IN SITU PERFORMANCE TESTING OF BIFACIAL PHOTOVOLTAIC PANELS

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1 IN SITU PERFORMANCE TESTING OF BIFACIAL PHOTOVOLTAIC PANELS Steven Anthony Sciara Brian W. Raichle Department of Technology and Environmental Design Department of Technology and Environmental Design Appalachian State University Appalachian State University Katherine Harper Hall Katherine Harper Hall Boone, NC Boone, NC ABSTRACT Bifacial photovoltaic (PV) panels offer potentially enhanced power output over conventional panels due to their reported ability to harvest reflected radiation increasing output up to an additional 30%; however, this enhancement has yet to be confirmed in the literature. The panels are comprised of a single crystalline layer enveloped by amorphous silicon thin film layers on both surfaces of the cell facilitating absorption from both upper and lower faces. Reflectivity and geometry of the backing surface, presumably, will determine panel output. Various reflecting and nonreflecting roofing surfaces with a range of array angles potentially moderate the degree of concentrated radiation the arrays will absorb. Additionally, purposeful provisions to the reflective roofing surfaces enhance the ability of the panels to perform to their maximum specifications. We report on a study comparing the power output of two nominally identical 700 W PV arrays utilizing equivalent system components and data logging equipment. This study was undertaken at the Appalachian State University Solar Research Laboratory, NC, which houses a Class 1 pyranometer and pyrheliometer. PV power will be reported under well-quantified irradiance conditions, including direct beam fraction. 1. INTRODUCTION The recent introduction of Sanyo HIT 195 Double Bifacial Photovoltaic Panels (PV) to the retail market suggests the need to investigate installation configurations in which they may be installed. The HIT Double Panels utilize the upper and lower faces of the module to generate electrical power. A power increase of up to 30% may be realized by properly installing the panels in locations where light may reach the lower face of the module (1). The additional power produced is determined by module orientation relative to reflecting surface, installation techniques, shadows produced by the racking system, weather, and reflective properties of the surface below the panels. Using eight Sanyo HIT 195 Double Panels, this study attempted to characterize power output by altering the reflectivity and configuration of the lower surface material in a non-traditional array and angle. As a result of this research, optimal materials may be identified for the lower reflective surface in canopy, as well as other installations. The use of a Unirac SolarMount rack system will suspend the panels above a pitched roof surface to permit an exchange of reflective materials below. 2. REVIEW OF LITERATURE The Sanyo HIT 195 Double Bifacial Photovoltaic Module permits the design of a system that produces higher power output per area by capturing solar radiation incident on both the top and bottom surfaces of the module. The panels may be installed at nearly any angle, but would be most effective in applications where nearby surfaces are reflecting. The panels are designed to allow a small percentage of light transmittance to assist with lower module surface absorption and to create an aesthetically pleasing detail for canopy installations. The area below the module remains partially illuminated by light transmittance through the clear glass (2), and the majority of transmitted light is diffused by the upper cells. 1

2 Traditionally, PV module performance is reported under Standard Test Conditions (STC), defined as an irradiance of 1,000 W/m 2 and a panel temperature of 25 C. STC does not prescribe nearby reflecting surfaces or their orientation. Bifacial panel manufacturers, therefore, report front-side performance under STC in the conventional way and additionally report a range of power enhancements that may be produced under certain conditions. For example, Sanyo rates the HIT 195 Double Module as 195 W, but indicates that with an additional 30% power produced by the lower surface, the panel may produce up to 249 W (2). Sanyo HIT 195 Bifacial Panels combine the use of single crystalline silicon (Si) with extremely thin amorphous silicon layers (a-si) that allow both the front and back side