Storage tank for liquid hydrogen with fill level indicator

Abstract

A storage tank (5) for liquid hydrogen (H2), comprising an outer tank (8), an inner tank (9) disposed inside the outer tank (8), a device (23) for indicating when a cell (25) has reached a predetermined filling level (24), the cell (25) being capable of being disposed inside the inner tank (9) and filled with a gas (G) that liquefies when the cell (25) is immersed in liquid hydrogen (H2), a pressure indicator device (30) that indicates a pressure drop in the cell (25) when the gas (G) liquefies, thus indicating that the predetermined filling level (24) has been reached, and a heating element (31) for continuously introducing heat (W) into the gas (G).

Description

[Technical Field]

[0001] The present invention relates to a storage tank for liquid hydrogen, comprising an outer tank, an inner tank arranged inside the outer tank, and a device for indicating when a predetermined filling level has been reached.Furthermore, the present invention relates to a method for indicating when a predetermined filling level has been reached in a storage tank for liquid hydrogen.

[0002] According to the applicant's internal findings regarding the outer and inner tanks, the storage tanks for liquid hydrogen in each case have a cylindrical shape with a tubular base and two curved cover sections that close the base at the end faces. Upon request, such storage tanks are filled with liquid hydrogen to a predetermined filling level. This can be achieved, for example, when the storage tank is approximately 88-95% filled with liquid hydrogen. The applicant is aware of internal prior art in which a filling level indicator is realized using what is known as the differential pressure method to detect or indicate when a predetermined filling level has been reached. Furthermore, according to the applicant's internal findings, capacitive measurement of the filling level is possible by changing the dielectric constant in liquid hydrogen. Due to the physical properties of hydrogen, a reliable filling level indicator can be technically implemented only in a very complex manner. In this case, different physical quantities must be measured and calculated in a complex manner.

[0003] Against this background, the object of the present invention is to provide a storage tank for liquid hydrogen of the type mentioned at the beginning, which storage tank is improved with regard to filling level measurement and an improved method for measuring the filling level of a storage tank for liquid hydrogen.

[0004] It is therefore proposed that a storage tank for liquid hydrogen has an outer tank, an inner tank arranged inside the outer tank, and a device for indicating when a predetermined filling level of liquid hydrogen has been reached, the device comprising a cell arranged inside the inner tank and filled with a gas that liquefies when the cell is immersed in liquid hydrogen, a pressure indicator device that indicates a pressure drop in the cell when the gas liquefies and thus indicates that the predetermined filling level has been reached, and a heating element that continuously introduces heat into the gas.

[0005] The provision of a heating element ensures that the gas in the cell does not liquefy before the liquid hydrogen flows around the cell or before a predetermined filling level is reached. Additionally, it ensures that the gas in the cell re-evaporates while the storage tank is being refilled, enabling reliable filling level measurement in closed, highly insulated tanks, such as this type of double-walled hydrogen storage tank. Furthermore, heat input is only required while the storage tank is being filled. Therefore, constant cryogenic leaks are ruled out. A filling level measurement device described in Utility Model No. S54153267, which does not have a heating device, is disclosed for an open Dewar vessel.

[0006] The device may also be referred to as an indicator device, a limit transducer, or a limit transducer device. The storage tank includes an outer tank, an inner tank disposed inside the outer tank, and preferably an insulating element disposed between the outer tank and the inner tank. The cell is disposed inside the inner tank. In addition to liquid hydrogen, the device may also be used for other cryogenic liquids. Examples of cryogenic fluids or liquids, or cryogens, include, for example, liquid helium, liquid nitrogen, or liquid oxygen.

[0007] Then, when a predetermined filling level is reached, the liquid hydrogen flows at least partially around the cell. In this case, "cell" is understood to mean a component that encloses an interior space in which a gas is contained. The term "cell" may also be referred to herein using the term "tank." The cell may also be referred to as a gas cell or gas tank.

[0008] The gas can be any gas that has the property of liquefying when the cell is immersed in liquid hydrogen. This phase transition of the gas from gas to liquid creates a pressure drop within the cell that can be detected or displayed by a pressure indicator device. The pressure drop is therefore an indication that a predetermined fill level has been reached.

