CN118481772A - Cold energy cascade utilization system - Google Patents

Abstract

The present invention relates to a cold energy cascade utilization system, comprising an organic Rankine power generation module, a seawater desalination module and a direct expansion power generation module. Liquid natural gas flows through the organic Rankine power generation module, the seawater desalination module and the direct expansion power generation module in sequence to realize the cascade utilization of cold energy; the organic Rankine power generation module absorbs high-grade cold energy from liquid natural gas in a primary cold storage heat exchanger to output electric energy through an organic Rankine cycle; the seawater desalination module absorbs low-grade cold energy from liquid natural gas after a heat exchange in the primary cold storage heat exchanger in a secondary cold storage heat exchanger to perform hydrate desalination seawater; the cold storage heat exchanger is filled with three layers of solid-liquid phase change materials whose phase change temperature increases in sequence along the flow direction of the liquid natural gas, so that the cold storage heat exchanger can store or/and release cold energy according to the flow rate of the liquid natural gas. The technical problems of insufficient utilization of liquid natural gas cold energy and fluctuation of natural gas regasification rate are solved.

Description

A cold energy cascade utilization system

Technical Field

The invention relates to the technical field of natural gas cold energy, in particular to a cold energy cascade utilization system.

Background Art

Natural gas is one of the cleanest fossil fuels. When natural gas is liquefied to -162℃, a large amount of cold energy is stored. This requires the liquid natural gas to be regasified before it can be used directly by the user end and releases a huge amount of cold energy. The cold energy of traditional liquefied natural gas is directly discharged into the sea, resulting in the problem of unutilized cold energy and energy waste.

In response to the above problems, most existing studies focus on utilizing liquid natural gas cold energy through power generation methods such as direct expansion and organic Rankine cycle. However, a single organic Rankine cycle power generation system only utilizes cold energy once, and the natural gas after heat exchange still has a large amount of cold energy that can be reused. Therefore, the utilization efficiency of liquid natural gas cold energy by traditional systems is not high. At the same time, during the utilization of liquid natural gas, changes in the user's demand for natural gas will cause fluctuations in the flow of liquid natural gas in the transmission pipeline, that is, there is a problem of fluctuations in the cold supply of liquid natural gas, which affects the operating state of the cold energy utilization system. The energy mismatch between this cold energy utilization system and user demand also needs to be solved. Therefore, there is an urgent need for a cold energy cascade utilization system that can fully utilize the cold energy of liquid natural gas and effectively adjust the fluctuation of cold supply.

Summary of the invention

In view of the deficiencies of the prior art, the present invention provides a cold energy cascade utilization system to solve the technical problems of insufficient cold energy utilization and fluctuation of natural gas regasification rate in the cold energy utilization process of liquefied natural gas.

The technical solution adopted by the present invention is as follows:

A cold energy cascade utilization system, the system comprising an organic Rankine power generation module, a seawater desalination module and a direct expansion power generation module, wherein liquid natural gas flows through the organic Rankine power generation module, the seawater desalination module and the direct expansion power generation module in sequence, thereby realizing the cascade utilization of cold energy;

The organic Rankine power generation module absorbs high-level cold energy from liquid natural gas in the primary cold storage heat exchanger to perform organic Rankine cycle to output electric energy; the seawater desalination module absorbs low-level cold energy from liquid natural gas after one heat exchange in the primary cold storage heat exchanger in the secondary cold storage heat exchanger to perform hydrate desalination seawater desalination;

The direct expansion power generation module is composed of multiple units connected in series, and each unit includes a seawater heater, an expander and a generator; the seawater heater uses warm seawater to heat the natural gas that enters the expander to do work after secondary heat exchange in the secondary cold storage heat exchanger, and the expander is connected to the generator power; the exhaust gas outlet of the expander of each unit is connected to the high-temperature side inlet of the seawater heater of the next unit;

The natural gas and cold seawater after heat exchange in the direct expansion power generation module are input into the seawater desalination module to perform the hydrate desalination seawater desalination;

The primary cold storage heat exchanger and the secondary cold storage heat exchanger are both plate heat exchangers, which include a plurality of heat exchange plate assemblies connected in series, each heat exchange plate assembly being filled with three layers of solid-liquid phase change materials with different melting points, the melting points of the three layers of solid-liquid phase change materials with different melting points increasing sequentially along the flow direction of the liquid natural gas, so that the cold storage heat exchanger can store and/or release cold energy according to the flow rate of the liquid natural gas flowing through.

