Co-generation system based on CO2 working medium and implementation method of co-generation system

Abstract

The invention discloses a cogeneration system based on a CO2 working medium and an implementation method of the cogeneration system, and belongs to the field of geothermal energy development. The system comprises a deep well heat exchange module, a heat exchange heat supply unit, a transcritical CO2 low-temperature power generation module, a gas-liquid separator, a ground cooling part, a variable coupling power module, a micro-power circulation module and a control system. Heat of a deep geothermal well is absorbed by liquid-phase CO2 through the well bottom and then is fed to the well, heat exchange is conducted through the heat exchange heat supply unit, the heat enters the transcritical CO2 low-temperature power generation module and is converted into electric energy for power generation, the micro-power circulation module is pushed through the variable coupling power module, and the liquid-phase CO2 is pressurized and pumped into the well; according to the invention, the whole heat and power cogeneration system can stably operate for a long time under the unattended or few-person attended condition; the method is not only suitable for a single well mode, but also can be expanded to a twin-well or multi-well network structure. The invention is applied to medium and low temperature terrestrial heat or waste heat occasions, waste oil and gas well transformation power generation, intelligent microgrids, comprehensive energy bases and the like.

Description

Technical Field

[0001] This invention relates to geothermal energy development technology, specifically to a combined heat and power system based on CO2 working fluid and its implementation method. Background Technology

[0002] As geothermal resource development extends into deeper strata, traditional deep-well thermal extraction methods using water as the working medium have revealed numerous problems, including low efficiency, severe corrosion, high risk of blockage, and rapid heat exchange decay. Supercritical CO2, due to its characteristics such as large density variation with temperature and pressure, strong self-circulation driving force, low viscosity, high heat transfer coefficient, and significant thermosiphon effect, has become an important alternative working medium for medium-deep geothermal development. However, the technology for low-temperature CO2 power generation using geothermal energy is currently lacking. Most geothermal utilization is now used for CO2 geological storage and high-temperature CO2 power generation, and most research focuses on isolated aspects such as wellbore heat exchange simulation, basic pump circulation experiments, and transcritical turbine unit verification, failing to form a complete engineering system structure of "deep well heat extraction—heat supply—low-temperature power generation—intelligent control." Existing technologies generally suffer from the following shortcomings: First, the actual heat exchange process in deep wells is difficult to monitor in real time. Traditional methods can only install temperature and pressure sensors at the wellhead or in a limited well section, failing to capture temperature changes along the horizontal and ascending sections. This makes it impossible to identify problems such as localized blockages inside the wellbore, instability in two-phase flow, scaling, or thermal decay of the rock mass. Consequently, the system lacks data support during long-term operation, hindering intelligent operation optimization. Secondly, the coupling between the geothermal heating end and the cryogenic power generation end is difficult, making it impossible to dynamically adjust the circulation volume, turbine intake enthalpy, or pump power distribution based on external heating load or power generation demand. This results in a significant waste of usable thermal energy due to poor system matching. Thirdly, although CO2 working fluid has self-circulation potential, in actual deep well circuit operation, it still relies on micro-power pumps to establish a minimum circulation pressure difference. Existing pumps are mostly motor-driven, with no power connection to the turbine, leading to long-term high-power operation of the pumps, which is detrimental to system energy efficiency. Fourth, due to the sensitivity of transcritical CO2 turbine units to high pressure ratio and inlet enthalpy, as well as the highly nonlinear characteristics of CO2 near-critical region, the mechanical work output of the turbine fluctuates greatly under different geothermal conditions, often resulting in a mismatch with pump demand. Insufficient pump inlet pressure can also lead to cavitation and even damage to the equipment. Fifth, existing systems mostly operate with fixed setpoints, lacking real-time predictive capabilities. They cannot adjust pressure, density, and circulation volume in advance based on changes in the wellbore temperature field, nor can they achieve rapid bypass switching and safe disconnection of the unit in abnormal situations. These shortcomings indicate that current technology has not yet formed a complete, engineering-ready combined heat and power system, lacking an integrated system solution.

[0003] Therefore, there is an urgent need for a full-chain CO2 geothermal heating-low temperature power generation system that can simultaneously solve problems such as difficulties in downhole monitoring, slow response of the circulation system, turbine-pump power contradiction, incoordination between heating and power generation, high pump power consumption and poor system safety. Summary of the Invention

[0004] To address the problems existing in the above-mentioned technologies, this invention proposes a deep-well CO2 combined heat and power system for engineering implementation. By adding online inversion based on fiber optic temperature field, a variable coupling power module, intelligent predictive control, and a two-mode switching mechanism, it achieves real-time perception of deep wells, adaptive power coupling, efficient energy utilization, and intelligent operation throughout the entire process.

