High efficiency Mo-Al2O3 cermet selective surfaces for high-temperature application

Highly efficient Mo-AL(2)O(3) cermet solar absorbers have been designed with a numerical model and deposited experimentally. The typical film structure is an Al2O3 anti-reflection layer on a double Mo-Al2O3 cermet layer on a Mo or Cu metal thermal reflector. In numerical calculations of the thermal emittance at high temperature for these selective surfaces, the temperature dependencies of the complex refractive indices of the metal reflector and cermet in the infrared region have been considered, and the dielectric functions of the cermet materials are evaluated using Sheng's approximation. An optimization calculation yields a photothermal conversion efficiency as high as 0.914 at 350 degrees C for a concentration factor of 26 for the film structure consisting of a double cermet layer on a Mo metal thermal reflector with an Al2O3 anti-reflection coating. The corresponding normal absorptance and hemispherical emittance at 350 degrees C are 0.96 and 0.11, respectively. Mo-Al2O3 cermet selective surfaces using the double cermet layer structure were deposited by vacuum co-evaporation, and an absorptance of 0.955 and near normal emittance of 0.032 at room temperature have been achieved. An emittance of 0.08 at 350 degrees C is estimated based upon room temperature experimental data for the film structure of a double cermet layer on a Cu metal thermal reflector with an Al2O3 anti-reflection coating.