DE102020119374A1 - Method for operating a fuel cell system - Google Patents

Abstract

The invention relates to a method for operating a fuel cell system that has at least one fuel cell, with a time profile of a target value of an electrical power of the at least one fuel cell being provided, with a periodic function for the power with successive half-waves being modulated onto the profile of the target value of the power is provided for each half-wave of the periodic function for the power, an amplitude, which is related to the course of the target value of the power, and a time interval are provided, wherein the electrical power is requested from the at least one fuel cell at a particular point in time, wherein a value of the power, which at the respective point in time results from the setpoint value of the power and the value of the periodic function, is set to be less than a permissible maximum limit value for the power and greater than a permissible minimum limit value for the power, wherein the respective half-waves of the periodic function for the power are modulated asymmetrically relative to the course of the nominal value of the power, the value of the power averaged over a period of the periodic function of the power essentially corresponding to the nominal value of the power.

Description

The invention relates to a method for operating a fuel cell system and an energy system.

A fuel cell system can be used as an electric power source for an apparatus.

A fuel cell system is from the publication U.S. 2010/092819 A1 known.

The pamphlet JP 2006/286405 A describes a control unit for a fuel cell.

A computer device for generating uniform heating in an exhaust system is in the reference DE 10 2004 009 211 A1 described.

Against this background, it was an object to operate a fuel cell system effectively.

This object is achieved by a method and an energy system having the features of the independent patent claims. Embodiments of the method and the energy system emerge from the dependent patent claims and the description.

The method according to the invention is provided for operating a fuel cell system that has at least one fuel cell, with a time profile of a target value of an electrical power being set and/or provided as an electrical operating parameter of the at least one fuel cell, with the profile of the target value of the power or for the power, a periodic function for the power with successive, alternating positive and negative half-waves is modulated. Due to the modulation, a respective positive half-wave runs above the course of the desired value of the power and a respective negative half-wave runs below the course of the desired value of the power, with an actual value of the power at a point in time being the sum of the desired value of the power at this point in time and a value of the periodic function at that point in time. In this case, it is provided that for each half-wave of the periodic function an amplitude, based on the course of the target value of the power, and a time interval or a time duration are set and/or provided. The electrical power is requested from the at least one fuel cell at a given point in time, usually by a control device and/or by at least one consumer. A value of the power, which results from the target value of the power and the value of the periodic function for the power at the respective point in time, continues to be smaller or lower than a permissible maximum limit value for the power and greater than a permissible minimum limit value for the power set. The respective half-waves of the periodic function for the power are asymmetrically modulated onto the progression of the target value for the power, with the value of the power modulated in this way averaged over a period of the periodic function or a corresponding mean value essentially corresponding to the target value for the power or corresponding to the target value is set. The positive half-waves and the negative half-waves can change in duration and length from period to period, with the positive half-wave and the negative half-wave being correlated with one another within a period.

In one embodiment, a power value that results from the power setpoint value plus an amplitude of a positive half-wave is set to be less than a permissible maximum limit for the power, with a power value that results from the power setpoint value minus an amplitude a negative half-wave of the periodic function for the power, is set to be greater than an allowable minimum limit for the power.

Depending on the definition, an absolute power or a total power resulting from the implementation of the method results from the target value of the power, which can be set as required or required, and the power, which is also set according to the asymmetrically modulated periodic function or is provided, whereby this additional power set due to the periodic function can also be referred to as relative power, depending on the definition, with the absolute or total power being derived from the nominal value of the power and the relative power due to the periodic function for the power results.

In addition, the same area resulting from mathematical integration of the respective half-wave is set for each positive as well as negative half-wave with respect to the profile of the nominal value of the power. In one embodiment, all half-waves of the periodic function are correlated with one another and have the same area within the usual tolerances, with the amplitude and duration of each half-wave being matched to one another, taking the area into account.

In a further embodiment, it is possible that the course of the respective target value of the power in a period of an order of magnitude of the time interval or the period of the periodic Function is usually approximately or largely constant.

The power requested, for example, by or from a control device in the at least one fuel cell, must be provided by the at least one fuel cell at a given point in time, with the at least one fuel cell having to deliver the power requested for it, with the electrical energy coming from the at least one fuel cell, e.g. a customer, is provided. In this case, the periodic function and thus its half-waves is or are asymmetrically modulated or modulated asymmetrically onto the time course of the reference value of the power. The power changes, e.g. oscillates, according to the periodic function for the power around the curve of the target value for the power within the two permissible limits or between the maximum and minimum limits for the power, which as the envelope is the power from the is to provide at least one fuel cell limit. A modulation of the periodic function for the power is usually only possible in the hardware power range.

