Numerical analysis of heat transfer performance in medium-depth coaxial casing heat exchangers considering seepage effects

Abstract

This study presents an innovative analytical model for a 3000-meter-deep coaxial borehole heat exchanger (CBHE) that simultaneously considers geothermal gradients, geological stratification, groundwater seepage, and the vertical permeability of the seepage layer. By focusing on the annular fluid and its temperature response within the borehole layer, the model incorporates factors often neglected in previous research, including the influence of groundwater flow and the thermal property variations across different soil and rock layers. The results show that seepage significantly enhances heat transfer via distinct mechanisms. For instance, when the seepage layer is located at a depth of 2000 m, conduction-dominated heat exchange is critical at a seepage velocity of 5 × 10⁻⁷ m/s, whereas exceeding 1 × 10⁻⁵ m/s shifts the dominant mechanism to convective heat transfer, thereby increasing the local heat exchange intensity in the annular fluid by 56.7 %. Furthermore, optimizing the inner pipe's insulation length significantly enhances system efficiency by increasing the outlet temperature and reducing thermal losses. The study also establishes a relationship between seepage parameters and the necessary buried depth for achieving a given heat extraction. These findings offer valuable theoretical guidance for improving geothermal system performance and hold significant engineering implications for sustainable geothermal energy utilization.