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), comprising an intake path (2) for supplying a fuel cell stack (3) with air, an exhaust gas path (4) for discharging exhaust gas escaping from the fuel cell stack (3), and a return path (5) for returning exhaust gas from the exhaust gas path (4) to the intake path (2), therefore, the air in the air inlet path (2) is humidified by the returned waste gas. According to the invention, the return circuit (5) is connected to the exhaust gas path (4) via a three-way valve, by means of which exhaust gas escaping from the fuel cell stack is fed to the exhaust gas path (4) and / or the return circuit (5). The invention also relates to an air system (1) and to a fuel cell system having an air system (1) according to the invention.

Description

TECHNICAL FIELD

[0001] The present application relates to a method for operating an air system. The present application furthermore relates to an air system which is suitable for carrying out the method according to the present application or which can be operated according to the method.

[0002] A preferred field of application of the present application is a fuel cell system. Fuel cell systems have an air system which supplies air to a fuel cell stack, the air serving as an oxygen donor. The present application therefore also relates to a fuel cell system having an air system according to the present application. BACKGROUND

[0003] Fuel cell systems convert fuel, for example hydrogen, and oxygen into electrical energy, heat and water. Conventionally, air, usually ambient air, is used as an oxygen donor. In practice, a plurality of fuel cells are arranged one above the other or side by side and connected into a fuel cell stack or reactor. The air supply to the reactor is carried out via an air system which comprises an intake path for the intake of air and an exhaust path for the discharge of exhaust air which escapes from the reactor.

[0004] Since the electrochemical reactions in the fuel cell require a certain air mass flow and a certain pressure level, the air is first compressed. For this purpose a compressor is integrated in the intake path. On compression the air is heated very strongly so that it has to be cooled. For this reason a cooling device is integrated in the intake path downstream of the compressor. In order to further regulate the temperature in the intake path, a device for humidifying the air can be provided. Humidifying the air should counteract drying of the fuel cell membrane. Since the proton conductivity of the membrane depends significantly on the fact that it stores a sufficient amount of water in its structure.

[0005] Different solutions for humidifying the air in the intake path of an air system are known from the prior art. The humidification can take place, for example, by means of a gas-to-gas membrane humidifier which is arranged in the intake path and which in operation transports the water produced from the exhaust gas side to the intake side in the form of water vapour through a membrane. Such a membrane humidifier, however, requires a large installation space which is limited, in particular in mobile applications. Furthermore, a membrane humidifier is comparatively sluggish in terms of heat and produces a pressure loss.

[0006] Other solutions for humidifying the air provide for injecting water into the intake path and / or for guiding moistened exhaust air back into the intake path. In order to guide back the exhaust air, the exhaust path has to be connected to the intake path via a guide-back path, wherein the moistened exhaust air can be mixed into the air in the intake path upstream or downstream of the compressor which is integrated in the intake path. In the former case a pressure difference between the intake path upstream of the compressor and the exhaust path for guiding back the exhaust air can be utilised. In the latter case a delivery unit for guiding back the exhaust air on the pressure side of the compressor has to be used additionally.

[0007] Active humidification of the air in the air supply path is required especially at low currents or low loads, since the product water from the fuel cell reaction is very low in this case. This means that the self-humidification effect of the membrane is not sufficient for setting the necessary membrane humidity at low currents or low loads. If active humidification is to be caused by the exhaust gas recirculation, the very small pressure losses occurring in the air system at low loads are a challenge. The exhaust gas recirculation is made difficult because of the pressure balance established. SUMMARY

[0008] The invention has the task of optimizing the exhaust gas recirculation for the humidification of the air in the air supply path. In particular, the exhaust gas recirculation should be simple to implement even at low currents or low loads.

[0009] To solve this task, a method according to the invention and an air system according to the invention are proposed. Advantageous further developments of the invention can be derived from the respective technical solutions. In addition, a fuel cell system having an air system according to the invention is given.

