Energy requirements of thin-film solar cell modules--a review

In this paper a number of energy analysis studies for thin-film solar cell modules are compared and reviewed. We start with a short introduction into methodological issues related to energy analysis (of PV systems) such as system boundary definition, treatment of different (secondary) energy types and the choice of functional unit. Subsequently we review results from 6 studies on a-Si modules and 3 studies on CdTe modules. The aim is to present results in a unified format, compare them and try to clarify observed differences. Although significant differences were found, many of these differences could be explained by the choice of materials for the module encapsulation. For categories with large observed differences, like indirect process energy and capital equipment energy, we performed additional analyses in order to gain a better understanding of these aspects. Finally we present best estimates of the energy requirement for present-day a-Si and CdTe thin film modules which are between 600 and 1500 MJ (primary energy) per mfn3 module area, depending on cell and encapsulation type. This means that the energy pay-back time is below two years for a grid-connected module under 1700 kWh[-45 degree rule]m2[-45 degree rule]yr irradiation. In the near future an energy pay-back time below one year seems feasible.