Method for operating a multi-stage air compression system, multi-stage air compression system, and fuel cell system

Abstract

The invention relates to a method for operating a multi-stage air compression system (1), comprising a first compressor (1.1) electrically driven and a second compressor (1.2) driven by a turbine (2), the compressors (1.1, 1.2) being arranged in an inlet path (3) of an air system for supplying air to a fuel cell stack (5), and the turbine (2) being arranged in an exhaust path (4). According to the invention, at the start of the air compression system (1), air compressed by the first compressor (1.1) is supplied to the fuel cell stack (5) via a bypass (6) for bypassing the second compressor (1.2), which is connected to the surroundings via at least one valve (7, 8) and / or throttle flap (9) on both the inlet and outlet sides. The invention further relates to the multi-stage air compression system (1) as well as to a fuel cell system having the multi-stage air compression system (1).

Description

Technical Field

[0001] The present invention relates to a method for operating a multi-stage air compression system, and further to a multi-stage air compression system operable based on this method. Additionally, a fuel cell system having a multi-stage air compression system based on the present invention is proposed.

Background Art

[0002] Hydrogen-based fuel cells are considered a future mobility concept. This is because they only emit water as exhaust gas and enable rapid refueling times. In addition to hydrogen, an oxidizing agent such as oxygen is required. As an oxygen supply source, air taken in from the surroundings can be utilized. Oxygen undergoes an electrochemical reaction with hydrogen in the fuel cell and is converted into electrical energy, heat, and water.

[0003] Since the electrochemical reaction in the fuel cell requires a certain air mass flow rate and a certain pressure level, the air supplied to the fuel cell is pre-compressed. In this case, an air compression system that may be manufactured in a single-stage or multi-stage manner, and also in a single-flow or multi-flow manner, is utilized. In mobile applications that require high efficiency and high failure safety, especially in commercial vehicles, a two-stage air compression system is often adopted. This includes a first stage having an electrically driven compressor and a second stage having a compressor driven by a turbine. The second compression stage is also called a turbomachine.

[0004] When starting a multi-stage air compression system, in order to start the turbomachine, a pressure higher than the inlet of the compressor of the turbomachine must be generated at the inlet of the turbine. However, at the start of the system, pressure is generated only through the first electrically driven compression stage. Therefore, in principle, the pressure at the inlet of the turbine is lower than the pressure at the inlet of the compressor of the turbomachine, and thus it does not start.

Summary of the Invention

[0005] The problem addressed by this invention is to eliminate, or at least mitigate, the starting problem when starting a multi-stage air compression system. [Means for solving the problem]

[0006] To solve this problem, a method having the constituent elements of claim 1 and an air compression system having the constituent elements of claim 8 are proposed. Preferred developments of the present invention can be read from the respective dependent claims. Furthermore, a fuel cell system having the air compression system according to the present invention is described.

[0007] A method for operating a multi-stage air compression system is proposed, comprising an electrically driven first compressor and a turbine-driven second compressor, with the compressors located in the intake path of the air system for supplying air to a fuel cell stack, and the turbine in the exhaust path. According to the present invention, at the start of the air compression system, air compressed by the first compressor is supplied to the fuel cell stack via a bypass that goes around the second compressor, which is connected to the surroundings via at least one valve and / or throttle flap on both the inlet and outlet sides.

[0008] Accordingly, at startup, air compressed by the electrically driven first compressor passes near the second compressor. Simultaneously, the pressure generated in the second compressor is reduced to the ambient pressure through its connection to the surroundings. This measure makes it possible to generate a pressure at the turbine inlet that exceeds the ambient pressure and, consequently, the pressure at the inlet of the second compressor.

[0009] In this way, the second compression stage can be started without any problems using only the first compression stage.

[0010] The connection of the second compressor to the surroundings can be implemented in various forms. For example, one valve each on the upstream and downstream sides of the second compressor, such as a four-way valve or a three-way valve, may be incorporated into the air supply path. The valve embodiment as a four-way valve has the advantage that a bypass can be connected to the air supply path via the same valve. As an alternative to the two valves, a valve on the upstream side of the second compressor and a throttle flap on the downstream side of the second compressor may be incorporated into the air supply path. Accordingly, the proposed method can be implemented simply and at low cost.

