Patent Application
20080152964
The invention generally relates to a technique and apparatus to regulate a reactant stoichiometric ratio of a fuel cell system.
A fuel cell is an electrochemical device that converts chemical energy directly into electrical energy. For example, one type of fuel cell includes a proton exchange membrane (PEM) that permits only protons to pass between an anode and a cathode of the fuel cell. Typically PEM fuel cells employ sulfonic-acid-based ionomers, such as Nafion, and operate in the 60° Celsius (C) to 70° C. temperature range. Another type employs a phosphoric-acid-based polybenziamidazole, PBI, membrane that operates in the 150° C. to 200° C. temperature range. 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:
H2→2H++2e− at the anode of the cell, and Equation 1
O2+4H++4e−→2H2O at the cathode of the cell. Equation 2
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.
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 fuel cell stack is one out of many components of a typical fuel cell system. For example, the fuel cell system may also include a cooling subsystem to regulate the temperature of the stack, a cell voltage monitoring subsystem, a control subsystem, a power conditioning subsystem to condition the power that is provided by the fuel cell stack for the system load, etc. The particular design of each of these subsystems is a function of the application that the fuel cell system serves.
The fuel cell system also may include a fuel processor that converts a hydrocarbon (natural gas, propane methanol, as examples) into the fuel for the fuel cell stack. To provide output power from the fuel cell stack, the reactant flows (i.e., the fuel and oxidant flows) to the stack must satisfy the appropriate stoichiometric ratios governed by the equations listed above. With respect to the fuel flow provided to the stack, the hydrogen stoichiometric ratio is defined as the ratio between the amount of hydrogen provided to the stack and the amount of hydrogen consumed by the stack. To maximize the efficiency of the stack, the hydrogen stoichiometric ratio should be minimized. Theoretically, the minimum hydrogen stoichiometric ratio is 1.1, which indicates that ten percent of the fuel provided to the stack is not consumed. In practice, however, the minimum achievable hydrogen stoichiometric ratio generally is greater than 1.1 and varies based on the output power provided by the stack to the load. To deal with this variation, a controller of the fuel cell system may monitor the output power of the stack and, based on the monitored output power, estimate the fuel flow to satisfy the hydrogen stoichiometric ratio. The controller regulates the fuel processor to produce this flow, and, in response to the controller detecting a change in the output power, the controller estimates a new rate of fuel flow and controls the fuel processor accordingly.
Due to non-ideal characteristics of the stack, it may be difficult to precisely predict the rate of fuel flow needed for a given output power. Moreover, as the fuel cell system ages, the fuel flow needed for a given output power may change. To take into account these uncertainties, the controller may build in a sufficient margin of error by causing the fuel processor to provide more fuel than is necessary to ensure that the cells of the stack receive enough fuel and, thus, are not starved. However, such a control technique may be quite inefficient, as the fuel cell stack typically does not consume all of the incoming fuel, leaving unconsumed fuel that may be burned off by an oxidizer of the fuel cell system. As the fuel cell system ages, this control technique may become even more inefficient as it does not take into account the degradation of the fuel cell system.
Thus, there is a continuing need for an arrangement and/or technique to address one or more of the problems discussed above.
In an embodiment of the invention, a technique useable with a fuel cell system that provides power to a load includes providing a reactant flow to a fuel cell stack, monitoring a plurality of operating parameters that are indicative of a reactant stoichiometric ratio, and determining a target range for at least one of the operating parameters based on an output power level provided by the fuel cell stack. The technique further includes determining a step size for adjusting the reactant stoichiometric ratio based on a difference between at least one of the monitored parameters and its target range. The technique also includes adjusting the reactant stoichiometric ratio in increments in accordance with the step size until a desired performance level of the system is reached. The desired performance level may be indicated when at least one of the operating parameters is approximately within its target range.
In another embodiment of the invention, a fuel cell system includes a fuel cell stack to provide power to a load, a fuel processor to provide a fuel flow to the fuel cell stack, and a circuit. The circuit is configured to monitor a plurality of operating parameters of the fuel cell system that are indicative of a hydrogen stoichiometric ratio, determine a target range for at least one of the operating parameters based on the output power level provided by a fuel cell stack, and determine a step size for adjusting the hydrogen stoichiometric ratio based on a difference between at least one of the monitored parameters and its target range. The circuit is further configured to adjust the hydrogen stoichiometric ratio in increments in accordance with the step size until at one of the monitored operating parameters is approximately within its target range.
