JP2011076816A - Fuel cell system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fuel cell system including constitution and control capable of reducing the hydrogen concentration exhausted from the system even in a fuel cell system generating electric power in such a condition that the stack outlet pressure is the normal pressure or close to it, by solving a problem that the control reducing the hydrogen concentration exhausted from the system by supplying hydrogen exhausted from the fuel cell stack to a stack air inlet line and then oxidizing is not effective because the pressure difference cannot be secured in the condition that the outlet pressure of the fuel cell stack is atmospheric pressure. <P>SOLUTION: The fuel cell system includes: a purge line having a valve capable of opening and closing and connecting from an air supply line between a fuel cell and an air blower to a hydrogen exhausting line between the fuel cell and an exhaust valve; and the control changing over the kinds of fluid flowing through the purge line and flowing direction. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a power generation system that uses power generated by a fuel cell.

  A fuel cell is an electrochemical device that directly converts fuel energy into electrical energy through an electrochemical reaction. Fuel cells are roughly classified into phosphoric acid fuel cells, molten carbonate fuel cells, solid oxide fuel cells, polymer electrolyte fuel cells (hereinafter abbreviated as PEFC), and alkaline fuel cells, depending on the charge carrier used. Is done.

  Among these various fuel cells, PEFC is capable of high current density power generation and operation at a relatively low temperature, and therefore is expected to be used in a wide range of applications such as mobile power supplies, stationary power supplies, and backup power supplies.

  As the PEFC electrolyte, a proton exchange ion exchange membrane having a film thickness of several tens to several hundreds of μm is used. An ion exchange membrane generally has a structure in which a side chain having a sulfonic acid group is bonded to a fluorocarbon constituting a main chain.

  The electrolyte membrane is a thin film, and an electrode is disposed on both sides of the electrolyte membrane, a gas diffusion layer is disposed on both sides thereof, and a separator is disposed on both sides thereof. This constitutes a power generation unit cell (single cell).

  The electrode is composed of carbon particles having a particle diameter of about several hundreds of nanometers as a carrier, a platinum catalyst in which platinum particles having a diameter of about several tens to several tens of thousands are dispersed on the surface, and a polymer having proton conductivity similar to that of an electrolyte as a binder As a result of binding and molding. Usually, the electrode has a thickness of about several to several tens of micrometers. The reason why carbon is used as a catalyst carrier is that it has good electron conductivity and high chemical stability. Further, the reason why the platinum is made fine is to increase the surface area of the platinum and increase the reaction rate of the electrochemical reaction.

  Further, a gas diffusion layer (GDL), which is a conductive porous body, is disposed on the surface of the electrode. The GDL plays a role of promptly supplying the fuel and oxidant gas contributing to the electrochemical reaction supplied from the separator to the electrode active point which is a reaction field, and discharging the reacted gas together with the product. GDL is generally a woven or non-woven fabric made of carbon fibers.

  Separators are arranged on both sides of the GDL. The separator has a function of separating the gas supplied to the adjacent cells and supplying the reaction gas to the electrode via the GDL. Further, since it is necessary to pass a current through the separator itself in order to take out a current generated by power generation, the separator is made of a material having good conductivity and corrosion resistance against the corrosive atmosphere in the battery. Generally, a carbon-based material or a metal material subjected to corrosion resistance treatment is used.

  Since the PEFC single cell has a generated voltage of 1 V or less, the single cells are stacked in series for boosting. A fuel cell system of several tens of cells is used in a fuel cell system for general households that is being developed, and a fuel cell in which several hundred cells are stacked in an automobile application.

