AIR SUPPLY DEVICE, FUEL CELL SYSTEM AND VEHICLE

Description

[0001] The invention relates to an air supply device for fuel cell systems with two-stage compression. Furthermore, the invention relates to a fuel cell system with such an air supply device and a vehicle with such a fuel cell system.

[0002] The use of fuel cell systems to generate electrical power, for example for stationary applications or, in particular, as electric drive power for vehicles, is known from the state of the art. PEM fuel cells are frequently used for this purpose, which are supplied with air on the cathode side as an oxygen supplier. Single-stage flow compressors are frequently used for the air supply. These flow compressors are particularly frequently connected to an exhaust air turbine and an electric machine, forming a so-called electric turbocharger or motor-assisted turbocharger. The energy contained in the exhaust air of the respective fuel cell can thus be at least partially recovered and used for the air supply.

[0003] It's always problematic that such setups, with their operating characteristics, operate very close to the surge limit. To ensure reliable supply to the fuel cell during partial load operation without exceeding the surge limit, a so-called bypass valve, also known as a wastegate valve, is typically installed.

[0004] In practice, this leads to a restriction on the free choice of operating points on the one hand and, in the event that already compressed supply air flows over the bypass valve, to a loss of power on the other hand, since only a part of the energy added during compression can be recovered in the exhaust air turbine.

[0005] To counteract this problem, WO 02 / 086997 A2 discloses a fuel cell air supply system designed as a two-stage system. This allows for high variability in volume flows and pressures, regardless of the surge limit. The design is such that the first stage is formed by an electrically driven flow compressor and the second compression stage by a turbocompressor. This is directly coupled to the exhaust air turbine via the shaft and is thus designed as a free-running unit. The exhaust air turbine has a variable turbine guide vane. Although a bypass valve is still provided in the design described there, it would in principle be unnecessary here. Furthermore, DE 10 2021 000329 also discloses an air supply device for fuel cell systems with two-stage compression for supplying fuel cell stacks connected electrically in parallel.

[0006] Fuel cell systems are increasingly being used to provide relatively high levels of power, for example, to power commercial vehicles such as buses, trucks, or the like. In order to achieve simple and, if necessary, scalable power adjustment to different sizes of commercial vehicles, several fuel cell stacks are electrically combined within the fuel cell system. A typical setup, for example, provides two fuel cell stacks electrically connected in parallel in such a setup. Since both fuel cell stacks are to be controlled independently of each other with regard to both the fuel supply and the air supply, two air supply systems are typically provided in parallel in the setups known from the general state of the art.These are usually two separate electric turbochargers in the sense described above, which then require two bypass valves, two inverters, two electric motors, and the like. The object of the present invention is to simplify this design, which is known in principle from the prior art, and to improve it with regard to the achievable variability in the air supply of the fuel cell systems.

[0007] According to the invention, this object is achieved by an air supply device for fuel cell systems having the features of claim 1, and in particular in the characterizing part of claim 1. Furthermore, a fuel cell system having such an air supply device achieves the object. A vehicle having such a fuel cell system can also achieve the object. Advantageous embodiments and further developments of the air supply device and the vehicle are also disclosed in the dependent subclaims.

[0008] The air supply device according to the invention provides a turbocompressor for each fuel cell stack to supply fuel cell stacks operated electrically in parallel. This turbocompressor is mechanically coupled to an exhaust air turbine for the respective fuel cell stack. At least one electrically driven flow compressor is provided upstream of the turbocompressor in the direction of compressed air flow, which supplies air to at least two of the turbocompressors in parallel.

[0009] The turbocompressors, each designed as a free-running unit in direct mechanical connection with an exhaust air turbine, form the second compressor stage of a two-stage air supply system. They are each assigned to one of the fuel cell stacks, which operate in parallel within the overall system and are typically electrically connected in parallel. However, an electrical series connection would also be conceivable in principle.

[0010] The first compression stage in the air supply device is now formed for at least two of the turbocompressors by an electrically driven flow compressor. In the typical setup with two fuel cell stacks, an electrically driven flow compressor as the first compression stage supplies two turbocompressors as the second compression stage, which in turn each supply a fuel cell stack assigned to them. The exhaust air from the cathode side of this fuel cell stack is then discharged into the environment via the exhaust air turbine connected to the respective turbocompressor, so that residual energy in the exhaust air drives the second compression stage for the respective fuel cell stack via the exhaust air turbine. The two turbocompressors then have a common flow compressor, which supplies them with air on the low-pressure side and thus forms the first compression stage for both turbocompressors together.

