Method for operating a fuel cell system

Abstract

Method for operating a fuel cell system (1), wherein the fuel cell system (1) comprises a fuel cell stack (101), an air line (10), an exhaust line (12), a fuel line (20), and a recirculation line, wherein the exhaust line (12) is connected to the air line (10) via an EGR line (30) with an EGR valve (31), and a purge line (40) with a purge valve (41) connects the recirculation line (50) to the exhaust line (12) or EGR line (30). The opening degree of the EGR valve (31) is determined, and this opening degree is taken into account when determining the H2 concentration in the exhaust line (12).

Description

[0001] The present invention describes a method for operating a fuel cell system with the features of the preamble of claim 1. State of the art

[0002] Hydrogen-based fuel cell systems are considered a mobility concept of the future, as they emit only water as exhaust gas and enable rapid refueling. Fuel cell systems require air and hydrogen for the chemical reaction within the cells. To provide the required amount of energy, the fuel cells within a fuel cell system are connected to each other to form so-called fuel cell stacks. The waste heat from the cells is dissipated via a cooling circuit and released into the environment. The hydrogen necessary for operating fuel cell systems is typically supplied to the systems from high-pressure tanks.

[0003] Hydrogen-based fuel cells are considered a promising mobility concept for the future, as they emit only water as exhaust gas and allow for rapid refueling. A typical fuel cell system requires air and hydrogen for the chemical reaction, with the waste heat from the stack being dissipated via a cooling circuit and transferred to the vehicle's main radiator.

[0004] In a current system topology, hydrogen is fed from a tank into the anode via a pressure reducer and an injection valve using a jet pump. The jet pump recirculates the gas mixture via a recirculation line. Air is supplied by an electric air compressor, whose air mass flow rate can be measured using a hot-film air mass flow sensor (HFM). A safety-related sensor (HYS-EX) monitors the H₂ concentration in the exhaust line downstream of the stack to prevent excessive hydrogen from being released into the environment.

[0005] During operation, nitrogen (N2) and water diffuse from the cathode side to the anode side, leading to an enrichment of H2 gas in the recirculation line. To maintain system efficiency, a purge line is installed, which periodically directs the gas from the recirculation line into the exhaust gas path, thus increasing the H2 concentration in the exhaust gas line. The purge process can be controlled based on time or modeling.

[0006] It is known from the prior art, in particular from the German application with file number 102020215558.5, that the hydrogen concentration in the exhaust gas line is detected during or after the purging process using the HYS-EX sensor and the purging strategy is controlled accordingly.

[0007] Determining the hydrogen concentration in the recirculation line is based on calculating the hydrogen concentration measured in the exhaust gas line. The dilution of the hydrogen due to mixing with the exhaust air must be taken into account. Disclosure of the invention

[0008] The invention relates to a method comprising the features of the independent claim. Further features and details of the invention are set forth in the respective dependent claims, the description, and the drawings.

[0009] The method according to the invention serves to provide an operating strategy for the precise determination of the hydrogen concentration in the exhaust gas line. This is particularly important for determining the exact hydrogen concentration that enters the exhaust gas line via the purge line. In particular, as described in the application with German file number 102020215558.5, an accurate determination of the hydrogen concentration is essential for adjusting the purge intervals and purge duration.

[0010] With an open or partially open EGR valve, some of the purge gas can flow into the air intake via the EGR line. Consequently, the H2 sensor can no longer detect the entire amount of purge gas. The invention aims to prevent this distortion of the hydrogen concentration measurement caused by the introduction of purge gas into the EGR line.

[0011] The dependent claims specify advantageous embodiments and further developments of the method according to the invention.

[0012] It is advantageous to consider the opening degree of the EGR valve when determining the H2 concentration because this allows for a more precise measurement of the hydrogen concentration, leading to a more accurate determination of the hydrogen concentration from the purge line.

[0013] It is advantageous to discard the measurement of the H2 concentration with the EGR valve open, because this prevents faulty measurements and thus increases the accuracy and reliability of the hydrogen concentration determination.

