Thermal-hydro-mechanical Coupled Model for Energy Pile in Mudstone Area

Energy piles, a novel energy-saving technology leveraging shallow geothermal resources, play a
crucial role in addressing energy shortages and environmental pollution. This technology
integrates the buried heat exchange pipes from conventional ground-source heat pump systems
with building pile foundations, forming underground heat exchangers that can both bear structural
loads and exchange thermal energy. This study, focusing on a shallow mudstone layer in the
Woodsmith project, examines the heat exchange efficiency, thermo-mechanical (TM) coupling,
and thermal-hydraulic-mechanical (THM) effects during temperature cycles of energy piles. A 3D
thermal-hydro (TH) coupling model was developed in COMSOL to evaluate their heat exchange
efficiency and longevity, and to investigate thermal interference in pile groups. A 3D TM model
was also created to assess the coupling effects in mudstone, while a 2D THM model explores the
mechanical responses and pore water pressure variations during temperature cycles.
Simulation results reveal that mudstone's low permeability minimally impacts the heat transfer
efficiency of energy piles due to density-driven pore water flow. Factors like mudstone’s thermal
properties, and the flow velocity of the working fluid inside the heat exchange pipes, influence the
efficiency of energy piles. TM coupling simulations indicate that thermal disturbances cause
notable displacements in energy piles, particularly cooling contraction or heating expansion, with
maximum displacements at the pile head and minimized, constrained displacements at the centre.
Mudstone's thermal expansion coefficient, elastic modulus, and thermal conductivity critically
influence these mechanical responses. THM coupling simulations show the pile head moves
upward during heating and downward during cooling, with minimal impact on the surrounding
layer's pore water pressure. The temperature setting of the heat exchanger crucially affects these
displacements and pressure changes