DE102017214966A1 - Method for operating a humidification system for a fuel cell system and motor vehicle with such - Google Patents

Abstract

The invention relates to a method for operating a humidification system for a fuel cell system (100), the humidification system comprising a humidifier (40) which is adapted to transfer moisture from a fluid flow passing through a second flow path (32) through a first flow path (FIG. 31) flowing fluid flow, wherein a content of liquid water in the humidifier (40) is determined by the measurement of a leakage current, and a motor vehicle designed for such a method.

Description

Fuel cells use the chemical transformation of a fuel with oxygen to water to generate electrical energy. For this purpose, fuel cells contain as a core component the so-called membrane electrode assembly (MEA for membrane electrode assembly), which is a microstructure of an ion-conducting (usually proton-conducting) membrane and in each case on both sides of the membrane arranged catalytic electrode (anode and cathode). The latter mostly comprise supported noble metals, in particular platinum. In addition, gas diffusion layers (GDL) can be arranged on both sides of the membrane-electrode arrangement on the sides of the electrodes facing away from the membrane.

As a rule, the fuel cell is formed by a multiplicity of stacked MEAs whose electrical powers add up. As a rule, bipolar plates (also called flow field plates or separator plates) are arranged between the individual membrane electrode assemblies, which ensure that the individual cells are supplied with the operating media, ie the reactants, and are usually also used for cooling. In addition, the bipolar plates provide an electrically conductive contact to the membrane-electrode assemblies.

During operation of the fuel cell, the fuel (anode operating medium), in particular hydrogen, is supplied to the anode via a flux field of the bipolar plate and electrochemically oxidized to protons with release of electrons (H ₂ → 2 H ⁺ + 2 e ^- ). About the electrolyte or the membrane, which gas-tight and electrically isolated from each other, the reaction chambers, a transport of protons from the anode compartment into the cathode compartment. The electrons provided at the anode are supplied to the cathode via an electrical line.

The cathode is supplied during operation of the fuel cell, oxygen or an oxygen-containing gas mixture (for example air) as a cathode operating medium, so that a reduction of O ₂ to O ^2- with absorption of the electrons takes place (½ O ₂ + 2 e ^- → O ^2-). At the same time, the oxygen anions in the cathode compartment react with the protons transported via the membrane to form water (O ^2- + 2 H ⁺ → H ₂ O).

In order to supply the fuel cell stack with the operating media, this has an anode supply and a cathode supply. The anode supply has an anode supply path for supplying the anode operating medium and an anode exhaust path for discharging an anode off-gas from the fuel cell stack. Similarly, the cathode supply includes a cathode supply path for supplying the cathode operating medium and a cathode exhaust path for discharging a cathode exhaust gas from the fuel cell stack.

The maximum power "P" of the fuel cell system is defined by a defined electrical voltage U _pmax and a defined electric current I _{pmax of} the fuel cell stack. In particular, the efficiency n _{pmax of} the fuel cell stack defines this current-voltage correlation. An important factor influencing n _{pmax of} the fuel cell system is the moistening of the fuel cell membrane. Too low a membrane moisture leads to a reduced proton conductivity of the membrane and, as a result, to a lower efficiency of the fuel cell. For a defined adjustment of the membrane moisture, the air supplied to the fuel cell is moistened with water. The humidification of the air is implemented within the fuel cell system with a humidifier. The air flowing out of the fuel cell (exhaust air) is moistened by the product water produced during operation inside the stack and then fed to the humidifier. Also, the humidifier is supplied with a dry air mass flow (supply air), which is humidified within the humidifier by the moist exhaust air of the fuel cell. The humidified supply air is supplied to the fuel cell.

The humidifiers used in fuel cell systems, such as membrane or hollow fiber humidifiers, extract gaseous water from the moist exhaust air of the fuel cell stack and transmit this to the supply air of the fuel cell stack. In order to set a variable inlet humidity of the supply air of the fuel cell, a bypass line to the humidifier and a bypass valve can also be used as a control device. The Befeuchtereinrichtungen used in fuel cell systems are subject to a significantly sinking humidity transfer capability η _{moisture transmission} over the lifetime t of the component. The aging of the humidifier depends on the damage to the membrane materials experienced during operation. High temperatures in combination with low gas humidities represent an increased potential for damage. In addition, the freezing of liquid water within the humidifier also leads to its damage. With respect to a humidification system used in a motor vehicle for humidifying a fuel cell system, this will be explained below.