[0014] It is advantageous to close the EGR valve before the next measurement of the H2 concentration because this ensures that the H2 concentration is measured under stable conditions and no purge gas enters the EGR line, which increases the accuracy of the measurement.

[0015] Leaving the H2 concentration measurement uncorrected with the EGR valve closed is a simple option that simplifies the measurement process and increases the efficiency of the system.

[0016] It is advantageous to apply a correction factor to the measured H2 concentration when the EGR valve is open. This correction factor takes into account the proportion of purge gas that flows through the EGR line to the air intake and is therefore not available for detection by the H2 sensor.

[0017] Determining the correction factor based on the EGR mass flow rate is a simple and accurate method because it can be assumed that the purge gas mass flow rate splits at the EGR line branch, analogous to the exhaust gas mass flow rate.

[0018] It is advantageous to determine the EGR mass flow rate by measuring the pressure drop and the opening angle of the EGR valve, as this is a precise and reliable method for determining the air mass flow rate. By using pressure sensors upstream and downstream of the EGR valve and verifying the opening angle, the EGR mass flow rate can be easily determined from a throttle characteristic curve.

[0019] It is advantageous to measure the air mass flow in the air duct both upstream and downstream of the EGR line, as this allows for an accurate calculation of the EGR mass flow and further improves the precision of the H2 concentration determination. Appropriate air mass sensors are usually installed at the beginning and end of the air duct before the fuel cell stack.

[0020] It is advantageous if the correction factor is determined by the formula M_AGR / M_exhaust air when the purge line is connected upstream of the EGR line to the exhaust line, where M_exhaust air is the mass flow rate in the exhaust line downstream of the EGR line and M_AGR is the EGR mass flow rate, i.e., the amount of exhaust gas flowing through the EGR line from the exhaust line into the air line.

[0021] It is advantageous if the correction factor is determined by a factor of 1 when the purge line is connected downstream of the EGR line to the exhaust line.

[0022] The method according to the invention can be used in particular in fuel cell-powered motor vehicles. However, use in other fuel cell-powered means of transport, such as cranes, ships, rail vehicles, aircraft, or even stationary fuel cell-powered objects, is also conceivable.

[0023] They show: Fig. 1 a schematic representation of a fuel cell system according to the invention in a first embodiment, Fig. 2 a schematic representation of a fuel cell system according to the invention in a second embodiment, Fig. 3 a schematic representation of a fuel cell system according to the invention in a third embodiment, Fig. 4 a flowchart of the individual steps of a method according to the invention in a first embodiment and Fig. 5 a flowchart of the individual steps of a method according to the invention in a second embodiment.

[0024] In the Fig. Figure 1 shows a schematic topology of a fuel cell system 1 according to a first embodiment of the invention, comprising at least one fuel cell stack 101. The at least one fuel cell system 1 has an air line 10, an exhaust line 12, and a fuel line 20. The at least one fuel cell stack 101 can be used for mobile applications with high power requirements, e.g., in trucks, or for stationary applications, e.g., in generators.

[0025] The air duct 10 serves as an air supply duct to deliver ambient air to a cathode 105 of the fuel cell stack 101 via an inlet 16. Components required for the operation of the fuel cell stack 101 are arranged in the air duct 10. An air compressor 11 and / or a compressor 11 is arranged in the air duct 10, which compresses or draws in the air according to the respective operating conditions of the fuel cell stack 101. A heat exchanger may be located downstream of the air compressor 11 and / or compressor 11, which heats the air in the air duct 10.

[0026] Additional components, such as a filter, humidifier, and / or valves, may be installed within air duct 10. Oxygen-rich air is supplied to the fuel cell stack 101 via air duct 10.

[0027] Furthermore, the fuel cell system 1 has an exhaust line 12 in which water, as well as other components of the air from the air line 10, are transported to the environment via an outlet 18 after passing through the fuel cell stack 101. A compressor 13 can be arranged in the exhaust line.

