Method for operating an air system, control unit

Abstract

The invention relates to a method for operating an air system (1) for supplying at least one fuel cell stack (2) with air, comprising an air supply path (3) and an air compression system (4) integrated into the air supply path (3) with two electrically operated air compression units (4.1, 4.2) connected in series, wherein at least the downstream second air compressor unit (4.2) comprises a compressor impeller (5.1, 5.2) which is coupled via a shaft (6.1, 6.2) to a turbine impeller (7.1, 7.2) of a turbine (9.1, 9.2) arranged in an exhaust air path (8). According to the invention, in order to limit an axial force acting on the second air compressor unit (4.2) while maintaining the air mass flow and the pressure in the supply air path (3), the intermediate pressure between the turbine (9.2) of the second air compressor unit (4.2) and a downstream of the turbine (9.2) further throttling device (10) integrated into the exhaust air path (8) in the form of a further turbine (9.1) connected to the first air compressor unit (4.1) or in the form of a pressure regulating valve is modified, in particular raised. The invention further relates to a control unit for carrying out the method or for carrying out steps of the method.

Description

[0001] The invention relates to a method for operating an air system with the features of the preamble of claim 1. The air system serves to supply at least one fuel cell stack of a fuel cell system with air. Furthermore, the invention relates to a control unit for carrying out the method or for carrying out individual steps of the method.

[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] In vehicles whose propulsion energy is generated using fuel cells, oxygen from the ambient air is typically used as the oxidizing agent. This oxygen reacts with hydrogen in the fuel cells to form water or water vapor, thereby generating electrical power through electrochemical conversion. To provide a variable air mass flow and a corresponding pressure level, ambient air is supplied to the fuel cells via an air conveying and compression system. High-performance air compression systems are known from the prior art; these are generally designed with multiple stages, thus enabling higher system pressures than single-stage air compression systems. Multi-stage air compression is usually achieved using several air compressor units connected in series.The air compressor units are each driven by an electric motor and may also include a turbine for energy recuperation. The air compressor units connected in series can be identical or different in design. Furthermore, a single air compressor unit can be single-stage or multi-stage and / or single- or multi-flow.

[0004] In two air compressor units connected in series, the second unit typically operates at a higher pressure than the first. To recover some of the energy used for compression, the second compressor unit often incorporates a turbine. Depending on the pressure conditions at the compressor and turbine, axial forces can act on a rotor connected to a shaft, and thus on the axial bearing of the air compressor unit's shaft. These forces can vary considerably. This means that the axial forces can cover a wide range. Furthermore, the direction of the axial force can reverse. As the axial forces increase, so do the temperatures in the axial bearing, causing it to age more quickly. This reduces the service life of the air compressor unit, as its functionality and safety are no longer guaranteed.

[0005] The present invention addresses the objective of limiting the axial forces acting on the shaft-rotor unit of the second air compressor unit during steady-state and dynamic operation, particularly under higher pressure requirements in the system. In this way, aging is to be minimized, service life increased, and the risk of axial bearing damage reduced.

[0006] To solve the problem, the method with the features of claim 1 is proposed. Advantageous embodiments of the invention are described in the dependent claims. Furthermore, a control unit for a fuel cell system is specified. Disclosure of the invention

[0007] A method for operating an air system to supply at least one fuel cell stack with air is proposed. The air system comprises a supply air path and an air compression system integrated into the supply air path, with two electrically driven air compression units connected in series, wherein at least the downstream second air compressor unit comprises a compressor impeller that is coupled via a shaft to a turbine impeller of a turbine arranged in an exhaust air path.According to the invention, in order to limit an axial force acting on the second air compressor unit while maintaining the air mass flow and the pressure in the supply air path, the intermediate pressure between the turbine of the second air compressor unit and a further throttling device integrated downstream of the turbine in the exhaust air path in the form of a further turbine connected to the first air compressor unit or in the form of a pressure control valve is changed, in particular increased.

[0008] The proposed method serves to protect components, specifically the second of two series-connected air compressor units in a multi-stage air compression system. This is because limiting the axial force leads to lower temperatures in the axial bearing of the shaft-rotor unit of the second air compressor unit, thus reducing its aging and minimizing the risk of axial bearing damage.

