Patent Application
20130189596
1. Field of the Invention
The invention relates to a fuel cell, and particularly to a decrease in a cell voltage.
2. Description of Related Art
A fuel cell including a Membrane-Electrode Assembly (MEA), in which catalytic electrode layers are provided on both sides of an electrolyte membrane, is used. In the fuel cell, electric power is generated by an electrochemical reaction caused using a fuel gas (for example, a hydrogen gas) and an oxidant gas (for example, air). It is known that in the fuel cell, the voltage of the fuel cell (i.e., a potential difference between a cathode and an anode: hereinafter, referred to as “cell voltage”) may decrease to a negative voltage due to the insufficiency of the amount of the fuel gas or the insufficiency of the amount of the oxidant gas. If the cell voltage becomes a negative voltage, for example, members (e.g., a platinum (Pt) catalyst and carbon) constituting a catalytic layer may deteriorate, resulting in significant deterioration of power generation performance. In this case, it is required to take a measure to increase the cell voltage from the negative voltage to a positive voltage. Because the measure to be taken varies depending on the cause of the negative voltage, there is demand for determining the cause of the negative voltage. Thus, Japanese Patent Application Publication No. 2008-130443 (JP-A-2008-130443) describes a method in which a sweep current is rapidly increased in stepwise manner in a fuel cell, the rate of decrease in a cell voltage is measured, and the cause of a negative voltage is determined based on the rate of decrease in the cell voltage
In the above-described technology in which the cause of the negative voltage is determined based on the rate of decrease in the cell voltage, there is only a small difference between the rate of decrease in the cell voltage when the amount of the fuel gas is insufficient, and the rate of decrease in the cell voltage when the amount of the oxidant gas is insufficient. Therefore, it is difficult to accurately determine the cause of the negative voltage. Further, it is assumed that the causes of the negative voltage include an increase in the resistance value of an electrolyte membrane (an increase in a resistance overvoltage) due to drying-up, in addition to the insufficiency of the amount of the fuel gas, and the insufficiency of the amount of the oxidant gas. However, in the above-described technology in which the cause of the negative voltage is determined based on the rate of decrease in the cell voltage, when the cause of the negative voltage is drying-up, it is not possible to determine that the cause of the negative voltage is drying-up.
The invention accurately determines the cause of a negative voltage of a fuel cell, when the voltage of the fuel cell is the negative voltage.
A first aspect of the invention relates to a fuel cell system. The fuel cell system includes a fuel cell; a voltage measuring portion that measures a voltage of the fuel cell; an electric current adjusting portion that adjusts an electric current flowing in the fuel cell; an electric current-voltage characteristic information obtaining portion that controls the electric current adjusting portion to change the electric current, and obtains electric current-voltage characteristic information that is information indicating a correspondence relation between an electric current value and a voltage value measured by the voltage measuring portion; and a negative voltage cause determining portion that determines, if the voltage of the fuel cell is a negative voltage, a cause of the negative voltage of the fuel cell, based on the obtained electric current-voltage characteristic information.
In the fuel cell system according to the first aspect, the electric current-voltage characteristic information is obtained by changing the electric current and measuring the voltage, and the cause of the negative voltage of the fuel cell is determined based on the obtained electric current-voltage characteristic information. Therefore, it is possible to accurately determine the cause of the negative voltage. Also, because the determination is performed based on the electric current-voltage characteristic information, it is possible to detect, as the cause of the negative voltage, drying-up, in addition to insufficiency of the amount of hydrogen, and insufficiency of the amount of oxygen.
In the fuel cell system according to the above-described aspect, in the fuel cell, a fuel gas containing hydrogen and an oxidant gas containing oxygen may be used as reaction gases; and the negative voltage cause determining portion may determine whether the cause of the negative voltage of the fuel cell is insufficiency of an amount of hydrogen, or a cause other than the insufficiency of the amount of hydrogen, based on the obtained electric current-voltage characteristic information.
In the above-described configuration, it is determined whether the cause of the negative voltage of the fuel cell is the insufficiency of the amount of hydrogen, or a cause other than the, insufficiency of the amount of hydrogen. Therefore, if it is determined that the cause is the insufficiency of the amount of hydrogen, it is possible to take an appropriate measure.
In the fuel cell system according to the above-described aspect, in the fuel cell, a fuel gas containing hydrogen and an oxidant gas containing oxygen may be used as reaction gases; when the electric current-voltage characteristic information is obtained, the electric current-voltage characteristic information obtaining portion may obtain the voltage value while changing the electric current in a predetermined positive electric current value range; the negative voltage cause determining portion may determine a zero current-time voltage value that is the voltage value at a time when the electric current is 0, based on the obtained electric current-voltage characteristic information, using extrapolation; if the zero current-time voltage value is lower than 0 volt, the negative voltage cause determining portion may determine that the cause is insufficiency of an amount of hydrogen; if the zero current-time voltage value is equal to or higher than 0 volt and lower than an open circuit voltage of the fuel cell at a normal time, the negative voltage cause determining portion may determine that the cause is insufficiency of an amount of oxygen; and if the zero current-time voltage value is equal to or higher than the open circuit voltage, the negative voltage cause determining portion may determine that the cause is drying-up.
In the above-described configuration, it is possible to accurately detect, as the cause of the negative voltage of the fuel cell, the insufficiency of the amount of hydrogen, the insufficiency of the amount of oxygen, and the drying-up. Also, the electric current value range is decreased, as compared to the case in which the upper limit value of the electric current value range is the same as the upper limit value in the above-described configuration, and the lower limit value is 0. Therefore, it is possible to decrease the time period required to obtain the electric current-voltage characteristic information. Thus, it is possible to decrease the time period required to determine the cause of the negative voltage.
