Comparing PV power plant soiling measurements extracted from PV module irradiance and power measurements

The accumulation of dust and other environmental contaminants on PV modules, also known as PV module soiling, is a significant source of lost potential power generation for PV installations. Designers and operators of utility-scale solar power plants are increasingly seeking methods to quantify soiling-related losses, in order to improve performance modeling and verification or to optimize washing schedules. Recently, soiling measurement equipment has been introduced based on the measurement of two co-planar PV modules, one of which is regularly cleaned, and the other of which naturally accumulates environmental contaminants. These measurements are used to determine a soiling ratio (SR), which may be applied as a derate factor in analysis of the PV system performance. In this work, we examine the difference between a soiling ratio metric calculated from measured temperature-corrected short-circuit current values (SRIsc), which represents the fraction of irradiance reaching the soiled modules, versus a soiling ratio calculated from measured temperature-corrected PV module maximum power values (SRPmax), which represents the fraction of power produced by the soiled modules compared to clean modules. We examine both techniques for CdTe and c-Si module technologies. This study is motivated by the fact that variations in module efficiency versus irradiance, as well as any non-uniformity of soiling, may introduce differences between the power losses estimated from short-circuit current values versus actual soiling-induced power losses. For CdTe, the SRIsc method is found to be a good proxy for the SRPmax method for nonuniform soiling levels up to 11%.