When the fuel cell system of a vehicle is switched off, depending on the operating strategy, the entire system is always freed from liquid water at critical points in the case of foreseeable frost operation. The goal is to ensure the successful restart of the system after freezing. Blocked valves, flaps or in this case a blocked humidifier flow field or humidifier hollow fibers are to be avoided or necessarily prevented. For the humidifier, freezing in addition to the disadvantage of increased flow resistance, reduced water transfer capability due to ice blocked flow field also damages the freezing process itself. The change in density during the phase change results in mechanical stress on the membrane material of the humidifier, up to cracking. This has a negative effect on the transferability and efficiency of the component and thus leads to accelerated humidification.

Blowing the humidifier off or in other states of the operating strategy may drain the critical liquid water and thus prevent damage. However, this requires a significant amount of energy to operate the ancillaries and extends, for example, the shutdown of the fuel cell system considerably.

The object of the invention is therefore to provide a method that makes it possible to initiate the blowing out targeted. In particular, the method is intended to provide a method of minimizing the energy and time required, as well as providing a means for determining the state of the component with respect to ease of freezability.

This object is achieved by a method having the features of the independent claim. Thus, a first aspect of the invention relates to a method of operating a humidification system for a fuel cell system. The humidification system includes a humidifier configured to transfer moisture from a fluid flow passing through a second flow path to a fluid flow passing through a first flow path. In this case, a content of liquid water, ie a liquid water content, in the humidifier is determined by the measurement of a leakage flow, the leakage flow being from a fluid flow flowing through the first flow path to a fluid flow that has flowed through the second flow path. The leakage current is thus directed in the opposite direction to the intended humidifying flow.

Against this background, the present invention provides a method for determining the liquid water content of a humidifier via a leakage rate measurement from high pressure to low pressure side of the component. As a result, a liquid water content can be set specifically, which prevents damage to the humidifier when freezing at low ambient temperatures. The aging of the humidifier can be effectively reduced or slowed down.

The inventive method thus enables the determination of the liquid water content within a humidifier of a fuel cell system by correlation against the leakage flow between high and low pressure side. This makes it possible to minimize the overrun or drying time of the humidifier for discharging liquid water and the associated savings in parasitic services of the secondary consumers. This is due to the measurable by the inventive method liquid water content of the humidifier, which can be selectively reduced. For example, the liquid water content is reduced only until damage during the freezing process is prevented. The aging of the humidifier can thus be slowed down by the method according to the invention without the need for frequent time-consuming and costly maintenance processes of the humidifier.

The leakage flow is a leakage fluid flow which flows from the first flow path through the humidifier into the second flow path and which is likewise preferably determined as volume or mass flow.

The humidifier is preferably a humidifier commonly used for placement in fuel cell systems. The humidifier preferably has at least one first flow channel for a fluid flow to be humidified with a first fluid supply line and a first fluid discharge and at least one second flow channel for a moist fluid flow with a second fluid supply line and a second fluid discharge line. In this case, the first flow channel is part of the above-mentioned first flow path and the second flow channel is part of the above-mentioned second flow path. The humidifier further comprises at least one water-permeable membrane for transferring moisture from the wet fluid stream to the fluid stream to be humidified.

Preferably, the first flow path and the second flow path are connected via the humidifier, in that the first flow path ends in the first flow channel and the second flow path ends in the second flow channel. In other words, in each case one of the first fluid supply and discharge and the second fluid supply and discharge at the beginning of the inventive method is closed. A connection between the first flow path and the second flow path thus exists over the at least one water-permeable membrane of the humidifier and over the at least one third flow path.

In another preferred embodiment of the method, the humidifier is disposed in a fuel cell system and the first flow path is a cathode supply path of the fuel cell system, the second flow path is a cathode exhaust path of the fuel cell system, and the at least one third flow path is a cathode supply path upstream of the fuel cell system Humidifier with Wastegate with the cathode exhaust gas downstream of the humidifier connecting. According to this embodiment, moreover, a compressor for adjusting the first fluid flow and the specific overpressure is arranged in the cathode supply path upstream of the humidifier. Further, a first shut-off means for blocking the first flow path is arranged downstream of the humidifier in the cathode supply path and a second shut-off means for blocking the second flow path upstream of the humidifier in the cathode exhaust path. In other words, the leakage current of a humidifier, which is arranged in a fuel cell system, is determined with the method according to the invention.

