Compressed carbon dioxide energy storage system and method of operation thereof

Abstract

The application provides a compressed carbon dioxide energy storage system and an operation method thereof, which comprises: low-pressure storage tanks, first throttles, first heat exchangers, compressor sets, coolers, high-pressure storage tanks, second throttles, reheaters, expander sets, second heat exchangers, pressure increasing devices and third heat exchangers which are sequentially connected to form a closed cycle; the low-pressure storage tanks are used for storing liquid carbon dioxide at normal temperature; the internal pressure of the low-pressure storage tanks is 3.5-7.2 MPa; the carbon dioxide at the outlet of the low-pressure storage tanks is stored in the high-pressure storage tanks in a liquid state or a supercritical state after being throttled, decompressed, compressed and cooled; the pressure of the high-pressure storage tanks is 7.4-25 MPa; the carbon dioxide at the outlet of the high-pressure storage tanks enters the low-pressure storage tanks in a normal-temperature liquid state after being throttled, reheated, expanded, cooled, pressurized and heat-exchanged; and the cold source required for cooling is obtained from the cold energy stored in the first heat exchangers and / or the third heat exchangers. The application can realize the liquid storage of carbon dioxide at normal temperature, reduces the dependence on low-temperature cold sources and improves the energy storage efficiency.

Description

Technical Field

[0001] This invention relates to the field of compressed carbon dioxide energy storage technology, specifically to a room-temperature liquid transcritical compressed carbon dioxide energy storage system and its operation method. Background Technology

[0002] With the continuous development of industry, people's demand for energy is also increasing. Currently, the supply of fossil fuels is becoming increasingly tight, and the excessive use of fossil fuels will damage the environment. Therefore, developing new energy sources has become an urgent social need. However, due to the intermittent and fluctuating nature of new energy sources, they cannot provide stable power for extended periods. If such unstable power is connected to the grid on a large scale, it will significantly impact the safe and stable operation of the grid and power quality. Therefore, developing energy storage technology is of great significance for solving the problem of grid connection of new energy sources.

[0003] Carbon dioxide is chemically stable, non-toxic, and low-cost, with a critical pressure of 7.38 MPa and a critical temperature of approximately 31°C, making it easily stored in liquid form at room temperature. Compressed carbon dioxide energy storage technology includes both storage and release processes. During storage, a compressor compresses and cools low-pressure gaseous carbon dioxide before storing it in a high-pressure tank. During release, the high-pressure carbon dioxide is heated and then enters an expander to perform work before being stored in a low-pressure tank. Current technologies use low-pressure tanks with relatively low pressures, requiring cryogenic cooling sources for condensation, resulting in significant energy losses and low storage efficiency. Summary of the Invention

[0004] To address the above problems, this invention provides a compressed carbon dioxide energy storage system and its operation method, which can achieve liquid storage of carbon dioxide at room temperature, reduce the system's dependence on low-temperature cold sources, and improve the system's energy storage efficiency and cold energy utilization rate.

[0005] This invention provides a compressed carbon dioxide energy storage system, comprising: a low-pressure storage tank, a first throttling valve, a first heat exchanger, a compressor unit, a cooler, a high-pressure storage tank, a second throttling valve, a reheater, an expander unit, a second heat exchanger, a pressurization device, and a third heat exchanger, connected in sequence to form a closed loop. The low-pressure storage tank stores liquid carbon dioxide at room temperature, with an internal pressure of 3.5–7.2 MPa. The pressure of the liquid carbon dioxide at the outlet of the low-pressure storage tank after being depressurized by the first throttling valve is not lower than its triple point pressure. The gas-liquid mixed carbon dioxide at the outlet of the first throttling valve exchanges heat with the heat exchange medium of the first heat exchanger and evaporates into gaseous carbon dioxide. The heat exchange medium of the first heat exchanger absorbs and stores the cooling energy released by the carbon dioxide. The gaseous carbon dioxide then enters the compressor unit and is compressed to a supercritical state. The supercritical carbon dioxide at the outlet of the compressor unit is cooled by the cooler and stored in the high-pressure storage tank in a liquid or supercritical state.

