A fuel cell engine anode exhaust method

Abstract

The invention discloses an anode exhaust method for a fuel cell engine, which belongs to the technical field of fuel cells. Specifically, the power generation loss of the fuel cell system caused by the increase of the nitrogen content in the anode loop is calculated first, the hydrogen emission loss caused by the exhaust is calculated, and then the total energy efficiency of the fuel cell system is calculated; then the total energy efficiency that changes with the anode nitrogen content under different stack currents and different initial average node voltages is calculated, and the corresponding single cell voltage drop when the total energy efficiency is the highest is calculated, and a map diagram is drawn; finally, the operating current and the initial average node voltage of the fuel cell stack are read, and the corresponding single cell voltage drop is queried in the map diagram, which is used as the exhaust trigger condition of the exhaust valve, and the switch of the exhaust valve is controlled to maintain the nitrogen content at an optimal value, so as to balance the hydrogen emission loss and the performance loss caused by the nitrogen content, so that the anode power generation efficiency of the fuel cell system is maximized.

Description

Technical Field

[0001] The invention belongs to the technical field of fuel cells, and in particular relates to an anode exhaust method for a fuel cell engine. Background Art

[0002] A fuel cell is an environmentally friendly, efficient, and long-life power generation device. Taking a proton exchange membrane fuel cell (PEMFC) as an example, the fuel gas enters from the anode side, hydrogen atoms lose electrons at the anode and become protons, the protons pass through the proton exchange membrane to the cathode, and the electrons also reach the cathode through an external circuit at the same time, and the protons and electrons at the cathode combine with oxygen to generate water. Fuel cells use a non-combustion method to convert chemical energy into electrical energy. Since they are not restricted by the Carnot cycle, their direct power generation efficiency can be as high as 45%. With the battery stack as the core power generation device, the fuel cell system integrates power management, thermal management and other modules, and has the characteristics of coordinated management of heat, electricity, water, and gas. Fuel cell system products have a wide range of applications, from fixed power stations to mobile power sources; from electric vehicles to spacecraft; from military equipment to civilian products.

[0003] The structure of a fuel cell system is generally as follows Figure 1 As shown, the anode loop of the fuel cell system mainly includes proportional valves, circulating pumps, exhaust valves, steam traps and other components. The high-pressure hydrogen entering the fuel cell system is adjusted to the set anode loop pressure value by the proportional valve, and mixed with the recycled hydrogen, and enters the battery stack to participate in the reaction. The unreacted gas carries out the product water vapor or liquid water, of which the liquid water is discharged from the anode loop through the steam trap. Pressure sensors P are installed in the hydrogen inlet manifold, hydrogen outlet manifold, and air inlet pipeline of the battery stack, and temperature sensors T are installed in the coolant inlet pipeline and coolant outlet pipeline.

[0004] When the fuel cell system is working, since the nitrogen partial pressure of the air on the cathode side is much higher than that on the anode side, the nitrogen in the air penetrates into the anode through the proton exchange membrane and continues to accumulate. Therefore, it is necessary to regularly open the exhaust valve set in the hydrogen outlet pipeline of the battery stack to exhaust in order to control the continuous accumulation of nitrogen. The accumulated nitrogen will have potential adverse effects on the operation of the fuel cell system: 1) Nitrogen dilutes the anode hydrogen, resulting in a drop in the node voltage and reduced power generation efficiency; 2) If the nitrogen content accumulates too high, it will increase the risk of hydrogen starvation and damage the membrane electrode. If the nitrogen concentration at the anode is controlled too low, the exhaust frequency will increase, and too much hydrogen will be discharged, resulting in hydrogen waste. Therefore, it is necessary to formulate a reasonable anode exhaust strategy to control the nitrogen content in the anode loop within a reasonable range to improve the power generation efficiency of the fuel cell system.

[0005] There have been many studies on the anode exhaust strategy of fuel cell systems.

[0006] The Chinese patent with publication number CN 114759233 A discloses a nitrogen exhaust valve control method and nitrogen exhaust valve system suitable for hydrogen fuel system, which uses neural network training to establish a model of current and anode nitrogen accumulation rate, and then calculates the anode nitrogen accumulation rate according to the current; uses neural network to establish a model of nitrogen exhaust valve internal pressure, atmospheric pressure difference and nitrogen exhaust valve nitrogen mass rate, and then calculates the nitrogen exhaust valve nitrogen mass rate according to the internal pressure of the nitrogen exhaust valve and atmospheric pressure difference; calculates the anode nitrogen accumulation mass in real time through the anode nitrogen accumulation rate, and when the anode nitrogen accumulation mass reaches M1, the nitrogen emission mass and emission time are cleared to 0, and the nitrogen exhaust valve is opened. The Chinese patent with publication number CN 116487653 A also discloses a similar scheme, specifically a method for controlling the anode exhaust valve of a fuel cell engine, which uses a model to calculate the permeation rate, and then formulates an exhaust strategy based on the cumulative permeation amount, specifically triggering the exhaust according to the calculated nitrogen concentration, but does not achieve the optimization of power generation efficiency, and the nitrogen accumulation rate depends on the training data or model accuracy, which will also cause the results to be greatly affected by the training data and model accuracy.

