Heat Transfer In Unsaturated Soil Research Articles

Fractal analysis on the effective heat conductivity of unsaturated soil

Because of the complicated structure, the traditional Euclidean geometry is difficult to accurately describe the inner distribution of pore soil. According to the fractal laws of inner structure in porous media, a popular fractal capillary model was presented to study the three phase heat transfer in unsaturated soil. Considering the soil pore size and its tortuosity, by using the fractal method about pore size distribution and tortuosity to depict the scaling relationships, the aim of this paper is to derive the effective heat conductivity of unsaturated three phase soil. The predicting values show good agreement with the experimental values of literature while the saturation values of water are among 0.2 to 0.9. Major factors that influence the effective heat conductivity of unsaturated soil were analyzed. The heat transfer mechanism of unsaturated soil was also analyzed according to the results. The conclusions of this study could help promote the improvement of the design of buried pipe heat exchanger in unsaturated soil.

Energy pile groups for thermal energy storage in unsaturated soils

• Energy pile groups provide superior thermal energy storage performance over boreholes. • Both energy pile geometry and number of internal heat exchangers are important. • Lower thermal conductivity of unsaturated soils leads to higher heat retention. • Transient decreases in degree of saturation were observed over several years. • Heat transfer in unsaturated soil can be enhanced in some cases via vapor diffusion. A coupled heat transfer and water flow model implemented in COMSOL and validated against measurements from a tank scale test was applied to investigate the application of energy pile groups for thermal energy storage in unsaturated soil layers. The novel focus of the investigation was understanding the long-term thermo-hydraulic response of the unsaturated soil within the energy pile group during heat injection at high temperature up to 90 °C and associated impacts on the heat storage performance. Unsaturated soil layers are advantageous for thermal energy storage due to enhanced convective heat transfer during injection associated with vapor diffusion and favorable insulation properties during storage associated with lower thermal conductivity of soils surrounding a heat storage system. Evolutions in temperature and degree of saturation in soil layers having different hydraulic properties and water table depths were simulated during five years of operating a group of five energy piles with inlet fluid temperatures of 90 °C during heat injection and 30 °C during heat extraction. Transient fluctuations in the degree of saturation were observed in all soil layers simulated, but a permanent decrease was only observed for a soil layer having a greater air entry suction after several cycles of heating and cooling. While the heat storage in energy pile groups in unsaturated soil layers was always between that of dry and saturated soils with no groundwater flow, the soil hydraulic properties and water table depth were found to control both the rate of heat transfer and the total heat stored. When comparing the performance of energy pile groups with a group of borehole heat exchangers commonly used in heat storage applications, the energy piles were approximately 1.2 times more effective in extracting heat with a faster response confirming their suitability for heat storage.

Coupled Thermally-Enhanced Bioremediation and Renewable Energy Storage System: Conceptual Framework and Modeling Investigation

This paper presents a novel method to couple an environmental bioremediation system with a subsurface renewable energy storage system. This method involves treating unsaturated contaminated soil using in-situ thermally enhanced bioremediation; the thermal system is powered by renewable energy. After remediation goals are achieved, the thermal system can then be used to store renewable energy in the form of heat in the subsurface for later use. This method can be used for enhanced treatment of environmental pollutants for which temperature is considered a limiting factor. For instance, this system can be used at a wide variety of petroleum-related sites that are likely contaminated with hydrocarbons such as oil refineries and facilities with above- and underground storage tanks. In this paper, a case-study example was analyzed using a previously developed numerical model of heat transfer in unsaturated soil. Results demonstrate that coupling energy storage and thermally-enhanced bioremediation systems offer an efficient and sustainable way to achieve desired temperature–moisture distribution in soil that will ultimately enhance the microbial activity.

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

Core IdeasWe validated a multiphase flow model for simulating heat transfer in an SBTES system.Convective/conductive heat transfer ratio was higher in the middle of heat storage domain.About 10% of the total heat transfer was in the form of latent heat.An asymmetric heat injection scheme can be useful to increase latent heat transfer.A constant effective thermal conductivity in the model can introduce significant errors.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.