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WO2001052339A1 - Liquid-fuel-cell system - Google Patents

Liquid-fuel-cell system Download PDF

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Publication number
WO2001052339A1
WO2001052339A1 PCT/EP2000/011587 EP0011587W WO0152339A1 WO 2001052339 A1 WO2001052339 A1 WO 2001052339A1 EP 0011587 W EP0011587 W EP 0011587W WO 0152339 A1 WO0152339 A1 WO 0152339A1
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WO
WIPO (PCT)
Prior art keywords
fuel
temperature
concentration
circuit line
anode
Prior art date
Application number
PCT/EP2000/011587
Other languages
German (de)
French (fr)
Inventor
Jens Thomas MÜLLER
Original Assignee
Daimlerchrysler Ag
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Daimlerchrysler Ag filed Critical Daimlerchrysler Ag
Priority to EP00985075A priority Critical patent/EP1252676A1/en
Priority to JP2001552460A priority patent/JP2003520399A/en
Publication of WO2001052339A1 publication Critical patent/WO2001052339A1/en

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Classifications

    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04082—Arrangements for control of reactant parameters, e.g. pressure or concentration
    • H01M8/04186—Arrangements for control of reactant parameters, e.g. pressure or concentration of liquid-charged or electrolyte-charged reactants
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04007—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids related to heat exchange
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/10—Fuel cells with solid electrolytes
    • H01M8/1009—Fuel cells with solid electrolytes with one of the reactants being liquid, solid or liquid-charged
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/30—Hydrogen technology
    • Y02E60/50—Fuel cells

Definitions

  • the invention relates to a fuel cell system and a method for operating such a fuel cell system with the features according to the preamble of patent claims 1 and 6, respectively.
  • a generic fuel cell system is known from DE 198 07 876 AI. There, a liquid methanol / water mixture is circulated on the anode side. To ensure a constant concentration of methano, the anode circuit is metered in from a methanol reservoir. The dosing amount is determined with the help of a concentration sensor in the anode circuit. Liquids or ionic or nonionic additives to the water with good frost protection properties are proposed as cooling agents. In particular for a DMFC, such suitable coolants are currently and probably not available in the foreseeable future.
  • the physical background is as follows:
  • the DMFC is usually operated at temperatures around 100 ° C.
  • the methanol concentration is typically between 0.5 and 2 mol / 1 or 1.6 and 6.4 percent by weight.
  • the cause is the methanol permeability of available membranes. If methanol is used in higher concentrations, the excess methanol diffuses through the membrane to the cathode. The result is a drastically reduced efficiency.
  • the cryoscopic constant of the water is 1.86 K kg / mol, which means that the freezing point drops by only 1.86 ° C per oil / kg of additive. Since this is a colligative property, this value is independent of the type of additive.
  • the freezing point of the commonly used water / methanol mixtures is around -1
  • an additive with a concentration of over 16 mol / kg is required.
  • Such an additive is currently not available. In principle, it will not be detectable in the long term either, because even a relatively small molecule with an assumed molar mass of 50 g / mol was required in a concentration of 800 g / kg. However, a mixture of this composition is no longer sufficient to stoichiometrically supply the water with water. However, water and methanol in a stochiometric ratio of 1: 1 are required for the anode reaction.
  • frost protection By increasing the fuel concentration in the anode circuit line as the temperature drops, the freezing point of the fuel / coolant mixture is increased and thus frost protection is ensured, while at the same time the efficiency in normal operation of the system does not deteriorate. With this measure, frost protection down to -35 ° C is possible.
  • the cold start behavior is improved by faster heating of the fuel cell, because the fuel increasingly diffuses through the membrane to the cathode due to the increased concentration and is oxidized there catalytically immediately after the start of the air supply with heat emission. This speeds up the cold start process considerably.
  • the frost resistance can be warrants ⁇ that the fuel concentration m the AnodenJ-creis ein either durcn continuous adjustment of the concentration setpoint is increased to the decreasing temperature or raised abruptly by comparing the detected temperature with a predetermined temperature threshold in a simple manner.
  • the additional amount of fuel required can be reduced despite adequate frost protection, thus improving overall efficiency.
  • the system is not always switched to maximum frost protection when the temperature falls below a threshold value, but the frost protection is adapted to the actual temperature.
  • the fuel cell designated overall by 1, consists of an anode compartment 2 and a cathode compartment 3, which are separated from one another by a proton-conducting membrane 4.
  • a liquid fuel / coolant mixture is led through the anode compartment 2 via an anode circuit line 5, which connects an anode compartment outlet 6 with an anode compartment inlet 7 of the fuel cell 1.
  • No suitable substance that is liquid and electrochemically oxidizable at room temperature can be used as the fuel.
  • the system described in the exemplary embodiment is operated with liquid methanol as the fuel and water as the coolant. Although only the use of a methanol / water mixture is described in the following, the scope of protection of this application should not be restricted to this exemplary embodiment.
  • Such a system operated with liquid methanol / water mixture is commonly referred to as direct methanol fuel cell (DMFC).
  • An oxygen-containing gas is fed into the cathode compartment 3 via a cathode feed line 8.
  • ambient air is used for this.
  • the fuel cell 1 the fuel is oxidized at the anode and the atmospheric oxygen at the cathode is reduced.
  • the proton-conducting membrane 4 is coated on the corresponding surfaces with suitable catalysts, such as, for example, high-surface-area precious metal tubes or supported catalysts.
  • suitable catalysts such as, for example, high-surface-area precious metal tubes or supported catalysts.
  • Protons can now migrate through the proton-conducting membrane 4 from the anode side and combine with the oxygen ions to form water on the cathode side.
  • This electrochemical reaction creates a voltage between the two electrodes.
  • the product produced at the anode outlet is a carbon dioxide gas enriched with water and methanol.
  • This liquid / gas mixture is discharged from the anode compartment 2 via the anode circuit line 5.
  • the cathode exhaust air containing residual oxygen and water vapor is discharged via a cathode exhaust gas device 9.
  • the ambient air in the cathode compartment 3 can preferably be provided with overpressure.
  • the methanol / water mixture is circulated through the anode circuit line 5 at a predetermined pressure with the aid of a pump 10.
