Abstract:
A fuel cell stack comprises a plurality of planar fuel cells in a spiral configuration. The fuel cells angularly offset from one another such that immediately adjacent cells only partially overlap one another. The cells are preferably of the unitized type. A manifold assembly is operatively adjacent to the fuel cells. The manifold assembly includes an inlet manifold and an outlet manifold for each of the gases that are in communication with the fuel cells. The unitized solid oxide fuel cell comprises a first planar interconnect, a planar ceramic cell adjacent the first planar interconnect, a second planar interconnect adjacent the opposite side of the planar ceramic cell, and a plurality of gas tubes adjacent the planar ceramic cell. The gas tubes are arranged to a first configuration, a second configuration, a third configuration and a fourth configuration with a cylindrical gas tube shape and a T-shape gas tube designs. These configurations provide a choice of co-flow, cross flow or counter flow patterns inside the cell to improve the thermal management and cell efficiency and provide more flexibilities to connect a manifold assembly. They also provide an option to directly combust the spent fuel just outside the fuel cell to eliminate a separate burner for residual energy recovery. These configurations thus provide means for improved fuel cell power system efficiency and lowered capital and operational costs.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation-in-part of U.S. patent application Ser. No. 09/546,308 filed Apr. 10, 2000. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The present invention generally relates to planar, solid oxide fuel cells and, more particularly, to an apparatus and method of stacking and manifolding unitized solid oxide fuel cells for ready connection and disconnection of gases to manifolds.  
           [0003]    A fuel cell is basically a galvanic conversion device that electrochemically reacts a fuel with an oxidant within catalytic confines to generate a direct current. A fuel cell typically includes a cathode material that defines the reaction for the oxidant and an anode material that defines the reaction for the fuel. An electrolyte is sandwiched between and separates the cathode and anode materials. An individual electrochemical cell usually generates a relatively small voltage. Thus, to achieve higher voltages that are useful, the individual electrochemical cells are connected together in series to form a stack. Electrical connection between cells is achieved by the use of an electrical interconnect between the cathode and anode of adjacent cells. The interconnect also normally contains gas passageways for the electrodes as well as ducts or manifolding to conduct the fuel and oxidant into and out of each cell in the stack.  
           [0004]    As the fuel and oxidant gases are continuously passed through their respective passageways, electrochemical conversion occurs at or near the three-phase boundary of the gas, the electrodes (cathode and anode) and electrolyte. The fuel is electrochemically reacted with the oxidant to produce a DC electrical output. The anode or fuel electrode enhances the rate at which electrochemical reactions occur on the fuel side. The cathode or oxidant electrode functions similarly on the oxidant side.  
           [0005]    Specifically, in a solid oxide fuel cell (SOFC), the fuel reacts with oxide ions on the anode to produce electrons and water, the latter of which is removed in the fuel flow stream. The oxygen reacts with the electrons on the cathode surface to form oxide ions that are conducted through the electrolyte to the anode. The electrons flow from the anode through an external circuit and then to the cathode. The circuit is closed internally by the transport of oxide ions through the electrolyte.  
           [0006]    In a SOFC, the electrolyte is in a solid form. Typically, the electrolyte is made of a nonmetallic ceramic, such as dense yttria-stabilized zirconia (YSZ) ceramic, that is a nonconductor of electrons that ensures that the electrons must pass through the external circuit to do useful work. As such, the electrolyte isolates the fuel and oxidant gases from one another and allows a potential to build up across it as a result of the difference in electrochemical potential between the fuel and the oxidant. The anode and cathode are generally porous, with the anode oftentimes being made of nickel/YSZ cermet and the cathode oftentimes being made of doped lanthanum manganite. In the solid oxide fuel cell, hydrogen or a hydrocarbon derived gas is commonly used as the fuel, while oxygen or air is used as the oxidant.  
           [0007]    As mentioned above, the voltage output of a single fuel cell is far too low for many applications. Thus, it frequently becomes necessary to connect multiple fuel cells in series to obtain high voltage power. Additionally, the power demands of many systems require that fuel cells frequently be connected in electrically parallel circuits, thereby providing a greater total current. The physical stacking of multiple fuel cells in series, parallel or series/parallel configuration, however, must incorporate gas-tight connections to allow for a safe and efficient flow of reaction gases. Typically, a group of individual fuel cells are welded, soldered or otherwise bonded together into a single unitary stack, thereby preventing the improper mixing of the reaction gasses, such as in U.S. Pat. No. 5,861,221.  
