Carbon dioxide energy storage system with seabed storage and high temperature thermal storage and operation method thereof

Abstract

The application discloses a seabed liquid storage and high-temperature heat storage coordinated carbon dioxide energy storage system and an operation method thereof, energy conversion and heat storage subsystems are arranged on a coast, a single-stage compression and expansion structure is adopted, and water and molten salt are combined to perform cascade heat storage, so that high-temperature storage and release of heat energy are realized; a floating flexible gas storage bag is arranged on a sea surface, a flexible liquid storage bag is moored to the seabed, a temperature difference between cold seawater on the seabed and hot seawater on the sea surface is utilized to drive carbon dioxide gas-liquid phase change, and constant-pressure liquid storage is realized; carbon dioxide is liquefied and compressed by using cold seawater on the seabed to increase the density of the carbon dioxide and weaken the buoyancy, so that reliable ballast of the seabed storage device is realized; meanwhile, the low-temperature environment of the deep sea is utilized to reduce the condensing pressure of carbon dioxide, the pressure gradient of carbon dioxide in the vertical direction is combined, the exhaust temperature of the compressor is reduced, and the reliability of the high-pressure ratio compressor and the low-melting-point molten salt is improved. The carbon dioxide energy storage system of the application cooperatively utilizes seabed constant-pressure liquid storage and high-temperature molten salt heat storage, and can significantly improve the round-trip efficiency of the system.

Description

Technical Field

[0001] This invention relates to the field of thermomechanical energy storage technology, and in particular to a carbon dioxide energy storage system that combines subsea liquid storage with high-temperature thermal storage, and its operation method. Background Technology

[0002] In recent years, with the rapid development of offshore wind power and photovoltaics, coastal areas have gradually become important regions for large-scale development of new energy sources. However, offshore new energy sources are intermittent and volatile, posing challenges to the stability of the power grid. Large-scale long-term energy storage technology plays an increasingly important role in balancing energy supply and demand and improving grid stability, providing solutions for power balance and peak shaving. Among various new long-term energy storage technologies, compressed carbon dioxide (CCCO) energy storage has become an important branch of thermomechanical energy storage due to its advantages such as long system life, low cost per kilowatt-hour, environmental friendliness, and no dependence on scarce materials. Among these, compressed carbon dioxide energy storage technology based on gaseous and liquid storage has been engineered and applied in inland and desert scenarios, attracting increasing attention and showing promise as a potential solution for efficient energy storage in coastal areas.

[0003] Compressed carbon dioxide energy storage systems achieve energy storage and release through compression, heat storage, heat release, and expansion processes. They also achieve electricity storage by simultaneously storing pressure energy and thermal energy. This involves core components such as compression heat storage, working fluid liquefaction, and heat recovery. Therefore, the key to improving system efficiency lies in avoiding the unnecessary loss of pressure energy and reducing heat loss. Irreversible dissipation. To avoid the system operating under variable pressure and reduced efficiency due to pressure changes during the charging and discharging process of high-pressure carbon dioxide stored in pressure vessels, Chinese invention patent application CN120062847A proposes a marine constant-pressure compressed carbon dioxide energy storage system and method coupled with ocean temperature difference. This system achieves gas-liquid conversion of carbon dioxide by utilizing deep-sea cold seawater and shallow-sea warm seawater, and uses seawater static pressure to achieve constant-pressure carbon dioxide storage. However, this scheme uses two-stage compression and low thermal energy storage temperature, which reduces the energy quality during the conversion of electrical energy into thermal energy, limiting the system's round-trip efficiency. Using a floating offshore platform to support the energy storage system requires the construction of an additional offshore platform, resulting in high investment costs. Wang Ding et al. (Performance Analysis of a Novel Gas-Liquid Phase Change Compressed Carbon Dioxide Energy Storage System [J]. Journal of Power Engineering, 2024.) proposed a compressed carbon dioxide energy storage system based on single-stage compression, which achieves high-temperature thermal energy storage through molten salt thermal storage. However, due to the lack of effective thermal management in the heat storage and release process, the compressor inlet temperature cannot rise to a higher temperature level, which hinders system performance. Furthermore, the use of ambient cold source to liquefy carbon dioxide results in high pressure, which in turn causes the compressor discharge temperature to be too high, affecting the normal operation of the compressor and easily leading to high-temperature decomposition of low-melting-point molten salt heat storage. Summary of the Invention

[0004] To overcome the shortcomings of low efficiency in existing two-stage compression systems and poor reliability in single-stage compression systems, this invention aims to provide a carbon dioxide energy storage system and its operation method that combines subsea liquid storage and high-temperature thermal storage. The energy conversion and thermal storage subsystem is located on the coast, employing a single-stage compression and expansion structure, combined with water and molten salt for tiered thermal storage, achieving high-temperature storage and release of thermal energy. The gas storage device uses a floating flexible gas storage bladder, positioned near the sea surface. The liquid storage device uses a flexible liquid storage bag, moored on the seabed, utilizing the temperature difference between the cold seawater on the seabed and the warm seawater on the surface to drive a phase change in carbon dioxide, achieving constant-pressure liquid storage. A bidirectional carbon dioxide transport pipeline connects the coastal energy conversion and thermal storage subsystem with the subsea gas-liquid phase change and liquid storage subsystem, reducing pipeline laying and lowering costs. This invention significantly increases the density of carbon dioxide by liquefying and compressing it with cold seawater on the seabed, thus reducing buoyancy and achieving reliable ballast for the subsea storage device. Simultaneously, the deep-sea environment reduces the condensation pressure of carbon dioxide, and combined with the vertical pressure gradient of carbon dioxide, effectively reducing the exhaust temperature of the compressor, improving the reliability of the high-pressure compressor and the low-melting-point molten salt. The carbon dioxide energy storage system proposed in this invention utilizes the constant pressure subsea liquid storage and high-temperature molten salt thermal storage in synergy, which can significantly improve the system's round-trip efficiency and provide an efficient energy storage solution for nearshore, island and other areas.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] A carbon dioxide energy storage system that combines subsea liquid storage and high-temperature thermal storage includes: a coastal energy conversion and thermal storage subsystem, a two-way carbon dioxide transport pipeline, a subsea gas-liquid phase change and liquid storage subsystem, and a near-shore floating gas storage bladder for storing low-pressure gaseous carbon dioxide.

