Method for operating gas reservoirs

Abstract

The invention relates to a method (100) for operating gas reservoirs. In particular, the invention relates to a method for filling gas reservoirs and removing gas from gas reservoirs, in particular for filling gas reservoirs and removing gas from gas reservoirs for use in a vehicle, wherein the gas is used as fuel for operating the vehicle. The invention also relates to a system of gas reservoirs for carrying out the method. According to a first aspect, the stated problem is solved by a method for removing a fuel from a pressure vessel system (10) having a first pressure vessel (11) with a first volume (V1) and a second pressure vessel (12) with a second volume (V2), wherein the first volume is smaller than the second volume, and wherein either at least part of the fuel is initially removed from the first pressure vessel, before the fuel is removed from the second pressure vessel, if the fuel in the second pressure vessel has a temperature (ϑ2) outside a previously defined permissible temperature window (TB), or at least part of the fuel is initially removed from the second pressure vessel, before the fuel is removed from the first pressure vessel, if the fuel in the second pressure vessel has a temperature within a previously defined temperature window, wherein the removal of fuel from the second pressure vessel is stopped and switched to the first pressure vessel as soon as the temperature of the fuel from the second pressure vessel leaves the previously defined temperature window.

Description

[0001] METHOD FOR OPERATING GAS STORAGE SYSTEMS Field of the Invention The invention relates to a method for operating gas storage systems. In particular, the invention relates to a method for filling gas storage systems and removing gas from gas storage systems, especially for filling gas storage systems and removing gas from gas storage systems for use in a vehicle, wherein the gas serves as fuel for operating the vehicle. The invention further relates to a device comprising a system of gas storage systems and a control unit for carrying out the method. Background of the Invention In recent years, interest in the use of alternative fuels in the automotive industry has increased enormously. For this reason, more and more vehicles powered by gaseous fuels such as natural gas, LPG, or hydrogen are coming onto the market. This applies to all types of vehicles, such as motor vehicles,This applies particularly to passenger vehicles, but also to commercial vehicles such as trucks or buses, or even rail vehicles, as well as watercraft, aircraft, and spacecraft. Furthermore, increasing production of natural gas and fracking gas, especially in countries without a corresponding pipeline network, necessitates storage in pressure vessels. Currently used cylindrical pressure vessels have a reinforcement layer made of fiber composite material embedded in a matrix material, which is wound as an outer layer onto an inner container (the so-called liner) of the pressure vessel, which acts as the winding core. While the inner container, for example, guarantees the gas-tightness of the pressure vessel,The reinforcement layer made of fiber composite material provides the pressure vessel with the necessary mechanical stability. Type 3 pressure vessels use a mechanical inner vessel (metallic liner), e.g., made of aluminum or steel; Type 4 pressure vessels have a plastic inner vessel (liner). However, pressure vessels are also known that consist of only one or more metallic materials. Pressure vessels are filled with gas. Since, especially at hydrogen filling stations, very high pressures and low temperatures prevail, and hydrogen is highly explosive in combination with oxygen,must meet these high requirements. Furthermore, filling stations must ensure safety when filling vehicles and filling various containers one after the other. The SAE TIR J 2601 guideline describes the refueling procedure for hydrogen-powered vehicles. Accordingly, the communication capability of the vehicle to be filled or its hydrogen tank is first determined. The hydrogen tank does not have to consist of a single tank. On the contrary, it is common for the hydrogen tank to consist of a system of interconnected individual tanks. The system usually has a check valve, although it is also common for each tank in a system with several individual tanks to have a check valve. A distinction is made between vehicles that can communicate various parameters via an electronic interface,such as the pressure and temperature of the hydrogen in the consumer's tank to the filling station, and vehicles that do not communicate electronically, but instead transmit the pressure in the tank to the filling station through a pressure query, i.e., applying pressure to the entire system. The hydrogen filling station then applies a pressure pulse. This pressure pulse opens all check valves, allowing pressure to be exchanged between the vehicle and the filling station. This pressure pulse corresponds to the pressure query for vehicles that do not communicate electronically. Due to the pressure exchange, the starting pressure for refueling the respective vehicle is established. A leak test of the system is then carried out to ensure safety. After this, the filling procedure begins, the parameters of which,In particular, the filling pressure is selected in a ramp-like manner depending on the starting pressure. Today, the average pressure rise rate method is commonly used, which determines a pressure rise rate (max. approx. 