Middle-deep geothermal well heat exchange system and method using supercritical carbon dioxide

Abstract

The invention relates to the technical field of geothermal energy utilization, and discloses a medium-deep geothermal well heat exchange system and method utilizing supercritical carbon dioxide. The density active regulation and control module comprises a micro pressurization injection unit, a thermoelectric refrigeration fine adjustment unit and a micro pressure relief buffer unit; and the intelligent control module is used for controlling the density active regulation and control module so as to regulate the natural circulation flow of the supercritical carbon dioxide in the closed circulation loop. The intelligent control module is arranged to predict the future trend of the natural circulation driving force of the system according to the real-time operation parameters, an instruction can be sent to the density active regulation and control module in advance based on the measurement result, and feedforward intervention is carried out before the system deviates from the optimal working point; therefore, the inherent time lag and overshoot problems in traditional feedback control are effectively inhibited, and the operation stability and the heat extraction efficiency of the whole heat exchange system are improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of geothermal energy utilization, in particular to a middle-deep geothermal well heat exchange system and method using supercritical carbon dioxide. BACKGROUND

[0002] As a clean, stable and renewable energy, geothermal energy is receiving increasing attention in the global energy structure transformation. In the development and utilization of middle-deep geothermal resources, closed cycle heat exchange systems are widely used due to their advantages of not directly extracting underground fluids, environmental friendliness, system reliability, etc.

[0003] To further improve the system performance, supercritical carbon dioxide (sCO2) is used as the heat transfer medium in the closed cycle system. Compared with traditional working fluids such as water, supercritical carbon dioxide has the characteristics of dramatic changes in near-critical region property parameters, large isobaric thermal expansion coefficient, and low viscosity, etc. These characteristics make it show unique advantages in geothermal systems driven by natural circulation (i.e. thermosyphon). By using supercritical carbon dioxide, the density difference formed by the decrease of density under heat in the well and the increase of density on the ground can drive the working fluid to flow continuously in the closed loop, thereby eliminating the need for mechanical circulation pumps, simplifying the system, and reducing energy consumption and operation and maintenance costs.

[0004] However, the existing supercritical carbon dioxide natural circulation geothermal system still has obvious technical bottlenecks in operation and regulation. First, the operation state of the system mainly depends on the passive balance between the formation temperature and the user-side heat load on the ground. When the user-side heat load changes, the system's response has a significant time lag, the adjustment of circulation flow and heat extraction power is slow, and it is difficult to match the real-time demand of users, and even may cause density wave oscillation and other unstable flow phenomena. SUMMARY

[0005] In view of the deficiencies of the prior art, the present application provides a middle-deep geothermal well heat exchange system and method using supercritical carbon dioxide, which solves the problems of adjustment lag, power regulation difficulty and unstable operation when the existing technology responds to changes in heat load due to reliance on passive balance.

[0006] To achieve the above purpose, the first aspect of the present application provides a middle-deep geothermal well heat exchange system using supercritical carbon dioxide, which comprises: a closed cycle loop for circulating supercritical carbon dioxide, the closed cycle loop comprising a downhole heat exchange subsystem for heat exchange underground and a surface heat exchanger for heat exchange on the ground; a density active regulation module arranged in the pipeline after the surface heat exchanger and before returning to the downhole heat exchange subsystem, for actively adjusting the density of supercritical carbon dioxide in the pipeline; The active density control module includes: A micro-pressurization injection unit is used to increase the density of supercritical carbon dioxide in the pipeline; A thermoelectric cooling fine-tuning unit is used to bidirectionally adjust the density of supercritical carbon dioxide in the pipeline; A micro-pressure relief buffer unit is used to reduce the density of supercritical carbon dioxide in the pipeline; An intelligent control module, connected to the density active regulation module, is used to control the density active regulation module to adjust the flow rate of supercritical carbon dioxide in the closed-loop circuit.

[0007] In one implementation, the intelligent control module includes a wellbore heat transfer model. This model calculates and predicts the density of supercritical carbon dioxide and the natural circulation driving force trend within the closed-loop system based on real-time parameters transmitted by sensors installed in the closed loop, and controls the density active regulation module based on the prediction results. The calculation of the wellbore heat transfer model is based on solving a set of partial differential equations describing the fluid state within the pipe, mainly including energy conservation equations and momentum conservation equations, expressed as follows: Energy conservation equation: ; Momentum conservation equation: ; In the formula, This refers to the mass flow rate of supercritical carbon dioxide. For supercritical carbon dioxide at depth Enthalpy at the location; This represents the vertical depth along the wellbore. This refers to the inner diameter of the heat exchange tube. For depth The overall heat transfer coefficient at the point is a function of the segments (insulated pipe segment, reinforced pipe segment, and counterweight pipe segment) corresponding to the composite heat exchanger pipe; For the strata at depth The original temperature at that location; For supercritical carbon dioxide at depth Temperature at that location; For supercritical carbon dioxide at depth Pressure at the location; For supercritical carbon dioxide at depth Density at that location; It is the acceleration due to gravity; The angle between the pipe and the vertical direction; Darcy friction factor; This represents the cross-sectional area of ​​the pipe.

