DE102007048317A1 - Method for operating a fuel cell system - Google Patents

Abstract

The invention relates to a method for operating a fuel cell system (1), which is started after a shutdown and during which a fuel is passed through the at least partially filled with air anode space (4) of a fuel cell (2) in the start - up phase, and during the Starting phase from the anode chamber (4) exiting anode gas mixture in a recirculation device (9) of the fuel cell system (1) is at least partially re-directed into the anode compartment (4), wherein the from a fuel storage (6) to the anode compartment (4) fed amount of fuel during the starting phase is at least temporarily varied.

Description

The The invention relates to a method for operating a fuel cell system, which is approached after a shutdown and in which in the Starting phase a fuel through which at least partially with Air-filled anode compartment of the fuel cell system passed becomes. An anode gas mixture emerging from the anode probe during the start-up phase is in a recirculation circuit of the fuel cell system at least partially re-directed into the anode compartment.

Such a method is known from DE 102 97 626 T5 known. Recirculation through the anode flow field exhaust gas recycle loop is initiated when this anode flow field gas 100 Air is. During the start-up phase, an anode exhaust vent valve is at least partially opened with a fan turned on to begin recirculating the anode flow field exhaust gases through the recycle circuit. A fuel flow valve disposed in a supply line between a hydrogen source and the anode is set in an unchangeable opening state during the start-up phase so that only a continuous, continuous flow of hydrogen-containing fuel can flow from the hydrogen source to the anode. In a further embodiment, it is provided that during the recirculation, the hydrogen and the anode exhaust gases necessarily flow through a plurality of burners.

From the DE 10 2004 037 097 A1 Furthermore, a method for operating a fuel cell system is known in which, in a start-up phase to avoid a shortage of a fuel cell with fuel, the amount of fuel to be supplied is added to the fuel cell system, taking into account a content of fuel still stored in the fuel cell. It is thus determined when starting the still existing in the anode amount of fuel and depending on then a still missing residual amount constantly supplied, so that there is a total amount of fuel by the sum of the still present in the fuel cell amount of fuel and the remaining amount supplied.

at The known method can solve the problem of hydrogen-oxygen gradient formation occurring be avoided only conditionally in the anode. When starting up a fuel cell is thus still given the problem that one in the anode directed hydrogen mass flow the remaining oxygen there repressed. The resulting hydrogen-oxygen gradient is the main reason for the premature aging of fuel cells. In particular, the catalyst of the fuel cell is too high Hydrogen oxygen gradients along a channel within a fuel cell damaged.

It It is an object of the present invention to provide a method of operation a fuel cell system in which the damage a fuel cell due to an excessive fuel-oxidant gradient at least it can be reduced.

These The object is achieved by a method which has the features according to claim 1, solved.

at a method according to the invention for operating of a fuel cell system becomes a shutdown fuel cell started again. In the start-up phase, a fuel is through the at least proportionally filled with air anode space passed the fuel cell system. During the start-up phase is an emerging from the anode chamber anode gas mixture in a Recirculation of the fuel cell system at least partially returned back to the anode compartment. The one by one Fuel storage to the anode compartment fed fuel is in its amount during the start-up phase at least temporarily varied. This means that during the start-up phase, at least temporarily no continuous supply of fuel from the Fuel storage is done to the anode compartment, but that this feeding at least once during a feeding process is changed in quantity.

By this approach can significantly reduce a fuel-oxidant gradient be achieved in the fuel cell. This can be a damaging mechanism of Fuel cell, which causes rapid aging, at least be clearly contained. Through this while the start-up phase varying dosage of fuel quantity supply from the fuel storage to the anode compartment may moreover also the startup process can be accelerated. start-ups the fuel cell system, such as stop-start processes, This also does not affect adversely on the life of the fuel cell system. In addition, you can also simplified and safe parking scenarios of the fuel cell system will be realized. For example, it is no longer required that the anode is shut off hermetically. Rather, it can even a residual hydrogen is consumed in the anode volume, so that the fuel cell system turned off without hydrogen overpressure or can be parked.

The varying metering of the fuel supplied allows for a very precise and optimally tuned at the current times the amount of fuel. It particularly efficiently prevents the avoidance of too high a fuel-oxidant gradient along a channel within the fuel cell.

Preferably becomes the fuel supplied from the fuel storage to the anode compartment in its quantity during the start-up phase at least temporarily elevated.

