METHOD FOR STARTING UP A FUEL CELL SYSTEM AFTER A STANDSTILL

Abstract

A fuel cell system is provided that includes a fuel cell with an assembly of multiple individual cells, each of which has an anode section, an electrolyte membrane, and a cathode section, an anode gas supply, which leads to an anode gas inlet and includes a fuel cell and a fuel metering device, a cathode gas supply, and a passive anode gas recirculation device, which connects an anode gas outlet to the recirculation gas inlet of a mixer arranged in the anode gas supply. The fuel cell system is started up after a standstill in that in a first phase, the fuel cell is activated while fuel is supplied from the fuel source, and the anode recirculation is suppressed without actively blocking the anode gas recirculation device, and in a second phase, anode gas is recirculated in addition to the supply of fuel from the fuel source.

Claims

1. A method for successfully powering up a fuel-cell system (1) after a stoppage, comprising providing: a fuel cell (1) having an arrangement of several individual cells provided respectively with an anode portion (7), an electrolyte membrane (11) and a cathode portion (9), an anode-gas supply that leads to an anode-gas inlet having a fuel source (25) and a fuel-metering device, a cathode-gas supply and a passive anode-gas recirculation device (21) that connects an anode-gas outlet with the recirculation-gas inlet of a mixer disposed in the anode-gas supply, the method having the following steps: in a first phase of powering-up (“initialization phase”), the fuel cell (3) is set in operation by feeding of fuel from the fuel source (25), wherein the anode-gas recirculation is suppressed, without active shutoff of the anode-gas recirculation device (21); and in a second phase of powering-up (“consolidation phase”) following the first phase in time, anode-gas recirculation takes place in addition to the feed of fuel from the fuel source (25).

2. The method of claim 1, wherein the consolidation phase directly follows the initialization phase.

3. The method of claim 1, wherein the mixer is realized by a jet pump (13).

4. The method of claim 1, wherein the jet pump (13) is operated in the initialization phase of powering-up without suction effect at the recirculation-gas inlet of the mixer.

5. The method of claim 1, wherein the equipment-related configuration of the anode-gas recirculation device (21) is identical in the initialization phase and in the consolidation phase of powering-up of the fuel-cell system.

6. The method of claim 1, wherein the anode-gas recirculation device (21) comprises a passive closure device (51), which is closed in the initialization phase and in contrast is opened in the consolidation phase.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0013] The present invention will be explained in more detail hereinafter on the basis of two exemplary embodiments illustrated in the drawing, wherein

[0014] FIG. 1 shows a schematic diagram of a first fuel-cell system suitable for carrying out the invention, wherein the fuel cell is symbolized on the basis of one of its individual cells,

[0015] FIG. 2 shows a schematic diagram of a second fuel-cell system suitable for carrying out the invention and

[0016] FIG. 3 shows, in an enlarged diagram, the passive closure device implemented in the fuel-cell system according to FIG. 2 in closed as well as in opened position.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] FIG. 1 schematically shows a fuel-cell system 1 suitable for carrying out embodiments of the invention. It comprises in particular a fuel cell 3 - symbolized by one of its individual cells - and a fuel-metering device in the form of a jet pump control-valve unit 5. Fuel cell 3 has, in conventional manner, an anode chamber 7, a cathode chamber 9 and an electrolyte membrane 11 separating anode chamber 7 and cathode chamber 9 from one another. Jet-pump control-valve unit 5 comprises a jet pump 13 - forming a mixer -and a fuel-gas control valve 15, and is connected via a suction port 17 and a pressure port 19 to anode chamber 7. It is used for metered charging of anode chamber 7 with fuel gas and, depending on operating phase and mode (see below), for recirculation of an anode gas via an anode-gas recirculation device 21. For this purpose, the fuel gas present under high pressure in fuel source 25 first passes an opened shutoff valve 27, before its pressure is reduced in a pressure regulator 29. Under control of fuel-gas control valve 15, the fuel gas, forming the propellant gas, then flows into jet pump 13, i.e. into its propellant jet nozzle.

