The recycle effect on the collector efficiency improvement of double-pass sheet-and-tube solar water heaters with external recycle

The paper presents a theoretical prediction of the performance of a double-pass sheet-and-tube solar water heater with external recycle and comparison with that of a conventional type collector. Analytical results show that the recycle effect can effectively enhance the collector efficiency compared with that in a single-pass device with the same flow rate. The desirable effect of increasing convective transfer rate and the undesirable effect of reducing heat-transfer driving force are the two conflict effects produced by the recycle operation. It was found that the increment of convective transfer rate by increasing the recycle ratio could generally compensate for the decrement of the temperature difference, leading to improved performance, especially for low flow rate. Also, the collector efficiency increases with increasing collector aspect ratio (i.e. either increasing the tube length or decreasing the number of tube pairs) for a specified collector area and the distance between tubes.