Thermal-Mechanical-Strength Coupled Design Framework for Safety of Coaxial Borehole Heat Exchangers

Abstract

Medium-deep coaxial borehole heat exchangers play a pivotal role in decarbonizing heating systems, yet the reliability of inner pipes, governing system safety and efficiency, remains a challenge. Current designs lack theoretical foundations due to variable multi-physics interactions and insufficient characterization of external pressure resistance, leading to recurrent inner pipe failures. Therefore, we conducted full-scale external pressure and tensile tests based on actual temperature-pressure conditions and developed an inner pipe design algorithm that couples heat transfer, hydraulics, tensile forces and two-dimensional strength. Results show temperature increase reduces the strength of PE-RT II pipes; borehole top is weak point for external pressure while bottom is vulnerable to tensile; formation temperature is the dominant factor affecting limits. Engineer-ready design charts considering design factors show flow-pressure-depth limits across geological scenarios. They show that flow limits decrease with depth at a reducing rate and pressure limits show a similar but more linear decreasing trend. By comparison analysis, Φ110×12.3 mm PE-RT II pipe demonstrates the highest pressure and limits. Maximum heat extraction capacity follows an S-shaped curve with depth, with greater performance differences between pipe sizes at larger depths. This work provides actionable guidelines for ground heat exchanger design and standardization to avoid risks in diverse settings.

Keywords: Medium-deep geothermal, coaxial borehole heat exchanger, inner pipe design, system safety, flow rate