Fuel cell system and method for operating a fuel cell system

Abstract

The invention relates to a fuel cell system with a fuel cell to which a compressed gas is supplied via a feed gas duct by means of a first gas conveying unit which comprises a first compressor which is drivingly connected to an exhaust gas turbine, and by means of a second gas conveying unit which comprises a second compressor which is drivingly connected to an exhaust gas turbine, said gas reacting in the fuel cell with a fuel, wherein exhaust gas is produced which is discharged via a exhaust gas duct by means of the exhaust gas turbines, wherein between the two gas conveying units a first gas-gas heat exchanger is arranged, wherein between the second gas conveying unit and the fuel cell a second gas-gas heat exchanger is arranged, and with an exhaust gas recirculation valve which is connected to a recirculation point in the feed gas duct. In order to make the operation of a fuel cell system more efficient, in particular without higher operating costs, the recirculation point is arranged in the feed gas duct between the first compressor and the second gas-gas heat exchanger.

Description

[0001] Description

[0002] title

[0003] Fuel cell system and method for operating a fuel cell system

[0004] The invention relates to a fuel cell system having a fuel cell to which a compressed gas is supplied via a supply line to a first gas delivery unit comprising a first compressor that is drivingly connected to an exhaust gas turbine, and to a second gas delivery unit comprising a second compressor that is drivingly connected to an exhaust gas turbine. This compressed gas reacts with a fuel in the fuel cell to produce exhaust gas that is discharged via an exhaust gas line to the exhaust gas turbines. A first gas-to-gas heat exchanger is arranged between the two gas delivery units, a second gas-to-gas heat exchanger is arranged between the second gas delivery unit and the fuel cell, and an exhaust gas recirculation valve is connected to a recirculation point in the supply line. The invention further relates to a method for operating such a fuel cell system.

[0005] State of the art

[0006] German patent application DE 10 2019 215 895 A1 discloses hydrogen-based fuel cells as the basis for a future mobility concept that emits only water and enables fast refueling times. Such fuel cells are electrochemical energy converters that are interconnected with a plurality of such electrochemical fuel cells to form a fuel cell stack in order to provide a correspondingly high total voltage. The reactants hydrogen and oxygen are converted into electrical energy, water, and heat.

[0007] For example, PEM fuel cells can be operated with air supplied to the cathode of the fuel cell, with oxygen as the oxidant, and hydrogen supplied to the anode of the fuel cell as fuel in an electrocatalytic electrode process to provide electrical energy with high efficiency.The German patent application DE 10 2018 205 886 A1 discloses a fuel cell system with a fuel cell, a supply line for supplying an oxidizing agent into the fuel cell and an exhaust gas line for discharging the oxidizing agent from the fuel cell, wherein a first compressor is arranged in the supply line, wherein a second compressor is arranged in the supply line downstream of the first compressor, wherein a cooler is arranged in the supply line between the first compressor and the second compressor, wherein an exhaust gas bypass is arranged in the exhaust gas line, in which exhaust gas bypass a control valve and an exhaust gas turbine are arranged.The international patent application WO 2019 / 166122 discloses a fuel cell system with a fuel cell, an air supply line for supplying an oxidizing agent into the fuel cell and an exhaust gas line for discharging the oxidizing agent from the fuel cell, wherein the fuel cell system has a cooling circuit through which a cooling medium can flow for cooling the fuel cell, wherein the cooling circuit has cooling channels arranged in the fuel cell, wherein the cooling circuit has a first heat exchanger arranged in the air supply line and wherein the cooling circuit has a second heat exchanger arranged in the exhaust gas line, wherein an exhaust gas turbine is arranged in the exhaust gas line, wherein the second heat exchanger is preferably arranged upstream of the exhaust gas turbine.

[0008] Disclosure of the invention

[0009] The object of the invention is to make the operation of a fuel cell system more efficient, in particular without increasing the operating costs.

