Method for operating a fuel cell unit

Abstract

The invention relates to a method for operating a fuel cell unit (12) of a fuel cell device (10), wherein a cathode inlet airflow of the fuel cell unit (12) is varied to control the temperature of the fuel cell unit (12), wherein in a method step (102) a cathode inlet airflow rate ṅ is controlled as a function of a load requirement of the fuel cell unit (12). It is proposed that in an adjustment method step (104) a cathode inlet airflow rate difference Δṅ is determined as a function of a temporal change of a fuel cell unit temperature parameter dTdter, with which the cathode inlet airflow rate ṅ is corrected.

Description

Prior ArtA method for operating a fuel cell unit has already been proposed.Disclosure of the InventionThe invention is based on a method for operating a fuel cell unit of a fuel cell device, wherein a cathode input air flow of the fuel cell unit is changed in order to regulate a temperature of the fuel cell unit, wherein in a method step a cathode input air flow rate ṅ is regulated as a function of a load requirement of the fuel cell unit.It is proposed that in an adaptation method step, a cathode input air flow rate difference Δegr is determined as a function of a change over time in a fuel cell unit temperature parameter, with which the cathode input air flow rate egr is corrected.A "fuel cell device" is to be understood to mean, in particular, at least a part, in particular a subassembly, of a fuel cell system, in particular of a solid oxide fuel cell system. In particular, the fuel cell device can also comprise an entire fuel cell unit, in particular the entire fuel cell system. The fuel cell device is preferably designed at least as part of a high-temperature fuel cell unit, in particular a high-temperature solid oxide fuel cell, SOFC for short, in particular as a high-temperature solid oxide fuel cell system. The at least one fuel cell device preferably comprises at least one fuel cell unit which is designed to react a fuel gas and oxygen to generate electrical energy. The at least one fuel cell device can have a plurality of fuel cell units, such as two, three, four or the like. A "fuel cell unit" is to be understood as meaning a fuel cell stack, in particular an SOFC fuel cell stack. Preferably, each fuel cell unit comprises at least one fuel cell, which is provided in particular for electrochemical conversion of fuel, in particular fuel gas, for generating electrical energy. Preferably, all fuel cells are of identical shape, in particular of identical dimensions. Preferably, each fuel cell comprises at least one anode. Preferably, each fuel cell comprises at least one cathode. Preferably, each fuel cell comprises at least one electrolyte which is arranged between the at least one anode and the at least one cathode. Preferably, the at least one electrolyte is connected to the at least one anode and the at least one cathode. The fuel cell device preferably comprises a computing unit which is provided for controlling and / or regulating components of the fuel cell device. The fuel cell device preferably comprises an air flow unit which is provided for supplying air to the at least one fuel cell stack and for supplying fuel, in particular fuel gas, to the at least one fuel cell stack. "Provided" is to be understood as meaning preferably specially configured, specially designed, specially designed and / or specially equipped. The fact that an object is provided for a specific function is preferably intended to mean that the object fulfills and / or carries out this specific function in at least one application state and / or operating state.A "cathode input air stream" is preferably to be understood as an air stream which is supplied to the cathodes of the fuel cells of the fuel cell unit for operation for generating an electrical energy. A "cathode input air flow rate ṅ" is to be understood as meaning the flow rate of air which is supplied to the cathode of the fuel cell unit.The cathode input airflow rate ṅ may be expressed, for example, in liters per minute. A "load requirement of the fuel cell unit" should preferably be understood to mean a load, in particular an electrical power, which is to be generated by the fuel cell unit. Preferably, in at least one method step, the currently generated electrical power of the at least one fuel cell unit is determined, in particular measured, preferably calculated. Preferably, in at least one method step, the currently generated electrical power of the at least one fuel cell unit is determined, in particular calculated, from the number of fuel cells of the at least one fuel cell unit, the voltage of the individual fuel cells of the at least one fuel cell unit and the current intensity of the individual fuel cells of the at least one fuel cell unit.A "cathode input air flow rate difference Δegr" is preferably to be understood as a correction value for the cathode input air flow rate egr. The cathode input airflow rate difference Δegr forms a correction value that takes into account a temporal change in the fuel cell unit temperature parameter, that is, a temporal change in the temperature of the fuel cell unit. The cathode input air flow rate difference Δegr as a correction value allows the cathode input air flow rate egr to be adjusted independently of a change in the power demand, merely on the basis of a change in the temperature of the fuel cell unit, in particular of the fuel cell unit temperature parameter. A "fuel cell unit temperature parameter" is preferably to be