Control method for controlling a fuel cell system and corresponding control unit

Abstract

A method for controlling a fuel cell system (6) is provided, wherein the method comprises: - measuring (S1) a voltage provided by the fuel cell system (6) and monitoring the voltage behavior, and - adjusting (S2) at least one physical parameter of the fuel cell system (6) based on the measured voltage and voltage behavior for adjusting the voltage provided by the fuel cell system (6) and / or the water content within the fuel cell system (6).

Description

[0001] 2025-12-10

[0002] Control method for controlling a fuel cell system and corresponding control unit

[0003] Description:

[0004] The present invention relates to a control method for controlling a fuel cell system according to claim 1 and a corresponding control unit according to claim 9.

[0005] During the operation of fuel cell systems, which may comprise one or more fuel cell stacks each of which comprises a plurality of membrane electrode assemblies (MEAs), which are sandwiched by so called bipolar plates (BPP), which provide a connection between anode and cathode electrodes of the MEAs, the components of the fuel cell system will degrade over time. For example, the catalyst in the cathode or anode electrodes can degrade in reversable and irreversible ways. Such a degradation influences the remaining lifetime of the fuel cell system. If the degradation can be reversed, the lifetime of the fuel cell system will not be reduced, but if the degradation is irreversible, as it is the case for example for catalyst dissolution or other types of corrosion, the lifetime of the fuel cell system will be reduced.

[0006] It is therefore object of the present invention to provide a possibility to reduce the degradation of a fuel cell system and thus to increase the lifetime of the fuel cell system.

[0007] This object is solved by a method for controlling a fuel cell system according to claim 1 and a corresponding control unit according to claim 9.

[0008] The method for controlling a fuel cell system comprises:

[0009] - measuring a voltage provided by the fuel cell system and monitoring the voltage behavior, and

[0010] - adjusting at least one physical parameter of the fuel cell system based on the measured voltage and voltage behavior for adjusting the voltage provided by the fuel cell system.

[0011] The voltage may be monitored directly by monitoring the voltage or may be monitored indirectly by measuring the current and / or power which are correlated with the voltage. In addition, the current and / or power may also be monitored and may be used for adjusting the voltage provided by the fuel cell system. In particular, the current and / or power may be controlled for adjusting the voltage.

[0012] It has been found that several factors have an influence on the degradation of components of a fuel cell system, such as catalyst dissolution or other types of corrosion: the water content and / or the voltage behavior, e.g., cycling of the voltage over defined limits or within defined ranges. In the context herein, catalyst dissolution is mentioned as one possible example for a degradation of the fuel cell system. However, it should be noted that this should be only considered as one example and the degradation of the fuel cell system may also refer to a degradation of other components of the fuel cell system such as catalyst support, bipolar plates, etc.

[0013] By adjusting and / or controlling at least one of these factors, the lifetime of the fuel cell system may be increased, or rather the degradation of the fuel cell system may be reduced and thus the lifetime of the fuel cell system will not be reduced. Controlling and / or adjusting the water content and / or the voltage behavior of the fuel cell system may be carried out by adjusting one or more physical parameters of the fuel cell system which influence the water content and / or voltage behavior of the fuel cell. Controlling in this context may comprise regulating and / or manipulating the water content and / or voltage behavior. This may also be done using a battery, dump load, capacitors, etc., which can directly adjust the voltage, current and / or load of the fuel cell system, which in this case would be the physical parameter.

[0014] Thus, by controlling and / or adjusting in a first step the voltage behavior of the voltage in the fuel cell system, the control method provides a reliable and easy way of increasing the lifetime of the fuel cell system. The voltage to be monitored may particularly be the voltage that the fuel cell system senses.

[0015] In a second step, the control method may further also monitor the water content of the fuel cell system.

[0016] Thus, the method for controlling the fuel cell system further comprises:

[0017] - determining, for example directly or indirectly measuring, a water content within the fuel cell system, and

[0018] - adjusting the at least one physical parameter of the fuel cell system based on the water content for adjusting the voltage provided by the fuel cell system and / or the water content within the fuel cell system.

