Coupled Simulation On Seepage Field And Temperature Field For The Single-Well Geothermal Energy Collection With Circulation Heat Exchange And Its Application

At present,the energy consumption of China's building for heating(cooling)accounts for about 18%of total energy consumption.With the advancement of urbanization,improvement of people's living standards and the continuous southward shift of heating demand lines,the energy consumption for heating and cooling is growing at a rate of 1%per year.However,in China coal-fired heating is the most important way to provide heating for buildings,which will bring great pressure on the adjustment of ecological environment and energy structure.The development of shallow geothermal energy is mainly to utilize ground source heat pump system which can elevate low-grade shallow geothermal energy extracted from subsurface for heating or cooling buildings.The large amount of shallow geothermal energy combined with ground source heat pump technology,with 1 part of clean electric energy,transporting more than 3 parts of heat to heat the building,saving about 40%compared with the traditional central air conditioning,not only save high grade such as coal or natural gas,but also improve the level of clean heating and the quality of the air environment.According to the analysis of the advantages and disadvantages of two traditional shallow geothermal energy systems such as borehole heat exchanger ground source heat pump system and groundwater heat pump system,this paper innovatively proposes a new generation system of extracting shallow geothermal energy with completely independent intellectual property rights based on the principle of absorbing each other's advantages and abandoning its shortcomings.That is the advanced single well circulation shallow geothermal energy system which has solved the problem of groundwater recharge difficulty,low heating efficiency,occupying large space,groundwater pollution and other worldwide problems in the development of shallow geothermal energy.Meanwhile,according to different hydrogeological conditions of project sites,three different single well circulation geothermal systems were developed,namely single well circulation geothermal energy system,single well circulation geothermal energy system with backfilling heat transfer particles,and single well circulation geothermal energy system under multi-aquifer conditions.The principle and characteristics of three different single well circulation geothermal energy systems are summarized.In addition,the well for the single well geothermal system has the function of pumping water and recharging water.In this case,the Dupuit formula and Theis formula cannot accurately describe the movement of groundwater in the confined aquifer with both pumping and recharge groundwater for the same single well.In this work,the mathematical model of the groundwater seepage field in the single well circulation geothermal system is established.The equation of movement of the groundwater to the single well with pumping function and recharging function is derived by employing the superposition principle.And this results are utilized in practical engineering application.On this basis,a two-dimensional coupled numerical model of seepage field and temperature field profile of shallow geothermal system for single well circulation geothermal system is presented,which is used to carried out the numerical simulation analysis of practical engineering in Zhongguancun Third Primary School,Haidian District.The variation trend of groundwater temperature,the change of underground temperature field and the variation rules of system COP in pumping area and recharge area under long-term operation conditions of the single well geothermal system are studied,and the actual engineering condition of single well circulation geothermal system is also evaluated.Finally,a three-dimensional coupled numerical model for seepage field and temperature field of single well circulation geothermal system is established.Through the generalization of the hydrogeological conditions of well groups of single well circulation geothermal systems in Zhongguancun Third Primary School,Haidian District,the physical generalization model and numerical model of well groups were established.Besides,Three-dimensional coupled simulation study on seepage field and temperature field of well group with single well circulation geothermal system was carried out.Considering four different working conditions on the basis of above,the numerical simulation is carried out to analyze the evolution trend of temperature field and seepage field of the single well circulation system under different pumping flow,operation in winter and stop operation in summer,operation in summer and stop operation in winter,and different hydraulic conductivities.The main research contents are as follows:(1)This paper Innovatively proposed a new generation of shallow geothermal energy extraction method with completely independent intellectual property rights,namely single well circulation geothermal energy system,and developed three different types of geothermal energy extraction system according to different hydrogeological conditions of engineering sites.The traditional groundwater heat pump systems have higher efficiency compared to other ground source heat pump systems but with the difficulty of groundwater recharging.Besides,although the borehole heat exchanger ground source heat pump systems do not consume water resource,but it also has some advantages like lower efficiency and occupying large space,etc.In this work,we adopted advantages of two traditional ground source heat pump systems,abandoned their shortcomings,and finally invented a new generation of shallow geothermal energy extraction methods with complete independent intellectual property rights,namely single well circulation geothermal energy systems.It solves the worldwide problems of the difficulty of groundwater recharging,low heat extraction efficiency,and occupying large space,which restrict the development of shallow geothermal energy.According to different hydrogeological conditions of the engineering site,three different types of geothermal energy extraction systems have been developed,namely single well circulation geothermal energy system,single well circulation geothermal energy system with backfilling heat transfer particles,and single well circulation geothermal energy system under multi-aquifer conditions.