Method for operating a fuel cell system, and control device

Abstract

The invention relates to a method for operating a fuel cell system (1) comprising an anode subsystem (3) for supplying at least one stack (2) with hydrogen in order to generate a stack stream, wherein the hydrogen is supplied in an anode gas which is enriched with nitrogen during operation, and the nitrogen concentration in the anode gas is controlled in that anode gas is intermittently let out of the anode subsystem (3) via a controllable purge valve (14) as part of a purging process and is exchanged with a supply of fresh hydrogen. The invention is characterized by the following steps: (a) ascertaining the hydrogen concentration in the anode gas, (b) inferring the actual anode stoichiometry from the hydrogen concentration and the stack stream, (c) comparing the actual anode stoichiometry with a target anode stoichiometry, and (d) in the event of a deviation between the actual anode stoichiometry and the target anode stoichiometry, indirectly adapting the hydrogen concentration by varying the duration and / or frequency of purge processes. The invention also relates to a control device for carrying out the method steps.

Description

[0001] Description

[0002] title

[0003] Method for operating a

[0004] The present invention relates to a method for operating a fuel cell system according to the preamble of claim 1. The invention further relates to a control device for carrying out steps of a method according to the invention.

[0005] State of the art

[0006] A fuel cell electrochemically converts hydrogen and oxygen into electrical energy, heat, and water. The core of a fuel cell is a membrane-electrode assembly. This comprises a membrane that separates an anode and a cathode side. The entire fuel cell system is therefore divided into an anode subsystem and a cathode subsystem. During operation, oxygen in the form of air is supplied to the fuel cell via the cathode subsystem and hydrogen via the anode subsystem. During operation, the hydrogen and oxygen react to form water, releasing energy. To scale the energy generated, individual fuel cells are usually combined into so-called stacks.

[0007] Since anode gas escaping from a stack usually still contains unused hydrogen, it is recirculated within the anode subsystem and fed back to the anode side of the stack. Over time, however, the recirculated anode gas becomes enriched with nitrogen and water, which can be in liquid or gaseous form. Liquid water is typically removed using a water separator. The water separator typically comprises a container in which the separated water is collected in liquid form. The container can be emptied by opening a valve, the so-called drain valve. The water flowing out via the drain valve is combined with the exhaust air from the cathode subsystem and removed from the fuel cell system.

[0008] The nitrogen contained in the anode gas is in gaseous form and cannot be effectively separated from the hydrogen. To remove nitrogen from the anode subsystem, anode gas is vented via a purge valve within or downstream of the water separator. The separated anode gas, which contains both nitrogen and hydrogen, is combined with the exhaust air from the cathode subsystem and any separated water and removed from the fuel cell system. The purge process is variable in terms of its duration and frequency and can be directly controlled. The disadvantage of the purge process is that hydrogen is lost, which could otherwise still be used as fuel.

[0009] The anode gas separated during the purge process is replaced within the anode subsystem by a supply of fresh hydrogen. However, this fresh hydrogen, stored in tanks, is generally contaminated with nitrogen. The hydrogen quality for fuel cell vehicles is specified in SAE J2719 as well as ISO 14687:2019. A maximum nitrogen concentration of 300 ppm is set for the fresh hydrogen. Therefore, the supply of fresh hydrogen to the anode subsystem always involves a supply of nitrogen.

[0010] Nitrogen is an inert gas for a fuel cell, reducing cell voltage and resulting in a loss of efficiency. Furthermore, an increased nitrogen concentration is accompanied by a reduced hydrogen concentration, which can lead to local undersupply within the fuel cell or stack, causing damage, particularly to the membrane, and subsequently to a reduced service life. This must always be taken into account when operating a fuel cell system. Therefore, during operation, more hydrogen is typically supplied to the anode side than would be required for the reaction with an applied current draw. The ratio of the amount of hydrogen supplied to the anode side to the amount of hydrogen required for the reaction is referred to as the anode stoichiometry, or simply as stoichiometry when considering the anode side in isolation.When determining the anode stoichiometry, it is particularly important to note that both too high and too low anode stoichiometry can cause damage or accelerated aging of the stack.