[0006] The pressure of the high-pressure storage tank is 7.4-25 MPa. The supercritical carbon dioxide at the outlet of the high-pressure storage tank is throttled to a pressure not lower than its triple point pressure by the second throttle valve, heated by the reheater, and then enters the expander unit to do work and generate electricity. The gaseous carbon dioxide after doing work enters the second heat exchanger and is cooled to liquid state by the second heat exchanger. The cold source of the second heat exchanger comes from the cold energy released by the carbon dioxide stored in the first heat exchanger and / or the third heat exchanger. The liquid carbon dioxide at the outlet of the second heat exchanger is pressurized to the rated pressure by the pressurization device and then enters the third heat exchanger to exchange heat and be heated to the rated temperature. The third heat exchanger stores the cold energy generated by the heat exchange of the pressurized liquid carbon dioxide. The liquid carbon dioxide after heat exchange, heating and pressurizing to the rated temperature and pressure enters the low-pressure storage tank.

[0007] This invention reduces the pressure of liquid carbon dioxide in a low-pressure storage tank to above the triple point pressure using a first throttling valve, and then stores the cold energy generated during the throttling process using a first heat exchanger. During energy release, a second heat exchanger uses the stored cold energy to condense the gaseous carbon dioxide into a liquid state, achieving the recycling of the system's cold energy and improving the system's energy storage efficiency. The carbon dioxide is stored in the low-pressure storage tank in a room-temperature liquid state, eliminating the need for an external low-temperature cold source to condense it, effectively improving the system's flexibility. The low-pressure storage tank does not require complex insulation measures, effectively reducing the system's energy loss.

[0008] In the optional technical solution of the present invention, a cold storage is also included. During the energy storage stage, the initial temperature of the cold storage is room temperature. The cold storage provides heat to the first heat exchanger to evaporate the gas-liquid mixed carbon dioxide at the outlet of the first throttle valve, and sends the cold energy released by the absorbed carbon dioxide into the cold storage for storage; and / or the third heat exchanger provides the heat required for heat exchange to the liquid carbon dioxide at the outlet of the pressurization device, and sends the cold energy generated by the heat exchange of the absorbed liquid carbon dioxide into the cold storage for storage; during the energy release stage, the initial temperature range of the cold storage is -55 to -20°C, and the cold storage is used to provide the second heat exchanger with gaseous carbon dioxide after the cold source has cooled and done work.

[0009] According to this technical solution, the cold storage setup simplifies the system's complexity, improves the flexibility of cold storage and release, saves energy, increases energy utilization, and reduces the system's dependence on external low-temperature cold sources, thus saving costs.

[0010] In an optional technical solution of the present invention, the compressor unit includes a two-stage compressor, and the cooler includes two-stage interstage coolers, with each interstage cooler located at the outlet of each stage compressor to form interstage cooling.

[0011] According to this technical solution, the low-pressure storage tank of this patent allows carbon dioxide to be stored at room temperature with a pressure of 3.5 to 7.2 MPa. The supercritical state can be achieved with a relatively low number of compression and cooling stages, which reduces the complexity of the system and the cost of equipment, and improves the efficiency of the system.

[0012] In an optional technical solution of the present invention, the compressor unit utilizes surplus electricity from the power grid or renewable energy to compress gaseous carbon dioxide to a supercritical state.

[0013] According to this technical solution, surplus electricity from the power grid and renewable energy sources can reduce the operating costs of the system and improve energy utilization.

[0014] In an optional technical solution of the present invention, the expander unit includes two-stage expanders and two-stage reheaters, with each stage of the reheater located at the inlet of each stage expander to form interstage reheating.

[0015] According to this technical solution, the pressure of the high-pressure storage tank is 7.4-25MPa. The relatively low number of expansion stages and heating stages can enable carbon dioxide to have a strong ability to do work, reducing the complexity of the system and the cost of equipment, and improving the efficiency of the system.

[0016] In an optional technical solution of the present invention, a thermal storage tank and a cold storage tank are also included. The inlet of the thermal storage tank is connected to the outlet of the interstage cooler, the outlet of the thermal storage tank is connected to the inlet of the interstage reheater, the inlet of the cold storage tank is connected to the outlet of the interstage reheater, and the outlet of the cold storage tank is connected to the inlet of the interstage cooler.

[0017] According to this technical solution, the thermal storage tank can collect the heat of compression and transfer it to the reheater. The low-temperature heat source generated by the reheater is stored in the cold storage tank, which improves the energy utilization rate and helps to save energy.

