Method for operating a fuel cell system

Abstract

Method for operating a fuel cell system (1) which is started up after being switched off and in which, during the start-up phase, a fuel is passed through the anode chamber (4) of a fuel cell (2) which is at least partially filled with air, and an anode gas mixture escaping from the anode chamber (4) during the start-up phase is at least partially returned to the anode chamber (4) in a recirculation device (9) of the fuel cell system (1), wherein the amount of fuel supplied from a fuel storage device (6) to the anode compartment (4) is varied at least temporarily during the start-up phase, wherein the amount of fuel supplied from the fuel storage device (6) to the anode compartment (4) is continuously increased at least temporarily during the start-up phase, wherein the anode gas mixture guided in the recirculation device (9) is at least temporarily discharged from the recirculation device (9) via a drain device (14, 15) during the start-up phase, wherein the anode gas mixture discharged from the recirculation device (9) via the drain device (14, 15) is supplied to the cathode chamber (3) of the fuel cell.

Description

[0001] The invention relates to a method for operating a fuel cell system that is started up after shutdown. During the start-up phase, a fuel is passed through the anode chamber of the fuel cell system, which is at least partially filled with air. An anode gas mixture escaping from the anode probe during the start-up phase is at least partially recirculated into the anode chamber in a recirculation circuit of the fuel cell system.

[0002] Such a method is known from DE 102 97 626 T5. Recirculation of the anode flow field exhaust gas through the recycling circuit is triggered when this anode flow field gas is 100% air. During the start-up phase, an anode exhaust gas vent valve is at least partially open, and a fan is switched on to begin recirculating the anode flow field exhaust gases through the recycling circuit. A fuel flow valve, which is arranged in a supply line between a hydrogen source and the anode, is set to a fixed open state during the start-up phase, so that only a permanent, continuous flow of hydrogen-containing fuel can flow from the hydrogen source to the anode. In a further embodiment, it is provided that the hydrogen and the anode exhaust gases necessarily flow through a plurality of burners during recirculation.

[0003] DE 10 2004 037 097 A1 further discloses a method for operating a fuel cell system, in which, during a start-up phase, the amount of fuel to be supplied to the fuel cell system is added, taking into account the amount of fuel still stored in the fuel cell, to avoid a fuel shortage in the fuel cell. Thus, during start-up, the amount of fuel still present in the anode is determined, and depending on this, a remaining amount is constantly added, so that the total amount of fuel is determined by the sum of the amount of fuel still present in the fuel cell and the remaining amount supplied.

[0004] WO 2006 / 134461 A1 discloses a fuel cell comprising an anode, a cathode, and a proton exchange membrane arranged between the anode and the cathode. An anode exhaust gas discharged from an anode outlet is recirculated to an anode inlet. Hydrogen is added to the anode exhaust gas before the anode exhaust gas reaches the inlet. In one method for starting the fuel cell, a purge valve at the outlet of the anode line is closed, recirculation is activated in an anode recirculation loop, and hydrogen is metered to remove oxygen from the anode line.

[0005] From DE 60 2005 002 407 T2, it is known to increase the anode operating pressure during fuel cell activation, similar to a conventional fuel cell system. Furthermore, when the exhaust valve is opened and the nitrogen gas and the mixed gas remaining in the anode are discharged, the fuel gas is discharged together with the nitrogen gas and the mixed gas, and the amount of fuel gas required to activate the fuel cell is correspondingly increased. Furthermore, the fuel gas is discharged together with the nitrogen gas and the mixed gas, and dilution equipment or combustion equipment is required to treat the fuel gas, making it difficult to simplify the configuration of the fuel cell system.

[0006] CA 2 629 627 A1 discloses a fuel cell system with a controller that estimates the air exchange state of a fuel electrode and a hydrogen circulation path while the operation of the fuel cell system is stopped. When the fuel cell system is started, the control unit changes the order in which the operation of a hydrogen circulation pump is started and a hydrogen pressure regulator is opened to start the supply of hydrogen gas based on the estimated air exchange state, thereby preventing deterioration due to an uneven distribution of air and hydrogen in the fuel electrode.

