JP2008047368A - Fuel cell system - Google Patents

Abstract

In an operating state where the output of a fuel cell changes transiently as in a fuel cell vehicle, the dry and wet state of an electrolyte membrane is quickly and accurately determined, and appropriate operation control is performed accordingly. Be able to implement.
Local current sensors (26a, 26b) are connected to at least two portions along a gas flow direction on at least one unit cell of a fuel cell stack (1). Then, when the output from the fuel cell stack 22 is transiently increased, the local current density of the fuel cell stack 22 is measured using these local current sensors 26a and 26b, and the local current is measured. The dry and wet state of the electrolyte membrane is determined from the density change behavior, and operation control is performed according to the determination result.
[Selection] Figure 2

Description

  The present invention relates to a fuel cell system that generates power using a fuel cell stack formed by stacking a plurality of unit cells.

  When a fuel cell stack (fuel cell stack) consisting of multiple unit cells is used as a power source for automobiles, the humidification system of the solid polymer electrolyte membrane is simplified for the purpose of reducing the volume of the fuel cell system It is generally done. The humidification capacity of the fuel cell system is designed so that the solid polymer electrolyte membrane does not become insufficiently humidified at the design operating point, but reaches an operating condition exceeding the designed operating point, for example, a temperature higher than the designed operating temperature. In some cases, the polymer electrolyte membrane may be insufficiently humidified. And when humidification insufficiency occurs, there exists a problem that a solid polymer electrolyte membrane will dry, and when drying advances excessively, it will lead to the perforation of a membrane.

  On the other hand, when the operating temperature of the fuel cell stack is low, such as immediately after starting to drive an automobile, the amount of water that can be taken out from the cathode gas passage or anode gas passage in the unit cell surface is small, so the cathode gas passage and anode gas The probability of occurrence of water clogging in the channel increases. Even when the operating temperature is sufficiently high, if the load (output) extracted from the fuel cell stack is high, the amount of water generated in the fuel cell stack increases accordingly, so that the solid polymer electrolyte membrane is wetted. It becomes an excessive state and water clogging is likely to occur in the cathode gas passage and the anode gas passage. When water is clogged in the cathode gas passage and the anode gas passage, there is a problem that the power generation efficiency of the fuel cell stack is lowered.

  Therefore, in order to prevent the perforation of the electrolyte membrane due to such excessive drying and the decrease in power generation performance due to water clogging, for example, when the operating temperature of the fuel cell stack reaches a predetermined temperature, Measures are taken such as limiting the output to be taken out from the battery stack, or determining the degree of dryness and wetness inside the fuel cell stack, and performing control so that the proper wet state is always maintained.

A number of methods for determining the dry and wet state of the electrolyte membrane inside the fuel cell stack have been disclosed so far. For example, there are the following documents. Patent Document 1 discloses a method of determining that an electrolyte membrane is dry when a cell voltage is less than a certain threshold value and a vibration is less than a predetermined range by looking at a change in voltage of a unit cell over time. . Patent Document 2 discloses a method for determining dryness of an electrolyte membrane based on a local current value at a specific site.
JP 2004-127915 A JP 2005-190997 A

  In the invention of Patent Document 1, drying is determined by utilizing the cell voltage oscillation phenomenon. In order to accurately determine the dry and wet state of the electrolyte membrane by this method, the output of the fuel cell stack is a predetermined value or more. And a constant output state is required to be continued. However, when considering how to use a fuel cell in a fuel cell vehicle, the condition that the output is constant above a certain output is, for example, when the vehicle continues uphill at a certain speed or continuously at the maximum speed. Appears only in limited driving conditions, such as uncertain conditions. For this reason, when considering how to use in a vehicle, it is difficult to accurately determine the dry and wet state with this method. In fact, if the cell voltage is below a certain threshold, it is determined to be dry. Even when it is determined that water is excessively wet and water, that is, avoidance control is to be performed, it may be erroneously determined to be dry and control of avoidance of drying may be performed.

  In the invention of Patent Document 2, drying is determined based on the local current density, but this determination method is effective when the current density is high, that is, when a high output is taken out. It is. For this reason, similarly to the invention of the above-mentioned Patent Document 1, considering the usage in a fuel cell vehicle, the driving conditions in which this method is effective are limited.

  Further, when the conventional examples disclosed in Patent Documents 1 and 2 are applied to a fuel cell vehicle, it is necessary to add and operate a safety factor at each threshold in order to avoid excessive drying. As a result, the operable temperature range or the like has to be narrower than the original ability, and there is a problem that the utilization efficiency of the fuel cell system is reduced due to excessive output limitation.

  In addition, in vehicles with a high frequency of operation that transiently increases or decreases the output of the fuel cell, it is possible to determine dry and wet when the output changes transiently, especially when it is determined to be dry It is desirable to be able to take a method that can avoid dry conditions during transient operation.