[0019] Thus, the at least one physical parameter of the fuel cell system may be adjusted to control, adjust and / or regulate the voltage provided by the fuel cell system not only based on the measured voltage and monitored voltage behavior, but also based on the water content. Thus, the physical parameter may be adjusted to meet the needs of the fuel cell system taking into account the fuel cell system conditions and / or environment.

[0020] The water content or humidity of the fuel cell system may be measured and monitored by sensors. Such sensors, for example humidity sensors, may be arranged for example outside a fuel cell stack of the fuel cell system, e.g., in the inlet or outlet of the fuel cell stack. Instead of directly measuring the water content, the water content may also be determined indirectly, for example by measuring the current.

[0021] The control method may control a fuel cell system having one or more fuel cell stacks as mentioned above or may control only one fuel cell stack of the fuel cell system.

[0022] The fuel cell system may be a fuel cell system connected to a reactant gas compressor, and / or a fuel cell system connected to a gas tank, and / or a fuel cell system connected to a mixed gas system of a fuel cell system connected to a hybrid system between a compressor and a gas tank. In those fuel cell systems, the voltage can be regulated by controlling the gas feed to help improve the fuel cell system's durability. Thus, the physical parameter to be adjusted may be parameters of the gas feed, such as gas pressure, gas temperature or gas flow rate.

[0023] A fuel cell system connected to, e.g., a mixed gas system such as enriched oxygen and nitrogen, can premix these gases in a gas mixture chamber prior to entering the fuel cell. The mixture chamber helps with premixing the gases to create a more homogenous gas mixture. In a mixed gas, the different gas can provide a more controlled concentration of the reactant gas concentration. Thus, the physical parameter to be adjusted may be a reactant gas concentration.

[0024] As explained, the current density and voltage of the fuel cell system is affected by a combination of many parameters, such as gases composition, pressure, flow rate, humidity, etc. In a fuel cell system operating with a cathode gas feed containing O2or being enriched with O2(in particular containing O2above 21 %), the current densities at a given voltage point will, in general, be higher than in a fuel cell system operating with a regular air-fed cathode. Thus, if operating at a given current density, if the O2concentration is increased, the voltage will increase, and if reducing the O2content, the voltage will decrease. Thus, one of the physical parameters may be the O2concentration. Instead of O2, air could be used, in particular with an oxygen O2content around 21 %.

[0025] It should be noted that one physical parameter or a combination of two or more physical parameters may be adjusted. Furthermore, other parameters of the fuel cell system may be adjusted such as the temperature of the fuel cell system itself.

[0026] According to a further embodiment, the method uses an optimization algorithm for adjusting the at least one physical parameter based on the measured voltage and voltage behavior and / or the determined water content. Such an optimization algorithm may take into account a currently measured voltage, voltage behavior and / or determined water content as well as a desired optimal voltage, voltage behavior and / or water content and may adjust the at least one physical parameter such that the optimal voltage, voltage behavior and / or water content is reached. The control method may use a neural network, artificial intelligence or the like for carrying out the optimization algorithm.

[0027] The control and adjustment of the at least one physical parameter may be based on historical data. These historical data may be for example data gathered during tests of the fuel cell system or of similar fuel cell systems. The historical data may indicate how which physical parameter influences the voltage, voltage behavior and / or water content etc. These collected historical information may be used as reference data inter alia for the optimization algorithm. Thus, the algorithm can adjust the operation condition according to the list of possible parameters for different voltages, voltage behaviors and / or water contents, for controlling the power and adjusting the voltage of the fuel cell system.

[0028] The method may further comprise comparing the determined water content with a predefined water content threshold and, if the determined water content is above the predefined water content threshold, adjusting the at least one physical parameter of the fuel cell system for reducing the water content within the fuel cell system.

