WO2022006606A1

WO2022006606A1 - Method for controlling moisture of a pem fuel cell system of a motor vehicle

Info

Publication number: WO2022006606A1
Application number: PCT/AT2021/060237
Authority: WO
Inventors: David ALDRIAN
Original assignee: Avl List Gmbh
Priority date: 2020-07-05
Filing date: 2021-07-05
Publication date: 2022-01-13
Also published as: DE112021001549A5; AT523992B1; AT523992A1

Abstract

The invention relates to a method for controlling moisture of a PEM fuel cell system, in particular of a cathode portion of a PEM fuel cell system, of a motor vehicle, wherein a cathode mass flow is controlled, characterised in that a cathode mass flow is reduced when the motor vehicle is operated in a minimum load range over a predetermined time period. The invention also relates to a PEM fuel cell system and to the use of such a PEM fuel cell system.

Description

Method for controlling the humidity of a PEM fuel cell system in a motor vehicle

The invention relates to a method for regulating humidity in a PEM fuel cell system, in particular a cathode section of a PEM fuel cell system, of a motor vehicle, with a cathode mass flow being regulated.

The invention further relates to a PEM fuel cell system which is designed to carry out such a method.

In addition, the invention relates to the use of such a PEM fuel cell system.

PEM fuel cell systems and their use in motor vehicles are known from the prior art. It is also known that in such fuel cell systems, moisture plays an important role in controlling and regulating the same. In particular, the sensitive membranes between the anode side and the cathode side in a fuel cell stack should not fall below a defined humidity level. At the same time, however, the moisture within the fuel cell stack must not exceed a certain maximum value in order to avoid an undesirably high level of condensation water in the fuel cell stack and thus flooding of the same.

If the power requirement of the motor vehicle with the fuel cell system is too low over a longer period of time, the fuel cells will dry out. If a motor vehicle is operated for a long period of time in a minimum load operation or idle (ie with only 10% to 20% of a possible maximum load), a larger amount of air must be directed to the cathode in order to still ensure the chemical reactions in the fuel cell stack. However, this leads to the membranes of the fuel cells drying out or the moisture in the fuel cells being reduced. However, such an increase in the air mass flow is necessary in order to get water droplets, which arise during the reaction in the fuel cell stack, out of a cathode section of the fuel cell stack, so that flooding of the fuel cell stack is avoided. In order to remove such water drops from the cathode, a certain pressure difference between a cathode inlet and a cathode outlet is necessary, which is why a cathode stoichiometry is increased when the vehicle is operated in the minimum load range. As explained above, an increase in the cathode stoichiometry leads to a reduction in cell moisture, which subsequently leads to degradation of the fuel cell stack.

In order to counteract this, it is known from the prior art to put the motor vehicle into a so-called stand-by mode when it is being operated in the minimum load range and to completely stop the air supply to the cathode in minimum load operation if an increase in the water content in is detected in the fuel cells, so that chemical reactions in the fuel cell stack are interrupted and the oxygen present there is still chemically converted. However, this has the disadvantage that, as a rule, an air conveying device has to be switched off in order to stop an air supply, which in turn affects the service life of the air conveying device or is at least time-consuming. This is where the invention comes in. The object of the invention is to specify a particularly efficient method for controlling humidity in a PEM fuel cell system of a motor vehicle during long-lasting minimum load operation, in particular degradation of individual components of the fuel cell system and/or the fuel cell stack being at least reduced.

Another goal is to specify a PEM fuel cell system which is designed to carry out such a method.

A further goal is to specify the use of such a fuel cell system.

The object is achieved in that, in a method of the type mentioned at the outset, a cathode mass flow is reduced when the motor vehicle is operated in a minimum load range for a predetermined period of time.

An advantage achieved in this way can be seen in particular in the fact that no complete interruption of the air supply to the cathode is necessary and/or provided.

The method according to the invention consequently regulates in particular a cathode section of the PEM fuel cell system. In the cathode section, air is conducted to a cathode of the fuel cell stack where it chemically reacts with fuel conducted in the anode section. As fuel, PEM fuel cell system uses gaseous hydrogen. Lines and elements downstream of the fuel cell stack can also belong to the cathode section and the anode section of the fuel cell system. The mass flow of the cathode is generally understood to be the mass of air moving to the cathode section per unit of time. In order to convey the air to the cathode section of the fuel cell stack, an air conveying device such as a compressor and/or a turbine is provided in particular, it also being possible for a fan to be arranged as an alternative. The air conveying device is arranged in particular upstream of a fuel cell stack. In order to reduce the cathode mass flow, the settings of the compressor and/or the turbine or the blower, in particular, are then changed, in particular their speed. These changes are specified in particular by a control device which is arranged in the motor vehicle.

In the context of the invention, it was found that with a continuous operation of approximately 11 minutes in minimum load operation between 10% and 20% of a maximum load, a system performance with a constant current drops by up to 540 watts or 6%.

