METHOD FOR OPERATING A FUEL CELL SYSTEM, FUEL CELL SYSTEM

Description

[0001] The invention relates to a method for operating a fuel cell system having the features of the preamble of claim 1. Furthermore, the invention relates to a fuel cell system which is suitable for carrying out the method according to the invention or can be operated according to the method according to the invention. State of the art

[0002] A fuel cell system can convert chemical energy into electrical energy using hydrogen and oxygen. The electrical energy generated in this way can be used, for example, to power a vehicle. The required hydrogen is stored in a suitable tank on board the vehicle. The remaining oxygen is taken from the ambient air.

[0003] Before the ambient air is fed to the at least one fuel cell, it is compressed using an air compression system to generate a certain air mass flow and pressure level. A thermal turbomachine, which can be single- or multi-stage and / or single- or multi-flow, can be used as the air compression system. For energy recovery, the air compression system can be coupled to a turbine or an exhaust gas turbocharger, to which the moist air or exhaust air flowing out of the at least one fuel cell is fed. If the air compression system is designed with multiple stages, a different technology can be used to create a compression stage instead of a thermal turbomachine, for example a piston engine, a screw compressor, a Roots compressor, or a scroll compressor.

[0004] Patent publications DE 10 2016 003 795 A1 and DE 10 2010 035 727 A2 each disclose a method for operating a fuel cell system and a fuel cell system, wherein a two-stage air compression system is located in the cathode supply air path. The present invention is based on the object of optimizing the operation of a fuel cell system with a multi-stage air compression system, in particular making it more efficient.

[0005] To achieve this object, the method having the features of claim 1 and the fuel cell system having the features of claim 7 are specified. Advantageous embodiments can be found in the respective subclaims.

[0006] The present invention is applicable not only to mobile fuel cell systems, but also to stationary fuel cell systems. Disclosure of the invention

[0007] A method for operating a fuel cell system is proposed, in which air is drawn in from the ambient air, compressed using an air compression system comprising a first compression stage and a second compression stage, and supplied to at least one fuel cell via a cathode supply air path. Furthermore, the method involves discharging exhaust air from the fuel cell via a cathode exhaust air path. According to the invention, a partial exhaust air mass flow from the cathode exhaust air path is at least temporarily introduced into the cathode supply air path via a connecting line.

[0008] Since the cathode exhaust air path carries moist air as exhaust air during operation of the fuel cell system, the cathode supply air can be humidified by introducing at least a portion of the moist air into the cathode supply air path. An additional humidification device in the cathode supply air path is therefore unnecessary. Humidifying the cathode supply air serves the purpose of preventing the proton-conducting membrane of the at least one fuel cell from drying out. In order to conduct protons, the membrane must be sufficiently moist. Since the risk of drying out is particularly high in the area where the cathode supply air path enters the fuel cell, the cathode supply air is generally humidified. The method according to the invention thus contributes to optimizing water management during operation of the fuel cell system.

[0009] Furthermore, the method according to the invention can be used to make the operation of the fuel cell system more efficient. By introducing a partial exhaust air mass flow from the cathode exhaust air path into the cathode supply air path, at least a portion of the exhaust air is recirculated and thus reused. The recirculated partial exhaust air mass flow can be used for system operation and / or system design, whereby the system design must take the entire operating range into account.

[0010] The proposed use of the exhaust air partial mass flow is particularly advantageous when the air compression system includes an exhaust gas turbocharger as the compression stage, which comprises a turbine impeller arranged on a shaft together with a compressor impeller. Since both impellers must always be operated at the same speed and under different thermodynamic conditions across a wide operating range, the coordination between the two impellers is not always optimal. This is particularly the case when a variable turbine geometry of the exhaust gas turbocharger is omitted for cost reasons. Depending on the system topology, measures that entail significant losses must therefore be taken at a variety of operating points, particularly at operating points outside of the full load point. For example, a partial exhaust air mass flow may be uselessly diverted via a turbine bypass in order to synchronize the two impellers.Alternatively or additionally, significant throttling can be achieved with the aid of a pressure control valve arranged downstream of the turbine. However, such loss-inducing measures are unnecessary if, according to the invention, a partial exhaust air mass flow is diverted from the cathode exhaust air path into the cathode supply air path. In this way, the method according to the invention can, in particular, achieve an improvement in the energy balance during dynamic operating conditions.

