Abstract:
A solid oxide fuel cell system comprising a plurality of electrochemically active fuel cell cassettes connected in electrical series and bonded together by a plurality of glass seals to form a fuel cell stack. A dummy cassette containing a thermocouple is disposed within the fuel cell stack. Each cassette may have at least one alignment tab for receiving a rod to maintain stack alignment during sintering, and each fuel cell cassette has electrical terminals extending from a side of the stack for performance testing. The distribution manifold is attached to stack, and a spring subassembly is disposed against the stack and is attached to the manifold by tie rods to maintain a compressive load on the stack through sintering and subsequent use to prevent unloading and rupture of the glass seals.

Description:
[0001]    The present invention was supported in part by a US Government Contract, No. DE-FC26-02NT41246. The United States Government may have rights in the present invention. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to solid oxide fuel cells; more particularly, to solid oxide fuel cell stacks having features for improving assembly, electrical certification of individual fuel cells, and long-term mechanical integrity; and most particularly, to an improved method and structure for forming a solid oxide fuel cell stack assembly having features for maintaining alignment of cassettes during sintering, for measuring temperatures at various locations within the stack, for measuring voltage performance of each cassette, and for maintaining compressive load on the stack after sintering and subsequent use of the fuel cell stack assembly. 
       BACKGROUND OF THE INVENTION 
       [0003]    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 to combine with hydrogen atoms to produce electricity and water; such fuel cells are referred to in the art as “solid oxide” fuel cells (SOFCs). 
         [0004]    In some applications, for example, as an auxiliary power unit (APU) for an automotive vehicle, an SOFC is preferably fueled by “reformate” gas, which is the effluent from a catalytic hydrocarbon oxidizing reformer. Reformate typically includes amounts of carbon monoxide (CO) as fuel in addition to molecular hydrogen. The reforming operation and the fuel cell operation may be considered as first and second oxidative steps, respectively, of the liquid hydrocarbon, resulting ultimately in water and carbon dioxide. Both reactions are exothermic, and both are preferably carried out at relatively high temperatures, for example, in the range of 700° C. to 1000° C. 
         [0005]    A prior art fuel cell stack assembly includes a plurality of individual fuel cell units known in the art as cassettes or repeating units. Typically, each cassette includes an interconnect which electrically connects the individual fuel cell to the next cassette in the stack to form one half of the fuel cell electric circuit. 
         [0006]    In addition, a typical prior art SOFC stack may be based upon a manifold that provides gas-tight attachment to the SOFC stack and distributes the fuel gas and combustion air streams to and from the stack&#39;s internal manifolds. 
         [0007]    In the prior art, as disclosed for example in U.S. Pat. No. 7,001,685, issued Feb. 21, 2006, a fuel cell stack is formed as a standalone unit in a load frame having self-contained spring means for maintaining a compressive load on the stack at all times as required by the sintered glass gas seals between the cassettes. The load frame subassembly is then mounted onto a manifold. A shortcoming of this system is that a bottom plate is required for the load frame, adding to the complexity and cost. Further, the finished height of the stack, and hence the compressive load within, is governed by spacer supports of fixed length within which a leaf spring arrangement is operative. Such supports also add to complexity and cost, and the compressive load will vary by thermal expansion according to the temperature of the stack. 
         [0008]    In the prior art, it is also known to provide at least one dummy cassette in a fuel cell stack, as disclosed in Published US Patent Application US 2009/0004532 (“the &#39;532 application”), published Jan. 1, 2009. It is disclosed to include an inoperative dummy cassette in place of a standard cassette at one or both ends of the stack. It has been observed in SOFC stacks formed of planar cassettes that the endmost cells in the stack perform substantially differently from those in the remainder of the stack. Specifically, the end cells typically exhibit 20-40% lower voltage output than do the rest of the cells. Such lower performing cells may limit the operation of the overall stack. For example, it is undesirable to operate a cell below about 0.5 volts for risk of damaging the cell. If the top and bottom cassettes of a stack are operating at 0.5 volts at a current level at which the rest of the cassettes are operating at 0.8 volts, the stack average voltage is well above the desirable average of 0.7 volts. No more current load may be imposed on the stack, which would be desirable to bring the stack average voltage to 0.7, without causing the top and bottom cassettes to operate at less than 0.5 volts. Failure of the top or bottom cassette due to its operating voltage being less than 0.5 volts can lead to overall stack failure. In the &#39;532 application, the dummy cassette is simply an inert spacer, and no broader use is contemplated in the cited reference. 
