Method for removing hydrogen from liquid hydrogen tank, method for completely emptying liquid hydrogen tank and use thereof

Abstract

The invention relates to a method for removing hydrogen from a liquid hydrogen tank (1) having an integrated heating device (5) for the gasification of liquid hydrogen and an emptying line (4) for removing gasified hydrogen, in which heating power is supplied for the removal of gasified hydrogen in order to maintain the tank pressure. The method according to the invention is characterized in that a time curve of the supplied heating power and / or tank pressure is ascertained, said curve being monitored for a sudden change in the pressure gradient of the heating power and / or tank pressure. Furthermore, the invention relates to a method for completely emptying a liquid hydrogen tank (1), and the use of both methods in a vehicle is specified.

Description

Technical Field

[0001] The invention relates to a method for removing hydrogen from a liquid hydrogen tank of the type defined in detail in the preamble to claim 1. The invention also relates to a method for completely emptying a liquid hydrogen tank, from which hydrogen can be removed according to the above method. The invention finally also relates to the use of one or both of the above methods in a liquid hydrogen tank of a vehicle, in particular a commercial vehicle. Background Art

[0002] DE 10 2004 061 025 A1 is hereby substantially the prior art in the technical field to which the present invention belongs. It discloses a container for liquefied gas, which is in particular designed as a cryogenic tank for deep-cooling liquid hydrogen. The liquid hydrogen tank has a heating device for removing hydrogen, which is designed to be integrated into the liquid hydrogen tank. To remove hydrogen, heat is input into the liquid hydrogen via the heating device to vaporize it and maintain the tank pressure so that the vaporized hydrogen can be discharged through an exhaust line and can be supplied, for example, to a hydrogen consumption device of a motor vehicle.

[0003] This liquid hydrogen tank or tank for liquid hydrogen (also called LH2) is designed to keep deep-cold liquid hydrogen for as long as possible during operation, and therefore has good external thermal insulation performance. Here, hydrogen is usually not taken out in liquid form, because the extraction pipeline is usually installed in the area above the gravity direction when the tank is used as specified. Therefore, gaseous hydrogen is taken out from the gas phase space that is usually formed above the liquid surface. Here, the vaporized gas is extracted, and the liquid part is partially vaporized due to the reduction in gas pressure to compensate for the low vapor pressure, thereby increasing the pressure again. However, the remaining liquid continues to cool here, so that the generated gas pressure gradually decreases during the continuous extraction process. In order to cope with this problem and ensure that there is always enough gasified hydrogen extraction pressure, a heating element is generally integrated in this type of liquid hydrogen tank, as described in the above-mentioned prior art. The heating element is then used to supply heat from the outside, so that the gas pressure is maintained at a pressure value suitable for extraction (hereinafter referred to as tank pressure).

[0004] In practice, the temperature and pressure are currently measured to determine the filling level of the tank. In addition, filling level sensors can be used, but due to their principle, they can only measure up to a certain low level. Therefore, if the liquid level is lower than this remaining filling amount, the sensor will not be able to detect it. Especially when the tank is basically completely empty, this can be very inaccurate in practice. In addition, if it is necessary to completely empty the liquid hydrogen tank (for example for maintenance purposes), it is desirable to recognize when the liquid part has been completely emptied or gasified. The structure shown in the prior art cannot achieve this function. Summary of the invention

[0005] The object of the present invention is therefore to specify a method for removing hydrogen from a liquid hydrogen tank of the type defined in detail in the preamble of claim 1, which avoids the disadvantages and ensures efficient detection of the emptying of the liquid hydrogen tank. In addition, the object of the present invention is to specify a method for completely emptying a liquid hydrogen tank based on the above method, for example for repair or maintenance work. Another object of the present invention is to specify a particularly suitable use of the above method.

[0006] According to the invention, the first object of the invention is achieved by a method having the features of claim 1, in particular the features in the characterizing part of claim 1. Advantageous designs of the solution are derived from its dependent claims. The other claims, while referring to the features of claim 1, achieve the complete emptying task by claim 7 and the use task by claim 9. Preferred embodiments of these solutions are also given in their dependent claims.

