Method for starting the normal operation

Abstract

A method for starting the normal operation of an electrical system with a fuel cell and a transducer from a stop mode is disclosed. The transducer absorbs the electrical power of the fuel cell, in which at least one reactant supply of the fuel cell was interrupted, where the interrupted reactant supply is resumed from a restart signal, and where a fuel cell voltage is prescribed and then regulated by the transducer. The prescribed fuel cell voltage is prescribed in a way that an electrical unloaded fuel cell supplied with reactants will exceed the prescribed fuel cell voltage in every case, and the current of the transducer necessary for maintaining the prescribed fuel cell voltage is measured, where the normal operation is released as of a prescribed current necessary to that effect.

Claims

1. A method for determining a start of a normal operation of an electrical system with a fuel cell and a transducer from a stop mode, comprising the steps of: receiving electrical power of the fuel cell by the transducer; interrupting a supply of reactant to the fuel cell; resuming the interrupted supply of reactant from a restart signal; and prescribing and regulating a fuel cell voltage by the transducer; wherein the prescribed fuel cell voltage is prescribed such that an electrical unloaded fuel cell supplied with reactants will exceed the prescribed fuel cell voltage, wherein the normal operation is determined to be started when a fuel cell current increases above a prescribed fuel cell current, and wherein the prescribed fuel cell current is set as approximately half an average fuel cell current in the normal operation at the prescribed fuel cell voltage.

2. The method according to claim 1, wherein the fuel cell consists of a pile of individual cells.

3. The method according to claim 2, wherein the prescribed fuel cell voltage is 800-900 mV per individual cell.

4. The method according to claim 1, wherein the transducer is a DC/DC-converter or a battery converter.

5. The method according to claim 1, wherein the step of interrupting is performed in the stop mode.

6. The method according to claim 1, wherein the reactant is air.

7. The method according to claim 1, wherein the electrical system supplies driving power for a vehicle.

8. A method for determining a start of a normal operation of an electrical system with a fuel cell and a transducer from a stop mode, comprising the steps of: receiving electrical power of the fuel cell by the transducer; interrupting a supply of reactant to the fuel cell; resuming the interrupted supply of reactant from a restart signal; and prescribing and regulating a fuel cell voltage by the transducer; wherein the prescribed fuel cell voltage is prescribed such that an electrical unloaded fuel cell supplied with reactants will exceed the prescribed fuel cell voltage, wherein the normal operation is determined to be started when a fuel cell current increases above a prescribed fuel cell current, and wherein the prescribed fuel cell current is between 0.02 and 0.05 A/cm.sup.2.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a vehicle indicated as a matter of principle with a fuel cell system; and

(2) FIG. 2 shows a diagram with signal curves when starting the normal operation of such a fuel cell system from a stop mode.

DETAILED DESCRIPTION OF THE DRAWINGS

(3) The illustration of FIG. 1 shows an implicitly indicated vehicle 1, which must be driven via an electrical travelling motor 2. The electrical travelling motor 2 drives in the representation illustrated here purely by way of example two drive wheels of the vehicle 1 designated with 4 and connected via a driven shaft 3. The electrical power for driving the vehicle 1 is delivered by an electrical system in the form of a fuel cell system 5. The fuel cell system 5 is indicated in the general normal and preferred form of embodiment represented here, purely by way of example. The core of the fuel cell system 5 consequently consists of a fuel cell 6 which is built-up typically as a pile of individual cells, as a so-called fuel cell pile or fuel cell stack. This fuel cell stack includes an anode side and a cathode side whereas a common anode chamber 7 and a common cathode chamber 8 are shown purely by way of example in the representation of FIG. 1. The anode chamber 7 is supplied with hydrogen from a compressed gas reservoir 9 via a pressure regulating and dosing unit 10. Non-consumed hydrogen returns in a manner known per se via a recirculation pipe 11 with a recirculation supply device 12 and is fed to the anode chamber 7 mixed with fresh hydrogen. The assembly is also designated as an anode circuit. Complementary and alternate to the recirculation supply device 12 illustrated as a blower, a gas jet pump could also be envisioned where the jet pump is driven by the fresh hydrogen gas from the compressed gas reservoir 9. To be able to discharge enriched water and enriched inert gases in the anode circuit from time to time, the recirculation pipe 11 moreover includes a water separator 13 with an exhaust valve 14. The assembly and its operating strategy are the known from the general state of the art so that it is not necessary to go into further details.

