Solar Coatings Based on Ag Infrared Reflector with High Stability at Medium and High Temperature

Abstract

The manufacturing of thermally stable solar coatings with high photo-thermal performance represents a key factor for the further deployment of the CSP technology. Since 2005, ENEA has been developing solar coatings suitable for medium and high temperature applications based on the technology of double nitride cermet, by employing silver and tungsten as infrared reflectors, respectively. Thanks to the high infrared reflectance of silver, the corresponding coatings have better optical performance than those with tungsten; however, the high diffusivity of silver compromises its use at high temperature. In order to improve the structural and chemical stability at medium and high temperature of coatings based on silver, this infrared reflector was placed between compact and uniform layers of metal and cermet manufactured by using high-energy and fast deposition processes. In particular, an Unbalanced Magnetron cathode was adopted to promote an ion-assisted deposition process that improved uniformity and compactness of the metal and cermet films. The new coating shows no photo-thermal parameters degradation after 25 years of service at the operating temperature of 400 °C, while its photo-thermal conversion efficiency decreases by only 1.5% after 25 years of service at an operating temperature of 514 °C.