[0011] In an advanced form of the present invention, it is proposed that after the start of the air compression system, the connection of the second compressor to the surroundings is closed by at least one valve, while the bypass remains open. Based on the pressure generated by the turbine, air flows backward through the bypass. In this way, the start of the turbine can be buffered, for example, to prevent over-rotation of the second compression stage.

[0012] To transition from the starting phase to normal operation, it is proposed that the bypass be closed after the connection of the second compressor to the surroundings is closed. Then the air compressed by the first compressor is supplied to the fuel cell stack via the second compressor, and no longer via the bypass. For pressure control, the bypass can be temporarily reopened in normal operation, thereby allowing at least a partial flow of compressed air to pass near the second compressor through the bypass.

[0013] Furthermore, in order to implement a pump protection function during the normal operation of the air compression system, it is proposed that a second compressor be connected to the surroundings at least temporarily via at least one valve and / or throttle flap. In this way, it is possible to prevent undesirable pump discharges that occur when the pump limits set by the characteristic maps of each compressor are exceeded, which would result in interruption of flow and, in some cases, reversal of flow.

[0014] Alternatively or as an addition, a bypass can be opened to circumvent the second compressor in order to implement a pump protection function during the normal operation of the air compression system.

[0015] Furthermore, it is proposed that a bypass be opened to bypass the second compressor when the air compression system stops. This measure allows the second compression stage to be decelerated more rapidly, thereby reducing bearing wear. This is particularly desirable because, since fuel cell systems typically require that the air be oil-free, the bearings are manufactured as gas bearings. When the rotational speed falls below the so-called levitation speed, which applies to starting and stopping, the "lubricating" air film is no longer present on the bearing, resulting in a higher frictional load on the bearing. If the stopping operation is shortened by the proposed measure, the frictional load will be alleviated.

[0016] To open the bypass, it is preferable that at least one valve or at least one other valve to which the bypass is connected in the supply air path is operated. As already mentioned, at least one valve to which the second compressor can be connected in the surroundings may be manufactured as a four-way valve, thereby enabling simultaneous connection of the bypass to the supply air path through it. Depending on the switching position of the four-way valve, air compressed by the first compressor is supplied to the second compressor and / or the bypass. Furthermore, the four-way valve can be switched to open the bypass and connect the second compressor in the surroundings. To implement the proposed method, in this case only two four-way valves are required, the first of which is incorporated into the supply air path upstream of the second compressor and the second of which is incorporated downstream.

[0017] Alternatively, the bypass may be connected to the air supply path by at least one other valve. In this case, it is preferable that this be manufactured as a three-way valve. Preferably, one three-way valve is incorporated into the air supply path upstream of the second compressor and downstream of the second compressor, and more preferably, one valve is incorporated upstream or downstream of the valve for connecting the second compressor to the surroundings. These may also be manufactured as three-way valves, so a total of four three-way valves are required, of which two are placed in the air supply path upstream of the second compressor and two are placed downstream.

[0018] Furthermore, the connection of the second compressor to the surroundings can be embodied through a valve, for example, a three-way valve, and a throttle flap. In this case, it is preferable that the throttle flap is incorporated into the intake path upstream of the second compressor, and the valve downstream. The bypass may be connected to the intake path via another valve, more preferably manufactured as a three-way valve, which is incorporated into the intake path upstream of the throttle flap. The bypass may be connected to the exhaust path upstream of the turbine via another throttle flap incorporated into the bypass, thereby bypassing not only the second compressor but also the fuel cell stack at the same time.

[0019] To address the problems mentioned at the beginning, a multi-stage air compression system is proposed in addition to the above, comprising an electrically driven first compressor and a turbine-driven second compressor. In this system, the compressor is located in the supply path of the air system for supplying air to the fuel cell stack, and the turbine is located in the exhaust path. The second compressor can be bypassed via a bypass. According to the present invention, the second compressor can be connected to the surroundings on either the inlet or outlet side via at least one valve and / or throttle flap.

