JP2009211919A - Fuel cell system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To accurately estimate in detail an inside state of a fuel cell and optimize the operation condition of the fuel cell based on the estimated contents. <P>SOLUTION: A fuel cell 10 is virtually divided into a plurality of small regions arranged in a line along the flow of hydrogen of a fuel cell. The generation of nitrogen or liquid water in each small region is calculated based on power generation environment in the small region, and the movement of nitrogen or liquid water to the downstream small region is also calculated. The amount of nitrogen and the amount of liquid water in each small region are periodically updated reflecting the generation or the movement of them. An ECU 4 controls the operation of a purge valve 6 or a hydrogen circulation valve 8 according to the estimated distribution state of nitrogen or liquid water in the fuel cell 10. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a fuel cell system, and more particularly to a fuel cell system that estimates the internal state of a fuel cell and optimizes the operating conditions of the fuel cell in accordance with the estimated contents.

  The power generation capacity of a fuel cell greatly depends on its internal state such as the concentration and humidity of the reaction gas. For this reason, for example, as described in the following patent documents, in some conventionally proposed fuel cell systems, the internal state of the fuel cell is estimated, and the operating conditions of the fuel cell are determined according to the estimated contents. Has been done to optimize. In these systems, the internal state of the fuel cell is associated with the signal state of a sensor provided in the system, and the internal state of the fuel cell is estimated from the sensor signal.

JP 2006-019121 A JP 2006-318784 A JP 2005-108673 A JP 2004-179000 A

  The power generation performance of a fuel cell depends not only on the internal state of the fuel cell but also on the local state. In particular, the location and amount of impurities such as nitrogen and liquid water greatly affect the power generation performance of the fuel cell. However, the systems described in each of the above-mentioned patent documents have not been able to estimate in detail up to what state inside the fuel cell. In order to further improve the fuel consumption by optimizing the operating conditions of the fuel cell, it has been desired to develop a technique that can estimate the state inside the fuel cell in more detail, and in particular, the impurity distribution state in detail.

  The present invention has been made in order to solve the above-described problems, so that the state inside the fuel cell can be estimated in detail with high accuracy, and the operating condition of the fuel cell can be optimized based on the estimated content. An object of the present invention is to provide a fuel cell system.

In order to achieve the above object, a first invention provides a fuel cell system comprising:
A fuel cell that generates electricity by receiving a supply of reactive gas;
Virtual dividing means for virtually dividing the fuel cell into a plurality of small regions arranged along the flow of the reaction gas;
Storage means for storing an estimated amount of impurities in each of the plurality of small regions;
Power generation environment estimation means for estimating a power generation environment of the fuel cell based on operating conditions of the fuel cell;
Impurity generation amount calculating means for calculating the generation amount of impurities in each of the plurality of small regions according to an impurity generation characteristic that defines the relationship between the power generation environment of the fuel cell and the generation amount of impurities;
For each of the plurality of small regions, the amount of impurities transferred from the small region to a small region located downstream of the flow of the reaction gas is defined as an impurity that defines the relationship between the power generation environment of the fuel cell and the amount of impurity transfer Impurity transfer amount calculation means for calculating according to the transfer characteristics;
Updating means for periodically updating the estimated impurity amount in each small region stored in the storage unit to reflect the amount of impurity movement between adjacent small regions and the amount of impurity generation in the small region;
Control means for changing the operating conditions of the fuel cell according to the distribution of impurities in the fuel cell;
It is characterized by having.

The second invention is the first invention, wherein
The reaction gas is an anode gas, and the impurity is an impurity present on the anode side of the fuel cell,
The control means changes operating conditions related to supply or discharge of the anode gas.

The third invention is the second invention, wherein
The impurities include nitrogen and liquid water,
The impurity generation amount calculating means calculates the generation amount of nitrogen and the generation amount of liquid water according to respective impurity generation characteristics,
The impurity transfer amount calculation means calculates the transfer amount of nitrogen and the transfer amount of liquid water according to different impurity transfer characteristics.

