Methods and conditions for rating bifacial PV modules

1. NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.
Methods and Conditions for
Rating Bifacial PV Modules
5th PV Performance Modeling
Workshop
Bill Marion
Chris Deline, Sara MacAlpine
May 9, 2016

2. 2
Acknowledgement
• This work funded by the DOE for FY2016-
FY2018 under the SunShot National
Laboratory Multiyear Partnership (SuNLaMP)
program
o “Performance Models for Bifacial PV Module
Technologies”
o Team – Sandia, NREL, University of Iowa

3. 3
Outline
• New IEC standard for
rating bifacial PV
modules
• Background on factors
that influence
performance
• Results for measuring
the available irradiance
for the rear side of the
PV module

4. 4
IEC Standard for Rating Bifacial PV Modules
• Under development by the IEC Technical Committee 82 Solar
Photovoltaic Energy Systems Working Group 2 Modules,
Non-Concentrating
Country Experts
Switzerland Vahid Fakhfouri, Project Leader
Austria Karl Berger
Canada Lorenzo Tiracchia
China Xuemeng Wang, Dengyuan Song, Qingzhu Wei
Germany Peter Seidel, Stefan Winter
Spain Petrina Jauregui Inigo
France Guillaume Razongles
United Kingdom Ralph Gottschalg
Republic of Korea Jaesun Lee, Seungkyu Ahn
United States Chris Deline (NREL), Eric Olson (Solar World)

5. 5
New Work Item Proposal
• IEC 60904-1-2 TS “Photovoltaic Devices – Part
1-2: Measurement of current-voltage
characteristics of bifacial PV devices”
• Terms and definitions
o Bifacial PV device – front and rear sides are used
for power generation
o Bifaciality – ratio between the short-circuit current
generated by the rear side and the front side

6. 6
Measurement of I-V Characteristics
• Using solar simulator with adjustable irradiance level
Step 1 - Flash front to determine Iscfront
Step 2 - Flash rear to determine Iscrear
Step 3 - Calculate a reflectivity compensated Isccomp
Isccomp = Iscfront + 0.2 x Iscrear
and, flash the front with an irradiance setting of Gcomp that
gives an Isc equal to Isccomp

7. 7
What Does the 0.2 Factor Represent?
Isccomp = Iscfront + 0.2 x Iscrear
• Irradiance for rear side relative to the front side
(initial estimate)
• Other and/or multiple factors (0.1, 0.2, 0.3, etc.)
under consideration to better represent bifacial
PV applications
• Rear side irradiance is primarily ground-
reflected

8. 8
Ground-Reflected Irradiance, Ir
Front Side
Ir = ½ (1 - cosβ) · albedo · GHI
Rear Side
Ir = ½ (1 - cos(180º - β)) · albedo · GHI
Configuration Factor:
Fraction of energy
leaving the source which
reaches the PV surface.
Source of Irradiance

9. 9
Shading Reduces Available Irradiance
• Field-of-view from the rear side includes shadows
and the PV modules in the next row
• For a typical multi-row system*, this reduces the
average irradiance at the ground by
o 45% for June
o 80% for December
* β = 30º, latitude = 33º, no row-to-row shading on December 21 from 9 to 3

10. 10
Rear Side Irradiance Trends GHI, Not POA
• Rear side irradiance will follow seasonal trends of
the GHI
• Rear side to front side irradiance ratio will follow
seasonal trends of the GHI to POA ratio

11. 11
Rear Side Irradiance Linear with Albedo
• Albedo is the ratio of the radiation reflected
from a surface to the incident radiation
Item Values
Grass 0.15 – 0.26
Black earth 0.08 – 0.13
Mojave Desert 0.24 – 0.28
White sand, New Mexico 0.60
Snow 0.55 – 0.98
Asphalt pavement 0.09 – 0.18
Concrete pavement 0.20 – 0.40
Surface Albedos

12. 12
Reflective Commercial Roofs
• An Energy Star rating for reflective roofs
requires an initial solar reflectance of 0.65
and a three-year aged reflectance of 0.50
• Oak Ridge National Laboratory tested
thermoplastic roof membranes at 5 US
locations
o After 3 years, the reflectance was reduced by 30%
to 50%
o Cleaning restored the reflectance to within 95% of
the original value

13. 13
Rear Side Irradiance Measurements
Rear Side Irradiance as a Percent of Front Side
SolarTAC PV Array
Ground Cover Panel Location of Reference Cell
Bottom Middle Top
Natural 6 4 4
Snow (partial) 8 7 10
Front Side
Reference Cell
Top
Middle
Bottom

14. 14
Rear Side Irradiance Measurements (cont)
Rear Side Irradiance as a Percent of Front Side
CU Engineering Center Roof
Roof Material Panel Location of Reference Cell
Bottom Middle Top
Beige, R= 50% 7.5 3.5 2.0
White, R= 80% 8.5 6.5 3.0
Bifacial roof top PV
systems would benefit
from an increased row-
to-row spacing and
elevated mounting.

15. 15
Summary
• IEC 60904-1-2 will provide a relatively
straightforward method for rating
bifacial PV modules using a solar
simulator with adjustable irradiance
level (outdoor method also being
evaluated).
• The surface albedo and the extent that
the PV panels shade the ground should
be considered for revising the “0.2
factor”.

16. Questions?
Bill.Marion@nrel.gov
Chris.Deline@nrel.gov
Sara.MacAlpine@nrel.gov