Characterisation and calibration methods for bifacial solar cells
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Characterisation of bifacial solar cells
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Characterisation and calibration methods for bifacial solar cells
- 1. Characterisation of bifacial solar cells Jochen Hohl-Ebinger, Wilhelm Warta Fraunhofer Institute for © Fraunhofer ISE Fraunhofer Institute for Solar Energy Systems ISE Bifi-Workshop Konstanz 23.04.2012 www.ise.fraunhofer.de
- 2. Calibration of bifacial solar cells Jochen Hohl-Ebinger, Wilhelm Warta Fraunhofer Institute for © Fraunhofer ISE Fraunhofer Institute for Solar Energy Systems ISE Bifi-Workshop Konstanz 23.04.2012 www.ise.fraunhofer.de
- 3. Frame and Outline Not only bifacial cells are bifacial! • Back contact solar cells • Back grid on standard cells (thin cells, cost savings in new cell concepts) © Fraunhofer ISE Context Estimation of bifaciality impact on current Experimental test Proposals to obtain inter-lab comparability
- 4. Context I: Standard Testing Conditions (STC) How to Reach International Comparability of Results? Usual path: Realize STC according to IEC 60904 Estimate uncertainty budget Spectral distribution Temperature 25°C Irradiance 1000 W/m² © Fraunhofer ISE 4 500 1000 1500 2000 2500 3000 3500 4000 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 spectralirradiance[Wm -2 nm -1 ] wavelength [nm] But: Bifaciality not addressed in STC AM1.5G Edition 2
- 5. Context II: Uncertainty of Reference Calibration Example: Traceability Chain at ISE CalLab PV Cells Planck spectrum, small diode Synthetic irradiation, small cell Cryoradiometer < 0.01% Photodiode < 0.1% Encapsulated 2x2 cm² Solar Cell < 0.7% © Fraunhofer ISE 5 Simulator irradiation, large area Contributions > 0.1 % count! Strong economic impact: 0.1 % of 20 GWp/a PV world production 200 Mill. € Industrial Solar Cell < 2.0%
- 6. Estimation of Bifaciality Impact Effect on Spectral Response and Short Circuit Current Light transmitted through cell: T(λ) Reflected by surface of measurement chuck R(λ) depends on individual chuck: Unpredictable differences between labs Solar Cell Chuck T(λ) E(λ) R(λ) © Fraunhofer ISE 6 Simple model: Non-linearities neglected - may be severe e.g. due to injection dependent surface recombination )()()()()()()( , λλλλλλλ backChuckCellfrontcontribbackfrontmeas SRRTSRSRSRSR +=+= λλλλλ ∂= ∫ )()()()(, backChuckCellcontribback SRRTEI
- 7. Estimation of Bifaciality Impact Set of Test Cells with Variable Bifaciality Measured Spectral Response 0.04 0.05 0.06 0.07 SR[A/W] Cell A Cell B Cell C Cell D Front side 0.04 0.05 0.06 0.07 SR[A/W] Cell A Cell B Cell C Cell D Back side Irradiation from front Irradiation from rear © Fraunhofer ISE 7 400 600 800 1000 1200 0.00 0.01 0.02 0.03 SR[A/W] Wavelength [nm] 400 600 800 1000 1200 0.00 0.01 0.02 0.03 SR[A/W] Wavelength [nm] Cell B: fully bifacial Cells A, C, D back contact cells with different metal coverage
- 8. 80 100 Reflection[%] black foil black anodized brown anodized Cu brass1 brass2 with25 30 35 Transmission[%] bifacial cell A B C D Estimation of Bifaciality Impact Additional Irradiation from Back Side Transmission through test cells Chuck reflection © Fraunhofer ISE 8 200 400 600 800 1000 1200 0 20 40 60 Reflection[%] Wavelength [nm] brass2 with ad. foil gold plated 400 500 600 700 800 900 1000 1100 1200 0 5 10 15 20 Transmission[%] Wavelength [nm] D
- 9. Estimation of Bifaciality Impact Calculated Impact Back current contribution 20 25 30 35 100 120 140 160 1.4 1.6 1.8 2.0 2.2 2.4 8 10 12 Cell B Cell C Backcurr.contrib.[%] black foil brown anod. brass1 gold plated black anod. Cu brass w. ad. foil Cell A Cell D front contribbackrel contribback SR SR SR , , = © Fraunhofer ISE 9 1000 1200 0 5 10 15 20 1000 1200 0 20 40 60 80 1000 1200 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1000 1200 0 2 4 6 Backcurr.contrib.[%] Wavelength [nm] Usual Cu or gold plated chucks may add significant uncertainty