  According to the first invention, the amount of impurities generated in each of the virtually divided small regions, and the amount of impurities transferred from each small region to the small region located downstream of the flow of the reaction gas, It can be determined based on the power generation environment in which the small area is located. For each small region, the amount of impurities generated in the small region, the amount of impurity transferred from the previous stage to the small region, and the amount of impurity transferred from the small region to the subsequent stage can be known, so that the amount of impurities in each small region can be accurately determined. Therefore, it is possible to grasp the distribution state of impurities in the fuel cell in detail. By accurately grasping the distribution state of impurities and changing the operating conditions of the fuel cell accordingly, the fuel consumption can be improved by optimizing the operating conditions.

  According to the second invention, it is possible to prevent the power generation performance of the fuel cell from being deteriorated due to the influence of impurities accumulated on the anode side of the fuel cell.

  According to the third invention, it is possible to accurately estimate each of the distribution state of nitrogen and the distribution state of liquid water on the anode side of the fuel cell.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. Note that the present invention is not limited to the following embodiments, and various modifications can be made without departing from the spirit of the present invention.

  FIG. 1 is a diagram for explaining the configuration of a fuel cell system according to an embodiment of the present invention. The fuel cell system shown in FIG. 1 includes a fuel cell stack 2 composed of a plurality of stacked fuel cells 10. An anode and a cathode are formed inside the fuel cell 10 with an electrolyte membrane interposed therebetween. On the anode side, there is formed a gas flow path for allowing a fuel gas containing hydrogen (in this embodiment, hydrogen gas) to flow in the plane. In addition, a gas flow path for allowing an oxidizing gas containing oxygen (in this embodiment, air) to flow in the plane is formed on the cathode side. Furthermore, each fuel cell 10 is isolated from the adjacent fuel cell by a current collecting plate having a cooling water passage formed therein. Since the configuration of the fuel cell 10 is known, further description is omitted here.

  The fuel cell stack 2 is controlled by the ECU 4 through a plurality of system auxiliary devices. The system auxiliary equipment includes a purge valve 6 for exhausting off-gas discharged from the anode outlet of the fuel cell stack 2 out of the system, and a hydrogen circulation pump 8 for circulating off-gas to the anode inlet again. The ECU 4 controls the operation of these devices 6 and 8 according to the in-plane state of each fuel cell 10, particularly the distribution state of nitrogen and liquid water at the anode of the fuel cell 10.

[Estimation of in-plane state of fuel cell]
The ECU 4 acquires information on the pressure and temperature in the fuel cell stack 2 from the outputs of various sensors not shown, and estimates the in-plane state of each fuel cell 10 based on the information. Hereinafter, a method for estimating the in-plane state of the fuel cell 10 employed in the fuel cell system of the present embodiment will be described.

  The ECU 4 estimates the in-plane state of the fuel cell 10 by simulation (hereinafter referred to as state estimation simulation). A model of the fuel cell 10 is used for the state estimation simulation. As shown in FIG. 1, this model is a one-dimensional model in which the fuel cell 10 is virtually divided into a plurality of small regions along the reaction gas flow path. As shown in this one-dimensional model, the fuel cell 10 of the present embodiment has a counter flow channel in which the cathode side gas and the anode side gas flow opposite to each other with the electrolyte membrane interposed therebetween.

  The fuel cell 10 generates power at a current density corresponding to the power generation environment within the plane. The “power generation environment” is a general term for a plurality of parameters that determine the power generation state of the fuel cell 10 among various parameters representing the in-plane state of the fuel cell 10. In the state estimation simulation by the ECU 4, the power generation environment in which each small area of the divided model is placed is considered to be uniform in each small area, and the power generation environment is estimated for each small area.

