GB2536706A

GB2536706A - Fuel cell system, vehicle and method for operating a fuel cell system

Info

Publication number: GB2536706A
Application number: GB1505238.4A
Authority: GB
Inventors: Kumar Mandal Manas; Sabharwal Mayank; Haladipur Madhukar Prasanna
Original assignee: Daimler AG
Current assignee: Mercedes Benz Group AG
Priority date: 2015-03-27
Filing date: 2015-03-27
Publication date: 2016-09-28
Also published as: GB201505238D0

Abstract

A fuel cell system for a vehicle comprises a fuel cell stack 12 with a first end plate 40 and a second end plate 42. The first end plate 40 and the second end plate 42 each have at least two fuel openings 14/48, 24/56 and at least two oxidant openings 16/50, 28/58. The fuel cell system comprises a control unit 60 configured to open one of the fuel openings 14/48, 24/56 and one of the oxidant openings 16/50, 28/58 to serve as fuel inlet and oxidant inlet respectively, and to open a further one of the fuel openings and of the oxidant openings to serve as fuel outlet and oxidant outlet respectively. The control unit 60 also closes the remaining fuel openings and oxidant openings. The control unit enables different fuel and oxidant openings to be used in various different configurations (figures 3-10), which allows a more even distribution of fuel cell degradation and hence increased lifetime of the fuel cell system. The fuel cell system is preferably used in a vehicle.

Description

Fuel cell system, vehicle and method for operating a fuel cell system The invention relates to a fuel cell system, in particular for a vehicle, comprising a fuel cell stack with a first end plate and a second end plate. The first end plate and the second end plate each have at least two fuel openings and at least two oxidant openings. The invention further relates to a vehicle with such a fuel cell system and to a method for operating a fuel cell system.

In a fuel cell system fuel cells such as proton exchange membrane (REM) fuel cells create electricity through the electro-chemical reaction that takes place when a fuel such as hydrogen and an oxidant such as oxygen are passed across opposite sides of an electrolyte membrane. Further a coolant or cooling fluid is typically used to remove the heat generated from this reaction.

The proton exchange membrane fuel cell comprises a membrane electrode assembly (MEA) which comprises an anode, a cathode and the proton exchange membrane arranged between these electrodes. This membrane electrode assembly is arranged between two separator plates, wherein one separator plate comprises channels for the distribution of the fuel and the other separator plate channels for the distribution of the oxidant. The respective channels facing the membrane electrode assembly build a channel structure which is called a flow field.

In a fuel cell stack a plurality of such unit cells comprising two separator plates and the membrane electrode assembly arranged between the separator plates are usually connected in series. In such a fuel cell stack instead of monopolar separator plates bipolar plates can be utilized, which are electrically conductive and act as an anode for one unit cell and as a cathode for the adjacent unit cell. Such a fuel cell stack typically comprises further two monopolar separator plates which form the delimiting components of the fuel cell stack. These plates are often called end plates and can be considerably different in construction from the bipolar plates.

Document US 6 663 995 B2 describes a fuel cell stack with a plurality of fuel cells arranged between two end assemblies. An upper end assembly comprises an upper end plate and a spacer plate arranged between the fuel cells and the upper end plate. In a like manner, a lower end assembly comprises a lower end plate and such a spacer plate. The upper end plate has one inlet for fuel, one inlet for oxidant and one inlet for a coolant. The upper end plate also has one outlet for fuel, one outlet for the oxidant and one outlet for the coolant. If the inlets and the outlets are connected to passageways for these fluids within the stacked fuel cells, the corresponding end of the fuel cell stack is also referred to as "wet end'. When there are no inlets and outlets going through the lower end assembly, the lower end of the fuel cell stack is also known as the "dry end".

Further, document JP 2000164238 A describes a fuel cell stack with an end plate having access holes for fuel, oxidant and cooling water. The end plate also has exit holes for discharging the fuel, the oxidant and the cooling water. The holes have an anticorrosive coating.

It is an object of the present invention to provide a fuel cell system of the initially mentioned kind, a vehicle with such a fuel cell system and a method for operating the fuel cell system, which provides for an improvement in the lifetime of the fuel cell stack.

This object is solved by a fuel cell system having the features of claim 1, by a vehicle having the features of claim 9 and by a method having the features of claim 10. Advantageous configurations with convenient developments of the inventions are specified in the dependent claims.

