Conference Paper

NREL/CP-520-45948

June 2009

Performance Comparison of a

BIPV Roofing Tile System in 

Two Mounting Configurations 

Preprint 

M.T. Muller, J. Rodriguez, and B. Marion

To be presented at the 34th IEEE Photovoltaic Specialists Conference

Philadelphia, Pennsylvania

June 7–12, 2009
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PERFORMANCE COMPARISON OF A BIPV ROOFING TILE SYSTEM IN TWO 

MOUNTING CONFIGURATIONS

Matthew Muller, Jose Rodriguez, Bill Marion

National Renewable Energy Laboratory (NREL), 1617 Cole Boulevard, Golden CO 80401, USA

ABSTRACT

This paper examines the performance of a 

building integrated photovoltaic (BIPV) roofing system 

commonly available to residential markets. In particular 

polycrystalline Si PV roofing tiles were integrated with 

concrete roofing tiles in two mounting configurations 

being used by roofing contractors. In the first 

configuration the tiles were directly mounted to the roof

sheeting allowing little to no airflow under the PV 

modules. In the second configuration furring strips were

attached to the roof deck to create a counter-batten 

system to which the roofing and PV tiles mount. This 

counter-batten system provides an air gap between the 

roof deck and the PV/concrete tiles which allows for 

convective cooling. A complete data acquisition system 

was applied to both mounting configurations and they 

were monitored for a summer period in Golden, 

Colorado. A performance comparison is presented for 

the systems while both are gauged against freestanding rack-mounted polycrystalline Si PV modules. 

As expected, modules mounted directly to the deck 

operated at higher temperatures and produced less 

power than those on a counter-batten system while both 

systems operated at higher temperatures than rackmounted modules.

INTRODUCTION

As the photovoltaic industry has grown 

exponentially in recent years, various companies have 

successfully created separate product lines to serve as 

building integrated photovoltaics (BIPV). The basic 

premise is to combine the power generation of PV within

common building materials such as roofing, glass, 

window shades, and other products. In attempts to 

provide an aesthetically pleasing roof, companies such 

as BP, GE, SunPower, Kyocera, Sharp, and Open 

Energy have all introduced PV tiles that integrate with 

standard concrete roofing tiles. Each of these brands 

has a slightly different appearance but each uses 

crystalline silicon PV cells built into modules which 

integrate with concrete roof tiles. 

It is well known that with crystalline silicon PV, 

both voltage and power have a negative relationship 

with temperature [1]. For this reason mounting PV too 

close to a hot roof reduces power output and therefore 

elevating an array off the roof is desired. The PVFORM 

model [2] indicates that modules mounted 6” off the roof 

deck have an installed nominal operating cell 

temperature (INOCT) that is 2°C greater than modules 

mounted on an free-standing rack. NJATC [1] suggests 

that roof mounted arrays should be elevated 3 to 6 

inches from the deck in order to optimize convective

cooling, power output, and wind loading. Although it is 

documented that a BIPV roof system will produce less 

power than a similar PV module in and elevated array, 

the literature is sparse in actually attaching numbers to 

available BIPV products.

This study will not only quantify performance 

for one of the aforementioned PV roofing tile products 

but also will compare installation techniques used in the 

concrete roofing industry. It is common to find concrete 

roof tiles installed in one of three methods. In the first 

and least expensive method, the tiles are directly 

screwed to roof sheeting, termed direct-mount. In the 

second method, equally spaced horizontal running 

battens are attached to the roof deck and then the tiles 

are attached to these battens. In the third method, 

(called counter-batten system) vertical battens are first 

attached to the roof deck to elevate the horizontal 

battens and therefore the tiles. Standards set by the 

tile roofing industry allow for different installation 

methods based on roof slope and climate. The counterbatten system is generally recommended as it provides 

air flow under the tiles and improved moisture drainage 

[3]. Testing from Oak Ridge National Laboratory 

documents additional benefits as the counter-batten 

system significantly reduces the home’s cooling load [4]. 

This report presents a side by side comparison 

of polycrystalline silicon (poly-Si) tiles mounted directly 

to the deck versus those elevated on counter-battens. 

