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

Abstract

Provide a method for efficiently recognizing that the liquid hydrogen tank has become empty. 【Solution means】The present invention relates to a method for removing hydrogen from a liquid hydrogen tank (1) comprising a heating device (5) for evaporating liquid hydrogen and a discharge line (4) for removing the evaporated hydrogen. For the removal of the evaporated hydrogen, heating power is supplied to maintain the tank pressure. The method according to the invention is characterized in that the temporal changes of the supplied heating power and / or the tank pressure are recorded and the changes are monitored for abrupt changes in the heating power and / or pressure gradient. The present invention also relates to a method for completely emptying the liquid hydrogen tank (1). Furthermore, the use of both methods in motor vehicles is provided.

Description

Technical Field

[0001] The present invention relates to a method for removing hydrogen from a liquid hydrogen tank according to a mode defined in detail in the first paragraph of claim 1. Furthermore, the present invention also relates to a method for completely emptying a liquid hydrogen tank from which hydrogen has been removed according to the above method. Finally, the present invention also relates to the use of either or both of the above methods in a liquid hydrogen tank of a motor vehicle, in particular a commercial vehicle.

Background Art

[0002] The following Patent Document 1 is essentially general prior art. This document shows a container for a compressed gas, which is designed in particular as a cryogenic tank for cryogenic liquid hydrogen. This liquid hydrogen tank has a heating device for removing hydrogen, and this heating device is incorporated inside the liquid hydrogen tank. In order to remove hydrogen, heat is introduced into the liquid hydrogen through this heating device to evaporate the liquid hydrogen, maintain the pressure of the tank, and the evaporated hydrogen is discharged through a discharge line and can be used, for example, for supply to a consumption device of a motor vehicle.

[0003] Such liquid hydrogen tanks, also known as LH2 tanks, are designed to hold cryogenic liquid hydrogen for as long as possible during operation, and for this purpose, they have extremely high external thermal insulation. Here, usually, hydrogen cannot be removed in liquid form. This is because the discharge line is typically provided in the upper region in the direction of gravity for the use of the tank. Therefore, the gaseous portion of hydrogen is removed from the gas cushion that normally forms above the liquid. There, the evaporated gas is removed, and the gas pressure decreases, so the liquid portion evaporates partially to compensate for the low vapor pressure, thereby causing the pressure to rise again. However, during this process, the remaining liquid is further cooled, and as a result, the gas pressure continues to decrease as the removal continues. In contrast, in order to always have sufficient pressure for the removal of the evaporated hydrogen, such liquid hydrogen tanks usually incorporate heating elements as described in the prior art above. These heating elements play a role in supplying heat from the outside to maintain the gas pressure at a pressure value suitable for removal (hereinafter referred to as the tank pressure).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] In practice, the temperature and pressure are recorded to determine the filling level of the tank. Additionally, a filling level sensor can be used, but in principle, it can only measure down to a lower level. And when the liquid level drops below this remaining filling amount, it can no longer be detected by the sensor. In fact, this is very inaccurate, especially when the tank is almost completely empty. Also, for example, when it is necessary to completely empty the liquid hydrogen tank for maintenance, it is desirable to be able to recognize when the liquid part has completely emptied or evaporated. The techniques shown in the prior art cannot achieve all of these.

[0006] From the above, the problem of the present invention is to provide a method for removing hydrogen from a liquid hydrogen tank in accordance with the manner specified in detail in the first paragraph of claim 1, avoiding the above-mentioned drawbacks, and enabling the efficient recognition that the liquid hydrogen tank has become empty. Further, the problem of the present invention is to provide a method for completely emptying a liquid hydrogen tank, for example, for repair or maintenance work, based on the above method. A further problem of the present invention is to provide a particularly suitable use of these methods.

Means for Solving the Problem

[0007] According to the present invention, the first problem of the present invention is solved by the method having the features of claim 1, here particularly the features of the characterizing part of claim 1. Advantageous embodiments of this solution are obtained from its dependent claims. Further claims including the features of claim 1 solve the problem by claim 7 regarding complete emptying and claim 9 regarding use. Here too, advantageous embodiments are obtained from the respective dependent claims.

