A method for estimating nitrogen content in anode of a fuel cell engine

Abstract

The present invention discloses a method for estimating nitrogen content in the anode of a fuel cell engine, which belongs to the technical field of fuel cells. Specifically, according to the anode flow field structure of the battery stack, the structural parameters of the inlet gas distribution area, the reaction area and the outlet gas distribution area are calculated; the hydrogen stoichiometric ratio, hydrogen humidity, coolant inlet temperature, coolant outlet temperature, anode inlet pressure and anode outlet pressure of the battery stack are obtained; by calculating the mixed gas viscosity of the inlet gas distribution area, the reaction area and the outlet gas distribution area, the nitrogen content in the anode of the fuel cell engine is iteratively estimated to meet the preset nitrogen content estimation accuracy. The present invention can realize accurate estimation of the anode nitrogen content to improve the efficiency and durability of the fuel cell engine, regardless of the age of the battery stack, and can avoid the problem that the accuracy of the traditional nitrogen estimation algorithm based on voltage performance deteriorates as the battery stack ages.

Description

Technical Field

[0001] The invention belongs to the technical field of fuel cells, and in particular relates to a method for estimating nitrogen content in an anode of 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, the nitrogen partial pressure of the air on the cathode side is much higher than that on the anode side. Therefore, 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 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) Excessive accumulation of nitrogen content will increase the risk of hydrogen starvation and damage the membrane electrode. Therefore, it is necessary to estimate the anode nitrogen concentration to improve the efficiency and durability of the fuel cell system.

[0005] A Chinese patent with publication number CN 115472876 A discloses a method for estimating the nitrogen concentration at the anode of a fuel cell stack based on voltage variation characteristics, and proposes that the anode nitrogen concentration can be estimated by collecting the voltage of the cell stack and using the voltage variation characteristics. This method draws a Map diagram by measuring the average voltage of the cell stack under different current densities and different nitrogen concentrations while only changing the anode nitrogen concentration. In practical applications, a Map diagram is drawn by obtaining the current density and average single-chip voltage of the fuel cell, and then the nitrogen concentration at the anode inlet of the fuel cell is estimated based on the Map diagram. This method has a good effect on new cell stacks, but as the cell stack ages, the cell stack performance and cell voltage consistency deteriorate, and the applicability of this method will gradually deteriorate. Summary of the invention

[0006] In view of the problem that the existing fuel cell stack anode nitrogen concentration estimation method has poor applicability to aging fuel cell stacks, the present invention provides a fuel cell engine anode nitrogen content estimation method, which estimates the nitrogen concentration by measuring observable parameters such as temperature and pressure, thereby avoiding the problem that the traditional voltage-based nitrogen estimation algorithm loses accuracy as the fuel cell stack ages.

[0007] The technical solution adopted by the present invention is as follows:

[0008] A method for estimating nitrogen content in an anode of a fuel cell engine comprises the following steps:

[0009] Step 1: 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 ;

[0010] Step 2: Obtain the hydrogen stoichiometric ratio st of the battery stack and the saturated vapor pressure P of the inlet gas distribution area sat,in , the saturated vapor pressure P in the reaction zone sat,mid , saturated steam pressure P of outlet gas distribution area sat,out , Average humidity of the inlet air distribution area Average humidity in the reaction zone Average humidity of outlet air distribution area Coolant inlet temperature T in , Coolant outlet temperature T out , reaction zone temperature T mid , anode stack pressure P in , anode stack pressure P out And the reaction zone pressure P mid ;

[0011] Step 3: Iteratively estimate the nitrogen content of the anode of the fuel cell engine. The specific process is as follows:

[0012] Step 3.1, let the number of iterations i = 1, the nitrogen content of the first iteration is

[0013] Step 3.2: In the i-th iteration, calculate the viscosity of the mixed gas in the inlet gas distribution area Viscosity of mixed gas in the reaction zone And the viscosity of the mixed gas in the outlet gas distribution area

[0014] Step 3.3: Calculate the estimated value of the anode nitrogen content in the i-th iteration The formula is:

[0015]

[0016] Where, F is the Faraday constant, which is 96485C / mol; I is the battery stack current; R is the gas constant, which is in J / (mol·K);

[0017] Step 3.4, judgment Is it less than the preset nitrogen content estimation accuracy ∈? If so, the estimation ends and the final anode nitrogen content estimation value is obtained. Otherwise, let β is the coefficient to prevent the algorithm from diverging, and let i=i+1, and return to step 3.2.