Builders use direct-deck mounting of concrete tiles as a 

means to cut bottom-line cost while counter-battens are 

considered the best practice and can improve home 

energy efficiency. As neither of these mounting 

techniques is ideal for PV, a baseline comparison is 

made against a free-standing, rack-mounted poly-Si

module from NREL’s performance energy rating testbed 

(PERT). Although Open Energy’s 34 watt roofing tile is 

the PV module used in this study, the results may be 

applicable to PV roofing tiles by other manufacturers if 

the products have similar temperature coefficients and 

are installed using methods described in this study 

This report proceeds as follows: Methods of design and 

installation are presented, the data acquisition system 

(DAS) is documented, a performance comparison is 

developed for each mounting method, and conclusions 

are drawn.

DESIGN AND INSTALLATION

The design of a PV/concrete tile roof system 

began with constraints provided by available roof space 

at NREL’s outdoor test facility. It was desired to have a 

test bed that was similar to a typical residential 

installation in that the roof should face south and have 

slope somewhere between 3/12 and 7/12. To meet 

these requirements the shed shown in Fig. 1 was 

chosen.

1
Fig. 1. NREL shed/BIPV test bed

The roof shown in Fig. 1 has a slope of 4/12 and faces 

due south. The workable area of the BIPV system is 

approximately 338” by 102” and after mounting, the 

modules will be tilted at roughly 15 degrees. The 

module pitch is a few degrees less than that of the roof 

tilt as the bottom of the modules are raised to rest on the 

top of the modules below them. NREL is located at 

39.74 latitude and -105.18 longitude. Although latitude 

tilt is typically desired for maximizing year round energy 

production, a reduced tilt is well suited for the summer 

months in which testing was completed. 

Initial plans were to have the two systems side 

by side, grid tied, and running on independent inverters. 

Design calculations with the following specifications 

from the Open Energy modules presented problems 

with this approach.

34 W Solar Save Roof Tile

 Pmax 34 Watts

 Imp 7.09 Amps

 Vmp 4.8 Volts

 Isc 7.76 Amps

 Voc 6.07 Volts

The roof space on the shed limited the number of tiles 

per side to 24 while still having a minimal border of 

concrete tiles. Calculations accounting for high 

temperatures at NREL indicate that Vmp could 

realistically drop to 3.82 volts. A string of 24 modules in 

series operating at MPP would only reach 92 volts. As 

an inverter that can peak power track at such a low 

voltage was not readily available, plans were changed. 

The design was modified to have 17 modules in each 

mounting configuration but connect all 34 modules in 

series to one grid-tied inverter. At standard test 

conditions this means the system would output 1156 DC 

watts at around 7 amps and 163 volts. The PV Powered 

1100EVR inverter was chosen as it can peak power 

track as low as 115 DC volts and its AC power rating 

makes it a good fit for this application. 

The modules from each side of the roof were

run into a junction box as an individual string and then 

connected together as a single string at a terminal block. 

This allowed for easy measurement of the voltages from 

each system while both systems were forced to operate 

at the same current. Because current is relatively 

unaffected by temperature changes, this configuration

provided a means to compare performance of the two 

mounting schemes.

Fig. 1 shows the shed roof just after rake metal

and 1x2 counter-battens were installed on the right side. 

The battens and metal were installed directly on top of 

existing asphalt shingles. While this is acceptable 

practice by the concrete tile association, a new home 

would have an underlayment material or appropriate 

membrane installed over the decking before battens are 

installed. The rake metal serves several purposes. It 

channels water down and off the roof, creates an 

aesthetic trim, and provides an airflow barrier between 

the two installation methods. Fig. 2 shows the right side 

of the shed with the complete counter-batten system in 

place.

Fig. 2. Counter-batten system/first row of concrete tiles

Fig. 2 also shows the first row of concrete tiles hung on 

one of the horizontal battens. The counter-battens run 

from eave to ridge and provide an unobstructed flow 

path for convective cooling. The horizontal batten at the 

eave has been covered by L metal to protect it from 

weather but Fig. 3 illustrates that air can still freely pass 

underneath between battens and through slots on the 

bottom of the concrete tiles. A typical installation 

progresses up the roof with the PV tiles being installed 

no different than concrete tile, with the exception that 

leads from each module must be plugged together as 

the installation progresses. 

Fig. 3. Air path to the bottom side of the tiles

2
Fig. 4 presents a view of modules integrated with 

concrete tiles. Module leads were connected as each 

tile is placed while the terminating leads come out at the 

eave and are maintained in an open circuit condition. 