[0007] The coastal energy conversion and thermal storage subsystem includes a compressor 3, a first thermal storage heat exchanger 4, a second thermal storage heat exchanger 5, a low-temperature water storage tank 6, a high-temperature water storage tank 7, a low-temperature molten salt storage tank 8, a high-temperature molten salt storage tank 9, a third thermal storage heat exchanger 19, a fourth thermal storage heat exchanger 20, and a turbine 21. During energy storage, the outlet of the gas storage tank 1 is sequentially connected to the compressor 3, the high-temperature side of the first thermal storage heat exchanger 4, and the high-temperature side of the second thermal storage heat exchanger 5. The outlet of the low-temperature water storage tank 6 is connected to the inlet of the high-temperature water storage tank 7 via the low-temperature side of the second thermal storage heat exchanger 5. The outlet of the low-temperature molten salt storage tank 8 is connected to the inlet of the first thermal storage heat exchanger 5 via the first thermal storage heat exchanger 6. The low-temperature side of the thermal storage heat exchanger 4 is connected to the inlet of the high-temperature molten salt storage tank 9. During energy release, the carbon dioxide outlet of the seabed gas-liquid phase change and liquid storage subsystem is connected in series with the low-temperature side of the third thermal storage heat exchanger 19 and the low-temperature side of the fourth thermal storage heat exchanger 20 through a bidirectional carbon dioxide transport pipeline. The outlet of the low-temperature side of the fourth thermal storage heat exchanger 20 is connected to the inlet of the turbine 21. The outlet of the high-temperature water storage tank 7 is connected to the inlet of the low-temperature water storage tank 6 through the high-temperature side of the third thermal storage heat exchanger 19. The outlet of the high-temperature molten salt storage tank 9 is connected to the inlet of the low-temperature molten salt storage tank 8 through the high-temperature side of the fourth thermal storage heat exchanger 20.

[0008] The subsea gas-liquid phase change and liquid storage subsystem includes a condenser 13, a first seawater pump 14, a liquid storage bag 15, a working fluid pump 16, an evaporator 17, and a second seawater pump 18. During energy storage, the high-temperature side outlet of the second thermal energy storage exchanger 5 is connected to the high-temperature side inlet of the condenser 13 via a bidirectional carbon dioxide transport pipeline, and the high-temperature side outlet of the condenser 13 is connected to the inlet of the liquid storage bag 15. The low-temperature side inlet of the condenser 13 is connected to a subsea pipeline via the first seawater pump 14. During energy release, the outlet of the liquid storage bag 15 is connected to the low-temperature side inlet of the evaporator 17 via the working fluid pump 16, and the low-temperature side outlet of the evaporator 17 is connected to the low-temperature side inlet of the third thermal energy storage exchanger 19 via a bidirectional carbon dioxide transport pipeline. The high-temperature side inlet of the evaporator 17 is connected to a surface pipeline via the second seawater pump 18.

[0009] The compressor 3 is connected to the motor 2 via a coupling, and the power inlet of the motor 2 is connected to the power outlet of the offshore wind power generation device; the turbine 21 is connected to the generator 22 via a coupling, and the power outlet of the generator 22 is connected to the power grid or the power inlet of the user.

[0010] The coastal energy conversion and thermal storage subsystem also includes a regenerator 23. The low-temperature side outlet of the evaporator 17 is connected to the low-temperature side inlet of the regenerator 23 via a bidirectional carbon dioxide transport pipeline. The low-temperature side outlet of the regenerator 23 is connected to the low-temperature side inlet of the third thermal storage heat exchanger 19. The turbine 21 outlet is connected to the inlet of the gas storage bladder 1 via the high-temperature side of the regenerator 23.

[0011] The heat storage temperature of the high-temperature water storage tank 7 and the low-temperature molten salt storage tank 8 is close, between 150°C and 300°C.

[0012] The bidirectional carbon dioxide transport pipeline includes a first reversing valve 10, a carbon dioxide pipeline 11, and a second reversing valve 12. The carbon dioxide pipeline 11 is divided into an insulated section and a non-insulated section. The insulated section is the part of the pipeline where the surrounding seawater temperature is lower than the set carbon dioxide evaporation temperature.

[0013] The liquid storage bag 15 is installed on the seabed and is used to store high-pressure liquid carbon dioxide under constant pressure.

[0014] The subsea pipeline is at a greater depth on the seabed than the storage bag 15 is at the same depth on the seabed.