The larger a respective amplitude of a respective half-wave is set, the shorter the respective time interval or the duration of the respective half-wave. The smaller a respective amplitude of a respective half-wave is set, the longer the respective time interval. Due to the time intervals of the two half-waves, there is a frequency of the periodic function and the power represented by it, which is correspondingly adjusted according to the asymmetric modulation. The respective area, which encloses a respective half-wave with the curve for the target value of the power, is the same for each half-wave. In one embodiment, a setpoint value for an electrical current can be specified as a further electrical operating parameter of the at least one fuel cell or requested from the at least one fuel cell, with a voltage being set as a further electrical operating parameter depending on the output. Conversely, a setpoint value for the voltage can also be specified, with the current being set accordingly or being requested or drawn off from the fuel cell accordingly. It is possible that the setpoint value of at least one of the electrical operating parameters mentioned remains essentially the same on average.

Power is the product of current and voltage. In this case, the current in the embodiment is the same on average, with the progression of the current including the same area for each half-wave with respect to the desired value. However, the voltage can be higher on average, since a continuously stimulated breakdown of a metal oxide in the at least one fuel cell, for example a PtOx breakdown process, leads to a slight increase in the mean value of the voltage. For example, if the current curve is sinusoidal, usually with increasing current, e.g. in an upward movement of the sinusoidal curve when the current increases, the voltage of the at least one fuel cell continues to fall, with metal oxide, e.g. PtOx, being gradually broken down . This happens with every change in current, usually periodic, and almost instantaneously. In a downward movement of the e.g metal oxide is slower than its degradation. This results in a higher mean voltage. An energy of the half-waves, which is the product of power and time, can be constant or variable. When a period of the periodic function is run through, the excess energy supplied briefly, namely in the positive half-wave, is stored, preferably by a hybrid manager in the battery, so that it can be made available to the vehicle in the negative half-wave when the fuel cell system is supplying less energy to be asked. As a result, an average voltage and an overall efficiency of the fuel cell system increase while maintaining the system limits, which are defined by the maximum limit value and the minimum limit value. As a result, efficient and gentle operation of the fuel cell system is possible, possibly through a low amplitude of the periodic function and a low frequency of the periodic function, resulting in a positive voltage balance.

Furthermore, a profile of an electrical voltage and/or a current as at least one further electrical operating parameter of the at least one fuel cell is dependent on a periodic function depending on the set electrical power that is to be supplied by the at least one fuel cell and/or is requested by it , adjusted, wherein the voltage and/or the current results or adjusts or results from the electrical power requested and provided in the at least one fuel cell. In this case, it is possible for the voltage and/or the current to be correspondingly asymmetrically modulated depending on the asymmetrically modulated power. In one refinement, only one of the two other electrical operating parameters, ie only the voltage or only the current, is modulated asymmetrically depending on the power that is required from the at least one fuel cell. Alternatively, both other electrical operating para meters are modulated depending on this power, whereby the voltage and the current can be mutually adjusted to each other. One of the two other electrical operating parameters, for example an average value of the operating parameter, is usually regulated as a function of the required power, as a result of which the other is adjusted accordingly.

In one embodiment, the power and thus also the voltage and/or the current is adjusted according to a load request by the control device to the at least one fuel cell and thus to the fuel cell system, it being possible for the control device to change load points of the at least one fuel cell, a load can also be requested according to an asymmetrically modulated periodic, e.g. sinusoidal, function.

The at least one fuel cell, usually a cathode and/or anode as at least one electrode of the at least one fuel cell, is or will be assigned a catalyst with or made of a metal, the catalyst of the at least one fuel cell or its at least one electrode during operation of the at least one fuel cell is loaded with the metal oxide of the metal by converting the metal into a metal oxide, loading of the catalyst with metal oxide or an amount of the metal oxide depending on the electrical power and/or can be adjusted by the resulting or resulting electrical voltage of the at least one fuel cell. In an embodiment, the loading can be dependent on the voltage resulting from the power and a dwell time in this voltage. The metal oxide can also be built up and broken down periodically. By varying the amplitude of the power, a frequency according to which the metal oxide is built up and broken down can also be varied.

A trigonometric function, in particular a sine function or a cosine function, is provided as the periodic function for the power and thus for the voltage and/or the current, which is asymmetrically modulated as part of the method. This means that the positive half-wave and the negative half-wave of a period have different amplitudes and durations.

The catalyst of the at least one fuel cell or the at least one electrode, e.g. cathode and/or anode, has a noble metal, e.g. platinum, palladium or ruthenium, with the catalyst having an oxide of this noble metal, e.g. platinum oxide , is loaded with palladium oxide or with ruthenium oxide.

With the asymmetrical modulation of the periodic function for the power, a constant mean value of the power is set with continuous regeneration of the at least one fuel cell, with the regeneration resulting from the voltage that results from the power. To do this, the areas of the positive and negative half-waves are set equal, and a requirement that the areas of the half-waves be equal is another possible aspect of setting the power.

