Patent Application
20090061263
1. Field of the Invention
The present invention relates to a fuel cell system and a method for estimating an output characteristic of a fuel cell.
2. Description of the Related Art
In order to improve the power generation efficiency of a fuel cell, a control in which the fuel cell output is determined taking the current-voltage characteristic (which is hereinafter referred to as “output characteristic”) of the fuel cell into account is conventionally performed. However, the current-voltage characteristic changes depending on the conditions of the fuel cell or over time. Thus, a related art is suggested in, for example, JP-A-2002-231295 in which the output characteristic of a fuel cell is estimated from the temperature of the fuel cell, hydrogen pressure or the like and the output of the fuel cell is controlled using the estimated output characteristic to operate the fuel cell efficiently.
In the above related art, however, the output characteristic of the fuel cell is derived from only the hydrogen supply pressure and the temperature of the fuel cell. Other operation parameters, which have a large influence on the output characteristic of the fuel cell, are not taken into account. Therefore, high estimation accuracy cannot necessarily be obtained.
The present invention improves the accuracy of estimation of an output characteristic of a fuel cell.
A first aspect of the present invention relates to a fuel cell system. The fuel cell system has: an operation parameter storing unit that stores the relation between an internal resistance of a fuel cell excluding both a diffusion resistance of the fuel cell that is generated depending on the flow rate of reaction gas to the fuel cell and a reaction resistance of the fuel cell that is generated depending on the pressure of the reaction gas, and at least one operation parameter indicating an operating state of the fuel cell; a detecting unit that detects the present value of the at least one operation parameter of the fuel cell; an internal resistance estimating unit that estimates the internal resistance of the fuel cell based on the detected value of the operation parameter, with reference to the relation stored in the operation parameter storing unit, and sets the estimated internal resistance as the basic internal resistance; an IR-free (internal resistance-free) output characteristic storing unit that stores the IR-free output characteristic, which is the relation between output current and output voltage of the fuel cell, excluding the influence of the internal resistance, at different flow rates and pressures of air supplied to the fuel cell, together with the flow rate and the pressure; a flow rate measuring unit that measures the flow rate of the air to be supplied to the fuel cell; a pressure measuring unit that measures the pressure of the air; a determining unit that determines the IR-free output characteristic of the fuel cell based on the flow rates and pressures of the air; and a setting unit that sets a basic output characteristic of the fuel cell based on the basic internal resistance and the IR-free output characteristic. In the above configuration, the fuel cell system may further have: an output current measuring unit that measures the output current of the fuel cell; an output voltage measuring unit that measures the output voltage of the fuel cell outputting the output current; and a calculating unit that corrects the basic output characteristic based on the output current and the output voltage.
According to the fuel cell system, the IR-free output characteristic is the relation between output current and output voltage of the fuel cell excluding the influence of the internal resistance at different flow rates and pressures of air supplied to the fuel cell. The IR-free output characteristic is stored together with the flow rate and the pressure. The flow rate and pressure of air to be supplied to the fuel cell is measured and an IR-free output characteristic of the fuel cell is determined based on the measured flow rate and pressure of the air. Therefore, the basic output characteristic of the fuel cell can be estimated accurately, and, consequently, the basic output characteristic can be corrected accurately.
The operation parameter may include at least one of the temperature of the fuel cell, the humidity in the fuel cell, and an impedance, which is measured across output terminals of the fuel cell using an alternating current (AC) signal with a predetermined frequency. According to this configuration, the basic internal resistance can be estimated based on the temperature of the fuel cell, the humidity in the fuel cell, or the impedance between the output terminals of the fuel cell.
The calculating unit may have a first correcting unit that corrects the basic internal resistance of the fuel cell based on the output current and the output voltage; and a second correcting unit that corrects the basic output characteristic with the corrected basic internal resistance. According to this configuration, the basic internal resistance of the fuel cell is corrected based on the output current and the output voltage, and the basic output characteristic is corrected. Therefore, the accuracy of the estimation of the basic output characteristic can be improved.
