JP2005267910A - Fuel cell system, and control method of the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve problems that poisonous substance in the air intrudes into a fuel cell and damages the power generating function thereof, because the pressure in the fuel cell is turned into negative against atmospheric pressure when the fuel cell is stopped by a conventional method, and the reliability of the fuel cell for a long period is lowered at the operation with repeated start and stop. <P>SOLUTION: The pressure in the fuel cell is prevented from getting into negative pressure while stopping, by turning the pressure in the fuel cell soon after the stoppage into a pressure of not less than a prescribed value. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a fuel cell system that generates power using a polymer electrolyte fuel cell and a control method thereof.

  A fuel cell basically includes an anode and a cathode that are a pair of electrodes that sandwich an electrolyte having ion conductivity, and an anode-side separator and a cathode-side separator that sandwich these. The anode side separator has a flow path for supplying fuel to the anode, and the cathode side separator has a flow path for supplying oxidant to the cathode. A fuel such as hydrogen gas or ethanol is supplied to the anode, an oxidant such as oxygen or air is supplied to the cathode, and the chemical energy of these reactants is generated by an oxidation or reduction reaction that occurs on each electrode. Convert to energy and extract current.

  In such fuel cells, a polymer membrane having hydrogen ion conductivity is used as an electrolyte, hydrogen or a mixed gas containing hydrogen as a main component is used as a fuel, and a gas such as oxygen or air is used as an oxidant. There is a type. In this fuel cell, hydrogen gas is oxidized on the anode by the reaction of formula (1) to generate electrons and hydrogen ions. Hydrogen ions move through the solid electrolyte membrane and reach the cathode side. On the other hand, electrons reach the cathode through an external circuit, and oxygen, electrons, and hydrogen ions at the cathode are reduced by the reaction of the formula (2) to generate water.

2H ₂ → 4H ⁺ + 4e (1)
4H ⁺ + O ₂ + 4e → 2H ₂ O (2)

  The solid polymer membrane that is the electrolyte of this fuel cell exhibits ionic conductivity only in a wet state. For this reason, in order to maintain high power generation performance, only the water generated by the reaction of formula (2) is insufficient, and it is necessary to replenish water from the outside. In general, a method of supplying moisture necessary for the operation of the fuel cell by passing a gas supplied to the fuel cell installed inside or outside the main body of the fuel cell through a device for humidifying is employed.

  Further, the operating temperature of this fuel cell is usually 90 ° C. or lower because it is restricted by the heat resistance performance of the solid polymer membrane as the electrolyte. However, since the reactions of the formulas (1) and (2) hardly occur in an environment of 90 ° C. or lower, the above-mentioned anode and cathode need to be provided with a catalyst having an action for activating these reactions. Therefore, platinum having high catalytic ability is used for the anode and cathode of this fuel cell.

  As an example of a conventional fuel cell system including this fuel cell, there is a system having the configuration shown in FIG. 1 (see, for example, Patent Document 1). That is, this system includes a fuel cell 10 that generates electricity by reacting hydrogen supplied from the hydrogen supply unit 11 and oxygen in the air supplied from the air supply unit 12 after being humidified through a humidifier 13. In order to recover the heat of the electrode reaction, a pump 16 that circulates cooling water through the fuel cell 10 and an inverter 25 that converts the direct current generated by the fuel cell 10 into alternating current are provided. The cooling water circulated by the pump 16 releases the heat energy obtained by the fuel cell by the heat exchanger 19. On the other hand, the water in the hot water tank 18 circulated by the pump 17 absorbs heat from the heat exchanger 19 and is stored in the hot water tank 18 as hot water.

  In this conventional system, three-way valves 21 and 22 are provided in a flow path that communicates with the fuel gas inlet 14a and a flow path that communicates with the air inlet 15a of the fuel cell 10, respectively. When the operation of the fuel cell 10 is stopped, an inert gas can be supplied from the inert gas cylinder 20 to the fuel gas passage and the air passage. 14b is an outlet for fuel gas, and 15b is an outlet for air.

  By the way, platinum used as a catalyst is known to be highly active in reactions other than the reactions represented by the formulas (1) and (2), which are target reactions, and is poisoned by various chemical substances. It is easy to be done. Therefore, when a substance having the property of poisoning platinum from the outside flows into the fuel cell, the reaction polarization of the electrode is remarkably increased, and thus the power generation performance of the fuel cell is impaired. Examples of substances that cause deterioration of the fuel cell and are generally present in an environment where the fuel cell is used include gases containing sulfur components such as nitrogen oxide, sulfur dioxide, and hydrogen sulfide, or ammonia gas. Can be mentioned.

