Method for monitoring a fuel cell system

Abstract

Method for monitoring a fuel cell system (2) for hydrogen leaks during temporary unattended operation of the fuel cell system (2) for the production and / or maintenance of predetermined system states, when hydrogen is supplied during the unattended operation of the fuel cell system (2), wherein a pressure profile over time in the hydrogen-carrying parts of the fuel cell system (2) is recorded in parallel with the supply of hydrogen and compared with a predetermined pressure profile, wherein in the event that the recorded pressure profile falls below the predetermined pressure profile, a disturbance signal is generated, at least parts of the fuel cell system (2) are surrounded by air via an air conveying device (6, 15), and the airflow is directed towards a hydrogen sensor (17), which generates a disturbance signal in the event of an increased hydrogen concentration.

Description

[0001] The invention relates to a method for monitoring a fuel cell system for hydrogen leaks according to the type defined in more detail in the preamble of claim 1.

[0002] Fuel cell systems, particularly those equipped with PEM fuel cells, which can be used to provide electrical drive power for vehicles, are known from the general state of the art.

[0003] A well-known problem with such fuel cells is that when the fuel cell system is started, and a hydrogen / oxygen front travels through the anode compartment, high potential differences occur along this front. These differences damage the catalyst in the anode compartment and adversely affect the fuel cell's lifespan. This problem typically arises because air has diffused into the anode compartment after a prolonged period of inactivity, and air and hydrogen are then added to the fuel cell during startup. This typically results in air or oxygen being present on the cathode side, while the added hydrogen flushes out any air already present in the cathode compartment. The resulting front travels across the entire electrochemically active area of ​​the anode compartment via the catalyst, while oxygen is already present on the opposite side in the cathode compartment.This results in the areas where hydrogen is already present having the regular voltage of the fuel cell, while the adjacent areas, where air is present on both sides, do not. The resulting potential difference is generally considered to damage the catalyst by oxidizing it or its support material.

[0004] To counteract this problem, it is known from the prior art that hydrogen remains in the anode compartment and, if applicable, in an anode circuit connected to the anode compartment when the fuel cell system is switched off. However, it is also the case that hydrogen diffuses completely out of the system over a very long period, so that the problem described above reappears. To counteract this, it can be provided that, depending on various mechanisms, for example, from time to time, depending on the pressure, or depending on measured concentrations, hydrogen is also added to the anode compartment during the standstill phase of the fuel cell system in order to maintain a hydrogen atmosphere in the anode compartment for a very long time.To enable this, unattended operation of the fuel cell system is necessary during the standby phase. This requires at least opening the hydrogen supply system and the hydrogen valve on a hydrogen storage unit to meter the hydrogen quantity as desired. Simultaneously or additionally, other processes can occur during the standby phase, such as drying the fuel cell system when the ambient temperature falls below a predetermined threshold, in order to prepare it for a later restart under freezing conditions. This process may also involve unattended operation, possibly requiring the hydrogen supply to be opened.The problem is that, especially in these situations, the unattended operation of the fuel cell system can lead to a safety deficit, as no user is present to intervene in the event of a malfunction.

[0005] US Patent 2010 / 0248060 A1 discloses a fuel cell system capable of performing a flow sweep to suppress degradation of a fuel cell during a fuel cell shutdown, the system comprising a control unit for correcting a hydrogen pressure, wherein during a shutdown associated with intermittent operation, in which, when the load of the fuel cell system is low, electrical energy is supplied from an energy storage unit in the fuel cell system to an auxiliary device and the like, and the energy generation of the fuel cell is temporarily stopped, the control unit corrects a hydrogen pressure in an anode of the fuel cell based on an amount of hydrogen consumed by the flow sweep.

[0006] The object of the invention is now to make such unattended operation of the fuel cell system with hydrogen dosing safer.

[0007] According to the invention, this problem is solved by the features in the characterizing part of claim 1. Further advantageous embodiments and developments of the method according to the invention are set forth in the dependent claims.

[0008] According to the inventive method, a pressure profile over time in the hydrogen-carrying parts of the fuel cell system is recorded in parallel with the supply of hydrogen and compared with a predetermined pressure profile. If the pressure profile falls below the predetermined pressure profile, a fault signal is generated. Such leakage monitoring based on a pressure profile is known, for example, from German patent DE 10 2007 055 486 A1, and a similar method is also known from DE 11 2005 002 230 T5. The inventors' idea is to use this leakage monitoring method, ideally in a simplified form, even during unattended operating phases, i.e., when the fuel cell system is actually at a standstill. This results in a significant increase in safety, since a fault signal can be generated and processed accordingly in the event of a hydrogen leak.

