GB2525697A

GB2525697A - Fuel cell stack, fuel cell system and vehicle

Info

Publication number: GB2525697A
Application number: GB1421294.8A
Authority: GB
Inventors: Elsa Assadian; Taryn Biggs; Caroline Roxanne Cloutier; Christian Martin Erdmann; David Kassebaum; Michael Medina; Alan Parkin; Jonas Stoll; Stephen Wade
Original assignee: Daimler AG
Current assignee: Mercedes Benz Group AG
Priority date: 2014-12-01
Filing date: 2014-12-01
Publication date: 2015-11-04
Also published as: DE102015015229A1; GB201421294D0

Abstract

Cells of a fuel cell stack comprise a membrane electrode assembly between bipolar plates 12. Each bipolar plate 12 comprises bonded first and second plates forming a coolant flow field between them comprising plural coolant channels 30, 32 forming an angle with reactant channels provided by the bipolar plate 12. Each coolant channel 30, 32 communicates with a coolant inlet port 44 provided in the bipolar plate 12 and with a coolant outlet port 46. Adjacent coolant outlet ports 46 are separated from each other by a partition 48 configured to enable a coolant flow through at least one partition opening (52, figure 7) such as by comprising a closing element (56) which may obstruct the opening (52). A control unit (88, figure 4) may operate the closing elements (56) and valve devices (62) at coolant inlet ports in response to a request for power output from the fuel cell stack of an associated vehicle, a parameter such as temperature or voltage of the stack, or environmental parameters. The closing element may be slideably engaged with a guide rail (110) of the partition 48 and may comprise a hole alignable with the opening (52).

Description

Fuel cell stack, fuel cell system and vehicle The invention relates to a fuel cell stack with a plurality of fuel cells. Each fuel cell comprises a membrane electrode assembly arranged between two bipolar plates. Each bipolar plate comprises a first plate and a second plate bonded together and forming a coolant flow field between the two plates. The coolant flow field comprises a plurality of coolant channels, which form an angle with reactant channels provided by the bipolar plate. The invention further relates to a fuel cell system with such a fuel cell stack and to a vehicle with such a fuel cell system.

In a fuel cell system fuel cells create electricity through the electrochemical reaction that takes place when a fuel such as hydrogen and an oxidant such as oxygen are passed across opposing sides of the membrane electrode assembly. In a polymer electrolyte membrane fuel cell (PEMEC) system the membrane is a polymer electrolyte membrane (PEM) or proton exchange membrane. Catalyst layers and gas diffusion layers form the electrodes of the membrane electrode assembly, namely an anode and a cathode located on each side of the membrane.

In the fuel cell the 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 comprises 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 the fuel cell stack a plurality of such unit cells comprising two separator plates and the membrane electrode assembly arranged between the separator plates are often connected in series. In such a fuel cell stack instead of monopolar separator plates bipolar plates can be utilized, which are electrically conductive and contact the anode of a first unit cell and the cathode of the adjacent unit cell. An anode plate and a cathode plate of the bipolar plate can be bonded together, for example by welding. In a space between the two plates the coolant channels are provided, which form the coolant flow field. Thus a coolant can be circulated through the bipolar plates in order to remove heat generated during the electrochemical reaction.

Document US 2006/0046117 Al describes a fuel cell stack with membrane electrode assemblies arranged between bipolar plates. The bipolar plates provide reactant flow fields with reactant channels extending in a first direction. The bipolar plates consist of two plates which are joined together, wherein grooves within each of the two plates form coolant channels in cooperation with each other. The coolant channels extend in a direction which is perpendicular to the first direction. A density or a width of the coolant channels can vary in the first direction. An increased removal of heat generated through the electrochemical reaction within the fuel cell stack can thus be achieved in central regions of the membrane electrode assemblies.

Document WO 201 2/131267 Al describes a fuel cell stack with a plurality of fuel cells comprising bipolar plates with internal coolant channels. Each coolant channel is equipped with a microvalve which allows obstructing a coolant flow through the respective coolant channel. The coolant channels within each bipolar plate are formed by two plates joined together.

