JP2008041624A - Fuel cell system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To change output limitation temperature of a fuel cell stack corresponding to degradation degree of the fuel cell stack, and operate in suitable operation temperature and humidification state corresponding to the fuel cell stack performance degradation. <P>SOLUTION: A collector board of an anode side of the fuel cell stack 2 is separated into an anode outlet collector board 2c at an anode outlet part and an anode collector board of other parts. An anode outlet local current sensor 13 measures electric current flowing through the anode outlet collector board 2c, a stack current sensor 14 measures electric current flowing through an anode collector board 2b. A control device 17 calculates stack current density and anode outlet local current density from the measured values of the current sensors 13, 14, when the difference between the stack current density and the anode outlet local current density is changed, the control device set to change the output limitation temperature. Then, when fuel cell operation temperature measured at a cooling liquid temperature sensor 11 becomes the output limitation temperature or more, the device controls output of the output fuel cell stack 2. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a fuel cell system that uses a solid electrolyte membrane in a wet state, and more particularly, to a fuel cell system that can set an output limit temperature according to deterioration of a fuel cell stack to achieve an appropriate operation state.

  When the fuel cell stack is used as a power source for automobiles, the humidification system has been simplified to reduce the volume of the fuel cell system, and the humidification capacity of the fuel cell system is insufficient at the design operating point. However, if the operating conditions exceed the design operating point, for example, a temperature higher than the design operating temperature is reached, insufficient humidification occurs, and the solid polymer electrolyte membrane dries. There is a problem of being connected.

  In order to prevent perforation of the membrane, for example, when the operating temperature of the fuel cell stack reaches a predetermined temperature, the output is limited or the single cell voltage or several cells of the entire stacked cell during the fuel cell stack operation is set to 1 The degree of dryness and wetness is determined based on the measurement value for each block or the local current measurement value in the power generation surface, and control is performed so as to always maintain an appropriate wet state.

  Many methods for maintaining the fuel cell stack in a proper wet state have been disclosed. For example, there are conventional examples shown below.

  According to Patent Document 1, the water balance is calculated by subtracting the moisture supplied to the fuel cell together with the oxidant gas, the moisture generated by power generation, and the moisture taken out from the fuel cell by the exhaust gas. A method of controlling to a certain range is disclosed.

Further, according to Patent Document 2, a method of measuring a local current of a portion that is easily dried in a fuel cell power generation surface and determining a dryness based on a comparison result between the measured local current value and a predetermined value is shown. Yes.
JP 2004-119052 (5th page, FIG. 2) Japanese Patent Laying-Open No. 2004-1000095 (page 10, FIG. 12)

  In the invention of Patent Document 1, a method for setting an appropriate control range after deterioration of the fuel cell is not shown, and no control method according to the degree of deterioration is shown.

  The invention of Patent Document 2 does not consider that the local current value deteriorates due to repeated operation of the fuel cell stack, and the local current value in a proper wet state changes, so depending on the progress of deterioration, There is a possibility of misjudging dry and wet.

  Accordingly, an object of the present invention is to change the output limit temperature of the fuel cell stack according to the degree of deterioration of the fuel cell stack, and to operate at an appropriate operating temperature and wet state according to the deterioration of the fuel cell stack performance. .

  In order to achieve the above-mentioned object, the present invention is formed by laminating a plurality of single cells having a structure in which a wet polymer electrolyte membrane is sandwiched between an anode to which a fuel gas is supplied and a cathode to which an oxidant gas is supplied. A fuel cell stack; operating temperature measuring means for measuring the operating temperature of the fuel cell stack; and output limiting means for limiting the output of the fuel cell stack when the fuel cell stack operating temperature is equal to or higher than a predetermined output limiting temperature. In the fuel cell system provided, the anode outlet local current measuring means for measuring the local current of the anode outlet portion of at least one cell of the cells constituting the fuel cell stack, and the anode outlet portion based on the measured local current An arithmetic means for calculating a local current density, a stack current measuring means for measuring the fuel cell stack current, and a measured scan Calculating means for calculating a stack current density based on a stack current, and the output limiting means outputs the output of the fuel cell stack based on a comparison result between the stack current density and the anode outlet local current density. The gist is to change the temperature limit.

