Anti-reflection coatings for solution-chemically derived nickel-alumina solar absorbers

In order to suppress reflections from a solar absorber surface an anti-reflection (AR) layer is usually deposited on top of the solar absorbing coating. For the AR coating to function well it should be made of a material with a lower refractive index than the underlying surface. Provided that the AR coating is sufficiently thin, it will not increase the thermal emittance value. Besides increasing the solar absorptance it is equally important that the AR layer is long term stable in order to create a successful solar selective coating. The objective was to create a highly efficient, flexible, inexpensive and durable AR coating for solar absorbers using simple techniques. Elastic properties are requested because it prohibits the formation of micro-cracks. Alumina, silica and hybrid silica AR coatings were synthesized using solution chemistry and sol-gel techniques. The AR coatings were deposited on the absorber surface using spin coating. The absorbers used without an AR layer had a normal solar absorptance of 0.79-0.81 and a normal thermal emittance of 0.03. The optimum anti-reflection coated sample acquired a normal solar absorptance of 0.93 while the thermal emittance remained unaltered. The study has shown that by adding a proper AR coating to a solar absorber the spectral selectivity can be considerably enhanced. (C) 2004 Elsevier B.V. All rights reserved.