Prepared for the U.S. General Services Administration

By National Renewable Energy Laboratory

January 2015

Photovoltaic-Thermal New 

Technology Demonstration

Jesse Dean (NREL)

Peter McNutt (NREL)

Lars Lisell (NREL)

Jay Burch (NREL)

Dennis Jones (Group14)

David Heinicke (Group14)
The Green Proving Ground program leverages GSA’s real estate portfolio 

to evaluate innovative sustainable building technologies and practices. 

Findings are used to support the development of GSA performance 

specifications and inform decision-making within GSA, other federal 

agencies, and the real estate industry. The program aims to drive 

innovation in environmental performance in federal buildings and help 

lead market transformation through deployment of new technologies.
Photovoltaic-Thermal New Technology Demonstration Page i 

Disclaimer 

This manuscript has been authored by employees of the Alliance for Sustainable Energy, LLC (“Alliance”) 

under Contract No. DE-AC36-08GO28308 with the U.S. Department of Energy (“DOE”).

This report was prepared as an account of work sponsored by the United States Government. Neither the 

United States Government nor any agency thereof, nor the authors of the report, nor any of their 

employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the 

accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or 

represents that its use would not infringe privately owned rights. Reference herein to any specific 

commercial product, process, or service by its trade name, trademark, manufacturer, or otherwise, does not 

constitute or imply its endorsement, recommendation, or favoring by the United States Government or any 

agency thereof, or the authors of the report. The views and opinions of authors expressed herein do not 

necessarily state or reflect those of the United States Government or any agency thereof. 

The work described in this report was funded by the U.S. General Services Administration and the Federal 

Energy Management Program of the U.S. Department of Energy under Contract Nos. GSA: PX013232, NREL: 

IAG-11-1827. 

Acknowledgements

United States General Services Administration (GSA) Region 1: Jeffrey Schetrompf, Roman Piaskoski, Sarah

Wenninger, Waikit Lui, Noretta Lleshi 

United States General Services Administration (GSA) Green Proving Ground program: Kevin Powell, Maria

Filios, Christine Wu

Tenfold Information Design: Andrea Silvestri

For more information contact:

Kevin Powell

Program Manager

Green Proving Ground

Office of the Chief Greening Officer

U.S. General Services Administration

50 United Nations Plaza

San Francisco, CA 94102-4912 

Email: kevin.powell@gsa.gov
Photovoltaic-Thermal New Technology Demonstration Page ii

Table of Contents 

I. Executive Summary............................................................................................................................ 1

II. Background ........................................................................................................................................ 5

A. Assessment................................................................................................................................................. 5

B. Opportunity................................................................................................................................................ 7

III. Technology Description ..................................................................................................................... 9

IV. M&V Evaluation Plan ....................................................................................................................... 17

A. Test Plan...................................................................................................................................................17

B. Facility Description...................................................................................................................................17

C. Technology Specification .........................................................................................................................18

D. Technology Deployment ..........................................................................................................................18

V. Summary Findings and Conclusions ................................................................................................ 24

A. Overall Technology Assessment at Demonstration Facility.....................................................................24

B. Ideal System Design Used for Modeling ..................................................................................................24

C. Thermal Model.........................................................................................................................................25

D. Results......................................................................................................................................................28

E. Barriers and Enablers to Adoption...........................................................................................................36

F. Market Potential within the GSA Portfolio ..............................................................................................36

G. Recommendations for Installation, Commissioning, Training and Change Management ......................36

VI. Appendices ....................................................................................................................................... 37

A. Detailed Technology Specification...........................................................................................................37

B. Glossary....................................................................................................................................................38
Photovoltaic-Thermal New Technology Demonstration Page 1 

I. Executive Summary

Background

Integrating renewable energy (RE) systems into the built environment and using available rooftop space 

for solar energy installations provide a number of financial and societal benefits for the U.S. General 

Services Administration (GSA). These benefits include cost savings, increased energy cost security, and 

reduction of greenhouse gas emissions. Utilization of available rooftop area also provides an 

opportunity to implement solar energy technologies such as photovoltaic (PV) and solar hot water 

(SHW) systems to generate electricity and hot water for those buildings. Given the fact that most of the

federally owned and leased facilities for which GSA is responsible are multi-story and located in densely 

populated urban environments, available rooftop area is a valuable commodity that should be optimized 

for PV and SHW installations. 

GSA is responsible for over 354 million square feet (ft2

) of federally owned and leased space in more 

than 9,600 buildings.

1

 A large percentage of those facilities are candidates for rooftop solar installations. 

The Energy Policy Act of 2005 (EPAct) requires that the Secretary of Energy ensure that, to the extent 

economically feasible and technically practicable, not less than 7.5% of the total electricity consumed by 

the Federal Government come from RE in FY 2013 and thereafter. The Energy Independence and 

Security Act of 2007 (EISA) also requires new federal buildings and major renovations meet 30% of their 

hot water demand with solar energy, provided it is cost-effective over the life of the system. According 

to the 2007 annual report to Congress, GSA ranks second in the Federal Government for the use of 

onsite RE with 228,791 megawatt-hours per year (MWh/year) of RE electricity production, or 8.2% of 

GSA’s total electricity usage. GSA plans to continue its proactive stance toward on-site RE 

implementation by identifying cost-effective, innovative technology solutions.

The solar energy industry is among the most dynamic industries in the energy sector, and is experiencing 

revolutionary reductions in installed costs and innovative technology offerings. Due to the liquidity of 

the market and the pace that new technologies enter the market, emerging technologies need to be

tested in the field to evaluate their actual in-situ performance. 

What is the technology?

Photovoltaic–thermal (PV-T) technology consists of a typical solar PV panel, coupled with a solar thermal 

collector installed on the back of the PV panel to pre-heat domestic hot water (DHW) or ventilation air.

This allows a larger portion of the solar energy incident on the collector to be turned into useful thermal 

and electrical energy. A primary feature of this system is that the efficiency of a PV panel decreases as 

the cell temperature increases. Water or air flowing through the thermal collector removes heat from 

the PV cells, allowing for more efficient operation. In addition, water or air heating and electricity can be

produced within the same footprint, resulting in more efficient use of valuable roof space.

There are a number of different types of PV-T technologies. Each technology has different target 

applications, installed costs and performance characteristics. For this demonstration, a hybrid PV-T 

solar energy system was installed at the Thomas P. O’Neill, Jr. Federal Building (O’Neill Federal Building)

in Boston, MA as part of a research effort to evaluate emerging technologies through the GSA Green 

Proving Ground (GPG) program. This type of thermal collector wicks heat from the backs of PV modules 

 

1 GSA Properties Overview, available at: http://www.gsa.gov/portal/content/104501
Photovoltaic-Thermal New Technology Demonstration Page 2 

as a means of pre-heating DHW for the facility. Funded by the American Recovery and Reinvestment Act 

of 2009 (ARRA), the project was the largest rooftop hybrid solar installation in the country when it was 

installed and the first of its kind on a federal building. While the PV-T technology has evolved over time 

and now covers domestic hot water, ventilation air and radiant floor heating, this evaluation is focused 

on a traditional solar hot water and PV application.

The system on the O’Neill Federal Building is rated at 31.5-kilowatt (kW) electric and 69-kW of thermal 

energy. The overall PV array consists of two subarrays of 75 modules and 150 modules in total. The PV 

panels are made of multi-crystalline silicon. They are rated at 210 watts (W) and have a rated electrical 

efficiency of 13.4%. The thermal system on the O’Neill Federal Building consists of three hot water 

loops, each of which has 48 thermal collectors and 160 gallons of storage in two 80-gallon tanks, for a 

total of 144 collectors and 480 gallons of storage. Of the 150 PV panels, 144 have solar thermal 

collectors installed on the back, leaving 6 PV panels without the thermal backing. The solar installer

installed 6 panels without thermal backing to provide a means of evaluating the performance of the PV 

panels without solar thermal panels. The original design goal for the SHW system was to meet 20% of 

the building’s DHW load.

Study design and objectives

The demonstration project was hosted by GSA Region 1, at the O’Neill Federal Building in Boston, MA. 

The O’Neill Federal Building is comprised of an 11-story high-rise section and a 5-story low-rise section 

connected by a 5-story interior courtyard covered by a continuous skylight. A total of 28 agencies 

occupy the facility, which has received an ENERGY STAR® certification. The PV-T system was installed on 

the roof of the facility’s five-story low-rise section. PV-T is an early commercial technology. One of the 

study’s primary goals was to identify what worked in the O’Neill Federal Building’s PV-T system and 

what needed improvement.

