Selective Coatings for Photothermal Conversion

Abstract

One of the simplest and most direct methods of harnessing solar energy is to convert the incident solar radiation into heat (called photothermal conversion). A variety of flat plate and concentrating type of collectors for this purpose have been designed and studied, and are being manufactured worldwide. One of the key components in photothermal conversion devices is the absorbing surface and its optical and thermal characteristics. With our present day understanding of the optical properties of surfaces, it is possible to tailor a surface by depositing a coating/film on it so that it yields literally any prescribed optical reflectance, transmittance, absorptance, or emittance spectra. Such surfaces/coatings having a selective response to solar spectrum are called solar selective coatings. Such coatings offer an important and cost effective way to increase the efficiency of photothermal collectors by providing a high solar absorptance (α ) in the visible and near i.r. part (0.3-2.5 μ m) of the spectrum and a low emittance (ɛ) to prevent radiation loss in the infrared (beyond 2.5 μ m) in which region a heated black surface is expected to radiate.