Degradation analysis of 1900 PV modules in a hot-dry climate: Results after 12 to 18 years of field exposure

If the annual degradation rate is known and is assumed to be linear for a PV module throughout its lifetime, then the lifetime can easily be calculated as the degradation limit is typically set at the time of procurement or the project initiation. The degradation rate is dependent on the module design, installation configuration and the environmental conditions of the site. The degradation rate of the individual modules in a module-string may also be influenced by the operating system voltage, grounding polarity (positive, negative or floating) and by the module position in the module-string; these degradations are commonly referred as PID, Potential Induced Degradation. The study presented in this paper was carried out in a PV power plant located in a hot-dry climate of Tempe, Arizona. This study was initiated to determine the annual degradation losses/rates of field aged modules and analyze if these losses are caused or influenced by the PID effects. In this study, about 1900 field exposed modules between 12 and 18 years from several manufacturers were evaluated. This includes modules with different construction schemes (glass/glass or Glass/polymer). In order to carry out this evaluation, the PV systems were shut down, the individual modules were electrically isolated from the string (but not physically removed from the installation) and the I-V curves of every individual module were collected on clear sunny days over several months. Assuming a linear degradation and the conventional 20-year warranty with 20% degradation limit from the nameplate rating, more than 80% of the tested modules in this hot-dry climate do not meet the warranty expectation.