Project Results/Findings

Because of complications in system design and conflicts in installation and commissioning, not 

uncommon in the deployment of early commercial technologies, the M&V process did not deliver 

results that would be representative of future installations. It did, however, provide numerous lessons in 

system design as well as a list of best practices. The lessons learned from the demonstration were used 

to inform future designs and a model was created for the “ideal” PV-T system design for this particular

facility. The ideal system was modeled per the manufacturer’s recommendations. TRNSYS was used to 

model the solar thermal system and SolOpt was used to model the impacts of panel cooling on PV 

production. The basic system characteristics of the ideal system are outlined below: 

• 48 solar thermal collectors installed on the back of typical crystalline silicon PV panels

• 480 gallons of storage 

• Typical Office building draw profile (1,300 gallons/day on weekdays and 0 gallons/day on 

weekend) 

• 150-Watt circulation pump

• Standalone boiler provides supplementary heating (electric boiler)

• Tank temperature set point 125 °F 

• Mixing valve set point temperature 120 °F 
Photovoltaic-Thermal New Technology Demonstration Page 3 

The system was modeled in six locations with different weather files, and utility rates, and of the six 

locations modeled (see Table 1), only Honolulu, HI, and Daggett, CA, which had an electric hot water 

backup system, met the federal life-cycle costing requirements, with a net present value greater than 

$0.00 over a 40-year analysis period. With a 30% Federal Tax Credit, all cities except Portland, OR and 

Boston, MA are life-cycle cost-effective. 

Table 1 - Ideal PV-T System Simple Payback Including Increased PV Production 

City

Electricity 

Rate 

($/kWh)

City Cost 

Adjustment 

Multiplier 

Solar 

Energy 

Production 

(kWh/yr)

Annual 

Cost 

Savings ($)

Installed 

Cost ($)

Simple 

Payback 

(yrs) 

Portland, OR 0.0868 0.992 6,698 $581 $56,765 98

Boston, MA 0.1476 1.172 6,331 $934 $67,065 72

Denver, CO 0.1083 0.943 11,063 $1,198 $53,961 45

Honolulu, HI 0.3454 1.173 10,097 $3,488 $67,123 19

Daggett, CA 0.1813 0.996 11,824 $2,144 $56,994 27

Phoenix, AZ 0.105 0.887 11,783 $1,237 $50,757 41

Future GSA installations should focus, therefore, on locations with characteristics similar to Hawaii or 

Daggett CA and which meet all of the following requirements:

Limited Roof Space – Facilities with limited roof space relative to the size of the electrical and thermal 

load that are looking to fill the entire roof with solar and maximize solar energy production should be 

targeted. These facilities are ideal candidates for PV-T as more energy can be produced from the same 

footprint than a separate PV and solar thermal system.

• High Energy Costs - The natural gas industry has experienced significant cost reductions over 

the last few years. The economics of the solar thermal system is sensitive to fuel source 

costs and the cost of electricity on a $/MMBtu basis is seven times higher than natural gas in 

Boston MA. In addition the modeling analysis showed that HI and Daggett CA were the only 

locations where the system was cost effective without federal incentives. For installations 

where the federal tax credit cannot be captured sites with electric rates of 30 cents/kWh or 

higher should be targeted, and for systems that can capture the federal tax incentives sites 

with electric rates of 15 to 20 cents/kWh or higher should be targeted.

• Hot Climates - PV-T systems were shown to have the best economics in warmer locations 

such as Phoenix AZ, Dagget CA, and Honolulu HI. A PV-T system will produce more hot 

water on an annual basis in these locations which increases the thermal energy savings. PVT systems were also shown to provide additional PV panel cooling benefits in hot locations 

since the panel temperatures are hotter in these locations and the impact on increased 

electrical production has a bigger impact on the overall economics. Systems with both hot 

climates and high utility rates should be targeted for future installations.

• Central Hot Water Systems – Facilities with centralized domestic hot water systems should 

be targeted for the PV-T technology and the baseline DHW load should be metered before 
Photovoltaic-Thermal New Technology Demonstration Page 4 

designing a solar thermal system in order to size the system properly. Facilities with small 

de-centralized point of use domestic hot water systems are not applicable for solar thermal 

installations. 

• Piping Costs – Piping costs need to be considered in future installations. The length of the 

home run and overall piping costs should as short as possible to minimize installation costs.

In addition to the specific requirements a number of general solar thermal design best practices and 

lessons learned are outlined below:

• Implement efficiency first – The existing DHW equipment should be analyzed prior to the 

installation of a SHW system. All applicable water conservation and energy efficiency 

opportunities should be implemented before sizing a solar thermal system.

• Use accurate component and system design tools to optimize performance – PV-T is a 

relatively new technology and design tools are still in development to model the solar 

thermal system and effects on the PV system. For future designs, a detailed hourly analysis 

should be conducted with SRCC-rated solar thermal panel performance data. This will aid in 

the correct sizing of the system and enable a more accurate economic analysis.

• Design systems with the same unit flow rate (gpm/ft2

) at which the panels were tested – The 

flow rates through each solar thermal panel should be set to SRCC test conditions, which in 

this case was a 0.0281 gpm/ft2 collector area.

• Carefully select heat exchangers – The solar thermal heat exchangers between the collection 

loops and storage tanks should be properly sized to avoid an adverse impact on system 

performance. The thermal tanks should be sized appropriately based on the solar thermal 

fraction, design inlet temperature, outlet temperature, and flow rates.

• Install sub metering – Sub metering should be installed and used to report thermal energy 

production and compare it to predicted energy production on a monthly basis. 

Conclusions

The advantages of the PV-T technology installed in Boston are associated with the ability to install a PV 

and solar thermal system in the same footprint, the ability to simultaneously increase PV electrical 

production, and the potential to meet the federal requirement that mandates 30% of hot water loads be 

satisfied with solar thermal in new construction and major renovations, where life-cycle cost-effective. 

The primary disadvantage of the technology is the lower thermal efficiency, which requires greater 

surface area to provide an equivalent amount of solar thermal energy. Ultimately, the total system cost 

must be 30% to 50% lower than the cost of traditional solar thermal systems to be cost competitive.

In the future, the technology should be evaluated against a standard solar thermal and separate PV 

system by an independent energy analyst using an hourly analysis tool. Installations should focus on 

hotter climates with good local solar incentives, facilities with electric hot water heaters, and high 

electric rates that can fully capture the benefits of panel cooling and apply the best practices listed 

above. Of the six locations modeled, only Honolulu, HI, which had an electric hot water backup system, 

met the federal life-cycle costing requirements, with a net present value greater than $0.00 over a 40-

year analysis period. 
Photovoltaic-Thermal New Technology Demonstration Page 5 

II. Background 

A. Assessment 

Solar energy technologies offer a number of strategic benefits to the United States. Replacing fossil fuelfired generators with solar energy reduces emissions of human-induced greenhouse gases (GHGs) and 

air pollutants. Sunlight is a free resource. Therefore, once solar technologies are installed, they have 

very low operating costs and require minimal non-solar inputs. This provides insurance against 

conventional fuel supply disruptions and price volatility. In addition, growing the domestic solar energy 

industry could establish the U.S. as a global leader in solar technology innovation and support a growing 

number of solar-related jobs.

Despite these benefits, solar energy currently supplies only a small fraction of U.S. energy needs, 

primarily because, historically, it has been more expensive than conventional energy sources. However, 

solar manufacturing costs and sales prices have dropped dramatically over the past few decades, and 

solar technologies are approaching energy-price parity with conventional generating sources in some 

regions of the U.S. and abroad. Further, experience accumulated by solar manufacturers and 

developers, utilities, and regulatory bodies has decreased the time and expense required to install a fully 

operating solar system. These gains have come through both research and development (R&D) and U.S. 

and global solar market stimulation. An additional strong, coordinated effort could enable solar energy 

technologies to become increasingly cost competitive with conventional electricity-generation 

technologies in the U.S. over the next decade.

Photovoltaic 

In 2010, solar energy provided less than 0.1% of U.S. electricity demand. This is comparable to the 

amount supplied by nuclear energy in 1960, which subsequently grew to 11% by 1980 and to 19% by 

1990. Over the past decade, U.S. solar deployment has lagged behind deployment in European and 

Asian countries, primarily because these countries instituted strong solar-promoting policies, while solar 

policies in the United States were limited and inconsistent.

Figure 1 shows the regional photovoltaic (PV) cell and module shipments from the U.S., Japan, Europe, 

China/Taiwan, and the rest of the world (ROW) over the past decade.2 Between 2000 and 2010, PV 

module shipments achieved a compound annual growth rate of 53%, reaching 17 gigawatts (GW) of 

annual module shipments in 2010, and bringing the cumulative global PV shipments to about 40 GW. 

Although the U.S. accounted for 30% of global shipments in 2000, the U.S. market share declined 

significantly during the past decade. In 2010, the U.S. accounted for only 6% (about 1,000 MW) of PV 

module supply and only 8% (or about 1,400 MW) of demand.

3 By mid-2011, continued R&D and market 

forces helped reduce PV prices sharply and, along with a mix of state and federal policies, have 

positioned the U.S. PV market for rapid future growth.

