Abstract:
A fuel cell system includes a fuel cell stack that receives a cathode feed gas and has an exhaust stream and a heat transfer stream flowing therefrom. A charge-air heat exchanger enables heat transfer between the heat transfer stream and the cathode feed gas. The charge-air heat exchanger also enables heat transfer between the heat transfer stream and the cathode feed gas to compensate for the adiabatic cooling effect. Furthermore, the charge-air heat exchanger vaporizes the liquid water to provide water vapor. The water vapor humidifies the cathode feed gas.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to fuel cell systems, and more particularly to humidifying charge air delivered to a fuel cell stack.  
       BACKGROUND OF THE INVENTION  
       [0002]     Fuel cell systems are increasingly being used as a power source in a wide variety of applications. Fuel cell propulsion systems have also been proposed for use in vehicles as a replacement for internal combustion engines. The fuel cells generate electricity that is used to charge batteries and/or to power an electric motor. A solid-polymer-electrolyte fuel cell includes a membrane that is sandwiched between an anode and a cathode. To produce electricity through an electrochemical reaction, a fuel, commonly hydrogen (H 2 ), but also either methane (CH 4 ) or methanol (CH 3 OH), is supplied to the anode and an oxidant, such as oxygen (O 2 ) is supplied to the cathode. The source of the oxygen is commonly air.  
         [0003]     In a first half-cell reaction, dissociation of the hydrogen (H 2 ) at the anode generates hydrogen protons (H + ) and electrons (e − ). The membrane is proton conductive and dielectric. As a result, the protons are transported through the membrane. The electrons flow through an electrical load (such as the batteries or the electric motor) that is connected across the membrane. In a second half-cell reaction, oxygen (O 2 ) at the cathode reacts with protons (H + ), and electrons (e − ) are taken up to form water (H 2 O).  
         [0004]     The relative humidity of the oxidant impacts durability and efficiency of the fuel cell system. Conventional strategies have been developed to humidify the oxidant flowing to the fuel cell. These strategies, however, present certain disadvantages. One disadvantage is that the achievable humidification level is limited. Other disadvantages include low durability, higher cost and increased space requirements.  
       SUMMARY OF THE INVENTION  
       [0005]     Accordingly, the present invention provides a fuel cell system. The fuel cell system includes a fuel cell stack that receives a cathode feed gas and has an exhaust stream and a heat transfer stream flowing therefrom. A charge-air heat exchanger enables heat transfer between the heat transfer stream and the cathode feed gas to adjust a feed gas temperature. The charge-air heat exchanger also enables heat transfer between the heat transfer stream and a liquid water to vaporize the liquid water providing water vapor. The water vapor humidifies the cathode feed gas. Preferably, the source of liquid water is a water condensate originating from within the fuel cell system. In one aspect, the heat transfer stream includes a fluid operable to heat and cool as needed. An important feature is cooling and therefore, the heat transfer stream is referred to as coolant for simplicity. It is appreciated, however, that it is not limited to cooling as it may also heat.  
         [0006]     In one feature, the fuel cell system further includes a condenser that condenses water vapor in the exhaust stream.  
         [0007]     In another feature, the fuel cell system includes an injector that injects the water condensate into the cathode feed gas. Preferably, the injector forms a part of the charge-air heat exchanger or is adjacent the charge-air heat exchanger.  
         [0008]     In still another feature, the fuel cell system further includes a compressor that compresses the cathode feed gas. The compressor receives a portion of the water condensate to humidify the cathode feed gas within the compressor. The compressor comprises an injector that injects the water condensate into the cathode feed gas. The water condensate is vaporized within the compressor during a compression process.  
         [0009]     In yet another feature, a portion of the water condensate is injected into the fuel cell stack to humidify the cathode feed gas within the fuel cell stack.  
         [0010]     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  
       [0011]     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0012]      FIG. 1  is a fuel cell system including charge air humidification according to the present invention;  
         [0013]      FIG. 2  is an alternative fuel cell system including charge air humidification according to the present invention; and  
         [0014]      FIG. 3  is another alternative fuel cell system including charge air humidification according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]     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.  
         [0016]     Referring now to  FIG. 1 , a fuel cell system  10  is shown. The fuel cell system  10  includes a fuel cell stack  12 , a coolant system  14 , a charge-air heat exchanger  16  and a compressor  18 . The coolant system  14  maintains the operating temperature of the fuel cell stack  12  at an appropriate level. Additionally, the coolant system  14  adjusts the temperature of fluids at various points in the fuel cell system  10  as explained in further detail below. The compressor  18  compresses oxidant that is supplied to the fuel cell stack  12 . More specifically, the oxidant is supplied as a cathode feed gas or charge air to a cathode side (not shown) of the fuel cell stack  12 . The cathode feed gas catalytically reacts with a hydrogen-rich reformate supplied to an anode side (not shown) of the fuel cell stack  12 . The oxidant is oxygen-rich air supplied by the compressor  18  and charge-air heat exchanger  16  at an appropriate operating state (i.e., temperature and pressure). The oxidant reacts with the hydrogen-rich reformate to produce electrical power and an exhaust stream.  
         [0017]     The exhaust stream is made up of reaction products including water (H 2 O) vapor and a small amount of liquid H 2 O depending on the operating strategy of the fuel cell stack  12 . The H 2 O vapor condenses as it travels through an exhaust conduit  20  to provide an H 2 O condensate. The exhaust conduit  20  can be configured to maximize the surface area over which the exhaust stream passes to enable condensation of the H 2 O vapor. Alternatively, a condenser  22  can be included to condense the H 2 O vapor to provide the H 2 O condensate. It is also anticipated that the source of H 2 O can be provided from a means other than the exhaust stream. For example, a separate water storage tank (not shown) can be used to supply liquid H 20 .  