understood as a temperature parameter by means of which a temperature to the interior of the fuel cell unit can be determined at least approximately. The fuel cell unit temperature parameter is preferably designed as a temperature which at least approximately corresponds to the temperature in the interior of the fuel cell unit, preferably corresponds exactly to the temperature in the interior of the fuel cell unit. Preferably, the fuel cell unit temperature parameter is formed as a temperature of a fluid flowing out of the fuel cell unit. Preferably, the fuel cell unit temperature parameter is formed as a temperature of the fluid flowing out of the cathode. The fuel cell unit temperature parameter is configured as a cathode outlet temperature. The fuel cell unit temperature parameter designed as a cathode outlet temperature would preferably be detected via a temperature sensor arranged in a cathode outlet. In principle, it would also be conceivable for the fuel cell unit temperature parameter to be designed as a temperature of a fluid flowing out of the anode. The fuel cell unit temperature parameter would be designed as an anode outlet temperature. The fuel cell unit temperature parameter designed as anode outlet temperature would preferably be detected via a temperature sensor arranged in an anode outlet. The cathode output temperature or the anode output temperature approximately corresponds to the temperature prevailing in the fuel cell unit itself. The cathode output temperature or the anode output temperature can therefore be used as an approximation of the temperature in the fuel cell unit. In principle, it would also be conceivable for a temperature within the fuel cell unit to be determined directly. In principle, it would be conceivable for this purpose for a temperature sensor to be provided, which is arranged in the fuel cell unit, and for a fuel cell unit temperature parameter, which directly reflects the temperature in the fuel cell unit.The embodiment of the method according to the invention advantageously reduces, preferably prevents, a risk of overheating or supercooling of the at least one fuel cell unit, in particular during transient power phases, in particular compared to classical closed-loop control models. A required feed rate for air to the at least one fuel cell unit, i.e. a cathode input air flow, can advantageously be set exactly such that a temperature of the fuel cell unit can be set particularly well, in particular can be kept at a predefined temperature. In particular, the temperature of the fuel cell unit can be maintained in particular independently of a state of the fuel cell unit, such as, for example, a current stack degradation, and other external influences, such as, for example, ambient temperature, or a composition of the fuel. In particular, an accurate setting of the temperature of the fuel cell unit, which is advantageously matched to a dynamic of the fuel cell device, can be achieved.It is further proposed that in the adaptation method step, a change in a fuel cell unit temperature parameter is compensated for by the correction of the cathode input air flow rate ṅ by the cathode input air flow rate difference Δṅ. As a result, the temperature of the fuel cell unit can be kept particularly advantageously in a range of the predefined setpoint temperature.It is further proposed that the cathode input airflow rate difference Δegr be determined from a product of the change in the fuel cell unit temperature parameter and a dynamic factor K.A "change in the fuel cell unit temperature parameter" is to be understood as a change over time in the fuel cell unit temperature parameter, i.e. the time derivative of the fuel cell unit temperature parameter. As a result, the cathode input air flow throughput difference Δegrcan be determined particularly easily.It is furthermore proposed that the dynamic factor K depends on a difference between a molar enthalpy of the cathode outlet air stream h air,out and a molar enthalpy of the cathode inlet air stream h air,in. A "molar enthalpy of the cathode outlet air stream h air,out" is preferably to be understood as a molar enthalpy of the fluid, in particular of the air, which flows into the cathode of the fuel cell unit. A "molar enthalpy of the cathode inlet air stream h air,in" is preferably to be understood as a molar enthalpy of the fluid, in particular of the exhaust air, which flows out of the cathode of the fuel cell unit during operation. As a result, the dynamic factor K can be formed particularly easily.Furthermore, it is proposed that the dynamic factor K depends on an application parameter α Can Dyn. which reflects an accuracy of a mathematical model of the fuel cell unit. An "application parameter α Can Dyn." is preferably to be understood as a parameter which reflects deviations of the mathematical model of the fuel cell unit from the real fuel cell unit and which is preferably provided to adapt a calculation of the dynamic factor to the real fuel cell unit. An "accuracy of a mathematical model" is preferably to be understood as a deviation of the mathematical model of the fuel cell unit, which is based on the calculation of the dynamic factor K, from the actual properties of the fuel cell unit. If mathematical model 1 to 1 were to correspond to the actual fuel cell unit, application parameter α Can Dyn. would have a value of 1. The application parameter α Can Dyn. preferably has a value of 0.1-2, particularly preferably of 0.5-1.5. The application parameter α Can Dyn. is preferably determined once