DE102016201611A1 - Method for drying a fuel cell system, method for switching off a fuel cell system and fuel cell system for carrying out these methods - Google Patents

Abstract

The invention relates to a method for drying a fuel cell system (100), comprising a fuel cell stack (10) with cathode compartments (13), anode compartments (12) and coolant channels. It is provided that for certain periods .DELTA.t1 and .DELTA.t2, the volume flow of a coolant flowing through the coolant channels and the cathode chambers (13) are flowed through with a certain mass flow of a cathode operating medium. During the specific period .DELTA.t1, a volume flow of the anode operating medium is prevented in the anode chambers (12), and in the specific period .DELTA.t2 the anode chambers (12) are flowed through by a specific volume flow of the anode operating medium. For the periods .DELTA.t1 and .DELTA.t2 a certain overpressure in the anode spaces (12) with respect to the cathode spaces (13) is maintained. The invention likewise relates to a method for switching off a fuel cell system, comprising drying the fuel cell system and a fuel cell system for carrying out these methods.

Description

The invention relates to a method for drying a fuel cell system, a method for switching off a fuel cell system and a fuel cell system for carrying out these methods.

Fuel cells use the chemical transformation of a fuel with oxygen to water to generate electrical energy. For this purpose, fuel cells contain as a core component the so-called membrane electrode assembly (MEA for membrane electrode assembly), which is a microstructure of an ion-conducting (usually proton-conducting) membrane and in each case on both sides of the membrane arranged catalytic electrode (anode and cathode). The latter mostly comprise supported noble metals, in particular platinum. In addition, gas diffusion layers (GDL) can be arranged on both sides of the membrane-electrode arrangement on the sides of the electrodes facing away from the membrane. As a rule, the fuel cell is formed by a multiplicity of stacked MEAs whose electrical powers add up. As a rule, bipolar plates (also called flow field plates or separator plates) are arranged between the individual membrane electrode assemblies, which ensure that the individual cells are supplied with the operating media, ie the reactants, and are usually also used for cooling. In addition, the bipolar plates provide an electrically conductive contact to the membrane-electrode assemblies.

During operation of the fuel cell, the fuel (anode operating medium), in particular hydrogen H ₂ or a hydrogen-containing gas mixture, is supplied to the anode via an anode-side open flow field of the bipolar plate, where an electrochemical oxidation of H ₂ to protons H ⁺ takes place with release of electrons (H ₂ → 2H ⁺ + 2e ^- ). Via the electrolyte or the membrane, which separates the reaction spaces gas-tight from each other and electrically isolated, takes place (water-bound or anhydrous) transport of the protons from the anode compartment into the cathode compartment. The electrons provided at the anode are supplied to the cathode via an electrical line. The cathode is supplied via a cathode-side open flow field of the bipolar plate oxygen or an oxygen-containing gas mixture (for example air) as a cathode operating medium, so that a reduction of O ₂ to O ^2- with absorption of the electrons takes place (½O ₂ + 2e ^- → O ^2-) , At the same time, the oxygen anions in the cathode compartment react with the protons transported via the membrane to form water (O ^2- + 2H ⁺ → H ₂ O).

The supply of the fuel cell stack with its operating media, ie the anode operating gas (for example hydrogen), the cathode operating gas (for example air) and the coolant, via main supply channels that enforce the stack in its entire stacking direction and of which the operating media on the bipolar plates, the single cells be supplied. For each operating medium at least two such main supply channels are present, namely one for feeding and one for discharging the respective operating medium.

Depending on the operating state of the fuel cell system, in particular of the fuel cell stack, the water formed in the reaction of the fuel in the cathode compartment can be in the form of vapor or in liquid form. The proportion of liquid product water depends on a variety of parameters and is difficult to predict. In addition, due to leaks in the fuel cell system as well as water permeability of the membranes, product water may pass from the cathode side to the anode side of the fuel cell stack and into the service sections of the fuel cell system. In addition to the temperature of the fuel cell stack therefore play in particular the pressures in the anode and cathode space, the operating life of the fuel cell stack and the operation of the fuel cell system for the amount and distribution of liquid water a crucial role. For example, the bypass of a humidifier in the cathode supply directly affects the proportion of liquid water in the cathode exhaust gas path.

A fuel cell system thus undergoes different states during operation with respect to the liquid water content, wherein neither the specific amount nor the location of the resulting liquid water can be predicted accurately. During operation, this results in the risk of blockage of the anode-side gas flow. When switching off the fuel cell system thereby the state in which the shutdown system is present, in particular its liquid water content, largely indeterminate. Thus, not only are unknown starting conditions for a restart of the system, under freezing conditions due to the liquid water also freeze damage occur.

Methods for drying and switching off a fuel cell stack are already known from the prior art.

The DE 10 2011 007 615 A1 discloses a method of drying the cathode compartment of a fuel cell stack during operation of a fuel cell system. In this case, the pressure in the cathode chamber is varied via the setting of a free flow cross section in the cathode exhaust gas path. The US 8,318,364 B2 also discloses a method for drying the cathode side of a fuel cell system. The drying takes place only if the fuel cell is to remain switched off for more than a predetermined reference time. Drying of the anode side and supply does not take place in these processes.

The WO 2006/030614 A2 discloses a method for drying a fuel cell system, wherein after switching off the fuel cell stack only the cathode side and only some time later also the anode side is rinsed with dry air. A similar procedure is in the JP-2012-074385 A disclosed. Meanwhile, it is known that when restarting a fuel cell stack even small amounts of oxygen on its anode side can lead to damage of the cathode. In addition, these methods require additional lines or funding for the dry air supply to the anode side.

