Fuel cell system and operating method for a fuel cell system
By using pressure sensors and calculation units in fuel cell systems, the flushing strategy is optimized, and the problems of nitrogen enrichment and hydrogen depletion are solved, achieving the robust and efficient operation of the system.
Patent Information
- Application Number
- CN202380072370.3
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Priority Date
- 2022-10-12
- Filing Date
- 2023-10-11
- Publication Date
- 2025-05-16
AI Technical Summary
Existing fuel cell systems are prone to nitrogen enrichment and hydrogen depletion during operation, resulting in damage to the catalyst layer, and due to cost reasons, the traditional flushing strategy is inefficient.
By installing a pressure sensor and calculation unit in the anode subsystem of the fuel cell system, the gas composition is inferred using the pressure curve, and the flush valve is operated according to the mass fraction, optimizing the flushing strategy to improve system efficiency.
The robust operation and efficient fuel utilization of the fuel cell system are achieved, the risks of nitrogen enrichment and hydrogen depletion are reduced, and the overall efficiency of the system is improved.
Smart Images
Figure CN120019510A_ABST
Abstract
Description
Technical Field
[0001] The proposed invention relates to a fuel cell system for converting energy and to a method for operating a fuel cell system according to the appended claims. Background Art
[0002] Hydrogen-based PEM fuel cells are considered to be future mobility concepts because they only emit water as exhaust gas and enable faster refueling times. PEM fuel cells are usually constructed with a closed anode circuit, which enables recirculation of the outflowing gas.
[0003] In this way, despite the operation of the anode in an overstoichiometric manner (which is provided, for example, to avoid local reactant undersupply), an optimal utilization of the supplied hydrogen can be set.
[0004] Furthermore, water diffused from the cathode side to the anode can be recirculated in steam form to ensure adequate wetting of the membrane at the anode inlet.
[0005] Both in operation and in the shut-down state, nitrogen diffuses through the membrane from the cathode side to the anode side. As a result, a nitrogen enrichment occurs in the anode circuit, so that the required recirculation power increases. At the same time, the hydrogen partial pressure decreases and thus the local maximum diffusion flow of hydrogen through the gas diffusion layer (GDL). As a result, local hydrogen depletion occurs, which leads to irreversible damage to the catalyst layer. The nitrogen transfer is difficult to estimate because it depends strongly on the operating point and the aging state of the membrane.
[0006] Over-enrichment is avoided by "flushing", ie by draining off the anode gas via a corresponding flushing valve, the so-called "purge valve". In this way, nitrogen-containing gases are removed from the anode circuit and fresh hydrogen is metered in, so that the nitrogen mole fraction is reduced.
[0007] Liquid water produced during operation of the fuel cell system can also be removed via a discharge valve (the so-called “bleed valve”) or removed together with the gas mixture via a common flush / bleed valve.
[0008] Frequently, for cost reasons, flushing and venting valves which are cyclically opened and closed are implemented as switching valves.
[0009] In order to optimize the flushing strategy, i.e. to open and close the flushing valve in a manner dependent on the operating point, the current gas composition in the anode circuit needs to be measured or calculated. Only in this way can the required recirculation power be minimized, the minimum partial pressure of hydrogen be maintained and excessive hydrogen losses be avoided, i.e. the system efficiency be maximized.
[0010] In principle, the gas composition can be measured directly by means of corresponding sensor devices. However, the sensors required for this purpose are expensive and require a large installation space. Summary of the invention
[0011] Within the scope of the present invention, a fuel cell system and an operating method for operating the fuel cell system are proposed. Further features and details of the invention can be derived from the corresponding claims, the description and the drawings. Here, the features and details described in conjunction with the operating method according to the invention naturally also apply in conjunction with the fuel cell system according to the invention, and vice versa, so that in the disclosure of the individual inventive aspects, reference is always made to each other or can always be made to each other.
[0012] The proposed invention serves, inter alia, to provide a robust and fuel efficient fuel cell system.
[0013] Therefore, according to a first aspect of the proposed invention, a fuel cell system for converting energy is proposed.
[0014] The fuel cell system comprises a fuel cell stack, which comprises a cathode subsystem and an anode subsystem, a pressure sensor arranged in the anode subsystem, a flushing valve for flushing the anode subsystem, and a computing unit. The computing unit is configured to deduce the composition of the gas flowing through the anode subsystem by means of a measured value determined by the pressure sensor and to control the flushing valve according to the mass fraction.
[0015] In the context of the proposed invention, a computing unit is to be understood as a computer, a processor, a control device or any other programmable circuit.
[0016] The proposed invention is based on the following principle: The composition of the gas flowing in the anode subsystem of the fuel cell system is inferred from the pressure curve in the anode subsystem. Given the known composition of the gas flowing in the anode subsystem, the flushing valve of the fuel cell system can be optimized (i.e. shortened if necessary) and controlled (i.e. opened) so that a particularly fuel-efficient operation of the fuel cell system is achieved.