before the fuel cell device is put into operation, for example in a test operation. In principle, it may also be conceivable for the application parameter α Can Dyn. to be adapted over the course of a service life of the fuel cell device in order to adapt the application parameter α Can Dyn. to the properties of the fuel cell device that change as a result of operation.As a result, the dynamic factor K can be determined particularly accurately, in particular as a function of the fuel cell unit.It is also proposed that the dynamic factor K is determined on the basis of the formula. The term "C stk" is preferably intended to mean the heat capacity of the fuel cell unit. As a result, the dynamic factor K can be determined particularly well.It is also proposed that the adaptation method step is carried out only in a limit operating case in which overheating or supercooling of the fuel cell unit is likely. A "limit operating case" is preferably to be understood as an operating case in which supercooling or overheating of the fuel cell unit is likely to occur. A limit operating case represents an operating state of the fuel cell unit in which at least one operating parameter of the fuel cell unit has already exceeded or fallen below defined limit values or will exceed or fall below them in the foreseeable time. A corresponding operating parameter of the fuel cell unit, which is undershot or exceeded in a limit operating case, is in particular the temperature of the fuel cell unit, that is to say of the fuel cell temperature parameters. As a result, an adaptation of the cathode input air flow throughput on the basis of the change over time of the fuel cell unit temperature parameter can advantageously be carried out only in the operating states in which it is necessary for safe operation of the fuel cell unit. In other operating cases in which a control based on the load requirement of the fuel cell unit is sufficient, a control based on the change over time of the fuel cell unit temperature parameter can be dispensed with, since this could lead to instability or to an excessive energy requirement of an air supply unit in certain operating cases.It is further proposed that operating characteristics of the fuel cell unit are evaluated in an evaluation method step in order to identify a limit operating case, wherein the adaptation method step is carried out only in the case of a detected limit operating case. As a result, a particularly simple method can be provided in order to identify a limit operating case in a particularly simple manner.In addition, it is proposed that in the evaluation method step, a limit operating case is detected if a change in a load requirement of the fuel cell unit exceeds an upper limit value and a temporal change in the fuel cell unit temperature parameter is greater than an upper limit value or a change in a load requirement of the fuel cell unit falls below a lower, negative limit value and a temporal change in the fuel cell unit temperature parameter is less than a lower, negative limit value. A "negative limit value" should preferably be understood to mean a limit value which has a negative sign. As a result, a possible overheating or supercooling of the fuel cell unit can be detected in a particularly advantageous manner and this can be prevented by changing the correction of the cathode input air flow rate ṅ.It is further proposed that in the evaluation method step, a limit operation case is detected when a difference of a measured fuel cell unit temperature parameter from a target value of the fuel cell unit temperature parameter is greater than an upper limit value and a time change of the fuel cell unit temperature parameter is greater than an upper limit value or when a difference of a measured fuel cell unit temperature parameter from a target value of the fuel cell unit temperature parameter is less than a lower, negative limit value and a time change of the fuel cell unit temperature parameter is less than a lower, negative limit value. As a result, overheating or supercooling of the fuel cell unit can be detected particularly advantageously in a simple and rapid manner and this can be counteracted particularly rapidly by changing the correction of the cathode input air flow rate ṅ.It is also proposed that in a normal operating state in which no limit operating case is detected, the cathode input air flow rate difference Δegr is set to the value "0". As a result, an adaptation of the cathode input air flow rate ṅ on the basis of the change over time of a fuel cell unit temperature parameter can be prevented in a particularly simple manner.It is furthermore proposed that the fuel cell unit temperature parameter is designed as a measured cathode output temperature or a measured anode output temperature. As a result, an actual temperature in the fuel cell unit can be approximated particularly easily by means of the fuel cell unit temperature parameter.In addition, it is proposed that the fuel cell unit temperature parameter T, which is designed as a measured cathode output temperature T air out or anode output temperature T An out is filtered using a low-pass filter.The low-pass filter is preferably designed to be parameterizable. A low-pass filter can be configured as a PT1 filter, for example. The low-pass filter can be used to evaluate the fuel cell unit temperature parameter T in a particularly advantageous manner.The fuel cell unit according to the invention is not intended to be limited to the application and embodiment described above. In particular, the