Method for operating a fuel cell system, fuel cell system

Abstract

The invention relates to a method for operating a fuel cell system (1), comprising at least one fuel cell stack (2) with a cathode (2.1) and an anode (2.2), wherein in normal operation - air is supplied to the cathode (2.1) via an air supply path (3), which air has previously been compressed by means of at least one air compressor (4) integrated into the air supply path (3), and wherein the exhaust air emerging from the cathode (2.1) is supplied via an exhaust air path (5) to a turbine (6) integrated into the exhaust air path (5) and operatively connected to the air compressor (4), - hydrogen is supplied to the anode (2.2) from a tank system (7) with at least one pressurized gas container. According to the invention, the tank system (7) is cooled before, during and / or after refueling, wherein air or exhaust air is used for cooling, which air is branched off from the exhaust air path (5) downstream of the turbine (6) by opening a valve (8) and is supplied to the tank system (7) via a tank cooling path (9). Furthermore, the invention relates to a fuel cell system (1) which is suitable for carrying out the method or can be operated according to the method.

Description

[0001] The present invention relates to a method for operating a fuel cell system. Furthermore, the invention relates to a fuel cell system that is suitable for carrying out the method or operable according to the method.

[0002] The preferred field of application of the invention is mobile fuel cell systems or fuel cell vehicles. State of the art

[0003] Hydrogen-based fuel cell systems are considered the mobility concept of the future because they emit only water as exhaust and enable fast refueling times. The electrochemical reaction in the fuel cells requires air and hydrogen, which are provided by subsystems. The waste heat generated during the electrochemical reaction is dissipated using a cooling circuit and released into the environment via a vehicle radiator.

[0004] In practice, a large number of fuel cells are stacked on top of each other and connected to form a fuel cell stack. To increase performance, a fuel cell system can also include more than one stack. In this case, it is called a multi-stack system.

[0005] The hydrogen required for the electrochemical reaction is stored in a tank system with one or more pressurized gas cylinders, for example, at a pressure of 700 bar. During refueling, the pressurized gas cylinders heat up. To prevent a specified temperature threshold of, for example, 85°C from being exceeded, the hydrogen is usually pre-cooled at the filling station to, for example, -40°C. Alternatively or additionally, the flow rate can be limited during refueling.

[0006] Since the pre-cooling of hydrogen at the filling station is cost-intensive and the flow limitation during refueling leads to long refueling times, the present invention is concerned with the task of providing a cooling of the tank system which does not have these disadvantages.

[0007] The object is achieved by the method having the features of claim 1 and the fuel cell system having the features of claim 7. Advantageous developments of the invention can be found in the respective subclaims. Disclosure of the invention

[0008] A method is proposed for operating a fuel cell system, comprising at least one fuel cell stack with a cathode and an anode. During normal operation of the fuel cell system, air is supplied to the cathode via an air supply path, which air has previously been compressed with the aid of at least one air compressor integrated into the air supply path. The exhaust air exiting the cathode is supplied via an exhaust air path to a turbine integrated into the exhaust air path and operatively connected to the air compressor. During normal operation, hydrogen is also supplied to the anode from a tank system with at least one compressed gas container. According to the invention, the tank system is cooled before, during and / or after refueling, wherein air or exhaust air is used for cooling, which is branched off from the exhaust air path downstream of the turbine by opening a valve and supplied to the tank system via a tank cooling path.

[0009] In the proposed process, cooling of the tank system is achieved using expanded and thus significantly cooled air or exhaust air. This is because the air or exhaust air is expanded in the turbine. This expansion leads to a significant cooling of the air or exhaust air (expansion cooling). For example, with a conventional turbine efficiency of 65% and an expansion from 4 bar to 1 bar, the exhaust air can be cooled from 30°C to -35°C.

[0010] The primary function of the turbine integrated into the exhaust air pad is to recover some of the energy used to compress the air. Using the proposed process, another function can be integrated into the turbine. The air or exhaust air, which is significantly cooled in the turbine by expansion cooling, can be diverted and fed into the tank system for cooling. The proposed cooling of the tank system eliminates the need for flow restriction during refueling, thus shortening refueling times. Under certain circumstances, pre-cooling of the hydrogen at the filling station may also be dispensed with, thus reducing the costs of providing hydrogen.

[0011] The proposed method can be performed before, during, and / or after refueling the tank system. For example, shortly before initiating a refueling process. Preferably, the tank system is cooled by expansion cooling at least during refueling. Furthermore, cooling is preferably continued for a certain period after refueling to reliably prevent exceeding a predetermined temperature threshold.