Within the scope of the invention, a minimum load range is understood to mean use between 0% and 20% of an available maximum load. This means much less current is drawn from the fuel cell than would be possible. In order not to allow the relative humidity of the fuel cells to drop, an increase in cathode stoichiometry is dispensed with within the scope of the invention. At constant current, a cathode stoichiometry changes in proportion to the change in the cathode mass flow. Basically, within the scope of the invention, the minimum load range is also to be understood as a range which is subject to a requirement for stable operation of the fuel cell system. The minimum load range is preferably operated as described with a low but at least approximately constant load.

It is advantageous if a cell voltage is measured. For this purpose, a CVM method (Cell Voltage Monitoring) is used in particular, for which purpose each individual cell is preferably connected to a sensor for determining a cell voltage. However, it can also be advantageous if the voltage of cells is measured in pairs rather than the voltage of each individual cell, or if, for example, a voltage of four or five cells is determined together. If the voltage of more than measured together in one cell, the difference between them is then calculated. In particular, a DC/DC converter, which outputs voltage values, is arranged downstream of the fuel cell stack. In principle, it is also possible to determine a cell voltage via THDA.

A cell voltage is particularly preferably also measured, in particular continuously, over a period of time. If this cell voltage changes frequently within a short time and a difference or a difference between a maximum voltage and a minimum voltage is greater than a predetermined value, for example 10 mV, the cathode side is flushed with air in particular. In the context of the invention, it was surprisingly found that the difference between a maximum voltage and a minimum voltage or between a mean value of a voltage and a minimum voltage (or generally a variance) is a good indicator of whether a fuel cell stack is flooded, i.e. whether there is too much water in it Fuel cell stack is.

It is preferred if the minimum load range is smaller than a minimum load range required for stable operation of the fuel cell system. To reach this minimum load range, the mass flow is reduced until the minimum load range is smaller than would be necessary for stable operation of the fuel cell system. The fuel cell system is thus intentionally placed in an unstable state, which is monitored in particular by measuring the cell voltage.

It is favorable here if a cathode mass flow is increased as soon as the cell voltage, in particular a difference between a maximum cell voltage and a minimum cell voltage, exceeds a predetermined limit value. An increase in the cathode mass flow again results in an increase in the cathode stoichiometry and is achieved in particular by changes to the air conveying device. For this purpose, a compressor is preferably started up briefly, for example for about one second, so that more air is conveyed through the cathode section. This flushes water or accumulated liquid droplets from the cathode. In principle, it can also be provided that the compressor is briefly accelerated several times in succession. By rinsing the cathode, the water that has accumulated there or floods the cathode is removed from the cathode. Alternatively or additionally, it is also possible to use the cathode mass to increase current when a predetermined period of time in the minimum load range is exceeded. The compressor is started up when either the time (for example 1 second) is exceeded or, preferably, when the difference between the maximum voltage and the minimum voltage falls below the specified limit value again. Another possibility is to open the stack bypass valve, in particular to increase the cathode mass flow, and to close it again when the mass flow increases.

The fuel cell system, which is operated in an unstable state (minimum load range), is advantageously brought back into a non-critical state by a countermeasure in good time before it collapses. In particular, a cathode mass flow is increased for this purpose, as a result of which the cathode is flushed or “purged” with air. As a result, drops of liquid in particular are removed from the cathode.

As soon as these steps (operating the fuel cell system in a critical state, increasing the cathode mass flow in order to stabilize the fuel cell system again) are completed, the process is restarted and the fuel cell system is operated in a minimum load range. A supplied air mass flow changes dynamically in this process, in which the fuel cell system is operated in a constant minimum load range. The mass flow preferably pulsates.

It is also advantageous if the specified limit value is determined in advance on a test stand. The connection between the cell voltage, in particular between a large difference between a maximum voltage and a minimum voltage or an average value and a minimum voltage and flooding of the fuel cell stack were determined on the test bench. If the cathode is flushed due to a time specification, this time value is also determined in advance on the test bench.

Although it can be beneficial to end the rinsing when a predetermined period of time has been reached, it is advantageous if the increase in the cathode mass flow is ended as soon as the cell voltage falls below the predetermined limit value again. The air purge of the cathode is then terminated since the water has been removed from the cathode and a desired humidity is restored. It is advantageous if the entire vehicle or at least the fuel cell system is calibrated in advance on a test bench. This is particularly favorable in order to obtain empirical values.

It is expedient if a pressure at a cathode inlet and a pressure at a cathode outlet are determined and a pressure difference is calculated. For this purpose, a pressure sensor is arranged in particular upstream of the fuel cell stack. Although a pressure sensor can also be provided downstream of the fuel cell stack, a mass flow is preferably measured at this point, from which a pressure is determined. In order to increase and/or decrease a cathode mass flow, a predetermined pressure difference between a cathode inlet and a cathode outlet is necessary.

It is advantageous if air is always directed to a cathode during the minimum load range. This means that the air supply is not stopped, only the mass flow is reduced. It is not necessary to switch off the air conveying device and there is no provision for so-called stand-by or stop operation of the vehicle.

The further objective is achieved when the PEM fuel cell system of the type mentioned at the outset comprises at least one fuel cell stack, an anode section and a cathode section with an air conveying device.

