Fuel cell system and method of controlling same

A fuel cell system includes a fuel cell having a stack composed of stacked cells that generate electrical power from anode gas and cathode gas; a cell voltage measuring device connected to the cells to measure the voltages of the cells, the cell voltage measuring device outputting a first predetermined value as a measurement value when the voltage of the cell being measured is at or below a detection limit; a stack voltage measuring device connected to the stack that measures the voltage of the stack; and a control unit that restricts the current flowing though the fuel cell when the difference between the sum of the individual voltages of the cells measured by the cell voltage measuring device and the voltage of the stack measured by the stack voltage measuring device exceeds a second predetermined value.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a fuel cell system provided with a fuel cell of a structure having a stack including a plurality of stacked cells, and a method of controlling same.

Priority is claimed on Japanese Patent Application No. 2005-42071, filed Feb. 18, 2005, the content of which is incorporated herein by reference.

2. Description of Related Art

In recent years, a fuel cell vehicle equipped with a fuel cell as a vehicle drive source has been proposed. As this type of fuel cell, there is known one having a stack including a plurality of cells, each cell consisting of a membrane and electrode assembly (MEA), in which a solid polymer electrolyte membrane is held between an anode and a cathode, and sandwiched by separators. By introducing hydrogen (fuel gas) to the anode and air (oxidizing gas) to the cathode, electricity is generated by an electrochemical reaction between the hydrogen and the oxygen. Moreover, water is produced with this generation of electrical energy (so-called “produced water”).

In this type of fuel cell, when, for example, generating electricity in a low-temperature environment, the reaction surface area of an MEA becomes covered with produced water and ice that is produced by freezing of the produced water. This leads to insufficient diffusion of the reactant gases, which may lower the cell voltage. Also, when an MEA is excessively dry, generation of electrical power does not fully take place, leading to a drop in cell voltage.

When the voltage of a cell drops, the conductivity of protons (hydrogen ions) produced at the anode side of the solid polymer electrolyte membrane of the MEA falls, leading to excessive heat generation due to movement of the protons from the anode side to the cathode side. As a result, the temperature of the fuel cell rises, causing degradation of the components (electrolyte membrane, catalyst layer, separators) constituting the fuel cell.

As technology aimed at prevention of this situation, Japanese Unexamined Patent Application, First Publication No. 2002-313396 proposes detecting drops in cell voltage and stopping the system when the cell voltage falls below a predetermined voltage.

When the conductivity of the aforementioned protons (hydrogen ions) falls in one cell that, constitutes the fuel cell (such a cell suitably being called a “failed cell”), the electromotive force generated at the other cells acts on that portion. This is equivalent to the state in which, for example, when one battery in a group connected in series acts as resistance without supplying electric power, the electromotive force of the other batteries ends up acting on the portion serving as resistance. Thereby, the drop in voltage at the failed cell acting as resistance increases, the voltage becoming extremely lower than during normal operation to be well below 0 V.

In this state, the performance of the failed cell suffers due to the heat generation that occurs at the failed cell due to the application of the electromotive force of the other cells, in addition to heat generation by-power generation. Accordingly, in order to prevent this state, it is extremely important to detect the voltage of each cell not only when in a normal state but also when in a state of power generation failure.

In order to detect a drop in the above-mentioned cell voltage, a sensor is therefore required that detects a wide range of voltages including the range of 0 V and less. But generally there is an inverse relationship between the detection accuracy and the detection range of a sensor. The problem therefore arises that when performing control using a sensor with a wide range of detection, the detection accuracy within the cell voltage range during normal operation falls. Also, installing another sensor just to detect drops in the cell voltage, apart from the sensor that detects the cell voltage during normal operation, complicates the entire system, leading to an increase in the number of components and cost as well as power consumption.

SUMMARY OF THE INVENTION

It is thus the object of the present invention to provide a fuel cell system that can suitably grasp the power generating state of a cell and ensure the power generation performance of a fuel cell while minimizing the number of parts, cost and power consumption, and a method of controlling the same.

The present invention provides a fuel cell system including: a fuel cell having a stack composed of stacked cells that generate electrical power from anode gas and cathode gas; a cell voltage measuring device connected to the cells to measure the voltages of the cells and output a first predetermined value as a measurement value when any of the cell voltages is at or below a detection limit; a stack voltage measuring device connected to the stack that measures the voltage of the stack; and a control unit that restricts the current flowing through the fuel cell when the difference between the sum of the individual voltages of the cells measured by the cell voltage measuring device and the voltage of the stack measured by the stack voltage measuring device exceeds a second predetermined value.

