JP2007242337A - Fuel cell system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fuel cell system capable of ensuring at the maximum a holding capacity of water removed by purge, and realizing smooth starting at below-zero temperatures. <P>SOLUTION: The fuel cell system 1 conducts purge control in the stop of power generation of a fuel cell stack 2, and after the purge is conducted for the prescribed time, cooling water is circulated and the temperature of the fuel cell stack 2 is quickly cooled to the prescribed temperature T1 lower than the temperature of purge gas. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a fuel cell system that performs purge control when power generation of a fuel cell stack is stopped, and more particularly, to a fuel cell system that maximizes the effect of purging and always realizes good freezing point starting.

  A fuel cell system is a device that directly converts chemical energy of fuel into electrical energy, and supplies a fuel gas containing hydrogen to an anode of a pair of electrodes provided with an electrolyte membrane interposed therebetween, and the other cathode An oxygen agent gas containing oxygen is supplied to the electrode, and electric energy is extracted from the electrode by using the following electrochemical reaction that occurs on the surface of the electrolyte membrane side of the pair of electrodes (see, for example, Patent Document 1). .

Anodic reaction: H ₂ → 2H ⁺ + 2e ⁻ (1)
Cathodic reaction: 2H ⁺ + 2e ⁻ + (1/2) O ₂ → H ₂ O (2)
As the fuel gas supplied to the anode, a method of directly supplying from a hydrogen storage device, a method of supplying a hydrogen-containing gas by reforming a fuel containing hydrogen, and the like are known. Examples of the hydrogen storage device include a high-pressure gas tank, a liquefied hydrogen tank, a hydrogen storage alloy tank, and the like. As the fuel containing hydrogen, natural gas, methanol, gasoline, and the like are conceivable.

  On the other hand, air is generally used as the oxygen agent gas supplied to the cathode.

  By the way, for example, when a fuel cell is used as a power source for an automobile or used for stationary use in a cold region, the fuel cell may be exposed to an atmosphere of 0 ° C. or lower. It is desired that the fuel cell can be started and can generate electricity as usual.

  However, in a low temperature state of 0 ° C. or lower, there is a problem that power generation cannot be performed because the generated water accompanying power generation of the fuel cell freezes and inhibits diffusion of the reaction gas. Therefore, in order to start the fuel cell from 0 ° C. or lower, it is necessary to continue the power generation for a certain period of time even if the generated water cannot be discharged as in the operation at the normal temperature.

Therefore, conventionally, when the fuel cell is stopped, the gas flow path is purged with a dry gas, and the membrane electrode assembly (MEA) is dried to secure a portion capable of holding the generated water in the MEA. A technique for allowing power generation to continue is known (see Patent Document 2 and Patent Document 3).
JP-A-8-106914 JP 2001-332281 A JP 2002-208422 A

  However, even if the dry purge is performed with a high-temperature dry gas as in the conventional fuel cell system described above, if the fuel cell stack is cooled after being left for several hours after the dry purge is performed, There was a problem that the produced water retention capacity of MEA would be extremely small.

  For example, considering the actual situation of a fuel cell mounted on a vehicle, even if the outside air temperature is below freezing point, it is difficult to imagine a situation where the fuel cell stack is rapidly cooled after purging. Under normal circumstances, the fuel cell stack is gradually cooled after maintaining a high temperature for a certain period of time due to the effects of heat mass, heat insulating material, etc. of the battery stack. Therefore, there is a problem in that the generated water retention capacity that can be secured by the dry purge cannot be utilized 100%, and the starting ability below freezing point is deteriorated.

  On the other hand, when cooling is started immediately after the dry purge is performed, the MEA can keep the generated water retention capacity removed by the dry purge as much as possible. It was necessary to actively cool the fuel cell stack.

  In order to solve the above-described problems, a fuel cell system according to the present invention includes a fuel cell stack using a solid polymer membrane, and in the fuel cell system in which purge control is performed when power generation of the fuel cell stack is stopped, the fuel cell After purging the stack, the temperature of the fuel cell stack is rapidly cooled to a predetermined temperature lower than the temperature of the purge gas.

  In the fuel cell system according to the present invention, after purging the fuel cell stack, the temperature of the fuel cell stack is rapidly cooled to a predetermined temperature lower than the temperature of the purge gas. As a result, it is possible to always realize a good freezing point start.

