Method for operating a fuel cell system and fuel cell system

Abstract

The invention provides a method for operating a fuel cell system, comprising: introducing (S1) a shutdown procedure and / or a shutdown state of the fuel cell system; reducing (S2) the production of product water at the anode of a fuel cell (BZ) of the fuel cell system (BS) to below or equal to a minimum specified value; actuating (S3) an anode bleed valve (DV) and opening the anode bleed valve (DV), thereby at least partially emptying a water separator and / or an anode recirculation loop (RZ) in the anode recirculation loop of the fuel cell (BZ) of the fuel cell system; monitoring (S4) an exhaust line from the anode recirculation loop by means of a sensor (S), wherein the exhaust line is connected to the anode bleed valve (DV) and the hydrogen concentration in the exhaust gas in the exhaust line is identified (S4a) after and / or while the product water is separated from the anode bleed valve (DV); and concluding (S5) that the water separator (WA) and / or the anode recirculation loop (RZ) is sufficiently emptied when the hydrogen concentration ascertained is greater than or equal to a predetermined limit value.

Description

Technical Field

[0001] The present invention relates to a method for operating a fuel cell system and a fuel cell system. Background Technology

[0002] In a fuel cell system, hydrogen is supplied from the medium-pressure region of the fuel cell in the anode loop. Here, a regulating valve for metering hydrogen on demand and a blower are typically used. Using a recirculation blower (hereinafter referred to as an ARB (Anode Recirculation Blower)), unused hydrogen in the fuel cell system can be drawn back from the fuel cell (stack) outlet to the inlet. This return loop can be part of the anode subsystem, which typically integrates a pre-separated water separator.

[0003] Polymer electrolyte membrane (PEM) fuel cell systems use oxygen to convert hydrogen into electrical energy, while simultaneously generating waste heat and water. The conversion of hydrogen here means that hydrogen molecules are consumed or removed on the anode side.

[0004] A fuel cell may include an anode supplied with hydrogen, a cathode supplied with air, and a polymer electrolyte membrane disposed between the two. Multiple such individual fuel cells can advantageously be stacked to increase the generated voltage. Within the stack, supply channels may exist that supply hydrogen and air to the individual cells and can remove depleted humid air and depleted anode exhaust gas. In other words, the medium can be diverted from the anode outlet back to the inlet, forming a closed loop for the anode medium, wherein byproducts such as nitrogen or water can be discharged via purge and vent valves.

[0005] Recirculation can typically be achieved using a recirculation blower or passively using a jet pump. A water separator can be used to separate liquid water from the gaseous components of the anode exhaust gas; the water separator can also store a certain volume of wastewater. Once a certain limit is reached, the water can be drained by opening a so-called vent valve, which is usually done at the anode outlet.

[0006] A storage tank with fittings and valves is described in DE 11 2006 003 013 B4. Summary of the Invention

[0007] The present invention provides a method for operating a fuel cell system according to claim 1 and a fuel cell system according to claim 8.

[0008] Advantageous improvements are the subject of the dependent claims.

[0009] Advantages of the present invention The present invention is based on the idea of ​​proposing a method for operating a fuel cell system and a fuel cell system, such as for a vehicle, wherein the extraction and removal of product water in the anode circuit can be improved.

[0010] Instead of measuring the amount of product water appearing in the anode using sensors, other standards can be considered to check for residual water in the system. One available standard could be the exhaust-hydrogen concentration, measured during the venting process, particularly in the exhaust path.

[0011] According to the present invention, in a method for operating a fuel cell system: a shutdown procedure and / or shutdown state of the fuel cell system is introduced; the production of product water at the anode of the fuel cell is reduced to below or equal to a minimum predetermined value (e.g., when there is no longer a significant amount of product water produced); an anode vent valve is operated and opened, thereby at least partially emptying the water separator and / or anode recirculation loop in the anode recirculation loop of the fuel cell system; an exhaust gas line from the anode recirculation loop is monitored by means of a sensor, wherein the exhaust gas line is connected to the anode vent valve, and the hydrogen concentration in the exhaust gas in the exhaust gas line is identified after and / or when product water is separated from the anode vent valve; and when the determined hydrogen concentration is greater than or equal to a predetermined limit value, sufficient emptying of the water separator and / or anode recirculation loop is deduced.

[0012] The operating point of this fuel cell can be largely known, therefore the operating parameters of the fuel cell system assigned to operation at that specific operating point can also be known.

[0013] According to a preferred embodiment of the method, the shutdown procedure on the fuel cell system includes shutting down the functional processes of the fuel cell system.

[0014] According to a preferred embodiment of the method, a minimum amount of process water to be discharged is allocated to the corresponding operating point of the fuel cell, and the opening time of the anode vent valve is matched or adapted to the minimum amount.

[0015] According to a preferred embodiment of the method, the reduction of product water production is achieved by reducing the operating current on the fuel cell to a predetermined value for minimum operation.

