Abstract:
A technique includes operating a fuel cell, which produces an effluent flow. The technique includes routing the effluent flow through an electrochemical pump to extract fuel from the effluent flow and providing the extracted fuel to the fuel cell.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to U.S. patent application Ser. No. 11/295,704, entitled, “HIGH EFFICIENCY FUEL CELL SYSTEM,” which is filed concurrently herewith. 
     BACKGROUND 
     The invention generally relates to a high efficiency fuel cell system. 
     A fuel cell is an electrochemical device that converts chemical energy directly into electrical energy. For example, one type of fuel cell includes a proton exchange membrane (PEM), that permits only protons to pass between an anode and a cathode of the fuel cell. Typically PEM fuel cells employ sulfonic-acid-based ionomers, such as Nafion, and operate in the 60° Celsius (C) to 70° temperature range. Another type employs a phosphoric-acid-based polybenziamidazole, PBI, membrane that operates in the 150° to 200° temperature range. At the anode, diatomic hydrogen (a fuel) is reacted to produce hydrogen protons that pass through the PEM. The electrons produced by this reaction travel through circuitry that is external to the fuel cell to form an electrical current. At the cathode, oxygen is reduced and reacts with the hydrogen protons to form water. The anodic and cathodic reactions are described by the following equations:
 
H 2 →2H + +2e −  at the anode of the cell, and  Equation 1
 
O 2 +4H + +4e − →2H 2 O at the cathode of the cell.  Equation 2
 
     A typical fuel cell has a terminal voltage near one volt DC. For purposes of producing much larger voltages, several fuel cells may be assembled together to form an arrangement called a fuel cell stack, an arrangement in which the fuel cells are electrically coupled together in series to form a larger DC voltage (a voltage near 100 volts DC, for example) and to provide more power. 
     The fuel cell stack may include flow plates (graphite composite or metal plates, as examples) that are stacked one on top of the other, and each plate may be associated with more than one fuel cell of the stack. The plates may include various surface flow channels and orifices to, as examples, route the reactants and products through the fuel cell stack. Several PEMs (each one being associated with a particular fuel cell) may be dispersed throughout the stack between the anodes and cathodes of the different fuel cells. Electrically conductive gas diffusion layers (GDLs) may be located on each side of each PEM to form the anode and cathodes of each fuel cell. In this manner, reactant gases from each side of the PEM may leave the flow channels and diffuse through the GDLs to reach the PEM. 
     The fuel cell stack is one out of many components of a typical fuel cell system, as the fuel cell system includes various other components and subsystems, such as a cooling subsystem, a cell voltage monitoring subsystem, a control subsystem, a power conditioning subsystem, etc. The particular design of each of these subsystems is a function of the application that the fuel cell system serves. 
     Irrespective of external humidification, with any PEM fuel cell stack there are two challenges with regard to the flow of gas inside the fuel cell stack: 1.) inert gas buildup; and 2.) water buildup. In the case of a pure hydrogen-fueled stack, over time, nitrogen and other inert gases diffuse from the cathode (air) side of the membrane to the anode (fuel) side of the fuel cell membranes. If the inert gases are not removed from the anode side of the membranes, then operation of one or more cells or the entire stack is eventually interrupted. In the case of all PEM stacks, water may build-up in the anode and/or cathode flow channels of the stack and over time, thereby causing instability of the cell or stack of cells. This condition is called flooding. In order to prevent the flooding condition, sufficient anode and cathode gas velocity must be provided to clear the water from the flow channels. 
     Thus, there exists a continuing need for a fuel cell system that prevents significant buildup of water and inert gases in a fuel cell stack of the system. 
     SUMMARY 
     In an embodiment of the invention, a technique includes operating a fuel cell, which produces an effluent flow. The technique includes routing the effluent flow through an electrochemical pump to extract fuel from the effluent flow and providing the extracted fuel to the fuel cell. 
     Advantages and other features of the invention will become apparent from the following drawing, description and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is a schematic diagram of a fuel cell system according to an embodiment of the invention. 
