Abstract:
Processes to shut down a fuel cell system are described. In one implementation ( 300 ), a load ( 215 ) is cyclically engaged and disengaged across a fuel cell stack ( 205 ) so as to deplete the fuel available to the system&#39;s fuel cells ( 205 ). Voltage and/or current thresholds may be used to determine when to engage and disengage the load ( 215 ) and when to terminate the shutdown operation. In another implementation ( 500 ), a variable load ( 405 ) is engaged and adjusted so as to deplete the fuel available to the system&#39;s fuel cells ( 205 ). As before, voltage and/or current thresholds may be used to determine when to adjust the load ( 405 ) and when to terminate the shutdown process. In still another implementation, a load ( 215  or  405 ) may be periodically engaged and disengaged during some portion of the shutdown process and engaged but adjusted during other portions of the shutdown process.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates a system and method for operating a fuel cell system and, more particularly, to a system and method for controlling fuel cell system shut-down operations. 
       BACKGROUND 
       [0002]    Fuel cells are electrochemical devices that convert chemical energy in fuels into electrical energy directly. In a typical operating cell, fuel is fed continuously to the anode (the negative electrode) and an oxidant is fed continuously to the cathode (positive electrode). Electrochemical reactions take place at the electrodes (i.e., the anode and cathode) to produce an ionic current through an electrolyte separating the electrodes, while driving a complementary electric current through a load to perform work (e.g., drive an electric motor or power a light). Though fuel cells could, in principle, utilize any number of fuels and oxidants, most fuel cells under development today use gaseous hydrogen as the anode reactant (aka, fuel) and gaseous oxygen, in the form of air, as the cathode reactant (aka, oxidant). 
         [0003]    To obtain the necessary voltage and current needed for an application, individual fuel cells may be electrically coupled to form a “stack,” where the stack acts as a single element that delivers power to a load. The phrase “balance of plant” refers to those components that provide feedstream supply and conditioning, thermal management, electric power conditioning and other ancillary and interface functions. Together, fuel cell stacks and the balance of plant make up a fuel cell system. 
         [0004]    Referring to  FIG. 1A , fuel cell  100  (shown in a top-down view) is configured to include anode inlet  105 , anode outlet  110 , cathode inlet  115 , cathode outlet  120 , coolant inlet  125  and coolant outlet  130 . Referring to  FIG. 1B , as noted above fuel cells (e.g., fuel cell  100 ) may be stacked to create fuel cell stack  135 , wherein each cell&#39;s anode, cathode and coolant passages are aligned. 
         [0005]    One operational issue unique to fuel cell systems concerns system start-up and shut-down operations. Unlike internal combustion power plants, fuel cell electrodes may be damaged if exposed to improper gases and/or gas mixtures. For example, an anode&#39;s exposure to air can be very damaging to the cell if not done properly. Similarly, shut-down operations that generate mixtures of gasses (e.g., hydrogen-air solutions) may detrimentally affect the fuel cell system during subsequent start-up operations. 
       SUMMARY 
       [0006]    In general, the invention provides methods to shutdown a fuel cell system. A method in accordance with one embodiment includes halting the flow of fuel and, thereafter, initiating the flow of an inert gas (e.g., nitrogen) to the anodes of a fuel cell stack while maintaining the flow of oxidizer to the cathodes. A load is then cyclically engaged and disengaged across the fuel cell stack so as to deplete the fuel available to the system&#39;s fuel cells. Voltage and/or current thresholds may be used to determine when to engage and disengage the load and when to terminate the shutdown operation. Once the fuel cells are substantially depleted of fuel, an oxidizer fluid may be flowed across both the anode and cathodes with the load engaged until a second voltage and/or current threshold is met. The oxidizer fluid flow may then be halted and the load disengaged. In another embodiment, a variable load is engaged and adjusted so as to deplete the fuel available to the system&#39;s fuel cells. As noted above, voltage and/or current thresholds may be used to determine when to adjust the load and when to terminate the shutdown process. In still another implementation, a load may be periodically engaged and disengaged during some portion of the shutdown process and engaged but adjusted during other portions of the shutdown process. 
