Exergy-based performance analysis of packed-bed solar air heaters

This paper presents an experimental investigation of the thermal performance of a solar air heater having its flow channel packed with Raschig rings. The packing improves the heat transfer from the plate to the air flow underneath. The dimensions of the heater are 0.9 m wide and 1.9 m long. The aluminium-based absorber plate was coated with ordinary black paint. The characteristic diameter of the Raschig rings, made of black polyvinyl chloride (PVC) tube, is 50 mm and the depth of the packed-bed in flow channel is 60 mm. Energy and exergy analyses were applied for evaluating the efficiency of the packed-bed solar air heater. The rate of heat recovered from the packed-bed solar air heater varied between 9.3 and 151.5 W m(-2), while the rate 2 of thermal exergy recovered from the packed-bed solar air heater varied between 0.04 and 8.77 W m(-2) during the charging period. The net energy efficiency varied from 2.05 to 33.78%, whereas the net exergy efficiency ranged from 0.01 to 2.16%. It was found that the average daily net energy and exergy efficiencies were 17.51 and 0.91%, respectively. The energy and exergy efficiencies of the packed-bed solar air heater increased as the outlet temperature of heat transfer fluid increased. Copyright (C) 2004 John Wiley Sons, Ltd.