This document discusses methods for analyzing and optimizing the efficiency of photovoltaic modules. It presents an analysis of series resistance losses, optical properties, and modeling of module power output. The analysis seeks to understand efficiency losses from solar cells to complete modules and identify sensitivities to improve module performance. Key factors investigated include electrical losses from string formation, optical losses and gains from layer interactions, and the impact of module design parameters.

1. PV Module efficiency analysis and
optimization
Ingrid Haedrich1, Harry Wirth1, Michael
Storz2, Gerhard Klingebiel2
1
Fraunhofer Institute for
Solar Energy Systems ISE,
2
Schmid Technology Systems
How to rate Solar Energy Efficiency to
maximize returns?
Webinar, 18th September 2012
www.ise.fraunhofer.de
© Fraunhofer ISE

2. Outline
 Fraunhofer ISE
 Introduction
 Determination of series resistance
 Analysis of optical properties
 Modeling of module power
 Sensitivity analysis
 Conclusions
Module Technology Center, interior
view
© Fraunhofer ISE

3. Photovoltaic Modules, Systems and Reliability
Fields of activity
Module technology Durability analysis and Quality Assurance
environmental Modules and Power
.
simulation Plants.
 Interconnection  Reliability Testing  Yield certificates
technology  Material analysis  Power plant inspec-
 Module efficiency tion and testing
 Testlab PV Modules
and new concepts  Monitoring
 Module Technology  CalLab PV Modules
Center (MTC)
© Fraunhofer ISE

4. Photovoltaic Modules, Systems and Reliability
Objectives
 Efficiency and electric
yield improvement
 Enabling of advanced cell
technologies
 Material cost reduction
 Production yield
improvement
Estimated cost structure for poly-Si PV module
Module technology responsible for:
35-40% of product cost, 15% of product efficiency, 95% of prod. reliability
© Fraunhofer ISE

5. Work areas and topics
 product development
 process development
 material proving
PC backsheet paste/adhesive
vac. carrier vac. carrier flip
4 PC backsheet
cells encapsulant (2)
encapsulant (1)
front glass
1 2 3 5
One-Layup process for back-contact module production
© Fraunhofer ISE

6. Work areas and topics
 Interconnection processes
 qualification of new cell metallization
 lead free/flux free soldering
 Al contacting
 Cell interconnector design
 back contact technology
Bi-metal effect, unilateral soldered, 160 µm
 stress reduction
cell
 electrical/optical efficiency
 Module efficiency
 optical, electrical improvements
 inactive area reduction
© Fraunhofer ISE

7. Introduction cell to module losses
 efficiency losses from solar cell to complete module:
typ. 10-15 percent
 especially for high efficiency cells it is important to
understand how to keep the efficiency inside a module
approach
 establish procedure for predicting module efficiency
from material and geometry data
 investigate effects on efficiency change for specific
module built-up
 analyse sensitivity of module power with respect to
material and design improvements
© Fraunhofer ISE

8. Introduction
Basic effects:
1. electrical losses generated by string
formation and cables
2. optical losses and gains generated by
the various interactions between cell,
encapsulant, glass and backsheet
3. module format including inactive areas
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9. Introduction
module Optical gains and electrical
format losses losses
© Fraunhofer ISE

10. Series resistance losses
Ploss ,mod = Ploss ,base + Ploss ,emitter + Ploss ,gap + Ploss ,sc + Ploss ,cable
© Fraunhofer ISE

11. Series resistance losses
Ploss ,mod = Ploss ,base + Ploss ,emitter + Ploss ,gap + Ploss ,sc + Ploss ,cable
© Fraunhofer ISE

12. Series resistance losses
Ploss ,mod = Ploss ,base + Ploss ,emitter + Ploss ,gap + Ploss ,sc + Ploss ,cable
© Fraunhofer ISE

13. Series resistance losses
Ploss ,mod = Ploss ,base + Ploss ,emitter + Ploss ,gap + Ploss ,sc + Ploss ,cable
© Fraunhofer ISE

14. Series resistance losses
Ploss ,mod = Ploss ,base + Ploss ,emitter + Ploss ,gap + Ploss ,sc + Ploss ,cable
© Fraunhofer ISE

15. Series resistance losses
Ploss ,mod = Ploss ,base + Ploss ,emitter + Ploss ,gap + Ploss ,sc + Ploss ,cable
2 l
Ploss ,base = I BB ⋅ ⋅ Rb ,eff
3
IBB current per busbar
l length of the cell
Rb,eff effective resistivity (ribbon, metallization)
 Assumption: continuous soldering joint over cell length
© Fraunhofer ISE

