Abstract:
A fuel cell system is disclosed including a fuel cell stack and pressure sensors, wherein bypass conduits having flow restriction devices disposed therein are provided for bypassing fluids around the fuel cell stack to militate against the accumulation of moisture in conduits in fluid communication with the pressure sensors.

Description:
FIELD OF THE INVENTION 
     The invention relates to a fuel cell system, and more particularly to a fuel cell system including a fuel cell stack and pressure sensors, wherein bypass conduits having flow restriction devices disposed therein are provided for bypassing fluids around the fuel cell stack to militate against the accumulation of moisture in conduits in fluid communication with the pressure sensors. 
     BACKGROUND OF THE INVENTION 
     Fuel cell assemblies convert a fuel and an oxidant to electricity. One type of fuel cell assembly employs a proton exchange membrane (hereinafter “PEM”) to facilitate catalytic reaction of fuels (such as hydrogen) and oxidants (such as air or oxygen) to generate electricity. The PEM is a solid polymer electrolyte membrane that facilitates transfer of protons from an anode to a cathode in each individual fuel cell normally deployed in the fuel cell assembly. 
     In a typical fuel cell assembly, individual fuel cell plates include channels through which various reactants, cooling fluids, and byproduct water formed by the reactants during operation of the assembly flow. When the fuel cell assembly is warmer than the ambient environment, water vapor in the fuel cell assembly may condense. In subzero ambient temperatures, the condensate may form ice in the fuel cell assembly. The presence of condensate and ice may affect the performance of the fuel cell assembly. 
     During operation of the fuel cell assembly, waste heat from the fuel cell reaction heats the assembly and militates against vapor condensation and ice formation in the assembly. However, condensate may flow through the system and accumulate in conduits throughout the fuel cell system, such as a conduit in fluid communication with a pressure sensor in the fuel cell system. Condensate blocking a fluid communication conduit to the pressure sensor may cause false pressure readings by the sensor resulting in a low reactant pressure within the fuel cell. Low reactant pressures can lead to an insufficient supply of the reactants needed to produce a required electrical output. Alternatively, false pressure readings by the sensors can result in a high reactant pressure. Pressure sensors are also susceptible to false readings when the fuel cell is operating at a subzero temperature. Frozen condensate can cause the false readings when the frozen condensate blocks communication between a reactant flow path and the sensor. 
     It would be desirable to develop a fuel cell system that militates against the accumulation of condensation or ice in a conduit in fluid communication with a pressure sensor, without affecting the pressure or stoichiometry of reactants flowing through a fuel cell stack of the fuel cell system. 
     SUMMARY OF THE INVENTION 
     Concordant and congruous with the present invention, a fuel cell system that militates against the accumulation of condensation or ice in a conduit in fluid communication with a pressure sensor, without affecting the pressure or stoichiometry of reactants flowing through a fuel cell stack of the fuel cell system, has surprisingly been discovered. 
     In one embodiment, the fuel cell system comprises a first fuel cell stack including at least a cathode inlet, a cathode outlet, an anode inlet, and an anode outlet, wherein an oxidant is caused to flow from a source of oxidant through the cathode inlet to the cathode outlet of said first fuel cell stack and a fuel is caused to flow from a source of fuel through the anode inlet to the anode outlet of said first fuel cell stack; a first bypass in fluid communication with the source of the fuel and the anode outlet; a second bypass in fluid communication with the source of the oxidant and the cathode outlet; a first pressure sensor in fluid communication with said first bypass and adapted to measure a pressure of the fuel caused to flow therethrough; and a second pressure sensor in fluid communication with said second bypass and adapted to measure a pressure of the oxidant caused to flow therethrough. 