           [0008]    For any given cell, defects can occur during processing. A cell can also become damaged during handling. Because some defects may have been undetected, their negative affects, such as poor performance and consequent effects on its neighboring cells or even the entire stack, are not realized until the cell is placed in the stack. Where adjacent cells are fused or bonded together into a single unitary stack, a single cell that is defectively formed cannot be removed and interchanged with a non-defective cell. At best, the performance of the fuel cell stack becomes impaired. At worst, the entire stack must be discarded due to the failure of a single cell.  
           [0009]    In addressing the above drawbacks, the assignee of the present invention has developed a unitized fuel cell that is the subject of U.S. patent application Ser. No. 09/419,343 filed Oct. 15, 1999. The unitized cell includes a first electrically conductive interconnect operatively connected to an anode of the fuel cell. The first interconnect has a first substantially planar portion and a first skirt portion. A second electrically conductive interconnect is operatively connected to a cathode of the fuel cell. The second interconnect has a second substantially planar portion and a second skirt portion, with the second skirt portion being juxtaposed to the first skirt portion. A first salient is formed by a portion of at least one of the first and second skirt portions, with the first salient being disposed at a first edge of the fuel cell. A second salient is formed by a portion of at least one of the first and second skirt portions, with the second salient being disposed at a second edge of the fuel cell. An insulating gasket is disposed between the first and second skirt portions and against the ceramic cell to seal the gases within their respective cell housings. The first and second salients can be attached to a gas manifold by attaching a tube to the skirt of the metal housing. Thus, the fuel cell can be electrically connected with other fuel cells in series and parallel configurations through contacts between metal housings and/or through metal gas manifold tubings. A series connection is made when the anode interconnect of one cell is made in contact with the cathode of its adjacent cell whereas a parallel connection can be made if a metal gas tubing is used to electrically connect similar electrodes of two different cells.  
           [0010]    While the use of unitized fuel cells solves many drawbacks in the prior art, design issues relating to the actual stacking and manifolding of fuel cells remain. For example, U.S. Pat. No. 5,298,341 describes prior art as including fuel cell stacks that are arranged in a block configuration. With the stacks positioned adjacent to one another, a manifold is attached to all gas channels of the same orientation. Another prior art design is described as manifolding each stack individually. However, both prior art designs are described as having numerous disadvantages. Thus, U.S. Pat. No. 5,298,341 provides a module having stacks of fuel cells. The fuel cells in each stack are arranged to provide an overall rectangular configuration to the stack. The stacks are oriented on edge and radially spaced apart around a central plenum. The fuel cells in the stacks have gas passageways that extend parallel and perpendicular to the longitudinal axis of the plenum. Circular manifold plates are positioned above and below the module. Each plate has gas flow apertures that coincide with the position of the stacks and a plenum aperture that coincides with the position of the central plenum. In this design, individual stacks may be replaced or repaired but it will be difficult to remove individual cells without affecting the integrity of the neighboring cells.  
           [0011]    In U.S. Pat. No. 4,048,385, manifolding is directed to planar, cylindrical shaped fuel cells. The cells include a central active portion surrounded by a frame portion. The frame portions contain duct openings so that when the cells are in a stack, the combined frame portions provide channels extending parallel to the longitudinal axis of the stack. The channels provide inlet and outlet means for different gases. Hollowed out portions in the frame portions allow the passage of gases between the channels and active portions. End plates are then used to sandwich the above components. In this design, holes around the perimeter of the cell can become weak spots that may cause the cell to fracture when placed under the stress of a stack assembly.  
           [0012]    Another example of manifolding is in U.S. Pat. No. 4,876,163 that discloses tubular shaped fuel cells with their longitudinal axes aligned parallel to one another. Having such parallel orientation, the fuel cells are arranged in either concentric circles, a spiral, or folded rows. Manifolds are located at the distal ends of the cells. The arrangement was intended to reduce the flow of heat from an interior location of the fuel cell stack to a peripheral location. It was also intended to enable series connection. This design, while being applicable to tubular cells, is not applicable to planar cells.  