[0018] The present invention further provides an operating method for the above-mentioned compressed carbon dioxide energy storage system, comprising the following steps:

[0019] Energy storage steps: Open the first throttle valve and close the second throttle valve. The pressure of the liquid carbon dioxide at the outlet of the low-pressure storage tank after being depressurized by the first throttle valve is not lower than its triple point pressure. The carbon dioxide at the outlet of the first throttle valve exchanges heat with the heat exchange medium of the first heat exchanger and evaporates into gaseous carbon dioxide. The heat exchange medium of the first heat exchanger absorbs and stores the cooling capacity released by the liquid carbon dioxide. The gaseous carbon dioxide then enters the compressor unit and is compressed to a supercritical state. The supercritical carbon dioxide at the outlet of the compressor unit is cooled by the cooler and stored in the high-pressure storage tank in a liquid or supercritical state. Close the first throttle valve, and the energy storage step ends.

[0020] Energy release steps: Close the first throttle valve and open the second throttle valve. The supercritical carbon dioxide from the high-pressure storage tank outlet is throttled through the second throttle valve to a pressure not lower than its triple point pressure. After being heated by the reheater, it enters the expander unit to generate electricity. The gaseous carbon dioxide after generating electricity enters the second heat exchanger and is cooled to liquid state. The cold source of the second heat exchanger comes from the cold energy released by the liquid carbon dioxide stored in the first heat exchanger and / or the third heat exchanger. The liquid carbon dioxide at the outlet of the second heat exchanger is pressurized to the rated pressure by the pressurization device and then enters the third heat exchanger to exchange heat and be heated to the rated temperature. The third heat exchanger stores the cold energy generated by the heat exchange of the pressurized liquid carbon dioxide. After being heated and pressurized to the rated temperature and pressure, the liquid carbon dioxide enters the low-pressure storage tank. The second throttle valve is then closed, and the energy release steps end. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the compressed carbon dioxide energy storage system in an embodiment of the present invention.

[0022] Figure 2 This is a schematic diagram of the heat exchange process structure of the first cold storage device in an embodiment of the present invention.

[0023] Figure 3 This is a schematic diagram of the operation method of the compressed carbon dioxide energy storage system in an embodiment of the present invention.

[0024] Figure label:

[0025] Low-pressure storage tank 1; First throttle valve 2; First heat exchanger 31; Normal temperature heat exchange medium 311; Low temperature heat exchange medium 312; Second heat exchanger 32; Third heat exchanger 33; Compressor unit 4; Compressor 41; Cooler 5; Interstage cooler 51; High-pressure storage tank 6; Second throttle valve 7; Reheater 8; Interstage reheater 81; Expander unit 9; Expander 91; Pressurization device 10; Thermal storage tank 11; Cold storage tank 12; Cold storage 13; First cold storage 131; Second cold storage 132. Detailed Implementation

[0026] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0027] Please see Figure 1As shown, this invention provides a compressed carbon dioxide energy storage system, comprising: a low-pressure storage tank 1, a first throttle valve 2, a first heat exchanger 31, a compressor unit 4, a cooler 5, a high-pressure storage tank 6, a second throttle valve 7, a reheater 8, an expander unit 9, a second heat exchanger 32, a pressurization device 10, and a third heat exchanger 33, which are sequentially connected to form a closed loop. The low-pressure storage tank 1 is used to store liquid carbon dioxide at room temperature, and the internal pressure of the low-pressure storage tank 1 is 3.5–7.2 MPa, which is the saturation pressure of carbon dioxide at room temperature. The pressure of the liquid carbon dioxide at the outlet of the low-pressure storage tank 1 after being depressurized by the first throttle valve 2 is not lower than its triple point pressure. The gas-liquid mixed carbon dioxide at the outlet of the first throttle valve 2 exchanges heat with the heat exchange medium of the first heat exchanger 3 and evaporates into gaseous carbon dioxide. The heat exchange medium of the first heat exchanger 3 absorbs and stores the cooling energy released by the carbon dioxide. The gaseous carbon dioxide then enters the compressor unit 4 and is compressed to the supercritical state. The supercritical carbon dioxide at the outlet of the compressor unit 4 is cooled by the cooler 5 and stored in the high-pressure storage tank 6 in the liquid or supercritical state.

