Preparation and characterization of absorbing tubes with spectrally selective coatings using economical methods for low- to mid-temperature solar thermal collectors

摘要

Absorbing surfaces with spectrally selective coatings having both high solar absorptance (alpha(S)) and low thermal emittance (epsilon(T)) are used in solar collectors. Selective absorbing surfaces can be made of specific materials or combinations of them with various methods. The common methods of fabrication are executed on the basis of the surface coating, whereas the solar absorptance depends on the black coated layer and the thermal emittance on the undercoating layer. In this study, absorbing surfaces from various materials were coated using electrodeposition, chemical conversion (such as the chromate conversion), anodizing and painting techniques in accordance to specified formulas and the existing operating conditions. Materials such as stainless steel 316, copper and aluminum have been deployed as substrates of the samples. The samples of the selective absorbing surface were considered as absorber-reflector tandems. According to the experimental results from the performed optical characterization of the fabricated samples, it can be deduced that most of them had high solar absorptance alpha(S) more than 0.80 (up to 0.940) and low thermal emittance epsilon(T) (100 degrees C) less than 0.12 (as low as 0.085). In addition, most samples had high photothermal conversion efficiency (eta) and selectivity factor (alpha(S)/epsilon(T)). Also, the morphology of surfaces and particular elements identification of the coatings were studied and evaluated by scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS) detector. Moreover, the cross-section image was used to determine the approximate thickness of the films formed in some of the prepared samples. (C) 2015 Elsevier B.V. All rights reserved.