[0009] The pressure indicator device is preferably located outside the storage tank so that it can be seen by a technician. The heating element is preferably an electric heating element. The heating element may be, for example, an electrical resistor. The cell is therefore heated and may be referred to as a heated cell. The fact that the heating element "continuously" introduces heat into the gas in this case means that the heating element is preferably energized during the entire process of filling the storage tank with liquid hydrogen, and thus introduces heat into the gas. Thus, the heating element has already introduced heat into the gas before the cell is immersed in liquid hydrogen. However, only enough heat is introduced into the gas to liquefy the cell when it is immersed in liquid hydrogen.

[0010] As soon as the liquid hydrogen no longer flows around the cell or the cell is no longer immersed in the liquid hydrogen, the gas is vaporized again by the heat introduced by the heating element. Pressure then rises again in the cell, indicating a drop in the liquid hydrogen level below the predetermined fill level. Thus, only enough heat is introduced into the gas to liquefy it when it is immersed in the liquid hydrogen and to vaporize it again as soon as the cell is no longer immersed in the liquid hydrogen.

[0011] For example, during the filling process, the heating element continuously introduces 5 W of heat power into the gas. After the filling process is completed, the heating element is preferably no longer energized. The heating element eliminates the need to introduce heat from outside the storage tank into the cell or into the gas to vaporize the gas. As a result, certain cold leaks, as described above, that would otherwise introduce heat into the cell from outside can be eliminated.

[0012] According to one embodiment, the gas is neon.

[0013] However, any other gas may also be used. In particular, the boiling point of the gas contained in the cell is higher than that of liquid hydrogen, so that the gas is liquefied when the cell is immersed in liquid hydrogen. The cell may be called a neon cell, or in particular a heated neon cell.

[0014] According to a further embodiment, the cells are spherical.

[0015] This allows the cell to have as large a volume as possible with as small a surface area as possible, but the cells may also be cylindrical or cuboid, or have any other shape.

[0016] According to a further embodiment, the device further comprises lines carrying the cells.

[0017] The lines are typically routed through the wall of the storage tank. The lines can be, for example, stainless steel lines. The cells can be suspended on the lines so that the lines absorb the weight of the cells. The lines keep the cells fixed in place. Therefore, the cells cannot float on the liquid hydrogen.

[0018] According to a further embodiment, the gas may be supplied to the cell by means of a line, through which a cable energizing the heating element is led to the cell.

[0019] The line can thus perform two functions: to supply gas to the cell and to house the cable leading to the heating element. In particular, the cable is housed in the center of the line. The cable can be housed in a protective tube located in the center of the line.

[0020] According to a further embodiment, the device further comprises a valve capable of supplying gas to the line.

[0021] A nozzle fitted with a valve may be provided on the line, which may be used to refill the gas before the storage tank is filled, or to introduce gas into the cell.

[0022] According to a further embodiment, the pressure indicator device is in fluid communication with the line.

[0023] The pressure indicator device is in fluid communication with the cell via a line, where "in fluid communication" means that gas can flow from the cell to the pressure indicator device via the line, and vice versa. The pressure indicator device can be attached to the aforementioned nozzle, which also carries a valve.

[0024] According to a further embodiment, the pressure indicator device is a pressure gauge.

[0025] This allows for a simple construction of the device: a technician filling the storage tank can easily recognize from the pressure drop that a predetermined filling level has been reached. However, the pressure indicator device can also be or include a pressure sensor.

[0026] According to a further embodiment, the heating element is arranged inside the interior space of the cell surrounded by the cell.

[0027] As a result, the heating element does not come into contact with the liquid hydrogen. Alternatively, however, the heating element can be provided outside the cell. However, the heating element is particularly preferably arranged in the interior space of the cell. The gas is also contained in the interior space.

[0028] According to a further embodiment, the device further comprises a control unit for controlling the heating element.

[0029] In the simplest case, the control unit may be a switch that allows energizing the heating element via a cable guided through the line. However, the control unit may also be or include a computer. If the pressure indicator device is or includes a pressure sensor, the control unit may also be suitable for detecting, evaluating and / or displaying the sensor signal of the pressure indicator device.

