Fuel cell hybrid power system suitable for cold start and its modeling method

Abstract

The present invention relates to a fuel cell hybrid power system suitable for cold start and a modeling method thereof. The system adds a channel from cathode air to anode, a channel bypassing an air cooler and a humidifier, and a heating resistor. The modeling method respectively establishes an air supply subsystem model, a cooling subsystem model, a battery low temperature model, a fuel cell monomer model, and a fuel cell stack model. The fuel cell monomer model takes into account the generation, phase change, ice accumulation, and transfer of water during the low temperature start of the fuel cell, providing a guarantee for the accuracy of the model. Compared with the prior art, the present invention models each component in the fuel cell separately and solves the mass transfer and heat transfer between each layer, which can reflect the temperature distribution difference between each layer of the battery, can better predict the output performance of the fuel cell during the low temperature start, and improves the accuracy and application value of the model.

Description

Technical Field

[0001] The invention relates to the field of electrochemical fuel cells, and in particular to a fuel cell hybrid power system suitable for cold start and a modeling method thereof. Background Art

[0002] The key factor for the success of a fuel cell cold start is the dynamic ratio of the battery's own temperature rise rate to the ice accumulation rate. The current mainstream cold start strategies mainly include insulation, shutdown purge, auxiliary heating, self-start and other methods. Shutdown purge can reduce the accumulation of ice inside the battery before the fuel cell is started, and is an indispensable part of the current cold start. Auxiliary heating and self-start both use a large amount of heat input to quickly heat up the fuel cell. The difference is that auxiliary heating uses batteries and heating resistors to heat the stack, while self-start uses the chemical reaction of the fuel cell itself to generate heat to increase the temperature of the stack.

[0003] The current cold start method, whether it is shutdown purging or using an external power supply to heat the fuel cell stack, requires modification of the fuel cell system, further increasing the cost of cold start control strategy testing. In order to reduce the time and cost of the trial and error process for cold start performance improvement, it is necessary to establish a fuel cell system model suitable for the fuel cell cold start process. As the core of the fuel cell system, the fuel cell stack has complex mass transfer and heat transfer phenomena during the cold start process, which is the focus and difficulty of system modeling.

[0004] The proton exchange membrane fuel cell is a complex system with nonlinearity, strong coupling and great environmental influence. Its output performance involves multiple fields such as fluid mechanics, thermodynamics, and electrochemistry. Its output characteristics are affected by many factors, especially the output characteristics under cold start conditions are more affected by water transmission and phase change. In order to study the output characteristics and startup strategies of fuel cells under low temperature conditions, it is necessary to establish corresponding models. Based on the spatial size of the model, the low-temperature simulation model of fuel cells can be divided into one-dimensional, two-dimensional, and three-dimensional models. The one-dimensional model mainly models the thickness direction of the fuel cell. Since the thickness direction is the main transmission direction of water and reaction gas, the one-dimensional model can more accurately reflect the heat and mass transfer phenomena in the battery. The two-dimensional model considers one more direction than the one-dimensional model, considering the transmission and distribution on a certain cut surface of the battery, and is generally used for material transmission phenomena in the flow channel direction. Whether it is a one-dimensional or two-dimensional model, it is based on the simplification of the three-dimensional model, and can provide more accurate simulation results under the premise of a certain amount of calculation. The three-dimensional model can accurately reflect the heat and mass transfer and distribution within the spatial range, and can obtain more accurate and comprehensive simulation results. However, the three-dimensional simulation model requires huge computing power and simulation time, so the three-dimensional model is generally used to simulate the single-channel monomer model and analyze its internal transfer mechanism.

[0005] Due to the limitations of computational complexity and simulation time, the modeling size and scale of the fuel cell three-dimensional model are limited, making it difficult to be used for the optimization of the fuel cell cold start control strategy.

[0006] At present, the low-temperature model of fuel cells is mainly based on commercial finite element software, and due to the limitation of computational complexity, the established models are mostly single-channel models. The single-channel finite element model can be used to study the material transfer phenomenon during the low-temperature start-up of the fuel cell, but the simulation model based on the single-channel model cannot be extended to the low-temperature start-up performance of the fuel cell stack, and the model cannot consider the real-time input of the fuel cell system to the fuel cell stack, and cannot be used to optimize the low-temperature start-up control strategy of the fuel cell.

[0007] In order to solve the above problems, one idea is to simplify and reduce the dimension of the three-dimensional model of the fuel cell to reduce the amount of calculation of the model. At present, the one-dimensional model of the fuel cell stack is mostly based on the finite element commercial software, which greatly reduces the amount of calculation of the fuel cell stack model. However, due to the information transmission barrier between the finite element commercial software and the control strategy formulation software, it brings great difficulties to the formulation and optimization of the low-temperature start-up control strategy. There is also a lumped parameter model based on MATLAB / SIMULINK, which assumes the fuel cell as a whole with uniform temperature and considers the heat exchange between the fuel cell and the outside world. The model can be used for the optimization and design of the fuel cell thermal management system, but the temperature is the key parameter that plays a decisive role in the low-temperature start-up process. The error between the model established by the lumped parameter method and the experimental data is large, which is not suitable for the formulation of the low-temperature start-up control strategy of the fuel cell.

