In-situ identification of axial borehole thermal conductivity from measured transient heat extraction
Energy and Buildings, vol.363, 2026 (SCI-Expanded, Scopus)
- Publication Type: Article / Article
- Volume: 363
- Publication Date: 2026
- Doi Number: 10.1016/j.enbuild.2026.117613
- Journal Name: Energy and Buildings
- Journal Indexes: Science Citation Index Expanded (SCI-EXPANDED), Scopus, Compendex, Environment Index, INSPEC, Public Affairs Index, Urban Studies Abstracts, Academic Search Ultimate (EBSCO), Engineering Source (EBSCO)
- Keywords: Borehole heat exchanger, Depth-dependent thermal conductivity, Ground source heat pump, Radial heat conduction model, Segment-wise inversion, Thermal response test
- Azerbaijan State University of Economics (UNEC) Affiliated: Yes
Abstract
This study presents a novel segment-wise inversion method for determining the depth-dependent thermal conductivity profile λ(z) of the subsurface using operational temperature measurements from a functioning ground source heat pump system. The method couples a fluid-side energy balance with a time-dependent 1D radial heat-conduction model and, in the present implementation, uses the borehole thermal resistance Rb obtained independently from a standard Thermal Response Test (TRT). This framework enables the reconstruction of high-resolution (10 m) thermal conductivity profiles from operational data without a dedicated depth-resolved TRT campaign. Results show that the reconstructed thermal conductivity profile clearly identifies major lithological transitions, including low-conductivity silty-clayey units and higher-conductivity sandy-gravel layers. The energy-weighted effective thermal conductivity deviates by less than ±2% from the TRT reference value, confirming that the method reproduces bulk thermal behaviour with TRT-level accuracy. A Monte Carlo noise analysis revealed that inversion stability strongly depends on the hydraulic loading: low mass flow rates reduce the signal-to-noise ratio and limit the interpretability of the segment-wise inversion, whereas higher flow rates yield stable, reproducible, and geologically meaningful profiles. The present field validation is limited to one 100 m borehole under conduction-dominated conditions; extension to groundwater-affected and multi-borehole settings requires further study.