[0009] Axial force limitation is achieved by altering, specifically increasing, the intermediate pressure between the turbine and a further throttling device integrated downstream of the turbine in the exhaust air path. This changes, and in particular reduces, the pressure differential across the turbine. Simultaneously, the pressure differential between the turbine impeller and the compressor impeller of the second air compressor unit decreases, and thus the axial force acting on the compressor unit is reduced.

[0010] To change, and in particular increase, the intermediate pressure between the turbine of the second air compressor unit and the throttle device, various measures can be taken – either individually or in different combinations.

[0011] A first measure to modify, in particular increase, the intermediate pressure between the turbine of the second air compressor unit and the further throttling device involves intensifying the throttling effect of the throttling device. How the throttling can be intensified depends on the design of the throttling device and is therefore topology-dependent. If the throttling device is formed by a turbine of the first compressor unit, the throttling can be intensified by further closing a bypass valve in a turbine bypass to circumvent the throttling device or the turbine connected to the first compressor unit. If the throttling device is formed by a pressure control valve, the pressure control valve can be closed further.

[0012] Another measure to change, in particular increase, the intermediate pressure between the turbine of the second air compressor unit and the further throttling device provides that the air mass flow in the exhaust air path upstream of the turbine of the second air compressor unit is unthrottled.

[0013] To reduce the throttling of the air mass flow, the following can be done: - a bypass valve in a turbine bypass to bypass the turbine of the second air compressor unit is opened further and / or - at least one outlet valve of a fuel cell stack is opened further.

[0014] The suggested steps can each be carried out individually or in combination.

[0015] Another measure to change, in particular increase, the intermediate pressure between the turbine of the second air compressor unit and the further throttling device provides that the drive torques of the two air compressor units are adjusted accordingly.

[0016] The proposed measures for modifying, in particular increasing, the intermediate pressure between the turbine of the second air compressor unit and the further throttling device utilize the degrees of freedom of both electrically driven air compressor units and the existing valves to reduce the axial force by means of balancing, while maintaining the target operating points of the fuel cell stacks. In particular, the degrees of freedom of the actuators, with unchanged requirements regarding the air mass flows and the pressures in the supply air path, enable the degree of freedom required for the component protection function.

[0017] In systems with two turbines, the intermediate pressure between the two turbines can be adjusted to reduce the axial force on the second air compressor unit. This means that the degree of freedom, or "balancing," between the two turbines is used to achieve component protection.

[0018] In systems with only one turbine assigned to the second air compressor unit and a pressure control valve downstream of the turbine, the intermediate pressure between the turbine and the pressure control valve can be changed to reduce the axial force on the second air compressor unit. This means that the degree of freedom, or "balancing," between the turbine and the pressure control valve is used to achieve component protection.

[0019] In a further development of the invention, it is proposed that the axial force acting on the second air compressor unit be calculated or estimated, and that a risk assessment be carried out based on the calculated or estimated axial force. Depending on the result of the risk assessment, at least one of the aforementioned measures can then be implemented to limit the axial force.

[0020] Alternatively or additionally, it is proposed that the temperature in the area of ​​an axial bearing of the second air compressor unit be measured and a risk assessment be carried out based on the measured temperature. Since the temperature in the axial bearing also increases with the axial force, the axial force in the axial bearing can be inferred from the measured temperature.

[0021] The proposed method can be implemented in both steady-state and dynamic operation of the air system or a fuel cell system with a corresponding air system. Advantageously, the method is carried out at higher pressures in the fuel cell stacks of the fuel cell system, for example, at pressures of 2.5 bar, 3 bar, or 4 bar. In this way, the axial force in the axial bearing of the second air compressor unit, which operates under higher loads, can be limited to a level that protects the components from rapid aging and minimizes the risk of axial bearing damage.

[0022] Furthermore, a control unit for a fuel cell system is proposed, wherein the control unit is configured to execute the method or steps of the method according to the invention. In particular, a risk assessment can be carried out with the aid of the control unit, based on which a decision is then made as to whether a measure to limit the axial force according to the proposed method is necessary. The respective measure can then also be initiated or executed by the control unit.

