Nanocoatings: Designed layers for solar thermal applications

Abstract

Solar energy is one of the most used renewable energy source in different applications nowadays. The direct method of harnessing solar energy is the solar thermal conversion method using solar absorbers. Solar thermal collectors are mainly used for heating water and for space heating. They capture incident solar radiation, convert it to usable thermal energy, and transfer the solar energy into a heat transfer fluid. All of these requirements should be accomplished economically with a minimum energy loss. One of the most important components of the solar thermal collector is the solar absorber. To be effective, the absorber should exhibit wavelength selectivity, i.e. have maximum solar absorbtance, minimum solar reflectance and thermal infrared emittance. A high solar absorbtance is required to collect as much of the incident solar radiation as possible and a low thermal infrared emittance contributes to minimize radiant energy losses. Usually, lightweight materials like, copper or aluminium, that have a high thermal conductivity, are used to transfer the absorbed energy from the surface to the media (fluid) which has to be heated. For high coefficient of efficiency of solar thermal collectors, the most important parts are: 1) spectral selective coatings that are composed of islands of metal embedded in a threedimensional matrix of dielectric with a solar absorbtance of α > 0.95 and an emittance of ε <= 0.05. Usually, those are TiNxOy coatings made by Physical Vapour Deposition (PVD) process, e.g. by combination of magnetron sputtering and cathodic arc evaporation deposition; and 2) enhanced construction solutions for better solar heat transmission from the absorber surface to the medium using the different absorbers' designs. In this context, the solar selective coatings are very important item on the absorbers' surface. They have advantages compared to galvanic or sprayed coatings used in solar applications. Their proper design and optimisation of layers order in multilayered structure of the coating and thickness dimensions of each layer is very important. Namely, transformation of solar irradiance to heat and transfer the heat from selective coating to the substrate materials such as: Cu, Al, Stainless steel, glass or polymers is very important step and it differ from substrate to substrate. Usually, these coatings are with thickness of several hundred nanometres, and each layer is with specific functional role within the coating. The general composition of selective coating consists of: protective layer, antireflective layer, several absorbting layers like, cermet layers, diffusion layer and infra red reflective layer. The main focus of this paper is optical properties of nanocoatings based on PVD selective coatings which allow maximum absorbtance, minimum emittance and as much as possible efficiency of the solar thermal collector that is assembled with coated absorbers with selective coatings.