JP2007053015A - Fuel cell system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To purge efficiently moisture inside a fuel cell and prevent freezing at operation stop of a fuel cell system. <P>SOLUTION: Two systems are provided at an exhaust system from a fuel cell stack 2. The first exhaust system comprises first exhaust pipes 6a, 6b, 6c, a humidity exchanger 4, and a pressure control valve 5. The second exhaust system comprises a second exhaust pipe 7. As an exhaust switching means, an exhaust switching valve 8 to switch over the first exhaust pipe 6a and the second exhaust pipe 7 is provided. At the time of operation stop of the fuel cell system, the exhaust switching valve 8 is switched over so as to open the second exhaust pipe 7. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

    The present invention relates to a fuel cell system having improved purge operation at the time of shutdown, and more particularly to a technique for improving the efficiency of purge at the time of shutdown for facilitating start-up from below freezing point.

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

Negative electrode reaction: H ₂ → 2H ⁺ + 2e ⁻ (1)
Positive electrode reaction: 2H ⁺ + 2e ⁻ + (1/2) O ₂ → H ₂ O (2)
As the fuel gas supplied to the negative electrode, a method of directly supplying from a hydrogen storage device, or a method of supplying a reformed hydrogen-containing gas by reforming a fuel containing hydrogen is known. Examples of the hydrogen storage device include a high-pressure gas tank, a liquefied hydrogen tank, and a hydrogen storage alloy tank. Air is generally used as the oxidant gas supplied to the positive electrode.

  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 normally. However, in a low temperature state of 0 ° C. or lower, moisture remaining in the cell freezes and the reaction gas flow path is blocked, or moisture remaining in the vicinity of the electrode freezes and inhibits diffusion of the reaction gas. There is a problem that power generation becomes impossible.

Therefore, in order to start the fuel cell from 0 ° C. or lower, it is necessary to remove moisture from the inside of the fuel cell in advance. For example, as shown in Patent Document 1, when the operation of the fuel cell is stopped, the gas is supplied to the fuel cell without being humidified and dried, and the operation is stopped when the fuel cell reaches a certain humidity. Technology is known.
JP 2001-332281 A (page 4, FIG. 1)

  However, a fuel cell system that is mainly mounted on an automobile has a humidity exchanger, a pressure control valve, and the like in the cathode side exhaust system, and has a long exhaust pipe for exhausting at the rear end of the automobile. Yes, even if the pressure regulating valve is fully opened, some pressure loss will occur. Therefore, when a large flow rate is flowed for the purpose of purging, the stack is in a pressurized state, and as shown in FIG. 5, the rotational speed of the compressor and the purge gas flow rate (volume flow rate) in the stack are not proportional. .

  Also, the purge flow rate (volume flow rate) for establishing sub-zero activation and the required purge time are in the relationship shown in FIG. 6, and the purge time increases as the purge flow rate becomes slower. There is a problem that the purge time becomes long because the flow rate that can be produced by the capacity of the compressor cannot be used to the maximum extent.

  An increase in the purge time means an increase in the time required for the stop operation of the automobile, resulting in a significant reduction in merchandise power.

  In order to solve the above problems, the invention according to claim 1 is a fuel cell main body comprising an anode and a cathode, air supply means for supplying air to the cathode, air supplied to the cathode, and the cathode. A humidity exchanger that humidifies the air supplied to the cathode by exchanging humidity with the discharged air, and a pressure adjusting valve that adjusts the cathode pressure by adjusting the air flow rate downstream of the humidity exchanger A first exhaust system that exhausts cathode exhaust from the cathode outlet via the humidity exchanger and the pressure regulating valve, a second exhaust system that branches from upstream of the humidity exchanger of the first exhaust system, An exhaust gas switching unit that selectively opens an exhaust system or a second exhaust system.