[0017] In short, at the moment of the flushing process, the anode subsystem can be regarded as a pressure volume from which the gas mixture is discharged through the flushing valve. In this case, the critical flow through the valve usually occurs initially at the narrowest cross section (area A). The resulting mass flow through the flushing valve It can be approximated by equation (1).
[0018]
[0019] In equation (1), ρ and p represent the density and total pressure of the gas mixture in the anode circuit, respectively. ψ represents the outflow function and is a constant in the case of critical flows.
[0020] Assuming an ideal gas mixture exists, the mass m of the gas mixture in the anode loop is calculated by equation (2):
[0021] m=pV / RT (2)
[0022] In equation (2), V denotes the volume of the anode circuit, T denotes the temperature in the anode circuit, and R denotes the specific gas constant of the gas mixture. The specific gas constant and therefore the density depend strongly on the composition of the gas. Now, using the conservation of mass according to equation (3)
[0023]
[0024] The pressure curve in the anode circuit during flushing or “purging” can be determined with the aid of differential equation (4):
[0025]
[0026] State 0 here characterizes the state variables at the time before the start of the flushing process. According to equations (5) and (6), the temperature and density development in the anode circuit can be regarded as an adiabatic expansion.
[0027]
[0028] The proportion of water vapor is neglected in the above considerations. At the usual operating point, complete saturation of the gas phase is assumed at the anode outlet. The partial pressure of water in the gas phase at the measured temperature is therefore known.
[0029] It may be provided that the calculation unit is configured to increase the actuation time of the flushing valve relative to a predetermined standard actuation value or shorten the closing time of the flushing valve relative to a predetermined standard closing value if the substance mass fraction in the gas is above a predetermined threshold value.
[0030] In order to reduce the high mass fraction of nitrogen in the anode subsystem, the purge valve may be actuated (ie opened) for a longer time and / or more frequently.
[0031] Furthermore, it can be provided that the computing unit is configured to reduce the actuation time for actuating the flushing valve or to increase the closing time for closing the flushing valve relative to a predetermined standard actuation value if the mole fraction of hydrogen in the gas is above a predetermined threshold value.
[0032] In order to minimize or prevent the emission of hydrogen by the flushing process, if the mole fraction of hydrogen is above a predetermined threshold value, the actuation time for actuating the flushing valve can be reduced or the closing time for closing the flushing valve can be increased relative to a predetermined standard actuation value.
[0033] Furthermore, it can be provided that the computing unit is configured to assign the measured value ascertained by the pressure sensor to a corresponding curve of a plurality of predetermined curves for different compositions of the gas.
[0034] By using a plurality of predetermined curves of the pressure in the anode subsystem over time, each of which is assigned to a specific gas composition, the corresponding curve can be assigned to the corresponding determined measured values by, for example, selecting a curve whose value or values are closest to a corresponding measured value or values of all curves.
[0035] Furthermore, it can be provided that the calculation unit is configured to calculate the amount of hydrogen flowing into the anode subsystem based on the position of the hydrogen metering valve, the pressure difference between a position upstream of the hydrogen metering valve and a position downstream of the hydrogen metering valve, and a cross section through which the inflowing hydrogen flows.
[0036] Usually, the position of the hydrogen metering valve or the HGI (Hydrogen Gas Injector) that regulates the flow of fresh hydrogen into the anode subsystem is used for pressure regulation in the anode subsystem. This can avoid potential mechanically damaging pressure differences on the membrane.
[0037] Furthermore, it can be provided that the computing unit is also configured to set the amount of hydrogen flowing into the anode subsystem as a function of the mole fraction of nitrogen in the gas flowing through the anode subsystem.
[0038] In the case of high nitrogen concentrations, a smaller amount of hydrogen must be supplied in order to maintain a predetermined harmless pressure level, or vice versa in the case of low nitrogen concentrations. The proposed fuel cell system determines the hydrogen mass flow through the hydrogen metering valve as a function of the pressure difference across the hydrogen metering valve and the cross section through which the flow occurs, which is a function of the valve position of the hydrogen metering valve. This presupposes that the valve geometry and the material properties of the pure hydrogen are known.
[0039] Furthermore, it can be provided that the fuel cell system comprises a discharge valve and a hydrogen metering valve for discharging water from the fuel cell system, wherein the computing unit is configured to determine the amount of liquid water present in the anode subsystem as a function of the time profile of the opening of the hydrogen metering valve during actuation of the discharge valve.
