FUEL CELL STACK

Description

[0001] The invention relates to a fuel cell stack according to the features of claim 1 and a vehicle according to the features of claim 9.

[0002] Fuel cells utilize the chemical conversion of a fuel with oxygen to water to generate electrical energy. For this purpose, fuel cells have a membrane electrode assembly (MEA) with a membrane electrode unit. The membrane electrode unit is formed by a proton-conducting membrane (PEM), with catalytic electrodes arranged on both sides. The membrane separates the anode compartment associated with the anode and the cathode compartment associated with the cathode, electrically insulating them. Gas diffusion layers can also be arranged on the sides of the electrodes not facing the membrane.

[0003] During operation of the fuel cell, a hydrogen-containing fuel is fed to the anode, where electrochemical oxidation of H2 to H+ occurs with the release of electrons. The protons H+ are transported from the anode compartment to the cathode compartment via the electrolytic membrane, either in water or without water. The electrons provided at the anode are transferred to the cathode via an electrical conductor.

[0004] An oxygen-containing operating medium is supplied to the cathode, where a reduction of O2 to O2 occurs, absorbing electrons. These oxygen anions react in the cathode chamber with the protons transported across the membrane to form water. A fuel cell stack is typically formed by a large number of MEAs arranged one above the other in a stack direction, whose electrical outputs add up. Bipolar plates are typically arranged between the membrane-electrode assemblies. These plates ensure that the individual MEAs are supplied with reactants and a coolant and act as an electrically conductive contact to the membrane-electrode assemblies.

[0005] Gaskets are arranged between the membrane electrode assemblies and the bipolar plates to seal the anode and cathode compartments from the outside and prevent the operating media from escaping from the stack. These gaskets are provided on the membrane electrode assemblies, the bipolar plates, or both of these components.

[0006] To permanently seal the stack and ensure electrical contact between the bipolar plates and membrane electrode assemblies, the fuel cell stack is pressed together before commissioning. Tension elements are also used to compress the fuel cell stack during operation.

[0007] Various tension elements are known from the prior art. For example, two end plates arranged at the ends of the fuel cell stack can be connected by means of tension elements. By introducing tensile forces into the end plates via the tension elements, the fuel cell stack is compressed. Threaded rods, tie rods, chains, or the like are used as tension elements.

[0008] Also known is the use of tensioned strip- or band-shaped elastic tensioning elements, which are either connected to the end plates or at least partially encircle the stack in at least one cross-section (in the stacking direction).

[0009] With regard to the design and fastening options of such tension and clamping elements, reference is made, for example, to EP 1 870 952 A2, DE 10 2012 000 266 A1, WO 2012 / 065890 A1 and DE 11 2014 001 941 T5, the contents of which are hereby incorporated by reference in their entirety.

[0010] Operational height changes can occur, particularly in the active areas of the fuel cell stack or the MEAs, which can vary, for example, with the temperature and moisture content of the fuel cell stack. This is also referred to as "breathing" of the stack. When using elastic clamping elements, the elasticity of these clamping elements can also decrease over time. Particularly with metallic clamping elements that encircle the fuel cell stack in a ring and thus have several distinct bends, the clamping elements can elongate, particularly in the area of ​​these bends.

[0011] Regardless of changes in the height of the fuel cell stack and the aging of the clamping elements, sufficient compression of the stack must always be ensured, especially to guarantee the sealing effect of the seals used. Therefore, state-of-the-art methods are already known to attempt to permanently ensure the compression of a fuel cell stack.

[0012] DE 10 2010 007 979 A1 discloses a fuel cell stack with fuel cells arranged between two end plates and at least one bracing means connecting the end plates for exerting a tensile force. A friction-reducing deflection means is arranged between the bracing means and at least one of the end plates. A change in the height of the stack is to be compensated for by compressed spring elements arranged between an end plate and a compression plate.

[0013] According to this state of the art, stress peaks due to stack expansion are to be prevented by elastic elements. Initially over-tensioning the elastic elements should even partially prevent a change in stack height. However, the elastic elements themselves are subject to aging, and at least in this respect, this passive option for adjusting the compression force is disadvantageous.

[0014] In known solutions with compressed spring elements arranged between the end plates and other stack components, recesses can be provided in the end plates to accommodate the spring elements. This requires additional space in the fuel cell stack, and the recesses also increase the tendency of the end plates to deform and damage under the applied compressive or tensile forces.

[0015] A further disadvantage of known solutions is that they often cannot ensure uniform clamping of the entire end plates. For example, known clamping elements often cannot be positioned near the inlet or outlet openings of the operating media without blocking them. An example of this is disclosed in US 2006 / 0093890 A1. This disadvantageously leads to bending of the end plate edges.