Further technical solutions are:

The phase change temperatures of the three layers of solid-liquid phase change materials with different melting points in the primary cold storage heat exchanger are -56.9°C, -114.4°C and -142.2°C respectively;

The phase change temperatures of the three layers of solid-liquid phase change materials with different melting points in the secondary cold storage heat exchanger are -29.7°C, -37.1°C and -53.7°C respectively.

The direct expansion power generation module is composed of four units connected in series.

The structure of the organic Rankine power generation module is:

It includes the first-stage cold storage heat exchanger, working fluid pump, evaporator, expander A and generator A;

The working fluid side outlet of the first-stage cold storage heat exchanger is connected to the working fluid side inlet of the evaporator through the working fluid pump, the working fluid side outlet of the evaporator is connected to the inlet of the expander A, the exhaust gas outlet of the expander A is connected to the working fluid side inlet of the first-stage cold storage heat exchanger, the natural gas side inlet of the first-stage cold storage heat exchanger is connected to an external gas source, and the natural gas side outlet of the first-stage cold storage heat exchanger is connected to the natural gas side inlet of the second-stage cold storage heat exchanger, so that the first-stage cold storage heat exchanger realizes heat exchange between liquid natural gas and working fluid, and the evaporator realizes heat exchange between working fluid and warm seawater.

The working fluid used in the organic Rankine power generation module is carbon dioxide, ammonia or R125fa.

The structure of the seawater desalination module is:

It includes the secondary cold storage heat exchanger, medium pump, hydrate crystal reactor, three-phase separator, hydrate crystal separator, and post-separation mixer;

The intermediate medium side of the secondary cold storage heat exchanger is connected to the intermediate medium side of the hydrate crystal reactor through the medium pump to form a loop, and the natural gas side outlet of the secondary cold storage heat exchanger is connected to the natural gas side inlet of the seawater heater of the primary unit of the direct expansion power generation module, so that the secondary cold storage heat exchanger realizes heat exchange between liquid natural gas and the intermediate medium;

The hydrate crystal reactor is used for reacting the heat-exchanged natural gas with cold seawater to generate a hydrate crystal solution. The hydrate crystal solution enters a three-phase separator and is separated into concentrated brine, hydrate crystals and unreacted natural gas. The hydrate crystals enter a hydrate crystal separator and are separated into fresh water and natural gas under the thermal stimulation of warm seawater, wherein the fresh water directly flows out and is collected, and the unreacted natural gas and the natural gas flowing out of the hydrate crystal separator continue to flow to a post-separation mixer to be mixed.

The hydrate crystal reactor is provided with a first inlet and a second inlet, wherein the first inlet is connected to the outlet of the seawater mixer, and the inlet of the seawater mixer is connected to the cold seawater outlets of the seawater heat exchangers at various stages;

The expander outlet of the final unit of the direct expansion power generation module is connected to the separator after direct expansion power generation, and at least a part of the natural gas separated by the separator after direct expansion power generation is input into the hydrate crystal reactor through the second inlet.

At least a portion of the natural gas separated by the separator after the direct expansion power generation is heated by a user-side heater and then delivered to the user.

The intermediate medium is ethylene glycol solution.

Two adjacent layers of solid-liquid phase change materials with different melting points are separated by a metal partition.

The beneficial effects of the present invention are as follows:

The present invention can utilize the solidification process of the solid-liquid phase change material to store cold when the flow rate of liquefied natural gas is large, and melt the solid-liquid phase change material to supply cold to the outside when the flow rate of liquefied natural gas is small, thereby effectively adjusting the fluctuation of liquefied natural gas cold supply, efficiently utilizing cold energy for power generation and seawater desalination, and realizing efficient, continuous and stable output of electricity and fresh water.

