systems that would fit within the building footprint. ZEBs are 

not feasible for the two-story buildings within their footprints unless their loads are reduced. 

• We can replicate the lessons learned from these case studies in future low-energy buildings. 

Even though every commercial building is unique, the lessons learned from these case studies 

can be applied to a wide variety of commercial buildings. The buildings and the lessons learned 

from them help to define a set of best practices. Best practices are proven real-world 

technologies and processes that lead to high-performance buildings. Understanding success and 

opportunities in the current generation of low-energy buildings can improve the energy efficiency 

of all commercial buildings—the best practices should be applied to future buildings. 

Best Practices for High-Performance Buildings 

The lessons learned from the six case studies have helped to define this set of best practices that should be 

applied to the design, construction, and operation of future low- and zero-energy buildings. 

1. Use a whole-building design process to design, construct, and operate future low-energy 

buildings. This includes: 

a. Set a specific, quantifiable energy design goal that is used to guide the design process and 

can be verified during operation. 

b. Include everyone involved in the project starting at the earliest stages of the design process. 

c. Use whole building simulation tools to determine the energy performance of the building at 

all stages of design, construction, and occupancy. The whole-building design process is a 

simulation-based, quantitative, and qualitative method to help architects and engineers create 

low-energy buildings. The energy use and energy cost of a building depend on the complex 

interaction of many parameters and variables—best studied with computerized energy 

ix
simulation software to thoroughly evaluate all interactions between the building envelope, 

HVAC system, and design features. 

d. Design the building envelope such that it can be used to meet as many loads as possible. The 

envelope should be the first method of creating low-energy buildings; the mechanical and 

lighting systems should then be sized to meet any remaining loads. Low-energy architecture 

is not effective if mechanical systems have to solve problems that result from the 

architectural design. 

e. Update the design simulations from predesign through occupancy to ensure the design 

simulations will measure up to actual performance. 

2. Provide for a postoccupancy energy performance evaluation (POE). Commissioning a new 

building is essential, but it ensures only that the building components operate as specified. All 

six case studies were commissioned either formally or informally. In general, the traditional 

commissioning did not necessarily translate to expected energy performance or verify low-energy 

design goals. Continual monitoring of the performance during the POE, or continuous system 

commissioning with key performance metrics, is important to ensure that the goals of the design 

are met under normal operating conditions. 

3. Implement measurement procedures such as those developed in DOE’s Performance 

Metrics Research Project. Obtaining reliable metrics for determining a building’s performance 

is one of the core challenges to achieving widespread adoption of high-performance buildings. 

Currently available procedures for determining performance metrics are available in the 

following documents: 

a. Procedure for Measuring and Reporting Commercial Building Energy Performance

(Barley et al. 2005) 

b. Procedure to Measure Indoor Lighting Energy Performance (Deru et al. 2005b)

c. Procedure for Measuring and Reporting the Performance of Photovoltaic Systems in 

Buildings (Pless et al. 2005)

d. Procedure for Developing a Baseline Simulation Model for a Minimally Code-Compliant 

Commercial Building (Pless, Deru, and Torcellini 2006)

e. Standard Definitions of Building Geometry for Energy Evaluation Purposes (Deru and 

Torcellini 2005)

f. Procedure to Determine Source Energy and Emissions for Energy Use in Buildings

(Deru and Torcellini 2006)

4. Integrate daylighting into the envelope and lighting systems. Controlling the electric lighting 

when daylighting is available works in all climates and in almost any type of building. A good 

daylighting design should result from an integrated design process. The daylighting system has 

to be integrated with the envelope and trade-offs with heating and cooling understood to 

maximize whole-building energy savings. Use the following best practices to integrate 

daylighting with the lighting systems: 

a. Design daylighting into all occupied zones adjacent to an exterior wall or ceiling. 

b. Provide integral glare mitigation techniques in the initial design. 

c. Provide automatic, continuously dimming daylighting controls for all daylit zones. 

d. Design interiors to maximize daylighting distribution (no dark surfaces). 

e. Integrate the electrical lights with the daylighting system. 

f. Commission and verify postoccupancy energy savings. 

5. Use economizers in conjunction with energy recovery ventilators (ERVs). The control and 

systems integration of economizers, natural ventilation, and ERVs needs to be very carefully 

x
designed and implemented. Natural ventilation or ERVs, as implemented in the buildings we 

studied, were not as good as the conventional economizers they replaced. Systems with ERVs 

and ground-source heat pumps should have economizers for when conditions warrant. ERVs 

should be operated when recovered energy is a net saving to the building. In addition, ERVs 

should be used only when needed, as part of a demand-responsive ventilation system. 

6. Use evaporative cooling systems in dry climates. All cooling requirements in dry climates can 

be met with evaporative cooling systems. If more thermal and latent control is needed, 

evaporative cooling can be used as a first stage. 

7. Design natural ventilation systems with the following best practices: 

a. Design natural ventilation to rely primarily on stack effect unless wind direction and speeds 

are reliable. 

b. Separate natural ventilation supply and relief from the fenestration and use relief dampers for 

the passive ventilation. 

c. Use automatic supply and relief controls that do not rely on occupant interaction. 

d. Minimize use of enclosed spaces. 

e. Do not use natural ventilation systems as a replacement for conventional economizers. 

8. Use demand-responsive controls that integrate on-site storage, daylighting, and energy 

production to reduce peak demand charges and increase load factors. An efficient building 

that uses very little energy could still have large demand charges. Load factors are low primarily 

because PV systems reduce energy use, but may not reduce peak demands. The same is true of 

daylighting and strategies to reduce ventilation. Demand-responsive strategies that integrate onsite generation with the thermal capacity of the building require controls that allow the building 

temperature to float based on instantaneous consumption and production. Using PV generation 

to further reduce peak demands is typically the most cost-effective use of this resource. 

Future Research Recommendations 

To help DOE reach its ZEB goal by 2025 and to continue to reduce the energy consumption in 

commercial buildings, we recommend the following, based on our experience with the six case study 

buildings. Details of each recommendation are included in Section 4.3. 

1. Standardized methods should be used to collect data and determine energy savings to report 

building performance. 

2. Future research efforts should start with the lessons learned from the six case studies. 

3. Future ZEB research efforts should contain an element to focus on building owners and 

developers. 

4. Develop integrated HVAC equipment, systems, and control packages. 

5. Develop integrated whole-building lighting system packages. 

6. Use simulation tools to develop technology option sets. 

7. Develop regional time-dependent valuations for a time-of-day source electricity conversion rate 

throughout the country. 

8. Investigate methods to further increase the energy efficiency of equipment and plug loads. 

9. Evaluate the technical viability of and marketability of key successful strategies for wide-scale 

deployment. 

10. Study the degradation of long-term whole-building energy performance. 

11. Study the ability for PV systems combined with on-site storage and advanced control to 

minimize peak demand charges. 

xi
ACRONYMS 

AC Alternating Current 

ASHRAE American Society of Heating, Refrigerating, and Air-Conditioning Engineers 

CBECS Commercial Building Energy Consumption Survey 

CBF Chesapeake Bay Foundation 

CMU Concrete Masonry Unit 

DAS Data Acquisition System 

DC Direct Current 

DEP Department of Environmental Protection 

DHW Domestic Hot Water 

DOE U.S. Department of Energy 

ECI Energy Cost Intensity 

EMS Energy Management System 

ERV Energy Recovery Ventilator 

EUI Energy Use Intensity 

HVAC Heating, Ventilation, and Air Conditioning 

HVAC&L Heating, Ventilation, Air Conditioning, and Lighting 

IESNA The Illuminating Engineering Society of North America 

LEED Leadership in Energy and Environmental Design 

LPD Lighting Power Density 

MPPT Maximum Power Point Tracker 

NPS National Park Service 

NREL National Renewable Energy Laboratory 

POE Postoccupancy Evaluation 

PV Photovoltaic 

SHGC Solar Heat Gain Coefficient 

TDV Time-Dependent Valuation 

TTF Thermal Test Facility 

UPS Uninterruptible Power Supply 

USGBC United States Green Building Council 

VSD Variable Speed Drive 

ZEB Zero Energy Building 

xii 
1 INTRODUCTION 

Commercial buildings have a significant impact on energy use and the environment. They account for 

approximately 18% (25.9 quads) of the total primary energy consumption in the United States 

(DOE 2005). The total for all buildings is more than one-third of the primary energy consumption and 

approximately 70% of the electricity consumption (see Figure 1-1). Buildings in the United States are 

responsible for 38% of U.S. and 9% of global carbon dioxide (CO2) emissions. 

Industry 

34% 

Transportation 

28% 

Buildings 

39% 

Figure 1-1 Energy use by sector in the United States 

Building energy consumption will continue to increase in the near future. Electricity consumption in the 

commercial building sector doubled between 1980 and 2000, and is expected to increase another 50% by 

2025 (EIA 2005a.) Furthermore, new buildings are added to the national building stock faster than old 

buildings are retired. The percentage share attributed to the building sector is also increasing because of 

reduced growth in the industrial sector. 

Reducing energy consumption in commercial buildings through energy efficiency and renewable energy 

technologies would significantly reduce primary energy consumption in the United States (EIA 2005a). 

Toward this end, the U.S. Department of Energy (DOE) has set an aggressive goal to create the 

technology and knowledge base for marketable and cost-effective commercial zero-energy buildings 

(ZEBs) by 2025. ZEBs are high-performance buildings designed to generate as much energy as they use 

over the course of a year to result in net-zero energy consumption. 

To help DOE reach its ZEB goal, the Buildings and Thermal Systems Center at the National Renewable 

Energy Laboratory (NREL) studied six high-performance buildings over the past four years to evaluate 

and understand their operating energy performance. Each building was designed to minimize energy and 

environmental impacts and used a variety of low-energy technologies. The buildings represent a range of 

climates and uses. They are all good energy performers. Understanding the energy performance of the 

current stock of high-performance buildings is an important step toward reaching the ZEB goal. 

1.1 Problem Definition 

A problem with getting high-performance buildings into the marketplace is that owners and designers 

hesitate to try new innovative technologies and processes that have not yet been adopted by the 

mainstream. Owners are unsure what potential energy savings goals are achievable. Someone always has 

to be first to use a technology. A few building owners and designers have made great strides to 

significantly change the way commercial buildings use energy. They have documented the performance 

of sustainable buildings with respect to energy and identified lessons learned from their experience. 

These case studies illustrate owners and designers who have been early adopters. Publishing case studies 

1 
and summarizing lessons learned encourages others to build low-energy buildings and can help to prevent 

errors from being repeated. Additionally, postoccupancy evaluation (POE) of these leading-edge 

buildings demonstrates the progress toward achieving DOE goals in real-world examples. 

1.2 Methodology 

NREL documented the design process and operating performance of the six buildings in separate detailed 

case study reports. The objectives of the case studies were to develop, analyze, evaluate, and document 

the technologies, processes, and methods by which high-performance buildings can be reliably produced. 

These case studies provide detailed building descriptions, documentation, and evaluation of monitored 

and simulated energy performance, as well as lessons learned about designing, operating, and evaluating 

each building. 

NREL completed the case study reports in FY 2005. The reports include: 

• Energy Performance Evaluation of an Educational Facility: The Adam Joseph Lewis Center for 

Environmental Studies, Oberlin College, Oberlin, Ohio 

www.nrel.gov/docs/fy05osti/33180.pdf

• Evaluation of the Low-Energy Design and Energy Performance of the Zion National Park 

Visitors Center 

www.nrel.gov/docs/fy05osti/34607.pdf

• Analysis of the Design and Energy Performance of the Pennsylvania Department of 

Environmental Protection Cambria Office Building 

www.nrel.gov/docs/fy05osti/34931.pdf

• Analysis of the Energy Performance of the Chesapeake Bay Foundation’s Philip Merrill 

Environmental Center 

www.nrel.gov/docs/fy05osti/34830.pdf

• Evaluation of the Energy Performance and Design Process of the Thermal Test Facility at the 

National Renewable Energy Laboratory 

www.nrel.gov/docs/fy05osti/34832.pdf

• Energy Design and Performance Analysis of the BigHorn Home Improvement Center 

www.nrel.gov/docs/fy05osti/34930.pdf

Each case study developed a list of lessons learned and recommendations relevant to that unique building. 

This report combines the overarching lessons learned. Although the case studies represent a variety of 

designs and climates, we looked at all six together to understand common successes and failures. We 

then distilled these and other wisdom acquired along the way into a set of lessons learned. 

The term lessons learned refers to positive and negative aspects of a project that have clear messages and 

might help subsequent low-energy building projects. The lessons learned are intended as 

recommendations either for changes to these buildings or for concepts that should be applied to the next 

generation of low-energy buildings. Members of future building projects should keep these lessons in 

mind and realize that they should be considered jointly along with the goals for saving energy. Lessons 

learned are key educational components. They should help design teams avoid repeating problems and 

identify where the process of delivering buildings needs to be changed to promote and realize low- and 

zero-energy buildings. 

We then used these buildings and their lessons learned to help identify and define a set of best practices 

—proven, real-world low-energy building technologies and processes that should be carried forward in 

future buildings. 

2 
1.3 Goals and Objectives 

The goal of this report is to combine and highlight the primary overarching lessons learned from the case 

studies to help inform and direct future design studies and implementations to work towards ZEBs. The 

specific objectives are to: 

• Summarize lessons learned from the six case studies and show common trends and gaps in 

technologies, design methodologies, energy performance, and operational strategies. 

• Identify the forces that drive decision making related to energy efficiency and integrated design. 

• Identify limitations in the application of innovative technologies and plan investigations of 

subtopics such as integrated daylighting, natural ventilation, performance degradation over time, 

and demand-responsive controls. 

• Evaluate instrumentation and analysis techniques for POEs. 

• Develop a list of best practices to guide the design of future high-performance buildings. 

• Identify research areas where the case study buildings and existing instrumentation could be 

leveraged for further study. 

1.4 Report Organization 

The report is presented in four sections: Section 1 introduces the problem definition and methodology 

used in this research; Section 2 describes the design, low-energy features, and energy performance of the 

six case study buildings; Section 3 describes the lessons learned as they relate to the low-energy design 

process, low-energy building technologies, and evaluation techniques; and Section 4 summarizes our 

findings, formulates a list of best practices, and offers recommendations for future research. 

Additional information on defining a zero energy building and a national daylighting potential analysis is 

located in the appendices. We also encourage the reader to consult the individual case study reports via 

the links provided in Section 1.2. 

3 
2 CASE STUDIES 

NREL analyzed the energy performance of six commercial buildings constructed over the past decade to 

understand how they performed and to verify their design goals. Each building design team included 

sustainability among its initial project goals and worked to minimize the energy and environmental 

impacts of the project. From the beginning, the teams set aggressive energy-saving goals that ranged 

from 40% better than code to net-zero energy performance. Some teams also had ambitious goals in other 

areas of sustainability such as water management, building materials selection, or obtaining a high 

LEED® (Leadership in Energy and Environmental Design) score (USGBC 2004). All buildings have 

thermal envelopes that exceeded the then-current versions of the building energy code (ANSI/ 

ASHRAE/IESNA Standard 90.1) at the time of construction. In addition, a variety of other energy-saving 

strategies were used, such as daylighting, radiant heating, natural ventilation, evaporative cooling, 

ground-source heat pumps, photovoltaic (PV) systems, and passive solar heating. The buildings studied 

include: 

• “Oberlin”—The Adam Joseph Lewis Center for Environmental Studies, Oberlin College 

• “Zion”—The Visitor Center at Zion National Park, Springdale, Utah 

• “Cambria”—The Cambria Department of Environmental Protection Office Building, Ebensburg, 

Pennsylvania

• “CBF”—The Philip Merrill Environmental Center, Chesapeake Bay Foundation, Annapolis, 

Maryland 

• “TTF”—The Thermal Test Facility, National Renewable Energy Laboratory, Golden, Colorado 

• “BigHorn”—The BigHorn Home Improvement Center, Silverthorne, Colorado 

NREL was directly involved during the design process for three of the buildings (BigHorn, TTF, and 

Zion). Computer simulations of energy performance were used to help guide each design: first by 

evaluating envelope options, then by designing mechanical systems that matched the building’s predicted 

loads. The case studies include an evaluation of the whole-building design process. We did not 

participate in the design process for Oberlin, CBF, or Cambria. 

Postoccupancy evaluations were performed for all six buildings, including intensive monitoring for at 

least one year. The monitoring procedure measured energy flows for the whole building, including major 

subloads, such as lighting, heating, ventilation, air-conditioning, and plug and equipment loads. In some 

cases, additional monitoring further disaggregated end-use loads to better characterize the building. 

Measured data were later used to calibrate simulation models of the building energy performance. Codecompliant baseline and calibrated as-built models were simulated to determine site, source, and energy 

cost savings. 

Performance metrics were established to evaluate and compare building energy performance. On-site 

energy generation from a PV system makes it important to distinguish between total and net energy use. 

Site energy refers to the total energy consumed by the building but does not include on-site generation. 

Net site energy refers to the total energy consumed by the building minus the energy produced by on-site 

generation; this is what the utility meter measures under a net-metering arrangement. Different fuel 

types, i.e., electricity and natural gas, make it necessary to distinguish between site and source energy. 

Net source energy represents primary energy use and is calculated from net site energy by applying 

national site-to-source conversion factors of 3.215 for electricity and 1.072 for natural gas (EIA 2005). 

All numbers are reported as facility totals and include plug loads and site lighting. 

A brief description of each building and its energy performance follows. A summary of the energy 

performance for all six buildings appears at the end of this section. 

4 
2.1 Adam Joseph Lewis Center for Environmental Studies, Oberlin College 

The Adam Joseph Lewis Center for Environmental Studies at Oberlin College in Oberlin, Ohio, is a twostory, 13,600-ft2

 (1,265-m2) classroom and laboratory building. The building contains four classrooms, a 

small auditorium, atrium, staff offices, and kitchenette. The design team envisioned a building with the 

potential to develop into a ZEB as technologies improve. The objective was to promote innovative 

technologies and create an educational tool for the Environmental Studies Program at the college. 

The integrated building design includes daylighting to offset lighting loads, natural ventilation to offset 

cooling loads, massive building materials to store passive solar gains, a ground-source heat pump system 

to meet the cooling and heating loads, an energy management system (EMS), and a system to process 

wastewater without sending the waste to the municipal sewage treatment plant. Because of the zeroenergy vision, the building was designed to be all electric, such that on-site energy generation could offset 

100% of the energy consumed. The roof is covered with a grid-tied, 60-kW PV array (Figure 2-1). 

Figure 2-1 South elevation of Oberlin showing roof-mounted PV array 

Measured annual site energy use was 29.8 kBtu/ft2 (338 MJ/m2), or 47% less than the whole-building 

simulation of an ASHRAE Standard 90.1-2001 comparable code-compliant building. PV panels provided 

45% of the total electric load of the building for a net site energy use of 16.4 kBtu/ft2 (186 MJ/m2

). As a 

point of reference, the energy use is less than half that of the average Midwestern educational building: 

79 kBtu/ft2

 (897 MJ/m2) (EIA 2005a). The source energy requirements of Oberlin are also very low at 

39.7 kBtu/ft2

 (451 MJ/m2), or 77% less than the code-compliant building. 

These results show that a high-performance academic building with heating and cooling loads is possible 

in a humid climate. However, a ZEB in this climate will be very difficult to realize, especially with onsite wastewater treatment loads. Additional PV capacity that extends beyond the footprint of the building 

and better control algorithms would be required to meet the zero-energy vision with today’s technology. 

The complete case study report (Pless and Torcellini 2005) is available at 

www.nrel.gov/docs/fy05osti/33180.pdf. 

2.2 Zion Visitor Center 

The Visitor Center Complex at Zion National Park in southwestern Utah exemplifies the National Park 

Service’s commitment to promote conservation and minimize impact on the natural environment. The 

building design incorporates energy-efficient features such as daylighting, natural ventilation, cooltowers, 

passive solar heating, solar load control with engineered overhangs, computerized building controls, and 

an uninterruptible power supply (UPS) system integrated with the 7.2-kW PV system (Figure 2-2). 

Two conditioned buildings were constructed: an 8,800-ft2

 (820-m2) main Visitor Center building that 

contains a retail bookstore, a visitor orientation area, and support areas; and a 2,756-ft2

 (256-m2) restroom 

facility. Landscaping in the outdoor exhibit areas and between the buildings creates outdoor rooms that 

increase the effective space available for visitor amenities without increasing energy consumption. 

5 
The innovative heating and cooling systems eliminated all ductwork and fuel storage from the project. 

Passive solar heating augmented with localized electric heating systems met the heating needs. The 

electric heating systems are controlled to purchase electricity when demand charges will not be incurred. 

The cooltowers eliminated the need for conventional air-conditioning. The cooltowers use a wetted 

medium at the top of a tower. Cool air naturally falls down the tower and into the building without fans. 

Two fractional horsepower water pumps drive the entire cooling system. Because there is less heating, 

ventilation, and air-conditioning (HVAC) equipment, the building was constructed at a slightly lower cost 

than a conventional national park visitor center. 

The building’s energy performance has been evaluated since May 2000. The integrated design resulted in 

a building complex that costs $0.43/ft2

 ($4.63/m2) to operate and consumes 27.0 kBtu/ft2 (307 MJ/m2). 

During the monitored year, the PV system produced a net 7,900 kWh or 8.5% of the annual energy use. 

The complete case study (Torcellini et al. 2005a) is available at www.nrel.gov/docs/fy05osti/34607.pdf. 

Figure 2-2 North elevation of the Zion Visitor Center showing cooltowers and plaza 

2.3 Cambria Office Building 

The Cambria Office Building in Ebensburg, Pennsylvania, has an area of 34,500 ft2

 (3,205 m2) and serves 

as the district office for Pennsylvania’s Department of Environmental Protection (DEP). The design team 

used the U.S. Green Building Council’s LEED 2.0 requirements as design guidelines and goals. Among 

the low-energy design features used in this building are ground-source heat pumps, an under-floor air 

distribution system, heat recovery ventilators, an 18.2-kW PV system, daylighting, motion sensors, 

additional wall and roof insulation, and high-performance windows (Figure 2-3). Pennsylvania’s DEP 

further reduces the impact of the building operations by purchasing 100% utility-based renewable energy. 

The selection of finishes, including carpets, walls, furniture, and paints, was based on recycled content 

and low emissions. 

The integrated energy design of this all-electric building produced an energy saving of 40% and an energy 

cost saving of 43% compared to ASHRAE Standard 90.1-2001. The lighting and HVAC efficiencies 

accounted for most of the savings. Some daylighting was used, but saved minimal energy. The PV array 

covers about 40% of the roof and provides approximately 2.7% of the annual energy. Operational 

problems with the PV system have been corrected and the energy production is expected to double. The 

complete case study (Deru et al. 2005) is available at www.nrel.gov/docs/fy05osti/34931.pdf. 

6 
Figure 2-3 South elevation of the Cambria Office Building 

2.4 Philip Merrill Environmental Center, Chesapeake Bay Foundation 

The Chesapeake Bay Foundation (CBF) in Annapolis, Maryland, is dedicated to restoring and protecting 

the resources of the Chesapeake Bay. CBF built the 31,000-ft2

 (2,900-m2) Philip Merrill Environmental 

Center on previously disturbed land. Native and recycled materials were used throughout the 

construction. 

CBF uses a ground-source heat pump system for heating and cooling. Forty-eight wells, each 300 ft 

(91 m) deep, use the ground as a heat sink in the summer and as a heat source in the winter. A glazed 

wall of windows on the south contributes daylight and passive solar heating. Operable windows are used 

for natural ventilation; the natural ventilation can be augmented with fans. The shed roof collects 

rainwater for fire protection, landscape watering, and clothes and hand washing (Figure 2-4). 

Composting toilets also minimize water use. 

Figure 2-4 North elevation of the Philip Merrill Environmental Center 

CBF was assessed by comparing measured performance to ASHRAE Standard 90.1-2001. For the 

monitoring period, the total site energy use saving was 24.5%, the source energy saving was 22.1%, and 

the energy cost saving was 12.1%. The complete report (Griffith et al. 2005) is available at 

www.nrel.gov/docs/fy05osti/34830.pdf. 

2.5 Thermal Test Facility 

The TTF at NREL in Golden, Colorado, is a 10,000-ft2

 (930-m2) steel-frame building that is typical of 

many small professional buildings, industrial parks, and retail structures. The building was designed to 

7 
be an example of an economical and simple low-energy building. Efficiency features include extensive 

daylighting through clerestory windows, two-stage evaporative cooling, overhangs to minimize summer 

solar gains, T-8 lamps, instantaneous water heaters, and a well-insulated thermal envelope (Figure 2-5). 

Figure 2-5 Thermal Test Facility at NREL 

The integrated design and energy features of the TTF have resulted in an energy cost saving of 51% and a 

site energy saving of 42% as compared to the Federal Energy Code 10 CFR 435 (DOE 1995). 

Daylighting provided the most significant energy saving. The lighting design and daylight harvesting 

reduced lighting energy by 75%. In this dry climate, two-stage evaporative cooling provides sufficient 

cooling capacity for less energy than conventional cooling systems. The building was built at a similar 

cost to a conventional building; however, an overall evaluation of the project by an independent estimator 

showed an approximate increase of 3.5% because of the energy features. Like the other projects, some of 

these features were integral to the architecture of the building. The complete case study (Torcellini et al. 

2005b) is available at www.nrel.gov/docs/fy05osti/34832.pdf. 

2.6 BigHorn Home Improvement Center 

The BigHorn Home Improvement Center in Silverthorne, Colorado, consists of an 18,400-ft2

 (1,710-m2) 

hardware store retail area and a 24,000-ft2

 (2,230-m2) warehouse (Figure 2-6). The owner was committed 

to using renewable energy and a building design optimized for minimal energy use. Reduced internal 

gains from lighting and smart envelope design in the retail area allowed for the use of natural ventilation 

to meet all cooling loads. The lighting load is reduced by extensive use of daylighting facilitated by the 

switching arrangement of the fluorescent lights. The retail area uses a hydronic radiant floor system with 

natural gas-fired boilers. An EMS controls the lights, natural ventilation, and heating system. A 

transpired solar collector and gas radiant heaters heat the warehouse. 

Figure 2-6 BigHorn Home Improvement Center 

8 
The integrated design of BigHorn yielded source energy savings of 54% and energy cost savings of 53%; 

annual energy costs were $0.43/ft2

 ($4.63/m2) as compared to ASHRAE Standard 90.1-2001. The 

lighting design and daylighting reduced electric lighting use by 93% in the warehouse and 67% in the 

retail and office areas. The PV system provides 2.5% of the annual electrical energy; the highest monthly 

percentage was 7.3% in July 2002. The additional building cost was approximately 10% greater than 

conventional construction. Some of the energy-related costs substantially enhanced the architecture of the 

building. The complete case study (Deru, Torcellini, and Pless 2005) is available at 

www.nrel.gov/docs/fy05osti/34930.pdf. 

2.7 Energy Performance Summary 

Several performance metrics are used to summarize the annual energy results in the six case studies. The 

absolute energy performance is summarized in terms of energy use intensity (EUI) and energy cost 

intensity (ECI) (see Figure 2-7). EUI and ECI are normalized by the building floor area. The net source 

EUI and ECI numbers include PV; site EUI does not. When considering net source EUI, keep in mind 

that Oberlin, Zion, and Cambria are all-electric buildings; CBF is primarily electric with propane backup; 

and TTF and Bighorn use natural gas heating systems. 

0

20

40

60

80

100

120

140

Site Energy Use Intensity

(EUI)

Net Source Energy Use

Intensity (EUI)

Energy Cost Intensity (ECI) 

Energy Use Intensity (kBtu/ft2·yr) 

and Energy Cost Intensity (¢/ft2·yr)

Oberlin

Zion

Cambria

CBF

TTF

BigHorn

Figure 2-7 Summary of energy performance metrics

Energy-efficient technologies are not the only influences on energy performance for these buildings. 

Although low site EUI is directly related to the efficiency measures in each building, source EUI and ECI 

are influenced by external factors. As shown in Figure 2-7, site EUIs over all the buildings range from 

27 kBtu/ft2 to 40 kBtu/ft2 (307 MJ/m2

 to 454 MJ/m2); ECIs and net source EUIs have a much greater 

distribution over the buildings. ECIs vary widely, ranging from $0.35/ft2

 to $1.04/ft2 ($3.77/m2 to 

$11.19/m2). The lowest source EUI is 53 kBtu/ft2 (602 MJ/m2) and the highest is 124 Btu/ft2

(1,408 MJ/m2

). Differences in utility rate structures, peak demand profiles, fuel types and associated 

source energy conversions, and on-site production contribute to the variability in ECI and source EUI 

metrics, despite comparatively consistent site energy use among the buildings. 

All three metrics are necessary for determining and comparing the energy performance of these buildings. 

However, the relative importance of each metric depends on the building’s design goals. TTF, BigHorn, 

9 
Cambria, and Zion were designed to reduce energy costs. Oberlin was designed to reduce net source 

energy. CBF was designed to obtain a LEED V1.0 Platinum rating. Further comparison of operating 

energy performance to design goals is provided in Section 3.1.2.2. 

Energy savings provide another layer of performance metrics to consider when evaluating and comparing 

energy performance. Relative energy performance for each of the case studies is summarized in terms of 

energy savings in Figure 2-8. Energy savings are determined by comparing the actual energy use to a 

standard benchmark, such as the energy use of a conventional building that just meets a minimum 

building energy code. Figure 2-8 shows net source, net site, site, and cost savings for each building. The 

net source, net site, and energy cost savings metrics include PV production, while the site energy savings 

metric does not include PV. 

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Oberlin Zion Cambria CBF TTF BigHorn

Percent Savings

Percent Net Source Energy Savings

Percent Net Site Energy Savings

Percent Site Energy Savings

Percent Energy Cost Savings

Figure 2-8 Summary of savings performance metrics 

All these buildings are considered low energy, with a wide range of savings, depending on the metric 

selected. Zion saved 60% to 67% over all the savings metrics, and is one of the better energy performers 

in this set. Aggressive demand management controls at Zion, which none of the other buildings used, 

resulted in significant energy cost savings. Additionally, the integrated design process was successful at 

Zion, which integrated the HVAC and lighting systems with the building envelope. Oberlin had the 

greatest net source energy savings, primarily because of the large PV system. CBF set and achieved a 

design goal of a LEED V1.0 Platinum certification. Saving energy was a design consideration, but was 

not the primary design goal. 

These buildings were further investigated to determine additional PV system requirements to meet ZEB 

goals. PV system array area and system capacity requirements for meeting a site, source, cost, and 

emissions ZEB goals were analyzed in each case. See Appendix A for details of these ZEB definitions as 

applied to the case studies and a discussion of the design impacts of the definition used. 

10 
Table 2-1 Summary of Energy Use, Savings, and Design Goals for each of the 

Six Case Study Buildings 

Metric Oberlin Zion Cambria CBF TTF1 BigHorn 

Benchmark ASHRAE Standard 

90.1 Version 2001 19952 2001 2001 19952 2001

Energy Cost $/ft2 0.84 0.43 0.87 1.04 0.35 0.43

Site Energy 

Consumption 

kBtu/ft2

 

(MJ/m2

) 

29.8

(338)

27.0

(307)

36.8

(418)

40.2 

(457) 

28.5 

(324) 

39.5

(449)

PV Production 

kBtu/ft2

· 

(MJ/m2

) 

13.4

(152)

2.3

(26)

0.9

(10)

0.3 

(3) 

04 

(0) 

0.4

(5)

Net Site Energy 

kBtu/ft2

· 

(MJ/m2

) 

16.4

(186)

24.7

(281)

36.0

(409)

39.9 

(453) 

28.5 

(324) 

39.2

(445)

Net Source Energy 

kBtu/ft2

· 

(MJ/m2

) 

52.7

(598)

79.3

(901)

115.75 

(1314)

124.0 

(1408) 

65.7 

(746) 

71.3

(810)

Annual 

Performance3

PV Contribution to 

Site Energy, % 45% 8.5% 2.7% 0.7% 0%4 2.3%

Net Source Energy 

Saving, % 79% 65% 42% 22% 45% 54%

Net Site Energy 

Saving, % 79% 65% 42% 25% 42% 36%

Site Energy 

Saving, % 47% 62% 40% 25% 42% 35%

Savings6

Energy Cost 

Saving, % 35% 67% 43% 12% 51% 53%

Design Goal or 

Predicted 

Performance 

Net site 

energy 

use: 

0.0 

kBtu/ft2

 

Energy 

Cost 

Saving: 

80% 

Energy 

Cost 

Saving: 

66%7

LEED 

1.0 

Platinum 

rating 

Energy 

Cost 

Saving: 

70%8

Energy 

Cost 

Saving: 

60%8

Project Goal 

Comparison

Measured or 

Simulated 

Performance 

Net site 

energy 

use: 16.4 

kBtu/ft2

 

Energy 

Cost 

Saving: 

67% 

Energy 

Cost 

Saving: 

44% 

LEED 

1.0 

Platinum 

rating 

Energy 

Cost 

Saving: 

51% 

Energy 

Cost 

Saving: 

53% 

11 
Notes: 

1. TTF was monitored for select periods. Actual data were used to calibrate simulations. 

2. Code used was 1995 Federal Energy Code, 10 CFR 435 (DOE 1995). Based on ASHRAE 90.1-1989 

(ASHRAE 1989) with more stringent lighting power densities. 

3. TTF annual performance data are based on simulations verified with actual data and run with typical weather. All 

other annual performance data are based on monitored performance. 

4. No PV installed on building. 

5. The Cambria office building purchases 100% green power (nonhydro renewable energy); therefore, the source 

energy could be calculated assuming only a 9% loss for transmission and distribution (EIA 2005). 

6. Oberlin, Bighorn, TTF, and Cambria energy savings were calculated with simulations of as-built and base-case 

buildings with typical weather data. Zion and CBF savings calculated with measured data and base-case 

simulations run with measured weather data. 

7. The predicted energy costs were calculated before construction and may not indicate future performance 

because of volatile energy prices. 

8. Goal was set on savings excluding plug loads. 

12 
3 LESSONS LEARNED 

This section discusses the lessons learned from the six case studies. Findings are presented and 

categorized as they relate to the low-energy design process, low-energy building technologies, and 

evaluation techniques. The term lessons learned refers to positive and negative aspects of a project that 

have clear messages and might help subsequent low-energy building projects. They are intended as 

observations that can help identify best practices, either for improvements to these buildings, or concepts 

that worked and should be applied to future buildings. Readers should remember that these lessons and 

best practices must be considered together with the goals for saving energy. They should help to identify 

where the process of delivering buildings needs to be changed to promote and realize extremely lowenergy buildings. 

3.1 Applying a Whole-Building Design Process 

In the traditional building design process, the architectural team works with the owner to creates a 

building program that specifies the needs for the building and parameters that should be considered in the 

design. The architect designs the building to satisfy the program requirements, and then the project 

engineers design the electrical and mechanical systems and evaluate compliance with energy codes and 

acceptable levels of environmental comfort. However, because many important architectural decisions 

are set at this point, few changes can be made that would improve energy performance. Typically, no 

performance goals are established. The architect and engineers may try to design efficient systems, but 

they have little interaction or goals to direct the design, so the results are usually mediocre. 

In contrast to the traditional building process, the whole-building design process requires the team—the 

architect, engineers (lighting, electrical, and mechanical), energy and other consultants, and the building’s 

owner and occupants—to work together to set and understand the energy performance goals. The 

purpose of whole-building design approach is to enable the full design team to interact throughout the 

design process to fully understand the building systems interdependences. The full design team focuses 

from the outset on energy and energy cost savings. The process relies heavily on energy simulation, 

includes design charrettes involving all members of the design team, and establishes energy goals early in 

the process. The whole-building design process begins with predesign, where often little more than the 

building size, type, location, and use are known. To be effective, the process must continue through 

design, construction, and commissioning. 

The best time to develop performance targets is at the beginning, before any design concepts have been 

developed. Once everyone has committed to an energy saving goal, the process can be used to guide the 

team toward good decision-making and trade-off analysis without sacrificing programmatic requirements. 

Team members must commit to an energy goal early in the process so they thoroughly understand the 

interdependencies of the architecture and systems and can create more efficient and cost-effective 

buildings. To realize low-energy buildings, the project team must not only set a low-energy performance 

goal, but must commit to it. Each member is encouraged to find solutions and offer suggestions that 

benefit other disciplines, the process, and ultimately the building design. 

Whole-building energy simulations guided the designs of all the buildings. Energy simulation in predesign helps to set quantified goals, and identifies areas with high potential for energy savings and peak 

reductions. In addition, energy simulations are a crucial tool for understanding the impact of design 

decisions on energy performance. Experience has shown that the chances of energy analysis affecting the 

design of a building decrease rapidly as the design proceeds. 

During the commissioning and occupancy phases, the design team cooperates to ensure that integrated 

architectural features such as daylighting, managed solar gains, and engineered building systems are built 

and function as originally designed. Ideally, once the building is occupied, the design team supports the 

operators as they learn how the building is intended to operate. Finally, the team must understand how 

the building operates over time so this and future buildings can realize energy savings over the long term. 

13 
The “green” building industry has generally recognized the value of whole-building design. The Whole 

Building Design Guide (NIBS 2006) offers up-to-date tools, design guidance, and technologies related to 

applying the whole-building design process. Further details of the whole building design process have 

been previously documented, and are outlined in Table 3-1 (Torcellini 1999). 

Table 3-1 Ten-Step Process for Designing and Constructing Low-Energy Buildings 

Predesign 

1. Set specific and measurable energy performance goals, which may include percent energy 

savings, percent energy cost savings, and emission reductions. The entire team must 

understand these goals and how they are affected by design features. Also at this stage, 

the design team should develop a thorough understanding of the building site, local weather 

patterns, and building functional requirements. At this point, the design team should 

brainstorm energy solutions, especially those that affect the architecture. Each building is 

unique and will have a different minimization strategy. Energy simulation at this stage helps 

to set quantified goals, and identifies areas with high potential for energy savings and peak 

reductions. 

2. Create a base-case building model to quantify base-case energy use and costs. The basecase building is solar neutral (equal glazing areas on all wall orientations) with equivalent 

floor area and meets the requirements of applicable energy efficiency codes such as 

ASHRAE Standard 90.1 (ASHRAE 2004). When building designs and operations change, 

the baseline has to evolve along with the building under consideration. 

3. Complete a parametric analysis of the base-case model to determine sensitivities to specific 

load components. Sequentially eliminate loads such as conductive losses, lighting loads, 

solar gains, and plug loads from the base-case building. 

4. Develop preliminary design solutions. The design team brainstorms possible solutions, 

which may include strategies to reduce lighting and cooling loads by incorporating 

daylighting or to meet heating loads with passive solar heating. 

Schematic Design 

5. Incorporate preliminary design solutions into a computer model of the proposed building 

design. The energy impact and cost effectiveness of each variant are determined by 

comparing the calculated energy performance with the original base-case building and to the 

other variants. Variants with the most favorable results should be incorporated into the 

building design. 

6. Prepare a preliminary set of construction drawings. These drawings are based on the 

decisions made in Step 4. Architectural decisions made during the schematic design phase 

can have the greatest impact on the long-term building energy performance. 

Design 

Development 

7. Identify the HVAC system that will meet the predicted loads. The HVAC system should 

complement the building architecture and exploit the specific climatic characteristics of the 

site for maximum efficiency. Often, the HVAC system capacity is much less than in a typical 

building. Verify that baseline and design simulations are updated with design changes. 

Construction 

Documents and 

Bid 

8. Finalize plans and specifications. Ensure that the building plans are properly detailed and 

that the specifications are accurate. The final design simulation should incorporate all costeffective features. Savings that exceed 50% from a base-case building are frequently 

possible with this approach. 

Construction 9. Rerun simulations before changes are made to the design during construction. Verify that 

changes will not adversely affect the building’s energy performance. 

Postoccupancy 

Evaluation 

10. Commission all equipment and controls. Educate building operators. Only a properly 

operated building will meet the original energy efficiency design goals. Building operators 

must understand how to properly operate the building to maximize its performance. 

Measure and evaluate actual energy performance to verify design goals were met. 

14 
Various styles of the whole-building design process were used to produce the buildings documented in 

the case studies. For Zion, BigHorn, and TTF, NREL was directly involved in the design process, 

providing energy analysis and facilitating the design process as it applied to energy performance. The 

whole-building design process was first documented and tested as part of the TTF design, and then further 

refined during the Zion and BigHorn designs (Torcellini et al. 1999). Oberlin, Cambria, and CBF each 

had a design teams consisting of energy consultants, architects, engineers, and whole-building design 

process facilitators. Results from POE of the buildings generated lessons learned and revealed common 

roadblocks in applying the process. 