[0007] With existing processes, the problem of hydrogen-oxygen gradient formation in the anode can only be partially avoided. When starting up a fuel cell, the problem still exists that a hydrogen mass flow fed into the anode displaces the oxygen still present there. The resulting hydrogen-oxygen gradient is the main cause of premature aging of fuel cells. The fuel cell catalyst, in particular, is damaged by excessively high hydrogen-oxygen gradients along a channel within a fuel cell.

[0008] It is an object of the present invention to provide a method for operating a fuel cell system in which the damage to a fuel cell due to an excessively high fuel-oxidant gradient can at least be reduced.

[0009] This object is achieved by a method having the features of claim 1.

[0010] In a method according to the invention for operating a fuel cell system, a switched off fuel cell is restarted. During the start-up phase, a fuel is passed through the anode chamber of the fuel cell system, which is at least partially filled with air. During the start-up phase, an anode gas mixture escaping from the anode chamber is at least partially recirculated back into the anode chamber in a recirculation circuit of the fuel cell system. The amount of fuel fed from a fuel reservoir to the anode chamber is varied at least temporarily during the start-up phase. This means that during the start-up phase, at least temporarily, there is no continuous supply of fuel from the fuel reservoir to the anode chamber, but that this supply is varied in quantity at least once during a supply process.

[0011] This approach can significantly reduce the fuel-oxidant gradient in the fuel cell. This can at least significantly limit a damage mechanism of the fuel cell that causes rapid aging. Furthermore, the start-up process can be accelerated by varying the dosage of fuel flow from the fuel storage to the anode compartment during the start-up phase. Starting processes of the fuel cell system, such as stop-start processes, no longer have a negative impact on the service life of the fuel cell system. Furthermore, simplified and safe shutdown scenarios for the fuel cell system can be implemented. For example, it is no longer necessary for the anode to be hermetically sealed.In fact, even residual hydrogen in the anode volume can be consumed so that the fuel cell system can be shut down or parked without hydrogen overpressure.

[0012] The varying dosage of the supplied fuel enables a very precise and optimized addition of the fuel quantity at the momentary point in time. This ensures particularly efficient prevention of an excessive fuel-oxidant gradient along a channel within the fuel cell.

[0013] The amount of fuel supplied from the fuel reservoir to the anode chamber is increased at least temporarily during the start-up phase. In particular, the amount of fuel supplied from the fuel reservoir to the anode chamber is continuously and steadily changed at least temporarily during the start-up phase. This means that, at least temporarily during the start-up phase, the amount of fuel supplied from the fuel reservoir to the anode chamber is continuously increased. This ensures a particularly advantageous procedure, on the one hand with regard to the fastest possible start-up of the fuel cell and, on the other hand, to avoid a damaging fuel-oxidant gradient. Preferably, the mass flow flowing through a valve is continuously increased as the start-up phase progresses by opening the valve further and further as the start-up phase progresses.

[0014] The anode gas mixture in the recirculation circuit is partially discharged from the recirculation circuit, at least temporarily during the start-up phase, via a discharge device coupled to the recirculation circuit. This procedure further improves the concentration of fuel and oxidizer that is currently required or to be adjusted during a start-up phase.

[0015] The anode gas mixture discharged from the recirculation circuit via the discharge device is fed to the cathode chamber of the fuel cell system. This supply can be provided either at the inlet of the cathode chamber or at the outlet of the cathode chamber.

[0016] Preferably, the duration of the discharge via the discharge device and / or the amount of anode gas mixture to be discharged via the discharge device is determined depending on the fuel concentration and / or the oxidant concentration in the anode chamber. This allows for particularly fine and precise adjustment of the amount of anode gas mixture supplied.

