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 supply air path (2) through which air is supplied to at least one fuel cell stack (3) and an exhaust air path (4) through which the air exiting the at least one fuel cell stack (3) is discharged, wherein the air in the supply air path (2) is compressed by means of an air compression system (5) comprising 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 downstream of the at least one turbine (6) after the at least one compression stage (5.1, 5.2) or between two compression stages (5.1, 5.2) using a gas-to-gas heat exchanger (7) with air from the exhaust air path (4). The invention further relates to an air system (1) and a fuel cell system (17) with an air system (1) according to the invention.

Description

[0001] The invention relates to a method for operating an air system that supplies 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.

[0002] The preferred application area of ​​the invention is mobile fuel cell systems or vehicles in which drive energy is generated with the help of fuel cells. State of the art

[0003] Fuel cells convert a fuel, such as hydrogen, and oxygen into electrical energy, heat, and water. Air, generally ambient air, typically serves as the oxygen source. The air supply is provided by an air system that includes an intake air path for supplying air and an exhaust air path for removing the air exiting the fuel cells.

[0004] Since the electrochemical reaction in the fuel cells requires a certain air mass flow and a certain pressure level, the air is compressed beforehand. For this purpose, an air compression system is integrated into the supply air path, comprising at least one thermal turbomachine driven by an electric motor and / or a turbine to compress the air. If present, some of the energy used for compression can be recovered using the turbine (energy recuperation).

[0005] High-performance air systems for supplying multiple fuel cell stacks include multi-stage, usually two-stage, air compression and one- or multi-stage energy recuperation from the air exiting the fuel cell stacks, using at least one turbine 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, especially in multi-stage air compression systems. To protect the components located in the supply air path, the air is cooled or intermediate-cooled after compression and, in multi-stage systems, typically also between compression stages. For cooling, a gas-to-gas heat exchanger integrated into the supply air path can be used, which utilizes the air exiting the fuel cell stacks to cool the air in the supply air path.Cooling using a gas-to-gas heat exchanger may not be sufficient, so that to protect the components located in the supply air path, the operating range must be restricted and / or a performance derating must be carried out. This can be particularly relevant in the following cases. - high ambient temperatures (driving in hot climates) - reduced air pressures (mountain travel) and / or - Full load / high load operation.

[0006] The present invention is concerned with the objective of optimizing the cooling, in particular the intermediate cooling of the air in the supply air path, so that the aforementioned limitations do not have to occur.

[0007] To solve the problem, the method with the features of claim 1 and the air system with the features of claim 5 are proposed. Advantageous embodiments of the invention can be found in the respective dependent claims. Furthermore, a fuel cell system with an air system according to the invention is described. Disclosure of the invention

[0008] A method for operating an air system is proposed, comprising a supply air path through which air is supplied to at least one fuel cell stack, and an exhaust air path through which the air exiting the fuel cell stack is discharged. The air in the supply air path is compressed by means of an air compression system comprising at least one compression stage and at least one turbine integrated into the exhaust air path as a drive mechanism. The compressed air is cooled downstream of the at least one turbine, either 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.

[0009] In the proposed method, the air in the supply air path is cooled by the air in the exhaust air path, utilizing the air downstream of the at least one turbine. Downstream of the at least one turbine, the air in the exhaust air path is coolest, thus increasing the cooling effect of the gas-gas heat exchanger and / or achieving better cooling or intercooling of the air in the supply air path. 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 respect 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.

[0010] If the compression system comprises only one compression stage, this stage is preferably driven by an electric motor and the turbine. The air in the exhaust air path downstream of the turbine then cools the air in the supply air path after compression, i.e., downstream of the single compression stage. The turbine impeller and the compressor impeller can be mechanically coupled via a common shaft.

[0011] If the compression system has multiple compression stages, one or more of these 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 provide intermediate cooling. This depends on the positioning of the gas-to-gas heat exchanger within the supply air path. For intermediate cooling, the gas-to-gas heat exchanger is integrated into the supply air path between two compression stages. For post-compression cooling, the gas-to-gas heat exchanger is integrated into the supply air path downstream of the last compression stage. This protects the fuel cell stack (at least one) from excessively high supply air temperatures.

[0012] The proposed method for cooling or intercooling the air in the supply air path can be combined with other cooling options, such as a water injection system, exhaust air recirculation, and / or connection to a coolant circuit using an additional heat exchanger. This allows for further enhancement of the cooling / intercooling of the air in the supply air path.