method according to the invention for operating a fuel cell unit in order to fulfil a mode of operation described herein can have a number which differs from a number of individual elements, components and units and method steps mentioned herein. In addition, in the value ranges specified in this disclosure, values lying within the stated limits should also be considered as disclosed and usable as desired.DRAWINGFurther advantages are evident from the following description of the drawings. The drawing shows an embodiment of the invention. The drawings, specification and claims contain numerous features in combination. The skilled person will expediently also consider the features individually and summarize them to form meaningful further combinations.The following are shown: FIG. 1 shows a highly schematic illustration of a fuel cell device according to the invention having a fuel cell unit for carrying out a method according to the invention, and FIG. 2 shows the method according to the invention in a schematic illustration.DESCRIPTION OF THE EMBODIMENTFIG. 1 shows a schematic illustration of a fuel cell device 10. The fuel cell device 10 is a sub-assembly of a fuel cell system. The fuel cell device 10 comprises, for example, a fuel cell unit 12. The fuel cell unit 12 is designed as an SOFC fuel cell stack comprising preferably a plurality of SOFC fuel cells 14. In principle, it would also be conceivable for the fuel cell device 10 to have a plurality of fuel cell units 12. The fuel cell unit 12 is designed to allow a fuel gas and oxygen to react to generate electrical energy, in particular electrical power. The fuel cell unit 12 comprises, for example, only one fuel cell 14 here. the fuel cell 14 is provided for electrochemical conversion of fuel, in particular a fuel gas, for generating electrical power. The fuel cell 14 shown as an example includes an anode 16. the fuel cell 14 includes a cathode 18. the fuel cell 14 includes an electrolyte 20. the electrolyte 20 is disposed between the anode 16 and the cathode 18. The electrolyte 20 is connected to the anode 16 and the cathode 18. The fuel cell unit 12 has an anode inlet 22. The anode inlet 22 is connected to the anodes of the fuel cells 14 of the fuel cell unit 12. During operation, an anode gas, in particular the fuel gas, flows via the anode inlet 22 into the fuel cells 14 of the fuel cell unit 12. The cathode inlet 24 is connected to the cathodes 18 of the fuel cells 14 of the fuel cell unit 12. During operation, a cathode gas, in particular the air, flows into the fuel cells 14 of the fuel cell unit 12 via the cathode inlet 24; the fuel cell device 10 comprises an air supply unit 26; the air supply unit 26 is designed to supply air to the at least one fuel cell unit 12. The air supply unit 26 is provided to supply a cathode fluid, i.e. the air, to the cathode inlet 24. The air supply unit 26 supplies the cathodes 18 of the fuel cells 14 of the fuel cell unit 12 with air via the cathode inlet 24. The fuel cell device 10 comprises a fuel supply unit 28. The fuel supply unit 28 is provided to supply an anode fluid, i.e. the fuel gas, to the anode inlet 22. The fuel supply unit 28 supplies the anodes 16 of the fuel cells 14 of the fuel cell unit 12 with fuel gas via the anode inlet 22.The fuel cell unit 12 has an anode outlet 30. During operation, an anode off-gas flowing out of the anode 16 of the fuel cells 14 of the fuel cell unit 12 is discharged via the anode outlet 30. The fuel cell unit 12 has a cathode outlet 32. During operation, a cathode off-gas flowing out of the cathode 18 of the fuel cells 14 of the fuel cell unit 12 is discharged via the cathode outlet 32.The fuel cell device 10 includes a cathode input temperature sensor 34. The cathode input temperature sensor 34 is disposed in the cathode input 24 of the fuel cell unit 12. The cathode inlet temperature sensor 34 is provided to detect a temperature of the cathode gas flowing in the cathode inlet, i.e., the air. The fuel cell device 10 includes a cathode output temperature sensor 36. The cathode output temperature sensor 36 is disposed in the cathode output 32 of the fuel cell unit 12. The cathode output temperature sensor 36 is provided to detect a temperature of the cathode off-gas flowing in the cathode output 32.The fuel cell device 10 comprises a computing unit 38. The computing unit 38 is provided for controlling and / or regulating components of the fuel cell device 10. The arithmetic unit 38 is provided for carrying out a method 100 to be described below. The arithmetic unit 38 is connected to the individual components of the fuel cell device 10, such as the air supply unit 26, the fuel supply unit 28, the cathode input temperature sensor 34, and the cathode output temperature sensor 36. The arithmetic unit 38 is provided for controlling and / or regulating the components of the fuel cell device 10. The computing unit 38 comprises in particular a processor and / or a processor unit, a memory unit and an operating, control and / or calculation program stored in the memory unit.The method 100 according to the invention for operating the fuel cell unit 12 of the fuel cell device 10 is described below. The method 100 is preferably designed as a continuous method in which the method steps of the method 100 proceed repeatedly and / or in parallel. The method 100 is carried out by the arithmetic unit 38 of the fuel cell device. The method 100 is designed to regulate or control a fuel cell unit of a fuel cell device 10 such that a temperature of the fuel