A thermo-hydro-mechanical finite element model of freezing in porous media-thermo-mechanically consistent formulation and application to ground source heat pumps
Freezing phenomena in porous media have attracted great attention in geotechnics,
construction engineering and geothermal energy. For shallow geothermal applications where heat
pumps are connected to borehole heat exchangers (BHEs), soil freezing around the BHEs is a
potential problem due to persistent heat extraction or inappropriate design which can sig-
nificantly influence the temperature distribution as well as groundwater flow patterns
in the subsurface, and even lead to frost heave. A fully coupled thermo-hydro-mechanical freezing
model is required for advanced system design and scenario analyses. In the framework
of the Theory of Porous Media, a triphasic freezing model is derived and solved with the finite
element method. Ice formation in the porous medium results from a coupled heat and mass transfer
problem with phase change and is accompanied by volume expansion. The model is able to capture
various coupled physical phenomena during freezing, e.g., the latent heat ef- fect, groundwater
flow with porosity change and mechanical deformation. The current paper is focused primarily on the
theoretical derivation of the conceptual model. Its numerical implementation is verified against analytical solutions of selected phenomena including pure phase
change and thermo-hydro-mechanical process couplings
