Stability of the performance of thin film modules during one year of operation

At the University of Agder in Grimstad, Southern Norway, a test bench for photovoltaic modules is in operation since December 2010. Currently, various c-Si as well as aSi, CIS and HIT modules are under investigation. This project is part of the Norwegian contribution to IEA PVPS Task13. The paper focuses on the stability of the modules´ MPP-performance in the course of one year. For this purpose, data on the DC response are logged together with irradiance and module temperature at one-minute intervals. Data analysis is performed in monthly blocks. The modeling involves two steps. First, the short-circuit current is expressed as function of irradiance and module temperature. Then, the MPP power is modeled as function of the short-circuit current and temperature. For c-Si and CIS technologies, a simple linear approach proved sufficient for the first step. For the aSi and HIT modules, the response of the short-circuit current has to take into account additional information on irradiance conditions - in this case the air mass (AM). The stability of the performance is analyzed by comparison of monthly parameter sets and the monthly stability of the model quality for a fixed parameter set. Whereas for the CIS and the HIT modules the model quality for the unique parameter set changes within the data uncertainty only, for the aSi module the variations are significant. The variability of the response characteristics in this case could be traced back to the first modeling step, giving short-circuit current as function of irradiance conditions.