A selective solar absorber for unconcentrated solar thermal panels

A new Selective Solar Absorber, designed to improve the Sun-to-thermal conversion
efficiency at mid temperatures in high vacuum flat thermal collectors, is presented. Efficiency has
been evaluated by using analytical formulas and a numerical thermal model. Both results have
been experimentally validated using a commercial absorber in a custom experimental set-up. The
optimization procedure aimed at obtaining Selective Solar Absorber is presented and discussed in
the case of a metal dielectric multilayer based on Cr2O3 and Ti. The importance of adopting a real
spectral emissivity curve to estimate high thermal efficiency at high temperatures in a selective solar
absorber is outlined. Optimized absorber multilayers can be 10% more efficient than the commercial
alternative at 250 C operating temperatures, reaching 400 C stagnation temperature without Sun
concentration confirming that high vacuum flat thermal collectors can give important contribution to
the energy transition from fossil fuels to renewable energy for efficient heat production.