Parametric study of a solar air heater with and without thermal storage for solar drying applications

A transient analytical model is presented for a flat-plate solar air heater with and without thermal storage. The flowing air temperature is assumed to vary with time and space coordinates. Analytical expressions are obtained for various temperatures of the air heater elements as well as for the temperature of the storage material. The performance of the air heater is investigated by computer simulation using the climatic conditions of Tanta (Lat. 30 degrees 47' N, Egypt). Effects of design parameters of the air heater such as length (L), width (b), gap spacing between the absorber plate and glass cover (d(f)), mass flow rate ((m) over dot) and thickness and type of the storage material (sand, granite and water) on the outlet and average temperatures of the flowing air are studied. It is found that as L and b increase the average temperature of flowing air (T-fav) increases up to typical values for these parameters. Typical values for L and b are obtained as 3 and 2 m, respectively. The outlet temperature (T-fo) of flowing air is found to decrease with increasing gap spacing and mass how rate of air. Improvements in the heater performance with storage have been achieved at the optimum thickness (0.12 m) of the storage material. Therefore, the air heater can be used as a heat source for drying agricultural products and the drying process will continue during night, instead of re-absorption of moisture from the surrounding air. Comparisons between experimental and theoretical results indicated that the proposed mathematical model can be used for estimating the thermal performance of flat-plate solar air heaters with reasonable accuracy. (C) 2000 Elsevier Science Ltd. All rights reserved.