Abstract:
An example method of operating a fuel cell system includes calculating the rate of water produced in the fuel cell stack, determining the rate of water exiting the system, and controlling the condenser temperature to maintain the cathode gas exit temperature from the condenser below the temperature required to maintain water balance in the fuel cell system. The method collects the condensed vapor as water and purges a portion of the collected water containing contaminants from the system.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the U.S. national phase of PCT/US2010/036276, filed May 27, 2010. 
     
    
     TECHNICAL FIELD 
       [0002]    This disclosure relates generally to a fuel cell system. In particular, this disclosure relates to removing contaminants from the fuel cell system. 
       DESCRIPTION OF RELATED ART 
       [0003]    Fuel cell assemblies are well known. One example fuel cell system includes multiple individual fuel cells arranged in a stack. Each individual fuel cell has an anode and a cathode positioned on either side of proton exchange membrane. A fuel, such as hydrogen, is supplied to the anode side of the proton exchange membrane. An oxidant, such as air, is supplied to the cathode side of the proton exchange membrane. Gas diffusion layers help distribute the fuel and the oxidant to the proton exchange membrane. As known, the chemical reactions within the fuel cell produce water that exits the fuel cell in liquid and vapor form. 
         [0004]    Some fuel cell assemblies communicate water produced by a fuel cell to a heat exchanger or condenser. The water carries thermal energy away from the fuel cell, which cools the fuel cell. The cooled water is then returned to the fuel cell to absorb more thermal energy. The water can contain ionic and other contaminants that can poison the individual fuel cells and degrade fuel cell performance. Many fuel cell assemblies communicate the water through a demineralizer bed to remove contaminants from the water before returning the water to the fuel cell. Demineralizer resin within the demineralizer bed must be periodically replaced. The demineralizer bed adds cost and complexity to fuel cell assemblies. 
       SUMMARY 
       [0005]    An example method of operating a fuel cell system includes calculating the rate of water produced in the fuel cell stack, determining the rate of water exiting the system, and controlling the condenser temperature to maintain the cathode gas exit temperature from the condenser below the temperature required to maintain water balance in the fuel cell system. The method collects the condensed vapor as water and purges a portion of the collected water containing contaminants from the system. 
         [0006]    An example method of removing contaminants from a fuel cell system includes condensing water vapor from the cathode exit stream and collecting the condensed vapor as water. The method also collects liquid water produced in the stack and holds the collected water in the system. The method purges a portion of the collected water containing contaminants from the system. 
         [0007]    Yet another example method of removing contaminants from a fuel cell system includes controlling the system such that the fuel cell stack rate of water production is greater than the amount of water exiting the fuel cell system. The method drains a portion of the system water containing contaminants from the system. 
         [0008]    These and other features of the example disclosure can be best understood from the following specification and drawings, the following of which is a brief description: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  shows schematic view of an example fuel cell system incorporating a condenser to remove thermal energy from water vapor provided by a fuel cell of the fuel cell system. 
           [0010]      FIG. 2  shows a schematic view of another example fuel cell system incorporating an intermediate heat exchanger to remove thermal energy from liquid water provided by a fuel cell of the fuel cell system. 
           [0011]      FIG. 3  shows a schematic view of another example fuel cell system incorporating a condenser to remove thermal energy from water vapor provided by a fuel cell of the fuel cell system. 
           [0012]      FIG. 4  shows a schematic view of another example fuel cell system incorporating a condenser to remove thermal energy from water vapor provided by a fuel cell of the fuel cell system. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    Referring to  FIG. 1 , an example fuel cell system  10  includes a fuel cell  14  having an anode  18  and a cathode  22  separated by a proton exchange membrane  26 . A fuel source  30  supplies a fuel, such as hydrogen, to the anode  18  of the fuel cell  14 . Some of the fuel is exhausted from the fuel cell  14  at a fuel exhaust  34 . An oxidant source  38  supplies an oxidant, such as air, to the cathode  22  of the fuel cell  14 . Some of the air is exhausted from the fuel cell  14  at an air exhaust  40 . The exhausted air carries water vapor from the fuel cell  14  as is known. Chemical reactions within the fuel cell  14  produce the water vapor carried by the exhausted air. 
         [0014]    Hydrogen-air PEM fuel cell systems, such as the system  10  shown in  FIG. 1 , produce water as a byproduct. If the total amount of water vapor exhausting from the system  10  equals the water that is produced by the fuel cell  14 , the system  10  can be said to be operating in water balance. 
