Abstract:
The present invention provides a fuel cell system including a first insulated enclosure enclosing a first interior space maintained at a temperature greater than ambient, a plurality of fuel cells maintained at an elevated temperature so as to maximize efficiency of an electrical current generating reaction, and a second insulated enclosure positioning within the first interior space and enclosing a second interior space. The second interior space can be maintained at a temperature greater than the first interior space and approximately equal to the elevated temperature of the stacks. The system can include non-superalloy metallic elements located in the first insulated enclosure. The temperature of the first interior space can be sufficiently low such that exposure of the non-superalloy metallic elements to one of an oxidizing gas stream and a reducing gas stream does not degrade the non-superalloy metallic elements.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Patent Application No. 60/898,583 filed on Jan. 31, 2007. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to fuel cells, and more specifically is directed to the construction and operation of fuel cell systems having solid oxide fuel cells. 
       SUMMARY 
       [0003]    Solid oxide fuel cells (SOFCs) are solid-state electrochemical devices that use a solid ceramic electrolyte to conduct oxygen ions from an oxidizing gas stream at a cathode end of the fuel cell to a reducing gas stream at the anode end of the fuel cell. The oxidizing flow can be air, while the fuel flow can be a hydrogen-rich gas created by reforming a hydrocarbon fuel source. 
         [0004]    The solid oxide fuel cell of the present invention can have a number of different constructions and chemistries, one of which is referred to as a planar solid oxide fuel cell. A planar SOFC can be constructed of a thin electrolyte with a cathode electrode on one surface and an anode electrode on the opposite surface. An interconnect can be used to electrically connect the anode of one fuel cell with the cathode of the adjacent cell in the stack. One set of flow channels in the interconnect can provide the fuel flow with access to the anode, and another set of flow channels in the interconnect can provide the air flow with access to the cathode. A flow manifold can be incorporated within the fuel cell stack in order to isolate the fuel flow from the oxidizing flow, and to evenly distribute the fuel flow to the anodes of the multiple cells in the stack. In some fuel cell designs of the present invention, a similar manifolding structure can be provided to distribute the air flow to the cathodes of the multiple cells in the stack (referred to as an internally manifolded stack), while in other fuel cell designs the cathode flow channels in each individual interconnect can have access to an inlet and an outlet face of the stack in order to provide an entrance and exit for the cathode air flow (referred to as an externally manifolded stack). 
         [0005]    The fuel cell, operating at a temperature typically between about 750° C. and about 1000° C., enables the transport of a negatively charged ion (O = ) from the cathode electrode to the anode electrode, where the ion combines with either free hydrogen or hydrogen in a hydrocarbon molecule to form water vapor, or with carbon monoxide to form carbon dioxide. The excess electrons from the negatively charged ion are routed back to the cathode side of the fuel cell through an electrical circuit completed externally between anode and cathode, resulting in an electrical current flow through the circuit. In some SOFC systems, multiple such cells are placed in an electrical series as one or more fuel cell stacks in order to provide an electrical current at a sufficiently high voltage. 
         [0006]    Such a fuel cell system can be used to produce useful electrical power by consumption of common hydrocarbon fuels, such as, for example natural gas, propane, liquified petroleum gas (LPG), gasoline, and diesel. This enables the use of a SOFC system as an alternative to conventional electrical power generation devices such as internal combustion engine based generator sets for use in a distributed power generation (DPG) system or auxiliary power unit (APU). A solid oxide fuel cell based DPG system or APU offers several advantages over traditional generator sets, including eliminating undesirable noise levels inherent in internal combustion engine operation, reducing or eliminating the emission of pollutants such as carbon monoxide, oxides of nitrogen, and unburned hydrocarbons, and providing higher power conversion efficiencies. 
         [0007]    There are substantial difficulties encountered in producing solid oxide fuel cell based distributed power generation systems or auxiliary power units at a cost level that is comparable to that of the traditional internal combustion engine based systems. One of the greatest such difficulties lies in producing the balance of plant componentry required for the proper operation of the solid oxide fuel cells. Proper operation of an SOFC system can require several processing steps to be performed, including one or more of the following: the recuperative transfer of thermal energy from the waste gas streams; chemical reforming of the hydrocarbon fuel into a hydrogen and carbon monoxide flow stream with minimal amounts of higher hydrocarbons; water recovery from waste gas streams; structural support of the fuel cell stacks; and combustion of remaining combustible species in the anode exhaust gas stream. 