[0016] According to a preferred embodiment of the method, the purge valve is kept closed while the anode vent valve is open.

[0017] According to a preferred embodiment of the method, the length of time until the water separator and / or anode recirculation loop is emptied is determined, and the fill factor present before emptying at the time of fuel cell system shutdown is determined.

[0018] According to a preferred embodiment of the method, a venting strategy for the fuel cell system is inferred from the fill level and / or the operating point of the fuel cell when the anode vent valve is open.

[0019] The shutdown process can be advantageously used to analyze whether there is residual water in the system, especially whether the residual water is small enough, in order to improve the operation of the fuel cell system and reduce or even avoid the disadvantages caused by the presence of product water, such as freezing in the fuel cell system.

[0020] Advantageously, during the shutdown procedure, little or no new product water can be generated, and flow dynamics and turbulence can be minimized, thus allowing it to be assumed that anode water can accumulate at the lowest point of the system. Opening the vent valve results in the removal of residual water first, followed by the removal of anode gas. If a sensor measuring the hydrogen concentration in the exhaust path indicates an increase in hydrogen, this can be an indicator of gas outflow with the vent valve open, meaning that there is no longer, or only a very small amount of, residual water in the anode system. During the vent valve's opening, the purge valve should be closed to allow for a clear assessment of the measurement signal. The time until the anode system is emptied can then be calculated back into the water volume. This allows for the venting strategy during operation to be quantified.

[0021] Advantageously, the product water that appears at the anode during fuel cell operation can be drained by appropriate control of the vent valve. The amount of product water produced may depend on many influencing variables, such as humidity, temperature, and load.

[0022] When the anode water separator is emptied or the amount of product water falls below the minimum specified value / minimum specified quantity, anode gas may flow out from the vent valve into the exhaust gas path, during which time the purge valve should be closed. The outflow of anode gas can be detected by an increase in the hydrogen concentration in the exhaust gas, which can be measured by a sensor located in the exhaust gas path. An increase in the hydrogen concentration in the exhaust gas indicates that the anode water separator has been completely emptied or emptied to a level below a predetermined limit.

[0023] Similarly, the fill level at the time of shutdown can be determined by the length of time until the fuel cell is emptied or reaches the minimum fill level. This leads to conclusions regarding the venting strategy during fuel cell operation.

[0024] For example, if it takes 10 seconds until the water separator is empty, historically it still contains x milliliters of water. If it takes 20 seconds (i.e., Y times the previous amount), then the water separator still contains Y*x milliliters of water. If the air side operates more humid than assumed, then potentially more water will be delivered to the anode. This causality can be evaluated during shutdown processes or, for example, whenever the "water separator should be empty" condition is met.

[0025] Therefore, assumptions can be made in the background based on the water model, and insights can be derived from them.

[0026] According to the present invention, a fuel cell system includes: a fuel cell having a cathode side and an anode side; a recirculation loop connected to the anode side; a discharge valve in the recirculation loop through which gaseous fuel can be discharged at least partially from the recirculation loop; a water separator and a vent valve in the recirculation loop through which product water from the fuel cell and from the water separator can be discharged at least partially from the recirculation loop; a hydrogen supply line connected to the recirculation loop, through which fresh hydrogen can be supplied to the recirculation loop; a sensor device arranged on the anode side and / or in the recirculation loop, by means of which the concentration of hydrogen in the recirculation loop and / or the amount of process water produced by the water separator can be determined; and a control device connected to the sensor device, the fuel cell, the discharge valve and the vent valve and configured to perform the method of the present invention.

[0027] The purge valve and the vent valve can have a common path toward the cathode.

[0028] Fuel cell systems can also be characterized by combining the features and advantages mentioned in this method, and vice versa.

[0029] Other features and advantages of the invention will become apparent from the following description with reference to the accompanying drawings. Attached Figure Description

[0030] The invention will now be explained in more detail based on the embodiments shown in the accompanying drawings.

[0031] The attached diagram shows: Figure 1 A schematic diagram of a fuel cell system according to an embodiment of the present invention is shown; Figure 2 A block diagram illustrating the steps of a method for operating a fuel cell system according to an embodiment of the present invention is shown.

[0032] In the accompanying drawings, the same reference numerals denote the same or functionally identical elements. Detailed Implementation

[0033] Figure 1 A schematic diagram of a fuel cell system according to an embodiment of the present invention is shown.

[0034] The fuel cell system BS shown includes: a fuel cell BZ having a cathode side K and an anode side A; a recirculation loop RZ connected to the anode side A; a water separator WA and a vent valve DV in the recirculation loop RZ, the vent valve being connected to an exhaust gas line, and product water from the fuel cell BZ and from the water separator WA being able to be discharged at least partially from the recirculation loop RZ through the vent valve; a sensor S connected to the exhaust gas line, by means of which the hydrogen concentration can be determined; and a control device SE connected to the sensor S, the fuel cell BZ and the vent valve DV and configured to perform the method of the present invention.