[0011] Advantageously, the exhaust air partial mass flow is introduced into the cathode supply air path between the two compression stages of the air compression system, preferably between the first compression stage and an intercooler upstream of the second compression stage. In the first compression stage, the air drawn in from the environment is compressed, simultaneously heating it. By introducing the exhaust air partial mass flow downstream of the first compression stage into the cathode supply air path, not only is the air humidified but also cooled. By introducing the exhaust air partial mass flow downstream of the first compression stage and upstream of an intercooler, a thorough mixing of moist and dry air is achieved. At the same time, the risk of water vapor contained in the moist air condensing and contributing to the formation of water droplets in the second compression stage is reduced.The cooling effect achieved by the recirculated exhaust air mass flow contributes to the optimization of the intercooling system. At the same time, the load on the intercooler and / or charge air cooler is reduced.

[0012] If the second compression stage is implemented by an exhaust gas turbocharger and the exhaust air partial mass flow to be recirculated is taken from a turbine bypass, the turbine power of the exhaust gas turbocharger decreases at full load with the recirculation mass flow. This results in a lower pressure ratio in the second compression stage area. This means that the first compression stage must provide a higher pressure ratio, or the power of the first compression stage must be increased. However, since the power of the first compression stage is simultaneously reduced by the recirculation mass flow, both effects largely cancel each other out. For a short time, this measure can achieve additional humidification at full load. At partial load points, in contrast, a mass flow that is diverted via the turbine bypass anyway is used for recirculation. Thus, no mass flow is lost at the turbine's partial load points.

[0013] Advantageously, the introduction of the exhaust air partial mass flow into the cathode supply air path is controlled by a controllable valve, preferably a regulating valve, particularly in the form of a proportional valve or a timed switching valve. A defined amount of moist air or exhaust air can thus be introduced into the cathode supply air path. Furthermore, the introduction of exhaust air into the cathode supply air path can be limited in time or carried out at specific times. If a proportional valve is used as the controllable valve, the exhaust air volume or the exhaust air partial mass flow can be adjusted via the opening cross-section of the valve. In the case of a timed switching valve, the same effect can be achieved via timing. Optionally, the exhaust air recirculation can also be carried out stationary or continuously.

[0014] Furthermore, a pressure level is preferably set in the cathode exhaust air path that is higher than the pressure level in the cathode supply air path between the two compression stages. This means that only the controllable valve needs to be opened for exhaust air from the cathode exhaust air path to flow via the connecting line into the cathode supply air path. Using suitable sensors and / or models, the pressure difference can be determined and taken into account when controlling the valve. Preferably, the valve only opens when a corresponding release is present via the pressure difference.

[0015] According to the invention, the partial exhaust air mass flow is diverted from the cathode exhaust air path via a bypass path to bypass a turbine. This means that a compression stage, preferably the second compression stage, is formed by an exhaust gas turbocharger with a turbine, which receives the exhaust air flow in the cathode exhaust air path. If the turbine does not have a variable turbine geometry, a partial exhaust air mass flow is generally diverted via the turbine bypass path to synchronize the turbine impeller and the compressor impeller arranged in the cathode supply air path. This partial exhaust air mass flow, or just a portion of it, can advantageously be recirculated and thus put to use. This operating mode also has the advantage that the power of the first compression stage does not need to be increased.

[0016] With the help of the proposed method, further operating modes can also be realized.

[0017] For example, temporary recirculation operation can be carried out with a recirculation rate that results in the air mass flow of the first compression stage being reduced by the recirculation mass flow. This is accompanied by a temporary reduction in the lambda value in the cathode intake air path, since the recirculation mass flow is oxygen-depleted. The lambda reduction is therefore limited in time.