60 g hydrogen / s) depending on the starting temperature and pressure, so that the filling time depends on the tank size. The maximum operating pressure, especially when filling a pressure vessel or pressure vessel system with hydrogen, can be, for example, between 200 and 1200 bar, typically between 350 and 875 bar, depending on the application. A fuel cell or internal combustion engine can then be supplied with hydrogen from this pressure vessel or pressure vessel system. During filling, the gas flowing into the pressure vessel or pressure vessel system heats up due to compression. If a fuel cell or internal combustion engine is to be started up after a pressure vessel or pressure vessel system has been filled,If the hydrogen is to be extracted from this pressure vessel or pressure vessel system, the tank pressure must be throttled down to the pressure of the fuel cell or combustion engine. This pressure is typically between 8 and 12 bar in the case of a fuel cell, and approximately 30 bar in the case of a combustion engine. The pressure reduction is usually achieved with a pressure reducer. Due to the throttling and the associated Joule-Thomson effect at the pressure reducer, the temperature of hydrogen, for example, increases depending on the pressure difference between high and low pressure. This temperature increase can be in the range of up to 40 K. If the temperature downstream of the pressure reducer exceeds the permissible limits of the valves or the fuel cell, operation must be interrupted until the gas has cooled sufficiently. The permissible upper limit temperature of a fuel cell, i.e., the maximum inlet temperature of the fuel gas,is typically approximately 85 °C. The filling gas does cool down during withdrawal, for example, to feed it to a fuel cell. Especially when a pressure vessel or pressure vessel system is to be filled quickly with hydrogen, this cooling is not sufficient, at least not shortly after filling, to bring the filling gas temperature below the upper limit temperature. Since the vehicle must be ready to drive immediately after filling, maintaining a corresponding waiting time for cooling is not desirable. Therefore, cooling devices are usually provided to cool the filling gas after filling to a temperature below the limit temperature. On the other hand, it can happen, for example, in very cold ambient temperatures, that the filling gas cools down too much due to the cooling during withdrawal. Fuel cells only operate efficiently within a certain temperature range.They can no longer operate below a lower limit temperature. Therefore, it is common practice to temperature-control fuel cells during operation, i.e., to heat or cool them as needed. However, if a vehicle with a fuel cell is put into operation in a very cold environment after a certain period of inactivity, it is first necessary to wait until the temperature control has heated the fuel cell above the lower limit temperature and the filling gas, which has cooled to a very low temperature due to the inactivity in the very cold environment, has warmed up sufficiently that it does not cool the fuel cell temperature below the lower limit temperature again during operation. Therefore, a minimum gas inlet temperature into the fuel cell is usually defined, which the storage system must maintain. This is, for example, -40°C. If the inlet temperature is too low, the fuel cell must be shut down.until the gas inlet temperature has reached the permitted range. Undesirable waiting times also arise in this case. Furthermore, it is naturally desirable for the gas inlet temperature to be as close as possible to, or even above, the limit temperature in order to minimize the fuel cell's energy requirement for tempering the gas to operating temperature. A further disadvantage of tempering the fuel cell and / or filler gas is that this tempering itself is complex and consumes energy during operation. Summary of the Invention The present invention is based on the object of providing an improvement or alternative to the prior art. According to a first aspect, the stated object is achieved by a method for removing a fuel from a pressure vessel system having a first pressure vessel with a first volume and a second pressure vessel with a second volume,wherein the first volume is smaller than the second volume, and wherein either the fuel is first at least partially withdrawn from the first pressure vessel before the fuel is withdrawn exclusively from the second pressure vessel, provided that the fuel in the second pressure vessel has a temperature outside a previously defined permissible temperature window, or the fuel is first at least partially withdrawn from the second pressure vessel before the fuel is withdrawn exclusively from the first pressure vessel, provided that the fuel in the second pressure vessel has a temperature within a previously defined temperature window, wherein the fuel withdrawal from the second pressure vessel is stopped and switched to the first pressure vessel as soon as the temperature of the fuel from the second pressure vessel leaves the previously defined temperature window. This also makes it possible,that the fuel is first extracted from both pressure vessels before the fuel is extracted exclusively from the first or second pressure vessel. The following terminology is explained in this regard: It is expressly pointed out that, within the scope of this patent application, indefinite articles and indefinite numerical expressions such as "one...", "two...", etc., are generally to be understood as "at least one...", "at least two...", etc., unless the context or the specific text of a particular passage indicates that only "exactly one...", "exactly two...", etc., are intended. Furthermore, all numerical expressions as well as information on process parameters and / or device parameters are to be understood in the technical sense, i.e., subject to the usual tolerances. Even the explicit specification of the