The performance of a fuel cell stack depends largely on the thermodynamic parameters of the respective fuel cells within the stack. In this regard, maintaining a homogenous temperature range within each fuel cell is rather challenging.

It is therefore an object of the present invention to provide a fuel cell stack of the initially mentioned kind, a fuel cell system with such fuel cell stack and a vehicle with such a fuel cell system, wherein the fuel cell stack has an improved cooling performance.

This object is solved by a fuel cell stack having the features of claim 1, by a fuel cell system having the features of claim 9 and by a vehicle having the features of claim 10.

Advantageous configurations with convenient developments of the invention are specified in the dependent claims.

The fuel cell stack according to the invention comprises a plurality of fuel cells, which each comprise a membrane electrode assembly arranged between two bipolar plates.

Each bipolar plate comprises a first plate and a second plate, which are bonded together.

The first and second plates form a coolant flow field between the two plates, and the coolant flow field comprises a plurality of coolant channels. The coolant channels form an angle with reactant channels provided by the bipolar plate. Each coolant channel fluidly communicates with a coolant inlet port provided in the bipolar plate. Each coolant channel further fluidly communicates with a coolant outlet port. Adjacent outlet ports are separated from each other by a partition device with at least one opening. The partition device comprises means configured to enable a coolant flow through the at least one opening.

Thus, the coolant provided to a coolant channel via a corresponding inlet port can be discharged through more than one outlet port. Therefore, a higher flow rate of the coolant through such a coolant channel can be realized. Further, the coolant flow rate through each one of the different coolant channels can be adapted to the cooling requirements of the fuel cell stack. Thus a particularly hornogenous heat distribution within each fuel cell of the fuel cell stack can be achieved, and the cooling performance within each unit fuel cell can be particularly exactly adapted to the specific requirements and the various areas of each unit fuel cell. The improved cooling performance leads to a better performance of the fuel cell stack.

This also results in energy efficiency gains for the fuel cell stack. Further, a particularly dynamic operation of each unit fuel cell and the fuel cell stack is possible. This is due to the fact that an increase in heat generation due to a more intense electrochemical reaction within the fuel cells can be accounted for by a better removal of the generated heat. Thus, the unit fuel cells and the fuel cell stack can be operated in a temperature range which is particularly beneficial for an optimum power output of each fuel cell.

In particular, high power applications of the fuel cell stack can be realized without the high power output leading to an unwanted overheating of the fuel cell stack. If the fuel cell stack is utilized in a fuel cell system of a vehicle, particularly dynamic operating conditions of the vehicle can thus be realized.

The coolant channels can in particular form an angle of about 90° with the reactant channels of the reactant flow fields. Then, a distance between the coolant inlet port and the coolant outlet port of each coolant channel is particularly small and heat can well be removed from the fuel cell in operation.

It is advantageous if the means are configured to vary and/or to disable the coolant flow through the at least one opening. Thus, the coolant flow through the individual coolant channels can be particularly well controlled. This allows operating the individual fuel cells or unit cells of the fuel cell stack in an electrochemically optimum operation mode which leads to a particularly high performance of the fuel cell stack.

Each coolant inlet port can fluidly communicate with a corresponding coolant inlet pipe, wherein the coolant inlet pipe is equipped with a valve device. Thus, the amount of coolant to be provided to each coolant channel can be controlled at the inlet side of each coolant channel.

Preferably, the fuel cell stack comprises a control unit which is configured to operate the means. In this way, a very rapid reaction to variations in the cooling requirements of the fuel cell stack can be performed by operating the means in a very comfortable manner.

The control unit can in particular be configured to operate the valve devices if the coolant inlet pipes, which are fluidly connected to the corresponding coolant inlet port, are equipped with such valve devices. In this case the control unit can also control the coolant flow towards the inlet ports according to the cooling requirements of the fuel cells.