In the fuel cell system according to the above-described aspect, the fuel cell may include a catalytic layer, and an oxidant gas supply passage through which the oxidant gas is supplied to the catalytic layer; if the zero current-time voltage value is 0 volt, the negative voltage cause determining portion may determine that the cause is the insufficiency of the amount of oxygen due to blockage of the oxidant gas supply passage; and if the zero current-time voltage value is higher than 0 volt and lower than the open circuit voltage, the negative voltage cause determining portion may determine that the cause is the insufficiency of the amount of oxygen in a vicinity of the catalytic layer.
In the above-described configuration, if the cause of the negative voltage is the insufficiency of the amount of oxygen, it is possible to determine the cause of the insufficiency of the amount of oxygen. Accordingly, it is possible to select and execute an appropriate measure in accordance with the cause of the insufficiency of the amount of oxygen.
In the fuel cell system according to the above-described aspect, a lower limit value in the predetermined positive electric current value range may be higher than the electric current value at an inflection point of dV/dI in a case where the cause is the insufficiency of the amount of oxygen or the drying-up, the dV/dI indicating a change in the voltage value with respect to a change in the electric current value.
In the above-described configuration, the zero current-time voltage obtained using extrapolation is controlled to be in an appropriate voltage range in accordance with the cause of the negative voltage. Thus, it is possible to more accurately determine the cause of the negative voltage, based on the zero current-time voltage.
In the fuel cell system according to the above-described aspect, in the fuel cell, a fuel gas containing hydrogen and an oxidant gas containing oxygen may be used as reaction gases; the negative voltage cause determining portion may determine dV/dI that indicates a change in the voltage value with respect to a change in the electric current value, based on the electric current-voltage characteristic information; if there is not an electric current value range in which the dV/dI is constant, the negative voltage cause determining portion may determine that the cause is insufficiency of an amount of hydrogen; if there is the electric current value range in which the dV/dI is constant and the voltage value at an inflection point of the dV/dI is lower than 0 volt, the negative voltage cause determining portion may determine that the cause is insufficiency of an amount of oxygen; and if there is the electric current value range in which the dV/dI is constant and the voltage value at the inflection point is equal to or higher than 0 volt, the negative voltage cause determining portion may determine that the cause is drying-up.
In the above-described configuration, it is possible to accurately detect, as the cause of the negative voltage of the fuel cell, the insufficiency of the amount of hydrogen, the insufficiency of the amount of oxygen, and the drying-up.
In the fuel cell system according to the above-described aspect, in the fuel cell, a fuel gas containing hydrogen and an oxidant gas containing oxygen may be used as reaction gases; the fuel cell may include a catalytic layer, and an oxidant gas supply passage through which the oxidant gas is supplied to the catalytic layer; the negative voltage cause determining portion may determine dV/dI that indicates a change in the voltage value with respect to a change in the electric current value, based on the electric current-voltage characteristic information; if there is not an electric current value range in which the dV/dI is constant, the negative voltage cause determining portion may determine that the cause is insufficiency of an amount of hydrogen; if there is the electric current value range in which the dV/dI is constant and there is not an inflection point of the dV/dI, the negative voltage cause determining portion may determine that the cause is insufficiency of an amount of oxygen due to blockage of the oxidant gas supply passage; and if there is the electric current value range in which the dV/dI is constant and the voltage value at the inflection point of the dV/dI is lower than 0 volt, the negative voltage cause determining portion may determine that the cause is the insufficiency of the amount of oxygen in a vicinity of the catalytic layer.
In the fuel cell system according to the above-described aspect, in the fuel cell, a fuel gas containing hydrogen and an oxidant gas containing oxygen may be used as reaction gases; the fuel cell may include a catalytic layer, and an oxidant gas supply passage through which the oxidant gas is supplied to the catalytic layer; the negative voltage cause determining portion may determine dV/dI that indicates a change in the voltage value with respect to a change in the electric current value, based on the electric current-voltage characteristic information; if there is not an electric current value range in which the dV/dI is constant, the negative voltage cause determining portion may determine that the cause is insufficiency of an amount of hydrogen; if there is the electric current value range in which the dV/dI is constant and there is not an inflection point of the dV/dI, the negative voltage cause determining portion may determine that the cause is insufficiency of an amount of oxygen due to blockage of the oxidant gas supply passage; if there is the electric current value range in which the dV/dI is constant and the voltage value at the inflection point of the dV/dI is lower than 0 volt, the negative voltage cause determining portion may determine that the cause is the insufficiency of the amount of oxygen in a vicinity of the catalytic layer; and if there is the electric current value range in which the dV/dI is constant and the voltage value at the inflection point is equal to or higher than 0 volt, the negative voltage cause determining portion may determine that the cause is drying-up.
In the above-described configuration, if the cause of the negative voltage is the insufficiency of the amount of oxygen, it is possible to determine the cause of the insufficiency of the amount of oxygen. Accordingly, it is possible to select and execute an appropriate measure in accordance with the cause of the insufficiency of the amount of oxygen.
The fuel cell system according to the above-described aspect may further include a temperature adjusting portion that adjusts a temperature of the fuel cell to 0° C. or higher, if it is determined that the cause is the insufficiency of the amount of oxygen due to the blockage of the oxidant gas supply passage; and an oxidant gas supply portion that increases an amount of the oxidant gas supplied to the fuel cell, if it has been determined that the cause is the insufficiency of the amount of oxygen due to the blockage of the oxidant gas supply passage and the temperature of the fuel cell has been adjusted to 0° C. or higher.
