Fuel cell system and associated operating procedure

Abstract

The invention relates to an operating method (17) for a fuel cell system (1) comprising a fuel cell (2) having an anode chamber (3), a cathode chamber (4) and an electrolyte membrane (5), an anode gas supply line (6) in which an anode gas supply valve (8) is arranged, a cathode gas supply line (9) in which a cathode gas supply valve (11) is arranged, and a cathode gas discharge line (12) in which a cathode gas discharge valve (14) is arranged. To open an iced-up cathode-side valve (11, 14), the operating method (17) proposes - that in order to switch on the fuel cell system (1), the two cathode-side valves (11, 14) are triggered to open, - that in the event that one cathode-side valve (11, 14) cannot be opened and thus forms a faulty valve, the other cathode-side valve (11, 14), which opens and thus forms a fault-free valve, is controlled to close again, - that after the fault-free valve has closed, the faulty valve is again or continues to be controlled to open and the anode gas supply valve (8) is also controlled to open, - that after closing the fault-free valve and opening the anode gas supply valve (8), it is waited until an opening pressure builds up in the cathode chamber (4), which supports the opening of the faulty valve which is still controlled to open.

Description

The present invention relates to an operation method for a fuel cell system. The invention also relates to a fuel cell system which can be operated according to the operating method.A fuel cell system typically includes a fuel cell having an anode compartment, a cathode compartment, and an electrolyte membrane separating the anode compartment from the cathode compartment. The fuel cell is usually formed by a stack of a plurality of fuel cell elements, each of which has an anode space, a cathode space and an electrolyte membrane which separates the anode space from the cathode space within the respective fuel cell element. The individual anode spaces of the fuel cell elements then together form the anode space of the fuel cell. The individual cathode spaces of the fuel cell elements then together form the cathode space of the fuel cell. The individual electrolyte membranes of the fuel cell elements then together form the electrolyte membrane of the fuel cell.A fuel cell system also includes a cathode gas supply line for supplying anode gas to the anode compartment, which is fluidly connected to an anode gas inlet of the fuel cell and in which an anode gas supply valve for controlling the anode gas supply line is disposed. Furthermore, a cathode gas supply line for supplying cathode gas is provided, which is fluidically connected to a cathode gas inlet of the fuel cell and in which a cathode gas supply valve for controlling the cathode gas supply line is arranged. Furthermore, a cathode gas discharge line for discharging cathode gas is provided, which is fluidically connected to a cathode gas outlet of the fuel cell and in which a cathode gas discharge valve for controlling the cathode gas discharge line is arranged.For efficient operation of the fuel cell system, the cathode gas contains a comparatively high water content, in particular in the form of water vapor. After the fuel cell system has been switched off, the water vapor can condense. At low ambient temperatures, the condensed water may freeze. If condensed water inside the cathode gas supply valve freezes, the cathode gas supply valve, which is closed when the fuel cell system is switched off, can freeze in the closed state, i.e. be blocked by the ice. The same applies to the cathode gas discharge valve. When the fuel cell system is switched on, the cathode-side valves, i.e. the cathode gas feed valve and the cathode gas discharge valve, must be opened. In order that the respective valve can be opened even in the frozen state, a corresponding actuating drive must be dimensioned comparatively large or powerfully in order to be able to generate a correspondingly large opening force. Since icing of the respective valve occurs only comparatively rarely, the respective actuating drive is oversized for the predominant majority of the actuating processes for opening and closing the respective valve. Such powerful actuating drives require comparatively much installation space and are comparatively expensive.The present invention is concerned with the problem of specifying an improved or at least one other embodiment for a fuel cell system or for an associated operating method, which embodiment is distinguished in particular in that a frozen valve can be opened even with smaller actuating drives.This problem is solved according to the invention by the subject matter of the independent claims. Advantageous embodiments are the subject of the dependent claims.The invention is based on the general idea of assisting the opening of a cathode-side valve by generating an opening pressure in the cathode chamber. In other words, the opening force that can be generated with the aid of an actuating drive is supported by an opening pressure provided on the cathode side, as a result of which, overall, a greater force for opening the respective valve can be generated. In any case, the force for opening the respective valve, which force is increased with the aid of the opening pressure, exceeds the opening force that can be generated by the respective actuating drive alone. As a result, it is possible to make the respective actuator smaller, thereby requiring less overall space, reducing costs, and increasing fuel cell system economics.In detail, it is proposed for the operating method according to the invention that, for switching on the fuel cell system, the closed cathode gas feed valve and the closed cathode gas discharge valve are controlled for opening. In the case that one of the cathode gas supply valve and the cathode exhaust valve cannot be opened and thereby forms a defective valve, the other of the cathode gas supply valve and the cathode gas discharge valve, which can be opened and thereby forms a defective valve, is driven to close again. After the fault-free valve has been closed, the faulty valve is actuated to open again