Fig. 4. Module integration and wiring

The completed installation on counter-battens is shown 

in Fig. 5. A thermocouple has been attached to the 

back side of two PV modules, one on the row closest to 

the eave and one on the full row closest to the ridge.

Fig. 5 Complete counter-batten installation

The counter-batten system allows air to enter at the 

eave and then exhausts at the gaps between the ridge 

tiles and the field tiles. The field tiles also promote 

breathing where they overlap and interlock as there are 

no air tight seals in concrete roofs. Underneath the 

ridge tiles a breathable vent tape is used to keep water 

out but let air through, as seen in Fig. 6.

Fig. 6. Vent tape applied under ridge tile

The installation of the direct-mount system on 

the left side of the roof precedes much the same as the 

right but without battens. Rather than hanging the tiles 

on battens, horizontal chalk lines are snapped as a 

guide for tile installation. A metal riser (L-shaped piece 

of sheet metal) is placed at the edge of the eave to lift 

up the bottom end of the first course of tiles. From here 

the installation proceeds exactly as before, integrating 

PV wiring and 2 thermocouples along the way. Ridge 

tape is applied at the top but this time the tape is solid 

and allows no breathing. Fig. 7 displays the completed 

side by side system.

Fig. 7. Completed side by side BIPV system

DATA ACQUISITION SYSTEM

A Campbell Scientific CR10X data logger in 

conjunction with Campbell Scientific AM416 multiplexer 

was used to read low-voltage signals every 5 seconds 

and record 15-minute average data. A Daystar DS-50 IV curve tracer was used to produce an I-V curve for 

each mounting configuration under field conditions. A 

Kipp and Zonen model CM11 pyranometer was used to 

measure plane-of-array (POA) irradiance, as seen 

mounted in Fig. 7. Ambient temperature was measured 

using a T-type thermocouple housed in a radiation 

shield. T-type thermocouples were also used to 

measure the temperature of the 4 previously mentioned 

modules, the inverter heat sink, and the data logger. A 

K-type thermocouple probe was used to monitor the 

shed’s attic temperature. Electrical data was measured 

using the Ohio Semitronic (O.H.) transducers in Table 1.

Table 1 DAS Transducers

Measurement Device Range

AC current O.H. CT8-016B 0-15 A

AC voltage O.H. VT-120A 0-150 V

DC current O.H. CT7-017B 0-20 A

DC voltage O.H. VT7-005B 0-150 V

All O.H. transducers were converted to low mV signals 

connecting 2.49 Kohm, +/-1%, resistors across the 

measurement terminals. Local wind speed data was 

measured separately by NREL’s Reference 

Meteorological and Irradiance System (RMIS). 

Transducers were bench calibrated and then the entire 

data acquisition system was assembled and calibrated 

3
as a complete unit using a Keithly 2000 multimeter, an 

HP3478A multimeter, and a Wavetek model 9100

calibrator. The data set examined for this report runs 

from 7/12/08 through 8/31/08 with the exception of a 

shut down on July 22nd and loss of grid power from 6-

8:45 AM on August 22nd. 

PERFORMANCE COMPARISON

As temperature is the primary driver between 

performance differences, the analysis begins here. As 

mentioned previously, two module temperatures were 

measured on each side of the roof. For the counterbatten installation, the eave module was an average of 

2.7  ̊C cooler than the module nearest to the ridge. For 

the direct-deck system, the eave module was only an 

average of 1  ̊C cooler than the module close to the 

ridge. The larger temperature gradient for the counterbatten system is consistent with the convective flow path 

under the modules. Cool air enters at the eave and is 

gradually warmed before exiting at the ridge. All 

performance comparisons and plots that follow report 

average temperature by installation method. In this 

case the eave and ridge module temperatures are 

averaged for the counter-batten and for the direct-mount 

systems. Fig. 9 plots the counter-batten system’s rise of 

ambient temperature against the rise above ambient 

temperature for the direct-mount system. Each point is 

a 15 minute average and all irradiances greater than 

zero have been plotted. Fig. 8 confirms that the 

counter-batten installation method consistently results in 

lower module temperatures but illustrates that there is

significant variation in this temperature difference. 

