Review of physical vapor deposited (PVD) spectrally selective coatings for mid- and high-temperature solar thermal applications

Solar energy is the most abundant source of renewable energy. The direct method of harnessing solar energy is the solar thermal conversion method using solar absorbers. The absorbers are coated with solar selective coatings with high absorptance and low thermal emittance. Spectrally selective coatings which are stable up to temperatures ≤300 °C (in air and vacuum) have been developed in the past. These coatings are mainly deposited from wet chemical routes (e.g., electrodeposition) and have been reviewed widely in the literature. Because of the environmental issues as well as low thermal stability of these wet chemical deposited coatings, researchers all over the world started looking for other alternative routes such as physical vapor deposited (PVD) coatings. A great deal of research has been carried out since 1990s to develop PVD coatings for both mid- and high-temperature applications. The mid-temperature coatings are used mainly for solar hot water and industrial process heat applications, whereas, the high-temperature absorber coatings are used in concentrating solar power systems for solar thermal power generation. It is well recognized that in order to increase the efficiency of solar thermal power plants, solar selective coatings with high thermal stability are required. In recent years, significant efforts have been made in the field of solar selective coatings to achieve high solar absorptance and low thermal emittance at higher temperatures (T≥400 °C). Transition metal based cermets have emerged as novel high temperature solar selective coatings, which are currently being used for solar thermal power plants for electricity generation. Solar selective coatings based on transition metal nitrides, oxides and oxynitrides also hold great potential for high-temperature applications because of their excellent mechanical and optical properties, which are yet to be commercialized. In this review, we present the state-of-the-art of the physical vapor deposited solar selective coatings used for solar thermal applications with an emphasis on sputter deposited coatings for high-temperature applications. A detailed survey, covering the period 1970-present, has been made for the PVD deposited solar selective coatings with high absorptance and low emittance. This review article also describes in detail about the commercially available PVD coatings for flat-plate/evacuated tube collectors and solar thermal power generation applications.