Fuel cell system and method for operating a fuel cell system

Abstract

The invention relates to a fuel cell system. The fuel cell system comprises a main fuel cell and a recirculation path. The main fuel cell comprises: an anode inlet through which fuel can flow into the main fuel cell; and an anode outlet from which an anode exhaust gas including unconsumed fuel can flow out. The recirculation path, in which the anode exhaust gas can flow as a recirculate, connects the anode inlet to the anode outlet. The recirculation path comprises an electrochemical filter and a bypass path. The electrochemical filter is designed to extract the unconsumed fuel from the recirculate. The extracted fuel can be recirculated to the anode inlet. The recirculate can be recirculated from the anode outlet to the anode inlet through the bypass path, while bypassing the electrochemical filter. The fuel cell system is designed to operate with a bypass ratio of more than 70 percent. The bypass ratio is defined as a ratio of a portion of the recirculate flowing through the bypass path to a total amount of the recirculate. The invention also relates to a corresponding method for operating a fuel cell system.

Description

[0001] Woodward L'Orange GmbH F&R Ref.: 46715-0170WO1 New PCT Application July 28, 2025

[0002] FUEL CELL SYSTEM AND METHOD FOR OPERATING A FUEL CELL SYSTEM

[0003] The present disclosure relates to a fuel cell system and a method for operating a fuel cell system.

[0004] A fundamental development goal for fuel cells in general, and especially for proton exchange membrane fuel cells (PEM fuel cells) or polymer electrolyte membrane fuel cells (PEM fuel cells), is to achieve the highest possible system efficiency of the fuel cell system.

[0005] A good way to achieve this is to use the fuel as efficiently as possible on the anode side of the fuel cell.

[0006] All common fuels for fuel cells are suitable, especially hydrogen and methanol. However, hydrogen-containing gases, such as reformate gas, which is produced, for example, from the reforming of methanol, ethanol, natural gas, or LPG (Liquefied Petroleum Gas), can also be used to operate fuel cells.

[0007] To ensure a good supply of fuel to the catalyst layers in all areas of the fuel cell (and thus increase system efficiency), the anode can be operated superstoichiometrically. In this mode, the anode is supplied with more fuel than would (theoretically) be required to maintain normal operation. Superstoichiometric operation ensures that even downstream areas of the anode, such as those near the anode outlet, are adequately supplied with fuel. However, the release of the resulting anode exhaust gas causes higher losses, as unused fuel is released into the environment. This reduces system efficiency. Woodward L'Orange GmbH F&R Ref.: 46715-0170WO1 New PCT Application July 28, 2025

[0008] Furthermore, releasing unused fuel is also undesirable for environmental reasons. Hydrocarbon-based fuels and (pure) hydrogen can contribute to atmospheric warming and damage the ozone layer. For example, in April 2022, the British government published a study showing that hydrogen released into the atmosphere accelerates global warming eleven times faster than the same amount of CO2.

[0009] To minimize the escape of unused fuel, the anode exhaust gas can be recirculated. This can be done passively (by a jet pump) or actively (by a blower). During recirculation, the anode exhaust gas is returned to the anode inlet of the anode compartment of a fuel cell via a flow path, the recirculation path, through the anode outlet. This allows the previously unused fuel to re-enter the anode compartment and thus be available for a further reaction.

[0010] The high fuel flow rate generated by the superstoichiometric operation of the fuel cell, including anode exhaust recirculation, does in principle make it possible to achieve high stoichiometry at the catalyst layers and thus a uniform fuel distribution without significant fuel losses. However, the recirculation leads to an enrichment of reaction products, such as nitrogen and water, and consequently to fuel depletion in the anode circuit. This undesirable enrichment also occurs in systems with recirculation on the anode side, even without superstoichiometric operation, because the reaction products diffuse from the cathode to the anode side via the fuel cell's membrane system.

[0011] This can have undesirable effects on the function of the fuel cell stack, such as a voltage drop during operation. To minimize or compensate for these effects and also to maintain or improve the longevity of the fuel cell system, an anode circuit purge can be performed. Woodward L'Orange GmbH F&R Ref: 46715-0170WO1

[0012] The new PCT application of July 28, 2025, will be used. The reaction products enriched in the recirculated fuel can thus be removed from the system by draining the mixture of fuel and reaction products contained in the recirculated fuel (preferably intermittently, but also continuously) from the recirculation path via a purging device, such as a purge valve, and replacing it with pure fuel.

[0013] This represents a significant loss of efficiency because unused fuel is released into the environment during this purging process. The fuel expelled from the system is no longer available for power generation in the fuel cell. Furthermore, this release of fuel into the environment is also undesirable due to its impact on the greenhouse effect.

[0014] One object of the present invention is therefore to prevent or minimize the accumulation of reaction products on the anode side and thereby avoid or reduce harmful emissions into the environment. More generally, the object of the invention is to provide a fuel cell system that can be operated efficiently and in an environmentally friendly manner.

[0015] The problem is solved by the fuel cell system and the method according to the independent claims. Advantageous embodiments of the present invention are the subject of the dependent claims.

[0016] In general, the enrichment of the anode recirculation or the anode side with reaction products can be counteracted by separating the liquid components of the reaction products from the anode recirculation using a liquid separator. For example, liquid water can be removed from the anode recirculation in this way.

[0017] On the other hand, the accumulation of reaction products can be reduced or even completely prevented by separating gaseous components of the reaction products (inert gas) from the gaseous fuel and flushing them out of the recirculation path. (See Woodward L'Orange GmbH F&R Ref: 46715-0170WO1)

[0018] A new PCT application dated July 28, 2025, states that for a hydrogen and atmospheric oxygen fuel cell, such as a PEM fuel cell, it is desirable to separate a gaseous hydrogen-nitrogen mixture into its components, hydrogen and nitrogen. This would allow the hydrogen to be recirculated to the anode compartment and the nitrogen to be purged from the system.

[0019] This would increase system efficiency and reduce or even eliminate unwanted emissions. Firstly, flushing processes would be less frequent and, ideally, unnecessary. Secondly, the substances flushed from the system would contain less fuel.

[0020] One technical approach to minimizing the presence of gaseous reaction products, in addition to liquid ones, in the recirculated fluid returned to the anode is mechanical or physical fuel separation. However, this technology has room for improvement, particularly regarding its efficiency.

[0021] Therefore, electrochemical fuel separation from the recirculated material is preferred.

[0022] The present invention is based on an electrochemical filter for fuel (electrochemical fuel filter, which can be, for example, an electrochemical hydrogen filter), such as hydrogen, or for the removal of reaction products, such as nitrogen and liquid water. Compared to mechanical alternatives, such as centrifuges, an electrochemical filter requires less power and is more efficient with regard to the overall efficiency of the anode side and thus of the fuel cell system as a whole. The electrochemical filter continuously removes reaction products from the anode recirculation path or the anode side of the fuel cell system, preventing high levels of reaction products from accumulating in the anode recirculation path, which increases the overall system efficiency and the lifetime of the fuel cell system.In comparison, a flushing valve without an upstream electrochemical filter operates intermittently. The intermittent operation of a flushing valve operating independently allows for the enrichment of larger quantities. Woodward L'Orange GmbH F&R Ref: 46715-0170WO1.

