Method for determining a hydrogen concentration in a housing of a fuel cell stack; fuel cell system

Abstract

Method for determining a hydrogen concentration in a housing of a fuel cell stack (101) in a fuel cell system (1), wherein air is supplied to the fuel cell stack (101) via an air path (10) and exhaust air emerging from the fuel cell stack (101) is discharged via an exhaust gas path (12), and wherein the housing is connected to the exhaust gas path via a purge air line. The following steps are carried out: - Closing a first shut-off valve in the air path; - Closing a second shut-off valve in the exhaust gas path; - Determining a hydrogen concentration by a hydrogen sensor in the exhaust gas path.

Description

The invention relates to a method for determining a hydrogen concentration in a housing of a fuel cell stack, having the features of the preamble of claim 1.Furthermore, the invention relates to a fuel cell system having the features of the preamble of claim 7.Prior ArtFuel cells are electrochemical energy converters. As reaction gases, it is possible in particular to use hydrogen (H 2) and oxygen (O 2). These are converted into electrical energy, water (H 2 O) and heat with the aid of a fuel cell. The core of a fuel cell is formed by a membrane electrode arrangement (MEA) which comprises a membrane which is coated on both sides with a catalytic material in order to form electrodes. During operation of the fuel cell, oxygen is supplied to one electrode, the anode, and hydrogen is supplied to the other electrode, the cathode.In order to increase the electrical power, in practice a multiplicity of fuel cells are connected to form a fuel cell stack or stack. In addition, a plurality of fuel cell stacks or fuel cell systems can be interconnected.A certain proportion of hydrogen always reaches the environment from a fuel cell stack. For this reason, the fuel cell stack is surrounded by a housing. The housing serves on the one hand to guarantee high-voltage safety, in particular contact protection, and on the other hand to minimize the risk of flammable gas mixtures. The housing is flushed with air, so that no undesired high concentration of ignitable gases occurs. As an additional protection, a sensor which monitors the hydrogen concentration can be located in the housing.The present invention is concerned with the object of providing an alternative method for determining the hydrogen concentration in a housing of a fuel cell system, and of making available a corresponding fuel cell system.In addition, the method can be used to check the functionality of the sensor in the housing of the fuel cell stack.Disclosure of the InventionThe method according to the invention for determining a hydrogen concentration in a housing of a fuel cell stack in a fuel cell system having at least one fuel cell stack, wherein air is supplied to the fuel cell stack via an air path and exhaust air exiting from the fuel cell stack is discharged via an exhaust gas path, and wherein the housing is connected to the exhaust gas path via a purge air line, has the following steps:closing a first shut-off valve in the air path;closing a second shut-off valve in the exhaust gas path;measuring a hydrogen concentration by a hydrogen sensor in the exhaust gas path;determining a hydrogen concentration in the housing by the hydrogen concentration measured with the hydrogen sensor.The proposed method can be used to determine the concentration of hydrogen in the housing of the fuel cell stack. A hydrogen sensor in the housing can be omitted, since the hydrogen sensor in the exhaust gas path is used to determine the hydrogen concentration, so that costs and installation space can be saved.The dependent claims describe advantageous embodiments and refinements of the fuel cell system and of the method according to the invention for humidifying an air path of a fuel cell system.It is advantageous if air is supplied from the air path to the housing via a ventilation line, since filters already present in the air path can be used to ensure a degree of purity of the air. This enables a compact and cost-effective construction of the fuel cell system.Activation of the air compressor in the air path in order to convey air into the housing is advantageous since no additional pumps or conveying units are required.In particular before starting the fuel cell system, it is advantageous if the first and second shut-off valves are closed and the air compressor in the air path is activated in order to check the hydrogen concentration in the housing. Checking the hydrogen concentration before the start is particularly advantageous since a particularly accurate measurement can be realized in this way since no hydrogen from the fuel cell stack is present in the exhaust gas path.After the fuel cell system has been stopped, the first and second shut-off valves are closed and the air compressor is activated again after a predefined time of standstill in order to carry out the most accurate possible measurement of the hydrogen concentration in the housing. It is advantageous that the first and the second valve are closed, since hydrogen from the fuel cell stack is prevented from distorting the measurement.A particular advantage results for the on-board diagnosis if the hydrogen concentration determined by the hydrogen sensor in the exhaust gas path is compared with the hydrogen concentration determined by a housing hydrogen sensor in order to perform a validation of the housing hydrogen sensor.It is advantageous if the purge air line has a substantially smaller cross section than the exhaust gas path, in particular a cross section which is at most half as large as the exhaust gas path, and / or is at least 1.5 times longer than the exhaust gas path, since in this way hydrogen escapes from the exhaust gas path more quickly than from the purge air line and thus does not lead to corruption of the measurement results.A throttle in the ventilation line or in the purge air line is advantageous since this increases the accuracy of the method since hydrogen escapes from the exhaust gas path more quickly than from the purge air line.Preferred Embodiments:The device according to the invention and the fuel cell system according to the invention are explained in more detail below with reference to FIG. 1.FIG. 1 shows a schematic topology of a fuel cell system 1 according to a first exemplary embodiment having at least one fuel cell stack 101. The at least one fuel cell stack 101 has an air path 10, an exhaust gas path 12 and a fuel line 20. The at least one fuel cell stack 101 can be used for