The invention relates to a method for flushing a fuel cell system during a power-down cycle.
Flushing fuel cell systems when they are powered down is generally known. As a result, the fuel cell is dried, for example by means of flushing out the cathode side with air, and flushing out the anode side with air or hydrogen, such that, when the temperature later drops to below the freezing point, the fuel cell cannot freeze, or at least not to such a great extent, and can accordingly be restarted more easily and quickly. Even at temperatures above the freezing point, drying out the fuel cell before restarting it, or when it is powered down, has crucial advantages in view of later restarting the cell, because this prevents the active surfaces from being wetted with condensed moisture, and thus prevents the active surfaces from being inhibited. The same applies to distribution channels, which can likewise be inhibited by drops of condensed liquid.
By way of example, reference can be made in this connection to DE 10 2007 026 330 B4, in which, in order to dry out the fuel cell, a flow compressor conveys air in part through the fuel cell itself and in part around the fuel cell in a “system bypass”, so as to always allow for sufficient air for diluting the hydrogen that is released on the anode side.
It can be a problem if unexpectedly high concentrations of hydrogen escape and there are therefore correspondingly low volumetric flow rates of air. In the cited document, these values are precisely specified, such that, for safety reasons, operation must always occur using a maximum amount of air, which is highly undesirable with regard to both the energy requirement of the air conveying device and the noise emissions thereof.
DE 603 07 595 T2 describes a method in which the amount of air is adjusted to the hydrogen concentration in the exhaust gas. However, this requires the air conveying device to be controlled in a corresponding manner, so as to always be able to provide a sufficient amount of air, according to the detected hydrogen concentration in the exhaust gas of the anode side. This is a crucial disadvantage in particular in a flow compressor, since, owing to the typically very high speeds thereof and the time required to change the speeds, the control of flow compressors is generally subject to delay, and the amount of air conveyed by the flow compressors, i.e., the air mass or the volumetric flow rate of air, cannot be adjusted very dynamically. A further disadvantage is that the noise emissions of an air conveying device of which the speed continuously changes are significantly higher than those of an air conveying device that operates at a constant speed, as in the document cited above, even if the speed has to be higher in this case.
The object of the present invention is that of providing a method for flushing a fuel cell system when it is powered-down, which method prevents the aforementioned disadvantages and further improves the methods according to the prior art.
In the method according to the invention, the outlet valve is cyclically closed and opened during power-down. The opening duration depends in this case on the amount of air that is conveyed. The outlet valve is therefore cyclically opened and closed during the entire power-down procedure of the fuel cell system, during which procedure air is conveyed by the air conveying device. The fact that the cyclical opening and closing is adjusted depending on the amount of air that is conveyed, i.e., a measured air mass or a measured volumetric flow of air, or even an amount of air estimated from the speed and/or driving power of the air conveying device, for example, makes it possible to achieve a maximum opening duration of the valve for the amount of air that is conveyed, without exceeding the critical limit values of hydrogen emissions. A maximum opening duration of the valve that is suitable for the corresponding amount of air can thus be achieved by adjusting the opening duration of the outlet valve. This results in the maximum possible amount of water being discharged from the anode side of the fuel cell.
A further crucial advantage of the method according to the invention is that the air, which is conveyed by the air conveying device and which correspondingly dilutes the emissions from the anode side in the region of the exhaust air line, has previously passed through the cathode side of the fuel cell and also dried out the cathode side, such that the entire structure of the fuel cell is safely and reliably dried during the power-down procedure.
At the same time, it is ensured that the hydrogen concentration in the region of the anode side of the fuel cell is as high as possible after the power-down cycle, since inert gases are also released through the opened valve. A hydrogen concentration of this type, which is as high as possible, at the anode side is a crucial advantage for the system that is later powered down, because restarting the fuel cell system when hydrogen is present at the anode side preserves the fuel cell much more effectively than restarting when oxygen from the air is present at the anode side. The “H2 protection time” is thus extended.
In order to achieve this as effectively as possible, according to an advantageous development of the concept, when the amount of air is larger, the opening duration is extended, and when the amount of air is smaller, the opening duration is correspondingly reduced, in order to achieve, on average, sufficient dilution, in each case, of the hydrogen that is typically also released at the anode side.
According to a very advantageous development of the method according to the invention, the conveyed air can also be conveyed, at least in part through a system bypass, from the delivery side of the air conveying device directly to the intake side of the turbine in the exhaust air line. A system bypass, which is typically present in any case, being used in this way reduces the amount of air which has to flow through the cathode side of the fuel cell, and the amount of air which optionally flows through other structures, such as humidifiers and the like. As a result, the pressure losses, and thus the amount of energy required to provide the air are reduced during the power-down procedure. In this case, appropriately positioning a valve device in the system bypass can ensure that a sufficient amount of air for drying flows through the cathode side and optionally through the humidifier, and also that there is a sufficient amount of air available for diluting any hydrogen emissions that may be present in the exhaust air line.
Another advantage of decreasing the amount of air that is guided through the cathode side of the fuel cell is that the voltage of the individual cells is typically limited during the power-down procedure. It is therefore necessary to continue to draw some load from the fuel cell. The limit on the voltage of the individual cells allows corrosion within the cells to be reduced, which has a positive effect on the service life of the fuel cell. The larger the amount of air in the cathode side, the more heavily the fuel cell has to be loaded with a suitable electrical load. At the same time, however, the increasing loading of the fuel cell produced more water, which is counter to the aim of drying the fuel cell. This problem can be mitigated by reducing the air mass that is guided through the cathode side by means of the system bypass.
Furthermore, according to another, very favorable embodiment of the method according to the invention, the air conveying device can also be operated at a constant speed. In particular, such a use of the air conveying device operated at a constant speed, and, according to the advantageous development described above, it being possible to adjust, if necessary, the amount of air flowing through the cathode side by means of the system bypass, allows a marked reduction in the disruptive acoustic emissions and in the vibrations caused by the air conveying device. Specifically, constant operation at a specified speed produces considerably less noise and fewer vibrations than would be the case if the speed of the air conveying device were continuously adjusted. Moreover, a sound absorber can ideally be provided on the air conveying device when it is operated at a constant speed, such that the noise emissions are minimized. At the same time, compared to dynamically changing operation, operation at a constant speed reduces the energy requirement of the air conveying device.
Further advantageous embodiments of the method and of the fuel cell system are explained by an embodiment which is described in greater detail below with reference to the drawing.