JP2006179242A - Fuel cell system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-durability fuel cell system capable of suppressing deterioration of an oxidizer electrode catalyst layer caused by oxidizer electrode potential variation and hardly causing deterioration with load change. <P>SOLUTION: In relation to fuel cell stacks of this application, the current density-voltage characteristic of a first fuel cell stack 2 is equivalent to that of a second fuel cell stack 3; and an effective power generation area ratio is m:n (m<n). When a load of this fuel cell system is less than m/(m+n) of the full load, switches 15 and 16 are opened; and a current is supplied to a load device 4 from the first fuel cell stack 2. When the load is set not less than m/(m+n) of the full load, the switches 15 and 16 are closed, and currents are supplied to the load device 4 from the first fuel cell stack 2 and the second fuel cell stack 3. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a fuel cell system, and more particularly to a fuel cell system in which two or more fuel cell stacks can be connected in parallel.

  In a fuel cell, a fuel gas such as hydrogen gas and an oxidizing gas containing oxygen are electrochemically reacted through an electrolyte, and electric energy is directly taken out between electrodes provided on both surfaces of the electrolyte. In particular, a polymer electrolyte fuel cell using a polymer electrolyte has attracted attention as a power source for electric vehicles because of its low operating temperature and easy handling. That is, a fuel cell vehicle is equipped with a hydrogen storage device such as a high-pressure hydrogen tank, a liquid hydrogen tank, or a hydrogen storage alloy tank in the vehicle, and reacts by supplying hydrogen supplied therefrom and air containing oxygen to the fuel cell. This is the ultimate clean vehicle that drives the motor connected to the drive wheels with the electric energy extracted from the fuel cell, and the only exhaust material is water.

  By the way, the change of the output voltage with respect to the fluctuation of the load current is relatively large in the fuel cell, and in order to supply a stable voltage to the load device, a DC / DC converter connected between the fuel cell and the load device, etc. The burden of the power converter is large.

As a conventional technique for reducing the burden of such a power conversion device, there is a technology that includes a plurality of fuel cell stacks, and the number of fuel cell stacks connected in parallel according to the size of a load is variable (for example, Patent Document 1). ).
JP2003-178786 (4th page, FIG. 1 (a))

  During operation of the fuel cell, particularly as an automobile power source, loads of various sizes and cycles are applied as the vehicle is accelerated and decelerated. Here, the fuel cell has the characteristic that the output voltage decreases as the load increases due to the influence of reaction resistance, electrical resistance, diffusion resistance of fuel and oxidant, etc. The potential fluctuation of the pole is large. As a result, when a load change is applied to the fuel cell, there is a problem that the deterioration of the oxidant electrode catalyst layer proceeds due to the change in the oxidant electrode potential.

  In order to solve the above problems, the present invention provides a first fuel cell stack, and at least one second fuel cell stack having the same current density-voltage characteristics as the first fuel cell stack and having a different effective power generation area. And a switch for connecting or releasing the second fuel cell stack in parallel with or opening the first fuel cell stack in response to a load change, wherein the unit cell voltage in each fuel cell stack is within a predetermined range. The gist of the invention is to control the switch so as to be within the range.

  The authors have been able to suppress the above-mentioned deterioration of the oxidizer electrode catalyst layer as a result of an experiment focusing on the deterioration caused by the decrease in the electrochemical reaction area accompanying the increase in the catalyst particle size in the catalyst layer of the polymer electrolyte fuel cell. I found the following conditions.

  For example, it was possible to significantly suppress the deterioration of the oxidizer electrode in a load change that would result in a unit cell voltage of about 0.85 [V] or less.

  On the other hand, it was possible to suppress deterioration of the fuel electrode catalyst layer, in particular, by holding the electrolyte membrane in a relatively wet state when there was no load.

  Next, when using a fuel cell as a power source for automobiles, in general driving in urban areas and suburbs, there is a result that the frequency of use at a relatively low output occupies most, depending on the set maximum output, In particular, it is considered necessary to suppress deterioration in the low output region.