[0010] The object is achieved in a fuel cell system with a fuel cell, to which a compressed gas is supplied via a supply line with a first gas delivery unit, which comprises a first compressor, which is drivingly connected to an exhaust turbine, and with a second gas delivery unit, which comprises a second compressor, which is drivingly connected to an exhaust turbine, which reacts with a fuel in the fuel cell, producing exhaust gas, which is discharged via an exhaust line with the exhaust turbines, wherein a first gas-gas heat exchanger is arranged between the two gas delivery units, wherein a second gas-gas heat exchanger is arranged between the second gas delivery unit and the fuel cell, and with an exhaust gas recirculation valve, which is connected to a recirculation point in the supply line,that the return point is arranged in the supply gas line between the first compressor and the second gas-gas heat exchanger. The fuel cell system comprises, in a known manner, several fuel cells arranged in a fuel cell stack, which is also referred to as a fuel cell stack or, for short, as a stack. The supplied gas is, for example, air containing oxygen that reacts with the fuel. The fuel is, for example, hydrogen. The term compressor also includes displacers with which the supplied gas, in particular the supplied air, is compressed. The compressors, which can also be referred to as air compressors, are, according to a preferred embodiment, provided with a drive comprising an electric machine. Furthermore, the two air compressors are each drivingly connected to at least one exhaust gas turbine.which is arranged in the exhaust gas duct. During compression, the supplied gas heats up. To protect the components downstream of the compressors, a heat exchanger is advantageously arranged downstream of each compressor. The heat exchanger can protect the downstream components from excessively high temperatures. The heat exchangers are preferably designed as supply-exhaust gas heat exchangers, which are also referred to as gas-gas heat exchangers for short. This design of the heat exchangers advantageously enables the relatively cool exhaust gas of the fuel cell to be used to cool the hot, compressed air. The claimed fuel cell system is advantageously equipped with several bypasses. The bypasses are each equipped with a bypass valve. Individual components, particularly in the exhaust gas duct, can be connected to the bypass via the bypasses.be bypassed if necessary. The bypasses in the exhaust gas system advantageously bypass a heat exchanger and an exhaust turbine. After the first compression, the air is heated, so it must be cooled by the first gas-to-gas heat exchanger. This is advantageously done by exhaust air,which is obtained by expansion in an exhaust turbine assigned to the first compressor. The second gas-to-gas heat exchanger advantageously ensures the cooling of the air heated by the second air compressor. The second gas-to-gas heat exchanger is cooled on the exhaust side with lukewarm exhaust gas from the fuel cell. This protects the fuel cell stack from excessively high temperatures. The claimed arrangement of the return point can effectively improve the component protection of the second compressor. This can extend the service life of the fuel cell system. Furthermore, the fuel cell stack can be more effectively protected from high temperatures. Furthermore, the efficiency of the fuel cell system can be improved through targeted intercooling upstream of the second compressor.that fuel consumption is reduced. Compressors can be single-stage or multi-stage. Therefore, the compressors can also be referred to as compressor stages.

[0011] A preferred embodiment of the fuel cell system is characterized in that a first turbine bypass with a first turbine bypass valve bypasses the second gas-to-gas heat exchanger and the exhaust gas turbine in the exhaust gas duct. This arrangement of the first turbine bypass provides functional advantages depending on the operating state, while deliberately accepting increased control effort given that the first turbine bypass essentially originates from the same fluidic branching as the exhaust gas recirculation.

[0012] A further preferred embodiment of the fuel cell system is characterized in that a second turbine bypass with a second turbine bypass valve bypasses the first gas-to-gas heat exchanger and the exhaust gas turbine in the exhaust gas duct. It is accepted that, when the second turbine bypass valve is open, the essential cooling effect on the air heated by the first compressor in the first gas-to-gas heat exchanger is significantly impaired. In this context, the claimed arrangement of the recirculation point has a strong effect on protecting the second compressor from overheating. A further preferred embodiment of the fuel cell system is characterized in that the exhaust gas recirculation originates from a branch arranged in the exhaust gas duct between the fuel cell and the second gas-to-gas heat exchanger. The first turbine bypass also advantageously originates from this branch.This has proven to be advantageous with regard to the desired cooling effect of the recirculated exhaust gas.

