DE102015213913A1 - Method and system for discharging anode exhaust gas of a fuel cell - Google Patents

Abstract

The technology disclosed herein relates to a method of operating at least one fuel cell. An anode exhaust gas to be discharged from an anode subsystem and water to be separated from the anode subsystem are separately withdrawn from the anode subsystem. The anode exhaust gas is fed to the cathode feed and / or the cathode exhaust gas. The water for humidification is injected into the cathode feed air.

Description

The technology disclosed herein relates to a method and system for venting anode exhaust gas of a fuel cell.

Fuel cell systems as such are known. For the fuel supply of fuel cell systems, for example, hydrogen is supplied to an anode input side of a fuel cell. To improve the hydrogen distribution within the fuel cell, a gas fraction is pumped from the anode outlet to the anode inlet via a recirculation pump. Alternatively, it is also possible to use a Venturi nozzle which generates a negative pressure on the anode outlet side of the fuel cell, so that a gas recirculation is also achieved in this way. During system start-up but also during operation nitrogen and liquid water accumulations occur in the anode subsystem. Both media, nitrogen and liquid water, must be taken from a certain amount of the anode circuit, which is referred to as purge process / purge process (for gases) or drain process / drainage process (for liquid water). To increase the H2 concentration of the resulting pressure drop is compensated by the addition of hydrogen. Venting / dewatering reduces the amount of nitrogen and liquid water in the anode subsystem and increases the concentration of hydrogen. Nitrogen and liquid water are introduced into an exhaust pipe of the fuel cell system. It must be ensured that there is sufficient cathode exhaust gas for dilution. Otherwise, an undesirably high concentration of hydrogen in the exhaust gas could occur.

In order to quickly bring the fuel cell system up to operating temperature during a cold start, the fuel cell system can be operated less efficiently at low temperatures. Especially during the cold start of the fuel cell system, therefore, the fresh air supply and thus also the exhaust gas mass flow is limited in order to achieve the shortest possible warm-up time of the fuel cell system. During the cold start, there may not be enough dilution air for the anode venting processes so that the maximum hydrogen concentration in the system exhaust gas could be exceeded, unless additional measures were provided.

The DE 10 2004 055 158 A1 shows a fuel cell system with a fuel cell stack. The anode exhaust gas is supplied to the cathode side. The anode-side exhaust pipe opens on the suction side of the compressor in a supply line of the cathode side. Further, the DE 10115336 A1 a fuel cell system having an anode exhaust gas recirculation to the cathode exhaust, and another pipe leading to an exhaust gas mixing device provided in the cathode exhaust gas line downstream of the cathode outlet. Also here water separators are disclosed, which separate water from the anode exhaust gas and the cathode exhaust, that can serve for humidification of the fuel cells supplied gases. From the WO 2008/052578 A1 a fuel cell system is known in which a purge line branches off from a recirculation circuit. The purge line serves both to remove water and to remove anode-side fuel cell exhaust gas. Furthermore, a change-over switch is provided which selectively applies the purge line to the cathode-side input or the cathode-side output.

It is an object of the technology disclosed herein to reduce or eliminate the disadvantages of the prior art solutions. Other objects arise from the beneficial effects of the technology disclosed herein. The object (s) is / are solved by the subject matter of claim 1. The dependent claims are preferred embodiments.

The technology disclosed herein relates to a method of operating at least one fuel cell of a fuel cell system. Such a fuel cell system comprises at least one fuel cell.

The fuel cell system is intended for example for mobile applications such as motor vehicles. In its simplest form, a fuel cell is an electrochemical energy converter that converts fuel and oxidant into reaction products, producing electricity and heat. The fuel cell includes an anode and a cathode separated by an ion selective separator. The anode has a supply for a fuel to the anode. Preferred fuels are: hydrogen, low molecular weight alcohol, biofuels, or liquefied natural gas. The cathode has, for example, a supply of oxidizing agent. Preferred oxidizing agents are, for example, air, oxygen and peroxides. The ion-selective separator can be designed, for example, as a proton exchange membrane (PEM). Preferably, a cation-selective polymer electrolyte membrane is used. Materials for such a membrane are, for example: Nafion ^®, Flemion ^® and Aciplex ^®. A fuel cell system comprises at least one fuel cell and peripheral system components (BOP components) that can be used during operation of the at least one fuel cell. As a rule, several fuel cells are combined to form a fuel cell stack or stack.