In the above-described configuration, in the case where the oxidant gas supply passage is blocked due to the accumulation of water generated by electric power generation in the fuel cell, the saturated vapor pressure is increased to make it easier to discharge the generated water that is accumulated, by adjusting the temperature of the fuel cell to 0° C. or higher. In addition, the amount of the supplied oxidant gas is increased after the temperature of the fuel cell is adjusted to 0° C. or higher to make it easier to discharge the generated water. Therefore, the generated water is easily discharged from the oxidant gas supply passage, and the amount of oxygen becomes sufficient.
The fuel cell system according to the above-described aspect may further include a malfunction detecting portion that detects that a malfunction has occurred in the fuel cell system, if the voltage value is a negative voltage value after the amount of the supplied oxidant gas is increased by the oxidant gas supply portion.
In the above-described configuration, if the oxidant gas supply passage is blocked due to a cause other than the generated water, it is possible to detect that a malfunction has occurred in the fuel cell system. Accordingly, for example, an appropriate measure is taken in response to the detection of the occurrence of a malfunction. For example, notification is provided to a manager, or a log is stored.
The fuel cell system according to the above-described aspect may further include an oxidant gas supply portion that increases an amount of the oxidant gas supplied to the fuel cell, if it is determined that the cause is the insufficiency of the amount of oxygen; a fuel gas supply portion that increases an amount of the fuel gas supplied to the fuel cell, if it is determined that the cause is the insufficiency of the amount of hydrogen; and a humidification portion that humidifies the fuel cell, if it is determined that the cause is the drying-up.
In the above-described configuration, it is possible to take an appropriate measure in accordance with the cause of the negative voltage. Accordingly, the voltage of the fuel cell is quickly increased from the negative voltage to a positive voltage. Thus, it is possible to reduce the possibility that a failure occurs, for example, members constituting the catalytic layer deteriorate, due to the negative voltage.
A second aspect of the invention relates to a method of determining a cause of a negative voltage of a fuel cell. The method includes obtaining an electric current-voltage characteristic information that is information indicating a correspondence relation between an electric current value and a voltage value, by changing an electric current flowing in the fuel cell, and measuring a voltage of the fuel cell; and determining the cause of the negative voltage of the fuel cell, based on the obtained electric current-voltage characteristic information.
In the method according to the second aspect, the electric current-voltage characteristic information is obtained by changing the electric current and measuring the voltage, and the cause of the negative voltage of the fuel cell is determined based on the obtained electric current-voltage characteristic information. Therefore, it is possible to accurately determine the cause of the negative voltage. Also, because the determination is performed based on the electric current-voltage characteristic information, it is possible to detect, as the cause of the negative voltage, drying-up, in addition to insufficiency of the amount of hydrogen, and insufficiency of the amount of oxygen.
In the method according to the above-described aspect, in the fuel cell, a fuel gas containing hydrogen and an oxidant gas containing oxygen may be used as reaction gases; and in determining the cause of the negative voltage of the fuel cell, it may be determined whether the cause of the negative voltage of the fuel cell is insufficiency of an amount of hydrogen or a cause other than the insufficiency of the amount of hydrogen, based on the obtained electric current-voltage characteristic information.
In the above-described configuration, it is determined whether the cause of the negative voltage of the fuel cell is the insufficiency of the amount of hydrogen, or a cause other than the insufficiency of the amount of hydrogen. Therefore, if it is determined that the cause is the insufficiency of the amount of hydrogen, it is possible to take an appropriate measure.
A third aspect of the invention relates to a program that determines a cause of a negative voltage of a fuel cell. The program causes a computer to implement a function of obtaining an electric current-voltage characteristic information that is information indicating a correspondence relation between an electric current value and a voltage value, by changing an electric current flowing in the fuel cell, and measuring a voltage of the fuel cell; and a function of determining the cause of the negative voltage of the fuel cell, based on the obtained electric current-voltage characteristic information.
In the program according to the third aspect, the electric current-voltage characteristic information is obtained by changing the electric current and measuring the voltage, and the cause of the negative voltage of the fuel cell is determined based on the obtained electric current-voltage characteristic information. Therefore, it is possible to accurately determine the cause of the negative voltage. Also, because the determination is performed based on the electric current-voltage characteristic information, it is possible to detect, as the cause of the negative voltage, drying-up, in addition to insufficiency of the amount of hydrogen, and insufficiency of the amount of oxygen.
A fourth aspect of the invention relates to a computer-readable storage medium that stores the program according to the third aspect.
In the above-described configuration, a computer reads the program from the storage medium, and, thus, the computer implements the functions.
Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:
The fuel cell stack 10 has a structure in which a plurality of unit cells 20 are stacked. Each unit cell is a polymer electrolyte fuel cell. In the fuel cell stack 10, an electrochemical reaction is caused at each electrode using pure hydrogen, which is a fuel gas, and oxygen in air, which is an oxidant gas, whereby an electromotive force is obtained. In the unit cell 20, both surfaces of a Membrane-Electrode Assembly (MEA) (not shown) are sandwiched between two gas diffusion layers. Further, the MEA and the gas diffusion layers are sandwiched between two separators: Two terminal plates 111, which are overall electrodes, are disposed at both ends of the stacked unit cells 20.
The hydrogen tank 61 stores a high-pressure hydrogen gas, and supplies the hydrogen gas, which is the fuel gas, to the fuel cell stack 10 via the fuel gas supply passage 81. For example, a tank that includes a hydrogen storing alloy therein may be employed as the hydrogen tank 61. In this case, hydrogen is stored in the hydrogen storing alloy, and thus, the tank stores hydrogen.
