Method for emptying liquid hydrogen tank

Abstract

The invention relates to a method for emptying a liquid hydrogen tank (13) by means of a tank heater (15) in a vehicle (1) having at least one hydrogen-driven drive train (2). The invention is characterized in that the drive train (2) is not operated during the emptying period, in which a coolant is circulated in a coolant circuit (3) of the drive train (2) or in a coolant circuit (3) which is indirectly connected to a cooling means of the drive train (2) in a heat-conducting manner, the heat in the coolant and the heat absorbed by the coolant from the ambient air are supplied directly or indirectly to the tank heater (15).

Description

TECHNICAL FIELD

[0001] The invention relates to a method for emptying a liquid hydrogen tank by means of a tank heater in a vehicle having at least one hydrogen drive power assembly. BACKGROUND

[0002] A hydrogen drive power assembly in the sense of the invention can be, for example, a fuel cell of a fuel cell system or a machine that burns hydrogen. This can be, for example, a hydrogen drive internal combustion engine or also a gas turbine.

[0003] The withdrawal of liquid hydrogen during the operation of a vehicle equipped with such a tank usually envisages the supply of heat to the liquid phase of the hydrogen in order to vaporize it. The vaporized gas under pressure can then be withdrawn from the tank. For this purpose various heat sources can be used. For example, US 2020 / 0298651 A1 describes a system in which the heat for vaporizing the hydrogen is taken from the environment or from the cooling circuit of a fuel cell depending on the season. This construction is relatively complex and is only suitable for stationary installations. Similar cases are also described in CN 115224306 A1.

[0004] In addition to the use of hydrogen as described in these two documents, for example, for feeding a fuel cell system or in principle also for feeding other hydrogen consumers, the emptying of a liquid hydrogen tank for maintenance purposes plays a decisive role, in particular in the field of vehicle technology. It is usual in vehicles for there to be an electric resistance heater or a dedicated tank circuit with a cooling medium to operate the tank heater. For this purpose, a liquid cooling medium at a temperature above the boiling point of hydrogen is passed through the tank heater. If the liquid hydrogen tank is now to be emptied of residual hydrogen in order to be able to carry out maintenance on it, the tank circuit is usually supplied with energy in order to heat the cooling medium and vaporize the hydrogen in the tank, so that the hydrogen can be withdrawn. The energy is usually provided by means of an electric heater, for example a heating resistor, since all other systems of the vehicle are not operated in such a case. In practice, a heating resistor is usually used, which is provided in the vehicle for generating electric braking power but the battery cannot absorb the generated power since it is already fully charged. In this case, the excess power is dissipated on the heating resistor. The heating resistor can be used for the energy input when the liquid hydrogen tank is emptied of residual hydrogen for maintenance purposes. However, this approach has a serious disadvantage. A relatively large amount of electric power is required, which is usually taken from the battery, so that after the tank has been emptied of residual hydrogen, the battery often exhibits a low or critical charge state or the electric energy must be supplied from the outside. SUMMARY

[0005] It is now the task of the invention to provide an improved method for emptying a liquid hydrogen tank by means of a tank heater for maintenance purposes, which removes the residual hydrogen from the tank in an energy-saving manner.

[0006] The task is accomplished by the method according to the application according to claim 1. Advantageous designs and refinements result from the dependent claims.

[0007] According to the method of the application, it is provided that the coolant-conveying device of the coolant circuit of the drive assembly or the coolant-conveying device of the coolant circuit, which is in indirect heat-conducting connection with the cooling device of the drive assembly, is operated when the drive assembly is not in operation, i.e. in a state in which it does not generate mechanical or electrical power. By means of the coolant-conveying device, it is then possible to utilize the heat present in the coolant and, once the coolant has cooled to a temperature below the ambient temperature, to utilize the heat from the environment. For this purpose, heat is input from the ambient air into the coolant by means of a cooling heat exchanger, which is provided for cooling the coolant as such. Thereby, it is possible to provide heat at a temperature level which corresponds at most to the temperature of the ambient air. This temperature will generally be in the order of magnitude of 5°C to 25°C, which is sufficient for evaporating the liquid cryogenic hydrogen in the tank, so that residual hydrogen can be removed before the tank is serviced. The method of the application thus utilizes the coolant circuit for supplying the tank heater, firstly with heat from its cooling medium and, subsequently, with heat input from the ambient air into the cooled cooling medium, directly or indirectly.

[0008] The cooling circuit can here be the cooling circuit of the fuel cell system or of the hydrogen engine, as such. In the case of a plurality of cooling circuits, for example operating at different temperature levels, it is the cooling circuit which comprises a cooling heat exchanger for discharging waste heat to the environment. Thereby, it is possible to input heat from the ambient air into the coolant by means of the cooling heat exchanger.

[0009] For this purpose, it is only necessary to operate the coolant-conveying device and, if necessary, a fan for flowing the ambient air through the cooling heat exchanger. This can be significantly more energy-efficient than the electrical heating of the tank heater for removing residual hydrogen. The method according to the application thus enables a complete emptying of the tank for servicing purposes in a very energy-efficient manner.

