Abstract:
A device of the present invention transfers the moisture and heat from an exhaust delivered from a fuel cell cathode to the air introduced to a fuel cell as a cathode reactant. The device includes at least one moisture exchange unit having reactant compartment, an exhaust compartment, and a polymer member permeable for water vapor separating these compartments. A reactant inlet manifold and a reactant outlet manifold of the device are in fluid communication through the reactant compartment of the moisture exchange unit. An exhaust inlet manifold and an exhaust outlet manifold of the device are also in fluid communication with the exhaust compartment the moisture exchange unit.

Description:
RELATED APPLICATIONS 
       [0001]    This non-provisional application claims priority to a provisional application Ser. Nos. 60/893,482 filed on Mar. 7, 2007 and incorporated by reference herein in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to an electrochemical energy conversion device, such as fuel cells, that produce electrical power, and more particularly the present invention related to a humidifier for a fuel cell assembly. 
       BACKGROUND OF THE INVENTION 
       [0003]    Hydrogen fuel cells convert the chemical energy stored in hydrogen and oxygen into electricity, heat, and water. One of the benefits of the fuel cell over, for example, a battery, is the ability of the fuel cell to operate virtually continuously as long as necessary flows are maintained. Unlike the battery, which store electrical energy chemically in a closed system, the fuel cells consume reactants, which must be replenished. Additionally, while the electrodes within the battery react and change as a battery is charged or discharged, the electrodes of the fuel cell are catalytic and relatively stable. 
         [0004]    Fuel cells employ an electrolyte disposed between two electrodes, such as a cathode and an anode. The electrodes generally comprise a porous, electrically conductive gas diffusion layer (GDL) material and an electrocatalyst disposed at the interface between the electrolyte and the electrode layers. The electrocatalyst enhances the electrochemical reactions: hydrogen oxidation and oxygen reduction reactions. Polymer electrolyte membrane (PEM) fuel cells, also called the solid polymer fuel cells, typically employ a membrane electrode assembly (MEA) consisting of a proton exchange membrane as electrolyte disposed between two electrode layers. The membrane, in addition of being ion-conductive material, also is an electrical insulator and a physical barrier for reactants mix. 
         [0005]    The MEA is typically interposed between two electrically conductive plates. The plates act as current collectors, and provide also mechanical support to the MEA. The current collector plates may have channels, or openings in one or both plate surfaces to direct the fuel and oxidant to the respective electrode layers, namely the anode on the fuel side and the cathode on the oxidant side. 
         [0006]    Typically fuel cells are assembled together in series into fuel cell stacks to increase the overall output power. In series arrangement, one side of a plate may serve as cathode plate for the adjacent cell, with the current collector plate functioning as a bipolar plate with the other side functioning as the anode. Such a bipolar plate may have flow field channels formed on both active surfaces. The fuel cell stack includes an inlet port and manifold for directing a coolant fluid to interior passages within the stack to absorb heat generated by the electrochemical reaction in the fuel cells. The stack also includes exhaust manifolds and outlet ports for expelling the non reacted fuel and oxidant, and water generated in the reaction. It may also have an exhaust manifold and outlet port for the coolant stream exiting the stack. The stack manifolds may be internal created through aligned openings formed in the separator layers and the MEAs, or may have external or edge manifolds, attached to the edges of the separator layers. 
         [0007]    The fuel cell stacks are compressed to enhance sealing and electrical contact between the surfaces of the plates and the MEAs, and between adjoining plates. In conventional fuel cell stacks, the fuel cell plates and MEAs are typically compressed and maintained in their assembled state between a pair of end plates by tie rods or tension members. The tie rods typically extend internally or externally to the stack through holes formed in the stack end plates, and have associated nuts or other fastening means to secure them in the stack assembly. 
         [0008]    An electrochemical reaction between hydrogen and the oxygen contained in the air produces the electrical current, water and heat as the reaction products. Water is removed from the cathode to make the catalytic layer accessible for the oxygen. On the other hand, the air introduced to the cathode supposed to be rich in water vapor to prevent drying out of the PEM, which results in failure of the fuel cell failure. In some fuel cell systems the hydrogen, delivered to the anode, is also subject for humidification. A humidifier of the fuel cell presents the main device to keep the correct water balance in the fuel cell, thereby transferring the moisture across an internal membrane permeable for water molecules from water carrier to gas introduced into the fuel cell as the reactant. The major sources of water intended for the humidification are DI water or an exhaust gas from the fuel cell cathode. 
