Device for storing cryogenic hydrogen, and motor vehicle

Abstract

The invention relates to a device (1) for storing cryogenic hydrogen, having at least one container (2). The device (1) according to the invention is characterized in that the inner surface of the container (2) has, at least in part, projections (4) that project into the container interior; and / or is provided with a rough surface or with elements (5) having a rough surface in order, by way of the roughness of the surface, to prevent superheating.

Description

[0001] Device for storing cryogenic hydrogen and motor vehicle

[0002] The invention relates to a device for storing cryogenic hydrogen with at least one container. The invention further relates to a motor vehicle with such a device.

[0003] The storage of liquid hydrogen, for example for use in motor vehicles, is well known in the art. The problem here is always that liquid hydrogen is extremely cold, so the storage tanks must be well insulated. Nevertheless, the hydrogen inevitably heats up over time, and once it has reached its gaseous phase and exceeds a predetermined maximum pressure in the tank, it must be released. This so-called boil-off gas is then typically catalytically converted to prevent hydrogen emissions into the environment. Nevertheless, this gas is lost for the intended application. The unavoidable heating in the tank used to store the cryogenic hydrogen also means that such hydrogen storage tanks have a relatively short retention time.Conventional tanks can be empty or significantly depleted after just a few days, without the hydrogen stored in them being able to be used in the desired way.

[0004] To counteract this problem, it is known, for example from DE 10 4342 210 02, to create a temperature stratification-free storage device for cryogenic liquids, such as hydrogen. The storage device is divided into two storage areas, a main chamber and a secondary chamber. Liquid hydrogen from the main chamber is pumped into the secondary chamber and from there, via a nozzle, distributed evenly over the surface of the hydrogen in the main chamber, above which a gas cushion has collected. This leads to cooling and ultimately to a longer storage time. However, the structure is comparatively complex and requires additional energy to pump the liquid hydrogen from the main chamber to the secondary chamber.

[0005] The object of the present invention is to provide an improved device for storing cryogenic hydrogen, which enables a longer storage time with a simple structure.

[0006] According to the invention, this object is achieved by a device having the features of claim 1, and in particular in the characterizing part of claim 1. Advantageous embodiments and further developments of the device according to the invention are set forth in the dependent claims. A motor vehicle, in particular a commercial vehicle, with such a device for storing cryogenic hydrogen also achieves this object.

[0007] According to the invention, it is now provided that the cryogenic hydrogen is stored in a container which has projections on its inner surface which project into the interior of the container and / or which is provided on its inner surface with a rough surface or with elements with a rough surface in order to prevent boiling delay due to the roughness of the surface.

[0008] In the device according to the invention, the inner surface of the container is at least partially provided with projections and / or has a rough surface or elements with a rough surface. This utilizes two different mechanisms identified by the inventors to achieve the most homogeneous heating possible of the liquid phase of the stored cryogenic hydrogen. If more heat can be absorbed via the liquid phase, the pressure increase slows down. This results in a longer retention time for the stored hydrogen in the container.

[0009] The projections protruding into the interior of the container, which can be designed in particular as ribs or pins and which, in typical container sizes such as those used for storing cryogenic hydrogen in commercial vehicles, can protrude into the container interior by a distance of a few millimeters to a few centimeters, have three positive effects. They enlarge the surface area of ​​the inner wall of the container and thus reduce the heat flow transferred per unit area from the outside into the interior of the container. The projections transport the heat flow, which cannot be completely avoided at the surface of the container, across the boundary layer between the container surface and the cryogenic hydrogen and comparatively far into the liquid hydrogen. This heats not only the boundary layer but also the hydrogen in an area somewhat distant from the container wall.This leads overall to a more homogeneous heating of the cryogenic hydrogen, which, as explained above, is associated with a longer holding time. On the other hand, the projections, especially when designed as ribs, impede flow in the boundary layer between the vessel wall and the cryogenic hydrogen. It is particularly advantageous if the ribs are designed perpendicular to gravity during intended use, thus preventing the upward flow of warmer hydrogen and thus reducing boundary layer flow and simultaneously reducing heat transfer between the liquid hydrogen and the vessel wall.

[0010] In addition to or as an alternative to such projections, a rough surface or elements with a rough surface can be used. Such elements have a surface roughness that prevents boiling delay. They can, for example, consist of sintered porous metal elements, especially metal layers, or of so-called boiling stones, which are well-known and commonly used in laboratory applications.

[0011] The surface roughnesses are in the order of magnitude of more than 0.3 pm to 0.5 pm, measured according to the version of the relevant standard valid on the date of application, in this case DIN EN ISO 4287.

