A review on different techniques used for performance enhancement of double pass solar air heaters

The performance of a conventional solar air heater (SAH) can be effectively improved by reducing the losses from the collector surface by providing the proper insulation and increasing the convective coefficient between heat collecting surface and working fluid by enhancing the heat transfer area which can be increased by double pass design. Various experimental and theoretical investigations have been considered to enhance the performance of double pass solar air heaters (DPSAHs) provided with performance enhancement techniques i.e. using packed bed materials (PBMs), extended surfaces and corrugated/grooved absorbing surfaces. These studies include the cost analysis, thermohydraulic characteristics and design of DPSAH. The objective of present study is to review the various investigations conducted on performance enhancement of double pass system. Based on the review, it is found that the most of the investigations were performed on double pass system having PBM and integrated with extended surfaces. Few studies were conducted on corrugated or grooved absorbing surface and very few studies have been presented on double pass systems having artificial roughness. Further, in order to compare the thermal performance of different designs of DPSAHs, an attempt has been made to generate the performance results using correlations developed by various investigators.