FIELD OF THE INVENTION  
         [0001]    The present invention relates to fuel cell systems and, more particularly, to fuel cell systems that have fuel cells that produce an anode effluent.  
         BACKGROUND OF THE INVENTION  
         [0002]    H 2 —O 2 (air) fuel cells are well known in the art and have been proposed as a power source for many applications. There are several types of H 2  —O 2  fuel cells including acid-type, alkaline-type, molten-carbonate-type, and solid-oxide-type. So called PEM (proton exchange membrane) fuel cells [a.k.a. SPE (solid polymer electrolyte) fuel cells] are of the acid-type, potentially have high power and low weight, and accordingly are desirable for mobile applications (e.g., electric vehicles). PEM fuel cells are well known in the art, and include a “membrane electrode assembly” (a.k.a. MEA) comprising a thin, proton transmissive, solid polymer membrane-electrolyte having an anode on one of its faces and a cathode on the opposite face. The MEA is sandwiched between a pair of electrically conductive elements which (1) serve as current collectors for the anode and cathode, and (2) contain appropriate channels and/or openings therein for distributing the fuel cell&#39;s gaseous reactants over the surfaces of the respective anode and cathode catalysts. A plurality of individual cells are commonly bundled together to form a PEM fuel cell stack.  
           [0003]    In PEM fuel cells hydrogen is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can either be in a pure form (i.e., O 2 ), or air (i.e., O 2  admixed with N 2 ). The solid polymer electrolytes are typically made from ion exchange resins such as perfluoronated sulfonic acid. The anode/cathode typically comprise finely divided catalytic particles (often supported on carbon particles) admixed with proton conductive resin.  
           [0004]    During the conversion of the anode reactant and cathode reactant to electrical energy, the fuel cell, regardless of the type, produces anode and cathode effluents that are exhausted from the fuel cell. The anode effluent typically contains unused hydrogen that represents an unused source of energy. The cathode effluent typically contains excess oxygen or air that was not consumed during the electricity production in the fuel cell. The amounts of hydrogen and oxygen remaining in the anode and cathode effluents is dependent upon a number of factors and will vary. For example, the efficiency of the fuel cell can impact the amount of hydrogen and oxygen that are exhausted in the respective anode and cathode effluents. Additionally, the stoichiometry of the fuel cell stack (i.e., the amounts of hydrogen and oxygen that are included in the respective anode and cathode reactants) will also effect the amount of remaining hydrogen and oxygen in the respective anode and cathode effluents.  
           [0005]    The hydrogen in the anode effluent represents a source of energy that can be converted into a more usable form. Typical fuel cell systems employ a tail gas combustor to convert the hydrogen in the anode effluent into heat that can be used in other parts of the fuel cell system. However, the conversion of the excess hydrogen to heat may not be the most efficient use of the energy contained in the anode effluent. The tail gas combustor produces emissions that may require additional processing before the emissions can be vented to the environment. The heat generated by the combustor, may only be needed during certain aspects of operating the fuel cell system, such as at start up, and thereafter become a source of lost energy in the form of heat that must be dissipated from the fuel cell system. The tail gas combustor operates at high temperature. The use of a tail gas combustor also requires additional controls and/or control schemes that differ from the controls and/or control schemes to operate the fuel cells. All of the above considerations increase the complexity of a fuel cell system incorporating a tail gas combustor. Therefore, it would be desirable to convert the energy in the anode effluent into a more useful form without the necessity of creating excess heat, emissions and/or requiring additional and/or different controls/control schemes.  
         SUMMARY OF THE INVENTION  
         [0006]    The present invention eliminates the need for the tail gas combustion process in a fuel cell system. The invention allows the excess hydrogen in the anode effluent to be converted directly to low voltage electricity with minimal controls and no excess combustion heat or emissions. The low voltage generated can be used for battery charging and/or other ancillary power needs within the fuel cell system and/or an apparatus within which the fuel cell system is operating.  
           [0007]    A fuel cell system according to the principles of the present invention comprises a first fuel cell having a first anode inlet that receives a first anode reactant flow and a first cathode inlet that receives a first cathode reactant flow. The first fuel cell reacts the first anode and cathode reactant flows to produce electricity, a first anode effluent exhausted from a first anode outlet and a first cathode effluent exhausted from a first cathode outlet. A second fuel cell has a second anode inlet that receives a second anode reactant flow and a second cathode inlet that receives a second cathode reactant flow. The second fuel cell reacts the second anode and cathode reactant flows to produce electricity, a second anode effluent that is exhausted from a second anode outlet and a second cathode effluent that is exhausted from a second cathode outlet. The first anode outlet is in fluid communication with the second anode inlet so that a portion (partial or entire) of the second anode reactant flow received in the second fuel cell is formed from a portion (partial or entire) of the first anode effluent exhausted from the first fuel cell.  
