The Effects of Radiation Properties of Surfaces and Coatings on the Performance of Solar Collectors

The present work suggests a more complete method for predicting the performance of solar collectors at intermediate temperatures, taking into account absorptance of the solar short-wave radiation in the glass covers and transmittance through them of the IR long wave radiation from the plate and other surfaces. The governing equations for energy balances of the shield and outer cover were developed, using a ray tracing method and radiation characteristics of slabs and coatings. The equations were solved numerically for the shield and cover temperatures, T_g and T_c, and then the net energy, Q_net, was found. Other heat fluxes were also obtained, to understand better the influence of each parameter on the performance. A parametric study has been carried out to study the effects of various coatings: heat mirrors (reflecting IR radiation) and anti-reflective for solar radiation. The influences of collector pressure, concentration, double glazing, and selective coatings on the absorbing plate have also been investigated. An important conclusion from the results is that there is no configuration of coatings which yields higher efficiencies over the whole temperature range. An “optimal” collector must be designed according to the intended consumer temperature.