Abstract:
A system and method for electrically interconnecting a plurality of fuel cells to provide dense packing of the fuel cells. Each one of a plurality of fuel cells has a plurality of discrete electrical connection points along an outer surface. Electrical connections are made directly between the discrete electrical connection points of adjacent fuel cells so that the fuel cells can be packed more densely. Fuel cells have at least one outer electrode and at least one discrete interconnection to an inner electrode, wherein the outer electrode is one of a cathode and an anode and wherein the inner electrode is the other of the cathode and the anode. In tubular solid oxide fuel cells the discrete electrical connection points are spaced along the length of the fuel cell.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of provisional patent application No. 60/750,569 filed on Dec. 15, 2005. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with Government support under DE-FC26-03NT41838 awarded by the U.S. Department of Energy. The Government has certain rights in this invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to solid oxide fuel cells, and, more particularly, to the electrical interconnection of solid oxide fuel cells. 
     BACKGROUND 
     Tubular solid oxide fuel cells (SOFCs) represent a significant advantage over planar-type SOFCs due to enhanced gas collection capability, ease of manufacture, and strength of the tubular design. Anode supported tubular SOFCs possess additional advantages over cathode or electrolyte supported cells due to lower cost, greater strength, and more intimate relationship with the critical gas component, i.e., the fuel. With this capture of the fuel, they also inherently have the ability to perform on-cell reformation of fuels rather than require external reforming equipment. 
       FIG. 1  is a cross-sectional view of a typical anode-supported tubular SOFC as known in the art. Generally speaking, an anode-supported tubular SOFC has a hollow, tubular inner anode layer  102 , an electrolyte layer  104  formed on a portion of the outside of the anode layer  102 , and a cathode layer  106  formed on a portion of the electrolyte layer. Current flows radially from the inside to the outside along the length of the tube. 
     As shown in  FIG. 2 , current collection in anode-supported tubular SOFCs typically involves anodic electrical take-off connections  202  and cathodic electrical take-off connections  206  located at one end of the tubular fuel cell adjacent a fuel input  220 . This arrangement allows mechanical ease of assembly, utilizing the gas distribution manifolds as current collection devices. For current collection, wires must be run between the manifold and the cathode and/or anode. Due to separation and connection constraints, the manifold must be designed to allow for sufficient spacing to accommodate these connections, resulting in a relatively large system. Additionally, this arrangement generally results in large electrical power losses, proportional to the length and thickness of the anode supported fuel cell. 
     One drawback of the current collection arrangement shown in  FIG. 2  is that the current needs to travel along the entire length of the tube. This can result in major power losses.  FIG. 3  shows a representation of current decreasing as a function of increasing tube length for an anode-supported tubular SOFC having anodic and cathodic current collectors at one end of the fuel cell. It is therefore desirable to reduce or minimize these losses to enhance cell performance and lower fuel cell costs. 
     Siemens Westinghouse describes the use of a single strip down the length of a cathode supported fuel cell, allowing current collection along the length, with only circumferential losses, although due to the design of their cathode-supported fuel cell, significant non-uniform circumferential stresses can be formed. With such a design, improved current collection is generally realized at the expense of a more complicated system design and greater variability in the packing of the tubular fuel cells. 
     SUMMARY OF THE INVENTION 
     A system and method for interconnecting bundled solid oxide fuel cells is disclosed. Each one of a plurality of fuel cells has a plurality of discrete electrical connection points along an outer surface. Electrical connections are made directly between the discrete electrical connection points of adjacent fuel cells so that a manifold does not need to be used in current collection and the fuel cells can be packed more densely. In this way, the manifold is not constrained by electrical requirements and therefore can be redesigned to improve fuel cell density. 
     Each fuel cell may include at least one outer electrode and at least one discrete interconnection to an inner electrode, wherein the outer electrode is one of a cathode and an anode and wherein the inner electrode is the other of the cathode and the anode. 
     The system may also include a current collector configured to directly connect electrical connection points of adjacent fuel cells, and bridge connection points of the fuel cell on which it is located while not shorting the cathode to the anode for any individual fuel cell. 
     Fuel cells may be aligned such that the cathode connection points of adjacent fuel cells are side-by-side and such that the anode connection points of adjacent fuel cells are side-by-side. Alternatively, fuel cells or the manufactured connections on the fuel cells may be staggered such that the cathode connection points of one fuel cell are side-by-side with the anode connection points of an adjacent fuel cell. The former configuration can be easily used to form serial or parallel electrical connections. The latter configuration is particularly useful for forming serial electrical connections, but can also be used to form parallel connections. In addition, these discrete connections provide for high-density packaging of fuel cells without hindering air flow between cells, as would a single connection running along the entire length of the cell (e.g., the Siemens Westinghouse connection type). 
