We studied the optical theory of multi-layer coatings and simulated the reflectance of the multi-layer coatings. The solar selective surface we studied featured high absorption of solar radiation and low emittance of infrared rays. The optical property and film thickness of thin film can be obtained by taking the transmittance or reflectance of the films, and applied the FB model in the regression process. Solar selective surfaces composed of Al and AIN were fabricated by DC or RF sputtering technique. Graded refractive index of the Al-AlN composite can be obtained by tuning the volume fraction of metal. The structure of solar selective surface with high solar absorptance can be simulated by the program of optical design and FB model. However, the maximum temperature that the solar selective surface can achieve depends on the ratio of solar absorptance and emittance. In this study, the surface with multi-layer cermet coatings can lead to a very high absorptance of 0.96, an emittance of 0.04, and the ratio of α/ε is 24. The surface with four cermet coatings can lead to an absorptance of 0.94 and a very low emittance of 0.03, which has the highest ratio of α/ε, 31.4.
In this study, we also coated a single cermet layer as solar selective surface. The efficiency of the film can be improved by heat treatment. The absorptance and emittance of the heated film are 0.88 and 0.03 respectively, and the ratio of α/ε can be achieved as high as 29.3 just by a single coating.
