DESC:
TECHNICAL FIELD
[1] The present disclosure relates generally to fuel-cell stack construction. In particular, the present disclosure relates to a fuel-cell stack clamping arrangement to provide uniform clamping force across the fuel-cell stack and over the entire active area of the fuel-cells.

BACKGROUND
[2] Background description includes information that may be useful in understanding the present disclosure. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed disclosure, or that any publication specifically or implicitly referenced is prior art.
[3] A fuel-cell is an energy converting device that converts the chemical energy of a source fuel into electric energy through an electrochemical reaction, without a process of converting the source fuel into heat by combustion. Fuel-cells may be utilized as power sources for vehicles and other industrial and domestic purposes.
[4] Polymer electrolyte membrane fuel-cells (PEMFC) are currently most widely researched as fuel-cells for vehicles. A PEMFC is operated at a relatively low temperature on the order of 50-100 °C and they offer a faster start up time, faster power converting reaction time, and higher energy density, compared to other types of fuel-cells. However, each unit cell only generates a low voltage. To generate a higher voltage, several unit cells are stacked on one another to form a fuel-cell stack.
[5] A fuel-cell stack includes a membrane-electrode assembly (MEA) as a main component. The MEA is positioned inside the stack and includes a solid electrolyte membrane that only allows movement of hydrogen ions, and electrode layers on either surface of the electrolyte membrane. The electrode layers include a cathode and an anode that are applied with a catalyst to ionise oxygen and hydrogen.
[6] A gas diffusion layer (GDL) and bipolar plate are placed at the outer portion of the MEA. The bipolar plate includes a flow field for the supply of a reaction gas (hydrogen as a fuel and oxygen or air as an oxidant). Also, cooling water may pass through the cavities so created between an anode and a cathode of an adjacent cell.
[7] The above construction forms a unit cell. A plurality of unit cells are stacked on one another and end plates are joined on the outmost portion thereof, thus completing a fuel-cell stack.
[8] Hydrogen and oxygen or air are supplied to the anode and cathode, respectively. The hydrogen at the anode is reduced to protons which passes through the MEA towards the cathode. The electrons generated during this reaction traverses an external circuit. The protons react with the oxygen and the electrons at the cathode to form water. This reaction can be described as follows,

H2? 2H+ + 2e- (at the anode)
O2 + 4H+ + 4e-? 2H2O (at the cathode)

[9] A conventional method of assembling and clamping a fuel-cell stack includes a bolt clamping method, a band clamping method, and a wire clamping method. In the bolt clamping method, end plates are joined on both ends of stacked cells and are then pressed. Long bolts (clamping rods) are then inserted through the end plates and are fastened by nuts so that the end plates do not move.
[10] In the band clamping method, end plates are joined on both ends of the stacked cells and are pressed. The end plates are then tied by a band, which is in turn fastened to the end plates by a bolt.
[11] When the fuel-cell stack assembly is being clamped using bolts in an optimized sequential manner, it is observed that few of the bolts get loosened, creating non-uniform clamping pressure over MEA surface, which ranges from inadequate clamping to over clamping. Further, variation in cumulative stiffness of end plate, bipolar plate, and gasket varies on individual bolts, causing uneven tension in the bolts while maintaining uniform pressure over MEA. Also, variation in thickness of components over active area and thermal expansion in clamp rods is observed in multi rod clamping method, which creates further uneven pressure.
[12] The surface pressure between two neighbouring cells has a considerable effect on the overall output of the fuel-cell stack. The surface pressure in the stack is directly associated with mass transfer resistance in the gas diffusion layer and ohmic loss due to an increase in contact resistance. When the surface pressure is too low, the contact resistance is increased between the bipolar plate, the gas diffusion layer and the MEA, and thus, a current-voltage drop occurs. When the surface pressure is too high, the gas diffusion layer is excessively compressed making it difficult to diffuse. As a result, stack output is lowered. Hence, for good performance of the stack, it is necessary to properly maintain an adequate and uniform clamping force.
[13] There is, therefore, a need in the art for a system and mechanism to maintain a uniform and adequate pressure across the fuel-cell stack at all times, that adapts to changes caused due to thermal expansion and other component deformations and continues to provide adequate and uniform clamping pressure, while also being easy to assemble and maintain.
[14] All publications herein are incorporated by reference to the same extent as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
[15] In some embodiments, the numbers expressing quantities of ingredients, properties such as concentration, reaction conditions, and so forth, used to describe and claim certain embodiments of the disclosure are to be understood as being modified in some instances by the term “about.” Accordingly, in some embodiments, the numerical parameters set forth in the written description and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by a particular embodiment. In some embodiments, the numerical parameters should be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of some embodiments of the disclosure are approximations, the numerical values set forth in the specific examples are reported as precisely as practicable. The numerical values presented in some embodiments of the disclosure may contain certain errors necessarily resulting from the standard deviation found in their respective testing measurements.
[16] As used in the description herein and throughout the claims that follow, the meaning of “a,” “an,” and “the” includes plural reference unless the context clearly dictates otherwise. Also, as used in the description herein, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise.
[17] The recitation of ranges of values herein is merely intended to serve as a shorthand method of referring individually to each separate value falling within the range. Unless otherwise indicated herein, each individual value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g. “such as”) provided with respect to certain embodiments herein is intended merely to better illuminate the disclosure and does not pose a limitation on the scope of the disclosure otherwise claimed. No language in the specification should be construed as indicating any non-claimed element essential to the practice of the disclosure.
