Abstract:
A fuel cell assembly having means for providing tempered air to, and removing spent air from, air-flow passages across the cathode. The air flow path includes means for reversing the direction of flow across the cathode periodically to reverse the roles of the leading and trailing edges of the cathode to prevent temperature differences across the cathode from exceeding 200° C., and thus to prevent damage to the cathode from thermally-induced stresses during startup heating and steady-state cooling.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates to hydrogen/oxygen fuel cells; more particularly, to stacks comprising a plurality of individual cells connected by interconnect elements; and most particularly, to a fuel cell assembly wherein internal temperature, especially temperature of the cell itself, is modulated by periodic reversal of the direction of air flow across the cathode.  
       BACKGROUND OF THE INVENTION  
       [0002]     Fuel cells which generate electric current by controllably combining elemental hydrogen and oxygen are well known. In one form of such a fuel cell, an anodic layer and a cathodic layer are deposited on opposite surfaces of a permeable electrolyte formed of a ceramic solid oxide. Such a fuel cell is known in the art as a “solid oxide fuel cell” (SOFC). Hydrogen, either pure or reformed from hydrocarbons, is flowed along the outer surface of the anode and diffuses into the anode. Oxygen, typically from air, is flowed along the outer surface of the cathode and diffuses into the cathode where it is ionized. The oxygen ions diffuse through the electrolyte and combine with hydrogen ions to form water. The cathode and the anode are connected externally through the load to complete the circuit whereby electrons are transferred from the anode to the cathode. When hydrogen is derived from “reformed” hydrocarbons, the reformate gas includes CO which is converted to CO 2  at the anode. Reformed gasoline is a commonly used fuel in automotive fuel cell applications.  
         [0003]     An SOFC operates at a temperature, typically, of about 750° C. or higher. The reaction is exothermic, so the SOFC requires active cooling during operation, typically by flowing cooler air across the cathode. Conversely, at startup from ambient temperatures, the SOFC requires heating for the catalytic electrolyte to begin ionizing oxygen, typically by flowing heated air across the cathode.  
         [0004]     A serious problem arises in thermal management within an SOFC. Because the cathode is highly vulnerable to cracking and consequent failure from thermal stresses, temperature differences greater than about 200° C. are unacceptable. Air flows through a fuel cell from introduction at an upstream edge of the cathode to discharge across a downstream edge, undergoing temperature change during such flow. Thus, the cathode experiences an inherent temperature difference between the upstream and downstream edges, and between itself and the temperature-modulating air. Since the permissible temperature difference (ΔT) between the temperature of the heating air and the internal temperature of the SOFC is limited, long warmup times on the order of several hours typically are required, whereas for automotive uses, startup times of about ten minutes or less are highly desirable.  
         [0005]     Similarly, large volumes of cooling air are required during operation because the permissible ΔT for cooling is limited. Providing such large volumes is parasitically consumptive of power being generated by the fuel cell, thereby reducing the net power output thereof, since it requires a relatively large blower having a relatively large electric motor.  
         [0006]     What is needed is a means for providing a higher difference between the average temperature of cathode entry air and the average temperature of cathode exit air for heating and cooling a fuel cell cathode to shorten the startup time and to reduce the volume of cooling air required.  
         [0007]     It is a principal object of the present invention to provide an improved thermal management method and apparatus for an SOFC wherein startup may be achieved in a short period of time.  
         [0008]     It is a further object of the invention to provide such a method and apparatus wherein lower volumes of cooling air are required.  
       SUMMARY OF THE INVENTION  
       [0009]     Briefly described, a fuel cell assembly in accordance with the invention has means for providing tempered air to, and removing spent air from, air-flow passages across the cathode(s). The air flow path includes means for reversing the direction of flow across the cathode(s) periodically to reverse the roles of the leading and trailing edges of the cathode(s) to prevent temperature differences across the cathodes(s) from exceeding 200° C., and thus to prevent damage to the cathode(s) from thermally-induced stresses during startup heating and steady-state cooling. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     These and other features and advantages of the invention will be more fully understood and appreciated from the following description of certain exemplary embodiments of the invention taken together with the accompanying drawings, in which:  
         [0011]      FIG. 1  is an exploded isometric view of a single solid oxide fuel cell, showing the various elements and the flow paths of fuel and oxygen through the cell;  
         [0012]      FIG. 2  is an isometric view of a fuel-cell stack comprising five cells like the cell shown in  FIG. 1 ;  
         [0013]      FIG. 3  is an isometric view like that shown in  FIG. 2 , partially exploded, showing the addition of current collectors, end plates, and bolts to form a complete fuel cell stack ready for use;  
         [0014]      FIG. 4   a  is a schematic view of a fuel cell assembly including tempering apparatus, showing flow of air through the fuel cell in a first direction; and  
         [0015]      FIG. 4   b  is a view like that shown in  FIG. 4   a , showing flow of air through the fuel cell in a second direction opposite to the first direction.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]     Referring to  FIGS. 1 and 2 , an individual fuel cell  11  includes a multilayer element  09  comprising an electrolyte  14  (E) having an anode  16  or positive element (P) deposited on a first surface thereof and a cathode  18  or negative element (N) deposited on a second surface thereof. Thus, element  09 , which is the actual “fuel cell,” is known in the art by the acronym PEN. Passage  24  for flow of fuel  21  across the free surface  20  of anode  16  is provided by first cut-out spacers  36  sealed to anode  16  by peripheral seal  37 , and passage  26  for flow of air  31  across the free surface of cathode  18  is provided by second cut-out spacers  38  sealed to cathode  18  by another peripheral seal  37 . Fuel  21 , typically in the form of hydrogen or reformate gas, is provided at a first edge  25  of anode surface  20  via supply conduits  23  formed in each element and is removed via exhaust conduits  27  provided at a second and opposite edge  29  of anode surface  20 . Oxygen, typically in the form of air, is provided via supply conduits  33  to passages  26  at a first edge  39  of cathode  18  and is removed via exhaust conduits  41  at a second and opposite edge  43  of cathode  18 .  
