Carbon dioxide geothermal battery system and method with common heat exchanger

Abstract

A kind of carbon dioxide heat battery system and method of common heat exchanger, including heat engine integrated machine, heat pump integrated machine and common heat exchanger component. Heat engine turbine branch is connected in parallel with heat pump compressor branch, heat engine compressor branch is connected in parallel with heat pump expander branch, and energy storage mode and energy release mode are realized by switching through shut-off valve. When energy is stored, working medium is formed heat pump cycle through heat pump compressor, molten salt-carbon dioxide heat exchanger, regenerator, heat pump expander, water-carbon dioxide heat exchanger, and heat energy is stored in molten salt and water;When energy is released, working medium is formed heat engine cycle through heat engine compressor, regenerator, molten salt-carbon dioxide heat exchanger, heat engine turbine, water-carbon dioxide heat exchanger, pre-cooler, and stored heat energy is converted into electric energy. System deeply shares molten salt-carbon dioxide heat exchanger, regenerator and water-carbon dioxide heat exchanger, reduces equipment cost, improves economy and market competitiveness.

Description

Technical Field

[0001] This invention belongs to the field of energy storage, specifically relating to a carbon dioxide thermal battery system and method with a shared heat exchanger. Background Technology

[0002] Carbon dioxide thermal batteries are a novel large-scale energy storage technology based on thermodynamic cycles, primarily composed of a heat pump cycle, a heat engine cycle, and a thermal storage system. As a highly promising physical energy storage technology, carbon dioxide thermal batteries offer significant advantages such as high energy density, compact system structure, fast response speed, and environmental friendliness throughout their entire lifecycle. Furthermore, they are not limited by geographical conditions (unlike pumped hydro storage and compressed air storage) and do not suffer from the capacity decay and safety hazards associated with chemical batteries, demonstrating extremely broad application prospects in areas such as renewable energy integration, grid peak shaving and valley filling, distributed energy systems, and multi-energy complementary microgrids.

[0003] In existing carbon dioxide thermal battery systems, the energy storage system and the energy release system are often equipped with independent and non-interchangeable heat exchange equipment. In traditional system designs, the heat pump cycle and the heat engine cycle use their own independent heat exchanger components, including heat exchangers and regenerators for high-temperature heat storage media and heat exchangers for low-temperature heat storage media. They are connected to each other through complex piping and operate their own independent heat exchange paths during energy storage and energy release processes.

[0004] However, the aforementioned existing technologies result in a large number of heat exchangers and complex piping connections within the system, significantly increasing manufacturing and material costs. They also lead to severe idleness and asset waste during off-peak hours. Since supercritical carbon dioxide heat exchangers (especially microchannel PCHEs or molten salt heat exchangers) account for a high percentage of the total investment, this design, with each heat exchanger equipped independently, results in excessively high initial investment costs and poor overall economic efficiency, becoming a key technological bottleneck restricting the commercial application of carbon dioxide thermal battery technology. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention discloses a carbon dioxide thermal battery system with a shared heat exchanger, characterized in that it includes an integrated heat engine, an integrated heat pump, and a shared heat exchanger assembly. The integrated heat engine includes a heat engine turbine, a generator motor, and a heat engine compressor, all three of which are coaxially connected. The integrated heat pump unit includes a heat pump compressor, an electric motor, and a heat pump expander, which are coaxially connected. The shared heat exchanger assembly includes a molten salt-carbon dioxide heat exchanger, a regenerator, and a water-carbon dioxide heat exchanger. The inlet pipe of the heat engine turbine is equipped with a first shut-off valve, and the outlet pipe is equipped with a second shut-off valve, forming a heat engine turbine branch. The heat pump compressor is equipped with a third shut-off valve on the inlet pipe and a fourth shut-off valve on the outlet pipe, forming a heat pump compressor branch. The heat engine turbine branch and the heat pump compressor branch are connected in parallel, and the two ends of the two branches are respectively connected to the high-temperature end pipeline of the molten salt-carbon dioxide heat exchanger and the low-pressure end hot side pipeline of the regenerator. The inlet pipe of the heat engine compressor is equipped with a heat engine pressure stabilizing tank, a precooler and a fifth shut-off valve, and the outlet pipe is equipped with a sixth shut-off valve, forming a heat engine compressor branch. The inlet pipe of the heat pump expander is equipped with a seventh shut-off valve, and the outlet pipe is equipped with a heat pump pressure stabilizing tank and an eighth shut-off valve, forming a heat pump expander branch. The heat engine compressor branch and the heat pump expander branch are connected in parallel, and the two ends of the two branches are respectively connected to the low-temperature side pipeline of the water-carbon dioxide heat exchanger and the high-pressure low-temperature side pipeline of the regenerator.

[0006] The present invention also discloses an operation method for a carbon dioxide thermal battery system based on the above-mentioned shared heat exchanger, characterized in that it includes an energy storage operation mode and an energy release operation mode; In energy storage operation mode, the first shut-off valve, the second shut-off valve, the sixth shut-off valve and the fifth shut-off valve are closed, and the electric motor is started to drive the heat pump compressor to work. The carbon dioxide working fluid flows sequentially through the heat pump compressor, the molten salt-carbon dioxide heat exchanger, the regenerator, the heat pump expander, the heat pump pressure tank, and the water-carbon dioxide heat exchanger before returning to the heat pump compressor, forming a heat pump cycle. In the energy release operation mode, the fourth, third, seventh, and eighth shut-off valves are closed. The generator motor is in motor mode when starting up, and switches to generator mode when the output power of the heat engine turbine is greater than the power consumption of the heat engine compressor. The carbon dioxide working fluid flows sequentially through the heat engine compressor, regenerator, molten salt-carbon dioxide heat exchanger, heat engine turbine, regenerator, water-carbon dioxide heat exchanger, precooler, and heat engine pressure stabilizing tank before returning to the heat engine compressor, forming a heat engine cycle.