  The inventors of the present invention have obtained the following two findings in research on solid polymer fuel cells. The first finding is that, as a result of repeated starting and stopping of the polymer electrolyte fuel cell, the cathode-side catalyst layer facing the anode outlet part deteriorates, resulting in power generation performance at the anode outlet part, that is, the anode outlet part. Current density is lower than other parts.

  The second finding is that the anode outlet site is in the cell plane regardless of the shape of the gas flow path in the fuel cell, whether the flow direction of the anode gas and the flow direction of the cathode gas are a counter flow or a parallel flow. This is the site where drying is most likely to occur.

  In the case where the gas flow is a counter flow, when hydrogen ions move through the electrolyte membrane from the anode to the cathode, an electroosmosis phenomenon in which moisture moves along with the hydrogen ions causes the anode gas from the anode inlet to the anode outlet. In addition to the gradual decrease in the amount of water, the anode outlet portion is close to the cathode inlet portion, so it is considered that moisture is rapidly diffused and lost to the cathode side.

  When the gas flow is a parallel flow, due to the electroosmosis phenomenon, the amount of water in the cathode gas gradually decreases from the anode inlet to the anode outlet. In addition, the anode outlet portion is a portion having the highest coolant temperature. Therefore, it is considered that the saturated water vapor pressure is high and the water easily evaporates.

  Based on the above knowledge, in the present invention, the local current and the stack current of the anode outlet part are measured, the anode outlet part local current density and the stack current density are calculated, and the comparison result between the anode outlet local current density and the stack current density is calculated. Based on the above, by changing the output limit temperature of the fuel cell stack, the wetness degree of the anode outlet part that is most likely to dry is properly maintained, and the deterioration of the output performance is suppressed even if the fuel cell stack is deteriorated be able to.

  According to the present invention, after the progress of deterioration of the fuel cell stack, when the difference or ratio between the stack current density and the anode outlet local current density is larger than that at the initial performance, the output limit temperature is set higher than at the initial performance. As a result, it is possible to reduce the frequency of the output restriction depending on the fuel cell temperature, and it is possible to suppress a decrease in output performance despite the deterioration of the fuel cell stack.

  Moreover, it can drive | operate with the output limitation temperature at the time of initial performance, and can prevent that an anode exit site | part becomes a state with excessive water | moisture content.

  With reference to FIG. 1, the structure of the fuel cell system which concerns on embodiment of this invention is demonstrated. Although not particularly limited, the present embodiment is a fuel cell system suitable for a fuel cell vehicle.

  In FIG. 1, a fuel cell system 1 according to this embodiment includes a plurality of unit cells each having a solid polymer electrolyte sandwiched between an anode supplied with a fuel gas containing hydrogen and a cathode supplied with an oxidant gas. The fuel cell stack 2 configured by the above is provided.

  The fuel cell system 1 also includes a fuel gas supply system that supplies fuel gas to the anode, an oxidant gas supply system that supplies oxidant gas to the cathode, and a cooling system that supplies coolant to the fuel cell stack 1. ing.

  The fuel gas supply system includes a hydrogen tank 3 that stores high-pressure hydrogen gas, a hydrogen pressure adjustment valve 4 that reduces the pressure of the high-pressure hydrogen gas to the operating pressure of the fuel cell, and an anode inlet of the fuel cell stack 2 by humidifying hydrogen. A hydrogen humidifier 5 to be supplied to the fuel cell, a hydrogen circulation pump 6 for circulating the anode off-gas containing unreacted hydrogen discharged from the anode outlet of the fuel cell stack 2 to the upstream side of the hydrogen humidifier 5, and an anode off-gas to the outside of the system And a purge valve 18. In addition to the hydrogen tank, the hydrogen source of the fuel gas supply system is one that generates hydrogen by a fuel reforming reaction from hydrocarbon-based raw fuels such as propane gas, city gas, gasoline, alcohol, etc. Hydrogen or the like obtained by electrolyzing water with electric power can be used.