Herein, it has proven advantageous if, by operating the valve device, the control unit is configured to vary a volume flow rate and/or a pressure of the coolant provided to each coolant channel. This allows to provide each coolant channel with coolant having different thermodynamic parameters and thus to individually control the coolant flow through the coolant channels.

In a further advantageous embodiment, the control unit is configured to operate the means and/or the valve device in dependence on a requested power output of the fuel cell stack. A sudden increase in the requested power output of the fuel cell stack leads to an increase of the amount of reactants provided to the fuel cell stack and therefore of the heat generated by the fuel cell stack. It is therefore highly beneficial to compensate for the increase in heat generation by adjusting the coolant flow through the individual coolant channels.

If the fuel cell stack is utilized as a power source in a vehicle, for example, an acceleration demand by the driver of the vehicle leads to an increase in the requested power output of the fuel cell stack, which can thus be accounted for. Also, a decrease in the requested power output resulting from decelerating the vehicle can be taken into account by the control unit in reducing the coolant flow through the coolant channels.

Alternatively or additionally, the control unit can be configured to operate the means and/or the valve device in dependence on at least one environmental parameter. For example, the ambient air temperature can have an influence on the cooling performance of a heat exchanger utilized to cool down the coolant discharged from the coolant outlet ports of the fuel cell stack. Therefore, taking into account such environmental parameters helps in maintaining the temperature within the individual fuel cells of the fuel cell stack within a desired temperature range throughout the fuel cell stack.

Further additionally or alternatively, the control unit can be configured to operate the means and/or the valve device in dependence on at least one parameter of the fuel cell stack. For example, the temperature of an individual fuel cell and/or a cell voltage can be utilized as a parameter to evaluate the status of the fuel cell stack with respect to its cooling requirements. Therefore, by taking into account such parameters of the fuel cell stack, the control unit can immediately react to variations of these parameters in adjusting the coolant flow through the individual coolant channels.

It has further proven advantageous if a first coolant channel which is closer to a reactant outlet side of the bipolar plate has a cross-section area capable of being passed by the coolant, which is smaller than the cross-section area of a second coolant channel which is closer to a reactant inlet side of the bipolar plate than the first coolant channel. Thus, more heat can be removed from an area at the reactant outlet side of the bipolar plate.

This is based on the finding that as a consequence of the electrochemical reaction taking place as the reactants flow from the reactant inlet side to the reactant outlet side, more and more heat is generated. By removing more heat from areas of the bipolar plate which are closer the reactant outlet side, a particular homogenous temperature distribution within the fuel cell can be achieved.

Alternatively or additionally, a number of coolant channels per surface area unit of the bipolar plate can be larger close to the reactant outlet side of the bipolar plate than close to the reactant inlet side of the bipolar plate. With such a higher density of coolant channels near the outlet side of the bipolar plate, the higher amount of heat at the outlet side, which is generated by the electrochemical reaction taking place within the fuel cell, can be readily removed.

The means can in particular comprise a driving rod with a closing element configured to obstruct the at least one opening, which can be provided in a wall element of the partition device. Herein, the closing element has at least one hole which can be aligned with the at least one opening. By aligning the hole with the opening, a coolant flow through the wall element and thus the partition device can be enabled. With such a partition device the coolant flow through the at least one opening can be particularly easily enabled or disabled. The hole can in particular have the same cross-section area and the same form as the opening provided in the wall element of the partition device.

The closing element can in particular be slidably engaged in a guide rail of the partition device. Thus, by sliding the closing element, the at least one opening in the wall element of the partition device can be gradually uncovered in order to enable the coolant flow through the at least one opening. Also, the amount of coolant passing through the at least one opening can be particularly well adjusted to the cooling requirements of the fuel cells within the fuel cell stack.

The fuel cell system according to the invention, which in particular can be employed in a vehicle, includes a fuel cell according to the invention. Herein an oxidant inlet and a fuel inlet can be located opposite an oxidant outlet and a fuel outlet of the fuel cell stack.

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.

The vehicle according to the invention includes a fuel cell system according to the invention.