[0007] Beneficial effects 1. Shared heat exchanger depth significantly reduces equipment costs. By configuring the heat engine turbine branch and the heat pump compressor branch to operate in parallel, and the heat engine compressor branch and the heat pump expander branch to operate in parallel, and by utilizing the first to eighth shut-off valves to achieve flexible switching between energy storage and energy release modes, the molten salt-carbon dioxide heat exchanger, regenerator, and water-carbon dioxide heat exchanger can be deeply shared in both operating modes. Compared to the traditional system where energy storage and energy release each have their own independent heat exchange equipment, this significantly reduces the number of high-pressure heat exchangers, lowers equipment procurement and manufacturing costs, and shortens the investment payback period.

[0008] 2. Real-time optimization of system performance coefficients to improve energy storage efficiency. In energy storage operation mode, the system's coefficient of performance (COP) is calculated by real-time monitoring of the molten salt-carbon dioxide heat exchanger's temperature and the heat pump compressor's power. The optimal control objective is to ensure the COP is zero, dynamically adjusting the heat pump compressor's speed. This algorithm ensures the heat pump cycle always operates near its maximum COP, effectively improving the thermal energy conversion efficiency during energy storage and reducing unit energy storage power consumption.

[0009] 3. Dynamic maintenance of the thermal efficiency of the heat engine cycle ensures efficient energy release and power generation. In the energy release operation mode, the inlet and outlet temperatures of the heat engine turbine are monitored in real time. The current cycle thermal efficiency is calculated based on the thermal efficiency formula, and the flow rate of the high-temperature molten salt pump is automatically increased to raise the turbine inlet temperature when the efficiency is lower than a preset threshold. This closed-loop control strategy overcomes the efficiency decline caused by operating condition fluctuations or heat exchanger performance degradation, ensuring that the energy release power generation process always maintains a high thermal efficiency.

[0010] 4. Precise control of regenerator efficiency optimizes system heat recovery. By monitoring the temperature and flow rate on both the hot and cold sides of the regenerator in real time, the actual efficiency of the regenerator is calculated and compared with the design target value. When the efficiency deviation exceeds the allowable range, the opening of the cold side bypass valve is adjusted to ensure that the heat exchange on both sides satisfies the energy balance equation. This control method maximizes the recovery of waste heat from turbine exhaust, reduces the need for external heat sources, and improves the overall thermodynamic perfection of the system.

[0011] 5. Mode switching pressure pre-balancing ensures safe system operation. Before switching between energy storage and energy release modes, the pressure difference at the junction of the heat pump compressor branch and the heat engine turbine branch is detected. A proportional-derivative control algorithm is used to dynamically adjust the opening of the bypass regulating valve until the pressure difference drops below a safe threshold before the shut-off valve is switched. This pressure pre-balancing step effectively avoids equipment shock and pipeline vibration caused by sudden pressure changes, improving the reliability and safety of system operation.

[0012] 6. Adaptive adjustment of low-temperature heat exchange efficiency enhances system environmental adaptability. In energy release operation mode, the effectiveness of the water-carbon dioxide heat exchanger is calculated in real time. When the effectiveness falls below the design minimum, the water supply flow to the cryogenic water tank is automatically increased. This adjustment mechanism ensures sufficient heat exchange on the cryogenic side, enabling the system to maintain stable waste heat discharge capacity under different ambient temperatures and water source conditions, thus guaranteeing the normal operation of the heat engine cycle. Attached Figure Description

[0013] Figure 1 This is a schematic diagram of the heat engine cycle of the carbon dioxide thermal battery system with a shared heat exchanger according to the present invention. Figure 2 This is a schematic diagram of the heat pump cycle of the carbon dioxide thermal battery system with a shared heat exchanger according to the present invention. Among them, there are: 1. Low temperature molten salt tank; 2. High temperature molten salt tank; 3. Low temperature water tank; 4. High temperature water tank; 5. Molten salt-carbon dioxide heat exchanger; 6. Regenerator; 7. Water-carbon dioxide heat exchanger; 8. Heat engine turbine; 9. Generator / motor; 10. Heat engine compressor; 11. Heat engine pressure stabilizing tank; 12. Precooler; 13. Heat pump compressor; 14. Motor; 15. Heat pump expander; 16. Heat pump pressure stabilizing tank; and 17-24 shut-off valves (the first shut-off valve is shut-off valve 18, the second shut-off valve is shut-off valve 19, the third shut-off valve is shut-off valve 20, the fourth shut-off valve is shut-off valve 17, the fifth shut-off valve is shut-off valve 23, the sixth shut-off valve is shut-off valve 22, the seventh shut-off valve is shut-off valve 21, and the eighth shut-off valve is shut-off valve 24). Detailed Implementation

[0014] Example 1 This embodiment provides a carbon dioxide thermal battery system with a shared heat exchanger, the structure of which is as follows: Figure 1 and Figure 2 As shown. Figure 1 This diagram illustrates the working fluid flow direction and valve status under the energy release operation mode (heat engine cycle). Figure 2 The diagram illustrates the working fluid flow and valve status in the energy storage operation mode (heat pump cycle). The equipment labels in both diagrams are identical, indicating that the same set of physical equipment is used in both modes.

[0015] The system in this embodiment includes an integrated heat engine unit, an integrated heat pump unit, a shared heat exchanger assembly, a high-temperature thermal storage subsystem, a low-temperature thermal storage subsystem, and eight shut-off valves (reference numerals 17 to 24) and corresponding connecting pipelines. The integrated heat engine unit consists of a heat engine turbine 8, a generator motor 9, and a heat engine compressor 10, all mechanically connected in series via a common shaft. The integrated heat pump unit consists of a heat pump compressor 13, a motor 14, and a heat pump expander 15, all mechanically connected in series via another shaft.

[0016] The shared heat exchanger assembly includes a molten salt-carbon dioxide heat exchanger 5, a regenerator 6, and a water-carbon dioxide heat exchanger 7. These heat exchangers all employ printed circuit board heat exchangers (PCHEs) capable of withstanding the high pressure of supercritical carbon dioxide, with the molten salt side of the molten salt-carbon dioxide heat exchanger 5 using a nickel-based alloy to prevent high-temperature molten salt corrosion.

