METHOD FOR OPERATING A FUEL CELL SYSTEM, AND FUEL CELL SYSTEM

Abstract

The invention relates to a method for operating a fuel cell system in which at least one fuel cell (1) is supplied with hydrogen via an anode path (2) and with oxygen via a cathode path, and in which anode exhaust gas exiting the fuel cell (1) is recirculated via a recirculation path (3), wherein steam contained in the anode exhaust gas is adsorbed by means of a zeolite container (4). According to the invention, the following steps are carried out in order to regenerate the zeolite container (4): a) separating the zeolite container (4) from the recirculation path (3) by closing at least one shut-off valve (5, 6) and/or switching a directional control valve (7), b) heating the zeolite container (4) by means of an electric heating device (8) such that previously adsorbed water is desorbed, and c) removing the desorbed water from the system by switching the directional control valve (7) again and/or by opening at least one flushing valve (9, 10). The invention additionally relates to a fuel cell system which is suitable for carrying out the method.

Claims

1. A method for operating a fuel cell system, in which at least one fuel cell (1) is supplied with hydrogen via an anode path (2) and oxygen via a cathode path, and in which anode exhaust gas escaping from the fuel cell (1) is recirculated via a recirculation path (3), wherein water vapor contained in the anode exhaust gas is adsorbed by means of a zeolite reservoir (4), wherein, for the regeneration of the zeolite reservoir (4), the following steps are carried out: a) separating the zeolite reservoir (4) from the recirculation path (3) by closing at least one shut-off valve (5, 6), and/or by switching a directional control valve (7), or both, b) heating the zeolite reservoir (4) by means of an electrical heating device (8), so that previously adsorbed water is desorbed, and c) removing desorbed water from the system by switching the directional control valve (7) again, and/or by opening at least one flushing valve (9, 10), or both.

2. The method according to claim 1, herein step a) is initiated when a maximum hydrogen concentration (X.sub.H2,.sub.max), and/or a maximum hydrogen partial pressure (p.sub.H2) is not reached in the recirculation path (3), or both .

3. The methodMethed according to claim 1, in step b), the zeolite reservoir (4) is heated to a temperature of about 250° C., at least one heating cartridge integrated into the zeolite reservoir (4) is used as an electrical heating device (8) for heating the zeolite reservoir (4), or both.

4. The methodMethed according to claim 1 wherein the pressure and/or the temperature in the zeolite reservoir (4) are measured, and, from the measured values, the amount of water desorbed in the zeolite reservoir (4) is deduced.

5. The method according to claim 1, wherein the heating of the zeolite reservoir (4) is ended when a prespecified maximum pressure and/or temperature limit value is reached in the zeolite reservoir (4).

6. The methodMethed according to claim 1 wherein, before step c) is initiated, preferably a check is made as to whether dilution conditions are present for opening a flushing valve (9, 10).

7. The methodMethed according to claim 1 wherein, in step c), desorbed water is introduced into a cathode exhaust gas path or discharged to the environment via the directional control valve (7) and/or the at least one flushing valve (9, 10).

8. The method according to claim 1 wherein, in step c), at least one shut-off valve (5, 6) is opened so that desorbed water from the zeolite reservoir (4) is routed to the at least one flushing valve (9, 10).

9. The methodMethed according to claim 1 wherein steps a) through c) are repeated wherein a first flushing valve (9) is opened during the first flushing, and a second flushing valve (10) is opened during repeated flushing.

10. A fuel cell system with at least one fuel cell (1), configured to be supplied with hydrogen via an anode path (2) and with oxygen via a cathode path, comprising a recirculation path (3) via which anode exhaust gas escaping from the fuel cell (1) can is recirculated, and also a zeolite reservoir (4) by means of which water vapor contained in the anode exhaust gas is adsorbed, wherein the zeolite reservoir (4) is connected or disconnected via at least one shut-off valve (5, 6) and/or a directional control valve (7).

11. The fuel cell system according to claim 10, wherein an electrical heating device (8), is integrated into the zeolite reservoir (4), so that the zeolite reservoir (4) can be heated for the desorption of water.

12. The fuel cell system according to claim 1, wherein the zeolite reservoir (4) is connected to a cathode exhaust gas path and/or to the environment via the directional control valve (7) and/or at least one flushing valve (9, 10), so that desorbed water from the zeolite reservoir (4) can be introduced into the cathode exhaust gas path or be discharged to the environment.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0040] The invention and its advantages are explained in more detail below with reference to the accompanying drawings. In the drawings:

[0041] FIG. 1 is a schematic view of a first fuel cell system according to the invention for carrying out the method according to the invention,

[0042] FIG. 2 shows the implementation of the method according to the invention with the aid of the fuel cell system of FIG. 1,

[0043] FIG. 3 is a schematic view of a second fuel cell system according to the invention for carrying out the method according to the invention,

[0044] FIG. 4 shows the implementation of the method according to the invention with the aid of the fuel cell system of FIG. 3,

[0045] FIG. 5 shows an alternative implementation of the method according to the invention with the aid of the fuel cell system of FIG. 3, and

[0046] FIG. 6 is a schematic view of a third fuel cell system according to the invention for carrying out the method according to the invention.

