JP5564315B2 - Fuel cell system - Google Patents

Description

  The present invention relates to a fuel cell system in which air supplied to a fuel cell is humidified by a humidifying means, and in particular, a technique for stably operating the fuel cell system by suppressing drying of the fuel cell and the humidifying means. About.

  As a fuel cell, a stacked body in which separators are stacked on both sides of a flat membrane electrode assembly (MEA) is used as a unit cell, and a fuel cell stack is formed by stacking several unit cells, for example, several hundred layers. A fuel cell configured as is known. The membrane electrode structure has a three-layer structure in which an electrolyte membrane made of an ion exchange resin or the like is sandwiched between a pair of electrodes constituting a positive electrode (air electrode, cathode) and a negative electrode (fuel electrode, anode). According to such a fuel cell, for example, a fuel gas (hydrogen) is allowed to flow through a gas flow channel facing the gas diffusion electrode on the fuel electrode side, and an oxidant gas ( When air is flowed, an electrochemical reaction occurs and power generation occurs.

  Here, in order to stabilize the electrochemical reaction as described above and maintain high power generation efficiency, it is necessary to maintain the electrolyte membrane in a saturated water-containing state to ensure the function as an ion exchange resin. For example, in Patent Document 1, the oxidant gas sent from the blower is circulated through the humidifying means, and the used oxidant gas discharged from the fuel cell stack in the humidifying means (hereinafter referred to as cathode offgas or offgas). And a fuel cell system in which moisture contained in fuel gas after use (hereinafter sometimes referred to as anode off-gas or off-gas) is supplied to unused oxidant gas, that is, water is exchanged. ing.

  However, the humidity of the off gas discharged from the fuel cell varies depending on the operating state of the fuel cell. For example, when the fuel cell is operating at a low load, the amount of water generated by the electrochemical reaction is small, and the gas flow rate is increased from the amount actually consumed to prevent the generated water from remaining in the fuel cell. The humidity of the off gas becomes low, and even if the moisture is exchanged with the unused gas as in Patent Document 1, the moisture that the off gas gives to the unused gas is insufficient. Further, even when the relative humidity of the off gas discharged from the fuel cell stack is less than 100% even at times other than such a low load, the moisture in the unused gas is similarly insufficient. Further, when the humidifying means is constituted by a hollow fiber membrane, if the humidifying means is kept in contact with the high-temperature and low-humidity gas, the water vapor in the pores formed in the hollow fiber membrane is lost, and the moisture is smoothly transferred. It will disappear and the humidification efficiency will drop significantly (dry up). Once this dry-up occurs, it takes time for the hollow fiber membrane to regain the moisture exchange function, during which time it will not function as a humidifying means, and as a result, the fuel cell cannot be operated at an appropriate humidification amount. was there.

  When the oxidant gas is humidified by the humidifying means as described above, the humidity of the fuel cell stack tends to decrease as the gas supply amount increases, and there is a correlation between the gas supply amount and the fuel cell stack voltage. In view of this, the stack humidity behavior is determined from the stack voltage behavior when the gas supply amount changes, and if it is determined that the humidification is insufficient, the stack is dried by reducing the air flow rate and humidifying the oxidant gas. A technique for preventing this is disclosed (for example, see Patent Document 2).

JP-A-6-132038 JP 2000-243418 A

  However, when the determination is made by measuring the fuel cell stack voltage, it is detected from the state in which the polymer electrolyte membrane has already started to dry, so even if the air flow rate is lowered at that time, it is slow and already dried. There is a problem that it takes a very long time to restore the polymer electrolyte membrane to its original humidified state. In addition, in a system in which the humidifier is made small in order to reduce the size of the system and reduce the cost, there is a problem that even the humidifier is dried when the drying of the stack is detected by voltage.

  Accordingly, an object of the present invention is to provide a fuel cell system that can suppress the drying of the fuel cell stack and the humidifier before starting and can operate stably.

