Performance of SolarPod TM Crown Dr. Mouli Vaidyanathan, PhD PE, Eagan Minnesota 55123

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1 Performance of SolarPod TM Crown Dr. Mouli Vaidyanathan, PhD PE, Eagan Minnesota Contents Contents... 1 Introduction:... 1 Purpose:... 1 Literature Review:... 2 Procedure:... 3 Field case 1 (SolarPod TM Crown and Open Air Solar system in same location):... 3 Field Case 2 (SolarPod TM Crown and Open Air Solar system in two different locations):... 4 Field Case 3 (CFD model of SolarPod TM Crown with 3 and 5.5 clearance from roof surface):... 4 Results:... 4 Field case 1:... 4 Field Case 2:... 5 Field Case 3:... 6 Discussion... 7 Conclusions... 8 Introduction: Cost reduction in solar is a key factor to its feasibility and application. In a highly competitive and commoditized energy market, small strategic advantages can provide significant benefits. The SolarPod TM Crown is a system that has been designed with no roof penetrations. Its design comprises a frame network that has a standoff clearance of (41mm) from the roof surface. There has been chatter in the solar community about the alleged insufficient air flow underneath the solar panels for SolarPod TM Crown installations. The contention is that the insufficient air flow could cause (1) lower solar energy generation and (2) potentially damage the Ethylene vinyl acetate (EVA) back-sheet. Both are alleged to higher back-sheet temperature in the SolarPod TM Crown. Conventional ground or flat roof solar installations have solar panel back frame exposed to good ventilation. Good panel ventilation provides no heat buildup. Reduced heat buildup provides highest solar energy generation. Many solar panel manufacturers require their solar panels to be mounted at a minimum standoff clearance between roof surface and solar panel frame to be minimum 100 mm (4inches) to satisfy UL Listing and allow for proper ventilation. This requirement protects EVA back-sheet from overheating. This required gap as indicated by the manuals have been between the solar module frames and the mounting surface. This means that the distance between the back-sheet and roof surface can be 5.5 inches or roughly 140 mm. Typical EVA lamination temperatures during assembly is between 140 to 155 C (285F to 311F). A significant concern in close to the roof installation is the possibility of the EVA back-sheet exceeding maximum temperature causing panel damage (delamination, broken hermetic seal, corrosion, etc.). Purpose: The purpose of this study was (1) to investigate the energy difference between the SolarPod TM Crown and conventional Grid Tied installations and (2) estimate through computational fluid dynamics model the typical temperature of the EVA back-sheet in the worst ambient temperature conditions for a typical home.

2 Literature Review: A study 1 by Guohui Gan conducted extensive numerical analysis on three variables, (1) air gap between roof and solar panels, (2) roof slope (or pitch) and (3) vertical number of solar panels. They used the air gap distance between roof surface and back-sheet of the solar panels instead of the roof surface and solar panel frame. The study was conducted under boundary conditions of bright sunshine and no wind. They found that To reduce the overheating potential, a minimum air gap of 0.125m between a very long PV panel (formed by mounting three modules continuously) and the building envelope would be required.. It was also found that Another study 2 conducted in 2013 by Orazio et al, was performed on three module mounting clearances between roof surface and module frame. They were 200mm (8 ), 40mm (1.625 ) and 0mm (0 ) clearance between the roof and solar panels. All of them had an 18 degree (4:12) roof pitch. They found a 40 mm gap to be sufficient for convective air flow and negligible difference between the 40 mm and 200 mm conditions. They also found a 0 mm gap to be insufficient for the convective air flow. Figure 1: Model used in the Orazio (Left) studies and in Guohui Gan (Top Right) studies. The studies from Guohui was purely numerical with no field or experimental results. The Orazio study included numerical, field and experimental studies. In a study by Airlangga Gunawan 3 et. Al. they investigated 4 different roof type installations and compared them to open air systems. The tilt angle was steeper at 35 degree than the Orazio study. Figure 2 provides the different roof configurations investigated. The maximum temperature on the back sheet or the number of rows of solar panels were not part of the Gunawan investigation. They found that there is an average 3% decrease in annual energy generation over above roof systems for solar panels with no ventilation underneath the solar panels. 1 Numerical determination of adequate air gaps for building-integrated photovoltaics, Guohui Gan, Solar Energy 83 (2009) M. D Orazioa*, C. Di Pernab, E. Di Giuseppea Performance assessment of different roof integrated photovoltaic modules under Mediterranean Climate, Energy Procedia 42 ( 2013 ) Airlangga Gunawan, Pritesh Hiralal, Gehan A.J. Amaratunga, KT Tan and Stuart Elmes The Effect of Building Integration on the Temperature and Performance of Photovoltaic Modules Photovoltaic Specialist Conference (PVSC), 2014 IEEE.