[0030] The storage tank is particularly suitable for marine applications. Therefore, the storage tank may be referred to as a marine storage tank. For example, the storage tank may be mounted on a vehicle, in particular a ship. Therefore, a vehicle, in particular a ship, having such a storage tank has also been proposed. The storage tank may also be referred to as a storage vessel, a hydrogen storage vessel, or a hydrogen storage tank. The storage tank is preferably suitable for containing liquid hydrogen. However, the storage tank may also be used for other cryogenic liquids. Examples of cryogenic fluids or liquids or cryogens include, for example, liquid helium, liquid nitrogen, or liquid oxygen.

[0031] After or during the filling of a storage tank with hydrogen, a gaseous region containing gaseous hydrogen and a lower liquid region containing liquid hydrogen are formed. An interface is formed between the gaseous and liquid regions. As soon as the interface reaches a predetermined filling level, the storage tank is filled. Therefore, after the storage tank is filled, the hydrogen has two distinct condensation states: liquid and gaseous. Hydrogen can transition from the liquid phase to the gaseous phase and vice versa.

[0032] According to one embodiment, the lines of the device carrying the cells are fixedly connected to the wall of the storage tank.

[0033] For example, the lines may be welded to the wall. The lines are preferably stainless steel lines.

[0034] As described above, the storage tank comprises an outer tank and an inner tank disposed inside the outer tank, and the cell is disposed inside the inner tank. Preferably, the outer tank and the inner tank are made of stainless steel. An insulating element for thermal insulation can be provided between the outer tank and the inner tank. The insulating element is used to insulate the inner tank. The aforementioned lines can be welded to the outer tank and the inner tank.

[0035] Furthermore, a method for indicating when a predetermined filling level has been reached in a storage tank for liquid hydrogen having an outer tank and an inner tank disposed inside the outer tank has been proposed, the method comprising the steps of: a) immersing a cell disposed inside the inner tank in liquid hydrogen when the predetermined filling level has been reached, whereby the cell is immersed in liquid hydrogen such that the hydrogen or liquid level of the liquid hydrogen inside the inner tank rises to the predetermined filling level and the liquid hydrogen at least partially flows around the cell, b) liquefying a gas contained in the cell using the liquid hydrogen, c) continuously introducing heat into the gas using a heating element, and d) using a pressure indicator device to indicate the pressure drop in the cell as the gas liquefies, thus indicating that the predetermined filling level has been reached.

[0036] As soon as liquid hydrogen flows at least partially around the cell, the gas contained in the cell is liquefied. In this case, only enough heat is introduced into the gas using a heating element, so that the gas liquefies as it flows around the cell, despite the heat input. The gas's phase transition from gas to liquid results in a pressure drop within the cell, which is indicated using a pressure indicator device. The pressure drop is therefore an indication that a predetermined fill level has been reached. However, as soon as the cell is no longer immersed in liquid hydrogen, the gas is vaporized again using a heating element. This phase transition results in a pressure increase within the cell, which is also indicated using a pressure indicator device. The pressure increase is an indication that the hydrogen level has dropped below the predetermined fill level.

[0037] According to one embodiment, heat is introduced into the gas during steps a), b), c) and d). This preferably means that heat is introduced into the gas during the entire filling process of the storage tank with liquid hydrogen. Preferably, the heating element introduces heat into the gas only during the filling process. Outside the filling process, the heating element preferably does not introduce heat into the gas. In the case of an electric heating element, the electric heating element is preferably only energized during the filling process. Therefore, the electric heating element is preferably not energized outside the filling process.

[0038] The embodiments and explanations given for the storage tank apply correspondingly to the method, and vice versa.

[0039] In this case, "a" should not necessarily be understood as limiting to exactly one element. Rather, several elements, such as two, three, or more, may be provided. Any other number word used herein should also not be understood as limiting to giving the exact number of elements referenced. Rather, unless otherwise indicated, the number may deviate upward or downward.

[0040] Further possible implementations of the storage tank and / or method also include combinations not expressly mentioned of the features or embodiments described above or below with respect to the exemplary embodiments. Those skilled in the art will also add individual aspects as improvements or additions to each of the basic forms of the storage tank and / or method. [Brief explanation of the drawings]

[0041] Further advantageous embodiments and aspects of the storage tank and / or method are the subject of the dependent claims as well as exemplary embodiments of the storage tank and / or method described below. The storage tank and / or method are described in more detail below on the basis of preferred embodiments and with reference to the accompanying drawings. [Figure 1] 1 is a schematic side view of an embodiment of a vehicle. [Figure 2] 2 is a schematic cross-sectional view of an embodiment of a storage tank for the vehicle according to FIG. 1; [Figure 3] This is a detailed view III in FIG. [Figure 4] 3 is a schematic block diagram of an embodiment of a method for indicating that a predetermined filling level of a storage tank has been reached according to FIG. 2;

[0042] In the figures, identical or functionally equivalent elements are designated by the same reference numbers unless otherwise indicated.