A fuel cell system according to the invention comprises a fuel cell stack with a first end plate and a second end plate. The first end plate and the second end plate each have at least two fuel openings and at least two oxidant openings. The fuel cell system comprises a control unit configured to open one of the fuel openings and one of the oxidant openings serving as fuel inlet and oxidant inlet respectively. The control unit is further configured to open a further one of the fuel openings and of the oxidant openings, which then serve as fuel outlet and oxidant outlet respectively, and to close the other fuel openings as well as the other oxidant openings. In other words the control unit opens just one of the fuel openings which shall be utilized as fuel inlet and one of the oxidant openings which shall be utilized as oxidant inlet, whereas the other fuel opening and oxidant opening in the end plate, which could serve as fuel inlet and oxidant inlet, respectively are closed. In a like manner, the control unit opens just one fuel opening and just one oxidant opening which shall be utilized as fuel outlet and oxidant outlet respectively, whereas the other fuel openings and oxidant openings in the end plate, which could serve as fuel outlet and oxidant outlet, respectively are closed. As a fuel conduit for providing fuel to the fuel cell stack and for discharging exhaust fuel from the fuel cell stack is connected to the fuel openings of the end plates the control unit can decide which fuel opening shall be used as fuel inlet and which fuel opening shall be used as fuel outlet by opening these fuel openings. In a like manner an oxidant conduit is connected to the oxidant openings of the end plates. And the control unit can decide which oxidant opening shall be used as oxidant inlet and which oxidant opening shall be used as oxidant outlet by opening these oxidant openings and by closing the remaining oxidant openings.

By selectively choosing one fuel opening of the at least two fuel openings per end plate as fuel inlet and one oxidant opening of the at least two oxidant openings per end plate as oxidant inlet, it is possible to vary among the openings serving as inlets for the fuel and the oxidant. Therefore, the flow of fuel and oxidant through the fuel cell stack does not follow the same pathways of flow paths throughout the lifetime of the fuel cell stack. Rather, the location where the fuel and the oxidant enter the fuel cell stack through the fuel inlet and the oxidant inlet respectively can be changed by the control unit. This allows a plurality of pathways for the fuel and the oxidant to be utilized within the fuel cell stack.

In this way, the usual flow related degradation of specific areas of the fuel cell stack can be more evenly distributed over larger areas of the fuel cell stack. As a result, the overall lifetime of the fuel cell stack can be extended. Moreover, the stack performance can be improved. By varying the pathways for the flow of the fuel and the oxidant through the fuel cell stack, also an increased residence time of these reactants in the fuel cell stack can be achieved. By slowing down the formation of degraded areas within the fuel cell stack, a better utilization of the fuel cells within the fuel cell stack can be achieved. Also, the effects correlated to the degradation of the fuel cell stack can be reduced.

Further, by selectively choosing the fuel opening and the oxidant opening of the first end plate as inlets in one operation mode of the fuel cell stack and of the second end plate in another operation mode of the fuel cell stack, the control unit enables an active switching of the flow directions of the reactants through the fuel cell stack. This also reduces the formation of areas within the fuel cell stack which are subject to intensified degradation.

Preferably, the at least two fuel openings are located in opposite edge regions of the first end plate and of the second end plate, respectively. When these fuel openings are utilized as fuel inlets or fuel outlets, the corresponding flow paths of the fuel through the fuel cell stack differ from each other to a particularly large extent. This distributes the areas of the fuel cell stack which are in more intense contact with fresh fuel during operation of the fuel cell system particularly evenly over the fuel cell stack.

It has further proven advantageous if the at least two oxidant openings are located in opposite edge regions of the first end plate and of the second end plate, respectively. Then also the pathways of the oxidant travelling through the fuel cell stack differ from one operation mode of the fuel cell stack to another operation mode, in which different ones of the oxidant openings are utilized as oxidant inlet or oxidant outlet, respectively.

It is further advantageous if the control unit is configured to open one fuel inlet and one oxidant inlet which are located in a first edge region of the first end plate and to open one fuel outlet and one oxidant outlet which are located in a second edge region of the second end plate. Herein, the first edge region is located opposite the second edge region. Such a configuration ensures that a particularly long pathway or flow path for the fuel and the oxidant is provided through the fuel cell stack. This leads to a particularly good utilization of the reactants and thus to a good performance of the fuel cell stack.