[0008] By numerically solving the above equations, the density profile along the wellbore depth can be obtained. Furthermore, the total driving head can be calculated. The calculation formula is as follows: ; In the formula, This represents the vertical depth of the geothermal well. and These are the supercritical carbon dioxide downcomer and supercritical carbon dioxide upcomer in the downhole heat exchange subsystem at depths of [insert depth here]. The supercritical carbon dioxide density at a given point is the key factor, and the magnitude of this total driving head directly determines the intensity of the supercritical carbon dioxide natural circulation within the heat exchange system. Therefore, by continuously calculating and analyzing the trend of this value, the intelligent control module can monitor and predict the trend of the natural circulation driving force in real time.

[0009] Furthermore, the downhole heat exchange subsystem includes a heat extraction pipe, which is composed of an insulated pipe section, a reinforced pipe section, and a counterweight pipe section from top to bottom. The wellbore heat transfer model is a distributed parameter model that matches the heat transfer characteristics of each pipe section of the heat extraction pipe.

[0010] In one specific implementation: the micro-pressurization injection unit includes a high-pressure pump, a one-way valve, and a flow regulating valve; the thermoelectric cooling fine-tuning unit includes a Peltier semiconductor cooling array in contact with the outer wall of the pipeline; and the micro-pressure relief buffer unit includes an electronic expansion valve and a buffer accumulator.

[0011] The intelligent control module is also used to perform specific controls under specific operating conditions: when heat exchange starts, it controls the thermoelectric cooling fine-tuning unit to actively cool the pipeline to establish an initial density difference in the closed loop; when the pressure in the closed loop exceeds a safety threshold, it prioritizes controlling the micro-pressure relief buffer unit to extract supercritical carbon dioxide from the pipeline; when heat exchange stops, it controls the density active regulation module to adjust the pressure and temperature in the closed loop to maintain the supercritical carbon dioxide in the closed loop in a supercritical state.

[0012] A second aspect of the present invention provides a method for heat exchange in medium-deep geothermal wells using supercritical carbon dioxide, comprising the following steps: Step 1: Establish a wellbore heat transfer model for the downhole heat exchange subsystem, and calculate and predict the density profile of supercritical carbon dioxide and the natural circulation driving force trend in the closed loop based on the real-time operating parameters of the heat exchange subsystem. Step 2: Based on the prediction results of Step 1, control the density active control module located on the pipeline after the surface heat exchanger and before the return to the downhole heat exchange subsystem. By adjusting the density of supercritical carbon dioxide in the pipeline, the natural circulation driving force is controlled. The adjustment of the density of supercritical carbon dioxide in the pipeline specifically includes one or more of the following operations: injecting supercritical carbon dioxide into the pipeline; cooling or heating the pipeline; and extracting supercritical carbon dioxide from the pipeline.

[0013] This invention provides a heat exchange system and method for medium-deep geothermal wells utilizing supercritical carbon dioxide. It offers the following advantages: 1. By setting up an intelligent control module and its built-in wellbore heat transfer model, this invention can predict the future trend of the system's natural circulation driving force based on real-time operating parameters. Based on the measurement results, the intelligent control module can issue instructions to the density active control module in advance and perform feedforward intervention before the system deviates from the optimal operating point, thereby effectively suppressing the inherent time delay and overshoot problems in traditional feedback control and improving the stability of the entire heat exchange system and the heat extraction efficiency.

[0014] 2. This invention can adjust the density of the circulating working fluid through a micro-pressurization injection unit, a thermoelectric cooling fine-tuning unit, and a micro-pressure relief buffer unit. It can not only greatly enhance the driving force through micro-pressurization injection, but also fine-tune through thermoelectric cooling and rapidly reduce the pressure through micro-pressure relief buffer, thereby flexibly adapting to heat load requirements and improving the system's operational stability. Attached Figure Description

[0015] Figure 1 This is a schematic diagram of the system architecture of the present invention; Figure 2 This is a schematic diagram of the density active control module structure of the present invention; Figure 3 This is a flowchart illustrating the working principle of the intelligent control module of the present invention; Figure 4 This is a schematic diagram of the method flow of the present invention. Detailed Implementation

[0016] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0017] To better understand the present invention, the above content will be described in detail below with reference to specific embodiments.

[0018] Example 1: Please refer to the appendix Figure 1 - Appendix Figure 3 This invention provides a heat exchange system for a medium-deep geothermal well utilizing supercritical carbon dioxide. The system includes: A closed-loop circulation system is used to support the supercritical carbon dioxide cycle. The closed-loop circulation system is divided into two parts: downhole and surface. The downhole heat exchange subsystem is used to absorb heat in the high-temperature formation deep underground, while the surface heat exchanger is used to exchange the absorbed heat to external users or transfer equipment.