The US 8,192,885 B2 discloses a method of drying a fuel cell stack wherein the flow directions of the cathode air stream and the anode gas stream are reversed to discharge water from the cathode or anode supply, respectively. The reversal of the flow directions can only be achieved with additional design effort.

The DE 10 2012 023 799 A1 discloses a method of drying a fuel cell system wherein liquid water on anode and cathode sides of a fuel cell stack is vaporized and discharged respectively by applying a negative pressure. Similar procedures are in the JP 2008103120 A and the DE 10061687 B4 disclosed. The vacuum pumps necessary for the generation of the negative pressure mean an increased space requirement.

The invention is based on the object to propose methods for drying and switching off a fuel cell system, which overcome the disadvantages of the prior art and allow the setting of a defined liquid water content in fuel cell systems. Furthermore, the method should ensure a long lifetime of the fuel cell stack and require no structural changes to the simple fuel cell system, in particular no increased space requirement.

• (a) Vermindern eines Volumenstroms von einem die Kühlmittelkanäle durchströmenden Kühlmittel für bestimmte Zeiträume Δt1 und Δt2;
• (b) Durchströmen der Kathodenräume mit einem bestimmten Massestrom eines Kathodenbetriebsmediums für die vorbestimmten Zeiträume Δt1 und Δt2;
• (c) Unterbinden von einem Volumenstrom eines Anodenbetriebsmediums durch die Anodenräume für den vorbestimmten Zeitraum Δt1;
• (d) Durchströmen der Anodenräume mit einem bestimmten Volumenstrom des Anodenbetriebsmediums für den bestimmten Zeitraum Δt2; und
• (e) Aufrechterhalten eines bestimmten Überdrucks in den Anodenräumen gegenüber den Kathodenräumen für die bestimmten Zeiträume Δt1 und Δt2.

The method according to the invention is directed to a fuel cell system comprising a fuel cell stack having cathode compartments, anode compartments and coolant channels. The method according to the invention for drying a fuel cell system comprises the steps:

• (A) reducing a volume flow of a coolant flowing through the coolant channels for certain periods .DELTA.t1 and .DELTA.t2;
• (b) flowing the cathode chambers with a certain mass flow of a cathode operating medium for the predetermined periods Δt1 and Δt2;
• (c) prohibiting a volume flow of an anode operating medium through the anode spaces for the predetermined time period Δt1;
• (d) flowing through the anode chambers with a specific volume flow of the anode operating medium for the specific period .DELTA.t2; and
• (e) maintaining a certain overpressure in the anode spaces with respect to the cathode spaces for the particular periods Δt1 and Δt2.

The starting point of the method according to the invention is an active fuel cell system, wherein it does not matter whether the fuel cell stack is started up in a startup phase or is already in continuous operation. In both cases, at the beginning of the process, the anode chambers of the fuel cell stack already with anode operating medium, the cathode chambers of the fuel cell stack already with cathode operating medium and the coolant channels of the fuel cell stack already flows through coolant.

As a first step of the method according to the invention, first the volume flow of the coolant, which flows through the fuel cell stack, is reduced. This leads advantageously to a reduced cooling of the stack. Thus, for the duration of the drying process, that is, for the predetermined periods .DELTA.t1 and .DELTA.t2, energetically favorable, the highest possible temperature of the fuel cell stack is obtained. This helps to evaporate the water present in liquid form. In addition, at the beginning of the method according to the invention by means of the anode operating medium, a specific target pressure is set in the anode chambers of the fuel cell stack. The value of the target pressure is chosen so that over the given period .DELTA.t1 in the anode chambers, an overpressure compared to the cathode chambers is realized.

Coolant is understood in the context of this application, a cooling liquid whose temperature is reduced by cooling. The cooling takes place, for example, by the cooling liquid flowing through a vehicle radiator. As a rule, the proportion of the coolant flowing through the cooling liquid can be adjusted by means of a thermostatic valve. If the thermostatic valve is closed, only a small or no part of the coolant flows through the vehicle radiator. Thus, no or only a small part of the coolant to coolant in the context of this application. Thus, the volume flow of a refrigerant flowing through the coolant channels can also be done by narrowing or closing of such a thermostatic valve. The coolant flow is accordingly reduced by reducing the volume flow of the coolant flowing through the fuel cell stack or by reducing the volume flow of the coolant flowing through the vehicle radiator.

If the anode chambers are filled with a volume of the anode operating medium which sets the target pressure in the anode chambers at the present temperature, a further volume flow of the anode operating medium in the anode chambers is prevented for the specific period .DELTA.t1. This is preferably achieved by closing actuating means, which are anyway in the anode supply. If the sequence in which the adjusting means are closed is adapted to the volume flow of the anode operating medium flowing through the anode chambers, then the target pressure in the anode chambers can also be adjusted. The overpressure in the anode chambers reduces the amount of liquid or gaseous water which has passed from the cathode side to the anode side during the period Δt1.