[0016] DE 10 2008 026 858 A1 discloses a fuel cell system with a compression maintenance system stretched between the ends of the fuel cell stack, which has a plurality of belts.

[0017] US 2009 / 0280359 A1 shows a fuel cell system with a fuel cell stack whose ends are clamped together by means of a plurality of springs.

[0018] DE 10 2004 027 694 A1 discloses a fuel cell stack whose end plates are tensioned by means of tensioning straps. Means for tensioning the tensioning straps are provided on the tensioning straps, with disc springs arranged between the means and the fuel cell stack. Neither the tensioning straps nor the disc springs are arranged in an end region of the end plates.

[0019] The invention is based on the object of overcoming the disadvantages of the prior art and providing a solution for the most uniform compression of fuel cell stacks possible, which can be integrated into existing fuel cell stacks in a space-saving manner.

[0020] The problem concerning the fuel cell stack is solved by a fuel cell stack having the features of claim 1. The problem concerning the vehicle is solved by a vehicle having the features of claim 9.

[0021] This object is achieved by a fuel cell stack comprising a first end plate and a second end plate, and a plurality of fuel cells arranged between the two end plates in the stacking direction. Furthermore, the fuel cell stack comprises at least one elastic tensioning or clamping element tensioned between the end plates in the stacking direction.

[0022] This at least one first tension or tensioning element is preferably a threaded rod, tie rod, chain, or the like. Likewise preferably, the first tension or tensioning element is a tensioned strip- or band-shaped elastic tensioning element that is either connected to the end plates or at least partially encircles the stack in at least one cross-section (in the stacking direction). In other words, the first tension or tensioning element is a conventional component known from the prior art and already regularly used to compress fuel cell stacks.

[0023] Inlet and outlet openings for operating media are arranged in first end regions of the first end plate and / or in second end regions of the second end plate. The end regions are always peripheral regions of the end plates, which thus always encompass at least one edge region of the end plate. Each end plate preferably has two end regions that are arranged on or encompass opposite edge regions. For example, rectangular end plates always have two pairs of opposite edge regions. The end regions are then preferably arranged on the pair of shorter edge regions.

[0024] The inlet and outlet openings are preferably inlet and outlet openings for an anode operating medium, a cathode operating medium, and a coolant. For example, a first end plate has inlet openings and outlet openings, whereas a second end plate has no such openings. Alternatively, a first end plate has only inlet openings and a second end plate only outlet openings. Likewise, a first end plate can have first inlet openings, for example for anode and / or cathode operating medium, and the second end plate can have second inlet openings, for example for a coolant. The arrangement of the inlet and outlet openings in the end plates depends significantly on the flow paths within the fuel cell stack and the arrangement of peripheral components, for example a coolant tank.

[0025] According to the invention, at least one (second) elastic tensioning element is tensioned between at least one first and at least one second end region of the end plates. The at least one (second) elastic tensioning element is different from the at least one (first) tension or clamping element and is arranged exclusively in the end regions of the end plates. A fuel cell stack that has first tension or clamping elements for clamping the entire stack additionally has, according to the invention, second elastic tensioning elements for clamping the end regions of the end plates.

[0026] Advantageously, the fuel cell stack according to the invention thus enables targeted clamping of the end regions of the end plates containing the operating fluid openings through the use of first tension or clamping elements and second clamping elements. This prevents bending of the end plates in the end regions, damage to the end regions due to excessive tensile forces, or blockage of the inlet / outlet openings.

[0027] According to the invention, a tensile stress applied by a tensioned (first) tension or clamping element differs from a tensile stress applied by a tensioned (second) elastic clamping element. According to the invention, the tensile stress applied by a second elastic clamping element is lower than the tensile stress applied by a first tension or clamping element. This advantageously prevents the end plates from bending and the end regions from becoming excessively tight. The elastic clamping element is preferably tensioned in the Hookian region and in a stacking direction (S) of the fuel cell stack.

[0028] The at least one second elastic tensioning element is preferably fixed to at least one end plate by a material, force, and / or form-fitting connection and / or by means of at least one tensioning device. To ensure easy disassembly of the fuel cell stack, the elastic tensioning element is preferably releasably fixed to at least one end plate of the stack.

[0029] Particularly preferably, the elastic second tensioning element is suspended from the end plate. For this purpose, the end plate preferably has at least one suspension hook for suspending the elastic tensioning element on one of its side surfaces or on its surface facing outward in the stacking direction. Alternatively, the elastic tensioning element has at least one suspension opening for suspension on a suspension hook.