The fuel cell system preferably has a fuel cell stack with a multiplicity of fuel cells and a cathode supply with a cathode supply path and with a cathode exhaust gas path. The humidifier is preferably disposed in the cathode supply path and in the cathode exhaust path. Further, a first shut-off means in the cathode supply path and a second shut-off means in the cathode exhaust path are arranged between the humidifier and the fuel cell stack. Upstream of the humidifier, a compressor is disposed in the cathode supply path. Further, a wastegate line connects the cathode supply path upstream of the compressor and upstream of the humidifier to the cathode exhaust path.

In a preferred embodiment of the invention it is provided that the moistening system additionally comprises pressure sensors, which are arranged upstream and downstream in the first and / or second flow path. The leakage current is determined by a difference between pressures measured upstream and downstream of the humidifier. If the leakage flow is measured directly as the differential pressure across the humidification flow paths of the humidified and / or wet gas stream, this directly allows correlation to the liquid water content within the humidifier. Such a determination of the leakage current represents a direct and also a simple and quickly accessible measurement of the leakage current. However, this requires additional sensors in the form of differential pressure sensors.

Alternatively, a method is used for determining a leakage current, which advantageously manages without these additional sensors and essentially comprises the following steps. A first step consists of connecting the first flow path and the second flow path via the humidifier and at least one third flow path to one another, in particular to connect them to one another in a fluid-conducting manner. The connection between the first and second flow paths consists in particular exclusively of the humidifier and the at least one third flow path. By definition, in a third flow path, there is essentially no chemical conversion of a fluid, in particular operating medium, flowing therein. Thus, at the beginning of the process according to the invention, any connection between the first and second flow paths, in which a chemical reaction of a fluid flowing therethrough, would be blocked.

Subsequently, a first fluid flow is adjusted in the first flow path or conveyed in this or in this. The first fluid flow is adjusted to set a certain overpressure from the first flow path to the second flow path. Subsequently, the first fluid flow actually flowing in the first flow path upstream of the humidifier and at least one second fluid flow actually flowing through the at least one third flow path are determined or detected. The determined or detected fluid flows are preferably fluid volume flows or fluid mass flows. Finally, the leakage flow through the humidifier is determined as the difference between the first fluid flow and the at least one second fluid flow. The leakage flow is a leakage fluid flow which flows from the first flow path through the humidifier into the second flow path and which is likewise preferably determined as volume or mass flow. With the leakage current is advantageously a previously unknown or previously unidentified characteristic value of a humidifier available.

Regardless of the determination of the leakage current, it is preferred that the humidification system also has a control system which is set up to receive signals from the humidifying system and / or from a fuel cell system connected thereto and to trigger the implementation of the method according to the invention. Advantageously, component tests form a data base on which exact amount of liquid water lead to damage of the components, for example during the freezing process. Based on this data basis, a correlation to the leakage current of the humidifier is preferably created, which then preferably deposited in the tax system. During operation of the fuel cell system or of the humidifier, the respectively measured leakage current is compared with the aforementioned correlation function within the control system. If this balancing results in potentially high liquid water content within the humidifier, the discharge of liquid water from the humidifier will be initiated, continued or extended in the manner and manner of the operating strategy intended therefor.

Such a control system also has the advantage that the method according to the invention is carried out automatically, in particular as a function of defined variables. One such variable is, for example, the operating time of the humidification system.

Thus, in a further preferred embodiment of the method according to the invention, it is provided that the method is executed after a defined number of operating hours. This has the advantage that the moisture content of the humidifier is determined at regular intervals.