[0015] The coastal energy conversion and thermal storage subsystem also includes an intermediate water tank 24 and an intermediate molten salt tank 25; the inlet of the intermediate water tank 24 is connected to the intermediate outlet on the low-temperature side of the second thermal storage heat exchanger 5, and the outlet of the intermediate water tank 24 is connected to the intermediate inlet on the high-temperature side of the third thermal storage heat exchanger 19; the intermediate molten salt tank 25 is located between the intermediate pipe on the low-temperature side of the first thermal storage heat exchanger 4 and the intermediate pipe on the high-temperature side of the fourth thermal storage heat exchanger 20, and the flow direction of the molten salt in the intermediate molten salt tank 25 needs to be determined according to the selected type of molten salt.

[0016] This invention also provides an operation method for a carbon dioxide energy storage system that combines subsea liquid storage with high-temperature thermal storage, including:

[0017] During energy storage, the low-pressure gaseous carbon dioxide in the gas storage bag 1 is compressed in a single stage by the compressor 3, and the heat of compression generated is recovered by molten salt heat storage and pressurized water heat storage in turn; while the carbon dioxide itself is cooled and transported to the condenser 13 through the carbon dioxide bidirectional transport pipeline, where it is completely condensed into liquid by the cold seawater on the seabed, and then stored at constant pressure in the liquid storage bag 15.

[0018] During energy release, the liquid carbon dioxide stored in the storage bag 15 is pressurized by the working fluid pump 16 and transported to the evaporator 17. It is evaporated into gaseous state by the hot seawater on the sea surface and transported to the regenerator 23 through the carbon dioxide bidirectional transport pipeline to absorb the waste heat of the exhaust gas at the turbine 21 outlet. Then, the preheated high-pressure carbon dioxide is heated to a high temperature state by pressurized water heat storage and molten salt heat storage in sequence, and enters the turbine 21 to expand and do work to generate electricity. The carbon dioxide at the turbine 21 outlet is cooled after being reheated by the regenerator 23 and then stored in the gas storage bag 1.

[0019] During energy storage, the molten salt thermal storage utilizes the low-temperature molten salt in the low-temperature molten salt storage tank 8 to recover the heat from the high-temperature carbon dioxide at the outlet of the compressor 3. The temperature of the low-temperature molten salt rises, becoming high-temperature molten salt, which is then stored in the high-temperature molten salt storage tank 9. The pressurized water thermal storage utilizes the low-temperature pressurized water in the low-temperature water storage tank 6 to further recover the heat from the carbon dioxide at the outlet of the first thermal exchanger 4. The temperature of the low-temperature pressurized water rises, becoming high-temperature pressurized water, which is then stored in the high-temperature water storage tank 7.

[0020] During energy release, the pressurized water thermal storage utilizes the high-temperature pressurized water in the high-temperature storage tank 7 to release heat to the carbon dioxide at the low-temperature side outlet of the regenerator 23. The temperature of the high-temperature pressurized water then decreases, becoming low-temperature pressurized water, which is then stored in the low-temperature storage tank 6. The molten salt thermal storage utilizes the high-temperature molten salt in the high-temperature molten salt storage tank 9 to further heat the carbon dioxide at the outlet of the third thermal exchanger 19. The temperature of the high-temperature molten salt then decreases, becoming low-temperature molten salt, which is then stored in the low-temperature molten salt storage tank 8.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0022] 1. The carbon dioxide energy storage system proposed in this invention, which combines subsea liquid storage and high-temperature thermal storage, can make full use of the geographical conditions of the coast (such as coastal areas or islands), placing the energy conversion and thermal storage subsystems on the coast without the need for additional offshore platforms; the atmospheric pressure gas storage bag floats on the nearshore sea surface without occupying land space; the carbon dioxide is liquefied and compressed using cold seawater on the seabed, significantly increasing its density and greatly reducing buoyancy; the liquid storage bag is moored on the seabed, achieving reliable ballast for constant pressure liquid storage; and the bidirectional transmission of gaseous carbon dioxide between the coastal and subsea equipment is achieved through a single pipeline and a reversing valve, reducing the investment cost of long-distance carbon dioxide pipelines.

[0023] 2. The carbon dioxide energy storage system proposed in this invention, which combines subsea liquid storage with high-temperature thermal storage, can fully leverage the performance advantages of high-pressure single-stage compression and expansion while ensuring reliable operation of the compressor equipment and the low-melting-point molten salt thermal storage medium. By utilizing the cold seawater on the seabed, carbon dioxide can be condensed at a lower temperature (pressure). Furthermore, by utilizing the pressure gradient of carbon dioxide in the vertical direction, the compressor outlet pressure and temperature can be further reduced, preventing the compressor and molten salt thermal storage from exceeding their reliable operating range due to excessively high temperatures.

[0024] 3. The carbon dioxide energy storage system proposed in this invention, which combines subsea liquid storage with high-temperature thermal storage, employs a two-stage thermal storage system using pressurized water and molten salt at both high and low temperatures. This segmented storage of heat at different temperatures avoids the efficiency loss caused by storing low-temperature heat in a high-temperature medium, resulting in higher system energy utilization. By incorporating a regenerator to enhance the self-regeneration process, the system utilizes the waste heat from turbine exhaust gas to preheat the low-temperature carbon dioxide, thereby improving the system's heat recovery rate and eliminating the need for additional radiators.

[0025] 4. This invention utilizes intermediate storage tanks to adjust the flow rates of pressurized water and molten salt, thereby improving the heat exchange matching between carbon dioxide and the heat storage medium during the heat storage and release process, enhancing the thermal management level, increasing the turbine inlet temperature, and improving the system's round-trip efficiency.