The present invention utilizes liquid natural gas cold energy to replace an external refrigeration cycle, thereby further reducing energy consumption in the seawater desalination process.

The direct expansion power generation module of the present invention can fully utilize the cold energy of liquid natural gas and output more electric energy externally.

Other features and advantages of the present invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a system structure of an embodiment of the present invention.

FIG. 2 is a schematic structural diagram of a cold storage heat exchanger according to an embodiment of the present invention.

FIG. 3 is a schematic structural diagram of a seawater desalination module according to an embodiment of the present invention.

FIG. 4 is a schematic diagram showing the principle of a seawater desalination module according to an embodiment of the present invention.

FIG5 is a schematic diagram of the structure of a four-stage direct expansion power generation module according to an embodiment of the present invention.

FIG6 is a temperature entropy diagram of a four-stage direct expansion power generation module according to an embodiment of the present invention.

In the figure: 1. User-side heater; 2. Liquid natural gas pump; 3. Primary cold storage heat exchanger; 4. Generator A; 5. Working fluid pump; 6. Expander A; 7. Evaporator; 8. Secondary cold storage heat exchanger; 9. Medium pump; 10. Hydrate crystal reactor; 11. Seawater mixer; 12. Three-phase separator; 13. Hydrate crystal separator; 14. Warm seawater pump; 15. Post-separation mixer; 16. Primary seawater heater; 17. Primary expander; 18. Primary Generator; 19. Secondary seawater heater; 20. Secondary expander; 21. Secondary generator; 22. Thirdary seawater heater; 23. Thirdary expander; 24. Thirdary generator; 25. Fourthary seawater heater; 26. Fourthary expander; 27. Fourthary generator; 28. Separator after direct expansion power generation; 29. Solid-liquid phase change material; 30. Metal plate; 31. Metal partition; 32. High-temperature phase change material; 33. Medium-temperature phase change material; 34. Low-temperature phase change material.

DETAILED DESCRIPTION

The specific implementation of the present invention is described below with reference to the accompanying drawings.

Referring to FIG. 1 and FIG. 2 , the cold energy cascade utilization system of this embodiment includes an organic Rankine power generation module, a seawater desalination module, and a direct expansion power generation module. Liquid natural gas flows through the organic Rankine power generation module, the seawater desalination module, and the direct expansion power generation module in sequence to achieve cascade utilization of cold energy.

The organic Rankine power generation module absorbs high-level cold energy from the liquid natural gas in the primary cold storage heat exchanger 3, and performs an organic Rankine cycle to output electrical energy; the seawater desalination module absorbs low-level cold energy from the liquid natural gas after the primary heat exchange in the primary cold storage heat exchanger 3 in the secondary cold storage heat exchanger 8, and performs hydrate desalination seawater desalination;

The direct expansion power generation module is composed of multiple units connected in series, and each unit includes a seawater heater, an expander and a generator; the seawater heater uses warm seawater to heat the natural gas that enters the expander to do work after secondary heat exchange in the secondary cold storage heat exchanger 8, and the expander is connected to the generator power; the exhaust gas outlet of the expander of each unit is connected to the high temperature side inlet of the seawater heater of the next unit;

The natural gas and cold seawater after heat exchange in the direct expansion power generation module are input into the seawater desalination module for hydrate desalination;

As shown in Figure 2, the first-stage cold storage heat exchanger 3 and the second-stage cold storage heat exchanger 8 are both plate heat exchangers, which include multiple heat exchange plate assemblies connected in series. Each heat exchange plate assembly is filled with three layers of solid-liquid phase change materials 29 with different melting points, including high-temperature phase change material 32, medium-temperature phase change material 33, and low-temperature phase change material 34. The melting points of low-temperature phase change material 34, medium-temperature phase change material 33, and high-temperature phase change material 32 increase successively, and the direction in which the melting points increase successively is consistent with the flow direction of liquid natural gas, so that the cold storage heat exchanger can store and/or release cold energy according to the flow rate of liquid natural gas flowing through, and effectively reduce the irreversible loss of the heat exchange process.

