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How Does a Thin-Film PV System Affect Cooling Loads on a Building?

May 15, 2009

ABSTRACT
Using calculated cooling loads and calculated
energy generated by a photovoltaic
(PV) system, it was determined that the
additional cooling load caused by a thinfilm
PV surface requires no more than 2.5%
of the electricity generated by the PV system
itself. The added cooling load is due to the
lower solar reflectance (SR) of the PV surface
itself. The minimal
penalty in added electricity
needed for the
higher cooling load is
based on a newly constructed
building with
code-compliant insulation
and new air conditioning
units with high
Coefficient of
Performance (COP) values.
In a renovated
building with a retrofit
roof, lower insulation
values will be encountered,
and a less efficient
air conditioning
unit will be in place.
Under those conditions,
even in the hottest part
of the country, with low
insulation and poor air
conditioning COP values,
less than 20% of
the electricity generated
by the thin-film PV unit is required to compensate
for the higher cooling load placed
on the building.
Based on these calculations, a thin-film
PV system can generate significantly more
than enough electricity for air conditioning
related to the higher cooling load and can
provide additional electricity for other uses
in the building.
Why Are PVs Gaining in Popularity?
When President Bush signed the Energy
Independence and Security Act in
December 2007, the federal government
authorized the formation of the Zero Net
Energy Commercial Buildings Initiative.
That program involves an alliance of industrial,
academic, and governmental representatives
working to transform energy per-
6 • IN T E R FA C E J U LY 2009
formance in commercial buildings. In order
to achieve zero energy, a building must be
designed with optimum efficiencies and
energy conservation measures in place to
reduce the energy demand as much as possible.
The remaining energy needs would
then be provided on site using renewable
energy sources.
Photovoltaic (PV) technology is one of
the more popular renewable forms of energy
because of society’s interest in the
impact on the environment and the rising
cost of fossil-fuel-based energy sources. Of
course, tax and financial incentives to use
PV systems are helping in many states.
Also, the technological improvements, production
efficiency improvements, and simple
economies of scale are making PVs more
attractive. Third- and fourth-generation PV
systems are under development to be even
more energy efficient, economical, and
practical.
The California Public Utility Commis –
sion recently announced a challenge to
have builders construct all-new commercial
structures as net zero energy by 2030.
Other states, such as Massachusetts,
Nevada, New Jersey, and New Mexico, have
passed legislation or are seriously considering
legislation that would require the construction
of net zero-energy buildings. The
use of thin-film, building-integrated PV systems
will play an important part of any zero
energy building initiative.
What Are PVs?
PV roof systems take advantage of a
renewable energy source for converting
sunlight into electricity. The generation of
electricity from PV technology is possible
through the interaction of sunlight with certain
“doped” semiconductor materials.
Electrons are released from these materials,
resulting in a current. That direct current is
then converted to an alternating current
with an inverter and provides electricity to
power the building. The most prevalent
material used in the production of PV
arrays is silicon. The basic building block of
PV technology is the solar “cell.”1
There are two primary types of cells
within silicon-based PV systems: crystalline
(mono and poly) and amorphous.
Crystalline silicon PV systems currently
represent 80% of the market. They typically
use 20 kg of silicon per 1 kW of PV. A sunlight-
to-electricity conversion efficiency of
15-20% is typical.2 However, the high cost
(energy and material) to refine and purify
crystalline silicon and the expensive and
The DOE Low-Slope Cool Roof Calculator6 can be used to evaluate the impact of the “darker,”
thin-film amorphous silicon PV systems on the heat gain into a building. The calculator allows
one to compare the cooling energy and cooling loads of a building with a roof of interest to that
of a building with a black roof as the reference in any location.
