A design method for a cycle system for a Carnot cell discharging process

Abstract

A design method for a cyclic system in the discharge process of a Carnot battery, belonging to the field of design optimization for the heat release process of energy storage systems, is disclosed. The cyclic components include an expander, pump, separator, throttling valve, regenerator, condenser, evaporator, and superheater. The cycle is decomposed into a non-heat exchange process and a heat exchange network. After data optimization, the systems are re-integrated to obtain a two-stage Carnot cycle integrated system with a binary working fluid and a two-stage Carnot cycle integrated system with a ternary working fluid. The working fluid is pressurized and then enters the heat exchanger to be heated to the two-phase region. The gas stream is heated to a superheated state and then enters the expander. The waste heat of the low-pressure exhaust gas in the expander is recovered by the regenerator. The liquid stream, after its waste heat is recovered by the regenerator, mixes with the gas stream and enters the condenser for condensation, completing one cycle. This invention optimizes the cycle structure by replacing the working fluid and using an integrated separation optimization method, breaking the structural upper limit of traditional optimization methods, achieving high matching degree of the working fluid selection and heat exchange curve, and reducing irreversible losses of the system.

Description

Technical Field

[0001] This invention belongs to the field of Carnot battery discharge process design, and relates to a cycle system and design method for the Carnot battery discharge process, and particularly to a two-stage cycle system of multi-component mixed working fluid based on the Carnot cycle and its design method. Background Technology

[0002] Due to global economic development and population growth, energy demand has been climbing at an annual rate of 2.4% over the past few decades. Carnot batteries are an emerging large-scale physical energy storage technology based on thermal energy storage and conversion. It is an "electricity-thermal-electricity" conversion and storage technology based on a reverse thermodynamic cycle. It does not directly store electrical energy, but rather converts electrical energy into thermal potential energy for storage, and then converts it back into electrical energy through a thermodynamic cycle when needed. It has advantages such as large energy storage capacity, long cycle life, and suitability for large-scale energy storage, and therefore has attracted widespread attention.

[0003] In some Carnot battery systems, LNG is used as the system's cold source, significantly improving the system's round-trip efficiency and energy density. LNG can reach temperatures as low as -162°C during vaporization. Utilizing this cooling capacity in the Carnot battery system not only enables efficient utilization of cold energy but also significantly reduces the cycle's condensation temperature, greatly improving system performance. However, due to the extremely low temperature and wide temperature variation range of LNG as the cold source, the heat transfer curves of traditional Rankine cycles using pure working fluids typically exhibit poor matching during phase transitions, resulting in significant irreversible heat transfer losses and limiting the overall energy conversion efficiency of the system.

[0004] Compared to the Rankine cycle, the Kalina cycle is more complex and difficult to operate, but it offers better heat exchange and performance. This is because the Kalina cycle incorporates additional component separation and mixing processes. The working fluid's gas-liquid ratio can be controlled by adjusting its temperature before separation, thus altering the gas flow rate, enhancing the matching of the heat exchange curves, reducing irreversible heat exchange losses, and improving the cycle's power generation efficiency. However, current research primarily focuses on the Kalina cycle's applications in geothermal energy utilization, ocean thermal energy conversion, and industrial waste heat recovery. For the Carnot battery discharge process using LNG as a cold source, a systematic optimization method for its cycle structure, working fluid, and key parameters is lacking.

[0005] Among existing optimization methods, the superstructure method is a widely used approach. It connects all possible paths in the circulation system and controls the path selection through a splitter. According to existing literature, the performance of supercritical carbon dioxide cycles improved by 12.58% after adopting the superstructure method. However, this method does not reconstruct and optimize the heat exchanger network and has a certain structural upper limit. Therefore, we need a systematic and innovative optimization design method that can self-optimize by computer and reconstruct the heat exchanger network. Summary of the Invention

[0006] To address the aforementioned problems in existing technologies, this invention proposes a two-stage multi-component mixed working fluid circulation system and its design method based on the Carnot cycle for the discharge process of Carnot batteries. This invention decomposes the system into a combination of two single-stage Carnot cycle systems. After optimization, a two-stage pre-defined heat exchange-free system is obtained. The heat exchange components are optimized through heat exchange network structure reconstruction. The combination of these two approaches yields the optimal two-stage mixed working fluid circulation system. This method models and optimizes the structure of specific circulation equipment and improves system performance through the collaborative solution of structure and parameters. The core of this invention lies in: (1) By introducing a thermodynamic breakpoint at the original heat exchanger location, the cycle is transformed into a structural model without heat exchange; (2) By setting up a split structure, the coupling between the two-stage cycle is transformed into a combined optimization problem of structural and thermodynamic state parameters; (3) Based on the breakpoint state parameters, construct a heat transfer network reconstruction model that satisfies the minimum heat transfer temperature difference constraint.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A design method for a cyclic system for the discharge process of a Carnot battery includes the following steps: Step 1: The single-stage Karina cycle system is divided into a single-stage system without heat exchange and a single-stage system with heat exchange. The single-stage Karina cycle system includes a first working fluid pump 1, a first separator 2, a first expander 3, a first throttle valve 4, an evaporator 5, a superheater 6, a regenerator 7, and a condenser 8. The specific process is as follows: High-pressure stream S1 enters the regenerator 7 and exchanges heat with the liquid stream S7 from the separator 2, absorbing heat and increasing in temperature. The preheated stream after absorbing heat is S2. The preheated stream S2 enters the evaporator 5 and becomes a two-phase stream S3. The two-phase stream S3 passes through the first separator 2 for gas-liquid separation. The gas stream S4 passes through the superheater 6 and becomes a superheated stream S5. The superheated stream S5 enters the first expander 3 to output power. The outlet of the first expander 3 is low-pressure exhaust steam S6. The separated liquid stream S7 passes through the regenerator 7 and releases heat, becoming a low-temperature liquid stream S8. The low-temperature liquid stream S8 enters the first throttling valve 4 for pressure reduction. The stream after pressure reduction is the pressure-reducing stream S9. The pressure-reducing stream S9 mixes with the expanded low-pressure exhaust steam S6 to become a low-pressure stream S10. The low-pressure stream S10 enters the condenser 8 for condensation. The condensed low-temperature liquid S11 re-enters the first working fluid pump 1 for pressurization and the next cycle. Evaporator 5, superheater 6, regenerator 7 and condenser 8 are defined as a single-stage heat exchange process system, and the first working fluid pump 1, first separator 2, first expander 3 and first throttle valve 4 are defined as a single-stage non-heat exchange process system.