         [0009]    A fuel cell humidifier is one of the important components to keep the correct water balance in the fuel cell. The major operational principle of the fuel cell humidifier is to transfer the moisture (across membrane permeable for water molecules) from the cathode exhaust leaving the cathode to the air introduced in the cathode of the fuel cell stack as the reactant. The most important humidifier performance characteristic is the approach temperature—the difference in the dew point temperature of the cathode exhaust and the reactant. The applicable approach temperature is 3-9° C. However, if the temperature exceeds this range, the fuel cell&#39;s lifespan and performance will be negatively impacted. 
         [0010]    The optimal value of the approach temperature in a given interval depends mainly on the operational conditions of the fuel cell stack (the reactant pressure, the air stoichiometric ratio, the fuel cell temperature). Like any power generating plant with a low efficiency, the fuel cell system incorporates the components responsible for heat withdrawal, which consume sufficient amount of power produced by the system. In case of a manned automotive application another 1-3 kW is spent to drive a conditioner. 
         [0011]    The overall current size and the cost of a modern fuel cell system makes it unpracticable and will increase the overall cost of the modern fuel cell is an air conditioning unit is added to the modern fuel cell as an integral part. Thus, there is a constant need in the area of the fuel cell art for an improved design of a fuel cell humidifier having an effective and low-cost humidifier installed therein. 
       SUMMARY OF THE INVENTION 
       [0012]    A humidifier device (the humidifier) of the present invention is used with a fuel cell for balancing fluids therein. The humidifier of the present invention transfers the moisture and heat to the air introduced the fuel cell as the cathode reactant. Simultaneously the device may produce cooling media and serves as cooling apparatus. The humidifier includes at least one moisture exchange cartridge separated into the reactant and exhaust compartments with a polymer membrane. The flow introduced to the exhaust compartment is an exhaust from the cathode at the dew point temperature close or even to the temperature of the fuel cell operation. The air, as a reactant, distributed into the reactant compartment is relatively dry. It is directed by either a blower or a compressor. In first case the reactant temperature is close to ambient, in another one it is supposed to be elevated. 
         [0013]    A polymer membrane used in the humidifier is permeable for water vapor. Mechanism of the water movement across the polymer membrane depends on its type. For the PEM, known as “Nafion”, the water transport associates with chemical reactivity between water molecules and sulfonic acid groups imbedded. In case of the membrane with micro-porous structure water is accommodated in pores on one side of a membrane and, then, realized in a gas stream from the other site. In both cases the water transport through the membrane is driven mainly by partial vapor pressure differential. The humidifier provides the water transport across the membrane from the exhaust saturated with water vapor to the reactant having lower water vapor pressure. This process is accompanied with the heat flow in the same direction. As result, on one hand, the exhaust temperature drops while the gas travels along the moisture exchange cartridge; on other hand, the partial pressure of water vapor and the temperature of the reactant flowing through the reactant compartment rise. The flows have to be directed in the countercurrent way to maintain the efficient gradient of heat and water vapor along the moisture exchange cartridge length. 
         [0014]    The humidifier design assumes that the membrane package, the configuration of compartments and the flow direction allow each portion of the introduced gases to be in contact with the membrane to order to be involved in the process of the heat and moisture exchange. From such point of view the most effective membrane package is plurality of hollow tubes arranged in a bundle (cylindrical or rectangular) which is inserted into a shell of the moisture exchange cartridge. The exhaust stream is directed, preferably, into the fiber tubes, the reactant flow passes the shell space over the external side of the tubes. At an inlet of the exhaust compartment of the cartridge (cartridges) there is an adjustable (manually or automatically) valve to divert an exhaust portion from entering in the moisture exchange cartridge which allowing the control of the amount of heat and water vapor introduced into the cartridge, and, as result, the maintenance of an optimal value of the approach temperature. The decrease in a volumetric proportion between the exhaust and the reactant participating in the moisture exchange in the cartridge (exhaust/reactant ratio) results in higher approach temperature (lower reactant vapor pressure). 
