Abstract
The time evolution of the performance of photovoltaic (PV) modules has attracted increasing interest in recent years and many methods exist to estimate the performance loss rates. In this work, we analyze the relative efficiency depending on temperature, irradiance and time. To analyze the temperature and irradiance behavior we use the model adopted by the Photovoltaic Geographical Information System (PVGIS). This might help to improve the PVGIS model to take into account the aging of PV modules. As the analysis of this model yields ambiguous results, we decided to investigate the degradation depending on either temperature or irradiance, separately. We investigated the time evolution of the temperature coefficients of power, current and voltage. With the help of the temperature coefficients, it is possible to find yearly power, current and voltage values at standard test conditions (STC), without taking the modules out of the field for laboratory inspection. With this approach, it is not possible to include spectral, annealing, light soaking and wind effects. The STC values can be used to study the performance loss of the analyzed modules. As a further step, we analyzed the degradation at different irradiance levels, which might help to identify the root causes of the degradation of the module. Whereas, it is not possible to find meaningful trends for the fitting parameters of the model used by PVGIS and the temperature coefficients, we see a definite irradiance dependency of the degradation rates. As a possible cause for this behavior, we identified a decrease of the shunt resistance.