[0013] 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 airflow direction of the supply air path is operated at a higher load than the at least one subsequent compression stage. This means that the pressure ratio is highest above the first compression stage. The first compression stage, or the compression stage to which ambient air is supplied, therefore performs the largest share of the air compression. Since air compression is more efficient from a lower temperature level than from a higher temperature level, an improvement in system efficiency can be achieved in this way. This is because the temperature level of the ambient air is below the temperature level of the at least one fuel cell stack in large and relevant areas of system operation.The ambient temperature level is not reduced by subsequent compression stages. This is particularly true when the cooling or intermediate cooling of the air in the supply air path is achieved using the air in the exhaust air path.

[0014] Furthermore, it is proposed that the air compression system comprise several compression stages and that at least the first compression stage in the airflow direction of the supply air path be driven by the at least one turbine. This applies particularly if the first compression stage operates at a higher load than the at least one subsequent 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 output. If not only the air compression but also the energy recuperation is multi-stage, it is proposed that the first compression stage in the airflow direction be driven by the turbine with the highest pressure ratio.This measure improves the impeller matching process between the impellers of the multi-stage compression and multi-stage energy recovery systems. This matching process requires fulfilling numerous criteria, particularly considering the axial and radial forces encountered during operation. The proposed matching increases the system's robustness, which in turn leads to benefits in terms of the unit's service life and, consequently, cost reduction.

[0015] Preferably, the air compression system comprises at least one compression stage which, alternatively or additionally to the turbine, is driven by an electric motor. This applies particularly if the air compression system has only one compression stage. In a multi-stage air compression system, preferably at least two compression stages are driven by electric motors so that balancing can be achieved.

[0016] Furthermore, to solve the aforementioned problem, an air system for a fuel cell system with at least one fuel cell stack is proposed. The air system comprises: - an air supply path through which air can be supplied to at least one fuel cell stack, - an exhaust air path through which the air exiting from the at least one fuel cell stack can be discharged, - an air compression system integrated into the supply air path with at least one compression stage and at least one turbine as a drive, which is integrated into the exhaust air path, as well as - a gas-to-gas heat exchanger integrated into the supply air path and the exhaust air path for cooling the air in the supply air path with the air in the exhaust air path.

[0017] According to the invention, the gas-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.

[0018] 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-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-gas heat exchanger, which is very advantageous for the design, cost, and service life of the components through which the air flows.

[0019] Advantageously, the gas-to-gas heat exchanger forms a single unit with the compressor unit of the air compression system. This allows for a very compact, space-saving arrangement.

[0020] According to a preferred embodiment of the invention, the air compression system has several compression stages, and the first compression stage in the airflow direction in the supply air path has at least one turbine, preferably in combination with an electric motor, as its drive. 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 an advantageous 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.

[0021] Furthermore, the exhaust air path preferably includes a turbine bypass with an integrated bypass valve to bypass the at least one turbine, preferably also the gas-to-gas heat exchanger. With this preferred variant, very low pressures can be achieved in the at least one fuel cell stack, for example, for drying processes during shutdown or when performing a freeze-start.

[0022] Furthermore, it is proposed that an additional gas-to-gas heat exchanger be provided for air cooling, 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. This additional gas-to-gas heat exchanger allows for post-compression air cooling, while the first gas-to-gas heat exchanger is used for intercooling. The additional gas-to-gas heat exchanger can also increase the enthalpy of the fluid at the turbine inlet. Alternatively or in addition to the additional gas-to-gas heat exchanger, the first gas-to-gas heat exchanger can also be combined with another cooling device, such as a water injection system, exhaust air recirculation, and / or a connection to a coolant circuit (preferably via a heat exchanger).

[0023] Since the air system according to the invention is particularly suitable for use in a fuel cell system, a fuel cell system with at least one fuel cell stack and an air system according to the invention is also proposed. The air system can 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 can be increased during air compression.

[0024] Preferred embodiments of the invention are explained in more detail below with reference to the accompanying drawings. These show: Fig. 1 a schematic representation of a fuel cell system with a first air system according to the invention, Fig. 2 a schematic representation of a fuel cell system with a second air system according to the invention and Fig. 3 a schematic representation of a fuel cell system with a third air system according to the invention. Detailed description of the drawings

[0025] The Fig. Figure 1 shows an exemplary fuel cell system 17 with a first air system 1 according to the invention. The air system 1 comprises a supply air path 2, through which several fuel cell stacks 3 can be supplied with air, and an exhaust air path 4 for removing the air exiting the fuel cell stacks 3. The air required by the fuel cell stacks 3 is taken from the environment 16 and purified of harmful particles and substances by means of an air filter 15 integrated into the supply air 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 supply air 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 it receives the air exiting the fuel cell stacks 3. 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.