         FIG. 2  is a schematic diagram of a hydrogen pump of  FIG. 1  according to an embodiment of the invention. 
         FIG. 3  is a schematic diagram of an anode exhaust subsystem for the hydrogen pump according to an embodiment of the invention. 
         FIG. 4  is a schematic diagram of a fuel cell stack of the hydrogen pump illustrating anode flows through the stack according to an embodiment of the invention. 
         FIG. 5  is a flow diagram depicting a technique to manage water inside a cascade of the hydrogen pump according to an embodiment of the invention. 
         FIG. 6  is an illustration of a flow plate of the hydrogen pump according to an embodiment of the invention. 
         FIG. 7  is a schematic diagram of a fuel cell stack of the hydrogen pump and thermal heating features of the stack according to an embodiment of the invention. 
         FIG. 8  is a schematic diagram of a combined power and hydrogen pump fuel cell stack illustrating anode flows of the stack according to an embodiment of the invention. 
         FIG. 9  is a schematic diagram of the combined power and hydrogen pump fuel cell stack illustrating cathode flows of the stack according to an embodiment of the invention. 
         FIG. 10  is a top view of an exemplary flow field plate of the power stack portion of the combined power and hydrogen pump fuel cell stack according to an embodiment of the invention. 
         FIG. 11  is a top view of an exemplary flow field plate of the hydrogen pump portion of the combined power and hydrogen pump fuel cell stack according to an embodiment of the invention. 
         FIG. 12  is a schematic diagram of a fuel cell system according to another embodiment of the invention. 
         FIG. 13  depicts waveforms of anode feedback flows of the fuel cell system of  FIG. 12  according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a fuel cell system  10  in accordance with embodiments of the invention includes a fuel cell stack (herein called a “power stack  20 ”) that produces electrical power for a load (not shown) in response to fuel and oxidant flows that are received by the power stack  20  at an anode intake inlet  22  and an oxidant intake inlet  24 , respectively. The fuel cell system  10  may include power conditioning circuitry  50  that is coupled to stack terminals  51  to convert the DC voltage of the power stack  20  into a regulated, lower DC voltage or to a regulated AC voltage, depending on the particular embodiment of the invention. Thus, the power conditioning circuitry  50  has output terminals  56  that provide the regulated DC or AC voltage to the load. 
     For purposes of ensuring that cells of the power stack  20  are not “starved” of fuel, the incoming fuel flow to the stack  20  exceeds the stoichiometric ratio that is set forth in Equations 1 and 2 above. Therefore, an anode exhaust flow (exiting the power stack  20  at an anode exhaust outlet  28 ) of the power stack  20  contains residual fuel. For purposes of recovering this residual fuel to improve the overall efficiency of the fuel cell system  10 , the system  10  includes an electrochemical hydrogen pump  30 . The hydrogen pump  30  1.) purifies the anode exhaust from the power stack  20  to produce a fuel feedback flow that is routed back to the anode intake inlet  22  of the stack  20 ; and 2.) establishes a fuel flow rate through the stack  20 , which is sufficient to keep the anode flow field channels of the stack  20  free of water blockages. 
     For purposes of simplifying the description herein, it is assumed that the power stack  20  and hydrogen pump  30  use polymer electrolyte membranes (PEMs). However, other embodiments of the invention are within the scope of the appended claims. For example, other types of fuel cell technologies other than PEM fuel cells are envisioned in other embodiments of the invention. Additionally, although an electrochemical hydrogen pump is described herein, it is understood that other types of electrochemical pumps may be used, in other embodiments of the invention. 