[0023] New PCT application July 28, 2025, of the reaction products in the anode recirculation path, leading to a reduction in fuel cell performance and other disadvantages.

[0024] To minimize the proportion of gaseous reaction products, such as nitrogen, in the recirculated fluid returned to the fuel cell anode, in addition to the proportion of liquid reaction products, such as water, media separation or filtration is advantageous. During operation of the fuel cell system, the exhaust gas stream from the fuel cell's anode outlet is fed to the anode of the electrochemical filter. In the electrochemical filter, the fuel, such as molecular hydrogen, is oxidized, releasing electrons. The fuel ions are transported through the proton-conducting membrane system of the electrochemical filter, which, for example, comprises a polymer membrane.The electrons flow via the electrically conductive components and the external circuit to the cathode of the electrochemical filter, where the fuel ions, such as protons, are reduced and recombine to form fuel, such as molecular hydrogen. Primarily fuel ions can be transported through the membrane system of the electrochemical filter. Other components of the fuel cell's anode exhaust remain essentially entirely on the anode side of the electrochemical filter and are removed from the recirculation path or the fuel cell system as anode exhaust from the electrochemical filter (and the fuel cell). A minor exchange of reaction product molecules, such as nitrogen and water molecules, occurs between the anode and cathode sides of the electrochemical filter through permeation processes.

[0025] The electrochemical filter is very similar in its operating principle to a fuel cell. Therefore, it can also be advantageous to integrate it into the fuel cell or fuel cell system. To distinguish it from the electrochemical filter, the fuel cell of the fuel cell system, in whose anode recirculation path the electrochemical filter is located, will be referred to as the main fuel cell in the following. Woodward L'Orange GmbH F&R Ref.: 46715-0170WO1 New PCT Application July 28, 2025

[0026] To solve the problem described above, the present invention comprises a fuel cell system with a main fuel cell and a recirculation path.

[0027] The main fuel cell comprises an anode inlet and an anode outlet. Fuel can flow into the main fuel cell through the anode inlet (during operation of the fuel cell system). In other words, the anode inlet is designed to supply fuel to the main fuel cell, specifically to the anode compartment of its anode. Anode exhaust gas, including unused fuel, can flow out of the anode outlet (during operation of the fuel cell system). In other words, the anode outlet is designed to discharge or expel the anode exhaust gas from the anode compartment.

[0028] The recirculation path connects the anode inlet to the anode outlet of the main fuel cell, allowing the anode exhaust gas to flow as recirculated fluid from the anode outlet to the anode inlet. In other words, the recirculation path is designed to (partially) return or recirculate anode exhaust gas from the anode compartment of the main fuel cell to it. The recirculation path includes an electrochemical filter and a bypass path. The electrochemical filter and the bypass path are located within the recirculation path. The electrochemical filter is designed to extract the unused fuel from the recirculated fluid. The fuel extracted or recovered by the electrochemical filter can then be directed to the anode inlet or returned (recirculated) to the anode compartment. The bypass path allows the recirculated fluid, a mixture of fuel and reaction products, to be directed to the anode inlet.The recirculated fluid is returned (recirculated) to the anode compartment. In other words, the bypass path allows the recirculated fluid to be recirculated from the anode outlet to the anode inlet of the main fuel cell, bypassing the electrochemical filter.

[0029] When reference is made to the anode exhaust of an anode, this refers to the composition of substances that flow out of the anode outlet. This composition can contain not only gaseous components but also liquid components. Woodward L'Orange GmbH F&R Ref.: 46715-0170WO1 New PCT Application July 28, 2025

[0030] When reference is made to a flow direction, this generally refers to the flow direction of the recirculated fluid from the anode outlet to the anode inlet, unless a different flow direction is defined in a specific case.

[0031] According to the invention, a bypass ratio is defined. The bypass ratio is the quotient of the volume fraction of the recirculated material flowing through the bypass path to the total quantity (total volume) of the recirculated material (more precisely, the exhaust gas discharged from the anode outlet). In other words, the bypass ratio is the ratio of the unfiltered exhaust gas volume passing through the electrochemical filter to the total exhaust gas volume discharged from the anode of the main fuel cell. The sum of the quotients of the bypass ratio and the exhaust gas flow exiting the anode outlet of the main fuel cell, as well as the quotients of the volume flow through the electrochemical filter and the exhaust gas flow exiting the anode outlet of the main fuel cell, is one.In other words, the sum of the bypass flow rate (which is the flow rate through the bypass path), the fuel flow rate extracted from the recirculated fluid (which is the flow rate released from the cathode outlet of the electrochemical filter), and the flow rate purged from the fuel cell system or the recirculation path (which is the flow rate released from the anode outlet of the electrochemical filter) equals the flow rate released from the anode outlet of the main fuel cell. The bypass flow rate does not need to be completely recirculated to the anode inlet of the main fuel cell.

[0032] According to the invention, the bypass ratio is over 70 percent. This means that in normal operation of the fuel cell system, less than 30 percent of the anode exhaust gas from the main fuel cell flows through the electrochemical filter. The fuel cell system is designed to operate with a bypass ratio of over 70 percent in normal (continuous) operation.

[0033] The relatively high bypass ratio results in a relatively low volume flow through the electrochemical filter. This in turn leads to a low Woodward L'Orange GmbH F&R Ref: 46715-0170WO1

[0034] New PCT registration date: July 28, 2025

[0035] The power consumption associated with operating the electrochemical filter. In other words, the high bypass ratio ensures that little power needs to be supplied to the electrochemical filter, which has a positive effect on the overall efficiency of the fuel cell system.

[0036] Preferably, the bypass ratio is in the range of at least 75%, preferably at least 80%, and even more preferably at least 85%, particularly 85 to 99%. Even more preferably, the bypass ratio is in the range of 90 to 98%. Even more preferably, the bypass ratio is in the range of 92.5 to 97.5%. Even more preferably, the bypass ratio is in the range of 95 to 97%. As described, the highest possible bypass ratio is desirable with regard to the overall system efficiency.

[0037] On the other hand, a certain volume flow through the electrochemical filter is desirable in order to extract unused fuel or to remove reaction products from the recirculated fluid.

[0038] Preferably, the recirculation path includes a purge path. The purge path is configured to discharge the recirculated fluid, depleted of the extracted fuel, from the recirculation path or the fuel cell system as it flows out of the electrochemical filter. In other words, the purge path is configured to purge the anode exhaust gas of the electrochemical filter. Accordingly, the purge path is fluidically connected to the anode outlet of the anode compartment of the electrochemical filter.

[0039] Optionally, the purging path includes a purging valve. The purging valve can be operated intermittently or continuously. The purging valve allows for metering the flow rate of the fluid purged from the fuel cell system.

[0040] Preferably, the main fuel cell comprises a main anode, a main cathode, and a main membrane system. The main membrane system is arranged between the main anode and the main cathode. Woodward L'Orange GmbH F&R Ref: 46715-0170WO1 New PCT Application July 28, 2025

[0041] The main membrane system can perform a (first) inert gas permeation (n IMain ). In this context, inert gas permeation generally refers to a molar flow rate (mmol / s) of an inert gas from one side of a cathode to one side of an anode across a membrane system. In the case of the main membrane system, the (first) inert gas permeation therefore refers to a molar flow rate of an inert gas, such as nitrogen, from one side of the main cathode (main cathode side) to one side of the main anode (main anode side) of the main fuel cell across the main membrane system.

