The effects of double-pass and external-recycle operations on the collector efficiency in solar air heaters have been investigated theoretically. It is found that considerable improvement in collector efficiency is obtainable if the operation is carried out with an external recycle, where the desirable effect of increasing fluid velocity to decrease the heat transfer resistance compensates for the undesirable effect of decreasing the driving force (temperature difference) of heat transfer, due to the remixing at the inlet by recycle operation. The enhancement increases with increasing reflux ratio, especially for operating at lower air flow rate. Moreover, performance of double-pass solar air heater with recycling device is better in comparison to single-pass solar air heater of the same size.
Journal of the Taiwan Institute of Chemical Engineers 42(5), pp.793-800