         [0009]    It is known in the art that maintaining alignment of fuel cell cassettes in a stack is an important requirement. It is further known that when sintering a stack of cassettes having glass seals the cassettes are prone to slipping out of alignment. 
         [0010]    Because a single defective fuel cell in a stack can lead to electrical failure of the entire stack, it is desirable to be able to test each fuel cell at will, both during manufacture of the stack and at any time during the working life of the stack. 
         [0011]    What is needed in the art is an SOFC assembly wherein the long-term compressive state is reliably and inexpensively maintained; wherein the thermal condition at various points within the fuel cell stack may be monitored at will; wherein the fuel cells are reliably and inexpensively aligned during manufacture; and wherein the electrical performance of each fuel cell in the stack may be tested at will. 
         [0012]    It is a principal object of the present invention to improve the manufacturability and working lifetime of a solid oxide fuel cell system. 
       SUMMARY OF THE INVENTION 
       [0013]    Briefly described, a solid oxide fuel cell system in accordance with the present invention comprises a plurality of fuel cell cassettes connected in electrical series and bonded together by a plurality of glass seals to form a gas-tight fuel cell stack. At least one dummy cassette containing at least one sensor, such as a thermocouple, is disposed within the fuel cell stack. Each fuel cell and dummy cassette has at least one location tab for receiving an alignment rod, which may be temporary, to maintain stack alignment during sintering in manufacture, and each fuel cell cassette has electrical terminals extending from a side of said stack for performance testing. The distribution manifold is attached to the cassette at a first end of the stack, and a spring subassembly is disposed against the stack at an opposite end thereof from the distribution manifold and is attached to the manifold by tie rods to maintain a compressive load on the stack after sintering and subsequent use to prevent unloading and rupture of the glass seals. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
           [0015]      FIG. 1  is an isometric view of an SOFC fuel cell system in accordance with the present invention; 
           [0016]      FIG. 2  is a plan view of a typical fuel cell cassette for use in the system shown in  FIG. 1 , showing voltage measurement terminals and sintering alignment tabs; and 
           [0017]      FIG. 3  is a plan view of a first plate in a dummy cassette in accordance with the present invention; 
           [0018]      FIG. 4  is an exploded isometric view from above of the first plate shown in  FIG. 3  and a second plate for a dummy cassette; and 
           [0019]      FIG. 5  is an exploded isometric view from below of the first plate shown in  FIG. 3  and a third plate for a dummy cassette. 
       
    
    
       [0020]    Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate currently preferred embodiments of the invention, and such exemplifications are not to be construed as limiting the scope of the invention in any manner. 
       DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0021]    Referring to  FIGS. 1 through 5 , a solid oxide fuel cell system  10  in accordance with the present invention comprises a stack  12  of fuel cell cassettes  14 , each having electrodes and an electrolyte layer as know in the art (also referred to herein as “electrochemically active cell repeating units”), and including at least one electrochemically inactive unit (hereinafter referred to as a “dummy cassette”)  26  positioned as described below, stack  12  having a longitudinal axis  13 ; a distributor manifold  16  supportive of stack  12 ; and a spring subassembly  18  including tie rods  20  for attaching spring subassembly  18  to distribution manifold  16 , thereby binding system  10  together. Assembly alignment tabs  22  and cassette electrical terminals  24  are also present as described below but are not visible in  FIG. 1 . 