[0042] Preferably, the electrochemical filter comprises a filter anode, a filter cathode, and a filter membrane system. The filter membrane system is arranged between the filter anode and the filter cathode.

[0043] The filter membrane system can have a (second) inert gas permeation.

[0044] Preferably, the second inert gas permeation is significantly smaller than the first inert gas permeation, so that the amount of inert gas passing through the filter membrane system (during normal operation of the fuel cell system) can be neglected.

[0045] Even more preferably, the second inert gas permeation is set up to be so much smaller than the first inert gas permeation that a ratio (SR) of a molar mass flow rate of the inert gas (n) EFPurge ), which (in normal operation of the fuel cell system) flows out of an anode outlet of the filter anode instead of via the filter membrane system, and the first inert gas permeation (hiMain ) is at least 1.5. Therefore, the following applies:

[0046] Even more preferred is SR > 1.75, and even more preferred is SR > 2.0. Woodward L'Orange GmbH F&R Ref.: 46715-0170WO1 New PCT application July 28, 2025

[0047] In other words, during normal operation of the fuel cell system, significantly more inert gas flows out of the anode outlet of the electrochemical filter (and thus out of the recirculation path or the fuel cell system) than inert gas reaches the main anode side via the main membrane system. This reduces or prevents the main anode side from becoming enriched with inert gas, such as nitrogen, or from becoming depleted of fuel.

[0048] The recirculation path can include a valve, such as a directional control valve, to control the bypass ratio. This valve allows direct control of the recirculated fluid composition, specifically the proportions of fuel and reaction products, and also directly influences the power consumption of the electrochemical filter. The operation and control of the valve can thus have a direct impact on the overall efficiency and lifespan of the fuel cell system. The valve can be designed simply, making it easy and relatively inexpensive to integrate into the fuel cell system.

[0049] Alternatively, the fuel cell system can be configured to control the bypass ratio (exclusively) by energizing the electrochemical filter. In other words, the bypass ratio can be controlled by varying, for example, the power output of the electrochemical filter. Preferably, no valve, such as a directional control valve, is provided or required to control the bypass ratio. This reduces the complexity, cost, and susceptibility to mechanical failure of the fuel cell system, while still offering similar advantages to the use of a valve (controllability of the bypass ratio and thus the overall efficiency and lifespan of the fuel cell system).

[0050] The recirculation path may include a recirculation drive. The recirculation drive is preferably a blower or a jet pump.

[0051] The electrochemical filter can be arranged in parallel with the recirculation drive in the recirculation path. Alternatively, the electrochemical filter can be connected in series with the Woodward L'Orange GmbH F&R Ref.: 46715-0170WO1 New PCT Application July 28, 2025

[0052] The recirculation drive can be located within the recirculation path. This allows for great flexibility in the design of the recirculation path.

[0053] By reducing the inert gas content or maintaining a low level of inert gas using the electrochemical filter during operation of the fuel cell system, a reduction in the power consumption of the recirculation drive can be achieved compared to a system without an electrochemical filter.

[0054] In general, the present invention is not limited to a particular type of fuel cell, but is preferably directed to PEM fuel cells.

[0055] Preferably, the fuel is hydrogen. Alternatively or additionally, the inert gas is nitrogen.

[0056] The main fuel cell is preferably a main fuel cell capable of operating in a superstoichiometric manner. The operation of the fuel cell system according to the invention, or of the main fuel cell comprised therein, is not limited to superstoichiometric operation, as long as the main fuel cell is capable of expelling unused fuel from the anode compartment, or as long as the main fuel cell expels unused or unconsumed fuel during operation.

[0057] Optionally, the fuel cell system or the recirculation path has a purging function (for the recirculated fuel depleted by the fuel extracted in the electrochemical filter).

[0058] (Main anode exhaust) no purge valve. The fuel cell system or recirculation path is configured to control purging via the bypass ratio (e.g., via the directional control valve described above) and / or the energizing of the electrochemical filter. This reduces the complexity (e.g., the number of components and / or the control system) of the fuel cell system or recirculation path, as well as the associated costs and efficiency losses, and increases the system's robustness with regard to mechanical disturbances. Woodward L'Orange GmbH F&R Ref.: 46715-0170WO1 New PCT application July 28, 2025

[0059] Preferably, the fuel cell system or recirculation path does not include a water separator (liquid separator). The fuel cell system or recirculation path is designed to remove any water (or liquid) contained in the recirculated fluid via the electrochemical filter. This reduces the complexity (e.g., the number of components) of the fuel cell system or recirculation path, as well as the associated costs and efficiency losses, and increases the system's robustness with regard to mechanical disturbances.

[0060] Preferably, the fuel cell system does not include a humidification device in the main fuel line. The main fuel line is defined as the flow path from the fuel (pressure) storage tank, via a metering device, towards the anode chamber, to which the fuel extracted in the electrochemical filter is added. Since the electrochemical filter can introduce more moisture into the extracted fuel than other separation devices for fuel and reaction products, a separate humidification device in the main fuel line is preferably omitted. This reduces the complexity (e.g., the number of components and / or the control system) of the fuel cell system, as well as the associated costs and efficiency losses, and increases the system's robustness with regard to mechanical disturbances.

[0061] To solve the problem described above, the present invention also includes a method for operating a fuel cell system.

[0062] The fuel cell system can be one of the fuel cell systems described above. The fuel cell system according to the invention comprises a main fuel cell and a recirculation path. The recirculation path connects an anode outlet of an anode compartment of an anode of the main fuel cell to an anode inlet of the anode compartment of the anode. The recirculation path includes an electrochemical filter.

[0063] The process according to the invention comprises the discharge and splitting of an exhaust gas as well as the extraction and supply of a fuel. Woodward L'Orange GmbH F&R Ref.: 46715-0170WO1 New PCT application July 28, 2025

[0064] The exhaust process involves releasing the exhaust gas from the anode chamber via the anode outlet into the recirculation path. This exhaust gas contains unburned fuel. The exhaust gas released from the anode outlet flows into the recirculation path as recirculated fuel towards the anode inlet.

[0065] The splitting process involves dividing the total recirculated flow into at least two partial flows. One (first) partial flow passes through the electrochemical filter. In other words, the first partial flow is recirculated (partially, with depleted exhaust gas not being recirculated) through the electrochemical filter (to the anode compartment of the main fuel cell). A (second) partial flow passes to the anode inlet, bypassing the electrochemical filter. That is, the second partial flow of the recirculated material is recirculated (to the anode compartment of the main fuel cell) without passing through or around the electrochemical filter.

[0066] Extraction involves extracting the unused fuel, which is located in the first partial stream of the recirculated fluid, through the electrochemical filter.

[0067] The feed process includes supplying the fuel extracted from the first partial stream of the recirculated fuel through the electrochemical filter to the anode inlet. The extracted fuel also contains reaction products, such as nitrogen, but in negligible amounts. The extracted fuel may also contain significant amounts of water. This can be conveniently used to moisten the fuel flowing in the main fuel line. The feed process further includes supplying the second partial stream, which contains unused fuel but also reaction products, to the anode inlet.