[0013] A further preferred embodiment of the fuel cell system is characterized in that the gas delivery units each comprise an electric motor drive. This allows the compression of the gas supplied to the fuel cell via the gas supply line, in particular the supplied air, to be adjusted and, if necessary, regulated with the utmost efficiency.

[0014] In a method for operating a fuel cell system described above, the above-mentioned object is achieved alternatively or additionally by recirculating exhaust gas from the fuel cell into the intake gas line via the exhaust gas recirculation valve at the return point in order to cool the compressed gas with the recirculated exhaust gas. The associated increased control complexity is deliberately accepted in order to effectively protect the individual components of the fuel cell system against overheating.

[0015] A preferred embodiment of the method is characterized in that the temperature at the return point in the intake duct is detected, whereby the exhaust gas recirculation valve is opened when the detected temperature exceeds a predetermined threshold and closed when the detected temperature no longer exceeds the predetermined threshold. Thus, both the second compressor and the fuel cell can be effectively protected against overheating in a relatively simple manner.

[0016] A further preferred embodiment of the method is characterized in that the opening and closing of the exhaust gas recirculation valve is controlled as a function of the temperature at the recirculation point in the intake air system. The control is carried out, for example, in a suitable control unit. The control advantageously also includes the opening and closing of the first turbine bypass valve in the first turbine bypass and / or the opening of the second turbine bypass valve in the second turbine bypass.

[0017] The invention further relates to a device configured to perform all steps of the method described above. The device advantageously comprises a control unit and / or a computer, or a computer combined with the control unit, in the fuel cell system.

[0018] The invention further relates to a computer program configured to perform all steps of the method described above. The invention also relates to a machine-readable storage medium having such a computer program stored thereon.

[0019] Further advantages, features and details of the invention will become apparent from the following description, in which various embodiments are described in detail with reference to the drawing.

[0020] Short description of the drawing

[0021] They show:

[0022] Figure 1 is a schematic representation of a fuel cell system and

[0023] Figure 2 is a flow chart illustrating a method for operating the fuel cell system shown in Figure 1.

[0024] Description of the embodiments

[0025] Figure 1 shows a fuel cell system 15 with a fuel cell stack 17 comprising a plurality of fuel cells 16. A gaseous oxidizing agent, in particular air containing oxygen, is supplied to the fuel cell stack 17 in the form of a compressor mass flow via a gas supply 18. A gas conveyor comprises two gas conveyor units 21, 22 in the supply line 18. The gas conveyor unit 21 comprises a first compressor 23. The first compressor 23 is drivable by a drive 24. The drive 24 comprises, for example, an electric machine that enables an electric motor drive of the first compressor 23. Furthermore, the first compressor 23 is drivingly connected to an exhaust turbine 25.

[0026] The exhaust gas turbine 25 represents a second exhaust gas turbine in the gas conveying unit 21. A first exhaust gas turbine 25 is arranged upstream of the second exhaust gas turbine 28 in an exhaust gas duct 19.

[0027] The exhaust turbine 28, in combination with a drive 27, is drivingly connected to a second compressor 26. The second compressor 26, together with the drive 27 and the exhaust turbine 28, constitutes a gas extraction unit 22 for gas extraction.

[0028] A first gas-to-gas heat exchanger 31 is arranged between the two gas delivery units 21 and 22. A second gas-to-gas heat exchanger 32 is arranged between the gas delivery unit 22 and the fuel cell stack 17.

[0029] The gas-to-gas heat exchanger 32 and the exhaust turbine 25 of the gas extraction unit 22 can be bypassed in the exhaust duct 19 by a turbine bypass 41 with a turbine bypass valve 43. The gas-to-gas heat exchanger 31 and the exhaust turbine 28 of the gas extraction unit 21 can be bypassed in the exhaust duct 19 by a turbine bypass 42 with a turbine bypass valve 44.

