Method for shutting down a fuel cell system

Abstract

Method for shutting down a fuel cell system (1) with at least one movable component (6, 8, 13, 14, 15, 16) which is arranged in a gas stream that is at least temporarily moist, in which a fuel cell (3) is shut down, wherein the at least one movable component (6, 8, 13, 14, 15, 16) is moved after the fuel cell (3) has been shut down, depending on the time since the fuel cell (3) was shut down and / or depending on the temperature, such that freezing of the movable component (6, 8, 13, 14, 15, 16) is prevented; characterized in that the at least one movable component (6, 8, 13, 14, 15, 16) is moved for a predetermined period of time.

Description

The invention relates to a method for shutting down a fuel cell system. Fuel cell systems are known from the general state of the art. They are used, for example, in vehicles to generate electrical power for on-board components and / or the vehicle's drive system. The fuel cell systems themselves comprise at least one fuel cell, typically a stack of individual cells, a so-called fuel cell stack. This fuel cell stack can, for example, be designed using PEM technology. It is known from the prior art that fuel cell systems are particularly susceptible to freezing when restarting under ambient conditions with sub-zero temperatures. This is because the product water generated in the fuel cell after operation can easily freeze in peripheral components of the fuel cell system surrounding the fuel cell. Within the fuel cell system, moving components such as valves, fans, and the like are especially vulnerable to this freezing problem. Therefore, it is known from the prior art to circulate air through the fuel cell system after it has been shut down, typically using supplied air, until these areas are sufficiently dry.Such a comparatively complex and energy-intensive shutdown process can be limited to situations where temperatures below freezing are expected until the system restarts. However, since the duration of the planned shutdown of the fuel cell system is unknown, this approach is very imprecise and requires a relatively large safety margin. Overall, this results in a very energy-intensive shutdown of the fuel cell system, which, while enabling a rapid restart, negatively impacts the overall efficiency of the fuel cell system. An alternative approach involves electrical heating, for example of valves, to thaw them if they are frozen before starting the fuel cell system. This method is described, for example, in DE 10 2004 055 728 A1. It is also very energy-intensive. Furthermore, it significantly delays the time between a start request and the possible restart of the fuel cell system, since frozen moving components must first be completely heated and thawed. Additional effort is required due to the heating elements and the wiring for sensors and power to these heating elements. DE 10 2007 056 542 A1 discloses a fuel cell device with a device for preventing ice formation of a product electrochemically converted during energy conversion in a fuel cell device, wherein the device comprises motion generators to locally move the product in a standstill state relative to the fuel cell device in order to prevent or reduce freezing of the product or to reduce an already frozen product. DE 11 2008 000 547 T5 discloses that a fuel cell system has an inlet shut-off valve provided in a main path that forms a supply flow path for oxidized gas, a humidifier bypass valve provided in a humidifier bypass path that is a path that bypasses the main path, a fuel cell bypass valve provided in a fuel cell bypass path that bypasses a fuel cell stack, and a valve control unit that releases valves stuck due to freezing by changing pressure. US 2010 / 0151291 A1 discloses that a fuel cell system comprises: a fuel cell with an anode and a cathode; an oxidation gas flow path that supplies the oxidation gas to the fuel cell and discharges the oxidation gas from the fuel cell; a first shut-off valve located upstream of the fuel cell and comprising a first valve body; a second shut-off valve located downstream of the fuel cell and comprising a second valve body; a cathode control unit for sealing the cathode; and a purge unit for purging the anode by supplying the oxidizing gas to the anode, wherein, prior to purging the anode using the purge unit, the cathode control unit unseals the cathode by opening the first shut-off valve and the second shut-off valve.The fuel cell system is able to prevent the valve bodies pressed against the seat sections from freezing even below freezing temperature and to avoid a situation in which the fuel cell system cannot be restarted in a switched-off state. The object of the present invention is to provide a method for shutting down a fuel cell system which avoids the aforementioned disadvantages and which ensures a safe and reliable restart of the fuel cell system at temperatures below freezing with minimal expenditure of components, installation space and energy. This problem is solved according to the invention by the features in the characterizing part of claim 1. Further advantageous embodiments of the method according to the invention are specified in the dependent claims. The inventive method utilizes the conventional and already existing control electronics in a fuel cell system, in which, as is customary, at least one moving component is arranged in an at least partially moist gas stream, to move this moving component depending on the time since the fuel cell was switched off and / or the temperature. The corresponding component, which can be, for example, a valve body, the impeller of a compressor, the fan of a hydrogen recirculation blower, or the like, is thus briefly moved again after the actual fuel cell has been switched off and after a certain time has elapsed and / or a certain temperature has been reached.This brief movement, for example of a valve body in a solenoid valve or a fan in a hydrogen recirculation blower, prevents these components from freezing solid at the point when the water is likely to freeze. This prevents at least one moving component from freezing. This is achieved with minimal energy expenditure, namely only the energy required to move the component for a short period. A restart of the fuel cell system is then possible quickly and efficiently, even after a long period of inactivity, without additional energy input or delay. In a particularly advantageous embodiment of the method according to the invention, the at least one movable component is moved after a predetermined time interval following the shutdown of the fuel cell. In this particularly preferred and simple method, the movement thus begins after a predetermined time interval. This time interval can, for example, be based on empirical values ​​that, in turn, estimate the cooling of the fuel cell system. The time interval can therefore be predetermined such that the movement of the at least one movable component occurs when it is not yet (completely) frozen solid, but is at risk of freezing solid. In a