Numerical simulation and experimental validation of the thermo-mechanical characteristics of phase change concrete energy pile

Energy shortage and environmental pollution have led to the wide use of clean and renewable energy. A ground source heat pump (GSHP) is extensively utilized in the new building of various renewable energy, because of its energy-saving, high efficiency and environmental friendliness. However, the current GSHP development is limited to a certain extent, due mainly to its high cost, and large land areas resulting from drilling boreholes. Alternatively, the concept of an energy pile was proposed to combine the GSHP exchanger of ground heat with the pile foundation of a building. However, the temperature change of the pile body can result in the thermal expansion and contraction of an energy pile, thereby inducing the pile deformation in the actual operation of energy piles. This variation of temperature has endangered the structural safety of energy piles, particularly on the thermal performance and bearing capacity. Therefore, a feasible energy pile with phase change concrete has been selected, where a type of phase change material (PCM) can serve as a part of filling material in a traditional energy pile. The thermal performance of the energy pile with phase change concrete was greatly enhanced, indicating the weak temperature variation and deformation during the heat exchange, due largely to the invariable temperature and the latent heat released during the phase change of PCM. In this study, a 3D numerical model of energy piles with phase change concrete was established to evaluate the thermo-mechanical characteristics of a whole energy pile under thermal-mechanical coupling loads. A systematic analysis was made to compare the thermo-mechanical characteristics of the traditional energy pile and the new one with phase change concrete. An experiment was conducted to explore the effects of the distance between two legs of U-tube, and the ratio of length to diameter of pile body on the thermo-mechanical behavior of an energy pile with phase change concrete. The results showed that the solid-liquid phase change of PCM increased the heat exchange rate at the per unit depth of pile by 10.3%, while reduced the changes in the temperature of the pile body. There was a reduced change in the displacement, axial force, and side friction resistance of the pile body caused by the temperature change. With the increase of distance between two legs of the U-tube, the heat transfer rate and heat influence range of soil increased in the energy pile, while the axial force of the pile decreased, as well as the pile displacement first increased and then decreased. An increase in the ratio of length to diameter improved the total heat exchange rate of energy piles, but reduced the heat exchange rate at the per unit depth of pile, while increased the pile top displacement, which was not conducive to the stability of the energy pile. The experimental validation showed that the predicted relative errors of middle temperature in the pile wall and displacement at the pile top were within 5.1% and 12%, respectively, and the average relative errors were 4.2% and 9.9%, respectively. Therefore, the developed numerical model of an energy pile can be used to simulate the thermo-mechanical characteristics of an energy pile with phase change concrete. © 2021, Editorial Department of the Transactions of the Chinese Society of Agricultural Engineering. All right reserved.