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

Therein show: Fig. 1 in a perspective view a fuel cell stack of a fuel cell system employed in a vehicle, wherein coolant inlet pipes are fluidly connected with corresponding coolant inlet ports provided in bipolar plates of the fuel cell stack and coolant outlet pipes are fluidly connected with corresponding coolant outlet ports; Fig. 2 one of the bipolar plates utilized in the fuel cell stack shown in fig. 1, wherein different dimensions of coolant channels extending in a perpendicular direction to a reactant flow across the bipolar plate are illustrated, and wherein coolant outlet pods corresponding with the coolant channels are created by partitioning an opening in the bipolar plates by a number of partition devices; Fig. 3 an enlarged and perspective view of a section of the bipolar plate shown in fig. 2; Fig. 4 schematically the fuel cell stack according to fig. 1, wherein a control unit is shown, which individually controls the flow of coolant provided to each coolant channel and which effects a movement of driving rods of the partition devices: Fig. 5 schematically a connection device with valves utilized to control the coolant flow into the inlet pipes, which are in fluid communication with the coolant inlet ports of the bipolar plates in the fuel cell stack; Fig. 6 a perspective view of the fuel cell stack according to fig. 1, wherein the coolant inlet ports and the coolant outlet ports are shown and the flow direction of a coolant through the fuel cell stack is indicated; Fig. 7 an enlarged perspective view of one of the partition devices utilized to control the coolant flow from one coolant outlet port to an adjacent coolant outlet port; Fig. 8 the partition device according to fig. 7 with the driving rod moved into a position in which openings in a wall of the partition device are completely obstructed; Fig. 9 the partition device according to fig. 7 with the driving rod moved into a position in which a coolant flow through the openings in the wall of the partition device is enabled; and Fig. 10 the partition device according to fig. 7 with the driving rod moved into a position in which holes in a closing member connected to the driving rod are aligned with the openings in the wall of the partition device such that a maximum coolant flow through the openings is enabled.

Fig. 1 shows a fuel cell stack 10, which is a component of a fuel cell system of a vehicle.

In the fuel cell stack 10, membrane electrode assemblies (not shown) are arranged between bipolar plates 12 (see fig. 2). Each bipolar plate 12 comprises a first plate 14 and a second plate 16 (see fig. 3), which are bonded together, for example by welding. The first plate 14 can face a cathode of the membrane electrode assembly of a first fuel cell within the fuel cell stack 10. Then the second plate 16 faces an anode of the membrane electrode assembly of an adjacent fuel cell within the fuel cell stack 10.

Each of the two plates 14, 16 has a corrugated structure and thus forms reactant channels 18, 22. A flow direction of, for example, air as an oxidant through oxidant channels 18 is indicated in fig. 3 by a first arrow 20. A flow direction of a fuel along fuel channels 22 formed by the corrugated second plate 16 is indicated in fig. 3 by a second arrow 24. Thus, the reactant channels 18, 22 formed by the bipolar plate 12 generally extend in a direction which runs from a reactant inlet side 26 to a reactant outlet side 28 of the bipolar plate 12 (see fig. 2).

In a space between the two plates 14, 16 the bipolar plate 12 further forms a coolant field, which comprises a plurality of coolant channels 30, 32 (see fig. 2). A width of each coolant channel 30, 32 is defined by a distance between two adjacent weld seams 34, i.e. lines along which welds 36 are arranged, which bond the metal plates 14, l6to each other (see fig. 2 and fig. 3).

As can be seen from fig. 2, the weld seams 34 are perpendicular to a length direction of the reactant channels 18, 22. Thus, the coolant channels 30, 32 formed in the space between the two plates 14, 16 form an angle, in the example shown in fig. 2 and fig. 3 an angle of 90°, with the reactant channels 18, 22. As can be further seen from fig. 2, a width of a first coolant channel 32, which is closer to the reactant outlet side 26 of the bipolar plate 12, is smaller than a width 38 of a second coolant channel 30, which is closer to the reactant inlet side 26 of the bipolar plate 12 (see fig. 3).