  • the ratio of water to methanol m of the anode circuit line 5 is adjusted with the aid of a sensor 11 which measures the methanol concentration m of the anode circuit line 5.
  • a concentration control for the methanol / water mixture the liquid methanol from one
  • Methanol reservoir 12 is fed via a feed line 13 and the anode circuit line 5 is injected with the aid of an injection nozzle 14 m (not shown in more detail).
  • the injection pressure is generated by an injection pump 15 arranged in the supply line 13.
  • the methanoidosing is carried out by a suitable control of the injection nozzle 14.
  • a control device 17 is provided, which is connected to the pump 10, the sensor 11, the injection pump 15, the injection nozzle 14 and possibly other components via dotted measurement or control lines.
  • a methanol / water mixture with a preferably constant methanol concentration is thus continuously supplied to the anode compartment 2.
  • a gas separator 16 is used to separate the carbon dioxide enriched with methanol and water vapor from the liquid / gas mixture m in the anode circuit line 5. Too much methanol discharge through the carbon dioxide gas is to be prevented, since otherwise the overall efficiency of the system is reduced and at the same time unburned methanol was released into the environment. Contrary to the gas separator shown in simplified form in the drawing, more complex devices are usually used for this purpose.
  • a device for determining a temperature T 1 ⁇ ⁇ is provided.
  • Conventional temperature sensors can be used for this. It is advantageous if the sensor 11 is designed as a combined concentration and temperature sensor. Additional components can thus be saved. However, it is of course, it is also possible to provide a separate temperature sensor.
  • the sensor 11 m of the anode circuit line 5 is arranged between the gas separator 16 and the pump 10. However, it is also possible to arrange the sensor 11 at a different location m in the anode circuit line 5 or also directly in the fuel cell 1. It is also possible to use a temperature sensor that measures the ambient temperature. However, the heat that was still present in the system after switching off could not be taken into account.
  • frost protection for the system is ensured by adapting the concentration K Me0H of the methanol / water mixture to the temperature T lst - m of the anode circuit line 5 or to the prevailing ambient temperature. If the temperature T drops , the concentration K Me oH is increased and thus the freezing point of the methanol / water mixture is lowered. This ensures frost protection.
  • the increased methanoconcentration K Me oH also leads to faster heating of the fuel cell 1, because the methanol diffuses more and more through the membrane 4 to the cathode 3 and is oxidized there catalytically immediately after the start of the air supply with heat emission. This speeds up the cold start process considerably.
  • the temperature monitoring and the associated concentration adjustment are preferably carried out only when the system is at a standstill because the temperatures are sufficiently high during operation of the fuel cell 1. However, for other applications, the temperature can also be monitored during operation.
  • the sensor 11 continuously monitors the temperature T actual and possibly the concentration K Me oH of the methanol / water mixture.
  • the measured temperature T actual with a predetermined temperature threshold value is then in the control device 17 S compared.
  • the temperature T falls below the temperature threshold T BLK ei ⁇ , for example below 0 ° C, the methanol concentration MeOH K is increased in the anode circuit line 5, by additional methanol is fed into the anode circuit line. 5
  • the injection pump 15 and the injection nozzle 14 are controlled accordingly by the control unit 17.
  • the concentration can be increased either by adding a predetermined amount of methanol once or by means of a control system by means of concentration monitoring.
  • the concentration sensor 11 is then preferably arranged upstream of the injection nozzle 14 in the anode circuit line 5, so that the setpoint K sol ⁇ for the methanol concentration is only reached when the concentration has spread over the entire anode circuit line 5 to the sensor 11.
  • a concentration target value K should predefined as a function of the current temperature T is and then the actual methanol concentration K Me0 H using a conventional control or regulating method by driving the injection pump 15 and the injector 14 set to the predetermined concentration setpoint K so u.
  • a control may be performed based on a stored in the control unit 17 characteristic map, for example, the map is predetermined injection amount for the methanol function of the measured temperature T and the current methanol concentration K Me oH contains in the anode circuit line. 5
  • T Schwe ⁇ _i several temperature thresholds T Schwe ⁇ _i can be specified, if, when decreasing Temperature T is below the next lower temperature threshold T SC hwei ⁇ _ ⁇ + ⁇ , a further predetermined amount of methanol is added or a higher methanol concentration K Me0 H is set.
  • the system is therefore not always immediately switched to maximum frost protection, but the frost protection is adapted to the actual temperature. As a result, the additional amount of methanol required can be reduced despite adequate frost protection and the overall efficiency can be improved.

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  • Life Sciences & Earth Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Manufacturing & Machinery (AREA)
  • Sustainable Development (AREA)
  • Sustainable Energy (AREA)
  • Chemical & Material Sciences (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Electrochemistry (AREA)
  • General Chemical & Material Sciences (AREA)
  • Fuel Cell (AREA)

Abstract

The invention relates to a fuel cell system comprising an anode chamber (2) and a cathode chamber (3), which are separated from one another by a proton-conductive membrane (4). A gas containing oxygen flows through the cathode chamber. A liquid fuel/coolant mixture, preferably a mixture of methanol and water is guided in a circuit through the anode chamber. To provide greater frost-protection and improved cold-starting capability, a temperature in the fuel cell system is monitored, even when said system is at a standstill and if a drop in temperature occurs, the concentration of fuel in the anode circuit is increased.

Description

FLUSSIGBRENNSTOFFZEL ENSYSTEM LIQUID FUEL TEL ENSYSTEM
Die Erfindung betrifft ein BrennstoffZellensystem und ein Verfahren zum Betreiben eines solchen Brennstoffzellensystems mit den Merkmalen gemäß dem Oberbegriff des Patentanspruchs 1 beziehungsweise 6.6. The invention relates to a fuel cell system and a method for operating such a fuel cell system with the features according to the preamble of patent claims 1 and 6, respectively.