The shutoff valve 71 is disposed at a hydrogen gas outlet (not shown) of the hydrogen tank 61. The shutoff valve 71 allows and stops the supply of the hydrogen gas. The pressure adjusting valve 72 is disposed in the fuel gas supply passage 81. The pressure adjusting valve 72 decreases the pressure of the high-pressure hydrogen gas discharged from the hydrogen tank 61 to a given pressure. The first circulation pump 62 is disposed in the bypass passage 83. The first circulation pump 62 supplies an anode-side off gas (i.e., a surplus hydrogen gas that has not been used in the electrochemical reaction), which has been discharged from the fuel cell stack 10 via the fuel gas discharge passage 82, to the fuel gas supply passage 81.
The air compressor 65 is disposed in the oxidant gas supply passage 87. The air compressor 65 pressurizes air, which has been taken into the air compressor 65 from an outside, and supplies the pressurized air to the fuel cell stack 10. The humidifier 66 is disposed to extend from the oxidant gas supply passage 87 to the oxidant gas discharge passage 88. In the humidifier 66, moisture is exchanged between relatively dry air supplied from the air compressor 65 and a relatively moist gas discharged from the oxidant gas discharge passage 88 (i.e., a cathode-side off gas containing generated water). Thus, the humidifier 66 humidifies air to be supplied to the fuel cell stack 10.
The radiator 30 is connected to the cooling medium supply passage 84 and the cooling medium discharge passage 85. In the radiator 30, heat is exchanged between a cooling medium (coolant, air, and the like) discharged from the fuel cell stack 10, and outside air. Thus, the radiator 30 supplies the cooling medium that has been subjected to heat exchange, to the fuel cell stack 10 via the cooling medium supply passage 84. The second circulation pump 31 is disposed in the cooling medium discharge passage 85. The second circulation pump 31 supplies the cooling medium discharged from the fuel cell stack 10, to the radiator 30. The temperature sensor 63 is disposed at the second circulation pump 31, and measures the temperature of the cooling medium discharged from the fuel cell stack 10. In the fuel cell system 100, the temperature measured by the temperature sensor 63 is used as the temperature of the fuel cell stack 10.
The cell monitor 40 is connected to the unit cells 20, and measures the cell voltage (i.e., a potential difference between a cathode electrode and an anode electrode) of each unit cell 20. The electric current sensor 41 is connected in series to the fuel cell stack 10, and measures the value of an electric current flowing in the fuel cell stack 10. The DC-DC converter 51 is connected in parallel to the fuel cell stack 10 and the secondary battery 52. The DC-DC converter 51 increases the voltage output from the secondary battery 52, and then supplies the increased voltage to the inverter 50. Also, in order to store surplus electric power generated by the fuel cell stack 10, the DC-DC converter 51 decreases the voltage output from the fuel cell stack 10, and then supplies the decreased voltage to the secondary battery 52. The inverter 50 is connected in parallel to the fuel cell stack 10 and the DC-DC converter 51. The inverter 50 converts a DC current supplied from the fuel cell stack 10 or the DC-DC converter 50, to an AC current, and supplies the AC current to a load L (for example, a motor for driving the vehicle).
The control unit 90 is electrically connected to the air compressor 65, the humidifier 66, the shutoff valve 71, the pressure adjusting valve 72, the first circulation pump 62, the second circulation pump 31, the inverter 50, and the DC-DC converter 51. The control unit 90 controls these elements, that is, the air compressor 65, the humidifier 66, the shutoff valve 71, the pressure adjusting valve 72, the first circulation pump 62, the second circulation pump 31, the inverter 50, and the DC-DC converter 51. Also, the control unit 90 is connected to the temperature sensor 63, and obtains the temperature measured by the temperature sensor 63.
Also, the control unit 90 includes a Central Processing Unit (CPU) 91, a Read Only Memory (ROM) 92, and a Random Access Memory (RAM) 93. The ROM 92 stores control programs (not shown) for controlling the fuel cell system 100. The CPU 91 functions as an electric current adjusting portion 91a, a voltage measuring portion 91b, a negative voltage cause determining portion 91c, a gas flow rate adjusting portion 91d, a humidification control portion 91e, a stack temperature adjusting portion 91f, and a malfunction occurrence notifying portion 91g, by executing the control programs using the RAM 93.
The electric current adjusting portion 91a controls the DC-DC converter 51, thereby adjusting the electric current flowing in the fuel cell stack 10. The voltage measuring portion 91b obtains the value of the voltage of each unit cell 20. The voltage measuring portion 91b is notified of the value of the voltage of each unit cell 20 by the cell monitor 40. When the voltage of the unit cell 20 is a negative voltage, the negative voltage cause determining portion 91c determines the cause of the negative voltage. The gas flow rate adjusting portion 91d adjusts the amounts of the hydrogen gas and air that are supplied to the fuel cell stack 10. More specifically, the gas flow rate adjusting portion 91d adjusts the amount of the hydrogen gas supplied to the fuel cell stack 10 by adjusting the opening degrees of the shutoff valve 71 and the pressure adjusting valve 72. Also, the gas flow rate adjusting portion 91d adjusts the amount of air supplied to the fuel cell stack 10 by controlling the rotational speed of the air compressor 65. The humidification control portion 91e adjusts the amount of humidification of air supplied to the fuel cell stack 10 by controlling the humidifier 66. The stack temperature adjusting portion 91f adjusts the temperature of the fuel cell stack 10 by controlling the second circulation pump 31 to adjust the flow rate of the cooling medium that flows in the cooling medium supply passage 84 and the cooling medium discharge passage 85. The malfunction occurrence notifying portion 91g detects the malfunction of the fuel cell system 100, and provides notification. More specifically, when the malfunction occurrence notifying portion 91g detects the malfunction of the system, the malfunction occurrence notifying portion 91g causes an operation panel (not shown) to indicate that a malfunction has occurred.
The value of the open-circuit voltage of the fuel cell stack 10 (i.e., cell voltage in the case where the load L is not connected to the fuel cell stack 10: OCV) at a normal time, and various threshold values are preliminarily stored in the above-described ROM 92.