Alternatively or additionally, it is provided that the method is carried out as part of a shutdown process and / or in a special conditioning of the fuel cell system shortly before the actual freezing process of the fuel cell system. The actual freezing process is represented, for example, by an autonomous state in the parked vehicle. If the liquid water content in the humidifier determined by the method according to the invention does not exceed a critical value, there is no additional conditioning, ie no additional discharge of liquid water from the humidifier. An already running water discharge can be stopped. This results in comparison to a merely time-dependent application of this freezing preconditioning the advantage of a reduced time requirement and minimizing the parasitic power. This is particularly advantageous in the application of the fuel cell system in a vehicle, since the liquid water content of the humidifier when parking the vehicle is always different and among other things from the previously worn load profile, the environmental conditions, inclinations and accelerations in the vehicle and also by aging effects such Deposits in hoses and components is affected. The inventive method is particularly robust in the aforementioned embodiments against such factors influencing the liquid water content of the humidifier, since a measurement in the respective present vehicle state takes place and the comparison to the freezing capability of the humidifier is based thereon.

Thus, another aspect of the invention relates to a motor vehicle having a humidification system that is configured to perform the method according to the invention.

Further preferred embodiments of the invention will become apparent from the remaining, mentioned in the dependent claims characteristics. The various embodiments of the invention mentioned in this application are, unless otherwise stated in the individual case, advantageously combinable with each other.

• 1 eine schematische Darstellung eines zum Durchführen eines Verfahrens gemäß der Erfindung eingerichteten Brennstoffzellensystems;
• 2 eine schematische Darstellung eines Befeuchters und
• 3 ein Entscheidungsdiagramm des erfindungsgemäßen Verfahrens in einer bevorzugten Ausgestaltung.

The invention will be explained below in embodiments with reference to the accompanying drawings. Show it:

• 1 a schematic representation of a set up for carrying out a method according to the invention fuel cell system;
• 2 a schematic representation of a humidifier and
• 3 a decision diagram of the method according to the invention in a preferred embodiment.

1 shows a fuel cell system generally designated 100 according to a preferred embodiment of the present invention. The fuel cell system 100 is part of a not further illustrated vehicle, in particular an electric vehicle having an electric traction motor, by the fuel cell system 100 is supplied with electrical energy.

The fuel cell system 100 comprises as a core component a fuel cell stack 10 containing a plurality of stacked single cells 11 having alternately stacked membrane-electrode assemblies (MEAs) 14 and bipolar plates 15 be formed (see detail). Every single cell 11 thus includes one MEA each 14 , which has an ion-conducting polymer electrolyte membrane (not shown in more detail here) and catalytic electrodes arranged on both sides, namely an anode and a cathode, which catalyze the respective partial reaction of the fuel cell conversion and in particular can be formed as coatings on the membrane.

The anode and cathode electrodes comprise a catalytic material supported on an electrically conductive high surface area support material, such as a carbon based material. Between a bipolar plate 15 and the anode is an anode compartment 12 formed and between the cathode and the next bipolar plate 15 a cathode compartment 13 , The bipolar plates 15 serve the supply of Operating media in the anode and cathode rooms 12 . 13 and further provide the electrical connection between the individual fuel cells 11 ago. Optionally, gas diffusion layers may be interposed between the membrane-electrode assemblies 14 and the bipolar plates 15 be arranged.

To the fuel cell stack 10 to supply with the operating media, the fuel cell system 100 on the one hand, an anode supply 20 and on the other hand, a cathode supply 30 on.

The anode supply 20 includes an anode supply path 21 which feeds an anode operating medium (the fuel), for example hydrogen, into the anode spaces 12 of the fuel cell stack 10 serves. For this purpose, the anode supply path connects 21 a fuel storage 23 with an anode inlet of the fuel cell stack 10 , The anode supply 20 further includes an anode exhaust path 22 containing the anode exhaust gas from the anode chambers 12 via an anode outlet of the fuel cell stack 10 dissipates. The anode operating pressure on the anode sides 12 of the fuel cell stack 10 is about an actuating means 24 in the anode supply path 21 adjustable.

In addition, the anode supply points 20 a fuel recirculation line 25 on which the anode exhaust path 22 with the anode supply path 21 combines. The recirculation of fuel is common to the fuel usually used more than stoichiometrically fuel cell stack 10 to be returned and used. In the fuel recirculation line 25 is a recirculation conveyor 26 arranged to get out of the fuel cell stack 10 discharged fuel back into the anode supply path 21 to lead.

The cathode supply 30 further includes a cathode supply path 31 for supplying an oxygen-containing cathode operating medium into the cathode compartments 13 of the fuel cell stack 10 , These are, for example, sucked from the environment air. The cathode supply 30 further includes a cathode exhaust path 32 , which the cathode exhaust gas (in particular the exhaust air) from the cathode compartments 13 of the fuel cell stack 10 dissipates and this optionally an exhaust system supplies.