 

2 Mints, P. (2011a). Photovoltaic Manufacturer Shipments, Capacity & Competitive Analysis 2010/2011. Palo Alto, CA: Navigant 

Consulting Photovoltaic Service Program. Report NPS-Supply6 (April 2011).

3 Mints, P. (2011b). Analysis of Worldwide Markets For Solar Products & Five-Year Application Forecast 2010/2011. Palo Alto, 

CA: Navigant Consulting Photovoltaic Service Program. Report NPS-Global6 (August 2011).
Photovoltaic-Thermal New Technology Demonstration Page 6 

Figure 1. Regional PV cell and module shipments, 2000 to 2010

Solar Thermal

According to the Energy Information Administration (EIA), water heating accounted for 6.7% of 

commercial building energy use, and solar energy supplied approximately 2% (0.05 Quads/year) in 2010. 6 EIA also estimated that 3% of buildings have solar thermal systems and, for the facilities that have solar 

thermal, approximately one third of the DHW load is supplied by the on-site solar thermal system.

Figure 2. Domestic hot water fuel sources
Photovoltaic-Thermal New Technology Demonstration Page 7 

Domestic hot water (DHW) systems are primarily served by electric and natural gas heating systems, 

with the majority of the heating coming from natural gas.

4

 Facilities switching to solar thermal that 

currently use electric hot water heating will have better economics than facilities with natural gas 

heating systems, due to the increased cost of electricity on a $/MMBtu basis.5

In 2009, there were 88 manufacturers or importers, or both, active in manufacturing, importing, and

exporting solar thermal collectors—an 18.9% increase from the 74 companies operating in 2008. These 

companies shipped 13.8 million square feet of solar thermal collectors in 2009, compared to 17.0 million 

square feet in 2008.6

Of the 88 companies reporting solar thermal collector shipments in 2009, many manufacturers also 

reported being involved in one or more of the following solar thermal-related activities: 

• 59 designed collectors or systems

• 27 developed prototype collectors

• 23 developed prototype systems

• 61 were involved in wholesale distribution

• 31 were involved in retail distribution

• 27 installed collectors.

In 2009, low-temperature and unglazed solar collector shipments in the U.S. totaled 10.5 million ft2, 

about 3.5 million ft2

less than the shipments in 2008. Nearly 94 percent of low-temperature collectors 

are used in the residential sector, primarily for pool heating. However, shipments to the pool heating 

market fell by more than 25 percent in 2009 compared with shipments in 2008, due partly to declines in 

U.S. home sales and prices, and the economic downturn.

U.S. shipments of medium-temperature collectors (defined in the Technology Description section) 

totaled 2.3 million ft2 in 2009, nearly 10 percent less than the shipments of approximately 2.6 million ft2

in 2008. The decrease in shipments is believed to be mainly due to the economic recession. 

Approximately 87 percent of medium-temperature collectors are used for hot water heating.6

B. Opportunity

The U.S. General Services Administration (GSA) is responsible for over 354 million ft2 of federally owned 

and leased space in more than 9,600 buildings.

7

 A large percentage of those facilities are candidates for 

rooftop solar installations. The Energy Policy Act of 2005 (EPAct) requires that the Secretary of Energy 

ensure that, to the extent economically feasible and technically practicable, not less than 7.5% of the 

total electricity consumed by the Federal Government come from renewable energy (RE) in FY 2013 and 

thereafter. According to the 2007 annual report to Congress, GSA ranks second in the Federal 

Government for the use of onsite RE with 228,791 MWh/year of RE electricity production, or 8.2% of 

GSA’s total electricity usage.

 

4 The DOE Buildings Energy Data Book, available at: http://buildingsdatabook.eren.doe.gov/TableView.aspx?table=3.1.4

5 MMBTu = One million British thermal units (BTU)

6 EIA Solar Thermal Collector Manufacturing Activities 2009, available at: 

http://www.energybc.ca/cache/solarthermal/www.eia.gov/renewable/annual/solar_thermal/solar.html

7 GSA Properties Overview, http://www.gsa.gov/portal/content/104501
Photovoltaic-Thermal New Technology Demonstration Page 8 

Targeting energy consumption in federal buildings, the Energy Independence and Security Act of 2007 

(EISA) also requires new federal buildings and major renovations to meet 30% of their hot water 

demand with solar energy, provided it is cost-effective over the life of the system. Federal facilities 

having financial difficulty meeting the EISA mandate (e.g., facilities with natural gas) can potentially take 

advantage of PV-T systems to meet both the EPAct 2005 and EISA 2007 renewable energy requirements. 

The PV-T technology is typically designed for lower solar fractions and operating temperatures; these 

systems are potentially capable of meeting the statutory requirements of 30% solar fractions costeffectively in certain locations and utility markets. The solar fraction is defined as the percentage of the 

overall DHW load over the course of a year that is supplied by an SHW system.

Although solar energy’s contribution to U.S. energy supply has been small to date, its technical potential 

is enormous. For example, one estimate suggested that the area required to supply an amount of 

electricity equivalent to all end-use electricity in the U.S. using PV is only about 0.6% of the country’s 

total land area.

8 A recent estimate of the total roof area suitable for PV in the U.S. is about 64.5 billion 

ft2

, even after eliminating 35% to 80% of roof space to account for panel shading (e.g., by trees) and 

suboptimal roof orientations. With current PV performance, this area has the potential for more than 

600 GW of PV capacity.

9

 The U.S. had 1,051 GW of electrical generating capacity in 2011.10

In a strategic attempt to reduce the installed cost of PV systems, the U.S. Department of Energy (DOE) 

launched the SunShot Initiative in February 2011.

11 This Initiative is a collaborative national effort to 

make solar energy cost competitive with other forms of energy by the end of the decade. Achieving this 

goal will require dramatic decreases in the cost structure of solar technologies—on the order of a 75% 

reduction—across all markets, including residential, commercial, and utility-scale deployments of solar. 

To do this effectively, the SunShot Initiative spans the full spectrum from basic science to applied 

research and development. It also spans across multiple DOE offices, including Energy Efficiency and 

Renewable Energy (EERE), Advanced Research Projects Agency- Energy (ARPA-E) and the Office of 

Science (SC).

The SunShot program is targeting a reduction in installed costs to $1.00/DC-Watt for utility scale solar in 

2020, and $1.25/DC-Watt for commercial rooftop solar. The 2010 benchmark prices for the SunShot 

vision study are $4.00/DC-Watt for utility scale solar in 2010 and $5.00/DC-Watt for commercial rooftop 

solar. Achieving the SunShot targets is projected to result in the cumulative installation of approximately 

302 GW of PV and 28 GW of concentrating solar power (CSP) by 2030. A recent study indicates that 

under these assumptions, solar electricity could contribute up to 14% and 27% of the total electricity 

demand by 2030 and 2050, respectively.12

These drastic reductions in installed costs will drive cost-competitive installations at federal facilities and 

provide a potential opportunity to marry the technology with a low cost solar thermal option that can 

meet 30% of the DHW needs of a facility.

 

8 This calculation is based on deployment/land in the entire United States (including Alaska and Hawaii).

9 Denholm, P.; Margolis, R. (2008a). “Land-Use Requirements and the Per-Capita Solar Footprint for Photovoltaic Generation in 

the United States.” Energy Policy; 36:3531–3543.

10 Electricity Generating Capacity, http://www.eia.gov/electricity/capacity/

11 DOE Sunshot Initiative, http://www1.eere.energy.gov/solar/sunshot/index.html 

12 DOE SunShot Vision Study, http://www.nrel.gov/docs/fy12osti/47927.pdf
Photovoltaic-Thermal New Technology Demonstration Page 9 

III. Technology Description

Traditional Solar Hot Water

Solar water heating systems use solar collectors to capture sunlight to heat water (or an antifreeze 

liquid) that is then moved from the collector to a storage tank (Figure 3). There are two types of 

systems, active and passive. Active systems use electricity to pump the fluid and have a reservoir or tank 

for heat storage and subsequent use. Passive systems rely on natural convection and water pressure 

during draw to move fluids and require no circulation hardware. The systems may be used to heat water 

in homes, businesses, and for industrial uses. In many climates, a SHW system can provide up to 80% or 

more of the energy needed to heat water. SHW systems almost always require a backup system for 

cloudy days and times of increased demand. Conventional natural gas or electric water heaters typically 

provide backup, so hot water is always available regardless of the weather or demand. 

Figure 3. Schematic of an active SHW system with freeze protection (Illustration by Jim Leyshon, NREL)

Collectors 

There are primarily three types of solar collectors used for common solar water heating systems: 

unglazed, flat plate, and evacuated tube. A fourth type of collector, parabolic trough, is only used to 

heat water for very large facilities or for high-temperature applications, including electric generation. 

