Abstract:
A solid-oxide fuel cell system having “hot” components, e.g., the fuel cell stacks, the fuel reformer, tail gas combuster, heat exchangers, and fuel/air manifold, contained in a “hot zone” within a thermal enclosure intended specifically for minimizing heat transfer to its exterior and having no significant structural or protective function for its contents. A two-part clamshell arrangement allows all piping and leads which must pass through the enclosure to do so at the join line between the parts, thus eliminating need for ports and fittings in the thermal enclosure. A separate and larger structural enclosure surrounds the thermal enclosure, defining a “cool zone” outside the thermal enclosure for incorporation of “cool” components, e.g., the air supply system and the electronic control system, and providing structural protection for all components of the fuel cell system.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates to hydrogen/oxygen fuel cells having a solid-oxide electrolytic layer separating an anode layer from a cathode layer; more particularly, to fuel cell assemblies and systems comprising a plurality of individual fuel cells in a stack wherein air and reformed fuel are supplied to the stack; and most particularly, to a fuel cell system wherein core components which operate at very high temperatures in a hot zone are contained in a thermal enclosure disposed within a separate and larger structural enclosure which also houses cool zone components.  
         BACKGROUND OF THE INVENTION  
         [0002]    Fuel cells which generate electric current by the electrochemical combination of hydrogen and oxygen are well known. In one form of such a fuel cell, an anodic layer and a cathodic layer are separated by an 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. Each O 2  molecule is split and reduced to two O −2  anions catalytically by the cathode. The oxygen anions transport through the electrolyte and combine at the anode/electrolyte interface with four hydrogen ions to form two molecules of water. The anode and the cathode are connected externally through a load to complete the circuit whereby four electrons are transferred from the anode to the cathode. When hydrogen is derived by “reforming” hydrocarbons such as gasoline in the presence of limited oxygen, the “reformate” gas includes CO which is converted to CO 2  at the anode via an oxidation process similar to that performed on the hydrogen. Reformed gasoline is a commonly used fuel in automotive fuel cell applications.  
           [0003]    A single cell is capable of generating a relatively small voltage and wattage, typically between about 0.5 volt and about 1.0 volt, depending upon load, and less than about 2 watts per cm 2  of cell surface. Therefore, in practice it is known to stack together, in electrical series, a plurality of cells. Because each anode and cathode must have a free space for passage of gas over its surface, the cells are separated by perimeter spacers which are selectively vented to permit flow of gas to the anodes and cathodes as desired but which form seals on their axial surfaces to prevent gas leakage from the sides of the stack. The perimeter spacers may include dielectric layers to insulate the interconnects from each other. Adjacent cells are connected electrically by “interconnect” elements in the stack, the outer surfaces of the anodes and cathodes being electrically connected to their respective interconnects by electrical contacts disposed within the gas-flow space, typically by a metallic foam which is readily gas-permeable or by conductive filaments. The outermost, or end, interconnects of the stack define electric terminals, or “current collectors,” which may be connected across a load.  
           [0004]    A complete SOFC system typically includes auxiliary subsystems for, among other requirements, generating fuel by reforming hydrocarbons; tempering the reformate fuel and air entering the stack; providing air to the hydrocarbon reformer; providing air to the cathodes for reaction with hydrogen in the fuel cell stack; providing air for cooling the fuel cell stack; providing combustion air to an afterburner for unspent fuel exiting the stack; and providing cooling air to the afterburner and the stack.  
           [0005]    An enclosure for a fuel cell system has two basic functions. The first is to provide thermal insulation for some of the components which must function at an elevated temperature (700-900° C.) to maintain them at that temperature for efficient operation, to protect lower temperature components, and to reduce the exterior temperature over the overall unit to a human-safe level. The second is to provide structural support for mounting of individual components, mounting the system to another structure such as a vehicle, protection of the internal components from the exterior environment, and protection of the surrounding environment from the high temperatures of the fuel cell assembly. Prior art fuel cell systems utilize a single enclosure to provide all functions, which can be complex and costly to fabricate and assemble, and consumptive of space.  