[0008] In the method for removing hydrogen from a liquid hydrogen tank according to the present invention, as in the prior art, a heating device incorporated in the liquid hydrogen tank operates to evaporate the liquid hydrogen, and the hydrogen gas thus generated is discharged through an exhaust line. There, generally, and particularly as also known from the above prior art, for the removal of hydrogen, heating power is supplied to maintain the tank pressure.

[0009] According to the present invention, the temporal changes in the supplied heating power and / or the tank pressure are recorded (detected), and the changes are monitored for abrupt changes in the heating power and / or the pressure gradient.

[0010] By monitoring for such abrupt changes in the heating power and / or the pressure gradient, it becomes possible to determine when the liquid hydrogen tank becomes empty or when the liquid hydrogen stored in the liquid hydrogen tank has completely evaporated. After the remaining gas is removed from the liquid hydrogen tank, the liquid hydrogen tank accordingly becomes empty. Here, different from the case of using a pressure-temperature sensor, by evaluating the temporal changes in the heating power and / or the pressure gradient, a very reliable and efficient method becomes possible for efficiently predicting the complete evaporation of the liquid hydrogen and thus that the tank will soon be completely empty. There, the temporal changes in the heating power and / or the pressure gradient are reliable and informative, different from the case of simply monitoring the temperature and the pressure. This is especially because the temperature inside the liquid hydrogen tank can vary greatly as the tank becomes empty and can vary very significantly depending on the position of the temperature sensor inside the liquid hydrogen tank.

[0011] According to a very advantageous development of the method according to the present invention, there, an electric heating element can be used as the heating device, the heating power of which is adjusted continuously or in a pulse-width modulation manner in accordance with the tank pressure, and the electric heating power is recorded and evaluated in the sense described at first. This is particularly simple and efficient. This is because the recording of the heating power of the electric heating element can be carried out simply and efficiently by measuring the electrical parameters of voltage and current.

[0012] In an advantageous development of the method according to the invention, in addition or alternatively, the heating device has a heat exchanger through which a heat medium flows, and the heating power can be recorded based on the volume flow rate of the heat medium and the temperature difference across the heat exchanger (upstream and downstream). Such heating can be carried out, for example, via the cooling system of an existing structure. For example, when hydrogen is converted to electricity in a fuel cell system, this fuel cell system usually has a cooling circuit. And the cooling medium of such a cooling circuit can be used as the heat medium of the heat exchanger for heating the liquid hydrogen tank. The waste heat of the fuel cell system generated in the process must be cooled somehow, but there it can be utilized without using additional energy. This type of heating can thus increase the overall efficiency. And here, in order to be able to record the applied heating power appropriately, the volume flow rate and the temperature difference can be recorded in the sense described above. In particular, the volume flow rate serves as a control parameter for setting the required tank pressure and can be adjusted accordingly. This can preferably be done by means of a conveying device for the heat medium whose speed can be continuously controlled, a valve device controlled by pulse width modulation or proportional control, etc.

[0013] In a highly preferred embodiment of the method according to the invention, during the monitoring of the heating power, the heating power is monitored for a sharp drop. This is usually based on the idea that in order to evaporate the hydrogen in the liquid hydrogen tank via the heating device in the liquid hydrogen tank, a supply of energy for evaporating the hydrogen is required. This so-called evaporation enthalpy is about 350 kJ / kg H2 there. Once the liquid hydrogen has completely evaporated and only gaseous hydrogen remains in the liquid hydrogen tank, there is no need to apply this evaporation enthalpy. That is, if the removal of hydrogen is constant and the tank pressure is constant, the supplied energy clearly decreases, which can be noticed by a sharp drop in the heating power for maintaining the tank pressure. That is, this sharp drop in the heating power is a reliable indicator indicating that the liquid hydrogen in the liquid hydrogen tank has completely evaporated.

