Scattering and specular reflection of solar reflector materials – Measurements and method to determine solar weighted specular reflectance

Abstract

Reflector materials directly affect the performance of concentrating solar applications such as solar thermal power plants or process heat installations. For concentrating collectors the specular reflectance and scattering of reflector materials in the near specular range is of special relevance for a good performance. In this article, we show characteristic bidirectional scatter and specular reflectance curves for solar materials, such as glass-based, polymer-based and aluminum-based reflectors, measured using the instrument VLABS, a commercial reflectometer and a spectrophotometer. The solar weighted specular reflectance is a relevant performance indicator, allowing the comparison of the different reflectors. However, usually it is not measured in solar laboratories. This is why this article presents a method for modelling of solar weighted specular reflectance based on hemispheric reflectance and single wavelength measurements. The model applies surface scatter theory and the concept of total integrated scatter. It allows to predict the specular reflectance for the solar spectral range. The application of the model to solar reflector materials with different surface roughness features and the impact of beam spread on the solar weighted specular reflectance is shown. The methodology allows for an improved evaluation and comparison of innovative reflector materials for solar applications.