Abstract:
A technique that is usable with a fuel cell stack includes providing a fuel flow and using at least some of the fuel flow to produce power with the fuel cell stack. A request is received to charge a battery. In response to the request, the technique includes determining if the remainder of the fuel flow is sufficient to produce additional power to charge the battery. Based on the determination, the remainder of the fuel flow is used to produce the additional power to charge the battery.

Description:
BACKGROUND  
         [0001]    The invention generally relates to a technique to control the charging of a battery using a fuel cell.  
           [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 polymer electrolyte membrane (PEM), often called a proton exchange 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  Equation 1  
           O 2 +4H + +4e − →2H 2 O at the cathode of the cell.  Equation 2  
           [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 more 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.  
           [0005]    A fuel cell system may include a fuel processor that converts a hydrocarbon (natural gas or propane, as examples) into a fuel flow for the fuel cell stack. For a given output power of the fuel cell stack, the fuel flow to the stack must satisfy the appropriate stoichiometric ratios governed by the equations listed above. Thus, a controller of the fuel cell system may determine the appropriate output power from the stack and based on this determination, estimate the fuel flow to satisfy the appropriate stoichiometric ratios. In this manner, the controller regulates the fuel processor to produce this flow, and in response to controller determining that the output power should change, the controller estimates a new rate of fuel flow and controls the fuel processor accordingly.  
           [0006]    The fuel cell system may provide power to an external load, such as a load that is formed from residential appliances and electrical devices that may be selectively turned on and off to vary the power that is consumed by the load. Thus, the power that is consumed by the load may not be constant, but rather, the power that is consumed by the load may vary over time and abruptly change in steps. For example, if the fuel cell system provides power to a house, different appliances/electrical devices of the house may be turned on and off at different times to cause the power that is consumed by the load to vary in a stepwise fashion over time.  
           [0007]    The fuel cell system may include a battery to temporarily supplement the power that the fuel cell stack provides to the load during times when the fuel processor does not provide a sufficient level of fuel to the stack to maintain the above-described stoichiometric equations. The battery may frequently need to be charged. However, the battery may need to be charged during times when the fuel cell stack is already providing the maximum amount of power that is possible with a given level of fuel flow from the fuel processor.  
           [0008]    Thus, there is a continuing need for an arrangement and/or technique to address one or more of the problems that are stated above.  
         SUMMARY  
         [0009]    In an embodiment of the invention, a technique that is usable with a fuel cell stack includes providing a fuel flow and using at least some of the fuel flow to produce power with the fuel cell stack. A request is received to charge a battery. In response to the request, the technique includes determining if the remainder of the fuel flow is sufficient to produce additional power to charge the battery. Based on the determination, the remainder of the fuel flow is used to produce the additional power to charge the battery.  
           [0010]    Advantages and other features of the invention will become apparent from the following description, drawing and claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0011]    [0011]FIG. 1 is a schematic diagram of a fuel cell system according to an embodiment of the invention.  
         [0012]    [0012]FIGS. 2 and 3 are flow diagrams depicting operation of the fuel cell system according to embodiments of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0013]    Referring to FIG. 1, an embodiment of a fuel cell system  10  in accordance with the invention includes a fuel cell stack  20  (a PEM-type fuel cell stack, for example) that is capable of producing power for an external load  50  (a residential load, for example) and parasitic elements (valves, fans, etc.) of the system  10  in response to fuel and oxidant flows that are provided by a fuel processor  22  and an air blower  24 , respectively. In this manner, the fuel cell system  10  controls the fuel production of the fuel processor  22  to control the fuel flow that is available for electrochemical reactions inside the fuel cell stack  22 . Control valves  44  of the fuel cell system  10  generally route most of this fuel flow to the stack  22 , with the remainder of the flow being diverted (via a conduit  55 ) to a flare, or oxidizer  38 .  
