Antireflective transparent covers for solar devices

Promising solutions for low-cost antireflective (AR) covers for solar recei vers are based on the principle of mixing bulk material with air on a subwa velength scale in order to obtain very low effective refractive indices. Po ssibilities to achieve this are given by porous media and by periodic or st ochastic subwavelength surface-relief structures. In this work, the mention ed approaches were investigated experimentally. Subwavelength surface-relie f structures were embossed in organically modified sol-ger 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 im proved by up to 6% with a porous sol-gel coating at normal incidence. This improvement of the solar transmittance increases with larger angles of inci dence. (C) 2000 Elsevier Science Ltd. All rights reserved.