Experimental and numerical analysis of heat transfer in unsaturated soil with an application to soil borehole thermal energy storage (SBTES) systems

Abstract

A promising energy storage option is to inject and store heat generated from renewable energy sources in geothermal borehole arrays to form soil-borehole thermal energy storage (SBTES) systems. SBTES systems involve direct circulation of heated fluid through closed-loop geothermal heat exchangers in closely spaced vertical borehole arrays. Although pilot programs are successfully utilizing SBTES systems, two of the main limitations in large-scale implementation of these systems are low system efficiency and high initial installation costs. An approach to potentially enhance the efficiency of SBTES systems is to install them in the vadose zone (the unsaturated zone of soil above the water table). In this case, it is possible to take advantage of phase change and convective heat transfer phenomena in the pore water to obtain greater heat injection and extraction rates, making the SBTES system more efficient. Although it is widely recognized that the movement of water in liquid and vapor forms through unsaturated soils is closely coupled to heat transfer, these coupled processes have not been considered in modeling of SBTES systems located in the vadose zone. Instead, previous analyses have assumed that the soil is a purely conductive medium with constant hydraulic and thermal properties. The goal of this dissertation was to better understand heat and mass transfer processes with an application to SBTES systems installed in the vadose zone through conducting experimental and numerical studies in different scales. To achieve this goal, three different phases using both experimental and numerical investigations are defined. The experimental study included both two- and three-dimensional bench and intermediate scale experiments, respectively. Experimental data were then used to validate a numerical model that solves for water and vapor flow and considers non-equilibrium phase change. In addition, a set of pore-scale numerical simulations using the phase field method were defined to study soil thermal and hydraulic properties. 2-D experimental and numerical results demonstrated the importance of simultaneously analyzing coupled heat and mass transfer in SBTES systems installed in the vadose zone. For the initial and boundary conditions assumed in the study, results indicated that convective heat flux is considerably larger than conductive heat flux, demonstrating the importance of including convective heat transfer in modeling of SBTES systems, especially in the unsaturated soils where water vapor is present. Results of the 3-D study 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 experiments with 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 implementing saturation-dependent effective thermal conductivity in SBTES numerical models rather than using constant values such as those obtained from system thermal response tests. Pore-scale simulations provide opportunities to understand the impact of smaller-scale phenomena on larger scale processes. Pore-scale simulation of heat conduction through unsaturated porous media showed an abrupt decrease in thermal conductivity at approximately 60% saturation. This abrupt decrease is most likely due to the disconnection of water-soil connections or “bridges” which in turn, diminished heat transfer through highly conductive water - soil pathways while the less conductive air - soil pathways dominated. In fact, a 4% decrease at approximately 60% saturation reduced the effective thermal conductivity by more than 30%. The pore scale model can be used to perform sensitivity analysis on several important properties, evaluate thermal and hydraulic properties in elevated temperatures and asses the validity of thermal-equilibrium assumption in continuum-scale models.