[0018] Furthermore, in step 3.2, the parameters are defined l=1,2,3, where l represents the region number. express The second area express The third area express According to Wilke's law, The formula is:

[0019]

[0020] Where N is the total number of components contained in the mixed gas introduced into the anode flow field; is the viscosity of the nth component gas in the lth region; is the molar content of the nth component gas in the lth region; is the molar content of the mth component gas in the lth region; is an intermediate coefficient with no special meaning, and its calculation formula is:

[0021]

[0022] In the formula, is the molar mass of the nth component gas in the lth region; is the molar mass of the mth component gas in the lth region; is the viscosity of the mth component gas in the lth region.

[0023] Furthermore, step 3.2 calculates according to Sutherland's law The formula is:

[0024]

[0025] In the formula, μ n,0 is the viscosity of the nth component gas at 273.15K; T 0 Indicates 273.15K; T l Indicates the temperature of the lth region in K. When l=1, 2, 3, it corresponds to T in , T mid and T out ; C n is the Sutherland's law constant for the nth component gas.

[0026] Furthermore, the flow field structure parameters in step 1 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.

[0027] Furthermore, the value of ∈ is 0.0001 to 0.01.

[0028] Furthermore, the value of β is 0.6 to 0.99.

[0029] The beneficial effects of the present invention are:

[0030] The present invention proposes a method for estimating the nitrogen content in the anode of a fuel cell engine. By measuring observable parameters such as temperature and pressure, the viscosity of the mixed gas in the inlet gas distribution area, reaction area and outlet gas distribution area in the anode flow field structure is calculated, thereby achieving accurate estimation of the anode nitrogen content to improve the efficiency and durability of the fuel cell engine. The present invention is independent of the age of the battery stack and can avoid the problem of the traditional nitrogen estimation algorithm based on voltage performance becoming less accurate as the battery stack ages. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 A schematic diagram of the structure of a fuel cell system;

[0032] Figure 2 A schematic diagram of the anode structure of a fuel cell stack in Example 1 of the present invention;

[0033] Figure 3 This is a schematic diagram of the anode flow field structure of a single cell of a fuel cell stack in Example 1 of the present invention;

[0034] Figure 4 This is a flow chart of the method for estimating nitrogen content in the anode of a fuel cell engine proposed in Example 1 of the present invention;

[0035] Figure 5 1 is a comparison chart of the estimated value of online nitrogen content and the given value at different currents in Example 1 of the present invention; among them, (a) is 198A; (b) is 297A; (c) is 396A; (d) is 495A; (e) is 594A; and (f) is 693A. DETAILED DESCRIPTION

[0036] 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.

[0037] Example 1

[0038] This embodiment provides a method for estimating nitrogen content in the anode of a fuel cell engine, based on the following reasonable assumptions:

[0039] (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.

[0040] (2) The operating pressure on the anode side usually does not exceed 3 bar, and the mixed gas conforms to the ideal gas law;

[0041] (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.

[0042] (4) Assume that the humidity is stabilized at 60% after the hydrogen enters the reaction zone;

[0043] (5) It is assumed that the flow of hydrogen in the anode flow field channel is laminar and reacts uniformly along the flow direction;

[0044] (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.

[0045] In this embodiment, the fuel cell stack is formed by stacking multiple single cells, and the anode structure of the fuel cell stack is as follows: Figure 2 As shown, the anode flow field of a single cell consists of three regions, such as Figure 3 As shown, the gas is evenly distributed 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 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 are evenly directed to the outlet gas distribution area of ​​the outlet manifold.

[0046] 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 P in , 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 .

[0047] The calculation formula for the flow channel length L in the three regions of the anode flow field is:

[0048]

[0049] 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.

[0050] 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:

[0051]

[0052] Where, d C is the flow channel depth of the corresponding area.

[0053] 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:

[0054]

[0055] 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.

[0056] 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:

[0057]

[0058] 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; f k 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.

[0059] 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.

[0060]

[0061] 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:

[0062]

[0063] 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.

[0064] 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 .