of the module to absorb light and produce electricity. Heterojunction with Intrinsic Thin Layer (HIT) Panels are reported to have a have high conversion efficiency, excellent temperature characteristics, and considerable output under diffuse and low light conditions (2). The HIT Double Module has many layers: a top electrode, p-type amorphous Si, intrinsic amorphous Si, crystalline Si (n-type), another layer of intrinsic amorphous Si, intrinsic amorphous Si, n-type amorphous Si, and a bottom electrode. This multi-layering effect allows light absorption from both sides. Compared with conventional solar cells, HIT solar cells have a better temperature coefficient and a higher-open circuit voltage (3). A hybrid solar thermal system using bifacial panels was tested using a transparent solar plane in the working spectral region of a PV module. It was determined that a bifacial PV module could be used for solar thermal and that the bifacial module produced approximately 40% more electrical energy than the conventional PV panels (4). In 2007, The European Photovoltaics Industrial Association, realizing that flat-panel crystalline silicon panels comprised 90% of photovoltaic devices produced, estimated that cell efficiency would need to increase from 12-16%, to 20% utilizing the back side of the module. This study, conducted with the use of silicon semiconductors, found that optimization of the lower portion of a bifacial module could produce an increase in efficiency potentially exceeding 21% (5). A recent study noted that the Sanyo HIT Double Module is capable of producing 10.9% more output than a single side HIT Module (6). In another experimental study, findings revealed that the increase in power conversion density that is achievable by using bifacial solar cells depends on the conversion efficiency of the cells under back illumination, which can be as high as 94 percent of the front efficiency, and on the amount of light that reaches the back surface (7). An experimental study done in Madrid, Spain in 1984 with bifacial photovoltaic panels found that they collected 59% more energy than monofacial panels when utilizing a white painted floor (8). Two case studies highlighted by Sanyo include Virginia Tech s Solar Decathlon Europe 2010 winning LUMENHAUS, and a solar canopy installation on an office building in Atlanta. What is not stated in either of the two case studies is what material was used below the panels (1). DuRock Alfacing International Manufacturing Company in Woodbridge, Ontario has mounted a 10 kw array of HIT 195 Double Panels at a 30 tilt angle on their flat roof. The reflective material used below the panels was TIOCOAT, a white roofing material. The bifacial panels produced on average 210 W, realizing a 7% increase (9). ISFH tested a white surface behind bifacial panels. They used the panels to shape the company acronym on the front of the building. A white background was placed behind the bifacial panels capable of reflecting light onto the back surface of the module. The panels used were backcontacted bifacial solar cells produced experimentally by ISFH. The power output per cell was expected to be equivalent to a 30% efficient monofacial cell of the same size (10). 3. METHODOLOGY Two nominally equivalent PV systems were installed, each consisting of three 190 W Sanyo HIT Double Panels, an SMA SunnyBoy 700 W inverter, and an Ohio Semitronics power transducer. Power output difference between the two arrays, each placed in a different configuration, was measured. Additionally, the solar resource was well quantified, including separate measurements of direct beam, diffuse beam, and plane of aperture irradiance using Hukseflux Class-1 pyranometers. All data were collected on a 30 second basis using Campbell Scientific CR-1000 data loggers. One minute averages of power and irradiance measurements were calculated for analysis. The experiment was conducted at the Appalachian State University Solar Research Laboratory in Boone, NC. Each array consisted of three panels mounted side-by-side using Unirac SolarMounts on a south facing shingled roof with a pitch of 36, which provides an array tilt angle equal to latitude for this location. Various mounting configurations and reflective roof coating were investigated. 2