[0068] The initial partial flow through the electrochemical filter, from which the fuel is extracted, is less than 30 percent of the total recirculated or exhaust gas flow leaving or being expelled from the anode compartment. The advantages of the comparatively low volume flow through the electrochemical filter, and the associated relatively high bypass ratio, with regard to the function and efficiency of the fuel cell system, have been explained above. (Woodward L'Orange GmbH F&R Ref.: 46715-0170WO1 New PCT Application July 28, 2025)

[0069] Preferably, the first partial flow that passes through the electrochemical filter and from which the fuel is extracted is at most 25, preferably at most 20, and even more preferably at most 15 percent of the total flow, in particular 1 to 15 percent of the total flow of the recirculated fluid or exhaust gas leaving the anode chamber, even more preferably 2 to 10 percent, even more preferably 2.5 to 7.5 percent and even more preferably 3 to 5 percent.

[0070] Preferably, the method also includes the continuous removal of an inert gas from the recirculated gas. The advantage gained from this is that the proportion of inert gas on the anode side of the fuel cell system can always be kept at a low level.

[0071] Preferably, the process also includes purging the portion of the recirculated fluid or exhaust gas flowing from the electrochemical filter, which has been depleted of the extracted fuel, from the recirculation path. Even more preferably, the purging takes place without the use of a purge valve. The advantages of being able to dispense with a purge valve have been explained above.

[0072] Preferably, the (second) inert gas permeation at the membrane system of the electrochemical filter (filter membrane system) is lower than the (first) inert gas permeation at the membrane system of the main fuel cell (main membrane system).

[0073] Even more preferred is a second inert gas permeation that is so small than the first inert gas permeation that the molar mass flow rate of the inert gas (during normal operation of the fuel cell system) through the filter membrane system is negligible.

[0074] Even more preferred (in normal operation of the fuel cell system) due to the low second inert gas permeation is a ratio of a molar mass flow rate of a Woodward L'Orange GmbH F&R Ref: 46715-0170WO1

[0075] New PCT registration date: July 28, 2025

[0076] The molar mass flow rate of the inert gas, which flows out of the anode outlet of the electrochemical filter (filter anode) instead of through the membrane system of the electrochemical filter, and the inert gas permeation at the membrane system of the main fuel cell (main membrane system) is at least 1.5, preferably at least 1.75, and even more preferably at least 2.0. The advantages of a comparatively high molar mass flow rate from the filter anode, relative to the inert gas permeation at the main membrane system, have been explained above.

[0077] Preferably, the total recirculation flow is divided by energizing or controlling the power of the electrochemical filter; even more preferably, exclusively by energizing or controlling the power. In the latter case, a valve, such as a directional control valve, for dividing the total flow, which is located upstream of the electrochemical filter in the recirculation path, can be omitted. This offers the advantages described above.

[0078] Preferably, water contained in the recirculated fluid is separated via the electrochemical filter. Even more preferably, the water is separated exclusively via the electrochemical filter. This eliminates the need for a separate water separation unit or one specifically designed for water removal. The associated advantages have been described above.

[0079] Preferably, the fuel supplied to the main anode is humidified via the electrochemical filter. Even more preferably, the fuel is humidified exclusively via the electrochemical filter. This eliminates the need for a separate fuel humidification device or one specifically designed for fuel humidification. The associated advantages have been described above.

[0080] In the method according to the invention, the main fuel cell is preferably a PEM fuel cell.

[0081] In the process according to the invention, the fuel is preferably hydrogen. Additionally or alternatively, the inert gas is preferably nitrogen. Woodward L'Orange GmbH F&R Ref: 46715-0170WO1 New PCT application July 28, 2025

[0082] In the process according to the invention, the main fuel cell is preferably operated superstoichiometrically. This has the advantage already described that all catalyst layers can be well supplied with fuel and the fuel cell can be operated efficiently.

[0083] Preferred embodiments of the present invention, which are not intended to limit it, are described in detail below. All features described above in general terms and below as part of the embodiments can be combined with one another as desired, as long as this appears technically or economically sensible.

[0084] FIG. 1 shows a schematic view of a fuel cell system.

[0085] FIG. 2 shows a schematic view of an electrochemical filter for the fuel cell system.

[0086] FIG. 3 shows a schematic view of a main fuel cell and an example of an anode side including recirculation path and main fuel string of the fuel cell system.

[0087] FIG. 4 shows a schematic view of a main fuel cell and another example of an anode side including recirculation path and main fuel string of the fuel cell system.

[0088] FIG. 5 shows a schematic view of a main fuel cell and another example of an anode side including recirculation path and main fuel string of the fuel cell system.

[0089] FIG. 6 shows, in a schematic view corresponding to that of FIG. 3, examples of material flows in the main fuel cell and on the main anode side. Woodward L'Orange GmbH F&R Ref: 46715-0170WO1

[0090] New PCT registration date: July 28, 2025

[0091] FIG. 7 shows a diagram illustrating the relationship between the volume flow rate through the electrochemical filter and the power consumption of the electrochemical filter.

[0092] The present invention relates generally to a fuel cell system. The fuel cell system comprises a main fuel cell and a recirculation path.

[0093] The main fuel cell includes an anode inlet through which fuel can flow into the main fuel cell, and an anode outlet from which an anode exhaust including unused fuel can flow out.

[0094] The recirculation path, through which the anode exhaust gas can flow as recirculated fluid, connects the anode inlet to the anode outlet. The recirculation path includes an electrochemical filter and a bypass path. The electrochemical filter is designed to extract the unused fuel from the recirculated fluid. The extracted fuel can then be recirculated to the anode inlet. The bypass path allows the recirculated fluid to flow from the anode outlet to the anode inlet, bypassing the electrochemical filter. The fuel cell system is designed for operation with a bypass ratio exceeding 70 percent. The bypass ratio is the ratio of the proportion of recirculated fluid flowing through the bypass path to the total amount of recirculated fluid.

[0095] The present invention also generally relates to a method for operating a fuel cell system comprising a main fuel cell and a recirculation path.

[0096] The recirculation path connects an anode outlet of an anode compartment of the main fuel cell to an anode inlet of the anode compartment. The recirculation path includes an electrochemical filter.

[0097] The process comprises a discharge, a split, an extraction, and a feed. Discharge is the discharge of exhaust gas containing unused fuel from the anode compartment via the anode outlet into the recirculation path as recirculated fuel. Splitting is the division of a total recirculated fuel flow into at least two partial flows: a first partial flow passing through the electrochemical filter and a second partial flow bypassing the electrochemical filter to the anode inlet. Extraction is the extraction of the unused fuel in the first partial flow through the electrochemical filter. Feeding is the feeding of the fuel extracted from the first partial flow and the second partial flow to the anode inlet. The first partial flow is less than 30 percent of the total flow.

[0098] In the following, identical parts are designated with the same reference symbols.

[0099] As described above, the fuel cell is not limited to a specific type. It has also been described that the fuel is not limited to a specific fuel. In the following embodiments, a PEM fuel cell system using hydrogen (H2) as the fuel is described as an example. Atmospheric oxygen (O2) serves as the oxidizing agent. The reaction products are accordingly mainly water (H2O) and nitrogen (N2).

