NUMERICAL EVALUATION OF BHE THERMAL RESISTANCE FOR GROUND COUPLED HEAT PUMP SYSTEMS

Heat transfer from the ground to a carrier fluid in ground coupled heat pump applications where vertical borehole heat exchangers (BHE) are present, is strongly influenced by the thermal resistance between the borehole surface and the fluid, and between the two (or more) pipes inside the borehole. In this systems the main contribution to the BHE resistance is due to the filling material (grout), and this resistance is often the most difficult to evaluate. Several authors have proposed empirical and theoretical correlations and models to evaluate these resistances. The present paper reviews the different approaches and presents some comparisons based on numerical simulations applied to single U and double U borehole configurations under different boundary conditions.