As a specific implementation, the solid-liquid phase change material 29 in the heat exchange plate assembly of the plate heat exchanger is arranged in a hollow structure surrounded by metal plates 30 , and two adjacent layers of phase change material are separated by a metal partition 31 .

As a specific implementation, the high-temperature, medium-temperature, and low-temperature phase change materials in the first-stage cold storage heat exchanger 3 are n-octane, ethanol, and cyclopentadiene (the phase change temperatures are -56.9°C, -114.4°C, and -142.2°C, respectively); the high-temperature, medium-temperature, and low-temperature phase change materials in the second-stage cold storage heat exchanger 8 are decane, 3-heptanone, and n-nonane (the phase change temperatures are -29.7°C, -37.1°C, and -53.7°C, respectively). It can be understood that other phase change materials can be selected according to actual needs.

The structure of the organic Rankine power generation module in this embodiment is:

It includes a primary cold storage heat exchanger 3, a working fluid pump 5, an evaporator 7, an expander A6 and a generator A4;

The working fluid side outlet of the first-stage cold storage heat exchanger 3 is connected to the working fluid side inlet of the evaporator 7 via the working fluid pump 5, the working fluid side outlet of the evaporator 7 is connected to the inlet of the expander A6, the exhaust gas outlet of the expander A6 is connected to the working fluid side inlet of the first-stage cold storage heat exchanger 3, the natural gas side inlet of the first-stage cold storage heat exchanger 3 is connected to an external gas source, the natural gas side outlet of the first-stage cold storage heat exchanger 3 is connected to the natural gas side inlet of the second-stage cold storage heat exchanger 8, the natural gas side inlet of the first-stage cold storage heat exchanger 3 is connected to the liquid natural gas pump 2, and the liquid natural gas pump 2 pumps external liquid natural gas into the first-stage cold storage heat exchanger 3.

The working principle of the organic Rankine power generation module is as follows: the working fluid in the evaporator 7 exchanges heat with the warm seawater, and the working fluid is output from the outlet of the evaporator 7 to the inlet of the expander A6. The exhaust gas after work flows out from the outlet of the expander A6 and enters the primary cold storage heat exchanger 3. The liquid natural gas in the primary cold storage heat exchanger 3 exchanges heat with the working fluid. After the working fluid is condensed, it enters the working fluid pump 5 for pressurization. The working fluid leaves the working fluid pump 5 and continues to enter the evaporator 7 again along the working fluid pipeline to circulate back and forth.

The working fluid used in the organic Rankine power generation module is carbon dioxide, ammonia or R125fa.

The structure of the seawater desalination module in this embodiment is:

It includes a secondary cold storage heat exchanger 8, a medium pump 9, a hydrate crystal reactor 10, a three-phase separator 12, a hydrate crystal separator 13, and a post-separation mixer 15;

The intermediate medium side of the secondary cold storage heat exchanger 8 is connected to the intermediate medium side of the hydrate crystal reactor 10 through the medium pump 9 to form a loop, and the natural gas side outlet of the secondary cold storage heat exchanger 8 is connected to the natural gas side inlet of the seawater heater of the primary unit of the direct expansion power generation module, so that the secondary cold storage heat exchanger 8 can realize heat exchange between liquid natural gas and the intermediate medium.

The hydrate crystal reactor 10 is provided with a first inlet and a second inlet, the first inlet is connected to the outlet of the seawater mixer 11, and the inlet of the seawater mixer 11 is connected to the cold seawater outlets of the seawater heat exchangers of each stage of the direct expansion power generation module;

The expander outlet of the final unit of the direct expansion power generation module is connected to the separator 28 after direct expansion power generation, and at least a part of the natural gas separated by the separator 28 after direct expansion power generation is input into the hydrate crystal reactor 10 from the second inlet.

At least a portion of the natural gas separated by the separator 28 after direct expansion power generation is heated by the user-side heater 1 and then delivered to the user.