[0029] A dissolution of the catalyst within the fuel cell system leads to losses of available catalyst area, which leads to a reduction of the cell performance. As described above, one factor contributing to increased dissolution rates in fuel cell systems is the water content in the system. This is because water helps to dissolve catalyst ions during catalyst oxide reduction, leaving the ions mobile. A high water content will promote catalyst dissolution, whereas dryer conditions lead to lower catalyst dissolution rates. Thus, by keeping the water content under a predefined water content threshold which does not contribute to catalyst dissolution or at least lowers the catalyst dissolution, the catalyst dissolution may be reduced. Similarly, this applies for example to carbon or metallic components (such as support or bipolar plates) which may also be affected by the water content within the fuel cell system. Thus, to reduce a degradation of such components, the water content may be determined, monitored and adjusted based on the at least one physical parameter. Further, there is an interaction between the water content within a fuel cell system and the voltage / voltage behavior. When the voltage of the fuel cell system crosses a predefined threshold (for example a predefined risky threshold such as the redox point of certain materials (catalyst, support, etc.) which will be described later in further detail), it will change the oxidation state of the materials within the fuel cell system. So, if the value of the voltage increases and crosses this threshold, this might lead to change to a higher oxidation state of the materials, while, if the voltage is reduced and crosses the threshold, it will reduce the oxidation state of the materials. Some of the materials may have several of these thresholds, for example redox points. When there is any water in the fuel cell system in liquid form being in contact with a material or component of the fuel cell system, when the respective material within the fuel cell system changes between an oxidation state above zero and the metal state (i.e., oxidation state zero) or between oxidation states, the water content may degrade the material.

[0030] For example, if the catalyst is metallic, crossing its redox point will lead to a switch between oxidation states, in particular between oxidation states and metal state, i.e., oxidation state zero, of the catalyst, and if there is water left and the oxidation state of the catalyst is reduced (e.g. back to metal state), this will promote dissolution and degradation of the catalyst material. Thus, adjusting the water content may also have an advantage in combination with adjusting the voltage and / or voltage behavior.

[0031] In the fuel cell system, the generated amount of water during operation at high load is relatively high. During start up and / or shutdown of the fuel cell system, the voltage may increase above OCV (open circuit voltage) level, which can introduce degradation of the fuel cell system due to the interaction of water with components of the fuel cell system, for example the catalyst during phase change from catalyst metal to catalyst oxidation state. In order to avoid the degradation, the water content may be removed or reduced as explained above before shutting down the fuel cell system. In an enriched 02 system, lowering the 02 concentration at the specific load (at low voltage) may further assist by reducing the water generation rate. However, it should be noted that the membrane electrode assembly (MEA) of the fuel cell system should be kept humidified when it is in shut-down state, but not flooded or too wet. A dry MEA would result in a degradation of the membrane. Thus, the water content of the fuel cell should be adjusted such that it is low enough to not degrade the fuel cell system but high enough to avoid a degradation of the membrane of the MEA.

[0032] A further cause of degradation of components within the fuel cell system, e.g., degradation of the catalyst, catalyst support, bipolar plates, etc., are repeated changes in the oxidation state, e.g., changes between metal and oxidation state (e.g., of the cathode or anode electrodes) or changes between different oxidation states (e.g., high oxidation state and low oxidation state). Even though the most common catalysts are noble metals, they will change their oxidation state when they experience cell voltages above a specific threshold voltage, for example ~0.8 V. When the cell voltage is below this threshold voltage, the catalyst will be in its metallic state. The potential at which the catalyst shifts its oxidation state is called the redox potential point. Repeated voltage shifts over this redox potential point can pose a significant risk of degradation of the components of the fuel cell system, such as catalyst dissolution, leading to losses of available catalyst area, which leads to reduction of cell performance. There exist further risky voltage thresholds which could cause or promote a degradation of the fuel cell system when being crossed by the voltage within the fuel cell system. These risky voltage thresholds may be different for different materials or components within the fuel cell system. The already described redox point, i.e., the voltage value which causes a shift between oxidation states of a material is one of them. It should be noted, as mentioned above, that the materials within the fuel cell system may have different redox points, wherein some materials may have more than one redox point, so that there may exist several risky voltage or risky voltage thresholds. There may exist further risky voltages such as a voltage being cause for carbon corrosion (e.g., above 1 V) which usually occurs during start-up / shutdown, a voltage being too low voltage which may promote formation of peroxide or other undesired byproducts that may affect the fuel cell system, etc. Thus, according to a further embodiment, the method comprises determining whether the measured voltage is above a predefined risky voltage threshold, in particular the redox potential or redox point of the fuel cell system, and if yes, adjusting the at least one physical parameter of the fuel cell system for reducing the voltage below the predefined risky voltage threshold.

