Thermal performance of double pass packed bed solar air heaters - A comprehensive review

The present work intended to conduct a comprehensive review on the thermal performance of double pass packed bed solar air heaters. Various aspects that govern the thermal performance of such solar air heaters has been highlighted and discussed in detail. A comprehensive review of reported studies indicate that the use of porous packed bed material in the upper channel of double pass solar air heaters significantly increases the thermal performance as compared to solar air heaters without packed bed and with packed bed when porous material is provided in the lower channel. Moreover, studies revealed that employment of parallel, counter and recyclic double air pass increases the thermal performance of the packed bed solar air heater as well by increasing the heat extraction rate of the flowing air through the packed bed duct. Although, it has been obtained that sufficient work using parallel and counter double air pass has been reported on packed bed solar air heaters, only few studies were reported using recyclic double air pass. However, results show that employment of recycle operation to the double pass packed bed solar air heaters momentously increases the thermal performance and can be used as an alternative thermal performance enhancement technique. Therefore, more investigations are required to be conducted using recyclic double pass packed bed solar air heaters.