METHOD FOR REGULATING THE HUMIDITY OF A MEMBRANE OF A FUEL CELL

Abstract

The invention relates to a method for regulating the humidity of a membrane (12) of a fuel cell, comprising the steps of compressing a cathode gas (2) by means of a compressor (22) and humidifying a cathode gas (2) by supplying water to the cathode gas (2) by means of a supply device, the supply device comprising an injection valve (26) by means of which the water is supplied to the already compressed cathode gas (2) on demand.

Claims

1. A method for regulating the humidity of a membrane (12) of a fuel cell, comprising the steps of: compressing a cathode gas (2) by means of a compressor (22), and thereafter humidifying the cathode gas (2) by supplying water to the cathode gas (2) by means of a supply device, wherein the supply device has an injection valve (26) via which the water is supplied to the already compressed cathode gas (2) according to requirements.

2. The method as claimed in claim 1, characterized in that the water that is supplied to the cathode gas (2) via the supply device according to requirements is obtained from exhaust air of the fuel cell.

3. The method as claimed in claim 1, characterized in that the water contained in the supply device is removed from the supply device at least partially before the fuel cell is switched off.

4. The method as claimed in claim 1, characterized in that a larger amount of water than necessary is optionally supplied to the membrane (12).

5. The method as claimed in claim 1, characterized in that the humidity of the cathode gas (2) is determined during operation of the fuel cell by means of a hygrometer (28), and the injection of water is regulated on the basis of the currently determined humidity.

6. The method as claimed in claim 1, characterized in that the humidity of the cathode gas (2) is first increased by a gas/gas exchanger (24) and then the amount of water that is still lacking from the fresh air is supplemented from the injection of water.

7. A system (1) for regulating the humidity of a membrane (12) of a fuel cell, comprising: a compressor (22) for compressing a cathode gas (2), and a supply device for humidifying the cathode gas (2) by supplying water to the cathode gas (2), wherein the supply device has an injection device (26) for supplying water to the cathode gas (2), and wherein the injection valve (26) is arranged between the compressor (22) and the cathode (14) of the fuel cell.

8. The system (1) as claimed in claim 7, characterized in that the system (1) has a hygrometer (28) for detecting a current humidity of the cathode gas (2), wherein the hygrometer (28) is arranged between the injection valve (26) and the cathode (14).

9. The system (1) as claimed in claim 7, characterized in that the system (1) has a reservoir (30) for storing and/or cooling water obtained from the exhaust air of the fuel cell.

10. The system (1) as claimed in claim 7, characterized in that the reservoir (30) and/or the lines that connect the reservoir (30) to the injection valve (26) have a heating device for heating the water, wherein the heating device is activatable.

11. The system (1) as claimed in claim 7, characterized in that the system (1) has a hygrometer (28) for detecting a current humidity of the cathode gas (2), wherein the hygrometer (28) is arranged between the injection valve (26) and the cathode (14) and is electrically connected to the supply device and/or the injection valve (26).

12. The system (1) as claimed in claim 7, characterized in that the system has a gas/gas exchanger (24).

13. The system (1) as claimed in claim 7, characterized in that the reservoir (30) and/or the lines that connect the reservoir (30) to the injection valve (26) have a heating device for heating the water, wherein the heating device is activatable at ambient temperatures around 0 C.

14. The method as claimed in claim 1, characterized in that the water that is supplied to the cathode gas (2) via the supply device according to requirements is obtained from exhaust air of the fuel cell, wherein the water is collected in a reservoir (30) before being supplied to the supply device.

15. The method as claimed in claim 1, characterized in that the water that is supplied to the cathode gas (2) via the supply device according to requirements is obtained from exhaust air of the fuel cell, wherein the water is cooled in a reservoir (30) before being supplied to the supply device.

16. The method as claimed in claim 1, characterized in that the water contained in the supply device is removed from the supply device completely before the fuel cell is switched off.

17. The method as claimed in claim 1, characterized in that air is evacuated from the supply device at least partially before the fuel cell is activated.

18. The method as claimed in claim 1, characterized in that air is evacuated from the supply device completely before the fuel cell is activated.

19. The method as claimed in claim 1, characterized in that a larger amount of water than necessary is optionally supplied to the membrane (12) when the reservoir (30) is substantially completely full.

20. The method as claimed in claim 1, characterized in that a larger amount of water than necessary is optionally supplied to the membrane (12) when the reservoir (30) is substantially completely full, at ambient temperatures in the region of 0 C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In the figures:

[0018] FIG. 1 shows a schematic representation of a system for regulating the humidity of a membrane according to the prior art;

[0019] FIG. 2 shows a schematic representation of a system according to the invention for regulating the humidity of a membrane.

[0020] In the figures, identical reference numerals are used for the same technical features.

