Abstract:
In a solid-oxide fuel cell assembly, at least one positive displacement air supply (PDAS) pump supplies at least a portion of the air required for various functional air streams through the assembly. Mass air flow through each PDAS pump is readily controlled to a predetermined flow by controlling the rotational speed of the pump, obviating the need for an MAF sensor and control valve. Preferably, each different air stream through the assembly is controlled by its own PDAS pump, sized for the required flow, allowing each to operate at its optimal pressure and thereby minimizing the parasitic electrical cost of providing air to the SOFC assembly.

Description:
TECHNICAL FIELD 
   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 stack assemblies and systems comprising a plurality of individual cells wherein air is supplied by a single air supply pump; and most particularly, to such fuel cell assemblies and systems wherein the incoming air is split into two streams, one to supply a reformer/anode loop of the system and the other to supply the cathode air loop, each stream being supplied by an independent positive displacement air pump. 
   BACKGROUND OF THE INVENTION 
   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 from “reformed” hydrocarbons, 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. 
   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 usual 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 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 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. 
   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. A complete SOFC assembly also includes appropriate piping and valving, as well as a programmable electronic control unit (ECU) for managing the activities of the subsystems simultaneously. 
   For control purposes, it is important to know the total mass air flow into the assembly. In a typical prior art assembly, a high-volume air pump supplies air past at least one mass air flow (MAF) sensor which regulates an air control valve to provide a predetermined mass flow of air into one or more functional areas of the assembly and through the various subsystem branches. Where different flows are needed in different areas, additional MAF sensors and valves are required in parallel for each branch, adding significant cost and complexity to the assembly. Further, all air must be pumped at the highest pressure required by any one of the subassemblies, increasing parasitic energy consumption by the assembly. 
   It is a principal object of the present invention to simplify the construction and reduce the cost of a solid-oxide fuel cell assembly. 
   It is a further object of the present invention to reduce the parasitic energy consumption of such a solid oxide fuel cell assembly. 
   BRIEF DESCRIPTION OF THE INVENTION 
   Briefly described, in a solid-oxide fuel cell assembly, a positive displacement air supply (PDAS) pump supplies at least a portion of the air required. Mass air flow through a PDAS pump is readily controlled to a predetermined flow by controlling the rotational speed of the pump, obviating the need for an MAF sensor and control valve. Preferably, each different air stream through the assembly is controlled by its own PDAS pump, allowing each to operate at its optimal pressure, thereby minimizing the parasitic electrical cost of providing air to the SOFC assembly. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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: 
       FIG. 1  is a schematic cross-sectional view of a two-cell stack of solid oxide fuel cells; 
       FIG. 2  is a schematic elevational view of two fuel cell stacks electrically connected in series; 
       FIG. 3  is a schematic mechanization diagram of a typical SOFC assembly; 
       FIG. 4  is a portion of a mechanization diagram like  FIG. 3 , showing division of the air supply into a plurality of subsystems, each of which is supplied by a PDAS pump; and 
       FIG. 5  is an isometric view of the fuel cell stacks shown in  FIG. 2 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   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. 
   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 a first 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 a second 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 the first 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 the second 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 to yield the desired height of the anode passageways  24  and cathode passageways  26 . 
   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. 
   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  ions 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 A and B 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. 
   Referring to  FIGS. 2 and 5 , the cells  11  are arranged side-by-side rather than in overlapping arrangement as shown in  FIG. 1 . Further, the side-by-side arrangement may comprise a plurality of cells  11 , respectively, such that each of first stack  44  and second stack  46  shown in  FIG. 2  is a stack of identical fuel cells  11 . The cells  11  in stack  44  and stack  46  are connected electrically in series by interconnect  47 , and the stacks are connected in series. 
   Referring to  FIG. 3 , the diagram of a solid-oxide fuel cell assembly  12  includes auxiliary equipment and controls for stacks  44 , 46  electrically connected as in  FIG. 2 . 
   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 a cooling shroud  58  surrounding pump  48 . 
   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 stack afterburner  66  via a second MAF sensor  68  and a burner cool air control valve  70 . 
   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 . 
   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 . 
   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. 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 . 
   Referring to  FIG. 4 , the improvement conferred by the invention is highlighted by oval  124  (other assembly elements as shown in  FIG. 3  are omitted for clarity but are assumed to be present). In a solid-oxide fuel cell assembly  13  in accordance with the invention, the single prior art air pump  48  and shroud  58  shown in  FIG. 3  are replaced by a first positive displacement air pump  126 , having a high output capacity for providing air to the high-volume cathode air functions via a first branched conduit  74 ′, and a second positive displacement air pump  128 , having a more modest output capacity for providing air to the low-volume anode air functions via a second branched conduit  74 ″. Such pumps, wherein their rotational speed can be controlled to cause them to deliver a predetermined metered air flow, are known, for example, as “roots style microblowers”. Other types of positive displacement pumps are vane or piston type. Gone also are the prior art first MAF sensor  54 , second MAF sensor  68 , and third MAF sensor  78 . A single MAF sensor  130  may be employed optionally for control of the rotational speed, and hence output flow and pressure, of high-output pump  126 . Also eliminated is silencer  56 , as the two smaller blowers are less noisy in total than a single large high-capacity air pump. Further, in the prior art arrangement, the single blower supplies air to a central plenum (not shown on the schematic drawing) from which the individual branches depart, whereas the individual blowers feed into direct ducts without a plenum. A plenum is not suppressive of pump noise, whereas the individual ducts are excellent sound suppressors. 
   Of course, the use of only two such positive-displacement air pumps is only exemplary. The use of a larger plurality is within the scope of the invention; any number may be used in combination to meet additional gas-flow needs in any specific SOFC assembly, for example, to supply cooling air  64  to burner  66 , as described above for the assembly shown in diagram  12  but not shown in exemplary  FIG. 4 . 
   An SOFC assembly in accordance with the invention is especially useful as an auxiliary power unit (APU) for vehicles  132  on which the APU may be mounted as shown in  FIG. 4 , 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 the SOFC assembly. 
   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. 
   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.