     In accordance with some embodiments of the invention, the hydrogen pump  30  is formed from a fuel cell stack that produces a relatively pure hydrogen flow at a cathode exhaust outlet  36  (of the pump  30 ) in response to the anode exhaust flow (received at an anode intake inlet  32  of the hydrogen pump  30 ) from the power stack  20  and received electrical power. In general, the hydrogen pump  30  may have the same overall topology of the power stack  30 , in that the hydrogen pump  30  contains PEMS, gas diffusion layers and flow plates that establish plenums and flow fields for communicating reactants to fuel cells of the hydrogen pump  30 . Furthermore, the hydrogen pump  30  may contain flow plates that route coolant through the pump  30 . However, unlike the power stack  20 , each fuel cell of the hydrogen pump  30  receives an electrical current (and serves as a load), and in response to the received current, hydrogen migrates from the anode chamber of the fuel cell to the cathode chamber of the fuel cell to produce hydrogen gas in the cathode chamber. 
     As described below, the fuel cell stack that forms the hydrogen pump  30  may be integrated with or separate from the power stack  20 , depending on the particular embodiment of the invention. Furthermore, as further described below, the fuel cell stack that forms the hydrogen pump  30  may be electrically connected to or isolated from the power stack  20 , depending on the particular embodiment of the invention. The hydrogen pump  30  is schematically depicted in  FIG. 1  as receiving its electrical power from electrical lines  54 . It is understood that the power to drive the cells of the hydrogen pump  30  may be furnished directly by a wide variety of different sources, such as the power stack  20  (for the case in which the cells of the stack  20  and the cells of the hydrogen pump  30  are part of the same stack), the power conditioning circuitry  50 , etc. 
     Among the other components of the fuel cell system  10 , the system  10  may include a hydrogen supply  11  (a hydrogen storage tank, for example) that has an outlet conduit  12 , which is connected to a pressure regulator  14 . The outlet of the pressure regulator  14  and the outlet of the hydrogen pump  30  are connected together to combine flows to produce the incoming fuel flow that is received ay the anode intake inlet  22  of the power stack  20 . The power stack  20  also includes a cathode exhaust outlet  26 , which may be connected to an oxidizer or a flare, in some embodiments of the invention. A controller  44  of the fuel cell system  10  may, for example, regulate the operation of the power conditioning circuitry  50 , as well as control operation of a flow control valve  40  that regulates when effluent is purged from the hydrogen pump  30 , as further described below. 
     The hydrogen pump  30  may be placed before or after the power stack  20  with respect to the direction that hydrogen is introduced into the system  10 , depending on the particular embodiment of the invention. It may be advantageous to place the hydrogen pump  30  such that the pump  30  receives inlet hydrogen first, as this may alleviate issues with startup. In accordance with some embodiments of the invention, the hydrogen pump  30  is immediately filled with hydrogen on power-up of the fuel cell system  10  so that the pump  30  can begin pumping. Another way to get around the startup issue is to command a flow control valve or auxiliary purge solenoid to fully open to release hydrogen from a single cell cascade stage  76  (described below in connection with  FIG. 2 ) while the cell is held at a constant, high voltage; and when the single cascade stage  76  cell sees hydrogen, its pumping voltage decreases, as the pump current simultaneously increases. At his point, the solenoid or flow control valve is commanded to close. It may also be advantageous to place the hydrogen pump  30  stack ahead of the power stack  20  for the purposes of scrubbing contaminants from a reformate flow. In such a configuration, waste hydrogen from the power stack, as well as fresh reformate enter the pump stack  20 , and only pure hydrogen returns to the power stack  20 . In this manner, the hydrogen pump  30  serves as both a purifier and a recirculator. 