[0014] Therefore, in monitoring the temporal variation of the heating power, a constant hydrogen removal and a constant tank pressure, which is adjusted by the heating power in particular, are prerequisite. Instead, when the heating power is supplied in a switching or pulsed manner, the pressure is also periodically adjusted in accordance with the oscillation of this heating power. This means that when it is heated, the pressure rises periodically, and when it is not heated, it drops. Also here, when the liquid hydrogen in the liquid hydrogen tank has completely evaporated, the evaporation enthalpy flows into the change in pressure. For this reason, when the heating power is periodically switched on and off, it becomes possible to monitor the pressure gradient. If it is heated now, the pressure rises faster than before, and the pressure gradient jumps upward. If it is not heated now, the heating power is switched off in accordance with its oscillation, the pressure drops much faster than before, and the pressure gradient jumps downward. Such a rapid change in the pressure gradient can be ideally utilized to determine that the liquid hydrogen in the liquid hydrogen tank has completely evaporated when the heating power is supplied in a pulsed manner in this particularly preferred development of the method according to the invention.

[0015] According to a very advantageous development of the method according to the invention, furthermore, a warning message can be generated when a rapid change in the heating power and / or the pressure gradient is detected. Such a warning message can, for example, warn that the liquid hydrogen tank is about to become empty so that it can be refilled immediately, or can indicate that the liquid hydrogen has completely evaporated, for example, when the liquid hydrogen tank is intentionally emptied for maintenance or decomposition.

[0016] And in the method for completely emptying the liquid hydrogen tank according to the present invention, in one of the described embodiments, hydrogen is removed from the liquid hydrogen tank according to the above method. However, after a rapid change is detected, for example, when a warning message is generated according to the last-described embodiment, the liquid hydrogen tank can be further heated to reach the limit temperature, if necessary, and then washed with an inert gas. This gas can be nitrogen in particular. Therein, the limit temperature is set above the boiling point of the inert gas. In the case of nitrogen, this is, for example, -196 °C. As a result, the nitrogen introduced for cleaning the liquid hydrogen tank exists as a gas without liquefying in the thermally well-insulated liquid hydrogen tank, expels the gaseous hydrogen remaining in the liquid hydrogen tank, and enables safe maintenance and disassembly of the liquid hydrogen tank.

[0017] Thus, the method for removing hydrogen from the liquid hydrogen tank described first is particularly suitable for removing hydrogen from the liquid hydrogen tank of a motor vehicle powered at least in part by the removed hydrogen. Therein, the removed hydrogen is supplied to a consumption device in the motor vehicle, for example, a combustion engine operating on hydrogen such as an internal combustion engine or a gas turbine, or is converted into electricity in a fuel cell system and used for the electric drive of the motor vehicle. The method according to the present invention is particularly ideally suitable for such applications. This makes it possible to safely, reliably, and in a timely manner detect that the liquid hydrogen tank is empty, almost independently of highly variable environmental parameters.

[0018] Further, when this method is combined in particular with the above method for completely emptying the liquid hydrogen tank, it can be emptied safely, for example, enabling maintenance and repair of a motor vehicle where no hydrogen must remain in the liquid hydrogen tank. This is because the maintenance directly involves the liquid hydrogen tank or the tank system, or because the maintenance time or downtime is expected to be long, during which, for example, hydrogen diffusion must be reliably prevented in a garage.

[0019] According to a further highly preferred embodiment of the present use in motor vehicles, in the case of a warning message, the remaining range of the motor vehicle can be estimated and transmitted together with the warning message. As already mentioned, a sudden change in the heating power and / or pressure gradient records the point in time when all the liquid hydrogen in the liquid hydrogen tank has evaporated. At this point, the liquid hydrogen tank is still filled with gaseous hydrogen, and at this point detected by the sudden change, an amount of gaseous hydrogen corresponding to the volume of the liquid hydrogen tank remains. This remaining amount depends on the volume and pressure and can be determined by a simple pressure measurement, making it possible to estimate the remaining range of the motor vehicle still achievable with this already evaporated gaseous hydrogen. That is, for a person driving the motor vehicle or, in the case of an autonomous or semi-autonomous vehicle, a warning to the control center enables appropriate measures to be taken, such as switching to another energy source or refueling with hydrogen.

