Research on the efficient development of shallow geothermal energy: Experimental and numerical investigations on the heat transfer characteristics of the soil-cement energy pile

The heating and cooling system of ground-source heat pump is currently the most important development and utilization method of shallow geothermal resources. In order to solve the bottleneck problems of low thermodynamic efficiency and large footprint of the traditional ground heat exchanger in ground-source heat pump system, this study proposes a new scheme of coupling the ground heat exchangers with the soil-cement pile composite foundation, a type of pile foundation where cement and the native soil are mixed on-site in the pile hole while drilling to form a pile, to improve the thermodynamic efficiency of the ground heat exchanger by utilizing the good thermodynamic properties of the ground treatment materials. By means of numerical simulation using COMSOL Multiphysic5.0 software and similarity model experiment with a similarity ratio of 1/2, in which, the diameter of the model pile is 300 mm, and the buried pipe diameter is DN16, the heat transfer mechanisms of the ground heat exchanger in the soil-cement pile and the soil are compared and studied, the spatio-temporal variation rules of temperature field distribution are discussed, and the calculation method of the soil-cement energy pile is established preliminarily. The research indicates that the soil-cement energy piles can effectively reduce the maximum thermal resistance in the heat transfer system of energy piles, greatly improving the thermodynamic efficiency of energy piles; the average heat exchange per linear meter of the soil-cement energy pile is 34.2% and 42.6% higher than that of the ground heat exchanger in soil when cooling in summer and heating in winter, respectively; However, simply increasing the flow velocity inside the ground heat exchanger cannot further enhance the heat exchange capacity of the soil-cement energy piles, the effective methods are to choose the appropriate flow velocity and pipe diameter. This new model of green heating and cooling for buildings, the soil-cement energy pile system, fully leverages the main advantages of both the wide applicability of the ground-source heat pump systems and the environmental protection and cost reduction of the soil-cement piles, providing a theoretical basis and experimental support for the efficient development of shallow geothermal energy.