Typically, unglazed collectors are used for heating pools and house a dark absorber plate (metal or 

plastic) without a cover. Conventional flat-plate collectors are insulated boxes with glass covers that 

contain a dark thin copper plate used to absorb the sun’s heat underneath. The terms single- and 

double-glazed collector come from the glass plate on the flat-plate collector. The collector housing is 

typically steel or aluminum. Evacuated-tube solar collectors use transparent glass tubes that contain a 

metal absorber tube attached to a fin. Most collectors sold in the U.S. today are flat-plate collectors, 

which constitute over 90% of the market. An illustration of the four solar thermal technologies is 

provided in Figure 4. 
Photovoltaic-Thermal New Technology Demonstration Page 10

Figure 4. Solar thermal technologies, with unique characteristics shown (Illustration by Jim Leyshon, NREL)

Storage Tanks

Typically, storage is required to couple the timing of the intermittent solar resource with the timing of 

the hot water load. Usually, one to two gallons of storage water per square foot of collector area is 

adequate. Storage can either be potable or non-potable water if a code-approved load side heat 

exchanger is used. For conventional small systems, storage is most often in the form of glass-lined steel 

tanks at line pressure. For large systems, unpressurized storage tanks made of polymers or using 

polymer liners are common. These can reduce storage cost per unit volume considerably compared to 

small pressurized tanks. 

Traditional Photovoltaic Systems

PV is a mature, commercially available renewable-energy technology (Figure 5). PV systems convert 

sunlight directly into electricity without moving parts and without producing air pollution. PV modules

increasingly use silicon semiconductor materials, but increasingly also use other materials to convert 

sunlight directly into electricity. Sunlight enters a PV module and is converted into direct-current (DC) 

electricity. 
Photovoltaic-Thermal New Technology Demonstration Page 11

Figure 5. PV System Schematic (Illustration by Jim Leyshon, NREL)

Traditional single-crystalline silicon (Si) solar modules are made with silicon cells, are usually flat-plate, 

and generally are the most efficient solar modules. Multi-crystalline Si modules are a similar technology, 

but may be slightly less efficient. A third type of module technology, called thin-film, is made from 

amorphous Si or a thin layer of non-silicon materials, such as cadmium telluride (CdTe) or copper indium 

gallium diselenide (CIGS). Thin-film solar cells use layers of semiconductor materials only a few 

micrometers thick. Table 4 lists some typical efficiencies for each type of module. Module efficiency is 

important because an array of high-efficiency modules can deliver from two to three times as much 

power as an array of low-efficiency modules for a given module footprint. Efficiency is measured by 

calculating the amount of electrical power produced by the PV module, measured in watts/meter2

(W/m2

), divided by the amount of solar energy striking the module, measured in W/m2. 

Table 2: Typical Efficiency Ranges for Different PV Module Technologies13

PV Module Type

Electrical 

Efficiency 

Range (%)

Single-crystalline silicon 14% to 19%

Multi-crystalline silicon 13% to 17%

Thin-film 6% to 11%

 

13 Implementing Solar PV Projects on Historic Buildings and in Historic Districts. A. Kandt, E. Hotchkiss, and A. Walker.

National Renewable Energy Laboratory; J. Buddenborg and J. Lindberg National Trust for Historic Preservation. NREL/TP-7A40-

51297. September 2011.
Photovoltaic-Thermal New Technology Demonstration Page 12

For applications that are connected to the electrical grid, an inverter transforms this DC electricity into 

the alternating-current (AC) electricity that can be used by a home or business, or can be exported to 

the utility grid. PV systems may be installed on the ground, rooftops, parking garages, and awnings. 

Rooftop PV systems range in size from a few kilowatts (kW) for residential systems to hundreds of 

kilowatts or megawatts (MW) on large commercial roofs.

PV-T Technology Overview

PV-T technology is intended to increase the amount of solar energy collected from a solar energy system 

by combining the PV and solar thermal panels into a single system of collectors. The thermal collector 

(absorber) is attached to the back of an off-the-shelf PV module, as illustrated in Figure 6. 

Figure 6. PV-T panel components

The sun does not hit the thermal collector directly; instead the solar energy that is not converted into 

electricity by the PV panel can be collected as useful heat by the solar thermal collector. This increases 

the overall efficiency of the system, which is characterized by Equation 1 and Equation 2: 

Equation 1: Total Energy Production

Ettttt = Epp + Quuuuu

Equation 2: Solar Efficiency

EE s = 

SSlr rr

Ettttt

PV 

module

thermal 

collector

foam 

insulation

outlet

inlet
Photovoltaic-Thermal New Technology Demonstration Page 13

Where

• Etotal = total collected solar energy (kWh or MMbtu)

• Epv = total collected electrical energy from PV cells (kWh or MMbtu)

• Quseful = total collected solar thermal energy (kWh or MMbtu)

• Effsys = solar efficiency of system (%)

• Solresource = incident solar radiation (kWh/ft2

) 

Equation 3: PV panel efficiency

EEEEEEEEEE = EE r ∗ (1 + T TpC ∗ (Tr − T )) 

Where

Efficiency = PV panel efficiency

EFFref = reference panel efficiency

Tempco = panel power temperature coefficient

Tref = reference temperature (77°F)

Tcell = calculated cell temperature

The cell temperature is calculated on an hourly basis in SolOpt using Equation 4: 

Equation 4: PV cell temperature

TC = TOO + SS Srii i i ∗ O − Tr

R ii i i

∗ (1 − EE r ) 

Where

Tcell = cell temperature

Toa = outside air temperature

Top = nominal operating temperature

CellTemp = calculated cell temperature

 Solarinsolation = solar insolation

Refinsolation = reference insolation

Effref = reference panel efficiency

Figure 7 illustrates how the technology is incorporated into a traditional PV module.
Photovoltaic-Thermal New Technology Demonstration Page 14

Figure 7. Thermal panel incorporated into a traditional panel14

The heat transfer fluid flows in between the thermal collector and PV module. The heat transfer fluid 

has a single inlet port and outlet port. The rigid insulation behind the thermal collector is 1⁄2-inch thick. 

The collector installed in Boston has a total area of 12.486 ft2 and an active collector area of 10.2257 ft2

. 

The PV-T system is an active system that uses pumps and a differential controller to start and stop the 

pumps based on the temperature difference between collector and tank. The Boston PV-T system uses a 

differential controller that activates the pumps once the temperature difference between the collector 

and tank is greater than 15°F.

The solar thermal collection device on the PV-T system operates using the same principles that an 

unglazed solar thermal collection device uses. The benefit of using unglazed collectors is that the 

technology has the potential to be less expensive than other types of collectors. Unglazed systems are 

typically less expensive due to the reduced installed cost of the collector and the fact that they are not 

subject to temperatures as high as glazed collectors, which also allows for the use of less expensive 

materials, such as PEX tubing. Additionally, there is no need to purge the thermal component as it will 

not overheat. In northern climates, the glycol loop assists in melting snow so that the array is able to 

capture electrical output for greater amount of time than a conventional PV array could.

PV-T systems require a backup heater to meet 100% of the instantaneous peak hot water demand on 

cloudy days or when the solar system output is insufficient. Conventional systems are most often sized 

to meet 60% to 80% of the water-heating load, covering most of the load during the summer when 

temperatures are high, but leaving the system unable to meet the entire winter load. PV-T systems are 

 

14 SunDrum Solar – Hybrid Solar Collector Assembly Guide REV 004
Photovoltaic-Thermal New Technology Demonstration Page 15

usually designed to meet 30% of the water heating load, because they operate at lower temperatures 

and have lower thermal efficiencies than traditional flat-plate or evacuated-tube collectors.

The solar thermal product can be installed on the back of PV modules in the field and has been designed 

to fit a number of different PV panels. Because PV modules are semiconductor devices, the power 

output of these modules decreases as temperature increases. PV panel manufactures provide a power 

temperature coefficient that can be used to characterize the effect of panel temperature on overall 

efficiency. Figure 8 lists the panel efficiencies as a function of cell temperature. The heavy dashed 

vertical line represents 77°F, the temperature at which all PV modules are rated.

Figure 8. PV efficiency versus cell temperature

Although panel efficiency is a function of panel temperature, a significant reduction in panel 

temperature is required to achieve a noticeable increase in electricity production. For example, if the PV 

cell temperature for the Evergreen solar panel is reduced from 122°F to 86°F, a 36°F reduction in panel 

temperature, the PV panel efficiency only increases from 13.1% to 14.2%. 

In summary, the combined PV-T panel provides the following potential synergies: 

• The PV module is typically 6% to 20% efficient (including thin film and high performance multijunction silicon, not shown in Figure 9). The lost energy (80% to 94% of the incident solar radiation) 

is mostly converted to heat and a portion of this energy can be used to satisfy thermal heating loads.

• PV modules operate more efficiently when PV surface temperatures are lower. The thermal 

collectors can lower the PV temperatures under the right conditions.

