Method for operating a fuel cell device and a fuel cell device

Abstract

The invention relates to a method (10a; 10b) for operating a fuel cell device (12a) which is connected to a power grid, wherein in at least one method step the provision of electrical current by the fuel cell device (12a) is adapted in the event of a short-term voltage drop of the power grid. It is proposed that a fluid supply (14a) to the fuel cell device (12a) be maintained at least partially during the short-term voltage drop of the power grid.

Description

State of the art

[0001] A method for operating a fuel cell device connected to a power grid has already been proposed, wherein in at least one process step the provision of electrical current by the fuel cell device is adapted in the event of a short-term voltage drop of the power grid. Disclosure of the invention

[0002] The invention relates to a method for operating a fuel cell device which is connected to a power grid, wherein in at least one method step the provision of electrical current by the fuel cell device is adapted in the event of a short-term voltage drop of the power grid.

[0003] It is proposed that a fluid supply to the fuel cell device be at least partially maintained during short-term voltage drops in the power grid. The fuel cell device is preferably an electrical generating unit or an electrical generating plant, particularly as defined in VDE-AR-N 4105. The fuel cell device preferably comprises at least one fuel cell unit. The at least one fuel cell unit comprises at least one fuel cell, preferably a plurality of fuel cells, which are interconnected, in particular in at least one fuel cell stack and / or a network of fuel cell stacks, and are particularly intended for joint operation.The at least one fuel cell is preferably designed as a solid oxide fuel cell, alternatively as a molten carbonate fuel cell, a polymer electrolyte fuel cell, a direct methanol fuel cell, or the like. The fuel cell device preferably comprises at least one inverter via which the at least one fuel cell unit is electrically connected to the power grid. Preferably, the at least one fuel cell unit and the inverter are part of a primary electrical network, specifically a single primary electrical circuit, of the fuel cell device. The power grid is preferably designed to connect the fuel cell device to at least one load. The power grid can be a public power grid, in particular an interconnected grid, or an island grid. Preferably, the power grid and the at least one inverter are part of a secondary electrical network.

[0004] A "short-term voltage drop" is defined as a voltage profile whose amplitude and duration lie within a voltage limit profile, and which, in particular, never falls below the voltage limit profile with respect to a specific voltage value. The voltage limit profile is preferably specified by a grid operator and defined, for example, in a national or regional grid code / transmission code, such as the grid and system rules of the German transmission system operators. Following a voltage drop, the voltage limit profile typically shows a return to a minimum voltage, for example, 90% of a nominal voltage, of the grid within a maximum of 5 seconds, preferably within a maximum of 3 seconds, and in particular within a maximum of 1.5 seconds or less. The voltage limit profile preferably defines whether a generating unit / generation plant is fault-ride-through (FRT) capable, i.e.,The generating unit / plant is only disconnected from the power grid if the voltage falls below the specified threshold, particularly for self-protection. A voltage in the power grid can fall below this threshold due to a decreasing voltage amplitude or a slow or non-existent return to the minimum voltage.

[0005] The fuel cell device preferably comprises a control unit for adjusting the electrical current of the fuel cell device. The electrical current of the fuel cell device is preferably the electrical current within the primary grid, in particular the electrical current through the at least one fuel cell. Preferably, the control unit contains an internal voltage limit profile which, at any given time, has an amplitude equal to or less than the corresponding amplitude of the grid operator's voltage limit profile. In particular, the internal voltage limit profile is identical to the grid operator's voltage limit profile. The internal voltage limit profile may, in certain sections, be on top of the grid operator's voltage limit profile or, with respect to amplitude, be lower, but it must not intersect the grid operator's voltage limit profile.Preferably, the control unit and / or the inverter separates the primary network from the secondary network when the voltage drop of the secondary network falls below the internal voltage limit.

