EP4150689A2

EP4150689A2 - Fuel cell system

Info

Publication number: EP4150689A2
Application number: EP21726616.2A
Authority: EP
Inventors: Oliver Harr; Philipp Hausmann; Benjamin Pieck
Original assignee: Cellcentric GmbH and Co KG
Current assignee: Cellcentric GmbH and Co KG
Priority date: 2020-05-15
Filing date: 2021-05-12
Publication date: 2023-03-22
Also published as: WO2021228915A8; JP7474353B2; JP2023524851A; WO2021228915A3; DE102020206156A1; WO2021228915A2; CN115552671A; US20230197988A1; KR20230013048A

Abstract

The invention relates to a fuel cell system (2) with at least one fuel cell stack (19) which comprises an anode chamber (20) and a cathode chamber (21), with at least one air conveying device (3) for the supply of the cathode chamber (21) with air via a feed air line (22), with an outlet air line (23) from the cathode chamber (21), with at least one fuel supply device (26) for the supply of the anode chamber (20) with fuel, with at least one anode circuit (28) for the recirculation of unused fuel around the anode chamber (20), furthermore with a cathode bypass (37). The fuel cell system according to the invention is characterized in that the cathode bypass line (37) branches off from the feed air line (22) upstream of or in the region of a valve device (35) in said feed air line (22), and opens into the outlet air line (23) downstream of or in the region of a further valve device (36) in said outlet air line (23), wherein a gas jet pump (38) which can be driven by the air which flows around the cathode chamber (21) is arranged in the cathode bypass (37), which gas jet pump (38) is connected switchably on the suction side to the anode chamber (20) and/or the cathode chamber (21).

Description

Fuel cell system

The invention relates to a fuel cell system according to the preamble of claim 1. It also relates to a method for operating such a fuel cell system.

Fuel cell systems with at least one fuel cell stack are known so far from the prior art. For example, they can be used to generate electrical drive power in vehicles. The fuel cells, in particular for this purpose, are typically so-called PEM fuel cells, that is to say low-temperature fuel cells with a membrane that is permeable to protons. This type of fuel cell is now relatively common. However, it has two serious problems with regard to its service life in regular operation. On the one hand, this is the constant need to keep the membranes sufficiently moist during operation, since drying out of the membranes can significantly shorten the service life of the fuel cells. Another problem is primarily associated with the supply of fuel and oxygen, typically hydrogen and atmospheric oxygen, to the fuel cells during start-up. After the fuel cell system has been idle for a long time, the hydrogen has often diffused out of the anode compartment of the fuel cell and air or oxygen has penetrated there. If the fuel cell is now started, a hydrogen / oxygen front runs over the catalyst in the anode compartment. This leads to increased oxidation of the same, which significantly shortens the service life of the fuel cell. With the above-described use of fuel cell systems in vehicles, however, it is now the case that a start takes place relatively frequently. This very often takes place after a long period of downtime, for example after the vehicle has been parked overnight. In this situation, therefore, this undesired type of starting of the fuel cell, which is also referred to as air / air start or air / air start, often occurs. Numerous measures therefore try to counteract this problem, for example by adding more hydrogen from time to time or by carrying nitrogen into compressed gas storage tanks in order to flush the system with nitrogen before it is started. All of this may work in test mode. All of these technologies are too complex, too expensive or have too many emissions for regular use in a large vehicle fleet.

An exemplary fuel cell system can be found in DE 102009 043 569 A1. This system provides on the one hand a system bypass for connecting the pressure side with the exhaust air side on the one hand and on the other hand a connection between the anode side and the cathode side via a blow-off line with a so-called blow-off or purge valve. In addition, a gas / gas humidifier that is customary in such fuel cell systems is indicated, which is used to humidify the supply air flow to the cathode compartment of the fuel cell by means of its humid exhaust air flow. In practice, however, these components are relatively large, complex and expensive.

The object of the present invention is to provide an improved fuel cell system which, due to its structure, can dispense with a gas / gas humidifier and avoid numerous operating situations that impair the safety and service life of the fuel cell.

According to the invention, this object is achieved by a fuel cell system having the features in claim 1. Advantageous refinements and developments of the fuel cell system result from the dependent claims. Claim 12 also specifies a particularly preferred method for operating such a fuel cell system.

The fuel cell system according to the invention comprises at least one fuel cell stack, which in turn comprises an anode compartment and a cathode compartment. The fuel cell stack can preferably be constructed as a PEM fuel cell stack from a multiplicity of individual cells. It is also known collectively as a fuel cell or fuel cell stack. The cathode compartment of this fuel cell is supplied with air via a supply air line via at least one air delivery device. The air then escapes through an exhaust duct the cathode compartment, for example, into the environment. In addition, at least one fuel supply device is provided for supplying the anode space with fuel. This can, for example, be a compressed gas storage device for hydrogen as fuel. The structure of the fuel cell system will also typically have what is known as an anode circuit, which is used to recirculate unused fuel, in particular hydrogen. This is recirculated around the anode compartment, i.e. returned from the exit of the anode compartment to the entrance. In most operating situations, it is mixed with fresh hydrogen and fed back into the anode compartment. The fuel cell system also includes a cathode bypass, that is to say, for example, a line which is formed parallel to the cathode.

According to the invention, this cathode bypass branches off in front of or in the area of a valve device in the supply air line and opens into the exhaust air line after or in the area of a further valve device. All of this can be built up on the system side around the cathode compartment or the fuel cell. However, it can also be fully or partially integrated into the fuel cell and / or its housing.

The cathode compartment can thus be shut off and the air actually flowing into the cathode compartment and flowing through it can be guided through the cathode bypass. Mixed forms of these two operating states are conceivable, possible and often also useful. In the case of the fuel cell system according to the invention, a gas jet pump driven by the air flowing around the cathode space is arranged in the cathode bypass. The gas jet pump is thus driven by this air as a propulsion jet in the event that air is passed around the cathode space. The gas jet pump is connected to both the anode compartment and the cathode compartment in a switchable manner on the suction side. In this way, gases and, if necessary, liquid can be sucked off from the volume of the anode space or the anode circuit as well as from the volume of the cathode space. In the ideal case, the suction takes place relatively evenly in order to avoid excessively high pressure differences between the cathode compartment and the anode compartment and thus to protect the membranes. Alone the possibility of being able to suck off gas both from the anode compartment and from the cathode compartment, optionally or jointly, via the cathode bypass according to the invention with the gas jet pump, creates a multitude of new application possibilities. In the method according to the invention, which is claimed in claim 12, this is described. If necessary, gases can be extracted from the anode compartment and / or the cathode compartment. This will be discussed in greater detail later and the possibilities and advantages that can be achieved with it will be explained in detail.