The cathode chambers of the fuel cell stack are traversed during the specific periods .DELTA.t1 and .DELTA.t2 with a certain mass flow of the cathode operating medium. At the time Δt1, the pressure in the cathode compartments, in particular at the cathode inlet, of the fuel cell is preferably as low as possible, preferably the pressure in the cathode compartments at time Δt1 is between ambient pressure and a pressure of 800 mbar above atmospheric pressure and particularly preferably at a pressure of 30 mbar above ambient pressure. The mass flow of the cathode operating medium causes a uniform distribution of liquid water and a flow-induced local pressure reduction in the cathode chambers and the cathode supply. As a result, the liquid water is finally evaporated and discharged as vapor from the cathode chambers of the fuel cell stack and the cathode supply. By discharging the moisture-laden cathode operating medium to the environment, the moisture content of the cathode side of the fuel cell system is reduced. The cathode operating medium is preferably outside air supplied to the fuel cell system and conditioned to a desired temperature and relative humidity. The mass flow of the cathode operating medium flowing through the cathode side is particularly preferably between 0.003 g / cell / s and 0.3 g / cell / s, preferably 0.04 g / cell / s.

During the period Δt2, the passage of the coolant channels with reduced coolant quantity and the flow through the cathode chambers is continued with a mass flow of the conditioned, that is to say to a desired temperature and a desired relative humidity, cathode operating medium. In addition, the anode spaces of the fuel cell stack are dried by flowing through a certain volume flow of the anode operating medium. The volume flow of the anode operating medium causes the uniform distribution of liquid product water and a flow-induced local pressure reduction in the anode chambers and the anode supply. As a result, the water is discharged from the anode side of the fuel cell system. The volume flow of the anode operating medium and the mass flow of the cathode operating medium are selected so that an overpressure of the anode compartments with respect to the cathode compartments is maintained during the period .DELTA.t2. Preferably, the volume flow of the coolant and the mass flow of the cathode operating medium from the period .DELTA.t1 are also maintained in the period .DELTA.t2. The volume flow rate of the anode operating medium flowing through the anode side in the period Δt1 is preferably between 0.001 l / min / cell and 2.7 l / min / cell, more preferably 1.45 l / min / cell. Further preferably, the overpressure of the anode chambers with respect to the cathode chambers in the periods Δt1 and Δt2 is between 3 mbar and 780 mbar, preferably 424 mbar.

Essential to the invention is the advantageous combination of the method steps of claim 1, with which an energetically favorable drying of a fuel cell system under exclusive use in common fuel cell systems already existing components is made possible. In this case, the degree of dryness of the fuel cell system or the remaining liquid water content of the fuel cell system is easily and selectively adjustable on the basis of a few variables. In addition, the anode side is kept free of oxygen-containing cathode operating medium during and preferably also after drying, which has a positive effect on the service life of the electrodes and counteracts air / air starts.

In a preferred embodiment of the method, the anode operating medium is selectively supplied to the anode chambers during a specific period .DELTA.t0 in order to supply them with the target pressure. Once the anode space operating medium has been supplied to the target pressure adjusting volume, it is particularly preferred to close it by closing a first actuating means disposed in an anode supply line and closing a second actuating means disposed in an anode exhaust gas line Anode spaces included. The closing of these actuating means preferably also takes place in the period .DELTA.t0. In a particularly preferred embodiment, the first and the second actuating means also remain closed for the periods .DELTA.t1 and .DELTA.t2, so that the anode spaces during the periods .DELTA.t1 and .DELTA.t2 form a largely fluid-tight closed volume, in particular with respect to the anode operating medium. Likewise particularly preferably, the anode operating medium supplied in the period Δt0 is circulated in this largely closed volume by means of a recirculation conveying device arranged in the anode supply for the period Δt2. Thus, the method according to the invention can be carried out almost exclusively with the anode operating medium fed in the period .DELTA.t0. Advantageously, this keeps the amount of fuel needed for drying as low as possible.

In detail, two different process routines for drying the anode operating medium are preferred. In a first preferred embodiment, the volume of the anode operating medium trapped in the anode compartments for the period .DELTA.t0 repeatedly flows through a water separator arranged in the anode supply during the circulation in the period .DELTA.t2. Thus, the liquid water loading of the anode operating medium is successively reduced. The liquid water is released from the water separator to the environment. In this embodiment, the first and second actuating means preferably remain closed even after the period Δt2 in order to maintain the overpressure on the anode side in the switched-off state of the fuel cell stack. Further preferably, the delivery of coolant and cathode operating medium is set at the expiration of the period Δt2. This embodiment allows drying of the fuel cell system and avoiding air / air start with a minimal amount of fuel.

An alternative embodiment of the method is particularly preferred above a certain limit of the liquid water content in the fuel cell system. In this case, the separation of liquid water in the water separator of the anode supply may not be sufficient. Preferably, the volume flow of the cathode operating medium is then prevented by the cathode compartments for a certain period .DELTA.t3. Thus, during this period .DELTA.t3, the recirculated and moisture-laden anode operating medium can be drained from the anode compartments without overpressurizing the anode compartments in the cathode compartments. Immediately after draining, anode operating medium is conveyed again into the anode chambers, in order to apply these again with a certain overpressure to the cathode chambers. Preferably, a first actuating means arranged in an anode supply line and a second actuating means arranged in the anode exhaust gas line are then closed in order to enclose the newly supplied anode operating medium in the anode chambers even during the switched-off state of the stack. The delivery of coolant and cathode operating medium is preferably set with expiration of the period Δt2. This embodiment allows a reliable drainage of the anode chambers and is particularly suitable for drying a fuel cell system after a start abort.

During the periods .DELTA.t1 and .DELTA.t2, maintaining an overpressure in the anode compartments with respect to the cathode compartments is essential for the drying process of the present invention. In order to ensure this overpressure throughout the duration of the process and to compensate for diffusion losses through the membranes as well as through leaks in the stack, in a preferred embodiment additional anode operating medium is fed into the anode supply even in the periods Δt1 and Δt2. For this purpose, the first and / or the second adjusting means are particularly preferably designed as a controllable non-return valve. Also preferably, the maintenance of an overpressure in the anode compartments with respect to the cathode compartments can take place by an adaptation of the mass flow of the cathode operating medium in the cathode supply.

