Technical Summary: Design and Heat-Transfer Performance of a Hybrid Insulated Inner-Pipe String for Deep Geothermal Coaxial Heat Exchangers

Problem Statement

Deep geothermal coaxial heat exchangers face a critical engineering trade-off regarding the inner-pipe string design. Conventional all-steel insulated pipes offer the necessary mechanical strength and high-temperature resistance for deep wells but impose excessive weight, straining wellhead loads and deployment operations. Conversely, polymer pipes offer low weight and low thermal conductivity but lack the long-term temperature resistance required for deep-well environments (often exceeding 120 °C). Existing solutions struggle to simultaneously satisfy the competing requirements of thermal insulation, high-temperature mechanical reliability, and lightweight design. Furthermore, radial heat transfer between the upward-flowing hot fluid and downward-flowing cold fluid within the wellbore causes "thermal short-circuiting," significantly reducing the system's effective heat extraction capacity.

Methodology

The study employed a multi-scale approach integrating numerical simulation, material science, structural design, and field validation:

1. Numerical Modeling: A transient coupled seepage–heat-transfer model was developed using COMSOL Multiphysics. The model simulated a deep coaxial heat exchanger, cement sheath, and surrounding formation (200 m × 200 m domain) based on the geological conditions of the Xi'an Depression. It incorporated the $k$–$\varepsilon$ turbulence model for wellbore flow and Darcy's law for groundwater seepage.
2. Model Validation: The numerical model was validated against field data from a 4050-m-deep geothermal well, showing an overall calculation error of 6.6%.
3. Parametric Analysis: The model evaluated the effects of inlet flow rate, inlet temperature, inner-pipe thermal conductivity, inner-pipe diameter, and insulated-section length on heat extraction performance.
4. Material and Structural Design:
   • Material Selection: Engineering polymers (PPS, PEEK, PEI) and aluminum alloys were evaluated. 7075-T6511 aluminum alloy was selected for the lower section due to its high strength-to-weight ratio and temperature resistance.
   • Insulation Structure: A composite insulation design combining a vacuum annular cavity with silica aerogel felt was developed.
   • Heat Transfer Enhancement: A spiral-fin structure was designed on the outer surface of the inner pipe to enhance convective heat transfer, optimized using a Performance Evaluation Criterion (PEC) balancing Nusselt number gains against friction factor increases.
   • Corrosion Mitigation: A thermally insulating and electrically isolating pup joint (using PEEK liners) was designed to prevent galvanic corrosion at the steel-aluminum interface.
5. Field Testing: A segmented hybrid inner-pipe string was deployed in a 4050-m-deep well. The configuration consisted of a P110 steel section (0–3685 m) and a 7075 aluminum-alloy section (3685–4001 m). Performance was monitored using Distributed Temperature Sensing (DTS) over a 6-day test and a three-month heating season.

Key Contributions

• Segmented Hybrid Design: The study proposes a novel "hybrid insulated inner-pipe string" that segments the well depth, utilizing steel for the upper section to handle the highest structural loads and lightweight aluminum alloy for the lower section to achieve weight reduction. This approach reconciles thermal insulation needs with structural weight reduction.
• Optimized Insulation and Enhancement: The development of a vacuum-aerogel composite insulation structure (achieving thermal conductivity as low as 0.008 W/(m·K)) and the optimization of a spiral-fin geometry (20 mm height, 4 mm thickness, 216.89 mm spacing) to maximize thermal-hydraulic performance.
• Comprehensive Validation: The research bridges the gap between material properties and system-level performance through laboratory testing, numerical simulation, and full-scale field validation in a deep geothermal well.

Results

• Numerical Findings:
  • Lower inner-pipe thermal conductivity and longer insulated sections significantly improve heat extraction by suppressing thermal short-circuiting.
  • Increasing the insulated-section length yields diminishing returns, suggesting that insulation in the upper well portion is most critical.
  • Higher inlet flow rates increase total heat extraction power but decrease outlet temperature; lower inlet temperatures improve the thermodynamic driving force.
• Material Performance:
  • The aluminum-alloy insulated pipe has a mass per unit length of 10.96 kg/m, which is only 36.9% of a comparable steel pipe (29.78 kg/m).
  • The aluminum alloy maintained a yield strength of 381 MPa at 150 °C.
  • Structural calculations confirmed the design meets collapse (58.1 MPa) and internal pressure (46.3 MPa) requirements for 4000 m depth.
• Field Performance:
  • The hybrid string reduced the total pipe string weight by 5.7% compared to an all-steel configuration.
  • DTS data showed a weighted average temperature drop of only 7.61 °C across the entire 4001 m inner pipe string.
  • The system achieved a heating Coefficient of Performance (COP) of 6.80.
  • The average heat extraction rate was 352 W/m, with an estimated annual saving of 770.73 tons of standard coal equivalent and a reduction of 2019.31 tons of CO₂ emissions.
  • Operating costs were significantly lower than gas-boiler and municipal district heating alternatives.

Significance

The paper claims that the proposed hybrid insulated inner-pipe string provides a viable system-level solution for deep geothermal coaxial heat extraction. By integrating lightweight design, thermal insulation, high-temperature mechanical reliability, and enhanced heat-transfer performance, the study demonstrates that it is possible to overcome the limitations of single-material pipe strings. The successful field test validates that this segmented approach effectively reduces structural weight while maintaining thermal insulation and mechanical integrity, offering a practical pathway for the large-scale utilization of deep geothermal resources for district heating.