[0030] Likewise, the at least one first tension or tensioning element preferably extends in a cross-section in the stacking direction at least substantially around a circumference of the fuel cell stack. A tension or tensioning element is fixed at at least one end region to another end region of the same or another tension or tensioning element. The tension or tensioning element is preferably designed in a closed, annular manner.

[0031] Particularly preferably, the end region of the at least one first tension or tensioning element is positively connected to another end region of the same tension or tensioning element, for example, by a crimp connection. Likewise preferably, an end region of the at least one tension or tensioning element is fixed to another end region of the same or another tension or tensioning element by means of a fastening device. Particularly preferably, an end region of the at least one tension or tensioning element is welded to another end region of the same or another tension or tensioning element.

[0032] In a likewise preferred embodiment, the fuel cell stack according to the invention has at least one rotatable mounting shaft arranged along an edge region of the first end region and / or the second end region. The mounting shaft is preferably integrated into the respective edge region and particularly preferably rotatably mounted in or on this edge region. According to this embodiment, an end section of at least one elastic clamping element is fixed to the mounting shaft, preferably by force-fitting, material-fitting, and / or form-fitting means, in order to prevent relative movement between the end section and the mounting shaft.

[0033] The edge region of an end plate refers to a transition between a first, outward-facing surface of the end plate and a second surface of the end plate perpendicular to the first surface. The second surface preferably extends along the stacking direction of the fuel cell stack or has a normal vector perpendicular to the stacking direction.

[0034] Particularly preferably, the at least one mounting shaft is guided partially through the end plate in the region of an edge area of ​​an end plate. In the case of a cylindrical mounting shaft, the end plate in this embodiment preferably has a cylindrical bore. This advantageously provides a simple rotatable mounting of the mounting shaft.

[0035] Furthermore, according to this embodiment, the end plate preferably has recesses or recesses in the edge region in which the mounting shaft is exposed. Particularly preferably, the at least one elastic tensioning element is attached to the mounting shaft in the region of the recess. This advantageously facilitates its winding onto the mounting shaft.

[0036] By fixing the end portion of at least one second elastic tensioning element to the mounting shaft, the tensile stress of the at least one elastic tensioning element can be adjusted by rotating the mounting shaft. The fuel cell stack according to the invention thus allows for precise re-tensioning of the at least one elastic tensioning element.

[0037] Preferably, a second end portion of the at least one elastic tensioning element is also fixed to the fuel cell stack. The second end portion is different from the first end portion, and each elastic tensioning element has a first and a second end portion.

[0038] The second end section is preferably fixed to another mounting shaft. This allows the elastic tensioning element to be advantageously re-tensioned on both sides, thus allowing it to be particularly evenly and precisely re-tensioned. Likewise, the second end section is preferably firmly fixed to any other section of the fuel cell stack, in particular an end region of an end plate. This is a particularly simple embodiment of the invention.

[0039] The tensile stress is preferably adjusted by winding the elastic tensioning element onto the mounting shaft by rotating the mounting shaft in a first direction of rotation. Thus, the at least one elastic tensioning element between the first and second end plates is effectively shortened, thereby increasing the tensile force generated by it. Advantageously, the elastic tensioning element can be unwound from the mounting shaft again by rotating the mounting shaft in an opposite second direction of rotation. Thus, the at least one elastic tensioning element between the first and second end plates is effectively lengthened, thereby reducing the tensile force generated by it. Thus, compression can be adjusted particularly precisely, particularly in the region of the end regions of the end plates.

[0040] In a likewise preferred embodiment of the fuel cell stack according to the invention, the at least one mounting shaft has an elongated radial groove. This preferably extends parallel to a rotational axis of the mounting shaft. The radial groove is preferably designed to accommodate the end portion of the at least one elastic clamping element. In other words, the end portion of the elastic clamping element is inserted into the radial groove and fixed there, for example, by a material fit, a form fit, and / or a force fit.

[0041] In a preferred embodiment of the fuel cell stack, the rotatable mounting shaft is connected to the fuel cell stack, in particular to an end region of one of the end plates, via at least one tensioned torsion spring. In other words, the torsion spring is fixed to the mounting shaft and additionally to the fuel cell stack, in particular to an end plate. Preferably, a first end of the torsion spring, for example a first end of a wire wound into a spring, is attached to the mounting shaft, and a second end of the torsion spring, for example a second end of the wire wound into a spring, is attached to the fuel cell stack. Thus, the spring tension or the torque of the tensioned torsion spring is transmitted to the rotatably mounted mounting shaft.Preferably, the torsion spring is connected to the mounting shaft in such a way that a torque exerted by the spring is opposed to a tensile stress exerted by the elastic tensioning element.