In a likewise preferred embodiment, the mean free flow cross-section of the cathode supply is increased during the periods .DELTA.t1 and .DELTA.t2. The free flow cross-section is the cross-sectional area through which the cathode operating medium flows without resistance at any cross-section of the cathode supply. The mean free flow cross section is to be understood as the arithmetic mean over all free flow cross sections of the cathode supply. Particularly preferably, the mean free flow cross-section of the cathode supply is increased by all the adjustable components contained therein, such as throttle valves, bypass valves and expander, are opened as far as possible. Thereby, the pressure required to convey the particular mass flow of the cathode operating medium through the cathode supply is set as low as possible, thereby facilitating the maintenance of an overpressure in the anode compartments with respect to the cathode compartments.

The reduction of the volume flow from the coolant channels flowing through the coolant is preferably carried out continuously by the reduction of a coolant conveying device subsidized amount of cooling liquid in total or by reducing the amount of coolant flowing through a cooling device. In this case, the continuously conveyed coolant flow is particularly preferably reduced to a value between 0 l / min / cell and 0.15 l / min / cell. Alternatively, a defined volume of coolant is pumped discontinuously, which corresponds to the volume of the entirety of all coolant channels. A reduction in the volume flow of coolant is then achieved by waiting times between the pumping of the coolant volume. The waiting time between the discontinuously pumped volumes is preferably between 1 s and 60 s, more preferably 5 s. The cyclic pumping of the coolant is energetically cheaper than its continuous promotion.

The most important control variables of the method according to the invention are the lengths of the periods .DELTA.t1 and .DELTA.t2. In a preferred embodiment of the method according to the invention, the periods .DELTA.t1 and .DELTA.t2 are determined as a function of a metrologically determined liquid water content of the fuel cell system. The liquid water content of the fuel cell system can be determined directly by means of one or more liquid sensors, indirectly via an impedance measurement on one or more cells of the fuel cell stack. The drying process is carried out the longer the more liquid water is contained in the fuel cell system. Likewise preferably, the periods Δt1 and Δt2 are determined as a function of a determined temperature of the fuel cell stack or the environment. The drying process is carried out the longer the lower the measured temperatures are. The determination of the periods Δt1 and Δt2 as a function of at least one current state of the fuel cell system advantageously makes it possible to set a defined degree of dryness. After drying process according to the invention, the fuel cell system is thus advantageous in a defined state, from which a restart is easily possible.

The period Δt1 is preferably between 1 s and 300 s, more preferably 15 s. The period Δt2 is determined as a function of the time period Δt1 and is preferably between 1 s and 120 s, particularly preferably 14 s. The periods Δt0, Δt1, Δt2 and Δt3 preferably follow each other directly or indirectly in this order. In particular, the period .DELTA.t0 is upstream of the period .DELTA.t1 and the time .DELTA.t3 is downstream of the period .DELTA.t2, wherein these sections can also follow each other directly or indirectly. Particularly preferably, after or between the individual periods, a further period of time is reserved for at least one further functionality of the fuel cell system.

This period is preferably between 0.01 s and 300 s and more preferably between 1 s and 10 s and additionally protects against accelerated degradation of the stack, in particular the electrodes, and for setting a defined state of the fuel cell system.

In a likewise preferred embodiment of the drying method according to the invention, the periods Δt1 and Δt2 are determined as a function of an operating time or an operating state of the fuel cell system. For a fuel cell system, three operating states are distinguished in this context, namely startup / startup, continuous operation and shutdown. In this case, a fuel cell can be switched off both from continuous operation and during startup / startup. In most cases, the liquid water content of a fuel cell located at startup will be higher than that of a continuously operated fuel cell. The operating time usually correlates with the operating state or allows conclusions to be drawn about it.

The invention likewise provides a method for switching off a fuel cell system, comprising the method steps: initializing a switch-off process by disconnecting a main electrical load from the fuel cell stack and performing a method for drying the fuel cell system, as described above, until a defined liquid water content of the fuel cell system is reached. The shutdown of a fuel cell system is regularly initiated by disconnecting the fuel cell stack from its main electrical load, for example, when switching off the ignition of a vehicle. Due to the separation of the main electrical load less and preferably no cathode and anode operating medium are consumed in the stack more. After separating the stack from its main load, the fuel cell system is dried as described above until a defined liquid water content of the fuel cell system is reached. The fuel cell stack is preferably used directly or indirectly for providing the electrical power necessary for the drying process, for example for the operation of electrically operated conveyors, adjusting means and / or control devices. At the end of the drying process, the fuel cell system is advantageously present in a defined drying state or with a defined liquid water content. This thus advantageously allows the setting of a defined shutdown state of the fuel cell system and thus a reliable restart of the same.

Likewise provided by the invention is a fuel cell system comprising a fuel cell stack with cathode compartments, anode compartments and coolant channels; a cathode supply having a cathode supply path with a compressor disposed therein and a cathode exhaust path; an anode supply comprising an anode supply path having a first actuator, an anode exhaust path having a second actuator, and a recirculation line connecting the anode exhaust path to the anode supply path and having a recirculation conveyor disposed therein; a coolant supply, comprising a coolant delivery device for supplying a coolant into the coolant channels; and a controller configured to perform a method of drying a fuel cell system according to the invention as described above.