3.1.1 Owners provide the main motivation for low-energy buildings 

The owner was the driving force in each case. The architect, engineer, or energy consultant alone did not 

provide the impetus for achieving a low-energy building. These low-energy designs required a 

committed owner who was willing to lead an integrated design process. Each project benefited from an 

owner who set aggressive goals and made educated decisions to keep the project on track. The owners 

pushed everyone involved in the design process to meet these goals. The architects and engineers had 

then to meet the goals and implement the vision. Additionally, the owners in each case were the primary 

decision makers in the low-energy design process. Dedicated owners made educated design decisions, 

and architectural and engineering firms were willing to use a whole-building design process to achieve 

low-energy goals. We can reasonably conclude that future owners and developers will be primary players 

in the industry and should be targeted with future research and development dollars for low-energy 

buildings. 

3.1.2 Setting measurable goals is crucial to achieve low-energy buildings 

The design of most buildings is typically driven by a set of minimum criteria including budget 

constraints, time scheduling, functionality requirements, safety regulations, and energy codes. This 

process typically produces buildings that meet only these criteria. The design team needs to focus on 

measurable energy performance goals to achieve better than average or exceptional performance. The 

goal-setting process should begin as early in the design process as possible for ease of implementation 

and best results. 

Establishing concrete energy goals early enabled the design teams to achieve greater energy savings. 

Concrete goals provided a true target for the design and a way to evaluate success. Measurable 

performance goals translate into efficient building performance. Setting such goals enables owners to 

make decisions that align with the goals. From the outset, all the owners and design teams set aggressive 

energy-saving goals that ranged from 40% better than code to net-zero energy performance. Some also 

had ambitious goals about other dimensions of sustainability such as water management, building 

materials selection, or obtaining a high LEED score. 

Effective goals are clearly stated and understood by the design team, construction staff, and occupants, 

and progress toward them should be measurable. Goals that are not clear and measurable are open to 

interpretation, which limits their effectiveness. Measuring success requires baselines or benchmarks and 

performance metrics to quantify the progress. Well-defined performance metrics allow the design team to 

easily evaluate its success throughout the design and operation of the building (Hitchcock 2003). 

3.1.2.1 Goal-setting process 

The process starts with a vision statement, such as: 

This project will design and construct a building that can be operated to provide a healthy and 

productive work environment and minimizes the use of nonrenewable material and energy 

resources in a cost-effective manner. 

15 
The vision statement should be used throughout the life of the building to help guide key decisions about 

material selection, design, and system operation and maintenance. It is a broad declaration about the final 

performance, which should be broken down into topic areas that can be implemented by the design team 

and later used by the operators to assess performance. For example, the above vision statement can be 

divided into the following areas: environmental loading, economics, service quality, and occupant health 

and safety. Each topic area should be further divided into subtopics. Examples of topic and subtopic 

areas are shown in Figure 3-1. The Whole-Building Design Guide (NIBS 2006) provides another example 

of dividing building performance into eight “objective areas” with further division into subtopic areas. 

Figure 3-1 Schematic of the performance-based design process 

Creating a single goal across topic areas is difficult. For example, water and energy use cannot be 

aggregated into a single value. However, financial comparisons, such as the costs of water and energy, 

can be made. The LEED rating system uses implicit weighting factors to combine the subtopic areas into 

a single performance indicator (USGBC 2004). Other tools such as GBTool and MCDM-23 rely on userselected weighting criteria to combine subtopic metrics into aggregate performance indicators (iiSBE 

2000, IEA 2002). 

The next step in the goal-setting process is to define objectives—general statements about the desired 

outcome—for each subtopic area. Specific goals are then developed from these statements. Developing 

16 
goals is often an iterative process during the conceptual design phase. Setting realistic goals involves 

engineering and economic analysis to determine what is possible and how much it will cost. 

Once the goals are established, the design team can identify obstacles and find solutions. This process 

can include computer simulations, consultations with technical experts, communications with product 

manufacturers, and research into similar projects. The goal-setting process can be summarized as 

follows: 

1. Develop a vision statement as a guide for design, construction, and operation. 

2. Divide the vision statement into topic and subtopic areas to address specific details. 

3. Define the objectives for each subtopic area. 

4. Establish clear and measurable goals (may be an iterative process). 

5. Define performance metrics to measure the progress toward achieving the goals. 

6. Develop and carry out a plan for monitoring performance throughout the design and operation of 

the building. 

The final, most important step requires the most effort and is often not carried out completely. If the 

performance is not verified, there is no way to know whether the goals are satisfied or how the building is 

performing. The monitoring plan should specify responsible parties and reporting requirements. The 

basic tenet behind this design process is that “you get what you ask for.” When a clear vision of the 

desired outcome is broken down into objectives and goals, there is a greater chance for producing a highperformance building. 

3.1.2.2 Buildings performance driven by design goals 

An important lesson learned from these projects is that performance is driven by design goals (Deru and 

Torcellini 2004). The design performance goals and measured building performance of the six projects 

are summarized in Figure 3-2 and Table 3-2. 

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Oberlin Zion Cambria CBF TTF BigHorn

Energy Savings .

Oberlin Net Source Energy Saving Design Goal

Net Source Energy Saving

Site Energy Saving

Energy Cost Saving

Energy Cost Saving Design Goal

Figure 3-2 Energy savings compared to design goals 

Note: PV production is included in the energy savings numbers, except for site energy savings. 

17 
Table 3-2 Performance Goals and Measured Performance 

Performance Category 

Building Performance Goals Net Energy 

Cost 

Net Energy 

Use 

Water

Use 

Material 

Selection 

Site 

Impact 

Oberlin Net zero energy building 

Zion 70% energy cost saving 

Cambria 

LEED Gold, 66% energy 

cost saving, 33% water 

saving, material 

selection, low site impact 

CBF 

LEED Platinum, 50% 

energy cost saving, low 

water consumption, 

material selection, low 

site impact 

TTF 70% energy cost saving 

BigHorn 60% energy cost saving, 

reduce site impact 

High Performance 

(50% or greater savings) 

Better Performance 

(20% to 49% savings) 

Standard Performance 

(0% to 19% savings) 

Note: Water use, material selection, and site impact metrics were not evaluated in the separate case 

studies; these performance categories were evaluated based on design documents and/or LEED 

submittals. 

All the buildings had aggressive energy saving goals, and each performs well with respect to the design 

goals. Three saved more than 50% in energy costs; Oberlin and Cambria saved less than 50% in energy 

costs. Cambria and CBF had aggressive water use reduction goals that strongly influenced the design 

process and goals for materials selection, which guided the selection of materials considered sustainable 

within the LEED framework. However, studies indicate that buildings that earn more LEED credits do 

not necessarily provide more environmental benefits than those that earn fewer credits 

(Stein and Reiss 2004). The United States Green Building Council (USGBC) is currently researching 

life-cycle assessment methods to enhance the LEED rating system to account for long-term energy and 

environmental impacts over the life of the building. 

A building that meets its goals might not fare as well compared to other buildings that used other metrics 

for goals. Each building performs with varying levels of success, depending on the metric the owner 

uses. As an example, CBF had a goal of constructing a LEED 1.0 Platinum building. It met this goal; 

however, energy was not a primary goal. The building is a good energy performer, but its energy use is 

greater than that of some of the other buildings. Oberlin had the most aggressive energy goal, which is 

evident in the results, but did not do as well in the energy cost category (which was not its goal). 

Oberlin’s goal was a net-zero energy building, which would result in 100% net source energy savings and 

100% net site energy savings. The building was not designed for high energy cost savings, which is 

shown in Figure 3-2. Figure 3-2 shows that the selecting site, source, or energy cost as a design goal can 

greatly affect the performance of the building. The design goals are also evident in the management 

philosophy. For example, Oberlin does not aggressively manage demand, and the large PV system does 

not reduce the substantial demand charges that affect overall costs. Zion is at the other end of the scale 

with few demand charges. 

18 
The design teams for Zion, TTF, BigHorn, and Oberlin established low energy as a priority at the outset. 

As shown in Figure 3-2, these aggressive energy goals produced buildings with excellent energy 

performance. In general, each building performs well in the areas that had clear goals set from the 

beginning of the design process. The performance in the other sustainability areas is slightly better than 

standard practice. These projects show that defining specific goals helps the design team achieve better 

results. 

3.1.3 Cost justification of integrated design 

Building owners make decisions based on values. Quite often they pay for features—especially 

architectural ones—they really want. Conversely, if an owner does not want a feature, cost is often used 

as the reason to eliminate it. In addition to energy cost savings, cost effectiveness should be expanded to 

include marketability of a green building, image, disaster resistance (for example, the ability to function if 

no grid power is available), and other value-added features. Each case study can justify low-energy 

solutions by means other than standard payback techniques. Regardless of the criteria for making 

decisions, all team members must understand the goals. The best way to reduce the cost of low-energy 

features and ensure successful integration is to incorporate them into the architecture early. If low-energy 

features such as daylighting clerestories or cooltowers can be integrated into the architectural statement, 

they can be included as part of the architecture costs. 

The cost of low-energy features can be reduced by using the architectural design and envelope to save 

energy and enhance comfort. Architectural features such as form, shading, space layout, envelope 

constructions, materials, sizing, orientation, and glass all have significant impacts on energy, lighting, and 

comfort. The total cost to construct the Zion Visitor Center was less than originally budgeted. 

Integrating energy features into the envelope at Zion increased total envelope cost; however, reducing 

infrastructure and mechanical systems (which included eliminating the mechanical room and duct work) 

reduced costs. 

Low-energy buildings often defy industry norms for equipment sizing, such as ft2

/ton. Lower loads 

resulting in downsized equipment can substantially reduce construction cost and the savings can often be 

used to pay for other energy features. There will be some trade-off during the design process between 

mechanical system decisions and architectural features. Although the energy design process may increase 

costs to design the building compared to the traditional design process, the increased cost is often offset 

by a reduction in errors and decreased mechanical system cost. Fewer errors occur because careful 

attention was paid throughout the design process and more emphasis is placed on checking and review. 

In addition, small mechanical systems require less space in the building, which reduces construction 

costs. The best example of this was at BigHorn. Internal gains were reduced by integrating daylighting, 

enhanced insulation, and natural ventilation into the envelope design. This eliminated the need for a 

conventional air-conditioning system. 

The BigHorn owners have successfully marketed the green image of the building to offset additional 

project costs. The construction costs were approximately 10% higher than for a conventional building. 

Some were energy related, but these were an integral part of the architectural image. Anecdotal evidence 

suggests that increased costs for low-energy features have been offset by increased customer traffic. The 

project received significant press coverage as the first low-energy hardware store in the area. This 

publicity increased customer traffic at the old store, even before the new “green” hardware store was 

constructed. The low-energy design has become a value-added feature, as it resulted in free advertising 

for BigHorn. 

At Zion, low-energy features are integrated into the high-value UPS. National Park Service staff 

members were concerned about the unreliable power at the park. A UPS was included in the original plan 

for the electrical system, which was designed to allow a grid-tied PV system to offset energy use and 

provide battery charging capacity during power outages. The PV-integrated UPS circuit allows the 

building to remain operational during daylight hours when no power is available by providing power to 

19 
the computer equipment, cash registers, window actuators, cooltower pumps, telephone switch, and BAS. 

The daylighting system provides enough light during most typical business hours. This value-added 

system allows the Visitor Center to continue normal business activities in a comfortable, lit space during 

the frequent power outages when every other business in the area is inoperable. 

Another value-added feature at Zion is the cooltowers. Most cooling systems are hidden and designed to 

be inconspicuous, but visitors to Zion congregate around the cool air outlets. They find the towers 

fascinating and give them the type of attention often given to large fireplaces in public areas. This 

interaction provides an unexpected amenity that a traditional cooling system would not. 

3.1.3.1 Construction costs 

This set of low-energy buildings represents a wide range of building types in different climates, at varying 

construction costs. Construction costs ranged from $93/ft2

 ($1,001/m2) to more than $400/ft2

($4,306/m2

), primarily because of programmatic requirements. The higher cost buildings such as Zion 

and Oberlin were more expensive—not solely because of energy features, but because of project 

requirements such as long life, high durability, or specialized laboratory spaces. In many “green” 

buildings, other architectural elements add to the cost, and separating architectural amenities from the 

“green” elements is difficult. 

At a construction cost of $93/ft2

 ($1,001/m2), the Cambria office building shows that extra attention can 

be paid to systems and materials to build a “green” office building within the same cost range as a 

conventionally constructed one. The building is economically viable in this location, as it was developed 

by a private developer who was looking for a good return on investment. This kept the costs in check. 

Experience with the TTF project showed that a low-energy commercial building can be constructed 

within the constraints of a fixed budget. Although construction analysis shows a 3.9% increase in costs 

directly related to energy efficiency improvements, the costs would probably have reached the maximum 

allowed under the fixed budget had the building been built without regard for energy consumption. The 

TTF represents an economically practical approach to achieving high performance, with construction 

costs at $119/ft2

 (1995 dollars) ($1,281/m2) and 52% energy cost savings. 

3.1.4 Using energy modeling throughout process 

Whole-building energy simulations guided the designs of all the buildings. Energy simulation in predesign helped to set measurable goals, and identified areas with high potential for energy savings and 

peak reductions. The design team used energy models to determine the minimum energy targets, 

incorporate the climate and the program goals, evaluate envelope options and architectural designs, and 

design mechanical systems that matched the predicted loads. Finally, NREL used simulations to evaluate 

postoccupancy performance and verify design goals. Simulation tools used for design analysis and POEs 

for these buildings included DOE-2.1E, DOE-2.2, and EnergyPlus. 

The energy use and costs of a building depend on the complex interaction of many parameters and 

variables. The problem is far too complex for rules of thumb or hand calculations. Computerized energy 

simulation software can thoroughly evaluate all interactions between the envelope, HVAC system, and 

design features. The best time to develop performance targets is at the beginning, before any design 

concepts have been developed, because energy analysis affects design less and less as the design 

proceeds. Yet energy analysis, if done at all, typically starts about the time of design development and is 

typically used for code compliance or selection of HVAC type—not for designing the building envelope. 

An “elimination” parametric analysis helps designers understand the sensitivity of total energy 

performance at specific loads. A parametric analysis is performed by eliminating loads—conduction 

losses, people, solar gains, and plug loads—one at a time from the simulation. As loads are eliminated, 

the designer determines their impact on the total energy performance by comparing energy use with and 

without each load. For example, if eliminating all conductive heat transfer through the envelope has little 

20 
effect on energy consumption and energy costs, there would be little reason to increase insulation levels 

to exceed code. Similarly, the exercise may demonstrate an upper limit to the amount of insulation before 

internal loads begin to increase cooling loads. The design team may discover that finding an optimal 

insulation level allows money otherwise spent on additional insulation to be used elsewhere, with a 

greater impact on the total energy picture. If, however, eliminating all solar gains greatly affects energy 

performance, solar-related issues such as window area, orientation, window solar heat gain coefficient, 

and facade shading geometry (such as window overhangs) are worth exploring. An energy goal for the 

climate and building type should be established based on this analysis, and all team members should 

agree on its feasibility. 

In all cases, energy simulations played an important part in understanding the forces that drive energy 

performance and allowed many design alternatives to be investigated. At BigHorn, additional windows 

were added that vastly improved daylighting in the building. At the TTF, clerestory heights and 

overhangs were optimized, minimizing cooling loads while maximizing daylighting. 

3.1.4.1 Setting energy baselines in the low-energy design process 

A standard performance metric for low-energy buildings is often percent savings. The first problem with 

this is determining the baseline or reference point for comparison. Most often, baselines are based on a 

comparable code-compliant building. For the six buildings studied, the design predictions used baselines 

that included ASHRAE 90.1-1989 (ASHRAE 1989), 1999 (ASHRAE 1999), 2001 (ASHRAE 2001), and 

10 CFR 435 (DOE 1995). For consistency among the case studies, actual energy savings were 

determined for most of the buildings in comparison with ASHRAE 90.1-2001. 

An essential element of the case studies has been defining and analyzing baseline models that are used to 

determine energy and energy cost savings. We used baselines to analyze energy performance for 

buildings from predesign stages to postoccupancy as-built buildings. When building designs and 

operations change, the baseline has to evolve along with the building under consideration. 

Three classifications of baselines were used depending on the design progress and purpose of the baseline 

(see Table 3-3). The baseline classifications used include: 

• The Predesign Baseline is used in the initial stages of the low-energy design process. This is a 

theoretical building based on basic information known about a proposed building in the pre-design 

phase. Basic inputs of building function, size, and location are used to define the predesign baseline, 

which is then used to estimate annual loads and peak electrical demands for heating, cooling, 

lighting, plug loads, and HVAC system fans and pumps. In general terms, a pre-design baseline 

building is “solar neutral.” For a typical commercial building, a predesign baseline design is a 

rectangular floor plan with an aspect ratio of 1.75 that uses a simplified zoning scheme with 

windows distributed equally at all four cardinal orientations. An “elimination” parametric analysis is 

performed to help building designers understand the driving forces and sensitivities of energy 

performance and energy costs in the climate under consideration. 

• A Proposed Design Baseline is used to determine the energy performance of a building during the 

design process. It represents a minimally code-compliant version of the proposed design. When the 

design of the proposed building has progressed to the point where floor plan, layout, and other 

physical fabric characteristics have been determined, the proposed design baseline is modeled in four 

orientations by rotating the baseline model 90° four times, and then averaging the results. This 

baseline includes features of the building design such as location, size, footprint, building use, fuel 

types, and expected schedules. Appendix G of ASHRAE 90.1-2004 provides a method for 

determining the annual energy performance of a proposed design baseline along with the proposed 

design (ASHRAE 2004). 

• The Existing Building Baseline can be used to evaluate the energy performance of existing 

buildings. The need to verify design goals during occupancy is increasing, especially for the current 

21 
generation of low-energy commercial buildings. Often, the as-built building has significantly 

different physical and operational characteristics than the building design. Thus, the baseline 

comparison for the existing building must be updated to reflect actual operation during typical 

occupancy. Assumptions made for the design baseline, such as occupancy schedules, equipment 

loads, weather, and set points, are measured in the existing building and included in the existing 

building baseline. Further evaluation techniques for existing buildings are discussed in Section 3.7. 

Table 3-3 Baseline use in the low-energy design process 

Rendering of Baseline Models Design Process Baseline Use 

Solar Neutral Predesign Baseline 

• To develop an understanding 

of the building site, local 

climate, and functional 

requirements. 

• To help set measurable design 

goals that are reasonable and 

attainable. 

• To complete a parametric 

analysis to determine 

sensitivities to specific load 

components. 

Proposed Design Baseline and 

Existing Building Baseline

• To evaluate preliminary design 

solutions. Energy and cost 

effectiveness of each energy 

design feature is determined 

by comparing the proposed 

design to the Proposed Design 

Baseline. 

• To determine predicted 

savings of the finalized design, 

which incorporates all energy 

saving features to be included 

in the construction documents. 

• To ensure changes made 

during construction do not 

adversely affect the energy 

performance. 

• To verify the existing building 

meets the design goals. 

Schematic 

Design 

Design 

Development 

Construction 

Documents 

Construction

Rotated four times 

and results averaged 

Occupancy

Predesign and 

Programming 

3.1.4.2 Modeling in postoccupancy evaluations 

Whole-building energy performance modeling was essential for determining energy savings and 

developing recommendations to address identified problems. To calculate the energy savings of a 

building, a model must be calibrated against actual building data. In most cases, too many changes 

occurred from design to occupancy to use the design-based models as accurate predictors of energy 

consumption. Schedules and plug loads vary widely from original assumptions. The base-case model 

must be modified to reflect the as-built schedules and plug loads. A calibrated as-built simulation 

compared to a conventional base case can provide a confident prediction of annual site, source, and cost 

22 
savings. Using typical meteorological year weather data allows long-term savings calculations with 

relatively short-term data. 

Energy saving uncertainties can be minimized when savings are determined from the comparison of one 

simulation to another (e.g., base-case to as-built). Because difficult-to-know inputs are held the same in 

both simulations, such comparisons remove much of the uncertainty inherent in an hourly building energy 

simulation. Variables that change throughout the year, such as inconsistent occupancy, set point changes, 

and equipment performance degradation, are difficult to account for in an annual building energy 

simulation. Comparing a base-case model to an as-built model with the same schedules reduces the 

uncertainty. Further lessons learned in using simulations for design and evaluation purposes are 

discussed in Section 3.7. 

3.1.5 Common roadblocks to successful integrated design process implementation 

Various styles of the whole-building design process were used to produce each building documented in 

NREL’s case studies. For Zion, BigHorn, and TTF, NREL was directly involved in the design process, 

providing energy analysis and facilitating the design process as it applied to energy performance. Results 

from each POE performed by NREL provided lessons learned and common roadblocks in applying the 

process. 

3.1.5.1 The energy models are not updated as design development progresses 

The simulations from predesign through occupancy must be maintained. Important changes to the design 

and siting of the Zion Visitor Center occurred late in the design development stage. Changes to the 

locations and orientations of offices reduced energy performance. For example, offices were located 

adjacent to the Trombe wall and far from the cooltowers, which caused localized comfort problems. In 

addition, cooltowers were removed. Had each design change been reanalyzed with detailed simulations 

and updated thermal zoning, the comfort problems may have been identified and corrected (e.g., by 

adding cooltowers that had been removed back into the design or moving the offices). 

3.1.5.2 Energy features are incorrectly installed 

We uncovered many examples of design features that were missing, installed incorrectly, or a lower 

quality product substituted. For example, the TTF foundation was missing insulation that designers 

specified. Perimeter slab insulation was specified on the inside of the foundation for ease of installation; 

however, this was not installed during construction. By the time it was discovered, adding it would have 

been too costly. As a result, a 6-in. (15-cm) thermal bridge around the perimeter of the concrete slab 

affects occupant comfort. We also determined that the construction team did not install the window and 

door frame thermal breaks as specified. The reasons are unclear for the error, but there are increased 

thermal losses. Door and window frames must be carefully specified with resistance values, and 

construction management must have a mechanism for evaluating such products in the field to ensure they 

are installed according to design. 

At Zion, the electrical lighting circuiting was not installed as designed, which limited the daylighting 

control strategy. The stepped daylighting controls have been problematic, as the stepped control resulted 

in uneven distribution and occupant complaints of on-off sequencing. The design of the stepped 

operation called for every other fixture to be on a separate circuit. Instead, every other row was installed 

on a separate circuit, which resulted in uneven light distribution during stepped control operation. 

Inspecting buildings with vigilant field verification of how the building is constructed with a concern for 

the building’s energy features can help to ensure that intended energy features are actually included in the 

building. An understanding of how the building differs from its design can be invaluable in assessing the 

reasons why the building displays a certain level of performance. In the case of TTF, knowing that proper 

foundation perimeter insulation was not installed helped designers understand how envelope heat losses 

could affect heating and comfort conditions. 

23 
3.1.5.3 Problems simulating innovative features in an integrated whole-building model 

A common limitation of the simulation tools was calculating energy costs. This was most evident when 

we tried to assess the impact of PV. To fully realize the cost implications in commercial buildings, PV 

must be integrated into the simulation engine to fully determine whether any demand savings are 

achieved. By using a simulation tool that models energy use, PV production, and peak demand on a 15-

minute time step, we can research and develop demand-responsive controls that enable on-site PV 

production to reduce peak demand and optimize energy costs. Neither DOE-2.1E nor DOE-2.2 can 

model PV power production directly. 

In the analysis of Zion, uncertainties with Trombe wall thermal models, difficulties modeling the as-built 

operation of subhourly demand-responsive controls with integrated PV production, and lack of buildingintegrated modeling techniques for the energy use and cooling capacity of cooltowers prevented the 

simulation of an as-built model of the Visitor Center and Comfort Station. With capabilities available 

today in EnergyPlus, models could be developed for further study of demand-responsive controls with onsite generation, water use in evaporative cooling systems, and enable an as-built to base-case simulation 

comparison for determining energy and cost savings (EnergyPlus 2005). These capabilities also enable a 

more realistic prediction of peak demands, which can be greater than 50% of the total energy costs. 

3.1.6 Measured versus predicted performance 

Comparing the measured performance to the design phase predicted performance is useful to the 

designers and energy researchers. The designers want to know how well the energy-efficient design 

features perform so they can design better buildings. The energy researchers are interested in how well 

the energy-efficient features perform and want to know how well the energy simulations can predict 

performance. Our experience with the low-energy buildings shows a disconnect between actual buildings 

and the simulated predictions. We have observed that real buildings, when compared to the design 

simulations, use more energy, produce less power with the PV system, and have worse controls. 

All six buildings used more energy and produced less energy than the design simulations predicted. 

Often this is due to the assumptions made in the simulations on building operation and schedules. These 

always vary from actual building operation. Simulations create idealistic controls—actual performance 

showed different set points and less setup and setback of space temperatures. Another reason is the lack 

of innovative controls for the innovative technologies. Appropriate control systems can successfully 

integrate the low-energy technologies and allow a building to operate at its design potential. The key is to 

maintain controls that function in harmony with building design intent. 

Another problem is that many energy-saving technologies have backups, so when they fail, there is no 

motivation to maintain the technologies or even verify their operation. Examples include natural 

ventilation and CO2 sensors at Oberlin that are not used, a desiccant heat recovery system at CBF that is 

not used, daylighting controls that are improperly used in some cases (lights are on when daylighting 

provides enough light). Additionally, PV systems were down for months at a time at Cambria and 

BigHorn because occupants were unaware of system status. See Section 3.4 for further discussion of 

performance degradation in PV systems. When these systems fail, detailed monitoring is required to 

detect the failures and ensure proper operation. 

Even though each building is a good performer, the energy performance was less than expected during 

design. Design teams were a bit too optimistic about the behavior of the occupants and their acceptance 

of systems. Energy consumption was higher and energy production was lower than simulations 

predicted. In particular, daylighting saving was less than predicted, which meant more electrical lighting 

was used. See Section 3.2 for a discussion of daylighting energy savings. 

In addition, plug loads were often greater than design predictions. The results from the CBF analysis 

indicate that the performance predictions made during design development were too optimistic, primarily 

from underestimating the amount of electricity drawn by plug loads. Plug and miscellaneous loads were 

24 
twice those assumed for the predictions. Underestimated loads included information technology 

equipment in the offices and server room and miscellaneous loads such as exterior lighting, mechanical 

room accessories, and the elevator. At BigHorn, a major omission in the design model was the lighting 

display area. During the design the lighting display area was not expected to be a large load; however, 

the installed display lights increased over time as light fixtures became large sale items. These data are 

hard to obtain in advance and need to be assumed for design. Efforts to improve the accuracy of 

predictions of whole-building energy performance during the design phase should focus on better 

methods of developing assumptions for receptacle and miscellaneous electrical loads. 

Total insulation values are often inflated during building design. Thermal bridging was partially 

accounted for in the models during the design process, but it is often an optimistic value compared to 

actual construction techniques. For example, the thermally broken window frames were not installed at 

TTF. In all the buildings, thermography indicated thermal leaks, especially at corners and areas where the 

building touches the ground—very difficult areas to insulate. In one case, a retaining wall was attached to 

the building for structural integrity and acted as a fin, reducing the integrity of the thermal envelope. 

These results are similar to those found by other researchers (Branco 2004, Norford 1994). 

3.1.6.1 Update design simulations from predesign through occupancy to ensure the design simulations 

will measure up to actual performance 

For Zion, an energy cost-saving goal of 70% was set at the beginning of the design process and used as an 

energy saving target throughout the process. Computer simulations showed that an energy saving goal of 

80% was achievable. However, actual measured data indicated that the building energy cost saving was 

67% because some construction and design changes were made before we could perform energy 

simulations to determine their energy impacts. Partway through the design development process step, the 

building site changed. The entrance to the building was relocated to the north side to facilitate pedestrian 

flow through the Visitor Center Complex, which allowed the south facade to be unobstructed and increase 

passive solar gains. The offices and break room were moved to the south side of the building, adjacent to 

the Trombe wall and far from the cooltowers. The number of cooltowers was reduced to three, two for 

the Visitor Center and one for the Comfort Station. The building engineer concluded that there would be 

enough airflow with fewer towers. For architectural reasons, the amount of north and west glass 

increased, although the tree canopy and building shading keep these surfaces shaded most of the summer. 

Finally, the outdoor cooltower was removed, so the two cooltowers were expected to condition inside and 

outside spaces. Had each design change been reanalyzed with detailed simulations and updated zoning, 

negative energy impacts may have been predicted and corrected. In addition, the design simulations 

would have more closely matched actual performance. The simulations must be maintained from 

predesign through occupancy to ensure the actual performance will match the design simulations and the 

energy goal. 

3.1.7 Monitoring leads to better management and improved performance 

Setting and following design goals does not guarantee that the goals will be satisfied in actual operations. 

The performance must still be tracked and verified. Many items were corrected because of the 

information gained through monitoring. The key is to maintain controls that function in harmony with 

building design intent. A good building quickly becomes a “bad” building with poor control strategies. 

Commissioning a new building is essential, but ensures only that the building components operate as 

specified. All six of these low-energy buildings were commissioned, formally or informally, before 

occupancy. Various degrees of continuous commissioning occurred during the postoccupancy monitoring 

as well. In general, the traditional commissioning process did not translate to expected energy savings, as 

we still found significant deficiencies. 

Continual monitoring of energy performance, or continuous commissioning with key energy-saving 

performance metrics, is important to ensure that the goals of the design are met under normal operating 

conditions. In all the buildings, adjustments in controls, equipment, and operation were made to better 

25 
align the performance with the design goals. Oberlin was a dramatic example of improved energy 

performance because of better information obtained from monitoring. Monitoring at Oberlin resulted in 

controls and equipment changes that reduced initial site energy use by 37%, from 47.5 to 29.8 kBtu/ft2

(539 to 338 MJ/m2

). The improvement in performance, shown in Figure 3-3, was due to correcting 

problem areas identified by the metering. Metering started in March 2001, reducing the initial excessive 

energy use as shown by the utility bills. 

0

200

400

600

800

1,000

1,200

1,400

1,600

Jan-00

Feb-00Mar-00

Apr-00

May-00

Jun-00

Jul-00

Aug-00Sep-00

Oct-00

Nov-00

Dec-00

Jan-01

Feb-01Mar-01

Apr-01

May-01

Jun-01

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Apr-02

May-02

Jun-02

Jul-02

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Dec-02

Jan-03

Feb-03

Daily Average Monthly Consumption and Production 

 (kWh/day)

Utility Bills

Equipment

Total Lights

Total Cooling

Total Heating

PV Production 

Figure 3-3 Oberlin monthly site energy use and production, January 2000–February 2003 

Achieving and maintaining a high-performance building requires a continual and concerted effort, which 

is absent in most buildings. Continually tracking building performance is expensive and requires a 

motivated staff. However, advances in metering technology, computerized communications, and 

automated controls are reducing the costs of monitoring building performance. Additional research is 

needed to further reduce costs, better optimize control strategies, and improve reliability to realize the full 

energy savings potential of high-performance buildings. 

3.2 Lighting and Daylighting Systems 

Lighting uses 30% of the total energy in commercial buildings, making it the largest single end use. 

From a national perspective, the potential for daylighting savings is significant. Based on the 

Commercial Building Energy Consumption Survey (CBECS) data set (EIA 2005a), nearly 80% of all 

commercial building floor area has an exterior ceiling or is within 15 ft (4.6 m) of an exterior wall, and 

therefore has the potential to be daylit. (See Appendix B for further discussion on determining the 

national daylighting potential in commercial buildings with the CBECS database.) Through toplit or 

sidelit daylighting designs, daylighting could be provided to a significant portion of the total floor area in 

the commercial building sector. With just toplit daylighting designs, 60% of all commercial buildings 

floor area has daylighting potential. Thus, lessons learned about applying daylighting design and controls 

to minimize lighting energy use are valuable to the industry and to future ZEB designs. 

26 
We believe daylighting is critical to high performance in buildings. All six buildings feature daylighting 

design, and all have experienced problems. Some lacked energy savings, experienced unacceptable glare, 

or had lower than anticipated lighting levels. Daylighting is a classic systems integration challenge; if 

five of the six elements necessary to good daylighting are done correctly, but one is not, energy savings 

may not result. Glazing and good lighting design are necessary, but are not enough to save substantial 

energy. Good daylighting can offer dramatic electrical energy savings benefits. Daylighting was the best 

resource for total energy savings at Zion, Oberlin, BigHorn, and TTF. 

Each daylighting and lighting system was evaluated as part of the POE. In each case, equipment or 

operational changes were made to the daylighting systems to increase energy savings and lighting quality. 

The goals of the daylighting evaluations in the case studies were to: 

• Determine the amount of electrical lighting offset by lighting design, daylighting, and occupancy 

controls compared to the baseline. 

• Analyze the operation of the lighting and daylighting controls and optimize their performance. 

• Document successes and weaknesses of the daylighting and lighting systems to expand the 

knowledge base in this area. 

The evaluation team performed an annual daylighting analysis based on a monitoring protocol developed 

by the International Energy Agency Solar Heating and Cooling Programme (IEA/SHC) Task 21 (Atif 

1997). This protocol offers guidelines for measuring daylighting performance, predicting performance, 

and evaluating control parameters. The performance measurement section of this protocol outlines 

recommended techniques for monitoring the daylighting contribution to indoor illuminance and the 

corresponding electrical lighting displacement. Per the daylighting analysis protocol, we measured 

horizontal illuminance in selected daylit zones during varying sky conditions for typical summer, winter, 

and fall/spring seasons. Recommended illuminance levels for each lighting zone were analyzed in 

accordance with the Lighting Handbook of the Illuminating Engineering Society of North America

(IESNA 2000). External horizontal illuminance and electrical lighting consumption must be 

simultaneously monitored to complete the daylighting measurements. 

3.2.1 Daylighting designs 

Toplit daylighting through clerestories and high windows is integral to each daylighting system. The 

daylighting design of all the buildings used clerestories, elongated east-west axes, and south glazing. 

South-facing clerestories are the primary sources of daylighting at Zion, Cambria, the BigHorn retail 

zones, and TTF (Figure 3-4), with some sidelit contribution. Sidelit daylighting is the primary 

daylighting source at CBF and Oberlin; north-facing clerestories are used for daylighting-specific zones. 

27 
Figure 3-4 TTF daylighting with clerestory

Daylighting is provided in the BigHorn warehouse primarily through skylights (insulated translucent 

glazing panels) along the ridgeline (Figure 3-5). One large dormer, two smaller dormers, and small 

windows on the south and east walls provide additional natural lighting to the warehouse space. 

Figure 3-5 Translucent skylights in the roof of the BigHorn warehouse 

The warehouse daylighting design is very effective, as it requires no electric lights during most of the 

daylight hours. The translucent skylights work well in this application where lighting is more important 

than the thermal issues of overheating in the summer and heat loss in the winter. This space was not 

cooled and only minimally heated. Daylighting strategies in the BigHorn retail area include north- and 

south-facing clerestory windows that run the length of the store; three dormer windows on the north side; 

high windows on the east and west ends; and a borrow window from the warehouse. The walls, floor, 

and vaulted ceiling of the retail area were painted white to distribute the daylight and make the space look 

brighter. 

Cambria’s east-west orientation is an important part of its daylighting system. Virtually all fenestration 

faces either north or south. The second floor is primarily open office plan and houses most of building 

occupants. Clerestory windows face north and south along the center of the building. The south-facing 

28 
clerestory windows are equipped with motorized sunscreens controlled by a photosensor to block directbeam radiation. Overhangs shade the second-floor windows on the south elevation. Light shelves are 

installed on the south-facing first-floor windows. These light shelves, combined with shading devices, 

help reflect the light to the ceiling plane and minimize direct gain through the view glass (see Figure 3-6). 

The interior finishes were selected to improve the light reflection and provide contrast. The first floor 

ceiling tiles have a light reflectance of 89%, the second floor has high vaulted white ceilings with an open 

truss construction, the bottom 2.5 ft (0.8 m) of the walls are a light, natural wood color, the top portion of 

the walls are painted off-white (light reflectance of 75%), and the cubicle dividers are off-white. The 

carpets and desktops are black (see Figure 3-7). 

Figure 3-6 Cambria first floor light shelves 

Figure 3-7 Cambria second floor clerestory showing dark floors and 

high vaulted white ceilings with an open truss 

29 
A combination of electric lighting and daylight illuminate TTF. The amount of electric light provided at 

any time depends on occupancy and the availability of daylight. The building’s clerestory or stair-stepped 

design is integral to the lighting plan. Daylight enters the building through a row of windows that line the 

south facade of the open office areas and two additional rows of clerestory windows in the mid- and highbays. To provide some bidirectional light, six 2.5-ft by 2.5-ft (0.76-m by 0.76-m) north windows are 

located in the high-bay. In addition, the service core south wall allows for reflection of mid-bay 

clerestory light into the office (low-bay) area. All windows were specified to maximize daylighting 

transmittance. NREL selected clerestory windows with a high visible transmittance of 72%. View glass 

windows in the open office area and conference room were chosen with a slightly gray tint with a visible 

transmittance of 38% to reduce glare. 

As shown in Figure 3-8, the primary source of light at Zion is daylight that enters through clerestory 

windows and a strip of windows high on the walls. Electric lighting provides additional light. We 

selected T-8 lights for most of the main open floor areas, with fixtures that are 88% indirect with 11% 

direct. 

Figure 3-8 Zion daylighting with fixtures that are 88% indirect and 11% direct 

At Oberlin, we calculated the percentage of daytime hours when daylighting alone could provide all the 

required illuminance (according to IESNA recommendations) to various spaces. The classrooms, shown 

in Figure 3-9, could be completely daylit for 52% to 73% of the daytime hours, depending on seasonal 

differences in available daylit and solar angles. The corridors and offices could be completely daylit for 

83% of the daytime hours over a year. Figure 3-10 shows the northern clerestory windows and secondfloor office windows.

30 
Figure 3-9 Oberlin second-floor daylit classroom 

Figure 3-10 Oberlin north facade 

Offices 

Clerestory 

3.2.2 Daylighting and lighting controls 

For daylighting to save energy, the electric lighting must be dimmed or turned off in response to natural 

light. All six of the daylighting systems had problems operating the lights in response to available 

daylight, from underdimming, overdimming, distracting on/off switching, or sensor installation problems. 

Each building used different daylighting control strategies: 

Cambria 

The luminaires in the open office areas are indirect fixtures with 32-W T-8 fluorescent lamps and an 

installed capacity of 0.75 W/ft2

 (8.1 W/m2). The LPD of the task lighting in the office areas is 

approximately 0.5 W/ft2

 (5.4 W/m2). The luminaires in the second-floor offices have dimmable ballasts 

controlled by lighting sensors in each office area. Under-cabinet task lighting in each cubicle is 

controlled by a motion sensor connected to a power strip. CFLs are used in other areas of the building 

such as the restrooms and lobby. Occupancy sensors are installed on the restroom lighting. Timing 

31 
circuits in the breaker boxes control the building ambient and exterior lighting systems with override 

switches near the main entrance. 

BigHorn 

Daylighting controls provided by the EMS use three light sensors (exterior, retail, and warehouse). The 

luminaires in the retail/office area and the warehouse use eight 42-W CFLs. The lamps can be controlled 

two at a time to provide five lighting level steps for each fixture as an energy-efficient way of matching 

the available daylighting. Sixteen circuits are available to control the lights: four are used for the exterior 

lights, four for the warehouse lights, seven for the retail/office lights, and one is reserved for future use. 

The lighting control circuits in the retail/office area are wired to provide even illuminance levels 

throughout the store. The lighting controls are configured to turn on some lights in the darker areas first 

(contractor sales and outside edges of the retail space), then more lights are turned on toward the center of 

the store as the natural light levels decrease. 

CBF 

The first-floor office lighting is provided by indirect T-8 lamps with electronic ballasts. The first row of 

lights on the south side of the building near the windows is dimmable and controlled by photocells. The 

second-floor office luminaires are 50% direct and 50% indirect. The first two rows of lamps on the south 

side are controlled by photocells. Task lighting is provided at all workstations. The lights in the normally 

occupied areas are enabled by the building automation system during occupied hours and have manual/ 

timed override switches. These controls are in parallel, such that if one is on, the lights are on. The timed 

override switches are push buttons—when the button is pushed, the switch enables the lights for a set 

time interval. 

