Pore-to-meter scale modeling of heat and mass transport applied to thermal energy storage: How local thermal and velocity fluctuations affect average thermal dispersivity

We use a dual-network model to simulate heat and mass transfer in porous media. The model captures pore-scale information in meter-scale simulations and allows for non-equilibrium between the solid and pore space. We apply the model to predict the effective thermal diffusivity in networks representing Bentheimer sandstone, Estaillades limestone and two random packings of monodisperse spheres. Non-Fourier transport at early times can lead to both higher and lower thermal dispersivity than currently assumed using a volume-weighted average of fluid and solid properties. Furthermore, we quantify the mechanical dispersion coefficient caused by differences in local flow velocity, which further contributes to thermal dispersion of the plume. We discuss the results in the context of the design and management of ATES (aquifer thermal energy storage). Ignoring pore-scale velocity and temperature fluctuations in the estimation of averaged properties can lead to errors of more than 50%. The work provides a framework to predict thermal properties of porous media under different flow conditions for more accurate prediction and design of thermal energy storage.