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

Office of Energy Efficiency & Renewable Energy

National Renewable Energy Laboratory 

Innovation for Our Energy Future 

Assessment of the Technical 

Potential for Micro-Cogeneration 

in Small Commercial Buildings 

across the United States 

Preprint 

B. Griffith 

National Renewable Energy Laboratory 

To be presented at the First International Conference and 

Workshop on Micro-Cogeneration Technologies and Applications 

(Micro-Cogen 2008) 

Ottawa, Canada 

April 29–May 1, 2008 

Conference Paper 

NREL/CP-550-43164 

May 2008 

NREL is operated by Midwest Research Institute ● Battelle Contract No. DE-AC36-99-GO10337 
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ASSESSMENT OF THE TECHNICAL POTENTIAL FOR MICRO-COGENERATION 

IN SMALL COMMERCIAL BUILDINGS ACROSS THE UNITED STATES 

Brent Griffith 

National Renewable Energy Laboratory 

Golden, CO 

United States 

ABSTRACT

This paper presents an assessment of the technical 

potential for micro-cogeneration in small commercial 

buildings throughout the United States. The 

cogeneration devices are simulated with the computer 

program EnergyPlus using models developed by Annex 

42, a working group of the International Energy 

Agency’s Energy Efficiency in Buildings and 

Community Systems (IEA/ECBCS). Although the 

Annex 42 models were developed for residential 

applications, this study applies them to small 

commercial buildings, assumed to have a total floor 

area of 500 m2

 or less. The potential for microcogeneration is examined for the entire existing stock 

of small U.S. commercial buildings using a bottom-up 

method based on 1,236 EnergyPlus models. The set of 

buildings used in the study represents all types of 

buildings in a range of U.S. weather locations and are 

based on samples used for the 2003 Commercial 

Building Energy Consumption Survey (CBECS) 

conducted by the U.S. Energy Information Agency 

(EIA). The cogeneration device is modeled in the 

context of whole-building, integrated systems, dynamic 

building simulation using annual runs with 15-minute 

time steps. Technical potential for micro-cogeneration 

is characterized by adding micro-cogeneration to 

individual baseline building models and quantifying the 

changes in a variety of whole-building, areanormalized, performance metrics including site energy, 

source energy, and energy cost. National implications 

for the United States are assessed by aggregating 

results using weighting factors from 2003 CBECS. 

INTRODUCTION

Micro-cogeneration devices, developed primarily for 

residential applications, may find applications in small 

commercial buildings. Assuming “small” to be those 

buildings smaller than 500 m2

 total floor area, U.S. 

survey data [EIA 2006] indicate that the existing stock 

of small commercial buildings constitutes a significant 

market segment with some 2.6 million buildings (56% 

of U.S. commercial buildings) or 656 million m2

 of 

floor area (11% of U.S. commercial floor area). This 

segment of the commercial sector includes a wide 

variety of businesses such restaurants, service, office, 

education, and retail stores. Each year, this subsector 

consumes 4.2×1017 J of electricity and 2.7×1017 J of 

natural gas. 

One of the challenges in modeling cogeneration 

devices is accounting for how thermal energy is used at 

the building site. Because thermal energy from 

cogeneration is intended for space heat and service 

water heating, it is important to include these systems 

in an integrated analysis when assessing cogeneration. 

IEA/ECBCS Annex 42 has developed new models for 

small, residential cogeneration that allow assessing 

micro-cogeneration devices within the context of 

detailed, annual building energy simulation programs 

[Kelly et al. 2008, Beausoleil-Morrison and Kelly 

2007]. The author implemented the Annex 42 models 

in EnergyPlus, a whole-building modeling program 

developed and maintained by the U.S. Department of 

Energy [DOE 2007]. This paper provides a summary 

of a study that used EnergyPlus with the Annex 42 

models to conduct a preliminary assessment of the 

technical potential for micro-cogeneration in small, 

U.S. commercial buildings. 

