• Malhotra, A.
;
• Garg, H. P.
;
• Rani, U.

Abstract

Based on available correlations, relations are found for the local maxima and minima in heat transfer as the gap spacing is varied in flat plate solar collectors. A criterion is proposed for evaluating the use of alternate mediums in the enclosed space. It is shown that the use of heavy gases such as argon can result in a 34% reduction in heat losses. Nusselt number correlations of a single gap are extended to a two-cover system. It is found that by using two covers there is an overall savings of more than 50% in convection losses. It is also found that heat transfer rates in the laminar and turbulent regions are relatively insensitive to the internal spacing of the covers but reduce on changing from the mid-way position in the initial regime. A new type of two-cover system is proposed in which the upper space is partially evacuated and it is shown that heat losses can be reduced by 85% on a one-tenth reduction of pressure.