In the fuel cell system 100, when the cell voltage of any one of the unit cells 20 constituting the fuel cell stack 10 is a negative voltage, negative voltage cause determining processing (described later) is executed to determine the cause of the negative voltage. Also, in the fuel cell system 100, cell voltage recovery processing (described later) is executed, whereby a measure is taken in accordance with the determined cause of the negative voltage. Thus, the cell voltage is increased from a negative voltage to a positive voltage.
The voltage measuring portion 91b may be regarded as the voltage measuring portion and the electric current-voltage characteristic information obtaining portion according to the invention. The gas flow rate adjusting portion 91d and the air compressor 65 may be regarded as the oxidant gas supply portion according to the invention. The gas flow rate adjusting portion 91d, the shutoff valve 71, and the pressure adjusting valve 72 may be regarded as the fuel gas supply portion according to the invention. The humidification control portion 91e and the humidifier 66 may be regarded as the humidification portion according to the invention. The malfunction occurrence notifying portion 91g may be regarded as the malfunction detecting portion according to the invention.
A2. Negative Voltage Cause Determining Processing
The electric current adjusting portion 91a decreases the electric current from the electric current value in the state in which the negative voltage is measured, to the electric current value of 0. The voltage measuring portion 91b measures the cell voltage at each electric current value, and causes the RAM 93 to store the cell voltage at each electric current value as an electric current-voltage characteristic (hereinafter, referred to as “I-V characteristic”) (step S105). At this time, the electric current adjusting portion 91a may decrease the electric current in a stepwise manner, or may continuously decrease the electric current. When the electric current is decreased in a stepwise manner, the voltage measuring portion 91b may obtain the voltage value in each step. Also, when the electric current is continuously decreased, the voltage measuring portion 91b may obtain the voltage value in each predetermined current interval. The cell voltage at each electric current value (i.e., I-V characteristic curve), which is obtained in step S105, may be regarded as the electric current-voltage characteristic information according to the invention.
The negative voltage cause determining portion 91c determines whether there is an electric current value range in which the change in the voltage with respect to the change in the electric current (dV/dI) is constant, based on the I-V characteristic obtained in step S105 (step S110).
As shown in
When the electric current is decreased in the case where the amount of oxygen is insufficient, dV/dI is constant and the voltage value linearly increases in a range from the operating point p0 to an operating point p1 (at which the electric current value is I1 (the electric current value=I1), and the voltage value is V1 (the voltage value=V1)), as shown by the curve L2. Note that the voltage value V1 at the operating point p1 is a negative voltage value. The operating point p1 is an inflection point, that is, dV/dI sharply increases from the operating point p1, and the voltage value becomes a positive voltage value. Then, dV/dI decreases, and the curve L2 coincides with the curve L0.
When the electric current is decreased in the dry-up state, dV/dI is constant and the voltage linearly increases in a range from the operating point p0 to an operating point p2 (at which the electric current value is I2 (the electric current value=I2), and the voltage value is V2 (the voltage value=V2)), as shown by the curve L3. Note that the voltage value V2 at the operating point p2 is a positive voltage value. The operating point p2 is an inflection point, that is, dV/dI decreases from the operating point p2, and the 1,3 coincides with the curve L0.
Accordingly, in the case where the amount of oxygen is insufficient or in the case where drying-up has occurred, there is the electric current value range in which dV/dI is constant. In the case where the amount of hydrogen is insufficient, there is not the electric current value range in which dV/dI is constant. The reason why the I-V characteristic curve varies according to the cause of the negative voltage will be described with reference to
Each of
As shown in the upper left graph in
H2→2H++2i− (1)
As shown in the upper right graph in
O2+4H++4e−→2H2O (2)
As shown in the lower left graph in
Then, the I-V characteristic curve (i.e., the curve L0) shown in the lower right graph is obtained by determining the potential difference between the electrode potentials, as the cell voltage, at each electric current value (at each absolute value of the electric current value).
In the case where the amount of hydrogen is insufficient, a reaction represented by the equation (3) described below occurs at the anode-side, instead of the reaction represented by the equation (1), as shown in the upper left graph in
C+2H2O→CO2+4H+4e− (3)
When the reaction represented by the equation (3) occurs, the anode potential becomes higher than the anode potential at the normal time (i.e., when the reaction represented by the equation (1) occurs). The degree of increase in the anode potential due to the increase in the electric current gradually increases. As shown in the upper right graph in
Once the amount of hydrogen becomes insufficient, the amount of hydrogen remains insufficient even when the value of the electric current is decreased to a value close to 0. Therefore, the I-V characteristic curve L1a does not contact the I-V characteristic curve L0 at the normal time for the following reason, as shown in the lower right graph in
In the case where the amount of oxygen is insufficient, a reaction represented by the equation (4) described below occurs at the cathode-side, instead of the reaction represented by the equation (2), as shown in the upper right graph in
2H++2e−→H2 (4)
When the reaction represented by the equation (4) occurs, the cathode potential becomes lower than the cathode potential in the normal state, and becomes a negative potential. As shown in the upper left graph in
In the dry-up state, the reactions, which are the same as the reactions at the normal time, occur in the anode-side and the cathode-side, respectively, as shown in the upper left graph and the upper right graph in
Referring to
If it is determined that there is the electric current value range in which dV/dI is constant (YES in step S110), the negative voltage cause determining portion 91c determines whether the voltage at the first inflection point obtained when the electric current is decreased, in the I-V characteristic curve obtained in step S105, is a negative voltage (step S120). If it is determined that the voltage at the first inflection point obtained when the electric current is decreased is a negative voltage (YES in step S120), the negative voltage cause determining portion 91c determines that the cause of the negative voltage is the insufficiency of the amount of oxygen (step S125), and causes the RAM 93 to store the determined cause (step S135). In contrast, if it is determined that the voltage at the first inflection point obtained when the electric current is decreased is equal to or higher than 0 volt (NO in step S120), the negative voltage cause determining portion 91c determines that the cause of the negative voltage is drying-up (step S130), and causes the RAM 93 to store the determined cause (step S135).