or further, wherein the anode gas feed valve is also actuated to open. Anode gas can thereby flow through the anode gas inlet into the anode space. After closing the fault-free valve and opening the anode gas feed valve, it is waited until an opening pressure builds up in the cathode chamber, which assists the opening of the fault-free valve which is still controlled to open. This procedure is based on the finding that anode gas, which is located in the anode space, penetrates into the cathode space by diffusion through the electrolyte membrane. Since both the faulty valve and the faulty valve are closed, the cathode chamber is closed off from the outside, so that the pressure in the cathode chamber increases due to the anode gas entering the cathode chamber. As a result, an opening pressure builds up in the cathode chamber, which assists the opening of the faulty valve which is still controlled for opening. It is of particular importance in this case that in this procedure virtually exclusively on-board components are used, that is to say components which are present in any case on the fuel cell system, with the result that this procedure can be realized virtually without additional costs for additional components. As soon as the closed, faulty valve can be opened with the aid of the opening pressure, the fuel cell system can be put into operation in the usual manner.According to an advantageous embodiment, the anode gas supply line can fluidically connect an anode gas tank for storing the anode gas, such as hydrogen or a hydrocarbon, to the anode gas inlet which leads to the anode space. The anode gas is stored in the anode gas tank in liquid and / or gaseous form at a storage pressure which is greater than an operating pressure with which the anode space is supplied with anode gas during normal operation of the fuel cell system. The anode gas tank present in the fuel cell system provides a pressure source which can be used for generating pressure first in the anode space and, owing to the diffusion, also in the cathode space with a time delay. In particular, it is thereby possible to generate an opening pressure in the cathode chamber that is greater than the operating pressure that is generated in the anode chamber during normal operation of the fuel cell system. The opening force of the respective valve can thus be significantly increased.In another advantageous embodiment, it can be provided that after closing the fault-free valve, the anode gas feed valve is controlled to open in such a way that the pressure in the anode space builds up so slowly that sufficient anode gas can diffuse through the electrolyte membrane into the cathode space, so that a differential pressure which builds up at the electrolyte membrane between anode space and cathode space remains below a predetermined critical differential pressure. The critical differential pressure can be determined in particular by the strength of the electrolyte membrane, which in turn can depend on the current temperature of the electrolyte membrane. In this embodiment, the pressure in the anode chamber is thus increased in a targeted manner such that the pressure increase in the cathode chamber can follow sufficiently quickly with a time delay without the critical differential pressure being reached. In particular, this prevents sudden pressurization of the anode chamber. The anode gas feed valve is preferably a proportional valve which is distinguished by a plurality of, in particular as many as desired, intermediate positions which lie between an open position and a closed position.According to another embodiment, it can be provided that, when the faulty valve is closed and controlled to open, after the fault-free valve is closed and the anode gas supply valve is opened, the pressure in the cathode chamber is increased only until the opening pressure suffices to open the faulty valve controlled to open, so that the switching on of the fuel cell system can be continued, or until a predetermined opening limit pressure is reached, which results in the switching on of the fuel cell system being aborted. As a result, the fuel cell system cannot be turned on. This embodiment is of interest in particular for cases in which the faulty valve is not frozen, but is jammed or otherwise blocked, so that the opening force of the actuator in conjunction with the opening pressure is not sufficient to open the valve. In this case, the fuel cell system cannot be operated. The opening limit pressure can also be used to take account of the case in which, by opening the faulty valve, a pressure drop occurs in the cathode chamber, which pressure drop can lead to a reverse pressure difference between anode chamber and cathode chamber at the electrolyte membrane. The opening limit pressure can be selected such that, in this case, the resulting differential pressure remains below the critical differential pressure. It is clear that the fuel cell has at least one pressure sensor with which the pressure in the cathode chamber and / or in the anode chamber can be detected in order to monitor the opening pressure and / or the differential pressure.According to another embodiment, it can be provided that in the event that both cathode-side valves, i.e. the cathode gas feed valve and the cathode gas discharge valve, cannot be opened and thereby each form a defective valve, the anode gas feed valve is controlled to open, while the two defective valves are controlled to open again or further. Thus, the opening pressure can be generated simultaneously at both faulty closed valves with the aid of the anode gas in the cathode chamber.If both closed, faulty valves then open simultaneously with the aid of the opening pressure, the fuel cell system can be put into operation in the usual manner.In the event that only one of the two closed, faulty valves initially opens by supporting the opening pressure, the open faulty valve now forms a faulty valve, so that the operating method can again be continued as described above. In other words, the previously defective, open and now defective valve is now controlled to close, while the defective valve that continues to be closed and the anode