[0018] A control unit C of the fuel-cell system acts in particular on the fuel-gas control valve 15. Via the corresponding effect, the feeding of fuel to jet pump 13 may be varied in multiple respects. Firstly, the fuel throughput (averaged over time), i.e. the average quantity of fuel per unit time, is adjustable. Secondly, the characteristic of the fuel supply is adjustable, and specifically within a considerable bandwidth. This ranges from a steady, continuous flow of fuel gas through fuel-gas control valve 15, which flow can be adjusted to different flow rates, to pulsed flow behavior with different frequency, different relation of the duration of opening and closing phases relative to one another as well as different opening and/or closing characteristics (e.g. rectangular profile, triangular profile, sawtooth profile, wave profile, etc.). By appropriate influence on the flow of fuel gas through fuel-gas control valve 15, it is possible to influence the suction behavior of jet pump 13, and, in fact, specifically in such a way that, in a first phase of powering-up (“initialization phase”), the fuel cell is set in operation with feeding of fuel from the fuel source, wherein, for lack of sufficient suction behavior of jet pump 13, recirculation of anode gas through anode-gas recirculation device 21 is suppressed and does not take place, in contrast to which, in a second phase of powering-up (“consolidation phase”) that takes place in time after the first phase, recirculation of anode gas through anode-gas recirculation device 21 takes place in addition to the feeding of fuel from fuel source 25, as a result of sufficient suction behavior of jet pump 13. In the consolidation phase, the fuel gas stream in the mixing chamber of jet nozzle 13 entrains - just as later in power operation of the fuel cell, after completion of powering-up - anode gas, which is sucked in through suction port 17 and mixed with (fresh) fuel gas to form mixed gas. The mixed gas exits jet pump 13 through pressure port 19 and flows past safety valve 35 and through an (optional) first condensate separator 37, before it flows into anode chamber 7 of fuel cell 3 through an anode-chamber inlet 39. In the region of anode-chamber inlet 39, state parameters of the mixed gas (e.g. temperature, pressure, gas-mixing ratio) relevant to control and operation are recorded by means of a sensor 41. The anode gas sucked out of anode chamber 7 through an anode-chamber outlet 43 passes a (second) condensate separator 45 used for separation of condensation water and flows past a flush valve 47, which permits removal of foreign gases (e.g. nitrogen) accumulated in the anode chamber. Condensation water collected in the first condensate separator 43 or second condensate separator 45 if such are provided may be drained via a condensate drain valve 49.

[0019] The fuel-cell system according to the second exemplary embodiment illustrated in FIG. 2 differs from that according to FIG. 1 only by an additional passive closure device 51 provided in anode-gas recirculation device 21. This comprises, as shown - partly schematically with respect to the size relationships - in FIG. 3, a housing 53 having an inlet 55 and an outlet 57. Inside housing 53, an arrangement of several of flexible rings 59 resembling angled cup springs in their shape is mounted together with an closure cap 61, which springs - in the absence of any noteworthy underpressure acting at outlet 57 - are held in contact with one another by means of a very soft spring 63 (shown at left). This inner chamber 65, bounded in sealingly closed manner in this way by the arrangement of rings 59 and closure cap 61, is in fluidic communication with inlet 55, whereas outer chamber 67 surrounding the said arrangement on the outside is in fluidic communication with outlet 57. If, due to appropriate operation of jet pump 13 (see above), a noteworthy underpressure develops at suction port 17 of jet pump 13, which is in fluidic communication with outlet 57 of closure device 51, the annular gaps between rings 59 are opened. A very large passage area for the recirculation gas is created abruptly, so that this is able to flow through anode-gas recirculation device 21 without noteworthy flow resistance. The recirculation flow that develops exerts suction - directed against the closing force of spring 63 - on closure cap 61, so that closure device 51 maintains its completely opened passing position (shown on the right) even with pressure conditions fluctuating within certain limits. For better clarity, guide and stop elements associated with rings 59 and cap 61, which bound the opening paths between the rings 59 and one another, between housing 53 and the ring adjacent thereto, and between cap 61 and the ring adjacent thereto and ensure guidance for the ring arrangement, are not illustrated.