METHOD FOR SHUTTING DOWN A FUEL CELL SYSTEM

Abstract

A method for shutting down a fuel cell system (2) having at least one fuel cell (3), which fuel cell comprises an anode chamber (10) and a cathode chamber (6), wherein after the shut-down hydrogen remains in the anode chamber (10) of the fuel cell (3) in order to prevent carbon corrosion and to ensure a hydrogen protection time. The invention is characterized in that when the hydrogen in the anode chamber (10) is largely used up directly or after a specified number of subsequent meterings of hydrogen at least into the anode chamber (10), the hydrogen protection time is actively terminated by air being actively admitted into the cathode chamber (6), the fuel cell (3) being actively cooled before air is actively admitted into the cathode chamber (6).

Claims

1. A method for shutting down a fuel cell system (2) having at least one fuel cell (3), which fuel cell comprises an anode chamber (10) and a cathode chamber (6), the method comprising shutting down the fuel cell system (2) wherein after the shut-down hydrogen remains in the anode chamber (10) of the fuel cell (3) in order to prevent carbon corrosion and to ensure a hydrogen protection time, actively terminating the hydrogen protection time by actively admitting air into the cathode chamber (6) when the hydrogen in the anode chamber (10) is largely used up directly or after a specified number of subsequent meterings of hydrogen at least into the anode chamber (10), wherein the fuel cell (3) is actively cooled before air is actively admitted into the cathode chamber (6).

2. The method according to claim 1, wherein the active cooling takes place until the temperature falls below a predetermined limit value and/or a predetermined period of time has elapsed.

3. The method according to claim 2, wherein the predetermined temperature limit value and/or the period of time are predetermined as a function of the ambient temperature or an expected ambient temperature at the time of active ventilation.

4. The method according to claim 1, wherein the fuel cell system (2) is used in a vehicle (1), wherein active cooling takes place after the vehicle (1) has been parked.

5. The method according to claim 1, wherein the fuel cell system (2) is used in a vehicle (1) together with an electrical energy store (24), wherein active cooling takes place before the vehicle (1) is parked when it approaches a destination, the vehicle (1) being supplied with power from the electrical energy store (24) during cooling.

6. The method according to claim 5, wherein an approach to a destination is detected via a navigation system, wherein programmed or learned destinations are used.

7. The method according to claim 5, wherein before the active cooling takes place, the electrical energy store (24) is charged by the fuel cell (3).

8. The method according to claim 1, wherein the number of subsequent meterings is specified as a function of a counter which counts the number of starts without active ventilation of the cathode chamber (6).

9. The method according to claim 1, wherein the number of subsequent meterings is predetermined depending on the ambient temperature or on the basis of an expected variation in the ambient temperature over time.

10. A method according to claim 4, wherein the number of subsequent meterings is predetermined as a function of the expected parking time, the expected parking time being estimated as a function of the destination, the time, the date and/or taking into account the vehicle user's calendar entries.

11. The method according to claim 1, wherein the active cooling commences when a system temperature drops to 35° C.

12. The method according to claim 1, wherein the active cooling takes place until a system temperature drops to 20° C.

13. The method according to claim 1, wherein the active cooling takes place when a system temperature reaches ambient temperature.

14. The method according to claim 1, wherein the active cooling commences at a time 10-15 hours after system shutdown.

15. The method according to claim 1, wherein the active cooling takes place prior to the end of the hydrogen protection time.

Description

[0024] Further advantageous refinements and developments of the method also result from the exemplary embodiment, which is explained in more detail below with reference to the figures.

[0025] These show:

[0026] FIG. 1 a graph of the concentrations over time in a first method according to the prior art;

[0027] FIG. 2 a graph of the concentrations over time in a second method according to the prior art;

[0028] FIG. 3 an exemplary fuel cell system which is suitable for carrying out the method according to the invention; and

[0029] FIG. 4 a graph of the concentrations over time in a method according to the invention;

[0030] FIG. 5 a graph of measured carbon dioxide peak values over temperature; and

[0031] FIG. 6 a flow chart of a possible embodiment of the method according to the invention.