DETAILED DESCRIPTION

[0021] FIG. 1 shows a schematic representation of a system 1 for regulating the humidity of a membrane 12 according to the prior art. The system 1 comprises a fuel cell with an anode 10 and a cathode 14 which are separated from one another by the membrane 12. For cooling the fuel cell, a cooling unit 16 comprising a cooling circuit 18 is arranged on the side of the cathode 14 of the fuel cell. Both the anode 10 and the cathode 14 are electrically connected to the membrane 12. During operation, the anode gas 2, which in the present case is hydrogen, flows around the anode 10 of the fuel cell. For this purpose, the hydrogen 2 is conducted into the anode supply line 8a by opening the shut-off valve 4. By regulated opening of the metering valve 6, the anode gas 2 flows through the second part of the anode supply line 8a arranged downstream of the metering valve 6, before the anode gas 2 reaches the anode 10. The cathode gas 2, in the present case oxygen-containing fresh air, is introduced on the opposite side to the anode gas 2. The air 2 is drawn in and first filtered in the air filter 20. Filtering the air serves both to protect the fuel cell components, in particular the catalyst material of the fuel cell, and to protect the remaining components of the fuel cell system from harmful particles and gaseous impurities from the air that is drawn in. After filtration, the cathode gas 2 is humidified with the aid of the gas/gas exchanger 24 by guiding the fresh air 2 past the exhaust air which likewise passes through the gas/gas exchanger 24 and is laden with product water. For air evacuation purposes, a bypass provided with a shut-off valve 4 is arranged at the gas/gas exchanger 24. The exhaust air leaves the fuel cell on the cathode side 14 via the cathode discharge line 8b and passes through the gas/gas exchanger 24. In this manner, the water formed on the side of the cathode 14 in the reaction is reused for humidifying the fresh air 2. After humidification of the fresh air 2 within the gas/gas exchanger 24, the air 2 is compressed by means of the compressor 22 before the humidified and compressed cathode gas 2 is supplied to the cathode 14 via the cathode supply line 8b and the chemical reaction between the anode and cathode gas 2, 2 takes place at the membrane. The anode gas 2 not consumed in the reaction in the meantime leaves the fuel cell via the anode discharge line 8a and is supplied to the cycle again via the anode supply line 8a. A problem with this type of design is in particular the fact that a very large amount of energy must be used for compressing the cathode gas 2 before it is supplied to the fuel cell. The high compression energy results from the heating of the cathode gas 2 during humidification in the gas/gas exchanger 24. Heating of the cathode gas 2 during humidification in the gas/gas exchanger 24 is in turn attributable to the exhaust air heated by the heat of reaction. In order to be able to introduce a sufficient amount of oxygen per volume despite the heating of the humidified cathode gas 2 and the resulting lowered density, the cathode gas 2 must be compressed in a manner that is expensive in terms of energy. Only in that manner is it possible to ensure optimum chemical conversion of the reactants and as constant an energy supply as possible.

[0022] FIG. 2 shows a schematic representation of a system 1 according to the invention for regulating the humidity of a membrane 12 in distinction to FIG. 1. In contrast to the system 1 shown in FIG. 1, the system 1 according to the invention does not have a gas/gas exchanger 24 for humidifying the fresh air 2 introduced on the cathode side of the fuel cell. Instead, an injection valve 26 is used for humidifying the fresh air 2. By means of the injection valve 26 and the hygrometer 28 it is possible to detect the current humidity of the fresh air 2 and control the humidification by the injection valve 26 according to the current humidity. The injection valve 26 receives the necessary water via a pump 32, which conveys the water from a reservoir 30 to the injection valve 26. The water obtained from the exhaust air is stored and preferably cooled in the reservoir 30. Between the pump 32 and the cathode gas supply line 8b there is further arranged a shut-off valve 34 which is preferably used for evacuating air from the supply device before the fuel cell is started. For controlling the system according to the invention, the individual system components are connected to one another via communication links, not shown here, preferably via a BUS system, in particular a CAN BUS system. Advantageously, the system 1 comprises a superordinate control device, likewise not shown, for controlling the system 1, in particular for controlling the admission of air, the evacuation of air, and the metering according to requirements. The superordinate control device can either be arranged separately or can be integrated into one of the other devices, preferably into the injection valve 26. Alternatively, the superordinate control device can also be arranged remote from the system 1 for regulating the humidity of a membrane 12 and can be connected to the individual components in a wired manner, preferably wirelessly, for communication and control. By using the injection valve 26 according to the invention instead of a gas/gas exchanger 24, not only can valuable space and weight be saved, but the power loss that must be applied for the compression of the supplied fresh air 2 can also be greatly reduced because the supplied fresh air 2 can be humidified by the injection valve 26 very efficiently and without heat transfer, so that the need for excessive compression of the cathode gas 2 is eliminated. In addition to the use of the injection valve 26 instead of a gas/gas exchanger 24, the compressor 22 is further arranged according to the invention not between the device 26 which serves to humidify the fresh air 2 and the cathode 14 but upstream of the injection valve 26. Because the process of compressing the cathode gas 2 necessarily takes place according to the invention before the cathode gas 2 is humidified that is to say when the cathode gas 2 is dry, a particularly efficient and energy-saving compression process is ensured.