Method for operating a fuel cell system, control unit

Abstract

The invention relates to a method for operating a fuel cell system (1), comprising at least one fuel cell stack (2) and an air system (3) for supplying the at least one fuel cell stack (2) with air, wherein the air is supplied to the at least one fuel cell stack (2) via an air supply path (4) of the air system (3) and the exhaust air exiting the fuel cell stack (2) is discharged via an exhaust air path (5) of the air system (3), and wherein, in the event of shutdown, the ingress of air into the at least one fuel cell stack (2) is prevented by closing a first shut-off valve (6) integrated into the air supply path (4) and a second shut-off valve (7) integrated into the exhaust air path (5). According to the invention, when the shut-off valves (6, 7) are closed, the sealing force of at least one shut-off valve (6, 7) is actively readjusted, in particular increased, to compensate for settling effects that influence the sealing effect. The invention further relates to a control unit for executing steps of the method.

Description

[0001] The present invention relates to a method for operating a fuel cell system. Furthermore, the invention relates to a control unit for implementing the method or for executing individual steps of the method.

[0002] The preferred field of application of the invention is mobile fuel cell systems or fuel cell vehicles, such as commercial vehicles. State of the art

[0003] Fuel cell systems comprise one or more fuel cell stacks. With the help of a stack, hydrogen and oxygen can be converted into electrical energy, heat, and water in an electrochemical reaction.

[0004] When the stack is shut down or in standby mode, the oxygen supply to the stack is interrupted using shutoff devices, and the stack is deliberately depleted of oxygen or rendered inert to prevent increased stack degradation. This is because the ingress of oxygen during the shutdown phase leads to an air-to-air start upon restart, i.e., a start in which oxygen is present on both the cathode and the anode of the stack. If the anode is then refilled with hydrogen, a hydrogen / oxygen front forms on the anode side, promoting degradation. The shutoff devices are therefore equipped with seals to prevent oxygen from entering the stack during shutdown.

[0005] Both shut-off valves and butterfly valves can be used as shut-off devices. The shut-off devices can be designed as simple on / off valves or flaps or as control devices with a sealing function. The latter has the advantage that the valves or flaps can be used for both shut-off and control. In a first operating mode (control mode), the sealing function is not activated and a mass flow > 0 flows through the valve or flap. In a second operating mode (shut-off mode), the sealing function is activated and there is no flow through the valve or flap. To enter this mode, the sealing pressure must be applied slowly to avoid damaging the seal. The same applies when the sealing effect is released before the valve or flap returns to control mode.

[0006] The shut-off valves or throttle valves are closed during shutdown and switched to shut-off mode. However, the seals, especially during extended downtimes, experience settling effects, leading to a loss of sealing force. This allows oxygen to reach the cathode side and, due to diffusion, also the anode side of the stack, resulting in an air-to-air start upon restart, resulting in increased stack degradation.

[0007] The present invention is concerned with the task of preventing or at least reducing increased degradation of the stack due to air-to-air starts.

[0008] To achieve this objective, the method with the features of claim 1 is proposed. Advantageous further developments of the invention are set forth in the subclaims. Furthermore, a control device for executing steps of the method is specified.

[0009] For the sake of simplicity, we will no longer distinguish between shut-off valves and throttle valves, but will refer only generally to shut-off valves. The term shut-off valve therefore also includes throttle valves. Disclosure of the invention

[0010] Method for operating a fuel cell system, comprising at least one fuel cell stack and an air system for supplying the at least one fuel cell stack with air, wherein the air is supplied to the at least one fuel cell stack via an air supply path of the air system and the exhaust air emerging from the fuel cell stack is discharged via an exhaust air path of the air system. wherein, in the event of a shutdown, the penetration of air into the at least one fuel cell stack is prevented by closing a first shut-off valve integrated into the air supply path and a second shut-off valve integrated into the exhaust air path. According to the invention, when the shut-off valves are closed, the sealing force of at least one shut-off valve is actively readjusted, in particular increased, to compensate for settling effects that influence the sealing effect.

[0011] Since settling effects on the seals of the shut-off valves generally lead to a reduction in the sealing force, the proposed method, or rather the active readjustment of the sealing force, primarily serves to increase the sealing force. By compensating for settling effects according to the proposed method, an inerted state of the fuel cell stack can be maintained or even prolonged during shutdown, thus counteracting the degradation of the fuel cell stack.