restriction "at least" or "at least" or similar must not be taken to meanthat the simple use of "one," i.e., without specifying "at least" or similar, means "exactly one." The terms "fuel," "filling gas," "gas," "fuel," etc. are used synonymously in this document. The invention can generally be used for the extraction of gaseous substances from a pressure vessel system. However, the invention can be used particularly advantageously when hydrogen is used as fuel for operating a fuel cell or for combustion in a corresponding internal combustion engine to power a vehicle. In this document, a "pressure vessel system" is understood to mean a plurality of interconnected pressure vessels. For example, hydrogen-powered vehicles typically have pressure vessel systems.The interconnected pressure vessels may have different volumes. Hydrogen pressure vessels must withstand high internal pressures of up to 1,200 bar and therefore usually have a spherical shape or a cylindrical central section, with the cylindrical central section closed by pole caps. For reasons of space, several pressure vessels are usually combined in vehicles to form a pressure vessel system, whereby the individual pressure vessels may have different volumes and are usually filled via a common filling line. The hydrogen is extracted via an extraction valve, with each pressure vessel usually having a separate extraction valve. The general term "vehicle" in this document refers to any vehicle, such as a motor vehicle, in particular a passenger car, or a commercial vehicle, such as a truck, van,Bus or agricultural vehicle, or also a rail-bound vehicle, as well as a watercraft, aircraft or spacecraft. Furthermore, the term should also include correspondingly powered work machines. The filling process, especially in vehicles, is also called "fueling". A "pressure reducer" is understood to be a pressure valve for installation in a hose or line system, which ensures that a certain output pressure is not exceeded on the output side despite different pressures on the inlet side. The pressure on the inlet side is also referred to as "high pressure" in this document, while the pressure on the outlet side is also referred to as "low pressure". The "Joule-Thomson effect" refers to the temperature change of a real, i.e. non-ideal,Gas during an isenthalpic pressure reduction. The direction and strength of the effect is determined by the strength of the attractive and repulsive forces between the gas molecules. Under normal conditions, for most gases and gas mixtures, e.g., air, the temperature drops upon expansion. In contrast, for hydrogen, for example, it increases. A "temperature window" refers to a temperature range between a lower and an upper limit temperature. When a pressure vessel is filled with a gas, the gas heats up due to the compression work performed. The compression work performed is proportional to the volume of the pressure vessel. The volume of a pressure vessel with an essentially cylindrical center section, in turn, is directly proportional to its length and disproportionately to its diameter, with the diameter contributing quadratically to the volume. Pressure vessel systems typically have a check valve, although it is also common tothat if there are several individual pressure vessels in a pressure vessel system, each pressure vessel has a check valve. When a pressure vessel system is filled, the prevailing pressure in the pressure vessel system is first queried. To do this, all check valves must be opened, for example, via a pressure surge on the entire system. For this purpose, all pressure vessels in the pressure vessel system are connected to a common filling line and are therefore usually filled in parallel during filling. If a pressure vessel system has a first pressure vessel with a first volume and a second pressure vessel with a second volume, where the first volume is smaller than the second volume,Thus, less compression work is performed in the first pressure vessel than in the second pressure vessel. As a result, the filling gas in the first pressure vessel heats up less than that in the second pressure vessel. Furthermore, the surface area to volume ratio is greater in the first pressure vessel than in the second pressure vessel. In other words, the pressure vessel with the smaller volume has more surface area relative to its volume than the pressure vessel with the larger volume. However, heat is exchanged with the environment via the surface. Since the filling gas usually heats up above the ambient temperature of the pressure vessel due to the compression work performed,Thus, more heat is dissipated from the filling gas to the environment in the first pressure vessel than in the second pressure vessel. The permissible temperature range for the vessel must not be exceeded. In addition, the temperature of the filling gas after the pressure regulator must not be outside the permissible range for the fuel cell. The two temperature ranges may differ if necessary. If the fuel in the second pressure vessel has a temperature outside a previously defined permissible temperature window, in particular a higher temperature than the upper limit temperature due to the compression work performed, which for example is 85 °C for conventional fuel cells, it may be advantageous to first withdraw at least partially from the first pressure vessel before also or exclusively withdrawing the fuel from the second pressure vessel. This allows the fuel in the second pressure vessel to have time toto cool below the upper limit temperature via heat exchange with the environment. At the very least, this allows any existing cooling device for the fuel to be dimensioned smaller. Furthermore, energy required for cooling the fuel can be minimized. In