         [0015]    If the operating conditions are such that the water vapor leaving the system  10  is higher than the water produced by the fuel cell  14 , the system  10  can be said to be in negative water balance. In some examples, an external source of water is needed to replenish lost water if the system  10  is operating in negative water balance. 
         [0016]    If operating conditions are chosen such that water vapor leaving the system  10  is less than the water produced, the system  10  can be said to be operating in positive water balance. In some examples, there is a build-up of liquid water in the system  10  requiring removal of the excess water in some form if the system  10  is operating in positive water balance. 
         [0017]    In this example, the exhausted air carrying water vapor is communicated to a condenser heat exchanger  42  along a path  44 . A fan  46  moves air across the condenser heat exchanger  42  to cool the exhausted air and condense the water vapor carried by the exhausted air. The condensate is separated from the exhausted air and communicated to an accumulator tank  50  at an outlet  52 . The remaining portions of the exhausted air are vented to the surrounding environment at an outlet  54 . 
         [0018]    Chemical reactions within the fuel cell  14  produce liquid water in addition to the water vapor carried by the exhausted air. In this example, a pump  56  is used to communicate the liquid water from the fuel cell  14  to the accumulator tank  50  along a path  58  to an outlet  59  or another conduit. The liquid water moves through the pump  56  in this example. 
         [0019]    The liquid water pumped from the fuel cell  14  combines with the condensate in the accumulator tank  50 . Both the liquid water pumped from the fuel cell  14  and the condensate communicated from the condensing heat exchanger  42  carry contaminants, such as ionic contaminants. 
         [0020]    The water level within the accumulator tank  50  rises as more water is added to the accumulator tank  50 . If the water level exceeds a certain level, the excess water drains from the accumulator tank  50  through an overflow valve  60  to the ground, for example. 
         [0021]    In this example, the accumulator tank  50  holds a first amount of water a 1 , and a second amount of water a 2  (if introduced to the accumulator tank  50 ) spills through the overflow valve  60  or another type of conduit. 
         [0022]    The first amount of water a 1  (i.e., the water held by the accumulator tank  50  below the overflow valve  60 ) is configured to move back to the fuel cell  14  along the path  62 . This water moves through the fuel cell  14  and absorbs thermal energy. Circulating liquid water from through the fuel cell  14  cools the fuel cell  14 . 
         [0023]    The second amount of water a 2  (i.e., the water discharged through the overflow valve) does not flow back to the fuel cell  14 . The second amount of water a 2  carries some contaminants away from the fuel cell system  10 . 
         [0024]    In this example, the fan  46  moves air across the condensing heat exchanger  42 . The moving air cools the water vapor and condenses liquid water from the water vapor. Increasing the speed of the fan  46  will more effectively cool the water vapor as more air is moving across the condensing heat exchanger  42 . Increased cooling of the water vapor increases the condensate removed from the water vapor. 
         [0025]    In some examples, a user increases the speed of the fan  46  to move more condensate into the accumulator tank  50  and thus increase liquid water within the fuel cell system  10 . 
         [0026]    As can be appreciated, increasing the amount of condensate in the accumulator tank  50  increases the second amount of water and thus the amount of water (and associated contaminants) exiting the fuel cell system  10  through the overflow valve  60 . 
         [0027]    Although the fuel cell system  10  is shown as having one fuel cell  14 , those skilled in the art and having the benefit of this disclosure will understand that other fuel cell assemblies may include several individual, water-cooled, individual fuel cells arranged in a fuel cell stack or still other fuel cell configurations. 
         [0028]    In this disclosure, like reference numerals designate like elements where appropriate, and reference numerals with the addition of one-hundred or multiples thereof designate modified elements. The modified elements incorporate the same features and benefits of the corresponding modified elements, expect where stated otherwise. 
         [0029]    Referring to  FIG. 2 , another example fuel cell system  110  includes a fuel cell  114  that exhausts air directly to the surrounding environment at an air exhaust  140 . The exhausted air carries water vapor from the fuel cell  14  as is known. 
         [0030]    A pump  156  moves liquid water from the fuel cell  114  to an intermediate heat exchanger  64  along a path  68 . After moving through the intermediate heat exchanger  64 , the liquid water is communicated to the accumulator tank  50  through an outlet  152 . The water level within the accumulator tank  50  rises as more water is added to the accumulator tank  50 . If the water level exceeds a certain level, the excess water drains from the accumulator tank  50  through the overflow valve  60  to the ground, for example. 