The fuel cell system of the present invention includes a fuel cell in which an oxidant gas and a fuel gas are supplied to a solid polymer film and generates electric power by an electrochemical reaction of these gases, and an oxidant that causes the oxidant gas to flow toward the fuel cell. A gas supply channel, an oxidant gas discharge channel through which the oxidant discharge fluid from the fuel cell flows, an oxidant gas supply channel and an oxidant gas discharge channel are spanned from the oxidant gas discharge fluid. Humidification means for transferring moisture to the oxidant gas, oxidant gas supply flow rate adjustment means provided on the oxidant gas supply flow path and upstream of the humidification means, and oxidant gas discharge flow path on the fuel cell Provided with a temperature detecting means for the oxidant gas discharge fluid provided on the downstream side and on the upstream side of the humidifying means , an output current detecting means for the fuel cell, and a control means for the oxidant gas supply flow rate adjusting means, the control means comprising: Fuel cell At large set area on the map than the temperature normal use of DENDEN flow value and the oxidant gas exhaust fluid, the boundary of either wet or dry humidifier, the temperature of the oxidant gas exhaust fluid Each output current threshold value is preset, and the flow rate is adjusted based on a comparison between the current output current value and the current threshold value at the current temperature of the oxidant gas discharge fluid.

  In the present invention, the oxidant gas supply flow rate adjusting means increases the flow rate of the oxidant gas when the humidifying means is determined to be wet, and the flow rate of the oxidant gas when the humidifying means is determined to be dry. It is a preferred embodiment to reduce the value.

Further, the fuel cell system of the present invention includes a fuel cell in which an oxidant gas and a fuel gas are supplied to a solid polymer membrane to generate power by an electrochemical reaction of these gases, and an oxidant gas is allowed to flow toward the fuel cell. The oxidant gas supply channel, the oxidant gas discharge channel through which the oxidant discharge fluid from the fuel cell flows, the oxidant gas supply channel, and the oxidant gas discharge channel are spanned and discharged. A humidifying means for transferring moisture from the fluid to the oxidant gas; an oxidant gas pressure adjusting means provided on the oxidant gas discharge flow path downstream of the humidification means; and an oxidant gas discharge flow path. A temperature detecting means for the oxidant gas discharge fluid provided on the downstream side of the fuel cell and on the upstream side of the humidifying means , an output current detecting means for the fuel cell, and a control means for the oxidant gas pressure adjusting means, the control means comprising: The fuel cell Temperature of the power generation current value and the oxidant gas exhaust fluid is in the region on the map that is larger than normal use, the boundary of either wet or dry humidifier, the temperature of the oxidant gas exhaust fluid Each output current threshold value is preset, and pressure adjustment is performed based on a comparison between the current output current value and the current threshold value at the current temperature of the oxidant gas discharge fluid.

  In the present invention, the oxidant gas pressure adjusting means lowers the oxidant gas pressure when the humidifying means is determined to be wet, and increases the oxidant gas pressure when the humidifying means is determined to be dry. This is a preferred embodiment.

  According to the fuel cell system having the above-described configuration, the moisture of the exhaust oxidant gas containing a large amount of moisture relative to the unused oxidant gas is humidified by the humidifying means, so that the oxidant gas having an increased relative humidity is used as the fuel. When the fuel cell output current exceeds the current threshold determined from the temperature at that time, that is, when it is determined that the fuel cell is in a dry state Further, since the operation is performed by increasing the water vapor partial pressure by reducing the oxidant gas supply flow rate or increasing the oxidant gas pressure, drying of the fuel cell and the humidifying means can be prevented. When the output current of the fuel cell is lower than the current threshold, that is, when it is determined that the fuel cell is in a wet state, the water vapor partial pressure is lowered by increasing the oxidant gas supply flow rate or decreasing the oxidant gas pressure. Since the operation is performed, excessive humidification of the fuel cell can be suppressed. In addition, the determination of the wet state / dry state is performed by detecting the output current before the actual situation, not the output voltage after the actual situation, so the fuel cell and the humidifying means are actually actually dried. There is an effect that it can cope before starting.