[0030] The turbine bypass 41 originates from a fluidic branch 35, which is arranged in the exhaust gas duct 19 between the gas-gas heat exchanger 32 and the fuel cell stack 17. An exhaust gas recirculation 34, in which an exhaust gas recirculation valve 33 is arranged, also originates from the fluidic branch 35. The exhaust gas recirculation 34 connects the exhaust gas recirculation valve 33 to a recirculation point 37, which is arranged in the intake gas duct 18 between the gas-gas heat exchanger 31 and the second compressor 26. The schematic representation of the fuel cell system 15 in Figure 1 includes a fluidic intersection 36. At the fluidic intersection 36, the exhaust gas recirculation 34 is not connected to the intake gas duct 18. A fluidic connection between the exhaust gas recirculation 34 and the supply gas guide 18 is provided in the fuel cell system 15 only at the return point 37 in the form of a fluidic branch.

[0031] Temperatures 1 to 12 at various locations in the fuel cell system 15 are illustrated in Figure 1 by circles numbered 1 to 12. Temperatures 1 to 12 change during operation of the fuel cell system 15. Further circles 13 and 14 in Figure 1 indicate that a first temperature 13, corresponding to temperature 3, and a second temperature 14, corresponding to temperature 5, are detected by sensors during operation of the fuel cell system 15.

[0032] During operation of the fuel cell system 15, the fuel cell stack 17 heats up due to the chemical reaction between the supplied air and the hydrogen fuel that is also supplied. The waste heat from the fuel cell stack 17 is dissipated, for example, via a cooling circuit connected to a main vehicle radiator. The air supplied to the fuel cell stack 17 via the intake duct 18 is compressed by the two electric air compressors 21 and 22.

[0033] As a result of compression, temperature 2 is significantly higher than temperature 1. Temperature 2 can be lowered to temperature 3 via the gas-to-gas heat exchanger 31. As a result of compression in the second compressor 26, temperature 4 increases again compared to temperature 3. Temperature 3 is lowered to temperature 5 via the gas-to-gas heat exchanger 32.

[0034] In the gas-to-gas heat exchanger 32, the relatively cool exhaust gas at temperature 10 from the fuel cell 16 is used for cooling. Therefore, temperature 9 is greater than temperature 10. Temperature 8 is lower than temperature 9 because the exhaust gas is expanded in the exhaust turbine 25. In the gas-to-gas heat exchanger 31, the cooled exhaust gas at temperature 8 is used for cooling; therefore, temperature 7 is greater than temperature 8. Due to the expansion in the exhaust turbine 28, temperature 6 is lower than temperature 7.

[0035] During normal operation of the fuel cell system 15, the fuel cell stack 17 is protected from excessively high temperatures by the illustrated combination of the gas feed units 21, 22 with the two gas-gas heat exchangers 31, 32

[0036] The fuel cell stack 17 is also referred to as stack 17 in the following. The exhaust gas turbines 25 and 28 are also referred to as turbines in the following. The gas-gas heat exchangers 31, 32 are also referred to as heat exchangers 31, 32 in the following.

[0037] If the second turbine 28 is bypassed or bypassed by opening the bypass valve 44, the first heat exchanger 31 on the exhaust air side is also bypassed or bypassed. Its cooling effect on the air heated by the first compressor 23 is then eliminated.

[0038] The compression in the second compressor 26 can cause the air to become very hot. This can lead to undesirable overheating in and behind the turbine 28. Since the temperature of a compressor wheel must not exceed certain temperatures, for example, 180 degrees Celsius, for material strength reasons, this condition can lead to damage, particularly to the second air compressor 26.

[0039] In the worst case, the second heat exchanger 32 can also become so overheated that it can no longer reduce the air temperature below the permissible temperature for stack 17. Then, stack 17 is also no longer protected from unwanted overheating and the associated damage.

[0040] Here, the cooling of the supply air supplied via the supply duct 18 with

[0041] The exhaust gas recirculation 34 from the branch 35 in the exhaust duct 19 to the return point 37 in the intake duct 18 offers significant advantages. Firstly, the component protection of the second compressor 26 is improved. Furthermore, the component protection of the stack 17 is improved.

[0042] Furthermore, the efficiency of the fuel cell system 15 can be effectively improved, particularly through improved intercooling upstream of the second compressor 26. This has a positive effect on the service life and operating costs of the fuel cell system 15. Furthermore, fuel consumption during operation of the fuel cell system 15 can be reduced.

