Abstract:
A fuel cell system wherein a plurality of fuel cells are arranged in a series of stages, the number of fuel cells decreasing in number in each stage from anode gas inlet to the anode gas outlet. The system allows for parallel flow to all of the cells in a given stage and series flow between the various stages. A similar configuration is present on a cathode side of the system. However, the direction of flow is reversed, providing a greater number of cells in the stage nearest the cathode outlet and a fewer number of cells in the stage near the cathode gas inlet. The invention further provides for the various stages to be configured such that the direction of flow of the anode gas of a given stage is generally opposite the direction of flow of the cathode gas of a given stage.

Description:
FIELD OF THE INVENTION 
     The present invention relates to fuel cells and, more particularly to controlling the relative humidity, air, and fuel distribution within fuel cells. 
     BACKGROUND OF THE INVENTION 
     Fuel cells are used as a power source for electric vehicles, stationary power supplies and other applications. One known fuel cell is the PEM (i.e., Proton Exchange Membrane) fuel cell that includes a so-called MEA (“membrane-electrode-assembly”) comprising a thin, solid polymer membrane-electrolyte having an anode on one face and a cathode on the opposite face. The MEA is sandwiched between a pair of electrically conductive contact elements which serve as current collectors for the anode and cathode, which may contain appropriate channels and openings therein for distributing the fuel cell&#39;s gaseous reactants (i.e., H 2  and O 2 /air) over the surfaces of the respective anode and cathode. 
     PEM fuel cells comprise a plurality of the MEAs stacked together in electrical series while being separated one from the next by an impermeable, electrically conductive contact element known as a bipolar plate or current collector. In some types of fuel cells each bipolar plate is comprised of two separate plates that are attached together with a fluid passageway therebetween through which a coolant fluid flows to remove heat from both sides of the MEAs. In other types of fuel cells the bipolar plates include both single plates and attached together plates which are arranged in a repeating pattern with at least one surface of each MEA being cooled by a coolant fluid flowing through the two plate bipolar plates. 
     The fuel cells are operated in a manner that maintains the MEAs in a humidified state. The level of humidity of the MEAs affects the performance of the fuel cell. Additionally, if an MEA is run too dry, the MEA can be damaged which can cause immediate failure or reduce the useful life of the fuel cell. To avoid drying out the MEAs, the typical fuel cells are operated in a condition wherein the humidity of the MEA is greater than 100% and liquid water is formed in the fuel cell during the production of electricity. Additionally, the cathode and/or anode reactant gases being supplied to the fuel cell are also humidified to prevent the drying of the MEAs in the locations proximate the inlets for the reactant gases. 
     The operation of the fuel cells with the MEAs humidified greater than 100%, however, limits the performance of the fuel cell stack. Specifically, the formation of liquid water impedes the diffusion of gas to the MEAs, thereby limiting their performance. The liquid water also acts as a flow blockage reducing cell flow and causing even higher fuel cell relative humidity which can lead to unstable fuel cell performance. Additionally, the formation of liquid water within the cell can cause significant damage when the fuel cell is shut down and is exposed to freezing conditions. That is, when the fuel cell is nonoperational and the temperature in the fuel cell drops below freezing, the liquid water therein will freeze and expand, potentially damaging the fuel cell. 
     Thus, it would be advantageous to control and operate the fuel cell in a manner that prevents and/or limits the formation of liquid water therein. It would be further advantageous if such a control or operation of the fuel cell resulted in the MEA being operated at a humidified state that results in optimum performance. 
     Controlling the operating conditions within the fuel cell, however, has proved to be difficult. Specifically, the measuring and controlling of the humidity of the gaseous reactant streams flowing into the fuel cell can be difficult. Traditionally, a water vapor transfer device (WVT) is utilized to humidify the cathode reactant gas prior to entering into the fuel cell. The operation of the WVT, however, is difficult to characterize and, as a result, the exact humidity of the cathode reactant gas flowing into the fuel cell may be difficult to ascertain. The WVT device and associated hardware also adds cost and volume to the fuel cell system. Thus, it would also be advantageous to control and operate a fuel cell with a reduced or no need for a WVT. 