[0042] By arranging the tensioned torsion spring between the mounting shaft and the fuel cell stack, particularly the end region of an end plate, a directed torque that opposes the tensile stress of the elastic clamping element is advantageously transmitted to the mounting shaft. Thus, when the tensile stress of the elastic clamping element decreases, for example, due to aging-related lengthening of the elastic clamping element, the spring advantageously leads to the retensioning of the elastic clamping element using the torque of the torsion spring.

[0043] According to this preferred embodiment, compression of the fuel cell stack, in particular of the end regions of the end plates clamped by elastic tensioning elements, is reliably ensured even over an extended period of time. Particularly advantageously, an elongated elastic tensioning element is automatically re-tensioned by winding the elastic tensioning element onto the mounting shaft using the potential energy stored in at least one torsion spring. Winding only occurs when the tensile stress of the elastic tensioning element falls below the torque of the torsion spring.

[0044] Likewise preferably, the at least one elastic second tensioning element, preferably each elastic tensioning element, is tensioned via a plurality of torsion springs connected to the at least one mounting shaft. For this purpose, the at least one elastic tensioning element is fixed to a mounting shaft, and the plurality of tensioned torsion springs are fixed to this mounting shaft and the fuel cell stack. Thus, the spring forces applied by the plurality of torsion springs add up to a total force.

[0045] In a further preferred embodiment, the at least one torsion spring is a helical spring, particularly preferably a helical spring wound from wire or spring steel. Likewise preferably, the torsion spring is a torsion bar spring or a torsion bar. Furthermore, such a torsion bar spring is preferably formed integrally with the mounting shaft, for example, as a section of the torsion bar spring connected to the fuel cell stack, preferably an end plate. Likewise preferably, the torsion spring is designed as a milled hollow spring ( machined spring ). The torsion spring is particularly preferably milled from a hollow cylinder made of spring steel. Such milled hollow springs advantageously allow for particularly precise adjustment of the spring force, especially over extended periods.

[0046] The connection between the torsion spring and the mounting shaft, or the torsion spring and the fuel cell stack, can be realized in a variety of ways. For example, the torsion spring is flanged to the fuel cell stack, preferably an end plate. The mounting shaft can also have a projecting flange area for fastening the torsion spring. Alternatively, the torsion spring is permanently connected to the mounting shaft, for example, welded. Furthermore, the torsion spring is preferably connected to the fuel cell stack and / or the mounting shaft by means of a positive connection, for example, via bayonet connection elements. Such connection elements are preferably designed to be self-locking, particularly taking into account the effective direction of the spring force of the torsion spring.

[0047] In a likewise particularly preferred embodiment, the fuel cell stack according to the invention further comprises at least one electric motor designed to electric-motorically drive the at least one mounting shaft. In other words, the at least one mounting shaft is rotatable by means of an electric motor. The motor preferably allows the mounting shaft to rotate in a first direction of rotation and particularly preferably in a first and an opposite second direction of rotation. This embodiment advantageously allows automated retensioning of the at least one torsion spring firmly connected to the mounting shaft. Thus, age-related tension loss can be counteracted by the torsion springs themselves. Furthermore, the electric motor also directly enables the second elastic tensioning element to be retensioned.

[0048] Particularly preferably, the fuel cell stack comprises at least one sensor configured to detect a tensile force exerted by at least one elastic tensioning element and / or the spring force exerted by at least one torsion spring. The sensor preferably detects the tensile force exerted by a specific elastic tensioning element and / or the spring force exerted by a specific torsion spring. For example, such a sensor is configured as a strain gauge. Particularly preferably, a sensor, in particular a strain gauge, is arranged on every second elastic tensioning element.

[0049] The use of such sensors in the fuel cell stack according to the invention advantageously enables the measurement of changes in the length of the elastic tensioning elements. Based on the change in length of the elastic tensioning element, a tensile force or change in the tensile force of the respective elastic tensioning element is preferably determined. This allows conclusions to be drawn that the elastic tensioning element needs to be retensioned or that one or more torsion springs are no longer sufficiently tensioned to compensate for the loss of tensile stress. By retensioning the elastic tensioning element and / or the springs, permanent compression of the end regions of the end plates is ensured.

[0050] Furthermore, the fuel cell stack according to the invention preferably has at least one control unit connected to the at least one sensor and configured to operate the at least one electric motor. The control unit is preferably configured to receive a sensor signal from the at least one sensor. Furthermore, the control unit is configured to output a control signal to the at least one electric motor. The embodiment advantageously enables automated retensioning of the elastic tensioning element and / or the torsion springs based on sensor values ​​for the tensile force of the elastic tensioning elements and / or based on sensor values ​​for the spring force of the torsion springs.

