JP4467415B2 - Fuel cell system - Google Patents

Description

  The present invention relates to a fuel cell system.

  A polymer electrolyte fuel cell is a cell (membrane electrode assembly) formed by joining and integrating an anode (fuel electrode) on one surface of a solid polymer electrolyte membrane and a cathode (air electrode) on the other surface, A plurality of layers are stacked with a cell sandwiched between a plate provided with a grooved fuel flow path on the surface facing the anode and a plate provided with a grooved oxidant flow path on the surface facing the cathode. A fuel cell stack is constructed by attaching an end plate and tightening with a through bolt. A fuel (hydrogen or hydrogen-based reformed gas) is circulated through the fuel channel, and an oxidant (usually air) is circulated through the oxidant channel, and electrochemically passed through the solid polymer electrolyte membrane. Direct current power is generated by causing a reaction.

  In such a polymer electrolyte fuel cell, the polymer electrolyte membrane functions properly in a saturated and wet state. Therefore, after the reaction gas (fuel and / or oxidizer) is humidified with a humidifier or the like, the flow path of the plate is passed through. The solid polymer electrolyte membrane is maintained in a saturated and wet state by flowing. The operating temperature of the polymer electrolyte fuel cell is about 80 ° C., but since the electrochemical reaction is an exothermic reaction, the temperature rises during power generation. In order to prevent this, a cooling plate is generally incorporated in the fuel cell stack, and cooling water is circulated through the channel to keep the fuel cell stack at the operating temperature.

In a conventional fuel cell system that operates a polymer electrolyte fuel cell, the flow of cooling water is stopped when the system is stopped (see Patent Document 1).
JP 2004-296340 A

  If the flow of the cooling water is stopped when the system is stopped and the fuel cell stack is naturally cooled, the fuel cell stack cools in order from the outside, and thus a temperature difference occurs in the fuel cell stack. Since the water vapor in the fuel cell stack is condensed first from the place where the temperature is low, the water distribution in each cell changes in the course of natural cooling. Thereby, the distribution of the reaction gas to each cell becomes non-uniform at the next start-up, and each cell voltage at the time of power generation becomes unstable. Also, if the temperature of the fuel humidifier or air humidifier connected by piping (water temperature) is higher than the battery temperature, even if the reaction gas is stopped, the steam diffuses from the fuel humidifier or air humidifier to the battery, Since condensation occurs in the battery, this also changes the water distribution in the battery.

  The present invention has been made in view of these problems, and an object thereof is to provide a technique for stabilizing the output when the fuel cell system is activated.

In one aspect of the present invention, a membrane electrode assembly in which an anode is joined to one surface of an electrolyte membrane and a cathode is joined to the other surface of the electrolyte membrane, and a fuel flow path for supplying fuel to the anode are provided. A fuel flow path plate provided with an oxidant flow path plate provided with an oxidant flow path for supplying an oxidant to the cathode, and a heat medium flow path plate provided with a heat medium flow path through which the heat medium flows. A fuel cell stack including the laminated body, a fuel humidifying means for humidifying the fuel by heat exchange with the heat medium, an oxidant humidifying means for humidifying the oxidant by heat exchange with the heat medium, The heat medium discharged from the fuel cell stack is cooled and then heat exchanged with the fuel humidifying means and the oxidant humidifying means, and the heat medium is inserted into the fuel cell stack to circulate the heat medium. And a control means for continuing the circulation of the heat medium until a predetermined cooling stop condition is satisfied, and the means for circulating the heat medium is configured such that after the load current is interrupted, the heat medium By circulating, the temperature of the fuel humidifying means or the temperature of the oxidant humidifying means is made equal to the temperature of the fuel cell stack.

  The fuel flow path plate, the oxidant flow path plate, and the heat medium flow path plate are not necessarily separate members. For example, the fuel flow path is provided on one surface of the bipolar plate, and the heat flow path is provided on the other surface. A configuration in which the fuel flow path plate and the heat medium flow path plate are realized by one member by providing the medium flow path is also included in the present invention.

  According to the above configuration, the distribution of the temperature distribution of each cell is suppressed by causing the heat medium to flow through the fuel cell stack even after the system is stopped, so that the variation of the water distribution of each cell is suppressed. As a result, the power generation amount of each cell at the next start-up becomes uniform, and the output is stabilized.

