Coupling of geothermal heat pumps with thermal solar collectors using double U-tube boreholes with two independent circuits

This study presents an analytical model to predict steady-state heat transfer in double U-tube boreholes with two independent circuits operating with unequal mass flow rates and inlet temperatures. The model predicts the fluid temperature profiles in both circuits along the borehole depth. It accounts for fluid and pipe thermal resistance and thermal interaction among U-tube circuits. The proposed model is used to study a novel double U-tube borehole configuration with one circuit linked to a ground-source heat pump operating in heating mode and the other to thermal solar collectors. The performance of this configuration is compared to a conventional ground-source heat pump system (without thermal recharge of the borehole) and to a single-circuit solar assisted ground-source heat pump system. All three systems are simulated over a 20-year period for a residential-type single-borehole configuration. Results indicate that winter solar recharging, either for the proposed configuration or the solar assisted ground-source heat pump system, reduces by 194 and 168% the amount of energy extracted from the ground by the heat pump. It is also shown that, for a ground thermal conductivity of 1.5 W m−1 K−1, the borehole length can be reduced by up to 17.6%, and 33.1% when the proposed configuration or the solar assisted ground-source heat pump system are used. The impact on the annual heat pump energy consumption is less dramatic with corresponding reductions of 3.5% and 6.5%.