In yet another embodiment of the invention, an article comprising a computer readable storage medium that is accessible by a processor-based system stores instructions. When executed by the processor-based system, the stored instructions cause the processor-based system to monitor a plurality of operating parameters of a fuel cell system that are indicative of a reactant stoichiometric ratio, determine a target range for at least one of the operating parameters based on an output power level provided by a fuel cell stack, and determine a step size for adjusting the reactant stoichiometric ratio based on a difference between at least one of the monitored operating parameters and its target range. The instructions further cause the processor-based system to adjust the reactant stoichiometric ratio in increments in accordance with the step size until at least one of the operating parameters is approximately within its target range.
Advantages and other features of the invention will become apparent from the following drawing, description and claims.
Referring to
In accordance with embodiments of the invention, a fuel processor 30 (a reformer, for example) of the fuel cell system 10 receives a hydrocarbon and produces a corresponding fuel flow (called “reformate”) to the fuel cell stack 20. The fuel flow from the fuel processor 30 may pass, for example, through a flow control 52 (one or more valves and/or a pressure regulator, as examples) to anode inlet 22 of the fuel cell stack 20. An air blower 34 may produce an air flow (i.e., the oxidant flow) that passes through the oxidant flow control 54 to a cathode inlet 24 of the fuel cell stack 20. The incoming oxidant flow to the fuel stack 20 passes through the oxidant flow channels of the fuel cell stack 20 to appear as cathode exhaust at a cathode outlet 28 of the stack 20, and the incoming fuel flow to the stack 20 passes through fuel flow channels of the fuel cell stack 20 to appear as anode exhaust at an anode outlet 26 of the stack 20.
In the embodiment illustrated in
In one embodiment, controller 40 may obtain indications representative of the various system operating parameters via, for example, communication bus 42 and communication bus 44. Controller 40 may provide control signals to various subsystems of system 10 in response to the indications of the monitored operating parameters via, for example, communication bus 46. For instance, the control signals may be provided to adjust the hydrogen stoichiometric ratio (referred to as the “H2 Stoic”), regulate the efficiency of system 10, recover from undesirable operating conditions, etc.
The H2 Stoic is the ratio between the amount of fuel provided to the stack 20 and the amount of fuel consumed by the stack 20 and, thus, also is an indicator of the operating efficiency of system 10. An optimal H2 Stoic is reached when substantially all of the fuel provided to the stack 20 is consumed by the stack 20. Theoretically, the optimal H2 Stoic is approximately 1.1, which indicates that ten percent of the fuel provided to the stack is not consumed. In practice, the optimal H2 Stoic is greater than 1.1 and varies based on the amount of power being provided to load 100. Typically, the optimal H2 Stoic at a high power output level is smaller than at a low power output level. In addition to varying with power output level, the optimal H2 Stoic also tends to increase as the fuel cell system 10 degrades. Accordingly, to achieve an optimal H2 Stoic for all operating conditions and as system 10 ages, the H2 Stoic may be adjusted while the system 10 is in operation based on indications of various system operating parameters.
In one embodiment, system operating parameters that may be used to guide the adjustment of the H2 Stoic are parameters that are indicative of the performance of stack 20 and the performance of fuel processor 30. Once an adjustment is made to the H2 Stoic, further adjustments may be implemented by observing the responses of various operating parameters to the initial adjustment. Operating parameters associated with the performance of stack 20 typically are indicated by the cell voltages measured by cell voltage monitoring system 32. For instance, various pieces of information derived from the cell voltages may indicate whether the stack 20 is starved of fuel, which indicates that the H2 Stoic is not at an optimal level.
With respect to parameters associated with the fuel processor 30, the fuel processor includes various subcomponents having operating parameters that are indicative of the H2 Stoic. As an example, the fuel processor 30 may include a steam mixing box 60 to mix the incoming fuel, air and steam streams before the mixture is heated and reacted in an autothermal reformer 62 of the fuel processor 30. In addition to the steam mixing box 60 and the autothermal reformer 62, the fuel processor 30 may include, for instance, a preferential oxidation reactor (PrOx) 64. If the temperature of any of the steam mixing box 60, the autothermal reformer 62, or the PrOx 64 is too low, this may be an indication that the fuel processor 30 may not be able to produce enough hydrogen to attain an optimal H2 Stoic or that high levels of carbon monoxide (i.e., carbon monoxide poisoning) may result in the stack 20. Thus, upon receipt of indications of parameters indicative of stack 20 performance or reformer 30 performance, controller 40 may implement a routine 200 to adjust certain system parameters and thereby adjust the H2 Stoic and/or the O2 Stoic to an optimal level that maximizes the efficiency and/or optimizes the performance of the fuel cell system 10.