[0012] If the fuel cell system comprises multiple fuel cell stacks, the readjustment is performed for at least one stack. This at least one stack must be shut down, i.e., at a standstill or in standby mode. Since this does not always apply to all stacks simultaneously, the proposed procedure is preferably performed on a stack-by-stack basis.

[0013] In a further development of the invention, it is proposed that, in the event of a shutdown, the sealing force of at least one further valve is readjusted. This valve is arranged in a transfer and / or inerting path that branches off from the supply air path downstream of the first shut-off valve and / or from the exhaust air path upstream of the second shut-off valve. A transfer and / or inerting path is a path via which, particularly in a multi-stack system, a media exchange can take place between two stacks, for example to humidify a medium supplied to the stack and / or to inertize a stack volume. In order to prevent air from entering the fuel cell stack via the transfer and / or inerting path in the event of a shutdown, this at least one further path must also be blocked by closing a valve.Since this valve can also be affected by settling effects, the sealing force of the valve is actively adjusted to compensate for these settling effects.

[0014] Preferably, the proposed method is triggered by the passage of time, a diagnostic result and / or a boundary condition which indicates a changed sealing effect of the at least one shut-off valve and / or the at least one further valve.

[0015] If the process is triggered by a time lapse, this can be the time elapsed since the shutoff valve or valve was switched to shutoff mode. Therefore, the time elapsed since the shutoff valve or valve was closed is considered. This will approximately correspond to the shutdown time of the respective fuel cell stack. To trigger the process, a minimum time can be specified, after which the process is then executed. This prevents the process from being executed too early or too frequently.

[0016] Alternatively or additionally, the time elapsed since the last readjustment of the sealing force of the shut-off valve or valve can be used as a basis, especially if the process has already been run through once or several times and the standby or standby mode of the fuel cell stack continues.

[0017] Another trigger can be a diagnostic result. This requires at least one diagnostic option. For example, there may be an option to diagnose the stack voltage and / or the individual cell voltage. Diagnosing the individual cell voltage enables, for example, the detection of a leak as well as the valve causing the leak. If, for example, the edge cells of a stack indicate a voltage while other cells do not yet indicate a voltage, the direction of penetration of the air or oxygen can be deduced from this, and thus the shut-off valve or valve causing the leak can be identified. In this case, the active readjustment of the sealing force can be limited to this one shut-off valve or valve.

[0018] A concentration sensor installed in the air system also provides a diagnostic option. This sensor can be used to detect a change in the inert state of a stack and thus a change in the sealing effect of a shut-off valve or valve.

[0019] Another trigger for readjusting the sealing force of a shut-off valve or valve can be a changing boundary condition, such as weather and / or temperature. This is especially true if the change results in a significant change in the pressure differential across one of the shut-off valves and / or another valve. If the ambient pressure is greater than the pressure in the enclosed volume, a leak can be counteracted by actively readjusting the sealing force.

[0020] It is further proposed that active readjustment of the sealing force be prevented if the end of the shutdown phase is detected by prediction. This prevents unnecessary stress on the seal of the shut-off valve or valve shortly before opening. Prediction is used in commercial vehicles, among other things, for planning operating and downtime periods. In this case, the prediction can also be used to prevent readjustment of the sealing force shortly before a restart of the shut-off fuel cell stack.

[0021] Alternatively or additionally, it is proposed that active readjustment of the sealing force be prevented if component limitations prohibit readjustment. For example, the sealing force of the shut-off valve or valve can be limited by the maximum actuating force of an actuator, which applies the sealing force to press the seal. This value can also vary, for example, depending on the load profile over its lifetime. For example, it can be provided that the maximum force is not fully utilized when the respective shut-off valve or valve is new, but only after a certain number of closing operations.

[0022] As a further development, it is therefore proposed that the respective load profile and / or the respective aging state be taken into account during the active readjustment of the sealing force of the at least one shut-off valve and / or the at least one further valve. In this way, premature readjustment of the sealing force can be prevented.