     In another embodiment, the fuel cell system comprises a first fuel cell stack including at least a cathode inlet, a cathode outlet, an anode inlet, and an anode outlet, wherein an oxidant is caused to flow from a source of oxidant through the cathode inlet to the cathode outlet of said first fuel cell stack and a fuel is caused to flow from a source of fuel through the anode inlet to the anode outlet of said first fuel cell stack; a second fuel cell stack including at least a cathode inlet, a cathode outlet, an anode inlet, and an anode outlet, wherein the fuel is caused to flow from the anode outlet of said first fuel cell stack to the anode inlet of said second fuel cell stack and the oxidant is caused to flow from the source of oxidant to the cathode inlet of said second fuel cell stack; a first bypass in fluid communication with the source of fuel, the anode outlet of said first fuel cell stack, and the anode inlet of said second fuel cell stack; a second bypass in fluid communication with the source of oxidant and the cathode outlet of the first fuel cell stack and the second fuel cell stack; a first pressure sensor in fluid communication with said first bypass and adapted to measure a pressure of the fuel caused to flow therethrough, wherein a portion of the fuel is caused to flow through said first bypass past said first pressure sensor, and wherein the fuel flowing through said first bypass militates against the accumulation of moisture in said first bypass; and a second pressure sensor in fluid communication with said second bypass and adapted to measure a pressure of the oxidant caused to flow therethrough, wherein a portion of the oxidant is caused to flow through said second bypass past said second pressure sensor, and wherein the oxidant flowing through said second bypass militates against the accumulation of moisture in said second bypass. 
     In another embodiment, the method of operating a fuel cell system comprises the steps of providing a first fuel cell stack including at least a cathode inlet in communication with a source of oxidant, a cathode outlet, an anode inlet in communication with a source of fuel, and an anode outlet; providing a first bypass in fluid communication with the source of fuel; providing a second bypass in fluid communication with the source of oxidant; providing a first pressure sensor adapted to measure a pressure of a fuel caused to flow from the source of fuel through the first bypass, providing a second pressure sensor adapted to measure a pressure of an oxidant caused to flow from the source of oxidant through the second bypass; causing a portion of the fuel to flow through the first bypass past the first pressure sensor, wherein the fuel flowing through the first bypass militates against the accumulation of moisture in the first bypass; and causing a portion of the oxidant to flow through the second bypass past the second pressure sensor, wherein the oxidant flowing through the second bypass militates against the accumulation of moisture in the second bypass. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above, as well as other advantages of the present invention, will become readily apparent to those skilled in the art from the following detailed description of a preferred embodiment when considered in the light of the accompanying drawings in which: 
         FIG. 1  is a schematic flow diagram of a fuel cell system according to an embodiment of the invention; and 
         FIG. 2  is a schematic flow diagram of a fuel cell system according to another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The following detailed description and appended drawings describe and illustrate various exemplary embodiments of the invention. The description and drawings serve to enable one skilled in the art to make and use the invention, and are not intended to limit the scope of the invention in any manner. In respect of the methods disclosed, the steps presented are exemplary in nature, and thus, the order of the steps is not necessary or critical. 
       FIG. 1  shows a fuel cell system  10  according to an embodiment of the invention. A typical fuel cell system may include several system components including a humidifier, a compressor, an exhaust system, and a heat exchanger. Such a fuel cell system is disclosed in commonly owned U.S. patent application Ser. No. 11/684,906, hereby incorporated herein by reference in its entirety. The fuel cell system  10  includes a plurality of pressure sensors  12 ,  12 ′ in fluid communication with a fuel cell stack  14 , a first bypass  16 , and a second bypass  17 . 
     In the embodiment shown in  FIG. 1 , the pressure sensor  12  is adapted to measure a pressure of an oxidant caused to flow from a cathode side of the fuel cell stack  14 , while the pressure sensor  12 ′ is adapted to measure a pressure of a fuel caused to flow from an anode side of the fuel cell stack  14 . The pressure sensor  12 ′ is in fluid communication with the first bypass  16  which is in fluid communication with a fuel source  28 . The pressure sensor  12  is in fluid communication with the second bypass  17  which is in fluid communication with a humidifier  18 . The humidifier  18  is in further fluid communication with a source of oxidant  26 . Both of the pressure sensors  12 ,  12 ′ are in electrical communication with a controller  13 . The controller  13  is adapted to provide a signal or data indicative of the pressure measurements from the sensors  12 ,  12 ′ to a computer or an operator so that the fluid flows may be adjusted to maintain an optimal operation of the fuel cell stack  14 . The controller  13  may be any device adapted to receive a signal generated by the pressure sensors  12 ,  12 ′ such as a programmable logic controller (PLC), for example. It is understood that the oxidant may be any fluid containing oxygen such as air, for example. The fuel may be any fuel such as hydrogen, for example. 