According to the method disclosed here, an anode exhaust gas to be discharged from the fuel cell is at least temporarily supplied exclusively to the cathode of the fuel cell during a first period of time. The anode exhaust gas is the gas mixture leaving the anode of the fuel cell. The anode exhaust gas to be discharged is the part of the exhaust gas that is not recirculated. In particular, the anode exhaust gas to be vented is the part of the anode exhaust gas which is taken from the anode subsystem via the vent valve or purge valve. By purging processes, this part of the anode exhaust gas, essentially nitrogen, water vapor and hydrogen, is taken from the anode exhaust gas. Recirculation in this context refers to the recycling of the anode exhaust gas into the anode feed air or the anode inlet. At times, during the venting operations, the exhaust gas to be vented can preferably be drained completely into the cathode.

Preferably, the first time period is the warm-up phase of the fuel cell. The warm-up phase is the phase in which the fuel cell system of the motor vehicle is heated to the (optimal) operating temperature. The warm-up begins i. d. R. with the activation of the fuel cell system and ends with the reaching of the operating temperature, from which the driving of the motor vehicle from the vehicle or a control is allowed. Particularly preferably, the warm-up phase of the fuel cell is activated before the ignition key or the starter button is actuated. For example, the warm-up phase can be initiated by a radio signal or by a timer. In one embodiment, the driver may, for example, start the warming up of the fuel cell system via an appropriate software of a mobile telephone. Alternatively, the warm-up phase may begin with the signal to unlock the central lock. Particularly preferably, a first period of time may be a cold or frost start phase of the fuel cell. When operating fuel cells in particular, the cold start (start at an ambient temperature of 0 ° C to 25 ° C) and the frost start (start at an ambient temperature below 0 ° C) is problematic. For this reason, there is a need to bring the fuel cell system as soon as possible to an operating temperature at which the system has a better efficiency. According to the technology disclosed herein, the anode exhaust gas to be discharged is at least partially admixed with the cathode exhaust gas during a second period of time. The cathode exhaust gas is the gas leaving the cathode. The second period of time is preferably an operating phase of the fuel cell following the warm-up phase of the fuel cell. The operating phase occurs after the fuel cell has reached its operating temperature. The operating temperature may be, for example, a predetermined temperature value. Preferably, the anode exhaust gas is fed exclusively to the cathode when the mass flow of air, which is the cathode to and / or discharged from this, below a threshold value. In particular, the limit value for the mass flow of air can be selected such that no critical hydrogen concentration in the cathode exhaust gas occurs. Further preferably, the anode exhaust gas is supplied exclusively to the cathode when the hydrogen concentration in at least a portion of the cathode exhaust gas is above a hydrogen limit. Instead of measuring the mass flow, therefore, the hydrogen concentration in the cathode exhaust gas could also be used directly. Sometimes it can be determined by other parameters than the fuel cell temperature, whether the anode exhaust gas to be discharged from the cathode (first period) or the cathode exhaust gas (second period) is supplied.

Preferably, water contained in the anode exhaust gas may be separated from the anode exhaust gas (drain, drainage). Preferably, the water is added to the cathode exhaust gas. More preferably, liquid water is added to the cathode exhaust gas during the warm-up of the fuel cell. Thus, advantageously, the risk can be reduced that liquid water accumulates in the fuel cell stack and disturbs a uniform air supply across all cells. Furthermore, it can be provided that at higher operating temperatures (> 80 ° C.), the liquid water of the anode side is used for moistening the cathode feed air.