           [0013]    As can be seen, there is a need for an improved solid oxide fuel cell stack and method of stacking such cells. Another need is for a planar, solid oxide fuel cell stack that provides improved stacking and manifolding. A further need is for a stack design that incorporates unitized fuel cells. Also needed is a fuel cell stack design that minimizes the footprint of the stack. Yet another need is for a fuel cell stack design that allows easy connection and disconnection of gases to the stack.  
         SUMMARY OF THE INVENTION  
         [0014]    In one aspect of the present invention, a unitized solid oxide fuel cell comprises a planar first interconnect that allows a first gas to flow therein; a planar ceramic cell adjacent the first interconnect; a planar second interconnect adjacent the ceramic cell, with the second interconnect allowing a second gas to flow therein; and a plurality of gas tubes in gas communication with the ceramic cell. The gas tubes comprise a first gas inlet affixed to the first interconnect; a second gas inlet affixed to the second interconnect; a first gas outlet in communication with the first gas inlet; and a second gas outlet in communication with the second gas inlet.  
           [0015]    These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    [0016]FIG. 1 is a perspective view of a unitized fuel cell that can be incorporated into a fuel cell stack according to the present invention;  
         [0017]    [0017]FIG. 2 is a perspective view of a partial fuel cell stack according to the present invention;  
         [0018]    [0018]FIG. 3A is a top view of one embodiment of a manifold assembly that can be used in a fuel cell stack according to the present invention;  
         [0019]    [0019]FIG. 3B is a top view of a second embodiment of a manifold assembly that can be used in a fuel cell stack according to the present invention;  
         [0020]    [0020]FIG. 3C is a top view of a third embodiment of a manifold assembly that can be used in a fuel cell stack according to the present invention;  
         [0021]    [0021]FIG. 4A is a perspective view of a unitized fuel cell having a first configuration of gas tubes wherein a co-flow pattern is provided according to an embodiment of the present invention;  
         [0022]    [0022]FIG. 4B is a perspective view of the unitized fuel cell of FIG. 4A wherein a cross flow pattern is provided according to another embodiment of the present invention;  
         [0023]    [0023]FIG. 4C is a diagrammatical view of the flow from the gas tubes shown in FIGS. 4A and 4C;  
         [0024]    [0024]FIG. 5A is a perspective view of a unitized fuel cell with a second configuration of gas tubes according to the present invention;  
         [0025]    [0025]FIG. 5B is a diagrammatical view of the flow from the gas tubes shown in FIG. 5A;  
         [0026]    [0026]FIG. 6A is a perspective view of a unitized fuel cell with a third configuration of gas tubes according to the present invention;  
         [0027]    [0027]FIG. 6B is a diagrammatical view of the flow from the gas tubes shown in FIG. 6A;  
         [0028]    [0028]FIG. 7A is a perspective view of a unitized fuel cell with a fourth configuration of gas tubes according to the present invention; and  
         [0029]    [0029]FIG. 7B is a diagrammatical view of the flow from the gas tubes shown in FIG. 7A. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0030]    While the present invention is described below in the context of solid oxide fuel cells, the present invention may also be used in the context of other types of fuel cells. Further, even though the present invention contemplates that each of the fuel cells  10  in a fuel cell stack  23  be identical, it is also contemplated that the cells  10  can be different. For example, one intermediate cell may serve a special purpose, such as for cooling, containing catalyst, gas conditioning, and others, and thus designed differently from the rest of the cells.  
         [0031]    [0031]FIG. 1 depicts one embodiment of a fuel cell  10  that can be utilized in a fuel cell stack  23  in accordance with the present invention. The cell  10  is characterized as being “unitized.” This is intended to generally mean a self-contained fuel cell that can be replaced from a fuel cell stack without impairing the performance of the overall stack, such as by damaging adjoining cells. In FIG. 1, the unitized fuel cell  10  is shown as having a rectangular and planar configuration. However, shapes other than rectangular are contemplated.  
         [0032]    The fuel cell  10  includes a first planar interconnect  11  that interfaces a second planar interconnect  12 . The first and second interconnects  11 ,  12  sandwich therebetween a first gas distribution structure  14 , a ceramic cell  13 , and a second gas distribution structure  15 . In general, and in the context of a solid oxide fuel cell, the ceramic cell  13  is constructed with an anode layer, a cathode layer, and an electrolyte layer therebetween according to any well-known construction in the art. The first and second gas distribution structures  14 ,  15  can be portions of the first and second interconnects  11 ,  12 , respectively. Alternatively, the first and second gas distribution structures  14 ,  15  can comprise electrodes, such as an anode and cathode, as is also known in the art.  