     SUMMARY OF THE INVENTION 
     The present invention includes a novel way of controlling the humidity of a fuel cell while at the same time providing a more efficient means of fuel and air distribution within the cell. Specifically, the present invention provides a system wherein a plurality of fuel cells are arranged in a series of stages, the number of cells decreasing in each stage from anode gas inlet to the anode gas outlet. The system allows for parallel flow to all of the cells in a given stage and series flow between the various stages. A similar configuration is present on the cathode side of the system. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  is a simplified schematic view of a fuel cell system showing cathode and anode gas flow passages according to the principles of the present invention; 
         FIG. 2  is a schematic representation of a partial fuel cell system illustrating the flow path of anode gasses in the system; 
         FIG. 3  is a schematic representation of a partial fuel cell system illustrating the flow path of cathode gasses in the system; and 
         FIG. 4  is a schematic representation of a fuel cell system including a tap stack. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description of the preferred embodiment is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
     With reference to  FIG. 1 , a schematic view is provided of a fuel cell system  10  constructed in accordance with the teachings of the present invention. The fuel cell system  10  includes a plurality of fuel cells  12 , each having a membrane electrode assembly (MEA)  14 , an anode gas distribution layer provided on an anode gas flow field  16  and a cathode gas distribution layer provided on a cathode gas-flow field  18 . The fuel cells  12  are arranged in a first stage  20  and a second stage  22 . An anode gas inlet manifold  24  provides an inlet for anode gas introduced to the fuel cell system  10 . An anode gas inlet/exhaust manifold  26  provides a connection for anode gas passing from the first stage  20  to the second stage  22 . An anode gas exhaust manifold  28  provides an outlet for anode gasses exiting the fuel cell system  10 . A cathode gas inlet manifold  30  provides an inlet for cathode gas introduced to the fuel cell system  10 . A cathode gas inlet/exhaust manifold  32  provides a connection for cathode gas passing from the second stage  22  to the first stage  20 . A cathode gas exhaust manifold  34  provides an outlet for cathode gasses exiting the fuel cell system  10 . The first stage  20 , by way of example, is comprised of four fuel cells  12  and the second stage  22  is comprised of two fuel cells  12 . The anode gas inlet manifold  24  is coupled to the first stage  20  and is in communication with the anode gas flow field  16  of each MEA  14 . The anode gas flow field  16  of each MEA  14  of the first stage  20  is coupled to the second stage  22  through the anode gas inlet/exhaust manifold  26 . The anode gas is then able to exit the second stage  22  through the anode gas exhaust manifold  28  which is in communication with the anode gas flow side  16  of each MEA  14  in the second stage  22 . The cathode gas inlet manifold  30  is coupled to the second stage  22  and is in communication with the cathode gas flow field  18 . The cathode gas flow field  18  of each fuel cell  12  of the second stage  22  is coupled to the first stage  20  through the anode gas inlet/exhaust manifold  32 . The cathode gas is then able to exit the first stage  20  through a cathode gas exhaust manifold  34  which communicates with the cathode gas flow field  18  of each fuel cell  12  of the first stage  20 . It should be noted that the inlet and outlet at the cathode side may be reversed, allowing cathode flow in generally the same direction as anode flow. 
       FIGS. 2 and 3  are simplified schematic views of the fuel cell system  10  with each Figure illustrating the separate anode and cathode sections, respectively. In  FIGS. 2 and 3 , the fuel cell system  10  includes a first stage  42 , a second stage  48  and a third stage  56 .  FIG. 2  depicts the anode section. The anode section includes an anode gas inlet valve  36 . One configuration of an anode gas inlet valve  36  would use a mechanical pressure regulator to reduce the fuel pressure from a fuel storage unit to control the fuel flow into the stack  10 . The regulator may be a dome-loaded design that will allow the pressure into the first stage to track the cathode inlet pressure entering the third stage  56 . The inlet valve  36  is in communication with the anode gas inlet manifold  38 , which is in communication with a series of anode flow field passages  44 , eight in the present example, in the first stage  42 . The anode flow field passages  44  are arranged in a parallel configuration, having an exit from the first stage  42  through a first anode gas inlet/exhaust manifold  46 . The first anode gas inlet/exhaust manifold  46  serves as an inlet to the second stage  48  in the fuel cell system  10 . 