Für die Alltagstauglichkeit von BrennstoffZellensystemen, insbesondere beim Einsatz m Fahrzeugen, sind Frostsicherheit und Kaltstarttauglichkeit wesentliche Kriterien. Für BrennstoffZeilensystem bedeutet dies wegen der vorhandenen Mengen an Wasser ein Problem. Auch für die sogenannte Direkt-Methanol-Brennstoffzelle (DMFC) , die wegen des Direktbetriebes mit fl ssigem Brenn ittel/Kuhlmittelgemisch sehr weitreichende Systemvereinfachungen erwarten laßt, ist das Frostschutz- und Kaltstartproblem bislang ungelöst.Frost safety and suitability for cold starts are essential criteria for the everyday suitability of fuel cell systems, especially when used in vehicles. This is a problem for the fuel line system because of the amount of water available. For the so-called direct methanol fuel cell (DMFC), which can be expected to make far-reaching system simplifications due to the direct operation with liquid fuel / coolant mixture, the frost protection and cold start problem has so far not been solved.
Ein gattungsgemaßes BrennstoffZellensystem ist aus der DE 198 07 876 AI bekannt. Dort wirα auf der Anodenseite ein flussiges Methanol/Wassergemisch im Kreislauf gefuhrt. Zur Gewährleistung einer konstanten Methanoikonzentration wird aus einem Vorratsbehalter Methanol m den Anodenkreislauf zudosiert. Die Do≤iermenge wird dabei mit Hilfe eines Konzentrationssensors im Anodenkreislauf ermittelt. Als Kuhlmittel werden Flüssigkeiten oder ionische beziehungsweise nichtionische Zusätze zum Wasser mit guten FrostSchutzeigenschaften vorgeschlagen. Insbesondere für eine DMFC sind solche geeigneten Kuhlmittel derzeit und wohl auch m absehbarer Zeit nicht verf gbar. Der physikalische Hintergrund ist der folgende:A generic fuel cell system is known from DE 198 07 876 AI. There, a liquid methanol / water mixture is circulated on the anode side. To ensure a constant concentration of methano, the anode circuit is metered in from a methanol reservoir. The dosing amount is determined with the help of a concentration sensor in the anode circuit. Liquids or ionic or nonionic additives to the water with good frost protection properties are proposed as cooling agents. In particular for a DMFC, such suitable coolants are currently and probably not available in the foreseeable future. The physical background is as follows:
Die DMFC wird üblicherweise bei Temperaturen um etwa 100° C betrieben. Die Methanolkonzentration liegt typischerweise zwischen 0,5 und 2 mol/1 beziehungsweise 1,6 und 6,4 Gewichtsprozent. Ursache ist die Methanolpermeabilitat verfugbarer Membranen. Wird Methanol m höheren Konzentrationen eingesetzt, diffundiert das überschüssige Methanol durch die Membran zur Kathode. Die Folge ist ein drastisch verringerter Wirkungsgrad. Anderseits betragt die kryoskopische Konstante des Wassers 1,86 K kg/mol, das heißt pro ol/kg zugesetzten Additivs sinkt der Gefrierpunkt um nur 1,86 °C. Da es sich um eine kolligative Eigenschaft handelt, ist dieser Wert unabhängig von der Art des Additives. Der Gefrierpunkt der üblicherweise verwendeten Wasser/Methanolgemische liegt damit bei etwa -1
Figure imgf000004_0001
The DMFC is usually operated at temperatures around 100 ° C. The methanol concentration is typically between 0.5 and 2 mol / 1 or 1.6 and 6.4 percent by weight. The cause is the methanol permeability of available membranes. If methanol is used in higher concentrations, the excess methanol diffuses through the membrane to the cathode. The result is a drastically reduced efficiency. On the other hand, the cryoscopic constant of the water is 1.86 K kg / mol, which means that the freezing point drops by only 1.86 ° C per oil / kg of additive. Since this is a colligative property, this value is independent of the type of additive. The freezing point of the commonly used water / methanol mixtures is around -1
Figure imgf000004_0001
Um zum Beispiel Frostschutz bis -30 °C zu gewahrleisten, wird jedoch ein Additiv m einer Konzentration von über 16 mol/kg benotigt. Ein solches Additiv ist gegenwartig nicht verfugbar. Ganz prinzipiell wird es auch langfristig nicht erfugbar sein, denn selbst ein relativ kleines Molekül mit einer angenommen Molmasse von 50 g/mol wurde m einer Konzentration von 800 g/kg benotigt. Ein Gemisch dieser Zusammensetzung reicht jedoch nicht mehr aus, um die Anode stochiometrisch mit Wasser zu versorgen. Für die Anodenreaktion werden jedoch Wasser und Methanol im stochiometπschen Verhältnis von 1:1 benotigt. Alle Salze, Sauren und Basen kommen als Frostschutzadditive nicht m Frage, weil sie die elektrische Leitfähigkeit des Kuhlwassers erhohen und somit unweigerlich zu Kurzschlußstromen im Stack fuhren. Es ist die Aufgabe der Erfindung ein mittels flussigem Brennmittel/Kuhlmittelgemisch betriebenes Brennstoffzellensystem und ein Verfahren zum Betreiben eines solchen BrennstoffZeilensystems mit verbesserten Frotschutz- und Kaltstarteigenschaften zu schaffen.For example, to ensure frost protection down to -30 ° C, an additive with a concentration of over 16 mol / kg is required. Such an additive is currently not available. In principle, it will not be detectable in the long term either, because even a relatively small molecule with an assumed molar mass of 50 g / mol was required in a concentration of 800 g / kg. However, a mixture of this composition is no longer sufficient to stoichiometrically supply the water with water. However, water and methanol in a stochiometric ratio of 1: 1 are required for the anode reaction. All salts, acids and bases do not come into question as anti-freeze additives because they increase the electrical conductivity of the cooling water and thus inevitably lead to short-circuit currents in the stack. It is the object of the invention to provide a fuel cell system operated by means of a liquid fuel / coolant mixture and a method for operating such a fuel cell system with improved frost protection and cold start properties.
Die Aufgabe wird erfmdungsgemaß durch die kennzeichnenden Merkmale des Patentanspruchs 1 beziehungsweise 6 gelost.The object is achieved according to the invention by the characterizing features of patent claims 1 and 6, respectively.