  The oxidant gas supply system includes a compressor 7 that takes in and compresses air, an air humidifier 8 that humidifies and supplies the compressed air to the cathode inlet of the fuel cell stack 2, and air pressure at the cathode outlet of the fuel cell stack 2. And an air pressure adjusting valve 9 for adjusting the air pressure.

  The coolant supply system supplies, for example, an antifreeze mixed with a freezing point depressant such as ethylene glycol and water to the fuel cell stack 2, and the coolant that pumps the coolant to the coolant inlet of the fuel cell stack 2. A pump 10, a coolant temperature sensor 11 that measures the coolant temperature at the coolant outlet of the fuel cell stack 2, a radiator 12 that releases the heat of the coolant to the outside of the system, and a cooling fan (not shown) are provided. The coolant temperature sensor 11 is an operating temperature measuring unit that measures the operating temperature of the fuel cell stack 2.

  The cathode current collector plate 2a of the fuel cell stack 2 is connected to a DC / DC converter 15 that extracts power from the fuel cell stack. The anode current collector plate of the fuel cell stack includes an anode current collector plate 2b that is a current collector plate at a portion other than the anode outlet portion, and an anode outlet current collector plate for measuring the local current at the anode outlet portion in the cell plane. It is divided into 2c. By dividing the anode current collector plate in this way, the influence of the current flowing in the direction of the current collector plate surface can be excluded, and the local current at the anode outlet can be accurately measured. The anode outlet current collector 2 c is connected to the DC / DC converter 15 via the anode outlet local current sensor 13, and the anode current collector 2 b is connected to the DC / DC converter 15 via the stack current sensor 14. .

  The DC / DC converter 15 converts the power generation voltage of the fuel cell stack 2 that fluctuates depending on the operating conditions and the magnitude of the load current into a constant voltage and supplies it to the load device 16. Further, the DC / DC converter 15 has an output limiting function for limiting the electric power extracted from the fuel cell stack 2 in accordance with an output limiting instruction from the control device 17.

  The control device 17 that controls the entire fuel cell system 1 includes an output unit that limits the output of the fuel cell stack 2 based on a comparison between the calculation means for calculating the current density and the operating temperature of the fuel cell stack 2 and the output limit temperature. It also serves as a limiting means.

  The control device 17 includes a stack current density calculation unit 20 that calculates the stack current density based on the measurement value of the stack current sensor 14, and an anode that calculates the anode outlet local current density based on the measurement value of the anode outlet local current sensor 13. Based on the comparison result by the outlet local current density calculation unit 21, the current density comparison unit 22 that compares the stack current density with the anode outlet local current density, and the comparison result by the current density comparison unit 22, the output limit temperature of the fuel cell stack 2 is determined. The output limit temperature setting unit 23 to be changed, the temperature comparison unit 24 that compares the fuel cell operating temperature measured by the coolant temperature sensor 11 with the output limit temperature, and the temperature comparison unit 24 based on the comparison result, the fuel cell stack 2 And an output limiting unit 25 for limiting the output.

  In the present embodiment, the control device 17 is composed of a microprocessor having a CPU, a program ROM, a working RAM, and an input / output interface.

Next, changes in the in-cell power generation performance distribution after deterioration of the fuel cell and changes in the dryout resistance will be described with reference to FIG. The horizontal axis in FIG. 2 indicates the position in the cell plane, with the left end being the anode inlet portion and the right end being the anode outlet portion. The vertical axis indicates the current density [A / cm ² ] at the position in each cell plane.

In the fuel cell stack, by repeatedly starting and stopping, the catalyst layer on the cathode side at the position corresponding to the anode flow path outlet in the cell surface deteriorates, and as a result, the power generation performance deteriorates at the anode outlet portion. For example, FIG. 2 shows the current density distribution during power generation of 1 [A / cm ² ] for the entire stack, but after the power generation performance deteriorates, the current density at the anode outlet portion decreases and the cell center Current density tends to increase.

