METHOD FOR STARTING A FUEL CELL SYSTEM AND A FUEL CELL SYSTEM

Abstract

The invention relates to a starting method for a fuel cell system (100), particularly for an air/air start of the fuel cell system (100). The method enables the reduction of damaging half-cell voltages in the fuel cell stack (10) through voltage limitation by means of a DC voltage converter. The homogeneous flushing of the fuel cell stack (10) required for this takes place by means of introduction of an anode operating medium into an anode inlet channel (17) of the otherwise sealed fuel cell stack (10) until a predetermined pressure is reached and flushing of the active areas of the fuel cells (11) of the stack (10) after said pressure is reached through opening of an anode discharge adjusting aid (26), preferably arranged in an exhaust coupling (29) connecting the anode exhaust line (22) and the cathode exhaust line (31). In preferred embodiments of the method according to the invention, a provision is to improve the mass flow of the anode operating medium in an anode supply (20) of the fuel cell stack (10) through suitable operation of a recirculation conveying mechanism (27). Another subject matter of the invention is also a fuel cell system (100) with a control unit (70) for implementing the method according to the invention.

Claims

1. A method comprising: starting a fuel cell system, the fuel cell system including a plurality of fuel cells, the starting including: blocking an anode discharge adjusting means in an anode exhaust line of an anode supply; detecting a pressure in an anode inlet channel of the fuel cell stack of the fuel cell system; introducing an anode operating medium into an anode supply line until reaching a threshold pressure in the anode inlet channel; evenly distributing the anode operating medium in the anode inlet channel; preventing the anode operating medium from entering the plurality of fuel cells; opening of the anode discharge adjusting means after reaching the threshold pressure in the anode inlet channel introducing of the anode operating medium by means of subcritical operation of an operating medium conveying mechanism; and converting to a supercritical operation of the operating medium conveying mechanism before or at the same time as the opening of the anode discharge adjusting means.

2. The method according to claim 1 further comprising: setting a voltage limit of a DC voltage converter and establishing an electrical connection between the DC voltage converter and at least one fuel cell of the plurality of fuel cells before the introducing of the anode operating medium into the anode supply line.

3. The method according to claim 1, further comprising creating an electrical current with depletion of oxygen by evenly flowing the anode operating medium into an active area of the plurality of fuel cells after the opening of the anode discharge adjusting means.

4. The method according to of claim 1, further comprising: activating of a recirculation conveying mechanism arranged in a recirculation line connecting the anode supply line and the anode exhaust line before or at the same time as the opening of the anode discharge adjusting means.

5. (canceled)

6. The method according to claim 4, further comprising: blocking the recirculation line or operating the recirculation conveying mechanism in reverse mode during the introducing of the anode operating medium; and unblocking the recirculation line or converting the recirculation conveying mechanism to normal mode before or at the same time as the opening of the anode discharge adjusting means.

7. The method according to claim 1, further comprising: recording a stack voltage according to a predetermined first time frame; determining a second time frame by means of the recorded stack voltage; and closing the anode discharge adjusting means after the first time frame and the second time frame.

8. The method according to claim 1, further comprising: blocking a first cathode separator, a second cathode separator, the first cathode separator in a cathode supply line and the second cathode separator in a cathode exhaust line; and activating of a compressor in the cathode supply line in response to a wastegate adjusting means opening before opening of the anode discharging adjusting means.

9. The method according to claim 8, further comprising: closing of the wastegate adjusting means by means of a control process; opening of the first cathode separator and of the second cathode separator; and converting to a controlled operation of the wastegate adjusting means.