This results in the same advantages which have been described in detail in connection with the method according to the invention. All of these features, advantages and effects naturally also apply in connection with the PEM fuel cell system according to the invention. The inventions to the invention PEM fuel cell system has at least one fuel cell stack with several 100 individual fuel cells comprising a cathode and an anode section. Furthermore, an anode feed section for introducing anode feed gas (hydrogen) into the anode section of the fuel cell stack and a cathode feed section for introducing cathode feed gas (air) into the cathode section are advantageously provided. The air conveying device is designed in particular as a special compressor. Spent anode off-gas is preferably discharged via an anode discharge section and runs through a water separator. In addition to discharge via a so-called purge valve to the environment and/or into the cathode discharge section, the anode waste gas can also be recirculated via a passive recirculation device, for example in the form of an ejector device. The water separator can also be used in recirculation be arranged onsabschnitt or in the anode feed section downstream of the Ejektorvorrich device. Next, a compressor for promoting Ka method gas (air) is advantageously provided.

Such a PEM fuel cell system is advantageously used in a motor vehicle. The motor vehicle can be a car, but it is advantageous if the motor vehicle is a truck, bus or the like.

Further advantages, features and details of the invention result from the following description in which an exemplary embodiment of the invention is described in detail with reference to the drawing. It shows schematically:

1 shows a schematic representation of the method according to the invention;

2 shows a schematic comparison between an effect of a method according to the invention and the prior art;

3 shows a schematic comparison between a further effect of a method according to the invention and the prior art.

1 shows a schematic representation of the method according to the invention. In the upper area of the figure, a cell voltage is plotted over time, in the middle the curve of a cathode stoichiometry and in the lower area a curve of a voltage difference over time. A diagram of an operation of a vehicle with a PEM fuel cell system in a minimum load operation is shown. It can be seen that a cell voltage in the minimum load range steadily decreases until a cathode stoichiometry is increased. The decrease in cell voltage increases the amount of air required, which subsequently leads to the cells drying out. The decrease in cell voltage therefore leads to an increase in cathode stoichiometry. According to the invention, the cathode stoichiometry is reduced to a minimum level in such a long-lasting minimum load range in order to prevent the cells from drying out. Now, however, water droplets can no longer be pumped out of the cells and there is a risk of the fuel cell stack being flooded. In order to be able to recognize this early, a cell voltage is monitored. It was found that the difference between a maximum voltage and a minimum voltage or between a mean value of a voltage and a minimum voltage (or generally a variance) is a good indicator of whether a fuel cell stack is flooded, i.e. whether too much water is ser in the fuel cell stack. Voltage values are therefore used as a reference for humidity or wetness in the cathode section. As soon as the voltage difference exceeds a predetermined value, a cathode stoichiometry is changed for a short time in the range of one or more seconds in such a way that an air mass flow is increased, as a result of which water droplets are removed from the cathode. This can be seen in the right part of FIG. The fuel cell stack current remains essentially constant.

2 and 3 each show an advantage achieved by the method according to the invention. The left part of FIG. 2 shows a development of a relative humidity in the case of methods known from the prior art and the development of relative humidity in the case of the method according to the invention is shown on the right-hand side. It is clear that the relative humidity in the fuel cell stack remains essentially constant over a long period of time as a result of the method steps according to the invention. 3 again shows a development of a system output power from the prior art on the left and one in the method according to the invention on the right. It can be seen that the system output power is increased by up to 6% by the method according to the invention.

In summary, the method according to the invention achieves the following advantages:

• Increase in relative humidity by up to 10% or more;

• increase a system output power in minimum load operation by up to 6% or more;

• Less dehydration of the cells.

Claims

patent claims

1. A method for controlling humidity of a PEM fuel cell system, in particular a cathode section of a PEM fuel cell system, of a motor vehicle, wherein a cathode mass flow is controlled, characterized in that a cathode mass flow is reduced when the motor vehicle lasts for a predetermined period of time in a Minimum load range is operated.

2. The method according to claim 1, characterized in that the minimum load range is smaller than a minimum load range required for stable operation of the fuel cell system.

3. The method according to claim 1 or 2, characterized in that a cell voltage is measured.

4. The method according to claim 3, characterized in that a cathode mass flow is increased as soon as the cell voltage, in particular a difference between a maximum cell voltage and a minimum cell voltage, exceeds a predetermined limit value.

5. The method according to claim 4, characterized in that the predetermined limit value is determined in advance on a test bench.

6. The method as claimed in claim 4 or 5, characterized in that the increase in the cathode mass flow is terminated as soon as the cell voltage falls below the predetermined limit value again.

7. The method as claimed in one of claims 1 to 6, characterized in that a pressure at a cathode inlet and a pressure at a cathode outlet are determined and a pressure difference is calculated.

8. The method according to any one of claims 1 to 7, characterized in that air is always directed to a cathode during the minimum load range.

9. PEM fuel cell system, which is designed to carry out a method according to any one of claims 1 to 8, characterized in that the PEM fuel cell system has at least one fuel cell station pel, an anode section and a cathode section with an air conveying device.

10. Use of a PEM fuel cell system according to claim 9 in a motor vehicle.