Shutdown purging method for fuel cell system

Abstract

The invention relates to the technical field of fuel cells, and particularly discloses a fuel cell system shutdown purging method, which comprises the following steps: modeling water diffusion and water phase change in the shutdown purging process of a fuel cell, during the shutdown purging period of the fuel cell; setting a fuel cell shutdown purging open-loop optimization control strategy, and adopting high-frequency impedance as the representation of the water content in the fuel cell in the open-loop optimization control strategy; a fuel cell shutdown purging closed-loop optimization control strategy is set to dynamically adjust purging parameters through a real-time feedback mechanism so as to ensure that the purging process is more stable and controllable. According to the invention, the change of water in the fuel cell is described more accurately, the energy consumption of the system is reduced to the greatest extent, and the required purging time is ensured to be met. According to the invention, the energy consumption is effectively reduced while the purging effect is ensured, and a more reliable and applicable purging solution is provided for the fuel cell.

Description

Technical Field

[0001] The invention relates to the technical field of fuel cells, in particular to a shutdown purge method for a fuel cell system. Background Art

[0002] The traditional fuel cell shutdown purge strategy mainly relies on the ambient temperature to switch between normal temperature and low temperature purge modes, and adopts a constant flow purge strategy. This is not only inefficient, but also difficult to ensure that the moisture inside the stack reaches the ideal residual state, and increases the risk of stack icing. Although there are currently improved strategies, such as using high-frequency impedance to estimate the internal water content and using this as a basis for open-loop regulation of the purge flow, open-loop regulation has its inherent limitations. Specifically, it cannot capture subtle changes in the internal state of the fuel cell in real time, which means that the purge strategy may not accurately match the actual needs of the fuel cell. In addition, open-loop regulation cannot dynamically adjust the purge parameters according to actual conditions, which may result in poor purge effects or excessive purges.

[0003] At the same time, during the fuel cell purge process, there is a close coupling relationship between the purge flow rate and the internal pressure of the fuel cell. If the purge pressure is too high, it will cause a large pressure difference between the anode and cathode of the fuel cell, which poses a direct threat to the membrane assembly of the fuel cell and may cause mechanical damage to the membrane or performance degradation. Therefore, in order to ensure the long-term stable operation of the fuel cell, the pressure inside the fuel cell during the purge process must be reasonably controlled. Summary of the invention

[0004] The object of the present invention is to provide a fuel cell system shutdown purge method to solve the problems raised in the above background technology.

[0005] To achieve the above object, the present invention provides the following technical solutions: A fuel cell system shutdown purge method, the method comprising: S1. Modeling of water diffusion and water phase change during the shutdown and purge process of the fuel cell. During the shutdown and purge period of the fuel cell, the water inside the stack exists in three forms: membrane water, water vapor and liquid water. S2. Setting the fuel cell shutdown purge open-loop optimization control strategy, using high-frequency impedance as a representation of the water content inside the fuel cell in the open-loop optimization control strategy; S3. Set the fuel cell shutdown purge closed-loop optimization control strategy, aiming to dynamically adjust the purge flow and purge pressure through a real-time feedback mechanism to ensure that the purge process is more stable and controllable.