  According to the present invention, at the time of purging when the fuel cell system is stopped, exhaust can be performed through the second exhaust system having no pressure loss element such as a humidity exchanger or a pressure regulating valve, so the flow velocity in the fuel cell stack at the time of purging The drainage from the gas flow path and the fuel cell main body is efficiently performed, and the purge time for preventing freezing can be shortened.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In addition, each Example demonstrated below is an example which applied this invention to the fuel cell vehicle.

  FIG. 1 is a system configuration diagram showing the configuration of Embodiment 1 of the fuel cell system according to the present invention. In the figure, a fuel cell system 1 includes a fuel cell stack 2 having an anode and a cathode, a compressor 3 that supplies air as an oxidant gas to the cathode, and a humidity exchanger that collects moisture in the cathode exhaust and humidifies the supply air. 4, a pressure adjusting valve 5 for adjusting the cathode pressure, a first exhaust pipe 6a from the cathode outlet 2b to the cathode exhaust inlet of the humidity exchanger 4, and a first from the cathode exhaust outlet 4d of the humidity exchanger 4 to the pressure adjusting valve 5 Exhaust pipe 6b, first exhaust pipe 6c from the pressure regulating valve 5 to the rearmost part of the automobile, second exhaust pipe 7 branched from the first exhaust pipe 6a, and exhaust for switching between the first exhaust pipe 6 and the second exhaust pipe 7 A switching valve 8 is provided. The first exhaust pipes 6a, 6b, and 6c are also collectively referred to as the first exhaust pipe 6.

  The first exhaust pipe 6, the humidity exchanger 4 and the pressure regulating valve 5 constitute a first exhaust system. The second exhaust pipe 7 constitutes a second exhaust system. The exhaust gas switching valve 8 is an exhaust gas switching means.

  The humidity exchanger 4 is a humidity exchanger that includes, for example, a hollow fiber membrane made of polyimide resin and moves water vapor inside and outside the hollow fiber membrane. In the humidity exchanger 4, the supply air inlet 4 a to which air is supplied from the compressor 3, the supply air outlet 4 b that is an outlet of the humidified air, and the cathode exhaust gas containing water vapor from the cathode outlet 2 b of the fuel cell stack 2 are the first. An exhaust inlet 4c and an exhaust outlet 4d supplied through the exhaust pipe 6a are provided. The humidity exchanger 4 exchanges humidity between the supply air introduced from the air supply inlet 4a and the cathode exhaust introduced from the exhaust inlet 4c. The humidified supply air is supplied from the supply outlet 4b of the humidity exchanger 4 to the cathode inlet 2b, and the anode exhaust with reduced humidity is supplied from the exhaust outlet 4d to the first exhaust pipe 6b, the pressure regulating valve 5, and the first exhaust. It is discharged from the rear part of the vehicle body via the pipe 6c.

  This figure shows only the parts related to the present invention, and an anode gas system, a cooling system and the like necessary for an actual fuel cell system are omitted.

  When the operation of stopping the operation of the fuel cell system is started, the compressor 3 reaches the number of revolutions necessary for purging, and at the same time, the exhaust pipe is switched from the first exhaust pipe 6 to the second exhaust pipe 7 by the exhaust switching valve 8. . As a result, the exhaust gas can be exhausted outside without passing through the pressure regulating valve 5 and the humidity exchanger 4 which are the causes of the pressure loss of the exhaust system, so that the flow rate of the purge gas in the fuel cell stack can be increased as shown in FIG. it can. As described above, by increasing the flow rate of the purge gas as shown in FIG. 6, the purge time required for starting below zero can be shortened, and the stop time of the fuel cell system can be shortened.

  Further, since the purge gas does not pass through the humidity exchanger 4 provided in the first exhaust pipe 6 at the time of purging, the moisture in the fuel cell stack 2 is not brought into the humidity exchanger 4 and the purge gas is unnecessary. It can prevent humidifying. Further, the amount of water remaining in the humidity exchanger 4 can be reduced, the amount of freezing when the temperature becomes below zero is reduced, and the humidity exchanger 4 can be quickly thawed at the next startup at below zero.