         [0007]    Methods in accordance with the invention may be performed by a programmable control device executing instructions organized into one or more program modules. Programmable control devices comprise dedicated hardware control devices as well as general purpose processing systems. Instructions for implementing any method in accordance with the invention may be tangibly embodied in any suitable storage device. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    Figure A shows the layout of a single fuel cell ( 1 A) and fuel cell stack ( 1 B) in accordance with conventional prior art fuel cell technology. 
           [0009]      FIG. 2  shows a fuel cell system in accordance with one embodiment of the invention. 
           [0010]      FIG. 3  shows a shutdown process in accordance with one embodiment of the invention. 
           [0011]      FIG. 4  shows a fuel cell system in accordance with another embodiment of the invention. 
           [0012]      FIG. 5  shows a shutdown process in accordance with another embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    The following description is presented to enable any person skilled in the art to make and use the invention as claimed and is provided in the context of the particular examples discussed below, variations of which will be readily apparent to those skilled in the art. More specifically, illustrative embodiments of the invention are described in terms of fuel cells that use gaseous hydrogen (H 2 ) as a fuel, oxygen (O 2 ) as an oxidant in the form of air (a mixture of O 2  and nitrogen, N 2 ) and proton exchange or polymer electrolyte membrane (“PEM”) electrode assemblies. The claims appended hereto, however, are not intended to be limited by the disclosed embodiments, but are to be accorded their widest scope consistent with the principles and features disclosed herein. 
         [0014]    Referring to  FIG. 2 , in one embodiment of the invention fuel cell system  200  includes fuel cell stack  205 , balance of plant  210 , load  215  and switch  220 . Fuel cell stack  205  includes a plurality of fuel cells, aligned as illustrated in  FIG. 1B , with unsealed anodes and cathodes. As used herein, the term “unsealed” means that the designated element (e.g., anode) cannot hold a vacuum and is, when not operating, at substantially ambient pressure. As discussed in more detail below, in one embodiment, switch  220  is periodically cycled (i.e., closed and opened) to permit substantially all of the fuel present at, and in, the stack&#39;s anodes to be consumed in a safe, convenient and relatively rapid manner. 
         [0015]    Referring to  FIG. 3 , in one embodiment shutdown operation  300  begins by terminating H 2  flow and, thereafter, initiating the flow of N 2  or some other inert gas across the anode (block  305 ). In one embodiment, a single anode&#39;s volume of nitrogen is used in this manner. In another embodiment, nitrogen flow is maintained for the process&#39; entire duration. In yet another embodiment, no nitrogen purge is used. The general purpose of using nitrogen in this way is to remove or purge much of the fuel present at the anode although, it will be recognized, relatively large amounts of H 2  may remain absorbed in the electrode&#39;s catalyst. In general, if nitrogen is available, the minimum amount of nitrogen used in this manner would be one anode&#39;s volume, while the maximum nitrogen flow would be continued for the entire duration of the hydrogen consumption. Following initiation of the N 2  purge and in light of the continued O 2 /air flow across the cathode, switch  220  is closed to engage load  215  (block  310 ). In practice, load  215  may be engaged before, simultaneously with or following the initiation of N 2  purge operations. 
         [0016]    It will be recognized that balance of plant  210  includes fuel cell stack sensors such as, for example, voltage and/or current sensors for monitoring the activity of each, most or some fuel cells in fuel cell stack  205 . These sensors may be used in accordance with the invention to determine when each discharge cycle (block  315 ) is complete and when all discharge cycles are complete (block  325 ). 