Oberlin 

Corridor lighting is controlled with an occupancy/daylighting hold sensor so the occupancy sensor will 

not turn on the lights if sufficient daylighting is available. Similar occupancy/daylighting hold sensors 

combined with manual dimmers control lights in the classroom and offices. As installed, the daylighting 

hold was not used and postoccupancy rewiring was required to enable the daylighting hold. The atrium 

and auditorium lights are manually controlled with a continuous dimmer. Restrooms and kitchen lights 

are controlled with occupancy sensors. 

TTF 

Daylit zones use ceiling-mounted occupancy sensors connected to the EMS. In addition, a single openloop analog photocell in the clerestory of the high-bay provides lighting levels to the EMS. The EMS 

uses the lighting level information to turn the lights on or off. At the time of design, dimming 

technologies were not available at a reasonable cost. Each zone was calibrated against the single 

photocell to provide set points for controlling the lights. When light levels drop below the threshold, 

lights turn on. The lights will turn off only if the minimum light level for each zone has been met for a 

certain time. This prevents short cycling on variably cloudy days. When motion is sensed, a zone is 

enabled to come on. After a programmed time delay of no motion, lights are turned off. The lights will 

come on only if the light level is below the set point and if there is motion. The EMS-based lighting 

controls provide the ability to change delay times and set points without accessing the sensor directly. It 

was important that the lighting controls be easily manipulated because of the ceiling heights and 

accessibility of the sensors. The exception to this control scheme is the hallway located between the 

offices and the service core. This hallway is served by an integrated motion and daylight sensor that 

provides security lighting for the building; its lights are not connected to the EMS. When the building is 

not occupied, no lighting is on. As soon as the front door is moved, the security lighting is triggered on, 

unless there is ample daylighting. Security lighting is off when daylighting is available. 

32 
Zion 

The EMS uses stepped controls to manage six zones of T-8 fluorescent strip lighting, which covers most 

of the open area of the visitor center. The design of the stepped operation called for every other fixture to 

be on a separate circuit; however, every other row was installed on a separate circuit, which resulted in 

uneven distribution during stepped control operation. Each zone has a minimum threshold for operation 

as well as a time delay to prevent excessive cycling. In addition, the EMS controls HID spotlights that 

highlight the Trombe wall, wall displays, and cooltowers. The offices, back hall, break area, storeroom, 

and restroom use fluorescent fixtures connected to motion sensor controls. Even though these are on 

daylighting control, the delay from on to off is long to account for the lamp type. HID is not 

recommended with daylighting systems because of the long restrike time and the lamp color shifting as 

the lamps come on. 

The Comfort Station uses fluorescent fixtures with T-8 lamps. Compact fluorescent cans are located in 

the entryway and over the sink area. The sink can be considered a task lighting area and these lights are 

left on during occupied periods. The entire Comfort Station is split into seven zones of lights controlled 

with the BAS based on occupancy and available daylighting. 

3.2.3 Lighting energy savings 

3.2.3.1 Lighting design energy saving 

The lighting design energy saving results from the design of the lighting design only with no occupancy 

or daylighting controls. It compares the maximum LPD allowed by code to the installed LPD. For the 

case studies, ASHRAE Standard 90.1-2001 was used to determine the maximum allowable LPDs. The 

lighting design energy saving is not the actual energy saving; it is only a measure of the effectiveness of 

the lighting design. Some of this saving is from the daylighting design that allows for less electrical 

lighting to be incorporated into the design. The lighting design savings were the greatest at Zion because 

of a relatively high LPD code requirement for retail/display lighting of 2.2 W/ft2

 (23.7 W/m2), as 

specified by 10 CFR 435. The actual LPD is near 1.0 W/ft2

 (10.8 W/m2), which significantly reduced 

installed lighting power. 

3.2.3.2 Daylighting and occupancy controls energy savings 

The daylighting energy saving represents the actual energy saving and includes the savings that result 

from the occupancy controls and the daylighting controls while maintaining the minimum illuminance 

levels as specified by IESNA. It can be calculated by two methods. 

• Use the installed LPDs and the operating schedule to compare the measured lighting energy use 

to the expected use. This approach uses measured data, but the accuracy of the expected energy 

use is based on an approximation of the annual operating schedule. 

• Calculate the energy savings from the calibrated whole-building energy simulations of the AsBuilt Baseline Model and the As-Built Model. This approach uses the same operating schedules, 

but the simulation may not exactly represent the as-built daylighting conditions. 

3.2.3.3 Total lighting energy saving 

For each building we calculated lighting energy savings from daylighting and lighting design. The total 

lighting energy saving is the sum of the lighting design and daylighting savings (see Figure 3-11 and 

Table 3-5). For Oberlin, Zion, TTF, and BigHorn, the total lighting saving represented the largest source 

of the total energy saving. Total lighting energy savings ranged from 93% in the BigHorn warehouse to 

30% at CBF. 

The total lighting energy saving at Cambria was solely due to the reduced LPD. Minimal daylighting 

savings were documented, as measurements have shown that the illuminance levels with full daylighting 

and undimmed electrical lights are at or below the IESNA recommended minimum illuminance level of 

33 
30 f.c. (323 lux) for a horizontal work surface in an office (IESNA 2000). The illuminance levels were 

low because of lower than expected daylight contributions combined with low LPDs, indirect lighting 

used throughout the building, and high ceilings on the second floor. On the second floor, light from the 

clerestory windows does not penetrate beyond the first row of cubicles. Too little light enters through the 

windows and the roof trusses block some of the light. The daylighting along the perimeter walls is not 

very good because the windows are not high enough, the glass area is too small, the large wood frames 

limit the glass area and block some of the light, and the first row cubicle walls are too high and block 

daylight entering these cubicles. In addition, the light shelves on the first floor do not provide enough 

light to the space. The ceilings are too low, there is not enough glass area, and the reflectance off the light 

shelves is too low to provide adequate light. Cambria demonstrates how all the necessary daylighting 

pieces were included in the design, but still failed to deliver significant lighting savings. Daylighting 

design should be analyzed carefully to ensure a high level of performance. The daylighting design at 

Cambria incorporated good daylighting design principles and elements (orientation, clerestories, high 

reflectance ceilings, open office design, and light shelves), but the details were not analyzed to determine 

likely performance. Small changes, such as more direct lighting (and less indirect lighting with high dark 

ceilings) and additional fenestration area could have improved the performance. 

All six high-performance buildings we studied use daylighting systems to offset electrical lighting energy 

use. In four cases, daylighting systems were responsible for the largest source of energy savings. The 

percentage of the total building energy savings that was attributed to daylighting ranged from 32% to 

79%, as shown in Table 3-4. Although all these daylighting systems realized various levels of savings, 

each had problems. Some lacked predicted energy savings, had unacceptable glare, or had lower than 

anticipated lighting levels. Postoccupancy evaluations addressed some of these problems, which 

increased energy savings and occupant satisfaction. 

74% Total 

Lighting Savings

93% Total 

Lighting Savings 30% Total 

Lighting Savings

48% Total 

Lighting Savings

75% Total 

Lighting Savings

84% Total 

Lighting Savings

68% Total 

Lighting Savings

0.0

2.0

4.0

6.0

8.0

10.0

12.0

Oberlin Zion Cambria CBF Bighorn Retail Bighorn

Warehouse

TTF

Lighitng Energy Use Intensity (kWh/ft2)

Reduced LPD Savings

Daylighting and/or Occupancy Controls Savings

Measured (or Simulated) Lighting Use

Figure 3-11 Lighting energy savings summary 

34 
35 

Table 3-4 Daylighting and Lighting Energy Savings 

 Oberlin Zion Cambria CBF TTF Bighorn

Baseline total site energy 

use 

(kBtu/ft2

) (MJ/m2)

56.2 

(638) 

71.1 

(807)

61.3 

(696)

53.6 

(609)

49.1 

(558)

60.8 

(691)

Total site energy use 

(kBtu/ft2

) (MJ/m2)

29.8 

(338)

27.0 

(307)

36.8 

(418)

40.2 

(457)

28.5 

(324)

39.5 

(449)

Lighting energy use 

(kBtu/ft2

) (MJ/m2)

2.0 

(23)

7.8 

(89)

8.5 

(97)

9.9 

(112)

5.5 

(62)

3.8 

(43)

Total energy savings 

(kBtu/ft2

) (MJ/m2)

26.4 

(638.2)

44.1 

(638.2)

24.5 

(638.2)

13.4 

(638.2)

20.6 

(638.2)

21.3 

(638.2)

Total lighting energy savings 

(kBtu/ft2

) (MJ/m2)

10.6 

(638.2)

23.2 

(638.2)

7.8 

(638.2)

4.4 

(638.2)

16.2 

(638.2)

15.7 

(638.2)

Percent of total savings due 

to lighting savings 40% 53% 32% 33% 79% 74% 

Daylighting largest source of 

total energy savings? Yes Yes No No Yes Yes 
Table 3-5 Lighting Energy Savings Summary 

Notes: 1. BigHorn lighting energy use displayed as Retail (R) and Warehouse (W), as these are significantly different space types

Bighorn 1 Annual Cambria CBF Oberlin TTF 

Lighting 

Metrics kWh kWh/ft2 kWh kWh/ft2 kWh kWh/ft2 kWh kWh/ft2 kWh k

Baseline Lighting Use 

R:123,700 

W:119,400 

R: 6.7 

W: 5.2 164,981 4.8 130,563 4.2 50,282 3.7 63,815 

Actual LPD Baseline Lighting Use 

R: 95,249 

W: 78,804 

R: 5.2 

W: 3.3 85,913 2.5 101,257 3.3 33,100 2.4 37,200 

Measured (or Simulated) Lighting Use 

R: 39,300 

W: 8,400 

R: 2.1 

W: 0.4 85,913 2.5 90,855 2.9 7,922 0.6 16,287 

Total Daylighting and Lighting Design Savings 

Retail: 68% 

Warehouse: 93% 48% 30% 84% 74% 

Actual LPD (W/ft2) 

Retail Warehouse Whole Building 

1.3 0.8 1.0 

Offices 0.75

1st-Floor Offices 2nd-Floor Offices Conference Room 

1.21.61.4

Offices Classrooms Corridors/Other Whole Building 

0.91.20.50.8

Offices and Lab 

Baseline LPD (W/ft2) 

Retail Warehouse Whole Building 

1.6 1.2 1.4 

Offices 1.3 

1st-Floor Offices 2nd-Floor Offices Conference Room 

1.51.51.5

Offices Classrooms Corridors/Other Whole Building 

1.51.60.71.2

Offices and Lab 

36 
3.2.4 Six elements of successful daylighting systems 

We have defined six elements necessary to achieve energy savings through daylighting. If five of the six 

elements necessary to good daylighting are done correctly, but one is not, energy savings may not result. 

A good daylighting design should result from an integrated design process. The daylighting system has to 

be integrated with the envelope and trade-offs with heating and cooling understood to maximize wholebuilding energy savings. Even with the good integrated design in each building, the following six 

elements of a daylighting system are needed to maximize daylighting savings: 

1. Design buildings to provide daylighting to all possible zones. 

Design daylighting systems into all occupied spaces in a one-story building. In multistory 

buildings, daylighting should be designed into all occupied spaces on the top floor and all other 

zones adjacent to an exterior wall. At Oberlin, areas such as the auditorium suffered from lack of 

daylighting design. At TTF, the nondaylit restrooms and service areas are highly used and could 

have been easily daylit with tubular daylighting devices and shared daylighting systems (glazing 

on the interior of the building). At Cambria, some daylighting was designed into all occupied 

spaces, but minimal savings have been realized because of problems with other daylighting 

elements. Designing buildings to provide daylighting to all possible zones is the first element to 

maximizing daylighting savings. 

Typically, all zones within 15 ft (4.6 m) of an exterior wall can use perimeter daylighting to 

reduce lighting loads. Locate minimally used spaces such as storage and mechanical rooms in 

zones that cannot be daylit. In many cases, restrooms, copier rooms, and hallways are used 

frequently and typically don’t need careful lighting control. These areas are often considered 

“minimal use spaces.” In reality, they are used during most of the building’s occupied hours. 

Daylighting systems can be designed to meet all the lighting requirements in the daylit zones for 

most of the daytime hours. The best way to realize lighting energy savings is to turn the lights 

off. It is better to overdesign daylighting systems so daylight can offset the entire lighting load 

for most of the daytime occupied hours. Generally, the daylighting should be designed to provide 

twice as much illuminance as the electrical lighting system. Sufficient daylighting can be 

achieved with relatively small aperture areas. Oversized apertures are counterproductive, 

especially in zones where cooling loads dominate. Daylight modeling and whole-building energy 

simulation tools are recommended for proper design of daylighting systems. 

Systems that require the full daylighting contribution and the electrical lights to operate and 

provide sufficient illuminance will be more likely to have inadequate lighting levels. For 

example, classrooms at Oberlin could be completely daylit (without electrical lighting) for 52% to 

73% of the daytime hours, depending on the time of year. In contrast, full electrical lighting 

combined with the full daylighting contribution at Cambria was required to provide sufficient 

illuminance levels in the offices. 

2. Provide for glare mitigation techniques in the initial design. 

All the case studies documented glare problems. Attempts to address these problems in response 

to occupant complaints often resulted in reduced daylighting contributions. Blinds are often 

manually adjusted in response to unwanted glare. Typically the blinds on the clerestories high 

above the occupants were completely closed. The problem occurred when they were not 

readjusted, as they were hard to access. This completely disabled the clerestory daylighting. 

Techniques that do not require occupant interaction for glare mitigation, such as diffusing glass, 

light-deflecting panels, full shading overhangs, and light shelves should be considered for all 

south-facing clerestories. Other daylighting techniques that do not have a direct-beam 

component, such as north-facing clerestories and tubular daylighting devices, should also be 

considered in the daylighting design. 

37 
The first glare example at Zion (Figure 3-12) shows excessive glare on the bookstore checkout 

counter and staff. This occurred during an early morning in December from direct solar gain 

through the clerestories. Glare can overwhelm staff and customers because the checkout stands, 

whose location was determined shortly before the building was occupied, are poorly placed 

relative to direct solar gain. In the rest of the areas, direct solar gain and glare do not cause 

problems because occupants are not focused on a single area and can move around to avoid glare. 

When the checkout counter glare was identified as a problem, diffusing film was placed on the 

clerestory glass to control direct solar gain. 

Figure 3-12 Zion bookstore checkout counter glare from clerestories 

The second glare example at Zion (Figure 3-13) shows excessive direct solar gain in the southeast 

office on a December afternoon. This glare is the result of direct gain through the unshaded 

southern windows above the Trombe wall. The occupants typically use a temporary shading 

device (cardboard) when glare is a problem. Adjustable blinds or diffusing film would provide a 

permanent solution. 

Figure 3-13 Zion office afternoon glare 

Daylighting at CBF penetrates across the second floor, but glare problems are evident, as shown 

by illuminance readings higher than 186 f.c. (2,000 lux) and observations of occupants setting up 

makeshift shading devices (see Figure 3-14). The CBF glare problem suggests that a more robust 

solution for reducing glare should be explored. The broad expanse of windows along the south 

38 
facade has more area than is needed for daylighting alone. (This wall probably also creates 

additional cooling and heating loads.) There are several solutions to this problem. Placing 

diffusing films on the glass is the easiest solution; however, this would reduce the view of the 

bay. The diffusing films and light-deflecting devices would help light the ceiling. Hanging 

translucent shades would minimize the glare. Flat screen computer monitors would reduce glare 

and plug loads. 

Figure 3-14 CBF glare mitigation solution 

At Cambria, the south-facing clerestory windows can cause undesirable lighting conditions. At 

low sun angles, they can admit direct beam radiation, and they can be very bright at other times, 

causing contrast and glare problems. Automatic sunshades, which are controlled by an exterior 

photosensor, are specified on the interior of the windows to block the direct beam radiation. The 

sunshades block too much light and defeat the purpose of the clerestory windows. Other options 

for these windows are to diffuse the incoming light with frosted or patterned glass or a lightdiffusing film on the glass, or direct the beam radiation to the ceiling with a louver system. 

The manually operated brushed-aluminum blinds on the TTF clerestory windows are further 

examples of misapplied glare mitigation techniques that reduce daylighting. The original plan for 

operating these blinds was to manually adjust the mid-bay blinds according to seasonal solar 

angles. In theory, this operation would deflect direct solar gains up to the ceiling during the 

winter and allow diffuse daylighting in the summer. In reality, the blinds have been difficult to 

operate as they are 12 ft (3.7 m) above the floor. The blinds remain closed and not adjusted, 

which defeats the mid-bay daylighting. In response, we installed diffusing films and lightdeflecting panels so the daylighting success does not depend on occupant interaction. 

If glare mitigation techniques are not addressed in the initial design, typical solutions such as 

blinds defeat the daylighting systems and there will be no benefit. Diffusing films and lightdeflecting panels have successfully eliminated most of the glare on work surfaces without 

defeating daylighting. Some types of deflecting panels will actually increase daylighting 

performance by reflecting more light deeper into the lighting zone. The drawback of these 

solutions is the view of the sky can be lost. 

39 
3. Provide automatic, continuously dimming daylighting controls for all daylit zones. 

Automatic, continuously dimming daylighting controls should be used to maximize savings for 

all daylit zones that have daylighting contributions. Occupants cannot be relied on to dim the 

lights in response to available daylight. The lighting controls in the open second-floor office at 

CBF are manual on/off (except for the first two rows, which are dimmable and connected to 

photocells). More of the lighting on the second floor could be controlled to harvest daylight. 

Daylighting provides considerable underused natural light to the second-floor offices, the 

conference pavilion, and the lobby. The operator currently switches banks of lights off 

(sometimes by removing lamps), but automatic controls would represent a better long-term 

solution. At Oberlin, appropriate manual control of the electrical lighting is necessary to realize 

daylighting savings in the classroom, corridor, atrium, and offices. The controls, as installed, 

would turn the lights on to 50% when the zones were occupied. The daylighting controls were 

then dependent on the occupants to dim the lights in response to daylighting. In the classrooms 

and corridors, the occupancy sensors were rewired for a daylighting hold so that the electric lights 

did not automatically turn on if there was enough daylighting. The manual dimming controls in 

the atrium and auditorium were generally managed appropriately, although periods of 

mismanagement were identified. For typical occupants who are less aware of their building 

operations, this type of daylighting control would not result in significant daylighting savings. 

Continuously automatic dimming daylighting controls, combined with occupancy sensors, would 

be the optimal control solution. 

Automatic controls that dim and energize lighting circuits according to available daylight are 

typically better than stepped controls. Dimmers that are fairly linear in the relationship of light 

output to energy use should be specified. The occupants tend to be distracted when the electric 

lights switch on and off in a stepped fashion. Stepped daylighting controls have had varying 

degrees of success at TTF, BigHorn, and Zion, but continuously dimming T-8 fluorescent 

systems would be better in each case. For intermittently occupied daylit zones such as corridors 

and restrooms, occupant sensors combined with a daylighting sensor have successfully reduced 

lighting energy use. In these areas stepped controls are typically acceptable. 

When a manual override for automatic daylighting controls is needed, a maximum illuminance 

set point should be included in the manual control strategy. This will ensure that even when the 

lights are manually controlled to be on, daylighting can still offset lighting energy use. 

Occupancy sensors should be used to disable the manual override when the zone is unoccupied. 

Central lighting controls tended to work better than distributed controls, and overly complex 

systems were problematic. The complex systems with sensors in each zone were difficult to 

calibrate, as oscillations frequently occurred when the sensors detected light from a neighboring 

zone. A simpler solution that used a single sensor to measure daylight availability was often 

easier to calibrate. This worked when the primary source of daylight was from the south-facing 

fenestration, and data from a single sensor could be used to provide separate control for each 

individual zone. The lighting level set points with respect to this sensor are unique for each 

lighting zone and set to provide necessary light levels. 

4. Design interiors to maximize daylighting contribution. 

Daylighting and indirect lighting fixtures benefit from lightly colored interior surfaces that reflect 

light. Dark colors are counter to design for daylighting. Dark ceilings and structural elements 

were common reasons for reduced savings and poor daylighting distribution. Figure 3-15 shows 

the relatively dark colors in CBF’s interior because of unfinished wood products, the rough 

surface of OSB and laminated beams, and exposed ductwork. Finishing the interior, especially 

the ceiling, would provide surfaces with higher reflectivity and brighten the space, which would 

allow for increased use of daylighting and less waste from the indirect fluorescents. 

40 
Measurements indicate that daylight is sufficient because of the large amount of glazed area. The 

dark colors absorb much of the light and provide contrast to the bright outdoors. This results in a 

visually difficult environment. The distribution of natural daylight and indirect electric lighting 

would be improved with lighter colored interior finishes. 

Figure 3-15 CBF second-floor ceiling, beams, and ducts 

The exposed beams in the ceiling structure often blocked daylighting from entering into the 

space. Dark structural elements reduced daylighting savings and distribution at Zion, Oberlin, 

CBF, Cambria, and TTF. The distribution of natural daylight and indirect electric lighting would 

be improved with lighter colored interior finishes that have a reflectance greater than 90% and 

can effectively diffuse direct beam daylighting. Highly specular interior surfaces should only be 

used to reflect direct beam up to the ceiling (such as a light shelf), where glare is not an issue. At 

Zion, the dark structural beams throughout the buildings were not considered when the 

daylighting features were designed. These beams absorb and block significant portions of the 

clerestory daylighting and uplighting (see Figure 3-16). 

Figure 3-16 Zion’s 88% uplighting with T-8 fixtures (note the dark ceilings and beams 

combined with minimal down lighting) 

41 
At TTF, ceilings, walls, furniture, and floors should be bright white to help reflect daylighting. If 

possible, smooth surfaces should be used to prevent shadowing on the ceilings, making the 

ceilings appear brighter. A high-reflective (greater than 80%) white finish was added on interior 

surfaces to support the daylighting features; however, cubicle walls, furniture, and carpeting 

placed in the open office area have a reflectance of less than 50%. 

Interior daylighting design should include high reflectivity nonspecular (diffuse) finishes to 

spaces that are daylit or have indirect light fixtures. In addition: 

• Cubicle partitions were not designed to maximize daylighting at the work surface. 

Lighter partition walls and cubicles that are oriented to optimize daylighting can result in 

increased daylighting savings. Cubicle walls, furniture, and carpeting should all have 

light, highly reflective colors. 

• Anticipate and design for unavoidable elements that will reduce daylighting aperture and 

transmittance, such as light absorbing merchandise and shelving in retail spaces, cubicle 

walls, window openers, frames, mullions, and screening. 

• Specify actual visible daylighting areas, not just fenestration areas. Frames, screens, 

window openers, and mullions can all reduce the visible fenestration area. 

5. Integrate the electrical lights with the daylighting system. 

The rows of electrical lighting circuits should run parallel to the daylighting source. Each circuit 

should be controlled separately so that they can be dimmed as necessary in response to the 

daylight levels that vary with distance from the source. 

Uplighting in daylit spaces with high ceilings should be used sparingly and not at all with high 

dark ceilings. Integrating uplighting into daylit systems with high ceilings has been difficult, as 

effectiveness and illuminance levels have suffered. If combined with downlighting, controls 

should be provided so that the uplighting can be separately controlled. Indirect lighting works 

best close to highly reflective ceilings. 

Lower LPDs are possible in daylit buildings, if task lighting is provided at the critical work 

surfaces. The daylighting system should be designed to exceed the recommended light levels, 

such that it is not noticed when the electric lighting is off. The electric lighting design should use 

LPDs for night use and should account for the contrast between dark exterior lighting and the 

interior environment. Task lighting should be used for critical lighting needs. Appropriate 

placement and use of occupant-controlled task lighting is essential for daylit buildings with 

reduced LPDs, especially for detailed task work. 

6. Commission and verify postoccupancy energy savings. 

Commissioning a daylighting system that consists of adjusting photosensors and ensuring proper 

sensor placement is required so the electric lighting system responds properly to daylight. 

Verifying lighting energy savings according to NREL’s lighting measurement procedure (Deru et 

al. 2005) is a good method to determine whether all six elements have been successfully 

implemented. Postoccupancy daylighting evaluations in all six case studies resulted in changes to 

the daylighting systems, which addressed occupant complaints and increased daylighting savings. 

Each building had glare problems. Clerestory glare problems were addressed with diffusing 

films, and adjustable blinds recommended for occasional use. We added more task lighting in 

critical areas, as the stepped controls, uplighting, and daylighting did not provide adequate 

illuminance levels for detailed work. 

Table 3-6 summarizes the success of these daylighting elements for each building. Daylighting elements 

that were partially successful are noted with a gray circle. For example, Oberlin was partially successful 

42 
in including daylighting in all possible zones, as the building was completely daylit except for the 

auditorium. 

Table 3-6 Six Elements Required to Maximize Daylighting Savings 

 Oberlin Zion Cambria CBF TTF Bighorn

1. Daylighting to all 

possible zones? 

2. Glare mitigation in 

initial design? 

3. Automatic 

daylighting controls? 

4. Interiors designed 

to maximize 

daylighting 

contribution? 

5. Lighting systems 

integrated with 

daylighting system? 

6. Postoccupancy 

energy savings 

commissioned and 

verified? 

 Yes 

Limited to partial application or success 

 No 

3.3 Integrating the Envelope and Mechanical Systems 

Heating is responsible for 25% of the total energy consumption in office buildings, cooling uses 9%, and 

ventilation uses 5% (Figure 3-17) (EIA 2005a). Combined HVAC loads are responsible for 39% of the 

total energy consumption in office buildings. Thus, lessons learned about applying efficient HVAC 

design and controls to minimize energy use are valuable to the industry and to future ZEB designs. 

An integrated approach to building design is the best way to lower energy use and cost. The building 

must be engineered as a system if the technologies are to be integrated. Each building first reduced 

HVAC loads with increased insulation and tightened envelopes, engineered windows and overhangs, 

reduced internal gains with daylighting, and an east-west axis with an optimal orientation. Envelopeintegrated HVAC technologies, including Trombe walls, natural ventilation, passive solar gains, and 

cooltowers, met some of the remaining HVAC loads. Additional energy-efficient HVAC technologies, 

including ground-source heat pumps, evaporative cooling, and energy recovery ventilators (ERVs) were 

then used to meet any remaining HVAC loads. 

43 
Space Heating

25%

Space Cooling

9%

Ventilation

5%

Water Heating

9%

Cooking

1%

Refrigeration

0%

Office 

Equipment

16%

Lighting

30%

Other

5%

Figure 3-17 Typical site end uses for office buildings (percent of total) (EIA 2005a) 

3.3.1 HVAC system descriptions 

The case study buildings use a variety of different HVAC systems. Ground-source heat pumps with 

ERVs are the primary heating and cooling sources for Oberlin, CBF, and Cambria. Water-source heat 

pumps exchange energy with the ground to provide heating and cooling at higher efficiencies than 

traditional air-source heat pumps. Various forms of natural ventilation were designed into the buildings 

and HVAC systems at Oberlin, BigHorn, CBF, and Zion. Direct and indirect evaporative cooing systems 

meet all the cooling needs for TTF and Zion. Oberlin and BigHorn use radiant heating in-floor slabs and 

unit heaters. Each of these systems, except for Cambria, is controlled with an EMS. Table 3-7 

summarizes the HVAC and control systems for each building. 

44 
45 

Table 3-7 HVAC Systems in High-Performance Buildings 

Building Heating Systems Cooling Systems Ventilation 

Systems Controls 

Ground-source heat pump loop with central 

variable speed drive (VSD) ground loop 

circulation pump 

Oberlin 

Hydronic radiant units 

Limited passive solar 

gains 

Limited natural 

ventilation 

100% outdoor air 

with ERVs 

Limited natural 

ventilation 

EMS under 

facilities control via 

a contractor 

Zion 

Trombe walls 

Electric radiant ceiling 

panels 

Passive solar gains 

Natural ventilation 

Direct evaporative 

cooltowers 

Natural ventilation EMS under 

researcher control 

Cambria 

Ground-source heat pump loop with 

distributed ground loop circulation pumps 

for each heat pump 

ERVs 

Independently 

controlled by zone 

thermostats under 

occupant control 

Ground-source heat pump loop with central 

VSD ground loop circulation pump 

CBF 

Propane boiler for 

hydronic fin and tube 

radiators 

Natural ventilation 

Natural ventilation 

ERVs 

EMS under local 

manager control 

(owner) 

TTF 

Campus hot water 

coils in variable air 

volume boxes 

Passive solar gains 

Direct/indirect 

evaporative 

cooling 

Dry-bulb 

economizer 

Mixed return and 

outdoor air 

EMS under 

campus facilities 

control (owner) 

BigHorn 

Gas-fired boilers for 

hydronic in-floor slab 

radiant heat 

Natural ventilation Natural ventilation EMS under owner 

control 

3.3.2 Using the architectural design and envelope to create low-energy buildings 

The architectural design and envelope have major impacts on building energy, lighting, and comfort 

performance. Envelope features include form, shading, daylighting, choice of materials, sizing, 

orientation, and glass specification. Too often, an inefficient envelope design must be compensated for 

with more energy-intensive (and costly) mechanical and lighting systems. Therefore, the envelope design 

should be the first step in creating low-energy buildings. The mechanical and lighting systems can then 

provide the remaining (smaller) thermal and lighting comfort needs. Low-energy architecture is not 

effective if mechanical and lighting systems have to solve problems created by inadequate envelopes. 
Each building reduced HVAC energy use through increased insulation levels and tightened envelopes, 

engineered windows and overhangs, reduced internal gains with daylighting, and an east-west axis with 

an optimal orientation. Envelope-integrated HVAC technologies, including Trombe walls, natural 

ventilation, passive solar gains, and cooltowers, met some of the remaining loads. 

By using the mechanical system to make up for what cannot be accomplished by architectural features 

and envelope alone, the mechanical system does not have to correct for an architectural design that is 

climatically ill conceived. The Zion Visitor Center is a good example of this concept: daylighting was 

designed into the roof structure through clerestories, the cooling system was integrated into the vertical 

tower elements, Trombe walls were designed into the south-facing walls, and natural ventilation is 

provided through operable windows. At BigHorn, a conventional cooling system was not needed because 

the building had increased envelope insulation and reduced internal gains. All cooling could then be 

provided through natural ventilation. 

3.3.2.1 Watch out for overglazing 

Glass has a very strong impact on energy, comfort, and lighting performance, which can be positive or 

negative depending on many factors. Simulations allow us to optimize glass optical and thermal 

properties, sizing, orientation, placement, and shading. Additional glass is often added to achieve 

transparency—a common architectural desire. However, the strong exterior contrast from the sun 

combined with reflections often prevents the desired transparency, even with lots of glass. Transparency 

creep is evident at Zion, Oberlin, and CBF. Zion is overglazed on the north façade, Oberlin on the east 

side, and CBF on the south side. Additional shading would help to reduce cooling requirements caused 

by excessive glazing at both CBF and Oberlin. The structure outside the south-facing glass at CBF might 

be refitted with additional shading. A vine trellis on the east side of Oberlin’s atrium was designed to 

provide summer shading, but was never installed. Before adding shading, a detailed analysis should be 

conducted that takes into account increased heating loads, reduced PV output, decreased daylighting, and 

glare problems. 

3.3.2.2 Minimize thermal bridging 

HVAC loads are affected by the overall thermal performance of the building envelope. Although high 

levels of thermal insulation are important, two- and three-dimensional heat flows through construction 

details can also have an important impact; these heat flows are referred to as thermal bridges. 

Although designers took care to develop designs that would minimize thermal bridging, four problem 

areas at the TTF were identified that add to its thermal loads. 

• The window frames form a thermal bridge. Figure 3-18 shows a sample of infrared images used 

to determine that the window and door frames installed were not thermally broken as specified. 

Compared to the baseline TTF model, as much as 13.6 MMBtu/yr (3,986 kWh/yr) are lost 

through the window frames because they are not thermally broken. 

Figure 3-18 TTF infrared thermal images showing heat loss through window frames 

46 
• There is a thermal bridge where a retaining wall meets the building. Although the impact of this 

thermal bridge on the annual heating and cooling performance is minimal, multiple incidences of 

thermal bridging can offset savings from the low-energy envelope. This flaw was built according 

to plan and should have been identified during the design phase. 

• During construction, the foundation insulation was relocated for structural reasons. As a result, 6 

in. (15 cm) of insulation was removed, which created a thermal bridge that is approximately 390 

ft (119 m) long by 6 in. (15 cm) wide. NREL estimates that an additional 4.3 MMBtu/yr (1,260 

kWh/yr) are lost through this thermal bridge. Figure 3-19 shows infrared thermal images that 

indicate heat loss through the foundation. 

>11.7°C

<-8.2°C

-5.0

0.0

5.0

10.0

>15.9°C

<-4.0°C

0.0

5.0

10.0

15.0

Figure 3-19 TTF infrared thermal images showing heat loss at the foundation 

• The high-bay garage door is a significant thermal bridge. Figure 3-20 shows the temperature 

distribution of the high-bay roll-up door during a summer morning. Incident morning sun heats 

this door, which causes summer comfort problems in this section of the high-bay. The interior 

surface of the east garage door in the high-bay of the TTF can exceed 110°F (38°C) during 

morning summer hours. As shown in Figure 3-20, the floor-to-ceiling temperature distribution 

can exceed 30°F (16.7°C) in the high-bay, which results in localized discomfort and additional 

cooling loads. A high-reflectivity and high-emissivity exterior paint, combined with a lowemissivity interior paint, would reduce the heat absorbed by the door and reduce heat emitted to 

the space. Reducing heat emitted from this door would improve thermal comfort at the east end 

of the high-bay. Other solutions include external shading. Although more expensive, replacing 

the door with a well-sealed R-25 (RSI-4.4) panel garage door would be the optimal solution, and 

would align with the design intent for the rest of the TTF envelope. 

*>110.0°F

*<72.0°F

80.0

90.0

100.0

110.0

Figure 3-20 TTF high-bay insulated roll-up door thermal bridging 

47 
Another example of a significant thermal bridge is in a small, unconditioned storeroom at Zion. This 

storeroom often overheats because its dark brown metal double doors collect heat during the morning. 

The internal temperature of the doors radiates to employees in the storeroom (see Figure 3-21). The doors 

could not be painted white because the exterior color of the building had to follow NPS standards, which 

is dark brown. To help cool and ventilate this zone, several small fans were installed along the floor 

vents. Designing the steel door with increased insulation and a radiant barrier would have been a better 

solution, in line with the design intent used for the rest of the building. A uniform thermal envelope that 

includes well-insulated storeroom and garage doors can eliminate comfort problems and unnecessary 

heating and cooling loads. 

85.0°F

149.0°F

100

120

140

Figure 3-21 Zion infrared image showing interior surface of storeroom door overheating 

from solar radiation 

Ground heat transfer is another area with complex three-dimensional heat flows. Many questions 

surround modeling heat transfer through slab-on-grade floors. The common practice of insulating only 

the perimeter of the slab is justified only if the ground under the center portion of the slab will eventually 

reach an equilibrium temperature under the influence of the controlled environment above the slab. 

Through STEM tests (Subbarao et al. 1989) and thermography, NREL found that more heat is lost 

through the floor and window frames than models predicted. Based on other work (Deru and 

Kirkpatrick 2001), researchers suspect that the ground under the center of the slab is not reaching 

equilibrium. Additional work would be required to determine the benefits and costs of insulating the 

entire slab. One impact of slab heat transfer losses is comfort. Based on thermal comfort models, a cold 

floor adversely affects foot temperature which is a critical component for occupant comfort. Insulating 

floors will result in warmer floors and may result in lower overall building temperatures because people 

with warmer feet can be more comfortable at lower zone temperatures. 

Designing the envelope to minimize thermal bridging is good practice. An otherwise well-insulated 

envelope can suffer if a single component lacks appropriate thermal properties. Additional consideration 

is typically needed to ensure that doors are well insulated (especially those that have significant direct 

gains), window frames are installed with thermal breaks, and the insulation at the interface with groundcoupled surfaces has been appropriately specified. Ensuring insulation details are included in the 

specifications and then implemented during construction is essential to achieving a thermally uniform 

envelope. 

48 
3.3.3 Passive solar heating 

Traditionally, passive solar heating is not beneficial for commercial buildings because they have abundant 

internal heat gains from large numbers of people, lights, and equipment, and, except for morning warmup, 

require cooling much of the year. However, this convention breaks down for daylit buildings in which 

the electric lights no longer provide significant heat gains because they are often turned off during the 

day. Zion, BigHorn, Oberlin, TTF, and Cambria are heating-dominated buildings. Passive solar heating 

through indirect gain Trombe walls provide a significant portion of the heating at Zion, and passive solar 

direct gains contribute to the heating at TTF. 

Many passive solar features were integrated into the TTF final design. The design team took advantage 

of Colorado’s sunny climate by carefully selecting, orienting, and placing windows and clerestories. The 

passive solar and daylighting design of the building incorporates 88% (1,134-ft2

 [105.4-m2]) of its total 

window area as a single row of view glass (492 ft2

 [45.7 m2]) and two rows of clerestories (642 ft2 [59.6 

m2]) along the southern facade. An additional 8% of the total view glass area is on the east (56 ft2 [5.2 

m2

]) and west (56 ft2 [5.2 m2]) facades; the remaining 1% is positioned on the north wall (38 ft2 [3.5 m2]). 

NREL engineered the building to provide passive solar gain during the winter months and minimize this 

gain during the summer months. The selected glass type allows solar energy to enter the building for 

passive heating energy. South-facing clerestory windows have a high solar heat gain coefficient (SHGC) 

of 0.68 (shading coefficient of 0.76); all others have a lower SHGC of 0.45 (shading coefficient of 0.51). 

The ground-level windows were designed for viewing and are larger than needed from an energy 

perspective. To avoid overheating and glare from direct gain through these windows during the winter, 

the view windows have a lower shading coefficient than the clerestory windows. All windows were 

engineered with a low-e coating to increase thermal resistance. Engineered overhangs were designed to 

block direct solar radiation during the warm summer months when sun angles are high. Although not a 

prominent design feature, thermal mass in the floor and north wall helps to minimize temperature swings. 

Opaque envelope components for the TTF were selected to be highly insulating and to expose thermal 

mass to the interior. 

Direct gains are used at the TTF to provide a portion of the heating needs. The building typically requires 

heating only during the early morning hours to warm up from nightly temperature setback. Although the 

temperature set point is set back to 64°F (19°C) every night, the temperature in the TTF rarely drifts that 

low. Heating loads on a cloudy day and a sunny winter day were examined to understand the direct solar 

gain benefit (see Figure 3-22). For both days, the set point was 64°F (19°C) from 5:00 p.m. to 12:00 

a.m.; the remainder of the day was set to 70°F (21°C). After the morning warmup, passive solar heating 

and internal gains met most of the building’s heating requirements. 

49 
0

50

100

150

200

250

1 4 7 10 13 16 19 22 1 4 7 10 13 16 19 22

Hour of Day

Heating (kBtu/hr)

0

15

30

45

60

75

Inside Temperature (°F) . 

Heating Tin

Cloudy Day Sunny Day

Figure 3-22 TTF heating loads on a cloudy day compared with 

heating loads on a sunny day 

3.3.3.1 Passive solar heating with Trombe walls 

The only building in the case studies to use a Trombe wall was the Zion Visitor Center. The Trombe wall 

was integrated into the envelope to provide a passive radiant heating source. Design details for this 

Trombe wall are shown in Figure 3-23. The 6-ft (1.8-m) high Trombe wall (740-ft2

 [68.7-m2] total area) 

is located on the entire length of south-facing walls. The wall makes up 44% of the south-facing wall 

area. The Trombe wall is constructed of 8-in (20-cm) grout-filled concrete masonry units (CMUs) with 

an R-value of 2.5 hr·ft2

·°F/Btu (0.4 K·m2/W). The other walls are 6-in (15-cm) framed walls with an Rvalue of R-16 hr·ft2

·°F/Btu (2.8 K·m2/W). The Trombe wall has a single layer of high transmittance 

patterned glass installed on a thermally broken storefront system. 

50 
Overhang Engineered for Summer 

Shading 

Daylighting and View 

Shotcrete Wall Finish Windows 

(Inside) Selective 

Surface 

Figure 3-23 Cross-section details of Zion Trombe wall 

3.3.3.2 Thermal and construction details of Trombe walls are important 

A major benefit of the Trombe wall is its ability to provide a uniform radiant heating source. This 

improves thermal comfort during the heating season. Also, as the heat transfer is governed by the thermal 

difference and the thermal resistance, the thermal resistance on the interior of the wall must be minimized. 

The interior of the Zion Trombe wall includes in the design cast-in place concrete projections. These 

projections could be used as shelves to display product. The hope was to minimize the occupant’s desire 

to place bookcases or other obstructions in front of the wall. This has worked fairly well; however, the 

occupants have had a strong urge to place objects in front of the wall. 

