JP2009032416A - Fuel cell system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fuel cell system capable of eliminating a fuel gas shortage in the fuel cell system generating power in such a state that fuel gas is stopped on the inside or the small amount of fuel offgas is exhausted. <P>SOLUTION: The occurrence of fuel gas shortage in part of a power generation region is estimated, and only when gas shortage is occurred, offgas of fuel gas is circulated in a supply passage of the fuel gas. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a fuel cell system, and more specifically to a fuel cell system that operates while stopping anode gas in the fuel cell or exhausting off-gas of the anode gas out of the minute amount system.

  Conventionally, as disclosed in, for example, the following patent documents, a fuel cell in which the off-gas of the anode gas used in the fuel cell is circulated again to the fuel cell to effectively use the hydrogen remaining in the off-gas. A system (circulation type system) is known. In this type of system, a circulation channel that connects an off-gas discharge channel and an anode gas supply channel is provided, and a pump is provided in the circulation channel. The pump is configured to circulate off-gas through the supply path.

  On the other hand, a fuel cell that employs a circulation type system as described above has a problem of poor fuel consumption because it requires power to drive the pump. Thus, a fuel cell system (circulation-less system) that generates power without circulating the anode off-gas has been proposed. As such a circulation-less fuel cell, an exhaust-less system that does not exhaust any anode off-gas and a small-volume exhaust system that exhausts a small amount of anode off-gas are known.

JP 2004-172026 A JP-A-2005-353569 JP 2004-327360 A JP 2004-342386 A JP 2005-203143 A

  However, in the exhaust-less system and the small-volume exhaust system, impurities other than the fuel gas sometimes accumulate in a part of the membrane electrode assembly because the flow rate of the reaction gas is small at the anode. When impurities are accumulated in part, the amount of anode gas may be insufficient at the location relative to the amount required for power generation. In such a state, there is a possibility that the power generation output is reduced and the fuel cell is deteriorated.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a fuel cell system that can reduce shortage of anode gas in a fuel cell system that employs an exhaust-less system or a small-volume exhaust system. There is to do.

In order to achieve the above object, a first invention is a fuel cell system,
A fuel cell comprising a membrane electrode assembly;
A gas supply path for supplying anode gas to the fuel cell;
A gas discharge path for discharging anode gas from the fuel cell;
A connection path connecting the gas discharge path to the gas supply path;
Connection switching means for switching blocking / communication of the connection path;
Gas flow means for causing a gas flow from the gas discharge path side to the gas supply path side when the connection path is in a communicating state;
A gas shortage estimating means for estimating a shortage of anode gas in a part of the membrane electrode assembly;
A connection control means for controlling the connection switching means so as to place the connection path in a communicating state when an anode gas shortage is estimated when the connection path is in a shut-off state;
It is characterized by providing.

The second invention is the first invention, wherein
Equipped with a gas shortage elimination estimating means for estimating that the anode gas shortage has been resolved,
The connection control means controls the connection switching means so as to put the connection path in a cut-off state when it is estimated that the shortage of anode gas is estimated.
It is characterized by that.

The third invention is the first or second invention, wherein
The fuel cell includes a plurality of membrane electrode assemblies,
A cell voltmeter for detecting the generated voltage of each of the plurality of membrane electrode assemblies,
The gas shortage estimating means estimates that the anode gas is short when the variation in the generated voltage of the membrane electrode assembly is a predetermined value or more.
It is characterized by that.

Moreover, 4th invention is set in 3rd invention,
The out-of-gas estimation means determines that the variation in the generated voltage is greater than or equal to a predetermined value when the difference between the highest voltage and the lowest voltage among the generated voltages of the membrane electrode assembly is greater than or equal to a predetermined value. It is characterized by.

The fifth invention is the first or second invention, wherein
An anode gas concentration detection means for detecting a correlation value of the anode gas concentration downstream of the membrane electrode assembly,
The gas shortage estimating means determines that the anode gas is insufficient when the anode gas concentration is a predetermined value or less.
It is characterized by that.

  According to the first invention, local shortage of anode gas can be solved by circulation.