To perform the calculations, the SR and thermal emittance of the roof surface, an R-value of
insulation, and the COP of an air conditioning unit are needed for any location selected. Newly
built structures would comply with the 2006 IECC code, which specifies higher R-values of roof
insulation entirely above deck, and higher efficiency of new air conditioning units. Since the radiant
properties of a roof can change over time, it is realistic to use aged values of SR and thermal
emittance when performing these calculations.
All calculations of the cooling load are based on a comparison against a black roof. Once the
calculation is made for a white reflective roof and a darker (thin-film, PV-covered) roof, the differences
between the two roofs can be determined.7
The calculations are based on the assumption that the roof properties pertain to 100% of
the roof surface area. However, in reality, a roof with laminated thin-film PV modules is never fully
covered. For example, the size of an individual UNI-SOLAR® panel made by United Solar Ovonics
is 18 ft long x 15.5 in wide, each rated at 136 Watts. If we use a 100,000 ft2 roof, measuring 80 ft
x 1250 ft, it would allow for 937 rows of PV panels laminated within the 16-in width of a standing
seam metal roof pan. Four panels would run from the eave to ridge and down again to the
other eave (72 ft in total length). With that layout, a total of 3,748 panels would be installed, each
23.25 ft2 in area, and generating 510 kW. (3,748 panels x 136 watts/panel). That would yield a total
PV surface area of 87,141 ft2, compared to the total roof surface area of 100,000 ft2 or an 87%
coverage factor.
The calculation must be modified to take into account the fact that the thin-film PV cooling
load applies to only 87% of the roof surface, and the cool white roof’s effect applies to the
remaining 13% of the surface. The result is an effective cooling load of the PV roof.
To determine the extra cooling load that the thin-film, PV-laminated roof creates as compared
to a white cool roof, we must subtract the effective cooling load of the PV-covered roof from the
scenario of th white roof that would cover 100% of the surface. In essence, this value becomes
the cooling load penalty attributed to the thin-film, PV-laminated product on the white roof.
The actual energy yields (kWh – AC) of PV systems cannot be determined strictly on the
nominal (kW – DC) rated power of a module. In addition, under outdoor conditions, the irradiance
and ambient temperatures are constantly changing.5 Even with the best solar-power systems
modeling, utilizing location-specific (historical), 30-year NREL climate data, some variation in predicted
output will occur.
To determine the energy yield from a PV system, a calculator developed by the National
Renewable Energy Laboratory is available for calculating the energy produced by a PV system in
any location on a monthly basis.8 The input parameters include the DC rating of the PV unit, the
DC-to-AC derate factor, the type of array, the array tilt, and the array azimuth. Using Version 1 of
this calculator provides estimates of monthly and annual energy generated by a thin-film PV system
in select cities. To calculate the actual energy generated by the PV module, an assumed roof
size of 100,000 ft2 is used. With the assumptions and values used for the scenarios, the Version 1
calculator yields the annual energy being generated by a PV system in that location.
J U LY 2009 I N T E R FA C E • 7
How is the
PV ENERGY OUTPUT vs. COOLING LOAD
Calculated?
complex process to turn silicon wafers into PV cells pose
serious problems to be cost competitive with various thinfilm
chemistries, which use far less material and energy to
create. On top of that, thin-film PV system technology is
advancing to the point where the peak-Watts power rating is
approaching that of crystalline silicon systems.
Conventional crystalline silicon PV cells are connected
to form a PV module, and many modules are linked together
to form a PV array. The modules consist of an assembly
of silicon wafers sandwiched between two layers of glass.
These panels are relatively heavy but can be mounted to
metal roofing with a special fastening device that does not
penetrate the roof surface. A typical 4-in silicon solar cell
can produce about one watt of direct current electricity.1
Alternatives to crystalline silicon PV modules are thinfilm
amorphous silicon PV laminates. These flexible PV laminates
are typically 0.12-in thick and are flexible because
they are deposited onto a coiled metal foil. Specifically,
amorphous silicon products are produced by depositing
films of doped silicon-germanium alloys to a thin sheet of
stainless steel and then encapsulating them with a strong
and flexible but highly light-transmissive polymer top layer.