     In accordance with another aspect of the invention there is provided a method of producing a fuel cell bundle. The method comprises coupling a plurality of fuel cells to a manifold, and electrically interconnecting each fuel cell directly to at least one adjacent fuel cell so that the manifold is not required for electrical connectivity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and advantages of the invention will be appreciated more fully from the following further description thereof with reference to the accompanying drawings wherein: 
         FIG. 1  is a cross-sectional view of a typical anode-supported tubular SOFC as known in the art; 
         FIG. 2  shows a representation of a standard anode-supported tubular solid oxide fuel cell having anodic and cathodic current collectors at one end of the fuel cell as known in the art; 
         FIG. 3  shows a representation of current decreasing as a function of increasing tube length for an anode-supported tubular SOFC having anodic and cathodic current collectors at one end of the fuel cell, as shown in  FIG. 2 ; 
         FIG. 4  is a schematic of a tubular fuel cell with three electrical connection points; 
         FIG. 5  shows a 4×4 fuel cell bundle; 
         FIG. 6  shows a close up of a serial connection between two adjacent fuel cells; 
         FIG. 7  shows a parallel connection using crimps or a welded joint; 
         FIG. 8  shows a serial connection using a crimp connection; 
         FIG. 9  shows a serial connection using a ceramic or metallic form; 
         FIG. 10  shows a serial connection using wire; 
         FIG. 11  shows prefabricated wire and clip segments; 
         FIG. 12  shows serial connections when the interconnection points on adjacent fuel cells are staggered; 
         FIG. 13A  shows fuel cells with a current collector connecting from the cathode at substantially a 90° angle from the fuel cells; and 
         FIG. 13B  shows fuel cells with a current collector connecting from the anode interconnection at substantially a 90° angle from the fuel cells. 
     
    
    
     DETAILED DESCRIPTION 
     U.S. provisional patent application No. 60/750,569 filed on Dec. 15, 2005 is incorporated by reference herein in its entirety. 
     Embodiments of the present invention use multiple electrical connection points along the outer surfaces of the fuel cells to make electrical connections directly between fuel cells so that the manifold does not need to be used in current collection. Among other things, such direct electrical connections allow multiple fuel cells to be closely packed, in part because the manifold design is not constrained by electrical requirements. By closely packing fuel cells, certain advantages, such as reduced size/volume (and therefore increased power/volume ratio), reduced weight (e.g., due to reduction in manifold and other materials), improved electrical efficiency (e.g., reduced resistance losses, reduced electrical losses between fuel cells, reduced voltage/current variability), improved thermal efficiency (e.g., lower thermal losses), ease of manufacture (e.g., the ability to connect fuel cells with serial and/or parallel electrical connections to achieve specific overall power requirements), and modularity (e.g., the ability to easily interconnect multiple bundles), can be realized. 
     Exemplary embodiments are described herein with reference to tubular anode-supported SOFCs having an inner anode, an intermediate electrolyte layer, and an outer cathode layer, although it should be understood that various aspects of the invention can apply to other types of anode-supported SOFCs (e.g., non-tubular) as well as other types of fuel cells that are not anode-supported. 
     In accordance with certain embodiments of the present invention, each anode-supported fuel cell may have multiple cathode and anode electrical connection points along the outer surface of the fuel cell, with the cathode being directly accessible for electrical connectivity by virtue of the cathode being the outer layer of the fuel cell, and with the anode being indirectly accessible for electrical connectivity, e.g., through an interconnection along the outer surface that is electrically coupled with the inner anode. As shown in  FIG. 4 , fuel cell  10  includes interconnections  12   a ,  12   b ,  12   c  accessible from one side of the fuel cell so as to allow relatively easy access to both the cathode and anode, although it should be noted that fuel cell  10  may have a greater or fewer number of interconnections. In addition, the interconnections are typically constructed and placed in such a manner as to improve electrochemical and manufacturing efficiency. A tubular shape is used in the exemplary embodiments described, but other shapes (triangles, squares, etc) may be utilized in a similar manner. 
     Serial and/or parallel electrical connections can be made between adjacent fuel cells by making electrical connections between the cathode and anode electrical connection points on one fuel cell and the cathode and anode electrical connection points on an adjacent fuel cell. In a serial connection, the cathode of one fuel cell is electrically connected to the anode interconnection of the adjacent fuel cell. In a parallel connection, the cathodes of adjacent fuel cells are connected to one another, and/or the anodes of the adjacent fuel cells are connected to one another. 
