Patent Publication Number: US-2003235723-A1

Title: Passive gas spring for solid-oxide fuel cell stack loading

Description:
RELATIONSHIP TO OTHER APPLICATIONS AND PATENTS  
     [0001] The present application claims priority from U.S. Provisional Patent Application, Serial No. 60/391,028, filed Jun. 24, 2002. 
    
    
     
       TECHNICAL FIELD  
       [0002] The present invention relates to hydrogen/oxygen fuel cells; more particularly, to fuel cell stacks comprising a plurality of individual fuel cell modules; and most particularly, to a method and apparatus for applying a compressive load to a fuel cell stack assembly and supply manifold during manufacture and for maintaining a compressive load thereupon during use.  
       BACKGROUND OF THE INVENTION  
       [0003] Fuel cells which generate electric current by controllably combining elemental hydrogen and oxygen are well known. In one form of such a fuel cell, an anodic layer and a cathodic layer are deposited on opposite surfaces of a permeable electrolyte formed of a ceramic solid oxide. Such a fuel cell is known in the art as a “solid oxide fuel cell” (SOFC). Hydrogen, either pure or reformed from hydrocarbons, is flowed along the outer surface of the anode and diffuses into the anode. Oxygen, typically from air, is flowed along the outer surface of the cathode and diffuses into the cathode where it is ionized. The oxygen ions diffuse through the electrolyte and combine with hydrogen ions to form water. The cathode and the anode are connected externally through the load to complete the circuit whereby electrons are transferred from the anode to the cathode. When hydrogen is derived from “reformed” hydrocarbons, the reformate gas includes CO which is converted to CO 2  at the anode. Reformed gasoline is a commonly used fuel in automotive fuel cell applications.  
       [0004] A single cell is capable of generating a relatively small voltage and wattage, typically between about 0.5 volt and about 1.0 volt, depending upon electrical load, and less than about 2 watts per cm 2  of cell surface. Therefore, in practice it is usual to stack together, in electrical series, a plurality of cells. Because each anode and cathode must have a free space for passage of gas over its surface, the cells are separated by perimeter spacers which are vented to permit flow of gas to the anodes and cathodes as desired but which form seals on their axial surfaces to prevent gas leakage from the sides of the stack. The perimeter spacers include dielectric layers to insulate the interconnects from each other. Adjacent cells are connected electrically by “interconnect” elements in the stack, the outer surfaces of the anodes and cathodes being electrically connected to their respective interconnects by electrical contacts disposed within the gas-flow space, typically by a metallic foam which is readily gas-permeable or by conductive filaments. The outermost, or end, interconnects of the stack define electric terminals, or “current collectors,” which may be connected across a load.  
       [0005] A complete SOFC assembly typically includes auxiliary subsystems for, among other requirements, generating fuel by reforming hydrocarbons; tempering the reformate fuel and air entering the stack; providing air to the hydrocarbon reformer; providing air to the cathodes for reaction with hydrogen in the fuel cell stack; providing air for cooling the fuel cell stack; providing combustion air to an afterburner for unspent fuel exiting the stack; and providing cooling air to the afterburner and the stack. These various subsystems typically are mated via mounting to an integrating manifold. A complete SOFC assembly also includes appropriate piping and valving, as well as a programmable electronic control unit (ECU) for managing the activities of the subsystems simultaneously.  
       [0006] During assembly of a fuel cell stack, a compressive load must be maintained during high-temperature sintering of the stack assembly seals. Further, a compressive load must also be maintained after the sintering process to ensure the integrity of the glass seals to the manifold during assembly and also afterwards during use of the finished fuel cell assembly.  
       [0007] The stack assembly is made from a variety of metallic and non-metallic materials, and the supporting structure fastening the stack to its manifold is constructed of, typically, a bolting material capable of withstanding high temperatures. At operating temperature, typically around 800° C., thermal growth of the stack does not match thermal growth of the bolting material because of differences in thermal expansion coefficients, which mismatch can result in loss of compressive load against the various seals.  