         [0033]    First gas channels are provided by the first gas distribution structure  14  and second gas channels are provided by the second gas distribution structure  15 . The first and second gas channels enable gases, such as a fuel and an oxidant, to flow therein. In this embodiment, the first and second gas channels are oriented perpendicular to one another to provide a cross flow of gases. Channels may also be oriented parallel to each other to provide co-flow or counterflow of gases. A first salient  16 , a second salient (not shown), a third -salient  17 , and a fourth salient (not shown) are formed between the outer perimeter of the fuel cell  10  and the first and second gas distribution structures  14 ,  15 .  
         [0034]    A pair of opposing salients enables a gas, such as an oxidant, to pass through the fuel cell  10  via an oxidant inlet  18  and an oxidant outlet  19 . Likewise, a gas, such as a fuel, is able to pass through the fuel cell  10  via a fuel inlet  20  and a fuel outlet  21 . Even though the inlets  18 ,  20  and the outlets  19 ,  21  are shown as being generally at the mid-point between the corners of the cell  10 , their positions can be different, such as at the corners. The entire cell  10  is then sealed by a gasket  22  between the first and second interconnects  11 ,  12  and against the edge of the ceramic cell  13 . The gasket  22 , along with the ceramic cell, also electrically insulates the first metal interconnect  11  from the second metal interconnect  12 .  
         [0035]    [0035]FIG. 2 depicts one embodiment of a fuel cell stack  23 . However, for ease of illustration, only a portion of the fuel stack  23  is depicted. The stack  23  includes a plurality of fuel cells  10  that are positioned in respective planes that are generally parallel to one another. In particular, the first and second interconnects  11 ,  12  of each cell  10  preferably lie in their respective plane. The overall configuration of the cells  10  may be generally described as spiral. In such a configuration, the cells  10  are angularly offset to one another about an axis that extends perpendicular to the planes in which the cells  10  lie. The amount of angular offset can vary, depending upon the desired cell  10  density. Thus, a higher cell  10  density will require a smaller offset, while a lower cell  10  density will allow a higher offset.  
         [0036]    The embodiment of FIG. 2 depicts each of the immediately adjacent cells  10  as only partially overlapping one another. In other words, the outer perimeter of one cell  10  does not completely match the angular position of the outer perimeter of an immediately adjacent cell  10 . The partial overlap provides ease of manifolding and thermal distribution as further described below. However, it can be appreciated that as the number of cells  10  increases, the cells  10  may eventually circle around in the spiral such that non-immediately adjacent cells  10  will completely overlap. For example, if it takes twenty cells  10  to complete a circle, the first cell  10  and the  20   th  cell  10  will completely overlap. While the foregoing is a preferred embodiment, it is also contemplated that immediately adjacent cells  10  can completely overlap. Similarly, the spiral of cells  10  does not have to be a complete circle. The repeating cycle may be within part of a circle. For example, cells  10  of a stack  23  may be divided into groups each consisting of a given number of cells  10 . The cells  10  in each group may be spiraled to just within a 90° span and each group is stacked directly over one another.  
         [0037]    The depiction of the fuel cell stack  23  in FIG. 2 is only partial insofar as the cells  10  are shown with only a single gas inlet  18  or  20  and a single gas outlet  19  or  21  for purposes of illustration. It can be seen that a gas tube  25  extends from the gas inlet of each cell  10  and to a manifold assembly  26  disposed below the cells  10 , when viewed from FIG. 2. Similarly, a gas tube  25  extends from the gas outlet of each cell  10  and to the manifold assembly  26 . The gas tube  25  from the gas inlet enables a gas, such as fuel, to flow from the manifold assembly  26  and to the cell  10 .  
         [0038]    [0038]FIG. 2 is also a partial depiction of the fuel cell stack  23  insofar as the manifold assembly  26 . The assembly  26  is shown as having a single inlet manifold  27  and a single outlet manifold  28 . However, the manifold assembly  26  actually includes a pair of inlet manifolds and a pair of outlet manifolds, as further described below in reference to FIGS.  3 A-C.  
