| The development and utilization of deep geothermal energy from HDR has gradually attracted people's attention.Aiming at an innovative scheme for large-scale deep geothermal mining—the enhanced geothermal system based on excavation technology(EGS-E),this thesis uses numerical simulation with COMSOL Multiphysics and programming calculation with MATLAB as tools to make a preliminary research on its geometry of component,performance estimation of fractured structures and heat extraction mechanism in the circulatory closed pipes on the basis of thermology and mechanics.The contents of this thesis are illustrated as follows.(1)The process of determining the reasonable section dimension of a bare tunnel—the most typical component of deep structure in EGS-E model is given,which can provide a geometrical basis for establishing models in subsequent more elaborate numerical simulation.It includes:(i)The lining thickness required for a bare tunnel with radius of 3 m is solved to be 590 mm through programming calculation based on the theory of multiply connected body of the axisymmetric plane-strain problem in elastic mechanics.(ii)Combined with the excavation experience of the deepest mine in the world,the detailed process of solving the instantaneous heat conduction problem with non-homogeneous boundary conditions through separation variable method to determine the thickness of the required insulation layer is given.In addition to(i)and(ii),the reasonable section dimension of tunnel should simultaneously be considered having enough space for the machinery during construction.(2)The concise and direct superposition estimation schemes for the early performance of large-scale deep geothermal mining is established based on EGS-E,which can be a viable alternative to supercomputing for subsequent quantitative studies of the relationship between typical deep engineering structure and its heat extraction efficiency.It includes:(i)Comparison is made between numerical solution and analytical solution of a radial plane problem that the transient heat conduction through high-temperature surrounding rock to tunnels covering 30 years,thus the reliability of numerical solution is verified.(ii)The effects of the fixed temperature at tunnel wall,the radius of tunnel and rock type on the annual heat extraction rate are also studied.The results show that at a certain initial field temperature,the annual heat extraction rate from tunnel wall increases with the lower inner boundary fixed temperature,the longer radius of tunnel or the greater thermal diffusivity of surrounding rock.(iii)And three different estimation methods of EGS-E efficiency with comb-shaped and chessboard-shaped underground tunnels are studied respectively to get the relative optimum estimation schemes.The research ideas for the estimation of EGS-E system with more complicated cobweb-shaped tunnels are also pointed out.(3)The steady heat transfer between the forced convection within the circulatory closed pipes and the constant-temperature flow in reservoir is simulated and the relationship between the flow rate of water in pipe and its annual heat extraction rate as well as the temperature of outlet water is studied respectively,which can be a reference for optimization of other influence factors(pipe diameter,fluid type,etc.)of heat transfer in EGS-E.The results show that when the other influence factors are constant,the temperature of outlet water will decrease to near its initial temperature with the increase of flow rate but the increase of annual heat extraction rate will get more and more close to zero.Thus the reasonable flow rate of water in pipe should be determined based on the comprehensive consideration about the annual heat extraction rate and the temperature of outlet water as well as the transportation cost of water varying with the flow rate. |
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