To reduce the thermal resistance, we verified construction of the CMU wall to ensure the concrete block 

cores were completely filled. This appears to have been effective. A problem area has been the shotcrete 

used as a finish material on the inside of the wall. Although a massive building material, there were air 

gaps between the material and the CMUs on the wall. These problems were detected using thermography 

as cooler temperatures on the wall surface. Placement of the footing insulation was also verified during 

the construction process to ensure proper installation. The location of this insulation is critical, as threedimensional heat transfer to the ground can diminish Trombe wall performance. By thermally decoupling 

the footings from the ground with insulation, unnecessary heat loss is avoided and more heat from the 

Trombe wall is supplied to the building. This was effective as shown by minimal stratification of 

temperatures through the Trombe wall. 

3.3.3.3 Trombe walls can meet a significant portion of the heat load 

The Zion Trombe wall daily performance during the 2001–2002 heating season is shown in Figure 3-24. 

The electric radiant heating system used 22,680 kWh during the course of one year and the Trombe wall 

contributed 20% of the total heating to the building. The Trombe wall imposed a heating load on the 

building for only two of the 151 days of the 2001–2002 heating season. For the other 149 heating days, 

the wall was net positive. The peak heat flux through the wall was 11.2 W/ft2

 (89 W/m2), or 8.3 kW over 

the entire Trombe wall area. The average efficiency of the wall over the 2001–2002 heating season 

(defined as the heat delivered to the building from the Trombe wall divided by the total solar radiation 

incident on the exterior of the wall) was 13%. 

8-in. Grout-Filled 

CMU Wall 

2-in. Air 

Gap 

5/32” 

Glazing 

Cast-in-Place Concrete Wall 

Projections 

1-1/2-in. Rigid Footing 

Insulation 

51 
During the first three months of the 2002–2003 heating season, the total electrical heating energy used 

was 5,389 kWh, while the Trombe wall provided approximately 41% of the energy or 3,800 kWh. This 

percentage was greater than the 2001–2002 heating season of only 20% because of improved controls of 

the electrical heating system and differences in weather. 

0

50

100

150

200

250

300

350

400

11/01/200111/08/200111/15/200111/22/200111/29/200112/06/200112/13/200112/20/200112/27/200101/03/200201/10/200201/17/200201/24/200201/31/200202/07/200202/14/200202/21/200202/28/200203/07/200203/14/200203/21/200203/28/2002

Daily Heating Energy Supplied to Visitor Center (kWh/day)

Heat Added by the

Trombe Wall

Heating System Use

2001-2002 Heating Season:

»5,470 kWh added by 

 Trombe Wall

»22,680 kWh added by

 Trombe Wall 

»20% of heating from 

 Trombe Wall

»8.3 kW Peak heating 

 from Trombe Wall

»17.2 kW Peak heating 

 from Heating System

Figure 3-24 Zion Visitor Center Trombe wall and heating system performance, 

2001–2002 heating season 

3.3.3.4 Consider net annual performance when designing Trombe walls 

A potential design issue to consider with the Trombe wall is overheating in the summer and swing 

seasons. The overhangs are designed to shade the Trombe walls during the cooling season. Even though 

the Trombe walls are shaded in the summer, the walls impose an additional cooling load on the buildings. 

This is because early morning and late afternoon radiation is not shaded and diffuse and reflected 

radiation is not negligible. Additionally, the insulation values of these walls are low. For Zion, the 

additional cooling loads do not impact performance, as the passive direct evaporative cooling system 

provides an abundance of cooling. The annual net effect of the wall has to be considered in its design, as 

the additional cooling loads can affect the cooling system performance. For buildings with conventional 

air-conditioning systems, careful Trombe wall design is essential so summer gains are minimized. If 

buildings are flushed with evaporative cooling, the summertime cooling loads generated by the Trombe 

walls do not pose a problem. 

3.3.4 Natural ventilation 

Four of the buildings used natural ventilation for cooling and providing outdoor air requirements when 

outdoor conditions are favorable. Table 3-8 provides control schemes and cooling types for each case 

study. Zion was designed to be entirely naturally ventilated with a stack effect that is primarily driven by 

52 
cool air provided by the cooltowers. The EMS mechanically operates windows in the clerestory. Natural 

downdraft cooltowers that require no fans are also considered natural ventilation; pumps are controlled by 

the EMS. Lower manually operated windows complete the scheme. 

CBF uses a similar strategy with manually operated lower windows and high windows operated by the 

EMS. Signs, lit by the EMS, help direct staff to open the lower windows (as shown in Figure 3-26). In 

practice, the so-called natural ventilation at CBF is often operated as a hybrid system with the aid of 

exhaust fans. 

BigHorn has motor-actuated clerestory windows that are controlled either by the EMS or by wall 

switches. The front doors, which open during normal business hours in the summer, provide supply air to 

complete the scheme. BigHorn has excellent stack effect, which makes the natural ventilation very 

effective and eliminates the need for conventional air-conditioning. 

Oberlin uses EMS-controlled windows for the atrium and second-floor hallway to provide natural 

ventilation to the atrium and corridors. Manually operated low windows in the classrooms can also 

provide limited natural ventilation. 

TTF had no natural ventilation provisions, although there was design intent to have some natural 

ventilation. The design intent of TTF was to use a high relief damper to release hot air from the building. 

However, during construction, this damper was installed much lower than desired, which renders the 

natural exhaust system ineffective. 

Active cooling was provided at Oberlin and CBF for backup if natural ventilation was not effective or 

could not meet all the cooling loads. Natural ventilation (or natural ventilation coupled to cooltowers) 

provides all the cooling at Zion and BigHorn. The only metric for measuring the performance of the 

natural ventilation is the comfort in the spaces: if the occupants are comfortable, the natural ventilation 

system is considered successful. 

Table 3-8 Natural Ventilation Controls in High-Performance Buildings 

Building High Window 

Control 

Low Window 

Control 

Active 

Cooling 

Oberlin EMS in north 

clerestory 

EMS in atrium, 

with manually 

operated windows 

in classrooms and 

offices 

Yes 

Zion EMS Manual No 

Cambria None None Yes 

CBF EMS Manual (EMS 

indicators) Yes 

TTF Relief damper 

(EMS) none Yes 

BigHorn 

EMS (with 

manual 

override) 

Manual (doors) No 

53 
3.3.4.1 Natural ventilation can provide significant fan and air-conditioning energy saving 

The natural ventilation systems at Zion and BigHorn were mostly successful, providing all the outdoor 

and cooling requirements (Zion’s cooltowers are an evaporative cooling system driven by natural 

ventilation). Savings were realized by not using fans to provide outdoor air to supply ventilation air or 

conditioned air. When combined with evaporative cooling, the energy savings were even greater, 

eliminating all fans and conventional air-conditioning equipment. Limited success was observed at CBF, 

as the system typically operated in hybrid mode with exhaust fans. The natural ventilation system is a 

hybrid or mixed-mode system in that fans are often used to help move ventilation air. These fans exhaust 

air on the north side. A fan on the second floor exhausts air at a rate of 2,800 cfm (1.32 m3

/s). A fan on 

the first floor exhausts air at a rate of 5,600 cfm (2.64 m3

/s). Minimal natural ventilation is used at 

Oberlin because of difficulties developing and implementing appropriate controls. 

3.3.4.2 Natural ventilation should be designed to rely primarily on stack effect 

Oberlin’s and BigHorn’s natural ventilation systems were designed to operate primarily on the stack 

effect, driven by temperature differences between warm air inside and cool air outside. The warm air in 

the room rises and exits at the ceiling or ridge, and enters via lower openings in the wall. At Zion, natural 

ventilation is combined with evaporatively cooled air, designed to operate by stack-driven temperature 

differences. 

The natural ventilation at CBF was designed to take advantage of prevailing winds off the bay that flow 

from south to north, as shown in Figure 3-25. The design of the building’s operable fenestration and 

intended natural ventilation airflow pathways were based on a single wind direction. Low- and midheight windows on the south side are grouped in banks of four and operated with hand cranks. Windows 

high on the north side have motorized operators that are controlled by the building’s EMS. In reality, this 

system has limited functionality because the wind direction is very changeable. Measured data confirmed 

that winds in the area tend to flow from the northwest when outdoor conditions are good for natural 

ventilation. They also come from the east as often as from the south. The monitoring data show that 

designers should not have assumed that the winds from the south (off the bay) would be the most 

important winds for natural ventilation cooling. Although winds do not come from the north, designing 

for cross ventilation in the east to west and west to east directions would have been an improvement. The 

discrepancy between expected and measured wind directions also suggests that engineered natural 

ventilation systems should be designed to operate by stack forces rather than by wind. Winds are 

important resources for daytime, natural cross ventilation. For CBF, passive cross ventilation should have 

been designed for multiple airflow paths and natural stack effect. 

Natural ventilation should be designed to rely primarily on stack effect unless wind direction and speeds 

are reliable and well understood. Even with reliable winds, passive cross-ventilation should be 

engineered for a variety of airflow pathways and wind directions. 

54 
Figure 3-25 Intended airflow pathways for natural ventilation from the design phase 

3.3.4.3 Separate natural ventilation supply and relief from the fenestration and use relief dampers for 

the passive ventilation 

Operable windows form an important part of the natural ventilation systems at Oberlin, CBF, Zion, and 

BigHorn. The windows near the ground at Zion and CBF are operated manually; clerestory windows are 

on automated actuators. Experience showed that occupants do not consistently operate the manual 

windows. In addition, windows are not considered typical HVAC equipment, so how the operable 

windows interface with the controls system was an issue in all cases. Commercial-grade window 

actuators were used at Zion, Oberlin, and CBF. Motorized window actuators were prone to failure. 

Custom wiring and control algorithms had to be developed to operate the windows. The EMS does not 

know when the windows are fully open or closed and does not shut off power to the device. The 

windows at Zion are cycled once daily in the summer to help ensure they are synchronized with the status 

as indicated by the EMS. Using dampers with typical HVAC control actuators would reduce maintenance 

and integration issues without reducing natural ventilation performance. 

Operable windows require more frame area than fixed windows, and the screens reduce both the effective 

open area and the visible transmittance important for the daylighting. In addition, the operable windows 

have very little open area when “fully open,” as the actuators were limited to about 6 in. (15 cm) of throw. 

Based on these lessons learned from the natural ventilation system at Zion, residential window actuators 

were specified at BigHorn. No actuator problems have been reported with this application. The operable 

windows are on the north side of the clerestory to minimize the effects of the screens and the larger frame 

area on the daylighting. 

55 
3.3.4.4 Use automatically controlled supply and relief controls that do not rely on occupant interaction 

The occupants of commercial buildings do not interact consistently with the windows as a natural 

ventilation control strategy. EMS typically control operable windows better than occupants do for natural 

ventilation purposes. For example, the operable lower windows at Zion are often not opened, even when 

outdoor conditions are favorable. Whether the problem is the height of the windows, security concerns, 

or lack of interest, the bottom line is that the windows are not opened. 

CBF experienced some of the same issues. Low- and mid-height windows on the south side are grouped 

in banks of four and operated with hand cranks. Windows high on the north side have motorized 

operators that are controlled by the EMS. Figure 3-26 shows a photograph of one of the signs in the 

office areas that reads “Open Windows.” It is used to inform occupants when conditions are appropriate 

to open the manually operated windows. Even when the EMS indicates to open the windows, occupants 

do this only occasionally. Interest in operating them has waned somewhat since the building first opened. 

There was more enthusiasm to work with the windows when the building was new. 

Figure 3-26 Signage at CBF directs occupants to operate windows 

At Oberlin, the HVAC system must be manually switched off when the classroom windows are manually 

opened. Provisions must be made to ensure HVAC systems do not turn on during natural ventilation 

operation. Experience shows that occupants do a poor job of operating the manual windows. 

Additionally, occupant control of windows can cause the HVAC system to operate when the windows are 

open. 

3.3.4.5 Provide multiple opportunities for passive airflow 

Carefully design interior partitions, floor plan, and fenestration to promote good circulation of naturally 

induced airflow and limit passive airflow disturbances. Enclosed spaces tend to overheat because of 

insufficient airflow caused by naturally induced low-pressure ventilation. 

As a special class of passive natural ventilation, the use of cooltowers requires careful design of interior 

partitions and fenestration to promote good circulation of cooled air. The enclosed offices at Zion tend to 

overheat because of insufficient airflow and continual heat gains from the Trombe wall. Exhaust fans 

56 
originally installed in the office area were not sufficient to move air through these spaces to counteract the 

heat gains from Trombe walls. As with many passive systems, moving air mechanically should be 

avoided. 

At BigHorn, the private offices on the mezzanine often do not receive enough natural ventilation. During 

design, the suggestion was made to add windows as high as possible to the office wall to use shared light 

from the clerestory windows. These windows would also allow ventilation air to circulate by natural 

convection. This suggestion was not incorporated and the offices require additional lights and fans during 

the summer to move the stuffy air. 

3.3.4.6 Natural ventilation systems should not replace conventional economizers 

One way to think about passive natural ventilation is that it essentially economizes without fans, and, if it 

is fan assisted, it is much like running an economizer. However, from a thermal comfort point of view, it 

is limited to moderate temperatures and humidity. An economizer-based system can control supply air 

temperature to provide thermal comfort for a wider range of outdoor temperatures. 

In practice, the natural ventilation at CBF is often operated as a hybrid system with the aid of exhaust 

fans, which run a significant amount of the time. The system essentially fits the role of an economizer, 

but without the controlled air distribution provided by ductwork and registers or the control afforded by 

controlling air temperature through mixing. This mode of hybrid operation could be more efficient 

because of lower static losses. The exhaust fans are used in the winter when the windows are closed and 

makeup air comes from infiltration. This type of system operation makes up for the lack of economizer 

operation. However, an economizer would be better because it is controlled to a mixed air set point and 

could be expected to provide more uniform comfort. The issue is that potentially cool air is not mixed 

with space air and cool drafts are felt. Infiltration does little to ensure that cooling effects are evenly 

distributed; it also increases static pressure, which causes the exhaust fan to work harder. Further analysis 

also found that considerable energy is being expended for heat pump cooling during the swing seasons 

and the winter, which indicates that natural ventilation systems do not always deliver cooling during the 

heating season. Measured data allowed NREL to examine how much of the cooling energy is consumed 

when outdoor air conditions would be favorable for economizer operation (were the building to have this 

feature). For this examination, we sorted and summed HVAC cooling energy use for times when the 

outdoor air temperatures fall below 58°F (14.3°C). The analysis revealed that of the 71.4 MWh of 

electricity used for cooling, 15% was used when the outdoor temperatures were favorable for economizer 

use and not for natural ventilation use. 

3.3.5 Evaporative cooling and cooltowers 

3.3.5.1 Use evaporative cooling systems in dry climates 

All cooling requirements were met with evaporative cooling systems in the dry climates at Zion and TTF. 

In a dry climate, direct/indirect evaporative cooling can provide sufficient cooling capacity with less 

energy use than refrigerant-based cooling systems. The TTF uses a direct/indirect variable speed supply 

fan evaporative cooling system, and Zion uses a passive downdraft cooltower system, which is 

functionally equivalent to a direct evaporative cooling without fans. 

The TTF evaporative cooling system was designed to meet the building’s normal cooling loads and keep 

humidity levels within the comfort range without conventional cooling coils. Figure 3-27 diagrams the 

main air handler. The main supply fan is variable speed and rated for 10,500 cfm (4,956 L/s). The 

evaporative cooler is a two-stage, direct/indirect unit sized for an airflow rate of 10,000 cfm (4,720 L/s). 

The direct section of the evaporative cooling unit has a rated effectiveness of 90%; the indirect portion 

has a rated effectiveness of 75% (S.A. 1993). Air for the secondary side of the indirect section is drawn 

from the building and then exhausted outside using a 5,300-cfm (2,501-L/s) variable-speed fan. The 

evaporative cooling system is equipped with two, 0.16-hp (0.12-kW) water pumps to wet the cooling 

medium. Separate pumps allow for independent control of the two evaporative stages. 

57 
The TTF main air handler is used only for cooling; it is turned off when cooling is not needed. There are 

four different operating modes for the air handler: (1) economizer, (2) direct evaporative cooling, (3) 

direct/indirect evaporative cooling, and (4) indirect evaporative cooling. When outside conditions are 

favorable, the air handler meets cooling loads by economizing where outside air and return air (drawn 

from the east mid-bay) are mixed to meet a control supply temperature. 

Figure 3-27 Main air handler with direct/indirect evaporative cooling 

Reducing the cooling load daylighting and enhanced envelope and then meeting the remaining cooling 

needs with the evaporative cooling system had the second-largest impact on TTF’s total energy savings. 

The cooling load energy cost was reduced by 77% compared to the baseline model, which saved $480 in 

annual cooling energy costs. The main contributors to the saving were daylighting, which reduced the 

internal gains from operating electric lights, and overhangs on south-facing windows, which block direct 

solar gains during the cooling season. Additional savings were achieved by using a two-stage evaporative 

cooler. 

Even though more fan energy use and airflow were required for the TTF evaporative cooling system, 

overall the system still saved significant energy costs. An advantage of using a 100% outdoor airconditioning system at TTF is that the system flushes the building and can handle very large loads (and 

variations of loads) that can be part of laboratory experiments. This would have been difficult with a 

traditional cooling system unless it were sized for 100% outside air. 

Both TTF and Zion evaporative cooling systems run all the time on hot days. This leads to issues about 

the comfort expectations of these systems. Although the buildings are maintained at ASHRAE comfort 

standards, the buildings are cooler in the mornings and warmer in the afternoons. Recovery from a 

mechanical or control issue can take 24 hours. 

3.3.5.2 Cooltowers work as well as a direct evaporative cooling system 

The Zion cooltowers function in two stages. The first stage occurs when the clerestory windows open and 

natural ventilation cools the space. When natural ventilation is inadequate, the first stage is augmented by 

58 
using the cooltowers to further reduce indoor temperature (second stage). We designed the cooltowers to 

operate on natural convection driven by buoyancy forces and prevailing winds. The cooltowers have 

evaporative cooling pads on all four sides at the top and large operable shutters on all four sides at the 

bottom. Air is cooled by pumping water over the evaporative cooling pads. This cool, dense air “falls” 

through the tower and exits through the large openings at the bottom of the towers, as shown in 

Figure 3-28. The cool air drawn into the building by the cooltowers causes the hot air inside the space to 

rise and exit the building through the open clerestory windows. 

There are no fans in either tower. The only energy required for each tower is a 1/3-hp (249-W) pump for 

water circulation to the evaporative pads. The building’s energy management computer controls the 

operable clerestory windows, shutter doors, and pumps. The shutter doors on the exterior of the building 

were intended for cooling the patio area, but they are rarely used because the air entering the patio area 

dissipates quickly and is not effective. In addition, using the exterior doors degrades the interior 

performance. Figure 3-29 and Figure 3-30 show the installed towers from outside and inside the building. 

Figure 3-28 Illustration of how the cooltowers work at the Zion Visitor Center 

Ceiling fans, controlled by the EMS, are in the main zone of the Visitor Center and in the break room. 

The fans are controlled based on zone temperatures, and help to keep air moving. We put small fans in 

the offices to provide additional circulation if needed. These fans exhaust air from the office and dump it 

into the exhibit area. During the winter, the cooltower doors are closed. 

59 
Figure 3-29 Natural downdraft cooltowers—exterior view 

Figure 3-30 Interior view of a cooltower showing operable shutters that allow cool air to 

enter the building through the bottom louvers 

The natural ventilation and cooltowers have provided adequate cooling for the Visitor Center Complex, 

with occasional periods of overheating and low flow rates. The Zion cooltowers work as well as a direct 

evaporative cooler. There were periods when the cooltowers could not meet the desired comfort range. 

This is typical of direct evaporative cooling. The system requires discharge into open areas because of 

the very low pressure drops. It does not work well in the enclosed offices at the fringe of the building. 

60 
Floor fans that blow air into these spaces have provided some relief. The problem is compounded with 

the Trombe wall summer gains (see Section 3.3.3). There are no issues with the Trombe wall and its 

interaction with the cooltowers in the open spaces. The building probably could have used a tower in the 

enclosed office area of the building, but it was eliminated during the design. 

The cooltowers produce more than 1.5 cfm/ft2

 (7.6 L/s/m2) of evaporatively cooled supply air, which is 

comparable to a fan-forced system. Measured airflows from each tower are approximately 6,000 to 8,000 

ft3

/min (2,382–3,776 L/s) during typical operation. (This number is only an approximation because of 

measurement difficulties for very low-pressure systems.) The cooltowers cool the Zion building with 

only a small water circulation pump and circulation fans that help push cool air into the enclosed office 

areas of the building. The cooling energy intensity was 1.28 kBtu/ft2

 (14.5 MJ/m2), which was 77% less 

than a typical building in the western United States that uses 5.5 kBtu/ft2

 (62.5 MJ/m2) (EIA 2005a). The 

cooltower in the comfort station works very well, as exact control is not needed. 

3.3.5.3 Provide multiple opportunities for passive airflow 

As a special class of passive natural ventilation, cooltowers require careful design of interior partitions 

and fenestration to promote good circulation of cooled air. The enclosed offices at Zion tend to overheat 

because of insufficient airflow and heat gains from the Trombe wall. Exhaust fans originally installed in 

the office area were not sufficient to move air through these spaces to counteract the heat gains from 

Trombe walls. As with many passive systems, moving air mechanically should be avoided. The Trombe 

wall is shaded in the summer, but the diffuse component of the solar radiation still heats the wall. In the 

initial building design, the Trombe walls were to be adjacent to open spaces, not to the enclosed offices. 

Late in the design process, the interior layout of the building was changed to place enclosed offices on the 

south side of the building adjacent to the Trombe walls. Even in the winter, this Trombe wall provides 

more heat than needed to the office spaces. As a result, circulation fans were installed between the public 

and private spaces to help induce additional air flow. These fans improved the comfort of the office 

spaces; however, they also increased the fan energy use and noise. 

3.3.5.4 Consider water use in an overall assessment of evaporative cooling systems 

Evaporative cooling systems use large quantities of water. The implications of water consumption for 

evaporative cooling versus water used at a power plant to generate electricity need to be factored into an 

overall assessment (Torcellini et al. 2003). Water consumption is a concern because of the semi-arid 

climate and the scarcity of water resources. From the installed water meters, 111,200 gal (420,938 L) of 

water were consumed during 2002 for the two cooltowers in the Visitor Center. Flaws in the design of 

the cooltower plumbing resulted in unnecessary water use, which occurred when the sump pit would 

overflow during on/off pump cycling. The pump cycling and sump pit overflow resulted in a 10% 

increase in water consumption. The cycling occurred during the swing season when the cooling was not 

required all the time. During this time, the pumps would run and saturate the pads and the sump pit 

would fill. The pumps would turn off because cooling was no longer needed and all the water in the pads 

would drain into the full sump pit and cause the pit to overflow. 

Another measure of cooltower success and excess water consumption is the effectiveness of delivering 

the evaporative cooled air to the building. By design, the dry air enters the building and picks up 

moisture. However, some of the air may exit through the pads into the environment driven by 

evaporation on the exterior of the pads or cross-flow through the top of the tower, which results in more 

evaporation than would be expected with traditional evaporative technologies. The towers contain a 

simple internal diagonal blocking device to minimize this effect (see Figure 3-31). 

No shading was designed into the cooltowers to shade the medium from direct exposure to sunlight, 

which increases the evaporation rate. Although the consequences were not quantified, researchers suspect 

that solar shading would reduce water consumption. The shading also reduces UV degradation of the 

media surface and the visible dissolved solids that accumulate on the exterior of the surface. 

61 
As an estimate, based on in-situ airflow, temperature, and humidity measurements, 50% of the total water 

used in the cooltowers is not used to cool the building. Much more detailed measurements are needed to 

validate and fine-tune this number. It does result in the conclusion that the efficiency of the towers 

should be improved with respect to water consumption. 

Figure 3-31 Plastic blocking device for cooltowers 

3.3.5.5 Verify installation and equipment quality of evaporative cooling systems 

Both the TTF and Zion evaporative cooling systems had setup problems related to the quality of the 

equipment and the installation. Evaporative systems must be designed for proper maintenance. The Zion 

system had problems with the size of the sump pit, the size of the drain lines, and the design of the frame 

holders, which allowed leaves to collect. The cooltowers trays that collect excess water extend about 1.5 

in. (3.8 cm) beyond the cooling medium. Leaves are caught in the trays and then migrate past the cooling 

medium at the corners and enter the water circulation system. The bleed valve has clogged on occasion 

because leaves collect in the system. Although improved details may reduce the debris, the water 

circulation system needs to be robust enough to handle it. Drains were poorly detailed and undersized, 

which resulted in overflows. To avoid wasting water, systems should be sized to avoid overflows. The 

cooltowers include a sump pit that was sized too small to hold all the in-transit water. The installed sump 

pit has a 20-gal (76-L) capacity. The trays and media, when wetted, hold about 20–30 gal (76–114 L) of 

water, which enables about 10–15 gal (38–57 L) of water to overflow the drain when the pump cycles off. 

This water is made up with fresh water when the pumps turn back on. 

The TTF evaporative cooling unit underwent many alterations to improve its efficiency and reliability. 

When the building was first occupied, the cooling system did not operate properly. The engineer 

responded that an evaporative system could not meet the load; however, NREL staff monitored the 

system and observed that the unit’s performance was significantly lower than specified by the 

manufacturer. Finally, NREL staff dismantled the unit to find that the direct section was not plumbed 

properly and the cooling medium was not wet. Only a small amount of the cooling medium was being hit 

with pumped water; the rest was completely dry. Although the direct suction pump operated normally, 

the nozzle responsible for spraying water onto the evaporative medium was never properly attached. The 

62 
investigation was complicated by the fact that the medium was not accessible as indicated in the 

specifications. This inaccessibility affects maintenance and commissioning of the building. 

The TTF’s evaporative cooling unit’s drain pan failed. The on-site fabricator of this drain pan used steel 

bolts to attach the pan to the evaporative unit. These bolts rusted through and resulted in a substantial 

leak. Although the mechanical room is outfitted with drains, expansion joints in the floor were located 

between the pan and the drain. Water leaked to the floor below the mechanical room and caused some 

damage to the copy room and restrooms. Stainless steel bolts and a replacement seal were used to repair 

the drain pan. All these failures can be attributed to system design—units such as these should be factory 

assembled and tested. None of these failures are attributed to the direct/indirect technology, which was 

successful in this building. 

3.3.5.6 Use outdoor air for the indirect section of an evaporative cooling system 

Several improvements could be made to the TTF’s evaporative cooling system. The indirect part of the 

system currently scavenges air from inside the building, but using outdoor air would be better. Because 

the TTF has a built-up central air handler, using indoor air was simpler than using outdoor air because it 

involves less ducting. However, the outdoor humidity is usually lower than that inside the TTF because 

of the direct evaporative cooling and process loads (especially when the desiccant test loop is operating). 

The indirect evaporative section would likely be more effective with outdoor air. The air handler draws 

outdoor air from the south side of the building, which is preheated by the roof. The air would be cooler if 

it were drawn from the north side. The evaporative cooler control is currently based on dry-bulb 

temperatures; more sophisticated controls based on a real-time psychrometric analysis of the various air 

streams could improve the determination of the most efficient means of cooling. They require minimal 

maintenance if drains are properly sized from the pads, pad trays are designed so that debris will not clog 

them, and pads can be examined and replaced if necessary. 

3.3.6 Ground-source heat pumps 

3.3.6.1 Carefully size heat pumps and ground loops 

Ground-source heat pumps with ERVs are the primary heating and cooling sources at Oberlin, CBF, and 

Cambria, and each heat pump system had performance issues related to either equipment or ground 

capacity sizing. 

The Cambria heat pump system has a slow response time, which results in small temperature setbacks and 

long startup periods. The heat pump cooling capacity is 54 tons (189 kW), which may be slightly 

undersized because it takes a long time to recover from a setback. This situation is compounded by the 

control of the ERVs, which should not be used on cold winter mornings until occupants arrive, and could 

be used in warmer weather to help cool the building before occupants arrive. 

A study of the effectiveness of the under-floor air distribution system at Cambria was completed by 

Lawrence Berkeley National Laboratory researchers in 2004 (Fisk et al. 2004). They found that the air 

change effectiveness was about the same as a zone with well-mixed air, but the pollutant removal 

efficiency for CO2 was 13% better than expected in a zone with well-mixed air. The thermal stratification 

in the zones during cooling mode operation was only 2°F–4°F (1°C–2°C) between just above the floor 

and the return air registers. This low thermal stratification is caused by higher than necessary supply 

airflow rates and low internal gains (partially occupied building). The thermal stratification would 

probably increase if the building were fully occupied or if there were a variable air volume system that 

reduced airflow with reduced zone loads. Thermal stratification is desirable because it can lead to energy 

savings during cooling. 

Based on the measured energy performance, the CBF ground well field may be undersized for the actual 

loads. This lesson stems from findings from the CBF analysis that shows temperatures in the ground loop 

fluctuate widely and are often higher than expected. The sizing calculations should be redone to include 

63 
the unused desiccant dehumidification system. Finally, follow-on efforts should verify that the field has 

been installed as designed and that all wells have the appropriate flow rates. 

The monitored data also provide a way to compare the ground-loop supply temperatures to the outside air 

temperature. The temperatures of heat sinks largely determine the thermodynamic efficiency of heat 

pump cooling. The purported advantage of a ground-loop arrangement that uses water-to-air heat pumps 

over rooftop packages with air-to-air direct expansion mechanical cooling is that ground-loop 

temperatures are expected to be lower than air temperatures and lead to more efficient cooling. NREL 

analyzed this assumption at CBF by comparing the ground loop and outdoor air temperature data for the 

model analysis year. We analyzed the data to determine how outdoor air dry-bulb temperatures compared 

to the ground-loop temperature. The data show that for 61% of the time, cooling occurred and the 

outdoor air was actually cooler than the ground loop that supplies the heat pumps. Analysis of energy use 

shows that of the 71.4 MWh of electricity used for cooling, 42% was used when the outdoor temperature 

was cooler than the ground-loop supply. 

As part of a follow-up evaluation of the CBF ground-source heat pump system, the addition of a cooling 

tower should be considered. Currently, outdoor air temperatures are cooler than the ground-loop 

temperatures 61% of the time (mostly during the winter and shoulder seasons) and about four times more 

heat is added to the ground for cooling than is extracted for heating. With such a California-style heat 

pump arrangement, the ground-source heat pumps would be used only when ground temperatures are 

more favorable than ambient conditions. Further analyses should be performed to study the rainwater 

collection system to determine whether a wet cooling tower would be feasible; otherwise, a dry cooling 

tower would need no water. 

The Oberlin heat pumps were not specified correctly, as they are not rated for ground-source water 

temperatures. Extended-range rather than standard-range heat pumps at Oberlin would have been the 

proper system for ground-source water temperatures. The installed standard range heat pumps typically 

operate outside the recommend ground-source water temperature range. In addition, a backup electric 

boiler is needed if the standard range heat pumps need additional capacity. Specifications for heat pumps 

must work with appropriate ground water temperatures. Appropriately rated ARI-330 ground-source heat 

pumps would increase the operational efficiency and provide operational capacity (ARI 2003). 

3.3.6.2 Use models with short-term response when designing ground-source heat exchangers 

Ground-source heat exchangers and heat pumps have been promoted for low-energy buildings. 

Experience with the CBF ground-source system indicates that although the system can deliver good space 

comfort, temperature fluctuations in the ground loop are wider than expected. Monitored data for fluid 

temperatures that return from the ground heat exchanger clearly show that loop temperatures can be high 

and show considerable seasonal and daily fluctuations. When modeling is used to design such systems, 

the models typically use constant fluid temperatures for each month. Although such analysis attempts to 

ensure that the field can perform over a long period (years), they do not capture the fluctuation of 

temperatures over the short term. The next-generation design tools for ground-source heat pumps, such 

as those now available in EnergyPlus (EnergyPlus 2005), should be applied as these systems are 

evaluated and designed. 

3.3.6.3 Provide for part-load groundwater pump control 

VSD pumps at Oberlin and CBF circulate the glycol-water mix through ground wells and to the heat 

pumps. The ground-loop pumps are controlled to provide a constant pressure difference between the 

water supply and return. As various heat pump packages cycle on or off, that portion of the water loop is 

opened or closed, which affects the pressure. When there is no demand for the ground-source loop, the 

VSD controls the pumps to 5% of full rated pump power. The groundwater circulation pumps at Cambria 

are mounted in series to match the flow and head requirements of each heat pump. This configuration is 

not the most efficient, but it is often used because pump sizes may have limited availability and many 

contractors want to use only one type of pump. The pumps are rated at 230 W each, but they were 

64 
measured to draw approximately 200 W each. The pumps run continuously; however, they need to 

operate only when the compressor is running. There was a concern that the capacitance effects of the 

ground might adversely affect the system when the pumps run in a cycling mode. According to a study 

done by Kavanaugh and Rafferty (1997), this is not a concern, and the best method of operating the 

circulation pumps in this configuration is to tie them to the compressor operation. A conservative 

estimate is that the compressors run an average of 50% of the time. The annual saving of linking the 

pumps with the compressor operation would be 18 MWh and approximately $1,300. 

As part of this recommendation, the control algorithms for VSD ground-loop pumps should be examined. 

Perhaps loop pump flow should be controlled based on supply temperatures (or temperature differences) 

with limits set by pressure. Such research should seek to balance pump energy, ground heat exchanger 

effectiveness, and heat pump efficiencies. 

3.3.7 Energy recovery ventilators 

ERVs were designed into the HVAC systems at Cambria, CBF, and Oberlin to recover energy from 

exhaust air to preheat or precool supply outdoor air. ERVs are often used to save energy required to 

condition incoming air. A properly controlled ERV can realize energy savings; the same ERV improperly 

controlled may actually increase energy use and cost. 

3.3.7.1 ERVs should be designed in conjunction with economizers 

Commercial buildings need a mechanism to utilize outdoor air for “free cooling”, especially when 

outdoor air is 50oF–70oF (10°C–21°C) and the building needs cooling. In this range, ERVs provide 

minimal heat recovery and add energy to systems that should use less energy. Design teams view ERVs 

as big energy savers and discard well-proven economizer concepts. Cambria, Oberlin, and CBF all had 

ERVs with no economizer cycle and high swing season loads. 

ERVs should include bypass dampers to use outside air for cooling for economizer operation without the 

overhead of the ERV fans. A schematic of an outdoor air heat pump system with an ERV bypass is 

shown in Figure 3-32. At Cambria, a conservative outdoor air temperature range for not operating the 

ERVs is 50°F–60°F (10°C–16°C), which represented about 13% of the weekday hours between 5:00 a.m. 

and 5:00 p.m. in 2002 and 2003. Above 55°F (13°C), the heat pumps may have to provide some cooling, 

but the ERV fans would not have to be used. Above 60°F (16°C), the outside air may have high moisture 

levels that would have to be removed by the heat pumps. Enthalpy control on the economizer would 

expand the economizer operation range and provide more hours for using outside air for cooling, although 

caution must be used with this option as enthalpy controls often are not reliable. Standard 90.1-2004 

(ASHRAE 2004) specifies economizer types for climates and HVAC systems. 

65 
Outdoor 

Air Heat 

Pump

ERV

Supply Air

Bypass Loop 

Damper

 Supply Fan

Return Air 

Outdoor 

Air

Exhaust 

Air

D X

Ground Source Loop

ERV Bypass for Economizer

Figure 3-32 Heat pump outdoor air system with ERV bypass for economization cycle 

The energy recovery desiccant wheel dehumidification system at CBF is not used as designed and its 

potential benefits should be evaluated. If the wheels are not going to be used, they should be removed 

because they increase the pressure drop and fan energy used to provide fresh air. One option is to use the 

wheels in a heat recovery mode without using the desiccant regeneration coil. If the wheels are retained 

for use and an economizer system is added, the air handler should be reconfigured to allow the wheels to 

be bypassed during economizer operation to minimize fan energy. 

3.3.7.2 Control ERVs to recover energy only when it is a net saving to the building and when outdoor 

air is needed 

ERVs can represent one of the largest factors in HVAC energy use. The ERV fans consume a 

considerable amount of fan energy to bring in large amounts of outside air, which must be conditioned. 

The first step in reducing energy use associated with outdoor air should be a demand-responsive 

ventilation system in conjunction with an economizer cycle that supplies outdoor air only when and 

where it is needed. An ERV that operates continuously can result in significant fan energy use. In 

addition, it may recover energy when outdoor air is not needed, thereby increasing heating or cooling 

energy use. ERVs integrated with a demand-responsive ventilation system can then save further energy 

when outdoor air is needed, contingent on an economizer cycle and controls that allow for appropriate 

operation. 

The Cambria ERVs initially provided outdoor air continuously. They were reprogrammed to run only 

during occupied hours, which reduced the fan and heat pump energy required to condition the extra 

outdoor air. This simple control change resulted in an estimated 13% energy savings as shown in 

Table 3-9. In addition, the ERVs at Cambria are more than twice as large as they need to be for outside 

air requirements. Reducing the ERV flow rate by one-half would reduce the fan energy consumption to 

about one-eighth of its current value because fan power is related to the cube of the fan flow rate. 

Additional savings could be achieved with a demand-responsive ventilation system that supplies outdoor 

air only when needed. A similar outdoor air system would also be effective at Oberlin. 

66 
Table 3-9 Cambria Energy Impacts of Changes to the ERV Control 

Model 

Annual 

Energy 

(MWh) 

Annual 

Energy Cost 

Energy 

Savings 

Cost 

Savings 

As-Built Model 344 $35,473 

As-Built Model with ERVs running 

continuously 397 $39,737 −13% −$4,260 

As-Built Model with ERVs replaced 

by airside economizers 315 $32,533 8% $2,940 

ERVs effectively precondition outside air, but should be used only if the recovered energy is greater than 

the energy required to operate the ERV unit and can recover energy at greater efficiency than the heating 

or cooling equipment. For example, at Oberlin, the power requirements of the supply fan, return fan, and 

enthalpy wheel in the ERV were 1.7 kW. As Figure 3-33 shows, the Oberlin ERV uses more energy than 

it recovers when the outdoor temperatures are 70°F−87°F (21°C–31°C). In addition, the outdoor air heat 

pump is rated at a coefficient of performance (COP) of 3.3. For the ERV to be a net energy benefit, the 

recovery efficiency has to be greater than the heat pump efficiency. In this case, the energy recovery 

efficiency is greater than the heat pump efficiency when the outdoor air temperature is less than 55°F–

60°F (13°C–16°C); therefore, it should be operated only at outdoor air temperatures lower than this. 

A portion of the 1.7 kW that the supply fan motor and enthalpy wheel motor of the ERV consumes is 

directly added to the supply air in the form of heat, because these motors are located in the supply air 

duct. This motor placement is beneficial during the heating season, as the heat from the motors is directly 

added to the supply air stream. During the cooling season, this added motor heat counteracts the cooling 

energy recovered from the exhaust air. When heat is added to the recovered cooling energy supply, it is 

only possible to recover more cooling energy than is required to operate the ERV at outdoor temperatures 

greater than 87°F (31°C). 

67 
0

25,000

50,000

75,000

100,000

125,000

10 20 30 40 50 60 70 80 90

Outdoor Temperature (°F)

ERV Energy Recovered (Btu/hr)

0

5

10

15

20

25

Coefficient of Performance (COP)

Before filter change

After filter change

Energy Required To Operate ERV (COP=1)

Rated Heat Pump 

Efficiency (COP=3.3)

Figure 3-33 Calculated Oberlin ERV energy recovery as a function of outdoor temperature

A properly controlled ERV can realize energy savings; the same ERV improperly controlled may increase 

energy use and cost. ERVs should not replace proper controls for the amount and scheduling of outdoor 

air, appropriate equipment sizing, or for conventional economizer cycles. ERV recovery should be used 

when recovered energy is a net saving and only be used when needed, integrated with a demandresponsive ventilation system with an economizer cycle. 

3.3.7.3 Consider energy recovery for all exhaust air streams 

The air flow balance between supply and exhaust in an ERV is critical to effective energy recovery. 

Often, energy in exhaust flows from bathrooms, kitchens, and process loads is not recovered. This can 

create an imbalance in the ERV unit that limits recovery effectiveness. In addition, the unrecovered 

energy from exhaust air can be significant compared to the outdoor air flow and reduce the overall energy 

recovery from a building’s exhaust flow. 