The technology disclosed herein further includes a fuel cell system having at least one fuel cell. At least one switching means is provided on the anode side of the fuel cell system. The switching means is configured to supply an anode exhaust gas of the fuel cell to be discharged at least temporarily only to a cathode of the fuel cell during a first period of time, and to at least partially mix the anode exhaust gas to be discharged with the cathode exhaust gas during a second period of time. The anode side, also called the anode subsystem, is the anode fluid circuit which, inter alia, comprises the anode inlet leading to the anode inlet, the anode outlet leading away from the anode outlet, and the recirculation line.

The technology disclosed here further comprises a method for operating at least one fuel cell, after which an anode exhaust gas to be discharged from an anode subsystem and water to be separated from the anode subsystem are removed separately, in particular offset in time, from the anode subsystem. Appropriately, the water is first drained and then the Anode exhaust gas. Another order would also be conceivable. Advantageously, the anode exhaust gas is supplied to the cathode feed air and / or the cathode exhaust gas. Furthermore, the water can be injected for humidification in the Kathodenzuluft.

The method may include the step of injecting the water and / or the anode exhaust gas upstream of a heat exchanger into the cathode feed. The increased air temperature of the compressed air (eg up to 200 ° C) can be used to evaporate the injected water. Injecting water increases the evaporation surface and also improves evaporation. Preferably, the water is injected adjacent to the oxidizer conveyor. Here the temperature of the cathode feed is highest.

The method may comprise the step of injecting the water and / or the anode off-gas into the cathode feed by means of a jet pump. Preferably, the at least one water injection device is thus designed as a jet pump. Jet pumps as such are known. Essentially, they represent a twisted prandl pitot tube, which, for example, can suck in water itself after the Venturi effect. Such jet pumps are also referred to as ejectors or jet pumps. The driving medium used here can be the oxidizing agent flowing past the jet pump, which expediently causes the jet pump to draw in water and / or anode exhaust gas. Such a configuration is particularly advantageous, since the jet pump can be configured such that it can do without external (eg electrical or pneumatic) energy.

It is particularly inexpensive and robust and also takes up little space to complete.

The method may include the step of removing the anode exhaust gas to be discharged and the water to be separated by a water separator. This has the advantage that only one component for flushing and dewatering is installed in the recirculation circuit.

The method may include the step of passing the separated water (hereinafter for simplicity only the term "water") in a drain line downstream of the water separator. Further, a water receptacle may store the water that is fluidly connected to the drainage line. The stored water can then be injected for humidification in the Kathodenzuluft.

The method may comprise the step of disposing the water separator and the water receiver, and optionally also any connecting pipes, so that the water can flow or flow by gravity into the water receiver.

The method may include the step of providing, in particular downstream of the water separator, at least one switching means which alters the flow of water or anode exhaust gas, for example the flow through various conduits downstream of the water separator.

The method may include the step of having the at least one switching means comprise a plurality of valves that may be disposed in different lines. The anode exhaust gas can be separated from the water by the actuation of the valves and / or by gravity due to the arrangement of the lines.

The at least one switching means may comprise at least one drain valve or drain valve. The method may include the step of having the drain valve downstream of the water receptacle and adjacent to the location of the water injection alter the supply of water for injection. This has the advantage that the injection can be controlled particularly well by the valve. The functions of water injection and dewatering of the anode subsystem are separated here and realized with only one drain valve, which could also be referred to as an injection valve. Alternatively or additionally, the or a further drain valve may be provided upstream of the water storage container. For example, if no gravimetric separation of water and anode exhaust gas is provided.

The method may include the step of passing the water into the cathode exhaust gas when the water receptacle is full. This can be provided, in particular during the warm-up phase, when the cathode supply air can still absorb comparatively little water. Thus, an excessive water entry into the cathode could be avoided.

The technology disclosed herein will now be explained with reference to the figures. It shows the 1 to 4 schematically the structure of a fuel cell system for carrying out the method described here.

