Fuel cell system, method for operating a fuel cell system

Abstract

The invention relates to a fuel cell system (1) comprising at least one fuel cell (5) which is connected to a cathode intake path (4) for supplying air and to a cathode exhaust path (6) for discharging exhaust, an air compression system (3) being arranged in the cathode intake path (4), the air compression system includes a first compression stage having a first compressor wheel (14) and a second compression stage having a second compressor wheel (24). A first turbine (11) and a second turbine (21) are arranged in the cathode exhaust gas path (6), the first compressor wheel (14) being drivable by the first turbine (11) and the second compressor wheel (24) being drivable by the second turbine (21). The invention also relates to a method for operating a fuel cell system (1).

Description

Technical Field

[0001] The invention relates to a fuel cell system having the features of the preamble of claim 1. The invention further relates to a method for operating a fuel cell system according to the invention. Background Art

[0002] With the help of a fuel cell system, chemical energy can be converted into electrical energy using hydrogen and oxygen. The electrical energy obtained in this way can be used, for example, to drive a vehicle. In this case, the required hydrogen is carried in a suitable tank on board the vehicle. The required oxygen is also taken from the ambient air.

[0003] Before the ambient air is supplied to at least one fuel cell, the ambient air is compressed by means of an air compression system to generate a certain air mass flow and pressure level. As an air compression system, in particular a thermal flow machine can be used, which can be constructed in a single-stage or multi-stage manner and / or in a single-flow or multi-flow manner. In order to recover energy, the air compression system can be coupled to a turbine or an exhaust gas turbocharger, to which the humid air or exhaust gas flowing out of at least one fuel cell is supplied. If the air compression system is implemented in multiple stages, in order to form a compression stage, other technologies besides thermal flow machines can also be used, such as piston engines, screw compressors, Roots compressors or scroll compressors. Summary of the invention

[0004] The object of the present invention is to optimize the operation of a fuel cell system having a multi-stage air compression system, in particular to configure it more efficiently.

[0005] To achieve this object, a fuel cell system is specified having the features of claim 1 and a method for operating a fuel cell system having the features of claim 7. Advantageous embodiments can be derived from the respective dependent claims.

[0006] The present invention is applicable not only to mobile fuel cell systems but also to stationary fuel cell systems.

[0007] A fuel cell system is proposed, in which air is drawn from the environment, compressed, and supplied to at least one fuel cell via a cathode intake path by means of an air compression system comprising a first compression stage with a first compressor impeller and a second compression stage with a second compressor impeller. In the fuel cell system, the exhaust gas flowing out of the fuel cell is also conducted via a cathode exhaust path. According to the invention, a first turbine and a second turbine are arranged in the cathode exhaust path, wherein the first compressor impeller can be driven by the first turbine and the second compressor impeller can be driven by the second turbine. Therefore, it is proposed to implement the first and second compression stages as exhaust gas turbochargers.

[0008] The fuel cell system according to the invention makes it possible to reduce fuel consumption, since less energy is required for operating the air compression system due to the connection of two turbines in the exhaust gas path.

[0009] The fuel cell system is more robust and reliable because, in the event of a failure of one of the two turbines, the energy in the exhaust gas can still be used to compress the air in the air path.

[0010] Another positive aspect is the improved energy recovery through the possibility of coupling in waste heat upstream of the turbine.

[0011] Advantageous developments and refinements of the fuel cell system according to the invention and of the method for operating a fuel cell system are specified in the dependent claims.

[0012] It is advantageous if the first compressor impeller can be driven by means of the first electric motor and / or the second compressor impeller can be driven by means of the second electric motor in order to establish a required air compression ratio in the air path.

[0013] The first bypass path, in which the first bypass valve is arranged, which is connected to the cathode exhaust path, is advantageous because the first bypass path is provided for bypassing the first turbine, thereby enabling the turbine to be coupled into or out of the air compression system. This increases the number of degrees of freedom in the operation management and generally leads to a reduction in the operating costs. The reduction in operating costs is so significant that the additional costs of the additional turbine are justified.

