Research On Solar Spectrally Selective Absorbing Coatings For Medium-high Temperature Application

Solar spectrally selective absorbing coating which is the most important on the solar energy collector restricts the collector efficiency directly. Majority of the coatings were only for medium-low temperature application, and some even could only be used in vacuum environment due to the influence of materials, structures, and processing methods. Currently, magnetron sputtering technology was used to prepare high-tempeature resistent metal ceramics and multi-layer gradient films, which can be applied to non-vacuum high-temperature surroundings but costly. Plasma sprying technology and sol-gel method with the advantages of simply processing, low cost were popular employed in coating preparation, and large-scale production can be achieved. However, the above methods being directly used to prepare solar selective absorbing coating are rarely reported. As a result, this article completed the experiments.During the experiment, Cr2O3,Al2O3 and SiO2 were taken as the main drugs, spray drying mathod was employed to obtain spherical powder agglomeration which is spraying powder with excellent mobility. Black chrome solar selective absorbing coating based on stainless steel was prepared by plasma spraying technology. Results indicated that the solar absorptance (α) and thermal emittance (ε) of the coatings were high. After being polished, the thermal emittance of the coatings fell to 0.76. The absorptance changed little but the emittance decreased to 0.50 after SnO2 films were fabricated. The composition did not change after thermal shock test by XRD analysis, it showed the coatings had excellent thermal stability and thermal erosion resistance.We also obtained the solar selective absorbing coating of Cu1.5Mn1.5O4 with spinel structure from CuCl2, Mn(CH3COO)2 and ethanol through sol-gel method. The dry gel TG-DSC analysis shows that the Cu1.5Mn1.5O4 phase begins to form at 450℃. As can be seen from the XRD patterns analysis that the precursor power prepared by annealing 1 h at 500℃contains Cu1.5Mn1.5O4 mostly, as well as a small amount of CuO and Mn2O3.The gel film-forming property improved with adding hydroxypropyl cellulose into the gel.The coating grains prepared under the sintering heating rate of 1℃/min appeared tightly packing, uniform size, and no visible defects on the surface. After pulled with the speed of 20mm/min twice, we got the coating with the thickness of 1μm, absorption rate of 0.88 and emissivity rate of 0.05. After the coating insulated among 1 to 30h at 500℃, CuO and Mn2O3 transformed into Cu1.5Mn1.5O4, and coating absorption and emissivity rate increased slightly, of the maximum values 0.91 and 0.09, respectively. The SEM images show the coating grains are without growth, and of no cracks or peeling, which means the coating gets a wonderful thermal stability.