Especially the gas supplied from the fuel storage to the anode compartment Fuel in its amount during the start-up phase, at least temporarily and continuously changed at times. Especially it is particularly advantageous if at least temporarily during the start-up phase of the fed from the fuel storage to the anode compartment Fuel in its amount is continuously increased. This ensures a particularly advantageous procedure on the one hand with a view to the fastest possible start the fuel cell and on the other hand to avoid a harmful fuel-Oxidationsmittelgradienten.

In the recirculation circuit of the fuel cell system is preferably arranged a fan, which in the start-up phase for Recycling the anode gas mixture into the anode compartment at least temporarily in a specific feedback mode is operated. This procedure becomes the return process the anode gas mixture very specific and on the current situation adapted carried out. In addition to the variation the mass flow of the fed from the fuel storage to the anode compartment Fuel can thus the situation-dependent adjustment of Concentrations of the fuel on the one hand and of the oxidizing agent on the other hand be refined again.

Preferably becomes in the return mode of the blower a Speed of the fan set, which is greater is the normal mode set after the start-up phase set the fuel cell system speed. Due to this increased speed can accelerate the recycling of the anode gas mixture and thereby increase of concentration of fuel in the anode compartment and reducing the concentration of the oxidizing agent be carried out particularly quickly in the anode compartment, without but that too high a fuel-oxidant gradient occurs would.

Especially it is preferably provided that in the return mode a maximum speed of the fan is set. The Blower is thus at least temporarily in the start-up phase operated at full load. It is preferably provided that in the return mode the speed of the fan essentially for the entire period of time is set substantially constant. However, it can also be provided that the speed in the return mode during the start-up phase of the fuel cell system at least is varied once. Through these procedures can turn Depending on the situation, the most effective mode of operation is selected and performed.

Preferably is the guided in the recirculation circuit anode gas mixture at least temporarily during the start-up period proportionally over a with the recirculation circuit coupled discharge device removed from the recirculation circuit. By this procedure will be another improvement with regard to during a start-up phase currently required or set concentration of the fuel and the oxidizing agent.

Preferably becomes the duration of the discharge via the discharge device and / or the amount of anode gas mixture to be discharged the discharge device depending on the fuel concentration and / or the oxidizer concentration in the anode compartment. This allows the vote to be taken with regard to the quantity of Anodengasgemisches particularly fine and accurate.

Preferably this is via the discharge from the recirculation circuit discharged anode gas mixture the cathode compartment of the fuel cell system fed. It can be provided that this feeder at the entrance of the cathode space or at the exit of the cathode space he follows.

It can also be provided that via the discharge device discharged from the recirculation circuit anode gas mixture into the environment or away from the cathode compartment Exhaust air line is supplied.

By continuously increasing the hydrogen concentration during a sufficiently high recirculation rate and the resulting mixing of the anode gas mixture harmful high fuel Oxidantmittelgradienten can be prevented. In particular, when the fuel is hydrogen and the oxidant is oxygen, a harmful high hydrogen-oxygen gradient can thus be avoided. The start-up or start-up of the fuel cell system is therefore characterized in particular by the fact that there is just no abrupt continuous supply of hydrogen, but that, at least temporarily, a gradual increase in the addition of hydrogen from the fuel reservoir occurs during the start-up phase. During this time should In particular, a high recirculation rate of the anode volume can be ensured, as a result of which in particular the recycling of the anode gas mixture takes place at least partially into the anode chamber. If the oxidant concentration in the anode compartment is then reduced and close to zero percent, the start-up phase is completed. The targeted opening of the recirculation circuit via the discharge device, the starting process can be additionally accelerated.

One Embodiment of the invention will be described below schematic drawings explained in more detail. there demonstrate:

1 a schematic representation of subcomponents of a fuel cell system, with which the inventive method is carried out;

2 a diagram in which the speed of the arranged in the recirculation fan is shown as a function of time; and

3 a diagram in which the concentration of the fuel and the oxidant as a function of time is shown.

In 1 is a schematic representation of a fuel cell system 1 which merely shows the components sufficient for the understanding of the invention. The fuel cell system 1 includes at least one fuel cell 2 , Preferably, a fuel cell stack is a plurality of fuel cells 2 intended. The fuel cell 2 includes an anode compartment 4 and one of them through a membrane 5 separated cathode compartment 3 , In the exemplary embodiment, the fuel cell 2 designed as a PEM fuel cell. However, this specific embodiment is not intended to be limiting of the present invention. In addition, the fuel cell system 1 provided as a mobile system and arranged in a vehicle.

