Three-dimensional Modeling And Application Analysis Of Pile Heat Exchangers

Ground source heat pump technology is the most effective way in the utilization of shallow geothermal energy, and it has been widely used in the world at present. The pile ground heat pump system has become the research hot spot as it has advantages in making full use of land area, reducing the initial investment and enhancing heat transfer performance of the heat exchangers.A three-dimensional numerical model of pile ground heat exchangers is established based on COMSOL Multiphysics software. It is verified through the comparison with ground thermal response test.An improved analytical model is proposed on the basis of the traditional analytical model, combining with the numerical model. The improved model can be used to obtain the ground thermal physical properties in inverse direction. The inverse results can be verified through the comparison with composite cylindrical heat source model. And as the setting of ground thermal physical properties in the numerical model does not affect the borehole thermal resistance, the improved model can be applied into the practical engineering directly. It is shown that the improved model can accurately reflect the heat exchange progress in pile ground heat exchangers and obtain the ground thermal physical properties?The numerical models of double-U(MOU),W(MOW) and spiral coil(MOS) pile ground heat exchangers are established, and the heat exchange performance of these three models are discussed in both summer and winter operational conditions. The influencing factors for the heat exchange performance of double-U pile ground heat exchangers are analyzed based on MOU model, including pile depth, fluid velocity, inlet temperature, ground initial temperature and ground thermal conductivity; The influencing factors for the heat exchange performance of spiral coil pile ground heat exchangers are analyzed based on MOS model, including coil distance and inlet types. The software SPSS is used in the multiple linear regression analysis which takes into consideration of influencing factors for the heat exchange performance of double-U pile ground heat exchangers, and the factors importance can be obtained. The regression model can be verified through the comparison with numerical simulation.The research results can provide a new tool for the analysis of energy piles' thermal performance and the test of ground thermal property. Meanwhile, it can provide reliable guideline for practical engineering.