  The gas supplied to the fuel cell is hydrogen and oxygen. Oxygen uses the atmosphere. Various methods of supplying hydrogen have been studied, but general ones are high pressure cylinders, storage alloys, reformers, and the like. The discharge of hydrogen supplied to the fuel cell is controlled by a system that recirculates the hydrogen discharged to the fuel cell stack inlet or by providing a valve downstream of the fuel cell stack. The hydrogen discharged from the system affects the power generation efficiency of the entire system and also relates to safety. In particular, the amount of hydrogen discharged from the system tends to increase when the system is started or stopped, or when the load on the fuel cell is greatly reduced. Even in the usage environment, when the fuel cell system is used in an enclosed space, hydrogen may accumulate, so the amount and concentration of discharged hydrogen must be reduced as much as possible. Therefore, in designing the fuel cell system, there is a problem of establishing safety by reducing the exhaust hydrogen concentration.

  On the other hand, in Japanese Patent Application Laid-Open Nos. 9-31167, 2005-203263, and 2008-177116, anode discharge hydrogen is introduced into the cathode and oxidized in the stack, so that the concentration of discharged hydrogen is reduced. Proposed reduction.

JP 9-31167 A JP 2005-203263 A JP 2008-177116 A

  However, the above proposal assumes the introduction of anode exhaust hydrogen into the cathode line. Therefore, the pressure relationship does not hold unless the anode discharge hydrogen pressure value is higher than the cathode line pressure value. In addition, since the fuel cell stack has a gas flow path formed therein, a pressure loss occurs during gas flow. Therefore, the pressure value is higher at the inlet than at the stack outlet. On the other hand, lowering the working gas is assumed as a power generation condition of the future fuel cell system (for example, “Promotion of solid polymer electrolyte fuel cell targets, research and development issues and evaluation methods” P5 (2007. 1)). In this system, the stack hydrogen line outlet is at atmospheric pressure and the cathode stack upstream is greater than atmospheric pressure by the stack pressure loss. A system in which the stack outlet pressure is equivalent to atmospheric pressure is defined as an atmospheric pressure system. In the normal pressure system, the pressure difference between the anode discharge hydrogen pressure value and the cathode line pressure value could not be secured, so that hydrogen could not be introduced to the cathode in order to reduce the hydrogen concentration, and the discharge hydrogen concentration could not be reduced.

  Therefore, an object of the present invention is to provide a fuel cell system having a configuration and control capable of reducing the system exhaust hydrogen concentration even in a fuel cell system that generates power under conditions where the stack outlet pressure is at or near normal pressure.

  The fuel cell system of the present invention includes a fuel cell, a hydrogen supply source that supplies hydrogen to the fuel cell via a hydrogen supply line, and a hydrogen pressure that is provided in the hydrogen supply line and is supplied from the hydrogen supply source. A pressure regulating valve for adjusting the pressure, a hydrogen discharge line for discharging hydrogen discharged from the fuel cell to the outside of the system, a discharge valve provided in the hydrogen discharge line, and a concentration of hydrogen discharged to the outside of the system A concentration sensor, an air blower for supplying air to the fuel cell via an air supply line, a humidifier provided in the air supply line for adding moisture to the air, and exhaust air discharged from the fuel cell An air discharge line for supplying the hydrogen to the hydrogen discharge line, a converter for converting the generated power, a control unit for controlling each auxiliary machine, and an air gap between the fuel cell and the air blower. A purge line connecting a hydrogen discharge line between the fuel cell and the discharge valve from the supply line is provided, and the control unit controls the opening and closing of the valve provided in the purge line and the discharge valve. The type of flowing fluid and its direction are switched. With this system, it is possible to effectively reduce the exhaust hydrogen concentration not only in the high pressure system but also in the normal pressure system as the power generation condition of the fuel cell system.

  Further, the control unit closes the purge valve, opens the discharge valve based on the information on the concentration of discharged hydrogen detected by the concentration sensor, and discharges the discharged hydrogen with the discharged air supplied from the air discharge line. A first stage of diluting, a second stage of opening the purge valve, opening the exhaust valve, diluting exhaust hydrogen by exhaust air supplied from the air exhaust line and air supplied from the bypass line, The exhaust valve is closed, the purge valve is opened, the pressure is increased by adjusting the pressure regulating valve, and the three-stage control of supplying the exhaust hydrogen to the upper part of the stack of the air supply line through the purge line is performed. It is characterized by that.

