Thermophysical and Heat Transfer Characteristics Based on Thermal Response and Thermal Recovery Test of a U-Pipe Borehole Heat Exchanger

Abstract

Effective ground thermal properties are the most important parameters in the design of borehole heat exchanger (BHE), which are usually can be obtained by thermal response test (TRT). They’re often vary with depth and influenced by heat injection. This paper presents a method combining TRT and thermal recovery test (TrT) to evaluate the ground thermal properties. The TRT and TrT conducted on a single U-pipe BHE by the TRT rig and two measurement lines each with ten three-wire Pt-100 temperature sensors. Thermal conductivity and borehole thermal resistance at different depths were inferred by the infinite line source model (ILSM) during the TRT while by combining the ILSM with the superposition principle and parameter estimation method during the TrT. Results reveal that, although the heat injection rate varies significantly in the TRT period, the average effective ground thermal conductivity and borehole thermal resistance inferred from the TRT are similar to those inferred from the TrT. However, the results at different depths show great variability and correspond to groundwater and geological information. In addition, TrT results showed that acceptable results (within 5% error) could be obtained for 22 hours in a TrT.