10. A fuel cell system comprising: a fuel cell stack with an anode inlet channel for supplying an anode operating medium to a plurality of fuel cells; an anode supply with an anode supply line for supplying the anode operating medium to the anode inlet channel and with an anode exhaust line; a cathode supply with a first cathode separating means arranged in a cathode supply line and with a second cathode separating means arranged in a cathode exhaust line; an exhaust coupling connecting the anode exhaust line and the cathode exhaust line, the exhaust coupling comprising an anode discharge adjusting means; a recirculation line connecting the anode supply line and the anode exhaust line, the recirculation line comprising a recirculation conveying mechanism; a compressor arranged in the cathode supply line; a wastegate line connecting the cathode supply line and the cathode exhaust line, the wastegate line comprising a wastegate adjusting means; a conveying mechanism arranged in the anode supply line; a sensor for detecting a pressure in the anode inlet channel; a voltage sensor for recording an electrical stack voltage; and a control unit that in operation: blocks the anode discharge adjusting means; detects a pressure in the anode inlet channel; introduces the anode operating medium into the anode supply line until a threshold pressure in the anode inlet channel is reached, the anode operating medium being evenly distributed in the anode inlet channel and the plurality of fuel cells being remaining free of the anode operating medium; and opens the anode discharge adjusting means after the threshold pressure in the anode inlet channel is reached.

11. The fuel cell system of claim 10 wherein the control unit in operation sets a voltage limit of a DC voltage converter that can be connected to the fuel cell stack and establishes an electrical connection between the DC voltage converter and at least one fuel cell of the plurality of fuel cells before the anode operating medium is introduced into the anode supply line.

12. The fuel cell system of claim 10 wherein the control unit activates the conveying mechanism before or at the same time as the anode discharge adjusting means is opened.

13. The method according to claim 1, further comprising: recording a stack voltage according to a predetermined first time frame; determining a second time frame by means of the recorded stack voltage; and closing the anode discharge adjusting means in response to exceeding a threshold concentration of the anode operating medium in the fuel cell stack, the anode exhaust line, or a cathode exhaust line.

14. The method according to claim 1, further comprising: recording a stack voltage according to a predetermined first time frame; determining a second time frame by means of the recorded stack voltage; and closing the anode discharge adjusting means in response to a voltage or current plateau.

Description

[0036] The invention is explained below in exemplary embodiments on the basis of the respective drawings. The following is shown:

[0037] FIG. 1 shows a block diagram of a fuel cell system according to a preferred embodiment;

[0038] FIG. 2 shows a sectional view of a fuel cell system according to a preferred embodiment;

[0039] FIG. 3 shows a schematic representation of a fuel cell vehicle according to a preferred embodiment; and

[0040] FIG. 4 shows a schematic block diagram of a method for starting a fuel cell system according to a preferred embodiment;

[0041] FIG. 1 shows a fuel cell system, denoted overall by 100, according to a preferred embodiment of the present invention. The fuel cell system 100 is part of a vehicle (not shown), in particular an electric vehicle, which has an electric traction motor, which is supplied with electrical energy by the fuel cell system 100.

[0042] The fuel cell system 100 comprises, as core components, a fuel cell stack 10, which has a plurality of individual cells 11, which are arranged in the form of a stack and which are formed by alternately stacked membrane electrode assemblies (MEAs) 14 and bipolar plates 15 (see detailed view). An individual cell 11 thus respectively comprises an MEA 14 with an ion-conductive polymer electrolyte membrane, not shown in more detail here, and catalytic electrodes arranged thereon on both sides. The electrodes, an anode, and a cathode catalyze the respective partial reaction of the fuel cell conversion and are respectively formed as a coating on the membrane.

[0043] The anode and cathode electrodes have catalytic material (for example platinum), which is supported on electrically conductive carrier material with a large specific surface (for example a carbon-based material). An anode chamber 12 is formed between a bipolar plate 15 and the anode, and the cathode chamber 13 is formed between a cathode and a bipolar plate 15. The bipolar plates 15 serve to supply the operating media to the anode and cathode chambers 12, 13 and also establish the electrical connection between the individual fuel cells 11. Optionally, gas diffusion layers are arranged between the MEA 14 and the bipolar plates 15.

[0044] In order to supply the fuel cell stack 10 with the operating media, the fuel cell system 100 has an anode supply 20 and a cathode supply 30.

[0045] The anode supply 20 comprises an anode supply line 21, which supplies an anode operating medium, for example hydrogen, as fuel to the anode chambers 12 of the fuel cell stack 10. To this end, the anode supply line 21 connects a fuel reservoir and a jet pump 24 to an anode inlet of the fuel cell stack 10 via a metering valve 23. The anode exhaust line 22 discharges the anode exhaust gas from the anode chambers 12 via an anode outlet of the fuel cell stack 10.

