High efficiency MoAl 2O 3 cermet selective surfaces for high-temperature application

Abstract

Highly efficient MoAl 2O 3 cermet solar absorbers have been designed with a numerical model and deposited experimentally. The typical film structure is an Al 2O 3 anti-reflection layer on a double MoAl 2O 3 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°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 Al 2O 3 anti-reflection coating. The corresponding normal absorptance and hemispherical emittance at 350°C are 0.96 and 0.11, respectively. MoAl 2O 3 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°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 Al 2O 3 anti-reflection coating.