44 castillo aguilella-multi-year_study_of_bifacial_energy_gains

Prism Solar Technologies – 2015 (All Rights Reserved)
Multi-Year Study of Bifacial
Energy Gains Under Various
Field Conditions
by Jose Castillo-Aguilella
j.castillo@prismsolar.com
4Th PV Performance Modelling and Monitoring Workshop
Cologne, Germany

Prism Solar Technologies
• Prism Solar founded in 2005.
• Researched bifacial technology and
applications since 2008.
• Manufacturing of bifacial modules since
2012.

Presentation Topics
• Bifacial module field data related to the
Bifacial Gain in Energy over similarly rated
monofacial modules.
• What is the true rating of a bifacial module:
Issues with standard STC module ratings when
they are applied to bifacial modules.

Traditional Bifacial Module Applications

Modules Used in the Study
Bifacial Modules: Prism Solar B245
Front STC = 245W (w/ black background); typ = 245-248W
Pmax Temp coefficient = -0.465%/C
Bifacial Ratio (BR) [%] = 95%*
*Bifacial Ratio [%] = 100*(REAR/FRONT) = 100*(PREAR@STC/PFRONT@STC)
Reference Module: Canadian Solar CS6P-245M
Front STC = 245W (+0-5W Positive Tolerance)
Pmax Temp coefficient = -0.45%/C
Bifacial Gain in Energy:
BGE [%] = (Total[kwh]-Front[kwh])/Front[kwh]

Test Conditions
Test
Condition
Date Range
Tilt Angle
(deg.)
Min
Height
Ratio h(m)
Albedo
(%)
Azimut
h
(deg)
BGE(%)
1
10/1/2012 to
4/28/2015
30 0.63 10% 180 17.7
2
5/10/2013 to
10/04/2013
30 0.76 77% 180 36.8
3
5/10/2013
to10/04/2013
30 0.2 77% 180 27.3
4
10/06/2013 to
01/14/2015
20 0.2 70% 180 18.41
5
10-31-2013 to
12/31/2014
20 0.2 68% 180 19.57
6
10-31-2013 to
12/31/2014
20 0.2 22% 180 12.31
7
1/1/2014 to
08/01/2015
10 0.3 75% 180 17.85
Vertical configurations pointing South, East and 45 deg from South will also be shown

Prism Solar Central Hudson Test Yard
Location: Highland, NY
Latitude: ~41.6°N
System Tilt: 30°
Azimuth: 180° (South)
Ground Reflectance: ≤10%
Test Condition Date Range
Tilt Angle
(deg.)
Min Height
Ratio h(m)
Albedo (%)
Orientation
(P/L)
Azimuth (deg)
Bifacial Gain in
Energy (%)
1
10/1/2012 to
4/28/2015
30 0.63 10% L 180 17.7
0%
5%
10%
15%
20%
25%
30%
35%
40%
0
1
2
3
4
5
6
O-12
D-12
F-13
A-13
J-13
A-13
O-13
D-13
F-14
A-14
J-14
A-14
O-14
D-14
F-15
A-15
BifacialGaininEnergy(%)
AverageDaily[kWh/kW]
Bifacial [kWh/kW_dc]/day Monofacial [kWh/kW_dc]/day
Bifacial Gain in Energy (%)

Prism Solar Tucson, AZ Test Yard
C6SP
CS6P
CS6P
CS6P
B245
B245
B245
B245
Location: Tucson, AZ
Latitude: ~32.18°N
System Tilt: ~30°
Ground Reflectance: 77%
CS6P-245M
Reference Array
B245 Bifacial
Array @ 0.76m
B245 Bifacial
Array @ 0.2m
Module Maximum Power
Rating of the (Pmax)
245 W 245 W 245 W
Inverter Model
PowerOne
Micro-0.3-I
OUTD-US
PowerOne
Micro-0.3-I
OUTD-US
PowerOne
Micro-0.3-I
OUTD-US
Inverter max AC power
output
300 W 300 W 300 W
Number of Modules 4 2 2
Mounting Angle 30 30 30
Data Range
5/10/2013 -
10/04/2013
5/10/2013 -
10/04/2013
5/10/2013 -
10/04/2013
Days 144 144 144
Normalized Energy
Generated
[kWh/kW_STC/day]
5.03 6.88 6.42
Gain - 36.8% 27.3%