         [0014]    The power that is produced by the fuel cell stack  22  is consumed by the load  50 , parasitic elements of the fuel cell system  20  and possibly a power grid  56  (when switches  57  and  58  are closed, a scenario not assumed for purposes of simplifying the following description). Thus, in this manner, if the fuel flow inside the fuel cell stack  22  is sufficient to satisfy the appropriate stoichiometric relationships (defined by Eqs. 1 and 2 above), the fuel cell stack  22  produces the appropriate level of power for its loads. Unconsumed, or unreacted, fuel passes through the fuel cell stack  22  to the oxidizer  38 .  
         [0015]    The fuel cell system  10  may include a battery  45  that provides power to supplement the power that is provided by the fuel cell stack  22  when the fuel flow through the fuel cell stack  22  is not sufficient to produce enough power for its loads. However, the power boost that is provided by the battery  45  is temporary in nature, as the battery  45  stores a finite amount of charge. Therefore, after the stored energy is depleted from the battery  45 , the battery  45  may need to be charged.  
         [0016]    In some embodiments of the invention, the battery  45  may include a bank  41  of battery cells (lead acid battery cells, for example) that store the energy for the battery  45  and is charged when the battery  45  is charged. The battery  45  may also include a battery monitoring circuit  43  that provides a signal (called CR) that when asserted (driven high, for example) indicates a request to charge the battery  45 , i.e., indicates a request to charge the bank  41 . The battery monitoring circuit  43  may determine when the bank  41  needs to be charged by monitoring a terminal voltage (called VDC) of the bank  41 , a voltage that decreases below a predetermined threshold to indicate that charging is needed. Alternatively, the battery monitoring circuit  43  may monitor the VDC voltage and a current of the bank  41  (via a current sensor  69 ) to monitor a net charge flowing out of the battery. In this manner, when the net charge exceeds a predetermined threshold, the battery monitoring circuit  43  asserts the CR signal. The battery monitoring circuit  43  may also determine when charging is complete by monitoring the current into the battery  41  (via the current sensor  69 ). In this manner, when the current approaches a predefined minimum threshold level, the battery monitoring circuit  43  deems the charging to be complete and de-asserts (drives low, for example) the CR signal.  
         [0017]    Regardless of the technique used to determine when the bank  41  needs to be charged, the fuel cell system  10  responds to the resultant charge request in a manner that coordinates the fuel that is available (if any) for charging with the charging of the bank  41 . In this manner, such control factors as whether the fuel cell system  10  charges the bank  41  when requested and the rate at which the fuel cell system  10  charges the bank  41  is a function of the available fuel from the fuel processor  22  at its current operating point. Attempting to charge the bank  41  when a sufficient level of fuel is not available would result in reducing the terminal voltage of the fuel cell stack  22  below acceptable levels.  
         [0018]    The fuel that is available for charging may vary over the operation of the fuel cell system  10 , leaving times in which the bank  41  maybe charged, times in which the bank  41  cannot be charged, and times in which the bank  41  may be charged at a rate less than a maximum charge rate. The changing level of available fuel may be a function of the power that is consumed by the load  50 . In this manner, the power that is consumed by the load  50  may vary over time, as the load  50  represents a collection of individual loads (appliances and/or electrical devices that are associated with a house, for example) that may each be turned on and off. As a result, the power that is consumed by the load  50  may change to produce a transient. In the context of this application, a “transient in the power consumed by the load  50 ” refers to a significant change in the power (that is consumed by the load  50 ) that deviates from the current steady state level of the power at the time the transient occurs. The transient may have a time constant that is on the same order or less than the time constant of the fuel processor  22 . In the context of the application, the phrase “down transient” refers to a negative transient in the power that is consumed by the load  50 , and the phrase “up transient” refers to a positive transient in the power that is consumed by the load  50 .  
         [0019]    For various reasons, the fuel processor  22  may not respond quickly to up transients, leaving times at which no additional fuel is available to produce power to charge the bank  41  should a charge request appear. As examples, the fuel processor  22  may incapable of rapidly adjusting to up transients and/or the rate at which the fuel processor  22  is permitted to increase its fuel flow output may be limited, for purposes of decreasing the level of carbon monoxide (CO) that is produced by the fuel processor  22 . However, regardless of the reason for the fuel processor  22  not immediately responding to up transients, after a up transient, a period of time may exist in which the fuel processor  22  supplies an insufficient fuel flow for charging the bank  41 .  