         [0031]    The liquid water carries thermal energy away from the fuel cell  114  to cool the fuel cell  114 . Within the intermediate heat exchanger  64 , the thermal energy moves from the water to another fluid circulating along a path  72 . The other fluid is a glycol solution in this example. 
         [0032]    A pump  76  moves the glycol solution along the path  72  between the intermediate heat exchanger  64  and an air-cooled radiator  80 . A fan  82  moves air across the radiator  80 . The moving air carries some of the thermal energy away from the glycol solution. 
         [0033]    In this example, a bypass valve  84  is used to control the operating temperature of the fuel cell  114  and thus the water balance of the fuel cell system  110 . More specifically, reducing flow through the bypass valve  84  increases flow of glycol solution through the intermediate heat exchanger  64 . 
         [0034]    As can be appreciated, more thermal energy is removed from the liquid water when more glycol solution moves through the intermediate heat exchanger  64 . Removing more thermal energy from the liquid water lowers the operating temperature of the fuel cell  114 . Operating the fuel cell  114  at the lower operating temperature decreases the amount of water exiting the fuel cell  114  as water vapor at an air exhaust  140 . Operating the fuel cell  114  at the lower operating temperature also increases the amount of water exiting the fuel cell  114  as liquid water at the path  68 . 
         [0035]    The liquid water exiting the fuel cell  114  contains contaminants. Removing some of the liquid water from the fuel cell system  110  through the overflow valve  60  removes contaminants from the fuel cell system  110 . Operating the fuel cell  114  at a water surplus facilitates removing liquid water from the fuel cell  114 . 
         [0036]    Referring now to  FIG. 3 , another example fuel cell system  210  includes a vacuum pump  88  and a vacuum break  90 . The vacuum pump  88  draws a vacuum to communicate liquid water away from a fuel cell  214  to a second accumulator tank  92 . When the liquid water reaches an established level within the second accumulator tank  92 , a level switch  94  actuates and opens the vacuum break  90 . The vacuum break  90 , when open, enables liquid water to flow or slump from the second accumulator tank  92  through a coolant return valve  96  to the accumulator tank  50 . The vacuum break  90  opens and closes intermittently, say every 1-2 seconds, which causes liquid water to pulse through the fuel cell  214 . 
         [0037]    In this example, the coolant return valve  96  contacts water, which can potentially freeze during cold shutdowns. The coolant return valve  96  is thus typically moved to a closed position during shutdown. A heater (not shown) is used in one example to thaw the coolant return valve  96  after start up of the fuel cell system  210 . 
         [0038]    As with the example of  FIG. 1 , an operator may increases the speed of the fan  46  to increase liquid water within the fuel cell system  210  by moving more condensate into the accumulator tank  50 . 
         [0039]    Again, increasing the amount of condensate in the accumulator tank  50  increases the second amount of water and thus the amount of water (and associated contaminants) exiting the fuel cell system  10  through the overflow valve  60 . 
         [0040]    Referring to  FIG. 4 , another example fuel cell system  310  is evaporatively cooled. The system  310  includes the vacuum pump  88  and the vacuum break  90 . The vacuum pump  88  draws a vacuum to communicate liquid water away from a fuel cell  314  to the second accumulator tank  92 . The vacuum break  90  opens and closes periodically to maintain a level of the liquid in the tank  92  at a level L 1 . The coolant return valve  96  opens periodically with the vacuum break  90  to allow the water (and associated contaminants) to pass through the valve  96  to be expelled from the system  310 . 
         [0041]    Features of the disclosed examples include controlling a fuel cell system to operate at a water surplus and then removing the surplus water (and associated contaminants) from the fuel cell system. Another feature of the disclosed examples includes removing contaminants from a fuel cell system without using a demineralizer bed. Automotive, bus, and stationary fuel cell powerplants could incorporate these features. 
         [0042]    Other features of the disclosed examples include operating a fuel cell system in positive water balance by selecting relevant heat exchanger and fuel cell operating parameters. As this extra liquid water in the system contains the contaminants in the same concentration as the general water in the coolant system, exhausting some of the liquid water will get rid of some of the contaminants. The extent to which the system is operated in positive water balance (i.e., the rate of water production minus the rate of water vapor removed through exhaust) can be adjusted based on the amount of contamination that is expected to leach out into the coolant system. 
         [0043]    Although a preferred embodiment has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.