First and Second Law Evaluation of Multipass Flat-Plate Solar Air Collector and Optimization Using Preference Selection Index Method

In this paper, different flow configurations of multipass flat-plate air collectors are explored. Multiple passes are formed from glass cover, absorber plate, and back plate. Five types of air collectors were analysed and optimized with respect to maximum efficiencies and minimum cost. The analytical prediction of the heat exchanger, pressure loss, and efficiencies was presented. The effects of mass flow rate from 0.01 to 0.02 kg/s, air channel depth from 15 to 30 mm, and collector length from 1.5 to 2.5 m on different configurations were examined and compared. The results of the parametric study show that the triple-pass type has the greatest efficiency, whereas the smallest efficiency is of the single-pass type. Among double-pass types, the type with two glass covers and natural convection heat transfer achieved the highest effective and exergy efficiencies due to a reduction in the top loss. Double-pass type with single glass cover is not recommended from both energy and exergy standpoints. As the collector length increases, the effective efficiency decreases, but the exergy efficiency increases. The exergy performance of the triple-pass type can reach up to 5% at the air flow rate of 0.005 kg/s. Finally, multiobjective optimization using the preference selection index method is conducted with three targets including effective efficiency, exergy efficiency, and number of plates. Optimal results show that the triple-pass type with the lowest air flow rate and the longest length is the best. The effective and exergy efficiencies for the best case were found to be about 52.1% and 4.7%, respectively. However, this type with the highest flow rate and the shortest length is the worst.