         [0008]    Because the fuel cell stacks themselves operate at an elevated temperature, many of these process operations, as well as the components that serve to deliver the gas streams between the different operations and components, are similarly exposed to elevated temperatures. This requires that the materials of construction for these balance of plant operations be capable of long-term operation while exposed to such temperatures. The materials generally considered to be both capable of long-term exposure to such temperatures and suitable for performing the required process operations are nickel-chromium based metallic “superalloys”, which exhibit advantageous properties such as high temperature creep resistance, long fatigue life, phase stability, and exceptional oxidation and corrosion resistance. The use of such materials, however, dramatically increases the cost of the fuel cell system. More conventional austenitic stainless steels, which have substantially lower nickel content, are available at a cost that is typically less than 10% of the cost of an equal quantity of superalloy material, but the properties of austenitic stainless steels make them unsuitable for use at a metal temperature exceeding approximately 600° C. Many of the balance of plant components have heat exchanger functionality, which requires that a substantial amount of heat transfer surface area and consequently a substantial amount of superalloy material be used. In addition, the conveyance of the fluid flows between the various processing components requires interconnecting piping that is similarly constructed of high temperature capable superalloys, and all of which can be connected using labor-intensive welding operations and/or expensive compression-fitting connections. This further increases the cost of an SOFC system. 
         [0009]    In some embodiments, the present invention provides a system and a method for reducing the cost of a solid oxide fuel cell system by, among other things, minimizing the amount of superalloy materials required in the construction of the fuel cell balance of the plant. 
         [0010]    In some embodiments, the present invention simplifies the construction of a solid oxide fuel cell system and minimizes the amount of superalloy materials required, thereby reducing the cost of a solid oxide fuel cell based distributed power generation system. 
         [0011]    In some embodiments, a fuel cell system includes a first insulated enclosure, the interior of which is maintained at a moderate elevated temperature over the surrounding ambient, the elevated temperature being suitably low to allow for the long-term exposure of austenitic stainless steel materials to both oxidizing and reducing gas streams at that temperature. The first insulated enclosure can contain a second insulated enclosure, the interior of which can be maintained at a temperature approximately equal to the operating temperature of solid oxide fuel cells. 
         [0012]    In some embodiments, the first insulated enclosure also contains a structure constructed of austenitic stainless steel or similar materials of construction, which structurally supports the second insulated enclosure and which delivers fuel cell process flows to and receives fuel cell process flows from the second insulated enclosure. In some embodiments, the aforementioned structure enables heat transfer required for proper operation of the fuel cell system between two or more of the fuel cell process flows therein. 
         [0013]    In some embodiments, the first insulated enclosure contains additional heat exchange components required for proper operation of the fuel cell system. 
         [0014]    In some embodiments, the second insulated enclosure contains a plurality of solid oxide fuel cell stacks. In some embodiments, the second insulated enclosure contains a fuel processing reformer. In some embodiments, the second insulated enclosure contains one or more high temperature heat exchangers. In some embodiments, the second insulated enclosure contains a flow manifold structure that provides structural support for the solid oxide fuel cell stacks and that routes flows to and/or from the solid oxide fuel cell stacks, the fuel processing reformer, and the one or more high temperature heat exchangers. 
         [0015]    In some embodiments, the air space inside the first insulated enclosure is filled with a gas comprised of cathode exhaust and combusted anode exhaust. In some embodiments, the gas is continuously vented from the first insulated enclosure and is replaced by more of the same gas from the second insulated enclosure during operation of the fuel cell system. 
