Free convection and shading due to gap spacing between an absorber plate and the cover glazing in solar energy flat-plate collectors

Abstract

Natural convection losses between an absorber plate and the cover glazing for different gap sizes and at different plate temperatures have been predicted theoretically by various empirical correlations developed by McAdams, 4 Mikheyev, 3 Tabor, 5 Buchberg et al. 6 and Hollands et al. 7 The free convection heat transfer coefficients have been evaluated for horizontal, vertical and tilted collectors. The natural convection coefficient decreases and the shading due to side walls increases with increase in gap size. Therefore, the shade correction factor has also been calculated for different collector inclinations for low and medium latitude stations. By taking into consideration both these factors—that is, convection losses and shading—the air gap between the absorber plate and the cover glazing has been optimised. It has been found that, for minimum shading and minimum convection losses, a gap of 4 to 5 cm should be maintained between the absorber plate and the cover glazing in typical flat-plate solar energy collectors.