[0100] Figure 1 shows a fuel cell system 1. The fuel cell system 1 comprises a fuel cell 3, hereinafter referred to as the main fuel cell 3, and a recirculation path 2 for the main fuel cell 3. The main fuel cell 3 has an anode compartment 4 on the anode side (to the left of the main fuel cell 3 in the representation of Figure 1) and a cathode compartment 5 on the cathode side (to the right of the main fuel cell 3 in the representation of Figure 1).

[0101] The cathode side of the main fuel cell 3 comprises a turbine-driven compressor, a cooling unit, a humidification unit, and a liquid separator (water separator). The cathode side of the main fuel cell 3 serves to supply and, preferably, recirculate the oxidizer (oxygen) to and from the main fuel cell 3. The cathode side of the main fuel cell 3 shown here represents a conventional cathode side of a known main fuel cell. Woodward L'Orange GmbH F&R Ref: 46715-0170WO1 New PCT Application July 28, 2025

[0102] The anode side of the main fuel cell 3 comprises a recirculation path 2 that recirculates fuel from an outlet 12 of the anode to an inlet 13 of the anode. Downstream of the outlet 12, relative to the flow direction of the recirculated fluid (indicated by arrows), which initially contains the fuel (hydrogen) and the reaction products (water and nitrogen), a separation device 6, such as a water separator, is arranged in the recirculation path 2 to separate the liquid (water) from the recirculated fluid. The separation device 6 is optional.

[0103] Downstream of the water separator 6, a branch 10 of the recirculation path 2 is arranged. The branch 10 is configured to direct the flow of the (now gaseous) recirculated fluid towards a conveying device 7 and a rinsing device 8. The branch 10 can direct the flow either to the conveying device 7 or to the rinsing device 8, or to both simultaneously.

[0104] The purging device 8 is arranged downstream of the branch 10 in the recirculation path 2. The purging device 8 can be a purging valve. The purging valve 8 is configured to purge recirculated fluid from the recirculation path 2 or the fuel cell system 1 continuously or intermittently. The purging valve 8 can operate intermittently. In the fuel cell system 1 shown in FIG. 1, the recirculated fluid contains inert gas (nitrogen) and gaseous fuel (hydrogen). Purging accordingly reduces the system efficiency due to hydrogen loss and, as described above, results in environmental disadvantages due to the release of hydrogen into the environment. Purging is nevertheless necessary in the fuel cell system 1 shown.This is accepted because otherwise the recirculated gas would become increasingly enriched with nitrogen, which diffuses from the air into the anode compartment 4 during operation of the main fuel cell 3 through the intervening membrane system of the main fuel cell 3, hereinafter referred to as the main membrane system. Consequently, due to the recirculation, the gas supplied to the anode compartment 4 becomes increasingly enriched with nitrogen, which impairs the operation of the main fuel cell 3, as described above. Woodward L'Orange GmbH F&R Ref: 46715-0170WO1 New PCT Application July 28, 2025.

[0105] Also downstream of branch 10, the conveying device 7, such as a recirculation blower, is arranged in the recirculation path 2. The recirculation blower 7 draws in the gaseous recirculated gas and expels it towards the anode chamber 4.

[0106] Any two of the three components, or all three components comprising the recirculation blower 7, the flushing valve 8 and the water separator 6, can be designed as an integrated unit in the recirculation path 2.

[0107] Downstream of the recirculation blower 7, a further branch 11 is arranged in the recirculation path 2. The branch 11 directs the recirculated fluid expelled by the upstream recirculation blower 7 towards the inlet 13 of the anode chamber 4. Also upstream of the branch 11 (in the direction of hydrogen flow), a valve 9, such as a metering valve, is shown in a main fuel line 14 of the anode chamber 4. This valve serves as a feed valve and meters and / or shuts off the hydrogen supplied from a fuel storage container 15 (e.g., a pressure vessel; see Figures 3-6) or allows it to pass through to the anode chamber 4. The branch 11 is configured to direct the flow of recirculated fluid and / or hydrogen from the main fuel line 14 towards the anode chamber 4. Branch 11 can be configured to interrupt the gas supply to the anode chamber 4. Branch 11 can, for example, power a jet pump 16 (see figure).Figures 3 and 6). The branch 11 connects the main fuel line 14 with the recirculation path 2 upstream of the inlet 13 of the anode of the main fuel cell 3 (hereinafter referred to as the main anode).

[0108] FIG. 2 shows an electrochemical filter which, according to the invention, is arranged in the recirculation path 2 of FIG. 1 and extracts unused fuel from the recirculate of the main anode, here also referred to as main anode recirculate.

[0109] The electrochemical filter 20 is very similar in its operating principle to a fuel cell, such as the main fuel cell 3. Woodward L'Orange GmbH F&R Ref: 46715-0170WO1

[0110] New PCT registration date: July 28, 2025

[0111] In order to keep the proportion of gaseous reaction products, such as the nitrogen content N2, as low as possible in the recirculated fluid returned to the main anode, in addition to the proportion of liquid reaction products, such as the water content H2O, media separation or filtration is advantageous.

[0112] As shown schematically in FIG. 2, during operation of the fuel cell system 1, the exhaust gas stream from the anode outlet 12 of the main fuel cell 3 is fed to the anode 21 of the electrochemical filter 20, also referred to herein as filter anode 21, via the anode inlet 24 of the electrochemical filter 20, also referred to herein as filter anode inlet 24. In the electrochemical filter 20, the molecular hydrogen H₂ is oxidized and releases electrons e⁻. The hydrogen ions H⁺ are transported through the proton-conducting membrane system 23 of the electrochemical filter 20 (also referred to herein as filter membrane system 23), which, for example, comprises a polymer membrane.The electrons e- flow via the electrically conductive components and the external circuit to the cathode 22 of the electrochemical filter 20 (also referred to here as filter cathode 22), where the hydrogen ions H+ (protons) are reduced again and recombine to form molecular hydrogen H2. The molecular hydrogen H2 then flows via the outlet 26 of the filter cathode 22, also referred to here as filter cathode outlet 26, towards the main fuel line 14 or the inlet 13 of the anode compartment 4 of the main anode.

[0113] Primarily hydrogen ions (H+) can be transported through the filter membrane system 23. Other components of the anode exhaust gas from the main fuel cell 3 preferably remain essentially entirely on the side of the filter anode 21 and are removed from the recirculation path 2 or the fuel cell system 1 as anode exhaust gas from the electrochemical filter 20 (filter anode exhaust gas) via the outlet 25 of the filter anode 21, also referred to here as the filter anode outlet 25. However, a slight exchange of nitrogen molecules (N2) and water molecules (H2O) takes place between the side of the filter anode 21 and the side of the filter cathode 22 due to permeation processes, as indicated in FIG. 2. It may be particularly advantageous with regard to the humidification of the fuel Woodward L' Orange GmbH F&R Ref.: 46715-0170WO1 New PCT Application July 28, 2025 to provide a filter membrane system 23 that allows a large quantity of water to pass through to the filter cathode 22.