[0012] In a further development of the invention, it is proposed that the at least one fuel cell stack is cooled with the aid of a cooling system which comprises a cooling circuit with a coolant pump for conveying a coolant and a main cooler. Before, during and / or after refueling of the tank system, an initial cooling of the air or exhaust air in the at least one fuel cell stack can then be achieved with the aid of the cooling system. The cooling system is preferably operated such that the coolant in the cooling circuit is brought to ambient temperature or almost to ambient temperature by dissipating heat to the environment in the main cooler. To support this, the delivery capacity of the coolant pump can be increased. The coolant cooled in this way then flows into the at least one fuel cell stack and likewise cools it to ambient temperature or almost to ambient temperature.

[0013] Furthermore, the air or exhaust air used to cool the tank system is preferably first compressed using the at least one air compressor and then cooled using a cooler integrated into the supply air path downstream of the at least one air compressor. The compression serves to provide a certain air or exhaust air mass flow. Since the air heats up considerably during compression, it is subsequently cooled using the downstream cooler. The at least one air compressor is preferably operated such that a high air compression ratio, for example, of 4 to 5, and a high air mass flow, for example, of 150 g / s, are achieved.

[0014] Furthermore, a heat exchanger integrated into the supply air path is preferably used as a cooler. The coolant of the cooling system or the exhaust air in the exhaust air path can flow through this heat exchanger. Cooling of the compressed air in the supply air path can therefore be achieved in different ways. Depending on the type of cooling, the heat exchanger can be designed as a gas-liquid heat exchanger or a gas-gas heat exchanger.

[0015] According to a first preferred embodiment of the invention, the cooler is designed as a gas-gas heat exchanger through which the air in the supply air path flows on the one hand, and the exhaust air in the exhaust air path on the other. Preferably, a first cooling of the air compressed by the at least one air compressor takes place in the gas-gas heat exchanger, followed by a second cooling in the at least one fuel cell stack with the aid of the cooling system. The exhaust air leaving the fuel cell stack then preferably has a temperature that almost corresponds to the ambient temperature. This exhaust air can then be used to cool the compressed air in the supply air path in the gas-gas heat exchanger, so that the exhaust air entering the turbine has a temperature that is slightly higher than the ambient temperature. In the turbine, the exhaust air is then cooled to a temperature of, for example, -30 to -35 °C due to expansion.

[0016] According to a further preferred embodiment of the invention, the cooler is designed as a gas-liquid heat exchanger through which the air in the supply air path flows on the one hand and the coolant of the cooling circuit flows on the other.

[0017] With the help of the coolant passed through the heat exchanger, the air can then be cooled to ambient temperature or almost to ambient temperature, so that the air no longer has to be passed through at least one fuel cell stack.

[0018] As a further development, it is therefore proposed that air be used to cool the tank system. This air is directed from the supply air path to the exhaust air path via a bypass path with an integrated bypass valve that bypasses the at least one fuel cell stack. This bypass valve is opened and a shut-off valve arranged in the supply air path is closed. Air, not exhaust air, is supplied to the turbine via the bypass path. The air has also been previously compressed with the aid of the at least one air compressor and cooled with a cooler integrated into the supply air path downstream of the at least one air compressor before being directed into the exhaust air path via the bypass path upstream of the turbine.

[0019] Preferably, the tank system and / or at least one temperature-critical component of the tank system is / are directly exposed to the air or exhaust air in the tank cooling path.

[0020] Alternatively, it is proposed that the air or exhaust air in the tank cooling path be passed through a heat exchanger integrated into the tank system.

[0021] The fuel cell system proposed to solve the above-mentioned problem also includes: - at least one fuel cell stack with a cathode and an anode, - an air system for supplying the cathode of the at least one fuel cell stack with air, wherein the air system has an air supply path with at least one air compressor integrated into the air supply path and an exhaust air path with a turbine integrated into the exhaust air path and operatively connected to the air compressor, and - a tank system with at least one compressed gas container for storing hydrogen.

[0022] According to the invention, a tank cooling path branches off from the exhaust air path downstream of the turbine, via which air or exhaust air can be supplied to the tank system for cooling.

[0023] The proposed fuel cell system is particularly suitable for implementing the method according to the invention described above or can be operated according to this method. Accordingly, the same advantages can be achieved with the proposed fuel cell system. In particular, by eliminating a flow restriction when refueling the tank system, refueling time can be shortened. Furthermore, the costs for providing hydrogen at the filling station can be reduced, since pre-cooling may be omitted under certain circumstances.