Fig. 8. Module temperature rise above ambient

Factors such as irradiance, time of day, wind speed, and 

attic temperature, can all interact and lead to variations 

in module temperature. Fig. 9 provides some insight 

into these variations over the course of a sunny day. 

The direct-mount and counter-batten modules are both 

several degrees below ambient temperature early in the 

morning, then peak at 43  ̊C and 33  ̊C above ambient 

respectively around 12:30 pm, and finally fall back to 

ambient at about 6:30. This rise and fall of module 

temperature obviously follows the rise and fall of 

irradiance but other subtle points can be made from this 

graph. First, ambient temperature is relatively flat as 

compared to the swing of the module temperatures. 

Although changes in ambient temperatures can cause a 

shift in module temperatures, irradiance is the stronger 

driver of module temperature. 

Fig. 9. Irradiance & Temperatures for Aug 28th

Second, over-night cooling of the modules and the attic 

space effectively nullifies any difference between the 

systems in the first few hours of the morning. Finally, 

the two installations do not show diverging temperatures 

until the attic temperature exceeds ambient. In the 

afternoon the high attic temperature appears to prolong 

the temperature difference between systems until the 

irradiance approaches zero. While this makes 

thermodynamic sense, it emphasizes the benefit of attic 

ventilation and suggests an area for further study. The 

research by Oak Ridge National Laboratory [4] 

documented that using a counter-batten installation 

method significantly reduced heat penetration into the 

attic space and therefore resulted in lower attic 

temperatures. As attic temperatures have some impact 

on module temperatures, an ideal study would have 

isolated attic space for each installation method. This 

study effectively has no battens on the majority of the

roof and therefore the counter-batten modules may have 

operated at slightly higher temperatures than an isolated 

installation. 

In an effort to better understand how various 

measured factors relate to module temperature, the data 

set was analyzed for correlations. Although wind speed 

and ambient temperature both impact module 

temperature, the recorded data set provided no clear 

correlation between these variables and (Tpv-Tamb). 

On the other hand weak to modest correlation was 

found between attic temperature and (Tpv-Tamb), 

(R2

=0.4 counter-battens, R2=0.5 direct-mount). The 

reduced correlation for the counter-batten system 

makes intuitive sense as the air gap should provide 

some isolation from attic conditions. Fig. 10 provides a 

visual of the relationship between the attic temperature

and (Tpv-Tamb) for the counter-batten system.

y = 1.2462x + 0.8058

R2 = 0.9897

0

5

10

15

20

25

30

35

40

45

50

0 10 20 30 40

(Tpv-Tamb) direct-deck mount (C)

(Tpv-Tamb) counter-battens (C)

0

10

20

30

40

50

60

70

80

90

100

300 400 500 600 700 800 900 1000 1100

Minutes from midnight

POA/10 

(W/m2)

Tpv(C) directmount

Tpv (C) 

counter-batten

T attic (C)

Tambient (C)

4
Fig. 10. Attic temp vs. (Tpv-Tamb) counter-battens

As expected there is a strong correlation, R2

=0.8 for 

both systems, between POA and (Tpv-Tamb) as seen in 

Fig. 11. The wide dispersion of data is not surprising as 

conditions readily occur that cause (Tpv-Tamb) to vary. 

Fig. 9 already demonstrated how equal irradiances from 

the morning and afternoon result in different module 

temperatures. Fig. 9 and 10 both support that attic 

temperature also impacts module temperature. Finally, 

many days will have rapidly changing irradiance as 

clouds come and go. In these events, the data will

disperse as the temperature transient is often much 

slower than the irradiance transient. 

Fig. 11. Temperature rise above ambient vs. POA

Because of the strong correlation between 

module temperature and POA, it is common practice to 

calculate the irradiance-weighted temperature and the 

temperature-rise coefficient given by Equation 1 and 2.

T POAEwhere

E

ET

weighted pv = = ∑

∑ ⋅ Eq(1)

( )

∑

∑ − = E

TT

rise coeff

ambpv T _ Eq(2)

The irradiance-weighted temperature parameters are 

presented in Table 2 for the entire data set as well as 

just the month of August. The August calculation was 

made to facilitate easy comparison to NREL PERT data 

which is broken into month blocks.