  In order to protect the fuel cell from poisonous substances contained in the atmosphere, measures such as providing a filter at the outside air inlet are taken in the fuel cell system. In addition, in order to remove the influence of nitrogen oxides and sulfur oxides mixed in the fuel cell for some reason and restore the cell performance, a method of reducing and removing these substances by flowing hydrogen into the cathode (For example, patent document 2), the method (for example, patent document 3) etc. which improve the poisoning resistance by adding an organic weak acid etc. to the solid polymer film | membrane which is electrolyte are proposed.

  In the prior art, in order to maintain the power generation performance of the fuel cell, measures such as preventing a poisonous substance from entering the fuel cell from the normal gas supply path during operation like a filter, or poisoned platinum Measures have been proposed to recover the electrodes. However, it is desirable to prevent the invasion of poisonous substances because the measures to recover the platinum electrode have disadvantages such as increased cost and complicated control by adding the function to the actual system. .

The fuel cell system represented by the example shown in FIG. 1 needs to change the operation output or repeat the start and stop according to the power demand of the supply destination in order to efficiently use the chemical energy of the fuel gas. There is. At this time, a method of sealing a fuel cell as a power generation source with an electromagnetic valve or a motor-operated valve has been proposed in order to prevent the entry of poisonous substances from the outside when stopped (for example, Patent Document 4).
JP-A-11-214025 JP 2000-260454 A JP2003-282092 JP-A-6-251788

  However, in such a fuel cell stopping method, when the temperature of the battery drops to near room temperature at the time of stopping, the volume of gas enclosed in the battery is reduced due to a temperature change. Further, as described above, since the humidified reaction gas is supplied to the fuel cell, condensation of water vapor in the gas occurs when the temperature of the cell decreases. For these reasons, the pressure inside the sealed battery is reduced and becomes negative with respect to atmospheric pressure. Also, when the operation is stopped with hydrogen remaining in the anode gas flow path and oxygen remaining in the cathode gas flow path, the hydrogen that has permeated the electrolyte reacts with oxygen on the cathode platinum to produce water. Since the reaction occurs, the pressure in the fuel cell further decreases.

  In this way, when the inside of the fuel cell is in a negative pressure relative to the outside air pressure, outside air containing poisonous substances directly enters the cell through the gap of the gasket without passing through the filter, etc. In addition, the performance of the fuel cell is problematic. That is, there has been a problem that the long-term reliability of the fuel cell is lowered by repeatedly stopping the fuel cell to have a negative pressure as in the conventional stopping method.

  The present invention solves the above-described conventional problem, and prevents the pressure inside the fuel cell from becoming negative with respect to the pressure outside the fuel cell while the fuel cell system is stopped, thereby repeating the start and stop. The object is to improve the long-term reliability of fuel cells.

In order to solve the above problems, a fuel cell system of the present invention includes a fuel cell, fuel gas supply means for supplying fuel gas to the fuel cell, and oxidant gas supply means for supplying oxidant gas to the fuel cell. The fuel cell is in a sealed state after the fuel cell system is stopped, and the pressure in the fuel cell is always equal to or higher than the pressure outside the fuel cell.
The present invention also includes a fuel cell, a fuel gas supply means for supplying a fuel gas to the fuel cell, an oxidant gas supply means for supplying an oxidant gas to the fuel cell, and a reaction gas flow path of an anode and a cathode. A fuel cell system control method comprising a device for controlling a gas flow rate on an inlet side or an outlet side, and having a process of supplying a certain amount of gas into the fuel cell when the fuel cell system is stopped A system control method is provided.
By adopting this system, it becomes possible to prevent the inside of the fuel cell from being negative with respect to the external pressure when the fuel cell is stopped, and poisonous substances do not flow into the fuel cell during the stop. A fuel cell system can be provided.

  According to the present invention, it is possible to prevent the internal pressure of the fuel cell from becoming a negative pressure, and thus it is possible to prevent poisonous substances from flowing into the fuel cell while the fuel cell system is stopped. Therefore, it becomes possible to suppress the performance deterioration of the fuel cell due to the start / stop. According to the present invention, it is possible to improve the reliability of the fuel cell system in a long-term operation involving starting and stopping.

