Patent Application
20120064424
1. Field of the Invention
This invention relates generally to a system and method for determining reactant gas flows in a fuel cell stack and, more particularly, to a system and method for identifying undesirable reactant gas flows in a fuel cell stack by applying a perturbation frequency to the fuel cell stack, measuring the stack current and stack voltage in response thereto and using the current and voltage measurements to determine the real and complex fuel cell impedance.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack by serial coupling to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input reactant gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen reactant gas that flows into the anode side of the stack. The stack also includes flow channels through which a cooling fluid flows.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between the two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
As a fuel cell stack ages, the performance of the individual cells in the stack degrade differently as a result of various factors. There are different causes of low performing cells, such as cell flooding, loss of catalyst, etc., some temporary and some permanent, some requiring maintenance, and some requiring stack replacement to exchange those low performing cells. Although the fuel cells are electrically coupled in series, the voltage of each cell when a load is coupled across the stack decreases differently where those cells that are low performing have lower voltages. Thus, it is necessary to monitor the cell voltages of the fuel cells in a stack to ensure that the voltages of the cells do not drop below a predetermined threshold voltage to prevent cell voltage polarity reversal, possibly causing permanent damage to the cell.
Typically, the voltage output of every fuel cell in the fuel cell stack is monitored so that the system knows if a fuel cell voltage is too low, indicating a possible failure. As is understood in the art, because all of the fuel cells are electrically coupled in series, if one fuel cell in the stack fails, then the entire stack will fail. Certain remedial actions can be taken for a failing fuel cell as a temporary solution until the fuel cell vehicle can be serviced, such as increasing the flow of hydrogen and/or increasing the cathode stoichiometry.
Fuel cell voltages are often measured by a cell voltage monitoring sub-system that includes an electrical connection to each bipolar plate, or some number of bipolar plates, in the stack and end plates of the stack to measure a voltage potential between the positive and negative sides of each cell. Therefore, a 400 cell stack may include 401 wires connected to the stack. Because of the size of the parts, the tolerances of the parts, the number of the parts, etc., it may be impractical to provide a physical connection to every bipolar plate in a stack with this many fuel cells, and the number of parts increases the cost and reduces the reliability of the system.
A total harmonic distortion (THD) of the fuel cell stack voltage can also be measured and used as a cell voltage detection signal. Typically, however, this method is not reliable as it does not produce a consistent signal, where it may be producing an increasing THD under some conditions, a decreasing THD under other conditions or no change in the THD under other conditions.
In accordance with the teachings of the present invention, a system and method for determining reactant gas flow through a fuel cell stack are disclosed to determine potential stack problems, such as a possible low performing fuel cell. The method includes applying a perturbation frequency to the fuel cell stack and measuring the stack current and stack voltage in response thereto. The measured voltage and current are used to determine the real and complex impedance of the stack fuel cells, which can then be compared to predetermined fuel cell impedance or ratio of impedances for normal stack operation. If an abnormal fuel cell impedance is detected, then the fuel cell system can take corrective action that will address the potential problem.
Additional features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.
The following discussion of the embodiments of the invention directed to a system and method for monitoring reactant gas flow in a fuel cell stack to determine stack abnormalities is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses.
As will be discussed in detail below, a perturbation frequency is applied to the stack 12 to determine fuel cell impedance, which can be an indication of proper reactant gas flow for both the cathode side and anode side of the fuel cell stack 12. A different frequency would be required to detect the flow through the anode and cathode sides of the stack 12. The reason that a different frequency is needed for the cathode side and the anode side of the fuel cell stack 12 has to do with the catalyst configuration at the electrodes of the MEAs in the fuel cells. The perturbation frequency will be a relatively low frequency, depending on the particular flow being determined. The particular frequency would depend on the stack technology being used, and would typically be determined experimentally. For current stack technologies, a frequency signal in the 2-5 Hz range may be applicable for hydrogen gas flow through the anode side of the fuel cell stack 12 and a frequency signal of about 50 Hz may be applicable for the air flow through the cathode side of the fuel cell stack 12.