<First Embodiment>
DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, a first embodiment of the invention will be described with reference to the drawings. FIG. 1 is a block diagram showing the configuration of the fuel cell system according to this embodiment.

  As shown in FIG. 1, a fuel cell system 1 according to this embodiment includes a fuel cell stack 2 that is supplied with fuel gas and an oxygen agent gas and generates electric power through an electrochemical reaction, and a control unit 3 that controls the fuel cell system 1. A hydrogen tank 4 that stores hydrogen gas that is a fuel gas, a compressor 5 that pressurizes air that is an oxygen agent gas and supplies it to the cathode of the fuel cell stack 2, and cooling that circulates cooling water to the fuel cell stack 2 The cooling water is circulated by connecting the water pump 6, the radiator 7 that radiates the cooling water to cool it, the cooling fan 8 that blows the cooling air to the radiator 7, the fuel cell stack 2, the cooling water pump 6 and the radiator 7. The cooling water flow path 9 to be switched, the three-way valve 10 for switching the cooling water flow path to the radiator 7 side and the cooling water flow path to the bypass side, and the temperature of the cooling water flowing into the fuel cell stack 2 It includes a cooling water inlet temperature sensor 11 for detecting the cooling water outlet temperature sensor 12 for detecting the temperature of cooling water flowing out from the fuel cell stack 2.

  Here, in the fuel cell system 1 described above, the fuel cell stack 2 is a solid polymer membrane type fuel cell in which a pair of electrodes are provided on both sides of the solid polymer electrolyte membrane, and hydrogen gas as a fuel gas is supplied to the anode. Then, the cathode is supplied with air, which is an oxygen agent gas, to generate electricity by an electrochemical reaction.

  In the hydrogen supply system that supplies hydrogen gas, which is fuel gas, hydrogen gas is supplied from the hydrogen tank 4 to the anode of the fuel cell stack 2 through a pressure reducing valve and a hydrogen supply valve (not shown). The high-pressure hydrogen supplied from the hydrogen tank 4 is mechanically reduced to a predetermined pressure by a pressure reducing valve, and then the pressure of the hydrogen gas in the fuel cell stack 2 is adjusted to a desired pressure by adjusting the opening of the hydrogen supply valve. It is controlled to become.

  On the other hand, in the air supply system for supplying air as oxygen agent gas, the air sucked from the outside is compressed and sent out by the compressor 5 and supplied to the cathode of the fuel cell stack 2. The air pressure at the cathode is detected by an air pressure sensor (not shown), and the detected value is fed back to the control unit 3. Based on the detected value, the control unit 3 rotates the rotational speed of the compressor 5 and the opening area of an air pressure regulating valve (not shown). Is adjusted to control the air pressure at the cathode.

  The control unit 3 controls the pressure and flow rate of hydrogen gas supplied to the anode and the pressure and flow rate of air supplied to the cathode based on detection values from various sensors (not shown). Further, based on the detection values of the cooling water inlet temperature sensor 11 and the cooling water outlet temperature sensor 12, the number of rotations of the cooling water pump 6 and the cooling fan 8 and the switching of the three-way valve 10 are controlled to control the temperature of the cooling water. ing.

  Note that the fuel cell system 1 of the present embodiment shown in FIG. 1 shows only the part related to the present invention, and other general equipment for establishing the system is omitted.

  Next, the purge control process by the fuel cell system 1 of the present embodiment will be described based on the flowchart of FIG. As shown in FIG. 2, when the stop trigger of the fuel cell system 1 is turned on, for example, by turning off the ignition key of the vehicle (S201), the control unit 3 determines whether or not a purge request is necessary. Performed (S202). In determining the purge request, whether or not to start below freezing at the next start is predicted based on the outside temperature, weather forecast data, the next scheduled start time, the location (latitude / longitude) of the fuel cell system 1, and the like. The control unit 3 determines whether a purge request is necessary.

  If it is determined that the purge is not required according to the determination of the purge request, the routine shifts to the normal stop mode. On the other hand, if it is determined that the purge is required, the compressor 5 is driven to supply the purge gas. Supplying to the cathode of the fuel cell stack 2 starts the purge (S203).