Durch eine Erhöhung der Brennmittelkonzentration m der Anodenkreisleitung bei sinkender Temperatur wird der Gefrierpunkt des Brennmittel/Kuhl ittelgemisches erhöht und somit Frostschutz gewährleistet, wobei sich gleichzeitig der Wirkungsgrad im Normalbetrieb des Systems nicht verschlechtert. Mit dieser Maßnahme wird ein Frostschutz bis -35° C möglich. Gleichzeitig wird das Kaltstartverhalten durch eine schnellere Erwärmung der Brennstoffzelle verbessert, weil das Brennmittel aufgrund der erhöhten Konzentration vermehrt durch die Membran zur Kathode diffundiert und dort nach dem Start der Luftversorgung sofort katalytisch unter Wärmeabgabe oxidiert wird. Dadurch wird der Kaltstartvorgang erheblich beschleunigt .By increasing the fuel concentration in the anode circuit line as the temperature drops, the freezing point of the fuel / coolant mixture is increased and thus frost protection is ensured, while at the same time the efficiency in normal operation of the system does not deteriorate. With this measure, frost protection down to -35 ° C is possible. At the same time, the cold start behavior is improved by faster heating of the fuel cell, because the fuel increasingly diffuses through the membrane to the cathode due to the increased concentration and is oxidized there catalytically immediately after the start of the air supply with heat emission. This speeds up the cold start process considerably.
Durch die Verwendung eines kombinierten Konzentrations- und Temperatursensors können Bauteile eingespart und somit die Kosten und der benotigte Bauraum reduziert werden.By using a combined concentration and temperature sensor, components can be saved and thus the costs and the required installation space can be reduced.
Die Frostsicherheit kann auf einfache Weise dadurch gewähr¬ leistet werden, daß die Brennmittelkonzentration m der AnodenJ-creisleitung entweder durcn kontinuierliche Anpassung des Konzentrations-Sollwertes an die sinkende Temperatur erhöht oder durch Vergleich der ermittelten Temperatur mit einer vorgegeben Temperaturschwelle sprunghaft angehoben wird. Durch den Einsatz mehrerer Temperaturschwellen kann die zusatzlich benotigte Brennmittelmenge trotz ausreichendem Frostschutz reduziert und somit der Wirkungsgrad insgesamt verbessert werden. In diesem Fall wird nämlich das System nicht immer sofort beim Unterschreiten eines Temperaturschwellwertes auf einen maximalen Frostschutz umgestellt, sondern der Frostschutz wird an die tatsachliche Temperatur angepaßt.The frost resistance can be warrants ¬ that the fuel concentration m the AnodenJ-creisleitung either durcn continuous adjustment of the concentration setpoint is increased to the decreasing temperature or raised abruptly by comparing the detected temperature with a predetermined temperature threshold in a simple manner. By using several temperature thresholds, the additional amount of fuel required can be reduced despite adequate frost protection, thus improving overall efficiency. In this case, the system is not always switched to maximum frost protection when the temperature falls below a threshold value, but the frost protection is adapted to the actual temperature.
Durch die Aktivierung der Temperaturuberwachung nur bei abgeschaltetem BrennstoffZeilensystem wird der Aufwand reduziert. Gleichzeitig bedeutet dies aber kein Nachteil, weil das System wahrend des Betriebs immer ausreichend warm ist und daher kein zusätzlicher Frostschutz notwendig ist.The effort is reduced by activating the temperature monitoring only when the fuel line system is switched off. At the same time, this means no disadvantage, because the system is always sufficiently warm during operation and therefore no additional frost protection is necessary.
Weitere Vorteile und Ausgestaltungen gehen aus den Unteranspruchen und der Beschreibung hervor. Die Erfindung ist nachstehend anhand einer Zeichnung, die den Prinzipaufbau eines vereinfacht dargestellten Brennstoffzellensystems zeigt, naher beschrieben.Further advantages and refinements emerge from the subclaims and the description. The invention is described below with reference to a drawing that shows the basic structure of a simplified fuel cell system.
Die insgesamt mit 1 bezeichnete Brennstoffzelle besteht aus einem Anodenraum 2 und einem Kathodenraum 3, die durch eine protonenleitende Membran 4 voneinander getrennt sind. Über eine Anodenkreisleitung 5, die einen Anodenraumausgang 6 mit einem Anodenraumeingang 7 der Brennstoffzelle 1 verbindet, wird ein flussiges Brennmittel/Kuhlmittelgemisch durch den Anodenraum 2 gefuhrt. Als Brennmittel kann nierbei ede geeignete, bei Zimmertemperatur fl ssige und elektrochemisch oxidierbare Substanz verwendet werden. Das im Ausfuhrungsbeispiel beschriebene System wird mit flussigem Methanol als Brennmittel und Wasser als Kuhlmittel betrieben. Obwohl im folgenden nur noch die Verwendung eines Methanol/Wassergemisches beschrieben wird, soll der Schutzbreich dieser Anmeldung jedoch nicht auf dieses Ausfuhrungsbeispiel beschrankt werden. Ein solches mit flüssigem Methanol/Wassergemisch betriebenes System wird üblicherweise als Direkt-Methanol-Brennstoffzelle (DMFC) bezeichnet.The fuel cell, designated overall by 1, consists of an anode compartment 2 and a cathode compartment 3, which are separated from one another by a proton-conducting membrane 4. A liquid fuel / coolant mixture is led through the anode compartment 2 via an anode circuit line 5, which connects an anode compartment outlet 6 with an anode compartment inlet 7 of the fuel cell 1. No suitable substance that is liquid and electrochemically oxidizable at room temperature can be used as the fuel. The system described in the exemplary embodiment is operated with liquid methanol as the fuel and water as the coolant. Although only the use of a methanol / water mixture is described in the following, the scope of protection of this application should not be restricted to this exemplary embodiment. Such a system operated with liquid methanol / water mixture is commonly referred to as direct methanol fuel cell (DMFC).