  Further, the present invention is characterized in that a control for increasing the air supply amount of the air blower with the transition from the first stage to the second stage is provided. With this control, even if air is supplied directly to the discharge part of the fuel cell stack through the bypass line in order to reduce the hydrogen concentration, the amount of air supplied to the fuel cell stack does not decrease, so the power generation amount can be maintained. .

  Moreover, the said control part is equipped with the control which restrict | limits the hydrogen amount per unit time supplied to the said air supply line from the said bypass line. With this control, even if the hydrogen discharged from the fuel cell stack and supplied again to the fuel cell stack through the bypass line generates heat due to an oxidation reaction in the battery, the stack temperature exceeds a predetermined value as determined by the control unit. By changing the opening / closing amounts of the purge valve and the exhaust valve, the amount of hydrogen supplied through the bypass line can be reduced, and the safety of the system can be further improved.

  According to the present invention, the exhaust hydrogen concentration can be reduced not only in a high pressure system in which the supply gas pressure is increased but also in a system under normal pressure conditions.

The figure which shows the system configuration | structure of this invention used in the Example. The figure which shows the structure of the system used by the comparative example.

  Embodiments of the present invention will be described according to the following examples.

〔Example〕
The system configuration of the embodiment is shown in FIG.

  Hydrogen supplied to the fuel cell 1 from the hydrogen supply source 2 through the hydrogen supply line 14 is adjusted in pressure by the pressure regulating valve 3 and introduced into the fuel cell. The hydrogen pressure is measured with a hydrogen inlet pressure gauge 5. Hydrogen discharged from the stack passes through a hydrogen discharge line 15 provided with a discharge valve 4, is diluted, and is discharged outside the system. The air supplied from the air blower 7 through the air supply line 16 is added with moisture by the humidifier 8 and introduced into the stack. The air pressure is measured by an air pressure gauge 9. After the oxygen supplied to the stack is consumed oxygen by power generation, surplus air passes from the stack through the air discharge line 17 to the hydrogen discharge line 15 and is used as a hydrogen diluent. The hydrogen concentration of the exhaust gas is monitored by the concentration sensor 6. A bypass line 11 connects the upstream of the humidifier and the downstream of the stack of the hydrogen line, and a purge valve 10 is disposed on the line. In the purge line, air can flow in the downstream direction of the stack and hydrogen can flow in the upstream direction of the humidifier, and the control is performed by the control unit 13. Based on the measured values of the hydrogen inlet pressure gauge 5 and the air pressure gauge 9, the measured value of the concentration sensor, the voltage / current value of the stack, and the temperature, the control unit 13 includes a pressure regulating valve 3, a discharge valve 4, an air blower 7, The purge valve 10 and the converter 12 that converts the generated power can be controlled.

  The contents of control during power generation in the system of the first embodiment shown in FIG. 1 will be described.

  It is desirable to dilute the hydrogen discharged from the stack to a concentration at which safety can be secured using the discharged air. Since discharging hydrogen as fuel reduces system efficiency, the discharge valve 4 is not always opened but is operated intermittently. The opening / closing operation of the discharge valve 4 is performed based on a signal from the control unit 13, and the valve is opened when the voltage value with respect to the current condition of the fuel cell falls below a predetermined value. The hydrogen outlet is open to the atmosphere and the pressure is atmospheric pressure. A concentration sensor 6 is disposed here, and the hydrogen concentration in the exhaust gas is monitored at all times when the system is in operation. When a predetermined upper limit hydrogen concentration value (for example, 0.25 vol%) is exceeded, the control unit determines to move to the next stage. The above is the first stage of the hydrogen concentration reduction control.