For promoting and compressing the cathode operating medium is a compressor 33 in the cathode supply path 31 arranged. This is for example as a mainly electric motor driven compressor 33 designed, the drive via a with a corresponding power electronics 35 equipped electric motor 34 he follows. The compressor 33 may also be through a in the cathode exhaust path 32 arranged turbine 36 supportively driven by a common shaft (not shown).

The cathode supply 30 also has a wastegate line 37 on which the cathode supply path 31 with the cathode exhaust path 32 connects, so a bypass of the fuel cell stack 10 represents. The wastegate pipe 37 allows excess air mass flow at the fuel cell stack 10 to pass without the compressor 33 shut down. One in the wastegate pipe 37 arranged adjusting means 38 serves to control the amount of the fuel cell stack 10 immediate operating medium.

The fuel cell system 100 also has a humidifier 40 on. The humidifier 40 is so in the cathode supply path 31 arranged to be flowed through by the cathode operating gas. On the other hand, it is so in the cathode exhaust path 32 arranged so that it can be flowed through by the cathode exhaust gas. The humidifier 40 typically has a plurality of water vapor permeable membranes formed either flat or in the form of hollow fibers. In this case, one side of the membranes is overflowed by the comparatively dry cathode operating gas (air) and the other side by the comparatively moist cathode exhaust gas (exhaust gas). Driven by the higher partial pressure of water vapor in the cathode exhaust gas, there is a transfer of water vapor across the membrane into the cathode working gas, which is moistened in this way.

The fuel cell system 100 also has a bypass line 41 on which the cathode supply path 31 upstream and downstream of the humidifier 40 connects with each other. In the bypass line 41 is a bypass valve 42 arranged. About the opening degree of the bypass valve 42 is adjustable how much of the cathode operating medium through the humidifier 40 while the remaining cathode operating medium humidifies and humidifies it 40 bypasses. Thus, the humidification of the cathode operating medium by means of the bypass valve 42 controllable.

The fuel cell system 100 is adapted to carry out a method according to the invention by placing it next to the wastegate line 37 , the wastegate valve 38 , the compressor 33 and the humidifier 40 Furthermore, a first shut-off 61 and a second shut-off means 62 having. In addition, the illustrated fuel cell system 100 a first air mass meter 51 and a second air mass meter 52 on. The first air mass meter 51 is immediately downstream of the compressor 33 in the cathode supply path 31 arranged. The second air mass meter 52 is upstream of the wastegate valve 38 in the wastegate line 37 arranged. The first barrier 61 is between humidifiers 40 and fuel cell stacks 10 in the cathode supply path 31 arranged and the second shut-off 62 is between humidifiers 40 and fuel cell stacks 10 in the cathode exhaust path 32 arranged.

The shut-off means 61 . 62 serve for a separation of the disconnected fuel cell stack 10 from the surroundings. Thus, in particular, the penetration of air in the first place in the cathode chambers 13 and from there to the anode rooms 12 of the shutdown stack 10 be prevented. In addition, the shut-off allow 61 . 62 together with the bypass actuator 38 setting a defined pressure gradient across the humidifier 40 by means of the compressor 33 , The air mass meter 51 . 52 allow the detection of all due to this pressure drop in the cathode supply 30 in particular in the cathode supply path 31 and the wastegate line 37 , generated fluid flows.

The in 2 shown humidifier has a first flow channel 43 with a first fluid supply line 44 and a first fluid discharge 45 in the cathode supply path (= first flow path) 31 of in 1 shown fuel cell system 100 is arranged. The humidifier also has a water-permeable membrane 46 on that the first flow channel 43 from a second flow channel 47 separates, a second fluid supply line 48 and a second fluid drain 49 and in the cathode exhaust path (= second flow path) 32 of in 1 shown fuel cell system 100 is arranged.

The course of the method according to the invention is described below with reference to FIGS 1 . 2 and 3 briefly explained.