In the present invention, after the fuel cell system is stopped, the fuel cell is controlled to be sealed and the pressure in the fuel cell is always equal to or higher than the pressure outside the fuel cell.
However, the pressure in the fuel cell immediately after the stop must, of course, be less than the pressure limit of the fuel cell gasket, and auxiliary machinery power must be used for sealing in a pressurized state with a blower or the like. Since it is necessary, pressurizing more than necessary increases the running cost of the fuel cell system. Therefore, desirably, after a sufficient time has elapsed, the internal pressure of the fuel cell that has reached an equilibrium state is set to be several percent higher than the external pressure.

In a preferred embodiment of the present invention, the fuel cell system further comprises a measuring device for measuring the pressure in the reaction gas channel on the inlet side or the outlet side of the anode and cathode reaction gas channel, When the system is stopped, control is performed so that gas is supplied until the pressure in the reaction gas flow path of the fuel cell reaches a preset reference value.
When the fuel cell system is stopped, it is preferable to have a mechanism for supplying an inert gas in order to discharge the fuel gas or oxidant gas remaining in the fuel cell.
Hereinafter, embodiments of the present invention will be described with reference to the drawings.

Embodiment 1
FIG. 2 is a configuration diagram showing the fuel cell system according to Embodiment 1 of the present invention. The fuel cell system in the present embodiment steam-reforms a solid polymer fuel cell 10 that generates power using fuel gas and oxidant gas, and a raw material such as natural gas, to generate a hydrogen-rich gas. A hydrogen supply means 11 for supplying the fuel cell 10, an air supply means 12 for taking in outside air as an oxidant gas, and a humidifier 13 for giving the humidity necessary for the fuel cell 10 to the taken-in air. In addition, a pump 16 that circulates cooling water for recovering heat generated by the fuel cell 10 during power generation, a heat exchanger 19 for recovering and storing thermal energy recovered by the cooling water, a hot water tank 18, and hot water storage A circulation pump 17 that circulates water in the tank 18 through the heat exchanger 19 and an inverter 25 that converts the direct current generated by the fuel cell 10 into alternating current are provided.
In the present embodiment, pressure gauges 41 and 42 are provided in the flow path on the fuel gas inlet 14a side and the flow path on the air inlet 15a side.

The stop sequence in the present embodiment is as follows.
When the power demand of the external circuit is lost and a stop signal is issued to the fuel cell system, the fuel cell system first opens an electric circuit to the inverter 25. Next, the setting is changed so that the flow rates of the fuel gas and the oxidant gas are minimized for the control of the system, and the flow rate of the fuel gas and the oxidant gas is maintained at the minimum flow rate for a certain period of time in order to stabilize the gas flow in the fuel cell. Thereafter, the electromagnetic valves 31b and 32b installed in the outlet gas flow path of the reaction gas are closed.

  In this state, the reaction gas is continuously supplied to the fuel cell. Then, after the pressures observed by the pressure gauges 41 and 42 installed in the flow path on the fuel gas inlet 14a side and the flow path on the air inlet 15a side reach preset reference values, the gas inlet of the fuel cell The electromagnetic valves 31a and 32a installed in the flow path on the side are closed. Next, the hydrogen supply unit 11 and the air supply unit 12 are stopped.

Here, the setting of the reference value of pressure will be described.
The reference value of the pressure in the present invention is a condition that the equilibrium is reached after the operation is stopped and that it is higher than the external atmospheric pressure, that is, the atmospheric pressure, and the pressure immediately after the stop is expressed by the following formula (3) and (4) must be satisfied.
anode;
0 ≦ P _eA
= {P _iA −2C _o (P _iC −f (T _dA )) − f (T _dA )} × (273.15 + T ₂ ) / (273.15 + T ₁ ) + f (T ₂ ) (3)
Cathode;
0 ≦ PeC
= {P _iC −C _o (P _iC −f (T _dC )) − f (T _dC )} × (273.15 + T ₂ ) / (273.15 + T ₁ ) + f (T ₂ ) (4)

Here, symbols and functions in the formula are as follows.
Saturated vapor pressure at temperature t (° C.): f (t) = 6.11 × 10 {7.5 t / (t + 237.3)}
Pressure immediately after stopping (cathode): _PiC
Pressure immediately after stopping (anode): _PiA
Pressure after stabilization (cathode): _PeA
Pressure after stabilization (anode): P _eC
Oxidant gas dew point: T _dC
Fuel gas dew point: T _dA
Fuel cell temperature immediately after stopping: T ₁
Fuel cell temperature after stabilization: T ₂
Oxygen fraction in oxidant gas: _Co

Equations (3) and (4) will be described.
First, 2C _o (P _iC −f (T _dA )) and C _o (P _iC −f (T _dC )) pass through the electrolyte membrane from the anode to the cathode, where they react with oxygen. Since the amount of gas decreases at the anode and the cathode, respectively, the pressure decreasing thereby is represented. When an inert gas is sealed in the anode or the cathode, that is, in a situation where water and oxygen cannot be generated, this term becomes zero.

f (T _dA ) and f (T _dC ) represent pressures that are reduced due to a decrease in the amount of gas due to condensation due to a decrease in the temperature of the fuel cell. When dry gas is sealed in the fuel cell, this term becomes zero.
Next, the fraction term integrated in the curly brackets of the first term of the above formula represents the pressure after the volume change as the temperature of the fuel cell decreases. The second term represents the saturated vapor pressure after the temperature has dropped and equilibrium has been reached.