[0009] According to a highly advantageous embodiment of the inventive idea, the disturbance signal can, for example, cause the hydrogen supply to be stopped by closing a valve device on the hydrogen storage tank. This makes it possible to prevent the escape of hydrogen and eliminate the danger posed by any potential hydrogen leaks.

[0010] According to the inventive method, at least parts of the fuel cell system are surrounded by air via an air supply device. Such an air supply device for surrounding parts of the fuel cell system with air helps in particular to dilute any hydrogen leakage that may occur, in order to avoid critical hydrogen concentrations in the vicinity of the fuel cell system.

[0011] According to the inventive method, the airflow is directed towards a hydrogen sensor, which generates a noise signal in the event of an increased hydrogen concentration. Particularly in fuel cell systems used in vehicles, hydrogen sensors are often positioned so that an increased hydrogen concentration can be detected during operation when hydrogen escapes and is blown towards the hydrogen sensor by the airflow generated by driving. Typically, the hydrogen sensors are therefore located behind and above the fuel cell system in the direction of travel. During unattended operation of the fuel cell system when stationary, escaping hydrogen may flow upwards and not reach the area of ​​the hydrogen sensor.This also prevents it from generating an interference signal, which would then be unusable in the manner described above. If the system is surrounded by air, this problem can be counteracted by directing the airflow in such a way that it essentially replaces the wind resistance in these situations.

[0012] In a further highly advantageous embodiment of the method according to the invention, it can also be provided that at least parts of the fuel cell system, in particular a fuel cell housing, are purged with air via an air conveying device. Such purging of components, especially the fuel cell housing, with air can also serve to remove any hydrogen that may escape or diffuse out, thus further increasing safety. Since this also occurs during unattended operating phases, the overall safety of the system can be increased. Here, too, it is possible in principle to direct the airflow in such a way that a hydrogen sensor can respond very easily and efficiently in the event of a malfunction and generate a fault signal.

[0013] The air can be supplied via the existing air supply system for the fuel cell system, such as a compressor or a flow compressor. Additionally or alternatively, a blower can be provided, for example, a 12 V blower, which can be powered, for instance, by a starter battery of a vehicle equipped with the fuel cell system. In a further advantageous embodiment, the blower can also be used to purge the fuel cell housing during normal operation, and, during unattended operation, additionally or alternatively to purging the housing, to also circulate air around components.

[0014] Further advantageous embodiments of the method according to the invention result from the exemplary embodiment described below, which is described in more detail with reference to the figure.

[0015] The only accompanying figure shows a vehicle with a fuel cell system, indicated in principle.

[0016] The single accompanying figure shows a vehicle 1, in principle, with a fuel cell system 2, which is intended to provide electrical drive power in the vehicle 1. The fuel cell system 2 is shown in a highly simplified manner, so that only the components necessary to explain the invention are visible. The core of the fuel cell system 2 is a fuel cell 3, which is intended to be constructed as a stack of individual PEM cells. Each of the individual cells comprises an anode area and a cathode area, whereby, by way of example, only a common anode compartment 4 and a common cathode compartment 5 are indicated in the figure. Air is supplied to the cathode compartment 4 via an air supply unit 6 as an oxygen source during normal operation.Hydrogen from a pressurized gas storage tank 7 is supplied to the anode chamber 5 via a tank valve 8, a so-called on-tank valve (OTV), and a pressure regulating and metering valve 9. Unused hydrogen returns via a recirculation line 10 with a recirculation pump 11 and is mixed with fresh hydrogen before being supplied to the anode chamber 5 again. A water separator 12 is also provided in the recirculation line 10 to separate water and to allow water and gases to be drained from the anode circuit periodically. This separator can be emptied periodically via a valve 13. The desired quantity of gas can then be released after the water has been removed. The fuel cell 3 is also housed in a casing 14, which is purged with air by a blower 15 during normal operation to remove any hydrogen leaks that may occur during fuel cell operation.The air can then be metered into the exhaust air or into the supply air flowing to the fuel cell, so that any hydrogen present can react on the catalysts of the cathode chamber 4.