[0008] Furthermore, the single-stage heat exchange-free system represents the pressurization, separation, expansion, and throttling processes in the cycle other than heat exchange. Its operation no longer includes heat exchange equipment; instead, thermodynamic breakpoints are set at the original heat exchanger locations, and the thermodynamic changes before and after heat exchange are calculated using the thermodynamic state parameters at these breakpoints. The thermodynamic breakpoint is defined as a virtual cross-section set at the inlet and outlet of the original heat exchanger, representing the changes in the thermodynamic state parameters of the working fluid before and after heat exchange. These thermodynamic state parameters include temperature, pressure, working fluid flow rate, working fluid composition, and thermodynamic properties. The thermodynamic state parameters at the thermodynamic breakpoint are calculated using the mass conservation equation, energy conservation equation, and phase equilibrium relationship. The thermodynamic breakpoint locations include: high-pressure stream S1 and preheated stream S2; preheated stream S2 and two-phase stream S3; gas stream S4 and superheated stream S5; liquid stream S7 and cryogenic liquid stream S8; and low-pressure stream S10 and cryogenic liquid S11. The heat exchangers at the points where the high-pressure stream S1 and preheated stream S2, and preheated stream S2 and two-phase stream S3 are adjacent and there are no other components between the breakpoints, are therefore merged into one point, namely, the high-pressure stream S1 and the two-phase stream S3. The operating principle of the single-stage heat exchange-free system is consistent with that of the single-stage Karina cycle system. The high-pressure stream S1 enters the heat exchanger at the breakpoint between the high-pressure stream S1 and the two-phase stream S3, absorbs heat and becomes the two-phase stream S3. After gas-liquid separation by the first separator 2, the gas phase stream S4 is heated by the heat exchanger at the breakpoint between the gas phase stream S4 and the superheated stream S5, becoming the superheated stream S5. The superheated stream S5 enters the first expander 3 to output power. The outlet of the first expander 3 is low-pressure exhaust steam S6. The separated liquid phase stream S7 passes through the liquid phase stream S7... After exotherming from the heat exchanger at the break point of the cryogenic liquid phase stream S8, it becomes cryogenic liquid phase stream S8. Cryogenic liquid phase stream S8 enters the first throttle valve 4 for depressurization. The depressurized stream becomes depressurized stream S9. Depressurized stream S9 mixes with the expanded low-pressure exhaust steam S6 to become low-pressure stream S10. Low-pressure stream S10 enters the heat exchanger at the break point of low-pressure stream S10 and cryogenic liquid S11 for condensation. The condensed cryogenic liquid S11 re-enters the first working fluid pump 1 for pressurization and the next cycle.

[0009] Step 2: Integrate two identical single-stage heat-exchange-free process systems (obtained from Step 1) to construct a two-stage pre-designed heat-exchange-free system. The two-stage pre-designed heat-exchange-free system includes a first-stage heat-exchange-free process system and a second-stage heat-exchange-free process system, corresponding to the first and second single-stage heat-exchange-free process systems, respectively. Each of the two single-stage heat-exchange-free process systems includes a working fluid pump, a separator, an expander, and a throttling valve, with its flow streams defined as the first and second flow streams, respectively. In the two-stage pre-designed heat-exchange-free system, a flow splitting structure is used to achieve coupling between the two single-stage heat-exchange-free process systems. Specifically, the flow splitters are located at the following positions: compression process connection, separation process connection, superheating process connection, expansion process connection, mixing process connection, regeneration process connection, and cooling process connection. Each flow splitter has two mutually exclusive flow splitting ratios, with a value of 0 or 1, and the sum of the flow splitting ratios for all paths at the same flow splitting location is 1. The flow divider is used to represent the selection of the actual pipeline connection method. Its flow division ratio of 0 or 1 corresponds to the closed or open state of the connection path, respectively. Ultimately, the specific circulation path is transformed into a mathematical model for subsequent optimization. Furthermore, to avoid heat loss due to differences in thermodynamic parameters, a new thermodynamic breakpoint exists at each connection point. Specifically: Step 2.1: A compression splitting structure is installed at the outlet of the second working fluid pump 9 to control the coupling mode between the primary and secondary non-heat exchange process systems during the compression process. The compression splitting structure includes a shared compression path and a non-shared compression path, corresponding to the shared compression splitter 13 and the non-shared compression splitter 20, respectively. These two paths satisfy mutual exclusion constraints, and their splitting ratios sum to 1. When the splitting ratio of the non-shared compression splitter 20 is 1, the fluid at the outlet of the second working fluid pump 9 enters the secondary cycle as an independent second high-pressure stream S12. When the splitting ratio of the shared compression splitter 13 is 1, the working fluid of the secondary non-heat exchange process system, after entering the second working fluid pump 9, mixes with the first cryogenic liquid S11 from the condenser outlet of the primary non-heat exchange process system at the same pressure. The mixed fluid then enters the first working fluid pump 1 for pressurization, forming the first high-pressure stream S1.

[0010] Step 2.2: After the pressurization process of the two-stage pre-set heat-exchange-free system is completed, the circulating working fluid enters the breakpoint positions corresponding to the separation process. The breakpoints are the state points of the two-phase flow S3 in the first-stage heat-exchange-free system before entering the first separator 2, and the state points of the second two-phase flow S13 in the second-stage heat-exchange-free system before entering the second separator 10. A separation and diversion structure is set before the first separator 2 and the second separator 10 to control the coupling mode between the first-stage and second-stage heat-exchange-free systems during the separation process. The separation and diversion structure includes a shared separation path and a non-shared separation path, corresponding to the shared separation diverter 14 and the non-shared separation diverter 21, which satisfy mutual exclusion constraints, and their diversion ratios sum to 1. Specifically: When the split ratio of the non-shared splitter 21 is 1, the second two-phase flow S13 of the secondary heat exchange-free system enters the second separator as an independent flow for gas-liquid separation, resulting in the separation of the second gas phase flow S14 and the second liquid phase flow S17.

[0011] When the split ratio of the splitter 14 is 1, the two-phase stream S3 of the first-stage system without heat exchange process and the second two-phase stream S13 of the second-stage system without heat exchange process are mixed under the same pressure conditions to form the first two-phase stream S3. The first two-phase stream S3 enters the first separator for gas-liquid separation to obtain the first gas stream S4 and the first liquid stream S7.