[0006] As a further technical solution of the present invention, step S1 includes a membrane water model, a water vapor model and a liquid water model; S1.1, membrane water model: The membrane water of fuel cells is mainly composed of the following parts: water generation, water diffusion and water phase change in the catalyst layer; Formula 1: ; in, is the source term of membrane water, It represents the reaction rate of the cathode catalyst layer, which is related to the concentration of the reactants. F represents the Faraday constant, J is the current density, and n is the electrical permeability coefficient. represents the molar mass of water, Represents the phase change process between liquid water and water vapor in the membrane; Formula 2: ; in, is the water content of the proton exchange membrane; Formula 3: ; in, represents the proton exchange membrane density, represents the proton exchange membrane equivalent, w represents the polymer in the catalyst layer, x represents the diffusion distance of the reaction gas, represents the water diffusion coefficient of the membrane and can be calculated by the following formula: Formula 4: ; in, is the fuel cell stack temperature, K is a coefficient affected by the water content of the proton exchange membrane The influence of is calculated by formula 5; Formula 5: ; The phase transition between liquid water and water vapor in the membrane is as follows: Formula 6: ; in, represents the change rate coefficient between non-frozen water and gaseous water in the film, represents the equilibrium membrane water content; Formula 7: ; a is the water activity of the fuel cell and can be expressed as: Formula 8: ; In the formula, represents the mole fraction of gaseous water in the pores, represents the mixed gas pressure in the pores, represents the water vapor saturation pressure, represents the liquid water volume fraction; S1.2, Water vapor model: Equation 9 is the water vapor mass model in the cathode, Indicates the input water vapor mass, is the output water vapor mass, is the mass of water vapor generated by the reaction, is the change between liquid water and water vapor in the cathode; Formula 9: ; in, is the cathode inlet air mass flow rate, , are the mass fractions of water vapor at the cathode inlet and cathode, respectively. represents the change between liquid water and water vapor in the cathode; Formula 10: ; in, is the water vapor condensation rate, is the evaporation rate of liquid water, represents the porosity, R represents the gas constant, is the density of water, represents the volume fraction of liquid water; The water vapor mass model in the anode is similar to that in the cathode: Formula 11: ; in, is the anode inlet gas mass flow rate, , are the mass fractions of water vapor at the anode inlet and anode, respectively. represents the change between liquid water and water vapor in the anode; S1.3, Liquid Water Model: Equation 12 is the liquid water mass model in the cathode, is the mass of cathode liquid water, is the change between liquid water and water vapor in the cathode, is the mass of liquid water transferred from the anode to the cathode by electroosmosis; Formula 12: ; represents the mass of liquid water diffused to the anode; Formula 13:

[0007] in, is the water diffusion coefficient of the membrane, y is the distance perpendicular to the membrane, is the water concentration of the membrane; Formula 14: ; in, is the density of the proton exchange membrane in the dry state, is the equivalent molar mass of the proton exchange membrane in a dry state; The liquid water mass model in the anode is similar to that in the cathode; Formula 15:

[0008] in, is the change between liquid water and water vapor in the anode, is the mass of liquid water at the anode; As a further technical solution of the present invention, in step S3, the high-frequency resistance of the membrane electrode is calculated as follows: Formula 16: ; Where HFR represents the high frequency resistance of the membrane electrode, represents the thickness of the film, represents the width of the membrane, represents the conductivity related to the water content of the membrane, obtained by solving Equation 17; Formula 17: ; The open-loop optimization control strategy is as follows: S3.1. Obtain the high frequency resistance of the fuel cell by electrochemical impedance analyzer, and find MAP according to the stack temperature and initial HFR to obtain the initial membrane water content; S3.2, setting the threshold of the water content of the fuel cell membrane after purging and the purging time; S3.3, simulate the shutdown purge of the fuel cell at different initial membrane water content, purge flow rate, and stack temperature, retain the results that meet the threshold and calculate the energy consumption during the shutdown purge; S3.4, generating an optimal purge flow MAP diagram for minimizing energy consumption of the fuel cell under different operating conditions; The open-loop control optimization objectives for fuel cell shutdown purge are as follows: ; in, represents the total energy consumption of the purge process, Indicates the purge time, Indicates the water content inside the fuel cell after purging.