  According to the present invention, since deterioration of the oxidant electrode catalyst layer due to fluctuations in the oxidant electrode potential can be suppressed, there is an effect that it is possible to provide a highly durable fuel cell system with little deterioration due to load fluctuations. is there.

  Embodiments of the present invention will be described below with reference to the drawings. Each embodiment described below is applied to a fuel cell system having a large load fluctuation from idling time (for example, a standby state where power corresponding to the power consumption of the fuel cell auxiliary machine is generated) to full load time, such as a fuel cell vehicle. This is a preferred embodiment.

  FIG. 1 is a schematic configuration diagram illustrating the configuration of a first embodiment of a fuel cell system 1 according to the present invention. In the figure, a fuel cell system 1 includes a first fuel cell stack 2, a second fuel cell stack 3 having a current density-voltage characteristic equivalent to that of the first fuel cell stack 2 and a different effective power generation area, Switches 15 and 16 that can connect the fuel cell stack 3 to the first fuel cell stack 2 in parallel are provided.

  The fuel electrode 5a and the oxidant electrode 6a of the first fuel cell stack 2 are always connected to the load device 4 via connection lines 11 and 12, respectively.

  The fuel electrode 5 b of the second fuel cell stack 3 is connected to the fuel electrode 5 a of the first fuel cell stack 2 and the load device 4 via the switch 15 and the connection line 13. The oxidant electrode 6 b of the second fuel cell stack 3 is connected to the oxidant electrode 6 a of the first fuel cell stack 2 and the load device 4 via the switch 16 and the connection line 14.

  The switches 15 and 16 are, for example, semiconductor switches that can be turned on / off by a control signal such as a power FET or IGBT.

  The switches 15 and 16 that connect the second fuel cell stack 3 to the second fuel cell stack 2 and the load device 4 are opened when the fuel cell load is lower than a predetermined value, and the fuel cell load becomes equal to or higher than the predetermined value. The switches 15 and 16 are closed, and the second fuel cell stack 3 is connected to the load device 4.

The first fuel cell stack 2 and the second fuel cell stack 3 are both solid polymer electrolyte fuel cells and are fuel cell stacks having different effective power generation areas. In particular, it is preferable that the first fuel cell stack 2 and the second fuel cell stack 3 have the same type of membrane electrode assembly (MEA) and have the same number of stacked cells. Here, the effective power generation area of both is
It is assumed that the relationship of effective power generation area of the first fuel cell stack 2 <effective power generation area of the second fuel cell stack 3 is established.

  The first fuel cell stack 2 is supplied with fuel gas from a fuel gas supply system (not shown) via a fuel gas supply pipe 7a and unreacted from the first fuel cell stack 2 via a fuel gas discharge pipe 8a. Fuel gas is discharged. Similarly, an oxidant gas is supplied to the first fuel cell stack 2 from an oxidant gas supply system (not shown) via an oxidant gas supply pipe 9a, and an oxidant gas discharge pipe 10a is connected from the first fuel cell stack 2 to the first fuel cell stack 2. The reacted oxidant gas is discharged.

  Similarly, the second fuel cell stack 3 is supplied with fuel gas from a fuel gas supply system (not shown) via a fuel gas supply pipe 7b and unreacted from the second fuel cell stack 3 via a fuel gas discharge pipe 8b. The fuel gas is discharged. Similarly, an oxidant gas is supplied to the second fuel cell stack 3 from an oxidant gas supply system (not shown) via an oxidant gas supply pipe 9b, and an oxidant gas discharge pipe 10b is supplied from the second fuel cell stack 3 to the second fuel cell stack 3. The reacted oxidant gas is discharged.

  As the gas supply system, one having a conventionally well-known configuration may be used, and there may be one system for each of the fuel gas and the oxidant gas, or an independent system for each fuel cell stack.