certain cases, it may even be possible to dispense with active fuel cooling entirely. Trucks, in particular, require long ranges with one filling, which leads to large fuel volumes being carried. On the other hand, the space available for the installation of pressure vessel systems is limited, so every available space is utilized for the installation of a pressure vessel, with the pressure vessels having different volumes, particularly due to different pressure vessel diameters.With the diameter ratios of pressure vessels in pressure vessel systems installed in trucks, the effects described result in temperature differences of 10 K to 25 K between the fuel in the pressure vessel with the largest volume and that in the pressure vessel with the smallest volume. If the fuel in the second pressure vessel has a temperature within a previously defined temperature window, the fuel can alternatively be initially at least partially withdrawn from the second pressure vessel before the fuel is also or exclusively withdrawn from the first pressure vessel, whereby the fuel withdrawal from the second pressure vessel can be stopped and switched to the first pressure vessel as soon as the temperature of the fuel from the second pressure vessel leaves the previously defined temperature window. This can be particularly advantageous in the casewhen a vehicle is to be refueled at very low ambient temperatures, where a pressure vessel system is installed in the vehicle, and where the vehicle is to be put into operation in a short time after the filling process, for example, immediately after the filling process. Such a scenario can occur, for example, in very cold regions of the world, such as Alaska. Ambient temperatures of -30°C are not uncommon there. The compression work performed during filling heats the fuel, but it also cools down quickly again due to the effects described above, particularly in the first pressure vessel. By first removing the fuel from the second pressure vessel, the vehicle can now be operated.While a temperature control device, if present, also keeps the fuel temperature in the first pressure vessel safely within the previously defined temperature range. This can ensure safe operation of vehicles even immediately after refueling, even under such extreme environmental conditions. These conditions can also be met by simultaneously withdrawing fuel from all vessels. Generally, the temperature of the filling gas in a large pressure vessel is higher than that in a smaller pressure vessel. To reduce the filling gas temperature in the larger vessel, the smaller pressure vessel can be at least partially filled after the larger one has been filled. Withdrawal from the respective pressure vessels can be stopped.when the respective temperature threshold is reached. In an advantageous embodiment of the method according to the invention, the first pressure vessel is filled with fuel from the second pressure vessel after being at least partially emptied. The filling of the first pressure vessel can take place without the temperature restrictions of the consumer, for example, a fuel cell or an internal combustion engine. As described above, temperatures in the first pressure vessel can adjust to the previously defined temperature range more quickly than in the second pressure vessel. This embodiment can be particularly advantageous when more than two pressure vessels with different volumes are present in the pressure vessel system, since a first pressure vessel with a smaller volume can always be filled from a second pressure vessel with a larger volume.wherein, at the same time, fuel can be removed from another first pressure vessel. In a further advantageous embodiment of the method according to the invention, the removal of fuel from the first pressure vessel or the second pressure vessel is controlled by a control unit, wherein the pressure vessels each have a removal valve and the control unit controls the removal valves. The control unit receives information about the temperature of the fuel in the first pressure vessel and in the second pressure vessel and compares these temperatures with a previously defined temperature window. Optionally, the temperature information can also be measured downstream of the pressure regulator or calculated by the control device using a model and included in the evaluation. The pressure vessels usually each have a removal valve.wherein each extraction valve in the pressure vessel system can be controlled separately. In this way, the method according to the invention can be easily implemented. According to a second aspect, the stated object is achieved by a device for carrying out the method according to the invention, wherein the device comprises a pressure vessel system with a first pressure vessel having a first volume and with a second pressure vessel having a second volume, wherein the first volume is smaller than the second volume, wherein the first pressure vessel and the second pressure vessel each have an extraction valve, and the device has a control unit configured to carry out the method according to the invention, wherein the extraction valves can be controlled by the control unit,wherein information about the temperature of the fuel in the first pressure vessel and in the second pressure vessel can be received by the control unit. For example, the pressure vessels can have temperature sensors that can transmit the temperature to the control unit. The control unit can be installed in the vehicle independently of the pressure vessel system in a space-saving manner. In an advantageous embodiment of the device according to the invention, the first pressure vessel and the second pressure vessel each have a substantially cylindrically shaped segment, wherein