Bifacial modules - promises and challenges 

1. Bifacial Modules - Promises and ChallengesBifacial Modules - Promises and Challenges
bifipv-workshop Konstanz 24th April 2012
Paul Grunow
PI Photovoltaik-Institut Berlin AG
Photovoltaik-Modultechnologie
Testing | Consulting | Research
Wrangelstr. 100, 10997 Berlin

2. PI Services in Detail
Clients Service
• Manufacturers Certificates, Re-Testing, Pre-testing,
bench-marking, Test-to-Failure tests
• Turn-key Suppliers same as above
• Component Suppliers Lamination service, screening, extended
IEC tests (double, triple)
• Wholesalers, OEM-Clients Factory Inspection, Bench Marking,
Quality Control, Certification, Analysis of
2
Quality Control, Certification, Analysis of
Field returns
• System developers, Owners Incoming Module Quality Control,
Systems engineering
• Banks, Investors Expertise in module failure probability
• Assurances Failure analysis, Module repair
• Universities, Institutes, Project partnering in industrial R&D
Industrial R&D teams projects
• ISO 17025 accredited CBTL-IECEE test laboratory in Berlin
• 35 employees in 2011
• Coming event: 2nd PV-Investor’s Day 9th May 2012 at PI Berlin

3. Efficiency increase through module design
Approach Effect Parameter potential
Remark/
Reference
Increased number of ribbons lower Rserie FF↑ 4% Day4
ARC/texturisation front glass lower reflection loss Isc ↑ 2,5%
ARC
supplier
Non-absorbing encapsulant higher transmission in UV Isc ↑ 1,5% e.g. Silicone
3
Others: concentration, frequency shifts via up and down conversion, active area increase
Passive Cooling via back sheets lower operating temperature Voc ↑ ? (0,4%/°C)
Bifacial module design (=new
cell+transparent substrate) additional use of albedo Isc ↑ 24%
Sanyo
2010
Total Efficiency increase: ∑ = 32%
-> Bifaciality is best in class

4. Higher efficiencies dilute area related costs
4
-> 32% higher module efficiencies converts to 20 % lower system cost
(bifi alone 24%-> 16%), if the overall cost per m² would remain constant.

5. Commercial Bifacial Module
5
Field installation in Geilenkirchen, Germany, Albedo 0,64, Sanyo HIT-DNKHE1: 70% back-to-front efficiency
-> Sanyo: 24% more energy yield compared to own standard module
[Sanyo data sheet 04/2010]

6. Bifaciality -> higher operating temperatures
6
+ 24% irradiation -> 6°C higher NOCT*
-> ≈ 3 % decrease in annual energy yield in Berlin
* Nominal Operating Conditions Temperature

7. Higher curents increases hot spot risk
0.6
0.8
1.0
normalisedI/A
current-limited cell
unshaded module
> 20% more current for bifacial modules
0.0
0.2
0.4
0.0 0.2 0.4 0.6 0.8 1.0
normalised V /V
normalisedI
Cell with the highst
leak current
Bypass diodes
turns on point
-> higher power dissipation in partly shaded cells (shading ≈ 25%)
-> more bypass diodes, i.e. shorter electrical strings
[Wendlandt 2010]

8. Higher currents, higher ribbon losses
-> Need to increase ribbon cross sections
-> or add more bus bars (thinner but same total cross section -> e.g. day4)
[Grunow 2010]
MPP
MPP
busbar
Cu
frontloss
U
I
twn
L
P
P
⋅
⋅⋅⋅
⋅=
3max
,
ρ

9. Low reflecting tunnel walls
STC measurements of bifacial modules:
Both sides independently with zero back
illumination
Bifacial Module
Openarea,noreflections
Removed
curtain
Openarea,noreflections
[Pasan manual SS3b]

10. Field: Irradiance on the module‘s back side
Maximum elevation of the sun 60°
in Europe (direct irradiance)
(Max. elevation at equator -> 0°module tilt)
11°60°
0,8m
String-to-string distance 4m: 11°(2m: 21°)
30°
Ground Albedo (20% … 60%)
glass/air reflection
-> no direct irradiance on the back
Albedo: Diffuse irradiance from the surrounding
“Module albedo”: an additional share of 11°/180°= 6%
from the modules in the rear of about 16% reflection (i.e.
6% cell +10% glass@ 60°incident angle, see graph )
-> “Module Albedo” < Standard Albedo of 20%
0,8m

11. Energy yield of bifacial modules using
standard simulation tools
Module 2
90°north
Module 1
30°south
30°
Ground Albedo (20% … 60%)
Location: Frankfurt am Main (meteonorm), Germany, tilt 30°, field installation
Shadowing angle with 1,6m module length and 4m distance:15°in all directions
Efficiency at the back (module 2): 80% of the front side (module 1)
Problems: module temperatures underestimated, angle of incidence of Module 2 not correct
Yieldbifacial = YieldModule1 + YieldModule2

