Environmental durability of PV modules — A model for accelerated testing

Primary critical factors in evaluating the long term environmental durability of photovoltaic modules are heat, light and moisture. In order to accelerate the effects of these stressors on the various degradation mechanisms occurring in PV modules, we use greater ranges of each and attempt to relate them to the effects of smaller variations in the real world. It would be highly desirable to determine the activation energies for each degradation process occurring during the outdoor exposure of PV modules. However, due to both the number of such processes and their interaction during the complex stresses encountered in the daily diurnal cycle, as well as in annual extremes, it has never been possible to accomplish such a feat. Instead, we have been obliged to use simplified models and to test laboratory acceleration practices against the effects anticipated from the field. Field data, however, requires years to obtain, making modeling our primary means of estimating test durations and protocols. In this study, we ignore the effects of humidity, but focus on the thermal effects of heat and light to estimate acceleration factors. Using detailed temperature and radiation data for the outdoor environments, we calculated what we term “equivalent temperatures” for thermal dark cycles and “effective temperatures”, irradiance-weighted average temperatures for light portions of a diurnal cycle to take into account the natural variability of the climatic data. For the accelerated laboratory portion of the data, we implemented an international standard for the weathering of plastics. It was then possible, using the model described, to calculate acceleration factors, dependent upon activation energies and reference locations. Acceleration factors ranging from 12 to 24 were obtained, which compare well with the ranges normally observed in our commercial practice.