Method and device for cathode air humidification in a fuel cell system, fuel cell system

Abstract

The invention relates to a method for humidifying air in an air intake path (2) of a fuel cell system (100) by means of a water injection device, in which product water produced on the cathode side is used, said product water being separated from wetted exhaust gas introduced into the exhaust gas path (3) by means of a water separator (4) integrated into the exhaust gas path (3), the portion of liquid water in the exhaust gas varies as a function of the load by the temperature of the exhaust gas. The invention further relates to a device for humidifying air in an air intake path (2) of a fuel cell system (1) and to a fuel cell system (1) having a device according to the invention.

Description

Technical Field

[0001] The invention relates to a method and a device for humidifying cathode air in a fuel cell system. The invention also relates to a fuel cell system having a device according to the invention for humidifying cathode air. Background Art

[0002] Fuel cells convert oxygen and fuel (such as hydrogen) into electricity, heat and water. Air extracted from the environment is usually used as the oxygen supply. Air is supplied to the cathode of the fuel cell and the fuel is supplied to the anode of the fuel cell.

[0003] In order to increase the power output, a large number of fuel cells are connected to form a fuel cell stack. The fuel cell stack is connected to an air intake path for air supply. Since the energy conversion process requires a certain air mass flow and a certain pressure level, the air introduced through the air intake path is first compressed by means of an air compressor arranged in the air intake path. The air is then conditioned to the correct humidity at a predetermined temperature. In particular, the water produced on the anode and / or cathode side during the energy conversion process, i.e., product water, can be used for humidification. The water can be collected in a water tank and metered into the air intake path as required by means of a suitable metering device.

[0004] Humidification of the cathode air with product water requires that a sufficient amount of product water is produced during operation of the fuel cell system so that there is always a certain amount of water in the water tank. However, depending on the load, a negative water balance may occur, resulting in water consumption exceeding the water inflow into the water tank. This is especially true at medium to low loads. Therefore, it is necessary to ensure that there is a sufficient supply of water in the water tank, especially during low load operation. Summary of the invention

[0005] The present invention is directed to this object. To achieve this object, a method having the features of claim 1 and a device having the features of claim 8 are proposed. Advantageous embodiments of the invention can be found in the corresponding dependent claims. In addition, the present invention also provides a fuel cell system with a device according to the invention.

[0006] Disclosure of the Invention

[0007] In the proposed method for humidifying the air in the intake path of a fuel cell system by means of a water injection device, product water produced on the cathode side is used. The product water produced on the cathode side is separated from the moist exhaust gas introduced into the exhaust path by means of a water separator integrated into the exhaust path. In this case, the liquid water content of the exhaust gas is varied depending on the load by the temperature of the exhaust gas.

[0008] The temperature of the exhaust gas has a significant influence on the water absorption capacity of the air. If the temperature of the exhaust gas is reduced, saturation is reached more quickly, so that the liquid water content of the exhaust gas increases. The amount of water separated by means of the water separator can therefore be controlled by varying the liquid water content. This is because the more liquid water the exhaust gas contains, the more product water is separated by means of the water separator, and the more water is available for humidifying the air in the intake path. Thus, by varying the liquid water content via the temperature of the exhaust gas, a negative water balance can be counteracted.

[0009] Since little product water is produced, especially at low loads, there is the risk of a negative water balance, a refinement of the invention provides that at medium to low loads the temperature of the exhaust gas is reduced so that the liquid water content of the exhaust gas is increased.

[0010] According to a preferred embodiment of the present invention, the temperature of the exhaust gas is controlled by the coolant temperature of the cooling circuit, in particular the coolant forward flow temperature (Kühlmittel-Vorlauftemperature), and the waste heat generated by the fuel cell system during operation is discharged through the cooling circuit. By adapting the coolant temperature or the coolant forward flow temperature, the exhaust gas temperature can be controlled, and the liquid water content of the exhaust gas can be controlled by the exhaust gas temperature. In load conditions with a negative water balance, in particular in medium to low load conditions, the coolant temperature or the coolant forward flow temperature is reduced, thereby increasing the liquid water content of the exhaust gas and at least compensating the water balance. In load conditions with a positive water balance, that is, high load conditions, the coolant temperature or the coolant forward flow temperature can be increased again. Alternatively, excess product water can be discharged together with the exhaust gas through the exhaust gas path.