With regard to the structural design of the fuel cell system, it can also be provided according to a very advantageous embodiment of the idea that a fan in the anode circuit is driven as a recirculation conveying device by an exhaust air turbine in the exhaust air line. In the fuel cell system according to the invention, energy in the exhaust air can thus be used. In contrast to many conventional fuel cell systems, this energy should not be used in an electric turbocharger to support the compression of the supply air, but rather to recirculate the anode exhaust gases in the anode circuit. This then enables, for example, an efficient exhaust gas recirculation despite the use of the energy contained in the exhaust air, which will be explained in more detail later.

The valve devices in the supply air line and / or exhaust air line, in the area of which the cathode bypass branches off, can, according to a very advantageous embodiment, each be designed as 3/2-way valves. In principle, a different design or just a 3/2-way valve and a shut-off valve in the other line would also be conceivable. In principle, it should only be ensured that the volume contained in the cathode space can be shut off while the air flows through the cathode bypass with the gas jet pump.

The air delivery device used in the fuel cell system according to the invention can preferably be designed in two stages. In particular with such a two-stage air delivery device, which is provided in this according to a very advantageous development of the fuel cell system according to the invention, a sufficiently high pressure can be achieved in order to be able to operate the gas jet pump in the cathode bypass very efficiently. In principle, any type of two-stage compression is conceivable, for example by means of two electrically driven flow compressors connected in series. According to an extremely favorable development of this idea, it can be provided that the two-stage air delivery device is designed in the form of a free-running turbocharger, which is connected on the turbine side to the pressure side of a first compressor wheel of an air compressor, and which on the compressor side is connected to the pressure side of a second compressor wheel of the same air compressor is. The air compressor in the fuel cell system according to the invention according to this embodiment thus has two compressor wheels which, according to an extremely favorable further development of this embodiment, are symmetrical and are arranged on a shaft with a common electric motor. This enables a structure in which the symmetrical arrangement of the compressor wheels and the electrical drive arranged in between enables a very good compensation of axial forces. This increases the efficiency, since the friction can be minimized. In addition, simpler and smaller axial bearings are possible, which is a further advantage. With this structure, the compressor side and the turbine side of a free-running turbocharger can now flow to. The compressor with the two symmetrical compressor wheels thus supplies the compressor side of the freewheel in order to implement register charging. At the same time, the turbine of the freewheel is flown through the other compressor wheel, so that this second compressor wheel drives the freewheel.

This is extremely simple and efficient and allows a few other advantages in addition to the possibility of generating a correspondingly high pressure, which is a decisive advantage for the operation of the gas jet pump in the cathode bypass. For example, if the electrically driven flow compressor provides a pressure level of 1.5 to 2.5 bar on the first compressor wheel, then this pressure can be increased further via the freewheel, for example up to 4.5 bar to supply the fuel cell. The structure also makes it possible to supply the fuel cell system with very humid gases, since the structure of the freewheel can be designed in such a way that, in the event that it freezes, it still allows sufficient air from the first electrically driven compressor wheel to pass through, as does the fuel cell system to be able to start in adverse weather conditions. This enables, for example, the recirculation of moist exhaust gas, which will be discussed in more detail later. According to a further very favorable embodiment of the fuel cell system according to the invention, it can furthermore be provided that a water separator is arranged in the exhaust air line in the flow direction of the exhaust air after the opening of the cathode bypass into the exhaust air line. In this way, the water that occurs in the area of the cathode compartment during regular operation can be separated and collected in this water separator. Since most of the product water accumulates in the area of the cathode compartment, this is the primary amount of water accumulated in the fuel cell system. By connecting the anode circuit or the anode compartment, preferably via the anode circuit and its blow-off or purge line or also purge / drain line, the water from this area of the fuel cell system reaches the cathode bypass and from here into the Water separator in the exhaust line. This therefore collects all of the water in the fuel cell system. In the variant in which a turbine for driving a fan as a recirculation conveying device is arranged in the exhaust air line, it can also be provided that the water separator is located in the flow direction in front of this exhaust air turbine. In this way, it can protect the exhaust air turbine from droplets that are potentially contained in the exhaust air and which could possibly damage the high-speed turbine.

An extremely favorable embodiment of the fuel cell system according to the invention can also provide that an exhaust gas recirculation line connects the exhaust air line after the opening of the cathode bypass and, if an exhaust air turbine is provided, also after this with a register line between the two compression stages. This structure allows exhaust gas recirculation in order to return exhaust air and in particular the moisture contained in the exhaust air to the cathode compartment. The second compressor stage, in particular the free-running turbocharger, can thus be used for the recirculation of exhaust air, which is now wholly or preferably partially recirculated and circulated through the second compressor stage, in particular the free rotor. As a result, the moisture which is carried along in the cathode exhaust air as a product of the reaction in the fuel cell and which has not already been separated out in the form of liquid water, if a water separator is provided, can be returned. In this way, on the one hand, the supply air to the cathode compartment can be humidified and, on the other hand, the oxygen content in the supply air to the cathode compartment can be reduced, in particular adjusted. Through this For example, when the electrical load on the fuel cell is low, the oxygen content in the cathode can be reduced. In this way, excessively high cell voltages and thus damage to the individual cells of the fuel cell can be prevented. This procedure is also known as oxygen depletion or air starvation. It allows the voltage of an individual cell to be kept below 0.9 V, for example, and thus on the one hand to protect the cells and on the other hand to ultimately limit the total voltage of a fuel cell stack. In the case of large fuel cell stacks in particular, such as those used in commercial vehicles, this enables the maximum voltage of the fuel cell stack to be reliably below a given limit value, which in turn means that, with a corresponding classification in a given high-voltage class, the given high-voltage class for this high-voltage class Voltage limit value can be reliably adhered to even with a larger number of individual cells. In this way, the number of individual cells and thus ultimately the performance of the fuel cell can be increased within the HV class. If the return of the exhaust air is not sufficient to lower the oxygen content sufficiently so that a correspondingly low voltage of the individual cells can be guaranteed, part of the air could also be led through the cathode bypass and the gas jet pump in this situation with the structure of the fuel cell system according to the invention. When the connection to the cathode is open, oxygen would then be actively sucked out of the cathode compartment, which further supports the limitation of the individual cell voltages in order to ensure compliance with the limit values for the individual voltages and the limit value for the entire fuel cell stack even more reliably.

