DE102016203466B4 - Cooling system for a fuel cell stack with sensing of a coolant level in an expansion tank by means of an electrical conductivity value - Google Patents

Abstract

Cooling system for a fuel cell stack (10), having: a coolant circuit (40) for conducting a coolant, a coolant delivery device (41), a coolant line (42) transporting the coolant, and an expansion tank (43), characterized in that a conductivity sensor (44) in the expansion tank (43) is arranged and a measuring tip (49) of the conductivity sensor (44) is arranged at the height of a minimum coolant level in the expansion tank (43).

Description

The invention relates to a cooling system for a fuel cell stack, a method for operating such a cooling system, a fuel cell system with such a fuel cell stack, a vehicle with such a fuel cell system and the use of a conductivity sensor arranged in a cooling system of a fuel cell stack to determine the electrical conductivity and the filling level of a coolant.

Fuel cells use the chemical reaction of a fuel with oxygen to form water to generate electrical energy. For this purpose, fuel cells have as a core component a so-called membrane electrode assembly (MEA—membrane electrode assembly) made of a proton-conducting membrane and electrodes arranged on both sides of it. During operation of the fuel cell, hydrogen H ₂ or a hydrogen-containing gas mixture is fed to the anode as fuel and electrochemically oxidized there with the release of electrons (H ₂ →72 H ⁺ + 2 e ^- ). The protons H ⁺ are transported from the anode compartment to the cathode compartment via the membrane, which insulates two reaction compartments gas-tight and electrically from one another. The electrons e ^- provided at the anode can be used to perform electrical work, after which they are conducted to the cathode. Oxygen or an oxygen-containing gas mixture is also supplied to this, so that the oxygen is reduced there, taking up the electrons (½ O ₂ + 2 e ^- → O ^2- ). The oxygen anions formed react in the cathode area with the protons transported across the membrane to form water (2 H ⁺ + O ² → H ₂ O).

In order to ensure optimum efficiency and a long service life for a fuel cell stack, it must be operated within a narrow temperature range. In the case of the fuel cells with proton-conducting membranes (PEM - fuel cells) preferably used for mobile applications, this temperature range is between 60°C and 90°C, for example. This results in the need to temper a fuel cell stack, which includes heating the stack before it is put into operation and cooling the stack while it is in operation.
As a rule, the fuel cell is formed by a large number of membrane-electrode assemblies arranged in a stack, the electrical power of which is added up. In this case, a bipolar plate is arranged between two membrane-electrode assemblies, which is used to supply the process gases to the anode and cathode of adjacent membrane-electrode assemblies. Furthermore, the bipolar plates are designed as electrical and thermal conductors. They are thus used for the process gas supply to the membrane electrode assemblies, their cooling and their electrical connection.

For the process gas supply to the electrodes, a bipolar plate has operating medium flow fields in its active area, in particular an anode-side open anode gas flow field and a cathode-side open cathode gas flow field. The anode gas and cathode gas flow fields are mostly in the form of trough-like channels. In addition, the active region has a closed coolant flow field, which is typically arranged between the cathode gas flow field and the anode gas flow field. A bipolar plate is often constructed from two partial plates which are connected to one another, in particular welded, with the coolant flow field being enclosed between two main surfaces of the partial plates which are partially connected to one another.

The fuel cell stack is supplied with its resources, ie the anode operating gas (e.g. hydrogen), the cathode operating gas (e.g. air) and the coolant, via main supply channels. These are formed by openings arranged in the bipolar plates and the membrane-electrode assemblies and aligned with one another in the fuel cell stack. Openings lying one above the other form the main supply channels penetrating the stack in its entire stacking direction. There are at least two such main supply channels for each piece of equipment, namely one for supplying and one for removing the respective equipment.

Exothermic fuel cell reactions can generate large amounts of heat in a fuel cell stack. In order to dissipate these to the environment, aqueous coolant systems with sufficient heat capacity are generally used. The use of mixtures of water, ethylene glycol as antifreeze and non-ionic corrosion inhibitors for cooling is known from internal combustion engines. In order to ensure that the heat is dissipated from the fuel cell stack, a sufficient quantity of cooling water must always be available. Numerous methods are known from the prior art for monitoring the filling level required for this purpose.