  The power generation environment includes the relative humidity of the anode side gas, the relative humidity of the cathode side gas, the anode pressure, the cathode pressure, the hydrogen concentration in the anode side gas, the oxygen concentration in the cathode side gas, the amount of liquid water in the anode, The amount of liquid water and the temperature of the fuel cell are included. The ECU 4 stores a map (power generation characteristic map) that defines the relationship between the power generation environment determined by these parameters and the current density. The ECU 4 also stores a map (consumption generation characteristic map) that defines the relationship between the current consumption and the oxygen consumption, hydrogen consumption, water generation amount, and current density. Further, the ECU 4 includes a map (water crossing movement characteristic map) that defines the relationship between the amount of water moving across the fuel cell from the cathode to the anode of the fuel cell and the power generation environment, and a fuel cell cathode to the anode. A map (nitrogen crossing movement characteristic map) that defines the relationship between the amount of nitrogen moving across the fuel cell and the power generation environment is stored. The consumption generation characteristic map and the water crossing movement characteristic map correspond to “impurity generation characteristics” related to liquid water. The nitrogen cross transfer characteristic map corresponds to the “impurity generation characteristic” related to nitrogen.

  If the power generation environment in which each small area is placed is specified, the current density generated in the power generation environment can be predicted by referring to the power generation characteristic map. In addition, by referring to the water crossing movement characteristics map, the amount of water crossing movement generated in the power generation environment can be predicted, and by referring to the nitrogen crossing movement characteristics map, nitrogen generated in the power generation environment can be predicted. The amount of movement can be predicted. When the current density is specified by the power generation characteristic map, the oxygen consumption amount, the hydrogen consumption amount, and the water generation amount in the power generation environment can be predicted by referring to the consumption generation characteristic map.

  Oxygen consumption, hydrogen consumption, water generation, water crossing movement, and nitrogen crossing movement are all factors affecting the power generation environment. In addition to these, other factors that affect the power generation environment include each gas component and each amount of liquid water moved from a small region located upstream of the reaction gas flow, and downstream of the reaction gas flow. Each movement amount of each gas component and liquid water to a small area is included. In the state estimation simulation, the values of these factors are acquired for the individual small areas, and the power generation environment in which the individual small areas are placed is updated based on the values.

[Specific method for renewing power generation environment]
The contents of the power generation environment update performed in the state estimation simulation will be specifically described with reference to FIG. FIG. 2 is a diagram for explaining a procedure for updating the power generation environment in which the n region is placed, taking an arbitrary small region (n region) of the division model as an example. In the state estimation simulation, each small region is divided into a cathode (Ca) side and an anode (An) side. Each small area has a state quantity (number of moles) of each component existing therein as an internal parameter. Specifically, as shown in FIG. 2, the n region has an oxygen amount n_O2_Ca, a nitrogen amount n_N2_Ca, a water vapor amount n_gasH2O_Ca, and a liquid water amount n_liqH2O_Ca as state quantities on the cathode side, and a hydrogen amount n_H2_An as state quantities on the anode side. , Nitrogen amount n_N2_An, water vapor amount n_gasH2O_An, and liquid water amount n_liqH2O_An.

  The n region also has the pressure P, volume V, and temperature T of the region as internal parameters. The volume V is a preset fixed value, and the temperature T is the same constant value as the cooling water temperature. The pressure P can be calculated by the equation of state (PV = nRT).

  In the state estimation simulation, the relative humidity of the anode-side gas, the relative humidity of the cathode-side gas, the hydrogen concentration in the anode-side gas, and the oxygen concentration in the cathode-side gas in the n region are calculated using the plurality of state quantities described above. To do. Each relative humidity can be calculated from the water vapor partial pressure P_H2O and the temperature T. The hydrogen concentration can be calculated from the ratio of the hydrogen amount n_H2_An to the amount (number of moles) of all gas components on the anode side. The oxygen concentration can be calculated from the ratio of the oxygen amount n_O2_Ca to the amount (number of moles) of all gas components on the cathode side.