This difference in the width 38 of the coolant channels 30, 32 goes along with a difference in a cross-section area capable of being passed by the coolant. Thus, for a given pressure at an inlet side of each coolant channel 30, 32 a higher coolant flow velocity can be realized in an area of the bipolar plate 12, which is closer to the reactant outlet side 28 of the bipolar plate 12, than in an area closer to the reactant inlet side 26 of the bipolar plate 12. A length direction of the reactant channels 18, 22 is illustrated in fig. 2 by another arrow 40. The width direction of the coolant channels 30, 32 is parallel to the length direction indicated by the arrow 40.

By varying the location of the weld seams 34, the coolant distribution within each bipolar plate 12 can be controlled. For example, the width 38 and thus the cross-section area capable of being passed by the coolant can be modified by appropriately choosing the spacing between the weld seams 34. Also, the number of coolant channels 30, 32 along the length direction of the bipolar plate 12 can be varied, wherein the length direction is indicated by the arrow 40 in fig. 2.

The coolant flow along the coolant channels 30, 32 is indicated in fig. 3 by another arrow 42. With respect to fig. 2, the coolant flow is thus oriented from a coolant inlet port 44 provided in the bipolar plate 12 towards a coolant outlet port 46. The coolant inlet ports 44 are openings in the bipolar plates 12, which form coolant manifolds when the plurality of bipolar plates 12 are stacked together in the fuel cell stack 10. In a like manner, the fuel outlet ports 46 form coolant manifolds.

The number of coolant inlet ports 44 provided in each bipolar plate 12 corresponds with the number of coolant channels 30, 32 provided in each bipolar plate 12. Also, the number of coolant outlet ports 46 corresponds to the number of coolant channels 30, 32.

However, the coolant outlet ports 46 are not formed by separate openings within the bipolar plate 12. Rather, one large opening provided in the bipolar plate 12 is divided or partitioned into separate compartments which form the coolant outlet ports 46. To achieve this, partition devices 48 are inserted into the large opening or large coolant outlet manifold provided by the stacked bipolar plates 12 (see fig. 6).

Although not explicitly shown in fig. 2, these partition devices 48 separate a first outlet port 46 corresponding to one coolant channel 30, 32 from a second outlet port 46 corresponding to the adjacent coolant channel 30, 32. Thus, each coolant channel 30, 32 has an individual inlet port 44 and an individual outlet port 46.

However, the partition devices 48 each comprise means to enable a coolant flow from one outlet port 46 to the adjacent outlet port 46. For this purpose, a wall element 50 of the partition device 48 has a plurality of openings 52 (see fig. 6 and fig. 7). These openings 52 can be at least partially opened or uncovered in order to allow a coolant flow through the wall element 50. However, the openings 52 can also be completely closed. To achieve this, the means can comprise a driving rod 54 configured to move a closing member 56 of the means. The closing member 56 also comprises a plurality of holes 58. The holes 58 in the closing member 56 can be brought in alignment with the openings 52 provided in the wall element 50 (see fig. 8 to fig. 10). By doing so, a coolant flow through the openings 52 provided in the partition device 48 can be varied. Also, by moving the driving rod 54 the openings 52 in the partition device 48 can be completely closed (see fig. 8). Thus, by manipulating the means comprising in the example shown the driving rod 54 and the closing member 56, the coolant flow through the coolant channels 30, 32 can be controlled.

However, the fuel cell stack 10 preferably comprises further features which allow an open loop control or a closed loop control of the coolant flow through the coolant channels 30, 32. As can be seen from fig. 1, the coolant inlet ports 44, which are aligned in the fuel cell stack 10 to form the coolant inlet manifolds, are provided with the coolant through a corresponding coolant inlet pipe 60. In the exemplary embodiment shown in fig. 1, there are eight inlet pipes 60 corresponding with the eight coolant inlet ports 44 of each bipolar plate 12. However, in variants of the fuel cell stack 10, a different number of coolant inlet pipes 60 and corresponding coolant inlet pods 44 can be provided.