mobile applications with high power requirements, for example in trucks, or for stationary applications, for example in generators.A high-pressure tank 21 and a shut-off valve 22 are located in the inlet of the fuel line 20, and further components can be arranged in the fuel line 20 in order to supply the fuel cell stack 101 with fuel as required.In order to always sufficiently supply the fuel cell stack 101 with fuel, there is the need for a superstoichiometric metering of fuel via the fuel line 20.To drive the flow in the recirculation line 50, various components, such as a jet pump 51 operated with the metered-in fuel or a blower 52, can be installed. A combination of jet pump 51 and blower 52 is also possible.The air path 10 serves as an air supply line for supplying air from the environment to the fuel cell stack 101 via an inlet 16. An air compressor 11 and / or compressor 11 is arranged in the air path 10, which compresses or draws in the air according to the respective operating conditions of the fuel cell stack 101.Further components such as, for example, a humidifier, a filter and / or a heat exchanger and / or valves can be provided within the air path 10. Oxygen-containing air is provided to the fuel cell stack 101 via the air path 10.In the air path 10, a first shut-off valve 14 is arranged upstream of the inlet to the fuel cell stack 101. By closing the first shut-off valve 14, the supply of air into the fuel cell stack 101 is interrupted.Furthermore, the fuel cell system 1 has an exhaust line 12 in which water and further constituents of the air from the air path 10 are transported to the environment via an outlet 18 after passage through the fuel cell stack 101. The exhaust gas of the exhaust gas line 12 may also contain hydrogen (H 2) because portions of the hydrogen may diffuse through the membrane of the fuel cell stack 101.A turbine 13 can be arranged in the exhaust gas path 10. The turbine 13 can be used to drive the compressor 11.In the exhaust gas path 12, a second shut-off valve 16 is arranged upstream of the outlet of the fuel cell stack 101. By closing the second shut-off valve 14, the discharge of exhaust air or exhaust gases from the fuel cell stack 101 into the environment is interrupted.Furthermore, a hydrogen sensor 35 is arranged in the exhaust gas path 12 which can measure the concentration of hydrogen in the exhaust air or exhaust gases flowing past the sensor. The hydrogen sensor 35 is disposed downstream of the second shut-off valve 16 and the optional turbine 13 in the exhaust path 12.The fuel cell stack 101 is arranged in a closed housing 36. The housing 36 is connected to the exhaust path 12 via a purge air line 34. The purge air line 34 is connected to the exhaust path 12 upstream of the hydrogen sensor 35 so that air from the housing 36, which can flow into the exhaust path via the purge air line 34, is introduced to the hydrogen sensor 35.The purge air line 34 can have a substantially smaller cross section than the exhaust gas path 12, a cross section which is at most half as large as the cross section of the exhaust gas path can preferably be selected. Alternatively or additionally, the purge air line 34 may be at least 1.5 times longer than the exhaust path 12. A throttle 33 can be arranged in the ventilation line 32.Since hydrogen leakage from the fuel cell stack 101 is to be expected, measures must be taken to avoid or detect an ignitable mixture of hydrogen (H2) and air in the housing 36.For this reason, a method for determining a hydrogen concentration in the housing 36 of the fuel cell stack 101 is proposed, wherein the following steps are carried out:closing the first shut-off valve 14 in the air path 10;closing the second shut-off valve 16 in the exhaust gas path 12;measuring a hydrogen concentration by the hydrogen sensor 35 in the exhaust gas path 12;determining a hydrogen concentration in the housing 36 by the hydrogen concentration measured by the hydrogen sensor 35.According to the exemplary embodiment shown in FIG. 1, air is supplied from the air path 10 to the housing 36 via a ventilation line 32. The ventilation line 32 is arranged between the air path 10 and the housing 36 downstream of the air compressor 11 so that air can be conveyed from the air line 10 into the ventilation line 32 and the housing 36 via the activated air compressor 11.The method according to the invention can be carried out before the fuel cell system 1 is started. In this case, before the start of the fuel cell system 1, the first and second shut-off valves 14, 16 are closed and the air compressor 11 in the air path 10 is activated. By activating the air compressor 11, air flows from the environment via the air path 10 and the ventilation line 32 into the housing 36 and is discharged again into the environment through the purge air line 34 and the exhaust path 12. When hydrogen is present in the housing, it is transported away by the air. The hydrogen sensor 35 in the exhaust gas path 12 can measure the hydrogen concentration and infer the hydrogen content in the housing on the basis of the measured hydrogen concentration.In an alternative exemplary embodiment, the method according to the invention can be carried out after the fuel cell system 1 has been stopped. In this case, after the fuel cell system 1 has been shut off, the first and second shut-off valves 14, 16 will be closed and the air compressor 11 will be activated again after a predefined time of standstill. By re-activating the air compressor 11, air flows from the environment via the air path 10 and the ventilation line 32 into the housing 36 and is discharged again into the environment via the purge air line 34 and the exhaust path 12. When hydrogen is present in the housing, it is transported away by the air. The hydrogen sensor 35 in the exhaust gas path 12 can measure the hydrogen concentration and infer the hydrogen content in the housing on the basis of the measured hydrogen concentration.In a further exemplary embodiment, the method according to the invention can be used for on-board diagnosis of an existing housing hydrogen sensor 37 in the housing 36 of the fuel cell stack 101. Here, the hydrogen concentration determined by the hydrogen sensor 35 in the exhaust path 12 is compared with the hydrogen concentration determined by a case hydrogen sensor 37 to perform validation of the case hydrogen sensor 37. If there is a deviation between the hydrogen concentration determined by the housing hydrogen sensor 37 and the hydrogen concentration determined by the hydrogen sensor 35 in the exhaust gas path 12, an error message is made.

