Collector efficiency of double-flow solar air heaters with fins attached

Abstract

A design for inserting an absorbing plate to divide the air duct into two channels (the upper and the lower) for double-flow operation in solar air heaters with fins attached over and under the absorbing plate has been investigated both experimentally and analytically. The present work is restricted to the case where the outside air is being heated directly, and the configuration investigated here will have lower collector efficiency if the inlet-air temperature is substantially higher than the ambient temperature because of the far greater potential for heat loss from the top. However, the double-flow device introduced here was designed for creating a solar collector with heat-transfer area double between the absorbing plate and heated air. This advantage may compensate for the heat loss from the top when the inlet-air temperature is higher than the ambient temperature. The agreement of the theoretical predictions with those measured values from the experimental results is fairly good. Considerable improvement in collector efficiency of solar air heaters with fins attached is obtained by employing such a double-flow device, instead of using a single-flow example and operating at the same total flow rate. Both the theoretical predictions and experimental results showed that the optimal fraction of airflow rate in upper and lower subchannels is around the value of 0.5. The effect of the flow-rate ratio of the two air streams of flowing over and under the absorbing plate on the enhancement of collector efficiency is also investigated.