Year round performance of double pass solar air heater with packed bed

The thermal performance of a double glass, double pass solar air heater with a packed bed (DGDPSAHPB) was investigated experimentally and theoretically. A suitable computer program was developed for the analytical solution of the energy balance equations for the various elements of the system. Numerical calculations were performed for typical summer days of 2003 using limestone and gravel as packed bed materials. To validate the proposed mathematical model, comparisons between experimental and theoretical results were performed. Good agreement was achieved. Furthermore, the effects of mass flow rate of air m˙f as well as that of the mass and porosity of the packed bed material on the outlet temperature of air Tflo, thermal output power Q˙u, pressure drop ΔP and thermohydraulic efficiency ηTH were also studied. Comparisons between the thermal performances of the system without and with the packed bed, either above or under the absorber plate were performed. Some experiments were also performed using iron scraps as a packed bed material. It was indicated that it is advisable to operate the system with a packed bed of low porosity above the absorber plate. The best performance was achieved with gravel as a packing material above the absorber plate when m˙f equals 0.05 kg/s or lower to provide a lower pressure drop across the system and, therefore, a higher thermohydraulic efficiency ηTH. Values of ηTH with gravel were found to be 22–27% higher than that without the packed bed. The annual averages of Tflo and ηTH were found to be 16.5% and 28.5% higher than those for the system without the packed bed; indicating an improvement of the heater performance on using a packed bed material, above or under the heater absorber, all year round.