Since fuel cell stacks used to generate electricity are inevitably exposed to high electrical voltages, the coolants used in them must meet additional requirements. In particular, a specific electrical conductivity of the coolant must not be exceeded, to avoid short circuits in the fuel cell system. In the case of fuel cell systems arranged in vehicles, there is also the risk of electrical voltages being transmitted to the bodywork via a coolant which is too electrically conductive and endangering the vehicle occupants. It is therefore problematic that ions are introduced into the coolant during operation of the fuel cell stack. The ions can be released from a contaminant, for example a flux of a soldered cooler, or directly from a stack component, for example an EPDM seal.

It is therefore necessary to monitor the electrical conductivity of a coolant used in a fuel cell system in order to rule out any danger to the fuel cell stack and the occupants. In addition, the coolant level must be monitored.

The JP 2006-114.413A describes a method for monitoring the quality of the water used in a fuel cell system for cooling. The electrical conductivity of the cooling water is constantly monitored and if the conductivity is too high, the cooling water is diluted with low-conductivity fresh water. In mobile applications in particular, however, there is generally no fresh water supply that is carried along.

The JP 2005-310.615A describes a fuel cell system in which the fill level of deionized water is determined by means of two electrodes immersed in the water. As the filling level increases, the electrode surface contributing to the current flow between the electrodes increases and a lower resistance is measured. A change in conductivity due to a change in filling level is difficult to distinguish from a change in conductivity due to a variation in the ion contamination.

The object of the invention is now to overcome the disadvantages of the prior art and to provide a cooling system for a fuel cell stack in which the fill level and the electrical conductivity of a coolant can be reliably monitored with the least amount of equipment.

The object is achieved by a cooling system for a fuel cell stack according to claim 1, a method for operating such a cooling system according to claim 6, a fuel cell system with such a fuel cell stack according to claim 8, a vehicle with such a fuel cell system according to claim 9 and the use of a conductivity sensor for Determination of falling below a minimum coolant fill level and the electrical conductivity of the cooling water according to Claim 10.

A cooling system according to the invention for a fuel cell stack has a coolant circuit for conducting a coolant, a coolant delivery device, a coolant line transporting the coolant and an expansion tank for the coolant. In particular, the coolant delivery device, the coolant line and the expansion tank are components of the coolant circuit. A conductivity sensor is arranged in the expansion tank, with a measuring tip of the conductivity sensor being arranged at the height of a minimum coolant level in the expansion tank. The expansion tank is a coolant reservoir which is connected to the coolant line in a fluid-carrying manner and is used to compensate for the amount of coolant in the coolant lines in the event of coolant losses, in particular due to leaks or evaporation, and in the event of thermal expansion. The coolant level in the expansion tank should not exceed a specific maximum fill level and should not fall below a specific minimum fill level in order to ensure the functionality of the fuel cell stack.

The conductivity sensor is a sensor for determining an electrical conductivity σ, the electrical conductivity being determined at least in the region of a measuring tip. Electrical conductivity can be measured by measuring an electrical current that flows when a specific voltage is applied to the coolant. For this purpose, the conductivity sensor preferably has two electrodes, at least in the area of the measuring tip. A specific voltage can be applied between the electrodes by means of an electronic circuit arranged in the sensor or connected to the sensor. Furthermore, the electronic circuit is preferably designed to measure a current flowing between the electrodes or to determine this, for example using a voltage drop across a shunt. The electronic circuit is preferably designed to determine the electrical resistance between the electrodes on the basis of the determined voltage and current values. A specific electrical resistance or the electrical conductivity of the medium arranged between the electrodes can be determined from this, taking into account preset values for the electrode spacing and electrode area. If the conductivity sensor is connected to a control unit, the functionalities of the electronic circuit described can alternatively also be implemented in the control unit.