[0026] In summary, the carbon dioxide energy storage system proposed in this invention utilizes the combined use of constant-pressure subsea liquid storage and high-temperature molten salt thermal storage, which can significantly improve the system's round-trip efficiency and provide an efficient energy storage solution for nearshore, island, and other areas. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the carbon dioxide energy storage system that combines subsea liquid storage with high-temperature thermal storage in Example 1.

[0028] Figure 2 This is a schematic diagram of the carbon dioxide energy storage system that combines subsea liquid storage with high-temperature thermal storage in Example 2.

[0029] Explanation of reference numerals in the attached diagram: 1 - Gas storage bag, 2 - Electric motor, 3 - Compressor, 4 - First heat storage heat exchanger, 5 - Second heat storage heat exchanger, 6 - Low-temperature water storage tank, 7 - High-temperature water storage tank, 8 - Low-temperature molten salt tank, 9 - High-temperature molten salt tank, 10 - First reversing valve, 11 - Carbon dioxide pipeline, 12 - Second reversing valve, 13 - Condenser, 14 - First seawater pump, 15 - Liquid storage bag, 16 - Working fluid pump, 17 - Evaporator, 18 - Second seawater pump, 19 - Third heat storage heat exchanger, 20 - Fourth heat storage heat exchanger, 21 - Turbine, 22 - Generator, 23 - Regenerator, 24 - Intermediate water storage tank, 25 - Intermediate molten salt tank. Detailed Implementation

[0030] To provide a better understanding of the structural features and effects of the present invention, a clear and complete description of the present invention will be given below in conjunction with the accompanying drawings.

[0031] Example 1

[0032] See Figure 1 A carbon dioxide energy storage system that combines subsea liquid storage and high-temperature thermal storage, comprising: a coastal energy conversion and thermal storage subsystem, a subsea gas-liquid phase change and liquid storage subsystem, a nearshore floating gas storage bladder 1, and a two-way carbon dioxide transport pipeline.

[0033] Nearshore floating air storage bladder 1: used to store low-pressure gaseous carbon dioxide; the air storage bladder 1 has a multi-layer composite membrane structure, the inner layer is made of a flexible polymer material that resists carbon dioxide permeation, so that its volume can be changed, and atmospheric pressure is used to maintain a constant pressure, so that its interior is always at normal pressure; the air storage bladder 1 floats on the nearshore sea surface by utilizing the buoyancy of seawater, without occupying land space, and at the same time, the bottom is in direct contact with seawater, and can be cooled to the ambient temperature by seawater.

[0034] Coastal Energy Conversion and Thermal Storage Subsystem: A compressor with a high pressure ratio, driven by offshore wind power, compresses low-pressure gaseous carbon dioxide into a high-temperature, high-pressure state. A dual thermal storage loop using pressurized water and molten salt is then employed for thermal storage and release. The high-pressure gaseous carbon dioxide expands to generate electricity, which is then converted into electrical energy and stably output to the grid or users. Waste heat from the high-pressure turbine outlet is used to preheat the low-temperature carbon dioxide in the gas storage tank 1, increasing internal heat recovery. This subsystem specifically includes an electric motor 2, a compressor 3, a first thermal storage heat exchanger 4, a second thermal storage heat exchanger 5, a low-temperature water tank 6, a high-temperature water tank 7, a low-temperature molten salt tank 8, and a high-temperature molten salt tank 9. A third thermal storage heat exchanger 19, a fourth thermal storage heat exchanger 20, a turbine 21, a generator 22, and a regenerator 23.

[0035] In the energy storage process, motor 2 is connected to compressor 3 via a coupling, and the power inlet of motor 2 is connected to the power outlet of the offshore wind power generation device; the outlet of gas storage bladder 1 is connected to the inlet of compressor 3, the outlet of compressor 3 is connected to the high-temperature side inlet of the first thermal heat exchanger 4, the high-temperature side outlet of the first thermal heat exchanger 4 is connected to the high-temperature side inlet of the second thermal heat exchanger 5, and the high-temperature side outlet of the second thermal heat exchanger 5 is connected to the carbon dioxide inlet of the seabed gas-liquid phase change and liquid storage subsystem via a bidirectional carbon dioxide transport pipeline; the outlet of the cryogenic water storage tank 6 is connected to the cryogenic side inlet of the second thermal heat exchanger 5, and the cryogenic side outlet of the second thermal heat exchanger 5 is connected to the inlet of the high-temperature water storage tank 7; the outlet of the cryogenic molten salt storage tank 8 is connected to the cryogenic side inlet of the first thermal heat exchanger 4, and the cryogenic side outlet of the first thermal heat exchanger 4 is connected to the inlet of the high-temperature molten salt storage tank 9. Offshore wind power generation is used to power motor 2 to drive compressor 3, which compresses low-pressure gaseous carbon dioxide in gas storage tank 1; the low-temperature molten salt in low-temperature molten salt storage tank 8 recovers the heat of high-temperature carbon dioxide from compressor 3 outlet, the temperature of low-temperature molten salt rises to become high-temperature molten salt, and is stored in high-temperature molten salt storage tank 9; the low-temperature pressurized water in low-temperature water storage tank 6 further recovers the heat of carbon dioxide from first heat exchanger 4 outlet, the temperature of low-temperature water rises to become high-temperature pressurized water, and is stored in high-temperature water storage tank 7.