[0007] A Chinese patent with publication number CN 116231008 A discloses a method for evaluating the effect of anode nitrogen content on stack performance in a fuel cell. When the cathode gas flow and cathode pressure of the fuel cell are constant, the anode pressure is changed to measure the stack voltage, and the effect of the anode nitrogen content on the stack performance is evaluated according to the change in the stack voltage to determine whether nitrogen removal is required. This method is not affected by the attenuation of the stack performance and has good adaptability. However, it requires increasing the anode pressure, increasing the anode-cathode pressure difference, and increasing the risk of mechanical fatigue of the proton membrane. At the same time, it does not achieve the optimization of power generation efficiency.

[0008] A Chinese patent with publication number CN 117976946 A discloses a control method for anode purge of a fuel cell. By using the molar concentration of anode nitrogen and the uniformity of the voltage of each single cell as control indicators of the purge strategy, the anode purge of the fuel cell is accurately controlled in real time, and while fully exhausting nitrogen, hydrogen waste is avoided to ensure that the fuel cell is in the optimal performance state. However, the selected nitrogen concentration does not correspond to the nitrogen concentration corresponding to the optimal efficiency. And as the battery stack ages, the voltage uniformity of each single cell in the battery stack will gradually decrease, resulting in poor applicability of this method. Summary of the invention

[0009] In response to the problems existing in the anode exhaust strategy of the existing fuel cell system, the present invention provides a fuel cell engine anode exhaust method, which explores the impact of nitrogen content on the power generation efficiency of the fuel cell system from two aspects: the impact of nitrogen content on performance and hydrogen loss in the exhaust process, and then determines the optimized nitrogen content range to balance the hydrogen emission loss and the performance loss caused by nitrogen content, so as to maximize the anode power generation efficiency of the fuel cell system.

[0010] The technical solution adopted by the present invention is as follows:

[0011] A fuel cell engine anode exhaust method comprises the following steps:

[0012] Step 1: Calculate the power generation loss ΔE of the fuel cell system due to the increase in nitrogen content in the anode circuit volt loss :

[0013] ΔE volt loss =∫(ΔE thermo +Δη+ΔV ir )N·I·dt

[0014] In the formula, ΔE thermo is the open circuit voltage difference caused by thermodynamics, which is related to the anode nitrogen content θ; Δη is the anode overpotential difference caused by reaction kinetics, which is related to the anode nitrogen content θ; ΔV ir is the change in internal resistance; N is the number of battery stacks; I is the battery stack current, in A; t is time;

[0015] Step 2: Based on the exhaust volume V of the exhaust valve when it is opened once purge , calculate the hydrogen emission loss caused by exhaust

[0016]

[0017] Where V purge The unit is L; The unit is kJ; is the molar fraction of hydrogen at the outlet of the battery stack; V 0 is the initial average node voltage when the nitrogen content is 0, in V; ΔH LHV is the lower heating value of hydrogen, which is 242 kJ / mol;

[0018] Step 3: Calculate the total energy efficiency η of the fuel cell system E :

[0019]

[0020] Step 4: According to the anode flow field structure of the battery stack, the flow field structure parameters are calculated, including the inlet gas distribution area structure parameters SPin , reaction zone structural parameters SP mid And the outlet gas distribution area structure parameters SP out ;

[0021] Step 5: Calculate the voltage drop ΔP of a single cell:

[0022] ΔP=P in -P out =ΔP in +ΔP mid +ΔP out

[0023]

[0024] Where P in is the anode stack pressure, in Pa; P out is the anode outlet pressure, in Pa; st is the hydrogen stoichiometric ratio of the battery stack; is the humidity of hydrogen; T in T is the temperature of the coolant entering the pile, in K; out The unit of temperature is K; ΔP in is the pressure drop of the inlet gas distribution area, in Pa; ΔP mid is the pressure drop in the reaction zone, in Pa; ΔP out is the pressure drop of the outlet gas distribution area, in Pa; P sat,in is the saturated steam pressure of the inlet gas distribution area, in Pa; P sat,mid is the saturated vapor pressure in the reaction zone, in Pa; P sat,out is the saturated steam pressure of the outlet gas distribution area, in Pa; is the average humidity of the inlet air distribution area; is the average humidity in the reaction zone; is the average humidity of the outlet air distribution area; T mid is the temperature of the reaction zone in K; P mid is the pressure in the reaction zone, in Pa;

[0025] Step 6: According to step 3, calculate different stack currents I and different initial average node voltages V 0 The total energy efficiency η changes with the anode nitrogen content θ under E , η E When the maximum value is reached, the corresponding θ is recorded as And according to step 5, calculate The corresponding single cell voltage drop ΔP is recorded as Draw a map;

[0026] Step 7: Obtain the operating current I of the fuel cell stack, and obtain the initial average node voltage V corresponding to the operating current I under steady-state conditions.0 ;

[0027] Step 8: Based on the read operating current I and initial average node voltage V 0 , query the corresponding

[0028] Step 9: Based on the result from step 8 is the exhaust valve exhaust trigger condition. When the anode nitrogen of the fuel cell stack accumulates to the anode stack pressure P in and anode outlet pressure P out The pressure drop ΔP between When the exhaust valve opens t dur time, and then shut down; when the anode nitrogen of the fuel cell stack accumulates again until the pressure drop ΔP reaches When the exhaust valve is opened and closed repeatedly, the nitrogen content of the anode of the fuel cell stack is maintained at Corresponding