particularly advantageous further development of this embodiment of the method according to the invention, it can also be provided that the time interval is predetermined as a function of a temperature or a temperature change. The time interval can, for example, be predetermined as a function of a temperature that is typically already being monitored, either within the fuel cell system or in the vicinity of the fuel cell system. This allows, for example, consideration of very low ambient temperatures, as it can be expected that switching off the fuel cell system or the fuel cell of the fuel cell system will lead to a relatively rapid cooling of the entire fuel cell system and thus necessitates the movement of the at least one movable component relatively quickly to prevent it from freezing.Alternatively or additionally, a temperature gradient, i.e., a change in temperature, can be used to parameterize the time interval. The temperature change thus measured can, in particular, be the temperature in a region within the fuel cell system. Depending on the operating temperature reached by the fuel cell system, which can vary depending on the operating time and load conditions, a cooling period can then be determined. From such a temperature gradient, a time interval can then be easily and efficiently specified within which the movement of at least one moving component should occur to prevent it from freezing. An alternative embodiment of the method according to the invention provides that the at least one movable component is moved below a predetermined temperature threshold. In this particularly simple variant, only a temperature sensor is necessary, which, via the existing control of the movable components, moves them from a certain temperature threshold. The moving component is moved for a predetermined period. This period can be relatively short, as movement at the crucial moment reliably prevents freezing, even if it only lasts for a few seconds. In a particularly advantageous further development of this principle, the operating time can be predetermined based on the temperature or temperature change. The period over which the moving components are operated can thus also be predetermined based on the temperature, ensuring that it is precisely as long as absolutely necessary to prevent freezing, either as a function of the temperature itself or, more specifically, as a function of a temperature gradient. This minimizes the power consumption and noise emissions caused by the movement of at least one moving component. In a further very favorable and advantageous embodiment of the method according to the invention, it is also provided that the fuel cell system to be switched off is a fuel cell system in a vehicle which is at least partially electrically driven with electrical power from the fuel cell system. The method according to the invention allows a fuel cell system to be shut down very quickly and restarted very energy-efficiently because moving components in the fuel cell system are moved at a critical point in time with regard to freezing, in order to prevent freezing. The method is therefore particularly suitable for fuel cell systems that are frequently exposed to such conditions and often have to be shut down and restarted at sub-zero temperatures. This applies especially to fuel cell systems used in vehicles, for example, in rail-bound or trackless land vehicles, ships, or the like, since these are frequently started and shut down and often remain at sub-zero temperatures until the fuel cell system is restarted. Further advantageous embodiments of the method according to the invention will become apparent from the remaining dependent subclaims and will be made clear by reference to the exemplary embodiment, which is described in more detail below with reference to the figures. The only accompanying figure shows a fuel cell system in a vehicle. The single accompanying figure shows a highly schematic representation of a fuel cell system 1, which is intended to be installed in a vehicle 2. The fuel cell system 1 essentially comprises a fuel cell 3, which is here designed as a PEM fuel cell. Proton-permeable membranes separate an anode compartment 4 from a cathode compartment 5 of the fuel cell 3. Air is supplied to the cathode compartment 5 in a manner known per se via an air conveying device 6, for example, a flow compressor, a Roots blower, or the like. The air then passes through a humidifier 7, where it is humidified, into the cathode compartment 5 of the fuel cell 3. The oxygen-depleted exhaust air from the cathode compartment 5 flows again through the humidifier 7 and releases the moisture absorbed in the fuel cell 3 in the form of vaporous product water into the supply air stream in the humidifier 7.The exhaust air can then be released into the environment via a turbine 8 to recover pressure energy. In the embodiment shown here, the turbine 8 and the air conveying device 6 are mounted on the same shaft. The energy recovered in the turbine 8 can thus drive the air conveying device 6. Since this energy will typically not be sufficient, an electric motor 9 is also provided on the shaft to supply the additional energy required for the air conveying device 6. If there is an energy surplus in the turbine 8, the electric motor 9 can also generate electrical energy for other uses. Hydrogen from a pressurized gas storage tank 10 is supplied to the anode compartment 4 of the fuel cell 3. The hydrogen passes from the pressurized gas storage tank 10 into the anode compartment 4 via a pressure regulating and metering unit 11. Typically, the anode compartment 4 is supplied with more hydrogen than is electrochemically converted within it. The residual hydrogen is then returned to the inlet area of ​​the anode compartment 4 via a recirculation line 12 and a recirculation pump 13, for example, a recirculation blower. Mixed with the fresh hydrogen from the pressure regulating and metering unit 11, it flows back into the anode compartment 4. Over time, product water accumulates in this so-called anode circuit or anode loop, as a small portion of the fuel cell 3's product water is also generated in the anode compartment 4.Furthermore, inert gases such as nitrogen diffuse through the membranes into the anode compartment 4. This ultimately leads to a decreasing hydrogen concentration over time in the area of ​​the recirculation line 12. To counteract this, gas and / or water is typically drained from the area of ​​the recirculation line 12 from time to time. A drain valve 14, which is located in an unspecified drain line, serves this purpose. Since the discharged mixture of water and gas always contains a certain amount of residual hydrogen, this is typically discharged into the area of ​​a catalytic unit where the hydrogen is converted. This can be, for example, a catalytic burner, which is located in the exhaust air stream upstream of turbine 8, or the catalytic unit can be a separate catalyst or the catalyst present in the cathode chamber 5. The fuel cell system 1 shown in the figure also has two additional