[0020] Further advantageous embodiments of the method according to the invention and their use are also obtained from the embodiments described in more detail below with reference to the drawings.

Brief Description of the Drawings

[0021]

Figure 1

Embodiments for Carrying out the Invention

[0022] Figure 1 shows a liquid hydrogen tank 1 entirely inside the heat insulating material 2. Such a liquid hydrogen tank 1 is also called a cryogenic tank or a cryo tank and is used to store liquid hydrogen at an appropriate low temperature. At least in the lower region of the liquid hydrogen tank 1 in the direction of the gravitational force Fg, this hydrogen exists as a liquid. The liquid hydrogen tank 1 can be replenished via a filling line 3. Hydrogen can be removed from the liquid hydrogen tank 1 via a discharge line 4. In the simplified diagram selected here, valve elements, filters, backflow prevention elements, etc. in those two lines 3, 4 are omitted. Usually, the liquid hydrogen tank 1 is also provided with a discharge chimney (not shown here), and in the case of a significant overpressure occurring inside the liquid hydrogen tank 1 that is critical for safety, gas can be discharged via a pressure relief valve, a rupture element, etc.

[0023] And during normal operation, or when, for example, the liquid hydrogen tank 1 itself or the maintenance of the tank system connected thereto requires the liquid hydrogen tank 1 to be completely emptied, hydrogen is removed from the liquid hydrogen tank 1. Usually, it is not intended and is impossible to completely remove the liquid hydrogen as a liquid. In particular, as shown here, the filling line 3 usually does not end in the lower region in the direction of gravity g in the liquid hydrogen tank 1 or its internal space for its intended use. Instead, it is the discharge line 4 that is responsible for the removal, and cryogenic hydrogen gas is removed from the liquid hydrogen tank 1 through the discharge line 4. This gaseous hydrogen accumulates as a gas cushion (g) above the liquid (f) in the liquid hydrogen tank 1 and usually always exists and can be removed through the discharge line 4. As a result, the gas pressure in the liquid hydrogen tank 1 decreases, and in order to compensate for the decrease in the tank pressure, a part of the liquid hydrogen evaporates. Thereby, the gas pressure rises again. However, the remaining liquid is further cooled due to the evaporation enthalpy required for evaporation, and if the removal continues, the resulting tank pressure also decreases more and more. When the tank pressure becomes less than atmospheric pressure, the removal becomes impossible in principle. In practice, the removal is usually carried out, for example, for using hydrogen in a fuel cell system. Here, since a certain hydrogen pressure is required in the system using hydrogen, the removal can only be carried out up to this required system pressure.

[0024] Therefore, in practice, it is common for the liquid hydrogen tank 1 to be equipped with a heating device 5 incorporated in its internal space, and heat can be supplied from the outside by this heating device 5 to evaporate the hydrogen. Thereby, the tank pressure can be maintained or adjusted to a desired pressure value. This makes it possible to continue removing hydrogen from the liquid hydrogen tank 1.

[0025] In order to determine whether the liquid hydrogen tank 1 is empty, it is necessary to recognize whether there is still liquid hydrogen in the liquid hydrogen tank 1 or only low-temperature hydrogen gas in the liquid hydrogen tank 1. The liquid hydrogen tank 1 is usually provided with a temperature sensor (not shown here) in addition to, for example, a pressure sensor 6 used to control the heating power of the heating device 5. However, this is only limitedly useful for determining that the tank is empty. This is especially true when it becomes empty rapidly because the temperature can vary greatly depending on the location. This is particularly the case when there is still liquid hydrogen remaining in the liquid hydrogen tank 1, and when the liquid hydrogen tank 1 is mounted on a vehicle, for example, due to the dynamic movement of the liquid hydrogen tank 1, the remaining liquid hydrogen may reach different regions of the internal space of the liquid hydrogen tank 1 by, for example, sloshing motion.

