Cascade cycle Carnot cell system of coupling coal-fired unit and optimization method

Abstract

The invention discloses a cascade cycle Carnot cell system of a coupling coal-fired unit and an optimization method. Relates to the technical field of coal power unit energy utilization, the system comprises a low-temperature circulation unit, a medium-temperature circulation unit and an energy storage release unit, the low-temperature circulation unit is used for recovering waste heat from flue gas of a coal-fired unit, and the waste heat is transmitted to the medium-temperature circulation unit after being heated and pressurized; the medium-temperature circulation unit is used for further heating and pressurizing and then transferring heat to the energy storage and release unit; the low-temperature circulating unit comprises a low-temperature evaporator, a compressor I, a heat exchanger I and a throttle valve I; low-temperature flue gas in the coal-fired unit and exhaust steam waste heat of the low-pressure cylinder are utilized in a gradient mode, the utilization rate of low-grade heat energy is effectively increased, and waste heat of the coal-fired unit is efficiently coupled and utilized; according to the performance optimization method disclosed by the invention, the energy efficiency index (RTE and efficiency) and the economic index (LCOE) of the system are considered, a group of Pareto optimal solutions can be obtained under various operating parameters and constraint conditions, and the global optimality and decision-making flexibility of system operation are enhanced.

Description

Technical Field

[0001] This invention relates to the field of energy utilization technology for coal-fired power units, and in particular to a cascaded cycle Carnot battery system coupled with a coal-fired power unit and its optimization method. Background Technology

[0002] To improve the flexibility of coal-fired power units in high-proportion renewable energy power systems, technologies such as thermoelectric decoupling and thermal energy storage for peak shaving are gaining increasing attention. Among them, the Carnot Battery, an "electric-thermal-electric" energy storage system, uses electricity to drive a high-temperature heat pump to store thermal energy in a thermal storage tank, and then releases it as electrical energy when needed through thermoelectric conversion equipment (such as a heat engine). Its operation does not rely on rare metals or chemical reactions, and it has sustainability and safety.

[0003] Traditional Carnot batteries are mostly single-stage heat pump-heat engine systems, which suffer from problems such as low heat storage temperature difference and limited cycle efficiency. Cascaded cycle Carnot battery systems improve the system's energy density and cycle efficiency through multi-stage temperature zone division. Currently, there are still many technical challenges in coupling Carnot battery systems with coal-fired power units, such as poor system matching, unreasonable energy level distribution, and insufficient optimization of thermodynamic cycle parameters. Therefore, there is an urgent need for a structurally sound, highly efficient, and highly adjustable cascaded cycle Carnot battery system, along with performance optimization methods, to achieve efficient coupling and coordinated operation with coal-fired power units, thereby improving the overall system's economy and flexibility. Summary of the Invention

[0004] To achieve efficient coupling and coordinated operation of the Carnot battery system and the coal-fired power unit, this application provides a cascaded cycle Carnot battery system coupled with the coal-fired power unit and an optimization method.

[0005] In a first aspect, this application provides a cascaded cycle Carnot battery system coupled to a coal-fired power unit, employing the following technical solution:

[0006] A cascaded cycle Carnot battery system coupled to a coal-fired power unit includes a low-temperature cycle unit, a medium-temperature cycle unit, and an energy storage and release unit. The low-temperature cycle unit is used to recover waste heat from the flue gas of the coal-fired power unit, and after heating and pressurizing, transfers it to the medium-temperature cycle unit. The medium-temperature cycle unit is used to further heat and pressurize the heat and then transfer it to the energy storage and release unit.

[0007] The low-temperature circulation unit includes a low-temperature evaporator, compressor I, heat exchanger I, and expansion valve I; the working fluid side of the low-temperature evaporator, the working fluid side of compressor I, the working fluid side of heat exchanger I, and expansion valve I are connected in series.