[0020] The possibility of connecting a second compressor to the surroundings allows the pressure to be reduced to ambient pressure, thereby enabling the electrically driven first compressor to generate a higher pressure at the turbine inlet than the pressure at the inlet of the second compressor when starting the air compression system, thereby allowing the second compression stage to be started by the first compression stage alone.

[0021] Accordingly, the proposed multi-stage air compression system is particularly suitable for carrying out the method according to the present invention as described above, or can be operated based on this method. Accordingly, the air compression system can realize the same advantages as when the present method is applied.

[0022] The connection of the second compressor to the surroundings is preferably realized by two valves, or by one valve and a throttle flap. If two valves are provided, the first valve is incorporated into the intake path upstream of the second compressor, and the second valve is incorporated downstream. If one valve and a throttle flap are provided, the throttle flap is incorporated into the intake path upstream of the second compressor, and the valve is incorporated downstream. Each of these valves may be manufactured as a directional control valve, particularly as a four-way valve or a three-way valve.

[0023] In a preferred embodiment of the present invention, the bypass can be connected to the air supply path simultaneously via at least one valve. In this case, it is not necessary to provide another valve or additional valve for connecting the bypass to the air supply path. Thus, this system can be implemented relatively simply and at low cost. Preferably, at least one valve is manufactured as a four-way valve in this case. That is, it has four passages or connections, two of which are connected to the air supply path, one to the bypass, and one to the perimeter.

[0024] In an alternative embodiment, the bypass is connected to the intake path via at least one additional valve. This at least one additional valve is preferably fabricated as a three-way valve and is incorporated into the intake path upstream of at least one valve and / or throttle flap for connecting the second compressor to the periphery. In this way, the bypass can be opened and / or the second compressor connected to the periphery completely independently of each other. Although this increases the number of valves required, these may be relatively simple to fabricate.

[0025] In order to reduce the number of valves, as a developmental measure, it is proposed that a throttle flap be incorporated into the air supply path upstream of the second compressor and a valve, preferably a three-way valve, be incorporated into the air supply path downstream of the second compressor. In this case, the throttle flap serves as an alternative to the valve, whereby the connection of the second compressor to the surroundings can be realized by just one valve and the throttle flap.

[0026] Furthermore, it is proposed that the bypass be connected to the exhaust path via another throttle flap in order to bypass the second compressor and the fuel cell stack. In that case, the air supply path and the exhaust path can be short-circuited via the bypass. The connection of the bypass is also effected in this case by just one valve and the throttle flap, thereby further reducing the number of valves.

[0027] Since a preferred application field of the multi-stage air compression system according to the invention is a fuel cell system, in addition to the above, a fuel cell system having a fuel cell stack and the multi-stage air compression system according to the invention for supplying air to the fuel cell stack is proposed. The multi-stage air compression system contributes to avoiding starting problems at startup. Furthermore, wear in the bearing area can be reduced, thereby improving fail-safe operation. Therefore, it is particularly preferred that the fuel cell system be a mobile fuel cell system.

[0028] Next, preferred embodiments of the invention will be described in detail with reference to the accompanying drawings. The drawings show the following.

