METHOD FOR OPERATING FUEL CELL SYSTEM, FUEL CELL SYSTEM

Abstract

A method of operating a fuel cell system (1), comprising at least one fuel cell (2) to which air is supplied via an air supply path (3), the air being compressed by an air compressor (4) integrated in the air supply path (3) before entering the fuel cell (2) and humidified by a valve unit (5) integrated in the air supply path (3) downstream of the air compressor (4) and through which water is supplied from a tank (7) via a first line by a pump (6). When the temperature of the compressed air in the air supply path (3) is below a required temperature level, the water in the first line is heated.

Description

[Technical Field]

[0001] The present invention relates to a method for operating a fuel cell system and to a fuel cell system suitable for carrying out or operable according to this method. [Background technology]

[0002] A fuel cell converts a fuel, such as hydrogen, and oxygen into electrical energy, heat, and water. Air, particularly ambient air, is used as the oxygen source. The air is supplied to the cathode of the fuel cell via an air supply path. Since the energy conversion process requires a certain air mass flow rate and a certain pressure level, the air supplied on the cathode side is pre-compressed by an air compressor arranged in the air supply path. Before entering the fuel cell, the air is further humidified to prevent the fuel cell membrane from drying out, which would otherwise risk damaging the fuel cell.

[0003] To achieve this, gas-to-gas membrane humidifiers can be used, which transport water, especially product water generated during fuel cell operation, from the outlet side of the fuel cell to the inlet side. Because of the large exchange surface required, such humidifiers require a considerable amount of space to transport the required moisture mass flow rate. Furthermore, water can only be transported if there is sufficient water on the outlet side. Water inevitably remains in the exchange area, which can lead to damage due to ice pressure during freezing. Additionally, these types of humidifiers lack direct control of moisture transport. Therefore, a bypass passage is typically provided on one of the two sides, allowing for adjustment of the humidity at the fuel cell inlet via a bypass flap. Such a concept is difficult to precisely control and is correspondingly expensive.

[0004] Alternatively, humidification can be achieved by spray metering. However, care must be taken to ensure that the air is hot enough. At high loads, this is achieved by the compressor's compression action. At low loads, this heat is not sufficient and the air must be cooled with additional water and subsequently brought back to the target temperature in a heat exchanger. The liquid water must then evaporate to achieve the desired target humidity. Such conditioners require a large design space and are difficult to design. Summary of the Invention [Problem to be solved by the invention]

[0005] The object of the present invention is therefore to provide a concept for humidifying air in the air supply path of a fuel cell system that allows for precise adjustment of the inlet humidity and inlet temperature of the air and that can be easily implemented. [Means for solving the problem]

[0006] To solve this problem, a method having the features of claim 1 and a method according to claim 1 are provided. 8 A fuel cell system is proposed having the following features: Preferred developments of the invention can be read from the respective dependent claims.

[0007] The method of the invention having the features of independent claim 1 and the independent claim 8 The fuel cell system of the present invention having the above configuration has the advantage that it can ensure that the water evaporates when injected into the air supply path under all operating points of the air compressor.

[0008] At low load points, the air in the air supply path is prevented from being cooled. Furthermore, the method of the present invention avoids the need to subsequently bring the air in the air supply path to the target temperature in a heat exchanger. Since no liquid water is generated in the air supply path, it does not have to be evaporated in a heat exchanger to achieve the appropriate target humidity. This results in high energy savings. Furthermore, the heat exchanger before the fuel cell can be designed to be compact, thereby saving costs and design space.

[0009] At certain operating points of the air compressor, particularly at low load points, the water is injected under high pressure to ensure that it is injected as steam or evaporates during injection, so that its boiling point exceeds the corresponding boiling point of water in the compressed air in the air supply path downstream of the air compressor. An air pressure of above approximately 3 bar is particularly preferred.

[0010] Preferably, the pump pumps water from the tank to a pressure level for injection that exceeds the gas pressure in the air supply line, so that when injected into the air supply line, the water can be finely atomized by at least one valve unit.

[0011] Furthermore, the compressed water can be heated in the first pipe to a temperature above the boiling temperature of water in the compressed air in the air supply path downstream of the air compressor, and thus the compressed water is heated in the first pipe to a temperature above the boiling temperature of water at a pressure corresponding to the current pressure of the compressed air in the air supply path downstream of the air compressor.

[0012] In this way, the valve unit can meter hot, but liquid (and therefore easy to meter) water into the air in the air supply path. Stress relief through the valve unit causes the water to evaporate, thereby providing a water supply as steam. This steam supply raises the temperature of the cathode gas.

[0013] At high load points, unheated water can still be supplied.

[0014] Preferably, the energy required to heat the water is provided by a heat pipe from the air compressor, since in this way already existing thermal energy can be used to heat the water at low cost.

[0015] A heater for heating water is a simple and therefore preferred alternative when the energy provided by a heat pipe is not sufficient.

[0016] The valve unit allows the water to be injected as a spray, i.e. in the form of fine droplets that facilitate evaporation. At the same time, this ensures a good distribution of the water in the air supply path. If the water is injected immediately after compression, the vortex at the outlet of the air compressor can be used to ensure optimal mixing of the water and air. At the same time, the air is heated so strongly immediately after the compressor that the water injected by the valve unit evaporates directly.