     The humidifier  18  is a water vapor transfer unit adapted to humidify the oxidant prior to entering the fuel cell stack  14 . The water vapor transfer unit includes a dry side and a wet side, separated by a water vapor permeable membrane (not shown) or the like. The dry side has an inlet  18   a  and an outlet  18   b , and the wet side has an inlet  18   c  and an outlet  18   d . The inlet  18   a  is in fluid communication with a compressor  24 . The outlet  18   b  is in fluid communication with a cathode inlet  14   a  of the fuel cell stack  14 . The inlet  18   c  is in fluid communication with a cathode outlet  14   b  of the fuel cell stack  14  and the second bypass  17 . The outlet  18   d  is in fluid communication with an exhaust system  30 . 
     The compressor  24  includes an inlet  24   a  and an outlet  24   b . The inlet  24   a  of the compressor  24  is in fluid communication with the source of oxidant  26 , and the outlet  24   b  of the compressor  24  is in fluid communication with the inlet  18   a  of the humidifier  18  and the second bypass  17 . The source of oxidant  26  is typically a source of air. It is understood that the source of oxidant  26  may be an oxygen storage tank or the atmosphere, for example, as desired. The compressor  24  may be any conventional compressor such as a centrifugal air compressor, a turbomachine, a centrifugal compressor, a mixed flow compressor, a blower, and a fan, for example. 
     The fuel cell stack  14  includes a stack of fuel cells. It is understood that the number of fuel cells in the fuel cell stack  14  may vary. Each fuel cell of the fuel cell stack  14  has a membrane electrode assembly (MEA) separated by an electrically conductive bipolar plate. The MEAs and bipolar plates are stacked together between clamping plates or end plates and end contact elements. The end contact elements and bipolar plates contain a plurality of grooves or channels for distributing the fuel and the oxidant gases to the MEAs. 
     The fuel cell stack  14  includes the cathode inlet  14   a , the cathode outlet  14   b , an anode inlet  14   c , and an anode outlet  14   d . The cathode inlet  14   a  is in fluid communication with the outlet  18   b  of the humidifier  18 . The cathode outlet  14   b  is in fluid communication with the inlet  18   c  of the humidifier  18  and the second bypass  17 . The anode inlet  14   c  is in fluid communication with the fuel source  28 . The anode outlet  14   d  is in fluid communication with the exhaust system  30  and the first bypass  16 . The number of inlets and outlets in the fuel cell stack  14  may vary based on the size of the fuel cell stack  14  in use, the amount of outlet energy required from the fuel cell stack  14 , and other design considerations. It is understood that the fuel source  28  may be a hydrogen storage tank or other system component, for example. It is also understood that the anode outlet  14   d  may be in fluid communication with the atmosphere, another fuel cell stack (not shown), or other system component, as desired. 
     The first bypass  16  is a conduit providing a flow of a desired amount of fluid from the fuel source  28 , to the pressure sensor  12 ′, and to the exhaust system  30 , thereby bypassing the fuel cell stack  14 . The first bypass  16  may include a flow restriction device  32  adapted to restrict the flow of fluid therethrough. It is understood that the flow restriction device  32  may be any device adapted to restrict the flow of a fluid such as an orifice cap and an orifice spud, for example. 
     The second bypass  17  is a conduit providing a flow of a desired amount of fluid from the outlet  24   b  of the compressor  24 , to the pressure sensor  12 , and to the inlet  18   c  of the humidifier  18 , thereby bypassing a humidification in the humidifier  18  and the fuel cell stack  14 . The second bypass  17  may include a flow restriction device  34  adapted to restrict the flow of fluid therethrough. It is understood that the flow restriction device  34  may be any device adapted to restrict the flow of a fluid such as an orifice cap and an orifice spud, for example. 