[0006] In at least one adaptive operating mode of the fuel cell device, the control unit adjusts the current within the primary network depending on the voltage of the secondary network during a brief voltage drop. Optionally, the fuel cell device includes at least one further operating mode in which the current, voltage, and / or electrical power of the primary circuit is regulated to a constant value during a brief voltage drop of the secondary network, for example, by using at least part of the electrical power of the primary circuit to charge an energy storage device of the fuel cell device, in particular a battery, a supercapacitor, or the like, or to operate a substitute load of the fuel cell device, for example, a heating element.The control unit preferably executes the adaptation operating mode when neither energy storage nor substitute load is available, for example because the fuel cell device does not include energy storage nor substitute load, for example because the energy storage and / or the substitute load cannot or should not absorb any further electrical power.

[0007] In at least one adaptive operating mode, the current within the primary network correlates, particularly positively, with a voltage profile in the secondary network, preferably also in the case of a short-term voltage drop. The at least one fuel cell is designed for the electrochemical conversion of a fuel. The conversion rate of the fuel by the at least one fuel cell preferably correlates with the current within the primary network. The fuel cell unit preferably comprises at least one fuel delivery unit for adjusting the flow rate of the fuel through the at least one fuel cell, in particular for adjusting the supply rate of fresh fuel.The fuel cell unit preferably comprises at least one recirculation pumping unit for adjusting the fuel flow rate through the at least one fuel cell, in particular for adjusting the return rate of exhaust gas from the at least one fuel cell to the fuel. The fuel pumping unit, the recirculation pumping unit, and / or the oxygen pumping unit are preferably designed as a pump, compressor, or blower, or alternatively as a proportional valve. The fuel cell unit preferably comprises at least one oxygen pumping unit for adjusting the flow rate of oxygen, in particular in the form of air, through the at least one fuel cell.Preferably, in at least one process step, the control unit adjusts the fuel delivery unit, the recirculation delivery unit, and / or the oxygen delivery unit, preferably by controlling at least one electrochemical parameter that describes the electrochemical reaction by the at least one fuel cell. The at least one electrochemical parameter is, for example, the current or electrical power through the at least one fuel cell, the fuel utilization of the at least one fuel cell, the temperature of the exhaust gas and / or the at least one fuel cell, the combustion-air ratio of the exhaust gas, or the like.

[0008] Preferably, in the event of a brief voltage drop, the fuel delivery unit, the recirculation delivery unit, and / or the oxygen delivery unit continues to operate, in particular with an unchanged delivery rate, with a reduced delivery rate relative to a maximum delivery rate, or is maintained at a value determined by controlling at least one electrochemical parameter. Preferably, at least one minimum flow rate of the fuel and / or oxygen, in particular a non-zero rate, is maintained in accordance with the control parameters or is unchanged.Preferably, the fuel delivery unit, the recirculation delivery unit and / or the oxygen delivery unit is only switched off or set to a delivery rate of zero and / or a fuel shut-off valve of the fuel cell device is closed if the voltage profile of the secondary network falls below the internal voltage limit profile.

[0009] The design according to the invention advantageously minimizes the dead time of the fuel cell device after a brief voltage drop. In particular, the fuel cell device is immediately ready for operation with respect to the fluid dynamics of fuel, oxygen, and exhaust gas. Specifically, the time required for pressure build-up and / or refueling of the fuel cell unit can be advantageously shortened, and in particular, avoided altogether. Furthermore, the provision of electrical current by the at least one fuel cell unit can be kept in compliance with the power grid, in particular with the voltage limit profile of the grid operator, so that the fuel cell device is considered FRT-capable, especially regardless of the presence of an energy storage system and / or a backup load.