the substantially cylindrically shaped segment of the first pressure vessel has a diameter,which is smaller than the diameter of the substantially cylindrically shaped segment of the second pressure vessel. Since the compression work performed is proportional to the volume of the pressure vessel, and the volume of a pressure vessel with a substantially cylindrical central section is in turn directly proportional to its length and disproportionately large to its diameter, with the diameter contributing quadratically to the volume, a greater effect can be achieved by changing the diameter than by varying the length of the pressure vessel. Regarding the surface of the pressure vessel, the effect is also greater by changing the diameter than by varying its length. In a further embodiment of the device according to the invention, the control unit has a data processing system,wherein the data processing system comprises means for carrying out the method according to the invention. A data processing system can ensure the automated implementation of the method at high speed. The previously described embodiments can be used individually or combined with one another as desired, even deviating from the claims. Detailed description of the invention The invention will be explained in more detail below using exemplary embodiments with reference to the drawings. Therein, Fig. 1 shows a flow diagram of the method 100 according to the invention; Fig. 2 shows a pressure vessel system 10 according to the invention. Fig. 1 shows a flow diagram of the method 100 according to the invention. The method 100 starts at the process start 110 with the commissioning of a consumer (not shown) to be supplied with fuel from a pressure vessel system 10, for example a vehicle with a fuel cell.The fuel is hydrogen, which is stored in the pressure vessel system 10. In the fuel cell, electricity is generated from the hydrogen, for example for an electric motor to drive the vehicle. The pressure vessel system 10 has a first pressure vessel 11 and a second pressure vessel 12 (see also Fig. 2). The temperature of the fuel in the second pressure vessel 12 is 12. The consumer, for example a fuel cell, requires the fuel at a temperature that lies within a consumer-specific temperature window TB, wherein the temperature window TB is defined by a lower limit temperature 12g,u and an upper limit temperature 12g,o. For a fuel cell, these temperatures are approximately 70°C for the lower limit temperature 12g,u and approximately 85°C for the upper limit temperature 12g,o. In the method according to the invention, immediately after the start in step 120, the following is queried:whether the fuel temperature ^2 in the second pressure vessel 12 lies within the previously defined temperature window TB, i.e., whether the fuel temperature ^2 in the second pressure vessel 12 lies between the lower limit temperature ^g, u and the upper limit temperature ^g, o, whereby the temperatures at the outlet of the pressure reducer can be queried: ^g, u ^ ^2 ^ ^g, o If this is the case, the Yes output Y of the query is selected, and in step 300 the fuel is removed from the second pressure vessel 12. If the fuel temperature ^2 in the second pressure vessel 12 is not between the lower limit temperature ^g, u and the upper limit temperature ^g, o, the No output N is selected in step 120, and in step 200 the fuel is removed from the first pressure vessel 11. Step 130 continually queries whether the process should be stopped, i.e., whether power generation in the fuel cell should be stopped. If this query is answered with Yes,After the "yes" output Y in step 140, the fuel extraction process is stopped. If, however, the question is answered with "no", the "no" output N is selected and the process continues with step 120, i.e., the query as to whether the fuel temperature ^2 in the second pressure vessel 12 is within the previously defined temperature window TB. It is also possible, if the temperature is predicted to fall below the limit temperature in the future, to reduce the power of the fuel cell and thus prevent or at least postpone shutdown. Fig. 2 shows a pressure vessel system 10 according to the invention. A first pressure vessel 11 and a second pressure vessel 12 are installed in a vehicle chassis 20. The vehicle chassis 20 is shaped such that the second pressure vessel 12 has a diameter D2 and the first pressure vessel 11 has a diameter D1.wherein the first diameter D1 is smaller than the second diameter D2. The first pressure vessel 11 has a first volume V1 and the second pressure vessel 12 has a second volume V2, wherein the first volume V1 is smaller than the second volume V2. If the fuel in the second pressure vessel 12 has a temperature ^2 outside of a permissible temperature window TB previously defined by a lower limit temperature ^g,u and an upper limit temperature ^g,o, in particular a higher temperature ^2 than the upper limit temperature ^g,o due to the compression work performed during filling, fuel is first at least partially removed from the first pressure vessel 11 before fuel is removed from the second pressure vessel 12. This allows the fuel in the second pressure vessel 12 time to cool below the upper limit temperature ^g,o to cool. At least in this way, any existing cooling device (not shown) for the fuel can be made smaller. Furthermore, energy required for cooling the fuel can be minimized. In certain cases, it may even be possible to dispense with active fuel cooling entirely. The embodiments shown here represent only examples of the present invention and should therefore not be understood as limiting. Alternative embodiments contemplated by those skilled in the art are equally encompassed within the scope of the present invention.