A warm seawater heat exchange pipe is provided in the hydrate crystal separator 13, into which warm seawater is continuously pumped through a warm seawater pump 14 to provide a heat source for the separation process of the hydrate crystals.

Referring to Figures 3 and 4, the working principle of the seawater desalination module is as follows:

The intermediate medium after heat exchange with liquid natural gas in the secondary cold storage heat exchanger 8 enters the hydrate crystal reactor 10 to provide low temperature and low pressure conditions. The hydrate crystal reactor 10 is used for the natural gas (guest molecule) after heat exchange to react with cold seawater to generate a hydrate crystal solution. The hydrate crystal solution enters the three-phase separator 12 to be separated into concentrated brine, hydrate crystals and unreacted natural gas. The hydrate crystals enter the hydrate crystal separator 13 and are separated into fresh water and natural gas under the thermal stimulation of warm seawater, wherein the fresh water directly flows out and is collected, and the unreacted natural gas continues to flow to the post-separation mixer 15 with the natural gas flowing out of the hydrate crystal separator 13 to be mixed.

The guest molecules cause the water molecules to undergo a phase change from liquid to solid, thereby removing the salt from the seawater. The water molecules form cage-shaped hydrate crystals around the guest molecules, effectively separating them from the brine solution at a temperature slightly above the normal freezing point of water. The hydrate crystals are essentially fresh water and guest molecules, and the separated guest molecules can be reused.

The intermediate medium is an antifreeze solution, and specifically, an ethylene glycol solution can be used.

The working principle of the cold storage heat exchanger in this embodiment:

When the demand for natural gas at the user end increases, the flow rate of liquid natural gas in the system increases. When the heat exchange rate between liquid natural gas and the working fluid and the intermediate medium remains unchanged, only a part of the solid-liquid phase change material in the cold storage heat exchanger melts, and the cold energy of liquid natural gas is transferred to the organic Rankine working fluid and the intermediate medium. The additional cold energy brought by the increase in the flow rate of liquid natural gas makes the other part of the solid-liquid phase change material remain in a solidified state, thereby storing the excess cold energy;

When the demand for natural gas at the user end decreases, the flow rate of liquid natural gas in the system decreases. At this time, the cold energy obtained by the organic Rankine power generation module and the seawater desalination module is insufficient due to the decrease in the flow rate of liquid natural gas. The temperature in the plate heat exchanger will increase, causing more solid-liquid phase change materials in the cold storage heat exchanger to melt and release the pre-stored cold energy, thereby maintaining the heat exchange rate between liquid natural gas and the working fluid and intermediate medium without significant changes, that is, ensuring that the cold energy provided by liquid natural gas to the organic Rankine power generation module and the seawater desalination module does not fluctuate significantly.

Therefore, when the flow rate of liquid natural gas is large, the phase change material solidifies to store cold energy, and when the flow rate of liquid natural gas is small, it melts to provide cooling, effectively addressing the problem of fluctuations in the cold energy of liquid natural gas.

The direct expansion power generation module of this embodiment adopts a four-stage expansion and inter-stage reheating design, and is preferably composed of four units in series, as shown in Figures 1 and 5, including a first-stage seawater heater 16, a first-stage expander 17, a first-stage generator 18, a second-stage seawater heater 19, a second-stage expander 20, a second-stage generator 21, a third-stage seawater heater 22, a third-stage expander 23, a third-stage generator 24, a fourth-stage seawater heater 25, a fourth-stage expander 26, and a fourth-stage generator 27.

Each level of expander is coaxially connected to the corresponding generator, and each level of seawater heater is located before the corresponding expander to heat the natural gas entering the expander. In the seawater heater, the liquid natural gas pipeline exchanges heat with the warm seawater pipeline. After the liquid natural gas is heated, it enters the expander along the liquid natural gas pipeline and performs work in the expander. The expander drives the generator to operate through the rotating shaft and outputs electrical energy to the outside, completing the first-level expansion process. After the work is done, the exhausted gas flows out from the expander outlet and flows to the seawater heat exchanger of the next expansion process. According to the above process, it goes through four-level direct expansion power generation processes in sequence.