[0008] The one-dimensional model of fuel cells mainly models the heat transfer in the thickness direction of the fuel cell and analyzes the influence of the battery operating parameters and structural parameters on the cold start performance based on the model. The one-dimensional water heat transfer model assumes that the layers inside the monomer are continuous and simplifies the heat transfer phenomenon between the layers inside the monomer. With the deepening of cold start research, the temperature difference between the layers in the fuel cell monomer has an increasingly important impact on the transmission and phase change of three-phase water. Therefore, it is particularly important to establish a one-dimensional cold start model that can accurately reflect the heat and mass transfer between the layers in the battery monomer during the cold start process. Moreover, most of the current one-dimensional models are based on simplified three-dimensional finite element simulation models, and the model simulation environment is mostly finite element simulation software. It can only simply analyze the influence of different operating parameters and battery structural parameters on the cold start performance based on the model, which is difficult to be used for the optimization of the cold start control strategy of the fuel cell. In order to make the established simulation model applicable to the solution of more cold start problems, it is more meaningful to establish a one-dimensional hierarchical simulation model in the MATLAB / SIMULINK environment. Summary of the invention

[0009] The purpose of the present invention is to overcome the defects of the above-mentioned prior art and provide a fuel cell hybrid power system suitable for cold start and a modeling method thereof that improves the cold start efficiency of the fuel cell hybrid power system.

[0010] The purpose of the present invention can be achieved by the following technical solutions:

[0011] A fuel cell hybrid power system suitable for cold start includes a fuel cell stack, which is respectively connected to a hydrogen supply subsystem, a gas supply subsystem, a battery subsystem and a cooling subsystem. The gas supply subsystem includes an air compressor, a first exchange interface, an air cooler and a humidifier connected in sequence. The humidifier is connected to the fuel cell stack, and the connection subsystem between the humidifier and the fuel cell stack is connected to the first exchange interface.

[0012] Furthermore, the connection line between the humidifier and the fuel cell stack is provided with a second exchange interface, the hydrogen supply subsystem comprises a high-pressure hydrogen tank, a pressure reducing valve, a third exchange interface, a hydrogen inlet valve and a hydrogen heat exchanger connected in sequence, the hydrogen heat exchanger is connected to the fuel cell stack, and the third exchange interface is connected to the second exchange interface;

[0013] When the fuel cell hybrid power system is shut down, the fuel cell stack is purged by passing the air of the air supply subsystem into the hydrogen supply subsystem.

[0014] Furthermore, the cooling subsystem includes a water supply tank, a water pump, a radiator, an electric heater and a thermostat; the fuel cell stack, the hydrogen heat exchanger, the air cooler, the radiator, the thermostat, the water pump, the water supply tank and the fuel cell stack are connected in sequence; one end of the electric heater is connected to the thermostat, and the other end is connected to the air cooler.

[0015] The present invention also provides a modeling method for a fuel cell hybrid power system suitable for cold start as described above, which is used to establish a fuel cell hybrid power system model, wherein the fuel cell hybrid power system model is provided with an air supply subsystem model, a cooling subsystem model, a battery low temperature model, a fuel cell monomer model and a fuel cell stack model, and the fuel cell stack model is constructed based on the fuel cell monomer model.

[0016] Furthermore, in the cooling subsystem model, the expression for the distribution of coolant flow in the cooling subsystem by the thermostat is:

[0017]

[0018] In the formula, To divert the coolant flow to the radiator, is the flow rate of coolant diverted to the bipolar plate, k is the opening of the thermostat valve, is the total coolant flow through the thermostat.

[0019] Furthermore, in the fuel cell monomer model, the calculation expression for the water content accumulated during the fuel cell reaction process is:

[0020]

[0021] In the formula, is the water content accumulated during the fuel cell reaction, It is the water generated by the reaction of the catalytic layer. is the amount of water absorbed and diffused through the proton exchange membrane, is the water content diffused into the gas diffusion layer.

[0022] Furthermore, in the fuel cell monomer model, the calculation expression for the ice accumulation in the fuel cell during low temperature startup is:

[0023]

[0024] Where s0 is the initial ice volume fraction in the fuel cell before low-temperature startup, v ice is the molar mass of ice, t is the current moment, ε CL is the porosity of the catalyst layer, δ CL is the thickness of the catalytic layer, and A is the reaction area of ​​the catalytic layer.

[0025] Furthermore, during the melting process of ice in the fuel cell, the calculation expression for the ice accumulation amount is:

[0026]

[0027] Where t1 is the time when the temperature of the fuel cell catalyst layer reaches the freezing point. is the heat generation rate of the fuel cell reaction, is the fuel cell heat dissipation rate, V CL is the volume of the catalyst layer, h sg It is the latent heat of phase change when water vapor condenses.

