Fuel cell system and operating method for operating the fuel cell system

Abstract

The presented invention relates to an operating method (100) for operating a fuel cell system (200), wherein the operating method (100) comprises: - measuring (101) an oxygen content in the air path of the fuel cell system (200) by at least one oxygen concentration sensor (203, 209) arranged in the air path of the fuel cell system (200), - determining (103) an air volume flow supplied to a fuel cell stack of the fuel cell system (200) based on the measured oxygen content, - Adjusting (105) the fuel cell system (200) depending on the determined air volume flow.

Description

[0001] The presented invention relates to an operating method for operating a fuel cell system, a fuel cell system and a program product according to the appended claims. State of the art

[0002] Known solid oxide fuel cell (SOFC) systems are based on feedforward control of characteristic parameters such as gas utilization, since these cannot be determined online. Disclosure of the invention

[0003] Within the scope of the invention presented, a fuel cell system and an operating method for operating the fuel cell system, as well as a program product, are presented. Further features and details of the invention emerge from the respective subclaims, the description, and the drawings. Features and details described in connection with the operating method according to the invention naturally also apply in connection with the fuel cell system according to the invention or the program product according to the invention, and vice versa, so that with regard to the disclosure of the individual aspects of the invention, reference is always made to each other.

[0004] The invention presented serves in particular to provide a possibility for fuel-efficient and robust operation of a fuel cell system.

[0005] Thus, according to a first aspect of the invention presented, an operating method for operating a fuel cell system is presented.

[0006] The presented operating method comprises measuring an oxygen content in the air path of the fuel cell system by at least one oxygen concentration sensor arranged in the air path of the fuel cell system, determining an air volume flow supplied to a fuel cell stack of the fuel cell system based on the measured oxygen content and adjusting the fuel cell system depending on the determined air volume flow.

[0007] In the context of the present invention, an air path is understood to be an air-conducting duct system of a fuel cell system. In particular, an air path comprises an air supply duct through which fresh air is supplied to a fuel cell stack, a cathode subsystem, and an exhaust duct through which a mixture of air and exhaust gases is removed from the fuel cell stack.

[0008] The presented operating procedure is based on measuring an oxygen content in the air path to determine an air volume flow supplied to the fuel cell stack as a control variable.

[0009] By using an oxygen concentration sensor to determine the air volume flow supplied to the fuel cell stack, a flow sensor for measuring the air volume flow supplied to the fuel cell stack can be omitted. Accordingly, the presented operating method enables the provision of a particularly compact, cost-effective, and robust fuel cell system.

[0010] It may be provided that a lambda probe is selected as the oxygen concentration sensor.

[0011] Lambda sensors, in particular broadband lambda sensors, as known, for example, from the automotive sector, are designed and constructed specifically for use in exhaust gases, which means that direct use of such a lambda sensor in the fuel cell system presented is possible without any adaptation.

[0012] Accordingly, using the presented operating procedure, an expensive and technically complex flow sensor can be replaced by an inexpensive and robust lambda sensor.

[0013] Broadband lambda sensors exhibit a pumping current of 1-5 mA in typical ambient air supplied to a fuel cell system with an oxygen content of 21%, for example, 4 mA (=4000 µA). Broadband lambda sensors also exhibit a resolution of at least 10 µA, for example. This allows the following calculation of the minimum resolution accuracy.

[0014] 4000 µA : 21% oxygen, divided by 400, gives 10 µA : 0.0525% oxygen as the minimum resolution of the oxygen content.

[0015] Assuming an oxygen concentration at the cathode outlet of a fuel cell system of, for example, 16%, particularly preferably less than 21%, this would result in a maximum value with a deviation of 16.0525%, which corresponds to an error of 0.328125% and thus represents a very high accuracy.

[0016] The volume flow of oxygen at the cathode outlet can be described by the following equations: λAn,a=xCO+xH2O+xCO22xCO+xH2O+xH2+2xCO2+4xCH4 n˙O2,Ka,out=n˙O2,Ka,in−IStack,on,out⋅NZelle2⋅F n˙O2,Ka,ein=n˙Luft,System,ein⋅xO2,System,ein n˙O2,Ka,out=n˙air,Ka,out⋅xO2,Ka,out

[0017] By substituting, we finally get equations (5) and (6). n˙Air,Ka,out=n˙Air,System,in⋅(1−xO2,System,in)+n˙O2,Ka,in−IStack,on,out⋅NZell2⋅F n˙O2,Ka,in=n˙Air,System,in⋅xO2,System,in((n˙Air,System,in⋅(1−xO2,System,in)+((n˙Air,System,in+xO2,System,in)−IStack,in,out⋅NZell2⋅F))⋅xO2,Ka,out)=(n˙Air,System,in⋅xO2,System,in)−IStack,in,out⋅NZell2⋅F

[0018] This means that the difference in oxygen concentrations in the cathode path before and after the fuel cell system is a function of the gas volume flow and the mass transfer, i.e., the electrical current through the fuel cell. The two oxygen sensors provide information about the oxygen concentration difference and thus allow the gas volume flow to be determined based on the known fuel cell operating current.