[0008] The low-temperature circulation unit is connected to the medium-temperature circulation unit through heat exchanger I, and to the energy storage and release system through the low-temperature evaporator; the medium-temperature circulation unit is connected to the energy storage and release system through heat exchanger II.

[0009] Optionally, the intermediate temperature circulation unit includes heat exchanger I, compressor II, and throttle valve II; the medium side of heat exchanger I is connected in series with compressor II, the working fluid side of heat exchanger II, and throttle valve II; the working fluid of the intermediate temperature circulation unit exchanges heat with the exhaust steam from the low-pressure cylinder of the coal-fired unit in heat exchanger II.

[0010] Optionally, the energy storage and release system includes heat exchanger II, compressor III, valve I, high-temperature storage tank, valve II, expander, three-way valve, and low-temperature evaporator; the medium side of heat exchanger II is connected in series with compressor III, valve I, high-temperature storage tank, valve II, expander, three-way valve, and low-temperature evaporator; the other inlet of the three-way valve is connected to the boiler flue gas outlet, and the steam side outlet of the low-temperature evaporator is connected to a condenser.

[0011] Optionally, the working fluid of the low-temperature cycling unit and the medium-temperature cycling unit is any one of the following: R1234yf, R1234ze(E), R1234ze(Z), R1224yd(Z), R1233zd(E), R1336mzz(Z).

[0012] Optionally, compressor I, compressor II and compressor III are centrifugal compressors or positive displacement compressors, and the expander is a turbine expander.

[0013] Secondly, this application also proposes an optimization method for a cascaded cycle Carnot battery system coupled with a coal-fired power unit, employing the following technical solution:

[0014] An optimization method for a cascaded cycle Carnot battery system coupled with a coal-fired power unit includes the following steps:

[0015] S1. Construct a physical model of the system;

[0016] S2. Determine the parameters and performance indicators that need to be optimized;

[0017] S3. Set various constraints;

[0018] S4. Multi-objective optimization based on NSGA-II algorithm;

[0019] S5. Determine the optimal operating point using the entropy weight method.

[0020] Optionally, in step S2, the parameters to be optimized include: low-temperature working fluid flow rate, medium-temperature working fluid flow rate, pinch point temperature difference of the three heat exchangers, compression ratio of each compressor and expansion ratio of the expander, compressor isentropic efficiency, expander isentropic efficiency; performance indicators include round-trip efficiency RTE, efficiency η, and levelized cost of electricity LCOE.

[0021] Optionally, the constraints mentioned in step S3 refer to the range of values ​​that the parameters must meet to ensure the normal operation of the system, including the compressor compression ratio setting must ensure a reasonable outlet working fluid temperature and a reasonable temperature constraint relationship between the working fluid and the heat exchange medium.

[0022] Optionally, the objective function of the multi-objective optimization in step 4 is... as follows

[0023] .

[0024] In summary, this application includes the following beneficial technical effects:

[0025] (1) The cascaded cycle Carnot battery system proposed in this invention constructs a cascaded cycle structure of low temperature cycle, medium temperature cycle and energy storage and release system, and utilizes the low temperature flue gas and low pressure cylinder exhaust waste heat in coal-fired units in a stepwise manner, which effectively improves the utilization rate of low-grade heat energy and efficiently couples and utilizes the waste heat of coal-fired units.

[0026] (2) The cascaded cycle Carnot battery system proposed in this invention utilizes the Carnot cycle principle and combines multi-stage compression and expansion processes to enable the system to maintain high thermodynamic efficiency during energy storage and release, thereby significantly improving the system's round-trip efficiency and efficiency, and enabling the Carnot battery system to maintain high-efficiency operation.

[0027] (3) This invention proposes a performance optimization method based on NSGA-II, which takes into account both system energy efficiency indicators (RTE and energy efficiency) and economic indicators (LCOE). It can obtain a set of Pareto optimal solutions under various operating parameters and constraints, enhancing the global optimality and decision-making flexibility of the system operation. The entropy weight method is introduced to comprehensively evaluate and weigh the results of multi-objective optimization, achieving scientific selection of the optimal operating point and providing accurate decision-making basis for system design and operation scheduling. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the overall structure of the system used in this application.

