4 2 castillo- aguilella - annual bifacial energy yield best-fit model

Prism Solar Technologies – 5th PVPMC Santa Clara CA 2016
Annual Bifacial Energy Yield
Best-Fit Model
by Jose Castillo-Aguilella
j.castillo@prismsolar.com
5Th PV Performance Modelling and Monitoring Workshop
Santa Clara, California

Prism Solar Technologies
• Prism Solar founded in 2005.
• Researched bifacial and holographic
technology and applications since 2008; 5
awarded patents in bifacial technology and
applications.
• Manufacturing of bifacial modules since
2012.

Rating of Bifacial Modules is
Generally Left to Manufacturers
Currently, 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.
This method does not address
how much additional light in
the rear it takes to reach that
additional power level.

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 black
background STC ratings and equal +30% bifacial ratings,
the performance under equal rear illumination conditions
would be different among them.

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.
BSTC conditions (TÜV and Prism guidelines) =
Cell temp 25°C, AM1.5, 1000W/m2 (front) + 300W/m2 (rear)

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]

B24
5
B245
CS6P
CS
6P
CS6
P
Modules Used in the Study

Variables Measured and Modeled
Total Bifacial Energy Yield [kWh] =
Front[kWh] + Rear[kWh]
BGE [%]] =
(Total Bifacial Energy Yield[kwh]-Front[kwh])/Front[kwh]
Front[kWh] = Monofacial
Total Bifacial Energy Yield[kwh] = Bifacial Measurement

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

Data Range Used for Model Generation
Test Condition Date Range
Tilt Angle
(degrees)
Min. Height
Ratio h(m)
Albedo (%)
Measured
BGE(%)
1
1/1/2014-
12/31/2014
30 0.63 10% 17.88%
2
5/10/2013-
10/04/2013*
30 0.76 80% 30.64%*
3
5/10/2013-
10/04/2013*
30 0.2 80% 22.75%*
4
1/1/2014-
12/31/2014
20 0.2 70% 18.85%
6
1/1/2014-
12/31/2014
20 0.2 25% 13.25%
7
1/1/2014-
12/31/2014
10 0.3 80% 17.66%

For Detailed Information on Data Set
1. J. E. Castillo-Aguilella and P. S. Hauser, "Multi-Variable Bifacial
Photovoltaic Module Test Results and Best-Fit Annual Bifacial Energy
Yield Model," in IEEE Access, vol. 4, pp. 498-506, 2016.
[DOI:10.1109/ACCESS.2016.2518399]
2. J E Castillo-Aguilella, “Multi-Year Study of Bifacial Energy Gains Under
Various Field Conditions”. 4th PV Performance Modelling Workshop.
Cologne, Germany on October 23rd, 2015.

Model for Deployed Systems
BGE(%) =
A*()+B*(h)+C*()
BGE(%) =
0.317/deg*()+12.145/m*(h)+0.1414/%*()
(Front [kWh]) * (BGE(%) + 100%)

Generalizing the Results
Main Assumptions:
 South oriented systems
 Systems in which the Bifacial Ratio (BR) is larger than 70%.
 Systems in which the minimum module height (h) varies from 0.15m
to 0.8m.
 Systems in which the module tilt angle () varies from 7.5 degrees to
35 degrees.
 Systems in which the ground albedo () varies from 10% to 90%.
 Systems in which the latitude range is from 21 to 51 degrees from
the equator.
 Systems which use non-hybrid bifacial cell technology.
 December 21st, 9am row shading conditions.

General Model
BGE’[%] =
BGE[%] *(BR’[%]/95%)
BGE’(%) =
[0.317/deg*()+12.145/m*(h)+0.1414/%*()]*(BR’[%]/95%)
(Front [kWh]) * (BGE’(%) + 100%)

Model Comparison to 3rd Party Data
Description
- Cases within Prism Solar modeling
conditions -
Bifacial
Ratio (%)
Tilt (deg.)
Height
Ratio
Albedo (%)
System
Prism Model
(%)
External Yield
Data (%)
Difference in
Yearly Energy
(%)
Yusufoglu Cairo - modelA 80 32 0.5 20 116.0 113.8 -1.93
Yusufoglu Cairo - modelA 80 34 0.5 50 120.1 128.8 7.20
PVGS over shells with snow
effects - measuredB 95 35 0.5 50 124.2 123.6 -0.52
PVGS over grass with snow
effects - measuredB 95 35 0.5 20 120.0 115.8 -3.50
bSolar Jerusalem - measuredC, D 75 30 0.7 50 119.8 115.0 -4.01
bSolar Jerusalem - modelC, D 75 30 0.7 90 124.3 126.0 1.39
bSolar Geilenkirchen Fraunhaufer
- measuredC, D 75 30 0.2 78 118.1 122.4 3.61
bSolar Packed Model BerlinC, D 75 30 0.25 25 112.7 109.0 -3.28
bSolar Packed Model BerlinC, D 75 30 0.25 50 115.5 115.0 -0.42
bSolar Packed Model BerlinC, D 75 30 0.25 80 118.8 124.0 4.35
bSolar Eilat-Eilot - measuredC, D 75 30 0.35 50 116.4 117.2 0.65
bSolar Adlershof - measuredC, D 75 30 0.4 35 115.3 111.0 -3.69
Average 79.2 31.3 0.425 49.8 118.4 118.5 -0.01
Median 75 30 0.45 50 118.5 116.5 -0.47
STD Dev 3.5

3rd Party Bifacial Data Set
A. U. Yusufoglu and A. Halm, "Modeling and simulation of annual energy yields of
bifacial modules at different climate zones.” 2nd Bifacial PV Workshop in Chambery,
France. May 27th 2014.
B. K. Sugibuchi, N. Ishikawa, S. Obara “Bifacial-PV Power Output Gain in the Field Test
Using EarthON High Bifaciality Solar Cells.” EU 28th PVSEC Proceedings, pages 4312 –
4317, 2013.
C. B-Solar Ltd., “Commercial Test Sites and Outdoor Field Results,” 2015. Available:
http://www.b-solar.com/Technology.aspx?Sel=Field%20Results
D. N. Eisenberg, L. Kreinin, N. Bordin, Y. Kofman, “High efficiency industrial PERT p-type
bifacial cell and field results.” 2nd Bifacial PV Workshop in Chambery, France. May
26th 2014.

Model Limitations
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]

Test Condition Date Range
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 (%)
Model Limitations - Continued

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 in a minimum
row configuration. GTM data to be published in Q2/Q3 2016.
2 sites currently operational (Phoenix, Arizona and Davis, California) since May
2015. For both sites, Prism Solar bifacial modules have reported a higher
energy yield (kWh/kW) than any other system including other bifacial modules.
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

Sandia Bifacial Gain Validation Study
Deploying 3 similarly configured bifacial systems in New Mexico, Nevada,
and Vermont. Other bifacial experiment configurations planned as well.

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.

Conclusions
• The field performance of bifacial modules under a variety
of field conditions was shown.
• A best fit model that covers common installation conditions
of bifacial models was introduced.
• The field data and annual energy model show that under
standard mounting conditions, bifacial modules exceeded
annual energy yield of monofacial reference modules by at
least 12% and up to +30%.
• Lack of bifacial standards makes a direct comparison
between bifacial manufacturers difficult, based both on IV
flash test conditions and rear side efficiency.

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

4 2 castillo- aguilella - annual bifacial energy yield best-fit model

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