WO2009068206A1

WO2009068206A1 - Fuel cell system and method of starting a fuel cell system in a cold start phase

Info

Publication number: WO2009068206A1
Application number: PCT/EP2008/009761
Authority: WO
Inventors: Matthias Jesse; Klaus Scherrbacher
Original assignee: Daimler Ag; Ford Global Technologie, Llc
Priority date: 2007-11-29
Filing date: 2008-11-19
Publication date: 2009-06-04
Also published as: DE102007057451A1

Abstract

The invention relates to a fuel cell system with at least one fuel cell (2), whose anode side (4) is connected to a fuel branch (6), via which fuel may be supplied to the anode side (4) by means of a metering unit (8) arranged in the fuel branch (6), wherein a cold start branch (18) branches off from the fuel branch (6) upstream of the metering unit (8), which cold start branch (18) leads into the fuel branch (6) downstream of the metering unit (8) and which is designed to meter fuel into the fuel branch (6) in a cold start phase of the fuel cell (2) in the event of the passage cross section of the metering unit (8) being at least reduced due to liquid freeze out. The invention also relates to a method of starting a fuel cell system in a cold start phase.

Description

FUEL CELL SYSTEM AND METHOD OF STARTING A FUEL CELL SYSTEM IN A

COLD START PHASE

The invention relates to a fuel cell system with at least one fuel cell, whose anode side is connected to a fuel branch, via which fuel may be supplied in a metered manner to the anode side by means of a metering unit arranged in the fuel branch. The invention furthermore relates to a method of starting a fuel cell system in a cold start phase.

An anode inlet unit for a fuel cell system is known from DE 10 2005 047 972 A1. The fuel cell system comprises a hydrogen source, a fuel cell stack and a hydrogen inlet unit, which responds to a hydrogen flow rate from the hydrogen source and provides a controlled hydrogen flow rate from the hydrogen source to the fuel cell stack, the inlet unit comprising at least one injector with a duty cycle which determines the throughput to the fuel cell stack. Furthermore, the fuel cell system comprises a valve for directing a hydrogen flow to another component in the fuel cell system other than the anode side of the fuel cell stack. With this valve the hydrogen may be conveyed to the cathode side of the stack for mixture with air, so as to provide combustion in the cathode side for heating up the stack in the event of a cold start. This procedure is very complex.

It is additionally known that specific components of the fuel cell system have at increased cost to be made capable of cold starting in a cold start phase, for example by means of an additional heating element.

Furthermore, fuel cell systems are known in which jet pumps are used on the anode side for metering in hydrogen, in order to be able to use the elevated hydrogen pressure of the tank system for recirculation in the anode of the fuel cell. If the fuel cell system is cooled down to temperatures of close to or below the zero degree boundary, small water droplets may freeze at or in the nozzle of the jet pump, so restricting the flow rate on restarting and thus the performance of the fuel cell system. Water droplets at the nozzle of the jet pump are particularly critical, since these directly restrict the narrowest cross section of the hydrogen supply and thus flow rate.

It is the object of the present invention to provide a fuel cell system and a method with which cold start behavior may be improved.

This object is achieved by a fuel cell system which comprises the features as claimed in claim 1 and a method which comprises the features as claimed in claim 13.

A fuel cell system according to the invention comprises at least one fuel cell, whose anode side is connected to a fuel branch, via which fuel may be supplied to the anode side by means of a metering unit arranged in the fuel branch. Upstream of the metering unit a cold start branch branches off from the fuel branch, which cold start branch leads into the fuel branch downstream of the metering unit and is designed to meter hydrogen into the fuel branch in a cold start phase of the fuel cell in the event of the passage cross section of the metering unit being at least reduced due to liquid freeze-out. The cold start branch thus in particular constitutes a sort of bypass round the metering unit in the fuel branch and thus in particular allows specific supply of fuel to the fuel cell in the event of an at least partially frozen-up metering unit passage. Cold start behavior may be improved significantly thereby.

Branching off of the cold start branch from the fuel branch may proceed on the one hand in that a line is arranged between the fuel receptacle and the metering unit from which the cold start branch branches off. Branching off of the cold start branch from the fuel branch is understood, however, also to mean an embodiment in which the cold start branch is guided away directly from the fuel receptacle. In principle, the fuel receptacle is associated with the fuel branch, such that this stated embodiment is also covered by the wording that the cold start branch branches off from the fuel branch upstream of the metering unit.