[0012] Regardless of whether elements such as boiling stones are incorporated or a targeted surface coating or treatment of the inner wall of the container, or if present also of the projections, is used, this correspondingly rough surface has the advantage of providing a large number of nucleating sites for the formation of vapor bubbles. Instead of a few large vapor bubbles, the rough surface causes the formation of a large number of small vapor bubbles, potentially distributed in a large number of locations within the container. Only a comparatively small amount of superheat is necessary to create these vapor bubbles, so that any incoming heat is absorbed very quickly by these vapor bubbles. These then rise in the direction of gravity through the remaining liquid, ensuring very effective mixing of the liquid, resulting in very homogeneous and even heating of the liquid portion of the hydrogen in the container.This also contributes to the longest possible holding time, as the entire liquid hydrogen is heated evenly and not just evaporated at the edges along the vessel wall, while the temperatures inside the vessel itself are much lower. This results in a less pronounced pressure increase in the gas phase of the vessel, so the boil-off gas does not need to be vented as frequently.

[0013] A further extremely advantageous embodiment of the device according to the invention provides for a tank heater arranged in the container for extracting hydrogen. In this case, the surface of the tank heater and the container has a lower surface roughness in the area of ​​the tank heater than the surrounding areas of the container. The idea of ​​greater surface roughness in order to generate a large number of small gas bubbles and to avoid delayed boiling under all circumstances is now modified in the area of ​​the tank heater so that the surface has very little roughness, i.e. is particularly smooth. This deliberately prevents nucleation points for the formation of vapor bubbles in order to achieve exactly the same result in the area of ​​the tank heater as is undesirable in other areas of the container. In fact, delayed boiling is intended to occur in the area of ​​the tank heater.The tank heater is only ever operated when hydrogen is to be withdrawn. In this case, the delayed boiling is advantageous because it ensures that the superheated hydrogen quickly forms large vapor bubbles. This causes the pressure in the gas phase to rise rapidly, which is desirable when hydrogen is being withdrawn. On the other hand, the delayed boiling significantly limits the heating of the liquid hydrogen around the very large gas bubbles created by the delayed boiling, so that despite the heating required to withdraw hydrogen, the adverse effect on the holding time can be reduced. This procedure ensures that essentially only the hydrogen in the immediate area of ​​the tank heater evaporates, and that little heat is introduced into the surrounding areas of the liquid hydrogen.Such a tank heater can now be ideally used, particularly in combination with the projections for heat conduction on the container surface and the design of the container surface, and in particular the projections with a correspondingly high surface roughness, to provide a multitude of nucleation sites. Such a structure can ensure that heat penetrating the container is distributed as evenly as possible both during operation, i.e., during the extraction of hydrogen, and during downtime phases in which no hydrogen is extracted. All of this has a positive effect on the longest possible retention time for the cryogenic hydrogen in the device according to the invention.

[0014] According to a very advantageous embodiment of the device according to the invention, the tank heater itself can be arranged at the bottom in the direction of gravity when the container is used as intended, so that it always comes into contact with liquid, even if the container is already relatively empty.

[0015] The projections, which, as mentioned above, are preferably designed as ribs or pins, and the sintered metal elements or boiling stones can be mounted anywhere in the tank, preferably anywhere where the tank heater is not located. As described above, they have their particular effect when they are within the liquid phase, so that, according to a very advantageous embodiment of the device according to the invention, they are also preferably arranged at the bottom in the direction of gravity when the tank is used as intended. Boiling stones or small sintered metal elements can, in principle, simply be "thrown" into the tank and will then collect at the bottom in the direction of gravity, ensuring the formation of nucleation sites.If they are to be distributed over larger areas of the tank, they can be attached to nets and held in the surface area, for example. On the other hand, a rough or porous layer can also be created as a surface treatment or surface coating. This applies both to the inner wall of the tank and to any projections that may be present there. These projections can, for example, also be in the form of sintered metal ribs that are attached to the inside of the tank surface, e.g. welded or glued. In principle, this device for storing cryogenic hydrogen can be used both stationary and mobile. However, a long storage time plays a crucial role, especially when used in vehicles such as commercial vehicles.Furthermore, due to limitations regarding installation space and weight, the options for insulating the device are more limited than is typical for stationary applications. Therefore, the inventive design of the device demonstrates its particular advantages here. A motor vehicle that achieves the above-mentioned object is thus equipped with such an inventive device in one of the described designs for storing liquid hydrogen. The motor vehicle itself can, in principle, use the liquid hydrogen to power a fuel cell, an internal combustion engine, or the like, or can also, in principle, transport it simply as a "tanker."

[0016] Further advantageous embodiments of the device according to the invention also emerge from the exemplary embodiments which are described in more detail below with reference to the figures.

