Spectrally selective tandem absorbers for photothermal conversion in high temperature solar thermal systems

Abstract

Solar energy is the inexhaustible and abundant energy resources on the earth which can be a best substitute for the fossil fuels. Over the past couple of decades, all the solar technologies are rising very steadily in two main branches including photovoltaics and solar thermal. One of the major components of solar thermal system is receiver, which plays an important role to enhance the photo-thermal efficiency by absorbing maximum amount of solar radiation with a minimum heat loss. In this regard, our objective was to fabricate a spectrally selective absorber coating for receiver which should have a high absorptance of and#8805; 0.95 in the solar spectrum (0.25-2.5 and#956;m) and a low thermal emittance of and#8804; 0.05 in the infrared region (2.5-25 and#956;m). In addition, while considering the real field applications, these coatings should exhibit exceptional thermal (gt 450 °C) and environmental stability in different operational conditions. In spite of the outstanding thermo-chemical and thermo-physical stability of the ultra-high temperature ceramics (UHTCs), there are only a few reports on the spectral selectivity of these materials. Therefore, in the first part, we have utilized DC and RF magnetron sputtering system to prepare TiB2/TiB(N)/Si3N4 -based multilayer absorber coating. A systematic investigation was carried out to understand the influence of various deposition parameters including target power, deposition time and reactive gas flow on the spectral selectivity of the coating. The optimal process parameters lead to a high absorptance of 0.964 and an emittance of 0.18. However, the film is inadequate in terms of environmental stability. We have subsequently developed W/WAlN/WAlON/Al2O3 -based multifunctional novel coating using magnetron sputtering. The rationale behind this specific coating design was based on good optical properties and high diffusion block ability of transition metal -based oxides and oxynitrides...