Abstract:
A fuel cell includes a fuel cell subsystem, a battery and a circuit. The fuel cell subsystem is adapted to furnish power to a load. The circuit is adapted to connect the battery to the load when the fuel cell subsystem substantially delays in responding to a change in the power.

Description:
This application claims the benefit under 35 U.S.C § 119(e) to U.S Provisional Patent Application Ser. No. 60/219,325, having a filing date of Mar. 31, 2000. 
    
    
     BACKGROUND 
     The invention relates to a fuel cell system that has an energy source backup. 
     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 →2 H   + +2 e   −  at the anode of the cell, and  
       
     
     
       
           O   2 +4 H   + +4 e   − →2 H   2   O  at the cathode of the cell.  
       
     
     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. 
     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. 
     The fuel cell stack may be part of a fuel cell stack system that supplies electrical power to an electrical load. For example, for a residential fuel cell system, the electrical load may be established by the various power consuming devices of a house. To furnish AC power to the house, the fuel cell system typically converts the DC voltage that is provided by the fuel cell stack into AC voltages. 
     Because the power that is demanded by the devices of the house may vary, the fuel cell system may control the rate at which the above-described electrochemical reactions occur for purposes of regulating the efficiency of the fuel cell stack. In this manner, the fuel cell system may include a fuel processor to convert a hydrocarbon (natural gas or propane, as examples) into a reformate that contains the hydrogen gas. The rate at which the fuel processor produces the hydrogen gas flow needs to be large enough to satisfy the stoichiometry that is dictated by the above-described equation. A larger power demand from the house typically requires a larger flow rate and thus, requires a higher rate of hydrogen production by the fuel processor. 
     A conventional fuel processor may have a relatively slow transient response that causes any increase in its rate of hydrogen production to significantly lag the increased demand for power. As a result, when the power that is demanded by the house suddenly increases, the cell voltages of the fuel cell stack may significantly decrease due to the lack of a sufficient hydrogen gas flow until the rate of hydrogen production by the fuel processor increases to the appropriate level. Due to the delayed response of the fuel processor, it is possible that the fuel cell stack may be damaged. For example, if an electrical load is placed on a fuel cell that is not adequately supplied with hydrogen, the cell can go from generating power to itself being an electrical load as water at the anode is electrolyzed to supply the protons passing through the membrane. In such a case, a negative voltage arises across the cell (the cell “goes negative”). This mode of operation in fuel cells has been known to irreparably damage the performance of such cells. Yet another problem arising from such a scenario is that the fuel cell is temporarily unable to meet the transient power demand. 
     Thus, there is a continuing need for an arrangement that addresses one or more of the problems that are stated above. 
     SUMMARY 
     In an embodiment of the invention, a fuel cell includes a fuel cell subsystem, a battery and a circuit. The fuel cell subsystem is adapted to furnish power to a load. The circuit is adapted to connect the battery to the load when the fuel cell subsystem substantially delays in responding to a change in the power. 
     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 
     FIG. 1 is a schematic diagram of a fuel cell system according to an embodiment of the invention. 
     FIGS. 2 and 3 illustrate voltages of the fuel cell system according to an embodiment of the invention. 
     FIG. 4 illustrates a cell voltage versus current characteristic. 
     FIG. 5 is a flow diagram depicting control of the stack current according to an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, an embodiment  10  of a fuel cell system in accordance with the invention includes a fuel cell stack  12  that consumes reactants (oxygen and hydrogen) to produce power for a load  20 . As an example, the fuel cell system  10  may be a residential fuel cell system that supplies power to a house. Thus, the load  20  may collectively represent the various power consuming devices of a house that are currently consuming power from the fuel cell system  10 . The power that is demanded by the house may vary over time and introduce power transients that may affect operation of the fuel cell stack  12 , as described below. 
