Method for removing hydrogen, for completely evacuating a 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 evaporating the liquid hydrogen and having an evacuation line (4) for removing the evaporated hydrogen, wherein heating power is supplied for the removal of the evaporated hydrogen, in order to maintain the tank pressure. The method according to the invention is characterised in that the temporal course of the supplied heating power and / or of the tank pressure is sensed, wherein the course is monitored for a sudden change in the heating power and / or in the pressure gradient of the tank pressure. The invention also relates to a method for completely evacuating a liquid-hydrogen tank (1). Also specified is the use of the two methods in a vehicle.

Description

[0001] Method for extracting hydrogen, for completely emptying a liquid hydrogen tank and its use

[0002] The invention relates to a method for withdrawing hydrogen from a liquid hydrogen tank according to the type defined in more detail in the preamble of claim 1. Furthermore, the invention relates to a method for completely emptying a liquid hydrogen tank from which hydrogen is withdrawn according to the method. Finally, the invention also relates to the use of one of the methods or both methods in a liquid hydrogen tank of a vehicle, in particular a commercial vehicle.

[0003] The generic state of the art here is essentially the

[0004] DE 10 2004 061 025 A1. This discloses a container for condensed gas, which is designed in particular as a cryogenic tank for cryogenic liquid hydrogen. This liquid hydrogen tank has a heating device for extracting hydrogen, which is integrated into the interior of the liquid hydrogen tank. To extract the hydrogen, heat is introduced into the liquid hydrogen via this heating device to evaporate it and maintain the tank pressure, so that the evaporated hydrogen can be discharged via a discharge line and used, for example, to supply a consumer of a motor vehicle.

[0005] Such liquid hydrogen tanks or tanks for liquid hydrogen, which is also referred to as LH2, are designed to retain the cryogenic liquid hydrogen for as long as possible during operation and are therefore very well thermally insulated from the outside. The hydrogen cannot typically be withdrawn in liquid form here, as the withdrawal line is usually located in the upper area of ​​the tank in the direction of gravity during normal use. Therefore, a gaseous portion of the hydrogen is withdrawn from a gas cushion that typically forms above the liquid. The vaporized gas is drawn off and the liquid portion will partially evaporate due to the falling gas pressure to compensate for the low vapor pressure, causing the pressure to rise again. However, the remaining liquid cools further, so that the resulting gas pressure increasingly drops with continuous withdrawal.To counteract this and ensure that sufficient pressure is always maintained for the extraction of vaporized hydrogen, such liquid hydrogen tanks typically incorporate heating elements, as described in the above-mentioned prior art. These then serve to supply heat from the outside and thus maintain the gas pressure at a pressure suitable for extraction, which is subsequently referred to as the tank pressure.

[0006] In practice, temperature and pressure are recorded to determine the tank's fill level. A fill level sensor can also be used, which, by design, can only measure down to a lower level. If the liquid level then falls below this residual fill level, the sensor can no longer detect it. This is very inaccurate in practice, particularly if the tank is almost completely empty. Furthermore, if a complete emptying of the liquid hydrogen tank is necessary, for example, for maintenance purposes, it would be desirable to detect when the liquid portion has been completely emptied or evaporated. The design shown in the prior art cannot achieve all of this.

[0007] It is therefore the object of the present invention to provide a method for withdrawing hydrogen from a liquid hydrogen tank according to the type defined in more detail in the preamble of claim 1, which avoids the aforementioned disadvantages and ensures efficient detection of the emptying of the liquid hydrogen tank. Furthermore, it is the object of the present invention to provide a method based thereon for completely emptying the liquid hydrogen tank, for example for repair or maintenance work. A further object of the invention is to provide a particularly suitable use of the method. According to the invention, the first-mentioned object of the present invention is achieved by a method having the features in claim 1, and here in particular in the characterizing part of claim 1. Advantageous embodiments of this solution emerge from the dependent claims dependent thereon.The further claims, incorporating the features of this claim 1, solve the problem through claims 7 regarding complete emptying and 9 regarding use. Here, too, advantageous embodiments arise from the respective dependent subclaims.