cell unit 12 can be kept as constant as possible. The method 100 is provided to let the temperature of the fuel cell unit deviate as little as possible from a predefined optimum temperature. The predefined optimum temperature is preferably designed as a temperature at which the fuel cell unit 12 operates most advantageously, preferably has its best efficiency. The method 100 is preferably provided in particular for controlling or regulating the fuel cell unit 12 in such a way that overheating or supercooling of the fuel cell unit 12 is prevented or this is counteracted as quickly as possible. Overheating of the fuel cell unit 12 occurs when the temperature of the fuel cell unit 12 exceeds an upper temperature limit value. Supercooling of the fuel cell unit 12 occurs when the temperature of the fuel cell unit 12 falls below a lower temperature limit value.In the method 100 for operating the fuel cell unit 12 of the fuel cell device 10, a cathode input air flow of the fuel cell unit 12 is changed in order to regulate the temperature of the fuel cell unit 12. In order to control the temperature of the fuel cell unit 12, a volume of air which is supplied to the cathodes 18 of the fuel cells 14 of the fuel cell unit 12 via the cathode inlet 24 is thus controlled. The air flowing through the fuel cell unit 12 cools the fuel cell unit 12 in that the air absorbs heat and conducts it out of the fuel cell unit 12 via the cathode outlet 32. During operation of the fuel cell unit 12, more air is conducted through the fuel cell unit 12 than would be necessary for generating current by reaction with the fuel gas. A large portion of the air conducted through the cathode inlet 24 into the fuel cell unit 12 serves for cooling the fuel cell unit 12, and it is thereby possible to vary how much the fuel cell unit 12 is cooled by the air by changing the cathode inlet air flow. By increasing the cathode input air flow, cooling of the fuel cell unit 12 can be increased. By reducing the cathode input air flow, cooling of the fuel cell unit 12 can be reduced.In a method step 102, a cathode input air flow rate ṅ, is determined. In method step 102, the cathode input air flow rate ṅ is determined, which is provided for cooling the fuel cell unit 12 to the predefined optimum temperature. The cathode input air flow rate ṅ is determined in method step 102 as a function of a load requirement of the fuel cell unit 12. The load requirement of the fuel cell unit 12 is the electrical power which is to be or is to be generated by the fuel cell unit 12. In principle, it is also conceivable for the load requirement of the fuel cell unit 12 to be formed or generated by a current intensity which is to be generated or is to be generated by the fuel cell unit 12. The load demand is preferably ascertained by the arithmetic unit 38 and the fuel cell device 10 is controlled or regulated accordingly. The load request can also be transmitted, for example, from the outside, from a further processing unit to the processing unit 38 of the fuel cell device 10. In method step 102, computing unit 38 ascertains cathode input air flow rate ṅ by means of a stationary feedforward control model on the basis of the load demand of fuel cell unit 12. The cathode input air flow throughput ṅ determined in method step 102 is preferably dependent only on the load requirement of the fuel cell unit. In method step 102, the cathode input air flow rate ṅ is regulated as a function of the load requirement of the fuel cell unit 12.The method 100 comprises an adaptation method step 104. In the adaptation method step 104, a cathode input air flow rate difference Δegris determined as a function of a change over time in a fuel cell unit temperature parameter. In the adaptation method step 104, the cathode input air flow rate ṅ determined in the method step 102 is corrected with the cathode input air flow rate difference Δṅ. The cathode input airflow rate difference Δegr forms a correction value with which the cathode input airflow rate egr is corrected in order to compensate for a change in the fuel cell unit temperature parameter, i.e., in particular a change in the temperature of the fuel cell unit 12. By correcting the cathode input air flow rate ṅ with the cathode input air flow rate difference Δṅ, cooling of the fuel cell unit 12 can be adapted such that the fuel cell unit temperature parameter T, in particular thus the temperature of the fuel cell unit 12, is controlled to the predefined optimum temperature. By adapting the cathode input air flow rate ṅ with the cathode input air flow rate difference Δṅ, a regulation of the cathode input air flow of the fuel cell unit 12 can react particularly quickly to changes in the fuel cell unit temperature parameter, i.e. in particular to a change in the temperature of the fuel cell unit 12.By calculating the cathode input air flow rate difference Δegr and correcting the cathode input air flow rate egr by the cathode input air flow rate difference Δegr, dynamics of the fuel cell unit temperature parameter dT, that is, dynamics of the temperature of the fuel cell unit 12 are canceled. By adapting the cathode input air flow rate ṅ by the cathode input air flow rate difference Δṅ, a change in the fuel cell unit temperature parameter, i.e. a change in the temperature of the fuel cell unit 12, is set equal to zero, i.e. the cathode input air flow rate difference Δṅ for correcting the cathode input air flow rate ṅ is determined in the adaptation method step 104 such that the change in the fuel cell unit temperature parameter