Abstract:
In a modular fuel cell cassette for forming a fuel cell stack, anode openings in the mounting plate and separator plate are separated and connected by modular spacer rings such that the cassette is incompressible at operating temperatures and compressive loads within the stack. The spacer rings are formed in modules wherein all of the rings required for all of the anode supply chimneys or all of the anode exhaust chimneys of any given cassette are ganged together and include a perimeter rail to which the rings are connected which automatically orients and positions the rings within the cassette during assembly thereof. The present invention eliminates the prior art need for individually positioning and spot welding each prior art ring in place prior to assembly of a prior art cassette. Two different structural embodiments for a spacer ring module are disclosed.

Description:
RELATIONSHIP TO OTHER APPLICATIONS AND PATENTS 
     The present application is a Continuation-In-Part of a U.S. patent application Ser. No. 11/027,095, filed Dec. 30, 2004 now U.S. Pat. No. 7,306,872 and published Jul. 6, 2006 as U.S. Published Patent Application 2006/0147786 A1 (now U.S. Pat. No. 7,306,872). 
    
    
     This invention was made with United States Government support under Government Contract/Purchase Order No. DE-FC26-02NT41246. The Government has certain rights in this invention. 
    
    
     TECHNICAL FIELD 
     The present invention relates to fuel cells; more particularly, to solid-oxide fuel cells; and most particularly, to modular fuel cell cassette spacers for use in assembling a fuel cell stack. 
     BACKGROUND OF THE INVENTION 
     Fuel cells for combining hydrogen and oxygen to produce electricity are well known. A known class of fuel cells includes a solid-oxide electrolyte layer through which oxygen anions migrate; such fuel cells are referred to in the art as “solid-oxide” fuel cells (SOFCs). 
     In some applications, for example, as an auxiliary power unit (APU) for a transportation application, an SOFC is preferably fueled by “reformate” gas, which is the effluent from a catalytic liquid or gaseous hydrocarbon oxidizing reformer, also referred to herein as “fuel gas”. Reformate typically includes amounts of carbon monoxide (CO) as fuel in addition to molecular hydrogen. 
     A complete fuel cell stack assembly includes fuel cell subassemblies and a plurality of components known in the art as interconnects, which electrically connect the individual fuel cell subassemblies in series. Typically, the interconnects include a conductive foam or weave disposed in the fuel gas and air flow spaces adjacent the anodes and cathodes of the subassemblies. 
     In the prior art, a fuel cell stack is assembled typically by first laying up each of the fuel cell subassemblies in a jig, forming a plurality of repetitive modular fuel cell units known in the art, and referred to herein, as “cassettes”. Typically, a fuel cell cassette comprises a ceramic solid-oxide electrolyte layer and a cathode layer coated onto a relatively thick, structurally significant anode element. In such a prior art assembly, each of the cassettes becomes a structural and load-bearing element of the stack. 
     At the elevated operating temperatures of an SOFC stack, typically in the range of about 700° C. to about 1000° C., most of the components of a cassette have very little inherent mechanical strength and would collapse if not for internal spacer rings disposed within each cassette around the anode fuel gas openings, collectively comprising supply and exhaust “chimneys” within a stack. Prior art spacer rings are fabricated so that they form a solid column of metal having radial openings to allow the anode fuel gas to flow into and out of the cassette. The assembly load of a stack thus is carried through the spacer rings. 
     A prior art spacer ring is a sheet metal part which is stamped and formed to achieve the desired geometry. This spacer ring is difficult to form, resulting in a part that is relatively expensive even with production tooling in high volumes. Further, each cassette requires a plurality of spacer rings (typically 8), and each ring must be tack welded into place to the cassette shell, accurately and firmly, prior to cassette assembly, adding further positioning and attachment cost and complexity to the assembly operation. 
     What is needed in the art is an improved spacer ring that is less expensive to manufacture and less expensive to install into a cassette during assembly thereof. 
     It is a principal object of the present invention to reduce the cost, difficulty, and complexity of mass-manufacturing fuel cell stack assemblies. 
