DE102013021472A1 - Method and device for monitoring an operating state of at least one fuel cell - Google Patents

Abstract

The invention relates to a method for monitoring an operating state of at least one fuel cell (2), in which an electrical resistance (R) of the at least one fuel cell (2) is determined. It is provided that at a first time (t0) a sudden change in current (.DELTA.I) of the at least one fuel cell (2) is performed and detected and dependent on the current change (.DELTA.I) temporal voltage change (.DELTA.U) of the at least one fuel cell (2). between the first time (t0) and a second time is detected, wherein the electrical resistance (R) is determined based on the voltage change (.DELTA.U). Furthermore, the invention relates to a device (8) for monitoring an operating state of at least one fuel cell (2).

Description

The invention relates to a method for monitoring an operating state of at least one fuel cell according to the preamble of claim 1. Furthermore, the invention relates to a device for monitoring an operating state of at least one fuel cell according to the preamble of claim 8.

From the DE 102 20 172 B4 a method for monitoring the operating state of an electrochemical device is known in which an impedance is measured by means of a measuring device at terminals of the electrochemical device. In an evaluation device, the operating state of the electrochemical device is monitored on the basis of the size of an imaginary part of the measured impedance.

Furthermore, from the DE 10 2010 005 400 A1 a device and a method for controlling the humidity in a fuel cell known, wherein a step response of the output voltage of a fuel cell with interrupted supply electrical consumers detected and determined from the detected step response parameter of the dynamic time behavior, whereby a capacity of the fuel cell is calculated. Depending on a possible deviation from a given capacity, a humidification control unit is caused to increase or decrease the humidification. This makes it possible to record the moisture balance of a fuel cell in situ.

The invention is based on the object to provide a comparison with the prior art improved method and an improved device for monitoring an operating condition of at least one fuel cell.

The object is achieved with respect to the method by the in claim 1 and in terms of the device by the features specified in claim 8.

Advantageous embodiments of the invention are the subject of the dependent claims.

In a method for monitoring an operating state of at least one fuel cell, an electrical resistance of the at least one fuel cell is determined. According to the invention, a sudden change in current of the at least one fuel cell is performed and detected at a first time and a voltage change of the at least one fuel cell dependent on the current change is detected between the first time and a second time, wherein the electrical resistance is determined based on the voltage change becomes.

By means of the method, a determination of the electrical resistance of the at least one fuel cell in a simple manner possible, wherein based on the electrical resistance, an electrical conductivity of a arranged in the at least one fuel cell membrane can be determined by means of a moisture content of a arranged in the at least one fuel cell Membrane can be determined. The method uses the well-known Ohm's law, in which a change in resistance of the fuel cell is determined from the detected voltage change. The electrical conductivity is defined as the reciprocal of the determined electrical resistance of the fuel cell. The method is preferably carried out for monitoring an operating state of a plurality of fuel cells, which are brought together to form a fuel cell module.

For a sudden change in current is the at least one fuel cell with an electrical load, eg. As an energy storage device coupled. For example, when arranging the at least one fuel cell in a vehicle as an electrical consumer, a battery of the vehicle may be used, so that no additional components for a current drain from the fuel cell are necessary.

The change in current preferably takes place in a time span of less than 10 μs, so that it is easily distinguishable from other operational changes in the fuel cell. Furthermore, this is advantageous since a transmission line for detecting the change in current and / or the voltage change is assigned a higher transmission speed for a limited period of time. Thus, the method can also take place during operation of the at least one fuel cell, so that no specific measurement operating state is necessary for carrying out the method.

Since a voltage change of a fuel cell by different sizes, eg. B. the electrical resistance, a passage overvoltage and / or a diffusion resistance, which have different time constants, wherein the electrical resistance has the smallest time constant, the voltage change is detected in less than 100 microseconds after the current change carried out.

Depending on the determined moisture of the membrane, this can be controlled and / or controlled. For example, an increase or a decrease in the moisture of the membrane is due to Setting certain operating parameters, such. As a humidification, a temperature, a stoichiometry and / or pressure of the at least one fuel cell, possible.

A device for monitoring an operating state of at least one fuel cell, by means of which an electrical resistance of the at least one fuel cell can be determined, comprises according to the invention at least one first detection unit and at least one second detection unit, which are coupled together and respectively to the at least one fuel cell, wherein by means of at least a first detection unit is a current change carried out at a first time and by means of the at least one second detection unit a voltage change resulting from the current change between the first time and a second time can be detected, and wherein the electrical resistance can be determined by means of a control unit based on the voltage change.