[0012] Step 2.3: After completing the separation process of the two-stage pre-set heat-exchange-free system, the circulating working fluid enters the breakpoint position corresponding to the superheating process. The breakpoints are the state points between the gas phase flow S4 and the first superheated flow S5 in the first-stage heat-exchange-free system, and the state points between the second gas phase flow S14 and the second superheated flow S15 in the second-stage heat-exchange-free system. An superheated flow splitting structure is set after the first gas phase flow S4 and the second gas phase flow S14 to control the coupling mode between the first-stage and second-stage heat-exchange-free systems during the superheating process. The superheated flow splitting structure includes a shared superheated path and a non-shared superheated path, corresponding to the shared superheated flow splitter 15 and the non-shared superheated flow splitter 22, which satisfy mutual exclusion constraints, and their splitting ratios sum to 1. Specifically: When the split ratio of the superheated non-shared splitter 22 is 1, the first superheated stream S5 and the second superheated stream S15 enter their respective expanders to expand and perform work. Specifically, the first superheated stream S5 expands in the first expander 3 to obtain the first low-pressure exhaust steam S6; the second superheated stream S15 expands in the second expander 11 to obtain the second low-pressure exhaust steam S16. The outlet of the second expander 11 is equipped with an expansion splitting structure to control whether the second non-heat-exchange process system is coupled with the first-stage non-heat-exchange process system. The expansion splitting structure includes a shared expansion path and a non-shared expansion path, corresponding to the shared expansion splitter 16 and the non-shared expansion splitter 23, respectively. These two paths satisfy mutual exclusion constraints, and the sum of their split ratios is 1.

[0013] When the split ratio of the non-shared expansion splitter 23 is 1, the second low-pressure exhaust steam S16 enters the subsequent mixing process as an independent stream. When the split ratio of the superheated shared expansion splitter 15 is 1, the working fluid of the secondary non-heat exchange process system enters the second expander 11 and expands to the same pressure as the first superheated stream S5. It mixes with the first superheated stream S5 from the outlet of the superheater of the primary non-heat exchange process system at the same pressure. The mixed fluids then enter the first expander 3 for further expansion to form the first low-pressure exhaust steam S6.

[0014] Step 2.4: After completing the pre-designed flow charts for the superheating and expansion processes of the two-stage pre-designed heatless system, the pre-designed flow chart for the liquid phase regeneration process of the two-stage pre-designed heatless system is then constructed. After regeneration, the first liquid phase stream S7 and the second liquid phase stream S17 will exit as the first low-temperature liquid phase stream S8 and the second low-temperature liquid phase stream S18, respectively. The regeneration process in the two-stage pre-designed heatless system is represented by breakpoints. The breakpoints corresponding to the regeneration process are the breakpoints between the liquid phase stream S7 and the low-temperature liquid phase stream S8 in the single-stage heatless system, and the breakpoints corresponding to the two-stage pre-designed heatless system are the breakpoints between the first liquid phase stream S7 and the first low-temperature liquid phase stream S8, and between the second liquid phase stream S17 and the second low-temperature liquid phase stream S18. The first cryogenic liquid phase stream S8 and the second cryogenic liquid phase stream S18, after being depressurized by the first throttle valve 4 and the second throttle valve 12, select their circulation system paths through the regenerating shared distributor 18 and the regenerating non-shared distributor 25. Specifically: When the split ratio of the regenerating shared splitter 18 is 1, the split ratio of the regenerating non-shared splitter 25 is 0, and the circulating working fluid of the primary non-heat exchange process system and the circulating working fluid of the secondary non-heat exchange process system are mixed to form the first depressurization stream S9.

[0015] When the split ratio of the regenerating shared splitter 18 is 0, the circulating working fluid of the first-stage non-heat exchange process system is not mixed with the circulating working fluid of the second-stage non-heat exchange process system, and the first depressurization stream S9 and the second depressurization stream S19 are calculated independently.

[0016] Step 2.5: After completing the pre-set flow chart for the liquid phase regeneration process of the two-stage pre-set heat exchange-free system, the pre-set flow chart for the mixing process of the two-stage pre-set heat exchange-free system is then constructed. A mixing splitting structure is set after the first depressurization stream S9 and the second depressurization stream S19 to control the coupling mode of the first and second stage heat exchange-free process systems on the low-pressure side. The mixing splitting structure includes a mixing shared path and a mixing non-shared path, corresponding to the mixing shared splitter 17 and the mixing non-shared splitter 24, which satisfy mutual exclusion constraints, and their splitting ratios sum to 1. Specifically: When the split ratio of the mixing and sharing splitter 17 is 1, the first low-pressure exhaust steam S6 and the second low-pressure exhaust steam S16 are mixed under the same pressure and temperature conditions, and then mixed with the first depressurization stream S9 to form the first low-pressure stream S10.

[0017] When the split ratio of the mixing and sharing splitter 17 is 0, the first low-pressure exhaust steam S6 and the second low-pressure exhaust steam S16 are mixed with the first pressure-reducing stream S9 and the second pressure-reducing stream S19 under the same pressure conditions after heat exchange, forming the first low-pressure stream S10 and the second low-pressure stream S20 respectively.

[0018] Step 2.6: After the two-stage pre-set heat-exchange-free system completes the low-pressure side mixing process, the second low-pressure stream S20 enters the cooling splitting structure to control the coupling mode between the first-stage and second-stage heat-exchange-free systems during the condensation process. The cooling splitting structure includes a shared cooling path and a non-shared cooling path, corresponding to the shared cooling splitter 19 and the non-shared cooling splitter 26. These two paths satisfy mutual exclusion constraints, and their splitting ratios sum to 1. Specifically: When the flow ratio of the non-shared cooling splitter 26 is 1, the first low-pressure stream S10 and the second low-pressure stream S20 enter their respective condensers for condensation, resulting in the first cryogenic liquid S11 and the second cryogenic liquid S21.

[0019] When the flow ratio of the cooling shared distributor 19 is 1, the second low-pressure stream S20 and the first low-pressure stream S10 mix under the same pressure conditions and enter the same condenser for condensation to obtain the first cryogenic liquid S11. This completes the closure of the two-stage preset heat exchange-free system.

[0020] Step 3: Optimize the parameters of the two-stage pre-designed heat-exchange-free system obtained in Step 2. The optimization objective is to maximize the net output power of the two-stage pre-designed heat-exchange-free system. The pressure, temperature, and working fluid composition of each stream in the two-stage pre-designed heat-exchange-free system are solved. The net output power of the two-stage pre-designed heat-exchange-free system is the difference between the sum of the output power of the first and second expanders and the sum of the power consumed by the first and second working fluid pumps. The optimization calculation is based on the mass conservation equation and the energy conservation equation. The thermodynamic state parameters of the streams at each discontinuity in the two-stage pre-designed heat-exchange-free system are calculated using Aspen Hysys. These thermodynamic state parameters include temperature, pressure, mass flow rate, and working fluid composition.