     In use, hydrogen gas is caused to flow from the fuel source  28  through the conduit  36  to the anode inlet  14   c  of the fuel cell stack  14 . A portion of the hydrogen gas from the fuel source  28  is caused to flow through the first bypass  16  to the pressure sensor  12 ′ and then to the exhaust system  30 . The humidity of the hydrogen flowing through the bypass  16  to the pressure sensor  12 ′ is minimized because the humidification of the hydrogen by the product water formed in the fuel cell stack  14  as the hydrogen flows therethrough is bypassed. A pressure measured by the pressure sensor  12 ′ is substantially equal to the pressure of the fluid from the anode outlet  14   d  of the fuel cell stack  14  because the flow of hydrogen gas through the first bypass  16  is a fluid flow parallel to the primary hydrogen gas flow through the fuel cell stack  14  that begins at the anode inlet  14   c  and ends at the anode outlet  14   d . The flow restriction device  32  disposed in the first bypass  16  introduces a restriction to the flow of the hydrogen gas therethrough, thereby causing a pressure drop. The size of the flow restriction device  32  is optimized to minimize the flow of hydrogen gas though the first bypass  16  to ensure that the amount of hydrogen gas caused to flow therethrough does not significantly reduce the stoichiometry of the reactants caused to flow through the fuel cell stack  14 . Because a portion of the first bypass  16  is disposed between the pressure sensor  12 ′ and the conduit  36  in communication with the exhaust system  30 , the size of the flow restriction device  32  is optimized to account for the pressure drop across the portion of conduit of the first bypass  16  after the pressure sensor  12 ′ to maximize the accuracy of the pressure measurement by the pressure sensor  12 ′. 
     The hydrogen flowing through the anode outlet  14   d  of the fuel cell stack  14  may contain product water generated by the reaction therein. Accordingly, water may accumulate at a junction  16   a  of the bypass conduit  16  and from the anode outlet  14   d  through the conduit  36  to the exhaust system  30 . The accumulation of moisture at the junction  16   a  may block the flow of the dry fluid through the bypasses  16 . However, the hydrogen gas caused to flow through the first bypass  16  militates against the accumulation of moisture in the first bypass  16  by creating a continuous flow of fluid therethrough. The continuous flow of the dry fluid militates against an inflow of humidified fluid. Because moisture may accumulate at the junction  16   a  when fluid is not flowing through the first bypass  16 , such as when the fuel cell assembly  10  is powered down, the fluid flowing through the bypass conduit  16  will evaporate the moisture when the fluid is again caused to flow therethrough, thereby militating against the accumulation of moisture therein. 
     Simultaneous to the flow of hydrogen gas through the fuel cell assembly  10 , air is caused to flow from the source of oxidant  26  and through the conduit  36  to the inlet  24   a  of the compressor  24 . In the compressor  24 , the volume of the air is reduced, thereby increasing the pressure thereof. A portion of the air from the outlet  24   b  of the compressor  24  flows to the first inlet  18   a  of the humidifier  18  and through the dry side of the humidifier  18  for humidification thereof. In the humidifier  18 , air having a higher moisture content than the air flowing through the dry side is caused to flow through the wet side. Water vapor is transferred through the membrane to the air flowing through the dry side. The air in the wet side is caused to flow through the second outlet  18   d  of the humidifier  18  and to the exhaust system  30 . The air in the dry side is caused to flow through the first outlet  18   b  to the cathode inlet  14   a  of the fuel cell stack  14 . 