[0009] As a further technical solution of the present invention, in step S3, an observer is used to estimate the cathode purge flow rate and pressure, and the observer state space equation used is as follows: ; ; ; ; in is the intake manifold pressure, is the intake manifold temperature, is the air compressor flow rate, R is the gas constant, is the intake manifold volume, is the molar mass of air, is the cathode inlet flow rate; The fuel cell cathode real-time pressure and cathode purge flow observed by the observer are used as feedback signals to compare with the target value obtained by open-loop control to solve the error, so as to adjust the speed of the air compressor and the opening of the back pressure valve in real time. The water activity, cathode pressure and inflow flow are selected as system state variables, that is, ; The controlled variables are the stack current, air compressor speed and back pressure valve opening, that is, ;Select the controlled output of the system as the water activity of the fuel cell, that is, y= , after Euler discretization, the discrete model of fuel cell shutdown purge is obtained: ; According to the optimization control idea of ​​model predictive control, it is necessary to obtain the prediction time domain at time k. The dynamic behavior of the system in the time domain, To predict the time domain, at the kth sampling time, the predicted output sequence of the system is defined as: ; At the same time, define the system optimization control vector sequence at time k : ; The selected performance function is as follows: ; Where: The term reflects the membrane water activity, i.e. safety; Reflects the stability of cathode pressure, It reflects the tracking effect of the purge flow rate. It reflects the soft constraints of the changes in the input quantities of the three systems; w1 , w2, w3 is the weight factor, The pressure difference between the anode and cathode shall not exceed 50kPa, which is determined by the anode pressure of the fuel cell and the pressure difference of the reduced pressure purge.

[0010] As a further technical solution of the present invention, the method also includes fuel cell shutdown purge control based on a multi-core chip: The fuel cell controller based on a multi-core processor distributes the task of shutting down and purging the fuel cell to the three cores of the processor, CPU0, CPU1 and CPU2, in parallel; The fuel cell shutdown optimal purge flow MAP optimization algorithm is assigned to the first core CPU0 of the multi-core processor in parallel, and the optimal purge flow and purge pressure calculated by the real-time updated MAP are written into the shared memory by CPU0; The fuel cell shutdown purge open-loop optimization algorithm is assigned to the second core CPU1 of the multi-core processor in parallel, and the feedforward compensation is continuously corrected by CPU1 reading the optimal purge flow and purge pressure updated in real time in the shared memory; The fuel cell shutdown purge closed-loop optimization algorithm is assigned to the third core CPU2 of the multi-core processor in parallel. CPU2 reads the optimal purge flow and purge pressure updated in real time in the shared memory to continuously correct the control target, and imports the purge data into CPU0 to update the MAP in real time.

[0011] Compared with the prior art, the beneficial effects of the present invention are: establishing a dynamic model to characterize the water diffusion and water phase change in the fuel cell during the purge process, and more accurately describing the changes of water inside the fuel cell.

[0012] The present invention has developed an open-loop purge optimization control strategy. This strategy uses MAP to find the initial high-frequency resistance and stack temperature to determine the optimal purge flow rate. Its purpose is to minimize system energy consumption while ensuring that the required purge time is met.

[0013] The present invention also combines open-loop control with closed-loop control to monitor and control the fuel cell cathode pressure and purge flow rate in real time online to ensure the stability of the purge process. This method effectively reduces energy consumption while ensuring the purge effect, providing a more reliable and applicable purge solution for fuel cells.

[0014] The present invention also proposes a multi-core processor parallel processing strategy, which effectively distributes multiple complex algorithms to multiple processor cores for parallel execution. This method not only significantly reduces the computing burden of the controller, but also greatly shortens the running time of the algorithm, thereby ensuring the real-time and accuracy of the control process. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present invention.

[0016] Figure 1 Graph showing membrane moisture content for various HFR and stack temperatures.

[0017] Figure 2 The diagram shows the optimal purge flow rate under various purge conditions.

[0018] Figure 3 Block diagram of the fuel cell shutdown purge control.

[0019] Figure 4 This is the shutdown purge control block diagram of a fuel cell based on a multi-core chip. DETAILED DESCRIPTION

[0020] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0021] The present invention aims to reduce the energy consumption during the shutdown purge process of the fuel cell, adjust the pressure and purge flow of the fuel cell in real time during the shutdown purge process, and ensure that the internal moisture content is maintained within the designed safety threshold after shutdown, thereby preventing ice formation and extending the battery life. To achieve this goal, the present invention will further explore the following three research directions: first, the water diffusion and water phase change mechanism during the shutdown purge process of the fuel cell; second, the open-loop optimization control strategy for the shutdown purge of the fuel cell; and third, the closed-loop optimization control method for the shutdown purge of the fuel cell.