By means of the device, a determination of the electrical resistance of the at least one fuel cell with a few additional components and in a simple manner possible. A complex measuring circuit is not necessary.

In one possible embodiment, the at least one first detection unit and / or the at least one second detection unit are coupled to one another and / or to the at least one fuel cell via a CAN bus. The CAN bus is already present in the arrangement of the at least one fuel cell in a vehicle, so that no additional transmission lines are necessary. In addition, a transmission speed of the CAN bus can be varied, so that a maximum transmission speed can be assigned to it for a short time for detecting the current and / or voltage change.

To reduce glitches in the change in current, is an electronic switch, for coupling the at least one fuel cell with an electrical load, for. As a metal-oxide-semiconductor field effect transistor, provided, which has an at least a reduced bounce compared to a mechanical switch.

Embodiments of the invention are explained in more detail below with reference to drawings.

Showing:

1 schematically a two-dimensional coordinate system with a current-time curve and a voltage-time curve and

2 schematically a simplified circuit diagram of an apparatus for monitoring an operating condition of a fuel cell.

Corresponding parts are provided in all figures with the same reference numerals.

In 1 is a Cartesian coordinate system 1 represented, which consists of two combined coordinate systems 1 is formed, the abscissa each time t is assigned. The ordinate of the one coordinate system 1 is an electrical voltage U and the ordinate of the other coordinate system 1 an electric current I assigned. Thus, the ordinate of the illustrated coordinate system 1 two parameters assigned.

In the coordinate system 1 is in a lower region a current-time curve SZK a in 2 illustrated fuel cell 2 which shows a detection of an electric current I of the fuel cell 2 represents. The current-time curve SZK has a value-discrete course, wherein at a first time t0 of the fuel cell 2 an electric current I is taken with a certain value, which is characterized in the current-time curve SZK by a sudden change in current .DELTA.I, in particular a current jump, to the specific value on which the current-time curve SZK remains in the course , The current drain from the fuel cell 2 can be done for example by coupling this with an energy storage device. The electric current I can z. B. are passed as a so-called load dump pulse in the energy storage device.

In an upper area of the coordinate system 1 is a voltage-time curve SpZK of the fuel cell 2 which shows a detection of an electrical voltage U of the fuel cell 2 represents and which at the first time t0 a sudden voltage change .DELTA.U, in particular a voltage drop, and subsequently has an exponential decaying course. The sudden voltage change .DELTA.U results according to the Ohm's law from the discharge of the electric current I from the fuel cell 2 ,

Based on the course of the voltage-time curve SpZK is a change over time of an electrical resistance R of the fuel cell with the help of Ohm's law 2 determinable, which for determining a in an unspecified electrolyte membrane of the fuel cell 2 existing moisture content can be used. According to Ohm's law, the electrical resistance R is determined as the quotient of the electric current I and the electrical voltage U. A temporal change of the electrical resistance R is thus a quotient of temporal Current change ΔI and voltage change ΔU can be determined.

In the present embodiment, in particular a proton exchange membrane fuel cell in short: PEM fuel cell, received, which in combination with other fuel cells 2 as fuel cell module is preferably used in fuel cell vehicles.

The fuel cell 2 consists of a membrane-electrode unit, which has two electrodes 2.1 . 2.2 , in particular an anode 2.1 and a cathode 2.2 , between which the electrolyte membrane is arranged.

The membrane-electrode unit is arranged between two bipolar plates, also not shown in detail, which distributes reactants 3 . 4 for the fuel cell 2 via the membrane-electrode assembly and the removal of the reactants 3 . 4 in this provided, in each case to the membrane electrode unit open towards channels, the removal of the heat of reaction and the production of an electrical connection between the anode 2.1 and the cathode 2.2 ,

As reactants 3 . 4 become a fuel 3 and an oxidizing agent 4 used. Usually, gaseous reactants 3 . 4 , short reaction gases, used, for. B. become the anode 2.1 via the bipolar plate hydrogen or a hydrogen-containing gas as fuel 3 fed.