  When the cathode flow direction and the coolant flow direction are parallel flows, the anode outlet tends to dry out regardless of whether the anode flow direction is opposite to the cathode flow direction or parallel flow. This is the beginning part. The reason for this is that when the anode flow direction and the cathode flow direction are counterflows, moisture in the fuel gas is gradually lost by electroosmosis from the anode inlet to the anode outlet, and the anode outlet part becomes the cathode inlet part. It is considered that water is rapidly diffused and lost to the cathode side.

  In addition, when the anode flow direction and the cathode flow direction are parallel flows, moisture in the fuel gas is lost by electroosmosis from the anode inlet to the anode outlet in the same way as in the counter flow, and at the anode outlet. This is probably because the coolant temperature is the highest.

  When the above-mentioned deterioration occurs when the fuel cell system is used for a long period of time, the local current density at the anode outlet where dryout begins decreases, so the amount of local electroosmotic water at the anode outlet decreases, resulting in insufficient water in the anode catalyst layer. It becomes difficult to fall. On the other hand, in the central part of the cell surface, the amount of power generation increases due to the reduced power generation at the anode outlet, but the central part is sufficiently wet compared to the anode outlet and anode inlet part, so there is no water shortage, As a result, the dryout resistance is improved as a whole cell. Conversely, when the current density increases at the anode outlet, the amount of electroosmotic water at the site where the amount of water is originally low increases, so that the anode catalyst layer is excessively dried and dryout is likely to occur.

  Therefore, when the anode outlet local current density is lowered, if the output limit temperature remains the same as compared with the initial state where the fuel cell stack is not deteriorated, the operation is performed in an excessively wet state. It will drive | operate in the state with high wetness. Therefore, by increasing the output limit temperature according to the degree of decrease in the initial performance of the anode outlet current density, it becomes possible to always operate at an appropriate output limit temperature, the cell voltage decreases after deterioration, Even when the output that can be taken out decreases, the output performance can be prevented from decreasing due to the increase in the output restriction temperature. In particular, in the fuel cell vehicle, it is possible to minimize the decrease in traveling speed during heat-resistant climbing.

  On the other hand, when the local current density at the anode outlet increases, the output limiting temperature is lowered as compared with the initial stage, so that dryout can be prevented and the reliability of the fuel cell system is improved.

  Next, processing contents of the control device 17 in the present embodiment will be described with reference to the control flowchart of FIG. This control flowchart is called and executed at regular intervals from the main routine of the control device 17.

  In FIG. 3, first, in step (hereinafter abbreviated as “S”) 10, the detected value Ia of the anode outlet local current sensor 13 and the detected value Is of the stack current sensor 14 are read into the control device 17. Next, in S12, the anode outlet local current density da = Ia / Sa and the stack current density ds = Is / Ss are calculated using the anode outlet local area Sa and the stack area Ss previously stored in the control device 17.

  Next, in S14, the anode outlet local current density da is compared with the stack current density ds. This comparison includes a method for obtaining a difference between the two and a method for obtaining a ratio between the two. When obtaining the difference in current density, the current density difference Di = ds-da is calculated. When obtaining the current density ratio, the current density ratio Ri = ds / da is calculated.

  Next, in S16, it is determined whether or not the current density comparison calculation value has changed from the previous value. When the comparison calculation value is the current density difference Di, Di is compared with its previous value Dio. When the comparison calculation value is the current density ratio Ri, Ri is compared with the previous value Rio. If the comparison calculation value has changed from the previous value (if Di ≠ Dio or Ri ≠ Rio), the process proceeds to S18.

If the comparison calculation value has not changed from the previous value (if Di = Dio or Ri = Rio), the process proceeds to S30.

