The history of solar thermal applications is longer than that of photovoltaic applications, commercial solar water heaters appeared in 1891 solar thermal applications are through the absorption of sunlight, light energy into thermal energy after direct use or storage can also be converted into electricity by heating steam-driven generators. Solar thermal applications can be divided into three categories according to the temperature range: low-temperature applications (<100C), mainly used for swimming pool heating, ventilation air preheating, etc., medium-temperature applications (100 ~ 400C), mainly used for domestic hot water and room heating, process heating in industry, etc.; high-temperature applications (>400C), mainly used for industrial heating, thermal power generation, etc.. With the promotion of the collector power generation system, resistance to medium and high temperature and environmentally resistant photothermal materials research has become a priority.
Thin film technology also plays an important role in solar thermal applications. Due to the low solar energy density at the surface (about 1kW/m² at midday, collectors need a large area to collect solar energy. The large area/thickness ratio of solar photothermal films results in films that are susceptible to aging, which affects the lifetime of solar photothermal equipment. The key requirements for solar thermal films are threefold: high energy efficiency, long lifetime and economical. Spectral selectivity is used to evaluate the energy efficiency of solar thermal films. A good solar thermal film needs to have excellent absorption over a wide range of solar radiation bands and low thermal emissivity. a/e coefficient is used to evaluate the spectral selectivity of the film, where a stands for solar absorptivity and e stands for thermal emissivity. The thermal performance of different films varies considerably. Early heat-absorbing films consisted of a black coating on a metal foil, which lost up to 45 per cent of the long-wavelength radiation emitted as it absorbed heat and warmed up, resulting in solar energy harvesting of only 50 per cent. The efficiency of photothermal films can be significantly improved by using spectrally selective thin-film materials such as platinum metal, chromium, or carbides and nitrides of some transition metals. Photothermal films are usually prepared by CVD or magnetron sputtering, and the thermal emissivity can be reduced to as little as 15 per cent for films with a collector efficiency of up to 80 per cent. Ideal spectrally selective collector films have an absorption coefficient of more than 0.98 in the main bands of the solar spectrum (<3um), and a thermal radiation coefficient of less than 0.05 in the 500C thermal radiation band (>3um), and are structurally and performance-stable at 500°C in an air atmosphere.
–This article is published by vacuum coating equipment manufacturer Guangdong Zhenhua Technology.
Post time: Aug-05-2023