       FIG. 5  shows a fuel cell bundle  14  in accordance with an exemplary embodiment of the present invention. Fuel is distributed to fuel cells  18 , which cells can be coupled to a manifold (not shown), specifically by allowing fuel to flow through the tubular anode. However, unlike prior art systems, connections from the cathode and anode are not returned to a manifold, such as, for example, the cell-holding manifold  16 . Rather, discrete connections are made directly between adjacent fuel cells, as discussed in greater detail below. In this way, the manifold is not constrained by electrical requirements and therefore can be redesigned to improve fuel cell density. 
     In particular,  FIG. 5  shows a serial connection between adjacent fuel cells  26  and  27 . Specifically, a current collector  24  (e.g., a wire) extends from cathode  20  of fuel cell  26  to cathode  21  of fuel cell  26 , but is raised above (i.e., bridged across) interconnection  23  of fuel cell  26  so that the wire does not contact interconnection  23 . As discussed below, an insulator  34  may be placed between the interconnection  23  and the current collector  24 . At the bridge point, the current collector  24  is coupled to interconnection  22  of adjacent fuel cell  27 . In this way, the cathode of fuel cell  26  is connected in series with the anode of adjacent fuel cell  27 . 
     As shown on  FIG. 5 , wire or braid ( 24 ) may be disposed adjacent YSZ (electrolyte) and wrapped with highly conductive windings, such as a silver wire winding, to create a cathode connection. The wire or braid  24  may extend along the length of the fuel cell and be disposed under a winding of another cathode portion so as to form a singular cathode current collector. Alternatively, the wire or braids disposed under the cathode connection windings may terminate in a pigtail so as to allow flexibility for connection to similar pigtails disposed under cathode connection windings of the same fuel cell or for connection to cathode or anode pigtails on an adjacent fuel cell based on a desired fuel cell interconnection arrangement. 
       FIG. 6  shows greater detail of a serial connection  28  of the type described above with reference to  FIG. 5 . Specifically, current collector  24  (e.g., a wire) is attached to cathode  20  of fuel cell  26  and to cathode  21  of fuel cell  26 , and may run along substantially the entire length of fuel cell  26 . At each interconnection  23 , the current collector  24  is detached from the fuel cell  26  so that it bridges the interconnection  23  of fuel cell  26 . An insulation layer  34  may be placed between current collector  24  and the interconnection  23  to prevent electrical contact between the two and, thus, to prevent shorting. A serial connection is made by connection of the bridged portion of current collector  24  with interconnection  22  of adjacent fuel cell  27 .  FIG. 6  also shows an interconnection clip  25 . Interconnection  23  comprises an interconnection material  36  that contacts the underlying, inner anode around which is placed a conductor layer, which can be, for example, thin wire-wrap or contact paste, or any other suitable contact material known in the art. Similarly, cathode regions  20  and  21  comprise a cathode material  32  that covers a portion of the electrolyte. An uncovered electrolyte gap  85  is shown in  FIG. 6 , separating the cathode and interconnection regions. The electrolyte layer is discontinuous where the interconnection material contacts the anode layer. 
     While  FIG. 5  and  FIG. 6  depict a current collector in the form of a wire that is bridged between two cathode segments over an anode interconnection, it should be understood that the present invention is not limited to this embodiment. Rather, many other types of electrical connections can be made. For example,  FIG. 13A  and  FIG. 13B  show wire “pigtails” that are preformed on the cathodes and anode interconnections, respectively, and then coupled as needed.  FIG. 13A  shows wire pigtails  90  formed on the cathodes.  FIG. 13B  shows wire pigtails  91  formed on the anode interconnections. These pigtails can be interconnected to form serial and/or parallel connections between fuel cells. For example, in order to form a serial connection, the cathode pigtails  90  on one fuel cell can be coupled to the anode pigtails  91  of an adjacent fuel cell, for example, by crimping, twisting, clip, wire, foam, or other means known in the art. In order to form a parallel connection, the cathode pigtails  90  of adjacent fuel cells can be coupled to one another, while the anode pigtails  91  of adjacent fuel cells can be coupled to one another, for example, by crimping, twisting, clip, wire, foam, or other means known in the art. 