[18] Groupings of alternative elements or embodiments of the disclosure disclosed herein are not to be construed as limitations. Each group member can be referred to and claimed individually or in any combination with other members of the group or other elements found herein. One or more members of a group can be included in, or deleted from, a group for reasons of convenience and/or patentability. When any such inclusion or deletion occurs, the specification is herein deemed to contain the group as modified thus fulfilling the written description of all Markush groups used in the appended claims.

OBJECTS
[19] It is a general object of the present disclosure to provide a clamping arrangement for securing a fuel-cell stack.
[20] It is another object of the present disclosure to provide a clamping arrangement for securing a fuel-cell stack, that distributes pressure uniformly over the entire surface of the end plates.
[21] It is yet another object of the present disclosure to provide a clamping arrangement that adapts to structural changes in the fuel stack assembly due to manufacturing constraints or during operation and continues to maintain a uniform pressure across the end plates.
[22] It is yet another object of the present disclosure to provide an arrangement for clamping a fuel-cell stack that handles the temperature variations and shocks efficiently.
[23] It is yet another object of the present disclosure to provide an arrangement for clamping a fuel-cell stack that is easy to assemble and maintain.
[24] It is yet another object of the present disclosure to provide an arrangement for clamping a fuel cell stack that allows the fuel cell stack to provide the required power output without compromising its functioning.

SUMMARY
[25] The present disclosure relates generally to fuel-cell stack construction. In particular, the present disclosure relates to a fuel-cell stack clamping arrangement to provide uniform clamping force across the fuel-cell stack.
[26] In an aspect, the present disclosure provides a single point clamping arrangement for a fuel-cell stack assembly, said assembly comprising: a fuel-cell stack comprising a plurality of fuel-cells stacked in series; a top end plate assembly configured at top of the fuel-cell stack; and a bottom end plate assembly.
[27] In another aspect, the bottom end plate assembly is configured at bottom of the fuel-cell stack, wherein the arrangement of the bottom end plate assembly and the top end plate assembly is configured to secure the fuel-cell stack.
[28] In another embodiment, at least one of the top end plate assembly and the bottom end plate assembly comprises an intermediate layer of a composite compressible element such that arrangement of the top end plate assembly and the bottom end plate assembly enables distribution of uniform pressure across the surface of each of the plurality of fuel-cells.
[29] In an embodiment, the fuel-cell stack assembly comprises at least two guiders at the top of the fuel-cell stack to guide the top end plate assembly on to the fuel-cell stack.
[30] In another embodiment, the plurality of fuel-cells are stacked vertically and in the same orientation.
[31] In another embodiment, the top end plate assembly and bottom end place assembly are secured by a plurality of mounting pillars using a plurality of fasteners.
[32] In another embodiment, the plurality of fasteners are operated to enable at least one of the top end plate assembly and the bottom end plate assembly to provide a force towards the fuel-cell stack. In another embodiment, the plurality of fasteners are operated such that force is provided at a pre-determined rate.
[33] In another embodiment, the composite compressible element is any, or a combination of elements selected from a group comprising a wedge-shaped pressure plate, tube springs, disc springs, silicone rubber and foam.
[34] In another embodiment, the composite compressible element comprises: a wedge-shaped pressure plate; one or more disc springs; a tensioner; and a disc spring loader. In another embodiment, the wedge-shaped pressure plate has a flat surface facing the fuel-cell stack and a narrow dome facing an opposite direction. In another embodiment, the disc spring loader is disposed at top of the one or more disc springs and is configured to transfer the provided force towards the fuel-cell stack. In another embodiment, the tensioner is configured to transfer the provided force to the domeof the wedge-shaped pressure plate as a single point force applied by the disc spring loader, such that the wedge-shaped pressure plate undergoes a swell action to enable uniform distribution of force on the fuel-cell stack. In another embodiment, the composite compressible element is encased within the top end plate assembly such that the top end plate assembly consisting of an end cover lies on the pressure plate.
[35] In another embodiment, the composite compressible element comprises: a spring plate comprising a plurality of grooves distributed uniformly on the spring plate, where each groove is configured to hold a tube spring. In another embodiment, the spring plate is configured to transfer the provided force to the fuel-cell stack such that the spring plate undergoes a swell action to enable uniform distribution of force on the fuel-cell stack.
[36] In another embodiment, the composite compressible element comprises: at least one layer of silicone rubber, where the silicone rubber is configured to transfer the provided force to the fuel-cell stack such that the silicone rubber undergoes a swell action to enable uniform distribution of force on the fuel-cell stack. In another embodiment, the at least one layer of silicone rubber has a honeycomb structure.
[37] In another embodiment, the composite compressible element comprises: at least one layer of foam, where the foam is configured to transfer the provided force to the fuel-cell stack such that the foam undergoes a swell action to enable uniform distribution of force on the fuel-cell stack.
[38] Various objects, features, aspects and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments, along with the accompanying drawing figures in which like numerals represent like components.

BRIEF DESCRIPTION OF THE DRAWINGS
[39] The accompanying drawings are included to provide a further understanding of the present disclosure and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the present disclosure and, together with the description, serve to explain the principles of the present disclosure.
[40] FIG. 1A illustrates a typical end plate used in the known art with locations shown for passing bolts through. The holes are numbered from 1 through 12 in the order in which the bolts are inserted and torqued.
[41] FIG. 1B illustrates the typical end plate used in the art with the locations and the relative placements of the twelve bolt sites.