[0043] Figure 2 shows an operating strategy in the form of a flowchart of the claimed method with method steps 51, 52, 53, and 54. Method step 51 relates to component protection against excessively high temperatures. In method step 52, a check is carried out to determine whether the first temperature 13 detected by the sensor exceeds the predetermined threshold. If this is the case, the exhaust gas recirculation valve 33 in the exhaust gas recirculation system 34 is opened in method step 53. In method step 53, the opening of the exhaust gas recirculation valve 33 in the exhaust gas recirculation system 34 can optionally be controlled.

[0044] If the first temperature 13 no longer exceeds the predetermined threshold, the exhaust gas recirculation valve 33 in the exhaust gas recirculation system 34 is closed in method step 54. In method step 52, the first temperature 13 is continuously monitored during operation of the fuel cell system.

[0045] By opening the exhaust gas recirculation valve 33, lukewarm exhaust air from the stack 17 is recirculated to the recirculation point 37 in the intake duct 18. This lowers all temperatures of the compressors 4; 5, 14 downstream of the second compressor 26. This protects the compressor from overheating. The second gas-to-gas heat exchanger 32 also does not overheat. This also protects the stack 17 from overheating.

Claims

Claims 1 . Fuel cell system (15) with a fuel cell (16), to which a compressed gas is supplied via a supply line (18) to a first gas delivery unit (21), which comprises a first compressor (23) which is drivingly connected to an exhaust gas turbine (28), and to a second gas delivery unit (22), which comprises a second compressor (26) which is drivingly connected to an exhaust gas turbine (25), which reacts with a fuel in the fuel cell (16), producing exhaust gas which is discharged via an exhaust gas line (19) to the exhaust gas turbines (25, 28), wherein a first gas-gas heat exchanger (31) is arranged between the two gas delivery units (21, 22), wherein a second gas-gas heat exchanger (32) is arranged between the second gas delivery unit (21) and the fuel cell (16), and having an exhaust gas recirculation valve (33) which is connected to a return point (37) in the feed guide (18), characterized in thatthat the return point (37) is arranged in the supply gas duct (18) between the first compressor (23) and the second gas-gas heat exchanger (32), 2. Fuel cell system according to claim 1, characterized in that a first turbine bypass (41) with a first turbine bypass valve (43) bypasses the second gas-gas heat exchanger (32) and the exhaust gas turbine (25) in the exhaust gas duct (19).

3. Fuel cell system according to one of the preceding claims, characterized in that a second turbine bypass (42) with a second turbine bypass valve (44) bypasses the first gas-gas heat exchanger (31) and the exhaust gas turbine (28) in the exhaust gas duct (19).

4. Fuel cell system according to claim 3, characterized in that the exhaust gas recirculation (34) originates from a branch (35) which is arranged in the exhaust gas duct (19) between the fuel cell (16) and the second gas-gas heat exchanger (32).

5. Fuel cell system according to one of the preceding claims, characterized in that the gas conveying units (21, 22) each comprise an electric motor drive (24, 27).

6. Method for operating a fuel cell system (15) according to one of the preceding claims, characterized in that exhaust gas from the fuel cell (16) is returned via the exhaust gas recirculation valve (33) in a targeted manner at the return point (37) into the supply gas guide (18) in order to cool the compressed gas with the recirculated exhaust gas.

7. The method according to claim 6, characterized in that the temperature at the return point (37) in the intake gas duct (18) is detected, wherein the exhaust gas recirculation valve (33) is opened when the detected temperature (13) exceeds a predetermined threshold value and is closed when the detected temperature (13) no longer exceeds the predetermined threshold value.

8. Method according to claim 6 or 7, characterized in that the opening and closing of the exhaust gas recirculation valve (33) is controlled as a function of the temperature (13) at the return point (37) in the intake gas duct (18).

9. Device designed to carry out all steps (51-54) of a method according to one of claims 6 to 8.

10. Computer program product configured to perform all steps (51 to 54) of a method according to any one of claims 6 to 8.

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