1 shows a fuel cell system according to the technology disclosed herein. The fuel cell system here comprises several fuel cells 100 which are merged into a fuel cell stack. On the anode side is a recirculation circuit 300 shown. As fuel is here Used hydrogen. The hydrogen flows through an anode feed line 310 into an anode inlet 134 the anode of the fuel cell 100 one. In the fuel cell 100 The previously known electrochemical reactions take place before the gas passes through the anode or anode compartment through the anode outlet 132 leaves again. Downstream of the anode outlet 132 is in the anode lead 312 a water cutter 330 arranged in which the proportion of water is separated from the anode exhaust gas. The recirculation pump 320 promotes the anode exhaust gas back into the anode feed line 310 (Recirculation). Instead of a pump 320 could also be a venturi 320 be provided.

Downstream of the water separator 330 here is a switching device 340 . 350 intended. This switching means 340 . 350 here includes two controllable valves 340 . 350 , The first switching valve 340 opens and closes a first connection line 342 , the upstream of the cathode in the cathode 210 empties. The second switching valve 350 opens and closes a second connection line 352 , which is downstream of the cathode in the cathode exhaust gas 250 empties. The first connection line 242 opens here in the Kathodenzuleitung. But you could also directly at the cathode inlet 122 lead. Further, the mouth could 344 also upstream of the heat exchanger 230 or the oxidizer promoter 220 the cathode feed into the cathode feed 210 lead. The estuary 354 the second connection line 352 ends in the cathode discharge 212 , During a purge process, the anode exhaust gas first flows through the second connection line 352 before it is finally mixed downstream of the cathode of the cathode exhaust air. A device for admixture is for example in the DE 10 2013 218 958 A1 which is hereby incorporated by reference. Not shown is the controller that the fuel cell system, in particular the first and second switching means 340 . 350 , controls. A device for mixing cathode exhaust gas and anode exhaust gas may also be provided in the cathode exhaust gas line. The mixture 410 From cathode exhaust and anode exhaust gas is then released by a suitable exhaust system into the environment. Also not shown are possibly provided sensors for determining any control variables such as temperature, air mass, and / or hydrogen concentration.

Suitably, during the warm-up phase (but also during the operating phase) of the fuel cell 100 the hydrogen concentration in the cathode exhaust gas or the mass air flow are measured. If the hydrogen concentration rises above a limit value or if the air mass flow drops below a limiting value, then the first switching valve could 340 opened and the second switching valve 350 getting closed. The anode exhaust gas then flows through the first connection line 342 and flows at the mouth 344 into the cathode feed line. The hydrogen reacts at the catalytic surfaces of the cathode. The hydrogen concentration in the cathode exhaust air can thus be reduced.

The reaction on the surfaces of the cathode releases heat which helps the fuel cell to reach operating temperature quickly. During a warm-up phase, the fuel cell is preferred 100 about 5% to about 30%, and more preferably about 10% to about 20% of the maximum hydrogen mass flow, which is obtained under full load of the fuel cell, through the first connecting line 342 drained. In normal Purge operations is usually only max. 1% of the max. Discharge hydrogen mass flow. Here, however, the amount of hydrogen could be actively increased for targeted heating of the fuel cell. Preferably, the amount of hydrogen vented can be increased until a hydrogen limit is reached in the cathode exhaust gas. This limit can be preset. For example, this limit could vary depending on the location (outdoors, in the garage). Thus, a particularly efficient heating of the fuel cell can be ensured without exhaust gases are discharged with a high hydrogen concentration in the environment.

Upon completion of the warm-up phase, the fuel cell is operated in "normal" operating mode. The first switching valve 340 can be closed and the second switching valve 350 can be open. The fuel then does not get into the cathode, but directly into the cathode exhaust gas 250 , This has the advantage that hydrogen contained in the anode exhaust gas does not react on the cathode side of the fuel cell and the fuel cell 100 additionally heated. However, it may also be useful during normal operation of the fuel cell in some load points, a portion of the anode exhaust gas via the first switching valve 340 upstream of the cathode. Advantageously, the first and second valves 340 . 350 activated in such a way that in the water separator 330 accumulated liquid water through the second connection line 352 directly into the cathode outlet 212 is drained. Thus, it is advantageously avoided that too much liquid water penetrates into the cathode.