According to an extremely advantageous embodiment, the fuel cell system according to the invention can have a liquid water system. At least one water separator is connected to a water tank or forms it directly. The water tank itself is then connected to a pressure water distributor, for example a common rail, via a water pump. Branch lines to consumption points for the liquid water then branch off from this pressurized water distributor. The liquid water can advantageously be heated, for example by means of waste heat from the fuel cell system or also by means of electrical heating. Such components are also used in the Combustion engines are used so that they are easily and cost-effectively available on the market.

According to a further very favorable embodiment of this idea, it can be provided that the consumption points include hydrodynamic bearings of the free-running turbocharger and / or at least one humidifier in the supply air. This humidifier can in particular be designed in the form of a one-fluid or two-fluid nozzle. The liquid water system therefore allows the freewheeler, if it is available, to be stored hydrodynamically.

This leads to an extremely simple and efficient mounting of this component, which ensures that the freewheel runs with very little friction. In contrast to an electrically driven turbocharger, it is completely uncritical if moisture or water escapes from the area of the hydrodynamic bearings, since this only gets into the exhaust air or into the supply air to the fuel cell. In both air currents, additional water or additional moisture does not play a role or does not represent a disadvantage. In the case of the supply air, this moisture would actually be an advantage.

The humidifiers can now be designed in a particularly simple manner in the form of a one-fluid or two-fluid nozzle. These humidifiers can be arranged in the supply air before and / or after the second compressor stage. As a result, the compression by the injected water, for example finely atomized water from a two-substance nozzle, is correspondingly moist and is atomized in the two-substance nozzle by the air flowing around the actual water nozzle. This atomized water helps to cool the air that becomes hot during compression and is evaporated in the air so that it is ideally humidified. In particular with an electric drive of the corresponding humidifier, humidification can take place independently of the operation of the fuel cell, which is another very decisive advantage compared to a much more complex, larger and more expensive gas / gas humidifier, which can be saved by this structure.

The fuel cell system in one or the other embodiment according to the invention now shows its particular advantages when it comes to achieving corresponding advantages by operating the gas jet pump in the cathode bypass and in particular to avoid adverse operating conditions for the service life of the fuel cell. The method according to the invention therefore provides that, if necessary, the cathode compartment and / or the anode compartment are connected to the gas jet pump and gases are thus sucked out of these areas.

As already mentioned, this can be used, for example, to support air starvation, which can be initiated primarily via the exhaust gas or exhaust air recirculation, if necessary, especially in system states in which there is no sufficient oxygen-depleted exhaust air available for recirculation.

Another aspect can be used, for example, to increase the safety of the fuel cell system when it is used in a vehicle. In the event of an accident in the vehicle, there is always the risk that if the anode compartment is still filled with hydrogen and the cathode compartment is still filled with oxygen, a correspondingly high voltage will be applied to the fuel cell, which can be problematic for both the occupants and any rescue workers. Typically, however, the compressor and turbocharger, if present, are still active at the time of the accident. If the vehicle's crash sensors detect an accident, the hydrogen supply can be interrupted and the flow through the cathode compartment blocked via the valves. When the compressor and / or turbocharger, which runs very quickly during operation, coasts to a stop, there is still a sufficient volume flow so that the cathode and the anode can be sucked off via the gas jet pump in the cathode bypass in order to reduce the voltage potential of the fuel cell as quickly as possible and the risk to occupants and to minimize rescue workers.

Another very favorable possibility of using the fuel cell system according to the invention is when preparing for a later start of freezing. Because the anode and the cathode can be sucked off via the gas jet pump in the cathode bypass, the pressure in the fuel cell stack can be lowered, preferably evenly, both in the anode compartment and in the cathode compartment. A pressure reduction to, for example, up to 100 mbar is possible. In practice, this means that water, which is in liquid form in the fuel cell and there, due to the risk of later freezing if the Temperatures dropping below freezing point, is not desirable, is already vaporized at very low temperatures of 20 to 40 °, in particular 25 to 30 ° C. This enables the fuel cell stack to be dried, with the membrane being able to be dried relatively gently because of the relatively low temperature. Drying at correspondingly higher temperatures, in particular at the operating temperatures of the fuel cell stack of approx. 80 ° C., would dry out the membranes very severely and thereby put them under extreme stress.

In the case of the preferred structure according to the invention with the liquid water system and the humidifiers, the membranes can also be humidified at a standstill if required. On the cathode side, liquid water available in the fuel cell system is metered into the fuel cell system via the humidifier, which can be designed, for example, in the form of an electrically driven injector with a single-fluid nozzle, and is brought to the membranes by inflowing air into the cathode compartment, which is (re) humidified as a result can.