[0029] Furthermore, the ΔE thermo The calculation formula is:

[0030]

[0031] Where R is the gas constant, with the unit of J / (mol·K); n is the number of electron transfers in the reaction, with the value of 2; F is the Faraday constant, with the value of 96485C / mol; is the hydrogen partial pressure in pure hydrogen state, in Pa; is the partial pressure of hydrogen in the nitrogen-containing mixed gas, in Pa; P mid is the pressure in the reaction zone, in Pa; P sat,mid is the saturated vapor pressure in the reaction zone, in Pa; is the average humidity in the reaction zone; θ is the nitrogen content at the anode.

[0032] Furthermore, the calculation formula of Δη is:

[0033]

[0034] Where cd is the actual working current density, in A / m 2 ; δ acl is the thickness of the anode catalyst layer, in m; a eff is the active specific surface area of ​​the anode catalyst layer, in m -1 ; The reference anode exchange current density is 1000A / m 3 ; α a and α cThey are the anode transfer coefficients, and their values ​​are all 0.5; is the hydrogen concentration in the reaction zone, in mol / m 3 ; is the reference hydrogen concentration, in mol / m 3 , the value is 40; is the molar concentration of hydrogen in the anode hydrogen-nitrogen mixed gas, in mol / m 3 ; T mid is the temperature of the reaction zone, in K.

[0035] Furthermore, the ΔV ir The calculation formula is:

[0036] ΔV ir =cd(rr mix )

[0037] Where r is the internal resistance when the anode is pure hydrogen; mix It is the internal resistance value when the anode is a mixture of hydrogen and nitrogen.

[0038] Furthermore, step 2 calculates V purge The specific process is:

[0039] During exhaust, the pressure of the anode circuit is kept basically constant under the regulation of the proportional valve, and the outlet pressure of the exhaust valve is approximately normal pressure. Then the gas flow rate Q through the exhaust valve is N The calculation formula is:

[0040]

[0041] In the formula, Q N The unit is m 3 / hr; P 1 is the pressure before the exhaust valve, which is approximately equal to the anode stack pressure P out , unit is bara; P 2 is the exhaust valve outlet pressure, in bara; ρ N It is the density of the fluid under standard conditions (101325Pa, 0℃), in kg / m 3 ; Kv is the exhaust valve flow coefficient, unit is m 3 / hr; ΔP v is the pressure difference on both sides of the valve, equal to P 1 -P 2 , unit is bara; M mix,out is the molar amount of mixed gas at the outlet of the battery stack, in g / mol;

[0042] Then calculate V purge :

[0043]

[0044] Where, t dur is the exhaust duration, in seconds.

[0045] Furthermore, the flow field structure parameters in step 4 are related to the flow channel length L and the equivalent hydraulic diameter d of the anode flow field. e , the flow area a of the flow channel, the friction coefficient f and the Reynolds number Re when the mixed gas flows through the flow field, and has nothing to do with the operating conditions, so it is a fixed value.

[0046] The beneficial effects of the present invention are:

[0047] Since the increase of nitrogen content in the anode loop will reduce the performance of the fuel cell and cause the loss of hydrogen emission to decrease, the present invention proposes a method for exhausting the anode of a fuel cell engine. Starting from the two aspects of the influence of nitrogen content on performance and hydrogen loss in the exhaust process, the influence of nitrogen content on the power generation efficiency of the fuel cell system is explored, and then the optimized nitrogen content range is determined to balance the hydrogen emission loss and the performance loss caused by nitrogen content; the relationship between the single cell pressure drop and the nitrogen content is used to trigger the switch of the exhaust valve, so that the nitrogen content is maintained at the optimal value and the anode power generation efficiency of the fuel cell system is maximized. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 A schematic diagram of the structure of a fuel cell system;

[0049] Figure 2 This is a flow chart of the anode exhaust method of a fuel cell engine proposed in Example 1 of the present invention;

[0050] Figure 3 This is a schematic diagram of the working state of the exhaust valve in Example 1 of the present invention;

[0051] Figure 4 The total energy efficiency at different currents varies with nitrogen content in Example 1 of the present invention; wherein (a) is 198A; (b) is 297A; (c) is 396A; (d) is 495A; (e) is 594A; (f) is 693A;

[0052] Figure 5 It is a parameter dynamic curve diagram of the fixed nitrogen content exhaust strategy at 198A in Example 1 of the present invention; wherein, (a) is the nitrogen content; (b) is the loss of average node voltage; and (c) is the voltage drop. DETAILED DESCRIPTION

[0053] In order to better understand the purpose and function of the present invention, the present invention is described in more detail below in conjunction with the accompanying drawings. In the following specific embodiments, many specific details are set forth to provide a thorough understanding of embodiments of the present invention. However, those skilled in the art will appreciate that embodiments of the present invention may be implemented without these specific details.