[0010] According to a very advantageous refinement of the method of the application, it is provided that the heat absorbed from the ambient air is transferred to the cooling medium of the tank circuit by means of a heat exchanger, by means of which the liquid hydrogen is heated and evaporated in the tank heater. This design utilizes a structure which is very common in fuel cell vehicles, which has, on the one hand, a coolant circuit for the fuel cell system and, on the other hand, a tank circuit with a further cooling medium for heating and removing hydrogen during operation. Generally, the two circuits are connected to one another by means of a heat exchanger, the electrical brake resistor mentioned above generally being part of the tank circuit. In particular, the air-conditioning circuit can also be heat-conductively connected to the tank circuit.

[0011] Thus, this particular design of the method according to the application now utilizes the cooling circuit, which can utilize its large-area cooling heat exchanger (commonly referred to as a cooler) to establish contact between its coolant and the ambient air. If the coolant is cooled by heating the liquid hydrogen to a temperature below the ambient temperature, the coolant thus absorbs heat from the ambient air in the cooling circuit and can transfer this heat to the cooling medium in the tank circuit accordingly by means of the heat exchanger between the two circuits, which in turn delivers the heat to the tank heater, for which here too only one delivery mechanism needs to be operated. The medium cooled in the tank heater is then heated again in the heat exchanger. For this, two delivery mechanisms (in one circuit and in the other circuit) are operated, and if necessary a fan to provide the cooling heat exchanger area of the cooling circuit with sufficient amounts of ambient air. No further energy is required, so that the hydrogen can be evaporated and removed from the tank extremely energy-efficiently.

[0012] In addition thereto, other heat sources can be utilized, which are connected to or can be connected to the cooling circuit and / or the tank circuit, in order to provide the tank heater with heat. Such heat sources are typically areas or circuits with media having a temperature at or above the level of the ambient air in a stopped vehicle, the interior air of the 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 thus also be incorporated. If other coolant circuits are in conductive contact with the cooling circuit, these circuits can also be operated in conjunction.

[0013] A further advantageous design of the method according to the application can also envisage that an electric resistance heater is provided in at least one of the circuits, which is operated when there is an additional heat requirement. Thus, there is also always the possibility of residual evacuation by electric auxiliary heating, for example when the ambient temperature is sufficiently low so that the cooling medium would freeze in at least one of the circuits, or when one of the cooling circuits is no longer or cannot be fully available due to a vehicle failure, for example after an accident.

[0014] The method according to the application can now be used for various tank arrangements, which can be used in various vehicles, for example commercial vehicles, light commercial vehicles, passenger cars, but also ships, rail vehicles or similar vehicles. BRIEF DESCRIPTION OF DRAWINGS

[0015] Further advantageous designs of the method according to the application also result from the embodiments described in detail below with reference to the drawings.

[0016] The sole drawing shows a vehicle with a fuel cell system and a liquid hydrogen tank in a schematic representation. DETAILED DESCRIPTION

[0017] In Figure 1In the illustration of Fig. 1 a vehicle 1 is schematically shown in dot-dash line. The vehicle 1 can be, for example, a commercial vehicle which is electrically driven by means of a fuel cell system. For the fuel cell system, in the embodiment shown here two fuel cells 2, i.e. so-called fuel cell stacks or fuel cell blocks, are shown in parallel connection. They constitute the powertrain 2 in the sense of the invention. However, they can also be replaced by internal combustion engines, gas turbines or the like, by means of which the vehicle 1 is mechanically driven or driven in a series hybrid form. The remainder is similar to the example described below with reference to the fuel cell vehicle 1.

[0018] The fuel cells 2 are here jointly arranged in a fuel cell cooling circuit 3 and can be cooled in parallel in normal operation by means of a volume flow of coolant which is conveyed by a coolant conveying device 4. A fuel cell bypass 5 is also provided. By means of this it is possible for the fuel cells 2 to be flowed through by coolant individually or jointly, or even not to be flowed through by coolant, for which purpose a bypass valve designated by 6 is provided in order to control the flow of coolant in accordance with the desired operating state. The waste heat of the fuel cells 2 is in normal operation discharged by means of a cooling heat exchanger 7. The cooling heat exchanger 7, which is also generally referred to as a cooler, can here consist of one or more elements which are generally so arranged on the vehicle 1 that they are flowed through by sufficient cooling air during the driving of the vehicle 1. If the cooling air is not sufficient, an additional air flow or an enhanced air flow through the cooling heat exchanger 7 can be produced by means of a fan wheel shown here and designated by 8. The cooling heat exchanger 7 is also provided with a cooler bypass 9 with a bypass valve 10 in order, for example, not to cool the slowly warming-up cooling medium in the case of a cold start of the fuel cell system, in order to heat the fuel cells 2 as quickly as possible. All of this is generally known to the specialist in the field of fuel cell systems. As an alternative to this cooling circuit 3, a plurality of coolant circuits can also be provided which are in heat-conducting connection with one another.

