Method for frost start-up of a fuel cell device, fuel cell device and motor vehicle having a fuel cell device

Abstract

The invention relates to a method for frost starting a fuel cell device (1) having a fuel cell stack (3), in which a plurality of electrically connected fuel cells (2) are compressed with a compression force of a tensioning device (23) between two end plates (22), comprising the following steps: determining the presence of frost starting conditions; reducing the compression force by means of the tensioning device (23), and operating the fuel cells (2) in a frost starting operation, in which a reduced compression force acting on the fuel cells (2) of the fuel cell stack (3) and relative to the compression force of a normal operation is maintained. The invention furthermore relates to a fuel cell device (1) and a motor vehicle having a fuel cell device (1).

Description

Technical Field

[0001] This invention relates to a method for frost-starting a fuel cell device having a fuel cell stack, wherein a plurality of fuel cells connected in series are compressed between two end plates by the compressive force of a tensioning device. The method includes the steps of: determining the presence of frost-start conditions; reducing the compressive force by means of the tensioning device; and operating the fuel cells under frost-start operation, wherein the reduced compressive force acting on the fuel cells of the fuel cell stack relative to the normal operating compressive force is maintained. The invention further relates to a fuel cell device and a motor vehicle having a fuel cell device. Background Technology

[0002] Fuel cell devices are used to chemically convert fuel and oxygen into water to generate electricity. For this purpose, a fuel cell includes a so-called membrane electrode unit (MEU) as its core component. The MEU is a composite consisting of a membrane that conducts protons and electrodes (anode and cathode) arranged on opposite sides of the membrane. Furthermore, a gas diffusion layer (GDL) can be arranged on either side of the MEU, on the sides of the electrodes facing away from the membrane. In the operation of a fuel cell device having multiple fuel cells assembled into a fuel cell stack, fuel, especially hydrogen (H2) or a hydrogen-containing mixture, is supplied to the anode, where H2 to H2 gas is converted into H2 upon the release of electrons. + Electrochemical oxidation. Proton H + The reaction proceeds from the anode chamber (with or without water) to the cathode chamber via a membrane that hermetically separates and electrically insulates the chambers. Electrons supplied at the anode are transported to the cathode via electrical conductors. Oxygen or an oxygen-containing mixture is supplied to the cathode to facilitate the O2 to O2 reaction upon electron reception. 2- The reduction occurs. Simultaneously, these oxygen anions react with protons transported across the membrane in the cathode chamber, forming water. This water must be extracted from the fuel cell and fuel cell stack until the humidity level required for the operation of the fuel cell system is reached.

[0003] Therefore, fuel cell devices require careful water management. On the one hand, it is necessary to prevent excessive water in the fuel cell or fuel cell stack, which can lead to blockage of the channels used to supply reactants. On the other hand, if there is too little water in the fuel cell, the proton conductivity of the membrane will be limited, thus requiring attention to adequate membrane humidity and water supply.

[0004] To provide sufficient oxygen from the air to the numerous fuel cells assembled in the fuel cell stack, the cathode space of the fuel cell stack is supplied with oxygen-containing air via a compressor during the cathode cycle. This results in relatively warm and dry compressed air, with insufficient humidity for use in the fuel cell stack for the membrane electrode assembly (MEA) unit. Therefore, a humidifier is used to facilitate the transfer of humidity to the drier medium in the presence of two gaseous media with different moisture contents. This is achieved by guiding the dry air supplied by the compressor through a humidifier membrane permeable to water vapor, with the moist exhaust gas from the fuel cell stack passing over the other side.

[0005] If frost conditions, i.e., conditions where water freezes, exist when starting up a fuel cell system, this is problematic. This can cause the necessary flow paths for reactant gases and product water to become blocked by ice; therefore, it is known to perform a drying procedure when shutting down a fuel cell unit. The same problem can arise during startup of a fuel cell unit because, during the preheating operation until it reaches its operating temperature, temperature differences exist and can occur in the fuel cell stack, related to the supply of the medium, i.e., the gaseous reactants and the cooling medium used for temperature control of the fuel cell stack, which typically consists of a cooling fluid with a heat capacity higher than that of the reactants.

[0006] In mobile applications, targeted attention is paid to ensuring that the startup of the fuel cell unit can be performed as quickly as possible, where startup is limited or delayed by the thermal mass of the fuel cell stack and coolant, by possible ice in the flow channels, and by the vehicle's maximum power consumption.

