Fuel cell stack with increased intrinsic frequency

Abstract

Fuel cell stacks (3), comprising: a plurality of fuel cells (4) arranged in a stack in a row, the stack in a row having a first side and a second side which are parallel to each other; an end plate (5, 6) at each end of the row stack; a compression belt (7, 8) adjacent to the first side of the stack of rows, and a compression belt (7, 8) adjacent to the second side of the stack of rows, wherein the opposing compression belts (7, 8) are in contact with the end plates (5, 6) at each end of the stack of rows and apply a compression pressure which presses the end plates (5, 6) together; and an electrically insulating support beam (9, 10) between each compression belt (7, 8) and the row stack, wherein the opposing compression belts (7, 8) exert a load on the first side and the second side of the row stack through the support beams (9, 10), wherein the support beams (9, 10) are flat along the sides adjacent to the stack of rows and curved along the sides adjacent to the compression belts (7, 8).

Description

Background / Area of ​​the invention

[0001] The present invention relates to designs and methods for increasing the natural frequency of fuel cell stacks and in particular for increasing the natural frequency in order to prevent damage in automotive fuel cell stacks caused by resonance. Description of the related prior art

[0002] Fuel cells, such as solid-state polymer electrolyte membrane fuel cells, electrochemically convert reactants—namely a fuel (such as hydrogen) and an oxidant (such as oxygen or air)—to generate electrical power. Solid-state polymer electrolyte fuel cells generally use a proton-conducting, solid polymer membrane electrolyte between cathodic and anodic electrodes. A structure comprising a solid-polymer membrane electrolyte positioned between these two electrodes is called a membrane electrode assembly (MEA).In a typical fuel cell, flow field plates, comprising numerous fluid distribution channels for the reactants, are provided on each side of a MEA to distribute the fuel and oxidant to the respective electrodes and to remove byproducts of the electrochemical reactions occurring within the fuel cell. Water is the main byproduct in a cell operating with hydrogen and air as reactants. Because the output voltage of a single cell is on the order of 1 V, for commercial applications, multiple cells are typically stacked in series to provide a higher output voltage. For use in automotive applications and the like, the fuel cell stacks can be further interconnected in groups of stacks linked together in series and / or parallel.

[0003] Fuel cell stacks therefore typically comprise numerous thin, fragile components. For various technical reasons related to cell function, the dimensions of individual cells in a solid-polymer electrolyte fuel cell stack can have high aspect ratios (e.g., length >> width >>> thickness). Similarly, in automotive or other vehicle applications, the desired dimensions of the fuel cell stacks can also have comparatively high aspect ratios for various reasons related to vehicle function and design. Particularly in stacks with numerous cells, the stack height can be much greater than the stack width / cell width.

[0004] As disclosed in US 5,789,091 A, to simplify assembly and minimize weight and volume, stacks can be secured in a compressed, assembled state by using one or more compression straps surrounding end-plate assemblies and the fuel cells arranged between them. One or more elastic components are preferably used, which interact with each compression strap to force the first end-plate assembly toward the second. This compressive force promotes sealing and electrical contact between the layers forming the fuel cell stack. Strips of an electrically insulating material can be inserted between the compression straps and the edges of the fuel cells.

[0005] In vehicle applications, onboard fuel cell stacks are subject to mechanical vibrations when the vehicle is in motion. Like other onboard components, the stack is susceptible to oscillation and potential damage if the frequency of these vibrations is close to the fuel cell stack's resonant frequency. This problem is disclosed in WO 2012 / 086 344 A1, and a design is proposed to raise the stack's resonant frequency without increasing the number of parts. This design incorporates a pair of reinforcing plates bonded to the stack's end plates, covering the outer peripheral sides of the fuel cell's layered composite.

[0006] US Patent 7,858,259 B2 discloses a clamping structure for a planar solid oxide fuel cell stack comprising a flexible film and a rigid, heat-insulating end block, wherein the flexible film is capable of bending into a substantially convex shape, the rigid, heat-insulating end block is formed as a rectangular base with a flat surface and an opposing surface that is substantially convex, the flexible film is arranged adjacent to the opposing surface of the rigid, heat-insulating end block, and the flexible film bends to obtain a substantially convex shape. US Patent 7,858,259 B2 also discloses a solid oxide fuel cell stack and a method for compressing the stack.

[0007] Further prior art is disclosed in US 4 020 244 A, US 7 858 224 B2 and US 2006 / 0 093 890 A1.