[0023] The invention and its advantages 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 an air system for supplying several fuel cell stacks with air, wherein the air system shown can be operated according to a method according to the invention, Fig. 2 a block diagram to illustrate a preferred sequence of a method according to the invention, Fig. 3 A graphic illustrating the pressures at the compressor and turbine impeller of the first air compressor unit of the air system of the Fig. 1, Fig. 4. A graphic illustrating the pressures at the compressor and turbine impeller of the second air compressor unit of the air system of the Fig. 1 and Fig. 5 a graphic illustrating the rotational speeds of the two air compressor units and the corresponding position of the two turbine bypass valves of the air system of the Fig. 1. Detailed description of the drawings

[0024] The Fig. Figure 1 shows an example of an air system 1 for a fuel cell system 20 with two fuel cell stacks 2. The air system 1 comprises an air supply path 3 through which ambient air 15 is supplied to the fuel cell stacks 2. The air is first fed to an air filter 16 via the air supply path 3 to remove harmful particles and chemical substances. The air is then compressed using a multi-stage air compression system 4. To implement a first compression stage, the air compression system 4 includes a first air compressor unit 4.1, which is driven by an electric motor and a turbine 9.1. Since the air heats up during compression, a cooling device 17 follows the first air compressor unit 4.1, in this example taking the form of a gas-to-gas heat exchanger.Since compression is more efficient at a lower temperature than at a higher temperature, the efficiency of the air compression system 4 can be increased in this way. The first compression stage is followed by a second compression stage, which is realized by a second compression unit 4.2, also driven by an electric motor and a turbine 9.2. The second air compressor unit 4.2 is followed by a further cooling unit 17, which in this case is also designed as a gas-to-gas heat exchanger. For further air conditioning, a humidification unit (not shown) and / or another cooling unit 17 (shown with dashed lines) may also be present. The [unclear text] Fig. The system shown in Figure 1 also features an exhaust air recirculation path 21 with an integrated shut-off valve 22 for recirculating the air exiting the fuel cell stacks 2 into the supply air path 3. Exhaust air recirculation can also be used to humidify and / or cool the air in the supply air path 3 and to inertize the air.

[0025] The turbines 9.1, 9.2 of the two air compressor units 4.1, 4.2 are arranged in an exhaust air path 8, through which the air exiting the fuel cell stacks 2 is discharged. Each of the turbines 9.1, 9.2 has a turbine impeller 7.1, 7.2, which is mechanically coupled to a compressor impeller 5.1, 5.2 via a shaft 6.1, 6.2. Furthermore, each of the turbines 9.1, 9.2 can be bypassed via a turbine bypass 11.1, 11.2 with an integrated bypass valve 12.1, 12.2. A stack bypass 18 with an integrated bypass valve 19 is provided to bypass the fuel cell stacks 2. Both fuel cell stacks 2 can also be isolated from the air system 1 by closing an inlet valve 13 and an outlet valve 14, respectively.

[0026] The second air compressor unit 4.2 is typically operated at a higher pressure level than the first air compressor unit 4.1. Therefore, high axial forces and consequently high temperatures can occur in the axial bearing (not shown) for supporting the shaft 6.2 of the second air compressor unit 4.2. These can lead to rapid aging and thus a reduced service life of the second air compressor unit 4.2. To prevent this, the following can be described using the Fig. The two methods described according to the invention are carried out.

[0027] The procedure is carried out during stationary or dynamic operation of air system 1, according to a control unit (not shown). In a first step S1, the control unit, which has the current operating data, initiates the procedure. In a step S2, an evaluation function for risk assessment is implemented. For this purpose, various data are provided to the control unit, for example, temperature data, which in a step S3 are in or on an axial bearing (in the Fig. 1 (not shown) of the second air compressor unit 4.2, and / or data on an axial force in the axial bearing, which was calculated or estimated in step S4 from measurement data (e.g., pressures, mass flows) and model data. The evaluation function in step S2 leads to step S5, in which the control unit decides whether a limitation of the axial force in the axial bearing of the second air compressor unit 4.2 is necessary. If not ("-"), step S1 is started again. If so ("+"), at least one measure to limit the axial force is initiated in step S6. Measure S6.1 involves opening the bypass valve 12.2 of the turbine bypass 11.2 further. Measure S6.2 involves closing the bypass valve 12.1 of the turbine bypass 11.1 further. Measure S6.3 provides that at least one outlet valve 14 of a fuel cell stack 2 is opened further. Measure S6.Section 4 provides that a pressure regulating valve (in the . Fig. (1 not shown) is further closed. Measure S6.4 is carried out if the first compressor unit 4.1 does not have a turbine 9.1 as a drive, so that the further throttling device 10 downstream of the turbine 9.2 of the second compressor unit 4.2 is implemented by a pressure regulating valve. Furthermore, measure S6.5 can be carried out, in which the drive torques of the electric motor drives of the two air compressor units 4.1, 4.2 are adjusted.