[0036] During the energy release process, generator 22 is connected to turbine 21 via a coupling, and the power outlet of generator 22 is connected to the power grid or the user's power inlet; the carbon dioxide outlet of the subsea gas-liquid phase change and liquid storage subsystem is connected to the low-temperature side inlet of regenerator 23 via a bidirectional carbon dioxide transport pipeline, the low-temperature side outlet of regenerator 23 is connected to the low-temperature side inlet of third thermal energy storage heat exchanger 19, the low-temperature side outlet of third thermal energy storage heat exchanger 19 is connected to the low-temperature side inlet of fourth thermal energy storage heat exchanger 20, and the low-temperature side outlet of fourth thermal energy storage heat exchanger 20 is connected to the low-temperature side inlet of fourth thermal energy storage heat exchanger 20. The high-temperature side outlet is connected to the inlet of turbine 21, the outlet of turbine 21 is connected to the high-temperature side inlet of regenerator 23, and the high-temperature side outlet of regenerator 23 is connected to the inlet of gas storage bladder 1; the outlet of the high-temperature water storage tank 7 is connected to the high-temperature side inlet of the third heat exchanger 19, and the high-temperature side outlet of the third heat exchanger 19 is connected to the inlet of the low-temperature water storage tank 6; the outlet of the high-temperature molten salt storage tank 9 is connected to the high-temperature side inlet of the fourth heat exchanger 20, and the high-temperature side outlet of the fourth heat exchanger 20 is connected to the inlet of the low-temperature molten salt storage tank 8. Low-temperature gaseous carbon dioxide from the seabed gas-liquid phase change and storage subsystem first enters the regenerator 23 for preheating, then sequentially enters the third and fourth heat exchangers 19 and 20 for further temperature increases, before entering the turbine 21 for expansion and work, resulting in a pressure reduction. High-temperature pressurized water in the high-temperature storage tank 7 releases heat to the carbon dioxide at the low-temperature outlet of the regenerator 23, causing the high-temperature pressurized water temperature to decrease, becoming low-temperature pressurized water, and is stored in the low-temperature storage tank 6. High-temperature molten salt in the high-temperature molten salt storage tank 9 further heats the carbon dioxide at the outlet of the third heat exchanger 19, causing the high-temperature molten salt temperature to decrease, becoming low-temperature molten salt, and is stored in the low-temperature molten salt storage tank 8. Low-pressure gaseous carbon dioxide at the turbine 21 outlet enters the regenerator 23 for heat exchange, improving internal heat recovery. After its own temperature decreases, it enters the gas storage bladder 1, thus avoiding the need for additional radiators.

[0037] The heat storage temperature of the high-temperature water storage tank 7 and the low-temperature molten salt storage tank 8 is close, between 150°C and 300°C; the molten salt can be solar salt or low-melting-point ternary molten salt, and the ternary molten salt is preferably Hitec molten salt.

[0038] A bidirectional carbon dioxide transport pipeline is used for bidirectional transport of high-pressure gaseous carbon dioxide between the seabed and the coast. It includes a first reversing valve 10, a carbon dioxide pipeline 11, and a second reversing valve 12. The carbon dioxide pipeline 11 is divided into an insulated section and a non-insulated section. The section of the pipeline where the surrounding seawater temperature is below the set carbon dioxide evaporation temperature (15℃-23℃) is insulated with conventional insulation materials (such as polyurethane foam or polyethylene foam). In this embodiment, the section of the pipeline approximately 230 meters below sea level is the insulated section. During energy storage, carbon dioxide from the coastal energy conversion and thermal storage subsystem enters the carbon dioxide pipeline 11 via the first reversing valve 10. It is cooled by seawater in the non-insulated section of the pipeline transported to the seabed, and then enters the seabed gas-liquid phase change and liquid storage subsystem via the second reversing valve 12. During energy release, carbon dioxide from the seabed gas-liquid phase change and liquid storage subsystem enters the carbon dioxide pipeline 11 via the second reversing valve 12. It is heated by seawater in the non-insulated section of the pipeline transported to the coast, and then enters the coastal energy conversion and thermal storage subsystem via the first reversing valve 10.

[0039] Subsea gas-liquid phase change and liquid storage subsystem: The system utilizes the hot seawater at the sea surface and the cold seawater at the seabed to achieve the phase change of carbon dioxide between the gaseous and liquid states, and utilizes the static pressure of seawater and a liquid storage bag made of flexible material to achieve constant pressure storage of liquid carbon dioxide; the subsystem specifically includes a condenser 13, a first seawater pump 14, a liquid storage bag 15, a working fluid pump 16, an evaporator 17, and a second seawater pump 18.

[0040] During the energy storage process, the high-temperature side outlet of the second heat exchanger 5 is connected to the high-temperature side inlet of the condenser 13 through a bidirectional carbon dioxide transport pipeline, and the high-temperature side outlet of the condenser 13 is connected to the inlet of the liquid storage bag 15; the low-temperature side of the condenser 13 is connected to the subsea pipeline through the first seawater pump 14; the high-pressure carbon dioxide from the high-temperature side of the second heat exchanger 5 enters the condenser 13 through the bidirectional carbon dioxide transport pipeline and is condensed into liquid by the cold seawater on the seabed, and then sent into the liquid storage bag 15 for constant pressure storage.