[0022] An embodiment of the present invention provides a fuel cell system shutdown purge method, the method comprising: S1. Modeling of water diffusion and water phase change during fuel cell shutdown and purge; A core challenge in the shutdown purge process of the fuel cell is to ensure that the moisture content inside the cell meets the safety threshold after the purge, thereby preventing cell damage caused by icing. To this end, it is crucial to build an accurate model of water changes inside the fuel cell. However, most of the existing purge models ignore the complex mechanisms of water transport and phase change during shutdown purge, such as the transfer of membrane water in the cathode and anode catalyst layers, and the phase change process between membrane water and water vapor. In view of this, the present invention comprehensively considers the above-mentioned key elements and innovatively establishes a proton exchange membrane fuel cell purge model that includes water phase change and gas diffusion dynamic processes.

[0023] S1.1, membrane water model: During the shutdown purge of the fuel cell, the water inside the stack exists in three forms: membrane water, water vapor and liquid water. Among them, the water produced by the chemical reaction will first dissolve into the ionomer in the cathode catalyst layer. Subsequently, the dissolved water is transported to other areas of the fuel cell under the action of electroosmotic resistance and reverse diffusion. The membrane water of fuel cells is mainly composed of the following parts: water generation, water diffusion and water phase change in the catalyst layer; Formula 1: ; in, is the source term of membrane water, It represents the reaction rate of the cathode catalyst layer, which is related to the concentration of the reactants. F represents the Faraday constant, J is the current density, and n is the electrical permeability coefficient. represents the molar mass of water, Represents the phase change process between liquid water and water vapor in the membrane; Formula 2: ; in, is the water content of the proton exchange membrane; Formula 3: ; in, represents the proton exchange membrane density, represents the proton exchange membrane equivalent, w represents the polymer in the catalyst layer, x represents the diffusion distance of the reaction gas, represents the water diffusion coefficient of the membrane and can be calculated by the following formula: Formula 4: ; in, is the fuel cell stack temperature, K is a coefficient affected by the water content of the proton exchange membrane The influence of is calculated by formula 5; Formula 5: ; The phase transition between liquid water and water vapor in the membrane is as follows: Formula 6: ; in, represents the change rate coefficient between non-frozen water and gaseous water in the film, represents the equilibrium membrane water content; Formula 7: ; a is the water activity of the fuel cell and can be expressed as: Formula 8: ; In the formula, represents the mole fraction of gaseous water in the pores, represents the mixed gas pressure in the pores, represents the water vapor saturation pressure, represents the liquid water volume fraction; S1.2, Water vapor model: Equation 9 is the water vapor mass model in the cathode, Indicates the input water vapor mass, is the output water vapor mass, is the mass of water vapor generated by the reaction, is the change between liquid water and water vapor in the cathode; Formula 9: ; in, is the cathode inlet air mass flow rate, , are the mass fractions of water vapor at the cathode inlet and cathode, respectively. represents the change between liquid water and water vapor in the cathode; Formula 10: ; in, is the water vapor condensation rate, is the evaporation rate of liquid water, represents the porosity, R represents the gas constant, is the density of water, represents the volume fraction of liquid water; The water vapor mass model in the anode is similar to that in the cathode: Formula 11: ; in, is the anode inlet gas mass flow rate, , are the mass fractions of water vapor at the anode inlet and anode, respectively. represents the change between liquid water and water vapor in the anode; S1.3, Liquid Water Model: Equation 12 is the liquid water mass model in the cathode, is the mass of liquid water at the cathode, is the change between liquid water and water vapor in the cathode, is the mass of liquid water transferred from the anode to the cathode by electroosmosis; Formula 12: ; represents the mass of liquid water diffused to the anode; Formula 13: ;