The PV “sheets” of material are then laminated to the flat
pan surface of a standing-seam metal roof panel.
In general, thin-film, amorphous, silicon-laminated PVs
reflect about 26% of incoming solar energy (i.e., SR = 0.26).
Only about 6.5% of the total solar energy that strikes the
surface is converted into electricity. Since the converted
energy is not absorbed, it can be considered part of an
“effective solar reflectance” of 32.5% (SRe =
0.325) In other words, from a thermal perspective,
a thin-film PV system is similar to
a cool roof surface with SR of approxi mately
0.30. That level of SR can be achieved with
many commercially available cool paint systems
used on metal roofing surfaces.3
Questions Remain
Building owners recognize that a white
reflective roof can significantly reduce the
cooling load placed on a commercial building
by reducing the solar heat gain.
However, if a thin-film PV system, which is
typically darker in color, is installed on
such a roof, the question arises, “Will the
building suffer a serious penalty in cooling
load, even though electrical energy is being
generated by the thin-film PV material
itself?”
When a thin-film amorphous silicon PV
system is installed on such a roof, 85% or
more of the roof surface may be covered
with a product that can have a lower SR
than the roof surface. A lower reflectance
value suggests that it will cause a higher
solar heat gain and create a “penalty” in the
cooling load of an otherwise cooler roof.
However, one must consider the fact that
any penalty due to just the difference in SR
may be offset by the PV’s conversion of solar
RCI Foundation Mission
To support research, education, and the dissemination of
information for issues important to the industry.
800-828-1902 www.rcifoundation.org
8 • IN T E R FA C E J U LY 2009
For a 100,000 ft2 low-slope roof
Assuming the PV’s DC rating is 5.1 kW/1000 ft2
With the energy being generated by the PV unit now known for the 100,000 ft2 assumed roof size, the total cooling load
for that same size roof must be calculated. Once that total cooling load is determined, it becomes simple to determine
what percentage of the total energy from the PV unit must be used to condition the added cooling load energy (the
penalty). This value can be expressed as a percentage of the total energy generated by the PV system.
SOLAR RADIATION AND AC ENERGY
Houston, TX
Month Solar Radiation AC Energy
(kWh/m2/day) (kWh)
January 3.11 35,824
February 3.70 38,203
March 4.56 51,687
April 5.06 54,524
May 5.62 61,218
June 6.06 62,802
July 5.86 62,140
August 5.62 60,138
September 5.18 54,364
October 4.66 51,013
November 3.65 39,421
December 2.79 31,704
YEAR 4.66 603,038
energy into electricity that can be used to
condition the added heat gain.
What Affects the Power Generation of PV
Systems?
The actual net power balance generated
by an installed PV system is affected by the
overall integrity of the roof, the size and efficiency
of the PV system, the local climate,
and the wind conditions.
Crystalline PV cells typically have a
higher peak-Watts power rating at room
temperature when compared to thin-film PV
technologies. This may lead one to assume
that the crystalline silicon will yield more
power than thin-film PV. However, thin-film
(amorphous silicon) PV cells offer outstanding
power generation characteristics at high
temperatures in comparison to crystalline
PV cells, which lose power production twice
as fast per degree of temperature.
Amorphous silicon layers in a multijunction
cell are doped to absorb red, green,
or blue light and are layered accordingly in
the cell. As a result, the inclination angle
has a significantly smaller effect on the generated
output of thin-film PV cells than that
of crystalline silicon PV panels. As a result,
thin-film PV can generate more power over
more hours per day, resulting in higher
power output per annum than crystalline
PV modules of identically rated peak output.
4,5
Compared to crystalline PV systems,
multijunction, thin-film, amorphous silicon
PV cells collect sunlight more efficiently
during low-light or diffuse conditions in
which light intensity is too low to activate
crystalline PV conduction. In the morning
and late afternoon hours, diffuse light can
dominate the available solar irradiance.
