Clamping system for a fuel cell stack and fuel cell system having such a clamping system

Abstract

The invention relates to a clamping system for a fuel cell stack comprising a plurality of disc springs and a fuel cell system having such a clamping system. According to the invention, it is provided that the disc springs are connected to one another in a materially and / or forcefully manner in a plane perpendicular to an axis of rotational symmetry extending through the center of a circle described by the disc.

Description

Technical Field

[0001] The invention relates to a clamping device for pressing together individual cells of a fuel cell stack and to a fuel cell system having such a clamping device. Background Art

[0002] Fuel cells utilize the chemical conversion of fuel and oxygen into water to generate electrical energy. For this purpose, fuel cells contain a so-called membrane-electrode assembly (MEA, i.e., membrane electrode assembly) as a core component, which is a structure consisting of an ion-conducting (mostly proton-conducting) membrane and a catalytic electrode (anode and cathode) arranged on both sides of the separator. The electrodes mostly include supported precious metals, especially platinum. In addition, gas diffusion layers (GDL) can be arranged on both sides of the membrane-electrode assembly on the side of the electrode facing away from the membrane. Usually, fuel cells are formed by a large number of MEAs arranged in a stack (fuel cell stack), and the electrical power of the MEAs is added. Bipolar plates (also called flow field plates or separator plates) are usually arranged between each membrane-electrode assembly, which ensure that each individual cell is supplied with an operating medium, i.e., a reactant, and are usually also used for cooling. In addition, the bipolar plate is responsible for the conductive contact with the membrane-electrode assembly.

[0003] During operation of the fuel cell, the fuel (anode operating medium), in particular hydrogen H 2 or a hydrogen-containing gas mixture, is supplied to the anode via an open flow field on the anode side of the bipolar plate, where the electrochemical oxidation of H 2 to protons H 2 occurs with the release of electrons. + (H2→2H + +2e – The transport of protons from the anode space (in combination with water or without water) to the cathode space is achieved via an electrolyte or a membrane (which separates the reaction spaces gas-tightly from each other and is electrically insulating). The electrons provided at the anode are directed to the cathode via an electrical line. Oxygen or an oxygen-containing gas mixture (e.g. air) is supplied to the cathode as a cathode operating medium via an open flow field on the cathode side of the bipolar plate, so that the reduction of O2 to O2 occurs when the electrons are accepted. 2- (1 / 2O2+2e – →O 2- At the same time, in the cathode space, oxygen anions react with protons transported via the membrane to form water (O 2- +2H + →H2O).

[0004] Fuel cell stacks typically have end plates at their opposite stack ends, which are connected to one another by means of clamping devices as part of a clamping system. The clamping devices transmit tensile forces that pull the end plates toward one another and press the individual cells arranged between the end plates, i.e. against one another.

[0005] In order to transmit the clamping force uniformly to the stack and to avoid damage to the stack, the clamping system comprises a spring system in addition to the clamping device.

[0006] DE 102 13 558 A1 and US 2014 / 0060143 A1 disclose the use of disc springs in clamping systems for fuel cell stacks, wherein a plurality of conical disc springs are arranged between an end plate and a support plate of the fuel cell system.

[0007] The disc spring has the shape of a flat truncated cone or spherical truncated outer surface. It is elastically pressed together between its edge circles (theoretically up to the maximum until it is deformed into a flat disk), wherein normal stresses (compressive and tensile stresses) are formed in the material as a reaction to the introduced forces.

[0008] Individual disc springs are relatively hard springs and are accordingly used where such disc springs are required. Softer springs are produced by stacking the discs in opposite directions ("series connection"). Springs that are harder than individual springs are produced by stacking them in the same direction ("parallel connection").

[0009] By stacking the disc spring columns, not only the force but also the spring travel can be increased multiple times.

[0010] The use of disc springs in clamping systems for fuel cell stacks results in a relatively uneven distribution of forces on the fuel cells or on the surface of the fuel cell stack. In addition, such clamping systems are sensitive to errors in the arrangement and number of the individual springs. Incorrect arrangement of the springs (within very small error tolerances) or incorrect orientation of at least one of the springs used leads to strong point loading of the fuel cell stack and often to the breaking or at least damage of some of the fuel cells. Summary of the invention

[0011] The object of the present invention is now to solve or at least alleviate the problems of the prior art. Furthermore, a fuel cell system is to be provided which is optimized with regard to the quality / yield ratio.