(2)The Dupuit formula and Theis formula of recharge hydrogeology have solved the problem of groundwater movement in the catchment structure with a single function(or pumping function or recharge function)under certain hypothetical conditions respectively under the condition of steady flow and unstable flow.However,they can not describe the movement of groundwater in aquifers to single catchment structures with both pumping function and recharge function,nor can they solve the problem of groundwater pumping at the same time,or Evaluation and prediction of seepage field of single catchment structure with double function irrigation.Based on the theory of superposition,the physical conceptual model and mathematical model of groundwater seepage field of single-well circulating geothermal energy acquisition system are established first time,and the motion equation of single water collecting building with simultaneous pumping and filling functions of groundwater is derived in this paper.The theoretical solution of groundwater depth reduction around single catchment building is solved analytically.The study and solution of this problem are of great theoretical significance and practical value in describing the groundwater movement in single-well circulation geothermal energy acquisition system.Two scenarios of aquifer thickness of 60m and 100m are designed for practical engineering project,and the influence of different aquifer thickness on the depth of groundwater in single-well circulation system is analyzed.It can be seen from the groundwater depth contour map of the single-well circulating that in the lower part of the pumping section of the single-well circulation system,the smaller the head is due to the pumping,the farther the distance from the shaft of the pumping section is,the smaller the depth of the groundwater is.In the recharge section,the head of the section is increased because of the reirrigation,and the longer the distance from the well axis,the smaller the absolute value of groundwater depth.Therefore,with the decrease of aquifer thickness,the absolute value of deep drop caused by pumping and recharging increases gradually.This result will be of enormous significance to understand the general rules groundwater movement in single-well circulation geothermal acquisition system,and has some guidance to practical engineering.(3)A two dimensional coupled numerical simulation model of seepage field and temperature field in shallow geothermal energy acquisition system with single-well circulation heat transfer is established,where the model is used to simulate the acquisition system of the demonstration project.Both the variation and trend of seepage field and temperature field,and the underground temperature field and system COP are analyzed under the condition of long-term operation of the collecting system.In addition,the status of single-well circulating heat transfer acquisition system is evaluated.It is possible to conclude that the project is in good condition,the COP value ranges from 3 to 4,and the system efficiency stays in a stable state.At the same time,1:1 model is constructed by Solidworks,then ANSYS ICEM is used to mesh the grid of model,and FLIUET is utilized to solve and show the cloud map of the flow field and temperature field in single-well circulation geothermal acquisition system.The contrasting analysis of the flow field map shows that with the accumulative time of recharge in the system,the amount of reclaimed water increases.However,due to the slow seepage velocity of porous in the aquifer,the recharge volume cannot meet the demand of water recharge,which leads to the difficulty of recharge in the system.The comparative analysis of temperature field cloud diagram shows that with the accumulation of operating time,the occurrence of heat conduction,thermal convection and thermal dispersion in the aquifer,the pumping temperature in the pumping area will soon be affected by the backwater temperature,and the system efficiency will be reduced as a result of the formation of the thermal transfixion phenomenon.At the same time,the general rules of the flow field and temperature field are concluded by analyzing the flow field and temperature field map.It clearly shows the tendency of forming thermal transfixion,and the process of influencing system efficiency.Hence,it is verified that the project runs well.The result is suggestive and instructive to slow down the efficiency reduction of single-well circulation underground heat transfer system.(4)Three-dimensional coupled simulation numerical model of seepage field and temperature field of shallow geothermal energy acquisition group wells with single-well circulation heat transfer is established,and parameters identification is carried out by numerical inversion method.Depending on the long-term observation data of well group of the demonstration project,the variation law of seepage field and temperature field of the acquisition system is simulated.Based on that,four separate operating scenarios were simulated and analyzed under the four working conditions of different pumping and irrigation flow.The evolutionary trend of the temperature field and permeation field of collecting system were simulated by summer outage in winter,winter shutdown in summer,and seepage coefficient of groundwater in different operating states.What's more,the heat transfer effect of the whole well collection system is measured as well.The research shows an important reference value for improving the energy saving and efficiency of the shallow geothermal energy acquisition system of single-well circulation heat transfer,and improving the service life of the acquisition system.