[0024] in, is the water diffusion coefficient of the membrane, y is the distance perpendicular to the membrane, is the water concentration of the membrane; Formula 14: ; in, is the density of the proton exchange membrane in the dry state, is the equivalent molar mass of the proton exchange membrane in a dry state; The liquid water mass model in the anode is similar to that in the cathode; Formula 15: ; in, is the change between liquid water and water vapor in the anode, is the mass of liquid water at the anode S2, setting the fuel cell shutdown purge open-loop optimization control strategy; The purge operation after the fuel cell is shut down can effectively remove internal residual moisture, significantly improve the success rate of cold start, and prevent accelerated battery aging due to icing. However, accurately evaluating the membrane water content before the fuel cell shutdown purge is a challenge. At present, the main methods for measuring water content are AC impedance method, neutron radiography, state observation method, etc. In contrast, the AC impedance method is widely used due to its real-time monitoring and feedback, high precision and reliability. Therefore, the present invention uses high-frequency impedance as a characterization of the internal water content of the fuel cell in the open-loop optimization control strategy; The high-frequency resistance calculation formula of the membrane electrode is as follows: Formula 16: ; Where HFR represents the high frequency resistance of the membrane electrode, represents the thickness of the film, represents the width of the membrane, represents the conductivity related to the water content of the membrane, obtained by solving Equation 17; Formula 17: ; The open-loop optimization control strategy is as follows: S2.1. Obtain the high frequency resistance of the fuel cell by electrochemical impedance analyzer, which is related to the temperature and membrane water content. Find MAP according to the stack temperature and initial HFR to obtain the initial membrane water content. S2.2, setting the threshold of the water content of the fuel cell membrane after purging and the purging time; S2.3, simulate the shutdown purge of the fuel cell at different initial membrane water content, purge flow rate, and stack temperature, retain the results that meet the threshold and calculate the energy consumption during the shutdown purge; S2.4, based on the above results, obtain the optimal purge flow MAP diagram for minimizing energy consumption of the fuel cell under different operating conditions; The open-loop control optimization objectives for fuel cell shutdown purge are as follows: ; in, It indicates the total energy consumption during the purge process. Since the main energy consumption during the shutdown and purge of the fuel cell system is the energy consumption of the air compressor, the energy consumption of the air compressor is used instead of the total energy consumption. Indicates the purge time, Indicates the water content inside the fuel cell after purging, Figure 1 The membrane moisture content diagram for various HFR and stack temperatures is shown in Figure 2. Figure 2 The diagram shows the optimal purge flow rate under various purge conditions.

[0025] S3. Set the fuel cell shutdown purge closed-loop optimization control strategy, aiming to dynamically adjust the purge flow and purge pressure through a real-time feedback mechanism to ensure that the purge process is more stable and controllable.

[0026] Currently, the shutdown purge strategy of fuel cells is generally implemented in an open-loop manner, but there are many challenges in this process. Especially during the purge period, the internal pressure and purge flow of the fuel cell are in a real-time fluctuating state, and may also encounter interference from the external environment. These factors together have a significant impact on the stability of the purge process. In view of this, it is particularly urgent and important to develop and implement a shutdown purge control strategy based on closed-loop regulation. This strategy aims to dynamically adjust the purge parameters through a real-time feedback mechanism to ensure that the purge process is more stable and controllable, thereby improving the overall performance and reliability of the fuel cell system. Figure 3 Shutdown purge control block diagram for fuel cell; Some key states of the fuel cell system, such as the cathode pressure and oxygen pressure in the stack, are often not directly measurable during actual operation. These states are critical to the performance and stability of the fuel cell, but measuring them directly can be difficult and expensive. Therefore, it is necessary to design a state observer using the measurable output of the system to estimate these unknown states; At the same time, the observer can accurately estimate key states such as cathode purge flow and pressure in real time, making it possible to achieve precise closed-loop control. In a fuel cell system, air pressure and air flow are crucial to the performance of the fuel cell. By obtaining accurate information about these states through the observer, a more precise control strategy can be designed to improve the stability and efficiency of the fuel cell. In step S3, the cathode purge flow and pressure are estimated using an observer, and the observer state space equation used is as follows: ; ; ; ; in is the intake manifold pressure, is the intake manifold temperature, is the air compressor flow rate, R is the gas constant, is the intake manifold volume, is the molar mass of air, is the cathode inlet flow rate; During the shutdown purge of the fuel cell, the difference in diffusion force generated by water concentration between the catalyst layer and the gas channel is too small to push the water generated by the catalyst layer into the gas channel within a limited time. Therefore, it is necessary to introduce a reduced pressure purge method to apply an additional force perpendicular to the membrane and the gas channel during the purge process. The specific implementation measures are achieved by adjusting the opening of the back pressure valve. When the back pressure valve is closed, the pressure of the catalyst layer and the channel will increase until the target pressure is reached. When the pressure reaches the target value, the back pressure valve opens and the accumulated gas is suddenly discharged outward. At this time, a pressure difference is generated between the catalyst layer and the fuel cell outlet; the catalyst layer pressure is high and the fuel cell outlet pressure is low. This pressure difference disappears in a short time. However, in the process, this sudden pressure imbalance causes water to move to the outside of the gas diffusion layer. When the accumulated gas is discharged, most of the liquid water in the channel is also discharged in a very short time. Therefore, the target cathode pressure is determined by the cathode pressure and the reduced pressure purge pressure difference in the present invention.

