Durability tests of solution-chemically derived spectrally selective absorbers

A promising novel solution-chemical method to fabricate spectrally selective solar absorber coatings has been developed. The objective was to create highly efficient, flexible, inexpensive and durable selective absorbers for solar thermal applications using simple techniques. The selectively absorbing film consists of a composite with nickel nano-particles embedded in a dielectric matrix of alumina. ti-reflection (AR) material should have the following properties: the proper refractive index, low thermal emittance, dense, flexible and long-term stable. The AR materials tested were silica, alumina and mixtures of silica–titania. The refractive indexes of the above-mentioned materials range from 1.4 (silica) to about 2.1 (50/50 molar ratio silica/titania mixture). Besides increasing the normal solar absorptance, αsol, it is equally important that the AR layer is long-term stable in order to create a successful solar selective coating. The anti-reflection coatings were synthesized using different solution-chemical methods and deposited on the absorbing layer by spin coating. Prepared samples were subjected to an accelerated lifetime test. In the test procedure the temperature of the environment was set to 40 °C and the relative humidity to 95%. Samples made with alumina as anti-reflection coatings failed the aging test. All other materials, silica and silica–titania mixtures proved to be very resilient. Samples that were coated with these anti-reflection materials showed no visible degradation of the sample surface even after 600 h of testing. -layer absorbers without an AR layer typically attain a normal solar absorptance of 0.80 and a normal thermal emittance of 0.03. Of the samples made with durable films a 70/30 silica/titania mixture showed the greatest increase of the αsol value, 0.91, while the thermal emittance remained unaltered.