Another procedural advantage is that the initially mentioned harmful air / air starts of the fuel cell can be prevented. One possible solution with the fuel cell system according to the invention provides that a vacuum is drawn at least for the cathode compartment and possibly also wholly or partially on the anode, so that the pressure difference typically remains less than 500 mbar. As a result, oxygen is sucked out in the area of the cathode. This process can be repeated from time to time, for example every 10 hours, or at least again when it has been recognized that the start of the fuel cell system is now imminent, similar to the previously customary and now no longer required replenishment of hydrogen . If the temperatures are above freezing point, this would in the long run dry out the membranes very much, since the water contained in them evaporates due to the negative pressure. For this reason, the above-mentioned moistening of the membranes, preferably at temperatures above freezing point, can be carried out from time to time, which has an advantageous effect on the overall service life of the fuel cell system or its fuel cell. In the embodiment of the fuel cell system with the exhaust gas recirculation line, which opens between the two stages of the compressor, circulation around the cathode is also possible. This enables the air in this circuit to be reduced to 0% oxygen. To do this, hydrogen can be introduced into the circuit around the cathode until the oxygen is completely consumed. This metering in of hydrogen can take place, for example, via the anode compartment, into which a certain amount of hydrogen is metered and then sucked out again via the gas jet pump in the cathode bypass. The hydrogen is then mixed with the oxygen in the air and can, for example, react on the catalyst of the cathode until the oxygen is completely consumed. As an alternative to this, it would also be conceivable to provide an additional catalyst, in particular after the gas jet pump in the direction of flow. If a negative pressure is then generated in the fuel cell, this oxygen-depleted air, which essentially consists of nitrogen, can flow into the area of the anode space by opening the purge valve. In this phase, flow can flow through the cathode compartment parallel to the cathode bypass and is then also filled with nitrogen. This enables an extremely gentle start, in which both the anode compartment and the cathode compartment are filled with nitrogen, without the nitrogen having to be carried along and / or (temporarily) stored. Rather, the special configuration of the fuel cell system according to the invention makes it possible to generate this nitrogen in the fuel cell system when required.

Another aspect of fuel cell systems is the poisoning of the anode-side catalytic converter with carbon monoxide that occurs during regular operation and increases with the operating time. So far, this has required an occasional air / air start in order to oxidize the carbon monoxide to carbon dioxide with the oxygen introduced and to remove it from the system. The reduction in the service life of the fuel cell must be accepted. As already mentioned several times above, however, such an air / air start should be avoided as far as possible due to the risk of a reduction in the service life of the fuel cell. The fuel cell system constructed according to the invention now enables an extremely gentle procedure here. The fuel cell itself and thus also its anode compartment, which is susceptible to poisoning the catalyst, is, as already described above, via the Using the cathode bypass with the gas jet pump evacuated. By opening the purge valve, oxygen-containing air flows backwards through the gas jet pump into the anode. This can now remain there passively or, preferably, if a recirculation fan is available, can be circulated by this. As a result, the catalytic converter can be refreshed and freed from CO impurities, even without an air / air start being necessary. Before the next start, oxygen and carbon dioxide are then sucked out of the anode compartment or the anode circuit.

A simple modification in the structure of the fuel cell system results in numerous advantages in terms of process management, so that an extremely efficient and long-lasting fuel cell system can be made available which enables the fuel cell to be operated very gently.

Further advantageous refinements of the fuel cell system according to the invention and its operating method also result from the exemplary embodiment, which is shown in more detail below with reference to the figure.

The only attached figure shows a possible embodiment of a fuel cell system according to the invention.

In the illustration in FIG. 1, an air compressor 1 for a fuel cell system 2 is shown. The air compressor 1 essentially consists of an electric drive motor 4, which is arranged on a common shaft 5 with two compressor wheels 6, 7. The compressor wheels 6, 7 are driven by the electric drive motor 4 arranged centrally between them on the shaft 5 and are designed essentially symmetrically. In this way, forces which act on the common shaft 5 in the axial direction are minimized. On the one hand, this helps to reduce friction losses and, on the other hand, allows a simple and efficient design of axial bearings. Air is sucked in by the compressor wheels 6, 7 via two separate or, optionally, a common suction path 8. The air filters that are typically present are not shown here. From the compressor wheel 6, the compressed air passes via a register line 9 to a compressor side 10 of a free-running turbocharger 11, which is also referred to as a free-wheel 11. In this freewheel 11, a common shaft 12 connects the compressor side 10 with a turbine side 13, which is connected to the pressure side of the compressor wheel 7 of the air compressor 1 and is accordingly driven by the air flow from this compressor wheel 7. After the turbine side 13 or its turbine, the expanded air, which had previously passed from the compressor wheel 7 to the turbine side 13 of the freewheel 11 via a turbine line 14, flows away again. In the illustration of the figure, the turbine-side discharge leads via the compressed gas reservoir 26 for hydrogen into the environment. In practice it is typically the case that this cools down very strongly when hydrogen is withdrawn, so that it can be kept ideally warm with the waste heat in the exhaust air of the turbine side 13. The air can also flush out any leaks by flowing, for example, through a housing arranged around the compressed gas reservoir 26 or the like. Another use of the exhaust air coming from the turbine side would also be conceivable, for example the heating of other heat-requiring components located in the fuel cell system 2, for example heating the water tank 42, which will be described in more detail later.

From the compressor side 10 of the freewheel 11, the now even more strongly compressed supply air reaches the fuel cell system 2. This structure enables a freewheel 11 to be used, based on the pressure generated by the compressor wheel 6 as the first compressor stage, for the Fuel cell system 2 required to generate pressure on the compressor side 10 of the freewheel 11. So it is a kind of register charging. The freewheel 11 and the air compressor 1 together thus form a two-stage air delivery device 3.

In addition, a bypass line 15 with a valve 16 is provided, which enables part of the air that has been compressed via the compressor wheel 7 of the air compressor 1 to be conducted from the turbine line 14 into the register line 9. In this way, for example, when the valve 16 is completely or partially open, a higher volume flow of air to the fuel cell system 2 can be achieved. At the same time, the air flow through the turbine side 13 of the freewheel 11 is correspondingly reduced, so that although a higher volume flow, but a lower pressure in the Fuel cell system 2 is present. With increasing closure of the valve 16 in the bypass line 15, the power on the turbine side 13 and thus also the compressor power on the compressor side 10 of the freewheel 11 increases accordingly, while at the same time the volume flow becomes smaller. This makes it possible to achieve a higher pressure with a lower volume flow. In addition, the direction of flow can also be such that more air flows to the turbine side 13 of the freewheel 11 via the bypass line. In the extreme case, all of the air in the air compressor 1 can be used to drive the freewheel 11. The air supply can therefore be controlled via the valve 16 in the bypass line 15. Even if the bypass line 15 with the valve 16 offers particular advantages, it is to be understood here as being purely optional and can in principle also be omitted.