         [0039]    In still referring to FIG. 2, it can be seen that for this embodiment, the inlet manifold  27  has circular configuration located below the lowermost fuel cell  10 , when viewed from FIG. 2. The manifold  27  can be a pipe, typically having a round cross section. It may be lying on a plane that is horizontal in position or that is in a tilted manner. The diameter of the inlet manifold  27  is preferably as much as or greater than the longest width of the fuel cells  10 . Thereby, the outer perimeter of the inlet manifold  27  is operatively adjacent the outer perimeters of the fuel cells  10 .  
         [0040]    The outlet manifold  28  is also planar and circular in configuration. It is disposed within and concentric to the inlet manifold  27 . A gas outlet  30  communicates with the outlet manifold  28  to allow the expulsion of a gas, such as a fuel or an oxidant, from at least one of the cells  10  via the tube  25 . Likewise, a gas inlet  29  communicates with the inlet manifold  27  to provide a supply of gas, such as a fuel or an oxidant, to at least one of the cells  10  via the tube  25 . Of course, it is preferred to have each of the cells  10  in gas communication with the inlet and outlet manifolds  27 ,  28 . Stack support  24  provides a stand for the stack  23  to elevate the stack  23  with respect to the manifolds  27 ,  28  and/or the ground.  
         [0041]    [0041]FIG. 3A represents one preferred embodiment of a manifold assembly  31 . In this embodiment, four planar and circular manifolds  32 - 35  are provided concentric to one another. The innermost manifold  32  has a diameter that is as much as or greater than the longest width of the fuel cells. The manifolds  32 - 35  can be either an inlet or outlet manifold to provide a pair of inlet manifolds and a pair of outlet manifolds. The manifolds  32 - 35  are respectively in gas communication with gas inlet/outlets  37 - 40 .  
         [0042]    A plurality of circumferentially spaced apart manifold openings  36  are provided in each of the manifolds  32 - 35  and are operatively adjacent to the outer perimeters of the fuel cells. The openings  36  provide gas communication between the gas tubes  25  and its respective manifold  32 - 35 . Although openings are shown in FIG. 3A as being equally spaced apart for any one manifold  32 - 35 , unequal spacing for one or more manifolds  32 - 35  is contemplated. Likewise, even though the openings  36  are shown as being in the same circumferential position from manifold-to-manifold, different circumferential positions are contemplated.  
         [0043]    [0043]FIG. 3B represents a second preferred embodiment of a manifold assembly  41 . In this embodiment, four planar and semicircular manifolds  42 - 45  are provided in pairs. The pairs of manifolds  42 - 45  are oriented in two concentric circles. As in FIG. 3A, manifold openings  46  are provided in the manifolds  42 - 45  and spaced apart circumferentially. The manifolds  42 - 45  can be either inlet or outlet manifolds and are connected to gas inlet/outlets  47 - 50 .  
         [0044]    [0044]FIG. 3C represents a third preferred embodiment of a manifold assembly  51 . In this embodiment, four planar and circular manifolds  52 - 55  are provided concentric to one another. In contrast to the above embodiments, the manifolds  52 - 55  have different widths in their radial directions. Further, whereas the above embodiments provided single spaced apart manifold openings, the manifold openings  56  in the manifolds  52 - 55  are spaced apart in a plurality of openings  56 . The manifolds  52 - 55  can be either inlet or outlet manifolds and are connected to gas inlet/outlets  57 - 60 .  
         [0045]    While the discussion above describes manifolding methods for bringing main line gases to the individual cells of a stack, the discussion below relates to methods of distributing the gases within the individual cells to the surface of the electrodes.  
         [0046]    [0046]FIG. 4A depicts a fuel cell  10  with a first configuration of gas tubes inside the fuel cell  10  according to an embodiment for distributing the gases within a cell according to the present invention. As in the above unitized cell embodiments, the fuel cell  10  includes a first planar interconnect  11  that interfaces a second planar interconnect  12 . The first interconnect includes sides  11   a ,  11   b ,  11   c , and  11   d , while the second interconnect includes sides  12   a ,  12   b ,  12   c , and  12   d . The first and second interconnects  11 ,  12  contain a first gas distribution structure  14  and a second gas distribution structure  15 , respectively. Together, they sandwich a ceramic cell  13 .  
