Offshore constant-pressure compressed carbon dioxide energy storage system coupled with ocean thermal gradient utilization, and method

Abstract

Disclosed in the present invention are an offshore constant-pressure compressed carbon dioxide energy storage system coupled with ocean thermal gradient utilization, and a method. The system comprises a working fluid constant-pressure storage device, a carbon dioxide energy storage loop, a carbon dioxide energy release loop, and a heat storage loop. In the present invention, in an energy storage stage, gaseous carbon dioxide is condensed into liquid carbon dioxide by means of deep-sea low-temperature seawater, and in an energy release stage, liquid carbon dioxide is evaporated into gaseous carbon dioxide by means of shallow-sea high-temperature seawater, thereby achieving comprehensive utilization of offshore renewable energy and ocean thermal gradient energy; by means of the hydrostatic pressure and buoyancy of seawater, and by using a flexible working fluid constant-pressure storage device to store low-pressure gaseous and high-pressure liquid carbon dioxide, a constant pressure of the working fluid in the working fluid constant-pressure storage device during charging and discharging processes can be ensured, thereby achieving efficient and stable operation of the system; and by further improving the energy storage and release system structure, the electrical-to-electrical round-trip efficiency or the comprehensive energy utilization rate of the system is improved. By synergistically utilizing the ocean thermal gradient and seawater hydrostatic pressure, the present invention achieves efficient gas-liquid phase transition and high-density storage of high-pressure carbon dioxide.

Description

Offshore constant-pressure compressed carbon dioxide energy storage system and method coupled with ocean temperature difference Technical Field

[0001] This invention relates to the field of thermomechanical energy storage technology, and in particular to a marine constant-pressure compressed carbon dioxide energy storage system and method coupled with ocean temperature differences. Background Technology

[0002] Due to the intermittent and fluctuating nature of wind and solar power, large-scale grid connection weakens grid resilience, and it is difficult to synchronize user-side energy consumption with renewable energy generation. Therefore, large-scale, long-term energy storage is of great significance for the construction of new power systems. With the development of solar and wind power, onshore solar and wind power are limited by land resources, while the ocean covers 71% of the Earth's surface. Therefore, offshore wind and solar power have emerged, demonstrating broad development prospects by fully utilizing ocean space and high-quality resources. As offshore wind and solar power develop towards larger scales and deeper waters, the problem of renewable energy consumption becomes more challenging. Transmitting deep-sea power generation to nearby onshore grids via cables has limitations; therefore, exploring on-site consumption of offshore wind and solar power is becoming increasingly important in the current context. To smooth the output of offshore wind and solar power and improve the economics of transmission, a certain capacity of energy storage system is needed. Therefore, it is urgent to develop new offshore energy storage technologies for on-site consumption of deep-sea wind and solar power.

[0003] Compressed gas energy storage, including compressed air and carbon dioxide energy storage, is a thermomechanical energy storage technology. Its principle is to use off-peak electricity to drive a compressor to compress gas and store it in a gas storage tank, converting electrical energy into the gas's internal energy. During peak electricity demand periods, the high-pressure gas is released, heated, and then used to drive a generator to produce electricity. Carbon dioxide has received increasing attention due to its excellent thermodynamic properties and environmental friendliness. Compared to air, carbon dioxide has higher density and thermal conductivity in the near-critical region, allowing it to store more energy in a smaller volume. Therefore, compressed carbon dioxide energy storage systems have significant advantages in improving energy conversion efficiency and volumetric energy density. Furthermore, compressed carbon dioxide energy storage can sequester large amounts of carbon-captured and enriched carbon dioxide, making it a typical carbon dioxide utilization technology. Depending on the working fluid storage state, compressed carbon dioxide energy storage can be divided into different types, such as high-pressure liquid storage / low-pressure gas storage and high-pressure liquid storage / low-pressure liquid storage. Among these, the subcritical gas-liquid storage scheme of high-pressure liquid storage / low-pressure gas storage has become the mainstream technology due to its high round-trip efficiency, low investment cost, and high technological maturity. Chinese invention patent CN112985144B proposes a compressed carbon dioxide energy storage device and method based on gas-liquid phase change. It utilizes a throttling valve and a water tank to achieve the gas-liquid conversion of carbon dioxide, and a constant-volume storage tank to store high-pressure liquid carbon dioxide. However, when the ambient temperature is too high to liquefy carbon dioxide near its critical point, an additional refrigeration unit is required to provide the necessary cooling energy, leading to reduced system efficiency and increased costs. Furthermore, when using a constant-volume storage tank for high-pressure carbon dioxide storage, the tank volume may not be fully utilized (utilization rate approximately 50%), or changes in storage pressure may cause the turbine and compressor to operate under non-design conditions, resulting in reduced energy density and round-trip efficiency. Summary of the Invention

[0004] To overcome the difficulties in liquefying high-pressure carbon dioxide and the inefficiency of maintaining constant pressure in existing technologies, this invention aims to provide a marine constant-pressure compressed carbon dioxide energy storage system and method coupled with ocean temperature differences. In the energy storage phase, the system utilizes deep-sea low-temperature seawater to condense gaseous carbon dioxide into liquid, and in the energy release phase, it utilizes shallow-sea high-temperature seawater to evaporate liquid carbon dioxide into gas, fully leveraging marine renewable energy and ocean temperature differences to achieve the gas-liquid conversion of carbon dioxide. By utilizing the hydrostatic pressure and buoyancy of seawater and employing a flexible, constant-pressure working fluid storage device to store low-pressure gaseous and high-pressure liquid carbon dioxide, the system can maintain a constant pressure of the internal working fluid during charging and discharging, achieving efficient and stable operation and high energy density. By constructing the compressed carbon dioxide energy storage system on-site at the marine renewable energy power generation side, the system fully utilizes the marine environment and marine energy, realizing the on-site utilization and in-depth development of marine resources.

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

[0006] A marine constant-pressure compressed carbon dioxide energy storage system coupled with ocean temperature difference includes: a working fluid constant-pressure storage device, a carbon dioxide energy storage loop, a carbon dioxide energy release loop, and a thermal storage loop.