[0032] FIGS. 1 and 2 show the methods according to the prior art. FIG. 1 shows a graph of different concentrations c, which are plotted logarithmically on the y-axis, specifically over time t in hours on the x-axis. The solid line c(O.sub.2) stands for the oxygen concentration in the fuel cell, the dash-dotted line c(H.sub.2) for the hydrogen concentration. In order to record the cathode corrosion, the dashed line 3 also shows the concentration c(CO.sub.2). The method in the illustration of FIG. 1 essentially corresponds to the method described in the prior art mentioned at the outset, in which the hydrogen is only enclosed in the anode chamber and, initially, no additional hydrogen subsequent metering takes place. This method can be used to achieve a hydrogen protection time up to the point in time t.sub.1, which is on the order of magnitude of approximately 10 hours, for example. After that, the concentration of carbon dioxide increases accordingly, which is an indication of increased carbon corrosion. The maximum CO.sub.2 peak value is around 4,200 ppm in the graph shown here. The fuel cell can cool down during the hydrogen protection time up to time t.sub.1. The cooling behavior is shown in the illustration of FIG. 1 below by the temperature T plotted over the same time axis t. The temperature can be, for example, the inlet temperature into the anode chamber. The differences are minimal, the quantitative behavior of all temperature curves measured in the fuel cell system or the fuel cell is essentially the same. The cooling takes place relatively quickly at first, and then very slowly thereafter. The system will therefore remain at a temperature of, for example, 30-35° C. for a very long period of time, for example when the vehicle is parked in a garage.

[0033] During the hydrogen protection time, air-hydrogen fronts are prevented by the excess pressure of hydrogen in the anode, and hydrogen can diffuse to the cathode. A positive side effect is that the hydrogen in the cathode chamber can reduce platinum oxide produced during operation and thus expose the catalyst surface in the cathode chamber again. If the hydrogen is largely consumed, the hydrogen protection time ends at time t.sub.1, since the hydrogen has, for example, escaped through leaks or has reacted with oxygen to form water. In this phase, there is now a passive transition from the hydrogen protection time to the subsequent time, during which in-plane currents form in the surface along the electrolyte membrane and slowly corrode the carbon carrier of the platinum catalyst and thus adversely affect the useful life of the fuel cell. In this phase, a maximum carbon dioxide concentration of approximately 4,200 ppm is recorded as a measure of the carbon corrosion that has occurred.

[0034] Nevertheless, there are positive side effects even in this phase. Oxygen that enters into the anode chamber not only causes corrosion there, but can also oxidize absorbed impurities and thus expose the surface of the catalyst in the anode chamber again.

[0035] The illustration of FIG. 2, which also shows a method according to the prior art and which uses the same division of the axes, shows the method in which a subsequent metering of hydrogen is made at time t.sub.1. The hydrogen protection time is thus extended until time t.sub.2. Between the times t.sub.1 and t.sub.2, the fuel cell can cool down further and is also protected from the corrosive potentials, in particular at elevated temperatures of the fuel cell, by the subsequently metered hydrogen. Here, too, air-hydrogen fronts are further prevented by the excess pressure of hydrogen, so that overall shutdown periods up to time t.sub.2 in the order of magnitude of 10 to 20 hours, for example approximately 14 to 16 hours, are possible before the formation of carbon dioxide, shown here again in dashed lines, suggests increasing carbon corrosion. As long as the restart of the fuel cell takes place within the period of time up to the point in time t.sub.2, this method is well-suited to effect a start that is protective of the useful life of the fuel cell by restarting within the hydrogen protection time. If not, a passive end of the hydrogen protection time occurs from time t.sub.2 onwards, comparable to the graph in FIG. 1 from time t.sub.1 onwards, and thus the feared carbon corrosion, which can again be measured here by a CO.sub.2 peak on the order of magnitude of approximately 4,200 ppm, this value representing a measure of the carbon corrosion that has occurred.

[0036] In the illustration of FIG. 3, a schematically indicated vehicle 1 is shown, which is to receive its electrical drive energy via a fuel cell system 2, likewise shown schematically. The core of the fuel cell system 2 is formed by a fuel cell 3, which is constructed as a stack of individual cells using PEM technology, a so-called fuel cell stack. Each of the individual cells comprises a cathode chamber 6 and an anode chamber 10, which are designed to be separated from one another by means of corresponding flow fields and gas distributors as well as an electrolyte membrane. Air is supplied to the cathode chamber 6 as an oxygen supplier via an air conveying device 4 and an air supply line 5. Unused exhaust air reaches the environment in regular operation via an exhaust air line 7 and an exhaust air turbine 8. In the exhaust air turbine 8, part of the pressure and thermal energy in the exhaust air is recovered. The exhaust air turbine 8 is in operative connection with the air conveying device 4 in order to drive it in a supporting manner. An electric machine 9, which is likewise in operative connection with the exhaust air turbine 8 and the air conveying device 4, can now be used on the one hand to drive the air conveying device 4, in particular to deliver the supporting drive power when the exhaust air turbine 8 does not deliver sufficient power, which will be the normal operating case. If there is an excess of power at the exhaust air turbine 8, the electric machine 9 can also be operated as a generator.

