From grains to aquifers: Under what conditions does Local Thermal Non-Equilibrium heat transport matter?

Accurate prediction of heat transport is fundamental to the design and performance assessment of underground thermal energy storage (UTES) systems. Most groundwater heat-transport models assume local thermal equilibrium (LTE) between the solid matrix and pore water, implying instantaneous interfacial heat exchange and a single temperature field. However, recent experimental and modelling studies show that this assumption can break down under conditions commonly encountered in permeable and heterogeneous aquifers.

Here we present a synthesis of a multi-scale research programme that identifies when and why local thermal non-equilibrium (LTNE) becomes relevant for subsurface heat transport, spanning grain-scale laboratory experiments and field-scale numerical modelling. At the grain scale, laboratory experiments resolving solid and fluid temperatures independently demonstrate that rate-limited interfacial heat exchange results in persistent solid–fluid temperature differences for coarse grains and elevated Darcy velocities representative of UTES operation. These effects are governed primarily by grain size and solid thermal properties and cannot be captured by LTE formulations.

At the aquifer scale, three-dimensional stochastic simulations show that LTNE-like behaviour can emerge even when pore-scale LTE holds, solely due to hydraulic conductivity heterogeneity. Preferential advection along high-permeability pathways accelerates thermal fronts, while delayed heat diffusion into low-permeability domains leads to effective thermal retardation that deviates fundamentally from predictions based on volumetric heat capacity. This field-scale LTNE depends systematically on the variance and correlation length of hydraulic conductivity and the thermal Péclet number.

Together, these results reveal a continuum of LTNE behaviour across scales: grain size controls interfacial heat exchange at the pore scale, while hydraulic conductivity heterogeneity governs delayed heat uptake at the aquifer scale. Ignoring either mechanism can potentially bias predictions of thermal plume migration, retardation, and heat recovery efficiency, with direct implications for UTES modelling, performance assessment, and design reliability.