Claims

Method for determining a hydrogen concentration in a housing (36) of a fuel cell stack (101) in a fuel cell system (1), wherein air is supplied to the fuel cell stack (101) via an air path (10) and exhaust air emerging from the fuel cell stack (101) is discharged via an exhaust gas path (12), and wherein the housing (36) is connected to the exhaust gas path (12) via a purge air line (34), characterized in that the following steps are carried out: - closing a first shut-off valve (14) in the air path (10); - closing a second shut-off valve (16) in the exhaust gas path (12); - measuring a hydrogen concentration by a hydrogen sensor (35) in the exhaust gas path (12); - determining a hydrogen concentration in the housing (36) by means of the hydrogen concentration measured by the hydrogen sensor (35).Method according to claim 1, wherein air is supplied from the air path (10) to the housing (36) via a ventilation line (32).Method according to one of claims 1 or 2, wherein an air compressor (11) in the air path (10) is activated in order to convey air into the housing (36).Method according to Claim 3, wherein, before starting the fuel cell system (1), the first and second shut-off valves (14, 16) are closed and the air compressor (11) in the air path (10) is activated.Method according to Claim 3, wherein after the fuel cell system (1) has been stopped, the first and second shut-off valves (14, 16) are closed and the air compressor (11) is activated again after a predefined time of standstill.Method according to one of the preceding claims, wherein the hydrogen concentration determined by the hydrogen sensor (35) in the exhaust gas path (12) is compared with the hydrogen concentration determined by a housing hydrogen sensor (37) in order to perform a validation of the housing hydrogen sensor (37).Fuel cell system (1) for determining a hydrogen concentration in a housing (36) of a fuel cell stack (101) in a fuel cell system (1), wherein air is supplied to the fuel cell stack (101) via an air path (10) and exhaust air emerging from the fuel cell stack (101) is discharged via an exhaust gas path (12), and wherein the housing (36) is connected to the exhaust gas path (12) via a purge air line (34), the fuel cell stack (101) is lockable to the air path (10) via a first shut-off valve (14) and to the exhaust gas path (12) via a second shut-off valve (16), and a hydrogen sensor (35) is arranged downstream of the purge air line (34) in the exhaust gas path (12), characterized in that a ventilation line (32) connects the air path (10) to the housing (36).Fuel cell system according to Claim 7, wherein the purge air line (34) has a substantially smaller cross section than the exhaust gas path (12), in particular a cross section which is at most half as large as the exhaust gas path (12), and / or is at least 1.5 times longer than the exhaust gas path (12).Fuel cell system according to either of Claims 7 and 8, characterized in that a throttle (33) is arranged in the venting line (32).Fuel cell system according to one of Claims 7 to 9, characterized in that the ventilation line (32) is connected to the air path (10) downstream of an air compressor (11).

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