Performance improvement of double-pass solar air heater - A state of art of review

This paper presents a comprehensive literature review of the various heat transfer techniques used to increase the performance of double-pass solar air heaters (SAHs). The heat transfer from absorber plate to air can be improved by increasing the convective heat transfer coefficient, the heat transfer area and by reducing thermal losses. These methodologies have guided the design and development of double-pass SAHs, and the results of several experimental studies show that double pass SAHs achieve better performance than single-pass SAHs. Literatures on double-pass SAHs have been reviewed to understand the different techniques used for performance improvement. The different operating and system parameters have been categorized to understand their performance behavior. The literature also reveals that numbers of techniques have been employed to improve heat transfer. These techniques include the incorporation of various features into the SAH design, such as extended surfaces, corrugated surfaces, artificial roughness, and packed beds. Furthermore, it is found that packed bed increase the heat transfer rate along with reduction in losses to environment because insolation is absorbed in the depth of packing material cause to lower the top layer temperature and, thereby, improvement in thermal losses.