     The first anode gas inlet/exhaust manifold  46  feeds a plurality of anode flow field passages  52 , four in the present example, in the second stage  48 . The number of anode flow field passages  52  in the second stage  48  is fewer in number than the number of anode flow field passages  44  in the first stage  42 . The remaining anode gasses from the second stage  48  exit the second stage  48  and travel to the third stage  56  through a second anode gas inlet/exhaust manifold  54 . 
     The remaining gasses then travel through a third plurality of anode flow field passages  58 , two in the present example, in the third stage  56 . The number of anode flow field passages  58  in the third stage  56  is fewer than the number of anode flow field passages  52  in the second stage  48 . The gasses passing through the anode flow field passages  58  of the third stage  56  may exit the system through an anode gas exhaust manifold  62 . An anode gas outlet valve  64  is in communication with the anode gas exhaust manifold  62  in order to assist in controlling system pressures. The anode gas outlet valve  64  may be an on/off solenoid or a proportional control valve. In the on/off configuration, the anode gas outlet valve  64  would be closed a majority of the time to allow inert gasses, which have diffused across the MEA  14  from the cathode or byproducts of fuel reforming, to build up in the third stage  56 . The anode gas outlet valve  64  is opened periodically to purge inert gasses and water from the third stage  56  based on either the voltage in the third stage  56  or by predicted inert gas concentrations in the third stage  56 . The cascaded design will result in an overall lower anode stack stoichiometry, compared to the stoichiometry in each stage of the stack. 
     The anode fuel gas is well humidified when it reaches the third stage  56 . Through diffusion and electro-osmonic drag, water vapor will cross over to the air in the cathode gas distribution layer  18  of the stack, reducing or eliminating the need for inlet cathode humidification. Using dry or partly humidified cathode gas, flowing counter flow in the third stage  56  will prevent the accumulation of water in the outlet of the anode gas distribution layer  16  of the stack due to water diffusion across the MEA  14  and improve stack performance. With the decreasing number of fuel cells  12  in each successive stage  42 ,  48 ,  56  of the system, the anode gas velocity will stay the same or increase from stage to stage. The stoichiometry of the anode gas may increase or decrease from stage to stage, depending on the number of cells in each stage. 
       FIG. 3  depicts the cathode section. The cathode section includes an inlet valve  66  in communication with a cathode gas inlet manifold  68  provided in the third stage  56 . The cathode gas inlet manifold  68  is in communication with the cathode gas flow field passages  72  of the third stage  56 . The third stage  56  contains a plurality of cathode flow field passages  72 , two in the present example. These passages are arranged parallel to one another, allowing gas to exit the third stage through a cathode gas inlet/exhaust manifold  76 . 
     The cathode gas inlet/exhaust manifold  76  is in communication with the second stage  48 . A second cathode gas inlet valve  84  is also in communication with the cathode gas inlet/exhaust manifold  76 . The second stage  48  includes a plurality of cathode flow field passages  82 , four in the present example, arranged in a parallel configuration. These cathode flow field passages  82  allow gasses to pass to the first stage  42  through a second cathode gas inlet/exhaust manifold  86 . 
     The second cathode gas inlet/exhaust manifold  86  is in communication with the first stage  42 . A third cathode gas inlet valve  90  is in communication with the second cathode gas inlet/exhaust manifold  86 . The first stage  42  contains a plurality of cathode flow field passages  92  arranged in a parallel configuration. The cathode flow field passages  92  communicate with a cathode gas exhaust manifold  94 . The cathode gas exhaust manifold  94  is in communication with a cathode gas exhaust valve  96 . The cascaded design should result in an overall lower cathode stack stoichiometry, compared to the stoichiometry in each stage of the stack. 
     The use of multiple cathode gas inlet valves  66 ,  84  and  90  provides for a lower cathode stack pressure drop than that in a cascaded system where all cathode flow is supplied to the third stage  56 . This overall lower cathode stack pressure drop will result in a reduced energy requirement for the pump/compressor within the fuel cell system  10 . It will also reduce the difference in pressure between the gas in the anode gas flow field passages and the gas in the cathode gas flow field passages in each cell  12  of the fuel cell system  10 . 