[0010] It is further proposed that during the brief voltage drop of the grid, the thermal efficiency of the fuel cell device is shifted at the expense of its electrical efficiency. The thermal or electrical efficiency of the fuel cell device preferably describes the ratio of the thermal or electrical energy produced by the at least one fuel cell unit to the chemical energy of the fuel. Preferably, the at least one fuel cell converts less fuel during the brief voltage drop than immediately before the brief voltage drop. The conversion rate of the at least one fuel cell during the brief voltage drop can correlate with a voltage profile in the secondary grid or be actively set to zero by the control unit.During the brief voltage drop, the fuel is preferably at least partially, and more preferably to a greater extent than immediately before the brief voltage drop, and in particular substantially completely, thermally converted by means of a burner unit of the fuel cell device, especially the fuel cell unit. The burner unit preferably comprises at least one burner, in particular an afterburner and / or a preheater, which is arranged together with the at least one fuel cell in a common housing of the fuel cell unit. Alternatively or additionally, the burner unit comprises a central burner to which several fuel cell units of the fuel cell device are connected.During the short-term voltage drop, the thermal conversion of the fuel can be distributed among the different burners of the burner unit or, preferably, carried out by only one of the burners, in particular the afterburner. The burner unit, in particular the afterburner, is designed to ensure at least substantially complete fuel conversion for at least the maximum duration of the short-term voltage drop, i.e., for the maximum duration of the internal voltage limit phase. "Substantially complete fuel conversion" preferably means at least one conversion of the fuel or fuel mixture, such that the exhaust gas from the conversion comprises a residue of at most 5%, preferably at most 1%, and most preferably at most 0.5% of the fuel based on the volume and / or mass of the exhaust gas.Preferably, the afterburner is designed to convert the fuel at least substantially completely for at least the maximum duration of the short-term voltage drop, particularly during continuous operation, at a predetermined maximum fuel flow rate through the fuel cell unit, and most preferably independently of the conversion rate of the at least one fuel cell. Alternatively, the afterburner is designed to depend on a minimum conversion rate of the at least one fuel cell, for example, if the internal voltage limit profile exhibits a non-zero minimum voltage profile. The design according to the invention allows a fuel supply rate to the fuel cell unit to be advantageously maintained at a high level during the short-term voltage drop without increasing the risk of fuel residue leaking from the fuel cell device.In particular, the fuel cell device can also be operated safely and advantageously during a short-term voltage drop.

[0011] It is further proposed that during the brief voltage drop of the grid, a thermal mass of the fuel cell device, in particular of an afterburner of the fuel cell device, is used to at least partially absorb the heat produced by the fuel cell device during the brief voltage drop. Preferably, the thermal mass is a heat capacity of the burner unit, in particular of the afterburner, which is designed to absorb heat generated, and in particular heat not carried away by the exhaust gas, during at least substantially complete conversion of the fuel by the burner unit, in particular by the afterburner.The thermal mass can be achieved through a specific component, preferably bonded to the burner unit, or through a design feature of the burner unit itself, such as wall thickness, overall volume, material used, or the like. Preferably, the temperature of the burner unit, particularly the afterburner, is increased by the fuel conversion during the brief voltage drop compared to the temperature immediately preceding the brief voltage drop. In particular, the temperature of the burner unit is negatively correlated with the voltage profile of the secondary network.Alternatively or additionally, the burner unit comprises a cooling or heat distribution system, such as a metallic connecting element to another thermal mass of the fuel cell device, a heat pipe, air cooling, or water cooling, to maintain the temperature of the burner unit below a maximum permissible temperature. The design according to the invention allows the fuel cell device to be advantageously operated independently of an external heat source.

[0012] It is further proposed that the fluid supply be switched off, in particular only, if at least one temperature parameter of the fuel cell device, especially the afterburner, exceeds a limit value. The temperature parameter preferably indicates the temperature of the burner unit, especially the afterburner, either by direct measurement at the afterburner or by measuring the temperature of the exhaust gas downstream of the burner unit. Particularly preferably, the thermal mass is designed to keep the temperature parameter below the limit value for at least a voltage drop that is just barely considered short-term and which, in particular, exhibits precisely the internal voltage limit profile. After a short-term voltage drop, the fuel cell device has a thermal stabilization phase in which the temperature parameter drops to a value corresponding to a stable target operating point of the fuel cell device.If a further brief voltage drop occurs within the thermal equilibrium phase, the temperature parameter may exceed the limit value. Preferably, if the limit value is exceeded, the control unit reduces the supply rate of fresh fuel and oxygen to the at least one fuel cell or shuts off the fluid supply. Preferably, the fluid supply within the thermal equilibrium phase is only shut off after the brief voltage drop has ceased. Particularly with the latter approach, the limit value is preferably lower than the maximum permissible temperature of the burner unit. The design according to the invention advantageously minimizes the risk of overheating of the fuel cell device while simultaneously maintaining its FRT capability.