[0008] The method according to the invention for withdrawing hydrogen from a liquid hydrogen tank provides, comparable to the prior art, that an integrated heating device is operated in the liquid hydrogen tank to evaporate liquid hydrogen and discharge the resulting hydrogen gas through a discharge line. As is generally known and in particular also from the aforementioned prior art, heating power is supplied for the withdrawal of hydrogen in order to maintain the tank pressure.

[0009] According to the invention, it is now provided that the temporal course of the supplied heating power and / or the tank pressure is recorded, wherein the course is monitored for a sudden change in the heating power and / or the pressure gradient.

[0010] Such monitoring for sudden changes in heating output and / or pressure gradient makes it possible to determine when the liquid hydrogen tank is running low, or when the liquid hydrogen stored in it has completely evaporated, so that after the remaining gas is removed from the liquid hydrogen tank, the tank is correspondingly empty. Unlike the use of pressure and temperature sensors, the evaluation of the temporal progression of heating output and / or pressure gradient provides a very reliable and efficient way to efficiently predict the complete evaporation of the liquid hydrogen and thus the impending complete emptying of the tank.The temporal progression of the heating power and / or the pressure gradient is reliable and meaningful, unlike the monitoring of temperatures and pressures alone, especially since the temperatures within the liquid hydrogen tank are subject to strong fluctuations as it becomes increasingly empty and can vary greatly depending on the position of the temperature sensor within the liquid hydrogen tank.

[0011] According to a highly advantageous development of the method according to the invention, an electric heating element can be used as the heating device, the heating output of which is continuously or pulse-width modulated to the tank pressure, with the electric heating output being recorded and evaluated as described above. This is particularly simple and efficient, since recording the heating output of an electric heating element is possible simply and efficiently by measuring the electrical quantities voltage and current.

[0012] Additionally or alternatively, in an advantageous development of the method according to the invention, the heating device can also have a heat exchanger through which a heating medium flows, wherein the heating output is determined based on the volume flow of the heating medium and the temperature difference before and after the heat exchanger. Such heating can, for example, be achieved via a cooling system of an already existing structure. If the hydrogen is converted into electrical power in a fuel cell system, for example, this fuel cell system typically has a cooling circuit. The cooling medium of such a cooling circuit can then be used as a heating medium for the heat exchanger to heat the heating device of the liquid hydrogen tank. The waste heat generated by the fuel cell system, which has to be cooled away anyway, is available without the need for additional energy.This type of heating can therefore increase the overall efficiency. In order to be able to record the introduced heating output accordingly, the volume flow and the temperature difference can be recorded as described above. In particular, the volume flow can serve as a control variable for setting the required tank pressure and can be regulated accordingly. This can preferably be achieved by a conveying device for the heating medium whose speed can be continuously controlled, a pulse-width modulated or proportionally controlled valve device, or the like. A very advantageous embodiment of the method according to the invention provides that when monitoring the heating output, it is monitored for any sudden drop.This is based on the consideration that energy is typically required to evaporate the liquid hydrogen in the liquid hydrogen tank via the heating device inside the liquid hydrogen tank. This so-called enthalpy of vaporization is approximately 350 kJ / kg H2. As soon as the liquid hydrogen has completely evaporated and only gaseous hydrogen remains in the liquid hydrogen tank, this enthalpy of vaporization no longer needs to be applied. The energy required therefore drops significantly with a constant hydrogen withdrawal and thus a constant tank pressure, which becomes noticeable through a sudden drop in the heating output to maintain the tank pressure. This sudden drop in heating output is therefore a reliable indicator that the liquid hydrogen in the liquid hydrogen tank has been completely evaporated.

