1. Ifte Choudhury & Ashwin Balabadhrapatruni
Use of Building Integrated Photovoltaic Roof Tiles in Residential Buildings
COBRA 2011: Salford, England
September 12-13

2. Introduction
BIPV is one of the most promising renewable energy technologies.
It allows buildings to generate all or part of their energy needs using photovoltaic (PV) panels that are integral part of the structure.
In BIPV systems, the PV array is part of the building’s roof, wall, or windows. A PV array directly converts solar radiation to electrical energy.
A residential PV system can be can be hooked up with utility grid, making it possible to export the excess energy to the utility company.
COBRA 2011: Salford, England
September 12-13

3. COBRA 2011: Salford, England
September 12-13

4. Introduction
Even though BIPV technology has been in existence for over a decade, cost issues have slowed down wide-spread acceptance and installation of the systems.
Cost-effectiveness of BIPV roof tiles, in comparison with asphalt roof shingles, for residential buildings has been analysed in this study.
The primary objective of the study was to find out the economic viability of the use of BIPV roof tiles for residential buildings in the United States.
COBRA 2011: Salford, England
September 12-13

5. Hypothesis Hypothesis 1 Hypothesis 2
It was hypothesized that net present value of the roof of a residential building using BIPV roof tiles is significantly different that of the roof of a residential building using asphalt shingles.
Hypothesis 2
It was hypothesized that net energy savings of a residential building using BIPV roof tiles is affected by the climatic location of a building, and number of heating and cooling degree days of the location.
COBRA 2011: Salford, England
September 12-13

6. Research Methodology Data Collection Procedure
Energy performance of BIPV roof tiles in different climatic locations of the United States was required to be ascertained for the study.
This was done through simulation by using Solar Advisor Model (SAM), also known as System Advisor Model, developed by the National Renewable Energy Laboratory.
COBRA 2011: Salford, England
September 12-13

7. COBRA 2011: Salford, England
September 12-13

8. Research Methodology
All data related to BIPV roof tiles was collected by using SAM.
The data included cost of BIPV roof tiles including their installation for all locations, operation and maintenance costs, cost of auxiliary devices such as inverters, and energy savings.
Cost of asphalt roof shingles for different locations in the United States was obtained from published sources.
The costs were adjusted for all different locations.
COBRA 2011: Salford, England
September 12-13

9. Research Methodology Location
Seventy locations were selected from the 5 different climatic zones of the United States, 35 each for buildings using BIPV roof tiles and asphalt roof shingles.
The climatic zones are:
Zone 1 (Cool),
Zone 2 (Temperate),
Zone 3 (Moderately temperate),
Zone 4 (Hot and arid), and
Zone 5 (Hot and humid).
COBRA 2011: Salford, England
September 12-13

10. Research Methodology Residential Prototype Building
A simple prototype residential building was designed by the authors for the study.
The roof area of the building was 1680 sq. ft.
Data on different variables was collected for the same building, assumed to be constructed in all the selected locations.
Data collection for buildings using BIPV roof tiles was done using SAM.
COBRA 2011: Salford, England
September 12-13

11. COBRA 2011: Salford, England
September 12-13

12. COBRA 2011: Salford, England
September 12-13

13. Research Methodology
Annual incident energy striking a roof surface is a function of solar altitude and azimuth angles.
SAM selected the part of the roof that would contribute to energy savings when BIPV roof tiles were installed.
Cost comparison was done based on only this part of the roof.
COBRA 2011: Salford, England
September 12-13

14. Research Methodology Variables Roof type (MATERIAL)
Net Present Value (NPV)
Location (LOCATION)
Energy savings (ENERGY)
Annual Cooling Degree Days (CDD)
Annual Heating Degree Days (HDD)
COBRA 2011: Salford, England
September 12-13

15. Analysis & Results Hypothesis 1
Hypothesis 1was tested using a General Linear Model available in SPSS statistical package.
The following model was used for the analysis
NPV = β0 + β1(MATERIAL) + β2(LOCATION) + e
COBRA 2011: Salford, England
September 12-13

16. Analysis & Results Variables Intercept Regression Coefficient t-value
p-value
-38.80
<0.0001
MATERIAL
ASPHALT
31.06
BIPV
LOCATION
ZONE 1
874.44
1.04
0.30
ZONE 2
543.38
0.64
0.52
ZONE 3
ZONE 4
-1.36
0.18
ZONE 5
-2.07
0.04
F =
p-value: <0.0001
Model R2 = 0.97
Adjusted R2 = 0.97
COBRA 2011: Salford, England
September 12-13

17. Analysis & Results Loc. Annual Energy 1 4000 8 4100 15 3900 22 4800 29
4700 2
4500 9
3800 16
3700 23 30
4200 3 10 17 24 31 4
3500 11 18 25
5200 32 5 12 19 26 33
4300 6 13
4900 20 27 34 7 14 21 28 35
Annual energy output (in kWh/kW peak rating) from BIPV roof tiles installed in prototype
COBRA 2011: Salford, England
September 12-13

18. Analysis & Results Loc. Annual ave. 1 939 8 1077 15 1010 22 1208 29
1199 2
1069 9
978 16
997 23
1062 30
1118 3
906 10
1190 17
911 24
1093 31
1146 4
890 11
1112 18
1040 25
1275 32
1102 5
1037 12
1049 19
1038 26
1164 33
1119 6 13
1216 20
972 27
1076 34
1226 7
1005 14
1203 21
1290 28
1101 35
1100
Average annual energy savings (in US $) for prototype residential building using BIPV roof tiles
COBRA 2011: Salford, England
September 12-13

19. Analysis & Results Hypothesis 2
The hypothesis was tested using a General Linear Model using SPSS statistical package. This test was done using the data only from 35 locations where BIPV roof tiles were used for the residential buildings.
The following model was used for the analysis
ENERGY = β0 + β1(LOCATION) + β2(CDD) + β3(HDD) + e
COBRA 2011: Salford, England
September 12-13

20. Analysis & Results Variables Intercept Regression Coefficient t-value
p-value
13.61
<0.0001
CDD
0.011
0.66
0.52
HDD
-0.05
-3.23
0.003
LOCATION
ZONE 1
-72.26
-1.56
0.13
ZONE 2
-3.87
0.001
ZONE 3
ZONE 4
-2.36
0.03
ZONE 5
-2.81
0.009
F = 6.39
p-value: <0.0001
Model R2 = 0.58
Adjusted R2 = 0.49
COBRA 2011: Salford, England
September 12-13

21. Conclusions
The findings of the study indicate the net present value of such buildings is significantly lower than that for buildings using asphalt roof shingles.
In other words, at current costs of materials and installation, BIPV systems are not economically attractive for use in residential buildings in the United States.
However, the results demonstrate that the use of BIPV roof tiles results in considerable saving in energy costs for the residential buildings.
The net energy cost savings are correlated with all but one climatic region in which a building is located and the annual heating degree days of that location.
COBRA 2011: Salford, England
September 12-13