           [0006]    What is needed is a means for separating the thermal requirements of an enclosure from the structural requirements.  
           [0007]    It is a principal object of the present invention to simplify the construction and reduce the cost and size of a solid-oxide fuel cell system.  
           [0008]    It is a further object of the invention to increase the reliability and safety of operation of such a fuel cell system.  
         BRIEF DESCRIPTION OF THE INVENTION  
         [0009]    Briefly described, in a solid-oxide fuel cell system, the “hot” components, e.g., the fuel cell stacks, the fuel reformer, tail gas combuster, heat exchangers, and fuel/air manifold, are contained in a “hot zone” within a thermal enclosure. The thermal enclosure is intended specifically for minimizing heat transfer to its exterior and has no significant structural or protective function for its contents. A two-part clamshell arrangement allows all piping and leads which must pass through the enclosure to do so at the join line between the parts, thus eliminating need for ports and fittings in the thermal enclosure. A separate and larger structural enclosure surrounds the thermal enclosure, defining a “cool zone” outside the thermal enclosure for incorporation of “cool” components, e.g., the air supply system and the electronic control system.  
       
    
    
     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]    [0011]FIG. 1 is a schematic cross-sectional view of a two-cell stack of solid oxide fuel cells;  
         [0012]    [0012]FIG. 2 is a schematic mechanization diagram of an SOFC system in accordance with the invention;  
         [0013]    [0013]FIG. 3 is an isometric view from above of a two-stack fuel cell assembly, shown connected electrically in series between two current collectors;  
         [0014]    [0014]FIG. 4 is an isometric view like that shown in FIG. 3, with a cover enclosing the stacks;  
         [0015]    [0015]FIG. 5 is an elevational cross-sectional view taken along line  5 - 5  in FIG. 4;  15  FIG. 6 is an elevational cross-sectional view taken along line  6 - 6  in FIG. 4;  
         [0016]    [0016]FIG. 7 is an equatorial cross-sectional view taken along line  7 - 7  in FIG. 4;  
         [0017]    [0017]FIG. 8 is an isometric view from above, showing a fuel cell assembly comprising the apparatus of FIG. 4 mounted on a manifold in accordance with the invention, along with reforming, combusting, and heat exchanging apparatus for servicing the fuel cell stacks;  
         [0018]    [0018]FIG. 9 is an isometric view from above, showing the fuel cell assembly of FIG. 8 mounted in the lower element of a thermal enclosure;  
         [0019]    [0019]FIG. 10 is an isometric view from above of an air supply assembly for controllably providing air to the fuel cell assembly shown in FIGS. 8 and 9;  
         [0020]    [0020]FIG. 11 is an exploded isometric view of a fuel cell system in accordance with the invention, showing the air supply assembly of FIG. 10 disposed in a structural enclosure, and showing the fuel cell assembly of FIG. 9 fully enclosed by both upper and lower elements of a thermal enclosure;  
         [0021]    [0021]FIG. 12 is an isometric view from above of a fully assembled fuel cell system in accordance with the invention;  
         [0022]    [0022]FIG. 13 is an exploded isometric view from the front, showing a multi-element basal manifold in accordance with the invention for distributing air and reformate fuel and exhaust products through and around the fuel cell stacks, as shown in FIG. 8;  
         [0023]    [0023]FIG. 14 is an isometric view from the rear, showing the manifold of FIG. 13 partially assembled;  
         [0024]    [0024]FIG. 15 is an isometric view from the rear, showing the manifold of FIG. 13 further assembled;  
         [0025]    [0025]FIG. 16 is a plan view of the lower level of chambers formed by the lower two elements shown in FIG. 13;  
         [0026]    [0026]FIG. 17 is a plan view of the upper level of chambers formed by the third and fourth elements shown in FIG. 13; and  
         [0027]    [0027]FIG. 18 is a plan view of the uppermost element shown in FIG. 13, showing the mounting surface for the apparatus shown in FIG. 8. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]    Referring to FIG. 1, a fuel cell stack  10  includes elements known in the art of solid-oxide fuel cell stacks comprising more than one fuel cell. The example shown includes two identical fuel cells  11 , connected in series, and is of a class of such fuel cells said to be “anode-supported” in that the anode is a structural element having the electrolyte and cathode deposited upon it. Element thicknesses as shown are not to scale.  
