Abstract:
A method and apparatus are provided for delivery and regulation of a process fluid to a multi-cell electrochemical device. A controller regulates the process fluid flow, after at least one of the process parameters indicates a drop in load current draw, based on the operating rate of the fuel cell and the rate of decrease in the load.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a system and method for delivering and regulating process gas streams to fuel cell stacks.  
         BACKGROUND OF THE INVENTION  
         [0002]    A fuel cell is an electrochemical device that produces an electromotive force by bringing the fuel (typically hydrogen) and an oxidant (typically air) into contact with two suitable electrodes and an electrolyte. A fuel, such as hydrogen gas, for example, is introduced at a first electrode where it reacts electrochemically in the presence of the electrolyte to produce electrons and cations in the first electrode. The electrons are circulated from the first electrode to a second electrode through an electrical circuit connected between the electrodes. Cations pass through the electrolyte to the second electrode. Simultaneously, an oxidant, such as oxygen or air is introduced to the second electrode where the oxidant reacts electrochemically in presence of the electrolyte and catalyst, producing anions and consuming the electrons circulated through the electrical circuit; the cations are consumed at the second electrode. The anions formed at the second electrode or cathode react with the cations to form a reaction product. The first electrode or anode may alternatively be referred to as a fuel or oxidizing electrode, and the second electrode may alternatively be referred to as an oxidant or reducing electrode. The half-cell reactions at the two electrodes are, respectively, as follows:  
           H2→2H++2 e− ½O 2 +2H++2 e−→H   2 O  
           [0003]    The external electrical circuit withdraws electrical current and thus receives electrical power from the cell. The overall fuel cell reaction produces electrical energy as shown by the sum of the separate half-cell reactions written above. Water and heat are typical by-products of the reaction.  
           [0004]    In practice, fuel cells are not operated as single units. Rather, fuel cells are connected in series, stacked one on top of the other, or placed side by side. A series of fuel cells, referred to as fuel cell stack, is normally enclosed in a housing. The fuel and oxidant are directed through manifolds to the electrodes, while cooling is provided either by the reactants or by a cooling medium. Also within the stack are current collectors, cell-to-cell seals and insulation, with required piping and instrumentation provided externally of the fuel cell stack. The stack, housing, and associated hardware make up the fuel cell module.  
           [0005]    The optimal operating level of components of the fuel cell system will depend upon the particular system operating level of the entire fuel cell system. Thus, for example, the optimal operating level of a blower for providing a process fluid to the fuel cell system will depend upon the particular system operating level of the fuel cell system. As the operating level of the fuel cell system increases, the optimal operating level of the blower will also increase. Analogously, as the operating level of the fuel cell decreases, the optimal operating level of the blower will decrease. In prior art systems, feedback from process parameters, such as cathode airflow, various temperatures and fuel cell voltages, are monitored and are used to either increase or decrease the operating level of individual components of the fuel cell system based upon the needs of the fuel cell system.  
         SUMMARY OF THE INVENTION  
         [0006]    In accordance with an aspect of the present invention, there is provided a method of operating a fuel cell system. The method comprises (a) operating a component of the fuel cell system at a component operating rate; (b) driving a load using the fuel cell; (c) measuring an operating rate of the fuel cell; (d) normally adjusting the component operating rate in dependence upon the operating rate of the fuel cell; and, (e) in response to selected changes in the operating rate of the fuel cell, indicative of corresponding changes in the demand from the load, delaying adjustment of the component operating rate.  
           [0007]    In accordance with a second aspect of the present invention, there is provided a fuel cell system comprising (a) a fuel cell for driving a load; (b) at least one measuring device for monitoring an operating rate of the fuel cell; (c) a controller for controlling an operation rate of a component of the fuel cell system based on the operating rate of the fuel cell; and, (d) means for detecting selected changes in the operating rate of the fuel cell, indicative of corresponding changes in the demand from the load, and in response thereto, delaying adjustment of the operation rate of the component. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    For a better understanding of the present invention and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings which show an embodiment of the present invention and in which;  
         [0009]    [0009]FIG. 1 is a schematic flow diagram of a first embodiment of a fuel cell gas and water management system in accordance with an aspect of the present invention; and,  
         [0010]    [0010]FIG. 2 is a block diagram of a controller for use in connection with the fuel cell gas and water management system of FIG. 1 in accordance with an embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0011]    For delivering and regulating process fluids, for example air and hydrogen gas streams, to a fuel cell stack, it is important to provide the process fluids in a required amount at a precise time. The following description will use as an example the delivery and regulation of air to a cathode portion of a fuel cell stack  12 . The same general principles can also be applied to other fluid deliveries, for example the hydrogen gas stream to the fuel cell stack.  