[0040] In fuel cell systems with a drain valve, in particular with a combined flushing / drain valve (so-called "purge / drain valve"), immediately after the valve is opened, a majority of liquid water or a two-phase gas mixture is discharged. Due to the initially higher density, the pressure in the anode subsystem changes nonlinearly, so that if pure water initially flows through the drain valve, a delay or even a reversal of direction in the pressure curve occurs. Only when liquid water has already flowed out and the gas phase is predominant in the anode subsystem or the drain valve can the pressure curve described above be determined and assigned to a predetermined curve.
[0041] Furthermore, it can be provided that the computing unit is configured to actuate the flushing valve and / or the drain valve as a function of the determined amount of liquid water.
[0042] In order to set a predetermined amount of liquid water in the fuel cell system, the flushing valve and / or the drain valve may be activated for a longer period of time or more frequently until the determined amount of liquid water corresponds to the predetermined amount of liquid water.
[0043] Furthermore, it may be provided that the computing unit is configured to evaluate a curve of a determined amount of liquid water over time and to output a fault report when the determined amount of liquid water decreases over time by more than a predetermined threshold value, the fault report comprising a message according to which a water transport characteristic of the fuel cell system is disturbed and / or an operating parameter of the fuel cell system is set according to the determined amount of liquid water.
[0044] The determination of the amount of liquid water can be used to set the duration of a flushing process or the frequency of a flushing process and / or to analyze the water transport through the membrane of the fuel cell system. In the event that the water transport characteristics change during the service life of the fuel cell system, for example, an excessive deterioration of the water transport can be observed and countermeasures can be taken. Thus, for example, operating parameters in the cathode subsystem, in particular pressure and chemical dosing, can be adapted to the new water transport characteristics, or in extreme cases a fault message can be output and the fuel cell stack replaced.
[0045] According to a second aspect, the proposed invention relates to an operating method for operating a possible embodiment of the proposed fuel cell system.
[0046] The proposed operating method includes determining the composition of a gas flowing through the anode subsystem of the fuel cell system by means of measured values ascertained by a pressure sensor in the anode subsystem and actuating a purge valve of the fuel cell system as a function of the determined composition. BRIEF DESCRIPTION OF THE DRAWINGS
[0047] Further advantages, features and details of the invention are apparent from the following description, in which exemplary embodiments of the invention are described in detail with reference to the accompanying drawings. The features mentioned in the claims and the description may be essential to the invention individually or in any combination.
[0048] It shows:
[0049] Figure 1 Schematic representation of a possible configuration of the proposed fuel cell system,
[0050] Figure 2 A diagram of possible pressure profiles for different gas compositions in an anode subsystem of a fuel cell system;
[0051] Figure 3 A schematic diagram showing a possible configuration of the proposed operating method,
[0052] Figure 4 Detailed illustration of possible aspects of the proposed operating method. DETAILED DESCRIPTION
[0053] Figure 1 1 shows a fuel cell system 100. The fuel cell system 100 comprises a fuel cell stack 101, which comprises a cathode subsystem 103 and an anode subsystem 105, a pressure sensor 107 arranged in the anode subsystem 105, a flushing valve 109 for flushing the anode subsystem 105, and a computing unit 111.
[0054] The computing unit 111 is configured to infer a composition, in particular a molar fraction of hydrogen and / or nitrogen, in the gas flowing through the anode subsystem 105 based on the measured values ascertained by the pressure sensor 107 and to actuate the purge valve 109 as a function of the molar fraction.
[0055] To this end, the calculation unit 111 may include, for example, a memory in which data based on Figure 2 A characteristic diagram or assignment scheme 200 of .
[0056] The assignment scheme 200 includes a plurality of curves 201, 203, 205, 207 and 209, each of which is assigned to a different gas composition. For example, curve 201 corresponds to a gas composed of pure hydrogen, curve 203 corresponds to a gas composed of 90% hydrogen and 10% nitrogen, curve 205 corresponds to a gas composed of 80% hydrogen and 20% nitrogen, curve 207 corresponds to a gas composed of 50% hydrogen and 50% nitrogen, and curve 209 corresponds to a gas composed of pure nitrogen.
[0057] Correspondingly, one of curves 201 , 203 , 205 , 207 or 209 and the gas composition determined therefrom may be associated with the respective measured value ascertained by pressure sensor 107 .
[0058] Figure 3 3 shows an operating method 300 for operating a fuel cell system. The operating method 300 comprises an opening step 301, in which a flushing valve of the fuel cell system is opened; a measuring step 303, in which the pressure in the anode subsystem of the fuel cell system is measured; a closing step 305, in which the flushing valve is closed; and an ascertaining step 307, in which the composition of the gas in the anode subsystem is ascertained with the help of the value measured in the measuring step 303.