results in zero.The cathode input airflow rate difference Δegris determined in the adjustment method step 104 from a product of the change in the fuel cell unit temperature parameter and a dynamic factor K. The cathode input airflow rate difference Δegr is thus determined by the formula. The dynamic factor K is dependent on a mathematical model of the fuel cell unit. The cathode input air flow rate difference Δegr for the different determined changes of the fuel cell unit temperature parameter can be determined by means of the dynamic factor K in order to compensate for the change of the fuel cell unit temperature parameter. The dynamic factor K depends on a difference of a molar enthalpy of the cathode outlet air stream h air,out and a molar enthalpy of the cathode inlet air stream h air,in. In addition, the dynamic factor is dependent on the heat capacity C Stk of the fuel cell unit 12. The dynamic factor K is dependent on an application parameter α can Dyn ·which represents an accuracy of a mathematical model of the fuel cell unit. The dynamic factor K is determined from the formula. The arithmetic unit 38 determines the cathode input air flow rate difference Δegr, i.e., preferably by using the formulaThe fuel cell unit temperature parameter T is designed as a measured cathode output temperature T Kat out. The cathode output temperature T air out is measured by the cathode output temperature sensor and evaluated by the arithmetic unit 38. The cathode output temperature T air out approximately corresponds to the actual temperature in the fuel cell unit 12. In principle, it would also be conceivable for the fuel cell unit temperature parameter T to be designed as a measured anode output temperature T An out which is measured in the anode output by an anode output temperature sensor, not shown in more detail. The anode output temperature T An out likewise approximately corresponds to the actual temperature in the fuel cell unit 12.The adaptation method step 104 is preferably carried out only in a limit operating case. The adaptation method step can become unstable in certain operating cases due to its dependence on the change over time of the fuel cell unit temperature parameter T or can lead to high fluctuations and / or high load requirements of the air supply unit 26. In a normal operating state, a regulation of the cathode input air flow of the fuel cell unit 12 by the cathode input air flow throughput ṅ determined in the method step 102 is sufficient to keep the temperature of the fuel cell unit 12 sufficiently accurately at the predefined optimum temperature. Therefore, in a normal operating state, the adaptation method step 104 can be dispensed with, or the cathode input air flow rate ṅ can be used without correction for regulating the air supply unit 26.The adaptation method step 104 is carried out only in a limit operating case in which overheating or supercooling of the fuel cell unit is likely. For evaluating whether a boundary operating case exists, the method 100 comprises an evaluation method step 106. In the evaluation method step 106, operating characteristics of the fuel cell unit 12 are evaluated in order to identify a limit operating case. Only in the case of a detected limit operating case is the adaptation method step then carried out.In the evaluation method step 106, at least one load requirement of the fuel cell unit I el and the fuel cell unit temperature parameter T are evaluated. As in the adaptation method step 104, the cathode output temperature T air out which is detected by the cathode output temperature sensor 36 is used as the fuel cell unit temperature parameter T. In the evaluation method step, a change in a load requirement of the fuel cell unit 12 is preferably evaluated. In the evaluation method step 106, the change of a load demand of the fuel cell unit 12 is matched with a lower, negative limit value for the change of the load demand and an upper limit value for the load demand. In the evaluation method step 106, the fuel cell unit temperature parameter T is matched with an upper temperature limit value and a lower temperature limit value for the temperature of the fuel cell unit 12.If the change in a load demand of the fuel cell unit 12 is above the upper limit value for the load demand, a possible limit operating case is therefore detected in the evaluation method step 106, in which the fuel cell unit 12 is imminent to overheat. In the evaluation method step 106, it is also checked whether the previous recognition is plausible. For this purpose, it is checked whether the change in the fuel cell unit temperature parameter is greater than a positive limit value for the change in the fuel cell unit temperature parameter, i.e. whether the temperature of the fuel cell unit 12 rises. If the load demand of the fuel cell unit 12 is above the upper limit value for the load demand and the change in the fuel cell unit temperature parameter is greater than a positive limit value for the change in the fuel cell unit temperature parameter, i.e., the fuel cell unit temperature parameter T rises, a limit operating case is detected in which overheating of the fuel cell unit 12 is imminent.If the change in a load demand of the fuel cell unit 12 is below the lower, negative limit value for the load demand, a possible limit operating case is therefore detected in the evaluation method step 106, in which supercooling of the fuel cell unit 12 is imminent. In the evaluation method step 106, it is also checked whether the previous recognition is plausible. For this