[0014] A second bypass valve is arranged in a second bypass path, wherein the bypass path is connected to the cathode exhaust gas path and is used to bypass the second turbine, and the second bypass path has the same advantages as the first bypass path.

[0015] If the first turbine is designed for a high load range and the second turbine is designed for a low load range, a particular advantage is obtained, because a variable turbine geometry can be approached by two turbines of different designs. In order to optimally operate different load ranges, a complex regulating mechanism such as a variable turbine geometry is not required. This leads to a reduction in manufacturing costs and a higher robustness of the turbine, because the regulating mechanism can be omitted. In addition, by flexibly coupling the two turbines, a wide operating range with a high recovery efficiency can be achieved, which leads to a reduction in hydrogen consumption. Here, it is advantageous if the first bypass valve and / or the second bypass valve are opened, closed or partially opened according to the mass flow required by the first turbine and / or the second turbine. When the fuel cell is operated in a low load range, especially below a first predetermined threshold, the first bypass valve is opened, and the exhaust gas is directed from the exhaust path through the second turbine. On the contrary, when the fuel cell is operated in a high load range, especially above a second predetermined threshold, the second bypass valve is opened, and the exhaust gas is directed from the exhaust path through the first turbine.

[0016] Since the cathode exhaust path conducts humid air as exhaust gas during operation of the fuel cell system, humidification of the cathode intake air can be achieved by introducing at least a portion of the humid air into the cathode intake path. Therefore, no additional humidifying device is required in the cathode intake path. Humidification of the cathode intake air is intended to prevent the proton conducting membrane of at least one fuel cell from drying out. Because in order to be able to conduct protons, the membrane must be sufficiently moist. Since the risk of drying out is particularly high in the area where the cathode intake path enters the fuel cell, the cathode intake air is usually humidified. Therefore, this achieves optimization of water management in the operation of the fuel cell system. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Preferred embodiments of the present invention will be described in more detail below with reference to the accompanying drawings. Figure 1 A schematic diagram of a fuel cell system according to the present invention is shown. DETAILED DESCRIPTION

[0018] from Figure 1 , a fuel cell system 1 according to the invention can be seen by way of example, which has a fuel cell 5 which can be supplied with oxygen via a cathode air inlet path 4. For this purpose, air is drawn in from the environment 2 and supplied to a multi-stage air compression system 3 via an optional air filter 13.

[0019] The multi-stage air compression system 3 has a first compression stage with a first compressor impeller 14 and a second compression stage with a second compressor impeller 24 .

[0020] The fuel cell is also connected to a cathode exhaust path 6. The cathode exhaust path 6 is used to discharge consumed air, water and exhaust gas from the fuel cell to the environment. In the cathode exhaust path 6, a first turbine 11 and a second turbine 21 are arranged.

[0021] A first compressor impeller 14 and a second compressor impeller 24 are each arranged on a shaft 18, wherein the first compressor impeller 14 can be driven by the first turbine 11 and the second compressor impeller 24 can be driven by the second turbine 21. This means that the first and second compression stages are each realized by an exhaust gas turbocharger.

[0022] The first turbine 11 has a turbine wheel which is arranged together with the compressor wheel 14 of the exhaust gas turbocharger on a common shaft 15. The turbine wheel of the first turbine 11 is struck by exhaust gas which, during operation of the fuel cell system 1, flows out of the fuel cell 5 and / or from the second turbine 21 and is conducted through the cathode exhaust gas path 6.

[0023] The second turbine 21 has a turbine wheel which is arranged together with the compressor wheel 24 of the exhaust gas turbocharger on a common shaft 15. The turbine wheel of the second turbine 21 is struck by exhaust gas which, during operation of the fuel cell system 1, flows out of the fuel cell 5 and is conducted through the cathode exhaust gas path 6.