[42] FIG. 2A illustrates a typical end plate used in the known art with locations shown for passing bolts through. Plane formation is illustrated considering combinations of three bolts selected arbitrarily.
[43] FIG. 2B illustrates an instance of bolting of a typical end plate known in the art, where back pressure from the gas diffusion layer (GDL) is also depicted. In this instance, bolts are to be inserted and torqued in the order shown-1, 2, 3 and 4, along the axes A34 and A12.
[44] FIGs. 3A and 3B depict the displacement plots of the top and bottom surfaces respectively of a typical top end plate known in the art, when twelve bolts are used to clamp the fuel-cell stack assembly.
[45] FIGs. 4A and 4B depict the displacement plots of the top and bottom surfaces respectively of a typical top end plate known in the art, when four bolts are used to clamp the fuel-cell stack assembly.
[46] FIG. 5A illustrates an exploded view of a fuel-cell stack assembly along with a clamping arrangement, in accordance with an embodiment of the present disclosure.
[47] FIG. 5B illustrates an exploded, sectional view of the components of the clamping arrangement, in accordance with an embodiment of the present disclosure.
[48] FIG. 5C illustrates an assembled fuel-cell stack secured with the clamping arrangement, in accordance with an embodiment of the present disclosure.
[49] FIGs. 6A illustrates a 3D model of the fuel-cell stack assembly secured with the clamping arrangement, in accordance with an embodiment of the present disclosure.
[50] FIG. 6B illustrates a longitudinal cross-section of the secured fuel-cell stack assembly with the components shown, in accordance with an embodiment of the present disclosure.
[51] FIG. 7A depicts a clamping force vs. deflection plot for two disc springs stacked in parallel, in accordance with an embodiment of the present disclosure.
[52] FIG. 7B depicts a comparison of the clamping force vs. deflection plots for systems with two disc springs stacked in parallel (a single point clamp), twelve bolts with disc springs and just twelve bolts.
[53] FIGs.8A and 8B depict displacement plots for an end plate and pressure plate respectively, in a single point clamp system, in accordance with an embodiment of the present disclosure.
[54] FIGs. 9A and 9B depict the deformation in the end plates of a fuel-cell stack assembly before clamping and after clamping using twelve bolts respectively.
[55] FIGs. 10A and 10B depict the deformation in the end plates of a fuel-cell stack assembly before clamping and after clamping using a single point clamping arrangement, in accordance with an embodiment of the present disclosure.
[56] FIGs. 11A and 11B illustrate exploded views of a clamping arrangement for fuel-cell stack assembly from different perspectives, in accordance with another embodiment of the present disclosure.
[57] FIG. 11C illustrates an exploded view of a fuel-cell stack assembly along with the clamping arrangement, in accordance with an embodiment of the present disclosure.
[58] FIG. 12A illustrates an exploded view of a fuel-cell stack assembly along with a clamping arrangement, in accordance with yet another embodiment of the present disclosure.
[59] FIG. 12B depicts the silicone rubber layer with the honeycomb structuring and the gripping features, in accordance with an embodiment of the present disclosure.
[60] FIG. 13 illustrates an exploded view of a fuel-cell stack assembly along with a clamping arrangement, in accordance with yet another embodiment of the present disclosure.
[61] FIGs. 14A and 14B illustrate the exploded view of the fuel-cell stack assembly and the fully assembled view of the fuel-cell stack assembly, respectively, secured with a clamping arrangement, in accordance with an embodiment of the present disclosure.
[62] FIGs. 14C and 14D illustrate the exploded view of the fuel-cell stack assembly and the fully assembled view of the fuel-cell stack assembly, respectively, secured with a clamping arrangement, in accordance with another embodiment of the present disclosure.
[63] FIGs. 14E and 14F illustrate the exploded view of the fuel-cell stack assembly and the fully assembled view of the fuel-cell stack assembly, respectively, secured with a clamping arrangement, in accordance with another embodiment of the present disclosure.
[64] FIGs. 14G and 14H illustrate an exploded view of the fuel-cell stack assembly and fully assembled view of the fuel-cell stack assembly, respectively, secured with a clamping arrangement, in accordance with another embodiment of the present disclosure.

DETAILED DESCRIPTION
[65] The following is a detailed description of embodiments of the disclosure depicted in the accompanying drawings. The embodiments are in such detail as to clearly communicate the disclosure. However, the amount of detail offered is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure as defined by the appended claims.
[66] Each of the appended claims defines a separate disclosure, which for infringement purposes is recognized as including equivalents to the various elements or limitations specified in the claims. Depending on the context, all references below to the "disclosure" may in some cases refer to certain specific embodiments only. In other cases, it will be recognized that references to the "disclosure" will refer to subject matter recited in one or more, but not necessarily all, of the claims.
[67] Various terms are used herein. To the extent a term used in a claim is not defined below, it should be given the broadest definition persons in the pertinent art have given that term as reflected in printed publications and issued patents at the time of filing.
[68] Fuel-cells are arranged over one another in series and this arrangement comprises a stack. Due to reasons such as, but not limited to, variations in cumulative stiffness of end plates, bipolar plates or gaskets, an uneven tension is caused in clamping arrangement of the stack and this results in variations in pressure maintained over the fuel-cell stack. In addition, other factors that affect uniform distribution of pressure are dimensional variations of components inherent to the manufacturing process, over an active layer and thermal expansion of clamps over the range of working temperature (30°C to 80°C).