[0008] As the development of commercial fuel cells progresses, vibration-related problems may be encountered more frequently. This is particularly true for vibrations caused by movement in vehicle applications. Consequently, there will be a need to solve these problems in a simple manner with minimal increase in weight, volume, and number of parts. The present invention addresses this need and provides further related advantages. Summary

[0009] The invention is defined in the independent claims. Further preferred embodiments are the subject of the dependent claims.

[0010] The present invention provides designs and methods for increasing the natural frequency of fuel cell stacks and, in particular, for preventing damage resulting from resonance in automotive fuel cell stacks.

[0011] Specifically, such fuel cell stacks comprise a plurality of fuel cells arranged in a row with first and second parallel sides. An end plate is provided at each side of the row stack, at least one compression belt is provided adjacent to the first side of the row stack, and at least one compression belt is provided adjacent to the second side of the row stack. The opposing compression belts are in contact with the end plates at each end of the row stack and apply a compression pressure that forces the end plates together. Additionally, an electrically insulating support beam is located between each compression belt and the row stack. The opposing compression belts, through the support beams, exert a load on the first and second sides of the row stack.

[0012] In an exemplary embodiment, the opposing compression belts are subsections of a single, endless compression belt which encloses the row stack and the end plates (that is, a single endless belt acts as the opposing compression belts).

[0013] Typically, two or more belts can be used at more than one location on a fuel cell stack. Therefore, for example, the fuel cell stack may include two endless compression belts encircling the row stack and end plates, and electrically insulating support beams between each endless compression belt and the row stack on each of the first and second sides of the row stack.

[0014] The invention is useful for fuel cell stacks in which the fuel cells are solid-state polymer electrolyte fuel cells. Furthermore, it is useful in stacks whose fuel cells are essentially rectangular, and particularly in stacks whose fuel cells have a relatively high aspect ratio. Such fuel cells have a longitudinal side dimension and a short side dimension (which represent the first and second sides of the fuel cells, respectively), wherein the longitudinal side dimension is greater than or approximately four times greater than the short side dimension.

[0015] Furthermore, the invention is useful in fuel cell stacks where the natural frequency of the stack is less than 50 Hz in the absence of support beams. This can be the case, for example, in stacks where the length of the row of fuel cells is greater than approximately 50 cm. However, the integration of suitable support beams can cause the natural frequency of the fuel cell stack to be greater than 50 Hz.

[0016] In embodiments of the invention, a suitable tensile load in the opposing compression belts can be greater than approximately 3 kN. A suitable shape for the support beams includes shapes that are flat along the sides adjoining the stack of rows and arcuate along the sides adjoining the compression belts. To provide sufficient stiffness, the support beams are of an appropriate thickness and can, for example, have a height at the center of between approximately 2.5 and 5 cm.

[0017] These and other aspects of the invention are evident with reference to the attached figures and the following detailed description. Brief description of the drawings Fig. Figure 1 is a perspective view of a prior art solid polymer electrolyte fuel cell stack with two compression belts surrounding the stack. Fig. Figure 2 shows an exemplary form of a support beam suitable for use in fuel cell stacks of the invention. Fig. Figure 3 is a perspective view of a fuel cell stack of the invention, comprising two endless compression belts and four support beams. Detailed description

[0018] In this description, words such as "a" and "encompasses" should be understood in an open sense, and should be understood to mean at least one, but not limited to just one.

[0019] In the present context, the expression “approximately” should be understood as ranging from plus 10% to minus 10%.

[0020] "Natural frequency" is the frequency at which an object will vibrate if it is not disturbed by an external force.

[0021] It has been found that, because the number of cells in typical, conventional solid polymer fuel cell stacks for use in vehicles is increasing, the natural frequency of the stacks becomes so low that they become unacceptably sensitive to vibration damage.

[0022] Fig.Figure 1 shows a perspective view of such a conventional fuel cell stack, in which two compression belts surround the stack (this figure was reproduced from US 5,789,091 A). The exemplary fuel cell stack 110 according to the prior art comprises end plate assemblies 115 and 120 and a plurality of fuel cell assemblies or fuel cells 125 interposed between the end plate assemblies 115, 120. Two compression belts 130 extend tightly around the end plate assemblies and fuel cells, fixing and securing the stack 110 in its assembled state. According to US 578,909 1 A, the end plate assemblies 115, 120 preferably have rounded edges 115a, 120a to reduce the stress on the belts.In the fuel cell stack, reactant and coolant fluid flows are supplied to and discharged from internal manifolds and passages in the stack 110 via a central fluid distribution plate 150. The compression bands 130 can be preferably formed from a rolled stainless steel strip, which can be pre-welded to the desired length to surround the stack. Strips of electrically insulating material (not shown) are arranged between the bands 130 and the edges of the fuel cells 125. The compression bands can be arranged on the stack in various ways, for example, by slightly excessive initial compression of the stack in a holding fixture during assembly, arranging and then welding the ends of the compression bands together around the stack, and finally releasing the stack from the holding fixture.