[0012] This object is achieved by a clamping system and a fuel cell stack with the features of the independent claims. Therefore, a first aspect of the invention relates to a clamping system for a fuel cell stack, which clamping system includes a plurality of disc springs, wherein the disc springs have a material-fitting and / or force-fitting connection in the plane of the disc, i.e. in a plane perpendicular to the axis of rotation extending through the center of the circle described by the disc. The disc spring connection according to the invention, on the one hand, leads to the fixing of the springs and thus to a more reliable uniform distribution of the springs on a surface, in particular the base surface of the fuel cell stack. In addition, a uniform distribution of forces on the plane is achieved via the material-fitting and / or force-fitting connection. Therefore, compared with similar known arrangements, disc springs of the same number and strength (thickness) have a higher spring force and in particular a higher stability in the design according to the invention. In turn, less material is required in the clamping system according to the invention for the same theoretical spring force, because the springs must be implemented less thick, which in turn leads to a lighter design of the clamping system according to the invention compared to the prior art. The use of the clamping system according to the invention in a fuel cell stack thus improves the power density with respect to the weight of the fuel cell system.

[0013] In the present case, a cohesive connection is also understood to mean a one-piece connection.

[0014] Furthermore, the force transmission to the fuel cell stack takes place via a larger surface, which is therefore more uniform and in turn results in a lower load on the stack. Furthermore, the arrangement of the disc springs in the clamping system according to the invention offers production advantages, since fewer work steps are required to place a plurality of disc springs in the fuel cell stack, and these fewer work steps place lower demands on dimensional stability during production.

[0015] In a preferred embodiment of the invention, it is provided that the disc springs are arranged in the form of a pack. Here, the pack comprises a layer, i.e. a plurality of springs connected in the plane defined above. Advantageously, a pack comprises or consists of a layer of disc springs, i.e. all disc springs arranged in a layer on the fuel cell stack. Alternatively, 2 or 4 packs are arranged in a layer, so that a pack comprises correspondingly fewer disc springs. The design as a pack improves the reproducibility of the arrangement of the springs relative to each other and in particular of their layers when arranged in the fuel cell stack.

[0016] Advantageously, the spring has an outer diameter in the range of 3 mm to 20 mm, preferably in the range of 5 mm to 15 mm, and is thus significantly smaller than the disc springs with a diameter of 5 cm to 6 cm typically used in the prior art. The thickness of the material used is preferably in the range of 0.05 mm to 2 mm, in particular in the range of 0.1 mm to 0.8 mm. These ranges exhibit an optimal ratio between weight and stability.

[0017] In the case of a plurality of groups, advantageously 4 to 6 springs are installed per group, and these springs are arranged so that the base surface of one group is rectangular, in particular square, and substantially corresponds to the base surface of the fuel cell or fuel cell stack. All this results in less material with the same spring force.

[0018] The plurality of disc springs used in the clamping system according to the invention are preferably designed as a whole in one piece, that is to say the disc springs connected to one another are particularly advantageously connected to one another via a material-bonded, in particular one-piece connection. This has the advantage that the plurality of disc springs, in particular a group, can be produced from a one-piece flat material, for example by stamping and / or bending material layers. Alternatively, the plurality of disc springs, in particular the group, is cast, in particular injection molded.

[0019] In the design of a one-piece or flat connection of a plurality of disc springs, one layer or a group is preferably stamped out of a material layer and / or formed by deforming a material layer. This provides production advantages of the connection as well as improved stability and durability.

[0020] In a preferred embodiment of the present invention, the connection between adjacent springs is implemented as a tab or as a planar, especially full-surface connection. The connection implemented via the tab leads to further material savings and thus less weight, wherein a planar, especially full-surface connection is more rigid and more stable.

[0021] If the springs are connected to one another via webs, each disc spring is preferably connected to four and / or six springs, which results in a rectangular or hexagonal arrangement of the springs. The webs form an equilateral triangle, with the centers of the disc springs serving as corner points. Connections to four disc springs are preferred in the case of large spacings of the disc springs relative to one another and in the case of low packing densities. Such an arrangement and connection results in a very regular and optimized use of space in one layer, which results in a higher stability and uniformity of the force transmission when arranged in a fuel cell stack.

[0022] Currently, web-type connections and flat connections are distinguished in particular by the ratio of the connection area to the free area or to the material area. In web-type connections, the material area between the disc springs is smaller than the area of ​​the gaps in the material.

[0023] The spacing between adjacent disc springs is preferably in the range of 40% to 70%, preferably in the range of 45% to 67%, of the outer diameter of one of the disc springs. In particular, an edge range of 45% (short flange) or 67% (long flange) of the outer diameter of the disc spring has proven to be particularly advantageous with regard to the material / force ratio.