[0027] The present invention uses the real-time cathode pressure and cathode purge flow of the fuel cell observed by the observer as feedback signals to compare with the target value obtained by open-loop control to solve the error, thereby adjusting the speed of the air compressor and the opening of the back pressure valve in real time. Combined with the above model, the present invention selects water activity, cathode pressure and stack flow as system state variables, that is, ; The controlled variables are the stack current, air compressor speed and back pressure valve opening, that is, ;Select the controlled output of the system as the water activity of the fuel cell, that is, y= , after Euler discretization, the discrete model of fuel cell shutdown purge is obtained: ; According to the optimization control idea of ​​model predictive control, it is necessary to obtain the prediction time domain at time k. The dynamic behavior of the system in the time domain, To predict the time domain, at the kth sampling time, the predicted output sequence of the system is defined as: ; At the same time, define the system optimization control vector sequence at time k : ; The optimization control goal of fuel cell shutdown purge is to reduce battery energy consumption while avoiding cold start failure caused by icing due to excessive residual water content. At the same time, the system input variable startup current curve and purge flow curve should be as smooth as possible to avoid large oscillations that bring difficulties to the underlying implementation. Therefore, the performance function is selected as follows: ; Where: The term reflects the membrane water activity, i.e. safety; Reflects the stability of cathode pressure, It reflects the tracking effect of the purge flow rate. It reflects the soft constraints of the changes in the input quantities of the three systems; w1 , w2, w3 is the weight factor, The pressure difference between the anode and cathode shall not exceed 50kPa, which is determined by the anode pressure of the fuel cell and the pressure difference of the reduced pressure purge.

[0028] See also Figure 4 In an embodiment of the present invention, the method further includes fuel cell shutdown purge control based on a multi-core chip: In view of the real-time tracking of purge flow and system energy consumption optimization requirements during the fuel cell shutdown purge control process, the performance of a single-core processor is often unable to meet the real-time requirements. Therefore, the present invention proposes a multi-core processor parallel processing strategy, which effectively distributes multiple complex algorithms to multiple processor cores for parallel execution. Through this method, not only the computing burden of the controller is significantly reduced, but also the running time of the algorithm is greatly shortened, thereby ensuring the real-time and accuracy of the control process. The fuel cell shutdown purge control based on a multi-core chip described in the present invention includes the following steps: The fuel cell controller based on a multi-core processor distributes the task of shutting down and purging the fuel cell to the three cores of the processor, CPU0, CPU1 and CPU2, in parallel; The fuel cell shutdown optimal purge flow MAP optimization algorithm is assigned to the first core CPU0 of the multi-core processor in parallel, and the optimal purge flow and purge pressure calculated by the real-time updated MAP are written into the shared memory by CPU0; The fuel cell shutdown purge open-loop optimization algorithm is assigned to the second core CPU1 of the multi-core processor in parallel, and the feedforward compensation is continuously corrected by CPU1 reading the optimal purge flow and purge pressure updated in real time in the shared memory; The fuel cell shutdown purge closed-loop optimization algorithm is assigned to the third core CPU2 of the multi-core processor in parallel. CPU2 reads the optimal purge flow and purge pressure updated in real time in the shared memory to continuously correct the control target, and imports the purge data into CPU0 to update the MAP in real time.