         [0018]     The coolant system  14  controls coolant flow through the fuel cell system  10  and includes a pump (not shown) and a radiator (not shown) that enables heat transfer to atmosphere. As used herein, the term coolant refers to a heat transfer fluid that is able to cool and heat as needed. For example, in a situation where the coolant is warmer than an adjacent fluid or structure, the coolant serves to heat that adjacent fluid or structure. Similarly, in a situation where the coolant is cooler than an adjacent fluid or structure, the coolant serves to cool that adjacent fluid or structure. Coolant is pumped through the fuel cell stack  12  to cool the fuel cell stack  12  and maintain an operating temperature of the fuel cell stack  12 . The coolant flows from the fuel cell stack  12 , through the charge-air heat exchanger  16  and back to the coolant system  14 . A regulator valve  23  is optionally provided to control the flow rate of coolant to the charge-air heat exchanger  16 . As described in further detail below, the heat of compression and heat transfer from the coolant enables vaporization of the H 2 O condensate. The heat exchanger adjusts the cathode feed gas to an appropriate temperature for reaction in the fuel cell stack  12 .  
         [0019]     The H 2 O condensate and coolant are directed to the charge-air heat exchanger  16  and cooperate to humidify the cathode feed gas. More particularly, an injector or multiple injectors  24  are provided to inject the H 2 O condensate into the cathode feed gas as it flows through the charge-air heat exchanger  16 . The coolant is in heat exchange relationship with the cathode feed gas and injected H 2 O condensate. Preferably, the adiabatic cooling effect occurs whereby the charge air temperature drops and the H 2 O condensate is vaporized to form H 2 O vapor. Additionally, heat transfer occurs from the coolant to the H 2 O condensate, vaporizing the H 2 O condensate. Concurrently, heat transfer occurs from the coolant to the cathode feed gas, reheating the cathode feed gas. As a result, the process in one embodiment is operable essentially at constant temperature and pressure (i.e., state) maintained by the coolant (i.e., working fluid).  
         [0020]     Depending upon the amount of the H 2 O condensate that must be injected to humidify the cathode feed gas to an appropriate level, a multi-stage humidification process is provided in one embodiment. The multi-stage humidification process includes a first stage with an injector  24  for injecting a first volume of the H 2 O condensate into the cathode feed gas. The first volume is vaporized within the cathode feed gas stream in the heat transfer process as described above. A second stage includes a second injector  24  for injecting a second volume of the H 2 O condensate into the partially humidified cathode feed gas. The second volume is vaporized within the cathode feed gas stream in the adiabatic heat transfer process as described above. Two or more stages (e.g., third and fourth stages) can be implemented to achieve the desired humidity level of the cathode feed gas.  
         [0021]     Referring now to  FIG. 2 , a fuel cell system  10 ′ is shown and includes humidification of the cathode feed gas within the compressor  18 . More specifically, a portion of the H 2 O condensate is fed to an inlet of the compressor  18 . The compressor includes an injector  26  that injects the H 2 O condensate into the cathode feed gas at the compressor suction side. The compression process generates sufficient heat to vaporize a part of the H 2 O condensate, humidifying the cathode feed gas. Thus, the fuel cell system  10 ′ of  FIG. 2  provides for humidification of the cathode feed gas at both the compressor  18  and the charge-air heat exchanger  16 , as described in detail above.  
         [0022]     The proportion of cathode feed gas humidification that occurs within the compressor  18  to that which occurs within the charge-air heat exchanger  16  can be controlled. Due to the limited available heat of compression and dwell time, a smaller portion of humidification can occur within the compressor  18 . As a result, the larger portion of humidification occurs within the charge-air heat exchanger  16  as detailed above. Alternatively, a larger portion of humidification can occur within the compressor  18 . As a result, the smaller portion of humidification occurs within the charge-air heat exchanger  16 . In such a case, the multi-stage humidification process may not be required depending on how much H 2 O condensate must be injected to sufficiently humidify the cathode feed gas.  
         [0023]     Referring now to  FIG. 3 , a fuel cell system  10 ″ is shown and includes humidification of the cathode feed gas within the compressor  18 , the cooler  16  and the fuel cell stack  12 . More specifically, a portion of the H 2 O condensate is fed to the compressor  18  for humidifying the cathode feed gas as described above with respect to  FIG. 2 . Additionally, a portion of the H 2 O condensate is fed to the fuel cell stack  12 . An injector  28  is provided to inject the H 2 O condensate into the cathode feed gas within the fuel cell stack  12 . Heat transfer occurs to vaporize the H 2 O condensate, humidifying the cathode feed gas within the fuel cell stack  12 . Thus, the fuel cell system  10 ″ of  FIG. 3  provides for humidification of the cathode feed gas at the compressor  18  and at the charge-air heat exchanger  16  as described in detail above, as well as within the fuel cell stack  12  itself. As described above with reference to  FIG. 2 , the proportion of humidification that occurs within the compressor  18 , the charge-air heat exchanger  16  and the fuel cell stack  12  can vary as design requirements dictate.  
         [0024]     The fuel cell systems of the present invention include several distinct advantages over conventional humidification strategies. One advantage is that overall system durability and efficiency is improved. This is a result of a higher achievable humidification level over conventional systems and a reduced heat load on the cooling system. The reduced heat load is a result of the heat that would otherwise be discharged through the coolant system being used to vaporize the H 2 O condensate within the cooler. As a result, lower system temperatures and a more stream-lined coolant system including a smaller radiator are achieved. Additionally, less liquid H 2 O exits the exhaust of the vehicle.  
         [0025]     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.