| Energy is always the power source for human survival and key factor for social development.With the development of science and the improvement of technology,the conventional energy structure dominated by coal and oil begins to transform to non-fossil energy.The utilization of renewable energy is increasing year by year,which will play an important role in improving the ecological environment,alleviating the energy crisis and promoting the safe use of energy.As a widely distributed,large reserves and environment-friendly energy,geothermal resources have made great progress in exploration,development,utilization,evaluation and protection in recent years.Compared with other renewable energy,geothermal energy has great advantages in stability,adaptability to local conditions and cascade utilization.Shallow geothermal energy is easy to develop and utilize,but its thermal quality is low.Deep geothermal resources have high thermal quality,but its development is difficult and the cost is high.And hydrothermal geothermal resources are facing the problems of recharge difficulty and groundwater pollution.The medium-deep rock and soil mass takes into account the advantages of high and low-quality thermal resources,which can effectively realize the sustainable development of resources and environment by using coaxial borehole heat exchanger with the mode of"extract heat and not water".Based on the above requirements,this paper focuses on clean heating in winter in cold regions.The heat extraction of deep coaxial borehole heat exchanger,as well as thermal reservoir enhancement of perforation and local stimulation is studied.The methods focus on the combination of theoretical analysis,field monitoring,laboratory test and numerical simulation.First of all,the feasibility analysis of deep coaxial borehole heat exchanger is carried out from the four parts of"source,reservoir,caprock and channel",which shows that the geothermal resource endowment of the medium-deep layers in the study area is excellent.The distributed optical fiber temperature sensor,thermal resistance and ultrasonic flowmeter are used to monitor the full depth of coaxial heat exchanger in real time,and the characteristics of ground temperature and the spatiotemporal evolutions of fluid temperature are conducted.The dynamic response process of rock-soil temperature is analyzed considering the non-heating period and intermittent operation mode.The results show that the average geothermal gradient of the study area is 0.0507°C/m,and the terrestrial heat flow can be estimated to be 126.75mw/m~2.The heat recovery rate during the first non-heating period can reach 96.96%.The coefficient of system performance of the coaxial heat exchanger can reach 8.04 and 6.14 in the initial and intermittent stage,respectively.The annular fluid temperature evolves nonlinearly during operation,and linearly increases similarly to the ground temperature characteristics during the stopping period.Secondly,according to the field monitoring data of coaxial borehole heat exchanger,the finite element model is established.The effects of heat extraction intensity,heat exchanger composition,thermal reservoir characteristics and circulation flow rate on fluid temperature evolution and dynamic response mechanism of rock-soil temperature are developed based on heat transfer theory and thermal resistance analysis.The results show that the fluid in the pipe is in turbulent state.The large heat load is not conducive to the long-term operation and heat recovery of the system.The increase and decrease of thermal conductivity for outer pipe and inner pipe can improve the system thermal performance,respectively.The increase and decrease of radius for outer pipe and inner pipe can improve the heat production,respectively.The pressure drop and Reynolds number can be reduced by increasing the inner pipe radius,and the the pump power consumption is declined.The grout with high thermal conductivity can reduce thermal resistance and increase heat production.The heat transfer between rock-soil and fluid is enhanced near the borehole druing the heat extraction process.The formation with high thermal conductivity,compactness and depth is more favorable to improve the system thermal performance.The longer operation time and the shorter stopping time in the intermittent operation mode is more unfavorable to the system thermal performance and the thermal recovery of rock-soil.The heat conduction between rock-soil and the fluid at the shallow formation is reverse.The impact scope expands with the increase of depth at the deep formation.And the impact scope can reach nearly 50m around the bottom borehole after operation of 20 years.Then,the thermal reservoir enhancement of closed-loop coaxial heat exchanger is conducted based on perforation technology,and the experimental studies on fluid flow and heat transfer in rock channel were carried out.Elastic wave velocity and uniaxial compressive strength tests are performed on multi-channel rock samples to analyze the influence mechanism of channel effect on heat transfer and mechanical damage.The results show that the fluid flow are mainly nonlinear and Darcy from channel of1?5 and 7?13,respectively.The increase of channel diameter and number can reduce the pressure drop and improve the average convective heat transfer coefficient.And the increase of Reynolds number and rock temperature can enhance heat transfer.The primary wave velocity,secondary wave velocity,dynamic elasticity modulus and dynamic Poisson's ratio are between 2.1 and 2.8km/s,1.2 and 1.5km/s,6.8 and10.5GPa,0.26 and 0.31,respectively.And these parameters decrease with the increase of the number of channels.The stress-strain profiles of specimens present four stages:compaction,linear elastic,plastic yield and strain softening,and follow the distribution law of stable failure.The increase of the channels number will significantly reduce uniaxial compressive strength.The rock samples develop tensile fractures,showing typical columnar splitting failure.Finally,the local stimulation method is used to further enhance the thermal reservoir on the basis of the channel formed by perforation.In view of the fact that thermal reservoir is equivalent to porous medium,the experimental study on convective heat transfer is conducted.The results show that the fluid flows nonlinearly in porous media,and the seepage resistance is mainly provided by inertial force.The pressure drop increases,decreases and increases with the increase of Reynolds number,temperature and confining pressure.And increasing Reynolds number,particle temperature and confining pressure can enhance heat transfer performance.For discrete fractured media,the rough fractured specimens are prepared based on joint roughness coefficient(JRC)and 3D printing technology.The experiments and numerical models on seepage and heat transfer are carried out considering the effects of proppant.And the effects of roughness on seepage and heat transfer are analyzed.The results show that the increase of confining pressure will significantly reduce flow rate and hydraulic aperture.The aperture is in millimeter level in seepage process,and proppant can double the equivalent hydraulic aperture.The increase of axial roughness will hinder the fluid flow,and the bulge formed by the radial roughness is more likely to form the dominant seepage path.The conductivity of fracture decreases with the increase of temperature.The proppant makes the stress on the rough fracture surface uneven easy to be damaged.The high flow rate makes the rock temperature decrease rapidly,and the increase of temperature and flow rate can improve the heat extraction rate.Roughness and directionality have great effects on heat transfer performance.The fluctuation of the axial roughness makes the fluid turbulent and enhances the heat transfer.The dominant path formed by radial roughness can reduce the heat transfer performance.The temperature in the fracture surface increases along the axial distance,and the shape of the cold front on the wall is zigzag.The surface characteristics formed by roughness will affect the velocity distribution and further affect the local heat transfer performance.The results in this study can improve theoretical direction and technical support for utilization of deep coaxial borehole heat exchanger and thermal reservoir enhancement. |
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