At Oberlin, we considered the overall recovery effectiveness of the total building exhaust and supply, 

assuming rated ERV recovery effectiveness and flow rates. The separate exhaust fans in the wastewater 

treatment, bathrooms, kitchen, and mechanical rooms are rated to exhaust conditioned room air directly 

outside; this accounts for up to 3,345 cfm (1,579 L/s) with no energy recovery. These exhaust fans create 

an airflow imbalance in the ERV that diminishes total building recovery capacity. Based on a balance of 

rated supply and exhaust flows into and out of the building, only half of the potential return air is 

exhausted through the energy recovery units. The room air that is exhausted through the wastewater 

treatment, kitchen, and bathrooms has no heat recovery, or a recovery effectiveness of 0.0. Combined, 

the total rated building exhaust recovery effectiveness is 46%. 

68 
ERVs must have balanced air flows for optimal exhaust recovery. This requires that restrooms, kitchens, 

and other exhaust flows be exhausted through the ERV. This may limit the type or configuration of 

ERVs used, as they could be staged between wheel types and fixed heat exchanger types to prevent crosscontamination. 

3.3.8 Electric resistance heating 

An electric radiant heating system was used at Zion because it offered several advantages. The Visitor 

Center Complex buildings require little heat, and a ducted air system was not used for cooling. Therefore, 

adding a mechanical air system and the associated ductwork for a small amount of heating was considered 

too costly and complex. We arranged electric radiant heaters to direct heat at the locations where staff 

would spend the most time. Results show that occupants allowed lower thermostat temperature set 

points; this is most likely because the radiant heating can provide comfort at lower dry-bulb temperatures. 

Electric radiant heaters also allow for a more cost-effective and precise temperature control because many 

thermal zones can be created. Although the electric heaters contribute to demand charges, they are simple 

to control. The costs of propane and associated transportation were key considerations in the creation of 

an all-electric building. The key is heating the building without incurring additional demand charges. 

This decision is highly dependent on the metrics used to measure success. The solution was successful on 

a cost basis, but was less favorable on a source energy basis. 

Electric boilers were originally designed into the hydronic under-floor radiant system at Oberlin. 

However, the design of the electric boiler hydronic system did not meet the design intent of the rest of the 

building. This design flaw, combined with original inadequate advanced controls, resulted in the initial 

limited energy savings and high demand charges. The unstaged 112-kW electronic boiler was replaced 

with water heating water-source heat pumps. An electric boiler was still used for backup capacity for the 

ground loop. Electric boilers can be used as a backup source, if they are used sparingly and they do not 

cause excessive demand charges on the building. Controls and staging are essential to integrate such 

limited use systems. 

3.3.9 HVAC controls 

3.3.9.1 Design the control system to be fully integrated with the capabilities of equipment and building 

operators 

Innovative controls are often crucial to achieving the potential of a low-energy building. In many cases, 

the energy savings comes from integration of standard technologies—the integration is accomplished with 

innovative sequencing and control strategies. Not having whole building control algorithms often results 

in non-optimal performance—that is, buildings that fall short of their design goals. In some cases a lowenergy building may have a unique design philosophy and nonstandard equipment and sensors that 

require special emphasis on designing control systems. A traditional approach to designing control 

systems may not meet the needs of coordinating HVAC and lighting systems in a low-energy building. 

For example, the TTF fans inside the fan-powered VAV boxes could not be turned off (these are used for 

heating, but not for cooling—this can also be considered a specification issue). The human-building 

relationship must also be considered during design and implemented properly so occupants and operators 

can interact with the low-energy features. 

There are trade-offs in using simple controls versus a complicated, yet flexible EMS. At Cambria, the 

simple HVAC controls included thermostats on each heat pump. The thermostats are less expensive and 

easier to program than a centralized control system. However, keeping the time and set points on all of 

the thermostats is difficult. The current system would work well if one or two people with proper training 

had access to the thermostats. On multiple occasions innovative systems were not used or optimized 

because of difficulties in understanding the complex algorithms needed to integrate the systems. For 

example, at Oberlin, a lack of staging on the electric boilers resulted in significant demand charges. 

Carbon Dioxide sensors were installed, but not programmed to control outdoor air. Natural ventilation 

69 
was designed into the atrium, but never used because of difficulties developing and implementing 

appropriate controls. In all cases, daylighting controls were tuned to increase lighting savings. In the 

three buildings with ERVs, controls were changed or changes recommended that would result in 

additional energy savings. 

Using the EMS for data collection was problematic, as discussed in Section 3.7.2.2, but direct access to 

the building controls was beneficial to researchers for understanding and influencing how the building 

was controlled. At Zion, researchers had access to and control of the EMS and were able to improve 

energy management strategies such as implementing and testing various demand-responsive strategies. 

Access to the EMS allowed us to reduce the demand charges to the building and facilitated continuous 

commissioning activities. Another advantage of EMS access and control is having a full understanding of 

the various set points and control strategies to use as inputs into building energy simulation. 

3.3.9.2 Budget for postcommissioning controls, tuning, and alterations 

The usual commissioning activities do little to improve the operation of a building beyond its design 

capabilities. Follow-up procedures that are intended to tune, alter, and adjust building operation are 

equally important. Experience with the TTF shows that after a building is constructed (and has been 

commissioned to verify its system components are as designed) there is a significant need to adjust or 

change lighting and HVAC controls to further reduce energy use and improve occupant satisfaction. 

Predictive capabilities during the design phase are not perfect, and once the building is up and running 

there should be an opportunity to revisit the design and make changes. Preprogrammed controllers do not 

always take full advantage of potential energy-saving measures and may need to be reprogrammed. 

The obvious expense of postcommissioning tuning and alterations, which for TTF took a “fleet of PhDs” 

as one engineer commented, suggests the need to develop more advanced control systems to facilitate 

such activities. The “black box” method of control could work with simple interfaces, if they are 

designed to maximize the energy saving potential. Robust controllers need to be developed that are 

related to technologies used in this building—evaporative cooling, daylighting systems, demandresponsive controllers, and variable-speed supply fan systems—for implementation in other buildings. 

3.4 Photovoltaic Systems 

For the ZEB goal to become a reality, energy production through PV systems on the roofs of buildings 

will have to produce more energy than the building uses. Future ZEBs will not only require efficient 

energy use, they will need to maximize energy production. Therefore, lessons learned and best practices 

related to maximizing PV systems energy output are valuable for future generations of ZEBs. Five of the 

six buildings in the case studies used PV systems connected to the building’s electrical system to offset 

electricity use, as shown in Figure 3-34. The Zion PV system is integrated with an uninterruptible power 

supply (UPS), as shown in Figure 3-35. PV system AC capacities are shown in Table 3-10. 

70 
Figure 3-34 Grid-connected PV system configuration with isolation transformers 

Figure 3-35 UPS grid-connected PV system configuration 

For each PV system, we compared measured system performance to predicted performance to better 

understand the effects of the system failures and sources of performance degradation. We determined 

expected production without degradation losses in each case. We simulated annual output with measured 

weather to determine performance with maximum power-point trackers (MPPTs), without shading, 

inverter faults, or standby losses. Degradation losses, measured output, and expected performance are 

71 
summarized in Table 3-10. All systems produced less than expected. Each system output was degraded 

because of balance of system components or operational problems and shading, which reduced overall 

output from 14 to 68%. PV system output ranged from 0.43 to 1.09 kWh/WR. 

Table 3-10 PV Systems Performance Summary 

Oberlin Zion Cambria CBF Bighorn 

PV System DC Rated Capacity 

(kWR) 60 kW 7.2 kW 17.2 kW 4.2 kW 8.9 kW 

Measured Annual Net PV 

Production (Total PV Production 

—Standby Losses) (kWh/yr)

55,154 7,861 7,912 2,676 3,800 

Measured PV Production per 

Rated Watt of DC Capacity 

(kWh/WR) 

0.92 1.09 0.46 0.64 0.43 

PV System Standby Use 

(kWh/yr) 

4,364 

(7% of 

total) 

minimal 

4,647 

(37% of 

total) 

minimal minimal 

Snow X X X 

Shading X X 

Inverter 

Operational 

Faults 

X X X X 

No Maximum 

Power Point 

Tracking 

X X 

Significant 

Performance 

Degradation 

Sources 

Standby Use X X 

Production Reduced due to 

System Degradation Losses 

(percent reduction) 

14% 19% 56% 48% 68% 

Expected Production per Rated 

Watt of DC Capacity without 

Degradation Losses (kWh/WR) 

1.07 1.35 1.04 1.22 1.34 

72 
3.4.1 Design grid-connected PV systems to have no parasitic standby loads 

During nighttime hours when the Oberlin or Cambria PV system was in standby mode, the inverters and 

transformers consumed electricity. The inefficiency of the isolation transformers in these systems results 

in a power draw of approximately 300 W per 15 kVA transformer. At Oberlin, this standby parasitic load 

of the three inverters and isolation transformers was a constant 900 to 1000 W during times of no PV 

production. The primary purpose of the isolation transformers was to transform the three-phase AC 208-

delta output of the inverters to utility-compatible, three-phase AC 208-wye/120. The Oberlin no-load 

transformer inefficiency of 2% of rated capacity resulted in a standby loss of 4,363.5 kWh/yr, or 7.3% of 

the total PV production. This does not include transformer losses when the PV system is generating 

power. 

Cambria’s inverter faults caused considerably greater standby losses. The causes of the inverter faults 

were a high AC voltage and high temperature. The high temperature fault was the most severe because 

the system would have to be manually reset. The inverter was removed and sent to the manufacturer in 

December 2003 and replaced with a new unit in February 2004. From May 31, 2002 to December 31, 

2003, the main PV system produced no energy on 50% of the days because of inverter problems. On 

many other days the system was operational for only part of the day because of inverter problems. From 

May 31, 2002 to December 31, 2003, the parasitic load on the PV system equaled 40% of the energy 

delivered to the building by the main PV system. Most of the parasitic load (37%) occurred when the PV 

system was down at night or because of an inverter fault; the other 3% were transformer losses during PV 

system operation. From the time the inverter was replaced on February 20, 2004 to December 31, 2004, 

the main PV system was down only three days because the whole system was shut down. During this 

same period, the parasitic load of the isolation transformer was 18% of the total energy delivered to the 

building. The monthly parasitic load varies from 11% in summer to more than 50% in winter. 

If the inverters have near optimal operation throughout the year, nighttime parasitic losses were 7% of the 

total PV output. With some additional downtime caused by snow and inverter faults, about 18% of the 

total output is needed for standby losses. A worst-case scenario, 37% of the total PV output was needed 

for standby losses. An automatic disconnect circuit that disconnects the PV system from the grid when 

the PV system is down and reconnects when the PV system is operational should be implemented. 

Disconnect controls should be added to these systems to avoid the large losses when the PV system is 

down, or an inverter system that does not have high parasitic loads should be used. 

3.4.2 Consider an automatic monitoring system for PV system operation 

Occupants and operators were often unaware of extended down times, which limits useful output. There 

is no way to know whether a grid-tied PV system is operating correctly without manually checking the 

inverter output on its display terminal or unless someone downloads and examines the power data from 

the detailed monitoring system. Continual manual monitoring is not practical, so a simple automated 

system should be put in place that alerts the building operators when the system is down. A simple 

method or monitoring technique that provides feedback to the owners and occupants about PV generation 

effectiveness or performance ratio would address the simple problem of inverter faults that require 

manual restarts. This fault, typically caused by system integration issues, has been a primary source of 

PV system downtime in the case studies. 

For example, the BigHorn PV system was estimated to be fully functional for only about one-third of the 

days during the two-year monitoring period and operated with only one of the three inverters for half the 

time. During periods of high output by the PV panels, the DC current exceeds the breaker limit (60 A) 

and trips the inverters offline. An additional intermittent problem, which became worse after the arrays 

were rewired, is that the battery circuitry occasionally fails to provide the proper power to the inverters 

when they are not producing power. This causes them to lose their memory and shut down. Because of 

the overcurrent and the battery circuit problems, one or both inverters shut down frequently. Because 

there is no automated means of alerting the building operators when an inverter has failed, they must be 

73 
manually checked. Relying on manual checking and resetting the inverters has led to long periods when 

the system is not fully operational. 

3.4.3 Locate PV panels where they will not be shaded 

Each PV array experienced shading that resulted in energy penalties of 2% to 44% output reduction. 

Sources of shade included trees, canyon walls, snow, and the building structure. PV panels should be 

located where they will not be shaded. The annual performance of the Zion PV system is degraded by 7% 

because a tree partially shades the west set of modules late in the afternoon (see Figure 3-36). CBF’s PV 

array is shaded by the building’s exterior structure (see Figure 3-37). The building shading accounts for 

all known system degradations, reducing output by 44%. Peak summer output of the CBF array has been 

reduced by shading to the point that the system provides more energy in the winter and swing seasons 

than the summer. 

Figure 3-36 Tree shading of Zion PV array 

Figure 3-37 PV array and shading caused by CBF building structure 

74 
Snow reduced the overall annual PV production by about 1.2% at Oberlin. Figure 3-38 is a photograph of 

accumulated snow on the flat sections of the Oberlin PV array, which typically have more snow 

accumulation than the tilted sections. Although not quantified, snow also shaded BigHorn’s and 

Cambria’s PV systems. The BigHorn PV system consists of amorphous silicon panels that are laminated 

onto the conventional standing-seam metal roofing. Some of the BigHorn PV panels showed signs of 

deteriorating after three years. The plastic laminate separated from the PV material in small areas spread 

over the PV panels. This problem occurred only on the 120-W panels in the area that has the most snow 

and ice coverage. 

Figure 3-38 Oberlin PV array partially covered with snow 

3.4.4 Consider specifications for how a PV-based UPS system transitions to and from utility power 

The Zion PV system is a high-value feature even though it displaces only a small part of the total energy 

load. During daylit hours, the system can provide power for business operations without relying on 

power stored in the UPS system battery bank. This system handled 40 power outages that ranged from 

1 second to 8 hours during the monitoring period. From February 2001 through May 2002, the building 

power was monitored to verify the operation of the UPS system. Because of the unreliable power in the 

area, the power shut off many times. A couple of times, the UPS system could not maintain clean power. 

If the grid shut down for less than 0.5 seconds, the scan rate on the EMS did not record the power failure. 

We believe that the panel had many split-second disturbances that the EMS could not record. The total 

time the grid was unavailable and the UPS system was functional during the evaluation period was 107.4 

hours or 2.6% of the time. The UPS maintained power to the building and the EMS during all but two of 

these instances. For example, in one case the power turned on and off 40 times in less than 5 seconds 

before disconnecting. The main UPS system has thus had difficulties determining when to disconnect 

from and reconnect to the utility power. The reliability of the PV system has been excellent, but its use as 

a UPS has not been satisfactory. As a result, some smaller, self-contained UPS systems were installed on 

critical devices, creating additional parasitic loads. Future efforts to use PV systems for building-wide 

UPS systems should carefully investigate how the equipment responds to the types of power failures the 

building is likely to experience. 

75 
3.4.5 Use maximum power-point tracking controllers 

A performance-limiting feature of the Zion PV system is the fixed-array voltage control required for UPS 

battery charging. The PV array operating voltage is set at 53.6 VDC, as this is the float-voltage set point 

of the inverters. The maximum power point (MPP) of the array is often greater than the fixed-voltage set 

point. An MPPT voltage controller would increase performance of the Zion PV system by 16% as 

compared to the fixed operating voltage control of 53.6 VDC. For summer weather conditions of 104°F 

(40°C) and a wind speed of 3.28 ft/s (1 m/s), the MPP is 54 VDC to 59 VDC, depending on solar 

insolation and cell temperature. For winter weather conditions of 32°F (0°C) and a wind speed of 9.8 ft/s 

(3 m/s), the MPP is 69 VDC to 73 VDC, depending on insolation and cell temperature. 

The BigHorn inverters also do not have MPPT ability. The inverters were not designed to operate in a 

grid-tied system; therefore, circuitry was included to couple the inverters to the grid. This circuitry 

includes four 12-V batteries used to maintain the inverter memories at night. The battery backup system 

is tied to the Phase-A inverter to maintain the battery charge. Charging the external battery backup 

requires the Phase-A inverter float voltage to be set at 50 VDC. The Phase-B and Phase-C inverters were 

also set at 50 VDC to match the Phase-A setting. Cell temperature affects the output of the amorphoussilicon PV modules only slightly; however, the peak power-point voltage drops as the cell temperature 

rises. Because the modules are integrated into the insulated roof of the BigHorn Center, there is little heat 

loss through the backs of the modules and cell temperatures can exceed 170°F (77°C). The annual 

average AC Generation Effectiveness values were 3.3% for the BigHorn PV system without inverter 

faults and 3.9% for the same PV array and an MPPT inverter, which is a 20% annual improvement with 

the MPPT inverters over the use of fixed voltage inverters. 

3.4.6 Grid-connected PV systems and inverters should be carefully designed as an integrated system 

Numerous problems with the original BigHorn PV system design have limited its useful output. We have 

recommended that the PV system inverters be replaced with ones that are designed to be grid-tied to 

significantly improve system reliability and performance. The Cambria inverter was replaced because of 

frequent faults that significantly reduced overall output. The new inverter has minimized downtime and 

increased system output. 

The Oberlin balance of system components have had limited operational issues. A performance 

degradation issue arose because the inverters shut down near peak operating limits, as illustrated in 

Figure 3-39. Occasionally, when the inverters operated at the maximum power output of 15 kW AC, they 

automatically tripped off, and then restarted after five minutes. To identify this inverter fault, minute data 

collected on PV system performance were considered. Because the automatic restart after an inverter 

fault of this type was 5 minutes, an identifiable pattern in PV production displayed by the minute was 

evident. Figure 3-39 shows a day when this occurred multiple times with all three inverters. Displaying 

the AC PV production in hourly averages does not identify this inverter fault pattern. To fix this problem, 

the inverter maximum allowable utility voltage output set point was increased, as the error code from the 

inverter indicated a high AC line voltage fault. 

76 
0

5

10

15

20

25

30

35

40

45

50

5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

Time of Day (hr)

AC and DC PV Power Produced (kW)

PV SubArrayA DC 

PV SubArrayB DC

Total AC PV output

Total AC PV Output

(Hourly average)

Figure 3-39 Oberlin inverter shutdown example with minute and hourly PV performance 

Other Oberlin PV system component faults included occasional AC PV circuit breaker trips and 

additional unidentified inverter faults. A single case of circuit breaker faults was documented. The cause 

of the breaker faults probably involved a high inrush current required to energize the isolation 

transformers during a loss of grid power to the PV system. We calculated that 707.9 kWh, or 1.2% of the 

annual PV production, was lost because of PV system downtime related to system component faults. 

This time was minimized because continuous monitoring detected trips and inverters were quickly reset 

by attentive occupants. Without monitoring, these downtimes may have been much longer. 

3.4.7 Integrate PV systems with demand-responsive controls and on-site thermal storage to maximize 

energy cost savings and payback 

The economic payback of PV systems depends on the net metering and interaction with demand charges. 

PV systems reduce building peak electrical demands very little. One of the largest variable electric loads 

in these daylit building is the lights, which are needed most when the sun is down. The peak summer 

building electric load is typically in the late afternoon to early evening; the PV peak output is in the 

middle of the day. This means that the PV system output does not correspond well with the building load 

profile. Therefore, the payback for the PV system is poor because it offsets only electricity energy 

charges, which are typically only $0.02–$0.05/kWh. See Section 3.5 for further discussion of peak 

demand profiles.

As PV typically does not significantly reduce peak demand charges, alternative methods for reducing 

peak demands in future ZEBs that have large PV systems will have to be considered. The Oberlin PV 

production does reduce energy consumption charges; however, the electricity demand costs in this allelectric building are not typically reduced. Cost justifying the PV system solely on energy cost reductions 

is difficult. There is a great potential for reducing peak demands and demand charges with the PV system 

in the case of the Oberlin’s 60-kW PV system. Reducing peak demands with Oberlin’s PV system would 

require demand responsive controls that limit electrical loads during periods of minimal PV production 

77 
(during cloudy periods or at night). Additional demand reduction cost savings that result from the PV 

system would increase the cost-justification of the PV system. 

In future ZEBs, energy use will decrease and site energy production will increase, which will result in 

significantly lower net site energy use. In contrast, peak electrical demands are not reduced at the same 

rate as the net site energy use, as PV does not offset peak demand as it does with energy use. The 

building’s peak electrical demand and associated charges will become even larger components of the 

overall energy costs. Therefore, technologies to reduce peak demands in future ZEBs, such as demandresponsive controls and ice storage systems, will become more important in realizing energy costs savings 

that are comparable to energy savings. 

The net metering rate structures will become even more important in future ZEBs, as the size of PV 

systems on buildings increases and energy use decreases. For net metering agreements that credit only 

exported PV energy at the same rate (or less) than the utility charges per kilowatt-hour, the full value of 

the PV energy is not realized. On-site storage systems such as ice storage combined with demandresponsive EMS will be required to realize the full economic value of the energy and power produced by 

the PV systems. With on-site storage, a significant PV system, and demand-responsive controls, future 

ZEBs will be able to use the full potential of PV systems to reduce demand charges and energy charges. 

In terms of a ZEB goal, on-site storage may be in conflict with this vision, as the primary purpose of 

storage is to reduce peak demands and energy costs, not energy use. Without net metering utility rates 

that do not account for the full value of on-site generation, using some amount of the PV energy to reduce 

demand first, and then supplying excess PV to the grid, is more cost effective. The balance of when to 

use PV for storage for reducing demand and when to supply excess PV to the grid to offset energy use to 

reach the ZEB goals should be investigated further. 

3.5 Peak Demand and Demand Management 

The easiest method to reduce energy cost in a commercial building is to reduce the peak demand charges. 

As seen in Figure 3-40, the Oberlin demand charges contribute more than 50% of the total costs. The 

BigHorn electrical demand charges and associated taxes constitute a significant part (59−80%) of the 

monthly electricity bills. An efficient building that uses very little energy could still have large demand 

charges. The peak demands and demand savings for each building are shown for heating and cooling 

seasons in Table 3-11. Evaporative cooling, reduced LPD, and HVAC equipment downsized to meet 

reduced thermal loads are the best ways to guarantee peak demand reductions. All these techniques 

reduce peak demands because even if they are all being used, the peak consumption is still less than the 

baseline equivalent. Technologies such as daylighting, Trombe walls, PV systems, and natural ventilation 

that depend on the solar resource are not always available to reduce energy use. Therefore, they cannot 

be relied on for peak demand reduction unless advanced controls are used to dynamically adjust building 

energy use to account for the variability of the solar resource. 

78 
$0

Mar-01

Apr-01

May-01

Jun-01

Jul-01

Aug-01Sep-01

$500

$1,000

$1,500

$2,000

$2,500

$3,000

Oct-01

Nov-01

Dec-01

Jan-02

Feb-02

Electricity Cost ($/month)

Fixed charge, taxes, distribution charges

Energy charge

Demand Charge

Figure 3-40 Oberlin monthly energy cost by charge type, March 2001–February 2002 

The greatest demand savings were at Zion—a primary reason for the largest energy cost savings. CBF 

had the lowest energy cost savings because it had higher demand charges than the baseline. CBF used 

heat pumps with propane backup; the baseline used a propane heating system as the primary heating 

source. Oberlin also had high heating demand charges because it occasionally used backup electric 

boilers that limited energy cost savings. When the electric boiler operated at 112 kW for 15 min, the 

demand charges of $8.59/kW resulted in high utility bills. The monthly demand charge for operating this 

unit for 15 minutes was $962.

79 
1. For Zion peak demands, PV does not directly reduce demand. However, demand-responsive controls used PV production and building thermal mass 

2. For CBF, baseline peak demands are lower than as-built because of the baseline propane heating system; the actual system was primarily heat pump

Table 3-11 Peak Demand Performance Summary 

All-Electric Buildings Electric an

Demand Metrics Oberlin Zion Cambria CBF B

winter 

peak 

summer 

peak 

winter 

peak 

summer

peak 

winter 

peak 

summer 

peak 

winter 

peak 

summer 

peak 

winte

peak 

136 kW 56 kW 108 kW 89 kW 249 kW 90 kW 64 kW 57 kW 131 kW 

Baseline Seasonal Peak Demand (Peak kW and W/ft2) 10.0 W/ft2 4.1 W/ft2 9.3 W/ft2 7.7 W/ft2 7.2 W/ft2 2.6 W/ft2 2.1 W/ft2 1.8 W/ft2 3.1 W/ft2 

142 kW 20 kW 28 kW 15 kW 99 kW 84 kW 136 kW 132 kW 68 kW 

As-Built Peak Demand (Peak kW and W/ft2) 10.4 W/ft2 1.4 W/ft2 2.4 W/ft2 1.3 W/ft2 2.9 W/ft2 2.4 W/ft2 4.4 W/ft2 4.3 W/ft2 2.2 W/Annual Energy Cost Savings 35% 65% 43% 12% Time of As-Built Peak Demand 1/4 9:00 am 8/9 4:00 pm 12/24 9:00 am 8/4 8:00 am N.A. N.A. 2/5 9:00 am 6/24 11:30 am 12/20 6:00 p

-6 kW 36 kW 80 kW 74 kW 150 kW 6 kW -72 kW -75 kW 63 kW 

Peak Demand Savings Including PV (kW savings and percent savings) -5% 65% 74% 83% 60% 6% -113% -132% 48% Load Factor (average monthly demand/peak 15-minute demand) 

0.09 0.06 0.40 0.53 0.45 0.42 0.26 0.34 0.35 

Peak Demand Savings due to PV 0.0 kW 10.0 kW NA NA NA NA 0.8 kW 0.9 kW 0.0 kW PV Demand Savings Performance Ratio (PV Demand Savings/AC PV capacity) 

0.00 0.22 NA NA NA NA 0.24 0.27 0.00 

80 
3.5.1 Low-energy buildings peak demand profiles 

Peak demands in low-energy buildings often occur at atypical times of the day. The conventional 

commercial building has summer peak demands in the late afternoon when cooling and lighting loads are 

the greatest. Well-daylit buildings reduce lighting and peak cooling loads by shifting peaks to atypical 

times of day. The TTF annual peak was shifted from the peak cooling day to a swing season day. The 

annual TTF peak occurred late in the day when some cooling was required combined the full lighting 

load, which occurred during a period of minimal daylighting. When energy efficiency technologies are 

combined with large PV systems, as at Oberlin, the demand profiles are almost opposite the energy use 

patterns in typical buildings. During the summer months, large PV systems in commercial buildings can 

export electricity from 8:00 a.m. to 6:00 p.m., as shown in Figure 3-41. From the utility perspective, the 

building was a net positive during daylight hours in the summer and provided power when it was most 

needed by the grid. Low-energy buildings can shift when a peak demand is met, but may not necessarily 

reduce the overall peak demand. 

-20

-15

-10

-5

0

5

10

15

20

25

0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400

Hour of the Day

Average Hourly Electricity Net Consumption (kW)

Spring '01

Summer '01

Fall '01

Winter '01-'02

Figure 3-41 Daily average net electricity consumption load shape by season, 

with PV production 

An examination of the peak demand days at BigHorn revealed that the peak demand typically occurs 

during one of four scenarios: 

• During normal business hours in the winter after the sun sets and most of the interior and some 

exterior lights automatically come on 

• During normal business hours when dark clouds cover the sun and more interior lights come on 

• After store hours when the cleaning crew turns on most of the interior and exterior lights 

81 
• If the electric forklift is plugged in for charging when the power draw is already high, which 

sometimes occurs at the same time as one of the first two scenarios. 

In the first two scenarios, little can be done to reduce the demand because the lights are required for 

normal business operation. Fortunately, these have the smallest peak demand. The third scenario is 

preventable with some training of the cleaning crew to avoid turning on all the lights at the same time; 

however, this requires retraining with each new crew. The fourth scenario is preventable by charging the 

forklift at night. However, this would require a timer on the charging station circuit or someone to come 

in late at night after the exterior lights are turned off. Each of the scenarios could also be addressed by 

using the energy management system to manage loads, such as the forklift and pumps. 

The electrical power profiles for typical peak demand days at BigHorn during the summer are shown in 

Figure 3-42. The heavy black line is the purchased electrical power. When this line is below the top of 

the graph, the PV system provided some of the building electrical load. In Figure 3-42, the PV system 

provides some power during the day; however, the PV system was operating on only one of three 

inverters on this day, and did not offset the demand when a peak was met. 

Figure 3-42 Electrical power profile on the peak demand day in July 2003 

The peak demand in Figure 3-42 occurred after the store was closed when the cleaning crew came in and 

turned on most of the lights, including the entire lighting display. Earlier in the day, the forklift was 

plugged in for charging right before closing time, which raised the power draw by nearly 10 kW but still 

allowed it to remain below peak. 

3.5.2 PV systems have had limited success reducing peak demands 

The PV systems in this set of low-energy buildings have not significantly reduced building demands. In 

these buildings, any small demand reduction from PV is from load diversity, as shown in Table 3-11. 

Figure 3-43 demonstrates that the Oberlin PV system did not significantly reduce peak electrical demand. 

82 
For July and August, the peak demand would have been 10 kW greater without a 60-kW PV system. The 

peak demand for these months was reduced because the peak consumption was coincident with PV 

production. For the other months of the year, the peak demand occurred when no PV production was 

present. Similar peak demand trends were documented for all the other PV systems in the case studies 

(see PV Demand Savings Performance Ratio in Table 3-11). Peak demands often occurred during 

atypical times that did not coincide with PV production, when daylighting was not available for demand 

reduction. 

142 142

87

45

22 20

89

139

125

100

125

140

0

20

40

60

80

100

120

140

160

Mar-01

Apr-01

May-01

Jun-01

Jul-01

Aug-01Sep-01

Oct-01

Nov-01Dec-01

Jan-02

Feb-02

15-min Peak Demand (kW)

Demand Savings Due to PV System

Peak Monthly Demand (kW)

Figure 3-43 Oberlin peak 15-minute demand by month (includes PV production)

3.5.3 Demand-responsive controls can save energy costs and increase load factors 

The largest energy cost savings of all the buildings was at Zion because of its aggressive demand 

management, which the other buildings did not use. The load factor was also consistently higher than the 

other buildings. Demand management combined with on-site storage (building mass) was the best use of 

on-site generation for reducing peak demands. Oberlin was the least successful at using PV for demand 

reduction, because of its high peak demands and large PV system that reduced energy use, but not at 

times of peak demand. This combination resulted in low load factors. 

At Zion, we analyzed all the building loads and determined typical use trends to minimize the demand 

costs. Once the end uses that typically contributed to peak demands were identified, we determined 

which loads might be shifted until a peak had passed. 

The cost associated with demand can be found by multiplying the maximum power draw over a 15-

minute window by the $8.10/kW demand charge. The typical peak demand during the winter can exceed 

44 kW, when all end uses, including the hot water, lighting, and some heating, were on at the same time. 

Although the key contributor to the increase in peak demand was turning on lights in the morning, the 

83 
heat was also on, partly because of the recovery from night setback. The hot water, lighting, and heating 

were at a maximum for the day during the demand peak. These types of loads offer the most potential for 

limiting the peak demand because they are not considered essential for short-term operation of the 

building. Hot water and heating systems can be temporarily turned off during potential periods of peak 

demand. During periods of low demand, the hot water and heating systems are turned on. The nighttime 

heating is used only if the building can be returned to a comfortable temperature without incurring a 

demand charge. This type of control strategy can also take advantage of on-site generation for potential 

demand reduction by offsetting the utility power draw with on-site generation. Demand is limited by 

continuously adjusting the heating set point of the Visitor Center based on the measured building demand. 

The EMS can reduce the temperature set point by 0.2°F (0.11°C) every 20 seconds when the demand is 

near the monthly peak demand. The maximum set point reduction is 10°F (12°C). 

The domestic hot water (DHW) units in the Zion Visitor Center and Comfort Station have demandresponsive controls. The DHW is disabled if the instantaneous building demand exceeds the weekly 10-

minute peak demand. These units remain off for at least 8 minutes in an attempt to shift the load. If the 

building demand is less than the weekly peak demand, the DHW units are enabled. The DHW is mainly 

used for cleaning, usually early in the morning. The hot water is not considered essential to the normal 

operation of the building. Before demand-responsive controls were installed, the cleaning crew emptied 

the water heater for cleaning, which caused a spike early in the morning when the system tried to recover 

just as other loads peaked. The strategy shifts the hot water recovery to a more advantageous time based 

on demand and rate structure. 

Figure 3-44 shows the energy consumption of a heating day after demand-responsive controls were 

implemented. Most noteworthy is that when the lights came on in the morning, the heating was reduced 

to maintain a flat profile. Heating consumption was decreased by 6 kW until daylighting reduced the 

electrical lighting loads and the PV system provided sufficient energy to limit demand. The heating 

energy consumption over the day is similar to that of a building without demand limiting because the load 

was shifted to a period when the heating system would not incur a peak demand. This load shifting 

strategy is an effective application of demand-responsive controls because the building still used the same 

energy but over a longer period; the controls were effective because the peak was reduced significantly. 

In addition, the DHW load, which can peak at 3.3 kW in the morning, was also shifted to later in the day. 

The building may be underheated when the demand-responsive controls reduce the heating set point. 

Zone temperature fluctuations were reduced during the load-shedding period by not setting back the 

nighttime set point, which allows for increased heat storage in the building thermal capacitance during 

periods of low demand. Again, the equipment and lighting loads could not be shifted or limited because 

they are necessary for building operation. 

84 
-5

0

5

10

15

20

25

30

35

000100200300400500600700800900

10001100120013001400150016001700180019002000210022002300

000

Time of Day (15 minute averages)

End Use, Purchased, and PV Production (kW) 

Net Used

Total PV

Total HVAC

Total Equipment

Total Lighting

Hot-Water Heaters

Figure 3-44 Zion demand-responsive controls example 

In general, by identifying short-term, nonessential end uses that contribute to peak demand charges, we 

developed a control strategy to shift these loads to significantly reduce demand charges. After 

overcoming some initial control implementation problems, we have been able to keep the peak demand at 

less than 30 kW for the entire year. Peak demands for previous heating seasons without demandresponsive controls exceeded 44 kW. Understanding the optimal demand controls involves understanding 

the interaction of on-site generation, daylighting, and precooling and preheating techniques to charge the 

thermal mass of the building during off-peak hours. 

When backup electric resistance heating is used in heat pumps, as reserve capacity for ground loops, or in 

radiant panels, these systems must be carefully staged and controlled to realize peak demand savings. If 

these systems can be used as part of a demand-responsive control system so they are not responsible for a 

building setting a peak demand, they can be successfully used as backup heating systems. The annual 

peak demand at Oberlin occurred during a high heating load day when the 112-kW electric boiler 

operated during the morning warmup period, as shown in Figure 3-45. PV production did not offset this 

peak demand at 9:00 a.m. Significant cost savings could be realized by sequencing the heating equipment 

to meet the heating loads and operate within demand-responsive constraints. This may use slightly more 

energy, but significantly even out the loads. The building could be preheated when PV demand is 

available maximizing the benefit of this system. Similar benefits could be achieved during the summer 

by precooling the building when extra on-site generation is available. 

85 
-40

-20

0

20

40

60

80

100

120

140

160

100200300400500600700800900

100011001200130014001500160017001800190020002100220023002400

Time of Day (15 minute averages)

PV Production, End Use Consumption,

 and Net Building Use (kW)

Electric Boiler 

Total Ventilation

Total HVAC Pumps

All Heat Pumps 

Total Lighting 

Total Equipment 

Net Use 

PV Production 

Maximum 15 Minute 

Demand of 142 kW 

at 9:00 a.m.

Figure 3-45 Oberlin peak heating season utility demand, PV production, and end use 

3.5.4 Low-energy buildings can have low load factors 

Building energy managers use peak electrical demand and load factor performance metrics to determine 

how well they balance the building energy use with peak demands; utilities use these to determine how 

much extra capacity they need to meet all the electrical loads of buildings. The peak electrical demand is 

typically defined as the highest average power over any 15-minute window in a month. The load factor is 

a dimensionless number and is the ratio of the average electrical energy and the peak electrical demand. 

Buildings often have load factors around 0.5, which means that the peak demand is twice as high as the 

average power. Utilities charge for peak demand to support the extra capacity needed in the system. 

Demand charges can be more than half the electric utility bill; therefore, reducing peak demands is in the 

best interest of the building owner. 

Energy-efficient buildings strive to lower energy consumption; however, they often do not reduce the 

demand by the same amount. If the energy use is reduced more than the peak demand, the load factor 

decreases. This means that the utilities will have to supply less energy, but there is no reduction in 

capacity requirements. Therefore, the utilities still have to maintain the same expensive generation, 

transmission, and distribution capacities. Keeping power plants in spinning reserve as they wait for 

higher demands requires energy, so even though a building may reduce site energy consumption, the 

source energy consumption may not be reduced by the same amount. Even buildings with on-site 

electricity generation from PV systems rarely reduce demand by more than a few percent. Energyefficient buildings should manage peak demand with demand-responsive controls and possibly on-site 

energy storage. 

Buildings that employ energy efficiency and PV systems to approach zero net energy can export 

electricity to the grid when the PV systems produce more than the buildings use. If a building exports 

86 
more energy than it uses over a month or a year, the load factor can be negative depending on how it is 

defined. A negative load factor has a very different meaning from a utility point of view. It tells the 

utility that the building exported electricity, but not the capacity needed to meet the building loads. 

The building closest to a ZEB (Oberlin) also has the worst load factor of all the buildings, with a load 

factor less than 0.1. The load factor, or the proportion of energy use to peak demands, is extremely 

unbalanced when PV systems offset significant energy use, but do not offset peak demands. For 

widespread adoption of future ZEBs, these buildings will need demand management controls integrated 

with on-site storage. This will ensure proportional energy and demand savings. Future ZEBs may shift 

peak demands and reduce absolute peak demands with efficiency technologies, but there will be 

significant unrealized utility impact reduction if the on-site generation technologies are not used to further 

help reduce a building’s impact on the grid. In addition, relying on the grid to provide annual storage for 

net meter accounting in a widespread ZEB adoption scenario may not be sufficient. Future work in this 

area is required to understand the utility infrastructure needed to allow for widespread adoption of netmetered buildings. Integrated storage at the building or grid level may be needed to allow a large number 

of net-metered buildings to be connected to the utility grid. 

Future work should focus on increasing load factors of ZEBs with significant PV systems. In addition, an 

effective load factor needs to be used in ZEBs that are net producers for any given month. Standard 

methods for calculating load factor when the average monthly energy use is negative do not provide 

useful results for understanding the demand-to-use ratio. Therefore, the effective load factor should 

consider either the building with no on-site generation or, alternatively, the ratio of energy supplied to the 

building to the actual peak demand. 

3.6 Plug and Equipment Loads 

Energy used in commercial buildings is very diffuse, with many types of end uses other than HVAC&L 

loads. In office buildings, energy used for office equipment is 16% of the total energy use, water heating 

uses 9%. Plug and equipment loads continue to grow in absolute energy use as they become a larger 

piece of the overall energy use pie as efficient HVAC&L systems become more prevalent. Thus, lessons 

learned and recommendations for future research concerning efficient plug and equipment load design 

and controls are becoming more valuable to the industry and to future ZEB designs. 

The plug and equipment loads in the case studies included non-HVAC&L loads such as computers, 

monitors, printers, task lights, server equipment, other office equipment, elevators, DHW, emergency 

equipment such as egress lighting, PV system standby loads, laboratory process loads, display lighting, 

forklift charging, refrigerators, vending machines, and miscellaneous plug loads. Table 3-12 shows the 

equipment and plug load use percent of the total source use, use intensity, and ratio of average night to 

average day use for each building. 

87 
Table 3-12 Plug and Equipment Loads in High-Performance Buildings 

Building 

Equipment Use 

Percent of Total 

Source Use 

Average Daily Peak 

and Nightly Minimum 

Equipment Use 

W/ft2 (W/m2) 

Average Nightto-Day 

Equipment Use 

Ratio 

Issues 

Oberlin 28% 0.32 night (3.4)

0.29 day (3.1)

1.1 

High lab process loads, 

PV system night standby 

losses 

Zion 33% 0.16 night (1.7) 

0.37 day (4.0) 

0.43 

Demand-responsive 

controls limit when DHW 

can operate 

Cambria 24% 0.26 night (2.8) 

0.49 day (5.3) 

0.5 

High nighttime load 

caused by electronic 

equipment—computers, 

monitors, printers, and 

copiers—that is being left 

on 

Even in standby mode, the 

sum of all equipment still 

consumes a significant 

amount of energy 

CBF 41% 0.34 night (3.7) 

0.66 day (7.1) 

0.52 

High information 

technology equipment 

loads in the offices and 

server room that operate 

continuously 

TTF 35% 0.25 (2.7) N.A. Equipment use based on 

simulation 

BigHorn 39% 0.16 night (1.7) 

0.50 day (5.4) 

0.32 High lighting display loads 

during the day 

3.6.1 Parasitic loads should be minimized 

Parasitic loads are problematic in commercial buildings because they use energy even in standby mode. 

