Key Takeaways & Executive Findings
- •• • Pump pressure drop scales linearly with depth: 622, 774, 924, and 1074 kPa at 2039, 2539, 3039, and 3539 m respectively for a 4.88 kg/s flow rate. This linearity confirms that frictional losses dominate over local losses, enabling predictable pump sizing for deeper wells without disproportionate hydraulic penalties. • • Comprehensive heat transfer power (including pump power) increases nearly linearly with depth and deviates from net heat transfer power by less than 1.5%. This marginal penalty indicates that pump energy consumption is not a limiting factor for deep systems, and heat extraction remains the primary performance driver. • • Per-unit-depth pump power decreases with increasing depth, meaning deeper wells dilute the parasitic pump load relative to total heat output. This favors deeper drilling from an energy efficiency standpoint, provided geological and drilling cost constraints are satisfied. • • The numerical model achieved a maximum relative error of 4.26% (17.8 kW) against field data over 72 hours, validating its reliability for predicting long-term performance. This accuracy supports the use of such models for design optimization and risk assessment in medium-deep geothermal projects.
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Abstract
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.
1. Introduction
Medium-deep geothermal energy offers a high-temperature, stable heat source for district heating, yet commercial deployment remains constrained by the parasitic energy consumption of circulation pumps and heat pumps. Existing research has predominantly focused on optimizing pipe geometry, backfill material thermal properties, and heat extraction rates, often neglecting the coupled influence of pump and heat pump power on overall system efficiency. This oversight leads to overestimating net energy gains, particularly for deep wells where hydraulic resistance escalates with depth. Field tests have demonstrated linear increases in heat extraction with depth, but the corresponding pump power penalties have not been systematically quantified, creating a design blind spot for engineers seeking to balance drilling costs against operational energy returns.
This study addresses that gap by constructing full-scale three-dimensional numerical models of coaxial casing-type deep buried pipes at depths of 2039, 2539, 3039, and 3539 m, simulating 121 days of continuous operation. By integrating pump pressure drop and heat pump performance coefficients into the heat transfer analysis, the work establishes a comprehensive performance metric that reflects real-world energy consumption. The model is validated against field data from a Xi'an project, achieving a maximum relative error of 4.26% in heat transfer power. The results quantify the marginal impact of pump power on total heat output and demonstrate that deeper wells exhibit reduced per-unit-depth pump power, providing a quantitative foundation for depth selection and system optimization in medium-deep geothermal applications.
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JIANG Chao, WU Jiale, LI Chao, XU Jiamin, WANG Jiachen, GUAN Yanling (2026). Comprehensive Heat Transfer Performance of Deep Buried Pipe Systems for Medium-Deep Geothermal Energy Utilization. Acta Energiae Solaris Sinica. https://doi.org/10.19912/j.0254-0096.tynxb.202608_9659
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Frequently Asked Questions
What is the maximum deviation between comprehensive heat transfer power and net heat transfer power when pump power is included, and what does this imply for system design?
The maximum deviation is no more than 1.5% across all depths (2039–3539 m) over a 121-day simulation. This indicates that pump power has a negligible impact on total heat output when flow rate is fixed, so designers can prioritize heat extraction optimization over pump efficiency for deep systems.
How does pump pressure drop scale with depth, and what are the exact values for the tested depths?
Pump pressure drop increases linearly with depth: 622 kPa at 2039 m, 774 kPa at 2539 m, 924 kPa at 3039 m, and 1074 kPa at 3539 m, for a flow rate of 4.88 kg/s. The linear trend is attributed to along-path friction dominating local losses, enabling predictable pump selection for deeper wells.
Does deeper drilling reduce the relative pump power burden, and what is the evidence?
Yes. The per-unit-depth pump power decreases with increasing depth, meaning the pump power proportion of total energy consumption diminishes. This is evidenced by the comprehensive heat transfer power increasing nearly linearly with depth while the pump power increment per meter declines, favoring deeper wells for better overall efficiency.
What is the validated accuracy of the numerical model against field data?
The model was validated over 72 hours against field experiments, showing a maximum heat transfer power difference of 17.8 kW and a maximum relative error of 4.26%. This high accuracy supports its use for long-term performance prediction and design optimization.
What are the key thermophysical parameters and boundary conditions used in the simulation?
The model uses a shallow constant-temperature layer of 20.524 °C, a geothermal gradient of 27.03 °C/km, and a bottom heat flux of 70.8 mW/m². Material properties include rock density 1800 kg/m³, thermal conductivity 3.0 W/(m·K), and specific heat 1200 J/(kg·K), with outer pipe conductivity 14.48 W/(m·K) and inner pipe conductivity 0.0312 W/(m·K).
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