In a further advantageous embodiment, the control unit is configured to open one fuel inlet and one oxidant inlet in the first end plate and to open one fuel outlet and one oxidant outlet in the first end plate. Further, the control unit is configured to close all the openings of the second end plate. By utilizing the fuel openings and oxidant openings of the first end plate only as inlets and outlets respectively, a particularly large difference in the pathways or flow paths for the reactants compared to utilizing openings in both end plates as inlets and outlets can be achieved.

The control unit can further be configured to control the provision of the fuel and/or of the oxidant to the corresponding inlets in a first direction and the discharge of the fuel and/or of the oxidant from the corresponding outlets in a second direction which coincides with the first direction. In other words, a co-current flow of all reactants entering the fuel cell stack and leaving the fuel cell stack can be achieved. This leads to a particularly good performance of the fuel cell stack.

Alternatively, the control unit can be configured to control the provision of the fuel and/or of the oxidant to the corresponding inlets in a first direction and the discharge of the fuel and/or of the oxidant from the corresponding outlets in a second direction which is opposite to the first direction. In other words, a counter-current flow of the reactants can be realized, which is beneficial for a particularly homogenous humidity distribution throughout the fuel cell stack.

In a further advantageous embodiment the control unit is configured to operate a plurality of first valve elements which are arranged in a fuel conduit connected to the fuel openings of the first end plate and of the second end plate. Herein the control unit is further configured to operate a plurality of second valve elements which are arranged in an oxidant conduit connected to the oxidant openings of the first end plate and of the second end plate. Such a valve arrangement makes it possible to particularly easily utilize each fuel opening either as a fuel inlet or as a fuel outlet and likewise to utilize each oxidant opening either as an oxidant inlet or as an oxidant outlet, as desired.

Finally, it has proven advantageous if the control unit is configured to open upon a startup of the fuel cell system another fuel opening and another oxidant opening serving as fuel inlet and oxidant inlet, respectively, and to open another fuel opening and another oxidant opening serving as fuel outlet and oxidant outlet, respectively, and to close the other fuel openings and oxidant openings of the end plates. The control unit in which such a routine is implemented therefore alters the fuel inlets and oxidant inlets as well as the fuel outlets and oxidant outlets after each restart of the fuel cell system. This is in particular useful if the fuel cell system is arranged in a vehicle. Then, during a drive with the vehicle equipped with the fuel cell system, no changes with respect to the openings which are utilized as inlets or outlets occur. Rather, these changes can easily be implemented when the fuel cell system is restarted to provide electrical energy for a subsequent drive of the vehicle.

Alternatively or additionally a change in the fuel openings to be utilized as fuel inlets and fuel outlets, respectively and a change in the oxidant openings to be utilized as oxidant inlets and oxidant outlets, respectively can be performed after a predetermined length of time of operation of the fuel cell system.

The vehicle according to the invention includes a fuel cell system according to the invention. Such a fuel cell system can include a plurality of further components usual in particular for fuel cell systems of vehicles, which presently do not have to be explained in detail.

In the method according to the invention for operating a fuel cell system comprising a fuel cell stack with a first end plate and a second end plate, wherein the first end plate and the second end plate each have at least two fuel openings and at least two oxidant openings, a control unit is provided which selectively opens and closes these openings. Herein, by means of the control unit one of fuel openings and one of the oxidant openings are opened to serve as fuel inlet and oxidant inlet, respectively. Also by means of the control unit a further one of the fuel openings and a further one of the oxidant openings are opened to serve as fuel outlet and oxidant outlet, respectively. And by means of the control unit the other fuel openings as well as the other oxidant openings are closed. By selectively opening and closing the corresponding openings, a dynamic flow direction and flow path control of the reactants through the fuel cell stack can be achieved. By altering the direction of the fuel flow and of the oxidant flow through the fuel cell stack, the formation of reactant flow related, more intensely degraded specific areas within the fuel cell stack can be avoided. This leads to an improvement in the lifetime of the fuel cell stack.

The advantages and preferred embodiments described for the fuel cell system according to the invention also apply to the vehicle according to the invention and to method according to the invention.