[0018] In another operating mode, a recirculation rate can be selected that results in the air mass flow of the first compression stage being reduced only to the extent that the oxygen content corresponds to the desired lambda value after the recirculation mass flow is introduced into the cathode supply air path. This requires that the air mass flow through the second compression stage be increased. In addition, a further bypass path for bypassing the fuel cell can be used to split the air mass flow, so that a partial mass flow is not fed to the fuel cell via the bypass path but is discharged into the cathode exhaust air path.

[0019] Furthermore, the recirculation of exhaust air from the cathode exhaust path can be completely suspended by keeping the controllable valve closed.

[0020] In a further development of the invention, it is proposed that the connecting line used for recirculating the exhaust air mass flow be blown dry before shutting down the fuel cell system. This prevents damage caused by ice pressure at low outside temperatures. The air required for blowing dry can be diverted from the cathode supply air path via a bypass. For this purpose, a bypass valve is opened in the bypass. Preferably, the controllable valve is operated in a cyclical manner during blowing, so that adhering water droplets / moisture are reliably removed.

[0021] The fuel cell system further proposed to achieve the aforementioned object comprises at least one fuel cell connected to a cathode supply air path for supplying air and to a cathode exhaust air path for discharging exhaust air. An air compression system comprising a first compression stage and a second compression stage is arranged in the cathode supply air path. According to the invention, a connecting line opens into the cathode supply air path and is connected directly or indirectly to the cathode exhaust air path via a bypass path for bypassing a turbine, so that a partial exhaust air mass flow from the cathode exhaust air path can be introduced into the cathode supply air path via the connecting line.

[0022] The proposed fuel cell system is therefore particularly suitable for carrying out the method according to the invention described above or can be operated according to the method according to the invention described above. Consequently, the same advantages can be achieved with the aid of the proposed fuel cell system. In particular, the partial exhaust air mass flow introduced into the cathode supply air path can be used to humidify the cathode supply air, thus eliminating the need for an additional humidification device. Furthermore, cooling of the cathode supply air is achieved, which leads to improved intercooling and / or to a reduction in the load on a further intercooler. A further advantage arises from the fact that exhaust air from the cathode exhaust air path is recirculated and thus put to use. As a result, the efficiency of the fuel cell system increases.

[0023] Preferably, the connecting line required for recirculating the exhaust air partial mass flow between the first compression stage and the second compression stage, preferably between the first compression stage and an intercooler upstream of the second compression stage, opens into the cathode supply air path. In this area, particularly effective humidification and cooling of the cathode supply air can be achieved. At the same time, a thorough mixing of moist and dry air is achieved. Furthermore, the risk of condensation of water vapor contained in the exhaust air is reduced.

[0024] Furthermore, a controllable valve is preferably arranged in the cathode exhaust air path, in the connecting line, or in the bypass path for bypassing the turbine. The controllable valve can be used to control the partial exhaust air mass flow to be recirculated. The controllable valve is preferably a control valve, in particular in the form of a proportional valve, or a pulsed switching valve, so that the partial exhaust air mass flow to be recirculated can be controlled via the opening cross-section of the proportional valve or the timing of the pulsed switching valve.

[0025] According to a preferred embodiment of the invention, the first compression stage is designed with multiple flow channels. In a multi-flow compressor, at least two compressor impellers are supplied with air in parallel. The compressor can thus have a symmetrical design, reducing the forces acting on the axial bearing. Furthermore, the compressor impellers can be made smaller, since with two compressor impellers, each impeller receives only half the mass flow. The rotor's moment of inertia is correspondingly reduced, which in turn has a beneficial effect on dynamics.

[0026] Alternatively or additionally, it is proposed that the second compression stage be designed as an exhaust gas turbocharger. The advantages of the invention become particularly apparent in this embodiment of a fuel cell system according to the invention, since during operation of the exhaust gas turbocharger—at least at partial load points—a partial exhaust air mass flow is generated that is guided past the turbine and can then be utilized using the method according to the invention.