[0017] A fan is preferably arranged in the recirculation circuit of the fuel cell system, which is operated at least temporarily in a specific recirculation mode during the start-up phase to return the anode gas mixture to the anode chamber. This procedure allows the recirculation of the anode gas mixture to be carried out very specifically and adapted to the current situation. In addition to varying the mass flow of the fuel supplied from the fuel storage tank to the anode chamber, the situation-dependent adjustment of the fuel concentrations and the oxidant concentrations can be further refined.

[0018] Preferably, in the fan recirculation mode, the fan speed is set to be higher than the speed set during normal operation of the fuel cell system after the start-up phase. This increased speed can accelerate the recirculation of the anode gas mixture, thus increasing the fuel concentration in the anode chamber and reducing the oxidant concentration in the anode chamber particularly quickly, without creating an excessively high fuel-oxidant gradient.

[0019] Particularly preferably, a maximum fan speed is set in the return mode. The fan is thus operated at full load at least temporarily during the start-up phase. Preferably, the fan speed is set essentially constant for the entire period in the return mode. However, it can also be provided that the speed is varied at least once in the return mode during the start-up phase of the fuel cell system. These procedures, in turn, allow the most effective operating mode to be selected and implemented depending on the situation.

[0020] By continuously increasing the hydrogen concentration during a sufficiently high recirculation rate and resulting mixing of the anode gas mixture, harmfully high fuel-oxidant gradients can be prevented.

[0021] Particularly when the fuel is hydrogen and the oxidant is oxygen, a harmful high hydrogen-oxygen gradient can be avoided. The start-up or startup of the fuel cell system is therefore characterized in particular by the fact that there is no abrupt, continuous supply of hydrogen, but rather that during the start-up phase there is a gradual increase in the addition of hydrogen from the fuel storage, at least temporarily. During this time, a high recirculation rate of the anode volume must be ensured, which in particular ensures that the anode gas mixture is at least partially returned to the anode compartment. Once the oxidant concentration in the anode compartment has been reduced and is close to zero percent, the start-up phase is complete. The start-up process can be further accelerated by deliberately opening the recirculation circuit via the vent device.

[0022] An embodiment of the invention is explained in more detail below with reference to schematic drawings. In the drawings: Fig. 1 a schematic representation of subcomponents of a fuel cell system with which the method according to the invention is carried out; Fig. 2 a diagram showing the speed of the fan arranged in the recirculation circuit as a function of time; and Fig. 3 a diagram showing the concentration of fuel and oxidizer as a function of time.

[0023] In Fig. 1 shows a schematic representation of a fuel cell system 1, which only shows those components sufficient for understanding the invention. The fuel cell system 1 comprises at least one fuel cell 2. Preferably, a fuel cell stack with a plurality of fuel cells 2 is provided. The fuel cell 2 comprises an anode chamber 4 and a cathode chamber 3 separated therefrom by a membrane 5. In the exemplary embodiment, the fuel cell 2 is designed as a PEM fuel cell. However, this specific embodiment is not to be understood as limiting the present invention. Furthermore, the fuel cell system 1 is intended as a mobile system and arranged in a vehicle.

[0024] The fuel cell system 1 comprises a fuel reservoir 6, which is connected to the anode chamber 4 via a supply line 7. Hydrogen or a hydrogen-containing gas is contained in the fuel reservoir 6 as fuel or can be generated therein. An element 8 for changing the flow cross-section of the supply line 7 is arranged in the supply line 7. In the exemplary embodiment, the element 8 is a valve, which is referred to below as valve 8.

[0025] Furthermore, the fuel cell system 1 comprises a recirculation device 9. This has a recirculation line 10, which extends from the outlet of the anode chamber 4 to an inlet 11. The recirculation line 10 thus opens into the supply line 7 between the valve 8 and the inlet of the anode chamber 4. A fan 12, which is connected to the recirculation line 10 and driven by a motor 13, is arranged in the device 9. Between the fan 12 and the outlet 11, a line 14 branches off from the recirculation line 10. A further element 15 is arranged in this line 14, which is designed to change the flow cross-section of the line 14. In the exemplary embodiment, the element 15 is designed as a valve, which is referred to below as valve 15.The line 14 and the valve 15 are assigned to a discharge device for discharging the anode gas mixture led from the anode chamber 4 via the recirculation device 9.

