Reliable long-term performance assessment of commercial photovoltaic modules tested under field conditions over 5 years

Due to their scalability and global abundance of sunlight, photovoltaic panels are a promising option as a renewable energy source. Implementation of photovoltaic technologies on a large scale requires a careful business-case assessment, aimed at the selection of the technological option most appropriate for the local conditions in terms of long-term performance. For this purpose, five types of modules representative of current options on the market were tested under field conditions for five years at a test facility in Germany. The degradation rates of module performance were computed from the obtained photovoltaic power normalized by both recorded and modeled solar irradiance. The results emphasize the relevance of using modeled irradiance data in addition to recorded solar irradiance in order to extract reliable degradation rates. The available methodological tools still have to be adapted to every dataset for the most accurate result. Eventually, robust degradation rates were extracted from experimental power data, based on modeled clear-sky irradiance, and a combination of aggregation and regression strategies. The results show distinctive degradation behaviors of the five available commercial photovoltaic modules in response to the local conditions.