[0011] At low loads, the increase in cooling capacity due to a reduction in the coolant temperature or coolant flow temperature is usually not a problem, because the cooling circuit is designed for high loads and the resulting waste heat. At the same time, waste heat can be reduced at high loads by water injection and liquid water evaporation. Since the cooling circuit has to be operated less dynamically, it is possible to relieve the cooling circuit at the same time.

[0012] Furthermore, it is proposed to determine the amount of water required for humidification using the coolant temperature of the cooling circuit, in particular the coolant upstream temperature. This means that the amount of water required for humidification is no longer predetermined by the temperature of the air in the intake path, but is determined by the coolant temperature or the coolant upstream temperature. Therefore, as long as the actual temperature of the air in the intake path is higher than the coolant temperature or the coolant upstream temperature, a smaller water requirement results.

[0013] Since the cooling circuit has a certain inertia, the coolant temperature or the coolant forward flow temperature is preferably pre-controlled, wherein preferably information about future load requirements and / or learned empirical values ​​are used in the pre-control. For example, information about future load requirements can be provided by a navigation system.

[0014] Preferably, the product water separated by means of the water separator is collected in a water tank and sprayed into the air intake path by means of a metering device. In order to always have enough water to humidify the air in the air intake path, it should be avoided that the water tank is completely emptied. Ideally, in addition to the product water produced on the cathode side, the product water produced on the anode side is also introduced into the water tank, so that all the product water produced during the operation of the fuel cell system can be used to humidify the air in the air intake path. Since there is usually a water separator on the anode side for removing liquid water from the anode gas, it is only necessary to connect the water separator to the water tank.

[0015] Furthermore, it is proposed to monitor the water level in the water tank. By means of the water level monitoring device, a continuous water supply can be ensured.

[0016] Furthermore, a device for humidifying the air in the air intake path of a fuel cell system by means of a water injection device is proposed. The device comprises:

[0017] - a water separator which is integrated into the exhaust gas path and serves to separate product water on the cathode side;

[0018] - a water tank for collecting the product water separated by means of the water separator; and

[0019] A metering device is arranged at the air intake path and is connected to the water tank via a water pipe.

[0020] The device is particularly suitable for implementing the method according to the invention described above, so that the same advantages can be achieved. In particular, by varying the temperature of the exhaust gas in accordance with the load, a negative water balance can be avoided.

[0021] A water pump is preferably integrated in the water pipe connecting the water tank and the metering device. With the help of the pump, the water supply to the metering device can be ensured. In addition, the water can be sprayed in under pressure, so that the water is atomized when sprayed in. In this way, the evaporation of the sprayed water can be promoted.

[0022] The water tank is preferably connected to a further water separator, which is arranged on the anode side and is used to separate the product water produced on the anode side. In this way, the product water produced on the cathode side and the anode side can both be used to humidify the air in the air intake path. Thus, a larger amount of water can be provided.

[0023] Furthermore, a fuel cell system is proposed which has a device according to the invention for humidifying the air in an air intake path through which a fuel cell stack can be supplied with air. With the aid of the device, the air supplied to the fuel cell stack can be adjusted in advance. In particular, the air can be set to the correct humidity at a predetermined temperature. As a result, the efficiency of the fuel cell stack is increased. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The present invention will be described in more detail below with reference to the accompanying drawings, which show:

[0025] Figure 1 a schematic representation of an air system of a fuel cell system according to the invention, and

[0026] Figure 2 Different diagrams showing a) the dosing rate of product water produced on the cathode side and the anode side, b) the water level in the water tank and the coolant feed temperature, and c) the power demand respectively as a function of time. DETAILED DESCRIPTION

[0027] The fuel cell stack 10 is Figure 1 The air system of the fuel cell system 1 shown supplies air. The air is taken from the environment and initially introduced into an air filter 11 via an air intake path 2. Since the electrochemical reaction in the fuel cell requires a certain air mass flow and a certain pressure level, a compressor 12 is integrated into the air intake path 2 downstream of the air filter 11. A metering device 6 is arranged downstream of the compressor 12, by means of which water can be sprayed into the air intake path 2 in order to humidify the air before it enters the fuel cell stack 10. A cooler 14 is arranged downstream of the metering device for further conditioning the air. The moist exhaust gas leaving the fuel cell stack 10 is discharged via an exhaust gas path 3, in which a water separator 4 is integrated. The liquid water content contained in the exhaust gas is separated by means of the water separator 4. Downstream of the water separator 4, a turbine 13 is integrated in the exhaust gas path 3, which is operatively connected to the compressor 12 and is used to recover energy. In the case of shutdown, the fuel cell stack 10 can be separated from the air system by means of shut-off valves 15, 16. Furthermore, a bypass path 17 with an integrated bypass valve 18 is provided for bypassing the fuel cell stack 10 .