     Referring back to  FIG. 1 , water is produced in the cathode gas distribution flow field passages  18  through an electrochemical reaction in the fuel cell  12 . A portion of this water will diffuse across the MEA  14  to the anode, while the remaining water will exit each stage  20  and  22  in the cathode exhaust. The humidified exhaust oxidant gas of the upstream stage is mixed with the additional oxidant gas prior to entry into the downstream stage  20  thus humidifying the newly introduced cathode gas. Carrying the inert gasses from the upstream  22  to downstream stage  20  will also allow for increased velocity in the flow field channels of the downstream stage  20 . An increase of velocity in the oxidant flow field has been shown to improve the removal of water droplets and improve stack performance, especially at low power. In the first stage  20 , the last stage of cathode flow, the water vapor in the cathode stream will diffuse across the MEA  14  humidifying the anode inlet stream and eliminating the need for external humidification of the hydrogen stream. 
     Referring back to  FIG. 3 , the first valve  66  in communication with the cathode gas inlet manifold  68  may be removed and air may be used directly from the air delivery system. The cathode exhaust leaving the third stage  56  will mix with fresh oxidant, controlled by the second cathode gas inlet valve  84 , and enter the second stage  48 , thus humidifying the fresh oxidant. The second cathode gas inlet valve  84  is sized to allow for a large variation in cathode stoichiometry going into the second stage  48  and to have a pressure drop equal to the cathode pressure drop in the third stage  56 . The cathode flow leaving the second stage  48  will mix with the fresh oxidant controlled by the third cathode gas inlet valve  90  prior to entering the first stage  42 . The third cathode gas inlet valve  90  should also be capable of allowing a wide range of cathode stoichiometry entering the first stage  42  and should have a pressure drop equal to the cathode pressure drop across stages two  48  and three  56 . The cathode gas outlet valve  96  is used to control back pressure for the cathode stack. A further benefit of the multiple valve design is an increase in system control flexibility. For example, if a stage becomes unstable due to flooding, the cathode stoichiometry in the problem stage can be increased to remove liquid water and dry the problem stage out. 
     With reference to  FIGS. 2 and 3 , the anode and cathode flow field passages  44 ,  52 ,  58 ,  72 ,  82  and  92  are arranged such that the flow of the anode gasses in the anode flow field passages  44 ,  52 ,  58  are generally opposite the direction of flow of the cathode gasses in the cathode gas flow field passages  72 ,  82  and  92 . These generally opposite flow directions facilitate the passage of excess water between the anode and cathode flow field passages. This results in the benefit of reduction or even elimination of the need for external humidification. It should be noted that it is sufficient that the flow direction of the anode and cathode gasses is in opposite directions relative to each stage and that the opposite flow direction across each MEA, although beneficial, is not required to still obtain many of the benefits of the present invention. 
     Another advantage of the present system is cost savings.  FIG. 4  comprises a system to supply power to a secondary receiver  98  directly from the fuel cell system  10  at a voltage lower than that of the total system. An example of this would be supplying power directly to a 12 or 42 volt DC/DC converter  100  from a supply at a voltage lower than that of the total system voltage. To achieve this the cells in the smaller stages, stages two  48  and three  56  in the present example, can be used as a tap stack  102  to supply power at lower voltage to the 12 or 42-volt converters  100 . To be able to sustain higher currents in the tap stack  102 , a higher cathode and anode flow would be required for these cells. The valves  66 ,  84  and  90  at each of the cathode stages could be utilized to increase the flow of oxidant locally for the cells in the tapped region of the stack that is electrically producing higher current. Using a greater number of cells to supply the low voltage power will also reduce the additional current being drawn from the tap stack  102 . As a result, the total flow needed locally for the tap stack  102  region is reduced. 
     A fuel cell system  10  that incorporates a 12 or 42-volt battery in addition to the tap stack  102 , and low voltage DCDC converter  100  will also have additional control flexibility through local load control. It has been shown that stable performance can be achieved in a cell by quickly dropping the load while maintaining the reactant flows. When a large voltage battery buffer  104  is utilized in combination with the tap stack  102 , and low voltage DCDC converter  100 , the current being drawn from the tap stack  102  can be quickly dropped using the battery buffer  104  to buffer the consumed power. 
     The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.