The fuel cell system preferably has sensors for determining a liquid water content of the fuel cell system, a temperature of the fuel cell system, a temperature of the environment, an electrical load, power, voltage or current strength of the fuel cell stack and / or an operating period of the fuel cell stack. The recirculation conveyor is preferably designed as a recirculation fan. The first and / or second adjusting means are preferably designed as metering valves or controllable non-return valves, which allow readjustment of the pressure of the anode operating medium in the anode chambers.

Further preferred embodiments of the invention will become apparent from the remaining, mentioned in the dependent claims characteristics.

The various embodiments of the invention mentioned in this application are, unless otherwise stated in the individual case, advantageously combinable with each other.

The invention will be explained below in embodiments with reference to the accompanying drawings, without being limited thereto. Show it:

1 a block diagram of a fuel cell system according to a preferred embodiment;

2 a schematic representation of state variables of the fuel cell system according to 1 during a first embodiment of a method of drying the same;

3 a block diagram of a fuel cell system according to a second preferred embodiment; and

4 a schematic representation of state variables of the fuel cell system according to 3 during a second embodiment of a method for the same.

1 and 3 show a total of 100 designated fuel cell systems according to preferred embodiments of the present invention. The fuel cell systems 100 are each part of a vehicle, not shown in detail, in particular an electric vehicle having an electric traction motor by the respective fuel cell system 100 is supplied with electrical energy.

The fuel cell systems 100 comprise as a core component a fuel cell stack 10 containing a plurality of stacked single cells 11 having alternately stacked membrane-electrode assemblies (MEAs) 14 and bipolar plates 15 be formed (see detail). Every single cell 11 thus includes one MEA each 14 , which has an ion-conducting polymer electrolyte membrane, not shown here, as well as on both sides arranged thereon catalytic electrodes, namely an anode and a cathode, which catalyze the respective partial reaction of the fuel cell reaction and in particular can be formed as coatings on the membrane. The anode and cathode electrodes comprise a catalytic material, such as platinum, supported on an electrically conductive high surface area support material, such as a carbon based material. Between a bipolar plate 15 and the anode thus becomes an anode compartment 12 formed and between the cathode and the next bipolar plate 15 the cathode compartment 13 , The bipolar plates 15 serve to supply the operating media in the anode and cathode rooms 12 . 13 and further provide the electrical connection between the individual fuel cells 11 ago. Optionally, gas diffusion layers may be interposed between the membrane-electrode assemblies 14 and the bipolar plates 15 be arranged.

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

The anode supply 20 in the 1 and 3 shown fuel cell systems 100 each includes an anode supply path 21 which feeds an anode operating medium (the fuel), for example hydrogen, into the anode spaces 12 of the respective fuel cell stack 10 serves. For this purpose, connect the anode supply paths 21 one each fuel storage 23 with an anode inlet of the respective fuel cell stack 10 , The anode supply 20 further includes an anode exhaust path 22 containing the anode exhaust gas from the anode chambers 12 via an anode outlet of the respective fuel cell stack 10 dissipates. The anode operating pressure on the anode sides 12 of the respective fuel cell stack 10 is about a first actuating means 24 in the anode supply path 21 adjustable. In addition, the anode supply points 20 in the 1 and 3 shown fuel cell systems as shown a recirculation line 25 on which the anode exhaust path 22 with the anode supply path 21 combines. The recirculation of fuel is common in order to return and utilize the fuel, which is mostly used in excess of stoichiometry, in the stack. In the recirculation line is in each case a recirculation conveyor 27 , preferably a recirculation blower, arranged. Further, in the anode exhaust path 22 one water separator each 28 Installed to get out of the fuel cell stack 10 Condensed liquid water to condense and dissipate.

In the anode exhaust gas line 22 of in 1 shown fuel cell system 100 is downstream of the recirculation line 25 a second actuating means 26 arranged. With the second actuating means 26 a recirculation circuit can be isolated from the environment. The first and second actuating means 24 . 26 can be used in common, an outflow of the anode operating medium from the anode chambers 12 largely to prevent. Further, downstream of the fuel cell stack 10 and upstream of the second actuator 26 a water separator 28 in the recirculation circuit of the cathode exhaust gas line 22 arranged.

In the in 3 shown fuel cell system 100 is downstream of the fuel cell stack 10 and upstream of the recirculation line 25 a three-way valve as a second actuating means 26 arranged. The anode exhaust gas line 22 this fuel cell system 100 is as a purge line 22 to the cathode exhaust path 32 educated. The second adjusting agent 26 can also be used here, together with the first actuating means 25 a recirculation circuit comprising a recirculation conveyor 27 and a water separator 28 to separate from the environment. In another position of the three-way valve 26 For example, the anode operating medium may be removed from the anode compartments 12 via the cathode exhaust gas line 32 be released to the environment.

The cathode supply 30 in the 1 and 3 shown fuel cell systems 100 each includes a cathode supply path 31 which is the cathode spaces 13 of the respective fuel cell stack 10 supplying an oxygen-containing cathode operating medium, in particular air which is drawn in from the environment. The cathode supply 30 further includes a cathode exhaust path 32 , which the cathode exhaust gas (in particular the exhaust air) from the cathode compartments 13 of the respective fuel cell stack 10 dissipates and optionally this feeds an exhaust system, not shown. For conveying and compressing the cathode operating medium is in the cathode supply path 31 a compressor 33 arranged. In the illustrated embodiments, the compressor 33 each as a mainly electric motor driven compressor 33 designed, the drive via a with a corresponding power electronics 35 equipped electric motor 34 he follows. The compressor 33 may also be through a in the cathode exhaust path 32 arranged turbine 36 (optionally with variable turbine geometry) are supported by a common shaft (not shown) driven.