  When a predetermined time elapses after the purge is started, it is determined that the purge is completed and the compressor 5 is stopped (S204), and at the same time, the cooling water pump 6 is operated to start the circulation of the cooling water (S205). .

  At this time, the three-way valve 10 is switched to the radiator 7 side, and the cooling water is supplied to the fuel cell stack 2 after passing through the radiator 7. Here, when the outside air temperature is high, if the vehicle is not running, the cooling capacity of the radiator 7 may be extremely reduced. However, when the temperature is required to start below freezing, the temperature rises during the day. However, since the outside air temperature is about 10 ° C. or less and the cooling fan 8 is driven, the temperature of the cooling water can be easily lowered to a predetermined temperature, so there is no problem.

  Next, when the circulation of the cooling water is started, it is determined whether or not the temperature of the cooling water measured by the cooling water outlet temperature sensor 12 is lower than a predetermined temperature T1 set in advance (S206).

  Here, the relationship between the standing temperature after purging and the amount of water retained in the fuel cell stack 2 will be described with reference to FIG. FIG. 3 is a graph showing the relationship between the temperature at which the fuel cell stack 2 is left after purging at the optimum temperature T2 (for example, 50 ° C.) for purging and the generated water retention capacity at the time of starting below freezing. As shown in FIG. 3, it can be seen that if the fuel cell stack 2 is left at the temperature T2 after purging at the temperature T2, the generated water retention capacity at the time of starting below the freezing point is reduced.

  On the other hand, after purging at the temperature T2, if it is allowed to cool to at least the temperature T1 (for example, about 30 ° C.) as soon as possible and left to stand, it can be understood that a larger capacity of retained water can be secured at the time of starting below the freezing point.

  Therefore, by obtaining the temperature T1 in advance by experiments and setting it as the predetermined temperature, it is possible to secure the generated water holding capacity at the time of starting below the freezing point to the maximum.

  Moreover, even if it cools to temperature T1 or less, since the generated water retention capacity at the time of starting below freezing is almost the same, if a temperature lower than temperature T1 is set as the predetermined temperature, the time required for cooling after purging becomes longer. Therefore, it is preferable to set the predetermined temperature to the temperature T1 in order to quickly complete the cooling after the purge.

  It is determined whether or not the temperature of the cooling water is lower than the predetermined temperature T1 with respect to the predetermined temperature T1 set in this way. If the temperature is equal to or higher than the predetermined temperature T1, the cooling water is continuously circulated. When the temperature of the cooling water becomes lower than the predetermined temperature T1, circulation of the cooling water is stopped (S207), the fuel cell system 1 is completely stopped, and the purge control process by the fuel cell system 1 of the present embodiment is finished. .

  However, in this embodiment, a method is adopted in which it is confirmed that the temperature of the fuel cell stack 2 has decreased to the predetermined temperature T1 by measuring the cooling water outlet temperature. You may employ | adopt the method of measuring directly, and another method.

  Thus, in the fuel cell system 1 of the present embodiment, when purging the fuel cell stack 2, the temperature of the fuel cell stack 2 is rapidly cooled to the predetermined temperature T1 lower than the temperature of the purge gas. As a result, it is possible to ensure the maximum capacity of the generated water that has been removed by the above-described operation, and thus it is possible to always achieve good freezing-point activation.

  Further, in the fuel cell system 1 of the present embodiment, the predetermined temperature T1 is set to approximately 30 ° C., so that it is possible to secure the maximum capacity of the generated water that has been removed by the purge, and to quickly complete the cooling after the purge. Can do.

<Second Embodiment>
Next, a second embodiment of the present invention will be described with reference to FIG. FIG. 4 is a flowchart showing a purge control process by the fuel cell system of the present embodiment. The configuration of the fuel cell system according to the present embodiment is the same as that of the first embodiment, and thus detailed description thereof is omitted.

  As shown in FIG. 4, the purge control process by the fuel cell system according to the present embodiment is performed when the stop trigger of the fuel cell system is turned on, for example, by turning off the ignition key of the vehicle (S401). Whether or not is necessary is determined (S402). In determining the purge request, it is predicted whether or not to start below freezing at the next start based on the outside temperature, weather forecast data, the next scheduled start time, the location (latitude / longitude) of the fuel cell system 1, and the like. The control unit 3 determines whether a purge request is necessary.