     The use of an electrochemical hydrogen pump in an anode exhaust feedback loop of a power fuel cell stack may include one or more of the following advantages. Exhaust gas recirculation for water management may be accomplished without the disadvantage of nitrogen buildup. The hydrogen pump  30  provides in-situ filtration of the gases circulating in the anode loop. With mechanical recirculation of exhaust gas, if the feed-gas (hydrogen) is contaminated with a diluent or contaminating species, this gas can result in loss of power generation stack performance because the impurities are not directly removed. With the hydrogen pump  30 , however, the diluent is constantly removed, resulting in a purified anode hydrogen loop. Additionally, the hydrogen pump  30  is a solid state device. This has several significant advantages over conventional exhaust gas recirculation methods, which include such devices as blowers and compressors. Because there are no moving parts, reliability is higher (i.e., an advantage over blowers and compressors). Hydrogen pumping is a more isothermal process than mechanical compression and therefore can achieve a higher efficiency, meaning lower power consumption for system auxiliaries. This can be particularly true at low loads where the hydrogen pump  30  may be efficiently turned down to a low flow with a near-linear response in pumping voltage (whereas a blower or compressor can continue to draw significant power). Because the hydrogen pump  30  has no moving parts, the issues associated with gas leakage at blower or compressor seals are eliminated. The hydrogen pump  30  preferentially selects hydrogen over nitrogen to re-circulate, which a mechanical system cannot do. When built as an integrated device with the power stack  20  (as further described below), this arrangement has lower costs due to the elimination of piping or hose connections. 
     Other and different advantages are possible in the various embodiments of the invention. 
     In accordance with some embodiments of the invention, a 1.2 hydrogen stoichiometric flow is provided to the power stack  20  to ensure that fuel cells of the stack  20  are not “starved” of hydrogen. This means that the hydrogen pump  30  circulates approximately a 0.2 hydrogen stoichiometric flow back to the anode intake inlet  22  of the power stack  20 . 
     As a more specific example, in accordance with some embodiments of the invention, the power stack  20  and the hydrogen pump  30  are part of the same stack of bipolar flow plates. Thus, the same current flows through the cells of the power stack  20  and the cells of the hydrogen pump  30 . Assume for purposes of this example that the power stack  20  has seventy cells, and the hydrogen pump  30  has fourteen cells. For a 0.2 hydrogen stoichiometric flow through the hydrogen pump  30  at a current density of 0.6 amperes per square centimeter (amps/cm 2 ), the individual cell voltage of the hydrogen pump  30  may be approximately 0.06 volt. Thus, the power requirement for the hydrogen pump  30  is approximately 132 watts (for 262 cm 2  active area). If the individual cells are controlled to operate at 0.6 amp/cm 2  at 0.09 volts by restricting the anode exhaust flow out of the hydrogen pump, the power requirement for the hydrogen pump  30  increases to approximately 198 watts. 
     Referring to  FIG. 2 , instead of the above-described approach in which serially-connected cells are used to implement the hydrogen pump, in accordance with some embodiments of the invention, the hydrogen pump  30  is formed from the interconnection of cascade stages  60 ,  68  and  76  that are electrically coupled together in series and also receive reactant flows in series, as described below. Each cascade stage  60 ,  68  and  76  functions as an electrochemical hydrogen pump. Thus, the cascade stage  60  produces a flow of hydrogen from the anode exhaust stream from the power stack  20 , leaving a second anode exhaust stream; the cascade stage  68  produces a flow of hydrogen from the second anode exhaust stream, leaving a third anode exhaust stream; and the cascade stage  76  produces a flow of hydrogen from the third anode exhaust stream. The hydrogen flows from the cascade stages  60 ,  68  and  76  are combined and appear at the cathode exhaust outlet  36  of the hydrogen pump  30 . 
     The cascade arrangement overcomes the cell-to-cell anode flow distribution problem that occurs with a parallel gas flow and series electrical current flow configuration. Referring now to the more specific details of the interconnections, in the cascade arrangement, the anode intake inlet  32  delivers the anode exhaust gas from the power stack  20  to the anode plenum of the stage  60 . In response to the anode exhaust flow from the power stack  20 , the cascade stage  60  creates a hydrogen gas flow in the cathode plenum of the cascade stage  60 , and this flow is routed through a cathode exhaust outlet  64  of the cascade stage  60  to the cathode exhaust outlet  36  of the hydrogen pump  30 . The anode exhaust from the cascade stage  60  passes through an anode exhaust outlet  62  to the anode plenum of the next cascade stage  68 . 