         [0015]    In prior art, according to U.S. Pat. No. 6,471,195, a desired dew point temperature of the humidified air is maintained by changing the number of water permeable device by a plurality of butterfly valves. In case, how it is shown in second embodiment, if the exhaust/reactant ratio is less than 0.7 there is a sufficient drop in the temperature of the exhaust leaving the moisture exchange cartridge due to the elevated heat loss to a value below the ambient temperature so that the given exhaust stream can serve as a coolant media. The way to maintain the exhaust/reactant ratio at value less than 0.7 is to prevent at least 30% of the exhaust from entering in the moisture exchange cartridge (cartridges) by means of the adjustable valve partially open. Other part of the exhaust, after passing the hollow tubes, is supposed to possess the cooling ability. 
         [0016]    Third embodiment of the invention assumes that in the humidifier at least two moisture exchange cartridges or a cartridge cascades (each cascade comprises, at least, two cascades connected in parallel regarding both to the reactant and the exhaust) is connected in parallel regarding to the reactant and in series regarding to the exhaust. Under the given connection the exhaust/reactant ratio is equal to 1/n (“n” is a number of the moisture exchange cartridges or the cartridge cascades in series regarding to the reactant). The desired reactant vapor pressure builds up gradually, in sequence of the moisture exchange cartridges or the cartridge cascades. Even at very low fuel cell air demand the reactant flow through any moisture exchange cartridge remains relatively high to be forced into the fiber bundle core to keep the moisture exchange at the proper level. In third embodiment the exhaust/reactant ratio is, at least, 0.5 (n=2). The exhaust passing, at least, the first moisture exchange cartridge (or cartridge cascade) regarding the reactant flow is supposed to be used for cooling purposes. The humidifier contains a water discharger to withdraw the liquid water (mainly, as product of condensation) from the reactant delivered to the fuel cell. The water discharger has two chambers separated with a membrane selectively permeable for water. First chamber is in fluid communication with a reactant outlet manifold of the humidifier and second one is open to an exhaust outlet manifold. If the reactant pressure exceeds the exhaust pressure, which is generally true, the discharger is able to drain the water from the outlet manifold of the reactant compartment preventing fuel cells against flooding. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    Other advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein: 
           [0018]      FIG. 1  is a cross sectional view of a humidifier of the present invention; 
           [0019]      FIG. 2  is a cross sectional view of the humidifier of a second embodiment of the invention; 
           [0020]      FIG. 3  is a perspective view of the humidifier of the second embodiment of the invention; and 
           [0021]      FIG. 4  is the cross sectional view of the humidifier of a third embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0022]    Referring to the Figures, wherein like numerals indicate like or corresponding parts, a humidifier is shown in  FIG. 1  and generally designated by the reference numeral  100  incorporates four moisture exchange units  110 . Each moisture exchange unit  110  designed as a bundle of polymer membrane hollow tubes  114  inserted into a shell  116  so that a space  118  between the polymer membrane hollow tubes themselves and between the hollow tubes and the shell  116  is filled with sealing media, preferably with an epoxy resin, on both ends of the moisture exchange unit  110 . 
         [0023]    An reactant inlet manifold  120  and a reactant outlet manifold  122  of humidifier  100  are in flow communication through a space  112  restricted with the bundle of polymer membrane hollow tubes  114  and the shell  116  of moisture exchange units  110 . An exhaust inlet manifold  124  and an exhaust outlet manifold  126  of humidifier  100  are in flow communication through internal capillaries of membrane hollow tubes of the bundle  114 , and through a by-pass line  130  which is secured with an adjustable valve  132 . The humidifier  100  incorporates a water discharger  140  comprising: a water collecting chamber  142 ; a water disposing chamber  144 ; a polymer water discharger membrane  146  permeable for water vapor separating the chambers  142  and  144 . The reactant outlet manifold  122  of humidifier  100  is in flow communication with the water collecting chamber  142  of the water discharger  140 ; the exhaust outlet manifold  126  of the humidifier  110  is in flow communication with a water disposing chamber  144  of the water discharger  140 . 
         [0024]    In humidification process utilizing the humidifier  100  a fuel cell cathode exhaust is distributed to the exhaust inlet manifold  124  and a reactant air is introduced by an air compressor (an air blower) to the reactant inlet manifold  120 . Part of the fuel cell cathode exhaust can be released by means of adjustable valve  132  from the exhaust inlet manifold  124  to the exhaust outlet manifold  126  through the by-pass line  130  without participation in the moisture and heat exchange. Other part of the fuel cell cathode exhaust flows to the exhaust outlet manifold  126  by internal capillaries of the polymer membrane hollow tubes combined in the bundles  114  of the moisture exchange units  110 . The reactant air moves from the reactant inlet manifold  120  to the reactant outlet manifold  122  of the humidifier  100  through the space  112  inside the moisture exchange units  110 . Along the moisture exchange units  110  water and heat are transferred from the fuel cell cathode exhaust to the reactant air. 