3 illustrates the inventive method in a preferred embodiment as a decision chart. In this embodiment, the method is implemented in the shutdown of the fuel cell system. It is checked according to method step I, whether a frost preconditioning is required. This can routinely be the case in the shutdown process or, for example, only after the expiration of a predefined number of operating hours. In method step II, the measurement of the current leakage current in the humidifier takes place 40 , The measured value is in step III by a correlation with reference data a liquid water content φ in the humidifier 40 assigned. The measured actual liquid water content φ that is the content of liquid water in the humidifier is then compared with a permissible content of liquid water φ _Max in the humidifier, which corresponds to method step IV. It is decided (V) whether the actual liquid water content φ is higher than the maximum permissible liquid water content φ _Max . If this is the case, a drying procedure is carried out, for example by blowing out the moisture in method step VI. If the liquid water content φ is smaller than the defined maximum liquid water content φ _Max , the placement process is ended and the fuel cell system deactivated (VII). Alternatively or additionally, the method steps II-VI are not carried out in the shutdown process but in a predefined interval, ie after a certain operating time of the humidifier and / or the fuel cell system and / or depending on other conditioning processes, such as shortly before an expected freezing process.

The leakage current is measured, for example, as a differential pressure via the humidification flow paths of the humidified and / or the moist gas stream or by the reference to 1 and 2 method described below.

During operation of the fuel cell system 100 will the in 2 shown humidifier to be humidified cathode operating medium with the mass flow ṁ ₁ flows through, the upstream of the humidifier 40 a pressure p ' ₁ , a temperature T ' ₁ and a moisture content φ ' ₁ has and downstream of the humidifier pressure p " ₁ , a temperature T " ₁ and a moisture content φ " ₁ having. In addition, the humidifier 40 in countercurrent of wet cathode resource with the mass flow ṁ ₂ flows through, the upstream of the humidifier 40 a pressure p ' ₂ , a temperature T ' ₂ and a moisture content φ ' ₂ has and downstream of the humidifier pressure p " ₂ , a temperature T " ₂ and a moisture content φ " ₂ having. In the humidifier there is a transition from water to a mass flow ṁ _̇H2O from the wet to the to be humidified resources.

In addition, depending on the operating time or the aging state of the humidifier flows 40 and in response to an overpressure in the first flow channel 43 to the second flow channel 47 dry cathode _{operating medium} with the leakage _current ṁ _LEAK via the water-permeable membrane into the second flow _channel 47 , The method is used to determine ṁ _LEAK for at least one specific overpressure between the first and second flow channels 43 . 47 ,

In a first step of the preferred method for determining the leakage flow, the first shut-off means 61 and the second shut-off means 62 closed. The shut-off means 61 . 62 are preferably designed as controllable flaps. Subsequently, by means of the compressor 33 a first cathode resource stream in the cathode supply path 31 set. In order to obtain accurate knowledge of the first cathode resource stream actually flowing in the cathode supply path, this is detected by means of the first air mass meter 51 detected. Because of the compressor 33 promoted first cathode resource flow, a certain overpressure from the Kathodenbetriebsmittelpfad 31 to the cathode exhaust path 32 and thus from the first flow channel 43 to the second flow channel 47 of the humidifier 40 one.

Due to the particular overpressure, a second cathode resource stream flows through the wastegate conduit 37 and the opened wastegate valve 38 from the cathode supply path 31 to the cathode exhaust path 32 , This second cathode resource flow is by means of the second air mass meter 52 detected. In addition, a leakage _current ṁ _LEAK flows into the humidifier as described above 40 , Because the shut-off 61 . 62 are closed and the Wastegateleitung 37 and the humidifier 40 represent the only fluid carrying connections between the cathode supply path and the cathode _{exhaust path} , the leakage _current ṁ _{LEAK corresponds to} the difference between the first and second cathode _{resource streams} and may be determined as these. The particular leakage current can be used to provide a current transfer efficiency of the humidifier 40 using a reference aging function. Alternatively or additionally, the leakage _flow ṁ _LEAK can be used to adapt the mass transport model published in [1].

The liquid water content in the humidifier influences the flow behavior of the gases passing through. A high proportion of liquid water forms a two-phase flow with increased pressure losses due to, for example, increased flow resistance compared to a "dry" humidifier flow field. This relationship also affects the leakage mass flow of the humidifier, which is reduced as the amount of liquid in the humidifier increases. A measurement of the leakage mass flow from high to low pressure side thus allows an indirect measurement of the liquid water content in the humidifier flow field.