[0037] Hydrogen is fed to the anode chamber 10, which, in the exemplary embodiment shown here, flows from a pressurized gas reservoir designated 11 via a pressure regulating and metering valve 12 and a hydrogen feed line 13 to the anode chamber 10. Hydrogen that has not been consumed reaches the hydrogen feed line 13 via a recirculation line 14 and a recirculation conveying device 15, which is designed here as a fan, and is fed back to the anode chamber 10, mixed with fresh hydrogen. This structure is known from the prior art and is referred to as the anode circuit. As an alternative to the fan shown here, the recirculation conveying device 15 can also be designed as a gas jet pump or as a combination of a fan and a gas jet pump.

[0038] Over time, water and inert gas collect in the anode circuit. The water is separated off via a water separator 16. Via a valve device 17, the water and inert gas that has accumulated in the anode circuit can be drained off via a drain line 18, for example from time to time or depending on the hydrogen concentration in the anode circuit. The exhaust gas laden with the water passes after the exhaust air turbine 8 into the part of the exhaust air line 19 there. This structure is known from the prior art, also with regard to its functionality. In practice, it will include additional components such as an intercooler, a humidifier or the like. These are of secondary importance for an understanding of the present invention and are therefore not shown. Nevertheless, they can be present accordingly, as those skilled in the art of fuel cell systems will appreciate.

[0039] A valve device 20, 21 is arranged in the supply air line 5 as well as optionally in the exhaust air line 7. Via these, the cathode chamber 6 can, if necessary, be blocked from a flow of air, in particular when the fuel cell system 2 is shut down. Preferably, only the valve device 20 is present in the supply air line 5, since this is less critical with regard to freezing.

[0040] In the illustration of FIG. 3, an electrical energy store 24 is also indicated in the vehicle 1, for example a battery. Like the fuel cell 3, this is electrically connected to power electronics 25, which supply the required electrical power P to the vehicle 1 in order to drive the vehicle 1 and to cover the energy requirements of secondary consumers.

[0041] In the illustration of FIG. 4, the sequence of the method according to the invention can again be seen in a graph with logarithmically plotted concentrations over the time t plotted in hours and the temperature curve T plotted below it over time t. The sections up to time t.sub.1 correspond to those of FIG. 1, the section up to time t.sub.2 to that of FIG. 2 and thus to the prior art. A special process now starts at time t.sub.2. Instead of a further subsequent metering of hydrogen, which would also be conceivable here once or twice, there is an active termination of the hydrogen protection time. For this purpose, the cathode chamber 6, which was previously blocked via the valve device 20 or, if present, via the valve device 20 and 21, is actively ventilated. The valve device 20 and 21 is opened for this purpose. The air conveying device 4 can in principle be operated in order to ventilate the cathode chamber 6 accordingly and to supply it with oxygen in order to actively terminate the hydrogen protection time. In order to be able to dispense with the operation of the relatively complex air conveying device 4 designed, for example, as a flow compressor, a fan 22 and/or a compressed air reservoir 23 can optionally be provided as an alternative. These are also suitable for ventilating the cathode chamber 6 at time t.sub.2 and thus actively terminating the hydrogen protection time.

[0042] Before the hydrogen protection time is actively terminated, the fuel cell system 2 or the fuel cell 3 is actively cooled. This can be seen from the temperature curve in FIG. 4 at the bottom: At point in time t.sub.1′, there is an active cooling, for example from a temperature level in the order of magnitude of 35° C. to a temperature level below 20° C. The active cooling of at least the fuel cell 3 of the fuel cell system 2 thus takes place before the termination of the hydrogen protection time. It can also take place much earlier, for example directly after parking or even before the vehicle 1 is parked. For this purpose, the vehicle 1 can be operated with energy from the battery or the electrical energy store 24, while the then inactive fuel cell 3 is cooled down. In order to prepare for this and to ensure a sufficient energy content of the electrical energy store 24, a full charge of the electrical energy store 24 can take place, for example, depending on a programmed or learned destination and the current position of the vehicle 1, which can be determined via GPS, for example in a navigation system, in order to then to have sufficient energy to supply the vehicle 1 with power P and to actively cool the fuel cell 3.

