Heat Collecting Materials For Solar Water Heaters

- Jan 30, 2019-

The sun mainly brings light and heat to the earth in the form of electromagnetic radiation. The wavelength of solar radiation is mainly distributed in the range of 0.25~ 2.5μm. From the light thermal effect, the infrared band in the solar spectrum directly produces thermal effects, and most of the light can not directly generate heat. We feel warm and hot in the strong sunlight, mainly because the clothes and skin absorb the sun's rays, resulting in photothermal conversion. From a physical point of view, black means that light is absorbed almost entirely, and the absorbed energy is converted into heat. Therefore, in order to maximize the photothermal conversion of solar energy, it seems that the use of black coating materials can be satisfied, but the actual situation is not so. This is mainly the material itself also has a thermal radiation problem. From the theory of quantum physics, the wavelength range of blackbody radiation is between 2~ 100μm, and the intensity distribution of blackbody radiation is only related to temperature and wavelength, and the peak of radiation intensity corresponds to the wavelength near 10μm . 

Thus, the wavelength distribution range of sunlight spectrum is basically not overlapping with thermal radiation, so to achieve the best solar thermal conversion, the material used must meet the following two conditions: ① absorbs a high degree of light in the sunlight spectrum, that is, has the highest absorption rate α;② in the thermal radiation wavelength range has the lowest possible radiation loss, That is, there is the lowest possible emissivity Gamma. Generally speaking, for the same wavelength, the absorption rate and emissivity of the material have the same value, that is, the absorption rate is high and the corresponding emissivity is also high. However, the absorption rate alpha and reflectivity Gamma and transmittance T meet the following relations: α+γ+ t= 1. For opaque materials due to t= 0, then α+γ= 1. For black objects, γ= 0, then α= 1. According to the above discussion, the most effective solar photothermal conversion material is in the sunlight spectrum range, that is, λ< 2.5μm, there is α≈1 (that is, γ≈0), 

while in λ> 2μm, that is, within the wavelength range of thermal radiation, there is ε= 0 (i.e. γ≈1 or α≈0), Coating materials with this characteristic are generally referred to as selective absorbent materials. If the above conditions are not fully met, the epsilon value is large in the wavelength range of thermal radiation, although the sunlight spectrum α≈1, there is still a large loss of thermal radiation, such materials are often referred to as non-selective coating materials. The construction of all selective absorption coatings is basically divided into two parts: Infrared reflection bottom (high infrared reflection ratio metals such as copper and aluminum) and solar spectral absorption layer (metal compounds or metal composites). The absorption coating produces a strong absorption near the wavelength (0.5μm) of the sunlight peak value, is free through the infrared band, 

and forms a selective coating with the help of the underlying high infrared reflection characteristics. In terms of concentrated light, due to the large wavelength coverage of sunlight, the high reflectivity or high transmittance wavelength of the focusing reflector or refraction mirror should cover 300~ 2500nm, so the mirror uses a new nano-coating, from indoor insulation coating to anti-anti-coating on the sunglasses, etc., these technologies will increase the efficiency of the collector by nearly 5%. From the recent research results of many nanotechnology, glass coating will get more rapid development.