3 Figure 2: Test conditions used in the Airlangga Gunawan et. Al investigation. Procedure: The SolarPod TM Crown design allows for a 41 mm clearance between the roof surface and Solar panel frame and 75 mm between the roof surface and solar panel back-sheet. In order to understand the effects of a 41 mm clearance between the roof surface and the solar module frame in this investigation, three distinct studies were conducted. Two of them were field system comparisons. And one system was a Computation fluid dynamics (CFD) model. To understand the effects of ventilation on energy generation and panel back-sheet temperature, we attempted to consider three field cases. Two of the field cases compare solar energy generation between ventilated systems (conventional) and restricted ventilation (SolarPod TM Crown). And one case was a computational fluid dynamics (CFD) model to understand the temperature on restricted ventilation (SolarPod TM Crown). Field case 1 (SolarPod TM Crown and Open Air Solar system in same location): The panels that were investigated are given in Figure 3. The two systems are in the same geographic location hence weather patterns differences are negligible. The power electrical line losses for the two systems are fairly identical. They both use Enphase Micro Inverters for the monitoring. Figure 3: Test systems showing the ground system and SolarPod TM Crown system for energy production.

4 The SolarPod TM Crown has more snow accumulation in winter because of the gable roof valley. The tilt angle for the two systems are different. The SolarPod TM Crown tilt is fixed at 45 degree flush to the roof. The SolarPod TM Grid Tied is 25 degree in summer and 45 degree in winter. Because the tilt angles are different, the Sun incidence angle is different in summer. For the SolarPod TM Grid Tied, the higher sun angle and lower solar panel tilt angle in summer provides an increase in power as compared to the SolarPod TM Crown. Field Case 2 (SolarPod TM Crown and Open Air Solar system in two different locations): The two systems that were compared are a flat roof top mounted public system on the Minneapolis Convention Center at Downtown Minneapolis against the SolarPod TM Crown installation at Marine on St. Croix, MN. They are 27 miles apart and are shown in Figure 4. Figure 4 : Comparison performed between 600 kw solar system and an 8.5kW SolarPod TM Crown system. Field Case 3 (CFD model of SolarPod TM Crown with 3 and 5.5 clearance from roof surface): A computations fluid dynamics model study was performed by the University of Minnesota 4. The studies main motivation was to understand the temperature at the back of the solar module when the module is placed at 5.5 clearance and 3 clearance. Conventional modules require a 4 clearance between the roof mounting surface and the module frame. Since the frame thickness is larger than the solar cell plane, the back-sheet of the solar modules is at 5.5. In the SolarPod TM Crown case, the clearance is (41mm) between the roof surface and the module frame. Since the frame thickness is larger than the solar cell plane, the back-sheet of the solar modules is at 3. Results: Field case 1: The observations were taken for each month between Aug and Aug (2 years of data). This is plotted in Figure 5 where each data point is the sum of energy for a month for the two systems. 4 Mechanical Engineering Department, Dr. John Gorman, University of Minnesota, Minneapolis Minnesota.

5 kwh/kw/day olarpod Crown (kwh/kw/month) The linear correlation is 92% accurate (R2= 0.921). This means there is a strong correlation between the two variables. The largest difference is in the winter months where the solar energy generation is low. Besides in these months the SolarPod TM Crown experiences snow accumulation while the open air system sheds snow faster. The dashed line splitting the chart in half is the line when both systems are identical in its performance. It is observed that in the higher energy production months (~ > 90 kwh/kw/month) the two systems behave almost identically. The difference is attributed to snow cover in the lower energy production months where snow removal and collection on the roof is cumbersome. Visually there are more data points towards the SolarPod Crown than Open Air System which indicates marginally higher performance by SolarPod TM Crown. Open Air v. SolarPod Crown Data between Aug & Sept Each point is sum for each month for the Crown and Ground mount y = x R² = Open Air (kwh/kw/month) Figure 5: Energy comparison for each month between the SolarPod Crown and ground mount solar system. The higher energy months show marginal difference between the two systems. Field Case 2: The two systems being compared are 27 miles apart, the weather patterns observed between the two systems are fairly identical. This can be observed in Figure 6 where the two systems follow identical energy patterns Daily energy production at MCC and SolarPod Crown SolarPod Crown MCC Downtown Figure 6: Daily energy production for two systems at Minneapolis Convention Center and SolarPod TM Crown. Separated by 27 miles both systems have similar energy generation pattern. The orange and blue prominence in specific regions of