  When the exhaust hydrogen concentration exceeds a predetermined value in the first stage, the process proceeds to the second stage shown below. As a situation in which the hydrogen concentration increases, it is conceivable that excessive hydrogen is generated when the instantaneous decrease in the amount of power generated by the stack is large. In addition, the amount of hydrogen discharged may increase when the system is started or stopped.

  In this case, the purge valve 10 and the discharge valve 4 of the bypass line 11 are opened. Since Example 1 is a normal pressure system, the hydrogen outlet pressure value of the stack is lower than the stack air inlet pressure value. Using this condition, air is allowed to flow from the upstream side of the humidifier 8 to the stack hydrogen outlet through the purge line 11. At the same time, the control unit 13 increases the operating rate of the air blower 7 and performs control so that the amount of air supplied to the stack is constant even when air branches and flows to the bypass line 11. By this control, the power generation output can be kept constant without causing a shortage in the amount of air supplied to the fuel cell stack. Since the air supplied through the bypass line 11 serves as a diluent for diluting hydrogen, the concentration of discharged hydrogen can be further reduced as compared with the first stage state.

  If the hydrogen concentration of the exhaust gas exceeds the predetermined value even in the second stage, the control unit 13 shifts to the third stage for reducing the hydrogen concentration. In this case, the control unit 13 closes the discharge valve 4 and opens the purge valve 10. The air blower 7 whose operating rate is increased in the second stage is shifted to normal operation. At the same time, the set secondary pressure of the pressure regulating valve 3 is set so that the stack outlet hydrogen pressure is at least higher than the cathode stack inlet pressure. As a result, the stack discharge hydrogen is introduced to the upstream portion of the humidifier 8 through the bypass line 11. The hydrogen mixed with the air line becomes water by an oxidation reaction with an electrode catalyst inside the stack. These are discharged out of the system together with the exhaust air. By utilizing the oxidation reaction in the stack, the hydrogen concentration in the exhaust gas can be greatly reduced.

  On the other hand, since hydrogen supplied to the air line undergoes an oxidation reaction in the battery, when a large amount of hydrogen is continuously introduced into the air line, there is a possibility that a rapid temperature rise of the stack is accompanied. Therefore, in the third stage, the control unit monitors the temperature change information of the stack, and when the temperature change exceeds a predetermined value, the control unit reduces the power generation load amount or shifts to the system stop.

  The system of the comparative example is shown in FIG. The comparative example has the same system configuration as that of the example, but hydrogen supplied from the hydrogen supply source 2 and pressure-adjusted by the pressure regulating valve 3 is consumed for the electrochemical reaction in the fuel cell stack 1, and surplus hydrogen is consumed in the fuel cell stack. 1 is not discharged out of the system, but is introduced to the upstream portion of the humidifier 8 of the air line through the bypass line 11 and the purge valve 10. The introduced hydrogen is added with moisture in the humidifier 8 together with the air from the air blower 2 and then supplied to the fuel cell stack 1. The hydrogen oxidized in the fuel cell stack 1 becomes water and is discharged out of the system together with surplus air. The hydrogen concentration of the exhaust gas is monitored by the concentration sensor 6.

  Here, attention is paid to the pressure of the gas line of the comparative example. The comparative example includes a hydrogen inlet pressure gauge 5 that measures the hydrogen pressure at the stack inlet, a hydrogen outlet pressure gauge 20 that measures the hydrogen pressure at the stack outlet, and an air inlet pressure gauge 9 that measures the air pressure at the stack inlet. Is placed. When comparing the pressure values of the gas lines during power generation under the rated condition of the comparative example, the hydrogen inlet pressure> the hydrogen outlet pressure> the air inlet pressure. The pressure difference at the inlet portion with hydrogen corresponds to the pressure loss of the hydrogen flow path of the fuel cell stack 1, and the relationship of hydrogen outlet pressure> air inlet pressure is for supplying discharged hydrogen to the air line. This is a necessary pressure condition. Therefore, the control unit 13 adjusts the hydrogen pressure value via the pressure regulating valve 3 so that the discharged hydrogen can be supplied to the upper part of the air line from the pressure value measured by each pressure gauge. Therefore, the hydrogen line inevitably needs to be pressurized, and when the pressure at the outlet of the fuel cell stack is normal pressure, exhaust hydrogen cannot be supplied to the air line, so power generation cannot be continued in the comparative example. .