[0002] List of used reference symbols 10 Pressure vessel system 11 First pressure vessel 12 Second pressure vessel 13 Withdrawal valve 20 Vehicle chassis 100 Method 110 Process start 120 Query as to whether the fuel temperature ^2 in the second pressure vessel 12 is within the previously defined temperature window TB ( ^g, u ^ ^2 ^ ^g, o) 130 Query as to whether the process should be stopped 140 Process stop 200 Withdrawal of fuel from the first pressure vessel 11 300 Withdrawal of fuel from the second pressure vessel 12 D1 First diameter D2 Second diameter V1 First volume V2 Second volume Y Yes output N No output ^1 Fuel temperature in the first pressure vessel ^2 Fuel temperature in the second pressure vessel ^g, u Lower limit temperature

Claims

1. A method (100) for removing a fuel from a pressure vessel system (10) having a first pressure vessel (11) with a first volume (V1) and a second pressure vessel (12) with a second volume (V2), wherein the first volume (V1) is smaller than the second volume (V2), characterized in that - the fuel is firstly at least partially removed from the first pressure vessel (11) before the fuel is removed exclusively from the second pressure vessel (12), provided that the fuel in the second pressure vessel (12) has a temperature (^2) outside a previously defined permissible temperature window (TB), or - that the fuel is firstly at least partially removed from the second pressure vessel (12) before the fuel is removed exclusively from the first pressure vessel (11),provided that the fuel in the second pressure vessel (12) has a temperature (^2) within a previously defined temperature window (TB), wherein the fuel withdrawal from the second pressure vessel (12) is stopped and switched to the first pressure vessel (11) as soon as the temperature (^2) of the fuel from the second pressure vessel (12) leaves the previously defined temperature window (TB).

2. Method (100) according to claim 1, characterized in that the first pressure vessel (11) is refilled with fuel from the second pressure vessel (12) after being at least partially emptied.

3. Method (100) according to one of the preceding claims, characterized in that the withdrawal of fuel from the first pressure vessel (11) or the second pressure vessel (12) is controlled via a control unit, wherein the pressure vessels (11, 12) each have a withdrawal valve and, the control unit controls the extraction valves, wherein the control unit receives information about the temperature of the fuel in the first pressure vessel (11) and in the second pressure vessel (12) and compares these temperatures with a previously defined temperature window (TB). 4.Device for carrying out the method (100) according to one of the preceding claims, characterized in that the device comprises a pressure vessel system (10) with a first pressure vessel (11) having a first volume (V1) and with a second pressure vessel (12) having a second volume (V2), wherein the first volume (V1) is smaller than the second volume (V2), wherein the first pressure vessel (11) and the second pressure vessel (12) each have a bleed valve, and the device comprises a control unit configured to carry out the method according to claim 1, wherein the bleed valves are controllable by the control unit, wherein information about the temperature of the fuel in the first pressure vessel (11) and in the second pressure vessel (12) is receivable by the control unit. 5.Device according to claim 4, characterized in that the first pressure vessel (11) and the second pressure vessel (12) each have a substantially cylindrically shaped segment, and the substantially cylindrically shaped segment of the first pressure vessel (11) has a diameter (D1) that is smaller than the diameter (D2) of the substantially cylindrically shaped segment of the second pressure vessel (12).

6. Device according to one of claims 4 or 5, characterized in that the control unit has a data processing system, the data processing system comprising means for carrying out the method according to one of claims 1 to 3.

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