[0013] Monitoring the temporal progression of the heating output therefore requires a consistent withdrawal of hydrogen and thus a consistent tank pressure, which is regulated in particular by the heating output. If this is not the case and the heating output is supplied in a switching or pulsating manner, then the pressure will also cyclically adapt to this pulsing of the heating output. This means that the pressure rises cyclically when heating is on and falls when heating is not on. Here, too, if the liquid hydrogen in the liquid hydrogen tank has completely evaporated, the enthalpy of vaporization flows into a change in pressure. For this purpose, the pressure gradient can be monitored for the case of cyclical switching of the heating output on and off. If heating is taking place at the current time, the pressure will rise more quickly than before, i.e. the pressure gradient will jump upwards.If heating is not taking place at the current time because the heating power is switched off at that time due to its pulsation, then the pressure drops much more quickly than before, i.e. the pressure gradient drops sharply. In the case of pulsating heating power, such a jump in the pressure gradient can be ideally used in this particularly advantageous development of the method according to the invention to determine that the liquid hydrogen in the liquid hydrogen tank has been completely evaporated. According to a very advantageous development of the method according to the invention, it can now further be provided that if a sudden change is detected, be it in the heating power and / or the pressure gradient, a warning message is generated. Such a warning message can, for example, warn of an empty liquid hydrogen tank and thus ensure that the liquid hydrogen tank can be refueled quickly.In the event of an intentional emptying of the liquid hydrogen tank, for example for maintenance purposes, disassembly or the like, it can indicate that the liquid hydrogen has been completely evaporated.

[0014] The inventive method for completely emptying a liquid hydrogen tank, from which hydrogen is extracted according to the method described above in one of the described embodiments, can now provide that after the sudden change is detected, for example, when the warning message is generated according to the last-described embodiment, the liquid hydrogen tank is further heated, if necessary, until a limit temperature is reached and then purged with an inert gas. This gas can be nitrogen, in particular. The limit temperature is specified to be greater than or equal to the boiling point of the inert gas. For nitrogen, this would be -196°C, for example.This ensures that the nitrogen introduced to flush the liquid hydrogen tank is not liquefied within the very well thermally insulated liquid hydrogen tank, but is in gaseous form in order to flush out the hydrogen remaining in gaseous form in the liquid hydrogen tank, so that safe maintenance, disassembly or the like of the liquid hydrogen tank is possible.

[0015] The method described above for extracting hydrogen from a liquid hydrogen tank is particularly suitable for extracting hydrogen from a liquid hydrogen tank of a vehicle powered at least partially by the extracted hydrogen. The extracted hydrogen is supplied to a consumer within the vehicle, for example, a hydrogen-powered combustion engine, such as an internal combustion engine, a gas turbine, or the like, or it is converted into electricity in a fuel cell system and used to electrically drive the vehicle. The method according to the invention is ideally suited for such applications. It enables the empty state of the liquid hydrogen tank to be determined in a timely and reliable manner, safely, reliably, and largely independently of environmental parameters, which can vary very slightly.

[0016] Furthermore, the method, particularly in combination with the previously described method for completely emptying a liquid hydrogen tank, enables safe emptying, for example, to carry out maintenance work, repair measures, or the like on the vehicle where no hydrogen should remain in the liquid hydrogen tank. This may be because the maintenance directly affects the liquid hydrogen tank or the tank system, or because a longer maintenance phase or downtime is expected, during which the diffusion of hydrogen, for example, within a vehicle hall, should be reliably prevented.

[0017] According to a further very advantageous embodiment of this use in a vehicle, it can be provided that in the event of a warning message, the remaining range of the vehicle is estimated and issued together with the warning message. As already mentioned, the sudden change in heating output and / or pressure gradient detects the point in time at which all of the liquid hydrogen within the liquid hydrogen tank has evaporated. At this point in time, the liquid hydrogen tank is still filled with gaseous hydrogen, so that at this point in time, which was detected via the sudden change, a residual amount of gaseous hydrogen corresponding to the volume of the liquid hydrogen tank is present. This residual amount, which depends on volume and pressure, can be determined using a simple pressure measurement and makes it possible to estimate the remaining range of the vehicle that can still be achieved with this already evaporated gaseous hydrogen.The warning to a person driving the vehicle or to a control center in the case of an autonomous or partially autonomous vehicle thus makes it possible to initiate appropriate measures, such as switching to another energy source, refueling with hydrogen, or the like. Further advantageous embodiments of the method according to the invention and their use also emerge from the exemplary embodiment, which is described in more detail below with reference to the figure.

