METHOD AND DEVICE FOR HUMIDIFYING CATHODE AIR IN A FUEL CELL SYSTEM AND FUEL CELL SYSTEM

Abstract

The invention relates to a method for humidifying air in a supply air path (2) of a fuel cell system (1) by means of water injection, wherein product water produced on the cathode side is used, with said product water being separated from the humid exhaust air introduced into the exhaust air path (3) with the aid of a water separator (4) integrated into the exhaust air path (3), wherein, depending on the load, the liquid water content of the exhaust air is varied by means of the temperature of the exhaust air.

The invention also relates to a device for humidifying air in a supply air path (2) of a fuel cell system (1) and to a fuel cell system (1) comprising a device according to the invention.

Claims

1. A method for humidifying air in a supply air path (2) of a fuel cell system (1) by means of water injection, wherein product water produced on the cathode side is used, with said product water being separated from the humid exhaust air introduced into the exhaust air path (3) with the aid of a water separator (4) integrated into the exhaust air path (3), wherein, depending on the load, the liquid water content of the exhaust air is varied by means of the temperature of the exhaust air.

2. The method according to claim 1, wherein, for medium to low loads, the temperature of the exhaust air is lowered so that the liquid water content of the exhaust air increases.

3. The method according to claim 1, wherein the temperature of the exhaust air is controlled via the coolant temperature of a cooling circuit via which the waste heat of the fuel cell system (1) generated during operation is removed.

4. The method according to claim 3, wherein the amount of water required for humidification is determined using the coolant temperature of the cooling circuit.

5. The method according to claim 3, wherein the coolant temperature, is pilot operated, preferably using information concerning future load requirements and/or learned pilot experience values.

6. A method according to claim 1, one of the preceding claims, wherein product water separated with the aid of the water separator (4) is collected in a water tank (5) and injected into the supply air path (2) with the aid of a metering device (6).

7. The method according to claim 6, wherein the fill level in the water tank (5) is monitored.

8. A device for humidifying air in a supply air path (2) of a fuel cell system (1) by water injection, comprising a water separator (4) that is integrated into an exhaust air path (3) to separate product water produced on the cathode side, a water tank (5) for collecting the product water separated using the water separator (4), as well as a metering device (6) located at the supply air path (2) and connected to the water tank (5) via a water line (7).

9. The device according to claim 8, wherein a pump (8) is integrated into the water line (7).

10. The device according to claim 8, wherein the water tank (5) is connected to a further water separator (9), which is located on the anode side for separating product water produced on the anode side.

11. A fuel cell system (1) comprising a device according to claim 8 for humidifying air in a supply air path (2) via which a fuel cell stack (10) is supplied with air.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The invention is explained in more detail below with reference to the accompanying drawings. Shown are:

[0024] FIG. 1 a schematic representation of an air system and a fuel cell system according to the invention and

[0025] FIG. 2 different diagrams illustrating a) the metering rate, product water produced on the cathode side and anode side, b) fill level in the water tank and coolant supply temperature, and c) power demand, each over time.

DETAILED DESCRIPTION

[0026] Air is supplied to a fuel cell stack 10 via the air system of a fuel cell system 1 shown in FIG. 1. The air is taken from the environment and first supplied to an air filter 11 via an air supply path 2. Downstream of the air filter 11, a compressor 12 is integrated into the supply air path 2, because the electrochemical reaction in the fuel cells requires a certain mass air flow and a certain pressure level. Downstream of the compressor 12, a metering device 6 is arranged by means of which water can be injected into the supply air path 2 to humidify the air prior to entering the fuel cell stack 10. To further condition the air, a cooler 14 is provided downstream of the metering device. The humid exhaust air exiting the fuel cell stack 10 is discharged via an exhaust air path 3, into which a water separator 4 is integrated. With the aid of the water separator 4, the liquid water content contained in the exhaust air is separated. Downstream of the water separator 4, a turbine 13 is integrated into the exhaust air path 3, which is operatively connected to the compressor 12 and serves for energy recovery. In the case of a shutdown, fuel cell stack 10 may be disconnected from the air system by way of check valves 15, 16. Furthermore, a bypass path 17 with an integrated bypass valve 18 is provided for bypassing the fuel cell stack 10.

[0027] The metering device 6 is connected via a water line 7 to a water tank 5, into which the product water separated from the exhaust air by means of the water separator 4 is introduced. Product water produced on the anode side can also be introduced into the water tank 5, which is separated with the aid of a water separator 9 located on the anode side. For this purpose, only a drain valve 19 located on the water separator 9 must be opened.

[0028] If a sufficient amount of water is present in the water tank 5, it may be supplied to the metering device 6 with the aid of a pump 8. Using the metering device 6, a certain amount of water may then be injected into the supply air path for humidifying the air. The metering can be effected, for example with the aid of a metering valve 20 of the metering device 6.

[0029] The amount of water present in water tank 5 varies depending on the load, so that there is a risk that the product water produced, or the water supply stored in water tank 5, is not sufficient to meet the water demand required for humidifying the air. According to the present invention, the water supply is therefore controlled on a load-dependent basis via the temperature of the exhaust air in the exhaust air path. Particularly at low loads, the temperature of the exhaust air is lowered so that the liquid water content of the exhaust air increases. To lower the exhaust air temperature, preferably the coolant supply temperature of a cooling circuit (not shown) is reduced, which serves to remove the waste heat of the fuel cell stack 10 generated during operation. With the liquid water content of the exhaust air, the water supply in the water tank 5 also increases.

[0030] The relationship between the coolant supply temperature T.sub.K and the water supply W.sub.T in the water tank 5 is exemplarily illustrated in the diagram of FIG. 2b). When the temperature T.sub.K is temporarily lowered compared to a nominal value T.sub.nom, the water supply W.sub.T in the water tank 5 increases. Accordingly, the amount of product water produced on the cathode side (curve A) and the metering rate (curve B) will vary, as shown in the diagram of FIG. 2a). The additional curve C of FIG. 2a) indicates the amount of product water produced on the anode side. The curves were determined at low load, that is, at a power demand of about 20% (see FIG. 2c)).