[0026] Since the air heats up during compression, it is cooled or intercooled after each compression stage 5.1, 5.2. Both cooling and intercooling are achieved using gas-to-gas heat exchangers 7 and 11, respectively. These gas-to-gas heat exchangers are integrated into the supply air path 2 and the exhaust air path 4, respectively. The first gas-to-gas heat exchanger 7, used for intercooling, utilizes the air downstream of the turbine 6, which has a lower temperature. Therefore, the air in the exhaust air path 4 is never cooler than downstream of the turbine 6. This increases the cooling effect of the gas-to-gas heat exchanger 7, resulting in particularly effective intercooling. Furthermore, since air compression from a lower temperature is more efficient than from a higher temperature, the overall system efficiency can be improved.Furthermore, a lower temperature level results in the supply air path downstream of the gas-gas heat exchanger, which is very advantageous for the design, cost, and service life of the components through which the air flows. Cooling the air after the second compression stage 5.2 using the additional gas-gas heat exchanger 11 ensures that acceptable temperatures are reached at the inlet of each fuel cell stack 3. It also increases the enthalpy of the fluid at the turbine inlet.

[0027] To bypass turbine 6 and the two gas-gas heat exchangers 7 and 11, a turbine bypass 9 with an integrated bypass valve 10 is provided. For this purpose, the turbine bypass 9 branches off from the exhaust air path 4 upstream of the second gas-gas heat exchanger 11 and rejoins the exhaust air path 4 downstream of the first gas-gas heat exchanger 7. Alternatively, the turbine bypass 9 can also connect to 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. Shut-off valves 14 are provided to isolate the fuel cell stacks 3 individually from the air system 1.

[0028] A modification (not shown) of air system 1 of the Fig. 1. A single-stage air compression system 5 can be used instead of the one in the Fig. The multi-stage air compression system 5 shown in Figure 1 is a "simple" system for moderate system pressures, saving installation space and costs. Furthermore, the second gas-to-gas heat exchanger 11 can be omitted.

[0029] Another modification of air system 1 of the Fig. 1 is the Fig. 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 means of a turbine 6. This means that multi-stage air compression and multi-stage energy recuperation are provided. The turbine bypass 9 is extended accordingly and has a further bypass valve 10, so that each turbine 6 can be bypassed individually. With the help of the turbine bypass 9, the two gas-gas heat exchangers 7, 11 can also be bypassed here, whereby the turbine bypass 9 – analogous to the Fig. 1 - can also flow back into the exhaust air path 4 upstream of the gas-gas heat exchanger 7 (see dashed line).

[0030] In the operation of the in the Fig. In the air system 1 shown in Figure 2, intercooling is achieved using the gas-to-gas heat exchanger 7, which utilizes the air in the exhaust air path 4 downstream of the turbines 6. Preferably, the first compression stage 5.1 is 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 respective higher pressure ratios therefore correspond directly to the ambient pressure.

[0031] Another air system 1 according to the invention is in the Fig. Figure 3 shows that a multi-stage air compression system 5 is provided for air compression, comprising a first compression stage 5.1 and a second compression stage 5.2, each driven by an electric motor 8 and a turbine 6, respectively. In contrast to the air system 1 of the Fig. 2 Here, the air exiting the fuel cell stacks 3 first flows through the turbine 6 coupled to the first compression stage 5.1 and exhibiting the higher pressure ratio, and only then through the turbine 6 coupled to the second compression stage 5.2. This means that the flow guidance or the sequential expansion by means of the two turbines 6 takes place in reverse order.

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 bythat the air compression system (5) comprises several 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 by 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 by 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 escaping from the at least one fuel cell stack (3) can be discharged, - an air compression system (5) integrated into the supply air path (2) with at least one compression stage (5.1, 5.2) and at least one turbine (6) as drive, which is integrated into the exhaust air path (4), and - a gas-gas heat exchanger (7) integrated into the supply air path (2) and the exhaust air path (4) for cooling the air in the supply air path (2) with the air in the exhaust air path (4), characterized bythat the gas-gas heat exchanger (7) is arranged in the supply air 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 by 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 by 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 bythat 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 by that a further gas-gas heat exchanger (11) is provided for cooling the air, 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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