The fuel cell system 2 comprises a fuel cell 19, which is typically a stack of individual cells. In this fuel cell stack 19, an anode space 20 and a cathode space 21 are indicated by way of example. The cathode chamber 21 is now supplied with air via a supply air line 22 via the air compressor 1 and the free rotor 11. Exhaust air arrives via an exhaust air line 23 to a valve device labeled 24, wherein this valve device 24 could also be referred to as an exhaust gas recirculation valve 24. Optionally, the exhaust air from the exhaust air line 23 can be wholly or partially returned via an exhaust air return line 25 to the register line 9 via this valve device 24, or via the section of the exhaust air line 23 labeled 23 ‘into the environment. In contrast to many conventional systems, the exhaust air no longer flows through an exhaust air turbine in section 23 ‘.

The anode space 20 is supplied with hydrogen from a compressed gas store 26. As an alternative to such a compressed gas storage device 26, other storage possibilities for the hydrogen would also be conceivable, for example cryogenic storage or hydride storage. Via a pressure regulating and metering device 27, this hydrogen reaches the anode compartment 20. Via an anode circuit 28 with a recirculation line labeled 29, in which a water separator 30 can be arranged, exhaust gas then returns from the outlet of the anode compartment 20 to its inlet and flows, in most operating states mixed with fresh hydrogen back into the anode compartment 20. In the recirculation line 29 can be known per se As an alternative or in addition to a gas jet pump (not shown), a recirculation fan 31 can be arranged. In the water separator 30 or alternatively in another area of the recirculation line 29, a blow-off line 17 with a so-called blow-off valve or purge valve 18 or purge valve is arranged, via which, for example, depending on the time, depending on the hydrogen concentration in the recirculation line 29 or, depending on other parameters, gas is drained from the recirculation line 29, optionally together with water from the water separator 30.

In this structure of the fuel cell system 2, it is now possible to recirculate exhaust air in whole or in part via the exhaust gas recirculation line 25 with the appropriate position of the valve device 24, so that the humidification of the supply air in the supply air line 22 to the cathode side 21 of the fuel cell 19 is supported. As an alternative or in addition to the use of a liquid water system 34, as indicated by dashed lines in the illustration of the figure and explained in greater detail later, this can contribute to the fact that a conventional gas / gas humidifier can be dispensed with. It is true that there is now the risk that moisture will get into the area of the freewheel 11. If the system comes to a standstill at temperatures below freezing point, this can lead to the freewheel 11 freezing. Unlike a freezing of the compressor wheels 6, 7 in the air compressor 1, this is relatively uncritical, since for the start of the fuel cell system 2 the air that is conveyed via the compressor wheel 6 and possibly via the compressor wheel 7 with the bypass valve 16 open and through the compressor side 10 of the Freewheel 11 is blown into the air supply line 22, is by far sufficient. It is thus sufficient if the freewheel 11 then resumes its operation when it is sufficiently thawed. The structure with the combination of air compressor 1 and freewheel 11 thus enables optimal operation with a high degree of controllability of pressure and volume flow of the supplied air - especially in combination with the liquid system 34 - a dispensing with a conventional gas / gas humidifier, since the exhaust gas recirculation becomes possible without the risk of the entire air delivery device 1 freezing in the case of temperatures below freezing point. In contrast to conventional electric turbochargers, in which the pressure energy from the fuel cell system 2 is relaxed and also serves to support the drive of the air compressor 1, this pressure cannot be used here for the air delivery device 3. Instead of an electrical drive for the recirculation fan 31, as is typically provided, it is now the case here that the exhaust air flows from the cathode chamber 21 of the fuel cell 19 via an exhaust air turbine 32 which is arranged in the exhaust air line 23 and which is coupled to the recirculation fan 31 in a power-transmitting manner, which is indicated here in the form of a common shaft 33. This makes it possible to use the energy contained in the exhaust air of the cathode chamber 21 of the fuel cell 19 to drive the recirculation fan 31 in order to recover this energy and thus make the overall system more energy-efficient. It is particularly favorable here if the coupling between the exhaust air turbine 32 and the recirculation fan 31 takes place magnetically. As a result, the two volumes, which on the one hand carry hydrogen or hydrogen-containing gas and on the other hand air, can easily be hermetically sealed against one another. This is indicated in the figure by the two lines in the area of the shaft 33.

It is now decisive for the structure of the fuel cell system 2 described here that a valve device 35 in the flow direction upstream of the cathode compartment 21 and a valve device 36 in both the supply air line 22 and the exhaust air line 23, namely here in each case relatively close to the cathode chamber 21 is arranged after the cathode chamber 21 in the flow direction. These valve devices 35, 36 can preferably, and this is how it is shown here, be designed as 3/2-way valves. However, they could essentially also be implemented by independent valve devices which are arranged both in the supply air line 22 and in the exhaust air line 23, and which would also be arranged in a cathode bypass 37. Essentially, the point is that the cathode bypass 37 can be switched via the valve devices 35, 36, to be precise when the cathode compartment 21 is closed or the volume comprising the cathode compartment 21 is closed. Unlike a pure system bypass, the cathode bypass 37 is provided with a gas jet pump 38 which, for example, can be designed in the manner of a Venturi tube. However, any other type of gas jet pump or ejector or jet pump is also conceivable, as long as through negative pressure effects and / or Pulse exchange can be sucked in by the air flowing around the cathode chamber 21. On the suction side, the gas jet pump 38 is connected to the blow-off line 17, which can be switched via the purge valve 18 in order to connect the blow-off line 17 to the gas jet pump 38. In this way, liquid and in particular gas can be sucked out of the anode circuit 28 and thus also out of the anode space 20.

Since the anode circuit 28 is otherwise tight and forms a closed volume when the hydrogen supply is switched off, a negative pressure can be achieved in the anode circuit 28, which is very favorable for the reasons explained below.

The gas jet pump 38 is also connected on the suction side via a cathode stub 39 and a cathode suction valve 40 arranged therein to the cathode compartment 21 or to the volume comprising the cathode compartment 21 located between valve devices 35 and 36. The cathode branch line 39 can be arranged both before and after the cathode space 21, that is to say with an opening into the supply air line 22 or the exhaust air line 23. In principle, a direct connection to the fuel cell stack 19 would also be conceivable, but this is technically far more complex than branching off from the corresponding line 22, 23. Here, too, gas can now be extracted from the cathode compartment with the cathode bypass 37 flowed through by the gas jet pump 38 with the cathode suction valve 40 open 21, which, when the valve devices 35 and 36 are closed, means that a negative pressure can also be generated in the cathode of the fuel cell 19. This will also be explained in more detail later with regard to the particularly advantageous use.