         [0016]    Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a partial cutaway perspective view of a solid oxide fuel cell system according to some embodiments of the present invention; 
           [0018]      FIG. 2A  is a schematic partial sectional view depicting certain features of the unit of  FIG. 1 , with cathode air flow movement within an insulated enclosure depicted; 
           [0019]      FIG. 2B  is a schematic partial sectional view depicting certain features of the unit of  FIG. 1  in a viewing direction perpendicular to that of  FIG. 2A , with cathode air flow movement within an insulated enclosure depicted; 
           [0020]      FIG. 3A  is a sectional view taken from line  3 A- 3 A in  FIG. 2B ; 
           [0021]      FIG. 3B  is a sectional view taken from line  3 B- 3 B in  FIG. 3A ; 
           [0022]      FIG. 4  is a perspective view of a manifolding structure and certain other components for use in the unit shown in  FIG. 1 ; 
           [0023]      FIGS. 5A and 5B  are views similar to  FIG. 3A , with  FIG. 5A  illustrating the flow of the anode feed and exhaust gases and  FIG. 5B  illustrating the flow of the cathode feed and exhaust gases; 
           [0024]      FIG. 5C  is a view similar to  FIG. 5B  depicting an alternate embodiment of the present invention; 
           [0025]      FIG. 6  is an enlarged perspective view showing selected portions of the structure shown in  FIG. 4 ; 
           [0026]      FIG. 7  is a perspective view of another embodiment of a heat exchange structure for use in the unit shown in  FIG. 1 ; 
           [0027]      FIG. 8  is a perspective view of features located on a bottom surface of the manifolding structure shown in  FIG. 4 ; 
           [0028]      FIG. 9  is a perspective view of a flow manifolding/heat exchange/structural support feature for use in the unit shown in  FIG. 1 ; 
           [0029]      FIG. 10  is a perspective view similar to that of  FIG. 9 , but with some components removed for clarity; 
           [0030]      FIG. 11  is a process flow schematic of a solid oxide fuel cell system embodying the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0031]    Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless specified or limited otherwise, the terms “mounted,” “connected,” “supported,” and “coupled” and variations thereof are used broadly and encompass both direct and indirect mountings, connections, supports, and couplings. Further, “connected” and “coupled” are not restricted to physical or mechanical connections or couplings. 
         [0032]      FIGS. 1 ,  2 A and  2 B illustrate a high temperature subsystem  9  for use in a fuel cell system based distributed power generation system or auxiliary power unit. The subsystem  9  includes an insulated outer enclosure  10  which contains a hotbox subsystem  100 , anode feed injection system  17  and heat exchange/flow manifolding/structural support component  20 . In some embodiments, the outer enclosure  10  serves to maintain the environment within at a moderately elevated temperature of approximately 300-450° C. In some embodiments, the insulated outer enclosure  10  also contains additional components, including but not limited to: an anode tailgas oxidation (ATO) reactor  12  connected to the hotbox subsystem  100  with piping  13 , an ATO air preheater  14 , and a reformer air preheater  15 . Other components that may be contained within the insulated outer enclosure  10  are explained in greater detail below. 
         [0033]    With reference to  FIGS. 2A and 2B , a cathode air stream, shown schematically by arrow  46 , enters the heat exchange/flow manifolding/structural support component  20  through inlet pipe  21 , which passes through the outer enclosure  10 , and is routed into the hotbox subsystem  100  as partially preheated cathode air, shown schematically by arrows  119 . An exhaust gas flow, shown schematically by arrow  124 , including the cathode exhaust and ATO exhaust is routed from the hotbox subsystem  100  through the heat exchange/flow manifolding/structural support component  20  and into an air space  49  located beneath component  20  and open to the air space within the outer insulated enclosure  10  at either end of component  20 , as is illustrated in  FIG. 1 . 
         [0034]    In some embodiments, a water vaporizer heat exchanger  16  is located within the air space  49  to transfer heat from the exhaust flow  124  to a water flow to be used for a reforming process within the hotbox subsystem  100 . Exhaust streams, shown schematically by arrows  41 , which include the exhaust gas flow  124 , exit the air space  49  and fill the cavity within the insulated outer enclosure  10 . The insulated outer enclosure  10  is vented through an exhaust pipe  11 , located at the upper region of enclosure  10 . The location of the exhaust pipe  11  causes the exhaust gas flow  41  to move in a generally upward direction through the enclosure  10 . As the exhaust gas flow  41  flows through the enclosure  10 , heat is removed from the flow in heat exchangers  14  and  15 . The pressure is maintained within the outer enclosure  10  by a flow of exhaust gas, shown schematically by arrow  42 , from the enclosure through exhaust piping  11 , the exhaust gas flow  42  being comprised of exhaust gas flow  124 . 
         [0035]    In some embodiments, the outer enclosure is sufficiently sealed so that the exhaust gas flow  42  is removed from the outer enclosure  10  at approximately the same rate as exhaust gas flow  124  enters the space  49 . In some embodiments, the exhaust gas flows  124 ,  41  and  42  are all in a temperature range of 300-450° C. 