[0029] It should be noted that, in this article, the term "comprises" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method, article or system including a series of elements includes not only those elements, but also includes other elements not explicitly listed, or also includes elements inherent to such process, method, article or system. In the absence of further restrictions, an element defined by the sentence "comprises a ..." does not exclude the existence of other identical elements in the process, method, article or system including the element.

[0030] The above are only preferred embodiments of the present invention, and are not intended to limit the patent scope of the present invention. Any equivalent structure or equivalent process transformation made using the contents of the present invention specification and drawings, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present invention.

Claims

1. A fuel cell system shutdown purge method, characterized in that: The method comprises: S1. Modeling of water diffusion and water phase change during the shutdown and purge process of the fuel cell. During the shutdown and purge period of the fuel cell, the water inside the stack exists in three forms: membrane water, water vapor and liquid water. S2. Setting the fuel cell shutdown purge open-loop optimization control strategy, using high-frequency impedance as a representation of the water content inside the fuel cell in the open-loop optimization control strategy; S3. Set the fuel cell shutdown purge closed-loop optimization control strategy, aiming to dynamically adjust the purge flow and purge pressure through a real-time feedback mechanism to ensure that the purge process is more stable and controllable.

2. A fuel cell system shutdown purge method according to claim 1, characterized in that: Step S1 includes a membrane water model, a water vapor model and a liquid water model; S1.1, membrane water model: The membrane water of fuel cells is mainly composed of the following parts: water generation, water diffusion and water phase change in the catalyst layer; Formula 1: ; in, is the source term of membrane water, It represents the reaction rate of the cathode catalyst layer, which is related to the concentration of the reactants. F represents the Faraday constant, J is the current density, and n is the electrical permeability coefficient. represents the molar mass of water, Represents the phase change process between liquid water and water vapor in the membrane; Formula 2: ; in, is the water content of the proton exchange membrane; Formula 3: ; in, represents the proton exchange membrane density, represents the proton exchange membrane equivalent, w represents the polymer in the catalyst layer, x represents the diffusion distance of the reaction gas, represents the water diffusion coefficient of the membrane and can be calculated by the following formula: Formula 4: ; in, is the fuel cell stack temperature, K is a coefficient affected by the water content of the proton exchange membrane The influence of is calculated by formula 5; Formula 5: ; The phase transition between liquid water and water vapor in the membrane is as follows: Formula 6: ; in, represents the change rate coefficient between non-frozen water and gaseous water in the film, represents the equilibrium membrane water content; Formula 7: ; a is the water activity of the fuel cell and can be expressed as: Formula 8: ; In the formula, represents the mole fraction of gaseous water in the pores, represents the mixed gas pressure in the pores, represents the water vapor saturation pressure, represents the liquid water volume fraction; S1.2, Water vapor model: Equation 9 is the water vapor mass model in the cathode, Indicates the input water vapor mass, is the output water vapor mass, is the mass of water vapor generated by the reaction, is the change between liquid water and water vapor in the cathode; Formula 9: ； in, is the cathode inlet air mass flow rate, , are the mass fractions of water vapor at the cathode inlet and cathode, respectively. represents the change between liquid water and water vapor in the cathode; Formula 10: ； in, is the water vapor condensation rate, is the evaporation rate of liquid water, represents the porosity, R represents the gas constant, is the density of water, represents the volume fraction of liquid water; The water vapor mass model in the anode is similar to that in the cathode: Formula 11: ； in, is the anode inlet gas mass flow rate, , are the mass fractions of water vapor at the anode inlet and anode, respectively. represents the change between liquid water and water vapor in the anode; S1.3, Liquid Water Model: Equation 12 is the liquid water mass model in the cathode, is the mass of cathode liquid water, is the change between liquid water and water vapor in the cathode, is the mass of liquid water transferred from the anode to the cathode by electroosmosis; Formula 12: ； represents the mass of liquid water diffused to the anode; Formula 13: ； in, is the water diffusion coefficient of the membrane, y is the distance perpendicular to the membrane, is the water concentration of the membrane; Formula 14: ； in, is the density of the proton exchange membrane in the dry state, is the equivalent molar mass of the proton exchange membrane in a dry state; The liquid water mass model in the anode is similar to that in the cathode; Formula 15: ； in, is the change between liquid water and water vapor in the anode, is the mass of liquid water at the anode.