[0028] Furthermore, in the fuel cell monomer model, the calculation expression of the low temperature output voltage of the fuel cell is:

[0029] U cell =E nernst +V act +V conc +V ohmic

[0030]

[0031]

[0032]

[0033]

[0034]

[0035]

[0036]

[0037] Where U cell is the low temperature output voltage of the fuel cell, E nernst is the ideal electromotive force of the fuel cell chemical reaction, V act is the activation overpotential of the fuel cell, V conc is the concentration difference overpotential, V ohmic is the ohmic overpotential, ΔG f is the change in Gibbs free energy, F is the Faraday constant, ΔG f 0 is the change in Gibbs free energy under standard conditions, R is the gas constant, T fc is the fuel cell temperature, is the hydrogen partial pressure, is the partial pressure of oxygen, is the water vapor partial pressure, s ice is the volume fraction of ice in the catalyst layer, s lq is the volume fraction of liquid water in the catalyst layer, α is the transmission coefficient, R is the gas constant, T m is the temperature of the proton exchange membrane, F is the Faraday constant, j * is the reaction rate of the anode and cathode catalyst layers of the fuel cell, δ CL is the thickness of the catalyst layer, c h is the concentration of the reaction gas, c ref is the reference gas concentration, δ GDL is the GDL thickness, is the gas diffusion coefficient in the GDL, δ CL is the thickness of the catalyst layer, is the gas diffusion coefficient in the catalyst layer, is the voltage loss due to electronic impedance, is the voltage loss caused by proton impedance, I is the fuel cell current, R elec is the impedance of electrons passing through the current collector, R proton is the resistance of protons passing through the proton exchange membrane, R internal is the total internal impedance of the fuel cell.

[0038] Furthermore, the fuel cell stack model divides the bipolar plates of the fuel cell stack into an anode gas flow channel layer, an anode bipolar plate layer, a cooling flow channel layer, a cathode bipolar plate layer and a cathode gas flow channel layer which are arranged in sequence.

[0039] Compared with the prior art, the present invention has the following advantages:

[0040] (1) The fuel cell hybrid power system of the present invention takes into account that the temperature of the air flowing out of the air compressor at low temperatures will not be too high, so a channel to bypass the air cooler and the humidifier is added. In addition, the relatively high temperature and high pressure air passing through the air compressor can be used as an external heat source input for the fuel cell stack, thereby accelerating the temperature rise process of the fuel cell stack.

[0041] (2) In order to meet the fuel cell shutdown purge requirement, the present invention adds a cathode air to anode channel in the fuel cell hybrid power system.

[0042] (3) In order to heat the coolant during the cold start process, the present invention adds a heating resistor in the small cycle of the cooling subsystem to use the relatively high temperature coolant to increase the temperature of the fuel cell stack.

[0043] (4) In the modeling method of the fuel cell hybrid power system of the present invention, the fuel cell monomer model takes into account the generation, phase change, accumulation, and transfer of water during the low-temperature startup of the fuel cell, and more completely reflects the transmission phenomenon of water in the fuel cell during the low-temperature startup, thus providing a guarantee for the accuracy of the model.

[0044] (5) The fuel cell stack stratification model divides the bipolar plates into layers, which can reflect the temperature distribution of each cell in the stack during low-temperature startup, and further reflect the uneven voltage distribution of each cell in the stack.

[0045] (6) The modeling method of the fuel cell hybrid power system of the present invention takes into account the water transmission, phase change and the influence of the ice volume fraction on the loss voltage during the low-temperature startup of the fuel cell, making the model physical process more complete. In addition, the model models each component in the fuel cell separately and solves the mass transfer and heat transfer between each layer, which can reflect the temperature distribution difference between each layer of the battery, and can better predict the output performance of the fuel cell during the low-temperature startup process, thereby improving the accuracy and application value of the model. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 It is a structural schematic diagram of the fuel cell hybrid power system of the present invention;

[0047] Figure 2 This is a schematic diagram of the low-temperature model structure of the fuel cell hybrid power system of the present invention;

[0048] Figure 3This is a schematic diagram of the low-temperature model structure of a battery;

[0049] Figure 4 It is a schematic diagram of the fuel cell output model structure;

[0050] Figure 5 It is a schematic diagram of the bipolar plate layering model;

[0051] Figure 6 This is a schematic diagram of the temperature stratification model of a proton exchange membrane fuel cell monomer;

[0052] Figure 7 This is a schematic diagram of the temperature stratification model for a fuel cell stack;

[0053] Figure 8 This is a comparison chart of the simulated output voltage of the fuel cell monomer when starting at 253.15K and the test data;

[0054] Fig. 9 This is the temperature variation diagram of each layer of the fuel cell monomer during the startup process at 253.15K;

[0055] Fig.10 This is a comparison chart of the simulated voltage and test data of the fuel cell monomer when starting at 270.15K;

[0056] Fig.11 The ice volume fraction of the catalyst layer and the membrane electrode temperature change diagram at 270.15K startup;

[0057] Fig.12 This is the temperature distribution diagram of the fuel cell stack before startup and shutdown at 253.15K (5 cells);

[0058] Fig.13 This is the simulation result diagram of the average output voltage of the 253.15K battery stack with different numbers of cells;

[0059] In the figure, 1. fuel cell stack, 2. hydrogen supply subsystem, 201. high-pressure hydrogen tank, 202. pressure reducing valve, 203. third exchange interface, 204. hydrogen inlet valve, 205. hydrogen heat exchanger, 3. air supply subsystem, 301. air compressor, 302. first exchange interface, 303. air cooler, 304. humidifier, 305. second exchange interface, 4. battery subsystem, 401. DC-DC converter, 402. battery, 403. motor, 5. cooling subsystem, 501. water tank, 502. water pump, 503. thermostat, 504. radiator, 505. electric heater, 6. hydrogen outlet valve. DETAILED DESCRIPTION

[0060] The present invention is described in detail below in conjunction with the accompanying drawings and specific embodiments. This embodiment is implemented based on the technical solution of the present invention, and provides a detailed implementation method and specific operation process, but the protection scope of the present invention is not limited to the following embodiments.