[0054] Example 1

[0055] This embodiment provides a method for exhausting the anode of a fuel cell engine, which calculates the total energy efficiency of the fuel cell system from two perspectives: the impact of nitrogen content on the power generation efficiency of the fuel cell system and the hydrogen energy loss during the exhaust process, and then formulates an exhaust strategy based on the anode pressure drop.

[0056] 1. Effect of nitrogen content on power generation efficiency of fuel cell system

[0057] As nitrogen permeation accumulates and the content gradually increases, it will cause a loss of average node voltage ΔV from three aspects: thermodynamics, reaction kinetics, and internal resistance, which will in turn cause a loss of power generation ΔE in the fuel cell system. volt loss .

[0058] 1) Performance degradation due to thermodynamics

[0059] Under the same operating pressure, the nitrogen content increases and the hydrogen partial pressure decreases, thereby reducing the theoretical open circuit voltage of the reaction at the thermodynamic level. The reduced value is quantitatively calculated by the Nernst equation, and the calculation formula for the open circuit voltage E is:

[0060]

[0061] In the formula, E 0 is the standard electromotive force of the battery, in V; R is the gas constant, in J / (mol·K); T mid represents the temperature of the reaction zone, in K; n is the number of electron transfers in the reaction, which is 2 in this embodiment; F is the Faraday constant, which is 96485 C / mol; is the hydrogen partial pressure in pure hydrogen state, in Pa; is the water vapor partial pressure, in Pa; is the oxygen partial pressure, in Pa;

[0062] Therefore, compared with the pure hydrogen state at the anode, the open circuit voltage difference ΔE caused by the increase in nitrogen content is thermo The calculation formula is:

[0063]

[0064] In the formula, is the partial pressure of hydrogen in the nitrogen-containing mixed gas, in Pa; Pmid is the pressure in the reaction zone, in Pa; P sat,mid is the saturated vapor pressure in the reaction zone, in Pa; is the average humidity in the reaction zone; θ is the nitrogen content at the anode.

[0065] 2) Performance degradation caused by reaction kinetics

[0066] The dilution effect of nitrogen also reduces the performance from the reaction kinetics perspective, which can be described by the first-order linearized Butler Volmer equation.

[0067] The calculation formula of transfer current density j is:

[0068]

[0069] In the formula, δ acl is the platinum thickness of the anode catalyst layer, in m; a eff is the active specific surface area of ​​the anode catalyst layer, in m -1 ; The reference anode exchange current density is 1000A / m 3 ; α a and α c They are the anode transfer coefficients, and their values ​​are all 0.5; is the hydrogen concentration in the reaction zone, in mol / m 3 ; is the reference hydrogen concentration, in mol / m 3 , the value is 40; η is the anode overpotential, the unit is V; m Pt is the platinum loading of the anode catalyst layer, in g / m 2 ; A c is the reactive area per unit mass of platinum, in m 2 / g;

[0070] At the same time, the transfer current density j is physically equivalent to the actual working current density cd (A / m 2 ) and the thickness of the anode catalyst layer platinum δ acl The ratio of, that is:

[0071]

[0072] Since the cathode operating conditions did not change during the entire test process, its potential was considered unchanged. The anode overpotential η was calculated when the nitrogen content was 0 and the maximum nitrogen content at each current, and the difference between the two was the effect of nitrogen content on the reaction kinetics.

[0073] The formula for the anode overpotential difference Δη caused by the reaction kinetics is:

[0074]

[0075] In the formula, is the molar concentration of hydrogen in the anode hydrogen-nitrogen mixed gas, in mol / m 3 ; T mid is the temperature of the reaction zone, in K.

[0076] 3) Performance degradation caused by changes in internal resistance

[0077] Under the same hydrogen stoichiometric ratio, the higher the nitrogen content and the faster the flow rate, the more water vapor the anode gas carries, which increases the ionic impedance of the proton exchange membrane and reduces its performance. This part of the performance loss can be obtained by multiplying the internal resistance change monitored by a 1kHz high-frequency impedance meter with the operating current to calculate the internal resistance change ΔV ir The formula is

[0078] ΔV ir =cd(rr mix )

[0079] Where r is the internal resistance when the anode is pure hydrogen; mix It is the internal resistance value when the anode is a mixture of hydrogen and nitrogen.

[0080] The internal resistance at different currents and nitrogen contents was measured experimentally, and the internal resistance was fitted as a function of current density and nitrogen content. The internal resistance at different nitrogen contents was determined through the function, so as to find out the performance degradation caused by the change of internal resistance due to the change of nitrogen content.

[0081] Based on the above calculations, the power generation loss of the fuel cell system caused by the increase in nitrogen content is ΔE volt loss The calculation formula is:

[0082] ΔE volt loss =∫(ΔE thermo +Δη+ΔV ir )N·I·dt

[0083] Where N is the number of battery cells; I is the battery current, in A; t is the time, in s.