[69] Surface pressure between two neighbouring cells has a considerable effect on overall output of the fuel-cell stack. The surface pressure on the stack is directly associated with mass transfer resistance in gas diffusion layer and ohmic loss due to an increase in contact resistance. When the surface pressure is too low, contact resistance between the bipolar plate, the gas diffusion layer and a membrane-electrode assembly (MEA) increases, causing a current-voltage drop. When the surface pressure is too high, the gas diffusion layer is excessively compressed making diffusion difficult and causing a lowered stack output. Hence, for optimum performance of a fuel-cell stack, it is necessary to maintain an adequate and uniform clamping force.
[70] FIGs. 1A to 4B depict clamping arrangements currently known and practiced in the art. Referring to FIG. 1A, wherein a typical end plate 100 known in the art is illustrated, a plurality of holes 102 is disposed along the periphery of the end plate 100, along with a plurality of slots 104 and ridges 106. The slots 104 and ridges 106 serve to provide stress relief from thermal expansion that occurs due to higher working temperature of the fuel-cell stack. Of the plurality of holes 102 machined on the periphery of the end plate 100, some can be used as locations for fastening the end plate 102 to a fuel-cell stack assembly (as shown in FIG. 5A). In an exemplary instance practised in the art, twelve bolts are used for fastening the end plate 100 to the fuel-cell stack. In FIG. 1A, the location of the bolt holes is illustrated, and the holes are numbered according to the sequence in which the bolts are torqued. For instance, the bolts are tightened in figure-of-eight pattern, according to the sequence shown as 1-12. Table 1 below tabulates size of bolts, tightening torque used and other relevant parameters in respect of the fuel-cell stack assembly using twelve bolts, illustrated in FIG. 1A.

Number of Bolts 12
Specification of Bolts M10X1.5, Grade 8.8
Tightening Torque 5.2Nm
Clamp Load per Bolt 2230N
Total Clamp Load of 12 Bolts 26670 N
Pressure applied on MEA 2.7 MPa
Pressure applied on End Plate 0.94 MPa

Table1:Data pertaining to fastening bolts in a typical fuel-cell stack.

[71] FIG. 1B depicts a typical end plate 100 used in the art. In the exemplary illustration, of the plurality of holes 102 provided at the periphery of the end plate, twelve of the holes are used for bolting the end plate 100 to the fuel-cell stack assembly. Linear distance between two adjacent bolt holes can be 50mm.
[72] As described in FIGs. 1A and 1B, when bolts are torqued in the specified sequence, the formation of planes occurs. Planes, in this context, are triangular areas of uniform stress formed between three bolts when they are torqued. When the plane is formed, there is uniform pressure within the plane, but the rest of the plate experiences uneven pressure compared to the plane. Due to the pressure being applied by the underlying layers of the fuel-cell stack, there is a displacement of the end plate 100 due to the non-uniform stress applied on it by the formation of the plane. This causes uneven clamping of the end plate 100 resulting in vibrations and distortions. This can eventually result in the loosening of the bolts causing inadequate clamping. In an exemplary instance, the formation of planes can cause inadequate clamping or over clamping, in a clamping pressure range of 0.37MPa-2.53MPa.
[73] FIG. 2A illustrates instances of plane formation when three arbitrarily chosen bolts are torqued. FIG. 2B illustrates the problem that arises with plane formation. Two end plates 100-1, 100-2 are depicted on either side of the fuel-cell stack assembly. The top end plate 100-1 is, in an instance, to be bolted using four bolts at the positions 1-4 shown, in the same sequence. The bolts 1,2 and 3,4 lie on the axes A34 and A12 respectively. When bolts 1, 2 and 3 are torqued, a triangular plane is formed with the three bolts at the vertices. While plane 1-2-3 is uniformly stressed, the rest of the plate isn’t. Due to the pressure being exerted by the fuel-cell stack from below the top end plate 100-1, the side of the plate with hole number 4 is slightly lifted. This causes an uneven clamping force to be spread across the plate. Subsequently, when the bolt 4 is torqued, the plate is sufficiently distorted that the clamping force is uneven across the surface of the plate and this causes uneven and inadequate clamping.
[74] Referring to FIG. 3A and 3B, wherein the displacement plots of the top and bottom sides of a typical end plate 100 used in the art are depicted, when it is bolted to the fuel-cell stack assembly using twelve bolts, as indicated by the positions of the bolts marked in FIG. 3A. It can be observed from the figures that the maximum displacement occurs at the centre of the plate and the displacement reduces slowly with decreasing radial distance from the centre. From this, it can be inferred that the pressure at the centre of the plate is maximum, and that it reduces with radial distance from the centre. The middle segments of the edges show little displacement. On the top surface of the end plate, displacement is seen at the position of the bolts, indicating that at least some localised deformation has occurred at the points of torque.
[75] Referring to FIG. 4A and 4B, wherein the displacement plots of the top and bottom sides of a typical end plate 100 used in the art are depicted, when it is bolted to the fuel-cell stack assembly using four bolts, as indicated by the positions of the bolts marked in FIG. 4A. It can be observed from the figures that the maximum displacement occurs at the centre of the plate and the displacement reduces slowly with decreasing radial distance from the centre. From this, it can be inferred that the pressure at the centre of the plate is maximum, and that it reduces with radial distance from the centre. The middle segments of the top and bottom edges show little displacement, while the edges with no bolts do not show any displacement. On the top surface of the end plate, displacement is seen at the position of the bolts, indicating that at least some localised deformation has occurred at the points of torque.
