The effect of internal recycle on the collector efficiency in upward-type flat-plate solar air heaters attached with fins has been investigated theoretically. The equations for predicting the outlet air temperature and the collector efficiency were derived from the energy balances on the absorbing plate and flow air. Considerable improvement in collector efficiency is obtainable if collector is attached with fins to extend the heat-transfer area, and if the operation is carried out with an internal recycle to increase the fluid velocity, leading to improved heat transfer coefficient. The enhancement in collector efficiency increases with increasing reflux ratio, especially for operating at lower air flow rate with higher inlet air temperature. It is found that more than 100% of improvement in collector efficiency is obtained by recycling operation. Furthermore, the performance in the device operated with internal recycle overcomes that in the same-size device operated with external recycle.
Journal of the Taiwan Institute of Chemical Engineers 43(2), pp.235-240