[0010] A method for operating an air system is proposed, the air system comprising an air supply path for supplying air to a fuel cell stack, an exhaust gas path for discharging exhaust gas escaping from the fuel cell stack, and a recirculation path for recirculating exhaust gas from the exhaust gas path to the air supply path in order to humidify the air in the air supply path with the recirculated exhaust gas. According to the invention, the recirculation path is connected to the exhaust gas path via a three-way valve, by means of which the exhaust gas escaping from the fuel cell stack is fed to the exhaust gas path and / or to the recirculation path.

[0011] By means of the three-way valve, the exhaust gas escaping from the fuel cell stack can be assigned to the exhaust gas path and / or to the recirculation path in a targeted manner. In particular, an exhaust gas assignment can be made which depends on the load and / or on the system, which takes into account the pressure losses in the system. In this way, sufficient exhaust gas recirculation in the system and thus humidification of the air in the air supply path can be ensured even at low loads or low pressure losses.

[0012] A further advantage of the proposed method is that only one valve, that is to say only one translationally and / or rotationally movable actuating element, has to be actuated in order to assign the exhaust gas to the exhaust gas path and / or to the recirculation path in a targeted manner. The number of parts and costs can thus be reduced.

[0013] Depending on the design of the three-way valve, different strategies for exhaust gas recirculation can be implemented.

[0014] A preferred first strategy setting opens the recirculation path at least partially by means of the three-way valve while closing the exhaust gas path at least partially. This means that the exhaust gas path is closed or throttled at least partially while the recirculation path is opened at least partially. Throttling the exhaust gas path requires the exhaust gas to be recirculated via the recirculation path, which is advantageous, inter alia, in the case of low loads and the associated small pressure losses. The partial opening of the recirculation path and / or the partial closing of the exhaust gas path preferably takes place continuously, for example proportionally to one another.

[0015] A preferred second strategy setting opens the recirculation path at least partially by means of the three-way valve while the exhaust gas path is completely open. In this case, throttling of the exhaust gas path does not take place during the at least partial opening of the recirculation path. In this way, the throttling losses can be minimised. Throttling of the exhaust gas path can be dispensed with, inter alia, at high loads, so that the method according to the application proves to be advantageous not only in the case of low loads.

[0016] The two strategies can also be combined. For example, the recirculation path can be continuously opened by means of the three-way valve, while the exhaust gas path is initially kept completely open, so that the throttling or pressure losses remain minimal. The exhaust gas path can then be at least partially closed by means of the three-way valve in a time-offset manner. The time-offset can be chosen so small that the at least partial closing of the exhaust gas path still takes place during the opening of the recirculation path or only after the complete opening. Furthermore, a time interval between the complete opening of the recirculation path and the start of the closing of the exhaust gas path can be observed, in order to create a defined transition phase. A preferred third strategy setting closes the exhaust gas path at least partially by means of the three-way valve while the recirculation path is completely open.

[0017] The advantages of the method according to the application are particularly evident when the recirculation of the exhaust gas is controlled exclusively via the pressure difference between the intake path and the exhaust gas path. According to a preferred embodiment of the application, it is therefore proposed that the exhaust gas is introduced from the exhaust gas path into the intake path upstream of a compressor integrated in the intake path via the recirculation path. Since the pressure upstream of the compressor is generally lower than the pressure in the exhaust gas path, this pressure drop can be used for the exhaust gas recirculation.

[0018] According to an alternative preferred embodiment of the application, the exhaust gas is introduced from the exhaust gas path into the intake path downstream of a compressor integrated in the intake path via the recirculation path, wherein the recirculation is actively supported by means of a delivery unit integrated in the recirculation path. Since the pressure downstream of the compressor is generally higher than the pressure in the exhaust gas path, this pressure difference can be overcome by means of the delivery unit.

[0019] Furthermore, an air system for a fuel cell system having a fuel cell stack is proposed. The air system comprises an intake path for supplying air to the fuel cell stack, an exhaust path for discharging exhaust air escaping from the fuel cell stack, and a return path for guiding exhaust air from the exhaust path back into the intake path in order to humidify the air of the intake path by the guided exhaust air. According to the invention, the return path is connected to the exhaust path via a three-way valve.