[0046] The anode operating pressure on the anode sides 12 of the fuel cell stack 10 can be adjusted via the metering valve 23 and a jet pump 24 in the anode supply line 21. Furthermore, the anode supply 20 has a fuel recirculation line 25, which connects the anode exhaust line 22 to the anode supply line 21. The recirculation of fuel is customary in order to return the overly-stoichiometric supplied fuel to the stack 10 and to use it. A recirculation conveying mechanism 27 is arranged in the fuel recirculation line 25.

[0047] An exhaust coupling 29, which connects the anode exhaust line 22 to the cathode exhaust line 32, branches into the anode exhaust line 22. An anode discharge adjusting means 26 is situated in the exhaust coupling 29. Thus, the anode exhaust and the cathode exhaust together can be discharged via a cathode exhaust line 32.

[0048] The cathode supply 30 comprises a cathode supply line 31 for supplying an oxygen-containing cathode operating medium, particularly from air suctioned from the environment, to the cathode chambers 13 of the fuel cell stack 10. The cathode supply 30 further comprises the cathode exhaust line 32, which discharges the cathode exhaust gas (particularly exhaust air) from the cathode chambers 13 of the fuel cell stack 10. The fuel cell stack 10 can be separated from the cathode supply 30 by means of a first cathode shut-off valve 40 situated in the cathode supply line 31 and a second cathode shut-off valve 41 situated in the cathode exhaust line 32, in which no gas exchange takes place between the fuel cell stack 10 and the cathode supply 30.

[0049] A compressor 33 for conveying and compressing the cathode operating medium is arranged in the cathode supply line 31. In the embodiment shown, the compressor 33 is designed as a compressor which is driven mainly by an electric motor, the drive of which is effected via an electric motor 34 equipped with a corresponding power electronics system 35.

[0050] The cathode supply 30 further has a wastegate line 37 for connecting the cathode supply line 31 to the cathode exhaust line 32, i.e., a bypass of the fuel cell stack 10. An air mass flow or an air volume flow can be moved to the fuel cell stack 10 by means of the wastegate line 37. A wastegate adjusting means 38 arranged in the wastegate line 37 serves to control the amount of the cathode operating medium bypassing the fuel cell stack 10.

[0051] The fuel cell system 100 further has a humidifier 39, which is permeable to water vapor. On one hand, the humidifier 39 is arranged in the cathode supply line 31 in such a way that cathode operating gas flows through it. On the other hand, it is arranged in the cathode exhaust line 32 such that the cathode exhaust gas can flow through it. In doing so, the comparatively dry cathode operating gas (air) flows over one side of the humidifier and the comparatively humid cathode exhaust gas (exhaust gas) flows over the other side. Driven by the higher partial pressure of the water vapor in the cathode exhaust gas, water vapors pass over the membrane into the cathode operating gas and this causes the humidification thereof.

[0052] FIG. 2 shows a sectional view of the fuel cell stack 10, which has a discernible plurality of stacked individual cells 11, which are formed by alternately stacked membrane electrode assemblies 14 and bipolar plates 15. The individual cells 11 are pressed together and held together, sealed in a fluid-tight manner, by two end plates 16. They are clamped together via clamping devices (not shown). In the representation according to FIG. 2, the left end plate 16 is formed as a media supply plate in that it has corresponding connections for the supply and discharge line 20, 30 of the anode operating medium. The end plate 16 downstream of the flow of operating medium is arranged on the right.

[0053] FIG. 2 further exemplary shows an anode inlet channel 17 for supplying an anode operating medium 19 for the fuel cell stack 10 as well as an anode outlet channel 18 for discharging the anode operating medium 19. The anode operating medium 19 is placed in the main supply channel 17 and flows through it using a corresponding connection on the media supply plate 16, shown on the left. From there, it is distributed to the individual cells 11 of the stack. After flowing over the active catalytic areas of the membrane electrode assembly 14 of the individual cells 11, the anode operating medium 19 flows from the individual cells 11 as exhaust gas into the anode outlet channel 18, from where it is discharged from the stack 10 via the end plate 16.