In den Kathodenraum 3 wird über eine Kathodenzuleitung 8 ein sauerstoffhaltiges Gas geleitet. GemäßAn oxygen-containing gas is fed into the cathode compartment 3 via a cathode feed line 8. According to
Ausfuhrungsbeispiel wird hierzu Umgebungsluft verwendet. In der Brennstoffzelle 1 wird das Brennmittel an der Anode oxidiert, der Luftsauerstoff an der Kathode reduziert. Hierzu wird die protonenleitende Membran 4 auf den entsprechenden Oberflachen mit geeigneten Katalysatoren, wie zum Beispiel hochoberflachige Edelmetallmohre oder getragene Katalysatoren beschichtet. Von der Anodenseite können nun Protonen durch die protonenleitende Membran 4 wandern und sich an der Kathodenseite mit den Sauerstoffionen zu Wasser verbinden. Bei dieser elektrochemischen Reaktion entsteht zwischen den beiden Elektroden eine Spannung. Durch Parallel- beziehungsweise Hintereinanderschaltung vieler solcher Zellen zu einem sogenannten Stack können auch höhere Spannungen und Stromstarken erreicht werden.Exemplary embodiment, ambient air is used for this. In the fuel cell 1, the fuel is oxidized at the anode and the atmospheric oxygen at the cathode is reduced. For this purpose, the proton-conducting membrane 4 is coated on the corresponding surfaces with suitable catalysts, such as, for example, high-surface-area precious metal tubes or supported catalysts. Protons can now migrate through the proton-conducting membrane 4 from the anode side and combine with the oxygen ions to form water on the cathode side. This electrochemical reaction creates a voltage between the two electrodes. By connecting many such cells in parallel or in series to form a so-called stack, higher voltages and currents can also be achieved.
Als Produkt entsteht am Anodenausgang ein mit Wasser und Methanol angereichertes Kohlendioxidgas . Dieses Flussigkeits/Gasgemisch wird über die Anodenkreisleitung 5 aus dem Anodenraum 2 abgeführt. Die Restsauerstoff und Wasserdampf enthaltende Kathodenabluft wird über eine Kathodenabgasle tung 9 abgeführt. Um einen guten Wirkungsgrad zu erhalten kann die Umgebungsluft im Kathodenraum 3 vorzugsweise mit Überdruck bereitgestellt werden.The product produced at the anode outlet is a carbon dioxide gas enriched with water and methanol. This liquid / gas mixture is discharged from the anode compartment 2 via the anode circuit line 5. The cathode exhaust air containing residual oxygen and water vapor is discharged via a cathode exhaust gas device 9. In order to obtain good efficiency, the ambient air in the cathode compartment 3 can preferably be provided with overpressure.
Auf der Anodenseite wird das Methanol/Wassergemisch mit Hilfe einer Pumpe 10 bei einem vorgegebenem Druck durch die Anodenkreisleitung 5 zirkuliert. Das Verhältnis von Wasser zu Methanol m der Anodenkreisleitung 5 wird mit Hilfe eines Sensors 11, der die Methanolkonzentration m der Anodenkreisleitung 5 mißt, eingestellt. In Abhängigkeit von diese Sensorsignal erfolgt dann üblicherweise eine Konzentrationsregelung für das Methanol/Wassergemisch, wobei das flussige Methanol aus einemOn the anode side, the methanol / water mixture is circulated through the anode circuit line 5 at a predetermined pressure with the aid of a pump 10. The ratio of water to methanol m of the anode circuit line 5 is adjusted with the aid of a sensor 11 which measures the methanol concentration m of the anode circuit line 5. In dependence of this sensor signal is then usually a concentration control for the methanol / water mixture, the liquid methanol from one
Methanolvorratsbehalter 12 über eine Zufuhrungsleitung 13 zugeführt und mit Hilfe einer nicht naher gezeigten Einspritzdüse 14 m die Anodenkreisleitung 5 eingespritzt wird. Der Einspritzdruck wird durch eine m der Zufuhrungsleitung 13 angeordneten Einspritzpumpe 15 erzeugt. Die Methanoidosierung erfolgt durch eine geeignete Ansteuerung der Einspritzdüse 14. Hierfür ist ein Steuergerat 17 vorgesehen, welches über gepunktet eingezeichnete Mess- beziehungsweise Steuerleitungen mit der Pumpe 10, dem Sensor 11, der Einspritzpumpe 15, der Einspritzdüse 14 und gegebenenfalls weiteren Komponenten verbunden ist. Dem Anodenraum 2 wird somit standig ein Methanol/Wassergemisch mit vorzugsweise konstanter Methanolkonzentration zugeführt. Es ist aber auch denkbar, auch wahrend des Betriebs des Brennstoffzellensystems die Methanolkonzentration zu variieren.Methanol reservoir 12 is fed via a feed line 13 and the anode circuit line 5 is injected with the aid of an injection nozzle 14 m (not shown in more detail). The injection pressure is generated by an injection pump 15 arranged in the supply line 13. The methanoidosing is carried out by a suitable control of the injection nozzle 14. For this purpose, a control device 17 is provided, which is connected to the pump 10, the sensor 11, the injection pump 15, the injection nozzle 14 and possibly other components via dotted measurement or control lines. A methanol / water mixture with a preferably constant methanol concentration is thus continuously supplied to the anode compartment 2. However, it is also conceivable to vary the methanol concentration even while the fuel cell system is in operation.
Auf der Anodenseite wird mit Hilfe eines Gasabscheiders 16 das mit Methanol- und Wasserdampf angereicherte Kohlendioxid aus dem Flussigkeits/Gasgemisch m der Anodenkreisleitung 5 abgetrennt. Dabei soll ein zu hoher Methanolaustrag über das Kohlendioxidgas verhindert werden, da sonst der Gesamtwirkungsgrad des Systems verringert wird und gleichzeitig unverbranntes Methanol an die Umgebung abgegeben wurde. Entgegen dem m der Zeichnung vereinfacht dargestellten Gasabscheider werden hierzu üblicherweise aufwendigere Vorrichtung eingesetzt.On the anode side, a gas separator 16 is used to separate the carbon dioxide enriched with methanol and water vapor from the liquid / gas mixture m in the anode circuit line 5. Too much methanol discharge through the carbon dioxide gas is to be prevented, since otherwise the overall efficiency of the system is reduced and at the same time unburned methanol was released into the environment. Contrary to the gas separator shown in simplified form in the drawing, more complex devices are usually used for this purpose.