[0028] The graphics of Fig. 3 to 5 are based on simulations and measurements on a test bench with an air system 1 according to the air system 1 of the Fig. 1. They show a limitation of the axial force in the axial bearing of the second air compressor unit 4.2 by a reduction of the pressure difference across the turbine 9.2 of the second air compressor unit 4.2.

[0029] The Fig. The pressures p_in and p_out can be taken from the compressor impeller 5.1 and the turbine impeller 7.1 of the first air compressor unit 4.1, respectively. Fig. Figure 4 shows the pressures p_in and p_out at the compressor impeller 5.2 and the turbine impeller 7.2 of the second air compressor unit 4.2, respectively. Different operating points result in different pressure differences across the turbine 9.2 (see arrows in the Fig. 4) The target values ​​for the two fuel cell stacks 2 with regard to air mass flow and pressure remained unchanged. The graph of the Fig. Figure 5 shows the rotational speed n of the air compressor units 4.1, 4.2 and the position of the turbine bypass valves 12.1, 12.2. The degrees of freedom of the actuators can therefore be used to limit the axial force.

Claims

[1] Method for operating an air system (1) for supplying at least one fuel cell stack (2) with air, comprising an air supply path (3) and an air compression system (4) integrated into the air supply path (3) with two series-connected, electric motor-driven air compression units (4.1, 4.2), wherein at least the downstream second air compressor unit (4.2) comprises a compressor impeller (5.1, 5.2) which is coupled via a shaft (6.1, 6.2) to a turbine impeller (7.1, 7.2) of a turbine (9.1, 9.2) arranged in an exhaust air path (8), characterized byin that, in order to limit an axial force acting on the second air compressor unit (4.2) while maintaining the air mass flow and the pressure in the supply air path (3), the intermediate pressure between the turbine (9.2) of the second air compressor unit (4.2) and a further throttle device (10) integrated downstream of the turbine (9.2) in the exhaust air path (8) in the form of a further turbine (9.1) connected to the first air compressor unit (4.1) or in the form of a pressure control valve is changed, in particular increased. [2] Method according to claim 1, characterized by in that in order to change, in particular increase, the intermediate pressure between the turbine (9.2) of the second air compressor unit (4.2) and the further throttle device (10), the throttling effected by means of the throttle device (10) is intensified. [3] Method according to claim 2, characterized by that to increase the throttling - a bypass valve (12.1) in a turbine bypass (11.1) is further closed to bypass the throttle device (10), in particular the turbine (9.1) connected to the first compressor unit (4.1), or - the throttle device (10), in particular the pressure control valve, is further closed. [4] Method according to one of the preceding claims, characterized by in that, in order to change, in particular increase, the intermediate pressure between the turbine (9.2) of the second air compressor unit (4.2) and the further throttle device (10), the air mass flow in the exhaust air path (8) upstream of the turbine (9.2) of the second air compressor unit (4.2) is dethrottled. [5] Method according to claim 4, characterized by that to de-throttle the air mass flow - a bypass valve (12.2) in a turbine bypass (11.2) is opened further to bypass the turbine (9.2) of the second air compressor unit (4.2) and / or - at least one outlet valve (14) of a fuel cell stack (2) is opened further. [6] Method according to one of the preceding claims, characterized by that in order to change, in particular increase, the intermediate pressure between the turbine (9.2) of the second air compressor unit (4.2) and the further throttle device (10), the drive torques of the two air compressor units (4.1, 4.2) are adjusted accordingly. [7] Method according to one of the preceding claims, characterized by that the axial force acting on the second air compressor unit (4.2) is calculated or estimated and a risk assessment is carried out on the basis of the calculated or estimated axial force. [8] Method according to one of the preceding claims, characterized bythat the temperature in the area of ​​an axial bearing of the second air compressor unit (4.2) is measured and a risk assessment is carried out on the basis of the measured temperature. [9] Control unit for a fuel cell system (20), wherein the control unit is configured to carry out the method or steps of the method according to one of the preceding claims.

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