[0051] In a particularly preferred embodiment, the fuel cell stack comprises a plurality of second elastic tensioning elements and a plurality of mounting shafts. Each mounting shaft is connected to the fuel cell stack, in particular to an end region of an end plate, by one or more tensioned torsion springs, and one or more elastic tensioning elements are fixed to each mounting shaft. Furthermore, the fuel cell stack preferably comprises a plurality of electric motors designed to drive at least one of the mounting shafts.

[0052] This embodiment enables tension control of individual groups of elastic tensioning elements or even of individual elastic tensioning elements. It is essential for this that different elastic tensioning elements are fixed to different mounting shafts. The mounting shafts are preferably rotated by a common electric motor. For example, two mounting shafts can be driven by a motor arranged centrally between these shafts. Likewise, several mounting shafts are preferably controllably connected to a common motor via respective couplings. The fuel cell stack preferably further comprises a plurality of sensors configured to measure the tensile stress of the elastic tensioning elements.

[0053] Furthermore, the at least one mounting shaft preferably has at least one ratchet wheel and at least one pawl engaging with the ratchet wheel.

[0054] The ratchet principle is therefore preferably used to allow rotation of the mounting shaft in only one direction. This advantageously relieves the load on the torsion springs and protects the electric motor bearings. Furthermore, manual re-tensioning and securing of the elastic clamping elements is advantageously possible.

[0055] The invention also relates to a vehicle, in particular an electric motor-driven vehicle, with a fuel cell stack according to the invention, as described above. The fuel cell stack serves, in particular, to power an electric motor of the vehicle.

[0056] Further preferred embodiments of the invention emerge from the remaining features recited in the subclaims. The various embodiments of the invention recited in this application can be advantageously combined with one another, unless otherwise stated in individual cases.

[0057] The invention is explained below in exemplary embodiments with reference to the accompanying drawings. They show: Figure 1 shows a schematic representation of a fuel cell system according to the prior art; Figure 2 shows a fuel cell stack with a plurality of clamping elements compressing the stack according to the prior art; Figure 3 shows a detailed representation of the fuel cell stack according to Fig.2 ; Figure 4 a detailed view of the fuel cell stack according to Fig.2 ; Figure 5 shows a fuel cell stack according to an embodiment; Figure 6 shows a detailed view of the fuel cell stack of the Fig. 5 ; Figure 7 shows a fuel cell stack according to an alternative embodiment; and Figure 8 shows two variants of milled hollow springs.

[0058] Figure 1shows a fuel cell system, designated overall by 100, according to the prior art. The fuel cell system 100 is part of a vehicle (not shown in detail), in particular an electric vehicle, which has an electric traction motor supplied with electrical energy by the fuel cell system 100.

[0059] The fuel cell system 100 comprises, as its core component, a fuel cell stack 10, which has a plurality of individual cells 11 arranged in a stacked form, formed by alternatingly stacked membrane electrode assemblies (MEAs) 14 and bipolar plates 15 (see detailed section). Each individual cell 11 thus comprises an MEA 14 with an ion-conductive polymer electrolyte membrane (not shown in detail here) and catalytic electrodes arranged on both sides thereof. These electrodes catalyze the respective partial reaction of the fuel conversion. The anode and cathode electrodes are formed as a coating on the membrane and comprise a catalytic material, for example, platinum, supported on an electrically conductive carrier material with a large specific surface area, for example, a carbon-based material.

[0060] As shown in the detailed illustration of the Figure 1As shown, an anode chamber 12 is formed between a bipolar plate 15 and the anode, and the cathode chamber 13 is formed between the cathode and the next bipolar plate 15. The bipolar plates 15 serve to supply the operating fluids to the anode and cathode chambers 12, 13 and also establish the electrical connection between the individual fuel cells 11. Optionally, gas diffusion layers can be arranged between the membrane-electrode assemblies 14 and the bipolar plates 15.

[0061] In order to supply the fuel cell stack 10 with the operating resources, the fuel cell system 100 has, on the one hand, an anode supply 20 and, on the other hand, a cathode supply 30.

[0062] The anode supply 20 of the Figure 1The fuel cell system 100 shown comprises an anode supply path 21, which serves to supply an anode operating medium (the fuel), for example hydrogen, into the anode chambers 12 of the fuel cell stack 10. For this purpose, the anode supply path 21 connects a fuel reservoir 23 to an anode inlet of the fuel cell stack 10. The feed pressure of the anode operating medium into the anode chambers 12 of the fuel cell stack 10 is adjusted via a metering valve 27.1. The anode supply 20 further comprises an anode exhaust gas path 22, which discharges the anode exhaust gas from the anode chambers 12 via an anode outlet of the fuel cell stack 10.

