Soiling of solar mirrors-impact of incidence angles on CSP plant performance

Soiling impacts energy harvesting of solar thermal power plants and operation and maintenance cost. Our research showed that reflectance of soiled glass reflectors diminishes significantly with increasing angles of incidence. The incidence angle dependent attenuation due to dust on the reflector surface can be modelled based on the Lambert-Beer law. In this study we show the effect of incidence angle dependent reflectance on the expected yield of exemplary power plants by integrating the model into ray tracing. Results illustrate the additional reduction of the solar yield in dependence of the mean incidence angles on the mirrors of a heliostat field, a parabolic trough field and a linear Fresnel collector array, depending on the power plants site and seasonal effect. Results show mean hourly incidence angles and optical performance for every hour of the year. In conclusion, not considering angle dependent reflectance can lead to on overestimation of the annual plant performance of up to 2%. For line focusing collectors the effect is dominant in winter, in the northern hemisphere, underlining the importance of cleaning during this season.