DETAILED DESCRIPTION

[0047] The fuel cell system shown in FIG. 1 comprises at least one fuel cell 1 which, on the anode side, can be supplied with an anode gas via an anode path 2, specifically with hydrogen from a tank 11. The supply of fresh hydrogen can be controlled via a valve 12. Since the anode exhaust gas escaping from the fuel cell 1 still contains a residue of hydrogen, it is recirculated via a recirculation path 3 or fed back into the anode path 2. In the present case, the recirculation is actively assisted by means of a recirculation fan 13. In the anode path 2, fresh hydrogen from the tank 11 is added to the recirculate.

[0048] Since the recirculated anode exhaust gas, in addition to hydrogen, also contains water, namely liquid and gaseous water or water vapor, the fuel cell system shown in FIG. 1 also has a zeolite reservoir 4 for dehumidifying the recirculate. The zeolite reservoir 4 is connected to or connected in parallel to the recirculation path 3 via a first shut-off valve 5 and a second shut-off valve 6 . In the open position of the shut-off valves 5, 6, recirculate flows through the zeolite reservoir 4, wherein the water contained therein is removed by means of adsorption. In this case, heat is produced, which can be used, for example, during a cold start of the system in order to bring the system up to operating temperature more quickly. In addition, the zeolite reservoir 4 can be heated by means of an integrated electrical heating device 8. This is advantageous in particular during a start under freezing conditions.

[0049] Since the anode gas can further accumulate nitrogen during operation of the fuel cell system, which, for example, diffuses from the cathode side (not shown) to the anode side, the anode path 2 and the recirculation path 3 must be flushed from time to time. For this purpose, a flushing valve 9 is provided on the outlet side, which preferably opens into a cathode exhaust gas path (not shown). The flushing quantity discharged via the flushing valve 9 is then replaced by fresh hydrogen from the tank 11.

[0050] The flushing valve 9 shown in FIG. 1 is also used in the present case for the regeneration of the zeolite reservoir 4. First, the zeolite reservoir 4 is separated from the recirculation path 3 and then brought to a temperature of about 250° C. by means of the electric heating device 8, so that previously adsorbed water is desorbed. The desorbed amount of water can then be introduced into the cathode exhaust gas path by opening the shut-off valve 6 and the flushing valve 9. As a rule, the heating and flushing of the zeolite reservoir 4 is repeated several times until the desorbed amount has been completely removed from the zeolite reservoir 4. If this is the case, the temperature and/or the pressure in the zeolite container 4 can be monitored. For this purpose, a temperature sensor 14 and a pressure sensor 15 are in each case provided in the zeolite reservoir 4.

[0051] The exact sequence of the adsorption and desorption phases of the zeolite reservoir 4 shown is explained below with reference to the diagram in FIG. 2.

[0052] Times t0 to t9 are plotted on the timeline. At time t0, the system requires water or water vapor to be removed from the recirculated anode exhaust gas and, if necessary, heat to be introduced, for example during a start under freezing conditions. The two shut-off valves 5, 6 are opened so that recirculated anode exhaust gas flows through the zeolite reservoir 4. At time t1, exothermic adsorption begins, wherein the zeolite reservoir 4 is heated until time t2 to about 160° C. Depending upon the requirement regarding dynamics and/or initial temperature, two operating modes can be differentiated: [0053] 1. without supply of electrical energy P.sub.electr. (solid line Tz), so that the zeolite reservoir 4 is heated solely via exothermic adsorption, and [0054] 2. with initial supply of electrical energy P.sub.electr. (dashed line Tz), so that the zeolite reservoir 4 is heated by exothermic adsorption and by the electrical energy P.sub.electr. supplied from the outside.

[0055] In principle, the kinetics of the adsorption process are sufficient to heat the zeolite reservoir 4, so that variant 1 can be followed. At particularly low outside temperatures, e.g., at -40° C., when the kinetics are very slow and the requirements of the system regarding the dynamic behavior of the zeolite reservoir 4 are otherwise not met, variant 2 proves advantageous.