The features and feature combinations mentioned above in the description as well as the features and feature combinations mentioned below in the description of figures and/or shown in the figures alone are usable not only in the respectively specified combination, but also in other combinations or alone, without departing from the scope of the invention. Thus, implementations are also to be considered as encompassed and disclosed by the invention, which are not explicitly shown in the figures or explained, but arise from and can be generated by separated feature combinations from the explained implementations.

Further advantages, features and details of the invention are apparent from the claims, the following description of preferred embodiments as well as based on the drawings in which features having analogous functions are designated with the same reference signs. Therein show: Fig. 1 schematically a fuel cell system for a vehicle comprising a control unit which is configured to change the flow paths of a fuel and of an oxidant through a fuel cell stack of the fuel cell system; Fig. 2 schematically the fuel cell stack of the fuel cell system according to fig. 1, wherein each end plate of the fuel cell stack has two fuel openings, two oxidant openings and one opening for a coolant; Fig. 3 a first way of directing the flow of fuel and oxidant through the fuel cell stack; Fig. 4 a second way of directing the flow of fuel and oxidant through the fuel cell stack; Fig. 5 a third way of directing the flow of fuel and oxidant through the fuel cell stack; Fig. 6 a fourth way of directing the flow of fuel and oxidant through the fuel cell stack; Fig. 7 a fifth way of directing the flow of fuel and oxidant through the fuel cell stack; Fig. 8 a sixth way of directing the flow of fuel and oxidant through the fuel cell stack; Fig. 9 a seventh way of directing the flow of fuel and oxidant through the fuel cell stack; Fig. 10 an eighth way of directing the flow of fuel and oxidant through the fuel cell stack; and Fig. 11 the fuel cell stack of the fuel cell system according to fig. 1, wherein a valve arrangement is shown which allows to realize the different flow paths of the fuel and the oxidant through the fuel cell stack shown in fig. 3 to fig. 10.

Fig. 1 schematically shows a fuel cell system 10 of a vehicle. The fuel cell system comprises a fuel cell stack 12 with a plurality of fuel cells 38 which are schematically shown in fig. 2. A fuel such as hydrogen is provided to the fuel cell stack 12 via a fuel inlet 14. Also, an oxidant such as oxygen or air is provided to the fuel cell stack 12 via an oxidant inlet 16. The air can be compressed by means of a turbocharger 18. Preferably an intercooler 20 cools the compressed air which is then humidified by a humidifier 22 before entering the fuel cell stack 12 through the oxidant inlet 16. After having participated in the electrochemical reactions taking place in fuel cells 38 of the fuel cell stack 12, the air leaves the fuel cell stack 12 through an oxidant outlet 28.

The exhaust air leaving the fuel cell stack 12 at the oxidant outlet 28 is preferably utilized to humidify the compressed air in the humidifier 22. As can be seen from fig. 1, the exhaust air can also be utilized in the intercooler 20, before it is discharged to a turbine 26 of the turbocharger 18. In a like manner the residual fuel leaves the fuel cell stack 12 through a fuel outlet 24. The fuel leaving the fuel cell stack 12 at the fuel outlet 24 can also be discharged to the turbine 26.

A coolant circuit 30 is provided to circulate a coolant through the fuel cell stack 12. The coolant circuit 30 comprises a cooler 32 configured to decrease the temperature of the coolant and a heater 34 configured to increase the temperature of the coolant if necessary. Heat can be provided to the heater 34 by a battery 36 of the vehicle. The battery 36 can store electrical energy provided by the fuel cell stack 12. The battery 36 and/or the fuel cell stack 12 can further provide electrical energy to an electric motor which can be part of a propulsion system of the vehicle.

As can be seen in more detail from fig. 2, the fuel cell stack 12 comprises a plurality of fuel cells 38, which are stacked between a first end plate 40 and a second end plate 42 of the fuel cell stack 12. The first end plate 40 comprises the fuel inlet 14 and the oxidant inlet 16, which are located in a first edge region 44 of the end plate 40. In a second edge region 46 of the first end plate 40 there is a second fuel inlet 48 and a second oxidant inlet 50.