  Thus, by specifying the values of the parameters in the n region, the power generation environment in which the n region is currently located is specified. In the state estimation simulation, the current density I in the n region is specified by comparing the specified power generation environment with the above-described power generation characteristic map. Then, by comparing the current density I with the above-described consumption generation characteristic map, the oxygen consumption amount and water generation amount on the cathode side in the n region and the hydrogen consumption amount on the anode side are specified. Further, the cross-H2O water crossing movement amount from the cathode side to the anode side in the n region is specified by collating the specified power generation environment with the water crossing movement characteristic map described above. Further, the cross-N2 amount of nitrogen crossing movement from the cathode side to the anode side in the n region is specified by collating the specified power generation environment with the above-described nitrogen crossing movement characteristic map. Note that the oxygen consumption, hydrogen consumption, water production, water crossing movement, and nitrogen crossing movement are all amounts per predetermined calculation interval.

  In the state estimation simulation, in parallel with the above calculation of each consumption generation amount, or before and after the calculation, the movement amount (moles) of each component from the n region to the n + 1 region downstream thereof per calculation time. Number). Specifically, as shown in FIG. 2, the oxygen transfer amount dn_O2_Ca, the nitrogen transfer amount dn_N2_Ca, the water vapor transfer amount dn_gasH2O_Ca, and the transfer amount dn_liqH2O_Ca are calculated on the cathode side, and the hydrogen transfer amount dn_H2_An and nitrogen transfer are calculated on the anode side. The amount dn_N2_An, the water vapor transfer amount dn_gasH2O_An, and the liquid water transfer amount dn_liqH2O_An are calculated. In calculating the movement amount of each component, a map (movement characteristic map) that defines the relationship between the movement amount and the power generation environment is used.

  It should be noted here that in the counterflow channel employed by the fuel cell 2 of the present embodiment, the n + 1 region on the cathode side and the n + 1 region on the anode side are not in a reverse relationship. The same applies to the n-1 region. The anode side and the cathode side have a front / back relationship across the fuel cell only in the n region to be calculated. In FIG. 2, since the gas flow on the cathode side and the gas flow on the anode side are drawn in the same direction, it seems that the cathode side and the anode side correspond to each other in the n + 1 region and the n−1 region. Actually, the n + 1 region on the anode side corresponds to the n + 1 region on the cathode side, and the n + 1 region on the anode side corresponds to the n-1 region on the cathode side. If the cathode side gas and the anode side gas flow in parallel, the n + 1 region on the cathode side and the n + 1 region on the anode side have a front / back relationship. The same applies to the n-1 region. The present invention can also be applied to a fuel cell having a coflow channel.

  In the state estimation simulation, the movement amount of each component downstream from the small region is calculated for each of the small regions. Therefore, as described above, the movement amount (outflow amount) of each substance from the n region to the n-1 region is calculated, and at the same time, the movement amount (inflow) of each substance from the upstream n-1 region to the n region. Amount) is also calculated.

  Through the above series of calculations, the amount of reaction gas consumed and the amount of water produced in the n region, the amount of movement of each component from the preceding n-1 region to the n region, and the movement of each component from the n region to the subsequent n + 1 region The amount of each component and the amount of movement of each component between the anode and the cathode in the n region are determined. By reflecting these consumption generation amount and movement amount on the state amount of each component in the n region, it is possible to grasp the latest state amount of each component.

  However, regarding water vapor and liquid water among the components, it is necessary to consider that condensation from water vapor to liquid water occurs due to the power generation environment in the n region, and conversely, evaporation from liquid water to water vapor occurs. is there. Assuming that all the water present in the cathode or anode in the n region is water vapor, if the relative humidity at that time exceeds 100%, condensation of water vapor occurs, and if the relative humidity is less than 100%, Water evaporation occurs. In the state estimation simulation, the liquid water generation amount or liquid water evaporation amount per calculation step time is calculated, and in addition to the above-mentioned various consumption generation amounts and movement amounts, the liquid water generation evaporation amount is reflected and n Update the state quantity of each component in the region. For the calculation of the liquid water generation amount and the liquid water evaporation amount, a map (liquid water generation evaporation characteristic map) that defines the relationship between them and the power generation environment is used.