       [0008] To compensate for this mismatch, it is known to use mechanical springs within the assembly. However, high operating temperatures can affect temper of spring materials, resulting in load failure. Further, spring constants typically diminish with increase in temperature, conditions under which an increase in spring force is desirable to compensate for increasing mismatch.  
       [0009] Another known approach is to carefully select the materials in the mounting mechanism to match the thermal characteristics of the stack. However, slight mismatches still can result in loss of compression or lateral shear between adjacent surfaces; also, this approach can undesirably limit the choice of materials and their combinations.  
       [0010] Further, a fuel cell assembly may comprise a plurality of fuel cell stacks disposed side-by-side within a single supporting structure, and different stacks may vary in height at different temperatures.  
       [0011] What is needed is a means for providing a compressive load to a fuel cell assembly at ambient and elevated temperatures to compensate for mismatches in the heights of multiple stacks and for the difference in thermal expansion between the stacks and the supporting structure.  
       [0012] It is a principal object of the present invention to compress a fuel cell assembly automatically under all required temperature conditions during manufacture.  
       [0013] It is a further object of the present invention to compress a fuel cell assembly automatically under all temperature conditions of use.  
       BRIEF DESCRIPTION OF THE INVENTION  
       [0014] Briefly described, in a fuel cell assembly comprising one or more fuel cell stacks and a supporting structure, a passive gas spring is disposed between the stacks and the supporting structure for maintaining compressive force on the stack and manifold seals. The spring includes a membrane formed of a metal alloy stable at the operating temperatures required of the fuel cell assembly. In one embodiment, the membrane is attached along a first edge to the fuel cell stacks and along a second edge to the supporting structure to form a closed chamber for retaining an amount of gas. As variation in temperature of the assembly and structure causes dimensional changes therein, the pressure within the gas spring also changes accordingly, thereby automatically maintaining a compressive load on the fuel cell stack over the full range of temperature variation, and in fact desirably increasing the load as temperature increases, unlike the prior art mechanical spring.  
       [0015] In a currently preferred embodiment of a gas pillow for inclusion in a fuel cell assembly, a closed frame element is formed having a trough shape that provides great resistance to radial expansion. Upper and lower metal membranes are laser-welded to the frame element to define a gas-filled space therebetween. Other configurations for capturing a gas-filled space are also comprehended by the invention, including one having a mechanical spring coupled within a gas spring.  
       [0016] As the temperature of the gas spring increases, the axial pressure exerted on the membranes by the captured gas, and therefore on the fuel cell stack, also increases in accordance with Boyle&#39;s Law. Thus, as height mismatches occur between the stacks and the supporting structure, the gas spring increases in force to maintain compressive load on the various assembly seals. It is especially beneficial that thermal expansion of the stack components and the gas are both thermally linear.  
       [0017] An advantage of the present invention is that the load applied by the gas spring is uniform over the operating area of the gas spring; thus, there are no high load concentrations against the fuel cell elements.  
       [0018] Another advantage of the present invention is that any desired load pattern may be provided simply by manipulating the areal shape of the gas pillow. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0019] These and other features and advantages of the invention will be more fully understood and appreciated from the following description of certain exemplary embodiments of the invention taken together with the accompanying drawings, in which:  
     [0020]FIG. 1 is an elevational cross-sectional view of a prior art SOFC assembly, showing a mechanical spring for maintaining compression of the assembly;  
     [0021]FIG. 2 is an elevational cross-sectional view of a portion of a first embodiment of a gas spring having an inward concave frame element in accordance with the invention;  
     [0022]FIG. 2 a  is the same view as FIG. 2, having an outward concave frame element;  
     [0023]FIG. 2 b  is an elevational cross-sectional view of a portion of a second embodiment of a gas spring in accordance with the invention;  
     [0024]FIG. 2 c  is an elevational cross-sectional view of a portion of a third embodiment of a gas spring in accordance with the invention;  
     [0025]FIG. 3 is an elevational cross-sectional view of an SOFC assembly like that shown in FIG. 1 but incorporating a first embodiment gas spring like that shown in FIG. 2;  
     [0026]FIG. 4 is an elevational cross-sectional view of an SOFC assembly like that shown in FIG. 3 but incorporating a fourth embodiment of gas spring; and  
     [0027]FIG. 5 is an elevational cross-sectional view of an SOFC assembly incorporating gas spring at the bottom of the fuel cell stack. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0028] The features and benefits of the present invention will be more fully understood by first considering a prior art fuel cell assembly utilizing a mechanical spring.  