gas supply valve are controlled to open, in order to build up the opening pressure in the cathode chamber to assist the opening of the defective valve.A fuel cell system according to the present invention includes a fuel cell having an anode space, a cathode space, and an electrolyte membrane separating the anode space from the cathode space. Here too, the fuel cell is usually formed by a stack of a plurality of fuel cell elements, each of which has an anode space, a cathode space and an electrolyte membrane which separates the anode space from the cathode space. The fuel cell system also includes an anode gas supply line for supplying anode gas to the anode compartment, which is fluidly connected to an anode gas inlet of the fuel cell and in which an anode gas supply valve for controlling the anode gas supply line is disposed. Further, the fuel cell system includes a cathode gas supply line for supplying cathode gas to the cathode space, which is fluidly connected to a cathode gas inlet of the fuel cell, and in which a cathode gas supply valve for controlling the cathode gas supply line is disposed. In addition, the fuel cell system has a cathode gas discharge line for discharging cathode gas from the cathode space, which is fluidically connected to a cathode gas outlet of the fuel cell and in which a cathode gas discharge valve for controlling the cathode gas discharge line is arranged. Further, the fuel cell system according to the present invention is provided with a controller which is coupled to the anode gas supply valve, the cathode gas supply valve, and the cathode gas discharge valve, and which is further configured such that the controller for turning on the fuel cell system performs the above-described operation method.In the present context, a "configuration" is synonymous with a "configuration" and / or "device" and / or "programming", such that the phrase "configured such that" is synonymous with the phrase "configured and / or configured and / or programmed such that".In an advantageous embodiment, the anode gas feed line can fluidically connect an anode gas tank for storing the anode gas to the anode gas inlet of the fuel cell leading to the anode space. In this way, the anode gas is provided in the anode gas tank at a storage pressure that can be used to generate the opening pressure.According to an advantageous embodiment, the fuel cell system can have a recirculation line which fluidically connects an anode gas outlet of the fuel cell to the anode gas feed line. In this case, it can expediently be provided that a connection point, in which the recirculation line is fluidically connected to the anode gas feed line, is arranged between the anode gas feed valve and the anode gas inlet. In conjunction with the recirculation line, it may be possible in particular to feed the anode gas to the anode space via the anode gas inlet and the anode gas outlet for the pressure build-up in the cathode space. Alternatively, a non-return valve can also be arranged in the recirculation line, which prevents a flow in the direction of the anode gas outlet.In another advantageous embodiment, it can be provided that the anode gas feed valve has an electromotive actuating drive coupled to the control device for opening and closing the anode gas feed valve, so that the control device for opening and closing the anode gas feed valve actuates the associated actuating drive. Additionally or alternatively, the cathode gas feed valve can have an electromotive actuating drive coupled to the control device for opening and closing the cathode gas feed valve, such that the control device for opening and closing the cathode gas feed valve actuates the associated actuating drive. Additionally or alternatively, the cathode gas discharge valve can have an electromotive actuating drive coupled to the control device for opening and closing the cathode gas discharge valve, so that the control device for opening and closing the cathode gas discharge valve actuates the associated actuating drive. Such electric motor actuators can be controlled particularly easily and reliably with the aid of the control device.According to an advantageous embodiment, the cathode gas discharge valve can have a valve inlet facing the cathode chamber, a valve outlet facing away from the cathode chamber and a valve member arranged between the valve inlet and the valve outlet, wherein the cathode gas discharge valve is configured such that the valve member is driven to open by an overpressure at the valve inlet relative to the valve outlet. Additionally or alternatively, the cathode gas feed valve may have a valve outlet facing the cathode chamber, a valve inlet facing away from the cathode chamber, and a valve member arranged between the valve inlet and the valve outlet, wherein the cathode gas feed valve is configured such that the valve member is driven to open by an overpressure at the valve outlet opposite the valve inlet. The designs or configurations proposed here supported the opening at the respective valve by the opening pressure. For example, flap valves in which a plate-shaped valve member is pivotable about a pivot axis can have a pivot axis arranged asymmetrically or eccentrically. With a symmetrically or centrally arranged pivot axis, a differential pressure acting on the valve member would drive the valve member equally for opening and closing, so that the pressure forces cancel each other out.According to another embodiment, the controller may be configured to monitor opening and closing of the cathode gas supply valve and the cathode gas discharge valve. In addition, the controller may be configured to identify the cathode gas supply valve as a faulty valve when the cathode gas supply valve remains closed although the cathode gas supply valve is driven to open. Additionally or alternatively, the controller may be configured to identify the cathode gas discharge valve as a faulty valve when the cathode gas discharge valve remains closed even though the cathode gas discharge valve is controlled to open. For example, the control device can monitor the current profile at the respective actuating drive of the respective valve and