Correction Methods and Stabilisation of Outdoor Measurements of Thin Film Modules

Stabilised electric performance data are necessary to compare different types of modules in the emerging thin film PV market and as basic information for energy yield calculations. The problems with accurate power measurements of thin film modules are well known. The module test standard IEC 61646 ed. 2 tries to take these into account by demanding light soaking and repeated STC measurements. Various types of CdTe, CIS and a-Si modules were installed on a outdoor test facility in Freiburg, Germany, facing south in 45° inclination. IV-Curves of all these modules are measured every ten minutes. Between the measurements of IV-curves the modules are held in MPP conditions by electronic loads. Isc, Voc, efficiency and filling factor are determined automatically. Measured irradiation, module temperature and air mass are used for analysis and comparison of the IV-curves. The influence of the respective air mass was higher than expected and therefore regarded more intensely. Measured data were corrected to 1000 W/m² irradiation and to 25°C module temperature according to standard IEC 60891 in order to make them comparable. Different procedures of filtering and correction have been compared. The results show that the best results are achieved when the data are selected in a small range of air mass and module temperature and corrected to the same irradiation. An additional correction to air mass was carried out. Some modules of the same types like the outdoor exposed modules were stored without irradiation for some years. Afterwards, they were exposed at the test site side by side with the other modules. Comparison of the module data of the same type allows the investigation of the stabilisation behaviour of the modules directly.