[0033] The predefined risky voltage threshold may be set for different materials or components of the fuel cell system, thus there may be more than one predefined risky voltage threshold. Preferably, the predefined risky voltage threshold can be set to be below or higher than the risky voltage values which would risk certain materials. Thus, in case of reaching that predefined threshold, adjusting the at least one physical parameter can help to avoid reaching the risky voltage.

[0034] By avoiding a voltage of the fuel cell above the predefined risky voltage threshold, for example the redox potential, repeated voltage shifts over the predefined risky voltage threshold may also be avoided. This may thus, for example, reduce the risk of catalyst dissolution or degradation of other components and thus may reduce the degradation of the fuel cell system.

[0035] Further, fluctuations of the voltage during operation, even without crossing the predefined risky voltage threshold, for example catalyst redox potential, can result in a certain level of irreversible and reversible losses. The reason for possible losses is that if there is a strong voltage shift, it can bring the fuel cell system to a new equilibrium state.

[0036] For this reason, the method may further comprise analyzing the voltage behavior and determining an amount of voltage fluctuations, and if the amount of voltage fluctuations is above a predefined amount threshold, adjusting the at least one physical parameter of the fuel cell system for reducing the voltage fluctuations. By reducing the amount of voltage fluctuations and thus the frequency of voltage state changes, the degradation of the fuel cell system may be reduced, and the durability may be extended. It should be noted that all thresholds as mentioned herein as well as the physical parameters depend on the specific controlled fuel cell system and the components within the fuel cell system, for example their materials. Thus, for example the thresholds may vary depending on the specific configuration of the fuel cell system.

[0037] In addition, degradation of components of the fuel cell system, for example catalyst dissolution, as a consequence of voltage sweeping over the predefined risky voltage threshold, e.g., the redox potential, might be more severe the larger the voltage sweep window is. For example, the catalyst dissolution will be more severe if the voltage is swept between 0.9 V and 0.6 V compared to when swept between 0.9 V and 0.7 V. Equally so, the dissolution will be more severe when swept between 0.6 V and 0.9 V, than when swept between 0.6 V and 0.8 V.

[0038] Therefore, the method may further comprise analyzing the voltage behavior and determining a range in which the voltage fluctuates, and if the range is beyond a predefined range, adjusting the at least one physical parameter of the fuel cell system for reducing the range of the voltage fluctuations. Narrowing the voltage fluctuation range can assist in reducing the risk of loss rate or any type of degradation and thus reducing the degradation of the fuel cell system.

[0039] In summary, adjusting the voltage and / or the water level or content has been shown as being important to reduce degradation of components of the fuel cell system, for example catalyst dissolution, and prolong the lifetime of the fuel cell system. For example, to increase the durability and the lifetime of the fuel cell system, a fuel cell system can be prevented from experiencing voltage shifts over a predefined risky voltage threshold, such as the catalyst redox point, a too high frequency of voltage shifts, a too high range of voltage shifts and / or a too high water content.

[0040] In the following, specific embodiments of adjusting parameters will be explained. However, it should be noted that these embodiments are only exemplary and are not intended to restrict the general idea of adjusting physical parameters in order to adjust the water content, voltage and / or voltage behavior.