6 SolarPod TM Crown (kwh/kw/day) the chart only portray small localized variation. The main point of this chart is to indicate the profiles for day to day generation are identical between the two systems (ventilated and SolarPod TM Crown). Figure 7 shows a scatter plot for SolarPod TM Crown and Open Air system s daily energy production. The two systems follow a linear trend as should be observed. Minor variations are present which are attributed to system installation, components variation and local weather. For the most part the correlation is very strong with R 2 at 90%. Further, visually there are more data points towards SolarPod TM Crown which indicates the SolarPod TM Crown may have marginally better energy performance than Open Air Systems for these two installations SolarPod TM Crown v. MCC Data between Jan. 1, 2016 and Oct. 13, 2016 Each point is energy production for a day y = x R² = Minneapolis Conv. Center (kwh/kw/day) Figure 7: Comparison between two solar systems, Minneapolis Convention Center and SolarPod Crown indicate no significant energy generation difference between the two. Field Case 3: Computational fluid dynamic model was built by Univ. of Minnesota Mechanical Engineering Department experts. The model included a two-solar panel model placed on the roof as given in Figure 8. Typical dimensions and thermal properties for building materials were used to obtain the heat flux (Figure 8). These were then simulated for the solar radiation and conditions from a day with no wind and temperature 5 (hottest day at the hottest time of the day) as in Phoenix, Arizona. The simulation results are presented in Table 1. The worst case temperature is on the top second row of panels at 4.8C increase. This overall 5 degree rise in temperature in Phoenix AZ is observed between the clearance and 3 clearances for the SolarPod TM Crown and conventional solar systems respectively. Table 1: Temperature increase in solar panels between conventional and SolarPod TM Crown in Phoenix, AZ. 3" Stand Off Conventional* 1.625" Stand Off SolarPod TM Crown* Bottom Row 0 ~+2 o Celsius 5 The weather was taken from June 21, at noon in Phoenix Az. The air temperature was 37.8 C ( K), the dew point temperature was 0.6 C, and the effective sky temperature, for sky radiation, was K. There was no wind, only natural convection.

7 Top Row 0 ~+4.8 o Celsius * 3 StandOff actually is 4.5 clearance and StandOff is 3 clearance between roof surface and back-sheet. Figure 8: Simulation conditions used in the computational fluid dynamic model to construct the SolarPod TM Crown vs the conventional solar system on a gable roof. Discussion: The results of the energy generation comparison between SolarPod TM Crown and conventional solar is extremely encouraging. The lower ventilation clearance in the SolarPod TM Crown has still proven that energy generation is identical to conventional solar installation. The cost reduction along with significantly higher reliability of the SolarPod TM Crown only makes the case even stronger to use the SolarPod TM Crown for on-site solar generation where there is no transmission losses while using existing roof tops over prime land. The CFD analysis was performed using typical thermal properties and dimensions for a home in Arizona. These are closer to actual application in the USA than using only theoretical models not close to US applications as in the Guihan Guo study. The only marginal increase in temperature on the SolarPod TM Crown at the worst day provides engineering proof that the EVA back-sheet cannot overheat even under the worst temperature conditions with no wind. This result obtained by an independent University study provides crucial data to the solar community for the application of SolarPod TM Crown on gable roof.

8 The CFD analysis was performed on a low pitch (18 degree) slope. A higher pitch will provide more convection. Further, on S Tile type of roof, the clearance between the roof surface and the solar panel back-sheet is higher because of the peak and valley of the S tiles providing for more convection between the roof and solar panels. Competitors 6 have introduced solar shingles that integrates into the roof boards. The intent of the shingles is to provide hermetic sealing as well as produce solar energy. These shingles have no ventilation under the cells hence can have lower solar energy generation. The SolarPod TM Crown optimizes ventilation, energy generation and aesthetics into the most optimal gable rooftop solution. The SolarPod TM Crown frame can be colored to match any roof top configuration. The solar shingle approach differentiates the solar panels into specific applications utility and residential. SolarPod TM Crown does not differentiate the solar panel technology. Hence a higher economy of scale can be achieved in SolarPod TM Crown while providing higher aesthetics and reliability at reduced cost. Conclusions: The purpose of this study was to investigate the energy generation difference between the SolarPod TM Crown and conventional Grid Tied installations. In the open air systems complete air circulation is possible behind the panels. The empirical studies performed by Orazio and Gunawan provide independent information that there is no significant energy generation issues with lower solar panel clearances. The SolarPod TM Crown despite a restricted air flow compared to open air ventilation performs equivalent to Open Air conventional systems. Computational fluid dynamic models on SolarPod TM Crown demonstrate a 2 to 5 degree rise in temperature over conventional solar systems under the worst case weather conditions in Phoenix Arizona with no wind. 6