[0017] The specific details of the pipeline connection are as follows (see Figure 1 or Figure 2 (solid and dashed lines in the pipeline) A shut-off valve 18 is installed on the inlet pipe of the heat engine turbine 8, and a shut-off valve 19 is installed on the outlet pipe. These two shut-off valves, together with the heat engine turbine 8, form the heat engine turbine branch. A shut-off valve 20 is installed on the inlet pipe of the heat pump compressor 13, and a shut-off valve 17 is installed on the outlet pipe. These two shut-off valves, together with the heat pump compressor 13, form the heat pump compressor branch. The heat engine turbine branch and the heat pump compressor branch are connected in parallel: one end of the two branches (i.e., upstream of shut-off valve 18 and downstream of shut-off valve 17) converges on the high-temperature end pipe of the molten salt-carbon dioxide heat exchanger 5; the other end of the two branches (i.e., downstream of shut-off valve 19 and upstream of shut-off valve 20) converges on the low-pressure end hot side pipe of the regenerator 6. This cross-parallel structure allows the shut-off valves 18 and 19 to open in the energy release mode, with the working fluid flowing through the heat engine turbine 8; and the shut-off valves 17 and 20 to open in the energy storage mode, with the working fluid flowing through the heat pump compressor 13.

[0018] Similarly, a heat engine pressure tank 11, a precooler 12, and a shut-off valve 23 are sequentially connected in series on the inlet pipe of the heat engine compressor 10, and a shut-off valve 22 is installed on the outlet pipe. These components, together with the heat engine compressor 10, form the heat engine compressor branch. A shut-off valve 21 is installed on the inlet pipe of the heat pump expander 15, and a heat pump pressure tank 16 and a shut-off valve 24 are sequentially connected in series on the outlet pipe. These components, together with the heat pump expander 15, form the heat pump expander branch. The heat engine compressor branch and the heat pump expander branch are connected in parallel: one end of the two branches (i.e., upstream of shut-off valve 23 and downstream of shut-off valve 24) converges on the low-temperature side pipe of the water-carbon dioxide heat exchanger 7; the other end of the two branches (i.e., downstream of shut-off valve 22 and upstream of shut-off valve 21) converges on the high-pressure low-temperature side pipe of the regenerator 6. In the energy release mode, shut-off valves 22 and 23 are opened, and the working fluid flows through the heat engine compressor 10; in the energy storage mode, shut-off valves 21 and 24 are opened, and the working fluid flows through the heat pump expander 15.

[0019] The high-temperature thermal energy storage subsystem includes a cryogenic molten salt tank 1 and a high-temperature molten salt tank 2. Cryogenic molten salt tank 1 stores molten salt at approximately 280°C (e.g., 60% NaNO3 + 40% KNO3), while high-temperature molten salt tank 2 stores molten salt at approximately 560°C. Both tanks are connected to the molten salt side of a molten salt-carbon dioxide heat exchanger 5 via molten salt pipelines equipped with molten salt pumps. During energy storage operation, the molten salt in cryogenic molten salt tank 1 is extracted, sent to the molten salt-carbon dioxide heat exchanger 5 to absorb heat from carbon dioxide, raising its temperature to approximately 560°C, and then enters the high-temperature molten salt tank 2. During energy release operation, the molten salt in high-temperature molten salt tank 2 is extracted, sent to the molten salt-carbon dioxide heat exchanger 5 to release heat to carbon dioxide, lowering its temperature to approximately 280°C, and then returned to cryogenic molten salt tank 1.

[0020] The cryogenic thermal energy storage subsystem includes a cryogenic water tank 3 and a high-temperature water tank 4. Cryogenic water tank 3 stores softened water at room temperature (approximately 25°C), while high-temperature water tank 4 stores hot water heated by a heat pump circulation system (up to approximately 90°C). Both tanks are connected to the water side of a water-carbon dioxide heat exchanger 7 via water pipes equipped with pumps. During energy storage operation, the hot water in high-temperature water tank 4 is extracted and sent to the water-carbon dioxide heat exchanger 7, where it transfers heat to the low-temperature, low-pressure carbon dioxide. The cooled water, after releasing heat, returns to the cryogenic water tank 3. During energy release operation, the cooled water in cryogenic water tank 3 is extracted and sent to the water-carbon dioxide heat exchanger 7 to absorb waste heat from the carbon dioxide. The heated hot water returns to the high-temperature water tank 4.

[0021] In this embodiment, the pipeline is specifically configured as follows: The outlet pipe from the carbon dioxide side of the water-carbon dioxide heat exchanger 7 is first connected to the shut-off valve 23, then sequentially connected to the precooler 12 and the heat engine pressure stabilizing tank 11, and finally connected to the inlet of the heat engine compressor 10. The precooler 12 is a shell-and-tube water-cooled heat exchanger, with cooling water sourced from an independent cooling tower system. The heat engine pressure stabilizing tank 11 is a volumetric buffer tank used to eliminate pressure pulsations at the compressor inlet. This sequence ensures that the compressor inlet can be cut off as quickly as possible during an emergency shutdown, while simultaneously ensuring that the temperature of the working fluid entering the compressor is precisely controlled by the precooler to be near the carbon dioxide critical point (31°C), thereby reducing compression work.

[0022] The working process of this embodiment is divided into energy storage operation mode and energy release operation mode.

[0023] Energy storage operation modes (see) Figure 2At this point, the system executes a heat pump cycle. Shut-off valves 18, 19, 22, and 23 are closed, while shut-off valves 17, 20, 21, and 24 are open. The motor 14 is started, driving the heat pump compressor 13. Carbon dioxide working fluid is drawn into the heat pump compressor 13 from the low-pressure hot-side pipeline of the regenerator 6 (passing through shut-off valve 20) and compressed to a high-temperature, high-pressure state (e.g., temperature 180°C, pressure 15MPa). The high-temperature, high-pressure carbon dioxide enters the molten salt-carbon dioxide heat exchanger 5, where it undergoes countercurrent heat exchange with molten salt from the low-temperature molten salt tank 1. After releasing heat, the carbon dioxide's temperature drops to approximately 150°C, and it then enters the low-pressure hot-side of the regenerator 6, where it further releases heat to the low-pressure cold-side working fluid. The cooled carbon dioxide enters the heat pump expander 15 (passing through shut-off valve 21), expands and decreases in pressure to approximately 8MPa and temperature to approximately 80°C, while simultaneously recovering some of the expansion work to directly drive the coaxial heat pump compressor 13. The low-pressure carbon dioxide from the heat pump expander 15 passes through the heat pump pressure stabilizing tank 16 and the shut-off valve 24, and enters the water-carbon dioxide heat exchanger 7. There, it absorbs heat from the hot water in the high-temperature water tank 4, raising its temperature to about 100°C. Then, it enters the high-pressure low-temperature side pipeline of the regenerator 6, where it is preheated by the high-temperature carbon dioxide from the outlet of the molten salt-carbon dioxide heat exchanger 5. Finally, the temperature rises to about 130°C and returns to the inlet of the heat pump compressor 13, completing a complete heat pump cycle.

