The effects of nickel-pigmented aluminium oxide selective coating over aluminium alloy on the optical properties and thermal efficiency of the selective absorber prepared by alternate and reverse periodic plating technique

Diffused reflectance measurements and Kubelka–Munk values were studied. The effects of the alumina layer and the nickel content on the optical properties of the selective absorber were discussed. In general and with respect to aluminium alloy substrate, the diffused reflectance decreased by 85.25–91.42% (±0.01%). The emissivity increased by a factor of 252.50–624.50 (±0.01). The average absorptivity increased by a factor of 4.99–5.35 (±0.01). The average thermal efficiency is increased by a factor of 4.51–4.80 (±0.01). The sample of nickel content 60 μg/cm2 was found to have the optimum optical properties as a good selective absorber. It has the highest solar selectivity by the two measurements. The range of hemispherical solar selectivity is 4.22–8.36 (±0.01); while for average solar selectivity is 7.410–17.456 (±0.001). We found that the thermal efficiency is increased by increasing the nickel content to a certain limit (60 μg/cm2) then it decreased after this limit due to the increase in the emissivity. Also, the total hemispherical optical properties are accordance with the averaged optical properties obtained from diffused reflectance measurements over the solar range. The nickel-pigmented aluminium oxide is one of the best selective absorbers for the photo thermal conversion and it is of high durability.