Double-Pass Flat-Plate Solar Air Heaters with External Recycle

A new device of inserting an absorbing plate to divide a flat-plate channel into two channels with fins attached and external recycling at the ends, resulting in substantially improving the heat transfer efficiency, has been investigated both experimentally and theoretically. The agreement of the theoretical predictions with those measured values from the experimental results is fairly good. The experimental and theoretical results are represented graphically and compared with that in the downward-type single-pass solar air heaters of the same size without recycling. Considerable improvement in heat transfer is obtained by employing double-pass operations with external recycling and fin attached over and under the absorbing plate. The influences of recycle ratio and absorbing plate location on the heat-transfer efficiency enhancement as well as on the power consumption increment have been also delineated