11.0%

12.0%

13.0%

14.0%

15.0%

16.0%

17.0%

18.0%

19.0%

20.0%

32 41 50 59 68 77 86 95 104 113 122

Panel Efficiency (%)

PV Cell Temperature (°F)

PV Panel Efficiency as a Function of Outside Air Temperature

Evergreen

Sanyo

Sharp

Suntech

SunPower

BP

Solon

Schott

Canadian Solar

Schuco
Photovoltaic-Thermal New Technology Demonstration Page 16

• By combining the two technologies, the efficiency of the PV-T system is increased, resulting in a 

higher power production per unit of solar collector area. This can be important in large commercial 

buildings, where the roof area may be the only space available for distributed-generation PV. 
Photovoltaic-Thermal New Technology Demonstration Page 17

IV. M&V Evaluation Plan

A. Test Plan

The PV-T system on the O’Neill Federal Building was studied to characterize its performance; collect 

performance data that can be used to compare the performance of the system to traditional PV and 

SHW systems; estimate energy usage and energy cost savings; and assess the technology for broader 

application by GSA. The study included seven months of continuous monitoring to determine system 

performance and identify potential performance issues. Although the system was monitored for seven 

months the results of the onsite measurement and verification effort are not included here because 

they were not representative of future installations and don’t accurately portray the potential energy 

savings of the given technology. Thus, the test plan and measurement and verification results were 

excluded from the final report.

B. Facility Description

The demonstration project was hosted by GSA Region 1 at the O’Neill Federal Building in Boston, MA

(Figure 9). The O’Neill Federal Building is comprised of an 11-story high-rise section and a five-story lowrise section connected by a five-story interior courtyard covered by a continuous skylight. The building is

670,818 ft2

.

Figure 9. Thomas P. O’Neill Jr. Federal Building

The office building is located in Boston next to North Station, one of Boston's main commuter rail 

stations. Constructed in 1986, the building has been well maintained and is in excellent condition. 

Below grade, on a single level, there is parking for 261 vehicles. Amenities include an auditorium, fitness 

center, cafeteria, and credit union. The building is not a designated historic building. 

The building is named after Thomas P. O'Neill Jr., also known as Tip O'Neill, who served as a 

Congressman from Boston for 34 years and Speaker of the House of Representatives for 10 years. Five 

federal agencies account for approximately 50% of the building's current occupancy, with the balance 

allocated to over 28 agencies. The facility has received an ENERGY STAR certification, and has 
Photovoltaic-Thermal New Technology Demonstration Page 18

implemented a number of sustainability measures, including the installation of a white reflective roof, 

low VOC paints, recycled content metal studding, and recycled content ceiling tiles.

C. Technology Specification

The PV-T system demonstration consists of a system of PV and solar thermal technologies. The PV 

components include PV panels, a ballasted mounting system, combiner boxes, wiring and inverters. The 

solar thermal consists of solar thermal collectors, distribution piping, pumps, storage tanks, heat 

exchangers, and an onsite boiler to provide make-up thermal energy. 

D. Technology Deployment

The O’Neill Federal Building was selected for this demonstration project because it had no renewable

energy systems prior to the demonstration and is also a regional headquarters for several federal 

agencies, which would expand the project’s potential visibility. Onsite personnel were originally 

considering a standalone PV system, but the current federal mandates and executive orders led them to 

consider a joint PV-T installation. 

The PV-T hybrid solar energy system that was installed is rated at 31.5-kW electric and an equivalent of 

69-kW of thermal energy. This ARRA-funded project was the largest rooftop hybrid solar installation in 

the country when it was installed in 2011 and the first of its kind on a federal building (Figure 10 and 

Figure 11). 
Photovoltaic-Thermal New Technology Demonstration Page 19

Figure 10. Aerial image of O’Neill Federal Building rooftop

Figure 11. O’Neill Federal Building PV-T system facing approximately south
Photovoltaic-Thermal New Technology Demonstration Page 20

The entire PV array consists of two sub arrays of 75 modules each (150 modules total). Each sub array

consists of five strings of 15 modules. The PV panels consist of multi-crystalline silicon PV panels. The 

panels are rated at 210 watts and have an electrical efficiency of 13.4%. The PV array utilizes a ballasted 

mounting system and has a tilt angle of 11 degrees. The azimuth angle of the array is approximately 145 

degrees, and the system experiences late afternoon shading from the upper section of the facility, with 

a weighted average annual solar access of 80%. A one-line electrical diagram is provided in Figure 12.

Figure 12. O’Neil Federal Building PV-T system electrical diagram

The array has a tilt angle of 11°. During the installation of the PV system, the modules were binned to 

improve the overall performance of the electrical system by reducing mismatch losses. The modules 

have a performance rating of -0/+5 watts of the rated capacity, which means that the top-rated modules 

have initial power ratings that are higher than the nameplate rating by as much as 5 watts. The PV 

modules were factory-tested and a specific power rating was identified for each panel. The higherperforming 75 modules are connected to a 15-kW inverter (rated at 15 kWAC) and the lower-performing 

modules are connected to a 13-kW inverter (rated at 12.3 kWAC). The inverters have a peak efficiency of 

94.5%. Figure 13 shows the inverters mounted in the penthouse with the DC and AC disconnects 

attached to the sides of the inverters.

combiner

box

DC

disconnect

AC fused

disconnect

15 kW 

inverter

277/480V

3p/4w out

 15 modules 

per string

5 strings 

(or 75 modules)

per subarray

DC fused

disconnect

combiner

box

DC

disconnect

AC fused

disconnect

 15 modules 

per string

DC fused

disconnect

panel

SPH

see electrical

riser diagram roof penthouse

5 strings 

(or 75 modules)

per subarray

13 kW 

inverter

277/480V

3p/4w out

utility

disconnect

100A

3p NF
Photovoltaic-Thermal New Technology Demonstration Page 21

Figure 13. Two inverters and disconnects

The thermal system is comprised of 144 units that are attached to the back of the PV panels. The 

thermal system on the O’Neill Federal Building consists of 3 loops, each with 48 thermal collectors and 

160 gallons of storage in two 80-gallon tanks, with a total of 144 collectors and 480 gallons of storage. 

The total active collector area for the system is rated at 1,472.5 ft2

. Of the 150 PV panels, 144 have solar 

thermal collectors installed on the back of the panel, and 6 PV panels have no thermal backing. Figure 14

shows the placement of the three main hot water loops, shown in yellow, purple, and blue, with the 6 

PV modules without solar thermal backing shown in orange.
Photovoltaic-Thermal New Technology Demonstration Page 22

Figure 14. PV-T System solar thermal loops and 6 PV panels without solar thermal

The three solar thermal loops have 6 collectors per string, as shown in Figure 15. The heat transfer fluid, 

propylene glycol, is coldest at the inlet to the first series of collectors. The average temperature gain 

across a panel is rated at 5°F, but depends on the panel temperature, ambient temperature, wind 

speed, and heat transfer fluid flow rate. As the heat transfer fluid flows through the string of 6 

collectors, the heat transfer fluid temperature increases, with a total design temperature rise of 30°F. 

The PV panels closest to the cold heat transfer fluid inlet experience the greatest cooling. There are a 

total of eight strings per sub array (48 panels) and a total of three sub arrays. 

Figure 15. Graphical representation of three solar thermal loops
Photovoltaic-Thermal New Technology Demonstration Page 23

The design goal for the DHW system is to meet 20% of the building DHW load. When the solar thermal 

system was installed, the facility consumed approximately 4,000 to 5,000 gallons of DHW per day, and 

the system was sized for 5,000 gallons per day. The target design temperature of the DHW tanks is 

110°F. The DHW system provides hot water for two hot water loads:

• Low-rise restrooms, which represent approximately 30% of the building’s DHW load; and

• Cafeteria and fitness center, which represent approximately 70% of the building’s DHW load.

Since the equipment was installed, the thermal DHW load has been reduced due to a new dishwashing 

system that has reduced the DHW load to around 1,000 GPD.

The system is intentionally designed to be undersized, i.e., to include more storage than would 

commonly be used for the given collector output. Therefore, the system will deliver cooler water to the 

collectors, keeping the PV panels cooler. The collector loop heat exchangers are at the bottom of each of 

the storage tanks and are double walled, to meet GSA safety specifications. A representative storage 

tank is shown in Figure 16.

15

Figure 16. Solar water heating storage tank with wrap-around heat exchanger

The heat transfer fluid is propylene glycol. The storage tanks and heat exchangers are installed in series. 

The PV-T system installed in Boston uses a differential controller that activates the pumps once the 

temperature difference between the collector and tank is greater than 15°F. 

 

15 SunEarth Inc, Solar Storage Tanks Graphic, http://www.sunearthinc.com/SolarTanks.html
Photovoltaic-Thermal New Technology Demonstration Page 24

V. Summary Findings and Conclusions

A. Overall Technology Assessment at Demonstration Facility 

The advantages of the PV-T technology installed in Boston are associated with the ability to install a PV 

and solar thermal system in the same footprint, the ability to simultaneously increase PV electrical 

production, and federal requirement to meet 30% of hot water loads with solar thermal loads that

provide a nice fit for the technology.