The above is the fuel cell activation sequence.
In the present embodiment, the system having the hydrogen supply means 11 is used. However, hydrogen may be directly supplied from outside the system, and humidified by a humidifier in the same manner as air, and supplied to the fuel cell.

  If the configuration and the stopping method of the fuel cell system in the present embodiment are taken, it is possible to prevent the internal pressure of the fuel cell after stopping from becoming a negative pressure. Therefore, it is possible to provide a highly reliable fuel cell system in long-term operation with start / stop.

Embodiment 2
FIG. 3 is a block diagram showing a fuel cell system according to Embodiment 2 of the present invention. In the fuel cell system according to the present embodiment, the following elements are added to the prior art system shown in FIG. That is, the flow rate of the inert gas between the inert gas cylinder 20 for purging the gas in the fuel cell at the time of stop and the switching valves 21 and 22 of the fuel gas inlet side flow path and the air inlet side flow path. Mass flow controllers 23 and 24 for controlling the above are provided. Pressure gauges 41 and 42 are provided in the flow path on the fuel gas inlet 14a side and the flow path on the air inlet 15a side.

  Here, the inert gas causes a redox reaction that can become a single electrode on platinum in a high humidity atmosphere of 0 ° C. to 100 ° C., such as rare gas such as helium or argon, nitrogen, natural gas after desulfurization, or water vapor. No gas.

The purge sequence at the time of stop in the present embodiment is as follows.
When the power demand of the external circuit disappears and a stop signal is issued to the fuel cell system, the fuel cell system first reduces the output to the minimum output. At this time, the flow rates of the mass flow controllers 23 and 24 are set to a controllable minimum flow rate value. In order to stabilize the gas flow in the fuel cell, the fuel cell is maintained at the minimum output state for a certain period of time. Next, the electric circuit to the inverter 25 is opened, the valves 21 and 22 are switched so that the inert gas is supplied to the fuel cell, and the hydrogen supply unit 11 and the air supply unit 12 are stopped.

In this state, the inert gas is continuously supplied to the fuel cell. Then, after the pressure observed by the pressure gauges 41 and 42 installed in the gas flow path on the inlet side reaches a preset reference value, the electromagnetic valves 21 and 22 in the gas flow path on the inlet side of the fuel cell are closed. The reference value at this time is a value calculated from the above equations (3) and (4).
The above is the fuel cell activation sequence.

  When the configuration of the fuel cell system and the purging method in the present embodiment are employed, as in the first embodiment, a highly reliable fuel cell system can be provided in long-term operation with start and stop.

Embodiment 3
FIG. 4 is a block diagram showing a fuel cell system according to Embodiment 3 of the present invention. The fuel cell system according to the present embodiment is different from the system according to the first embodiment in the following points. That is, as a mechanism for purging the gas in the fuel cell when the operation is stopped, natural gas introduced from outside the system by the booster pumps 51 and 52 can be supplied to the fuel cell as an inert gas instead of the inert gas cylinder. ing.

The purge sequence at the time of stop in the present embodiment is as follows.
When the power demand of the external circuit disappears and a stop signal is issued to the fuel cell system, the fuel cell system first reduces the output to the minimum output. At this time, the booster pumps 51 and 52 are operated. In order to stabilize the gas flow in the fuel cell, after maintaining a minimum output state for a certain time, the electric circuit to the inverter 25 is opened, and the valves 21 and 22 are switched so that natural gas is supplied to the fuel cell. Then, the hydrogen supply means 11 and the air supply means 12 are stopped.

In this state, the natural gas is continuously supplied to the fuel cell, and after the pressure observed by the pressure gauges 41 and 42 installed in the gas flow path on the inlet side reaches a preset reference value, After the solenoid valves 31b and 32b installed in the gas flow path are closed, the supply of natural gas is stopped. The reference value at this time is a value calculated from the above equations (3) and (4).
The above is the fuel cell activation sequence.