[0021] The thermodynamic state parameters at all thermodynamic breakpoints are input into the Aspen Energy Analyzer to obtain the structure of the two-stage pre-defined heat exchange system, including the number of heat exchangers, the connection relationships between heat exchange units, and the heat load distribution of each heat exchange unit, thereby constructing the optimal heat exchange network structure that satisfies the constraints. The two-stage pre-defined heat exchange system is constructed based on the following constraints: (1) The energy conservation constraint of the hot and cold flow streams corresponds to the following formula (1); (2) Minimum temperature difference constraint of heat exchanger, corresponding to the following formula (2); (3) The mass conservation constraint of the hot and cold streams in the heat exchange process corresponds to the following formula (3).

[0022] The constraint relationship described above can be expressed as: (1) (2) (3) In the formula and The heat exchange capacity of the cold or hot stream of the k-th heat exchanger, where the superscripts h and c represent the hot and cold streams respectively, and the subscript k indicates the k-th heat exchanger, with the unit being kW; It is the minimum heat transfer temperature difference of the k-th heat exchanger. It is the minimum heat transfer temperature difference of a two-stage preset heat exchange system. and These are the inlet and outlet temperatures of the heat stream in the k-th heat exchanger, respectively. and These are the inlet and outlet temperatures of the cold stream of the k-th heat exchanger, respectively. The superscript k indicates the k-th heat exchanger, the subscript h represents the hot stream, and the subscripts in and out represent the inlet and outlet temperatures of the stream, all in °C. It is the mass flow rate of the k-th branch stream at the outlet of the i-th splitter. It is the mass flow rate of the k-th branch stream at the inlet of the i-th splitter. It is the total mass flow rate of the circulating working fluid flowing to the i-th splitter, where the superscripts in and out represent the inlet and outlet of the splitter, the subscript i is the i-th splitter, and the subscript k is the k-th branch stream, and the unit is kg / s.

[0023] Step 4: Based on the flow information of the two-stage preset heat exchange-free system obtained in Step 3 and the constructed optimal heat exchange network structure, connect the hot and cold flow streams at each break point to the corresponding heat exchangers, so that the two-stage preset heat exchange-free system and the two-stage preset heat exchange system form a complete closed loop structure, thereby obtaining a complete two-stage mixed working fluid loop system.

[0024] The beneficial effects of this invention are: (1) Using alkanes as working fluid avoids the possible solidification problem of ammonia water under low temperature conditions, and the temperature slip range of the alkane mixed working fluid is close to that of the ammonia water working fluid, thus achieving efficient temperature matching of the entire cycle.

[0025] (2) The cycle is divided into heat exchange process and non-heat exchange process, which breaks the structural upper limit of the cycle design, reduces the a priori influence of the superstructure method, further improves the net output power of the system, and reconstructs the heat exchange network for the heat exchange process to reduce the irreversible loss of temperature difference in the heat exchange process. Attached Figure Description

[0026] Figure 1 This is a flowchart of a single-stage Karina loop system.

[0027] Figure 2 This is a flowchart of a single-stage system without a heat exchange process.

[0028] Figure 3 The flowchart is for a two-stage pre-set heat exchange-free system.

[0029] Figure 4 The flowchart shows the optimized two-stage pre-set heat exchangerless system.

[0030] Figure 5 The flowchart is for a simplified two-stage pre-set heat exchanger-free system.

[0031] Figure 6 This is a flowchart of the optimized two-stage pre-set heat exchange system.

[0032] Figure 7 This is a flowchart of a two-stage circulation system with a mixed working fluid.

[0033] Figure 8 This is a flowchart of the Carnot battery energy storage system.

[0034] In the diagram: 1 First working fluid pump; 2 First separator; 3 First expander; 4 First throttle valve; 5 Evaporator; 6 Superheater; 7 Regenerator; 8 Condenser; 9 Second working fluid pump; 10 Second separator; 11 Second expander; 12 Second throttle valve; 13 Compression shared distributor; 14 Separation shared distributor; 15 Superheat shared distributor; 16 Expansion shared distributor; 17 Mixing shared distributor; 18 Regenerator shared distributor; 19 Cooling shared distributor; 20 Compression non-shared distributor; 21 Separation non-shared distributor; 22 Superheat non-shared distributor; 23 Expansion non-shared distributor. 24 Mixing non-shared splitter; 25 Regenerating non-shared splitter; 26 Cooling non-shared splitter; 28 First superheater; 29 Second superheater; 30 First regenerator; 31 Second regenerator; 32 Third regenerator; 33 First evaporator; 34 Fourth regenerator; 35 Fifth regenerator; 36 Sixth regenerator; 37 First condenser; 38 Seventh regenerator; 39 Eighth regenerator; 40 Heat pump system evaporator; 41 Heat pump system compressor; 42 Heat pump system condenser; 43 Heat pump system throttling valve; 44 Thermal storage system cold water tank; 45 Thermal storage system hot water tank. Detailed Implementation

[0035] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings and technical solutions.

[0036] Example 1 In this embodiment, the initial pressure of liquefied natural gas (LNG) is 7000 kPa, the temperature is -162°C, and the throughput is 90.47 kg / s. Its molar composition is: methane 91.33%, ethane 5.36%, propane 2.14%, n-butane 0.47%, isobutane 0.46%, n-pentane 0.01%, isopentane 0.01%, and nitrogen 0.22%. The heat source is simulated through flue gas, with a pressure of 101.325 kPa, a temperature of 300°C, and a throughput of 226.5 kg / s. Its molar composition is: nitrogen 76.42%, carbon dioxide 7.56%, oxygen 9.48%, and water 6.54%. The working fluid required for the system's circulation process is a mixture of methane (R50) and propane (R290).

[0037] In the heat-exchange-free process, based on a two-stage pre-designed heat-exchange-free system, the thermodynamic parameters of each stream and the split ratio of each distributor are optimized to obtain a two-stage heat-exchange-free system. The simplified circulation path is as follows: The circulating working fluid, after being pressurized by the first working fluid pump 1, enters the cryogenic heating process, that is, it completes heat absorption at the thermodynamic breakpoint corresponding to the first high-pressure stream S1 and the first two-phase stream S3, forming the first two-phase stream S3. The first two-phase stream S3 enters the first separator 2 for gas-liquid separation, obtaining the first gas phase stream S4 and the first liquid phase stream S7. The first gas phase stream S4, after undergoing a heating process (corresponding to the thermodynamic breakpoint between the first gas phase stream S4 and the first superheated stream S5), enters the expansion process and is divided into two sub-cycles: the first superheated stream S5 enters the first expander 3 and discharges the first low-pressure exhaust steam S6; the second superheated stream S15 enters the second expander 11 and discharges the second low-pressure exhaust steam S16. The first liquid phase stream S7, after undergoing a liquid phase reheating process (corresponding to the thermodynamic breakpoint between the first liquid phase stream S7 and the first cryogenic liquid phase stream S8), forms the first cryogenic liquid phase stream S8. After being depressurized by the first throttle valve 18, stream S8 splits into two streams: the first depressurized stream S9 and the second depressurized stream S19. The first low-pressure exhaust steam S6 and the second low-pressure exhaust steam S16, after being reheated, mix with the corresponding depressurized streams S9 and S19 to form the first low-pressure stream S10 and the second low-pressure stream S20, respectively. The first low-pressure stream S10 and the second low-pressure stream S20 then enter a condensation process (corresponding to the thermodynamic breakpoint between the first low-pressure stream and the first cryogenic liquid), resulting in the first cryogenic liquid S11. The first cryogenic liquid S11 returns to the inlet of the first working fluid pump 1 to complete the cycle.