     Another portion of the air from the outlet  24   b  of the compressor  24  is caused to flow through the second bypass  17  to the pressure sensor  12  and then to the second inlet  18   c  of the humidifier  18 . The humidity of the air flowing through the second bypass  17  to the pressure sensor  12  is minimized because the humidifier  18  is bypassed, thereby bypassing humidification of the air therein. A pressure measured by the pressure sensor  12  is substantially equal to the pressure of the fluid from the cathode outlet  14   b  of the fuel cell stack  14  because the flow of air through the second bypass  17  is a fluid flow parallel to the primary air flow through the fuel cell stack  14  that begins at the cathode inlet  14   a  and ends at the cathode outlet  14   b . The flow restriction device  34  disposed in the second bypass  17  introduces a restriction to the flow of the air therethrough, thereby causing a pressure drop. The size of the flow restriction device  34  is optimized to minimize the flow of air though the second bypass  17  to ensure that the amount of air caused to flow therethrough does not significantly reduce the stoichiometry of the reactants caused to flow through the fuel cell stack  14 . Because a portion of the second bypass  17  is disposed between the pressure sensor  12  and the conduit  36 , the size of the flow restriction device  34  is optimized to account for the pressure drop across the portion of conduit of the second bypass  17  after the pressure sensor  12  to maximize the accuracy of the pressure measurement by the pressure sensor  12 . 
     The air flowing through the cathode side of the fuel cell stack  14  is humidified in the humidifier  18  prior to entering the fuel cell stack  14 . The air exiting the cathode outlet  14   b  of the fuel cell stack  14  may contain product water generated by reaction therein. Accordingly, water may accumulate at a junction  17   a  of the bypass conduit  17  and the conduit  36  to the wet side of the humidifier  18 . The accumulation of moisture at the junction  17   a  may block the flow of the dry fluid through the second bypasses  17 . However, the air caused to flow through the second bypass  17  militates against the accumulation of moisture in the second bypass  17  by creating a continuous flow of air therethrough. The continuous flow of the dry fluid militates against an inflow of humidified fluid. Because moisture may accumulate at the junction  17   a  when a dry fluid is not flowing through the second bypass  17 , such as when the fuel cell assembly  10  is powered down, the fluids flowing through the second bypass conduit  17  will evaporate the moisture when the fluids are again caused to flow therethrough, thereby militating against the accumulation of moisture therein. 
     The pressure measurement by the pressure sensor  12  and the pressure measurement by the pressure sensor  12 ′ are electrically communicated to the controller  13 . The controller  13  compares the pressure measurements from the sensors  12 ,  12 ′ and provides a signal or data indicative of the pressure measurements to a computer or an operator so that the reactant fluid flows may be adjusted to maintain the stoichiometry of reactants in the fuel cell stack  14  and to maintain an optimal operation of the fuel cell stack  14 . 
     In the fuel cell stack  14 , the oxygen in the air electrochemically reacts with the hydrogen to generate power to drive a vehicle or other system as is known in the art. Unreacted hydrogen is caused to flow out of the fuel cell stack  14 , through the anode outlet  14   d , and to the exhaust system  30 . Unreacted oxygen is caused to flow through the cathode outlet  14   b  to the atmosphere. 
       FIG. 2  shows a fuel cell system  110  according to another embodiment of the invention. The fuel cell system  110  includes a pressure sensor  112  in fluid communication with a first fuel cell stack  114 , a second fuel cell stack  115 , a first bypass  116 , and a second bypass  117 . 
     In the embodiment shown in  FIG. 2 , the pressure sensor  112  is adapted to measure a pressure of an oxidant caused to flow from a cathode side of the fuel cell stack  114 , while the pressure sensor  112 ′ is adapted to measure a pressure of a fuel caused to flow from an anode side of the fuel cell stack  114 . The pressure sensor  112 ′ is in fluid communication with the first bypass  116  which is in fluid communication with a fuel source  128  and an anode outlet  114   d  of the first fuel cell stack  114 . The pressure sensor  112  is in fluid communication with the second bypass  117  which is in fluid communication with a compressor  124  and a humidifier  118 . Both of the pressure sensors  112 ,  112 ′ are in electrical communication with a controller  113 . The controller  113  is adapted to provide a signal or data indicative of the pressure measurements from the sensors  112 ,  112 ′ to a computer or an operator so that the fluid flows may be adjusted to maintain an optimal operation of the fuel cell stacks  114 ,  115 . The controller  113  may be any device adapted to receive a signal generated by the pressure sensors  112 ,  112 ′ such as a PLC, for example. It is understood that the oxidant may be any fluid containing oxygen such as air, for example. The fuel may be any fuel such as hydrogen, for example. 