[0019] The density active control module is installed in the connecting pipeline between the surface heat exchanger and the downhole heat exchange subsystem, specifically in the cold pipe section where supercritical carbon dioxide returns to the well. The density active control module is used to actively adjust the density of supercritical carbon dioxide in this pipeline, thereby directly controlling the driving force that drives the natural circulation.

[0020] The density active regulation module includes: Micro-pressurization injection unit: used to increase local pressure by injecting a small amount of supercritical carbon dioxide into the pipeline, thereby increasing the density of supercritical carbon dioxide in the pipeline and thus increasing the driving force of the natural circulation of supercritical carbon dioxide.

[0021] Thermoelectric cooling fine-tuning unit: Used to actively cool or heat the pipeline through Peltier semiconductor cooling array to adjust the local temperature, thereby finely regulating the density of supercritical carbon dioxide and thus regulating the driving force of the natural circulation of supercritical carbon dioxide.

[0022] Micro-pressure relief buffer unit: Used to draw supercritical carbon dioxide from the pipeline to the buffer accumulator through an electronic expansion valve, reduce local pressure, thereby reducing the density of supercritical carbon dioxide in the pipeline, and thus reducing the driving force of the natural circulation of supercritical carbon dioxide.

[0023] The system also includes an intelligent control module, which is electrically connected to the density active regulation module. Its main function is to control the three regulation units to work together according to the system's operating status (such as predicted driving force trends) in order to achieve stable and precise regulation of the natural circulation flow of supercritical carbon dioxide in the entire closed loop.

[0024] In a specific embodiment: To achieve predictive control over the physical state of supercritical carbon dioxide within a closed-loop circulation system, this embodiment establishes a wellbore heat transfer model that reflects the actual downhole physics.

[0025] Specifically, this cylinder heat transfer model discretizes the entire downhole heat extraction pipe along its depth direction into multiple control units in a digital environment, and assigns each control unit physical properties matching those of the actual pipe section. For example, the portions of the cylinder heat transfer model corresponding to the insulated pipe section, the reinforced pipe section, and the counterweight pipe section are each assigned their own overall heat transfer coefficient (BTU). , , The model's calculations are based on the material properties and parameters, thus fundamentally ensuring the accuracy of the calculations.

[0026] To drive the heat transfer model calculations, the intelligent control module continuously receives and processes real-time parameters transmitted by sensors located at multiple key nodes in the closed-loop circuit. These sensors include, but are not limited to: temperature and pressure sensors that can be installed at the wellhead to measure the entry and exit of supercritical carbon dioxide into the downhole heat exchange subsystem, and temperature and pressure sensors installed at the inlet and outlet of the surface heat exchanger. By collecting the real-time parameters from these sensors, the inlet temperature entering the downhole heat exchange subsystem can be obtained. Inlet pressure and the outlet temperature of the heat exchange subsystem flowing out of the well. Export pressure This constitutes the dynamic boundary conditions necessary for solving the heat transfer model of the cylinder.

[0027] After obtaining the boundary conditions, the intelligent control module calculates the state inside the wellbore by solving a set of partial differential equations. This set of equations mainly includes the energy conservation equation and the momentum conservation equation. Among them: The energy conservation equation describes the heat exchange process between supercritical carbon dioxide and the surrounding geological formation when it flows through a pipeline. Its expression is: ; The momentum conservation equation describes the pressure change during the flow of supercritical carbon dioxide. This change is determined by both gravity and pipe wall friction, and its expression is: ; In the formula, This refers to the mass flow rate of supercritical carbon dioxide. For supercritical carbon dioxide at depth Enthalpy at the location; This represents the vertical depth along the wellbore. This refers to the inner diameter of the heat exchange tube. For depth The overall heat transfer coefficient at the point is a function of the segments (insulated pipe segment, reinforced pipe segment, and counterweight pipe segment) corresponding to the composite heat exchanger pipe; For the strata at depth The original temperature at that location; For supercritical carbon dioxide at depth Temperature at that location; For supercritical carbon dioxide at depth Pressure at the location; For supercritical carbon dioxide at depth Density at that location; It is the acceleration due to gravity; The angle between the pipe and the vertical direction; Darcy friction factor; This represents the cross-sectional area of ​​the pipe.

[0028] Therefore, by solving the above equations using numerical methods such as finite difference, the intelligent control module can calculate the temperature and pressure profiles at any depth within the entire wellbore at the current moment. Furthermore, by using standard supercritical carbon dioxide physical property data, the intelligent control module calculates the temperature and pressure profiles, thereby obtaining a more accurate density profile of supercritical carbon dioxide, which is crucial for control. The temperature, pressure, and density profiles are composed of the temperature, pressure, and density of supercritical carbon dioxide at different depths.

[0029] By integrating the density profile to obtain the total driving force and combining it with trend analysis, a decision-making basis can be provided for subsequent active control actions.