The solution shown here is particularly suitable, selectively via the first connection line 342 drain the anode exhaust gas upstream of the cathode and also via the second connection line 352 Drain anode exhaust into an area downstream of the cathode.

The system configuration proposed here thus sees two anode vent valves 340 . 350 in front. vent valve 340 the anode gas mixture leads to the cathode inlet 122 the fuel cell 100 , This valve 340 This is used during system startup because it often limits the amount of fresh air. The hydrogen from the venting process is in the fuel cell 100 and is not diluted in the exhaust gas to be released into the environment. If a sufficiently high air supply mass flow and thus a sufficiently high exhaust gas mass flow are present, the anode venting can also be achieved via a valve 350 for diluting the hydrogen gas into the exhaust gas 250 be made.

2 shows another fuel cell system according to the technology disclosed herein. In the following, the components and features that are identical to the previous figure will not be discussed further in the discussion of the following figure. Instead, more attention is paid to the different or additional components and features.

In the anode subsystem, a jet pump is provided here, which supplies the recirculated anode exhaust gas into the anode feed line 310 brings. The anode exhaust gas line 312 is with a water separator 330 connected. The water separator 330 the liquid water separates from the anode exhaust gas. Below the water separator 330 here is a connection line 332 provided, which _aufgabelt in two lines L _purge and L _drain . The line L _purge or its branch point from the connecting line 332 is preferably higher with respect to the vertical in the installed position than the line L _drain or its branch point of the connecting line 332 ,

In the purge line L _purge here is a switching means 340 . 350 arranged. The switching means 340 . 350 can distribute the anode exhaust gas to at least one of the three port ports Port 1, Port 2 and / or Port 3. This distribution can be done by a multi-way valves or several individual valves. Port 1 and port 2 are upstream of the cathode in the cathode 210 arranged.

The port 2 represents an alternative to port 1, wherein the admixing in front of the heat exchanger can have the advantage of achieving a more complete mixing of the anode exhaust gas with the cathode feed air. Port 3, however, is downstream of the cathode in the cathode exhaust air 250 intended. As in the embodiment according to the 1 Here, during the warm-up phase of the fuel cell, the anode exhaust gas can be introduced upstream of the cathode and during the operating phase at least partially downstream of the cathode.

In the cathode 210 is between the oxidizer conveyor 220 and the charge air heat exchanger 230 an injection device 270 provided, which injects the water from the anode subsystem. The water can do so in a water receptacle 370 get saved. A pump 380 here promotes the water to the injector 270 , Preferably, the water receptacle 370 in the installation position of the fuel cell system below the water separator 330 arranged and preferably siphon-free with the water separator 330 connected. Other mounting positions are also conceivable. In the drain line L _drain is upstream of the water tank 370 a drain valve 360 arranged. Further, in purge valve 340 . 350 provided in the purge line L _purge . For dewatering the anode subsystem 300 becomes the drain valve 360 open. The water then flows into the water tank 370 , Thereafter, to purge the anode exhaust, the purge valve 340 . 350 be opened. The anode exhaust gas then flows, depending on the circuit of the purge valve 340 . 350 in at least one of the three ports 1, 2, 3.

In the system configuration shown here, two valves are provided, both below the anode water separator 330 are arranged. The first drain valve 360 Alone gives the liquid water content from the separator 330 in a water collection container 370 , from which the cathode water injection 270 is supplied. The anode purge valve 340 . 350 will not be opened until the liquid is removed and will discharge the residual anode gas (H2, N2, water vapor) either to the cathode inlet side (Port 1 or Port 2) or to the cathode exhaust side (Port 3). Advantage of this system interconnection is the possible use of the deposited anode water for a cathode gas humidification and the non-occurring hydrogen emission via the cathode exhaust gas.