         [0017]    Generally speaking, with load  215  engaged the voltage across each fuel cell will decrease as fuel at and within the cell&#39;s anode is consumed. For those implementations which monitor cell voltages, while the measured voltages remain above a specified first threshold (the “No” prong of block  315 ), load  215  remains engaged. When the measured voltages drop to this first specified threshold (the “Yes” prong of block  315 ), load  215  is disengaged via switch  220  (block  320 ). If all discharge cycles have not been completed (the “No” prong of block  325 ), a pause is provided to allow fuel cell voltages to equalize (block  330 ) before load  215  is reengaged (block  310 ). When the monitored fuel cell voltages indicate all discharge cycles have been completed (the “Yes” prong of block  325 ), N 2  flow across the anode is halted (if it is still active), load  215  is engaged and O 2 /air flow is initiated across the anode (while maintaining O 2 /air flow across the cathode) until all monitored fuel cell voltage&#39;s are below another specified threshold. At this point, fuel cell system  200  has been prepared for shutdown and all O 2 /air flow and further monitoring may be terminated (block  335 ). 
         [0018]    In one embodiment, a cycle is considered completed when any monitored (typically minimum) fuel cell&#39;s voltage drops to a specified value. Illustrative specified values include 0, 5, 10, 20, 50 and 75 millivolts (“mv”). In like manner, all discharge cycles may be considered complete when any monitored (typically minimum) fuel cell&#39;s voltage reaches a specified lower-limit value (e.g., 0, 5, 30, 50 or 75 mv) and the maximum monitored fuel cell&#39;s voltage is at or below a specified upper-limit voltage (e.g., 100, 150 or 200 mv). In another embodiment, the total stack voltage is monitored to determine when all hydrogen has been consumed (e.g., when the total stack voltage falls to a specified level or voltage—although it will be understood that it is presently important to ensure that no monitored cell&#39;s voltage drops below typically, zero mv). In accordance with the acts of block  335 , air flow is then initiated to the anode (recall, air flow is already provided to the cathode) with load  215  engaged until all monitored fuel cell voltages&#39; drop to yet another threshold (e.g., 10, 25, 50 or 75 mv). While the values provided here are illustrative, one of ordinary skill in the art will recognize that the precise values applicable to any given implementation will be dependent on a number of design factors such as, for example, the number of fuel cells in fuel cell stack  205 , the type of electrode used, the type of fuel and oxidant employed, the electrical resistance provided by load  215  and the age, age distribution and homogeneity of the fuel cells in fuel cell stack  205 . 
         [0019]    By way of example only, in a fuel cell system employing H 2  fuel, O 2 /air oxidant, a  220  cell fuel cell stack, PEM electrode assemblies and a 10 ohm (“Q”) load, a cycle is considered complete whenever any single monitored fuel cell&#39;s voltage drops to 0 mv. All discharge cycles are considered complete when any single monitored fuel cell&#39;s voltage drops to 25 mv and the maximum voltage measured at any monitored fuel cell is 200 mv. Following detection of this “all discharge cycles complete” condition, the load is engaged and air flow is initiated to both the anode and cathode until all monitored fuel cells register a voltage of 50 mv or less. Beginning with a substantially fully-charged fuel cell stack, an inter-cycle pause of between 1 to 2 seconds is typical. Start to finish, the described shutdown operation on the system identified here takes approximately 300 seconds, with load  215  engaged for about 60 seconds of this time over approximately 100 cycles. 
         [0020]    Referring to  FIGS. 4 and 5 , in another embodiment fuel cell system  400  utilizing variable load  405  may be shutdown in accordance with procedure  500 . In this approach, variable load  405  is continuously engaged and periodically adjusted so as to reduce the monitored fuel cell voltages&#39; to a specified shutdown value. Referring again to  FIG. 5 , in this approach fuel flow is terminated and a purge using N 2  or some other inert gas is initiated across the anode (block  505 ). Next, and while O 2 /air flow across the cathode is maintained, switch  220  is closed to engage variable load  405  (block  510 ). As before, load  405  may be engaged before, simultaneously with or following the initiation of N 2  purge operations. Initially, variable load  405  is set to a relatively high value so that little current flow is extracted from fuel cell stack  205 . In general, load  405  would initially be set to a relatively low value and slowly increased with time based on keeping the minimum monitored cell&#39;s voltage above a specified lower threshold (e.g., 0, 5, 30, 50 or 75 mv). While the fuel cells have not been depleted of residual fuel (the “No” prong of block  515 ), load  405  may be periodically adjusted (block  520 ). When the measured fuel cell voltages drop to a first specified threshold (the “Yes” prong of block  515 ), the N 2  purge is terminated and air flow across the anode is initiated. When the monitored fuel cell voltages are at a second threshold, load  405  is disengaged via switch  220  and air flow to both the anode and cathode is terminated (block  525 )—completing shutdown operation  500 . 