A regenerative effect and thus a degradation of platinum oxide (PtOx) for the at least one fuel cell depends on a level or level of the voltage set as part of the method that has been reached. A flatter positive half-wave with a small amplitude and consequently with a longer holding time or duration of the positive half-wave has a different effect than a higher positive half-wave with a large amplitude and thus with a short holding time or duration. By setting a flatter and therefore longer half-wave, the fuel cell system can be operated closer to the upper or lower system limit. With an upper system limit of 100 kW, for example, with symmetrical modulation, the setpoint power can be 90 kW, for example, with the positive half-wave then being allowed to have a maximum amplitude of 10 kW. In the case of asymmetrical modulation, on the other hand, a higher target power of 95 kW can also be permitted, with the positive half-wave then having an amplitude of a maximum of 5 kW, but being longer or lasting. Such a flexible adjustment of the power or a corresponding equalization of the amplitude by adjusting the duration of a respective half-wave is not possible with symmetrical modulation. A recovery effect is subject to compliance with the upper, maximum permissible power limit.

For example, a sinusoidal modulation of the power with respect to its target value can be used and/or implemented for low target values of the power, with the modulation of the power for low target values having a greater effect than for higher target values. In the case of low target values, an upper half-wave is limited less than a negative half-wave, a negative half-wave with a comparatively small amplitude lasting longer or being maintained for setting the same areas. In one embodiment, it is possible for a regenerative effect to be subordinated to an equality of the areas, with a hybrid manager as a control device being able to tolerate a deviation from the intended target value for the power.

A fuel cell is an electrochemical energy converter, at whose terminals at Leer theoretically the Nernst voltage is present during operation (without external wiring), although in practice a voltage of around 950 mV is to be expected due to internal losses. If a customer and/or consumer, for example an electrical machine, is now connected and current flows as a result, the voltage at the terminals drops as a function of the load, i.e. as a function of the magnitude of the current. The load point, i.e. the level of the current, is set in technical applications via a controller. The voltage adjusts accordingly and falls to a full load point, with a maximum designed current flow, in a range of approx. 0.6 V. Although a higher current flow is possible, it is no longer useful due to reduced efficiency. Alternatively, instead of controlling the current to the desired value provided for this purpose, it would also be possible to control the voltage to the desired value provided for this purpose. In this case, a variable current is drawn from the consumer or requested from the at least one fuel cell, which results in the desired voltage at the terminals of the at least one fuel cell. Irrespective of which form of regulation of the at least one electrical operating parameter is used, a respective product of the currently required current and the voltage that is set for this results in the current output of the fuel cell system. If the fuel cell system is used for a vehicle, with electrical energy from the fuel cell system being used to drive the vehicle, provision is made for the vehicle to request or specify the target value for the power. Either the current or the voltage is controlled to achieve the desired power setpoint. When the power is modulated, the current or the voltage is also modulated, depending on the respective regulation. Irrespective of this, a level of the voltage of the at least one fuel cell is constantly changed, which is accompanied by a conversion reaction of the metal oxide.

The energy system according to the invention has a fuel cell system that has at least one fuel cell and a control device, e.g an embodiment of the method. The monitoring device is designed to set a time profile of a target value for an electrical output of the at least one fuel cell and to modulate a periodic function with successive positive and negative half-waves onto the profile of the target value and an amplitude for each half-wave of the periodic function that is based on the profile of the Target value of the power is related or is, and set and / or provide a time interval. The monitoring device is also designed to request the electrical power from the at least one fuel cell at a particular point in time and to set a value for the power that is at the particular point in time from the target value for the power and the value of the periodic function of the power to be less than or equal to 100%. less than an allowable maximum power limit and greater than an allowable minimum power limit. In addition, the control device is designed to modulate the respective half-waves asymmetrically relative to the progression of the desired power value and to set the mean value of the power, which is to be averaged over a period of the periodic function, essentially corresponding to the desired power value. By providing the asymmetric modulation for the power, the allowable power limits foreseen, i.e. H. the system limits, are complied with, whereby the maximum permissible limit for the power is not exceeded and the minimum permissible limit is not fallen below.

The energy system has at least one consumer for electrical energy, e.g. a battery or an accumulator and/or a consumer, e.g asymmetric modulation of the power resulting from the fuel cell system is supplied. If the at least one consumer is designed as an electrical machine, for example, which converts the electrical energy from the at least one fuel cell into mechanical energy, the setpoint value for the power can be adapted to the respective needs of the electrical machine and can therefore also be changed over time. With the asymmetrical modulation of the periodic function, a time-dependent change in the desired value of the power and thus a change over time in the profile of the desired value of the power is taken into account, with the desired value of the power changing over time over a period of the periodic function, in particular relative to the periodic function, only little changes.

The energy system is provided for a vehicle, for example a motor vehicle, with a battery and an electrical machine being supplied or to be supplied with electrical energy by the energy system and thus by the fuel cell system, with the electrical machine being designed to supply electrical energy of the power system into mechanical energy to propel the vehicle.