16. Series resistance losses
Resistivity of emitter and base busbar
TLM measurement results for a commercial mc cell
 2 mOhm/cm effective for base busbar (including aluminium screen print)
 11 mOhm/cm for emitter busbar
© Fraunhofer ISE

17. Series resistance losses
 polycrystalline full square 6” cell
 STC power from cell flasher: 4,54 W
 short circuit current: 8,35 A
total power loss due to stringing electrical resistivity: 0,156 W/cell
power loss 60-cell module with 3 mm cell distance: 9,9 W
© Fraunhofer ISE

18. Analysis of optical properties
 Effective reflection and absorption losses of
glass and encapsulation material
 Direct coupling gain due to encapsulation
air
EVA
 Optical gains due to an increasing cell space
© Fraunhofer ISE

19. Analysis of optical properties
Effective transmission-, reflection and absorption coefficient
From sheet to material properties
R01 = R10 = R12 =
( n1 − 1) 2 τ measured sheet transmissivity
(1 + n1 ) 2 ρ measured sheet reflectivity
Txy transmissivity from
T01 2 ⋅ R12 ⋅ t 2
ρ = R01 + 2 2
medium x (air) to medium y
1 − R01 ⋅ t
Rxy reflectivity at the surface
T01 2 ⋅ t from medium x (air) to medium y
τ= 2
1 − R01 ⋅ t 2 t bulk transmissivity
nx refractive index of medium x
© Fraunhofer ISE

20. Analysis of optical properties
Effective transmission-, reflection and absorption coefficient
Relative spectral response / AM 1.5
0,20 1,0
Absorption / Reflection
0,8
0,15
0,6
0,10
0,4
0,05
0,2
0,00 0,0
400 600 800 1000 1200
Wavelength
air glass reflectivity relative spectral response
bulk absorptivity glass AM 1.5 spectrum standardized
bulk absorptivity EVA
© Fraunhofer ISE

21. Analysis of optical properties
Effective transmission-, reflection and absorption coefficient
Determined optical properties for glass and encapsulation
bulk reflectivity
bulk absorptivity
transmission air/medium
glass 99,2% 4,1% 0,3%
encapsulant 97,9% 3,2% 2,1%
© Fraunhofer ISE

22. Analysis of optical properties
Coupling effects due to encapsulation
Change of Isc
air
cell Isc,air
air
encapsulant
Isc,EVA cell
© Fraunhofer ISE

23. Analysis of optical properties
Coupling effects due to encapsulation
The measured change of Isc results from several optical effects:
 reflection at air/EVA surface
 effective bulk absorption of EVA material
 direct coupling gain, due to increased refraction
index at cell interface
coupling effects
 indirect coupling gain, due to multiple reflection
between cell surface and encapsulant/air interface
© Fraunhofer ISE

24. Analysis of optical properties
Coupling effects due to encapsulation
coupling gain
(Isc,encapsulated-Isc,air)/Isc,air [% ]
10%
8% measured total current gains/losses
6%
4%
2%
0%
-2%
-4%
-6%
Poly (Typ A) Mono (Typ B) Mono (Typ C) Mono (Typ D)
© Fraunhofer ISE

25. Analysis of optical properties
Optical gains due to cell spacing with white backsheet
2,0%
Isc change
(Isc,enc-Isc,air)/Isc,air [%]
1,0% Linear (Isc change)
y = 0,0046x - 0,0174
0,0%
-1,0%
-2,0%
-3,0%
0 1 2 3 4 5 6
cell distance [mm]
 Measurement on 8 samples with increasing cell distances
 average Isc gain: 0.5 %/mm spacing
© Fraunhofer ISE

26. Modeling of module power
 The predicted module power was found to be within the range of module
flasher measurement uncertainty = +/- 3%
© Fraunhofer ISE

27. Sensitivity analysis
3,0% cross section area cell connector ribbon
reduction width module border
2,5%
reduction width cell distance
∆ module efficiency
reduction resistivity emitter busbar
2,0%
1,5%
1,0%
0,5%
0,0%
0% 10% 20% 30%
parameter increase [% ]
Sensitivity of module efficiency of four different parameters
© Fraunhofer ISE

28. Conclusion
 Major effects and sensitivities have been shown
 Efficiency analysis tool has been set up
 Module efficiency depends strongly on cell/module interaction
 New cell technologies will require adapted module materials
 Outlook: additional evaluation of electric yield
this work has been performed in cooperation with:
Schmid Technology Systems
© Fraunhofer ISE

29. Thank You Very Much for Your Attention!
Fraunhofer Institute for Solar Energy Systems ISE
Ingrid Haedrich
www.ise.fraunhofer.de
Ingrid.haedrich@ise.fraunhofer.de
© Fraunhofer ISE