Temperature and moisture evolution near energy walls in partially saturated soils: preliminary physical and numerical modelling

Shallow energy geostructures enable heat exchange with a soil mass which is usually partially saturated, whereas analytical and numerical models used for preliminary assessment and design are generally based on dry or fully saturated soil conditions. To investigate the coupled thermo-hydraulic process that occurs in partially saturated conditions, and specifically the significance of thermally induced moisture migration, laboratory physical tests on a reduced-scale energy wall and finite element analyses were performed. The laboratory tests allowed for the monitoring of temperature and water content evolution during thermal loading, under controlled initial soil density and moisture. The numerical analyses provided insights into the system’s thermal and moisture response under various soil conditions, thermal loads and process duration. Both experiments and numerical simulations show that heating induces a gradual migration of water from the energy wall to cooler areas. However, moisture redistribution develops slowly and can be limited if the timescale is limited, resulting in only minor variations in soil thermal conductivity. As a result, moisture migration has marginal effect on heat transfer under the tested conditions, indicating that the thermal and hydraulic balance equations may be solved independently without compromising the accuracy of thermal predictions.