The effect of internal-recycle operation on the collector efficiency in flat-plate solar air heaters has been investigated theoretically. It is found that considerable improvement in collector efficiency is obtainable if the operation is carried out with an internal 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 effect at the inlet by recycle operation. The enhancement increases with increasing reflux ratio, especially for operating at lower air flow rate, as well as with higher solar radiation incident and inlet air temperature. Further, the performance in a solar air heater operated with internal recycle overcomes that in the same-size device operated with external recycle.
Journal of the Taiwan Institute of Chemical Engineers 42(2), pp.286-291