From Normal-Incidence Light Transmission Loss to Outdoor PV Soiling Loss under Solar Radiation

The reading of a soiling sensor can be calibrated against normal-incidence soiling loss, but it cannot be directly calibrated against the PV soiling loss in the field because of the incidence angle effect. Previous laboratory studies have shown that PV soiling loss at an arbitrary incidence angle can be related to the normal-incidence soiling loss through a model involving the secant of the incidence angle, but the applicability of this relationship has not been verified until this study. Multiple soiling samples were assessed in this study for their soiling loss on PV reference cell under outdoor solar radiation and for their normal-incidence soiling loss under a solar simulator. The model previously developed from laboraotry studies was found to be a accurate decription of the relationship between outdoor PV soiling loss and the normal-incidence soiling loss. This finding confirms that once a soiling sensor is calibrated for normal-incidence soiling loss, its reading can be used to assess the PV soiling loss under solar radiation through the incidence angle secant model. The finding provides critical guidance on the use of soiling sensors for PV soilng loss assessment..