[0024] Particularly advantageously, the clamping system has a plurality of layers of connected disc springs which are arranged one above the other. The disc springs can be arranged vertically one above the other, or alternatively the layers can be arranged offset relative to one another. The latter design offers the advantage that the forces transmitted to the fuel cells by the clamping elements of the clamping device are distributed more evenly over the base surface of the fuel cell stack, for example when used in a fuel cell stack.

[0025] Particularly preferably, the layers are arranged relative to one another in such a way that the layers arranged one above the other are oriented in a spring-superimposed manner, ie in particular in a vertical superposition. In this case, the known cylindrical group construction of a plurality of disk-shaped lenses described above has known advantages.

[0026] It is further preferred that the springs of the layers arranged one above the other are at least partially oriented in the same direction. At present, a distinction is to be made between a same orientation and a reverse orientation in a layer or stack of adjacent disc springs, wherein a same orientation of the disc springs is to be understood as bending in the same direction, and a reverse orientation corresponds to an orientation of adjacent disc springs in opposite directions.

[0027] The bulge of the disc spring is preferably designed to be conical or spherical, in particular hemispherical. A conical, i.e. truncated cone-shaped bulge is preferred here because it is more rigid than the alternative designs. It has been shown that the force / deflection diagram has a different course than a spherical disc-shaped lens, which has a favorable effect on use in a fuel cell stack.

[0028] Another aspect of the invention is a fuel cell stack and a fuel cell system having a clamping system according to the invention in one of the described embodiments. Such a fuel cell stack or fuel cell system has a better weight-to-yield ratio and is also more fracture-stable.

[0029] The singulation of a plurality of disc springs according to the invention is preferably carried out by punching, laser cutting or water jet cutting, whereby the most efficient method for singulation is provided for the respectively required number of pieces.

[0030] Further preferred embodiments of the invention result from the features mentioned in the remaining dependent claims.

[0031] The different embodiments of the invention mentioned in this application can advantageously be combined with one another, unless they are implemented separately. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The invention is explained below in the form of an exemplary embodiment with reference to the accompanying drawings.

[0033] Figure 1 A schematic diagram showing a fuel cell stack according to a preferred design solution;

[0034] Figure 2 A schematic diagram showing a section of a group in a first embodiment of the present invention;

[0035] Figure 3 A schematic diagram showing a group with multiple disc springs in a second design of the present invention;

[0036] Figure 4 A preferred design scheme of the present invention is shown according to Figure 2 A schematic diagram of a stack of multiple groups of the design shown in ; and

[0037] Figure 5 The diagram shows the course of the force versus spring deflection in four preferred embodiments relative to known arrangements. DETAILED DESCRIPTION

[0038] Figure 1 A schematic diagram shows a fuel cell stack according to a preferred embodiment of the present invention, generally designated 100. Fuel cell stack 100 is part of a vehicle (not shown in detail), in particular an electric vehicle, which has an electric traction motor which is supplied with electrical energy via fuel cell stack 100.

[0039] The fuel cell stack 100 comprises a plurality of membrane-electrode assemblies 10 and polar plates 12 which are arranged alternately on their flat sides (stacked). Thus, a plurality of stacked individual cells 11 form the fuel cell stack 100 overall, wherein not only an individual cell of each individual cell 11 but also the fuel cell stack 100 can be generally referred to as a fuel cell.

[0040] The polar plates 12 can be designed as bipolar plates, as long as they are arranged between the membrane-electrode assemblies 10. Two polar plates 12 arranged between the membrane-electrode assemblies 10 and the end plates 18 of the fuel cell stack 100 are called monopolar plates. An anode space and a cathode space (not shown) are arranged between the polar plates 12 and the respective membrane-electrode assemblies 10, which are delimited by a surrounding seal 20. In particular, in order to establish the sealing function of the seal 20, the fuel cell stack 100 is pressed together (compressed) in the stacking direction S by means of a clamping system. The clamping system comprises an external clamping device 22 and a spring system arranged between the external polar plates and the end plates 18. In order to build up external stresses (which are transmitted to the fuel cell stack via the spring system), an elongated tension body (Zugkoerper) 24 of the external clamping device 22 transmits a tensile force between the two end plates 18, so that the end plates 18 are pulled toward each other by means of the tension body 24. For this purpose, the tensioning body 24 extends in the stacking direction S of the fuel cell stack 100. The elongated tensioning body 24 and at least a part of the fixing means 26 are designed as one piece.