[0007] Working fluid constant pressure storage device: including a low-pressure gas storage bladder for constant pressure storage of low-pressure gaseous carbon dioxide and a high-pressure liquid storage bladder for constant pressure storage of high-pressure liquid carbon dioxide;

[0008] Carbon dioxide energy storage circulation loop: The outlet of the low-pressure gas storage bladder is connected to the inlet of the high-pressure liquid storage bladder through the carbon dioxide energy storage circulation loop. The power input shaft of the carbon dioxide energy storage circulation loop is connected to the power output shaft of the motor. The power inlet of the motor is connected to the power outlet of the offshore new energy power generation. The offshore new energy power generation supplies power to the motor, which drives the compressor to compress the low-pressure gaseous carbon dioxide to a high-pressure state. Then, the high-pressure carbon dioxide is condensed into liquid using deep-sea low-temperature seawater and stored in the high-pressure liquid storage bladder.

[0009] Carbon dioxide energy release circulation loop: The inlet of the low-pressure gas storage bladder is connected to the outlet of the high-pressure liquid storage bladder through the carbon dioxide energy release circulation loop. The power output shaft of the carbon dioxide energy release circulation loop is connected to the power input shaft of the generator. The power outlet of the generator is connected to the power inlet of the energy consumption side. High-temperature seawater from shallow seas is used to evaporate high-pressure liquid carbon dioxide into gaseous state. The expansion of high-pressure gaseous carbon dioxide is used to do work, and the mechanical energy generated is converted into electrical energy and stably delivered to the energy consumption side. At the same time, the expanded low-pressure gaseous carbon dioxide is stored in the low-pressure gas storage bladder.

[0010] Thermal storage loop: During the energy storage process, the carbon dioxide energy storage loop is connected to the thermal storage loop through a cooler. The stored low-temperature thermal storage medium absorbs the heat of compression generated during the carbon dioxide energy storage loop and is transformed into a high-temperature thermal storage medium for storage. During the energy release process, the carbon dioxide energy release loop is connected to the thermal storage loop through a heater. The stored high-temperature thermal storage medium heats the gaseous carbon dioxide in the carbon dioxide energy release loop and is transformed into a low-temperature thermal storage medium for storage.

[0011] Both the low-pressure air reservoir and the high-pressure liquid reservoir are made of flexible materials. The low-pressure air reservoir floats on the sea surface, with its internal pressure remaining constant at atmospheric pressure. The high-pressure liquid reservoir is moored on the seabed at a depth of 450 to 600 meters, utilizing the hydrostatic pressure of the seawater to maintain its internal pressure at a constant level of approximately 4.5 to 6 MPa. Even better results are achieved at depths of 500-550 meters, maintaining an internal pressure of 5 to 5.5 MPa.

[0012] The thermal storage circulation loop includes: a cold water storage tank, a hot water storage tank, a first cooler, a second cooler, a first heater, and a second heater. The outlet of the cold water storage tank is connected to the low-temperature inlet of both the first and second coolers, and the low-temperature outlets of both the first and second coolers are connected to the inlet of the hot water storage tank. The outlet of the hot water storage tank is connected to the high-temperature inlet of both the first and second heaters, and the high-temperature outlets of both the first and second heaters are connected to the inlet of the cold water storage tank. The cold water storage tank is used to store the low-temperature thermal storage medium, and the hot water storage tank is used to store the high-temperature thermal storage medium. The thermal storage medium in both the cold water and hot water storage tanks is pressurized water or heat transfer oil.

[0013] The carbon dioxide energy storage cycle includes: a low-pressure compressor, a high-pressure compressor, and a condenser; the power input shafts of both the low-pressure and high-pressure compressors are connected to the power output shaft of the electric motor, the power inlet of the electric motor is connected to the power outlet of the offshore new energy power generation, and the offshore new energy power generation supplies power to the electric motor to drive the two compressors; the outlet of the low-pressure gas storage bladder is connected to the inlet of the low-pressure compressor, the outlet of the low-pressure compressor is connected to the high-temperature side inlet of the first cooler, the high-temperature side outlet of the first cooler is connected to the inlet of the high-pressure compressor, the outlet of the high-pressure compressor is connected to the high-temperature side inlet of the second cooler, the high-temperature side outlet of the second cooler is connected to the high-temperature side inlet of the condenser, and the high-temperature side outlet of the condenser is connected to the inlet of the high-pressure liquid storage bladder; the low-pressure gaseous carbon dioxide is compressed in two stages by the low-pressure and high-pressure compressors to a high-pressure state, and the heat of compression is stored in the heat storage cycle. After being condensed into liquid by the low-temperature seawater in the deep sea through the condenser, the liquid carbon dioxide is transported to the high-pressure liquid storage bladder for constant pressure storage.

[0014] The deep-sea low-temperature seawater used in the carbon dioxide energy storage cycle is transported to the low-temperature side inlet of the condenser through a pipeline, and the temperature of the deep-sea low-temperature seawater is below 15°C, and more preferably below 10°C.

[0015] The carbon dioxide energy release cycle includes a working fluid pump, an evaporator, a high-pressure turbine, a low-pressure turbine, and a heat exchanger. The outlet of the high-pressure storage tank is connected to the inlet of the working fluid pump, the outlet of the working fluid pump is connected to the low-temperature side inlet of the evaporator, the low-temperature side outlet of the evaporator is connected to the low-temperature side inlet of the first heater, the low-temperature side outlet of the first heater is connected to the inlet of the high-pressure turbine, the outlet of the high-pressure turbine is connected to the low-temperature side inlet of the second heater, the low-temperature side outlet of the second heater is connected to the inlet of the low-pressure turbine, the outlet of the low-pressure turbine is connected to the inlet of the heat exchanger, and the outlet of the heat exchanger is connected to the inlet of the low-pressure storage tank. The power output shafts of both the high-pressure and low-pressure turbines are connected to the power input shaft of the generator, and the power outlet of the generator is connected to the power inlet on the energy consumption side. The stored liquid carbon dioxide absorbs heat from the high-temperature shallow seawater in the evaporator and evaporates into a gaseous state. After being heated by the first and second heaters, the high-pressure carbon dioxide is expanded to a low-pressure state by the high-pressure and low-pressure turbines. The low-pressure gaseous carbon dioxide is cooled by the heat exchanger and then transported to the low-pressure storage tank for constant-pressure storage.