[0030] Specifically, after obtaining accurate calculation results of the current state, the state is predicted using a wellbore heat transfer model, and predictive control decisions are made based on the prediction results.

[0031] Specifically, the intelligent control module integrates the density profile of supercritical carbon dioxide inside the wellbore obtained through continuous calculation to obtain the driving force of the natural circulation, i.e., the driving head, which characterizes the macroscopic circulation capacity of the entire heat exchange system. The calculation formula is as follows: ; In the formula, This represents the vertical depth of the geothermal well. and These are the supercritical carbon dioxide downcomer and supercritical carbon dioxide upcomer in the downhole heat exchange subsystem at depths of [insert depth here]. The supercritical carbon dioxide density at a given point is the key factor, and the magnitude of this total driving head directly determines the intensity of the supercritical carbon dioxide natural circulation within the heat exchange system. Therefore, by continuously calculating and analyzing the trend of this value, the intelligent control module can monitor and predict the trend of the natural circulation driving force in real time.

[0032] In addition to the total driving force, to ensure the smooth operation of the system, the intelligent control module further evaluates the flow stability of the heat exchange system based on model calculation results. This process involves calculating dimensionless flow stability parameters. This parameter characterizes the response relationship between driving force and flow resistance to changes in mass flow rate, and its expression is: ; in, This represents the total flow pressure drop of the heat exchange system. When this parameter... When the predicted value approaches the critical value, there is a risk of unstable flow such as density wave oscillation in the quantified heat exchange system. For the main drive head mass flow The partial derivatives of the driving force characterize the response characteristics of the driving force as the flow rate changes; Total flowing pressure drop mass flow The partial derivative of the coefficient characterizes the response characteristics of the total system resistance to changes in flow rate.

[0033] Therefore, based on the aforementioned predictions of the total driving head and flow stability parameters, a feedforward-feedback composite control strategy can be implemented. In this strategy, the intelligent control module issues control commands based on the predictions of the aforementioned model, and these predictions constitute the feedforward signal in the control loop. For example, when the model predicts that the total driving head will decrease in the near future due to changes in user-side heat load, the intelligent control module will intervene proactively in advance.

[0034] Meanwhile, the actual system operating parameters measured by the sensors (such as the actual value of the outlet temperature) serve as feedback signals to correct and fine-tune the control process, thereby eliminating the effects of model bias or unmodeled dynamics.

[0035] Furthermore, the control action is triggered in advance by the feedforward signal, rather than after the measured value becomes abnormal. This makes the control action predictable and can actively adjust the heat exchange system before it deviates from the optimal operating point, thereby eliminating potential operational fluctuations and unstable flow. This ensures that the entire heat exchange system always operates in a highly efficient and stable state.

[0036] When the intelligent control module formulates a control decision based on its feedforward-feedback composite control strategy that requires enhancing the driving force of natural circulation, it will trigger corresponding regulatory actions. Specifically, the intelligent control module predicts, through the wellbore heat transfer model, that the user-side heat load will soon increase, or predicts that the current and future driving force trends of natural circulation will be unable to meet the system's set heat extraction power target.

[0037] Once the above conditions are met, the intelligent control module generates and sends a control command to the micro-pressurization injection unit in the density active control module. This command controls the micro-pressurization injection unit to inject supercritical carbon dioxide into the pipeline located after the surface heat exchanger and before the return to the downhole heat exchange subsystem. This injection process is executed collaboratively by the high-pressure pump and flow control valve within the micro-pressurization injection unit. The injection method can be high-frequency, pulsed micro-injection to achieve precise control of the injected mass flow rate.

[0038] The technical principle behind this control action lies in the instantaneous increase in fluid pressure within a local pipe section by injecting a minute amount of supercritical carbon dioxide into the pipeline. Because supercritical carbon dioxide has a very high isothermal compressibility in the near-critical region, even a small pressure increase... This will cause a significant increase in density. This relationship can be quantitatively described by the following formula: ; In the formula: The initial density of supercritical carbon dioxide within this local pipe section; is the isothermal compressibility coefficient of supercritical carbon dioxide at the temperature within this local pipe section; This refers to the local pressure increment caused by the injection action.

[0039] After supercritical carbon dioxide is injected, it circulates in the closed-loop pipeline, thereby increasing the average supercritical carbon dioxide density in the downcomer of the downhole heat exchange subsystem. This, in turn, increases the driving force for the natural circulation of supercritical carbon dioxide. Subsequently, the increased driving force disrupts the original balance between the driving force and the total flow resistance of the system. At this point, the heat exchange system automatically enters a new equilibrium steady-state operating point, where the increased driving force matches the flow resistance at higher flow velocities. This is specifically manifested as an increase in the steady-state mass flow rate of supercritical carbon dioxide in the closed-loop loop, and the relationship can be qualitatively expressed by the following equation: ; In the formula, This represents the total resistance coefficient of the heat exchange system. An increase in mass flow rate means that more mass of supercritical carbon dioxide flows through the downhole heat exchange subsystem per unit time for heat exchange, thereby directly increasing the heat extraction power of the downhole heat exchange subsystem.