[0024] According to a preferred embodiment of the invention, a valve, for example a 3 / 2-way valve, is integrated into the exhaust air pad downstream of the turbine, via which the tank cooling path is or can be connected to the exhaust air path. By opening the valve, air or exhaust air from the exhaust air path is supplied to the tank system for cooling via the tank cooling path. In the 3 / 2-way valve design, the connection of the tank cooling path to the exhaust air path can be implemented particularly easily with just one valve.

[0025] According to a further preferred embodiment of the invention, the at least one fuel cell stack is integrated into a cooling circuit of a cooling system comprising a coolant pump for conveying a coolant and a main cooler. With the aid of the cooling system, the at least one fuel cell stack can be cooled, not only during normal operation, but also before, during, and / or after refueling the tank system in order to pre-cool the exhaust air supplied to the turbine.

[0026] Furthermore, it is proposed that a cooler be integrated into the supply air path downstream of the at least one air compressor. With the aid of the cooler integrated into the supply air path, the air previously compressed by the at least one air compressor can be cooled, since this air heats up considerably during compression. The cooler is preferably designed as a heat exchanger through which the coolant of the cooling circuit or the exhaust air in the exhaust air path flows. In the former case, the heat exchanger is preferably designed as a gas-liquid heat exchanger. In the latter case, the heat exchanger is preferably designed as a gas-gas heat exchanger.

[0027] Advantageously, the supply air path and the exhaust air path are connected or connectable via a bypass path with an integrated bypass valve that bypasses the at least one fuel cell stack. The air from the supply air path can be fed directly into the exhaust air path via the bypass path, bypassing the at least one fuel cell stack. Thus, air and not exhaust air is supplied to the turbine. Preferably, the air is first compressed using the at least one air compressor and then cooled using the cooler integrated into the supply air path downstream of the air compressor.

[0028] Preferably, a shut-off valve is integrated into the supply air path to separate the at least one fuel cell stack from the supply air path. If only air and no exhaust air is to be supplied to the turbine, the shut-off valve can be closed while the bypass valve integrated into the bypass path is opened at the same time.

[0029] Cooling the tank system using the air or exhaust air diverted from the exhaust air path can be achieved in various ways. For example, a heat exchanger can be integrated into the tank system, through which the tank cooling path is routed. The cryogenic air or exhaust air supplied to the heat exchanger via the tank cooling path then absorbs the heat generated during refueling in the heat exchanger and dissipates it.

[0030] Alternatively, it is proposed that at least one temperature-critical component of the tank system be integrated into the tank cooling path, so that it is directly exposed to the cryogenic air or exhaust air in the tank cooling path. In this case, an additional heat exchanger can be omitted. Drawings

[0031] Preferred embodiments of the invention are described below with reference to the accompanying drawings, in which: 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, Fig. 3 a flow chart illustrating a preferred process sequence before, during and / or after refueling of the tank system of the Fig. 1 shown fuel cell system and Fig. 4 a flow chart illustrating a preferred process sequence before, during and / or after refueling of the tank system of the Fig. 2 shown fuel cell system. Detailed description of the drawings

[0032] The Fig. 1 shows a fuel cell system 1 according to the invention, which comprises a fuel cell stack 2 with a cathode 2.1 and an anode 2.2. In addition, the fuel cell system 1 has an air system 20 for supplying the cathode 2.1 of the fuel cell stack 2 with air. The air system 20 comprises an air supply path 3, into which an electric motor-driven first air compressor 4 and a second air compressor 4 driven by a turbine 6 are integrated. The second air compressor 4 can - as in the Fig. 1 - optionally additionally have an electric motor as a drive. The order of the air compressors 4 can also be swapped. The air system 20 further comprises an exhaust air path 5 for discharging the exhaust air exiting the cathode 2.1.

[0033] Since the air heats up considerably during compression, a cooler 14 is integrated into the supply air path 3 downstream of the second air compressor 4. The cooler 14 is in the Fig. 1 is designed as a gas-gas heat exchanger through which the exhaust air flows in the exhaust air path 6. The air in the supply air path 3 is therefore cooled by the exhaust air in the exhaust air path 5. This is possible because the fuel cell stack 2 is cooled with the aid of a cooling system 10, which comprises a cooling circuit 11, a coolant pump 12 integrated into the cooling circuit 11 for conveying a coolant, and a main cooler 13 integrated into the cooling circuit 11. To ensure that the exhaust air exiting the cathode 2.1 is cooler than the air supplied to the cathode 2.2, the delivery rate of the coolant pump 12 can be increased so that more coolant is passed through the main cooler 13. At the same time, a bypass path 18 bypassing the main cooler 13 is blocked by closing a bypass valve 19. In this way, the fuel cell stack 2 and the cathode 2.1 of the fuel cell stack 2 is cooled to a temperature that is almost the same as the ambient temperature.

