INVESTIGATION ON THE PERFORMANCE OF SOLAR AIR HEATER USING PACKED BED DUCT

Abstract

Solar air heaters find applications in space heating, drying of grains, food and artificial crop and many systems requiring low grade thermal energy. Inspite of its simplicity and low cost, their application has been limited in contrast to solar water collectors, mainly because of lower heat transfer coefficient between the absorber and the carrier fluid i.e. air. In order to enhance the rate of heat transfer in solar air heaters, packed beds in the form of pebbles or rocks are provided in the duct of the solar air heater with the aim of producing turbulence in the flow as well as increasing the heat transfer area, thus, increasing the thermal efficiency of the system. However, the use of packed beds results in higher friction factor requiring more pumping power. As a result, there is need to optimize the system to obtain maximum benefit from the packed bed while still keeping the pumping power at minimum possible level. Besides, there is need for the analysis of such systems in order to develop simple relationships to predict the performance of packed bed solar air heaters. In the present work, various aspects of heat transfer and fluid flow characteristics of packed bed solar air heaters have been investigated. An optimization parameter has been proposed for the thermohydraulic optimization of the bed. This parameter has been evaluated as function of Reynolds number, porosity and the ratio of bed depth to bed particle size. It has been found that for optimum conditions the porosity should be around 0.61. V Based on the critical examination of the heat transfer mechanism both inside the bed from bed to air and outside the bed from bed and cover to ambient air, expressions have been developed for collector efficiency factor, overall loss coefficient and other thermal performance parameters. These expressions have been used to investigate the effect of bed and operating parameters viz, porosity, bed depth, bed particle size, fluid flow rates, fluid inlet temperature and solar radiation intensity on the thermal performance of such heaters. Thermal efficiency has been found to vary with increasing flow Reynolds number, increasing inlet fluid temperature and increasing porosity. It ha. been found that whcrcas the flow Reynolds number has significant effect on the thermal efficiency of the packed bed systems, the intensity of radiation and bed depth to particle size has negligible effect at given values of porosity and fluid inlet temperature. The comparison of thermal efficiency of packed bed system with that of a similar sized smooth duct system under similar operating conditiont:. has shown substantial enhancement of performance of solar air heaters having a packed bed duct, the percentage enhancement decreasing with an increase in superficial flow Reynolds number. It can therefore be concluded that a considerable improvement in the thermal efficiency of solar air heaters can be brought about by using a packed bed and that the pumping power could also be kept at reasonable level by judicious choice of system (bed) parameters based on the thermohydraulic optimization