Additional heating elements and the like are thus no longer necessary with this embodiment. Furthermore, this cold start branch allows the necessary fuel feed to proceed more rapidly and precisely than is the case in an embodiment in which the metering unit has firstly to be heated up by means of heating elements.

Preferably, a cold start unit for metering the hydrogen is arranged in the cold start branches. The supply of hydrogen via the cold start branch may thereby be further improved and made more exact.

Preferably, the cold start unit has a larger passage cross section than the metering unit. Precisely as a result of this configuration, fuel may always be supplied to the fuel cell via the cold start branch in a particularly preferred way even at low ambient temperatures. Freezing up of the cold start unit may thus be virtually prevented also at very low temperatures.

Preferably, the cold start branch, in particular the cold start unit, is only opened up to allow fuel to pass in the cold start phase. A component of the fuel cell system is thus provided which is activated in a manner specific to the operating phase and thus is not needed when the fuel cell system is operating normally. Since this component does not therefore have to be designed for normal operation, it may be specially designed for cold starting. This may be achieved in a relatively simple, non-complex manner for example in particular by the already stated relatively large passage cross section compared with the metering unit. Furthermore, the configuration in which the cold start branch and in particular the cold start unit is opened up for the passage of fuel only during the cold start phase also prevents water from accumulating undesirably in the cold start branch in a relatively large quantity, which could then also once again freeze out at low temperatures. This may be prevented by this specific embodiment, such that the cold start branch is always fully usable.

Preferably, the cold start branch and in particular the cold start unit is constructed without a jet pump and furthermore preferably with a correspondingly large passage cross section, such that for each cold start the necessary fuel may be made available.

Preferably, the cold start unit is a valve. As a result of this configuration, a relatively simple part may be provided which is economical as regards installation space and which, furthermore, is functionally reliable. Preferably, the cold start branch leads into the fuel branch upstream of the fuel cell in the flow direction of the fuel.

Provision may also be made for the cold start branch to lead into a feed line of the fuel branch downstream in the flow direction of the fuel of a junction of a recirculation branch of the anode side associated with the fuel branch and into the feed line upstream of the fuel cell in the flow direction of the fuel.

In a further embodiment, provision may be made for the cold start branch to lead into the fuel branch downstream of the fuel cell in the flow direction of the fuel.

It is likewise possible for the cold start branch to lead into the recirculation branch of the anode side.

Particularly preferably, provision is made for the cold start branch to lead into the recirculation branch or into the feed line of the fuel branch downstream of a separator in the flow direction of the fuel. This is particularly advantageous because the smallest amount of liquid water is usually present downstream of the separator.

Provision may also be made for the cold start branch, in particular the cold start unit, to be coupled to a component to which heat may be applied in the cold start phase. In such an embodiment it is no longer necessary for a valve to be provided explicitly as a cold start unit. The cold start unit may thus also differ structurally from a valve, wherein a different valve principle or indeed a heating device may be provided in this regard. In particular, a thermal coupling to the fuel cell itself may also be provided.

Preferably, the quantity of fuel supplied via the cold start branch is dependent on the quantity of fuel suppliable via the metering unit as a result of the reduction in passage cross section. This is dependent in particular on the instantaneously maximum quantity of fuel suppliable by the metering unit. This configuration thus makes possible very precise quantity metering through the cold start branch, which is metered appropriately at virtually every instant in the cold start phase. The supply of too much or too little fuel via the cold start branch may thus be prevented. Once the cold start phase is at an advanced stage and the passage cross section of the metering unit has thawed again at least in places, which results in a relatively large fuel flow rate, the quantity of fuel through the cold start branch may then be modified, in particular reduced, as a function thereof.

The metering unit may also be arranged directly at the junction of the recirculation branch with the fuel branch.

In the case of a method according to the invention for starting a fuel cell system in a cold start phase, fuel is fed into the fuel branch in the cold start phase by means of a cold start branch branching off from the fuel branch upstream of the metering unit and leading into the fuel branch downstream of the metering unit in the event of a passage cross section being at least reduced as a result of liquid freeze out of a metering unit arranged in a fuel branch, which is connected to the anode side of a fuel cell, for metered feed of the fuel to the anode side. The cold start behavior of the fuel cell may be improved significantly thereby. Even in the case of an at least partially frozen-up metering unit, a sufficient quantity of fuel may thus always be conveyed to the fuel cell.

Preferably, the cold start branch is only opened up for the supply of fuel in the cold start phase. This particular mode of operation thus makes possible optimum use of the cold start branch and additionally prevents the accumulation of undesired water in the cold start branch, which would lead to freezing when the system is in the "off' state at low ambient temperatures.