[0011] The present invention is therefore concerned with the fine adjustment of the hydrogen concentration in the anode gas and, consequently, with an adjustment of the anode stoichiometry.

[0012] To achieve this objective, the method is proposed with the features of claim 1. Advantageous further developments of the invention are set forth in the subclaims. Furthermore, a control device for executing steps of the method is specified.

[0013] Disclosure of the invention

[0014] A method is proposed for operating a fuel cell system comprising an anode subsystem for supplying at least one stack with hydrogen to generate a stack current, wherein the hydrogen is supplied in an anode gas that becomes enriched with nitrogen during operation. The nitrogen concentration in the anode gas is controlled by temporarily purging anode gas from the anode subsystem via a controllable purge valve during a purge process and replacing it with fresh hydrogen. According to the invention, the following steps are carried out:

[0015] (a) Determination of a hydrogen concentration in the anode gas,

[0016] (b) Inference from the hydrogen concentration and the stack current to an actual anode stoichiometry,

[0017] (c) Comparing the actual anode stoichiometry with a target anode stoichiometry, (d) Indirect adjustment of the hydrogen concentration by varying the duration and / or frequency of the purge process if the actual anode stoichiometry deviates from the target anode stoichiometry.

[0018] With the proposed method, the hydrogen concentration is regulated by specific control of the purge processes, thus enabling compliance with a specified target anode stoichiometry. The determination of the hydrogen concentration in the anode gas described in step (a) can be carried out mathematically or by sensory means. Step (b) uses a relationship that exists between hydrogen concentration, stack current, and actual anode stoichiometry. This relationship must be determined empirically. The anode stoichiometry A is defined as where mH2,tats is the amount of hydrogen actually supplied to the anode side and m H2, st represents the amount of hydrogen required for complete combustion at a given stack current draw. The former is typically adjusted by setting an anode gas mass flow. Due to the permanent enrichment of the anode gas with nitrogen and the associated successive reduction in the hydrogen concentration, the amount of hydrogen actually supplied to the anode side varies constantly. Consequently, the actual anode stoichiometry also continuously diverges from the target anode stoichiometry as the nitrogen enrichment of the anode gas progresses. The amount of this divergence at a specific time is determined in step (c).

[0019] The hydrogen concentration control described in step (d) is achieved by actively using the purge process, more precisely by changing its frequency and / or duration, to create capacity for fresh hydrogen in the anode gas by removing nitrogen from the fuel cell system. This ensures full control over the nitrogen concentration in the anode gas at all times, thus preventing the negative consequences of an excessively high nitrogen concentration. Furthermore, it enables the fuel cell system to operate permanently at the desired operating point, so that the conditions used for control better match the actual conditions. The beneficial consequence is an increase in the precision with which the fuel cell can be controlled to a specific operating point.

[0020] Furthermore, it is proposed that, to carry out method step (a), the nitrogen concentration in the anode gas is balanced as the sum of a maximum possible nitrogen content in the fresh hydrogen and a nitrogen content transferred via diffusion processes within the at least one stack, and that the minimum existing hydrogen concentration is mathematically estimated from the nitrogen concentration in the anode gas thus determined. This preferred embodiment is advantageous because it allows an estimate of the hydrogen concentration in the form of a worst-case scenario to be carried out on the safe side without the use of additional components such as measuring devices. Uncertainties in the determination of state variables, such as tolerance fields, have no negative effects due to the estimate on the safe side.This preferred embodiment is based on purely simulative considerations and can also be used well in combination with other methods as an upper limit for the nitrogen content or lower limit for the hydrogen content in the anode gas.

[0021] Furthermore, it is proposed that, to carry out process step (a), the hydrogen concentration be determined using sensors at the inlet and / or outlet of at least one stack. Typically, such sensors are already installed to monitor operation, thus drawing on existing data. At the same time, measurements usually provide the most realistic value and thus better reflect reality than computational approaches.

