Analytic models and effective resistances for coaxial ground heat exchangers

•Review of analytical models for coaxial ground heat exchanger.•Evaluation of the effective resistance for coaxial boreholes.•Impact of the different configurations on thermal performance is presented.•Particular features of coaxial systems are discussed. Ground heat exchangers (GHE) have been extensively studied in the past decades in order to optimize the heat exchange with the ground in particular when they are coupled with a building equipped with heat pumps. Ground heat exchangers can be categorized in several ways: vertical, horizontal or even pile heat exchangers. In the special case of vertical heat exchangers, the single U-tube and double U-tube are the most common configurations but coaxial boreholes have drawn attention in recent years. This paper provides some new insight on this configuration based on analytical models that were validated with numerical simulations. The outcome of the study gives new comprehension on the effective borehole resistance associated with coaxial configurations in ground heat exchangers design.