Characterisation of soiling on glass surfaces and their impact on optical and solar photovoltaic performance

Photovoltaic (PV) module soiling, i.e., the accumulation of soil deposits on the surface of a PV module, directly affects the amount of solar energy received by the PV cells in that module and has also been suggested as a mechanism that can give rise to additional heating, leading to significant power generation losses or even physical degradation, damage and lifetime reduction. Investigations of PV soiling are challenging and limited. We present results from an extensive outdoor experimental testing campaign of soiling, apply detailed characterisation techniques, and consider the resulting losses. Soil from sixty low-iron glass coupons was collected at various tilt angles over a study period of 12 months to capture monthly, seasonal and annual variations. The coupons were exposed to outdoor conditions to mimic the upper surface of PV modules. Transmittance measurements showed that the horizontal coupons experienced the highest degree of soiling. The horizontal wet-season, dry-season and full-year samples experienced a relative transmittance decrease of 62 %, 66 %, and 60 %, respectively, which corresponds to a predicted relative decrease of 62 %, 66 %, and 60 % in electrical power generation. An analysis of the soiling matter using an X-ray diffractometer and a scanning electron microscope showed the presence of particulate matter with diameters <10 μm (PM10), which was the most prevalent in the studied region. The findings of this study lay the groundwork for research into soiling mitigation practices.