Method for emptying a tank for liquid hydrogen

Abstract

The invention relates to a method for emptying a tank (13) for liquid hydrogen by means of a tank heater (15) in a vehicle (1) with at least one hydrogen-operated drive assembly (2). The invention is characterized in that the drive assembly (2) is not operated during the emptying, wherein a coolant is circulated in a coolant circuit (3) of the drive assembly (2) or in a coolant circuit (3) indirectly in thermally conductive contact with a cooling of the drive assembly (2), in order to supply heat in the coolant, and heat taken up from the ambient air by the coolant, directly or indirectly to the tank heater (15).

Description

[0001] Method for emptying a tank for liquid hydrogen

[0002] The invention relates to a method for emptying a tank for liquid hydrogen by means of a tank heater in a vehicle with at least one hydrogen-powered drive unit.

[0003] Hydrogen-powered drive units within the meaning of the invention can be, for example, fuel cells of a fuel cell system or hydrogen-burning machines. This could be, for example, a hydrogen-powered combustion engine or a gas turbine.

[0004] The extraction of liquid hydrogen during operation of a vehicle equipped with such a tank typically involves adding heat to the liquid phase of the hydrogen to vaporize it. The vaporized, pressurized gas can then be extracted from the tank. Various heat sources can be used for this purpose. US 2020 / 0298651 A1 describes a system in which, depending on the season, the heat for liquefying the hydrogen is extracted either from the environment or from the fuel cell's cooling circuit. The setup is accordingly complex and suitable only for stationary systems. Something similar is also described in CN 115224 306 A1.

[0005] In addition to the removal of hydrogen described in these two documents, for example to supply it to a fuel cell system or, in principle, to another hydrogen consumer, the emptying of the liquid hydrogen tank for maintenance purposes plays a crucial role, especially in the field of automotive technology. In vehicles, an electrical resistance heater or a separate tank circuit with a cooling medium is typically provided to operate the tank heater. The liquid cooling medium, which has a higher temperature than the boiling point of the hydrogen, flows through this. If a liquid hydrogen tank is to be completely emptied for maintenance, energy is typically supplied to this tank circuit to heat the cooling medium and evaporate the hydrogen in the tank so that it can be removed.This energy is typically provided via an electric heater, such as an electric heating resistor, since all other vehicle systems are no longer in operation in such a situation. In practice, a heating resistor is usually used. This is provided in vehicles for the case of regenerative braking during operation, but a battery is not able to absorb the resulting power because it is already fully charged. In this case, the excess power is consumed by the heating resistor. If a liquid hydrogen tank is completely emptied for maintenance purposes, this heating resistor can then be used to input heat. However, this approach has a serious disadvantage.A relatively high amount of electrical power is required, which is typically taken from the battery, so that after the tank is completely empty, the battery often has a very low or critical charge level or the electrical energy has to be provided externally.

[0006] The object of the present invention is to provide an improved method for emptying a tank for liquid hydrogen by means of a tank heater for maintenance purposes, which method removes the residual hydrogen from the tank in an energy-efficient manner.

[0007] This object is achieved by the inventive method according to claim 1. Advantageous embodiments and further developments emerge from the dependent claims.

[0008] The method according to the invention provides that when the drive unit is not in operation, i.e. in a state in which it does not generate any mechanical or electrical power, a coolant conveying device of a coolant circuit of the drive unit or of a coolant circuit indirectly connected to a cooling system of the drive unit is nevertheless operated. This coolant conveying device can then be used to utilize the heat present in the coolant and, as soon as the coolant has cooled to a temperature below the ambient temperature, heat from the environment. For this purpose, heat from the ambient air is introduced into the coolant via the cooling heat exchangers otherwise provided for cooling the coolant. This allows heat to be provided at a temperature level that corresponds at most to the temperature of the ambient air.This temperature, which will typically be in the range of 5 to 25°C, is sufficient to evaporate the liquid cryogenic hydrogen in the tank, allowing the residual hydrogen to be removed before tank maintenance. The method according to the invention thus utilizes the coolant circuit to first transfer the heat in its cooling medium and then the heat introduced from the ambient air into the cooled cooling medium directly or indirectly to the tank heater.

[0009] The cooling circuit can be the cooling circuit of, for example, a fuel cell system or a hydrogen engine. If there are multiple cooling circuits, operating at different temperature levels, the cooling circuit that includes a cooling heat exchanger to dissipate waste heat to the environment is the one that is used. This makes it possible to transfer heat from the ambient air into the coolant via the cooling heat exchanger.

[0010] For this purpose, only the coolant delivery system and, if necessary, a fan for flowing ambient air through the cooling heat exchangers need to be operated. This is far more energy-efficient than electrically heating the tank heater to remove the residual hydrogen. The method according to the invention thus makes it possible to completely empty the tank for maintenance purposes in a very energy-efficient manner.