[0024] Energy release operation mode (see) Figure 1 At this point, the system is executing a heat engine cycle. Shut-off valves 17, 20, 21, and 24 are closed, while shut-off valves 18, 19, 22, and 23 are open. The generator motor 9 starts as the motor, driving the heat engine compressor 10. Carbon dioxide working fluid is drawn into the heat engine compressor 10 from the heat engine pressure tank 11 via shut-off valve 23 and precooler 12, and compressed to high pressure (e.g., 20 MPa, temperature approximately 120°C). The high-pressure carbon dioxide enters the high-pressure, low-temperature side pipeline of the regenerator 6, absorbing waste heat from the exhaust gas of the heat engine turbine 8, raising its temperature to approximately 300°C. It then enters the molten salt-carbon dioxide heat exchanger 5, where it is heated to approximately 500°C by molten salt from the high-temperature molten salt tank 2. This high-temperature, high-pressure carbon dioxide enters the heat engine turbine 8 (passing through shut-off valve 18) and expands, doing work to drive the coaxial generator motor 9 to generate electricity (automatically switching to generator mode when the turbine output power exceeds the compressor's power consumption). The expanded low-pressure carbon dioxide (approximately 8 MPa, temperature approximately 200°C) enters the low-pressure hot side of the regenerator 6 through the shut-off valve 19, releasing heat to the working fluid on the high-pressure side, and its own temperature drops to approximately 120°C. It then enters the water-carbon dioxide heat exchanger 7, releasing heat to the cold water from the cryogenic water tank 3, further reducing its temperature to approximately 40°C. Finally, it passes through the precooler 12 to discharge the trace amounts of unusable waste heat to the environment, and the cooled carbon dioxide (approximately 35°C) returns to the inlet of the heat engine compressor 10 via the heat engine pressure stabilizing tank 11, completing the heat engine cycle.

[0025] 1. Pressure balance control during mode switching An electric bypass regulating valve, equipped with a differential pressure sensor, is installed between the two junction points of the heat pump compressor branch and the heat engine turbine branch. Before switching, the pressure difference across the junction points is detected. (Unit: Pascal), The bypass valve opening is dynamically adjusted based on the proportional-derivative control algorithm:

[0026] in, This is the adjustment increment of the bypass valve opening (dimensionless, value range 0~1). The measured pressure difference is expressed in Pa. This is the proportionality coefficient, in units of... In this embodiment, the value is 0.05; These are the differential coefficients, in units of... In this embodiment, the value is 0.01; Time is measured in seconds (s). The shut-off valve will only be switched on after the pressure drops below the preset safety threshold of 0.05 MPa and remains stable for 5 seconds. This control method avoids water hammer and protects the valve and pipeline.

[0027] 2. Real-time optimization of system performance coefficient in energy storage mode System performance coefficient Defined as the ratio of the heat power transferred to the high-temperature molten salt to the electrical power consumed by the heat pump compressor. The following parameter is collected in real time: the mass flow rate of carbon dioxide through the molten salt-carbon dioxide heat exchanger. (Unit: kg / s), Average isobaric specific heat capacity of carbon dioxide (Unit: J / (kg·K)), molten salt side outlet temperature of molten salt-carbon dioxide heat exchanger and inlet temperature (Unit: Kelvin, K), and the real-time electrical power consumption of the heat pump compressor. (Unit: Watt, W). Calculated by the following formula:

[0028] When measured Below the historical high When the speed reaches 90%, the rotational speed of the heat pump compressor 13 is increased in increments of 1%. And recalculate until two consecutive times The rate of change is less than 0.1%. This optimization ensures the system is always at its maximum. Operating nearby.

[0029] 3. Dynamic maintenance of heat engine cycle thermal efficiency under energy release mode Thermal efficiency of heat engine cycle Determined by the inlet and outlet temperatures of the heat engine turbine 8:

[0030] in and These are the carbon dioxide temperatures at the turbine inlet and outlet of the heat engine, respectively, both in Kelvin (K). A minimum thermal efficiency threshold is set. When measured At the same time, increasing the speed of the high-temperature molten salt pump increases the molten salt flow rate into the molten salt-carbon dioxide heat exchanger, thereby improving... ,make It has rebounded above the threshold.

[0031] 4. Precise adjustment of regenerator efficiency The efficiency of regenerator 6 Defined as the ratio of actual heat exchange to maximum possible heat exchange:

[0032] in , , These represent the temperatures at the hot-side inlet, hot-side outlet, and cold-side inlet of the regenerator, respectively, all in Kelvin (K). Design target value. Allowable deviation .when At the same time, adjust the opening of the cold-side bypass valve of the regenerator to change the cold-side mass flow rate. (Unit: kg / s), while simultaneously monitoring the hot-side mass flow rate. (Unit: kg / s), ensuring that the heat exchange on both sides satisfies the energy balance equation:

[0033] in and The isobaric specific heat capacity of carbon dioxide on the hot and cold sides are respectively (unit: J / (kg·K)). This is the cold-side outlet temperature (unit: K). This adjustment ensures the regenerator always operates near its highest efficiency.

[0034] 5. Adaptive control of low-temperature heat exchange effectiveness In energy release mode, the effectiveness of water-carbon dioxide heat exchanger 7 Defined as:

[0035] in , These are the water-side inlet and outlet temperatures (unit: K). Temperature at the carbon dioxide inlet (unit: K). Design minimum. When measured At the same time, increase the speed of the water supply pump of the low temperature water tank 3 to increase the water flow rate, thereby enhancing heat exchange and restoring the effectiveness to above 0.6.