     The cascade stage  68  recovers hydrogen from the incoming anode exhaust flow to form a hydrogen gas flow in the cathode plenum of the stage  68 ; and the hydrogen flow is routed through a cathode exhaust outlet  72  of the stage  68  to the cathode exhaust outlet  36  of the hydrogen pump  30 . The anode exhaust from the cascade stage  68  passes through an anode exhaust outlet  70  of the stage  68  to the anode plenum of the last cascade stage  76 . 
     The cascade stage  76  recovers hydrogen from the incoming anode exhaust flow to form a hydrogen gas flow in the cathode plenum of the stage  76 ; and the cascade stage  76  routes the hydrogen flow through a cathode exhaust outlet  78  of the stage  76  to the cathode exhaust outlet  36  of the hydrogen pump  30 . The anode exhaust from the cascade stage  76  is routed to. the anode exhaust outlet  34  of the hydrogen pump  30 . 
     Although  FIG. 2  depicts three cascade stages, the hydrogen pump  30  may have fewer or more stages, depending on the particular embodiment of the invention. Furthermore, each cascade stage may have a different number of cells, in accordance with some embodiments of the invention. Neither the current density, nor the active area, need be identical between the cascade stages Although convenient to build the cascade stages as an integrated stack of flow plates, additional scrubbing of hydrogen from the anode exhaust flow may be accomplished by pumping the flow through a small single cell, perhaps one with a 50 cm 2  active area, in some embodiments of the invention. 
     As a more specific example, in accordance with some embodiments of the invention, the cascade stage  60  contains ten cells, the cascade stage  68  contains three fuel cells and the stage  76  contains one fuel cell. The first two stages  60  and  68  may have, for example, a hydrogen stoichiometric flow in excess of 1.2, and the flow of the single cell stage  76  is controlled by the flow control valve  40  (see  FIG. 1 ). In this arrangement, all of the circulation flow of about 15.3 liters per minute passes through the anode chambers of the cascade  60 . At 0.6 amp/cm 2  (about 157 amps for this example) the cascade stage  60  pumps about 10.9 liters per minute of hydrogen back to the anode intake inlet  22  (see  FIG. 1 ) of the power stack  20 . The individual cells of the cascade stage  68  have cell voltages of about 0.06 volt each for a total power input into the cascade stage  60  of about 94 watts. 
     For the cascade stage  68 , an anode exhaust of about 4.5 liters per minute is directed to the anode chambers of the stage  68 . At 0.6 amp/cm 2  (about 157 amps for this example) the stage  68  pumps about 3.3 liters per minute of hydrogen back to anode intake inlet  22  of the power stack  20 . The individual cells of the cascade stage  68  have cell voltages of about 0.06 volt each for a power input of about 28 watts. 
     For the single cell, cascade stage  76 , the anode exhaust (about 1.2 liters per minute) from the stage  68  is directed to the anode intake inlet of the stage  76 . At a current density of 0.6 amp/cm 2  (about 157 amps for this example) the stage  76  pumps about 1.1 liters of hydrogen back to the fuel cell anode intake inlet  22 . The cell of the stage  76  has a voltage of about 0.11 volts for a power input of about 17 watts. The voltage of this cell is regulated by the controller  44  (see  FIG. 1 ) to be about 0.11 volt via the flow control valve  40 . Thus, in response to the cell voltage decreasing below some threshold voltage near 0.11 volts, the controller  44  opens the flow control valve  40  to purge gas from the cell to raise the voltage back to 0.11 volts. This arrangement may be replaced by an appropriately-sized bleed orifice, in other embodiments of the invention. In this case, the bleed is on the order of about 0.1 liters per minute with a hydrogen content of approximately 1%, or 1 cm 3  per minute. In some embodiments of the invention, this bleed flow is routed back to the cathode intake inlet  24  of the power stack  20 . 
     By using the above-described cascade arrangement, the anode flow distribution problem that occurs with a non-cascaded cell stack hydrogen pump is avoided, and the power that is needed to operate the hydrogen pump  30  is reduced from approximately 198 watts to approximately 139 watts (for the example described above). 
