VERY LOW-EMITTANCE SOLAR SELECTIVE SURFACES USING NEW FILM STRUCTURES

New cermet film structures suitable for selective solar absorbers, composed of two cermet sublayers with identical metal volume fractions in each sublayer, on metal reflectors with an antireflection dielectric layer coating are described. The absorbing cermet sublayers have thickness and volume fractions such that solar radiation is absorbed internally and by phase cancellation interference. They are substantially transparent in the thermal infrared region. Modeling of the new cermet layer structures has revealed a number of different selective surfaces, involving a variety of practical cermet materials, with better performance than recent published results. For example, the best predicted ratio of absorptance to normal emittance alpha/epsilon at room temperature is 46 for a Cu-SiO2 cermet, 49 for a Cu-Al2O3 cermet and a Au-Al2O3 cermet. The best laboratory results of alpha/epsilon at room temperature thus far are 46 for a Cu-SiO2 cermet, and 45 for a Au-Al2O3 cermet. From computer modeling, using published experimental dielectric functions of a Co-Al2O3 cermet, absorptance of 0.93 and normal emittance of 0.026 at 50-degrees-C (alpha/epsilon = 36) could be obtained for a two-cermet-sublayer film on a Mo reflector with an Al2O3 antireflection layer.