Method for operating an air system, air system, and fuel cell system

Abstract

The invention relates to a method for operating an air system (1) with a feed air path (2), via which air is fed to at least one fuel cell stack (3), and an exhaust air path (4), via which the air exiting from the at least one fuel cell stack (3) is discharged, wherein the air in the feed air path (2) is compressed with the aid of an air compression system (5) which comprises at least one compression stage (5.1, 5.2) and at least one turbine (6) integrated as drive into the exhaust air path (4), and wherein the compressed air is cooled downstream of the at least one compression stage (5.1, 5.2) or between two compression stages (5.1, 5.2) using a gas-gas heat exchanger (7) with air from the exhaust air path (4) downstream of the at least one turbine (6). The invention further relates to an air system (1) and to a fuel cell system (17) having an air system (1) according to the invention.

Description

[0001] Description

[0002] The invention relates to a method for operating an air system which serves to supply at least one fuel cell stack of a fuel cell system with air. Furthermore, the invention relates to an air system for a fuel cell system with at least one fuel cell stack and to a fuel cell system with an air system according to the invention.

[0003] The preferred field of application of the invention is mobile fuel cell systems or vehicles in which drive energy is generated with the help of fuel cells.

[0004] State of the art

[0005] Fuel cells convert a fuel, such as hydrogen, and oxygen into electrical energy, heat, and water. The oxygen source is typically air, generally ambient air. The air supply is provided by an air system that includes an air supply path for supplying air and an exhaust path for removing the air exiting or exhausting the fuel cells.

[0006] Since the electrochemical reaction in the fuel cells requires a certain air mass flow and a certain pressure level, the air is first compressed. For this purpose, an air compression system is integrated into the supply air path. This system comprises at least one thermal turbomachine driven by an electric motor and / or a turbine to compress the air. If available, a portion of the energy used for compression can be recovered with the help of the turbine (energy recuperation). High-performance air systems for supplying multiple fuel cell stacks comprise multi-stage, usually two-stage, air compression and single- or multi-stage energy recuperation from the air or exhaust air exiting the fuel cell stacks with the help of at least one turbine, which is mechanically coupled to a compressor impeller via a shaft.Since the air heats up considerably during compression, high temperatures can occur in the supply air path, particularly in multi-stage air compression. To protect the components arranged in the supply air path, the air is therefore cooled or intercooled after compression and, in multi-stage systems, usually also between compression stages. For cooling purposes, a gas-to-gas heat exchanger integrated into the supply air path can be provided, which uses the air escaping from the fuel cell stacks or exhaust air to cool the air in the supply air path. However, cooling with the aid of a gas-to-gas heat exchanger may not be sufficient, so that the operating range must be restricted and / or power derating must be applied to protect the components arranged in the supply air path. This can particularly apply at high ambient temperatures (hot climate driving), reduced air pressures (uphill driving) and / or full-load / high-load operation.

[0007] The present invention is concerned with the task of optimizing the cooling, in particular the intermediate cooling of the air in the supply air path, so that the above-mentioned restrictions do not have to occur.

[0008] To achieve this objective, the method having the features of claim 1 and the air system having the features of claim 5 are proposed. Advantageous developments of the invention can be found in the respective subclaims. Furthermore, a fuel cell system with an air system according to the invention is specified.

[0009] Disclosure of the invention

[0010] A method is proposed for operating an air system having an air supply path, via which air is supplied to at least one fuel cell stack, and an exhaust air path, via which the air exiting the at least one fuel cell stack is removed. The air in the air supply path is compressed using an air compression system comprising at least one compression stage and at least one turbine integrated into the exhaust air path as a drive. The compressed air is cooled after the at least one compression stage or between two compression stages using a gas-to-gas heat exchanger with air from the exhaust air path downstream of the at least one turbine.