     SUMMARY OF THE INVENTION 
     Briefly described, a modular fuel cell cassette for use in assembling a fuel cell stack is a sheet metal assembly comprising a metal separator plate and a metal cell-mounting plate so formed that when they are joined at their perimeter edges to form the cassette, a cavity is formed between them which can contain a gas stream that feeds a fuel cell subassembly attached within the cassette to the mounting plate. Outboard of the fuel cell subassembly, the separator plate and cell-mounting plate are perforated by openings to form chimney-type manifolds for supplying fuel gas to the anode and air to the cathode, and for exhausting the corresponding gases from the stack. The fuel cell subassembly is attached to, and insulated from, the mounting plate by a dielectric seal. The mounting plate includes an opening through which one of the electrodes is accessible, preferably the cathode, and through which a conductive interconnect element extends to make contact with the outer surface of the next-adjacent cassette in a stack. 
     The anode openings in the mounting plate and separator plate are separated and connected by modular spacer rings such that the cassette is incompressible. The spacer rings include radial openings which allow fuel gas to flow from the anode supply chimney into the anode gas channel in the cassette and also back into the anode exhaust chimney. In accordance with the present invention, the spacer rings are formed in modules wherein all of the rings required for all of the anode supply chimneys or all of the anode exhaust chimneys of any given cassette are ganged together and include a perimeter element to which the rings are connected which automatically orients and positions the rings within the cassette during assembly thereof. The present invention eliminates the prior art need for individually positioning and spot welding each ring in place prior to assembly of a cassette. Two different structural embodiments for a spacer ring module are disclosed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  is an exploded isometric view of a prior art fuel cell cassette; 
         FIG. 2  is an isometric view of a prior art fuel cell stack comprising three cassettes as shown in  FIG. 1 ; 
         FIG. 3  is a plan view of a prior art separation ring, as stamped from sheet stock; 
         FIG. 4  is a plan view of the prior art separation ring shown in  FIG. 3 , folded for use in a fuel cell cassette as shown in  FIG. 1 ; 
         FIG. 5  is a cross-sectional view taken along line  5 - 5  in  FIG. 4 ; 
         FIG. 6  is an isometric view of a first embodiment of a spacer ring module in accordance with the invention; 
         FIG. 7  is an isometric view of a second embodiment of a spacer ring module in accordance with the invention; 
         FIG. 8  is an isometric view of the second embodiment as shown in  FIG. 7 , after folding into a planar configuration for use; and 
         FIG. 9  is an exploded isometric view of a fuel cell stack comprising a cassette formed with spacer rings in accordance with the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention is directed to a modular spacer ring element which may be substituted in an otherwise prior art fuel cell cassette  100  and fuel cell stack, in a greatly simplified assembly procedure. Therefore, it is useful to review here such a prior art fuel cell cassette  100  to understand how an improved modular spacer ring element  326 , 426  may be used to replace the prior art individual spacer rings  126 . 
     Prior art fuel cell cassette  100  is substantially as disclosed in the parent patent application referenced hereinabove and made public in US Published Patent Application No. 2006/0147786 A1(now U.S. Pat. No. 7,306,872), the relevant disclosures of which are incorporated herein by reference. 
     Referring to  FIG. 1 , a prior art fuel cell cassette  100  includes a cassette housing  101  including a fuel cell mounting plate  102  and a separation plate  104 . Mounting plate  102  includes a large central electrode opening  106  for receiving a fuel cell subassembly  128  as described below. Outboard of central electrode opening  106  are cathode air inlets  108   a , cathode air outlets  110   a , fuel gas inlets  112   a , and fuel gas outlets  114   a . Separation plate  104  is provide with similar and mating air and fuel openings  108   b , 110   b , 112   b , and  114   b , respectively, said electrode and separation plate inlets and outlets defining respective supply and exhaust chimneys for air and fuel gas. Separation plate  104  is formed as a shallow tray  115  such that a cavity is created between plates  102 , 104  for receiving fuel cell components and fuel gas as described below. Preferably, the mounting and separation plates are formed as by stamping or drawing from thin sheet stock (0.1 to 1.0 mm) of a ferritic stainless steel, although other materials such as austenitic stainless steel or high temperature alloys may also be acceptable. During assembly, prior art plates  102 , 104  are joined to define a cassette housing by formation of a metallurgical bond at their edges and around each of the air inlets and outlets such that only openings  112 , 114  have access to the interior of the cassette. 