  According to the second invention, the consumption of energy by the gas flow means can be suppressed by terminating the circulation at an appropriate timing.

  According to the third invention, it is possible to estimate the shortage of anode gas generated only in a part of the cells, and to solve the shortage of anode gas.

  According to the fourth invention, since the variation in the power generation of the cells can be easily detected, it is possible to more reliably estimate that the anode gas shortage has occurred only in some of the cells and eliminate the anode gas shortage. it can.

  According to the fifth aspect of the present invention, the shortage of anode gas generated in the downstream portion of any cell can be estimated, and the shortage of anode gas can be resolved.

Embodiment 1 FIG.
[Configuration of Embodiment 1]
FIG. 1 is a diagram schematically showing the configuration of the fuel cell system according to Embodiment 1 of the present invention. The fuel cell system is a system that generates power by a fuel cell and supplies the electric power to an electric load such as a motor. The fuel cell of this embodiment is used as a fuel cell stack 10 formed by stacking a plurality of unit fuel cells (cells). Although not shown, the unit fuel cell has a structure in which a membrane electrode assembly is sandwiched between a pair of separators. The membrane electrode assembly is obtained by integrating catalyst electrodes on both sides of a solid polymer electrolyte membrane, and further, a gas diffusion layer made of a carbon sheet or the like is integrated on each side. Each unit fuel cell is supplied with hydrogen as a fuel gas (anode gas) at the anode, and is supplied with air at the cathode to generate power.

  A hydrogen supply path 14 for supplying hydrogen from the high-pressure hydrogen tank 12 to the fuel cell stack 10 is connected to the fuel cell. In the middle of the hydrogen supply path 14, a shut valve 16 and a modulatable pressure valve 18 are sequentially arranged from the upstream. Hydrogen is depressurized by the adjustable pressure valve 18 and adjusted to a desired pressure before being supplied to the fuel cell stack 10. Hydrogen supplied to the fuel cell stack 10 is distributed to the anode of each unit fuel cell by a supply manifold formed in the fuel cell stack. The gas that has passed through the anode of each unit fuel cell is collected in an exhaust manifold formed in the fuel cell and discharged to the anode gas discharge path 20. An exhaust valve 22 is disposed in the anode gas discharge path 20. The exhaust valve 22 is controlled to open only when a predetermined purge condition is satisfied with the closed state as its basic state.

  Furthermore, the fuel cell system of the present embodiment includes a connection path 24 that connects the anode gas discharge path 20 to the hydrogen supply path 14. The connection path 24 connects the upstream side of the exhaust valve 22 in the anode gas discharge path 20 to the downstream side of the adjustable pressure valve 18 in the hydrogen supply path 14. A small circulation pump 26 is disposed in the connection path 24. The circulation pump 26 can flow the anode off gas from the anode gas discharge path 20 to the hydrogen supply path 14 by its operation, and can block the flow of the connection path by stopping. That is, the circulation pump 26 functions as a “connection switching unit” that switches between disconnection / communication of the connection path 24 and gas from the anode gas discharge path 20 side to the hydrogen supply path 14 side when the connection path 24 is in a communication state. It also functions as a “gas flow means” that causes the flow of gas. The output performance of the circulation pump 26 is such that the anode off gas from the anode gas discharge passage 20 to the hydrogen supply passage 14 is opposed to the pressure difference between the gas pressure in the hydrogen supply passage 14 and the gas pressure in the anode gas discharge passage 20. As long as it can flow. The circulation pump 26 is controlled so as to operate only when a predetermined operation condition is satisfied with the stop state as its basic state.

  Each cell is connected to a terminal of a voltmeter 28 for monitoring the generated voltage. The voltmeter 28 is connected to the ECU 30, and the ECU 30 controls the above-described circulation pump 26 and the exhaust valve 22 based on information from the voltmeter 28.