  The present invention was devised in view of the above-described conventional situation, and the electrolyte membrane is dried in an operating state in which the output of the fuel cell changes transiently as applied to a fuel cell vehicle. An object of the present invention is to provide a fuel cell system capable of quickly and accurately determining a wet state and performing appropriate operation control according to the wet state.

  In the present invention, local current measuring means is connected to at least two portions along the gas flow direction of the unit cell constituting the fuel cell stack, and when the output from the fuel cell stack is increased, these local Solving the above-mentioned problems by determining the dry and wet state of the electrolyte membrane according to the local current density change for each part measured by the current measuring means, and controlling the system operation according to the determination result To do.

  According to the fuel cell system of the present invention, in an operation state in which the output of the fuel cell changes transiently, the dry and wet state of the electrolyte membrane is quickly and accurately determined, and appropriate operation control is performed accordingly. be able to.

  Hereinafter, specific embodiments of a fuel cell system to which the present invention is applied will be described in detail with reference to the drawings.

  First, a schematic configuration of the entire fuel cell system will be briefly described with reference to FIG. The fuel cell system includes a fuel cell stack 1 that generates power, a hydrogen supply system that supplies hydrogen gas as a fuel gas to the fuel cell stack 1, and an air supply system that supplies air as an oxidant gas to the fuel cell stack 1 And as a main component.

  As the fuel cell stack 1, for example, a solid polymer fuel cell using a solid polymer membrane as an electrolyte is used. The solid polymer electrolyte membrane is made of an ion (proton) conductive polymer membrane such as a fluororesin ion exchange membrane, and has a property of functioning as an ion conductive electrolyte when saturated with water. A polymer electrolyte fuel cell has a power generation cell in which a membrane electrode structure in which an electrode catalyst layer is formed on both sides of a solid polymer electrolyte membrane is sandwiched between a pair of separators as a unit, and these unit cells are stacked in multiple stages. A stack structure. Then, hydrogen gas from the hydrogen supply system is supplied to the anode electrode side of each unit cell, and air from the air supply system is supplied to the cathode electrode side, so that power is generated by an electrode reaction between the anode and cathode of each unit cell. Is generated. For the electrode catalyst layer of each unit cell, for example, a catalyst in which a catalyst such as platinum fine particles is supported on a catalyst carrier such as carbon black is used.

  The hydrogen supply system includes, for example, a high-pressure hydrogen tank 2 that stores high-pressure hydrogen gas, the hydrogen gas taken out from the high-pressure hydrogen tank 2 is adjusted to a desired pressure by a pressure reducing valve or a hydrogen pressure regulating valve, and then a fuel cell. The stack 1 is supplied to the anode electrode side. The configuration of the hydrogen supply system is not limited to the above example, and any conventionally known configuration can be employed in this type of fuel cell system.

  The air supply system includes, for example, an air compressor 3 that sucks in outside air, compresses and discharges the air, and the air compressed by the air compressor 3 is sufficiently humidified by a humidifier 4 or the like, and then the fuel cell stack. 1 is supplied to the cathode electrode side. The configuration of the air supply system is not limited to the above example, and any conventionally known configuration can be employed in this type of fuel cell system. In addition, in a fuel cell system mounted as a power source in a fuel cell vehicle, in addition to the above hydrogen supply system and air supply system, a cooling system for maintaining the fuel cell stack 1 at an appropriate operating temperature is provided as appropriate. As the cooling system and the like, any conventionally known configuration can be adopted in this type of fuel cell system.

  In the fuel cell system having the above-described configuration, the operation of each part in the system is comprehensively controlled by the control unit 10. That is, the control unit 10 monitors the detection values of various sensors (pressure sensor, flow sensor, temperature sensor, voltage sensor, current sensor, etc.) provided in the system to grasp the operation status of the system and Understands the amount of power generation required for the fuel cell system according to the operating conditions, etc., and oversees the operation of the hydrogen supply system and the air supply system so that the fuel cell stack 1 can provide an output that matches the required power generation amount Control. In particular, in the fuel cell system of the present embodiment, the dry and wet state of the solid polymer electrolyte membrane of the fuel cell stack 1 is determined in an operation state in which the output of the fuel cell stack 1 changes transiently, and the determination result is determined accordingly. The control unit 10 executes various controls for preventing overdrying of the solid polymer electrolyte membrane and preventing water clogging. The control unit 10 also performs the determination of the dry and wet state and the operation control according to the determination result. Is done.

  Next, a specific configuration example of the fuel cell stack 1 used in the fuel cell system of the present embodiment will be described with reference to FIG. As shown in FIG. 2A, the fuel cell stack 1 used in the fuel cell system of the present embodiment has a plurality of unit cells 21a, 21b,... Each having an electrolyte membrane, a cathode electrode, and an anode electrode. The current collector plates 23a and 23b are connected to both end faces of the fuel cell stack 22, and these are clamped and held by the clamping structures 24a and 24b.

