Transient heat transfer performance of a vertical double U-tube borehole heat exchanger under different operation conditions

The transient thermal performance of a vertical double U-tube borehole heat exchanger (BHE) was numerically studied by validated heat transfer model. Further, the influence of several operation parameters, including inlet velocity, temperature and operation interval, on radial/axial soil temperature distribution were investigated. The simulation result showed that the increased amplitude of the BHE's heat transfer rate was similar when charging temperature was increased under the same velocity conditions. Meanwhile, the difference of the heat transfer rate between 0.1 and 0.3 m/s was greater than that of the 0.3 and 0.5 m/s when the inlet temperature kept constant. The charging temperature had a more vital influence than the flow velocity on soil temperature lifting, and the flow velocity around 0.3 m/s was recommended under the conditions in this work. Moreover, the heat charging time had a more obvious effect on the heat transfer sensitive zone in the radial direction than the other two parameters. Finally, the choice of charging temperature, appropriate interval, space and depth of borehole were discussed. This study could contribute to the comprehensive understanding of the dynamic thermal behaviour and operation parameter optimization of BHE and its further application in the field of borehole thermal energy storage.