detect, on the basis of the current consumption, whether the respective valve opens or not. By monitoring the opening state and the closing state of the respective valve, the control device operates particularly reliably.Preferably, the cathode gas supply valve may include a sensor coupled to the controller and configured to monitor opening and closing of the cathode gas supply valve and communicate the current state of the cathode gas supply valve to the controller. Additionally or alternatively, the cathode gas discharge valve may comprise a sensor coupled to the control device and configured to monitor the opening and closing of the cathode gas discharge valve and to communicate the current state of the cathode gas discharge valve to the control device. Such a sensor-based monitoring of the opening state and of the closing state of the respective valve operates particularly reliably.Further important features and advantages of the invention are evident from the dependent claims, from the drawings and from the associated description of the figures with reference to the drawings.It is understood that the features mentioned above and those still to be explained below can be used not only in the respectively specified combination, but also in other combinations or alone, without departing from the scope of the invention as defined by the claims. The above-mentioned components of a superordinate unit, such as a device, a device or an arrangement, which are designated separately, can form separate components or components of this unit or can be integral regions or sections of this unit, even if this is illustrated differently in the drawings.Preferred exemplary embodiments of the invention are illustrated in the drawings and are explained in more detail in the following description, wherein the same reference numerals refer to the same or similar or functionally the same components.They show, in each case schematically, FIG. 1 shows a greatly simplified schematic diagram of a fuel cell system during a starting process in the case of a valve which is icy in the closed position, FIG. 2 is a view similar to FIG. 1, but showing the opening of the icy valve, FIG. 3 is a view as in FIGS. 1 and 2, but after the icy valve has been opened, FIG. 4 is a flow chart illustrating an operating method for opening an icy valve, FIG. 5 is a flow chart as in FIG. 4, but in another embodiment.According to FIGS. 1 to 3, a fuel cell system 1 comprises a fuel cell 2, which is usually designed as a fuel cell stack, which has a plurality of fuel cell elements. The fuel cell 2 has an anode chamber 3, a cathode chamber 4 and an electrolyte membrane 5 which separates the anode chamber 3 from the cathode chamber 4. The fuel cell system 1 further comprises an anode gas feed line 6 for feeding anode gas, preferably hydrogen, to the anode space 3, which is fluidically connected to an anode gas inlet 7 of the fuel cell 2. In the anode gas supply line 6, an anode gas supply valve 8 for controlling the anode gas supply line 6 is disposed. Furthermore, the fuel cell system 1 has a cathode gas feed line 9 for feeding cathode gas, preferably air, to the cathode space 4, which is fluidically connected to a cathode gas inlet 10 of the fuel cell 2. In the cathode gas supply line 9, a cathode gas supply valve 11 for controlling the cathode gas supply line 9 is disposed. In addition, the fuel cell system 1 has a cathode gas discharge line 12 for discharging cathode gas from the cathode space 4, which is fluidically connected to a cathode gas outlet 13 of the fuel cell 2. In the cathode gas discharge line 12, a cathode gas discharge valve 14 for controlling the cathode gas discharge line 12 is disposed. Furthermore, the fuel cell system 1 is equipped with a control device 15 which is coupled via corresponding control lines 16 to the anode gas feed valve 8, to the cathode gas feed valve 11 and to the cathode gas discharge valve 14. The control device 15 is configured such that it carries out an operating method 17 explained in more detail below with reference to FIGS. 4 and 5 in order to switch on the fuel cell system 1.According to FIGS. 1 to 3, the fuel cell system 1 can also have an anode gas tank 18 for storing the anode gas, which is fluidically connected to the anode gas inlet 7 via the anode gas feed line 6. The anode gas inlet 7 communicates with the anode space 3. A conveying device for driving the anode gas can be arranged in the recirculation line 19, but is not shown here. In addition, a non-return valve can be arranged in the recirculation line 19, which valve blocks a flow in the direction of the anode gas outlet 20 and which is likewise not illustrated here. A connection point 21, in which the recirculation line 19 is fluidically connected to the anode gas feed line 6, can preferably be arranged between the anode gas feed valve 8 and the anode gas inlet 7.The anode gas feed valve 8 can have an electromotive actuating drive 22 which is coupled to the control device 15 via one of the control lines 16 and which serves for opening and closing the anode gas feed valve 8. In this way, the control device 15 for opening and closing the anode gas feed valve 8 can correspondingly actuate the associated actuating drive 22. The cathode gas feed valve 11 also has an electromotive actuating drive 23 which is coupled to the control device 15 via one of the control lines 16 and which serves for opening and closing the cathode gas feed valve 11. In this way, the control device 15 for opening and closing the cathode gas supply valve 11 can correspondingly actuate the associated actuating drive 23. In addition, the cathode gas discharge valve 14 can have an electromotive actuating drive 24, which is coupled to the control device 15 via one of the control lines 16 and which is designed for opening and closing the cathode gas discharge valve 14. In this way, the control device 15 for opening and closing the cathode gas discharge valve 14 can correspondingly actuate the associated actuating drive 24.A configuration is preferred in which the cathode gas discharge valve 14 has a valve inlet 25 facing the cathode chamber 4, a valve outlet 26 facing away from the