| Solar energy is inexhaustible in quantity and environmentally friendly and can be converted to various forms of energy to be used. Conversion of solar radiation into useful heat is widely used on solar collectors, solar absorber surfaces are the most important part of thermal collectors. It is essential for an absorber to absorb the incoming solar radiation as much as possible and at the same time keep the collected heat. Selective absorbers can accomplish this requirement by having high solar absorptivity and high thermal reflectivity simultaneously. Currently, various preparation methods of solar spectrum selective absorption coatings have their own limitations in the practical application, so developing absorbers with comparable optical properties through a cheaper and simpler manufacturing method is the actual need of the solar industry. Among all the selectivity mechanism, preparing a composite layer on a metallic matrix which is highly reflecting in the thermal infrared will be the most flexible method which is easy to control and achieve. Optical properties of a composite layer can be easily modified by changing the thickness, constituent volume fraction, and the particle size.In this paper, we developed solar spectrum selective absorption composite coatings by plasma spraying, sol-gel and wet chemistry method. Wet chemistry method is a new type of preparation method of solar spectrum selective absorption coatings. The structure and surface topography of absorbing coatings were analyzed by XRD, FTIR, EDS and SEM. Optical properties of absorbing coatings were analyzed and researched by UV-Vis-NIR spectrophotometer of UV-3600 type and Bruker infrared spectrometer of Tensor 27 type.The results showed that Ni-Al2O3 metal-ceramic composite coatings which were prepared by plasma spraying method under optimal experimental conditions, reached a solar absorptance of 0.81 and a thermal emittance of 0.68, and the quality factorα/εwas about 1.19, selective absorption property of the coating was poor. NiO-Al2O3 composite films which were prepared on the surface of stainless steel substrate by sol-gel dip-coating method under optimal experimental conditions, reached a solar absorptance of 0.76 and a thermal emittance of 0.25, and the quality factorα/εreached 3. Adding an anti-reflection layer of SiO2 on top of the absorbing layer enhanced the performance of the absorber. The optimum anti-reflection coated sample reached a solar absorptance of 0.84 and a thermal emittance of 0.30. Ni-NiO-Al2O3 composite films which were prepared on the surface of stainless steel substrate by wet chemistry method under optimal experimental conditions, reached a solar absorptance of 0.85 and a thermal emittance of 0.12, and the quality factor a/s was about 7.1, selective absorption property of the coating was the best. Adding an anti-reflection layer of SiO2 on top of the absorbing layer enhanced the performance of the absorber. The optimum anti-reflection coated sample reached a solar absorptance of 0.93 and a thermal emittance of 0.18.The results of surface topography of absorbing coatings showed that spectral selective absorption properties of coatings were not only related to the property of the material itself, but also related to the surface structure of the coating. The surface of coatings which were prepared by plasma spraying method was rough and not dense, they also had many large holes which had a role of trap absorption to infrared spectrum and thus resulted in a large emittance of coatings, selective absorption property of the coating was poor. Films prepared by sol-gel and wet chemistry method all had a micro-rough surface structure. This special surface structure could improve the spectral selective absorption property of thin films. |