[0011] In the proposed method, the air in the supply air path is cooled with the air in the exhaust air path, with the air downstream of the at least one turbine being used. Downstream of the at least one turbine, the air in the exhaust air path is coolest, so that the cooling effect of the gas-gas heat exchanger is increased and better cooling or intercooling of the air in the supply air path is achieved. For this purpose, the gas-gas heat exchanger is integrated into the exhaust air path downstream of the at least one turbine, or in the case of multiple turbines, downstream of the multiple turbines. With regard to the supply air path, the gas-gas heat exchanger is integrated into the supply air path after the at least one compression stage or between two compression stages.

[0012] If the compression system comprises only one compression stage, this is preferably driven by an electric motor and the turbine. The air in the exhaust air path downstream of the turbine is then used to cool the air in the supply air path after compression, i.e., downstream of the one compression stage. The turbine impeller and the compressor impeller can be mechanically coupled via a common shaft.

[0013] If the compression system has multiple compression stages, one or more compression stages can be turbine-driven. Furthermore, the gas-to-gas heat exchanger can be used to either cool the air in the supply air path after compression or to intermediately cool the air in the supply air path. This depends on the positioning of the gas-to-gas heat exchanger in the supply air path. If intermediate cooling is to be achieved, the gas-to-gas heat exchanger is integrated into the supply air path between two compression stages. If cooling of the air after compression is to be achieved, the gas-to-gas heat exchanger is integrated into the supply air path downstream of the last compression stage. This protects at least one fuel cell stack from excessively high supply air temperatures.

[0014] The proposed method for cooling or intermediate cooling of the air in the supply air path can be combined with further cooling options, such as a water injection device, an exhaust air recirculation and / or a connection to a coolant circuit with the aid of an additional heat exchanger. In this way, the cooling / intermediate cooling of the air in the supply air path can be further increased.

[0015] In a further development of the invention, it is proposed that the air compression system comprises several compression stages, and that the first compression stage in the direction of air flow in the supply air path is operated at a higher load than the at least one subsequent compression stage. This means that the pressure ratio above the first compression stage is greatest. The first compression stage, or the compression stage to which ambient air is supplied, therefore assumes the greatest share of the air compression. Since air compression from a lower temperature level is more efficient than from a higher temperature level, this can improve the system efficiency. This is because the temperature level of the ambient air is below the temperature level of the at least one fuel cell stack in wide and relevant areas of system operation.The ambient temperature level is not undercut by subsequent compression stages. This is especially true if the cooling or intermediate cooling of the air in the supply air path is achieved by the air in the exhaust air path.

[0016] Furthermore, it is proposed that the air compression system comprise multiple compression stages, and that at least the first compression stage in the direction of air flow in the supply air path be driven by the at least one turbine. This applies in particular if the first compression stage is operated at a higher load than the at least one further compression stage. The energy recuperation achieved by the turbine can then be used to reduce the energy consumption of the compressor stage with the highest compressor power.If not only the air compression but also the energy recuperation is implemented in multiple stages, it is proposed that the first compression stage in the direction of air flow be driven by the turbine with the highest pressure ratio. This measure can improve the impeller assignment, the so-called "matching", between the impellers of the multi-stage compression and the multi-stage energy recuperation. This is because a variety of criteria must be met during the assignment, in particular the axial and radial forces occurring during operation must be taken into account. The proposed assignment increases the robustness of the system, which in turn brings advantages in terms of the service life of the units and thus costs.

[0017] The air compression system preferably comprises at least one compression stage, which, alternatively or in addition to the turbine, is driven by an electric motor. This applies in particular if the air compression system has only one compression stage. In a multi-stage air compression system, at least two compression stages are preferably driven by electric motors, allowing balancing.

[0018] To achieve the aforementioned objective, an air system for a fuel cell system with at least one fuel cell stack is also proposed. The air system comprises: an air supply path, via which air can be supplied to the at least one fuel cell stack; an exhaust air path, via which the air exiting the at least one fuel cell stack can be discharged; an air compression system integrated into the air supply path, having at least one compression stage and at least one turbine as a drive, which is integrated into the exhaust air path; and a gas-gas heat exchanger integrated into the air supply path and the exhaust air path for cooling the air in the air supply path with the air in the exhaust air path.