[0084] 2. Hydrogen energy loss during exhaust process

[0085] Excessive nitrogen content will have a negative impact on the operation of the battery stack, and an exhaust strategy is needed to control the accumulation of nitrogen. During exhaust, the pressure of the anode circuit is basically kept constant under the regulation of the proportional valve, and the outlet pressure of the exhaust valve is approximately normal pressure. The gas flow rate Q through the exhaust valve is N The calculation formula is:

[0086]

[0087] In the formula, Q N The unit is m 3 / hr; P 1 is the pressure before the exhaust valve, which is approximately equal to the anode stack pressure P out , unit is bara; P 2 is the exhaust valve outlet pressure, in bara; ρ N It is the density of the fluid under standard conditions (101325Pa, 0℃), in kg / m 3 ; Kv is the exhaust valve flow coefficient, unit is m 3 / hr; ΔP v is the pressure difference on both sides of the valve, equal to P 1 -P 2 ,unit is bara;;M mix,out It is the molar amount of mixed gas at the outlet of the battery stack, in g / mol.

[0088] M mix,out According to material balance, the formula is:

[0089]

[0090] In the formula, is the water vapor mole fraction at the outlet of the battery stack; is the hydrogen mole fraction at the outlet of the battery stack; is the nitrogen mole fraction at the outlet of the battery stack; is the molar mass of water, which is 18 g / mol; is the molar mass of hydrogen, which is 2 g / mol; is the molar mass of nitrogen, which is 28 g / mol; P sat,out is the saturated steam pressure of the outlet gas distribution area; is the average humidity of the outlet gas distribution area; st is the hydrogen stoichiometric ratio of the battery stack.

[0091] Exhaust volume V when the exhaust valve is opened once purge (unit: L) is Q N With exhaust duration t dur (unit: s) is a function of the following formula:

[0092]

[0093] The hydrogen emission loss caused by exhaust The calculation formula is:

[0094]

[0095] Where V 0 is the initial average node voltage when the nitrogen content is 0, in V; ΔH LHV It is the lower heating value of hydrogen, which is 242 kJ / mol.

[0096] Furthermore, the overall energy efficiency η of the fuel cell system E The calculation formula is:

[0097]

[0098] 3. An exhaust strategy based on anode pressure drop

[0099] Based on the following reasonable assumptions:

[0100] (1) The fuel cell engine uses high-purity hydrogen (≥99.97%) as the gas source, with a low impurity content. Most of the impurity gas is nitrogen that permeates from the cathode through the proton membrane. It is assumed that the mixed gas in the anode loop contains only three components: nitrogen, hydrogen, and water vapor.

[0101] (2) The operating pressure on the anode side usually does not exceed 3 bara, and the mixed gas conforms to the ideal gas law;

[0102] (3) Due to the small hydrogen flow rate, the temperature difference between the hydrogen and the cooling water after preheating in the manifold is very small. When the hydrogen enters the single cell, its temperature is equal to the cooling water temperature at the corresponding position.

[0103] (4) Assume that the humidity is stabilized at 60% after the hydrogen enters the reaction zone;

[0104] (5) It is assumed that the flow of hydrogen in the anode flow field channel is laminar and reacts uniformly along the flow direction;

[0105] (6) Assume that the anode liquid water is discharged quickly, and ignore the effect of the liquid water occupying the gas flow area on the gas flow.

[0106] In this embodiment, the fuel cell stack is formed by stacking multiple single cells, and the anode flow field of the single cell includes three areas, which evenly distribute the gas from the inlet manifold to the inlet gas distribution area of ​​the reaction zone, the reaction area of ​​the parallel straight flow channel (including M flow channels), and the outlet gas distribution area of ​​the outlet manifold to evenly guide the unreacted hydrogen, accumulated inert gas (the purity of automotive hydrogen is more than 99.97%, and it originally contains nitrogen, methane, carbon dioxide, etc., but very little, mainly permeated nitrogen. This part is assumed to be nitrogen for the convenience of calculation), water vapor and condensed liquid water to the outlet gas distribution area.

[0107] Take the case where hydrogen flows countercurrently with cooling water and air and cooling water flow in the same direction, that is, the cooling water inlet temperature is equal to the hydrogen outlet temperature, and the cooling water outlet temperature is equal to the hydrogen inlet temperature. The measured values ​​of the anode inlet fluid pressure and temperature are marked as Pin 、T out , the anode stack fluid pressure and temperature measurement values ​​are marked as P out 、T in The pressure and temperature of the reaction zone are approximately the average of the inlet and outlet pressures, marked as P mid 、T mid .

[0108] The calculation formula for the flow channel length L in the three regions of the anode flow field is:

[0109]

[0110] Where A is the area of ​​the corresponding region (m 2 ), Q is the number of flow channels in the corresponding area, w C is the hydrogen flow channel width of the corresponding area, w L is the ridge width of the corresponding area.

[0111] According to the definition, the equivalent hydraulic diameter of the flow channel de is the ratio of 4 times the flow area to the wetted perimeter:

[0112]

[0113] Where, d C is the flow channel depth of the corresponding area.

[0114] The anode nitrogen content θ is defined as the nitrogen content of the dry gas (i.e. hydrogen and nitrogen) in the mixed gas entering the stack. Since the residence time of the reaction gas flowing through the battery stack is short, the effect of the nitrogen permeating from the cathode on the nitrogen concentration can be ignored. In a single estimation, the nitrogen content is assumed to be unchanged, and the calculation formula for the anode nitrogen content θ is:

[0115]

[0116] In the formula, is the molar flow rate of nitrogen in the mixed gas entering the reactor; is the molar flow rate of hydrogen in the mixed gas entering the reactor.

