Abstract:
An apparatus includes a fuel cell subsystem, a first circuit and a second circuit. The fuel cell subsystem produces power for a load in response to receiving a flow of a reactant, and the first circuit is coupled to the fuel cell subsystem to halt the flow to the fuel cell stack and isolate the fuel cell stack from the load after the flow is halted. The second circuit monitors a characteristic of the fuel cell subsystem and receives power from the stack to dissipate a portion of the reactant that remains in the stack while the flow is halted.

Description:
BACKGROUND  
         [0001]    The invention generally relates to residual fuel dissipation for a fuel cell stack.  
           [0002]    A fuel cell is an electrochemical device that converts chemical energy produced by a reaction directly into electrical energy. For example, one type of fuel cell includes a proton exchange membrane (PEM), often called a polymer electrolyte membrane, that permits only protons to pass between an anode and a cathode of the fuel cell. At the anode, diatomic hydrogen (a fuel) is reacted to produce hydrogen protons that pass through the PEM. The electrons produced by this reaction travel through circuitry that is external to the fuel cell to form an electrical current. At the cathode, oxygen is reduced and reacts with the hydrogen protons to form water. The anodic and cathodic reactions are described by the following equations:  
           H 2 →2H + +2e −  at the anode of the cell, and  
           O 2 +4H + +4e − →2H 2 O at the cathode of the cell.  
           [0003]    A typical fuel cell has a terminal voltage near one volt DC. For purposes of producing much larger voltages, several fuel cells may be assembled together to form an arrangement called a fuel cell stack, an arrangement in which the fuel cells are electrically coupled together in series to form a larger DC voltage (a voltage near 100 volts DC, for example) and to provide a larger amount of power.  
           [0004]    The fuel cell stack may include flow plates (graphite composite or metal plates, as examples) that are stacked one on top of the other, and each plate may be associated with more than one fuel cell of the stack. The plates may include various surface flow channels and orifices to, as examples, route the reactants and products through the fuel cell stack. Several PEMs (each one being associated with a particular fuel cell) may be dispersed throughout the stack between the anodes and cathodes of the different fuel cells. Electrically conductive gas diffusion layers (GDLs) may be located on each side of each PEM to form the anode and cathodes of each fuel cell. In this manner, reactant gases from each side of the PEM may leave the flow channels and diffuse through the GDLs to reach the PEM. The PEM and its adjacent pair are often assembled together in an arrangement called a membrane electrode assembly (MEA).  
           [0005]    A fuel cell system may automatically or manually be shut down for purposes of repairing the system or for performing routine maintenance on the system. However, such a shut down may pose problems due to the residual fuel that is left in the fulel cell stack after the shut down. For example, the residual fuel is effectively potential energy that may deliver an electrical shock to a technician attempting to service the system. As another example of the problems posed by the residual fuel, the residual fuel may not satisfy the appropriate stoichiometric ratios and thus, may cause some of the cells of the fuel cell stack to exhibit negative voltages and enter unstable and potentially unsafe states in which these cells may produce hydrogen on the wrong side of the cells.  
           [0006]    Thus, there is a continuing need for an arrangement and/or technique to address one or more of the problems that are recited above.  
         SUMMARY  
         [0007]    In an embodiment of the invention, an apparatus includes a fuel cell subsystem, a first circuit and a second circuit. The fuel cell subsystem produces power for a load in response to receiving a flow of a reactant, and the first circuit is coupled to the fuel cell subsystem to halt the flow to the fuel cell stack and isolate the fuel cell stack from the load after the flow is halted. The second circuit monitors a characteristic of the fuel cell subsystem and receives power from the stack to dissipate a portion of the reactant that remains in the stack while the flow is halted.  
           [0008]    Advantages and other features of the invention will become apparent from the following description, from the drawing and from the claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0009]    [0009]FIG. 1 is a schematic diagram of a fuel cell system according to an embodiment of the invention.  
         [0010]    [0010]FIG. 2 is a flow diagram depicting a technique to dissipate residual fuel of a fuel cell stack of the system of FIG. 1 according to an embodiment of the invention.  
         [0011]    [0011]FIG. 3 is a schematic diagram of a cell voltage measuring circuit of the system of FIG. 1 according to an embodiment of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0012]    Referring to FIG. 2, an embodiment of a fuel cell system  10  in accordance with the invention includes a fuel cell stack  20  that, when the system  10  is in a power production mode, is capable of producing power for a load  50  (a residential load, for example) in response to reactant fuel and oxidant flows that are provided by a fuel processor  22  and an air blower  24 , respectively. It is possible that over the normal course of operation of the fuel cell system  10 , the system  10  may transition from the power production mode to a shut down mode, a mode in which the system  10  shuts off the fuel processor  22  and the air blower  24  to halt the flows of oxidant and fuel to the fuel cell stack  20 .  