[0019] According to the invention, the gas-to-gas heat exchanger is arranged in the supply air path downstream of the at least one compression stage or between two compression stages, and in the exhaust air path downstream of the at least one turbine. The proposed air system is particularly suitable for carrying out the previously described method according to the invention or can be operated according to this method, so that the same advantages can be achieved. In particular, the cooling or intercooling of the air in the supply air path can be improved with the aid of the gas-to-gas heat exchanger. This simultaneously increases the system efficiency. In addition, a lower temperature level results in the supply air path downstream of the gas-to-gas heat exchanger, which is very advantageous for the design, costs, and service life of the components through which the air flows.

[0020] Advantageously, the gas-to-gas heat exchanger forms a single unit with a compressor unit of the air compression system. This allows for a very compact, space-saving arrangement.

[0021] According to a preferred embodiment of the invention, the air compression system has multiple compression stages, and the first compression stage in the direction of air flow in the supply air path has at least one turbine as its drive, preferably in combination with an electric motor. In this case, the first compression stage can be operated at a higher load than the at least one subsequent compression stage, which also has a beneficial effect on the system efficiency during air compression. This is because the air can be compressed more efficiently from a lower temperature level than from a higher temperature level.

[0022] Furthermore, the exhaust air path preferably has a turbine bypass with an integrated bypass valve for bypassing the at least one turbine, preferably for bypassing the at least one turbine and the gas-to-gas heat exchanger. With the aid of the preferred variant, very low pressures can be achieved in the at least one fuel cell stack, for example, for drying processes during shutdown or during a freeze start.

[0023] Furthermore, it is proposed that a further gas-gas heat exchanger be provided for cooling the air, which is integrated into the supply air path downstream of the air compression system and into the exhaust air path upstream of the at least one turbine. With the aid of the further gas-gas heat exchanger, cooling of the air after compression can be achieved, while the first gas-gas heat exchanger is used for intermediate cooling. Furthermore, with the aid of the further gas-gas heat exchanger, an increase in the enthalpy of the fluid at the turbine inlet can be achieved. Alternatively or in addition to a further gas-gas heat exchanger, the first gas-gas heat exchanger can also be combined with another cooling device, for example a water injection device, an exhaust air recirculation system, and / or a connection to a coolant circuit (preferably by means of a heat exchanger).

[0024] Since the air system according to the invention is particularly suitable for use in a fuel cell system, a fuel cell system comprising at least one fuel cell stack and an air system according to the invention is also proposed. The air system can be used to supply the at least one fuel cell stack with air. The advantages of the air system according to the invention also extend to the fuel cell system. In particular, the system efficiency during air compression can be increased.

[0025] Preferred embodiments of the invention are explained in more detail below with reference to the accompanying drawings. These show:

[0026] Fig. 1 is a schematic representation of a fuel cell system with a first air system according to the invention,

[0027] Fig. 2 is a schematic representation of a fuel cell system with a second air system according to the invention and

[0028] Fig. 3 is a schematic representation of a fuel cell system with a third air system according to the invention.

[0029] Detailed description of the drawings

[0030] Figure 1 shows, by way of example, a fuel cell system 17 with a first air system 1 according to the invention. The air system 1 comprises an air supply path 2, via which a plurality of fuel cell stacks 3 can be supplied with air, and an exhaust air path 4 for removing the air or exhaust air escaping from the fuel cell stacks 3. The air required by the fuel cell stacks 3 is taken from the environment 16 and freed of harmful particles and substances with the aid of an air filter 15 integrated into the air supply path 2. Since the electrochemical reaction in the fuel cells of the fuel cell stacks 3 requires a specific air mass flow and a specific pressure level, a multi-stage air compression system 5 for compressing the air is integrated into the air supply path 2. The air compression system 5 is designed in two stages in this case to enable higher system pressures. A first compression stage 5.1 is driven by an electric motor 8 and a turbine 6. The turbine 6 is integrated into the exhaust air pad 4, so that the air escaping from the fuel cell stacks 3 is fed to it. In this way, the turbine 6 enables partial recovery of the electrical energy used for compression (energy recuperation). A second compression stage 5.2 has only an electric motor 8 as its drive.