[0007] US 2005 / 058 865A1 describes a fuel cell stack in which the fuel cell integrates an electric heating element between surrounding end plates in case of frost conditions during startup.

[0008] DE 10 2016 118 511 A1 describes an electrical contact device for a fuel cell, the contact area of ​​which can be used as a resistance heating element to generate heat.

[0009] JP 2008-262 822A describes a method for stopping and starting a fuel cell unit for a fuel cell vehicle. At each stop, the electrolyte membrane is compressed in the stacking direction of the membrane electrode assembly by increasing pressure via a compression device, wherein this compressed state is maintained throughout the duration until a new start. The compression of the electrolyte membrane prevents it from absorbing water, thus providing water absorption capacity again in the event of a frost start, because at each new start, the compression of the electrolyte membrane is terminated by a pressure reduction until the increased pressure reaches zero, i.e., completely and accurately reversing the pressure increase that occurred at the time of stop.

[0010] In KR 2013 0 017 960A, the compression unit is used to increase the resistance at the edge side of the fuel cell stack due to the reduction of the compressive force acting on it, thereby causing it to operate and heat at a deteriorated efficiency. Summary of the Invention

[0011] The objective of this invention is to provide a method for achieving more uniform heating across the entire fuel cell stack. Furthermore, it is to provide an improved fuel cell device and an improved motor vehicle.

[0012] This task is accomplished by the method, fuel cell device, and motor vehicle having the features of the claims. Advantageous designs of the invention with suitable improvements are given in the dependent claims.

[0013] The method described at the beginning is characterized by an increase in the individual contact resistance (ohmic resistance) between the fuel cells in the fuel cell stack during frost start-up operation, due to the reduction of compressive force by the tensioning device.

[0014] Advantages in this configuration include continuous or clock pulse-based detection of the fuel cell stack temperature, and the setting of the compression force by means of the tensioning device depending on the detected temperature. Therefore, the tensioning device can actively control and / or adjust the compression force based on the fuel cell stack temperature or ambient temperature.

[0015] During frost start-up operation, the compression force is preferably increased continuously or in stages by the tensioning device until the predetermined normal operating temperature of the fuel cell stack is reached. By increasing the compression force, the ohmic resistance across the entire fuel cell stack decreases again.

[0016] Upon reaching the normal operating temperature, the fuel cell unit can transition to normal operation, during which an increased compression force compared to the compression force applied during frost start-up is applied to the fuel cell. Therefore, the increased compression force during normal operation ensures the necessary conductivity between the cell units of the fuel cell stack.

[0017] For additional heating of the stack, the fuel cell can also operate in an oxygen-consuming manner during frost start-up operation.

[0018] To provide additional heating for the stack, it is feasible for the fuel cell to operate at a reduced voltage relative to normal operation during the frost start-up run. For this purpose, a voltage reduction of less than 0.4 volts per cell is considered.

[0019] The compression force that effectively tightens / compresses the cell and improves the adjustment effect can be caused, for example, by adjusting the compression force on the cell stack based on the pressure in the air bellows.

[0020] The effective tensioning / compression of the cell and the improved adjustment of the compression force can alternatively be caused, for example, by the tensioning device having a tensioning band whose tension is adjusted by an electric actuator.

[0021] The effective tensioning / compression of the cell and the improved adjustment of the compression force can alternatively be caused, for example, by the tensioning device having a pull rod whose tension is adjusted by an electric actuator.

[0022] The aforementioned effects and advantages also apply, in a sense, to fuel cell devices having an adjustable tensioning device for adjusting the compressive force on the fuel cells stacked between two end plates, which is configured to perform one of the above methods and to motor vehicles having such fuel cell devices.

[0023] The features and combinations of features mentioned in the above description, as well as the features and combinations of features mentioned and / or shown separately in the accompanying drawings, can be used not only in the combinations described herein, but also in other combinations or individually, without departing from the scope of the invention. Therefore, the following embodiments, which are not explicitly shown or explained in the drawings but are known and can be produced from the individual combinations of features in the explained embodiments, should also be considered as included and disclosed by the invention. Attached Figure Description

[0024] Further advantages, features, and details of the invention will become apparent from the claims, the following description of preferred embodiments, and the accompanying drawings. Wherein:

[0025] Figure 1 A schematic diagram of a fuel cell device is shown, and

[0026] Figure 2 The graphs show the voltage-current characteristic lines of the fuel cell unit during normal operation (normal operation) as dashed lines, and the voltage-current characteristic lines of the fuel cell unit during frost start operation with reduced compression force on the cell as solid lines. Detailed Implementation

[0027] Figure 1 The diagram schematically shows a fuel cell device 1, which includes multiple fuel cells 2 assembled in a fuel cell stack 3.

