INVESTIGATION ON THE ENHAN.U..MENI OF THtMAL PERFORMANCE .Q.F PACKED BED SOLAR AIR HEATER

Abstract

Solar air heaters find applications in space heating, drying of grains, timber seasoning process and other devices requiring low grade thermal energy.. Such heaters have simple design and low manufacturing , operation and maintenance costs but have low thermal efficiency due to low heat transfer coefficient between absorber plate and the air leading to higher plate temperature and resulting greater heat losses. Several methods have been proposed to improve the thermal performance of these heaters ; one of these methods being the enhancement of heat transfer capability from absorber to air by packing the duct with optically semi-transparent materials and pebbles. It is also desired that a solar energy utilization system should also have heat storage effect in order to compensate for an irregular supply of solar radiation. In the present investigation, thermal performance characteristics of a solar air heater having its duct packed with optically semi-transparent materials and blackened stone pebbles have been experimentally investigated. An experimental set-up having a packed bed as well as plane solar air heaters, complete with flow straightener, instrumentation and capable of experimentation under actual outdoor conditions was designed and fabricated in order to evaluate the thermal performance and also the enhancement achieved as a result of packing thern duct. The experimentaton included the (iv) investigation of the effect of system (packing material characteristics) and operating (mass flow rates) parameters on the thermal performance in terms of thermal efficiency and the storage characteristics. Results show that the solar air heaters with packed bed.. absorbers yield higher thermal efficiencies, collector efficiency factor and collector heat removal factor in comparison to the'.. corresponding plane collector operating under similar conditions ; the corresponding maximum values of the ratios of these parameters for packed bed to that for plane collector has been found to be 1.876, 1.856 and 1.824 respectively. The semi-transparent bed comparising glass beads has been found to lead to. best performance in comparison to stone pebbles and the glass tubes, both in terms -, of thermal efficiency as also the energy storage characteristics; the glass beads of smaller size being better than larger sized-: beads. It can therefore be concluded that the thermal performance of solar air heater can be considerably enhanced by packing the flow passage with semi-transparent materials and this enhancement is strong function of particle size, thermophysical properties of materials and the operating parameters.