  In this embodiment, a total of two stacks of the first fuel cell stack 2 and one second fuel cell stack 3 can be connected in parallel. However, if the current density-voltage characteristics of each stack are equivalent. For example, the number of second fuel cell stacks may be arbitrarily increased according to the required maximum output. However, each second fuel cell stack must be connected to an openable / closable switch.

  At the minimum load, for example, when the fuel cell vehicle is idling, only the first fuel cell stack 2 is operated, and the unit cell voltage in the first fuel cell stack 2 at this time is preferably about 0.85 [V] or less. The power generation area of the first fuel cell stack 2 is determined so as to be held.

  When the load increases and reaches the set maximum output (power or current) of the first fuel cell stack 2, the switches 15 and 16 are closed, and the power generation of the second fuel cell stack 3 is started. At this time, the current density in the first fuel cell stack 2 and the second fuel cell stack 3 is equal, and the power generation area of the second fuel cell stack 3 is such that the unit cell voltage is about 0.85 [V] or less. It is determined. When the load further increases, the power generation amount of the first fuel cell stack 2 and the second fuel cell stack 3 increases to the set maximum output.

  FIG. 2 is a diagram showing the relationship of the stack voltage with respect to the stack current (current density display) in the first and second fuel cell stacks 2 and 3. As shown by a broken line in FIG. 2, all the fuel cell stacks are operated in a use region in which the unit cell voltage is equal to or lower than a predetermined voltage of 0.85 [V].

FIG. 3 shows the stack area ratio as follows: first fuel cell stack: second fuel cell stack = m: n
It is a figure explaining the load current for every fuel cell stack with respect to request | requirement load current, and operation of switch 15,16.

  In FIG. 3, only the first fuel cell stack 2 generates power in the load range from idling to switching, and when the required load is 100% or more after switching, the first fuel cell stack 2 and the second fuel cell stack 3 A current is supplied to the load device 4 in parallel.

  In the power generation state immediately before switching, the first fuel cell stack 2 is 100% of the stack capacity, and the second fuel cell stack 3 is 0%. Immediately after switching, the power generation states of the first fuel cell stack 2 and the second fuel cell stack 3 are 100 × m / (m + n) [%] of the respective stack capacities. For example, if m = 1 and n = 3, then 100 × m / (m + n) [%] = 25 [%].

  The method for obtaining the effective power generation area ratio of these two fuel cell stacks is as follows. For example, load time variation pattern data is collected, and a load model variation pattern is created based on the collected data. Then, in this model variation pattern of the load, by determining the effective power generation area ratio to a value that minimizes the frequency with which the cell voltage of the second fuel cell stack passes the predetermined unit cell voltage, the oxidant catalyst layer It is possible to further suppress the deterioration of the fuel cell stack and further enhance the durability of the fuel cell stack.

  In the case of a fuel cell system used as a power source for an electric vehicle, it is obvious that the time variation pattern data of the load can be obtained from the running pattern data of the vehicle.

  This makes it possible to provide a fuel cell system that is highly resistant to deterioration of the oxidant electrode catalyst layer and has high durability in applications as a power source for automobiles with a relatively low load and a high frequency of fluctuation.

  Next, the difference between the present embodiment and the comparative example according to the prior art will be described with reference to FIGS.

  FIG. 4 is a time chart showing an example of time variation of load variation and unit cell voltage in a conventional fuel cell system substantially including one fuel cell stack, and FIG. 5 is a diagram showing load variation and unit in the first embodiment. It is a time chart which shows the example of a time change of a cell voltage.

  In the comparative example of FIG. 4, the fuel cell stack voltage fluctuates in accordance with the load variation, and individual unit cell voltages that generate the fuel cell stack voltage also fluctuate. For example, a predetermined value (for example, 0.85 [V] indicated by a broken line) ) Is repeatedly crossed up and down, and the deterioration of the fuel cell stack proceeds.