[0017] It is known from the general state of the art that even when the fuel cell system 2 is not actually in operation, it can briefly operate unattended, for example, to dry the fuel cell system 2 or to maintain the hydrogen supply to the anode compartment 5 during a standstill phase of the fuel cell system 2, the so-called soak period. This prevents a hydrogen / oxygen front from passing over the catalyst of the anode compartment 5 when the fuel cell system 2 is restarted later. A problem with such unattended operation is that in the event of a fault in the hydrogen supply, hydrogen could escape uncontrollably from the fuel cell system 2 or the vehicle 1, since manual shutdown is not possible because no user is present.

[0018] Therefore, additional safety routines are performed in the vehicle 1 or fuel cell system 2 shown here. During the supply of hydrogen to the anode compartment 5, for example when the fuel cell system 2 is switched off or during standby conditioning in unattended operation of the fuel cell system 2, a hydrogen leak test is simultaneously carried out, which is performed analogously to the type described in DE 10 2007 055 486 A1. Essentially, the pressure of the hydrogen in the hydrogen-carrying parts of the fuel cell system 2 is recorded over time and compared with a predefined curve. If the pressure drops faster than predicted in the predefined curve, there is a risk of a leak.In this case, a disturbance signal can be generated, which ultimately leads to the closing of the on-tank valve 8 on the compressed gas storage tank 7 as a hydrogen storage tank, in order to safely and reliably prevent the escape of further hydrogen.

[0019] Additionally, even during unattended operation while the vehicle 1 is stationary, an airflow can be generated, for example, via the blower 15. This airflow flushes the housing 14 and, if a valve 16 is appropriately positioned, also circulates air A around the hydrogen-carrying components of the fuel cell system 2. This ensures that any hydrogen leaks are sufficiently diluted to prevent an ignitable or explosive mixture. Furthermore, a hydrogen sensor 17, which is integrated into the fuel cell system 2 as a safety sensor, is exposed to the airflow A surrounding the anode-side components of the fuel cell system 2, thus generating a corresponding alarm signal in the event of an increased hydrogen concentration.Since the hydrogen sensor 17 is typically positioned so that the airflow during vehicle operation carries any hydrogen leaks into its vicinity, its function is typically not guaranteed during unattended operation of the fuel cell system 2 when the vehicle is stationary. An additional hydrogen sensor would entail considerable expense. Therefore, the airflow A delivered by the blower 15 can be used in such a way that it not only dilutes any leaks but also carries them towards the hydrogen sensor 17 by directing the airflow. It effectively replaces the airflow and can thus enable the existing hydrogen sensor 17 to be used for safety monitoring during unattended operation of the fuel cell system 2 when the vehicle 1 is stationary.With minimal additional control effort, the safety of the unattended operation of the fuel cell system 2 in the vehicle 1 is thus significantly increased.

Claims

[1] Method for monitoring a fuel cell system (2) for hydrogen leaks during a temporary unattended operation of the fuel cell system (2) for the production and / or maintenance of predetermined system states, when hydrogen is supplied during the unattended operation of the fuel cell system (2), wherein a pressure profile over time in the hydrogen-carrying parts of the fuel cell system (2) is recorded in parallel with the supply of hydrogen and compared with a predetermined pressure profile, wherein in the event that the recorded pressure profile falls below the predetermined pressure profile, a disturbance signal is generated, at least parts of the fuel cell system (2) are surrounded by air via an air conveying device (6, 15), and the airflow is directed towards a hydrogen sensor (17), which generates a disturbance signal in the event of an increased hydrogen concentration. [2] Method according to claim 1,characterized by , that at least parts of the fuel cell system (2) are purged with air via an air conveying device (6, 15), in particular a housing (14) around a fuel cell (3) of the fuel cell system (2). [3] Method according to one of claims 1 or 2, characterized by that the air is conveyed via an air conveying device (6) that is already present for the air supply of the fuel cell system (2). [4] Method according to claim 1, 2, or 3, characterized by , that the air is conveyed via a blower (15). [5] Method according to claim 4, characterized by , that the blower (15) is used in the regular operation of the fuel cell system (2) to purge a housing (14) around a fuel cell (3) of the fuel cell system (2). [6] Method according to any one of claims 1 to 5, characterized by, that in the event of a fault signal the hydrogen supply is stopped by closing a valve device (8) on the hydrogen storage (7). [7] Use of the method according to any one of claims 1 to 6 for monitoring a fuel cell system (2) which is used in a vehicle (1) to provide electric drive power.

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