The coolant inlet pipes 60, which fluidly communicate with the corresponding coolant inlet ports 44, are equipped with individual valves 62 schematically shown in fig 4 and in fig. 5.

These valves 62 can be provided in a connection device 64 having one inlet 66 and a number of outlets corresponding with the number of coolant inlet pipes 60 (see fig. 5). By providing the valves 62, a coolant flow into each one of the coolant channels 30, 32 fluidly communicating with the corresponding coolant inlet manifold formed by inlet ports 44 of the stacked bipolar plates 12 can be individually controlled. By also providing the partition device 48 with the means configured to enable the coolant flow through the openings 52, the coolant flow through the individual coolant channels 30, 32 can be alternatively or additionally controlled by varying the flow through the openings 52.

As can be further seen from fig. 1, the driving rods 54 of the partition devices 48 extend through a cover plate 68 of the fuel cell stack 10. Further, coolant outlet pipes 70 corresponding with each one of the coolant outlet manifolds formed by the coolant outlet ports 46 of the stacked bipolar plates 12 preferably extend through the cover plate 68.

The fuel cell stack 10 also comprises a fuel inlet 72 and an oxidant inlet 74 on the reactant inlet side 26 of the bipolar plates 12. Accordingly, the bipolar plates 12 comprise a fuel inlet port 76 and an oxidant inlet port 78 located on the reactant inlet side 26 of the bipolar plates 12. The fuel inlet ports 76 and the oxidant inlet pods 78 form manifolds in the fuel cell stack 10, in which the bipolar plates 12 and the membrane electrode assemblies are stacked together.

The fuel cell stack 10 further comprises a fuel outlet 80 and an oxidant outlet 82 (see fig. 1). The fuel outlet 80 and the oxidant outlet 82 are located on the reactant outlet side 26 of the bipolar plates 12 and fluidly communicate with fuel outlet ports 84 and oxidant outlet ports 86 forming manifolds as the bipolar plates 12 and the membrane electrode assemblies are stacked together in the fuel cell stack 10.

As can be seen from fig. 4, the valves 62, which control the coolant amount to be fed into the individual coolant channels 30, 32, can be controlled by a control unit 88, which operates the valves 62. Thus, a volume flow rate and a pressure of the coolant provided to each coolant channel 30, 32 can be adjusted. For example, each coolant channel 30, 32 can be opened or closed as required. The pressure within a coolant circuit 90 is provided by a pump device such as a compressor 92. In controlling the compressor 92 and the pressure it creates within the cooling circuit 90, the control unit 88 preferably takes into account environmental parameters such as an ambient air temperature captured, for example, by a sensor 94.

Further, the control unit 88 takes into consideration parameters of the fuel cell stack 10, for example a cell voltage and a temperature of the fuel cells within the fuel cell stack 10.

The corresponding information to be processed by the control unit 88 can be provided to the control unit 88 by a communication line 96.

The control unit 88 further takes into account a driving behavior of a driver of the vehicle equipped with the fuel cell system. For example, a sensor 98 can capture whether the driver accelerates or brakes the vehicle. The corresponding driving behavior has an influence on the requested power output of the fuel cell stack 10 and thus the quantity of reactants provided to the fuel cell stack 10. This influences the cooling requirements of the fuel cell stack 10. By taking into account these parameters, the control unit 88 can regulate or control the temperature of the fuel cell stack 10 and thus operate the fuel cell stack 10 in a temperature range which provides for an optimum performance of all the fuel cells within the fuel cell stack 10.

The coolant circuit 90 also comprises a heat exchanger 100. By considering environmental parameters such as the ambient air temperature, the heat exchange performance of the heat exchanger 100 can be taken into account by the control unit 88.