The calculating unit may further have a correction value storing unit that stores the corrected basic internal resistance as a new basic internal resistance; a third correcting unit that further corrects the basic internal resistance based on the output current and the output voltage when a basic internal resistance has been stored in the storing unit; and a fourth correcting unit that corrects the basic output characteristic with the further corrected basic internal resistance. According to this configuration, the corrected basic internal resistance is stored as a new basic internal resistance, and the basic internal resistance is further corrected based on the output current and the output voltage when a basic internal resistance has been stored in the correction value storing unit. Therefore, the accuracy of the estimation of the basic output characteristic.
The detecting unit may have an impedance measuring unit that measures an impedance across output terminals of the fuel cell using an AC signal with a predetermined frequency; and an AC impedance storing unit that stores the measured impedance, and the calculating unit may have a comparing unit that compares the impedance measured by the impedance measuring unit with the impedance measured previously and stored in the impedance storing unit; and an output characteristic correcting unit that corrects the basic internal resistance when the difference between the measured impedance and the previously measured impedance is equal to or greater than a predetermined value as a result of comparison by the comparing unit and corrects the IR-free output characteristic when the difference is smaller than a predetermined value.
According to this configuration, the impedance measured by the measuring unit and an impedance measured previously and stored in the impedance storing unit are compared. The basic internal resistance is corrected when there is a difference equal to or greater than a predetermined value between the measured impedance and the previously measured impedance. The IR-free output characteristic is corrected when there is not such a difference. Therefore, the object to be corrected can be limited and the accuracy of the correction can be improved.
A second aspect of the present invention relates to a method for estimating an output characteristic of a fuel cell. This method includes: detecting a present value of at least one operation parameter that indicates the operating state of the fuel cell; estimating the internal resistance of the fuel cell based on the detected value of the operation parameter with reference to the relation between an internal resistance of the fuel cell excluding both a diffusion resistance of the fuel cell that is generated depending on the flow rate of reaction gas to the fuel cell and a reaction resistance of the fuel cell that is generated depending on the pressure of the reaction gas, and the operation parameter; and setting the estimated internal resistance as a basic internal resistance; measuring the flow rate of air to be supplied to the fuel cell; measuring the pressure of the air; determining an IR-free output characteristic of the fuel cell based on the flow rate and pressure of the air; and setting a basic output characteristic of the fuel cell based on the basic internal resistance and an IR-free output characteristic, which is the relation between the output current and output voltage of the fuel cell, excluding the influence of the internal resistance, at different flow rates and pressures of air to be supplied to the fuel cell. In the above configuration, the method may further include: measuring the output current of the fuel cell and measuring the output voltage of the fuel cell outputting the output current; and correcting the basic output characteristic based on the output current and the output voltage.
The operation parameter may include at least one of temperature of the fuel cell, humidity in the fuel cell, and an impedance, which is measured across output terminals of the fuel cell using an AC signal with a predetermined frequency.
The method may further include: correcting the basic internal resistance of the fuel cell based on the output current and the output voltage; and correcting the basic output characteristic with the corrected basic internal resistance.
The method may further include: storing the corrected basic internal resistance as a new basic internal resistance; correcting the basic internal resistance based on the output current and the output voltage when a basic internal resistance has been stored; and correcting the basic output characteristic with the further corrected basic internal resistance.
The method may further include: measuring an impedance across output terminals of the fuel cell using an AC signal with a predetermined frequency; comparing the measured impedance with a previously measured impedance; and correcting the basic internal resistance when the difference between the measured impedance and the previously measured and stored impedance is equal to or greater than a predetermined value, and correcting the IR-free output characteristic when the difference is smaller than the predetermined value.
The foregoing and further objects, features and advantages of the invention will become apparent from the following description of preferred embodiments with reference to the accompanying drawings, wherein like numerals are used to represent like elements and wherein:
Description is hereinafter made of fuel cell systems according to the best modes for carrying out the present invention (which are hereinafter referred to as “embodiments”) with reference to the drawings. The configurations of the embodiments are illustrative, and the present invention is not limited to the configurations of the embodiments.
