Thermal energy storage in borehole arrays installed in unsaturated soils

In the last decades, much work has been performed to provide sustainable solutions for energy-related needs of society due to the increase in energy demands and concerns about warming of the climate. This includes using the subsurface as thermal energy sources such as in the Borehole Thermal Energy Storage (BTES) systems, an innovative approach to provide heating and cooling of the buildings through geothermal heat exchangers installed in the subsurface. This study focuses on the role of unsaturated soils on coupled thermo-hydraulic response of a BTES system, and specifically highlights how the coupled heat transfer and water flow processes and coupled thermo-hydraulic constitutive properties of soils may be exploited to optimize the performance of the BTES systems. A comprehensive study including characterization of constitutive properties of thermo-hydraulic properties and transient laboratory and field-scale responses of BTES systems, is performed. Then, the results from laboratory and full-scale field experiments are used to validate a three-dimensional finite element model to characterize heat transfer and heat storage within the BTES system. In addition, the economic and environmental impacts of the BTES systems are evaluated using a Life Cycle Assessment approach. The results indicate that the BTES systems can efficiently reduce energy consumption and CO2 emissions that make these systems more attractive and environmentally friendly.