The effect of soiling on the reflectance of solar reflector materials - Model for prediction of incidence angle dependent reflectance and attenuation due to dust deposition
The accumulation of dust and soil on the surface of solar reflectors is an important factor reducing the effective use of solar energy, especially for concentrating solar power plants. In this study, we investigated glass-based mirrors exposed in the desert and mirrors soiled in the laboratory. We achieved artificial, controlled soiling with different soil types by use of an in-house dusting device and measured scattering, hemispherical and specular reflectance. Results showed that small-angle scattering due to dust is noticeable and severely reduces reflectance of mirrors with otherwise high specularity. Prominent result is a significant decrease of reflectance for increasing local solar incidence angles, for example from 0.91 at 8° incidence angle to 0.84 at 70° incidence angle. Therefore, we present a model to predict specular reflectance for different levels of cleanliness. The model applies the Lambert-Beer law and relates the incidence angle dependent attenuation of solar radiation to the dust layer. An application of our model to optical yield prediction indicated a significant relative decrease in predicted annual optical efficiency of up to 1.5% for parabolic trough power plants. In future research, a similar method could be adapted to predict optical losses of photovoltaic modules with flat cover glass.