[0027] 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 first fuel cell system according to the invention, Fig. 2 a schematic representation of a second fuel cell system according to the invention and Fig. 3 a schematic representation of a third fuel cell system according to the invention. Detailed description of the drawings

[0028] The Fig. 1 1 shows, by way of example, a fuel cell system 1 according to the invention comprising a fuel cell 5 which can be supplied with oxygen via a cathode supply air path 4. For this purpose, air is drawn in from an environment 2 and fed via an air filter 13 to a first compressor stage 3.1 and a second compressor stage 3.2 of a multi-stage air compression system 3. Each compressor stage 3.1, 3.2 has a compressor impeller 14 arranged on a shaft 15. The compressor impeller 14 of the first compression stage 3.1 is driven by an electric motor 16, and the compressor impeller 14 of the second compression stage 3.2 is driven by a turbine 11. This means that the second compression stage 3.2 is implemented by an exhaust gas turbocharger.The turbine 11 has a turbine impeller 17, which is arranged on the shaft 15 together with the compressor impeller 14 of the exhaust gas turbocharger, so that the turbine impeller 17 is subjected to exhaust air flow, which exits the fuel cell 5 during operation of the fuel cell system 1 and is discharged via a cathode exhaust air path 6. To synchronize the two impellers of the exhaust gas turbocharger, a bypass path 10, 24 is provided, via which a partial mass flow can be guided past the respective impeller. A bypass valve 12, 25 is arranged in each bypass path 10, 24. Bypass 24 with bypass valve 25 is optional. According to an alternative embodiment (not shown), the bypass 24 and the bypass valve 25 are omitted. Furthermore, a pressure control valve 18 for throttling the exhaust air mass flow is arranged in the cathode exhaust air path 6 downstream of the turbine 11. The pressure control valve 18 can, if necessary,are omitted if the turbine 11 of the exhaust gas turbocharger has a variable turbine geometry.

[0029] The fuel cell system 1 of the Fig. 1 has two intercoolers 8 in the cathode supply air path 4. A first intercooler 8 is arranged between the two compression stages 3.1, 3.2, and a second intercooler 8 is arranged downstream of the second compression stage 3.2. The first intercooler 8 is optional. The second intercooler 8 can also be replaced by a water injection device (not shown). Upstream of the first intercooler 8, a connecting line 7 opens into the cathode supply air path 4, the other end of which is connected to the cathode exhaust air path 6. A controllable valve 9 is also arranged in the connecting line 7. When the valve 9 is opened, exhaust air or moist air flows from the cathode exhaust air path 6 via the connecting line 7 into the cathode supply air path 4 due to the prevailing pressure conditions. The cathode supply air is humidified and cooled in this way. At the same time, part of the exhaust air is recirculated and used.

[0030] A bypass path 19 with a bypass valve 20 is provided to bypass the fuel cell 5. If a partial supply air mass flow is to be diverted from the cathode supply air path 4 into the cathode exhaust air path 6, the bypass valve 20 can be opened. The bypass valve 20 can also be opened to blow the connecting line 7 dry in the event of a shutdown.

[0031] The Fig. 2 A second fuel cell system 1 according to the invention can be seen. This has a dual-flow compressor as the first compression stage 3.1. This means that the compressor has two compressor impellers 14 on a common shaft 15, which are driven by an electric motor 16. Instead of the second intercooler 8, a heat exchanger 23 is also provided, which uses the cooler exhaust air from the cathode exhaust air path 6 to cool the heated cathode supply air. The heat exchanger 23 is arranged here between the bypass path 19 for bypassing the fuel cell 5 and the fuel cell 5. In addition, a first shut-off valve 21 is provided in the cathode supply air path 4 upstream of the fuel cell 5, and a second shut-off valve 22 is provided in the cathode exhaust air path 6 downstream of the fuel cell 5.By closing the shut-off valves 21, 22 and opening the bypass valve 20 arranged in the bypass path 19, the cathode supply air can be completely bypassed by the fuel cell 5.