         [0036]    Turning now in greater detail to the hotbox subsystem  100 , as best seen in  FIGS. 3A and 3B , the hotbox subsystem  100  includes an insulating enclosure  102 , a flow manifolding structure  101 , a number of solid oxide fuel cell stacks  106 , a reformer  105 , and a cylindrical cathode recuperator heat exchanger  107 . In the illustrated embodiment, the reformer  105  is of a cylindrical monolithic catalytic reactor type and is located at the center of the hotbox subsystem  100 . The anode feed injection system  17  is located at the top of the hotbox subassembly  100  in the illustrated embodiment, and is connected to the reformer  105  in such a manner as to allow fluids to flow from the injection system  17  to the reformer  105 . A cylinder  108 , concentric with and larger in diameter than the cylindrical reformer  105 , is provided in order to isolate gas flows in the reformer from the air space inside the enclosure  102 . The cylinder  108  extends from the anode feed injection system  17  to the flow manifolding structure  101 , and is connected to both the flow manifolding structure  101  and the anode feed injection system  17  in order to prevent the leakage of flow. The connection between cylinder  108  and flow manifolding structure  101  is preferably a metallurgical bond, such as can be achieved by welding or brazing, although other methods of connection may also or alternatively be used. The connection between cylinder  108  and anode feed injection system  17  is can be a serviceable joint, such as, for example, a bolted flange connection with a suitable gasketing material. 
         [0037]    In the illustrated embodiment, the cylindrical heat exchanger  107  is larger in diameter than, and located concentric to, cylinder  108 , so that a first annular flow passage is created between the inner surface of cylindrical heat exchanger  107  and the outer surface of cylinder  108 . The illustrated embodiment can also or alternatively house a cylinder  109  which is larger in diameter than, and located concentric to, cylindrical heat exchanger  108 , so that a second annular flow passage is created between the outer surface of cylindrical heat exchanger  107  and the inner surface of cylinder  109 . 
         [0038]    As best seen in  FIGS. 3A ,  3 B, and  4 , the illustrated embodiment can also or alternatively include a top plate  129 , a first pair of parallel side walls  110 , and a second pair of parallel side walls  125  oriented perpendicular to the first pair of side walls  110 . The first pair of side walls  110 , second pair of side walls  125 , top plate  129 , cylindrical heat exchanger  107  and manifolding structure  101  are connected by a method, such as, for example, welding and/or brazing, so that a gas flow in the aforementioned first annular flow passage and a gas flow in the aforementioned second annular flow passage are kept isolated from one another. 
         [0039]    With reference to  FIG. 5A , a hydrocarbon fuel flow, shown schematically by arrow  113 , a reformer air flow, shown schematically by arrow  112 , and steam flow, shown schematically by arrow  114 , are delivered through separate plumbing lines (not shown) to the anode feed injection system  17 . In some embodiments, the hydrocarbon fuel flow  113  is a vapor. In other embodiments, the hydrocarbon fuel flow  113  is a liquid hydrocarbon and the anode feed injection system  17  is of a design capable of atomizing the fuel flow, including but not limited to a gas-assisted injector, multipoint impingement injector, piezoelectric injector, or other type of injector known to those skilled in the art of liquid fuel injection. The flow streams  112 ,  113 , and  114  together comprise a reformer feed stream shown schematically by arrow  115 . The reformer feed stream  115  passes through the catalytic reformer  105 , wherein the hydrocarbon fuel is chemically reformed by catalytic partial oxidation and steam reforming to produce a reformate flow which is comprised primarily of hydrogen (H 2 ), carbon monoxide (CO), carbon dioxide (C0 2 ), water vapor (H 2 O), and nitrogen (N 2 ). In some embodiments, the ratios of steam and of oxygen in the supplied air to carbon in the hydrocarbon fuel are regulated in order to provide a desired balance between the exothermic catalytic partial oxidation reaction and the endothermic steam reforming reaction, so that the temperature of the reformate exiting the catalytic reformer  105  is kept within a desired temperature range. As one example of such an embodiment, the hydrocarbon fuel flow  113  may include liquid diesel fuel, the atomic oxygen to carbon molar ratio may be maintained at approximately 1.0, and the steam to carbon molar ratio may be maintained at approximately 0.65. It should be noted that the desired steam to carbon and oxygen to carbon ratios can vary greatly depending on, among other factors, the type of hydrocarbon fuel and the type of catalyst used. Moreover, no limitation to the ranges or ratios of steam to carbon and oxygen to carbon is intended in this disclosure. In certain embodiments, the present invention can be operated without any steam flow to the reformer. 
