Antireflective transparent covers for solar devices

Promising solutions for low-cost antireflective (AR) covers for solar receivers are based on the principle of mixing bulk material with air on a subwavelength scale in order to obtain very low effective refractive indices. Possibilities to achieve this are given by porous media and by periodic or stochastic subwavelength surface-relief structures. In this work, the mentioned approaches were investigated experimentally. Subwavelength surface-relief structures were embossed in organically modified sol - gel materials or in acrylic materials, and porous sol - gel coatings were produced by dip-coating. It is shown that the solar transmittance of a transparent cover can be improved by up to 6% with a porous sol - gel coating at normal incidence. This improvement of the solar transmittance increases with larger angles of incidence.