In the case where the amount of oxygen is insufficient, the voltage value V1 at the first inflection point (the operating point p1) is a negative voltage value as shown by the curve L2 in
A3. Cell Voltage Recovery Processing
The negative voltage cause determining portion 91c obtains the cause of the negative voltage from the RAM 93 (step S205), and determines whether the cause of the negative voltage is the insufficiency of the amount of hydrogen (step S210). If it is determined that the cause of the negative voltage is the insufficiency of the amount of hydrogen (YES in step S210), the gas flow rate adjusting portion 91d increases the amount of hydrogen gas supplied to the fuel cell stack 10, by controlling the shutoff valve 71 and the pressure adjusting valve 72 (step S215). Then, the gas flow rate adjusting portion 91d determines whether the cell voltage of the unit cell 20 has become higher than 0 volt due to the increase in the amount of the supplied hydrogen gas (step S220). The gas flow rate adjusting portion 91d repeats the processes in steps S215 and S220 until the cell voltage becomes higher than 0 volt. If the cell voltage has become higher than 0 volt, the cell voltage recovery processing ends.
If it is determined that the cause of the negative voltage is not the insufficiency of the amount of hydrogen (NO in step S210), the negative voltage cause determining portion 91c determines whether the cause of the negative voltage is the insufficiency of the amount of oxygen (step S225). If it is determined that the cause of the negative voltage is the insufficiency of the amount of oxygen (YES in step S225), the gas flow rate adjusting portion 91d increases the amount of air supplied to the fuel cell stack 10 by controlling the rotational speed of the air compressor 65 (step S230). Then, the gas flow rate adjusting portion 91d determines whether the cell voltage of the unit cell 20, which was the negative voltage, has become a positive voltage due to the increase in the amount of supplied air (step S235). The gas flow rate adjusting portion 91d repeats the processes in steps S230 and S235 until the cell voltage becomes a positive voltage. If the cell voltage has become a positive voltage, the cell voltage recovery processing ends.
If it is determined that the cause of the negative voltage is not the insufficiency of the amount of oxygen (NO in step S225), the humidification control portion 91e increases the amount of humidification of air supplied to the fuel cell stack 10, by controlling the humidifier 66 (step S240). By executing this process, the amount of moisture inside the unit cell 20 is increased, and the unit cell 20 may be no longer in the dry-up state. Then, the humidification control portion 91e determines whether the cell voltage of the unit cell, which was the negative voltage, has become higher than 0 volt as a result of the unit cell 20 being no longer in the dry-up state (step S245). The humidification control portion 91e repeats the processes in steps S240 and S245 until the cell voltage becomes higher than 0 volt. If the cell voltage has become higher than 0 volt, the cell voltage recovery processing ends.
In the fuel cell system 100 that has been described, the I-V characteristic curve is obtained by measuring the voltage value while the electric current flowing in the fuel cell stack 10 is decreased to 0; and it is determined whether the cause of the negative voltage is the insufficiency of the amount of hydrogen, the insufficiency of the amount of oxygen, or drying-up, by determining whether there is the electric current value range in which dV/dI is constant in the obtained I-V characteristic curve, and determining whether the voltage at the first inflection point is a negative voltage, if there is the electric current value range in which dV/dI is constant. Accordingly, it is possible to accurately determine the cause of the negative voltage, and to accurately determine the cause even when the negative voltage is caused due to drying-up.
In addition, after the cause of the negative voltage is determined, the cell voltage recovery processing is executed to execute an appropriate process in accordance with the determined cause. Therefore, it is possible to increase the cell voltage from a negative voltage to a positive voltage in a short time period. Accordingly, it is possible to reduce the possibility that members constituting the catalytic layer deteriorate due to the negative voltage.
More specifically, as shown in
The negative voltage cause determining portion 91c determines the voltage value at a time when the electric current value is 0 (hereinafter, the voltage value may be referred to as “zero current-time voltage value”), based on the I-V characteristic obtained in step S305, using extrapolation (step S310). The method of extrapolation may be, for example, a method in which straight-line approximation is performed on the I-V characteristic obtained in step S305, and the zero current-time voltage value is determined by assigning 0 to the electric current value in the obtained straight line.
In
As shown in
As shown in
As shown in
After the process in step S310 shown in
If it is determined that the zero current-time voltage value Ve is equal to or higher than 0 (NO in step S315), the negative voltage cause determining portion 91c determines whether the zero current-time voltage value Ve is equal to or higher than V0 (OCV) (step S325). If it is determined that the zero current-time voltage value Ve is lower than V0 (NO in step S325), the negative voltage cause determining portion 91c determines that the cause of the negative voltage is the insufficiency of the amount of oxygen (step S330), and causes the RAM 93 to store the determined cause (step S340).
If it is determined that the zero current-time voltage value Ve is equal to or higher than V0 (YES in step S325), the negative voltage cause determining portion 91c determines that the cause of the negative voltage is drying-up (step S335), and causes the RAM 93 to store the determined cause (step S340).
The fuel cell system with the above-described configuration in the second embodiment has the same advantageous effects as those of the fuel cell system 100 in the first embodiment. In addition, because the lower limit value employed when the electric current value is decreased in step S305 is higher than 0, and higher than the electric current values at the inflection points p1 and p2, it is possible to complete the process of measuring the voltage value in a short time period, as compared to the case where the lower limit value is 0. Accordingly, the cause of the negative voltage is determined in a shorter time period, and thus, the negative voltage recovery processing is executed more quickly after the cell voltage becomes a negative voltage. Thus, it is possible to reduce the possibility that the catalyst deteriorates.