METHODOLOGY

What might be achieved if all small commercial 

buildings throughout the United States were retrofitted 

with micro-cogeneration devices and hydronic heating 

systems? This study developed quantitative answers to 

this question using a methodology based on a large set 

of detailed energy performance simulations of 

individual buildings. Alternate scenarios are evaluated 

by comparing simulation results across the entire set of 

models where each is changed to reflect the addition of 

micro-cogeneration devices or a change in control 

strategy. 

The set of 1,236 building models used in this study was 

drawn from a separate study [Griffith et al. 2007; 

Griffith et al. 2008] that developed models for the 

entire U.S. commercial sector. The models were 

derived from 2003 CBECS public use data [EIA 2006] 

1
and represent a statistically robust, random sample of 

U.S. commercial buildings. Each sample is an 

individual building of a specific type in a specific 

location and includes a weighting factor to represent 

how many such buildings exist in the United States. 

Additional details that are needed to generate complete 

EnergyPlus models, but are not available from 2003 

CBECS data sets, were generated synthetically using a 

mixture of literature data, iteration, and random number 

generation. The subset of 1,236 models was derived 

from the full set of 4,820 models by filtering out only 

those buildings with floor area smaller than 500 m2

. 

The details of model generation are provided in Griffith 

et al. [2008]. 

The current study involves running the 1,236 

EnergyPlus models under six different scenarios. Table 

1 summarizes the scenarios by the name used in this 

paper, the type of heating, ventilation, and airconditioning (HVAC) system for space conditioning, 

the type of plant, and how the micro-cogeneration 

devices are controlled. The scenarios include: 

• EXISTING STOCK is a scenario that models 

the stock of small U.S. commercial buildings 

as of 2003. The set of models is a recent 

update of those from Griffith et al. [2008]. 

These are a mix of vintages and have all 

different kinds of HVAC systems. Some of 

these buildings lack heating and/or cooling. 

• Boiler PTAC is a scenario where the entire 

stock of buildings is retrofit with new HVAC 

systems that use hot water heating. This is 

considered the baseline scenario for evaluating 

micro-cogeneration. Although only 44% of 

the subsector appears to have natural gas, this 

scenario assumes that all buildings could use 

natural gas. The hydronic HVAC systems 

selected are packaged terminal air 

conditioners (PTACs) with hot water heating 

coils. These are through-the-wall direct 

expansion cooling and ventilating devices 

with individual zone control. The hot water 

system is heated by a central boiler with 80% 

efficiency (with respect to higher heating 

value) but otherwise uses the same plant 

topology used to model the micro-cogenerator 

scenarios (see Figure 1). All of the models are 

fully heated and cooled and thereby provide a 

higher level of service in terms of comfort and 

ventilation than what is in the Existing Stock. 

• CHP Therm PTAC is a scenario where a 

micro-cogenerator (CHP) is used and 

controlled to turn on when there is a thermal 

load. A net metering arrangement is assumed 

so if the building’s electrical demand is less 

than the electric generation, then excess 

electricity is fed back into the grid. 

• CHP Therm Elect Limit PTAC is a scenario 

where a micro-cogenerator is operated to turn 

on when there is a thermal load, however 

overall production is also limited by the 

current electrical demand of the building. No 

excess electricity is allowed to be exported 

back into the grid. 

• CHP Elect PTAC is a scenario where a 

micro-cogenerator is operated to follow the 

building’s electrical demand. No excess 

electricity is fed back into the grid. 

• CHP Baseload PTAC is a scenario where a 

micro-cogenerator is operated to run 

continuously in a baseline mode. 

This study modeled one type of micro-cogeneration 

device using the Annex 42 model for combustion 

devices with input data from laboratory measurements 

[Beausoleil-Morrison and Ferguson 2007]. The 

Senertech unit, a production internal combustion 

micro-cogenerator, was chosen for this study because 

of the availability of robust laboratory test data from 

Annex 42 and because it was one of the better 

performing units that was measured. The device is a 

natural gas-powered, internal combustion cogenerator 

with nominal electrical capacity of 5,500 W. The 

electrical efficiency was measured at 27% and the 

thermal efficiency at 66%, (with respect to lower 

heating value). In this simulation study, the control and 

modulation capabilities were extended to model “What 

If” scenarios; not all the operating schemes modeled 

were necessarily available in the actual production unit. 