[0005] One object of the present invention is to provide a combined heat and power system based on CO2 working fluid.

[0006] The CO2-based combined heat and power system of the present invention includes: a deep well heat exchange module, a heat exchange heating unit, a transcritical CO2 cryogenic power generation module, a gas-liquid separator, a surface cooling section, a variable coupling power module, a micro-power circulation module, and a control system; wherein, the deep well heat exchange module is located underground; the outlet of the deep well heat exchange module is connected to the heat exchange heating unit via a pipeline; the heat exchange heating unit is connected to the transcritical CO2 cryogenic power generation module via a pipeline; the transcritical CO2 cryogenic power generation module is connected to the gas-liquid separator via a pipeline; the gas-liquid separator is connected to the surface cooling section via a pipeline; the surface cooling section is connected to the micro-power circulation module via a pipeline; the micro-power circulation module is connected to the inlet of the deep well heat exchange module via a pipeline; the transcritical CO2 cryogenic power generation module and the micro-power circulation module are coupled through the variable coupling power module; the variable coupling power module is connected to the control system.

[0007] The deep well heat exchange module uses a U-shaped wellbore to deliver the heat and pressure of high-temperature, high-pressure CO2 from downhole to the surface. It consists of a downcomer (injection pipe), a riser (production pipe), and an intermediate horizontal pipe, with a well depth ≥2500 meters and an internal spiral turbulence enhancement structure. The downhole CO2 is in a liquid state, with an initial relative temperature of 0.30~0.80 and an initial relative pressure of 0.45~0.77. After heat absorption at the bottom of the well, the CO2 relative temperature rises to 2.5~3.5, and the relative pressure increases to 1.3~2.4. The well depth is determined based on geological conditions, and enhanced heat exchange fins are installed downhole.

[0008] The heat exchange heating unit employs a heat exchanger, specifically a heat pipe-type microchannel enhanced heat transfer structure filled with a low-temperature, high-thermal-conductivity porous medium. The working medium can be water, ammonia, or propane, or alternatively, a plate heat exchanger or a shell-and-tube heat exchanger. It further includes a flow stabilizer, positioned between the outlet of the deep well heat exchange module and the inlet of the heat exchanger, to stabilize the outlet pressure. The outlet of the deep well heat exchange module is connected to the CO2 inlet of the heat exchange heating unit, and the CO2 outlet of the heat exchange heating unit is connected to the transcritical CO2 cryogenic power generation module. The working fluid inlet and outlet of the heat exchange heating unit are connected to the user end. In the heat exchange heating unit, high-temperature, high-pressure CO2 from downhole exchanges heat with the working medium to provide heating to the user. Single-stage or multi-stage heat exchangers are used for heat recovery, utilizing supercritical CO2 heat in stages to provide heating above 65°C.

[0009] The transcritical CO2 cryogenic power generation module employs a centripetal turbine generator. High-temperature, high-pressure CO2 from underground, after heat exchange in the heat exchange unit, remains in a supercritical state before entering the centripetal turbine generator, where it expands and generates electricity, reaching liquid phase CO2. This forms a complete closed-loop cycle, providing power to the centripetal turbine generator. The turbine inlet CO2 operating pressure is ≥8MPa, and the isentropic efficiency is ≥85%. The CO2 flow state within the centripetal turbine generator is a two-phase transition from supercritical to liquid.

[0010] The ground cooling system utilizes a compact condenser and a super cooler.

[0011] The micro-power circulation module uses a micro-power circulation pump. It also includes an external motor, which is connected to the micro-power circulation module. When the electrical energy provided by the transcritical CO2 cryogenic power generation module is insufficient to drive the micro-power circulation module, the external motor provides drive to the micro-power circulation module, slowly injecting liquid CO2 into the U-shaped wellbore inside the deep geothermal well through the wellhead casing.

[0012] The variable coupling power module includes a clutch, a turbine drive rod, and a micro-power drive rod. The transcritical CO2 cryogenic power generation module is connected to the turbine drive rod, and the micro-power circulation module is connected to the micro-power drive rod. The clutch couples the turbine drive rod and the micro-power drive rod. The clutch is connected to the control system. The clutch can be an electromagnetic or hydraulic clutch. The variable coupling power module controls the coupling between the transcritical CO2 cryogenic power generation module and the micro-power circulation module in three coupling modes: full coupling mode, partial slip mode, and complete disengagement mode. Under steady-state conditions, the transcritical CO2 cryogenic power generation module uses full coupling mode to directly drive the micro-power circulation module to compensate for pump power. During heating priority or start-up phases, partial slip mode is used to reduce pump speed through partial slip. In abnormal conditions, complete disengagement mode is used to avoid overspeed, cavitation, or shaft torque impact by disengaging the clutch.