[0040] Corresponding to the aforementioned method for enhancing driving force, this invention also provides an adjustment mechanism that weakens the natural circulation driving force, thereby achieving bidirectional control of the heat extraction power of the heat exchange system. Specifically, when the intelligent control module predicts, through the wellbore heat transfer model, that the user-side heat load is about to decrease, or predicts that the current heat extraction power is too high and needs to be reduced, it will execute a weakening adjustment of the driving force.

[0041] Under this operating condition, the intelligent control module sends a command to the thermoelectric cooling fine-tuning unit in the density active regulation module. Specifically, the thermoelectric cooling fine-tuning unit enters the heating mode. This is achieved by the intelligent control module reversing the direct current supplied to the Peltier semiconductor cooling array of this unit. Utilizing the inverse Peltier effect, the function of heat absorption at the cold end and heat release at the hot end is reversed, thereby actively and slightly heating the tightly contacted outer wall of the pipe.

[0042] This heating process transfers heat through the pipe wall to the supercritical carbon dioxide flowing inside the pipe, causing its temperature to rise as it flows through that section. Since the density of supercritical carbon dioxide is highly sensitive to temperature changes, this temperature rise will lead to a corresponding decrease in its density. This physical process can also be quantitatively described: ; In the formula: The initial density of supercritical carbon dioxide in this pipe section; This is the isobaric thermal expansion coefficient of supercritical carbon dioxide within this pipe section; This refers to the local temperature change caused by the thermodynamic adjustment action of the thermoelectric cooling fine-tuning unit.

[0043] The low-density fluid created in the cold pipeline will enter the downcomer of the downcomer subsystem along with the main fluid, thereby reducing the overall average density of supercritical carbon dioxide in the downcomer.

[0044] According to the formula for calculating the total drive head, a decrease in the average density of the downcomer will directly reduce the density difference between it and the riser, thus affecting the total drive head of the system. This reduces the driving force of the natural circulation of supercritical carbon dioxide. Ultimately, the reduced driving force will decrease the flow rate of the downhole heat exchange subsystem, thereby reducing the heat extraction power of the downhole heat exchange subsystem.

[0045] Furthermore, the thermoelectric cooling fine-tuning unit has bidirectional adjustment capability. When the well heat transfer model of the intelligent control module predicts that the natural circulation driving force needs to be increased, the DC current of the Peltier semiconductor cooling array is positive, which enables it to actively cool the outer wall of the closely contacting pipe, thereby quickly removing heat and forcibly reducing the temperature of the supercritical carbon dioxide flowing through the pipe section inside the pipe.

[0046] The physical principle of this process is the opposite of the aforementioned heating process. It involves a localized temperature drop caused by the cooling action. (at this time (The value is negative), and through the isobaric thermal expansion effect, the local density of supercritical carbon dioxide increases at this time ( (Increase). This relationship is also described by the following formula: ; In the formula: The initial density of supercritical carbon dioxide in this pipe section; This is the isobaric thermal expansion coefficient of supercritical carbon dioxide within this pipe section; This refers to the local temperature change caused by the thermodynamic adjustment action of the thermoelectric refrigeration fine-tuning unit. When the density of supercritical carbon dioxide is increased, the local high-density fluid generated in the cold pipeline will also enter and increase the overall average density in the downcomer of the downhole heat exchange subsystem, thereby regulating and enhancing the driving force of the natural circulation of supercritical carbon dioxide.

[0047] Therefore, the method of enhancing driving force through cooling can serve as a supplement or fine-tuning to the effect of the micro-boost injection unit. When a large-scale power increase is required, the intelligent control module can instruct both units (micro-boost injection unit and thermoelectric cooling fine-tuning unit) to work together; while when only a small and precise power increase is needed, this method can be used alone.

[0048] In another embodiment, when the intelligent control module receives a heat exchange start command, it first sends a command to the thermoelectric cooling fine-tuning unit to control it to enter the maximum power cooling mode and actively cool the pipeline, thereby reducing the temperature of the stagnant supercritical carbon dioxide in the local pipe section.

[0049] Because the fluid inside the pipes is static during the initial startup phase, cooling will create localized high-density areas, known as cold plugs. This, in turn, generates an initial hydrostatic head in the vertical direction, known as the initial driving head. Its size can be estimated by the following formula: ; In the formula: This represents the density of supercritical carbon dioxide in the local pipe section after intense cooling. The initial static density of supercritical carbon dioxide in the remaining part of the circuit; It is the acceleration due to gravity; The effective operating length of the thermoelectric cooling fine-tuning unit along the pipeline axis.

[0050] The intelligent control module precisely controls the cooling power and duration of the unit to ensure the generated initial drive head. It is large enough to overcome the starting resistance of the entire closed-loop circuit (including fluid inertia and static friction resistance). Once the driving head exceeds the starting resistance threshold, the stationary working fluid begins to flow, thereby rapidly establishing a supercritical carbon dioxide natural cycle.