[0020] The proposed air system is particularly suitable for carrying out or can be operated according to the method according to the invention explained before, so that the same advantages can be achieved. In particular, it is also possible to achieve a return rate of exhaust air which ensures a sufficient humidification of the air in the intake path even in the case of low loads. Furthermore, parts and costs can be saved, since the advantages are achieved by only one valve.

[0021] Preferably, the three-way valve has a variable opening cross section which can be controlled by means of a translationally and / or rotationally movable actuating element. In this way, it is possible to continuously close the exhaust path and / or to continuously open the return path via the three-way valve.

[0022] Furthermore preferably, a compressor is integrated in the intake path and the return path leads into the intake path upstream of the compressor. In this configuration of the air system, the existing pressure drop can be utilized for guiding the exhaust air, so that no additional transport unit is required.

[0023] Alternatively, it is proposed that a compressor is integrated in the intake path and the return path leads into the intake path downstream of the compressor, wherein a transport unit is integrated in the return path. The additional transport unit supports the guiding of the exhaust air.

[0024] In a further preferred configuration of the air system, an exhaust turbine is integrated in the exhaust path downstream of the three-way valve. Via the exhaust turbine, it is possible to at least partially recover the energy which was previously used for compressing the air.

[0025] Since the preferred field of application of the invention is a fuel cell system, a fuel cell system having a fuel cell stack and an air system for supplying air to the fuel cell stack according to the invention is also proposed. BRIEF DESCRIPTION OF DRAWINGS

[0026] The preferred embodiments of the invention are explained in further detail below with reference to the drawings.

[0027] The drawings show:

[0028] Figure 1 a schematic view of a first air system according to the invention,

[0029] Figure 2 a schematic view of a second air system according to the invention,

[0030] Figure 3 The graph shows the first possible trend of change in the effective opening cross-section of the three-way valve caused by the actuation stroke of the actuator.

[0031] Figure 4 A graph showing the second possible trend of change in the effective opening cross-section of the three-way valve caused by the actuation stroke of the actuator, and

[0032] Figure 5 A graph showing the third possible trend of change in the effective opening cross-section caused by the actuation stroke of the actuator of the three-way valve. Detailed Implementation

[0033] exist Figure 1 The air system 1 according to the invention, exemplarily illustrated, is used to supply air to a fuel cell stack 3. Air is drawn from the surrounding environment and delivered to the fuel cell stack 3 via an intake path 2 of the air system 1. Air or exhaust gas escaping from the fuel cell stack 3 is discharged via an exhaust path 4 of the air system 1. To bypass the fuel cell stack 3, a bypass path 10 with an integrated bypass valve 11 is provided.

[0034] The air supplied to the fuel cell stack 3 via intake path 2 is pre-compressed by compressor 7. Compressor 7 is currently electrically driven and supported by exhaust turbine 9 integrated into exhaust path 4. To humidify the air in intake path 2, a return path 5 is provided, through which humidified exhaust gas from exhaust path 4 is supplied to intake path 2. The attachment of return path 5 to exhaust path 4 is achieved via a three-way valve 6. Because return path 5 enters intake path 2 upstream of compressor 7, exhaust gas can be returned under pressure control. This means that an additional conveyor unit 8 is not required. However, when—as Figure 2 As exemplarily shown, such a conveyor unit is required when the return path 5 is introduced into the intake path 2 downstream of the compressor 7.

[0035] If the attachment to the exhaust gas path 4 is made using a three-way valve 6, the method according to the invention can be implemented regardless of the specific configuration of the attachment of the return path 5 to the intake path 2. However, what is relevant to the specific design of the three-way valve 6 is the exhaust gas return strategy to which it can be applied. Preferred strategies are described below according to... Figures 3 to 5 The following graphs will be used for explanation. In all three graphs, the X-axis represents the actuation stroke of the actuator, and the Y-axis represents the effective opening cross-section of the three-way valve 6. In this case, the effective opening cross-section in the direction of the exhaust gas path 4 (variation curve A) should be different from the effective opening cross-section in the direction of the return path 5 (variation curve B), and these opening cross-sections depend on the actuation stroke of the actuator.

