Abstract

This study focuses on the application of ground source heat pump systems in subway stations, specifically investigating the thermal-mechanical coupled response and heat transfer mechanisms of energy slabs. Currently, the thermal performance and structural safety of such structures under actual operating conditions have not been sufficiently studied.

Based on the Dalian Metro Line 4 project, two sets of in-situ field tests were conducted: one employed a constant inlet water temperature mode, focusing on analyzing the effects of inlet water temperature, flow rate, groundwater conditions, and operation modes on heat exchange performance; the other adopted a constant heating power mode, exploring the influence of heating power and flow rate variations on heat transfer characteristics.

Experimental results indicate that increasing both the inlet water temperature and flow rate contributes to enhanced heat exchange efficiency. Under intermittent operation conditions, the average heat flux per unit area reached 57 W/m², representing a 14.7% improvement compared to continuous operation. The thermal-mechanical response is significantly influenced by inlet water temperature, flow rate, and stratum permeability.

During the pump-off phase of intermittent operation, a reverse temperature gradient and the consequent rise in thermally-induced compressive stress were observed. Uneven flow distribution in heterogeneous soil layers tends to cause localized heat transfer enhancement while reducing overall efficiency. During long-term operation, the structural response remains within the elastic range, with a maximum thermal stress of 4.15 MPa and a maximum vertical displacement of 2.84 mm, both well below the limits specified in current codes.

This study systematically reveals the working mechanism of energy slabs under multi-field coupling effects, providing theoretical basis and experimental support for their design and application in thermally activated underground structures.

Keywords: energy foundation; shallow geothermal energy; heat exchange power; thermo-mechanical coupling; subway station; in-situ test