Weiterhin ist eine Vorrichtung zur Ermittlung einer Temperatur T1Ξ^ vorgesehen. Hierzu können übliche Temperatursensoren verwendet werden. Vorteilhaft ist es, wenn der Sensor 11 als kombinierter Konzentrations- und Temperatursensor ausgeführt ist. Dadurch können zusatzliche Bauteile eingespart werden. Es ist jedoch selbstverständlich auch möglich einen separaten Temperatursensor vorzusehen. Gemäß Ausfuhrungsbeispiel ist der Sensor 11 m der Anodenkreisleitung 5 zwischen dem Gasabscheider 16 und der Pumpe 10 angeordnet. Es ist jedoch auch möglich, den Sensor 11 an einer anderen Stelle m der Anodenkreisleitung 5 oder auch direkt m der Brennstoffzelle 1 anzuordnen. Möglich ist es auch, einen Temperatursensor zu verwenden, der die Umgebungstemperatur mißt. Damit konnte allerdings die nach dem Abschalten m dem System noch enthaltene Warme nicht berücksichtigt werden.Furthermore, a device for determining a temperature T 1Ξ ^ is provided. Conventional temperature sensors can be used for this. It is advantageous if the sensor 11 is designed as a combined concentration and temperature sensor. Additional components can thus be saved. However, it is of course, it is also possible to provide a separate temperature sensor. According to the exemplary embodiment, the sensor 11 m of the anode circuit line 5 is arranged between the gas separator 16 and the pump 10. However, it is also possible to arrange the sensor 11 at a different location m in the anode circuit line 5 or also directly in the fuel cell 1. It is also possible to use a temperature sensor that measures the ambient temperature. However, the heat that was still present in the system after switching off could not be taken into account.
Erfmdungsgemaß wird der Frostschutz für das System dadurch gewährleistet, daß die Konzentration KMe0H des Methanol/Wassergemischs an die Temperatur Tlst- m der Anodenkreisleitung 5 beziehungsweise an die herrschende Umgebungstemperatur angepaßt wird. Fallt die Temperatur Tlst, so wird die Konzentration KMeoH erhöht und somit der Gefrierpunkt des Methanol/Wassergemisches erniedrigt. Dadurch wird der Frostschutz gewährleistet. Beim Kaltstart des Systems fuhrt die erhöhte Methanoikonzentration KMeoH außerdem zu einer schnelleren Erwärmung der Brennstoffzelle 1, weil das Methanol vermehrt durch die Membran 4 zur Kathode 3 diffundiert und dort nach dem Start der Luftversorgung sofort katalytisch unter Wärmeabgabe oxidiert wird. Dadurch wird er Kaltstartvorgang erheblich beschleunigt. Vorzugsweise erfolgt die Temperaturuberwachung und die damit verbundene Konzentrationsanpassung nur im Stillstand des Systems, weil im Betrieb der Brennstoffzelle 1 die Temperaturen ausreichend hoch sind. Allerdings kann für andere Anwendungsfalle die Temperatur auch wahrend des Betriebs überwacht werden.According to the invention, frost protection for the system is ensured by adapting the concentration K Me0H of the methanol / water mixture to the temperature T lst - m of the anode circuit line 5 or to the prevailing ambient temperature. If the temperature T drops , the concentration K Me oH is increased and thus the freezing point of the methanol / water mixture is lowered. This ensures frost protection. When the system is cold started, the increased methanoconcentration K Me oH also leads to faster heating of the fuel cell 1, because the methanol diffuses more and more through the membrane 4 to the cathode 3 and is oxidized there catalytically immediately after the start of the air supply with heat emission. This speeds up the cold start process considerably. The temperature monitoring and the associated concentration adjustment are preferably carried out only when the system is at a standstill because the temperatures are sufficiently high during operation of the fuel cell 1. However, for other applications, the temperature can also be monitored during operation.
Der Sensor 11 überwacht permanent die Temperatur Tlst und gegebenenfalls die Konzentration KMeoH des Methanol/Wasser- gemisches. Im Steuergerat 17 wird dann die gemessene Temperatur Tlst mit einem vorgegebenen Temperaturschwellwert Schweiι verglichen. Sobald im Stillstand die Temperatur Tist unter den Temperaturschwellwert TschWeiι fallt, zum Beispiel unter 0°C, wird die Methanolkonzentration KMeoH in der Anodenkreisleitung 5 erhöht, indem zusätzliches Methanol in die Anodenkreisleitung 5 zugeführt wird. Hierzu wird die Einspritzpumpe 15 und die Einspritzdüse 14 vom Steuergerät 17 entsprechend angesteuert. Die Konzentrationserhöhung kann entweder durch einmaliges Zugeben einer vorgegebenen Methanolmenge oder anhand einer Regelung durch eine Konzentrationsüberwachung erfolgen. Im zweiten Fall ist es vorteilhaft, das Methanol/Wassergemisch in der Anodenkreisleitung 5 zumindest während des Regelvorganges zusätzlich mit Hilfe der Pumpe 10 umzuwälzen, damit die Konzentration laufend ausgeglichen wird. Außerdem ist dann der Konzentrationssensor 11 vorzugsweise stromauf der Einspritzdüse 14 in der Anodenkreisleitung 5 angeordnet, damit bei der Regelung der Sollwert Ksolι für die Methanolkonzentration erst dann erreicht wird, wenn sich die Konzentration über die gesamte Anodenkreisleitung 5 bis zum Sensor 11 ausgebreitet hat.The sensor 11 continuously monitors the temperature T actual and possibly the concentration K Me oH of the methanol / water mixture. The measured temperature T actual with a predetermined temperature threshold value is then in the control device 17 S compared. Once at rest, the temperature T falls below the temperature threshold T BLK eiι, for example below 0 ° C, the methanol concentration MeOH K is increased in the anode circuit line 5, by additional methanol is fed into the anode circuit line. 5 For this purpose, the injection pump 15 and the injection nozzle 14 are controlled accordingly by the control unit 17. The concentration can be increased either by adding a predetermined amount of methanol once or by means of a control system by means of concentration monitoring. In the second case, it is advantageous to additionally circulate the methanol / water mixture in the anode circuit line 5, at least during the control process, with the aid of the pump 10, so that the concentration is constantly balanced. In addition, the concentration sensor 11 is then preferably arranged upstream of the injection nozzle 14 in the anode circuit line 5, so that the setpoint K sol ι for the methanol concentration is only reached when the concentration has spread over the entire anode circuit line 5 to the sensor 11.