[0028] Each fuel cell 2 includes an anode, a cathode, and a proton-conducting membrane separating the anode and cathode. This membrane is formed of an ionomer, preferably a sulfonated polytetrafluoroethylene (PTFE) polymer or a perfluorosulfonic acid (PFSA) polymer. Alternatively, the membrane may also be formed as a sulfonated hydrocarbon membrane.

[0029] Additionally, a catalyst may be incorporated at the anode and / or cathode, wherein the membrane is preferably coated on its first side and / or its second side with a catalyst layer consisting of a noble metal or a mixture including noble metals such as platinum, palladium, ruthenium or the like, which serves as a reaction promoter in the reaction of each fuel cell 2.

[0030] Fuel (e.g., hydrogen) can be supplied to the anode from fuel tank 13 through the anode chamber. In a polymer electrolyte membrane fuel cell (PEM fuel cell), the fuel or fuel molecules are split into protons and electrons at the anode. The PEM allows protons to pass through but is impermeable to electrons. For example, at the anode, the following reaction occurs: 2H₂ → 4H₂ + +4e - (Oxidation / Electron Emission). When protons reach the cathode through the PEM, electrons are conducted to the cathode or energy storage device through an external circuit.

[0031] A cathode gas (e.g., oxygen or oxygen-containing air) can be supplied to the cathode through the cathode chamber so that the following reaction occurs on the cathode side: O₂ + 4H₂O + +4e - →2H2O (reduction / electron absorption).

[0032] In the fuel cell stack 3, multiple fuel cells 2 are assembled into a battery bank between two end plates 22. These fuel cells are subjected to compressive force by a tensioning device 23 to ensure the necessary sealing and contact pressure required for the stack's conductivity. Since the multiple fuel cells 2 must be supplied with a sufficiently large amount of cathode gas, a large cathode gas mass flow or fresh gas flow is provided via compressor 18, where the temperature increases significantly as a result of cathode gas compression. The regulation of the cathode gas or fresh gas flow, i.e., its adjustment in terms of desired temperature and humidity within the fuel cell stack 3, is carried out in a booster air cooler 5 located after compressor 18 and a humidifier 4 located after it. This causes the membranes of the fuel cells 2 to become saturated with moisture to improve their efficiency, as this is beneficial for proton transport. To remove the heat generated during the normal operation of the fuel cells 2, the fuel cell stack 3 is traversed by coolant conduits, which form part of a coolant circuit (not shown in more detail).

[0033] The compression force on the battery pack can be adjusted by tensioning device 23, whose tensioning device, such as a tie rod, tensioning belt, or similar actuator, can be electrically driven (not shown in more detail). Due to this possibility of adjusting the compression force, the fuel cell unit 1 can switch between frost start-up operation and normal operation.

[0034] The method for frost start-up utilizes Figure 2 The charts illustrate this in more detail. Here, the U / I characteristic line for frost start operation with reduced compression of the battery pack is lowered compared to the normal operating U / I characteristic line with conventional compression of the battery pack. The waste heat portion A1, the electrical power portion B1, the waste heat portion A2, the electrical power portion B2, and the calorific value H (H2) of the fuel are also shown here.

[0035] The method includes the following steps: determining the presence of frost start conditions; reducing the compressive force by means of tensioning device 23; and operating the fuel cell 2 under frost start operation, under which the reduced compressive force acting on the fuel cell 2 of the fuel cell stack 3 relative to the normal operating compressive force is maintained.

[0036] Preferably, the temperature of the fuel cell stack 3 is continuously or pulsed using a temperature sensor, wherein the compression force is adjusted by a tensioning device 23 depending on the detected temperature. The compression force is then continuously or in stages increased by the tensioning device 23 until a predetermined normal operating temperature is reached for the fuel cell stack 3. Upon reaching the normal operating temperature, the fuel cell unit 1 transitions to normal operation, in which an increased compression force compared to the compression force used during frost-free start-up is applied to the fuel cell 2 to achieve adequate contact between the cell units.