[0016] The shallow-sea high-temperature seawater used in the carbon dioxide energy release cycle is transported to the high-temperature side inlet of the evaporator through a pipeline, and the temperature of the shallow-sea high-temperature seawater is above 20°C, and more preferably above 25°C.

[0017] This invention also provides an operation method for a marine constant-pressure compressed carbon dioxide energy storage system coupled with ocean temperature differences, including the energy storage process and the energy release process:

[0018] During the energy storage process, the low-pressure gaseous carbon dioxide stored in the low-pressure gas storage bladder undergoes two-stage compression to a high-pressure state. The heat energy generated by the compression of carbon dioxide is absorbed by the low-temperature heat storage medium in the cold water storage tank, and the low-temperature heat storage medium is transformed into a high-temperature heat storage medium and stored in the hot water storage tank. The low-temperature high-pressure carbon dioxide after releasing heat exchanges with the low-temperature seawater in the deep sea through the condenser, causing the low-temperature high-pressure carbon dioxide to condense into liquid and be stored in the high-pressure liquid storage bladder.

[0019] During the energy release process, the high-pressure liquid carbon dioxide stored in the high-pressure liquid storage bladder is transported to the evaporator by the working fluid pump to absorb the heat of the high-temperature seawater in the shallow sea and evaporate into a gaseous state. It then rises to the sea level and further absorbs the heat energy of the high-temperature heat storage medium in the hot water storage tank. It then generates electricity through two-stage expansion. The low-pressure gaseous carbon dioxide after releasing heat is stored in the low-pressure gas storage bladder. The high-temperature heat storage medium after absorbing heat energy is transformed into a low-temperature heat storage medium and stored in the cold water storage tank.

[0020] During the energy storage process, a portion of the low-temperature thermal storage medium flowing out of the cold water storage tank exchanges heat through the first cooler, absorbing the heat of compression of the carbon dioxide after the first compression; another portion of the low-temperature thermal storage medium flowing out of the cold water storage tank exchanges heat through the second cooler, absorbing the heat of compression of the carbon dioxide after the second compression; after absorbing heat, the temperature of the two portions of low-temperature thermal storage medium rises, and they mix and flow into the hot water storage tank for storage.

[0021] During the energy release process, a portion of the high-temperature thermal storage medium flowing out of the hot water storage tank exchanges heat with the carbon dioxide flowing through the first heater; another portion of the high-temperature thermal storage medium flowing out of the hot water storage tank exchanges heat with the carbon dioxide flowing through the second heater. After releasing heat, the temperature of these two portions of high-temperature thermal storage medium decreases, and they mix and flow into the cold water storage tank for storage.

[0022] Compared with existing technologies, the advantages of this invention are as follows:

[0023] 1. The marine constant-pressure compressed carbon dioxide energy storage system coupled with ocean temperature difference proposed in this invention utilizes the low temperature of deep-sea seawater to condense and liquefy gaseous carbon dioxide during the energy storage process, and utilizes the relatively high temperature of shallow-sea seawater to evaporate and vaporize liquid carbon dioxide during the energy release process. It makes full use of ocean temperature difference resources, realizes efficient carbon dioxide gas-liquid phase conversion, and overcomes the disadvantage of traditional compressed carbon dioxide energy storage systems that require an additional refrigeration unit.

[0024] 2. The offshore constant-pressure compressed carbon dioxide energy storage system coupled with ocean temperature difference proposed in this invention utilizes underwater hydrostatic pressure to provide a stable external pressure for the high-pressure liquid storage bladder, achieving constant pressure in the bladder and ensuring constant pressure between the compressor outlet and turbine inlet. This eliminates the need for sliding pressure operation, allowing the entire unit to operate under stable conditions, while fully utilizing the space in the high-pressure liquid storage bladder. Compared to traditional compressed carbon dioxide energy storage systems, this system improves round-trip efficiency and operational stability, while reducing the volume of the liquid storage device and investment costs.

[0025] 3. The offshore constant-pressure compressed carbon dioxide energy storage system coupled with ocean temperature difference proposed in this invention reduces the maximum pressure of the entire system to the range of 4.5 to 6 MPa by utilizing the seawater temperature difference, thereby improving economic efficiency; it utilizes seawater buoyancy to allow the low-pressure gas storage bladder to float on the sea surface, without occupying the space of the offshore platform, thus increasing the volumetric energy density; by utilizing ocean temperature difference to achieve gas-liquid conversion of carbon dioxide, the energy storage and release structure is improved to enhance the system's electric-electric round-trip efficiency or comprehensive energy utilization rate, making the system independent of other low-grade waste heat, thus becoming an independent energy storage system for deep-sea offshore wind power and photovoltaic power.

[0026] In summary, this invention addresses the on-site utilization of new energy sources such as wind and solar power in deep-sea areas, integrating ocean thermal energy resources and overcoming the technical limitations of traditional compressed carbon dioxide energy storage. By utilizing ocean thermal differences and seawater hydrostatic pressure, it achieves efficient liquefaction and high-density storage of high-pressure carbon dioxide, improving the efficiency and energy density of the energy storage system and providing a feasible technical path for the development and utilization of new energy sources in deep-sea areas. Attached Figure Description

[0027] Figure 1 is a schematic diagram of the structure of the marine constant-pressure compressed carbon dioxide system coupled with ocean temperature difference in Example 1.

[0028] Figure 2 is a schematic diagram of the marine constant-pressure compressed carbon dioxide system coupled with ocean temperature difference in Example 2.

[0029] Figure 3 is a schematic diagram of the marine constant-pressure compressed carbon dioxide system coupled with ocean temperature difference in Example 3.

[0030] Figure 4 is a schematic diagram of the marine constant-pressure compressed carbon dioxide system coupled with ocean temperature difference in Example 4.

[0031] Figure 5 is a typical schematic diagram of how seawater temperature changes with seawater depth.

