Transient heat transfer in a coaxial borehole heat exchanger

•Proposed model calculates transient vertical temperature profiles in coaxial borehole heat exchanger.•Distributed temperature sensing (DTS) system verifies model in a test borehole during a thermal response test.•Model matches both short-time and long-time data in test borehole.•Proposed model estimates the borehole thermal resistance taking into account temperature profiles.•Proposed model estimates borehole resistance better than conventional methods.

Ground-source heat pumps often use vertical boreholes to exchange heat with the ground. A transient heat transfer model has been developed for a thermal response test on a pipe-in-pipe coaxial borehole heat exchanger. The analytical model calculates the vertical temperature profiles in the fluid flowing through the pipes, which are coupled to the surrounding grout and ground. The model is verified against measured vertical temperature profiles in the circulating fluid during a distributed thermal response test. The comparison with measured data indicates that the proposed model gives a more accurate estimate of the borehole thermal resistance than the conventional analytical model that uses a mean temperature approximation. The model demonstrates how strongly the shapes of the temperature profiles are dependent on the thermal resistance of the internal pipe wall and the flow direction.