         [0029]    Each fuel cell  11  includes an electrolyte element  14  separating an anodic element  16  and a cathodic element  18 . Each anode and cathode is in direct chemical contact with its respective surface of the electrolyte, and each anode and cathode has a respective free surface  20 , 22  forming one wall of a respective passageway  24 , 26  for flow of gas across the surface. Anode  16  of one fuel cell  11  faces and is electrically connected to an interconnect  28  by filaments  30  extending across but not blocking passageway  24 . Similarly, cathode  18  of fuel cell  11  faces and is electrically connected to interconnect  28  by filaments  30  extending across but not blocking passageway  26 . Similarly, cathode  18  of a second fuel cell  11  faces and is electrically connected to a cathodic current collector  32  by filaments  30  extending across but not blocking passageway  26 , and anode  16  of fuel cell  11  faces and is electrically connected to an anodic current collector  34  by filaments  30  extending across but not blocking passageway  24 . Current collectors  32 , 34  may be connected across a load  35  in order that the fuel cell stack  10  performs electrical work. Passageways  24  are formed by anode spacers  36  between the perimeter of anode  16  and either interconnect  28  or anodic current collector  34 . Passageways  26  are formed by cathode spacers  38  between the perimeter of electrolyte  14  and either interconnect  28  or cathodic current collector  32 . Anode spacer  36  and cathode spacer  38  are formed from sheet stock in such a way as to yield the desired height of the anode passageways  24  and cathode passageways  26 .  
         [0030]    Preferably, the interconnect and the current collectors are formed of an alloy, typically a “superalloy,” which is chemically and dimensionally stable at the elevated temperatures necessary for fuel cell operation, generally about 750° C. or higher, for example, Hastelloy, Haynes 230, or a stainless steel. The electrolyte is formed of a ceramic oxide and preferably includes zirconia stabilized with yttrium oxide (yttria), known in the art as YSZ. The cathode is formed of, for example, porous lanthanum strontium manganate or lanthanum strontium iron, and the anode is formed of, for example, a mixture of nickel and YSZ.  
         [0031]    In operation (FIG. 1), reformate gas  21  is provided to passageways  24  at a first edge  25  of the anode free surface  20 , flows parallel to the surface of the anode across the anode in a first direction, and is removed at a second and opposite edge  29  of anode surface  20 . Hydrogen and CO diffuse into the anode to the interface with the electrolyte. Oxygen  31 , typically in air, is provided to passageways  26  at a first edge  39  of the cathode free surface  22 , flows parallel to the surface of the cathode in a second direction which can be orthogonal to the first direction of the reformate (second direction shown in the same direction as the first for clarity in FIG. 1), and is removed at a second and opposite edge  43  of cathode surface  22 . Molecular oxygen gas (O 2 ) diffuses into the cathode and is catalytically reduced to two O −2  anions by accepting four electrons from the cathode and the cathodic current collector  32  or the interconnect  28  via filaments  30 . The electrolyte ionically conducts or transports O −2  anions to the anode electrolyte innerface where they combine with four hydrogen atoms to form two water molecules, giving up four electrons to the anode and the anodic current collector  34  or the interconnect  28  via filaments  30 . Thus cells  11  are connected in series electrically between the two current collectors, and the total voltage and wattage between the current collectors is the sum of the voltage and wattage of the individual cells in a fuel cell stack.  
         [0032]    Referring to FIG. 2, a schematic mechanization diagram of a solid-oxide fuel cell system  12  in accordance with the invention includes auxiliary equipment and controls.  
         [0033]    A conventional high speed inlet air pump  48  draws inlet air  50  through an air filter  52 , past a first MAF sensor  54 , through a sonic silencer  56 , and through a cooling shroud  58  surrounding pump  48 .  
