The moderate temperature solar collector such as sheet-and-tube water heater is a convenient and simple device to be used in domestic water system. The collector efficiency of double-pass sheet-and-tube solar water heaters with attaching internal fins on tube wall and external recycle has been investigated theoretically in this study. The influences of fin attached numbers Nf, recycle ratio R, incident solar radiation I0 and mass flow rate m on the collector efficiency were discussed. However, the power consumption increment is a significant magnitude to be taken into account to optimize the collector efficiency under the economic sense. The operation of such a double-pass device with external recycle is represented graphically and compared with those in the single-pass device without external recycle under the same working dimensions. There exists an optimal aspect ratio of considering both the collector efficiency and power consumption with number of pair ducts as parameter for a specified collector area. Considerable improvement of collector efficiency is obtained by operating such a recyclic device with internal fins attached, under I0 = 1.0 kJ m-2 s-1, m = 9 kg s-1, R = 1, n = 4 and Nf = 2 instead of using the single-pass device without recycle.
Relation:
淡江理工學刊=Tamkang journal of science and engineering 10(4)，頁323-334