[0019] It can be provided that the at least one oxygen concentration sensor is arranged in the exhaust tract of the fuel cell system and / or at a cathode inlet of the fuel cell stack.

[0020] The oxygen concentration can be measured at the inlet and / or outlet of the fuel cell system.

[0021] By combining several oxygen concentration sensors, e.g. at the inlet and outlet of the fuel cell system, an estimate of the oxygen content of the ambient air present at the inlet of the fuel cell system can be dispensed with in favor of an accurate measurement, so that the determined air volume flow can be determined with particular accuracy.

[0022] Furthermore, it can be provided that the fuel cell system is a solid oxide fuel cell system and is operated with natural gas.

[0023] The presented operating procedure is particularly suitable for solid oxide fuel cell systems, which are controlled in particular by the supplied air volume flow.

[0024] Analogously, the presented operating procedure can also be used in a PEM fuel cell system to determine or monitor the oxygen content in the cathode air. Ambient air containing oxygen and nitrogen is fed to the cathode. Although the transport of hydrogen ions creates water vapor at the cathode and the air content decreases, the oxygen concentration sensor selectively measures only pure oxygen during normal operation, while water or H2O is not converted or measured.

[0025] It should be noted that two water molecules are produced at the cathode of a PEM fuel cell system for every oxygen molecule consumed. This increases the overall volume of the air flow, and the oxygen concentration also decreases due to a dilution effect. However, this can be mathematically compensated for by taking into account the electrical stack current provided by the fuel cell stack. The molar flow of water produced is directly calculated from the stack current, allowing the volume flow of the intake air to be accurately determined.

[0026] Accordingly, it can be provided that, in order to adjust the fuel cell system, the air volume flow supplied to the fuel cell stack is regulated as a function of the measured oxygen content.

[0027] Through closed-loop control, especially in a closed-loop system, the fuel cell system can be adjusted quickly and precisely. The air flow rate can be directly adjusted based on the measured oxygen content, for example, using an allocation scheme or a characteristic map.

[0028] It can further be provided that, in order to adjust the fuel cell system, the air volume flow supplied to the fuel cell stack is adjusted as a function of the determined air volume flow.

[0029] By calculating the air volume flow based on the measured oxygen concentration, the air volume flow can be adjusted particularly precisely.

[0030] It can further be provided that, in order to adjust the fuel cell system, the air volume flow supplied to the fuel cell stack is regulated as a function of a first oxygen concentration sensor arranged in the air supply tract of the fuel cell system and a second oxygen concentration sensor arranged in the exhaust tract of the fuel cell system.

[0031] By using two oxygen concentration sensors, the influence of ambient air quality can be determined and taken into account when determining the air volume flow.

[0032] It may further be provided that a machine learner or a machine learning method is used to determine an error between a real value and a value determined by a sensor of the fuel cell system, wherein the value determined by the sensor is corrected based on the value determined by the machine learning method.

[0033] In order to improve the presented operating procedure with the aim of achieving higher accuracy, the presented operating procedure can be combined with a machine learning method in the sense of a hybrid system.

[0034] The machine learning method or the machine learner can be used in particular to detect an error between the real value h s and a value measured with the sensor concepts shown h s,Sensor and thus improve the latter values.

[0035] To do this, the machine learner only needs to be trained in advance with training data to estimate the error at various operating points. The machine learner can be set up as a function of the measured variables of the oxygen concentration sensor, such as pump current, pump voltage, temperature, and Nernst voltage, as well as other variables (temperatures, pressures, volume flows, etc.) from the fuel cell system in which the oxygen concentration sensor is used. The sensor error then represents the output variable. In addition to multivariate linear regression, the application of a neural network and, in particular, the application of a Gaussian process represent suitable methods for implementing the machine learner.

[0036] According to a second aspect, the presented invention relates to a fuel cell system for converting energy, wherein the fuel cell system comprises a number of fuel cell stacks, at least one oxygen concentration sensor arranged in the air system of the fuel cell system, and a computing unit, wherein the computing unit is configured to execute a possible embodiment of the presented operating method. The fuel cell system for converting energy has the same advantages as the operating method for operating a fuel cell system according to the first aspect of the invention.

