1. This document discusses power measurements of bifacial photovoltaic modules. It outlines difficulties in predicting energy yields for bifacial modules due to dependencies on albedo. 2. Outdoor measurements were taken of a bifacial module's maximum power (Pmax) under different albedo backgrounds and compared to Pmax measurements in a lab under standardized test conditions. 3. Specific energy yield comparisons between the bifacial module and a standard reference module showed good agreement when using the bifacial module's Pmax that was measured under a white background, indicating this is a suitable value for predicting energy yields.

1. 1 N. Ferretti, Investorentag PI-Berlin, 30.10.20131 N. Ferretti, BifiPV Workshop, 26.05.2014
Nicoletta Ferretti, Giulia Mancini, Juliane Berghold, Paul Grunow (PI-Berlin)
ferretti@pi-berlin.com, Tel.: +49 30 8145264-119
PI Photovoltaik-Institut Berlin AG, Wrangelstraße 100, 10997 Berlin
Power measurements of bifacial
modules

2. 2 N. Ferretti, Investorentag PI-Berlin, 30.10.20132 N. Ferretti, BifiPV Workshop, 26.05.2014
Content
• The output power of bifacial modules in the field depends on the albedo
Difficulty for energy yield predictions.
• STC measurement of bifacial module outdoor and in the lab taking albedo impact
into account
Measurements at different and defined backgrounds.
• Outdoor power logging of a bifacial and a standard module as reference.
• Comparison of specific energy yields with respect to Pmax of bifacial module.
• Conclusions and outlook.

3. 3 N. Ferretti, Investorentag PI-Berlin, 30.10.20133 N. Ferretti, BifiPV Workshop, 26.05.2014
Bifacial modules
The output power depends on the albedo
(reflection ratio from the ground)
Difficulty for energy yield predictions.
No standards for I-V curve measurement
at STC of bifacial modules
Bifacial modules can not be compared
and economically evaluated.

4. 4 N. Ferretti, Investorentag PI-Berlin, 30.10.20134 N. Ferretti, BifiPV Workshop, 26.05.2014
Field Installation
Field installation in Germany, Albedo 64%, Sanyo HIT-DNKHE1: 70% back-to-front
efficiency, Sanyo data sheet 04/2010
Sanyo bifacial: 24% more energy yield compared to own standard module.
Need to define Pmax evaluation for energy yield prediction and for economical
evaluation of the bifacial modules.

5. 5 N. Ferretti, Investorentag PI-Berlin, 30.10.20135 N. Ferretti, BifiPV Workshop, 26.05.2014
Outdoor STC: Pmax vs. background
Pmax measured at the flasher by covering the
not measured side by a black foil:
Pmax Front=236 W
Pmax Back= 189 W
Bifaciality=79%
Pmax is changing depending on
the background
Height=55 cm
Angle= 30°
Area background: 7m x 5m

6. 6 N. Ferretti, Investorentag PI-Berlin, 30.10.20136 N. Ferretti, BifiPV Workshop, 26.05.2014
Indoor STC
Background for bifacial modules
(2.6 m x 3.4 m )
Background Albedo Ground
Grey 25% Grass
Silver 33% Sand
White 58% Snow
Albedo measured with
a cell at the flasher
[Pasan manual SS3b] No-reflecting walls
Bifacial module

7. 7 N. Ferretti, Investorentag PI-Berlin, 30.10.20137 N. Ferretti, BifiPV Workshop, 26.05.2014
STC comparison Outdoor/flasher
Background
Pmax Outdoor
(W)
Pmax Flasher
(W)
Deviation
(%)
WHITE 282.9 285.1 -0.8
SILBER 274.3 271.3 1.1
GREY 265.0 268.6 -1.3
Reference 251.6 251.9 -0.1
Repeatability bifacial outdoor=0.9%
Repeatability Reference outdoor=0.5%
Uncertainty Pmax Flasher =2.9%
A standard module is also tested both outdoor and at the flasher as reference.

8. 8 N. Ferretti, Investorentag PI-Berlin, 30.10.20138 N. Ferretti, BifiPV Workshop, 26.05.2014
Bifacial modules measurements at flasher
Producer
Pmax front
(W)
Bifaciality
(%)
a1 287.6 54.6
a2 285.1 53.0
a3 284.2 52.7
b1 255.4 91.6
b2 255.8 91.4
b3 255.8 91.5
b4 255.0 91.6
c1 246.9 76.5
c2 247.2 76.9
c3 79.90 79.1
The Pmax deviation measured with defined albedo (white/grey background) is not directly related to
the evaluated bifaciality.
Single side measurents covering the not illuminated side with a black foil.
Pmax measurements with background
[Nsolar project results]

9. 9 N. Ferretti, Investorentag PI-Berlin, 30.10.20139 N. Ferretti, BifiPV Workshop, 26.05.2014
Outdoor power logging
Height=30 cm
Angle= 25°
Area white background: 7m x 5m
Outdoor power logging performed for both the reference and the bifacial module
Specific yields comparison to evaluate if for bifacial modules Pmax measured at a defined
albedo is suitable for energy yield predictions.
Reference module installed with same conditions.

10. 10 N. Ferretti, Investorentag PI-Berlin, 30.10.201310 N. Ferretti, BifiPV Workshop, 26.05.2014
Specific Yield comparison
Days
Spec. Yield Bif. vs.
Spec. Yield Ref. (%)
11-18 Dec -5.2
6-13 Jan -1.8
1-21 Feb -1.3
3-7 Mar -2.5
26 Jan
Snow
+1.6
4 Feb Direct Irrad.,
max 800 W/m²
+0.9
Specific Yield (KWh/KWp) comparison of bifacial module with reference module.
Bifacial Pmax with white background = 285 W
Diffuse Irrad. > Direct Irrad. (except 4 Feb.)

11. 11 N. Ferretti, Investorentag PI-Berlin, 30.10.201311 N. Ferretti, BifiPV Workshop, 26.05.2014
Irradiance December and January
Days
Bif. vs.
Ref. (%)
11-18
Dec
-5.2
6-13
Jan
-1.8
Specific yield comparison

12. 12 N. Ferretti, Investorentag PI-Berlin, 30.10.201312 N. Ferretti, BifiPV Workshop, 26.05.2014
Irradiance February and March
Days
Bif. vs.
Ref. (%)
1-21
Feb
-1.3
3-7
Mar
-2.5
Specific yield comparison

13. 13 N. Ferretti, Investorentag PI-Berlin, 30.10.201313 N. Ferretti, BifiPV Workshop, 26.05.2014
Conclusions and outlook
• Measurement of a bifacial module at STC outdoor under different albedos
(up to +20% on Pmax).
• Pmax value measured outdoor reproduced at the flasher at defined backgrounds.
• Comparison of specific yields of a standard module with the bifacial
good match when considering the Pmax value measured with background.
• Next step: Comparison of measured energy yield with simulation results.