1 is a system diagram showing a fuel cell system according to a first embodiment of the present invention. It is a systematic diagram which shows the fuel cell system of 2nd Embodiment of this invention. It is a systematic diagram which shows the fuel cell system of 3rd Embodiment of this invention. It is a flowchart figure which shows the control method of the control means of the fuel cell of this invention. It is a graph which shows the relationship between a fuel cell output current and air exit temperature (oxidant gas discharge | emission fluid temperature).

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
First Embodiment FIG. 1 shows a fuel cell system according to a first embodiment of the present invention. Reference numeral 10 denotes a fuel cell stack, and the fuel cell stack 10 is configured by stacking a number of unit cells of a fuel cell. Yes. A humidifier (humidifying means) 12 is connected to the oxidant gas supply port of the fuel cell stack 10. The fuel cell stack 10 and the humidifier 12 are connected by a first pipe (oxidant gas supply channel) 20 so that air in the atmosphere as an oxidant gas flows into the humidifier 12 from the first pipe 20. It has become. An air pump (oxidant gas supply flow rate adjusting means) 11 is connected to the upstream side of the humidifier 12 on the first pipe 20, and the oxidant supplied to the humidifier 12 and the fuel cell stack 10. The gas flow rate can be adjusted.

  A fuel tank (not shown) is connected to the fuel gas supply port of the fuel cell stack 10 by a second pipe (fuel gas supply flow path) 21, and hydrogen gas as a fuel gas is supplied from the second pipe 21 to the fuel cell stack 10. It has come to be.

  In addition, the fuel cell stack 10 and the humidifier 12 are connected by a third pipe (oxidant gas discharge channel) 22 so that the cathode off-gas discharged from the fuel cell stack 10 is supplied to the humidifier 12. .

  Here, the humidifier 12 is configured by incorporating a plurality of hollow fiber membranes for exchanging moisture in parallel in a chamber. The hollow fiber membrane is a hollow thin yarn membrane that has the property of preventing the permeation of gas but allowing the permeation of moisture, that is, water molecules. In this case, moisture permeates through the hollow fiber membrane from the direction of high water vapor partial pressure to the low direction. Therefore, when a gas with a low relative humidity is circulated inside the hollow fiber membrane and a gas with a high relative humidity is circulated outside, the moisture permeates from the outside to the inside of the hollow fiber membrane and diffuses into the gas with a low relative humidity. And increase its humidity. The same effect can be obtained even if a gas having a high relative humidity is circulated inside the hollow fiber membrane and a gas having a low relative humidity is circulated outside.

  In the present invention, either the first pipe 20 or the third pipe 22 is connected to the inside of the hollow fiber membrane, and the other is outside the hollow fiber membrane and connected to the inside of the chamber. Therefore, air that is not humidified and has a low relative humidity and cathode offgas that is 100% or close to the relative humidity after use of the fuel cell are supplied to the inside and outside of the hollow fiber membrane, respectively, and are included in the cathode offgas in the hollow fiber membrane. Moisture is exchanged for the air supplied to the fuel cell.

  A temperature sensor (temperature detection means) 14 is connected to the upstream side of the humidifier 12 on the third pipe 22, and the temperature of the oxidant gas discharge fluid discharged from the fuel cell stack 10 is measured. .

  A back pressure valve (oxidant gas pressure adjusting means) 13 is connected to the downstream side of the humidifier 12 on the third pipe 22, and the humidifier 12 and the third pipe upstream of the valve. 22 and the oxidant gas passage in the fuel cell stack 10 and the pressure in the first pipe 20 can be adjusted.

  Furthermore, a current sensor (output current detection means) 15 is connected to the output circuit 30 from which the electric power of the fuel cell stack 10 is taken, and the output current of the fuel cell stack 10 is measured.

