Soiling of solar-field heliostats during operation in concentrating solar thermal plants

• Face-down stow of heliostats shows considerable advantage for soiling protection • Seven times lower soiling rate compared to worst case face-up stow • Creation of whole-year soiling database under realistic operation conditions • Linear correlation found to translate between portable field reflectometer readings Solar energy conversion is highly affected by soiling of collector surfaces, potentially decreasing the yield of running systems. Design, operation, and maintenance strategies of solar energy plants have the potential to mitigate the most severe consequences of soiling. In this study, the soiling of tracked heliostats of concentrating solar thermal technologies based on central receiver systems is investigated and how the selection of the stow position during inactivity impacts the soiling of the reflectors used. For this, a long duration campaign was conducted, covering more than a whole year at a central tower system of the CIEMAT-Plataforma Solar de Almería, with regular measurements on two heliostats during realistic operation. Two stow positions were chosen to compare between the heliostats: the reflector side facing-up towards the sky and down towards the ground. Important benefits in soiling protection were detected for the face-down option, resulting in a nearly seven times lower soiling rate for this case compared to the face-up position. This results in a higher yearly average cleanliness of 0.984 for face-down compared to 0.895 for face-up stow. Further key parameters were investigated, such as tracking-stow time ratio, inclination and height above ground, and the used measurement equipment, and their impact on soiling was quantified. The results of this study are intended to serve as an input parameter for future developments, concerning new heliostat designs, operational strategies and measurement procedures.