  The current collector plates 23 a and 23 b have a function of collecting current generated by the fuel cell stack 22. In the fuel cell stack 1 used in the fuel cell system of the present embodiment, in order to measure the local current density of the fuel cell stack 22, one of the current collector plates 23a and 23b is insulated. It divides | segments into the several site | part using the thing 25a, 25b. Specifically, for example, as shown in FIGS. 2A and 2B, both end portions along the flow direction of the oxidant gas on the unit cell of one current collector plate 23a, that is, the oxidant gas. And a portion near the cathode electrode inlet, where the air flows into the cathode electrode, and a portion near the cathode electrode outlet, where the air as the oxidant gas flows out from the cathode electrode. It is electrically divided by 25b. A bus bar 27 is connected to each part of the current collector plate 23a, and by collecting the bus bars 27 connected to each part into one, the current collected by the entire current collector plate 23a is taken out. . A stack current sensor 28 is connected to the bus bar 27 combined into one, and the stack current sensor 28 measures the entire current generated by the fuel cell stack 22. Further, local current sensors 26a and 26b are connected to a portion near the cathode electrode inlet and a portion near the cathode electrode outlet of the current collector plate 23a electrically divided by the insulators 25a and 25b, respectively. , 26b, the local current extracted from each divided part is measured. Then, the current density of each divided part, that is, the local current density of the fuel cell stack 22 is measured from the area of each part that is grasped in advance.

  As described above, in the fuel cell stack 1 used in the fuel cell system according to the present embodiment, the portion near the cathode electrode inlet of the current collector plate 23a that collects the current generated by the fuel cell stack 22 and the cathode electrode outlet. The fuel cell laminate is constructed with a relatively simple configuration by electrically dividing the neighboring parts by the insulators 25a and 25b and measuring the local currents of the two divided parts by the local current sensors 26a and 26b. The local current density of 22 can be measured, but the present invention is not limited to this, and the portion where the current flows in the form of the flow direction of the gas on one or more unit cells. May be divided into a plurality of parts in the unit cell surface, and the local current density flowing through a specific part in the unit cell surface may be measured using the local current sensors 26a and 26b. Moreover, you may make it provide a local electric current with the same structure using the other collector plate 23b connected to the opposite end surface of the fuel cell laminated body 22 instead of the collector plate 23a. . Of course, local current density may be obtained at three or more sites.

  In addition, in FIGS. 2A and 2B, a configuration example in the case of taking a so-called straight line or a flow path shape in the cell according to the so-called straight line is shown, but the present invention is not limited to this. For example, as shown in FIG. 3 (a) and FIG. 3 (b), the present invention can also be applied to a case where a so-called serpentine shape or a flow path shape in a cell according to the shape is adopted. Also in this case, the part near the cathode electrode inlet and the part near the cathode electrode outlet of the current collector plate 23a are electrically divided using the insulators 25a and 25b. And the local current taken out from each divided | segmented each site | part is measured by the local current sensors 26a and 26b, and a local current density is calculated | required from the area of each site | part. Thereby, the local current density of the fuel cell stack 22 can be measured with a relatively simple configuration.

  In the fuel cell system according to the present embodiment, when the output from the fuel cell stack 22 is increased transiently, the fuel according to the dry and wet state of the electrolyte membrane of each single cell constituting the fuel cell stack 22 Focusing on the fact that a characteristic appears in the local current density change of the battery stack 22, from the local current density change when the output from the fuel cell stack 22 is transiently increased, the electrolyte membrane The dry and wet state is determined. Here, the state of local current density change when the output of the fuel cell stack 22 is transiently increased will be described with reference to FIGS. 4 and 5. FIG. 4 and 5, [1], [2], [3], and [4] indicate the order of elapsed time from when the output of the fuel cell stack 22 starts to change.

  First, with reference to FIG. 4A and FIG. 4B, a typical behavior of the local current at the time of transition when the electrolyte membrane is in a dry state will be described. When the electrolyte membrane is in a dry state, the rate of increase in local current density differs at different sites along the gas flow direction of the fuel cell stack 1. Specifically, when the output of the fuel cell stack 22 transiently increases when the electrolyte membrane is in a dry state, the stack current increases from [1] to [4], and the local current near the cathode electrode outlet and The local current in the vicinity of the cathode electrode entrance also increases corresponding to the increase in the stack current. However, comparing the local current increase rate near the cathode electrode exit and the local current increase rate near the cathode electrode entrance, the current increase rate is lower at the cathode electrode entrance than at the cathode electrode exit. It becomes difficult to increase.