[0013] Distributed optical fiber temperature sensing units are laid along the entire length of the wellbore outer wall of the deep well heat exchange module to continuously acquire the temperature distribution along the wellbore. The distributed optical fiber is connected to the optical fiber demodulation device located at the wellhead on the ground. The ground optical fiber demodulation device is connected to the control system, and the acquired temperature field data is input to the control system.

[0014] The control system includes a fiber optic temperature measurement and parameter inversion module and an intelligent prediction and mode switching module. The fiber optic temperature measurement and parameter inversion module acquires fiber optic temperature field data and, based on the thermodynamic control equations, inverts the pressure distribution, density changes, and equivalent heat transfer coefficient of the wellbore to obtain the fiber optic inversion results. The intelligent prediction and mode switching module uses the fiber optic inversion results in real time to correct the outlet temperature and pressure prediction parameters, achieving full-process perception of the wellbore's dynamic changes and controlling the switching of coupled modes. The outlet temperature and pressure prediction parameters include the fiber optic temperature field, real-time surface pressure, temperature, and flow rate, and turbine rotational speed.

[0015] Further, it includes a safety relief structure and an automatic bypass system. The safety relief structure uses a pressure relief valve, installed on the pipeline connecting the transcritical CO2 cryogenic power generation module and the gas-liquid separator. The automatic bypass system includes a generator bypass and a cooler bypass. The generator bypass is located before the transcritical CO2 cryogenic power generation module and after the gas-liquid separator, used to bypass the transcritical CO2 cryogenic power generation module. The cooler bypass is located after the gas-liquid separator and before the micro-power circulation module, used to bypass the ground cooling section. Valves are installed at the inlets of the generator bypass, the transcritical CO2 cryogenic power generation module, the cooler bypass, and the ground cooling section, respectively. The control system switches between these modes by controlling the safety relief structure and the automatic bypass system. When the system experiences abnormal pressure, turbine stall, pump cavitation, or abnormal fluctuations in the fiber optic temperature field, the control system immediately switches to bypass mode and adjusts the clutch state to ensure safe and stable system operation. For example, when an external cold source is suddenly lost, the heat exchange capacity of the surface cooling section drops sharply, or two-phase fluctuations occur at the turbine outlet, the fiber optic temperature field and pressure change rate can be quickly detected. The control system can execute the bypass valve action within milliseconds to seconds, allowing CO2 to bypass the transcritical CO2 cryogenic power generation module and directly enter the surface cooling section or bypass the surface cooling section for reinjection into the wellbore, avoiding turbine damage due to unstable aerodynamic loads. In the event of structural anomalies in the wellbore, such as a sudden temperature drop or rise exceeding a safety threshold shown in the fiber optic temperature field, the system can automatically reduce the circulation rate or switch to a shutdown cooling mode by controlling the safety relief structure to protect the wellbore structure and formation stability. Combined with a compact heat exchanger, surface cooling section, and geothermal wellbore insulation measures, this invention forms a deep-well CO2 energy utilization system with real-time monitoring, parameter inversion, power matching, coupling mode switching, and intelligent full-process control capabilities. Compared to traditional solutions, it can significantly increase exploitable heat, reduce circulation power consumption, improve power generation efficiency, and enhance system reliability.

[0016] This invention is applicable not only to single-well models but can also be extended to dual-well or multi-well network structures. In multi-well models, the fiber optic temperature fields of multiple well sections can be merged and processed through a unified control system to form regional dynamic monitoring of geothermal reservoirs; by adjusting the circulation rate and surface heat exchange of each well, coordinated exploitation of geothermal resources can be achieved. In practical engineering applications, the depth, temperature field, and permeability of geothermal wells vary significantly across regions. The fiber optic temperature measurement and parameter inversion module and the intelligent prediction and mode switching module of this invention can adaptively adjust the outlet temperature and pressure prediction parameters based on real-time fiber optic temperature field data, enabling the system to automatically match the optimal operating strategy under different geological conditions.