[0022] In a further development of the invention, it is proposed that, to carry out method step (a), the composition of the anode gas is modeled, and the hydrogen concentration is determined computationally using the data model. This preferred embodiment, which can be used both alone and in combination with other methods for determining the hydrogen concentration, generates a generally very realistic picture of the system states without the need for new sensors. Particularly in conjunction with sensor-based determination of the hydrogen concentration, operating errors can be detected via a discrepancy between model predictions and measured values.

[0023] It is further proposed that, to carry out method step (a), a pressure loss within a component of the anode subsystem, for example, the at least one stack or a water separator, is determined as a function of the stack current, and the hydrogen concentration is inferred from this, preferably using a system-specific characteristic curve map created in advance. This preferred embodiment uses a relationship between pressure loss in a component and the stack current. This relationship must be determined in advance for each individual system. Advantageously, once determined and recorded in a characteristic curve, it can be used at any time during operation to determine the hydrogen concentration. No computationally intensive modeling is necessary for this.To increase the accuracy of this preferred embodiment, a temperature signal from a suitable sensor is preferably used at at least one location in the anode circuit.

[0024] Furthermore, it is proposed that, to carry out method step (a), the hydrogen concentration is determined using a system-specific characteristic field based on a predetermined stack current and a predetermined switching frequency of the purge valve. This characteristic field represents the respective course of the hydrogen concentration in the anode gas over the stack current for different switching frequencies of the purge valve. This embodiment is advantageous in that the hydrogen concentration is derived directly from two parameters that can be freely adjusted during operation. Complex switching strategies for the purge valve can also be stored in the characteristic field. The characteristic curves are available at any time during operation and provide the required hydrogen concentration quickly and without any modeling or computational effort.

[0025] It is further proposed that, in order to carry out method step (b), the actual anode stoichiometry is deduced from the stack current using a characteristic curve field created in advance for the individual system, which represents the relationship between the stack current and the actual anode stoichiometry for various hydrogen concentrations in the anode gas. Carrying out step (b) with the aid of a characteristic curve field is advantageous because, after initial definition, these are available at any time during operation and are dependent neither on other components nor on computationally intensive modeling. Preferably, a desired anode stoichiometry ASOLL is also mapped onto the characteristic curve field, which links the stack current to an actual anode stoichiometry AIST, from which the comparison required in step (c) is directly determined in the form of a difference AIST - ASOLL.

[0026] Furthermore, it is proposed that in process step (d), if the target anode stoichiometry is detected to be exceeded by the actual anode stoichiometry, the frequency and / or duration of the purge process be reduced. This is based on the following causal relationship, whereby a system state defined by the target anode stoichiometry ASOLL is abbreviated as the target state, and a system state defined by the actual anode stoichiometry AIST is abbreviated as the actual state:

[0027] If A IST > S0L L follows from the previously presented definition of anode stoichiometry: mH2 tats IST m H2 actual TARGET mH2_st mH2_st where mH2_st represents the amount of hydrogen required for a given current consumption, which is the same in both the actual and the target state, and mH2_tats_isT and mH2_tats_soLL represent the actually supplied amount of hydrogen in the actual and target states, respectively. Consequently, m H2_tats_IST > mH2_tats_SOLL , which means that in the actual state more hydrogen is available for combustion than required by the target state. This situation is corrected by reducing the duration and / or frequency of the purge process, thus removing less nitrogen from the fuel cell system. As a result, the nitrogen concentration in the anode gas increases at the expense of the hydrogen concentration and the amount of hydrogen supplied to the anode side decreases. which means that in the actual state less hydrogen is supplied than is required in the target state. In this case, it is concluded that there is an increased nitrogen content, which is removed from the fuel cell system by increasing the duration and / or frequency of the purge process.