[76] In the case of FIGs. 3A and 3B, twelve bolts are used of type M10, Grade 8.8 and each bolt is torqued at 5Nm. In the case of FIGs. 4A and 4B, four bolts are used of the type M8, Grade 5.8 and each bolt is torqued at 18.3Nm. From the two figures, however, it is observed that the displacement plots and values are similar. This leads to an inference that a four-bolt system can instead be employed. The advantage of this is that the number of planes formed will be fewer and this, in turn, will reduce the tendency of the bolts to loosen. This, further, leads to better pressure distribution and more efficient clamping.
[77] The present disclosure relates to an assembly and a method for securing fuel-cell stacks by providing a clamping arrangement that distributes pressure uniformly over the surface of the fuel-cell stack. In one embodiment, the clamping assembly exerts a single point clamping force on the fuel-cell stack, while maintaining uniform pressure across the surface of the fuel-cell stack. In other embodiments, other layers are introduced between the end plates and fuel-cell stack in order to enhance the uniform distribution of pressure across the surface of the fuel-cell stack.
[78] In an aspect of the present disclosure, the clamping assembly comprises two end plates or end covers-one on either end of the fuel-cell stack, plurality of mounting sleeves or supporting pillars and a plurality of bolts for mounting the end plates on the mounting sleeves or supporting pillars. In an embodiment of the disclosure, the mounting sleeves or supporting pillars are six in number.
[79] In another aspect of the present disclosure, the end plates-one or both, are integrated with a clamping assembly which applies a uniform pressure over the surface of the fuel-cell stack. In various embodiments, the assembly for a clamping arrangement comprises either a single point clamping with disc springs, or an array of tube springs, or an array of disc springs, or a honeycomb structured silicone rubber or a foam layer.
[80] In another aspect of the present disclosure, the clamping assemblies provide a mechanism to adapt to causes like variations in cumulative stiffness of the end plates, bipolar plates or gaskets, the dimensional inconsistencies of the components and the thermal distortion of the clamping components by incorporating media with lower stiffness that can, by action of swelling, maintain a uniform pressure distribution over the surface of the fuel-cell stack. In various embodiments, this medium can be a pressure plate-disc springs assembly, an array of tube springs, an array of disc springs, a honeycomb structured silicone rubber or a foam layer.
[81] Referring to FIGs. 5A through 5C, wherein one of the proposed clamping arrangement to clamp a fuel-cell stack assembly (also referred to as assembly hereinafter) and maintain a uniform pressure is disclosed, in accordance with an embodiment of the present disclosure. FIG. 5A illustrates an exploded view of the fuel-cell stack assembly and the currently embodied system of clamping. The fuel-cell stack assembly comprises afuel-cell stack 502, supported on either side by pressure end covers 510-1 and 510-2 (also collectively referred to as end covers 510 or individually referred to as end cover 510). The end covers 510 are mounted onto the assembly through a plurality of mounting sleeves or supporting pillars 504. In an exemplary instance, there are six supporting pillars or mounting sleeves. The top end cover 510-1 is modified to hold a single point clamping arrangement for clamping the fuel-cell stack 502 and for maintaining a uniform pressure across the fuel-cell stack 502. The fuel-cell stack 502 has a pair of guiders 518 at the top for guiding the single point clamping arrangement.
[82] Referring to FIG. 5B, where the proposed single point clamping arrangement is illustrated in greater detail, the clamping arrangement comprises a disc spring locator or loader 516, a wedge-shaped pressure plate 506, a dome 508 made of a material with a low co-efficient of friction, a tensioner 512 and a pair of parallelly stacked disc springs 514. In an exemplary instance, the dome 508 is made of polytetrafluoroethylene (PTFE). The PTFE dome is placed atop the pressure plate 506, over which are placed the tensioner 512, the parallelly stacked disc springs 514 and the disc spring locator or loader 516, in that sequence. This assembly is then encased within the top end cover 510-1, such that the end cover 510-1 also lies on the pressure plate 506. These components are aligned using the guiders 518 present on the fuel-cell stack 502. Once assembled, the disc spring locator or loader 516 is torqued, and this load is transmitted to the pressure plate 506 through the disc springs 514, tensioner 512 and the PTFE dome 508. The PTFE dome 508 forms a single point of contact with the pressure plate 506. Upon increase in load on the pressure plate 506, swelling action of the plate 506 occurs and this transmits and maintains a uniform pressure across the fuel-cell stack 502. FIG. 5C illustrates the assembled fuel-cell stack 500 with the single point clamping arrangement, in accordance with an embodiment of the present disclosure. The two pressure end covers 510 are bolted to the assembly 500 through six bolts each, through the mounting sleeves or supporting pillars 504.
[83] FIG. 6A illustrates a 3D model of the fuel-cell stack assembly 500 with a single point clamping arrangement, in accordance with an embodiment of the present disclosure. The model also shows the inlets for the fuel (hydrogen) 602 and the oxidant (oxygen or air) 604. FIG. 6B illustrates a longitudinal cross-section of the fuel-cell stack assembly 500 with the single point clamping arrangement, in accordance with an embodiment of the present disclosure. Shown herein is the fuel-cell stack 502 placed and bolted between the two pressure end covers 510. The single point clamping assembly is also seen, with the wedge-shaped pressure plate 506, the PTFE dome 508, the tensioner 512, the disc springs 514 and the disc spring locator or loader 516. The single point of contact between the PTFE dome 508 and the tensioner 512 is indicated. As the disc spring locator or loader 516 is torqued, the load is transmitted gradually to the pressure plate 506 through the disc springs 514, the tensioner 512 and the PTFE dome 508. On loading, the pressure plate 506 swells and this causes the uniform distribution of pressure across the fuel-cell stack 502.
