Why Natural Cleaning of Solar Collectors Cannot be Described Using Simple Rain Sum Thresholds

Abstract

This study investigates parameters describing the modelling of natural cleaning by precipitation in soiling models for solar collectors. Approaches assuming that the solar collector is cleaned completely if a daily rain sum threshold is exceeded are investigated with the help of more than three years of soiling and natural cleaning data at CIEMAT's Plataforma Solar de Almería in Spain. It is found that the natural cleaning completeness by rain cannot be described well by using only a threshold of the daily rain sum. We discuss the dependence on further parameters, e.g. the existing soiling levels, the rain direction dependent on wind speed, the collector orientation, the kind of soiling, the collector surface properties and several more. Considering the rain/wind and collector orientation is especially important for the modelling of natural cleaning of tracked collectors as the effective rain sum hitting the collector and the runoff speed is expected to change drastically with the orientation of the collector.