Table 2 Irradiance-weighted temperatures

August July12-Aug31

Tweighted(counter-batten) 48.16  ̊C 49.34  ̊C

Tweighted (direct-mount) 53.9 ̊C 55.37  ̊C

Trise_coeff (PERT rack) 21.3 ̊C/kW/m2

Trise_coeff(counter-batten) 28.7 ̊C/kW/m2 27.7 ̊C/kW/m2

Trise_coeff (direct-mount) 37.1 ̊C/kW/m2 36.3 ̊C/kW/m2

As expected, rack-mounted modules will operate at the 

lowest temperatures, followed by the counter-batten 

system, and then direct-deck mounted modules. In 

other words, at noon on a sunny day it would be 

expected that modules on counter-battens would 

operate roughly 8-9  ̊C cooler than with no battens and 7 

 ̊C hotter than a rack-mount system. 

If the temperature-rise coefficient for August is 

considered a proxy for INOCT, the temperature 

differences between counter-batten mounting and rack 

mounting, and direct mounting and rack mounting were 

observed to be 6.6 C and 5.2 C less than expected by

PVFORMs prediction for a 1 inch standoff and directmount respectively [2]. This may be a consequence of 

direct-mounted tiles having their lower edge raised, 

which permits air flow from the sides. Likewise, the 

counter-batten installation may have more effective air 

flow than the one inch standoff height would indicate. 

Additionally, the temperature-rise coefficients for the 

roof-mounted tiles may be lower than PVFORM would 

indicate due to sub-ambient attic temperature seen in 

the first couple hours of the morning. The thermal 

model used by PVFORM makes the assumption that

surfaces behind the modules (typically the ground or 

roof) will always be in a range that is bounded by the 

ambient and module temperatures [5]. Sub-ambient 

attic temperatures violate this assumption and help keep 

modules cooler than would be otherwise expected.

Although it is important to quantify each 

temperature-rise coefficient, it may be appropriate for

the consumer, to quantify system differences in terms of 

power produced. As a precursor to comparing power 

measurements, it is first necessary to compare baseline 

measurements for each string. Prior to installation on 

the shed roof, all modules were tagged and 

characterized indoors using a SPIRE 240A. By Spire 

measurements at STC, the modules connected to form 

the counter-batten string should have a VMP of 81.696 

volts while the direct-mount string’s VMP should be 

81.475 volts. If both strings operate at the lowest IMP of 

all the modules this would result in a 0.27% difference in 

power between the strings. As a secondary check that 

the strings were well matched in voltage, the outdoor 

data were filtered for occurrences when the POA was 

between 920-1150 W/m2 and module temperatures 

were within +/- 1.1  ̊C of each other. This data resulted 

in a VMP of 66.15 volts for the counter-batten string and 

66.1 volts for the direct-mount string, only a 0.07% 

difference. With the voltage difference being very small 

for both of these baselines, it is fair to conclude that 

-20

-10

0

10

20

30

40

0 10 20 30 40 50 60

attic temperature (C)

Tpv-Tamb (C) -10

0

10

20

30

40

50

0 200 400 600 800 1000 1200

POA W/m2

Tpv - Tamb 

counter battens

direct mount

5
deviations in power will be the result of thermal 

conditions resulting from mounting configurations.

The previous thermal results made it clear that 

POA is the main variable driving a temperature delta 

between each string. In conjunction with this 

information, it is useful to compare average system 

power for data with POA > 100 W/m2 against data with 

POA > 900 W/m2

. POAs less than 100 W/m2 were 

dropped as little to no power is produced below this 

point and POAs above 900 W/m2 should show the 

greatest divergence between the two installations.

The results for these POA ranges are given in Table 3. 

Table 3. Average System Data

POA range

Averaged Data > 100 W/m2 > 900 W/m2

T_ambient ( ̊C) 27.5 28.6

POA (W/m2) 577.7 996.8

Wind (m/s) 2.07 2.49

T_pv_battens ( ̊C) 44.0 59.0

T_pv_direct ( ̊C) 48.9 67.4

V_mp_battens (V) 72.4 66.6

V_mp_direct (V) 70.7 63.5

DC_Watts_batten (W) 288.9 468.5

DC_Watts_direct (W) 279.5 446.7

In simple terms the counter-batten installation produced 

3.4% more DC watts for all irradiances greater than 100 

W/m2 and 4.9% more DC watts in the peak hours when 

irradiance was above 900 W/m2

. 