The primary disadvantage of the technology is associated with the lower thermal efficiencies, which 

require greater surface area to provide an equivalent amount of solar thermal energy. Ultimately, the

total system costs need to come in at a fraction of the costs of traditional solar thermal systems to be 

competitive. 

Because of conflicts in the installation and commissioning of the PV-T system, the M&V process did not 

deliver definitive results. However, it did provide valuable lessons learned and best practices. Modeling, 

based on ideal PV-T system design, provided information on where the technology could be most 

suitably deployed.

B. Ideal System Design Used for Modeling

The lessons learned that were identified at the demonstration in Boston were incorporated into an 

“ideal system” design in order to understand how the system would perform in future commercial 

installations. The ideal system was modeled per the manufacturer’s recommendations using TRNSYS to 

model the solar thermal system and SolOpt to model the impacts of panel cooling on PV production. 

The basic system characteristics of the ideal system are outlined below.

System Characteristics

• 48 solar thermal collectors installed on the back of typical crystalline silicon PV panels

o Sub array consist of three panels in series (string), and 16 strings in parallel

o Flow rate per string 0.5 gallons/minute

• 480 gallons of storage 

o Case #1: storage tank modeled with an immersed heat exchanger

o Case #2: storage tank modeled with an external plate and frame heat exchanger

• Typical Office building draw profile

o 1,300 gallons/day on weekdays and 0 gallons/day on weekend

• 150-Watt circulation pump

• Standalone boiler provides supplementary heating (electric boiler)

Control System

• Tank temperature set point 125 °F 

• Mixing valve set point temperature 120 °F 

• Pump turns on when panel temp is 15 °F hotter than tank and turn off when tank is within 4 °F 

of set point

A complete list of input parameters are outlined in Table 3. 
Photovoltaic-Thermal New Technology Demonstration Page 25

Table 3 -Key Input Parameters for the Solar Thermal System

Ideal Commercial PV-T System

Variable Collector Array

PV Module Area 16.9 ft2

Number in Series 3 -

Number of Modules 48 -

Array Area 811.17 ft2

Tilt angle 45 Deg

Azimuth 0 Deg

PV efficiency 13 %

A10 0.337 -

A10 0.00756 -

A20 10.1245 -

A21 0.02415 -

Variable Piping

Pipe length 100 ft

Pipe diameter 0.8 in

Pipe insulation thickness 0.5 in

Insulation conductivity 0.02 Btu/h-ft-F 

Variable Storage Tank

Tank Volume 480 gal

Tank height 6.56 ft

Insulation k value 0.23 Btu/h-ft-F

Insulation thickness 2.5 in

Tank U value 0.069 Btu/h-ft2-f

Number of nodes 20 -

Variable Circulation System

Delta T to Activate Pump 15 F

Delta T to Turn Off Pump 4 F

Pump power 150 W

Solar loop flow rate 8 gpm

C. Thermal Model

The ideal solar thermal system was modeled in TRNSYS. TRNSYS (Transient System Simulation Tool) 

graphically based software environment is used to simulate the behavior of transient systems. TRNSYS 

is a time-series hourly simulation program that can simulate the performance of photovoltaic and SHW 

systems using TYM2 or TMY3 weather data. TMY2 and TMY3 weather datasets are assembled as 15-

year average hourly weather data sets for a specific location. TRNSYS is the most accurate and robust 
Photovoltaic-Thermal New Technology Demonstration Page 26

hourly simulation program for solar thermal systems and the TRNSYS model is available upon request.16 

The thermal model constructed for the PV-T system is shown in Figure 17. The solar loop is shown in 

red, the potable water lines are shown in blue, and the boiler loop is shown as magenta.

Figure 17 - Thermal Model. The solar loop is shown in red, and the water loops are shown in blue. The boiler 

loop is shown as magenta.

The collectors here are modeled as in ISO9806/SRCC OG100. The collector efficiency equation is given 

as:

ηcoll = A1,0(1-A1,1Vwind)K(θ) – A2,0(1 + A2,1Vwind)/Inet 

where Ai,j are coefficients determined by the OG100 test, Vwind is the wind velocity, K(θ) is the effective 

incidence angle modifier, and Inet is the net radiation. Inet is calculated as:

Inet = Isun – (ε/α)absorberInet,IR, where

Inet,IR = net long-wave infrared exchange with sky = σ(Tamb4

– Tsky4).

Tsky is the effective black body sky temperature, as calculated using the methods of Martin-Berdahl (2). 

Inet,IR goes to zero under 100% cover, and can be up to 150 W/m2 under clear skies with low humidity.

The incident angle modifier function as supplied by SRCC is shown in Figure 18. The values are 

impossible for a flat plate PV with a glass or polymer cover sheet. The IAMs shown here are very typical 

for an evacuated tube collector without a back reflector; despite assurances from SRCC that these are 

the right values, the data was not used because the curves are not physically possible for flat plate 

collector systems. Rather, the IAMs from other similar PV/T systems were examined, which showed very 

flat behavior out to about 60 degrees, where it began to drop off to zero at 90 degrees. The function 

adopted here is also shown in Figure 18. 

 

16 http://www.trnsys.com 
Photovoltaic-Thermal New Technology Demonstration Page 27

Figure 18 Incidence angle modifier function.

Note: The SRCC supplied curve is the data on the certification report, and the assumed curve was taken from similar 

systems and used here.

System Metrics

Qsolar is the energy delivered by the solar system to the downstream (unspecified) auxiliary and the 

water heating loads:

Qsolar = ṁcp(Tout,tank – Tmains,in) 

Qsolar,net is the net energy including parasitic power consumed (only the pump/controller here):

Qsolar,net = Qsolar – Qparasitic 

Related metrics are the collector efficiency, the system efficiency (which includes pipe and tank losses), 

and the solar fraction Fsol = Qsolar,net/Qload, where Qload = ṁcp(Tset – Tmains,in).

System Parameters

The SRCC thermal collector test report is given in Appendix A. It was tested without the PV active. 

Subsequent changes to OG100 have clarified that the PV must be switched on when testing for thermal 

performance. This rule makes sense because the PV will always be operative during normal operation, 

and when PV is on irradiance that would have been thermalized it is instead used to promote electron 

excitation and electrical power. Thus, the unit produces less thermal energy than when the PV is off. 

Because the PV was off during the thermal collector test, the PV energy during normal operation must 

be subtracted from the available energy. Here, NREL subtracted the PV energy production directly from 

the incidence, because that energy is never thermalized. That is,

Isun,effective incidence = (1 – ηPV)Isun,incident 

0

0.25

0.5

0.75

1

1.25

1.5

0 15 30 45 60 75 90

IAM [-]

Incidence Angle [Degrees]

SunDrum IAM Functions

SRCC supplied

Assumed
Photovoltaic-Thermal New Technology Demonstration Page 28

Draw Profile

The draw profile used is shown in Figure 19. The water draw is constant from 7AM to 6 PM for 

weekdays, and is zero for weekends. The NREL analysis did not use the more complex ASHRAE office 

building draw profile, which has some variation throughout the workday, because the large storage

capacity leads to the exact profile of usage being of no relevance; only the daily total volume is 

significant. NREL thus chose the simpler but equivalent draw profile of Figure 19 - Water draw time 

profile. The draw is a constant value on weekdays from 7 AM to 6 PM, and is set to zero otherwise. The 

total daily draw volume was set to 1,300 gal/day on weekdays, and 0 gal/day on weekends. The year 

starts with Monday, and holidays are not considered.

Figure 19 - Water draw time profile. The draw is a constant value on weekdays from 7 AM to 6 PM, and is set to 

zero otherwise.

The weather files used are TMY3 files. The variables used include the solar beam/diffuse/total on 

horizontal, the ambient temperature, the humidity, the cloud cover, and the wind velocity. No account 

was taken for presence of snow cover, as this field is not present in TMY3 weather files. The humidity 

and cloud cover are used to compute the net sky infrared flux. The weather sites chosen are for Boston, 

MA; Dagget, CA; Denver, CO; Honolulu, HA; Phoenix, AZ; and Portland, OR.

D. Results

The total domestic hot water load, heat into the tank, and total solar energy are provided in Figure 20. 

The total solar energy term is the total energy into the tank minus the solar loop pumping energy. As 

expected, the energy delivered is larger in the sunnier climates like Phoenix and Denver, and lower in 

the more cloudy climates like Portland and Boston.