The micro-generators are modeled using the hot water 

plant topology diagrammed in Figure 1. A hot water 

storage tank serves hot water coils in PTAC units 

located in each thermal zone. Service water heating 

uses are also connected to the same tank. The tank is 

modeled as a stratified tank with ten nodes in the 

vertical direction. For the boiler and thermal load 

following scenarios, the hot water storage tank requests 

indirect heating when its lower control point drops 

below the tank’s set point. If the upper setpoint is not 

met, then the tank uses backup heat. For the electrical 

load following scenarios, the recovery loop runs 

whenever the generator runs and the water heater vents 

any excess heat. 

2
The EnergyPlus models are all complete building 

models. Each model includes a full complement of 

energy-related characteristics including: internal gains, 

infiltration, occupancy, schedules and HVAC systems. 

They are annual runs with 15-minute time steps. The 

models are located across 170 different weather file 

locations. The geometries are relatively simple with 

rectangular floor plans and five thermal zones per floor. 

A pre-release compile of EnergyPlus version 2.2 was 

used for the study. 

Table 1. Summary of scenarios examined in study 

No. Scenario Name HVAC Plant 

Cogeneration 

control mode 

1 EXISTING STOCK Various Various none 

2 BOILER PTAC 

Packaged Terminal Air 

Conditioners with hot 

water coils 

Boiler indirectly 

heating HW tank, 

backup in stratified 

tank 

none 

3 CHP THERM PTAC 

Packaged Terminal Air 

Conditioners with hot 

water coil 

MicroCHP indirectly 

heating HW tank, 

backup in stratified 

tank 

Thermal Load 

Following (with 

Net Metering) 

4 CHP THERM ELECT 

LIMIT PTAC 

Packaged Terminal Air 

Conditioners with hot 

water coil 

MicroCHP indirectly 

heating HW tank, 

backup in stratified 

tank 

Thermal Load 

Following but 

limited by building 

electric demand 

(no export) 

5 CHP ELECT PTAC 

Packaged Terminal Air 

Conditioners with hot 

water coil 

MicroCHP indirectly 

heating HW tank, 

backup in stratified 

tank 

Electric load 

Following 

(no export) 

6 CHP BASELOAD PTAC 

Packaged Terminal Air 

Conditioners with hot 

water coil 

MicroCHP indirectly 

heating HW tank, 

backup in stratified 

tank 

Baseload 

(continuous rated 

capacity ) 

Figure 1. Hot Water Plant Topology in Models 

ANALYSIS

The thousands of EnergyPlus results are reduced using 

weighting factors to combine results across the 

subsector. Whole-building performance intensity 

metrics are used to quantify energy use across different 

buildings. Predicted energy use is normalized by 

3
building floor area in the following metrics: total site 

energy use intensity, net site energy use intensity, net 

source energy use intensity, total site electricity use 

intensity, total site gas use intensity, electricity 

generated intensity, and energy cost intensity. The 

annual whole building energy use and cost results are 

based on higher heating value of natural gas. The 

model input data include site-to-source energy 

conversion factors from Deru and Torcellini [2007] that 

are resolved by U.S. states. The model input includes a 

comprehensive set of utility tariffs, resolved at the state 

level, for calculating energy costs (assuming 

availability of net metering). Sector-wide averages are 

mean values weighted by floor area rather than number 

of buildings. 

The large number of samples also allows examining 

how energy performance is distributed across the 

subsector. Analysis of results includes generating 

probability density functions using a histogram analysis 

with a bin size of 300 MJ/m2

-yr. 

RESULTS

Table 2 provides a summary of the results. Comparing 

the Boiler PTAC scenario to the Existing Stock 

scenario shows that the installation of new PTACbased HVAC systems with hot-water-based heating 

would improve the thermal conditioning while also 

improving the average energy performance of the 

existing stock by 4.7% in terms of site energy and 22% 

in terms of source energy. 