[0023] Instead of surrounding the stack with continuous or endless compression bands, one uses a method that is in Fig. An alternative arrangement shown in Figure 1 features compression belts on each side of the fuel cell stack, which are connected at opposite ends to each end plate arrangement 115 and 120. This arrangement is generally less preferred because the fastening devices increase the number of parts and slightly increase weight and volume.

[0024] The fuel cell stack in Fig.1 comprises fuel cells whose lengths are much greater than their widths. This allows for a simple, desirable flow field arrangement within them and can also be a desirable profile for integration into a vehicle. Furthermore, the stack height (the dimension in the stack direction) is also much greater than the cell widths or stack width. Although such stack designs can be desirable for operational and integration reasons, they can be susceptible to damage resulting from vibration caused by driving. For example, for automotive applications, a component is generally considered unsuitable if its natural frequency is less than about 50 Hz, because such frequencies can be encountered in typical driving duty cycles.It is therefore necessary to somehow increase the natural frequency of such components to be greater than about 50 Hz, and preferably considerably greater than 50 Hz, in order to use them in automotive applications.

[0025] As illustrated in the following examples, fuel cell stacks such as the one in Fig. 1 shown, have an unsuitably low natural frequency when enough fuel cells are included in the stack and lead to stack heights on the order of 50 cm or more.

[0026] However, an elegant solution to this problem involves inserting a simple, suitably shaped, electrically insulating support beam between the compression belts and the fuel cell stack, preferably on each side of the stack, so that the opposing compression belts, through the support beams, exert sufficient load on the opposite sides of the stack to substantially increase its natural frequency.

[0027] Fig. Figure 2 shows an exemplary form of a support beam, which is suitable for use in fuel cell stacks such as the one in Fig.The support beam 2 shown in Figure 1 is suitable. The support beam 2 has an arc shape along one side 2a, which is the side that will abut the compression belts when inserted into the stack. The support beam 2 is flat along one side 2b, which is the side that will abut the row of fuel cells when inserted into the stack. The support beam 2 is formed from a suitable electrically insulating material (so that it does not electrically short-circuit the cells in the stack) that is also capable of handling the sustained loads involved. For example, polymers such as polypropylene, polyethylene, glass-fiber reinforced nylon, and the like can be used. The length and width of the support beam 2 correspond to those of the stack (or a large part of the stack) and / or those of the compression belts used.The height of the support beam 2 is chosen such that, in combination with the tensile load applied by the opposing compression straps, the load applied to the sides of the stack increases the stack's natural frequency to a sufficient value. In actual embodiments, as shown in the following examples, the support beam 2 can be of sufficient size if its height at the center 2c is between approximately 2.5 and 5 cm. Larger (or smaller) heights can, of course, be considered and may be appropriate in other circumstances. It is assumed that larger or smaller heights will result in a greater or smaller increase, respectively, in the stack's natural frequency. Although a greater increase in natural frequency is preferred, larger support beams can add unnecessary volume and weight.

[0028] Fig.Figure 3 shows a perspective view of a fuel cell stack comprising two endless compression belts, into which four support beams have been inserted between the belts and the sides of the row of fuel cells, similar to those found in Fig. 2 are shown. Fig. 3 comprises a fuel cell stack 3, a row stack of fuel cells 4 between opposing end plates 5, 6. Two endless compression belts 7, 8 encircle the stack 3 and secure the components in a compressed state. Support beams 9, 10 are inserted between the compression belts 7, 8 and the row stack of cells 4. Similar support beams are used on the opposite side of the fuel cell stack 3, but are in Fig. 3 not visible. (Coordinate axes x, y, and z are in Fig.3 are provided to help indicate the orientation of the support beams, and they correspond to the coordinate axes which are in Fig. (2 are shown.) Note that, as shown, the compression belts 7, 8 are similar to, but not identical to, those in Fig. Figure 2 shows the following. Here, although this is not easily visible in the figure, the compression straps 7, 8 near the end of the stack at the end plate 6 have been slightly shortened. Furthermore, although this is not easily visible in the figure, the compression straps 7, 8 have been notched at their other end to accommodate them in receiving grooves formed in the end plate 5. Additionally, as shown, two further endless compression straps 11, 12 encircle the stack 3 to assist in securing the components in a compressed state. However, no support beams are used between the compression straps 11, 12 and the stack of cells 4.