         [0021]    In still another embodiment, applicable to both of the above described operations, anode fluid (e.g., N 2  or another inert gas) may be recirculated so as to pass the same fluid over the anode multiple times. Doing this tends to keep fuel cell voltages more constant and as a result, the load (e.g.,  215  and  405 ) may be left engaged for longer periods of time—all other factors remaining the same. In yet another embodiment, maximum value cell voltages may be ignored. For example, as noted above a minimum fuel cell threshold may be used to determine when a cycle is complete and an average voltage level may be used to determine when the shutdown operation is complete (e.g., block  325  and  515 ). Implementations of this sort may simplify the process by performing a specified number of cycles. In yet another implementation, loads may be engaged and disengaged for specified amounts of time and for a specified number of cycles. 
         [0022]    In some embodiments, a fuel cell operational parameter other than voltage may be used to control the load. In principal, any fuel cell operational parameter indicative of the fuel cell&#39;s capacity to produce power may be used. For example, shutdown procedure  300  may use the rate of voltage decline during load engagement or the amount of current drawn from fuel cell stack  205  to determine when each or all discharge cycles are complete. It will be further recognized, shutdown procedure  500  may use similar operational parameter tests during the acts of block  515 . 
         [0023]    It will be recognized that using materials currently available, it is desirable to maintain monitored fuel cell voltages above zero to minimize carbon corrosion of the fuel cells&#39; electrodes. As different materials become available, this consideration may become less significant. As a result, fuel cell voltages may be allowed to drop closer to zero or even go “negative” before determining that each cycle (e.g., block  315 ) or all cycles (e.g.,  325  and  515 ) are complete. 
         [0024]    Various changes in the materials, components, circuit elements, as well as in the details of the illustrated operational methods are possible without departing from the scope of the following claims. For instance, the illustrative systems of  FIGS. 2 and 4  are not limited to hydrogen fueled, air oxidized fuel cell systems. In addition, switch  220  may be of any type practical—e.g., electromechanical or electronic. Further, the embodiments of  FIGS. 3 and 5  are illustrative only. For example, aspects of both shutdown operations  300  and  500  may be combined; a load may be periodically engaged and disengaged during one epoch and continuously engaged during a second epoch of the shutdown operation—either approach may be used first. In addition, acts in accordance with  FIGS. 3 and 5  may be performed by a programmable control device executing instructions organized into one or more program modules. Further, the systems of  FIGS. 2 and 4  and the processes of  FIGS. 3 and 5  are applicable to sealed anode and/or cathode systems. A programmable control device may be a single computer processor, a special purpose processor (e.g., a digital signal processor, “DSP”), a plurality of processors coupled by a communications link or a custom designed state machine. Custom designed state machines may be embodied in a hardware device such as an integrated circuit including, but not limited to, application specific integrated circuits (“ASICs”) or field programmable gate array (“FPGAs”). Storage devices suitable for tangibly embodying program instructions include, but are not limited to: magnetic disks (fixed, floppy, and removable) and tape; optical media such as CD-ROMs and digital video disks (“DVDs”); and semiconductor memory devices such as Electrically Programmable Read-Only Memory (“EPROM”), Electrically Erasable Programmable Read-Only Memory (“EEPROM”), Programmable Gate Arrays and flash devices.