         [0040]    The reformate flow, shown schematically by arrows  116 , enters the flow manifolding structure  101  and is distributed through the manifold structure to the fuel cell stacks  106 . The reformate flow  116  enters anode inlet manifolds internal to the fuel cell stacks  106 , wherein the reformate flow is distributed to the anode sides of the individual fuel cells that comprise the fuel cell stacks  106 . The anode exhaust gas, shown schematically by arrows  118 , is returned to the flow manifolding structure  101  by way of anode exit manifolds internal to the fuel cell stacks  106 , and is routed within the flow manifolding structure  101  to two anode exhaust ports  28 , through which the anode exhaust gas  118  is removed from the hot box subassembly  100 . 
         [0041]    With reference to  FIG. 5B , the partially preheated cathode air  119  enters the hot box subassembly  100  through a plurality of cathode air inlet ports  32  connected to the flow manifolding structure  101 . The flow manifolding structure  101  directs the partially preheated cathode air  119  to flow through the previously described second annular flow passage formed by the outer surface of cylindrical heat exchanger  107  and the inner surface of cylinder  109 . During operation of the fuel cells, substantial waste heat is generated by internal electrical resistances in the fuel cell stacks. This heat must be removed at a sufficient rate to maintain the stack operating temperature at a desired level. In order to accomplish this cooling, sufficient cathode air must be supplied to the fuel cell stacks  106 , and must be preheated to a temperature that is sufficiently high to prevent damage to the stacks due to thermal shock, but low enough to prevent overheating of the stacks. As the air flow  119  flows along the outer surface of cylindrical heat exchanger  107 , the air is further preheated to a temperature appropriate for the fuel cells. 
         [0042]    Sufficient space is provided between plate  129  and the top edge of cylinder  109  to allow the now fully preheated air flow  120  to return back down to the manifolding structure  101  through a flow area bounded by the outer surface of cylinder  109  and the inside surfaces of walls  110  and  125 . As the air flow moves along the walls  110 , it accomplishes some portion of the required stack cooling by removing heat that is radiated from the stacks  106  to the walls  110 , thereby also preventing distortion of the structure due to a difference in the thermal expansion of walls  110  relative to the other portions of the structure. The cathode air  120  is routed through the flow manifolding structure  101  to the fuel cell stacks  106 . As both the cathode air  120  and the reformate  116  move through the manifolding structure  101 , thermal energy is exchanged between them so that any temperature differences between the flow streams is reduced, thereby decreasing any thermal stress due to fluid temperature differences experienced by the fuel cell stacks  106 . 
         [0043]    In the embodiment illustrated in  FIG. 5B , the fuel cell stacks  106  are of an internally manifolded cathode type. The cathode air  120  is thus routed from the flow manifolding structure  101  to enter the cathode inlet manifolds internal to the fuel cell stacks  106 , which distribute the cathode air to the cathode sides of the individual fuel cells that comprise the fuel cell stacks  106 . The cathode exhaust, shown schematically by arrows  122 , is removed from the cathode exit manifolds internal to the fuel cell stacks  106  at the top portions of the stacks, where it enters the air space inside of the insulated enclosure  102 . The cathode exhaust  122  and ATO exhaust flow  121  ( FIG. 11 ) are combined in a mixing region  111 , best seen in  FIG. 3A , located between the plate  129  and the insulated enclosure  102 , to comprise an exhaust gas flow shown schematically by arrows  123 . The exhaust gas  123  flows through the previously described first annular flow passage formed by the inner surface of cylindrical heat exchanger  107  and the outer surface of cylinder  108 , wherein heat is convectively transferred to the cathode air  119  through the cylindrical heat exchanger  107 . The cooled exhaust gas, shown schematically by arrows  124 , is removed from the hotbox subassembly  100  through a plurality of exhaust ports  33  that are connected to the flow manifolding structure  101  and pass through the insulated enclosure  102 . 