(4) The cathode chamber 8 of the fuel cell 6 is supplied with air via a pneumatic feeding machine 15 as an oxygen supplier. The air thus flows into the cathode chamber 8 via an optional humidifier 16 in which it is humidified. Humid exhaust air, depleted of oxygen, leaves the cathode chamber 8 and flows back via the optional humidifier 16, so as to discharge the humidity comprised therein, at least partially to the air intake. It then flows into the surrounding atmosphere through a turbine 17. The turbine 17 forms together with an electrical machine 18 and the pneumatic feeding machine 15 a so-called electrical turbo charger which is designed for the best energy-efficient air supply of the fuel cell 6. The outlet of the pneumatic feeding machine 15 and the inlet to the turbine 17 can again be connected together via a system bypass 19 with a system bypass valve 20, so that the system bypass valve 20 can be open in certain situations so as to avoid or to limit the ingress of air into the cathode chamber 8, even if the pneumatic feeding machine 15 is still running. This can be the case due to a very high rotational speed shown by the pneumatic feeding machine designed as a flow compressor in regular operation, when in overrun.

(5) The electrical power of the fuel cell 6 is received by a transducer 22 via the electrical lines 21 suggested here. The transducer 22 which can be designed by way of example as a DC/DC-converter or as a battery converter. The converter is in contact with an optional high volt battery 23 as an electrical energy storage device. It is moreover in contact with further power electronics 24, by way of example a DC/AC converter which is formed to provide driving power for at least the travelling motor 2.

(6) Such a fuel cell system 5 in a vehicle 1 is now often driven in such a way that it is used with a so-called start-stop strategy. If the vehicle 1 does not require, or hardly, any driving power which can be made available without any problems via the battery 23, for example when the vehicle is driving uphill and is stopped at a red light, then the fuel cell system 5 is switched into a so-called stop mode to save energy and to reduce the emissions of noise in these stop phases.

(7) Typically, the air supply of the fuel cell 6 is to do so interrupted while the hydrogen supply continues at reduced level. The residual oxygen in the fuel cell 6 then reacts with the still present hydrogen at least partially, according to the length of the stop phase so that the fuel cell voltage U.sub.BZ falls to zero at least after a certain time t. This is suggested accordingly in the right-hand section designated with A, in the top diagram of FIG. 2, which shows the voltage U.sub.BZ and the current I.sub.BZ of the fuel cell 6 over the time period t. The voltage U.sub.BZ is hence illustrated with a solid line in section A at zero. The alternative dotted representation, at which the stop phase has not lasted as long, shows the still persisting drop of voltage almost to zero up to the end of phase A. The diagram represents the condition of the stop mode below the diagram with voltage U and current I. The condition is set on one in the area A, the fuel cell system 5 is thus in stop mode. The requirement of the stop mode changes from one to zero when switching from area A to area B, which at the end of the day corresponds to a restart signal for the fuel cell system 5.

(8) Accordingly, the interrupted air supply is resumed and the fuel cell 6 supplied with air increasingly. Simultaneously, a fuel cell voltage U.sub.BZ,1 is prescribed, which is slightly lower than the idle voltage U.sub.OCV of the supplied fuel cell 6. The voltage, which for example is in the order of magnitude of 850 mV per individual cell of the fuel cell 6, is regulated accordingly by the transducer 22 inasmuch as it draws the current I.sub.BZ, represented by the dash-dotted line from the fuel cell 6. In order to maintain the fuel cell voltage U.sub.BZ at or below the prescribed voltage value U.sub.BZ,1, the current must rise accordingly as the supply of the fuel cell 6 with oxygen and hydrogen increases. Should the fuel cell current I.sub.BZ increases above a prescribed value I.sub.BZ,1, the system switches from restart operation B into normal operation C of the fuel cell 6, in which current I.sub.BZ and voltage U.sub.BZ are adjusted by the vehicle 1 according to the power requirements.

(9) Typically, a fuel cell current I.sub.BZ of approx. 10 A as an average current I.sub.BZ during normal operation can take place with a fuel cell 6 composed of approx. 350-500 individual cells. The prescribed current value 1.sub.BZ,1 as of which the normal operation C is released again is set more or less at half the value, i.e., around 5 A, which corresponds to a current density of around 0.035 A/cm.sup.2.

(10) As soon as the fuel cell current I.sub.BZ, which is necessary to maintain the fuel cell voltage U.sub.BZ below the prescribed voltage U.sub.BZ,1, exceeds the prescribed current value 1.sub.BZ,1, the complete performance of the fuel cell system 5 is again available so that the normal operation C can be released. No voltage interruption is expected any longer when the fuel cell 6 is subjected to higher stress so that safe and reliable release of the normal operation is possible via simple current measurement.