[0028] The pressure of the high-pressure storage tank 6 is 7.4-25 MPa. The supercritical carbon dioxide at the outlet of the high-pressure storage tank 6 is throttled to a pressure not lower than its triple point pressure by the second throttle valve 7, heated by the reheater 8, and then enters the expander unit 9 to do work and generate electricity. The gaseous carbon dioxide after doing work enters the second heat exchanger 32 and is cooled to liquid state by the second heat exchanger 32. The cold source of the second heat exchanger 32 comes from the cold energy released by the liquid carbon dioxide stored in the first heat exchanger 31 and / or the third heat exchanger 33. The liquid carbon dioxide at the outlet of the second heat exchanger 32 is pressurized to the rated pressure by the pressurization device 10 and then enters the third heat exchanger 33 to exchange heat and be heated to the rated temperature. The third heat exchanger 33 also stores the cold energy generated by the heat exchange of the pressurized liquid carbon dioxide. The liquid carbon dioxide after heat exchange, heating and pressurization to the rated temperature and pressure enters the low-pressure storage tank 1.

[0029] This invention reduces the pressure of liquid carbon dioxide in low-pressure storage tank 1 to above the triple point pressure (0.52 MPa) through the first throttling valve 2, and then stores the cold energy generated during the throttling process using the first heat exchanger 31. During the energy release process, the second heat exchanger 32 uses the stored cold energy to condense the gaseous carbon dioxide into a liquid state. The temperature range after condensation is -55 to -20°C, realizing the recycling of the system's cold energy. The third heat exchanger 33 exchanges heat with the pressurized liquid carbon dioxide, causing the carbon dioxide to heat up to room temperature and storing the cold energy generated by the heat exchange, further improving the system's energy storage efficiency and energy utilization rate. Carbon dioxide is pressurized by the pressurization device 10, increasing the pressure at which liquid carbon dioxide enters the low-pressure storage tank 1, thereby raising the condensation temperature of the carbon dioxide. After heat exchange and temperature rise with the third heat exchanger 33, the carbon dioxide can be stored in the low-pressure storage tank 1 in a room-temperature liquid state. This embodiment of the invention does not require an external low-temperature cold source to condense the carbon dioxide, thus achieving room-temperature storage of liquid carbon dioxide. Compared to gaseous storage of carbon dioxide, liquid / supercritical state storage improves energy storage density and the efficiency of the energy storage system. Furthermore, the use of both low-pressure and high-pressure storage tanks allows for flexible switching between energy storage and release without relying on external environmental factors, enhancing system flexibility. The low-pressure storage tank 1 does not require complex insulation measures, effectively reducing system energy loss and cost.

[0030] In a preferred embodiment of the present invention, a cold storage 13 is also included. During the energy storage stage, the initial temperature of the cold storage 13 is ambient temperature. The cold storage provides heat to the first heat exchanger 31 to evaporate the gas-liquid mixed carbon dioxide at the outlet of the first throttle valve 2, and sends the cold energy released by the absorbed carbon dioxide into the cold storage 13 for storage. And / or the third heat exchanger 33 provides the heat required for heat exchange to the liquid carbon dioxide at the outlet of the pressurization device 10, and sends the cold energy generated by the heat exchange of the absorbed liquid carbon dioxide into the cold storage 13 for storage. During the energy release stage, the initial temperature range of the cold storage 13 is -55 to -20°C. The cold storage 13 is used to provide gaseous carbon dioxide after the cold source has done work to the second heat exchanger 32.

[0031] By adopting the above methods, the setup of cold storage simplifies the complexity of the system, improves the flexibility of cold storage and release, saves energy, improves energy utilization, and reduces the system's dependence on external low-temperature cold sources, thus saving costs.

[0032] In a preferred embodiment of the present invention, the cold storage 13 includes a first cold storage 131 and a second cold storage 132. The first cold storage 131 is shared with the first heat exchanger 31 and the third heat exchanger 33, and the second heat exchanger is connected to the second cold storage 132. The arrangement of the first cold storage 131 and the second cold storage 132 improves the convenience of the system, allowing for energy storage or release as needed.