[0032] In the fuel cell system 1 of the Fig. 2 The connecting line 7 for recirculating a partial exhaust air mass flow is connected to the bypass path 10 for bypassing a turbine 11 of an exhaust gas turbocharger forming the second compression stage 3.2. The connection to the cathode exhaust air path 6 is thus only indirectly via the bypass path 10.

[0033] A modification of the fuel cell system 1 of the Fig. 2 is in the Fig. 3shown. Here, the connection of the connecting line 7 to the bypass path 10 is made via the bypass valve 12, which simultaneously forms the controllable valve 9. This eliminates the need for a valve. The heat exchanger 23 is also arranged such that the fuel cell 5 can be bypassed via the bypass path 19, but not the heat exchanger 23.

[0034] The invention is not limited to the illustrated embodiments. Further modifications are possible, particularly concerning the arrangement of the various bypass paths, the arrangement and / or provision of a heat exchanger and / or a second intercooler. The topology variation is thus comparatively high.

Claims

1. Method for operating a fuel-cell system (1), in which air is sucked in from the surroundings (2), is compressed with the aid of an air-compression system (3) comprising a first compression stage (3.1) and a second compression stage (3.2), and is fed to at least one fuel cell (5) via a cathode supply-air path (4), and in which exhaust air exiting the fuel cell (5) is discharged via a cathode exhaust-air path (6), wherein an exhaust-air partial mass flow is introduced at least temporarily into the cathode supply-air path (4) from the cathode exhaust-air path (6) via a connecting line (7), characterized in that the exhaust-air partial mass flow is branched off from the cathode exhaust-air path (6) via a bypass path (10) for bypassing a turbine (11).

2. Method according to Claim 1, characterized in that the exhaust-air partial mass flow is introduced into the cathode supply-air path (4) between the two compression stages (3.1, 3.2), preferably between the first compression stage (3.1) and an intermediate cooler (8) which is arranged before the second compression stage (3.2).

3. Method according to Claim 1 or 2, characterized in that the introduction of the exhaust-air partial mass flow into the cathode supply-air path (4) is controlled by means of an actuatable valve (9), preferably by means of a regulating valve, in particular in the form of a proportional valve, or clocked switching valve.

4. Method according to one of the preceding claims, characterized in that, in the cathode exhaust-air path (6), a pressure level which is above the pressure level in the cathode supply-air path (4) between the two compression stages (3.1, 3.2) is set.

5. Method according to one of the preceding claims, characterized in that the connecting line (7) is blown dry before the fuel-cell system (1) is shut down, wherein preferably the actuatable valve (9) is operated in a clocked manner.

6. Fuel-cell system (1) comprising at least one fuel cell (5) which, for feeding of air, is connected to a cathode supply-air path (4) and, for discharge of exhaust air, is connected to a cathode exhaust-air path (6), wherein, in the cathode supply-air path (4), there is arranged an air-compression system (3) comprising a first compression stage (3.1) and a second compression stage (3.2), characterized in that a connecting line (7) which is connected to the cathode exhaust-air path (6) directly or indirectly via a bypass path (10) for bypassing a turbine (11) opens out into the cathode supply-air path (4), so that an exhaust-air partial mass flow is able to be introduced into the cathode supply-air path (4) from the cathode exhaust-air path (6) via the connecting line (7).

7. Fuel-cell system (1) according to Claim 6, characterized in that the connecting line (7) opens out into the cathode supply-air path (4) between the first compression stage (3.1) and the second compression stage (3.2), preferably between the first compression stage (3.1) and an intermediate cooler (8) which is arranged before the second compression stage (3.2).

8. Fuel-cell system (1) according to Claim 6 or 7, characterized in that an actuatable valve (9), preferably a regulating valve, in particular in the form of a proportional valve, or a clocked switching valve, is arranged in the cathode exhaust-air path (6), in the connecting line (7) or in the bypass path (10) for bypassing the turbine (11).

9. Fuel-cell system (1) according to one of Claims 6 to 8, characterized in that the first compression stage (3.1) is of multiple-channel design and / or the second compression stage (3.2) is designed as an exhaust-gas turbocharger.