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

• The selective absorbing surfaces were considered as absorber–reflector tandems.
• Methods of electrodeposition, chemical conversion, anodizing and painting were used.
• The best optical properties of samples are αS=0.94αS=0.94 and εT=0.085εT=0.085.
• Economical methods were used in manufacture of solar absorbing surfaces.
• The samples are useful to achieve low- to mid-temperature ranges in solar collectors.

Absorbing surfaces with spectrally selective coatings having both high solar absorptance (αSαS) and low thermal emittance (εTεT) 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 αSαS more than 0.80 (up to 0.940) and low thermal emittance εTεT (100 °C) less than 0.12 (as low as 0.085). In addition, most samples had high photothermal conversion efficiency (η  ) and selectivity factor (αS/εT)(αS/εT). 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.