Numerical investigation on the influence of groundwater flow on long-term heat extraction performance of deep borehole heat exchanger array

Groundwater convection is commonly observed in real-world projects, particularly in coastal and groundwater-abundant regions. To accurately evaluate the heat extraction capacity of the deep borehole heat exchanger (DBHE) considering groundwater flow, a conduction–convection coupled numerical model of the DBHE is established by OpenGeoSys (OGS) software. Then, the variation of the DBHE circulation temperature and the heat extraction capacity affected by different groundwater conditions, including Darcy velocity, location of the aquifer, and porosity of the aquifer, are quantitatively analyzed. The results show that the porosity and location of the aquifer have a limited effect on the heat extraction capacity of the DBHE. With the given scenario in this study, when the Darcy velocity reaches more than 1×10-7m/s, it has a distinguishable effect on the heat extraction capacity of DBHE under the influence of groundwater. In addition, long-term simulations of multiple DBHEs considering the characteristics of the ground pipe network are performed in different directions of groundwater flow. The results indicate that groundwater flow can alleviate cold accumulation around the boreholes, and the thermal plume is pushed much towards the downstream direction. The cross-flow groundwater results in a higher circulation temperature than the parallel flow for the DBHE array. The maximum temperature difference between the two configurations is 1.98∘C, which occurs at the end of the 15th operating year based on the given parameters. The results of this study can be used as a guide for project engineers and decision-makers to accurately assess the heat extraction capacity of DBHE and strategize the layout of the DBHE array, taking into account the influence of groundwater flow.