The fuel cell system comprises at least one fuel cell stack and accordingly the at least one fuel cell for providing energy, for example starting energy, and for buffering power peaks with the battery and thus a corresponding battery system. The fuel cell system can be combined with the control device, e.g. a higher-level hybrid manager. It is known that the electrical power provided by a fuel cell system is not consistently high in constant power operation. With the same setpoint current, the voltage of the fuel cell stack and thus of the fuel cell system drops slightly as the operating time increases. The drop in the voltage of the fuel cell stack can be attributed, among other things, to unwanted loading of the catalyst of the at least one fuel cell, including platinum oxide or different species of platinum oxide, with a cathode of the at least one fuel cell usually being contaminated with the catalyst, here coated with platinum. This platinum oxide or this platinum oxide species forms or forms alternatively or additionally on the cathode of the at least one fuel cell during operation and is voltage-driven, with a build-up and a breakdown of platinum oxide being a function of a half-cell voltage of the cathodes and is thus a function of the cell voltage and thus the voltage of the at least one fuel cell. Such a build-up process cannot be prevented and is part of normal operation of the at least one fuel cell. The greater the PtOx or platinum oxide loading, the greater the voltage losses. A respective loss or a respective reduction or decrease in voltage behaves according to a logarithmic function, with the voltage changing most strongly in the first few seconds. After that, the voltage changes only slowly and insidiously. By changing the power or a load point by the control device, a new voltage of the at least one fuel cell is set, with conversion processes of the platinum oxide (PtOx) taking place. By changing to a higher voltage, more PtOx is built up. By changing to a lower voltage, PtOx is at least partially removed. Such a conversion process, which includes a build-up process and a breakdown process, is never complete, but strives logarithmically towards a new electrochemical equilibrium. A change to a high load point and consequently to a lower voltage of the fuel cell stack can also be interpreted and/or referred to as regeneration, since part of an undesired load of PtOx is reduced. The PtOx build-up processes described above and the regenerative effect due to load changes mean that a higher load point is in principle more favorable for the operation of the vehicle, which is supplied with electrical energy by the fuel cell stack. The fact that higher load points cannot be consistently implemented during operation of the vehicle and that PtOx is also slowly built up in these load points is countered with the presented method and the presented energy system, with which a strategy for dealing with PtOx loads is provided. A strategy provided as part of the method for dealing with the mechanism described above with regard to the loading of the cathode or the catalyst with PtOx or a quantitative change in an amount of PtOx, ie the build-up and breakdown of PtOx, includes a modulation of a periodic course, usually a sine or a sinusoidal progression to the power target value, with constant conversion and thus a corresponding change in the amount of PtOx, the desired power, ie the power target value, being achieved on average. In a refinement of the method, the raising and lowering of a stack voltage and/or the voltage of the at least one fuel cell is limited by modulating the periodic, usually sinusoidal course of the target value for the power of the fuel cell system by the minimum and maximum power provided for the at least one fuel cell. It is taken into account that the amplitude of the modulation of the usually sinusoidal curve or a sinusoidal modulation of the curve of the desired value of the power at low load points is not less than the minimum permissible power and at high load points not greater than the maximum permissible power of the fuel cell system or the at least is or will be a fuel cell. Such a sine modulation is usually only possible in the hardware area.

When an embodiment of the method is implemented, PtOx is partially reduced when the load changes or when the load changes to a higher load point. If the load is then switched back to the initial state in the event of a corresponding load change, a higher voltage is available again in the fuel cells. This recovery or recovery effect lasts for a limited time until the PtOx loading of the catalyst and thus the cathode coated and/or loaded with PtOx has again reached the initial state or a corresponding initial level. The sinusoidal modulation of the setpoint value of the power and thus the resulting voltage addresses or takes this fact into account, but only works to a limited extent near the power limits or the limits of the power of the fuel cell system, since the amplitude of a sinusoidal oscillation of the power curve in the embodiment minimum power not fall below or not exceed the maximum power.

Due to the asymmetrical modulation provided according to the invention of the periodic, e.g a maximum limit or threshold value of the power of the fuel cell system is not exceeded by a respective positive half-wave of the resulting periodic curve of the power (and thus also the voltage) or its amplitude and the lower limit or the minimum limit or threshold value of the Performance of the fuel cell system is not fallen short of by a respective negative half-wave of the resulting periodic curve or its amplitude. Such a flexible adjustment is not possible with a symmetrical modulation.

