Research On Ground Heat Exchanger Deformation Due To Frost Heave In Earth Energy Application

The structure stability of underground heat exchangers are becoming a hot issue withthe application of underground thermal energy storage (UTES) and the development ofresearch effort. The operating temperature of ground heat exchange pipe usually sustainsbelow0°C, when a larger load occurs during cool thermal storage or ground source heatpump heating process. The sustaining low temperature can lead to pore water freeze andvolume expansion in soil, which is called frost heave. The frost heave can not only changethe soil pore structure to influencing the heat transfer, but also squeeze the buried heatexchange pipe and result in a pipe deformation when the frost heave force is large enough.The pipe deformation can bring about harm to the circulating system, e.g. fluid overflow andflow resistance increase etc., and the circulating system even can not work in a serioussituation. Consequently, the deformation of ground heat exchange pipe due to frost heave isgetting more and more attention.For this purpose, based on the issue of ground heat exchanger operating capabilitydecay and failure proposed firstly in the Natural Science Foundation Project of China,combined with theoretical analysis, experiment and simulation, this study focused on themechanism, evolution and influence factors of pipe-soil structure frost heave anddeformation at the temperature below0°C. This work is beneficial to improving the designtheory of ground heat exchanger, maintaining the ground heat exchange system operating inan efficient status. Furthermore, this study lays a theoretical foundation of scientific, healthyand sustainable development on shallow geothermal energy application technology.The main research works included in this paper are as follows:The pipe-soil structure frost heave experimental system was established on the basis ofsimplifying the real ground heat exchange system and other restrictive conditions. The soilfreezing front tracking, pipe strains and pipe inner measuring were proposed as the methods to investigating the frost heave and pipe deformation. The experimental system lays afoundation of further experimental and simulative research.The experimental work investigated the basic variation of U pipe-soil structure duringfrost heave, including freezing area, pipe deformation, soil pressure on pipe, pipe volumeand circulating flow resistance. The three influence factors on frost heave, temperatureoperating mode, initial pipe deformation and soil grain size, were also analyzed using thefrost heave experimental system. The experimental work could provide some reference inthe course of choosing running mode and backfill material, as well as the dealing with pipeinjury.The basic process of soil frost heave and its influence factors were stated. During thefrost heave, soil porosity variation, pipe-soil structure deformation, phase change heattransfer in freezing soil and contact surface interaction between pipe and soil were discussedin detail. The computing model of pipe-soil structure frost heave and deformation was builtbased on simulation platform ABAQUS and thermal-mechanical coupled subroutine.Meanwhile, this computing model was verified by frost heave experiment, the verificationincluded mainly soil freezing temperature field and pipe deformation strain. The result ofverification indicated this model could be applied for the study of pipe-soil structure frostheave and deformation. The computing model could lay the theory and applicationfoundation of ground heat exchanger deformation due to frost heave.The typical U pipe, double U pipe and U elbow pipe were taken as examples toinvestigate the basic characteristics of pipe-soil structure, e.g. freezing area temperature, icepercentage, soil strain, pipe displacement, pipe deformation and pressure on pipe, based onthe built computing model. This simulative work was carried out in a continuous temperaturedrop mode and considering the temperature difference between inlet pipe and outlet pipe.Based on the experiment, this simulative study could make a more comprehensiveunderstanding in soil freezing area development and pipe deformation during frost heave.Moreover, based on the computing model of U pipe-soil structure frost heave anddeformation, this paper also investigated three factors in depth, which could influence the frost heave and pipe deformation. The three factors were heat exchange pipe operatingcondition (continuous temperature drop mode, constant temperature mode and temperaturedifference between inlet pipe and outlet pipe), pipe structure parameter (pipe centre distanceand pipe diameter) and soil characteristic parameter (initial stress, elastic modulus and initialwater content). The simulative work could provide some reference for determining theoperating mode and pipe structure reasonably, as well as predicting the frost heave and pipedeformation in different backfill environment.