[0013] It is further proposed that the fluid supply be kept at least substantially constant during the brief voltage drop. The fluid supply preferably includes a subordinate control system for adjusting an operating parameter, in particular speed, torque, flow rate, or the like, to a fluid supply setpoint specified by the control unit. The phrase "keeping the fluid supply constant" preferably means that the control unit sets a constant value for the fluid supply setpoint for the duration of the brief voltage drop. The control unit preferably determines the fluid supply setpoint as a manipulated variable within the framework of controlling at least one electrochemical parameter.Preferably, the control unit uses as a constant value either a value last determined before the voltage drop or an average value of the fluid supply setpoint within a time window up to the voltage drop. Alternatively, the control unit sets the fluid supply setpoint to a predetermined error standard value. Preferably, the control unit suspends, at least by means of the fluid supply, preferably completely during the voltage drop. Alternatively, the control unit continues the control of the electrochemical parameter unchanged even during the brief voltage drop and adjusts the fluid supply setpoint during the brief voltage drop, in particular depending on the electric current through the at least one electrochemical cell.The design according to the invention advantageously minimizes the risk of over-regulation of the electrochemical parameter, particularly due to the relatively rapid changes in the electric current through the at least one fuel cell during the voltage drop. In particular, the risk of a damaging excess of oxygen in the at least one fuel cell during resumption or ramp-up of the electrochemical reaction can be advantageously minimized.

[0014] It is further proposed that, as already explained above, during the brief voltage drop, the fluid supply is adjusted to a value determined by an operating point of the fuel cell device, which was established within a time window prior to the brief voltage drop. The operating point is preferably a result of the control of the electrochemical parameter. In particular, the operating point links the fluid supply setpoint to the electrochemical parameter of the fuel cell device. The design according to the invention advantageously allows the fluid dynamics within the fuel cell device to be kept constant during the brief voltage drop. In particular, parameter fluctuations, especially pressure and / or density fluctuations, within the fuel cell device during the brief voltage drop can be advantageously minimized.

[0015] It is further proposed that fluid supply control is resumed after a brief voltage drop when the electric current supplied by the fuel cell device corresponds at least substantially to a predetermined threshold. Fluid supply control here refers to determining the target fluid supply value within the framework of controlling the electrochemical parameter. The subordinate control of the fluid supply operating parameter, as a function of the fluid supply parameter, is preferably continuously active during fluid supply operation. In the simplest case, control is resumed when the electric current exceeds the threshold. Preferably, control is resumed when the electric current remains within a tolerance band around a target current value, which is limited by the threshold, for a defined period of time.The current setpoint is preferably included in the operating point on which the fluid supply setpoint for the duration of the short-term voltage drop is determined. The threshold value can be specifically defined by a particular configuration of the fuel cell device, in particular depending on the susceptibility to, or tolerance of, the control of the electrochemical parameter to a deviation of the fuel cell device temperature from a predetermined operating temperature. The configuration according to the invention advantageously allows the fluid dynamics within the fuel cell device to be kept constant during the short-term voltage drop. In particular, parameter fluctuations, especially pressure and / or density fluctuations, within the fuel cell device after the short-term voltage drop can be advantageously minimized.

[0016] It is further proposed that an inverter, particularly the one already mentioned, of the fuel cell device determines whether a voltage drop is to be considered momentary. Preferably, the inverter is FRT-capable. Preferably, the inverter sends at least one grid signal, particularly one triggered or continuous, to the control unit, which depends on a voltage of the secondary grid. Preferably, the grid signal encodes at least two, and preferably exactly two, states of the secondary grid. Preferably, the grid signal encodes a normal state of the secondary grid in which the secondary grid operates at least substantially at nominal voltage or in which there is at most a momentary voltage drop of the secondary grid. Preferably, the grid signal encodes a fault state of the secondary grid in which a voltage drop exceeds a momentary voltage drop.Coding can be implemented digitally, as a TTL level, by transmitting / not transmitting the network signal, or similar methods. Optionally, the network signal further distinguishes between a momentary voltage drop and normal operation of the secondary network at nominal voltage. Preferably, the control unit evaluates a fluctuation in the electrical current and / or voltage of the primary network as a momentary voltage drop of the secondary network only if the network signal indicates the normal state or does not indicate a fault state. If the fluctuation in the electrical current and / or voltage of the primary network lies outside a permissible range defined by the control of the electrochemical quantity, without the inverter's network signal indicating the normal state, or if the network signal indicates a fault state, the control unit preferably shuts down the fuel cell device.If the fluctuation of the electrical current and / or voltage of the primary network lies outside a permissible range defined by the control of the electrochemical quantity, the control unit allows the fuel cell device to operate, particularly according to the process steps described above, as long as the network signal indicates the normal state or does not indicate a fault state. The design according to the invention advantageously allows for a simple and reliable distinction between control-compliant fluctuations of the primary network, a short-term voltage drop in the secondary network, and a more serious fault state of the fuel cell device and / or the network.