Spectral measurements of the fuel cell stack 12 are provided at box 26, which represents a voltage meter that measures the voltage across the stack 12, or at least a series of fuel cells in the stack 12, and a current meter that measures the current flow through the stack 12 or the current flow through a series of the fuel cells in the stack 12. The voltage and the current measurements from the box 26 are provided to an impedance calculation algorithm at box 28 that uses those measurements to calculate the real and complex impedance of the cells in the stack 12 or the group of series connected cells being measured. The impedance calculation algorithm uses the calculated impedance and, depending on whether it is the cathode air or the anode hydrogen gas being monitored, determines whether the calculated impedance is the optimal impedance by a comparison process, or ratio of impedances, for the fuel cells at the current system operating conditions. If the impedance of the fuel cells is not the desired impedance for those operating conditions, then the impedance calculation algorithm sends a signal to the summation junction 18 to adjust the desired spectral measurements on the line 16 so that the reactant control algorithm at the box 20 changes the reactant flow at the box 24. The reactant control algorithm will know which of the cathode or the anode side of the fuel cell stack 12 is currently being monitored and will for that time adjust only one or the other of the compressor or the hydrogen gas injectors, if necessary.
In addition, the system controller can take other remedial or corrective actions to improve the cell impedance, such as adjusting the humidification of the cathode inlet air, adjusting the coolant flow through and/or temperature of the fuel cell stack 12, reducing the stack load current, etc. Thus, in this manner, the system 10 is able to monitor cell voltages to detect abnormal operating conditions with only two connections to the fuel cell stack 12 for the voltage meter and the current meter, instead of the many connections that were typically required to measure fuel cell voltages to detect low performing cells.
In addition to detecting abnormal or improper system operating conditions, the system and method discussed herein can be used to trim or minimize the cathode air flow and the hydrogen gas flow to the fuel cell stack 12. Particularly, by identifying the minimum cathode air flow and/or anode gas flow to the stack 12 for the current stack power request or load, determining the cell impedance in the manner as discussed above can be used to ensure that this minimal flow is being achieved for efficient system operation. Thus, the compressor speed can be minimized and the amount of hydrogen provided at the stack 12 can be minimized for efficient operation.
The present invention contemplates any suitable technique for providing the perturbation frequency to the stack 42 for determining cell impedance in the manner as discussed above. In this non-limiting embodiment, the system 40 includes a load 54 having a certain resonate frequency, such as a suitable resistor, and a MOSFET switch 56 electrically coupled to the lines 46 and 48 across the stack 42, as shown. When power is being provided by the stack 42, the switch 56 is opened and closed at the desired frequency, i.e., the resonate frequency of the load 54, so that an AC frequency signal is applied to the stack 42 on top of the DC power signal provided by the stack 12. The voltage across the stack 42 and the current through the stack 42 are measured at the frequencies that the switch 56 is opened and closed. These measurements are used to determine both the real and reactive impedance of the cells 44 in the stack 42 in a manner that is well understood to those skilled in the art. The measurement of the voltage and current at the frequencies that the switch 56 is opened and closed to determine cell impedance has to do with the electrodes in the MEAs discharging as a capacitance when the switch 56 is opened. Further, each different catalyst material would provide a different cell impedance. When the cathode airflow is being determined, then the switch 56 is opened and closed at one desirable frequency and when the anode fuel flow is being determined, the switch 56 is opened and closed at a different frequency. In an alternate embodiment, the switch 56 may be some device that is able to provide both the cathode frequency and the anode frequency simultaneously.
In the discussion above, the perturbation frequency was provided by elements that were added to the system for that particular purpose. In alternate designs, the load 54 may be an existing component in the fuel cell system 10, such as end cell heaters, power converters, DC/DC boost converters, etc.
The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.
