A fully transient model for a direct expansion borehole heat exchanger was modeled by means of the thermal resistance and capacity model approach. The fluid defined in terms of pressure and enthalpy is coupled to the borehole heat exchanger and the surrounding ground described in terms of temperature. This model was compared against the well known infinite cylindrical source model and showed very good agreements for the mean fluid temperature, borehole wall temperature and borehole resistance. For supercritical CO2 applications, the work demonstrates that both the fluids inlet pressure and mass flow rate influences the useful length of the BHE.