  As described above, according to the fuel cell system of the present invention, the purge line includes a purge line that has an openable and closable valve and connects the hydrogen supply line between the fuel cell and the discharge valve from the air supply line between the fuel cell and the air blower. A fuel cell system capable of effectively reducing exhaust hydrogen concentration not only in a high-pressure system but also in a normal pressure system by performing control to switch hydrogen and air flowing through the air including its flow direction We were able to.

DESCRIPTION OF SYMBOLS 1 Fuel cell 2 Hydrogen supply source 3 Pressure regulating valve 4 Discharge valve 5 Hydrogen inlet pressure gauge 6 Concentration sensor 7 Air blower 8 Humidifier 9 Air inlet pressure gauge 10 Purge valve 11 Bypass line 12 Converter 13 Control part 14 Hydrogen supply line 15 Hydrogen discharge line 16 Air supply line 17 Air discharge line 20 Hydrogen outlet pressure gauge

Claims (6)

A fuel cell;
A hydrogen supply source for supplying hydrogen to the fuel cell via a hydrogen supply line;
A pressure regulating valve provided in the hydrogen supply line for adjusting the pressure of hydrogen supplied from the hydrogen supply source;
A hydrogen discharge line for discharging hydrogen discharged from the fuel cell to the outside of the system, a discharge valve provided in the hydrogen discharge line,
A concentration sensor that detects the concentration of hydrogen discharged outside the system;
An air blower for supplying air to the fuel cell via an air supply line;
A humidifier provided in the air supply line for adding moisture to the air;
An air discharge line for supplying exhaust air discharged from the fuel cell to the hydrogen discharge line;
A converter for converting generated power;
A control unit for controlling each auxiliary machine,
A purge line connecting a hydrogen discharge line between the fuel cell and a discharge valve from an air supply line between the fuel cell and the air blower;
The fuel cell system according to claim 1, wherein the controller switches the type and direction of the fluid flowing through the purge line by controlling opening and closing of a valve and a discharge valve provided in the purge line. The fuel cell system according to claim 1, wherein
The control unit, based on the information on the concentration of exhaust hydrogen detected by the concentration sensor,
A first stage of closing the purge valve, opening the exhaust valve, and diluting exhaust hydrogen with exhaust air supplied from the air exhaust line;
A second stage of opening the purge valve, opening the discharge valve, and diluting discharged hydrogen by the discharged air supplied from the air discharge line and the air supplied from the bypass line;
The three-stage control of the third stage that closes the discharge valve, opens the purge valve, increases the pressure by adjusting the pressure regulating valve, and supplies the discharged hydrogen to the upper part of the stack of the air supply line through the purge line. A fuel cell system comprising:   3. The fuel cell system according to claim 2, wherein control is performed to increase an air supply amount of the air blower along with the transition from the first stage to the second stage. 4.   2. The fuel cell system according to claim 1, wherein the control unit performs control for limiting a hydrogen amount per unit time supplied from the bypass line to the air supply line. 3. The fuel cell system according to claim 2, wherein
The fuel cell system according to claim 3, wherein the control unit performs control for reducing a power generation load amount or stopping the system based on temperature change information of the fuel cell in the third stage.   2. The fuel cell system according to claim 1, wherein the pressure of hydrogen supplied from the hydrogen supply source is adjusted so that the pressure of hydrogen discharged from the fuel cell is in the vicinity. system.

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