  Next, with reference to FIG. 5A and FIG. 5B, a typical behavior of the local current at the time of transient when the electrolyte membrane is in an excessively wet state (during flooding) will be described. Even when the electrolyte membrane is in an excessively wet state, the rate of increase in the local current density differs at different sites along the gas flow direction of the fuel cell stack 2. Specifically, when the output of the fuel cell stack 22 is transiently increased in the wet excess state, the stack current increases from [1] to [4], and the local current near the cathode electrode outlet and the cathode electrode are increased. The local current near the inlet also increases corresponding to the increase in stack current. However, comparing the local current increase rate near the cathode electrode exit and the local current increase rate near the cathode electrode entrance, the current increase rate is lower at the cathode electrode exit than at the cathode electrode entrance. It becomes difficult to increase.

  Thus, during drying, the local current density on the cathode outlet side increases relatively quickly, and the local current density on the cathode inlet side increases slowly. In contrast, during flooding, the local current density on the cathode inlet side increases relatively quickly, and the local current density on the cathode outlet side increases slowly. By utilizing such a feature, when the output of the fuel cell stack 2 is transiently increased from a low load state to a high load state, the vicinity of the cathode inlet and the cathode from when the stack current starts to increase until a predetermined time is obtained. By comparing the rate of increase of the local current density at least at two locations in the vicinity of the outlet, it is possible to accurately determine the dry and wet state of the electrolyte membrane at an earlier stage than in the conventional method. In addition, since it can be determined at a relatively early stage (for example, stage [2]) when the current is transiently increased, during the subsequent transient operation, that is, [2] to [4] It is also possible to take avoidance measures such as avoiding a voltage drop by limiting the rate of increase of the transient current during the phase. Furthermore, by limiting the current increase rate and slowing down the load change rate, it is possible to secure the time required to move the water from the high moisture content part to the low part using the water movement inside the electrolyte membrane. The degree of voltage drop due to drying can be alleviated.

  Next, with reference to FIG. 6, the mechanism of the moisture content change in the electrolyte membrane 31 when the current taken out from the fuel cell stack 1 is transiently increased when the electrolyte membrane is in a dry state will be described. The phenomenon shown in FIG. 4A and FIG. 4B described above can be explained by the change with time of the moisture content in the electrolyte membrane 31. Below, the characteristic of the moisture content change at the time of the transition in each site | part in the electrolyte membrane 31 is demonstrated.

  First, the change of the moisture content of the site | part [A] of FIG. 6 is demonstrated. Part [A] is the vicinity of the cathode electrode entrance. When the electrolyte membrane 31 is in a dry state, the moisture content of the portion [A] is low. When the output current is increased, the water content of the part [A] is low, so that the current of the part [A] is difficult to increase. This is because moisture brought in from the outside is necessary to increase the moisture content. When the moisture content of the part [A] increases after a predetermined time has elapsed, the drug water moves from the anode electrode and the moisture content increases.

  Next, the change of the moisture content of the site | part [B] of FIG. 6 is demonstrated. Part [B] is the vicinity of the cathode electrode outlet. When the electrolyte membrane 31 is in a dry state, the moisture content of the part [B] is low. When the output current is increased, water generated on the upstream side of the air (part [A] side) is carried, so that the amount humidified by the air is larger than that of the part [A]. Therefore, the moisture content rises at an early stage, and as a result, the current in the part [B] tends to increase.

  Next, the change in the moisture content of the part [C] in FIG. 6 will be described. Part [C] is the vicinity of the anode electrode outlet. When the electrolyte membrane 31 is in a dry state, the moisture content of the part [C] is low. If the output current is increased, the water content of the part [A] is low, so that the current is difficult to increase. Even so, the water content is further lowered because water flows from the site [A] to the site [C] as drug water only for the current that flows. In the reverse diffusion of water, since the water concentration difference is the driving force, there is a time delay until the water content increases due to the reverse diffusion. For this reason, it will be in the state which an electric current does not increase easily.

  Next, the change of the moisture content of the site | part [D] of FIG. 6 is demonstrated. Part [D] is the vicinity of the anode electrode entrance. When the electrolyte membrane 31 is in a dry state, the moisture content of the portion [D] is low. When the output current is increased, the moisture content on the part [B] side tends to increase, so that the moisture content tends to increase due to reverse diffusion of water. For this reason, the moisture content increases at an early stage, and as a result, the current tends to increase.

  As described above, the vicinity of the cathode electrode inlet and the vicinity of the cathode electrode outlet are places where the characteristics of drying and wetting are most likely to appear in the current density change during the transition. In addition, here, as an example for explaining the mechanism, the case of the counter flow in which the so-called anode gas and the cathode gas are opposed to each other is shown. However, even when another flow direction is selected, the flow is not changed. Accordingly, in the case where there are characteristically easy-to-dry parts and easy-to-wet parts, it is possible to use the same mechanism and the current density change behavior at that time.