Four scenarios replaced the boiler with a microcogenerator using different operating schemes. The 

thermal load following schemes reduced net source 

energy. The electric load following scheme did not 

change source energy use much. The baseload scheme 

increased source energy use. Energy costs increased in 

all cases but only the baseload scheme led to dramatic 

increases in energy costs compared to energy costs for 

the current existing stock. The cogenerators lead to a 

substantial shift away from electricity and toward using 

more natural gas. 

Table 2. Summary of Scenario Results for Average Energy Use Intensity Metrics 

Whole Building, 

Whole Sector, 

Annual Intensities units 

Existing Stock

Boiler PTAC 

CHP Therm 

PTAC 

CHP Therm 

Elect Limit 

PTAC 

CHP Elect 

PTAC

CHP Baseload 

PTAC 

Total Site EUI MJ/m2-yr 1,318 1,256 1,919 1,414 2,060 3,140 

Net Site EUI MJ/m2-yr 1,318 1,256 1,644 1,258 1,750 2,548 

Net Source EUI MJ/m2-yr 3,039 2,365 2,314 2,100 2,376 2,746 

Electricity Generated MJ/m2-yr 0 0 275 156 310 592 

Energy Cost Intensity $/m2-yr $27.88 $22.90 $29.18 $24.47 $30.57 $40.63 

Total Electricity EUI MJ/m2-yr 934 589 591 591 591 583 

Total Natural Gas EUI MJ/m2-yr 374 667 1,328 824 1,468 2,557 

4
0.00

0.05

0.10

0.15

0.20

0 1000 2000 3000 4000 5000

Net Source Energy Use Intensity (MJ/m2-yr)

Probability Density

Existing Stock

Boiler PTAC

CHP Therm PTAC

CHP Therm Elect Limit PTAC

CHP Elect PTAC

CHP Baseload PTAC

Figure 2 Probability Density Functions for Scenarios 

Table 3. Average Results for Energy End Uses 

Average Energy End Uses units 

Existing Stock

Boiler PTAC 

CHP Therm 

PTAC 

CHP Therm 

Elect Limit 

PTAC 

CHP Elect 

PTAC

CHP Baseload 

PTAC 

Cooling Electricity MJ/m2-yr 182 161 164 163 157 158 

Heating Gas MJ/m2-yr 226 491 0 0 0 0 

Fans Electricity MJ/m2-yr 342 81 74 74 73 64 

Pumps Electricity MJ/m2-yr 1 6 7 7 15 15 

Lighting Electricity MJ/m2-yr 160 160 160 160 160 160 

Plug/process Electricity MJ/m2-yr 88 88 88 88 88 88 

Process Gas MJ/m2-yr 101 101 101 101 101 101 

Refrigeration Electricity MJ/m2-yr 98 98 98 98 98 98 

Water Heater Gas MJ/m2-yr 77 75 64 65 95 24 

Generator Gas MJ/m2-yr 0 0 1163 658 1272 2432 

Figure 2 plots probability density functions that show 

how source energy performance is distributed across 

the subsector. Performance in the subsector varies 

widely. The higher probability of low energy buildings 

in the Existing Stock is because many of those 

buildings lack heating and/or cooling systems but in the 

other scenarios, all the buildings are completely heated 

and cooled. The density functions show that the CHP 

Baseload PTAC scenario is the worst and that the CHP 

Therm Elect Limit PTAC scenario is the best. 

Table 3 lists end uses that show the breakdown for 

where energy used at site is consumed, on average. 

The hot water storage tank always used a small amount 

of gas for backup heating. All of the CHP scenarios 

were effective at replacing the energy used for space 

5
heating. The operating scheme that followed thermal 

loads with electrical demand limit ran the CHP unit the 

least but was, on average, able to meet the heating 

loads. This indicates that the other operating schemes 

are not able to use all the thermal energy being 

produced. 