[0061] Example 1

[0062] like Figure 1 As shown, this embodiment provides a fuel cell hybrid power system suitable for cold start, namely a fuel cell-battery hybrid system, including a fuel cell stack 1, which is respectively connected to a hydrogen supply subsystem 2, a gas supply subsystem 3, a battery subsystem 4 and a cooling subsystem 5.

[0063] In order to meet the needs of fuel cell shutdown purge, this embodiment adds a cathode air to anode channel in the fuel cell-battery hybrid system, and considering that the temperature of the air flowing out of the air compressor will not be too high at low temperatures, a bypass channel for the air cooler and humidifier is added, and the relatively high temperature and high pressure air passing through the air compressor can be used as an external heat source input for the fuel cell stack to accelerate the temperature rise process of the fuel cell stack.

[0064] In order to heat the coolant during the cold start process, this embodiment adds a heating resistor in the small cycle of the cooling cycle to use the relatively high temperature coolant to increase the temperature of the battery stack. The energy source of the heating resistor can be an external power supply, or directly connected to the battery for heating.

[0065] The specific structure is as follows:

[0066] The air supply subsystem 3 includes an air compressor 301 , a first exchange interface 302 , an air cooler 303 and a humidifier 304 which are connected in sequence. The humidifier 304 is connected to the fuel cell stack 1 , and the connection subsystem between the humidifier 304 and the fuel cell stack 1 is connected to the first exchange interface 302 .

[0067] The connection line between the humidifier 304 and the fuel cell stack 1 is provided with a second exchange interface 305, and the hydrogen supply subsystem 2 includes a high-pressure hydrogen tank 201, a pressure reducing valve 202, a third exchange interface 203, a hydrogen inlet valve 204 and a hydrogen heat exchanger 205 connected in sequence, the hydrogen heat exchanger 205 is connected to the fuel cell stack 1, and the third exchange interface 203 is connected to the second exchange interface 305; the first exchange interface 302, the second exchange interface 305 and the third exchange interface 203 are all three-way valves.

[0068] When the fuel cell hybrid power system is shut down, the fuel cell stack 1 is purged by passing air from the air supply subsystem 3 into the hydrogen supply subsystem 2 .

[0069] The cooling subsystem 5 includes a water supply tank 501, a water pump 502, a radiator 504, an electric heater 505 and a thermostat 503. The fuel cell stack 1, the hydrogen heat exchanger 205, the air cooler 303, the radiator 504, the thermostat 503, the water pump 502, the water supply tank 501 and the fuel cell stack 1 are connected in sequence. One end of the electric heater 505 is connected to the thermostat 503, and the other end is connected to the air cooler 303.

[0070] The electric heater 505 is powered by an external power source or the battery subsystem 4 .

[0071] The battery subsystem 4 includes a DC-DC converter 401, a battery 402 and a motor 403. The fuel cell stack 1 is connected to the motor 403 via the DC-DC converter 401, and the battery 402 is connected to the motor 403. The fuel cell stack 1 is also connected to a hydrogen outlet valve 6.

[0072] The present embodiment also provides a modeling method of the above-mentioned fuel cell hybrid power system, which is used to establish a fuel cell hybrid power system model, wherein the fuel cell hybrid power system model is provided with an air supply subsystem model, a cooling subsystem model, a battery low temperature model, a fuel cell monomer model and a fuel cell stack model, and the fuel cell stack model is constructed based on the fuel cell monomer model.

[0073] The model established by this modeling method includes: the gas supply subsystem solves the flow, temperature, pressure and other parameters of the reaction gas, the cooling subsystem solves the coolant flow and temperature change, the battery model is established based on experimental data to solve the low-temperature input and output performance, the fuel cell low-temperature model is established to solve the output performance during the low-temperature start-up process, and the system structure is modified based on the current low-temperature start-up strategy, which is suitable for the simulation and optimization of the fuel cell low-temperature start-up strategy. The system model structure can be applied to the simulation study of the low-temperature start-up strategy of the fuel cell. As the core component of the hybrid system, the ice inside the fuel cell will continue to accumulate before the battery temperature reaches above the freezing point, and may even completely cover the fuel cell catalyst layer, causing the battery to shut down and unable to complete the low-temperature start-up. This modeling method considers the water transmission, phase change and ice volume fraction on the loss voltage during the low-temperature start-up of the fuel cell, making the model physical process more perfect. In addition, the model models each component in the fuel cell separately and solves the mass transfer and heat transfer between each layer, which can reflect the temperature distribution differences between the layers of the battery, and can better predict the output performance of the fuel cell during the low-temperature start-up process, improving the accuracy and application value of the model.

[0074] The following is a detailed description of each model.

[0075] 1. Air supply subsystem model

[0076] like Figure 2As shown, this embodiment models the key components and pipelines in the fuel cell gas supply subsystem, solves the gas state equation, and obtains the changes in parameters such as temperature, humidity, and pressure of the reaction gas in the gas supply system.

[0077] The temperature and pressure of the gas after passing through the air compressor are represented by adiabatic compression:

[0078]

[0079] Among them, T1 and p1 are the temperature and pressure of the air at the inlet of the air compressor, and T2 and p2 are the temperature and pressure of the air at the outlet of the air compressor.

