Patent Publication Number: US-2007111068-A1

Title: Compliant feed tubes for planar solid oxide fuel cell systems

Description:
FEDERAL RESEARCH STATEMENT  
      This invention was made with Government support under Contract No. DE-FC26-01 NT41245 awarded by the Department of Energy. The Government may have certain rights in this invention. 
    
    
     TECHNICAL FIELD  
      The present invention relates generally to power systems using solid oxide fuel cells and more particularly relates to compliant gas feed tubes for an external manifold and a solid oxide fuel cell stack.  
     BACKGROUND OF THE INVENTION  
      A fuel cell is a galvanic conversion device that electrochemically reacts a fuel with an oxidant to generate a direct current. The fuel cell generally includes a cathode material, an electrolyte material, and an anode material. The electrolyte material is a non-porous material sandwiched between the cathode and the anode materials. The anode and the cathode generally will be referred to as electrodes. An individual electrochemical cell usually generates a relatively small voltage. Thus, the individual electrochemical cells are connected together in series to form a stack so as to achieve higher voltages that are practically useful.  
      The anode, the electrolyte, and the cathode structures are substantially planar, or flat, in a planar fuel cell. To create a fuel cell stack, an interconnecting member is used to connect the adjacent fuel cells together in electrical series. A fuel cell stack is typically accompanied by one or more master manifolds so as to supply fuel and/or oxidant to the stack and to remove the spent fuel or air as well. Most fuel cell stack designs typically allow the fuel and the oxidant flow chambers of each cell in the stack to communicate individually with the corresponding master manifold. In internally manifolded fuel cell stack designs, the master manifolds are integral with the fuel cell stack and may be directly connected to the individual flow chambers. In externally manifolded fuel cell stack designs, the master manifold is substantially separated from the fuel cell stack and feed tubes or passages are provided to connect the master manifold to the cells in the fuel cell stack. One or more feed tubes may carry the same fluid (fuel or oxidant) to each fuel cell or the same feed tube may supply one or more fuel cells. Feed tubes may similarly be used to carry spent fuel or oxidant away from the fuel cell into an appropriate exhaust master manifold. The present invention relates to the design of such feed tubes in an externally manifolded fuel cell stack.  
      An external master manifold may be formed a number of ways. In one way, the manifold may include a pre-fabricated tube. In another method, stacking individual manifold “slices” may form the master manifold. In such a construction, appropriate manifold seals are required between these individual manifold slices to avoid leakage of the fluid carried through the master manifold.  
      A compressive load normal to the plane of the cells in a solid oxide fuel cell stack (the axial direction) generally is used. This axial compressive load performs several functions at three interfaces: (1) reduces area specific resistance by maintaining contact between a cell and an interconnect, (2) reduces leakage by maintaining compression on the perimeter seal of a cell, and (3) reduces leakage by maintaining compression on the manifold seal. Given the variety of materials used at each of these interfaces, and the variation in their behavior at different times in the stack lifecycle, the amount of axial deflection at each interface is different. Specific issues include manufacturing tolerances, seal compression, loss of interfacial filler materials (bond paste), relative thermal expansion, etc. Several of these conditions are reoccurring while some are only present at the initial assembly of the stack. Varying axial loads therefore may be required at each interface at various times. Excessive compression on the cell could lead to cell failure while insufficient compression could lead to reduced performance.  
      There is a need therefore for a means to apply an axial load to a solid oxide fuel cell stack while accommodating the differing characteristics of the elements that make up the stack as a whole. The load should be applied without compromising system efficiency.  
     SUMMARY OF THE INVENTION  
      The present application thus describes a solid oxide fuel cell system. The solid oxide fuel cell system may include a number of fuel cells placed under load in a fuel cell stack, a number of manifold slices placed under load in a manifold column, and a number of compliant feed tubes connecting the fuel cells and the manifold slices.  
      The manifold column may be placed under load separately from the fuel cell stack. The mechanical load applied to the fuel cell stack and the mechanical load applied to the manifold column may be substantially isolated by the number of compliant feed tubes. The manifold column may include a number of seals with one of the seals positioned between a pair of the manifolds. The seals may include mica or vermiculite based gaskets. One or more of the compliant feed tubes electrically isolates the respective fuel cell and the manifold slice. The manifold slices may be integral with or separate from the compliant feed tubes. The fuel cells include a number of interconnects such that the interconnects are in communication with the compliant feed tubes.  
      The compliant feed tubes may include a metallic or ceramic material in whole or in part. The compliant feed tubes may include a corrugated material or a bent feed tube. The manifold slices may have a coating of an alumina, yttria stabilized zirconia, or a ceramic.  
      The present application further describes a method of manufacturing a fuel cell system. The method may include assembling a sub-stack of a number of fuel cells, a number of manifold slices, and a number of compliant feed tubes, heating the sub-stack such that the number of compliant feed tubes sets, and assembling the sub-stacks into the solid oxide fuel cell system. The method further may include placing the fuel cells and the manifold slices under load independently, isolating the mechanical load applied to the manifold and to the fuel cell stack by deflection of the compliant feed tubes, and integrally fabricating the manifolds and the compliant feed tubes.  
      The present application further may describe a solid oxide fuel cell system. The solid oxide fuel cell system may include a number of fuel cells placed under load in a fuel cell stack and a number of manifold slices placed under load in a manifold column such that the manifold column is placed under load separately from the fuel cell stack. A number of compliant feed tubes may connect the fuel cells and the manifold slices. The compliant feed tubes may include a metallic or ceramic material in whole or in part. The load applied to the fuel cell stack and load applied to the manifold column may be substantially isolated by the compliant feed tubes.  
      These and other features of the present application will become apparent to one of ordinary skill in the art upon review of the following detailed description when taken in conjunction with the drawings and the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       FIG. 1  is a perspective view of a solid oxide fuel cell stack as is described herein.  
       FIG. 2  is a perspective view of an alternative embodiment of a solid oxide fuel cell stack. 
    