[0018] The only attached Figure 1 shows the schematic representation of a liquid hydrogen tank with which the methods according to the invention can be carried out and which is particularly suitable for use in a vehicle.

[0019] In the illustration of Figure 1, a liquid hydrogen tank, designated in its entirety by 1, can be seen within an indicated thermal insulation 2. Such liquid hydrogen tanks 1, which are also referred to as cryogenic tanks or cryo-tanks, are used to store liquid hydrogen at correspondingly low temperatures, so that this hydrogen remains liquid at least in the direction of gravity F glower area of ​​the liquid hydrogen tank 1. The liquid hydrogen tank 1 can be refueled via a filling line 3. Hydrogen can be withdrawn from the liquid hydrogen tank 1 via a withdrawal line 4. In the simplified representation chosen here, valve elements, filters, check elements and the like are omitted within these two lines 3, 4. Typically, a liquid hydrogen tank 1 also has a discharge chimney (not shown here), through which gas can be discharged through a pressure relief valve, a bursting element or the like in the event of a safety-critical overpressure occurring in the liquid hydrogen tank 1.

[0020] Hydrogen is now withdrawn from the liquid hydrogen tank 1 during regular operation, or also in the event that it has to be completely emptied, for example in order to service the liquid hydrogen tank 1 itself or a tank system connected to it. Complete withdrawal of the liquid hydrogen as a liquid is typically neither intended nor possible, in particular because the filling line 3, as indicated here, typically does not end in the lower region of the liquid hydrogen tank 1 or its internal volume in the direction of gravity g during intended use. Rather, the withdrawal is carried out by the withdrawal line 4, via which cold gaseous hydrogen is withdrawn from the liquid hydrogen tank 1. This gaseous hydrogen, which collects as a gas cushion (g) above the liquid (f) in the liquid hydrogen tank 1, is typically always present and can be withdrawn via the withdrawal line 4.As a result, the gas pressure in the liquid hydrogen tank 1 drops, and a portion of the liquid hydrogen evaporates to compensate for the falling tank pressure. This causes the gas pressure to rise again. However, the remaining liquid continues to cool due to the evaporation enthalpy required for evaporation, so that with continuous withdrawal, the resulting tank pressure also continues to drop. As soon as the tank pressure drops below atmospheric pressure, withdrawal is, in principle, no longer possible. In practice, withdrawal typically occurs for the consumption of the hydrogen, for example, in a fuel cell system. Since a certain hydrogen pressure is required within the system consuming it, withdrawal would only be possible up to this necessary system pressure.

[0021] In practice, it is therefore generally customary for the liquid hydrogen tank 1 to include a heating device 5 integrated into its interior volume, with which heat can be supplied from the outside to vaporize hydrogen. This allows the tank pressure to be maintained or adjusted to a desired pressure value. This makes the continuous withdrawal of hydrogen from the liquid hydrogen tank 1 possible.

[0022] To determine whether the liquid hydrogen tank 1 has been emptied, it is necessary to detect whether liquid hydrogen is still present in the liquid hydrogen tank 1 or whether only cold hydrogen gas remains in the liquid hydrogen tank 1. In addition to a pressure sensor 6, which is used, for example, to regulate the heating output of the heating device 5, a temperature sensor (not shown here) is typically also present in the liquid hydrogen tank 1. However, this is only of limited use for detecting an empty tank, since the temperature can vary greatly locally, especially during rapid emptying.This applies in particular if residues of liquid hydrogen are present in the liquid hydrogen tank 1, which, for example, due to dynamic movements of the liquid hydrogen tank 1, if it is installed in a vehicle, can also reach various areas of the internal volume of the liquid hydrogen tank 1, for example due to sloshing movements or the like.