In the fuel cell system shown in
In this embodiment, the ECU 20 has a memory (which may be regarded as an operation parameter storing unit), in which the relation between the cell stack temperature and the internal resistance, which was measured experimentally in advance, is stored as a map. Therefore, the ECU 20 can detect the cell stack temperature and determine the internal resistance by monitoring the output signal from the temperature sensor 6. The internal resistance, which is determined based on the cell stack temperature according to the map stored, is referred to as “basic internal resistance R0.”
(2) The ECU 20 determines the IR-free output characteristic of the fuel cell body 1 based on the pressure on the air electrode side in the fuel cell body 1 and the airflow rate to the air electrode. The IR-free output characteristic of the fuel cell body 1 is also measured experimentally in advance and stored in the memory (which may be regarded as an output characteristic storing unit) of the ECU 20 as a map.
Such a map may be obtained by, for example, the following procedure. The output characteristic, that is, the relation between the output current and the output voltage of the fuel cell, is measured with the pressure on the air electrode side and the airflow rates to the air electrode varied in advance by an experiment. Then, the internal resistance of the fuel cell is estimated based on the current cell stack temperature according to a map of the relation between the cell stack temperature and the internal resistance as shown in
If the output current Ifc exceeds a certain limit, the output voltage Vfc drops rapidly. This is because insufficient reaction gas to generate the output current Ifc is supplied, and therefore the gas diffusion resistance, which is generated when gas is diffused in the electrodes, appears prominently as a resistance value. When the amount of reaction gas (the airflow rate on the air electrode side, for example) is sufficiently large to generate the output current Ifc, the influence of the reaction gas diffusion resistance is small and the drop of the output voltage is small.
Also, it is known that even when the air is supplied at the same flow rate, as the pressure on the air electrode side is higher, the output voltage Vfc corresponding to an output current Ifc is higher as shown in
(3) The ECU 20 estimates the current-voltage characteristic of the fuel cell body 1 including the internal resistance at that instant from the relation between the basic internal resistance R0 and the IR-free output characteristic obtained as described in (1) and (2), respectively. The current-voltage characteristic is referred to as “basic output characteristic” in this embodiment.
The ECU 20 then calculates the correction amount AR for correcting the basic internal resistance R0 based on the difference in the output current and output voltage actually measured by the current sensor 8 and the voltage sensor 9, respectively, and the basic output characteristic to obtain a corrected internal resistance R1=R0+ΔR. Then, the ECU 20 newly sets a current-voltage characteristic using the corrected internal resistance R1. The thus set current-voltage characteristic is used as a reference to set the amount of reaction gas that is supplied, or other operations with respect to the amount of electric power demanded from the fuel cell body 1. The ECU 20 repeatedly performs the above processes (1) to (3) at predetermined intervals.
The ECU 20 then calculates a basic internal resistance R0 based on the temperature Tfc of the fuel cell body 1 (S101: the ECU 20, which performs this step, may be regarded as an internal resistance estimating unit). Then, the ECU 20 calculates an IR-free output characteristic Virf(Ifc), which is measured in advance based on the airflow rate Fair and the air pressure Pair (S102: the ECU 20, which performs this step, may be regarded as a determining unit). The Virf(Ifc) is the function representing the relation between the output current Ifc and the IR-free voltage Virf, and may be represented as pairs of an output current Ifc and an IR-free voltage Virf, for example. The relation between the output current Ifc and the IR-free voltage Virf may, however, be represented as an experimental formula.
The ECU 20 then calculates the basic output characteristic (S103: the ECU 20, which performs this step, may be regarded as a setting unit). The basic output characteristic is calculated by adding the effect of the basic internal resistance R0 to the above IR-free output characteristic, to be derived as Virf(Ifc)−Ifc×R0.