[0065] 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:

[0066]

[0067] 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:

[0068]

[0069] Where ΔP in is the pressure drop of the inlet gas distribution area; ΔP mid is the pressure drop in the reaction zone; ΔP out is the pressure drop of the outlet gas distribution area; P sat,in is the saturated steam pressure of the inlet gas distribution area; P sat,mid is the saturated vapor pressure in the reaction zone; P sat,out is the saturated steam pressure of the outlet gas distribution area; 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; P mid is the pressure in the reaction zone; F is the Faraday constant, which is 96485C / mol; I is the stack current; R is the gas constant of all gases, which is in J / (mol·K); and st is the hydrogen stoichiometric ratio of the stack.

[0070] Then, the expression of the anode nitrogen content θ can be obtained, which is specifically related to the flow field structure parameter SP, observable parameters (i.e., hydrogen stoichiometric ratio st, temperature T, pressure P, humidity and operating parameters such as current I), namely:

[0071]

[0072] Through the above analysis, a method for estimating nitrogen content in the anode of a fuel cell engine proposed in this embodiment has the following process: Figure 4 As shown, the specific steps include:

[0073] Step 1: 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 ;

[0074] Step 2: Obtain the hydrogen stoichiometric ratio st of the battery stack and the saturated vapor pressure P of the inlet gas distribution area sat,in , the saturated vapor pressure P in the reaction zone sat,mid , saturated steam pressure P of outlet gas distribution area sat,out , Average humidity of the inlet air distribution area Average humidity in the reaction zone Average humidity of outlet air distribution area Coolant inlet temperature T in , Coolant outlet temperature T out , reaction zone temperature T mid , anode stack pressure P in , anode stack pressure P out And the reaction zone pressure P mid ;

[0075] Step 3: Iteratively estimate the nitrogen content of the anode of the fuel cell engine. The specific process is as follows:

[0076] Step 3.1, let the number of iterations i = 1, the nitrogen content of the first iteration is

[0077] Step 3.2: In the i-th iteration, calculate the viscosity of the mixed gas in the inlet gas distribution area Viscosity of mixed gas in the reaction zone And the viscosity of the mixed gas in the outlet gas distribution area

[0078] Defining parameters l=1,2,3, where l represents the region number. express The second area express The third area express According to Wilke's law, The formula is:

[0079]

[0080] Where N is the total number of components contained in the mixed gas introduced into the anode flow field; is the viscosity of the nth component gas in the lth region; is the molar content of the nth component gas in the lth region; is the molar content of the mth component gas in the lth region; is an intermediate coefficient with no special meaning, and its calculation formula is:

[0081]

[0082] In the formula, is the molar mass of the nth component gas in the lth region; is the molar mass of the mth component gas in the lth region; is the viscosity of the mth component gas in the lth region;

[0083] Calculation based on Sutherland's law The formula is:

[0084]

[0085] In the formula, μ n,0 is the viscosity of the nth component gas at 273.15K; T 0 Indicates 273.15K; T l Indicates the temperature of the lth region in K. When l=1, 2, 3, it corresponds to T in , T mid and T out ; C n is the Sutherland's law constant of the nth component gas;

[0086] When the components of the mixed gas introduced into the anode flow field are hydrogen, nitrogen and water vapor, the corresponding μ n,0 are 8.45E-06, 1.656E-05 and 1.006E-05Pa s, respectively, corresponding to C n They are 66.8, 104.7 and 523.3 respectively;

[0087] Step 3.3: Calculate the estimated value of the anode nitrogen content in the i-th iteration The formula is:

[0088]

[0089] Step 3.4, judgment Is it less than the preset nitrogen content estimation accuracy ∈? If so, the estimation ends and the final anode nitrogen content estimation value is obtained. Otherwise, let β is the coefficient to prevent the algorithm from diverging, and let i=i+1, and return to step 3.2.

[0090] In this embodiment, the value of ∈ is 0.0001; the value of β is 0.95.

[0091] Taking a 12-cell short stack as an example, the inlet gas distribution area structural parameter SP in and outlet gas distribution area structural parameters SP out Equal to 10.07×10 12 , reaction zone structural parameters SPmid =7.63×10 12 .

