ANALYTICAL INVESTIGATIONS OF THE PERFORMANCE OF PACKED BED SOLAR AIR HEATERS

Abstract

The flat plate collector is the simplest and least expensive device used for solar energy collection. It is easy to fabricate, install and maintain and is capable of using both diffuse and beam radiations unlike concentrat-ing collector which utilizes only beam radiations. This is used for heating both air and water. The efficiency of air heater is low due to its lower heat transfer coefficient. Considerable efforts have been made to improve the performs mance of such collectors by increasing the intensity of incident radiations# reducing the thermal losses and increasing the heat transfer capability from absorber plate to fluid. The effect of various methods on the per-formance of flat plate air heater has been investigated by a number of investigators In the present work, a packed bed solar air heater packed with a semitransparent bed material has been investigated* Heat transfer equations have been modified in terms of solid and gas temperatures and various infiuenc•. ing parameters for quasi-steady state condition. The heat balance equations for a q"asi steady state condition have been numerically solved using a finite difference method to obtain temperature profiles both for the bed and the gas as function of space and other parameters. The thermal eff-iciency of the packed bed collector has been calculated and compared with that of the plane collector. It is found that there is considerable enhancement of the efficiency (upto 300.) when a packed bed is used* Comparison of thermal efficiency values obtained by varying the mass flow rates, inlet air temperature,, dis- meter of the bed material and the depth showed .th'ut the mass flow rater bed depth and the inlet temperature are more significant influencing parameters as compared to the diameter of bad material. Lower values of mass flow rat. and higher depth* have been found to give maximum enhance- ment, It is noted that the diameter of the bed material to not an Important parameter as far as the thermal eff-iciency is concerned* The effect of mass flow rate, total bed depth and diameter of bed material on the pressure drop is found to be significant, The pressure drop decreases with increase in the depth and diameter of material while it Increases with Increase In the mass flow rate.. It can be concluded that considerable improvement in the thermal performance of the solar air heater can be brought about by using a packed bed