In one application, this relates to the restriction of the positive half-wave of the phase of the periodic curve when approaching the upper limit or the upper threshold value of the power of the fuel cell system and the restriction of the negative half-wave when approaching the lower limit or the lower limit value of the power of the fuel cell system. If the amplitude is reduced, however, the time interval or a length or duration of the half-wave is consequently lengthened, with the area between the course of the nominal value of the power and the respective half-wave, which can be calculated by integration, and thus the energy remaining the same or being retained, since the Time interval is adjusted to the amplitude. The at least one fuel cell can thus be operated longer at higher load points and consequently at lower voltages, and builds up less PtOx. A loss or a reduction in voltage due to PtOx is thus reduced and the desired nominal value of the power is achieved on average. The excess energy briefly supplied when passing through the phase or the sine wave (ie when passing through the positive half-wave) is transferred from the fuel cell system into the battery or into the battery system by the control device, which can be designed or referred to as a hybrid manager transferred and stored therein, this stored energy then in the second part of the sine wave (ie when passing through the negative half-wave), when the fuel cell system delivers less energy, the vehicle is made available. This method increases the average voltage in the fuel cell stack as well as the overall efficiency of the fuel cell system, while maintaining limits on the performance of the fuel cell system.

In an embodiment of the method, an asymmetric sinusoidal modulation and thus an asymmetric modulation of the phase of the periodic profile of the desired value of the power is carried out in the vicinity of the limits of the fuel cell system. The positive half-wave of the phase, usually the sine wave, is restricted when the upper, maximum power limit of the fuel cell system is approached, and the negative half-wave of the phase is restricted when the lower, minimum power limit of the fuel cell system is approached.

The fuel cell system is or is combined with a sufficiently powerful battery system and controlled by a hybrid manager, i. H. regulated and/or controlled. With the asymmetric sinusoidal modulation of the power to the target value of the power or a target power value, a constant PtOx conversion with regenerative effect of the stack voltage of the fuel cell system is achieved with at the same time mild load change dynamics to protect the entire fuel cell system.

The method also enables gentler operation of mechanical components, e.g. B. a compressor, the entire fuel cell system realized while maintaining the limits of the fuel cell system. With the method, an efficiency of the fuel cell system can be increased without additional components. The method can be used for fuel cell vehicles, for example for fuel cell range extender vehicles.

The method can also be implemented for stationary systems for providing electricity based on fuel cell systems and for different vehicles such as aircraft, drones and submarines with fuel cell drives.

The setpoint for the power can change and/or be constant at times. The total power resulting from asymmetric modulation must be between a maximum permissible value and a minimum permissible value of the power and thus within the limits of the fuel cell system, for example between 100 kW and 0 kW. A, for example, trigonometric, sinusoidal or cosinusoidal function, depending on the definition, is asymmetrically modulated onto the desired power value, with the asymmetrically modulated power on average corresponding to the respectively provided and/or required desired power value. If the target value of the power is, for example, 90 kW, the amplitude of the positive half-wave can be a maximum of 10 kW, with the total maximum permissible limit of the power of 100 kW results, whereas the amplitude of the negative half-wave can be up to 90 kW, at least theoretically, without falling below the minimum permissible limit of 0 kW. However, the positive half-wave is then correspondingly longer and the negative half-wave correspondingly shorter in order to achieve the target value of 90 kW on average. The situation is reversed if the nominal power value is 20 kW, for example. In this case, the amplitude of the positive half-wave can be up to 80 kW, but the amplitude of the negative half-wave is at most 20 kW, where the positive half-wave can be shorter and have a larger amplitude, and the negative half-wave can be longer and have a smaller amplitude can. The usually asymmetrically modulated function means that the limits of the fuel cell system are adhered to and at the same time the target value of the power is provided on average. The maximum permissible value of the power and the minimum permissible value of the power as limits of the fuel cell system represent envelopes for the modulation of the power.

It goes without saying that the features mentioned above and those still to be explained below can be used not only in the combination specified in each case, but also in other combinations or on their own, without departing from the scope of the present invention.

• 1 zeigt in schematischer Darstellung eine Ausführungsform des erfindungsgemäßen Energiesystems zum Durchführen einer Ausführungsform des erfindungsgemäßen Verfahrens.
• 2 zeigt ein erstes Diagramm zu der Ausführungsform des erfindungsgemäßen Verfahrens.
• 3 zeigt ein zweites Diagramm zu der Ausführungsform des erfindungsgemäßen Verfahrens.

The invention is shown schematically on the basis of embodiments in the drawing and is described schematically and in detail with reference to the drawing.

• 1 shows a schematic representation of an embodiment of the energy system according to the invention for carrying out an embodiment of the method according to the invention.
• 2 shows a first diagram for the embodiment of the method according to the invention.
• 3 shows a second diagram for the embodiment of the method according to the invention.

The figures are described coherently and comprehensively. The same reference symbols are assigned to the same components.