         [0012]    Referring to FIG. 1, this shows a schematic flow diagram of a fuel cell gas management system  10  in accordance with an aspect of the present invention. The fuel cell gas management system comprises a fuel supply line  20 , an oxidant supply line  30 , a cathode exhaust recirculation line  40  and an anode exhaust recirculation line  60 , all connected to the fuel cell  12 . It is to be understood that the fuel cell may comprise a plurality of fuel cells (a fuel cell stack) or just a single fuel cell. For simplicity, the fuel cell  12  described herein operates on hydrogen as fuel and air as oxidant and can be a Proton Exchange Membrane (PEM) fuel cell. However, the present invention is not limited to this type of fuel cells and is applicable to other types of fuel cells that rely on other fuels and oxidants.  
         [0013]    The fuel supply line  20  is connected to a fuel source  21  for supplying hydrogen to the anode of the fuel cell  12 . A hydrogen humidifier  90  is disposed in the fuel supply line  20  upstream from the fuel cell  12  and an anode water separator  95  is disposed between the hydrogen humidifier  90  and the fuel cell  12 . The oxidant supply line  30  is connected to an oxidant source  31 , e.g. ambient air, for supplying air to the cathode of the fuel cell  12 . An enthalpy wheel  80  is disposed in the oxidant supply line  30  upstream of the fuel cell  12  and also in the cathode recirculation line  40 . A cathode water separator  85  is disposed between the enthalpy wheel  80  and the fuel cell  12 . The enthalpy wheel  80  comprises porous material with a desiccant. In known manner, a motor  81  drives either the porous materials or a gas diverting element to rotate around the axis of the enthalpy wheel so that gases from the oxidant supply line  30  and the oxidant recirculation line  40  alternately pass through the porous materials of the enthalpy wheel. Dry ambient air enters the oxidant supply line  30  and first passes through an air filter  32  that filters out the impurity particles. A blower  35  is disposed upstream of the enthalpy wheel  80 , to draw air from the air filter  32  and to pass the air through a first region of the enthalpy wheel  80 . The enthalpy wheel  80  may be any commercially available enthalpy wheel suitable for fuel cell system, such as the one described in the applicant&#39;s co-pending U.S. patent application Ser. No. 09/941,934.  
         [0014]    A fuel cell cathode exhaust stream contains excess air, product water and water transported from the anode side, the air being nitrogen rich due to consumption of at least part of the oxygen in the fuel cell  12 . The cathode exhaust stream is recirculated through the cathode exhaust recirculation line  40  connected to the cathode outlet of the fuel cell  12 . The humid cathode exhaust stream first passes through the hydrogen humidifier  90  in which the heat and humidity is transferred to incoming dry hydrogen in the fuel supply line  20 . The humidifier  90  can be any suitable humidifier, such as that commercially available from Perma Pure Inc, Toms River, N.J. It may also be a membrane humidifier and other types of humidifier with either high or low saturation efficiency. In view of the gases in the anode and cathode streams, an enthalpy wheel or other device permitting significant heat and humidity interchange between the two streams cannot be used.  
         [0015]    From the hydrogen humidifier  90 , the fuel cell cathode exhaust stream continues to flow along the recirculation line  40  and passes through a second region of the enthalpy wheel  80 , as mentioned above. As the humid cathode exhaust passes through the second region of the enthalpy wheel  80 , the heat and moisture is retained in the porous paper or fiber material of the enthalpy wheel  80  and transferred to the incoming dry air stream passing through the first region of the enthalpy wheel  80  in the oxidant supply line  30 , as the porous materials or the gas diverting element of the enthalpy wheel  80  rotate around its axis. Then the cathode exhaust stream continues to flow along the recirculation line  40  to an exhaust oxidant water separator  100  in which the excess water, again in liquid form, that has not been transferred to the incoming hydrogen and air streams is separated from the exhaust stream. Then the exhaust stream is discharged to the environment along a discharge line  50 .  