[0032] Explanation of reference numerals in the attached diagram: 1 - Low-pressure gas storage tank, 2 - Electric motor, 3 - Low-pressure compressor, 4 - First cooler, 5 - High-pressure compressor, 6 - Second cooler, 7 - Cold water storage tank, 8 - Hot water storage tank, 9 - Condenser, 10 - High-pressure liquid storage tank, 11 - Working fluid pump, 12 - Evaporator, 13 - First heater, 14 - High-pressure turbine, 15 - Second heater, 16 - Low-pressure turbine, 17 - Generator, 18 - Heat exchanger, 19 - Cold seawater pump, 20 - Warm seawater pump, 21 - Cold salt storage tank, 22 - Hot salt storage tank. Detailed Implementation

[0033] 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. Example

[0034] Referring to Figure 1, a marine constant-pressure compressed carbon dioxide energy storage system coupled with ocean temperature difference is shown. The system includes: a constant-pressure working fluid storage device, a carbon dioxide energy storage circulation loop, a carbon dioxide energy release circulation loop, and a heat storage circulation loop.

[0035] A constant-pressure working fluid storage device includes a low-pressure gas storage bladder 1 and a high-pressure liquid storage bladder 10, used for constant-pressure storage of low-pressure gaseous carbon dioxide and high-pressure liquid carbon dioxide. The low-pressure gas storage bladder 1 stores low-pressure gaseous carbon dioxide at constant pressure, and the high-pressure liquid storage bladder 10 stores high-pressure liquid carbon dioxide at constant pressure. The outer shell of the high-pressure liquid storage bladder 10 is made of a flexible material, allowing its volume to change while maintaining a constant internal pressure. The high-pressure liquid storage bladder 10 is moored at a depth of approximately 450 to 600 meters on the seabed, utilizing the hydrostatic pressure of seawater to maintain a constant internal pressure of approximately 4.5 to 6 MPa. More preferably, at a depth of 500-550 meters, the pressure is 5-5.5 MPa. The inner layer of the low-pressure gas storage bladder 1 is also made of a flexible material, allowing its volume to change while maintaining a constant internal pressure. The interior of the low-pressure gas storage bladder 1 is at atmospheric pressure, maintaining a constant low pressure using atmospheric pressure, while simultaneously floating on the sea surface using seawater buoyancy, without occupying space on an artificial floating platform.

[0036] The thermal energy storage loop includes a hot water storage tank 8, a cold water storage tank 7, a first cooler 4, a second cooler 6, a first heater 13, and a second heater 15. The outlet of the cold water storage tank 7 is connected to the low-temperature side inlets of the first cooler 4 and the second cooler 6, respectively, and the low-temperature side outlets of the first cooler 4 and the second cooler 6 are both connected to the inlet of the hot water storage tank 8. The outlet of the hot water storage tank 8 is connected to the high-temperature side inlets of the first heater 13 and the second heater 15, respectively, and the high-temperature side outlets of the first heater 13 and the second heater 15 are both connected to the inlet of the cold water storage tank 7. The cold water storage tank 7 is used to store the low-temperature thermal energy storage medium, and the hot water storage tank 8 is used to store the high-temperature thermal energy storage medium. During the energy storage process, the low-temperature thermal energy storage medium in the cold water storage tank 7 absorbs the compression heat generated during the operation of the carbon dioxide thermal energy storage loop, transforming it into a high-temperature thermal energy storage medium, which is stored in the hot water storage tank 8. During the energy release process, the high-temperature thermal energy storage medium in the hot water storage tank 8 heats the gaseous carbon dioxide evaporated by the evaporator 12, transforming it into a low-temperature thermal energy storage medium, which is stored in the cold water storage tank 7. The thermal energy storage medium in the thermal energy storage loop is pressurized water.

[0037] Carbon dioxide energy storage loop: Low-pressure gaseous carbon dioxide is compressed into a high-pressure state using new energy sources such as offshore wind power, and then condensed into a liquid state using deep-sea low-temperature seawater; including a low-pressure compressor 3, a high-pressure compressor 5, and a condenser 9; the power input shafts of both the low-pressure compressor 3 and the high-pressure compressor 5 are connected to the power output shaft of the motor 2, the power inlet of the motor 2 is connected to the power outlet of the new energy source such as offshore wind power, and the new energy source such as offshore wind power supplies power to the motor 2 to drive the two compressors; the outlet of the low-pressure gas storage bladder 1 is connected to the inlet of the low-pressure compressor 3, the outlet of the low-pressure compressor 3 is connected to the high-temperature side inlet of the first cooler 4, the high-temperature side outlet of the first cooler 4 is connected to the inlet of the high-pressure compressor 5, the outlet of the high-pressure compressor 5 is connected to the high-temperature side inlet of the second cooler 6, the outlet of the second cooler 6 is connected to the high-temperature side inlet of the condenser 9, and the high-temperature side outlet of the condenser 9 is connected to the inlet of the high-pressure liquid storage bladder 10. Low-pressure gaseous carbon dioxide is compressed in two stages by low-pressure compressor 3 and high-pressure compressor 5 to a high-pressure state. The heat generated by compression is stored in the heat storage circulation loop and then condensed into liquid by deep-sea low-temperature seawater through condenser 9. The high-pressure liquid carbon dioxide is then transported to the high-pressure liquid storage bladder 10 for constant-pressure storage.

[0038] Carbon dioxide energy release cycle: High-pressure liquid carbon dioxide is evaporated into gaseous state using high-temperature shallow seawater. The expansion of the high-pressure gaseous carbon dioxide generates electricity, converting the mechanical energy into stable electrical energy output to the energy-consuming side. The system includes a working fluid pump 11, an evaporator 12, a high-pressure turbine 14, a low-pressure turbine 16, and a heat exchanger 18. The outlet of the high-pressure storage tank 10 is connected to the inlet of the working fluid pump 11, the outlet of the working fluid pump 11 is connected to the low-temperature side inlet of the evaporator 12, the low-temperature side outlet of the evaporator 12 is connected to the low-temperature side inlet of the first heater 13, the low-temperature side outlet of the first heater 13 is connected to the inlet of the high-pressure turbine 14, the outlet of the high-pressure turbine 14 is connected to the low-temperature side inlet of the second heater 15, and the low-temperature side outlet of the second heater 15 is connected to the low-temperature side inlet of the second heater 16. The high-pressure turbine 16 is connected to the inlet, and the low-pressure turbine 16 is connected to the inlet of the heat exchanger 18. The outlet of the heat exchanger 18 is connected to the inlet of the low-pressure gas storage bladder 1. The power output shafts of both the high-pressure turbine 14 and the low-pressure turbine 16 are connected to the power input shaft of the generator 17. The power outlet of the generator 17 is connected to the power inlet on the energy consumption side. The stored high-pressure liquid carbon dioxide absorbs heat from the high-temperature shallow seawater through the evaporator 12 and evaporates into a gaseous state. After the temperature is raised by the first heater 13 and the second heater 15, the high-pressure gaseous carbon dioxide undergoes two-stage expansion through the high-pressure turbine 14 and the low-pressure turbine 16, expanding to a low-pressure state. After further heat dissipation through the heat exchanger 18, the low-pressure gaseous carbon dioxide is transported to the low-pressure gas storage bladder 1 for constant-pressure storage.