  Here, an example of the principle that the anode gas shortage (hereinafter referred to as hydrogen shortage) occurs in the fuel cell system configured as described above will be described. FIG. 2 is a schematic diagram showing the flow of hydrogen gas when the connection path is interrupted. In FIG. 2, for simplicity, the fuel cell stack 10 is composed of five cells, and the hydrogen inlet and outlet are the central portions of the manifolds 34 and 36. Hereinafter, the hydrogen inlet side may be referred to as upstream and the outlet side may be referred to as downstream. When the connection path is interrupted, the flow rate of the reaction gas in the anode surface is small. Further, when a fuel cell stack is configured by stacking a plurality of cells, the pressure loss of the reaction gas in each cell may vary. For this reason, the off-gas that has passed through the cell having a small pressure loss may flow backward to the cell having a large pressure loss. When the reverse flow occurs, a portion (A in FIG. 2) where the gas flow is almost lost is formed, and impurities accumulate in the portion.

  Note that impurities refer to substances other than hydrogen. Specifically, it is nitrogen flowing from the cathode side through the electrolyte membrane to the anode side or water vapor generated by power generation.

  FIG. 3 is a diagram showing the hydrogen gas concentration distribution in the anode surface of each cell when the gas flow is as shown in FIG. Hydrogen gas is supplied only from the side to which the hydrogen tank is connected, and is used for power generation in the plane. On the other hand, nitrogen gas flows from the cathode to the anode through the electrolyte membrane. For this reason, the hydrogen concentration generally decreases as it goes downstream in the cell. Moreover, in the cell with a high pressure loss, the gas which passed through the other cell flows in as mentioned above. As a result, there is no gas flow in a part, and hydrogen is not supplied even if the hydrogen in the part is consumed. Therefore, it becomes a part (hydrogen deficiency) where the hydrogen concentration is extremely low. Since power generation cannot be performed in the hydrogen deficient portion, the power generation performance is reduced in a cell deficient in hydrogen. On the other hand, in a cell having a small pressure loss, there is a flow of a reaction gas over the entire cell, so that a lack of hydrogen hardly occurs. Therefore, when hydrogen deficiency occurs due to this mechanism, it is considered that the generated voltage varies between cells.

  FIG. 4 is a schematic diagram showing the flow of hydrogen gas when the connection path is in a communication state and the off-gas is circulated. Since the off gas circulates through the communication path to the supply path, the flow rate of the reaction gas in each cell increases. As a result, gas flows from the hydrogen inlet side to the hydrogen outlet side even in a cell having a large pressure loss, so that the remaining impurities are washed away and hydrogen deficiency is eliminated.

[Specific Processing in First Embodiment]
FIG. 5 is a flowchart of the process according to the first embodiment. In the system according to the first embodiment, when the connection path 24 is cut off, the routine shown in FIG. 5 is performed every predetermined time. First, the ECU 30 determines whether the cell voltage obtained by the voltmeter 28 varies (S101). Specifically, the difference between the highest voltage and the lowest voltage among the cell voltages is taken, and the voltage difference is compared with the first threshold value. If the voltage difference is less than or equal to the predetermined value, it is determined that there is no variation in the cell voltage and no lack of hydrogen has occurred, and the routine is terminated.

  If the voltage difference is greater than or equal to a predetermined value, it is determined that hydrogen shortage has occurred. In that case, by operating the circulation pump, the connection path is brought into a communication state, and the anode off gas is circulated through the supply path (S103). After the predetermined time has elapsed, the ECU acquires the cell voltage again, calculates the voltage difference in the same manner as in step S101, and compares the voltage difference with the second threshold value (S105). If the voltage difference is greater than or equal to the second threshold, it is determined that the lack of hydrogen has not been resolved and the circulation is continued. On the other hand, if the voltage difference is equal to or smaller than the second threshold, it is determined that the lack of hydrogen has been resolved, and the circulation is terminated (S107).

  Note that the second threshold value is the same value as the first threshold value or a value smaller than the first threshold value, and is set in advance as a value with which it can be determined that the variation in the generated voltage has been eliminated.

[Effect of Embodiment 1]
According to the present embodiment, it is possible to eliminate the lack of hydrogen due to variations in pressure loss between cells that occur in the manufacturing process. In addition, since the circulation pump operates only under predetermined conditions, the pump consumes less energy than a system that constantly circulates.