         [0025]    The adjustable valve  132  controls the amount of heat and water vapor introduced into the moisture exchange units  110 , and, as result, is means to maintain an optimal value of the approach temperature (the reactant vapor pressure) for the specific fuel cell operational condition. Water condensate derived from the reactant air is collected on the bottom of the reactant outlet manifold  120  due to the gravity, then, transported through the water collecting chamber  142  of the water discharger  140  to the water disposing chamber  144  through the water-permeable polymer water discharger membrane  146  under the pressure difference which equals, in general, a sum of the pressure drops for the reactant air across the fuel cell and for the fuel cell cathode exhaust along the moisture exchange units  110  of the humidifier  100 . 
         [0026]    In second embodiment referring to the drawing, the humidifier shown in  FIGS. 2 and 3  and generally designated by the reference numeral  200  incorporates four moisture exchange units  210 . Each moisture exchange unit  210  designed as a bundle of polymer membrane hollow tubes  214  inserted into a shell  216  so that a space  218  between the polymer membrane hollow tubes themselves and between the hollow tubes and the shell  216  is filled with sealing media, preferably with an epoxy resin, on both ends of the moisture exchange unit  210 . 
         [0027]    An reactant inlet manifold  220  and a reactant outlet manifold  222  of humidifier  200  are in flow communication through a space  212  restricted with the bundle of polymer membrane hollow tubes  214  and the shell  216  of moisture exchange units  210 . An exhaust inlet manifold  224  is in flow communication with a coolant outlet manifold  228  of humidifier  200  through internal capillaries of membrane hollow tubes of the bundle  214 , and with the exhaust outlet manifold  226  of humidifier  200  through a by-pass line  230  which is secured with an adjustable valve  232 . The humidifier  200  incorporates a water discharger  240  comprising: a water collecting chamber  242 ; a water disposing chamber  244 ; a polymer water discharger membrane  246  permeable for water vapor separating the chambers  242  and  244 . 
         [0028]    The reactant outlet manifold  222  of humidifier  200  is in flow communication with the water collecting chamber  242  of the water discharger  240 ; the exhaust outlet manifold  226  of the humidifier  200  is in flow communication with a water disposing chamber  244  of the water discharger  240 . In humidification process utilizing the humidifier  200  a fuel cell cathode exhaust is distributed to the exhaust inlet manifold  224  and a reactant air is introduced by an air compressor (an air blower) to the reactant inlet manifold  220 . Part of the fuel cell cathode exhaust can be released by means of adjustable valve  232  from the exhaust inlet manifold  224  to the exhaust outlet manifold  226  of the humidifier  200  through the by-pass line  230  without participation in the moisture and heat exchange. Other part of the fuel cell cathode exhaust flows to the coolant outlet manifold  228  by internal capillaries of the polymer membrane hollow tubes combined in the bundles  214  of the moisture exchange units  210 . The reactant air moves from the reactant inlet manifold  220  to the reactant outlet manifold  222  of the humidifier  200  through the space  212  inside the moisture exchange units  210 . Along the moisture exchange units  210  water and heat are transferred from the fuel cell cathode exhaust to the reactant air. The adjustable valve  232  controls the amount of heat and water vapor introduced into the moisture exchange units  210 , and, as result, is means to maintain an optimal value of the approach temperature (the reactant vapor pressure) for the specific fuel cell operational condition. 
         [0029]    In case if the portion of the fuel cell cathode exhaust directed into the moisture exchange units  210  by adjustment adjustable valve  232  is less than 70% of the total fuel cell exhaust the flow from the coolant outlet manifold  228  can be distributed then as the coolant due to the elevated heat loss to a value below the ambient temperature occurred in the fuel cell cathode exhaust flowing along the moisture exchange units  210 . Water condensate derived from the reactant air is collected on the bottom of the reactant outlet manifold  220  due to the gravity, then, transported through the water collecting chamber  242  of the water discharger  240  to the water disposing chamber  244  through the water-permeable polymer water discharger membrane  246  under the pressure difference which equals, in general, a sum of the pressure drops for the reactant air across the fuel cell and for the fuel cell cathode exhaust along the moisture exchange units  210  of the humidifier  200 . In third embodiment referring to the drawing, the humidifier shown in  FIG. 4  and generally designated by the reference numeral  300  incorporates four moisture exchange units  310 . 