     The humidifier  118  is a water vapor transfer unit adapted to humidify the oxidant prior to entering the first fuel cell stack  114 . The water vapor transfer unit includes a dry side and a wet side, separated by a water vapor permeable membrane (not shown) or the like. The dry side has an inlet  118   a  and an outlet  118   b , and the wet side has an inlet  118   c  and an outlet  118   d . The inlet  118   a  is in fluid communication with the compressor  124 . The outlet  118   b  is in fluid communication with a cathode inlet  114   a  of the first fuel cell stack  114  and a cathode inlet  115   a  of the second fuel cell stack  115 . The inlet  118   c  is in fluid communication with the bypass conduit  117 , a cathode outlet  114   b  of the first fuel cell stack  114 , and a cathode outlet  115   b  of the second fuel cell stack  115 . The outlet  118   d  is in fluid communication with the exhaust system  130 . 
     The compressor  124  includes an inlet  124   a  and an outlet  124   b . The inlet  124   a  of the compressor  124  is in fluid communication with the source of oxidant  126 , and the outlet  124   b  of the compressor  124  is in fluid communication with the inlet  120   a  of the humidifier  118  and the second bypass  117 . The source of oxidant  126  is typically a source of air. It is understood that the source of oxidant  126  may be an oxygen storage tank or the atmosphere, for example, as desired. The compressor  124  may be any conventional means for compressing a fluid such as a centrifugal air compressor, a turbomachine, a centrifugal compressor, a mixed flow compressor, a blower or a fan, for example. 
     The fuel cell stacks  114 ,  115  each include a stack of fuel cells, as previously discussed herein. It is understood that the number of fuel cells in the fuel cell stacks  114 ,  115  may vary. Each fuel cell of the fuel cell stacks  114 ,  115  has a membrane electrode assembly MEAs (not shown) separated by an electrically conductive bipolar plate (not shown). The MEAs and bipolar plates are stacked together between clamping plates or end plates (not shown) and end contact elements (not shown). The end contact elements and bipolar plates contain a plurality of grooves or channels for distributing the fuel and the oxidant. 
     The first fuel cell stack  114  includes the cathode inlet  114   a , the cathode outlet  114   b , an anode inlet  114   c , and the anode outlet  114   d . The cathode inlet  114   a  is in fluid communication with the outlet  118   b  of the humidifier  118 . The cathode outlet  114   b  is in fluid communication with the inlet  118   c  of the humidifier  118  and the pressure sensor  112 . The anode inlet  114   c  is in fluid communication with a hydrogen source  128  and the first bypass  116 . The anode outlet  114   d  is in fluid communication with an anode inlet  115   c  of the second fuel cell stack  115  and the pressure sensor  112 . The number of inlets and outlets in the first fuel cell stack  114  may vary based on the size of the stack in use, the amount of outlet energy required from the stack, and other design considerations. It is understood that the hydrogen source  128  may be a fuel tank or other system component, for example, as desired. 
     The second fuel cell stack  115  includes the cathode inlet  115   a , the cathode outlet  115   b , the anode inlet  15   c , and an anode outlet  115   d . The cathode inlet  115   a  is in fluid communication with the outlet  118   b  of the humidifier  118 . The cathode outlet  115   b  is in fluid communication with the inlet  118   c  of the humidifier  118 . The anode inlet  115   c  is in fluid communication with the anode outlet  114   d  of the first fuel cell stack  114  and the first bypass  116 . The anode outlet  115   d  is in fluid communication with the exhaust system  130 . The number of inlets and outlets in the second fuel cell stack  115  may vary based on the size of the stack in use, the amount of outlet energy required from the stack, and other design considerations. It is understood that the anode outlet  115   d  may be in fluid communication with the atmosphere, another fuel cell stack (not shown), or other system component, as desired. 
     The first bypass  116  is a conduit providing a flow of a desired amount of fluid from the hydrogen source  128  to the pressure sensor  112 ′, thereby bypassing the first fuel cell stack  114 . The first bypass  116  may include a flow restriction device  132  adapted to restrict the flow of fluid therethrough. It is understood that the flow restriction device  132  may be any device adapted to restrict the flow of a fluid such as an orifice spud, for example, as desired. 
