DE102019200509A1 - Method for operating a fuel cell system and fuel cell system - Google Patents

Abstract

The invention relates to a method for operating a fuel cell system (2), in particular a motor vehicle (4), air (L) from the environment being fed to a separator (8) to increase its oxygen content and the air (L) is then compressed with an increased oxygen content , or wherein air (L) compresses from the environment and then the compressed air (L) is fed to the separator (8) to increase its oxygen content, and wherein the compressed air (L) with increased oxygen content is supplied to a fuel cell stack (18) for providing electrical Energy is supplied. The invention further relates to such a fuel cell system (2) and a motor vehicle (4) with such a system.

Description

The invention relates to a method for operating a fuel cell system, the fuel cell system having a fuel cell stack for providing electrical energy. Furthermore, the invention relates to such a fuel cell system and a motor vehicle with such.

A fuel cell system has a number of fuel cell stacks, also called a stack, by means of which chemical energy is converted into electrical energy during operation. For this purpose, a fuel and an oxidizing agent (oxidant) are fed to the fuel cell stack, the fuel being oxidized and the oxidizing agent being reduced. The oxidation reaction and the reduction reaction take place spatially separated from one another on the electrodes of the respective fuel cell stack. The electrodes are typically separated from one another by means of an electrolyte, a polymer electrolyte membrane (PEM), for example, being used for a fuel cell system of a motor vehicle.

Gaseous hydrogen is suitably used as fuel and gaseous oxygen as oxidizing agent. For example, air containing the oxygen required for the chemical reaction is fed to the fuel cell stack.

The electrical energy provided by a fuel cell stack and accordingly the electrical power provided by it can be increased by increasing the pressure of the fuel gas and the oxidizing agent.

For this purpose, the air is first of all compressed by means of a compressor, for example in the form of a screw machine, and then fed to the fuel cell stack. The compressor has a comparatively large dimension in accordance with the performance requirements for the fuel cell stack that occur during operation and an associated amount of air that is compressed. The compressor is therefore comparatively expensive and requires a comparatively large installation space.

The invention is based on the object of specifying a suitable method for operating a fuel cell system with a fuel cell stack, the efficiency of the fuel cell system in particular being as high as possible. Furthermore, such a fuel cell system and a motor vehicle with such a are to be specified.

With regard to the method, the object is achieved according to the invention by the features of claim 1. With regard to the fuel cell system and the motor vehicle, the object is achieved according to the invention by the features of claims 5 and 9, respectively. Advantageous refinements and developments are the subject of the dependent claims. The explanations in connection with the method also apply mutatis mutandis to the fuel cell system and to the motor vehicle and vice versa.

In the method for operating a fuel cell system, in particular a motor vehicle, air is supplied in a first step to a separator from the surroundings of the fuel cell system, in particular the fuel cell stack, or from the surroundings of the motor vehicle in order to increase its oxygen content. In a second, subsequent step, the air, the oxygen content of which has been increased by means of a separator, is compressed.

According to an alternative embodiment of the method, the air from the environment is compressed in the first step. In the subsequent second step, the compressed air is fed to the separator to increase its oxygen content.

At most, ie according to both variants, in a third step the compressed air with an increased oxygen content is fed to a fuel cell stack of the fuel cell system. When the fuel cell stack is in operation, in particular on electrodes of the fuel cell stack, the oxygen of the compressed and enriched air is (chemically) reduced in a reduction reaction and a fuel, in particular hydrogen, is oxidized in an oxidation reaction. Chemical energy is converted into electrical energy and the electrical energy or a corresponding electrical power is made available to a load (consumer).

Enriched air is here and in the following briefly referred to as air with an increased proportion of oxygen compared to the air from the environment.

In the fuel cell stack, only the oxygen from the air is used for the chemical reaction. The remaining components of the air, especially nitrogen, do not take part in the chemical reaction for the supply of electrical energy. According to the first variant of the method, in which air is compressed with an already increased oxygen content, comparatively few such constituents, in particular nitrogen, are compressed. As a result, the energy that would be required to compress these components does not have to be used. As a result, an efficiency of the fuel cell system is advantageously increased.