12. Energy yield of bifacial modules with standard
simulation tools
Albedo 20% (= Standard value)
e.g. grass

13. Energy yield of bifacial modules with standard
simulation tools
Yield increase:
Albedo 20% -> 60%:
e.g. grass -> white concrete
Albedo 20% 60% 20->60% increase
Standard 904 kWh/kWp/d 919 kWh/kWp/d 1,7%
Bifacial 1048 kWh/kWp/d 1117 kWh/kWp/d 6,6%
Sanyo demo system Geilenkirchen 2009: 24,9% extra yield under similar conditions

14. 90°east/west?
N
a) Bifacial 30°south
vs.
a)
b)
Module orientation Yield Albedo 0,2 deviation Yield Albedo 0,6 deviation
South 30°tilt 1048 kWh/kWp - 1117 kWh/kWp -
East/west 90°tilt 835 kWh/kWp -20,3% 1065 kWh/kWp -4,6%
b) Bifacial 90°east/west
-> standard 30°south still better, because of lower shadowing angle
(break even with single row installations o°shadowing angle, e.g. sound barriers at north/south highways
but 90°east/west offers peak-shaving effect
N
a)

15. Transparent back sheet
= isolation
layer for 1000V
system voltage
15
-> Stable transparent back sheets more costly by 20%-30% vs. glass
many plastics are made UV-durable by adding opacifying pigments
[Honeywell 2010]
= weatherability layer

16. 50€/kg for stable
barrier films like
Tedlar™ (PVF)
Transparent back sheet
Tedlar alternatives
< 10€/kg
1€/kg for PET
isolation layer with
low weatherability
< 10€/kg
[Wallner, Workshop »Trends in der Solarmodulfertigung« 2011, Schkopau, Germany]

17. Glass-Glass-Modules
Cons
• More weight per m²
• Longer lamination cycle
• increased thickness makes
framing difficult
• high breakage risk during
PQ 10/40
AEG 1984
EVA
2mm glass-glass
17
-> glass-glass is a design breaker for most module makers
• high breakage risk during
lay-up of the 2nd glass
Pros
• less tensile stress on the cells
• cheaper (then transparent
polymers)
[PI 2007]
2mm glass-glass
steel frame
Degradation: 0,3%/a

18. Summary
• Higher efficiencies dilute area related costs
• Proven yield increase of > 20%
• NOCT higher
• Hot spot risk higher
• Ribbon losses higher, like for high efficiency cells
• Bifacial yields can be estimated with standard simulation programs• Bifacial yields can be estimated with standard simulation programs
• Standard albedo of 0,2 still effective with ca. 16% vs standard yield
• East/west orientations makes sense with high albedos of ≥ 0,6
• Glass-Glass is an cost effective alternative to transparent back sheet
• Constant cost level per m² for bifacial module designs with glass-glass
Vielen Dank
grunow@pi-berlin.com
18

19. Module 2
90°north
Module 1
30°south
30°
Height above ground
corrective action 1: The yield of module 2 with albedo =0 is substracted from the yield of module 2 excluding the direct impact of the diffuse irradiation on it
corrective action 2: The area of module 2 is giving the cross sectionfor the albedo. The heigth above ground is enlarging this cross section.
albedo 0 0,1 0,2 0,3 0,4 0,5 0,6 0,7
Module 1: Standard 30°south 913 kWh/kWp 917,4 kWh/kWp 938,3 kWh/kWp 942,6 kWh/kWp 946,8 kWh/kWp 951 kWh/kWp 955,2 kWh/kWp 959,5 kWh/kWp
Module 2: 89,9°north 297 kWh/kWp 331,4 kWh/kWp 365,9 kWh/kWp 400,3 kWh/kWp 434,6 kWh/kWp 469 kWh/kWp 502,8 kWh/kWp 536,7 kWh/kWp
Addendum
-> Additional yield scales better with albedo now, but approach is still a work around
Module 2: 89,9°north 297 kWh/kWp 331,4 kWh/kWp 365,9 kWh/kWp 400,3 kWh/kWp 434,6 kWh/kWp 469 kWh/kWp 502,8 kWh/kWp 536,7 kWh/kWp
Table1: Additional yield of an bifacial module vs. an standard module 30°south with
albedo ?
Height above ground/m ?
0 0,1 0,2 0,3 0,4 0,5 0,6 0,7
0,2 0% 1,7% 3,2% 4,8% 6,4% 7,9% 9,4% 10,9%
0,4 0% 2,0% 3,9% 5,8% 7,6% 9,5% 11,3% 13,1%
0,6 0% 2,3% 4,5% 6,7% 8,9% 11,1% 13,2% 15,3%
0,8 0% 2,6% 5,2% 7,7% 10,2% 12,7% 15,1% 17,5%
1,0 0% 3,0% 5,8% 8,6% 11,5% 14,2% 17,0% 19,7%
1,2 0% 3,3% 6,4% 9,6% 12,7% 15,8% 18,9% 21,9%
1,4 0% 3,6% 7,1% 10,6% 14,0% 17,4% 20,8% 24,1%
1,6 0% 4,0% 7,7% 11,5% 15,3% 19,0% 22,6% 26,3%
Bifaciality factor 70%