  Here, in actual control, the detected values of the anode outlet local current sensor 13, the stack current sensor 14, and the coolant temperature sensor 11 include measurement errors, and an extremely small change in the output limit temperature is as follows. Since there is no meaning in terms of control, the determination whether or not the current density comparison calculation value in S16 has changed from the previous value is | Di-Dio |> ε1 (or | Ri-Rio |> ε2). Judgment by whether or not. Here, ε1 (ε2) is a small positive number, and it is assumed that the change in measurement accuracy and output limit temperature of each sensor is appropriately set according to a value indicating a significant difference in control.

  In S18, the previous value Dio (Rio) of the comparison calculation value is replaced with the current comparison calculation value Di (Ri) to obtain the previous value of the comparison calculation value when this control routine is called next time. Next, in S20, the output limit temperature Tlim of the fuel cell stack 2 is calculated from the comparison calculation value Dio (Rio) with reference to a control map or the like, and the output limit temperature is changed based on the calculation result.

In S30, the previous value Dio (Rio) of the comparison calculation value is replaced with the current comparison calculation value Di (Ri), and the process returns to the main routine as the previous value of the comparison calculation value when this control routine is called next time. FIG. 4A is an example of a control map for obtaining the output limit temperature Tlim from Di, which is the difference between the stack current density and the anode outlet local current density. FIG. 4B is an example of a control map for obtaining the output limiting temperature Tlim from Ri, which is the ratio of the stack current density to the anode outlet local current density. In either case, if the value of the comparison calculation value of the current density increases, the output limit temperature is changed and set. In addition, when the value of the comparison calculation value becomes smaller, the output limit temperature is changed and set.

  4 (a) and 4 (b) show that the deterioration degree of the catalyst layer on the cathode side located at the anode channel outlet of the test fuel cell stack is determined by the accelerated deterioration test in which the start and stop of the test fuel cell system is repeated. Change the output limit temperature to obtain the upper limit temperature at which the anode outlet does not dry out or maintain proper wetness for each different value (or ratio) of the stack current density and the anode outlet local current density. It is what. In this test fuel cell stack, for example, an electrode for measuring the AC impedance of the membrane electrode assembly is provided, and it is possible to determine whether the wetness is appropriate or the dry-out based on the measured AC impedance.

  In S20, after calculating and setting the output limit temperature Tlim, the process proceeds to S22, and the detected value T of the coolant temperature sensor 11 is read into the control device 17 as the operating temperature of the fuel cell stack 2. Next, in S24, it is determined whether or not the fuel cell operating temperature T is equal to or higher than the output limit temperature Tlim. If it is determined in S24 that the fuel cell operating temperature T is equal to or higher than the output limit temperature Tlim, the process proceeds to S26, the fuel cell output is limited, and the process returns to the main routine. As described above, according to this embodiment, when the difference (or ratio) between the stack current density and the anode outlet local current density becomes large after the progress of deterioration of the fuel cell stack, the output limit temperature is set higher than that at the initial performance. As a result, it is possible to reduce the frequency of the output restriction, and the output performance is improved. Moreover, it can drive | operate with the output limitation temperature at the time of initial performance, and can prevent generating electric power in an unnecessarily wet state. Further, according to this embodiment, when the difference (ratio) between the stack current density and the anode outlet local current density is small, the output limiting temperature can be appropriately reduced, and the electrolyte membrane perforated due to excessive drying. And the reliability of the fuel cell stack can be improved.

  If it is determined in S24 that the fuel cell operation temperature T is lower than the output limit temperature Tlim, the process proceeds to S28, and the process returns to the main routine without limiting the fuel cell output.

  Here, the fuel cell output limitation of S26 is achieved by limiting the target extraction current or target extraction power that the DC / DC converter 15 extracts from the fuel cell stack 2, for example.

It is a block diagram explaining the structure of the fuel cell system which concerns on this invention. It is a figure explaining the change of the current density distribution in a cell produced before and after the deterioration by the repetition of starting and stopping of a fuel cell. 3 is a control flowchart for explaining output limitation due to fuel cell temperature in the fuel cell system of the present invention. (A) Example of control map showing relationship between stack current density and anode outlet local current density (difference) and output limit temperature, (b) Comparison value (ratio) of stack current density and anode outlet local current density and output It is an example of the control map which shows the relationship of a limit temperature.