[0028] Similarly, it is proposed that in process step (d) if the actual anode stoichiometry falls below the target anode stoichiometry, the frequency and / or duration of the purge process be increased. In this case, A IST < A S0L L and es the previously listed causal chain with inverted operator: m H2_tats_lST < m H2_tats_SOLL , which means that less hydrogen is being supplied in the actual state than is planned in the target state. In this case, it is concluded that there is an increased nitrogen content, which is eliminated from the fuel cell system by increasing the duration and / or frequency of the purge process. With these two preferred embodiments presented, the regulation of the hydrogen concentration in the anode gas is based on a straightforward, traceable causal relationship.

[0029] Furthermore, a control unit is proposed that is configured to execute steps of a method according to the invention. This control unit preferably stores the characteristic curves determined for each system, evaluates measurement data, and / or compares the actual air ratio with the desired air ratio. Since a fuel cell system is typically already equipped with a control unit that exercises extensive control over controllable components of the fuel cell system, the control of a method according to the invention is preferably located there.

[0030] A preferred embodiment of the invention is illustrated in more detail below with reference to the figures. Figure 1 shows a schematic representation of a fuel cell system.

[0031] Figure 2 schematic representation of a process sequence according to the invention

[0032] Character description

[0033] Fig. 1 shows, by way of example, a schematic representation of a fuel cell system 1, the central component of which is stack 2. This comprises an anode side 5 belonging to an anode subsystem 3 and a cathode side 6 belonging to a cathode subsystem 4. Hydrogen is supplied to stack 2 via anode subsystem 3. Within stack 2, this hydrogen reacts with air supplied on cathode side 6 to form water. During this process, water and nitrogen diffuse over to anode side 5 and enrich the anode gas accordingly. The anode gas, enriched with water and nitrogen and still containing unused hydrogen, is discharged from stack 2 for recirculation. Water is separated from the anode gas in a water separator 8, and the water separator 8 is emptied by temporarily opening a drain valve 9.The separated water is introduced into an exhaust air path 10 located in the cathode subsystem 4 and leaves the fuel cell system 1 via an exhaust system 11 together with air discharged from the cathode side 6.

[0034] The anode gas, largely free of water, is then discharged from the water separator 8 and recirculated via a jet pump 12. Instead of a jet pump 12, an active pump module or a combination of both can be used. In the jet pump 12, fresh hydrogen is fed to the anode gas from a tank 7. The hydrogen stored in tank 7 is contaminated with nitrogen and typically contains a maximum nitrogen concentration of 300 ppm. The hydrogen is fed via a controllable hydrogen metering valve 13.

[0035] Due to both the diffusion processes within the stack 2 and the contamination of the fresh hydrogen, the anode gas is constantly enriched with nitrogen. To remove this nitrogen from the fuel cell system 1, a so-called purge process is provided. During this purge process, anode gas is vented within or downstream of the water separator by temporarily opening a purge valve 14. The anode gas vented via the purge valve 14 is introduced into the exhaust air path 10 of the cathode subsystem 4 and expelled via an exhaust system 11. The hydrogen concentration in the exhaust gas is monitored by a measuring device 15 to prevent explosive hydrogen-air mixtures. Within the anode subsystem 3, the anode gas removed during the purge process is compensated by supplying fresh hydrogen via the hydrogen metering valve 13. The duration and frequency of the purge process are variable and controllable.

[0036] During operation, the correct adjustment of the hydrogen concentration in the anode gas and thus the anode stoichiometry is of particular importance. This has a particular impact on the service life of the fuel cells within a stack 2. In a method according to the invention, the hydrogen concentration in the anode gas is controlled by adjusting the frequency and / or duration of the purge processes. Such a method according to the invention is illustrated by way of example in Fig. 2. In a first method step S1, the hydrogen concentration in the anode gas is determined. This is preferably done via a corresponding sensor system 16 at the inlet and / or outlet of the anode side 5 of the stack 2.In other preferred embodiments, process step S1 is carried out by balancing the nitrogen mass flows entering the anode gas and estimating the resulting minimum hydrogen concentration present, by modeling the anode gas composition or by other means.