[0038] The specific heat exchange flow information for the two-stage heat exchangerless system is shown in Table 1: Table 1: Optimized process flow information for the two-stage heat exchangerless system

[0039] To facilitate the explanation of the two-stage heat exchange system, the thermodynamic breakpoints in the two-stage non-heat exchange system are numbered as follows: the thermodynamic breakpoint between the first high-pressure stream S1 and the first two-phase stream S3 is cold stream 1; the thermodynamic breakpoint between the first gas phase stream S4 and the first superheated stream S5 is cold stream 2; the breakpoint at the connection between the first superheated stream S5 and the second superheated stream S1 is cold stream 3; and the breakpoint at the connection between the second low-pressure exhaust steam S16 and the first low-pressure exhaust steam S6 is hot stream. The following are the heat transfer streams: 1. The two disconnections between the mixing shared flow divider 17 and the first low-pressure flow stream S10 are heat transfer streams 2 and 3. The disconnection between the first liquid phase flow stream S7 and the first cryogenic liquid phase flow stream S8 is a heat transfer stream 4. The disconnection between the second low-pressure flow stream S20 and the cooling shared flow divider 19 is a heat transfer stream 5. The disconnection between the cooling shared flow divider 19 and the first cryogenic liquid flow stream S11 is a heat transfer stream 6. The disconnection between the regenerating shared flow divider 18 and the second depressurizing flow stream S19 is a heat transfer stream 7. The specific two-stage heat exchange system is as follows: Cold stream 1 passes sequentially through: the eighth regenerator 39 and hot stream 7 for heat exchange; the sixth regenerator 36 and hot stream 4 for heat exchange; the fifth regenerator 35 and hot stream 3 for heat exchange; the fourth regenerator 34 and hot stream 5 for heat exchange; then it splits into two branches: one enters the first evaporator 33 and exchanges heat with the heat source; the other enters the third regenerator 32 and exchanges heat with hot stream 2. Cold stream 2 passes sequentially through: the seventh regenerator 38 and hot stream 1 for heat exchange; the second regenerator 31 and hot stream 2 for heat exchange; the first regenerator 30 and hot stream 1 for heat exchange; the second superheater 29 and heat source for heat exchange. Cold stream 3 exchanges heat with the heat source through the first superheater 28. The cold source exchanges heat with hot stream 6 through the first condenser 37.

[0040] In the heat flow streams, except for heat flow stream 1, heat flow stream 2, and the heat source which pass through multiple heat exchangers, all other heat flow streams have only one heat exchanger. Heat flow stream 1 passes sequentially through: the first regenerator 30 and cold flow stream 2; the seventh regenerator 38 and cold flow stream 2. Heat flow stream 2 passes sequentially through: the second regenerator 31 and cold flow stream 2; the third regenerator 32 and cold flow stream 1. The heat source passes sequentially through: the first superheater 28 and cold flow stream 1; the second superheater 29 and cold flow stream 2; the first evaporator 33 and cold flow stream 1.

[0041] The specific heat exchanger data for the secondary heat exchange system are shown in Table 2.

[0042] Table 2: Heat exchanger data of the optimized two-stage heat exchange system

[0043] By matching the structural layout of the two-stage non-heat exchange system with the heat exchanger layout of the two-stage heat exchange system, a complete two-stage circulation system of mixed working fluid is obtained.

[0044] After being pressurized by the first working fluid pump 1, the circulating working fluid is preheated by passing through the eighth regenerator 39, the sixth regenerator 36, the fifth regenerator 35 and the fourth regenerator 34 in sequence. Then it is divided into two streams: one stream enters the first evaporator 33 to exchange heat with the heat source; the other stream enters the third regenerator 32 to exchange heat with the hot flow stream. After the heat exchange, the two fluids merge to form the first two-phase flow stream S3.

[0045] The first two-phase stream S3 enters the first separator 2 for gas-liquid separation, resulting in a first gas stream S4 and a first liquid stream S7. The first gas stream S4 is heated sequentially by the seventh regenerator 38, the second regenerator 31, the first regenerator 30, and the second superheater 29, and then splits into two streams: one is the first superheated stream S5, and the other is heated by the first superheater 28 to become the second superheated stream S15. Specifically, the first superheated stream S5 enters the first expander 3 to expand and perform work, discharging the first low-pressure exhaust steam S6; the second superheated stream S15 enters the second expander 11 to expand and perform work, discharging the second low-pressure exhaust steam S16.

[0046] The first liquid phase stream S7 is reheated by the sixth regenerator 36 to form the first low-temperature liquid phase stream S8. The first low-temperature liquid phase stream S8 is divided into two streams after being depressurized by the first throttle valve 18: one stream enters the eighth regenerator 39 for reheating to form the second depressurized stream S19; the other stream, as the first depressurized stream S9, enters the low-pressure side mixing process.

[0047] The second low-pressure exhaust steam S16 is reheated sequentially through the first regenerator 30 and the seventh regenerator 38, then mixed with the first low-pressure exhaust steam S16. After further reheating through the second regenerator 31 and the third regenerator 32, it splits into two streams: one enters the fifth regenerator 35 for heat exchange, forming the first low-pressure stream S10; the other mixes with the second depressurization stream S19, forming the second low-pressure stream S20. The second low-pressure stream S20, after heat exchange in the fourth regenerator 34, mixes with the first low-pressure stream S10 and the first depressurization stream S9, and enters the first condenser 37 for condensation, yielding the first cryogenic liquid S11. The first cryogenic liquid S11 returns to the inlet of the first working fluid pump 1 for pressurization, completing the cycle.