[0017] To optimize the humidity distribution, the compressed and humidified air is preferably guided through a mixing section. In this way, it is possible to simultaneously ensure that the proportion of liquid water remaining at the end of the mixing section is reduced to an acceptable level, depending on the length of the mixing section. The mixing section is preferably integrated into the air supply path between at least one valve unit and the heat exchanger. In this case, the proportion of liquid water remaining in the air at the end of the mixing section can be vaporized when flowing through the heat exchanger.

[0018] Thus, the proposed fuel cell system has all the components necessary to carry out the inventive method described above, and can accordingly be operated in accordance with such method, thereby achieving the same advantages as the inventive method described above.

[0019] If the second valve unit is connected to the pump and tank via a second pipe, this offers a particular advantage, since water can be injected through the second valve unit at high load points of the air compressor without the need to heat the water.

[0020] A heat exchanger is preferably integrated into the air supply path downstream of at least one valve unit. This allows the air to be brought to the required target temperature before entering the fuel cell. Furthermore, any remaining liquid water present in the air evaporates as it flows through the heat exchanger. In this way, the heat exchanger helps to completely evaporate the injected water. [Brief explanation of the drawings]

[0021] [Figure 1] 1 is a schematic diagram showing a fuel cell system according to the present invention, based on a first embodiment; [Figure 2] FIG. 3 is a schematic diagram showing a fuel cell system according to the present invention, based on a second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0022] 1 shows a schematic topology of a fuel cell system 1 according to a first embodiment of the invention, which comprises at least one fuel cell stack 2 with a cathode 11 and an anode 12. The cathode 11 can be supplied with air via an air supply path 3. The anode 12 can be supplied with fuel, in particular hydrogen, via an anode circuit not shown in detail. Furthermore, the fuel cell 2 has a connection 13 to a cooling circuit, via which the waste heat of the fuel cell 2 can be removed. In addition, connections 14 for electrical components not shown in detail are provided.

[0023] The air supplied to the fuel cell 2 via the air supply line 3 is taken from the surroundings and supplied via an air filter 21 to an air compressor 4 integrated into the air supply line 3. The air is compressed by the air compressor 4 and heated in the process. A valve unit 5 arranged in the air supply line 3 downstream of the air compressor 4 subsequently injects water into the air supply line 3 for humidifying the air. For this purpose, the valve unit 5 is connected via a first pipe 30 and a pump 6 to a tank 7 in which water is stored. This may be deionized water, in particular. By using the valve unit 5 to humidify the air in the air supply line 3, the water can be very finely atomized during injection. This means that tiny droplets are formed that easily evaporate in the heated air and are thus evenly distributed.

[0024] To facilitate mixing, the air supply path 3 forms a mixing section 9 downstream of the valve unit 5. This leads to a heat exchanger 8, which allows the temperature of the compressed and humidified air to be adjusted before it enters the fuel cell 2.

[0025] In the simplest embodiment, the mixing section 9 may be a simple pipe. To facilitate the distribution of the water droplets, fittings (not shown) may be provided which lead to swirling of the air flowing through.

[0026] The air or exhaust air leaving the fuel cell 2 is fed via an exhaust line 15 to a turbine 16 to recover some of the energy previously injected during compression. Before the exhaust air enters the turbine 16, a water separator 17 integrated in the exhaust line 15 removes water from the exhaust air, helping to protect the turbine 16.

[0027] As an alternative to the view of FIG. 1, the valve unit 5 can also be arranged inside the mixing section 9 .

[0028] The valve unit 5 mixes finely atomized water into the air, turning it into moist air and cooling it down.

[0029] The first pipe 30 has a heating device 33 arranged between the pump 6 and the valve unit 5. The heating device 33 serves to heat the water.

[0030] The heating device 33 may be configured as a heat pipe 31, which transports thermal energy from a heat source to the water in the first pipe 30. In one embodiment of the invention, the temperature level required to heat the water in the first pipe 30 by the heat pipe 31 may be provided by the air compressor 4. Alternatively, the heat pipe 31 may be connected to another high temperature component inside the fuel cell system 1.

[0031] In an alternative embodiment, an electric additional heating element 32 may be arranged inside the first pipe 30 to heat the water. This electric additional heating element may also serve to supplement the heat pipe 31 if the temperature level provided by the heat pipe 31 is not sufficient to heat the water in the first pipe 30.

[0032] The heating device 33 can therefore be configured as a heat pipe 31 and / or an electric heating element 32 .

[0033] When the temperature of the compressed air in the air supply line 3 downstream of the air compressor 4 is below a required temperature level, the water in the first pipe 30 is heated.

[0034] The temperature of the compressed air in the air supply path 3 downstream of the air compressor 4 can be determined by a temperature sensor in the air supply path 3. In one embodiment of the invention, the temperature of the compressed air in the air supply path 3 downstream of the air compressor 4 is determined with reference to the load point of the air compressor 4.

