In Air Durability Study of Solar Selective Coating for Parabolic Trough Technology

The effectiveness of Parabolic Trough CSP (Concentrated Solar Power) is strongly dependent on the Heat Collector Element (HCE) and specifically on the thermal stability of solar absorber at high temperature. For Molten Salt applications, operative temperature up to 600 degrees C can be reached with vacuum solar receiver tubes (HCE). Nevertheless operation in air is also possible: such operative mode requires yet a deep knowledge of the possible degradation mechanisms of the solar selective coating. Here we report on the results of the optical and thermal investigation performed by CEA LITEN on Archimede Solar Energy (ASE) absorber coating according to the established procedures and the experimental facilities settled in the framework of the STAGE-STE project. A complete study of the accelerating thermal ageing test in air at high temperatures (up to 500 degrees C) of ASE solar absorber is presented. The relation between optical parameter, such as solar absorbance and emittance, and temperature as function of time is used to study the thermal stability of coating. To this aim, we use a performance criterion defined as the variation of solar emittance solar absorbance during the ageing test. The degradation mechanism at elevated temperatures has been identified as oxidation of the metallic part of the cermet layer, W to WO3 and WOx. By means of an Arrhenius analysis, an activation energy of approximately 150 kJ/mol is calculated for this degradation process. Such a value has been found consistent with the expects from literature. Additionally, the lifetime prediction in the case of CSP plant in Ouarzazate is presented. The results of this analysis show a good thermal stability of samples up to 375 degrees C for 25 years of uses in CSP plants working in air.