[0063] In addition, the anode supply 20 of the Figure 1The fuel cell system 100 shown has a recirculation line 24 that connects the anode exhaust gas path 22 to the anode supply path 21. Fuel recirculation is common practice to return the over-stoichiometrically used fuel to the fuel cell stack 10. A recirculation conveying device 25, preferably a recirculation fan, and a flap valve 27.2 are arranged in the recirculation line 24.

[0064] A water separator 26 is also installed in the anode supply 22 of the fuel cell system to drain the product water resulting from the fuel cell reaction. An outlet of the water separator can be connected to the cathode exhaust line 32, a water tank, or an exhaust system.

[0065] The cathode supply 30 of the Figure 1The fuel cell system 100 shown comprises a cathode supply path 31, which supplies an oxygen-containing cathode operating medium, in particular air drawn in from the environment, to the cathode chambers 13 of the fuel cell stack 10. The cathode supply 30 further comprises a cathode exhaust gas path 32, which removes the cathode exhaust gas (in particular the exhaust air) from the cathode chambers 13 of the fuel cell stack 10 and, if appropriate, feeds it to an exhaust system (not shown).

[0066] To convey and compress the cathode operating fluid, a compressor 33 is arranged in the cathode supply path 31. In the illustrated embodiment, the compressor 33 is designed as a primarily electric motor-driven compressor 33, which is driven by an electric motor 34 equipped with corresponding power electronics 35.

[0067] The Figure 1The fuel cell system 100 shown further comprises a humidifier module 39 arranged upstream of the compressor 33 in the cathode supply line 31. The humidifier module 39 is arranged, on the one hand, in the cathode supply path 31 such that the cathode operating gas can flow through it. On the other hand, it is arranged in the cathode exhaust gas path 32 such that the cathode exhaust gas can flow through it. A humidifier 39 typically comprises a plurality of water vapor-permeable membranes, which are either flat or in the form of hollow fibers. The comparatively dry cathode operating gas (air) flows over one side of the membranes, and the comparatively moist cathode exhaust gas (exhaust gas) flows over the other side. Driven by the higher partial pressure of water vapor in the cathode exhaust gas, water vapor passes through the membrane into the cathode operating gas, which is thus humidified.

[0068] The fuel cell system 100 further comprises a humidifier bypass 37 connecting the cathode supply line upstream and downstream of the humidifier 39, with a flap valve arranged therein as a bypass control means 38. Furthermore, flap valves 27.3 and 27.4 are arranged upstream of the fuel cell stack 10 in the anode supply line 31 and downstream of the fuel cell stack 10 in the anode exhaust line 32, respectively.

[0069] Various further details of the anode and cathode supply 20, 30 are in Figure 1 Not shown for reasons of clarity. For example, the anode exhaust line 22 can open into the cathode exhaust line 32, so that the anode exhaust and the cathode exhaust are discharged via a common exhaust system.

[0070] The Figure 2 shows a representation of a fuel cell stack according to the prior art, which is used in the fuel cell system 100 of the Figure 1The fuel cell stack 10 has a plurality of fuel cells stacked flat on top of one another in the stacking direction S.

[0071] In the stacking direction S, the fuel cell stack 10 is delimited by a first end plate 51 and an opposite second end plate 52. In a first direction transverse to the stacking direction S, the fuel cell stack 10 is delimited by side panels 53, 54. In a second direction transverse to the first direction and transverse to the stacking direction S, the fuel cell stack 10 is delimited by side panels 56.

[0072] The Figure 2The fuel cell stack 10 shown is compressed by a plurality of a total of five clamping elements 55. Each clamping element 55 completely surrounds a cross-section of the fuel cell stack 10, sweeping over the end plates 51, 52 and the side panel 56. The clamping elements 55 bear against the end plates 51, 52 and the side panel 56. Each clamping element 55 is welded to itself in a region of the first end plate 51. To compress the fuel cell stack 10, the clamping elements 55 are welded to themselves while the clamping elements 55 are under tensile stress.

[0073] A re-tensioning of the tensioning elements 55 is possible in the fuel cell stack 10 according to the Figure 2 not possible. Instead, as described in the Figure 3 shown, between the end plate 51 and a compression plate (not shown) of the fuel cell stack 10 of the Figure 2A plurality of compression springs 59 are arranged. If the tensioning elements 55 become elongated due to age, this is compensated for by the compression springs 59. The springs 59 are arranged in inward-facing recesses of the end plate 51. Thus, either additional space is required for the springs 59, for example, by making the end plate 51 thicker, or the end plate 51 must be made thinner in the area of ​​the recesses, making it less stable. Both variants lead to specific disadvantages of the respective design.