In order to manipulate the driving rods 54, the control unit 88 can, for example, control individual motors 102, which actuate the driving rods 54. By opening and closing the openings 52 provided in the partition devices 48, which separate the coolant outlet ports 46 from each other, a homogenization and/or a separation of the coolant discharged from the fuel cell stack 10 can be achieved. Thus, by controlling the coolant flow through the fuel cells of the fuel cell stack 10, an energy efficient cooling of the fuel cell stack 10 is achieved, which allows a highly dynamic operation of the fuel cell stack 10.

From fig. 6 it can particularly well be seen how the coolant inlet ports 44, the coolant outlet ports 46, the fuel inlet ports 76, the oxidant inlet ports 78, the fuel outlet ports 84 and the oxidant outlet ports 86 form manifolds in a perimeter region of the bipolar plates 12 stacked together within the fuel cell stack 10. Further, an arrow 104 illustrates the flow direction of the coolant into the fuel cell stack 10, whereas a further arrow 106 illustrates the coolant flow through the coolant channels 30, 32 and thus in a direction which is substantially perpendicular to the flow of the reactants across the flow fields comprising the reactant channels 18, 22. A further arrow 108 in fig. 6 indicates the flow direction of the coolant as it is discharged from the fuel cell stack 10.

In the enlarged perspective view of a section of the fuel cell stack 10, which is shown in fig. 7, components of the partition device 48 are shown in detail. The partition device 48 separates the large opening which is provided in the bipolar plates 12 into the distinct coolant outlet ports 46. The wall element 50 with the openings 52 divides the large opening in the bipolar plate 12 in a direction which is parallel to the orientation of the weld seams 34. The partition device 48 further has two guide rails 110, which guide the closing member 56 when it slides along a length direction of the driving rod 54. This sliding movement of the closing member 56 in the length direction of the driving rod 54, which is effected by translational movement of the driving rod 54, is illustrated in fig. 7 by two further arrows 112.

Fig. 8 shows the partition device 48 in a side view in which the bolos 58 provided in the closing member 56 do not overlap with the openings 52 provided in the wall member 50.

Thus, the openings 52 are completely closed and the outlet ports 46 are fluidly separated from each other. Upon movement of the closing member 56, the holes 58 provided in the closing member 56 partially overlap with the openings 52 provided in the wall member 50.

Thus, the openings 52 can, for example, be uncovered by 30 % (see fig. 9). By moving the closing member 56 in the corresponding position, a low coolant flow through the openings 52 is therefore enabled. The coolant flow through the openings 52 can be gradually or continuously increased.

lithe closing member 56 is further moved by pushing or pulling the driving rod 54, the holes 58 can be brought into complete overlap with the openings 52, which can in particular have the same dimensions as the holes 56. Thus, the openings 52 are completely uncovered or opened and a maximum coolant flow from one outlet port 46 to the adjacent outlet port 46 is enabled. As the driving rods 54 are operated independently, the coolant flow can be particularly well adjusted to the cooling requirements of the fuel cell stack 10.

As each coolant inlet port 44, which can be called a side feed port, fluidly communicates with only one corresponding coolant channel 30, 32, the coolant flow into each coolant channel 30, 32 can be particularly well adjusted by operating the valves 62. Further, the slide mechanism or such a means for enabling a coolant flow through the openings 52 in the partition devices 48 can be utilized to control the coolant flow through the coolant channels 30, 32. For example, by opening or uncovering all the openings 52 provided in the partition devices 48, one large coolant discharge port can be provided at the coolant outlet side of the bipolar plates 12. Alternatively, all openings 52 in the partition devices 48 can be closed such that each compartment or coolant outlet port 46 is in fluid communication with one corresponding coolant outlet pipe 70 only.

However, by operating the means enabling a coolant flow through the openings 52, a coolant flow from one coolant outlet port 46 to adjacent or neighboring coolant outlet ports 46 is possible, even though the number of the coolant inlet pipes 60 corresponds to the number of coolant outlet pipes 70. As the coolant flow through the fuel cells of the fuel cell stack 10 can be controlled according to the specific cooling requirements, the fuel cell stack 10 can be operated particularly efficiently and dynamically.