           [0008]    The invention also discloses a method of converting an anode effluent exhausted from a primary fuel cell in a fuel cell system into an electrical current. The method includes the steps of: (1) routing a portion of the anode effluent exhausted from a primary fuel cell to a secondary fuel cell; (2) supplying the secondary fuel cell with a cathode reactant flow; and (3) converting the portion of the anode effluent and the cathode reactant flow to electricity in the secondary fuel cell.  
           [0009]    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  
       [0010]    The present invention will become more fully understood from the detailed description and the accompanying drawing, wherein:  
         [0011]    [0011] 
         [0012]    [0012]FIG. 1 is a schematic representation of a first preferred embodiment of the fuel cell system according to the principles of the present invention utilizing a single secondary fuel cell. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0013]    The following description of the preferred embodiments is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.  
         [0014]    Referring to FIG. 1, a fuel cell system  20  in accordance with the principles of the present invention is shown. The fuel cell system  20  has a primary fuel cell  22  that converts an anode reactant  24  and a cathode reactant  26  into electricity to power a primary electrical load  28 . The primary electrical load  28  can take a variety of forms depending upon the application within which the fuel cell system  20  is employed. For example, the primary electrical load  28  can be electric motors that are used to propel a vehicle, or other apparatuses that require an electrical current to be operated. The process of converting the anode reactant  24  and the cathode reactant  26  into electricity also produces an anode effluent  30  and a cathode effluent  32  that are exhausted from the primary fuel cell  22 . The anode reactant  24  is a fuel source that contains hydrogen (H 2 ) and the cathode reactant  26  is an oxidation agent that contains oxygen (O 2 ). The oxygen in the cathode reactant  26  can be in the form of pure O 2  or can be air (O 2  admixed with N 2 ). The anode reactant  24  supplies an anode feed stream or reactant flow  34  to the primary fuel cell  22  and the cathode reactant  26  supplies a cathode feed stream or reactant flow  36  to the primary fuel cell  22  that are converted to electricity. Cathode reactant flow  36  can be provided via an optional compressor  38 . The above described operation of the primary fuel cell  22  is known in the art and will not be described further.  
         [0015]    The H 2  that is supplied as the anode reactant  24  can come from a variety of sources. For example, the H 2  can come from a pure H 2  source, such as liquid hydrogen from a storage tank, or can be reformed from another fuel source, such as gasoline, methanol, ethanol, or other fuel sources as is known in the art.  
         [0016]    As was stated above, when the primary fuel cell  22  uses the anode reactant flow  34  and the cathode reactant flow  36  to produce electricity, an anode effluent  30  and a cathode effluent  32  are also produced and exhausted by the primary fuel cell  22 . Due to the operating conditions and fuel cell efficiencies, the anode effluent  30  typically contains unused H 2  and the cathode effluent  32  contains unused O 2 . The fuel cell system  20  of the present invention uses the anode effluent  30  to produce additional electricity for the fuel cell system  20  or the apparatus within which the fuel cell system  20  is employed. To accomplish this, the fuel cell system  20  employs a secondary fuel cell  40  that, like the primary fuel cell  22 , takes an anode reactant flow and a cathode reactant flow and converts them to electricity. That is, the secondary fuel cell  40  is provided with a secondary anode feed stream or reactant flow  42  and a secondary cathode feed stream or reactant flow  44  that the secondary fuel cell  40  converts into electricity that can be used to provide electrical current to a secondary electrical load  46  or a storage device such as a battery. The secondary fuel cell  40  uses the H 2 -containing anode effluent  30  exhausted by the primary fuel cell  22  as at least a portion of the secondary anode reactant flow  42  so that the unused H 2  within the anode effluent  30  can be converted to electricity. Preferably, the entire secondary anode reactant flow  42  is provided by the anode effluent  30 .  
         [0017]    Optionally, however, the secondary anode reactant flow  42  can be supplemented by the anode reactant flow  34 . That is, a portion  48  of the anode reactant flow  34  can be routed to the secondary fuel cell  40  as a part of the secondary anode reactant flow  42 . Preferably, a control valve  50  controls the portion  48  of the anode reactant flow  34  that is routed to the secondary fuel cell  40 . The control valve  50  is operated so that the amount of H 2  that is provided to the secondary fuel cell  40  from the anode reactant flow  34  via the portion  48  is low enough so that the secondary fuel cell  40  can convert most or all of the H 2  contained within the secondary anode reactant flow  42  into electricity.  