The ECU 20 then determines an operation point (Ifc, Vfc) on the basic output characteristic Virf(Ifc)−Ifc×R0 based on the output demanded to the fuel cell system, and controls the voltage across the output terminals of the fuel cell body 1 to Vfc (S104). More specifically, the ECU 20 controls the voltage across the output terminals to Vfc via a DC-DC converter (not shown), for example. At this time, when the amount of reaction gas (hydrogen or air) corresponding to the output of the fuel cell is not supplied to the fuel cell and therefore insufficient, the current decreases. Thus, although not shown in
The ECU 20 then reads the actual output current Ifc_m and output voltage Vfc_m of the fuel cell from the current sensor 8 and the voltage sensor 9, respectively (S105). Then, the ECU 20 derives a value Vfc0=Virf(Ifc_m)−Ifc_m×R0 on the basic output characteristic corresponding to the output current Ifc_m (S106). When the IR-free output characteristic Virf(Ifc) is given as an experimental formula, for example, the value is obtained by adding the voltage drop caused by the basic internal resistance R0 to a value calculated using the formula. When the IR-free output characteristic Virf(Ifc) is given with a plurality of current-voltage pairs (I, V), the value is obtained by adding the voltage drop caused by the basic internal resistance R0 to the output voltage value at the time when I=Ifc_m obtained by interpolation.
The ECU 20 then determines whether the difference between the output voltage Vfc0 obtained from the basic output characteristic and the output voltage Vfc_m obtained from the voltage sensor 9 is within a predetermined range (S107). When it is determined that the difference is not within the predetermined range and there is a large difference, the ECU 20 corrects the basic output characteristic as follows to obtain a new output characteristic.
That is, the ECU 20 calculates a correction value for the basic internal resistance R0 according to the equation ΔR=(Vfc0−Vfc_m)/Ifc_m (S108). Then, the ECU 20 corrects the basic internal resistance R0 according to the equation R1=R0+ΔR to obtain a new internal resistance R1 (S109: the ECU 20, which performs this step, may be regarded as a first correcting unit).
Also, the ECU 20 calculates an output characteristic Vfc(Ifc)=Virf(Ifc)−R1×Ifc using the newly obtained internal resistance R1 (S110: ECU 20, which performs this step, may be regarded as a calculating unit and a second correcting unit).
As described above, according to the fuel cell system of the embodiment, the changes in the limit present value and in the gas diffusion resistance caused by change in the airflow rate and the change in the reaction resistance caused by changes in the air pressure are reflected in the basic output characteristic. Therefore, an output characteristic in which the changes in the airflow rate and the air pressure are reflected is estimated, and an output characteristic may be estimated with higher accuracy.
In the above embodiment, the detected internal resistance difference value AR is used as it is as a correction value for the internal resistance as shown in step S109 in
R1=R0+Kp×ΔR+Ki×∫TΔRdt;
where, Kp and Ki are the proportional gain and the integral gain, respectively.
Also, in the flowchart shown in
Referring to
The ECU 20 has a map of the relation between the temperature Tfc and the internal resistance of the fuel cell body 1 with the amount of humidification to the fuel cell body 1 as a parameter in the memory. Here, the humidification amount is the humidity that is measured when the air is humidified. Therefore, the ECU 20 obtains an air humidification amount Sair from a detection signal from the humidity sensor 12 and the temperature Tfc of the fuel cell body 1 from a detection signal from the temperature sensor 6. Then, the ECU 20 calculates a basic internal resistance R0 based on the humidification amount Sair and the temperature Tfc.
When the temperature of the fuel cell body 1 is high, because the temperature of the polymer electrolyte membrane is high and the temperature of air supplied at this time is also high, the saturated vapor pressure of the air is high. Therefore, the change in the amount of humidification to the air has a large influence on the amount of water in the polymer electrolyte membrane. Thus, as shown in
When the temperature of the fuel cell body 1 is low, because the temperature of air supplied at this time is low, the saturated vapor pressure of the air is low. Therefore, changing the amount of humidification to the air has less influence on the amount of water in the polymer electrolyte membrane than at high temperatures. Thus, as shown in
Also, as shown in
The ECU 20 then calculates a basic internal resistance R0 based on the temperature Tfc of the fuel cell body 1 and the humidification amount Sair (S201). The subsequent steps are the same as those in and after S102 in
As described above, in the fuel cell system of this embodiment, a basic internal resistance R0, which is attributed to the proton conductivity of the membrane, is estimated from the fuel cell temperature Tfc and the air humidification amount Sair. That is, the humidification degree of the air is reflected in the estimation of the basic internal resistance R0. Therefore, the output characteristic may be estimated with higher accuracy than in the system of the first embodiment.