In another aspect of the present invention, a membrane electrode assembly in which an anode is joined to one surface of an electrolyte membrane and a cathode is joined to the other surface of the electrolyte membrane, and a fuel flow path for supplying fuel to the anode are provided. A fuel flow path plate provided, an oxidant flow path plate provided with an oxidant flow path for supplying an oxidant to the cathode, and a heat medium flow path plate provided with a heat medium flow path through which the heat medium flows. A fuel cell stack including a combined laminate, fuel humidifying means for humidifying the fuel by heat exchange with the heat medium, and oxidant humidifying means for humidifying the oxidant by heat exchange with the heat medium; Means for exchanging heat with the fuel humidifying means and oxidant humidifying means after cooling the heat medium discharged from the fuel cell stack, and circulating the heat medium into the fuel cell stack; and a predetermined cooling stop when the system is stopped. The circulation of the heat medium is continued until the condition is satisfied, the supply of at least one of the fuel or the oxidant is continued, and the load current is gradually decreased from the system stop to satisfy a predetermined cooling stop condition. Control means for interrupting the load current, and the means for circulating the heat medium is the temperature of the fuel humidifying means or the oxidant humidifying means by circulating the heat medium after the load current is interrupted. And the temperature of the fuel cell stack are made equal to each other.

  According to the above configuration, since the electrode is suppressed from being oxidized after the system is stopped, the durability and output stability of the fuel cell can be improved.

  In the above configuration, the control means is configured to calculate the temperature of the heat medium discharged from the fuel cell stack and the temperature of the heat medium input to the fuel cell stack between the time when the system is stopped and a predetermined cooling stop condition is satisfied. The circulation amount of the heat medium may be adjusted so that the temperature difference falls within a predetermined range. According to this, the water distribution of each cell in the fuel cell stack can be made more uniform.

  In the above configuration, the control means may determine that the temperature of the heat medium discharged from the fuel cell stack is lower than a value determined by a function of the outside air temperature as a cooling stop condition. The control means may determine that a predetermined time has elapsed since the system stop as a cooling stop condition. According to this, power consumption can be reduced by terminating the circulation of the heat medium in an appropriate period.

  A combination of the above-described elements as appropriate can also be included in the scope of the invention for which patent protection is sought by this patent application.

  According to the present invention, it is possible to improve the output stability when starting the fuel cell system.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 shows an overall configuration of a fuel cell system 10 according to an embodiment of the present invention. The fuel cell system 10 includes a fuel cell stack 20, a fuel supply means 30, a fuel humidifier 40, an air supply means 50, an air humidifier 60, a heat exchanger for heat medium 70, a pipe 80, a control valve 86, a circulation pump 90, and A control unit 100 is provided.

  The fuel cell stack 20 is provided with a membrane electrode assembly in which an anode is joined to one surface of a polymer electrolyte membrane and a cathode is joined to the other surface of the electrolyte membrane, and a fuel flow path for supplying fuel to the anode. The fuel flow path plate, the oxidant flow path plate provided with the oxidant flow path for supplying the oxidant to the cathode, and the heat medium flow path plate provided with the heat medium flow path through which the heat medium flows are combined. Includes laminates. The fuel cell stack 20 may have a known configuration, and a typical example thereof is the configuration shown in FIGS. 1 and 2 of Japanese Patent Application Laid-Open No. 2004-185938 or FIG. 1 of Japanese Patent Application Laid-Open No. 2004-185934. Configuration. In the fuel cell stack 20 of the present embodiment, the flow direction of the fuel and air used for power generation and the flow of the heat medium used for cooling the anode and / or cathode are parallel flows in the direction of gravity. In this embodiment, water is used as the heat medium, but other liquids and gases can be used as long as heat can be transferred. Hereinafter, water used as a heat medium is referred to as cooling water.

  The fuel supply means 30 is means for supplying hydrogen as fuel. For example, the fuel supply means 30 includes a fuel tank that stores a hydrocarbon gas such as natural gas or methane gas, a desulfurizer that removes sulfur components from the hydrocarbon gas supplied from the fuel tank, and a hydrocarbon system after desulfurization. It consists mainly of a reformer that reforms gas and extracts hydrogen.

  The fuel humidifier 40 humidifies the fuel supplied from the fuel supply means 30. Specifically, the fuel humidifier 40 includes a fuel humidification tank 42 and a fuel heat exchanger 44, and is used for bubbling with water that is put in the fuel humidification tank 42 and heated by the fuel heat exchanger 44. The fuel is humidified by the method, and the relative humidity of the fuel is set to 100% RH.