[0026] Preferably, the line 14 is directed to the cathode chamber 3 of the fuel cell 2. The portion of the anode gas mixture that is conducted from the recirculation line 10 via the discharge device 14, 15 is thus directed into the cathode chamber 3.

[0027] It can also be provided that the discharge device 14, 15 opens into the environment or into an exhaust air path leading away from the outlet of the cathode chamber 3.

[0028] In particular, the fuel cell system 1 also has a control unit (not shown) designed to control the valve 8. Preferably, this control unit is also designed to control the valve 15 and the motor 13 in order to be able to adjust the speed of the fan 12.

[0029] In addition, the fuel cell system 1 comprises a sensor system (not shown) in order to detect the fuel concentration and the oxidant concentration in the fuel cell 2.

[0030] The operation of the fuel cell system 1 is explained in more detail below. After the fuel cell system 1 is shut down, it is subsequently started and then initially enters a startup phase. In the shut-down state, an oxygen concentration has accumulated in the anode chamber 4, so that a subsequent abrupt addition of hydrogen fuel would create a high hydrogen-oxygen gradient, which could damage the fuel cell 2.

[0031] Therefore, after starting the fuel cell system 1 in the start-up phase, the hydrogen contained in the fuel reservoir 6 is at least temporarily supplied in a changed amount via the supply line 7 to the anode chamber 4. In the exemplary embodiment, it is provided that the valve 8 is controlled by the control unit such that the mass flow flowing through the valve 8 is continuously increased as the start-up phase progresses. For this purpose, the valve 8 is continuously opened wider as the start-up phase progresses. Thus, at least temporarily during the start-up phase, the amount of hydrogen supplied from the fuel reservoir 6 to the anode chamber 4 is continuously increased, and the dosage is continuously changed.

[0032] This is shown in the diagram according to Fig. 3. The diagram shows the course of the oxygen concentration and the hydrogen concentration in the anode chamber 4.

[0033] At the beginning of a start-up phase at time t0, the hydrogen concentration is zero. As time progresses, the hydrogen concentration in the anode chamber 4 increases steadily from time t2 onward.

[0034] This is achieved in particular by the fact that the valve 8 is continuously opened further and further, thereby continuously increasing the hydrogen mass flow flowing from the fuel storage 6 to the anode chamber 4.

[0035] In addition, according to the diagram in Fig.2, which shows the speed of the blower 12 as a function of time, the blower 12 is started at time t0. In the exemplary embodiment, the blower 12 is operated in a recirculation mode during the start-up phase, in that the anode gas mixture located in the anode chamber 4 is returned to the inlet of the anode chamber 4 via the return line 10 of the recirculation circuit. For this purpose, in the exemplary embodiment, full-load operation is set in the return mode of the blower 12, whereby the maximum speed D1 is set for this purpose. This maximum speed D1 is maintained constant until a time t1, at which the start-up phase is ended and the oxygen concentration in the anode chamber 4 is essentially zero percent. After the start-up phase has ended, the blower 12 is controlled via the control unit such that the speed is reduced from the maximum speed D1 to a lower speed D2.This speed D2 characterizes the normal operation of the fan 12 during operation of the fuel cell system 1 following the start-up phase.

[0036] It can also be provided that a lower speed than the maximum speed D1 is set during the start-up phase. However, it is preferably provided that the speed D1 of the fan 12 set during the start-up phase and thus in the return mode is greater than the speed D2 set during normal operation. It can also be provided that the speed D1 in the return mode of the fan 12 is not constant over the entire period, but is changed at least temporarily during the start-up phase.