According to the present invention, since the detectable range of the cell voltage measuring device can be limited to the voltage range at the time of normal operation of the cell or plurality of cells, the measurement accuracy of the cell voltage measuring device can be sufficiently maintained even during normal power generation. The stack voltage measuring device also has sufficient accuracy required for measurement of the stack voltage.

When the conductivity of protons (hydrogen ions) falls in one cell that makes up the stack, and the electromotive force from the other cells acts on this cell, causing the voltage to fall significantly lower than during normal operation (that is, when the cell becomes a “failed cell”), the voltage of this failed cell falls to the detectable voltage limit or lower of the voltage measuring device. For this reason, the measured value of the failed cell becomes a fixed value. Meanwhile, the voltage which fell extremely due to the cell failure is reflected in the stack voltage measured by the stack voltage measuring device. Therefore, when the difference between the sum voltage of one or a plurality of cells measured by the cell voltage measuring device and the voltage of the stack measured by the stack voltage measuring device exceeds the first predetermined value, it can be determined that cell failure has occurred. Thereupon, the cell or cell group (plurality of cells) in which the fixed value is output by the cell voltage measuring device can be specified as a failed cell or failed cell group. Moreover, by restricting the current flowing through the fuel cell, excessive heat generation by the failed cell or cell group can be suppressed, thereby preventing degradation of the parts constituting the fuel cell and enabling continued operation of the system. In this manner, a failed cell can be detected without newly installing a sensor just to detect drops in the cell voltage. Thereby, the power generating state of the cell can be suitably grasped and the power generation performance of the fuel cell can be secured while minimizing the number of components and cost as well as power consumption. Here, the first predetermined value is a value which should be ensured to prevent degradation of a fuel cell. In other words, electrical power generation is permitted as long as the difference between the sum of the cell voltages and the stack voltage is equal to the first predetermined value or less.

The control unit may stop power generation of the fuel cell when the current flowing through the fuel cell cannot be restricted.

In this case, even when the current value cannot be restricted at the occurrence of the failed cell, by stopping power generation by the fuel cell, heat generation by the failed cell and heat generation caused by the electromotive force of other cells acting on the failed cell can be eliminated. This can prevent excessive heating of the failed cell and ensure the power generation performance of the fuel cell.

The present invention also provides a fuel cell system including: a fuel cell having a stack composed of stacked cells that generate electrical power from anode gas and cathode gas; a cell voltage measuring device connected to the cells to measure the voltages of the cells and output a first predetermined value as a measurement value when any of the cell voltages is at or below a detection limit; a stack voltage measuring device connected to the stack that measures the voltage of the stack; and a control unit that stops power generation by the fuel cell when the difference between the sum of the individual voltages of the cells measured by the cell voltage measuring device and the voltage of the stack measured by the stack voltage measuring device exceeds a second predetermined value.

Furthermore, the present invention provides a vehicle provided with a vehicle body, wheels attached to the vehicle body, and the above-mentioned fuel cell system.

According to this invention, since a failed cell can be detected without newly installing a sensor just to detect drops in the cell voltage, any required increase in space can be suppressed. Since cost and power consumption can be minimized, an increase in fuel consumption can be prevented. Also, the power generating state of the cells can be suitably grasped. This can serve to improve the reliability of running performance of vehicles in which fluctuations in air temperature, humidity or required production of electricity per unit time are great.

Moreover, the present invention provides a method of controlling a fuel cell system equipped with a fuel cell having a stack composed of stacked cells that generate electricity from anode gas and cathode gas, the method including a step to measure the voltages of the cells; a step to output a first predetermined value as a measurement value when the voltage of the cell being measured is at or below a detection limit; a step to measure the voltage of the stack; and a step to restrict the current flowing through the fuel cell when the difference between the sum of the voltages of the cells and the voltage of the stack-exceeds a second predetermined value.

Moreover, the present invention provides a method of controlling a fuel cell system with a fuel cell having a stack composed of stacked cells that generate electricity from anode gas and cathode gas, the method including a step to measure the voltages of the cells; a step to output a first predetermined value as a measurement value when the voltage of the cell being measured is at or below a detection limit; a step to measure the voltage of the stack; and a step to stop power generation of the fuel cell when the difference between the sum of the voltages of the cells and the voltage of the stack exceeds a second predetermined value.

DETAILED DESCRIPTION OF THE INVENTION

The fuel cell system in the embodiment of the present invention and method of controlling the same is described below with reference to the accompanying drawings. The present embodiment describes a fuel cell system in which a fuel cell is mounted in a vehicle.