Brief Description of the Drawings

[0029] [Figure 1] It is a schematic diagram showing a first fuel cell system according to the invention at startup. [Figure 2] It is a schematic diagram showing the fuel cell system of FIG. 1 immediately after startup. [Figure 3] It is a schematic diagram showing the fuel cell system of FIG. 1 in normal operation. [Figure 4] Figure 1 is a schematic diagram showing the fuel cell system in the first pump protection mode. [Figure 5] Figure 1 is a schematic diagram showing the fuel cell system in the second pump protection mode. [Figure 6] This is a schematic diagram showing the fuel cell system in the stopped state (Figure 1). [Figure 7] This is a schematic diagram showing a second fuel cell system according to the present invention. [Figure 8] This is a schematic diagram showing the third fuel cell system according to the present invention at the start. [Figure 9] This is a schematic diagram showing the fuel cell system in Figure 8 immediately after startup. [Figure 10] Figure 8 is a schematic diagram showing the fuel cell system in normal operation. [Figure 11] Figure 8 is a schematic diagram showing the fuel cell system in the first pump protection mode. [Figure 12] Figure 8 is a schematic diagram showing the fuel cell system in the second pump protection mode. [Figure 13] This is a schematic diagram showing the fuel cell system in the stopped state (Figure 8). [Modes for carrying out the invention]

[0030] The fuel cell system shown in Figure 1 has a fuel cell stack 5 connected to an air system having an air intake path 3 and an exhaust path 4 for air supply. The air system incorporates a multi-stage air compression system 1, which includes an electrically driven first compressor 1.1 and a second compressor 1.2 driven by a turbine 2. Compressors 1.1 and 1.2 are incorporated into the air intake path 3, and the turbine 2 is incorporated into the exhaust path 4, thereby supplying the turbine 2 with exhaust gases from the fuel cell stack 5. In this way, the turbine 2 can recover some of the energy previously used to compress the air. Since the air is heated during compression, an intercooler 13 is placed after the first compressor 1.1, and a cooler 14 and a heat exchanger 15 are placed after the second compressor 1.2. The heat exchanger 15 is connected to a cooling circuit 16 which incorporates a coolant pump 17 and a vehicle cooler 18. The fuel cell stack 5 is simultaneously cooled through the cooling circuit 16.

[0031] The electrochemical reaction in the fuel cell of fuel cell stack 5 requires hydrogen in addition to air, so an anode circuit 19 is connected to fuel cell stack 5, and hydrogen is supplied through this circuit.

[0032] When the fuel cell system starts up, the multi-stage air compression system 1 also starts up simultaneously. To do this, the first electrically driven compressor 1.1 is activated first. The air compressed by the first compressor 1.1 is supplied to the second compressor 1.2 via the air supply path 3. However, at startup, the compressed air is diverted to a bypass 6 via valve 7, which is manufactured as a four-way valve in this example and incorporated into the air supply path 3 upstream of the second compressor 1.2, and then reintroduced into the air supply path 3 downstream of the second compressor 1.2 via another valve 8, also manufactured as a four-way valve. In this way, the second compressor 1.2 is bypassed. At the same time, valves 7 and 8 are switched so that the second compressor 1.2 is connected to the surroundings, thereby reducing the pressure in the second compressor 1.2 to the ambient pressure. This means that the pressure at the inlet of turbine 2 can be adjusted by compressed air, and this pressure is higher than the pressure at the inlet of the second compressor 1.2, thereby allowing turbine 2 to start without problems.

[0033] As illustrated in Figure 2, immediately after starting, the connection of the second compressor 1.2 to the surroundings can be closed again by the appropriate switching of valves 7 and 8, while the bypass 6 remains temporarily open. Pressure is generated in the second compressor 1.2, which results in the bypass 6 flowing backward. This creates a buffering effect during startup, thereby preventing over-rotation.

[0034] Subsequently, when valves 7 and 8 are switched to close bypass 6, the system can transition from the starting phase to normal operation. This switching position of valves 7 and 8 is shown as an example in Figure 3. To clearly indicate the closed bypass 6, the connection of valves 7 and 8 representing bypass 6 is not shown.

[0035] One variation of normal operation is shown in Figure 4. Bypass 6 (not shown) remains closed, but a corresponding connection of valve 8 located downstream of the second compressor 1.2 is opened, establishing a connection to the surroundings. In this way, undesirable pump discharge from compressor 1.2 can be addressed. Accordingly, opening the connection of valve 8 leads to pump protection.

[0036] As illustrated in Figure 5, the pump protection function can be further enhanced by temporarily reopening bypass 6. In this way, not only undesirable pump discharge but also over-rotation can be addressed simultaneously.