  In the present embodiment, the circulation pump 26 is stopped in the basic state, and circulation is not performed except when the variation in the cell voltage is estimated. When the circulation is not performed, since the flow rate of the anode gas is small, impurities accumulate in the downstream portion (the discharge manifold 36 and the anode gas discharge path 20) in the anode. If the exhaust valve 22 is opened only when impurities are accumulated downstream, the impurities can be discharged while suppressing the discharge of hydrogen. Corresponding to this, in the first embodiment, it is preferable not to open the exhaust valve 22 during the circulation and for a certain time after the end of the circulation. When circulating, impurities are circulated together with hydrogen, so hydrogen and impurities are mixed. This is because if the exhaust valve is opened in a mixed state, hydrogen will also be discharged. Therefore, the predetermined time may be set to a time during which the hydrogen concentration in the downstream portion is estimated to be sufficiently high.

[Modification of Embodiment 1]
In the first embodiment, the exhaust valve 22 is opened only when predetermined purge conditions are met. However, the present invention is not limited to this. That is, a small amount of off gas may be continuously exhausted through the exhaust valve 22 (continuous small amount exhaust system). In a continuous small exhaust system, the exhaust valve 22 is basically open. As described above, since impurities accumulate in the downstream when not circulating, the impurities are continuously exhausted. The overall configuration is the same as that of the first embodiment shown in FIG.

  FIG. 6 is a flowchart of the process in the case of continuous small amount exhaust. The determination of lack of hydrogen or its elimination is performed in the same manner as in the first embodiment (S111, S115). However, as described above, since impurities and hydrogen gas mix when circulated, it is preferable to stop the exhaust during the circulation. Therefore, the exhaust valve is closed at the start of circulation (S113), and the exhaust valve is opened again after the end (S117). In this modification, the exhaust is stopped only during the circulation, but it is preferable to stop the exhaust for a certain time after the end of the circulation. This is because immediately after the circulation, the hydrogen concentration is somewhat high even in the downstream portion, and if exhaust is performed in this state, hydrogen is discharged.

  FIG. 7 is a schematic diagram showing the configuration of another modification of the first embodiment. In the first embodiment, the exhaust system is a small amount exhaust system that performs exhaust under a predetermined purge condition. However, the present modification does not include an exhaust valve. This modification is an exhaust-less system that does not exhaust at all. Even in the case of no exhaust, hydrogen deficiency may occur due to variations in pressure loss between cells, and the hydrogen deficiency can be eliminated by circulation. Specific processing is the same as that in the first embodiment.

  In the first embodiment, in step S105, the circulation is terminated when the voltage difference falls below a predetermined value. However, the present invention is not limited to this. For example, the circulation may be executed for a predetermined time and terminated when the time has elapsed. The predetermined time can be determined in advance as the time that is expected to eliminate the hydrogen shortage by circulation. In any case, the circulation may be stopped when it is estimated that the lack of hydrogen is eliminated.

  In the first embodiment, in step S101, when the difference between the highest voltage and the lowest voltage among the cell voltages is taken and the difference is a predetermined value or more, there is a variation (there is a lack of hydrogen). However, it is not limited to this. For example, there may be variations when the standard deviation of the cell voltage is greater than or equal to a predetermined value. Note that, from the viewpoint of finding a cell in which the power generation output is reduced, it is preferable to determine the variation based on the voltage of the cell having the lowest output voltage. As a method based on the voltage of the cell with the lowest output voltage, in addition to the method of the first embodiment, the difference between the average value of the voltages and the lowest voltage is taken, and there is variation when the difference is equal to or greater than a predetermined value. You can also.

[Others]
As described above, since the power generation capacity of the cell in which hydrogen deficiency occurs is reduced, the occurrence of hydrogen deficiency can be detected by looking at the difference in cell voltage. On the other hand, the power generation capacity in a specific cell is not limited to lack of hydrogen. For example, a case where flooding or dry-up occurs only in some cells can be considered. According to the method of the first embodiment, at least when hydrogen deficiency occurs, the operation is switched to the circulation operation, and therefore step S101 corresponds to the gas deficiency estimation means.