[0043] 1 shows a highly simplified schematic side view of one embodiment of a vehicle 1. The vehicle 1 may be, for example, a marine vessel, in particular a ship. The vehicle 1 may be called a marine vehicle. In particular, the vehicle 1 may be a marine passenger ferry. Alternatively, the vehicle 1 may also be a land vehicle. However, it is assumed below that the vehicle 1 is a ship.

[0044] The vehicle 1 comprises a buoyant hull 2. A bridge 3 is provided in or on the hull 2. The vehicle 1 preferably runs on hydrogen. To this end, the vehicle 1 may have a fuel cell 4. In this case, "fuel cell" is understood to mean a galvanic cell that converts the energy of a chemical reaction between a continuously supplied fuel, in this case hydrogen, and an oxidant, in this case oxygen, into electrical energy. The electrical energy obtained can drive an electric motor (not shown), for example, which drives a ship's propeller for driving the vehicle 1.

[0045] To supply hydrogen to the fuel cell 4, a storage tank 5 is provided that stores liquid hydrogen. The storage tank 5 is rotationally symmetrical about a central axis or axis of symmetry 6. The storage tank 5 may be arranged, for example, inside the hull 2, in particular inside the machinery room, on the bridge 3 of the hull 2 ​​that serves as the foundation 7, or on the deck. The axis of symmetry 6 is oriented along the direction of gravity g. That is, the storage tank 5 is arranged upright or vertically. The axis of symmetry 6 is therefore perpendicular to the foundation 7. If the vehicle 1 is, for example, a vehicle that will be converted to hydrogen power, the storage tank 5 may also be arranged, for example, in a chimney or smokestack of the vehicle 1.

[0046] FIG. 2 shows a schematic cross-sectional view of one embodiment of the above-mentioned storage tank 5. The storage tank 5 may also be called a storage vessel. As mentioned above, the storage tank 5 is suitable for storing liquid hydrogen H2 (boiling point at 1 bara: 20.268 K = -252.882 °C). Therefore, the storage tank 5 may also be called a hydrogen storage tank or hydrogen storage vessel. However, the storage tank 5 may also be used for other cryogenic liquids. Examples of cryogenic fluids or liquids, or cryogens for short, in addition to the above-mentioned liquid hydrogen H2, are liquid helium He (boiling point at 1 bara: 4.222 K = -268.928 °C), liquid nitrogen N2 (boiling point at 1 bara: 77.35 K = -195.80 °C), or liquid oxygen O2 (boiling point at 1 bara: 90.18 K = -182.97 °C).

[0047] The storage tank 5 is rotationally symmetrical about an axis of symmetry 6. The storage tank 5 comprises an outer tank 8 which is rotationally symmetrical about the axis of symmetry, and an inner tank 9 which is rotationally symmetrical about the axis of symmetry 6. The inner tank 9 is arranged completely inside the outer tank 8. Between the outer tank 8 and the inner tank 9 an insulating element 10 is provided. The insulating element 10 may be or be designed to be a multilayer insulating layer (MLI).

[0048] The outer tank 8 comprises a tubular or cylindrical base 11 having a rotationally symmetrical design relative to the axis of symmetry 6. The base 11 is closed at both ends by a cover part 12 and a bottom part 13. In the orientation of FIG. 2 or along the direction of gravity g, the cover part 12 is arranged above the bottom part 13. The cross section of the base 11 may have a circular or approximately circular shape. The cover part 12 and the bottom part 13 are curved. The cover part 12 and the bottom part 13 are curved in opposite directions such that the cover part 12 and the bottom part 13 are curved outward relative to the base 11. The outer tank 8 is liquid-tight and in particular airtight.