         [0018]    Optionally, but preferably, a control valve  52  is disposed between the primary fuel cell  22  and the secondary fuel cell  40  and controls the amount of anode effluent  30  that is exhausted by the primary fuel cell  22 . In operation, the control valve  52  can regulate the amount of anode effluent  30  exhausted from the primary fuel cell  22 . In one mode of operation, the control valve  52  is used to prevent the exhaust of anode effluent  30  from the primary fuel cell  22 . In this mode, the control valve  52  is closed while the primary fuel cell  22  converts the reactants within the primary fuel cell  22  into electricity and periodically opens to “burp” the primary fuel cell  22 . The burping of the primary fuel cell  22  is performed to increase the residence time of the anode reactant flow  34  and to increase the efficiency of the primary fuel cell  22  so that the anode effluent  30  contains a minimal amount of H 2 .  
         [0019]    Preferably, the secondary fuel cell  40  is supplied with a secondary cathode reactant flow  44  that is drawn from the ambient air within which the fuel cell system  20  is employed. When the secondary cathode reactant flow  44  is drawn from the ambient air, a blower (not shown) and/or other hardware may be needed to propel the flow of the ambient air into the secondary fuel cell  40  via the secondary cathode reactant flow  44 . A blower is preferred over the use of a compressor due to the decreased energy consumption associated with the operation of a blower. Optionally, the secondary cathode reactant flow  44  can be supplemented with a portion  54  of the cathode effluent  32  that is exhausted from the primary fuel cell  22 . The portion  54  of the cathode effluent  32  that is used to supplement the secondary cathode reactant flow  44  can be either an entire portion of the cathode effluent  32  or less than the entire portion of the cathode effluent  32 . When it is desired to provide less than the entire portion of the cathode effluent  32  to supplement the secondary cathode reactant flow  44 , a control valve  56  is preferably disposed between the cathode effluent  32  and the secondary cathode reactant flow  44 . The control valve  56  can regulate the amount of cathode effluent  32  that is used to supplement the secondary cathode reactant flow  44 .  
         [0020]    Alternatively, and/or additionally, the secondary cathode reactant flow  44  can also be supplemented from the cathode reactant flow  36  that is provided via the compressor  38 . That is, a portion  58  of the cathode reactant flow  36  can be routed to the secondary fuel cell  40  via the secondary cathode reactant flow  44 . When the secondary cathode reactant flow  44  is supplemented by the portion  58  of the cathode reactant flow  36 , a control valve  60  is disposed between the cathode reactant flow  36  and the secondary cathode reactant flow  44 . The control valve  60  controls the portion  58  of the cathode reactant flow  36  that is used to supplement the secondary cathode reactant flow  44 . Optionally, the secondary cathode reactant flow  44  can be provided entirely by the portion  54  of the cathode effluent  32  and/or the portion  58  of the cathode reactant flow  36 . A valve  61  can be provided on the ambient air intake to the secondary fuel cell  40 . Valve  61  can be closed to prevent portion  54  of cathode effluent  32  and/or portion  58  of cathode reactant flow  36  from being exhausted from fuel cell system  20  via the air intake when the secondary cathode reactant flow  44  is being supplemented by portion  54  of cathode effluent  32  and/or portion  58  of cathode reactant flow  36 . Valve  61  can be opened to allow secondary cathode reactant flow  44  to be drawn from the ambient air within which fuel cell system  20  is employed. Therefore, the secondary fuel cell  40  can be provided with a secondary cathode reactant flow  44  that is comprised of ambient air and/or the portion  54  of the cathode effluent  32  and/or the portion  58  of the cathode reactant flow  36 .  