[0034] Downstream of the gas-gas heat exchanger, the turbine 6 is integrated into the exhaust air path 5. In the turbine 6, the exhaust air is expanded, where it cools down considerably. In certain cases, this cryogenic exhaust air is not discharged to the environment, but is branched off into a tank cooling path 9 by controlling a valve 8 integrated into the exhaust air path 5 downstream of the turbine 6. The tank cooling path 9 connects the exhaust air path 5 to a tank system 7, in which the hydrogen required by the anode 2.2 of the fuel cell stack 2 is stored. With the help of the cryogenic exhaust air supplied via the tank cooling path 9, the tank system 7 can be cooled before, during and / or after refueling. For this purpose, the following in connection with the Fig. 3 described steps must be carried out: In step S10, refueling of the tank system 7 is initiated. In the subsequent step S11, the air compressors 4 are then operated such that the compression ratio is increased. Depending on the cathode-side pressure, the anode-side pressure level and / or the pressure level in the cooling circuit 11 may also need to be increased in a step S12. This is because if the pressure difference is too high, there is a risk of leaks. In step S13, the coolant temperature is then brought to ambient temperature by closing the bypass valve 19 and / or increasing the delivery rate of the coolant pump 12. In step S14, the coolant pumped through the fuel cell stack 2 by the coolant pump 12 ensures appropriate cooling of the fuel cell stack 2. In an optional step S15, current can be drawn to prevent excessively high cell voltages and thus premature degradation of the cells of the fuel cell stack 2.The exhaust air exiting the fuel cell stack 2 is then fed to the turbine 6 via the gas-to-gas heat exchanger and, in step S16, is significantly cooled in the turbine 6 by means of expansion cooling. Subsequently, in step S17, the valve 8 is opened, and the exhaust air, significantly cooled by expansion cooling, is fed to the tank system 7 for cooling via the tank cooling path 9. In step S18, which is performed during and / or after refueling, a check is performed to determine whether the temperature in the tank system 7 is below a defined temperature threshold. If this is the case, the cooling of the tank system 7 can be terminated in step S19 by closing the valve 8.

[0035] The Fig. 2 shows a further fuel cell system 1 according to the invention. This differs from that of Fig. 1 essentially in that the cooler 14 integrated into the supply air path 3 downstream of the two air compressors 4 is designed as a gas-liquid heat exchanger. This is also integrated into the cooling circuit 11, so that the coolant flows through the heat exchanger as a liquid. Furthermore, downstream of the cooler 14, a bypass path 15 with an integrated bypass valve 16 branches off to bypass the fuel cell stack 2. The bypass path 15 opens into the exhaust air path 5 upstream of the turbine 6. In addition, two shut-off valves 17 are provided, by means of which the fuel cell stack 2 can be separated from the air system 20. With the aid of the Fig. 2, cooling of the tank system 7 before, during and / or after refueling can also be achieved by means of expansion cooling. For this purpose, the tank system 7 is supplied with cryogenic air, expanded by means of the turbine 6, rather than exhaust air that is expanded by means of the turbine 6. The method is described below in connection with the Fig. 4 described: In step S20, refueling of the tank system 7 is initiated. In the subsequent step S21, the shut-off valve 17 is closed and the bypass valve 16 is opened, so that the fuel cell stack 2 is separated from the air system 20. In step 22, the two air compressors 4 are then operated such that the compression ratio is increased. The air, which heats up considerably during compression, is then cooled in step S23 with the help of the cooler 14 downstream of the air compressors 4, so that the temperature of the air preferably corresponds to the ambient temperature. The compressed and cooled air then passes from the supply air path 3 directly into the exhaust air path 5 and thus to the turbine 6 via the bypass path 15. In step S24, the air is then further cooled considerably by means of expansion cooling in the turbine 6. In step S25, the valve 8 is then opened, and the greatly cooled air is fed to the tank system 7 for cooling via the tank cooling path 9.In step S26, which is performed during and / or after refueling, a check is made to determine whether the temperature in the tank system 7 is below a defined temperature threshold. If this is the case, cooling of the tank system 7 can be terminated in step S27 by closing the valve 8.