         [0034]    Air output  60  from pump  48 , at a pressure sensed by pressure sensor  61 , is first split into branched conduits between a feed  62  and a feed  72 . Feed  62  goes as burner cooling air  64  to a tail gas afterburner  66  having an igniter  67  via a second MAF sensor  68  and a burner cool air control valve  70 .  
         [0035]    Feed  72  is further split into branched conduits between an anode air feed  74  and a cathode air feed  75 . Anode feed  74  goes to a hydrocarbon fuel vaporizer  76  via a third MAF sensor  78  and reformer air control valve  80 . A portion of anode air feed  74  may be controllably diverted by control valve  82  through the cool side  83  of reformate pre-heat heat exchanger  84 , then recombined with the non-tempered portion such that feed  74  is tempered to a desired temperature on its way to vaporizer  76 . Downstream of vaporizer  76  is a start-up combustor  77  having an igniter  79 . During start-up, when the reformer is cold or well below operating temperature, vaporized fuel is ignited in combustor  77  and the burned gas is passed directly through the reformer to warm the plates therein more rapidly. Obviously, the start-up combustor is deactivated during normal operation of the system.  
         [0036]    Cathode air feed  75  is controlled by cathode air control valve  86  and may be controllably diverted by cathode air preheat bypass valve  88  through the cool side  90  of cathode air pre-heat heat exchanger  92  on its way to stacks  44 , 46 . After passing through the cathode sides of the cells in stacks  44 , 46 , the partially spent, heated air  93  is fed to burner  66 .  
         [0037]    A hydrocarbon fuel feed pump  94  draws fuel from a storage tank  96  and delivers the fuel via a pressure regulator  98  and filter  100  to a fuel injector  102  which injects the fuel into vaporizer  76 . The injected fuel is combined with air feed  74 , vaporized, and fed to a reformer catalyst  104  in main fuel reformer  106  which reforms the fuel to, principally, hydrogen and carbon monoxide. Reformate  108  from catalyst  104  is fed to the anodes in stacks  44 , 46 . Unconsumed fuel  110  from the anodes is fed to afterburner  66  where it is combined with air supplies  64  and  93  and is burned. When gases are below self ignition temperature, they are ignited by igniter  67 . The hot burner gases  112  are passed through a cleanup catalyst  114  in main reformer  106 . The effluent  115  from catalyst  114  is passed through the hot sides  116 ,  118  of heat exchangers  84 ,  92 , respectively, to heat the incoming cathode and anode air. The partially-cooled effluent  115  is fed to a manifold  120  surrounding stacks  44 , 46  from whence it is eventually exhausted  122 .  
         [0038]    Still referring to FIG. 2, a first check valve  150  and a first oxygen getter device  124  are provided in the conduit feeding reformate  108  to the anodes (not visible) in stacks  44 , 46 . A second check valve  152  and second oxygen getter device  126  are similarly provided in the conduit feeding spent reformate  110  from the anodes to afterburner  66 . As described above, during cool-down of the fuel cell stacks after shutdown of the assembly, it is important to prevent migration of oxygen into anode passages  24  wherein anode surface  20 , comprising metallic nickel, would be subject to damaging oxidation. Each check valve includes a typical frusto-conical valve seat  154  receptive of a valve ball  156 . Preferably, each valve  150 , 152  is oriented within assembly  12  such that the ball is held in the seat by gravity when reformate is flowed through the system in the forward direction. Thus, fuel flow opens the valve sufficiently for fuel to pass in the forward direction. When assembly  12  is shut down, each valve is closed by gravity. The valves may not be identical, as oxygen flows opposite to the reformate in valve  152 , but in the same direction as the reformate in valve  150 ; the so the balls and seats may require different weights and/or sizes to function as intended. Each getter  124 , 126  includes a passageway  128  having an inlet  130  and an outlet  132  through which reformate is passed during operation of the fuel cell assembly. Within the passageway is a readily-oxidized material  134  (oxygen-reducing means), for example, nickel metal foam, nickel wire or nickel mesh, which is capable of gettering oxygen by reaction therewith but which does not present a significant obstruction to flow of reformate through the chamber. Nickel in the getters reacts with oxygen to produce nickel oxide, NiO, when the assembly is shut down, thus protecting the nickel-containing anodes from oxidation. When the assembly is turned back on, reformate is again produced which, in passing through the getters, reduces the NiO back to metallic nickel, allowing the getters to be used repeatedly.  
