Abstract:
A fuel cell stack is disclosed including a non-fuel cell cassette having temperature sensing elements disposed therein. The temperature sensing elements are disposed in one or more void spaces in the non-fuel cell cassette, which void spaces are connected to openings in the side of the non-fuel cell cassette for lead wires to communicate information from the temperature sensing elements to components outside of the fuel cell stack.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This patent application is a continuation-in-part of U.S. patent application Ser. No. 11/823,618, filed Jun. 28, 2007, which is incorporated herein by reference in its entirety. 
     
    
     RELATIONSHIP TO GOVERNMENT CONTRACTS 
       [0002]    This invention was made with Government support under DE-FC26-02NT41246 awarded by DOE. The Government has certain rights in this invention. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    In practical fuel cell systems, the output of a single fuel cell is typically less than one volt, so connecting multiple cells in series is required to achieve useful operating voltages. Typically, a plurality of fuel cell stages, each stage comprising a single fuel cell unit, are mechanically stacked up in a “stack” and are electrically connected in series electric flow from the anode of one cell to the cathode of an adjacent cell via intermediate stack elements known in the art as interconnects and separator plates. 
         [0004]    A solid oxide fuel cell (SOFC) comprises a cathode layer, an electrolyte layer formed of a solid oxide bonded to the cathode layer, and an anode layer bonded to the electrolyte layer on a side opposite from the cathode layer. In use of the cell, air is passed over the surface of the cathode layer, and oxygen from the air migrates through the electrolyte layer and reacts in the anode with hydrogen being passed over the anode surface, forming water and thereby creating an electrical potential between the anode and the cathode of about 1 volt. Typically, each individual fuel cell is mounted, for handling, protection, and assembly into a stack, within a metal frame referred to in the art as a “picture frame”, to form a “cell-picture frame assembly”. 
         [0005]    To facilitate formation of a prior art stack of fuel cell stages wherein the voltage formed is a function of the number of fuel cells in the stack, connected in series, a known intermediate process for forming an individual fuel cell stage joins together a cell-picture frame assembly with an anode interconnect and a metal separator plate to form an intermediate structure known in the art as a fuel cell cassette (“cassette”). The thin sheet metal separator plate is stamped and formed to provide, when joined to the mating cell frame and anode spacers, a flow space for the anode gas. Typically, the separator plate is formed of ferritic stainless steel for low cost. In forming the stack, the cell-picture frame assembly of each cassette is sealed to the perimeter of the metal separator plate of the adjacent cassette to form a cathode air flow space and to seal the feed and exhaust passages for air and hydrogen against cross-leaking or leaking to the outside of the stack. 
         [0006]    In order to monitor operating conditions and/or control operating parameters of a fuel cell stack, it is desirable to be able to measure internal temperatures at one or more locations in the stack. However, placement of temperature measuring sensor such as thermocouples into the stack can cause a number of problems, such as disrupting gas flow through the gas flow spaces, electrically shorting adjacent cell repeating units, and/or providing a potential path for gas leaks. 
         [0007]    Accordingly, it would be desirable to provide a way to effectively measure internal stack temperature at one or more locations in the fuel cell stack while mitigating the above-identified problems. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention provides a fuel cell stack comprising repeating fuel cell cassette units, and further comprising a cassette unit that does not include a fuel cell. This non-fuel cell cassette unit comprises:
       (a) a planar electrically and thermally conductive housing having first and second opposing horizontal planar surfaces in electrical contact with adjacent fuel cell cassette units in the stack assembly;   (b) at least one opening disposed on a vertical side surface disposed between said first and second opposing horizontal planar surfaces, this opening leading to a void space within the housing;   (c) a temperature sensor disposed in the void space; and   (d) a lead wire disposed connected to the temperature sensor in the void space and extending through the opening and away from the fuel cell stack assembly for connection to a temperature monitoring device.       
 
         [0013]    The non-fuel cell cassette described herein provides accurate measurement of internal temperatures in a fuel cell stack without exposing the temperature sensors to fuel and/or tail gas in the stack and with reduced potential for leaks along temperature sensor lead pathways, while still providing electrical continuity between fuel cells in adjacent cassettes in the fuel cell stack. These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0015]      FIG. 1  is a schematic drawing of an SOFC mounted in a frame; 
           [0016]      FIG. 2  is an exploded isometric drawing of a portion of a fuel cell stack employing a plurality of single-cell cassettes; 
           [0017]      FIG. 3  is a plan view of a functional fuel cell cassette; 
           [0018]      FIG. 4  is a plan view of a non-functional fuel cell cassette in accordance with the present invention; 
           [0019]      FIG. 5  is an exploded isometric view of the non-functional cassette shown in  FIG. 4 ; and 
           [0020]      FIG. 6  is an exploded isometric view of a complete fuel cell stack employing a non-functional cassette. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    Referring now to the Figures, the invention will be described with reference to specific embodiments, without limiting same. Where practical, reference numbers for like components are commonly used among multiple figures. 