valves 15, 16. These are arranged in the airflow in the area of ​​the cathode supply air and the cathode exhaust air, respectively. They can be closed, in particular after the fuel cell system 1 has been switched off and after the residual oxygen in the cathode area has been used up, in order to prevent adverse effects on the fuel cell during restart. This is known from the prior art and is described, for example, in DE 10 2007 059 999 A1. Typically, control units are provided to control, for example, the air delivery system 6 and the turbine 8, as well as the electric motor 9 and the valves designated here as 15 and 16, and to control other components. Two such control units 17 and 18 are shown here as examples. The first control unit 17 is designed as a "cathode" control unit and controls the air supply to the cathode chamber 5 and the valves 15 and 16, which are optionally indicated here. The other control unit 18 is designed as an "anode" control unit and controls the hydrogen metering, the recirculation delivery system 13, and the drain valve 14. Of course, the control units can be combined or divided into further individual control units. It is also conceivable that these interact with other control units, particularly those of the vehicle, at a higher or lower level. The fuel cell system 1 described so far is known from the general state of the art. A problem arises when this fuel cell system 1 is switched off when the vehicle 2 is turned off. Typically, the fuel cell is switched off first, for example, in the manner described in the aforementioned DE 10 2007 059 999 A1. Other configurations and possibilities are, of course, also conceivable. The problem is that in all piping systems containing moist gases, there is a risk that moisture contained in the gas will condense, settle, and freeze solid at temperatures below freezing. Since the moisture in the area of ​​the fuel cell system 1 typically originates from the product water of the fuel cell 3, it is highly pure and therefore freezes immediately at temperatures below freezing. In particular, moving components in such moist gas flows, such as the valve bodies of valves 14, 15, and 16, but also rotating components like the fan or impeller in the recirculation conveying device 13, the turbine 8, and to a certain extent also rotary pistons or compressor impellers in the area of ​​the air conveying device 6, can be affected. Typically, there is no moist gas present in the area of ​​the air conveying device 6, or only moist gas under adverse conditions, so the problem mainly relates to the other moving components mentioned. If, after the fuel cell system is switched off at sub-zero temperatures, these components cool down to sub-zero temperatures and water is present in the vicinity of the components, then even a single freezing droplet can be enough to cause the moving component to freeze solid. For example, valve bodies can freeze to the valve seat, or in the area of ​​turbine 8 or the recirculation blower 13, the impeller can freeze solid, or at least droplets can freeze to the walls, thus blocking the free movement of the impeller. To prevent this, the following procedure is used: depending on a predetermined time interval after the fuel cell 3 is switched off and / or depending on a temperature, the moving components, in particular components 8, 13, 14, 15, and 16, are moved. The usual control units 17 and 18 can be used to control the respective components accordingly. For example, the electric machine 9 can be controlled to move the turbine 8, causing the turbine and typically the air conveying device 6, which is arranged on the same shaft, to move accordingly.This movement, especially when it occurs during the critical phase of the liquid water freezing in the fuel cell system 1, then safely and reliably prevents the moving components from freezing solid, so that they remain freely movable even at temperatures below freezing and can be used immediately in the event of a restart of the fuel cell system 1. To minimize the energy required for moving the components and to reduce the noise emissions, which are unavoidable during this process, the time interval after fuel cell 3 is switched off, after which the components are moved, is to be specified as precisely as possible. This interval can, for example, be based on estimated values ​​and thus simply define a time period. Additionally, this specification can be parameterized using temperature values ​​if necessary. For instance, an outdoor temperature sensor 19 can be used to determine whether temperatures are below freezing and whether such movement of the components is necessary. Alternatively or additionally, temperatures inside the fuel cell system 1 or in the area of ​​the moving components themselves can also be measured.Two temperature sensors 20 and 21 in the area of ​​the control units 17 and 18 are shown here as purely exemplary examples. In addition to simply monitoring the temperature, temperature changes can also be taken into account. For example, it is sufficient to measure the temperature inside the fuel cell system 1, for instance, using temperature sensor 21. A change in this temperature, i.e., the temperature gradient, allows conclusions to be drawn about a cooling gradient of the fuel cell system 1 and thus makes it possible to specify, with relative precision, the time interval after which the moving components must be moved. Alternatively or additionally, the movement can also be initiated solely based on temperature. This is particularly advantageous if the temperature is measured directly at the moving component itself, thus ensuring a reliable indication of when conditions conducive to freezing occur. The time period for which the moving components are to be moved can be specified almost arbitrarily. Experience has shown that typically even a very short period is sufficient to reliably prevent the moving components from freezing to walls, stationary counter-elements, or even moving counter-elements. The period can preferably be specified as a function of temperature or a temperature gradient, since different temperatures may require different durations to reliably prevent freezing. Different components may also require different durations. In general, however, just a few seconds are sufficient to reliably prevent freezing. Since the cooling of fuel cell system 1 cannot be predicted solely based on temperature and time, an operating history can also be included if necessary. This allows for parameterization of either the time span, the period, and / or the temperature threshold at which the moving components are activated. Such an operating history can, for example, consider the power drawn, the cooling load incurred during operation, and other parameters. This allows for the incorporation of information on both the amount of product water generated, and thus ultimately the humidity of the gas streams, and the expected cooling of the moving components into the most efficient and energy-optimized strategy for moving the components after fuel cell 3 is shut down.

