Performance of SolarPodTM Crown
Contents
Contents................................................................................................................................................................. 1
Introduction: .......................................................................................................................................................... 1
Purpose: ................................................................................................................................................................. 1
Literature Review:.................................................................................................................................................. 2
Procedure: ............................................................................................................................................................. 3
Field case 1 (SolarPodTM Crown and Open Air Solar system in same location): ................................................ 3
Field Case 2 (SolarPodTM Crown and Open Air Solar system in two different locations): ................................. 4
Field Case 3 (CFD model of SolarPodTM 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 SolarPodTM Crown is a system that has been designed with no roof penetrations. Its design comprises a frame
network that has a standoff clearance of 1.625” (41mm) from the roof surface. There has been chatter in the
solar community about the alleged insufficient air flow underneath the solar panels for SolarPodTM 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 SolarPodTM 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 SolarPodTM 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.
Literature Review:
A study1 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 study2 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 Gunawan3 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)
1253–1273
2
M. D’Orazioa*, C. Di Pernab, E. Di Giuseppea “Performance assessment of different roof integrated photovoltaic modules
under Mediterranean Climate”, Energy Procedia 42 ( 2013 ) 183 – 192
3
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.
Figure 2: Test conditions used in the Airlangga Gunawan et. Al investigation.
Procedure:
The SolarPodTM 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 (SolarPodTM Crown). And one case was a computational fluid dynamics
(CFD) model to understand the temperature on restricted ventilation (SolarPodTM Crown).
Field case 1 (SolarPodTM 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 SolarPodTM Crown system for energy production.
The SolarPodTM Crown has more snow accumulation in winter because of the gable roof valley.
The tilt angle for the two systems are different. The SolarPodTM Crown tilt is fixed at 45 degree flush to the roof.
The SolarPodTM 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 SolarPodTM Grid Tied, the higher sun angle and lower solar
panel tilt angle in summer provides an increase in power as compared to the SolarPodTM Crown.
Field Case 2 (SolarPodTM 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 SolarPodTM 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 SolarPodTM Crown system.
Field Case 3 (CFD model of SolarPodTM Crown with 3” and 5.5” clearance from roof surface):
A computations fluid dynamics model study was performed by the University of Minnesota4. 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 SolarPodTM Crown case, the clearance is 1.625” (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. 2014 and Aug. 2016 (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.
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 SolarPodTM 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 SolarPodTM Crown.
Open Air v. SolarPod Crown
SolarPod Crown (kWh/kW/month)
Data between Aug. 2014 & Sept. 2016
Each point is sum for each month for the Crown and Ground mount
150
140
130
120
110
100
90
80
70
60
50
40
30
20
y = 1.0512x - 6.6598
R² = 0.9202
20
30
40
50
60
70
80
90
100
110
120
130
140
150
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
8.00
SolarPod Crown
MCC Downtown
kWh/kW/day
7.00
6.00
5.00
4.00
3.00
2.00
1.00
0.00
Figure 6: Daily energy production for two systems at Minneapolis Convention Center and SolarPodTM Crown.
Separated by 27 miles both systems have similar energy generation pattern. The orange and blue prominence in
specific regions of 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 SolarPodTM Crown).
Figure 7 shows a scatter plot for SolarPodTM 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 R2 at
90%. Further, visually there are more data points towards SolarPodTM Crown which indicates the SolarPodTM
Crown may have marginally better energy performance than Open Air Systems for these two installations.
SolarPodTM Crown v. MCC
Data between Jan. 1, 2016 and Oct. 13, 2016
Each point is energy production for a day
SolarPodTM Crown (kWh/kW/day)
8.00
7.00
6.00
5.00
4.00
y = 0.9888x + 0.1518
R² = 0.9
3.00
2.00
1.00
0.00
0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
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 temperature5
(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 1.625” clearance
and 3” clearances for the SolarPodTM Crown and conventional solar systems respectively.
Table 1: Temperature increase in solar panels between conventional and SolarPodTM Crown in Phoenix, AZ.
3" Stand Off Conventional* 1.625" Stand Off SolarPodTM Crown*
~+2o Celsius
Bottom Row
0
Top Row
0
~+4.8o Celsius
* 3” StandOff actually is 4.5” clearance and 1.625” StandOff is 3” clearance between roof surface and back-sheet.
5
The weather was taken from June 21, at noon in Phoenix Az. The air temperature was 37.8 C (310.95 K), the dew point
temperature was 0.6 C, and the effective sky temperature, for sky radiation, was 284.58 K. There was no wind, only natural
convection.
Figure 8: Simulation conditions used in the computational fluid dynamic model to construct the SolarPodTM
Crown vs the conventional solar system on a gable roof.
Discussion:
The results of the energy generation comparison between SolarPodTM Crown and conventional solar is extremely
encouraging. The lower ventilation clearance in the SolarPodTM Crown has still proven that energy generation is
identical to conventional solar installation. The cost reduction along with significantly higher reliability of the
SolarPodTM Crown only makes the case even stronger to use the SolarPodTM 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 SolarPodTM 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 SolarPodTM Crown on gable roof.
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.
Competitors6 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 SolarPodTM Crown optimizes ventilation, energy generation
and aesthetics into the most optimal gable rooftop solution. The SolarPodTM 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.
SolarPodTM Crown does not differentiate the solar panel technology. Hence a higher economy of scale can be
achieved in SolarPodTM Crown while providing higher aesthetics and reliability at reduced cost.
Conclusions:
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 purpose of this study was to investigate the energy generation difference between the SolarPodTM Crown and
conventional Grid Tied installations. In the open air systems complete air circulation is possible behind the panels.
The SolarPodTM Crown despite a restricted air flow compared to open air ventilation performs equivalent to Open
Air conventional systems.
Computational fluid dynamic models on SolarPodTM 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.
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