[0033] Furthermore, such as Figure 2As shown, taking the first heat exchanger 31 as an example, the flow of its heat exchange medium is explained. The first heat exchanger 31 includes a main inlet, a main outlet, an auxiliary inlet, and an auxiliary outlet. The main inlet is connected to the outlet of the first throttle valve 2, and the main outlet is connected to the inlet of the compressor 41. The auxiliary inlet and auxiliary outlet are both connected to the first cold storage 131. The initial temperature of the first cold storage 131 is room temperature. The room temperature heat exchange medium 311 in the first cold storage 131 enters the first heat exchanger 31 through the auxiliary inlet and exchanges heat with the gas-liquid mixed carbon dioxide in the main line. The carbon dioxide in the main line absorbs heat and evaporates into gaseous carbon dioxide. The room temperature heat exchange medium 311 in the auxiliary line absorbs the cold energy released by the liquid carbon dioxide and becomes a low temperature heat exchange medium 312, which enters the first cold storage 131 through the auxiliary outlet. As the energy storage process continues, the temperature of the first cold storage 131 gradually decreases to -55 to -20℃. Conversely, the initial temperature of the second cold storage 132 is -55 to -20°C. The low-temperature heat exchange medium 312 in the second cold storage 132 cools the gaseous carbon dioxide at the outlet of the expander 91 and gradually heats up by absorbing the heat from the carbon dioxide before entering the second cold storage 132. When the temperature of the first cold storage 131 reaches -55 to -20°C, it can act as a low-temperature cold source (second cold storage 132) to cool the gaseous carbon dioxide at the outlet of the expander 91. When the temperature of the second cold storage 132 rises to room temperature, it can also act as a high-temperature heat source (first cold storage 131) to evaporate the liquid carbon dioxide at the outlet of the first throttle valve 2. The first cold storage 131 and the second cold storage 132 work alternately in a cycle, improving the convenience of the system and realizing the recycling of cold energy, thus saving energy. In a preferred embodiment of the present invention, the heat exchange medium is an organic material; in some embodiments, the heat exchange medium can also be LNG.

[0034] In a preferred embodiment of the present invention, the compressor unit 4 includes a two-stage compressor 41, and the cooler 5 includes two-stage interstage coolers 51, with each interstage cooler 51 located at the outlet of each stage compressor 41 to form interstage cooling.

[0035] Through the above method, the low-pressure storage tank 1 of this patent allows carbon dioxide to be stored at room temperature with a pressure of 3.5–7.2 MPa. A lower number of compression and cooling stages is sufficient to achieve a supercritical state, reducing system complexity and equipment costs while improving system efficiency. Furthermore, since the pressure of carbon dioxide at the inlet of compressor unit 4 is higher than its triple point pressure, the temperature after compression is related to the efficiency and compression ratio of compressor 41.

[0036] In a preferred embodiment of the present invention, compressor unit 4 utilizes surplus electricity from the power grid or renewable energy to compress gaseous carbon dioxide to a supercritical state. Through this method, surplus electricity from the power grid and renewable energy can reduce system operating costs, improve energy utilization, and enhance the system's environmental friendliness.

[0037] In a preferred embodiment of the present invention, the expander unit 9 includes two-stage expanders 91, and the reheater 8 consists of two-stage interstage reheaters 81. Each interstage reheater 81 is located at the inlet of each expander 91 to form interstage reheating. Through this method, the interstage reheater 81 heats carbon dioxide through a heat storage medium during energy release, increasing its work-producing capacity. The pressure of the high-pressure storage tank 6 is 7.4-25 MPa. A relatively low number of expansion stages and heating stages is sufficient to enable carbon dioxide to perform work, reducing system complexity and equipment costs, and improving system efficiency. Furthermore, after expansion, the outlet pressure of the final expander 91 is higher than the triple point pressure of carbon dioxide, ranging from 0.52 to 2 MPa. The temperature after expansion is determined by the inlet temperature of the expander 91 and the expansion ratio.

[0038] In a preferred embodiment of the present invention, a heat storage tank 11 and a cold storage tank 12 are also included. The inlet of the heat storage tank 11 is connected to the outlet of the interstage cooler 51, the outlet of the heat storage tank 11 is connected to the inlet of the interstage reheater 81, the inlet of the cold storage tank 12 is connected to the outlet of the interstage reheater 81, and the outlet of the cold storage tank 12 is connected to the inlet of the interstage cooler 51.

[0039] In this way, the heat storage tank 11 can collect the heat of compression and transfer it to the reheating device 8. The low-temperature cold source generated by the reheating device 8 enters the cold storage tank 12 for storage. The low-temperature cold source in the cold storage tank is used to cool the carbon dioxide at the outlet of the compressor 41, realizing the recycling of energy, improving the energy utilization rate, and helping to save energy.