C1. Negative Voltage Cause Determining Processing
The negative voltage cause determining processing in the third embodiment is different from the negative voltage cause determining processing shown in
As shown in
As shown in
If it is determined that there is no inflection point in step S405 shown in
C2. Cell Voltage Recovery Processing
If it is determined that the insufficiency of the amount of oxygen is not due to the blockage of the passage, that is, if it is determined that the amount of oxygen is insufficient in the vicinity of the catalytic layer (NO in step S505), the amount of supplied air is increased until the cell voltage becomes higher than 0 volt (steps S230 and S235).
In contrast, if it is determined that the amount of oxygen is insufficient due to the blockage of the passage (YES in step S505), the stack temperature adjusting portion 91f increases the temperature of the fuel cell stack 10 to a predetermined temperature (for example, 0° C.) or higher (step S510). Then, the gas flow rate adjusting portion 91d increases the amount of air supplied to the fuel cell stack 10 (step S515). The gas flow rate adjusting portion 91d determines whether the cell voltage has become higher than 0 volt (step S520). If the cell voltage has become higher than 0 volt, the cell voltage recovery processing ends. If it is determined that the cell voltage is equal to or lower than 0 volt (NO in step S520), the gas flow rate adjusting portion 91d determines whether the amount of supplied air has been increased to a predetermined amount or larger (step S525). If it is determined that the amount of supplied air has not been increased to the predetermined amount or larger (NO in step S525), the gas flow rate adjusting portion 91d executes the processes in the above-described steps S515 to S525. In contrast, if it is determined that the amount of supplied air has been increased to the predetermined amount or larger (YES in step S525), the malfunction occurrence notifying portion 91g notifies a manager that a malfunction has occurred, by causing the operation panel (not shown) to indicate that a malfunction has occurred (step S530).
If the amount of oxygen is insufficient due to the blockage of the passage, the amount of air supplied to the fuel cell stack 10 is increased after increasing the temperature of the fuel cell stack 10, for the following reason. If water generated due to electric power generation is accumulated in the oxidant gas supply passage, and the passage is blocked due to the accumulated water, the temperature is increased to increase the saturated vapor pressure. Accordingly, the accumulated water is transformed to vapor that is easily discharged. Therefore, when the amount of supplied air is increased thereafter, the accumulated water (vapor) is discharged using the flow of air, and in addition, the amount of air becomes sufficient.
If the cell voltage remains equal to or lower than 0 volt although the temperature of the fuel cell stack 10 has been increased and the amount of supplied air has been increased to the predetermined amount or larger, there is a high possibility that the passage is blocked for a reason other than the accumulation of the generated water. In this case, the manager needs to determine the cause of the blockage, and to perform maintenance. Therefore, the fuel cell system in the third embodiment is configured to notify the manager that a malfunction has occurred.
The fuel cell system with the above-described configuration in the third embodiment has the same advantageous effects as those of the fuel cell system 100 in the first embodiment. In addition, in the fuel cell system in the third embodiment, it is determined whether the amount of oxygen is insufficient due to the blockage of the passage, or the amount of oxygen is insufficient in the vicinity of the catalytic layer. Therefore, it is possible to select and execute an appropriate process in accordance with the cause of the insufficiency of the amount of oxygen, in the cell voltage recovery processing.
If the amount of oxygen is insufficient due to the blockage of the passage, the amount of supplied air is increased after the temperature of the fuel cell stack 10 is increased. Therefore, if the passage is blocked because the water generated due to the electric power generation is accumulated in the passage, the accumulated water is discharged, and the amount of oxygen quickly becomes sufficient.
In the case where the amount of oxygen is insufficient due to the blockage of the passage, if the cell voltage remains equal to or lower than 0 volt although the amount of supplied air has been increased to the predetermined amount or larger after increasing the temperature of the fuel cell stack 10, the manager is notified that a malfunction has occurred. Therefore, the manager determines the cause of the blockage of the passage, and performs maintenance, based on the notification. Accordingly, even when the blockage of the passage is not due to the generated water, maintenance is quickly performed, and the amount of oxygen quickly becomes sufficient.
The negative voltage cause determining processing in the fourth embodiment shown in
In step S325, if it is determined that the zero current-time voltage value Ve is lower than V0 (OCV) (NO in step S325), the negative voltage cause determining portion 91c determines whether the zero current-time voltage value Ve is 0 volt (step S605). If it is determined that the zero current-time voltage value Ve is 0 volt, the negative voltage cause determining portion 91c determines that the amount of oxygen is insufficient due to the blockage of the passage (step S610), and if it is determined that the zero current-time voltage value Ve is not 0 volt (that is, the zero current-time voltage value Ve is higher than 0 volt and lower than V0 (OCV)), the negative voltage cause determining portion 91c determines that the amount of oxygen is insufficient in the vicinity of the catalytic layer (S615).