[0023] However, the typical procedure for withdrawing hydrogen from the liquid hydrogen tank 1 can now be utilized, since as long as there is still liquid hydrogen in the liquid hydrogen tank 1, it must be evaporated via a continuous heat supply in order to maintain the pressure in the area of ​​the pressure sensor 6. The required energy supply corresponds to the evaporation enthalpy of approximately 350 kJ / kg H2, which must be applied via the heating device 5. As soon as the liquid hydrogen in the liquid hydrogen tank 1 has completely evaporated and only cold gaseous hydrogen remains in the volume of the liquid hydrogen tank 1, this evaporation enthalpy no longer needs to be applied. The energy required to be supplied, and thus the heating power required to maintain the tank pressure, drops dramatically.

[0024] It is therefore proposed to combine a uniform withdrawal of hydrogen via the withdrawal line 4 with the measurement of the required heating power curve at the heating element 5, so that the temporal progression of the supplied heating power can be used to determine when the liquid hydrogen has completely evaporated and the liquid hydrogen tank is therefore close to being emptied. The uniform withdrawal can be recorded accordingly during regular operation and, if necessary, adjusted mathematically. In the event of withdrawal to empty the liquid hydrogen tank 1 for maintenance reasons, withdrawal via an orifice with a fixed cross-section is recommended, thus ensuring a constant withdrawal mass flow.

[0025] The power can then be measured via a tank control unit, whereby, in the case of heating element 5 being heated, for example, via a coolant in a vehicle's cooling circuit, the power is determined indirectly. In this case, the supplied heating power is calculated using the volume flow of this coolant and the difference between the flow and return temperatures, i.e., the temperature of the heating medium before and after the heating device 5, which in this case is designed as a heat exchanger. Alternatively, when using a heating device 5 designed as an electrical heating element, the heating power can be measured directly using the electrical variables.

[0026] Regardless of how the heating power supplied to the heating device 5 to maintain the tank pressure is measured, the situation is that with continuous tank pressure or continuous withdrawal, or, in the case of dynamic withdrawal, even with values ​​mathematically converted to such continuous withdrawal, the moment the liquid hydrogen in the liquid hydrogen tank 1 has been completely evaporated, the evaporation enthalpy no longer needs to be applied. The required amount of heat thus drops abruptly, so that the heating power required for continuous withdrawal or to maintain the tank pressure also drops abruptly. This abrupt drop in heating power is then an indication that the liquid hydrogen has been completely evaporated. The liquid hydrogen tank 1 is therefore empty except for the remaining cold gaseous hydrogen in the volume of the liquid hydrogen tank 1.

[0027] If the heating power is applied in a pulsating manner, i.e. is repeatedly switched on and off, the pressure within the volume also rises cyclically whenever heating is taking place, and falls whenever heating is not taking place. In this case, the point at which the liquid hydrogen in the volume has completely evaporated can be detected by a jump in the pressure gradient. If heating is taking place at the current time, the pressure rises faster than before, i.e. the pressure gradient jumps upwards. If heating is not taking place at the current time, the pressure drops correspondingly faster than before, i.e. the pressure gradient jumps downwards.

[0028] By means of a sudden change in the heating power or the pressure gradient, when the heating power is applied in a pulsating manner, the described condition, in which the liquid hydrogen has completely evaporated and the liquid hydrogen tank 1 is therefore close to being empty, can be detected. In this case, a warning message can be generated, for example, if the liquid hydrogen tank 1 is used in a vehicle to provide hydrogen for a fuel cell system. The warning message can then be issued to the driver or, in the case of an autonomous vehicle, to a control center. This is particularly efficient because, in this condition of the liquid hydrogen tank 1, a level indicator based on pressure and temperature is relatively useless for the reasons already described above.The cold hydrogen gas contained in the liquid hydrogen tank 1 can still be used to cover a short distance, thus enabling emergency operation. The warning message issued may, in particular, include a warning about the empty liquid hydrogen tank 1 as well as an estimated remaining range based on the remaining gas quantity in the liquid hydrogen tank 1. When estimating the remaining range, it is also conceivable and sensible to consider the charge level of a typically present hybrid battery, the current vehicle weight, route topography, or similar factors.