  According to the present embodiment described above, when purging when the fuel cell system is stopped, exhaust can be performed through the second exhaust system without pressure loss elements such as a humidity exchanger and a pressure regulating valve. The flow rate in the stack can be increased, drainage from the gas flow path and the fuel cell body can be performed efficiently, and the purge time for preventing freezing can be shortened.

  Next, FIG. 2 shows a cross-sectional view of an essential part of Embodiment 2 of the present invention. The difference between the second embodiment and the first embodiment is that, as an exhaust gas switching means, a second exhaust gas that opens and closes the second exhaust pipe 7 instead of the exhaust gas switching valve 8 of the first exhaust pipe 6 and the second exhaust pipe 7. This is the point that a tube opening / closing valve 9 is provided. Other configurations are the same as those of the first embodiment shown in FIG.

  When the operation of stopping the operation of the fuel cell system is started, the compressor 3 has a rotational speed necessary for purging, and the second exhaust pipe opening / closing valve 9 that is closed during normal operation is opened. As a result, the gas flows through both the first exhaust pipe 6 and the second exhaust pipe 7, so that the pressure loss can be further reduced and the purge flow rate can be increased.

  According to the present embodiment described above, when purging when the fuel cell system is stopped, exhaust can be performed through the second exhaust system without pressure loss elements such as a humidity exchanger and a pressure regulating valve. The flow rate in the stack can be increased, drainage from the gas flow path and the fuel cell body can be performed efficiently, and the purge time for preventing freezing can be shortened.

  Next, FIG. 3 shows a cross-sectional view of an essential part of Embodiment 3 of the present invention. The difference between the third embodiment and the first and second embodiments is that, in addition to the second exhaust pipe on / off valve 9, a first exhaust pipe on / off valve 10 is provided as an exhaust gas switching means. Other configurations are the same as those of the first embodiment shown in FIG.

  When the operation of stopping the operation of the fuel cell system is started, the compressor 3 has the number of revolutions necessary for purging. First, the second exhaust pipe opening / closing valve 9 that is closed in the normal operation state is opened, and conversely, the normal operation is performed. The first exhaust pipe on / off valve 10 that is open in the state is closed, and after a predetermined time, the first exhaust pipe on / off valve 10 is opened. As a result, when the amount of water taken out from the fuel cell stack at the beginning of the purge is large, gas is not allowed to flow into the humidity exchanger 4, and when the amount of water taken out from the fuel cell decreases after a predetermined time. The first exhaust pipe opening / closing valve 10 is opened, and the pressure loss can be further reduced without bringing water into the humidity exchanger 4. The first exhaust pipe opening / closing valve 10 of this embodiment may be replaced with the pressure adjustment valve 5.

  According to the present embodiment described above, each of the first exhaust system and the second exhaust system has a valve for opening and closing the respective exhaust system, and after the stop operation of the fuel cell system is started, When the exhaust system closes, the second exhaust system open / close valve opens, and after purging for a predetermined time, the first exhaust system open / close valve opens. Can prevent gas from flowing through the humidity exchanger. When the amount of water taken out from the fuel cell decreases after a predetermined time, the first exhaust system opening / closing means is opened, so that both the first and first exhaust systems can be used without bringing water into the humidity exchanger. The gas can be discharged from the gas, and the pressure loss can be further reduced.

  Next, a fourth embodiment of the present invention will be described with reference to FIG. The configuration of this embodiment is the same as the configuration of Embodiments 1 to 3 shown in FIGS.