[0035] The water in the first pipe 30 is pumped by a pump 6 to a pressure level that exceeds the gas pressure in the air supply line 3 downstream of the air compressor 4 .

[0036] The control of the target humidity at the inlet of the fuel cell 2 can be realized by a water mass flow rate that can be variably adjusted by the valve unit 5. In this way, the water supply can be optimally metered and the respective target humidity can be adjusted in every load state.

[0037] A schematic topology of a fuel cell system 1 according to a second embodiment of the present invention is shown in Figure 2. The fuel cell system 1 shown in Figure 2 corresponds to the topology of Figure 1, except for the differences explained below.

[0038] A second valve unit 51 is arranged in the air supply line 3 downstream of the air compressor 4. Water can also be injected into the air supply line 3 via the second valve unit 51 in order to humidify the air. 51 is connected to the pump 6 and the tank 7 via the second pipe 35 .

[0039] A second valve unit 51 that can be opened or closed can be arranged in the second line 35. The second valve unit 51 serves to meter unheated water into the air supply line 3 at high load points of the compressor 4. [Explanation of symbols]

[0040] 1. Fuel cell system 2 fuel cell 3 Air supply path 4. Air compressor 5 Valve Unit 6. Pump 7. Tank 8 Heat exchanger 9 Mixed Section 30 First Pipe 31 Heat pipe 32 Heating section 33 Heating device 51 Second valve unit

Claims

1. A method for operating a fuel cell system (1), the fuel cell system (1) comprising at least one fuel cell (2) to which air is supplied via an air supply path (3), the air being compressed by an air compressor (4) incorporated in the air supply path (3) before entering the fuel cell (2), and humidified by a valve unit (5) incorporated in the air supply path (3) downstream of the air compressor (4), to which water is supplied from a tank (7) via a first pipe (30) by a pump (6), the method comprising: when the temperature of the compressed air in the air supply line (3) downstream of the air compressor (4) is below a required temperature level, the water in the first pipe (30) is heated by a heating device (33); The water in the first pipe (30) is heated to a temperature above the boiling point of water under a pressure corresponding to the pressure of the air compressed by the air compressor (4), so that the water is injected as steam from the valve unit (5) or evaporates when injected from the valve unit (5). A method, characterized in that

2. 2. A method according to claim 1, characterized in that the pump (6) pumps water to a pressure level above the gas pressure in the air supply line (3) downstream of the air compressor (4).

3. 2. A method according to claim 1, characterized in that the temperature of the compressed air in the air supply path (3) downstream of the air compressor (4) is determined with reference to the load point of the air compressor (4).

4. 4. Method according to any one of claims 1 to 3, characterized in that the energy required to heat the water is provided from the air compressor (4) by means of a heat pipe (31).

5. A method as described in claim 4, characterized in that a heating section (32) is used to heat water when the energy provided by the heat pipe (31) is insufficient.

6. 4. The method according to claim 1, wherein the compressed and humidified air is passed through a mixing section (9) integrated into the air supply line (3) between the valve unit (5) and a heat exchanger (8) in order to optimize the humidity distribution.

7. A fuel cell system (1) including at least one fuel cell (2) capable of supplying air via an air supply path (3), wherein an air compressor (4) for compressing air and at least one valve unit (5) for humidifying air downstream of the air compressor (4) are incorporated in the air supply path (3), and a first valve unit (5) is connected to a tank (7) for storing water via a first pipe (30) and a pump (6), wherein the first pipe (30) has a heating device (33) capable of heating water, the heating device heats the water in the first pipe (30) when the temperature of the compressed air in the air supply path (3) downstream of the air compressor (4) is below a required temperature level; The water in the first pipe (30) is heated to a temperature above the boiling point of water under a pressure corresponding to the pressure of the air compressed by the air compressor (4), so that the water is injected as steam from the valve unit (5) or evaporates when injected from the valve unit (5). A fuel cell system comprising:

8. 8. The fuel cell system (1) according to claim 7, characterized in that the heating device (33) is configured as a heat pipe (31) and / or an electric heating element (32).

9. 9. The fuel cell system (1) according to claim 7 or 8, characterized in that a mixing section (9) is incorporated into the air supply path (3) downstream of the valve unit (5), the mixing section (9) being arranged between the valve unit (5) and a heat exchanger (8).

10. 9. The fuel cell system (1) according to claim 8, characterized in that a second valve unit (51) is connected to the pump (6) and the tank (7) via a second pipe (35), and water is injected through the second valve unit (51) under high load points of the air compressor (4).

11. A fuel cell system (1) comprising at least one fuel cell (2) capable of supplying air via an air supply path (3), the air supply path (3) incorporating an air compressor (4) for compressing air and at least one valve unit (5) for humidifying the air downstream of the air compressor (4), a first valve unit (5) connected to a tank (7) for storing water via a first pipe (30) and a pump (6), the first pipe (30) having a heating device (33) capable of heating water, a second valve unit (51) connected to the pump (6) and the tank (7) through a second pipe (35), and water is injected through the second valve unit (51) under high load points of the air compressor (4); A fuel cell system comprising:

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