[0041] In the illustrated design, at least one of the clamping devices 22 includes, in addition to the elongated pulling body 24, at least one fixing device 26 connected to the pulling body 24. The fixing device 26 is designed to fix at least one component 29 adjacent to the fuel cell stack 100. The adjacent component 29 can be, for example, an insulating part of the stack or, as shown, a cable. The cable can extend from the current collector toward the opposite end of the fuel cell stack 100 and is used to conduct the electrical power of the fuel cell stack 100.

[0042] The contact pressure generated by the external clamping device 22 is usually transmitted to the fuel cell stack 100 via a spring system. According to the invention, the spring system comprises a plurality of disk springs 41 which are connected to one another in a materially and / or force-fitting manner.

[0043] The section of the spring system 21 according to the invention is Figure 2 Shown in. Figure 2 A plurality of disc springs 41 are shown, which are connected to one another in a materially and / or forcefully manner in a plane 47. In a design in which a plurality of disc springs are connected to one another both in a materially and forcefully manner, it is provided, for example, that some of the disc springs are connected to one another in a materially and forcefully manner, while other disc springs 41 are connected to one another in a forcefully manner. The plane 47 in which the disc springs 41 are connected to one another is in the plane 47. Figure 2 The plane 47 extends perpendicularly to the axis of rotation 48 , which in turn extends through the center 46 of the circle described by the disc spring 41 .

[0044] The connection 42 between the disc springs 41 is preferably designed as a web 43, but alternatively (in Figure 3 In the design scheme shown in FIG. 4 , the connecting portion 44 is designed as a flat surface. Figure 2 In the design shown in , each disc spring 41 is connected to 6 other adjacent disc springs via a web 43. Alternatively, the disc spring is connected to 4 adjacent disc springs via a web 43. The number of connecting webs 42 per disc spring 41 corresponds essentially to the number of adjacent disc springs 41 for each disc spring 41.

[0045] Figure 3 The design shown in the figure has in particular a full-surface connection 44 of the disc springs 41. The design shown of the flat connection 44 between the disc springs 41 is in particular Figure 3 The result of the full-surface connection 44 shown in FIG. 4 is that not only adjacent disc springs are connected to one another, but that almost all disc springs of a group 45 have a material-locking connection.

[0046] exist Figure 3 45 shows a group which comprises a plurality of disc springs 41. The number of disc springs 41 shown arranged side by side along an edge is to be understood here merely as an example and in no way as limiting, and the same applies to the spacing and arrangement of the disc springs 41 relative to one another. It becomes clear that the disc springs arranged along the edge of the group 45 have fewer adjacent disc springs 41. The same also applies to a group 45 whose plurality of disc springs 41 are connected to one another via webs 43, whereby the disc springs 41 arranged along the edge have a reduced number of connections 43 corresponding to their reduced number of adjacent disc springs.

[0047] The disc springs 41 preferably have an outer diameter in the range of 0.05 mm to 2 mm. The spacing between the disc springs 41 is preferably in the range of 40% to 70% of the outer diameter, in particular in the range of 45% to 67%, the last-mentioned edge range being particularly preferred. The percentages mentioned relate to the spacing between the centers 46 of the disc springs 41.

[0048] When arranged in a fuel cell stack 100, preferably a plurality of the above-described groups 45 are arranged one above the other, in particular stacked, the group consisting of a respective plane 47 of a plurality of mutually connected disc springs 41. Figure 4 A schematic cross-sectional view of such a stack is shown in FIG. Figure 4 A section of a group 45 is shown in which adjacent disk springs 41 are connected to one another via webs 43. Furthermore, the features and embodiments shown and described above and below can be transferred accordingly to a group 45 with flat connections 44 between the individual disk springs 41.

[0049] The sectional view shows four disc springs 41 stacked one on top of the other, wherein in the embodiment shown, a stack of disc springs 41 each comprises two sub-stacks 51, 52. By way of example, the stacked disc springs 41 of the first sub-stack 51 are arranged laterally, in other words, the bulges of adjacent disc springs 41 in this sub-stack 51 are oriented in opposite directions. As an alternative, a sub-stack 52 is shown, wherein adjacent disc springs 41 in the sub-stack 52 are oriented in the same direction, i.e., their bulges point in the same direction, which results in the surfaces of the disc springs 41 being parallel.