3. A fuel cell system shutdown purging method according to claim 2, characterized in that: In step S3, the high-frequency resistance of the membrane electrode is calculated as follows: Formula 16: ； Where HFR represents the high frequency resistance of the membrane electrode, represents the thickness of the film, represents the width of the membrane, represents the conductivity related to the water content of the membrane, obtained by solving Equation 17; Formula 17: ； The open-loop optimization control strategy is as follows: S3.

1. Obtain the high frequency resistance of the fuel cell by electrochemical impedance analyzer, and find MAP according to the stack temperature and initial HFR to obtain the initial membrane water content; S3.2, setting the threshold of the water content of the fuel cell membrane after purging and the purging time; S3.3, simulate the shutdown purge of the fuel cell at different initial membrane water content, purge flow rate, and stack temperature, retain the results that meet the threshold and calculate the energy consumption during the shutdown purge; S3.4, generating an optimal purge flow MAP diagram for minimizing energy consumption of the fuel cell under different operating conditions; The open-loop control optimization objectives for fuel cell shutdown purge are as follows: ； in, represents the total energy consumption of the purge process, Indicates the purge time, Indicates the water content inside the fuel cell after purging.

4. A fuel cell system shutdown purge method according to claim 3, characterized in that: In step S3, the cathode purge flow and pressure are estimated using an observer, and the observer state space equation used is as follows: ； ； ； ； in is the intake manifold pressure, is the intake manifold temperature, is the air compressor flow rate, R is the gas constant, is the intake manifold volume, is the molar mass of air, is the cathode inlet flow rate; The fuel cell cathode real-time pressure and cathode purge flow observed by the observer are used as feedback signals to compare with the target value obtained by open-loop control to solve the error, so as to adjust the speed of the air compressor and the opening of the back pressure valve in real time. The water activity, cathode pressure and inflow flow are selected as system state variables, that is, ; The controlled variables are the stack current, air compressor speed and back pressure valve opening, namely ;Select the controlled output of the system as the water activity of the fuel cell, that is, y= , after Euler discretization, the discrete model of fuel cell shutdown purge is obtained: ； According to the optimization control idea of ​​model predictive control, it is necessary to obtain the prediction time domain at time k. The dynamic behavior of the system in the time domain, To predict the time domain, at the kth sampling time, the predicted output sequence of the system is defined as: ； At the same time, define the system optimization control vector sequence at time k : ； The selected performance function is as follows: ； Where: The term reflects the membrane water activity, i.e. safety; Reflects the stability of cathode pressure, This reflects the tracking effect of the purge flow rate. It reflects the soft constraints of the changes in the input quantities of the three systems; w1 , w2, w3 is the weight factor, The pressure difference between the anode and cathode shall not exceed 50kPa, which is determined by the anode pressure of the fuel cell and the pressure difference of the reduced pressure purge.

5. A fuel cell system shutdown purge method according to claim 1, characterized in that: The method also includes fuel cell shutdown purge control based on a multi-core chip: The fuel cell controller based on a multi-core processor distributes the task of shutting down and purging the fuel cell to the three cores of the processor, CPU0, CPU1 and CPU2, in parallel; The fuel cell shutdown optimal purge flow MAP optimization algorithm is assigned to the first core CPU0 of the multi-core processor in parallel, and the optimal purge flow calculated by the real-time updated MAP is written into the shared memory by CPU0; The fuel cell shutdown purge open-loop optimization algorithm is assigned to the second core CPU1 of the multi-core processor in parallel, and the feedforward compensation is continuously corrected by CPU1 reading the optimal purge flow and purge pressure updated in real time in the shared memory; The fuel cell shutdown purge closed-loop optimization algorithm is assigned to the third core CPU2 of the multi-core processor in parallel. CPU2 reads the optimal purge flow and purge pressure updated in real time in the shared memory to continuously correct the control target, and imports the purge data into CPU0 to update the MAP in real time.

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