Abstract
A method for preparing a fuel cell system in a vehicle for starting, for which purpose a starting preparation routine is carried out after the vehicle has been shut down depending on a temperature limit value. In the method, in the event that the fuel cell had not reached its normal operating temperature during the previous operation and a temperature falls below the predetermined temperature limit value, the fuel cell system is operated until it has reached its normal operating temperature and after which the starting preparation routine is subsequently carried out.
Claims
1. A method for preparing a fuel cell system in a vehicle for starting, comprising the steps of: when the fuel cell system did not reach a normal operating temperature of the fuel cell system during a previous operation and a current temperature falls below a predetermined temperature limit value, operating the fuel cell system until the fuel cell system reaches the normal operating temperature and subsequently performing a starting preparation routine, wherein the starting preparation routine comprises drying the fuel cell system, wherein the starting preparation routine is performed only after the fuel cell system has cooled down to the predetermined temperature limit value after reaching the normal operating temperature.
2. The method according to claim 1, wherein the normal operating temperature is 60° C. to 70° C.
3. The method according to claim 1, wherein the predetermined temperature limit value is a value within a range of 0° C. to 10° C.
4. The method according to claim 1, wherein the current temperature is measured in an interior of the fuel cell system.
5. The method according to claim 4, wherein the current temperature is measured in a region of a fuel cell of the fuel cell system and/or in a region of a coolant for the fuel cell.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) FIG. 1 illustrates a vehicle with a fuel cell system suitable for carrying out the method according to the invention; and
(2) FIG. 2 is a diagram describing the method according to the invention on the basis of a temperature curve T over time t.
DETAILED DESCRIPTION OF THE DRAWINGS
(3) In the depiction in FIG. 1, a vehicle 1 is indicated in a highly schematic way. The vehicle 1 can, for example, be designed as a passenger car, a lorry, a rail-bound vehicle or an industrial vehicle for logistics purposes. The vehicle 1 could just as easily be used, for example, as a ship or an aircraft. In the vehicle 1, there is a fuel cell system 2 indicated in principle, the core of which is a fuel cell 3. This fuel cell 3 is to be constructed as a stack of PEM single cells, a so-called fuel cell pile or fuel cell stack. Symbolically, a cathode compartment 4 and an anode compartment 5 are indicated within the fuel cell 3. For regular operation, air is supplied to the cathode compartment 4 via an air conveyor 6 as an oxygen supplier. Exhaust air exits the fuel cell system 2 via an exhaust line 7. Hydrogen is supplied to the anode compartment 5 of the fuel cell 3 from a compressed gas storage unit 8 via a pressure control and metering unit 9. Unused hydrogen as well as inert gases and water which are produced in the area of the anode compartment 5 are returned via a recirculation line 10 and can be fed back to the anode compartment 5 mixed with fresh hydrogen. A recirculation conveyor 11 is arranged in the recirculation line 10, which in the exemplary embodiment depicted here is designed as a hydrogen recirculation blower (HRB). The recirculation conveyor 11 could just as well be implemented as a gas jet pump or as a combination of a gas jet pump and a blower. In addition, a water separator 12 is located in the recirculation line 10, which is connected to the exhaust air line 7 from the fuel cell system 2 via a drain line 13 with a valve device 14. Water can thus be collected via the water separator 12 and the valve device 14 and discharged from time to time, for example. It is just as well conceivable to drain the water depending on a filling level in the water separator or depending on concentrations in the so-called anode circuit. Together with the water, gas can also be discharged, since inert gas, which has diffused through the membranes of the fuel cell 3 from the cathode compartment 4 into the anode compartment 5, accumulates in the anode circuit over time. Since this would reduce the hydrogen concentration in the anode circuit, which is constant in its volume, this gas must also be discharged. This can be done via a separate line or together with the water via the drain line 13 and the valve device 14.
(4) In the depiction of FIG. 1, a connecting line 15 with a valve device 16 can now also be seen, which connects the anode circuit with an air supply line 17 to the cathode compartment 4 of the fuel cell 3. A connection between the cathode side and the anode side of the fuel cell system 2 can thus be created via the connecting line 15 when the valve device 16 is open. Depending on the arrangement and design, the connecting line 15 can also be used, for example, to discharge the gas in parallel with the discharge of water via the water separator 12 and the discharge line 13, in which case the branching point would typically be arranged between the water separator 12 and the recirculation feed device 11. The introduction of the discharged gas into the supply air line 17 is generally known and customary in this case, since in this way any hydrogen, which is typically always discharged along with it in small quantities, reacts on the catalyst of the cathode chamber 4, and hydrogen emissions to the environment can thus be avoided.