         [0020]    Likewise, the fuel processor  22  may not respond quickly to down transients, leaving times in which additional fuel is available to produce the additional power needed for charging the bank  41 . Therefore, if the request for charging is generated during these times, the fuel cell system  10  may grant the request and charge the battery  41  at the appropriate rate.  
         [0021]    Even though a sufficient fuel flow may not be available when a charge request is generated, the fuel cell system  10  may, in response to the request, begin a process to increase the fuel output of the fuel processor  22  and defer the charging of the bank  41  until a sufficient fuel flow is available.  
         [0022]    Thus, in general, the fuel cell system  10  may use a technique  100  (depicted in FIG. 2) to respond to requests to charge the bank  41 . In the technique  100 , the fuel cell system  10  determines (diamond  102 ) whether a charge request has been generated. If not, control returns to diamond  102  until a charge request is received. Otherwise, if a charge request has been received, the fuel cell system  10  determines (diamond  104 ) whether there is available fuel for charging the bank  41 . The fuel cell system  10  may determine this by examining the power that is consumed by the load  50  and parasitic elements of the fuel cell system  10 ; and the fuel output of the fuel processor  22 . If fuel is available for charging, then the fuel cell system  10  regulates charges the bank  41 , as indicated in block  106 . If fuel is not available for charging, then the fuel cell system  10  returns to diamond  102  until the bank  41  can be charged.  
         [0023]    Referring back to FIG. 1 to describe more specific features of the fuel cell system  10 , in some embodiments of the invention, the fuel cell system  10  includes a controller  60  to process charge requests; monitor the power that is consumed by the load  50  and parasitic elements of the fuel cell system  10 ; and regulate the charging of the bank  41  accordingly. More particularly, in some embodiments of the invention, the controller  60  monitors the power that is consumed by the load  50  and the parasitic elements of the system  10  by monitoring the cell voltages, the terminal stack voltage (called “V TERM ”) and an output current (called I 1 ) of the fuel cell stack  20 . From these measurements, the controller  60  may detect up and down transients and determine the power that is being consumed from the fuel cell stack  20 .  
         [0024]    The controller  60  regulates the charging of the bank  41  by controlling (via an electrical communication line  53 ) a terminal voltage (called V DC ) of the bank  41  via a voltage regulator  30 , a regulator  30  that is coupled between a main output terminal  31  of the fuel cell stack  20  and the battery  45 . The controller  60  controls the output of the fuel processor  22  via electrical communication lines  46 .  
         [0025]    To obtain the above-described power measurements and monitor the cells of the fuel cell stack  20 , the fuel cell system  10  may include a cell voltage monitoring circuit  40  to measure the cell voltages of the fuel cell stack  20  and the VTERM stack voltage; and a current sensor  49  to measure the I 1  output current. The cell voltage monitoring circuit  40  communicates (via a serial bus  48 , for example) indications of the measured cell voltages to the controller  60 . The current sensor  49  is coupled in series with the output terminal  31  of the fuel cell stack  20  to provide an indication of the output current (via an electrical communication line  52 ). With the information about the power being consumed, the controller  60  may execute a program  65  (stored in a memory  63  of the controller  60 ) to process charge requests and control the charging of the bank  41 .  
         [0026]    Referring to FIG. 3, in some embodiments of the invention, the program  65 , when executed by the controller  60 , may cause the controller  60  to perform a technique  150  to process the charge requests. In the technique  150 , the controller  60  determines (diamond  152 ) whether a charge request needs to be processed. In this manner, a charge request may be pending until the controller  60  determines that sufficient fuel is available to charge the bank  41 . If no charge request needs to be processed, control returns to diamond  152 .  
         [0027]    If a charge request needs to be processed, then the controller  60  determines (block  154 ) the fuel (if any) that is available for charging. If the controller  60  determines (diamond  155 ) that sufficient fuel is not available, the controller  60  may operate the control valves  44  (via control lines  66 ) to route more fuel to the fuel cell stack  20  or control the fuel processor  22  to produce more fuel, and control returns to diamond  152 .  