         [0047]    However, unlike the embodiments above, a fuel inlet gas tube  20  may then be affixed at the side  11   c  such that the inlet tube  20  extends within the first interconnect  11  along the entire length of the side  11   a  for purposes of illustration. Via a plurality of openings or holes  61  in the inlet tube  20  that may extend along its entire length within the interconnect  11 , an inlet gas in the tube  20  is in gas communication with the first gas structure  14  (FIG. 4C). A fuel outlet gas tube  21  may then be affixed at the side  11   d  such that the outlet tube  21  extends within the first interconnect  11  along the entire length of the side  11   b  for purposes of illustration. Via a plurality of openings or holes  61  in the outlet tube  21  that may extend along its entire length with the interconnect  11 , an outlet gas in the tube  21  is in gas communication with the first gas structure  14 .  
         [0048]    Similarly, an oxidant inlet gas tube  18  may be affixed at the side  12   c  such that the inlet tube  18  extends within the second interconnect  12  along the entire length of the side  12   b  for purposes of illustration. Via openings or holes  61  in the inlet tube  18 , an inlet gas in the tube  18  is in gas communication with the second gas structure  15  (FIG. 4C). An oxidant outlet gas tube  19  may then be affixed at the side  12   d  such that the outlet tube  19  extends within the second interconnect  12  along the entire length of the side  12   a  for purposes of illustration. Via openings or holes  61  in the outlet tube  19 , an outlet gas in the tube  19  is in gas communication with the second gas structure  15 .  
         [0049]    As such, the fuel gas tubes  20 ,  21  may be parallel to the oxidant gas tubes  18 ,  19 . The gas tubes are preferably made of metals that fit inside the interconnect skirt and are shorter in height compared to the gas structures  14 ,  15 . The interconnect skirt refers to the vertical wall that surrounds sides  11   a - d  and  12   a - d  of each interconnect. The inside diameters of the gas tubes, depending on the cell size and gas flow requirements, can range between about 0.030 to 0.120 inches. The gas tubes typically are cylindrical with a straight body shape. Like the gas tubes, the inside diameter of the gas tube openings  61  also depend on the gas flow requirements, and typically can range between about 0.005 to 0.090 inches. The openings  61  are in gas communication with the gas outlet  20 . Thereby, a gas (such as a fuel) may enter the inlet tube  21  and flow out of the openings  61  therein. The gas may then flow through the first gas distribution structure  14  and into the openings  61  of the outlet tube  20  for eventual discharge. In a similar fashion, a gas (such as an oxidant) may flow into inlet tube  18 , out of the openings  61 , across the second gas distribution structure  15 , into the openings  61  of the outlet tube  19  for eventual discharge. The shapes of the gas tubes  18 - 21  and gas tube openings  61  are shown as cylindrical in FIGS. 4A and C since it is a common and economical shape to manufacture. However, gas tubes with different cross sections other than circular are contemplated.  
         [0050]    Although the gas tubes are described as separate pieces inserted within the interconnect structures, they can also be formed as an integral part of the interconnects. For example, using sheet metal forming techniques, the sides of interconnects may be folded into tubes and the ends welded to the base for closure. Holes are then bored on the folded tubes.  
         [0051]    In the embodiment of FIG. 4A, the cell  10  provides a co-flow of gases as shown by the arrows. Co-flow of gases is preferred over other flow patterns in some SOFCs for thermal management reasons. However, different types of flow patterns can also be used when preferred. Traditionally, spent fuel gas from the fuel outlet gas tube  21  and spent oxygen gas from the oxidant outlet gas tube  19  are combusted in a separate burner (not shown) to recover residual energy for enhancing system efficiency.  
         [0052]    [0052]FIG. 4B depicts another embodiment of a first gas tube configuration for a manifold assembly of the present invention. This embodiment is the same as that of FIG. 4B, except that the placement of the tubes  18 ,  19  have changed relative to the second interconnect  12 . In this embodiment, the inlet tube  18  extends into the interconnect  12  at side  12   a  and outlet tube  19  extends out of the side  12   b . With such configuration of perpendicular oriented tubes, the gas flow pattern is cross flow as shown by the arrows.  