[0037] In the context of the invention presented, a computing unit is understood to mean a computer, a processor, a control unit or any other programmable circuit.

[0038] According to a third aspect, the presented invention relates to a program product, wherein the program product comprises program code means which, when the program product is executed on a computing unit, configure the computing unit to execute a possible embodiment of the presented operating method.

[0039] Further advantages, features, and details of the invention will become apparent from the following description, in which exemplary embodiments of the invention are described in detail with reference to the drawings. The features mentioned in the claims and in the description may be essential to the invention individually or in any combination.

[0040] They show: Fig. 1 a schematic representation of a possible design of the presented operating procedure, and Fig. 2 a possible design of the presented fuel cell system.

[0041] In Fig. 1 shows an operating method 100 for operating a fuel cell system.

[0042] The operating method 100 comprises a measuring step 101 in which an oxygen content in the air path of the fuel cell system is measured by at least one oxygen concentration sensor arranged in the air path of the fuel cell system, a determining step 103 in which an air volume flow supplied to a fuel cell stack of the fuel cell system is determined based on the measured oxygen content and an adjusting step 105 in which the fuel cell system is adjusted depending on the determined air volume flow.

[0043] In Fig. 2 shows a fuel cell system 200 for converting energy.

[0044] The fuel cell system 200 comprises a fuel cell stack 201, a first oxygen concentration sensor 203 in the supply air tract 205 and a second oxygen concentration sensor 209 arranged in the exhaust gas tract 207, and a computing unit 211.

[0045] The fuel cell stack 201 is supplied with fresh air via an air supply system 213.

[0046] In the present case, the fuel cell system 200 is shown as an example of a solid oxide fuel cell system and comprises a burner 215 and a reformer 217 arranged in a recirculation path 219.

[0047] The computing unit 211 is configured to execute the operating method 100 according to Fig. 1 and, for example, to set an air volume flow to be supplied to the fuel cell stack 201 based on the determined air volume flow.

Claims

[1] Operating method (100) for operating a fuel cell system (200), wherein the operating method (100) comprises - measuring (101) an oxygen content in the air path of the fuel cell system (200) by at least one oxygen concentration sensor (203, 209) arranged in the air path of the fuel cell system (200), - determining (103) an air volume flow supplied to a fuel cell stack of the fuel cell system (200) based on the measured oxygen content, - Adjusting (105) the fuel cell system (200) depending on the determined air volume flow. [2] Operating method (100) according to claim 1, characterized by that a lambda probe is selected as the oxygen concentration sensor (203, 209). [3] Operating method (100) according to claim 1 or 2, characterized bythat the at least one oxygen concentration sensor (203, 209) is arranged in the exhaust tract (207) of the fuel cell system (200) and / or at a cathode inlet of the fuel cell stack (201). [4] Operating method (100) according to one of the preceding claims, characterized by that the fuel cell system (200) is a solid oxide fuel cell system and is operated with natural gas. [5] Operating method (100) according to one of the preceding claims, characterized by in that, in order to adjust the fuel cell system (200), the air volume flow supplied to the fuel cell stack (201) is regulated as a function of the measured oxygen content. [6] Operating method (100) according to one of claims 1 to 5, characterized by in that, in order to adjust the fuel cell system (200), the air volume flow supplied to the fuel cell stack (201) is regulated as a function of the determined air volume flow. [7] Operating method (100) according to one of claims 1 to 6, characterized in that, in order to adjust the fuel cell system (200), the air volume flow supplied to the fuel cell stack (201) is regulated as a function of a first oxygen concentration sensor (203) arranged in the air supply tract (205) of the fuel cell system (200) and a second oxygen concentration sensor (209) arranged in the exhaust tract (207) of the fuel cell system (200). [8] Operating method (100) according to one of the preceding claims, characterized by that a machine learner is used to determine an error between a real value and a value determined by a sensor of the fuel cell system (200), wherein the value determined by the sensor is corrected based on the value determined by the machine learner. [9] Fuel cell system (200) for converting energy, the fuel cell system (200) comprising: - a number of fuel cell stacks (201), - at least one oxygen concentration sensor (203, 209) arranged in the air system of the fuel cell system (200), - a computing unit (211), wherein the computing unit (211) is configured to execute an operating method (100) according to one of claims 1 to 8. [10] A program product, wherein the program product comprises program code means which, when the program product is executed on a computing unit, configure the computing unit to execute an operating method (100) according to any one of claims 1 to 8.

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