In the 1 schematically illustrated embodiment of the energy system according to the invention is arranged in a vehicle 2, which is designed here as a motor vehicle. The energy system has a fuel cell system 4, which has at least one fuel cell. In addition, the energy system has a control device 6 which is designed here to monitor, ie to control and/or regulate, operation of the fuel cell system 4 and the embodiment of the method according to the invention for operating the fuel cell system 4 . Furthermore, the energy system is integrated into an on-board electrical system of the vehicle 2 and is connected via this to consumers 8, 10 for electrical energy. Electrical energy, which the fuel cell system 4 generates during its operation, is made available to these customers 8, 10 by the fuel cell system 4. At least one first consumer 8, 10 is designed as an electrical energy store, which is designed as a battery or as an accumulator, for example as a high-voltage battery. At least one second consumer 8, 10 is designed here as an electrical consumer, here as an electrical device of the vehicle 2 or as an electrical machine. The electrical machine is designed to convert electrical energy from the fuel cell system 4 into mechanical energy, with which the vehicle 2 is driven or moved.

Each fuel cell of the fuel cell system 4 has a cathode and an anode as electrodes. At least one electrode, ie the cathode and/or the anode, has a catalyst made of a noble metal, here platinum for example, with which it is coated. During the operation of a respective fuel cell, oxidation or electron release (2H ₂ → 4H ⁺ + 4e ^- ) is carried out as a chemical reaction _{at the anode, whereas reduction or electron acceptance (O 2} + 4H ⁺ + 4e ^- → 2H ₂ O) is carried out.

The two diagrams from 2 and 3 each have a time axis as abscissa 20. Values of an electrical voltage which is applied to a respective fuel cell of the fuel cell system 4 are plotted above this along an ordinate 22 of a respective diagram. During the embodiment of the method, it is provided that the control device 6 changes load points of the respective fuel cell, with the control device 6 requesting an electrical power to be delivered or provided by the fuel cell in the fuel cell, with the method for the fuel cell changing one in the chart off 2 shown first time-dependent curve 24 of the voltage according to a first periodic function and in the diagram 3 shows the second time-dependent profile 40 of the voltage according to a second periodic function. It is possible that the abscissa 20 or time axis is a level of a mean value and/or a target value of the respective curve 24, 40 of the voltage or is at the level that can correspond to the intended mean value and/or target value.

During operation of the respective fuel cell, the catalyst is loaded, particularly at the cathode, with an oxide of the noble metal and thus with a metal oxide, here for example with platinum oxide (PtOx), with a change in the amount of metal -Oxide, ie a build-up and a breakdown of the metal oxide, is correlated with the voltage applied to the respective fuel cell, with the voltage and the change in quantity affecting each other and being mutually dependent on one another. A load point of the respective fuel cell is set with the control device 6 and the electrical power required from the fuel cell is specified, with either the current or the voltage being regulated to achieve the desired target power. The voltage applied to the fuel cell and thus also the amount of metal oxide is continuously changed by the asymmetrical modulation of the setpoint power value provided according to the invention, in particular by an asymmetrical sinusoidal modulation, ie the metal oxide is constantly being converted. Metal oxide is built up when the voltage is increased and depleted when the voltage is decreased or reduced.

As part of the method, the power requested from the fuel cell by the control device 6 and thus a power to be delivered by the fuel cell is changed by changing the load point according to a periodic function, here a sine function, whereby depending on the underlying regulation, the voltage is also changed according to a respective periodic Function is changed, which is shown on the basis of a respective course 24, 40 in the diagrams. The electrical voltage U is usually proportional to the electrical power P, with the voltage U and the power P being related via the electrical current I according to P=U*I.

The curve 24 of the voltage in the diagram is off 2 divided into a first section (arrow 25) and a second section (arrow 27). The here periodic, in particular sinusoidal curve 24 results in the first section (arrow 25) when the power requested by the control device 6 in the fuel cell or a target value of the specified power to be delivered by the fuel cell is measured by the control device 6 according to a periodic, in particular, a sinusoidal function is modulated and thus set, the resulting mean value being indicated by the abscissa 20 or the time axis. If the power required from the fuel cell should not be modulated according to the sinusoidal function, the voltage curve results in the second section (arrow 27), with a mean value for the voltage indicated by a line 26 resulting, which is indicated by a double arrow 28 indicated value of the mean value of the sinusoidally modulated curve 24 deviates. Accordingly, the mean value of the voltage can be increased and the efficiency of the fuel cell can be increased by the sinusoidal modulation of the power and a resulting modulation of the voltage of the fuel cell.

The diagram also shows an upward transient 30 extending from a minimum to a maximum of the curve 24 and a downward transient 32 extending from the maximum to the next minimum. When the voltage increases (upward transient 30), metal oxide is built up. Furthermore, when the voltage is reduced (down transient 32), metal oxide is decomposed. A change in the amount of metal oxide according to alternating build-up and breakdown can behave logarithmically.