[0017] The control system of this invention can interface with district heating networks, oilfield gathering and transmission pipelines, or industrial heat load systems via communication protocols to acquire real-time demand changes and automatically schedule heat distribution. Through CO2 self-circulation capability, variable coupling power modules, and intelligent control methods, this invention enables the entire combined heat and power system to operate stably for extended periods under unattended or minimally staffed conditions.

[0018] Another objective of this invention is to propose a method for implementing a combined heat and power system based on CO2 working fluid.

[0019] The method for implementing a combined heat and power system based on CO2 working fluid according to the present invention includes the following steps:

[0020] 1) CO2 is injected into a medium-deep geothermal well through a deep well heat exchange module. The deep well heat exchange module exchanges heat with the rock mass. The liquid CO2 relies on its own density difference to achieve downhole pressurization and temperature increase. After absorbing heat at the bottom of the well, it is brought up to the well as high-temperature supercritical CO2.

[0021] 2) After being brought to the well, the high-temperature supercritical CO2 transfers heat to the working medium of the fluid that needs to be heated through the heat exchange heating unit. The supercritical CO2 remains in a supercritical state when it is cooled down.

[0022] 3) Cooled supercritical CO2 enters the transcritical CO2 cryogenic power generation module, providing power for expansion and work. The CO2 flow state in the transcritical CO2 cryogenic power generation module is a two-phase transformation from supercritical to liquid, which is converted into electrical energy. The transcritical CO2 cryogenic power generation module generates electricity.

[0023] 4) Two-phase CO2 enters the gas-liquid separator, where gas and liquid are separated, and gaseous CO2 is stored in a gas storage tank from the gas outlet located at the top.

[0024] 5) Liquid CO2 is switched via valves to either enter the surface cooling section or directly enter the micro-power circulation module; in the surface cooling section, the liquid CO2 is further cooled to the operating temperature required for downhole operation;

[0025] 6) The transcritical CO2 cryogenic power generation module uses a variable coupling power module to drive a micro-power circulation module to pressurize liquid CO2 and inject it into the well.

[0026] Advantages of this invention:

[0027] This invention employs an environmentally friendly working fluid suitable for medium- and low-temperature geothermal energy: CO2 has a low critical point (31.1℃ / 7.38MPa), making it suitable for medium- and low-temperature geothermal applications; it features a fully enclosed CO2 cycle with no direct emissions, resulting in 100% water savings compared to water-based working fluids; it utilizes a high-efficiency heat exchanger to optimize heat transfer, increasing the heat transfer coefficient to 8000W / (m²·K); it achieves a breakthrough in heating-low-temperature power generation efficiency: the power generation cycle efficiency reaches over 50% (under geothermal temperature of 150℃), with a comprehensive energy efficiency of 78% (compared to ≤55% for traditional distributed power systems); and it boasts excellent economic performance: due to the high heat density of CO2, despite the high pressure, the unit investment is 17% lower than that of traditional ORC cycles, with a dynamic payback period of 4.2 years (IRR 21.3%), compared to 6.8 years for traditional systems. This invention can be applied to medium- and low-temperature geothermal or waste heat applications, abandoned oil and gas well conversion for power generation, smart microgrids, and integrated energy bases. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of an embodiment of the CO2-based cogeneration system of the present invention;

[0029] Figure 2 This is a flowchart of an embodiment of the cogeneration system based on CO2 working fluid of the present invention. Detailed Implementation

[0030] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0031] like Figure 1As shown, the CO2-based combined heat and power system of this embodiment includes: a deep well heat exchange module, a flow stabilizer, a heat exchange heating unit, a transcritical CO2 cryogenic power generation module, a gas-liquid separator, a supercooler, a variable coupling power module, a micro-power circulation module, and a control system. The deep well heat exchange module is located underground using a U-shaped wellbore. The outlet of the deep well heat exchange module is connected to the heat exchange heating unit via a pipeline through the flow stabilizer. The heat exchange heating unit uses a heat exchanger. The outlet of the deep well heat exchange module is connected to the CO2 inlet of the heat exchanger via a pipeline, and the CO2 outlet of the heat exchanger is connected to the transcritical CO2 cryogenic power generation module. The working fluid inlet and outlet of the heat exchanger are connected to the user end. The transcritical CO2 cryogenic power generation module uses a centripetal turbine generator and is connected to the inlet of the gas-liquid separator via a pipeline. The liquid outlet at the bottom of the gas-liquid separator is connected to the supercooler via a pipeline, while the gas outlet at the top of the gas-liquid separator is connected to the gas storage tank via a pipeline. The supercooler is connected to the micro-power circulation module via a pipeline, and the micro-power circulation module uses a micro-power circulation pump. The micro-power circulation pump is connected to the inlet of the deep well heat exchange module via a pipeline. The transcritical CO2 cryogenic power generation module is coupled to the micro-power circulation module via a variable coupling power module. The micro-power circulation module is also connected to an external motor. When the electrical energy provided by the transcritical CO2 cryogenic power generation module is insufficient to drive the micro-power circulation module, the external motor provides drive to the micro-power circulation module, slowly injecting liquid CO2 into the U-shaped wellbore of the deep geothermal well through the wellhead casing. The variable coupling power module is electrically connected to the control system.