     [0029] Referring to FIG. 1, a prior art fuel cell assembly  10  includes a fuel cell stack  12  comprising a plurality of individual fuel cell modules  14 . Surrounding stack  12  is a supporting structure or load frame  16  comprising a base plate  18 ; a spring holder  20  for transferring spring force to stack  12 ; a mechanical spring  22  having first and second leaves  24 ; and a spring retaining plate  26  providing a mechanical stop for spring  20 . Also shown in FIG. 1 is a current collector  28 , power lead  30 , and housing  32 .  
     [0030] Bolts  34  extend through ears  36  on retaining plate  26  and through bores  38  in base plate  18  and are threadedly received in supply and exhaust manifold  40 . The fuel cell stack, spring, and base plate are thus sandwiched between the retaining plate and the manifold. Tension on the bolts serves to provide compression of the stack, spring, and base plate.  
     [0031] Passageways  39 , formed by aligned apertures in supply and exhaust manifold  40 , gasket element  42 , base plate  18  and stack  12  serve to carry oxygen or hydrogen to active surfaces of fuel cell modules  14  as known in the art. The purpose of spring  20  is to keep the bolts under tension, and thus the stack under compression, at all conditions. Modules  14  are sealed to each other and to current collector  28  by thin glass seals (not visible in FIG. 1). Base plate  18  is sealed to manifold  40  by gasket element  42 . For these seals to remain intact at all conditions, and for the integrity of the passageways to be retained, the assembly must be maintained in compression, by maintaining tension on bolts  34 .  
     [0032] Mechanical spring  22  is disposed between spring holder  20  and retaining plate  26 . Deflection of leaves  24  is intended to provide a continuous compressive load despite differences in thermal growth between stack  12  and load frame  16 . Preferably, the entire assembly is held in a jig at predetermined elevated temperature and pressure for a predetermined time to sinter the various seals, and then the bolts are torqued by a predetermined amount to establish the preload on the spring and assembly.  
     [0033] Referring to FIGS. 2 and 3, in accordance with the invention, in improved assembly  10 ′ a novel gas spring  44  is substituted directly for mechanical spring  22 . Gas spring  44  comprises a closed frame element  46  having axis  47  and preferably is formed in a trough shape to resist radial deformation under load. Frame element  46 , 46 ′ may be concave inwards as shown in FIG. 2 or outwards as shown in FIG. 2 a  to equal effect. Frame element  46 , 46 ′ includes first and second axial surfaces  48 , 50  to which first and second membranes  52 , 54 , respectively, are continuously attached as by laser welding  56  to form a flattened pillow enclosing a chamber  58 . Preferably, membranes  52 , 54  are formed of a flexible high-temperature metal alloy, for example, Haynes 160, 214, 230, 825, or 901; a Hastelloy; or Inconel DS, 625, or 718. Preferably, membranes  52 , 54  are between about 0.005 inch and 0.010 inch in thickness. Chamber  58  is filled with a gas  60 , preferably air, which may be installed in known fashion at any desired pressure above or below atmospheric for any specific application; one atmosphere is currently preferred for fuel cell uses.  