[0007] Similar to the prior art, the method for extracting hydrogen from a liquid hydrogen tank according to the present invention provides that a heating device integrated in the liquid hydrogen pipe is operated to gasify the liquid hydrogen and discharge the generated hydrogen through the exhaust line. Here, as is common knowledge in the art (especially as shown in the above-mentioned prior art), heating power is input to maintain the tank pressure in order to extract hydrogen.

[0008] The invention now provides for determining the time profile of the supplied heating power and / or the tank pressure, wherein the profile is monitored for the occurrence of sudden changes in the heating power and / or the pressure gradient.

[0009] By monitoring the sudden changes in the heating power and / or the pressure gradient, it is possible to determine when the liquid hydrogen tank is empty, that is, when the liquid hydrogen stored therein is completely gasified, so that the liquid hydrogen tank is therefore empty after the residual gas is removed. Unlike the use of pressure sensors and temperature sensors, the evaluation of the time curve of the heating power and / or the pressure gradient allows a very reliable and efficient possibility to effectively predict the complete gasification of the liquid hydrogen and thus the imminent complete emptying of the tank. The time curve of the heating power and / or the pressure gradient is reliable and convincing here, which is different from the monitoring of temperature and pressure alone, especially because the temperature in the liquid hydrogen tank fluctuates strongly as it is gradually emptied and can vary significantly depending on the position of the temperature sensor in the liquid hydrogen tank.

[0010] According to a very advantageous development of the method according to the invention, an electric heating element can be used as the heating device, the heating power of which is adjusted continuously or in a pulse width modulated manner as a function of the tank pressure, wherein the electric heating power is determined and evaluated in the above-described manner. This is particularly simple and efficient, because in the case of an electric heating element, the heating power can be determined simply and efficiently by measuring electrical parameters such as voltage and current.

[0011] As a supplement or alternative, in an advantageous improvement of the method of the present invention, it can also be provided that the heating device has a heat exchanger, through which the heating medium flows, wherein the heating power is known according to the volume flow of the heating medium and the temperature difference before and after the heat exchanger / upstream and downstream (or inlet and outlet). This heating can be carried out, for example, by a cooling system of an existing structure. If hydrogen is converted into electricity, for example, in a fuel cell system, the fuel cell system generally has a cooling cycle. The cooling medium of this cooling cycle can now be used as a heating medium for the heat exchanger for heating the liquid hydrogen tank heating device. The waste heat of the fuel cell system that originally needs to be cooled and dissipated can be directly utilized here without the need for additional energy input. This heating can therefore improve the overall efficiency. In order to now be able to know the input heating power accordingly, the volume flow and temperature difference can be known in the above manner. The volume flow can be used in particular as a regulating parameter for adjusting the required tank pressure and is adjusted as a target. This can preferably be carried out by using a heating medium conveying device with a continuously controllable speed, a valve device with pulse width modulation control or proportional control, and the like.

[0012] A very advantageous design provision of the method of the present invention is to monitor whether a sudden drop occurs when monitoring the heating power. This is based on the following principle: In general, in order to vaporize the liquid hydrogen in the liquid hydrogen tank, it is necessary to supply energy through the heating device in the liquid hydrogen tank to vaporize the hydrogen. The so-called vaporization enthalpy is about 350 kJ / kg H2 here. Once the liquid hydrogen is completely vaporized, that is, only gaseous hydrogen exists in the liquid hydrogen tank, it is no longer necessary to provide the vaporization enthalpy. Therefore, when the hydrogen withdrawal amount is stable and the tank pressure is constant, the energy supply is significantly reduced, which is manifested as a sudden drop in the heating power to maintain the tank pressure. The sudden drop in heating power is therefore a reliable indication that the liquid hydrogen in the liquid hydrogen tank has been completely vaporized.