As long as the measuring tip of the conductivity sensor is in contact with the coolant, it measures an electrical conductivity in the order of magnitude of the electrical conductivity of the respective coolant. Changes in the measured electrical conductivity of the coolant correlate almost exclusively with the ion concentration of the coolant. If the coolant level falls below a minimum coolant level, the measuring tip is no longer in contact with coolant. Then the electrical conductivity of air, in particular humid air, is measured and the electrical conductivity measured by the measuring tip drops abruptly, preferably by several orders of magnitude. On the basis of the abrupt drop in the conductivity determined by means of the measuring tip, it is thus recognized that the coolant level has fallen below a critical level.

With the cooling system according to the invention, the electrical conductivity and the falling below a minimum fill level of a coolant used in a fuel cell stack can advantageously be determined using only one conductivity sensor and only one electronic evaluation system. These two pieces of information are essential to ensure long-term operation of the fuel cell stack, in particular to prevent overheating of the stack or short circuits in the stack. A sensor for monitoring the fill level and a sensor for determining the electrical conductivity of the coolant are therefore arranged in common cooling systems for fuel cell stacks. However, continuous monitoring of a current coolant fill level is generally unnecessary as long as it can be reliably determined that the level has fallen below a certain limit. With the cooling system according to the invention, the outlay on equipment and the space requirement are thus reduced in that the conductivity of a coolant and the presence of a sufficient coolant fill level are monitored using only one sensor and only one electronic circuit.

The measuring tip is preferably dimensioned and/or the conductivity sensor is arranged in the expansion tank in such a way that partial covering of the measuring tip with coolant only lasts for a short time when the coolant level falls and/or is largely ruled out. Thus, a correlation of the electrode surface of the probe tip that is in contact with coolant and the electrical conductivity measured by it, as described in the appreciation of the prior art, is avoided. The specific electrical conductivity of the coolant is thus almost independent of its fill level. The conductivity sensor particularly preferably protrudes through the underside of the expansion tank and in its rest position vertically upwards, with the measuring tip being arranged at its upper end. This arrangement has proven particularly advantageous in mobile applications, since there is contact between the measuring tip and the coolant even when cornering.

The coolant delivery device is preferably adapted to the type of coolant, and it is particularly preferably a circulating pump. The coolant line is preferably designed as a pipe or hose and is adapted to the type of coolant used. The line means are designed in such a way that they can guide the coolant without being damaged by it and without coolant escaping from the line means in an uncontrolled manner.

A coolant is preferably used that has an electrical conductivity of less than 50 μS/cm, particularly preferably less than 10 μS/cm and likewise preferably less than 5 μS/cm. The conductivity refers to the pure coolant without ion contamination. In this case, a conductivity sensor is arranged in the expansion tank, which is designed to measure an electrical conductivity of less than 50 μS/cm, particularly preferably less than 10 μS/cm and also preferably less than 5 μS/cm. With such a coolant, the risk of electrical short circuits between the individual cells of the fuel cell stack is minimal. At the same time, a significant difference is ensured between the electrical conductivity of the coolant and the electrical conductivity of air, which is between 3E-11 µS/cm and 8E-11 µS/cm depending on its relative humidity. Thus, the abrupt drop in the conductivity determined with the measuring tip described above can be used as a level indicator. A coolant composed of water, preferably deionized water, an antifreeze, preferably glycol, and corrosion inhibitors, preferably nonionic corrosion inhibitors, is particularly preferably used. The coolant may contain other additives.