  When the configuration of the fuel cell system and the purging method in the present embodiment are employed, as in the first embodiment, a highly reliable fuel cell system can be provided in long-term operation with start and stop.

Next, examples of the present invention will be described.
A fuel cell system was actually manufactured as Embodiments 1 to 3 for Embodiments 1 to 3 shown in FIGS. 2 to 4 and the effects of the invention were confirmed. Further, the fuel cell system having the configuration shown in FIG. Further, in the system of FIG. 2, a system that operates according to a sequence in which the pressure inside the fuel cell is stopped in the same state as the atmospheric pressure is set as Comparative Example 2.

  In the examples and comparative examples, a hydrogen cylinder was used as the hydrogen supply means 11, and a blower (VB-004-DN manufactured by Hitachi, Ltd.) was used as the air supply means 12 and the booster pumps 51 and 52. The fuel cell stack used had an electrode area of 8 cm × 10 cm and an outer dimension of the separator of 10 cm × 20 cm. 100 such single cells were stacked and used as a fuel cell stack. In Example 2, nitrogen was used as the inert gas, and in Example 3, city gas was used as the inert gas.

In order to confirm the effect of the embodiment, an experiment of a start / stop cycle according to the following sequence was performed. In this sequence, control was performed using an external load so that power is generated at a current density of 0.5 A / cm ² during power generation. Further, in this sequence, since the influence of the temperature change on the durability of the fuel cell stack is taken into consideration, the time required for the temperature of the fuel cell stack to drop to near room temperature after the operation is stopped is measured as 3.2 ± 0. It was found to be 4 hours. Therefore, the stop time was set to 4.0 hours.

  Power generation (2.0 hours) → Stop or stop purge (1.0 hour) → Power generation (2.0 hours) → ... (repeated)

  Examples 1 to 3 and Comparative Examples 1 and 2 were repeatedly operated in the sequence described above, and the transition of the average voltage during operation was examined. The result is shown in FIG. In FIG. 5, the voltages of Comparative Example 1 and Comparative Example 2 are monotonously decreasing, whereas in Examples 1 to 3, no significant change in voltage is observed even after 3000 cycles. Thereby, the effect of the present invention was confirmed.

  The fuel cell system of the present invention is useful as a home cogeneration system because of its high reliability in long-term operation with start and stop. It can also be applied as an energy source for motor vehicles for vehicles such as passenger cars, buses and scooters.

It is a schematic block diagram of the conventional fuel cell system. It is a figure which shows the structure of the fuel cell system of Embodiment 1 of this invention. It is a figure which shows the structure of the fuel cell system of Embodiment 2 of this invention. It is a figure which shows the structure of the fuel cell system of Embodiment 3 of this invention. It is a graph which shows transition of the generated voltage in the cycle test of the fuel cell system of the Example of this invention, and a comparative example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Fuel cell 11 Hydrogen supply means 12 Air supply means 13 Humidifier 14a Fuel gas inlet 15a Air inlet 16, 17 Pump 18 Hot water storage tank 19 Heat exchanger 20 Inert gas cylinder 25 Inverter 21, 22, 31a, 31b, 32a, 32b Valve 23, 24 Mass flow controller 41, 42 Pressure gauge 51, 52 Booster pump

Claims (4)

  A fuel cell system comprising: a fuel cell; a fuel gas supply means for supplying fuel gas to the fuel cell; and an oxidant gas supply means for supplying an oxidant gas to the fuel cell, wherein the fuel cell system comprises: After stopping, the fuel cell is in a sealed state, and the pressure in the fuel cell is always equal to or higher than the pressure outside the fuel cell.   The fuel cell system according to claim 1, further comprising a mechanism for supplying an inert gas in order to discharge fuel gas or oxidant gas remaining in the fuel cell when the fuel cell system is stopped.   A fuel cell; a fuel gas supply means for supplying a fuel gas to the fuel cell; an oxidant gas supply means for supplying an oxidant gas to the fuel cell; and an inlet side or an outlet side of an anode and cathode reaction gas flow path And a device for controlling the gas flow rate, the fuel cell system comprising a step of supplying a certain amount of gas into the fuel cell when the fuel cell system is stopped. How to control the system.   The fuel cell system further comprises a measuring device for measuring the pressure in the reaction gas channel on the inlet side or the outlet side of the reaction gas channel of the anode and the cathode, and the reaction of the fuel cell is stopped when the fuel cell system is stopped. 4. The method of controlling a fuel cell system according to claim 3, further comprising a step of supplying gas until the pressure in the gas flow path reaches a preset reference value.

Images