[0017] Furthermore, a fuel cell device with at least one control unit, in particular one of those already mentioned, is proposed for carrying out a method according to the invention. A "control unit" is understood to be, in particular, a unit with at least one control electronics unit. A "control electronics unit" is understood to be, in particular, a unit with a processor unit and a memory unit, as well as with an operating program stored in the memory unit. The fuel cell device preferably includes the inverter. The fuel cell device preferably includes the at least one fuel cell unit. Optionally, the fuel cell device includes several, in particular structurally identical, fuel cell units.In an embodiment with multiple fuel cell units, the fuel cell device can have at least one central inverter to which several, in particular all, fuel cell units are connected in parallel. Alternatively, at least one, in particular each, of the fuel cell units comprises a specific inverter, which is arranged, in particular together with at least one fuel cell of the fuel cell unit, within a housing of the fuel cell unit. The at least one fuel cell unit preferably comprises at least the fuel delivery unit and / or the oxygen delivery unit as part of the fluid supply of the fuel cell device. The at least one fuel cell unit preferably comprises the burner unit, in particular the afterburner, which is fluidly connected to the at least one fuel cell.The fuel cell unit preferably includes further components for fluid handling and / or heat management, such as in particular exhaust gas heat exchangers, shut-off valves, metering valves, a fuel reformer, a steam injection unit or the like.

[0018] "Intended" should be understood in particular to mean specifically programmed, designed, and / or equipped. The fact that an object is intended for a specific function should be understood in particular to mean that the object fulfills and / or performs this specific function in at least one application and / or operating state.

[0019] The design according to the invention enables the provision of a fuel cell device which can advantageously be operated even without an electrical energy storage device and / or backup load during a short-term voltage drop in the grid. In particular, an advantageously cost-effective, low-component, compact, and FRT-capable fuel cell device can be provided.

[0020] The method and / or the fuel cell device according to the invention are not to be limited to the application and embodiment described above. In particular, the method and / or the fuel cell device according to the invention may, to achieve a functionality described herein, comprise a different number of individual elements, components, units, and process steps than those specified herein. Furthermore, values ​​within the specified limits of the value ranges stated in this disclosure shall also be considered disclosed and freely usable. Drawings

[0021] Further advantages will become apparent from the following description of the drawings. The drawings illustrate two exemplary embodiments of the invention. The drawings, the description, and the claims contain numerous features in combination. Those skilled in the art will also expediently consider the features individually and combine them into useful further combinations.

[0022] They show: Fig. 1 a schematic representation of a fuel cell device according to the invention, Fig. 2 a schematic flowchart of a method according to the invention and Fig. 3 a schematic flowchart of an alternative embodiment of a method according to the invention. Description of the embodiments