[0039] The marine constant-pressure compressed carbon dioxide energy storage system coupled with ocean temperature difference utilizes the hydrostatic pressure and buoyancy of seawater to store both high-pressure liquid carbon dioxide and low-pressure gaseous carbon dioxide at constant pressure in a flexible working fluid constant-pressure storage device. During the energy storage and release process, seawater at different depths is coupled as a cold source and a heat source to achieve gas-liquid conversion of carbon dioxide under subcritical conditions.

[0040] The cold source is deep-sea cold seawater below 15°C, which is coupled into the energy storage process; specifically, cold seawater located at a depth of approximately 450 to 600 meters below sea level, or deeper than the location of the high-pressure liquid storage bag 10, is introduced into the low-temperature side of the condenser 9. The heat source is shallow-sea hot seawater above 20°C, which is coupled into the energy release process; specifically, hot seawater located at the shallow sea level is introduced into the high-temperature side of the evaporator 12. Referring to Figure 5, taking advantage of the characteristic that seawater temperature changes with depth, the seawater temperature decreases as the depth increases; the shallow hot seawater and the deep low-temperature seawater are used synergistically as the cold and heat sources for the gas-liquid conversion process.

[0041] The heat storage cycle circuit, the low-pressure compressor 3 and the high-pressure compressor 5 in the carbon dioxide energy storage cycle circuit, the high-pressure turbine 14, the low-pressure turbine 16 and the heat exchanger 18 in the carbon dioxide energy release cycle circuit, as well as the motor 2 and the generator 17 are all installed on the offshore platform.

[0042] In the heat storage circulation loop, the heat storage medium is pressurized water or heat transfer oil; the hot water storage tank 8 is insulated by wrapping it with heat insulation material and placed on an offshore platform; the cold water storage tank 7 does not need to be insulated and can float on the sea surface, with the low-pressure air bladder 1 providing additional buoyancy support.

[0043] The present invention also provides an operation method for a marine constant-pressure compressed carbon dioxide energy storage system coupled with ocean temperature difference, including an energy storage process and an energy release process;

[0044] During the energy storage process, the low-pressure gaseous carbon dioxide stored in the low-pressure gas storage bladder 1 is compressed in two stages to increase the carbon dioxide pressure to a high-pressure state. The heat energy generated by the compression of carbon dioxide is absorbed by the low-temperature pressurized water in the cold water storage tank 7, and the low-temperature pressurized water is transformed into high-temperature pressurized water and stored in the hot water storage tank 8. The low-temperature high-pressure carbon dioxide after releasing heat is sent into the deep sea through pipelines to exchange heat with the low-temperature seawater in the deep sea, causing the low-temperature high-pressure carbon dioxide to condense into liquid, and then stored in the deep sea using the flexible high-pressure liquid storage bladder 10.

[0045] Specifically, a portion of the low-temperature pressurized water flowing out of the cold water storage tank 7 exchanges heat through the first cooler 4, absorbing the heat from the carbon dioxide after the first compression; another portion of the low-temperature pressurized water flowing out of the cold water storage tank 7 exchanges heat through the second cooler 6, absorbing the heat from the carbon dioxide after the second compression; after absorbing heat, the temperature of the two streams of low-temperature pressurized water rises, and they mix and flow into the hot water storage tank 8 for storage.

[0046] During the energy release process, firstly, the high-temperature seawater in the shallow sea is sent into the sea through a pipeline. The liquid carbon dioxide stored in the high-pressure liquid storage bladder 10 is transported to the evaporator 12 by the working fluid pump 11 to absorb the heat of the high-temperature seawater in the shallow sea and evaporate into gas. Then it rises to the sea surface and further absorbs the heat energy of the high-temperature pressurized water in the hot water storage tank 8. Then it generates electricity through two-stage expansion. The low-pressure gaseous carbon dioxide after heat release is stored in the low-pressure gas storage bladder 1. The high-temperature pressurized water after absorbing heat energy is transformed into low-temperature pressurized water and stored in the cold water storage tank 7.

[0047] Specifically, a portion of the high-temperature pressurized water flowing out of the hot water storage tank 8 passes through the first heater 13, transferring heat to the carbon dioxide flowing through the first heater 13; another portion of the high-temperature pressurized water flowing out of the hot water storage tank 8 passes through the second heater 15, transferring heat to the carbon dioxide flowing through the second heater 15. After releasing heat, the temperature of the two portions of high-temperature pressurized water decreases, and after mixing, they flow into the cold water storage tank 7 for storage. Example

[0048] Referring to Figure 2, compared with Embodiment 1, this embodiment provides a cold seawater pump 19 at the low-temperature side inlet of the condenser 9 to pump cold seawater to the low-temperature pipeline of the condenser 9, and a warm seawater pump 20 at the high-temperature side inlet of the evaporator 12 to pump warm seawater to the high-temperature pipeline of the evaporator 12. Example

[0049] As shown in Figure 3, this embodiment improves the carbon dioxide energy release cycle loop, so that the pressure of high-pressure carbon dioxide decreases to atmospheric pressure after one expansion, generating low-temperature cold energy while generating electricity, and saving some heat storage, which can be used for heating, realizing the combined supply of cooling, heating and electricity, and improving the comprehensive energy utilization rate.