The already mentioned liquid water system 34 is shown in dashed lines in the figure. It can preferably be filled with water which is recovered from the fuel cell system 2. The fuel cell system 2 typically has the water separator 30 in the recirculation line 29 and another water separator 41 in the area of the exhaust air line 23, and here, if possible, upstream of the exhaust air turbine 32. In the exemplary embodiment of the fuel cell system 2 shown here, the water from the water separator 30 arrives via the gas jet pump 38 and the cathode bypass 37 also into the water separator 41. Alternatively, a parallel line from the water separator 30, for example into the water separator 41 or directly into a water tank 42 of the Liquid water system 34 conceivable, in which the entire water of all water separators 30, 41 of the fuel cell system 2 then collects. As indicated by the arrow labeled 43, this water tank 42 is now supplied with heat. In principle, this can involve electrical heating, as is generally known and customary for such water tanks in the area of vehicles with internal combustion engines. Alternatively or in addition to this, waste heat from the fuel cell system 2 can also be used, for example waste heat from the fuel cell 19 itself, or waste heat in the exhaust air, in particular in the exhaust air flowing out of the turbine side 13 of the freewheel 11, which, however, can alternatively be used to heat the compressed gas reservoir 26 could.

The water stored in the water tank 42 ideally has a temperature of approx. 80 ° C., the water tank 42 therefore has thermal insulation 44 in order to prevent the water tank 42 from cooling down unnecessarily and quickly. The water from the water tank 42 is then passed via a water pump 45 into a pressurized water distributor 50, for example in the form of a so-called common rail. Individual branch lines then branch off from this pressurized water distributor 50. A first branch line 46 leads to a first humidifier 47. This is designed as a simple humidifier 47 which atomizes the water with a single-fluid nozzle or a two-fluid nozzle. It can be operated, for example, with electrical energy, and thus independently of the operation of the fuel cell 19, and controlled with regard to the humidification. In this way, together with the exhaust gas recirculation during operation, an expensive conventional gas / gas humidifier can be dispensed with in any case. A further humidifier 48, which is constructed in a comparable manner, is optionally provided. This is located in the register line 9 and is supplied with the pressurized water of the pressurized water distributor 50 via a second branch line 49. As already mentioned, this previous structure of the liquid water system 34 is also used in internal combustion engine drives, in particular internal combustion engines with gasoline injection. The components such as the water pump 45, the heatable water tank 42 and the humidifiers 47, 48 are therefore available on the market as adequately tested parts in large numbers and accordingly inexpensively. Via two further branch lines 51 and 52, two hydrodynamic bearings 53,

54 of the free-running turbocharger 11 is supplied with water so that it is mounted on water. In the fuel cell system 2, there will typically be sufficient water to manage both the humidification of the supply air flow and the storage of the free-running turbocharger 11 so that the water can be supplied without an external supply of water.

Such a fuel cell system 2 with the cathode bypass 37 and the gas jet pump 38 arranged therein, driven by the air flowing parallel to the cathode chamber 21, which can switchably suck out both the cathode chamber 21 and the anode chamber 20, now enables numerous advantageous options, via which some problems are solved which could not be solved in previous fuel cell systems or which could not be solved in a comparable manner and which have adversely affected the safety and in particular the longevity of the individual cells in the fuel cell stack 19.

As already mentioned, such a fuel cell system now allows particular advantages in terms of operational management. With an appropriately set exhaust gas recirculation valve 24, when the freewheel 11 is in operation, its compressor side 10 can be used to recirculate exhaust air around the cathode space 21. At the same time, some of this recirculated air can flow through the cathode bypass 37 and thus through the gas jet pump 38. This makes it possible, for example, to suck out gases from the anode space 20 and / or the cathode space 21 when the purge valve 18 or the cathode suction valve 40 are accordingly open. Different purposes are conceivable. For example, in the event of an accident, if crash sensors of a vehicle, not shown here, preferably having the fuel cell system 2, detect this accident, the hydrogen supply can be stopped. With the remaining volume flow when the air compressor 1 and the freewheel 11 are coasting down, gas can then be sucked out of the shut-off cathode space 21 and the anode circuit 28 and thus the anode space 20. As a result, the (no-load) voltage of the fuel cell 19 can be reduced very quickly when the load is dropped and the current of the fuel cell 19 is reduced to zero, so as to prevent the occupants of the vehicle and rescue workers from being endangered. The same applies to the reaction to the actuation of an emergency stop switch or a recognized emergency in the fuel cell system 2 itself. This can also be applied analogously to stationary fuel cell systems.

Furthermore, the oxygen content in the fuel cell 19 can be reduced in order to limit the cell voltage, for which a corresponding amount of oxygen-depleted exhaust air is returned via the exhaust gas recirculation valve 24 and the exhaust gas recirculation line 25 and also supports the humidification of the supply air. If this is not sufficient, oxygen can also be actively sucked out of the cathode compartment 21 with the cathode suction valve 40 open by leading part of the supply air via the cathode bypass 37 and the gas jet pump 38 in order to reduce the voltage in the individual cells or the voltage of the entire cell To limit fuel cell stack 19 further and more reliably controllable.

Two very decisive points for the operation of the fuel cell system 2 relate to a preparation for a freeze start, a so-called FSU (Freeze Start Up) preparation. Because it is possible to lower the pressure in the anode compartment 20 and in the cathode compartment 21, for example to up to 100 mbar, it is possible to evaporate water present in both the anode compartment 20 and in the cathode compartment 21 and to actively suck it off via the gas jet pump 38. This can take place, for example, in a temperature window of 25 to 35 ° C. of the fuel cell 19. In contrast to higher temperatures, the membranes are largely prevented from drying out, so that the fuel cell 19 can be dried very gently. If the temperatures later drop below freezing point, the fuel cell can be prevented from freezing beyond an intended or tolerable level. If the temperatures rise again above freezing point, active humidification can be carried out even without the fuel cell 19 being actively started, since liquid water is available via the liquid water system 34 and, for example, via the humidifier 47, which in particular is an electrically operated humidifier can be designed with a single-fluid nozzle, can be easily and efficiently introduced into the supply air. As already mentioned, this can be circulated via the exhaust gas recirculation valve 24, on the one hand to keep the membranes sufficiently moist and on the other hand to be prepared for a freeze start at any time. A previously common strategy for preparing for the freezing start provides for the longest possible time to be achieved in which an air / hydrogen front is prevented in the anode compartment when the fuel cell system is started. This always occurs when the hydrogen has diffused from the anode space 20 and air has penetrated. If fresh hydrogen is now replenished, this dreaded front occurs, which correspondingly damages the cathode and has an extremely disadvantageous and strong effect on the service life of the fuel cell 19. The fuel cell system 2 in the embodiment variant shown here now has several options for preventing such an air / air start.