In a preferred embodiment, the cooling system also has a control unit connected to the conductivity sensor. In addition, the control unit can also be connected to further elements of the cooling system or a fuel cell system in which the cooling system is arranged. If the fuel cell system is arranged in a vehicle, the control unit can be a control unit of the vehicle. The control unit is set up to generate a first output signal when the electrical conductivity σ determined by the conductivity sensor exceeds a first limit value σ ^th ₁ . The control unit is also set up to send a second output signal generate when the electrical conductivity σ determined by the conductivity sensor falls below a second limit value σ ^th ₂ . The control unit receives an electrical conductivity σ determined by the conductivity sensor or a current I measured by the conductivity sensor as a function of a specific voltage U as an input signal. The control unit preferably compares the input signal received with a predetermined first limit value σ ^t 1 and with a predetermined second limit value σ ^th 2 , for example by means of a comparator circuit. Depending on the result of these comparisons, the control unit generates a first output signal or a second output signal. The first and/or second output signal can be binary signals, for example in the form of an output voltage which assumes the values GND or _VPP . The control unit also preferably has communication means in order to transmit the output signals to other elements of the cooling system, a fuel cell system or a vehicle system. In a particularly simple embodiment, the control unit is part of the conductivity sensor.

In a likewise preferred embodiment, the cooling system also has an ion separator arranged in the cooling system, in particular the coolant circuit. The ion separator has a deionizing agent through which the coolant can flow or at least overflow. The deionizing agent can be, for example, an ion exchange resin which can absorb the ions (cations and anions) dissolved in the coolant and release H ⁺ and OH ^- ions. In this case, the ion separator is designed as an ion exchanger. The deionizing agent can be in bulk directly in the ion separator or in the form of a filter element that can be inserted into the ion separator. The capacity of common deionizing agents is limited, so they have to be replaced at regular intervals. For this reason, the placement of the deionizing agent in a replaceable filter element is particularly preferred. In addition, a filter element ensures safe separation of the deionizing agent and the coolant circuit. Alternatively, however, the entire ion separator can also be removed from the cooling system at regular intervals, emptied and filled with new deionizing agent. The ion separator preferably has means for determining an operating state of the ion separator as a function of the remaining capacity of the deionizing agent. The capacity of the deionizing agent can preferably be determined via an impedance measurement on the deionizing agent. The ion separator preferably has a housing through which the coolant can flow and in which the deionizing agent is arranged. The coolant flows through the ion separator, in particular its housing, continuously or controllably discontinuously. A continuous flow through the ion separator preferably takes place automatically depending on the pressure loss of the ion separator. In order to control a discontinuous flow, the ion separator preferably has at least one controllable adjusting means. This control means and the means for determining an operating state of the ion separator are preferably connected to the control unit of the cooling system.

The invention also relates to a method for operating a cooling system, as described above, the method having the following method steps: detecting a value of an electrical conductivity σ using the conductivity sensor, generating a first output signal if the electrical conductivity σ determined by the conductivity sensor has a exceeds the first limit value σ ^th ₁ and generates a second output signal if the electrical conductivity σ determined by the conductivity sensor falls below a second limit value σ ^th ₂ . The method according to the invention advantageously enables the determination of two essential operating states of a cooling system for a fuel cell stack, namely exceeding a maximum conductivity o ^th ₁ of the coolant and falling below a minimum coolant level (σ ^th ₂ ) of the coolant in an expansion tank of the coolant system. The outlay in terms of apparatus and circuitry for carrying out the method is advantageously minimal, since only one measured conductivity value has to be compared with two limit values. The method preferably includes comparing the electrical conductivity σ determined by the conductivity sensor with the first limit value σ ^t 1 and with the second limit value o ^th ₂ . The method is also preferably carried out by means of the control unit and also has the method step: reading the value of the electrical conductivity σ detected by means of the conductivity sensor into the control unit.

In a preferred embodiment of the method according to the invention, a cooling system having an ion separator and preferably a control unit, as described above, is operated. The method also includes determining an operating state of the ion separator in response to the first output signal. If it is determined that the ion separator is inactive, ie coolant has not yet flowed through it, the ion separator is activated, preferably by the control unit. For this purpose, an adjusting agent is preferably adjusted in such a way that the ion separator and the deionization arranged therein are medium flows through by the coolant. For this purpose, the adjusting means is preferably connected to the control unit. If, on the other hand, it is determined that the ion separator is already active but coolant has only partially flowed through it up to now and the deionizing agent arranged in the ion separator still has capacity to absorb further ions, the quantity of coolant flowing through the ion separator is increased. This is preferably also done by setting an adjusting means in the coolant circuit, particularly preferably also by the control unit. Thus, a first response to the operating condition of the ion separator can be summarized as increasing the amount of coolant flowing through the ion separator. If, on the other hand, it is determined that the ion separator is already active but the maximum amount of coolant is already flowing through it or the deionizing agent has almost no capacity to take up further ions (or both), as a second reaction to the operating state of the ion separator and preferably by means of the control unit third output signal generated. This preferred embodiment of the method according to the invention enables the ion load of a coolant to be minimized in a simple manner on the basis of the detected conductivity value and a third output signal to be output if the ion load of the coolant can no longer be reduced.