[0051] In another embodiment, in addition to using a thermoelectric cooling fine-tuning unit for driving force adjustment, the present invention also provides a method for rapidly reducing driving force based on mass extraction. Specifically, when the wellbore heat transfer model of the intelligent control module predicts a sudden drop in user-side heat load, or when it is necessary to rapidly reduce the system's heat extraction power for other reasons, this step can be executed. Specifically: The intelligent control module then sends a command to the micro-pressure relief buffer unit in the density active regulation module. This micro-pressure relief buffer unit specifically controls the electronic expansion valve within it to open to a specific degree and remain open for a preset time. The opening of the electronic expansion valve causes a portion of supercritical carbon dioxide to be drawn from the pipeline and enters a connected buffer accumulator with a fixed volume.

[0052] At this point, the pressure of supercritical carbon dioxide in the pipeline can be rapidly reduced. Due to the physical properties of supercritical carbon dioxide, the pressure drop will cause a momentary decrease in local density through isothermal compression. This relation can be expressed as: ; In the formula, The initial density of supercritical carbon dioxide in this pipe section; is the isothermal compressibility coefficient of supercritical carbon dioxide under this state; This represents the change in local pressure caused by the pressure relief action.

[0053] The formation of this localized low-density fluid region reduces the density of supercritical carbon dioxide within the pipeline. Subsequently, the low-density supercritical carbon dioxide enters the downcomer of the downcomer subsystem along with the main flow, thereby reducing the average density within the downcomer.

[0054] Ultimately, based on the calculation formula for the total driving head, the density difference between the downcomer and the riser decreases, thereby rapidly weakening the driving force of the natural circulation. This, in turn, reduces the mass flow rate of supercritical carbon dioxide, thus enabling a rapid reduction in heat extraction power. The supercritical carbon dioxide extracted to the buffer accumulator can be reinjected into the loop by the micro-pressurization injection unit when the heat exchange system requires increased driving force, thus achieving zero-loss management of the working fluid.

[0055] Furthermore, the density active control module can prioritize sending a command to the micro-pressure relief buffer unit to perform pressure relief when the real-time pressure value measured by any pressure sensor exceeds a preset safety threshold that is lower than the set pressure of the system's mechanical safety valve. Specifically, the intelligent control module controls the opening degree and duration of the electronic expansion valve based on the difference between the current pressure value and the safety threshold using a closed-loop control algorithm (such as a PID controller). This improves the long-term operational reliability of the heat exchange system.

[0056] Under a specific operating condition, namely planned shutdown, when the intelligent control module receives a shutdown command, it coordinates with the three units within the active control module to proactively manage the pressure and temperature of the entire closed-loop circuit.

[0057] Specifically, the intelligent control module first predicts the final pressure of the system after natural cooling based on the wellbore heat transfer model. If the predicted value is lower than the critical pressure of carbon dioxide (e.g., 7.38 MPa), it instructs the micro-pressurization injection unit to inject supercritical carbon dioxide to ensure that the overall pressure of the heat exchange system remains above the critical pressure even after it has reached equilibrium with the ambient temperature.

[0058] During the cooling process, the intelligent control module continuously monitors the surface pipeline temperature. When the temperature approaches the critical temperature of carbon dioxide (30.98℃), it instructs the thermoelectric cooling fine-tuning unit to enter the low-power heating mode to provide compensatory heating to the pipeline and prevent the local temperature from falling below the critical point.

[0059] If an abnormal increase in pressure occurs during the above adjustment process, the micro-pressure relief buffer unit can be activated for fine adjustment.

[0060] Through this coordinated control of pressure and temperature, all supercritical carbon dioxide in the loop is ultimately maintained in a preset, homogeneous supercritical state, avoiding phase change and two-phase flow problems during shutdown cooling, and creating ideal initial conditions for the next rapid and reliable start-up.

[0061] Example 2: Please refer to the appendix Figure 4 This invention provides a method for heat exchange in medium-deep geothermal wells using supercritical carbon dioxide, the method comprising: Step 1: Establish a wellbore heat transfer model for the downhole heat exchange subsystem, and calculate and predict the density profile of supercritical carbon dioxide and the natural circulation driving force trend in the closed loop based on the real-time operating parameters of the heat exchange subsystem.

[0062] First, the downhole heat extraction pipe is discretized into multiple control units along the depth direction, and each unit is assigned corresponding physical properties, such as different overall heat transfer coefficients corresponding to insulation, reinforcement, or counterweight pipe sections. Then, real-time parameters from sensors deployed at key nodes of the closed-loop circuit are continuously collected and processed, mainly including the inlet and outlet temperatures and pressures at the wellhead, as well as the inlet and outlet temperatures and pressures of the surface heat exchanger. The real-time parameters can be used to construct the dynamic boundary conditions for solving the wellbore heat transfer model.