[0032] The U-shaped wellbore can reach a depth of 2800m, with a horizontal section length of 1000m. The temperature gradient of the wellbore rock mass can range from 3.6 to 4.2℃ / (100m). The wellbore is constructed with a double-layer structure or high-temperature and corrosion-resistant materials, and distributed fiber optic temperature sensing units are deployed along the pipe axis on the outer surface of the casing, ensuring good mechanical adhesion and thermal response of the optical fibers throughout the wellbore. CO2 gradually heats up in the descending section due to gravity and temperature changes at the bottom of the well. It undergoes sufficient heat exchange with the rock mass in the horizontal section, and its temperature further increases upon entering the ascending section, reaching 70-90℃ and a pressure of 14-20MPa at the wellhead, forming a natural upward backflow. This self-circulation capability is guaranteed by the significant temperature variation of CO2 density, allowing the dynamic pressure difference from the downhole to the wellhead to maintain basic circulation without the need for a large pump.

[0033] A fiber optic demodulation device is installed at the wellhead to output the temperature changes throughout the well in real time to the surface in the form of T(z,t), where T is temperature, z is altitude, and t is time. The control system uses the fiber optic temperature field as a boundary condition. By constructing a one-dimensional or quasi-one-dimensional set of mass, momentum, and energy conservation equations, and combining them with a real property table of CO2 or an improved equation of state, it performs online inversion of the friction pressure p(z), density ρ(z), and the equivalent heat transfer coefficient h(z) of the wellbore. The inversion employs recursive least squares, extended Kalman filtering, or a data-driven nonlinear fitting strategy to ensure that the deviation between the calculated temperature T_calc(z,t) and the fiber-meas(z,t) converges within a controllable range, thereby achieving real-time identification of the actual heat transfer process inside the wellbore. The fiber optic inversion results are not only used for wellbore condition monitoring, but also for correcting the outlet temperature and pressure prediction parameters on the surface side, enabling the control system to predict the temperature, pressure and flow rate changes in the next 10 to 60 minutes in advance, and achieve feedforward control.

[0034] The high-temperature, high-pressure CO2 returning from the wellhead first enters the heat exchange heating unit, which includes a multi-stage compact heat exchanger to provide high-quality heat energy to urban heating water, industrial heat users, or oilfield crude oil gathering and transportation pipelines. The CO2, still possessing a high enthalpy after heat exchange, enters the transcritical CO2 cryogenic generator for expansion and power generation. The turbine inlet conditions of the generator are typically 40-60°C at a temperature and 14-20 MPa at a pressure. Because CO2 exhibits strong property changes in the transcritical region, this embodiment includes a flow stabilizer before the turbine inlet to maintain the turbine inlet pressure fluctuation rate within an acceptable range, thereby avoiding nonlinear loads on the blades and efficiency reduction caused by pressure pulsation. The turbine outlet typically presents a two-phase or low-quality vapor state; therefore, a high-pressure gas-liquid separator is installed at the outlet to allow the liquid phase CO2 to enter the pump suction inlet of the micro-power circulating pump, preventing pump cavitation; the gas phase can be cooled back to a liquid state before recirculation.

[0035] To achieve self-compensation and dynamic matching of pump power, this invention connects the transcritical CO2 cryogenic power generation module and the micro-power circulation pump through a variable coupling power module, which includes a clutch, a turbine drive rod, and a micro-power drive rod. Under normal heating and power generation conditions, the mechanical work generated by the expansion of supercritical CO2 in the generator is directly transmitted to the pump through the clutch, thereby significantly reducing the external motor load on the pump. When the heating demand increases or the external load changes suddenly, the clutch creates partial slip by changing the coupling degree, allowing the speed of the micro-power circulation pump to be independent of the generator output, thus meeting the circulation volume requirements. During system startup, when the pump inlet conditions are insufficient, or when the generator needs to be isolated, the clutch can be completely disengaged, allowing the micro-power circulation pump to be driven by an external motor to ensure stable circulation.

