High solar performance selective surface using bi-sublayer cermet film structures

Abstract

Numerical modelling calculations have been used to design solar selective absorbers with new cermet film structures, composed of two cermet sublayers, each having a different metal volume fraction, located between a conventional metal reflector and a dielectric anti-reflection layer. These selective surfaces may use a variety of practical cermet materials and all achieve better solar thermal performance than any published results. For example, our best predicted ratio of absorptance to normal emittance α/e at room temperature is 46 for a CuSiO2 cermet. The best experimental result of α/e at room temperature is also 46 for the same cermet. From a computer optimazation using published experimental dielectric functions of CoAl2O3 cermets, we clearly show improved performance is obtained by using two cermet layers rather than one. For example, an absorptance of 0.90 and normal emittance of 0.024 at 50°C (α/e = 37) could be obtained for a film composed a two cermet sublayers on a Mo reflector with an Al2O3 antireflection layer. For an optimized low emittance double cermet coating, hemispherical emittance at 350–400°C is very close to hemispherical emittance at room temperature.