  The air supply means 50 is a means for supplying air containing oxygen as an oxidant. For example, the air supply means 50 includes a blower that takes in outside air and an air filter that is provided as necessary.

  The air humidifier 60 humidifies the air supplied from the air supply means 50. Specifically, the air humidifier 60 includes an air humidification tank 62, and uses water contained in the air humidification tank 62 to humidify the air by a bubbling method so that the relative humidity of the air becomes 100% RH. .

  The heat exchanger for heat medium 70 lowers the temperature of the cooling water discharged from the fuel cell stack 20 through heat exchange with outside air or the like. With the heat exchanger for heat medium 70, the temperature of the cooling water discharged from the fuel cell stack 20 can be lowered efficiently.

  The pipe 80 has a configuration capable of circulating the cooling water such that the cooling water discharged through the heat medium flow path provided in the fuel cell stack 20 is supplied to the heat medium flow path again. Specifically, the cooling water discharged from the fuel cell stack 20 is first guided to the heat medium heat exchanger 70, and at a branch point 82 provided downstream of the heat medium heat exchanger 70, the fuel humidifier Branches into a line toward 40 and a line toward the air humidifier 60 at a predetermined distribution ratio. A part of the cooling water discharged from the fuel cell stack 20 flows through the fuel heat exchanger 44 of the fuel humidifier 40, and the rest of the cooling water discharged from the fuel cell stack 20 goes to the air humidifier 60. Supplied directly. The cooling water after flowing through the fuel heat exchanger 44 joins at the junction 84 with the cooling water flowing on the line toward the air humidifier 60 described above upstream of the air humidifier 60. The combined cooling water is discharged from the air humidifier 60 after flowing through the air humidification tank 62 of the air humidifier 60. The circulation pump 90 pumps up the cooling water discharged from the air humidifier 60 and sends it to the fuel cell stack 20 as a predetermined amount of cooling water.

  The control valve 86 is a valve having a variable opening / closing degree provided between the junction 82 and the junction 84. By adjusting the opening degree of the control valve 86, the distribution ratio of the cooling water can be corrected. The installation of the control valve 86 is not indispensable, and is unnecessary when it is not necessary to correct the distribution ratio of the cooling water according to the operating conditions.

  In addition to controlling the amount of power generated by the fuel cell stack 20, the control unit 100 controls the amount of cooling water by adjusting the opening of the control valve 86 and the circulation pump 90. Further, the control unit 100 controls the fuel supply amount from the fuel supply unit 30 and the air supply amount from the air supply unit 50 as necessary.

(Operation when the system is stopped)
The operation of the fuel cell system 10 when the system is stopped will be described. In the following description, the temperature near the cooling water inlet provided in the fuel cell stack 20 is referred to as a cooling water inlet temperature (T1), and the temperature near the cooling water outlet provided in the fuel cell stack 20 is cooled. Called water outlet temperature (T2). Further, the temperature of the fuel humidified by the fuel humidifier 40 is called a humidified fuel temperature (T3), and the temperature of the air humidified by the air humidifier 60 is called a humidified air temperature (T4). Furthermore, the dew point near the fuel inlet provided in the fuel cell stack 20 is called a fuel dew point (T5), and the dew point near the air inlet provided in the fuel cell stack 20 is called an air dew point (T6). T1, T2, T3, T4, T5, and T6 are measured by a temperature sensor (not shown) as necessary, and the measured values are transmitted to the control unit 100.

  When the system is stopped, the control unit 100 interrupts the load current while continuing the circulation of the cooling water by the circulation pump 90. The circulation of the cooling water is continued until a cooling stop condition described later is satisfied. Next, the control unit 100 stops the supply of fuel from the fuel supply unit 30 and the supply of air from the air supply unit 50. Either the fuel supply stop or the air supply stop may be performed first, or both may be stopped simultaneously. The control unit 100 determines whether the cooling stop condition is satisfied after the load current is interrupted. As the cooling stop condition, for example, any one of the battery temperature, the cooling water outlet temperature (T2), the humidified fuel temperature (T3), or the humidified air temperature (T5) has become a set temperature. In this case, the set temperature is determined by a function of the outside air temperature such as (outside air temperature + 5) ° C., for example. In addition, as a cooling stop condition, it may be adopted that a certain time has elapsed after the load current is cut off. By stopping the circulation of the cooling water when such a cooling stop condition is satisfied, it is possible to reduce power consumption while making the water distribution of each cell uniform.