0%
5%
10%
15%
20%
25%
30%
35%
40%
45%
0
2
4
6
8
10
12
14
16
18
20
5/11
5/18
5/25
6/1
6/8
6/15
6/22
6/29
7/6
7/13
7/20
7/27
8/3
8/10
8/17
8/24
8/31
9/7
9/14
9/21
9/28
10/5
DailyBifacialGaininEnergy(%)
kWh/kWperday
AVG Energy B245 30" [kWh/kW]
AVG Energy B245 8" [kWh/kW]
AVG Energy CS6P [kWh/kW]
Daily Bifacial Energy Gain 8" (%)
Daily Bifacial Energy Gain 30" (%)
100
120
140
160
180
200
220
240
260
280
300
5/11
5/18
5/25
6/1
6/8
6/15
6/22
6/29
7/6
7/13
7/20
7/27
8/3
8/10
8/17
8/24
8/31
9/7
9/14
9/21
9/28
10/5
PeakModulePower(W)
Peak Daily Power 30" Bifacial (Watts)
Peak Daily Power 8" Bifacial (Watts)
Peak Daily Power CS6P 245 (Watts)
Tilt Angle
(deg.)
Min Height
Ratio h(m)
Albedo (%)
Orientation
(P/L)
Azimuth (deg)
Bifacial Gain in
Energy (%)
2
5/10/2013 to
10/04/2013
30 0.76 77% L 180 36.8
3
5/10/2013
to10/04/2013
30 0.2 77% L 180 27.3

0
20
40
60
80
100
120
140
160
180
200
220
240
260
280
5 7 9 11 13 15 17 19
AverageModulePower[W]
5/11/2013 - Time of day
CS6P-245M Average
B245 Average
0%
25%
50%
75%
100%
125%
150%
5 7 9 11 13 15 17 19
BGE(%)
5/11/2013 - Time of day
5 min BGE (%)
Daily BGE (%)
Annual BGE (%)
Test Condition 2 values for (LEFT) 5 minutes average power values for the B245 modules,
set at =77%, h=0.76m, =30 , and CS6P-245M modules on 5/11/2013. (RIGHT) Average
BGE values for 5 minute, daily and annual average for 5/11/2013 – Please note the large
variation of the BGE in relation to the time of study.
BGE [%]] = (Total[kwh]-Front[kwh])/Front[kwh]

B245
B245
CS6P
CS6P
CS6P
Tilt Angle
(deg.)
Min Height
Ratio h(m)
Albedo (%)
Orientation
(P/L)
Azimuth (deg)
Bifacial Gain in
Energy (%)
4
OCT 2013 to
JAN 2015
20 0.2 70% L 180 18.41
0%
5%
10%
15%
20%
25%
30%
0
1
2
3
4
5
6
7
8
O-13
N-13
D-13
J-14
F-14
M-14
A-14
M-14
J-14
J-14
A-14
S-14
O-14
N-14
D-14
BifacialGaininenergyBGE(%)
DailyAveragekWh/kW
Bifacial (kWh/kW) Mono (kWh/kW) BGE (%)

Tilt Angle
(deg.)
Min Height
Ratio h(m)
Albedo (%)
Orientation
(P/L)
Azimuth (deg)
Bifacial Gain in
Energy (%)
5
10-31-2013 to
12/31/2014
20 0.2 68% P 180 19.57
6
10-31-2013 to
12/31/2014
20 0.2 22% P 180 12.31
Location: Tucson
Latitude: ~32.18°N
System Tilt: 20° Portrait
Ground Reflectance: 68% and 22%
0%
5%
10%
15%
20%
25%
30%
35%
0
1.5
3
4.5
6
7.5
O-13
N-13
D-13
J-14
F-14
M-14
A-14
M-14
J-14
J-14
A-14
S-14
O-14
N-14
D-14
BifacialGaininEnergy(%)
DailyAveragekWh/kW
0.68 Bifacial (kWh/kW) 0.22 Bifacial (kWh/kW)
Monofacial (kWh/kW) 0.68 Bifacial Gain in Energy (%)
0.22 Bifacial Gain in Energy (%)

B245
B245
CS6P
CS6P
CS6P
CS6P
Test
Condition
Date
Range
Tilt Angle
(deg.)
Min
Height
Ratio
h(m)
Albedo
(%)
Orientati
on (P/L)
Azimuth
(deg)
Bifacial
Gain in
Energy
(%)
7
JAN
1/2014 to
AUG
01/2015
10 0.3 75% P 180 17.85
0%
2%
4%
6%
8%
10%
12%
14%
16%
18%
20%
0
1
2
3
4
5
6
7
8
J-14 F-14 M-14 A-14 M-14 J-14 J-14 A-14 S-14 O-14 N-14 D-14 J-15 F-15 M-15 A-15 M-15 J-15 J-15 A-15
BGE(%)
AverageDaily[kWh/kW]
Bifacial [kWh/kW] Monofacial [kWh/kW] BGE(%)

Vertical Testing - Prism Solar Tucson, AZ
0
1
2
3
4
5
6
7
8
J-13
F-13
M-13
A-13
M-13
J-13
J-13
A-13
S-13
O-13
N-13
D-13
J-14
F-14
M-14
A-14
M-14
J-14
J-14
S-14
O-14
N-14
D-14
J-15
AverageDailykWh/kw
C6SP
South
(Wh)
B245
South
(Wh)
C6SP East
(Wh)
B245 East
(Wh)
B245 -45
(Wh)
Vertical C6SP
South (REF)
Vertical B245
South
Vertical C6SP East Vertical B245 East Vertical B245 -45
Average Annual Energy (kWh/kW) 1186 1555 854 1796 1737
Energy Gain Jan 2013 – Jan 2015 31.34% -27.93% 51.55% 46.54%
For an additional comparison, the 10 degree tilt monofacial reference system of Test Condition 7 produced an annual yield 1744 kWh/kW

Notes on Bifacial Testing for Tucson, AZ
• Dry and hot environment at an elevation of 730m
above sea level, and therefore less air mass.
• The observed results would tend to understate the
bifacial gain in locations with a higher proportion of
diffuse light, under similar conditions.