[0063] After obtaining the boundary conditions, the temperature and pressure profiles at any depth within the entire wellbore are calculated by numerically solving the partial differential equations for energy and momentum conservation. Based on the physical properties of standard supercritical carbon dioxide, the density profile, which is crucial for control, is further calculated from the temperature and pressure profiles.

[0064] Subsequently, by integrating the density profiles along the depth of the downcomer and riser of the downhole heat exchange subsystem, the total driving head of natural circulation, characterizing the system's circulation capacity, can be calculated in real time. By analyzing the time series data of this total driving head and other key parameters (such as flow stability parameters), the intelligent control module can predict their changing trends over a future period.

[0065] Step 2: Feedforward-feedback composite control and active regulation based on prediction results.

[0066] Based on the prediction results from Step One, this step involves the intelligent control module sending instructions to the density active regulation module located in the surface cooling pipeline section. Through a combination of one or more of the following methods, the density of supercritical carbon dioxide within the pipeline is actively adjusted to achieve predictive regulation of the natural circulation driving force. Specifically, this includes: 1. Enhanced regulation of driving force: When the wellbore heat transfer model predicts an impending increase in user-side heat load, or predicts that the future natural circulation drive force will be insufficient to meet the preset heat extraction power target, the intelligent control module will implement enhanced regulation of the drive force. Specifically, this can be achieved through the following methods: By using a high-pressure pump and a flow regulating valve, a small amount of supercritical carbon dioxide is injected into the pipeline in a high-frequency pulse manner to increase the local pressure. The high isothermal compressibility of supercritical carbon dioxide is used to increase the density of the local fluid.

[0067] The system enters cooling mode via a thermoelectric cooling fine-tuning unit. Its Peltier semiconductor cooling array actively cools the outer wall of the pipe and lowers the temperature of the supercritical carbon dioxide flowing through that section. Due to the isobaric thermal expansion effect of supercritical carbon dioxide, the temperature reduction directly leads to an increase in its density. Whether generated through injection or cooling, the high-density fluid flows with the main stream into the downcomer of the downcomer subsystem, thereby increasing the overall average density within the downcomer. According to the formula for calculating the total drive head, the increased density difference between the downcomer and riser effectively enhances the driving force of natural circulation, ultimately resulting in increased system mass flow rate and heat extraction power.

[0068] 2. Modulation of weakening driving force: When the wellbore heat transfer model predicts that the user-side heat load will decrease, or predicts that the current heat extraction power is too high and needs to be reduced: By reversing the direct current supplied to the Peltier array, the outer wall of the pipe is actively and slightly heated. This causes the temperature of the supercritical carbon dioxide flowing through it to rise, resulting in a corresponding decrease in its density due to its physical properties.

[0069] In situations requiring a rapid and significant reduction in driving force (such as a sudden drop in user-side load), a specific opening is achieved via an electronic expansion valve, extracting a portion of supercritical carbon dioxide from the pipeline into a connected buffer accumulator. This instantly reduces the local pressure in the pipeline. Utilizing the high isothermal compressibility of supercritical carbon dioxide, the local fluid density drops sharply. The low-density fluid region created through heating or depressurization flows with the mains into the downcomer of the downcomer subsystem, thereby reducing the overall average density within the downcomer and decreasing the density difference with the riser. Ultimately, this rapidly weakens the natural circulation driving force, achieving the goal of reducing the system's heat extraction power.

[0070] Step 3: Proactive management methods under special working conditions: In addition to adjustments during the stable operation phase, this method also includes: Heat exchange start-up method: When a heat exchange start-up command is received, the fluid in the loop is initially static, and there is no natural circulation driving force. At this time, the pipeline is subjected to strong active cooling to form a local high-density area (i.e., a cold plug) in the static fluid, thereby establishing an initial driving head in the vertical direction that is sufficient to overcome the start-up resistance of the heat exchange system, thus realizing the ice-breaking start-up of natural circulation.

[0071] Furthermore, throughout the entire operation of the heat exchange system, the readings of the pressure sensor can be continuously monitored. If the pressure value at any point exceeds the preset safety threshold (which is lower than the set pressure of the mechanical safety valve), the system will actively release pressure to adjust it back to the safe range, thereby improving the reliability of the heat exchange system.

[0072] Furthermore, during planned shutdowns, to prevent the supercritical carbon dioxide from undergoing a phase change due to temperature and pressure drops during system cooling, the final pressure of the heat exchange system after complete cooling is first predicted. If it is lower than the critical pressure, supercritical carbon dioxide is added to the pipeline. During cooling, if the surface pipeline temperature is detected to be close to the critical temperature, heating is initiated. Through the combined regulation of pressure and temperature, it can be ensured that the carbon dioxide in the entire loop is always maintained in a supercritical state, creating conditions for the next startup.