[0026] However, here, a typical procedure for removing hydrogen from the liquid hydrogen tank 1 can be utilized. This is because as long as there is still liquid hydrogen in the liquid hydrogen tank 1, this liquid hydrogen must be evaporated by continuous heat supply so that the pressure in the region of the pressure sensor 6 is maintained. There, the required energy supply corresponds to the evaporation enthalpy of about 350 kJ / kg H2, which must be applied via the heating device 5. And when the liquid hydrogen in the liquid hydrogen tank 1 is completely evaporated and only low-temperature gaseous hydrogen remains in the space of the liquid hydrogen tank 1, there is no need to apply this evaporation enthalpy. Therefore, the supplied energy, and thus the heating power required to maintain the tank pressure, drops rapidly.

[0027] Therefore, it is proposed to combine a constant removal of hydrogen via the discharge line 4 with the measurement of the necessary heating change of the heating element 5, so that it is possible to determine, via the time change of the supplied heating power, when the liquid hydrogen is completely evaporated and the liquid hydrogen tank is about to become empty. Therein, the constant removal can be appropriately recorded during normal operation and can be mathematically adjusted as necessary. In the case of removal to empty the liquid hydrogen tank 1 for maintenance, it is recommended to remove via a diaphragm having a constant cross-sectional area to ensure a constant discharge mass flow rate.

[0028] At that time, the measurement of the heating power can be carried out via the tank control device. In the case of heating the heating element 5, the heating power is measured indirectly, for example, via the coolant of the cooling circuit of an automobile. In this case, the calculation of the supplied heating power is performed by the volume flow rate of this coolant and the temperature difference between the forward and return temperatures, that is, in this case, the temperature difference of the heat medium before and after the heating device 5 designed as a heat exchanger. Alternatively, when using the heating device 5 designed as an electric heating element, the heating power can be directly recorded by electrical parameters.

[0029] Regardless of how the heating power supplied to the heating device 5 to maintain the tank pressure is recorded, in the case of continuous tank pressure or continuous removal, or even in the case of dynamic removal, a value mathematically converted to such continuous removal, at the moment when the liquid hydrogen in the liquid hydrogen tank 1 is completely evaporated, there is no need to apply the evaporation enthalpy. Therefore, the required amount of heat rapidly decreases, and the heating power required for continuous removal or for maintaining the tank pressure also rapidly decreases. And this rapid decrease in heating power indicates that the liquid hydrogen has completely evaporated. That is, the liquid hydrogen tank 1 is empty except for the remaining low-temperature gaseous hydrogen remaining in the space of the liquid hydrogen tank 1.

[0030] When the heating force is applied in a pulsed manner, i.e., when on and off are repeated, the pressure in the space also periodically rises constantly when being heated and drops constantly when not being heated. In this case, the fact that the liquid hydrogen in the space has reached a state of complete evaporation can be detected by a sudden change in the pressure gradient. If it is being heated now, the pressure rises faster than before and the pressure gradient jumps upward. If it is not being heated now, the pressure drops faster than before accordingly and the pressure gradient jumps downward in this case.

[0031] When the heating force is introduced in a pulsed manner, the above state (the state just before becoming empty) where the liquid hydrogen is completely evaporated and the liquid hydrogen tank 1 is about to become empty can be detected by a sudden change in the heating force or the pressure gradient. In this case, for example, when the liquid hydrogen tank 1 is used to supply hydrogen to a fuel cell system in a vehicle, a warning message can be generated. At that time, the warning message can be transmitted to the driver or, in the case of an autonomous vehicle, to the control center. This is particularly efficient. In this state of the liquid hydrogen tank 1, the level indicator based on pressure and temperature is of relatively little value for the reasons already described above. There, the low-temperature hydrogen gas in the liquid hydrogen tank 1 can still be used for a short-distance drive, and thus an emergency operation is possible. The transmitted warning message can particularly include a warning that the liquid hydrogen tank 1 is about to become empty and the estimated remaining range based on the remaining amount of gas in the liquid hydrogen tank 1. Therefore, when estimating the remaining range, it is also conceivable and useful to consider the charge state of the normally existing hybrid battery, the current vehicle weight, the terrain of the route, etc.