[0054] FIG. 3 shows a schematic representation of a fuel cell vehicle 200 according to a preferred embodiment of the present invention. The vehicle 200 shown in FIG. 3 has the fuel cell system 100 described in detail with reference to FIG. 1, the electronic control unit 70, an electrical power system 60, as well as a vehicle drive system 50.

[0055] The electrical power system 60 comprises a voltage sensor 61 for detecting a voltage generated by the fuel cell stack 10 and a current sensor 62 for detecting a current generated by the fuel cell stack 10. The electrical power system 60 further comprises an energy storage unit 64, such as a high-voltage battery or a capacitor. A converter 65, designed in triport topology (triport converter), is further arranged in the power system 60. The battery 64 is connected to a first side of the double DC/DC converter 65. The DC/DC converter 65 can be operated as a voltage limiter and can be switched for this purpose.

[0056] All traction network components of the drive system 50 are connected to a second side of the converter 65, with a fixed voltage level. In the same or a similar manner, the auxiliary units of the fuel cell system 100, for example the electric motor 34 of the compressor 33, or other electrical consumers of the vehicle, for example a compressor for a climate-control system or the like, are connected to the power network.

[0057] The drive system 50 comprises an electric motor 51, which serves as traction motor of the vehicle 200. To this end, the electric motor 51 drives a drive axle 52 with drive wheels 53 arranged thereon. The traction motor 51 is connected to the electrical power system 60 of the fuel cell system 100 via an inverter 63 and represents the main electrical consumer of the fuel cell system 100.

[0058] The electronic control unit 70 controls the operation of the vehicle 200 and particularly the fuel cell system 100, including its anode and cathode supply 20, 30, the electrical power system 60, and the traction motor 51. For this purpose, the control unit 70 receives different input signals, such as the voltage V of the fuel cell 10, detected using the voltage sensor 61, the current I of the fuel cell stack 10, detected using the current sensor 62 (from which the control unit calculates the current stack power P.sub.stack(t)), information about the temperature T of the fuel cell stack 10, the pressures p in the anode and/or cathode chamber 12, 13, the charge state SOC of the energy storage unit 64, and the pressure p.sub.in in the anode inlet channel.

[0059] FIG. 4 shows a schematic block diagram of a preferred embodiment of the method according to the invention while using the fuel cell system 100 arranged in the vehicle 200, as shown in FIGS. 1 to 3.

[0060] The method according to the invention starts with the receiving of an “ON” control signal in the control unit 70 that shows a desired start of a fuel cell system.

[0061] In step S1, the control unit 70 closes the anode discharge adjusting aid 26 and the first and second cathode separating aid 40, 41. Subsequently, the control unit 70 opens the waste gate adjusting aid 38 and activates the compressor 33 positioned in the cathode supply 30. In doing so, the waste gate adjusting aid 38 is completely open such that an air mass flow of about 50 g/s is completely supplied via the wastegate line 37 of the cathode exhaust line 32. In addition, the control unit 70 connects the DC voltage converter 65 to the fuel cell stack 10 and sets an upper voltage limit of the DC voltage converter 65.

[0062] In step S2, the control unit 70 opens the metering valve 23 to a hydrogen reservoir, which is not shown, such that the mass flow exiting remains low to the extent that the conveying mechanism formed as the jet pump 24 is operated at subcritical level. Thus, the anode inlet channel 17 of the fuel cell stack 10 is filled with the hydrogen by means of a first pressure ramp of low rise. The air previously penetrating into the fuel cell stack 10 is pushed out of the anode inlet channel 17 and into the active areas of the fuel cells 11 and compressed in the anode outlet channel 18. Simultaneously, the control unit 70 activates the recirculation conveying mechanism 27 in a reverse mode, i.e. with a conveying direction from the anode supply line 31 toward the anode exhaust line 32. This additionally prevents this air, during the subcritical operation of the conveying mechanism 24, from being suctioned by means of the recirculation line 25 and supplied back into the stack 10.