[0023] Furthermore, it is proposed that an active increase in the sealing force of the at least one shut-off valve and / or the at least one further valve be carried out only temporarily. This applies in particular if the trigger is a condition or boundary condition that is not permanent. In this case, an increase in the sealing force caused by readjustment can also be reversed to protect the seal of the shut-off valve or valve.

[0024] Preferably, in order to readjust the sealing force of the at least one shut-off valve and / or the at least one further valve, the actuating force of an actuator assigned to the respective shut-off valve and / or the respective valve is changed, in particular increased.

[0025] Furthermore, a control unit for a fuel cell system is proposed. The control unit is configured to carry out steps of the method according to the invention. In particular, the control unit can be used to actuate an actuator, via whose actuating force the sealing force of the at least one shut-off valve and / or the at least one further valve can be readjusted.

[0026] The invention and its advantages are described in more detail below with reference to the accompanying drawings. These show: Fig. 1 a schematic representation of an air system of a fuel cell system suitable for carrying out the method according to the invention, Fig. 2 a schematic representation of an air system of another fuel cell system suitable for carrying out the method according to the invention, and Fig. 3 a flowchart illustrating the sequence of a method according to the invention. Detailed description of the drawings

[0027] The Fig. 1 shows a first fuel cell system 1 suitable for carrying out the method according to the invention or operable according to the method. The fuel cell system 1 comprises a fuel cell stack 2 and an air system 3 for supplying the fuel cell stack 2 with air. The air system 3 comprises an air supply path 4 and an exhaust air path 5. The air is supplied to the fuel cell stack 2 via the air supply path 4. For this purpose, an air conveying and compression system 12 is integrated into the air supply path 4. The air or exhaust air escaping from the fuel cell stack 2 is discharged via the exhaust air path 5.

[0028] During shutdown, the air supply to the fuel cell stack 2 is interrupted by means of a shut-off valve 6 integrated into the supply air path 4. Furthermore, a further shut-off valve 7 integrated into the exhaust air path 5 is closed, preventing any further air from entering the fuel cell stack 2. The volume between the two shut-off valves 6, 7, including the stack volume, is thus enclosed, so that it can be kept as oxygen-poor or inert as possible during the shutdown phase. This prevents an air-to-air start when the fuel cell stack 2 is restarted.

[0029] A further fuel cell system 1 which is suitable for carrying out the method according to the invention or which can be operated according to the method is shown in Fig. 2. This differs from that of the Fig. 1 in that it comprises more than just one fuel cell stack 2. This means that the fuel cell system 1 of the Fig. 2 is a multi-stack system. Shown are a first fuel cell stack 2.1 and a second fuel cell stack 2.2. Both are supplied with air via the air system 3, whereby the illustration of the Fig. 2 is limited to the air supply of the fuel cell stack 2.1.

[0030] Based on the Fig. 2 primarily illustrates that a fuel cell system 1 can additionally have at least one transfer and / or inerting path 10, 11, which branches off from the supply air path 4 downstream of the shut-off valve 6 and / or from the exhaust air path 5 upstream of the shut-off valve 7. In this case, at least one further valve 8, 9 is required to enclose the volume between the shut-off valves 6, 7, including the stack volume. In this case, in addition to the shut-off valves 6, 7, the additional valves 8, 9 are required. A transfer and / or inerting path 10, which connects the two fuel cell stacks 2.1, 2.2, can be opened via the valve 8. A transfer and / or inerting path 11 can be released via the valve 9, which connects one of the two fuel cell stacks 2.1, 2.2 with an exhaust air area 13 of the other fuel cell stack 2.2, 2.1 for inerting.

[0031] The following based on the Fig. 3 described method according to the invention can be used with the fuel cell system 1 of the Fig. 1 as well as with the Fig. 2. In the latter case, any readjustment of the sealing force also includes valves 8 and 9.