Claims

Method for shutting down a fuel cell system (1) with at least one movable component (6, 8, 13, 14, 15, 16) which is arranged in a gas stream that is at least temporarily moist, in which a fuel cell (3) is shut down, wherein the at least one movable component (6, 8, 13, 14, 15, 16) is moved after the fuel cell (3) has been shut down, depending on the time since the fuel cell (3) was shut down and / or depending on the temperature, such that freezing of the movable component (6, 8, 13, 14, 15, 16) is prevented; characterized in that the at least one movable component (6, 8, 13, 14, 15, 16) is moved for a predetermined period of time. Method according to claim 1, characterized in that the at least one movable component (6, 8, 13, 14, 15, 16) is moved after a predetermined period of time from the switching off of the fuel cell (3). Method according to claim 2, characterized in that the time interval is specified depending on a temperature or a temperature change. Method according to claim 1, characterized in that the at least one movable component (6, 8, 13, 14, 15, 16) is moved below a predetermined limit temperature. Method according to claim 1, characterized in that the period is predetermined depending on a temperature or a temperature change. Method according to one of claims 1 to 5, characterized in that the temperature in the fuel cell system (1) is measured, in particular in the area of ​​the at least one movable component (6, 8, 13, 14, 15, 16). Method according to one of claims 1 to 6, characterized in that the period, time span and / or limit temperature is specified depending on the operating history of the fuel cell system (1) before the fuel cell (3) is switched off. Method according to one of claims 1 to 7, characterized in that at least one of the following components is moved as at least one movable component (6, 8, 13, 14, 15, 16): - valve body - turbine wheel - impeller of a flow compressor - fan of a blower - rotary piston of a rotary piston blower. Method according to one of claims 1 to 8, characterized in that the fuel cell system (1) to be switched off is a fuel cell system (1) in a vehicle (2) which is at least partially electrically driven with electrical power from the fuel cell system (1).

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