[0117] The pressure drop of a single cell on the anode side of the battery stack mainly includes two parts. The first part is the resistance loss along the flow path of the anode flow field, and the second part is the local resistance loss formed by the flow at the diversion, confluence, diameter change and elbow. The pressure drop is calculated according to the Bernoulli equation. The formula is:

[0118]

[0119] Where K is the total number of straight sections in the anode flow channel; J is the total number of local resistance losses in the anode flow channel; ρ k is the density at the kth straight pipe section; fk is the friction coefficient of the kth straight pipe section; L k is the length of the kth straight pipe section; d ek is the equivalent hydraulic diameter of the kth straight pipe section; u k is the flow velocity at the kth straight pipe section; j is the jth local resistance loss coefficient; ρ j is the density at the jth local resistance loss; u j is the flow velocity at the jth local resistance loss.

[0120] In the laminar flow section, the product of the friction coefficient f and the Reynolds number Re is a constant coef, which is related to the width and depth of the flow channel.

[0121]

[0122] Therefore, when calculating the voltage drop of a single cell, the definition of the Reynolds number Re can be substituted into the above formula and the The local resistance loss term of the term can be obtained by the anode voltage drop expression:

[0123]

[0124] In the formula, coef k is the constant coef at the kth straight pipe section; μ k is the viscosity of the mixed gas at the kth straight pipe section.

[0125] It can be seen that the single cell voltage drop ΔP is related to the flow rate u k is proportional to the first power of .

[0126] Due to the constant coef, the flow channel length L, the flow channel equivalent hydraulic diameter d e , flow channel flow area a are inherent structural parameters related to the anode flow field design and have nothing to do with the operating conditions, so the flow field structural parameters SP (Structural parameters) are defined as:

[0127]

[0128] The single cell pressure drop ΔP is the sum of the pressure drops in the inlet gas distribution area, the reaction area, and the outlet gas distribution area. The pressure drops in these three areas are calculated separately according to the hydrogen-oxygen reaction equation and the mass conservation equation. The expression of the single cell pressure drop ΔP is:

[0129]

[0130] Where P in is the anode stack pressure, in Pa; P out is the anode outlet pressure, in Pa; st is the hydrogen stoichiometric ratio of the battery stack; is the humidity of hydrogen; T in T is the temperature of the coolant entering the pile, in K; out The unit of temperature is K; ΔP in is the pressure drop of the inlet gas distribution area, in Pa; ΔP mid is the pressure drop in the reaction zone, in Pa; ΔP out is the pressure drop of the outlet gas distribution area, in Pa; P sat,in is the saturated steam pressure of the inlet gas distribution area, in Pa; P sat,mid is the saturated vapor pressure in the reaction zone, in Pa; P sat,out is the saturated steam pressure of the outlet gas distribution area, in Pa; is the average humidity of the inlet air distribution area; is the average humidity in the reaction zone; is the average humidity of the outlet air distribution area; T mid is the temperature of the reaction zone in K; P mid is the pressure in the reaction zone, in Pa.

[0131] As the nitrogen content in the anode loop increases, the performance of the fuel cell stack decreases and the power generation efficiency decreases, but the hydrogen emissions decrease and the hydrogen utilization rate increases. Therefore, there is a nitrogen concentration value or nitrogen concentration range to achieve the maximum anode power generation efficiency. Based on the relationship between the single cell pressure drop ΔP and the nitrogen concentration value, an exhaust strategy based on the anode pressure drop can be formulated.

[0132] Through the above analysis, a fuel cell engine anode exhaust method proposed in this embodiment has the following process: Figure 2 As shown, the specific steps include:

[0133] Step 1: Calculate the power generation loss ΔE of the fuel cell system due to the increase in nitrogen content in the anode circuit volt loss , the specific process is:

[0134] Step 1.1. Calculate the open circuit voltage difference ΔE caused by thermodynamics thermo :

[0135]

[0136] Step 1.2, calculate the anode overpotential difference Δη caused by reaction kinetics:

[0137]

[0138] Step 1.3: Calculate the change in internal resistance ΔV ir :

[0139] ΔV ir =cd (rrmix ) (7)

[0140] Step 1.4: Based on ΔE thermo , Δη and ΔV ir , calculate ΔE volt loss :

[0141] ΔE volt loss =∫(ΔE thermo +Δη+ΔV ir )N·I·dt (8)

[0142] Step 2: Calculate the hydrogen emission loss caused by exhaust The specific process is:

[0143] Step 2.1: Gas flow rate Q through the exhaust valve N The calculation formula is:

[0144]

[0145] Step 2.2: Calculate the exhaust volume V of the exhaust valve when it is opened once purge :

[0146]

[0147] Step 2.3, based on V purge ,calculate

[0148]

[0149] Step 3: Calculate the total energy efficiency η of the fuel cell system E :

[0150]

[0151] Step 4: According to the anode flow field structure of the battery stack, the flow field structure parameters are calculated, including the inlet gas distribution area structure parameters SP in , reaction zone structural parameters SP mid And the outlet gas distribution area structure parameters SP out ;

[0152] Step 5: Calculate the voltage drop ΔP of a single cell:

[0153]