         [0021]    As was stated above, the secondary fuel cell  40  uses the secondary anode reactant flow  42  and the secondary cathode reactant flow  44  to produce electricity to power the secondary electrical load  46 . The production of electricity within the secondary fuel cell  40  results in the production of a secondary anode effluent  62  and a secondary cathode effluent  64  that are exhausted from the secondary fuel cell  40 . The secondary anode and cathode effluents  62 ,  64  are exhausted to the environment within which the fuel cell system  20  is operating. The secondary fuel cell  40  is sized and/or operated so that all of the H 2  contained within the secondary anode reactant flow  42  is consumed during the electricity production within the secondary fuel cell  40  so that the secondary anode effluent  62  is substantially free of H 2 . The fuel system  20  will thereby consume a majority or all of the H 2  that is supplied by the anode reactant flow  34  and result in a fuel cell system  20  that exhausts little or no unused H 2 . As a result, the fuel system  20  does not employ a tail gas combustor to extract energy from the H 2  exhausted by conventional fuel cell systems  20 . Preferably, the secondary anode effluent  62  exhausted by the secondary fuel cell  40 , passes through a check valve  65  prior to being exhausted to the environment. The check valve  65  prevents back flow within the anode portions of the secondary fuel cell  40  and the primary fuel cell  22  and thereby prevents contamination of the secondary fuel cell  40  and the primary fuel cell  22 .  
         [0022]    Preferably, the primary fuel cell  22  is a PEM fuel cell. However, it should be understood that the primary fuel cell  22  can be any type of fuel cell that uses H 2  as a reactant and O 2  (or air) as an oxidant to produce electricity, and still be within the scope of the present invention. Preferably, the secondary fuel cell  40  is the same type of fuel cell as the primary fuel cell  22 . However, it is not necessary for the secondary fuel cell  40  and the primary fuel cell  22  to be the same type of fuel cell to be within the scope of the present invention. That is, the secondary fuel cell  40  can be a different type of fuel cell from the primary fuel cell  22  provided that the secondary fuel cell  40  also utilizes H 2  as a reactant and O 2  (or air) as an oxidant in the reaction within the secondary fuel cell  40  to produce electricity. Preferably, the secondary fuel cell  40  is sized to be a lower power fuel cell than the primary fuel cell  22 . Furthermore, the secondary fuel cell  40  can also be designed to operate differently from the primary fuel cell  22 . For example, the secondary fuel cell  40  may have different pressure requirements, temperature requirements, cooling requirements, efficiencies, etc.  
         [0023]    Regardless of the type or size of the primary and secondary fuel cells  22 ,  40 , the fuel cell system  20  is designed so that the secondary anode effluent  62  that is exhausted by the secondary fuel cell  40  is substantially free of H 2  so that the fuel cell system  20  is more efficient and very little or no H 2  provided by the anode reactant flow  34  is exhausted to the environment. The size of the primary fuel cell  22  and the secondary fuel cell  40  will be dependent upon the application within which the fuel cell system  20  is employed. That is, the primary fuel cell  22  will be sized to provide the primary electrical load  28  with enough electricity to operate within the design parameters and the secondary fuel cell  40  will be sized to provide a secondary anode effluent  62  that is substantially free of H 2 . Because it is preferred that the secondary fuel cell  40  be a lower power fuel cell than the primary fuel cell  22 , it is expected that the secondary fuel cell  40  will be used to provide electricity to a secondary electrical load  46  that is an ancillary component to the fuel cell system  20  and/or the apparatus within which the fuel cell  20  is operating which will require less power from the secondary fuel cell  40 . For example, the secondary electrical load  46  may be a battery that is contained within the apparatus within which the fuel cell system  20  is employed and the secondary fuel cell  40  is used to recharge the battery. However, it should be understood that the primary and secondary electrical loads  28 ,  46  will vary depending upon the application within which the fuel cell system  20  is employed and that the primary and secondary electrical loads  28 ,  46  can take on a variety of forms and still be within the scope of the present invention.  
         [0024]    The fuel cell system  20  illustrated in FIG. 1 utilized one secondary fuel cell  40 . Optionally, the fuel cell system  20  can employ multiple secondary fuel cells  40 . The plurality of secondary fuel cells  40  may be arranged in a parallel configuration, in a series configuration or in a combination of parallel and series configuration. In such an arrangement, each of the plurality of secondary fuel cells  40  are used as a source of electricity for a plurality of secondary electrical loads  46  and designed to consume substantially all of the H 2  supplied to the fuel cell system  20  by the anode reactant flow  34 . When a plurality of secondary fuel cells  40  are utilized, similar hardware and mechanization will be needed.  
         [0025]    When a parallel configuration is utilized, the plurality of secondary fuel cells  40  are arranged downstream of the primary fuel cell  22  and in parallel with one another such that the secondary fuel cells  40  divide the anode effluent  30  exhausted by the primary fuel cell  22  for use as an anode reactant flow to each of the secondary fuel cells  40 . More specifically, the anode effluent  30  exhausted by the primary fuel cell  22  is routed to all of the secondary fuel cells  40  so that each of the secondary fuel cells  40  use different portions of the anode effluent  30  as an anode reactant flow to produce electricity.  