[0019] By means of the heat exchanger 11, the at least one fuel cell cooling circuit 3 is now connected with the cooling medium circuit 12 of the liquid hydrogen tank 13. The tank circuit 12 also comprises a transport mechanism 14 for the cooling medium circulating therein. In addition, a tank heater 15 is part of the tank circuit 12. This is controllably extendable by means of a bypass valve 16 and, if required, can be flow bypassed through the tank 13 by means of a bypass 17, precisely in the portion of the tank 13 which, in the intended use, is located below in the direction of the gravitational force. The tank heater 15 should be in contact with the liquid hydrogen and accordingly heat the liquid hydrogen. Bubbles then rise to the upper region of the tank 13 in the intended use and are discharged from the tank by means of a take-off device 18 and, in the normal operation of the vehicle 1, are provided, for example, as a reactant to the fuel cell 2. In most cases, a heating resistor 19 is also in heat-conducting contact with the tank circuit 12, which heating resistor can be provided for heating the coolant in the tank circuit 12. It can thus assist the tank heater 15. The heating resistor 19 is also generally used in the case of energy recovery when the vehicle 1 brakes, in which case more electrical power is generated than can be absorbed by the battery not shown here. The excess electrical power is then accordingly consumed on the heating resistor 19.

[0020] Other heat sources can be provided within the tank circuit 12 or also at the fuel cell cooling circuit 3. Here, purely by way of example, other heat sources, such as power electronics to be cooled, electric motors to be cooled, air conditioners, etc., should be symbolized by the block designated with 20, which are in heat transfer connection with one of the circuits 3, 12, in this example with the tank circuit 12.

[0021] The operation of such a system within a vehicle 1 in normal operation is generally known and familiar to the person skilled in the art, so that it is not necessary to discuss this further.

[0022] The situation on which this is based should now be as follows: the transport means 1 has failed, for example, at least in the region of the hydrogen feed line and / or in the region of the tank 13. In the case of a standstill of the transport means 1 and the fuel cell 2 no longer being operational or not being operable, the residual hydrogen of the tank 13 should now be extracted in an energy-saving manner. Instead of the heat required for evaporating the liquid residual hydrogen being provided by the heating resistor 19 as was previously customary, both the transport means for the coolant in the fuel cell circuit 3, namely the transport means 4, and the transport means for the cooling medium in the tank circuit 12, namely the transport means 14, can now be operated. In the embodiment shown here, the fuel cell circuit 3 can be operated, for example, in such a way that only the heat exchanger 11, the cooling heat exchanger 7, the fuel cell bypass 5 and the coolant transport means 4 are flowed through. Flowing through the fuel cell 2 itself is possible in principle, but here, if a corresponding fuel cell bypass 5 is present, which is usually the case, it is not necessary in principle in this example. When the transport means 14 are operated in the tank circuit 12 and the bypass valve 16 is correspondingly set, the cooling medium can now be transported through the tank heater 15 and evaporate the liquid residual hydrogen in the tank 13 while itself being cooled. The coolant of the tank circuit 12 can now take up heat again in the region of the heat exchanger 11 in order to evaporate more liquid residual hydrogen in the tank heater 15. The heat can now be provided simply in such a way that the coolant of the fuel cell cooling circuit 3 is circulated in such a way that it takes up heat from the ambient air in the cooling heat exchanger 7 when it has been cooled to a temperature below the ambient temperature in the heat exchanger 11 and transports it to the heat exchanger 11. The heat is then transferred to the coolant in the tank circuit 12 and used to evaporate the liquid residual hydrogen, which can be extracted by the extraction device 18.

[0023] In addition to operating the two transport means 4, 14 and, if necessary, the fan 8 to provide a greater amount of ambient air and thus a higher amount of heat, no additional energy requirement is necessary here, in particular no energy-intensive electric auxiliary heating has to be carried out. If other heat sources are available here, which are symbolized by the block 20, it is of course also possible to utilize the residual heat present in the region of these heat sources to heat the cooling medium in the tank circuit 12 in order to evaporate the liquid residual hydrogen.

[0024] In addition, the heat can also be supplied to the cooling medium of the tank circuit 12 by the resistor heater 19, if necessary, but this should only be carried out when all other measures do not work, for example because of a failure of the fuel cell cooling circuit or the like.

Claims

1. A method for emptying a liquid hydrogen tank (13) in a vehicle (1) having at least one hydrogen-powered powertrain (2) by means of a tank heater (15), Its characteristics are, The powertrain (2) is not operated during the emptying period, wherein the coolant in the coolant circuit (3) of the powertrain (2) or the coolant in the coolant circuit (3) indirectly connected to the cooling system of the powertrain (2) is circulated to directly or indirectly supply the heat in the coolant and the heat absorbed by the coolant from the ambient air to the tank heater (15).

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

3. The method according to claim 2, characterized in that, Other 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. The method according to claim 2 or 3, characterized in that, A heating resistor (19) is provided in at least one circuit (3, 12), which operates when there is an additional heat demand.

5. The method according to any one of claims 1 to 4, characterized in that, At least one fuel cell (2) is used as the powertrain.

6. The method according to any one of claims 1 to 4, characterized in that, The powertrain (2) uses at least one hydrogen-burning machine.

Images