In a like manner, the fuel outlet 24 and the oxidant outlet 28 of the second end plate 42 are arranged in a first edge region 52 of the second end plate 42. In an opposite edge region 54 the second end plate 42 also has a second fuel outlet 56 and a second oxidant outlet 58. A control unit 60 of the fuel cell system 10 determines which one of the fuel openings in the form of the fuel inlets 14, 48 and the fuel outlets 24, 56 shall be utilized to provide the fuel into the fuel cell stack 12 and to discharge the fuel from the fuel cell stack 12, respectively.

For example, the control unit 60 can open the fuel inlet 14 and close the fuel inlet 48 of the first end plate 40 while the fuel is discharged through the fuel outlet 24 of the second end plate 42 and while the other fuel outlet 56 of the second end plate 42 is closed. In a like manner, the control unit 60 can open the oxidant inlet 16 of the first end plate 40 and close the oxidant inlet 50 of the first end plate 40 while the exhaust oxygen is discharged through the oxidant outlet 28 of the second end plate 42 and the other oxidant outlet 58 of the second end plate 42 is closed. Such an operation mode of the fuel cell stack 12, which is controlled by the control unit 60, is illustrated in fig. 3.

As can be further seen from fig. 2, the end plates 40, 42 further comprise at least one coolant opening each, for example a coolant inlet 62 provided in the first end plate 40 and a coolant outlet 64 provided in the second end plate 42.

As can be seen from fig. 3, by opening the fuel inlet 14 and the oxidant inlet 16 in the first end plate 40 and by opening the oxidant outlet 28 and the fuel outlet 24 in the second end plate 42, a reactant flow from the edge region 44 of the first end plate 40 to the edge region 52 of the second end plate 42 can be realized within the fuel cell stack 12. However, the reactants in the form of the fuel and the oxidant are in this case distributed to the different fuel cells 38 from a manifold region 66. The reactants that have taken part in the electrochemical reaction within the fuel cells 38 are then collected in another manifold region 68 of the fuel cell stack 12, before they are discharged through the fuel outlet 24 and the oxidant outlet 28, respectively.

In fig. 3 the directions in which the fuel and the oxidant are provided to the fuel inlet 14 and the oxidant inlet 16 are indicated by arrows 70. Also, the directions in which the exhaust fuel and the exhaust oxidant are discharged from the oxidant outlet 28 and the fuel outlet 24 respectively are indicated by further arrows 72. Accordingly, the directions indicated by the arrows 70, 72 in fig. 3 coincide.

As can be seen from fig. 4, the fuel inlet 14 can also be utilized as fuel outlet, whereas the fuel outlet 24 can also be utilized as fuel inlet. In a like manner, the oxidant can enter the fuel cell stack 12 through the fuel outlet 28 provided in the second end plate 42, and the exhaust oxidant can be discharged from the fuel cell stack 12 through the oxidant inlet 16 of the first end plate 40. However, as in the operation mode shown in fig. 3 the control unit 60 keeps the other fuel openings and the other oxidant openings closed, which are provided in the end plates 40, 42. Also, in fig. 4 the directions in which the reactants are provided to the corresponding inlets and the direction in which the reactants are discharged from the corresponding outlets coincide.

In the operation mode illustrated in fig. 5, other fuel openings and other oxidant openings are closed and opened by the control unit 60. Here, the fuel openings in the form of the fuel inlet 14 and the fuel outlet 24 are closed. In a like manner, the oxidant openings in the form of the oxidant inlet 16 and the oxidant outlet 28 are closed. However, the fuel opening which in another operation mode of the fuel cell stack 12 can serve as the fuel outlet 56 is utilized as fuel inlet and the second fuel opening in the form of the (potential) fuel inlet 48 is utilized as fuel outlet. In a like manner, the second oxidant outlet 58 of the second end plate 42 is utilized as oxidant inlet and the second oxidant inlet 50 of the first end plate 40 is utilized as oxidant outlet. However, also in this operation mode of the fuel cell stack 12, the directions indicated by the arrows 70, 72 coincide.