  Reference numeral 16 denotes a control unit (control means) of the air pump 11, which receives a temperature detection signal 40 and an output current detection signal 41 indicated by broken lines, and transmits a command signal 42 to the air pump 11 to transmit the command signal 42 of the air pump 11. Rotational speed control is performed and the oxidant gas supply flow rate is adjusted.

  FIG. 4 shows a flowchart of the operation of the fuel cell system according to the first embodiment having the above-described configuration. First, the control unit 16 detects the current fuel cell temperature based on the temperature detection signal 40 from the temperature sensor 14. A humidifier drying current threshold value is calculated from the current temperature. This current threshold is a value that is separately experimentally determined in advance and is uniquely determined from the temperature of the fuel cell, and is a temperature at which it is determined that when the current reaches this value and the operation is continued, a dry state is reached. . For example, as illustrated outside the chart of FIG. 4, the current threshold when the fuel cell is 90 ° C. is determined to be 300A. In addition, the membrane drying limit air outlet temperature (stack outlet temperature) of the humidifier is measured in advance for each fuel cell current value, and a temperature-limit current value map is created from that, and the current threshold value at each temperature is set as a curve. The plotted graph is shown in the curve of FIG.

  Then, the control unit 16 detects the current operation output current of the fuel cell based on the output current detection signal 41 from the current sensor 15. The current output current value is compared with the current threshold value, and when the output current is less than the current threshold value, it is determined that the state is wet, and the oxidant gas supply flow rate by the air pump 11 is normally set by the command signal 42. Operation continues at the value.

  On the other hand, when the output current is equal to or greater than the current threshold, it is determined that the air is being dried, and the oxidant gas supply flow rate by the air pump 11 is set to a humidifier dry prevention flow rate (a value smaller than normal) by the command signal 42. The air pump 11 is operated at a reduced rotational speed. By reducing the oxidant gas flow rate with the air pump 11, the amount of water vapor removed from the fuel cell and the humidifier can be reduced, and drying is suppressed. In the first embodiment, the opening degree of the back pressure valve 13 is fixed, and only the rotation speed of the air pump 11 is adjusted.

  Furthermore, although not shown in the graph or the flowchart, when the output current is less than the current threshold value and the wet state is desired to be more appropriate, the command signal 42 increases (rotates) the oxidant gas supply flow rate by the air pump 11. By adjusting in the direction of increasing the number, it is possible to avoid an excessively wet state, which is more preferable.

Second Embodiment FIG. 2 shows a fuel cell system according to a second embodiment of the present invention. Here, description of components common to the first embodiment will be omitted, and only the changes will be described. Reference numeral 17 in the second embodiment denotes a control unit (control means) for the back pressure valve 13, which receives the input of the temperature detection signal 40 and the output current detection signal 41 indicated by a broken line, and issues a command signal 43 to the back pressure valve 13. Is transmitted to open / close the back pressure valve 13 to adjust the oxidant gas pressure.

  A flowchart of the operation of the fuel cell system of the present invention having the above configuration will be described with reference to FIG. 4 again. First, the control unit 17 detects the current fuel cell temperature based on the temperature detection signal 40 from the temperature sensor 14. A humidifier drying current threshold value is calculated from the current temperature. Then, the control unit 17 detects the current operation output current of the fuel cell based on the output current detection signal 41 from the current sensor 15. The current output current value is compared with the current threshold value, and when the output current is less than the current threshold value, it is determined that the state is wet, and the oxidant gas pressure by the back pressure valve 13 is normally set by the command signal 43. Operation continues at the value.