[0080] The gas supply pipeline model is established according to the mass conservation equation as follows:

[0081]

[0082] Among them, F cp,out is the mass flow rate into the cathode gas supply pipe, F sp,out is the flow rate out of the cathode gas supply pipeline, m sp is the mass of gas accumulated in the cathode gas supply pipeline.

[0083] The differential equation for the gas supply pipeline pressure is:

[0084]

[0085] Among them, p sp is the pressure in the supply pipe, t is the time, V sp is the supply pipe volume, R a is the air gas constant, T sp is the temperature of the supply pipe, γ is the specific heat ratio of air, T cp,out is the gas temperature at the outlet of the air compressor, F sp,out is the gas flow rate out of the supply pipeline.

[0086] Relative humidity of the reaction gas after passing through the air cooler The calculation is as follows:

[0087]

[0088] in, is the relative humidity of air at normal pressure, p cl is the pressure of the air in the cooler, p sat is the saturated water vapor pressure, T atm is the temperature of air at normal pressure, p atm is the normal pressure of ambient air, T cl is the gas temperature in the cooler.

[0089] Assuming that the temperature of the air does not change after passing through the humidifier, the water vapor partial pressure p v,hm The calculation formula is as follows:

[0090]

[0091] In the formula, is the relative humidity of the gas in the humidifier, p sat is the saturated water vapor pressure, T hm is the gas temperature in the humidifier.

[0092] 2. Cooling subsystem model

[0093] The cooling subsystem consists of a coolant circulation pump, an air cooler, a fuel cell stack, a thermostat, and a radiator. In order to adapt to the cold start strategy, the cooling cycle designed in this paper adds an electric heater to heat the coolant to assist the cold start of the fuel cell. During the model establishment process, it is assumed that the coolant flow rate in the cooling cycle remains unchanged.

[0094] According to the first law of thermodynamics, the temperature change model of the air cooler is established as follows:

[0095]

[0096] In the formula, ρ cool is the coolant density, V inc is the cooling system volume, C cool is the specific heat capacity of the coolant, T inc is the temperature of the coolant in the cooling system, is the coolant flow rate, T cool,inc,out is the coolant temperature leaving the cooling system, T cool,inc,in is the coolant temperature at the cooling system inlet, is the heat transfer power between coolant and air.

[0097] The temperature change model of the reaction gas passing through the hydrogen heat exchanger is as follows:

[0098]

[0099] Where V ex is the volume of hydrogen heat exchanger, T ex is the temperature of the hydrogen heat exchanger, T cool,ex,in is the temperature of the coolant before it flows into the heat exchanger, is the heat transfer power between hydrogen and coolant.

[0100] The distribution of coolant flow by the thermostat can be expressed by the following function:

[0101]

[0102] In the formula, To divert the coolant flow to the radiator, is the flow rate of coolant diverted to the bipolar plate, k is the opening of the thermostat valve, is the total coolant flow through the thermostat.

[0103] 3. Battery low temperature model

[0104] The Rint model is modified based on the low-temperature test data of a specific battery. The model can reflect the input and output performance of the battery in different temperature ranges, such as key performance parameters such as battery capacity, charge and discharge open circuit voltage, charge and discharge internal resistance, etc. The input and output of the model and its structure are as follows: Figure 3 shown.

[0105] 4. Fuel cell monomer model

[0106] like Figure 4 As shown, for the low-temperature start-up process of the fuel cell, this embodiment focuses on the influence of the water balance, heat balance and water-heat distribution of the fuel cell on the performance output of the fuel cell during the low-temperature start-up process of the fuel cell.

[0107] The main reason for the failure of fuel cell low temperature startup is that the water produced by the reaction during the low temperature startup freezes and blocks the catalyst layer, causing the reaction to fail, which in turn causes a sharp drop in output voltage and startup failure. The model established by the present invention takes into account the generation, phase change, accumulation, and transfer of water during the low temperature startup of the fuel cell, and more completely reflects the transmission of water in the fuel cell during the low temperature startup, providing a guarantee for the accuracy of the model. The water content accumulated during the fuel cell reaction It can be expressed by the following formula:

[0108]

[0109] in, It is the water generated by the reaction of the catalytic layer. is the amount of water absorbed and diffused through the proton exchange membrane, is the water content diffused into the gas diffusion layer.

[0110] The amount of ice accumulated in the fuel cell during low-temperature startup, i.e., the ice volume fraction, can be expressed by the following formula:

[0111]

[0112] Where s0 is the initial ice volume fraction in the fuel cell before low-temperature startup, v ice is the molar mass of ice, t is the current moment, ε CL is the porosity of the catalyst layer, δ CLis the thickness of the catalytic layer, and A is the reaction area of ​​the catalytic layer.

[0113] The model not only includes the process of ice volume fraction accumulation, but also models the ice melting process after the battery temperature rises above the freezing point. If the fuel cell temperature can rise above the freezing point before the catalyst layer is completely covered by ice, the heat generated by the electrochemical reaction is used to melt the accumulated ice, and the battery temperature will not continue to rise. The melting rate and time required for ice can be calculated by the following formula:

[0114]

[0115] Where t1 is the time when the temperature of the fuel cell catalyst layer reaches the freezing point. is the heat generation rate of the fuel cell reaction, is the fuel cell heat dissipation rate, V CL is the volume of the catalyst layer, h sg It is the latent heat of phase change when water vapor condenses.

