FUEL CELL SYSTEM AND METHOD OF OPERATING A FUEL CELL SYSTEM

A fuel cell system includes a fuel cell having an anode region and a cathode region. The fuel cell defines an anode inlet to permit feeding the anode region with hydrogen and defines a cathode inlet to permit feeding the cathode region with oxygen. The fuel cell has an anode outlet and has a cathode outlet. An anode outlet conduit receives anode offgas at the anode outlet and a catalytic converter is arranged so as to permit the anode offgas in the anode outlet conduit to flow therethrough. A cathode outlet conduit is arranged for receiving cathode offgas at the cathode outlet. A cathode branch conduit connects the cathode outlet conduit to the anode outlet conduit upstream of the catalytic converter.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority of German patent application no. 10 2023 114 075.2, filed May 30, 2023, the entire content of which is incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to a fuel cell system that can be utilized, for example, in an electrically operated vehicle to generate the electrical energy required for operation of the vehicle.

BACKGROUND

In the case of such fuel cell systems having one or more fuel cells in the form of PEM fuel cells, for example, purge operations are conducted to some degree at the start of fuel cell operation and also to some degree during fuel cell operation. In these purge operations, the anode region of a fuel cell is purged with hydrogen which is introduced into the anode inlet region of the fuel cell and released again therefrom at an anode outlet region, such that air and/or nitrogen that accumulate in the anode region are discharged therefrom in order thus to ensure efficient fuel cell operation with unimpaired hydrogen concentration in the anode region.

The hydrogen released from the anode region with the anode offgas in such a purge operation is a fundamentally environmentally harmful gas, and the release of hydrogen to the environment can lead to a potentially critical situation in terms of explosion risk.

SUMMARY

It is an object of the present disclosure, in a simple and compact construction configuration of a fuel cell system, to reliably minimize the amount of hydrogen released to the environment.

In a first aspect of the present disclosure, this object is achieved by a fuel cell system, especially for a vehicle, including:at least one fuel cell,an anode region which is to be fed with hydrogen at an anode inlet region of the at least one fuel cell,a cathode region which is to be fed with oxygen at a cathode inlet region of the at least one fuel cell,an anode outlet conduit which accepts anode offgas at an anode outlet region of the at least one fuel cell,at least one catalyst unit through which the anode offgas can flow in the anode outlet conduit,a cathode outlet conduit which accepts cathode offgas at a cathode outlet region of the at least one fuel cell,a cathode branch conduit that connects the cathode outlet conduit to the anode outlet conduit upstream of at least one catalyst unit.

A significant contribution to reduction in the level of hydrogen released to the environment is made by the at least one catalyst unit at which, especially in the case of performance of purge operations, the hydrogen released at the anode outlet region is converted to water in a catalytic reaction with oxygen. The oxygen required for this catalytic conversion is provided in that a portion of the (residual) oxygen-containing cathode offgas which is released at the cathode outlet region is branched off and introduced together with the anode offgas into the at least one catalyst unit or, if two or more such catalyst units are provided, into at least one of the catalyst units.

Since, in the case of the fuel cell system configured in accordance with the disclosure, it is not the whole stream of the cathode offgas that is mixed with the anode offgas, the occurrence of a very low hydrogen concentration in the region of the at least one catalyst unit can be avoided. This avoids the necessity of having to provide the catalyst unit with a large catalyst volume and a large catalyst surface area, via which it is possible to keep firstly build size and secondly build costs low. Secondly, an excessively large mass flow rate through the at least one catalyst unit that leads to a relatively high pressure drop is avoided.

In order to divide the stream of the cathode offgas, the cathode branch conduit may be assigned a flow-directing arrangement for directing a portion of the cathode offgas introduced into the cathode outlet conduit into the cathode branch conduit.

In order to be able to achieve suitable mixing of hydrogen and of oxygen present in the cathode offgas for the catalytic reaction in accordance with purge operations to be conducted in various states of operation or in various phases of operation, the proportion of the cathode offgas introduced into the cathode branch conduit may be variable via the flow-directing arrangement.

In a particularly advantageous configuration, a cathode offgas demoisturizing arrangement may be disposed in the cathode outlet conduit upstream of a branch of the cathode branch conduit from the cathode outlet conduit or/and a cathode offgas demoisturizing arrangement may be disposed in the cathode offgas branch conduit. By virtue of the use of such a demoisturizing arrangement in the form of a condenser unit or water separator, for example, moisture is withdrawn from the portion of the cathode offgas to be mixed with the anode offgas, which firstly has an advantageous effect on the conversion characteristics of the catalyst unit on performance of the catalytic reaction and secondly prevents excessively rapid aging of the catalytically active material in particular.

In the fuel cell system constructed in accordance with the disclosure, a fuel cell offgas system may be provided to accept the portion of the cathode offgas that has not been branched off from the cathode outlet conduit and the mixture of the anode offgas with the portion of the cathode offgas that has been branched off from the cathode outlet conduit, the mixture having been released from the at least one catalyst unit after performance of the catalytic reaction.