During cloudy conditions, diffuse light is
also the main form of irradiance. In some
northern climates, the majority of the solar
irradiance is from diffuse lighting. Since
thin-film PV systems produce energy under
lower light levels than crystalline silicon
can, and because they are efficient for
greater amounts of time under a wider
available spectrum of light, they generate
more power per installed peak-Watt (DC).
How Much Excess Electrical Energy Can a PV
System Generate After Additional Cooling?
With the Department of Energy’s Low-
Slope Roof Calculator and the PV Watts calculator9
from the National Renewable
Energy Laboratory, a cooling load penalty
and PV energy can be calculated for different
cities across the nation. These calculations
show that there is in fact an added
cooling load when a dark, thin-film PV system
is laminated to an otherwise cool reflective
roof surface.
Using the process described in the sidebar
on page 7, calculations for thin-film PV
systems installed in various locations and
different climate zones are summarized in
the table above.8 The results show that in all
of the practical examples for new construction,
less than 2.5% of the energy generated
by the thin-film PV modules was needed to
compensate for the added cooling load. The
high R-value insulation required by code
and the cool roof surface covering 13% of
the roof surface area help to minimize the
heat gain from the darker, thin-film PV surface.
The high COP of 3.0 for new air conditioning
units also helps to significantly
reduce the cooling energy load of new buildings.
This means a typical net gain of 97-
98% of the PV-produced electricity, for modern
construction, after considering the
additional cooling load.
The energy generated by the PV system
is more than enough to provide the electricity
to offset the added cooling load from the
For 100,000 ft2 roof surface
2006 IECC Extra Annual Annual PV % of PV Energy
ASHRAE Above-Deck Cooling Load from Energy Generated Used to Compensate for
City Climate Zone Insulation Thin-Film PV (kWh) (kWh) Cooling Load Penalty
Miami 1 R-15 16,600 664,716 2.5%
Houston 2 R-15 13,100 603,038 2.2%
Phoenix 2 R-15 18,000 775,105 2.3%
Charleston 3 R-15 11,500 644,200 1.8%
Los Angeles 3 R-15 4,300 709,351 0.6%
San Francisco 3 R-15 700 693,585 0.1%
St. Louis 4 R-15 9,000 604,301 1.5%
Chicago 5 R-20 3,800 564,717 0.7%
Minneapolis 6 R-20 3,400 587,153 0.6%
www.rci-online.org
10 • I N T E R FA C E J U LY 2009
SUMMARY OF CALCULATIONS FORTHIN-FILM PV COOLING LOAD VS. COOL ROOF LOADS
lower SR of the PV module, compared to a
cool white surface and the resulting higher
cooling load. This suggests that a buildingintegrated,
thin-film PV system can generate
a net positive flow of electricity to power
air conditioners and other energy loads in
new commercial low-sloped roofed buildings.
Taking this to the extreme or worst-case
scenario and using the procedure described
above, a very low insulated building with R-
5 and a low COP air conditioner located in
an intense solar radiance location (Phoenix)
suggested that about 20% of the energy
generated by the thin-film PV modules
would be required to compensate for the
added cooling load from the penalty of the
dark surface of the PV product.
Note that the calculations that were performed
in this study focused only on the
annual cooling loads determined by the
DOE Low-Slope Roof Calculator. In colder
climates, the darker surface of the thin-film
laminates may be beneficial in lowering the
overall annual combined cooling/heating
energy savings.
CONCLUSIONS
For newly constructed buildings, less
than 2.5% of the energy generated by thinfilm
PV modules is needed to compensate
for the added cooling load caused by the
darker PV product’s surface. Even in the
worst-case scenario representing an older
building with a retrofit roof, less than 20%
of the energy generated by the thin-film PV
modules would be needed to offset the
added cooling load.