         [0044]    In another embodiment illustrated in  FIG. 5C , the fuel cell stacks  106  are of an externally manifolded cathode type. In externally manifolded cathode fuel cells, the passages that deliver air to the cathodes of the individual fuel cells that comprise the fuel cell stack are all open to an inlet face of the stack and an opposite exit face of the stack. In this embodiment, a plurality of additional blocks  140  of ceramic or similar material are used to create an inlet air plenum  143  between stack inlet faces  141  and the inside end wall  117  of insulated enclosure  102  at either end of the hotbox subassembly  100 . Cathode air  120  enters the air inlet plenums  143  from the flow manifolding structure  101 , and flows through the cathode channels in the fuel cell stacks  106 . The cathode exhaust  122  exits the fuel cell stacks  106  and discharges into an exit plenum  144  between stack exit faces  142  and walls  110  at either end of the hotbox subassembly  100 , from where the cathode exhaust gas  122  is able to flow into the mixing region  111 . 
         [0045]    It should be appreciated that while it is desirable to minimize the amount of air leakage from the insulated enclosure  102 , an advantage of the present invention is that a small amount of air leakage from the insulated enclosure  102  is tolerable since the inner insulated enclosure  102  is contained within the outer insulated enclosure  10 . This minimizes the extent to which the inner enclosure  102  needs to be of a welded or equivalently sealed construction, thereby allowing for a lower cost of construction. It should further be appreciated that the structure as described minimizes the number of fluid connections that must be made, and allows for a thermally unconstrained design that obviates the need for thermal expansion bellows or similar features, thereby reducing the overall system cost. 
         [0046]    Turning now in greater detail to the construction of the flow manifolding structure  101 , as best seen in  FIG. 4  in the illustrated embodiment, the flow manifolding structure  101  includes a pair of stack mounting surfaces  130  upon which the fuel cell stacks  106  are supported. Each of the stack mounting surfaces  130  have one or more anode feed exit ports  127 , whereby the anode feed  116  is delivered from the flow manifolding structure  101  into the anode inlet manifolds internal to the fuel cell stacks  106 , and one or more anode exhaust inlet ports  128 , whereby the anode exhaust  118  is delivered from the anode exhaust manifolds internal to the fuel cell stacks  106  into the flow manifolding structure  101 . It should be appreciated that while two exit ports  128  and two inlet ports  127  are depicted for each fuel cell stack  106 , the number of such ports can be more than two or less than two, depending on the construction details of the fuel cell stacks. It should further be appreciated that the locations of the ports  128  and  127  can be at any location within the footprint of the fuel cell stacks  106 . In the illustrated embodiment, each of the stack mounting surfaces  130  of the flow manifolding structure  101  further includes one or more cathode air exits  126 , whereby the cathode air  120  is delivered from the flow manifolding structure  101  into the cathode inlet manifolds internal to the fuel cell stacks  106  or externally manifolded fuel cell cathode air inlet plenums  143 . 
         [0047]    With reference to  FIG. 6 , which shows some aspects of the construction of the flow manifolding structure  101  depicted in  FIG. 4  in greater detail, the flow manifolding structure  101  includes a laminated plate assembly  137  through which the anode flows  116  and  118  are routed on internal layers, the internal passages being capped by a top plate  138  of the laminated plate assembly  137 , and a bottom plate  139  of the laminated plate assembly  137 . In some embodiments, the laminated plate assembly  137  is fabricated as a leak-free structure by a nickel vacuum brazing process. The manifolding structure  101  is further comprised of a porous cathode air flow structure  130  that allows for the passage of the cathode air  120  with minimal pressure drop while simultaneously providing structural support for the fuel cell stacks  106 . In the embodiment illustrated in  FIG. 6 , the porous cathode air flow structure  130  includes a corrugated metal fin structure  133  with a top plate  131  and a bottom plate  132  metallurgically bonded to either side. The manifolding structure  101  can also or alternatively include a number of tubes  134  that are bonded to the laminated plate assembly  137  and pass through the porous cathode air flow structure  130 . The tubes  134  are fluidly connected to the internal passages within the laminated plate assembly  137 , and provide the anode feed exit ports  127  and anode exhaust inlet ports  128  for the flow manifolding structure  101 . 