[0048] According to Aspen Hysys simulation software, the net output power of the two-stage mixed working fluid circulation system in this embodiment is 18979.68 kW, and the average heat exchange temperature difference is 30.67 ℃. Compared with the traditional two-stage Karina circulation system using ammonia water as the working fluid, the net output power increases by approximately 42.71% and the average heat exchange temperature difference decreases by 39.04% under the same operating conditions. This embodiment achieves a structural topology design without a two-stage heat exchange system and optimization of the two-stage heat exchange system.

[0049] The Carnot battery energy storage system operates in a charging phase during off-peak electricity demand and a discharging phase during peak demand. It comprises a heat pump system, a thermal storage system, and a two-stage mixed-working-fluid circulation system. During the charging phase, the heat pump system converts electrical energy into heat energy. During the discharging phase, the system employs a two-stage mixed-working-fluid circulation system. Specifically, the heat pump system is connected to the thermal storage system via a heat pump system condenser 42, used to transfer heat to the thermal storage medium during the charging phase, completing the conversion from electrical energy to heat energy. The thermal storage system, acting as a heat source, is connected to the two-stage mixed-working-fluid circulation system to provide heat to the system during the discharging phase.

[0050] The heat pump system includes a heat pump evaporator 40, a heat pump compressor 41, a heat pump condenser 42, and a heat pump expansion valve 43. The outlet of the heat pump evaporator 40 is connected to the inlet of the heat pump compressor 41; the outlet of the heat pump compressor 41 is connected to the inlet of the heat pump condenser 42; the outlet of the heat pump condenser 42 is connected to the inlet of the heat pump expansion valve 43; and the outlet of the heat pump expansion valve 43 is connected to the inlet of the heat pump evaporator 40, forming a heat pump working fluid circulation loop. The heat pump condenser 42 is also connected to a heat storage system to transfer the heat released by the heat pump working fluid to the heat storage medium. The heat pump evaporator 40 is used to absorb low-temperature waste heat. The heat pump compressor 41 is used to compress the working fluid using externally input electrical energy during the charging phase to increase the working fluid's pressure and temperature. The heat pump condenser 42 is used to transfer heat to the heat storage medium. The heat pump expansion valve 43 is used to reduce the working fluid pressure.

[0051] The thermal storage system includes a cold water storage tank 44 and a hot water storage tank 45. The outlet of the cold water storage tank 44 is connected to the inlet of the thermal storage medium of the condenser 42 of the heat pump system. The outlet of the thermal storage medium of the condenser 42 is connected to the inlet of the hot water storage tank 45. The outlet of the hot water storage tank 45 serves as the heat source for a two-stage mixed working fluid circulation system and is connected to the system. The outlet of the heat source of the two-stage mixed working fluid circulation system is connected to the inlet of the cold water storage tank 44, forming a thermal storage medium circulation loop. The hot water storage tank 45 stores the high-temperature thermal storage medium after heat absorption; the cold water storage tank 44 stores the low-temperature thermal storage medium after heat release. During the charging phase, the heat storage medium absorbs the heat released by the heat pump system through the heat pump system condenser 42 and flows from the heat storage system cold water tank 44 into the heat storage system hot water tank 45. During the discharge phase, it releases heat to the mixed working fluid two-stage circulation system and flows back from the heat storage system hot water tank 45 to the heat storage system cold water tank 44, thereby realizing the storage and release of heat.

[0052] The above-described embodiments are merely illustrative of the implementation methods of the present invention, but should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the protection scope of the present invention.

Claims

1. A design method for a cyclic system for the discharge process of a Carnot battery, characterized in that, The design method includes the following steps: Step 1: Construct a single-stage Karina cycle system consisting of a first working fluid pump (1), a first separator (2), a first expander (3), a first throttle valve (4), an evaporator (5), a superheater (6), a regenerator (7), and a condenser (8), and divide it into a single-stage non-heat exchange process system and a single-stage heat exchange process system; define the evaporator (5), superheater (6), regenerator (7), and condenser (8) as a single-stage heat exchange process system, and define the first working fluid pump (1), the first separator (2), the first expander (3), and the first throttle valve (4) as a single-stage non-heat exchange process system; Step 2: Integrate two identical single-stage heat exchange-free systems to construct a two-stage pre-designed heat exchange-free system, including a first-stage heat exchange-free system and a second-stage heat exchange-free system, which correspond to the first single-stage heat exchange-free system and the second single-stage heat exchange-free system, respectively. Both single-stage heat-exchange-free process systems include a working fluid pump, a separator, an expander, and a throttling valve, and their flow streams are defined as the first flow stream and the second flow stream, respectively. In the two-stage pre-designed heat-exchange-free system, the coupling connection between the two single-stage heat-exchange-free process systems is achieved by setting a flow splitting structure. Step 3: Optimize the parameters of the two-stage preset heat exchange-free system obtained in Step 2. With the goal of maximizing the net output power of the two-stage preset heat exchange-free system, solve for the pressure, temperature and working fluid composition of each stream in the two-stage preset heat exchange-free system. The net output power of the two-stage preset heat exchange-free system is the difference between the sum of the output power of the first expander and the second expander and the sum of the power consumed by the first working fluid pump and the second working fluid pump. Step 4: Based on the flow information of the two-stage preset heat exchange-free system obtained in Step 3 and the constructed optimal heat exchange network structure, connect the hot and cold flow streams at each break point to the corresponding heat exchangers, so that the two-stage preset heat exchange-free system and the two-stage preset heat exchange system form a complete closed loop structure, thereby obtaining a complete two-stage mixed working fluid loop system.

2. The design method of a cyclic system for the discharge process of a Carnot battery according to claim 1, characterized in that, In the single-stage Karina circulation system of step 1: The high-pressure stream S1 enters the regenerator (7) and exchanges heat with the liquid stream S7 from the first separator (2), absorbing heat and increasing in temperature. The preheated stream after absorbing heat is S2. The preheated stream S2 enters the evaporator (5) and becomes a two-phase stream S3. After the two-phase stream S3 passes through the first separator (2) for gas-liquid separation, the gas stream S4 passes through the superheater (6) and becomes a superheated stream S5. The superheated stream S5 enters the first expander (3) to output power. The first expander (3) The outlet is low-pressure exhaust steam S6. After separation, the liquid phase stream S7 passes through the regenerator (7) to release heat and becomes low-temperature liquid phase stream S8. The low-temperature liquid phase stream S8 enters the first throttle valve (4) for pressure reduction. The stream after pressure reduction is the pressure-reducing stream S9. The pressure-reducing stream S9 mixes with the expanded low-pressure exhaust steam S6 to form a low-pressure stream S10. The low-pressure stream S10 enters the condenser (8) for condensation. The condensed low-temperature liquid S11 re-enters the first working fluid pump (1) for pressurization and the next cycle. The single-stage heat exchange-free system is used to represent the pressurization, separation, expansion, and throttling processes in the cycle other than heat exchange. Its operation does not include heat exchange equipment. A thermodynamic breakpoint is set at the original heat exchanger location, and the thermodynamic changes before and after heat exchange are calculated using the thermodynamic state parameters at the breakpoint. The thermodynamic breakpoint is defined as a virtual cross-section set at the inlet and outlet of the original heat exchanger, used to represent the changes in the thermodynamic state parameters of the working fluid before and after heat exchange. These thermodynamic state parameters include temperature, pressure, working fluid flow rate, working fluid composition, and thermodynamic property parameters. The thermodynamic state parameters at the thermodynamic breakpoint are calculated using the mass conservation equation, energy conservation equation, and phase equilibrium relationship.