[0056] At time t2, the system-side requirement to store water in the zeolite reservoir 4 is withdrawn, since, for example, no more water can be stored, or there is no longer any such requirement. Since the shut-off valves 5, 6 are still open, the flow through the zeolite reservoir 4 continues. This is because a suitable point in time is awaited for closing the shut-off valves 5, 6. This is arrived at, for example, when a maximum hydrogen concentration X.sub.H2,.sub.max in the zeolite reservoir 4 is not reached. In this way, hydrogen losses during the subsequent regeneration of the zeolite reservoir 4 can be kept low. After the closing of the two shut-off valves 5, 6 at time t.sub.shut-off, the gas composition in the zeolite reservoir 4 initially no longer changes.

[0057] At time t3, the zeolite reservoir 4 is to be regenerated. For this purpose, the zeolite material is heated to about 250° C. by means of the electrical heating device 8 in order to desorb water from the zeolite reservoir 4. Since both shut-off valves 5, 6 are closed, heat losses are kept at a minimum. The heating of the zeolite reservoir 4 is also possible independently of the system.

[0058] At time t4, the desorption temperature of 250° C. is reached, and the zeolite reservoir 4 releases the previously adsorbed water again to the volume of the zeolite reservoir 4 as water vapor. The result of this is that the pressure in the zeolite reservoir 4 rises, which can be used as a measured variable for the desorbed water quantity.

[0059] At time t5, a maximum pressure and/or a maximum temperature in the zeolite reservoir 4 is or are exceeded, so that the electrical heating device 8 is switched off. Furthermore, a query is made to the system as to whether the required dilution conditions (dilution constraint) are present for flushing the system. If there is a positive response, the shut-off valve 6 and the flushing valve 9 are opened, and the hydrogen/water vapor mixture is flushed out of the zeolite reservoir 4. In this phase, operation of the fuel cell system is not possible, or possible only to a limited extent.

[0060] After a first flushing operation, the shut-off valve 6 is closed again at time t6, and the processes of heating the zeolite reservoir 4 and flushing are repeated until the desired amount of water is expelled from the zeolite reservoir 4, and the regeneration of the zeolite reservoir 4 is completed. The characteristic behavior of the temperature rise and pressure rise in the zeolite reservoir 4 during the heating phase from t3 to t4 or from t6 to t7, etc., can be used as a termination criterion. This is because, with increasing regeneration of the zeolite reservoir 4, the pressure increases less strongly in comparison with the temperature.

[0061] The successive phases are denoted in FIG. 2 by A for adsorption phase, B for valve closing phase, C for heating phase, and D for desorption phase. This can be followed by phase E of shutdown, or further operation of the system.

[0062] FIG. 3 shows a modification of the system of FIG. 1. The modification consists of a further flushing valve 10 being provided, which opens into an additional flushing path 16. The further flushing valve 10 can be opened for the regeneration of the zeolite reservoir 4 independently of the first flushing valve 9, and thus independently of the operation of the fuel cell system. The further flushing valve 10 thus enables more degrees of freedom during operation of the fuel cell system.

[0063] The sequences during operation of the fuel cell system of FIG. 3 are shown in FIG. 4. The adsorption phase A, the valve closing phase B, and the heating phase C run analogously to the corresponding phases in FIG. 2, so that reference is made to the description of FIG. 2. Differences exist only with regard to the desorption phase D. The discharge of the hydrogen/water vapor gas mixture takes place here via the further flushing valve 10 into the additional flushing path 16. Here, too, certain dilution conditions must be observed, but these can differ from those mentioned above. Depending upon the design of the zeolite reservoir 4 and the choice of X.sub.H2,max, it is even possible to discharge the gas mixture directly to the environment.

[0064] Alternatively, a combined flushing strategy can also be applied with the aid of the system of FIG. 3. This is shown by way of example in FIG. 5. Here, both flushing valves 9, 10 are opened in the desorption phase D, and indeed with a time offset. At high hydrogen concentrations in the zeolite reservoir 4, flushing is initially carried out via first flushing valve 9 - at least during the first flushing operation. In the second and in each further flushing operation (with a low to negligible hydrogen concentration), flushing takes place via the further flushing valve 10. This strategy is optimal for ensuring the required dilution of the residual hydrogen. This is because, with the opening of the first flushing valve 9 and introduction of the flushing quantity into the cathode exhaust gas path (not shown), the flushing quantity mixes with the air present there. However, normal operation of the fuel cell system is interrupted or disturbed. It is therefore advantageous if, in the further course of regeneration of the zeolite reservoir 4, the flushing quantity is discharged via the further flushing valve 10 and the flushing path 16. This is because this process does not affect the operation of the fuel cell system.

[0065] A further modification of the fuel cell system according to the invention is shown in FIG. 6. Here, the functions of the shut-off valve 6 and of the further flushing valve 10 are realized by a 3/2-way control valve 7. The construction of the fuel cell system can thereby be simplified, since a valve is conserved.