[0024] In addition, the shaft 18 of the first compressor impeller 14 has an electric motor 15, by means of which the first compressor impeller 14 can be driven. The shaft 18 of the second compressor impeller 24 also has an electric motor 25, by means of which the second compressor impeller 14 can be driven.

[0025] The fuel cell system 1 has a first bypass path 10 in the cathode exhaust path 6, in which a first bypass valve 17 is arranged. The first bypass path 10 is used to bypass the first turbine 11, so that the exhaust gas can flow from the cathode exhaust path 6 to bypass the first turbine 11. The first bypass valve 17 can be fully or partially opened or closed according to the power demand of the fuel cell 5. If the bypass valve 17 is fully opened, the entire exhaust gas flows around the first turbine 11, so that the first turbine cannot contribute to driving the first compressor impeller 14.

[0026] The fuel cell system 1 has a first bypass path 10 in the cathode exhaust path 6, in which a first bypass valve 17 is arranged. The first bypass path 10 is used to bypass the first turbine 11, so that the exhaust gas can flow from the cathode exhaust path 6 to bypass the first turbine 11. The first bypass valve 17 can be fully or partially opened or closed according to the power demand of the fuel cell 5. If the bypass valve 17 is fully opened, the entire exhaust gas flows around the first turbine 11, so that the first turbine 11 cannot contribute to driving the first compressor impeller 14.

[0027] The fuel cell system 1 has a second bypass path 20 in the cathode exhaust path 6, and a second bypass valve 27 is arranged in the second bypass path 20. The second bypass path 20 is used to bypass the second turbine 21, so that the exhaust gas can flow from the cathode exhaust path 6 to bypass the second turbine 21. The second bypass valve 27 can be fully or partially opened or closed according to the power demand of the fuel cell 5. If the bypass valve 27 is fully opened, the entire exhaust gas flows around the second turbine 21, so that the second turbine cannot contribute to driving the first compressor impeller 24.

[0028] According to the inventive method for operating a fuel cell system, the first compressor impeller 14 can be driven by the first electric motor 15 and / or by the first turbine 11. In the case of being driven only by the electric motor 15, the first bypass valve 17 is fully opened. In the case of being driven only by the turbine 11, the first electric motor 15 is not working. In addition, it can also be a combination of being driven by the first electric motor 15 and the first turbine 11. In this case, the first bypass valve 17 is partially opened. Similar operation can also be achieved in the second compressor impeller 24, because the second compressor impeller can be driven by the first electric motor 25 and / or by the second turbine 21.

[0029] Depending on the respective power requirement of the fuel cell system, the first bypass valve 17 and / or the second bypass valve 27 can be opened, closed or partially opened depending on the required mass flow through the first turbine 11 and / or the second turbine 21 .

[0030] Since the first compressor impeller 14 can be operated in any mode via the actuator (first turbine 11 and first electric motor 15), and the second compressor impeller 24 can be operated in any mode via the actuator (second turbine 21 and second electric motor 25), any desired power requirement can be met by the two compression stages of the air compression system 3.

[0031] The degrees of freedom of the actuators make it possible to optimize the operating strategy so that the total power PeLEAC=PeLEAC1+PeLEAC2 of the two electric motors 15 and 25 is minimized for each required total mass flow and each required pressure level over the entire operating range. This makes it possible to achieve a significant reduction in consumption, which makes the investment in the electric motors 15, 25 and the two turbines 11, 21 worthwhile from a cost point of view.

[0032] According to another embodiment of the invention, the first turbine 11 has a fixed geometry which is preferably designed for a high load range, and the second turbine 21 has a fixed geometry which is preferably designed for a low load range.

[0033] In this other embodiment, when the fuel cell 1 is operated in the low load range, the first bypass valve 17 is opened so that the exhaust gas is directed from the exhaust path only through the second turbine 21. If necessary, the first compressor impeller 14 can be driven only by the first motor 15. Accordingly, when operating in the high load range, the second bypass valve 27 is opened and the exhaust gas is directed from the exhaust path only through the first turbine 11. If necessary, the second compressor impeller 24 can be driven only by the second motor 25.

