Long-term performance and reliability assessment of 8 PV arrays at Sandia National Laboratories

In the last decade, c-Si module degradation rates of <1%/year have been reported. It is unclear if this degradation rate extends directly to the string level and what is the expected statistical spread of degradation rates. Nine photovoltaic (PV) arrays totaling nearly 100 kW at Standard Reporting Conditions are currently being used at Sandia National Laboratories (SNL) primarily for inverter testing. The measured power degradation of these arrays at the string level varied from no change over three years within measurement error to greater than 25% in three years. This paper outlines the methodology used to test the DC output, outlines analysis techniques used to evaluate the array performance, provides a current reliability assessment, presents the comparative data for up to five years of use and exposure, and discusses the methods used to track down the causes of unexpected string-level degradation.