         [0016]    A drain line  42  may optionally be provided in the recirculation line  40  adjacent the cathode outlet of the fuel cell to drain out any liquid water remaining or condensed out. The drain line  42  may be suitably sized so that gas bubbles in the drain line actually retain the water in the drain line and automatically drain water on a substantially regular basis, thereby avoiding the need of a drain valve that is commonly used in the field to drain water out of gas stream. Such a drain line can be used anywhere in the system where liquid water needs to be drained out from gas streams. Pressure typically increases with gas flow rate and water regularly produced or condensed, and a small flow rate of gas is not detrimental such as cathode exhaust water knockout separator and drain line  42 .  
         [0017]    The humidified hydrogen from the hydrogen humidifier  90  flows along the fuel supply line  20  to the anode water separator  95  in which excess water is separated before the hydrogen enters the fuel cell  12 . Likewise, the humidified air from the enthalpy wheel  80  flows along the oxidant supply line  30  to the cathode water separator in which excess liquid water is separated before the air enters the fuel cell  12 .  
         [0018]    Fuel cell anode exhaust comprising excess hydrogen and water is recirculated by a pump  64  along an anode recirculation line  60  connected to the anode outlet of the fuel cell  12 . The anode recirculation line  60  connects to the fuel supply line  20  at a joint  62  upstream from the anode water separator  95 . The recirculation of the excess hydrogen together with water vapor not only permits utilization of hydrogen to the greatest possible extent and prevents liquid water from blocking hydrogen reactant delivery to the reactant sites, but also achieves self-humidification of the fuel stream since the water vapor from the recirculated hydrogen humidifies the incoming hydrogen from the hydrogen humidifier  90 . This is highly desirable since this arrangement offers more flexibility in the choice of hydrogen humidifier  90  as the humidifier  90  does not then need to be a highly efficient one in the present system. By appropriately selecting the hydrogen recirculation flow rate, the required efficiency of the hydrogen humidifier  90  can be minimized. For example, supposing the fuel cell  12  needs 1 unit of hydrogen, hydrogen can be supplied from the hydrogen source in the amount of 3 units with 2 units of excess hydrogen recirculated together with water vapor. The speed of pump  64  may be varied to adjust the portion of recirculated hydrogen in the mixture of hydrogen downstream from joint  62 . The selection of stoichiometry and pump  64  speed may eventually lead to the omission of the hydrogen humidifier  90 .  
         [0019]    In practice, since air is used as oxidant, it has been found that nitrogen crossover from the cathode side of the fuel cell to the anode side can occur, e.g. through the membrane of a PEM fuel cell. Therefore, the anode exhaust actually may contain some nitrogen and possibly other impurities. Recirculation of anode exhaust may result in the build-up of nitrogen and poison the full cell. Preferably, a hydrogen purge line  70  branches out from the fuel recirculation line  60  from a joint or connection  74  adjacent the fuel cell cathode outlet. A purge control device  72  is disposed in the hydrogen purge line  70  to purge a portion of the anode exhaust out of the recirculation line  60 . The frequency and flow rate of the purge operation is dependent on the power at which the fuel cell  12  is running. When the fuel cell  12  is running at high power, it is desirable to purge a higher portion of anode exhaust. The purge control device  72  may be a solenoid valve or other suitable device.  
         [0020]    The hydrogen purge line  70  runs from the position  74  to a joint or connection  92  at which it joins the cathode exhaust recirculation line  40 . Then the mixture of purged hydrogen and the cathode exhaust from the enthalpy wheel  80  passes through the exhaust water separator  100 . Water is condensed in the water separator  100  and the remaining gas mixture is discharged to the environment along the discharge line  50 . Alternatively, either the cathode exhaust recirculation line  40  or the purge line  70  can be connected directly into the water separator  100 .  
         [0021]    Preferably, water separated by the anode water separator  95 , the cathode water separator  85 , and the exhaust water separator  100  is not discharged, but rather the water is recovered, from these separators respectively, along a line  96 , a line  84  and a line  94  to a product water tank (not shown).  
