Preventing water temperature drop in a flow-rate-controlled ground source heat pump system using a lining borehole ground heat exchanger

In cold regions, antifreeze fluids are used as heat-transfer mediums in borehole heat exchangers of ground source heat pump (GSHP) systems. Water represents a more cost-effective and environmentally friendly alternative to antifreeze fluids. However, to prevent damage to equipment and ensure stable operation of the GSHP systems that use water, special care is needed to avoid the freezing of circulating water. In this study, we proposed a flow-rate control method to prevent the circulating water temperature in a GSHP system from falling below the freezing point. We developed a lining borehole ground heat exchanger (LBHE) and assessed its control performance via a heating-operation simulation using measured data. The simulation confirmed that temperature-drop prevention control maintained the circulating water temperature above the freezing point. Notably, the effectiveness of the control depended on the borehole diameter and length. Under conditions in which increased flow rate caused the internal flow of the LBHE to transition from laminar to turbulent, the difference between the LBHE length required for freeze prevention under control and that under uncontrolled case was larger. Compared with the uncontrolled case, the required length was at most 7 m shorter. Based on these results, circulating water in a GSHP system can be maintained above the freezing temperature even if the heating load increases more than expected by incorporating flow-control strategies when designing LBHEs with relatively large borehole diameters, while maintaining laminar internal flow within the LBHE. Overall, this study provides valuable insights for developing sustainable and stable GSHP systems.