[0049] Like the outer tank 8, the inner tank 9 includes a tubular or cylindrical base 14 having a rotationally symmetrical design relative to the axis of symmetry 6. In the orientation of FIG. 2, the base 14 is closed at the top by a cover part 15 and at the bottom by a bottom part 16. The cross section of the base 14 may have a circular or approximately circular shape. The cover part 15 and the bottom part 16 are curved. In particular, the cover part 15 and the bottom part 16 are curved in opposite directions such that the cover part 15 and the bottom part 16 are curved outward relative to the base 14. The inner tank 9 is liquid-tight, and in particular air-tight. The outer tank 8 and / or the inner tank 9 may have a blow-off valve (not shown).

[0050] The two cover parts 12, 15 form a cover 17 of the storage tank 5 with the insulating element 10 arranged between them. The two bottom parts 13, 16 form a bottom 18 of the storage tank 5 with the insulating element 10 arranged between them, and the two bottom parts 11, 14 form a wall 19 of the storage tank 5 that extends rotationally symmetrically about the axis of symmetry 6 with the insulating element 10 arranged between them.

[0051] Liquid hydrogen H2 is contained in the inner tank 9. As long as the hydrogen H2 is in a two-phase region, a gas region 20 having vaporized hydrogen H2 and a liquid region 21 having liquid hydrogen H2 can be provided in the inner tank 9. Therefore, after being filled into the inner tank 9, the hydrogen H2 has two phases in different condensation states, namely, liquid and gas. That is, an interface 22 between the liquid hydrogen H2 and the gaseous hydrogen H2 exists in the inner tank 9.

[0052] Once the storage tank 5 is filled with liquid hydrogen H2 in this way, it is necessary to be able to detect when the storage tank 5 is completely filled or when a predetermined filling level in the storage tank 5 has been reached, thereby preventing the liquid hydrogen H2 from overflowing. For example, filling level detection can be achieved by applying a differential pressure method. Furthermore, a capacitance measurement can be performed by changing the dielectric constant in the liquid hydrogen H2. However, these methods are complex. As a result of the physical properties of hydrogen H2, they either do not provide reliable filling level detection or can only be technically implemented in a very complex manner due to the need to measure and calculate different physical quantities in a complex manner.

[0053] Figure 3 shows detail III in Figure 2. The storage tank 5 is equipped with a device 23 (shown only in Figure 3) that indicates a predetermined filling level 24 of liquid hydrogen H2 in the storage tank 5. When the storage tank 5 is completely filled, the predetermined filling level 24 corresponds to the interface 22. The predetermined filling level 24 is defined as a predetermined position of the liquid column of liquid hydrogen H2 contained in the storage tank 5. The predetermined filling level 24 can be achieved, for example, when the storage tank 5 is filled to 88% to 95% with liquid hydrogen H2. The predetermined filling level 24 can also be called a limit value or a predetermined limit value.

[0054] The device 23 comprises a cell 25 that is at least partially or completely immersed in liquid hydrogen H2 when a predetermined filling level 24 is reached. The cell 25 is arranged inside the inner tank 9 in the area of ​​the cover 17. In this case, "cell" is understood to mean a closed volume. The cell 25 may be made of stainless steel. The cell 25 may be spherical. Alternatively, the cell 25 may also be rectangular or cylindrical. The shape of the cell 25 is arbitrary.

[0055] The cell 25 encloses an interior space 26 that contains a gas G that liquefies when the cell 25 comes into contact with liquid hydrogen H2. The gas G may be, for example, neon Ne (boiling point at 1 bara: 27.07 K = -246.08 °C). Therefore, the cell 25 may be referred to as a neon cell. However, other suitable gases may also be used. The boiling point of the gas G contained in the cell 25 is higher than the boiling point of liquid hydrogen H2, so that the gas G liquefies when the cell 25 is immersed in liquid hydrogen H2.

[0056] The cell 25 is fixedly mounted on the storage tank 5 by means of a line 27. The line 27 is led through the inner tank 9, the insulating element 10, and the outer tank 8. For example, the line 27 is welded to the storage tank 5. The gas G can be introduced into the cell 25 through the line 27. For this purpose, a nozzle 28 having a valve 29 can be provided on the line 27. The cell 25 can be filled with the gas G via the valve 29. The line 27 can be made of stainless steel.

