Comparative thermo-hydraulic and thermo-economic analysis of coaxial and U-shaped medium-deep borehole heat exchangers: Flow-rate-dependent trade-offs

Abstract

Medium-deep borehole heat exchangers (MDBHEs) are promising for low-carbon urban heating. However, existing comparative studies on coaxial (C-MDBHE) and U-shaped (U-MDBHE) configurations often yield contradictory conclusions under varying geological and operational conditions, hindering configuration selection. To establish a generalized decision-making framework, this study systematically compares the thermo-hydraulic and thermo-economic performance of parallel C-MDBHE and U-MDBHE systems. Results indicate that, at an identical burial depth, the C-MDBHE exhibits 41.79% lower total heat extraction than the U-MDBHE, yet 22.24% higher heat extraction per unit length. With an equivalent buried pipe length, the parallel C-MDBHE achieves 1.45–8.63% higher heat extraction than the U-MDBHE but consumes approximately 2.8 times the ground-loop pumping power. This contrast drives a fundamental flow-rate-dependent thermo-economic trade-off between the two configurations. A parallel C-MDBHE is favored at lower flow rates (<10.48 kg·s −1 ), whereas the U-MDBHE becomes preferable at higher flow rates (>20.96 kg·s −1 ). Within the intermediate range, the superior configuration depends on the chosen economic metric, and the crossover flow rate shifts in response to local electricity price fluctuations. Crucially, this overall thermo-economic trade-off pattern remains remarkably robust across diverse geological stratifications and initial investment costs. Additionally, this study clarifies criteria for simplifying geological models: Homogeneous models yield reliable predictions for formations with alternating thermal conductivity (<3.1% deviation), whereas stratified models are necessary for strongly depth-dependent formations (>6.8% deviation). Although based on relative first-year thermal performance rather than absolute long-term predictions, this framework reconciles existing discrepancies regarding MDBHE superiority and provides a reliable basis for engineering decisions.

The authors' abstract, as published at the source. Applied Thermal Engineering, 2026 · DOI ↗