[0029] Tension in the compression straps 7, 8 ensures a distributed load on the support beams 9, 10, which in turn preferentially provide a uniform load on the sides of the cells in the stack. As illustrated in the following examples, tensile loads of more than approximately 3 kN in the compression straps can provide sufficient stress in actual exemplary stacks to produce a satisfactory increase in the natural frequency. This is achieved in a tidy manner without an undue increase in the weight or volume of the fully assembled stack.

[0030] The in Fig.The embodiment shown in Figure 3 represents a currently preferred stacking configuration and utilizes support beams with a specifically effective choice of geometry. However, the person skilled in the art will recognize that other embodiments may have a significantly different stacking configuration and may include additional belts in a different tension range. Furthermore, such embodiments may use support beams with significantly different geometries and relative dimensions.

[0031] The following examples illustrate the invention, but should not be considered limiting in any way. Examples / Calculated Examples

[0032] The natural frequency of an exemplary automotive fuel cell stack was calculated as a function of the stack length (or alternatively, the number of cells in the stack). The cells were assumed to be essentially rectangular solid-polymer electrolyte fuel cells with long and short dimensions of approximately 40 cm and 10 cm, respectively. The length of the rows of cells in the stack was assumed to vary from approximately 45 to 65 cm. The overall stack length is approximately 4 cm greater than the height of the rows of cells, so the stack height was assumed to vary from approximately 49 to 69 cm. The stack was similar to the one in Fig. 3 shown, but only used two endless compression belts without any support beams.

[0033] It was assumed that the fuel cell stack behaves like a simply supported and uniform beam, characterized by the following parameters: a shear modulus of 27 MPa and an area moment of inertia of 6260 cm². 4 A mass / length ratio of 0.977 kg / cm and a length L in the stacking direction were used. Using the appropriate equation for the natural frequency from Marks' Standard Handbook for Mechanical Engineers, McGraw Hill Professional, 1987, the natural frequencies for such a stack in the stacking direction were calculated for various values ​​of length L. The results are tabulated in Table 1 below. Table 1. Calculated natural frequencies versus stack length Stack length (cm) Length of stacked cells (cm) Natural frequency (Hz) 49 45 54 54 50 45 58 54 39 64 60 32 69 65 27

[0034] For this exemplary stack, it was calculated that the natural frequency is unacceptably low (below 50 Hz) for stack lengths of more than approximately 50 cm.

[0035] Next, the natural frequency of a fuel cell stack like the one above was calculated, assuming the use of different support beams between each of the two compression belts and on both sides of the fuel cell stack, in the form shown in Fig. The figures shown are similar. It was assumed that the stack length was always 58 cm and the tension in the compression belts 3750 N. However, it was taken into account that the height at the center of the support beams varies from 2.5 to 5 cm. Table 2 shows the calculated range of the compressive load applied to the support beams by the compression belts and, conversely, the range of the load from the support beams to the stacked cells, along with the calculated natural frequencies in the stacking direction. Table 2. Calculated natural frequencies relative to the height of the support beam Support beam height (cm) Calculated load from the belt on the beams (kPa) Calculated load from the beams onto the cells (kPa) Natural frequency (Hz) 2,5 8.7 to 11.4 80 61 3,8 9.8 to 14.5 100 68 5 10 to 16 110 72

[0036] The natural frequency increases significantly as the height of the support beams increases. In all cases considered, the natural frequency of the stack was acceptably increased to considerably more than 50 Hz. Actual example

[0037] An experimental model of an automotive fuel cell stack was assembled, as in the preceding calculated examples, with a stack length of 58 cm. The stack was mounted on a vibration table, and sinusoidal excitation was performed to determine its actual natural frequency along the longitudinal axis of the stack. Here, and as in Fig. As shown in Figure 3, the longitudinal side dimension of the fuel cells lies along the z-axis, the short side dimension lies along the x-axis, and the stack length dimension lies along the y-axis.

[0038] Support beams like the ones in Fig.The two shown, each with a height of 3.6 cm at its center, were then inserted between the two compression belts and the rows of cells on both sides of the stack. The stack was again mounted on the vibrating table, and a sinusoidal excitation was performed to determine the natural frequency along the longitudinal axis of the actual fuel cell stack.

