Solar energy is important source of renewable energy. This energy are used in solar air heater. Solar air heater is one of the valuable heat sources with variety of applications such as space heating, industrial process heating and drying of fruits and vegetables etc. The thermal efficiency of a conventional solar air heater (SAH) can be improved by reducing the losses from the collector surface by using proper insulation and increasing the convective coefficient between heat collecting surface and working fluid by increasing the heat transfer area which can be increased by double pass design. Some 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 like fins, and corrugated/grooved absorbing surfaces. The objective of present study is to review the various investigations conducted on performance enhancement of solar air heater. 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. Thermal efficiency of double pass solar air heater with porous media is higher than single pass and double pass solar air heater without porous media. Effect of various parameters of porous media like pitch, number of layers , bed depth, porosity, thermal conductivity, pitch to wire diameter ratio have been studied. Few studies were conducted on corrugated or grooved absorbing surface and very few studies have been presented on double pass systems having artificial roughness.
solar air heater, fins, thermal efficiency, porous media.
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