According to the second variant, in which the air is first compressed and then fed to the separator, a compressor used for the compression is used in particular to deliver the air to the separator with increased pressure and / or with an increased flow rate, that is to say the amount of air per unit time. feed. The compressor therefore fulfills a double function here.

In both variants of the method, the fuel cell stack is advantageously supplied with a comparatively large amount of oxygen, that is to say a comparatively large amount of oxidizing agent, for the chemical reaction generating the electrical energy due to the increased oxygen content of the air. A smaller amount of air must therefore be made available to the fuel cell stack for a given output. As a result, it is possible to make the compressor smaller for a predetermined output, that is to say space-saving and weight-saving, and / or with a comparatively low output, or alternatively to increase the output of the fuel cell system with the same amount of air supplied.

For example, in the first variant of the method, in which the oxygen portion of the air from the environment is first increased by means of the separator, the air is again fed to the or another separator after compression in order to further increase the oxygen portion.

According to a suitable development, such a separator is used, which passively separates (separates) at least part of the nitrogen from the air supplied to it. As a result of the separation of the nitrogen, the proportion of oxygen in the air is increased. Passive is to be understood here to mean that the separator has no driven or controllable mechanisms, for example flaps. The separator is suitably used as a gas separation membrane, in particular a hollow fiber membrane, to which the air to be enriched is merely supplied and through which it flows. The nitrogen is at least partially separated from the air due to selective permeation. The separator, which has in particular the gas separation membrane, is therefore not actively driven, which is why no additional energy has to be provided for it by the fuel cell system or by an external energy source such as a battery. As a result, a fuel cell system with a comparatively high efficiency is advantageously implemented. The separated nitrogen is supplied to the environment of the fuel cell system, for example. If a motor vehicle has the fuel cell system, the separated nitrogen is fed directly to the surroundings of the motor vehicle or introduced into an exhaust device of the motor vehicle.

According to a suitable development, the oxygen content of the air supplied to the fuel cell stack is increased depending on the power required by the load (consumer), which load is supplied by the fuel cell stack. So the amount of oxygen content is set depending on the power required by the load. For example, an oxygen fraction of the air is assigned to a power range of the fuel cell system. For example, an oxygen fraction is assigned to idling, a partial load and a full load of the fuel cell system. The oxygen fractions assigned to the power ranges are therefore discrete, that is, they cannot be continuously set and adjusted. In this way, depending on the performance range, a flow rate of air per unit time that is compressed and conveyed is set accordingly. This makes it possible to use the compressor in its most efficient (optimal) operating range even with different performance ranges.

A fuel cell system, which is particularly suitable for carrying out the method in one of the variants shown above, has a number of separators which are provided and set up to increase the oxygen content of air supplied by the separators. A number of separators is understood to mean a single separator or more than one separator. The fuel cell system is provided and set up in particular for a motor vehicle.

Furthermore, the fuel cell system has a compressor for compressing air and a fuel cell stack for providing electrical energy based on the compressed and enriched air supplied to it. The separator or the separators are connected upstream of the compressor in terms of flow technology. Additionally or alternatively, the separators are connected between the compressor and the fuel cell stack. If the separator is connected upstream of the compressor in terms of flow technology, in particular the method according to the first variant is carried out, according to which air is first fed to the separator from the surroundings of the fuel cell system, in particular the fuel cell stack, or from the surroundings of the motor vehicle and is subsequently compressed. If the separator or the separators is or are connected between the compressor and the fuel cell stack, the fuel cell system is particularly suitable for carrying out the method according to the second variant.