In the operation mode illustrated in fig. 6, the second fuel inlet 48 of the first end plate 40 is opened, whereas the first fuel inlet 14 is closed. Also, the second oxidant inlet 50 of the first end plate 40 is opened, whereas the first oxidant inlet 16 is closed. Correspondingly, the first fuel outlet 24 of the second end plate 42 and the first oxidant outlet 28 of the second end plate 42 are closed. However, the second fuel outlet 56 of the second end plate 42 and the second oxidant outlet 58 of the second end plate 42 are opened. Therefore, the reactants that enter the fuel cell stack 12 in the edge region 46 are distributed to the fuel cells 38 via the manifold region 68 and are then collected in the manifold region 66. From this manifold region 66 the reactants leave the fuel cell stack 12 through the fuel outlet 56 and the oxidant outlet 58. Here again the arrows 70, 72 indicate the coinciding directions in which the reactants are provided to the corresponding inlets and discharged from the corresponding outlets.

Although not shown in fig. 3 to fig. 5, instead of a co-current flow of the reactants through the fuel cell stack 12, a counter-current flow can be realized. In this case the fuel enters one of the end plates 40, 42 through a fuel inlet and the oxidant leaves this end plate 40, 42 through an oxidant outlet adjacent to the fuel inlet.

In the operation mode shown in fig. 7, all the fuel openings and the oxidant openings of the first end plate 40 are closed, whereas all the fuel openings and all the oxidant openings of the second end plate 42 are opened. For example, the fuel can enter the second end plate 42 through the fuel outlet 56 and leave the fuel cell stack 12 through the fuel outlet 24. Also, the oxidant enters the fuel cell stack 12 through the oxidant opening which serves as the oxidant outlet 58 in another operation mode of the fuel cell stack 12, and the oxidant can leave the fuel cell stack 12 through the oxidant outlet 28.

In the operation mode illustrated in fig. 8, the oxidant enters the fuel cell stack 12 through the oxidant opening which serves as the oxidant outlet 28 in another operation mode of the fuel cell stack 12, and the oxidant leaves the fuel cell stack 12 through the oxidant outlet 58. In a like manner, the fuel enters the fuel cell stack 12 through the fuel opening which can serve as the fuel outlet 24, but which is utilized as fuel inlet in the operation mode illustrated in fig. 8. The exhaust fuel is discharged from the second end plate 42 through the second fuel outlet 56 of the second end plate 42. Here again, all the fuel openings and oxidant openings of the first end plate 40 are kept closed by the control unit 30.

Fig. 9 and fig. 10 show operation modes which are inverse to the operation modes described with respect to fig. 7 and fig. 8. Here, all the oxidant openings and fuel openings provided in the second end plate 42 are closed, whereas all the oxidant openings and all the fuel openings in the first end plate 40 are opened. For example, the oxidant can enter the fuel cell stack 12 through the oxidant inlet 50 and be discharged through the oxidant opening which can also serve as the oxidant inlet 16 (see fig. 9). In a like manner, the fuel can enter the fuel cell stack 12 through the fuel inlet 48 and leave the fuel cell stack 12 through the fuel opening which can also be utilized as the fuel inlet 14.

In fig. 10, however, the fuel and the oxidant are provided to the fuel cell stack 12 through the fuel inlet 14 and the oxidant inlet 16, respectively, and the reactants are discharged through the other fuel opening and oxidant opening of the first end plate 40.

In the operation modes illustrated in fig. 7 to fig. 10, the directions indicated by the arrows 70 are opposite to the directions indicated by the arrows 72. Further, also in these operation modes a counter-current flow of the fuel with respect to the oxidant can be realized.

Fig. 11 shows a possibility which enables the control unit 60 to open and close the various oxidant openings and fuel openings in the two end plates 40, 42. For example, a fuel conduit 74 which is connected to a fuel source 76 is also connected to the various fuel openings of the first end plate 40 and the second end plate 42. If a first valve 78 and a second valve 80 arranged in the fuel conduit 74 are closed, whereas a third valve 82 arranged in the fuel conduit 74 is opened, the fuel is provided to the fuel inlet 14.

However, by also closing the third valve 82 and opening a fourth valve 84, the fuel can also be provided to the fuel outlet 56 in the second end plate 42, which then serves as a fuel inlet. If the fuel enters the fuel cell stack 12 through the fuel outlet 56, the fuel inlet 48 can be utilized to discharge the fuel from the fuel cell stack 12. In this case, the first valve 78 can be closed and the fuel flows from the fuel inlet 48 through a branch 86 of the fuel conduit 74 to a discharge section 88 of the fuel conduit 74. To prevent the fuel from leaving the fuel cell stack 12 through the fuel outlet 24, a fifth valve 90 can be closed in this operation mode, whereas a sixth valve 92 arranged in the branch 86 can be opened.