Since the voltage is changed the most in the first section (arrow 25), a maximum amplitude of the voltage is also reached at a respective load point. The higher the frequency of the periodic function, according to which the power is requested from the fuel cell by changing the load points, the breakdown of the metal oxide is significantly faster in relation to the build-up of the metal oxide, with a regenerative effect on the catalyst being much greater is greater than a renewed loss of performance due to a renewed build-up of the metal oxide in the lower range of a load point that has been passed through for a short time.

Metal oxide (PtOx) is usually partially broken down when the load changes to a higher load point. If there is then a change back to an initial state, the at least one fuel cell again (briefly) has a higher voltage available. Such a recovery effect lasts for a limited time until the loading with metal oxide has again reached an initial level. The loading with metal oxide is minimized by constantly changing the load states or load points and by repeatedly raising and lowering the voltage of the at least one fuel cell. Conversion processes of the metal oxide can be much faster than the load change itself, so that theoretically a highly dynamic load change would be possible. In order not to damage the entire fuel cell system and, among other things, to protect the mechanical components of the fuel cell system from excessive load dynamics, a low or soft and constant load change, e.g. B. corresponding to the sinusoidal curve 24 from 2 , carried out, whereby short abrupt load steps are avoided. The frequency and the amplitude of the course 24 determine the load change dynamics and thus a Regeneration effect for the metal oxide. The greater the amplitude in a first semi-arc or a first half-wave, the greater the build-up of the metal oxide in the up-transient 30 and consequently a power regenerated in the down-transient 32 . Since the first few seconds show the greatest change in voltage due to the logarithmic behavior, the greatest voltage for this load point is also reached in this area. The higher the frequency of curve 24, the greater the effective regeneration benefit. For a second half-arc or a second half-wave, the downward transient 32 is initially continued and then returned to the origin. Since a degradation process for the metal oxide is orders of magnitude faster in relation to a build-up process, the regeneration effect is much greater than a renewed loss of performance due to a renewed build-up of the metal oxide in a lower load point range briefly passed through. After a period, in this case a sine wave, has been completed, the at least one fuel cell provides a higher voltage with the same current flow.

The metal oxide is always built up towards a thermodynamic and electrochemical equilibrium according to a current voltage level and a holding time or length of a period, but also depending on temperature, humidity and other operating conditions for the at least one fuel cell.

In addition, the electrochemical equilibrium shifts as the holding time increases, with the current loading of the at least one fuel cell having to be continuously adjusted with metal oxide being forced. This results in the already mentioned logarithmic course of the voltage, in this case a loss of voltage, with the voltage never reaching a limit value.

The greater the difference between a momentary loading with metal oxide and a desired equilibrium constellation for the loading, the more the build-up of the metal oxide is forced. Therefore, the largest build-up of metal oxide occurs in the first few seconds and then weakens more and more. The build-up process is slow in relation to the break-down process and lasts from several seconds to minutes. Electrochemically, the degradation process takes place much faster and lasts on the order of a few milliseconds. Such a time difference results in a positive voltage balance, with the voltage increasing.

The fuel cell or a fuel cell stack consisting of a plurality of fuel cells of the fuel cell system 4 provides a higher voltage with the same current flow after a sine curve or a phase of the periodic curve 24 consisting of two half waves has been completely run through. The excess electrical energy that is briefly provided when the sine wave or phase passes through can be transmitted from the control device 6 to one of the consumers 8 , 10 .

In the method provided, the power required for the fuel cell according to the periodic, in particular sinusoidal function is asymmetrically modulated by the control device 6 and/or with the control device 6, with the load being changed in a correspondingly asymmetrically modulated manner and a time sequence of the load points being changed accordingly . This results in the in the diagram 3 course 40 shown of the voltage of the fuel cell. This curve 40 has the form of a periodic, here sinusoidal, function, with each phase having a positive half-cycle 42 with positive values for the voltage and a negative half-cycle 44 with negative values for the voltage. The diagram shows 3 , as in the case of curve 24 in the diagram 2 that the required power and thus the resulting voltage can be set by symmetrical modulation 48, with an amplitude and a frequency of the respective curve 24, 40 of the voltage remaining largely constant or be specified or adjusted constantly.

Furthermore, the diagram shows off 3 , how when the embodiment of the method according to the invention is carried out, the power requested by the control device 6 in the fuel cell is or is asymmetrically modulated relative to a power setpoint by means of a periodic, in particular sinusoidal function provided for this purpose, i.e. the requested power is modulated asymmetrically by the periodic function shown on a curve of the power setpoint, from which a corresponding curve 40 of the voltage with a corresponding asymmetrical modulation 46 results or for which an asymmetrical modulation 46 results, with the curve 40 of the voltage oscillating around the setpoint of the voltage, the is indicated here by the abscissa 20 and is constant at least in a period of time shown here. In this case, an amplitude and a length or a duration or a time interval and thus a frequency of the curve or the sinusoidal function of the required power and thus the voltage from half-wave 42, 44 to half-wave 42, 44 are adjusted.