         [0026]    When a series configuration is utilized, the secondary fuel cells  40  are arranged downstream of the primary fuel cell  22  with each of the secondary fuel cells  40  arranged in a series configuration with the anode effluent exhausted by a preceding upstream secondary fuel cell  40  used as an anode reactant flow for a subsequent downstream secondary fuel cell  40 . Each of the plurality of secondary fuel cells  40  also receives a cathode reactant flow.  
         [0027]    The plurality of secondary fuel cells  40  can all be the same type of fuel cell or can each be different types of fuel cells that use H 2  as an anode reactant and O 2  (or air) as a cathode reactant. Preferably, the plurality of secondary fuel cells  40  are lower power fuel cells than the primary fuel cell  22 .  
         [0028]    Referring to FIG. 1, the operation of the fuel cell system  20  of the present invention will now be discussed. The primary fuel cell  22  is provided with an anode reactant flow  34  from the anode reactant  24  and also a cathode reactant flow  36  from the cathode reactant  26  via the optional compressor  38 . The primary fuel cell  22  then converts the anode and cathode reactant flows  34 ,  36  into electricity to meet the primary electrical load  28 . The production of electricity within the primary fuel cell  22  produces anode and cathode effluents  30 ,  32 . The anode effluent  30  is routed to the secondary fuel cell  40  where it is used as a secondary anode reactant flow  42 . The routing of the anode effluent  30  from the primary fuel cell  22  to the secondary fuel cell  40  is controlled by the control valve  52  which can be used to “burp” the primary fuel cell  22 . Optionally, the secondary anode reactant flow  42  can be supplemented by routing a portion  48  of the anode reactant flow  34  directly to the secondary fuel cell  40  to mix with the anode effluent  30  exhausted from the primary fuel cell  22  to form the secondary anode reactant flow  42 . The supplementing of the secondary anode reactant flow  42  with a portion  48  of the anode reactant flow  34  is controlled by the control valve  50 . Typically, the secondary anode reactant flow  42  will not be supplemented with the portion  48  of the anode reactant flow  34 .  
         [0029]    The cathode effluent  32  produced by the primary fuel cell  22  is preferably exhausted to the environment and the secondary fuel cell  40  will use a secondary cathode reactant flow  44  that is air drawn from the environment within in which the fuel cell system  20  is employed. However, it may be desirable to supplement and/or replace the air that comprises the secondary cathode reactant flow  44  with the cathode effluent  32  and/or the cathode reactant flow  36 . Therefore, the cathode effluent  32  exhausted by the primary fuel cell  22  can be routed via control valve  56  to the cathode inlet of the secondary fuel cell  40  to act as part or all of the secondary cathode reactant flow  44 . Also, a portion  58  of the cathode reactant flow  36  can be routed to the cathode inlet of the secondary fuel cell  40  via control valve  60  to form a part or all of the secondary cathode reactant flow  44 . The secondary fuel cell  40  then uses the secondary anode reactant flow  42  and the secondary cathode reactant flow  44  to produce electricity. The production of electricity within the secondary fuel cell  40  produces secondary anode and cathode effluents  62 ,  64  that can be exhausted to the environment. The secondary anode effluent  62  is routed through a check valve  65  to prevent back flow contamination of the secondary fuel cell  40  and the primary fuel cell  22 .  
         [0030]    The secondary fuel cell  40 , is designed to enable the secondary fuel cell  40  to consume most or all of the H 2  contained within the secondary anode reactant flow  42  so that the secondary anode effluent  62  exhausts by the secondary fuel cell  40  is substantially free of H 2 . The fuel cell system  20  thereby provides a means of using most or all of the H 2  exhausted by the primary fuel cell  22  in the anode effluent  30  to produce useful energy in the form of electricity.  
         [0031]    The above described fuel cell system  20  made according to the principals of the present invention provides a fuel cell system  20  that can meet the primary electrical load  28  while producing a secondary anode effluent  62  that contains very little or no unused H 2 .  
         [0032]    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.
A fuel cell system having first and second fuel cells that each receive anode reactant flows and cathode reactant flows. Each of the fuel cells uses the reactant flows to produce electricity. The electricity production by the fuel cells produces respective first and second anode and cathode effluents that are exhausted from the respective fuel cells. The second fuel cell is connected to and downstream from the first fuel cell so that the anode reactant flow to the second fuel cell is formed from a portion of the anode effluent exhausted from the first fuel cell.