[0017] Showing:

[0018] Fig. 1 is a schematic cross-section through the inner container of a device for storing liquid hydrogen in a first possible embodiment according to the invention;

[0019] Fig. 2 is a schematic cross-section through the inner container of a device for storing liquid hydrogen in a second possible embodiment according to the invention; and

[0020] Fig. 3 is a schematic longitudinal section through the inner container of a device for storing liquid hydrogen in a third possible embodiment according to the invention.

[0021] Figure 1 shows a schematic cross-section through a container 2 of a device 1 for storing cryogenic hydrogen. This container 2 is typically insulated from the environment by a vacuum and / or other isolation mechanisms. It is located within the device 1 and is therefore also referred to as the inner container. In this container 2, hydrogen is present in a liquid phase fl on the one hand and in a gaseous phase g on the other. A supercritical state of the hydrogen is also possible in part of the volume. The liquid phase fl separates from the gaseous phase g according to gravity F. g , which is shown schematically in the illustration in Figure 1, so that when the device 1 is used as intended, the liquid phase fl lies in the area shown below and the gas phase g lies above it.

[0022] In practice, heat will inevitably be present in the outer region of the container 2, whereby heat here means everything which, in terms of its temperature, is above the temperature of the hydrogen in the liquid phase fl. In order to introduce this heat as evenly as possible into the hydrogen, and here in particular into the liquid phase fl, and at the same time to prevent boundary layer flow between the liquid hydrogen and the interior of the container wall 2, projections 4, here in the form of ribs or rods, are distributed on the inner surface of the container 2. These can be distributed around the entire circumference of the container 2, but have their special effect particularly in the region of the liquid phase fl of the hydrogen, so that they are particularly effective in the direction of gravity F during intended use. g lower part of the container 2.

[0023] These ribs or rods 4 protrude by a few millimeters to a few centimeters into the interior of the container 2. They thus enlarge the surface area of ​​the inner wall of the container 2 and thus reduce the heat flow to be transferred per area from the outside into the interior of the container and thus essentially into the liquid hydrogen. In addition, the ribs or rods direct the heat flow not only into the boundary layer between the hydrogen in the liquid phase fl and the inner wall of the container 2, but further into the liquid phase fl of the hydrogen. This not only heats the boundary layer, but also heats the hydrogen more evenly. Overall, this leads to a reduction in the temperature stratification within the hydrogen in the container 2, which increases the holding time in the container 2 or the device 1.With a corresponding geometric arrangement of the ribs, in addition to reducing the boundary layer flow due to heat conduction further into the liquid, they can also impede such a boundary layer flow purely mechanically, for example by the ribs running horizontally along the container 2, as shown here, and thus preventing flows against the direction of gravity F. gfrom bottom to top accordingly. Figure 2 shows a further variant for modifying the container 2. Here, too, a gaseous phase g and a liquid phase fl of the hydrogen can be seen. The ribs 4 are now replaced in the area of ​​the inner wall of the container 2 by a surface or elements 5 with a high surface roughness. Examples of corresponding elements 5 along the surface are shown here. These can be, for example, sintered or porous metal layers or an increase in surface roughness through mechanical processing. Alternatively or in addition, porous or rough boiling stones can be introduced into the tank as elements 5, for example, held along the surface by a net. Here, too, the arrangement in the intended use is in the direction of gravity F gat the bottom of the container 2 is particularly useful, so that it would also be conceivable, for example, to simply place the boiling stones loosely into the container.

[0024] The appropriately roughened surfaces or the porous / rough surfaces of the boiling stones ensure that no boiling delay can occur, so that a large number of nucleation sites for vapor bubbles are present. This then leads to a correspondingly large number of vapor bubbles, which can form anywhere in the area of ​​the porous or rough surface and thus contribute to very even heating. The vapor bubbles, which are shown schematically here in the liquid phase fl and some of which are provided with the reference number 6, also ensure that the hydrogen in the liquid phase fl is mixed, so that a very homogeneous temperature distribution in the hydrogen is achieved. At the same time, the many small vapor bubbles reduce the pressure increase in the gaseous phase compared to large vapor bubbles, which could occur, for example, in the case of boiling delay, i.e. when there are no or too few nucleation sites.This minimizes the need to vent hydrogen once the maximum pressure of the container 2 or the device 1 has been reached. Increasing the surface roughness or introducing boiling stones or other similarly effective elements 5 also ensures a longer retention time of the hydrogen in the device 1.

[0025] Although the two mechanisms are shown separately in Figures 1 and 2, it is understood that they can also be used in combination. For example, a corresponding coating can be provided on both the inner wall of the container 2 and the projections 4, or the projections 4 can be made directly from a suitable porous material, for example, from sintered metal elements.

[0026] All these measures can preferably be arranged distributed over the entire inner surface of the container, and they can also be combined with each other as desired. As already mentioned above, it is particularly advantageous to use them appropriately in the area of ​​the liquid phase fl, so that they preferably move in the direction of gravity F during the intended use of the device 1. g in the lower region. The lower region can refer to the region that is typically covered by the liquid phase fl after the device 1 has been refueled with liquid hydrogen, which can also be a large part of the volume of the container 2.