[0114] As described above, the electrochemical filter 20 consumes comparatively little power and is efficient with regard to the overall efficiency of the main anode side or the fuel cell system 1. The electrochemical filter 20 can continuously remove nitrogen N2 from the main anode side or from the recirculation path 2, so that low nitrogen levels can be maintained on the main anode side, which contributes to improving the overall efficiency of the fuel cell system 1, the performance of the main fuel cell 3 and its lifetime.

[0115] Figures 3-6 show the part of the fuel cell system 1 that is outlined with a dashed line in FIG. I and that essentially comprises the main fuel cell 3 of the fuel cell system 1 as well as the anode side of the main fuel cell 3 together with recirculation path 2 and main fuel string 14.

[0116] FIG. 3 shows an example of one side of the main anode where the electrochemical filter 20 is used to extract unused fuel from the main anode recirculation.

[0117] The anode outlet 12 of the main fuel cell 3 is connected to a line 30. During operation of the fuel cell system 1, the exhaust gas from the anode chamber 4 of the main anode flows as recirculated gas into the line 30.

[0118] At branch 33, line 30 splits into lines 31 and 32. Line 31 leads to the electrochemical filter 20, which is located in the recirculation path 2, and more precisely to the filter anode inlet 24. The recirculated fluid, along with the fuel it contains, enters the electrochemical filter 20 via line 31 so that the fuel can be extracted from the recirculated fluid. Line 32 runs past or under the electrochemical filter 20, directly towards the main fuel line 14 or the anode inlet 13 of the main fuel cell 3. The Woodward L'Orange GmbH F&R Ref: 46715-0170WO1

[0119] New PCT registration date: July 28, 2025

[0120] Recirculated fuel, including the unextracted fuel contained therein, back to the anode compartment 4 or in the direction of the main fuel line 14.

[0121] The total volume flow of the exhaust gas from the main anode, split at branch 33 (where 100 percent of the exhaust gas volume is present at anode outlet 12 and in line 30, respectively), divides into two volume flows: a (first) partial volume flow (partial flow) that flows through line 31 towards the electrochemical filter 20, and a second partial volume flow (partial flow) that flows through line 32 and bypasses the electrochemical filter 20. With respect to the total volume flow (i.e., the 100 percent at anode outlet 12 and in line 30, less than 30 percent of the volume flow passes into line 31 and more than 70 percent passes into line 32 during operation of the fuel cell system 1.

[0122] It is conceivable to provide a valve, such as a directional control valve, in recirculation path 2 instead of branch 33.

[0123] The processes described above with reference to FIG. 2 take place in the electrochemical filter 20.

[0124] The filter cathode outlet 26 is connected to line 34. This leads to branch 35, which connects line 34 to line 38, which is described below. During operation, the extracted fuel (hydrogen) returns via line 34 to the anode chamber 4 of the main fuel cell 3 or can mix with the (pure) fuel (hydrogen) supplied via the main fuel line 14 from the fuel storage 15 (the fuel source) and flow into the anode chamber 4 of the main fuel cell 3.

[0125] The portion of the recirculated fluid (or the exhaust gas from the main anode) that has been depleted by the electrochemical filter 20 is discharged from the electrochemical filter 20 via the filter anode outlet 25 during operation. The filter anode outlet 25 is connected to a line 36. In this example, line 36 is part of a purge path. The purge path includes an optional purge valve 8, which is provided in line 36. During operation, the Woodward L'Orange GmbH F&R Ref: 46715-0170WO1 is discharged via line 36.

[0126] New PCT application July 28, 2025: depleted part of the recirculated fluid (which is not recirculated) or (more precisely) the depleted part of the exhaust gas from the main anode is diverted or purged from recirculation path 2 or fuel cell system 1.

[0127] The main fuel line 14 comprises a fuel source, which in this example is a fuel storage tank (e.g., a pressure accumulator) 15, a line 37 connected to the fuel storage tank 15, and an optional metering valve 9 (metering device). During operation, (fresh) fuel flows from the fuel storage tank 15 towards the anode chamber 4 of the main fuel cell 3 to operate the main fuel cell 3, preferably in an amount that enables superstoichiometric operation of the main fuel cell 3.

[0128] In the example shown in FIG. 3, the main fuel line 14 leads to, or includes, a jet pump 16, which also serves as a recirculation drive during operation. The jet pump 16 acts as a branch where the two lines 32 and 37 meet. In other words, during operation, the fresh fuel from the fuel storage 15 and the recirculated fuel containing the unused fuel are combined or mixed in the jet pump. The jet pump 16 may include a convergent-divergent passage to promote the mixing of the flows and to minimize the total pressure loss across the jet pump 16, or to increase the fuel pressure at the jet pump 16 outlet.

[0129] The jet pump 16 is connected to line 38 at its outlet. During operation, the mixture of fresh fuel and non-depleted recirculated fuel flows via line 38 to the anode compartment 4 of the main fuel cell.

[0130] In the example shown, the electrochemical filter 20 and the jet pump 16 are arranged in parallel in the recirculation path 2.

[0131] In the example shown, the bypass path corresponds to lines 32 and 38 with any components located therein, such as the jet pump 16. Woodward L'Orange GmbH F&R Ref: 46715-0170WO1

[0132] New PCT registration date: July 28, 2025

[0133] The two lines 34 and 38 meet at branch 35. During operation, the mixture exiting the jet pump 16 and the essentially pure fuel extracted by the electrochemical filter 20 are combined or mixed at branch 35 and flow onward toward the anode inlet 13 or the anode chamber 4 of the main fuel cell 3. It is conceivable to provide a mixing device, such as another jet pump, instead of branch 35, where the two lines 34 and 38 are joined.

[0134] Due to the electrochemical filter 20 used, a dedicated humidification device on the main anode side of the fuel cell system 1 is not required. However, such a humidification device can be provided if necessary.

[0135] The embodiment shown in FIG. 3 enables a simple and cost-effective implementation of the concept according to the invention, particularly when no directional control valve is provided instead of the branch 33, no purge valve 8 is required, and no humidification device is needed. The provision of a passive recirculation drive in the form of the jet pump 16 also allows for a simple design and control of the volume flow through the recirculation path 2. Because the jet pump 16 has no active drive and therefore consumes no power during operation, the overall efficiency of the fuel cell system 1 can be improved.

[0136] FIG. 4 shows another example of one side of the main anode where the electrochemical filter 20 is used to extract unused fuel from the main anode recirculation.

[0137] The part of the fuel cell system 1 shown in FIG. 4 corresponds to that shown in FIG. 3, except that instead of the jet pump 16, the branch 11 is provided and the recirculation blower 7 is provided as the recirculation drive in the recirculation path 2 or is arranged on the line 32. Woodward L'Orange GmbH F&R Ref: 46715-0170WO1

[0138] New PCT registration date: July 28, 2025

[0139] This means that, unlike the operation described with reference to FIG. 3, in the fuel cell system 1 according to FIG. 4, the recirculated fuel diverted at branch 33 (i.e., more than 70 percent of the total volume of the exhaust gas from the anode compartment 4 of the main fuel cell 3) flows through the recirculation fan 7 to branch 11 during operation. Furthermore, the fresh fuel from the fuel storage 15 flows via the optional metering valve 9 to branch 11, where it mixes with the unleached portion of the recirculated fuel. The mixture then flows towards branch 35, where it mixes with the essentially pure fuel extracted by the electrochemical filter. The mixture then flows towards the anode inlet 13 or the anode compartment 4 of the main fuel cell 3.