The four-stage expansion power generation of this embodiment is compared with the conventional one-stage expansion power generation. The temperature of the liquid natural gas entering the first-stage seawater heater 16 is 248.9K, the pressure is 30MPa, and the mass flow rate is 1kg/s. Ignoring the pressure drop of the heater, the temperature of the natural gas at the outlet of the seawater heater is 288.2K. The isentropic efficiency of the expander is taken as 92%. During the four-stage expansion process, the natural gas is expanded to 19MPa, 10.9MPa, 9MPa and 7MPa in sequence. The gas after each expansion is heated to 288.2K by passing through the seawater heater once. The one-stage expansion power generation directly expands the natural gas from 30MPa to 7MPa. The output power of each stage of the expander is calculated, and the total power output of the expander in the two processes is compared. Table 1 shows the thermodynamic parameters of the main state points of the four-stage direct expansion power generation in Figure 5.

Table 1

Table 2 gives the thermodynamic parameters of the main state points of conventional first-stage direct expansion power generation.

Table 2

Figure 6 shows the temperature entropy change of liquefied natural gas during the four-stage direct expansion power generation process. The output power of the first to fourth stage expanders in the four-stage direct expansion power generation calculation process is as follows:

W _Exp1 =m (h _L2 -h _L3 ) = 1·(590.73-545.7) = 45.03kW

W _Exp2 =m (h _L4 -h _L5 ) = 1·(626.32-591.26) = 35.06kW

W _Exp3 =m (h _L6 -h _L7 ) = 1·(702.35-685.51) = 16.84kW

W _Exp4 =m (h _L8 -h _L9 ) = 1·(727.1-707.95) = 19.15kW

The total output power is 116.08kW.

Output power of conventional one-stage direct expansion power generation calculation process:

W _Exp =m(h _L2a -h _L3a )=1·(590.73-486.63)=104.1kW

The total power generation in the two modes is 104.1 kW and 116.08 kW respectively, indicating that the four-stage expansion power generation in the embodiment can make fuller use of the cold energy of liquid natural gas and output more electric energy.

Those skilled in the art can understand that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention is described in detail with reference to the aforementioned embodiments, those skilled in the art can still modify the technical solutions recorded in the aforementioned embodiments or replace some of the technical features therein by equivalents. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims (10)

