In this study, we numerically and experimentally evaluated heat transfer in soils under unsaturated conditions in the context of simulating a laboratory-scale, three-dimensional soil-borehole thermal energy storage (SBTES) system. Most previous studies assumed that soil thermal and hydraulic properties are constant and that heat transfer in soil occurs only in the form of conduction, neglecting convective and latent heat transfer. In addition, there is a lack of data from controlled experiments to validate multiphase numerical models that can be used to better study SBTES systems installed in the vadose zone. The goal of this study was to evaluate the significance and impact of variable soil thermal and hydraulic properties, as well as different heat transfer mechanisms, in unsaturated soils. Four laboratory experiments were performed using a three-dimensional laboratory-scale SBTES system, incorporating sensors to collect soil temperature and moisture data at high spatial and temporal resolutions. Experimental data were then used to validate a numerical model that solves for water and vapor flow and considers nonequilibrium phase change. Results revealed that for the test conditions studied, convective heat transfer was higher than conductive heat transfer in the middle of the borehole array. Moreover, for the experiments on unsaturated sand, about 10% of the total heat transfer was in the form of latent heat. Simulation results demonstrated the importance of including both convection and latent heat in SBTES system modeling. Results also revealed a need for using saturation-dependent effective thermal conductivity in modeling SBTES systems in unsaturated soils rather than using constant values such as those obtained from system thermal response tests.

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Heat Transfer in Unsaturated Soil with Application to Borehole Thermal Energy Storage

Although siting of thermal energy storage systems in the vadose zone may be beneficial due to the low thermal conductivity of unsaturated soils, water phase change and vapor diffusion in so...

/pdf/role-of-nonequilibrium-water-vapor-diffusion-in-thermal-3rt25b9wv4.pdf

Role of nonequilibrium water vapor diffusion in thermal energy storage systems in the vadose zone

Application of changes in temperature to soils and rocks may lead to a wide range of flow processes and physical phenomena. This chapter focuses on the fundamental aspects of coupled heat transfer and water flow in saturated and unsaturated soils and rocks, thermal pressurization of pore fluids, thermal volume change, thermal softening of the preconsolidation stress, thermal hydro-shearing, and desiccation cracking. Established applications are also presented, including energy piles, barriers for radioactive waste repositories, and thermal energy storage. Emerging research areas including the role of thermal processes in climate change and elevated temperature landfills are also discussed.

Thierry Traore

Papers

Physical modeling of coupled heat transfer and water flow in soil-borehole thermal energy storage systems in the vadose zone