Thermal performance improvement of evacuated u-pipe solar collectors using passive elements

This experimental study investigates the effects of different passive turbulence promoters on the thermal performance of an evacuated U-pipe solar collector (EUpSC). Despite the extensive exploration of passive heat transfer enhancement (HTE) methods in flat plate collectors within extant literature, studies on such applications in EUpSC remain scarce. The present study investigates three different turbulence promoter elements, namely twisted tape (TT), wire coil (WC), and sealing wire coil (SWC). Collector efficiency and pipe pressure losses are evaluated together. Notably, the present study introduces SWC as a passive HTE element in this type of system. Experiments were conducted under a solar simulator at four inlet temperatures and four different fluid flow rates. The results showed correlation between increased fluid flow rate and improvement in thermal efficiency, with optimum performance achieved in the range of 40–45 L/h. The highest efficiency was observed for collectors employing TT, with a percentage of 93.7 %, followed by SWC at 92.5 %, and WC at 90.9 %. The enhancement in efficiency ranged from 2 to 7 % compared with the standard collector. However, along with the increase in efficiency, a significant rise in pressure drop was observed, ranging from 280 % to 980 %. It can be concluded that achieving an optimal balance between thermal efficiency and hydrodynamic losses is a fundamental consideration in the design of the system. The findings demonstrate the significance of selecting the most appropriate passive elements and optimum fluid flow rate in EUpSC. It is anticipated that this study will further research on the efficiency and enhancement of EUpSC.