Thermal analysis of borehole thermal energy storage in unsaturated soil

The thermal performance of soil borehole thermal energy storage (SBTES) systems in unsaturated soils is investigated to address three primary objectives: (1) to explore the impact of subsurface moisture content condition on the SBTES thermal performance, (2) to assess the effect of seasonal surface pressure variation on the SBTES thermal performance, and (3) to examine the influence of soil porosity on the thermal performance of SBTES in homogeneous and layered soils. Results indicate that a higher subsurface moisture content leads to a lower average temperature in the thermal storage domain. Notable hydrodynamic fluctuations are observed in the shallow depth of SBTES domain because of seasonal surface pressure variations although the average temperature and heat storage in SBTES remain mostly unaffected. The results show that a low soil porosity leads to superior SBTES thermal performance, while high porosity soils experienced more pronounced hydrodynamic fluctuations under surface pressure variation conditions. The impact of soil porosity on SBTES thermal performance is minimal in layered soil, except when one soil layer dominates and has a high volume fraction.