[0116] The low-temperature fuel cell model proposed in the present invention does not need to solve the charge conservation formula, but modifies the Tafel formula according to the low-temperature starting characteristics of the fuel cell to calculate the low-temperature output voltage U of the fuel cell. cell , which is calculated as follows:

[0117] U cell =E nernst +V act +V conc +V ohmic

[0118] In the formula, E nernst is the ideal electromotive force of the fuel cell chemical reaction, also known as the Nernst voltage, which can be expressed by the change in Gibbs free energy. The expression is as follows:

[0119]

[0120] Among them, ΔG f is the change in Gibbs free energy, F is the Faraday constant, ΔG f 0 is the change in Gibbs free energy under standard conditions, R is the gas constant, T fc is the fuel cell temperature, is the hydrogen partial pressure, is the partial pressure of oxygen, is the water vapor pressure.

[0121] V act is the activation overpotential of the fuel cell, which is caused by the activation energy of the reaction that electrons need to overcome when passing through the proton exchange membrane. It is calculated as follows:

[0122]

[0123]

[0124]

[0125] Among them, s ice ,s lq are the volume fractions of ice and liquid water in the catalyst layer, respectively. The introduction of the volume fractions reflects the effect of the condensation of ice and liquid water on the activation overpotential. The volume fraction of ice is calculated by the water balance model. α is the transmission coefficient, R is the gas constant, and T m is the temperature of the proton exchange membrane, F is the Faraday constant, j * is the reaction rate of the anode and cathode catalyst layers of the fuel cell, δ CL is the thickness of the catalyst layer, c h is the concentration of the reaction gas, c ref is the reference gas concentration.

[0126] V conc is the concentration difference overpotential, which is calculated as follows:

[0127]

[0128]

[0129] Where F is the Faraday constant, c h is the concentration of the reaction gas, δ GDL is the GDL thickness, is the gas diffusion coefficient in the GDL, δ CL is the thickness of the catalyst layer, is the gas diffusion coefficient in the catalyst layer.

[0130] V ohmic is the Ohm overpotential, and the calculation formula according to Ohm's law is as follows:

[0131]

[0132] in, is the voltage loss due to electronic impedance, is the voltage loss caused by proton impedance, I is the fuel cell current, R elec is the impedance of electrons passing through the current collector, R proton is the resistance of protons passing through the proton exchange membrane, R internal is the total internal impedance of the fuel cell.

[0133] In order to more accurately reflect the low-temperature start-up performance of fuel cells, the simulation and prediction of the temperature distribution inside the fuel cell during the low-temperature start-up process is particularly important. The present invention models each component in the fuel cell separately and solves the mass transfer and heat transfer formulas between each layer. The model can reflect the temperature distribution in the thickness direction of the fuel cell during the low-temperature start-up process, and can reflect the temperature difference between each component in the fuel cell monomer, and can more accurately predict the low-temperature start-up performance of the fuel cell. Moreover, unlike the low-temperature finite element model of the fuel cell established based on finite element commercial software, the low-temperature stratified model of the fuel cell established by the present invention based on MATLAN / SIMULINK can be applied to the solution of engineering problems such as the formulation and optimization of the low-temperature start-up control strategy of the fuel cell, and has a high engineering application significance.

[0134] The heat generated during the low-temperature startup of the fuel cell is mainly the reversible heat generated by the chemical reaction in the catalyst layer and the proton exchange membrane, the irreversible heat caused by the polarization phenomenon, and the phase change heat of water. The heat generation power of the fuel cell is expressed as follows:

[0135]

[0136] In the formula, is the heat generation rate of the fuel cell, U o is the open circuit voltage, h sg is the latent heat of phase change when water vapor condenses, F is the Faraday parameter, T is the fuel cell temperature, i is the current density, A is the reaction area, U cell is the fuel cell output voltage.

[0137] The model is based on the principles of heat transfer. It models and solves the heat transfer between the components in each layer of the fuel cell and the heat exchange between the fuel cell and the ambient air. It can reflect the heat transfer and temperature distribution of each component in the fuel cell.

[0138] 5. Fuel cell stack model

[0139] Based on the fuel cell monomer stratification model, the present invention establishes a fuel cell stack stratification model, which can reflect the temperature distribution of each monomer in the stack during low-temperature startup, and further reflect the uneven voltage distribution of the monomers in the stack. The model divides the bipolar plate into five layers: anode gas flow channel layer, anode bipolar plate layer, cooling flow channel layer, cathode bipolar plate layer, and cathode gas flow channel layer, and uses the bipolar plate as the boundary for heat exchange between monomers in the stack. The bipolar plate stratification diagram is shown in the figure. Figure 5 shown.

[0140] The model also takes into account the different structures and heat transfer methods of the monomers in contact with the end plates at both ends of the fuel cell stack, and fully considers the heat transfer and temperature distribution of monomers at different positions in the fuel cell stack. It can be applied to the study of engineering problems such as low-temperature start-up strategies for fuel cells.

[0141] 6. Specific implementation

[0142] The battery structure and related parameters are as follows: the battery reaction area is 235cm2, the length and width of the cell is 100mm*100mm, the thickness of the proton exchange membrane is 0.178mm, the thickness of the catalyst layer is 0.01mm, and the thickness of the gas diffusion layer is 0.2mm. The equivalent mass of the proton exchange membrane is 1100kg / kmil, the density of the proton exchange membrane is 1980kg / m3, the density of CL, GDL, and BP is 1000g / m3, and the specific heat capacity of the proton exchange membrane, CL, GDL, and BP is 833, 3300, 568, and 1580J / (kg*K), respectively. The input current density used in the simulation increases uniformly to 40mA / cm2 within 80s and then remains stable. The simulation time is set to 300s.