     [0034] As the temperature of captive gas  60  rises, increased outward axial pressure is exerted on element  46  and membranes  52 , 54  in accordance with Boyle&#39;s Law, urging the membranes apart axially as shown by phantom membranes  52 ′, 54 ′ in FIG. 2. When installed in assembly  10 ′, membranes  52 , 54  are restrained by spring holder  20  and spring retaining plate  26 . Thus, thermal expansion of gas spring  44  urges spring holder  20  toward base plate  18 , keeping stack  12  under compression, and urges retaining plate  26  away from manifold  40 , thus keeping bolts  34  under tension and gasket element  42  under compression.  
     [0035] The operative principle of the invention is the use of a captive gas volume to maintain seal compression over a range of temperatures. A volume of gas in accordance with the invention may be captured in a wide variety of structures, all of which are comprehended by the invention. For example, referring to assembly  10 ″ in FIG. 4, in some applications a separate gas spring structure  44  may be omitted and a chamber  58 ′ created simply by flexibly sealing the space between spring holder  20  and spring retaining plate  26 . A corrugated, flexible membrane  62  is attached continuously along a first edge  64  to spring holder  20  and along a second edge  66  to spring retaining plate  26  of the fuel cell stack. Thus gas  60  is captured and the spring holder and spring retaining plate are urged away from each other in response to increase in the temperature of gas  60 .  
     [0036] Referring to FIG. 2 b , a simplified gas spring  44 ′ in accordance with the invention may be formed for use in some applications by omitting frame element  46  from spring  44  and directly sealing membrane  52  to membrane  54  as by laser welds  56  to form a gas-filled pillow.  
     [0037] Referring to FIG. 2 c , a combined mechanical—gas spring  44 ″ in accordance with the invention may be formed using mechanical spring  57  enclosed in two-part membrane spring of the type shown in FIG. 2 b . In this embodiment, even at lower temperatures when the pressure of gas  60  inside chamber  58  is reduced, forces exerted by mechanical spring  57  outwardly against membranes  52 , 54  will maintain a compressive load on the fuel cell stack. Optional stop element  59 , attached, for example, to the inside surface of membranes  52 , 54  serves to prevent excessive inward travel of membranes  52 , 54  under low temperature conditions and over-deflection of spring  57  beyond its yield limit. While FIG. 2 c  discloses a particular type of spring, any spring means that applies a compressive load to the stacks, used in conjunction with a gas spring, is comprehended by this invention. Also, while FIG. 2 c  discloses a two-part membrane gas spring used in conjunction with a mechanical spring, it is understood that a mechanical spring can be used in conjunction with any of gas spring embodiments shown in FIGS. 2 a ,  2   b  and  4 , and be in accordance with this invention. Further, while FIG. 2 c  discloses the gas and mechanical springs to be functionally in parallel with each other and one set of stops operating for both springs, it is understood that the gas and mechanical springs can be functionally in series with each other, such as by incorporating the mechanical spring outside of chamber  58 , and for each spring to have its own set of stops.  
     [0038] Referring to FIG. 5, yet another embodiment of the current invention is shown wherein a gas spring such as, for example, one of the two part construction shown in FIG. 2 b  is positioned below the fuel cell stacks rather than above the fuel cell stacks, replacing gasket element  42 . In this position, the gas spring provides compensation for both a loss of compression and lateral shear caused by the differing thermal growth of materials.  
     [0039] As shown in FIG. 5, membrane  52  is first sealably joined to membrane  54 , such as by laser welding  56 . Chamber  58  formed therebetween is filled with gas  60 , preferably air, to form a gas spring  144 . Apertures  61 , formed in edge regions of gas spring  144 , align with similarly shaped apertures in supply and exhaust manifold  40 , base plate  18 , and stack  12 , and provide passageways  39  for carrying oxygen and hydrogen to fuel modules  14 . By locating gas spring  144  between manifold  40  and base plate  18 , spring  144  serves both to add compressive force to the stack and to seal around oxygen or hydrogen passageways  39 .  
     [0040] While the invention has been described by reference to various specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but will have full scope defined by the language of the following claims.