         [0053]    [0053]FIG. 5A depicts an embodiment of a second gas tube configuration for the present invention. In FIG. 5A, the fuel inlet gas tube  20  may be inserted into and near the middle of the first interconnect  11  at the side  11   c  while the oxidant inlet gas tube  18  may be inserted into and near the middle of the second interconnect  12  at the side  12   d . The gas inlet tubes  18 ,  20  may be arranged in parallel to each other and directly over one another. Further, the tubes  18 ,  20  may extend through the entire lengths of the respective interconnects  12 ,  11 . The tubes  18 ,  20  may each be provided with a plurality of openings  61  that can extend along the entire portions of the tubes  18 ,  20  that are disposed within the interconnects  11 ,  12 , respectively. Thereby, gases can exit along the tubes  18 ,  20  and radiate out from the tubes in a co-flow pattern, as shown by the arrows in FIGS. 5A and 5B. As in the first gas tube configuration, the openings  61 , depending on the gas flow requirements, may be on the order of about 0.005 to 0.090 inches in diameter. As the gases flow through the first and second gas distribution structures  14 ,  15 , the gases may then exit through openings  62  in paired sides of the first and second interconnects  11 ,  12 .  
         [0054]    In the embodiment of FIG. 5A, there are no gas outlet tubes to collect spent gases since the spent gases are allowed to exit the cell  10  at two ends or sides that are open. In this embodiment, the sides or skirts  11   a  and  11   b  in the first interconnect  11  include openings  62  on their surfaces (not shown for side  11   a ). Likewise, the sides or skirts  12   a  and  12   b  in the second interconnect  12  include openings  62  on their surfaces (not shown for side  12   a ). These openings  62  may typically have inside diameters between about 0.005 to 0.090 inches. Consequently, the spent fuel gas exits from the sides of the cell  10  and then gets combusted by the spent oxidant along those sides whereby energy is recovered for enhanced system efficiency, but without the need for a separate burner.  
         [0055]    [0055]FIGS. 6A and 6B depict an embodiment of a third gas tube configuration with stubbed T-shaped gas inlet tubes affixed to and near the middle area of opposing sides of the interconnects  11 ,  12 . The stubbed T-shaped gas inlet tubes can have a cross member portion and a base or inlet portion. The cross member portion is preferably perpendicular to the inlet portion. The cross member portion can be disposed within the first and second interconnects  11 ,  12 , and may be disposed immediately adjacent the respective inlet portions of the first and second gas inlets  20 ,  18 . The stubbed T-shaped gas tubes are preferably made of metals with inside diameters typically between about 0.030 to 0.120 inches. Openings  61  in the cross member portion can typically have inside diameters between about 0.030 to 0.120 inches. The openings  61  deliver inlet gases across the first gas distribution structure  14  and the second gas distribution structure  15  of the first interconnect  11  and the second interconnect  12  respectively.  
         [0056]    Thus, in FIG. 6A, a fuel inlet gas tube  20  is affixed to the side or skirt  11   a  of the first interconnect  11  while an oxidant inlet gas tube  18  is affixed to the same side or skirt  12   a  of the second interconnect  12 . The fuel and oxidant gases may then exit from the cross member portion of the tubes  18 ,  20  disposed at respective sides or skirts  12   a  and  12   b , flow through the first and second gas distribution structures  14 ,  15 , and then exit at the opposite end of the cell  10  via the openings  62  in the sides or skirts  11   b  and  12   b . This configuration can provide a co-flow pattern.  
         [0057]    [0057]FIGS. 7A and 7B depict an embodiment of a fourth gas tube configuration having an extended T-shape affixed to and near the middle area of same sides of the interconnects  11 ,  12 . The extended T-shaped gas tubes can have a cross member portion and a base or inlet portion. The cross member portion is preferably perpendicular to the inlet portion. The cross member portion may be disposed within the first and second interconnects  11 ,  12 , but away from the inlet portions of the first and second gas inlet tube  20 ,  18  portions. The extended T-shaped gas tubes are preferably made of metals with inside diameters typically between about 0.030 to 0.120 inches. Like the stubbed T-shaped gas tubes in FIGS. 6A and 6B, the openings  61  are located on the surface of the cross member portion of the extended T-shape gas tubes. These openings  61  deliver inlet gases across the first gas distribution structure  14  and the second gas distribution structure  15  of the first interconnect  11  and the second interconnect  12 , respectively.  