[0029] Explanation of reference numerals in the attached figures:

[0030] 1. Low-temperature evaporator; 2. Compressor I; 3. Heat exchanger I; 4. Throttling valve I; 5. Compressor II; 6. Heat exchanger II; 7. Throttling valve II; 8. Compressor III; 9. Valve I; 10. High-temperature storage tank; 11. Valve II; 12. Expander; 13. Three-way valve. Detailed Implementation

[0031] The following is in conjunction with the appendix Figure 1 This application will be described in further detail.

[0032] This application discloses a cascaded cycle Carnot battery system coupled to a coal-fired power unit, including a low-temperature cycle unit, a medium-temperature cycle unit, and an energy storage and release unit;

[0033] The low-temperature circulation unit includes a low-temperature evaporator 1, a compressor I2, a heat exchanger I3, and a throttling valve I4; the working fluid side of the low-temperature evaporator 1, the working fluid side of the compressor I2, the heat exchanger I3, and the throttling valve I4 are connected in series.

[0034] The intermediate temperature circulation unit includes heat exchanger I3, compressor II, heat exchanger II6 and throttle valve II117; the medium side of heat exchanger I3 is connected in series with the working fluid side of compressor II, heat exchanger II6 and throttle valve II117 in sequence; the working fluid of the intermediate temperature circulation unit exchanges heat with the exhaust steam of the low-pressure cylinder of the coal-fired unit in heat exchanger II6.

[0035] The energy storage and release system includes heat exchanger II6, compressor III8, valve I9, high-temperature storage tank 10, valve II11, expander 12, three-way valve 13, and low-temperature evaporator 1. The medium side of heat exchanger II6 is connected in series with compressor III8, valve I9, high-temperature storage tank 10, valve II11, expander 12, three-way valve 13, and low-temperature evaporator 1. The other inlet of three-way valve 13 is connected to the boiler flue gas outlet, and the steam side outlet of low-temperature evaporator 1 is connected to a condenser.

[0036] The low-temperature circulation unit is connected to the medium-temperature circulation unit through heat exchanger I3 and to the energy storage and release system through low-temperature evaporator 1. The medium-temperature circulation unit is connected to the energy storage and release system through heat exchanger II6. The low-temperature circulation unit is used to recover waste heat from the flue gas of the coal-fired unit, and after heating and pressurizing, it is transferred to the medium-temperature circulation unit. The medium-temperature circulation unit is used to further heat and pressurize and then transfer the heat to the energy storage and release unit. After collecting the exhaust gas from the low-pressure cylinder of the coal-fired unit, the gas is heated by the heat of the medium-temperature circulation unit and stored to achieve the effect of charging.

[0037] The working fluid for the low-temperature cycling unit and the medium-temperature cycling unit is any one of the following: R1234yf, R1234ze(E), R1234ze(Z), R1224yd(Z), R1233zd(E), R1336mzz(Z).

[0038] Compressors I2, II and III8 are centrifugal compressors or positive displacement compressors, and expander 12 is a turbine expander 12.

[0039] Its overall operating method includes:

[0040] (1) Energy storage process: During off-peak hours or when there is excess power in the grid, valve I9 ​​is ​​opened and valve II11 is closed. The system initiates a multi-stage compression energy storage process, the specific process of which is as follows:

[0041] The low-temperature working fluid first enters the low-temperature evaporator 1, absorbs the waste heat from the low-temperature flue gas of the coal-fired unit, and evaporates into low-temperature, low-pressure steam; then it enters the compressor I2 and is compressed into high-temperature, medium-pressure steam, and enters the heat exchanger I3 to release heat. After releasing heat, it is throttled and depressurized by the throttling valve I4 and returns to the low-temperature evaporator 1 to complete the first stage of the cycle.