[0023] Fig. Figure 1 shows a fuel cell device 12a. The fuel cell device 12a preferably comprises at least one fuel cell unit 22. The fuel cell unit 22a preferably comprises a fuel cell array 24a of fuel cells, which are particularly preferably connected in series for common operation. The fuel cell array 24a is functionally represented here by a single fuel cell with a fuel electrode 26a and an oxygen electrode 28a. The fuel cell array 24a is preferably designed for the electrochemical conversion of a fuel with the supply of oxygen, in particular atmospheric oxygen.The fuel cell unit 22a preferably comprises a fluid supply 14a for supplying the fuel cell assembly 24a with the fuel, for example methane, hydrogen and / or ammonia as a pure substance or as a mixture, in particular natural gas or biogas, and with oxygen, in particular air. The fuel cell device 12a comprises at least one control unit 20a. The control unit 20a is particularly preferably designed to adjust the fluid supply 14a, in particular a flow rate of the fuel and / or the oxygen. The control unit 20a is designed to carry out a method 10a or 10b, which is described in the... Fig. 2 or 3 are explained in more detail below. The fuel cell unit 22a preferably comprises an inverter 18a for feeding electrical power provided by the fuel cell assembly 24a into a power grid.

[0024] The fluid supply 14a preferably comprises at least one fuel delivery unit 30a for delivering the fuel to the fuel electrode 26a. The fuel delivery unit 30a is preferably designed as a blower. The fuel cell unit 22a preferably comprises at least one fuel line 44a, which leads from the fluid supply 14a to the fuel electrode 26a.The fuel cell unit 22a comprises, for example, along the fuel line 44a, a recirculation pumping unit 36a for adjusting a feed-back rate of exhaust gas into the fuel line 44a, a fuel recuperator 38a for transferring heat from the re-fed exhaust gas to the fuel in the fuel line 44a, a fuel preheater 40a for transferring heat from an afterburner exhaust gas leaving the fuel cell unit 22a to the fuel in the fuel line 44a and / or a reformer 42a for reforming the fuel, particularly in this order with respect to a flow direction of the fuel.

[0025] The fluid supply 14a preferably comprises a fuel shut-off valve 32a for coupling or decoupling the fuel delivery unit 30a from the fuel line 44a. Optionally, the fluid supply 14a comprises a further fuel branch 34a for feeding a further fuel into the fuel line 44a, in particular a starter fuel such as pure hydrogen for commissioning the fuel cell unit 22a. The further fuel branch 34a preferably comprises a further fuel shut-off valve 35a for coupling or decoupling a source of the further fuel from the fuel line 44a.

[0026] The fuel cell unit 22a preferably comprises at least one afterburner 16a for the conversion of fuel residues in the exhaust gas exiting the fuel electrode 26a. The afterburner 16a is preferably connected downstream of the fuel cell assembly 24a to the fuel electrode 26a and to the oxygen electrode 28a. An afterburner outlet of the afterburner 16a is preferably connected to the fuel preheater 40a.

[0027] The fuel cell unit 22a preferably comprises a recirculation line 48a for feeding back a portion of the exhaust gas exiting the fuel electrode 26a. The recirculation line 48a is preferably connected in parallel to the afterburner 16a at the fuel electrode 26a. The recirculation line 48a preferably includes the fuel recuperator 38a. The recirculation line 48a preferably includes a recirculation oxygen preheater 50a for transferring heat from the recirculated exhaust gas to the oxygen. The outlet of the recirculation line 48a into the fuel line 44a is preferably arranged upstream of the recirculation pumping unit 36a with respect to the fuel, and particularly downstream of the fuel pumping unit 30a.

[0028] The fluid supply 14a preferably comprises an oxygen delivery unit 52a for delivering oxygen, in particular air, to the oxygen electrode 28a. The oxygen delivery unit 52a is preferably designed as a blower. The fuel cell unit 22a preferably comprises a main oxygen line 54a, which connects the fluid supply 14a to the oxygen electrode 28a. The main oxygen line 54a preferably comprises an exhaust gas oxygen preheater 56a for transferring heat from the afterburner exhaust gas of the afterburner 16a to the oxygen. The fuel cell unit 22a preferably comprises a bypass line 58a, which includes the exhaust gas oxygen preheater 56a. The bypass line 58a includes, for example, an electric heating element for heating the oxygen. The fuel cell unit 22a preferably comprises a preheating line 60a, in which the regenerative oxygen preheater 50a is arranged.The preheating line 60a preferably opens into the main oxygen line 54a upstream of the exhaust gas oxygen preheater 56a. The fluid supply 14a preferably comprises an oxygen control unit 62a for coupling or decoupling the oxygen delivery unit 52a with the main oxygen line 54a, with the bypass line 58a and / or the preheating line 60a. The oxygen control unit 62a is functionally represented here by three individual valves and can be configured as a single valve or as a multi-way valve.