     Thus, current collector  24  can be made from a variety of materials including, but not limited to, Ag, Au, Pt, Pdt coated metals, or conductive ceramics. Interconnections can be formed of a ceria-based, Fe-based, Cr-based or other gas-tight, dual-atmosphere ceramic conductor, such as, for example LaCrO 3 . Interconnection-to-interconnection, cathode-to-cathode, and interconnection-to-cathode connections can be made by, for example, the following: crimp  40  (for example, as shown in  FIG. 7 ); clip  42  (for example, as shown in  FIG. 8 , wherein current collecting wire  82  adjacent cathode  80  of a first fuel cell is connected to a similar wire collecting current from another cathode  81  on the same cell, these wires being joined by clip  42  to a bridging wire that connects to the interconnection of a second adjacent fuel cell, where gaps  85  separate the interconnection from nearby cathode on each cell and where an optional insulator  84  can be positioned between the bridging wire and interconnection  83  of the same first cell); ceramic or metallic form  44  (for example, as depicted in  FIG. 9 , where cathodes  80  and  81  are connected by the form  44  that connects to the interconnection of a second adjacent fuel cell, where gaps  85  and interconnection  83  are as described in  FIG. 9 ); metallic wire  46  (for example, as depicted in  FIG. 10 , wherein cathodes  80  and  81  are electrically connected by wire  46  that connects to the interconnection of a second adjacent fuel cell, where gaps  85 , insulator  84  and interconnection  83  are as described in  FIG. 9 ); prefabricated wire/clip segments  48  (for example, as depicted in  FIG. 11 ); or combinations thereof. 
       FIG. 7  shows parallel electrical connections using crimped wire pigtails  50 , in accordance with an exemplary embodiment of the present invention. Also shown in  FIG. 7  (and in  FIG. 13B ), a wire or braid  91  can be disposed under an interconnection winding, interconnection chip or other electrically contacting means  92 . The wire or braid can extend outboard of the winding to form pigtail in the manner discussed above to enable interconnection with other anode interconnection portions on the same fuel cell or to enable interconnection with other anode or cathode interconnection portions on one or more adjacent cells based on a desired fuel cell interconnection arrangement. 
     In the exemplary embodiments shown and described above with reference to  FIGS. 5-10 , electrical interconnection of fuel cells may be facilitated by aligning the anode interconnections (and, therefore, also aligning the cathodes) of adjacent fuel cells. In an alternative embodiment shown in  FIG. 12 , adjacent fuel cells or the manufactured connection on adjacent fuel cells are staggered so that the cathodes  20  of one fuel cell are immediately adjacent to the anode interconnections  22  of the adjacent fuel cell. In this staggered orientation, serial electrical connections between interconnections  22  and cathodes  20  can easily be made using current collectors  30 , which can also act as spacers. By choosing cells of different connection spacings, serial/parallel connections can be constructed with minimal manufacturing effort. 
     As shown in  FIG. 5 , when parallel and series connections are completed, a uniform bundle of, for example, four cells by four cells can be constructed to have the voltage of four fuel cells and the current of four fuel cells, with fuel required for all sixteen. A four by four bundle is used for example only. Bundles of varying sizes may also be created to obtain the desired voltage and current. It will be understood from the above that bundles having at least two fuel cells in each of two dimensions or axes may be formed. 
     In addition, it is possible to form bundles in this manner as a subset of a larger system. One bundle can be attached to a second bundle either by the same means within the bundle, or through use of interconnecting plates or wires that can be welded, crimped, sintered, or twisted. 
     Construction of a fuel cell bundle can utilize on-bench fixturing. Such fixturing can be easily duplicated or automated to allow for many such bundles to be constructed in parallel, minimizing production time. Fixtures may include the use of perforated sheet at the ends of the bundle weights to maintain the bundle placement, and side-wall constraints. The fixtures would allow the formation of a green unsintered bundle body, as well as the sintering and fixing of that body through temperature and or gas processing. Once formation of the green body and fixing of that body is complete, the bundle should be self-supporting, requiring only fixturing as might be needed in support of the fuel cell system requirements such as gas flow or power control. The bundle may be sintered prior to full system assembly, or may be sintered in situ, as processing would dictate. 
     Exemplary embodiments of the invention utilize 1.5 cm diameter anode-supported fuel cells with three anode interconnections each. However, similar methods and materials may be applied to any diameter with at least one discrete interconnection without substantive modification. 
     While exemplary embodiments of the invention have been described, it should be understood that the present invention is not limited to the exemplary embodiments. The present invention is not limited to anode-supported fuel cells, to tubular fuel cells, to any particular alignment of fuel cells, or to any particular way of making electrical connections between fuel cells. The present invention may be embodied in other specific forms without departing from the true scope of the invention. The described embodiments are to be considered in all respects only as illustrative and not restrictive.