Characterization and thermal aging tests of Cr based multilayer for unconcentrated solar thermal applications

•New absorber designed for unconcentrated solar thermal applications under vacuum.•Custom apparatus reproducing a flat plate collector under vacuum used to characterize.•Optical characteristics measured as function of temperature (up to 573 K).•Thermal aging tests to predict the service lifetime of absorber.•New Performance Criterion for absorbers working in mid-high temperature applications. Optical characterization and thermal aging tests are performed on a sputter-deposited coating, consisting of SiO2/Cr2O3/Cr/Cr2O3 layers, designed and developed as a selective solar absorber to be used for unconcentrated solar thermal applications. Both measurements are performed by using a home-made apparatus, which mimics a flat plate collector under high vacuum. A Performance Criterion (PCη(T)), based on absorber efficiency is proposed, and a forecast of service lifetime is obtained. As a result of the thermal aging tests, the selective solar absorber under study appears to be highly efficient at mid temperatures (up to 573 K) and thermally stable at temperatures (up to 690 K).