[0041] As mentioned above, in a fuel cell system based on gas compressors as a source of reactant flow, it is possible to alter the physical parameters that help and / or assist with maintaining the proper functionality of the fuel cell system, such as pressure, stoic / flow and RH to influence the cell voltage values in case the system is about to pass a predefined risky voltage threshold, such as the redox point, which can alter the oxidation state of the material of a component within the fuel cell system or which can cause formation of unwanted materials such as hydrogen peroxide and other unwanted byproducts, or if the system is experiencing large voltage fluctuations as explained above.

[0042] Further, the relative humidity (RH) may be adjusted. The relative humidity is water vapor when the water in the fuel cell system is in the gas phase and is mixed with the overall gas in the environment. RH depends on the temperature, pressure, dew point temperature, and water content of the system of interest. Thus, to control the water content, the relative humidity may be adjusted.

[0043] The reactive gas concentration has a strong influence on regulating the voltage when a current load is pulled. For a given power level, increasing or decreasing the reactant concentration will influence the voltage. By altering the reactant concentration when power demand is low, the voltage can be forced to stay below the redox potential. Thus, an optimal voltage range can be maintained, and voltage levels that might promote dissolution can be avoided.

[0044] During operation at a certain power demand, the voltage fluctuation range can be large. Keeping the voltage fluctuation range narrow can help to lower the degradation of the fuel cell. Dynamic change of the gas mixture ratio can help sustain a narrower voltage range.

[0045] Further, the gas flow (stoichiometry) rate can help regulate the voltage by changing the reactant amount in a fuel cell. Increasing or reducing flows can result in a relatively fast shift of the voltage. If the voltage is close to a predefined risky voltage threshold, such as the redox potential, the stoichiometry / flow can be reduced to force the voltage to decrease and to keep the voltage below the predefined risky voltage threshold. This method can also help during strong voltage fluctuation, by controlling the dynamic of the gas flows accordingly to the voltage fluctuation. This way it is possible to narrow down the fluctuation range.

[0046] Furthermore, the pressure in the fuel cell can have a major effect on the voltage and is relatively fast to shift. Lowering the pressure will result in a lower cell voltage, which can also be utilized to keep the voltage at optimal voltage levels. In particular, it can be beneficial when a low load is pulled, and the voltage is close to the predefined risky voltage threshold such as the catalyst redox potential or redox potentials of other materials. During high loads and different gas mixtures, controlling and setting the pressure to a certain value can set a specific voltage range that the cell will operate during high loads.

[0047] Further, the RH point can be set. This method may be slower to control but has an impact on the voltage due to effects on the conductivity. Setting an RH point to a certain level can help determine a voltage range that can help to regulate the voltage better while shifting the other physical parameters.

[0048] As explained above, the sweep between the redox points during operation should be avoided to prolong the catalyst lifetime. Therefore, it may be preferred to keep the voltage below the redox point during low / mid power demand time. Since many physical parameters during operation can influence the system voltage, it is possible to keep the voltage in a range below the threshold point while running operating the fuel cell system.

[0049] Controlling the physical parameters may be preferred to keep the voltage in the required range. Some of the physical parameters have a quick response time, which can alter the voltage and keep it below the threshold point.

[0050] As mentioned above, during operation, the power will vary depending on the load demand, which can reach voltage values that can result in minor degradation that can accumulate over time. Therefore, it may be preferred to keep the voltage value in a certain range and can be done by controlling parameter with a fast respond time.

[0051] For example, during system operation, the fuel cell system is under pressure from the gas, which is called back pressure. The voltage is influenced by the back pressure in the fuel cell system; higher pressure will increase the voltage value and vice versa for each selected load point. Hence, in case of reaching the threshold during low power demand, the pressure in the fuel cell system can be reduced, which will assist with lowering the voltage value.

[0052] In an enriched O2environment, sensing the voltage level during mid / low power demand will result in a voltage increase. When the threshold is reached, it is possible to reduce the O2percentage (increase N2or reduce O2flow). It is also possible just to increase the flow to O2, which will increase the overall flow and the percentage. The response time is relatively quick, which will help to maintain the voltage value below the threshold.

