The energy pay-back time of crystalline silicon PV-modules varies from 2.58 (multi c-Si) and 2.66 (mono c-Si) years for the sunbelt region to 4.92 (multi c-Si) and 5.07 (mono c-Si) years for continental climate, high latitude regions in the state-of-the-art production technology. It varies from 1.4 (sunbelt) to 2.67 (continental climate, high latitude) years for a medium term projected scenario based on thinner wafers and better material yields, higher efficiencies and frameless modules. In the long term for efficiencies of 20%, pay-back times are reduced to 1.22 and 2.33 years for the corresponding regions.

Pay-back times are obtained in this study are rather conservative because the kWh-content of semiconductor grade silicon is accounted for in full. A sensitivity analysis shows that a reduction of the kWh-content of semiconductor grade Si with nearly 50%, results in a 30-40% reduction in pay-back times in all scenarios: for the actual scenario, energy payback time would go down to 1.63 (multi c-Si) or 1.74 (mono c-Si) years for the sunbelt region and 3.12 (multi c-Si) or 3.33 (mono c-Si) years for continental climate, high latitude regions. For the medium term projection, these figures would even go down further to about 0.9 years for the sunbelt region and about 1.7 years for continental climate, high latitude regions.

Considering semiconductor grade as waste material from the electronic industry (as is currently utilized) with no accountable kWh-content even reduces the pay-back times by 70-83%: for the actual scenario, the enrgy payback time would decrease to 0.51 (multi c-Si) or 0.65 (mono c-Si) years for the sunbelt region and 0.97 (multi c-Si) or 1.24 (mono c-Si) years for continental climate, high latitude regions. For the medium term these figures would decrease further to around 0.3 years (4 months) for the sunbelt region and around 0.5-0.6 years (6-7 months) for continental climate, high latitude regions.

from Advances in Solar Energy, An Annual Review of Research and Development, Volume 11, American Solar Energy Society, Edited by Karl W. Böer, 1997

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