OPTIMAL DESIGN OF PACKED BED = . SOLAR AIR HEATERS

Abstract

Conventional solar air heaters used for heating air in general, have low thermal efficiency, because of poor heat- transfer rate from absorber plate to air flowing in a duct on the lower side of the absorber plate. Packing elements are employed to pack the duct of solar air heaters to enhance the thermal efficiency. However, in such systems the pumping power needed to overcome the friction losses can reduce the benefits of increased useful heat gain. Thus the design of packed bed solar air heaters with high heat transfer rates and low friction losses is of particular interest. Both the heat transfer coefficient and friction factor are. found to be strong functions of bed parameters. The effective efficiency is . a useful parameter to take into account the effect of enhanced useful heat gain and friction losses. By maximizing the effective efficiency, the bed parameters can be optimized with regards to high useful heat gain and low friction loss. In this work solar air heaters packed with wire mesh matrix are optimized for maximum effective efficiency with respect to, bed parameters i.e. is number of layer, bed porosity and pitch to diameter ratio of wire mesh. The results of optimization are presented in the form of design plots for packed bed solar air heaters in terms of bed parameters. These plots can be used for selecting the optimal bed parameters and predicting the thermal efficiency of these solar air heaters for a given temperature rise and