Soiling model for spectral reflectance of solar mirrors

In concentrating solar power plants, mirrors are used to redirect solar radiation onto a receiver. The optical yield of a plant is related to the mean value of the near-specular reflectance in the range of interest of incidence and acceptance angles for that plant. To avoid the need for an exhaustive measurement of the solar mirror at various incidence and acceptance angles, a predictive method based on the Equivalent Model Algorithm for Solar Mirrors was recently proposed. Whilst developed for clean surfaces, unfortunately that model is inaccurate for soiled solar mirrors. In this paper, an additional model to describe the spectral reflectance loss caused by soiling of solar mirrors is presented and successfully tested on a set of artificially and naturally-soiled specimens. The model is based on absorption, scattering and diffraction. Its parameters are obtained by best-fitting a set of experimental hemispherical and near-specular reflectance spectra, both in soiled and cleaned state. The best-fit software developed is now published as open source software, providing access also to non-specialized users. The model also allows to correlate simple handheld reflectometer readings used in the solar field (with typically limited spectral and acceptance angle information) to the most meaningful parameter determining the plant yield: the solar-weighted near-specular spectral reflectance of the soiled reflector. Such correlations are specific for the plant, the kind of local dust and the chosen portable reflectometer.