  If it is determined that purging is not necessary based on the determination of the purge request, the routine shifts to the normal stop mode. On the other hand, if it is determined that purging is necessary, the cooling water pump 6 is driven. Cooling water is circulated at a predetermined flow rate (S403). At the same time, the compressor 5 is driven to supply the purge gas to the cathode of the fuel cell stack 2 to perform the purge (S404), and this purge is continuously performed for a predetermined time as the first purge. This first purge generates latent heat of vaporization at the time of purging, but since the circulating cooling water takes most of this latent heat, the temperature of the fuel cell stack 2 does not decrease, and is necessary at the time of starting below freezing point. A product water retention capacity can be created.

  When the predetermined time has elapsed, the cooling water pump 6 is stopped and the circulation of the cooling water is stopped (S405), but the supply of the purge gas is continued (S406), and this purge is continued as the second purge for a predetermined time. And do it. Since the cooling water is stopped in the second purge, the cooling water does not take most of the latent heat of vaporization at the time of purging, and the fuel cell stack 2 can be rapidly cooled by this latent heat. In addition, by rapidly lowering the temperature of the fuel cell stack 2 after drying by the first purge, it is possible to secure the generated water retention capacity at the time of starting below the freezing point, and to realize a favorable sub-freezing starting. .

When the predetermined time elapses, the compressor 5 is stopped to complete the purge (S407), the fuel cell system 1 is completely stopped, and the purge control process by the fuel cell system of this embodiment is finished.

  Thus, in the fuel cell system according to the present embodiment, the cooling water is circulated when purging the fuel cell stack 2, so that the fuel cell stack 2 can be cooled more quickly.

  Further, in the fuel cell system of the present embodiment, when the cooling water is circulated to the fuel cell stack 2, the cooling water is circulated for a predetermined time at the initial stage of the purge out of the total purge time, and the remaining time in the latter purge period is the cooling water. Therefore, the fuel cell stack 2 can be rapidly cooled by the latent heat cooling that occurs during the evaporation of water in the latter stage of the purge. In addition, since the cooling water is circulated in the initial stage of the purge, the circulating cooling water takes most of the latent heat, so that the temperature of the fuel cell stack 2 does not decrease and is generated when starting below the freezing point. Water retention capacity can be created efficiently.

<Third Embodiment>
Next, a third embodiment of the present invention will be described with reference to FIG. As shown in FIG. 5, the fuel cell system 51 of the present embodiment has an air introduction valve 52 for introducing the air supplied to the cathode into the cooling water flow path 9 and the cooling water circulating through the cooling water flow path 9. The second embodiment is different from the second embodiment in that a tank 53 to be discharged and a drain valve 54 for discharging cooling water to the tank 53 are provided. Omitted.

  In the fuel cell system 51 of the present embodiment, in the purge control process of the second embodiment described above, the cooling water is extracted into the tank 53 when the second purge is performed. It is different from control processing.

  Next, the purge control process by the fuel cell system 51 of this embodiment will be described with reference to FIG. As shown in FIG. 6, the purge control processing by the fuel cell system 51 of the present embodiment is performed by the control unit 3 when the stop trigger of the fuel cell system 1 is turned on (S601) by turning off the ignition key of the vehicle, for example. A determination is made as to whether a purge request is necessary (S602). In determining the purge request, whether or not to start below freezing at the next start is predicted based on the outside temperature, weather forecast data, the next scheduled start time, the location (latitude / longitude) of the fuel cell system 51, and the like. The control unit 3 determines whether a purge request is necessary.

  If it is determined that purging is not necessary based on the determination of the purge request, the routine shifts to the normal stop mode. On the other hand, if it is determined that purging is necessary, the cooling water pump 6 is driven. Cooling water is circulated at a predetermined flow rate (S603). At the same time, the compressor 5 is driven to supply purge gas to the cathode of the fuel cell stack 2 to perform purge (S604), and this purge is continuously performed for a predetermined time as the first purge. This first purge generates latent heat of vaporization at the time of purging, but since the circulating cooling water takes most of this latent heat, the temperature of the fuel cell stack 2 does not decrease, and is necessary at the time of starting below freezing point. A product water retention capacity can be created.