0

0.2

0.4

0.6

0.8

1

Relative Draw Flow Rate

0 24 48 72 96 120 144 168

Time During Week [Hours]

Water Draw Profile- 1 Week
Photovoltaic-Thermal New Technology Demonstration Page 29

0

10,000

20,000

30,000

40,000

50,000

60,000

Portland Boston Denver Honolulu Dagget Phoenix

Energy (kWh/year)

PV-T Annual Energy Production

DHW Load

Heat into Tank

Total Solar Energy

Figure 20 –PV-T Annual Energy Production

Two cases were run for the ideal system, one case with an immersed heat exchanger and one case for 

an external heat exchanger. The solar thermal fraction and annual solar thermal collector efficiency are 

provided in Figure 21 and Figure 22. The solar thermal fraction (percent of domestic hot water load met 

by the solar thermal system) varies from 10% to 26%. The annual average collector efficiency ranges 

from 5.7% to 7.6% depending on location.

Portland Boston Denver Honolulu Dagget Phoenix

Sol Frac Imm. Hx. 10% 11% 17% 20% 22% 24%

Sol Frac Ex. Hx. 10% 11% 18% 22% 23% 26%

0%

5%

10%

15%

20%

25%

30%

Annual Solar Thermal Fraction 

PV-T Solar Thermal Fraction

Figure 21 –PV-T Solar Thermal Fraction
Photovoltaic-Thermal New Technology Demonstration Page 30

Portland Boston Denver Honolulu Dagget Phoenix

Coll. Eff Imm. Hx 5.7% 5.8% 7.3% 5.8% 6.1% 6.0%

Coll. Eff Ext. Hx 6.0% 6.0% 7.6% 6.3% 6.5% 6.5%

0%

1%

2%

3%

4%

5%

6%

7%

8%

Annual Solar Thermal Efficiency 

PV-T Solar Annual Thermal Efficiency

Figure 22 –PV-T Solar Thermal Efficiency

The thermal collector manufacturer provided revised installed cost estimates that estimated the 

installed cost of the solar thermal portion of the system at $57,223 (Table 4). 

Table 4 -Key Input Parameters for the Solar Thermal System

Installed Costs

PV-T System Components & Labor Price Dealer 

Price 

SDK400 kit $43,200 $32,400 

Collector Installation Labor $1,440 $1,080 

Solar Storage Tank/w heat 

exchanger $4,800 $3,600 

External H/E (Pool) $400 $300 

Plumber (Tank Connections) $2,100 $1,575 

Plumber (HE Connections) $1,400 $1,050 

PEX + cladding $400 $300 

PEX Installation labor $400 $300 

Contingency (2%) $3,083 $2,312 

Total PV-T Components & Labor $57,223 $42,917 
Photovoltaic-Thermal New Technology Demonstration Page 31

The revised installed cost estimates were then used to calculate a simple payback in each location (Table 

5 and Table 6). The simple payback was calculated using blended electric rates for the commercial sector 

for each state from the Energy Information Administration (EIA).17 The city cost adjustment multiplier 

comes from the construction estimation database RSMeans and is applied to the installed cost estimate 

for each location.18 The simple payback does not include any local or federal incentives and is a nonincentivized payback. 

Table 5 –Energy Savings and Economics for Immersed Heat Exchanger Case

City

Electricity 

Rate 

($/kWh)

City Cost 

Adjustment 

Multiplier

Solar 

Energy 

Production 

(kWh/yr) 

Annual 

Cost 

Savings ($) 

Installed 

Cost ($)

Simple 

Payback 

(yrs) 

Portland, OR 0.0868 0.99 5,934 $515 $56,765 110

Boston, MA 0.1476 1.17 5,934 $876 $67,065 77

Denver, CO 0.1083 0.94 10,063 $1,090 $53,961 50

Honolulu, HI 0.3454 1.17 8,780 $3,033 $67,123 22

Daggett, CA 0.1813 1 10,529 $1,909 $56,994 30

Phoenix, AZ 0.105 0.89 10,384 $1,090 $50,757 47

Table 6 –Energy Savings and Economics for External Heat Exchanger Case

City

Electricity 

Rate 

($/kWh)

City Cost 

Adjustment 

Multiplier 

Solar 

Energy 

Production 

(kWh/yr)

Annual 

Cost 

Savings ($)

Installed 

Cost ($) 

Simple 

Payback 

(yrs) 

Portland, OR 0.0868 0.99 6,349 $551 $56,765 103

Boston, MA 0.1476 1.17 5,934 $876 $67,065 77

Denver, CO 0.1083 0.94 10,605 $1,149 $53,961 47

Honolulu, HI 0.3454 1.17 9,614 $3,321 $67,123 20

Daggett, CA 0.1813 1 11,269 $2,043 $56,994 28

Phoenix, AZ 0.105 0.89 11,247 $1,181 $50,757 43

The energy savings and economics are slightly better with the external heat exchanger.

 

17 Energy Information Administration, http://www.eia.gov

18 RS Means, http://rsmeans.reedconstructiondata.com
Photovoltaic-Thermal New Technology Demonstration Page 32

Modeled Increase in PV Production

It also is necessary to account for the synergies that exist from simultaneously cooling the PV modules. 

The PV system was modeled using SolOpt in the same 6 locations. The PV system production is 

calculated in SolOpt using panel properties from a database of panel types and manufacturers. This 

subset of panel options was taken from the SAM database of panels. The production calculation 

algorithms are based on an efficiency model.

19 The PV module output is based on the temperatureadjusted efficiency of the module. The cell temperature of the module is calculated on an hourly basis, 

and the efficiency of the module is adjusted accordingly.

20

The efficiency of the PV panel is calculated with the Equation 5. 

Equation 5: PV panel efficiency

EEEEEEEEEE = EE r ∗ (1 + T TpC ∗ (Tr − T ))

Where

Efficiency = PV panel efficiency

EFFref = reference panel efficiency

Tempco = panel power temperature coefficient

Tref = reference temperature (77°F)

Tcell = calculated cell temperature

The cell temperature is calculated on an hourly basis in SolOpt using Equation 6

Equation 6: PV cell temperature

TC = TOO + SS Srii i i ∗ O − Tr

R ii i i

∗ (1 − EE r )

Where

Tcell = cell temperature

Toa = outside air temperature

Top = nominal operating temperature

CellTemp = calculated cell temperature

Solarinsolation = solar insolation 

Refinsolation = reference insolation

Effref = reference panel efficiency

 

19 Menicucci, D.F. “Photovoltaic Array Performance Simulation Models.” Photovoltaics and Insolation Measurements 

Workshop, Vail, CO, U.S.A., June 30 – July 3, 1985. Albuquerque, NM: Sandia National Laboratories, 1985.

20 Fuentes, M. K. “A Simplified Thermal Model of Photovoltaic Modules.” SAND85-0330 Albuquerque, NM: Sandia National 

Laboratories, 1985.
Photovoltaic-Thermal New Technology Demonstration Page 33

The SolOpt code was modified to analyze the effects of panel temperature on an hourly basis at each 

location. The code was modified to reduce the cell temperature by 3°F, 5°F, 10°F, and 20°F for every 

hour that the system is producing electricity. This provides a conservative estimate of increased 

electricity production. In real life an actual installation would have periods of time that the solar 

thermal pumps are not activated or would need to warm up before the control signal turns on the 

pumps, and thus the PV panels would not experience a consistent reduction in panel temperature each 

hour of the year. The increase in PV output is fairly constant from one location to the next, as shown in 

Figure 23. 

0.0%

1.0%

2.0%

3.0%

4.0%

5.0%

6.0%

Boston MA Honolulu HI Daggett CA Phoenix AZ Denver CO Portland OR

Percent Increase (%)

Percent Increase in Annual Electricity Prodcution

3 °F

5 °F

10 °F

20 °F

Figure 23. Percent increase in annual production

The average percent increase in annual electricity production across each climate zones is 0.8% at 3°F, 

1.3% at 5°F, 2.5% at 10°F, and 5.1% at 20°F. 

NREL recalculated the simple payback for each location using the external plate and frame heat 

exchanger case assuming the solar thermal panels would reduce the PV panel temp 10 °F (which is a 

very conservative estimate). The economics are presented in Table 7 for the non-incentivized case and 

for a case that includes a 30% investment tax credit (Table 8). 
Photovoltaic-Thermal New Technology Demonstration Page 34

Table 7 - Ideal PV-T System Simple Payback Including Increased PV Production

City

Electricity 

Rate 

($/kWh)

City Cost 

Adjustment 

Multiplier 

Solar 

Energy 

Production 

(kWh/yr)

Annual 

Cost 

Savings ($)

Installed 

Cost ($)

Simple 

Payback 

(yrs) 

Portland, OR 0.0868 0.992 6,698 $581 $56,765 98

Boston, MA 0.1476 1.172 6,331 $934 $67,065 72

Denver, CO 0.1083 0.943 11,063 $1,198 $53,961 45

Honolulu, HI 0.3454 1.173 10,097 $3,488 $67,123 19

Daggett, CA 0.1813 0.996 11,824 $2,144 $56,994 27

Phoenix, AZ 0.105 0.887 11,783 $1,237 $50,757 41

Honolulu HI and Daggett CA are the only locations (of the 6 analyzed) that have a payback period that 

meets the federal life cycle costing requirements and has a payback period under 40 years. Future GSA 

installations that can’t capture the federal tax credits should focus on installations in HI with electric hot 

water back up tanks until other markets mature and the technology is shown to be cost effective in 

other locations.

