ANALYTICAL PERFORMANCE PREDICTION OF SOLAR AIR HEATERS WITH POROUS ABSORBER

Abstract

Solar energy has assumed great importance and appears to be promising among renewable energy resources. Solar air heaters are efficient means of harnessing solar energy for low temperature applications . e.g. timber seasoning, drying of grains and chemical industries etc. Although conventional flat plate type solar air heaters are cheap, easy to maintain and operate, these are not economical owing to their low thermal efficiency. The thermal performance of these air heaters can be enhanced by several methods including porous absorber..; Porous absorber absorbs solar radiations in depth. Such air heaters also has high heat transfer rate due to increased heat transfer area and modified flow regimes. Effect of various flow configurations and media material on solar air heater performance have been investigated by several investigators. In a porous absorber solar air heater, heat transfer is a complex phenomenon, as all three modes, namely conduction, convection and radiation are involved. In the present investigation, an analytical model of the system has been proposed to predict the performance of such solar air heaters. A two dimensional model has been considered. The equations obtained using energy balance for the system have been solved numerically to predict the performance of such solar air heaters. v • The effect of important system and operating parameters i.e. ambient temperature, air flow rate, intensity of radiations, bed depth and bed material was studied. The results indicate that collector efficiency. increases with an increase in air flow rate but the rate of increase of efficiency reduces with an increase in the flow rate. It has been found that the variation in the intensity of solar radiations, ambient temperature and bed depth have negligible effect on the efficiency, although 'these parameters significantly affect thea outlet air temperature. Of the three materials considered; • iron screens have been found to be the most efficient followed by glass beads, while the stone pebble bed was the least efficient. It can therefore be concluded that the design of the packed bed must be based on the quantitative effect of bed and operating parameters.