[0032] In a first step S1, the method according to the invention is initiated. The system is in normal operation, so that the shut-off valves 6, 7 are open. In the next step S2, the system switches from normal operation to standby mode, and the shut-off valves 6, 7 are closed. If valves 8, 9 are present, they are also closed. In step S3, a check is then performed to determine whether the sealing force of at least one of the shut-off valves 6, 7 and / or the other valves 8, 9 needs to be readjusted due to a trigger event. A trigger or trigger event can be: T1 Time elapsed after switching off the fuel cell stack 2 and / or closing the shut-off valves 6, 7 T2 Time elapsed after final readjustment of the sealing force T3 Diagnostic result based on sensor data, in particular related to the stack voltage and / or single cell voltage, which indicates a change in the sealing effect of one of the shut-off valves 6, 7 and / or the other valves 8, 9 T4 Change of a boundary condition in the stack and / or valve environment

[0033] If a trigger event occurs, it is necessary to check whether a condition or circumstance exists that prohibits readjustment of the sealing force. Such prohibitive circumstances may include: V1 component limitations V2 Prediction of an impending commissioning of the fuel cell stack

[0034] Other factors to be considered during the review may include: F1 Load profile and / or aging condition of at least one shut-off valve 6, 7 and / or further valve 8, 9 F2 Comparison data from other systems, for example, provided via a cloud

[0035] The test can be carried out separately for each shut-off valve 6, 7 and / or each additional valve 8, 9, so that, depending on the result of the test, the readjustment of the sealing force can be specifically limited to the valve whose sealing force is no longer sufficient.

[0036] If the evaluation of the test in step S4 indicates that readjustment of the sealing force is indicated ("+"), the actuating force of an actuator assigned to the shut-off valve 6, 7 or valve 8, 9 whose sealing force is to be readjusted is increased in a further step S5. If the sealing effect is still inadequate, step S5 can be repeated. A repetition is excluded if commissioning of the fuel cell stack 2 is initiated in step S6. For this purpose, the shut-off valves 6, 7 are opened in the subsequent step S7.

[0037] If the evaluation of the test in step S4 shows that readjustment of the sealing force is not indicated (“-”), the test is repeated in step S3, for example at specific time intervals or event-triggered, as long as the fuel cell stack 2 is switched off.

Claims

[1] Method for operating a fuel cell system (1), comprising at least one fuel cell stack (2) and an air system (3) for supplying the at least one fuel cell stack (2) with air, wherein the air is supplied to the at least one fuel cell stack (2) via an air supply path (4) of the air system (3) and the exhaust air emerging from the fuel cell stack (2) is discharged via an exhaust air path (5) of the air system (3), and wherein, in the case of shutdown, the penetration of air into the at least one fuel cell stack (2) is prevented by closing a first shut-off valve (6) integrated into the air supply path (4) and a second shut-off valve (7) integrated into the exhaust air path (5), characterized by that when the shut-off valves (6, 7) are closed, the sealing force of at least one shut-off valve (6, 7) is actively readjusted, in particular increased, in order to compensate for settling effects that influence the sealing effect. [2] Method according to claim 1, characterized by in that, in the event of shutdown, the sealing force of at least one further valve (8, 9) is readjusted, which is arranged in a transfer and / or inerting path (10, 11) which branches off from the supply air path (4) downstream of the first shut-off valve (6) and / or from the exhaust air path (5) upstream of the second shut-off valve (7). [3] Method according to claim 1 or 2, characterized by that the method is triggered by the passage of time, a diagnostic result and / or a boundary condition which indicates a changed sealing effect of at least the at least one shut-off valve (6, 7) and / or the at least one further valve (8, 9). [4] Method according to one of the preceding claims, characterized bythat active readjustment of the sealing force is prevented if it is detected by prediction that the end of the shutdown phase is imminent and / or component limitations prohibit readjustment. [5] Method according to one of the preceding claims, characterized by that the respective load profile and / or the respective ageing state is / are taken into account during the active readjustment of the sealing force of the at least one shut-off valve (6, 7) and / or the at least one further valve (8, 9). [6] Method according to one of the preceding claims, characterized by that an active increase in the sealing force of the at least one shut-off valve (6, 7) and / or the at least one further valve (8, 9) is only carried out temporarily. [7] Method according to one of the preceding claims, characterized byin that, in order to readjust the sealing force of the at least one shut-off valve (6, 7) and / or the at least one further valve (8, 9), the actuating force of an actuator assigned to the respective shut-off valve (6, 7) and / or the respective valve (8, 9) is changed, in particular increased. [8] Control unit for a fuel cell system (1), wherein the control unit is designed to carry out steps of a method according to one of the preceding claims.

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