[0028] The exhaust line 12 is connected via an EGR line 30 to the air line 10 upstream of the air compressor 11 and / or compressor. An EGR valve 31 is located inside the EGR line 30. When the EGR valve 31 is opened, exhaust gas from the exhaust line 12 can flow back into the air line 10.

[0029] The exhaust gas from exhaust line 12 can also contain hydrogen (H2) because some of the hydrogen can diffuse through the membrane of the fuel cell stack 101. Hydrogen diffusion also occurs when the fuel cell system 1 is switched off. Hydrogen can also be transported into exhaust line 12 via a purge line 40 with a purge valve 41. The purge line 40 connects the recirculation line 40 to exhaust line 12 upstream of the EGR line 30. Opening the purge valve 41 allows a gas mixture containing hydrogen to flow into exhaust line 12.

[0030] Downstream of the purge line 40, the EGR line 30, and the optional compressor 13, an H2 sensor 14 is located. The H2 sensor 14 can determine the concentration of hydrogen in the exhaust gas.

[0031] A first valve 61 is arranged in the air line 10 and a second valve 62 in the exhaust line 12. By closing the first valve 61 and the second valve 62, a volume V around the cathode is sealed off, so that no air can flow from the air line 10 into the volume V or exhaust gases can escape from the volume.

[0032] The fuel cell system 1 can further include a cooling circuit, which is designed to cool the fuel cell stack 101. The cooling circuit is located in the Fig. 1 is not shown because it is not part of the invention.

[0033] At the inlet of the fuel line 20 is a high-pressure tank 21, which stores a fuel gas, in particular hydrogen. Downstream of the high-pressure tank 21 is a shut-off valve 22. Further components can be arranged in the fuel line 20 to supply an anode 103 of the fuel cell stack 101 with fuel as needed.

[0034] To ensure a sufficient fuel supply to the fuel cell stack 101 at all times, it is necessary to meter fuel above its normal level via the fuel line 20. The excess fuel, as well as certain amounts of water and nitrogen that diffuse through the cell membranes to the anode side, are returned in a recirculation line 50 and mixed with the metered fuel from the fuel line 20.

[0035] Various components, such as a jet pump 51 powered by the metered fuel or a blower 52, can be installed to drive the flow in the recirculation circuit 50. A combination of jet pump 51 and blower 52 is also possible.

[0036] In the Fig. Figure 2 shows a schematic topology of a fuel cell system 1 according to a second embodiment of the invention. The topology and the components arranged therein in the second embodiment correspond to the topology and components of the first embodiment.

[0037] The topology differs in the arrangement of the purge line 30, which connects the recirculation line 50 and the exhaust gas line 12 downstream of the EGR line 30.

[0038] In the Fig. Figure 3 shows a schematic topology of a fuel cell system 1 according to a third embodiment of the invention. The topology and the components arranged therein in the third embodiment correspond to the topology and components of the first embodiment.

[0039] The topology differs in the arrangement of the purge line 30, which connects the recirculation line 50 and the EGR line 30 upstream of the EGR valve 31.

[0040] According to the inventive method, the opening degree of an EGR valve 31 is taken into account when determining the H2 concentration by an H2 sensor 14 in the exhaust gas line 12. By considering the opening degree of the EGR valve 31, the hydrogen content in the recirculation line 50 can be calculated with higher accuracy. Furthermore, the purge duration or purge interval can be adjusted with greater accuracy.

[0041] Fig. Figure 4 shows a flowchart of the individual steps of a method according to the invention for operating a fuel cell system 1 according to a first embodiment.

[0042] In process step 100, the degree of opening of the EGR valve 31 is determined.

[0043] In process step 200, it is checked whether the degree of opening is equal to zero, which corresponds to a tightly closed EGR valve 3, or whether the degree of opening is greater than zero, which corresponds to a partially or fully open EGR valve 31.

[0044] If the opening degree is zero and the EGR valve 31 is tightly closed, the H2 concentration is measured by the H2 sensor 14 in process step 300. A correction factor is not determined, as the entire purge mass flow bypasses the H2 sensor 14.

[0045] The process is terminated in step 400.