cathode chamber 4, and a valve member 27 which is arranged between the valve inlet 25 and the valve outlet 26 and is drivingly connected to the associated actuating drive 24. The cathode gas discharge valve 14 is expediently now configured such that the valve member 27 is driven to open by an overpressure at the valve inlet 25 relative to the valve outlet 26. Additionally or alternatively, the cathode gas feed valve 11 can have a valve outlet 28 facing the cathode chamber 4, a valve inlet 29 facing away from the cathode chamber 4, and a valve member 30. The valve member 30 is also arranged here between the valve inlet 29 and the valve outlet 28 and is expediently drive-connected to the associated actuating drive 23. Further, the cathode gas supply valve 11 is preferably configured such that the valve member 30 is driven to open by an overpressure at the valve outlet 28 opposite the valve inlet 29.The control device 15 is expediently configured such that it monitors the opening and closing of the cathode-side valves, that is to say of the cathode gas feed valve 11 and of the cathode gas discharge valve 14. Further, the controller 15 is configured to identify the cathode gas supply valve 11 as a defective valve when the cathode gas supply valve 11 remains closed although the cathode gas supply valve 11 is driven to open by the controller 15. Additionally or alternatively, the control device 15 can be configured such that it identifies the cathode gas discharge valve 14 as a faulty valve if the cathode gas discharge valve 14 remains closed, even though the cathode gas discharge valve 14 is controlled to open by the control device 15. For example, the cathode gas supply valve 11 may have a sensor 31 which is coupled to the control device 15 via a corresponding signal line 32. Here, the sensor 31 is suitably configured to monitor the opening and closing of the cathode gas supply valve 11 and to communicate the current state of the cathode gas supply valve 11 to the controller 15. The cathode gas discharge valve 14 can expediently also have a sensor 33, which is likewise coupled to the control device 15 via a signal line 32. Furthermore, this sensor 33 can also be configured such that it monitors the opening and closing of the cathode gas discharge valve 14 and transmits the current state of the cathode gas discharge valve 14 to the control device 15.A cathode-side valve that properly opens when commanded to open is identified as a non-defective valve.According to FIGS. 4 and 5, the operating method 17 comprises the steps explained in more detail below with reference to the flowcharts of FIGS. 4 and 5. The operating method 17 presented here is used when starting the fuel cell system 1 and is carried out by the control device 15. The operating method 17 represents only a part at the beginning of a starting process for starting up or for starting up the fuel cell system 1.According to FIGS. 4 and 5, the operating method 17 begins in a field 34, which represents a start of the switch-on process or a start procedure. When the fuel cell system 1 is turned off, the anode gas supply valve 8, the cathode gas supply valve 11 and the cathode gas discharge valve 14 are closed. The fuel cell 2 is thus hermetically separated from an environment in order to avoid emission of anode gas, in particular of hydrogen, into the environment. The cathode gas supply valve 11 and the cathode gas discharge valve 14 may also be referred to as cathode-side valves 11, 14 for short.After the start in step 34, a step 35 follows in which, in order to switch on the fuel cell system 1, the closed cathode gas supply valve 11 and the closed cathode gas discharge valve 14 are controlled to open. In a subsequent step 36, it is checked whether both cathode-side valves 11, 14 can be opened. If both cathode-side valves 11, 14 can be opened, the interrogation ends positively, which is symbolized by (+) in FIGS. 4 and 5. In this case, the method follows a path 37 and reaches a field 38 in which the operating method 17 presented here ends and a conventional method for starting the fuel cell system 1 begins. In other words, starting from the field 38, the city procedure is continued in the usual manner in order to put the fuel cell system 1 into operation.If, on the other hand, the query in step 36 is negative, at least one of the cathode-side valves 11, 14 cannot be opened. In the simplified example of FIG. 4, it is assumed that only one of the two cathode-side valves 11, 14 cannot be opened and thus forms a faulty valve, while the other cathode-side valve 11, 14 can be opened and thus forms a faulty valve.A more complex example is explained below with reference to FIG. 5, in which both cathode-side valves 11, 14 cannot be opened and each form a faulty valve. The two examples otherwise correspond.If the result of the query in step 36 is now negative, which is indicated by (-) in FIGS. 4 and 5, at least one of the cathode-side valves 11, 14 cannot accordingly be opened. If it is clear that this is only one of the two cathode-side valves 11, 14, the method follows a path 39, which leads to a step 40, in which the defective valve and the defect-free valve are determined by the two cathode-side valves 11, 14. In the example of FIGS. 1 to 3, the cathode gas discharge valve 14 is frozen, which is indicated in FIGS. 1 and 2 by a snowflake symbol 41. Accordingly, when the fuel cell system 1 is switched on, the cathode gas discharge valve 14 cannot be opened here, so that the cathode gas discharge valve 14 forms the defective valve here. In contrast, it opens. In the example, the cathode gas supply valve 11 so that it forms the defect-free valve.After the defective valve 14 and the defective valve 11 have been determined in step 40, according to FIGS. 4 and 5, in a subsequent step 42 the defective valve 14 is still controlled to open, while the defective valve 11 is controlled to close. In addition, in a step 43, the anode gas supply valve 8 is driven to open. Anode gas is thereby supplied to the anode space 3. This state is shown in FIG. 1. An arrow 53 represents the anode gas feed to the anode space 3. The anode gas thus reaches the anode space 3.According to FIGS. 