3 Approximately two weeks of data were collected during each trial during fall 2011 through spring The following subsections describe the configuration details and experimental objective of each trial. 3.1 Trial 1 Objective: Determine if back-side power production differs due to partial shading of the array sides. Surface: brown shingles Note: one unconnected panel on either side of the array Surface: brown shingles The upper array had a non-functioning panel mounted on either side (total of five panels). Trial 1 data was collected between 18 Nov and 2 Dec Trial 2 Objective: Determine if roof coating effects back-side power production difference due to partial shading of the array sides. Surface: TIOCOAT with SWARCO glass beads Note: one unconnected panel on either side of the array Surface: TIOCOAT with SWARCO glass beads The upper array had a non-functioning panel mounted on either side (total of five panels). Two coats of TIOCOAT paint were applied to a cotton canvas tarp that covered the roof shingles. SWARCO glass beads were cast onto the wet coating at a rate of 1.4 ounces per square foot of tarp. The tarp extended 8 beyond the top and bottom panel edges and 40 beyond the right and left panel edges. Trial 2 data was collected between 3 Dec and 31 Dec Trial 3 Objective: Determine the effect on power output difference due to varying panel mounting orientation Mount: flush to roof with 6 spacing Surface: TIOCOAT with SWARCO glass beads Mount: horizontal, with the bottom edge of the array elevated above the roof Surface: TIOCOAT with SWARCO glass beads Trial 3 data was collected between 1 Jan and 29 Jan Trial 4 Objective: Determine the power output difference from differing reflective coatings below the arrays. Mount: flush to roof with 6 spacing Surface: Benjamin Moore Weatherproof Aluminum Paint Mount: flush to roof with 6 spacing Surface: TIOCOAT with SWARCO glass beads Three coats of Benjamin Moore Weatherproof Aluminum Paint were applied to a canvas tarp that covered the roof shingles. The tarp extended 8 beyond the top and bottom panel edges and 40 beyond the right and left panel edges. Trial 5 data was collected between 23 Feb and 9 Mar Data analysis procedure One minute power differences were binned in time, plane of aperture irradiance, direct beam irradiance, diffuse beam irradiance, and direct beam fraction. 4. PRELIMINARY RESULTS 4.1 Trial 1 Results The P vs. time graph is shown in Figure 1. Known shading issues appear before 10:30 and after 14:00 and are very evident in the graph. In all subsequent analyses only data between 11:00 and 14:00 will be considered. Between 10:30 and 14:00, the two arrays have very similar outputs, agreeing to within 2 W. It is likely that the dark colored shingles effectively suppress reflection and the majority of the power is produced by the front side of the shingles. 3

4 The effects of DNI and GDIFF can be combined by defining a Direct Beam Fraction (DBF), which is the ratio of DNI to total irradiance. The graph of P vs. DBF is shown in Figure 4. For both low and high values of DBF, the non side shielded array outperformed, and for moderate values of DBF the two array outputs are quite similar. Fig. 1: The P vs. time graph. Figure 2 shows P vs. Direct Normal Irradiance. The largest power difference is seen under the highest DNI conditions, when the non side shielded array outperforms the side shielded array by 4 W. Fig. 4: The P vs. DBF graph. Power difference vs. plane of aperture (POA) irradiance is shown in Figure 5. This graph follows the trends seen in the P vs. DNI graph. Fig. 2: The P vs. DNI graph. Figure 3 shows P vs. global diffuse radiation. For low values of GDIFF the non side shielded array outperforms the side shielded array by around 2 W, but under higher values of GDIFF the side shielded array outperforms the non side shielded array by around 8 W. Fig. 5: The P vs. POA graph. 4.2 Trial 2 Results Figure 6 shows the measured power difference vs. time. Throughout most of the day, the upper (side shielded) array outperformed the lower array, with the largest difference around noon. This behavior differs significantly from Trial 1; the change can be attributed to the addition of the reflective roof cover. Fig. 3: The P vs. GDIFF graph. 4