[0011] According to a very advantageous development of the method according to the invention, it is provided that the heat absorbed from the ambient air is transferred via a heat exchanger to a cooling medium of a tank circuit, by means of which the liquid hydrogen in the tank heater is heated and vaporized. This design uses the design very frequently found in fuel cell vehicles for the method according to the invention, with a coolant circuit for the fuel cell system on the one hand and a tank circuit of a further cooling medium for heating and removing the hydrogen during operation on the other. Typically, these two circuits are coupled to one another via a heat exchanger; the electrical braking resistor already mentioned above is frequently part of the tank circuit. In particular, an air conditioning circuit can also be coupled to this tank circuit in a heat-transfer manner.

[0012] This particular embodiment of the method according to the invention therefore utilizes the cooling circuit, which, with its large-surface cooling heat exchangers, colloquially also referred to as coolers, can establish contact between its coolant and the ambient air. Once the coolant has cooled to a temperature below ambient temperature by heating the liquid hydrogen, this coolant absorbs heat from the ambient air in the cooling circuit and can transfer this heat via the heat exchanger between the two circuits to the cooling medium in the tank circuit, which in turn conveys the heat to the tank heater, for which, again, only one conveying device needs to be operated. The medium cooled in the tank heater is then reheated in the heat exchanger.For this purpose, both conveyor systems are operated in both circuits, as well as fans, if necessary, to provide a sufficient amount of ambient air in the area of ​​the cooling circuit's heat exchangers. No additional energy is required, so the hydrogen can be evaporated and removed from the tank with exceptionally high energy efficiency.

[0013] In addition, additional heat sources that are or can be connected to the cooling circuit and / or the tank circuit can be used to provide heat for the tank heater. In a switched-off vehicle, such heat sources are typically areas or circuits containing a medium with a temperature at or above the ambient air level, the interior air of a passenger compartment, or the like. For example, heat from the air conditioning circuit of an air conditioning system, which is typically connected to the tank circuit, can also be incorporated. If additional coolant circuits are in heat-conducting contact with the cooling circuit, these can also be operated.

[0014] Another very advantageous embodiment of the method according to the invention can further provide for an electric resistance heater to be provided in at least one of the circuits, which is operated when additional heat is required. Thus, as before, it is always possible to carry out the residual emptying via an additional electric heater, for example, if the ambient temperatures are so low that the cooling medium in at least one of the circuits would freeze, or if one of these cooling circuits is no longer or no longer fully available due to a vehicle defect, for example, after an accident.

[0015] The method according to the invention can now be used for various tank systems which can be used in various vehicles, for example commercial vehicles, light commercial vehicles, passenger cars, but also in ships, rail vehicles or the like.

[0016] Further advantageous embodiments of the method according to the invention also emerge from the exemplary embodiment which is described in more detail below with reference to the figure.

[0017] The only attached figure shows a schematically indicated vehicle with a fuel cell system and a tank for liquid hydrogen.

[0018] In Figure 1, a vehicle 1 is schematically indicated by a dash-dotted line. This vehicle 1 can, for example, be a commercial vehicle that is electrically powered by a fuel cell system. For this fuel cell system, two parallel fuel cells 2, so-called fuel cell stacks, are indicated in the exemplary embodiment shown here. They form the drive unit 2 within the meaning of the invention. However, they could also be replaced by an internal combustion engine, a gas turbine, or the like, via which the vehicle 1 would then be powered mechanically or in the form of a serial hybrid. The rest then results analogously to the example described below using the fuel cell vehicle 1.

[0019] The fuel cells 2 are arranged together in a fuel cell cooling circuit 3 and can be cooled in parallel by a volume flow of a coolant conveyed via a coolant conveying device 4 during regular operation. A fuel cell bypass 5 is also provided. This makes it possible for the coolant to flow through the fuel cells 2 individually, jointly, or not at all, for which purpose bypass valves designated 6 are provided to control the flow of the coolant depending on the desired operating state. The waste heat from the fuel cells 2 is dissipated via a cooling heat exchanger 7 during regular operation. The cooling heat exchanger 7, which is often also referred to as a cooler, can consist of one or more elements, which are typically arranged on the vehicle 1 in such a way that sufficient cooling air flows through them while the vehicle 1 is traveling.If this cooling air is insufficient, an additional air flow or an increased air flow through the cooling heat exchanger 7 can be generated via a fan impeller indicated here and designated 8. This cooling heat exchanger 7 also has a cooler bypass 9 with a bypass valve 10 to prevent the slowly warming cooling medium from cooling down, for example, in a cold-start scenario of the fuel cell system, and thus heat the fuel cells 2 as quickly as possible. All of this is generally known to a person skilled in the art of fuel cell systems. As an alternative to the single cooling circuit 3, several coolant circuits could also be provided, which are in thermally conductive connection with one another.