The invention also relates to a fuel cell system, comprising a fuel cell stack with cathode spaces, anode spaces and coolant channels; a cathode supply comprising a cathode supply path with a compressor disposed therein and a cathode exhaust path; an anode supply comprising an anode supply path and an anode exhaust path, and a cooling system as described above.

The invention also relates to a vehicle having a fuel cell system or a cooling system, as described above, and a driver information system. The driver information system is configured to generate a first warning signal in response to the third output signal. The third output signal is generated when the ion separator has almost run out of capacity to accept additional ions. A driver of the vehicle is thus informed by the first warning signal that the maximum capacity of the ion separator, in particular of the deionizing agent, will soon be reached. The driver then still has enough time to arrange a workshop appointment to change the ion separator or the deionizing agent it contains, or to carry out the change himself. The driver information system is further configured to generate a second warning signal in response to the second output signal. The second warning signal informs the driver that the coolant level in an expansion tank has fallen below a minimum coolant level. The driver thus knows that new coolant must be refilled in order to avoid overheating of the fuel cell stack. The minimum coolant level is selected so that there is still enough time to fill up the expansion tank. The driver information system is preferably an instrument cluster in the cockpit of the vehicle that is visible to the driver, and the warning signals are preferably acoustic and/or visual warning signals. The driver information system is preferably connected to the control unit.

The invention also relates to the use of a conductivity sensor arranged in an expansion tank of a coolant system for a fuel cell stack to determine the electrical conductivity of a coolant present in the coolant system and to determine whether the coolant fill level in the expansion tank of the coolant system falls below a minimum.

Further preferred configurations of the invention result from the remaining features mentioned in the dependent claims.

Unless stated otherwise in the individual case, the various embodiments of the invention mentioned in this application can advantageously be combined with one another.

• 1 eine schematische Darstellung eines Brennstoffzellensystems mit einem erfindungsgemäßen Kühlsystem,
• 2 eine schematische Darstellung eines Ausgleichsbehälters eines erfindungsgemäßen Kühlmittelsystem, und
• 3 ein schematisches Ablaufdiagramm eines erfindungsgemäßen Verfahrens zum Betrieb eines erfindungsgemäßen Kühlsystems.

The invention is explained below in exemplary embodiments with reference to the associated drawings. Show it:

• 1 a schematic representation of a fuel cell system with a cooling system according to the invention,
• 2 a schematic representation of an expansion tank of a coolant system according to the invention, and
• 3 a schematic flowchart of a method according to the invention for operating a cooling system according to the invention.

1 12 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 that is not shown in any more detail, in particular an electric vehicle that has an electric traction motor that is supplied with electrical energy by the respective fuel cell system 100 .

The fuel cell system 100 comprises a fuel cell stack 10 as a core component, which has a multiplicity of individual cells 11 arranged in stack form, which are formed by alternately stacked membrane electrode assemblies (MEA) 14 and bipolar plates 15 (see detailed section). Each individual cell 11 thus comprises an MEA 14 with an ion-conductive polymer electrolyte membrane (not shown in detail here) and catalytic electrodes arranged on both sides of it. These electrodes catalyze the respective partial reaction of the fuel conversion. The anode and cathode electrodes are formed as a coating on the membrane and comprise a catalytic material, for example platinum, supported on an electrically conductive support material with a large specific surface area, for example a carbon-based material.

