Numerical approaches for the design and optimization of energy geostructures: assessment of the thermal resistance of energy piles

The shallow geothermal energy is a renewable source, which in recent years has found widespread use in the energy requirements of buildings. Using the natural thermal energy present in the soil, structural foundation elements, as piles, walls, or tunnel linings, can be exploited as ground heat exchangers. It consists of a multifunctional technology which requires multidisciplinary approaches. In the design of energy geostructures, structural problems, heat and mass transfers, as well as coupled hydro-thermo-mechanical response of geomaterials, must be taken into account. Actually, as it is an innovative technology developed over the past 20 years, there is no unified codes and framework, for the analysis and design of energy geostructures. Therefore, the aim of this work is to provide a numerical approach for the design and optimization of energy piles. In particular, the energy efficiency of the piles has been analysed using the concept of the thermal resistance. The latter is an important parameter, which allows to understand and quantify the heat exchange capacity of a pile. At first, the governing equations and fundamentals of heat and mass transfer in the context of energy piles, have been studied. Then, 3D numerical models have been implemented in the FEM software Lagamine, and the time evolution of the thermo-hydraulic coupled phenomena, in the operation of energy piles, has been analysed. A number of simulations have been performed on a single energy pile, to understand the key controlling geometrical and thermo-hydraulic parameters of the pile thermal resistance. Afterwards, a groundwater flow has been applied to the models, to investigate the influence on the pile and on the temperature field around it. In conclusion, a more real case of an energy pile group has been defined, to analyse both the thermal interaction between the piles and the pile screen effect with a flow applied.