         [0022]    As is known to those skilled in the art, a coolant loop  14  runs through the fuel cell  12 . A pump  13  is disposed in the cooling loop  14  for circulating the coolant. The coolant may be any coolant commonly used in the field, such as any non-conductive water, glycol, etc. An expansion tank  11  can be provided in known manner. A heat exchanger  15  is provided in the cooling loop  14  for cooling the coolant flowing through the fuel cell  12  to maintain the coolant within an appropriate temperature range. FIG. 1 shows one variant, in which a secondary loop  16  includes a pump  17 , to circulate a secondary coolant. A heat exchanger  18 , e.g. a radiator, is provided to maintain the temperature of the coolant in the secondary loop and again, where required, an expansion tank  19  is provided. The coolant in the cooling loop  16  may be any type of coolant as the coolants in cooling loop  14  and  16  do not mix. However, it is to be understood that the second cooling loop is not essential.  
         [0023]    In the invention, as exemplified for the cathode air delivery, a time delay is introduced when a demand for spooling down blower  35  is generated during operation of the fuel cell system. When demand from a load  200  connected to the fuel cell  12  drops off, i.e. the current draw requirements, measured by amperemeter  250  (FIG. 2) go down, the flow of air is held high for a certain pre-set time (for example  10  seconds) at the earlier higher load conditions. This is done so that the fuel cell system  10  can quickly be responsive to any immediate load increase demand shortly after the load demand has decreased. A situation like this might arise when the fuel cell system is powering a moving vehicle and the driver has ceased accelerating, but immediately after slowing down again presses an accelerator pedal, or uses some other means, to increase speed again.  
         [0024]    A controller  300  of the system  10  of FIG. 1 is shown in FIG. 2, and compares the previous load level with the current load level, and holds the system air throughput at this level (corresponding to the previous load and operating level of the fuel cell system). The load changes that fall into this category of “abrupt” load changes are changes that occur at at least a pre-determined rate: thus “substantial” changes over “short” periods of time. The actual definition of change rate, “substantial” and “short” will depend on the application the fuel cell system is used in.  
         [0025]    By using a system according to the invention near instantaneous transient power output back to a previous load level is possible. In practical use, one transient in power demand (load current draw) may often be followed by another transient in power demand in the opposite direction. Transient power demand is typical for city driving conditions for a vehicle as mentioned earlier, for instance in stop-and-go traffic. In such a situation, a transient reduction in a power demand, resulting from a vehicle stopping or slowing down at a stoplight or due to traffic, may be followed very shortly by a transient increase in power as the way ahead clears for the vehicle and the driver applies more pressure to the accelerator. If the blower is operating at a low rate due to the prior reduction in load, the fuel cell system will be less able to quickly increase power output to meet increased demand. Thus, the controller controls the operation of the fuel cell system in a way that anticipates flow demands that may arise from probable fuel cell system user behavior.  
         [0026]    Referring to FIG. 2, the controller  300  is illustrated in a block diagram. The controller  300  includes a storage module  302  for storing a selected time lag and a selected rate of decrease in the load. Both the selected time lag and the selected rate of decrease in the load are selected based on the particular application of the fuel cell system, and may be subsequently modified to improve performance. A linkage module  306  of controller  300  is linked to amperemeter  250 , thereby enabling the controller  300  to monitor the load  200  placed on the fuel cell system  10 . As demand from the load  200  diminishes, the rate of decrease in the demand is communicated from the amperemeter  250  to the linkage module  306 , and from the linkage module  306  to a processor or logic module  308 . The processor or logic module  308  then determines whether the actual rate of decrease in the load  200  exceeds the threshold rate of decrease stored in the storage module  302 . If the rate of decrease in the load  200  does not exceed the threshold rate of decrease stored in the storage module  302  then the logic module  308  via linkage  306  will reduce the operating level of the blower  35  to correspond to the lower operating level of the fuel cell system  10  needed for load  200 . If, however, the rate of decrease in the load  200  exceeds the threshold rate of decrease stored in the storage module  302 , then the logic module  308  will delay reducing the operating level of the blower  35  by a period of time equal to the time lag stored in the storage module  302 . After this time lag, the logic module  308  will lower the operating level of the blower  35  to correspond to the lower operating level of the fuel cell system  10  needed for load  200 .  
         [0027]    Other variations and modifications of the invention are possible. For example, instead of, or in addition to, the operating rate of the blower  35  being regulated, the operating rate of the hydrogen recirculation pump  64 , and/or the operating rate of the coolant pump  13  as well as other components may be regulated. All such modifications are variations are believed to be within the sphere and scope of the invention as defined by the claims appended hereto.