[0143] Figure 6 The schematic diagram of the fuel cell monomer layered model and the heat transfer between the layers are shown. Figure 7 A schematic diagram of the fuel cell stack layered model structure is shown. Based on the model schematic diagram, each cell in the stack and each layer within the cell are modeled separately and the mass transfer and heat transfer formulas between the layers are solved.

[0144] Example Figure 8 The figure shows the comparison between the simulation voltage results and the experimental results of the fuel cell monomer startup at 253.15K and 243.15K. It can be concluded from the figure that the simulation results are in good agreement with the experimental results, and the model can more accurately reflect the performance output of the fuel cell monomer at low temperature startup.

[0145] Fig. 9 The temperature trend of each layer in the fuel cell monomer during the 253.15K startup process is shown. As the low-temperature startup process progresses and heat is generated, the temperature of each component in the battery rises steadily, but fails to reach above freezing before the catalyst layer is completely covered by ice, resulting in startup failure. Fig. 9 It can also obtain the temperature distribution differences between the components in the fuel cell, which has high research significance.

[0146] Fig.10 The comparison between the simulation results and the test results when the 270.15K startup is successful is shown. The results show that the fitting effect is good. Fig.11 The changes in battery temperature and ice volume fraction during the successful low-temperature start are shown, which is consistent with theoretical expectations, indicating that the model established by the present invention can not only simulate failed low-temperature starts, but also reflect successful low-temperature starts.

[0147] Fig.12It is the temperature distribution in the thickness direction of a battery stack with 5 cells when it fails to start in a low-temperature environment of 253.15K. According to the simulation results, it can be known that the temperature of the cells in the middle is higher than the temperature of the cells on both sides, and because the cells at both ends of the battery stack are in direct contact with the end plates, there is an obvious gap between their temperatures and those in the middle.

[0148] Fig.13 It is the average voltage output of fuel cell stacks with different numbers of cells when they are started in a low-temperature environment of 253.15K. It can be found that as the number of fuel cell stacks increases, the average voltage output of the fuel cell stack increases at the same time, and the startup duration is extended.

[0149] The preferred specific embodiments of the present invention are described in detail above. It should be understood that a person skilled in the art can make many modifications and changes based on the concept of the present invention without creative work. Therefore, any technical solution that can be obtained by a person skilled in the art through logical analysis, reasoning or limited experiments based on the concept of the present invention on the basis of the prior art should be within the scope of protection determined by the claims.