  It should be noted that even when flooding or dry-up occurs in some cells, it can be solved by the present embodiment. For example, in the case of flooding, the flow rate of gas in the cell is increased by circulating operation, so that the water present in the cell can be discharged out of the cell. Therefore, flooding is eliminated.

  Drying up is also eliminated by the flow of gas in the cell by circulation and the movement of water in the cell according to the gas flow. In particular, when the cathode gas and the anode gas flow in opposite directions across the electrolyte screen (so-called counterflow), the gas flows toward the downstream part of the anode (upstream part of the cathode), which is the most dry part. , Dry-up is eliminated.

  As described above, since the first embodiment detects a variation in power generation between cells, it can also detect a cause other than a lack of hydrogen. And it is expected that such variation will be eliminated by carrying out the circulating operation.

Embodiment 2. FIG.
[Configuration of Embodiment 2]
FIG. 8 is a diagram schematically showing the configuration of the fuel electronic system of the second embodiment. In this embodiment, the anode gas discharge path 20 is provided with a hydrogen concentration 32 sensor. On the other hand, no cell voltmeter is provided. In addition, the present embodiment is an exhaust-less system that generates power without exhausting any anode off-gas.

  Also in the present embodiment, the connection path 24 is cut off in the basic state, and the circulation operation is performed only when a predetermined condition is satisfied. When the circulating operation is not performed, impurities gradually accumulate on the downstream side of the fuel cell. Then, even in the cell, the concentration of impurities may increase in the downstream portion, resulting in lack of hydrogen. Such a lack of hydrogen can occur when power is generated with most of the reaction gas remaining inside, regardless of whether the cell pressure loss varies.

  As described above, since impurities tend to accumulate in the downstream portion of the fuel cell, hydrogen deficiency tends to occur in the downstream portion of the cell in the gas flow direction. Therefore, the lack of hydrogen in the cell can be estimated by detecting the hydrogen concentration in the downstream portion of the fuel cell. Then, when hydrogen depletion occurs, if the off-gas is circulated with the connection path 24 in communication, hydrogen and impurities are mixed by circulation, so that the hydrogen depletion can be eliminated.

[Specific Processing of Embodiment 2]
FIG. 9 is a flowchart of the process according to the second embodiment. In the system of the second embodiment, when the connection path 24 is shut off, the routine shown in FIG. 9 is performed every predetermined time. First, the ECU 30 compares the hydrogen concentration acquired by the hydrogen concentration sensor 32 with a predetermined threshold value (S121). If the hydrogen concentration is greater than or equal to the predetermined value, it is determined that no hydrogen deficiency has occurred in the cell, and the process ends.

  When the hydrogen concentration is less than or equal to a predetermined value, it is estimated that hydrogen deficiency has occurred in the cell. In that case, the ECU 30 activates the circulation pump to bring the connection path into a communication state and circulate the anode off gas through the supply path (S123). While performing the circulating operation, the ECU acquires the hydrogen concentration every predetermined time, and determines whether or not the change in the hydrogen concentration has ceased (S125). Specifically, the acquired hydrogen concentration is compared with the hydrogen concentration acquired one time before, and it is determined whether the difference is equal to or less than a predetermined threshold value. If it is equal to or less than the threshold value, it is considered that the hydrogen and the impurities are sufficiently mixed. Therefore, the ECU determines that the lack of hydrogen has been resolved, and ends the circulation (S127). On the other hand, if it is equal to or greater than the predetermined threshold, the circulation is continued.

[Effect of Embodiment 2]
According to the present embodiment, the lack of hydrogen in the downstream portion of the cell can be eliminated. In addition, since the circulation pump operates only under predetermined conditions, the pump consumes less energy than a system that constantly circulates.

[Modification of Embodiment 2]
In the second embodiment, in step S125, the circulation is terminated when the change in the hydrogen concentration disappears. However, the present invention is not limited to this. For example, the circulation may be executed for a predetermined time and terminated when the time has elapsed. In this case, the predetermined time can be determined in advance as the time expected to eliminate the hydrogen shortage by circulating. Alternatively, the process may be terminated when the hydrogen concentration becomes higher than a predetermined threshold value. Moreover, it is good also as termination conditions combining these. In any case, the circulation may be stopped when it is estimated that the lack of hydrogen is eliminated.