In a like manner, the fuel can be provided to the fuel outlet 24, which then serves as a fuel inlet. To achieve this, the first valve 78 can be closed and the second valve 80 can be opened. In this case, it is desirable that the fuel exits the fuel cell stack 12 through the fuel inlet 14. This can be achieved by closing a seventh valve 94 and the third valve 82, whereas an eighth valve 96 located in a further branch 98 of the fuel conduit 74 is opened.

In a like manner, the air or such an oxidant coming from the humidifier 22 is provided to the fuel cell stack 12 through an oxidant conduit 100. The control unit 60 can open a valve 102 and close further valves 104, 106, 108 in order to provide the oxidant to the fuel cell stack 12 through the oxidant inlet 16. However, by closing the valve 102 and opening the valve 104, the oxidant outlet 58 can be utilized as oxidant inlet. In a like manner, the valve 106 can be opened, whereas the valves 102, 104, 108 are closed in order to utilize the oxidant inlet 50 in the first end plate 40 to provide the oxidant to the fuel cell stack 12. Here again, by closing the valve 106 and opening the valve 108, the oxidant outlet 28 in the second end plate 42 can be utilized as an inlet for the oxidant.

If the oxidant enters the fuel cell stack 12 through the oxidant inlet 16, a further valve 110 can be opened in order to allow the exhaust oxidant to be discharged through the oxidant outlet 28 and further to a discharge section 112 of the oxidant conduit 100. Herein another valve 113 which is located between the oxidant outlet 58 and the discharge section 112 is preferably closed.

However, if the oxidant enters the fuel cell stack through the oxidant outlet 58 and leaves the fuel cell stack 12 through the oxidant inlet 50, the valve 110 can be closed and a further valve 114 located in a branch 116 of the oxidant conduit 100 can be opened to discharge the exhaust oxidant to the discharge section 112. In this case, preferably a further valve 118 is also closed by the control unit 60.

The valve 110 is closed if the oxidant enters the fuel cell stack 12 through the oxidant outlet 28 and is discharged through the oxidant inlet 16. In this operation mode the further valve 118 located in a branch 120 of the oxidant conduit 100 is opened. Thus the exhaust oxidant is discharged through the branch 120 and further towards the discharge section 112 of the oxidant conduit 100.

The arrangement and operation of the valves shown in fig. 8 can differ in variants of the fuel cell system 10. In particular, at junctions of the fuel conduit 74 and the oxidant conduit 100 three-way valves can be provided in order to reduce the number of valves utilized in the arrangement shown in fig. 11.

The operation of the valves shown in fig. 11 is controlled by the control unit 60. This allows to actively switch the direction of the fuel flow and of the oxidant flow through the fuel cell stack 12. By changing the location at which the fuel and the oxidant enter the fuel cell stack 12, the formation of more intensely degraded areas within the fuel cell stack 12 can be avoided. Rather, the degradation of the areas within the fuel cell stack 12 is particularly evenly distributed throughout the stack. This leads to an overall lifetime extension of the fuel cell stack 12. Also, an improvement of the performance of the fuel cell stack 12 is achieved. The control unit 60 thus enables a particularly dynamic control of the reactant flow through the fuel cell stack 12.

List of reference signs fuel cell system 12 fuel cell stack 14 fuel inlet 16 oxidant inlet 18 turbocharger intercooler 22 humidifier 24 fuel outlet 26 turbine 28 oxidant outlet coolant circuit 32 cooler 34 heater 36 battery 38 fuel cell end plate 42 end plate 44 edge region 46 edge region 48 fuel inlet oxidant inlet 52 edge region 54 edge region 56 fuel outlet 58 oxidant outlet control unit 62 coolant inlet 64 coolant outlet 66 manifold region 68 manifold region arrow 72 arrow 74 fuel conduit 76 fuel source 78 valve valve 82 valve 84 valve 86 branch 88 discharge section valve 92 valve 94 valve 96 valve 98 branch oxidant conduit 102 valve 104 valve 106 valve 108 valve valve 112 discharge section 113 valve 114 valve 116 branch 118 valve branch