[0037] In a subsequent method step S2, the actual anode stoichiometry on the anode side 5 within the stack 2 is determined from the hydrogen concentration in the anode gas. In a preferred embodiment, the actual anode stoichiometry is determined from the stack current using a characteristic curve map created in advance for each system, which represents the relationship between the stack current and the actual anode stoichiometry for different hydrogen concentrations in the anode gas. In a subsequent method step S3, the actual anode stoichiometry determined in method step S2 is compared with a target anode stoichiometry. The target anode stoichiometry is derived from a target operating point of the fuel cell system that defines the control or regulation objective.

[0038] If the actual anode stoichiometry is greater than the target anode stoichiometry, the hydrogen concentration in the anode gas is above its target value. In this case, the frequency and / or duration of the

[0039] Purge processes are reduced. This increases the nitrogen concentration in the anode gas and decreases the hydrogen concentration.

[0040] Conversely, if the actual anode stoichiometry is smaller than the target anode stoichiometry, the hydrogen concentration in the anode gas is below its target value.

[0041] In this case, the frequency and / or duration of the purge processes is increased in process step S4.2. This decreases the nitrogen concentration in the anode gas and increases the hydrogen concentration.

Claims

Claims 1. A method for operating a fuel cell system (1) comprising an anode subsystem (3) for supplying at least one stack (2) with hydrogen to generate a stack current, wherein the hydrogen is supplied in an anode gas which is enriched with nitrogen during operation, and wherein a nitrogen concentration in the anode gas is controlled by temporarily discharging anode gas from the anode subsystem (3) via a controllable purge valve (14) during a purge process and replacing it by supplying fresh hydrogen, characterized by the following steps: (a) Determination of a hydrogen concentration in the anode gas, (b) Inference from the hydrogen concentration and the stack current to an actual anode stoichiometry, (c) comparing the actual anode stoichiometry with a target anode stoichiometry, (d) Indirect adjustment of the hydrogen concentration by varying the duration and / or frequency of the purge process if the actual anode stoichiometry deviates from the target anode stoichiometry.

2. Method according to claim 1, characterized in that for carrying out method step (a) the nitrogen concentration in the anode gas is balanced as the sum of a maximum possible nitrogen content in the fresh hydrogen and a nitrogen content transferred via diffusion processes within the at least one stack and the minimum existing hydrogen concentration is mathematically estimated from the nitrogen concentration in the anode gas thus determined.

3. Method according to one of the preceding claims, characterized in that for carrying out method step (a) the hydrogen concentration is determined by means of a sensor (16) at the input and / or at the output of the at least one stack (2).

4. Method according to one of the preceding claims, characterized in that, in order to carry out method step (a), the composition of the anode gas is modeled and the hydrogen concentration is determined mathematically with the aid of the model.

5. Method according to one of the preceding claims, characterized in that, in order to carry out method step (a), a pressure loss within a component of the anode subsystem, for example the at least one stack (2) or a water separator (8), is determined as a function of the stack current and the hydrogen concentration is deduced therefrom, preferably using a family of characteristics created in advance for the individual system.

6. Method according to one of the preceding claims, characterized in that, for carrying out method step (a), the hydrogen concentration is determined on the basis of a predetermined stack current and a predetermined switching frequency of the purge valve (14) via a characteristic curve field which is previously created individually for the system and which represents the respective course of the hydrogen concentration in the anode gas over the stack current for different switching frequencies of the purge valve (14).

7. Method according to one of the preceding claims, characterized in that, in order to carry out method step (b), the actual anode stoichiometry is inferred from the stack current via a characteristic field created in advance for the individual system, which represents the relationship between the stack current and the actual anode stoichiometry for different hydrogen concentrations in the anode gas.

8. Method according to one of the preceding claims, characterized in that in method step (d) if it is determined that the target anode stoichiometry is exceeded by the actual anode stoichiometry, the frequency and / or duration of the purge process is reduced.

9. Method according to one of the preceding claims, characterized in that in method step (d) if the actual anode stoichiometry falls below the target anode stoichiometry, the frequency and / or duration of the purge process is increased.

10. Control device configured to carry out steps of a method according to the invention.

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