For example, the fuel cell offgas system may include a demoisturizing arrangement in order to withdraw further moisture or water from the fuel cell offgas before release to the environment. Alternatively or additionally, the fuel cell offgas system may include a sound absorber in order to suppress the release of noise that may arise, for example, in the region of a compressor that conveys air into the cathode region into the environment.

In a further aspect of the present disclosure, the object is achieved by a method of operating a fuel cell system, especially a fuel cell system constructed in accordance with the disclosure, where anode offgas released at an anode outlet region of a fuel cell is directed through at least one catalyst unit to reduce the hydrogen content in the anode offgas, and a portion of cathode offgas released at a cathode outlet region of a fuel cell is added to the anode offgas upstream of at least one catalyst unit.

For adaptation to various states of operation, the portion of the cathode offgas added to the anode offgas may be variable.

In particular, it may be the case that the amount of the cathode offgas added to the anode offgas is adjusted depending on the hydrogen content in the anode offgas released at the anode outlet region.

For efficient catalytic conversion with minimized volume flow rate through the at least one catalyst unit, it is proposed that the amount of the cathode offgas added to the anode offgas be adjusted such that an at least stoichiometric, preferably superstoichiometric, oxygen/hydrogen ratio is provided for catalytic reaction in the at least one catalyst unit. Especially operation of the at least one catalyst unit with a superstoichiometric oxygen/hydrogen ratio ensures that essentially all the hydrogen present in the anode offgas can be converted to water.

The occurrence of a critical hydrogen concentration with regard to the occurrence of an explosive hydrogen/oxygen gas reaction in the region of the at least one catalyst unit in the mixture of anode offgas and cathode offgas directed through the at least one catalyst unit can be avoided in that the amount of the cathode offgas added to the anode offgas is adjusted such that the mixture of anode offgas and cathode offgas fed to the at least one catalyst unit has a hydrogen content below a threshold hydrogen content.

It is particularly advantageous here when the threshold hydrogen content is in the range from 4% by volume to 8% by volume, and hence an ignition ratio that permits such a reaction is not attained.

For further treatment of the various offgas streams, the portion of the cathode offgas not added to the anode offgas and the mixture of the anode offgas and of the portion of the cathode offgas added to the anode offgas that leaves the at least one catalyst unit after performance of the catalytic reaction are directed into a fuel cell offgas system. In such a fuel cell offgas system, further moisture or further water may be withdrawn from this flowing gas mixture. It is also possible for such a fuel cell offgas system to include a sound absorber in order to suppress the release of noise that may arise in particular in the region of the compressor that conveys air into the cathode region to the outside.

DETAILED DESCRIPTION

InFIG.1, a fuel cell system used in a vehicle for generation of electrical energy for example is generally labeled10. The fuel cell system10includes, as central unit, a fuel cell12in the form of a PEM fuel cell, for example, or a fuel cell stack, having an anode region14and a cathode region16. The anode region14is supplied at an anode inlet region18with gaseous hydrogen H2, for example from a cryogenic hydrogen tank. At a cathode inlet region20, for example, via a compressor22, the cathode region16is supplied with air L and hence oxygen O2present in the air. In fuel cell operation of the fuel cell12, hydrogen protons diffuse through the membrane24of the fuel cell12into the cathode region16, where they react with oxygen introduced into the cathode region16to give water, which is released together with residual oxygen and nitrogen present in the air L supplied to the cathode region16into a cathode outlet conduit28at a cathode outlet region26.

In order to remove air, that is, essentially oxygen and nitrogen, accumulating in the anode region14from the anode region14when the fuel cell12is not activated, or to discharge nitrogen that accumulates in the anode region14via diffusion through the membrane24from the anode region14during fuel cell operation, purge operations are conducted, for example, before startup of the fuel cell12or during fuel cell operation, in which an anode outlet region30is opened and the anode region14is purged by hydrogen introduced into the anode region14, or nitrogen and/or oxygen accumulating therein are directed from the anode region14via the anode outlet region30into an anode outlet conduit32.

The anode offgas A which is released in particular in such purge operations in the anode outlet conduit32contains hydrogen, the release of which to the environment is fundamentally undesirable. For that reason, a catalyst unit34is disposed in the anode outlet conduit32, in which the hydrogen present in the anode offgas A is reacted with oxygen to yield water in a catalytic reaction.

In order to be able to provide the amount of oxygen required for this catalytic reaction, the cathode outlet conduit28is assigned a flow-directing arrangement36in the form of a valve or flow flap, integrated into the cathode outlet conduit28in the configuration example shown. Alternatively, the flow-directing arrangement36assigned to the cathode outlet conduit28may be integrated into a cathode branch conduit38that branches off from the cathode outlet conduit28and opens into the anode outlet conduit32upstream of the catalyst unit34. The flow-directing arrangement36allows a portion of the cathode offgas K released at the cathode region16to be directed into a cathode branch conduit38, or the cathode branch conduit38can be electively opened or shut off.