[0029] Fig. Figure 2 shows the method 10a for operating the fuel cell device 12a, which is connected to the power grid. The method 10a preferably comprises a regular operation 64a of the fuel cell device 12a. In the regular operation 64a, the control unit 20a preferably performs a control of an electrochemical parameter, for example, fuel utilization, of the electrochemical conversion of the fuel in the fuel cell assembly, which is known in particular per se. In the regular operation 64a, a power setpoint of the supplied electrical power, in particular of an electrical current associated with the supplied electrical power, is preferably specified. The control unit 20a preferably attempts to maintain the fuel cell device 12a at an operating point dependent on the power setpoint by adjusting a setting of the fluid supply 14a.In particular, the control unit 20a determines at least one fluid supply setpoint, for example, a fuel supply setpoint, a recirculation rate setpoint, and / or an oxygen supply setpoint, as a function of a control deviation defined by the operating point. Preferably, the control unit 20a outputs the at least one fluid supply setpoint to a control unit of the fluid supply component 14a to be adjusted. The component to be adjusted can, in particular, be the fuel delivery unit 30a, the recirculation delivery unit 36a, the oxygen delivery unit 52a, the fuel shut-off valve 32a, the further fuel shut-off valve 35a, and / or the oxygen control unit 62a.

[0030] Method 10a preferably comprises a voltage monitoring system 66a of the power grid. Preferably, the inverter 18a monitors the voltage of the power grid in the voltage monitoring system 66a. Preferably, the inverter 18a outputs a grid signal to the control unit 20a, which encodes a state of the power grid. The grid signal can be configured as an enable signal or as a fault signal, depending on which the control unit 20a continues to operate the fuel cell unit 22a or whether the control unit 20a initiates a shutdown process 70a of the fuel cell unit 22a. Preferably, the grid signal indicates whether the voltage of the power grid, apart from any short-term voltage drops, is within a tolerance band around a nominal voltage of the power grid, or whether a voltage profile of the power grid exceeds a short-term voltage drop, in particular whether it falls below a predetermined voltage limit.Inverter 18a specifies whether a voltage drop is to be considered short-term.

[0031] Method 10a preferably includes a fault evaluation 68a, which is preferably performed when the control unit 20a receives the grid signal as an enable signal or as long as it does not receive the grid signal as a fault signal. In the fault evaluation 68a, the control unit 20a preferably checks whether a fluctuation of the electric current through the fuel cell array 24a is within a permissible fluctuation range of the electrochemical parameter control. If a fluctuation of the electric current through the fuel cell array 24a is within a permissible fluctuation range of the electrochemical parameter control, the control unit 20a preferably continues normal operation 64a.If a fluctuation in the electrical current through the fuel cell array 24a is outside a permissible fluctuation range for the control of the electrochemical parameter, the control unit 20a preferably executes a fault operation 72a of method 10a. The control unit 20a preferably executes the fault operation 72a as long as the control unit 20a receives the grid signal as an enable signal, or as long as it does not receive the grid signal as a fault signal and the deviation of the electrical current from the power setpoint through the fuel cell array 24a is outside the permissible fluctuation range. The fault operation 72a is preferably executed when there is a brief voltage drop in the power grid.

[0032] In fault mode 72a, the control unit 20a preferably suspends the control of the electrochemical parameter. This ensures that, in the event of a short-term voltage drop in the power grid, the electric current through the fuel cell device 12a correlates with the short-term voltage drop. The control unit 20a maintains at least a partial supply of fluid 14a to the fuel cell device 12a during the short-term voltage drop in the power grid. During the short-term voltage drop, the control unit 20a keeps the fluid supply 14a at least substantially constant by specifying at least a constant fluid supply setpoint for the duration of fault mode 72a.During the brief voltage drop, the control unit 20a determines a fluid supply setpoint value that is defined by an operating point of the fuel cell device 12a, which was determined within a time window prior to the brief voltage drop. Specifically, during fault operation 72a, the control unit 20a sets the fluid supply setpoint to the last determined fluid supply setpoint of normal operation 64a. The control unit 20a reduces the fluid supply setpoint in fault operation 72a compared to normal operation 64a, specifically only if at least one temperature parameter of the fuel cell device 12a, particularly of the afterburner 16a, exceeds a limit value.