  Here, in the fuel cell system of the present embodiment, a series of processing in which the control unit 10 determines the dry and wet state of the electrolyte membrane using a local current change at the time of a transient response, and performs operation control according to the determination result. The flow will be described with reference to the flowchart of FIG.

  When the flow of FIG. 7 starts, the control unit 10 first determines in step S1 whether or not the increase rate of the current density of the stack current measured by the stack current sensor 28 is equal to or greater than a predetermined value A0. Then, when the increasing rate of the stack current density is equal to or greater than the predetermined value A0, the process proceeds to step S2, and the determination of the dry and wet state of the electrolyte membrane characteristic of the present invention is performed. Here, the predetermined value A0 may be set in advance to a certain fixed value, or may take a function form in consideration of stack operation parameters such as stack temperature and pressure.

  Next, in step S2, the control unit 10 determines whether or not the increase rate of the current density of the local current in the vicinity of the cathode electrode outlet measured by the local current sensor 26b is equal to or greater than the threshold value A1. If the local current density increase rate in the vicinity of the cathode electrode outlet is greater than or equal to the threshold A1, the process proceeds to step S3, and if less than the threshold A1, the process proceeds to step S5. Here, the threshold A1 may be set in advance to a certain fixed value, or may be a function of the stack current density increase rate. Further, a function form may be taken in consideration of not only the increasing rate of the stack current density but also the stack operating parameters such as the stack temperature and pressure. Also, two values (threshold value A1-1 and threshold value A1-2) are prepared as the threshold value A1, and if the local current density increase rate in the vicinity of the cathode electrode outlet is equal to or greater than the threshold value A1-1, go to step S3. If it is less than the threshold value A1-2, the process proceeds to step S5. If it is greater than or equal to the threshold value A1-2 and less than A1-1, the process may return to step S1. At this time, the relationship between the threshold A1-1 and the threshold A1-2 is A1-1> A1-2. Thus, even when two values are prepared as the threshold A1, each threshold takes into consideration not only a constant value, a function of the stack current density increase rate, and a stack operation parameter such as a stack temperature and pressure, but also a current density increase rate. Any form of function may be used.

  If the determination in step S2 is YES and the process proceeds to step S3, the control unit 10 determines that the increase rate of the current density of the local current in the vicinity of the cathode electrode inlet measured by the local current sensor 26a is equal to or less than the threshold value A2 in step S3. It is determined whether or not. If the determination in step S3 is YES, that is, the rate of increase in local current density in the vicinity of the cathode electrode outlet is equal to or greater than the threshold A1, and the rate of increase in current density of local current in the vicinity of the cathode electrode entrance. Is less than or equal to the threshold value A2, the process proceeds to step S4, where it is determined that the electrolyte membrane 31 is in a dry state. On the other hand, if the determination in step S3 is NO, that is, if the rate of increase in the current density of the local current in the vicinity of the cathode electrode entrance is greater than or equal to the threshold value A2, the process returns to step S1. Here, the threshold value A2 may be set to a certain fixed value in advance, as with the previous threshold value A1, or may be a function of the stack current density increase rate. Further, a function form may be taken in consideration of not only the increasing rate of the stack current density but also the stack operating parameters such as the stack temperature and pressure.

  When it is determined in step S4 that the electrolyte membrane 31 is in a dry state, the control unit 10 thereafter performs various operation controls for avoiding that the dry state of the electrolyte membrane 31 proceeds to become an overdried state. . As an example of the operation control for avoiding overdrying, for example, setting the upper limit of the increase rate of the stack current to a threshold value A4 or less can be mentioned. In addition to this, the control temperature (temperature control target value) of the fuel cell stack 1 is lowered, the gas flow rate control target value is lowered to lower the target SR (stoichiometric ratio), and the output itself of the fuel cell stack 22 is restricted. It is also possible to perform operation control generally known in order to avoid an overdried state. The threshold value A4 for limiting the increase rate of the stack current may be set to a certain fixed value in advance, or may take a function form taking into consideration stack operation parameters such as stack temperature and pressure. May be.