[0154] Step 6: According to formula (13), calculate different stack currents I and different initial average node voltages V 0 The total energy efficiency η changes with the anode nitrogen content θ under E , η E When the maximum value is reached, the corresponding θ is recorded as According to formula (14), calculate The corresponding single cell voltage drop ΔP is recorded as Draw a map;

[0155] Step 7: Obtain the operating current I of the fuel cell stack, and obtain the initial average node voltage V corresponding to the operating current I under steady-state conditions. 0 ;

[0156] Step 8: Based on the obtained operating current I and initial average node voltage V 0 , query the corresponding

[0157] Step 9: Based on the result from step 8 The exhaust valve exhaust trigger conditions, such as Figure 3 As shown in the figure, when the anode nitrogen of the fuel cell stack accumulates to the anode inlet pressure P in and anode outlet pressure P out The pressure drop ΔP between When the exhaust valve opens t dur time, and then shut down; when the anode nitrogen of the fuel cell stack accumulates again until the pressure drop ΔP reaches When the exhaust valve is opened and closed repeatedly, the nitrogen content of the anode of the fuel cell stack is maintained at Corresponding

[0158] For example, the anode tail gas is pressurized by a volumetric circulation pump and then returned to the inlet of the battery stack for reuse. For a volumetric pump, at a fixed speed and fixed pressure, the total mass of the tail gas circulated by the circulation pump is fixed, that is, the nitrogen that penetrates continues to increase, and the corresponding hydrogen flow in the anode tail gas will decrease. At the same time, since the mass passing through the circulation pump is constant, the mass flowing through the battery stack is not affected by the accumulation of nitrogen, and the internal resistance does not change. Therefore, in this example, only the performance degradation caused by thermodynamics and reaction kinetics when the nitrogen content changes needs to be considered.

[0159] In this example, The value is 40 mol / m 3 ; Anode transfer coefficient α a =α c =0.5; m Pt The value is 0.05g / cm 2 ; Anode catalyst layer platinum thickness δ acl The value of is 2μm; A c The value is 50m 2 / g 1 .

[0160] The fuel cell stack used in the fuel cell engine contains 340 single cells, and the total volume of the anode flow channel, manifold and pipeline is V anode The flow coefficient Kv of the exhaust valve is 0.337m 3 / hr, exhaust duration t dur The total stoichiometric ratio of hydrogen and nitrogen, hydrogen outlet pressure, hydrogen inlet humidity, air stoichiometric ratio, average air side pressure, initial voltage when nitrogen content is 0, and coolant inlet and outlet temperatures under different currents are shown in Table 1.

[0161] Table 1

[0162]

[0163] Each current runs for 1800s. Structural parameters of the inlet gas distribution area SP in and outlet gas distribution area structural parameters SP out Equal to 10.07×10 12 , reaction zone structural parameters SP mid =7.63×10 12 .

[0164] The total power generation efficiency η of 198A, 297A, 396A, 495A, 594A and 693A under different anode nitrogen contents is calculated according to formula (13): E The total power generation efficiency η in the 1800s simulation time is E The change of nitrogen content entering the battery stack is plotted, such as Figure 4 As shown, Figure 4 (a) is 198A, Figure 4 (b) is 297A, Figure 4 (c) is 396A, Figure 4 (d) is 495A, Figure 4 (e) is 594A, Figure 4 (f) is 693 A. It can be seen from the figure that as the current increases, the nitrogen content corresponding to the peak power generation efficiency decreases from 17% at 198 A to 12.5% ​​at 297 A, 11% at 396 A, 9% at 495 A, 8% at 594 A, and further to 6% at 693 A. And it is relatively flat near the peak efficiency, indicating that the anode exhaust method of the fuel cell engine proposed in this embodiment has good robustness.

[0165] Taking 198A as an example, the test results show Figure 5 The parameter dynamic curve diagram of the fixed nitrogen content exhaust strategy at 198A is shown, where Figure 5 (a) is the nitrogen content; Figure 5 (b) is the loss of average node voltage; Figure 5(c) is the voltage drop. As the nitrogen content gradually increases from 0 to 17%, the average cell voltage of the battery stack gradually decreases by about 5.6V. The corresponding anode voltage drop The exhaust trigger condition is that the exhaust valve opens and discharges part of the nitrogen. The accumulated amount of nitrogen that permeates is equal to the amount of nitrogen discharged by the exhaust valve last time. When the anode voltage drop reaches the threshold again The exhaust valve is opened again to allow the nitrogen content to reach a balanced value. Eventually, the nitrogen content stabilizes at around the controlled 17%, and the average cell voltage drop of the battery stack also reaches a balance.

[0166] The above embodiments are only used to illustrate the technical method of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical method of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical method of the present invention.