[0032] When the liquid hydrogen tank 1 is completely emptied, a sudden change in the heating force and / or pressure gradient indicates that the liquid hydrogen in the liquid hydrogen tank 1 has completely evaporated. Here, in order to further empty it, the liquid hydrogen tank 1 can be filled with a non-combustible inert gas such as nitrogen to expel the remaining hydrogen and ensure a safe state. For this purpose, usually, after the liquid hydrogen has completely evaporated, it is necessary to further heat the liquid hydrogen tank 1 until the gaseous hydrogen remaining in the liquid hydrogen tank 1 is replaced by the non-combustible gas. Usually, the hydrogen gas in the tank after the liquid phase has completely evaporated is around -230°C. At this temperature, the introduced nitrogen may liquefy or solidify, so the gas and thus the liquid hydrogen tank 1 must be heated to at least the boiling point of nitrogen, -196°C, before filling with nitrogen. This temperature may vary accordingly when using other inert or non-combustible gases or mixed gases. Alternatively, when using helium or, although limited, neon, since its boiling point is lower than -230°C, heating can be completely omitted. However, noble gases have the disadvantage of being correspondingly costly.

[0033] And finally, the liquid hydrogen tank 1 is purged with an inert gas, and this purging is carried out until the internal temperature of the liquid hydrogen tank 1 is adjusted to the ambient temperature and the concentration of hydrogen remaining in the liquid hydrogen tank 1 becomes low enough so that it no longer forms an explosive or flammable mixture upon contact with air.

Claims

1. A method for removing hydrogen from a liquid hydrogen tank (1) comprising a heating device (5) for evaporating liquid hydrogen and a discharge line (4) for removing the evaporated hydrogen, the method for maintaining the tank pressure by supplying heating power for removing the evaporated hydrogen, wherein the supplied heating power and / or the temporal change in the tank pressure are recorded, and the change is monitored for abrupt changes in the heating power and / or pressure gradient A method characterized by this.

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

3. A heat medium is flowed through a heat exchanger as the heating device (5), and the heating power is recorded based on the volume flow rate of the heat medium and the temperature difference before and after the heat exchanger The method according to claim 1 or claim 2, characterized by this.

4. When the heating power is adjusted continuously, the heating power is monitored for an abrupt decrease when the hydrogen removal is constant The method according to any one of claims 1 to 3, characterized by this.

5. When the heating power is supplied in a pulsed manner, the pressure gradient is monitored, and depending on whether it is currently being heated, the pressure gradient is monitored for an abrupt increase or an abrupt decrease The method according to any one of claims 1 to 3, characterized by this.

6. When an abrupt change is detected, a warning message is generated and transmitted The method according to any one of claims 1 to 5, characterized by this.

7. A method for completely emptying the liquid hydrogen tank (1), in a method in which hydrogen is removed according to the method according to any one of claims 1 to 6, wherein After an abrupt change is detected, the liquid hydrogen tank (1) is heated until it reaches the critical temperature and then washed with an inert gas Method.

8. The critical temperature is set to be equal to or higher than the boiling point of the inert gas The method according to claim 7, characterized by this.

9. In the use of the method according to any one of claims 1 to 6 for removing hydrogen and / or the method according to claim 7 or claim 8 for emptying a liquid hydrogen tank (1) of a motor vehicle powered at least in part by the removed hydrogen, wherein The removed hydrogen is supplied for combustion or conversion to electricity in a fuel cell system Use.

10. In the case of a warning message, the cruising range of the motor vehicle is estimated and transmitted together with the warning message Use according to claim 9, characterized in that

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