         [0039]    For clarity of presentation and to enhance the reader&#39;s understanding, the numbers of elements of the invention as presented further below are grouped in century series depending upon the functional assembly in which the elements occur; therefore, elements recited above and shown in FIGS. 1 and 2 may have different numerical designators when shown and discussed below, e.g., stacks  44 , 46  become stacks  344 , 346 .  
         [0040]    Referring to FIGS. 3 through 7, in a fuel cell stack assembly  300  in accordance with the invention, the cells  311  are arranged side-by-side and may comprise a plurality of cells  311 , respectively, such that each of first stack  344  and second stack  346  is a stack of identical fuel cells  311 . The plurality of cells is preferably about  30  in each of the two stacks. The cells  311  in stack  344  and stack  346  are connected electrically in series by interconnect  347 , and the stacks are connected in series with cathode current collector  332  and anode current collector  334  on the bottom of the stacks. The current collectors are sized to have a “footprint” very close to the same dimension as a cover-sealing flange  340 . The current collectors preferably are adhesively sealed to a stack mounting plate  338 , and the stacks preferably are in turn adhesively sealed to the current collectors. The sealing flange  340  for the cover  342  and top  343  is then mounted and sealed to the current collector plates. A gasket  341  between flange  340  and the current collectors is a dielectric so that flange  340  does not cause a short between the current collectors. Power leads  350 , 352  are attached to current collectors  332 , 334 , respectively, through strong, reliable and highly conductive metallurgical bonds, such as brazing. In this manner, the current collectors may pass under the cover mounting flange  340 , with no additional sealing or power lead attachment required, and do not have to pass undesirably through the cover itself, as in some prior art stack assemblies. Passing leads through the cover makes the assembly more complex and less reliable.  
         [0041]    Referring to FIG. 8, a fuel cell assembly  400  in accordance with the invention comprises stack assembly  300  operatively mounted on an integrated fuel/air manifold assembly  500  which also supports first and second cathode air heat exchangers  600  and an integrated fuel reformer and waste energy recovery unit (“reforWER”)  1100 . Assembly  400  receives air from air supply system  900  (FIGS.  10 - 12 ) as described below and selectively preheats air going to the reformer. ReforWER  1100  reforms hydrocarbon fuel, such as gasoline, into reformate fuel gas comprising mostly hydrogen, carbon monoxide, and lower-molecular weight hydrocarbons, tempers the air and reformate entering the stacks, selectively burns fuel not consumed in the stacks, recovers heat energy generated in various internal processes which would otherwise be wasted, and exhausts spent air and water, all in order to efficiently generate DC electric potential across power leads  350 , 352  (not visible in FIG. 8).  
         [0042]    Referring to FIGS. 9 and 11, enclosure of the fuel cell assembly comprises two nested enclosures: a thermal enclosure  700  and a structural enclosure  800 . Fuel cell assembly  400  is first disposed in a “clam-shell” type thermal enclosure  700 , comprising a bottom portion  702  and a top portion  704 , which in turn is disposed in a structural enclosure  800 . The split line  706  between bottom portion  702  and top portion  704  is easily arranged such that all pipes, manifolds, shafts, power leads, etc., which need to pass between the “hot zone”  716  within the thermal enclosure and the “cool zone”  816  within the structural enclosure, do so in the middle of split line  706 . This provides for easy assembly of the hot components into the thermal enclosure. First, all hot zone components, included in assembly  400 , are nestled into in bottom portion  702 , which may be provided with a conforming well  708  for securely holding and cushioning assembly  400 , as shown in FIG. 9. The mating surface  710  of bottom portion  702 , along split line  706 , is configured as required to accommodate the lower halves of the components extending through enclosure  700 . Top portion  704  is configured to matingly engage bottom portion  702 . Top portion  704  is placed onto bottom portion  702  and may be sealed thereto along line  706  as desired. Thermal enclosure  700  may be formed of any suitable high-temperature high-efficiency insulating material, as is known in the insulating art, and may be a composite including a light-weight metal case. The range of suitable insulating materials is expanded by removing the constraint of overall structural integrity afforded by providing a separate structural enclosure  800 .  
