INVESTIGATION OF THERMAL PERFORMANCE OF SOLAR AIR HEATING COLLECTORS USING PACKED BED IN THE DUCT

Abstract

Solar energy is the most promising renewable energy resource. It can be economically harnessed by flat plate air heating collectors for low temperature applications. Their usefulness has been limited because of low thermal efficiencies primarily as a result of lower• convective heat transfer coefficient between the absorber plate and air. Attempts have been made to enhance the performance of solar air heaters by using packed bed in the duct. The bed can be packed by materials like wire mesh, glass beads, pebbles, iron fillings and glass balls. Packed bed porous •absorbers have high heat transfer rates, resulting in an increase in thermal efficiency of the collector. In the present work, the bed system and operating parameters effecting the enhancement of the performance were identified as mass flow parameter (X1), depth to bed element size ',.ratio (X2), porosity (X3) and the thermal conductivity of the packing material (kp). Using these parameters the heat transfer correlations were formulated and a computer program was developed for the prediction of performance using governing equations available in the literature. The effect of these parameters_, on the thermal efficiency was examined. Efficiency versus temperature rise plots have been prepared for different values of mass flow parameter and depth to bed element size ratio. It has been found that values of thermal efficiency increase with an increase in mass flow parameter apparently because of an increase in the convective heat transfer coefficient. The enhancement in the performance gradually decreases as the mass flow parameter increases. The performance of the system also improved with increase in depth to - bed element size ratio, as a result of increase in surface area per unit, volume. The enhancement in the efficiency gradually decreases with an increase in depth to bed element size ratio value.. The other parameters -porosity • and thermal conductivity of the packing material have been found to have negligible effect on the thermal performance of the system. In view of the significant influence of mass flow -parameter and depth to bed element ratio, the designer has to select these values in such a way that it result in maximum practicable values of mass flow parameter and depth to bed element size ratio to yield . maximum -thermal efficiency. The performance plots prepared in this work can be utilized by the designer for this purpose.