[0036] In summary, this embodiment solves the problems of high cost and idle equipment in the prior art; at the same time, by introducing the above-mentioned control algorithm based on thermodynamic theory, the system's operating efficiency, safety and environmental adaptability are further improved.

[0037] Example 2 This embodiment provides an operation method for a carbon dioxide thermal battery system based on the shared heat exchanger described in Embodiment 1. This method executes different valve state switching and working fluid circulation processes in energy storage and energy release operation modes, respectively. Furthermore, it introduces several dynamic optimization control strategies based on thermodynamic principles, including real-time optimization of the system performance coefficient, dynamic maintenance of the heat engine cycle thermal efficiency, precise adjustment of the regenerator efficiency, pressure pre-balancing control before mode switching, and adaptive adjustment of the low-temperature heat exchange effectiveness. The following is in conjunction with the appendix... Figure 1 and attached Figure 2 The method will be explained in detail.

[0038] I. Basic Process and Control Strategy of Energy Storage Operation Mode See Figure 2 In energy storage operation mode, the control system first confirms that the first shut-off valve 18, the second shut-off valve 19, the sixth shut-off valve 22, and the fifth shut-off valve 23 are in the closed state, while simultaneously confirming that the fourth shut-off valve 17, the third shut-off valve 20, the seventh shut-off valve 21, and the eighth shut-off valve 24 are in the open state. Then, the motor 14 is started to drive the heat pump compressor 13 to work. The carbon dioxide working fluid flows sequentially through the heat pump compressor 13, the molten salt-carbon dioxide heat exchanger 5, the regenerator 6, the heat pump expander 15, the heat pump pressure stabilizing tank 16, and the water-carbon dioxide heat exchanger 7, finally returning to the heat pump compressor 13, forming a heat pump cycle. During this process, hot water from the high-temperature water tank 4 releases heat to the working fluid in the water-carbon dioxide heat exchanger 7, and after cooling, enters the low-temperature water tank 3; molten salt from the low-temperature molten salt tank 1 absorbs heat from the working fluid in the molten salt-carbon dioxide heat exchanger 5, and after heating up, enters the high-temperature molten salt tank 2 to complete the heat storage.

[0039] In energy storage operation mode, to maximize energy storage efficiency, this embodiment performs real-time optimization control of the system performance coefficient (COP). The control system collects the following parameters at a sampling frequency of once per second: the mass flow rate of carbon dioxide flowing through the molten salt-carbon dioxide heat exchanger 5. The unit is kilograms per second (kg / s), measured by a mass flow meter installed at the carbon dioxide inlet of the heat exchanger; the average isobaric specific heat capacity of carbon dioxide. The unit is joules per kilogram Kelvin (J / (kg·K)), obtained by real-time interpolation from the carbon dioxide property database based on measured temperature and pressure; molten salt side outlet temperature of molten salt-carbon dioxide heat exchanger 5. and inlet temperature All units are Kelvin (K), measured by armored thermocouples; real-time power consumption of heat pump compressor 13. The unit is watts (W), obtained from the inverter output power reading of motor 14. The system performance coefficient COP is calculated by the following formula:

[0040] The physical meaning of this formula is: the numerator is the thermal power obtained by the molten salt, and the denominator is the electrical power consumed by the compressor. A higher COP value indicates a more efficient energy storage process. This embodiment sets a performance coefficient optimization objective: to make the COP as close as possible to the maximum value under the current operating conditions. Since the rotational speed N (in revolutions per minute, rpm) of the heat pump compressor 13 is the most direct and controllable parameter affecting the COP, and the COP changes with N in a unimodal function (with a unique maximum value), the control system uses the gradient ascent method for optimization. The specific execution steps are as follows: The first step is to record the current rotational speed. The COP value below is denoted as The second step is to use a step size. After increasing the rotational speed in the positive direction and running stably for 30 seconds, calculate the new COP value and record it as follows. The third step is comparison. and :like If so, continue to increase the rotational speed by the same amount of time; if If the difference between the two is less than 0.001, the optimal speed is considered to have been reached, and the adjustment is stopped. Fourth, when the measured COP is lower than 90% of the historical highest COP, the above optimization procedure is immediately initiated to ensure the system always operates within the high-efficiency range.

[0041] The theoretical basis for this optimized control is the upper limit of the Carnot efficiency of the heat pump cycle: ,in This refers to the average temperature on the high-temperature side of the molten salt-carbon dioxide heat exchanger. This represents the average temperature on the low-temperature side of the water-carbon dioxide heat exchanger. By adjusting the rotational speed to change the working fluid flow rate and pressure ratio, the irreversible losses between the actual cycle and the Carnot cycle can be reduced, allowing the COP to approach the theoretical maximum value.

[0042] II. Basic Process and Control Strategy of Energy Release Operation Mode See Figure 1 In the energy release operation mode, the control system first confirms that the fourth shut-off valve 17, the third shut-off valve 20, the seventh shut-off valve 21, and the eighth shut-off valve 24 are in the closed state, while simultaneously confirming that the first shut-off valve 18, the second shut-off valve 19, the sixth shut-off valve 22, and the fifth shut-off valve 23 are in the open state. When the generator motor 9 starts, it initially operates as a motor, driving the heat engine compressor 10 to build up pressure. Once the output power of the heat engine turbine 8 exceeds the power consumption of the heat engine compressor 10, the generator motor 9 automatically switches to generator mode to supply power. The carbon dioxide working fluid flows sequentially through the heat engine compressor 10, the regenerator 6, the molten salt-carbon dioxide heat exchanger 5, the heat engine turbine 8, the regenerator 6, the water-carbon dioxide heat exchanger 7, the precooler 12, and the heat engine pressure stabilizing tank 11, finally returning to the heat engine compressor 10, forming a heat engine cycle. During this process, the high-temperature molten salt from the high-temperature molten salt tank 2 releases heat to the working medium in the molten salt-carbon dioxide heat exchanger 5, and after cooling, it enters the low-temperature molten salt tank 1; the cold water from the low-temperature water tank 3 absorbs the waste heat of the working medium in the water-carbon dioxide heat exchanger 7, and after being heated, it enters the high-temperature water tank 4 to complete the heat release.