4 and 5, step 43 is followed by a step 44 in which it is queried whether the faulty valve 14 is already open. If this query is positive, a path 45 leads again to the field 38, after which the operating method 17 presented here ends and a conventional starting procedure for starting up the fuel cell system 1 begins or continues.If, on the other hand, the query in step 44 is negative, the operating method 17 follows a path 46, which results in a further query in a step 47. In this step 47, it is checked whether an opening pressure building up in the cathode chamber 4 exceeds a predetermined opening limit pressure. If the query is positive, the method follows a path 48 and reaches a field 49, which causes the starting procedure to be aborted. The fuel cell system 1 cannot then be started.If, on the other hand, the result of the determination in step 47 is negative, the opening pressure in the cathode chamber 4 is still below the opening limit pressure. Then, the method can follow a path 50 which returns to step 43 so that anode gas is still supplied to the anode space 3, as a result of which ultimately the pressure in the cathode space 4 can be further increased.The pressure in the cathode chamber 4 is increased until either the faulty valve 14 opens or until the predetermined opening limit pressure is reached. If the faulty valve 14 opens, the operating method 17 ends in field 38 so that the starting procedure can be continued and the fuel cell system 1 can be put into operation. If, on the other hand, the predetermined opening limit pressure is exceeded, the operating method 17 presented here ends in the field 49, which results in the starting procedure being ended and the fuel cell system 1 not being able to be started.In FIG. 2, it is shown how the anode gas supplied to the anode compartment 3 diffuses through the electrolyte membrane 5 and thus enters the cathode compartment 4, whereby the pressure gradually increases in the cathode compartment 4. The opening pressure which builds up in the cathode chamber 4 assists the actuating drive 24 in opening the faulty valve 14 which is still controlled to open. In the state of FIG. 2, the opening pressure which builds up in the cathode chamber 4 is not yet sufficient to bring about the opening of the faulty valve 14.In the state of FIG. 3, on the other hand, such a high opening pressure is achieved in the cathode chamber 4 that it can support the actuating drive 24 to such an extent that the faulty valve 14 can now be opened. In FIG. 3, an arrow 52 indicates the outflow of anode gas from the cathode chamber 4 through the cathode gas discharge valve 14 that is now successfully opened.In FIGS. 2 and 3, an arrow 51 indicates the diffusion movement of the anode gas through the electrolyte membrane 5. In FIGS. 1 and 2, an arrow 53 indicates the anode gas supply to the anode space 3.The anode gas is stored in the anode gas tank 18 under a storage pressure which is greater than an operating pressure with which the anode space 3 is supplied with anode gas during normal operation of the fuel cell system 1. The opening pressure generated in the cathode chamber 4 can be greater than the operating pressure and is at any rate less than the storage pressure. In any case, the predetermined opening limit pressure can be selected to be greater than the operating pressure. The opening limit pressure cannot be selected to be arbitrarily high, since depending on the design of the respective valve, the opening of the faulty valve can take place abruptly, as a result of which the opening pressure in the cathode chamber 4 can be abruptly reduced, as a result of which a comparatively large differential pressure between anode chamber 3 and cathode chamber 4 can abruptly bear against the electrolyte membrane 5. This differential pressure must not exceed a predetermined critical differential pressure in order to avoid damage to the electrolyte membrane 5 when the respective faulty valve 14 is opened due to pressure. It can expediently be provided that the respective faulty cathode-side valve 11, 14 does not open completely abruptly, but initially opens only slightly or opens only slowly during the pressure-induced opening. Here too, it can be provided that the respective cathode-side valve 11, 14 is configured as a proportional valve. Thus, when the faulty valve is opened with pressure assistance, a strong, rapid pressure drop in the cathode chamber 4 can be avoided, whereby the critical pressure difference at the electrolyte membrane can be avoided.Likewise, the pressure build-up in the anode chamber 3 takes place slowly by opening the anode gas feed valve 8, in such a way that sufficient anode gas can always diffuse through the electrolyte membrane 5 into the cathode chamber 4. In particular, the pressure build-up in the anode chamber 3 takes place so slowly that the differential pressure which builds up at the electrolyte membrane 5 between the anode chamber 3 and the cathode chamber 4 remains below the predetermined critical differential pressure.The pressure build-up in the anode chamber 3 and thus in the cathode chamber 4 takes place only until the opening pressure in the cathode chamber 4 is sufficient to open the faulty valve 14 or until the predetermined opening limit pressure is reached.The method described above is realized in a corresponding manner also in the case where, when the fuel cell system 1 is switched on, the cathode gas supply valve 11 turns out to be a faulty valve, while the cathode gas discharge valve 14 forms a faulty valve.In the following, a modification of the operating method from FIG. 4 will be explained with reference to FIG. 5, which takes into account the rare case in which both cathode-side valves 11, 14 are simultaneously frozen and cannot be opened.In step 36, it is queried whether both cathode-side valves 11, 14 open. If this is the case, method 17 follows path 37 and reaches the positive end of method 17 according to field 38, so that the starting procedure can be continued and fuel cell system 1 can be put into operation. If, on the other hand, the query in step 36 is negative, the path 39 according to FIG. 5 leads to a further query in a step 