Im Falle einer Regelung der Methanolkonzentration KMe0H wird im Steuergerät 17 ein Konzentrations-Sollwert Ksoll in Abhängigkeit der aktuellen Temperatur Tis vorgegeben und dann die tatsächliche Methanolkonzentration KMe0H anhand üblicher Steuer- oder Regelverfahren durch Ansteuerung der Einspritzpumpe 15 und des Einspritzventils 14 auf den vorgegebenen Konzentrations-Sollwert Ksou eingestellt. Eine Steuerung kann beispielsweise anhand eines im Steuergerät 17 abgelegten Kennfeldes erfolgen, wobei das Kennfeld vorgegebene Einspritzmengen für das Methanol in Abhängigkeit von der gemessenen Temperatur Tist und der aktuellen Methanolkonzentration KMeoH in der Anodenkreisleitung 5 enthält.In the case of control of the methanol concentration K Me0 H is in the control unit 17, a concentration target value K should predefined as a function of the current temperature T is and then the actual methanol concentration K Me0 H using a conventional control or regulating method by driving the injection pump 15 and the injector 14 set to the predetermined concentration setpoint K so u. A control may be performed based on a stored in the control unit 17 characteristic map, for example, the map is predetermined injection amount for the methanol function of the measured temperature T and the current methanol concentration K Me oH contains in the anode circuit line. 5
Alternativ können auch mehrere Temperaturschwellen Tschweιι_i vorgegeben werden, wobei dann, wenn bei sinkender Temperatur Tlst die nächst niedrigere Temperaturschwelle TSChweiι_ι+ι unterschritten wird, jeweils eine weitere vorgegebene Methanolmenge zugegeben oder eine höhere Methanolkonzentration KMe0H eingestellt wird. Damit wird das System nicht immer sofort auf einen maximalen Frostschutz umgestellt, sondern der Frostschutz wird an die tatsächliche Temperatur angepaßt. Dadurch kann die zusätzlich benötigte Methanolmenge trotz ausreichendem Frostschutz reduziert und somit der Wirkungsgrad insgesamt verbessert werden.Alternatively, several temperature thresholds T Schwe ιι_i can be specified, if, when decreasing Temperature T is below the next lower temperature threshold T SC hweiι_ι + ι, a further predetermined amount of methanol is added or a higher methanol concentration K Me0 H is set. The system is therefore not always immediately switched to maximum frost protection, but the frost protection is adapted to the actual temperature. As a result, the additional amount of methanol required can be reduced despite adequate frost protection and the overall efficiency can be improved.
Neben der Brennstoffzelle 1 selbst können auf diese Art und Weise gegebenenfalls noch weitere gefährdete Komponenten in dem System durch Zugabe von Methanol in einer für den Frostschutz ausreichenden Konzentration in Abhängigkeit von der momentanen Temperatur geschützt werden. In addition to the fuel cell 1 itself, other vulnerable components in the system can also be protected in this way by adding methanol in a concentration sufficient for frost protection depending on the current temperature.

Claims

Patentanspruche Patent claims
1. Brennstoffzellensystem (1) mit einem Anodenraum (2) und einem Kathodenraum (3), die durch eine protonenleitende Membran (4) voneinander getrennt sind, mit einer Kathodenzuleitung (8) zur Zufuhr von sauerstoffhaltigem Gas zum Kathodenraum (3) , mit einer Kathodenabgasleitung (9) , einer Anodenkreisleitung (5) zur Kreislauffuhrung eines flussigen Brennmittel/Kuhlmittelgemisches zwischen dem Anodenraumausgang (6) und dem Anodenraumeingang (7), mit einer Vorrichtung (11) zur Bestimmung der Brennmittelkonzentration (KMeθH) m der Anodenkreisleitung1. Fuel cell system (1) with an anode compartment (2) and a cathode compartment (3), which are separated from one another by a proton-conducting membrane (4), with a cathode feed line (8) for supplying oxygen-containing gas to the cathode compartment (3), with a Cathode exhaust gas line (9), an anode circuit line (5) for circulating a liquid fuel / coolant mixture between the anode chamber outlet (6) and the anode chamber inlet (7), with a device (11) for determining the fuel concentration (K MeθH ) m of the anode circuit line
(5), mit einem Brennmittelvorratsbehalter (12), mit einer Leitung (13) zur Zufuhr von Brennmittel aus dem Brennmittelvorratsbehalter (12) in die Anodenkreisleitung (5), mit einer m der Leitung (13) angeordneten Vorrichtung (14) zur Dosierung des zugefuhrten Brennmittels m Abhängigkeit von der Brennmittelkonzentration (KMe0H) / d a d u r c h g e k e n n z e i c h n e t , daß eine Vorrichtung (11) zur Ermittlung einer Temperatur (Tlst-) vorgesehen ist und daß die Vorrichtung (14) zur Dosierung des zugefuhrten Brennmittels bei sinkender Temperatur (Tls1-) zur Erhöhung der Brennmittelkonzentration (KMeoH) m der Anodenkreisleitung (5) ansteuerbar ist.(5), with a fuel storage container (12), with a line (13) for supplying fuel from the fuel storage container (12) into the anode circuit line (5), with a line (13) arranged in the device (14) for metering the supplied fuel m as a function of the fuel concentration (K Me0 H) / characterized in that a device (11) for determining a temperature (T actual -) is provided and that the device (14) for metering the supplied fuel when the temperature drops (T ls1 -) to increase the fuel concentration (K Me oH) m of the anode circuit line (5) can be controlled.
2. Brennstoff zellensystem nach Anspruch 1, d a d u r c h g e k e n n z e i c h n e t , daß eine Vorrichtung (17) zum Vergleichen der ermittelten Temperatur (Tlst) mit einem vorgegebenen2. Fuel cell system according to claim 1, characterized in that a device (17) for comparing the determined temperature (T lst ) with a predetermined
Temperaturschwellwert (T3Chweu) vorgesehen ist und daß die Vorrichtung (14) zur Dosierung des zugefuhrten Brennmittels beim Unterschreiten des Temperaturschwellwertes (TschWeiι) zur Erhöhung der Brennmittelkonzentration (KMeoH) in der Anodenkreisleitung (5) ansteuerbar ist.Temperature threshold (T 3Chwe u) is provided and that the device (14) for metering the supplied fuel if the temperature falls below the temperature threshold (T schW eiι) to increase the fuel concentration (K Me oH) in the anode circuit line (5) can be controlled.