The first possibility is that the cathode space 21 can be evacuated accordingly. If there is no oxygen in this, the front cannot develop its damaging effect even if oxygen is present on the anode side and is displaced by the hydrogen flowing in at start-up. This simple possibility can, for example, provide for the cathode to be kept permanently free of oxygen, which, given the tightness that usually occurs in the system, requires the cathode compartment 21 to be evacuated again, for example every 10 hours or the like. Since such a recurring evacuation is relatively risky for the membranes, since they can dry out, this procedure can be accompanied in particular with the moistening of the membranes described above if the temperatures are above freezing point and a safe and reliable start even with a certain residual moisture in the fuel cell 19 is possible.

A second possibility of avoiding an air / air start of the fuel cell 19 is that the air, which also entered the anode compartment 20 while the fuel cell system 2 was at a standstill, is sucked out of the anode compartment 20 again before the fuel cell 19 is started to evacuate it. For this purpose, air is conveyed and flows via the cathode bypass 37 and the gas jet pump 38. When the purge valve 18 is open, the air which has penetrated into the anode space 20 during the standstill can be sucked off. This makes it possible to at least significantly reduce the oxygen content in the volume of the anode space 20 and ultimately also of the anode circuit 28 before the start of the Fuel cell 19 is metered in hydrogen. This also enables a gentle start to be achieved and the service life of the fuel cell 19 to be extended.

The third option uses the generation of nitrogen or oxygen-depleted air, in particular air with an oxygen content of 0%, in order to achieve a very gentle start. The circulation around the cathode space 21 is used for this. Hydrogen metered into the anode circuit 28 or residual hydrogen still present in it is sucked in via the gas jet pump 38 when the purge valve is open and thus enters the circuit together with the oxygen-containing air, which is caused by operation of the air compressor 1, which flows against the turbine side 13 of the freewheel 11 and so that the compressor side 10 moves, maintained. The air then flows in a circle around the cathode compartment 21. It flows partly through the cathode compartment 21 and partly through the cathode bypass 37. Then it flows via the exhaust air line 23 and the exhaust gas recirculation valve 24 as well as the exhaust gas recirculation line 25 back into the register line 9 and through there the compressor side 10 of the freewheel driven back to the valve device 35 in the air supply line 22. The mixing of hydrogen and air in this operation now leads to a reaction of the hydrogen and the oxygen, for example on the catalysts of the anode compartment 21 or in the area of a special one provided catalyst 55, which can be arranged for example in the cathode bypass 37 after the gas jet pump 38, as shown here. In the case of the additional catalytic converter 55, the cathode chamber 21 does not have to be flowed through constantly in order to generate the nitrogen. This reduces the dehydration of the membranes and protects them. If necessary, however, they could also be re-moistened, as explained above.

The fourth possibility to avoid an air / air start represents a combination of the second and the third possibility. In addition to the valve of the pressure regulating and metering device 27, which is now in each case multi-stage switchable, a hydrogen metering line 56 is required, via which hydrogen can be metered onto the cathode side. Similar to or as an alternative to the purge line 17, this hydrogen metering line 56 is connected to the gas jet pump 38 in the cathode bypass 37, as can be seen in the illustration of the figure. This makes it possible to supply hydrogen via the hydrogen metering line 56 to the To meter the cathode side of the fuel cell system 2 without this hydrogen having to flow through the anode space 20 beforehand. The above-mentioned catalytic converter 55, which is connected downstream of the gas jet pump 38 in the circuit around the cathode chamber 21, can thus consume oxygen in the air. This is then autarkically recirculated in this circuit with the aid of the valve devices 35, 36, the valve 16 and the exhaust gas recirculation valve 24 and through the operation of the freewheel 11. This takes place until the oxygen content in the original air with the aid of the catalyst and the hydrogen entering the circuit via the hydrogen metering line 56 in the area of the mixing point in the gas jet pump 38 is less than 1 percent by volume, in particular to about 0 percent by volume is reduced. The then recirculated gas is thus virtually free of oxygen and essentially consists of nitrogen.

As a result of the recirculation via the freewheel 11, this gas is heated at the same time, which promotes the catalytic reaction in the catalytic converter 55 in order to efficiently convert oxygen and hydrogen. A temperature range of approx. +60 to + 80 ° C is ideal for this. This allows the catalytic conversion to be regulated very well in order to avoid undesired nitrogen oxides within the closed volume. These nitrogen oxides as a by-product are undesirable because of the emissions of the same which occur later, but would not further impair the handling of the fuel cell 19, which is gentle on the service life.

After a while, all of the oxygen is used up if sufficient hydrogen is available or if it has been appropriately replenished. In the entire circuit there is now gas that has been depleted to 0% oxygen. This is essentially nitrogen, if carbon dioxide and some noble gases, which however do not adversely affect the process, are disregarded. Now that there is nitrogen in the circuit, the purge valve 18 can be opened, the freewheel 11 and thus ultimately the air compressor 1 can be switched off. The cathode suction valve 40 and / or the valve devices 35, 36 are opened. The nitrogen then flows back into the fuel cell 19 via the purge line and the cathode stub line and / or supply air line, so that it is filled with nitrogen. This enables an extremely gentle start of the fuel cell during next start process without the damaging mechanisms of the air / air start occurring.