[0050] The arched portion of the disc spring 41 can be as follows Figure 4 As shown in , it is designed to be conical, that is, in the form of a flattened truncated cone. In this case, the outer peripheral surface of the truncated cone forms the disc portion of the disc spring 41. Alternatively, the disc portion of the disc spring 41 is formed by a spherical segment, especially a hemisphere, wherein a circular gap is arranged in the center.

[0051] Figure 5 A force / deformation diagram is shown, wherein the deformation of the respective disc spring is given in millimeters on the abscissa. To determine the deformation, the reduction in the height of the curvature of the disc spring is measured in millimeters. The deformation is plotted against the force (in Newtons) acting on the disc spring, wherein the force acting on the disc spring is shown in the diagram on the ordinate.

[0052] The line diagram shows five curves, wherein the quadrilateral measuring points correspond to a plurality of disc springs which are connected by a planar connection, wherein the spacing between the disc springs is in the following preferred range, in the present case 45% of the outer diameter of the disc springs. Likewise, the planar connection is based on the measuring points marked with "X", wherein the spacing between the disc springs is in the range of 67% of the outer diameter of the disc springs. The measurement of a plurality of disc springs connected to one another by webs shows approximately the same slope (at least in the range of 0 mm to 0.25 mm compression of the disc springs), wherein the webs are again connected to one another in the same way (the measuring points are marked with stars). The measuring points marked with triangles correspond to the results of the deformation of a plurality of disc springs which are connected to one another only by webs.

[0053] Compared to the base line (rectangular measuring points), all four measured designs of the connected disc springs according to the invention show a steeper curve. The connected disc springs are therefore stiffer for the same force transmission at the same material thickness. Therefore, less material can be used in the design according to the invention, which in turn leads to a reduction in weight.

[0054] List of reference numerals:

[0055] 10. Film-Electrode-Assembly

[0056] 11 Single battery

[0057] 12 Plates

[0058] 18 End plate

[0059] 20 Seals

[0060] 21 Spring system

[0061] 22 Clamping device

[0062] 24 Slender pulling body

[0063] 26 Fixing device

[0064] 28 Cable guide

[0065] 29 Components

[0066] 41 disc spring

[0067] 42 Connection

[0068] 43 splice

[0069] 44-sided connection

[0070] 45 groups

[0071] 46 Center

[0072] 47 Plane

[0073] 48 Rotation axis

[0074] 50 stacks

[0075] 51 Reverse orientation

[0076] 52 Same direction orientation

[0077] 100 Fuel Cell Stack

[0078] S Stacking direction.

Claims

1. A clamping system for a fuel cell stack (100), the clamping system comprising a plurality of disc springs (41), characterized in that: The disc springs (41) are integrally formed in one piece and are connected to one another in a disc plane (47) by means of a one-piece connection (42), wherein the connection between adjacent disc springs (41) is configured as a web (43), wherein the disc plane (47) is a plane perpendicular to an axis of rotation extending through the center of a circle described by the disc springs (41), wherein the disc springs (41) are located in the disc plane (47) and the one-piece connection (42) is also located in the disc plane (47), wherein each disc spring (41) is connected to four or six adjacent disc springs (41), and wherein the material area between the disc springs (41) is smaller than the area of ​​the gaps in the material.

2. The clamping system according to claim 1, characterized in that The plurality of disc springs are produced by punching out a material piece or by forming a material piece.

3. The clamping system according to claim 1 or 2, characterized in that: Adjacent disc springs (41) are arranged in a hexagonal or rectangular manner relative to one another. 4 . The clamping system according to claim 1 , comprising at least two layers of a plurality of disc springs ( 41 ) arranged one above the other.

5. The clamping system according to claim 4, characterized in that The layers are arranged relative to one another in such a way that the disk springs (41) of the layers arranged one above the other are oriented in a superimposed manner.

6. The clamping system according to claim 5, characterized in that The disc springs (41) of the layers arranged one above the other are at least partially oriented in the same direction.

7. The clamping system according to claim 5, characterized in that The disc springs (41) of the layers arranged one above the other are oriented alternately in the same direction and in opposite directions.

8. The clamping system according to claim 5, characterized in that The disc springs (41) of the layers arranged one above the other are oriented alternately in the same direction and in opposite directions at defined intervals.

9. The clamping system according to claim 1 or 2, characterized in that: The disc spring (41) has a conical or spherical truncated shape.

10. The clamping system according to claim 1 or 2, characterized in that: The disc spring (41) has a hemispherical shape.

11. A fuel cell stack (100) having a clamping system according to any one of the preceding claims 1 to 10.

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