[0036] The control system of this invention utilizes fiber optic inversion results, real-time ground pressure, temperature and flow rate, turbine speed and pump inlet pressure, and, supported by an outlet temperature and pressure prediction model, performs coordinated control on clutch coupling degree, pump frequency, valve opening degree and turbine guide vane angle to achieve turbine inlet enthalpy control, circulation optimization and switching between heating priority / power generation priority modes.

[0037] Further safety relief structures and automatic bypass systems are incorporated. The multi-stage safety relief structure includes a fast-response electronic pressure relief valve; the automatic bypass system includes generator bypass and cooler bypass. In the event of turbine stall, excessively rapid pressure rise, or abnormal disturbances in the fiber optic temperature field, the control system rapidly activates the bypass loop, allowing CO2 to bypass the turbine and return directly to the cooling or downhole module, ensuring the system maintains thermodynamic and structural safety under all operating conditions.

[0038] The ground cooling section uses a super cooler that automatically adjusts its cooling capacity according to ambient temperature and operating conditions, maintaining CO2 at a suitable inlet state before reinjecting it into the wellbore.

[0039] The control system includes a fiber optic temperature measurement and parameter inversion module and an intelligent prediction and mode switching module. By fusing multi-source data such as fiber optic temperature field, real-time surface pressure, temperature, flow rate, and turbine rotation speed, a prediction model for the outlet temperature and pressure of a CO2 deep well thermoelectric cycle is constructed. This model is used to calculate the predicted outlet temperature and pressure parameters, including the fiber optic temperature field, real-time surface pressure, temperature, flow rate, and turbine rotation speed. The prediction model comprehensively considers the pressure drop along the wellbore, the drastic changes in CO2 properties in the transcritical region, the gradual decay of formation temperature over time, the temperature fluctuations of the working fluid on both sides of the heat exchanger, and the changes in the supercooler's operating conditions. This allows the model to predict the trends of temperature, pressure, flow rate, and turbine inlet enthalpy within the system over the next 10 to 60 minutes. The prediction results, used as feedforwards to the control system, are input together with the fiber optic inversion results, enabling the system to respond quickly and stably even under drastic load changes.

[0040] The variable coupling power module control operates in three coupling modes between the transcritical CO2 cryogenic power generation module and the micro-power circulation module: full coupling mode, partial slip mode, and complete separation mode. Under steady-state conditions, the full coupling mode is used, where the clutch fully engages the turbine drive rod and the micro-power drive rod, resulting in a rigid connection and high-efficiency joint output. During the heating priority or start-up phases, the partial slip mode is used, where the coupling degree between the turbine drive rod and the micro-power drive rod is between the full coupling and complete separation modes. In abnormal conditions, the complete separation mode is used, where the clutch completely disconnects the turbine drive rod and the micro-power drive rod, decoupling the two modules and enabling independent and safe operation. The generator does not drive the micro-power circulation; instead, it is used entirely for power generation. In this embodiment, the outlet temperature and pressure prediction model and the control system work together to control the switching process between the heating priority and power generation priority operating modes. When external heating demand is high, the control system reduces the turbine inlet mass flow rate or adjusts the guide vane angle to allow more heat to be utilized in the heat exchanger. The clutch coupling is kept low to ensure the micro-power circulation pump maintains sufficient circulation volume, thus achieving stable heating. When power generation demand increases or heating load decreases, the control system increases the clutch coupling, enabling the generator to transfer more mechanical energy to the micro-power circulation pump, reducing external motor power. Simultaneously, it increases the turbine inlet enthalpy, allowing the generator to achieve higher expansion work. When heating and power generation demands conflict, the control system uses fiber optic temperature measurement and parameter inversion modules to identify the maximum sustainable heat power provided by the wellbore, automatically selecting the operating mode and adjusting the circulation volume to achieve optimal heat distribution between heating and power generation.

[0041] In a further embodiment, the automatic bypass system and safety relief structure of the present invention can provide full-process protection under extremely rapidly changing operating conditions. For example, when an external cold source is suddenly lost, the heat exchange capacity of the supercooler drops sharply, or two-phase fluctuations occur at the turbine outlet, the fiber optic temperature field and pressure change rate can be detected rapidly. The control system can execute the bypass valve action within milliseconds to seconds, allowing CO2 to bypass the generator and directly enter the supercooler or reinjection wellbore, preventing damage to the generator due to unstable aerodynamic loads. When structural anomalies occur in the wellbore, such as a sudden drop or rise in temperature exceeding a safety threshold as shown by the fiber optic temperature field, the system can automatically reduce the circulation rate or switch to a shutdown cooling mode to protect the wellbore structure and formation stability.