  When the determination in step S2 is NO and the process proceeds to step S5, the control unit 10 determines that the increase rate of the current density of the local current in the vicinity of the cathode electrode inlet measured by the local current sensor 26a is greater than or equal to the threshold A3 in step S5. It is determined whether or not. If the determination in step S5 is YES, that is, the local current density increase rate in the vicinity of the cathode electrode outlet is less than the threshold A1, and the current density increase rate of the local current in the vicinity of the cathode electrode entrance. Is greater than or equal to the threshold value A3, the process proceeds to step S6, where it is determined that the electrolyte membrane 31 is in an excessively wet (flooding) state. On the other hand, if the determination in step S5 is NO, that is, if the rate of increase in the current density of the local current in the vicinity of the cathode electrode entrance is less than the threshold value A3, the process returns to step S1. Here, the threshold value A3 may be set to a certain fixed value in advance as with the previous threshold value A1, or may be a function of the stack current density increase rate. Further, a function form may be taken in consideration of not only the increasing rate of the stack current density but also the stack operating parameters such as the stack temperature and pressure. Further, the relationship between the threshold value A2 and the threshold value A3 described above is A3> A2.

  When it is determined in step S6 that the electrolyte membrane 31 is in the flooding state, the control unit 10 thereafter performs various operation controls for eliminating the flooding and avoiding water clogging in the gas flow path. As the operation control for eliminating the flooding, for example, the control temperature (temperature control target value) of the fuel cell stack 1 is increased, or the target SR (stoichiometric ratio) is increased by increasing the gas flow rate control target value. It is possible to perform generally known operation control in order to eliminate flooding.

  In addition to the above examples, the current density increase rate in the vicinity of the cathode electrode entrance is determined before the current density increase rate in the vicinity of the cathode electrode exit. Various combinations are conceivable, such as when determining the rate of increase in current density at the cathode electrode inlet vicinity and the cathode electrode outlet vicinity at the same time. In the flow of FIG. 7, as an example, the case where the increase rate of the current density in the vicinity of the cathode electrode outlet is determined first is shown, but the combination of the determination order is not limited to the example of FIG. 7. Any feasible order can be employed.

  The fuel cell system according to the present embodiment has been described above with specific examples. However, in this fuel cell system, the following functions and effects are obtained by the characteristic configuration of the present invention and the processing by the control unit 10. It is done.

  That is, in the fuel cell system of the present embodiment, the local current sensors 26a and 26b are connected to at least two parts along the flow direction of the oxidant gas on at least one unit cell of the fuel cell stack 1, and the control unit 10 shows the behavior of local current density for each part measured using these local current sensors 26a and 26b when the output from the fuel cell stack 22 of the fuel cell stack 1 is increased transiently. Therefore, the dry and wet state of the electrolyte membrane 31 is determined. Thus, even when the fuel cell system is used as a vehicle driving source, such as when the output is transiently increased or decreased frequently, the dry and wet state of the electrolyte membrane 31 can be quickly and accurately determined. It is possible to determine and perform appropriate operation control according to the determination.

  Further, in the fuel cell system of the present embodiment, the local current sensors 26a and 26b are provided at a portion in the vicinity of the cathode electrode inlet and a portion in the vicinity of the cathode electrode outlet, which are representative portions where the current change at the time of transition is characteristic. Since they are connected and the current change behavior at each of these parts is measured, the dry and wet state of the electrolyte membrane 31 can be determined very accurately.

  Further, in the fuel cell system of the present embodiment, the current collector plate 23a having a function of collecting the current generated by the fuel cell stack 22 is electrically divided into a plurality of parts by the insulators 25a and 25b. Since the local current sensors 26a and 26b are connected to the electrically divided portions of the current collector plate 23a to measure the current collected at each portion, the fuel can be measured in a relatively simple manner. It is possible to accurately measure the local current density of the battery stack 22.

  In the fuel cell system of the present embodiment, the current generated by the fuel cell stack 22 increases at an increase rate of a predetermined value or more, and is measured using the local current sensor 26b connected to the vicinity of the cathode electrode outlet. The current density increases and the current density increase rate measured using the local current sensor 26a connected to the vicinity of the cathode electrode entrance is predetermined with respect to the current increase rate generated by the fuel cell stack 22. When the ratio is less than or equal to the ratio, the control unit 10 determines that the electrolyte membrane 31 is in a dry state. Therefore, it can be quickly and accurately determined that the electrolyte membrane 31 is in a dry state. That is, the membrane electrode structure (MEA) of each unit cell of the fuel cell stack 1 has a property that the current increases from the portion having the smallest resistance when the current is taken out. This resistance varies greatly depending on the dry and wet state in the electrolyte membrane 31, and becomes smaller in a wet place and larger in a dry place. Due to the nature of this MEA, the increase in local current density is slow during transients at sites that are most likely to dry, and the increase in current density is rapid at sites that are most difficult to dry. In the fuel cell system of the present embodiment, the control unit 10 makes use of such MEA characteristics to make a dry determination based on a local current change at the time of transient. In addition, it is possible to determine drying at an early timing, and as a result, it is possible to shorten the time period in which the drying is performed, and it is possible to improve the performance in an actual driving state.