[0035] Hydrogen can be supplied from the hydrogen storage system to the anode subsystem via the hydrogen supply pipeline GS.

[0036] In addition, the hydrogen injector HGI can be connected to the hydrogen supply line GS, which can meter hydrogen into the fuel cell system BS.

[0037] The water separator WA can advantageously separate the recycle material from the liquid water on the anode side.

[0038] In one embodiment of the invention, the recirculation blower GB can be connected upstream of HGI in the recirculation loop RZ.

[0039] The vent valve DV (or a vent hole connected only to this valve) can be located at the lowest point of the recirculation loop RZ (e.g., viewed vertically) and connected to the exhaust gas path AG. The sensor S can be arranged in this exhaust gas path, which can be located in or connected to the cathode exhaust path (of the cathode K). The fuel cell BZ can have a connection terminal connected to the cooling device KH and an electrical connection terminal EL.

[0040] A pressure sensor P can also be installed between HGI and anode A.

[0041] Figure 2 A block diagram illustrating the steps of a method for operating a fuel cell system according to an embodiment of the present invention is shown.

[0042] In this method: S1 is introduced as a shutdown procedure and / or shutdown state of the fuel cell system; S2 is reduced to below or equal to a minimum specified value of product water production at the anode of the fuel cell system; S3 is operated and the anode vent valve is opened, thereby at least partially emptying the water separator and / or anode recirculation loop in the anode recirculation loop of the fuel cell system; S4 is monitored by means of a sensor from the exhaust gas line of the anode recirculation loop, wherein the exhaust gas line is connected to the anode vent valve, and the hydrogen concentration in the exhaust gas in the exhaust gas line of S4a is identified after and / or when product water is separated from the anode vent valve; and when the determined hydrogen concentration is greater than or equal to a predetermined limit value, S5 is inferred as sufficient emptying of the water separator and / or anode recirculation loop.

[0043] Although the present invention has been fully described above based on preferred embodiments, the present invention is not limited thereto and can be modified in various ways.

Claims

1. A method for operating a fuel cell system (BS), comprising the following steps: Introduce (S1) the shutdown procedure and / or shutdown state of the fuel cell system; Reduce (S2) the production of product water at the anode of the fuel cell (BZ) of the fuel cell system (BS) to below or equal to a minimum specified value; Operate (S3) the anode vent valve (DV) and open the anode vent valve (DV), thereby at least partially emptying the water separator and / or the anode recirculation loop (RZ) of the fuel cell (BZ) of the fuel cell system (BS); The waste gas line from the anode recirculation loop is monitored (S4) by means of a sensor (S), wherein the waste gas line is connected to the anode vent valve (DV), and the hydrogen concentration in the waste gas in the waste gas line is identified (S4a) after and / or when the product water is separated from the anode vent valve (DV); and When the hydrogen concentration is greater than or equal to the predetermined limit value, it is deduced (S5) that the water separator (WA) and / or the anode recirculation loop (RZ) are fully emptied.

2. The method according to claim 1, wherein, The shutdown procedure on the fuel cell system includes the shutdown of the functional processes of the fuel cell system.

3. The method according to claim 1 or 2, wherein, The minimum amount of process water to be discharged is allocated to the corresponding operating point of the fuel cell (BZ), and the opening time of the anode vent valve (DV) is matched or adapted to the minimum amount.

4. The method according to any one of claims 1 to 3, wherein, The reduction (S2) in the production of product water is achieved by reducing the operating current on the fuel cell (BZ) to a specified value for minimum operation.

5. The method according to any one of claims 1 to 4, wherein, The purge valve (PV) remains closed while the anode vent valve is open.

6. The method according to any one of claims 1 to 5, wherein, Calculate the length of time until the water separator (WA) and / or the anode recirculation loop (RZ) are emptied, and calculate the fill factor present at the time of shutdown of the fuel cell system prior to the emptying.

7. The method according to claim 6, wherein, The venting strategy for the fuel cell system is inferred from the fill level and / or the operating point of the fuel cell (BZ) when the anode vent valve (DV) is open.

8. A fuel cell system (BS), comprising: A fuel cell (BZ) having a cathode side and an anode side (A); A recirculation loop (RZ) is connected to the anode side (A) and includes a water separator (WA) and a drain valve (DV) in the recirculation loop (RZ), the drain valve being connected to an exhaust gas line, and product water from the fuel cell (BZ) and from the water separator (WA) being able to be discharged at least partially from the recirculation loop (RZ) through the drain valve; A sensor (S) connected to the exhaust gas pipeline, wherein the hydrogen concentration can be determined by means of the sensor; and A control device (SE) is connected to the sensor (S), the fuel cell (BZ), and the vent valve (DV), and is configured to perform the method according to any one of claims 1 to 7.

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