The level of roof insulation has a significant
impact on the effective roof cooling
load and the cooling load penalty from the
thin-film PV system. Other variables, such
as wind speed and direction and solar irradiance,
can complicate the evaluation of the
PV energy needed to offset the higher cooling
load.
The effective SR and thermal emittance
values of modern thin-film PV modules are
similar to those of other steep-slope cool
metal roof surfaces. Installing a thin-film
PV module on a cool metal roof is prudent
to minimize the heat gain from those areas
of the roof that are not covered with PV
modules.
Test your knowledge of building envelope
consulting with the follow ing ques tions devel –
oped by Donald E. Bush, Sr., RRC, FRCI, PE,
chairman of RCI’s RRC Examination Develop –
ment Subcommittee.
1. What components of a roof
make up its dead load?
2. What is a roof’s live load?
3. What is the minimum
number of roof drains or
scuppers required per
roof?
4. When using drains with a
diameter of less than 6 in
or scuppers less than 8-in
width, what is the mini –
mum number of drains or
scuppers required per
10,000 sq ft?
5. When using roof drains
that are 6 in or greater in
diameter, what would be
the required minimum
number of drains on a
30,000-sq-ft roof?
6. What should be the
minimum rainfall intensity
used to determine roof
drainage design
provisions?
Answers on page 12
􀁳􀀀􀀷􀁁􀁔􀁅􀁒􀁔􀁉􀁇􀁈􀁔􀀀􀁒􀁏􀁏􀁆􀀀􀁁􀁔􀁔􀁁􀁃􀁈􀁍􀁅􀁎􀁔􀁓􀀌􀀀􀁏􀁖􀁅􀁒􀀀􀀒􀀑􀀌􀀐􀀐􀀐􀀀
􀁉􀁎􀁓􀁔􀁁􀁌􀁌􀁅􀁄􀀀􀁎􀁁􀁔􀁉􀁏􀁎􀁗􀁉􀁄􀁅􀀀􀁗􀁉􀁔􀁈􀁏􀁕􀁔􀀀􀁁􀀀􀁓􀁉􀁎􀁇􀁌􀁅􀀀􀁌􀁅􀁁􀁋
􀁳􀀀􀀰􀁒􀁏􀁊􀁅􀁃􀁔􀀀􀁓􀁐􀁅􀁃􀁉􀁬􀀀􀁃􀀀􀁅􀁎􀁇􀁉􀁎􀁅􀁅􀁒􀁉􀁎􀁇􀀀􀁉􀁎􀁃􀁌􀁕􀁄􀁅􀁄
􀁳􀀀􀀭􀁏􀁄􀁕􀁌􀁁􀁒􀀌􀀀􀁂􀁏􀁌􀁔􀀍􀁔􀁏􀁇􀁅􀁔􀁈􀁅􀁒􀀀􀁄􀁅􀁓􀁉􀁇􀁎 􀀣􀁁􀁌􀁌􀀀􀁕􀁓􀀀􀁔􀁏􀁌􀁌􀀀􀁆􀁒􀁅􀁅􀀀􀁁􀁔
􀀘􀀖􀀖􀀍􀀲􀀯􀀯􀀦􀀳􀀣􀀲􀀥􀀥􀀮
􀀘 􀀖 􀀖 􀀍 􀀗 􀀖 􀀖 􀀍 􀀓 􀀗 􀀒 􀀗
􀁒􀁏􀁏􀁆􀁓􀁃􀁒􀁅􀁅􀁎􀀎􀁃􀁏􀁍
When you really need your
equipment covered,
call RoofScreen!
J U LY 2009 I N T E R FA C E • 1 1
REFERENCES
1. I. Melody, Photovoltaics: A Question
and Answer Primer, Florida Solar
Energy Center, Publication Number
FSEC-EN-11-83, February 1985.