[0059] In the comparison step 309, the amount of at least one component of the composition determined in the determination step 307 is compared with a predetermined threshold value. If, for example, the amount of hydrogen is too high, the flushing interval (i.e., the time for operating the flushing valve of the fuel cell system) is shortened in the first setting step 311. If, for example, the amount of nitrogen is too high, the flushing interval (i.e., the time for operating the flushing valve of the fuel cell system) is increased in the second setting step 313.
[0060] In the last storage step 315 , the flushing interval determined in the first setting step 311 or the second setting step 313 is stored and used as a standard value for subsequent flushing processes.
[0061] Figure 4 4 shows a diagram 400 which is stretched over time on its abscissa and over pressure and manipulated variables and pilot control values on its ordinate.
[0062] Curve 401 shows the progression over time of a pilot control value of a hydrogen metering valve of a fuel cell system.
[0063] Curve 403 shows the change in pressure in the anode subsystem of the fuel cell system over time.
[0064] Curve 405 shows the correction performed by adjusting the position of the hydrogen metering valve over time.
[0065] Based on diagram 400 , it can be seen that despite the sudden increase in pilot control, the hydrogen metering valve opens slightly later and with a flat slope during the opening of the purge valve.
[0066] Depending on the gas composition or in the presence of liquid water, the regulator can also initially be deflected in the opposite direction. In this case, the pressure does not change for a short time despite the open flushing valve. In this case, there is a large amount of liquid water in the anode subsystem, which can be discharged, for example, by temporarily increasing the actuation frequency of the flushing valve.
Claims
1. A fuel cell system (100) for converting energy, in, The fuel cell system (100) comprises: - a fuel cell stack (101), said fuel cell stack comprising a cathode subsystem (103) and an anode subsystem (105), - a pressure sensor (107) arranged in the anode subsystem (105), - a flushing valve (109) for flushing the anode subsystem (105), - a computing unit (111), Therein, the computing unit (111) is configured to ascertain a composition of a gas flowing through the anode subsystem (105) using a measured value ascertained by the pressure sensor (107) and to actuate the flushing valve (109) as a function of the ascertained composition.
2. The fuel cell system (100) according to claim 1, It is characterized in that The calculation unit (111) is configured to, if the mass fraction of nitrogen in the gas is higher than a predetermined threshold, increase the actuation time of the flushing valve (109) relative to a predetermined standard actuation value, or shorten the closing time of the flushing valve (109) relative to a predetermined standard closing value.
3. The fuel cell system (100) according to claim 1 or 2, It is characterized in that The calculation unit (111) is configured to reduce the actuation time of the flushing valve (109) or increase the closing time of the flushing valve (109) relative to a predetermined standard actuation value if the mass fraction of hydrogen in the gas is higher than a predetermined threshold.
4. The fuel cell system (100) according to any one of the preceding claims, It is characterized in that The computing unit (111) is configured to assign the measured value ascertained by the pressure sensor to a corresponding curve from among a plurality of predetermined curves for different compositions of the gas.
5. The fuel cell system (100) according to any one of the preceding claims, It is characterized in that The calculation unit (111) is configured to calculate the amount of hydrogen flowing into the anode subsystem (105) based on the position of the hydrogen metering valve, the pressure difference between a position upstream of the hydrogen metering valve and a position downstream of the hydrogen metering valve, and a cross-section through which the inflowing hydrogen flows.
6. The fuel cell system (100) according to claim 5, It is characterized in that The calculation unit (111) is further configured to set the amount of hydrogen flowing into the anode subsystem (105) according to the mole fraction of nitrogen in the gas flowing through the anode subsystem (105).
7. The fuel cell system (100) according to any one of the preceding claims, It is characterized in that The fuel cell system (100) comprises a discharge valve and a hydrogen dosing valve for discharging water from the fuel cell system (100), Therein, the calculation unit (111) is configured to determine the amount of liquid water located in the anode subsystem (105) based on a time curve of the opening of the hydrogen metering valve during the actuation of the discharge valve.
8. The fuel cell system (100) according to claim 7, It is characterized in that The computing unit (111) is configured to actuate the flushing valve (109) as a function of the determined amount of liquid water.
9. The fuel cell system (100) according to claim 7 or 8, It is characterized in that The calculation unit (111) is configured to evaluate a curve of the determined amount of liquid water over time and, if the determined amount of liquid water decreases over time by more than a predetermined threshold, output a fault report, the fault report comprising a message according to which the water transport characteristics of the fuel cell system (100) are disturbed and / or an operating parameter of the fuel cell system (100) is set according to the determined amount of liquid water.
10. A method (300) for operating a fuel cell system (100) according to any one of claims 1 to 9, in, The operating method (300) comprises: - determining (301) the composition of the gas flowing through the anode subsystem (105) of the fuel cell system (100) by means of measured values ascertained by a pressure sensor (107) in the anode subsystem (105), and - actuating (303) a flushing valve (109) of the fuel cell system (100) as a function of the determined gas composition.