Preferably, the fuel supplied via the cold start branch is introduced into a recirculation branch of the anode side or a waste gas line of the fuel branch after disconnection of the fuel branch in the flow direction of the fuel.

Preferably, the quantity of fuel supplied via the cold start branch is metered as a function of the quantity of fuel suppliable via the metering unit as a result of the reduction in passage cross section.

Advantageous configurations of the fuel cell system according to the invention may be regarded as advantageous configurations of the method according to the invention. Exemplary embodiments of the invention are explained in more detail below with reference to a schematic drawing. The single figure shows a schematically represented fuel cell system.

The fuel cell system 1 takes the form of a mobile fuel cell system and is provided for use in a vehicle. The fuel cell system 1 comprises at least one fuel cell 2, in particular a fuel cell stack with a plurality of such fuel cells, the fuel cell 2 preferably taking the form of a PEM fuel cell.

The fuel cell 2 comprises a cathode side 3, which is separated from an anode side 4 by a membrane 5.

Furthermore, the fuel cell system 1 comprises a fuel branch 6, which comprises a receptacle 7 which contains hydrogen or a hydrogen-containing gas as fuel. Furthermore, the fuel branch 6 comprises a metering unit 8, which in the exemplary embodiment comprises components 9 and 10 connected in parallel in the form of valves, which are provided for flow pressure regulation. The metering unit 8 preferably takes the form of a jet pump.

The fuel branch 6 comprises a feed line 15 leading from the receptacle 7 to the anode side 4 and a waste gas line 13 leading away from the anode side 4.

Furthermore, the fuel branch 6 comprises a recirculation branch 11 of the anode side 4. The recirculation branch 11 branches away from the waste gas line 13 at the branch point 12 and leads into the feed line 15 at the junction 14. In the recirculation branch 11 there is arranged a separator 16, with a blower 17 being arranged downstream thereof. The metering unit may also be arranged directly at the junction 14.

A cold start branch 18 branches off from the fuel branch 6 upstream of the metering unit 8 in the flow direction of the fuel. In the exemplary embodiment, provision is made, to this end, for the cold start branch 18 to branch off from the feed line 15 between the receptacle 7 and the metering unit 8 at the branch point 19. Provision may also be made for the cold start branch 18 to branch off directly from the receptacle 7 (dashed line). In a first embodiment, the cold start branch 18 leads into the feed line 15 at a junction 20. In the flow direction of the fuel, the junction 20 is thus downstream of the metering unit 8 and upstream of the anode side 4 and thus also upstream of the fuel cell 2.

In a further embodiment, provision may also be made for the cold start branch 18 to lead into the fuel branch 6 downstream of the fuel cell 2 (dashed line). In such an embodiment, provision may for example be made for the junction 21 to be provided downstream of the anode side 4, which opens into the waste gas line 13.

In a particularly preferred embodiment, provision is made for the cold start branch 18 to lead into the recirculation branch 11 at the junction 22, as per the alternative dashed representation. This is provided in particular at a location downstream of the separator 16.

In principle, provision may also be made for the separator 16 to be arranged in the waste gas line 13 and for the junction 22 then to lead into the waste gas line 13 downstream of the separator 16 in the flow direction of the waste gas.

In the exemplary embodiment, the cold start branch 18 comprises a cold start unit 23, which may take the form simply of a valve. The cold start unit 23 has a larger passage cross section than do components 9 and 10.

The cold start branch 18 is opened up only during the cold start phase of the fuel cell system 1 , and fuel may thus be conveyed from the receptacle 7 via the cold start branch 18 to the anode side 4 of the fuel cell 2 only in this cold start phase. This is the case in particular only when the passage cross sections of the components 9 and 10 are at least partially reduced as a result of liquid freeze out and thus the fuel supply to the anode side 4 via the metering unit 8 is limited or prevented in this cold start phase.

It goes without saying that the cold start branch 18 may comprise a plurality of cold start units 23, which may then also in turn preferably be connected in parallel with one another. Apart from a configuration of the cold start unit 23 as a simple valve, a heating element may also be provided. Provision may likewise be made for the cold start branch 18 to be thermally coupled to a component of the fuel cell system 1 to which heat may be applied in the cold start phase. In particular, a thermal coupling to the fuel cell 2 may here also be provided. Furthermore, provision may also be made for the metering unit 8 to be coupled to at least one heating element, which is provided for applying heat to the metering unit 8 particularly in the cold start phase and thus for thawing the components 9 and 10 if they are at least partially frozen up.