  Further, in the fuel cell system of the present embodiment, when the control unit 10 determines that the electrolyte membrane 31 is in a dry state, by providing a predetermined limit on the increase rate of the current generated by the fuel cell stack 22 Further, the dry state of the electrolyte membrane 31 proceeds to become an overdried state, and the disadvantage of damaging the electrolyte membrane 31 can be prevented. In order to wet the dry part of the electrolyte membrane 31, it is necessary to increase the reverse diffusion in the electrolyte membrane 31, but the driving source of the reverse diffusion is a moisture concentration difference. There will be a time delay until Here, in the fuel cell system of the present embodiment, since the dryness determination can be performed at the initial stage during the transition using the above-mentioned properties of the MEA, the subsequent load change speed is slowed down and the despreading is performed. Utilization of the wetness can be promoted, and the electrolyte membrane 31 can be recovered from the dry state.

  Further, in the fuel cell system of the present embodiment, when the control unit 10 determines that the electrolyte membrane 31 is in a dry state, the temperature is controlled by the temperature control target value of the fuel cell stack 22 so that the gas is supplied to the fuel cell. The amount of moisture taken out from the stack 1 can be reduced, and the amount of water generated by power generation can be reduced outside the fuel cell stack 1. As a result, the water content of the electrolyte membrane 31 can be increased, and the electrolyte membrane 31 can be prevented from being overdried.

  Further, in the fuel cell system of the present embodiment, when the control unit 10 determines that the electrolyte membrane 31 is in a dry state, the gas flow rate is reduced by reducing the target value of the gas flow rate control target value, thereby reducing the gas to the fuel. The amount of moisture taken out from the battery stack 1 can be reduced, and the amount of water generated by power generation can be reduced outside the fuel cell stack 1. As a result, the water content of the electrolyte membrane 31 can be increased, and the electrolyte membrane 31 can be prevented from being overdried.

  Further, in the fuel cell system of the present embodiment, when the control unit 10 determines that the electrolyte membrane 31 is in a dry state, by reducing the limit value of the maximum output value generated by the fuel cell stack 22, The amount of water movement accompanying proton movement in the electrolyte membrane 31 can be reduced, and the reduction in the water content of the electrolyte membrane 31 in the vicinity of the anode electrode outlet that is most likely to dry can be suppressed. Thereafter, when the water content of the electrolyte membrane 31 as a whole increases, the water content near the anode electrode outlet also increases due to reverse diffusion or the like, thereby avoiding an overdried state.

  In the fuel cell system of the present embodiment, the current generated by the fuel cell stack 22 increases at an increase rate of a predetermined value or more, and is measured using the local current sensor 26a connected to the vicinity of the cathode electrode inlet. The current density increases, and the current density increase rate measured using the local current sensor 26b connected to the vicinity of the cathode electrode outlet is predetermined with respect to the current increase rate generated by the fuel cell stack 22. When the ratio is less than or equal to the ratio, the control unit 10 determines that the electrolyte membrane 31 is in an excessively wet (flooded) state, so that it can be quickly and accurately determined that the electrolyte membrane 31 is in a flooded state. it can. That is, when the electrolyte membrane 31 is sufficiently wet, flooding is likely to occur on the cathode electrode outlet side when the amount of generated water is increased by extracting current. When flooding occurs, it becomes difficult for the gas necessary for the battery reaction to reach the catalyst part, and the local current density is unlikely to increase. As the rebound, the local current density tends to increase on the cathode electrode entrance side. In the fuel cell system according to the present embodiment, the control unit 10 makes use of such a property to determine whether the electrolyte membrane 31 is wet based on a local current change at the time of transient. It can be determined with high accuracy.

  Further, in the fuel cell system according to the present embodiment, when the control unit 10 determines that the electrolyte membrane 31 is in the flooding state, the gas flow rate increases the target stoichiometric ratio by increasing the gas flow rate control target value, so that the gas is fueled. The amount of moisture taken out from the battery stack 1 can be increased, excess moisture can be taken outside, and excess moisture in the electrolyte membrane 31 can be eliminated, so that flooding can be eliminated.

  Further, in the fuel cell system according to the present embodiment, when the control unit 10 determines that the electrolyte membrane 31 is in the flooding state, the temperature is controlled by increasing the temperature control target value of the fuel cell stack 22 so that the gas is supplied to the fuel cell. The amount of moisture taken out of the stack 1 can be increased, and the amount of excess water taken out of the fuel cell stack 1 itself can be increased. As a result, excess water in the electrolyte membrane 31 can be eliminated, and flooding can be eliminated.

  Note that the above embodiment exemplifies an application example of the present invention, and the technical scope of the present invention is not limited to the contents disclosed in the description of the above embodiment, and from these disclosures. Of course, various alternative techniques that can be easily derived are also included.