Explanation of symbols

1: Fuel cell system 2: Fuel cell stack 2a: Cathode collector plate 2b: Anode collector plate 2c: Anode outlet collector plate 3: Hydrogen tank 4: Hydrogen pressure regulating valve 5: Hydrogen humidifier 6: Hydrogen circulation pump 7 : Compressor 8: Air humidifier 9: Air pressure adjustment valve 10: Coolant circulation pump 11: Coolant temperature sensor 12: Radiator 13: Anode outlet local current sensor 14: Stack current sensor 15: DC / DC converter 16: Load device 17: Controller 18: Purge valve 20: Stack current density calculation unit 21: Anode outlet local current density calculation unit 22: Current density comparison unit 23: Output limit temperature setting unit 24: Temperature comparison unit 25: Output limit unit

Claims (10)

A fuel cell stack in which a plurality of single cells having a structure in which a wet polymer electrolyte membrane is sandwiched between an anode to which a fuel gas is supplied and a cathode to which an oxidant gas is supplied, and an operating temperature of the fuel cell stack A fuel cell system comprising: an operating temperature measuring means for measuring the fuel cell stack; and an output limiting means for limiting the output of the fuel cell stack when the fuel cell stack operating temperature is equal to or higher than a predetermined output limiting temperature.
An anode outlet local current measuring means for measuring a local current at an anode outlet portion of at least one of the cells constituting the fuel cell stack;
A computing means for computing the local current density at the anode outlet site based on the measured local current;
Stack current measuring means for measuring the fuel cell stack current;
Computing means for calculating the stack current density based on the measured stack current,
The fuel cell system, wherein the power limiting means changes a power limiting temperature of the fuel cell stack based on a comparison result between the stack current density and the anode outlet local current density.   2. The fuel cell system according to claim 1, wherein an output limit temperature of the fuel cell stack is changed based on a difference between the stack current density and the anode outlet local current density.   When the difference between the stack current density and the anode outlet local current density is larger than the difference between the stack current density and the anode outlet local current density before deterioration of the fuel cell, the output limit temperature is increased. The fuel cell system according to claim 2, characterized in that:   When the difference between the stack current density and the anode outlet local current density is smaller than the difference between the stack current density and the anode outlet local current density before the deterioration of the fuel cell, the output restriction temperature is reduced. The fuel cell system according to claim 2, characterized in that:   2. The fuel cell system according to claim 1, wherein an output restriction temperature of the fuel cell stack is changed based on a ratio between the stack current density and the anode outlet local current density.   When the ratio between the stack current density and the anode outlet local current density is larger than the ratio between the stack current density and the anode outlet local current density before deterioration of the fuel cell, the output limit temperature is increased. 6. The fuel cell system according to claim 5, wherein:   When the ratio between the stack current density and the anode outlet local current density is smaller than the ratio between the stack current density and the anode outlet local current density before deterioration of the fuel cell, the output limit temperature is reduced. 6. The fuel cell system according to claim 5, wherein:   The output limit temperature is an upper limit that can maintain the wet state of the anode outlet, which is obtained for each different value of the difference between the stack current density and the anode outlet local current density by changing the degree of deterioration of the test fuel cell stack in advance. The fuel cell system according to any one of claims 2 to 4, wherein the fuel cell system is a temperature.   The anode outlet local current measuring means divides the anode current collector plate of the fuel cell stack into a current collector plate in the vicinity of the anode outlet and a current collector plate in the other part, and a current flowing through the current collector plate in the vicinity of the anode outlet. The fuel cell system according to claim 1, wherein the fuel cell system is measured.   2. The fuel cell system according to claim 1, wherein the operating temperature measuring means measures the temperature of the coolant outlet of the fuel cell stack.

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