[0140] In the example shown, the electrochemical filter 20 and the recirculation fan 7 are arranged in parallel in the recirculation path 2.

[0141] In the example shown, the bypass path corresponds to lines 32 and 38 with any components that may be arranged therein, such as the recirculation fan 7.

[0142] The embodiment shown in FIG. 4 enables a simple and cost-effective implementation of the concept according to the invention and also offers the advantage of an active recirculation drive in the form of the recirculation blower 7. This allows the volume flow through the recirculation path 2 to be actively controlled and adapted or optimized for the operation of the main fuel cell 3 and / or the electrochemical filter 20. This can improve the overall efficiency of the fuel cell system 1 under certain operating conditions or at certain operating points of the fuel cell system 1.

[0143] FIG. 5 shows another example of one side of the main anode where the electrochemical filter 20 is used to extract unused fuel from the main anode recirculation. Woodward L'Orange GmbH F&R Ref: 46715-0170WO1

[0144] New PCT registration date: July 28, 2025

[0145] The part of the fuel cell system 1 shown in FIG. 5 corresponds to that shown in FIG. 4, except that the recirculation blower 7 is arranged upstream of the branch 33 or on the line 30 and the bypass path 32 and the line 34 connected to the filter cathode outlet 26 are joined at a branch 39.

[0146] This means that, unlike the operation described with reference to FIG. 4, in the fuel cell system 1 according to FIG. 5, the total volume flow of the exhaust gas from the main anode flows through the recirculation fan 7 during operation. This can be advantageous, for example, if the recirculation fan 7 has an integrated water separator and it is desired to separate as much water as possible from the recirculated fluid. The recirculated fluid then flows downstream to the branch 33, where it is split into two partial flows: one flowing towards the electrochemical filter (first partial flow) and the other into the bypass path, which corresponds to line 32 with any components optionally arranged therein (second partial flow). The split ratio is, as described above, such that less than 30 percent of the total volume flow constitutes the first partial flow, which flows through the electrochemical filter 20.

[0147] During operation of the fuel cell system 1, the fuel separated in the electrochemical filter 20 as described above flows into line 34 via the filter cathode outlet 26, is combined or mixed with the bypass volume flow from line 32 at branch 39 and then flows in a common line 41 to branch 40.

[0148] At branch 40, the pure fuel from line 37 or the main fuel line 14 is combined or mixed with the mixture from bypass path 32 and the filter cathode 22. The mixture then flows via line 42 to the anode inlet 13 of the main anode.

[0149] It is also conceivable that the three volume flows from line 37 or the main fuel line 14 (pure fuel), from the bypass path or line 32 (recirculated with unused fuel) and from line 34 or from the Woodward L'Orange GmbH F&R Ref.: 46715-0170WO1 New PCT application July 28, 2025

[0150] The filter cathode outlet 26 (extracted pure fuel) can meet or mix at the same branch 39 or 40. In this case, one of the branches 39 and 40, as well as the line 41 located between them, can be omitted. A valve for controlling the partial flows or a jet pump can also be provided at such a point. This can improve the mixing of the flows and / or the controllability of the fuel cell system 1.

[0151] In the example shown, the electrochemical filter 20 and the recirculation fan 7 are arranged in series in the recirculation path 2.

[0152] The embodiment shown in FIG. 5 enables a simple and cost-effective implementation of the concept according to the invention and also offers the advantage of an active recirculation drive in the form of the recirculation blower 7, through which the entire volume flow in the recirculation path 2 flows. This allows the volume flow through the recirculation path to be actively controlled and adapted or optimized for the operation of the main fuel cell 3 and / or the electrochemical filter 20. This can improve the overall efficiency of the fuel cell system 1 under certain operating conditions or at certain operating points of the fuel cell system 1. Furthermore, water can be separated from the entire recirculated fluid if the recirculation blower 7 has an integrated water separator.

[0153] FIG. 6 corresponds to the example shown in FIG. 3 and schematically illustrates the material or volume flows that occur during the operation of the fuel cell system 1. Similar representations based on FIGS. 4 and 5 are apparent to those skilled in the art.

[0154] In the main fuel cell 3, FIG. 6 shows the mass flow rate of the fuel (h). FMain ) via the main membrane system from the main anode compartment 4 to the main cathode compartment 5. Furthermore, the mass flow rate of the inert gas (n IMain in mmol / s) across the main membrane system from the main cathode chamber 5 to the main anode chamber 4. The main membrane system is configured such that the desired fuel flow rate is significantly greater than the undesired inert gas flow rate, as indicated by the respective arrow lengths. Woodward L'Orange GmbH F&R Ref: 46715-0170WO1 New PCT Application July 28, 2025

[0155] The total volume flow rate (V GesThe exhaust gas flow from the main anode is indicated in FIG. 6 by a relatively wide line, from which a relatively thin line branches off at the junction 33, symbolizing the small initial partial volume flow through the electrochemical filter 20. An example is shown here in which the initial partial volume flow is less than 15 percent of the total volume flow. However, as described, the initial partial volume flow can also be less than 30 percent of the total volume flow.

[0156] The mass flow rate of the fuel passing through the filter membrane system is essentially equal to the volume flow rate exiting the filter cathode outlet 26, since the mass flow rate of the reaction products passing through the filter membrane system is negligible (and therefore not shown in the illustration of FIG. 6).

[0157] The exhaust gas from the main anode, depleted of the extracted fuel, flows during operation essentially as a mass flow of the inert gas (n EFPurge ) from the filter anode outlet 25 into the line 36 or into the flushing path.

[0158] As described above, the following applies

[0159] The ratio SR indicates that the side of the main anode always has a low level of inert gas during operation of fuel cell system 1. This is even more true for SR > 1.75 and SR > 2.0.

[0160] For the sake of completeness, it should be mentioned that FIG. 6 also shows the second partial flow (bypass flow) of the total volume flow (V). Ges The exhaust gas from the main anode shows a significantly wider (because it is significantly larger) current compared to the first partial flow. Woodward L'Orange GmbH F&R Ref: 46715-0170WO1

[0161] New PCT registration date: July 28, 2025

[0162] Furthermore, FIG. 6 shows the fuel flow of the pure fuel in the main fuel line 14 upstream of the jet pump 16 in the direction of fuel flow.

[0163] FIG. 7 illustrates the disproportionate (approximately exponential) increase in the power consumption of the electrochemical filter 20 with increasing volume flow rate of the recirculated fluid through the electrochemical filter 20. FIG. 7 shows, by way of example, that at small volume flow rates (of the first partial volume flow through the electrochemical filter) up to about 15 percent, the relationship between the volume flow rate and the power consumed is approximately linear to a very good degree. Depending on the type of fuel cell and / or the operating mode or operating point, the relationship between the volume flow rate and the power consumed can also be approximately linear to a very good degree even at volume flow rates (of the first partial volume flow through the electrochemical filter) below 30 percent. Due to the approximately linear relationship, a good or even positive relationship exists in this range (i.e., below 30 and preferably below 15 percent) of the volume flow rates of the first partial volume flow.A satisfactory balance between the amount of extracted fuel or filtered-out inert gas, such as the nitrogen content (at 15 percent of the total volume fraction of the main anode exhaust gas, the nitrogen content can be reduced to near zero) and the power consumption, thereby improving or optimizing the system efficiency.