purpose, it is checked whether the change in the fuel cell unit temperature parameter is less than a negative limit value for the change in the fuel cell unit temperature parameter, i.e. whether the temperature of the fuel cell unit 12 falls. If the change in the load demand of the fuel cell unit 12 is below the lower, negative limit value for the load demand and the change in the fuel cell unit temperature parameter is less than a negative limit value for the change in the fuel cell unit temperature parameter, i.e. the fuel cell unit temperature parameter T falls, a limit operating case is detected in which undercooling of the fuel cell unit 12 is imminent.If a difference of the fuel cell unit temperature parameter T, i.e. the cathode output temperature T air out and a setpoint value of the fuel cell unit temperature parameter, is above the upper limit value for the temperature difference, a limit operating case is detected in the evaluation method step 106, in which there is overheating of the fuel cell unit 12. The arithmetic unit 38 in this case quasi recognizes that the measured fuel cell unit temperature parameter T is far above an actual setpoint temperature. In the evaluation method step 106, it is also checked whether the previous recognition is plausible. For this purpose, it is checked whether the change in the fuel cell unit temperature parameter is greater than a positive limit value for the change in the fuel cell unit temperature parameter, i.e. whether the temperature of the fuel cell unit 12 rises. If the difference of the fuel cell unit temperature parameter T, i.e. the cathode output temperature T air out and a setpoint value of the fuel cell unit temperature parameter is above the upper limit value for the temperature difference and the change of the fuel cell unit temperature parameter is greater than a positive limit value for the change of the fuel cell unit temperature parameter, i.e. the fuel cell unit temperature parameter T rises, a limit operating case is detected in which the fuel cell unit 12 is just overheating.If a difference of the fuel cell unit temperature parameter T, i.e. the cathode output temperature T air out and a setpoint value of the fuel cell unit temperature parameter, is below the lower limit value for the temperature difference, a limit operating case is detected in the evaluation method step 106, in which supercooling of the fuel cell unit 12 is likely to occur. The arithmetic unit 38 in this case quasi recognizes that the measured fuel cell unit temperature parameter T is far below an actual setpoint temperature. In the evaluation method step 106, it is also checked whether the previous recognition is plausible. For this purpose, it is checked whether the change in the fuel cell unit temperature parameter is less than a negative limit value for the change in the fuel cell unit temperature parameter, i.e. whether the temperature of the fuel cell unit 12 falls. If the difference of the fuel cell unit temperature parameter T, i.e. the cathode output temperature T air out and a setpoint value of the fuel cell unit temperature parameter is below the lower limit value for the temperature difference and the change of the fuel cell unit temperature parameter is smaller than a negative limit value for the change of the fuel cell unit temperature parameter, i.e. the fuel cell unit temperature parameter T falls, a limit operating case is detected in which the fuel cell unit 12 is just undercooled.In the evaluation process step 106, the checks of the change of a load requirement of the fuel cell unit 12, the check of the change of the fuel cell unit temperature parameter, and the check of the fuel cell unit temperature parameter T are each provided with a turn-off delay. The exceeding of the change of a load requirement of the fuel cell unit 12, the change of the fuel cell unit temperature parameter and the fuel cell unit temperature parameter T above or below the respective limit value is only recognized as correct after a predefined time interval. Only if the respective exceeding or falling below the respective limit value remains over the entire time interval is a limit operating case detected in the evaluation method step 106. This advantageously makes it possible to exclude that a brief exceeding of a limit value leads to a detection of a limit operating case.If a limit operating case is detected in the evaluation method step 106, the adaptation method step 104 is carried out and the cathode input air flow rate ṅ determined in the method step 102 is corrected with the cathode input air flow rate difference Δṅ. The arithmetic unit controls the air supply unit 26 with a control signal adapted to the cathode input air flow rate ṅ corrected with the cathode input air flow rate difference Δmn. Preferably, the cathode input airflow rate difference Δegris filtered by a parameterizable low pass filter in the matching method step 104. Preferably, in the method 100, the fuel cell unit temperature parameter T, i.e. the cathode output temperature T air out are also filtered by a parameterizable low-pass filter, for example a PT1 filterIn a normal operation state in which no limit operation case is detected, the cathode input airflow rate difference Δegr is set to "0". Thereby, the correction of the cathode input airflow rate ṅ by the cathode input airflow rate difference Δṅ can be easily turned off. The cathode input air flow rate ṅ is thereby regulated in the normal operating state merely as a function of the load requirement of the fuel cell unit 12.

