Upward-Type Solar Air Heaters with Internal Recycle for Improved Performance

Abstract

The influence of internal recycle on the collector efficiency in upward-type solar air heaters 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 the operation is carried out with an internal recycle, where the desirable effect of increase in fluid velocity leading to improved heat transfer, overcomes the undesirable effect of decrease in driving force (temperature difference) of heat transfer due to remixing. The enhancement in collector efficiency based on the device of same size without recycle, increases with increasing reflux ratio, especially for operating at lower air flow rate with higher inlet air temperature and radiation incident. It is found that more than 150% of improvement in collector efficiency is obtained by recycle operation. Further, the performance in a upward-type solar air heater operated with internal recycle overcomes that in the same-size device operated with external recycle.