         [0043]    Structural enclosure  800  preferably is fabricated from thicker metal, for example, to provide structural strength and a simple shape, such as a box with a removable lid, for ease of fabrication. Features such as brackets, studs, electrical connectors, studs, weld-nuts, air intake ducts, and exhaust ducts, for example, may be part of the structural enclosure for mounting internal components thereto and for connecting the system to external structures. Features for vibration and shock isolation (not shown) may also be provided with the enclosure.  
         [0044]    The air control assembly  900  is connected to elements of fuel cell assembly  400  projecting through split line  706 , and assemblies  700 , 900  are then installed within structural enclosure  800 , as shown in FIG. 12, to form a fuel cell system  1000  in accordance with the invention. Preferably, control system  200  (shown schematically in FIG. 2 as power conditioner  202 , circuit protection I/O  204 , drivers  206 , and electronic control unit  208 , but not visible in FIG. 12) is also installed onboard the system within cool zone  816  to minimize the number of discrete signals  210  which must be passed through enclosure  800  via connector  820 . Note also that high current capacity power leads also pass through enclosure  800  via dual connectors  821 .  
         [0045]    Referring to FIGS. 13 through 18, an integrated fuel/air manifold assembly  500  receives air via flexible bellows elements from air supply assembly  900  and reformed fuel from reformer assembly  1100  and conveys high temperature air, exhaust, and hydrogen-rich reformate fuel to and from the core components of the system. Basal manifold assembly  500  is shown in FIG. 13 as comprising a three-dimensional assembly of three perforated plates and two partitioned elements which are easily and inexpensively formed and which comprise a two-level network of passageways which allow for the mounting, close-coupling, and integration of critical fuel cell system components, including heat exchangers, combustors, fuel reformers, solid-oxide fuel cell stacks, check valves, threaded inserts, and catalyzed and non-catalyzed filters. Of course, while a five-component manifold is shown for simplicity, within the scope of the invention any two of the perforated plates obviously may be incorporated into the partitioned elements, through appropriate and obvious casting or moulding processes, such that the manifold comprises only three elements.  
         [0046]    It should be noted that manifold  500  is actually two mirror image manifolds  500 - 1 , 500 - 2  sharing some common features, for example, cathode air return from the stacks. Thus, reformate fuel flows from reforWER unit  1100  in two parallel streams to stacks  344  and  346  and is returned to reforWER  1100  in two parallel streams.  
         [0047]    Likewise, cathode air flow from air supply assembly  900  is divided into two parallel streams and enters into each manifold  500 - 1 , 500 - 2  via mirror image couplings  902 - 1  and  902 - 2  (FIGS.  8 - 10  and  13 ). Fuel cell assembly  400  thus is seen to have its fuel cell stacks  344 , 346  connected in series electrically but serviced by gas flows in parallel.  
         [0048]    For simplicity of presentation and discussion, except where functions are unique, the following construction and function is directed to manifold  500 - 1  but should be understood to be equally applicable to mirror-image manifold  500 - 2 .  
         [0049]    Bottom plate  502  is the base plate for the manifold and forms the bottom for various chambers formed by combination of plate  502  with lower partitioned element  504 , defining a lower distribution element  505 , as shown in FIG. 16. Intermediate plate  506  completes the chambers in element  504  and forms the bottom plate for upper partitioned element  508 , defining an upper distribution element  509 . Top plate  510  completes the chambers in element  508  and forms the mounting base for fuel cell assembly  300 , heat exchangers  600 , and reforWER unit  1100 , as described above.  