[0053] For example, a fuel cell system in an enriched O2environment can generate high water content during high power demand. Then, if the power demand is switched from high to medium / low power, the voltage will increase. If the power demand is low enough, the voltage will be swept back and forward over the catalyst redox point and redox points of other components, resulting in a back and forward sweep between oxidation states, hence the catalyst will dissolve more due to the higher water content. In order to avoid or minimize such events, parameters such as the O2concentration can be regulated in the system, as described above. O2concentration changes have a fast response time in the system.

[0054] In an exemplary scenario, the system is operated with higher O2concentrations at high current densities where the voltage is below the redox point. Further, the load point is then lowered such that the resultant current density would result in a voltage above the redox potential if the O2concentration is kept the same. To avoid this unwanted voltage increase, the O2concentration can be reduced in the cathode gas feed, such that the resultant voltage for the given current density is still below the redox point, thus keeping the catalyst in its oxidation state. A threshold voltage can be implemented in the system control, where if this voltage threshold is reached, the O2concentration is reduced to keep the voltage at an optimal value avoiding catalyst dissolution.

[0055] According to a further aspect, a control unit for controlling a fuel cell system is provided. The control unit comprises a measuring module for measuring a voltage provided by the fuel cell system and monitoring the voltage behavior, and a control module for adjusting at least one physical parameter of the fuel cell system based on the measured voltage and voltage behavior for adjusting the voltage provided by the fuel cell system and / or the water content within the fuel cell system.

[0056] The respective entity, e.g., the measuring module and the control module, may be implemented in hardware and / or in software. If said entity is implemented in hardware, it may be embodied as a device, e.g., as a computer or as a processor or as a part of a system, e.g., a computer system. If said entity is implemented in software, it may be embodied as a computer program product, as a function, as a routine, as a program code or as an executable object.

[0057] According to an embodiment, the measuring module is further configured to determine, for example indirectly or directly measuring, a water content within the fuel cell system, and the control module is further configured to adjust the at least one physical parameter of the fuel cell system based on the determined water content for controlling the voltage provided by the fuel cell system and / or the water content within the fuel cell system.

[0058] The monitoring module may comprise one or more sensors which are configured to measure the voltage and / or the water content. For example, such a sensor may be a humidity sensor or a voltage sensor. Further, the monitoring module may comprise sensors for measuring the pressure, flow, humidity, current, etc. Such sensors may be used for monitoring the at least one physical parameter and then can alter the relevant physical parameter by the current condition or needs. The embodiments of the herein described control method apply also to the described control unit and vice versa.

[0059] An even further aspect of the present invention relates to a computer program product comprising a computer program code which is adapted to prompt a control unit, e.g. a computer, and / or a computer to perform the above discussed steps of the control method.

[0060] The computer program product may be provided as memory device, such as a memory card, USB stick, CD-ROM, DVD and / or may be a file which may be downloaded from a server, particularly a remote server, in a network. The network may be a wireless communication network for transferring the file with the computer program product.

[0061] Further preferred embodiments are defined in the dependent claims as well as in the description and the figures. Thereby, elements described or shown in combination with other elements may be present alone or in combination with other elements without departing from the scope of protection.

[0062] In the following, preferred embodiments of the invention are described in relation to the drawings, wherein the drawings are exemplarily only, and are not intended to limit the scope of protection. The scope of protection is defined by the accompanied claims, only.

[0063] The figures show:

[0064] Fig. 1 : a schematic block diagram of a control unit; and

[0065] Fig. 2: a schematic flow diagram of a control method.

[0066] In the following same or similar functioning elements are indicated with the same reference numerals. Fig. 1 shows a control unit 1 which is configured to control a fuel cell system 6. The fuel cell system 6 may comprise one or more fuel cell stacks, which may thus also be controlled by the control unit 1.