[0011] This design now makes it possible to utilize the advantages of two-stage compression, thus providing a high degree of freedom in selecting operating points without running the risk of exceeding the surge limit. The design is also compact and efficient, as it relies on simple and efficient free-running compressors without an electric motor in the area of ​​the two turbocompressors as the second stage. The first compression stage uses a similarly simple flow compressor, which can, for example, have a larger compressor wheel circumference than the two turbocompressors, in order to provide the required air volume at moderate speeds and a correspondingly simple and cost-effective drive motor.

[0012] The overall design then requires only a single electric motor with the corresponding converter for both parallel fuel cell stacks, and thanks to the two-stage compression, the blow-off valves can be dispensed with. This results in significant cost and space savings due to the reduced number of components, particularly the electrical components and their control systems.

[0013] According to an extremely advantageous development of the air supply device according to the invention, at least one of the exhaust air turbines has a variable turbine guide vane. According to an advantageous development, all of the exhaust air turbines can preferably have this. By means of such a variable turbine guide vane in the region of the exhaust air turbines, the energy provided via the exhaust air from the fuel cell in the respective exhaust air turbine can be varied accordingly in order to achieve a high degree of variability in the drive of the turbocompressor in the second compression stage. As a result, both fuel cell stacks operated in parallel can be controlled largely independently of one another with regard to the amount of air with which they are supplied. This enables very advantageous operation of the respective fuel cell stack with regard to its air supply, largely independent of the other fuel cell stack.Together with a simple controllable dosing valve for the hydrogen on the anode side, each of the fuel cell stacks can be operated independently of the other fuel cell stack.

[0014] Another crucial aspect is that the at least one variable turbine geometry and an electric drive motor of the flow compressor are controlled via a common controller, thus reducing the control effort. In particular, the controller can also control the dosing of hydrogen to the anode side of the respective fuel cell stack, thereby enabling the electrical power provided in the at least two fuel cell stacks operating in parallel to be individually controlled.

[0015] As already mentioned, according to a particularly advantageous embodiment, the structure can have exactly one flow compressor and exactly two turbo compressors and is intended to supply air to two fuel cell stacks operated in parallel.

[0016] A further advantageous embodiment further provides that a charge air cooler is arranged downstream of the turbo compressor in the flow direction in order to cool the charge air, which is hot and typically dry after compression in the two-stage compression process, accordingly and, if necessary, to humidify it via a humidifier downstream of the charge air cooler or in combination with the charge air cooler.

[0017] A fuel cell system according to the invention can now comprise at least two fuel cell stacks and an air supply device of the type described. This allows ideal use of the advantages described with regard to the air supply and the possibilities opened up by the air supply for the efficient operation and individual control of at least two fuel cell stacks within a fuel cell system.

[0018] A vehicle according to the invention accordingly comprises such a fuel cell system with at least two, preferably exactly two, fuel cell stacks. The vehicle can be designed as a commercial vehicle, for example, as a truck or bus.

[0019] Further advantageous embodiments of the air supply device according to the invention and of the fuel cell system and / or vehicle according to the invention also emerge from the exemplary embodiment, which is explained in more detail below with reference to the figures.

[0020] Showing: Fig. 1 shows a representation of a fuel cell system with two fuel cell stacks and two air supply systems according to the prior art; and Fig. 2 shows a structure analogous to that in Fig. 1 with an air supply device in a possible embodiment according to the invention.

[0021] In the presentation of the Figure 1a vehicle 1 is shown very schematically, for example a commercial vehicle. It is intended to obtain its electrical drive power, or at least part of its electrical drive power, from two fuel cell stacks 2, 3. The fuel cell stacks 2, 3 are operated in parallel for this purpose and are in particular electrically connected in parallel. The structure has a common hydrogen source 4, which can be designed, for example, as a compressed gas storage device, as a cryogenic storage device or the like. Hydrogen is fed from the hydrogen source 4 via two separate pressure control and metering valves 5, 6 into the anode chambers 7, 8 of the two fuel cell stacks 2, 3. In the exemplary embodiment shown here, unused hydrogen is released into the environment together with exhaust gases.

[0022] This is merely a schematic representation. It will be clear to those skilled in the art that additional components, such as anode circuits, may be arranged here, or that measures may be taken to feed the exhaust gas into the exhaust air, for example, and dilute it accordingly. However, since the anode side is of secondary importance for the present invention, it will not be discussed further.