Claims

1. A modeling method for a fuel cell hybrid power system suitable for cold start, wherein the fuel cell hybrid power system comprises a fuel cell stack (1), wherein the fuel cell stack (1) is respectively connected to a hydrogen supply subsystem (2), a gas supply subsystem (3), a battery subsystem (4) and a cooling subsystem (5), wherein: The air supply subsystem (3) comprises an air compressor (301), a first exchange interface (302), an air cooler (303) and a humidifier (304) which are connected in sequence, the humidifier (304) is connected to the fuel cell stack (1), and the connection subsystem between the humidifier (304) and the fuel cell stack (1) is connected to the first exchange interface (302); The connection line between the humidifier (304) and the fuel cell stack (1) is provided with a second exchange interface (305); the hydrogen supply subsystem (2) comprises a high-pressure hydrogen tank (201), a pressure reducing valve (202), a third exchange interface (203), a hydrogen inlet valve (204) and a hydrogen heat exchanger (205) connected in sequence; the hydrogen heat exchanger (205) is connected to the fuel cell stack (1); and the third exchange interface (203) is connected to the second exchange interface (305); When the fuel cell hybrid power system is shut down, the fuel cell stack (1) is purged by passing air from the air supply subsystem (3) into the hydrogen supply subsystem (2); The cooling subsystem (5) comprises a water supply tank (501), a water pump (502), a radiator (504), an electric heater (505) and a thermostat (503); the fuel cell stack (1), the hydrogen heat exchanger (205), the air cooler (303), the radiator (504), the thermostat (503), the water pump (502), the water supply tank (501) and the fuel cell stack (1) are connected in sequence; one end of the electric heater (505) is connected to the thermostat (503), and the other end is connected to the air cooler (303); The modeling method is used to establish a fuel cell hybrid power system model, wherein the fuel cell hybrid power system model is provided with a gas supply subsystem model, a cooling subsystem model, a battery low temperature model, a fuel cell monomer model and a fuel cell stack model, and the fuel cell stack model is constructed based on the fuel cell monomer model; The modeling process of the gas supply subsystem model includes: modeling the components and pipelines of the gas supply subsystem model, solving the gas state equation, and obtaining the parameter changes of the reaction gas in the gas supply system. The specific process includes: The temperature and pressure of the gas after passing through the air compressor are represented by adiabatic compression: Wherein, T1 and p1 are the temperature and pressure of the gas at the inlet of the air compressor, and T2 and p2 are the temperature and pressure of the gas at the outlet of the air compressor; The gas supply pipeline model is established according to the mass conservation equation as follows: Among them, F cp,out is the mass flow rate into the cathode gas supply pipe, F sp,out is the flow rate out of the cathode gas supply pipeline, m sp is the mass of gas accumulated in the cathode gas supply pipeline; The differential equation for the gas supply pipeline pressure is: Among them, p sp is the pressure in the supply pipe, t is the time, V sp is the supply pipe volume, R a is the air gas constant, T sp is the temperature of the supply pipe, γ is the specific heat ratio of air, T cp,out is the gas temperature at the outlet of the air compressor, F sp,out is the gas flow rate out of the supply pipeline; Relative humidity of the reaction gas after passing through the air cooler The calculation is as follows: in, is the relative humidity of air at normal pressure, p cl is the pressure of the air in the cooler, p sat is the saturated water vapor pressure, T atm is the temperature of air at normal pressure, p atm is the normal pressure of ambient air, T cl is the gas temperature in the cooler; Assuming that the temperature of the air does not change after passing through the humidifier, the water vapor partial pressure p v,hm The calculation formula is as follows: In the formula, is the relative humidity of the gas in the humidifier, p sat is the saturated water vapor pressure, T hm is the gas temperature in the humidifier; The modeling process of the cooling subsystem assumes that the flow rate of the coolant remains constant in the cooling cycle. The specific process includes: According to the first law of thermodynamics, the temperature change model of the air cooler is established as follows: In the formula, ρ cool is the coolant density, V inc is the cooling system volume, C cool is the specific heat capacity of the coolant, T inc is the temperature of the coolant in the cooling system, is the coolant flow rate, T cool,inc,out is the coolant temperature leaving the cooling system, T cool,inc,in is the coolant temperature at the cooling system inlet, is the heat exchange power between coolant and air; The temperature change model of the reaction gas passing through the hydrogen heat exchanger is as follows: Where V ex is the volume of hydrogen heat exchanger, T ex is the temperature of the hydrogen heat exchanger, T cool,ex,in is the temperature of the coolant before it flows into the heat exchanger, is the heat transfer power between hydrogen and coolant; The thermostat distributes the coolant flow through the following function: In the formula, To divert the coolant flow to the radiator, is the flow rate of coolant diverted to the bipolar plate, k is the opening of the thermostat valve, is the total coolant flow through the thermostat; The battery low temperature model modifies the Rint model based on the low temperature test data of a specific battery to reflect the input and output performance of the battery in different temperature ranges; In the fuel cell monomer model, the calculation expression for the water content accumulated during the fuel cell reaction is: In the formula, is the water content accumulated during the fuel cell reaction, It is the water generated by the reaction of the catalytic layer. is the amount of water absorbed and diffused through the proton exchange membrane, is the water content diffused into the gas diffusion layer; In the fuel cell monomer model, the calculation expression for the ice accumulation in the fuel cell during low-temperature startup is: Where s0 is the initial ice volume fraction in the fuel cell before low-temperature startup, v ice is the molar mass of ice, t is the current moment, ε CL is the porosity of the catalyst layer, δ CL is the thickness of the catalytic layer, A is the reaction area of ​​the catalytic layer; During the melting of ice in the fuel cell, the calculation expression for the ice accumulation amount is: Where t1 is the time when the temperature of the fuel cell catalyst layer reaches the freezing point. is the heat generation rate of the fuel cell reaction, is the fuel cell heat dissipation rate, V CL is the volume of the catalyst layer, h sg is the latent heat of phase change when water vapor condenses; In the fuel cell monomer model, the calculation expression of the low temperature output voltage of the fuel cell is: U cell =E nernst +V act +V conc +V ohmic Where U cell is the low temperature output voltage of the fuel cell, E nernst is the ideal electromotive force of the fuel cell chemical reaction, V act is the activation overpotential of the fuel cell, V conc is the concentration difference overpotential, V ohmic is the ohmic overpotential, ΔG f is the change in Gibbs free energy, F is the Faraday constant, ΔG f 0 is the change in Gibbs free energy under standard conditions, R is the gas constant, T fc is the fuel cell temperature, is the hydrogen partial pressure, is the partial pressure of oxygen, is the water vapor partial pressure, s ice is the volume fraction of ice in the catalyst layer, s lq is the volume fraction of liquid water in the catalyst layer, α is the transmission coefficient, R is the gas constant, T m is the temperature of the proton exchange membrane, F is the Faraday constant, j * is the reaction rate of the anode and cathode catalyst layers of the fuel cell, δ CL is the thickness of the catalyst layer, c h is the concentration of the reaction gas, c ref is the reference gas concentration, δ GDL is the GDL thickness, is the gas diffusion coefficient in the GDL, δ CL is the thickness of the catalyst layer, is the gas diffusion coefficient in the catalyst layer, is the voltage loss due to electronic impedance, is the voltage loss caused by proton impedance, I is the fuel cell current, R elec is the impedance of electrons passing through the current collector, R proton is the resistance of protons passing through the proton exchange membrane, R internal is the total internal impedance of the fuel cell.

2. A modeling method for a fuel cell hybrid power system adapted for cold start according to claim 1, characterized in that: The fuel cell stack model divides the bipolar plate of the fuel cell stack into an anode gas flow channel layer, an anode bipolar plate layer, a cooling flow channel layer, a cathode bipolar plate layer and a cathode gas flow channel layer which are arranged in sequence.

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