[0050] The outlet of the first heater 13 at low temperature is connected to the inlet of the high-pressure turbine 14, and the outlet of the high-pressure turbine 14 is directly connected to the low-pressure turbine 16. The power output shafts of the high-pressure turbine 14 and the low-pressure turbine 16 are connected to the power input shaft of the generator 17. High-pressure gaseous carbon dioxide from the seabed enters the first heater 13 and is heated to a high temperature by a portion of the heat storage medium from the hot water storage tank 8. Then, it enters the high-pressure turbine 14 and the low-pressure turbine 16 in sequence to continuously expand and do work, driving the generator 17 to rotate and generate electricity for power supply. After releasing all the heat, the heat storage medium enters the cold water storage tank 7 for storage. Another portion of the heat storage medium is not used to heat the carbon dioxide, and its thermal energy can be released in the second heater 15 for other purposes, such as direct heating or for heat-driven power generation, refrigeration, seawater desalination and other devices. The high-pressure carbon dioxide continuously expands to atmospheric pressure in the high-pressure turbine 14 and the low-pressure turbine 16, and the temperature of the carbon dioxide at its outlet is as low as zero, generating low-temperature cold energy. The cold energy is exchanged with the low-temperature carbon dioxide by a refrigerant flowing through the high-temperature side of the heat exchanger 18 for cooling.

[0051] In addition, in this embodiment, the high-pressure liquid storage bladder 10 is moored on the seabed, and multiple liquid storage bladders are arranged in parallel to reduce longitudinal pressure changes and improve reliability; at the same time, a two-way carbon dioxide transport pipeline is used to transport high-pressure carbon dioxide between the offshore platform and the seabed, which can reduce pipeline investment costs. Example

[0052] As shown in Figure 4, this embodiment further improves the carbon dioxide energy storage cycle loop, the thermal storage cycle loop, and the energy release cycle loop. It adopts a single-stage high-pressure ratio compression and expansion system structure and couples water and molten salt cascade thermal storage to achieve high-temperature storage and release of compression heat, thereby further improving the electric-electric round-trip efficiency of the energy storage system.

[0053] In addition to the system structure of Embodiment 1 or Embodiment 2, this embodiment also includes a cold salt storage tank 21 and a hot salt storage tank 22 in the thermal storage loop. During energy storage, the outlet of the gas storage bladder 1 is connected to the inlet of the low-pressure compressor 3, the outlet of the low-pressure compressor 3 is connected to the inlet of the high-pressure compressor 5, the outlet of the high-pressure compressor 5 is connected to the high-temperature side inlet of the first cooler 4, and the high-temperature side outlet of the first cooler 4 is connected to the high-temperature side inlet of the second cooler 6. The outlet of the cold salt storage tank 21 is connected to the low-temperature side inlet of the first cooler 4, and the low-temperature side outlet of the first cooler 4 is connected to the inlet of the hot salt storage tank 22. During energy release, the low-temperature side outlet of the heat exchanger 18 is connected to the low-temperature side inlet of the first heater 13, the low-temperature side outlet of the first heater 13 is connected to the low-temperature side inlet of the second heater 15, the low-temperature side outlet of the second heater 15 is connected to the inlet of the high-pressure turbine 14, and the outlet of the high-pressure turbine 14 is connected to the inlet of the low-pressure turbine 16. The outlet of the hot salt storage tank 22 is connected to the high-temperature side inlet of the second heater 15, and the high-temperature side outlet of the second heater 15 is connected to the inlet of the cold salt storage tank 21. During the energy storage process, the low-pressure gaseous carbon dioxide stored in the gas storage bladder 1 is continuously compressed at a high pressure ratio using a low-pressure compressor and a high-pressure compressor, resulting in a higher grade of heat energy. The generated high-temperature heat energy is then recovered through molten salt heat storage and pressurized water heat storage. During the energy release process, the high temperature of the carbon dioxide exhaust gas at the turbine outlet is used to preheat the high-pressure gaseous carbon dioxide from the seabed. The preheated high-pressure gaseous carbon dioxide absorbs the heat energy stored in pressurized water and molten salt to raise its temperature, and then continuously expands to atmospheric pressure in the high-pressure turbine and low-pressure turbine.

[0054] In addition, this embodiment places the energy storage, energy release and thermal storage circulation loop on the coast, making full use of the geographical conditions of the coast (such as coastal areas or islands) without the need to build an additional offshore platform; on the other hand, the low-pressure gas storage bladder floats on the near-shore sea surface without occupying land space.

[0055] In summary, the system of the present invention can improve the efficiency and energy density of energy storage systems, realize the in-depth development of marine resources, and provide a high-efficiency, long-term energy storage technology that can be deployed on a large scale for nearshore, island, and deep-sea platforms.