A fifth possibility, in combination with the previously customary way of keeping the hydrogen in the system, can also be ideally used in the construction of the fuel cell system according to the invention. Ideally, using a slight static overpressure compared to the air pressure in the surrounding atmosphere, the volumes of both the anode compartment 20 and the cathode compartment 21 are filled with hydrogen and kept under a slight overpressure in order to completely inert the volume through a hydrogen concentration of approximately 100 percent realize. Before the regular start, the residual hydrogen present in the cathode compartment 21 can now be removed again via the gas jet pump 38 and its operation through the supply air that has already been pumped but not flowing into the cathode compartment 21, in that the hydrogen is completely sucked out of the cathode compartment 21 before the cathode compartment 21 by opening the valve device 35 in the direction of the cathode chamber 21, oxygen or the air containing the oxygen is then acted upon in order to be able to start the fuel cell system 2 or its fuel cell 19.

In order to get oxygen into the anode chamber 20 from time to time in order to oxidize the accumulated CO poisoning there, the fuel cell 19 can be evacuated again using the gas jet pump 38 in the cathode bypass 37. With the purge valve 18 open, air can then be used with the air compressor switched off or oxygen-containing gas reach the area of the anode space 20. In principle, the passive oxidation of carbon monoxide to carbon dioxide is conceivable. It becomes more efficient if the recirculation conveying device 31 is operated, for example by operating the air compressor 1 again with the purge valve 18 closed after air has flowed over into the anode circuit 28, in order to shape the recirculation conveying device 31 via the exhaust air turbine 32 in the exemplary embodiment shown here to drive the fan. The refresh of the catalytic converter is then completed after a short time, for example on the order of less than a minute. The oxygen-containing gas can then be sucked out of the anode circuit again by opening the purge valve 18 again and the system can be filled with nitrogen in the manner described above, for example, in order to prepare it for the next start.

LIST OF REFERENCE NUMERALS Air compressor 28 Anode circuit, fuel cell system 29 Recirculation line, air delivery device 30 Water separator, electric motor 31 Recirculation delivery device, shaft 32 Exhaust air turbine, second compressor wheel 33 Shaft, first compressor wheel 34 Liquid water system, suction path 35 Valve device, register line 36 Valve device, compressor side 37 Cathode bypass, Free-running turbocharger, water separator, cathode line, 41, Turbocharger, cathode tank line, 42, Turbocharger, cathode tank line, 40, Turbocharger, gas jet pump, shaft 43 Arrow blow-off line 44 thermal insulation purge valve 45 water pump fuel cell stack 46 branch line anode compartment 47 humidifier cathode compartment 48 humidifier supply air line 49 branch line exhaust air line 50 pressure water distributor exhaust gas recirculation valve 51 branch exhaust gas recirculation line 52 branch line fuel 53 hydrodynamic bearing supply device 54 hydrodynamic bearing pressure control and 55 catalyst dos i device 56 hydrogen metering line

Claims

1. Fuel cell system (2) with at least one fuel cell stack (19) which comprises an anode space (20) and a cathode space (21), with at least one air delivery device (3) for supplying the cathode space (21) with air via an air supply line (22) , with an exhaust air line (23) from the cathode space (21), with at least one fuel supply device (26) for supplying the anode space (20) with fuel, with at least one anode circuit (28) for the recirculation of unused fuel around the anode space (20), furthermore with a cathode bypass (37), characterized in that the cathode bypass (37) branches off in front of or in the area of a valve device (35) in the supply air line (22) and after or in the area of another valve device (36) in the exhaust air line (23) ) opens into this, with a gas jet pump (38) which can be driven by the air flowing around the cathode space (21) and which is also arranged on the suction side in the cathode bypass (37) the anode compartment (20) and / or the cathode compartment (21) is switchably connected.

2. Fuel cell system (2) according to claim 1, characterized in that in the anode circuit (28) a fan as a recirculation conveyor (31) is driven by an exhaust air turbine (32) in the exhaust air line (23).

3. Fuel cell system (2) according to claim 1 or 2, characterized in that at least one of the valve devices (35, 36) in the supply air line (22) and / or the exhaust air line (23) is designed as a 3/2-way valve.

4. Fuel cell system (2) according to claim 1, 2 or 3, characterized in that the air delivery device (3) is designed in two stages.

5. Fuel cell system (2) according to claim 4, characterized in that the two-stage air delivery device (3) is designed in the form of a free-running turbocharger (11) which is connected on the turbine side to the pressure side of a first compressor wheel (7) of an air compressor (1), and which on the compressor side is connected to the pressure side of a second compressor wheel (6) of the same air compressor (1).

6. Fuel cell system (2) according to claim 5, characterized in that the first and the second compressor wheel (6, 7) are symmetrical and are arranged on a shaft (5) with a common electric motor (4).

7. Fuel cell system (2) according to one of claims 1 to 6, characterized in that a water separator (41) is arranged in the flow direction of the exhaust air after the mouth of the cathode bypass (37) in the exhaust air line (23), and in the execution according to claim 2 in particular in the flow direction of the exhaust air upstream of the exhaust air turbine (32).

8. Fuel cell system (2) according to one of claims 4 to 7, characterized in that an exhaust gas recirculation line (25), the exhaust air line (23) after the mouth of the cathode bypass (37), and in an embodiment according to claim 2 in particular after the exhaust air turbine (32 ), with a register line (9) switchably connects between the two stages of compression.

9. Fuel cell system (2) according to claim 7 or 8, characterized in that the at least one water separator (30, 41) with a water tank (42) is connected or forms, wherein the water tank (42) is connected via a water pump (45) to a pressure water distributor (50), from which branch lines (46, 49, 51, 52) branch off to consumption points.

10. Fuel cell system (2) according to claim 9, characterized in that the consumption points hydrodynamic bearings (53, 54) of the free-running turbocharger (11) and / or at least one humidifier (47, 48) in the supply air, in particular in the form of a or two-fluid nozzle.

11. Fuel cell system according to claim 10 and according to one of claims 4 to 8, characterized in that the at least one humidifier (47, 48) is arranged in the supply air before and / or after the second compressor stage.

12. The method for operating a fuel cell system according to one of claims 1 to 11, characterized in that, if required, gases from the cathode compartment (31) and / or the anode circuit (28) are sucked off via the gas jet pump (38) in the cathode bypass line (37) .