Even when a parasitic load is off, it still uses energy. The ratio of average night-to-day equipment and 

plug loads in Table 3-12 can indicate excessive consumption during unoccupied periods. The isolation 

transformers on PV systems at Oberlin and Cambria are prevalent parasitic loads. At Oberlin, the nightly 

PV system standby load results in more equipment and plug load energy use at night, which results in a 

night-to-day use ratio greater than 1.0 (see Figure 3-46).. See Section 3.4.1 for further PV system 

parasitic load analysis. 

88 
0.0

0.1

0.2

0.3

0.4

100200300400500600700800900

100011001200130014001500160017001800190020002100220023002400

Hour of the Day

Average Hourly Equipment Use Intensity (W/ft2

)

Emergency Plug Loads Elevator DHW

Plug Loads LM Equipment PV Transformers

Total Equipment

Figure 3-46 Oberlin equipment use intensity load shape 

Equipment and plug loads at CBF include energy use for the elevator, the exterior and garage lighting, 

plug loads, and the mechanical room and storage room receptacles. Equipment and plug loads are the 

largest energy end-use category at CBF, followed closely by HVAC. Miscellaneous loads are significant 

and show seasonal dependence that indicates there could be heating loads associated with the elevator (oil 

heating) and the ground-floor receptacles. The results in Figure 3-47 show that CBF off-hour equipment 

loads are about half of what they are during the day, which indicates that equipment is being left on when 

not in use. Presumably, this is because a large amount of information technology equipment operates in 

the offices and server room. The off-hour plug loads in the Merrill Center are typical of contemporary 

offices, but they could be considered an opportunity for improving the energy performance of the 

building. Methods such as communicating with employees about the value of shutting down equipment, 

deploying new network-based software products that monitor and reduce computer power, and replacing 

equipment with energy-saving sleep mode, should be investigated to reduce off-hour plug loads. 

Weekday daily load profiles at Cambria for the process and equipment loads are shown in Figure 3-48. 

The total load is very consistent and varies between 9 kW at night (normalized to 0.26 W/ft2

 [2.8 W/m2]) 

and on weekends to approximately 17 kW (0.49 W/ft2

 [5.3 W/m2]) during the occupied hours. The plug 

load profile closely reflects the occupant behavior. Most of the occupants arrive between 7:00 a.m. and 

8:00 a.m. and leave between 3:00 p.m. and 4:00 p.m. The lunch break is consistently from about 11:30 

a.m. until 1:00 p.m. The nighttime load is approximately half the daytime load, which is much higher 

than was anticipated during the design phase of the building. The high nighttime load is caused by 

electronic equipment including computers, monitors, printers, and copiers that are left on. Much of this 

equipment goes into a standby mode, which still consumes a significant amount of energy when all the 

equipment in the building is considered. The weekend/holiday plug daily load profile is flat and at the 

same level as the weekday nighttime load profile. 

89 
0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0:00 4:00 8:00 12:00 16:00 20:00

Time of Day

Equipment and Plug Loads Use (W/ft 2

)

Figure 3-47 CBF average equipment and plug load use intensity 

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0 3 6 9 12 15 18 21 2

Hour

Plug Load Weekday Electrical Power (W/ft2

)

4

Oct 2002

Jan 2003

Apr 2003

Jun 2003

Figure 3-48 Cambria daily plug load profiles for weekdays 

90 
A significant part of the equipment and plug loads in all these buildings comprises idle computers. 

Energy savings can be realized by making the machines ENERGY STAR® enabled to shut them down 

during periods of inactivity. However, standby modes can consume a substantial amount of energy. The 

numbers and types of equipment left on or in standby mode were noted during an after hours survey. 

Most desktop computers were left on and were in standby mode. Their exact power draw is not known, 

but most desktop computers draw 40 to 80 W in normal operation. Most operating systems have a power 

management feature that will put the computer in standby mode after a certain period; however, these 

features reduce the power consumption by only about one-third when they are in operation. In addition, 

power management software could be installed on all computers. This type of program can be easily 

installed and controlled over a local area network. Annual energy savings are 100 to 300 kWh per 

computer. 

Many CRT monitors in standby mode use 3 to 10 W; however, older monitors do not have a standby 

mode and use around 40 W when turned on with a blank screen. The computer and monitor energy 

consumption can be reduced with manual or automatic control. Manual control relies on users to turn off 

their computers and monitors at night. If this approach is not successful, power management software 

options can be implemented and controlled over a local network. Computer power management software 

can effectively reduce energy consumption during unoccupied periods. Energy savings average 200 

kWh/yr per computer. Flat screen LCD monitors should also be used for added energy savings. 

Computers are not the only large plug load; there are numerous printers at Cambria—nearly every 

occupant on the second floor has one. Each printer consumes energy even on standby. Replacing 80% of 

these printers with five central networked printers would save money in the purchasing, maintenance, and 

operation of the printers. One more opportunity to save energy is to power of vending machines. Savings 

can be substantial, but vary with the type of vending machine (Deru et al. 2003). 

A major electrical end use at BigHorn is the lighting display area, which displays all the lighting fixtures 

and other interior decorating items. The displays are often changed to show new fixtures and new light 

bulbs. Even with efforts to control the energy consumption of this area with compact fluorescent lights 

(CFLs), the energy consumption is about the same as the lighting for the entire retail and office spaces. 

BigHorn has the lowest night-to-day ratio of equipment use, as display lighting is a large daytime use 

without a parasitic load, as these lights are turned off nightly. There are considerable plug and other 

electrical loads at BigHorn, including 37 computers; 22 CRT monitors; 8 LCD monitors; 7 printers; 17, 9-

pin dot matrix printers; 2 copiers; 3 cash registers; accent lighting; 2, 10-gal domestic water heaters; 1, 

500-W air conditioner in the server room; ceiling fans; 500 W of roof ice melt; 2 electric space heaters; 2 

refrigerators; 3 vending machines; 2 microwave ovens; and other plug loads. These loads change 

continually as new equipment and plug-in lighting are added or removed. The accent lighting load 

comprises display lights and other plug-in lights that are used to accent products or features. Electric ice 

melt is required on some parts of the roof and roof drain structures to avoid ice damming, which can 

damage the roof and prevent proper drainage. At the beginning of the monitoring period, there was 500 

W of roof ice melt; however, this number increased to approximately 6 kW because of persistent ice 

damming problems. Designing roofs to minimize ice dams is typically a better solution than melting the 

ice with electric resistance heaters. 

Equipment and plug loads continue to grow in absolute energy use as well becoming a greater piece of the 

overall building energy use pie as efficient HVAC&L systems become more prevalent. Parasitic loads 

that use energy during standby or off modes are of particular concern, as they have been problematic in 

each of the case studies. Even in standby mode, equipment can consume a substantial amount of energy. 

Future work is needed in this area to understand the energy penalty of each plug load that includes an ACto-DC conversion device and investigate potential solutions for limiting energy use when equipment is 

not needed. Potential energy-saving technologies, such as switching transformers or a DC plug load 

electrical infrastructure that leverages on-site DC generation, should be investigated. 

91 
3.6.2 Domestic hot water loads are small 

DHW loads are fairly small compared to other end uses in these buildings, as demonstrated in the Oberlin 

and Zion end use data (see Figure 3-50 and Figure 3-51). DHW loads were responsible for 0.3 kBtu/ft2

(3.4 MJ/m2

) at Oberlin and 1.0 kBtu/ft2 (11.4 MJ/m2) at Zion. Tankless water heaters at the TTF and 

solar collectors at CBF have successfully met the DHW loads. 

Two natural gas tankless water heaters provide DHW for the kitchen and restrooms at the TTF. These 

units have an advertised thermal efficiency of 80%. These heat-on-demand systems were selected 

because they reduce energy consumption by diminishing standby tank losses, which can be 15% to 20% 

of a conventional system. 

The solar hot water system at CBF consists of four arrays of 30 evacuated-tube solar collectors on the 

roof of the main building (see Figure 3-49). A glycol loop circulates fluid from the roof to two hot water 

tanks that are plumbed in parallel. One tank is for potable and the other for nonpotable water. The tanks 

have electric backup heaters. The system provides DHW for sinks, showers, two dishwashing machines, 

and a clothes washing machine. Because the overall water use is very low, all the DHW needs of the 

building are met by the solar collectors; the backup heating coils have never come on. 

Figure 3-49 CBF evacuated-tube solar collector arrays 

92 
Receptacles

Emergency receptacles

PV system consumption

DHW

Auditorium lights

Indoor room lights

Hydronic circulation pumps 3-6

VSD Hydronic circulation 

pumps 1-2

Room Heat Pumps

Classroom energy recovery 

unit

Auditorium energy recovery 

unit

Emergency lights

Sidewalk lights

Parking lot lights

Classroom ventilation heat 

pump

Auditorium heat pump

Hydronic system electric boiler

Wastewater treatment

Elevator

Total 

Equipment

28%

Total Lights

13%

Total HVAC

59%

Figure 3-50 Oberlin energy end uses, 2002 

Visitor Center Heating

Fee Station

Ceiling and Exhaust Fans

Comfort Station Cooling

Comfort Station Plug Loads

Visitor Center Domestic Hot 

Water Comfort Station Domestic 

Hot Water

Visitor Center Plug Loads

Outdoor 

Comfort Station Visitor Center Switch 

Controlled

Visitor Center Computer 

Controlled

Visitor Center Cooling

Comfort Station Heating

Figure 3-51 Zion energy end uses, 2002 

Equipment/

Other

33%

HVAC

36%

Lighting

31%

93 
3.7 Postoccupancy Evaluation Techniques 

The purpose of the postoccupancy evaluation (POE) case studies was to determine actual energy 

performance of current generation of low-energy buildings to verify design goals and document realworld energy savings. Additionally, the POEs provided lessons learned in the design, technologies, 

operation, and analysis techniques to ensure these and future buildings operate at a high level of 

performance over time. A whole-building POE should consist of a combination of monitored energy use 

data and hourly building simulations. The POE for each case study included measuring actual energy 

performance of the building for at least a year during typical occupancy, simulating calibrated as-built and 

baseline models, and calculating whole-building energy savings. Additionally, specific technologies, 

including daylighting, HVAC, and PV systems, were evaluated in each building. Each POE provided 

valuable feedback to maintain or improve building performance. A POE is similar to equipment 

commissioning, but on a whole-building scale during occupancy. Standard equipment commissioning is 

valuable, but it does not guarantee each component will perform properly as part of an efficient system. 

3.7.1 Determining whole-building energy savings 

When discussing low-energy buildings, it has become common to characterize performance with the term 

percent savings. A calibrated as-built simulation compared to a conventional base-case model can 

provide a confident prediction of annual site, source, and cost savings. For each POE, we attempted to 

determine energy savings according to the following method: 

1. Monitor site weather conditions and energy use for the whole building and at each end use over a 

year during typical occupancy. Aggregate end uses by HVAC&L and equipment/plug loads. 

(See Section 3.7.2 for details on measuring energy use.) 

2. Update the design model to reflect as-built features and operation. Verify that the as-built model 

represents actual performance, which may require calibrating the as-built simulation, driven by 

measured weather data, to ensure confidence in the model. (See Section 3.7.1.3 for details on the 

calibration process.) 

3. Extract calibrated schedules and equipment loads from the as-built model for use in the baseline 

model. Process loads, operational schedules, and set points should be the same in the baseline 

and as-built models to ensure a fair comparison. No credit should be taken for differences in 

building function or operation. 

4. Determine annual site energy use, source energy use, and energy costs for both the calibrated asbuilt model and the updated baseline for a typical weather year. 

5. Compare annual predictions of the calibrated as-built and baseline models to determine site 

energy use savings, source energy use savings, and energy cost savings. 

This method is outlined in Figure 3-52. 

94 
Calibrated As-built 

Model with TMY2 

weather

Measured Data Models

As-built Source Use

Simulated Results 

with TMY2 weather

Percent Savings

Site End Uses

Site Weather

Calibrated As-built 

Model with site 

weather

Base-Case Model 

with site weather

Base-Case Model 

with TMY2 weather

Base-Case Energy Costs 

As-built Site Use

Base-Case Site Use

Base-Case Source Use

Source Use Savings

Site Use Savings

Energy Costs Savings

Extract 

calibrated 

schedules

As-built Energy Costs 

Calibrate 

As-built 

model

Figure 3-52 Evaluation flow chart comparing as-built to baseline models 

Energy cost savings were set as the design goal for four of the six buildings. Energy cost saving design 

goals from 60% to 80% were set for Zion, BigHorn, TTF, and Cambria. Oberlin set a site energy savings 

goal and CBF set a LEED platinum design goal. (See Section 3.1.2.2 for further discussion of 

performance-based design goals.) If the team sets energy cost saving goals during design, verifying these 

design goals during actual operation may be difficult because energy prices can change. At BigHorn, 

natural gas prices varied by 40% during the three-year monitoring period and electrical prices varied 

widely, mainly because of new pollution regulations and a partial shift from coal to natural gas for 

electricity production. Site or source energy savings goals are easier to measure and typically are less 

susceptible to external fluctuations such as energy prices. 

In each case where currency units are used, the years during which the expenditures occurred, as well as 

the utility rates during the period of energy costs reporting, should be noted. When energy costs are 

compared between two buildings that have been analyzed at different times or in different places, several 

factors may bias the comparison. These include: 

• Changes in energy prices over time 

• Differences in energy prices from place to place 

• General inflation during the time interval. 

3.7.1.1 Source energy 

There are two forms of site energy use, net and total, where net gives credit for on-site production from 

PV panels. Source energy use is determined from net site energy use with assumed multipliers. One way 

we have estimated these multipliers is to use yearly national averages for different energy sources. 

Electricity conversion efficiency was assumed at 31.1%. Natural gas and propane conversion efficiency 

is assumed at 93.3%. The level of these assumptions was determined from data in the Energy 

Information Agency’s Annual Energy Outlook (EIA 2005). In this report, source energy consumed for 

electricity is determined from net (purchased) electricity, which gives credit for the PV generated on site. 

(See further discussion on complexities of calculating accurate source energy multipliers in Appendix A.) 

The Procedure to Determine Source Energy and Emissions for Energy Use in Buildings (Deru and 

95 
Torcellini 2005a) provides additional source energy multipliers by the three interconnect regions in the 

United States. 

Table 3-13 2004 Site-to-Source Energy Conversions 

Energy Source-to-Site 

Efficiency 

Site-to-Source 

Conversion 

Natural Gas 93.3% 1.072 

Electricity 31.1% 3.215 

Comparing the consumption of different forms of energy at the building site is difficult. For example, 

heating with gas, electricity, or district heat cannot be compared, because electricity and district heat are 

nearly 100% efficient. This comparison does not account for the energy used to generate and deliver the 

electricity or district heat to the site. A better comparison is to calculate the source (or primary) energy 

used to generate and deliver the energy to the site. But many of the important issues necessary to 

calculate source energy, including energy source mix, generation efficiencies, and distribution and 

transmission efficiencies, are often unknown at the local utility level. Time-of-day and season can also 

change the source-to-site conversion efficiency (CEC 2005). A solution is to use primary energy based 

on total energy generation and consumption data for the United States. Using national average data is a 

good approach because the energy distribution network in the United States is highly interconnected. 

National energy data are compiled by EIA (2005). The site-to-source conversion efficiencies for natural 

gas and electricity are shown in Table 3-13.

The delivery efficiency of natural gas for 2004 was approximately 93.3%, or the source energy is 1.072 

times the site energy consumption. This number represents the total natural gas delivered to the 

consumers divided by the total consumption of natural gas. The inefficiency in delivery represents 

natural gas consumed at the well, in processing plants, and in distribution. However, the number does not 

account for other energy forms such as gasoline, diesel fuel, and electricity, which are consumed in 

extracting, processing, and distributing products. The magnitude of these other energy forms is not easily 

obtainable and would require a full life-cycle assessment of the natural gas extraction-to-delivery process. 

The information used here comes from Diagram 3, Natural Gas Flow, 2004 in the Annual Energy Review 

2004 (EIA 2005). 

The average generation and delivery efficiency of electricity for 2004 was approximately 31.1%, or 

source energy is 3.215 times site energy consumption. Electricity efficiency is calculated by dividing 

end-use electricity by energy consumed to generate electricity and accounts for conversion, transmission, 

and distribution losses. It is based on the average of all sources of electricity generation and distribution 

in the nation, as reported in the 2004 Annual Energy Review (EIA 2005). This number does not account 

for precombustion energy—energy to extract, transport, and process fuels used to generate electricity. 

3.7.1.2 Baseline modeling 

Everyone wants to report the percent savings for a building as a metric. The first problem with this is 

determining the baseline. Most often, baselines are based on a comparable code-compliant building. For 

the six buildings studied, this baseline varied from ASHRAE 90.1-1999 (ASHRAE 1999) to 10 CFR 435. 

(See Table 3-14 for the baseline energy code used for each case study.) The baselines included all 

process, equipment, and plug loads that were in the as-built model, even if design goals were determined 

against HVAC&L. For consistency, most of the buildings were compared to ASHRAE 90.1-2001. 

Although it is consistent, it forces one to ask, “Is it fair to evaluate a building against a standard that is 

higher than the original code that the building was designed against?” On the other hand, evaluating a 

building for its performance against old standards will not move low-energy design forward. The lack of 

a standard comparison technique points to the conclusion that there is a real need to create a standard 

benchmark. DOE’s residential program has established a benchmark for Building America, and a similar 

96 
tool is needed for commercial buildings. A first step in establishing a standard commercial building’s 

benchmark is a procedure developed in the Performance Metrics task, Procedure for Developing a 

Baseline Simulation Model for a Minimally Code-Compliant Commercial Building (Pless, Deru, and 

Torcellini 2005). This procedure documents assumptions and identifies modeling inputs necessary for 

simulating a baseline for comparison purposes, from predesign through postoccupancy. We used 

baselines to analyze energy performance for buildings from predesign stages to postoccupancy as-built 

buildings. When building designs and operations change, the baseline has to evolve along with the 

building under consideration. 

Energy savings uncertainties can be minimized when savings are determined from the comparison of one 

simulation to another simulation (e.g., baseline to as-built). Because difficult-to-know inputs are held the 

same in both simulations, such comparisons remove much of the uncertainty inherent in an hourly 

building energy simulation. Variables that change throughout the year, such as inconsistent occupancy, 

set point changes, and equipment performance degradation, are difficult to account for in an annual 

building energy simulation. Comparing a baseline model to an as-built model with the same schedules 

reduces the uncertainty. 

3.7.1.3 Model calibration process 

To calculate energy savings of a building, a model must be calibrated against actual building data. Too 

many changes occurred to use the design-based models as accurate predictors of energy consumption. 

Schedules and plug loads vary widely from original assumptions. The baseline model must also be 

modified to reflect the as-built schedules and plug loads. A calibrated as-built simulation compared to a 

conventional baseline can provide a confident prediction of annual site, source, and cost savings. Using 

typical meteorological year weather data allows long-term savings calculations with relatively short-term 

data. 

To verify that an as-built model and accompanying assumptions were adequate, the as-built model’s 

energy performance was compared to the actual measured energy performance over a full year. For a 

proper comparison, the measured local weather data from this year was used as the weather file in the asbuilt simulation. Each of the primary measured end uses (HVAC&L and equipment) was compared to 

the simulated end uses for model calibration. To calibrate the model, assumptions such as heating and 

cooling schedules, occupancy schedules, and unoccupied infiltration were slightly tuned until the energy 

performance of the calibrated as-built model described the measured energy performance. We also used 

daily load shape profile comparisons of the lighting, equipment, and HVAC to ensure appropriate as-built 

model representation of daily use patterns. These calibrated schedules were also used in the comparison 

baseline model. For the model to describe the actual building energy performance within expected 

simulation accuracy, the difference between modeled and measured monthly energy totals should be less 

than 10%. 

Where appropriate, specific measured equipment performance parameters were compared to simulated 

predictions. For example, in the Oberlin heat pump ground loop, actual flow rates, ground supply, and 

return temperatures were compared to simulated ground temperatures and flow rates. The highly variable 

and relatively unknown thermal properties of the ground were adjusted to ensure the simulated groundsource temperatures were similar to measured values. In general, the calibration process allows for 

greater confidence in the many modeling assumptions required to simulate whole-building performance. 

3.7.1.4 Energy saving uncertainty 

The uncertainty of the annual performance metrics based on simulations, such as the site energy saving 

and site EUI for a typical weather year, are difficult to estimate with direct calculations. The processes 

used in the POEs attempted to reduce uncertainty related to building simulations. To reduce the 

uncertainty of the annual simulation metrics, NREL calibrated the models with measured end uses and 

site weather. NREL considers a building simulation to be calibrated when the simulated monthly end 

uses (heating, cooling, lighting, equipment) is within ±10% of the measured monthly energy use. This 

97 
±10% criterion can be assumed to represent a base level of uncertainty in annual performance metrics 

based on simulation results. 

Input perturbation methods can also be used to model the uncertainty in whole-building energy 

simulations, but these methods require an extensive level of effort to prepare hundreds of input files and 

process results. Such efforts have produced error estimates of about ±14% (Griffith et al. 2005). Based 

on these two methods, NREL estimates the uncertainty of the annual performance based on simulation 

(site energy use, source use, and energy costs) to be ±12%. Although not quantified, uncertainties are 

much lower for percent saving metrics (site energy savings, source energy savings, and energy cost 

savings), which result from comparing one simulation to another. 

3.7.1.5 Alternative methods 

For each POE, the ideal whole-building evaluation technique was to compare the baseline site energy use, 

source energy use, and energy costs to an as-built model to calculate savings for a typical weather year. 

Although this method is very straightforward, several issues surfaced when we attempted to determine 

energy savings for each building. For Zion and CBF, we were only able to compare the actual 

measurements to a well-calibrated baseline model because of limitations in the whole-building simulation 

tools. We used monitoring data for weather and operational schedules to develop a baseline model of a 

conventional energy code-compliant building. We attempted to account for the inherent uncertainty 

involved in comparing measured data to a simulation by calibrating the baseline model with measured 

weather data, measured equipment loads, and operation schedules. Table 3-14 shows the energy savings 

calculation method used for each case study. 

At CBF, an as-built model of the building could not be developed for this project because of the 

complexity of the HVAC systems and the late addition of water source heat pumps in EnergyPlus. In the 

analysis of Zion, an as-built model was not completed because of limitations in DOE-2.1E modeling 

capabilities. These limitations include nonexistent building integrated modeling techniques for the energy 

use and cooling capacity of cooltowers, difficulties modeling the as-built operation of subhourly demandresponsive controls with integrated PV production, and uncertainties with Trombe wall thermal models. 

The flow chart in Figure 3-53 shows how the measured data were used in the models and the process used 

to obtain the simulation results for Zion and CBF. 

Measured Data Models Measured Results

Simulated Results Percent Savings

Site End Uses

Site Weather

Base-Case Model with 

Site Weather with AsBuilt Geometry

Measured Site Use

Measured Net 

Source Use

Measured Net 

Site Use

Base-Case Energy Costs 

Base-Case Site Use

Source Use Savings

Site Use Savings

Energy Costs Savings

Extract Equipment 

Loads and Schedules

PV Production Measured PV Production

Utility Bills Monthly Energy Costs

Base-Case Source Use

Base-Case Error 

Analysis Using Input 

Perturbation Methods

Figure 3-53 Evaluation flow chart comparing measured data to baseline model 

98 
99 

Difficulties with modeling as-built energy costs at Oberlin resulted in a similar evaluation technique. 

Measured energy costs were compared to baseline energy costs to determine energy cost savings. 

Although we were able to adequately model as-built and baseline site and source energy use, the 

limitations of the DOE-2 simulation program prevented us from modeling the as-built energy costs and 

accurately predicting peak demands on a 15-minute time step with integrated on-site generation. The 

peak demand modeling limitation also prevented us from developing and quantifying savings of demandresponsive controls that enable on-site PV production to reduce peak demand and optimize energy costs. 

Another method of characterizing energy savings levels is to consider only the energy used for HVAC&L 

by removing that energy used by the occupants. This method of comparing energy use was used in the 

Energy Cost Budget of ASHRAE Standard 90.1-2001 (ASHRAE 2001). The USGBC has also used this 

method in its original LEED V1.0 rating system. However, some researchers and professionals in the 

building industry disapprove of this method because it tends to overstate the level of savings and overlook 

critical energy efficiency opportunities in reducing plug and equipment loads. In addition to wholebuilding energy savings, HVAC&L savings (without equipment loads) were determined for the TTF and 

CBF for an assessment of design predictions. The energy savings design goals for the TTF and CBF were 

determined without considering plug loads. A significant difficulty arises when we try to remove plug 

loads from the energy costs because of their contribution to demand charges. Therefore, energy costs 

without plug loads were modeled by subtracting the contribution from plug loads from the overall costs 

using a virtual electricity rate that was determined from a year of billing data. 

Although the methods used to determine actual energy savings used are straightforward, several issues 

arose during the evaluation of the case studies, limiting the application and consistency of the evaluation 

techniques. These issues points to the need to develop a standardized process that can be consistently 

applied to measuring energy savings in commercial buildings. A standardized process would include 

guidance for combining Appendix G of 90.1-2004 (ASHRAE 2004) with measured energy performance 

end use data to report measured energy savings of occupied buildings. 
Table 3-14 Postoccupancy Evaluation Techniques Summary 

Oberlin Zion Cambria CBF TT

Site, Source, and Cost Savings Calculation Methods 

For site and source savings calibrated as-built model compared to baseline model with typical weather. For energy cost savings, utility bills compared to baseline model run with measured weather file 

Measured energy use compared to baseline model run with measured weather file 

Calibrated as-built model compared to baseline model run with typical weather file 

Measured energy use compared to baseline model run with measured weather file 

Calibrated model combaseline mwith typical file 

Whole-Building Simulation Tools 

DOE-2.2 (PowerDOE) DOE-2.1E DOE-2.2 (PowerDOE) EnergyPlus V1.1 DOE-2.1E 

Baseline Codes ASHRAE 90.1-2001 ASHRAE 90.1-1999 ASHRAE 90.1-2001 ASHRAE 90.1-2001 

1995 FedeEnergy CodCFR 435 

Energy Monitoring Methods 

Two dedicated data loggers EMS 

Dedicated data logger for PV and indoor space conditions PowerLogic and utility meters for electrical measurements 

Two dedicated data loggers 

EMS combPC for data 

100 
3.7.2 Measuring energy use 

Monitoring end uses over a full year can be expensive, but it is critical for identifying problem areas and 

providing feedback to facility operators and building designers on actual performance versus predicted 

performance. Measured performance is necessary for documenting real-world energy savings and 

provides a means for validating energy simulation models. Each building was monitored for at least a 

year during typical occupancy. 

3.7.2.1 Develop a performance monitoring plan 

A key first step for each monitoring system installed was to determine a performance monitoring plan. 

The monitoring plan should be carefully laid out early, beginning with a list of specific questions the 

evaluation will address. The most suitable performance metrics are then chosen, which leads to the data 

and analysis techniques required. This plan ensures that unneeded data and the associated expense of 

excessive measurements will not be incurred. Guidance for monitoring and reporting the energy 

performance of commercial buildings can be found in the Procedure for Measuring and Reporting 

Commercial Building Energy Performance. 

An example of an energy monitoring plan follows: 

The overall goal of the energy monitoring analysis was to measure and evaluate the building energy use 

patterns. This goal was broken down into the following objectives for the energy-monitoring plan: 

1. Evaluate the whole-building energy performance and compare this with the design expectations. 

2. Analyze the monthly electrical demand and cost profiles. 

3. Evaluate the lighting system performance, including the effects of daylighting. 

4. Evaluate the PV system performance. 

5. Compare the building energy performance to a building that meets the minimum standards of the 

energy code. 

6. Generate a list of lessons learned to apply to other buildings. 

To satisfy these objectives, a data monitoring plan was developed and the following measurements were 

taken: 

• Surveys of electrical equipment in the building, including spot measurements of power 

• Monthly building utility bills for natural gas and electricity 

• Total electrical energy at 15-minute increments 

• Sub-metering of major electrical energy end uses at 15-minute increments 

• Electrical energy delivered to the building by the PV system in 15-minute increments 

• Temperature and flow of the radiant floor water loop 

• Solar radiation incident on the PV system and temperature of the PV cells. 

3.7.2.2 Data logging systems 

The data acquisition systems (DAS) used to monitor energy performance in the case study evaluations at 

Oberlin, BigHorn, Cambria, and CBF consisted of Campbell Scientific data loggers with battery backup, 

various pulse and voltage measurement multiplexers, a network or modem interface, and all the necessary 

sensors for measuring electrical and gas energy flows. The monitoring equipment for the electrical 

measurements consisted of current transformers (CTs) and watt-hour transducers. The CTs were sized 

based on the expected load on the circuits from the building electrical drawings, rather than breaker 

capacity. The watt-hour transducers have a pulse output relative to the energy consumed by the circuit. 

101 
The data loggers take measurements every 20 seconds and report totaled or averaged results every 15 

minutes. Electrical systems should be monitored with watt-hour (energy) meters and not pure watt 

(power) meters. Watt meters that are read every 15 minutes provide only a snapshot of the instantaneous 

power and do not give a true measure of the energy used over the previous 15 minutes. 

At Oberlin, during the two-year period of monitoring minute and hourly end-use data, 0.6% of the 

collected hourly data were incomplete or missing using dedicated data loggers. Of the 56 variables that 

were collected on an hourly basis, only 6,000 data points were missing or incomplete over the two years. 

At CBF, the measurements with the dedicated data loggers were reliable with no periods when data were 

lost or uncollected. 

We used the EMSs at Zion and TTF to collect energy performance data. Table 3-14 shows the data 

logging system used for each case study. For the monitoring systems that used the EMS, frequent 

downloads to personal computers were required to archive continuous energy use data because the EMS 

had limited memory dedicated to data storage. Additional data parsing and management were also 

required because some data formats were difficult to process. At the TTF, we had difficulties with the 

long-term reliability of custom archiving routines and EMS to PC communications. We had limited 

success at Zion with long-term data archiving to a remote PC, which required considerable operator 

oversight. The process required automatically scheduled download of trend logs twice daily, processing 

trend files, and archiving performance data in a usable format. 

EMS data collection, logging, and retrieval methods were problematic and should be avoided. With 

current EMS and building automation systems there appears to be a low probability of obtaining 

contiguous, error-free, measured data sets over a long period. A better solution is to use dedicated, selfcontained DAS that are designed for unattended remote operation and do not use operating systems 

designed for PCs. 

Initially, the watt-hour transducers at BigHorn were connected to the building EMS for data logging 

purposes. However, using the EMS to log data caused many difficulties. First, the format of the data was 

difficult to process. Second, downloading and archiving the data were difficult. Finally, the system has 

limited memory dedicated to data storage, which resulted in lost data. For these reasons, a dedicated data 

logger was used for all the instrumentation installed by NREL. It was connected to a cellular phone for 

remote access and the all the data storage and retrieval operations were automated. 

Although using the EMS for datalogging was problematic, direct access to the building controls was 

beneficial to researchers for understanding and influencing how the building was being controlled. At 

Zion, researchers had access to the EMS. Researchers were able to improve energy management 

strategies such as implementing and testing various demand-responsive strategies. Access to the EMS 

allowed us to reduce the demand charges to the building and facilitated continuous commissioning 

activities. Another advantage is that EMS access and control provide a full understanding of the various 

set points and control strategies to use as inputs into building energy simulation. 

3.7.2.3 Data acquisition system accuracy 

Setting up the monitoring system to allow for an energy balance calculation was important for ensuring 

monitoring system accuracy. An energy balance at the main distribution panel was used as the error 

checking procedure to verify that each meter was measuring properly. The energy balance helps identify 

any errors in the metering installation and DAS programming. Figure 3-54 illustrates an example of this 

check and shows the overall accuracy of the metering system. 

102 
y = 1.0143x

R2

 = 0.9992

0

5

10

15

20

25

30

35

0 5 10 15 20 25 30 35

Utility Meter Hourly Average Use (kW)

Sum of End Use Meters Hourly Average Use (kW)

Figure 3-54 Energy balance of calculated building electricity end uses versus 

utility meter electricity supply 

The expected accuracy of the sensors used in each monitoring system is determined from product 

specifications and is shown in Table 3-15. Individual electricity measurements are ±0.5% based on the 

manufacturer’s data. Totaled values are a summation of individual measurements, but the errors are 

assumed to be independent and to not increase the level of uncertainty. Based on the expected uncertainty 

of the energy use measurements and the reliability of the long-term DAS, NREL expected the uncertainty 

of the annual performance metrics based on measured energy use to be ±1%. 

Table 3-15 Measurement Type Accuracy 

Measurement Type Sensor Accuracy 

WattNode watt-hour meter for electrical end 

use measurements ±0.5% 

Standard type T thermocouple for PV cell 

temperatures, ERV air temperatures, and 

heat pump ground loop temperatures 

±0.5°C 

Temperature and RH probes for ambient, 

atrium, and wastewater treatment 

environmental conditions 

±3% RH for 10–90% RH, 

±6% for 90–100% RH, 0.5°C 

Empro DC current shunts for DC current of 

PV Systems ±0.25% 

Voltage dividers for DC voltage of PV 

system ±1.0% 

LiCor solar radiation pyranometer for 

outdoor horizontal and vertical insolation ±5.0% 

103 
3.7.2.4 Site weather conditions 

Weather information is important for high-performance building projects that are often more weather 

dependent. Preferably, weather data should be measured on site, but a nearby reliable weather station 

with the required data can also be used. On-site weather data are needed in the as-built model calibration 

process and was valuable for modeling baseline performance when an as-built model was not simulated. 

Monitored weather at each building included: 

• Dry-bulb ambient temperature 

• Relative humidity 

• Global horizontal solar radiation 

• Wind speed and direction. 

The global horizontal solar radiation is the most useful solar radiation measurement. Most simulation 

programs are written to use this value of solar radiation along with direct normal and horizontal diffuse, 

both of which can be estimated from the global horizontal value combined with a cloud cover variable. If 

the measurement plan includes a PV system, measuring the solar radiation in the plane of the PV panels 

may be useful. If two pyranometers are available, both the global horizontal solar radiation and the PV 

plane solar radiation should be measured. 

3.7.2.5 Plan for monitoring system during design 

The electrical panels should group like loads, such as HVAC, lighting, and plug loads. The electrical 

panels at BigHorn are not well organized, which made it difficult to operate and maintain the building and 

extremely difficult to monitor energy performance by end use. 

Because CBF and Oberlin were built before the monitoring effort was conceived, flow meters could not 

be installed on the ground loop. Flow rates and loop temperatures are required to measure energy 

rejection or extraction from the ground. The initial measurement plan called for ultrasonic flow meters to 

monitor the flow rates, but the pipe configuration was not suitable. Future monitoring efforts on 

buildings with ground-source loops should attempt to change or otherwise influence mechanical 

plumbing designs so that relatively long, straight, horizontal sections of pipe are available to form suitable 

locations for ultrasonic flow meters. An alternative is to design and install flow meters as part of the 

system design. 

3.7.2.6 Filling missing data 

Using a dedicated monitoring system to collect evaluation data was functional, practical, and very 

reliable. The reality of long-term experimental research is that missing data are possible even with 

reliable and dedicated equipment and sensors. A typical error was a missing hour of all end-use variables. 

To account for this type of error, NREL inspected diurnal, weekly, and seasonal patterns in the end uses, 

combined with a driving variable, such as weather and occupancy, to estimate the missing data points. 

3.7.3 Daylighting evaluations 

The lighting systems in each building were evaluated to determine the energy savings and to evaluate the 

quality of the light delivered by the lighting design. The Procedure to Measure Indoor Lighting Energy 

Performance (Deru et al. 2005b) provides performance metrics for evaluating lighting design, including 

daylighting. The goals of the daylighting evaluations were to: 

• Measure the energy consumption by the lighting systems. 

• Determine the energy savings that result from the lighting design without daylighting controls. 

• Determine the amount of electric lighting offset by daylighting and the energy saved in lighting. 

• Analyze the operation of the lighting design and optimize its performance. 

104 
• Quantitatively assess the quality of the lighting and daylighting designs. 

• Document the successes and weakness of the lighting design. 

Seasonal illuminance measurements were taken as recommended in the International Energy Agency 

protocol established under Daylight in Buildings Task 21 (Atif et al. 1997). Daylighting was measured 

with photometers to evaluate how well the building is lit by daylight that enters the building through the 

windows. In contrast to continuous monitoring over a year, we obtained these ancillary measurements 

over short periods and used a mixture of handheld and continuous measurements, which were closely 

supervised by researchers. The intent of this analysis is to understand how well this resource is being 

used by determining the extent to which daylight might meet the lighting needs. Recommended 

illuminance levels for each lighting zone were determined in accordance with the Lighting Handbook of 

the Illuminating Engineering Society of North America (IESNA 2000). Recommended lighting levels are 

30 to 50 f.c. (300 to 500 lux) depending on space type, and where daylight can provide the recommended 

lighting levels, the analysis shows where electric lighting can be reduced. 

Daylighting was measured over the course of three site visits at different times of the year. The 

measurements include horizontal illuminance in selected daylit zones during varying sky conditions for 

typical summer, winter, and fall/spring seasons. External horizontal illuminance and electrical lighting 

energy use had to be monitored simultaneously to complete the daylighting measurements. During each 

visit, data loggers were set up to continuously collect data from photometers at 5-minute intervals. 

Additional measurements were taken with handheld photometers to collect supplementary data that cover 

a wider distribution of locations in the building. 

As each building has a low LPD, ensuring that the daylighting and lighting systems can provide 

recommended illuminance levels is important. We want to avoid claiming daylighting savings when 

lights are off and illuminance levels are lower than recommended. The seasonal illuminance 

measurements, both handheld and 5-minute continuous interval, were necessary to verify that the lighting 

systems (daylighting combined with electrical lighting) could provide sufficient lighting levels. 

3.7.4 PV systems evaluations 

The PV systems in each building were evaluated to determine the energy they produced, their effect on 

the building purchased electrical energy, and their performance. The Procedure for Measuring and 

Reporting the Performance of Photovoltaic Systems in Buildings (Pless et al. 2005) provides guidance on 

evaluating the performance of PV systems in the built environment. Additional measurements were taken 

for a more detailed evaluation of the system performance. Evaluations of PV systems in future buildings 

should: 

• Measure the delivered energy production from the PV system. 

• Determine the percentage of the building electrical energy consumption offset by the PV system. 

• Determine building electrical demand offset by the PV system and the energy cost savings. 

• Determine the performance of the PV system compared to the expected performance. 

Bidirectional electrical meters are required to determine net PV energy production. Standby losses can be 

a significant portion of the total PV system production that requires electrical energy meters that can 

measure energy to and from PV systems. Additional PV measurements that are valuable for system 

analysis included insolation in the plane of the PV array and PV module operating temperature. Shortterm current-voltage measurements were valuable for understanding maximum power point settings. 

All PV systems were modeled with an annual modeling tool to evaluate measured energy production and 

to identify and quantify system degradations. We used the PV system simulation tool PVSyst v3.3 

(Mermoud 1996) to calculate the expected annual performance of the PV system and determine the 

105 
effects of degradations on total output. Inputs to this model include PV panel size and operational 

characteristics, inverter size and operational characteristics, array configuration and wiring details, array 

tilt and azimuth, and hourly weather data. System specific inputs such as shading and array wiring losses 

were also simulated. The simulation approximated the cell temperatures based on the default thermal 

properties of the specific mounting configuration. When available, measured PV cell temperature was 

used to calibrate the site-specific thermal properties of the mounting configuration. 

Hourly or 15-minute data were collected for all PV systems. One-minute data were also collected for 

system diagnostic purposes. Careful examination of the Oberlin PV system performance shows the value 

of collecting PV performance data on a 1-minute interval. On some days, the inverters would shut down 

repeatedly for short periods (Figure 3-55). Daily hourly average power production data appeared to show 

that a cloud had passed by in mid-afternoon, not that there was a system performance issue. The system 

performance problem was seen when the minute-by-minute performance data were reviewed. On cool, 

sunny days, the PV system supplied more power to the inverter than it could invert. The result was an 

overvoltage fault that temporarily shut down one, two, or all three inverters for 5 minutes at a time. 