[0164] Depending on the type of fuel cell and / or the operating point, the exponential curve becomes noticeable from approximately 15 or 30 percent of the volume flow rate, and any further increase in extracted fuel comes at the cost of an undesirably high power consumption by the electrochemical filter. This negatively impacts the overall efficiency of fuel cell system 1. This is particularly evident at 15 percent of the volume flow rate.

[0165] According to the invention, the area of ​​the first partial volume flow through the electrochemical filter 20 is therefore avoided from 30 percent and preferably already above 15 percent of the total volume flow of the main anode exhaust gas.

Claims

Woodward L'Orange GmbH F&R Ref: 46715-0170WO1 New PCT application July 28, 2025 REQUIREMENTS 1. Fuel cell system (1) comprising a main fuel cell (3) comprising an anode inlet (13) through which fuel can flow into the main fuel cell and an anode outlet (12) from which an anode exhaust including unused fuel can flow out; and a recirculation path (2) in which the anode exhaust can flow as recirculate, connecting the anode inlet to the anode outlet and comprising: an electrochemical filter (20) for extracting the unused fuel from the recirculate, wherein the extracted fuel is recirculable to the anode inlet, and a bypass path (32, 38) through which the recirculate is recirculable from the anode outlet to the anode inlet bypassing the electrochemical filter;wherein the fuel cell system is set up for operation with a bypass ratio, which is a quotient of the proportion of recirculated fluid flowing through the bypass path to the total amount of recirculated fluid, of over 70 percent.

2. Fuel cell system according to claim 1, wherein the bypass ratio is in a range of at least 75, preferably at least 80, and more preferably at least 85 percent, in particular 85 to 99 percent, preferably 90 to 98 percent, more preferably 92.5 to 97.5 percent and more preferably 95 to 97 percent.

3. Fuel cell system according to claim 1 or 2, wherein the recirculation path comprises a purge path (36) which is configured to discharge the recirculated fluid, depleted of the extracted fuel, flowing out of the electrochemical filter from the recirculation path, wherein the purge path optionally comprises a purge valve (8). Woodward L'Orange GmbH F&R Ref: 46715-0170WO1 New PCT application July 28, 2025 4. Fuel cell system according to any one of the preceding claims, wherein the main fuel cell comprises a main anode, a main cathode and an intermediate main membrane system comprising a first inert gas permeation, which is a molar mass flow of an inert gas from one side of the main cathode to one side of the main anode via the main membrane system; and the electrochemical filter comprises a filter anode (21), a filter cathode (22) and an intermediate filter membrane system (23) comprising a second inert gas permeation, which is a molar mass flow of the inert gas from one side of the filter cathode to one side of the filter anode via the filter membrane system;wherein the second inert gas permeation is configured to be lower than the first inert gas permeation such that the ratio of the molar mass flow rate of the inert gas, which flows out of an anode outlet (25) of the filter anode instead of via the filter membrane system, to the first inert gas permeation is at least 1.5, preferably at least 1.75 and more preferably at least 2.

0.

5. Fuel cell system according to one of the preceding claims, wherein the recirculation path further comprises a valve for controlling the bypass ratio; or the fuel cell system is configured to control the bypass ratio by energizing the electrochemical filter, and the recirculation path for controlling the bypass ratio preferably i) does not have a valve, and / or ii) the fuel cell system is configured to control the bypass ratio exclusively by energizing the electrochemical filter.

6. Fuel cell system according to one of the preceding claims, wherein the recirculation path comprises a recirculation drive (7, 16), preferably a blower (7) or a jet pump (16); and the electrochemical filter is arranged parallel to the recirculation drive in the recirculation path; or Woodward L' Orange GmbH F&R Ref: 46715-0170WO1 New PCT application July 28, 2025, the electrochemical filter is arranged in series with the recirculation drive in the recirculation path.

7. Fuel cell system according to one of the preceding claims, wherein the main fuel cell is a proton exchange membrane fuel cell; and / or the fuel is hydrogen and / or the inert gas is nitrogen; and / or the main fuel cell is a superstoichiometrically operable main fuel cell.

8. Fuel cell system according to one of the preceding claims, wherein the recirculation path for purging does not have a purge valve and is configured to control the purging via the bypass ratio and / or the current flow to the electrochemical filter; and / or the recirculation path does not have a water separator and is configured to separate any water contained in the recirculated fluid via the electrochemical filter; and / or the fuel cell system does not have a humidification device in a main fuel line leading to the anode inlet.

9. Method for operating a fuel cell system (1) comprising a main fuel cell (3) and a recirculation path (2) connecting an anode outlet (12) of an anode compartment (4) of an anode of the main fuel cell to an anode inlet (13) of the anode compartment and comprising an electrochemical filter (20), the method comprising: releasing an exhaust gas containing unused fuel from the anode compartment via the anode outlet into the recirculation path as recirculated fuel; splitting a total flow of the recirculated fuel into at least two partial flows, a first partial flow flowing through the electrochemical filter and a second partial flow flowing to the anode inlet bypassing the electrochemical filter; extracting the unused fuel in the first partial flow through the electrochemical filter; and Woodward L' Orange GmbH F&R Ref: 46715-0170WO1 New PCT application July 28, 2025: supplying the fuel extracted from the first partial stream and the second partial stream to the anode inlet, wherein the first partial stream is less than 30 percent of the total stream.

10. A method for operating a fuel cell system according to claim 9, wherein the method further comprises continuously removing an inert gas from the recirculated gas in order to keep the proportion of inert gas on an anode side of the fuel cell system at a low level; and / or the first partial flow is at most 25, preferably at most 20, and more preferably at most 15 percent of the total flow, in particular between 1 and 15 percent of the total flow, preferably 2 to 10 percent, more preferably 2.5 to 7.5 percent, and more preferably 3 to 5 percent; and / or the method further comprises purging the exhaust gas, depleted of the extracted fuel, flowing out of the electrochemical filter from the recirculation path, preferably without using a purge valve;and / or the inert gas permeation at a membrane system (23) of the electrochemical filter is so low as the inert gas permeation at a membrane system of the main fuel cell that the ratio of the molar mass flow rate of an inert gas, which flows out of an anode outlet (25) of an anode of the electrochemical filter instead of via the membrane system of the electrochemical filter, to the inert gas permeation at the membrane system of the main fuel cell is at least 1.5, preferably at least 1.75 and even more preferably at least 2.0; and / or the separation is carried out by energizing the electrochemical filter, preferably exclusively by energizing; and / or water contained in the recirculated gas is separated via the electrochemical filter, preferably exclusively via the electrochemical filter; and / or the fuel is moistened via the electrochemical filter, preferably exclusively via the electrochemical filter;and / or the main fuel cell is a proton exchange membrane fuel cell; and / or the fuel is hydrogen and / or the inert gas is nitrogen; and / or the main fuel cell is operated superstoichiometrically.;

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