[84] FIGs. 7A and 7B depict plots of clamping force vs. deflection in respect of different conventional clamping methods described above, and the proposed single point clamping arrangement. FIG. 7A depicts a plot of clamping force vs. deflection for a single point clamping arrangement 702 using a pair of disc springs. The highlighted area indicated on the curve is the operating range of the disc springs, and the dot indicates the optimum design consideration. In this instance, the optimum design value is 1.8MPa. It can be observed from this plot that for a specific operating range, the clamping force does not change greatly over deflection. This means that, even if there are irregularities over the surface of the fuel-cell stack 502 or clamping components due to reasons such as, but not limited to, thermal expansion or variations in thicknesses of components, the clamping force remains uniform and, importantly, adequate.
[85] FIG. 7B depicts a comparison of clamping force vs. deflection plots of conventional methods, namely: clamping with 12 bolts 706 and clamping with 12 bolts along with a disc spring 704, vis-a-vis the proposed single point clamp 702. It can be seen from the plots that the 12-bolt clamp 706 shows maximum variation in clamping force with deflection. This means that any causes of deformation of clamping components causes a drastic change in the clamping force resulting in non-uniform distribution of pressure across the fuel-cell stack. The 12-bolt clamp with disc spring 704 shows reasonable variation of clamping force with displacement. It is, however, evident that the proposed system of single point clamp 702 is best suited to adapt to deformations and other shocks.
[86] FIGs. 8A and 8B depict displacement plots for the top pressure end cover 510-1 and the pressure plate 506 of the single point clamping arrangement respectively, in accordance with an embodiment of the present disclosure. From FIG. 8A, it can be observed that, for the top pressure end cover 510-1 under load, the maximum displacement occurs at the centre. From FIG. 8B, it can be observed that, for the pressure plate 506 under load, the maximum displacement occurs at the edges. This means that irrespective of the component of the clamping arrangement that undergoes any deformation or shock, a compensating mechanism exists between the end cover 510-1 and the pressure plate 506 that continues to maintain uniform pressure across the fuel-cell stack 502.
[87] FIGs. 9A and 9B illustrate deformation of the end plates before and after clamping, respectively, in respect of the conventional 12-bolt clamping arrangement. It can be observed that the net deformation is to the tune of 0.036mm due to a top plate deformation of 0.018mm and a bottom plate deformation of 0.018mm. Apart from the net deformation value, this serves to further illustrate how the thickness variation across the fuel-cell stack surface is non-uniform.
[88] FIGs. 10A and 10B illustrate deformation of the end plates before and after clamping, respectively, using the proposed single point clamping arrangement, in accordance with an embodiment of the present disclosure. It can be observed that the net deformation is to the tune of 0.028mm, which is lower than for a 12-bolt clamping arrangement. Moreover, the deformation is due to a top plate deformation of (-)0.027 mm and a bottom plate deformation of 0.055mm. This results in a lower net deformation, and a more uniform variation in thickness across the fuel-cell stack surface.
[89] Table 2 below tabulates a comparison of 12-bolt clamping arrangement, 4-bolt clamping arrangement and the proposed single point clamping arrangement based on parameters such as pressure distribution, adaptability to thermal expansion, adaptability to variations in gasket thickness and adaptability to assembly misalignment.

Description 12 Bolt clamping
(Existing) 4 Bolt Clamping Single Point Clamping
(Proposed)
Uniform Pressure distribution Good Good Good
Adaptability to thermal expansion Poor Average Excellent
Adaptability to variation in thickness of Gaskets, MEA and plates Poor Poor Excellent
Adaptability to misalignment of parts during assembly Poor Poor Excellent

Table2: A comparison of 12 bolt clamping, 4 bolt clamping and single point clamping arrangement.

[90] It is evident from the above table that the performance of the proposed single point clamping arrangement, in accordance with an embodiment of the present disclosure, is superior to those of the others.
[91] Referring to FIGs. 11A to 11C, wherein an alternate embodiment for clamping arrangement is disclosed, the fuel-cell stack assembly 500 comprises a fuel-cell stack, supported on either side by end plates 1106. The end plates 1106 are mounted onto the assembly through a plurality of mounting sleeves or supporting pillars 1108. In an exemplary instance, there are six supporting pillars or mounting sleeves. The application and maintenance of uniform pressure across the fuel-cell stack 502 is achieved through the use of an array of tube springs 1102. The tube spring 1102 array and the bolting mechanism of the end plates 1106 together constitute a clamping arrangement to secure a fuel-cell stack 502 and maintain uniform pressure across the end plates 1106, in accordance with an embodiment of the present disclosure. FIG. 11A and 11B illustrate two different perspectives of the tube spring 1102 array, in accordance with an embodiment of the present disclosure. The tube spring 1102 array comprises of a base plate 1104 with grooves for holding a plurality of tube springs 1102. This tube spring 1102 array is placed in between the end plate 1106 and the fuel-cell stack 502, on both ends of the fuel-cell stack 502.