  On the other hand, in Example 1 of FIG. 5, when the load is less than the threshold value Lth, only the first fuel cell stack generates power, and when the load exceeds the threshold value Lth, the first fuel cell stack In addition, the second fuel cell stack is also controlled to generate power. For this reason, the unit cell voltage of the first fuel cell stack is always equal to or less than a predetermined value (for example, 0.85 [V]) indicated by a broken line, and the unit cell voltage of the second fuel cell stack has a load Lth Every time crossing, a broken line having a predetermined value is crossed.

  Therefore, according to the present embodiment, the frequency with which the unit cell voltage of the first and second fuel cell stacks passes a predetermined value is significantly reduced, and deterioration of the oxidant electrode catalyst layer due to oxidant electrode potential fluctuation is suppressed. Therefore, there is an effect that it is possible to provide a highly durable fuel cell system with little deterioration due to load fluctuation.

  Next, a second embodiment of the fuel cell system according to the present invention will be described. The second embodiment is characterized in that the first fuel cell stack 2 and the second fuel cell stack 3 include a fuel gas supply system and an oxidant gas supply system that can be controlled independently. The connection relationship between the first fuel cell stack 2 and the second fuel cell stack 3 is the same as that of the first embodiment shown in FIG.

  The first fuel cell stack 2 and the first fuel cell stack 2 have the same current density-voltage characteristics and different effective power generation areas.

  FIG. 6 is a time chart illustrating fuel cell system control in the second embodiment. In this embodiment, when the load on the fuel cell is less than the threshold value Lth, the fuel gas and the oxidant gas are supplied only to the first fuel cell stack 2, and the fuel gas and the oxidation gas are supplied to the second fuel cell stack 3. Do not supply agent gas. Then, when the load on the fuel cell becomes equal to or higher than the threshold value Lth, control is performed so that the fuel gas and the oxidant gas are supplied to the first fuel cell stack 2 and the second fuel cell stack 3.

  This makes it possible to minimize the gas supply amount at low load, and to provide a highly durable fuel cell that has a short response time against load fluctuations and is resistant to deterioration of the oxidizer electrode catalyst layer. Become.

  When the second fuel cell stack 3 is not generating power, a high relative humidity fuel gas and an oxidant gas with high relative humidity are supplied, or these gases are sealed in the fuel electrode 5b and the oxidant electrode 6b. It may be. Thereby, there exists an effect that the catalyst layer deterioration at the time of no load can be suppressed.

  FIG. 7 is a schematic configuration diagram illustrating the configuration of Example 3 of the fuel cell system 1 according to the present invention. In the figure, a fuel cell system 1 includes a first fuel cell stack 2, a second fuel cell stack 3 having a current density-voltage characteristic equivalent to that of the first fuel cell stack 2 and a different effective power generation area, Switches 15 and 16 that can connect the fuel cell stack 3 to the first fuel cell stack 2 in parallel are provided.

  The fuel electrode 5a and the oxidant electrode 6a of the first fuel cell stack 2 are always connected to the load device 4 via connection lines 11 and 12, respectively.

  The fuel electrode 5 b of the second fuel cell stack 3 is connected to the fuel electrode 5 a of the first fuel cell stack 2 and the load device 4 via the switch 15 and the connection line 13. The oxidant electrode 6 b of the second fuel cell stack 3 is connected to the oxidant electrode 6 a of the first fuel cell stack 2 and the load device 4 via the switch 16 and the connection line 14.

  The switches 15 and 16 that connect the second fuel cell stack 3 to the second fuel cell stack 2 and the load device 4 are opened when the fuel cell load is lower than a predetermined value, and the fuel cell load becomes equal to or higher than the predetermined value. The switches 15 and 16 are closed, and the second fuel cell stack 3 is connected to the load device 4.

  The first fuel cell stack 2 and the second fuel cell stack 3 are fuel cells that have the same type of membrane electrode assembly (MEA), have the same number of stacked cells, but have different effective power generation areas.

  Moreover, in FIG. 7, the high load prediction means 17 which estimates that the load with respect to the fuel cell system 1 transfers from a low load to a high load is provided. Here, the high load is a load that also generates power in the second fuel cell stack 3, and when the stack area ratio m: n of Example 1 is used, 100 × m / (m + n) [% of the total load of the fuel cell system. ] The above load.