[0057] Furthermore, the device 23 includes a pressure indicator device 30 that indicates the pressure drop in the cell 25 when the gas G liquefies and thus indicates that the predetermined filling level 24 has been reached. The pressure indicator device 30 may be a pressure gauge. However, the pressure indicator device 30 may also be or include a pressure sensor. The pressure indicator device 30 may be attached to the nozzle 28, i.e., both the valve 29 and the pressure indicator device 30 may be attached to the nozzle 28. The nozzle 28 may, for example, be welded or soldered to the line 27.

[0058] The device 23 further comprises a heating element 31 suitable for introducing heat W into the cell 25 or into the gas G. In this way, the cell 25 is heated. The cell 25 may therefore also be called a heated cell, in particular a heated neon cell. The heating element 31 may be an electric heating element. The heating element 31 may be arranged in the interior space 26. Alternatively, the heating element 31 may also be provided, for example, outside the cell 25. The heating element 31 is energized by means of a cable 32 guided through a line 27. The cable 32 is guided centrally within the line 27 so that the gas G can be supplied around the cable 32 through the annular opening of the cell 25.

[0059] A control unit 33 arranged external to the storage tank 5 may be assigned to the heating element 31. In the simplest case, the control unit 33 is a switch that is actuated before or during filling of the storage tank 5 in order to energize the heating element 31. However, the control unit 33 may also be or comprise a computer. If the pressure indicator device 30 is or includes a pressure sensor, the control unit 33 may also be suitable for processing the signals of the pressure indicator device 30.

[0060] The function of device 23 is explained below. Depending on requirements, the predetermined filling level 24 can be selected so that when it is reached, the storage tank 5 is filled to 88-95%. In order to prevent an overflow of liquid hydrogen H2 when the storage tank 5 is filled, the filling must be terminated when the predetermined filling level 24 is reached. Before the storage tank 5 is filled, the cell 25 is filled with gas G or the cell 25 is already filled with gas G.

[0061] The heating element 31 is energized during the filling of the storage tank 5, thereby continuously providing a heat output of approximately 5 W and introducing heat W into the gas G. When the cell 25 is immersed in liquid hydrogen H2, i.e., when the predetermined filling level 24 is reached and the liquid hydrogen flows at least partially around the cell 25, the gas G liquefies, causing a pressure drop in the interior space 26 of the cell 25, which can be indicated using the pressure indicator device 30. The heat output of the heating element 31 is such that the gas G liquefies when the cell 25 is immersed in liquid hydrogen H2, regardless of the introduced heat W. If the hydrogen level is decreasing, the heating element 31 ensures that the liquid gas G vaporizes again. This results in an increase in pressure in the cell 25. The pressure indicator device 30 can then be used to recognize when the hydrogen level has dropped below the predetermined filling level 24.

[0062] Furthermore, the heating element 31 ensures that the required heat input W is only required during the measurement task, thus eliminating constant low temperature leaks. The device 23 allows for a clear indication of when a predetermined filling level 24 has been reached.

[0063] 4 is a schematic block diagram of an embodiment of a method for indicating when a predetermined filling level 24 has been reached. The method is carried out using a device 23. In step S1, a cell 25 arranged inside a storage tank 5 is immersed in liquid hydrogen H2. Immersion occurs when the predetermined filling level 24 is reached, i.e., when the hydrogen level in the storage tank 5 rises to the predetermined filling level 24 and the liquid hydrogen flows at least partially around the cell 25. Immersion of the cell 25 is therefore not achieved by moving the cell 25. The cell 25 is arranged in a stationary manner inside the inner tank 9.

[0064] In step S2, the gas G contained in the cell 25 is liquefied by the liquid hydrogen H2. The liquid hydrogen H2 flows at least partially around the cell 25 and removes heat from the gas G as a result of the gas liquefaction. At the same time, in step S3, heat W is continuously introduced into the gas G by means of the heating element 31. The heat power is, for example, 5 W. However, the introduced heat W does not prevent the liquefaction of the gas G when the cell 25 is immersed in the liquid hydrogen H2. As soon as the cell 25 is no longer immersed in the liquid hydrogen H2, or when the liquid hydrogen no longer flows around or leaves the surroundings, the gas G is vaporized again by the introduced heat W.

