Coaxial tube heat exchangers for medium-deep geothermal utilization: numerical analysis and layout optimization

Medium-deep geothermal energy resources have been extensively researched all over the world owing to their environmental friendliness, large reserves and renewability. However, the lack of field experimental data remains a major obstacle to model validation and engineering application. Based on field measurements, a model for a single well of a medium-deep coaxial tube heat exchanger (MDCTHE) was established in this study via the application of the finite element method. The impacts of key parameters, including fluid inlet temperature, volumetric flow rate, and thermal conductivity, on thermal performance were systematically investigated. Simulation results reveal that as the inlet temperature rises from 14 °C to 44 °C, the heat transfer capacity significantly drops by 74.6% (from 603.52 kW to 153.28 kW) due to the diminished thermal driving force. Conversely, while increasing the inlet volumetric flow rate reduces fluid residence time, the enhanced convective heat transfer coefficient dominates, leading to a net increase in total heat extraction. Meanwhile, a multi-well pipe group heat transfer model was constructed using the MDCTHE simulation data, and the impact of drilling layout on system efficiency was then examined. This study establishes a theoretical and practical basis for promoting the production and use of renewable energy, and provides the empirical evidence and the theoretical framework necessary for the engineering design and the developmental research of medium-deep geothermal resources.