0

10

20

30

40

50

5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

Time of Day (hr)

AC PV Power Produced (kW)

Total AC PV Output

(Minute Average)

Total AC PV Output

(Hour Average)

 

Figure 3-55 PV AC power output for Oberlin’s three subarrays on April 28, 2001 

3.7.5 Performance metrics 

Obtaining reliable metrics to determine a building's performance is one of the core challenges to 

achieving widespread adoption of high-performance buildings. Building professionals should plan to 

implement the measurement procedures as described in DOE’s Performance Metrics Research Project. 

The current suite of procedures was developed in parallel with the case studies, as the need for 

standardized procedures and metrics was evident early in the POE process. 

Currently available procedures for determining performance metrics include: 

Procedure for Measuring and Reporting Commercial Building Energy Performance 

The purpose of this procedure is to establish a standard method for monitoring and reporting on the 

energy performance of commercial buildings. It determines the energy consumption, electrical energy 

demand, and on-site energy production in existing commercial buildings of all types for the facility and 

end uses. The performance metrics determined here may be compared against benchmarks to evaluate 

performance and verify that performance targets have been achieved. This procedure includes definitions 

of the performance metrics obtained, detailed steps for quantifying performance, and a list of suggested 

monitoring equipment (Barley et al. 2005). Uses may include: 

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• Compare performance with the design intent. 

• Compare performance with other buildings. 

• Evaluate building performance rating systems. 

• Perform economic analysis of energy-efficient strategies in buildings. 

• Establish long-term performance records that enable maintenance staff to monitor trends in 

energy performance. 

Procedure to Measure Indoor Lighting Energy Performance 

This document provides standard definitions of performance metrics and methods to determine them for 

the energy performance of building interior lighting systems. It can be used for existing and proposed 

buildings. Typical results from the use of this procedure are the monthly and annual energy used for 

lighting, energy savings from occupancy or daylighting controls, and the percent of the total building 

energy use that is used by the lighting system. The document is not specifically intended for retrofit 

applications. However, it does complement measurement and verification protocols that do not provide 

detailed performance metrics or measurement procedures (Deru et al. 2005b). 

Procedure for Measuring and Reporting the Performance of Photovoltaic Systems in Buildings 

This procedure provides a standard method for measuring and characterizing the long-term energy 

performance of PV systems in buildings and the resulting implications to the building’s energy use. The 

performance metrics determined here may be compared against benchmarks for evaluating system 

performance and verifying that performance targets have been achieved (Pless et al. 2005). Uses may 

include: 

• Compare performance with the design intent. 

• Compare with other PV systems in buildings. 

• Economic analysis of PV systems in buildings. 

• Establish long-term performance records that enable maintenance staff to monitor trends in 

energy performance. 

Procedure for Developing a Baseline Simulation Model for a Minimally Code-Compliant Commercial 

Building 

This procedure provides a standard method for developing a baseline simulation model of a minimally 

code-complaint commercial building for the purposes of whole-building energy simulations. It provides 

various classifications of baselines based on the design and operation status of the building under 

construction. Three major baselines are defined: Pre-Design, Proposed Design, and Existing Building. 

The procedure also tells users how to use each of these three baselines with an integrated design process 

(Pless, Deru, and Torcellini 2006). 

Standard Definitions of Building Geometry for Energy Evaluation Purposes 

This procedure provides definitions and metrics of building geometry for use in building energy 

evaluation. Building geometry is an important input in the analysis process, yet there are no agreed-upon 

standard definitions of these terms for use in energy analysis (Deru and Torcellini 2005). The metrics can 

be used to: 

• Characterize building geometry. 

• Calculate energy performance metrics. 

• Conduct energy simulations. 

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Procedure to Determine Source Energy and Emissions for Energy Use in Buildings 

This procedure provides the fuel and emission factors to calculate the source (primary) energy and 

emissions for electricity, fuels, and thermal energy delivered to a facility, and combustion of fuels at a 

facility on an annual basis. The fuel and emission factors provided in this procedure account for the 

energy and emissions associated with extracting, processing, and delivering the fuels to the electrical 

power plants or directly to the buildings. In addition, the breakdown of the fuel used to generate 

electricity is provided on the national level, three interconnection levels, and the state level for Alaska and 

Hawaii (Deru and Torcellini 2006). 

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4 CONCLUSIONS 

Each of these six commercial buildings has its unique purposes and functions, and all have 

commonalities. All must provide protection against varying climatic conditions and acceptable comfort 

for the occupants. The six buildings in this study are successful in these respects, and they are all good 

energy performers. All had owners who pushed low-energy or sustainability goals and considered energy 

efficiency as part of the decision-making process. The architects and engineers then met and 

implemented the vision. This vision required an integrated design process to achieve the goals. All the 

buildings use innovative energy technologies to reduce their energy use and minimize their environmental 

impacts. Some use technologies that may seem inapplicable to other types of buildings. However, there 

is substantial evidence that these buildings represent a broad cross-section of the commercial sector. 

Direct replication is not possible, as each commercial building is unique. One must think of how these 

technologies might adapt to the design of other commercial buildings. 

Examples of this replication thought process might include: 

• TTF is a steel frame building typical of much of the commercial one-story building stock. 

Daylighting, evaporative cooling, enhanced thermal envelope, and overhangs would work well in 

similar structures. 

• The Zion Visitor Center is an excellent example of a small retail space, and of museum and park 

visitor centers that can easily be replicated. 

• The Zion Comfort Station is a good example of a rest area facility and any restroom in any 

building. 

Significant energy savings were realized for this set of low-energy buildings constructed in a wide range 

of climates in the United States. We found that the buildings’ energy consumption was 25% to 70% 

lower than code. At current levels of performance, the one-story buildings—Zion, BigHorn, and TTF—

could accomplish the ZEB goal with PV systems on their roofs. ZEBs are not feasible for the two-story 

buildings within their footprints unless their loads are reduced further. Further ZEB analyses and 

definitions are provided in Appendix A. 

Each case study report developed a list of lessons learned and recommendations relevant to that unique 

building. This report combines the overarching lessons learned. Although the case studies represent a 

variety of designs and climates, we stepped back and looked at all six together to understand common 

successes and failures. We then distilled successes and failures and other wisdom acquired along the way 

into a set of lessons learned. 

The term lessons learned refers to positive and negative aspects of a project that have clear messages and 

might help subsequent low-energy building projects. The lessons learned are intended as 

recommendations and best practices, either for changes to these buildings or for concepts that should be 

applied to the next generation of low-energy buildings. Members of future building projects should keep 

these lessons in mind and realize that they should be considered jointly along with the goals for saving 

energy. Lessons learned are key educational components. They should help design teams avoid 

repeating problems and identify where the process of delivering buildings needs to be changed to promote 

and realize low- and zero-energy buildings. 

We then used these buildings and their lessons learned to help identify and define a set of best practices— 

design elements and techniques that should be avoided in commercial buildings, as well as elements that 

should be carried forward to additional buildings. Best practices are proven, real-world technologies and 

processes that can lead to high-performance commercial buildings. 

4.1 Lessons Learned 

Lessons learned from these projects, from the design process through the operation of the building, will 

help to improve all buildings. Based on good design and effective operation, buildings can be constructed 

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that use significantly less energy than code-compliant buildings. As with all building projects, not 

everything the design teams desired was achieved. 

4.1.1 Applying a whole-building energy design process 

Each building established energy goals from the beginning of the design process and saved energy as a 

result. Whole-building energy simulations were a great help in goal setting, informing the design process, 

and evaluating the impact of design and construction decisions. The envelope was designed first, and the 

remaining loads were met with HVAC equipment. Successful daylighting was vital to reducing lighting 

and HVAC loads. 

Measured data provided valuable feedback to building owners and design teams about the success of their 

buildings. The monitoring results were used to compare results against the goals and identify areas that 

needed to be corrected to improve the energy performance. All the buildings’ energy performance 

benefited from postoccupancy fine-tuning of the system operations. 

Based on our evaluations, here are the significant lessons learned related to the whole-building design 

process: 

• Owners provide the main motivation for low-energy buildings. The owner was the driving 

force in each case. Each owner set the goals and made decisions to keep the project on track. 

The architects and engineers strived to meet the goals of the building owners, which resulted in 

the whole-building design process. 

• Setting measurable energy saving goals at the onset of the project is crucial to realizing lowenergy buildings. In the case studies, all the owners and design teams set aggressive energy 

saving goals at the outset. The goals ranged from 40% better than code to net-zero energy 

performance. Some also had ambitious goals about other dimensions of sustainability such as 

water management, building materials selection, or obtaining a high LEED score. People are 

motivated to achieve when they set the goals. Setting such goals enables owners to make 

decisions that align with the goals. In general, the teams that set the strongest energy 

performance goals and used energy simulation to understand the energy impacts of design 

decisions had the best energy performance. These projects show that defining specific, 

measurable energy saving goals early in the design process helps the design team achieve better 

results. 

• Many decisions are not motivated by cost. Building owners make decisions based on values. 

Quite often owners will pay for features they really want in a building—this is especially true of 

architectural features. Conversely, if an owner does not want a feature, cost is often used as the 

reason to eliminate it. 

• Today’s technologies can substantially change how buildings perform. Properly applied offthe-shelf or state-of-the-shelf technologies are available to achieve low-energy buildings. 

However, these strategies must be applied together and properly integrated in the design and 

operation to realize energy savings. There is no single efficiency measure or checklist of 

measures to achieve low-energy buildings. 

• A whole-building design approach is a good way to lower energy use and cost. An integrated 

whole-building approach begins with a design team that is committed to the energy goals. The 

building must be engineered as a system if the technologies are to be integrated in design and 

operation. The integrated design process starts with predesign and is applied through 

construction and operation, ensuring that the building is built to plan. Whole-building energy 

simulation is a great help in setting goals, informing the design process, and evaluating the energy 

impacts of design and construction decisions. The envelope should meet as many needs as 

possible and be the first method of creating a low-energy building. The mechanical and lighting 

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systems should provide the remaining thermal and lighting comfort needs. Low-energy 

architecture is not achievable if mechanical systems have to solve problems from inadequate 

envelopes. 

The best way to reduce the cost of low-energy features and ensure successful integration is to 

incorporate them into the architecture early. If low-energy features such as daylighting, 

clerestories, or cooltowers can be integrated into the architectural statement, they can be included 

as part of the architecture costs. 

The weak point in realizing low energy is not necessarily in the technologies, but rather in the 

lack of a widely used and cost-effective design and construction processes that can integrate these 

technologies from a systems engineering perspective. This process includes integrating the 

technologies with advanced control hardware and control sequences. The final step in the wholebuilding design process includes verifying postoccupancy performance so the building operates 

as designed. The probability that a low-energy building will be achieved is improved by adopting 

the whole-building design process. 

• Low-energy buildings do not always operate as they were designed. The design community 

rarely goes back to see how their buildings perform after they have been constructed. 

Measurements in all six buildings showed that they used more energy and produced less energy 

than predicted in the design/simulation stage. Several reasons were documented. 

3⁄4 There was often a lack of control software or appropriate control logic to allow the 

technologies to work well together. Appropriate control systems can successfully integrate 

the low-energy technologies and allow a building to operate at its design potential. 

3⁄4 Design teams were a bit too optimistic about the behavior of the occupants and their 

acceptance of systems. 

3⁄4 Energy savings from daylighting were substantial, but were less than expected. 

3⁄4 Plug loads were often greater than design predictions. 

3⁄4 Effective insulation values are often inflated when comparing the actual building to the 

designed building. 

3⁄4 Thermal bridging was partially accounted for in the models during the design process, but 

construction details and specifications are not always implemented as designed; in some 

cases the designs did not include specifications about thermal bridging. 

3⁄4 Operational failures in many energy-saving technologies go unnoticed because they have 

backups. Therefore, when these systems do not operate as intended, comfort is not affected. 

For example, if a daylighting sensor fails, sufficient lighting is still provided with the 

electrical lighting system, but the expected daylighting energy savings are not realized. 

Sophisticated submetering is often required for detection and to keep the systems operating 

properly. 

3⁄4 PV systems experienced a range of operational performance degradations. Common 

degradation sources included snow, inverter faults, shading, and parasitic standby losses. 

• Monitoring leads to better management and improved performance. Setting and following 

design goals or traditional commissioning does not guarantee that the goals will be satisfied in 

actual operations. The whole-building energy performance must still be tracked and verified. 

Many items, such as daylighting controls, PV system faults, and equipment sequencing were 

corrected because of the knowledge gained through metering and submetering. Especially with 

Oberlin, there was a dramatic improvement in performance with monitoring and correcting some 

problem areas identified by the metering. There is a performance gap between what today’s 

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technologies have provided and the potential performance of state-of-the-shelf technologies. 

Commissioning a new building is essential, but it ensures only that the building components 

operate as specified. All six of these low-energy buildings were commissioned, formally or 

informally, before occupancy. Various degrees of advanced commissioning occurred during the 

postoccupancy monitoring as well. In general, the traditional commissioning did not translate to 

expected energy performance or verify low-energy design goals. Continual monitoring of the 

performance, or continuous commissioning with key performance metrics, is important to ensure 

that the goals of the design are met under normal operating conditions. In all the buildings, 

controls, equipment, and operation were all adjusted to better align the performance with the 

design goals. 

• This set of current generation low-energy buildings shows progress toward achieving a ZEB 

goal in actual buildings. Each of these buildings saved energy, with energy use 25% to 70% 

lower than code. Although each building is a good energy performer, additional energy 

efficiency and on-site generation is required for these buildings to reach DOE’s ZEB goal. At 

current levels of performance, the one-story buildings—Zion, BigHorn, and TTF—could 

accomplish the ZEB goal with PV systems that would fit within the building footprint. ZEBs are 

not feasible for the two-story buildings within their footprints unless their loads are reduced. 

• We can replicate the lessons learned from the case studies in the future generation of lowenergy buildings. Even though every commercial building is unique, the lessons learned from 

the case studies can be applied to a wide variety of commercial buildings. The buildings and their 

lessons learned define a set of best practices. This includes design elements that should be 

avoided in commercial buildings as well as elements that should be carried forward to additional 

buildings. Understanding success and opportunities in the current generation of low-energy 

buildings can improve the energy efficiency of all commercial buildings—the best practices 

should be applied to future buildings. 

4.1.2 Lighting and daylighting systems 

Lighting is the largest single end use in commercial buildings, at 30% of the total energy used. From a 

national perspective, the potential for daylighting savings is significant. Based the 1999 CBECS data set, 

nearly 80% of the total floor area in commercial buildings has an exterior ceiling or is within 15 ft (4.6 m) 

of an exterior wall and therefore has the potential to be daylit. 

We believe daylighting is critical to creating low-energy buildings. Good daylighting can offer dramatic 

electric energy savings benefits. All six high-performance buildings use daylighting systems to offset 

electrical lighting energy use. Top-lit daylighting through clerestories and high windows are integral to 

each daylighting system. Daylighting is the best resource for total energy savings at Zion, Oberlin, 

BigHorn, and TTF. All six buildings feature daylighting design, and all have experienced problems. 

Some lacked the expected energy savings, experienced unacceptable glare, or had lower than anticipated 

lighting levels. Postoccupancy evaluations addressed some of these problems, increasing energy savings 

and occupant satisfaction. 

Six elements were defined to achieve energy savings through daylighting. If five of the six elements 

necessary to good daylighting are done correctly, but one is not, energy savings may not result. A good 

daylighting design should result from an integrated design process. The daylighting system has to be 

integrated with the envelope and trade-offs with heating and cooling understood to maximize wholebuilding energy savings. Even with the good integrated design in each building, the following six 

elements of a daylighting system are needed to maximize daylighting savings. 

• Design buildings to provide daylighting to all possible zones. 

• Provide for glare mitigation techniques in the initial design. 

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• Provide automatic dimming daylighting controls for all daylit zones. 

• Interior designs should compliment the daylighting. 

• Integrate the electrical lights with the daylighting system. 

• Commission and verify postoccupancy energy savings. 

4.1.3 Integrating the envelope and mechanical systems 

Energy use in commercial buildings is very diffuse. Some larger components are combined with many 

smaller end uses that add up to significant energy use at the whole building level. Therefore, there is no 

single technology or even a set of innovative technologies for realizing low-energy buildings. A diffuse 

set of solutions in the design and operation must be successfully applied and integrated. Low-energy 

buildings must be system engineered from a whole-building perspective to integrate the innovative 

technologies throughout the design process through occupancy and operation. 

We learned the following lessons from working with the building envelopes and mechanical systems of 

the six case study buildings. 

• Use the envelope as the first method of creating low-energy buildings. Engineer the envelope 

to reduce loads with features such as increased insulation, window tuning, and self-shading, and 

at the same time integrate passive solar heating, cooling, daylighting, and natural ventilation 

technologies. Then size HVAC equipment to meet the remaining loads. Use the mechanical 

system to make up for what cannot be accomplished by architectural form and envelope, not to 

correct for an architectural design that is climatically ill conceived. 

• Design the envelope to minimize thermal bridging. An otherwise well-insulated envelope can 

be ineffective if a single component lacks appropriate thermal properties. Additional 

consideration is typically needed to ensure the doors are well insulated (especially those that 

experience direct solar gains), window frames are installed with thermal breaks, and the 

insulation at the interface with ground-coupled surfaces has been appropriately specified. 

Ensuring proper insulation details are included in the specifications and then implemented during 

construction is essential to achieving a thermally uniform envelope. 

• Consider Trombe walls and direct solar gains for passive heating. We found Trombe walls to 

be an effective integrated envelope technology that can provide a significant portion of the 

heating needs. The annual net effect has to be considered when designing a Trombe wall, as the 

additional cooling loads can affect the cooling system performance. Properly designed direct 

solar gains can also provide space heating, if direct beam is acceptable. Zion, BigHorn, Oberlin, 

TTF, and Cambria are clearly heating dominated buildings. Passive solar heating has not been 

used often for commercial buildings because conventional wisdom indicates that such buildings 

are cooling dominated. However, low-energy commercial buildings tend to require more heating 

because of reduced internal and solar heat gains. 

• Natural ventilation can provide significant fan and air-conditioning energy savings. Four of 

the buildings used natural ventilation for cooling and ventilation. The systems at Zion and 

BigHorn were mostly successful. They provided all the outdoor and cooling requirements 

(Zion’s cooltowers are considered an evaporative cooling system driven by natural ventilation). 

Limited success was observed at CBF, as the system typically operated in hybrid mode, using 

exhaust fans. Minimal natural ventilation is used at Oberlin because of difficulties developing 

and implementing appropriate controls. 

• Use evaporative cooling systems in dry climates. All cooling requirements were met with 

evaporative cooling systems in the dry climates of Zion and TTF. In a dry climate, two-stage 

(direct plus indirect) evaporative cooling can provide sufficient cooling capacity with less energy 

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use than refrigerant-based cooling systems. TTF uses a two-stage evaporative cooling system 

with a variable speed supply fan, and Zion uses a passive downdraft cooltower system that is 

functionally equivalent to direct evaporative cooling. 

• Consider electric resistance heating carefully. Electric resistance heating can be used 

successfully in low-energy buildings, if it is used sparingly and does not cause excessive demand 

charges on the building. No ductwork or mechanical rooms are required with radiant panels, and 

radiant heat can be provided directly to where it is needed. Electric radiant heating panels 

worked at Zion because Trombe walls were a significant heating source, and demand-responsive 

controls ensured electric heating did not cause excessive peak demands. Heat pumps are often 

better for the primary heating system, with electric resistance as backup. 

• Use ERVs in conjunction with economizers and only when recovered energy is a net energy 

saving. A properly controlled ERV can realize energy savings; the same ERV improperly 

controlled may actually increase energy use and cost. ERVs should not replace proper controls 

for the amount and scheduling of outdoor air, appropriate equipment sizing, or conventional 

economization cycles. Systems with ERVs and ground-source heat pumps suffered from the lack 

of an economizer. ERV recovery should be used when recovered energy is more than the fan 

energy for the unit and only when the recovered energy benefits the building. These outdoor air 

systems should also be incorporated with an economizer cycle. 

• Carefully size heat pumps and ground loops. Ground-source heat pumps with ERVs are the 

primary heating and cooling sources at Oberlin, CBF, and Cambria; each heat pump system had 

performance issues related either to equipment or to ground capacity sizing. Part-load controls 

for groundwater loop pumps should be considered to save on pumping energy when capacity is 

not needed. Additional work is needed to understand the ground characteristics as it is believed 

wells are underperforming. Additional work is needed to control the systems to minimize pump 

energy. In some cases, ground temperatures were less favorable than outdoor air temperatures. 

• Design the control system to be fully integrated with the capabilities of equipment and 

building operators. Innovative controls are often the critical glue for integrating a diffuse set of 

energy efficiency solutions, allowing a low-energy building to reach its potential. As each lowenergy building is a prototype that includes state-of-the-shelf efficiency technologies, the lacking 

technologies are often in the whole-building control algorithms (software) required to ensure the 

building operates as a system. 

4.1.4 Photovoltaic systems 

For DOE’s ZEB goal to become a reality, buildings will have to produce more energy than they use with 

PV systems on the roofs. Future ZEB will not only require efficient use of energy, but they will also need 

to maximize energy production. All systems produced less than expected because of performance 

degradation sources such as shading, parasitic losses, inverter faults, and lack of power-point tracking. 

Each system output was degraded because of balance of system component integration problems, 

operational problems, and shading. Combined, they reduced overall output 14% to 68%. 

Our experience with PV systems in buildings taught us the following: 

• Design grid-connected PV systems to not have parasitic standby loads. During nighttime 

hours when the Oberlin or Cambria PV system was in standby mode, the inverters and 

transformers consumed electricity. Nighttime parasitic PV system losses accounted for 7% to 

37% of the total PV output depending on how often each system was in standby mode. An 

automatic circuit that disconnects the PV system from the grid when the PV system is down and 

reconnects when the PV system is operational should be implemented when isolation 

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transformers are used. This strategy applies for systems where loads are created by the PV 

system at night, usually a result of the choice of inverter. 

• Consider an automatic monitoring system for PV system operation. Occupants and operators 

were often unaware of extended downtimes; this lack of awareness can result in even longer 

downtimes. When the system is down, it is not easily noticed as the grid power meets any 

building load. A simple method or monitoring technique that provides feedback to the owners 

and occupants about PV generation effectiveness or performance ratio would address the simple 

problem of inverter faults that require manual restarts. This fault, typically caused by system 

integration issues, has been a primary source of PV system downtime in the case studies. 

• PV panels should be located where they will not be shaded. Each PV array experienced 

shading, resulting in a range of energy penalties from 2% to 44% output reduction. Sources of 

shade included trees, canyon walls, and the building structure. 

• Consider specifications for how a PV-based UPS system transitions to and from utility 

power. The Zion PV/UPS system is a high-value feature, even though it displaces only a part of 

the total energy load. During daylit hours, the system can provide power for business operations 

without relying on power stored in the UPS system battery bank. Additionally, this central UPS 

system entirely eliminates the need for additional small UPS systems in the building—a source of 

additional plug loads because of their poor efficiency. However, the Zion UPS system has had 

some difficulties determining when to disconnect from and reconnect to the utility power. Future 

efforts to use PV systems for a building-wide UPS should carefully investigate how the 

equipment responds to the types of power failures the building is likely to experience. 

• Use maximum power-point tracking controllers. An MPPT voltage controller would increase 

performance of the Zion PV system by 16% compared to the fixed operating voltage control of 

53.6 VDC. The annual average AC Generation Effectiveness values were 3.27% for the BigHorn 

PV system without inverter faults and 3.92% for the same PV array and an MPPT inverter, which 

is a 20% annual improvement with the MPPT inverters over the use of fixed voltage inverters. 

• Integrate PV systems with demand-responsive controls and on-site thermal storage to 

maximize energy cost savings and payback. The economic payback of PV systems depends on 

the net metering and interaction with demand charges. PV systems contribute very little to 

reducing building peak electrical demands. Therefore, the payback for the PV system is poor 

because it offsets only electricity energy charges, typically $0.02–$0.05/kWh. Additional 

demand reduction cost savings because of the PV system would increase the cost justification of 

the PV system. On-site storage systems such as ice storage, combined with demand-responsive 

EMS, will be required to realize the full economic value of the energy and power produced by PV 

systems in future ZEBs. 

4.1.5 Peak demand and demand management 

The easiest method to reduce energy cost in a commercial building is to reduce the peak demand charges. 

Electric demand charges can contribute to more than 50% of the total utility costs. An efficient building 

that uses very little energy could still have large demand charges with poor control. 

The following lessons learned provide valuable insight into understanding peak demands in low-energy 

buildings: 

• Low-energy buildings do not have typical load shape profiles. Peak demands in low-energy 

buildings are often occur at atypical times of the day. The conventional commercial building has 

summer peak demands in the late afternoon when cooling and lighting loads are the greatest. 

Well-daylit buildings reduce lighting and peak cooling loads, shifting peaks to atypical times. 

Low-energy buildings can shift when a peak demand is met, but they do not necessarily reduce 

115 
the overall peak demand. Evaporative cooling, reduced LPD, and HVAC equipment that have 

been downsized to meet reduced thermal loads are the best ways to guarantee peak demand 

reductions. Each of these techniques reduces peak demands because even if they are all on, the 

peak consumption is still less than the baseline equivalent. Technologies that depend on a solar 

resource such as daylighting, Trombe walls, PV systems, and natural ventilation are not always 

available to reduce energy use. Therefore, they cannot be relied on for peak demand reduction, 

unless controls are used to adjust building energy use to account for the solar availability. 

• PV systems have had limited success reducing peak demands. The PV systems in this set of 

low-energy buildings have not significantly reduced the building demand. In these buildings, any 

small demand reduction caused by PV is from load diversity. Peak demands often occurred 

during atypical times that were not coincident with PV production, when daylighting was not 

available for demand reduction. 

• Demand-responsive controls can save energy costs and increase load factor. The largest 

energy cost savings of all the buildings was at Zion because of aggressive demand management, 

which the other buildings did not use. The load factor was also consistently higher than the other 

buildings. Demand management combined with on-site storage (building mass) was the best use 

of on-site generation for reducing peak demands. Oberlin was the least successful at using PV for 

demand reduction because of high peak demands and a large PV system that reduced energy use 

but not peak demands, which resulted in low load factors. 

• Low-energy buildings can have low load factors. Energy-efficient buildings strive to lower 

energy consumption; however, they often do not reduce the demand by the same amount. If the 

energy use is reduced more than the peak demand, the load factor decreases. A low load factor 

means that the utilities will have to supply less energy, but there is no reduction in capacity 

requirements. Therefore, the utilities still have to maintain the same capital structure: generation, 

transmission, and distribution capacities. Keeping power plants in spinning reserve while they 

wait for higher demands requires energy, so even though a building may reduce site energy 

consumption, the source energy consumption may not be reduced by the same amount. Even 

buildings with on-site electricity generation from PV systems rarely reduce demand by more than 

a few percent. Energy-efficient buildings should manage peak demand with demand-responsive 

controls and possibly on-site energy storage to ensure demand savings are in line with energy 

savings. 

4.1.6 Plug and equipment loads 

Minimize parasitic loads. A significant part of the equipment and plug loads in all these buildings 

consists of idle computers. Energy savings can be realized by turning equipment off or forcing it into 

“sleep mode” during periods of inactivity. Power management software should be installed on all 

computers. Flat screen monitors should also be used for added energy savings. Vending machines can be 

turned off during periods of inactivity. EnergyMisers save 30% to 50% in energy use and typically have 

a payback period of two years. 

4.1.7 Postoccupancy evaluation techniques 

The following lessons learned are derived from our experiences with applying POE processes: 

• Determine whole-building energy savings with as-built and base-case model comparisons. 

Ideally, the base-case site energy use, source energy use, and energy costs should be compared to 

an as-built model to calculate savings for a typical weather year. Energy savings uncertainties 

can be minimized when savings are determined from the comparison of one simulation to another 

simulation (e.g., base case to as-built). Because difficult-to-know inputs are held the same in 

both simulations, such comparisons remove much of the uncertainty inherent in an hourly 

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building energy simulation. Variables that change throughout the year, such as inconsistent 

occupancy, set-point changes, and equipment performance degradation are difficult to account for 

in an annual building energy simulation. By comparing a base-case model to an as-built model 

with the same schedules, the uncertainty caused by these inconsistent variables is reduced. 

• Modeling tools that can simulate baseline and as-built low-energy buildings should be used 

to ensure consistent evaluation techniques. To perform complete POEs, a whole-building 

simulation tool should be used that can adequately model all features and operations in an 

integrated manner, including subhourly energy use, innovative HVAC and control systems, site 

utility costs, and on-site production. A common limitation of the simulation tools was calculating 

energy costs. This problem was most evident when we tried to assess the impact of PV. To fully 

realize the cost implications in commercial buildings, PV must be integrated into the simulation 

engine to fully determine demand savings potential. 

• Measure performance against design goals. Energy savings depend on how savings are 

defined. A complete high-performance building should result in significant energy savings in the 

following metrics: site energy savings, source energy savings, and energy cost savings. A 

building can excel in the energy performance indices most important to the building owners, but 

fall short or exaggerate other performance indices. Oberlin performed well in site savings, 

realized minimal energy cost savings, and excelled in source savings (mostly because of PV 

production and benchmarking techniques). A building energy performance evaluation should 

focus on the metric the building was designed to optimize, but it should also consider other 

significant performance indices. If the team sets energy cost savings goals during design, 

verifying these design goals during actual operation may be difficult because energy prices may 

change. Site or source energy savings goals are easier to measure and typically less susceptible 

than energy cost savings to external fluctuations such as energy prices. 

• Use performance metrics procedures to standardize measurement and reporting for POEs. 

The commercial building industry needs to develop standardized and repeatable whole-building 

POE techniques. As more commercial buildings are designed to be “green,” the industry will 

need to validate real-world energy savings. An integral part of a standardized POE is a 

commercial buildings benchmark. 

• Use dedicated, self-contained data acquisition equipment to monitoring energy 

performance. Performance monitoring systems need to be robust and maintained by someone 

locally or monitored remotely, with someone local to handle problems. Personal computers 

should not be exclusively relied upon for data retrieval and archiving, as they are unreliable 

compared to systems dedicated to data logging. A dedicated DAS for monitoring energy 

performance can provide 99% data availability over a two-year monitoring period. A complete 

energy balance on metering is essential to find faults in monitoring. Missing data can be 

minimized with error checking routines and reasonable data filling techniques. Fifteen-minute 

data logging is appropriate for a wide-range of evaluations. In cases where equipment issues 

were found (such as a five-minute PV inverter cutout), data sets that are more detailed were 

collected for limited periods. 

• The current methods for calculating electricity source energy use are too general. 

Additional work is needed to account for hourly, daily, and regional variations in electricity 

generation heat rates and supply mix. Regional time-dependent valuations for determining timeof use source energy are needed throughout the country to account for variations in when and 

where energy is used. 

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4.2 Best Practices for High-Performance Buildings 

Much can be learned from these case studies. Each of these six commercial buildings has its unique 

purpose and function, and all have commonalities. All must stand up to climatic conditions, and all must 

provide acceptable comfort for the occupants. All are successful in these respects, and all are good 

energy performers. All had owners who pushed low-energy goals and considered energy efficiency as 

part of the decision-making process. The architects and engineers then met and implemented the vision. 

This vision required an integrated design process to achieve the goals. All the buildings use innovative 

energy technologies to reduce their energy use. 

We captured a set of best practices that result from the lessons learned from these six buildings. Best 

practices are not lessons learned; rather, the best practices are derived from the lessons learned. Best 

practices are proven real-world technologies and processes that lead to high-performance buildings. This 

list is the result of our research experience with the six case study buildings—this is not an attempt to 

create an exhaustive list. 

The following best practices should be applied to future low- and zero-energy buildings: 

1. Use a whole-building design process to design, construct, and operate future low-energy 

buildings. This includes: 

a. Set a specific, quantifiable energy design goal that is used to guide the design process and can 

be verified during operation. 

b. Include everyone involved in the project at the earliest stages of the design process. 

c. Use whole building simulation tools to determine the energy performance of the building at 

all stages of design, construction, and occupancy. The whole-building design process is a 

simulation-based, quantitative, and qualitative method to help architects and engineers create 

low-energy buildings. Low-energy design is not intuitive. The energy use and energy cost of 

a building depend on the complex interaction of many parameters and variables. The 

problem is far too complex for “rules of thumb” or hand calculations. The interactions are 

best studied with computerized energy simulation software to thoroughly evaluate all 

interactions between the building envelope, HVAC system, and design features. 

d. Design the building envelope such that it can be used to meet as many of the loads as 

possible. The envelope should be the first method of creating low-energy buildings; the 

mechanical and lighting systems should then be sized to meet any remaining loads. Lowenergy architecture is not effective if mechanical systems have to solve problems from the 

architectural design. 

f. Update the design simulations from predesign through occupancy to ensure the design 

simulations will measure up to actual performance. 

2. Plan for an energy performance POE. A POE includes disaggregating the energy use of major 

end use categories. The best time to ensure ease of end use monitoring is in the design and 

construction phase, when like loads can be arranged together in the wiring panels. Providing for 

a POE also allows evaluators access to the energy management system to develop and test 

innovative control strategies. Commissioning a new building is essential, but it ensures only that 

the building components operate as specified. All six case studies were commissioned either 

formally or informally. In general, the traditional commissioning did not necessarily translate to 

expected energy performance or verify low-energy design goals. Continual monitoring of the 

performance during the POE, or continuous system commissioning with key performance metrics 

is important to ensure that the goals of the design are met under normal operating conditions. In 

all the buildings, adjustments were made to better align the performance with the design goals. In 

addition, feedback on actual performance to the engineers and architects is important for 

improving future designs. 

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3. Implementing measurement procedures such as those developed in DOE’s Performance 

Metrics Research Project. Obtaining reliable metrics for determining a building’s performance 

is one of the core challenges to achieving widespread adoption of high-performance buildings. 

Currently available procedures for determining performance metrics are available in the 

following documents: 

a. Procedure for Measuring and Reporting Commercial Building Energy Performance 

b. Procedure to Measure Indoor Lighting Energy Performance 

c. Procedure for Measuring and Reporting the Performance of Photovoltaic Systems in 

Buildings 

d. Procedure for Developing a Baseline Simulation Model for a Minimally Code-Compliant 

Commercial Building 

e. Standard Definitions of Building Geometry for Energy Evaluation Purposes 

f. Procedure to Determine Source Energy and Emissions for Energy Use in Buildings. 

(See Section 3.7.5 for details and references of these procedures.) 

4. Integrate daylighting into the envelope and lighting systems. Controlling the electric lighting 

when daylighting is available works in all climates and in almost any type of building. A good 

daylighting design should result from an integrated design process. The daylighting system has 

to be integrated with the envelope and trade-offs with heating and cooling understood to 

maximize whole-building energy savings. Use the following best practices to integrate 

daylighting with the lighting systems: 

a. Design daylighting into all occupied zones adjacent to an exterior wall or ceiling. Design 

daylighting systems into all occupied spaces in a one-story building. In multistory buildings, 

daylighting should be designed into all occupied spaces on the top floor and all other zones 

adjacent to exterior walls. Typically, all zones within 15 ft (4.6 m) of an exterior wall can use 

perimeter daylighting to reduce lighting loads. Greater daylighting penetration depths can be 

achieved with architectural elements such as light shelves or other devices that reflect light 

onto ceilings. Sufficient daylighting can be achieved with relatively small aperture areas. 

Making openings larger than needed for daylighting is counterproductive, especially in 

locations where cooling loads dominate. Daylight modeling and whole-building energy 

simulation tools are recommended for proper design of daylighting systems. 

b. Provide for integral glare mitigation techniques in the initial design. Techniques such as 

diffusing glass, light-deflecting panels, full shading overhangs, and light shelves that do not 

require occupant interaction for glare mitigation should be considered for all south-facing 

clerestories. Other daylighting techniques such as north-facing clerestories and tubular 

daylighting devices that do not have direct solar-beam components should also be considered 

c. Provide automatic, continuously dimming, daylighting controls for all daylit zones. For 

all daylit zones that have a daylighting contribution, continuously dimming controls should 

be used to maximize daylighting savings. Occupants cannot be relied on to control the lights 

in response to available daylight. Automatic controls that dim and energize lighting circuits 

according to available daylight are typically better than stepped controls. Dimmers should be 

specified that are fairly linear in the relationship of light output to energy use. When a 

manual override for automatic daylighting controls is needed, a maximum illuminance set 

point should be included in the manual control strategy. This control sequence will ensure 

that even when the lights are manually controlled to be on, daylighting can still offset lighting 

energy use. Occupancy sensors should be used to disable the manual override when the zone 

is unoccupied. On/off daylighting controls are acceptable for hallways, restrooms, and areas 

where constant lighting levels are not critical. 

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d. Design interiors to maximize daylighting distribution. Install highly reflective 

nonspecular finishes in all daylit zones, especially when indirect lighting is used. Dark 

ceilings and structural elements are common reasons for poor daylighting distribution and 

reduced savings. Anticipate for how occupants, furniture, and other features such as light 

absorbing merchandise and shelving in retail spaces, window frames, actuators, mullions, and 

screening will reduce daylighting aperture, transmittance, and distribution. 

e. Integrate the electric lights with the daylighting system. Design the daylighting system to 

exceed the recommended light levels, such occupants do not notice when the electric lighting 

is off. The rows of electrical lighting fixtures (and circuits) should run parallel to the 

daylighting fenestration. Each circuit should be controlled separately so that they can be 

dimmed as necessary in response to the daylight levels that vary with distance from the 

fenestration. Use uplighting sparingly in daylit spaces with high ceilings and not at all with 

high dark ceilings. Lower LPDs are possible in daylit buildings if task lighting is provided at 

the critical work surfaces. The electric lighting design should use lower LPDs for night use 

and should account for the contrast between the dark exterior and interior environment. This 

implies that reduced lighting levels are adequate at night, compared to the day. Appropriate 

placement and use of occupant-controlled task lighting is essential for daylit buildings with 

reduced LPDs, especially for detailed task work. 

f. Commission and verify postoccupancy energy savings. Commissioning a daylighting 

system that consists of adjusting photosensors and ensuring proper sensor placement is 

required so the electric lighting system responds properly to daylight. Verifying lighting 

energy savings according to NREL’s lighting measurement procedure (Deru et al. 2005) is a 

good method to determine whether all six elements have been successfully implemented. 

Postoccupancy daylighting evaluations in all six case studies resulted in changes to the 

daylighting systems, which addressed occupant complaints and increased daylighting savings. 

5. Use economizers in conjunction with ERVs. The control and systems integration of 

economizers, natural ventilation, and ERVs needs to be very carefully designed and implemented. 

Natural ventilation or ERVs, as implemented in the buildings we studied, were not as good as the 

conventional economizer cycle they replaced. Systems with ERVs and ground-source heat 

pumps should have economizers for when conditions warrant. ERVs should be operated when 

recovered energy is a net saving to the building. In addition, ERVs should be used only when 

needed, as part of a demand-responsive ventilation system. 

6. Use evaporative cooling systems in dry climates. All cooling requirements in dry climates can 

be met with evaporative cooling systems. If more thermal and latent control is needed, 

evaporative cooling can be used as a first stage of cooling. 

7. Design natural ventilation systems with the following best practices: 

a. Design natural ventilation to rely primarily on stack effect unless wind direction and 

speeds are reliable. Even with reliable winds, passive cross-ventilation should be 

engineered for a variety of airflow pathways and wind directions. 

b. Separate natural ventilation supply and relief from the fenestration and use relief 

dampers for the passive ventilation. Use dampers with typical HVAC control actuators to 

reduce maintenance and integration issues. Motorized window actuators are prone to failure 

and are difficult to interface with the controls system. Furthermore, operable windows do 

not fully open and window screens reduce the effective opening, limiting both daylighting 

and natural ventilation. 

c. Use automatic supply and relief controls that do not rely on occupant interaction. The 

case studies showed that occupants cannot be relied on to operate the manual windows. 

120 
Failure of the occupants to control windows properly can cause the HVAC system to operate 

when the windows are open. 

d. Minimize the use of enclosed spaces. Carefully design interior partitions, floor plan, and 

fenestration to promote good circulation of naturally induced airflow. Enclosed spaces tend 

to overheat because naturally induced low-pressure ventilation has difficulties providing 

sufficient airflow. 

e. Do not use natural ventilation systems as replacements for conventional economizers. 

One way to think about passive natural ventilation is that it essentially economizes without 

fans. If it is fan assisted, it is much like running an economizer. However, from a thermal 

comfort point of view, it is limited to moderate temperatures. An economizer-based system 

can control supply air temperature through mixing to provide more uniform thermal comfort 

for a wider range of outdoor temperature and humidity, especially in buildings with high 

internal gains. 