54; there, a query is made as to whether both cathode-side valves 11, 14 are closed. If the answer to the query of step 54 is negative, it is clear that one of the two cathode-side valves 11, 14 is closed and one of the two cathode-side valves 11, 14 is open. A path 55 then leads the method 17 to step 40, so that the method 17 can continue to run as described above with reference to FIG. 4. If, on the other hand, the answer to the query of step 54 is positive, it is clear that both cathode-side valves 11, 14 are closed. In this case, the method 17 follows a path 56, which leads to a step 40'. Starting here, the method 17 follows a secondary branch which is very similar to the main branch already described, so that similar steps are provided with the same reference numerals, but are identified by a prime point ('). Accordingly, in step 40' both cathode-side valves 11, 14 are identified as faulty valves. In the subsequent step 42', the two faulty valves 11, 14 are controlled to open. In step 43', the anode gas supply valve 8 is slowly opened to increase the pressure in the cathode space 4 and to build up the opening pressure. In the interrogation 44', it is checked whether at least one of the faulty valves 11, 14 is already open. If so, a path 57 leads back to the field 35. There, both cathode-side valves 11, 14 are again controlled to open.If, on the other hand, the query 44' is negative, the method follows the path 46', so that a query is made thereafter as to whether the current opening pressure exceeds the opening limit pressure. If this is the case, the method 17 follows the path 48' and reaches the field 49, in which the starting procedure is terminated. If, on the other hand, this query 44' is negative, the method follows the path 50' and the anode gas supply valve 8 is opened further. The pressure is thus also increased in this minor point until at least one of the faulty valves 11, 14 opens or until the opening limit pressure is exceeded in the cathode chamber 4.If both faulty valves 11, 14 open quasi simultaneously, query 44' is positive and method 17 returns to step 35. In the subsequent step 36, both cathode-side valves 11, 14 are then also open, so that the method 17 reaches the positive end in field 38.If only one of the two faulty valves 11, 14 opens, the query 44' is also positive and the method 17 returns to step 35. In the subsequent step 36, not both cathode-side valves 11, 14 are open and in the subsequent step 54, not both cathode-side valves 11, 14 are closed, so that the method 17 returns to the main branch according to the path 55 and continues as described above with reference to FIG. 4.If, on the other hand, both faulty valves 11, 14 do not open, the opening limit pressure is exceeded at some point, so that method 17 follows path 48' and the starting procedure is terminated in field 49. The fuel cell system 1 cannot then be switched on.

Claims

Operating method (17) for a fuel cell system (1), - wherein the fuel cell system (1) comprises at least the following components: - a fuel cell (2) which comprises an anode space (3), a cathode space (4) and an electrolyte membrane (5) which separates the anode space (3) from the cathode space (4), - an anode gas feed line (6) for feeding anode gas to the anode space (4) which is fluidically connected to an anode gas inlet (7) of the fuel cell (2) and in which an anode gas feed valve (8) for controlling the anode gas feed line (6) is arranged, - a cathode gas feed line (9) for feeding cathode gas which is fluidically connected to a cathode gas inlet (10) of the fuel cell (2) and in which a cathode gas feed valve (11) for controlling the cathode gas feed line (9) is arranged, a cathode gas discharge line (12) for discharging cathode gas, which is fluidically connected to a cathode gas outlet (13) of the fuel cell (2) and in which a cathode gas discharge valve (14) for controlling the cathode gas discharge line (12) is arranged, - wherein the operating method (17) has at least the following steps: - for switching on the fuel cell system (1), the closed cathode gas feed valve (11) and the closed cathode gas discharge valve (14) are controlled to open, - in the case that the one of the cathode gas feed valve (11) and the cathode gas discharge valve (14) cannot be opened and thereby forms a faulty valve, the other of the cathode gas feed valve (11) and the cathode gas discharge valve (14), which opens and thereby forms a faulty valve, is controlled to close again, after the fault-free valve is closed, the faulty valve is actuated again or further to open and the anode gas feed valve (8) is likewise actuated to open, after the fault-free valve is closed and the anode gas feed valve (8) is opened, an opening pressure is allowed to build up in the cathode chamber (4), which assists the opening of the fault-type valve which is also actuated to open.Operating method (17) according to Claim 1, characterized - in that the anode gas feed line (6) fluidically connects an anode gas tank (18) for storing the anode gas to the anode gas inlet (7) which leads to the anode space (3), - in that the anode gas is stored in the anode gas tank (18) in liquid and / or gaseous form under a storage pressure which is greater than an operating pressure with which the anode space (3) is supplied with anode gas during normal operation of the fuel cell system (1).Operating method (17) according to Claim 2, characterized - in that the opening pressure is greater than the operating pressure and is less than the accumulator pressure.Operating method (17) according to one of the preceding claims, characterized - in that after the fault-free valve has been closed, the anode gas feed valve (8) is controlled to open in such a way that the pressure in the anode space (3) builds up so slowly that sufficient anode gas can diffuse through the electrolyte membrane (5) into the cathode space (4) so that a differential pressure building up at the electrolyte membrane (5) between the anode space (3) and the cathode space (4) remains below a predetermined critical differential pressure.Operating method (17) according to one of the preceding claims, characterized - in that, when the faulty valve