The fuel cell system 1 includes a fuel storage 6 , which via a supply line 7 with the anode compartment 4 connected is. In the fuel storage 6 is as fuel, hydrogen or a hydrogen-containing gas or can be generated therein. In the supply line 7 is an element 8th for changing the flow cross section of the supply line 7 arranged. In the embodiment, the element 8th a valve, which subsequently as a valve 8th referred to as.

Furthermore, the fuel cell system includes 1 a recirculation device 9 , This has a recirculation line 10 on which from the exit of the anode compartment 4 up to a junction 11 enough. The recirculation line 10 thus flows between the valve 8th and the entrance of the anode compartment 4 in the supply line 7 , In the facility 9 is one with the recirculation line 10 connected fan 12 arranged, which has a motor 13 is driven. Between the blower 12 and the mouth 11 branches a line 14 from the recirculation line 10 from. In this line 14 is another element 15 arranged, which for changing the flow cross section of the line 14 is trained. In the embodiment, the element 15 designed as a valve, which subsequently as a valve 15 referred to as. The administration 14 and the valve 15 are a discharge device for draining the anode space 4 via the recirculation device 9 associated anode gas mixture.

It is preferably provided that the line 14 to the cathode compartment 3 the fuel cell 2 is guided. The proportion of the anode gas mixture, which of the recirculation line 10 via the discharge device 14 . 15 is guided, is thus in the cathode compartment 3 directed.

It can also be provided that the discharge device 14 . 15 into the environment or into one from the exit of the cathode compartment 3 leading away exhaust air line opens.

In particular, the fuel cell system 1 Also, a control unit, not shown, which for controlling the valve 8th is trained. It is preferably provided that this control unit also for controlling the valve 15 and the engine 13 is designed to increase the speed of the fan 12 to be able to adjust.

In addition, the fuel cell system includes 1 a sensor, not shown, to the fuel concentration and the oxidant concentration in the fuel cell 2 to be able to detect.

Below is the operation of the fuel cell system 1 explained in more detail. After the fuel cell system 1 If it is in a switched-off state, it is subsequently started and is then initially in a start-up phase. In the parked state has become in the anode room 4 accumulated an oxygen concentration, so that in a subsequent abrupt feeding of the fuel hydrogen, a high hydrogen-oxygen gradient would arise, which the fuel cell 2 can harm.

Therefore, after starting the fuel cell system 1 in the start-up phase in the fuel storage 6 contained hydrogen at least temporarily in an altered amount on the feed management 7 to the anode compartment 4 fed. In the embodiment, it is provided that the valve 8th is controlled via the control unit so that with the progressive start-up phase via the valve 8th flowing mass flow is continuously increased. This is the valve 8th continuously opened continuously as the start-up phase progresses. Thus, at least at times during the start-up phase of the fuel storage 6 to the anode room 4 Continuously increased in its amount of hydrogen fed, and the dosage changed continuously.

This is in the diagram according to 3 shown. The diagram shows the course of the oxygen concentration and the hydrogen concentration in the anode space 4 ,

At the beginning of a start-up phase at time t ₀ , the concentration of hydrogen is zero. As the time progresses, the concentration of hydrogen in the anode space then increases from time t ₂ 4 steadily.

This is achieved in particular by the fact that the valve 8th steadily continues to open and thereby the fuel storage 6 to the anode room 4 flowing hydrogen mass flow steadily increases.

In addition, according to the diagram in 2 , which is the speed of the fan 12 as a function of time shows, at time t _0, the fan 12 started. In the embodiment, during the start-up phase, the blower 12 operated in a recycle mode by the anode compartment 4 Anodengasgemisch located via the return line 10 the recirculation cycle back to the input of the anode compartment 4 is returned. For this purpose, in the embodiment in the return mode of the blower 12 set the full load operation, to which the maximum speed D1 is set. Up to a time t ₁ , in which the start-up phase is completed and the oxygen concentration in the anode compartment 4 is substantially zero percent, this maximum speed D1 is maintained constant. After completing the start-up phase, the blower 12 controlled via the control unit so that the speed is reduced from the maximum speed D1 to a lower speed D2. This speed D2 characterizes the normal operation of the blower 12 in the following on the start-up phase of the fuel cell system 1 ,

It can also be provided that a lower rotational speed than the maximum rotational speed D1 is set in the starting phase. Preferably, however, it is provided that the set in the start-up phase and thus in the return mode speed D1 of the blower 12 is greater than the set in normal operation speed D2. It can also be provided that the rotational speed D1 in the return mode of the blower 12 is not constant over the entire duration but is at least temporarily changed during the start-up phase.