As in the detail of the 1 As shown, an anode compartment 12 is formed between a bipolar plate 15 and the anode, and the cathode compartment 13 is formed between the cathode and the next bipolar plate 15 . The bipolar plates 15 serve to feed the operating resources into the anode and cathode chambers 12, 13 and also establish the electrical connection between the individual fuel cells 11. Gas diffusion layers can optionally be arranged between the membrane electrode assemblies 14 and the bipolar plates 15 .

In order to supply the fuel cell stack 10 with the resources, the fuel cell systems 100 have an anode supply 20 on the one hand and a cathode supply 30 on the other.

The anode supply 20 of the in 1 The fuel cell system 100 shown includes an anode supply path 21 which serves to supply an anode resource (the fuel), for example hydrogen, into the anode compartments 12 of the fuel cell stack 10 . For this purpose, the anode supply paths 21 connect a fuel reservoir 23 to an anode inlet of the fuel cell stack 10. The anode supply 20 also includes an anode exhaust gas path 22, which discharges the anode exhaust gas from the anode chambers 12 via an anode outlet of the fuel cell stack 10. The anode operating pressure on the anode sides 12 of the fuel cell stack 10 can be adjusted via a first adjustment means 24 in the anode supply path 21 .

In addition, the anode supply 20 of the 1 fuel cell system shown on a recirculation line 25 which connects the anode exhaust gas path 22 to the anode supply path 21. The recirculation of fuel is customary in order to return the fuel, which is mostly used superstoichiometrically, to the fuel cell stack 10 . A recirculation conveying device 27, preferably a recirculation fan, is arranged in the recirculation line 25. A second adjusting means 26 is arranged downstream of the recirculation line 25 in the anode exhaust gas path in order to adjust the proportion of the recirculated anode exhaust gas. Furthermore, a water separator 28 is installed in the anode off-gas path 22 in order to condense and drain liquid water discharged from the fuel cell stack 10 .

The cathode supply 30 of the in 1 The fuel cell system 100 shown includes a cathode supply path 31, which supplies the cathode chambers 13 of the fuel cell stack 10 with an oxygen-containing cathode resource, in particular air that is sucked in from the environment. The cathode supply 30 also includes a cathode exhaust gas path 32 which discharges the cathode exhaust gas (in particular the exhaust air) from the cathode chambers 13 of the fuel cell stack 10 and optionally feeds this to an exhaust gas system (not shown). A compressor 33 is arranged in the cathode supply path 31 for conveying and compressing the cathode operating medium. In the exemplary embodiment shown, the compressor 33 is configured as a compressor 33 driven primarily by an electric motor, the drive of which takes place via an electric motor 34 equipped with corresponding power electronics 35 . The compressor 33 can also be driven in support of a common shaft (not shown) by a turbine 36 (possibly with variable turbine geometry) arranged in the cathode exhaust gas path 32 .

This in 1 The fuel cell system 100 shown also has a humidifier module 39 . The humidifier module 39 is arranged on the one hand in the cathode supply path 31 in such a way that the cathode operating gas can flow through it. On the other hand, it is arranged in the cathode exhaust gas path 32 in such a way that the cathode exhaust gas can flow through it. A humidifier 39 typically has a plurality of water vapor-permeable membranes, which are either flat or in the form of hollow fibers. The comparatively dry cathode operating gas (air) flows over one side of the membranes and the comparatively moist cathode exhaust gas (exhaust gas) flows over the other side. Driven by the higher partial pressure of water vapor in the cathode exhaust gas, water vapor passes over the membrane into the cathode operating gas, which is humidified in this way. The cathode supply 30 also has a bypass line 37 which the cathode supply line 31 is connected to the cathode supply line 31 in such a way that the humidifier module 39 upstream of the fuel cell stack 10 is not flowed through. A control means 38 arranged in the bypass line 37 is used to control the quantity of the cathode operating medium bypassing the humidifier 39 .