         [0017]     Referring to  FIG. 3 , a plurality of fuel cells  11  may be stacked together to form a stack  12 , five such cells being shown in  FIG. 2 . In a complete working fuel cell  13 , stack  12  is sandwiched between an anodic current collector  34  and a cathodic current collector  32  which in turn are sandwiched between a top plate  15  and a gas-manifold base  17 , the entire assembly being bound together by bolts  19  extending through bores in top plate  15  and threadedly received in bores in base  17 . Air is provided to base  17  for supply to conduits  33  via a first connector  44 .  
         [0018]     Referring to  FIG. 4   a , an air tempering and flow control system  46  for supplying combustion air and thermal maintenance of an SOFC stack  13  in accordance with the invention is connected across the stack between first connector  44  and second connector  44   a . System  46  and stack  13  together define a fuel cell assembly  10 . Incoming air  48  is introduced at a controlled volume flow by conventional flow control means such as a fan or compressor (not shown) and is passed through an air tempering device  50 , shown as a cathode air heater in  FIGS. 4   a  and  4   b , by means of which the temperature of air  52  exiting device  50  may be controlled to a desired setpoint temperature.  
         [0019]     A flow path selector  54 , for example, a rotary four-port valve having ports A, B, C, and D connected to tempered air  52 , input connector  44 , output connector  44   a , and exhaust  56 , respectively, and responsive to conventional programmable control means  47 , is shown in  FIGS. 4   a  and  4   b . Of course, other means for making and controllably selecting such connections as may occur to those of ordinary skill in the art are fully comprehended by the present invention. Selector  54  includes a shaped rotor  58  rotatably disposed in a housing  60  to form first and second chambers  59  and  61 . Rotor  58  is capable of being rotated about an axis  62  between a first position as shown in  FIG. 4   a , wherein port A is connected to port B and port C is connected to port D, and a second position as shown in  FIG. 4   b , wherein port A is connected to port D and port B is connected to port C. With rotor  58  in the first position, tempered air  52  flows through stack  13  in a first direction from port  44  to port  44   a  and thence to exhaust  56 ; and in the second position in the reverse direction from port  44   a  to port  44  and thence to exhaust  56 . To control the flow direction of the tempered air, control means  47  can be programmed to rotate rotor  58  in one direction or in either direction between the first position and the second position, and at varied duty cycles to achieve the desired fuel cell temperature.  
         [0020]     By reversing the flow of air through SOFC  13  across the cathode surfaces thereof, and thereby alternating the effective supply and exhaust edges  39 , 43  of the cathodes, flow control system  46  prevents establishment of a significant and dangerous temperature difference between these edges.  
         [0021]     It should be noted that air tempering device  50  may be programmed to do no tempering (i.e., turned off) and thus can supply ambient temperature air as well as heated air. Thus, tempering system  46  may be used both for heating of the SOFC during startup and for cooling of the SOFC during operation, and can make the transition from one mode to the other, all while minimizing thermal imbalances within the SOFC.  
         [0022]     It should be further noted that preferably rotor  58  is rotated in only a single direction, either clockwise or counterclockwise, to simplify actuation mechanisms, and that non-50% duty cycles are fully comprehended by the invention.  
         [0023]     As noted above, in prior art tempering, the vulnerability of the cathode to failure from thermal expansion imposes very modest limits on the temperature of the air which may be used to heat or cool the SOFC, i.e., ΔT&lt;200° C. Improved control system  46  permits use of much greater ΔT values, the maximum permissible values for any given SOFC being readily determinable without undue experimentation. Higher permissible ΔT values confer two very important benefits over prior art systems, particularly for automotive uses wherein a fuel cell may be required to start repeatedly on short notice and wherein net electric output is critically important. First, warmup times from ambient temperatures may be significantly shortened. Second, parasitic electric losses may be reduced by providing heating and cooling air at substantially lower volume and higher ΔT; hence, the size and power of the air blower may be reduced.  
         [0024]     Rotor  58  may be alternated between the first and second positions on any desired periodicity. Preferably, the reversal frequency is selected to be relatively high with respect to the thermal time constant of the fuel cell (e.g., 2 Hz) but is low with respect to the time it takes for the flow to move from port A to port C. Preferably, multiple air volume changes occur between ports A and C between reversals of flow. Since flow reversal is fast with respect to the thermal time constant, the cell does not respond to the high gradients associated with much higher ΔT air, and thus a much smaller volume of much hotter or much cooler air can be used to heat or cool, respectively, the fuel cell more evenly. Temperature differences exceeding 200° C. may be employed without damage to the cathode.  
         [0025]     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.