[0041] In the energy release process, the outlet of the liquid storage bag 15 is connected to the low-temperature inlet of the evaporator 17 via a working fluid pump 16. The low-temperature outlet of the evaporator 17 is connected to the low-temperature inlet of the regenerator 23 via a bidirectional carbon dioxide transport pipeline. The high-temperature inlet of the evaporator 17 is connected to a sea surface pipeline via a second seawater pump 18. The liquid carbon dioxide in the liquid storage bag 15 enters the evaporator 17 via the working fluid pump 16, is evaporated into a gaseous state by the hot seawater transported by the second seawater pump 18, and then enters the regenerator 23 for preheating via the bidirectional carbon dioxide transport pipeline. The liquid storage bag 15 is used to store high-pressure liquid carbon dioxide at constant pressure. Its outer shell is made of multi-layer flexible material, which allows its volume to change while maintaining a constant internal pressure. It is moored at a depth of about 450-500 meters on the seabed, where the static pressure of the seawater maintains an internal pressure of about 4.5-5.5 MPa.

[0042] The temperature of the hot seawater at the sea surface is between 25°C and 30°C; the temperature of the cold seawater at the seabed is below 10°C, and the low-temperature side of the condenser 13 draws seawater from a depth deeper than the location of the storage bag 15 through the first seawater pump 14.

[0043] The carbon dioxide energy storage system proposed in this invention, which combines subsea liquid storage with high-temperature thermal storage, utilizes cold seawater from the seabed to condense carbon dioxide, achieving a lower condensation temperature than in terrestrial environments and reducing the system's high pressure. Simultaneously, by leveraging the pressure difference of carbon dioxide in the vertical direction, the outlet pressure of compressor 3 is further reduced, thereby lowering the compressor exhaust temperature when using single-stage compression to below 450°C, keeping the compressor within a feasible temperature range. This also facilitates the use of low-melting-point molten salts, preventing their high-temperature decomposition. Furthermore, the carbon dioxide energy storage system of this invention utilizes cold seawater from the seabed to achieve liquid storage of high-pressure carbon dioxide. Liquefaction increases the density of carbon dioxide, reducing the density difference with seawater, thus weakening buoyancy and improving the reliability of the storage bag 15 when moored on the seabed.

[0044] This invention also provides an operation method for a carbon dioxide energy storage system that combines subsea liquid storage with high-temperature thermal storage, including an energy storage process and an energy release process;

[0045] During the energy storage process, the low-pressure gaseous carbon dioxide stored in the gas storage bladder 1 is compressed by a single-stage high-pressure compressor 3 to generate higher heat quality. The generated high-temperature heat energy is recovered by molten salt heat storage and pressurized water heat storage in sequence. After being cooled, the carbon dioxide is transported to the condenser 13 through a bidirectional carbon dioxide transport pipeline. During the transport process, the carbon dioxide is cooled by seawater and its temperature decreases. As the depth increases, the pressure further increases. It is completely condensed into liquid by the cold seawater on the seabed and then stored at constant pressure in the liquid storage bag 15.

[0046] The molten salt thermal storage utilizes the low-temperature molten salt in the low-temperature molten salt storage tank 8 to recover the heat from the high-temperature carbon dioxide at the outlet of the compressor 3. The temperature of the low-temperature molten salt rises, becoming high-temperature molten salt, which is then stored in the high-temperature molten salt storage tank 9. The pressurized water thermal storage utilizes the low-temperature pressurized water in the low-temperature water storage tank 6 to further recover the heat from the carbon dioxide at the outlet of the first thermal exchanger 4. The temperature of the low-temperature pressurized water rises, becoming high-temperature pressurized water, which is then stored in the high-temperature water storage tank 7.

[0047] During the energy release process, hot seawater from the sea surface is first piped into the evaporator 17 on the seabed. Liquid carbon dioxide stored in the storage bag 15 is pressurized by the working fluid pump 16 and transported to the evaporator 17 to be evaporated into gas. Then, it is transported to the coast through a bidirectional carbon dioxide transport pipeline. During the transport process, the pressure of carbon dioxide decreases as the depth decreases. It is preheated by seawater in the pipeline near the sea surface and then enters the regenerator 23 to absorb the waste heat of the exhaust gas from the turbine 21 outlet. Then, pressurized water heat storage and molten salt heat storage are used in sequence to heat the preheated high-pressure carbon dioxide to a high temperature state. It then enters the turbine 21 to expand and do work to generate electricity. The carbon dioxide at the turbine 21 outlet is cooled after reheating and enters the floating gas storage bag 1 near the sea surface for storage. During storage, it is cooled to room temperature by seawater and air.

[0048] The pressurized water thermal storage utilizes the high-temperature pressurized water in the high-temperature storage tank 7 to release heat to the carbon dioxide at the low-temperature side outlet of the regenerator 23. The temperature of the high-temperature pressurized water then decreases, becoming low-temperature pressurized water, which is then stored in the low-temperature storage tank 6. The molten salt thermal storage utilizes the high-temperature molten salt in the high-temperature molten salt storage tank 9 to further heat the carbon dioxide at the outlet of the third thermal storage heat exchanger 19. The temperature of the high-temperature molten salt then decreases, becoming low-temperature molten salt, which is then stored in the low-temperature molten salt storage tank 8.

[0049] Example 2

[0050] like Figure 2 As shown, in addition to the system structure of Embodiment 1, the thermal storage device of the coastal energy conversion and thermal storage subsystem also includes an intermediate water tank 24 and an intermediate molten salt tank 25; by adjusting the structure of the thermal storage heat exchanger, the thermal storage medium side (pressurized water and molten salt) has an intermediate inlet or outlet.

