Potential of quarterwave interference stacks for colored thermal solar collectors

The architectural integration of thermal solar collectors into buildings is often limited by their black color and the visibility of tubes and corrugations of the absorber sheets. A certain freedom in color choice would be desirable, but the colored appearance should not cause excessive energy losses. Multilayered interference filters on the collector glazing can produce a colored reflection, hiding the corrugated metal sheet, while transmitting the non-reflected radiation entirely to the absorber. We investigate the potential of quarterwave stacks by simulation of their optical behavior, yielding the visible reflectance RVIS, the solar transmittance Tsol, a figure of merit M = RVIS/Rsol, and the CIE color coordinates. The necessary number of individual layers in the multilayer stack as well as the choice of refractive indices and thus of thin film materials are discussed. Finally, examples for realistic multilayer designs are proposed.