The in the 1 and 3 shown fuel cell systems 100 also have a humidifier module 39 on. The humidifier modules 39 are on the one hand in each case in the cathode supply path 31 arranged to be flowed through by the cathode operating gas. On the other hand, they are so in the cathode exhaust path 32 arranged so that they can be flowed through by the cathode exhaust gas. A humidifier 39 typically has a plurality of water vapor permeable membranes formed either flat or in the form of hollow fibers. In this case, one side of the membranes is overflowed by the comparatively dry cathode operating gas (air) and the other side by the comparatively moist cathode exhaust gas (exhaust gas). Driven by the higher partial pressure of water vapor in the cathode exhaust gas, there is a transfer of water vapor across the membrane in the cathode operating gas, which is moistened in this way.

The cathode supply 30 according to the in 1 illustrated embodiment further comprises a bypass line 37 on which the cathode supply line 31 with the cathode supply line 31 so that connects the humidifier module 39 upstream of the fuel cell stack 10 is not flowed through. The cathode supply 30 according to the in 3 illustrated embodiment further comprises a bypass line 37 on which the cathode exhaust gas line 32 with the cathode exhaust gas line 32 so that connects the humidifier module 39 downstream of the fuel cell stack 10 is not flowed through. One each in the bypass line 37 arranged adjusting means 38 is used to control the amount of the humidifier 39 immediate cathode operating medium.

For cooling the fuel cell stack 10 have the in the 1 and 3 shown fuel cell systems 100 also a coolant circuit 40 on. This is outside the respective fuel cell stack 10 through a conduit carrying a coolant 42 formed with the coolant inlet openings and coolant outlet openings of the respective fuel cell stack 10 connected is. In the fuel cell stack are between the coolant inlet openings and coolant outlet openings coolant channels in the bipolar plates 15 arranged. For conveying the coolant through the coolant line 42 and the coolant channels of the fuel cell stack 10 is in the coolant circuit 40 a coolant conveyor 41 arranged. Coolant in the context of this application is a coolant whose temperature is reduced by cooling. For cooling, an unillustrated vehicle radiator is provided here as a cooling device, wherein a controllable thermostatic valve is arranged between the fuel cell stack and the radiator upstream of the radiator and downstream of the fuel cell stack.

All adjusting means 24 . 26 . 38 the fuel cell systems 100 can be designed as controllable or non-controllable valves or flaps. Corresponding further actuating means can be in the lines 21 . 22 . 31 and 32 be arranged to the fuel cell stack 10 isolate from the environment.

In the 2 are state variables of the fuel cell system 100 according to 1 during a first embodiment of a method of drying the same. In particular, the temperature T of the fuel cell stack 10 and the volume flow V. _K of the stack 10 promoted coolant shown. Furthermore, the pressure p _Ka in the cathode compartments 13 and the mass flow m. _{Ka of} the cathode operating medium through the cathode compartments 13 shown. Also shown are the pressure p _A in the anode compartments 12 and the volume flow V. _{A of} the anode operating medium through the anode chambers 12 ,

Like in the 2 shown take during a startup phase of the fuel cell system 100 or at startup of the fuel cell stack 10 the mass flow m. _{Ka of} the cathode operating medium through the cathode compartments 13 and the volume flow V. _{A of} the anode operating medium through the anode chambers 12 continuously too. This increases the pressure p _Ka in the cathode chambers 13 and the pressure p _A in the anode compartments 12 globally. Due to the onset of reactions in the stack 10 its temperature T increases, which is due to an increase in the coolant volume flow V. _{K is} accompanied.

During the continuous operation of the fuel cell system 100 are the currents m. _Ka , V. _A and V. _K , the pressures p _Ka and p _A and the temperature T largely constant. The fuel cell stack 10 is with an overpressure of the anode chambers 12 opposite the cathode compartments 13 operated by 200 mbar. During continuous operation, the fuel cell stack is 10 heated and there is little liquid water in it.

By disconnecting the fuel cell stack 10 from the main electric load by switching off ignition of the fuel cell system 100 equipped vehicle, the reaction reactions of anode and cathode operating medium come to a large halt and the inventive method for drying the fuel cell system 100 Is initiated. Based on the service life of the fuel cell system 100 determines a controller (not shown) that the stack 10 was in continuous operation. Based on this information and a measured temperature of the fuel cell stack 10 and the environment, the control unit determines by means of a value table a period Δt1 of 15 s and derived therefrom a period Δt2 of 4 s

Immediately after the initiation of the drying process, that is, at the beginning of the period .DELTA.t1, the volume flow is V. _K of continuously through the stack 10 reduced coolant to a minimum of 0.018 l / min / cell, this value being chosen so that overheating of the membranes 14 is just avoided. The reduced volume flow V. _K is maintained for the periods .DELTA.t1 and .DELTA.t2, so that the temperature T of the fuel cell stack 10 only slowly decreases, especially in comparison to the gradient of heating of the stack 10 during the starting phase. The reduction of the coolant volume flow can take place in this case by reducing the volume flow of the fuel cell stack flowing through the cooling medium by means of the coolant conveying device or by reducing the volumetric flow of the coolant flowing through the vehicle radiator by means of the thermostatic valve.