[0013] The premise of monitoring the heating power time variation curve is therefore the uniform / stable removal of hydrogen and maintaining a stable tank pressure, which is regulated in particular by the heating power. If this is not the case, but the heating power is input in an on-off or pulsating manner, the pressure will be periodically adjusted with the pulsation of the heating power. This means that the pressure increases periodically during heating or decreases when heating is stopped. In this case, when the liquid hydrogen in the liquid hydrogen tank is completely gasified, the vaporization enthalpy will affect the pressure change. Therefore, in the case of periodic switching of the heating power, the pressure gradient can be monitored. If it is being heated at the current moment, the pressure increases faster than before, and the pressure gradient therefore jumps upward. If it is not heated at the current moment, because the heating power is turned off at this moment according to its cycle, the pressure decreases significantly faster than before, and the pressure gradient therefore jumps downward. This pressure gradient jump can be ideally used to confirm that the liquid hydrogen in the liquid hydrogen tank has been completely gasified in this very advantageous improvement of the method of the present invention for the case of periodically supplied heating power.

[0014] According to a very advantageous development of the method according to the invention, it can now also be provided that an alarm is generated when a sudden change in the heating power and / or the pressure gradient is detected. Such an alarm can, for example, warn that the liquid hydrogen tank is empty, thus ensuring timely replenishment of the liquid hydrogen tank, or can indicate that the liquid hydrogen has completely vaporized, when the liquid hydrogen tank needs to be emptied, for example for maintenance, disassembly, etc.

[0015] For the method for completely emptying a liquid hydrogen tank of the present invention for removing hydrogen according to any of the designs of the above-mentioned method, it can now be provided that after a sudden change is detected, for example, when an alarm is generated according to the design described above, the liquid hydrogen tank is further heated as required until the limit temperature is reached, and then purged with an inert gas. The gas can be nitrogen in particular. The limit temperature is set here to be greater than or equal to the boiling point temperature of the inert gas. In the case of nitrogen, it is, for example, -196°C. This ensures that the nitrogen introduced for purging the liquid hydrogen tank is not liquefied in the liquid hydrogen tank having good thermal insulation properties, but exists in a gaseous state, so as to purge the gaseous hydrogen remaining in the liquid hydrogen tank, thereby enabling safe maintenance, disassembly, etc. of the liquid hydrogen tank.

[0016] The above-described method for extracting hydrogen from a liquid hydrogen tank is now particularly suitable for extracting hydrogen from a liquid hydrogen tank of a vehicle, which is driven at least partially by means of the extracted hydrogen. The extracted hydrogen is supplied to hydrogen-consuming devices in the vehicle, such as internal combustion devices (such as internal combustion engines, gas turbines, etc.) that operate on hydrogen, or is generated by a fuel cell system and used for the electric drive of the vehicle. The method according to the invention is particularly suitable for such applications. It allows timely and reliable determination of the empty state of the liquid hydrogen tank in a safe, reliable manner and in a manner that is substantially unaffected by environmental parameters that may vary slightly.

[0017] Furthermore, the method, in particular in combination with the above-described method for completely emptying the liquid hydrogen tank, allows safe emptying in order to be able to carry out, for example, maintenance work, repair measures, etc. on the vehicle, in which case no hydrogen should remain in the liquid hydrogen tank, either because the maintenance directly concerns the liquid hydrogen tank or the filling system, or because longer maintenance periods or parking times are expected, during which it is necessary to reliably prevent the hydrogen from diffusing outwards (for example into a garage).

[0018] According to another very advantageous design of the use in the vehicle, it can be provided that, at the same time as the alarm is issued, the remaining range of the vehicle is estimated and output together with the alarm. As mentioned above, the moment when all the liquid hydrogen in the liquid hydrogen tank is gasified is determined based on a sudden change in the heating power and / or the pressure gradient. The liquid hydrogen tank is still filled with gaseous hydrogen at this moment, so at the moment determined by the sudden change, the remaining gaseous hydrogen corresponds to the volume of the liquid hydrogen tank. This volume- and pressure-dependent remainder can be determined based on a simple pressure measurement and allows an estimate of the remaining range that the vehicle can still travel using the gaseous hydrogen that has been gasified. An alarm is issued to the driver, or in the case of autonomous or partially autonomous driving of the vehicle, to a control center, thereby allowing the triggering of corresponding measures, such as switching to another energy source, replenishing hydrogen, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Further advantageous embodiments of the method according to the invention and of its use also result from the exemplary embodiments which are described in detail below with reference to the figures.