         [0048]    In some embodiments, heat transfer surface enhancement features are incorporated on one or both sides of the cylindrical heat exchanger  107 .  FIG. 7  illustrates such an embodiment, with a first convoluted fin structure  146  metallurgically bonded to the inside surface of cylinder  107  to provide enhanced convective heat transfer for the exhaust gas  123  flowing there through, and with a second convoluted fin structure  145  metallurgically bonded to the outside surface of cylinder  107  to provide enhanced convective heat transfer for the cathode air  119  flowing there through. Although the heat transfer surface enhancement features illustrated in  FIG. 7  are of a serpentine plain fin type, it should be appreciated that any variety of heat transfer surface enhancements known to those skilled in the art, such as but not limited to louvered fins, herringbone fins and lanced and offset fins, can also or alternatively be employed. 
         [0049]    Turning now to the bottom surface of the flow manifolding structure  101 , as illustrated in  FIG. 8 , it can be seen that the bottom plate  139  of the flow manifolding structure  101  contains a number of air inlet ports  32  in a predominantly circular arrangement through which the cathode air  119  enters the hot box subassembly  100 , and a plurality of exhaust ports  33  in a predominantly circular arrangement located concentric to and radially inward from the arrangement of air inlet ports  32  through which the exhaust gas  124  exits the hot box subassembly  100 . The bottom plate  139  of the flow manifolding structure  101  further contains two anode exhaust ports  28  through which the anode exhaust gas exits the hot box subassembly  100 . The bottom plate  139  of the flow manifolding structure  101  further contains a plurality of structural supports  147  formed out of bent sheet metal. In some embodiments, one of the structural supports  147  is located more or less directly beneath each one of the fuel cell stacks  106 . It some embodiments, the ports  28 ,  32 , and  33  and the structural supports  147  provide only a minimal pathway for the undesirable conduction of heat out of the high temperature hotbox subassembly  100 . 
         [0050]    It should be noted that, while the illustrated embodiments show two fuel cell stacks  106  side by side at either end of the hotbox subassembly  100 , the invention is not limited in this regard and more or fewer fuel cell stacks can be implemented without affecting the merits of the invention. 
         [0051]    The construction of the heat exchange/flow manifolding/structural support component  20  will now be described in greater detail. Principal aspects of component  20  will be explained with reference to  FIGS. 9 and 10 , which illustrate the heat exchange/flow manifolding/structural support component  20  along with the laminated plate assembly  137  and the bottom portion of the insulating enclosure  102  in an upside-down orientation consistent with the orientation of  FIG. 8 . The heat exchange/flow manifolding/structural support component  20  can be formed from austenitic stainless steel construction, and includes a top plate  50 , a bottom plate  40 , two side walls  36  and two end walls  35 . Although not fully illustrated, it should be understood that the top plate  50  is in direct contact with the surfaces  103  of the structural supports  147  illustrated in  FIG. 8 . The heat exchange/flow manifolding/structural support component  20  can also or alternatively include two support legs  39 , which provide the air space  49  below the heat exchange/flow manifolding/structural support component  20 . The bottom plate  40  contains a centrally located circular opening  37 , through which the exhaust flow  124  enters the air space  49  from a cylindrical plenum  26  bounded by the top plate  50  and a cylindrical wall  34 . The plurality of tubes  33  are attached to the top plate  50  in such a manner as to prevent leakage, and allow the exhaust gas flow  124  to enter the cylindrical plenum  26  from the hotbox subassembly  100 . 
         [0052]    The heat exchange/flow man folding/structural support component  20  contains a pair of cathode air preheater heat exchangers  23  to preheat the cathode air  46  by transferring heat from the anode exhaust gas flow  118 . Although it should be understood that the heat exchangers  23  can be of many different types of heat exchanger construction known to those skilled in the art, one embodiment is illustrated in  FIG. 10 . The illustrated embodiment includes a number of tubes  31  through which the cathode air flow  46  passes. The heat exchange/flow manifolding/structural support component  20  includes an air inlet opening  27  to provide entry of the cathode air flow  46  into the structure  20  from the air inlet pipe  21 . The cathode air flow  46  fills an air space  24  around the inside periphery of the structure  20 , which distributes the flow  46  to the inlets of the heat exchange tubes  31 . The anode exhaust gas flow  118  enters the heat exchange/flow manifolding/structural support component  20  from the hotbox subassembly  100  through the two anode exhaust tubes  28 . The two anode exhaust tubes  28  connect to inlet tanks  29  on the two heat exchangers  23  and flow over the outsides of the heat exchange tubes  31 , transferring heat to the cathode air  46 . The anode exhaust exits the heat exchangers  23  as a cooled anode exhaust flow  51  through exit tanks  30 . The cooled anode exhaust flow  51  subsequently flows into the piping  22 , which brings the anode exhaust flow  51  out of the heat exchange/flow manifolding/structural support component  20  and out of the high temperature subsystem  9  through the insulating enclosure  10 . Although the anode exhaust flow  118  enters the structure  20  at a temperature approximately equal to the temperature of the fuel cell stacks  106 , the components  28  and  29  through which the anode exhaust gas flow  118  passes are located directly within the air space  24  through which the cold cathode air  46  passes. As a result, the temperature of components  28 ,  29  and the other metallic components within the structure  20  which are exposed to the anode exhaust flow  118  can be maintained at a temperature below the acceptable temperature limit for austenitic stainless steel. 