[0051] For the pressurized water thermal storage loop, the inlet of the intermediate water tank 24 is connected to the intermediate outlet on the low-temperature side of the second thermal storage heat exchanger 5, and the outlet of the intermediate water tank 24 is connected to the intermediate inlet on the high-temperature side of the third thermal storage heat exchanger 20. During the energy storage process, the pressurized water in the low-temperature water tank 6 enters the second thermal storage heat exchanger 5 and is heated. When heated to the intermediate temperature, a portion of the pressurized water flows out from the intermediate outlet and enters the intermediate water tank 24 for storage, while the remaining pressurized water is further heated to a higher temperature and then enters the high-temperature water tank 7 for storage. During the energy release process, the pressurized water in the high-temperature water tank 7 is cooled by carbon dioxide to the same temperature as the intermediate water tank 24. After the two are mixed, they are further cooled by carbon dioxide and then enter the low-temperature water tank 6 for storage.

[0052] For molten salt thermal storage loops, the principle of setting up the intermediate molten salt tank 25 is the same as that of pressurized water thermal storage loops. However, the flow direction of the molten salt in the intermediate molten salt tank 25 needs to be determined according to the selected type of molten salt. Since the specific heat of carbon dioxide increases slightly in the high-temperature region, when the specific heat of the molten salt decreases with increasing temperature, not changing the flow rate will lead to temperature mismatch in the heat exchange process. In this embodiment, the flow direction of the molten salt in the intermediate molten salt tank 25 is given when using a typical low-melting-point ternary salt, Hitec. During the energy storage process, a portion of the molten salt enters the low-temperature side of the first thermal storage heat exchanger 4 from the intermediate molten salt tank 25. During the energy release process, a portion of the molten salt is drawn from the high-temperature side of the fourth thermal storage heat exchanger 20 and enters the intermediate molten salt tank 25. The decrease in specific heat is balanced by increasing the flow rate of the Hitec salt in the high-temperature region.

[0053] Since the specific heat capacities of carbon dioxide, pressurized water, and molten salt change differently with temperature, large temperature difference heat exchange can cause significant heat exchange mismatch, which is not conducive to efficient heat recovery of the system. This invention adds an intermediate heat storage tank to the pressurized water heat storage circuit and the molten salt heat storage circuit respectively, which can split the water and molten salt during the energy storage and release process. By controlling the temperature and flow rate of the heat storage medium (water and molten salt) at the split state point, the specific heat capacities of carbon dioxide and heat storage medium change with temperature, adjust the temperature curves of the heat storage and release process, improve the heat exchange temperature matching, enhance the thermal management level, and thus further increase the inlet temperature of turbine 21 and improve the round-trip efficiency of the entire system.

Claims

1. A carbon dioxide energy storage system combining subsea liquid storage and high-temperature thermal storage, characterized in that, include: Coastal energy conversion and thermal storage subsystem, bidirectional carbon dioxide transport pipeline, subsea gas-liquid phase change and liquid storage subsystem, and nearshore floating gas storage bladder for storing low-pressure gaseous carbon dioxide (1). The coastal energy conversion and thermal storage subsystem includes a compressor (3), a first thermal storage heat exchanger (4), a second thermal storage heat exchanger (5), a low-temperature water storage tank (6), a high-temperature water storage tank (7), a low-temperature molten salt storage tank (8), a high-temperature molten salt storage tank (9), a third thermal storage heat exchanger (19), a fourth thermal storage heat exchanger (20), and a turbine (21). During energy storage, the outlet of the gas storage bladder (1) is sequentially connected to the compressor (3), the high-temperature side of the first thermal storage heat exchanger (4), and the high-temperature side of the second thermal storage heat exchanger (5). The outlet of the low-temperature water storage tank (6) is connected to the inlet of the high-temperature water storage tank (7) through the low-temperature side of the second thermal storage heat exchanger (5). The low-temperature molten salt storage tank (8) The outlet is connected to the inlet of the high-temperature molten salt storage tank (9) via the low-temperature side of the first thermal heat exchanger (4); during energy release, the carbon dioxide outlet of the seabed gas-liquid phase change and liquid storage subsystem is connected in series to the low-temperature side of the third thermal heat exchanger (19) and the low-temperature side of the fourth thermal heat exchanger (20) via a bidirectional carbon dioxide transport pipeline, and the outlet of the low-temperature side of the fourth thermal heat exchanger (20) is connected to the inlet of the turbine (21); the outlet of the high-temperature water storage tank (7) is connected to the inlet of the low-temperature water storage tank (6) via the high-temperature side of the third thermal heat exchanger (19); the outlet of the high-temperature molten salt storage tank (9) is connected to the inlet of the low-temperature molten salt storage tank (8) via the high-temperature side of the fourth thermal heat exchanger (20). The coastal energy conversion and thermal storage subsystem also includes an intermediate water tank (24) and an intermediate molten salt tank (25); the inlet of the intermediate water tank (24) is connected to the intermediate outlet on the low-temperature side of the second thermal heat exchanger (5), and the outlet of the intermediate water tank (24) is connected to the intermediate inlet on the high-temperature side of the third thermal heat exchanger (19); the intermediate molten salt tank (25) is located between the intermediate pipe on the low-temperature side of the first thermal heat exchanger (4) and the intermediate pipe on the high-temperature side of the fourth thermal heat exchanger (20), and the flow direction of the molten salt in the intermediate molten salt tank (25) needs to be determined according to the type of molten salt selected; The subsea gas-liquid phase change and liquid storage subsystem includes a condenser (13), a first seawater pump (14), a liquid storage bag (15), a working fluid pump (16), an evaporator (17), and a second seawater pump (18). During energy storage, the high-temperature outlet of the second heat exchanger (5) is connected to the high-temperature inlet of the condenser (13) via a bidirectional carbon dioxide transport pipeline, and the high-temperature outlet of the condenser (13) is connected to the inlet of the liquid storage bag (15). The low-temperature inlet of the condenser (13) is connected to the subsea pipeline via the first seawater pump (14). During energy release, the outlet of the liquid storage bag (15) is connected to the low-temperature inlet of the evaporator (17) via the working fluid pump (16), and the low-temperature outlet of the evaporator (17) is connected to the low-temperature inlet of the third heat exchanger (19) via a bidirectional carbon dioxide transport pipeline. The high-temperature inlet of the evaporator (17) is connected to the surface pipeline via the second seawater pump (18).