     To regulate the fuel cell stack  12  for purposes of responding to these transients, the system  10  includes a controller  40  that monitors the output power of the fuel cell system  10  and controls a fuel processor  14  (of the system  10 ) accordingly to regulate a rate at which the fuel processor  14  produces hydrogen gas containing reformate that is provided to the fuel cell stack  12 . However, this control may not be adequate, as it is possible that the fuel processor  14  may have a relatively slow transient response. More specifically, when the power that is demanded by the load  20  increases suddenly, the fuel processor  14  may not respond quickly enough to prevent the fuel cells of the stack  12  from receiving a sufficient hydrogen gas flow. The deprivation of an adequate hydrogen gas flow, in turn, may cause the voltages of the fuel cells of the stack  12  to decrease to unacceptable levels, if not for the features of the fuel cell system  10  that are described below. 
     More particularly, the fuel cell stack  12  produces a terminal voltage (called V s ) that is the algebraic sum of all of the cell voltages of the stack  12 . During the times when a transient does not occur, a voltage (called V I ) that is received by a voltage regulator  35  is approximately equal to the V s  voltage. The voltage regulator  35  uses the V I  voltage to produce a regulated DC voltage (called V R ). An inverter  18 , in turn, converts the V R  voltage into AC voltages that are furnished to (via electrical lines  20 ) the load  20 . 
     Because the fuel cells of the stack  12  are electrically coupled together in series, the cells have the same current. In this manner, each fuel cell may be characterized by a cell voltage versus cell current plot, called a polarization curve, such as an exemplary polarization curve  220  that is depicted in FIG.  4 . As shown, the terminal voltage of the cell generally decreases with an increasing cell current. An exemplary operating point of the fuel cell is depicted in FIG. 4, an operating point at which the cell has a terminal voltage called V CELL  and a current called I CELL . The V CELL  voltage is above a threshold voltage called V TH  (−0.35 to −0.5 volts, for example) that approximately defines a breakpoint between a healthy fuel cell and an unhealthy fuel cell. 
     For example, contrary to what was previously believed (that a cell going negative to any extent would damage the cell), it was determined that for a fuel cell system with a Gore™ membrane electrode assembly utilizing between 13 and 130 standard liters per minute of reformate, fuel starving the system under an electrical load to produce a negative voltage up to −0.35 volts for 30 seconds produces no significant fuel cell damage. However, it is also noted that allowing the system to go negative to −0.5 volts will generally produce noticeable irreversible degradation. Thus, in this example −0.35 volts might be taken as V TH . Alternatively, as an additional example the value might also be taken in the range of up to −0.5 volts. In this manner, the health, efficiency and general safety of the fuel cell stack  32  may suffer if one of the cell voltages decreases below the V TH  voltage. 
     It is noted that the terminal voltage(s) of one or more of the fuel cells may go negative when a power transient occurs, as depicted in FIG.  4 . However, it has been discovered that a negative cell voltage is acceptable as long as the cell voltage remains above V TH , which, as an example, may be approximately −0.35 volts. 
     Referring back to FIG. 1, to keep each fuel cell voltage within an acceptable range, in some embodiments of the invention, the fuel cell system  10  includes an energy source, such as a battery  22 , to supplement the power that is furnished by the fuel cell stack  12  when a power transient occurs. This supplemental power, in turn, reduces the power drain on the fuel cell stack  12  until the fuel processor  14  recovers from the power transient to produce the correct flow of reformate. As an example, in some embodiments of the invention, the battery  22  and fuel cell stack  12  are both coupled to a node  30  via a circuit  24  that includes diodes  26  and  28 . The circuit  24  selectively switches the battery  22  and the fuel cell stack  12  to the node  30  to keep the V s  voltage within a predefined range. In this manner, the diode  28  has its anode coupled to the positive terminal of the battery  22 , and the cathode of the diode  28  is coupled to the node  30 . The anode of the diode  26  is coupled to positive terminal of the fuel cell stack  12 . The cathode of the diode  26  is coupled to the node  30 . The node  30  furnishes the V I  voltage. 
     Referring also to FIG. 2, due to this arrangement, the battery  22  keeps the V s  stack voltage from decreasing below a predefined voltage level (called V TH ). For example, at time T 0 , the V s  voltage may be near a voltage level (called V L ), the normal voltage level when a power transient has recently occurred. For this voltage level, the diode  26  conducts to couple the node  30  to the positive terminal of the fuel cell stack  12 . However, at the V L  voltage level, the diode  28  does not conduct and isolates the battery  22  from the node  30 . At time T 0 , a power transient may occur and cause the V s  voltage to decrease due to the finite response time of the fuel processor  14 , as depicted in FIG.  2 . As shown, near time T 1 , the V s  voltage approaches a voltage called V B , the terminal voltage of the battery  22 . When the V s  and V B  voltages are approximately equal (such as at time T 1 ), both the diodes  26  and  28  conduct so that the battery  22  and the fuel cell stack  12  are both furnishing current to the node  30 . 