[0042] After absorbing the heat released by the low-temperature working fluid in heat exchanger I3, the medium-temperature working fluid enters compressor II and is further compressed into high-temperature medium-pressure steam. Then it enters heat exchanger II6 to release heat. After releasing heat, it is throttled and depressurized by throttling valve II117 and returns to the inlet of heat exchanger I3 to complete the second stage of the cycle.

[0043] The exhaust steam from the low-pressure cylinder enters the heat exchanger II6, absorbs the heat released by the medium-temperature working fluid, and then enters the compressor III88 to be compressed into high-temperature and high-pressure steam. Finally, it is sent to the high-temperature storage tank 10 through valve I for storage, completing the energy storage process.

[0044] During this process, the system uses a three-stage compression cycle to recover and integrate the waste heat from the flue gas of the coal-fired unit and the waste heat from the exhaust steam of the low-pressure cylinder. It converts the surplus electrical energy and the low-grade thermal energy in the system into high-grade thermal energy and pressure energy, which are stored in the high-temperature storage tank 10, thus achieving efficient energy transfer and storage.

[0045] (2) Energy release process: During peak power hours or when there is a peak in electricity consumption, valve II11 is opened and valve I9 ​​is ​​closed, and the system enters the energy release stage. The specific process is as follows:

[0046] High-temperature, high-pressure steam from high-temperature storage tank 10 first enters expander 12 to expand and perform work, driving generator rotation and converting stored thermal and pressure energy into electrical energy output, effectively supplementing the grid load during peak power periods. After expansion, the steam pressure and temperature decrease significantly, becoming exhaust steam in a medium-to-low pressure state. This exhaust steam then mixes with low-temperature flue gas from the boiler tail in three-way valve 13, and the mixed gas flow enters low-temperature evaporator 1 together, serving as a heat source to heat the low-temperature working fluid. After completing heat transfer, the exhaust steam temperature further decreases, finally entering the condenser to complete phase change condensation, with the condensate being recycled by the system. At this point, the entire energy release process is complete.

[0047] This energy release process not only efficiently converts the stored high-grade thermal energy and pressure energy into electrical energy output through expansion work, but also achieves the cascade recovery and comprehensive utilization of waste heat by mixing exhaust steam and low-temperature flue gas in the low-temperature evaporator 1, which significantly improves the overall energy utilization efficiency of the system.

[0048] This invention also provides a performance optimization method for a cascaded cycle Carnot battery system coupled with a coal-fired power unit, comprising the following steps:

[0049] Step 1: Build the system physical model.

[0050] Step 2: Determine the parameters and performance metrics that need to be optimized.

[0051] Step 3: Set various constraints.

[0052] Step 4: Perform multi-objective optimization based on the NSGA-II algorithm.

[0053] Step 5: Determine the optimal operating point using the entropy weight method.

[0054] The parameters to be optimized in step 2 include: low temperature working fluid flow rate, medium temperature working fluid flow rate, pinch temperature difference of the three heat exchangers, compression ratio of each compressor and expansion ratio of expander 12, isentropic efficiency of compressor, and isentropic efficiency of expander 12; performance indicators include round-trip efficiency (RTE), efficiency η, and levelized cost of electricity (LCOE).

[0055] The constraints in step 3 refer to the range of values ​​that the parameters must meet to ensure the normal operation of the system. For example, the compressor compression ratio setting must ensure a reasonable outlet working fluid temperature and a reasonable temperature constraint relationship between the working fluid and the heat exchange medium.

[0056] The objective function for multi-objective optimization in step 4 is as follows:

[0057] .

[0058] The above are all preferred embodiments of this application, and are not intended to limit the scope of protection of this application. Therefore, all equivalent changes made in accordance with the structure, shape and principle of this application should be covered within the scope of protection of this application.