Claims

Claims 1. Fuel cell system, in particular for a vehicle, comprising a fuel cell stack (12) with a first end plate (40) and a second end plate (42), wherein the first end plate (40) and the second end plate (42) each have at least two fuel openings (14, 24, 48, 56) and at least two oxidant openings (16, 28, 50, 58), characterized in that the fuel cell system (10) comprises a control unit (60) configured to open one of the fuel openings (14, 24, 48, 56) and one of the oxidant openings (16, 28, 50, 58) serving as fuel inlet and oxidant inlet respectively and to open a further one of the fuel openings (14, 24, 48, 56) and of the oxidant openings (16, 28, 50, 58) serving as fuel outlet and oxidant outlet respectively and to close the other fuel openings (14, 24, 48, 56) as well as the other oxidant openings (16, 28, 50, 58).
2. Fuel cell system according to claim 1, characterized in that the least two fuel openings (14, 24, 48, 56) are located in opposite edge regions (44, 46, 52, 54) of the first end plate (40) and of the second end plate (42), respectively.
3. Fuel cell system according to claim 1 or 2, characterized in that the least two oxidant openings (16, 28, 50, 58) are located in opposite edge regions (44, 46, 52, 54) of the first end plate (40) and of the second end plate (42), respectively.
4. Fuel cell system according to any one of claims 1 to 3, characterized in that the control unit (60) is configured to open one fuel inlet and one oxidant inlet which are located in a first edge region (44) of the first end plate (40) and to open one fuel outlet and one oxidant outlet which are located in a second edge region (52) of the second end plate (42), wherein the first edge region (44) is located opposite the second edge region (52).
5. Fuel cell system according to any one of claims 1 to 4, characterized in that the control unit (60) is configured to open one fuel inlet and one oxidant inlet in the first end plate (40) and to open one fuel outlet and one oxidant outlet in the first end plate (40), and to close all the openings of the second end plate (42).
6. Fuel cell system according to any one of claims 1 to 5, characterized in that the control unit (60) is configured to control the provision of the fuel and/or of the oxidant to the corresponding inlets in a first direction and the discharge of the fuel and/or of the oxidant from the corresponding outlets in a second direction which coincides with the first direction or which is opposite to the first direction.
7. Fuel cell system according to any one of claims 1 to 6, characterized in that the control unit (60) is configured to operate a plurality of first valve elements (78, 80, 82, 84, 90, 92, 94, 96) arranged in a fuel conduit (74) connected to the fuel openings (14, 24, 48, 56) of the first end plate (40) and of the second end plate (42) and to operate a plurality of second valve elements (102, 104, 106, 108, 110, 113, 114, 116, 118) arranged in an oxidant conduit (100) connected to the oxidant openings (16, 28, 50, 58) of the first end plate (40) and of the second end plate (42).
8. Fuel cell system according to any one of claims 1 to 7, characterized in that the control unit (60) is configured to open upon a startup of the fuel cell system (10) another fuel opening (14, 24, 48, 56) and another oxidant opening (16, 28, 50, 58) serving as fuel inlet and oxidant inlet respectively and to open another fuel opening (14, 24, 48, 56) and another oxidant opening (16, 28, 50, 58) serving as fuel outlet and oxidant outlet respectively and to close the other fuel openings (14, 24, 48, 56) and oxidant openings (16, 28, 50, 58) of the end plates (40, 42).
9. Vehicle with a fuel cell system (10) according to any one of claims 1 to 8.
10. Method for operating a fuel cell system (10), in particular of a vehicle, comprising a fuel cell stack (12) with a first end plate (40) and a second end plate (42), wherein the first end plate (40) and the second end plate (42) each have at least two fuel openings (14, 24, 48, 56) and at least two oxidant openings (16, 28, 50, 58), characterized in that by means of a control unit (60) of the fuel cell system (10) one of the fuel openings (14, 24, 48, 56) and one of the oxidant openings (16, 28, 50, 58) are opened to serve as fuel inlet and oxidant inlet respectively and a further one of the fuel openings (14, 24, 48, 56) and a further one of the oxidant openings (16, 28, 50, 58) are opened to serve as fuel outlet and oxidant outlet respectively and the other fuel openings (14, 24, 48, 56) as well as the other oxidant openings (16, 28, 50, 58) are closed.