Such a routine 200 is illustrated in the flow diagram of
The routine 200 illustrated in
Having determined the target range for the operating parameters, those parameters are observed (block 208). If any one or more of the operating parameters are outside of the target range (diamond 210) (e.g. above or below the target range), then controller 40 determines an appropriate step size for adjusting the reactant stoichiometric ratio to thereby bring the operating parameters within the target range (block 212). In one possible embodiment, the step size may be a fixed step size that has been predetermined. In other embodiments, an adaptive or variable step size may be used which may result in better overall system efficiency. For instance, the larger the difference is between the monitored operating parameters and the target range, the larger the step size of the adjustment can be. By adapting the step size based on a comparison between the target range and the monitored value, the reactant stoichiometric ratio may be more quickly brought to its optimal value, thus maximizing the efficiency of the operation of fuel cell system 10.
Once the step size of the reactant stoichiometric ratio adjustment has been determined, controller 40 then provides the appropriate control signals to adjust the reactant stoichiometric ratio. For instance, if the monitored operating parameters indicate that a non-optimal amount (either too much or too little) of fuel is being provided to stack 20, then controller 40 may provide a control signal to fuel processor 30 or flow control 52 to increase or decrease the fuel flow as needed. Alternatively, if the monitored operational parameters indicate that a temperature of a subsystem of the fuel processor 30 is out of range, such that either fuel starvation or carbon monoxide poisoning may result, controller 40 may provide an appropriate control signal to fuel processor 30 to increase or decrease the temperatures of the subsystems and/or to increase or decrease the flow of fuel provided to stack 20 as needed. Yet further, if the monitored operating parameters indicate that a non-optimal amount (either too much or too little) of oxidant is being provided to stack 20, then controller 40 may provide a control signal to air blower 34 or flow control 54 to increase or decrease the oxidant flow as needed.
After making the adjustment, the controller 40 observes a response of the system to the adjustment (block 215) and determines whether the monitored operating parameters have been brought within their target range (thus indicating that the desired performance level of system 10 has been achieved) (diamond 216). If not, then controller 40 continues to increment the adjustment until the target range is reached. Once the operating parameters are within the target range, controller 40 continues to monitor the operating parameters to determine whether further adjustments are needed while the system 10 is operating.
In some embodiments of the invention, circuitry other than the controller 40 may be used to perform one or more parts of the routine 200. For instance, in some embodiments, the cell voltage monitoring circuit 32 may determine whether a parameter is out of range and indicate to the controller 40 whether to increase or decrease the reactant stoichiometric ratio based on this determination. In other embodiments, the fuel processor 30 may determine whether an operating parameter is out of range and indicate to the controller 40 whether to increase or decrease the reactant stoichiometric ratio based on this determination. For purposes of simplifying the description below, it is assumed that the controller 40 determines whether the reactant stoichiometric ratio can be improved, although other variations are possible.
As mentioned above, there are numerous ways for the controller 40 to determine whether the reactant stoichiometric ratio is at an optimal level. For example,
After determining the step size, controller 40 provides control signals to adjust the fuel provided to stack 20 in accordance with the determined step size (block 314). Controller 40 then observes the response of the cell voltages to the adjusted fuel flow (block 315) and continues to adjust the fuel flow until the cell voltages are within the target range (diamond 316 and block 314). Once the cell voltages are within the target range, controller 40 returns to monitoring the cell voltages at block 308.
In addition to or as an alternative to observing the cell voltages, controller 40 may look at a cell ratio, which is derived from the measured cell voltages, to determine whether the H2 Stoic is at an optimal level. The cell ratio is the ratio between the lowest cell voltage in the stack 20 and the average of all the cell voltages. As with a cell voltage being outside of a target range, the cell ratio may be indicative of a non-optimal fuel flow provided to the stack 20 and, thus, a non-optimal H2 Stoic. A standard deviation of the cell voltages also may be examined to determine whether the H2 Stoic should be adjusted. Generally, the standard deviation may be used as an indicator of carbon monoxide poisoning, which would affect the manner in which the H2 Stoic may be adjusted.
It should be understood that routines 300 and 400 may be implemented separately or in conjunction with each other, various of the steps may be performed in different orders, and fewer or additional steps than those shown in the figures may be performed. In addition, other control loops may be used in combination with either of routine 300 or 400. For example, the controller 40 may adjust the fuel flow in response to a monitored output power of the fuel cell stack 20. However, the controller 60 continues to implement the control provided by the general routine 200 to obtain an optimal reactant stoichiometric ratio and thus maximize the efficiency and/or performance of the fuel cell system 10.
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.