         [0022]    Referring to  FIGS. 1 through 3 , an exemplary functional SOFC fuel cell module  10  comprises an electrode  11  including cathode layer  12 , an electrolyte layer  14  formed of a solid oxide and bonded to the cathode layer  12 , and an anode layer  16  bonded to the electrolyte layer  14  on a side opposite from the cathode layer. Air  18  is passed over the surface  34  of the cathode layer  12 , and oxygen from the air migrates through the electrolyte layer  14  and reacts in the anode layer  16  with hydrogen anode gas  20  being passed over the anode surface  31  to form water, thereby creating an electrical potential between the anode and the cathode of about 1 volt. Each individual fuel cell module  10  is mounted, for handling; protection, and assembly into a stack, within a metal frame  22  referred to in the art as a “picture frame”, the frame having a central opening or “window”  23 , to form a “cell-picture frame assembly”  24 . 
         [0023]    To facilitate formation of a stack  26  of individual fuel cells connected in series wherein the voltage formed is a function of the number of individual fuel cell modules in the stack, an intermediate process joins together each cell-picture frame assembly  24  with a separator plate  28  and a first solid (anode) interconnect  30  to form an intermediate structure known as a fuel cell cassette  32 . The thin sheet metal separator plate  28  is stamped and formed to provide, when joined to the mating cell frame  22  and inlet and outlet anode spacers  29   a ,  29   b , a flow space for the anode gas  20 . Preferably, the separator plate  28  is formed of ferritic stainless steel for low cost. Anode interconnect  30  is placed between the separator plate  28  and the anode surface  31  of the cell within the cassette  32 . The anode interconnect  30  is typically a woven wire mesh of uniform thickness and is solid in the direction perpendicular to the cell surface in a multitude of points. 
         [0024]    A second solid (cathode) interconnect  35 , installed during final assembly against cathode surface  34 , provides a cathode air flow space. Interconnect  35  also is typically a woven wire mesh of uniform thickness and solid in the direction perpendicular to the cell surface in a multitude of points. During the final prior art stack assembly process, a glass perimeter seal  42  is disposed between adjacent of the cassettes  32 , and the stack under pressure is brought to operating temperature and allowed to settle to its final form. The separator plate and cell frame may deform slightly, providing a compliant assembly, until the cells and interconnects are resting on one another, under load, which prevents further motion. 
         [0025]    Referring now to  FIG. 4 , a non-fuel cell cassette  132  in accordance with an exemplary embodiment of the invention comprises a substantially planar housing  122  having a first or lower surface (not seen in this view) and an upper or second surface  123 . Openings  135  are disposed in a side surface disposed between the upper surface  123  and the lower surfaces. Channels or void spaces  137  connected to the openings  135  are disposed in the interior of the otherwise solid housing  122 . The openings  135  connected to channels  137  may be formed by any known technique, such as by drilling or laser-cutting holes in the side member of the housing  22 . Temperature sensors (not shown) are disposed inside of channels  137  with wire leads (not shown) running through the openings  135 , from where they connect to conventional temperature sensing components such as a sending unit connected to an electronic control unit. Grommets or sealant may be used at the openings  135  to seal and retain the wire leads in place. In an exemplary embodiment where the non-fuel cell cassette is used as a cassette in a fuel cell stack as depicted in  FIG. 2 , the non-fuel cell cassette  132  will include air supply openings  140  for transmitting supply air between adjacent cassettes in the stack, fuel supply openings  142  for transmitting fuel between adjacent cassettes in the stack, exhaust air openings  150  for transmitting spent air collected from adjacent cassettes in the stack, and tail gas openings  152  for transmitting exhaust air collected from adjacent cassettes in the stack. In one exemplary embodiment, the channels  137  are connected in fluid communication with the exhaust air openings  150  (e.g., with a pilot hole or channel) in order to prevent heat-induced pressure buildup that could occur in the channels if they are completely sealed off and enclosed. 
         [0026]    The housing  122  may be formed from any material such as steel that has suitable electrical conductivity so as to conduct electricity from adjacent fuel cells in cassettes on either side of the non-fuel cell cassette  132 , and has suitable thermal conductivity so as to provide an accurate temperature reading from the temperature sensors in the channels  137 . 