[0033] In fault mode 72a, the control unit 20a shifts a thermal efficiency of the fuel cell device 12a at the expense of an electrical efficiency of the fuel cell device 12a. In fault mode 72a, a thermal mass of the fuel cell device 12a, in particular of an afterburner 16a of the fuel cell device 12a, is used to at least partially absorb heat produced by the fuel cell device 12a during the brief voltage drop.

[0034] Regular operation 64a is resumed after the brief voltage drop if, in the fault assessment 68a, the electrical current provided by the fuel cell device 12a is at least substantially equal to a predetermined threshold.

[0035] In the Fig. Figure 3 shows a further embodiment of the invention. The following descriptions and drawings are essentially limited to the differences between the embodiments, whereby, with regard to identically designated components, in particular components with the same reference numerals, reference is also generally made to the drawings and / or the description of the other embodiments, in particular the Fig. 1 to 2, reference can be made. To distinguish the embodiments, the letter a is the reference numeral of the embodiment in the Fig. 1 to 2 followed. In the exemplary embodiment of the Fig. 3, the letter a is replaced by the letter b.

[0036] Fig. Figure 3 shows an alternative method 10b for operating a fuel cell device connected to a power grid. In at least one fault operation 72b of method 10b, the supply of electrical current by the fuel cell device is adjusted in the event of a short-term voltage drop in the power grid. A fluid supply to the fuel cell device is at least partially maintained during the short-term voltage drop in the power grid. In the fault operation 72b, the control of an electrochemical parameter of a normal operation 64b of method 10b is maintained, in particular unchanged. Preferably, a control unit of the fuel cell device adjusts at least one fluid supply setpoint during the fault operation 72b as a function of an electrical current through the fuel cell device.

[0037] For further features of procedure 10b, refer to the Fig.2 and their description.

Claims

[1] Method (10a; 10b) for operating a fuel cell device (12a) which is connected to a power grid, wherein in at least one method step a provision of electrical current by the fuel cell device (12a) is adapted in the event of a brief voltage drop in the power grid, characterized by that a fluid supply (14a) of the fuel cell device (12a) is at least partially maintained during the short-term voltage drop of the power grid. [2] Method (10a; 10b) according to claim 1, characterized by that during the short-term voltage drop of the network, a thermal efficiency of the fuel cell device (12a) is shifted at the expense of an electrical efficiency of the fuel cell device (12a). [3] Method (10a; 10b) according to claim 1 or 2, characterized bythat during the brief voltage drop of the network, a thermal mass of the fuel cell device (12a), in particular of an afterburner (16a) of the fuel cell device (12a), is used to at least partially absorb heat produced by the fuel cell device (12a) during the brief voltage drop. [4] Method (10a; 10b) according to claim 3, characterized by that the fluid supply (14a) is reduced, in particular only when at least one temperature parameter of the fuel cell device (12a), in particular of the afterburner (16a), exceeds a limit value. [5] Method (10a) according to one of the preceding claims, characterized by that during the short-term voltage drop the fluid supply (14a) is kept at least substantially constant. [6] Method (10a) according to one of the preceding claims, characterized bythat during the brief voltage drop, the fluid supply (14a) is set to a value which is predetermined by an operating point of the fuel cell device (12a) which was determined in a time window before the brief voltage drop. [7] Method (10a) according to one of the preceding claims, characterized by that a control of the fluid supply (14a) is resumed after the brief voltage drop when an electrical current provided by the fuel cell device (12a) is at least substantially equal to a predetermined threshold value. [8] Method (10a; 10b) according to one of the preceding claims, characterized by that an inverter (18a) of the fuel cell device (12a) specifies whether a voltage drop is to be assessed as short-term. [9] Fuel cell device (12a) with at least one control unit (20a) for carrying out a method (10a; 10b) according to one of the preceding claims.

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