Soiling Rates of PV Modules vs. Thermopile Pyranometers

Performance monitoring of solar facilities depends on quantifying both irradiance and soiling losses, among other factors. Frequently thermopile pyranometers are used for irradiance measurement in conjunction with PV module-based soiling monitoring stations. However, thermopile pyranometers are also subject to soiling and must be cleaned regularly to maintain accuracy. In this paper we compare the rate of soiling of a thermopile pyranometer to a PV module at a site in the western US. The results for this site indicate that the thermopile pyranometer soils at less than half the rate of the PV module. The data can be used to establish cleaning guidelines for thermopile pyranometers. In addition, the results show that soiling of thermopile pyranometers should not be used as a proxy for the soiling of a PV array.