2. T. Parker and G. Moine, “Amorphous
Silicon and Crystalline Modules:
Similarities and Differences,”
Powerpoint information from UNISOLAR.
3. W.A. Miller, A.O. Desjarlais, S.
Kriner, “The Thermal Performance of
Painted and Unpainted Standing
Seam Metal Roof Systems Exposed
to Two Years of Weathering,” presented
at Thermal Performance of
the Exterior Envelopes of Whole
Buildings VIII, Clearwater, FL,
December 2001.
4. Mitsubishi Heavy Industries, Ltd.,
“Photovoltaic Power Generation
Utilizing Renewable Energy
Available in Unlimited Supply,”
www.mhi.co.jp/env/csr/csr04_e
.html.
5. J.A. Eikelboom and M.J. Jansen,
Characterisation of PV Modules of
New Generations, ECN-C-00-067,
June 2000.
6. Department of Energy, Cool Roof
Calculator, www.ornl.gov/sci/roofs+
walls/facts/CoolCalcEnergy
.htm.
7. M. Thimons and S. Kriner, “Thin-
Film Photovoltaics and Their Impact
on a Commercial Building’s Cooling
Load,” Architectural West, January/
February 2009, p. 18-23.
8. National Renewable Energy
Laboratory, PV Watts Calculator,
www.pvwatts.org.
Scott Kriner is president of Green Metal Consulting, Inc. and
serves the Metal Construction Association (MCA) as technical
director. He is also a consultant for manufacturers and suppliers
of metal roofing and wall systems. Scott is technical
editor for EcoStructure magazine and is an accredited LEED
Professional. Prior to establishing his consulting firm, he
served as technical marketing manager of building products
for Akzo-Nobel Coatings, Inc. He started his career with
Bethlehem Steel in the coated steel research department at
Homer Laboratories. Kriner has more than 27 years’ experience in the domestic and
international metal and coatings industry and has held numerous positions of responsibility
on the board of directors of the NCCA and MCA, was chairman of the Cool Roof
Ratings Council’s Weathering Farm Task Group, and chairman of the Zinc and
Aluminum Coaters Association. He is the founding chairman of the Cool Metal Roofing
Coalition, which was formed in 2002. Scott also serves on the Advocacy Committee of
the Lehigh Valley Branch of the Delaware Valley Green Building Council. Scott has
bachelor’s and master’s degrees in metallurgy and materials engineering from Lehigh
University. He holds a patent of improvement on 55% Al-Zn alloy-coated steel.
Scott Kriner
Answers to questions from page 11:
1. Permanent or fixed
components, including
supporting members, deck,
insulation, roof covering,
gravel, and suspended or
supported ceilings or
equipment.
2. The weight allowance for
temporary or movable
loads, such as rain, snow,
construction materials,
equipment, and workers.
3. Two.
4. One drain per 10,000 sq ft
of roof area.
5. Two drains: one per 15,000
sq ft of roof area.
6. A rainfall intensity of at
least a one-hour event
with a 100-year mean
recurrence interval (MRI).
Rainfall intensity is
expressed in inches or
millimeters per hour.
REFERENCE:
FM Global Loss Prevention
Data Sheet 1-54
12 • I N T E R FA C E J U LY 2009
Several errors were made in reporting the speech of President David Hawn at the
Annual Meeting of the Members, published in the May/June 2009 issue of
Interface. Hawn actually said he was fortunate in the “marriage department,” not
the managing department, when he married Carol, who has no connection or
involvement in his business, a solely owned, limited liability company (LLC). Also,
he worked at Iowa State University from 1982 to 1985; at Professional Service
Industries from 1985-1995; ATEC/ATC from 1995-1997; and started Dedicated
Roof and Hydro-Solutions in 1997. The stint during which he “filled up 2½ passports”
occurred during his years (1998 – 2000) as a personal service contractor to
the U.S. Department of State. We regret the errors.
Correction