4.3 Vacuum evaporated thin films for solar collectors

Abstract

Selectively absorbing surfaces are important for the efficient use of solarenergy. The main characteristics aimed at for this type of coatings are the highest possible absorptivity for solar radiation and the lowest possible emissivity for thermal radiation. (Temperatures up to 100–200°C). There are several ways to create surfaces with these properties. In the present work evaporated semiconductor films with suitable anti-reflection coatings were used on different metals. For the laboratory tests, the metals were vacuum evaporated on glass substrates. The evaporations were performed with an electron beam gun in a diffusion pumped system. The evaporated thicknesses and the rate of evaporation were controlled by a quartz crystal monitoring system. The selective coatings were studied on flat and rough metal surfaces. Sinusoidal, metal covered gratings with different periods were used as the rough absorber surfaces. This simplified the measurements of absorptivity and emissivity and gave a more thorough understanding of the basic thin film phenomena. It is possible to use gratings because the general rough surface can be looked upon as a superposition of different sinusoidal gratings. A comparison is made between the properties of the layers on flat and rough surfaces. The coatings were subjected to high humidity and high temperature tests. They were also cycled between the intended working temperature and ambient temperature to check possible degradation. Results concerning film adherence and stability during these environmental tests are given.