It is a figure which shows schematic structure of the whole fuel cell system. It is a figure which shows the specific structural example of the fuel cell stack used with the fuel cell system to which this invention is applied, (a) is a schematic diagram which shows the whole fuel cell stack, (b) is the structure of a current collecting plate. It is a schematic diagram shown. It is a figure which shows the other structural example of the fuel cell stack used with the fuel cell system to which this invention is applied, (a) is a schematic diagram which shows the whole fuel cell stack, (b) shows the structure of a current collecting plate. It is a schematic diagram. It is a figure explaining the typical behavior of the local current at the time of transient when the electrolyte membrane is in a dry state, the behavior of the current density on the cathode inlet side and the current on the cathode outlet side when the stack current is transiently increased It is a graph which shows the behavior of a density. It is a figure explaining the typical behavior of the local current at the time of transient when the electrolyte membrane is in an excessively wet state. The behavior of the current density on the cathode inlet side and the cathode outlet side when the stack current is increased transiently. It is a graph which shows the behavior of current density. It is a schematic diagram for demonstrating the mechanism of the transient behavior when the electrolyte membrane is drying. It is a flowchart which shows the flow of a series of processes which a control unit determines the dry / wet state of an electrolyte membrane using the local current change at the time of a transient response, and performs operation control according to the determination result.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Fuel cell stack 10 Control unit 21a, 21b, ... Unit cell 22 Fuel cell laminated body 23a, 23b Current collecting plate 25a, 25b Insulation part 26a, 26b Local current sensor 28 Stack current sensor

Claims (12)

A fuel cell system for generating power by a fuel cell laminate comprising a plurality of unit cells each having an electrolyte membrane, a cathode electrode and an anode electrode,
Local current measuring means connected to each of at least two sites along the gas flow direction on at least one unit cell and measuring a local current density of the fuel cell stack;
Determining the dry and wet state of the electrolyte membrane according to a change in local current density for each part measured by the local current measuring means when the output from the fuel cell stack is increased, and the electrolyte A fuel cell system comprising: control means for controlling system operation in accordance with a determination result of a dry and wet state of the membrane.   2. The fuel cell system according to claim 1, wherein one of the local current measuring means is connected to a portion where the current density is less likely to increase when the output is increased compared to other portions. .   The part to which one of the local current measuring means is connected is in the vicinity of the cathode electrode inlet where the gas flows into the cathode electrode, and the part to which the other one of the local current measuring means is connected. The fuel cell system according to claim 2, wherein the fuel cell system is near a cathode electrode outlet, which is a position where the gas flows out from the cathode electrode.   Current collectors that collect current generated by the fuel cell stack are connected to both end faces of the fuel cell stack, and the current collector is divided into a plurality of parts by an insulator, and the local current 4. The fuel cell system according to claim 1, wherein the measurement unit is connected to a part divided by the insulator and measures a current density collected at the part. 5.   When the current generated by the fuel cell stack increases at an increase rate of a predetermined value or more, the current density measured by the local current measuring means connected in the vicinity of the cathode electrode outlet increases, and the cathode electrode When the increase rate of the current density measured by the local current measuring means connected in the vicinity of the inlet is equal to or less than a predetermined ratio with respect to the increase rate of the current generated by the fuel cell stack, the control means includes the electrolyte 4. The fuel cell system according to claim 3, wherein it is determined that the membrane is in a dry state.   6. The fuel cell according to claim 5, wherein the control unit sets a predetermined limit on an increase rate of a current generated by the fuel cell stack when it is determined that the electrolyte membrane is in a dry state. 7. system.   6. The fuel cell system according to claim 5, wherein the control unit decreases a temperature control target value of the fuel cell stack when it is determined that the electrolyte membrane is in a dry state. 7.   6. The fuel cell system according to claim 5, wherein when the electrolyte membrane is determined to be in a dry state, the control unit decreases the target flow rate control value of the gas.   6. The fuel cell system according to claim 5, wherein, when it is determined that the electrolyte membrane is in a dry state, the control unit reduces a limit value of a maximum output value generated by the fuel cell stack. .   When the current generated by the fuel cell stack increases at an increase rate of a predetermined value or more, the current density measured by the local current measuring means connected in the vicinity of the cathode electrode inlet increases, and the cathode electrode When the increasing rate of the current density measured by the local current measuring unit connected in the vicinity of the outlet is equal to or less than a predetermined rate with respect to the increasing rate of the current generated by the fuel cell stack, the control unit includes the electrolyte 4. The fuel cell system according to claim 3, wherein it is determined that the membrane is in an excessively wet state.   11. The fuel cell system according to claim 10, wherein the control means increases the flow rate control target value of the gas when it is determined that the electrolyte membrane is in an excessively wet state.   11. The fuel cell system according to claim 10, wherein the control unit increases a temperature control target value of the fuel cell stack when it is determined that the electrolyte membrane is in an excessively wet state.

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