Discrepancies in using CO₂ or water as heat-carrier fluid on the output temperature of deep coaxial borehole heat exchanger

Conventional deep coaxial borehole heat exchanger (DCBHE) may experience notable discrepancies with water or CO₂ as heat-carrier fluid. In this paper, a numerical model was established by considering the temperature-dependent properties of CO₂ and water. The model's effectiveness was validated against the demonstration project and fit the demonstrated data. The results indicating that the suitable inlet pressure of CO₂ under benchmark condition was determined to be 10 MPa, the output CO₂ temperature in DCBHE was high and fluctuated from 42℃ to 44℃ in a supercritical state. With respect to heat transfer rate varying from 80 W/m to 120 W/m and inlet velocity changing from 0.5 m/s to 0.9 m/s, it was recommended to select CO₂ as heat-carrier fluid with inlet pressure of 10 MPa, and water as the heat carrier fluid with inlet pressure of 15 MPa when the depth of DCBHE was 2500 m. In terms of depth lengthening from 2000 m to 3500 m under the benchmark conditions, selecting water as the heat carrier fluid with inlet pressure of 15 MPa was suggested. And choosing water as heat-carrier fluid with inlet pressure of 10 MPa was proposed only when the depth was 2000 m and 3500 m. The findings answered the choices of using CO₂ or water as heat-carrier fluid of DCHBE and achieve a mini step in the precise design of DCBHE in ground source heat pump systems.