Correlation between Solar Mirror Degradation and Colorimetric Measurement of Protective Back Layer

AbstractBy means of large mirrors, CSP (Concentrated Solar Power) technologies concentrate the solar energy on an absorber where it is collected as thermal energy. The decrease of the kWh cost and the insurance of a 25 years lifetime minimum are the key points to make these technologies cost competitive and ensure their large deployment. The solar field is the first expense item (20 to 30% of the investment) and is composed by the reflectors which concentrate the solar flux on the absorber. The durability of the mirrors is therefore of principal interest and has to be studied since the conception of the power plant. Actually, the majority of this technology uses glass mirrors made with silver and copper layers for a high reflectance. The protection of the silver layer from oxidation is ensured both by the copper layer and backside paint. In service conditions, the mirrors are submitted to arid climate where temperature, humidity and solar irradiation are extreme. Accelerated climatic tests in controlled environment are used to create the same degradations than the ones happening on site. Specular reflectance being the primary function of the mirrors, this is the parameter usually characterized to evaluate the aging of a mirror. Depending on the aging test and specifically for outdoor exposure, it might take a long time before a loss of reflectance occurs. We present here a new methodology based on the monitoring of the protective back layer degradation that can be used to anticipate the reflectance loss. Based on the activated energy calculated via an Arrhenius law, the lifetime prediction in normal condition can be established