[0043] In the energy release operation mode, to maintain high power generation efficiency, this embodiment implements dynamic maintenance control of the heat engine cycle thermal efficiency in real time. The control system monitors the carbon dioxide temperature before the turbine inlet 8 of the heat engine in real time. and the carbon dioxide temperature at the outlet All units are Kelvin (K). Thermal efficiency of a heat engine cycle. Calculated by the following formula:

[0044] This formula is based on the thermal efficiency expression for an ideal Brayton cycle, where and Absolute temperatures are used in all cases. Considering the various irreversible losses that occur in actual cycles, this embodiment sets a minimum thermal efficiency threshold. The control system calculates at a frequency of once per second. and with Comparison. When When the control system determines that the heat source temperature is insufficient, it immediately performs the following adjustment actions: increases the speed of the high-temperature molten salt pump and increases the mass flow rate of molten salt entering the molten salt-carbon dioxide heat exchanger 5. (Unit: kg / s). Increasing the molten salt flow rate increases the heat transfer coefficient on the hot side of the heat exchanger, thereby improving... .because It is mainly affected by back pressure (determined by the condensing pressure of the water-carbon dioxide heat exchanger 7), and the changes are relatively small. An increase will directly improve The control system uses a proportional-integral controller to control the speed of the molten salt pump. Set as:

[0045] in This is a proportionality coefficient, with a value of 100 rpm (dimensionless efficiency difference). This is the integral coefficient, with a value of 10 rpm / s. When... Once the temperature rises above 0.35 and remains stable for one minute, adjustment is stopped. This control strategy ensures that the power generation efficiency remains above the design lower limit during energy release, avoiding economic losses caused by a drop in molten salt temperature or heat exchanger fouling.

[0046] III. Precise Adjustment Methods for Regenerator Efficiency In energy storage operation mode, the efficiency of the regenerator 6 directly affects the energy efficiency of the heat pump cycle. This embodiment monitors four temperature measurement points of the regenerator 6 in real time: the hot-side inlet temperature... Hot side outlet temperature Cold side inlet temperature and cold side outlet temperature All units are Kelvin (K). Regenerator efficiency Defined as the ratio of actual heat exchange to theoretical maximum heat exchange, it is calculated by the following formula:

[0047] This formula holds true only if the heat capacity flow rates on the cold and hot sides are equal (in this embodiment, the design ensures that the mass flow rates on both sides are similar), at which point the efficiency is determined solely by the inlet temperature. Design target value. Allowable deviation When the calculation is obtained When the absolute value of the difference from 0.85 is greater than 0.02, the system performs the following adjustment: adjusts the opening of the electric bypass valve installed on the cold side inlet bypass pipe of regenerator 6 to change the cold side mass flow rate. (Unit: kg / s). Simultaneously, the control system monitors the hot-side mass flow rate. (Unit: kg / s), and the regulation effect was verified using the energy balance equation:

[0048] in and These are the isobaric specific heat capacities of carbon dioxide on the hot and cold sides, respectively, both in joules per kilogram of Kelvin (J / (kg·K)). The right side of the equation represents the heat absorbed on the cold side, and the left side represents the heat released on the hot side; their equality is a fundamental requirement of energy conservation. If the calculated heat values ​​on both sides deviate by more than 5% after adjustment, further fine-tune the bypass valve opening until the deviation is less than 2%. After adjustment, recalculate. It was confirmed that the value fell within the range of 0.83 to 0.87. This control method is based on the efficiency-number of heat transfer units method in heat transfer. By changing the cold-side flow rate, the heat capacity flow rate ratio can be changed, thereby adjusting the efficiency to near the design value and achieving optimal heat recovery.

[0049] IV. Pressure pre-balancing control before mode switching When the system needs to switch from energy storage operation mode to energy release operation mode, or vice versa, a significant pressure difference may exist at the parallel junction of the heat pump compressor branch and the heat engine turbine branch. Directly operating the shut-off valve will generate a severe pressure surge (water hammer effect), which may damage the valve, pipeline, and heat exchanger. Therefore, this embodiment performs the following pressure pre-balancing control steps before executing any shut-off valve switching action: The first step is to detect the pressure difference at the intersection of the heat pump compressor branch outlet and the heat engine turbine branch inlet. Specifically, a pressure sensor is installed downstream of shut-off valve 17 (near the junction) and upstream of shut-off valve 18 (near the junction), and the measured values ​​are respectively... and All units are Pascals (Pa). The formula for calculating pressure difference is:

[0050] The second step involves the control system dynamically adjusting the opening of the electric bypass regulating valve installed between the two junction points based on the pressure difference. Let the current opening be... (Dimensionless, value range 0~1), opening adjustment amount Determined by the proportional-derivative control algorithm:

[0051] in This is a proportionality coefficient, measured in Pascals (1 / Pa), and in this embodiment, it is set to 0.05. The value is the differential coefficient, expressed in seconds per pascal (s / Pa), and is set to 0.01 in this embodiment. The time unit is seconds (s). The introduction of the differential term can predict the trend of pressure difference changes and avoid overshoot. The control system executes this algorithm with a control cycle of 0.1 seconds, calculating each value... Multiply by 0.1 (smoothing factor) and add to the current opening to prevent the valve from operating too abruptly.

[0052] The third step, when Less than the preset safety pressure difference And this state continues for more than a set time threshold. At 1 second, the control system determines that the pressure has reached equilibrium. Subsequently, according to the requirements of the target mode, it sequentially closes the shut-off valves of the current mode (e.g., shut-off valves 17, 20, 21, and 24 in the energy storage mode) and then opens the shut-off valves of the target mode (e.g., shut-off valves 18, 19, 22, and 23 in the energy release mode). The entire switching process is completed within 1 second. Because the pressure has been pre-balanced, no impact pressure wave is generated when the shut-off valves actuate.

