Laboratory study on the thermal performance of vertical U-tube ground heat exchanger during short-term borehole thermal energy storage (BTES) and heat extraction process

To explore the thermal performance of vertical U-tube ground heat exchanger (GHE) during short-term thermal energy storage (TES) and heat extraction process, a novel and well-controlled sandbox with a plate heat exchanger at the bottom of GHE was proposed and established. According to the experiments of short-term TES and heat extraction under different inlet temperature with Re between 2677 and 2823, we found that the thermal performance of GHE was mainly determined by the dynamic soil temperature distribution around GHE, which was induced by the inlet temperature and operation process of GHE. Furthermore, the energy was concentrated within a limited radius (0.4 m) around the GHE during the 8 hours TES and heat extraction process, which was of benefit to the efficient operation of GHE and the thermal environment of soil. Moreover, compared to the simple extraction process C10 (−/8°C) without TES, the heat extraction process of C5 (35/8°C) obtained a 47.8% increase of average heat extraction rate. In addition, approximately 35% of the injected heat was left in the experimental soil under thermal balance conditions C3, C5, and C7. These results show the superiority and characteristic of short-term TES and heat extraction for GHE operation.