At the anode 2.1 the fuel is oxidized, the thereby released electrons via an electrical load to the cathode 2.2 be dissipated. The protons of the fuel become the cathode via the electrolyte membrane 2.2 transferred. There the protons recombine with that of the cathode 2.2 supplied oxidizing agent 4 , z. As oxygen, and via the electrical load to the cathode 2.2 migrated electrons to water. Thus, the electrolyte membrane provides a closed circuit in the fuel cell 2 for sure. Usually, the electrolyte membrane is formed of organic polymers, so that an electrical conductivity of the electrolyte membrane is dependent on its water content or moisture content. The lower the moisture content of the electrolyte membrane, the higher the electrical resistance R and the lower the electrical conductivity of the electrolyte membrane. The electrical conductivity is defined as the reciprocal of the electrical resistance.

To a low electrical resistance R and thus comparatively high voltages U of the fuel cell 2 At high electric currents I, a corresponding moisture content in the electrolyte membrane is necessary.

The moisture content of the electrolyte membrane can by a choice of favorable operating conditions, which in particular a temperature and pressure of the fuel cell 2 , Mass flows, pressure losses in the fuel cell module, and / or temperature differences between an input and an output of the fuel cell module, are maintained at an at least approximately constant level.

If an excessively low moisture content of the electrolyte membrane is determined on the basis of the change over time of the electrical resistance R, it is possible to initiate appropriate measures which determine the moisture content of the electrolyte membrane of the fuel cell 2 increase or decrease. This includes z. B. additional humidification in the fuel cell 2 initiated reactants 3 . 4 For example, by a recirculation of a wet anode exhaust gas or by placing an additional gas humidifier 5 . 9 on the cathode side. Furthermore, by a targeted change of operating parameters of the fuel cell 2 , z. As a temperature, humidification, stoichiometry or pressure in the fuel cell 2 a moisture content of the membrane can be varied.

As already described, the determination of the moisture content of the electrolyte membrane can be determined on the basis of the temporal voltage change .DELTA.U. The in the coordinate system 1 shown current change .DELTA.I can alternatively be replaced by a current pulse, z. B in the form of a delta pulse or rectangular pulse. It is also possible, a temporal voltage change .DELTA.U after stopping the flow of electricity from the fuel cell 2 capture.

The electrical voltage U of the fuel cell 2 is characterized by different sizes, such. B. the electrical resistance R, a passage overvoltage and / or a diffusion resistance, which have different time constants, so that corresponding changes in the time course of the electrical voltage U in different time periods after the first time t0 can be detected.

In this case, the electrical resistance R has the smallest time constant of the stated variables, which is less than 100 μs, so that the change in the electrical resistance R can be detected in less than 100 μs after the first time t0 on the basis of the voltage change ΔU.

The fuel cell is used to detect the voltage change ΔU 2 with a first detection unit 6 for detecting a current change .DELTA.I coupled, which detects in particular fast current changes .DELTA.I with a time duration of less than 10 microseconds and then a corresponding signal to an also with the fuel cell 2 coupled via a CAN bus second detection unit 7 for detecting a voltage change .DELTA.U outputs.

So is from the first detection unit 6 detects a correspondingly high and fast current change ΔI, followed by the detection of the voltage change .DELTA.U. The amount of current change ΔI can be limited to a few amperes.

Since the CAN bus has only a limited transmission speed, this is during the detection of the voltage change .DELTA.U a maximum transmission speed for a very short period, eg. B. 100 ms assigned. For this purpose, it is necessary that the current change ΔI does not fall short of a period of 10 μs, preferably the time duration of the current change ΔI is less than 10 μs. The current change .DELTA.I preferably has no glitches, which z. B. may occur due to bounce when using a mechanical relay. Therefore, it is advantageous, instead of a mechanical relay, an electronic relay, z. B. a metal oxide semiconductor field effect transistor, also referred to as MOS-FET to use.

After the voltage change .DELTA.U has been detected, a slower transmission speed can again be assigned to the CAN bus.

The method described above may be during operation of the fuel cell 2 be carried out, with a special Messbetriebszustand is not mandatory. Alternatively or additionally, the method can also be used when starting or shutting down the fuel cell 2 and / or in non-operation of the fuel cell 2 respectively.

Furthermore, the method is not limited to the operation of a single fuel cell 2 rather, it is more suitable for operating a plurality of fuel cells 2 , which are combined to one or more fuel cell modules. It can the individual fuel cells 2 each separately a first detection unit 6 , a second detection unit 7 and / or a gas humidifier 5 . 9 be assigned or a fuel cell module is each with a first and a second detection unit 6 . 7 and / or a gas humidifier 5 . 9 coupled.

