The invention concerns a method for starting the normal operation of an electrical system with a fuel cell and a converter receiving the electrical power of the fuel cell.
With fuel cell systems, in particular with fuel cell systems, which can be used to provide electrical driving power in vehicles, a so-called start-stop operation is meaningful, similarly to vehicles with combustion engines in the meantime. It means that in case when the vehicle needs no or very little electrical power, the fuel cell system changes to an idle mode or a switched-off operation, so-called stop mode, so as to re-start in normal operation, if again a matching quantity of power is necessary. Typically, the air supply to the fuel cell is stopped in stop mode, so that the residual oxygen reacts in the fuel cell with the hydrogen typically supplied at lower level, so that the voltage of the fuel cell drops to zero after a certain period of time. Before the fuel cell can again deliver power in its entirety after leaving the stop mode, it must first of all be supplied with air or oxygen again. Only then, the full power can be required by the fuel cell. In other words, normal operation of the fuel cell is only possible once all the conditions necessary to that end are met.
It is now usual from the state of the art to provide certain waiting times, which are necessary according to certain parameters after starting the air supply until it is possible to change into normal operation without causing voltage interruption further to premature and excessive power requirements.
Additionally or alternately thereto, further operation parameters, such as for example cathode pressure or air mass flow are monitored in the general state of the art, to influence the waiting time. The shortcoming of this possibility then consists in that the observed criteria are partially costly and unreliable to observe and to measure, that they only constitute partial factors in terms of stressability of the fuel cell, and that they however do not examine the load capacity itself. It may happen that the corresponding criteria are admittedly met, but the fuel cells are still not stressable. The result is an extremely undesirable voltage interruption, when power is required by the electrical system.
Document US 2012/0141895 A1 handles the problem to know as of when a fuel cell which again in normal operation can be loaded completely, after the system has re-started from a stop mode. To do so, the voltage of the fuel via a converter connected to the fuel cell, in such a case a DC/DC-converter, is maintained at a preset voltage level. In the meantime, the condition of the fuel cell system is monitored continuously and observed to that effect to know whether the monitored parameters stabilize themselves. The monitored parameters can be for example voltages and the supplied volume flow rate of air. If all the values have stabilized themselves accordingly, it is assumed that the fuel cell is again stressable and the normal operation is released.
Similarly to the general state of the art described above, the method has the shortcoming that it uses values with the air mass flow which on the one hand can be measured only with difficulty and not overly reliably and which on the other hand are admittedly indicators that the fuel cell is again stressable, but do not reflect this condition with certainty. Consequently, there is also the risk that in spite of stable values a voltage interruption crops up for an according load of the fuel cell.
The object of the present invention is then to provide a method for starting an electrical system with fuel cell and converter from a stop mode, which avoids these shortcomings and guarantees quite simply and reliably that the fuel cell works safely and reliably once the restart has been released under normal conditions.
In the method according to the invention, the interrupted reactant supply is conducted as in the state of the art, from a restart signal and a fuel cell voltage is prescribed, which is regulated accordingly by the transducer. It is provided according to the invention that the prescribed fuel cell voltage is prescribed in such a way that an electrical unloaded fuel cell supplied with reactants will exceed the prescribed fuel cell voltage in every case. It is also provided that the current of the transducer necessary for maintaining the prescribed fuel cell voltage is measured after which the normal operation is released as of a prescribed current necessary to that effect. The method according to the invention also uses an upper limit for a fuel cell voltage which is prescribed in such a way that the voltage is below the idle voltage of a functional fuel cell supplied with reactants. In particular, the prescribed fuel cell voltage can be preset in such a way that critical corrosion effects at their catalysts are avoided in terms of lifetime of the fuel cell. Current is drawn from the fuel cell via the transducer so as to maintain the voltage delivered by the fuel cell below the prescribed threshold, at the start of the reactant supply of the fuel cell. As soon as the current drawn and measured at the transducer reaches a preset value, which is necessary to maintain the current voltage at or below the prescribed voltage, the fuel cell should be stressable again. Once the prescribed current has been reached, the normal operation of the fuel cell is released, without having to fear shortcomings as regards the operation, in particular a voltage interruption due to an early load of the fuel cell after starting.
In a very advantageous further development of the idea of the invention it is moreover provided that the prescribed necessary current is preset approximately at half the average current typically cropping up at the upper voltage limit. Once such an amperage of the fuel cell has been reached, which corresponds approximately to half the average current cropping up typically in normal operation, the performance of the fuel cell is, to the best of the inventor's knowledge, unrestricted, so that the fuel cell is loaded accordingly and the normal operation can be released after restarting the fuel cell.
The fuel cell can be designed as a pile of individual cells, as generally known and normal. In an advantageous further development of the method according to the invention it is provided that the prescribed fuel cell voltage is preset with 800-900 mV, preferably 850 mV, per individual cell of the fuel cell pile. Such a voltage value in the order of magnitude of 800-900 mV per individual cell is ideal to guarantee on the one hand the operation of the fuel cell safely and reliably and on the other hand to minimize corrosion effects caused by higher voltages, whereas longer lifetime of the fuel cell can be reached.
According to a very favorable further embodiment of the method according to the invention, it can therefore be provided that the prescribed current related to the active cell face, from which the normal operation is released, is prescribed between 0.02 and 0.05 A/cm2, preferably between 0.03 and 0.04 A/cm2. Such a current value of 0.035 A/cm2 for example, has proven as ideal according to the experience and experiments of the inventor. The normal operation of the electrical system can thus be released as soon as it happens to maintain the voltage of the fuel cell at or below the prescribed voltage.
In a favorable embodiment of the method of the invention it is thus provided that the oxygen supply of the fuel cell is interrupted in stop mode, which oxygen supply can be realized by supplying the fuel cell with air. Such interruption of the oxygen supply, ideally when maintaining the hydrogen supply of the fuel cell, the requirement for the stop mode is ideal since the air supply device, comparatively energy-intensive and noise-emitting, is stopped or can be brought into idle mode, so that energy can be saved and emissions can be avoided in stop mode. If for instance a flow compressor is used as a pneumatic feeding machine, an idle rotational speed is ideally meaningful during stop mode, since it guarantees a very quick restart. Any postflowing compressed air can be blown out in such a case for instance via an additional valve so that the air is not guided into the fuel cell, so that no electrical power on the one hand is generated and the fuel cell is not dried out in stop mode on the other hand.
The method according to the invention is now ideal for electrical systems composed of fuel cell and the transducer, in particular fuel cell systems with a transducer, which are often operated in stop mode, for example to reduce energy consumption and emissions in phases, in which there is no power requirement or only a small one. Resorting to such start and stop strategies is in particular meaningful with vehicles so that the preferred usage of the method according to the invention sets forth that the electrical system is used to provide driving power for a vehicle.
Further advantageous embodiments of the method according to the invention can be derived from the remaining dependent sub-claims and which are described more in detail below in the light of the exemplary embodiment and with reference to the figures.