[92] FIG. 11C illustrates an exploded view of the fuel stack assembly 500 with the clamping arrangement of FIGs. 11A and 11B. The upper end plate 1106-1 and lower end plate 1106-2 are bolted onto the fuel-cell stack 502 assembly using six bolts each. As the bolts are gradually torqued, pressure is applied across the fuel-cell stack 502 through the tube spring 1102 array. On continued application of the load, the tube springs 1102 swell and through this swelling, any irregularities in pressure distribution is adjusted and rectified. Once the end plate 1106 bolts are fully torqued, the uniform pressure is maintained across the fuel-cell stack 502.In an exemplary embodiment, the array of tube springs 1102 can be replaced by array of disc springs 1110 as illustrated in figure 11C.
[93] Referring to FIGs. 12A and 12B, where yet another embodiment for clamping arrangement to clamp the fuel-cell stack 502 and maintain a uniform pressure across it is disclosed, the fuel-cell stack assembly 500 comprises a fuel-cell stack 502, supported on either side by end plates 1106. The end plates 1106 are mounted onto the assembly through a plurality of mounting sleeves or supporting pillars 1108. In an exemplary instance, there are six supporting pillars or mounting sleeves. The application and maintenance of uniform pressure across the fuel-cell stack 502 is achieved by inserting a silicone rubber layer 1202 that has a honeycomb structuring between the end plate 1106 and the fuel-cell stack 502. The honeycomb structured silicone rubber layer 1202 and the bolting mechanism of the end plates 1106 together constitute a clamping arrangement to secure a fuel-cell stack 502 and maintain uniform pressure across the end plates 1106, in accordance with an embodiment of the present disclosure.
[94] FIG. 12A illustrates an exploded view of the fuel stack assembly 500 with the clamping arrangement that applies and maintains a uniform pressure across the fuel-cell stack 502, in accordance with an embodiment of the present disclosure. The upper end plate 1106-1 and lower end plate 1106-2 are bolted onto the fuel-cell stack assembly 500 using six bolts each. FIG. 12B illustrates the currently embodied honeycomb structured silicone rubber layer 1202. On either side of the layer are anti-slip features that mesh in with grooves and slots present on the fuel-cell stack 502 surface as well as on the under surface of the end plates 1106. The honeycomb structure of the silicone rubber allows the layer 1202 to distort without compromise to its structural integrity. The honeycomb structured silicone rubber layer 1202 is inserted between the end plates 1106 and the fuel-cell stack 502, on both sides of the fuel-cell stack 502. As the bolts of the endplate 1106 are gradually torqued, pressure is applied across the fuel-cell stack 502 through the honeycomb structured silicone rubber layer 1202. On continued application of the load, the honeycomb structured silicone rubber layer 1202 swells and through this swelling, any irregularities in pressure distribution is adjusted and rectified. Once the end plate 1106 bolts are fully torqued, the uniform pressure is maintained across the fuel-cell stack 502.
[95] Referring to FIG. 13, where still another embodiment for clamping arrangement clamp the fuel-cell stack 502 and maintain a uniform pressure is disclosed, the fuel-cell stack assembly 500 comprises afuel-cell stack 502, supported on either side by end plates 1106. The end plates 1106 are mounted onto the assembly through a plurality of mounting sleeves or supporting pillars 1108. In an exemplary instance, there are six supporting pillars or mounting sleeves. The application and maintenance of uniform pressure across the fuel-cell stack 502 is achieved by inserting a foam layer 1302 between the end plate 1106 and the fuel-cell stack 502. The foam layer 1302 and the bolting mechanism of the end plates 1106 together constitute a clamping arrangement to secure a fuel-cell stack 502 and maintain uniform pressure across the end plates 1106, in accordance with an embodiment of the present disclosure. FIG. 13 illustrates an exploded view of the fuel-cell stack assembly 500 with the currently embodied clamping arrangement that applies and maintains a uniform pressure across the fuel-cell stack 502. The upper end plate 1106-1 and lower end plate 1106-2 are bolted onto the fuel-cell stack assembly 500 using six bolts each. The foam layer 1302 is inserted between the end plates 1106 and the fuel-cell stack 502, on both sides of the fuel-cell stack 502. As the bolts of the endplate 1106 are gradually torqued, pressure is applied across the fuel-cell stack 502 through the foam layer 1302. On continued application of the load, the foam layer 1302 swells and through this swelling, any irregularities in pressure distribution is adjusted and rectified. Once the end plate 1106 bolts are fully torqued, the uniform pressure is maintained across the fuel-cell stack 502.
[96] FIGs. 14A and 14B illustrate the exploded view of the fuel-cell stack assembly and the fully assembled view of the fuel-cell stack assembly 1402, respectively, secured with a clamping arrangement, in accordance with an embodiment of the present disclosure. FIGs. 14C and 14D illustrate the exploded view of the fuel-cell stack assembly and the fully assembled view of the fuel-cell stack assembly 1404, respectively, secured with another clamping arrangement, in accordance with an embodiment of the present disclosure. FIGs. 14E and 14F illustrate the exploded view of the fuel-cell stack assembly and the fully assembled view of the fuel-cell stack assembly 1406, respectively, secured with yet another clamping arrangement, in accordance with an embodiment of the present disclosure. FIGs. 14G and 14H illustrate the exploded view of the fuel-cell stack assembly and the fully assembled view of the fuel-cell stack assembly 1408, respectively, secured with still another clamping arrangement, in accordance with an embodiment of the present disclosure. These embodiments represent clamping arrangements that serve to secure fuel-cell stacks 502 and also apply and maintain a constant and uniform pressure across the fuel-cell stack 502. Variations in cumulative stiffness of the end plates, bipolar plates or gaskets, the dimensional inconsistencies of the components and the thermal distortion of the clamping components can cause irregularities and unevenness in the clamping assembly and, consequently, inadequate and non-uniform clamping pressure across the fuel-cell stack 502. The above clamping arrangement adapt to these issues by incorporating media between the endplates 1106 and the fuel stack 502 that can, by action of swelling, continue to maintain a uniform pressure distribution across the fuel-cell stack 502.