         [0053]    In some embodiments, the heat exchangers  23  include heat transfer surface augmentation features attached to the inside surfaces of the heat transfer tubes  31 . In these and other embodiments, the heat exchangers  23  can include heat transfer surface augmentation features attached to the outside surfaces of the heat transfer tubes  31 . 
         [0054]    The heat exchange/flow manifolding/structural support component  20  further contains an air exit plenum  25  comprised of the exit faces of the heat exchangers  23 , first and second side walls  37 ,  38  spanning the distance between the two heat exchangers  23 , the top plate  50  and the cylindrical wall  34 . The partially preheated cathode air flow  119  flows from the heat exchanger tubes  31  into the air exit plenum  25 . The plurality of air inlet ports  32  are attached to the top plate  50  in such manner as to prevent leakage, and provide for a fluid connection to the air exit plenum  25 , allowing the partially preheated cathode air flow  119  to exit the heat exchange/flow manifolding/structural support component  20  and enter into the hotbox subassembly  100 . 
         [0055]    Certain componentry required for operation of the fuel cell system, such as the fluid connections between some of the components within the high temperature subsystem  9  and the electrical buswork that electrically connects the fuel cell stacks to the remainder of the fuel cell system, have not been expressly described within this detailed description and the accompanying drawings, but it should be understood that these and other elements can also or alternatively be included within the high temperature subsystem  9  of one or more embodiments of the present invention. In some embodiments of the invention, all or substantially all of the required fluid and other penetrations through the insulated outer enclosure  10  are located on a common face of the enclosure  10  to facilitate assembly and sealing of the high temperature subsystem  9 . 
         [0056]      FIG. 11  is a schematic representation of the previously described high temperature subassembly  9  within a fuel cell system  1 , and showing the various flows through the high temperature subassembly  9  in relation to each of the major components of the high temperature subassembly  9 .  FIG. 11  also shows an anode exhaust condenser  3  as an additional component in the fuel cell system  1  that can be employed to condense and remove water vapor formed by the fuel cell anode reactions from the anode exhaust stream  51  exiting the high temperature subassembly  9 , after which the now cooled and condensed anode exhaust flow  47  is returned to the high temperature subassembly  9  to be combusted in the anode tailgas oxidizer  12 .  FIG. 11  also shows a water reservoir  4  to receive the condensed water from the condenser  3 , and a water pump  5  to provide a flow of water  48  to the water vaporizer  16  from the water reservoir  4 . In a preferred embodiment, the rate at which water is recovered from the condenser  3  exceeds the flow rate at which the water flow  48  is supplied to the vaporizer  16 , so that the fuel cell system  1  can be operated in a water-neutral state, that is a state in which a store of makeup water is not required for proper operation of the fuel cell system  1 .  FIG. 11  also shows an optional fuel tank  7  and fuel pump  2  to provide a fuel flow  113  to the anode feed injection system  17  in the high temperature subsystem  9 . Additionally shown in  FIG. 11  is an exhaust heat recovery device  6  that receives the exhaust flow  42  from the high temperature subsystem  9  and extracts product heat such as for space heating or other heating use, and produces a fully cooled exhaust flow  43  which is exhausted from the fuel cell system  1 . 
         [0057]    The embodiments described above and illustrated in the figures are presented by way of example only and are not intended as a limitation upon the concepts and principles of the present invention. As such, it will be appreciated by one having ordinary skill in the art that various changes are possible.