For defined adjustment of the amount of the cathode offgas K directed via the cathode branch conduit38into the anode output conduit32and hence also into the catalyst unit34, the flow-directing arrangement36is subject to actuation by an actuation unit40, which can also be utilized for actuation of the fuel cell12itself or the compressor22.

If a purge operation is to be conducted, it is possible in unchanged operation of the compressor22, for example, and hence with an unchanged amount of air L introduced into the cathode region16, to actuate a valve (not shown) assigned to the anode outlet region30in order to open it and to allow the anode offgas A to flow into the anode outlet conduit32. Since the opening of the anode outlet region30causes the pressure in the anode region14to drop, the conveying output of the compressor22can be lowered during such a purge operation in order to maintain uniform pressure conditions. The flow-directing arrangement36can be actuated with synchronization to the introduction of the hydrogen-containing anode offgas A into the anode outlet conduit32in such a way that a suitable amount of the cathode offgas K is branched off from the cathode outlet conduit28and introduced into the anode outlet conduit32. In order to ensure that there is not at any time too small an amount of oxygen present in the catalyst unit34for the performance of the catalytic reaction, it may be the case, for example, that the flow-directing arrangement36, even before the directing of hydrogen-containing anode offgas A into the anode outlet conduit32, feeds a portion of the cathode offgas K via the cathode branch conduit38into the anode outlet conduit32and hence into the catalyst unit34. With an amount of hydrogen released from the anode region14that then increases in the purge operation, a hydrogen/oxygen mixture suitable for complete conversion of the hydrogen is established in the catalyst unit. Since the level of the amount or concentration of hydrogen in the anode offgas A is generally also known, it is also possible to ensure via corresponding actuation of the flow-directing arrangement36that the amount of cathode offgas K suitable for the establishment of a defined ratio of hydrogen to oxygen is branched off from the cathode outlet conduit28.

The first important factor in the performance of the catalytic reaction in the catalyst unit34is that essentially no hydrogen that has not reacted with oxygen to give water leaves the catalyst unit34. This means that the hydrogen/oxygen ratio must be at least stoichiometric. In order to reliably prevent the occurrence of unconverted hydrogen, the oxygen/hydrogen ratio is preferably superstoichiometric, such that the reaction can proceed with an excess of oxygen.

Moreover, it has to be ensured that the percentage by volume of hydrogen in the mixture of anode offgas A and cathode offgas K which is fed to the catalyst unit34is sufficiently low that an ignition ratio that entails the risk of a hydrogen/oxygen explosion is not attained. For that reason, it is advantageous when the amount of the cathode offgas K branched off from the cathode offgas K is adjusted such that, taking account of the expected hydrogen content in the anode offgas A in a purge operation, the hydrogen content in the mixture of anode offgas A and cathode offgas K which is then generated does not exceed a threshold hydrogen value in the range from 4% by volume to 8% by volume. It is possible here in particular to determine the amount of the cathode offgas K added to the anode offgas A such that the temperature that arises in the catalyst unit owing to the heat of reaction when the catalytic reaction is in progress lies within an optimal range that assists this reaction.

The anode offgas A leaving the catalyst unit34, which ideally contains virtually no hydrogen but does contain water, can be fed together with the proportion of the cathode offgas K that has not been branched off from the cathode outlet conduit28to a fuel cell offgas system42, in which, for example, further water can be withdrawn from the mixture of anode offgas A and cathode offgas K that flows through it. It is also possible for the fuel cell offgas system42to contain one or more sound absorbers, via which it is then possible to emit the fuel cell offgas B essentially free of hydrogen and with only a comparatively low water content to the environment.

An alternative configuration of the fuel cell system10is shown inFIG.2. In this configuration of the fuel cell system10, a cathode offgas demoisturizing arrangement44is provided in the cathode branch conduit38downstream of the flow-directing arrangement36. This may include, for example, a condenser or a water separator in order to draw off at least a portion of the water transported in the cathode offgas K. The effect of this is that the water content or relative moisture content of the mixture of anode offgas A and cathode offgas K fed to the catalyst unit34is reduced, which increases the efficiency of the catalyst unit34and prevents excessively severe aging thereof.

In a further alternative configuration of the fuel cell system as shown inFIG.3, the cathode offgas demoisturizing arrangement44is disposed upstream of the flow-directing arrangement36in the cathode outlet conduit28.

The fuel cell system of the disclosure, with its simple configuration in terms of construction, reliably ensures that hydrogen emitted especially during purge operations from the anode region of one or more fuel cells can be converted reliably to water in a catalytic reaction with oxygen. Since the anode offgas is mixed only with a portion of the cathode offgas, the volume flow rate put through the catalyst unit is comparatively small, which also contributes to a smaller and hence less costly construction of the catalyst unit. Since, moreover, a sufficiently high concentration of hydrogen can be provided for the catalytic conversion, this leads to a greater adiabatic temperature increase and hence to a higher reaction rate, which can increase the efficiency of the catalyst unit.