2. The carbon dioxide energy storage system combining subsea liquid storage and high-temperature thermal storage according to claim 1, characterized in that: The compressor (3) is connected to the motor (2) via a coupling, and the power inlet of the motor (2) is connected to the power outlet of the offshore wind power generation device; the turbine (21) is connected to the generator (22) via a coupling, and the power outlet of the generator (22) is connected to the power grid or the power inlet of the user.

3. The carbon dioxide energy storage system combining subsea liquid storage and high-temperature thermal storage according to claim 1, characterized in that: The coastal energy conversion and thermal storage subsystem also includes a regenerator (23). The low-temperature outlet of the evaporator (17) is connected to the low-temperature inlet of the regenerator (23) through a bidirectional carbon dioxide transport pipeline. The low-temperature outlet of the regenerator (23) is connected to the low-temperature inlet of the third thermal storage heat exchanger (19). The turbine (21) outlet is connected to the inlet of the gas storage bladder (1) through the high-temperature side of the regenerator (23).

4. The carbon dioxide energy storage system combining subsea liquid storage and high-temperature thermal storage according to claim 1, characterized in that: The heat storage temperature of the high-temperature water storage tank (7) is close to that of the low-temperature molten salt storage tank (8), which is between 150°C and 300°C.

5. The carbon dioxide energy storage system combining subsea liquid storage and high-temperature thermal storage according to claim 1, characterized in that: The bidirectional carbon dioxide transport pipeline includes a first reversing valve (10), a carbon dioxide pipeline (11), and a second reversing valve (12); the carbon dioxide pipeline (11) is divided into an insulated section and a non-insulated section, and the insulated section is the part of the pipeline where the surrounding seawater temperature is lower than the set carbon dioxide evaporation temperature.

6. The carbon dioxide energy storage system combining subsea liquid storage and high-temperature thermal storage according to claim 1, characterized in that: The storage bag (15) is set on the seabed for storing high-pressure liquid carbon dioxide under constant pressure.

7. The carbon dioxide energy storage system combining subsea liquid storage and high-temperature thermal storage according to claim 1, characterized in that: The depth of the subsea pipeline on the seabed is greater than the depth of the liquid storage bag (15) on the seabed.

8. The operation method of the carbon dioxide energy storage system combining subsea liquid storage and high-temperature thermal storage as described in any one of claims 1-7, characterized in that, include: During energy storage, the low-pressure gaseous carbon dioxide in the gas storage bag (1) is compressed in a single stage by the compressor (3), and the heat generated by compression is recovered by molten salt heat storage and pressurized water heat storage in turn; while the carbon dioxide itself is cooled and transported to the condenser (13) through the carbon dioxide bidirectional transport pipeline, and is completely condensed into liquid by the cold seawater on the seabed, and then stored in the liquid storage bag (15) under constant pressure. During energy release, the liquid carbon dioxide stored in the storage bag (15) is pressurized by the working fluid pump (16) and transported to the evaporator (17). The hot seawater is evaporated into gaseous state and transported to the regenerator (23) through the carbon dioxide bidirectional transport pipeline to absorb the waste heat of the exhaust gas at the turbine (21) outlet. Then, the preheated high-pressure carbon dioxide is heated to a high temperature state by pressurized water heat storage and molten salt heat storage in sequence, and enters the turbine (21) to expand and do work to generate electricity. The carbon dioxide at the turbine (21) outlet is cooled after being reheated by the regenerator (23) and then stored in the gas storage bag (1).

9. The operation method of the carbon dioxide energy storage system combining subsea liquid storage and high-temperature thermal storage according to claim 8, characterized in that, During energy storage, the molten salt thermal storage utilizes the low-temperature molten salt in the low-temperature molten salt storage tank (8) to recover the heat of the high-temperature carbon dioxide at the outlet of the compressor (3). The temperature of the low-temperature molten salt rises, becoming high-temperature molten salt and stored in the high-temperature molten salt storage tank (9). The pressurized water thermal storage utilizes the low-temperature pressurized water in the low-temperature water storage tank (6) to further recover the heat of the carbon dioxide at the outlet of the first thermal heat exchanger (4). The temperature of the low-temperature pressurized water rises, becoming high-temperature pressurized water and stored in the high-temperature water storage tank (7). When releasing energy, the pressurized water heat storage is achieved by using the high-temperature pressurized water in the high-temperature water storage tank (7) to release heat to the carbon dioxide at the low-temperature side outlet of the regenerator (23). The temperature of the high-temperature pressurized water then decreases, becoming low-temperature pressurized water and stored in the low-temperature water storage tank (6). The molten salt heat storage is achieved by using the high-temperature molten salt in the high-temperature molten salt storage tank (9) to further heat the carbon dioxide at the outlet of the third heat exchanger (19). The temperature of the high-temperature molten salt then decreases, becoming low-temperature molten salt and stored in the low-temperature molten salt storage tank (8).

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