         [0027]    The embodiment shown in  FIG. 4  may be formed from a solid piece of metal, but other embodiments may also be utilized with different fabrication techniques. For example,  FIG. 5  depicts a two-piece exemplary embodiment of a non-fuel cell cassette  132  formed from lower housing plate  122 ′ and upper housing plate  122 ″. In this embodiment, channels  137  having a depth less than the thickness of lower housing plate  122 ′ are cut into the upper surface of lower housing plate  122 ′ using suitable tools such as a router or a laser or by chemical etching. Temperature sensors are disposed in the channels  137  with wire leads extending out of the channels  137  on the side of lower housing plate  122 ′. Any sort of known temperature sensor, such as a thermocouple, may be used. Spots of contact paste or brazing  136  are applied to the upper surface of the lower housing plate  122 ′, and the upper housing plate  122 ″ is adhered to the lower housing plate  122 ′ and welded or brazed along the periphery to form a tight seal. The channels  137  can be disposed in fluid communication with the air exhaust openings  150  by leaving spaces between the spots of contact paste  136  to create a fluid flow path between the upper housing plate  122 ″ and the lower housing plate  122 ′, running between channels  137  and openings  150 , and configuring the openings  140 ′,  140 ″,  142 ′,  142 ″,  152 ′, and  152 ″ to sealingly connect with one another while configuring the openings  150 ′ and  150 ″ to allow for fluid flow along the fluid flow path between the housing plates  122 ′,  122 ″. 
         [0028]    In another exemplary embodiment,  FIG. 6  depicts a three-piece embodiment of a non-fuel cell cassette  132  formed from middle housing plate  122 ′, upper housing plate  122 ″, and lower housing plate  122 ′″. In this embodiment, channels  137  are cut through the entire thickness of middle housing plate  122 ′ using suitable tools such as a router or a laser or by chemical etching. Either upper housing plate  122 ″ or lower housing plate  122 ′″ are adhered to the middle housing plate  122 ′, and temperature sensors are disposed in the channels  137  with wire leads extending out of the channels  137  on the side of middle housing plate  122 ′. Spots of contact paste or brazing  136  are applied to the other surface of the lower housing plate  122 ′, and the other of the upper housing plate  122 ″ or lower housing plate  122 ′″ is adhered to the middle housing plate  122 ′ and welded or brazed along the periphery to form a tight seal. The channels  137  can be disposed in fluid communication with the air exhaust openings  150  by leaving spaces between the spots of contact paste  136  to create a fluid flow path between either of the upper or lower housing plates  122 ″,  122 ′″ and the lower housing plate  122 ′, running between channels  137  and openings  150 , and configuring the openings  140 ′,  140 ″,  140 ′″,  142 ′,  142 ″,  142 ′″,  152 ′,  152 ″, and  152 ′″ to sealingly connect with one another while configuring the openings  150 ′,  150 ″, and  150 ′″ to allow for fluid flow along the fluid flow path between the middle housing plates  122 ′ and the upper or lower housing plates  122 ″,  122 ′″. 
         [0029]    In yet another exemplary embodiment,  FIG. 7  depicts a two-piece clamshell configuration of a non-fuel cell cassette  132  with lower clamshell component  122 ′ and upper clamshell component  122 ″. Internal stand-offs  43  provide bracing support to maintain the structural integrity of the clamshell structure. Upper clamshell component  122 ″ can be a flat plate structure as shown in  FIG. 7  or it can be a three-dimensional structure having depth and stand-offs like the lower clamshell component  122 ′. No channels are necessary, as the clamshell configuration provides ample internal void space for the disposition of temperature sensors  148 . Wire leads  147  run from the temperature sensors  148  through openings  135 . The clamshell components  122 ′ and/or  122 ″ will include air supply channels  140 ′ for transmitting supply air between adjacent cassettes in the stack, fuel supply channels  142 ′ for transmitting fuel between adjacent cassettes in the stack, exhaust air channels  150 ′ for transmitting spent air collected from adjacent cassettes in the stack, and tail gas channels  152 ′ for transmitting exhaust air collected from adjacent cassettes in the stack. Fluid communication for venting can be provided between the void space inside the clamshell structure and the exhaust air by the inclusion of a small hole or slit  155  in the side of the exhaust air channel  150 ′. 
         [0030]    Any number of non-functional cassettes as described herein may be used in a stack at any of a number of locations in the stack, essentially any place where it is desired to measure temperature in the stack. Additionally, the non-functional cassettes may have any number of temperature sensors located at any of a number of locations along the plane of the non-functional cassette. Of course, the temperature sensor location profile of various non-functional cassettes in the fuel cell stack may be different from one another, depending on the temperature profile information that is desired at the particular level in the stack where the particular non-functional cassette is located. 
         [0031]    While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description.