  When the predetermined time has elapsed, the cooling water pump 6 is stopped to stop the circulation of the cooling water, and the cooling water is extracted from the fuel cell stack 2 (S605). As this extraction method, the air introduction valve 52 and the drainage valve 54 are opened, and the purge water supplied from the compressor 5 to the cathode is introduced into the cooling water passage 9 so that the cooling water is drained into the tank 53 and extracted. To do.

  The purge gas is continuously supplied even after the cooling water is extracted (S606), and this purge is continuously performed for a predetermined time as the second purge. Since the cooling water is extracted in the second purge, the cooling water does not take away the latent heat of vaporization, and the heat capacity of the fuel cell stack 2 is reduced, so that it can be cooled to the predetermined temperature T1 earlier. .

  When the predetermined time elapses, the compressor 5 is stopped to complete the purge (S607), the fuel cell system 51 is completely stopped, and the purge control process by the fuel cell system 51 of this embodiment is finished.

  However, in this embodiment, air is introduced into the cooling water passage 9 to extract the cooling water, but the cooling water may be extracted by other methods. For example, the cooling water may be extracted from the fuel cell stack 2 using gravity. In the flowchart of FIG. 6, the cooling water is extracted before the second purge, but the cooling water may be extracted from the fuel cell stack 2 while performing the second purge.

  As described above, in the fuel cell system 51 of the present embodiment, when the cooling water is circulated to the fuel cell stack 2, the cooling water is supplied from the fuel cell stack 2 for the remaining purge time only for a predetermined time. Since it is extracted and purged, the heat capacity of the fuel cell stack 2 during latent heat cooling can be reduced, whereby the fuel cell stack 2 can be rapidly cooled in a shorter time.

  Further, in the fuel cell system 51 of the present embodiment, when cooling water is extracted from the fuel cell stack 2, air is introduced into the cooling water channel 9 and the cooling water is extracted, so the purge gas supplied to the cathode is used. The cooling water can be extracted, and the cooling water can be efficiently extracted by a simpler method.

  Although the fuel cell system of the present invention has been described based on the illustrated embodiment, the present invention is not limited to this, and the configuration of each part is replaced with an arbitrary configuration having the same function. Can do.

1 is a block diagram showing a configuration of a fuel cell system according to a first embodiment of the present invention. It is a flowchart which shows the purge control process by the fuel cell system which concerns on the 1st Embodiment of this invention. It is a figure which shows the relationship between the temperature which left the fuel cell stack after purging, and the produced water holding capacity at the time of starting below freezing. 6 is a flowchart showing purge control processing by a fuel cell system according to a second embodiment of the present invention. It is a block diagram which shows the structure of the fuel cell system which concerns on the 3rd Embodiment of this invention. 7 is a flowchart showing a purge control process by a fuel cell system according to a third embodiment of the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1,51 Fuel cell system 2 Fuel cell stack 3 Control part 4 Hydrogen tank 5 Compressor 6 Cooling water pump 7 Radiator 8 Cooling fan 9 Cooling water flow path 10 Three-way valve 11 Cooling water inlet temperature sensor 12 Cooling water outlet temperature sensor 52 Air introduction valve 53 Tank 54 Drain valve

Claims (6)

In a fuel cell system comprising a fuel cell stack using a solid polymer membrane, wherein purge control is performed when power generation of the fuel cell stack is stopped,
The fuel cell system, wherein after purging the fuel cell stack, the temperature of the fuel cell stack is rapidly cooled to a predetermined temperature lower than the temperature of the purge gas.   The fuel cell system according to claim 1, wherein the predetermined temperature is approximately 30 ° C.   When purging to the fuel cell stack, the first purge is performed while circulating the cooling water to the fuel cell stack. After the first purge is performed, the second purge water is not circulated. 2. Therefore, by performing the second purge, the temperature of the fuel cell stack is rapidly cooled to a predetermined temperature lower than the purge gas temperature. 3. The fuel cell system according to any one of 2 above.   4. The fuel cell system according to claim 3, wherein the second purge is performed by stopping the circulation of the cooling water.   The fuel cell system according to claim 3, wherein the second purge is performed by removing cooling water from the fuel cell stack.   6. The fuel cell system according to claim 5, wherein the cooling water is extracted from the fuel cell stack by introducing air into the cooling water flow path.

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