3. The design method of a cyclic system for the discharge process of a Carnot battery according to claim 2, characterized in that, The thermodynamic breakpoint locations include: high-pressure stream S1 and preheating stream S2, preheating stream S2 and two-phase stream S3, gas stream S4 and superheated stream S5, liquid stream S7 and cryogenic liquid stream S8, and low-pressure stream S10 and cryogenic liquid S11. Among them, the heat exchangers of high-pressure stream S1 and preheating stream S2, and preheating stream S2 and two-phase stream S3 are adjacent and there are no other components between the breakpoints. These two breakpoints are merged into one. The operating principle of the single-stage heat exchange process-free system is the same as that of the single-stage Karina cycle system.

4. The design method of a cyclic system for the discharge process of a Carnot battery according to claim 1, characterized in that, In step 2: The distributor is located at the following positions: compression process connection, separation process connection, superheating process connection, expansion process connection, mixing process connection, regeneration process connection, and cooling process connection. Each splitter has two mutually exclusive split ratios, which are either 0 or 1, and the sum of the split ratios of all paths at the same split location is 1. The splitter is used to represent the selection of the actual pipeline connection method, and its split ratio of 0 or 1 corresponds to the closed or open state of the connection path, respectively. Finally, the specific circulation path is transformed into a mathematical model for subsequent optimization. In addition, in order to avoid heat loss caused by differences in thermodynamic parameters, a new thermodynamic breakpoint exists at each connection point.

5. The design method of a cyclic system for the discharge process of a Carnot battery according to claim 4, characterized in that, Step 2 specifically involves: Step 2.1: A compression splitting structure is set at the outlet of the second working fluid pump (9) to control the coupling mode between the first-stage non-heat exchange process system and the second-stage non-heat exchange process system during the compression process; the compression splitting structure includes a compression shared path and a compression non-shared path, which correspond to the compression shared splitter (13) and the compression non-shared splitter (20) respectively. The two satisfy mutual exclusion constraints, and the sum of their splitting ratios is 1. Step 2.2: After the pressurization process of the two-stage pre-set heat exchange-free system is completed, the circulating working fluid enters the breakpoint position corresponding to the separation process. The breakpoints are the state point of the two-phase flow S3 in the first-stage heat exchange-free system before entering the first separator (2) and the state point of the second two-phase flow S13 in the second-stage heat exchange-free system before entering the second separator (10). A separation and diversion structure is set in front of the first separator (2) and the second separator (10) to control the coupling mode of the first-stage heat exchange-free system and the second-stage heat exchange-free system in the separation process. The separation and diversion structure includes a separation shared path and a separation non-shared path, corresponding to the separation shared diversion device (14) and the separation non-shared diversion device (21). The two satisfy mutual exclusion constraints, and the sum of their diversion ratios is 1. Step 2.3: After completing the separation process of the two-stage preset heat exchange-free system, the circulating working fluid enters the breakpoint position corresponding to the superheating process. The breakpoints are the state point between the gas phase stream S4 and the first superheated stream S5 in the first-stage heat exchange-free system, and the state point between the second gas phase stream S14 and the second superheated stream S15 in the second-stage heat exchange-free system. An overheating split structure is set after the first gas phase flow stream S4 and the second gas phase flow stream S14 to control the coupling mode between the first-stage non-heat exchange process system and the second-stage non-heat exchange process system during the overheating process; the overheating split structure includes an overheating shared path and an overheating non-shared path, corresponding to an overheating shared splitter (15) and an overheating non-shared splitter (22), which satisfy mutual exclusion constraints and the sum of their split ratios is 1; Step 2.4: After completing the pre-designed flow charts for the superheating and expansion processes of the two-stage pre-designed heatless system, the pre-designed flow chart for the liquid phase regeneration process of the two-stage pre-designed heatless system is then constructed. After regeneration, the first liquid phase stream S7 and the second liquid phase stream S17 will exit as the first low-temperature liquid phase stream S8 and the second low-temperature liquid phase stream S18, respectively. The regeneration process in the two-stage pre-designed heatless system is represented by breakpoints, and the breakpoints corresponding to the regeneration process are the liquid phase streams of the single-stage heatless system. At the break point between S7 and the low-temperature liquid phase stream S8, the break point corresponding to the two-stage preset no-heat exchange system is the break point between the first liquid phase stream S7 and the first low-temperature liquid phase stream S8 and the break point between the second liquid phase stream S17 and the second low-temperature liquid phase stream S18; after the first low-temperature liquid phase stream S8 and the second low-temperature liquid phase stream S18 are depressurized by the first throttle valve (4) and the second throttle valve (12), the circulation system path is selected through the regenerative shared flow divider (18) and the regenerative non-shared flow divider (25); Step 2.5: After completing the pre-set process construction of the two-stage pre-set liquid phase regeneration process without heat exchange system, construct the pre-set process construction of the two-stage pre-set mixing process without heat exchange system. A hybrid flow splitting structure is set after the first pressure-reducing flow stream S9 and the second pressure-reducing flow stream S19 to control the coupling mode of the primary and secondary non-heat exchange process systems on the low-pressure side; the hybrid flow splitting structure includes a hybrid shared path and a hybrid non-shared path, corresponding to a hybrid shared flow splitter (17) and a hybrid non-shared flow splitter (24), which satisfy mutual exclusion constraints and the sum of their splitting ratios is 1; Step 2.6: After the two-stage preset heat exchange-free system completes the low-pressure side mixing process, the second low-pressure stream S20 enters the cooling split structure to control the coupling mode between the first-stage heat exchange-free system and the second-stage heat exchange-free system during the condensation process; the cooling split structure includes a cooling shared path and a cooling non-shared path, corresponding to the cooling shared splitter (19) and the cooling non-shared splitter (26), which satisfy mutual exclusion constraints, and the sum of their split ratios is 1.