    
     DETAILED DESCRIPTION  
      Referring now to the drawing, in which like numerals reflect like elements throughout the view,  FIG. 1  shows a solid oxide fuel cell (“SOFC”) system  100  as is described herein. The SOFC system  100  includes a fuel cell stack  110  with a number of fuel cells  120 . The SOFC stack  110  may have any desired number of fuel cells  120  therein. The fuel cells  120  may be of largely conventional design. The fuel cells  120  within the SOFC stack  110  may be connected by a number of interconnects. As is well known, the interconnects may be two or more layers of metal joined together to form flow passages for fuel and/or oxidant.  
      The SOFC system  100  may have a master manifold  130  positioned adjacent to the SOFC stack  110 . The master manifold  130  may have any number of manifold slices  140  positioned therein. The manifold slices  140  are used to deliver fuel and oxidant to the interconnects of the fuel cells  120 . Generally, one manifold slice  140  is used for each of the fuel cells  110 . It is possible to have one manifold slice  140  supply several fuel cells  120  as well.  
      A seal  150  may be positioned within each of the manifold slices  140  of the manifold column  130 . The seals  150  may be high temperature compressive gaskets such as mica or vermiculite based gaskets. Glass seals also may be used. Other types of high temperature resistant materials may be used herein. The seals  150  also may be made out of an insulating material so as to provide electrical insulation. Alternatively, the surface of the manifold slices  140  may be covered with an insulating coating such as alumina, yttria stabilized zirconia, a general ceramic, or another appropriate type of coating material resistant to high temperature operation.  
      The fuel cells  120  of the SOFC stack  110  may be in communication with the manifold slices  140  of the master manifolds  130  via a number of compliant feed tubes  160 . Specifically, each of the fuel cells  120  may be in communication with the master manifold  130  via one or more of the compliant feed tubes  160 . The compliant feed tubes  160  may include metallic or ceramic tubes or tubes that are metallic in some regions and ceramic in other regions along the length. The compliant feed tubes  160  may be circular or non-circular in cross-section. The compliant feed tubes  160  may deliver fuel or oxidant from the appropriate master manifold  130  to the fuel cells  120  or deliver spent fuel or air from the fuel cell  120  to the appropriate master manifold  130 . The compliant nature of the feed tubes  160  substantially isolates the mechanical loads applied to the SOFC stack  110  and the manifold column  130 .  
      The required compliance in the feed tubes  160  may be achieved by one of several methods, including but not limited to: appropriate design of the length and cross-section of the feed tubes  160 , corrugating at least a portion of the length of the feed tubes  160 , or providing one or more appropriately designed bends in the feed tubes  160 . Other methods may be used herein. The compliant feed tubes  160  also may provide electrical insulation between the fuel cell  120  and the master manifold  130 .  
      The compliant feed tubes  160  may be integral with the manifold slices  140  of the manifold column  130 . Alternatively, the feed tubes  160  may be separately fabricated and then attached to the fuel cells  120  on one end and the manifold slices  140  on the other end. One or more feed tubes  160  may arise from each manifold slice  140 . Additional layers of feed tubes  160  and manifold slices  140  may be stacked on top of one another to form the master manifold or manifold column  130 . The seals  150  may be placed between the manifold slices  140  in order to prevent leakage of gas from the master manifold  130  formed by stacking the manifold slices  140 . Likewise, the other end of each of the compliant feed tubes  160  may be attached to a fuel cell  120 . Additional fuel cells  120  may be stacked one on top of the other so as to form the SOFC stack  110 . The appropriate mechanical load then may be applied to the SOFC stack  110  and the manifold column  130 . The master manifold  130  may be placed under load independently of the SOFC stack  110 .  
      Instead of completing the entire SOFC stack  110  or the entire manifold column  130 , a sub-stack  170  may be created. The sub-stack  170  then may be heated to cause at least some of the one time relative axial deflections between the SOFC stack  110  and the manifold column  130 . This heating also may cause the compliant feed tubes  160  to develop a permanent set corresponding to this deflection. The sub-stacks  170  then may be assembled into a full stack system  100 . The use of the sub-stacks  170  limits or reduces the mechanical load required to deflect the compliant feed tubes  160 .  
      The use of the external manifold column  130  and the compliant feed tubes  160  thus allows the fuel cell stack  110  to be isolated of the mechanical loads and deflections. The compliant feed tubes  160  also may have a permanent set in the final state such that deflection loads may be relieved. The compliant feed tubes  160  and the manifold column  130  also may be integrally fabricated so as to reduce manufacturing steps and the number of joints required. The use of the external manifold column  130  also allows for a detachable and durable seal.  
       FIG. 2  shows a further embodiment of a SOFC stack  200 . In this embodiment, the manifold column  130  is not a unitary structure. Rather, a number of separate manifold slices  210  may be used. Specifically, three (3) manifold slices  210  are shown surrounding the fuel cell  120 . The fuel cell  120  thus is connected three compliant feed tubes  160 . The manifold slices  210  thus may be stacked into three (3) manifold columns. One column may provide fuel inlet, one column may provide fuel outlet, and one column may provide air inlet. Any desired number of manifold slices  210  and columns may be used.  
      It should be apparent that the foregoing relates only to the preferred embodiments of the present application and that numerous changes and modifications may be made herein by one of ordinary skill in the art without departing from the general spirit and scope of the invention as defined by the following claims and the equivalents thereof.