[0067] The control unit 1 comprises a measuring module 2 for measuring a voltage provided by the fuel cell system 6. The measuring module 2 is further configured to monitor the voltage behavior. In addition, or as an alternative, the measuring module 2 is configured to determine, for example directly or indirectly measuring, a water content within the fuel cell system 6.

[0068] It has been found that the water content and / or the voltage or voltage behavior within a fuel cell system 6 have a significant impact on the degradation of the fuel cell system 6 and thus its lifetime. This is particularly the case as the water content and the voltage or voltage behavior influence the degradation of components of the fuel cell system 6, such as the catalyst dissolution or corrosion of components of the fuel cell system 6, which contributes to the degradation of the fuel cell system 6.

[0069] Thus, in order to enhance the lifetime of the fuel cell system 6 and to reduce the degradation of the fuel cell system 6, for example by reducing the catalyst dissolution, the control unit 1 further comprises a control module 4.

[0070] The control module 4 is configured to adjust at least one physical parameter of the fuel cell system 6 based on the measured voltage and voltage behavior and / or the determined water content. By adjusting the physical parameter(s), the voltage provided by the fuel cell system 6 and / or the water content within the fuel cell system 6 are adjusted which in turn influences the degradation of components of the fuel cell system 6, such as catalyst dissolution, and thus the degradation of the fuel cell system 6.

[0071] The physical parameters may be several different parameters that help to maintain the fuel cell system operation, like gas concentration, pressure, temperature and the like. The parameters may depend on the specific design of the controlled fuel cell system 6. Fig. 2 shows a schematic flow diagram of a control method, which may be carried out by the control unit 1 of Fig. 1.

[0072] In a first step S1 , the method measures a voltage provided by the fuel cell system 6 and monitors the voltage behavior, and / or determines a water content within the fuel cell system 6.

[0073] In a second step S2, the measured voltage and voltage behavior are used for adjusting the voltage provided by the fuel cell system 6 and / or the water content within the fuel cell system 6. This may be done by adjusting at least one physical parameter of the fuel cell system 6 as described above.

[0074] In case the water content has been determined in step S1 , the method can use the determined water content in a third optional step S3 for adjusting the at least one physical parameter to change in particular the determined water content. It should be noted that the water content may be directly measured using for example a humidity sensor or may be indirectly measured for example by measuring the current and calculating the water content based on the current (which is influenced by the water content). Steps S2 and S3 may also be carried out simultaneously or in parallel.

[0075] The control method may use an optimization algorithm for adjusting the at least one physical parameter in an optimal manner to adjust the voltage and voltage behavior and / or the determined water content so that the degradation of components of the fuel cell system, e.g., catalyst dissolution, is reduced.

[0076] For example, the determined water content can be compared in step S3 with a predefined water content threshold and, if the determined water content is above the predefined water content threshold, the at least one physical parameter of the fuel cell can be adjusted such that the water content within the fuel cell is reduced. This may be done for example by changing the O2concentration which influences the water content. Further, the method further can determine in step S2 whether the measured voltage is above a predefined risky voltage threshold, such as the redox potential of the fuel cell system 6. If yes, the at least one physical parameter of the fuel cell system 6 can be adjusted such that the voltage is reduced below the predefined risky voltage threshold. This is advantageous as it has been found that a voltage above one or more predefined risky voltage thresholds, e.g., the redox potential, has an impact on the degradation of the fuel cell system 6 and that, when the voltage is kept below the predefined risky voltage threshold, the degradation may be reduced.

[0077] Further, as voltage fluctuations have also an impact on the degradation of the fuel cell system 6, either fluctuations crossing the predefined risky voltage threshold or fluctuations within a high range or a high frequency of fluctuations, the control method may further control these fluctuations.

[0078] For example, the method can analyze the voltage behavior in step S2 and determine an amount of voltage fluctuations, and if the amount of voltage fluctuations is above a predefined amount threshold, can adjust the at least one physical parameter of the fuel cell system for reducing the voltage fluctuations. Further, the method can analyze the voltage behavior in step S2 and determine a range in which the voltage fluctuates, and if the range is beyond a predefined range, can adjust the at least one physical parameter of the fuel cell system for reducing the range of the voltage fluctuations.