[0031] Since the air heats up during compression, it is cooled or intercooled after each compression stage 5.1, 5.2. Both the cooling and the intercooling are achieved in this case with the help of a gas-gas heat exchanger 7, 11. The gas-gas heat exchangers 7, 11 are integrated into the supply air path 2 on the one hand and the exhaust air path 4 on the other. The first gas-gas heat exchanger 7, provided for intercooling, uses the air downstream of the turbine 6, which has a low temperature level. At no point in the exhaust air path 4 is the air cooler than downstream of the turbine 6. This simultaneously increases the cooling effect of the gas-gas heat exchanger 7, so that particularly effective intercooling is achieved. Since air compression from a lower temperature level is more efficient than from a higher temperature level, the efficiency of the system can also be increased.In addition, a lower temperature level results in the supply air path downstream of the gas-to-gas heat exchanger, which is very advantageous for the design, costs, and service life of the components through which the air flows. Cooling the air after the second compression stage 5.2 with the aid of the additional gas-to-gas heat exchanger 11 ensures that acceptable temperatures are achieved at the inlet of each fuel cell stack 3. It also increases the enthalpy of the fluid at the turbine inlet. A turbine bypass 9 with an integrated bypass valve 10 is provided to bypass the turbine 6 and the two gas-to-gas heat exchangers 7, 11. For this purpose, the turbine bypass 9 branches off from the exhaust air path 4 upstream of the further gas-gas heat exchanger 11 and flows back into the exhaust air path 4 downstream of the first gas-gas heat exchanger 7. Alternatively, the turbine bypass 9 can also flow into the exhaust air path 4 upstream of the gas-gas heat exchanger 7 (see dashed line).Furthermore, a stack bypass 12 with an integrated bypass valve 13 is provided to bypass the fuel cell stacks 3. Shutoff valves 14 are provided to individually isolate the fuel cell stacks 3 from the air system 1.

[0032] A modification (not shown) of the air system 1 of Figure 1 can have a single-stage air compression system 5 instead of the multi-stage air compression system 5 shown in Figure 1. This "simple" system for moderate system pressures saves installation space and costs. Furthermore, the second gas-to-gas heat exchanger 11 can be omitted.

[0033] A further modification of the air system 1 in Figure 1 can be seen in Figure 2. The second air compression stage 5.2 of the air compression system 5 is driven here – analogous to the first compression stage 5.1 – by an electric motor and by a turbine 6. This means that multi-stage air compression and multi-stage energy recuperation are provided. The turbine bypass 9 is correspondingly extended and has an additional bypass valve 10 so that each turbine 6 can be bypassed individually. With the help of the turbine bypass 9, the two gas-to-gas heat exchangers 7, 11 can also be bypassed here, whereby the turbine bypass 9 – analogous to Figure 1 – can also flow back into the exhaust air path 4 upstream of the gas-to-gas heat exchanger 7 (see dashed line).

[0034] During operation of the air system 1 shown in Figure 2, the intercooling is achieved with the aid of the gas-gas heat exchanger 7, which uses the air in the exhaust air path 4 downstream of the turbines 6 for this purpose. The first compression stage 5.1 is preferably operated at a higher load than the second compression stage 5.2. This means that the pressure ratio of the first compression stage 5.1 is greater than the pressure ratio of the second compression stage 5.2. In this way, the efficiency of the compression system 5 can be increased, since air compression from a lower temperature level is more efficient than from a higher temperature level. The same preferably applies to the pressure ratio of the turbines 6. This means that the pressure ratio across the turbine 6 coupled to the first compression stage 5.1 is greater than across the turbine 6 coupled to the second compression stage 5.2.The impellers with the higher pressure ratio therefore correspond directly to the.

[0035] Ambient pressure.