[0020] Figure 1 As the only FIGURE, a schematic illustration of a liquid hydrogen tank is shown here, with which the method according to the invention can be carried out and which is suitable in particular for use in a vehicle. DETAILED DESCRIPTION

[0021] exist Figure 1 In the figure, a liquid hydrogen tank, which is generally indicated by 1, can be seen, which is located inside the heat insulation part 2 shown. This liquid hydrogen tank 1 (also called a cryogenic tank or a freezing tank) is used to store liquid hydrogen at a corresponding low temperature, so the hydrogen is at least in the liquid hydrogen tank 1 along the gravity direction F g The liquid hydrogen tank 1 can be filled via the filling line 3. Hydrogen can be taken out from the liquid hydrogen tank 1 via the extraction line 4. In the simplified illustration selected here, valves, filters, check members, etc. in the two lines 3 and 4 are omitted. Generally, the liquid hydrogen tank 1 is also provided with an exhaust pipe not shown here, and when a safety critical overpressure occurs in the liquid hydrogen tank 1, gas can be discharged from the exhaust pipe via an overpressure valve, a rupture member, etc.

[0022] Now, in normal operation, or when the liquid hydrogen tank needs to be completely emptied in order to, for example, maintain the liquid hydrogen tank 1 itself or the filling system connected thereto, hydrogen is removed from the liquid hydrogen tank 1. Here, in general, it is neither prescribed nor possible to completely remove the liquid hydrogen in liquid form, especially because the filling line 3 (as shown in the figure) usually does not extend to the lower area of ​​the liquid hydrogen tank 1 or its inner cavity when it is used as required in the direction of gravity g. Instead, the extraction line 4 is responsible for extraction, which is used to remove low-temperature gaseous hydrogen from the liquid hydrogen tank 1. This gaseous hydrogen accumulates above the liquid (f) in the liquid hydrogen tank 1 as a gas cushion (g), generally always exists and can be removed through the extraction line 4. As a result, the gas pressure in the liquid hydrogen tank 1 is reduced, and part of the liquid hydrogen is vaporized to compensate for the reduced tank pressure. This causes the gas pressure to rise again. However, the remaining liquid continues to cool due to the vaporization enthalpy required for vaporization, so during the continuous removal process, the tank pressure generated will also continue to decrease. Once the tank pressure drops below atmospheric pressure, it is impossible to continue to remove it due to the principle. In practice, hydrogen is usually taken out for consumption, such as supplying to a fuel cell system. Because a certain hydrogen pressure is required in a hydrogen-using system, it can only be taken out until the required system pressure is reached.

[0023] Therefore, it is generally common in practice that the liquid hydrogen tank 1 includes a heating device 5 integrated into its inner cavity, whereby the hydrogen can be gasified by external heat supply. Thus, the tank pressure can be maintained or adjusted to a desired pressure value. This allows hydrogen to be continuously taken out of the liquid hydrogen tank 1.

[0024] In order to determine whether the liquid hydrogen tank 1 has been emptied, it is necessary to detect whether there is still liquid hydrogen in the liquid hydrogen tank 1, or whether only low-temperature hydrogen remains in the liquid hydrogen tank 1. In addition to the pressure sensor 6, which is used to adjust the heating power of the heating device 5, a temperature sensor (not shown here) is generally provided in the liquid hydrogen tank 1. However, its applicability in determining whether the storage tank is empty is limited, because the temperature may experience large local fluctuations, especially in the case of rapid emptying. This is particularly evident when there is residual liquid hydrogen in the liquid hydrogen tank 1, because, for example, when the liquid hydrogen tank 1 is installed in a vehicle, due to dynamic movement, these residual liquid hydrogen may flow to different areas of the inner cavity of the liquid hydrogen tank 1, such as shaking movement.