[0020] Via a heat exchanger 11, this at least one fuel cell cooling circuit 3 is now connected to a cooling medium circuit 12 of a tank 13 for liquid hydrogen. This tank circuit 12 also comprises a conveying device 14 for the cooling medium circulating therein. A tank heater 15 is also part of this tank circuit 12. It runs through the tank 13 in a controllable manner via a bypass valve 16 and, if necessary, via a bypass 17, bypassing it, preferably in a section of the tank 13 that lies at the bottom in the direction of gravity during normal use. The tank heater 15 is intended to come into contact with liquid hydrogen and heat it accordingly. Gas bubbles then rise into the upper region of the tank 13, which is the upper region during normal use, and are removed from the tank via a removal device 18. During regular operation of the vehicle 1, they are made available, for example, to the fuel cells 2 as a reactant.In most cases, an electrical heating resistor 19 is also in thermally conductive contact with this tank circuit 12, which can be provided for heating the coolant in the tank circuit 12. It can thus support the tank heater 15. This heating resistor 19 is often also used when more electrical power is generated during recuperation during braking of the vehicle 1 than can be absorbed by a battery (not shown here). The excess electrical power is then consumed accordingly by the electrical heating resistor 19.

[0021] Additional heat sources can be arranged in the tank circuit 12 or also on the fuel cell cooling circuit 3. Purely by way of example, the box designated 20 symbolizes additional heat sources, such as power electronics to be cooled, electric motors to be cooled, an air conditioning system, or the like, which are in heat-transfer connection with one of the circuits 3, 12, here the tank circuit 12.

[0022] The operation of such a system in a vehicle 1 during regular operation is generally known and familiar to the person skilled in the art, so that it need not be discussed further here.

[0023] The underlying scenario here is the following. The vehicle 1 is defective, for example, or at least also in the area of ​​hydrogen-carrying lines and / or in the area of ​​the tank 13. With the vehicle 1 parked and the fuel cells 2 no longer operating or operable, the residual hydrogen in the tank 13 is to be extracted in an energy-efficient manner. Instead of providing the heat required to evaporate the liquid residual hydrogen via the electrical heating resistor 19, as was previously customary, the two delivery devices 4 for the coolant in the fuel cell circuit 3 and 14 for the cooling medium in the tank circuit 12 can now be operated. In the exemplary embodiment shown here, the fuel cell circuit 3 can, for example, be operated such that only the heat exchanger 11, the cooling heat exchanger 7, the fuel cell bypass 5, and the coolant delivery device 4 are flowed through.While flow through the fuel cells 2 themselves is fundamentally possible, it is not necessary here if a corresponding fuel cell bypass 5 exists, which is typically the case. Through the operation of the conveying device 14 in the tank circuit 12, with the bypass valve 16 set accordingly, the cooling medium is now conveyed through the tank heater 15 and evaporates the residual liquid hydrogen in the tank 13, cooling in the process. This coolant of the tank circuit 12 can then absorb heat again in the area of ​​the heat exchanger 11 to evaporate further residual liquid hydrogen in the tank heater 15.This heat can now be easily provided by circulating the coolant of the fuel cell cooling circuit 3 in the manner described. Once it has been cooled in the heat exchanger 11 to a temperature below ambient temperature, it absorbs heat from the ambient air in the cooling heat exchanger 7 and transports it to the heat exchanger 11. This heat is then transferred to the coolant in the tank circuit 12 and serves to evaporate the residual liquid hydrogen, which can be removed via the removal device 18.

[0024] Apart from the operation of the two conveying devices 4, 14 and, if necessary, the operation of the fan 8 to provide a larger amount of ambient air and thus a greater amount of heat, no additional energy is required here; in particular, there is no need for energy-intensive electrical heating. If additional heat sources, symbolized here by box 20, are available, the residual heat present in the area of ​​these heat sources can of course also be used to heat the cooling medium in the tank circuit 12 for the evaporation of the residual liquid hydrogen.

[0025] In addition, if necessary, heat could also be supplied to this cooling medium of the tank circuit 12 via the electrical resistance heater 19, but this should only be done if all other measures do not work, for example due to a defective fuel cell cooling circuit or the like.

Claims

Patent claims 1. A method for emptying a tank (13) for liquid hydrogen by means of a tank heater (15) in a vehicle (1) with at least one hydrogen-powered drive unit (2), characterized in that the drive unit (2) is not operated during emptying, wherein a coolant is circulated in a coolant circuit (3) of the drive unit (2) or a coolant circuit (3) which is indirectly connected in a heat-conducting manner to a cooling system of the drive unit (2), in order to supply heat in the coolant and heat absorbed by the coolant from the ambient air directly or indirectly to the tank heater (15).

2. Method according to claim 1, characterized in that the heat from the coolant of the coolant circuit (3) is transferred via a heat exchanger (11) to the cooling medium of a cooling medium circuit (12) of the tank (13), by means of which the liquid residual hydrogen is then heated and evaporated by the tank heater (15).

3. Method according to claim 2, characterized in that further heat sources (20) in contact with the coolant circuit (3) and / or the cooling medium circuit (12) of the tank (13) are used to heat the cooling medium in the cooling medium circuit (12) of the tank (13).

4. Method according to one of claims 2 or 3, characterized in that in at least one of the circuits (3, 12) an electrical heating resistor (19) is provided, which is operated when additional heat is required.

5. Method according to one of claims 1 to 4, characterized in that at least one fuel cell (2) is used as the drive unit.

6. Method according to one of claims 1 to 4, characterized in that at least one engine which burns hydrogen is used as the drive unit (2).

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