     Referring to FIGS.  1  and  3 - 5 , a prior art spacer ring  126  is provided within each cassette  100  for each anode fuel gas inlet  112   a,b  and each anode fuel gas outlet  114   a,b . In the prior art embodiment shown here for forming prior art spacer rings  126 , a pair of rings  120   a,b  having radial tabs  118  extending from rings  120   a,b  are connected by a link  122 . Radial tabs  118  are folded inward and line up with one another when the two rings  120   a , 120   b  are folded over at link  122  to form solid columns of metal, as shown in  FIG. 4 . Link  122  provides a convenient tab for tack welding of each ring  126  to the cassette shell during assembly. The spaces between the tabs  118  form openings  124  which allow fuel gas to flow from the fuel gas inlets  112  into the anode gas channel (space contained within the cassette), and into the fuel gas outlets  114  from the anode gas channel. The folded spacer rings  126  form solid metal spacers between mounting plate  102  and separator plate  104 , thus defining and maintaining a constant spacing therebetween despite assembly and operational loads on the cassette. Prior art rings  126  are formed by stamping from sheet materials similar to those disclosed for forming the mounting plate and separator plate. 
     Referring to  FIG. 2 , a fuel cell stack  200  is formed by literally stacking together a plurality of individual fuel cell cassettes  100 . The cassettes are bonded together outboard of central opening  106  in a pattern surrounding the air and fuel gas inlets and exhausts. 
     Referring now to  FIGS. 6 and 9 , a first embodiment of a modular spacer ring element  326  comprises a plurality of identical individual spacer rings  326   a , 326   b , 326   c , 326   d  oriented and attached via individual tethers  380   a , 380   b , 380   c , 380   d  to a common rail  382 . Each spacer ring  326   a - d  has radial anode fuel gas flow passages  324  formed into one surface of the ring. The flow passages  324  are separated by columnar ring segments  318  corresponding to prior art tabs  118  which are the full thickness of the ring and therefore can act as structural support columns around the anode fuel gas openings after assembly of a cassette and stack. Spacer ring element  326  can be simply placed into the cassette during the cassette assembly process. The rings are automatically positioned and oriented, and no welding is required. Rail  382  is sandwiched between the abutting edges of fuel cell mounting plate  102  and a separation plate  104  ( FIGS. 1 and 9 ), thereby securing rings  326   a - d  in position. The axial faces of rings  326   a - d  are sealed to the fuel cell mounting plate  102  and separation plate  104  by compression during assembly of the cassettes into a fuel cell stack. 
     Modular spacer ring element  326  is readily formable as a monolith in known fashion via, for example, photochemical machining, powdered metal fabrication, coining, or forging. Two such elements  326 , one for anode fuel gas supply and one for anode fuel gas exhaust, are required for each cassette  300 . Preferably, element  326  is formed by photochemical machining. Although photochemically machined parts are typically more expensive than simple stampings, a single photochemically machined element  326  is less expensive than the corresponding four stamped prior art rings  126  currently in use (in addition to the assembly savings already described). 
     Referring now to  FIGS. 7 and 8 , a second embodiment of a modular spacer ring element  426  comprises a sub-element  426 ′ having a plurality of identical individual spacer sub-rings  426   a   1 , 426   a   2 , 426   b   1 , 426   b   2 , 426   c   1 , 426   c   2 , 426   d   1 , 426   d   2  oriented and attached via individual tethers  480   a   1 , 480   a   2 , 480   b   1 , 480   b   2 , 480   c   1 , 480   c   2 , 480   d   1 , 480   d   2  to a common rail  482  formed in two parts,  482 - 1 , 482 - 2 , and foldable at points  484 . 