3 shows a similar structure as in the 1 is shown. The drain valve 360 is here downstream of the water storage tank 370 arranged. The water receptacle 370 is here in the installation position of the fuel cell system below the water separator 330 arranged and may preferably siphon-free with the water 330 be connected. The separated water of the anode subsystem can thus gravimetrically into the water receptacle 370 stream. A drain valve upstream of the water receiver 370 is not planned here. The drain valve is preferred 360 adjacent to the injector 270 educated. Here, the drain valve can precisely meter the amount of water injected by injection into the cathode 210 is removed from the anode subsystem. The functions of injecting water and purges of anode exhaust gas are realized in this embodiment with two valves which operate independently of each other. In particular, water can continuously flow into the intermediate store here. The easy one Anode exhaust can then be drained from time to time by actuation of the purge valve.

The 4 shows a further embodiment of the fuel cell system. Here is an injection device, a jet pump 270 intended. A branch of the purge line L _purge opens here in a port 1 between the jet pump 270 and the drain valve 360 , which is also downstream of the water tank 370 is provided. The steel pump 270 can suck in the water and / or the anode exhaust gas here. Depending on the circuit of the valves 340 . 350 . 360 the jet pump sucks in water and / or anode exhaust gas. On a pump, remove the water from the water receptacle 370 to the jet pump 270 promotes, can be dispensed with. Thus, the cost, the required differential pressure between the anode and cathode and the space can be reduced.

The features disclosed in connection with the figures can also be combined with one another. For example, the jet pump is the 4 also in the embodiments of the 2 and 3 applicable. Furthermore, in connection with the 1 described circuitry during the warm-up also applicable to the other figures.

The foregoing description of the present invention is for illustrative purposes only, and not for the purpose of limiting the invention. Various changes and modifications are possible within the scope of the invention without departing from the scope of the invention and its equivalents.

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 102004055158 A1 [0004]
• DE 10115336 A1 [0004]
• WO 2008/052578 A1 [0004]
• DE 102013218958 A1 [0025]

Claims (10)

Method for operating at least one fuel cell ( 100 ), - wherein an anode exhaust gas to be discharged from an anode subsystem and water to be separated from the anode subsystem are separated from the anode subsystem ( 300 ), the anode exhaust gas being supplied to the cathode ( 210 ) and / or the cathode exhaust gas ( 250 ), and - whereby the water is supplied to the cathode for humidification ( 210 ) is injected. The method of claim 1, wherein the water and / or the anode exhaust gas upstream of a heat exchanger ( 230 ) into the cathode ( 210 ) are injected. A method according to claim 1 or 2, wherein the water and / or the anode exhaust gas by means of a jet pump ( 270 ) into the cathode ( 210 ) are injected. Method according to one of the preceding claims, wherein the anode exhaust gas and the water to be separated by a water separator ( 330 ). Method according to one of the preceding claims, wherein the water in a drain line (L _drain ) is guided, and wherein a water receptacle ( 370 ) stores the water in the drainage line (L _drain ) before the water enters the cathode for humidification ( 210 ) is injected. Process according to claim 5, wherein the water separator ( 330 ) and the water receptacle ( 370 ) are arranged so that the water by gravity into the water receptacle ( 370 ) flows. Method according to one of the preceding claims, at least one switching means ( 340 . 350 . 360 ) is provided, which changes the flow of water or anode exhaust gas. Method according to one of the preceding claims, wherein the at least one switching means ( 340 . 350 . 360 ) several valves ( 340 . 350 . 360 ), which are arranged in different lines (L _purge , L _drain ), wherein the anode exhaust gas is separated from the water by the actuation of the valves and / or by gravity due to the arrangement of the lines (L _purge , L _drain ). Method according to one of the preceding claims, wherein the at least one switching means ( 340 . 350 . 360 ) at least one drain valve ( 360 ), wherein the drain valve ( 360 ) downstream of the water receptacle ( 370 ) and adjacent to the location of the water injection changed the supply of water for injection. Method according to one of the preceding claims, wherein during a warm-up phase of the fuel cell, the anode exhaust gas of the fuel cell ( 100 ) at least temporarily only the cathode ( 210 ) and / or the cathode of the fuel cell ( 100 ) is supplied.

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