1. A cold energy cascade utilization system, characterized in that the system comprises an organic Rankine power generation module, a seawater desalination module and a direct expansion power generation module, and liquid natural gas flows through the organic Rankine power generation module, the seawater desalination module and the direct expansion power generation module in sequence to achieve cascade utilization of cold energy; The organic Rankine power generation module absorbs high-level cold energy from liquid natural gas in the primary cold storage heat exchanger (3) to perform an organic Rankine cycle to output electrical energy; the seawater desalination module absorbs low-level cold energy from the liquid natural gas after a heat exchange in the primary cold storage heat exchanger (3) in the secondary cold storage heat exchanger (8) to perform hydrate desalination seawater desalination; The direct expansion power generation module is composed of multiple units connected in series, each unit comprising a seawater heater, an expander and a generator; the seawater heater uses warm seawater to heat the natural gas that enters the expander to perform work after secondary heat exchange in the secondary cold storage heat exchanger (8), and the expander is connected to the generator power; the exhaust gas outlet of the expander of each unit is connected to the high temperature side inlet of the seawater heater of the next unit; The natural gas and cold seawater after heat exchange in the direct expansion power generation module are input into the seawater desalination module to perform the hydrate desalination seawater desalination; The first-stage cold storage heat exchanger (3) and the second-stage cold storage heat exchanger (8) are both plate heat exchangers, and the plate heat exchangers include a plurality of heat exchange plate assemblies connected in series, each heat exchange plate assembly is filled with three layers of solid-liquid phase change materials with different melting points, and the melting points of the three layers of solid-liquid phase change materials with different melting points increase successively along the flow direction of the liquid natural gas, so that the cold storage heat exchanger can store and/or release cold energy according to the flow rate of the liquid natural gas flowing through. 2. The cold energy cascade utilization system according to claim 1, characterized in that the phase change temperatures of the three layers of solid-liquid phase change materials with different melting points in the primary cold storage heat exchanger (3) are -56.9°C, -114.4°C and -142.2°C respectively; The phase change temperatures of the three layers of solid-liquid phase change materials with different melting points in the secondary cold storage heat exchanger (8) are -29.7°C, -37.1°C and -53.7°C respectively. 3. The cold energy cascade utilization system according to claim 1 is characterized in that the direct expansion power generation module is composed of four units connected in series. 4. The cold energy cascade utilization system according to claim 1, characterized in that the structure of the organic Rankine power generation module is: It includes the first-stage cold storage heat exchanger (3), a working fluid pump (5), an evaporator (7), an expander A (6) and a generator A (4); The working fluid side outlet of the first-stage cold storage heat exchanger (3) is connected to the working fluid side inlet of the evaporator (7) via the working fluid pump (5), the working fluid side outlet of the evaporator (7) is connected to the inlet of the expander A (6), the exhaust gas outlet of the expander A (6) is connected to the working fluid side inlet of the first-stage cold storage heat exchanger (3), the natural gas side inlet of the first-stage cold storage heat exchanger (3) is connected to an external gas source, and the natural gas side outlet of the first-stage cold storage heat exchanger (3) is connected to the natural gas side inlet of the second-stage cold storage heat exchanger (8), so that the first-stage cold storage heat exchanger (3) realizes heat exchange between liquid natural gas and working fluid, and the evaporator (7) realizes heat exchange between working fluid and warm seawater. 5. The cold energy cascade utilization system according to claim 4, characterized in that the working fluid used in the organic Rankine power generation module is carbon dioxide, ammonia water or R125fa. 6. The cold energy cascade utilization system according to claim 4, characterized in that the structure of the seawater desalination module is: It includes the secondary cold storage heat exchanger (8), a medium pump (9), a hydrate crystal reactor (10), a three-phase separator (12), a hydrate crystal separator (13), and a post-separation mixer (15); The intermediate medium side of the secondary cold storage heat exchanger (8) is connected to the intermediate medium side of the hydrate crystal reactor (10) through the medium pump (9) to form a loop, and the natural gas side outlet of the secondary cold storage heat exchanger (8) is connected to the natural gas side inlet of the seawater heater of the primary unit of the direct expansion power generation module, so that the secondary cold storage heat exchanger (8) realizes heat exchange between liquid natural gas and the intermediate medium; The hydrate crystal reactor (10) is used for reacting the natural gas after heat exchange with cold seawater to generate a hydrate crystal solution. The hydrate crystal solution enters a three-phase separator (12) and is separated into concentrated brine, hydrate crystals and unreacted natural gas. The hydrate crystals enter a hydrate crystal separator (13) and are separated into fresh water and natural gas under the thermal stimulation of warm seawater, wherein the fresh water directly flows out and is collected, and the unreacted natural gas and the natural gas flowing out of the hydrate crystal separator (13) continue to flow to a post-separation mixer (15) to be mixed. 7. The cold energy cascade utilization system according to claim 6, characterized in that the hydrate crystal reactor (10) is provided with a first inlet and a second inlet, the first inlet is connected to the outlet of the seawater mixer (11), and the inlet of the seawater mixer (11) is connected to the cold seawater outlets of the seawater heat exchangers at various stages; The expander outlet of the final unit of the direct expansion power generation module is connected to the direct expansion power generation post-separator (28), and at least a portion of the natural gas separated by the direct expansion power generation post-separator (28) is input into the hydrate crystal reactor (10) through the second inlet. 8. The cold energy cascade utilization system according to claim 7 is characterized in that at least a portion of the natural gas separated by the separator (28) after the direct expansion power generation is heated by the user-side heater (1) and then delivered to the user. 9. The cold energy cascade utilization system according to claim 6, characterized in that the intermediate medium is an ethylene glycol solution. 10. The cold energy cascade utilization system according to claim 1, characterized in that two adjacent layers of solid-liquid phase change materials with different melting points are separated by a metal partition (31).