[0040] In a preferred embodiment of the present invention, valves (not shown in the figure) are provided at the inlet and outlet of the low-pressure storage tank 1, the inlet and outlet of the high-pressure storage tank 6, the inlet and outlet of the interstage cooler 51, and the inlet and outlet of the interstage reheater 81, so that the flow rate of carbon dioxide can be adjusted as needed.

[0041] like Figure 3 As shown, the present invention further provides an operating method for the above-mentioned compressed carbon dioxide energy storage system, comprising the following steps:

[0042] Energy storage steps: Open the first throttle valve 2 and close the second throttle valve 7. The pressure of the liquid carbon dioxide at the outlet of the low-pressure storage tank 1 after being depressurized by the first throttle valve 2 is not lower than its triple point pressure. The gas-liquid mixed carbon dioxide at the outlet of the first throttle valve 2 exchanges heat with the heat exchange medium of the first heat exchanger 31 and evaporates into gaseous carbon dioxide. The heat exchange medium of the first heat exchanger 31 absorbs and stores the cooling energy released by the evaporation of liquid carbon dioxide. The gaseous carbon dioxide then enters the compressor unit 4 and is compressed to the supercritical state. The supercritical carbon dioxide at the outlet of the compressor unit 4 is cooled by the cooler 5 and stored in the high-pressure storage tank 1 in the liquid or supercritical state. Close the first throttle valve 2, and the energy storage step ends.

[0043] Energy release steps: Close the first throttle valve 2 and open the second throttle valve 7. The liquid or supercritical carbon dioxide from the outlet of the high-pressure storage tank 6 is throttled through the second throttle valve 7 to a pressure not lower than its triple point. After being heated by the reheater 8, it enters the expander unit 9 to generate electricity. The gaseous carbon dioxide after generating electricity enters the second heat exchanger 32 and is cooled to liquid state by the second heat exchanger 32. The cold source of the second heat exchanger 32 comes from the cold energy released by the evaporation of the liquid carbon dioxide stored in the first heat exchanger 31 and / or the cold energy released by the heat exchange of the liquid carbon dioxide stored in the third heat exchanger 33. The liquid carbon dioxide from the outlet of the second heat exchanger 32 is pressurized to the rated pressure by the pressurization device 10 and then enters the third heat exchanger 33 to exchange heat and be heated to the rated temperature. The third heat exchanger 33 stores the cold energy generated by the heat exchange of the pressurized liquid carbon dioxide. The liquid carbon dioxide after heat exchange, heating and pressurization to the rated temperature and pressure enters the low-pressure storage tank 1. The second throttle valve 7 is closed, and the energy release steps end.

[0044] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A compressed carbon dioxide energy storage system, characterized in that, include: The following components are connected in sequence to form a closed-loop system: a low-pressure storage tank, a first throttle valve, a first heat exchanger, a compressor unit, a cooler, a high-pressure storage tank, a second throttle valve, a reheater, an expander unit, a second heat exchanger, a pressurization device, and a third heat exchanger. The low-pressure storage tank is used to store liquid carbon dioxide at room temperature. The internal pressure of the low-pressure storage tank is 3.5~7.2MPa. The pressure of the liquid carbon dioxide at the outlet of the low-pressure storage tank after being depressurized by the first throttle valve is not lower than its triple point pressure. The gas-liquid mixed carbon dioxide at the outlet of the first throttle valve exchanges heat with the heat exchange medium of the first heat exchanger and evaporates into gaseous carbon dioxide. The heat exchange medium of the first heat exchanger absorbs and stores the cooling capacity released by the liquid carbon dioxide. The gaseous carbon dioxide then enters the compressor unit and is compressed to a supercritical state. The supercritical carbon dioxide at the outlet of the compressor unit is cooled by the cooler and stored in the high-pressure storage tank in a liquid and / or supercritical state. The pressure of the high-pressure storage tank is 7.4-25 MPa. The supercritical carbon dioxide at the outlet of the high-pressure storage tank is throttled to a pressure not lower than its triple point pressure by the second throttle valve, heated by the reheater, and then enters the expander unit to generate electricity. The gaseous carbon dioxide after generating electricity enters the second heat exchanger and is cooled to liquid state by the second heat exchanger. The cold source of the second heat exchanger comes from the cold energy released by the carbon dioxide stored in the first heat exchanger and the third heat exchanger. The liquid carbon dioxide at the outlet of the second heat exchanger is pressurized to the rated pressure by the pressurization device and then enters the third heat exchanger to exchange heat and be heated to the rated temperature. The third heat exchanger stores the cold energy generated by the heat exchange of the pressurized liquid carbon dioxide. The room-temperature liquid carbon dioxide after heat exchange, heating and pressurization to the rated temperature and pressure enters the low-pressure storage tank. It also includes a cold storage unit. During the energy storage phase, the initial temperature of the cold storage unit is ambient temperature. The cold storage unit provides heat to the first heat exchanger to evaporate the gas-liquid mixed carbon dioxide at the outlet of the first throttle valve, and sends the cold energy released by the absorbed carbon dioxide into the cold storage unit for storage. During the energy release phase, the third heat exchanger provides the heat required for heat exchange to the liquid carbon dioxide at the outlet of the pressurization device, and sends the cold energy generated by the heat exchange of the absorbed carbon dioxide into the cold storage unit for storage. The initial temperature range of the cold storage unit is -55 to -20°C. The cold storage unit is used to provide a cold source to the second heat exchanger to cool the gaseous carbon dioxide after the work has been done.