Claims

An offshore constant pressure compressed carbon dioxide energy storage system coupled to ocean temperature difference, characterized in that, include: Working fluid constant pressure storage device, carbon dioxide energy storage cycle loop, carbon dioxide energy release cycle loop and heat storage cycle loop; Working fluid constant pressure storage device: includes a low pressure gas storage bladder (1) for constant pressure storage of low pressure gaseous carbon dioxide and a high pressure liquid storage bladder (10) for constant pressure storage of high pressure liquid carbon dioxide. Carbon dioxide energy storage loop: The outlet of the low-pressure gas storage bladder (1) is connected to the inlet of the high-pressure liquid storage bladder (10) through the carbon dioxide energy storage loop. The power input shaft of the carbon dioxide energy storage loop is connected to the power output shaft of the motor (2). The power inlet of the motor (2) is connected to the power outlet of the offshore new energy power generation. The offshore new energy power generation supplies power to the motor (2). The motor (2) drives the compressor to compress the low-pressure gaseous carbon dioxide to a high-pressure state. Then, the high-pressure gaseous carbon dioxide is condensed into liquid by deep-sea low-temperature seawater and stored in the high-pressure liquid storage bladder (10). Carbon dioxide energy release circulation loop: The inlet of the low-pressure gas storage bag (1) is connected to the outlet of the high-pressure liquid storage bag (10) through the carbon dioxide energy release circulation loop. The power output shaft of the carbon dioxide energy release circulation loop is connected to the power input shaft of the generator (17). The power outlet of the generator (17) is connected to the power inlet of the energy user side. The high-pressure liquid carbon dioxide is evaporated into gaseous state by using high-temperature seawater in shallow sea. The high-pressure gaseous carbon dioxide is expanded to do work, and the generated mechanical energy is converted into electrical energy and stably delivered to the energy user side. At the same time, the expanded low-pressure gaseous carbon dioxide is stored in the low-pressure gas storage bag (1). Thermal storage loop: During energy storage, the carbon dioxide energy storage loop is connected to the thermal storage loop via a cooler. The stored low-temperature thermal storage medium absorbs the compression heat generated during the carbon dioxide energy storage loop process and transforms into a high-temperature thermal storage medium for storage. During energy release, the carbon dioxide energy release loop is connected to the thermal storage loop via a heater. The stored high-temperature thermal storage medium heats the gaseous carbon dioxide during the carbon dioxide energy release loop process and transforms into a low-temperature thermal storage medium for storage. The thermal storage loop includes: a cold water storage tank (7), a hot water storage tank (8), a first cooler (4), a second cooler (6), and a third cooler (7). A heater (13) and a second heater (15); the outlet of the cold water storage tank (7) is connected to the low-temperature side inlet of the first cooler (4) and the second cooler (6), respectively, and the low-temperature side outlets of the first cooler (4) and the second cooler (6) are both connected to the inlet of the hot water storage tank (8); the outlet of the hot water storage tank (8) is connected to the high-temperature side inlet of the first heater (13) and the second heater (15), respectively, and the high-temperature side outlets of the first heater (13) and the second heater (15) are both connected to the inlet of the cold water storage tank (7); the cold water storage tank (7) is used to store low-temperature heat storage medium, and the hot water storage tank (8) is used to store high-temperature heat storage medium. The offshore constant-pressure carbon dioxide compression energy storage system coupled with ocean temperature difference according to claim 1, characterized in that, The inner layers of both the low-pressure air reservoir (1) and the high-pressure liquid reservoir (10) are made of flexible materials; the low-pressure air reservoir (1) floats on the sea surface and its internal pressure is constant at atmospheric pressure; the high-pressure liquid reservoir (10) is set at a depth of 450 to 600 meters below sea level and its internal pressure is constant at 4.5 to 6 MPa. The offshore constant-pressure carbon dioxide compression energy storage system coupled with ocean temperature difference according to claim 1, wherein, The carbon dioxide energy storage cycle includes: a low-pressure compressor (3), a high-pressure compressor (5), and a condenser (9); the power input shafts of both the low-pressure compressor (3) and the high-pressure compressor (5) are connected to the power output shaft of the motor (2), the power inlet of the motor (2) is connected to the power outlet of the offshore new energy power generation, and the offshore new energy power generation supplies power to the motor (2) to drive the two compressors; the outlet of the low-pressure gas storage bladder (1) is connected to the inlet of the low-pressure compressor (3), and the outlet of the low-pressure compressor (3) is connected to the high-temperature side inlet of the first cooler (4), and the high-temperature side of the first cooler (4) is connected to the inlet of the first cooler (4). The outlet is connected to the inlet of the high-pressure compressor (5), the outlet of the high-pressure compressor (5) is connected to the high-temperature side inlet of the second cooler (6), the high-temperature side outlet of the second cooler (6) is connected to the high-temperature side inlet of the condenser (9), and the high-temperature side outlet of the condenser (9) is connected to the inlet of the high-pressure liquid storage bladder (10). The low-pressure gaseous carbon dioxide is compressed in two stages by the low-pressure compressor (3) and the high-pressure compressor (5) to a high-pressure state. After the compression heat is stored in the heat storage circulation loop, it is condensed into liquid by the deep-sea low-temperature seawater through the condenser (9). The liquid carbon dioxide is transported to the high-pressure liquid storage bladder (10) for constant pressure storage. The offshore constant-pressure carbon dioxide compression energy storage system coupled with ocean temperature difference according to claim 3, characterized in that, The deep-sea low-temperature seawater used in the carbon dioxide energy storage cycle is transported to the low-temperature side inlet of the condenser (9) through a pipeline, and the temperature of the deep-sea low-temperature seawater is below 15°C. The offshore constant-pressure carbon dioxide compression energy storage system coupled with ocean temperature difference according to claim 3 or 4, characterized in that, The thermal storage loop also includes a cold salt storage tank (21) and a hot salt storage tank (22); during the energy storage process, the outlet of the low-pressure gas storage bladder (1) is connected to the inlet of the low-pressure compressor (3), the outlet of the low-pressure compressor (3) is connected to the inlet of the high-pressure compressor (5), the outlet of the high-pressure compressor (5) is connected to the high-temperature side inlet of the first cooler (4), and the high-temperature side outlet of the first cooler (4) is connected to the high-temperature side inlet of the second cooler (6); the outlet of the cold salt storage tank (21) is connected to the low-temperature side inlet of the first cooler (4), and the low-temperature side outlet of the first cooler (4) is connected to the inlet of the hot salt storage tank (22); during the energy release process, the low-temperature side outlet of the heat exchanger (18) is connected to the low-temperature side inlet of the first heater (13), the low-temperature side outlet of the first heater (13) is connected to the low-temperature side inlet of the second heater (15), and the low-temperature side outlet of the second heater (15) is connected to the high-pressure turbine. (14) The outlet of the high-pressure turbine (14) is connected to the inlet of the low-pressure turbine (16); the outlet of the hot salt storage tank (22) is connected to the high-temperature side inlet of the second heater (15), and