         [0022]    In an operating SOFC stack, measuring the gas stream temperatures can be inaccurate and misleading, since there is averaging of gas stream temperature and heat loss from a cell to an adjacent thermocouple. Inserting a thermocouple into an electrochemically active cell unit may affect stack performance, may electrically short adjacent cell repeating units, and may lead to gas leaks. However, knowing the actual cell temperature and temperature distribution at one or more levels of a stack improves understanding of stack operation and can improve stack control. 
         [0023]    Stack  12  incorporates one or more dummy cassettes  26  (combination of  26   a ,  FIGS. 4 , and  26   b ,  FIG. 5 ) that contains one or more sensors  28  ( FIG. 3 ), preferably thermocouples, in a plane transverse of longitudinal axis  13  that is hermetically isolated from the stack gas streams. Dummy cassette  26  passes electrical current from one adjacent electrochemically active cell repeating unit  14  to the other electrochemically active cell repeating unit  14  with minimal voltage loss. The thermocoupled dummy cassette  26  is mostly solid, thin, and metallic so that it has minimal influence on the temperature distribution of adjacent cell repeating units. Since the electrically and thermally conductive faces of the thermocoupled dummy cassette are in very close proximity to the operating cells, the temperatures and temperature distribution across the plane of the thermocoupled dummy cassette are almost exactly the same as in the adjacent active cells. Thus, providing a plurality of sensors  28 , such as thermocouples, with their measuring tips at different locations in the plane of dummy cassette  26  affords insight into temperature distributions within the stack during stack design development; and ultimately, one or more thermocouples can be used in a production stack for stack operation feedback for system control purposes. 
         [0024]    In one aspect of the invention, thermocoupled dummy cassette  26  (TDC) consists of two or three principal components. 
         [0025]    A first or carrier plate  30  is thicker than the sheathed thermocouple  28  and has channels  32  formed into it that locate the thermocouples. One end of each channel is located at the desired measurement point, the other end opens to an edge  34  of plate  30 . The channels follow a smooth path from the edge to the measuring point so that the thermocouples can be easily inserted after the TDC plates are joined into an assembly, or replaced if necessary. Sensor leads  29  extend from the ends of channels  32 . First plate  30  is substantially solid sheet metal, with the exception of the narrow thermocouple channels, for good electrical and thermal conductivity. A second or upper plate  36  provides an electrical contacting surface and sealing surface to one adjacent active repeating unit above the TDC. In cases where channels  32  are formed through carrier plate  30 , a third or lower plate  38  may be used to provide an electrical contacting surface and sealing surface to the other adjacent active repeating unit below the TDC. All three plates  30 , 36 , 38  have through-holes  40  that correspond to the gas supply and return chimneys in the active repeating units of stack  12 . The two or three plates are metallurgically bonded together (brazed or welded) so that the holes for the gas supply and return chimneys are hermetically sealed; therefore, the gas streams pass through the TDC without leaking into or out of the TDC. The plates are also joined in such a way as to provide a highly electrically conductive path through the TDC. Since the TDC is relatively thin in the vertical or axial (Z) direction, it is a very good thermal conductor in the Z direction, but relatively poor in the X-Y plane; this enables the TDC to accurately reflect the temperature and temperature distribution in the X-Y plane of the adjacent active repeating units. 
         [0026]    Stack  12  is assembled on a distribution manifold  16  preferably fabricated by casting from stainless steel (to match the CTE of the stack components) and finish-machined to final dimensions. A dummy cassette  26 , is assembled adjacent an electrochemically active cell repeating unit  14  with a glass seal interposed therebetween. The glass seal is the same type as is used between functional electrochemically active cell repeating units  14  in the SOFC stack; the seal contains the fuel gas and air streams between repeating units and provides an adhesive, electrically insulating, mechanical bond between repeating units. The glass seal provides the same gas-tight bond joint between manifold  16  and the first component of the stack which effectively bonds the completed stack to the manifold. Manifold  16  then serves as the build platform for stack  12 , a supporting carrier for stack  12  after assembly, and as a mounting interface between stack  12  and the SOFC system hardware. Manifold  16  also provides for a simple gas-tight attachment to the SOFC system plumbing and distributes the fuel gas and air streams to the stack internal manifolds. 
