Acceleration Factor for Damp Heat Testing of PV Modules

Interconnect Metallization System (IMS) degradation of photovoltaic (PV) modules is one among the major degradation modes in the field caused by higher operating temperatures and daily/seasonal/cloud cyclic temperatures. Usually, the acceleration factor (AF) and activation energy (E-a) of IMS degradation are determined based on power degradation data. Using power degradation data may not be fully representative of a specific mechanism since the power drop could be caused by multiple degradation mechanisms In this paper, we have used the series resistance (R-s) increase, instead of power degradation, to obtain the AF and E-a for IMS degradation mechanism in the damp heat test (85 degrees C/85% RH). The degradation data were sourced from our qualification damp heat test database with 94 crystalline silicon modules, and two field databases of Arizona and New York climates with 615 and 236 crystalline silicon modules, respectively. A 3-step approach was implemented to determine the AF for the damp heat testing. First, the AF for the field-to-field degradation was determined based on R-s degradation rates of the modules in Arizona and New York. Second, the E-a was determined based on the AF, and hourly differences in field-to-field module temperature and relative humidity. Third, the AF was determined based on R-s increase in the damp heat test and the E-a determined using the field data in the second step. A model based on modified Peck's equation was used to determine a generic AF for the R-s increase in qualification damp heat testing. This approach is useful to predict the service lifetime and reliability of PV modules for specific climatic regions.