CONCLUSIONS

This study applied the Annex 42 model for 

combustion-based micro-cogeneration devices to assess 

how one such device would affect energy use in the 

subsector composed of small U.S. commercial 

buildings. This study leads to the following 

conclusions: 

• EnergyPlus is useful for analyzing the annual 

energy performance of micro-cogeneration 

devices in a modeling framework that includes 

complete hot water systems and the building’s 

thermal loads. 

• The production internal combustion microcogenerator evaluated has the potential to 

reduce net source energy use in the subsector 

by an average of 11%. 

• Over the course of a year, the electricity 

demands of most of the subsector could be 

met by micro-cogenerators operated 

continuously with a 5.5-kW electrical output. 

• The most favorable operating scheme from a 

cost and source energy viewpoint is for the 

micro-cogenerator to be operated to follow 

thermal loads. 

• Micro-cogeneration is effective at shifting 

from electricity to natural gas, but does not 

decrease net site energy use or energy costs. 

• Some operating schemes will produce excess 

thermal energy that cannot be used at the site. 

ACKNOWLEDGEMENTS

This document was prepared under the auspices of 

Drury Crawley of the Building Technologies (BT) 

Program within the Office of Energy Efficiency and 

Renewable Energy (EERE) at the U.S. Department of 

Energy (DOE). 

REFERENCES

Kelly, N.; Ferguson, A.; Griffith, B.; Weber, A. (2008). 

The Development of a Generic Systems-Level Model 

for Combustion-Based Domestic Cogeneration. 

Proceedings of the Micro-Cogeneration 2008 

conference held in Ottawa, Canada, April 28 to May 1, 

2008. 

Beausoleil-Morrison, I. and Ferguson, A. (2007). Intermodel Comparative Testing and Empirical Validation 

of Annex 42 Models for Residential Cogeneration 

Devices. IEA/ECBCS Annex 42. Available at 

www.cogen-sim.net

Beausoleil-Morrison, I.; Kelly, N. eds (2007). 

Specifications for Modelling Fuel Cell and 

Combustion-Based Residential Cogeneration Devices 

within Whole-Building Simulation Programs. 

IEA/ECBCS Annex 42. Available at www.cogensim.net

DOE. (2007). EnergyPlus Version 2.1. Computer 

program available at www.energyplus.gov

Griffith, B.; Long, N.; Torcellini, P.; Judkoff, R.; 

Crawley, D.; Ryan, J. (2007). Assessment of the 

Technical Potential for Achieving Net-Zero-Energy 

Buildings in the Commercial Sector. NREL Report No. 

TP-550-41957. Golden, CO: NREL. 

Griffith, B.; N. Long; P. Torcellini; R. Judkoff; D. 

Crawley, J. Ryan. (2008). Methodology for Modeling 

Building Energy Performance across the Commercial 

Sector. NREL Report No. TP-550-41956. Golden, CO: 

NREL. 

Deru, M. and Torcellini, P. (2007). Source Energy and 

Emission Factors for Energy Use in Buildings. Golden, 

CO: NREL/TP-550-38617. Revised June 2007. 

EIA. (2006). 2003 Commercial Buildings Energy 

Consumption Survey. Washington, DC: EIA. 

Available from www.eia.doe.gov/emeu/cbecs/ 

cbecs2003/introduction.html. 

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14. ABSTRACT (Maximum 200 Words)

This paper presents an assessment of the technical potential for micro-cogeneration in small commercial buildings 

throughout the United States. The cogeneration devices are simulated with the computer program EnergyPlus using 

models developed by Annex 42, a working group of the International Energy Agency’s Energy Efficiency in Buildings 

and Community Systems. Although the Annex 42 models were developed for residential applications, this study 

applies them to small commercial buildings, assumed to have a total floor area of 500 square meters or smaller. The 

potential for micro-cogeneration is examined for the entire existing stock of small U.S. commercial buildings using a 

bottom-up method based on 1,236 EnergyPlus models. 

15. SUBJECT TERMS

micro-cogeneration; cogeneration; energyplus; commercial buildings 

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