CONCLUSIONS

This work has examined the thermal and power 

characteristics of a BIPV roofing system using two 

installation techniques, counter-batten and direct-mount. 

Thermal data showed that in early morning and late 

evening the two systems operated near the same 

temperature. On the other hand, in mid-day, as the 

irradiance increased and the attic temperature exceeded 

ambient temperature the counter-batten system 

provided convective cooling to the modules that resulted 

in temperatures that were as much as 10  ̊C lower than 

the direct-mount system. Irradiance-weighted 

temperature coefficients were compared for both BIPV 

installations in comparison to PERT data for a rackmounted poly Si module. Results showed that the 

temperature-rise coefficient was 7.4  ̊C/kW/m 2 higher for 

the counter-batten modules as compared to the freestanding, rack-mounted module. The direct-mount 

modules were another 8.4  ̊C/kW/m 2 higher than the 

counter-batten modules. The temperature-rise 

coefficients were 5-6  ̊C less than expected based on 

minimum standoff height; perhaps a consequence of tile 

roofing systems providing better air flow.

As expected the thermal differences translated 

into a delta in power for each installation method. 

Overall the counter-batten system produced 3.4% more 

DC power than the direct-mount system. When only 

irradiances above 900 W/m2 were considered, the 

counter-batten system produced 4.9% more DC Watts.

This work provides a clear contrast between 

installation techniques for a BIPV concrete tile roof but 

was limited to a side by side system over a single attic 

space. Future work could benefit by testing installation 

methods over isolated attic spaces. This could also be 

combined with an analysis of the reduction in home 

cooling load that results with use of the counter batten 

installation method. Accounting for attic temperature 

might also lead to improved model estimates of PV 

module temperature.

ACKNOWLEDGEMENTS 

Appreciation is extended to Open Energy Corporation 

(renamed Applied Solar) and Eagle Roofing for 

supplying materials and expertise for completing this 

project. This work was supported by the U.S. 

Department of Energy under Contract No. DOE-AC36-

08GO28308 with the National Renewable Energy 

Laboratory.

REFERENCES

[1] Dunlop Jim, NJATC, Photovoltaic Systems. 

American Technical Publishers: Illinois, 2007

[2] Menicucci D. F., Fernandez J. P., User’s Manual for 

PVFORM: A Photovoltaic System Simulation Program 

For Stand-Alone and Grid-Interactive Applications, 

Sandia Report, 1988

[3] Tile Roofing Institute, Concrete and Clay Roof Tile 

Installation Manual for Moderate Climate Regions, ICCES ESR-2015P, July 2006

[4] Miller William, Steep-slop Assembly Testing of Clay 

and Concrete Tile with and without Cool Pigmented 

Colors, Oak Ridge National Laboratory, 2005

[5] Fuentes Martin, A Simplified Thermal Model for FlatPlate Photovoltaic Arrays, Sandia Report, 1987

 

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Performance Comparison of a BIPV Roofing Tile System in Two 

Mounting Configurations: Preprint

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13. SUPPLEMENTARY NOTES

14. ABSTRACT (Maximum 200 Words)

This paper examines the performance of a building-integrated photovoltaic (BIPV) roofing system commonly 

available to residential markets. In particular, polycrystalline Si PV roofing tiles were integrated with concrete roofing 

tiles in two mounting configurations being used by roofing contractors. In the first configuration, the tiles were directly 

mounted to the roof sheeting, allowing little to no airflow under the PV modules. In the second configuration, furring 

strips were attached to the roof deck to create a counter-batten system to which the roofing and PV tiles mount. This 

counter-batten system provides an air gap between the roof deck and the PV/concrete, tiles which allows for 

convective cooling. A complete data acquisition system was applied to both mounting configurations and they were 

monitored for a summer period in Golden, Colorado. A performance comparison is presented for the systems while 

both are gauged against free-standing, rack-mounted polycrystalline Si PV modules. As expected, modules mounted 

directly to the deck operated at higher temperatures and produced less power than those on a counter-batten 

system, while both systems operated at higher temperatures than rack-mounted modules.

15. SUBJECT TERMS

PV; building-integrated photovoltaic (BIPV); roofing tiles; polycrystalline silicon; modules; counter-batten system; 

temperature; convective cooling; roofing configuration

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