  In addition, the control unit 100 circulates the circulating pump 90 so that the difference between the cooling water outlet temperature (T2) and the cooling water inlet temperature (T1) becomes a predetermined temperature, for example, 2 ° C. or less, during the cooling water circulation after the load current is interrupted. May be used to adjust the amount of cooling water. According to this, since the temperature distribution in the flow direction of the cooling water in the fuel cell stack 20 becomes gentle, the difference in the water distribution in each cell can be made difficult to occur.

FIG. 2 is a graph showing a temperature change when cooling water circulation is continued after load interruption (hereinafter referred to as forced cooling). FIG. 4 is a graph showing a temperature change (comparative example) when the cooling water circulation is stopped and natural cooling is performed simultaneously with load interruption (hereinafter referred to as natural cooling). The temperature was measured at a total of four points: two locations on the battery surface, the surface of the fuel humidifier 40, and the surface of the air humidifier 60. As a result, it was confirmed that during forced cooling, each temperature did not vary within 1 hour after the load current was interrupted, and quickly decreased to 40 ° C. or lower. On the other hand, at the time of natural cooling, it can be seen that each temperature is kept at 40 ° C. or more and variation is observed in each temperature even if 4 hours or more have passed after the system is stopped.

FIG. 3 is a graph showing changes in cell current and voltage of each cell when the system is started after forced cooling. FIG. 5 is a graph showing changes in cell current and voltage of each cell (comparative example) when the system is started after natural cooling. Variations are observed in each cell voltage after system startup after natural cooling. At the time of natural cooling, the fuel cell stack 20 cools in order from the outside, so that a temperature difference occurs in the fuel cell stack 20. The water vapor in the fuel cell stack 20 condenses first from a place where the temperature is low, so that the water distribution in each cell changes in the course of natural cooling. Thereby, the distribution of the reaction gas to each cell becomes non-uniform at the next start-up, and each cell voltage at the time of power generation becomes unstable. In addition, when the temperature (water temperature) of the fuel humidifier 40 or the air humidifier 60 connected by the pipe 80 is higher than the battery temperature, the steam is generated from the fuel humidifier 40 or the air humidifier 60 even if the reaction gas is stopped. Since it diffuses in the cell stack 20 and dew condensation occurs in the fuel cell stack 20, the water distribution in the fuel cell stack 20 also changes.

  On the other hand, in each cell voltage after the system startup after forced cooling, there was no variation as in the comparative example, and it was confirmed that the output was stable. This is because the water circulation in the fuel cell stack 20 becomes uniform as a result of suppressing the temperature difference in the fuel cell stack 20 during the cooling process by continuing the circulation of the cooling water even after the load is cut off. It is thought that. Further, since the cooling water circulation is continued even after the load is cut off, the temperature of the fuel humidifier 40 or the air humidifier 60 and the fuel cell stack 20 changes to be equal. Therefore, the fuel cell stack from the fuel humidifier 40 or the air humidifier 60 Suppression of vapor diffusion to 20 is also a factor in stabilizing the output.

  The present invention is not limited to the above-described embodiments, and various modifications such as design changes can be added based on the knowledge of those skilled in the art. Embodiments to which such modifications are added Can also be included in the scope of the present invention.

  For example, in the above-described embodiment, the supply of fuel and air is also interrupted after the load current is interrupted. However, the supply stop of at least one of fuel and air may be made on the condition that the cooling stop condition is satisfied. The amount of fuel or air supplied after the load current is interrupted may be the same as that during operation, or may be gradually decreased. By continuing the fuel supply after the load current is cut off, the anode electrode is suppressed from being oxidized. At this time, the fuel supply means 30 replaces the fuel with nitrogen and supplies it, so that the fuel can be expelled from the fuel cell stack 20 and the fuel supply pipe without mixing oxygen. Oxidation is suppressed, and the fuel concentration at the next start-up can be kept appropriate, and the output stability can be further improved. Further, since the air supply means 50 replaces the air with nitrogen and supplies the oxygen, the oxygen can be expelled from the fuel cell stack 20 and the air supply pipe, so that the oxidation of the cathode electrode is suppressed and the durability of the fuel cell is improved. Performance and output stability can be further improved.

