Nanostructured spectrally selective coatings for high-temperature solar thermal applications

Abstract

A spectrally selective TiAlC/TiAlCN/TiAlSiCN/TiAlSiCO/TiAlSiO coating was deposited on stainless steel substrate and silicon substrates by unbalanced magnetron sputtering system. Each individual layer of the tandem absorber was optimized by varying the reactive gas flow rates (C2H2, N2 and O2) and target power densities (Ti, Al and Si). The optimized tandem absorber shows a solar absorptance of 0.960 and an emittance of 0.15 at 82°C, measured using solar spectrum reflectometer and emissometer, respectively. The refractive indices and extinction coefficients of the tandem absorber were studied using the phase-modulated spectroscopic ellipsometry. Absorption coefficient of each layer was calculated from the extinction coefficient of the layer. The results indicate that the first three layers (i.e., TiAlC, TiAlCN, and TiAlSiCN) are absorbing in nature, while TiAlSiCO and TiAlSiO act as intermediate and antireflection layers. The obtained refractive indices and extinction coefficients of the tandem absorber were used to simulate the reflectance of the deposited tandem absorber using SCOUT software. Simulated reflectance data of the tandem absorber showed a good agreement with the experimental data. The angular dependence of the selective properties of the tandem absorber was studied by measuring the reflectance spectra of the tandem absorber at different incident angles. The thermal emittance of the tandem absorber at high temperatures (80–500°C) was also studied in detail. At the temperature of 200°C, 300°C, 400°C and 500°C the tandem absorber shows the emittance of 0.152–0.157, 0.181–0.19, 0.214–0.246 and 0.251–0.275, respectively with an absorptance of ~0.930. These results show the good selectivity of the tandem absorber even at high operating temperatures (e.g., 500°C). The performance evaluation of the tandem absorber has been evaluated by heating it in air at 325 °C for 600 h and in vacuum upto 600°C for 910 h under cycling heating conditions without any significant degradation of the optical properties. Thus demonstrating its suitability for high-temperature solar thermal power generation applications.