  On the other hand, when the output current is equal to or greater than the current threshold value, it is determined that the drying state is being approached, and the oxidant gas pressure by the back pressure valve 13 is set to the humidifier drying prevention pressure (a value larger than normal) by the command signal 43. Thus, the back pressure valve 13 is adjusted in the closing direction. By increasing the oxidant gas pressure by the back pressure valve 13, the water vapor pressure increases, and the humidity of the gas supplied to the fuel cell and the humidifier increases, so that drying is suppressed. In the second embodiment, the rotation speed of the air pump 11 is fixed, and only the opening degree of the back pressure valve 13 is adjusted.

  Further, although not shown in the graph or the flowchart, when the output current is less than the current threshold value and the wet state is desired to be more appropriate, the oxidant gas supply pressure by the back pressure valve 13 is decreased by the command signal 43 ( By adjusting the opening in the direction), an excessively wet state can be avoided, which is more preferable.

Third Embodiment FIG. 3 shows a fuel cell system according to a third embodiment of the present invention. Here, description of components common to the first and second embodiments is omitted, and only the changed points are described. Reference numeral 18 in the third embodiment denotes a control unit (control means) for both the air pump 11 and the back pressure valve 13, which receives the input of the temperature detection signal 40 and the output current detection signal 41 indicated by broken lines, and receives the air pump 11 and the back pressure valve 41. The command signals 42 and 43 are transmitted to the control unit 13 to perform these adjustments, and the oxidant gas supply flow rate and the oxidant gas pressure can be adjusted simultaneously or individually, respectively.

  According to the fuel cell system of the third embodiment, the control unit 18 determines that the fuel cell stack is in a wet state when the output current of the fuel cell stack is less than the current threshold value. The oxidant gas pressure by the pressure valve 13 is set to the normal set value by the command signals 42 and 43, and the operation is continued. On the other hand, when the output current is equal to or higher than the current threshold, it is determined that the air pump is going to the dry state. The oxidant gas supply flow rate by 11 and the oxidant gas pressure by the back pressure valve 13 are adjusted to the humidifier drying prevention state quantity (flow rate value and pressure value) by the command signals 42 and 43.

  As described above, the temperature detection and output current detection of the first to third embodiments are continuously performed during the operation of the fuel cell, and the current value at each operating temperature exceeds the threshold value, that is, as shown in FIG. In the upper right region of the curve, the operation is performed with the humidifier drying prevention state quantity.

  In the above description, a humidifier having a hollow fiber membrane is used, but a known humidifier such as one having a water vapor permeable membrane as described in Patent Document 1 can be applied to the present invention. . Further, in the above description, the output current is detected to determine the wet state / dry state. However, the determination may be made using the required current to the fuel cell stack.

  According to the fuel cell system configured as described above, when the output current of the fuel cell exceeds the current threshold determined from the temperature at that time, that is, when it is determined that the fuel cell is in a dry state, the oxidant gas supply flow rate is reduced or the oxidation current is reduced. Since the operation is performed by increasing the water vapor partial pressure by increasing the agent gas pressure, drying of the fuel cell and the humidifier can be prevented. Further, when the output current of the fuel cell is lower than the current threshold, that is, when it is determined that the fuel cell is in a wet state, the water vapor partial pressure is decreased by increasing the oxidant gas supply flow rate or decreasing the oxidant gas pressure. Since the operation is performed, excessive humidification of the fuel cell can be suppressed. In addition, since the determination of the wet state / dry state is performed by detecting the output current before the actual situation, not the output voltage after the actual situation, the fuel cell and the humidifying means are actually used. There is an effect that it is possible to cope with before starting to dry. Furthermore, no drying of the fuel cell / humidifier occurs, and there is no operating situation where the fuel cell deteriorates due to drying, and the humidifier can be designed to be small, reducing the size and cost of the system. Can do.

  In the present invention, when the operating current exceeds the current threshold at each temperature of the fuel cell, the operation is performed with the oxidant gas state quantity that can prevent the humidifier from being dried. Since drying is prevented in advance and stable operation of the fuel cell is ensured, it is extremely promising when applied to a fuel cell system for automobiles that require strict reliability.