[0092] Uniformly distributed current points are selected within the design operating current range of the battery stack, which are 198A, 297A, 396A, 495A, 594A and 693A respectively. At each current point, several groups of nitrogen flow rates are tested, and each nitrogen flow rate is tested for a period of time, starting from 0 standard liters per minute (slpm) and gradually increasing. After the highest nitrogen flow rate test is completed, the nitrogen flow rate is reduced to 0slpm and continues to run for a period of time, and the test of this current point is completed. Switch to the next current point and repeat the above process until all 6 current points are tested. During the entire experiment, only the nitrogen flow rate entering the anode changes, and other test conditions, such as the gas flow rate of the anode and cathode, the humidity entering the stack, the pressure entering the stack (the cathode entering the stack pressure is controlled to be equal to the anode exiting the stack pressure in the experiment), and the cooling water inlet temperature, circulating water flow and other test conditions are kept constant. The detailed experimental conditions are shown in Table 1.

[0093] Table 1

[0094]

[0095] The nitrogen content in the mixed gas entering the reactor is estimated by this algorithm, and the estimated value is compared with the given value, such as Figure 5 As shown, Figure 5 (a) is 198A, Figure 5 (b) is 297A, Figure 5 (c) is 396A, Figure 5 (d) is 495A, Figure 5 (e) is 594A, Figure 5 (f) is 693A, which shows that the method for estimating the anode nitrogen content of the fuel cell engine proposed in this embodiment has good accuracy and dynamic response capability.

[0096] In the example, the sensor is installed on the hydrogen manifold and the pressure drop of hydrogen flowing through the manifold is ignored. If the hydrogen inlet and outlet stack pressure sensor is installed on the pipeline, it can also be calibrated through the test platform to obtain the total pressure drop from the sensor to the manifold and in the manifold. After correction, the method of this embodiment is also applicable.

[0097] 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 estimating nitrogen content in anode of a fuel cell engine, characterized in that: The following steps are involved: Step 1: 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 2: Obtain the hydrogen stoichiometric ratio st of the battery stack and the saturated vapor pressure P of the inlet gas distribution area sat,in , the saturated vapor pressure P in the reaction zone sat,mid , saturated steam pressure P of outlet gas distribution area sat,out , Average humidity of the inlet air distribution area Average humidity in the reaction zone Average humidity of outlet air distribution area Coolant inlet temperature T in , Coolant outlet temperature T out , reaction zone temperature T mid , anode stack pressure P in , anode stack pressure P out And the reaction zone pressure P mid ; Step 3: Iteratively estimate the nitrogen content of the anode of the fuel cell engine. The specific process is as follows: Step 3.1, let the number of iterations i = 1, the nitrogen content of the first iteration is Step 3.2: In the i-th iteration, calculate the viscosity of the mixed gas in the inlet gas distribution area Viscosity of mixed gas in the reaction zone And the viscosity of the mixed gas in the outlet gas distribution area Step 3.3: Calculate the estimated value of the anode nitrogen content in the i-th iteration The formula is: Where, F is the Faraday constant; I is the battery stack current; R is the gas constant; Step 3.4, judgment Is it less than the preset nitrogen content estimation accuracy ∈? If so, the estimation ends and the final anode nitrogen content estimation value is obtained. Otherwise, let β is the coefficient to prevent the algorithm from diverging, and let i=i+1, and return to step 3.

2.

2. The method for estimating nitrogen content at the anode of a fuel cell engine according to claim 1, characterized in that: Define the parameters in step 3.2 l=1,2,3, where l represents the area number. express express express According to Wilke's law, The formula is: Where N is the total number of components contained in the mixed gas introduced into the anode flow field; is the viscosity of the nth component gas in the lth region; is the molar content of the nth component gas in the lth region; is the molar content of the mth component gas in the lth region; is an intermediate coefficient with no special meaning, and its calculation formula is: In the formula, is the molar mass of the nth component gas in the lth region; is the molar mass of the mth component gas in the lth region; is the viscosity of the mth component gas in the lth region.

3. The method for estimating nitrogen content at the anode of a fuel cell engine according to claim 2, characterized in that: Step 3.2 Calculate according to Sutherland's law The formula is: In the formula, μ n,0 is the viscosity of the nth component gas at 273.15K; T0 represents 273.15K; T l Indicates the temperature of the lth region in K. When l=1, 2, 3, it corresponds to T in , T mid and T out ; C n is the Sutherland's law constant for the nth component gas.

4. The method for estimating nitrogen content in the anode of a fuel cell engine according to any one of claims 1 to 3, characterized in that: The value of ∈ is 0.0001~0.

01.

5. The method for estimating nitrogen content in the anode of a fuel cell engine according to any one of claims 1 to 3, characterized in that: The value of β is 0.6~0.99.

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