     Referring to  FIG. 3 , in accordance with some embodiments of the invention, a subsystem  80  may be alternatively used to vent exhaust from the cascade stage  76 . Thus, the subsystem may be connected to the exhaust outlet  34  of the hydrogen pump  30  in place of the flow control valve  40  (see  FIG. 1 ). The subsystem  80  includes a water trap  82  that is connected to the exhaust outlet  34  to remove water from the exhaust. An outlet  84  of the water trap  82  is connected to a flow restricting orifice  86 , and an outlet  88  of the orifice  86  is connected to a purge solenoid valve  90 , which may be controlled by the controller  44  (see  FIG. 1 ). An outlet  94  of the solenoid valve  90  is in communication with the ambient environment, in some embodiments of the invention. 
     In operation, the solenoid valve  90  is opened in response to a voltage of the fuel cell of the cascade stage  76  dropping below a predetermined threshold voltage. Otherwise, the solenoid valve  90  remains closed. In other embodiments of the invention, the solenoid valve  90  may transition between open and closed states at a certain duty cycle, and in some embodiments of the invention, the duty cycle may be controlled to regulate the voltage of the cell. Thus, many embodiments are possible and are within the scope of the appended claims. 
     Referring to  FIG. 4 , the cascade stages  60 ,  68  and  76  may be formed in the same fuel cell stack  31  in accordance with some embodiments of the invention.  FIG. 4  depicts the internal anode flow paths inside the stack  31 . For the cascade stage  60 , the flow plates have openings that align to collectively form a plenum  102  to communicate an incoming fuel flow  150  (i.e., the anode exhaust flow from the power stack  20 ) to the cells of the cascade  60 . The flow field plates of the cascade stage  60  also have openings that align to form a plenum  108  to communicate an anode exhaust flow  158  from the cascade stage  60 . 
     The anode exhaust plenum  108  is aligned with an anode intake plenum  109  of the cascade stage  68 . Thus, the anode exhaust flow  158  from the cascade  60  serves as the incoming anode flow for the cascade stage  68 . The flow  158  is communicated through the anode flow fields of the cascade stage  68 , and the cascade stage  68  includes an anode exhaust plenum  122  that communicates a resulting anode exhaust flow  154  from the cascade  68 . 
     The anode exhaust plenum  154  is aligned with an anode intake plenum  123  of the cascade stage  76 . Thus, the anode exhaust flow  154  from the cascade  68  serves as the incoming anode flow for the cascade stage  76 . The flow  154  is communicated through the anode flow fields of the cascade stage  76 , and the cascade stage  76  includes an anode exhaust plenum  134  that communicates a resulting anode exhaust flow  160  from the cascade  76  and to anode exhaust outlet  34  (see  FIG. 1 ) of the hydrogen pump  30 . 
     The use of the cascade arrangement may cause water to build up in the anode exhaust plenums in the middle cascade stages, such as the cascade stage  68 . Thus, water may collect in the plenum  122  and may possible cause instability in the cells of the cascade stages, which, in turn, may disrupt operation of the cascade stage  68 . One way to manage the water buildup is to make the flow plates of the cascade stage  68  relatively thick, as compared to the other flow plates of the hydrogen pump  30 . A relatively large plate thickness allows larger cross-sectional areas for the anode flow channels of the cascade stage  68 , thereby increasing the ability to accommodate water. 
     Referring to  FIG. 5 , therefore, in general a technique  180  to manage water in the hydrogen pump  30  includes forming (block  182 ) flow plates of the cascade stage  60  and  76  at a first thickness and forming (block  184 ) flow plates of the middle cascade stage  68  at a second, increased thickness. 
     Alternatively or in combination with the thicker flow plates, a membrane that wicks away liquid water but separates gas streams, might be used to transport liquid water that builds up in upper cascades down to lower cascades for removal. For example, a Supor™ brand membrane might be used for this application. However, other membranes may be used, in other embodiments of the invention. 