     Second embodiment  426  as formed initially ( FIG. 7 ) is one-half the thickness of first embodiment  326 . Each spacer sub-ring  426   a   1 - d   2  has a plurality of curves defining an annular pattern of alternating inwardly- and outwardly-extending arcs  486 - 1 , 486 - 2 . Arcs  486 - 1  are angularly shifted from arcs  486 - 2  by one-quarter cycle (in the present example, by 30°) with respect to tethers  480  such that when first rail portion  482 - 1  is folded at points  484 , defining a folding line  485 , onto second rail portion  482 - 2 , as shown in  FIG. 8 , a fully formed spacer  426  is formed having the same thickness as first embodiment  326 . The annular geometry of the two rows of sub-rings is such that, when folded into superposition, radial openings  488  are formed therebetween for passage of anode fuel gas into and out of the stack chimneys. The fully-formed spacer rings  426   a - d  ( FIG. 8 ) define columns  418  where the sub-rings overlap, corresponding to prior art tabs  118  which are the full thickness of the ring and therefore can act as structural support columns around the anode fuel gas openings after assembly of a cassette and stack. Spacer ring element  426  can be simply placed into the cassette during the cassette assembly process. The rings are automatically positioned and oriented, and no welding is required. Rail  482  is sandwiched between the abutting edges of fuel cell mounting plate  102  and a separation plate  104 , thereby securing rings  426   a - d  in position. The axial faces of rings  426   a - d  are sealed to the fuel cell mounting plate  102  and separation plate  104  by compression during assembly of the cassettes into a fuel cell stack. 
     It will be observed that portions  426 - 1  and  426 - 2 , shown in  FIG. 7 , are not mirror images but rather inverted images; that is, portion  426 - 2  may be derived from a second portion  426 - 1  by simply turning portion  426 - 1  end-for-end. Thus, sub-element  426 ′ may be formed either by stamping as a single sheet from sheet stock, for folding as described at points  484 , or by two identical portions  426 - 1  oriented as just described for attachment at points  484 . 
     Modular spacer ring element  426  is readily formable in known fashion via, for example, photochemical machining, powdered metal fabrication, coining, or forging. Two such elements  426 , one for anode fuel gas supply and one for anode fuel gas exhaust, are required for each cassette  300 . Preferably, element  426  is formed by stamping and folding from sheet stock. 
     Referring to  FIG. 9 , a portion  500  of a completed fuel cell stack in accordance with the invention comprises first and third cassettes  500   a , 500   c  completed in accordance with the invention on either side of an intermediate exploded second cassette  500   b.    
     Second cassette  500   b  includes a cassette housing  501  including a fuel cell mounting plate  502  and a separation plate  504 . Mounting plate  502  includes a large central electrode opening for receiving a cathode mesh air baffle  503 . Outboard of the central electrode opening are cathode air inlets  508   a , cathode air outlets  510   a , fuel gas inlets  512   a , and fuel gas outlets  514   a . Separation plate  504  is provide with similar and mating air and fuel openings, respectively, said electrode and separation plate inlets and outlets defining respective supply and exhaust chimneys for air and fuel gas. Separation plate  504  is formed as a shallow tray such that a cavity is created between plates  502 , 504  for receiving fuel cell components and fuel gas. A first anode modular spacer ring element  526 -A is installed adjacent anode fuel gas inlets  512   a , and a second anode modular spacer ring element  526 -B is installed adjacent anode fuel gas outlets  514   a . An anode mesh fuel baffle  505  is disposed between ring elements  526 -A, 526 -B. A contact paste layer  507  electrically connects the cathode mesh  503  to the surface of the cathode layer in mounting plate  502 . A contact paste layer  509  electrically connects the anode mesh fuel baffle  505  to the separator plate  504 . A fusible glass seal  511  seals cassette  500   b  to cassette  500   a . (A similar glass seal is required but not shown between cassette  500   c  and cassette  500   b .) 
     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.