Study On Long-Term Heat Transfer Characteristics And Application Of Multiple Deep Borehole Heat Exchangers

In recent years,the closed medium-deep ground source heat pump technology has gained more and more attention and application with the advantages of large single-borehole heat extraction,smaller footprint and high system performance.However,because of the complexity of the heat transfer process and the immature theoretical development in the multiple medium-deep borehole heat exchanger（MMBHE）,it faces problems such as complex and difficult underground heat transfer calculations,huge numerical calculation workload,slow calculation speed,little research and poor understanding of MMBHE long-term heat transfer characteristics,and lack of theoretical guidance for engineering applications.To this end,a three-dimensional unsteady heat transfer model for MMBHE was established,a fast and efficient dimension reduction algorithm for solving the model was given,and long-term heat transfer characteristics and application studies of MMBHE were carried out based on the developed model and algorithm.A three-dimensional unsteady-state heat transfer model for MMBHE was developed.The model takes into account the unsteady heat transfer processes and the influence of the heat capacity of each material in borehole,taking into account the influence of many practical conditions such as terrestrial heat flow rate,initial non-uniform temperature distribution,inhomogeneous physical properties of rock and soil,and non-uniform heat flow on the borehole wall.It has been mathematically demonstrated that the linear superposition principle can be applied to the heat transfer calculations of MMBHE when multiple vertical boreholes are in the same formation with the same geometrical and physical conditions（thermal properties and terrestrial heat flow rate）and the distance between any two boreholes is much greater than the radius of each borehole.This means that the temperature response at any point in the MMBHE subsurface geotechnical soil is equal to the sum of the temperature response of each single-borehole at zero initial conditions and the initial temperature distribution.This provides a theoretical basis for turning the three-dimensional heat transfer problem of MMBHE into a two-dimensional problem.Based on this,a numerical heat transfer model for a three-dimensional MMBHE was developed.The model decomposes the solution of the 3D heat transfer problem of the medium-deep borehole cluster into the superposition of the solutions of a number of 2D sub-problems of single boreholes,thus realising a reduced dimensional calculation of the complex three-dimensional problem and significantly reduces the difficulty and computational workload of the MMBHE heat transfer calculation.Simulations with ANSYS FLUENT and Open Geo Sys have demonstrated that the model can increase the computational speed by several orders of magnitude compared to existing methods while maintaining sufficient computational accuracy,and significantly reduces the computational hardware requirements,making it easy to carry out relevant scientific research and engineering applications.The close relationship between the heat extraction capacity of MMBHE and the terminal heat load pattern was pointed out.The index Q_i（i=1,2,3）,which characterises the heat extraction capacity of MMBHE,and the decay rateζ_i（i=1,2,3）of the heat extraction capacity index were proposed,which are clear in concept and easy to calculate and analyse,and provide a quantitative basis for evaluating the decay of the heat extraction capacity of MMBHE.In particular,the decaying heat extraction load Q₃,which describes the decay of a MMBHE during long-term operation,is more intuitive and clearer,and is more suitable for analysing variations in the heat extraction capacity and variations in the decay rate of the heat extraction capacity of MMBHE.By calculating and analysing the influences of various factors,such as load characteristics,borehole geometry parameters,number of boreholes,geotechnical and borehole material thermal properties parameters,operating parameters and intermittent thermal storage operation during the non-heating season on the Q_iandζ_iof the MMBHE,the long-term heat transfer performance change pattern of MMBHE was clarified.Based on the results of the study of Q_iandζ_i,recommendations were given for the design and operation regulation of boreholes to improve the heat extraction capacity of MMBHE and maintain the long-term efficient operation of MMBHE,as well as the concept and calculation method of the thermal storage fluid temperature T_hrequired to slow down/eliminate the decay of the heat extraction capacity of MMBHE was presented to provide theoretical basis and guidance for engineering applications.Taking a hospital MMBHE geothermal energy heating project as an example,the calculation and scheme comparison analysis of the coaxial heat exchanger implementation scheme and the borehole arrangement scheme were carried out using numerical calculation simulations,and design principles for coaxial heat exchanger and borehole arrangement were given.The analysis shows that when the medium-deep borehole ground source heat pump is operated with intermittent heat storage in the non-heating season,and the temperature of the heat storage fluid is not lower than the temperature of the heat storage fluid T_hrequired to make the basic balance between heat extraction and heat storage in the medium-deep borehole heat exchanger,the borehole spacing can be significantly reduced,and its surface area can be correspondingly reduced.