[0028] The metering device 6 is connected to the water tank 5 via a water pipe 7, into which the product water separated from the exhaust gas by means of the water separator 4 is introduced. In addition, the product water produced on the anode side can be introduced into the water tank 5, and the product water is separated by means of the water separator 9 arranged on the anode side. For this purpose, it is only necessary to open the drain valve 19 arranged on the water separator 9.

[0029] If there is enough water in the water tank 5, it can be supplied to the metering device 6 by means of the pump 8. Then, a determined amount of water can be sprayed into the air intake path by means of the metering device 6 for humidifying the air. For example, metering can be achieved by means of a metering valve 20 of the metering device 6.

[0030] The amount of water in the water tank 5 varies depending on the load, so that the produced product water or the water supply stored in the water tank 5 is insufficient to cover the water demand required for humidifying the air. Therefore, according to the present invention, the water supply is controlled according to the load via the temperature of the exhaust gas in the exhaust path. In particular, at low loads, the temperature of the exhaust gas decreases, thereby increasing the liquid water content of the exhaust gas. In order to reduce the exhaust gas temperature, it is preferred to reduce the coolant forward flow temperature of a cooling circuit (not shown) for conducting away the waste heat generated by the fuel cell stack 10 during operation. As the liquid water content of the exhaust gas increases, the water supply in the water tank 5 also increases.

[0031] Coolant front flow temperature T K The water supply W in the water tank 5 T For example, the relationship between Figure 2 b) is shown in the diagram. K Relative to the rated value T nom When it temporarily drops, the water supply W in the water tank 5 T The amount of product water produced on the cathode side (curve A) and the dosing rate (curve B) also change accordingly. Figure 2 a) is shown in the diagram. Figure 2 The other curve C of a) shows the amount of product water produced on the anode side. These curves are obtained at low load, that is, at a power demand of about 20% (see Figure 2 c)).

Claims

1. A method for humidifying air in an air intake path (2) of a fuel cell system (1) by means of a water injection device, wherein: Product water produced on the cathode side is separated from moist exhaust gas introduced into the exhaust gas path (3) by means of a water separator (4) integrated into the exhaust gas path (3), wherein the liquid water content in the exhaust gas varies depending on the temperature of the exhaust gas in accordance with the load.

2. The method according to claim 1, characterized in that At medium to low loads, the temperature of the exhaust gas decreases, thereby increasing the liquid water fraction of the exhaust gas.

3. The method according to claim 1 or 2, characterized in that: The temperature of the exhaust gas is controlled by means of the coolant temperature, in particular the coolant feed temperature, of a cooling circuit, through which the waste heat generated by the fuel cell system (1) during operation is dissipated.

4. The method according to claim 3, characterized in that The amount of water required for the humidification is ascertained using the coolant temperature of the cooling circuit, in particular using the coolant feed temperature.

5. The method according to claim 3 or 4, characterized in that: The coolant temperature, in particular the coolant forward flow temperature, is pre-controlled, wherein preferably information about future load requirements and / or learned empirical values ​​are used for the pre-control.

6. The method according to any one of the preceding claims, characterized in that The product water separated by means of the water separator (4) is collected in a water tank (5) and sprayed into the air inlet path (2) by means of a metering device (6).

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

8. A device for humidifying air in an air intake path (2) of a fuel cell system (1) by means of a water injection device, comprising: - a water separator (4) which is integrated into the exhaust gas path (3) and is used to separate the product water produced on the cathode side; - a water tank (5) for collecting the product water separated by means of the water separator (4); as well as - a metering device (6), which is arranged on the air inlet path (2) and connected to the water tank (5) via a water pipe (7).

9. The device according to claim 8, characterized in that A water pump (8) is integrated into the water pipe (7).

10. The device according to claim 8 or 9, characterized in that The water tank (5) is connected to another water separator (9), which is arranged on the anode side and is used to separate product water produced on the anode side.

11. A fuel cell system (1) having a device according to any one of claims 8 to 10 for humidifying air in an air intake path (2) through which a fuel cell stack (10) can be supplied with air.

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