List of reference signs fuel cell stack 12 bipolar plate 14 first plate 16 second plate 18 oxidant channel arrow 22 fuel channel 24 arrow 26 reactant inlet side 28 reactant outlet side coolant channel 32 coolant channel 34 weld seam 36 weld 38 width arrow 42 arrow 44 coolant inlet port 46 coolant outlet port 48 partition device wall element 52 opening 54 driving rod 56 closing member 58 hole inlet pipe 62 valve 64 connection device 66 inlet 68 cover plate outlet pipe 72 fuel inlet 74 oxidant inlet 76 fuel inlet port 78 oxidant inlet port fuel outlet 82 oxidant outlet 84 fuel outlet port 86 oxidant outlet port 88 control unit coolant circuit 92 compressor 94 sensor 96 communication line 98 sensor heat exchanger 102 motor 104 arrow 106 arrow 108 arrow guiderail 112 arrow

Claims

Claims Fuel cell stack with a plurality of fuel cells which comprise a membrane electrode assembly arranged between two bipolar plates (12), wherein each bipolar plate (12) comprises a first plate (14) and a second plate (16) bonded together and forming a coolant flow field between the two plates (14, 16), wherein the coolant flow field comprises a plurality of coolant channels (30, 32) which form an angle with reactant channels (18, 22) provided by the bipolar plate (12), characterized in that each coolant channel (30, 32) fluidly communicates with a coolant inlet port (44) provided in the bipolar plate (12) and with a coolant outlet port (46), wherein adjacent coolant outlet ports (46) are separated from each other by a partition device (48) with at least one opening (52) and with means (54, 56, 58) configured to enable a coolant flow through the at least one opening (52).
2. Fuel cell stack according to claim 1, characterized in that the means (54, 56, 58) are configured to vary and/or to disable the coolant flow through the at least one opening (52).
3. Fuel cell stack according to claim 1 or 2, characterized in that each coolant inlet port (44) fluidly communicates with a corresponding coolant inlet pipe (60) equipped with a valve device (62).
4. Fuel cell stack according to any one of claims 1 to 3, characterized by a control unit (88) configured to operate the means (54, 56, 58) and/or the valve device (62).
5. Fuel cell stack according to claim 4, characterized in that by operating the valve device (62) the control unit (88) is configured to vary a volume flow rate and/or a pressure of the coolant provided to each coolant channel (30, 32).
6. Fuel cell stack according to claim 4 or 5, characterized in that the control unit (88) is configured to operate the means (54, 56, 58) and/or the valve device (62) in dependence on -a requested power output of the fuel cell stack (10) and/or -at least one environmental parameter and/or -at least one parameter, in particular comprising a temperature and/or a cell voltage, of the fuel cell stack (10).
7. Fuel cell stack according to any one of claims 1 to 6, characterized in that -a first coolant channel (32) which is closer to a reactant outlet side (28) of the bipolar plate (12) has a cross-section area capable of being passed by the coolant, which is smaller than the cross-section area of a second coolant channel (30) which is closer to a reactant inlet side (26) of the bipolar plate (12) than the first coolant channel (32) and/or -a number of coolant channels (30, 32) per surface area unit of the bipolar plate (12) is larger close to a reactant outlet side (28) of the bipolar plate (52) than close to a reactant inlet side (26) of the bipolar plate (12).
8. Fuel cell stack according to any one of claims 1 to 7.characterized in that the means comprise a driving rod (54) with a closing element (56) configured to obstruct the at least one opening (52) in a wall element (50) of the partition device (48), wherein the closing element (56), which is in particular slideably engaged in a guide rail (110) of the partition device (48), has at least one hole (58) which can be aligned with the at least one opening (52).
9. Fuel cell system, in particular for a vehicle, with a fuel cell stack (10) according to any one of claims 1 to 8, wherein an oxidant inlet (74) and a fuel inlet (72) are located opposite an oxidant outlet (82) and a fuel outlet (80) of the fuel cell stack (10).
10. Vehicle with a fuel cell system according to claim 9.