Table 8 - Ideal PV-T System Simple Payback Including Increased PV Production and Federal Tax Credit

City

Electricity 

Rate 

($/kWh)

City Cost 

Adjustment 

Multiplier 

Solar 

Energy 

Production 

(kWh/yr)

Annual 

Cost 

Savings ($)

Installed 

Cost ($)

Simple 

Payback 

(yrs) 

Portland, OR 0.0868 0.992 6,698 $581 $39,736 68

Boston, MA 0.1476 1.172 6,331 $934 $46,946 50

Denver, CO 0.1083 0.943 11,063 $1,198 $37,773 32

Honolulu, HI 0.3454 1.173 10,097 $3,488 $46,986 13

Daggett, CA 0.1813 0.996 11,824 $2,144 $39,896 19

Phoenix, AZ 0.105 0.887 11,783 $1,237 $35,530 29

The simple payback for the case of Honolulu, HI with the federal tax credit is 13 years and the Daggett 

CA also showed a payback under 20 years in this case. 

As the first large-scale installation of this specific PV-T technology, a primary motivation for the project 

was to outline the design details that need to be addressed in future installations. Future PVT 

installations should focus on sites that meet all of the following requirements: 

• Limited Roof Space – Facilities with limited roof space relative to the size of the electrical 

and thermal load that are looking to fill the entire roof with solar and maximize solar energy 

production should be targeted. These facilities are ideal candidates for PVT as more energy 

can be produced from the same footprint than a separate PV and solar thermal system.
Photovoltaic-Thermal New Technology Demonstration Page 35

• High Energy Costs - The natural gas industry has experienced significant cost reductions over 

the last few years. The economics of the solar thermal system is sensitive to fuel source 

costs and the cost of electricity on a $/MMBtu basis is seven times higher than natural gas in 

Boston MA. In addition the modeling analysis showed that HI was the only location where 

the system was cost effective without federal incentives. For installations where the federal 

tax credit cannot be captured sites with electric rates of 30 cents/kWh or higher should be 

targeted, and for systems that can capture the federal tax incentives sites with electric rates 

of 15 to 20 cents/kWh or higher should be targeted.

• Hot Climates - PV-T systems were shown to have the best economics in warmer locations 

such as Phoenix AZ, Dagget CA, and Honolulu HI. A PV-T system will produce more hot 

water on an annual basis in these locations which increases the thermal energy savings. PVT systems were also shown to provide additional PV panel cooling benefits in hot locations 

since the panel temperatures are hotter in these locations and the impact on increased 

electrical production has a bigger impact on the overall economics. Systems with both hot 

climates and high utility rates should be targeted for future installations.

• Central Hot Water Systems – Facilities with centralized domestic hot water systems should 

be targeted for for PV-T technology and the baseline DHW load should be metered before 

designing a solar thermal system to size the system properly. Facilities with small decentralized point of use domestic hot water systems are not applicable for solar thermal 

installations 

• Piping Costs – Piping costs need to be considered in future installations. The length of the 

home run and overall piping costs should as short as possible to minimize installation costs.

In addition to the specific requirements a number of general solar thermal design best practices and 

lessons learned are outlined below:

• Implement efficiency first – The existing DHW equipment should be analyzed prior to the 

installation of a SHW system. All applicable water conservation and energy efficiency 

opportunities should be implemented before sizing a solar thermal system.

• Use accurate component and system design tools to optimize performance – PV-T is a 

relatively new technology and design tools are still in development to model the solar 

thermal system and effects on the PV system. For future designs, a detailed hourly analysis 

should be conducted with SRCC-rated solar thermal panel performance data. This will aid in 

the correct sizing of the system and enable a more accurate economic analysis.

• Design systems with the same unit flow rate (gpm/ft2

) at which the panels were tested – The 

flow rates through each solar thermal panel should be set to SRCC test conditions, which in 

this case was a 0.0281 gpm/ft2 collector area. 

• Carefully select heat exchangers – The solar thermal heat exchangers between the collection 

loops and storage tanks should be properly sized to avoid an adverse impact on system 

performance. The thermal tanks should be sized appropriately based on the solar thermal 

fraction, design inlet temperature, outlet temperature, and flow rates.
Photovoltaic-Thermal New Technology Demonstration Page 36

• Install sub metering – Sub metering should be installed and used to report thermal energy 

production and compare it to predicted energy production on a monthly basis. 

E. Barriers and Enablers to Adoption

The two primary barriers to market adoption within the GSA building stock consist of the installed costs 

of the PV and solar thermal system and the magnitude and consistency of the DHW load. Given that the 

system is less than half as efficient as traditional solar thermal systems, a larger solar thermal system is 

needed to meet a given solar thermal load, and the installed costs of this specific PV-T system need to 

be 30% to 50% lower than a traditional solar thermal system sized to meet the same loads. 

Facilities with electric resistance back-up heating systems should be targeted for future installations and 

systems with natural gas back up should be avoided unless the local utility incentives can overcome the 

reduced fuel costs. In the future, the site should require the use of a third-party energy analyst to 

analyze the economics of a standard PV and standard solar thermal system against the PV-T system 

using an hourly analysis tool such as TRNSYS. Given that most GSA facilities have intermittent DHW loads 

and are typically heated with natural gas, a large portion of the GSA building stock may not fit the 

criteria required for a cost-effective solar thermal project. 

Federal and utility rebates that can pay for as much as 75% of the installed costs of PV and solar thermal 

system and locations with both PV and solar thermal incentives should be targeted for PV-T installations.

F. Market Potential within the GSA Portfolio

The majority of the existing buildings in the GSA building stock have relatively small DHW loads and are 

primarily heated with natural gas. These buildings may not be appropriate candidates for PV-T 

technology unless significant solar thermal incentives are available. The facilities that should be targeted 

for PV-T technology are smaller facilities with larger DHW loads. The larger the array, the more 

challenging it is for the solar thermal design engineer to design a system with the correct flow rate per 

panel. However, if roof space is limited and there are smaller hot water loads, PV-T could still be a viable 

solution using fewer thermal panels, provided that the thermal component is properly sized.

In assessing the effect of PV panel cooling, NREL found a larger benefit in the hotter climates with a 

better solar resource, such as Phoenix AZ, Daggett CA, and Honolulu HI. Facilities that are roof 

constrained in these locations should be targeted for future installations.

Because EISA 2007 requires that 30% of the hot water demand in new federal buildings (and major 

renovations) be met with SHW equipment (provided it is life-cycle cost-effective), new construction and 

major renovation should be a target market for the technology.

G. Recommendations for Installation, Commissioning, Training and Change Management

Before installing a PV-T system, the site should install temporary metering on the DHW system to 

accurately capture the daily hot water usage and load profile. This information is critical to sizing the 

system correctly. In addition, all of the lessons learned and best practices should be incorporated into 

future designs that will be life-cycle cost-effective. The operation and maintenance costs are similar to 

traditional PV and solar thermal technologies and the design and integration of the technology also 

follows the same general procedures.
Photovoltaic-Thermal New Technology Demonstration Page 37

VI. Appendices

A. Detailed Technology Specification 

A copy of the SRCC test results are provided in Figure 24.

Figure 24. SunDrum SRCC rating
Photovoltaic-Thermal New Technology Demonstration Page 38

B. Glossary 

AC Alternating current

ARRA American Recovery and Reinvestment Act of 2009

CdTe Cadmium Telluride 

CIGS Copper Indium Gallium Diselenide 

DAS Data acquisition system

DC Direct current

DHW Domestic hot water

DOE Department of Energy

EIA Energy Information Administration

EISA Energy Independence and Security Act of 2007

EPAct Energy Policy Act of 2005

FSEC Florida Solar Energy Center

GHG Greenhouse gas

GPD Gallons per day

GPM Gallons per minute

GPG Green Proving Ground program

GSA General Services Administration

GW Gigawatts

Hx Heat exchanger

IAM Incident angle modifier

kBtu Thousand British thermal units

kW Kilowatt 

M&V Measurement and verification

MMBtu Million British thermal units 

MW Megawatt

MWh Megawatt hour

NREL National Renewable Energy Laboratory

PV Photovoltaic

PV-T Photovoltaic – thermal

RE Renewable energy

R&D Research and development 

SAM Solar advisor model
Photovoltaic-Thermal New Technology Demonstration Page 39

Si Silicon 

SHW Solar hot water

SRCC Solar Rating and Certification Company

TMY Typical Meteorological Year

TRNSYS Transient System Simulation Tool

VOC volatile organic compound

W Watt