Low Cost, High Performance Solar Selective Paints

Abstract

Terrestrial solar radiation is a low-intensity, variable energy source arriving at about 1000 wm^-2. The economic feasibility of solar energy utilization depends upon efficient collection, conversion and storage. The efficient utilization of solar energy for heating, cooling, and process applications requires the use of flat-plate or focussing collector systems which first capture as much as possible of incoming radiation and deliver a high fraction of the captured energy to the working fluid. The conversion efficiency of a collector system is limited by the thermal losses from the heated absorber due to conduction, convection and radiation. The losses become increasingly significant at higher temperatures. The economical and efficient utilization of thermal energy derived from solar radiation using solar collectors requires an efficient and low cost ‘solar selective coating’ or ‘selective surface’. An efficient solar selective surface is defined as having a high absorptance over the solar spectrum (0.30–2.0 microns) and, in addition also having a low emittance to reduce the thermal radiative heat losses. The achievement of such a surface with wavelength selective properties has been possible due to the fact that the solar spectrum and the thermal infrared spectrum of heated bodies do not overlap to any appreciable extent (for temperatures below 500°C, 98% of the thermal infrared radiation occurs at wavelengths greater than 2 microns). A parameter that has been used to characterise a solar selective surface is the ratio of solar absorptance to thermal emittance. However, the trade-off between absorptance and emittance for various collector systems has been discussed by various authors and it has been shown that an increase of absorptance is more effective in improving the operating efficiency than a corresponding decrease in emittance.