[0074] As further stated in the Figure 4As shown, due to the use of circumferential clamping elements 55, clamping of the first end plate 51 in the first inlet / outlet regions 57 of the fuel cell stack 10, where the operating media are supplied or removed, is not possible. Thus, in the prior art, the first end plates 51 disadvantageously bend in their first end regions 57.

[0075] Figure 5 shows a schematic representation of a fuel cell stack 10 according to a first embodiment of the present invention. Figure 6 shows a detailed view of this fuel cell stack 10, which is described below, as it differs from the fuel cell stack according to the Figures 2 to 3 differs.

[0076] The fuel cell stack 10 has a first end plate 51 and an opposite second end plate 52. The first end plate 51 has first end regions 57 with inlet and outlet openings located therein, and the second end plate 52 has second end regions 58 with inlet and outlet openings located therein. The end plates 51, 52 each have a rectangular shape with a pair of short and a pair of long side edges. The first and second end regions 57, 58 each completely encompass the short side edges and only partially encompass the long side edges.

[0077] The first and second end plates 51, 52 are clamped along their long side edges by means of a plurality of first clamping elements 55. The clamping elements 55 completely encircle the cross-section of the fuel cell stack 10. The clamping elements 55, which are thus formed in a closed circle, are welded to themselves in the region of the first end plate 51. The first clamping elements 55 thus correspond to the Figure 2 already explained clamping elements known from the state of the art.

[0078] In addition, the fuel cell stack 10 has second elastic tensioning elements 60, each of which is tensioned between a first end region 57 of the first end plate 51 and a second end region 58 of the second end plate 52. Each end region 57, 58 has a first edge region 62, which is part of a long side edge of the end plate 51, 52. Furthermore, each end region 57, 58 has a second edge region 63 extending perpendicular to the first edge region, which comprises a short side edge of the end plate 51, 52. In the Figure 5 the elastic tensioning elements 60 are tensioned between the first edge regions 62 of the end plates 51, 52.

[0079] As detailed in the Figure 6As shown, a mounting shaft 70 is rotatably mounted in the region of the first edge region 62 of the end plates 51, 52. For this purpose, the end plates 51, 52 each have recesses 53 in the first edge regions 62, in which the respective mounting shaft 60 is exposed. The end sections 61 of each elastic tensioning element 60 are fixed to the mounting shafts 70, with one end section 61 being fixed to a mounting shaft 70. By fixing, i.e. by materially, force-fitting, and / or positively fastening the end sections 61 of the tensioning elements 60 to the mounting shafts 70 in the region of recesses 53 of the end plates 51, 52, the tensioning elements 60 can be advantageously wound onto the mounting shafts 70 by rotating them and thus re-tensioned, without the end plates 51, 52 hindering this winding process.

[0080] Within the recesses 53, torsion springs 71 are also fixed both to the mounting shaft 70 and to the respective end plate 51, 52. In particular, the torsion springs 70 are welded to the respective mounting shaft 70 and fastened by means of a flange (not shown) to an inwardly facing wall of the respective recess 53. Each end section 61 of each clamping element 60 in each recess 53 is surrounded by two torsion springs 71. Thus, one end section 61 of a clamping element 60 and two torsion springs 71 are arranged on each section of a mounting shaft 70 exposed within a recess 53.

[0081] The torsion springs 71 are tensioned such that a force exerted by the torsion springs 71 on the mounting shaft 70 or a torque exerted on the mounting shaft 70 is opposite to a force or torque exerted by the tensioning elements 60 on the mounting shaft 70. In particular, a tensioning element 60 wound at least once around the mounting shaft 71 transmits a torque to the mounting shaft 71.

[0082] Due to the rotatable mounting of the mounting shafts 70, there is always a kinematic equilibrium between the torsion springs 71 and the clamping elements 60. Thus, a reduction in the tensile stress of the clamping elements 60, for example due to age-related lengthening, is compensated for by the clamping elements 60 winding up onto the mounting shafts 70, which is caused by the torque of the torsion springs 71.

[0083] With the fuel cell stack 10 according to the invention, a targeted compression of the fuel cell stack 10 in the end regions 57, 58 of the end plates 51, 52 is permanently ensured by means of the clamping elements 60, the mounting shafts 70, and the torsion springs 71, independent of the compression by the clamping elements 60. The mounting shaft 71 enables re-tensioning of the clamping elements 70, and the torsion springs 71 enable the permanent maintenance of a specific tension of the clamping elements 70.

[0084] Preferably, the mounting shafts 70 each have an elongated radial groove (not shown). In other words, a groove is formed in the mounting shaft 70 in the radial direction along a longitudinal extent of the mounting shaft 70. The radial groove is designed to receive an end portion 61 of one of the clamping elements 60.