Component tests can provide the data basis, which exact amount of liquid water leads to damage of the component during the freezing process. Based on these measurements, the necessary correlation to the leakage mass flow of the humidifier (III) can be established and compared to the currently measured leakage mass flow during operation of the fuel cell system on the control system (IV). If this balancing results in a potential excessively high liquid water content within the humidifier, the discharge of liquid water from the component is continued or extended within the intended manner of the operating strategy (VI). As a result, the application of the invention within the vehicle operation is apparent, for example, within the shutdown process or possibly in a special conditioning of the fuel cell system shortly before the actual freezing process (for example, by an autonomous state in the parked vehicle). If the liquid water content φ, measured by the method presented, does not exceed the critical value φ _max , no additional conditioning must take place, or the latter can be terminated. This results in comparison with a merely time-dependent application of this freezing preconditioning the advantage of a reduced time requirement and minimizing the parasitic power.

This is particularly clear from the fact that the liquid water content of the humidifier is always different when parking the vehicle and is influenced by the previously worn load profile, the environmental conditions, inclinations and accelerations in the vehicle and also by aging effects, such as deposits in hoses and components. Against this and all other possible influencing factors on the liquid water content within the humidifier, the presented approach is robust, since a measurement is carried out in each case present vehicle state and the comparison to the freezing capability of the humidifier based on it. This also guarantees the component protection of the humidifier.

LIST OF REFERENCE NUMBERS

100

The fuel cell system

10

fuel cell stack

11

single cell

12

anode chamber

13

cathode space

14

Membrane electrode assembly (MEA)

15

Bipolar plate (separator plate, flow field plate)

20

anode supply

21

Anode supply path

22

Anode exhaust gas path

23

fuel tank

24

actuating means

25

Brennstoffrezirkulationsleitung

26

recirculation conveyor

30

cathode supply

31

Cathode supply path (first flow path)

32

Cathode exhaust path (second flow path)

33

compressor

34

electric motor

35

power electronics

36

turbine

37

Waste gate line

38

Wastegate valve

40

humidifier

41

bypass line

42

bypass valve

43

first flow channel

44

first fluid supply line

45

first fluid discharge

46

water-permeable membrane

47

second flow channel

48

second fluid supply line

49

second fluid discharge

51

first air mass meter

52

second air mass meter

61

first shut-off device

62

second shut-off device

Claims (9)

A method of operating a humidification system for a fuel cell system (100), the humidification system including a humidifier (40) configured to transfer moisture from a fluid flow passing through a second flow path (32) to a fluid flow passing through a first flow path (31) wherein a liquid water content in the humidifier (40) is determined by the measurement of a leakage flow and the leakage flow is a fluid flow passing through the first flow path (31) to a fluid flow through the second flow path (32). Method according to Claim 1 wherein the humidification system additionally comprises pressure sensors located upstream and downstream of the first and / or second flow paths (31, 32), characterized in that the leakage flow is determined by a difference between pressures measured upstream and downstream of the humidifier (40) becomes. Method according to Claim 1 characterized in that the leakage flow is determined by a method comprising the steps of: connecting the first flow path (31) and the second flow path (32) via the humidifier (40) and at least one third flow path (37); Adjusting a first fluid flow and pressure to the second flow path (32) in the first flow path (31); Detecting or determining the first fluid flow in the first flow path (31); Detecting or determining at least one second fluid flow through the at least one third flow path (37) and determining the leakage flow as the difference between the first fluid flow and the at least one second fluid flow. Method according to one of the preceding claims, characterized in that the moistening system further comprises a control system which is adapted to receive measurement signals from the moistening system and / or a fuel cell system (100) connected thereto and to trigger the implementation of the method as a function thereof. Method according to one of the preceding claims, characterized in that the method is carried out after a defined number of operating hours. Method according to one of the preceding claims, characterized in that the method is carried out as part of a shutdown of the fuel cell system. Method according to one of the preceding claims, characterized in that the method, in particular within a defined time interval, is carried out before a freezing process of the fuel cell system. Method according to one of the preceding claims, characterized in that the method is carried out when a defined maximum content of water in the humidifier (40) is exceeded. A motor vehicle comprising a humidification system configured to carry out a method according to any one of the preceding claims.

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