In 2 is a device 8th for operation of the fuel cell 2 shown in the form of a simplified block diagram. The fuel cell 2 indicates the anode 2.1 , the cathode 2.2 and a tempering device 2.3 for temperature control of the fuel cell 2 on which of a temperature control M, z. B. water, is flowed through. The fuel cell 2 can also be considered as a fuel cell module as already described.

The fuel cell 2 is still with the already described first and second detection devices 6 . 7 coupled, the detection of a current change .DELTA.I and a voltage change .DELTA.U serve. The determination of the change in the electrical resistance R can be effected by means of a control unit, not shown.

Furthermore, the fuel cell 2 the gas humidifier 5 . 9 upstream. The gas humidifier 5 . 9 serves for at least partial humidification of the oxidizing agent 4 so that this is enough water for optimal operation of the fuel cell 2 contains.

The gas humidifier 5 . 9 is a cooling unit in the present embodiment 10 upstream, which for cooling the oxidizing agent 4 after conveyance by means of a compressor 11 serves. The compressor 11 is with an expander 12 Connected, by means of which the exhaust gases of the fuel cell 2 be at least partially relaxed. In addition, the compressor 11 by means of the expander 12 at least partially supplied a drive power.

For dehumidifying a cathode exhaust gas is already described gas humidifier 5 . 9 intended.

The anode 2.1 is to supply the fuel 3 a recirculation device 13 , z. B. a gas jet pump for conveying a hydrogen-rich fluid provided.

LIST OF REFERENCE NUMBERS

1

coordinate system

2

fuel cell

2.1

anode

2.2

cathode

2.3

tempering

3

Reactant, fuel

4

Reactant, oxidizer

5.9

gas humidifier

6

first detection unit

7

second detection unit

8th

contraption

10

cooling unit

11

compressor

12

expander

13

recirculation

t

Time

t0

first time

I

electrical current

M

tempering

R

electrical resistance

SCC

Current-time curve

SpZK

Voltage-time curve

U

electrical voltage

.DELTA.I

current change

.DELTA.U

voltage change

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 10220172 B4 [0002]
• DE 102010005400 A1 [0003]

Claims (10)

Method for monitoring an operating state of at least one fuel cell ( 2 ), in which an electrical resistance (R) of the at least one fuel cell ( 2 ) is determined, characterized in that at a first time (t0) a sudden change in current (.DELTA.I) of the at least one fuel cell ( 2 ) is carried out and detected, and a time change of voltage (ΔU) of the at least one fuel cell (ΔU) which depends on the change in current ( 2 ) is detected between the first time (t0) and a second time, wherein the electrical resistance (R) is determined based on the voltage change (ΔU). A method according to claim 1, characterized in that based on the electrical resistance (R) is an electrical conductivity of a in the at least one fuel cell ( 2 ) arranged membrane is determined. A method according to claim 1 or 2, characterized in that for a sudden change in current (.DELTA.I), the at least one fuel cell ( 2 ) is coupled to at least one electrical load. Method according to one of the preceding claims, characterized in that the current change (ΔI) takes place in a time span of less than 10 μs. Method according to one of the preceding claims, characterized in that the voltage change (ΔU) is detected in less than 100 μs after the current change (ΔI) has taken place. Method according to one of claims 2 to 5, characterized in that based on the determined electrical conductivity, a moisture of the membrane is determined. A method according to claim 6, characterized in that regulated and / or controlled in dependence of the determined moisture. Contraption ( 8th ) for monitoring an operating state of at least one fuel cell ( 2 ), by means of which an electrical resistance (R) of the at least one fuel cell ( 2 ) can be determined, characterized by at least one first detection unit ( 6 ) and at least one second detection unit ( 7 ), which together and in each case with the at least one fuel cell ( 2 ), wherein by means of the at least one first detection unit ( 6 ) a current change (ΔI) carried out at a first time (t0) and by means of the at least one second detection unit ( 7 ) is detected as a function of the current change (.DELTA.I) resulting voltage change (.DELTA.U) between the first time (t0) and a second time and wherein by means of a control unit, the electrical resistance (R) based on the voltage change (.DELTA.U) can be determined. Contraption ( 8th ) according to claim 8, characterized in that the at least one first detection unit ( 6 ) and / or the at least one second detection unit ( 7 ) with each other and / or with the at least one fuel cell ( 2 ) are coupled via a CAN bus. Contraption ( 8th ) according to claim 8 or 9, characterized in that for coupling the at least one fuel cell ( 2 ) is provided with at least one electrical load an electronic switch.

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