[0037] There is an additional possibility that fuel cell 2 may operate in an oxygen-consuming mode during frost start-up operation. Furthermore, it is feasible for fuel cell 2 to operate at a voltage higher than the normal operating voltage during the frost start-up operation.

[0038] In the fuel cell unit 1 with a control device, the control device is configured to execute a method that provides the aforementioned advantages for frost start-up, and the degradation of the fuel cell stack 3 can be avoided or at least mitigated. This is particularly suitable when the fuel cell unit 1 is installed in a motor vehicle.

[0039] This method can improve the efficiency of motor vehicle drive systems, thereby reducing relative fuel costs, improving lifecycle assessments, and increasing range. This further improvement in efficiency in fuel cell 2 will make the already challenging freeze-start process even more difficult in the future, due to a further decrease in thermal power.

[0040] This invention counteracts this effect by temporarily reducing the efficiency of fuel cell 2, thereby increasing the waste heat generated when heating fuel cell stack 3 with the same electrical power. This increases the freedom in designing the frost start procedure, and fuel cell stack 3 can be heated more quickly. For the user, this means the vehicle is ready to drive more quickly.

[0041] List of reference numerals

[0042] 1. Fuel Cell Device

[0043] 2. Fuel Cell

[0044] 3 fuel cell stack

[0045] 4 Humidifier

[0046] 5. Boost Air Cooler

[0047] 6. Bypass wire

[0048] 7 Humidifier - Bypass Valve

[0049] 8. Fresh air metering valve

[0050] 9. Cathode Fresh Gas

[0051] 10. Cathode exhaust gas conductor

[0052] 11. Cathode exhaust valve

[0053] 12 Fuel wire

[0054] 13 fuel tanks

[0055] 14. Circulating wires

[0056] 15. Circulating blower

[0057] 16 heat exchangers

[0058] 18 compressors

[0059] 19 Fuel Metering Valve

[0060] 20 water separator

[0061] 21 Coolant wire

[0062] 22 end plates

[0063] 23 Tensioning device

[0064] A1 waste heat (frost start-up operation)

[0065] B1 section electrical power (frost start-up operation)

[0066] A2 section waste heat (normal operation)

[0067] B2 section power (normal operation)

[0068] H (calorific value of fuel / H2)

Claims

1. A method for frost-starting a fuel cell device (1), the fuel cell device having a fuel cell stack (3) wherein a plurality of electrically connected fuel cells (2) are compressed between two end plates (22) by a compressive force of a tensioning device (23), the method comprising the steps of: determining the presence of frost-starting conditions; reducing the compressive force by means of the tensioning device (23) such that the individual contact resistance between each fuel cell (2) in the fuel cell stack (3) increases; operating the fuel cells (2) under frost-starting operation, under which the reduced compressive force acting on the fuel cells (2) in the fuel cell stack (3) relative to the compressive force under normal operation is maintained; continuously or in a clock pulse manner detecting the temperature of the fuel cell stack (3); and adjusting the compressive force by means of the tensioning device (23) depending on the detected temperature.

2. The method according to claim 1, characterized in that, The compressive force is continuously or in stages increased by the tensioning device (23) until the predetermined normal temperature of the fuel cell stack (3) is reached.

3. The method according to claim 2, characterized in that, When the normal temperature is reached, the fuel cell device (1) transitions to normal operation, during which an increased compression force compared to the compression force during the frost start-up operation is applied to the fuel cell (2).

4. The method according to any one of claims 1 to 3, characterized in that, The fuel cell (2) also operates in an oxygen-consuming mode under the frost start-up operation.

5. The method according to any one of claims 1 to 3, characterized in that, The fuel cell (2) also operates at a reduced voltage relative to the normal operating voltage during the frost start-up operation.

6. The method according to any one of claims 1 to 3, characterized in that, The tensioning device (23) has a tensioning band whose tension is adjusted by an electric actuator.

7. The method according to any one of claims 1 to 3, characterized in that, The tensioning device (23) has a pull rod whose tension is adjusted by an electric actuator.

8. A fuel cell device (1) having an adjustable tensioning device (23) for adjusting the compressive force on a fuel cell (2) stacked between two end plates (22), and having a control device configured to perform the method according to any one of claims 1 to 7.

9. A motor vehicle having a fuel cell device (1) according to claim 8.

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