Claims

[1] Method for operating a fuel cell system (1), comprising at least one fuel cell stack (2) with a cathode (2.1) and an anode (2.2), wherein in normal operation - air is supplied to the cathode (2.1) via an air supply path (3), which air has previously been compressed by means of at least one air compressor (4) integrated into the air supply path (3), and wherein the exhaust air emerging from the cathode (2.1) is supplied via an exhaust air path (5) to a turbine (6) integrated into the exhaust air path (5) and operatively connected to the air compressor (4), - hydrogen is supplied to the anode (2.2) from a tank system (7) with at least one pressurised gas container, characterized bythat the tank system (7) is cooled before, during and / or after refueling, wherein air or exhaust air is used for cooling, which air is branched off from the exhaust air path (5) downstream of the turbine (6) by opening a valve (8) and is supplied to the tank system (7) via a tank cooling path (9). [2] Method according to claim 1, characterized by that the at least one fuel cell stack (2) is cooled by means of a cooling system (10) which comprises a cooling circuit (11) with a coolant pump (12) for conveying a coolant and a main cooler (13). [3] Method according to claim 1 or 2, characterized bythat the air or exhaust air used to cool the tank system (7) is previously compressed with the aid of the at least one air compressor (4) and cooled with the aid of a cooler (14) integrated into the supply air path (3) downstream of the at least one air compressor (4), wherein preferably a heat exchanger integrated into the supply air path (3) is used as the cooler (14), through which the coolant of the cooling system (10) or the exhaust air in the exhaust air path (5) flows. [4] Method according to one of the preceding claims, characterized by that for cooling the tank system (7) air is used which is guided from the supply air path (3) into the exhaust air path (5) via a bypass path (15) bypassing the at least one fuel cell stack (2) with an integrated bypass valve (16) by opening the bypass valve (16) and closing a shut-off valve (17) arranged in the supply air path (3). [5] Method according to one of the preceding claims, characterized bythat the tank system (7) and / or at least one temperature-critical component of the tank system (7) is / are directly exposed to the air or exhaust air in the tank cooling path (9). [6] Method according to one of claims 1 to 4, characterized by that the air or exhaust air in the tank cooling path (9) is passed through a heat exchanger integrated into the tank system (7). [7] Fuel cell system (1), comprising - at least one fuel cell stack (2) with a cathode (2.1) and an anode (2.2), - an air system (20) for supplying the cathode (2.1) of the at least one fuel cell stack (2) with air, wherein the air system (20) has an air supply path (3) with at least one air compressor (4) integrated into the air supply path (3) and an exhaust air path (5) with a turbine (6) integrated into the exhaust air path (5) and operatively connected to the air compressor (4), and - a tank system (7) with at least one compressed gas container for storing hydrogen, characterized by that downstream of the turbine (6) a tank cooling path (9) branches off from the exhaust air path (5), via which air or exhaust air can be supplied to the tank system (7) for cooling. [8] Fuel cell system (1) according to claim 7, characterized by that downstream of the turbine (6) a valve (8), for example a 3 / 2-way valve, is integrated into the exhaust air pad (5), via which the tank cooling path (9) is or can be connected to the exhaust air path (5). [9] Fuel cell system (1) according to claim 7 or 8, characterized by that the at least one fuel cell stack (2) is integrated into a cooling circuit (11) of a cooling system (10) which comprises a coolant pump (12) for conveying a coolant and a main cooler (13). [10] Fuel cell system (1) according to one of claims 7 to 9, characterized bythat downstream of the at least one air compressor (4) a cooler (14) is integrated into the supply air path (3), which cooler is preferably designed as a heat exchanger through which the coolant of the cooling circuit (11) or the exhaust air in the exhaust air path (5) flows. [11] Fuel cell system (1) according to one of claims 7 to 10, characterized by that the supply air path (3) and the exhaust air path (5) are connected or can be connected via a bypass path (15) bypassing the at least one fuel cell stack (2) with an integrated bypass valve (16). [12] Fuel cell system (1) according to one of claims 7 to 11, characterized by that a shut-off valve (17) for separating the at least one fuel cell stack (2) from the supply air path (3) is integrated into the supply air path (3). [13] Fuel cell system (1) according to one of claims 7 to 12, characterized bythat a heat exchanger is integrated into the tank system (7), via which the tank cooling path (9) is guided. [14] Fuel cell system (1) according to one of claims 7 to 12, characterized by that at least one temperature-critical component of the tank system (7) is integrated into the tank cooling path (9).

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