[0043] As can be seen in the graph in FIG. 4, this active termination causes a significant increase in the oxygen concentration according to the solid line with a simultaneous decrease in the hydrogen concentration according to the dash-dotted line. Here, too, carbon corrosion inevitably occurs, as indicated by the concentration of carbon dioxide with the dashed line. However, it is now recognizable, although the logarithmic scaling of the y-axis makes it much less visible than it would be with a linear representation, that the maximum amount of carbon dioxide, i.e. the CO.sub.2 peak, is significantly lower than with the prior art methods shown in FIGS. 1 and 2. This would already be the case even without active cooling of the fuel cell 3, as is known from the German patent application 10 2018 008794.9, which was not previously published.

[0044] By actively cooling the fuel cell 3, this CO.sub.2 peak can be reduced even further. The measured CO.sub.2 peak depends on the temperature of the fuel cell 3. The graph shown in FIG. 5 shows, by way of example, three measured values of the CO.sub.2 peak using the example of a method according to FIG. 1 at different temperatures of 20° C., 35° C. and 40° C., for example. The CO.sub.2 peak is reduced from the above-mentioned values in the order of magnitude of 4200 ppm at 40° C. to approximately 4000 ppm at 35° C. and ultimately to less than 3000 ppm at 20° C. If the active termination of the hydrogen protection time is now combined with a previous cooling of the fuel cell 3, this then leads to a further significant reduction in the CO.sub.2 peak when the hydrogen protection time is actively terminated.

[0045] Measured on the same fuel cell stack on which the test values in FIGS. 1 and 2 are based, the CO.sub.2 peak here without active cooling equaled approximately 1,100 ppm, and with active cooling only approximately 800 to 900 ppm, i.e. on the order of magnitude of less than a quarter of the CO.sub.2 peak in the prior art methods. By actively terminating the hydrogen protection time, carbon corrosion can thus be significantly reduced.

[0046] For this purpose, a gas exchange is actively carried out in the cathode chamber 6 at point in time t.sub.2, in that it is correspondingly ventilated, for example, via a small fan 22 that can be operated from the starter battery. This gas exchange then also leads to a change in the anode chamber 10 through permeation, as a result of which oxygen-hydrogen fronts, which slowly develop, are avoided. By avoiding the slowly developing oxygen-hydrogen fronts or air-hydrogen fronts, the high level of carbon corrosion from the passive termination of the hydrogen protection time can be significantly reduced, which is a decisive advantage, especially if, as mentioned above, the active termination does not take place in every shutdown period of the fuel cell system 2 or of the vehicle 1, but only if the restart was not carried out within the hydrogen protection time that can be provided with a few subsequent meterings.

[0047] As already mentioned, however, an air/air start from time to time, especially if it can take place without a critical, slowly developing oxygen-hydrogen front, can be advantageous in order to remove contamination such as, for example, accumulated carbon monoxide from the catalyst of the anode chamber 10. For this reason, a counter can be used by means of which the number of starts without active ventilation of the cathode chamber 6, that is to say without an actively terminated hydrogen protection time, is captured. After a specified number of times, the subsequent metering can be stopped or the number reduced in order to achieve an air/air start that is desired in this case. In addition, the temperature is taken into account accordingly here, so that the air/air start, regardless of whether it is desired or if the hydrogen protection time is terminated after (repeated) subsequent metering, can take place at a low temperature and with a previously cooled fuel cell system 2.

[0048] One possibility for implementing the method accordingly therefore results from the flowchart shown in FIG. 6. If such a counter is available, the count variable is used to determine whether an air/air start is necessary or not. If this is not the case, it is checked whether the hydrogen protection time is likely to be exceeded, which can be estimated, for example based on the location, learned parking times, the times of day, calendar entries of the vehicle user or the like. If this is not the case, the hydrogen protection time is extended until the restart, if necessary, also by a higher number of hydrogen subsequent meterings.

[0049] If it is to be expected that the hydrogen protection time will be exceeded, it is checked in the same manner as in the case of an affirmed air/air start whether a longer parking time is planned. If this is the case, the battery is charged while driving, especially in the penultimate section of the drive. In the last section of the drive, the electrical energy store 24, for example a battery, is then used as the only source for the power P of the vehicle 1 and the passively switched fuel cell 3 is actively cooled. If a longer parking period is not foreseen, this step is saved. After shutdown, it is then checked whether the fuel cell 3 is below the predetermined temperature value for active cathode chamber ventilation when it is shut down. If this is the case, no further cooling has to take place and the active termination of the hydrogen protection time is initiated, if necessary, after a predetermined number of hydrogen subsequent meterings. If this is not the case, the fuel cell is actively cooled again beforehand in order to ensure that the temperature of the fuel cell 3 is as low as possible for the active termination of the hydrogen protection time that then takes place.