     Referring to  FIG. 6 , in accordance with some embodiments of the invention, a cascade separator plate  200  may be used between the cascade stages  60  and  68  for purposes of water management. The cascade separator plate  200  includes an opening  201  that is aligned with the anode exhaust plenum  122  (see  FIG. 4 ) of the cascade  68  and the anode intake plenum  102  of the cascade  60 . A membrane  202  that separates the anode plenums  102  and  122  resides in the opening  201  but separates the gas streams in the plenums  102  and  122 . The membrane  202  serves as a wick to collect water from the anode exhaust plenum  122  and route the water to the anode intake plenum  102 . As depicted in  FIG. 6 , a recessed region  204  may exist above the membrane  202  for purposes of creating a local region of low flow velocity to facilitate knock-out of water droplets. 
     Other variations may be used to collect water from the cascade stage  68  in other embodiments of the invention. For example, in other embodiments of the invention, a float valve or a U-trap may be used to collect water from the anode exhaust plenum  120 . As another example, a water leveling-sensing solenoid valve may be used to remove from the cascade stage  68 . 
     Measures may also be used to prevent the generation of water in the cascade stage  68 . More specifically, in accordance with some embodiments of the invention, precise thermal regulation may be used to prevent the accumulation of water in the hydrogen pump  30 . Although the hydrogen pump  30  technically generates heat because it is an electrical load, the stack that forms the pump  30  radiates more than enough heat to the surrounding environment to keep itself cool. Thus, the “coolant” flow to the hydrogen pump  30  actually serves to put heat into the stack, as the stack radiates more than enough heat to the surrounding environment to keep itself cool. Therefore, in accordance with some embodiments of the invention, thermal energy is applied to the hydrogen pump  30  to raise the pump&#39;s operating temperature to the dewpoint of hydrogen to minimize if not prevent the condensation of water. 
     Referring to  FIG. 7 , to accomplish this, heater pads  262  may distributed through a stack  250  in which the hydrogen pump  30  is formed. As depicted in  FIG. 7 , in some embodiments of the invention, the heater pads  262  are located at the cascade stage boundaries. However, in other embodiments of the invention, the heater pads  262  may be located between every cell of the stack are at a set cell spacing (every fourth cell, for example) throughout the stack. Thus, many variations are possible and are within the scope of the appended claims. 
     The hydrogen pump  30  may be heated in a number of different ways including but not limited to, the pad heaters that are located between, such as the pad heaters  262 ; pad heaters that surround the stack  250 ; and pad heater that heat the entire enclosure. Pad heaters may also be placed between cells as un-insulated resistive heaters, and the stack may be heated using current that passes through the resistive heaters. Passive heating may also be accomplished, for example, by using waste heat from the power stack  20 . Additionally, all of the exit coolant from the power stack  20 , at perhaps 5° C. higher than the inlet, may be fed to the stack of the hydrogen pump  30 , thereby raising the pump stack&#39;s operating temperature nearer to dewpoint of its inlet hydrogen stream. 
     In accordance with some embodiments of the invention, the hydrogen pump  30  and the power stack  20  may be integrated together in the same stack. Thus, although  FIG. 1  depicts explicit conduits  29  and  37  communicating exhaust and fuel input flows between the power stack  20  and the hydrogen pump  30 , these “conduits” may be internal plenums of the same stack in accordance with some embodiments of the invention. 
     As a more specific example,  FIG. 8  depicts internal anode flow paths of a stack  300  that forms the power stack  20  and the hydrogen pump  30  in accordance with some embodiments of the invention. Referring to  FIG. 8  in conjunction with  FIG. 4 , the anode intake plenum  102  of the cascade stage  60  is aligned with and is in fluid communication with an anode exhaust plenum  310  ofthe power stack  20 . An incoming anode flow  304  enters an anode intake plenum  308  of the power stack  20 , and the portion of the flow that is not consumed by electrochemical reactions forms the anode exhaust flow  150  that is processed by the hydrogen pump  30 . 