         [0050]    In operation, air enters a first bottom chamber  512  via coupling  902 - 1 , flows upwards through slots  514 - 1 , 514 - 2 , 514 - 3  into heat exchanger  600 - 1 , through the heat exchanger conventionally where the air is heated as described below, downwards through slot  516 - 3  into a first upper chamber  518 , thence through opening  520  in plate  506  into a second lower chamber  522 . In chamber  518 , the heated air is controllably mixed with cool air entering the chamber via bypass connection  904 - 1  from air supply assembly  900 . The tempered air flows upwards from chamber  522  through opening  524  in plate  506  into a chamber  526  which defines a cathode supply plenum for supplying reaction and cooling air upwards through slotted openings  528  to the cathode air flow passages in stack  344 . Spent air is returned from the cathodes via slotted openings  530  into a cathode return plenum  532  and flows downwards through an opening  534  in plate  506  into a common cathode air return runner  536  leading into a tail-gas combustor  1102  within reforWER  1100 .  
         [0051]    Hot reformate from reforWER  1100  enters manifold  500 - 1  via opening  538  in top plate  510  and flows into chamber  540 , thence downwards through opening  542  into a feed runner  544 , and upwards through opening  546  into a chamber  548  defining an anode supply plenum for stack  344 .  
         [0052]    Preferably, opening  546  defines a seat for a valve having a ball  550  (FIG. 14), preferably held in place by gravity, for allowing flow of reformate during operation but preventing flow of oxygen into the anodes when the system is shut down. Further, preferably, chamber  544  and/or  548  contains an oxygen-reactive material (not shown here but indicated as  134  in FIG. 2), such as nickel wool, through which reformate may easily pass but which can scavenge any oxygen passing by ball  550  on its way to the anodes.  
         [0053]    Preferably, cathode supply chamber  522  and anode supply chamber  544  are configured to maximize the area of the common wall between them, such that chambers  522 , 544  define a co-flow heat exchanger which tends to decrease the temperature difference between the cathode supply air and the anode supply reformate.  
         [0054]    From chamber  548 , reformate flows upwards through slots  552  into the anode flow passages in stack  344 . Spent reformate (“tail gas”) flows downwards through slots  554  into an anode return plenum  556  and thence downwards through opening  558  into a reformate return runner  560 . From runner  560 , spent reformate flows upwards through opening  562  into elongate chamber  564  common with manifold  500 - 2  and thence through openings  566  into the tail-gas combustor in reforWER  1100 . Preferably, opening  562  is also formed as a check valve seat like opening  546  for receiving a check ball  563  preferably held in place by gravity for preventing reverse flow of oxygen into the anodes when the system is shut down. Further, preferably, chamber  556  and/or  560 , like chamber  548 , contains an oxygen-reactive material (not shown here but indicated as  134  in FIG. 2), such as nickel wool, through which the tail gas may easily pass but which can scavenge any oxygen passing by ball  563  on its way to the anodes.  
         [0055]    Burned tail gas from the combustor enters manifold  500 - 1  via slot  568 - 3  and flows via slots  568 - 2 , 568 - 1  into bottom chamber  570  and thence through opening  572  into chamber  574  which acts as a supply plenum for cathode air heat exchanger  600 - 1 .  
         [0056]    Burned tail gas flows upward from chamber  574  through openings  576  and through heat exchanger  600 - 1 , thus heating incoming cathode air, returning through openings  578  into chamber  580  and thence via openings  582  into a tempering jacket space  354  (FIG. 7) surrounding stack  344  between the fuel cells  311  and cover  342 . The stack is thus tempered by the exhaust gas. The burned tail gas returns from jacket  354  via openings  584  into an exhaust plenum comprising openings  586 - 3 , 586 - 2 , 586 - 1  which is vented to the atmosphere by exhaust pipe  588  and pipe flange  590 .  
         [0057]    An SOFC system  1000  in accordance with the invention is especially useful as an auxiliary power unit (APU) for vehicles  136  (FIG. 12) on which the APU may be mounted, such as cars and trucks, boats and ships, and airplanes, wherein motive power is supplied by a conventional engine and the auxiliary electrical power needs are met by an SOFC system.  
         [0058]    An SOFC assembly in accordance with the invention is also useful as a stationary power plant such as, for example, in a household or for commercial usage.  
         [0059]    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.