A Brief Review and Evaluation of Available 1-Dimensional Models for the Borehole Thermal Performance Prediction in a Ground-Coupled Heat Pump System

The borehole thermal performance prediction is critical in the performance assessment and economical utilization of the ground source heat pump (GSHP). Hence, it is inevitable to evaluate these correlations and reveal their accuracy limits before adapting to the thermal design of the ground heat exchangers. The present article launches an evaluation for the available correlations of the borehole thermal resistance predictions in the direct ground exchange (DX) condenser when it circulates R-410A refrigerant. Eleven elected correlations in the open literature were examined to predict the thermal resistance of vertical single and double U-tube heat exchangers for various geometry configurations. A hypothetical (3.5) kW cooling unit of (3.6) COP was employed to investigate the borehole thermal resistance and depth of the ground copper tubing DX condenser in a single and two-loops design. A large scatter was evident for the borehole thermal resistance and depth as predicted by these correlations. It was as much as double for the examined single and double U-tube geometries. The principal outcome of the present study proved that the utilization of these correlations is only in the preliminary thermal design of the ground source heat pumps (GSHPs). They cannot be adopted for a finalized thermal structure of the borehole without being supported by experimental data due to their predictions' wide range of variations. Many correlations showed good interaction with the borehole configuration, tube diameter (do), tube center-center spacing (Sp), and borehole size (DB). All of the examined models showed that increasing the grout thermal conductivity enhances the thermal performance of the borehole and minimizes the depth for given operating conditions.