At the same time to reduce the volume flow V. _{K of} the coolant is at the beginning of the period .DELTA.t1 in the anode supply 20 first the second actuating means 26 and then the first actuating means 24 closed and thereby an overpressure p _A - p _Ka of 424 mbar in the anode chambers 12 opposite the cathode compartments 13 set. As the anode chambers 12 no new anode operating medium from the memory 23 is supplied, the volume flow V decreases. _A at the beginning of the period .DELTA.t1 to a minimum, substantially to zero, from. This condition of anode rooms 12 is maintained for the entire period Δt1.

In the cathode supply 30 At the beginning of the period .DELTA.t1 all adjusting means are adjusted so that the mean free flow cross-section of the cathode support 30 increased. In particular, the control means becomes 38 the bypass line 37 opened to the humidifier 39 to circumvent and thus reduce the flow resistance for the cathode operating medium and to prevent its humidification. By setting all the actuating means, the pressure p _Ka in the cathode chambers 13 despite the promotion of a reduced mass flow m. _{Ka of} the cathode operating medium of 0.042 g / s / cell through the cathode compartments 13 can be significantly reduced, which is the maintenance of the overpressure p _A - p _Ka of 424 mbar in the anode chambers 12 ensures. The mass flow m. _{Ka of} the cathode operating medium dries the cathode compartments 13 of the pile 10 while the overpressure in the anode chambers 12 a transfer of liquid water there greatly reduced or prevented. The vapor-loaded cathode operating medium is via the exhaust pipe 32 delivered to the environment. The mass flow m. _Ka and the pressure p _Ka are maintained for the entire periods Δt1 and Δt2.

At the beginning of the period Δt2, the recirculation conveyor 27 on the anode side 20 put into operation while the adjusting means 24 . 26 stay closed. This will already do so in the anode chambers 12 Anode operating medium with a volume flow of 1.45 l / min / cell circulated through it, resulting in a global increase in pressure in the anode chambers 12 leads. The circulated anode operating medium causes liquid water to escape from the anode compartments 12 discharged and in the water separator 28 deposited. Thus, in the period .DELTA.t2 also a drying of the anode chambers takes place 12 , wherein an overpressure p _A - p _{Ka of} the anode spaces 12 630 mbar.

At the end of the drying process according to the invention at the end of the period .DELTA.t2 is the fuel cell system 100 with a defined liquid water content. The compressor 33 , the coolant conveyor 41 and the recirculation fan 27 are turned off and the adjusting means 24 . 26 kept closed. The system 100 thus lies with a defined state and with an overpressure in the anode chambers 12 in the off state before.

In the 4 are state variables of the fuel cell system 100 according to 3 during a second embodiment of a method of drying the same. The state variables shown correspond to those of 2 and show during a starting phase with reference to 2 Behavior described above.

In the in 4 As shown embodiment, the drying method according to the invention already during the start-up phase of the fuel cell system 100 initiated. The separation of the pile 10 from an electric main load takes place in particular at the beginning of a period .DELTA.t0.

During this period .DELTA.t0 is also the flow rate V. _K of the stack 10 reduced coolant to zero. Also the volume flow V. _{A of} the anode operating medium through the anode chambers 12 is reduced to zero during the period Δt0, while first the second actuating means 26 to the purge line 22 and then the first actuating means 24 be closed to a certain pressure p _A in the Anodenbetriebsmedium filled anode spaces 12 adjust. Also during the period Δt0, all adjusting means in the cathode supply 30 set to increase the mean free flow area as described above, and becomes the compressor 33 so controlled that a certain mass flow m. _{Ka of} the cathode operating medium through the cathode compartments 13 is encouraged. Because of all these steps, there is an overpressure p _A -p _Ka of 215 mbar at the end of the period Δt0.

During the period Δt1, all state variables of the anode spaces become 12 and the cathode rooms 13 maintained. That is, there is a drying of the cathode compartments 13 while the volumetric flow in the anode compartments 12 continues to be zero. After the expiration of a waiting period of 3 s from the end of the period Δt0, a volume of the coolant, which corresponds to the sum of the volumes of all the coolant channels, is introduced into the fuel cell stack during a period of 5 s 10 pumped, causing an exchange of the in the stack 10 located coolant takes place.

At the beginning of the period Δt2, the recirculation fan 27 in the anode supply 20 activated and by circulating the in the anode chambers 12 located anode flow medium generates a volume flow, as described above. This results in a global increase in pressure in the anode compartments compared to the period Δt1 12 , for setting an overpressure p _A - p _Ka of 424 mbar and for drying the anode chambers 12 as described above. In addition, in the period Δt2 after a waiting time of 4 s after the end of the last operation of the coolant conveyor 41 again pumped coolant.

At the end of the period .DELTA.t2 or at the beginning of the period .DELTA.t3, the compressor 33 and the coolant delivery device 41 deactivated and that in the cathode supply 30 discharged cathode operating medium to the environment. As a result, the pressure in the cathode supply drops 30 to zero. Also at the beginning of the period .DELTA.t3 in the anode supply is the actuating means 26 placed so that the previously circulated and moisture-laden anode operating medium via the purge line 22 and the cathode exhaust gas line 32 delivered to the environment. This also drops the pressure in the anode chambers 12 briefly to zero. Subsequently, the adjusting agent 26 completely closed and becomes the actuating means 24 briefly open to new anode operating medium in the anode chambers 12 feed again until an overpressure p _A - p _Ka of 424 mbar is set again. The system 100 is thus at the end of the period .DELTA.t3 in a defined state and with an overpressure in the anode chambers 12 off.