[0036] In the graph according to Figure 3 the effective opening cross section to the exhaust gas path 4 direction continuously decreases (change curve A), while the effective opening cross section to the return path 5 direction continuously increases (change curve B). Furthermore, the change curve A is proportional to the change curve B. This means that the exhaust gas path 4 is closed to the same extent as the return path 5 is opened. In this way a continuous throttling of the exhaust gas path 4 is achieved, which is advantageous, in particular at low loads or small pressure losses.

[0037] In the graph according to Figure 4 the return path 5 is continuously opened, while the exhaust gas path 4 remains completely open. This means that initially no throttling of the exhaust gas path 4 takes place. If the actuating element reaches the position PI, the return path 5 is completely open. The continuous closing of the exhaust gas path 4 is only carried out as position P2 is reached, so that the throttling takes place in time staggered manner. The time necessary for the actuating element to reach the position P2 from the position PI defines the transition phase. If this time is not present, the closing of the exhaust gas path 4 can already be started as the actuating element reaches the position PI, while the complete opening of the return path 5 is only carried out as position P2 is reached, as is exemplarily shown in the graph according to Figure 5

[0038] The staggered throttling of the exhaust gas path 4 is advantageous, in particular at high loads.​

Claims

1. A method for operating an air system (1), comprising an intake path (2) for supplying air to a fuel cell stack (3), an exhaust path (4) for discharging exhaust gas escaping from the fuel cell stack (3), and a return path (5) for guiding the exhaust gas from the exhaust path (4) back to the intake path (2) so that the air in the intake path (2) is humidified by the returned exhaust gas, characterized in that, The return path (5) is connected to the exhaust path (4) via a three-way valve, and the exhaust gas escaping from the fuel cell stack is transported to the exhaust path (4) and / or the return path (5) by means of the three-way valve (6).

2. The method according to claim 1, characterized in that, The three-way valve (6) is used to at least partially open the return path (5) and at least partially close the exhaust path (4).

3. The method according to claim 1 or 2, characterized in that, With the aid of the three-way valve (6), the return path (5) is at least partially opened while the exhaust path (4) is fully open.

4. The method according to any one of the preceding claims, characterized in that, The exhaust gas path (4) is at least partially closed only when the return path (5) is fully open, thanks to the three-way valve (6).

5. The method according to any one of the preceding claims, characterized in that, The exhaust gas is introduced from the exhaust gas path (4) via the return path (5) upstream of the compressor (7) integrated in the intake path (2).

6. The method according to any one of claims 1 to 4, characterized in that, The exhaust gas is introduced from the exhaust gas path (4) via the return path (5) downstream of the compressor (7) integrated in the intake path (2), wherein the return is actively supported by a conveyor unit (8) integrated in the return path (5).

7. An air system (1) for a fuel cell system having a fuel cell stack (3), comprising an intake path (2) for supplying air to the fuel cell stack (3), an exhaust path (4) for discharging exhaust gas escaping from the fuel cell stack (3), and a return path (5) for guiding the exhaust gas from the exhaust path (4) back to the intake path (2) so that the air in the intake path (2) is humidified by the returned exhaust gas, characterized in that, The return path (5) is connected to the exhaust gas path (4) via a three-way valve (6).

8. The air system (1) according to claim 7, characterized in that, The three-way valve (6) has a variable opening cross-section, which can be controlled by an actuator capable of translational and / or rotational movement.

9. The air system (1) according to claim 7 or 8, characterized in that, The compressor (7) is integrated in the intake path (2) and the return path (5) enters the intake path (2) upstream of the compressor (7).

10. The air system (1) according to claim 7 or 8, characterized in that, The compressor (7) is integrated in the intake path (2) and the return path (5) is introduced into the intake path (2) downstream of the compressor (7), wherein a conveyor unit (8) is integrated in the return loop (5).

11. The air system (1) according to any one of claims 7 to 10, characterized in that, Downstream of the three-way valve (6), the exhaust gas turbine (9) is integrated into the exhaust gas path (4).

12. A fuel cell system having a fuel cell stack (3) and an air system (1) according to any one of claims 7 to 11, the air system being used to supply air to the fuel cell stack (3).

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