In the cold start phase, provision may be made for the supply of fuel to the anode side 4 to proceed via the cold start branch 18 until the fuel cell 2 or the fuel cell system 1 has been started and/or the metering unit 8 has been thawed so as to supply sufficient fuel to the anode side 4.

List of reference signs

1 Fuel cell system

2 Fuel cell

3 Cathode side

4 Anode side

5 Membrane

6 Fuel branch

7 Receptacle

8 Metering unit

9,10 Components

11 Recirculation branch

12,19 Branch point

13 Waste gas line

14,20,21 ,22 Junction

15 Feed line

16 Separator

17 Blower

18 Cold start branch

23 Cold start unit

Claims

Daimler AG andFord Global Technologies, LLCPatent Claims

1. A fuel cell system with at least one fuel cell (2), whose anode side (4) is connected to a fuel branch (6), via which fuel may be supplied to the anode side (4) by means of a metering unit (8) arranged in the fuel branch (6), characterized in that a cold start branch (18) branches off from the fuel branch (6) upstream of the metering unit (8), which cold start branch (18) leads into the fuel branch (6) downstream of the metering unit (8) and which is designed to meter fuel into the fuel branch (6) in a cold start phase of the fuel cell (2) in the event of the passage cross section of the metering unit (8) being at least reduced due to liquid freeze out.

2. The fuel cell system as claimed in claim 1 , characterized in that a cold start unit (23) for metering hydrogen is arranged in the cold start branch (18).

3. The fuel cell system as claimed in claim 2, characterized in that the cold start unit (23) has a larger passage cross section than the metering unit (8).

4. The fuel cell system as claimed in claim 2 or 3, characterized in that the cold start unit (23) is a valve.

5. The fuel cell system as claimed in one of the preceding claims, characterized in that the cold start branch (18) leads into the fuel branch (6) upstream of the fuel cell (2) in the flow direction of the fuel.

6. The fuel cell system as claimed in one of the preceding claims, characterized in that the cold start branch (18) leads into a feed line (15) of the fuel branch (6) downstream in the flow direction of the fuel of a junction (14) of a recirculation branch (1 1) of the anode side (4) associated with the fuel branch (6) and into the feed line (15) upstream of the fuel cell (2) in the flow direction of the fuel.

7. The fuel cell system as claimed in one of the preceding claims, characterized in that the cold start branch (18) leads into the fuel branch (6) downstream of the fuel cell (2) in the flow direction of the fuel.

8. The fuel cell system as claimed in one of the preceding claims, characterized in that the cold start branch (18) leads into the recirculation branch (11) of the anode side (4).

9. The fuel cell system as claimed in one of the preceding claims, characterized in that the cold start branch (18) leads into the recirculation branch (1 1) or the feed line (15) downstream of a separator (16) in the flow direction of the fuel.

10. The fuel cell system as claimed in one of the preceding claims, characterized in that the cold start branch (18), in particular the cold start unit (23), is opened up to allow fuel to pass only in the cold start phase.

11. The fuel cell system as claimed in one of the preceding claims, characterized in that the cold start branch (4), in particular the cold start unit (23), is thermally coupled to a component to which heat may be applied in the cold start phase.

12. The fuel cell system as claimed in one of the preceding claims, characterized in that the quantity of fuel supplied via the cold start branch (18) is dependent on the quantity of fuel suppliable via the metering unit (8) as a result of the reduction in passage cross section.

13. A method of starting a fuel cell system (1) in a cold start phase, in which fuel is fed into the fuel branch (6) in a cold start phase by means of a cold start branch (18) branching off from the fuel branch (6) upstream of the metering unit (8) and leading into the fuel branch (6) downstream of the metering unit (8) in the event of a passage cross section being at least reduced as a result of liquid freeze out of a metering unit (8) arranged in a fuel branch (6), which is connected to the anode side (4) of a fuel cell (2), for feeding the fuel to the anode side (4).

14. The method as claimed in claim 13, characterized in that the cold start branch (18) is opened up to supply fuel only in the cold start phase.

15. The method as claimed in claim 13 or 14, characterized in that the fuel supplied via the cold start branch (18) is introduced downstream in the flow direction of the fuel of a separator (16) in a recirculation branch (11) into said recirculation branch (9) or into a waste gas line (13) of the fuel branch (6).

16. The method as claimed in one of claims 13 to 15, characterized in that the quantity of fuel supplied via the cold start branch (18) is metered as a function of the quantity of fuel suppliable via the metering unit (8) as a result of the reduction in passage cross section.