Improved method and case study of thermal response test for borehole heat exchangers of ground source heat pump system

Thermal response test (TRT) is crucial for the determination of the ground thermal conductivity and the evaluation of the thermal performance of borehole heat exchangers (BHEs) of ground source heat pump (GSHP) system. This paper presented a novel constant heating-temperature method (CHTM) for TRT. Further, a type of improved TRT equipment was developed and the mathematical model to deal with test data was presented. Based on the measurement of the natural ground temperature distribution, an in situ TRT case was carried out. The experimental results showed that, compared with the conventional TRT with constant heating-flux method (CHFM), CHTM has an obvious advantage of reducing the time period reaching a steady heat-transfer state between the BHE and its surrounding soils. This improved TRT equipment can effectively operate under both heat-injection and heat-extraction modes, and its test data can accurately reflect the thermal properties of the soils as well as the thermal performance of the BHE under different operation conditions. Finally, the advantages and disadvantages between CHTM and CHFM were compared, which can provide a useful reference for the design of GSHP system.