Claims

1. A cascaded cycle Carnot battery system coupled to a coal-fired power unit, characterized in that: It includes a low-temperature circulation unit, a medium-temperature circulation unit, and an energy storage and release unit. The low-temperature circulation unit is used to recover waste heat from the flue gas of the coal-fired unit, and after heating and pressurizing, it is transferred to the medium-temperature circulation unit. The medium-temperature circulation unit is used to transfer heat to the energy storage and release unit after further heating and pressurization; The low-temperature circulation unit includes a low-temperature evaporator, compressor I, heat exchanger I, and expansion valve I; The working fluid side of the low-temperature evaporator, the working fluid side of compressor I, the working fluid side of heat exchanger I, and throttling valve I are connected in series in sequence; The low-temperature circulation unit is connected to the medium-temperature circulation unit through heat exchanger I, and to the energy storage and release system through the low-temperature evaporator; the medium-temperature circulation unit is connected to the energy storage and release system through heat exchanger II.

2. The cascaded cycle Carnot battery system coupled to a coal-fired power unit according to claim 1, characterized in that: The intermediate temperature circulation unit includes heat exchanger I, compressor II, and throttle valve II; the medium side of heat exchanger I is connected in series with compressor II, the working fluid side of heat exchanger II, and throttle valve II in sequence; the working fluid of the intermediate temperature circulation unit exchanges heat with the exhaust steam from the low-pressure cylinder of the coal-fired unit in heat exchanger II.

3. The cascaded cycle Carnot battery system coupled to a coal-fired power unit according to claim 2, characterized in that: The energy storage and release system includes heat exchanger II, compressor III, valve I, high-temperature storage tank, valve II, expander, three-way valve, and low-temperature evaporator; the medium side of heat exchanger II is connected in series with compressor III, valve I, high-temperature storage tank, valve II, expander, three-way valve, and low-temperature evaporator; the other inlet of the three-way valve is connected to the boiler flue gas outlet, and the steam side outlet of the low-temperature evaporator is connected to a condenser.

4. A cascaded cycle Carnot battery system coupled to a coal-fired power unit according to claim 3, characterized in that: The working fluid of the low-temperature cycle unit and the medium-temperature cycle unit is any one of the following: R1234yf, R1234ze(E), R1234ze(Z), R1224yd(Z), R1233zd(E), R1336mzz(Z).

5. A cascaded cycle Carnot battery system coupled to a coal-fired power unit according to claim 4, characterized in that: The compressors I, II and III are centrifugal compressors or positive displacement compressors, and the expander is a turbine expander.

6. An optimization method for a cascaded cycle Carnot battery system coupled to a coal-fired power unit, used to optimize the cascaded cycle Carnot battery system coupled to a coal-fired power unit as described in claim 5, characterized in that: Includes the following steps: S1. Construct a physical model of the system; S2. Determine the parameters and performance indicators that need to be optimized; S3. Set various constraints; S4. Multi-objective optimization based on NSGA-II algorithm; S5. Determine the optimal operating point using the entropy weight method.

7. The optimization method for a cascaded cycle Carnot battery system coupled with a coal-fired power unit according to claim 6, characterized in that: In step S2, the parameters that need to be optimized include: low-temperature working fluid flow rate, medium-temperature working fluid flow rate, pinch point temperature difference of the three heat exchangers, compression ratio of each compressor and expansion ratio of the expander, compressor isentropic efficiency, and expander isentropic efficiency; performance indicators include round-trip efficiency RTE, efficiency η, and levelized cost of electricity LCOE.

8. The optimization method for a cascaded cycle Carnot battery system coupled with a coal-fired power unit according to claim 7, characterized in that: The constraints mentioned in step S3 refer to the range of values ​​that the parameters must meet to ensure the normal operation of the system, including the compressor compression ratio setting must ensure a reasonable outlet working fluid temperature and a reasonable temperature constraint relationship between the working fluid and the heat exchange medium.

9. The optimization method for a cascaded cycle Carnot battery system coupled with a coal-fired power unit according to claim 8, characterized in that: The objective function of multi-objective optimization in step 4 as follows 。

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