     Referring also to FIG. 3, for this example, the transient lasts until time T 2 , the time at which the fuel processor  14  recovers to produce a sufficient level of reformate, and due to this recovery, the V s  voltage rises above the V B  voltage. During times T 1  to T 2 , the V s  voltage may tend to further decrease, as depicted by the curve  50 , if not for the circuit  24  (see FIG.  1 ). In this manner, the diode  26  does not conduct between times T 1  to T 2  to keep the V s  voltage form decreasing below the V B  voltage level. However, the diode  28  conducts during the T 1  to T 2  time interval to keep the V I  voltage limited to the V B  voltage, as depicted in FIG.  3 . 
     Thus, the circuit  24  and the battery  22  perform at least two functions. First, this arrangement keeps the V s  voltage (i.e., the terminal voltage of the of the fuel cell stack  12 ) from decreasing below the V TH  voltage level, a voltage level at which some of the fuel cell voltages may decrease below an acceptable voltage. Secondly, this arrangement places a minimum voltage limit on the V I  voltage so that the voltage regulator  35  is able to maintain regulation of the V R  voltage. 
     When the battery  22  is not connected to the node  33 , in some embodiments of the invention, the controller  40  activates a charger  32  (that is coupled to the terminals of the battery  22 ) to charge the battery  22 . The controller  40  may base activation of the charger  22  on a monitored stack voltage that is provided by a cell voltage measuring circuit  36 . 
     Referring to FIG. 1, besides the circuit  24  and the battery  22 , the fuel cell system  10  may have additional features to limit the cell voltages. For example, in some embodiments of the invention, the controller  40  places a maximum limit on the cell current to keep the minimum cell voltage (among all of the cell voltages of the fuel cell stack  12 ) from decreasing below the V TH  threshold voltage. The controller&#39;s choice of the maximum current limit may not be static, but rather, the choice of the maximum current limit may depend on other parameters, such as the minimum cell voltage, as described below. 
     In this manner, the polarization curve  220  that is depicted in FIG. 4 is for a particular hydrogen flow. As an example, for this given hydrogen flow, the controller  40  may set the maximum current limit near I TH , the current level at the V TH  threshold voltage. However, the polarization curve for a given fuel cell is not static, but rather, changes in the hydrogen flow cause the polarization curve for a given fuel cell to generally translate, or shift, along the current axis of the polarization curve. Using the polarization curve  220  as an example, an increase in the hydrogen flow shifts the curve  220  in a positive direction along the cell current axis to generally increase the level of the I TH  threshold current level. A decrease in the hydrogen flow shifts the curve  220  in a negative direction along the cell current axis to generally decrease the level of the I TH  threshold current level. Thus, an increase in the hydrogen flow generally causes the controller  40  to increase the maximum cell current limit, and a decrease in the hydrogen flow generally causes the controller  40  to decrease the maximum cell current limit. 
     To accomplish the above-described current limiting, in some embodiments of the invention, the controller  40  sets a maximum current limit of the voltage regulator  35  (see FIG. 1) of the fuel cell system  10 . In this manner, for purposes of setting the maximum current limit, the controller  40  furnishes a signal (called MAX) that may be, for example, a current or voltage signal that is received by the regulator  35  and indicates the maximum cell current limit. In this manner, the MAX signal is used to set the maximum output current of the regulator  35 , thereby establishing the maximum limit on the cell current. 