In
In
After the process in step S610 or S615 is executed, the process in step S340 (i.e., the process of causing the RAM 93 to store the determined cause) is executed, and the negative voltage cause determining processing ends. The cell voltage recovery processing in the fourth embodiment is the same as the cell voltage recovery processing in the third embodiment in
The fuel cell system with the above-described configuration in the fourth embodiment has the same advantageous effects as those of the fuel cell system 100 in the first embodiment. In addition, in the fuel cell system in the fourth embodiment, it is determined whether the amount of oxygen is insufficient due to the blockage of the passage, or the amount of oxygen is insufficient in the vicinity of the catalytic layer, and therefore, it is possible to execute an appropriate process in accordance with the cause of the insufficiency of the amount of oxygen in the cell voltage recovery processing. Also, because the lower limit value employed when the electric current value is decreased in step S305 is higher than 0, and higher than the electric current values at the inflection points p1 and p2, it is possible to complete the process of measuring the voltage value in a short time period, as compared to the case where the lower limit value is 0. Accordingly, the cause of the negative voltage is determined in a shorter time period, and thus, the negative voltage recovery processing is executed more quickly after the cell voltage becomes a negative voltage. Thus, it is possible to reduce the possibility that the catalyst deteriorates.
The invention is not limited to the above-described embodiments. Thus, the invention may be implemented in various manners without departing from the scope of the invention. For example, modifications described below may be made.
In each of the first and third embodiments, in step S105, the value of the electric current is decreased from the electric current value in the state in which the negative voltage is measured, to the electric current value of 0. However, the invention is not limited to this configuration. For example, the configuration may be such that the value of the electric current is decreased from the electric current value in the state in which the negative voltage is measured; each time the voltage value is obtained, it is determined whether there is an inflection point; and if the inflection point is caused, the decrease of the electric current is stopped, and the process in step S105 ends. In this configuration, when the cause of the negative voltage is the insufficiency of the amount of oxygen or drying-up, it is not necessary to decrease the electric current value up to 0, and therefore, it is possible to decrease the time period required to execute the negative voltage cause determining processing.
In each embodiment, in the cell voltage recovery processing, if the cause of the negative voltage is the insufficiency of the amount of hydrogen, the amount of supplied hydrogen gas is increased. In addition, the fuel cell stack 10 may be electrically separated from the load L. Also, at this time, electric power may be supplied to the load L from the secondary battery 52. In this configuration, the fuel cell stack 10 can be, placed in the OCV state. Therefore, when the amount of supplied hydrogen gas is increased, the anode-side catalytic layer is exposed to a sufficient amount of hydrogen gas, and thus, the catalyst, which has been inactivated, is brought to the active, state. That is, when the amount of supplied hydrogen gas is increased, it is possible to remove the oxide of the catalyst such as platinum, which has been generated in the anode-side catalytic layer due to the insufficiency of the amount of hydrogen. Also, because electric power is supplied from the secondary battery 52 to the load L, the cell voltage recovery processing is executed without stopping the electric vehicle.
In the cell voltage recovery processing in the third embodiment, if the cause of the negative voltage is the insufficiency of the amount of oxygen in the vicinity of the catalytic layer, the amount of supplied air is increased in step S230. In addition to increasing the amount of supplied air, the temperature of the fuel cell stack 10 may be increased. Similarly, in the first embodiment as well, if the cause of the negative voltage is the insufficiency of the amount of oxygen, the temperature of the fuel cell stack 10 may be increased, in addition to increasing the amount of supplied air (step S230). In the configuration, in the case where the amount of oxygen is insufficient in the vicinity of the catalytic layer due to the accumulation of the generated water in the vicinity of the catalytic layer, the generated water is removed, and the amount of oxygen becomes sufficient.
In each embodiment, the stack temperature adjusting portion 91f adjusts the temperature of the fuel cell stack 10, by controlling the second circulation pump 31 to adjust the flow rate of the cooling medium. However, the invention is not limited to this configuration. For example, the configuration may be such that a heater is provided in the cooling medium supply passage 84 or the cooling medium discharge passage 85, and the temperature of the fuel cell stack 10 is adjusted by increasing the temperature of the cooling medium using the heater. Also, the configuration may be such that a heater is provided to directly heat the unit cells 20, and the temperature of the fuel cell stack 10 is adjusted using the heater.
In each embodiment, the malfunction occurrence notifying portion 91g causes the operation penal (not shown) to indicate that a malfunction has occurred. Instead of, or in addition to the indication in the operation panel, notification may be provided using sound such as buzzer, or a lamp may be lit or may be caused to flash to notify that a malfunction has occurred. Also, instead of notifying that a malfunction has occurred, a log indicating that a malfunction has occurred may be stored in the RAM 93. In this configuration as well, the manager is aware that a malfunction has occurred by seeing the log stored in the RAM 93. Thus, the manger can determine the cause of the negative voltage, and can perform, for example, maintenance.
In each embodiment, it is determined whether the cause of the negative voltage is the insufficiency of the amount of hydrogen, the insufficiency of the amount of oxygen, or drying-up. However, the invention is not limited to this configuration. For example, it may be determined whether the cause of the negative voltage is the insufficiency of the amount of hydrogen, or a cause other than the insufficiency of the amount of hydrogen. More specifically, for example, in the negative voltage cause determining processing in the first embodiment, the processes in step S120 and subsequent steps may be omitted. In this configuration as well, it is possible to determine whether the cause of the negative voltage is the insufficiency of the amount of hydrogen, or a cause other than the insufficiency of the amount of hydrogen. Therefore, if the cause of the negative voltage is the insufficiency of the amount of hydrogen, it is possible to take an appropriate measure (for example, it is possible to execute the processes in steps S215 and S220 in
In each embodiment, the fuel cell system is provided in an electric vehicle. However, the fuel cell system according to the invention may be employed in various movable bodies such as a hybrid vehicle, a ship, and a robot, instead of the electric vehicle. Also, the fuel cell stack 10 may be used as a stationary power source, and the fuel cell system may be employed in architectural structures such as a building and a house.
In each embodiment, a portion of the configuration implemented by software may be replaced by hardware. On the other hand, a portion of the configuration implemented by hardware may be replaced by software.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2010-216647 | Sep 2010 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IB2011/002183 | 9/23/2011 | WO | 00 | 3/25/2013 |