The separators each have a gas separation membrane or a plurality of gas separation membranes. According to a suitable embodiment, the separators each have a number of hollow fiber membranes, preferably a plurality of hollow fiber membranes, by means of which part of the nitrogen is separated from the air supplied to them. The air supplied to the separator permeates due to a partial pressure difference between the retentate side, that is, the inside of the hollow fiber, and the permeate side, that is, the outside of the hollow fiber. The permeability for different gases in the air is different. In particular, the permeability of the nitrogen is comparatively small, so that it reaches the permeate side in a comparatively small amount. As a result, the nitrogen is at least partially separated from the supplied air. The hollow fiber membrane is formed in particular by means of a polymer, for example by means of polysulfone, polyacrylonitrile, polyamide, polyphenylene sulfide or by means of a polyphthalamide. As an alternative to this, the hollow fiber membrane is formed by means of a ceramic or by means of a sintered metal.

According to a suitable embodiment, the separators each have an outlet for the air with an increased proportion of oxygen, two of the outlets being connected in parallel or in series with one another. In other words, the output for the air with increased oxygen content is connected to the input of another separator, provided that they are connected in series. When the separators are connected in parallel, their outputs are connected directly to a common air duct.

If the separators are connected in series, the oxygen content of the air supplied to them is successively increased by means of each of the separators. The proportion of oxygen is therefore increased comparatively strongly. If the separators are connected in parallel, it is possible to enable a comparatively large flow rate of air per time unit, that is to say a comparatively large flow rate. If the fuel cell system has more than two separators, these can be connected to one another in parallel, to one another in series, or to one another in a combination of series and parallel connection.

The interconnection of the separators can expediently be changed. For example, a valve and a bypass line are used for this purpose, by means of which a series connection can be converted into a parallel connection and vice versa. In this way, the oxygen content of the air supplied to the fuel cell stack and its flow rate can be set. In particular, it is possible in this way to increase the oxygen content as a function of the power required by the load (consumer).

The fuel cell stack is preferably designed as a low-temperature fuel cell stack, for example as a so-called PEM fuel cell stack. When the fuel cell system is in operation, the fuel cell stack has, for example, a temperature between 80 ° C. and 95 ° C. The water generated from the hydrogen and oxygen in the fuel cell stack is accordingly both liquid and gaseous. According to an advantageous embodiment, the compressor is designed as an, in particular electrical, turbocharger. The generated gaseous water is fed to the turbine. In this way, the gaseous water is used to operate the compressor. The efficiency of the fuel cell system is thus advantageously further increased.

According to an advantageous embodiment, a motor vehicle has a fuel cell system in one of the variants shown above, the fuel cell system preferably being operated according to one of the variants of the method shown above. According to an advantageous development, the separator or the separators are arranged in a flow-through space formed by means of the body of the motor vehicle.

Such a flow space is, for example, an engine compartment, which is formed by means of the body. Alternatively, the flow-through space is arranged between an outer door panel of a door of the body and an inner lining of the door. If the separator or the separators is or are connected upstream of the compressor in terms of flow technology, the flow area can expediently be flowed through by the air from the surroundings of the motor vehicle. In other words, the flow-through space is fluidly connected to the surroundings of the motor vehicle.

In this case, an installation space in the motor vehicle that is no longer used is preferably used as the throughflow space. As a result, no additional space is required for the separator (s). In particular, if the separator is arranged between the inner lining and an outer shell facing the surroundings of the motor vehicle, for example an outer panel, the body, noise is damped due to the separator, that is, acoustic insulation of the motor vehicle is improved.

• 1a,b schematisch ein Brennstoffzellensystem mit einem Separator, mit einem Verdichter und mit einem Brennstoffzellenstapel, wobei der Sauerstoffanteil von Luft der Umgebung mittels des Separators erhöht und anschließend die Luft mittels des Verdichters verdichtet wird bzw. wobei Luft der Umgebung zunächst verdichtet und anschließend deren Sauerstoffanteil erhöht wird,
• 2a schematisch eine perspektivische Schnittdarstellung des Separators, welcher eine Anzahl an Hohlfasermembranen aufweist,
• 2b in einer perspektivischen Ansicht einen Längsschnitt einer der Hohlfasermembranen, wobei Stickstoff durch selektive Permeation aus der dieser zugeführten Luft abgetrennt wird,
• 3 schematisch vier Separatoren, wobei jeweils zwei der Separatoren strömungstechnisch in Serie geschaltet sind, und wobei die in Serie geschalteten Separatoren strömungstechnisch parallel zueinander geschaltet sind, und
• 4 schematisch in einer perspektivischen Ansicht ein Kraftfahrzeug mit dem Brennstoffzellensystem, wobei die Separatoren in mittels der Karosserie des Kraftfahrzeugs gebildeten Durchströmungsräumen angeordnet sind.