         [0013]    In the shut down mode, the fuel cell system  10  opens a switch  29  (a relay, for example) to break an electrical connection between the fuel cell stack  20  and the load  50  and thus, isolate the load  50  from the stack  20 . The shut down mode may be needed, for various reasons, such as for purposes of shutting off power production from the fuel cell system  10  so that repairs or routine maintenance may be performed on the system  10 .  
         [0014]    It is possible that even though the fuel processor  22  is shut down, residual fuel may remain in the fuel cell stack  20  for a significant period of time, if not for the safety scheme that is imposed by the system  10 , as described below. Thus, described herein is a safety scheme for rapidly removing the residual fuel from the fuel cell stack  20  to quickly bring the stack  20  into a safe state.  
         [0015]    For purposes of dissipating the residual fuel in the stack  20 , one such safety scheme, or technique, in accordance with the invention, includes the continued operation of circuitry of the system  10  after the system  10  enters the shut down mode. In this manner, this circuitry may be used to perform some function during the power production mode of the system  10 . For example, the circuitry may be used to monitor a characteristic (cell voltages, for example) of the fuel cell stack  20 , monitor other operations of the fuel cell system  10  and/or generally aid in controlling the operation of the system  10  during the power production mode of the system  10 . To perform these functions, the circuitry draws power from the fuel cell stack  20 . Although during the shut down mode the circuitry may not be used to perform the functions that the circuitry performs during the power production mode, the circuitry continues to serve as a power sink during the shut down mode to dissipate the residual fuel in the fuel cell stack  20 .  
         [0016]    A possible advantage of this arrangement is that because of the rapid dissipation of the residual fuel, the fuel cell stack  20  may reach a safe state more rapidly than stacks of conventional fuel cell systems. Other and different advantages are possible.  
         [0017]    More specifically, in some embodiments of the invention, the above-described circuitry may include a cell voltage monitoring circuit  18 . The cell voltage monitoring circuit  18  is coupled to one or more of the cell terminals of the fuel cell stack  20  (via electrical lines  21 ) to receive its operating power and to receive indications of the cell voltages. In this manner, during the power production mode of the system  10 , the circuit  18  scans the cell voltages of the stack  20  and provides indications of the cell voltages and/or status of the stack  20  to a system controller  16  (of the system  20 ) via a serial bus  19 . As an example, the controller  16  may include a microcontroller or microprocessor.  
         [0018]    When a conventional fuel cell system transitions from its power production mode into the shut down mode, the conventional system powers down all circuitry that is used to operate the system in the power production mode. In this manner, the conventional system may include a power good circuit to generate a power status signal that shuts down such circuitry when the terminal voltage of the stack decreases below a predefined operating level.  
         [0019]    However, unlike conventional arrangements, the fuel cell system  10  permits the cell voltage monitoring circuit  18  to continue to operate in the shut down mode, as if the system  10  were in the power production mode. As the residual fuel in the fuel cell stack  20  dissipates, the terminal voltage of the stack  20  decreases to the point where the supply voltage or voltages that are received by the cell voltage monitoring circuit  18  may fall out of regulation, thereby causing the cell voltage monitoring circuit  18  to behave erratically. However, during the shut down mode, the purpose of the continued operation of the cell voltage monitoring circuit  18  is to draw power to dissipate the residual fuel in the fuel cell stack  20 . Therefore, the controller  16  is programmed to ignore the voltages and/or stack status that is indicated by the cell voltage monitoring circuit  18  during the shut down mode.  
         [0020]    In some embodiments of the invention, the fuel cell system  10  performs a technique  100  that is depicted in FIG. 2 to dissipate the residual fuel. As an example, the controller  16  may store a program  17  (in a memory of the controller  16 , for example) that the controller  16  executes to control the appropriate circuitry to perform the technique  100 . In this manner, the program  17  may be a routine that the controller  16  executes when the fuel cell system  10  is to be automatically or manually shut down. As examples, this shut down may be needed to prepare the fuel cell system  10  for scheduled maintenance or the shut down may be in response to a detected failure of the system  10 . Other reasons for the shut down of the fuel cell system  10  are possible.  