     The second bypass  117  is a conduit providing a flow of a desired amount of fluid from the compressor  124  to the pressure sensor  112 , thereby bypassing a humidification in the humidifier  118  and the fuel cell stacks  114 , 115 . The second bypass  117  may include a flow restriction device  134  adapted to restrict the flow of fluid therethrough. It is understood that the flow restriction device  134  may be any device adapted to restrict the flow of a fluid such as an orifice spud, for example, as desired. 
     In use, hydrogen gas is caused to flow from the hydrogen source  128  through the conduit  136  to the anode inlet  114   c  of the first fuel cell stack  114 . A portion of the hydrogen gas from the fuel source  128  is caused to flow through the first bypass  116  to the pressure sensor  112 ′ and to the anode inlet  115   c  of the second fuel cell stack  115 . The humidity of the hydrogen flowing through the bypass  116  to the pressure sensor  112 ′ is minimized because the humidification of the hydrogen by the product water formed in the first fuel cell stack  114  as the hydrogen flows therethrough is bypassed. A pressure measured by the pressure sensor  112 ′ is substantially equal to the pressure of the fluid from the anode outlet  114   d  of the first fuel cell stack  114  because the flow of hydrogen gas through the first bypass  116  is a fluid flow parallel to the primary hydrogen gas flow through the first fuel cell stack  114  that begins at the anode inlet  114   c  and ends at the anode outlet  114   d . The flow restriction device  132  disposed in the first bypass  116  introduces a restriction to the flow of the hydrogen gas therethrough, thereby causing a pressure drop. The size of the flow restriction device  132  is optimized to minimize the flow of hydrogen gas though the first bypass  116  to ensure that the amount of hydrogen gas caused to flow therethrough does not significantly reduce the stoichiometry of the reactants caused to flow through the first fuel cell stack  114 . Because a portion of the first bypass  116  is disposed between the pressure sensor  112 ′ and the conduit  136 , the size of the flow restriction device  132  is optimized to account for the pressure drop across the portion of conduit of the first bypass  116  after the pressure sensor  112 ′ to maximize the accuracy of the pressure measurement by the pressure sensor  112 ′ 
     The hydrogen flowing from the anode outlet  114   d  of the first fuel cell stack  114  may contain product water generated by the reaction therein. Accordingly, water may accumulate at a junction  116   a  of the bypass conduit  116  and from the anode outlet  114   d  through the conduit  136  to the exhaust system  130 . The accumulation of moisture at the junction  116   a  may block the flow of the dry fluid through the bypasses  116 . However, the hydrogen gas caused to flow through the first bypass  116  militates against the accumulation of moisture in the first bypass  116  by creating a continuous flow of fluid therethrough. The continuous flow of the dry fluid militates against an inflow of humidified fluid. Because moisture may accumulate at the junction  116   a  when dry fluids are not flowing through the first bypass  116 , such as when the fuel cell assembly  110  is powered down, the fluids flowing through the bypass conduit  116  will evaporate the moisture when the fluids are again caused to flow therethrough, thereby militating against the accumulation of moisture therein. 
     Simultaneous to the flow of hydrogen gas through the first fuel cell stack  114 , air is caused to flow from the source of oxidant  126  and through the conduit  136  to the inlet  124   a  of the compressor  124 . In the compressor  124 , the volume of the air is reduced, thereby increasing the pressure thereof. A portion of the air from the outlet  124   b  of the compressor  124  flows to the first inlet  118   a  of the humidifier  118  and through the dry side of the humidifier  118  for humidification thereof. In the humidifier  118 , air having a higher moisture content than the air flowing through the dry side is caused to flow through the wet side. Water vapor is transferred through the membrane to the air flowing through the dry side. The air in the wet side is caused to flow through the second outlet  118   d  of the humidifier  118  and to the exhaust system  130 . The air in the dry side is caused to flow through the first outlet  118   b  to the cathode inlet  114   a  of the first fuel cell stack  114 . 