[0037] Preferably, the time period between the times t0 and t2 is minimal, since immediately after starting the fuel cell system 1, hydrogen is supplied from the fuel storage 6 via the valve 8 to the anode chamber 4.

[0038] At time t1, the maximum hydrogen concentration in the anode chamber 4 is reached, which is then essentially maintained thereafter.

[0039] The start-up process of the fuel cell system 1 is initiated by recirculating the volume in the anode chamber 4. A recirculation rate comparatively high to normal operation is set by operating the fan 12 in the specific recirculation mode. Simultaneously or staggered with the start of recirculation, the continuously increasing amount of hydrogen is added from the fuel storage 6. This also achieves a correspondingly slower pressure buildup on the anode side. This prevents the formation of undesirable high hydrogen-oxygen gradients in the anode chamber 4.

[0040] The amount of anode gas mixture supplied via inlet 11 can also be varied. This can be achieved, on the one hand, by controlling the speed of the fan 12 and, on the other hand, by discharging a portion of the anode gas mixture via line 14 and valve 15.

[0041] The duration of the discharge and / or the amount of anode gas mixture to be discharged via line 14 and valve 15 is preferably determined depending on the hydrogen concentration measured by a suitable sensor and / or the oxygen concentration in the anode chamber 4. This allows the start-up behavior to be optimally adjusted and, moreover, accelerated.

[0042] It can be provided that the supply of fuel from the fuel storage 6 to the anode chamber 4 is started simultaneously with the return of the anode gas mixture via the recirculation device 9 to the anode chamber 4. However, a time-delayed start of these two processes can also be provided, and the fuel supply from the fuel storage 6 to the anode chamber 4 can be started earlier or later than the return of the anode gas mixture via the recirculation circuit.

[0043] Through the preferred interaction of the medium supply to the anode chamber from the fuel reservoir 6 and the recirculation device 9, with coordinated, variable addition of the respective quantities, an undesirable fuel-oxidant gradient can be avoided. This can be achieved even more precisely, particularly in further interaction with the discharge devices 14, 15.

Claims

[1] Method for operating a fuel cell system (1) which is started up after being switched off and in which, in the start-up phase, a fuel is passed through the anode chamber (4) of a fuel cell (2) which is at least partially filled with air, and an anode gas mixture emerging from the anode chamber (4) during the start-up phase is at least partially passed back into the anode chamber (4) in a recirculation device (9) of the fuel cell system (1), wherein the amount of fuel supplied from a fuel storage (6) to the anode chamber (4) is varied at least temporarily during the start-up phase, wherein the amount of fuel supplied from the fuel storage (6) to the anode chamber (4) is continuously increased at least temporarily during the start-up phase, wherein the anode gas mixture conducted in the recirculation device (9) is at least temporarily discharged from the recirculation device (9) via a discharge device (14, 15) during the start-up phase, wherein the anode gas mixture discharged from the recirculation device (9) via the discharge device (14, 15) is fed to the cathode chamber (3) of the fuel cell. [2] Method according to claim 1, wherein as the start-up phase progresses, a mass flow of fuel flowing through a valve (8) is continuously increased by continuously opening the valve (8) further and further as the start-up phase progresses. [3] Method according to one of the preceding claims, wherein a blower (12) is arranged in the recirculation device (9), which blower is operated at least temporarily in a specific recirculation mode in the start-up phase for returning the anode gas mixture to the anode chamber (4). [4] Method according to claim 3, wherein in the return mode a speed of the fan (12) is set which is greater than the speed set in the normal operation of the fuel cell system (1) set after the start-up phase. [5] Method according to claim 4, wherein in the return mode a maximum speed of the fan (12) is set. [6] Method according to one of the preceding claims, wherein the duration of the discharge and / or the quantity of the anode gas mixture to be discharged via the discharge device (14, 15) is determined as a function of the fuel concentration and / or the oxidizing agent concentration in the anode chamber (4).

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