LIST OF REFERENCE NUMBERS

100

The fuel cell system

10

fuel cell stack

11

single cell

12

anode chamber

13

cathode space

14

Membrane electrode assembly (MEA)

15

Bipolar plate (separator plate, flow field plate)

20

anode supply

21

Anode supply line

22

Anode exhaust gas line

23

fuel tank

24

First actuating means

25

recirculation

26

Second actuating means

27

recirculation conveyor

28

water

30

cathode supply

31

Cathode supply line

32

Cathode exhaust gas line

33

compressor

34

electric motor

35

power electronics

36

turbine

37

Bypass line

38

actuating means

39

humidifier

40

Coolant circuit

41

Coolant delivery device

42

Coolant line

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 102011007615 A1 [0008]
• US 8318364 B2 [0008]
• WO 2006/030614 A2 [0009]
• JP 2012-074385A [0009]
• US 8192885 B2 [0010]
• DE 102012023799 A1 [0011]
• JP 2008103120A [0011]
• DE 10061687 B4 [0011]

Claims (10)

Method for drying a fuel cell system ( 100 ) comprising a fuel cell stack ( 10 ) with cathode spaces ( 13 ), Anode spaces ( 12 ) and coolant channels, the method comprising the steps of: (a) reducing a volume flow of a coolant flowing through the coolant channels for certain periods Δt1 and Δt2; (b) flow through the cathode compartments ( 13 ) with a certain mass flow of a cathode operating medium for the specific periods Δt1 and Δt2; (c) preventing a volume flow of an anode operating medium through the anode spaces ( 12 ) for the specific period Δt1; (d) flow through the anode chambers ( 12 ) with a certain volume flow of the anode operating medium for the specific period .DELTA.t2; and (e) maintaining a certain overpressure in the anode compartments ( 12 ) opposite the cathode compartments ( 13 ) for the specific periods Δt1 and Δt2. The method of claim 1, further comprising the method steps: - supplying the anode operating medium into the anode chambers ( 12 ) during a certain time interval Δt0 preceding the period Δt1 for charging the anode compartments ( 12 ) with the specific overpressure; and at least one of the following method steps: closing one in an anode supply line ( 21 ) arranged first actuating means ( 24 ) and one in an anode exhaust line ( 22 ) arranged second actuating means ( 26 ) during the period Δt0 and for the periods Δt1 and Δt2; and - circulating the anode operating medium supplied in the period Δt0 by means of a in the anode supply ( 20 ) arranged recirculation conveyor ( 27 ) for the specific period Δt2. The method of claim 1 or 2, further comprising the method steps: - Preventing a volume flow of the cathode operating medium through the cathode chambers ( 13 ) for a certain period of time Δt3, which is subsequent to the period Δt2; - Draining the circulated anode operating medium from the anode chambers ( 12 ) during the determined period Δt3; and - supplying anode operating medium into the anode chambers ( 12 ) for charging the anode chambers ( 12 ) with a certain overpressure to the cathode spaces ( 13 ) during the determined period Δt3 and closing one in an anode supply line ( 21 ) arranged first actuating means ( 24 ) and one in an anode exhaust line ( 22 ) arranged second actuating means ( 26 ). Method according to one of the preceding claims, characterized in that the determined volume flow of the anode operating medium in the period Δt2 one in the anode supply ( 20 ) arranged water separator ( 28 ) flows through. Method according to one of the preceding claims, characterized in that the overpressure in the anode spaces ( 12 ) opposite the cathode compartments ( 13 ) during the particular periods Δt1 and Δt2 by feeding additional anode operating medium into the anode supply ( 20 ) and / or by adaptation of the mass flow of the cathode operating medium in the cathode supply ( 30 ) is maintained. Method according to one of the preceding claims, characterized in that the mean free flow cross-section of the cathode supply ( 30 ) is increased during the periods Δt1 and Δt2. Method according to one of the preceding claims, characterized in that a volume of all coolant channels corresponding volume of coolant during the periods .DELTA.t1 and .DELTA.t2 cyclically into the fuel cell stack ( 10 ) is pumped. Method according to one of the preceding claims, characterized in that the periods Δt1 and Δt2 in dependence of a determined liquid water content of the fuel cell system ( 100 ), a determined temperature of the fuel cell stack ( 10 ), a determined ambient temperature, an operating time of the fuel cell system ( 100 ) and / or an operating state of the fuel cell system ( 100 ). Method for switching off a fuel cell system ( 100 ) comprising a fuel cell stack ( 10 ) with cathode spaces ( 13 ), Anode spaces ( 12 ) and coolant channels, the method comprising the steps of: - initializing a shutdown process by disconnecting a main electrical load from the fuel cell stack; and - performing the method for drying the fuel cell system according to one of claims 1 to 8 until reaching a defined liquid water content of the fuel cell system. Fuel cell system ( 100 ), comprising - a fuel cell stack ( 10 ) with cathode spaces ( 13 ), Anode spaces ( 12 ) and coolant channels; A cathode supply ( 30 ), comprising a cathode supply path ( 31 ) with a compressor arranged therein ( 33 ) and a cathode exhaust path ( 32 ); An anode supply ( 20 ), comprising an anode supply path ( 21 ) with a first actuating means ( 24 ), an anode exhaust path ( 22 ) with a second actuating means ( 26 ) and an anode exhaust path ( 22 ) with the anode supply path ( 21 ) connecting recirculation line ( 25 ) with a recirculation conveyor device ( 27 ); A coolant supply ( 40 ), comprising a coolant delivery device ( 41 ) for supplying a coolant into the coolant channels; and a controller adapted to perform a method according to any one of claims 1 to 8.

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