the high-temperature side outlet of the second heater (15) is connected to the inlet of the cold salt storage tank (21); during the energy storage process, the low-pressure gaseous carbon dioxide stored in the low-pressure gas storage bladder (1) is continuously compressed with a high pressure ratio by the low-pressure compressor and the high-pressure compressor, so that the generated heat energy is of higher quality, and the generated high-temperature heat energy is recovered by molten salt heat storage and pressurized water heat storage in sequence; during the energy release process, the exhaust gas temperature of the carbon dioxide at the turbine outlet is high, which is used to preheat the high-pressure gaseous carbon dioxide from the seabed. The preheated high-pressure gaseous carbon dioxide absorbs the heat energy stored in pressurized water and molten salt in sequence to raise the temperature, and then continuously expands to atmospheric pressure in the high-pressure turbine (14) and the low-pressure compressor (3). The offshore constant-pressure carbon dioxide compression energy storage system coupled with ocean temperature difference according to claim 1, characterized in that, The carbon dioxide energy release cycle includes a working fluid pump (11), an evaporator (12), a high-pressure turbine (14), a low-pressure turbine (16), and a heat exchanger (18). The outlet of the high-pressure reservoir (10) is connected to the inlet of the working fluid pump (11), the outlet of the working fluid pump (11) is connected to the low-temperature side inlet of the evaporator (12), the low-temperature side outlet of the evaporator (12) is connected to the low-temperature side inlet of the first heater (13), the low-temperature side outlet of the first heater (13) is connected to the inlet of the high-pressure turbine (14), the outlet of the high-pressure turbine (14) is connected to the low-temperature side inlet of the second heater (15), the low-temperature side outlet of the second heater (15) is connected to the inlet of the low-pressure turbine (16), and the outlet of the low-pressure turbine (16) is connected to the inlet of the second heater (15). The heat exchanger (18) is connected to the inlet, and the heat exchanger (18) outlet is connected to the inlet of the low-pressure gas storage bladder (1). The power output shafts of the high-pressure turbine (14) and the low-pressure turbine (16) are both connected to the power input shaft of the generator (17). The power outlet of the generator (17) is connected to the power inlet of the energy user. The stored liquid carbon dioxide absorbs heat from the shallow seawater at high temperature through the evaporator (12) and evaporates into a gaseous state. After the temperature is raised by the first heater (13) and the second heater (15), the high-pressure carbon dioxide is expanded to a low-pressure state through the high-pressure turbine (14) and the low-pressure turbine (16). After the low-pressure gaseous carbon dioxide is cooled by the heat exchanger (18), it is transported to the low-pressure gas storage bladder (1) for constant pressure storage. The offshore constant pressure carbon dioxide compression energy storage system coupled with ocean temperature difference according to claim 6, characterized in that, The shallow seawater with high temperature used in the carbon dioxide energy release cycle is transported to the high temperature side inlet of the evaporator (12) through a pipeline, and the temperature of the shallow seawater with high temperature is above 20°C. The offshore constant pressure carbon dioxide compression energy storage system coupled with ocean temperature difference according to claim 6 or 7, characterized in that, In the carbon dioxide energy release loop, the outlet of the high-pressure turbine (14) is directly connected to the low-pressure turbine (16). High-pressure gaseous carbon dioxide from the seabed enters the first heater (13) and is heated to a high temperature by a portion of the heat storage medium from the hot water storage tank (8). Then, it enters the high-pressure turbine (14) and the low-pressure turbine (16) in sequence to expand and do work, driving the generator (17) to rotate and generate electricity for power supply. The heat storage medium enters the cold water storage tank (7) for storage after releasing the heat. Another portion of the heat storage medium is not used to heat the carbon dioxide. Its heat energy is released in the second heater (15) and used directly for heating or for heat-driven power generation, refrigeration, and seawater desalination. The high-pressure carbon dioxide expands to atmospheric pressure in the high-pressure turbine (14) and the low-pressure turbine (16). The temperature of the carbon dioxide at its outlet is as low as zero, generating low-temperature cold energy. The refrigerant flows through the high-temperature side of the heat exchanger (18) to exchange heat with the low-temperature carbon dioxide for cooling. An operation method for a marine constant-pressure compressed carbon dioxide energy storage system coupled with ocean temperature difference, employing the marine constant-pressure compressed carbon dioxide energy storage system as described in any one of claims 1-6, characterized in that it includes an energy storage process and an energy release process: During the energy storage process, the low-pressure gaseous carbon dioxide stored in the low-pressure gas storage bladder (1) undergoes two-stage compression to a high-pressure state. The heat energy generated by the compression of carbon dioxide is absorbed by the low-temperature heat storage medium in the cold water storage tank (7), and the low-temperature heat storage medium is transformed into a high-temperature heat storage medium and stored in the hot water storage tank (8). The low-temperature high-pressure carbon dioxide after heat release exchanges heat with the low-temperature seawater in the deep sea through the condenser (9), causing the low-temperature high-pressure carbon dioxide to condense into a liquid state and be stored in the high-pressure liquid storage bladder (10). During the energy release process, the high-pressure liquid carbon dioxide stored in the high-pressure liquid storage bladder (10) is transported to the evaporator (12) by the working fluid pump (11) to absorb the heat of the high-temperature seawater in the shallow sea and evaporate into gas. Then it rises to the sea surface and further absorbs the heat energy of the high-temperature heat storage medium in the hot water storage tank (8). Then it generates electricity through two-stage expansion. The low-pressure gaseous carbon dioxide after heat release is stored in the low-pressure gas storage bladder (1). The high-temperature heat storage medium after absorbing heat energy is transformed into a low-temperature heat storage medium and stored in the cold water storage tank (7). The operation method of the offshore constant-pressure compressed carbon dioxide energy storage system coupled with ocean temperature difference according to claim 9 is characterized in that: During the energy storage process, a portion of the low-temperature heat storage medium flowing out from the cold water storage tank (7) exchanges heat through the first cooler (4) and absorbs the heat of compression of carbon dioxide after the first compression. Another portion of the low-temperature heat storage medium flowing out from the cold water storage tank (7) exchanges heat through the second cooler (6) and absorbs the heat of compression of carbon dioxide after being compressed for the second time. After absorbing heat, the temperature of the two low-temperature heat storage media rises, and after mixing, they flow into the hot water storage tank (8) for storage. During the energy release process, a portion of the high-temperature heat storage medium flowing out of the hot water storage tank (8) exchanges heat with the carbon dioxide flowing through the first heater (13) through the first heater (13); another portion of the high-temperature heat storage medium flowing out of the hot water storage tank (8) exchanges heat with the carbon dioxide flowing through the second heater (15) through the second heater (15). After the heat is released, the temperature of the two portions of high-temperature heat storage medium decreases, and after mixing, they flow into the cold water storage tank (7) for storage.

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