Referring to
In general, the humidification amount Sair increases as the airflow rate Fair in the air supply passage L1 increases in the water vapor exchange membrane type humidifier 13 as shown in
When the fuel cell temperature Tfc is low, the saturated vapor pressure of the air decreases and the humidification amount decreases as compared when the fuel cell temperature Tfc is high. Therefore, the relation among the airflow rate Fair, the temperature Tfc of the fuel cell body 1, and the humidification amount Sair is stored in advance as a map in the memory of the ECU 20 so that the humidification amount Sair can be calculated based on the results of measurement of the airflow rate Fair and the temperature Tfc of the fuel cell body 1.
The ECU 20 then calculates the air humidification amount Sair based on the temperature Tfc of the fuel cell body 1 and the airflow rate Fair according to the map stored in the memory (S210).
The ECU 20 then calculates a basic internal resistance R0 based on the temperature Tfc of the fuel cell body 1 and the humidification amount Sair (S201). The subsequent steps are the same as those in and after S102 in
As described above, in the fuel cell system of this embodiment, the air humidification amount Sair can be obtained from the temperature Tfc of the fuel cell body 1 and the airflow rate Fair without the humidity sensor 12. Therefore, the fuel cell system is advantageously applicable to a vehicle or the like which is required to have a long service life because the humidity sensor 12 is unnecessary.
Referring to
In the fourth embodiment, a fuel cell system is described in which an AC signal is input into the output terminals of the fuel cell body 1 and the necessity of correcting the basic internal resistance R0 is estimated based on the impedance between the output terminals. The other configurations and functions are the same as those of the first embodiment. The same components as those of the first to third embodiments are therefore denoted by the same reference numerals, and their description is not repeated.
As a method for measuring an internal resistance of a fuel cell, a method in which an AC impedance of a fuel cell is measured is disclosed in, for example, JP-A-2003-297408. The AC impedance of the fuel cell may be obtained by frequency analysis of the current and voltage, which are measured with a sine wave superimposed on the output current (or output voltage) of the fuel cell.
The ohmic resistance in this case is generally attributed to the resistances of the polymer electrolyte membrane and the separator part and constitutes a primary element of the internal resistance of the fuel cell. Thus, by monitoring whether the ohmic resistance fluctuates, it can be known whether the internal resistance fluctuates.
The ECU 20 then reads the actual output current Ifc_m and output voltage Vfc_m of the fuel cell from the current sensor 8 and the voltage sensor 9, respectively (S105).
The ECU 20 then measures an AC impedance and obtains the real part Zre(ω) of the AC impedance (S301: the ECU 20, which performs this step, may be regarded as a detecting unit and an AC impedance measuring unit). The real part of the AC impedance is hereinafter referred to simply as “AC impedance” in this embodiment.
The ECU 20 then derives a value Vfc0=Virf(Ifc_m)−Ifc_m×R0 on the basic output characteristic corresponding to the output current Ifc_m (S106).
The ECU 20 then determines whether the difference between the output voltage Vfc0 obtained from the basic output characteristic and the output voltage Vfc_m obtained from the voltage sensor 9 is within a predetermined range (S107). When it is determined that the difference is not within the predetermined range and there is a large difference, the ECU 20 corrects the basic output characteristic as follows to obtain a new output characteristic.