[97] While the foregoing describes various embodiments of the disclosure, other and further embodiments of the disclosure may be devised without departing from the basic scope thereof. The scope of the disclosure is determined by the claims that follow. The disclosure is not limited to the described embodiments, versions or examples, which are included to enable a person having ordinary skill in the art to make and use the disclosure when combined with information and knowledge available to the person having ordinary skill in the art.
ADVANTAGES
[98] The present disclosure provides a clamping arrangement for securing a fuel-cell stack.
[99] The present disclosure provides a clamping arrangement for securing a fuel-cell stack, that distributes pressure uniformly over the entire surface of the end plates.
[100] The present disclosure provides a clamping arrangement that adapts to structural changes in the fuel stack assembly due to manufacturing constraints or during operation and continues to maintain a uniform pressure across the end plates.
[101] The present disclosure provides an arrangement for clamping a fuel-cell stack that handles the temperature variations and shocks efficiently.
[102] The present disclosure provides an arrangement for clamping a fuel-cell stack that is easy to assemble and maintain.
[103] The present disclosure provides an arrangement for clamping a fuel cell stack that allows the fuel cell stack to provide the required power output without compromising its functioning.
,CLAIMS:

1. A single point clamping arrangement for a fuel-cell stack assembly, said assembly comprising:
a fuel-cell stack comprising a plurality of fuel-cells stacked in series;
a top end plate assembly configured at top of the fuel-cell stack; and
a bottom end plate assembly configured at bottom of the fuel-cell stack, wherein the arrangement of the bottom end plate assembly and the top end plate assembly is configured to secure the fuel-cell stack, and
wherein at least one of the top end plate assembly and the bottom end plate assembly comprises an intermediate layer of a composite compressible element such that arrangement of the top end plate assembly and the bottom end plate assembly enables distribution of uniform pressure across the surface of each of the plurality of fuel-cells.
2. The fuel-cell stack assembly as claimed in claim 1, wherein the fuel-cell stack assembly comprises at least two guiders at the top of the fuel-cell stack to guide the top end plate assembly on to the fuel-cell stack.
3. The fuel-cell stack assembly as claimed in claim 1, wherein the plurality of fuel-cells are stacked vertically and in the same orientation.
4. The fuel-cell stack assembly as claimed in claim 1, wherein the top end plate assembly and bottom end place assembly are secured by a plurality of mounting pillars using a plurality of fasteners.
5. The fuel-cell stack assembly as claimed in claim 1, wherein the plurality of fasteners are operated to enable at least one of the top end plate assembly and the bottom end plate assembly to provide a force towards the fuel-cell stack.
6. The fuel-cell stack assembly as claimed in claim 5, wherein the plurality of fasteners are operated such that force is provided at a pre-determined rate.
7. The fuel-cell stack assembly as claimed in claim 1, wherein the composite compressible element is any, or a combination of elements selected from a group comprising a wedge-shaped pressure plate, tube springs, disc springs, silicone rubber and foam.
8. The fuel-cell stack assembly as claimed in claim 1, wherein the composite compressible element comprises:
a wedge-shaped pressure plate with a flat surface facing the fuel-cell stack and a narrow dome facing an opposite direction;
one or more disc springs;
a tensioner disposed between the dome of the pressure plate and the one or more disc springs;
a disc spring loader disposed at top the one or more disc springs, configured to transfer the provided force towards the fuel-cell stack,
wherein the tensioner is configured to transfer the provided force to the dome of the wedge-shaped pressure plate as a single point force applied by the disc spring loader, such that the wedge-shaped pressure plate undergoes a swell action to enable uniform distribution of force on the fuel-cell stack.
9. The fuel-cell stack assembly as claimed in claim 8, wherein the composite compressible element is encased within the top end plate assembly such that the top end plate assembly consisting of an end cover lies on the pressure plate.
10. The fuel-cell stack assembly as claimed in claim 1, wherein the composite compressible element comprises:
a spring plate comprising a plurality of grooves distributed uniformly on the spring plate, each groove configured to hold a tube spring,
wherein the spring plate is configured to transfer the provided force to the fuel-cell stack such that the spring plate undergoes a swell action to enable uniform distribution of force on the fuel-cell stack.
11. The fuel-cell stack assembly as claimed in claim 1, wherein the composite compressible element comprises:
at least one layer of silicone rubber,
wherein the silicone rubber is configured to transfer the provided force to the fuel-cell stack such that the silicone rubber undergoes a swell action to enable uniform distribution of force on the fuel-cell stack.
12. The fuel-cell stack assembly as claimed in claim 11, wherein the at least one layer of silicone rubber has a honeycomb structure.
13. The fuel-cell stack assembly as claimed in claim 1, wherein the composite compressible element comprises:
at least one layer of foam,
wherein the foam is configured to transfer the provided force to the fuel-cell stack such that the foam undergoes a swell action to enable uniform distribution of force on the fuel-cell stack.