Analysis of the Reflection Properties of Solar Mirror Material by Measuring the BRDF to Enhance the Precision of State of the Art Methods

Abstract

Big mirrors are used in Concentrating Solar Power to concentrate sunlight onto an absorber where the produced heat drives an ordinary steam turbine cycle and electricity is generated. New mirror materials currently entering the market exhibit a higher microscopic surface roughness than the conventional glass mirrors, which causes a fraction of the reflected light to scatter away from the specular direction and occasionally miss the absorber. In order to quantify the amount that can contribute to the power generation process, the knowledge about the scatter distribution above the mirror is indispensable. In this diploma thesis I show how the roughness of a mirror affects the reflected light to scatter by reducing the surface profile to a minimum set of 2 statistical parameters, namely the root mean square roughness Rq and the auto correlation length T. A quantum mechanical approach yields an expression of the scatter distribution in terms of Rq and T. Around the specular peak the estimation shows good agreement with measurements carried out externally using a goniometer. In order to provide the ability of measuring the scatter distribution directly, a complete design of a new photogoniometer, the Mirror Reflectance Analyzer (MIRA) is devised. When assembled it will facilitate measuring the distribution in the entire 2 hemisphere with a nominal resolution of 1.3 msr (1.3 mrad per angular direction) in the order of seconds where usual goniometers need hours.