For cooling the fuel cell stack 10, the 1 Fuel cell system 100 shown also has a coolant circuit 40 . This is formed outside of the fuel cell stack 10 by a line 42 carrying a coolant, which is connected to coolant inlet openings and coolant outlet openings of the respective fuel cell stack 10 . In the fuel cell stack, coolant channels are arranged in the bipolar plates 15 between the coolant inlet openings and coolant outlet openings. A coolant delivery device 41 is arranged in the coolant circuit 40 for delivering the coolant through the coolant line 42 and the coolant channels of the fuel cell stack 10 . In addition, a compensating tank 43 is arranged in the coolant system 40 and is connected to a coolant line 42 in a fluid-carrying manner. A conductivity sensor 44 for measuring an electrical conductivity of the coolant by means of an in 1 arranged measuring tip not shown.

The cooling system also has an ion separator 45 which is filled with a deionizing agent (not shown) and can be connected to the coolant lines 42 of the cooling circuit 40 via a controllable third adjusting means 46 . The third adjusting means 46 is designed as a controllable three-way valve. If the coolant flows through the ion separator 45, the deionizing agent contained therein absorbs ions (cations and anions) dissolved in the coolant and releases H ⁺ and OH ^- ions in return, which react in the coolant to form water.

In the 2 a detailed view of the expansion tank 43 is shown. The expansion tank has a cap 50 on its upper side, which can be opened in order to fill in new coolant. A conductivity sensor 44 pierces a side wall of the surge tank 43 and is disposed in the surge tank 43 inclined downward so that a probe tip 49 of the conductivity sensor 44 is located at a minimum coolant level in the surge tank 43 . The minimum coolant level does not correspond to a minimum level marking (min) on the expansion tank, which is only for the information of the user and is intended to ensure that the expansion tank is filled up in good time. The expansion tank 43 also has a maximum level mark (max) for the information of the user.

The in the 1 and 2 The conductivity sensor 44 shown determines an electrical conductivity by measuring the current I flowing between the electrodes of the measuring tip 49 as a function of the voltage U applied between these electrodes. The conductivity sensor 44 can, as in 3 shown, determine the electrical conductivity σ of the coolant. Alternatively, the conductivity sensor 44, as in 1 shown, which output the values of voltage U and current I to a control unit 47 (ECU) also arranged in fuel cell system 100, electrical conductivity σ of the coolant being determined in control unit 47.

As in 3 shown, the electrical conductivity σ of the coolant is compared in the control unit 47 with a first limit value σ ^t 1 and a second limit value σ ^th 2 . If this comparison shows that the first limit value σ ^t 1 is not exceeded and the second limit value σ ^th 2 is not exceeded, the conductivity sensor 44 continues to determine the electrical conductivity σ of the coolant, as in FIG 3 indicated by arrows. As in the 1 and 3 shown, the control unit 47 also receives information from the ion separator 45. This is information about the operating state of the ion separator 45, in particular the position/the degree of opening of the third adjusting means 46, or about the capacity of the deionizing agent contained in the ion separator 45 to absorb ions.

If the comparison in the control unit 47 shows that the conductivity value σ determined by the conductivity sensor 44 exceeds the first limit value σ ^t ₁ , the control unit 47 outputs a first output signal and, as a reaction thereto, checks the data received from the ion separator 45 in a next step . Based on this data, the control unit 47 determines the appropriate reaction to the excessive ionic contamination of the coolant. If the control unit 47 determines that the ion separator has been inactive up to now, i.e. the third control means 46 is closed, the control unit 47 opens the third adjusting means 46 to the ion separator 45. If the control unit 47 determines that the adjusting means 46 has so far only been partially opened, the control unit 47 increases its degree of opening and thus the quantity of coolant flowing through the ion separator 45 . If control unit 47 determines that the amount of coolant flowing through ion separator 45 is already at its maximum and/or the deionizing agent contained in the ion separator has almost no capacity to absorb further ions, control unit 47 generates a third output signal and transmits this to a driver information system 48 (BCM) of the vehicle, not shown. In response to the third output signal, the driver information system 48 generates and outputs a first warning signal to the driver.