[0036] Another air system 1 according to the invention is shown in Figure 3. For air compression, a multi-stage air compression system 5 is provided with a first compression stage 5.1 and a second compression stage 5.2, both of which are driven by an electric motor 8 and a turbine 6. In contrast to the air system 1 in Figure 2, here the turbine 6, which is coupled to the first compression stage 5.1 and has the higher pressure ratio, is first flowed through by the air emerging from the fuel cell stacks 3, and only then the turbine 6, which is coupled to the second compression stage 5.2

[0037] Turbine 6. This means that the flow guidance or the sequential expansion by means of the two turbines 6 takes place in reverse order.

Claims

Claims 1. Method for operating an air system (1) with an air supply path (2), via which air is supplied to at least one fuel cell stack (3), and an exhaust air path (4), via which the air emerging from the at least one fuel cell stack (3) is discharged, wherein the air in the air supply path (2) is compressed with the aid of an air compression system (5) which comprises at least one compression stage (5.1, 5.2) and at least one turbine (6) integrated into the exhaust air path (4) as a drive, and wherein the compressed air is cooled after the at least one compression stage (5.1, 5.2) or between two compression stages (5.1, 5.2) using a gas-gas heat exchanger (7) with air from the exhaust air path (4) downstream of the at least one turbine (6).

2. Method according to claim 1, characterized in that the air compression system (5) comprises a plurality of compression stages (5.1, 5.2) and the first compression stage (5.1) in the flow direction of the air in the supply air path (2) is operated at a higher load than the at least one subsequent compression stage (5.2).

3. Method according to claim 1 or 2, characterized in that the air compression system (5) comprises a plurality of compression stages (5.1, 5.2) and at least the first compression stage (5.1) in the flow direction of the air in the supply air path (2) is driven by means of the at least one turbine (6), preferably by means of the turbine (6) which has the highest pressure ratio.

4. Method according to one of the preceding claims, characterized in that the air compression system (5) comprises at least one compression stage (5.1, 5.2) which, alternatively or additionally to the turbine (6), is driven by means of an electric motor (8).

5. Air system (1) for a fuel cell system (17) with at least one fuel cell stack (3), comprising an air supply path (2) via which air can be supplied to the at least one fuel cell stack (3), an exhaust air path (4) via which the air emerging from the at least one fuel cell stack (3) can be discharged, an air compression system (5) integrated into the air supply path (2) with at least one compression stage (5.1, 5.2) and at least one turbine (6) as a drive, which is integrated into the exhaust air path (4), and a gas-gas heat exchanger (7) integrated into the air supply path (2) and the exhaust air path (4) for cooling the air in the air supply path (2) with the air in the exhaust air path (4), characterized in that the gas-gas heat exchanger (7) in the air supply path (2) downstream of the at least one compression stage (5.1, 5.2) or between two compression stages (5.1, 5.2) and in the exhaust air path (4) downstream of the at least one turbine (6).

6. Air system (1) according to claim 5, characterized in that the gas-gas heat exchanger (7) forms a structural unit with a compressor unit of the air compression system (5).

7. Air system (1) according to claim 5 or 6, characterized in that the air compression system (5) has a plurality of compression stages (5.1, 5.2) and the first compression stage (5.1) in the flow direction of the air in the supply air path (2) has at least one turbine (6), preferably in combination with an electric motor (8), as a drive.

8. Air system (1) according to one of claims 5 to 7, characterized in that the exhaust air path (4) has a turbine bypass (9) with an integrated bypass valve (10) for bypassing the at least one turbine (6), preferably for bypassing the at least one turbine (6) and the gas-gas heat exchanger (7).

9. Air system (1) according to one of claims 5 to 8, characterized in that a further gas-gas heat exchanger (11) for Cooling of the air is provided, which is integrated into the supply air path (2) downstream of the air compression system (5) and into the exhaust air path (4) upstream of the at least one turbine (6).

10. Fuel cell system (17) with at least one fuel cell stack (3) and an air system (1) according to one of claims 5 to 9.

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