     The above-described control loop prevents the cells of the fuel cell stack  12  from “starving” 0  from a lack of a sufficient hydrogen flow due to an increase in the power that is demanded by the load. In this manner, the maximum limit that is placed on the cell current accommodates the transient response of the fuel processor  14  and allows time for the fuel processor  14  to “catch up” in its production of reformate. As the fuel processor  14  increases the production reformate in response to an increase in the power demand, the controller  14  may increase the maximum current limit. Thus, the fuel cell system  10  has at least two control loops: a first quick response control loop to limit sudden changes in the output power, an event that may “starve” the cells of fuel cell stack  12  due to the relatively slow response of the fuel processor  14 ; and a second possibly slower control loop to control the fuel processor  14  to regulate the hydrogen flow into the fuel cell stack  12 . 
     As described above, the controller  40  may at least partially base the maximum current limit on the minimum cell voltage. For purposes of monitoring the cell voltages and determining the minimum cell voltage, the controller  40  may use the voltage monitoring circuit  36 . The voltage monitoring circuit  36  is coupled to the fuel cell stack  12  to continually measures the cell voltages and to also provide indications of the measured cell voltages to the controller  40 . Either the voltage monitoring circuit  36  or the controller  40  (depending on the particular embodiment) may determine the minimum cell voltage, a voltage that the controller  40  uses to regulate the power output of the fuel cell system  10 . 
     In this manner, the controller  40  may execute a program  13  (stored in a read only memory (ROM) of the controller  40 , for example) that causes the controller  40  to perform a technique  112  that is depicted in FIG.  5 . In accordance with this technique, the controller  40  uses the voltage monitoring circuit  36  to measure (block  113  of FIG. 5) all of the cell voltages of the fuel cell stack  12 . From this information, the program  13  may also cause the controller  40  to select (block  114 ) the cell that has the minimum voltage, compare (block  116 ) the minimum cell voltage to the V TH  threshold voltage, determine (block  117 ) the cell current (via a current sensor  34  (see FIG.  1 ), for example) and set (block  118 ) the cell current limit based on these factors. In some embodiments, the controller  40  may not use the cell current as a factor when setting the maximum current limit, and in some embodiments, the controller  40  may periodically retrieve indications of the measured voltages and/or an indication of the lowest cell voltage from the voltage monitoring circuit  36 . 
     In some embodiments, the controller  40  regulates the minimum cell voltage to keep this voltage within an error band of voltages about a setpoint voltage, a voltage near the minimum threshold voltage. In the course of this regulation, when the minimum cell voltage becomes high enough to rise above the error band, the controller  40  increases the current limit. Conversely, when the lowest cell voltage becomes low enough to decrease below the error band, the controller  40  decreases the current limit. 
     When changing the current limit, in some embodiments of the invention, the controller  40  may adjust the current limit by an incremental amount that is a predetermined percentage (five percent, for example) of the maximum rated stack current. In other embodiments, the controller  40  may base the amount of change on other criteria or may base the change on a predetermined value. 
     In some embodiments, the controller  40  may use a proportional-integral-derivative (PID) control scheme, a heuristic control scheme or a look-up table (LUT)  11  (see FIG. 1) to perform the above-described dynamic current limiting. As examples, the LUT  11  may be stored in a read only memory (ROM) or a random access memory (RAM) of the fuel cell system  10 , such as a memory of the controller  40 . In some embodiments, the controller  40  may also be connected to a hydrogen sensor (not shown) in the anode exhaust. As an example, if the hydrogen concentration in the anode exhaust falls below a predetermined threshold (e.g., eight percent), the controller  40  can decrease the current limit. In this way, the hydrogen sensor can provide additional feedback to prevent the current draw on the stack  12  from starving the cells. 
     The cell voltages may vary over the lifetime and/or operating conditions of the fuel cell stack  12 . Thus, over time, different cells may furnish the minimum cell voltage. However, because the controller  40  bases its control on the most recently determined minimum voltage cell, the control by the controller  40  accounts for this occurrence. Thus, in some embodiments, the controller  40  dynamically determines the minimum cell as voltage during the course of regulation. 
     The fuel cell system  10  may use other techniques to counteract a sudden change in demand for output power. For example, in some embodiments of the invention, the fuel cell system  10  includes a hydrogen tank  44  (see FIG. 1) that stores hydrogen to supplement the reformate flow from the fuel processor  14  to counteract transients. In this manner, the controller  40  may control a hydrogen control valve  42  to regulate the introduction of the hydrogen into the reformate flow that enters the stack  12 . 
     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.