     Another portion of the air from the outlet  124   b  of the compressor  124  is caused to flow through the second bypass  117 , to the pressure sensor  112 , and to the second inlet  118   c  of the humidifier  118 . The humidity of the air flowing through the second bypass  117  to the pressure sensor  112  is minimized because the humidifier  118  and the stacks  114 , 115  are bypassed, thereby bypassing humidification of the air therein. A pressure measured by the pressure sensor  112  is substantially equal to the pressure of the fluid from the cathode outlets  114   b ,  115   b  of the fuel cell stacks  114 ,  115  because the flow of air through the second bypass  117  is a fluid flow parallel to the primary air flow through the fuel cell stacks  114 ,  115  that begins at the cathode inlets  114   a ,  115   a  and ends at the cathode outlets  114   b ,  114   b . The flow restriction device  134  disposed in the second bypass  117  introduces a restriction to the flow of the air therethrough, thereby causing a pressure drop. The size of the flow restriction device  134  is optimized to minimize the flow of air though the second bypass  117  to ensure that the amount of air caused to flow therethrough does not significantly reduce the stoichiometry of the reactants caused to flow through the fuel cell stacks  114 ,  115 . Because a portion of the second bypass  117  is disposed between the pressure sensor  112  and the conduit  136 , the size of the flow restriction device  134  is optimized to account for the pressure drop across the portion of conduit of the second bypass  117  after the pressure sensor  112  to maximize the accuracy of the pressure measurement by the pressure sensor  112 . 
     The air flowing through the cathode side of the fuel cell stacks  114 ,  115  is humidified in the humidifier  118  prior to entering the fuel cell stacks  114 ,  115 . The air exiting the cathode outlets  114   b ,  115   b  of the fuel cell stacks  114 ,  115  may contain product water generated by reaction therein. Accordingly, water may accumulate at a junction  117   a  of the bypass conduit  117  and the conduit  136  to the wet side of the humidifier  118 . The accumulation of moisture at the junction  117   a  may block the flow of the dry fluid through the second bypasses  117 . However, the air caused to flow through the second bypass  117  militates against the accumulation of moisture in the second bypass  117  by creating a continuous flow of air therethrough. The continuous flow of the dry fluid militates against an inflow of humidified fluid. Because moisture may accumulate at the junction  117   a  when a dry fluid is not flowing through the second bypass  117 , such as when the fuel cell assembly  110  is powered down, the fluids flowing through the second bypass conduit  117  will evaporate the moisture when the fluids are again caused to flow therethrough, thereby militating against the accumulation of moisture therein. 
     The pressure measurement by the pressure sensor  112  and the pressure measurement by the pressure sensor  112 ′ are electrically communicated to the controller  113 . The controller  113  compares the pressure measurements from the sensors  112 ,  112 ′ and provides a signal or data indicative of the pressure measurements to a computer or an operator so that the reactant fluid flows may be adjusted to maintain the stoichiometry of reactants in the fuel cell stacks  114 ,  115  and to maintain an optimal operation of the fuel cell stack  114 ,  115 . 
     In the first fuel cell stack  114 , the oxygen in the air electrochemically reacts with the hydrogen to generate power to drive a vehicle or other system as is known in the art. Unreacted hydrogen is caused to flow out of the first fuel cell stack  114 , through the anode outlet  114   d , and to the anode inlet  115   c  of the second fuel cell stack  115 . Unreacted oxygen is caused to flow through the cathode outlet  114   b , through the conduit  136 , to the second inlet  118   c , and through the humidifier  118  to the exhaust system  130 . 
     In the second fuel cell stack  115 , the oxygen in the air electrochemically reacts with the hydrogen to generate power to drive a vehicle or other system as is known in the art. Unreacted hydrogen is caused to flow out of the second fuel cell stack  115 , through the anode outlet  115   d , and to the exhaust system  130 . Unreacted oxygen is caused to flow through the cathode outlet  115   b , through the conduit  136 , to the second inlet  118   c , and through the humidifier  118  to the exhaust system  130 . 
     The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.