[0042] The system described in this invention is not only applicable to single-well mode but can also be extended to dual-well or multi-well network structures. In multi-well mode, the fiber optic temperature fields of multiple well sections can be merged and processed through a unified prediction-inversion-optimization control platform to form regional dynamic monitoring of geothermal reservoirs; by adjusting the circulation volume and surface heat exchange of each well, coordinated exploitation of geothermal resources can be achieved. In practical engineering applications, the depth, temperature field, and permeability of geothermal wells vary greatly from place to place. The parameter inversion module and prediction control module of this invention can adaptively adjust the model coefficients based on real-time fiber optic data, enabling the system to automatically match the optimal operating strategy under different geological conditions.

[0043] Under pilot-scale engineering conditions, the system configuration of this invention includes a high-pressure CO2 shell-and-tube heat exchanger, a plate heat exchanger, a jet-type supercooler, a compact transcritical generator, a micro-power circulating pump, an electromagnetic or hydraulic clutch, and a high-speed data acquisition device for wellbore fiber optic data demodulation. During the pilot-scale phase, a full-size heat exchanger thermal testing rig, a low-pressure section cooling performance testing platform, a platform for the trial production and debugging of core components of the transcritical CO2 turbine, and an experimental platform for simulating the dynamics of geothermal well reservoirs with an adjustable heat source are set up. This invention can utilize these pilot-scale platforms to verify the turbine inlet enthalpy control strategy, the variable coupling pump drive strategy, the accuracy of fiber optic temperature field inversion, the model prediction capability, and the overall stability of the system under multiple operating conditions, and further provide a data foundation for engineering projects.

[0044] In engineering deployment, the system of this invention is installed in the geothermal well area. The well casing adopts a corrosion-resistant and high-pressure-resistant structure and is equipped with optical fibers. The surface component includes a heat exchange station, generator room, cooling device, energy storage or peak-shaving device, electrical control and monitoring platform, etc. The control system of this invention can interface with the district heating network, oilfield gathering and transmission pipelines, or industrial heat load system through communication protocols to obtain real-time demand changes and automatically schedule heat distribution. Through CO2 self-circulation capability, variable coupling power module, and intelligent control method, this invention enables the entire combined heat and power system to operate stably for a long time under unattended or minimally-attended conditions.

[0045] The implementation method of the combined heat and power system based on CO2 working fluid in this embodiment is as follows: Figure 2 As shown, it includes the following steps:

[0046] 1) CO2 is injected into a medium-deep geothermal well through a deep well heat exchange module. The deep well heat exchange module exchanges heat with the rock mass. The liquid CO2 relies on its own density difference to achieve downhole pressurization and temperature increase. After absorbing heat at the bottom of the well, it becomes high-temperature supercritical CO2 at the surface, with the relative temperature rising to 2.5~3.5 and the relative pressure rising to 1.3~2.4.

[0047] 2) After being brought to the well, the high-temperature supercritical CO2 transfers heat to the working medium of the fluid that needs to be heated through the heat exchange heating unit. The supercritical CO2 remains in a supercritical state when it is cooled down.

[0048] 3) Cooled supercritical CO2 enters the generator, providing power for expansion and work. The CO2 flow state inside the generator is a two-phase transformation from supercritical to liquid, which is converted into electrical energy. The generator generates electricity and transmits it to the user. The relative pressure at the outlet drops to about 0.5.

[0049] 4) Two-phase CO2 enters the gas-liquid separator, where gas and liquid are separated, and gaseous CO2 is stored in a gas storage tank from the gas outlet located at the top.

[0050] 5) Liquid CO2 is switched via valves to either enter the surface cooling section or directly enter the micro-power circulation pump; in the surface cooling section, the liquid CO2 is further cooled to the operating temperature required for downhole operation;

[0051] 6) The mechanical work generated by the expansion of supercritical CO2 in the generator is directly transmitted to the micro-power circulation pump through the clutch to pressurize the liquid CO2 and pump it into the well.

[0052] Finally, it should be noted that the purpose of disclosing the embodiments is to help further understand the present invention. However, those skilled in the art will understand that various substitutions and modifications are possible without departing from the spirit and scope of the present invention and the appended claims. Therefore, the present invention should not be limited to the content disclosed in the embodiments, and the scope of protection of the present invention is defined by the claims.