- 10. Estimation of Bifaciality Impact Calculated Impact Increase of Current back current contribution Cell A Cell B Cell C Cell D black plastic foil 0.00% 0.05% 0.01% 0.03% black anod. 0.01% 0.22% 0.06% 0.11% brown anod. 0.00% 0.18% 0.05% 0.09% © Fraunhofer ISE 10 Usual Cu or gold plated chucks may add significant uncertainty brown anod. 0.00% 0.18% 0.05% 0.09% Cu 0.02% 0.66% 0.17% 0.33% brass1 0.02% 0.69% 0.19% 0.34% brass w. ad. foil 0.02% 0.77% 0.20% 0.38% gold plated 0.03% 1.07% 0.30% 0.52%
- 11. Experimental Test Special Chuck for Bifacial Cells Exchangeable surface plates Measured reflectivity 30 40 50 60 black plastic foil brown anodized black anodized grey anodized Reflection[%] © Fraunhofer ISE 11 200 400 600 800 1000 1200 0 10 20 Reflection[%] Wavelength [nm]
- 12. Experimental Test Comparison to Simple Model Measurement with black foil as reference Model fits for brown surface Measurement 25% higher for black and grey anodized surfaces 12 14 16 18 20 22 24 26 28 backcurr.contrib.[%] calculated brown anodized black anodized grey anodized measured brown anodized black anodized © Fraunhofer ISE 12 Likely reason: SR for calculation measured with small irradiation angle Anodized surfaces reflect highly diffusive Light trapping increases current 200 400 600 800 1000 1200 0 2 4 6 8 10 12 backcurr.contrib.[%] Wavelength [nm] black anodized grey anodized
- 13. Reference Chuck Reflection black or white? 200 400 600 800 1000 1200 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 black backcurr.contrib.[%] black (R = 0%) Simple realization with plastic foil No further definition necessary © Fraunhofer ISE 13 200 400 600 800 1000 1200 Wavelength [nm] 200 400 600 800 1000 1200 0 20 40 60 80 100 R[%] Wavelength [nm] Paint Paper PTFE deco-foil gold plated w-module foil white (R = 100%) Realization with 80 - 90% possible (paint, gold, foil) Tight definition of albedo incl. angular distribution of reflection necessary Higher c2m-loss
- 14. White as Reference Reflections (1000-1200nm): 65 to 95% 60 80 100 © Fraunhofer ISE 14 200 400 600 800 1000 1200 0 20 40 60 R[%] Wavelength [nm] Paint Paper PTFE deco-foil gold plated w-module foil
- 15. 80 100 Reference chuck reflection How to achieve comparable measurements? Reflections (1000-1200nm): 65 - 95% 80 - 90 % 80 100 black foil black anodized brown anodized back current contribution: 0.8 - 1.2% 1 - 1.13% © Fraunhofer ISE 15 200 400 600 800 1000 1200 0 20 40 60 80 R[%] Wavelength [nm] Paint Paper PTFE deco-foil gold plated w-module foil 200 400 600 800 1000 1200 0 20 40 60 80 Reflection[%] Wavelength [nm] brown anodized Cu brass1 brass2 with ad. foil gold plated
- 16. Reference Chuck Reflection Black, White or Grey? R = 0% R = 100% © Fraunhofer ISE 16 R = 100% R ≈ 90%
- 17. Reference Chuck Reflection Black, White or Grey? R = 0% Easy definition Easy to realize (Rfoil ≈ 4%) High lab inter-lab comparability Higher internal reflection improves calibration value not energy yield © Fraunhofer ISE 17 R = 100% R ≈ 85% Hard to realize (R ≈ 80-90% realistic) Angular and spectral distribution makes influence Improving calibration value improves energy yield
- 18. Conclusions Impact of bifaciality can be significant Efficiencies should be stated with relevant information: area definitionmodule ready or not © Fraunhofer ISE 18 η(STC) = 29 % t, ap, damr, nmr bl, w, av. 92% reflection of back sheet cusi, sf contact unit shadow included/ shadow free
- 19. Thank You! © Fraunhofer ISE 19 Fraunhofer ISE CalLab PV Cells