This document presents the results of experiments characterizing bifacial solar modules. Bifacial modules were able to generate additional energy from their back sides by collecting light reflected from the ground or other surfaces below. Testing was conducted at outdoor test fields in Jerusalem and Geilenkirchen, Germany on bifacial and standard monofacial modules over time periods ranging from days to months. The energy gain provided by bifacial modules depended on factors like diffuse to global radiation ratio, sun position, albedo of the underlying surface, and module elevation and spacing. Annual energy gains over 23% were typical for bifacial modules in field tests in Germany.

1. Naftali Eisenberg, Lev Kreinin, Ninel Bordin, Asher Karsenty, Avishai Drori
Konstanz, Apr. 2012
OUTDOOR BIFACIAL MODULE CHARACTERIZATION:
ENERGY GENERATION AND GAIN

2. Outline
1. INTRODUCTION
2. EXPERIMENTAL
2
2. EXPERIMENTAL
3. TEST RESULTS
4. CONCLUSIONS

3. THE BEAUTY OF BIFACIAL SOLAR MODULE IS IN THE
COLLECTION OF ADDITIONAL ENERGY BY THE CELL BACK
1. INTRODUCTION
3

4. FACTORS AFFECTING THE BACK CONTRIBUTION
1. ILLUMINATION CONDITIONS
1. INTRODUCTION
4
• Sun elevation
• Diffused/global radiation ratio
• Albedo of underlying surface

5. 2. MODULE AND SYSTEM PARAMETERS
• Back/front efficiency
1. INTRODUCTION
FACTORS AFFECTING THE BACK CONTRIBUTION
5
• Back/front efficiency
• Module inclination (tilt)
• Distance between rows
• Stand alone/field system
• Module elevation above underlying surface
• Distance between modules in the row

6. 1. INTRODUCTION
ROOFTOP TEST FIELD IN JERUSALEM
6

7. TEST SYSTEM
Panel 1
and Mount
Panel 2
and Mount
Global
Irradiance
pyranometer
Diffuse
Irradiance
pyranometer
Wind
speed
sensor
Panel
temperature
sensors
Reference cells,
Pyranometers
Meteorological station
1. INTRODUCTION
pyranometer pyranometer sensor sensors
Pyranometers
Data Logger
Laptop >>> Database >> Website
Multiplexer
IV
Tracers
Input Irradiance
Profile
Measurement
Sensor Array

8. • Module orientation in a fixed position: south, tilt: 300. The distances between
rows (S-N) and between modules (E-W) were 150 and 20 cm. Elevation of the
module lower edge was 70 cm.
• The albedo was ~50 %
• I-V characteristics of a bifacial and a mono-facial modules inside the "field“ were
monitored simultaneously as well the module temperature and direct and
1. INTRODUCTION
8
monitored simultaneously as well the module temperature and direct and
diffused radiation.
• All the Energy data were normalized by nominal front power Pmax
of the given module, i.e. we measured : kWh (module) / kWp (front)
BIFACIAL MODULE GAIN OVER MONOFACIAL:
G = { kWh/ kWp, front} bifacial - { kWh/ kWp, front} monofacial

9. ANALYSIS OF FRONT SIDE MODULE PARAMETERS WHICH
COULD AFFECT THE COMPARATIVE ENERGY GENERATION
2. EXPERIMENTAL
1. How to cover the back side for perfect front characterization
9

10. 2. EXPERIMENTAL
2. Front illumination is not a discriminating factor in our test site
ANALYSIS OF FRONT SIDE MODULE PARAMETERS WHICH
COULD AFFECT THE COMPARATIVE ENERGY GENERATION
10
a) b)
Front power as a function of irradiance for a mono-facial (a) and a bifacial (b) module

11. 2. EXPERIMENTAL
3. Incident angle is not a discriminating factor in our site
ANALYSIS OF FRONT SIDE MODULE PARAMETERS WHICH
COULD AFFECT THE COMPARATIVE ENERGY GENERATION
11
Angular dependence of short circuit current for mono-facial and bifacial modules

12. 3. TEST RESULTS
SIMULTANEOUS MONITORING OF MONO AND BIFACIAL MODULES
12
Daily energy gain of a bifacial vs. a mono-facial module

13. 3. TEST RESULTS
SIMULTANEOUS MONITORING OF MONO AND BIFACIAL MODULES
13Monthly energy gain of a bifacial vs. a mono-facial module

14. 3. TEST RESULTS
HOURLY DEPENDANCE OF ENERGY OUTPUT FOR MONO
AND BIFACIAL MODULES IN A FIELD
14
Monitoring for sunny day 01.05.2011 with
diffused/global radiation ratio 11 %

15. 3. TEST RESULTS
HOURLY DEPENDANCE OF ENERGY OUTPUT FOR MONO
AND BIFACIAL MODULES IN A FIELD
15
Monitoring for cloudy day 17.09.2010 with
diffused/global radiation ratio 88 %

16. 3. TEST RESULTS
NON UNIFORMITY OF BACK IRRADIANCE VS. PANEL ELEVATION
16
Back side irradiance for a 30o tilted
module (30.05.10)
•Albedo 0.55
•Global Irradiation 1006 W/m2
•Diffuse Irradiation 111 W/m2
Elevation 58 cm
Elevation 108 cm
Elevation 8 cm

17. 3. TEST RESULTS
NON UNIFORMITY OF BACK IRRADIANCE VS. PANEL ELEVATION
17Back side irradiance E (min, max) and max power gain vs. module elevation

18. 3. TEST RESULTS
EFFECT OF DIFFUSED/GLOBAL RADIATION RATIO
18
Daily energy gain of a bifacial vs. a mono-facial module as a function of
diffuse to global radiation ratio for two seasonal sun positions

19. Berlin-Dahlem, Tilt=30°, NS=2.32m. BIFACIALITY 70 %
Annual Bifacial Gain and Equivalent Cell Efficiency
for various Field Designs

20. ROOFTOP TEST FIELD IN GEILENKIRCHEN
3. TEST RESULTS
• Bifacial modules vs.
monofacial reference modules.
• Inverter - x2 1.5kW units, per
20
System monitored by Fraunhofer/ISE
• Inverter - x2 1.5kW units, per
string
• Az = 145 deg
• Ground reflectance - 78%
• Tilt=15°
• Height = 30 cm
• NS= 2.5m

21. ROOFTOP TEST FIELD IN GEILENKIRCHEN
3. TEST RESULTS
21

22. ROOFTOP TEST FIELD IN GEILENKIRCHEN
3. TEST RESULTS
22

23. 4. CONCLUSIONS
THE ENERGY GAIN IN USING BIFACIAL MODULES IS HIGLY
DEPENDANT ON :
• DIFFUSE TO GLOBAL RADIATION RATIO,
• SEASONAL AND TIME-OF-DAY SUN POSITION
• ALBEDO OF UNDERLYING SURFACE
• MODULE ELEVATION AND TILT, DISTANCE BETWEEN MODULES
23
TYPICAL ANNUAL GAIN MEASURED IN GERMANY
WAS ABOVE 23 %.
A BIFACIAL MODULE WITH FRONT POWER OF 250 W
WILL GENERATE ENERGY AS A STANDARD MODULE OF 307 W

24. 24