  By having this high load predicting means 17, when a transition from a low load to a high load is predicted, a predetermined amount of fuel gas and oxidant gas are supplied to the second fuel cell stack 3 in advance. The high load predicting means 17 compares the magnitude of the current flowing to the load device 4 and the time change with the data of the current magnitude and the time change obtained in advance through experiments or the like, for example. The transition to load shall be predicted.

  According to the present embodiment, it is provided with a high load predicting means for predicting a transition from a low load to a high load, and when predicted to shift to a high load by the high load predicting means, a fuel gas having a predetermined flow rate and By supplying the oxidant gas to the stack where power generation is not performed, the fuel cell at the time of load change or deterioration of the catalyst layer due to lack of oxidant is suppressed, and the fuel cell with high durability and little deterioration due to load change Can be provided.

  While preferred embodiments have been described above, they are not intended to limit the invention. For example, the number of second fuel cell stacks is not limited to one, and two or more second fuel cell stacks may be provided, and the ratio of the effective power generation area of each second fuel cell stack to the effective power generation area of the first fuel cell stack is changed. You can also

BRIEF DESCRIPTION OF THE DRAWINGS It is a schematic block diagram explaining the structure of Example 1 of the fuel cell system based on this invention. It is a figure which shows the relationship of the stack voltage with respect to a stack current density. It is a figure explaining the connection load of the demand load in Example 1, the load for every stack, and the 2nd fuel cell stack. It is a time chart which shows the relationship between the load fluctuation | variation and unit cell voltage in the conventional structure. 3 is a time chart showing the relationship between load fluctuation and unit cell voltage in Example 1. It is a time chart which shows the relationship between the load fluctuation | variation in Example 2, and oxidant gas flow volume. It is a schematic block diagram explaining the structure of Example 3 of the fuel cell system which concerns on this invention. It is a time chart explaining the relation between load fluctuation and supply gas in Example 3, and load fluctuation prediction.

Explanation of symbols

1: Fuel cell system 2: First fuel cell stack 3: Second fuel cell stack 4: Load devices 5a, 5b: Fuel electrodes 6a, 6b: Oxidant electrodes 7a, 7b: Fuel gas supply pipes 8a, 8b: Fuel gas Exhaust pipes 9a, 9b: Oxidant gas supply pipes 10a, 10b: Oxidant gas exhaust pipes 11, 12, 13, 14: Connection lines 15, 16: Switch 17: High load prediction means

Claims (6)

A first fuel cell stack;
At least one second fuel cell stack having a current density-voltage characteristic equivalent to that of the first fuel cell stack and having a different effective power generation area;
A switch for connecting or releasing the second fuel cell stack in parallel with or opening the first fuel cell stack in response to load fluctuations,
A fuel cell system, wherein the switch is controlled so that a unit cell voltage in each fuel cell stack falls within a predetermined range.   2. The fuel cell system according to claim 1, wherein fuel gas and oxidant gas are not supplied to a fuel cell stack that is not generating power at a low load. 3. Comprising a high load prediction means for predicting that the load of the fuel cell system shifts from a low load to a high load;
The fuel gas and the oxidant gas at a predetermined flow rate are supplied in advance to a fuel cell stack where power generation is not performed when it is predicted that the high load prediction unit will shift to a high load. The fuel cell system according to claim 2.   The effective power generation area ratio between the first fuel cell stack and the second fuel cell stack is set to a value that minimizes the frequency of passing a predetermined unit cell voltage when a required load is extracted from each fuel cell stack. The fuel cell system according to any one of claims 1 to 3, wherein the fuel cell system is set.   The fuel cell system according to claim 4, wherein the predetermined unit cell voltage is about 0.85 [V].   2. The fuel cell system according to claim 1, wherein a gas having a high relative humidity is supplied or enclosed in a fuel cell stack in which power generation is not performed.

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