[0034] In another embodiment, the impellers of the first turbine 11 and the second turbine 21 have different geometries and are designed for different load ranges, wherein the impeller of the first turbine 11 is optimized for a low load range and the impeller of the second turbine 21 is optimized for a high load range.

[0035] In an optional embodiment, the fuel cell system 1 may have two intercoolers 8 in the cathode intake path 4. The first intercooler 8 is arranged between the two compressor impellers 14 and 24, and the second intercooler 8 is arranged downstream of the second compressor impeller 24. The first intercooler 8 is optional. The second intercooler 8 can also be replaced by a water spray device (not shown). Upstream of the first intercooler 8, a connecting pipe 7 merges into the cathode intake path 4, and the other end of the connecting pipe is connected to the cathode exhaust path 6. A controllable valve 9 is also arranged in the connecting pipe 7. If the valve 9 is opened, due to the pressure ratio, the exhaust gas or humid air flows from the cathode exhaust path 6 through the connecting pipe 7 into the cathode intake path 4. In this way, the cathode intake air is humidified and cooled. At the same time, part of the exhaust gas is recirculated and utilized.

Claims

1. A fuel cell system (1) comprising at least one fuel cell (5) which is connected to a cathode air inlet path (4) for air supply and to a cathode exhaust path (6) for exhaust gas discharge, wherein: An air compression system (3) is arranged in the cathode intake path (4), and the air compression system includes a first compression stage having a first compressor impeller (14) and a second compression stage having a second compressor impeller (24), characterized in that a first turbine (11) and a second turbine (21) are arranged in the cathode exhaust path (6), wherein the first compressor impeller (14) can be driven by the first turbine (11), and the second compressor impeller (24) can be driven by the second turbine (21).

2. The fuel cell system (1) according to claim 1, characterized in that: The first compressor impeller (14) can be driven by means of a first electric motor (15) and / or the second compressor impeller (24) can be driven by means of a second electric motor (25).

3. The fuel cell system (1) according to claim 1, characterized in that: A first bypass path (10) is connected to the cathode exhaust path (6), a first bypass valve (17) being arranged in the first bypass path, wherein the first bypass path (10) is configured to bypass the first turbine (11).

4. The fuel cell system (1) according to claim 1, characterized in that: A second bypass path (20) is connected to the cathode exhaust path (6), a second bypass valve (27) being arranged in the second bypass path, wherein the second bypass path (20) is provided for bypassing the second turbine (21).

5. The fuel cell system (1) according to any one of the preceding claims, characterized in that The first turbine (11) is designed for a low load range, and the second turbine (21) is designed for a high load range.

6. The fuel cell system (1) according to any one of the preceding claims, characterized in that A connecting pipe (7) flows into the cathode air inlet path (4), and the connecting pipe is directly or indirectly connected to the cathode exhaust path (6), so that a partial mass flow of exhaust gas can be introduced from the cathode exhaust path (6) into the cathode air inlet path (4) through the connecting pipe (7).

7. Method for operating a fuel cell system (1) according to any one of the preceding claims, characterized in that The first compressor impeller (14) is driven by the first electric motor (15) and / or by the first turbine (11), and the second compressor impeller (24) is driven by the second electric motor (25) and / or by the second turbine (21).

8. The method according to claim 7, characterized in that The first bypass valve (17) and / or the second bypass valve (27) are opened, closed or partially opened depending on the required mass flow through the first turbine (14) and / or the second turbine (24).

9. The method according to claim 5, characterized in that When the fuel cell (5) is operated in a low load range, the second bypass valve (27) is opened and exhaust gas is directed from the exhaust path through the first turbine (11).

10. The method according to claim 7, characterized in that When the fuel cell (5) is operated in a high load range, the first bypass valve (17) is opened and exhaust gas is directed from the exhaust path through the second turbine (21).

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