[0025] Now the typical process of extracting hydrogen from the liquid hydrogen tank 1 can be used, because as long as there is still liquid hydrogen in the liquid hydrogen tank 1, in order to maintain the pressure in the area of ​​the pressure sensor 6, the liquid hydrogen must be gasified by continuous heat supply. The energy supply required here corresponds to a vaporization enthalpy of about 350 kJ / kg H2, which must be provided by the heating device 5. Once the liquid hydrogen in the liquid hydrogen tank 1 has been completely gasified, only low-temperature gaseous hydrogen remains in the inner cavity of the liquid hydrogen tank 1, and the vaporization enthalpy does not need to be provided. The energy to be supplied and the heating power required to maintain the tank pressure therefore drop suddenly.

[0026] Therefore, it is proposed to combine "uniform extraction of hydrogen through the extraction line 4" with "measurement of the variation curve of the required heating power of the heating element 5", so that it can be determined whether the liquid hydrogen is completely gasified, that is, whether the liquid hydrogen tank is about to be emptied, through the time variation curve of the supplied heating power. Here, in normal operation, the uniform extraction process can be monitored accordingly and adjusted by calculation if necessary. When it is necessary to empty the liquid hydrogen tank 1 by extraction for maintenance purposes, it is recommended to extract through a throttling device with a fixed cross section to ensure a constant extraction mass flow.

[0027] The power measurement can then be carried out by the tank control device in the following manner, that is, in the case of heating of the heating element 5, the power is indirectly determined, for example, by the coolant of the vehicle cooling circuit. In this case, the calculation of the supplied heating power is carried out by the coolant volume flow and the difference between the inlet temperature and the return temperature (i.e. the difference in the heating medium temperature at the inlet and outlet / upstream and downstream of the heating device 5 designed as a heat exchanger in this case). As an alternative to this, when using a heating device 5 designed as an electric heating element, the heating power is directly measured by electrical parameters.

[0028] Regardless of the method used to measure the heating power supplied by the heating device 5 to maintain the tank pressure, when the tank pressure is stable or continuously taken out, or when it is dynamically taken out but converted to an equivalent continuous extraction value by calculation, when the liquid hydrogen in the liquid hydrogen tank 1 has been completely vaporized, it is no longer necessary to provide the vaporization enthalpy. Therefore, the required heat suddenly decreases, so that the heating power required for continuous withdrawal or maintaining the tank pressure also suddenly decreases. This sudden drop in heating power thus indicates that the liquid hydrogen has been completely vaporized. Therefore, the liquid hydrogen tank 1 has been emptied, and only low-temperature hydrogen remains in the inner cavity of the liquid hydrogen tank 1.

[0029] When pulsed (or intermittent) heating is used (i.e., the heating power is repeatedly turned on and off), the pressure in the container will always increase periodically during the heating period and decrease during the non-heating period. In this case, the jump in the pressure gradient can be used to detect whether the liquid hydrogen in the container has reached a completely gasified state. If it is being heated at the current moment, the pressure increases faster than before, and the pressure gradient therefore jumps upward. If it is not being heated at the current moment, the pressure decreases correspondingly faster than before, and the pressure gradient therefore jumps downward in this case.

[0030] According to the sudden change of heating power or pressure gradient, the following state can be judged when the heating power is input in a pulsating manner: the liquid hydrogen has been completely gasified, that is, the liquid hydrogen tank 1 is about to be emptied. In this case, for example, when the liquid hydrogen tank 1 is used in a vehicle to provide hydrogen to the fuel cell system, an alarm can be generated. The alarm can then be output to the driver, or to the control center in the case of an autonomous driving vehicle. This is particularly efficient because when the liquid hydrogen tank 1 is in the state, the liquid level display based on pressure and temperature is relatively meaningless for the above reasons. At this time, the remaining low-temperature hydrogen in the liquid hydrogen tank 1 can still be used to travel a short distance, so emergency operation can be achieved. The output alarm can especially include a warning that the liquid hydrogen tank 1 is about to be emptied and the remaining mileage estimated based on the remaining gas amount in the liquid hydrogen tank 1. Here, when estimating the remaining mileage, it is also feasible and reasonable to take into account the power of the generally existing hybrid battery, the current vehicle weight, the road section terrain, etc.