         [0058]    In this embodiment, the fuel inlet gas tube  20  is affixed to the side or skirt  11   b  of the first interconnect  11 , extends through the first gas distribution structure  14 , and a cross member portion of the tube  20  is disposed adjacent the side or skirt  11   a  wherein such cross member portion includes a plurality of openings  61 . Similarly, the oxidant inlet gas tube  18  is affixed to the side or skirt  12   b , extends through the second gas distribution structure  15 , and a cross member portion of the tube  18  is disposed adjacent the side or skirt  12   a  wherein such cross member portion includes openings  61 . Thus, fuel and oxidant gases are flowed in from one side of the cell  10  and enter the gas distribution structures  14 ,  15  from a side of the cell that is opposite the entry. The gases then exit at the side of the cell  10  from where the gases entered. This provides a co-flow pattern as shown by the arrows in FIGS. 7A and 7B.  
         [0059]    With the foregoing configuration of FIGS. 7A and 7B, a section of each gas tube extends through a combustion zone or hot section that is outside of the cell  10  along the sides  11   b  and  12   b  before being distributed to the cell  10 . A heat exchange process takes place whereby the cold inlet gases in the tubes  18  and  20  pick up heat from the combusting gases oustside of the cell  10 . After the gas tubes enter the cell, they run through the entire width of an active area  10   a  of the cell. Electrochemical reactions taking place in the active area  10   a  also generate heat that further heats up the gases in the gas tubes. In other words, the length and path of the tubes allow the gases inside to pick up enough heat so that when the gases come out of openings  61 , their temperature is already close to the cell operating temperature (i.e., temperature at  10   a ). Without this heat exchange process, the temperature difference across the cell width, that is, between sides  11   a  and  12   a  and sides  11   b  and  12   b  will be significantly higher and create high thermal stresses between the two sides and within the cell that can damage the cell materials.  
         [0060]    In view of the above, it can be seen that the present invention also provides a method of making a fuel cell stack  23 . The method includes juxtaposing a plurality of planar fuel cells  10  to one another. Thereby, one interconnect of one fuel cell  10  oppositely faces another interconnect of an adjacent fuel cell. Further, the pair of interconnects in any one fuel cell  10  are positioned in a respective plane and the planes of all of the fuel cells  10  are substantially parallel to one another. The method further includes orienting the fuel cells  10  in a spiral configuration and interfacing a manifold assembly  26  to the fuel cells  10 . Thereafter, the fuel cells  10  are placed in gas communication with the manifold assembly  26 .  
         [0061]    As can be appreciated by those skilled in the art, the present invention provides an improved solid oxide fuel cell stack and method of stacking such cells. Furthermore, the invention provides an improved gas communication path between the fuel cell stack  23  and manifold assembly  26 . The fuel cell stack  23  design of the present invention incorporates unitized fuel cells  10  and minimizes the footprint of the stack  23 . This is accomplished by having the manifold assembly  26  set directly below the stack  23 . Another aspect of the present invention is a fuel cell stack  23  design that allows easy connection and disconnection of gases to the stack  23 . By the use of convenient fittings, each of the gas tubes  24 ,  25  can be easily connected or disconnected to the manifold assembly  26  which allows any particular fuel cell  10  to be removed or replaced with little disturbance to adjacent cells  10 .  
         [0062]    The present invention further minimizes the thermal gradient that otherwise exists in a stack of cells. During power generation, a cell will be colder in the gas inlets  18 ,  20  because of the colder gases, and hotter in the gas outlets  19 ,  21  because of the heat generated from the reaction throughout the cell  10 . If the cells  10  are stacked in a conventional way, one completely overlaying on the other, the outlet corners will be heated to much higher temperature due to combined generated heat from multiple fuel cells  10 , that is usually a hundred of degrees Celcius or higher than the gas inlet corners. This can create tremendous thermal stress and material property gradient across the fuel cell stack  23 . By setting the cells  10  into spiral configuration, the outlets are positioned offset from one to another and is capable of dispersing and distributing heat.  
         [0063]    In a further aspect of the present invention, perforated gas tubes within the unitized fuel cells  10  arranged in different manners and orientation, provide a wide variety of gas distribution patterns on the ceramic cell surfaces  13 .  
         [0064]    It should be understood, of course, that the foregoing relates to preferred embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.