If the comparison in control unit 47 shows that conductivity value σ determined by conductivity sensor 44 falls below second limit value σ ^th ₂ , control unit 47 generates a second output signal and transmits it to driver information system 48 (BCM). The driver information system 48 issues a second warning signal to the driver in response to the second output signal.

Reference List

100

fuel cell system

10

fuel cell stack

11

single cell

12

anode room

13

cathode room

14

Membrane Electrode Assembly (MEA)

15

Bipolar plate (separator plate, flow field plate)

20

anode supply

21

anode supply line

22

anode exhaust line

23

fuel tank

24

first means of adjustment

25

recirculation line

26

second adjusting agent

27

recirculation conveyor

28

water separator

30

cathode supply

31

cathode supply line

32

cathode exhaust line

33

compressor

34

electric motor

35

power electronics

36

turbine

37

bypass line

38

setting means

39

humidifier module

40

coolant circuit

41

coolant delivery device

42

coolant line

43

surge tank

44

conductivity sensor

45

ion separator

46

third adjusting agent

47

control unit

48

driver information system

49

measuring tip

50

sealing cap

Claims (10)

Cooling system for a fuel cell stack (10), having: a coolant circuit (40) for conducting a coolant, a coolant delivery device (41), a coolant line (42) transporting the coolant, and an expansion tank (43), characterized in that a conductivity sensor (44 ) is arranged in the expansion tank (43) and a measuring tip (49) of the conductivity sensor (44) is arranged at the height of a minimum coolant level in the expansion tank (43). cooling system after claim 1 , characterized in that the coolant has an electrical conductivity σ of less than 50 µS/cm and the conductivity sensor (44) is designed to measure an electrical conductivity σ of less than 50 µS/cm. cooling system after claim 1 or 2 , characterized in that the coolant contains water, an antifreeze and at least one corrosion inhibitor. Cooling system according to one of the preceding claims, further comprising: a control unit (47) connected to the conductivity sensor (44) and configured to generate a first output signal when the electrical conductivity σ determined by the conductivity sensor (44) exceeds a first limit value σ ^t ₁ , and to generate a second output signal when the electrical conductivity σ determined by the conductivity sensor (44) falls below a second limit value σ ^th ₂ . Cooling system according to one of the preceding claims, further comprising an ion separator (45) which is arranged in the coolant circuit (40) and has a deionizing agent through which the coolant can flow. Method for operating a cooling system according to one of Claims 1 until 5 , having the method steps: detecting a value of an electrical conductivity σ by means of the conductivity sensor (44); and one of the following: generating a first output signal if the electrical conductivity σ determined by the conductivity sensor (44) exceeds a first limit value σ ^t ₁ ; and generating a second output signal when the electrical conductivity σ determined by the conductivity sensor (44) falls below a second limit value σ ^th ₂ . procedure after claim 6 to operate a cooling system claim 5 , further comprising the steps of: determining an operational state of the ion separator (45) in response to the first output signal; and one of: increasing the amount of coolant flowing through the ion separator (45); and generating a third output signal if, when the first output signal is received, the amount of coolant flowing through the ion separator (45) is already at its maximum and/or the deionizing agent contained in the ion separator (45) has almost no capacity to take up further ions. Fuel cell system (100), comprising a fuel cell stack (10) with cathode spaces (12), anode spaces (13) and coolant channels; a cathode supply (30) comprising a cathode supply path (31) with a compressor (33) disposed therein and a cathode exhaust path (32); an anode supply (20) having an anode supply path (21) and an anode exhaust path (22); and a cooling system (40) according to any one of Claims 1 until 5 . Vehicle, having a cooling system according to one of Claims 1 until 5 or a fuel cell system (100). claim 8 and further comprising a driver information system (48) arranged to generate a first warning signal in response to the third output signal and to generate a second warning signal in response to the second output signal. Use of a conductivity sensor (44) arranged in an expansion tank (43) of a coolant system for a fuel cell stack (10) to determine the electrical conductivity of a coolant present in the coolant system and to determine whether the coolant level in the expansion tank (43) of the coolant system falls below a minimum level.

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