[0037] When the air compression system 1 is paused or stopped, valves 7 and 8 can be switched so that only bypass 6 is opened—as shown in Figure 6 as an example—in order to protect the gas bearings from increased wear. In this way, a reduction in the stopping phase is achieved, which in turn leads to the preservation of the gas bearings.

[0038] Figure 7 shows another fuel cell system according to the present invention, which differs from that of Figure 1 only in the presence of valves 7, 8, 9, and 10. This is because four three-way valves are provided instead of two four-way valves. In other words, valves 7 and 8 are manufactured as three-way valves in this example. In addition to these, there are two other three-way valves, namely valves 9 and 10, through which the bypass 6 is connected to the air supply path 3. The same functions described above in relation to the fuel cell system of Figure 1 can also be realized with the fuel cell system of Figure 7, so we will refer back to the explanation given above.

[0039] Figure 8 shows another fuel cell system according to the present invention, which enables all the functions described above in the same way but with different piping. Instead of two four-way valves or four three-way valves, there are two valves 8, 10 fabricated as three-way valves and two throttle flaps 9, 12. The second compressor 1.2 is connected to the surroundings via a first throttle flap 9 incorporated into the intake path 3 upstream of the compressor 1.2, and via a first valve 8 incorporated into the intake path 3 downstream of the second compressor 1.2. Bypass 6 is connected to the intake path 3 via another valve 10 and to the exhaust path 4 via another throttle flap 12. Accordingly, bypass 6 bypasses not only the second compressor 1.2 but also the fuel cell stack 5 at the same time.

[0040] At startup, valve 10 is switched so that air compressed by the first compressor 1.1 is guided from the intake path 3 to the bypass 6 and introduced into the exhaust path 4 via the throttle flap 12 upstream of the turbine 2. As a result, the pressure at the inlet of the turbine 2 increases, while the pressure in the second compressor 1.2 decreases. This is because the second compressor 1.2 is simultaneously connected to the surroundings via the throttle flap 9 and valve 8. In this way, the turbine 2 can be started without any problems.

[0041] Immediately after starting, valve 8 can be switched so that the connections to the surroundings are closed, while bypass 6 remains open for the time being via valve 10. This leads to pressure generation in the second compressor 1.2, and bypass 6 flows in the opposite direction. In this way, a sudden increase in rotational speed and the resulting over-speed are dealt with (see Figure 9).

[0042] To transition to normal operation, the bypass 6 can be closed by switching valve 10, thereby supplying all of the air compressed by the first compressor 1.1 to the second compressor 1.2 and, consequently, to the fuel cell stack 5 (see Figure 10). To clearly show that the bypass 6 is closed, the illustration of the bypass 6 is omitted in Figure 10.

[0043] To implement the pump protection function—as illustrated in Figure 11—valve 8 can be switched to open the connection to the surroundings during normal operation. Bypass 6 remains closed (and is therefore not shown).

[0044] Alternatively, as shown in Figure 12, the bypass 6 can be opened by appropriately switching valve 10, thereby allowing the bypass to flow in the opposite direction, similar to Figure 9. In this way, in addition to the pump protection function, protection against over-speed can also be achieved at the same time.

[0045] Furthermore, when the system is idle or stopped, an additional deceleration effect can be achieved by the appropriate switching of valve 10 under the open bypass 6, which disconnects the second compressor 1.2 from the first compressor 1.1 (see Figure 13). [Explanation of Symbols]

[0046] 1. Multi-stage air compression system 1.1 First Compressor 1.2 Second Compressor 2 Turbines 3. Air supply path 4. Exhaust path 5. Fuel cell stack 6 Bypass 7,8 valves 9. Throttle flap 10 Another valve 11 Another valve 12 Throttle flaps