         [0028]    If sufficient fuel is available for charging, then the controller  60  regulates (block  156 ) the VDC terminal voltage of the bank  41  at the appropriate level to accept a predetermined charge rate. In this manner, the controller  60  may adjust the VDC voltage of the bank  41  to set the rate at which the bank  41  charges. In some embodiments of the invention, if enough fuel is available to provide the additional power needed for charging the bank  41  at a predefined maximum charging rate, then the controller  60  charges the bank  41  at the maximum rate. Otherwise, the controller  60  downwardly adjusts the rate based on the fuel that is available.  
         [0029]    During the charging, the controller  60  regularly examines the CR signal to determine (diamond  158 ) if the bank  41  is charged. If so, control returns to diamond  152 . Otherwise, the controller  60  determines (diamond  160 ) if the power that is consumed from the fuel cell stack  20  has significantly changed during the charging. If so, control returns to block  154  to determine if changes in the charging rate or a halt of the charging needs to occur. Otherwise, control returns to block  156 .  
         [0030]    Referring back to FIG. 1, among the other features of the fuel cell system  20 , the system  20  may include the DC-to-DC voltage regulator  30  that regulates the V TERM  stack voltage to produce the V DC  voltage that may be used to charge the bank  41  and may be converted into an AC voltage for the load  50 . In this manner, the fuel cell system  20  includes an inverter  33  that converts the V DC  into an AC voltage that appears on output terminals  32  of the inverter  33  and system  10 . Besides being controlled by the controller  60  to divert some of the fuel flow that is received by the fuel cell stack  20  to the oxidizer  38  via the flow line  55 , the control valves  44  may also provide emergency shutoff of the oxidant and fuel flows to the fuel cell stack  20 . The control valves  44  are coupled between inlet fuel  37  and oxidant  39  lines and the fuel and oxidant manifold inlets, respectively, to the fuel cell stack  20 . The inlet fuel line  37  receives the fuel flow from the fuel processor  22 , and the inlet oxidant line  39  receives the oxidant flow from the air blower  24 . The fuel processor  22  receives a hydrocarbon (natural gas or propane, as examples) and converts this hydrocarbon into the fuel flow (a hydrogen flow, for example) that is provided to the fuel cell stack  20 .  
         [0031]    The fuel cell system  10  may include water separators, such as water separators  34  and  36 , to recover water from the outlet and/or inlet fuel and oxidant ports of the fuel cell stack  20 . The water that is collected by the water separators  34  and  36  may be routed to a water tank (not shown) of a coolant subsystem  54  of the fuel cell system  10 . The coolant subsystem  54  circulates a coolant (de-ionized water, for example) through the fuel cell stack  20  to regulate the operating temperature of the stack  20 . The fuel cell system  10  may also include the oxidizer  38  to burn any fuel from the stack  22  that is not consumed in the fuel cell reactions.  
         [0032]    For purposes of isolating the load  50  from the fuel cell stack  20  during a shut down of the fuel cell system  10 , the system  10  may include a switch  29  (a relay circuit, for example) that is coupled between the main output terminal  31  of the stack  20  and an input terminal of the current sensing element  49 . The controller  60  may control the switch  29  via an electrical communication line  51 .  
         [0033]    In some embodiments of the invention, the controller  60  may include a microcontroller and/or a microprocessor to perform one or more of the techniques that are described herein when executing the program  65 . For example, the controller  60  may include a microcontroller that includes a read only memory (ROM) that serves as the memory  63  and a storage medium to store instructions for the program  65 . Other types of storage mediums may be used to store instructions of the program  65 . Various analog and digital external pins of the microcontroller may be used to establish communication over the electrical communication lines  46 ,  51 ,  52  and  53 ; and the serial bus  48 . In other embodiments of the invention, a memory that is fabricated on a separate die from the microcontroller may be used as the memory  63  and store instructions for the program  65 . Other variations are possible.  
         [0034]    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.