That is, the ECU 20 compares the AC impedance Zre(ω) obtained in S301 and an AC impedance Zre(ω)_old measured in the previous time and determines whether their difference is within a predetermined range (S302: the ECU 20, which performs this step, may be regarded as a comparing unit). If it is determined that the difference is not within the predetermined range and there is a large difference, the ECU 20 determines that the internal resistance of the fuel cell has changed to a value different from the basic internal resistance R0 and corrects the internal resistance and the output characteristic as in the case shown in
When it is determined in S302 that the difference between the AC impedances is within the predetermined range and there is only a small change, it is determined that the IR-free output characteristic has changed. Then, the ECU 20 calculates the difference ΔV between the output voltage Vfc0 obtained based on the basic output characteristic and the actual output voltage Vfc_m measured by the voltage sensor 9 (S303).
Then, the ECU 20 corrects the IR-free output characteristic according to the following equation shown below (S304).
Virf′=Virf+ΔV;
The ECU 20 then obtains an output characteristic of the fuel cell based on the corrected IR-free output characteristic (S305).
Vfc(If)=Virf′(If)+R0×If;
After the above process, the ECU 20 stores the measured AC impedance as the AC impedance measured in the previous time (S306: the ECU 20, which performs this step, may be regarded as an impedance storing unit). After that, the ECU 20 ends the output characteristic estimating operation. The ECU 20, which performs steps S108 to S110 or S303 to S305 may be regarded as an output characteristic correcting unit.
As described above, according to the fuel cell system of this embodiment, it is determined whether the difference between the basic output characteristic and the current-voltage characteristic actually measured is attributed to the internal resistance or the IR-free output characteristic, and the output characteristic of the fuel cell is estimated. Therefore, the output characteristic of the fuel cell can be estimated with higher accuracy than in the first embodiment. As a result, because the accuracy of an IR-free output characteristic, which is obtained in advance, is allowed to include some degree of error, the number of steps to obtain it can be significantly reduced.
The internal resistance R1 corrected in S109 in
Vfc(If)=Virf′(If)+R1×If;
Referring to
In this embodiment, the relation among the humidification amount, the AC impedance and the basic internal resistance R0 is stored in advance as a map in the memory of the ECU 20. Then, the basic internal resistance R0 is determined based on the results of measurement of the humidification amount and the AC impedance.
The ECU 20 then measures the AC impedance Zre(ω) of the fuel cell. This step is the same as S301 of the fourth embodiment shown in
The ECU 20 then calculates the humidification amount Sair based on the flow rate from the humidity sensor or the humidifier (S210). The ECU 20 then refers a basic internal resistance R0 from the map (see
As described above, in this embodiment, a basic internal resistance R0 is determined from the AC impedance of the fuel cell and the air humidification amount with reference to a map of the relation among the AC impedance, the air humidification amount and the basic internal resistance R0. Therefore, the output characteristic of the fuel cell may be estimated with higher accuracy than in the second embodiment, in which the basic internal resistance R0 is determined simply from the humidification amount. Also, the output characteristic of the fuel cell may be estimated with higher accuracy than in the fourth embodiment, in which a measurement value of the AC impedance is used to determine whether the internal resistance has changed.
Referring to
The other configuration and function of this embodiment are the same as those of the fourth and fifth embodiments. The same components as those of the fourth and fifth embodiments are therefore denoted by the same reference numerals, and their description is not repeated.
The ECU 20 then measures the AC impedance Zre(ω) of the fuel cell. The ECU 20 also calculates the air humidification amount Sair (S210). The ECU 20 then refers a basic internal resistance R0 from the map (see
As described above, in this embodiment, a basic internal resistance R0 is calculated based on the AC impedance of the fuel cell and the air humidification amount, and the output characteristic of the fuel cell is corrected based on the amount of variation in the AC impedance of the fuel cell. Therefore, the output characteristic of the fuel cell can be calculated with higher accuracy than in the fourth or fifth embodiment.
While the invention has been described with reference to embodiments thereof, it is to be understood that the invention is not limited to the described embodiments or constructions. To the contrary, the invention is intended to cover various modifications and equivalent arrangements. In addition, while the various elements of the embodiments are shown in various example combinations and configurations, other combinations and configurations, including more, less or only a single element, are also within the scope of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006-021486 | Jan 2006 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IB2007/000212 | 1/30/2007 | WO | 00 | 7/29/2008 |