8. Use demand-responsive controls that integrate on-site energy production, energy storage, 

and daylighting to reduce peak demand charges and increase load factors. An efficient 

building that uses very little energy could still have large demand charges. Load factors in the 

current generation of low-energy buildings are often lower than typical buildings because of a 

disproportionate ratio of average energy use to peak demand. Load factors are low primarily 

because PV systems reduce energy use, but may not reduce peak demands. Demand-responsive 

strategies that integrate on-site generation with the thermal capacity of the building require 

advanced controls that allow the temperature of the building to float based on instantaneous 

consumption and production. Using PV generation with demand-responsive controls to further 

reduce peak demands is typically the most cost-effective use of this resource. 

9. Minimize parasitic loads. A significant fraction of the equipment and plug loads in all these 

buildings is made up of idle computers. Energy savings can be realized by making the machines 

EnergyStar enabled to shut them down during periods of inactivity. Power management software 

should be installed on all computers. Flat screen monitors should also be used for added energy 

savings. Vending machines should be turned off during unoccupied hours. 

4.3 Recommended Future Research 

To help DOE reach its ZEB goal by 2025 and to continue to help owners and design teams realize highperformance buildings, we recommend the following based on our experience with the six case study 

buildings: 

1. Standardized methods should be used to collect data and determine energy savings to 

report building performance. There is currently no industry standard for reporting building 

energy savings in the built environment. A POE should include calculating site energy, source 

energy, and energy cost savings in a consistent and repeatable manner. An important part of a 

standardized energy savings calculation method is the comparison benchmark. Standard 

benchmark buildings need to be developed to identify consistent starting points and performance 

metrics. A standardized method, similar to that used for the residential program (Building 

America) should be used to create a fixed minimally code-compliant building. These methods 

will form a starting point for evaluating energy performance that does not change as energy 

efficiency standards become more stringent. 

2. For future research efforts, start with the lessons learned from the six case studies. We do 

not need to relearn these lessons. Future test buildings should use the best practices developed as 

part of these case studies as a starting point. Additional test buildings should focus on showing 

the industry that we can construct and operate commercial ZEB. Future case studies may identify 

additional best practices. 

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3. Include an element in future ZEB research efforts to focus on building owners and 

developers. Future ZEB research is needed to move the ZEB concept forward from prototype to 

market adoption. Future work should contain an element that is directed toward building owners, 

as they have had the greatest influence on producing the current generation of low-energy 

buildings. 

4. Develop integrated HVAC equipment, system, and control packages. The weak point in 

integrating the HVAC system in the case studies was the control system. Innovative systems 

often require integrated controls to realize energy savings. Controls problems were primarily 

software and algorithm related, and not hardware related. Each HVAC system is a unique 

application that requires specialized controls to integrate each component to realize energy 

savings. Detailed POEs identified EMS reprogramming needs for each case. Integrated HVAC 

packages designed for energy savings could remove the prototype application in future ZEBs. An 

integrated HVAC system design package could also reduce the need for detailed postoccupancy 

tuneups to realize savings. 

5. Develop integrated whole-building lighting system packages. The best method to ensure the 

electrical lighting system is integrated with the daylighting is to develop lighting system 

packages. These design packages would provide a system-engineered envelope and daylighting 

design, glare mitigation practices, interior designs, electrical lighting design, and the controls 

necessary to ensure the lights turn off when sufficient daylighting is available. 

6. Use simulation tools to develop technology option sets. Optimization research and tools are 

needed to developed technology option sets that provide robust support information to help 

designers make good decisions that affect energy performance. Within the goal of increasing the 

market penetration of high-performance buildings, technology option sets are envisioned as a 

means of improving the convergence of building energy analysis tools and the building design 

process. Optimization based on pre-defined options has the potential to provide more useful 

results, and at the same time, be more accessible to the building design team. Technology option 

sets should consist of specific guidance on optimal combinations of energy design measures for 

fabric and equipment and be determined by using optimization methods on building models that 

include the program and form details available during design development. 

7. Develop regional time-dependent valuations for a time-of-day source electricity conversion 

rate throughout the country. Additional work is needed to account for hourly, daily, and 

regional variations in electricity generation heat rates and supply mix. The current methods for 

calculating source energy use are too general. TDVs were developed by the California Energy 

Commission to determine the hourly value of delivered energy in California (CEC 2005). 

Regional TDVs are needed to accurately calculate source energy use and savings throughout the 

country to provide a more accurate way to credit the value of energy savings than traditional flat 

valuation methods because they account for variations in source energy conversion rates related 

to time of day, seasons, geography, and fuel type. Buildings designed under TDVs will be more 

economical for building owners because they will consume less energy during peak conditions 

and reduce the need for additional power plants and distribution systems. 

8. Investigate methods to further increase the energy efficiency of equipment and plug loads. 

Equipment and plug loads continue to grow in absolute energy use and become a greater piece of 

overall building energy use as efficient HVAC&L systems become more prevalent. A particular 

concern is parasitic loads that use energy during standby or off modes. Future work in this area is 

needed to understand the energy penalty of each plug load that includes an AC-to-DC conversion 

device and potential solutions for limiting energy use when equipment is not needed. Potential 

energy savings technologies such as switching transformers or a DC plug load electrical 

infrastructure that leverages on-site DC generation should be investigated. In addition, as these 

122 
loads were often underestimated in the design, future efforts should focus on improving the 

accuracy of the equipment and plug load assumptions made during the design phase. 

9. Evaluate the technical viability of and marketability of key successful strategies for widescale deployment. Technologies such as cooltowers, natural ventilation, and daylighting should 

be investigated further to determine optimal design solutions and appropriate applications. There 

is a need in the design community to identify better designs and controls for natural ventilation 

systems. We need to identify the research and development needs to facilitate the wide-scale 

deployment of daylighting, as the technology and controls have improved to the point were 

almost 80% of the commercial floor area could be daylit. As more buildings produce energy 

from PV, the grid implications of large amounts of exported PV energy must be investigated. 

This evaluation would also include developing better methods to evaluate each technology and 

the energy savings potential. 

10. Study the degradation of long-term, whole-building energy performance. Determine how 

low-energy buildings perform over time and investigate how continuous commissioning and 

operator education can ensure continued energy savings over the long-term operation of a lowenergy building. Identify technologies that suffer from performance degradation over time. For 

example, this could include understanding the long-term capacity of the ground wells in the 

ground-source heat pump systems. This study should also include maintainability of the systems. 

11. Study the ability of PV systems to minimize demand with controls. Further analysis of the 

potential value added by demand-responsive controls integrated with on-site generation is needed 

to understand the best use of excess PV energy production. This work could include developing 

peak demand-shifting algorithms for minimizing electrical cost to fully benefit from energy 

efficiency devices (especially daylighting) and on-site generation capacity. These algorithms 

could be researched with the planned releases of EnergyPlus that will have the capability to 

model demand-responsive controls integrated with thermal storage, daylighting, and PV systems 

on a 15-minute time step. As more buildings have PV production, it is also necessary to 

understand the grid implications of large amounts of exported PV energy. 

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Energy Performance of the Zion National Park Visitor Center. www.nrel.gov/docs/fy05osti/34607.pdf. 

Technical Report NREL/TP-550-34607. Golden, CO: National Renewable Energy Laboratory, 156 pp. 

Torcellini, P.; Pless, S. (2004). Trombe Walls in Low-Energy Buildings: Practical Experiences; 

Preprint. www.nrel.gov/docs/fy04osti/36277.pdf. Conference Report NREL/CP-550-36277. Presented 

at the World Renewable Energy Congress VIII, Denver, CO, August 29–September 3, 2004. Golden, 

CO: National Renewable Energy Laboratory, 8 pp. 

Torcellini, P.; Pless, S.; Griffith, B.; Judkoff, R. (2005b). Evaluation of the Energy Performance and 

Design Process of the Thermal Test Facility at the National Renewable Energy Laboratory. 

www.nrel.gov/docs/fy05osti/34832.pdf. Technical Report NREL/TP-550-34832. Golden, CO: National 

Renewable Energy Laboratory, 144 pp. 

Torcellini, P.; Long, N.; Judkoff, R. (2003). Consumptive Water Use for U.S. Power Production. 

www.nrel.gov/docs/fy04osti/35190.pdf. Conference Report NREL/CP-550-35190. Presented at the 

ASHRAE Winter Meeting, Anaheim, California, January 24−28, 2004, Paper #4677. Golden, CO: 

National Renewable Energy Laboratory, 11 pp. 

USGBC. (2004). Leadership in Energy & Environmental Design Version 1.0. United States Green 

Building Council. www.usgbc.org. Last accessed September 2005. 

126 
APPENDIX A: ZERO-ENERGY BUILDINGS: DEFINITIONS AND ANALYSIS 

In concept, a net zero energy building (ZEB) is a building with greatly reduced energy needs through 

efficiency gains such that the balance of the energy needs can be supplied by renewable technologies. 

Despite our use of the phrase “zero energy,” we lack a common definition—or a common understanding 

—of what it means. In this appendix, we use our sample of current generation low-energy buildings to 

explore the concept of zero energy—what it means, why a clear and measurable definition is needed, and 

how we have progressed toward the ZEB goal. 

Using ZEB design goals takes us out of designing low-energy buildings with some percent energy savings 

goal into the realm of a sustainable energy endpoint. The goals that are set and how those goals are 

defined are critical to the design process. The definition of the goal will influence designers who strive to 

meet it (Deru and Torcellini 2004). Because design goals are so important to achieving high-performance 

buildings, the way a ZEB goal is defined is crucial to understanding the combination of applicable 

efficiency measures and renewable energy supply options. 

A.1 Boundary Definitions and Energy Flows 

At the heart of the ZEB concept is the idea that buildings can meet all their energy requirements from 

low-cost, locally available, nonpolluting renewable sources. At the strictest level, a ZEB generates 

enough energy annually to equal or exceed its annual energy use. The following concepts and 

assumptions have been established to help guide definitions for ZEBs: 

A.1.1 Grid connection is allowed and necessary for energy balances 

A ZEB typically uses traditional energy sources such as the electric and natural gas utilities when on-site 

generation does not meet the loads. When the on-site generation is greater than the building’s loads, 

excess electricity is exported to the utility grid. By using the grid to account for the energy balance, 

excess production can offset later energy use. Achieving a ZEB without the grid would be very difficult, 

as the current generation of storage technologies is limited. Despite the electric energy independence of 

off-grid buildings, they usually rely on outside energy sources such as propane (and other fuels) for 

cooking, space heating, water heating, and backup generators. Off-grid buildings cannot feed their excess 

energy production back onto the grid to offset other energy uses. As a result, the energy production from 

renewable resources must be oversized. In many cases (especially during the summer), excess generated 

energy cannot be used. 

We assume that excess on-site generation can always be sent to the grid. However, in high market 

penetration scenarios, the grid may not always need the excess energy. In this scenario, on-site energy 

storage would become necessary. 

A.1.2 Prioritize supply-side technologies to those available on site and within the footprint 

Various supply-side renewable energy technologies are available for ZEBs. Typical examples of 

technologies available today include PV, solar hot water, wind, hydroelectric, and biofuels. All these 

renewable sources are favorable over conventional energy sources such as coal and natural gas; however, 

we have developed a ranking of renewable energy sources in the ZEB context. Table A-1 shows this 

ranking in order of preferred application. The principles we have applied to develop this ranking are 

based on technologies that: 

• Minimize overall environmental impact by encouraging energy-efficient building designs and 

reducing transportation and conversion losses. 

• Will be available over the lifetime of the building. 

• Are widely available and have high replication potential for future ZEBs. 

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Table A-1 ZEB Renewable Energy Supply Option Hierarchy 

Option 

Number ZEB Supply-Side Options Examples 

0 Reduce site energy use through lowenergy building technologies 

Daylighting, high-efficiency HVAC equipment, 

natural ventilation, evaporative cooling, etc. 

On-Site Options 

1 Use renewable energy sources available 

within the building’s footprint 

PV, solar hot water, and wind located on the 

building. 

2 Use renewable energy sources available 

at the site 

PV, solar hot water, low-impact hydro, and wind 

located on-site, but not on the building. 

Off-Site Options 

3 Use renewable energy sources available 

off site to generate energy on-site 

Biomass, wood pellets, ethanol, or biodiesel that 

can be imported from off site, or waste streams 

from on-site processes that can be used on-site 

to generate electricity and heat. 

4 Purchase off-site renewable energy 

sources 

Utility-based wind, PV, emissions credits, or 

other “green” purchasing options. Hydroelectric 

is sometimes considered. 

This hierarchy is weighted toward renewable technologies that are available within the building footprint 

and at the site. Rooftop PV and solar water heating are the most applicable supply-side technologies for 

widespread application of ZEBs. Other supply-side technologies such as parking lot-based wind or PV 

systems may be available for limited applications. It can be argued that renewable energy resources from 

outside the boundary of the building site could also be used to achieve a ZEB. While this approach may 

achieve a building with net zero energy consumption, it is not the same as one that generates the energy 

on site and should be classified as such. We will use the term “off-site ZEB” for those buildings that use 

renewable energy from sources outside the boundaries of the building site. 

A good ZEB definition should first encourage energy efficiency, and then use renewable energy sources 

available on site. A building that buys all of its energy from a wind farm or other central location has 

little incentive to reduce building loads, which is why we refer to this as an off-site ZEB. Efficiency 

measures or energy conversion devices such as daylighting or combined heat and power devices cannot 

be considered on-site production in the ZEB context. Fuel cells and microturbines do not generate 

energy; rather they typically transform purchased fossil fuels into heat and electricity. Passive solar 

heating and daylighting are demand-side technologies and are considered efficiency measures. Energy 

efficiency is usually available for the life of the building; however, efficiency measures must have good 

persistence and should be “checked” to make sure they continue to save energy. It is almost always easier 

to save energy than to produce energy. 

Determining a project’s boundary, which can be substantially larger than the building footprint, is an 

important part of defining on-site generation sources. The question arises as to whether this larger area 

should be considered for on-site renewable energy production. Typically, the only area available for onsite energy production that a building has guaranteed as “its own” over its lifetime is within its footprint. 

To ensure this area is available for on-site production, many states, counties, and cities have existing solar 

access ordinances, which declares that the right to use the natural resource of solar energy is a property 

right. For example, the city of Boulder, CO has a solar access ordinance, which guarantees access to 

sunlight for homeowners and renters in the city. This ordinance protects solar access of existing buildings 

by limiting the amount of shadow new development may cast on neighboring buildings, maintaining the 

potential for using renewable energy systems in buildings (City of Boulder 2006). Using a neighboring 

128 
field to generate electricity is not as favorable as a roof-mounted PV system; the area outside the 

building’s footprint cannot be guaranteed to provide long-term generation because of the possibility of 

future development. 

Wind resources for ZEBs are limited because of structural, noise, and wind pattern considerations, and are 

not typically installed on buildings. Some parking lots or adjacent areas may be used to produce energy 

from wind, but this resource is site specific and not widely available. Similar to PV generation in an 

adjacent parking lot, the wind resource is not necessarily guaranteed because it could be superseded by 

future development. 

Renewable sources imported to the site, such as wood pellets, ethanol, or biodiesel can be valuable, but 

do not count as on-site renewable sources. Biofuels such as waste vegetable oil from waste streams and 

methane from human and animal wastes can also be valuable energy sources, but these materials are 

typically imported for the on-site processes. 

The final option for supply-side renewable energy sources includes purchasing “green credits” or 

renewable sources such as wind power or utility PV systems that are available to the electrical grid. 

These central resources require infrastructure to move the energy to the building and are not always 

available. Buildings employing resources 3 and 4 in Table A-1 to achieve zero energy are considered offsite ZEBs. For example, a building can achieve an off-site ZEB for all of these definitions by purchasing 

wind energy. Although becoming an off-site ZEB can have little to do with design and a lot to do with 

the different sources of purchased off-site renewable energy, an off-site ZEB is still in line with the 

general concept of a ZEB. 

A.2 Definitions 

A zero energy building can be defined in several ways, depending on the boundary and the metric. 

Different definitions may be appropriate depending on the project goals and the values of the design team 

and building owner. For example, building owners typically care about energy costs. Organizations, 

such as DOE, are concerned with national energy numbers, and are typically interested in primary or 

source energy. A building designer may be interested in site energy use for energy code requirements. 

Finally, those who are concerned about pollution from power plants and the burning of fossil fuels may 

be interested in reducing emissions. Four commonly used definitions are: net zero site energy, net zero 

source energy, net zero energy costs, and net zero energy emissions. 

• Net Zero Site Energy: A site ZEB produces at least as much energy as it uses in a year, when 

accounted for at the site. 

• Net Zero Source Energy: A source ZEB produces at least as much energy as it uses in a year, 

when accounted for at the source. Source energy refers to the primary energy used to generate 

and deliver the energy to the site. To calculate a building’s total source energy, imported and 

exported energy is multiplied by the appropriate site-to-source conversion multipliers. 

• Net Zero Energy Costs: In a cost ZEB, the amount of money the utility pays the building owner 

for the energy the building exports to the grid is at least equal to the amount the owner pays the 

utility for the energy services and energy used over the year. 

• Net Zero Energy Emissions: A net-zero emissions building produces at least as much 

emissions-free renewable energy as it uses from emissions-producing energy sources. 

Each definition uses the grid for net use accounting and has different applicable renewable energy 

sources. Note that the definitions do apply for grid independent structures. For all definitions, supplyside option 2 can be used if this resource will be available for the life of the building. Off-site ZEBs can 

be achieved by purchasing renewable energy from off-site sources, or in the case of an off-site zero 

emissions building, purchasing emissions credits. In support of DOE’s ZEB research needs, the 

following definition discussion refer to ZEBs that use supply side options available on site. For ZEBs 

129 
that have a portion of the renewable generation supplied by off-site sources, these buildings are referred 

to as “off-site ZEBs.” 

A.3 Low- and Zero-Energy Buildings: Examples 

To study the impacts of these ZEB definitions, we examined the six low-energy commercial buildings 

with respect to these definitions. These buildings were further investigated to determine additional PV 

system requirements to meet the ZEB goals. PV system array area and system capacity requirements for 

meeting site and source ZEB goals were determined in each case (see Table A-2). Annual electricity and 

natural gas site-to-source conversion multipliers (3.2 for electricity and 1.07 for natural gas) were applied 

to each building to determine source energy use (EIA 2005). For the all-electric buildings (Oberlin, Zion, 

and Cambria), the site ZEB and source ZEB are the same. CBF used a minimal amount of propane, and 

the TTF and BigHorn used natural gas for heating and water heating. Zion, TTF, and BigHorn are singlestory buildings; Oberlin, Cambria, and CBF are two stories. We used the PV system simulation tool 

PVSyst v3.3 (Mermoud 1996) to calculate the expected annual performance of the PV system. Singlecrystalline PV modules were modeled with 0.0° tilt, as we assumed the PV system would be mounted on 

a flat roof of each building. These modules provide the best available output per unit area of 

commercially available PV modules. 

Table A-2 ZEB Example Summary 

A.4 How Definition Determines Design 

Depending on the ZEB definition, the results can vary substantially. Each definition has advantages and 

disadvantages, which are discussed below. 

A.4.1 Net zero site energy building 

A site ZEB produces as much energy as it uses, when accounted for at the site. Generation examples 

include roof-mounted PV or solar hot water collectors (Table A-1, Option 1). Other site-specific on-site 

generation options such as small-scale wind power, parking lot-mounted PV systems, and low-impact 

Building and PV 

System (DC 

Rating Size) 

Site Energy 

Use (w/o 

PV) 

(MWh/yr) 

Source 

Energy Use 

(w/o PV) 

(MWh/yr) 

Actual Roof 

Area 

(footprint) 

(ft2) (m2)

Flat Roof Area (ft2) 

(m2) Needed for 

Source ZEB and 

Site ZEB with PV 

PV System DC Size 

Needed for Source 

ZEB and Site ZEB 

Oberlin-60 kW 118.8 380.2 8,500 

(790) 10,800 (1,003) 120 kW 

Zion-7.2 kW 91.6 293.1 11,726 

(1,089) 6,100 (567) 73 kW 

Cambria-17.2 kW 372.1 1,190.7 17,250 

(1,603) 37,210 (3,457) 415 kW 

CBF-4.2 kW 365.2 1,142.0 15,500 

(1,440)

25,316 (2,352) 

Source ZEB 

25,640 (2,382) 

Site ZEB 

282 kW Source ZEB 

286 kW Site ZEB 

TTF-No PV 83.5 192.5 10,000 

(929)

4,010 (373) 

Source ZEB 

5,550 (516) 

Site ZEB 

45 kW Source ZEB 

62 kW Site ZEB 

BigHorn-8.9 kW 490.4 901.0 38,923 

(3,616)

18,449 (1,714)

Source ZEB 

31,742 (2,949) 

Site ZEB 

206 KW Source ZEB 

354 kW Site ZEB 

130 
hydro (Table A-1, Option 2), may be available. As discussed earlier, it is preferable to have the on-site 

generation be within the building footprint. 

A limitation of a site ZEB definition is that the values of various fuels at the source are not considered. 

For example, one energy unit of electricity used at the site is equivalent to one energy unit of natural gas 

at the site, but electricity is more than 3 times as valuable at the source. For all-electric buildings, a site 

ZEB is equivalent to a source ZEB. For buildings with significant gas use, a site ZEB will need to 

generate much more on-site electricity than a source ZEB. As an example, the TTF would require a 62-

kWDC PV system to be a site ZEB, but only a 45-kWDC PV system for a source ZEB (Table A-2); this 

is because gas heating is a major end use. The net site definition encourages aggressive energy efficiency 

designs because on-site generated electricity has to off set gas use on a 1 to 1 basis. 

A site ZEB can be easily verified through on-site measurements, whereas source energy or emissions 

ZEBs cannot be measured directly because site-to-source factors need to be determined. An easily 

measurable definition is important to accurately determine the progress toward meeting a ZEB goal. 

A site ZEB has the fewest external fluctuations that influence the ZEB goal, and therefore provides the 

most repeatable and consistent definition. This is not the case for the cost ZEB definition because 

fluctuations in energy costs and rate structures over the life of a building affect the success in reaching net 

zero energy costs. For example, at BigHorn, natural gas prices varied 40% during the three-year 

monitoring period and electricity prices varied widely, mainly because of a partial shift from coal to 

natural gas for utility electricity production. Similarly, source energy conversion rates may change over 

the life of a building, depending on the type of power plant or power source mix the utility uses to provide 

electricity. However, for all the ZEB definitions, the impact of energy performance can affect the success 

in meeting a ZEB goal. 

A building could be a site ZEB but not realize comparable energy cost savings. If peak demands and 

utility bills are not managed, the energy costs may or may not be similarly reduced. This was the case at 

Oberlin, which realized a 79% energy saving, but did not reduce peak demand charges. Uncontrolled 

demand charges resulted in a disproportionate energy cost saving of only 35%. 

An additional design implication of a site ZEB is that this definition favors electric equipment that is 

more efficient at the site than its gas counterpart. For example, in a net site ZEB, electric heat pumps 

would be favored over natural gas furnaces for heating because they have a coefficient of performance 

from 2 to 4, while natural gas furnaces are about 90% efficient. This was the case at Oberlin, which had a 

net site ZEB goal that influenced the design decision for an all-electric ground source heat pump system. 

A.4.2 Net zero source energy building 

A source ZEB produces as much energy as it uses as measured at the source. To calculate a building’s 

total source energy, both imported and exported energy are multiplied by the appropriate site-to-source 

energy factors. To make this calculation, power generation and transmission factors are needed. Source 

Energy and Emission Factors for Energy Use in Buildings (Deru and Torcellini 2006) used a life cycle 

assessment approach and determined national electricity and natural gas site-to-source energy factors of 

3.37 and 1.12. Site gas energy use will have to be offset with on-site electricity generation on a 3.37 to 1 

ratio (one unit of exported electricity for 3.37 units of site gas use) for a source ZEB. This definition 

could encourage the use of gas in as many end uses as possible (boilers, domestic hot water, dryers, 

desiccant dehumidifiers) to take advantage of this fuel switching and source accounting to reach this ZEB 

goal. For example, the higher the percent of total energy used at a site that is gas, the smaller the PV 

system required to be a source ZEB. At BigHorn, for a source ZEB, 18,500 ft2

 of PV are required; 

however, 31,750 ft2

 of PV are required for a site ZEB (Table A-2). In this case, almost twice the amount 

of PV is needed for a source ZEB over a site ZEB. 

This definition also depends on the method used to calculate site-to-source electricity energy factors. 

National averages do not account for regional electricity generation differences. For example, in the 

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Northwest, where hydropower is used to generate significant electricity, the site-to-source multiplier is 

lower than the national number. In addition, national site-to-source energy factors do not account for 

hourly variations in the heat rate of power plants or how utilities dispatch generation facilities for peak 

loading. Electricity use at night could have fewer source impacts than electricity used during the peak 

utility time of day. Further work is needed to determine how utilities dispatch various forms of 

generation and the corresponding daily variations of heat rates and source rates. Regional time-dependent 

valuations (TDVs) for determining time-of-use source energy is one method used to account for 

variations in how and when energy is used. TDVs have been developed by the California Energy 

Commission to determine the hourly value of delivered energy for 16 zones in California (CEC 2005). 

Similar national TDVs would be valuable to accurately calculate source energy use to determine a 

building’s success in reaching a source ZEB goal. A first step in understanding regional site-to-source 

multiplier differences is available (Deru and Torcellini 2006), with multipliers provided for the three 

primary grid interconnects and for each state. 

There may be issues with the source ZEB definition when electricity is generated on site with gas from 

fossil fuels. The ZEB definitions state that the building must use renewable energy sources to achieve the 

ZEB goal; therefore, electricity generated on site from fossil fuels cannot be exported and count toward a 

ZEB goal. However, this is unlikely, because buildings are unlikely to need more heat than electricity 

and the inefficiencies of on-site electricity generation and exportation make this economically very 

unattractive. The best cost or energy pathways will determine the optimal combination of energy 

efficiency, on-site cogeneration, and on-site renewable energy generation. 

The issue of unmanaged energy costs in a site ZEB is similar for a source ZEB. A building could be a 

source ZEB and not realize comparable energy cost savings. If peak demands and utility bills are not 

managed, the energy costs may or may not be similarly reduced. 

A.4.3 Net zero energy cost building 

A cost ZEB receives as much financial credit for exported energy as it is charged on the utility bills. The 

credit received for exported electricity (often referred to net energy generation) will have to offset energy, 

distribution, peak demand, taxes, and metering charges for electricity and gas use. A cost ZEB provides a 

relatively even comparison of fuel types used at the site as well as a surrogate for infrastructure. 

Therefore, the energy availability specific to the site and the competing fuel costs would determine the 

optimal solutions. However, as utility rates can vary widely, a building with consistent energy 

performance could meet the cost ZEB goal one year and not the next. 

In wide-scale implementation scenarios, this definition may be ineffective because utility rates will 

change dramatically. As energy-efficient building technologies and renewable energy installations 

increase, the effects of large numbers of energy-efficient buildings must be considered in a given utility’s 

service area. In addition to purchasing fuel to generate electricity, electric utilities must provide 

dependable service, maintain capacity to meet potential loads, meet obligations for maintaining and 

expanding infrastructure, and provide profitability for shareholders. The fixed costs associated with these 

activities result in rate structures that provide only limited incentive for consumers to create cost ZEBs. 

Trends in other utility sectors, such as water districts, indicate that as buildings become more efficient, 

and consequently have lower consumptive charges, the costs associated with infrastructure are increased. 

If significant numbers of buildings achieved a zero energy cost, then financial resources would not be 

available to maintain the infrastructure, and the utility companies would have to raise the fixed and 

demand charges. 

For commercial buildings, a cost ZEB is typically the hardest to reach, and is very dependent on how a 

utility credits net electricity generation and the utility rate structure the building uses. One way to reach 

this goal in a small commercial building might be to use a utility rate that minimizes demand charges. 

For example, at Zion, for a site and source ZEB at current performance levels (about 65% energy savings 

without PV), a 73-kW PV system is needed. To be a cost ZEB, with the utility providing credit for net 

132 
electricity generation at avoided generation costs, a 100-kW PV system would be needed. A cost ZEB 

may be technically possible in this case, but the following characteristics would all be required to achieve 

this ZEB definition: 

• High energy savings (Zion’s measured energy savings approach 65%). 

• Aggressive demand management to allow PV to help offset demand. Without demandresponsive controls, PV systems cannot be relied upon to reduce peak demand charges. 

Additionally, the low peak demands enable the building to qualify for the small commercial rate 

structure. 

• A favorable utility rate structure weighted toward energy use, not peak demand charges. 

Standard commercial rate structures often result in electricity charges that are typically split 

between peak demand and energy charges. The small building commercial rate structure for 

Zion, which has comparatively low peak demand rates and higher consumption rates, would not 

apply if the building used more than 35 kW for any 15-minute period over any time of the year. 

This small commercial rate includes a low demand charge of $6.30/kW for all usage that exceeds 

15 kW, and an energy charge of $0.08/kWh for the first 1500 kWh and $0.045/kWh for all 

additional kilowatt-hours. A time-of-use rate would also be advantageous for a cost ZEB. 

• A net-metering agreement that credits excess electricity generation at avoided generation costs 

($0.027/kWh in this case), without capacity eligibility limits to PV system sizes. Avoided 

generation costs refer to how the utility credits the customer for net generation and is based on the 

costs associated with the utility not having to generate this energy. A far more favorable netmetering agreement would credit the net generation at the full retail rate. This is considered 

“true” net metering, and would be the favored net metering arrangement in a cost ZEB. The netmetering agreement also must allow the excess generation credit to be used for offsetting energyrelated and nonenergy charges, such as monthly meter charges, demand charges, and taxes. 

In the Zion net cost ZEB example, a PV system 30% larger than a site or source ZEB PV system would 

be required to reach the net cost ZEB goal. For utility rates that do not allow the net generation credit to 

be applied to nonenergy charges, a net cost ZEB would not be possible, irrespective of the size of the PV 

system, the energy or demand savings, or how the rates weight energy and nonenergy charges. 

If demand charges account for a significant portion of the utility bills, a net cost ZEB becomes difficult. 

For example, Oberlin’s rate structure is not weighted toward energy rates combined with minimal demand 

savings. A 430-kW PV system would be required for a cost ZEB at Oberlin at current levels of 

performance. This is 3.6 times the size of the PV system Oberlin would need to be a site or source ZEB. 

For this 13,600 ft2

 building to be a net cost ZEB, a PV system approaching 40,000 ft2 would be required—

much larger than the building footprint. 

If two-way or net metering is not available, on-site energy storage and advanced demand-responsive 

controls to manage peak demand charges should be included in the design and operation of cost ZEBs. It 

may be more effective to store excess PV energy and use it at a later time to reduce demand charges 

rather than export the energy to the grid. 

A.4.4 Net zero energy emissions building 

An emissions-based ZEB produces at least as much emissions-free renewable energy as it uses from 

emissions-producing energy sources. An on-site emission ZEB offsets its emissions using supply-side 

options 1 and 2 in Table A-1. If an all electric building gets all of its electricity from an off-site zero 

emissions source (such as hydro, nuclear, or large scale wind farms), then it is already zero emissions and 

does not have to generate any on-site renewable energy to offset emissions. However, if the same 

building uses natural gas for heating, then it will need to generate and export enough emissions-free 

133 
renewable energy to offset the emissions from the natural gas use. Purchasing emissions offsets from 

other sources would be considered an off-site zero emissions building. 

Success achieving an emissions ZEB depends on the generation source of the electricity used. Emissions 

are highly variable depending on the source of electricity, ranging from nuclear, coal, hydro, and other 

utility generation sources. One could argue that any building built in an area that has a large hydro or 

nuclear contribution to the regional electricity generation mix would have fewer emissions than a similar 

building in a region with a predominantly coal-fired generation mix. Therefore, an emissions ZEB would 

need a smaller PV system in areas with a large hydro or nuclear contribution as compared to a similar 

building supplied by a utility with a large coal-fired generation contribution. 

 The net zero emissions ZEB definition has similar calculation difficulties previously discussed with the 

source ZEB definition. Many of these difficulties are related to the uncertainty in determining the 

generation source of electricity. Like the source definition, one would need to understand the utility 

dispatch strategy and generation source ratio to determine emissions from each of these sources. 

A.4.5 ZEB definitions applied to a sample of current generation low-energy buildings 

Each of these leading-edge case study buildings demonstrates the progress toward achieving ZEB goals in 

real-world examples. The one-story buildings—Zion, BigHorn, and TTF—could achieve ZEB within 

their roof areas for all the definitions except cost ZEB. ZEB is not feasible for the two-story buildings 

unless their loads are further reduced. For Oberlin (the building currently closest to meeting a ZEB goal), 

the annual PV production is still less than the best-case energy consumption scenario. Oberlin is 

currently installing another 100-kW PV system in the parking lot (total installed DC capacity will be 160 

kW), which will be tied into the building’s electrical system. We expect that the building will achieve a 

site, source, and emissions ZEB, but that a cost ZEB will be difficult to reach without further demand 

management controls. To accomplish a ZEB, the PV system has been extended past the building 

footprint. 

None of the buildings could clearly be cost ZEBs with the current rate structures. Zion could be the 

closest because of its aggressive demand management, favorable utility rate structure, and efficient use of 

energy. A cost ZEB is the most difficult ZEB goal to reach because typical commercial rate structures do 

not allow for net metering such that exported electricity can offset all other utility charges. To reach a 

cost ZEB goal, the credit received for exported electricity would have to offset energy, distribution, peak 

demand, taxes, and metering charges for both electricity and gas use. 

A.4.6 The ZEB definition selected can have an impact on future ZEB designs 

The zero energy definition affects how buildings are designed to achieve the goal. It can emphasize 

energy efficiency, supply-side strategies, purchased energy sources, utility rate structures, or whether 

fuel-switching and conversion accounting can help meet the goal. Table A-3 highlights key 

characteristics of each definition. 

A source ZEB definition can emphasize gas end uses over the electric counterparts to take advantage of 

fuel switching and source accounting to reach a source ZEB goal. Conversely, a site ZEB can emphasize 

electric heat pumps for heating end uses over the gas counterpart. For a cost ZEB, demand management 

and on-site energy storage are important design considerations, combined with selecting a favorable 

utility rate structure with net metering. An emissions ZEB is highly dependent on the utility electric 

generation source. Off-site ZEBs can be reached just by purchasing off-site renewable energy, no 

demand or energy savings needed. Consistent ZEB definitions are needed for those who research, fund, 

design, and evaluate ZEBs. 

134 
Table A-3 ZEB Definitions Summary 

Definition Pluses Minuses Other Issues 

Site ZEB 

• Easy to implement. 

• Verifiable through on-site 

measurement. 

• Conservative approach to 

achieving ZEB. 

• No externalities affect 

performance, can track 

success over time. 

• Easy for the building 

community to understand 

and communicate. 

• Encourages energyefficient building designs. 

• Requires more PV export to offset 

natural gas. 

• Does not consider all utility costs 

(can have a low load factor). 

• Not able to equate fuel types. 

• Does not account for nonenergy 

differences between fuel types 

(supply availability, pollution). 

Source 

ZEB 

• Able to equate actual 

value of fuel types used 

at the site. 

• Better model for impact 

on national energy 

system. 

• Easier ZEB to reach. 

• Does not account for nonenergy 

differences between fuel types 

(supply availability, pollution). 

• Source calculations too broad (do 

not account for regional or daily 

variations in electricity generation 

heat rates). 

• Source energy use accounting 

“gaming” can have a larger impact 

than efficiency technologies. 

• Does not consider all energy costs 

(can have a low load factor). 

• Need to develop 

site-to-source 

conversion 

factors that 

require significant 

amounts of 

information to 

define. 

Cost ZEB 

• Easy to implement and 

measure. 

• Market forces result in a 

good balance between 

fuel types. 

• Encourages demandresponsive control. 

• Verifiable from utility bills. 

• May not reflect impact to national 

grid for demand, as extra PV 

generation can be more valuable 

for reducing demand with on-site 

storage than exporting to the grid. 

• Requires net-metering agreements 

such that exported electricity can 

offset energy and nonenergy 

charges. 

• Highly volatile energy rates make 

for difficult tracking over time. 

• Offsetting 

monthly service 

and infrastructure 

charges require 

going beyond 

ZEB. 

• Net metering is 

not well 

established, often 

with capacity 

limits and at 

buyback rates 

lower than retail 

rates. 

Emissions 

ZEB 

• Better model for green 

power. 

• Accounts for nonenergy 

differences between fuel 

types (greenhouse gases, 

pollution). 

• Easier ZEB to reach. 

• Need appropriate 

fuel factors for 

emissions. 

135 
APPENDIX B: NATIONAL DAYLIGHTING POTENTIAL 

We used currently available CBECS buildings and national weighting factors (EIA 2005a) to determine 

total square footage in all commercial buildings in the United States that have daylighting potential. Data 

used in CBECS include total floor area, number of floors, and the building-weighting factor. We used an 

internally developed CBECS analysis tool “NREL CBECS Data Analyzer, Version 1.0.0.12” with 

CBECS Public Use Data for 1999. The CBECS Data Analyzer program enabled us to extract and analyze 

data on 5,430 commercial buildings and apply results with weighting factors to the full commercial 

building sector. 

The assumptions required to use the CBECS data set included: 

• Aspect ratio of 1, 3, or a random aspect ratio of 1 to 3 applied to each of the CBECS buildings, 

evenly distributed (average of 2) 

• Daylighting potential in all 15-ft (4.6-m) perimeter zones (shown in Figure B-1) 

• 100% daylighting potential on top floor, through toplit and sidelit daylighting 

• No daylighting potential in first- and second-floor core zones 

• For CBECS buildings classified as 15−25 floors, random number of floors between 15 and 25 

• For CBECS buildings with more than 25 floors, a 50-floor building is assumed 

• Identical floor area for each floor 

• 1999 CBECS square footage and number of floors data combined with weighting factors to 

determine national commercial building stock daylighting potential 

From the CBECS dataset and associated weighting factors, the following daylighting potential results are 

provided: 

• Seventy-seven to eighty-two percent of all square footage in the commercial building stock has 

the potential to be daylit through a combination of toplit and sidelit systems, in zones within 15 

feet (4.6 m) of an exterior wall or the top floor with a roof. 

• The range of potentially daylit square footage depends on assumed aspect ratio. If all buildings in 

CBECS were square (aspect ratio of 1), 77% of all square footage could be daylit. If all buildings 

had an aspect ratio of 3, the percentage increases to 82%. Using a random distribution of aspect 

ratio (with an average of 2), 80% of all square footage has daylighting potential. 

• Sixty percent of all square footage in commercial buildings could be toplit through an exterior 

ceiling, as shown in Figure B-2. 

136 
Figure B-1 Perimeter and core zones daylighting potential 

0%

5%

10%

15%

20%

25%

30%

35%

40%

45%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15-

25

>25

Number of Floors

Percent of Total Commercial Building 

Floor Area

No Daylighting Potiential

Sidelit Daylighitng Potiential

Toplit and/or Sidelit Daylighting Potential

Figure B-2 Toplit and sidelit daylighting potential by number of floors 

Top Floor with Toplit 

or Sidelit Daylighting 

Potential 

Core Zones 

without Daylighting 

Potential 

Perimeter Zones with 

Sidelit Daylighting 

Potential Core Zones

without Daylighting 

Potential 

137 
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4. TITLE AND SUBTITLE

Lessons Learned from Case Studies of Six High-Performance 

Buildings 

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NREL/TP-550-37542 

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BEC61012 

6. AUTHOR(S)

P. Torcellini, S. Pless, M. Deru, B. Griffith, N. Long, and R. Judkoff 

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13. SUPPLEMENTARY NOTES

14. ABSTRACT (Maximum 200 Words)

Commercial buildings have a significant impact on energy use and the environment. They account for approximately 

18% (17.9 quads) of the total primary energy consumption in the United States (DOE 2005). The energy used by the 

building sector continues to increase, primarily because new buildings are added to the national building stock faster 

than old buildings are retired. Energy consumption by commercial buildings will continue to increase until buildings 

can be designed to produce more energy than they consume. As a result, the U.S. Department of Energy’s (DOE) 

Building Technologies Program has established a goal to create the technology and knowledgebase for marketable 

zero-energy commercial buildings (ZEBs) by 2025. 

15. SUBJECT TERMS

zero energy building; ZEB; energy consumption; commercial building; zion national park visitor center; oberlin; 

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