A new cermet film structure of solar thermal absorber is presented. The typical film is similar to that of the single cermet layer case, except the single cermet layer is replaced by two isotropic cermet sublayers. The calculated results have been shown that low- and high-temperature performances using this new selective surface structure are excellent. The value of the ratio of absorptance to normal emittance α/ε, 46, for deposited film has been achieved.

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Articles you may be interested inLow-emittance electron bunches from a laser-plasma accelerator measured using single-shot X-ray spectroscopy AIP Conf. Proc. 1507, 278 (2012); 10.1063/1.4773707 A lowenergy and lowemittance ion beam source for use with the supermicrobeam production system (abstract) Rev. Sci.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 a/e at room temperature is 46 for a Cu-SiO, cermet, 49 for a Cu-A&O3 cermet and a Au-A&O, cermet. The best laboratory results of a/~ at room temperature thus far are 46 for a Cu-SiO, cermet, and 45 for a Au-Al,O, cermet. From computer modeling, using published experimental dielectric functions of a Co-A&O3 cermet, absorptance of 0.93 and normal emittance of 0.026 at 50 "C! (a/~=36) could be obtained for a two-cermet-sublayer film on a MO reflector with an AlZ03 antireflection layer. 3013

A physical model of the optical behaviour of aluminium nitride cermet solar coatings has been used to optimize the metal volume fraction and layer thickness of the coatings. A modified photo-thermal conversion efficiency for solar collector tubes is presented and used. The cermet layers are generally deposited by reactive sputtering in a gas mixture of argon and nitrogen. Sputtered aluminium Alsp is used as a metallic component in the cermet and its refractive index, evaluated in this study, is employed. Due to oxygen contamination, aluminium oxynitride (AlON) is used as a ceramic component in the cermet. Bruggeman approximations are used to calculate the dielectric function for composite materials. An initial ten-layer grade film optimized to one nearly identical to a double cermet film structure when maximizing photo-thermal conversion efficiency at 80 °C under a concentration of 1. The optimized films, F10Lm (for initial ten-layer graded film) and F3Lm (for three-layer film) have an identical solar absorptance of 0.957 and an identical hemispherical emittance of 0.048 at 80 °C. The optimized film consists of one anti-reflection layer and two cermet layers with metal volume fractions of 0.093 and 0.255, and thicknesses of 30 nm and 93 nm, respectively, going from the anti-reflection coating to the infrared reflector layer. The solar performance can be further improved using a lower refractive index anti-reflection layer and a lower emittance infrared reflector. For example, using an Al2O3 anti-reflection layer, the solar absorptance increases to 0.974, and using a Cu infrared reflector, hemispherical emittance decreases to 0.033 at 80 °C. For these different anti-reflection and infrared reflector materials, optimized calculations have also predicted that the double cermet layer film structures have the highest photo-thermal conversion efficiency.

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