[0063] In step S3, which starts at the same time as step S2, the control unit 70 monitors the pressure of the hydrogen in the anode inlet channel 17 by means of the pressure sensor 71. When a predetermined pressure is reached in the anode inlet channel 17, which is about 1.3 bar in the present case, the control unit 70 proceeds to the next process step S5.

[0064] In process step S4, the control unit 70 converts the conveying mechanism 24 from the subcritical to a supercritical operation by means of a fastest-possible pressure ramp and, in doing so, increases the mass flow being conveyed from the recirculation line 25 by the conveying mechanism 24 significantly. Simultaneously, the control unit 70 opens the anode discharge adjusting aid 26 and the second cathode separating aid 41. The established pressure means that the hydrogen that was previously evenly present in the anode inlet channel 17 is now introduced into the exhaust coupling 29 and into the cathode exhaust line 32 within a short time due to the active areas of the fuel cells 11 and by means of the anode outlet channel 18. The hydrogen is mixed with the air mass flow recirculated by the compressor 33 there and safely discharged into the environment.

[0065] In step S5, which takes place simultaneously with step S4, the control unit 70 measures an electronic voltage of the fuel cell stack 10 after a first predetermined time frame by means of the voltage sensor 61. The control unit 70 estimates the quantity of the air that previously penetrated from the measured voltage. Using this estimate, the control unit 70 determines a second time frame, after the elapsing of which it can be expected that the air will be pushed from the entire anode region and be replaced by a sufficient quantity of hydrogen. The total from the first time frame and the second time frame at present is between 40 and 500 ms. Based on the voltage limit set by the DC voltage converter 65, an electric current then flows, which limits the high cell voltages and thus the degradation of the fuel cell stack 10. Due to the not yet activated supply of the cathode side of the fuel cell stack 10, the oxygen still present is quickly depleted such that the electrical current is limited in a natural manner.

[0066] After the first and the second time frames have expired, the control unit 70 closes the anode discharge adjusting aid 26 and proceeds to process step S6 in that it initially closes the wastegate adjusting aid 38 in a controlled manner, which opens the first cathode separator 40 and then routes the airflow from the compressor 33 through the fuel cell stack 10. Subsequently, the control unit 70 transitions to a controlled operation of the wastegate adjusting aid 38 that discharges air to the fuel cell stack 10 via the wastegate line 37 according to a current power query. Once process step S6 is complete, the controllable air supply of the fuel cell stack 10 is then assured such that the electrical current then increases as soon as the hydrogen/air front on the anode has passed the stack 10. Thus, the control unit 70 transitions to normal mode of the fuel cell stack 10.

REFERENCE LIST

[0067] 100 Fuel cell system [0068] 10 Fuel cell stack [0069] 11 Individual cell [0070] 12 Anode chamber [0071] 13 Cathode chamber [0072] 14 Membrane electrode assembly (MEA) [0073] 15 Bipolar plate (separator plate, flux field plate) [0074] 16 End plate/media supply plate/downstream plate [0075] 17 Anode inlet channel [0076] 18 Anode outlet channel [0077] 19 Operating medium/anode operating medium/hydrogen [0078] 20 Anode supply [0079] 21 Anode supply line [0080] 22 Anode exhaust line [0081] 23 Metering valve [0082] 24 Conveying mechanism/jet pump [0083] 25 Recirculation line [0084] 26 Anode discharge adjusting aid [0085] 27 Recirculation conveying mechanism [0086] 29 Exhaust coupling [0087] 30 Cathode supply [0088] 31 Cathode supply line [0089] 32 Cathode exhaust line [0090] 33 Compressor [0091] 34 Electric motor [0092] 35 Power electronics [0093] 37 Wastegate line [0094] 38 Wastegate adjusting aid [0095] 39 Humidifier module [0096] 40 First cathode separator [0097] 41 Second cathode separator [0098] 42 Air bypass [0099] 50 Drive system [0100] 51 Traction motor [0101] 52 Drive axle [0102] 53 Drive wheels [0103] 60 Electrical power system [0104] 61 Voltage sensor [0105] 62 Current sensor [0106] 63 Inverter [0107] 64 Energy storage unit [0108] 65 DC converter [0109] 70 Control unit [0110] 71 Pressure sensor [0111] S Stack direction