6. The design method of a cyclic system for the discharge process of a Carnot battery according to claim 5, characterized in that, In step 2: In step 2.1: when the split ratio of the non-shared compressor splitter (20) is 1, the fluid at the outlet of the second working fluid pump (9) enters the subsequent process of the secondary cycle as an independent second high-pressure stream S12; when the split ratio of the shared compressor splitter (13) is 1, the working fluid of the secondary non-heat exchange process system, after entering the second working fluid pump (9), mixes with the first low-temperature liquid S11 from the outlet of the condenser of the primary non-heat exchange process system at the same pressure, and the mixed fluid enters the first working fluid pump (1) together for pressurization to form the first high-pressure stream S1; In step 2.2: when the split ratio of the non-shared splitter (21) is 1, the second two-phase flow S13 of the secondary non-heat exchange process system enters the second separator as an independent flow for gas-liquid separation, and the second gas phase flow S14 and the second liquid phase flow S17 are obtained; when the split ratio of the shared splitter (14) is 1, the two-phase flow S3 of the primary non-heat exchange process system and the second two-phase flow S13 of the secondary non-heat exchange process system are mixed under the same pressure conditions to form the first two-phase flow S3, and the first two-phase flow S3 enters the first separator for gas-liquid separation, and the first gas phase flow S4 and the first liquid phase flow S7 are obtained. In step 2.3: when the split ratio of the superheated non-shared splitter (22) is 1, the first superheated stream S5 and the second superheated stream S15 enter their respective expanders to expand and do work; wherein, the first superheated stream S5 enters the first expander (3) to expand and obtain the first low-pressure exhaust steam S6; the second superheated stream S15 enters the second expander (11) to expand and obtain the second low-pressure exhaust steam S16; the outlet of the second expander (11) is provided with an expansion splitting structure to control whether the second non-heat exchange process system is coupled with the first-stage non-heat exchange process system; the expansion splitting structure includes an expansion shared path and an expansion non-shared path, corresponding to the expansion The shared splitter (16) and the expansion non-shared splitter (23) satisfy mutual exclusion constraints, and their split ratios are summed to 1. When the split ratio of the expansion non-shared splitter (23) is 1, the second low-pressure exhaust steam S16 enters the subsequent mixing process as an independent stream. When the split ratio of the superheated shared splitter (15) is 1, the working fluid of the secondary non-heat exchange process system enters the second expander (11), expands to the same pressure as the first superheated stream S5, and mixes with the first superheated stream S5 from the outlet of the superheater of the primary non-heat exchange process system at the same pressure. The mixed fluid enters the first expander (3) together for expansion to form the first low-pressure exhaust steam S6. In step 2.4: when the split ratio of the regenerating shared splitter (18) is 1, the split ratio of the regenerating non-shared splitter (25) is 0, and the circulating working fluid of the first-stage non-heat exchange process system and the circulating working fluid of the second-stage non-heat exchange process system are mixed to form the first depressurization stream S9; when the split ratio of the regenerating shared splitter (18) is 0, the circulating working fluid of the first-stage non-heat exchange process system is not mixed with the circulating working fluid of the second-stage non-heat exchange process system, and the first depressurization stream S9 and the second depressurization stream S19 are calculated independently; In step 2.5: when the split ratio of the mixing and sharing splitter (17) is 1, the first low-pressure exhaust steam S6 and the second low-pressure exhaust steam S16 are mixed under the same pressure and temperature conditions, and then mixed with the first pressure-reducing stream S9 to form the first low-pressure stream S10; when the split ratio of the mixing and sharing splitter (17) is 0, the first low-pressure exhaust steam S6 and the second low-pressure exhaust steam S16 are mixed with the first pressure-reducing stream S9 and the second pressure-reducing stream S19 respectively under the same pressure conditions after heat exchange, to form the first low-pressure stream S10 and the second low-pressure stream S20 respectively. In step 2.6: when the split ratio of the cooling non-shared splitter (26) is 1, the first low-pressure stream S10 and the second low-pressure stream S20 enter their respective condensers for condensation, respectively obtaining the first cryogenic liquid S11 and the second cryogenic liquid S21; when the split ratio of the cooling shared splitter (19) is 1, the second low-pressure stream S20 and the first low-pressure stream S10 are mixed under the same pressure conditions and enter the same condenser for condensation, obtaining the first cryogenic liquid S11; thus, the closure of the two-stage preset heat exchange-free system is completed.

7. The design method of a cyclic system for the discharge process of a Carnot battery according to claim 6, characterized in that, In step 3: The optimization calculation is based on the mass conservation equation and the energy conservation equation. It calculates the thermodynamic state parameters of the stream at each breakpoint in the two-stage pre-set heat exchange-free system. The thermodynamic state parameters include temperature, pressure, mass flow rate and working fluid composition.

8. The design method of a cyclic system for the discharge process of a Carnot battery according to claim 7, characterized in that, Step 3 specifically involves: The thermodynamic state parameters at all thermodynamic breakpoints are input into the Aspen Energy Analyzer to obtain the structure of the two-stage pre-defined heat exchange system, including the number of heat exchangers, the connection relationships between heat exchange units, and the heat load distribution of each heat exchange unit. An optimal heat exchange network structure satisfying the constraints is then constructed. The two-stage pre-defined heat exchange system is constructed based on the following constraints: The energy conservation constraint of the hot and cold flow streams corresponds to the following formula (1). The minimum temperature difference constraint of the heat exchanger corresponds to the following formula (2); The mass conservation constraint of the hot and cold streams in the heat exchange process corresponds to the following formula (3). （1） （2） （3） In the formula, and The heat exchange capacity of the cold or hot stream of the k-th heat exchanger, where the superscripts h and c represent the hot and cold streams respectively, and the subscript k indicates the k-th heat exchanger, with the unit being kW; It is the minimum heat transfer temperature difference of the k-th heat exchanger. It is the minimum heat transfer temperature difference of a two-stage preset heat exchange system. and These are the inlet and outlet temperatures of the heat stream in the k-th heat exchanger, respectively. and These are the inlet and outlet temperatures of the cold stream of the k-th heat exchanger, respectively. The superscript k indicates the k-th heat exchanger, the subscript h represents the hot stream, and the subscripts in and out represent the inlet and outlet temperatures of the stream, all in °C. It is the mass flow rate of the k-th branch stream at the outlet of the i-th splitter. It is the mass flow rate of the k-th branch stream at the inlet of the i-th splitter. It is the total mass flow rate of the circulating working fluid flowing to the i-th splitter, where the superscripts in and out represent the inlet and outlet of the splitter, the subscript i is the i-th splitter, and the subscript k is the k-th branch stream, and the unit is kg / s.

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