FIG. 1is an overall configuration diagram of the fuel cell system in the embodiment of the present invention. A fuel cell2shown in the drawing consists of a stack4composed of a plurality of cells3stacked together, with the stack4being sandwiched by terminal plates5. Each cell3has a membrane and electrode assembly (MEA), in which a solid polymer electrolyte membrane made of, for example, a solid polymer ion-exchange membrane is held between an anode and a cathode, with this membrane and electrode assembly being sandwiched by separators. Hydrogen is supplied to the anode of each cell as fuel, while air including oxygen is supplied to the cathode as oxidant. Hydrogen ions produced at the cathode by catalytic reaction pass through the electrolyte membrane to the cathode, where they react electrochemically with oxygen to generate electricity.

Water is produced at the cathode side during power generation, and since a portion of the produced water that occurs at the cathode side back-diffuses through the electrolyte membrane to the anode side, produced water is also present at the anode side.

Hydrogen supplied from a hydrogen tank6passes through a hydrogen supply route8via a cutoff valve7and a regulator (not shown) to be supplied to the anode of the fuel cell2.

Also, a hydrogen off-gas discharge path11is connected to a dilution box (not shown), with used hydrogen off-gas being discharged from the hydrogen off-gas discharge path11to the dilution box.

Air is fed to an air supply path14by a compressor13, to be supplied to the cathode of the fuel cell2. The air supplied to the cathode of the fuel cell2, after being supplied to power generation, is discharged as off-gas, along with residual water such as produced water on the cathode side, from the fuel cell2to an air off-gas discharge path15.

The air off-gas discharge path15is connected to the aforementioned dilution box (not shown), and air off-gas discharged from the air off-gas discharge path15is mixed with hydrogen off-gas in the dilution box. Hydrogen off-gas discharged from the hydrogen off-gas discharge path11is thereby diluted by the dilution box to a predetermined concentration or less.

Moreover, the fuel cell2is provided with a cooling water path (not shown) equipped with a circulating pump that circulates the cooling water. Circulating cooling water during operation of the fuel cell2maintains the fuel cell2at a temperature suitable for electrochemical reaction (for example, 80° C.).

Also, the fuel cell2is connected via wiring17to a load16such as a motor that drives the vehicle, with power obtained by electric power generation by the fuel cell2being supplied to the load16via the wiring17.

In the present embodiment, a current sensor22is provided in the path of the wiring17connected to the load16in order to measure a current I of the fuel cell2. In addition, a stack voltage sensor23that measures a voltage Vs of the fuel cell stack4and cell voltage sensors25that measure a voltage Vci (i=1 to n, with n being the number of cells) of the cells3are also provided.

A control unit24for controlling the fuel cell system1is provided in the fuel cell system1. The ignition switch (IG SW) is connected to this control unit24. The ignition ON and OFF (IG-ON, IG-OFF) signal from the ignition switch, and the output values from the stack voltage sensor23and the cell voltage sensors25are input into the control unit24. The control unit24outputs a signal to drive the cutoff valve7, the compressor13, and a hydrogen purge valve12based on these values and signal that were input.

Each cell voltage sensor25is constituted to output a fixed value (for example, 0 V) as a measurement value when the voltage value of the respective cell is at or below a detection limit. This ensures the measurement accuracy required during normal power generation of the cells3(for example, 0 to 1.3 V) (seeFIG. 4). Also, the stack voltage sensor23is one having sufficient accuracy for measuring the voltage Vs of the stack4.

FIG. 2is a flowchart showing the process of cell voltage determination of the fuel cell system shown inFIG. 1.

First, in step S10, a stack voltage A (=Vs) is measured by the stack voltage sensor23. Next, each cell voltage (Vci) is measured by its respective cell voltage sensor25, and the sum B (=ΣVci) of the cell voltages is computed. Then in step S14, the difference between the sum B of the cell voltages and the measured value A of the stack voltage is calculated, and a determination is made as to whether this difference is greater than or equal to a predetermined value ΔV1. If the determination result is YES, the process proceeds to step S16, and if the determination result is NO, the process returns to the process of step S10.

The meaning of this determination process will now be explained with reference. toFIGS. 3A and 3B.FIG. 3Ais an explanatory drawing showing the state of normal power generation, andFIG. 3Bis an explanatory drawing showing the state when a failed cell has occurred. First, as shown inFIG. 3A, when each cell3is generating electrical power normally, since each cell voltage sensor25measures the voltage of its respective cell3with sufficient accuracy, the measured values are equivalent to the actual voltages. Also, the sum of the voltages generated by the cells3is reflected by the voltage measured by the stack voltage sensor23. Therefore, when each cell3is normally generating power, the difference between the sum ΣVci of the voltages of the cells3measured by the cell voltage sensors25and the stack voltage Vs measured by the stack voltage sensor23is within the predetermined value ΔV1.