3. Brennstoff zellensystem nach Anspruch 1, d a d u r c h g e k e n n z e i c h n e t , daß ein Sensor (11) zur Erfassung der Umgebungstemperatur (Tls ) oder der Temperatur (Tlst) des3. Fuel cell system according to claim 1, characterized in that a sensor (11) for detecting the ambient temperature (T ls ) or the temperature ( T ls t) of
Brennmittel/Kuhlmittelgemisches m der Anodenkreisleitung (5) vorgesehen ist.Fuel / coolant mixture m of the anode circuit line (5) is provided.
4. Brennstoff zellensystem nach Anspruch 1, d a d u r c h g e k e n n z e i c h n e t , daß ein kombinierter Konzentrations- und Temperatursensor (11) m der Anodenkreisleitung (5) vorgesehen ist.4. A fuel cell system according to claim 1, that a combined concentration and temperature sensor (11) m of the anode circuit line (5) is provided.
5. Brennstoff zellensystem nach Anspruch 3 oder 4, d a d u r c h g e k e n n z e i c h n e t , daß der Sensor (11) zwischen dem Anodenraumausgang (6) und der Dosiervorrichtung (14) in der Anodenkreisleitung (5) angeordnet ist.5. Fuel cell system according to claim 3 or 4, so that the sensor (11) is arranged between the anode compartment outlet (6) and the metering device (14) in the anode circuit line (5).
6. Verfahren zum Betreiben eines BrennstoffZellensystems mit einem Anodenraum (2) und einem Kathodenraum (3), die durch eine protonenleitende Membran (4) voneinander getrennt sind, wobei der Kathodenraum (3) mit einem sauerstoffhaltigen Gas beaufschlagt wird, wobei ein flüssiges Brennmittel/Kuhlmittelgemisch mit Hilfe einer Anodenkreisleitung (5) durch den Anodenraum (2) gefuhrt wird, und wobei die Brennmittelkonzentration (KMΘ0H) wahrend des Betriebs des Brennstoffzellensystems (1) auf einem vorgegebenen Konzentrations-Sollwert (Ksoι ) gehalten wird, d a d u r c h g e k e n n z e i c h n e t , daß die Umgebungstemperatur (Tlst) und/oder die Temperatur (T1S,-) des Brennmittel/Kuhlmittelgemisches m der Anodenkreisleitung (5) ermittelt wird und daß bei sinkender Temperatur (Tlst) zur Gewährleistung eines ausreichenden Frostschutzes die Brennmittelkonzentration (KMeoH) in der Anodenkreisleitung (5) erhöht wird.6. Method for operating a fuel cell system with an anode compartment (2) and a cathode compartment (3), which are separated from one another by a proton-conducting membrane (4), the cathode compartment (3) being acted upon by an oxygen-containing gas, a liquid fuel / Coolant mixture is passed through the anode compartment (2) with the aid of an anode circuit line (5), and the fuel concentration (K MΘ0 H) is kept at a predetermined concentration setpoint (K so ι) during operation of the fuel cell system (1), characterized in that that the ambient temperature (T ls t) and / or the temperature (T 1S , -) of the fuel / coolant mixture m of the anode circuit line (5) is determined and that as the temperature drops (T lst ) to ensure sufficient Frost protection the fuel concentration (K Me oH) in the anode circuit line (5) is increased.
7. Verfahren nach Anspruch 6, d a d u r c h g e k e n n z e i c h n e t , daß der Konzentrations-Soliwert (KSGn) in Abhängigkeit der ermittelten Temperatur (T1S- vorgegeben wird.7. The method according to claim 6, characterized in that the concentration soli value (K SG n) in dependence on the determined temperature (T 1S - is predetermined.
8. Verfahren nach Anspruch 6, d a d u r c h g e k e n n z e i c h n e t , daß laufend die ermittelte Temperatur (Tlsr) mit einem vorgegebenen Temperaturschwellwert (TschWeiι) verglichen wird, und daß dann, wenn die ermittelte Temperatur (T1S- den Temperaturschwellwert (TSChv,e_ unterschreitet, die Brennmittelkonzentration (Kveoκ) in der Anodenkreisleitung (5) erhöht wird.8. The method according to claim 6, characterized in that the determined temperature (T lsr ) is continuously compared with a predetermined temperature threshold (T schW eiι), and that when the determined temperature (T 1S - the temperature threshold (T SC hv, e_ falls below, the fuel concentration (Kv e oκ) in the anode circuit line (5) is increased.
9. Verfahren nach Anspruch 8, d a d u r c h g e k e n n z e i c h n e t , daß mehrere Temperaturschwellen (Tschweiι_ι) vorgegeben werden, wobei dann, wenn bei sinkender Temperatur (Tlst) die nächst niedrigere Temperaturschwelle (TschWeiι_ι+ι) unterschritten wird, die Brennmittelkonzentration (KMΘOH) in der Anodenkreisleitung (5) jeweils weiter erhöht wird.9. The method according to claim 8, characterized in that a plurality of temperature thresholds (T sch weiι_ι) are specified, when, when the temperature drops (T lst ) the next lower temperature threshold (T schW eiι_ι + ι) falls below, the fuel concentration (K MΘ OH) is further increased in each case in the anode circuit line (5).
10. Verfahren nach Anspruch 8 oder 9, d a d u r c h g e k e n n z e i c h n e t , daß bei Unterschreiten eines vorgegeben Temperaturschwellwertes ( T3crra^ι ) eine vorgegebene Brennmittelmenge in die Anodenkreisleitung (5) zugegeben oder der vorgegebene Konzentrations-Sollwert (KΞCι erhöht wird.10. The method according to claim 8 or 9, characterized in that when falling below a predetermined temperature threshold (T 3crra ^ ι) a predetermined amount of fuel in the anode circuit line (5) is added or the predetermined concentration setpoint (K ΞC ι is increased.
11. Verfahren nach Anspruch 6, d a d u r c h g e k e n n z e i c h n e t , daß die Temperaturüberwachung nur bei abgeschaltetem Brennstoffzellensystem (1) aktiviert ist. 11. The method of claim 6, d a d u r c h g e k e n n z e i c h n e t that the temperature monitoring is only activated when the fuel cell system (1) is switched off.
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