Exemplary embodiments of the invention are explained in more detail below with reference to a drawing. In it show:

• 1a, b schematically shows a fuel cell system with a separator, with a compressor and with a fuel cell stack, wherein the oxygen content of air in the environment is increased by means of the separator and then the air is compressed by the compressor or air is initially compressed in the environment and then its oxygen content is increased,
• 2a schematically a perspective sectional view of the separator, which has a number of hollow fiber membranes,
• 2 B 3 shows a perspective view of a longitudinal section of one of the hollow fiber membranes, nitrogen being separated from the air supplied to it by selective permeation,
• 3rd schematically four separators, wherein two of the separators are connected in series in terms of flow technology, and wherein the separators connected in series are connected in parallel with one another in terms of flow technology, and
• 4th schematically in a perspective view of a motor vehicle with the fuel cell system, the separators being arranged in flow-through spaces formed by means of the body of the motor vehicle.

Corresponding parts and sizes are always provided with the same reference symbols in all figures.

In the 1a is a fuel cell system 2nd shown, which is a component of a motor vehicle in the embodiment shown here 4th is. However, the fuel cell system 2nd according to variants not shown, can also be used in other devices for providing electrical energy.

The fuel cell system 2nd has an air filter 6 on what (ambient) air L _U from the surroundings of the motor vehicle 4th is fed. By means of the air filter 6 dust and dirt particles from the air L _U filtered out of the environment (ambient air). The air filter 6 is fluidic with a separator 8th coupled, by means of which the oxygen content of the air L _U the environment is increased. This is done using the separator 8th at least part of the nitrogen in the air L _U passively separated from the environment. As especially in the 2a and 2 B the separator shows 8th a number of gas separation membranes designed as hollow fiber membranes 10th on.

The separated nitrogen becomes an exhaust device 12th fed. The air L _A whose oxygen content by means of the separator 8th has been increased, a compressor designed according to this exemplary embodiment as an electrical turbocharger 14 supplied on the compressor side. The compressor 14 also has an electric motor 16 by means of which the compression, in particular when the fuel cell system has comparatively high performance requirements or when the compressor is started 14 can be supported. In summary, the compressor designed as an electrical turbocharger is used 14 the air L _A compressed with increased oxygen content. The compressed air L _AV with an increased oxygen content then becomes a fuel cell stack 18th fed.

The fuel cell stack 18th also becomes gaseous hydrogen H ₂ supplied as fuel. Inside the fuel cell stack 18th it will be in the air L _AV existing oxygen is reduced and the hydrogen H ₂ oxidized, producing liquid and gaseous water H ₂ O. Here chemical energy is converted into electrical energy, with the electrical energy at connections 20 is provided for a load (consumer) not shown.

The gaseous water H ₂ O becomes the compressor designed as a turbocharger 14 fed on the turbine side, so that the compression is supported by the generated water, i.e. the gaseous water for operating the compressor 14 is used.

The gaseous water H ₂ O then becomes the exhaust device 12th fed. The separated nitrogen and the water H ₂ O are via the exhaust device 12th to the surroundings of the motor vehicle 4th derived.

According to a variant not shown, the compressor has no turbines. For example, this is designed as a compressor, a screw or scroll compressor. Accordingly, the gaseous water is fed directly to the exhaust device.

