Application of Ni-Al2O3 cermet coating on aluminium fin as the solar absorber in evacuated tube collector (ETC)

Abstract

In a commercial evacuated tube collector (ETC) with a double layer tube, a solar absorber coated on outer surface of the inner glass of a vacuum tube, and aluminium (Al) fin is used for transferring heat from the solar absorber to the heat pipe. In the present study, the Al fin inside the ETC was initiated to be a solar absorber and heat transfer medium together with using a clear double layer evacuated tube. Various thicknesses of Ni-pigmented alumina (Ni-Al2O3) layer over the Al fin were prepared using the anodic anodization process. The thickness and solar absorptance (α) of all samples were investigated. The collector thermal efficiency (η) of the ETC with different thicknesses of Ni-Al2O3 coating over Al fin as the solar absorber was evaluated, following standard ISO 9806-1. The thicknesses of Ni-Al2O3 coatings were 11, 12, and 16 µm. For each thickness, the α over the whole wavelength of the solar spectrum had an identical value (∼0.94). The maximum collector thermal efficiency, FR (τα), of the new prototype ETC at each thickness was 0.71, 0.72 and 0.81. The -FRUL term for each thickness was -15.66, -15.83 and -18.56 W/m2·°C.