Claims

1. A heat exchange system for a medium-deep geothermal well utilizing supercritical carbon dioxide, characterized in that, include: A closed-loop circulation system for supercritical carbon dioxide circulation, the closed-loop circulation system comprising a downhole heat exchange subsystem for heat exchange underground and a surface heat exchanger for heat exchange at the surface. The density active control module is installed in the pipeline after the surface heat exchanger and before the return to the downhole heat exchange subsystem, and is used to actively adjust the density of supercritical carbon dioxide in the pipeline. The active density control module includes: Micro-pressurization injection unit; used to increase the density of supercritical carbon dioxide in the pipeline, thereby increasing the driving force of the natural circulation of supercritical carbon dioxide; Thermoelectric cooling fine-tuning unit; used to adjust the density of supercritical carbon dioxide in the pipeline, thereby adjusting the driving force of the natural circulation of supercritical carbon dioxide; Micro-pressure relief buffer unit; used to reduce the density of supercritical carbon dioxide in the pipeline, thereby reducing the driving force of the natural circulation of supercritical carbon dioxide; An intelligent control module, connected to the density active regulation module, is used to control the density active regulation module to adjust the natural circulation flow rate of supercritical carbon dioxide in the closed-loop circuit.

2. The heat exchange system for a medium-deep geothermal well utilizing supercritical carbon dioxide according to claim 1, characterized in that, The intelligent control module includes a wellbore heat transfer model, which is used to calculate the density of supercritical carbon dioxide and the natural circulation driving force trend in the closed circulation loop based on the real-time parameters transmitted by the sensors set in the closed circulation loop, and to control the density active regulation module based on the predicted supercritical carbon dioxide density in the circulation loop.

3. A medium-deep geothermal well heat exchange system utilizing supercritical carbon dioxide according to claim 2, characterized in that, The downhole heat exchanger includes a heat extraction pipe consisting of an insulated pipe section, a reinforced pipe section, and a counterweight pipe section from top to bottom; the wellbore heat transfer model is a distributed parameter model that matches the heat transfer characteristics of each pipe section of the heat extraction pipe.

4. A medium-deep geothermal well heat exchange system utilizing supercritical carbon dioxide according to claim 1, characterized in that, The micro-pressurization injection unit includes a high-pressure pump, a one-way valve, and a flow regulating valve. The intelligent control module controls the micro-pressurization injection unit to inject supercritical carbon dioxide into the pipeline to increase the density of supercritical carbon dioxide in the pipeline.

5. A medium-deep geothermal well heat exchange system utilizing supercritical carbon dioxide according to claim 1, characterized in that, The thermoelectric cooling fine-tuning unit includes a Peltier semiconductor refrigeration array, which is in contact with the outer wall of the pipeline. The intelligent control module controls the thermoelectric cooling fine-tuning unit to actively cool or heat the pipeline in order to adjust the density of supercritical carbon dioxide in the pipeline.

6. A medium-deep geothermal well heat exchange system utilizing supercritical carbon dioxide according to claim 1, characterized in that, The micro-pressure relief buffer unit includes an electronic expansion valve and a buffer accumulator. The intelligent control module controls the micro-pressure relief buffer unit to extract supercritical carbon dioxide from the pipeline to reduce the density of supercritical carbon dioxide in the pipeline.

7. A medium-deep geothermal well heat exchange system utilizing supercritical carbon dioxide according to claim 1, characterized in that, The intelligent control is also used to regulate the pressure and temperature in the closed loop when heat exchange stops, so as to maintain the supercritical carbon dioxide in the closed loop in a supercritical state.

8. A medium-deep geothermal well heat exchange system utilizing supercritical carbon dioxide according to claim 5, characterized in that, The intelligent control module is also used to control the thermoelectric cooling fine-tuning unit to actively cool the pipeline during heat exchange startup, so as to establish an initial density difference in the closed loop.

9. A medium-deep geothermal well heat exchange system utilizing supercritical carbon dioxide according to claim 6, characterized in that, The intelligent control module is also used to: when the pressure in the closed-loop circuit exceeds the safety threshold, prioritize controlling the micro-pressure relief buffer unit to extract supercritical carbon dioxide from the pipeline.

10. A method for heat exchange in medium-deep geothermal wells utilizing supercritical carbon dioxide, based on the heat exchange system for medium-deep geothermal wells utilizing supercritical carbon dioxide as described in any one of claims 1-9, characterized in that, Includes the following steps: Step 1: Establish a wellbore heat transfer model for the downhole heat exchange subsystem, and calculate and predict the density profile of supercritical carbon dioxide and the natural circulation driving force trend in the closed loop based on the real-time operating parameters of the heat exchange subsystem. Step 2: Based on the prediction results of Step 1, control the density active regulation module installed on the pipeline after the surface heat exchanger and before the return to the downhole heat exchange subsystem. By adjusting the density of supercritical carbon dioxide in the pipeline, the natural circulation driving force is regulated. Specifically, adjusting the density of supercritical carbon dioxide in the pipeline includes: Inject supercritical carbon dioxide into the pipeline; The pipeline is cooled or heated; Supercritical carbon dioxide is extracted from the pipeline.

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