The middle temperature solar collector, sheet-and-tube water heater heated by solar radiation, is a convenient and simple device. Sheet-and-tube solar water heater is often used in domestic water system. The collector efficiency of double-pass sheet-and-tube solar water heaters with attaching internal fins to tube wall and external recycle has been investigated theoretically. The influences of fin attached numbers Nf, recycle ratio R, incident solar radiation I0 and mass flow rate m on collector efficiency were discussed. The power consumption increment by operating such a recyclic device with the same working dimensions was also delineated. There exists an optimal aspect ratio of numbers of pair ducts for a specified collector area. Considerable improvement of collector efficiency is obtained by employing such a recyclic device with internal fins attached instead of using the device with recycle.