FIG. 3Bshows a drop in conductivity of the protons (hydrogen ions) in one of the cells constituting the stack4. In this case, the electromotive force from the other cells3acts on that cell, causing its voltage to fall far lower than during normal operation. When this happens, (that is, when a failed cell3′ has occurred), the power generation voltage of this failed cell3′ is lower than the voltage detectable by the cell voltage sensor25(0 V). For this reason, the voltage value of the failed cell3′ in the cell voltage sensor25becomes a fixed value (0V).

Meanwhile, the voltage (−15 V) that fell extremely due to the failed cell3′ is reflected in the stack voltage Vs measured by the stack voltage sensor23. Therefore, when the difference between the sum ΣVci (=B) of the voltages of the cells3(including the voltage of the failed cell3′) measured by the cell voltage sensors25and the stack voltage Vs (=A) measured by the stack voltage sensor23exceeds the predetermined value ΔV1, it can be determined that the failed cell3′ has occurred, and the cell3′ in which the fixed value has been output by the cell voltage sensor25can be specified as a failed cell. Here, the predetermined value ΔV1is a value which should be ensured to prevent degradation of the fuel cell2. That is, power generation is permitted as long as the difference between the sum ΣVci of the cell voltages and the stack voltage Vs is within the predetermined value ΔV1.

In step S16, a determination is made as to whether it is possible to restrict the current1sent to the load16. If this determination result is YES, the process proceeds to step S18, and if the determination result is NO, the process proceeds to step S20. In step S18, the current I sent to the load16is restricted, and the process returns to step S10. As ways of restricting the current I, the value of the current I itself may be decreased, or the time of sending the current I may be restricted. By performing this process, heat generated by the failed cell3′ itself can be suppressed, and by reducing the electromotive force acting on the failed cell3′, heat generated by the electromotive force can be suppressed. Thereby, power generation can be continued while minimizing degradation of the failed cell3′.

In step S20, the fuel cell system1is stopped. Specifically, the cutoff valve7is closed to cut off the supply of hydrogen to the anode of the fuel cell2, and the compressor13is stopped to cut off the supply of air to the cathode of the fuel cell2. The process in the flowchart is then terminated.

In the present embodiment, since the fuel cell system1is a vehicle, although stoppage of the fuel cell system1means stoppage of the vehicle, when running is possible by another means, the process of step S20may be replaced with stopping the power generation of the fuel cell2.

As explained above, since the detectable range of the cell voltage sensor25can be limited to the voltage range during normal operation of the cells3according to the present embodiment, the measurement accuracy of the cell voltage sensor25can be maintained at accuracy sufficient during normal power generation. The stack voltage sensor23can also be one with a sufficient accuracy for measurement of the voltage Vs of the stack4.

Moreover, by restricting the current flowing through the fuel cell2, excessive heat generation by the failed cell3′ can be suppressed. Thereby, degradation of the parts constituting the fuel cell2can be prevented and operation of the fuel cell system1can be continued. Also, in the present invention, a failed cell3′ can be detected without newly installing a sensor just to detect drops in the cell voltage. Thereby, the power generating state of the cells3can be suitably grasped and the power generation performance of the fuel cell2can be secured while minimizing the number of components and cost as well as power consumption.

Next, a different process of cell voltage determination of the fuel cell system of the present invention than the one shown inFIG. 2will be explained usingFIG. 5. The point of difference inFIG. 5from the process ofFIG. 2is that, as shown in step S14′, it is determined whether or not the difference between the sum B of the cell voltages and the measured value A of the stack voltage is equal to or greater than the predetermined value ΔV2. If the determination result is YES, the process proceeds to step S20and the fuel cell system1is stopped.

In this manner, since heat generation of the failed cell3′ itself and heat generation due to the electromotive force of the other cells3can be immediately eliminated, the failed cell3′ is protected from excessive heat generation, and the power generation performance of the fuel cell3can be secured. Here, the predetermined value ΔV2is a value which should be ensured to prevent degradation of the fuel cell2similarly to the predetermined value ΔV1. In the sense of continuing power generation for as long as possible, it is preferable to set the predetermined value ΔV2to be equivalent to or slightly greater than the predetermined value ΔV1.

While preferred embodiments of the invention have been described and illustrated above, it should be understood that these are exemplary of the invention and are not to be considered as limiting. For example, the embodiment described the fuel cell being mounted in a vehicle, but it may be applied to a fixed-type fuel cell system other than for a vehicle. Also, the embodiment explained the case of measuring the voltage of each cell, but the present invention can also be applied to the case of treating a plurality of cells as one cell group and measuring the voltage of that cell group.

Additions, omissions, substitutions, and other modifications can be made without departing from the spirit or scope of the present invention. Accordingly, the invention is not to be considered as being limited by the foregoing description, and is only limited by the scope of the appended claims.