  In the time chart of FIG. 4, when the operation of stopping the operation of the fuel cell system is started, the rotation speed of the compressor 3 is increased, and the pressure adjusting valve 5 provided in the first exhaust pipe opening / closing valve 10 or the first exhaust pipe 6. Is closed and the pressure inside the fuel cell stack 2 is increased. After a predetermined time T, or after the pressure inside the fuel cell stack 2 exceeds a predetermined value, the first exhaust pipe opening / closing valve 10 or the pressure regulating valve 5 and the second exhaust pipe opening / closing valve 9 are fully opened, and the pressure is increased all at once. Lower. As a result, the flow velocity inside the fuel cell stack 2 when the pressure drops all at once becomes extremely high, and the water in the gas flow path, gas diffusion layer (GDL), and catalyst layer inside the fuel cell stack 2 can be efficiently removed. Therefore, the purge time can be shortened.

  The predetermined time T is, for example, a time during which the pressure in the space from the outlet of the compressor 3 to the exhaust pipe opening / closing valve 10 or the pressure regulating valve 5 does not increase even if the operation of the compressor 3 is continued. The volume from the outlet to the exhaust pipe opening / closing valve 10 or the pressure regulating valve 5 and the air compression capacity of the compressor 3 are determined. Actually, it can be determined by an experiment with an actual machine or a computer simulation of a hydrodynamic numerical model from the outlet of the compressor 3 to the exhaust pipe opening / closing valve 10 or the pressure regulating valve 5.

  According to the present embodiment described above, after the stop operation of the fuel cell system is started, the pressure control valve of the first exhaust system is closed, and after the pressure inside the fuel cell stack is increased, the second exhaust system is opened. Therefore, the pressure inside the fuel cell stack is suddenly released, and an extremely fast gas flow is generated for a moment, so that drainage from the gas flow path, the gas diffusion layer, and the like is performed efficiently.

  In each of the above embodiments, the second exhaust pipe 7 may be provided with a silencer for reducing noise when a large flow rate of gas flows.

1 is a system configuration diagram illustrating Example 1 of a fuel cell system according to the present invention. It is principal part sectional drawing explaining Example 2 of the fuel cell system which concerns on this invention. It is principal part sectional drawing explaining Example 3 of the fuel cell system which concerns on this invention. It is principal part sectional drawing explaining Example 4 of the fuel cell system which concerns on this invention. It is a figure which shows the relationship between purge flow velocity (volumetric flow rate) and compressor rotation speed. It is a figure which shows the relationship between the purge flow rate (volume flow rate) and the purge time required for starting below zero.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Fuel cell system 2 Fuel cell stack 3 Compressor 4 Humidity exchanger 5 Pressure control valve 6a, 6b, 6c 1st exhaust pipe 7 2nd exhaust pipe 8 Exhaust switching valve 9 2nd exhaust pipe on-off valve 10 1st exhaust pipe on-off valve

Claims (4)

A fuel cell body comprising an anode and a cathode;
Air supply means for supplying air to the cathode;
A humidity exchanger that humidifies the air supplied to the cathode by exchanging humidity between the air supplied to the cathode and the air discharged from the cathode;
A pressure regulating valve that regulates the cathode pressure by regulating the air flow downstream of the humidity exchanger;
A first exhaust system for discharging cathode exhaust from the cathode outlet via the humidity exchanger and the pressure regulating valve;
A second exhaust system that branches from upstream of the humidity exchanger of the first exhaust system;
Exhaust switching means for selectively opening the first exhaust system or the second exhaust system;
A fuel cell system comprising:   2. The fuel cell system according to claim 1, wherein when the operation of the fuel cell system is stopped, purge is performed by opening the second exhaust system by the exhaust gas switching unit. The exhaust gas switching means includes first exhaust system opening / closing means for opening / closing a first exhaust system, and second exhaust system opening / closing means for opening / closing a second exhaust system,
The first exhaust system opening / closing means is closed, the second exhaust system opening / closing means is opened after the fuel cell system is stopped, and the first exhaust system opening / closing means is opened after purging for a predetermined time. The fuel cell system described.   After starting the stop operation of the fuel cell system, the pressure regulating valve is closed, air is supplied from the air supply means to increase the pressure inside the fuel cell stack, and then at least the second exhaust system is opened. The fuel cell system according to claim 1.

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