Comprehensive Heat Transfer Performance of Deep Buried Pipe Systems for Medium-Deep Geothermal Energy Utilization
The utilization of medium-deep geothermal energy is primarily achieved through deep buried pipe closed-loop heat exchange systems, where the heat transfer efficiency is governed by the coupled effects of pipe depth, pump power, and heat pump energy consumption. Based on a casing-type deep buried pipe heat exchange project in Xi'an, three-dimensional full-scale numerical models with depths of 2039, 2539, 3039, and 3539 m were established to simulate heat extraction, pump power, and heat pump energy consumption over a 121-day operational period, thereby evaluating the comprehensive heat transfer performance. Results indicate that the comprehensive heat transfer power, accounting for pump power, increases approximately linearly with depth, with a maximum deviation of no more than 1.5% from the net heat transfer power. The pump pressure drop required to achieve a flow rate of 4.88 kg/s increases linearly with depth, reaching 622, 774, 924, and 1074 kPa for the four depths, respectively. The per-unit-depth pump power decreases with increasing depth, indicating that greater burial depth reduces the pump power proportion and enhances the overall heat transfer efficiency. The numerical model was validated against field experimental data, showing a maximum relative error of 4.26% in heat transfer power over a 72-hour period. These findings provide a quantitative basis for optimizing deep buried pipe system design and assessing the trade-offs between heat extraction and parasitic energy consumption in medium-deep geothermal applications.