  In the second embodiment, in step S121, the circulation is started when the hydrogen concentration becomes lower than the predetermined threshold value, but the present invention is not limited to this. For example, it may be circulated when a predetermined time has elapsed since the end of the previous circulation operation. In that case, the predetermined time may be set in advance to a time at which hydrogen depletion occurs in the cell, corresponding to the speed at which nitrogen moves from the cathode side through the electrolyte membrane to the anode side. Since the moving speed depends on the temperature of the electrolyte membrane, the time may be set according to the temperature.

  In the second embodiment, the hydrogen concentration sensor 32 is provided in the discharge path. However, the present invention is not limited to this, and it is sufficient if the hydrogen concentration sensor 32 is provided in the downstream portion in the reaction gas flow direction from the power generation region of the fuel cell. For example, you may provide in the discharge side manifold inside a fuel cell laminated body.

  In the second embodiment, the voltmeter for measuring the cell voltage is not provided, but the present invention is not limited to this. For example, the process of the first embodiment and the process of the second embodiment are provided such that a voltmeter and a hydrogen concentration sensor are provided to circulate when either a voltage variation or a downstream hydrogen concentration decrease is detected. May be combined.

It is a figure which shows the structure of Embodiment 1 of this invention. It is a figure which shows typically the flow of the gas in Embodiment 1 of this invention. It is a figure which shows the density | concentration distribution of the anode gas in Embodiment 1 of this invention. It is a figure which shows typically the flow of the gas in Embodiment 1 of this invention. It is a flowchart which shows the process of Embodiment 1 of this invention. It is a flowchart which shows the process of the modification of Embodiment 1 of this invention. It is a figure which shows the structure of the modification of Embodiment 1 of this invention. It is a figure which shows the structure of Embodiment 2 of this invention. It is a flowchart which shows the process of Embodiment 2 of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Fuel cell laminated body 12 Hydrogen tank 14 Hydrogen supply path 16 Shut valve 18 Modulatable pressure valve 20 Anode gas discharge path 22 Exhaust valve 24 Connection path 26 Circulation pump 28 Voltmeter 30 ECU
32 Hydrogen concentration sensor 34 Supply manifold 36 Exhaust manifold

Claims (5)

A fuel cell comprising a membrane electrode assembly;
A gas supply path for supplying anode gas to the fuel cell;
A gas discharge path for discharging anode gas from the fuel cell;
A connection path connecting the gas discharge path to the gas supply path;
Connection switching means for switching blocking / communication of the connection path;
Gas flow means for causing a gas flow from the gas discharge path side to the gas supply path side when the connection path is in a communicating state;
A gas shortage estimating means for estimating a shortage of anode gas in at least a part of the membrane electrode assembly;
A connection control means for controlling the connection switching means so as to place the connection path in a communicating state when an anode gas shortage is estimated when the connection path is in a shut-off state;
A fuel cell system comprising: Equipped with a gas shortage elimination estimating means for estimating that the anode gas shortage has been resolved,
The connection control means controls the connection switching means so as to put the connection path in a cut-off state when it is estimated that the shortage of anode gas is estimated.
The fuel cell system according to claim 1. The fuel cell includes a plurality of membrane electrode assemblies,
A cell voltmeter for detecting the generated voltage of each of the plurality of membrane electrode assemblies,
The gas shortage estimating means estimates that the anode gas is short when the variation in the generated voltage of the membrane electrode assembly is a predetermined value or more.
The fuel cell system according to claim 1 or 2, wherein The out-of-gas estimation means determines that the variation in the generated voltage is greater than or equal to a predetermined value when the difference between the highest voltage and the lowest voltage among the generated voltages of the membrane electrode assembly is greater than or equal to a predetermined value. The fuel cell system according to claim 3. An anode gas concentration detection means for detecting a correlation value of the anode gas concentration downstream of the membrane electrode assembly,
The gas shortage estimating means estimates that the anode gas is insufficient when the anode gas concentration is a predetermined value or less.
The fuel cell system according to claim 1 or 2, wherein

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