         [0030]    Each moisture exchange unit  310  designed as a bundle of polymer membrane hollow tubes  314  inserted into a shell  316  so that a space  318  between the polymer membrane hollow tubes themselves and between the hollow tubes and the shell  316  is filled with sealing media, preferably with an epoxy resin, on both ends of the moisture exchange unit  310 . The humidifier  300  combines two cascades  301   a ,  301   b  connected regarding to the reactant air in series and in parallel regarding to the fuel cell cathode exhaust. The cascades  301   a  and  301   b  combine, consequently, the moisture exchange units  310   a,b  and  310   c,d . The moisture exchange units of each cascade are connected in parallel regarding to both the reactant air the fuel cell cathode exhaust. 
         [0031]    A reactant inlet manifold  320   a  ( 320   b ) of the cascade  301   a  ( 320   b ) is in fluid communication with a reactant outlet manifold  322   a  ( 322   b ) of cascade  301   a  ( 301   b ) through a space  312  restricted with the bundle of polymer membrane hollow tubes  314  and the shell  316  of moisture exchange units  310   a,b  ( 310   c,d ). 
         [0032]    An exhaust inlet manifold  324  of humidifier  300  is in flow communication: with an coolant outlet manifold  328  of humidifier  300  through the moisture exchange units  310   a,b  of cascade  301   a ; with an exhaust outlet manifold  326  of humidifier  300  through the moisture exchange units  310   c,d  of cascade  301   b  and through a by-pass line  330  which is secured with an adjustable valve  332 . The humidifier  300  incorporates a water discharger  340  comprising: a water collecting chamber  342 ; a water disposing chamber  344 ; a polymer water discharger membrane  346  permeable for water vapor separating the chambers  342  and  344 . The reactant outlet manifold  322   b  of cascade  301   b  is in flow communication with the water collecting chamber  342  of the water discharger  340 ; the exhaust outlet manifold  326  of the humidifier  300  is in flow communication with a water disposing chamber  344  of the water discharger  340 . 
         [0033]    In humidification process utilizing the humidifier  300  a fuel cell cathode exhaust is distributed to the exhaust inlet manifold  324  of the humidifier  300  and a reactant air is introduced by an air compressor (an air blower) to the reactant inlet manifold  320  of the cascade  301   a . Part of the fuel cell cathode exhaust can be released by means of adjustable valve  332  from the exhaust inlet manifold  324  to the exhaust outlet manifold  226  of the humidifier  300  through the by-pass line  330  without participation in the moisture and heat exchange. Other part of the fuel cell cathode exhaust flows along the moisture exchange units  310  by internal capillaries of the polymer membrane hollow tubes combined in the bundles  314 . The reactant air moves from the reactant inlet manifold  320   a  of the cascade  301   a  to the reactant outlet manifold  322   a  along the moisture exchange units  310   a,b , and, then from the reactant inlet manifold  320   b  of the cascade  301   b  to the reactant outlet manifold  322   b  along the moisture exchange units  310   c,d . Along the moisture exchange units  310  water and heat are transferred from the fuel cell cathode exhaust to the reactant air. 
         [0034]    The adjustable valve  332  controls the amount of heat and water vapor introduced into the moisture exchange units  310 , and, as result, is means to maintain an optimal value of the approach temperature (the reactant vapor pressure) for the specific fuel cell operational condition. As of the fuel cell cathode exhaust directed into the moisture exchange units  310   a,b  is twice less of the total fuel cell exhaust flowing through the moisture exchange units  310   a,b  the flow from the coolant outlet manifold  328  of the cascade  301   a  can be distributed then as the coolant due to the elevated heat loss to a value below the ambient temperature occurred in the fuel cell cathode exhaust flowing along the moisture exchange units  310   a,b.    
         [0035]    Water condensate occurring in the reactant outlet manifold  322   b  of the cascade  302   a  from the reactant air is collected on the bottom of the reactant outlet manifold  322   b  due to the gravity, then, transported through the water collecting chamber  342  of the water discharger  340  to the water disposing chamber  344  through the water-permeable polymer water discharger membrane  346  under the pressure difference which equals, in general, a sum of the pressure drops for the reactant air across the fuel cell and for the fuel cell cathode exhaust along the moisture exchange units  310   c,d  of the cascade  302   a.    
         [0036]    While the invention has been described with reference to an exemplary embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.