[0023] To supply air to the two cathode chambers 9, 10 of the two fuel cell stacks 2, 3, this design incorporates state-of-the-art electric turbochargers 11, 12. They each have an electric drive motor 13, 14 and a turbocompressor 15, 16. An exhaust air turbine 17, 18 of the electric turbocharger is also arranged on a common shaft with these two components. A charge air cooler 21, 22 is also provided in an air supply line 19, 20 to the respective fuel cell stack 2, 3.The supply air lines 19, 20 to the two fuel cell stacks 2, 3 are also connected to the respective exhaust air lines 25, 26 of the fuel cell stacks 2, 3 via so-called bypass valves 23, 24, also referred to as wastegate valves, in order to be able to blow off compressed air downstream of the respective turbocompressor 17, 18 as needed when the respective fuel cell stack 2, 3 is operating at partial load. This is necessary to reliably prevent the surge limit from being exceeded at certain operating points. The design enables the two fuel cell stacks 2, 3 to be supplied with air independently of each other, but is relatively complex in terms of the required components and their control.

[0024] In the presentation of the Figure 2A structure with an air supply device 27 is shown, which offers decisive advantages here. The two fuel cell stacks 2, 3 with their supply and discharge lines as well as the two charge air coolers 21, 22 are analogous to the illustration in Figure 1 presented and understood. This structure, as well as the entire anode side of the fuel cell system, which is also only shown schematically and as an example, will not be discussed further.

[0025] The air supply device 27 serves to supply air to the two fuel cell stacks 2, 3 independently of one another and is designed as a two-stage air supply device 27. The first stage for both fuel cell stacks 2, 3 is a flow compressor 28 with an electric drive motor 29. This is controlled via a common control 30, as are other components mentioned later. The air flow of the flow compressor 28 is then divided into two parallel intake lines 31, 32 and supplies two turbo compressors with air, which are again designated by the reference numerals 15, 16, analogous to the illustration in Figure 1 These two turbocompressors 15, 16 now form the second compression stage and supply the air compressed by them as an oxygen supplier via the charge air coolers 21, 22 through the supply air lines 19, 20 into the cathode chambers 9, 10 of the two fuel cell stacks 2, 3.

[0026] In contrast to the structure in Figure 1 According to the state of the art, they are designed as so-called free-running turbines, which are arranged on a common shaft with their respective exhaust air turbines 17, 18. Without the electric machine of the two electric turbochargers 12, 13 according to the state of the art, however, this design is much simpler and more efficient to implement. In addition to the previously developed Figure 1In the structure described above, both exhaust air turbines 17, 18 each have a variable turbine guide vane 33, 34. Control of these turbine guide vanes 33, 34 is provided independently of one another via the common control system 30. This makes it possible, with a common first stage of compression via the flow compressor 28 via the two turbo compressors 15, 16, to adjust the air quantity supplied to the respective fuel cell stack 2, 3 or its cathode chamber 9, 10 independently of one another. Comparable to the structure in the prior art of Figure 1Despite the simpler design with fewer components, both fuel cell stacks 2, 3 can be controlled independently of each other with regard to their air supply. Together with the control of the metering valves 5, 6, which is already quite easy to implement independently, this enables largely independent operation of the two fuel cell stacks 2, 3. The two-stage design also makes it possible to dispense with the bypass valves 23, 24, although these could, in principle, still be provided if they were needed for other reasons, for example, as a system bypass or the like.

Claims

1. An air supply device (27) for fuel cell systems with a two-stage compression, characterized in that for supply of fuel cell stacks (2, 3) electrically connected in parallel, a turbo compressor (15, 16) is provided for each of the fuel cell stacks (2, 3), which is mechanically coupled to an exhaust gas turbine (17, 18) for the respective fuel cell stack (2, 3), wherein, in flow direction of the compressed air upstream of each turbo compressor (15, 16), at least one electrically driven flow compressor (28) is provided, which supplies air to at least two of the turbo compressors (15, 16) in parallel.

2. The air supply device (27) according to claim 1, characterized in that at least one of the exhaust gas turbines (17, 18) comprises a variable turbine nozzle (33, 34).

3. The air supply device (27) according to claim 1 or 2, characterized in that each of the exhaust gas turbines (17, 18) comprises a variable turbine nozzle (33, 34).

4. The air supply device (27) according to claim 1, 2 or 3, characterized in that the at least one variable turbine nozzle (33, 34) and a drive motor (29) of the flow compressor (27) are controlled via a common control unit (30).

5. The air supply device (27) according to any one of claims 1 to 4, characterized in that one flow compressor (28) and exactly two turbo compressors (15, 16) are provided.

6. The air supply device (27) according to any one of claims 1 to 5, characterized in that in flow direction downstream of the respective turbo compressor (15, 16), an intercooler (21, 22) is arranged in each case.

7. A fuel cell system with at least two fuel cell stacks (2, 3) and an air supply device (28) according to any one of claims 1 to 6.

8. A vehicle (1) with a fuel cell system according to claim 7.

9. The vehicle (1) according to claim 8, characterized by its configuration as a commercial vehicle.