is closed and controlled to open, after the fault-free valve is closed and the anode gas feed valve (8) is opened, the pressure in the cathode space (4) is increased only until the opening pressure is sufficient to open the faulty valve controlled to open or until a predetermined opening limit pressure is reached.Operating method (17) according to one of the preceding claims, characterized - in that, in the event that the cathode gas feed valve (11) and the cathode gas discharge valve (14) cannot be opened and thereby each form a faulty valve, the anode gas feed valve (8) is controlled to open and the two faulty valves are controlled to open again or further.Operating method (17) according to Claim 6, characterized - in the event that the one faulty valve opens, the open faulty valve now forms a fault-free valve and is actuated for closing.A fuel cell system (1), comprising: a fuel cell (2) having an anode compartment (3), a cathode compartment (4) and an electrolyte membrane (5) separating the anode compartment (3) from the cathode compartment (4); an anode gas supply line (6) for supplying anode gas to the anode compartment (3) and fluidly connected to an anode gas inlet (7) of the fuel cell (2) and in which an anode gas supply valve (8) for controlling the anode gas supply line (6) is disposed; a cathode gas supply line (9) for supplying cathode gas to the cathode compartment (4) and fluidly connected to a cathode gas inlet (10) of the fuel cell (2) and in which a cathode gas supply valve (11) for controlling the cathode gas supply line (9) is disposed, a cathode gas discharge line (12) for discharging cathode gas from the cathode space (4), which is fluidically connected to a cathode gas outlet (13) of the fuel cell (2) and in which a cathode gas discharge valve (14) for controlling the cathode gas discharge line (12) is arranged, a control device (15) which is coupled to the anode gas feed valve (8), to the cathode gas feed valve (11) and to the cathode gas discharge valve (14) and which is configured such that the control device (15) for switching on the fuel cell system (1) carries out the operating method (17) according to one of the preceding claims.Fuel cell system (1) according to Claim 8, characterized - in that the anode gas feed line (8) fluidically connects an anode gas tank (18) for storing the anode gas to the anode gas inlet (7) of the fuel cell (2) leading to the anode space (3).Fuel cell system (1) according to Claim 8 or 9, characterized - in that a recirculation line (19) fluidically connects an anode gas outlet (20) of the fuel cell (2) to the anode gas feed line (6), - in that a connection point (21), in which the recirculation line (19) is fluidically connected to the anode gas feed line (6), is arranged between the anode gas feed valve (8) and the anode gas inlet (7).Fuel cell system (1) according to one of Claims 8 to 10, characterized - in that the anode gas feed valve (8) has an electromotive actuating drive (22), which is coupled to the control device (15), for opening and closing the anode gas feed valve (8), such that the control device (15) actuates the associated actuating drive (22) for opening and closing the anode gas feed valve (8), and / or - in that the cathode gas feed valve (11) has an electromotive actuating drive (23), which is coupled to the control device (15), for opening and closing the cathode gas feed valve (11), such that the control device (15) actuates the associated actuating drive (23) for opening and closing the cathode gas feed valve (11), and / or - the cathode gas discharge valve (14) has an electromotive actuating drive (24), coupled to the control device (15), for opening and closing the cathode gas discharge valve (14), so that the control device (15) actuates the associated actuating drive (24) for opening and closing the cathode gas discharge valve (14).Fuel cell system (1) according to one of Claims 8 to 11, characterized - in that the cathode gas discharge valve (14) has a valve inlet (25) facing the cathode space (4), a valve outlet (26) facing away from the cathode space (4) and a valve member (27) arranged between the valve inlet (25) and the valve outlet (26), wherein the cathode gas discharge valve (14) is configured such that the valve member (27) is driven to open by an overpressure at the valve inlet (25) with respect to the valve outlet (26), and / or - in that the cathode gas feed valve (11) has a valve outlet (28) facing towards the cathode space (4), a valve inlet (29) facing away from the cathode space (4) and a valve member (30) arranged between the valve inlet (29) and the valve outlet (28), wherein the cathode gas feed valve (11) is configured such that, the valve member (30) is driven to open by an overpressure at the valve outlet (28) relative to the valve inlet (29).Fuel cell system (1) according to any of claims 8 to 12, characterized in - that the control device (15) is configured to monitor the opening and closing of the cathode gas supply valve (11) and the cathode gas discharge valve (14), - that the control device (15) is further configured to identify the cathode gas supply valve (11) as a faulty valve if the cathode gas supply valve (11) remains closed although the cathode gas supply valve (11) is driven to open, and / or to identify the cathode gas discharge valve (14) as a faulty valve if the cathode gas discharge valve (14) remains closed although the cathode gas discharge valve (14) is driven to open.Fuel cell system (1) according to claim 13, characterised in - that the cathode gas supply valve (11) has a sensor (31) which is coupled to the control device (15) and is configured to monitor the opening and closing of the cathode gas supply valve (11) and to communicate the current state of the cathode gas supply valve (11) to the control device (15), and / or - that the cathode gas discharge valve (14) has a sensor (33) which is coupled to the control device (15) and is configured to monitor the opening and closing of the cathode gas discharge valve (14) and to communicate the current state of the cathode gas discharge valve (14) to the control device (15).

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