[0085] The end section 61 is then fixed in the radial groove, for example, by means of a material bond using an adhesive. It is also conceivable for the end section 61 to have a thickened end portion that corresponds to an internal widening of the radial groove. The end section 61 of the clamping element 60 can then be inserted laterally into the mounting shaft 70, i.e., in the direction of the longitudinal extension of the shaft 70, and would thus be positively fixed in the radial groove. It is also conceivable for the radial groove to be narrowed after the end section 61 has been inserted, for example, by placing a snap ring on the shaft 70. This would result in a force-fitting fixation of the end section 61 in the shaft 70.

[0086] A fuel cell stack 10 according to a further embodiment is shown in Figure 7. This differs from the first embodiment of the Figures 4 to 6in that the elastic tensioning elements 60 are tensioned between second edge regions 63 of the end plates 51, 52. In particular, two elastic tensioning elements 60 are each tensioned between a second edge region 63 of a first end region 57 of the first end plate and a second edge region 63 of a second end region 58 of the second end plate 52. In total, four elastic tensioning elements 60 are thus tensioned between the first and second end plates 51, 52 in addition to the five circumferential tension and tensioning elements 55. Alternatively, only one wide elastic tensioning element 60 could be tensioned between end regions 57, 58 of the end plates 51, 52.

[0087] In Figure 8 Two preferred embodiments of a torsion spring 80 are shown as milled hollow springs 81. Figure 8 (A)a first torsion spring 80 milled from a hollow cylinder of spring steel. This has an internal thread with which the torsion spring 80 can be screwed onto a mounting shaft 60, wherein the thread direction causes the spring 80 to be self-locked on the mounting shaft 60. In the simplest case, the torsion spring 80 is fixed to the fuel cell stack 10, in particular to an inner wall of the recess 53 of an end plate 51, 52, by a force fit of the compressed torsion spring 80 with an end plate 51, 52 and a form fit of a stop lug of the end plate 51, 52 engaging in the milled recess of the spring 80. Figure 8 (B)shows a second torsion spring 80 milled from a hollow cylinder of spring steel. This spring has through holes at both ends as connecting elements for attachment to the mounting shaft 60 or the fuel cell stack 10, in particular an end plate 51, 52. The connecting elements are designed as a guide for a locking pin.

Claims

1. A fuel cell stack (10), comprising a first end plate (51) and a second end plate (52); a plurality of fuel cells (11) arranged between the end plates (51, 52); at least one first tension or tensioning element (55) tensioned in the stacking direction (S) between the end plates (51, 52); wherein inlet and outlet openings for operating media are arranged in first end regions (57) of the first end plates (51) and / or in second end regions (58) of the second end plate (52), wherein at least one elastic second clamping element (60) is clamped between a first end region (57) and a second end region (58), characterized in that a tensile stress applied by a tensioned first tension or tensioning element (55) differs from a tensile stress applied by an elastic second tensioning element (60), and in that the tensile stress applied by the second tensioning element (60) is lower than the tensile stress applied by the first tension or tensioning element (55).

2. The fuel cell stack (10) according to claim 1, wherein the end regions (57, 58) have first edge regions (62) and second edge regions (63) perpendicular to the first edge regions (62), wherein the at least one elastic second tensioning element (60) is tensioned between first edge regions (62) and / or second edge regions (63).

3. The fuel cell stack (10) according to any one of the preceding claims, further comprising at least one rotatable mounting shaft (70) arranged along an edge region (62, 63) of a first and / or second end region (57, 58), wherein an end portion (61) of at least one elastic second tensioning element (60) is fixed to the mounting shaft (70).

4. The fuel cell stack (10) according to claim 3, wherein a tensile stress of the elastic second tensioning element (60) can be adjusted by rotating the mounting shaft (70).

5. The fuel cell stack (10) according to claims 3 or 4, wherein the rotatable mounting shaft (70) is connected to the fuel cell stack (10) via at least one tensioned torsion spring (71).

6. The fuel cell stack (10) according to claim 5, wherein the at least one torsion spring (71) is designed as a helical spring, torsion bar spring or milled hollow spring (81).

7. The fuel cell stack (10) according to any one of claims 3 to 6, further comprising at least one electric motor configured to electromotively drive the at least one mounting shaft (70).

8. The fuel cell stack (10) according to any one of claims 3 to 7, wherein the at least one mounting shaft (70) comprises at least one ratchet wheel and at least one pawl in engagement with the ratchet wheel.

9. A vehicle with a fuel cell stack (10) according to any one of claims 1 to 8.