Hydrogen store having a liquid piston

Abstract

The invention relates to a hydrogen compressor and / or displacer, in particular a hydrogen store (1) for storing hydrogen (2), comprising: a hydrogen volume filled with hydrogen (2) and a working-fluid volume filled with a working fluid (3), wherein the working fluid (3) is in the form of a piston for changing the hydrogen volume, characterized in that the working fluid (3) comprises a poly-alpha-olefin. The invention also relates to a method for displacing and / or compressing hydrogen (2), comprising the steps of: (a) providing the hydrogen compressor and / or displacer according to the invention, and (b) adding working fluid (3) into the hydrogen compressor and / or displacer so that the working-fluid volume of the hydrogen compressor and / or displacer is expanded and the hydrogen volume of the hydrogen compressor and / or displacer is reduced.

Description

Hydrogen storage with liquid piston The invention relates to a hydrogen compressor and / or displacer, preferably a hydrogen storage device for storing hydrogen or a housing-piston unit, comprising: a hydrogen volume filled with hydrogen, and a working fluid volume filled with a working fluid, wherein the working fluid is designed as a liquid piston for changing, namely for increasing and / or reducing, the hydrogen volume. Furthermore, the invention relates to a method for displacing and / or compressing hydrogen. Furthermore, the invention relates to a housing-piston arrangement, comprising: a housing, preferably a cylinder, wherein the housing encloses a working volume for accommodating hydrogen, a solid-state piston which is movable relative to the housing to change the working volume, and a seal for sealing the movement of the solid-state piston relative to the housing, wherein a sealing fluid is provided as the seal. Furthermore, the invention relates to a rotor comprising: a rotation axis, a rotation drive, and such a housing-piston arrangement. Finally, the invention relates to a method for sealing the movement of a solid-state piston relative to a housing. A first embodiment of the invention provides a hydrogen compressor or hydrogen displacer and a corresponding method in which hydrogen is compressed or displaced by means of a working fluid in direct contact with the hydrogen, referred to here as the working fluid. The hydrogen compressor or displacer can be designed in a first variant according to the invention as Hydrogen storage can be designed from which hydrogen can be transferred, for example, into a hydrogen tank of a vehicle. In the hydrogen storage, the working fluid acts as a liquid piston in order to compress or displace the hydrogen in direct contact with the hydrogen. In a second variant according to the invention, the hydrogen compressor or displacer can be designed as a housing-piston unit with a, in particular cylindrical, housing, in which the working fluid, likewise as a liquid piston in direct contact with the hydrogen, is moved to compress or displace the hydrogen, in particular moved back and forth in an axial direction of the housing. US 2007 / 258828 A1 describes a hydrogen storage device in which a working fluid is used as a liquid piston. In this prior art, the working fluid is preferably an ionic fluid, a hydraulic oil with a high boiling point, or a fluid with a very low vapor pressure; examples include vacuum pump oils, molten salts and metals with a low melting point, as well as fluids with a gas solubility of less than 10-4 mol / 1 bar (US 2007 / 258828 A1,

[0017] ) . US 2007 / 258828 Al focuses on ionic fluids - these are consistently described as advantageous, e.g. in paragraph.

[0018] ,

[0019] ,

[0021] -

[0023] ,

[0047] and

[0050] . The use of vacuum pump oil, however, is only mentioned once in US 2007 / 258828 A1, namely in paragraph

[0017] , without highlighting any particular advantages associated with it. Therefore, if a person skilled in the art were to choose a working fluid based on US 2007 / 258828 A1, it would be an ionic fluid. US 2007 / 258828 Al does not mention which ionic fluids could be used. From US 2010 / 154924 Al, the expert knows that not all ionic fluids are suitable as liquid pistons (US 2010 / 154924 Al,