5 Fig. 6: The P vs. time graph. Measurements of DNI and GDIFF were not available during Trial 2. Figure 7 presents the measured power difference binned in POA irradiance. Generally, the power difference increases with increasing POA irradiance, with the upper array consistently outperforming the lower array. Fig. 8: The P vs. time graph. Measurements of DNI and GDIFF were not available during Trial 3. Figure 9 shows P vs. POA irradiance. At the highest irradiances the upper array is producing almost 30% more power. At this time of year in Boone the sun elevation angle is around 45, resulting in near normal incidence on the flush mounted array, thus maximizing the array s effective area and reflective gain. This sun angle results in a large incidence angle on the horizontal array, thus decreasing the array s effective area. Little reflected radiation is captured by the horizontal array s back side, since much of the reflected radiation was diffused from passing through the panels. Fig. 7: The P vs. POA graph. 4.3 Trial 3 Results The P vs. time graph is shown in Figure 8. The upper (flush mounted) array significantly outperforms the lower (horizontally mounted) array throughout the day. The power difference is almost 15% of rated power. Clearly, more transmitted radiation was reflected to the back side of the flush mounted array compared to the scattered radiation that was reflected to the back side of the horizontal array. This difference likely has a strong seasonal dependence, as the elevation angle will affect the scattering. Fig. 9: The P vs. POA graph. 4.4 Trial 5 Results Figure 10 shows the P vs. POA graph. The upper array (silver reflecting) always outperforms the white coating. 5

6 (2) Sanyo Energy Corporation. (2010, April 1). HIT Double 195 Spec. Sheet. Retrieved July 5, 2100, from Sanyo: 0Consumer/HIT%20Double%20195%20w_ pdf (3) Zhao, L., Zhou, C., Li, H., Diao, H., & Wang, W. (2008). Design optimization of bifacial HIT solar cells on P- type silicon substrates by simulation. Solar Energy Materials & Solar Cells, 92, Fig. 10: The P vs. time graph. No irradiance measurements are available for this trial. The calculated average power difference is 13.2 W, with the silver coating outperforming the white coating. 5. SUMMARY The various trials clearly indicate that modifying the geometry and reflectivity of surfaces behind bifacial panels affect power output. Trial 1, with dark shingles behind the panels, had very little power difference under any irradiance conditions. Trial 2, which duplicated trial 1 with a reflecting roof cover, showed around a 25 W power difference. Trial 3 had the most dramatic results, with nearly 200 W more power from a flush mounted array compared to a horizontally mounted array. Trial 5 suggests that silver reflective paint results in more power output from bifacial panels than does white paint. 6. ACKNOWLEDGEMENTS The authors would like to thank Sanyo Corporation for donating the bifacial panels used in this study, DuROCK ALFACING INTERNATIONAL Manufacturing Company for providing TIOCOAT and Boone Paint for supplying Benjamin Moore Waterproof Aluminum Paint, SWARCO glass beads, and canvas. The authors would also like to thank the Appalachian State University Renewable Energy Initiative (REI) for its support and for providing space and equipment that was crucial for the completion of the project. (4) Robles-Ocampo, B., Ruiz-Vasquez, E., Canseco- Sanchez, H., Corneho-Meza, R., Trapaga-Martinez, G., Garcia-Rodriguez, F., Vorobiev, Y. V. ( ). Photovoltaic/thermal solar hybrid system with bifacial PV module and transparent plane collector. Solar Energy Materials & Solar Cells, 91 (5) Untila, G., Kost, T. N., Chebotareva, A. B., Zaks, M. B., Sitnikov, A. M., & Solodukha, O. I. (2005). A New Type of High-Efficiency Bifacial Silicon Solar Cell with External Busbars and a Current-collecting Wire Grid. Semiconductors, 39(11), (6) Mishima, T., Taguchi, M., Sakata, H., & Maruyama, E. (2011). Development status of high-efficiency HIT solar cells. Solar Energy Materials & Solar Cells(95), (7) Cuevas, A., Luque, A., Eguren, J., & Del Alamo, J. (1982). 50 Percent More Output Power From An Albedo- Collecting Flat Panel Using Bifacial Solar Cells. Solar Energy, 29(5), (8) Luque, A., Lorenzo, E., & Sala, G. (1984). Diffusing Reflections for Bifacial Photovoltaic Panels. Solar Cells(13), (9) SolarTown. (2011). Sanyo Bifacial Solar Panels-A New Approach to Extracting More Energy from your Rooftop. Retrieved from SolarTown: (10) Hezel, R. (2003). Novel Applications of Bifacial Solar Cells. Progress in Photovoltaics: Research and Applications, 11, doi: /pip REFERENCES (1) Sanyo Energy Corporation. (2010, April 1). Sanyo HIT Products. Retrieved July 5, 2011, from Sanyo: 6