[0079] After step S2 and / or S3, i.e., after adjusting one or more of the physical parameters, the control method can return to step S1 for repeating the method flow by measuring a voltage provided by the fuel cell system and monitoring the voltage behavior, and / or determining a water content within the fuel cell system 6. The method can repeat steps S1 and S2 / S3 continuously or in predefined intervals during the operation of the fuel cell system 6.

[0080] In summary, the described control unit and control method provide an improved possibility of controlling a fuel cell system using physical parameters so that the lifetime of a fuel cell system can be enhanced. Reference numerals

[0081] 1 control unit

[0082] 2 measuring module

[0083] 4 control module

[0084] 6 fuel cell system

[0085] S1-S3 method steps

Claims

Method for controlling a fuel cell system and corresponding control unitClaims:

1. A method for controlling a fuel cell system (6), wherein the method comprises:- measuring (S1) a voltage provided by the fuel cell system (6) and monitoring the voltage behavior, and- adjusting (S2) at least one physical parameter of the fuel cell system (6) based on the measured voltage and voltage behavior for adjusting the voltage provided by the fuel cell system (6) and / or the water content within the fuel cell system (6).

2. The method according to claim 1 , wherein the method further comprises:- determining (S1) a water content within the fuel cell system (6), and- adjusting (S3) the at least one physical parameter of the fuel cell system (6) based on the determined water content for adjusting the voltage provided by the fuel cell system (6) and / or the water content within the fuel cell system (6).

3. The method according to claim 1 , wherein the physical parameter is at least one of gas pressure, reactant gas concentration, temperature, fuel concentration, in particular H2or O2concentration, and gas flow rate.

4. The method according to any one of the preceding claims, wherein the method uses an optimization algorithm for adjusting (S2, S3) the at least one physical parameter based on the measured voltage and voltage behavior and / or the determined water content.

5. The method according to any one of the preceding claims, wherein the method further comprises comparing the determined water content with a predefined water content threshold and, if the determined water content isabove the predefined water content threshold, adjusting (S3) the at least one physical parameter of the fuel cell system (6) for reducing the water content within the fuel cell system (6).

6. The method according to any one of the preceding claims, wherein the method further comprises determining whether the measured voltage is above a predefined risky voltage threshold, in particular the redox potential of the fuel cell system (6), and if yes, adjusting (S2) the at least one physical parameter of the fuel cell system (6) for reducing the voltage below the predefined risky voltage threshold.

7. The method according to any one of the preceding claims, wherein the method further comprises analyzing the voltage behavior and determining an amount of voltage fluctuations, and if the amount of voltage fluctuations is above a predefined amount threshold, adjusting (S2) the at least one physical parameter of the fuel cell system (6) for reducing the voltage fluctuations.

8. The method according to any one of the preceding claims, wherein the method further comprises analyzing the voltage behavior and determining a range in which the voltage fluctuates, and if the range is beyond a predefined range, adjusting (S2) the at least one physical parameter of the fuel cell system (6) for reducing the range of the voltage fluctuations.

9. A control unit (1 ) for controlling a fuel cell system (6), wherein the control unit (1 ) comprises:- a measuring module (2) for measuring a voltage provided by the fuel cell system (6) and monitoring the voltage behavior, and- a control module (4) for adjusting at least one physical parameter of the fuel cell system (6) based on the measured voltage and voltage behavior for adjusting the voltage provided by the fuel cell system (6) and / or the water content within the fuel cell system (6).

0. The control unit according to claim 9, wherein the measuring module (2) is further configured to determine a water content within the fuel cell system (6), and wherein the control module (4) is further configured to adjust the at least one physical parameter of the fuel cell system(6) based on the determined water content for adjusting the voltage provided by the fuel cell system (6) and / or the water content within the fuel cell system (6).

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