10 ... Fuel cell stack,
11 ... Air pump (oxidant gas supply flow rate adjusting means),
12 ... Humidifier (humidifying means),
13 ... Back pressure valve (oxidant gas pressure adjusting means),
14 ... temperature sensor (temperature detection means),
15 ... Current sensor (output current detection means),
16-18 ... Control part (control means) of air pump and / or back pressure valve,
20 ... 1st piping (oxidant gas supply flow path),
21 ... 2nd piping (fuel gas supply flow path),
22 ... 3rd piping (oxidant gas discharge flow path),
23. Fourth pipe (fuel gas discharge passage).
30 ... Output circuit,
40 ... temperature detection signal,
41 ... Output current detection signal,
42 ... Flow command signal,
43 ... Pressure command signal.

Claims (4)

A fuel cell in which an oxidant gas and a fuel gas are supplied to a solid polymer film and an electric power is generated by an electrochemical reaction of these gases;
An oxidant gas supply flow path for flowing an oxidant gas toward the fuel cell;
An oxidant gas discharge passage for passing an oxidant discharge fluid from the fuel cell;
Humidification means that moves over the oxidant gas supply flow path and the oxidant gas discharge flow path to move moisture from the oxidant gas discharge fluid to the oxidant gas;
An oxidant gas supply flow rate adjusting means provided on the upstream side of the humidifying means on the oxidant gas supply flow path;
A temperature detection means for the oxidant gas discharge fluid provided on the oxidant gas discharge flow path and downstream of the fuel cell and upstream of the humidification means ;
An output current detecting means of the fuel cell;
Control means for the oxidant gas supply flow rate adjusting means;
With
The control means includes
In the region on the map where the generated current value of the fuel cell and the temperature of the oxidant gas discharge fluid are set larger than the normal use state, a boundary between the humidifying means and the wet state is defined as the boundary of the oxidation state. Preset as the output current threshold for each temperature of the agent gas discharge fluid,
The fuel cell system, wherein the flow rate is adjusted based on a comparison between a current output current value and a current threshold value at a current temperature of the oxidant gas discharge fluid.   The oxidant gas supply flow rate adjustment unit increases the flow rate of the oxidant gas when the humidification unit is determined to be in a wet state, and the flow rate of the oxidant gas when the humidification unit is determined to be in a dry state. The fuel cell system according to claim 1, wherein the fuel cell system is reduced. A fuel cell in which an oxidant gas and a fuel gas are supplied to a solid polymer film and an electric power is generated by an electrochemical reaction of these gases;
An oxidant gas supply flow path for flowing an oxidant gas toward the fuel cell;
An oxidant gas discharge passage for passing an oxidant discharge fluid from the fuel cell;
Humidification means that moves over the oxidant gas supply flow path and the oxidant gas discharge flow path to move moisture from the oxidant gas discharge fluid to the oxidant gas;
An oxidant gas pressure adjusting means provided on the oxidant gas discharge channel and downstream of the humidifying means;
A temperature detection means for the oxidant gas discharge fluid provided on the oxidant gas discharge flow path and downstream of the fuel cell and upstream of the humidification means ;
An output current detecting means of the fuel cell;
Control means for the oxidant gas pressure adjusting means;
With
The control means includes
In the region on the map where the generated current value of the fuel cell and the temperature of the oxidant gas discharge fluid are set larger than the normal use state, a boundary between the humidifying means and the wet state is defined as the boundary of the oxidation state. Preset as the output current threshold for each temperature of the agent gas discharge fluid,
The fuel cell system, wherein the pressure adjustment is performed based on a comparison between a current output current value and a current threshold value at a current temperature of the oxidant gas discharge fluid.   The oxidant gas pressure adjusting unit decreases the oxidant gas pressure when the humidifying unit is determined to be in a wet state, and increases the oxidant gas pressure when the humidifying unit is determined to be in a dry state. The fuel cell system according to claim 3.

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