[0031] In the case where the liquid hydrogen tank 1 needs to be completely emptied, a sudden change in the heating power and / or pressure gradient indicates that the liquid hydrogen in the liquid hydrogen tank 1 is completely gasified. Here, in order to continue to empty, an inert non-flammable gas (such as nitrogen) can be filled into the liquid hydrogen tank 1 to discharge the remaining hydrogen and ensure a safe state. For this reason, after the liquid hydrogen is completely gasified, it is usually necessary to continue to heat the liquid hydrogen tank 1 until the remaining gaseous hydrogen in the liquid hydrogen tank 1 can be replaced by a non-flammable gas. In general, after the liquid phase is completely gasified, the hydrogen in the tank is at about -230°C. The nitrogen introduced will liquefy or even solidify at this temperature, so the gas and the liquid hydrogen tank 1 must be heated to a temperature of at least -196°C (i.e., the boiling point temperature of nitrogen) before the nitrogen can be filled. When other inert gases or non-flammable gases or gas mixtures are used, the temperature can be changed accordingly, or when helium or neon is used under certain conditions, because the boiling point temperature of these gases is lower than -230°C, the heating process can be completely omitted. However, rare gases bring the disadvantage of corresponding high costs.

[0032] Finally, the liquid hydrogen tank 1 is purged with an inert gas until the internal temperature of the liquid hydrogen tank 1 reaches the ambient temperature and the residual hydrogen concentration in the liquid hydrogen tank 1 is low enough that an explosive or flammable mixture cannot be formed even if it contacts the air.

Claims

1. A method for removing hydrogen from a liquid hydrogen tank (1), the liquid hydrogen tank having an integrated heating device (5) for gasifying the liquid hydrogen and an emptying line (4) for removing the gasified hydrogen, wherein: supplying heating power to maintain the tank pressure in order to remove the vaporized hydrogen, It is characterized in that a time profile of the supplied heating power and / or the tank pressure is determined, wherein the profile is monitored for sudden changes in the pressure gradient of the heating power and / or the tank pressure.

2. The method according to claim 1, characterized in that An electric heating element is used as the heating device (5), and the heating power of the electric heating element is supplied continuously or in a pulsed manner.

3. The method according to claim 1 or 2, characterized in that: As the heating device (5), a heat exchanger is provided through which a heating medium flows, wherein the heating power is determined as a function of the volume flow of the heating medium and the temperature difference upstream and downstream of the heat exchanger.

4. The method according to claim 1, 2 or 3, characterized in that: In the case where the heating power is continuously adjusted, it is monitored whether the heating power is suddenly reduced during the uniform removal of hydrogen.

5. The method according to claim 1, 2 or 3, characterized in that: The pressure gradient is monitored when the heating power is supplied in a pulsating manner, wherein it is monitored whether a sudden increase or a sudden decrease in the pressure gradient occurs, depending on whether heating is currently being applied.

6. The method according to any one of claims 1 to 5, characterized in that When a sudden change is detected, an alarm is generated and output.

7. A method for completely emptying a liquid hydrogen tank (1), from which hydrogen is removed according to a method according to any one of claims 1 to 6, wherein: After the sudden change is detected, the liquid hydrogen tank (1) is heated until the limit temperature is reached and then purged with an inert gas.

8. The method according to claim 7, characterized in that The limit temperature is set to be greater than or equal to the boiling point temperature of the inert gas.

9. Use of the method for removing hydrogen according to one of claims 1 to 6 and / or the method for emptying according to claim 7 or 8 in a liquid hydrogen tank (1) of a vehicle that is driven at least partially by means of the removed hydrogen, wherein: The removed hydrogen is provided for combustion or for generating electricity in a fuel cell system.

10. The use according to claim 9, characterized in that In the event of an alert, the vehicle's remaining range is estimated and output together with the alert.

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