[0053] The theoretical basis of this pressure pre-balancing control is the water hammer equation in fluid mechanics: the pressure impact amplitude is proportional to the rate of change of flow velocity. By actively balancing the pressure difference, the flow velocity change before and after the switch can be made close to zero, thereby eliminating water hammer. Experimental results show that after adopting this method, the maximum pressure peak during the switching process is reduced to less than 1.1 times the rated pressure, while the unbalanced control may generate more than 2 times the impact pressure.

[0054] V. Adaptive Control of Low-Temperature Heat Exchange Effectiveness In the energy release operation mode, the water-carbon dioxide heat exchanger 7 is responsible for discharging waste heat from the working fluid to the cold water. Due to changes in ambient temperature, cold water inlet temperature, and load, the effectiveness of the heat exchanger may deviate from the design value, affecting the system condensing pressure and heat engine cycle efficiency. Therefore, this embodiment monitors the following temperatures of the water-carbon dioxide heat exchanger 7 in real time: water-side inlet temperature. Water-side outlet temperature Carbon dioxide side inlet temperature All units are Kelvin (K). Heat exchanger effectiveness Defined as:

[0055] This formula reflects the ratio of the actual temperature drop of the water to the theoretical maximum temperature drop; a higher efficiency indicates more thorough heat exchange. (Design minimum value) The control system calculates at a frequency of once per second. When the measured value is below 0.6, the following adjustment is performed: increase the speed of the water supply pump in low-temperature water tank 3 to increase the volumetric flow rate of water (in cubic meters per second, m³ / s). Increased water flow rate will increase the pipe-side Reynolds number, enhance the convective heat transfer coefficient, and thus increase effectiveness. To prevent over-adjustment, the control system adopts incremental step adjustment: each time the water supply pump speed is increased by 5%, and after waiting 30 seconds, the calculation is recalculated. If it remains below 0.6, continue increasing until... If the efficiency decreases after increasing the rotation speed (indicating that the heat transfer may have deteriorated), the speed will automatically revert to the previous value. The theoretical basis for this control is the ε-NTU relationship of the heat exchanger: for a given heat exchanger, there is an optimal value for the efficiency as the heat capacity flow rate ratio (water-side heat capacity flow rate / carbon dioxide-side heat capacity flow rate) changes, and the efficiency can be maintained within a reasonable range by adjusting the water flow rate.

[0056] VI. System-wide collaborative logic and security protection The aforementioned control strategies do not operate independently but are coordinated by a central controller. The controller allocates computing resources according to priority: pressure pre-balancing control related to mode switching has the highest priority (control cycle 0.1 seconds); followed by thermal efficiency maintenance and COP optimization control (control cycle 1 second); regenerator efficiency regulation and low-temperature heat exchange effectiveness regulation have lower response time requirements (control cycle 5-10 seconds). When multiple control objectives conflict (e.g., COP optimization requires increasing compressor speed, while pressure pre-balancing requires limiting speed changes), the controller follows the principle of "safety first, energy efficiency second," temporarily freezing the energy efficiency optimization algorithm until the mode switch is completed or the pressure stabilizes.

[0057] In addition, this embodiment also includes the following safety protection logic: when any temperature measuring point exceeds the equipment's allowable limit (e.g., the carbon dioxide side outlet temperature of the molten salt-carbon dioxide heat exchanger 5 exceeds 550°C), an emergency shutdown is immediately triggered, all shut-off valves are restored to the closed state, and all pumps and compressors are stopped. All control parameters and measurement data are recorded in the historical database of the distributed control system for subsequent analysis and optimization.

[0058] Many specific details have been set forth in the foregoing description to provide a thorough understanding of the present invention. However, the above description is merely a preferred embodiment of the present invention, and the present invention can be implemented in many other ways different from those described herein. Therefore, the present invention is not limited to the specific embodiments disclosed above. Furthermore, any person skilled in the art can make many possible variations and modifications to the technical solutions of the present invention, or modify them into equivalent embodiments, using the methods and techniques disclosed above, without departing from the scope of the present invention. Any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention, without departing from the content of the present invention, shall still fall within the protection scope of the present invention.

Claims

1. A carbon dioxide thermal battery system with a shared heat exchanger, characterized in that, This includes integrated heat engine units, integrated heat pump units, and shared heat exchanger assemblies; The integrated heat engine includes a heat engine turbine, a generator motor, and a heat engine compressor, all three of which are coaxially connected. The integrated heat pump unit includes a heat pump compressor, an electric motor, and a heat pump expander, which are coaxially connected. The shared heat exchanger assembly includes a molten salt-carbon dioxide heat exchanger, a regenerator, and a water-carbon dioxide heat exchanger. The inlet pipe of the heat engine turbine is equipped with a first shut-off valve, and the outlet pipe is equipped with a second shut-off valve, forming a heat engine turbine branch. The heat pump compressor is equipped with a third shut-off valve on the inlet pipe and a fourth shut-off valve on the outlet pipe, forming a heat pump compressor branch. The heat engine turbine branch and the heat pump compressor branch are connected in parallel, and the two ends of the two branches are respectively connected to the high-temperature end pipeline of the molten salt-carbon dioxide heat exchanger and the low-pressure end hot side pipeline of the regenerator. The inlet pipe of the heat engine compressor is equipped with a heat engine pressure stabilizing tank, a precooler and a fifth shut-off valve, and the outlet pipe is equipped with a sixth shut-off valve, forming a heat engine compressor branch. The inlet pipe of the heat pump expander is equipped with a seventh shut-off valve, and the outlet pipe is equipped with a heat pump pressure stabilizing tank and an eighth shut-off valve, forming a heat pump expander branch. The heat engine compressor branch and the heat pump expander branch are connected in parallel, and the two ends of the two branches are respectively connected to the low-temperature side pipeline of the water-carbon dioxide heat exchanger and the high-pressure low-temperature side pipeline of the regenerator.

2. The system according to claim 1, characterized in that, It also includes a high-temperature thermal storage subsystem, which includes a low-temperature molten salt tank and a high-temperature molten salt tank, and the low-temperature molten salt tank and the high-temperature molten salt tank are respectively connected to the molten salt side pipeline of the molten salt-carbon dioxide heat exchanger.

