Influence of Ni-Al coating thickness on spectral selectivity and thermal performance of parabolic trough collector.

This study investigates the influence of Ni-Al coating thickness on the spectral selectivity and thermal performance of a parabolic trough collector (PTC). Three thicknesses of Ni-Al coating for use as solar absorber material were successfully prepared on the outer surface of a stainless steel 316L (SS) tube by flame spray. The phase, morphology, and reflectance (R) of the Ni-Al coatings were characterized using several techniques. The PTC and solar receiver tube were specially designed and constructed for observing the collector thermal performance by following ASHRAE 93-1986. Looking at the results, the actual average thicknesses of the three Ni-Al coatings turn out to be 195, 215, and 299 μm. The morphology and chemical composition of all three thicknesses are similar. The chemical composition in the cross-sectional view exhibits non-uniform distribution. The three thicknesses of the coating are composed of NiO and Al₂O₃ phases, which also corresponded to the results of SEM-EDX mapping. The differences in a solar absorptance (α) of the three thicknesses of Ni-Al coating are not statistically significant, with an average α value of 0.74-0.75. However, there are differences in thermal efficiency of the PTC depending on the thickness of the Ni-Al coating. Of the three samples, the thickest one (299 µm) demonstrates the highest ability to convert solar radiation into thermal energy. [ABSTRACT FROM AUTHOR]