[0029] In the event that the liquid hydrogen tank 1 is to be completely emptied, the sudden change in heating power and / or pressure gradient indicates that the liquid hydrogen in the liquid hydrogen tank 1 has completely evaporated. For further emptying, the liquid hydrogen tank 1 can now be filled with a non-flammable inert gas, such as nitrogen, to flush out the remaining hydrogen and ensure a safe condition. For this purpose, it is usually necessary, after the liquid hydrogen has completely evaporated, to further heat the liquid hydrogen tank 1 until the remaining gaseous hydrogen in the liquid hydrogen tank 1 can be replaced by the non-flammable gas. Typically, the hydrogen gas within the tank is present at approximately -230°C after the liquid phase has completely evaporated.Introduced nitrogen would liquefy or even solidify at these temperatures, which is why the gas, and thus the liquid hydrogen tank 1, must be heated to a temperature of at least -196°C, the boiling point of nitrogen, before the nitrogen can be added. When using other inert or non-flammable gases or gas mixtures, this temperature may change accordingly; or, when using helium or, to a limited extent, neon, heating may be completely unnecessary, as their boiling point is below -230°C. However, noble gases have the disadvantage of correspondingly high costs.Finally, the liquid hydrogen tank 1 is then flushed with the inert gas until the internal temperature of the liquid hydrogen tank 1 has adjusted to the ambient temperature and the residual concentration of hydrogen within the liquid hydrogen tank 1 is so low that no explosive or flammable mixture can form upon contact with air.

Claims

Claims: A method for extracting hydrogen from a liquid hydrogen tank (1) comprising an integrated heating device (5) for vaporizing the liquid hydrogen and a discharge line (4) for extracting the vaporized hydrogen, wherein heating power is supplied to maintain the tank pressure during the extraction of the vaporized hydrogen, characterized in that the temporal profile of the supplied heating power and / or the tank pressure is recorded, the profile being monitored for abrupt changes in the heating power and / or the pressure gradient of the tank pressure. A method according to claim 1, characterized in that an electric heating element is used as the heating device (5), the heating power of which is supplied continuously or in pulses.A method according to claim 1 or 2, characterized in that a heat exchanger (5) is used as the heating device, through which a heating medium flows, wherein the heating power is determined based on the volume flow rate of the heating medium and the temperature difference before and after the heat exchanger. A method according to claim 1, 2 or 3, characterized in that, in the case of a continuously adjusted heating power, this is adjusted to the abrupt change. A method is used to monitor for pressure drops during constant hydrogen withdrawal. A method according to one of claims 1, 2, or 3, characterized in that the pressure gradient is monitored when heating power is applied in pulses, and the pressure gradient is monitored for a sudden increase or decrease depending on whether heating is currently taking place or not. A method according to one of claims 1 to 5, characterized in that a warning message is generated and issued in the event of a detected sudden change. A method for completely emptying a liquid hydrogen tank (1) from which hydrogen is withdrawn according to the methods according to one of claims 1 to 6, wherein, after a detected sudden change, the liquid hydrogen tank (1) is heated until a limit temperature is reached and then purged with inert gas.The method of claim 7, characterized in that the limiting temperature is specified as greater than or equal to the boiling point of the inert gas. Use of the method of any one of claims 1 to 6 for extracting hydrogen and / or the method of claim 7 or 8 for emptying a liquid hydrogen tank (1) of a vehicle powered at least partially by the extracted hydrogen, wherein the extracted hydrogen is supplied to combustion or power generation in a fuel cell system. Use of claim 9, characterized in that. In the event of a warning message, the remaining range of the vehicle is estimated and displayed along with the warning message.

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