3. The system according to claim 1, characterized in that, It also includes a low-temperature thermal storage subsystem, which includes a low-temperature water tank and a high-temperature water tank, and the low-temperature water tank and the high-temperature water tank are respectively connected to the water-side pipeline of the water-carbon dioxide heat exchanger.

4. The system according to claim 1, characterized in that, The heat engine pressure stabilizing tank and the precooler are connected in series on the inlet pipe of the heat engine compressor, and the fifth shut-off valve is located upstream or downstream of the precooler.

5. A method for operating a carbon dioxide thermal battery system based on a shared heat exchanger as described in any one of claims 1 to 4, characterized in that, This includes energy storage operation mode and energy release operation mode; In energy storage operation mode, the first shut-off valve, the second shut-off valve, the sixth shut-off valve and the fifth shut-off valve are closed, and the electric motor is started to drive the heat pump compressor to work. The carbon dioxide working fluid flows sequentially through the heat pump compressor, the molten salt-carbon dioxide heat exchanger, the regenerator, the heat pump expander, the heat pump pressure tank, and the water-carbon dioxide heat exchanger before returning to the heat pump compressor, forming a heat pump cycle. In the energy release operation mode, the fourth, third, seventh, and eighth shut-off valves are closed. The generator motor is in motor mode when starting up, and switches to generator mode when the output power of the heat engine turbine is greater than the power consumption of the heat engine compressor. The carbon dioxide working fluid flows sequentially through the heat engine compressor, regenerator, molten salt-carbon dioxide heat exchanger, heat engine turbine, regenerator, water-carbon dioxide heat exchanger, precooler, and heat engine pressure stabilizing tank before returning to the heat engine compressor, forming a heat engine cycle.

6. The method according to claim 5, characterized in that, In energy storage operation mode, the carbon dioxide side outlet temperature of the molten salt-carbon dioxide heat exchanger and the real-time speed of the heat pump compressor are monitored in real time, and the speed of the heat pump compressor is dynamically adjusted according to the deviation between the real-time system performance coefficient and the maximum system performance coefficient. The system performance coefficient is calculated by the following formula: in, The mass flow rate of carbon dioxide flowing through the molten salt-carbon dioxide heat exchanger is expressed in kilograms per second. The specific heat capacity of carbon dioxide at constant pressure is expressed in joules per kilogram of Kelvin. This refers to the molten salt side outlet temperature of the molten salt-carbon dioxide heat exchanger. The values ​​represent the molten salt inlet temperature, all in Kelvin. This refers to the real-time electrical power consumption of the heat pump compressor, measured in watts. When the measured COP is lower than the set threshold of the maximum historical COP, the speed of the heat pump compressor is gradually increased until the measured COP reaches the threshold.

7. The method according to claim 5, characterized in that, In the energy release operation mode, the carbon dioxide temperature before the turbine inlet of the heat engine and the molten salt flow rate at the outlet of the high-temperature molten salt tank are monitored in real time, and the speed of the high-temperature molten salt pump is dynamically adjusted according to the calculation formula of the heat engine cycle thermal efficiency. The thermal efficiency of the heat engine cycle is given by the following formula: in, The inlet carbon dioxide temperature of the heat engine turbine. The values ​​represent the carbon dioxide temperature at the turbine outlet of the heat engine, all in Kelvin. when Below the preset minimum thermal efficiency threshold At the same time, increasing the speed of the high-temperature molten salt pump increases the molten salt flow rate into the molten salt-carbon dioxide heat exchanger, thereby improving... until rebounded to above.

8. The method according to claim 5, characterized in that, In energy storage operation mode, the inlet temperature of the hot side, the outlet temperature of the hot side, and the inlet temperature of the cold side of the regenerator are monitored in real time, the actual efficiency of the regenerator is calculated, and the opening of the cold side bypass valve of the regenerator is adjusted according to the efficiency deviation. The efficiency of the regenerator is defined by the following formula: in, This refers to the hot-side inlet temperature of the regenerator. This refers to the hot-side outlet temperature. These are the cold-side inlet temperatures, all in Kelvin. when Compared with the design target value The absolute deviation exceeds the permissible deviation At this time, adjust the opening of the cold side bypass valve to ensure that the heat exchange on both sides of the regenerator satisfies the following balance equation: in, and These are the carbon dioxide mass flow rates on the hot and cold sides, respectively, in kilograms per second. and These are the specific heat capacities at constant pressure on the corresponding sides, in joules per kilogram Kelvin. This is the cold-side outlet temperature, in Kelvin.

9. The method according to claim 5, characterized in that, Before switching from energy storage operation mode to energy release operation mode, perform the pressure pre-balancing control step: The first step is to detect the pressure difference between the outlet of the heat pump compressor branch and the inlet of the heat engine turbine branch. The calculation formula is: in, This refers to the carbon dioxide pressure at the outlet of the heat pump compressor branch. The pressure is carbon dioxide at the inlet of the turbine branch of the heat engine, and the unit is Pascal. The second step is to dynamically adjust the opening of the bypass regulating valve located between the intersection points of the two parallel branches based on the pressure difference. Determined by the following formula: in, This is the proportional adjustment factor, expressed in Pascals. This is the differential adjustment coefficient, measured in Pascals per second. Time, in seconds; The third step, when Less than the preset safety pressure difference And the duration exceeds the set time threshold. When the time comes, the shut-off valves corresponding to the energy storage operation mode are closed sequentially, and the shut-off valves corresponding to the energy release operation mode are opened sequentially.

10. The method according to claim 5, characterized in that, In the energy release operation mode, the water-side outlet temperature and carbon dioxide-side outlet temperature of the water-carbon dioxide heat exchanger are detected in real time, the effectiveness of the heat exchanger is calculated, and the water supply flow of the low-temperature water tank is adjusted according to the effectiveness deviation. The effectiveness of the water-carbon dioxide heat exchanger is defined by the following formula: in, The water inlet temperature, The water-side outlet temperature, The values ​​represent the carbon dioxide inlet temperature, all in Kelvin. when Below the design minimum At this time, increase the speed of the water supply pump in the low-temperature water tank to increase the water flow rate and restore the effectiveness to the design range.

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