Claims

1. A method for operating a multi-stage air compression system (1), comprising an electrically driven first compressor (1.1) and a second compressor (1.2) driven by a turbine (2), wherein the compressors (1.1, 1.2) are located in an air supply path (3) of the air system for supplying air to a fuel cell stack (5), and the turbine (2) is located in an exhaust path (4), in this method, When the air compression system (1) is started, the air compressed by the first compressor (1.1) is supplied to the fuel cell stack (5) or the turbine (2) via a bypass (6) that bypasses the second compressor (1.2), the inlet side of the second compressor (1.2) is connected to the surroundings via a valve (7) or throttle flap (9) provided on the inlet side of the second compressor (1.2), and the outlet side of the second compressor (1.2) is connected to the surroundings via a valve (8) provided on the outlet side of the second compressor (1.2), thereby reducing the pressure in the second compressor (1.2) to ambient pressure. method.

2. The method according to claim 1, characterized in that after the start of the air compression system (1), the connection of the second compressor (1, 2) to the surroundings is closed by at least one of the valves (7, 8), while the bypass (6) remains open.

3. The method according to claim 2, characterized in that the bypass (6) is closed after the connection portion of the second compressor (1.2) to the surroundings is closed, thereby supplying the air compressed by the first compressor (1.1) to the fuel cell stack (5) via the second compressor (1.2).

4. The method according to any one of claims 1 to 3, characterized in that the second compressor (1, 2) is at least temporarily connected to the surroundings via at least one of the valves (7, 8) and / or the throttle flap (9) in order to implement a pump protection function during the normal operation of the air compression system (1).

5. The method according to any one of claims 1 to 3, characterized in that the bypass (6) for bypassing the second compressor (1.2) is opened in order to implement the pump protection function during the normal operation of the air compression system (1).

6. The method according to any one of claims 1 to 3, characterized in that the bypass (6) for bypassing the second compressor (1.2) is opened when the air compression system (1) stops.

7. The method according to any one of claims 1 to 3, characterized in that, in order to open the bypass (6), at least one of the valves (7, 8) or at least one other valve (10, 11) connected to the air supply path (3) is operated.

8. A multi-stage air compression system (1) comprising an electrically driven first compressor (1.1) and a second compressor (1.2) driven by a turbine (2), wherein the compressors (1.1, 1.2) are located in an air supply path (3) of the air system for supplying air to a fuel cell stack (5), and the turbine (2) is located in an exhaust path (4), and the second compressor (1.2) can be bypassed via a bypass (6), in an air compression system, When the air compression system (1) is started, the bypass (6) supplies the air compressed by the first compressor (1.1) to the fuel cell stack (5) or the turbine (2) by bypassing the second compressor (1.2), the inlet side of the second compressor (1.2) is connected to the surroundings via a valve (7) or throttle flap (9) provided on the inlet side of the second compressor (1.2), and the outlet side of the second compressor (1.2) is connected to the surroundings via a valve (8) provided on the outlet side of the second compressor (1.2), thereby reducing the pressure in the second compressor (1.2) to ambient pressure. Air compression system.

9. The air compression system (1) according to claim 8, characterized in that the bypass (6) can be connected to the air supply path (3) simultaneously via at least one of the valves (7, 8), and at least one of the valves (7, 8) is manufactured as a four-way valve.

10. The air compression system (1) according to claim 8, characterized in that the bypass (6) is connected to the air intake path (3) via at least one other valve (10), the other valve being manufactured as a three-way valve and being incorporated into the air intake path (3) upstream of at least one of the valves (7, 8) and / or the throttle flap (9) for connection of the second compressor (1, 2) to the surroundings.

11. The air compression system (1) according to claim 8 or 10, characterized in that the throttle flap (9) is incorporated into the intake path (3) upstream of the second compressor (1.2), and the valve (8) is incorporated into the intake path (3) downstream of the second compressor (1.2).

12. The air compression system (1) according to claim 11, characterized in that the bypass (6) is connected to the exhaust path (4) via another throttle flap (12) to bypass the second compressor (1.2) and the fuel cell stack (5).

13. A fuel cell system comprising a fuel cell stack (5) and a multi-stage air compression system (1) according to any one of claims 8 to 10 for supplying air to the fuel cell stack (5).

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