It is provided that at a given point in time, a sum of a setpoint value for the power and a periodic function for the power, which is sinusoidal in this case, according to which the power is asymmetrically modulated, is less than a maximum permissible limit for the power and higher than a minimum permissible limit for the Power is where also the voltage trace 40 is less than a maximum allowable voltage limit and higher than a minimum allowable voltage limit. The power and thus the voltage changes or oscillates between a respective permissible maximum value and a respective permissible minimum value for the power or for the voltage. By changing the amplitude from half-wave 42, 44 to half-wave 42, 44 due to the asymmetric modulation 46, the time interval or the duration of the respective half-wave 42, 44 is also modulated asymmetrically, with an area 50 here being covered by each half-wave 42, 44 and is enclosed by the time axis or abscissa 20 and is calculated by integration, is constant, with the electrical energy being retained or remaining constant despite asymmetrical modulation 46 of the power. The time interval then results as a function of the amplitude and the area 50 set to be constant, which is surrounded by a half-wave 42, 44 and the time axis. The larger the amplitude of a half-wave 42, 44 is set, the shorter its duration or time interval becomes. The lower an amplitude of a half-wave 42, 44 is set, the longer its duration or time interval becomes. It is possible to modulate the build-up and breakdown of metal oxide in the catalyst in a variable manner and to operate the fuel cell gently.

The asymmetrical modulation carried out in the method makes it possible to adapt an amplitude and a duration of a respective half-wave 42, 44 to the fuel cell system 4 flexibly and as required. This is not possible with symmetrical modulation in this form, since the amplitude and the duration of a respective half-wave of a period of curve 24 are the same for the two half-waves forming one period.

Reference List

2

vehicle

4

fuel cell system

6

control device

8, 10

customer

20

abscissa

22

ordinate

24

course

25

arrow

26

line

27

arrow

28

double arrow

30

up transient

32

down transient

40

course

42

positive half-wave

44

negative half-wave

46

asymmetric modulation

48

symmetrical modulation

50

surface

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• US2010092819A1 [0003]
• JP2006286405A [0004]
• DE 102004009211 A1 [0005]

Claims (10)

Method for operating a fuel cell system (4) which has at least one fuel cell, a time profile of a target value for an electrical power of the at least one fuel cell being provided, a periodic function for the power with successive half-waves being modulated onto the profile of the target value for the power , wherein for each half-wave of the periodic function for the power, an amplitude, which is related to the curve of the setpoint value of the power, and a time interval are provided, wherein the electrical power is requested from the at least one fuel cell at a respective point in time, with a value of the power, which results at the respective point in time from the nominal value of the power and the value of the periodic function, is set to be less than a permissible maximum limit value for the power and greater than a permissible minimum limit value for the power, with the respective Ha lb waves of the periodic function for the power are modulated asymmetrically relative to the progression of the target value of the power, the value of the power averaged over a period of the periodic function of the power substantially corresponding to the target value of the power. procedure after claim 1 , in which for each half-wave the same area resulting from mathematical integration is set between the respective half-wave and the progression of the target value of the power. procedure after claim 1 or 2 , in which a course of an electrical voltage of the at least one fuel cell is set according to a periodic function depending on the set electrical power. Method according to one of the preceding claims, in which the power is adjusted according to a load requirement on the at least one fuel cell. Method according to one of the preceding claims, in which the at least one fuel cell is assigned a catalyst with a metal, the catalyst of the at least one fuel cell being loaded with the metal oxide of the metal during operation of the fuel cell by converting the metal into a metal oxide is, wherein a loading of the catalyst with metal oxide is set depending on the electrical power. Method according to one of the preceding claims, in which a sine function is provided as the periodic function for the power with respect to the course of the reference value for the power. Procedure according to one of Claims 5 or 6 , In which the catalyst of the at least one fuel cell has a noble metal. Energy system that has a fuel cell system (4) with at least one fuel cell and a control device (6), wherein the control device (6) is designed to set a time profile of a target value of an electrical output of the at least one fuel cell and to the profile of the target value to modulate the periodic function for the power with successive half-waves and to set an amplitude for each half-wave of the periodic function for the power, which is related to the progression of the nominal value of the power, and a time interval, the control device (6) being designed to To request power from the at least one fuel cell at a particular point in time and a value of the power that results from the target value of the power and the value of the periodic function of the power at the particular point in time, is less than a permissible maximum limit value for the power and greater than a permissible minimum limit value for the power, the control device (6) being designed to modulate the respective half-waves of the periodic function of the power asymmetrically relative to the course of the desired value of the power, the value of the power over a period of the periodic function of the power averaging and adjusting substantially to the target value of the power. energy system after claim 8 Having at least one buyer (8, 10) for electrical energy from the fuel cell system (4). Energy system according to one of Claims 8 or 9 , which is provided for a vehicle (2).

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