[0046] If the degree of opening is greater than zero, a possible measurement signal from the H2 sensor 14 is discarded in process step 250 before the process is terminated in process step 400.

[0047] In an alternative embodiment of the invention, process step 251 is performed instead of process step 250. In process step 251, the EGR valve 31 is closed before proceeding to process step 300. In process step 300, the H2 concentration is measured by the H2 sensor 14. A correction factor is not determined because the entire purge mass flow passes by the H2 sensor 14.

[0048] Fig. Figure 5 shows a flowchart of the individual steps of a method according to the invention for operating a fuel cell system 1 according to a second embodiment.

[0049] In process step 100, the degree of opening of the EGR valve 31 is determined.

[0050] In process step 200, it is checked whether the opening degree is zero, which corresponds to a tightly closed EGR valve 3, or whether the opening degree is greater than zero, which corresponds to a partially or fully open EGR valve 31. If the opening degree is greater than zero, the process proceeds to process step 250. If the opening degree is zero, the process proceeds directly to process step 300.

[0051] In process step 250, a correction factor is calculated. The correction factor is calculated using the EGR mass flow rate flowing through the EGR line 30.

[0052] According to a first embodiment, the EGR mass flow rate (M_AGR) can be determined by the pressure drop across the EGR valve 31 and the degree of opening of the EGR valve 31.

[0053] However, according to a second embodiment, the air mass flow can also be determined by determining the air mass flow in the air line 10 upstream and downstream of the EGR line 30.

[0054] In process step 300, the H2 concentration is measured by the H2 sensor 14.

[0055] In process step 400, the measured H2 concentration is adjusted by a correction factor. If the opening degree of the EGR valve is zero, the correction factor is 1, since the entire purge mass flow bypasses the H2 sensor 14. If the opening degree of the EGR valve is greater than zero, the correction factor calculated in process step 250 is used.

[0056] The process is terminated in step 400.

Claims

[1] Method for operating a fuel cell system (1) wherein the fuel cell system (1) comprises a fuel cell stack (101), an air line (10), an exhaust line (12), a fuel line (20) and a recirculation line (50), wherein the exhaust line (12) is connected to the air line (10) via an EGR line (30) with an EGR valve (31) and a purge line (40) with a purge valve (41) connects the recirculation line (50) to the exhaust line (12) or EGR line (30), characterized by , that an opening degree of the EGR valve (31) is determined, and this opening degree is taken into account when determining the H2 concentration in the exhaust gas line (12). [2] Method according to claim 1, wherein if the opening degree of the EGR valve (31) is greater than zero, the H2 concentration determined via an H2 sensor (14) is discarded. [3] Method according to claim 2, characterized by, that the EGR valve (31) will be closed before the next measurement of the H2 concentration. [4] Method according to claim 1, wherein, when the opening degree of the EGR valve (31) is zero, in particular when the EGR valve (31) is closed, the H2 concentration determined via an H2 sensor (14) is not corrected. [5] Method according to claim 1, wherein, when the opening degree of the EGR valve (31) is greater than zero, in particular when the EGR valve (31) is partially or fully open, the H2 concentration determined via an H2 sensor (14) is changed by a correction factor. [6] Method according to claim 1, characterized by , that the correction factor is determined using an EGR mass flow through the EGR line (30). [7] Method according to claim 6, wherein the EGR mass flow is determined by the pressure drop across the EGR valve (31) and the degree of opening of the EGR valve (31). [8] Method according to claim 6, wherein the EGR mass flow is determined by determining the air mass flow in the air line (10) upstream and downstream of the EGR line (30). [9] Method according to any of the preceding claims wherein the correction factor is M_AGR / M_exhaust air when the purge line (40) is connected upstream of the EGR line (30) to the exhaust line (12), wherein M_exhaust air is the mass flow in the exhaust line (12) downstream of the EGR line (30). [10] Method according to one of the preceding claims, wherein the correction factor is 1 when the purge line (40) is connected downstream of the EGR line (30) to the exhaust line (12), wherein M_exhaust air is the mass flow in the exhaust air line (12) downstream of the EGR line (30).

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