       FIG. 9  depicts internal cathode flow paths of the stack  300  in accordance with an embodiment of the invention. As shown, the power stack  20  includes a cathode intake plenum  374  that receives an oxidant intake flow  372 . The oxidant intake flow  372  is routed through the oxidant flow channels of the power stack  20  to produce an oxidant exhaust flow  378  in the cathode exhaust plenum  379 . A bleed flow from the anode exhaust of the hydrogen pump  30  is routed via an orifice  380  into the cathode intake plenum  374 . Alternatively, a bleed flow is routed from the anode exhaust of the hydrogen pump  30  to the cathode exhaust plenum  379 . 
       FIGS. 10 and 11  depict top views of exemplary flow plates  400  and  420  of the power stack  20  and hydrogen pump  30 , respectively, in accordance with some embodiments of the invention. Referring to  FIG. 10 , anode inlet  402  and anode outlet  412  plenum openings in the flow plate  400  are diagonally opposed to each other. Additionally, cathode inlet  406  and cathode outlet  408  plenum openings in the flow plate  400  are also diagonally opposed to each other; and the flow plate  400  includes a coolant inlet opening  404  and a coolant outlet opening  410 . Referring to both  FIGS. 10 and 11 , the anode inlet  402  of the flow plate  400  aligns with a cathode outlet  424  of the flow plate  420 ; the cathode inlet  406  of the flow plate  400  aligns with an anode outlet  428  of the flow plate  420 ; and the anode outlet  412  of the flow plate  400  aligns with an anode inlet  436  of the flow plate  420 . The hydrogen pump  30  in these embodiments of the invention has no plumbing connection directly above the cathode outlet  408  of the power stack  20 , so this plenum serves as an intermediary exchange port for transfers between cascades. 
     Referring to  FIG. 12 , in accordance with some embodiments of the invention, a fuel cell system  500  may be used in place of the fuel cell system  10  of  FIG. 1 . The fuel cell system  500  has common components with the fuel cell system  10  and is described with like reference numerals, with the differences being pointed out below. 
     Among these differences, the fuel cell system  500  includes an additional anode exhaust recirculation flow. In particular, the fuel cell system  500  uses a venturi  520  to establish another recirculation flow path between the anode exhaust outlet  28  of the power stack  20  and the anode intake inlet  22  of the stack  20 . In this regard, an inlet  508  of the venturi  520  is coupled to receive an incoming fuel flow, such as a fuel flow provided by the hydrogen supply  11 . An outlet  510  of the venturi  520  is connected to the anode intake inlet  22 , and a flow path or conduit  504  couples the anode exhaust outlet  28  to a feed inlet  509  of the venturi  520 . A pressure regulator  522  that remotely senses the pressure at the anode intake inlet  22  regulates the incoming fuel flow to the inlet  508 . 
     Due to this arrangement, a relatively constant feedback flow is created through the flowpath or conduit  37  from the cathode exhaust outlet  36  of the hydrogen pump  37 . This flow may be generally represented by a waveform  602  in  FIG. 13 , a waveform that depicts the hydrogen pump flow versus system power or “motive” flow, meaning the flow of fresh hydrogen fuel to the power stack  20 . Referring to  FIG. 12  in conjunction with  FIG. 13 , during the initial startup of the fuel cell system  500  when the system power is low, the feedback flow through the venturi  520  generally establishes the overall recirculation of anode exhaust back to the anode intake inlet  22  of the power stack  20 , as depicted by waveform  600 , which is the total recirculated anode exhaust that is fed back to the inlet  22 . 
     As can be seen from  FIG. 13 , during the initial startup of the fuel cell system  500 , the flow from the hydrogen pump  30  is relatively low (as compared to the feedback through the venturi  520 ). However, after the initial startup phase of the fuel cell system  500 , the feedback flow from the hydrogen pump  30  dominates to significantly improve the overall efficiency of the fuel cell system  500 . 
     While the invention has been disclosed with respect to a limited number of embodiments, those skilled in the art, having the benefit of this disclosure, will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of the invention.