Preferably, the time period between the times t ₀ and t _{2 is} minimal, since immediately after starting the fuel cell system 1 a supply of hydrogen from the fuel storage 6 over the valve 8th to the anode room 4 he follows.

At time t ₁ then the maximum hydrogen concentration in the anode compartment 4 achieved, which is then maintained substantially in the following.

The starting process of the fuel cell system 1 is due to a recirculation of the volume in the anode compartment 4 initiated. In this case, a comparatively high recirculation rate for normal operation is set by the blower 12 operated in the specific feedback mode. At the same time or offset in time to the beginning of the recirculation is then the continuously increasing addition of the amount of hydrogen from the fuel storage 6 , As a result, a correspondingly slower anode-side pressure build-up is achieved. An undesirable high hydrogen oxygen gradient formation in the anode space 4 can be prevented.

Also the over the confluence 11 supplied amount of the anode gas mixture can be varied. On the one hand, this can be done via the speed control of the blower 12 and additionally, on the other hand, via the removal of a portion of the anode gas mixture via the line 14 and the valve 15 be enabled.

The duration of the discharge and / or the amount of anode gas mixture to be discharged via the line 14 and the valve 15 is preferably dependent on the measured by a suitable sensor concentration of hydrogen and / or the concentration of oxygen in the anode compartment 4 certainly. As a result, the starting behavior can be optimally tuned and beyond accelerated.

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

Due to the preferred interaction of the medium supply to the anode chamber from the fuel storage 6 and the recirculation device 9 with coordinated variable amounts of each addition, an undesirable fuel-Oxidationsmittelgradient can be avoided. Especially in the further interaction with the discharge device 14 . 15 this can be done even more precisely.

1

The fuel cell system

2

fuel cell

3

cathode space

4

anode chamber

5

membrane

6

fuel storage

7

feed

8th

Valve

9

recirculation

10

recirculation

11

junction

12

fan

13

engine

14 15

discharge device

D1, D2

rotation speed

t0 t1, t2

time

QUOTES INCLUDE IN THE DESCRIPTION

This list The documents listed by the applicant have been automated generated and is solely for better information recorded by the reader. The list is not part of the German Patent or utility model application. The DPMA takes over no liability for any errors or omissions.

Cited patent literature

• - DE 10297626 T5 [0002]
• DE 102004037097 A1 [0003]

Claims (9)

Method for operating a fuel cell system ( 1 ), which is started after a shutdown and in which in the start-up phase, a fuel through the at least partially filled with air anode space ( 4 ) a fuel cell ( 2 ), and a during the start-up phase from the anode compartment ( 4 ) exiting anode gas mixture in a recirculation device ( 9 ) of the fuel cell system ( 1 ) at least partially back into the anode space ( 4 ), characterized in that the fuel storage ( 6 ) to the anode compartment ( 4 ) supplied amount of fuel during the start-up phase is at least temporarily varied. A method according to claim 1, characterized in that the of the fuel storage ( 6 ) to the anode compartment ( 4 ) supplied amount of fuel during the start-up phase is at least temporarily increased. A method according to claim 1 or 2, characterized in that the of the fuel storage ( 6 ) to the anode compartment ( 4 ) supplied amount of fuel during the start-up phase is at least temporarily changed continuously. Method according to one of the preceding claims, characterized in that in the recirculation device ( 9 ) a blower ( 12 ) is arranged, which in the start-up phase for returning the anode gas mixture in the anode space ( 4 ) is operated at least temporarily in a specific feedback mode. A method according to claim 4, characterized in that in the return mode, a speed of the blower ( 12 ) is set, which is greater than the set in the after-starting phase normal operation of the fuel cell system ( 1 ) set speed. A method according to claim 5, characterized in that in the return mode, a maximum speed of the blower ( 12 ) is set. Method according to one of the preceding claims, characterized in that in the recirculation device ( 9 ) guided anode gas mixture at least temporarily during the start-up pro rata via a discharge device ( 14 . 15 ) from the recirculation device ( 9 ) is discharged. A method according to claim 7, characterized in that the duration of the discharge and / or the amount to be discharged of the anode gas mixture via the discharge device ( 14 . 15 ) depending on the fuel concentration and / or the oxidant concentration in the anode space ( 4 ) is determined. A method according to claim 7 or 8, characterized in that via the discharge device ( 14 . 15 ) from the recirculation device ( 9 ) discharged anode gas mixture the cathode compartment ( 3 ) is supplied to the fuel cell.