Additional Independent Testing
TÜV/GTM Comparative Energy Yield (CEY) program which is deploying systems at
4 sites world-wide, each with 10 module manufacturer’s products in a direct
comparison of various PV commercially available technologies, each
manufacturer with 2kW-2.5kW.
2 sites currently operation (Phoenix, Arizona and Davis, California). For both
sites, Prism Solar bifacial modules have reported a higher monthly energy
(kWh/kW) yield than any other system including other bifacial modules, since
testing started in May 2015.
All systems were deployed over
low albedo surfaces and with
partial racking obstructions of
the rear of some of the bifacial
modules in the array

Bifacial Energy Estimation –
Prism Solar Design Guide
A simplified annual yield model based on these
experiments is presented as the Prism Solar
Bifacial Design Guide.
http://www.prismsolar.com/pdf/Design_guide.pdf
Customers can estimate the additional annual
backside energy performance of Prism Solar
modules (Bifacial Ratio ~0.90) under some of
the most common field installation conditions.

What is the True Rating of a Bifacial
Module?

What is the True Rating of a Bifacial?
UL, IEC and other standards only account for the front side at
STC conditions to rate the modules, bifacial or not. The
background and angular illumination for a bifacial STC test
conditions are not defined.
• Fails to address the true energy generation potential of
bifacial modules. Wiring, fuses/protection and inverter
may be undersized as a consequence of STC sizing.
• Customers cannot compare bifacial manufacturers.
Prism Solar modules are IV-tested using a collimated light source and black
material behind the module, as to not add opposite-side power contribution
to the side being tested. This produces a lower STC flash than someone using
a white background or/and an un-collimated flash tester.

Rating of Bifacial Modules is
Generally Left to Manufacturers
Most manufacturers only
present the STC rating and the
performance of the module
when it achieves 10%, 20% or
30% of additional power from
the backside. None describe
the background or illumination
conditions for their STC flash.
Unfortunately, this method
doesn’t address the issue of
how much light it takes to
reach that additional power.

What is the True Rating of a
Bifacial Module?
Manufacturer
Parameter A B C D E
STC Rating (W) 270 270 270 270 270
Equivalent Rating at +30% Power (W) 351 351 351 351 351
Backside Efficiency Ratio (Rear/Front) 10% 50% 75% 90% 100%
Rear Illumination Needed for +30% in
(W/m^2) 3000 600 400 333 300
Field Average Illumination Level (W/m^2) 300 300 300 300 300
Power at Field illumination Level (W) 278.1 310.5 330.8 342.9 351
Although all these manufactures would have equal STC
ratings and equal +30% bifacial ratings, the performance
under equal rear illumination conditions would be
different among them.

BSTC = Bifacial STC
Prism used a BSTC rating for protection and wire sizing since
2013, and certified its modules with the independently
developed bifacial TÜV guidelines at TÜV PTL (Phoenix, AZ) in
2015.
BSTC conditions (TÜV guidelines) =
Cell temp 25°C, AM1.5, 1000W/m2 (front) + 300W/m2 (rear)
Prism Solar modules tested with TÜV underwent full UL, IEC
and NREL’s Qualification Plus testing at BSTC amperage levels to
fully certify the safety aspects of the bifacial design.

BSTC = Bifacial STC
BSTC provides additional
relevant information
about the bifacial
performance of a module.
By comparing the BSTC
and Front STC ratings, the
rear STC module efficiency
can be estimated.
Alternatively, if the Front
STC and Rear STC is given,
the BSTC can be
determined.
Bifacial ratings, safety
protections and wiring
sizing should account for
bifacial effects.

Conclusions
• The field performance of bifacial modules under a variety of field
conditions was shown.
• Under standard mounting conditions, bifacial modules exceeded
annual energy yield of monofacial reference modules by at least 12%
and up to +30%.
• Vertically mounted bifacial modules have the potential to generate
as much energy as traditionally mounted monofacial modules.
• We have shown how the current lack of bifacial standards makes a
direct comparison between bifacial manufacturers difficult, based
both on IV flash test conditions and rear side efficiency.
• In the case of protection of bifacial systems, safety issues could arise
from the additional current and power not being accounted for
under STC conditions.

Future Work
• Add data set from TÜV/GTM Comparative
Energy Yield program to bifacial model.
• Joint bifacial work with Sandia National Labs
with 3 sites in the US with multiple installation
configurations at each site and add data set to
model.
• Work with Sandia/NREL on bifacial standards.

Thank You
Additional questions:
j.castillo@prismsolar.com

44 castillo aguilella-multi-year_study_of_bifacial_energy_gains

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