[Control flow of fuel cell stack by ECU]
According to the state estimation simulation described above, for each small region obtained by virtually dividing the fuel cell 10, each amount of nitrogen and liquid water generated at the anode and each movement of nitrogen and liquid water between adjacent small regions Since the amount is periodically calculated, the amount of nitrogen and liquid water that affect the power generation performance can be estimated with high accuracy. And if the amount of nitrogen and liquid water in the anode is known for each small area, it is possible to accurately grasp at which position in the surface the nitrogen and liquid water are accumulated and how much. .

  FIG. 3 is a flowchart of a routine executed by the ECU 4 in order to reflect the estimation result by the state estimation simulation in the control of the fuel cell stack 2. The routine shown in FIG. 3 is started when the fuel cell system is started, and is repeatedly executed with the above-described calculation step time as one cycle.

  In the first step S101 of the routine shown in FIG. 3, a state estimation simulation is executed. In the next step S102, it is determined whether or not the power generation performance of the fuel cell stack 2 is reduced. The power generation performance of the fuel cell stack 2 can be determined from the voltage value, current value, or IV characteristics.

  As a result of the determination in step S102, when the power generation performance of the fuel cell stack 2 is degraded, the determination in step S103 is performed next. In step S <b> 103, it is determined whether or not the place where nitrogen or liquid water is accumulated is near the anode inlet of the fuel cell 10 based on the estimation result by the state estimation simulation. When these impurities are accumulated in the vicinity of the anode inlet, it takes time to discharge from the inside of the fuel cell 10, but the pulsation effect is large because the pressure is high. Therefore, when it is estimated that nitrogen or liquid water has accumulated near the anode inlet, the process of step S105 is executed. In step S105, the operations of the hydrogen circulation pump 8 and the purge valve 6 are controlled in a direction in which the effect of discharging the anode gas from the fuel cell 10 is greatly increased.

  On the other hand, if the location where the impurities are accumulated is not near the anode inlet, the determination in step S104 is performed next. In step S104, it is determined whether the amount of nitrogen or liquid water accumulated exceeds a predetermined threshold based on the estimation result obtained by the state estimation simulation. When these impurities are accumulated in a large amount, the pulsation effect is small and the effect of increasing the gas flow rate is small. Therefore, when it is estimated that a large amount of nitrogen or liquid water is accumulated, the process of step S106 is executed. In step S106, the operations of the hydrogen circulation pump 8 and the purge valve 6 are controlled so that the discharge effect of the anode gas from the fuel cell 10 increases.

  The control content of the operation of the hydrogen circulation pump 8 includes increasing the rotational speed of the hydrogen circulation pump 8 to increase the circulation flow rate, adding pulsation with the hydrogen circulation pump 8, changing the frequency of the pulsation, For example, changing the control time. Further, the contents of control of the operation of the purge valve 6 include opening the purge valve 6 and changing the opening degree. The ECU 4 has a two-dimensional map of location and amount for each of nitrogen and liquid water, and determines the operation contents of the hydrogen circulation pump 8 and the purge valve 6 in light of the estimation result by the state estimation simulation.

  The processing in step S105 or step S106 described above is continuously performed until the power generation performance of the fuel cell stack 2 is restored. As a result of the determination in step S102, when the power generation performance of the fuel cell stack 2 is recovered, the process in step S107 is executed. In step S107, the control of the hydrogen circulation pump 8 and the purge valve 6 is returned to the standard control.

  By executing the above routine, it is possible to avoid a decrease in the power generation performance of the fuel cell stack 2 due to accumulation of nitrogen and liquid water. In addition, since driving under special driving conditions can be kept to the minimum necessary, fuel consumption can be improved by driving under optimized driving conditions.

  Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention. For example, each small region of the divided model shown in FIG. 2 may be represented by a one-dimensional model further divided in the cross-sectional direction, and the position and amount of nitrogen and liquid water in the cross-sectional direction may be grasped. Considering the distribution of nitrogen and liquid water in the cross-sectional direction, the operating conditions of the fuel cell can be further optimized.

  The present invention is also applicable to a fuel cell system that uses an ejector instead of a hydrogen circulation pump. Even in such a fuel cell system, the operating conditions can be optimized by changing the hydrogen supply amount, the supply pressure, and the like.

  In the above-described embodiment, the fuel cell operating conditions are optimized by focusing on nitrogen and liquid water accumulated in the anode. However, the liquid water accumulated in the cathode may also be focused. According to the state estimation simulation described above, the distribution state of liquid water at the cathode can also be accurately estimated.

It is a figure which shows the structure of the fuel cell system of embodiment of this invention. It is a figure which shows the model which divided | segmented the surface of the fuel cell along the flow path of the reactive gas used in the state estimation simulation of embodiment of this invention. It is a flowchart of the routine which ECU performs in order to reflect the estimation result by a state estimation simulation in control of a fuel cell stack.

Explanation of symbols

2 Fuel cell stack 4 ECU
6 Purge valve 8 Hydrogen circulation pump 10 Fuel cell I Current density
n_O2_Ca Oxygen content
n_H2_An Hydrogen amount
n_N2_Ca, n_N2_An Nitrogen content
n_liqH2O_Ca, n_liqH2O_An Liquid water volume
n_gasH2O_Ca, n_gasH2O_An amount of water vapor
dn_O2_Ca Oxygen transfer
dn_H2_An Hydrogen transfer amount
dn_N2_Ca, dn_N2_An Nitrogen transfer
dn_liqH2O_Ca, dn_liqH2O_An Liquid water transfer amount
dn_gasH2O_Ca, dn_gasH2O_An Water vapor transfer amount
cross_H2O Cross-water travel
cross_N2 Nitrogen crossing travel

Claims (3)

A fuel cell that generates electricity by receiving a supply of reactive gas;
Virtual dividing means for virtually dividing the fuel cell into a plurality of small regions arranged along the flow of the reaction gas;
Storage means for storing an estimated amount of impurities in each of the plurality of small regions;
Power generation environment estimation means for estimating a power generation environment of the fuel cell based on operating conditions of the fuel cell;
Impurity generation amount calculating means for calculating the generation amount of impurities in each of the plurality of small regions according to an impurity generation characteristic that defines the relationship between the power generation environment of the fuel cell and the generation amount of impurities;
For each of the plurality of small regions, the amount of impurities transferred from the small region to a small region located downstream of the flow of the reaction gas is defined as an impurity that defines the relationship between the power generation environment of the fuel cell and the amount of impurity transfer Impurity transfer amount calculation means for calculating according to the transfer characteristics;
Updating means for periodically updating the estimated impurity amount in each small region stored in the storage unit to reflect the amount of impurity movement between adjacent small regions and the amount of impurity generation in the small region;
Control means for changing the operating conditions of the fuel cell according to the distribution of impurities in the fuel cell;
A fuel cell system comprising: The reaction gas is an anode gas, and the impurity is an impurity present on the anode side of the fuel cell,
2. The fuel cell system according to claim 1, wherein the control means changes operating conditions relating to supply or discharge of the anode gas. The impurities include nitrogen and liquid water,
The impurity generation amount calculating means calculates the generation amount of nitrogen and the generation amount of liquid water according to respective impurity generation characteristics,
3. The fuel cell system according to claim 2, wherein the impurity transfer amount calculation means calculates the transfer amount of nitrogen and the transfer amount of liquid water according to different impurity transfer characteristics.

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