The 1b shows an alternative embodiment of the fuel cell system 2nd . This is done using the air filter 6 filtered air L _U the surroundings of the motor vehicle 4th the compressor designed as a turbocharger 14 supplied on the compressor side. The compressed air as a result L _V becomes that between the compressor 14 and the fuel cell stack 18th switched separator 8th supplied so that the oxygen content of the compressed air L _V is increased. In an analogous manner to the configuration according to the 1a becomes the compressed air L _AV with an increased oxygen content then the fuel cell stack 18th supplied to provide the electrical energy.

According to a further embodiment of the fuel cell system, not shown further 2nd is a separator 8th fluidically both in front of the compressor 14 as well as between the compressor 14 and the fuel cell stack 18th switched.

In the 2a is a sectional view of the separator 8th shown. The separator 8th is in this case essentially cylindrical. According to alternative variants not shown, the separator has 8th another shape, for example a cuboid or prism shape. The separator 8th has an entrance 22 to supply air L _U , in an embodiment of the fuel cell system 2nd according to 1a , or in an embodiment of the fuel cell system 2nd according to 1b of air L _V , as well as an exit 24th for using the separator 8th enriched air L _A or. L _AV on.

In addition, the separator 8th another second exit 26 for those components L _R the air, which is not through the hollow fiber membranes to the exit 24th permeate. Like in the 2 B is shown, these components flow through the respective hollow cylindrical hollow fiber membrane. The permeability of the air L _V or. L _U The nitrogen present is smaller than that of the oxygen contained in the air, so that the nitrogen reaches the permeate side of the hollow fiber membrane in a comparatively small amount. Thus are the components of the air L _R essentially nitrogen. In summary, this way, at least part of the airborne L _V or. L _U existing nitrogen separated and this via the second outlet 26 the exhaust device 12th fed. The increase in the proportion of oxygen in the air L _V or. L _U is done passively by selective permeation, i.e. without actively driving the separator 8th .

In the 3rd is a fluidic connection of the separators 8th shown. Here are a first separator 8a and a second separator 8b the separators 8th connected in series. In an analogous way there are a third separator 8c and a fourth separator 8d the separators 8th connected in series. The two series connections are connected in parallel to each other. With the series connection is the output 24th of the separator 8a to the input of the separator 8b , and the exit 24th of the separator 8c to the input of the separator 8d connected. When the separators connected in series are connected in parallel 8a , 8b and 8c , 8d are the exits 24th the separators 8b and 8d with a common air duct 28 connected. Due to the series connection of the separators 8a and 8b or. 8c and 8d the oxygen percentage is first by means of the separator 8a or. 8c increased and then by means of the separator 8c or. 8d further increased. In summary, the separators are connected in series 8th the oxygen content of the air supplied to them L _U , in a configuration of the fuel cell system 2nd according to 1a , or the air supplied to them L _V , in a configuration of the fuel cell system 2nd according to 1b , comparatively greatly increased. Compared to the series connection, the separators are connected in parallel 8th a flow rate in air increased.

The interconnection of the separators 8a to 8d is changeable. For this purpose, bypass lines are shown in dashed lines 30th and valves 32 used. Depending on the valve position of the valves 32 are the separators 8a and 8c parallel or in series to the separator 8c or. 8d switched. In this way, the oxygen content and the flow rate per unit time is that of the fuel cell stack 18th supplied air L _AV adjustable and adjusted. According to the in the 3rd shown variant are the valve positions of the valves 32 , and thus the oxygen content and / or the flow rate of the fuel cell stack 18th supplied air L _AV depending on the electrical power required by the load (consumer) P set. The setting of the valve position of the respective valve 32 depending on the required performance P is shown here schematically by means of an arrow.