[0027] Hydrogen is typically withdrawn in gaseous form from such a device 1. For this purpose, the gas cushion of the gaseous phase g that forms due to the unavoidable heating above the liquid phase fl can be used. In general, however, this will not be sufficient for the required amount of hydrogen and to provide the required pressure, for example an operating pressure of 4 to 6 bar for a fuel cell system. For this reason, and this is also common practice in the general state of the art, a tank heater 7 is provided to withdraw hydrogen. This tank heater 7 evaporates hydrogen, increases the pressure in the gaseous phase g and thus offers the possibility of withdrawing hydrogen in gaseous form from the device 1 at the desired pressure.

[0028] In the illustration in Figure 3, a longitudinal section through the container 2 can be seen. In the illustration in Figure 2 on the left, there is a schematically indicated tank heater 7, which, when used as intended, is directed in the direction of gravity F gis arranged right at the bottom in order to still be in the region of the liquid phase fl even when the container 2 is already largely empty. Unlike in the areas surrounding the tank heater 7, in which the projections 4 are preferably arranged and in particular there is a correspondingly high surface roughness or the use of boiling stones or other elements 5 as described above, the surfaces in the area of ​​the tank heater 7 are designed with a very low roughness, in particular less than 0.3 pm. For example, a heating coil which is heated electrically or by a correspondingly preheated medium, as well as the areas surrounding it, can be polished. By having such a very low surface roughness in the area of ​​the tank heater 7, exactly what is to be avoided at other points within the container 2 is achieved in the tank heater 7.Since there are no or only extremely few nucleation points for the formation of vapor bubbles here, a boiling delay typically occurs in the area of ​​the tank heater 7 due to the low surface roughness, so that the liquid hydrogen in this area is correspondingly overheated and then suddenly forms very large vapor bubbles. This is desired in the area of ​​the tank heater 7 in the design of the device 1. Unlike in the other areas, heat is to be introduced here and vaporous or gaseous hydrogen is to be generated at high pressure. This is achieved by nucleate boiling with boiling delay, so that only a few large vapor bubbles 8 rise. In Figure 3, this is indicated by the large vapor bubbles 8 in the area above the tank heater 7 and in comparison to the small vapor bubbles 6, analogous to Figure 2. This leads to the desired pressure increase in the gas phase g in order to remove the hydrogen from the container 2 orfrom the device 1. At the same time, the heat input into the liquid phase fl due to the large vapor bubbles 9 formed during boiling delay is very low, since the heat largely remains in these gas bubbles and does not heat the surrounding areas or does not heat them excessively.

[0029] A tank heater 7 with very low surface roughness on the one hand, and a container 2 with high surface roughness and / or heat-conducting projections 4, such as ribs or pins, on the other hand, can thus extend the holding time both during operation and when no hydrogen is being drawn from the device 1. All structures are correspondingly simple to implement and provide the desired effect passively, i.e., without additional energy input, cooling circuit, or the like. This is extremely simple, efficient, and energy-saving.

Claims

Patent claims 1. A device (1) for storing cryogenic hydrogen with at least one container (2), characterized in that the inner surface of the container (2) at least partially has projections (4) which project into the container interior; and / or is provided with a rough surface or with elements (4) with a rough surface in order to prevent boiling delay due to the roughness of the surface.

2. Device (1) according to claim 1, characterized by a tank heater (7) arranged in the container (2) for removing hydrogen, wherein the surface of the container (2) in the region of the tank heater (7) and the surface of the tank heater (7) have a lower surface roughness than the surrounding regions of the container (2).

3. Device (1) according to claim 2, characterized in that the tank heater (7) in the intended use of the container (2) in the direction of gravity (F g ) is located below.

4. Device (1) according to claim 1, 2 or 3, characterized in that the rough surface is produced by a coating or mechanical processing of the surface.

5. Device (1) according to one of claims 1 to 4, characterized in that the projections (4) are designed in the form of ribs or pins.

6. Device (1) according to one of claims 1 to 5, characterized in that the elements (5) are made of sintered metal and / or as boiling stones.

7. Device (1) according to claim 6, characterized in that the elements (5) are held by a net in the region of the surface of the container (2).

8. Device (1) according to one of claims 1 to 7, characterized in that the projections (4) are provided with a rough surface, in particular are designed as sintered metal elements.

9. Device (1) according to one of claims 1 to 8, characterized in that the projections (4), the elements (5) and / or the rough surface are oriented in the direction of gravity (F g ) are arranged below.

10. Motor vehicle with a device (1) according to one of claims 1 to 9, for storing hydrogen, in particular for a fuel cell drive system.

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