Claims

Method for operating a fuel cell unit (12) of a fuel cell device (10), wherein a cathode input air flow of the fuel cell unit (12) is changed in order to regulate a temperature of the fuel cell unit (12), wherein in a method step (102) a cathode input air flow rate ṅ is regulated as a function of a load requirement of the fuel cell unit (12), characterized in that in an adaptation method step (104) a cathode input air flow rate difference Δṅ is determined as a function of a temporal change of a fuel cell unit temperature parameter d T d t with which the cathode input air flow rate ṅ is corrected.Method according to Claim 1, characterized in that, in the adaptation method step (104), a change in a fuel cell unit temperature parameter d T d t is compensated for by the correction of the cathode input air flow throughput ṅ by the cathode input air flow throughput difference Δṅ.Method according to Claim 1 or 2, characterized in that the cathode input air flow rate difference Δegr is determined from a product of the change in the fuel cell unit temperature parameter d T d t and a dynamic factor K.Method according to one of the preceding claims, characterized in that the dynamic factor K depends on a difference between a molar enthalpy of the cathode outlet air stream h air,out and a molar enthalpy of the cathode inlet air stream h air,in.Method according to one of the preceding claims, characterized in that the dynamic factor K depends on an application parameter α Can Dyn. which reflects an accuracy of a mathematical model of the fuel cell unit (12).Method according to one of the preceding claims, characterized in that the dynamic factor K is determined on the basis of the formula K = α C a n D y n ≥ C S t k h a i r, o u t - h a i r i n .Method according to one of the preceding claims, characterized in that the adaptation method step (104) is carried out only in a limit operating case in which overheating or supercooling of the fuel cell unit (12) is likely.Method according to one of the preceding claims, characterized in that in an evaluation method step, operating characteristics of the fuel cell unit (12) are evaluated in order to identify a limit operating case, wherein the adaptation method step (104) is carried out only in the case of a detected limit operating case.Method according to Claim 8, characterized in that, in the evaluation method step, a limit operating case is detected if a change in a load requirement of the fuel cell unit exceeds an upper limit value and a temporal change in the fuel cell unit temperature parameter is greater than an upper limit value or a change in a load requirement of the fuel cell unit falls below a lower, negative limit value and a temporal change in the fuel cell unit temperature parameter is less than a lower, negative limit value.Method according to Claim 8, characterized in that, in the evaluation method step, a limit operating case is detected if a difference of a measured fuel cell unit temperature parameter from a setpoint value of the fuel cell unit temperature parameter is greater than an upper limit value and a change over time of the fuel cell unit temperature parameter is greater than an upper limit value or if a difference of a measured fuel cell unit temperature parameter from a setpoint value of the fuel cell unit temperature parameter is less than a lower, negative limit value and a change over time of the fuel cell unit temperature parameter is less than a lower, negative limit value.Method according to one of the preceding claims, characterized in that, in a normal operating state in which no limit operating case is detected, the cathode input air flow throughput difference Δegr is set to the value "0".Method according to one of the preceding claims, characterized in that the fuel cell unit temperature parameter T is designed as a measured cathode output temperature T air out or a measured anode output temperature T An out.Method according to one of the preceding claims, characterized in that the fuel cell unit temperature parameter T, which is designed as a measured cathode output temperature T air out or anode output temperature T An out is filtered using a low-pass filter.

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