In the 4th is the motor vehicle 4th shown which is the fuel cell system 2nd according to the in the 1a variant shown. The fuel cell system shows 2nd a number of separators 2nd , of which only two connected in parallel to one another are shown for the purpose of an improved overview. The separators 8th are each in one by means of the body 34 of the motor vehicle 4th flow space formed 36 arranged by an air flow of air L _U from the surroundings of the motor vehicle 4th is flowed through. The flow areas 36 are therefore fluidly related to the surroundings of the motor vehicle 4th connected. The flow areas 36 are here by means of the body 34 engine compartment formed and enclosed by this as well as a space between an outer door panel 38 a door of the body 34 and their inner lining 40 , which is hatched for the purpose of improved clarity. For better visibility of the flow areas 24th is the body 34 displayed transparently in their area.

The invention is not restricted to the exemplary embodiments described above. Rather, other variants of the invention can also be derived therefrom by a person skilled in the art without departing from the subject matter of the invention. In particular, all of the individual features described in connection with the exemplary embodiments can also be combined with one another in other ways without departing from the subject matter of the invention.

Reference list

2nd

Fuel cell system

4th

Motor vehicle

6

Air filter

8, 8a to 8d

separator

10th

Gas separation membrane

12th

Exhaust device

14

compressor

16

Electric motor

18th

Fuel cell stack

20

Connection for one consumer

22

Separator input

24th

Output of the separator for air with increased oxygen content

26

second output of the separator

28

Air duct

30th

Bypass line

32

Valve

34

body

36

Flow space

38

Door outer panel

40

interior panelling

H ₂

hydrogen

L _A

Air with increased oxygen content

L _AV

compressed air with increased oxygen content

L _R

Components of the air

L _U

Air from the surroundings of the motor vehicle

L _V

compressed air

P

required electrical power

Claims (10)

Method for operating a fuel cell system (2), in particular a motor vehicle (4), - air (L _U ) from the environment being fed to a separator (8) to increase its oxygen content and the air (L _A ) is then compressed with an increased oxygen content, or - where air (L _U ) compresses from the environment and then the compressed air (L _V ) is fed to the separator (8) to increase its oxygen content, and - wherein the compressed air (L _AV ) with increased oxygen content is fed to a fuel cell stack (18 ) is supplied to provide electrical energy. Procedure according to Claim 1 , characterized in that such a separator is used to increase the oxygen content, which separates at least part of the nitrogen from the air (L _U or L _V ) passively, in particular only by means of selective permeation. Procedure according to Claim 1 or 2nd , characterized in that a gas separation membrane (10), in particular a hollow fiber membrane, is used to increase the oxygen content. Procedure according to one of the Claims 1 to 3rd , characterized in that the oxygen content of the air (L _AV ) supplied to the fuel cell stack (18) is set as a function of an electrical power (P) which is required by a load supplied by the fuel cell stack (18). Fuel cell system (2), in particular for performing the method according to one of the Claims 1 to 4th , comprising - a number of separators (8) for increasing the oxygen content of air (L _U or L _V ), - a compressor (14) for compressing air (L _A or L _U ), and - a fuel cell stack (18 ) to provide electrical energy on the basis of the compressed air (L _AV ) supplied with an increased oxygen content, - the separators (8) connected in terms of flow technology upstream of the compressor (14) and / or between the compressor (14) and the fuel cell stack (18) are. Fuel cell system (2) after Claim 5 , characterized in that the separators (8) each have a number of hollow fiber membranes (10). Fuel cell system (2) after Claim 5 or 6 , characterized in that the separators (8) each have an outlet (24) for the air (L _A or L _AV ) with increased oxygen content, two of the outlets (24) being connected in terms of flow technology in parallel or in series with one another. Fuel cell system (2) according to one of the Claims 5 to 7 , characterized in that the Compressor (14) is an, in particular electric, turbocharger, to which gaseous water generated in the fuel cell stack (18) is fed on the turbine side. Motor vehicle (4) with a fuel cell system (2), which according to one of the Claims 5 to 8th is formed and which in particular according to one of the Claims 1 to 4th is operated. Motor vehicle (4) after Claim 9 , characterized in that the separator (8) or the separators (8) are each arranged in a through-flow space (36) formed by means of the body (34).

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