Thermal performance of a packed bed double-pass solar air heater

The thermal performance of a double-glass double-pass solar air heater with a packed bed (DPSAHPB) above the heater absorber plate was investigated experimentally and theoretically. Suitable computer program was developed for the analytical solution of the energy-balance equations for the various elements of the system. Limestone and gravel were used as packed bed materials. Numerical calculations were carried out, on typical summer days of 2003, to study the effect of different operational and configurational parameters on the heater performance. Effects of the mass flow rate of air m˙f and the mass and porosity of the packed bed material were also studied. It was inferred that for increasing the outlet temperature Tflo of the flowing air after sunset, it is advisable to use the packed bed materials with higher masses and therefore with low porosities. The thermohydraulic efficiency ηTH   was found to increase with increasing m˙f until a typical value of 0.05 kg/s beyond which the increase in ηTH becomes insignificant. It is recommended to operate the system with packed bed with values of m˙f equal 0.05 kg/s or lower to have a lower pressure drop across the system. To validate the proposed mathematical model, comparisons between experimental and theoretical results showed that good agreement was achieved.