  In addition to the supply stop of at least one of fuel and air being conditioned on the condition that the cooling stop condition is satisfied, when the system is stopped, the load current is gradually decreased and the condition that the cooling stop condition is satisfied is satisfied. The load current may be cut off. According to this, since the oxygen in the fuel cell stack 20 is consumed quickly, the oxidation of the electrode is suppressed more quickly.

It is a figure showing the whole fuel cell system composition concerning an embodiment. It is a graph which shows the temperature change at the time of forced cooling. It is a graph which shows the change of the cell current and the voltage of each cell when starting a system after forced cooling. It is a graph which shows the temperature change (comparative example) at the time of natural cooling. It is a graph which shows the change (comparative example) of a cell current and the voltage of each cell when starting a system after natural cooling.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 Fuel cell system, 20 Fuel cell stack, 30 Fuel supply means, 40 Fuel humidifier, 50 Air supply means, 60 Air humidifier, 70 Heat exchanger for heat medium, 80 Piping, 86 Control valve, 90 Circulation pump, 100 Control unit.

Claims (6)

A membrane electrode assembly in which an anode is joined to one surface of an electrolyte membrane, and a cathode is joined to the other surface of the electrolyte membrane; and a fuel channel plate provided with a fuel channel for supplying fuel to the anode; A fuel comprising a laminate in which an oxidant flow path plate provided with an oxidant flow path for supplying an oxidant to the cathode and a heat medium flow path plate provided with a heat medium flow path through which the heat medium flows are combined. A battery stack,
Fuel humidifying means for humidifying the fuel by heat exchange with the heat medium;
An oxidizing agent humidifying means for humidifying the oxidizing agent by heat exchange with the heat medium;
Means for exchanging heat with the fuel humidifying means and the oxidant humidifying means after cooling the heat medium discharged from the fuel cell stack, and circulating the heat medium into the fuel cell stack;
A control means for continuing circulation of the heat medium until a predetermined cooling stop condition is satisfied after the load current is interrupted when the system is stopped;
The means for circulating the heat medium equalizes the temperature of the fuel humidifying means or the temperature of the oxidant humidifying means and the temperature of the fuel cell stack by circulating the heat medium after the load current is interrupted. A fuel cell system.   2. The fuel cell according to claim 1, wherein the control unit continues supplying at least one of the fuel and the oxidant after the load current is interrupted until the predetermined cooling stop condition is satisfied. 3. system. A membrane electrode assembly in which an anode is joined to one surface of the electrolyte membrane, and a cathode is joined to the other surface of the electrolyte membrane; and a fuel channel plate provided with a fuel channel for supplying fuel to the anode; A fuel comprising a laminate in which an oxidant flow path plate provided with an oxidant flow path for supplying an oxidant to the cathode and a heat medium flow path plate provided with a heat medium flow path through which the heat medium flows are combined. A battery stack,
Fuel humidifying means for humidifying the fuel by heat exchange with the heat medium;
An oxidizing agent humidifying means for humidifying the oxidizing agent by heat exchange with the heat medium;
Means for exchanging heat with the fuel humidifying means and the oxidant humidifying means after cooling the heat medium discharged from the fuel cell stack, and circulating the heat medium into the fuel cell stack;
When the system is stopped, the circulation of the heat medium is continued until a predetermined cooling stop condition is satisfied, and the supply of at least one of the fuel or the oxidant is continued, and the load current is gradually decreased from the system stop, Control means for interrupting the load current when a predetermined cooling stop condition is satisfied,
The means for circulating the heat medium equalizes the temperature of the fuel humidifying means or the temperature of the oxidant humidifying means and the temperature of the fuel cell stack by circulating the heat medium after the load current is interrupted. A fuel cell system comprising:   The control means includes a temperature between a temperature of the heat medium discharged from the fuel cell stack and a temperature of the heat medium charged into the fuel cell stack between the system stop and a predetermined cooling stop condition. The fuel cell system according to any one of claims 1 to 3, wherein the circulation amount of the heat medium is adjusted so that the difference falls within a predetermined range.   5. The cooling stop condition as defined in claim 1, wherein the control means determines that the temperature of the heat medium discharged from the fuel cell stack is lower than a value determined by a function of an outside air temperature as the cooling stop condition. 2. The fuel cell system according to item 1. The fuel cell system according to any one of claims 1 to 4, wherein the control unit determines that the predetermined time has elapsed since the system stop as the cooling stop condition.

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