[0039] For comparison, the natural frequencies for each of the investigated stacks were calculated along the stack's longitudinal axis as described above. The results obtained for this actual fuel cell stack are presented in tabular form in Table 3. Table 3. Calculated and actual natural frequencies for stacks with and without support beams along the stack's longitudinal axis. Calculated natural frequency without support beam (Hz) Actual natural frequency without support beam (Hz) Calculated natural frequency with support beam (Hz) Actual natural frequency with support beam (Hz) 39 37 75 90

[0040] In one actual embodiment, the calculated natural frequency was the actual natural frequency along the important stacking longitudinal direction (y-axis in the Fig. 2 and Fig. 3) reasonably close. And the introduction of support beams according to the invention led to a significant increase in the natural frequency (from about 37 to 90 Hz) in the longitudinal direction of the stack.

Claims

[1] Fuel cell stack (3), comprising: a plurality of fuel cells (4) arranged in a stack in a row, the stack in a row having a first side and a second side which are parallel to each other; an end plate (5, 6) at each end of the row stack; a compression belt (7, 8) adjacent to the first side of the stack of rows, and a compression belt (7, 8) adjacent to the second side of the stack of rows, wherein the opposing compression belts (7, 8) are in contact with the end plates (5, 6) at each end of the stack of rows and apply a compression pressure which presses the end plates (5, 6) together; and an electrically insulating support beam (9, 10) between each compression belt (7, 8) and the row stack, wherein the opposing compression belts (7, 8) exert a load on the first side and the second side of the row stack through the support beams (9, 10), wherein the support beams (9, 10) are flat along the sides adjacent to the stack of rows and curved along the sides adjacent to the compression belts (7, 8). [2] Fuel cell stack (3) according to claim 1, wherein the opposing compression belts (7, 8) are subsections of a single, endless compression belt (7, 8) which encloses the row stack and the end plates (5, 6). [3] Fuel cell stack (3) according to claim 2, comprising: two endless compression belts (7, 8) which enclose the stack of rows and the end plates (5, 6); and an electrically insulating support beam (9, 10) between each endless compression belt (7, 8) and the row stack on each of the first side and the second side of the row stack. [4] Fuel cell stack (3) according to claim 1, wherein the fuel cells (4) are substantially rectangular with a longitudinal side dimension and a short side dimension. [5] Fuel cell stack (3) according to claim 4, wherein the longitudinal side dimension is about 4 times larger than the short side dimension. [6] Fuel cell stack (3) according to claim 1, wherein the tensile load in the opposing compression belts (7, 8) is greater than about 3 kN. [7] Fuel cell stack (3) according to claim 1, wherein the length of the row stack of fuel cells (4) is greater than about 50 cm. [8] Fuel cell stack (3) according to claim 1, wherein the height in the center of the support beams (9, 10) is between about 2.5 and 5 cm. [9] Fuel cell stack (3) according to claim 1, wherein the natural frequency of the fuel cell stack (3) in the absence of the support beams (9, 10) is less than 50 Hz and the natural frequency of the fuel cell stack (3) with the support beams (9, 10) is greater than 50 Hz. [10] Fuel cell stack (3) according to claim 1, wherein the fuel cells (4) are solid polymer electrolyte fuel cells. [11] Method for increasing the natural frequency of a fuel cell stack (3), wherein the fuel cell stack (3) comprises a plurality of fuel cells (4) arranged in a row stack, the row stack having a first side and a second side parallel to each other, an end plate (5, 6) at each end of the row stack, and a compression belt (7, 8) adjacent to the first side of the row stack and a compression belt (7, 8) adjacent to the second side of the row stack, wherein the opposing compression belts (7, 8) are in contact with the end plates (5, 6) at each end of the row stack and apply a compression pressure which presses the end plates (5, 6) together, the method comprising: Inserting an electrically insulating support beam (9, 10) between each compression belt (7, 8) and the row stack such that the opposing compression belts (7, 8) exert a load on the first side and the second side of the row stack through the support beams (9, 10), and Selecting a geometry for the support beams (9, 10) which is flat along the sides adjacent to the row stack and arc-shaped along the sides adjacent to the compression belts (7, 8). [12] Method according to claim 11, comprising selecting a tensile load in the opposing compression belts (7, 8) and selecting a geometry for the support beams (9, 10) such that the natural frequency of the fuel cell stack (3) is increased from less than 50 Hz to more than 50 Hz. [13] Method according to claim 12, comprising selecting a tensile load in the opposing compression belts (7, 8) which is greater than about 3 kN. [14] Method according to claim 12, comprising selecting a geometry for the support beams (9, 10), wherein the height at the center of the support beams (9, 10) is between about 2.5 and 5 cm.

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