STORAGE EFFECT IN PACKED BED SOLAR AIR HEATER

Abstract

The solar air heaters have been widely used for low temperature applications, like drying of grains, timber seasoning, space heating etc. but they have disadvantage of low ,thermal efficiency owing to low heat transfer coefficient between absorber plate and air. One of the methods to improve the thermal efficiency of solar air heater is the use of packed bed absorber. The packed bed absorbs the incoming solar radiations 'in depth' and have high surface area for heat transfer per unit volume of bed. They also have advantage of storing energy to make it available during off sunshine hours. This report analyses the heat transfer in packed bed solar air heater. The transient heat transfer equations have been solved by utilizing implicit Crank-Nicolson scheme. The time variation of temperature distribution both in the bed and air is discussed. Then the time change of outlet air temperature is also found out The decay of outlet air temperature with time when radiations are cutoff, which shows storage characteristics of the heater is also studied. The effect of various parameters like number of glass covers, thickness of insulation, porosity of the bed, diameter of packing, and mass flow rate of air on outlet air temperature variation with time showing storage effect has been studied. The effect of above parameters on amount of energy available is also discussed. All these investigations have been done with the help of a computer program and finally the main conclusions are presented.