Comparison of Analysis Methods for the Calculation of Degradation Rates of Different Photovoltaic Technologies

Abstract

The degradation rates of crystalline silicon (c-Si) and thin-film photovoltaic (PV) systems of different manufacturers and different technologies were calculated and compared for the systems’ first five years of outdoors exposure, by applying a host of different analysis methods, in order to quantify the differences between each method. These include linear regression using linear least squares (LLS), classical seasonal decomposition (CSD) and seasonal-trend decomposition by Loess (STL) on daily and monthly time series of two performance metrics, performance ratio (PR) and PR with temperature correction (PR-TC). The comparison of the resulting degradation rates for each PV group (c-Si and thin-film) showed that the monthly PR-TC-STL method provided the lowest standard deviation and a mean degradation rate of 1.12 %/year for the c-Si PV systems. On the other hand, the daily PR-TC-LLS method demonstrated the lowest standard deviation and an average degradation rate of 2.47 %/year for the thin-film PV systems. Linear regression using LLS produced the lowest degradation rates overall, but when temperature correction was applied, the calculated degradation rates were increased by 0.4 %/year. LLS also showed the lowest standard deviation for the thin-film PV systems, which was further reduced by applying temperature correction.