         [0021]    Thus, upon executing the program  17 , the controller  16  shuts down (block  102  of FIG. 2) the fuel processor  22  to halt the flow of fuel to the fuel cell stack  20  and place the fuel cell system  10  in the shut down mode. The controller  16  also isolates (block  104 ) the load  50  from the fuel cell stack  20 . For example, in some embodiments of the invention, the controller unit  16  may open (via a switch control line  31 ) the switch  29 , a switch that is coupled to a main output terminal  39  of the stack  20  and controls when power is routed from the fuel cell stack  20  to the load  50 . In this manner, the output terminal  39  provides a stack terminal voltage (called V TERM ) that power conditioning circuitry  41  of the system  10  receives and converts to the appropriate AC voltage for the load  50 . Because the switch  29  is coupled between the terminal  39  and this power conditioning circuitry  41 , the opening of the switch  29  isolates the load  50  from the fuel cell stack  20 , and the closing of the switch  29  couples the load  50  to the fuel cell stack  20 .  
         [0022]    Therefore, when the controller  16  opens the switch  29 , the power conditioning circuitry  41  becomes disabled. However, the voltage monitoring circuit  18  continues to operate and thus, continues to draw power from the stack  20  to dissipate the residual fuel. Although the power that is provided by the stack  20  may eventually be insufficient to keep the cell voltage monitoring circuit  18  functioning properly, the circuit  18  continues to draw power until the residual fuel is dissipated, as depicted in diamond  107  of FIG. 2.  
         [0023]    Referring to FIG. 1, in some embodiments of the invention, the power conditioning circuitry  41  includes a current sensor  27  that is coupled to the terminal switch  29  opposite from the terminal  39 . Thus, when the switch  29  is closed, the current sensor  27  provides (via an electrical line  33 ) an indication of the current being drawn from the fuel cell stack  20 . Therefore, via the current sensor  27  and the cell voltage monitoring circuit  18 , the controller  16  may monitor the cell voltages and states of the stack  20  during the power production mode.  
         [0024]    A voltage regulator  28  of the power conditioning circuitry  41  has its input terminal coupled to the output terminal of the current sensor  27 . When the system  10  is in the power production mode, the voltage regulator  28  receives approximately the V TERM  DC voltage from the terminal  39  of the fuel cell stack  20  and converts this DC input voltage into a regulated DC output voltage that the regulator  28  provides to an inverter  30  (of the power conditioning circuitry  41 ). Output terminals  32  of the inverter  30  furnish an AC wall voltage to the load  50 .  
         [0025]    Among the other features of the fuel cell system  10 , in some embodiments of the invention, the system  10  includes a coolant subsystem  40  that circulates a coolant through the fuel cell stack  20  to regulate a temperature of the stack  20 . The coolant subsystem  40  receives its operating power from the fuel cell stack  20  and may be shut down when the fuel cell system  10  enters the shut down mode, in some embodiments of the invention. The fuel cell system  10  may also include control valves  26  that regulate the oxidant and air flows into the fuel cell stack  20  and provide for emergency shut off of the flows. The fuel cell system  10  may also include gas/water separators  36  and  38  to remove water from flows of the system  10 , such as the outlet oxidant and fuel flows from the fuel cell stack  20 , for example. The gas/water separators  36  and  38  may route the collected water to a water tank of the coolant subsystem  40 , in some embodiments of the invention. The fuel cell system  10  may also include an oxidizer  34  to oxidize exhaust fuel (from the fuel cell stack  20 ) that is not consumed in cell reactions.  
         [0026]    Referring to FIG. 3, in some embodiments of the invention, the cell voltage monitoring circuit  18  may include several voltage regulators  150 , each of which receives an input voltage formed from the voltage across one or more cells and forms a DC voltage at its output terminal  154 . The voltages at the output terminals  154 , in turn, are received by various components of the cell voltage monitoring circuit  18  to power these components. As an example, these components may include voltage scanning units  156 , each of which scans the voltages of a particular group of cells. The voltage scanning units  156  may have grounds that are referenced to different cell terminals of the fuel cell stack  20 , as described in U.S. Pat. No. 6,140,820, entitled, “MEASURING CELL VOLTAGES OF A FUEL CELL STACK,” granted on Oct. 31, 2000. The cell voltage monitoring circuit  18  may also include an interface  158  that receives indications of the scanned voltages from the voltage scanning units  156 . This interface  158  may be coupled to a serial bus interface  162  (via a bus  161 ) of the circuit  18 . In this manner, a microprocessor  160  (of the circuit  18 ) may be coupled to the bus  161  to control the serial bus interface  162  to communicate indications of the cell voltages and/or the status of the stack  20  over the serial bus  19  to the controller  16 .  
         [0027]    Other embodiments are within the scope of the following claims. For example, circuitry other than the cell voltage measuring circuit  18  may be used to dissipate the residual fuel in other embodiments of the invention. In this manner, the circuitry may operate to perform some function in the power production mode and operate in the shut down mode to dissipate the residual fuel in the fuel cell stack  20 .  
         [0028]    While the invention has been disclosed with respect to a limited number of embodiments, those skilled in the art, having the benefit of this disclosure, will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of the invention.