Claims

1. A combined heat and power system based on CO2 working fluid, characterized in that, The combined heat and power (CHP) system includes: a deep well heat exchange module, a heat exchange heating unit, a transcritical CO2 cryogenic power generation module, a gas-liquid separator, a surface cooling section, a variable coupling power module, a micro-power circulation module, and a control system. The deep well heat exchange module is located underground. The outlet of the deep well heat exchange module is connected to the heat exchange heating unit via a pipeline. The heat exchange heating unit is connected to the transcritical CO2 cryogenic power generation module via a pipeline. The transcritical CO2 cryogenic power generation module is connected to the gas-liquid separator via a pipeline. The gas-liquid separator is connected to the surface cooling section via a pipeline. The surface cooling section is connected to the micro-power circulation module via a pipeline. The micro-power circulation module is connected to the inlet of the deep well heat exchange module via a pipeline. The transcritical CO2 cryogenic power generation module and the micro-power circulation module are coupled via the variable coupling power module. The variable coupling power module is connected to the control system.

2. The combined heat and power system according to claim 1, characterized in that, It also includes a flow stabilizer, which is installed between the outlet of the deep well heat exchange module and the inlet of the heat exchanger.

3. The combined heat and power system according to claim 1, characterized in that, It also includes a safety relief structure, which is a pressure relief valve installed on the pipeline connecting the transcritical CO2 cryogenic power generation module and the gas-liquid separator.

4. The combined heat and power system according to claim 1, characterized in that, It also includes an automatic bypass system, which includes a generator bypass and a cooler bypass; wherein, the generator bypass is located before the transcritical CO2 cryogenic power generation module and after the gas-liquid separator; the cooler bypass is located after the gas-liquid separator and before the micro-power circulation module.

5. The combined heat and power system according to claim 1, characterized in that, The variable coupling power module includes a clutch, a turbine drive rod, and a micro-power drive rod; wherein, the transcritical CO2 cryogenic power generation module is connected to the turbine drive rod, and the micro-power circulation module is connected to the micro-power drive rod; the clutch couples the turbine drive rod and the micro-power drive rod; the clutch is connected to the control system.

6. The combined heat and power system according to claim 5, characterized in that, The variable coupling power module controls the coupling between the transcritical CO2 cryogenic power generation module and the micro-power circulation module in three coupling modes: full coupling mode, partial slip mode, and complete separation mode. The full coupling mode is used under steady-state conditions, the partial slip mode is used during the heating priority or start-up phase, and the complete separation mode is used under abnormal conditions.

7. The combined heat and power system according to claim 1, characterized in that, It also includes a distributed optical fiber temperature sensing unit, which is laid along the entire length of the outer wall of the wellbore of the deep well heat exchange module.

8. A method for implementing a combined heat and power system based on CO2 working fluid according to any one of claims 1 to 7, characterized in that, The implementation method includes the following steps: 1) CO2 is injected into a medium-deep geothermal well through a deep well heat exchange module. The deep well heat exchange module exchanges heat with the rock mass. The liquid CO2 relies on its own density difference to achieve downhole pressurization and temperature increase. After absorbing heat at the bottom of the well, it is brought up to the well as high-temperature supercritical CO2. 2) After being brought to the well, the high-temperature supercritical CO2 transfers heat to the working medium of the fluid that needs to be heated through the heat exchange heating unit. The supercritical CO2 remains in a supercritical state when it is cooled down. 3) Cooled supercritical CO2 enters the transcritical CO2 cryogenic power generation module, providing power for expansion and work. The CO2 flow state in the transcritical CO2 cryogenic power generation module is a two-phase transformation from supercritical to liquid, which is converted into electrical energy. The transcritical CO2 cryogenic power generation module generates electricity. 4) Two-phase CO2 enters the gas-liquid separator, where gas and liquid are separated, and gaseous CO2 is stored in a gas storage tank from the gas outlet located at the top. 5) Liquid CO2 is switched via valves to either enter the surface cooling section or directly enter the micro-power circulation module; in the surface cooling section, the liquid CO2 is further cooled to the operating temperature required for downhole operation; 6) The transcritical CO2 cryogenic power generation module uses a variable coupling power module to drive a micro-power circulation module to pressurize liquid CO2 and inject it into the well.

9. The implementation method according to claim 8, characterized in that, In step 6), the variable coupling power module controls the coupling between the transcritical CO2 cryogenic power generation module and the micro-power circulation module in three coupling modes: full coupling mode, partial slip mode, and complete separation mode; the full coupling mode is used under steady-state conditions; the partial slip mode is used during the heating priority or start-up phase; and the complete separation mode is used in abnormal conditions.

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