[0065] In step S4, the pressure drop in the cell 25 when the gas G liquefies is indicated by means of the pressure indicator device 30. The pressure drop thus indicates that the predetermined filling level 24 has been reached. The pressure drop results from the phase transition of the gas G from the gaseous state to the liquid state. Conversely, as soon as the cell 25 is no longer immersed in liquid hydrogen H2, the pressure rises and the gas G is vaporized by means of the heating element 31. Heat W is preferably continuously introduced into the gas G during all steps S1 to S4.

[0066] Although the present invention has been described with reference to exemplary embodiments, the invention can be varied in many ways within the scope of the appended claims. [Explanation of symbols]

[0067] 1 vehicle 2. Hull 3 Bridge 4 fuel cell 5. Storage Tanks 6 Axis of Symmetry 7 Basics 8 Outer Tank 9 Inner Tank 10. Insulating Elements 11 Base 12 Cover 13 Bottom 14 Base 15 Cover part 16 Bottom 17 Cover 18 bottom 19 Wall 20 Gas Region 21 Liquid area 22 Interface 23 devices 24 Fill Level 25 cells 26 Interior Space 27 Line 28 nozzles 29 Valve 30 Pressure Indicator Device 31 Heating element 32 Cable 33 Control Unit g direction of gravity G gas H2 Hydrogen S1 Step S2 Step S3 Step S4 Step W heat

Claims

1. A storage tank (5) for liquid hydrogen (H2), comprising an outer tank (8), an inner tank (9) arranged inside the outer tank (8), and a device (23) for indicating when a predetermined filling level (24) of the liquid hydrogen (H2) has been reached, the device (23) comprising a cell (25), the cell (25) being arranged inside the inner tank (9) and filled with a gas (G) that liquefies when immersed in the liquid hydrogen (H2), the device (23) further comprising a pressure indicator device (30) that indicates a pressure drop in the cell (25) when the gas (G) liquefies, thus indicating that the predetermined filling level (24) has been reached, and a heating element (31) for continuously introducing heat (W) into the gas (G), A storage tank (5) in which the heating element (31) is disposed in an internal space (26) of the cell (25) surrounded by the cell (25).

2. 2. The storage tank (5) according to claim 1, wherein the gas (G) is neon.

3. 3. The storage tank (5) according to claim 1 or 2, wherein the cells (25) are spherical.

4. The storage tank (5) according to any one of claims 1 to 3, further comprising a line (27) carrying said cell (25).

5. 5. The storage tank (5) according to claim 4, wherein the gas can be supplied to the cell (25) using the line (27), and a cable (32) that energizes the heating element (31) is led to the cell (25) through the line (27).

6. 6. A storage tank (5) according to claim 4 or 5, further comprising a valve (29) capable of supplying said gas (G) to said line (27).

7. The storage tank (5) according to any one of claims 4 to 6, wherein the pressure indicator device (30) is in fluid communication with the line (27).

8. The storage tank (5) according to any one of claims 1 to 7, wherein the pressure indicator device (30) is a pressure gauge.

9. The storage tank (5) according to any one of the preceding claims, further comprising a control unit (33) for activating said heating element (31).

10. 10. The storage tank according to any one of claims 1 to 9, wherein the lines (27) of the device (23) carrying the cells are fixedly connected to the wall (19) of the storage tank (5).

11. 1. A method for indicating when a predetermined filling level (24) has been reached in a storage tank (5) for liquid hydrogen (H2), comprising an outer tank (8) and an inner tank (9) arranged inside the outer tank (8), comprising: a) a step (S1) of immersing cells (25) arranged inside the inner tank (9) in the liquid hydrogen (H2) when the predetermined filling level (24) is reached, whereby the cells (25) are immersed in the liquid hydrogen such that the liquid level of the liquid hydrogen inside the inner tank (9) rises to the predetermined filling level and the liquid hydrogen flows at least partially around the cells (25); b) liquefying the gas (G) contained in the cell (25) using the liquid hydrogen (H2) (S2); c) a step (S3) of continuously introducing heat (W) into the gas (G) by means of a heating element (31), the heating element (31) being arranged in an interior space (26) of the cell (25) surrounded by the cell (25); d) using a pressure indicator device (30) to indicate the pressure drop in the cell (25) when the gas (G) liquefies, and thus to indicate when the predetermined filling level (24) has been reached.

12. 12. The method of claim 11, wherein the heat (W) is introduced into the gas (G) during steps a), b), c), and d).

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