2. The compressed carbon dioxide energy storage system according to claim 1, characterized in that, The compressor unit includes a two-stage compressor, and the cooler includes two-stage interstage coolers. Each stage of the interstage cooler is located at the outlet of each stage of the compressor to form interstage cooling.

3. The compressed carbon dioxide energy storage system according to claim 2, characterized in that, The compressor unit uses surplus electricity from the power grid or renewable energy to compress gaseous carbon dioxide to a supercritical state.

4. The compressed carbon dioxide energy storage system according to claim 2, characterized in that, The expander unit includes two-stage expanders, and the reheater is a two-stage interstage reheater. Each stage of the interstage reheater is located at the inlet of each stage of the expander to form interstage reheating.

5. The compressed carbon dioxide energy storage system according to claim 4, characterized in that, It also includes a thermal storage tank and a cold storage tank. The inlet of the thermal storage tank is connected to the outlet of the interstage cooler, the outlet of the thermal storage tank is connected to the inlet of the interstage reheater, the inlet of the cold storage tank is connected to the outlet of the interstage reheater, and the outlet of the cold storage tank is connected to the inlet of the interstage cooler.

6. A method for operating a compressed carbon dioxide energy storage system as described in any one of claims 1 to 5, characterized in that, Includes the following steps: Energy storage steps: Open the first throttle valve and close the second throttle valve. The pressure of the liquid carbon dioxide at the outlet of the low-pressure storage tank after being depressurized by the first throttle valve is not lower than its triple point pressure. The carbon dioxide at the outlet of the first throttle valve exchanges heat with the heat exchange medium of the first heat exchanger and evaporates into gaseous carbon dioxide. The heat exchange medium of the first heat exchanger absorbs and stores the cooling capacity released by the carbon dioxide. The gaseous carbon dioxide then enters the compressor unit and is compressed to a supercritical state. The supercritical carbon dioxide at the outlet of the compressor unit is cooled by the cooler and stored in the high-pressure storage tank in a liquid or supercritical state. Close the first throttle valve, and the energy storage step ends. Energy release steps: Close the first throttle valve and open the second throttle valve. The supercritical carbon dioxide from the outlet of the high-pressure storage tank is throttled through the second throttle valve to a pressure not lower than its triple point. After being heated by the reheater, it enters the expander unit to generate electricity. The gaseous carbon dioxide after generating electricity enters the second heat exchanger and is cooled to liquid state. The cold source of the second heat exchanger comes from the cold energy released by the liquid carbon dioxide stored in the first and third heat exchangers. The liquid carbon dioxide from the outlet of the second heat exchanger is pressurized to the rated pressure by the pressurization device and then enters the third heat exchanger to exchange heat and rise to the rated temperature. The third heat exchanger stores the cold energy generated by the heat exchange of the pressurized liquid carbon dioxide. After heat exchange, heating, and pressurization to the rated temperature and pressure, the room-temperature liquid carbon dioxide enters the low-pressure storage tank. The second throttle valve is then closed, and the energy release step ends.

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