(5) The connecting line 15 with the valve device 16 can also be omitted in the event that the side of the anode compartment 5 is flushed with hydrogen during drying, which is then appropriately diluted by means of the air conveyed through the cathode compartment 4 when it is discharged into the environment via the exhaust line 7.
(6) The fuel cell system 2 in the vehicle 1 may further comprise a so-called system bypass 17, which makes it possible to connect the pressure side of the air conveying device 6 and the exhaust air line 7 via a bypass valve 8. In addition, an exhaust air turbine (not shown here) may be arranged in the exhaust air line 7, which may be mechanically connected to the air conveying device 6 and preferably to an electric machine as a motor/generator or only to a generator and via the latter electrically to a motor for the air conveying device 7. Such a structure is also known from the general prior art and is referred to as an electric turbocharger or motor-assisted turbocharger.
(7) A starting preparation routine is now carried out in a manner known per se by means of flushing the system and/or drying the system by heating and/or negative pressure as soon as the temperatures, for example the temperature in the environment or in particular the temperature in the fuel cell system, and very preferably in the region of the fuel cell stack, i.e., the fuel cell 3, fall below a predetermined temperature limit value of, for example, 5° C.
(8) In order to ensure that sufficient drying always takes place with a standardized starting preparation routine, which is largely independent of any input parameters and is therefore simpler than in the prior art, the following procedure is now followed. The process sequence can be seen in particular in FIG. 2 in the graph of temperature T versus time t. The first section between the point in time t.sub.0 and the point in time t.sub.1 describes a normal operation in which the operating temperature T, which can preferably be measured in a cooling water circuit of the fuel cell system 2, since temperature sensors are installed there anyway, fluctuates around an average value T.sub.1. The temperature value should, for example, be below the usually occurring temperature value T.sub.0, which occurs during normal operation of the fuel cell system 2 or of the vehicle 1, during a short-distance trip. It may be 40% for example. At the point in time t.sub.1, driving operation is ended and the system is shut down. It then cools down to a limit temperature T.sub.G, which is minimally above the freezing point of water, which is drawn here with a 0° C. line. If this temperature limit T.sub.G is reached during the course of the temperature T, in this case at the point in time t.sub.2, then a starting preparation routine is usually triggered in order to dry out the fuel cell system 2, because when the limit temperature T.sub.G is reached it must be assumed that the temperature will drop further and, in particular, will fall below the freezing point of 0° C. However, if the previous operation of the fuel cell system 2 was such that the normal operating temperature T.sub.0 was not reached at all, then no starting preparation routine is triggered at the point in time T.sub.2, but the fuel cell system 2 is started up while the vehicle 1 is stationary. Accordingly, it heats up during this automated operation starting from the limit temperature T.sub.G. It is operated in such a way that it reaches the normal operating temperature T.sub.0 of, for example, 65° C. for a certain time during the operating phase at the point in time t.sub.3. After this, the automatically started fuel cell system 2 is automatically stopped again, and cooling starts anew. At the point in time t.sub.4, the limit temperature T.sub.G of, for example, 5° C. is reached again. Now, however, cooling takes place as usual from the normal temperature level T.sub.0 of the fuel cell system 2. At the point in time t.sub.4, therefore, a system preparation routine starts which dries the fuel cell system 2 accordingly, in particular by one or more of the measures already described above and known from the prior art.
(9) In the exemplary embodiment shown here, the temperature then drops even further, in particular to a temperature well below the freezing point at the point in time t.sub.5, which is only minimally above the ambient temperature T.sub.a, which is also very low here. At the point in time t.sub.5, a freeze starting routine is then initiated to start the fuel cell system 2, in the course of which the temperature T rises relatively quickly back to the normal operating temperature T.sub.0 and the fuel cell system is ready for operation. This is possible without any problems due to the starting preparation routine started at the point in time t.sub.4, which has dried out the fuel cell system 2, since blockages due to ice can be efficiently prevented due to the procedure described.