         [0027]    During the stack assembly process, the glass seals are added in a green state: unsintered glass particles in an organic binder carrier material. The glass seals, along with the stack assembly, are then subjected to a high temperature sintering process to achieve their final dimension and gas-tight bonding properties. During this process, the seals shrink substantially in thickness and become somewhat liquidous as the organic carrier is destroyed. Therefore the stack assembly shrinks substantially in height, and all the repeating units must be restrained to prevent them from “floating” laterally out of their intended positions. To achieve good locational control of the stack assembly components (active repeating units, dummy cassettes, current collectors, etc.), at least one, and preferably two, assembly alignment tab  22  is added to the exterior perimeter of the components; for example, a first tab with a hole and a second tab with a slot at opposite locations to assure correct orientation of each cassette. Ceramic rods (not shown) are inserted through these features and into a locating hole in a reference element such as the base manifold to provide guiding and locating during the sintering process. The rods may be removed after sintering if desired. 
         [0028]    After sintering, stack  12  is tested at elevated temperature to verify proper function of the stack. In order to do so, the voltage of each repeating unit must be measured at open circuit and with electrical load. To accomplish this, each repeating unit  14  is provided with at least one, and preferably two, voltage terminal  24  formed from the metal structure of that unit. For testing, a mating terminal (not shown) is preassembled with a mechanical joint or metallurgical bond to the voltage sensing test equipment, preferably computerized, using high volume wiring harness assembly techniques. The mating terminal is also mechanically joined or metallurgically bonded to voltage terminal  24 . A low resistance (particularly at the high operating temperature of the stack) joint between the sensing wire and repeating unit is required for accurate voltage measurement (to 0.01 volt). After stack assembly and test, some or all of the voltage leads may be left in place for stack performance monitoring in the SOFC system during usage thereof. 
         [0029]    After sintering, the glass seals provide sturdy bonded joints between the components of the completed stack assembly. However, when stack  12  is cooled to room temperature from its operating temperature of 700° C. to 800° C., residual temperature gradient-induced stresses within the components may cause tensile stresses within the glass joints that exceed the tensile strength of the joint. Since the glass seal joints are much stronger in compression than in tension, it is desirable to maintain a compressive load on the SOFC stack (and thus on the seal joints) at all times through the remainder of its life. In the prior art, this was accomplished with an end plate held in place with bolts and torqued to provide a clamping load. However, due to the large difference in coefficient of thermal expansion (CTE) between the SOFC stack operating temperature and room temperature, even a relatively small difference in CTE between the bolts and the stack could result in either an excessively high clamp load or no clamp load at all. To overcome this problem, the present stack assembly is provided with a low profile spring subassembly  18  that provides a continuous compressive load even at SOFC operating temperatures. The present arrangement is simplified considerably over a prior art arrangement disclosed in U.S. Pat. No. 7,001,685 and described above. The present arrangement is bolted directly to the distribution manifold rather than to a base plate and comprises first and second leaf springs  42 , 44  fabricated from metal alloys with high temperature creep resistance that are assembled one on top of the other. Two springs are used to achieve the desired spring rate while keeping the spring stresses below the creep limit. Depending on load and spring rate requirements, one spring may be sufficient or more than two may be required. In any case, the uppermost spring  44  is larger than the footprint of stack  12 , and the end is formed, or a stiffener added, to prevent bending perpendicular to the desired bending direction. Tie rods  20  which pass through upper spring  44  and are is anchored to distribution manifold  16  and are tensioned to pull the ends of spring  44  to a desired deflection to thereby load the spring assembly. Tie rods  20  may be screws, threaded rod, or headed fixed length rods fabricated from a high temperature metal alloy. Lower spring  42  applies load to a stiff load plate  46  fabricated from a high temperature metal alloy or low cost ceramic (such as alumina or ZTA) to distribute the spring load uniformly over the stack footprint area. 
         [0030]    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.