Claims

1. A method for exhausting the anode of a fuel cell engine, characterized in that: The following steps are involved: Step 1: Calculate the power generation loss ΔE of the fuel cell system due to the increase in nitrogen content in the anode circuit voltloss : DE volt loss =∫(ΔE thermo +Δη+ΔV ir )N·I·dt In the formula, ΔE thermo is the open circuit voltage difference caused by thermodynamics, which is related to the anode nitrogen content θ; Δη is the anode overpotential difference caused by reaction kinetics, which is related to the anode nitrogen content θ; ΔV ir is the change in internal resistance; N is the number of battery stack nodes; I is the battery stack current, in A; t is time, in seconds; Step 2: Based on the exhaust volume V of the exhaust valve when it is opened once purge , calculate the hydrogen emission loss caused by exhaust Where V purge The unit is L; The unit is kJ; is the molar fraction of hydrogen at the outlet of the battery stack; V0 is the initial average node voltage when the nitrogen content is 0, in V; ΔH LHV is the lower heating value of hydrogen, in kJ / mol; Step 3: Calculate the total energy efficiency η of the fuel cell system E : Where, F is the Faraday constant, which is 96485C / mol; Step 4: According to the anode flow field structure of the battery stack, the flow field structure parameters are calculated, including the inlet gas distribution area structure parameters SP in , reaction zone structural parameters SP mid And the outlet gas distribution area structure parameters SP out ; Step 5: Calculate the voltage drop ΔP of a single cell: Where R is the gas constant, the unit is J / (mol·K); P in is the anode stack pressure, in Pa; P out is the anode outlet pressure, in Pa; st is the hydrogen stoichiometric ratio of the battery stack; T in T is the temperature of the coolant entering the pile, in K; out is the coolant outlet temperature, in K; ΔP in is the pressure drop of the inlet gas distribution area, in Pa; ΔP mid is the pressure drop in the reaction zone, in Pa; ΔP out is the pressure drop of the outlet gas distribution area, in Pa; P sat,in is the saturated steam pressure of the inlet gas distribution area, in Pa; P sat,mid is the saturated vapor pressure in the reaction zone, in Pa; P sat,out is the saturated steam pressure of the outlet gas distribution area, in Pa; is the average humidity of the inlet air distribution area; is the average humidity in the reaction zone; is the average humidity of the outlet air distribution area; T mid is the temperature of the reaction zone, in K; P mid is the pressure in the reaction zone, in Pa; Step 6: According to step 3, calculate the total energy efficiency η with the change of anode nitrogen content θ under different stack currents I and different initial average node voltages V0. E , η E When the maximum value is reached, the corresponding θ is recorded as And according to step 5, calculate The corresponding single cell voltage drop ΔP is recorded as Draw a map; Step 7, obtaining the operating current I of the fuel cell stack and the initial average node voltage V0 under the corresponding steady-state condition; Step 8: According to the read operating current I and initial average node voltage V0, query the corresponding Step 9: Based on the result from step 8 is the exhaust valve exhaust trigger condition. When the anode nitrogen of the fuel cell stack accumulates to the anode stack pressure P in and anode outlet pressure P out The pressure drop ΔP between When the exhaust valve opens t dur time, and then shut down; when the anode nitrogen of the fuel cell stack accumulates again until the pressure drop ΔP reaches When the exhaust valve is opened and closed repeatedly, the nitrogen content of the anode of the fuel cell stack is maintained at Corresponding 2. The fuel cell engine anode exhaust method according to claim 1, characterized in that: The ΔE thermo The calculation formula is: Where R is the gas constant, the unit is J / (mol·K); n is the number of electron transfers in the reaction; F is the Faraday constant, the value is 96485C / mol; is the hydrogen partial pressure in pure hydrogen state, in Pa; is the partial pressure of hydrogen in the nitrogen-containing mixed gas, in Pa; P mid is the pressure in the reaction zone, in Pa; P sat,mid is the saturated vapor pressure in the reaction zone, in Pa; is the average humidity in the reaction zone; θ is the nitrogen content at the anode.

3. The fuel cell engine anode exhaust method according to claim 2, characterized in that: The calculation formula of Δη is: Where cd is the actual working current density, in A / m 2 ; δ acl is the thickness of the anode catalyst layer, in m; a eff is the active specific surface area of ​​the anode catalyst layer, in m -1 ; is the reference anode exchange current density; α a and α c are the anode transfer coefficients; is the hydrogen concentration in the reaction zone, in mol / m 3 ; is the reference hydrogen concentration, in mol / m 3 ; is the molar concentration of hydrogen in the anode hydrogen-nitrogen mixed gas, in mol / m 3 ; T mid is the temperature of the reaction zone, in K.

4. The fuel cell engine anode exhaust method according to claim 3, characterized in that: The ΔV ir The calculation formula is: ΔV ir =cd(r-r mix ) Where r is the internal resistance when the anode is pure hydrogen; mix It is the internal resistance value when the anode is a mixture of hydrogen and nitrogen.

5. The fuel cell engine anode exhaust method according to claim 4, characterized in that: Step 2 Calculate V purge The specific process is: Gas flow rate Q through the exhaust valve N The calculation formula is: In the formula, Q N The unit is m 3 / hr; P1 is the pressure before the exhaust valve, in bara; P2 is the pressure at the outlet of the exhaust valve, in bara; ρ N is the density of the fluid under standard conditions, in kg / m 3 ; Kv is the exhaust valve flow coefficient, unit is m 3 / hr; ΔP v is the pressure difference between the two sides of the valve, equal to P1-P2, in bara; M mix,out is the molar amount of mixed gas at the outlet of the battery stack, in g / mol; Then calculate V purge : In the formula, r dur is the exhaust duration, in seconds.

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