[0017] ). In the search for suitable ionic fluids, the expert would come across Kermani et al., Int. Journal of Hydrogen Energy, 45(33), 2020 - in which a study is conducted to determine suitable ionic fluids as liquid pistons. Consequently, the Based on the cited documents, the expert can select an ionic fluid identified as suitable in Kermani et al., Int. Journal of Hydrogen Energy, 45(33), 2020 as the working fluid for a storage device. However, the ionic fluid is electrically conductive, which creates the problem of the electrochemically effective series when—as in the applications according to the invention—dissimilar metals come into contact. This could lead to unintentional coating flow between the metals via the ionic fluid. US Pat. No. 6,066,604 A discloses a vacuum pump oil whose base oil can be a synthetic oil or a mineral oil. A polyalphaolefin with 4-14 carbon atoms per molecule can be used as the synthetic oil. Paraffinic mineral oils (especially isoparaffins) or naphthenic mineral oils can be used as the mineral oil (US Pat. No. 6,066,604 A, column 1, line 59 to column 2, line 41). EP 3 508 559 A1 also concerns vacuum pump oils, although the use of a synthetic oil (e.g., polyalphaolefin) in a vacuum pump oil is only mentioned in combination with a mineral oil. According to EP 3 508 559 A1, care must be taken to ensure that the effects are not impaired by the presence of the synthetic oil. Accordingly, EP 3 508 559 A1 prefers that a synthetic oil be present only in a small amount or not at all in the vacuum pump oil (EP 3 508 559 A1,

[0134] -

[0136] ) . However, it would not be obvious to the person skilled in the art to use a vacuum pump oil with polyalphaolefin according to US 6 066 604 A or EP 3 508 559 A1 as the working fluid of US 2007 / 258828 A1. Although US 6 066 604 A mentions the use of a polyalphaolefin as a vacuum pump oil, US 6 066 604 A focuses on achieving good vacuum quality and simplified starting of pumps in low-temperature applications (US 6 066 604 A, column 1, lines 4-7). Due to this different application area, a person skilled in the art would not have Reason to consider US 6 066 604 A for applications in hydrogen storage. The vacuum pump oils are subject to entirely different stresses than the working fluid of the hydrogen compressor or displacer according to the invention. The vacuum pump uses a rotor which is sealed using vacuum pump oil. Such a rotary pump requires a high throughput at low working pressure to achieve the vacuum. With the hydrogen compressor or displacer it is precisely the opposite. Typically, the hydraulically driven hydrogen compressor (in contrast to a piston compressor with a crank drive) uses an axial piston or radial piston pump which places high demands on the oil films, which are subject to considerable surface pressures, and on their load-bearing capacity. In contrast, vacuum pumps do not have high surface pressures, so that much lower demands are placed on wear protection. On the other hand, in vacuum pumps, the vapor pressure is crucial at moderate ambient and thus operating temperatures, particularly between -20 °C and +60 °C, and especially between 0 °C and 30 °C, since boiling at low pressures must be avoided. In hydrogen compressors, however, the working fluid should evaporate only slightly at high temperatures or pressures. This is primarily due to the fact that hydrogen gas for technical applications often has high purity requirements and is usually compressed to high pressures due to its low density. In view of the different requirements for a vacuum pump oil (sealing at high throughput, low pressure and low temperatures) compared to the hydrogen compressor according to the invention (sealing at relatively low volumetric throughput, high pressure with considerable surface pressures and high temperatures), the person skilled in the art would not use the known vacuum pump oils for the use according to the invention in the hydrogen compressor. A second embodiment of the invention provides a cylinder-piston arrangement and a corresponding method provided in which the movement of a solid-state piston in a housing of a housing-piston arrangement is sealed by a sealing fluid. US 4,750,409 A discloses a device for compressing gas. For this purpose, a piston having a sealing groove is moved in a bore. Lubricant is located on the top of the piston and enters the sealing groove via grooves running from the top of the piston to the sealing groove. The lubricant ensures smooth movement of the piston and lubricates the seal located in the sealing groove, thereby reducing its wear. Hydraulic oil is used as the lubricant. US 2016 / 145523 A discloses a compressor oil and a process for producing it. The compressor oil comprises a hydrocarbon oil with a sulfur content of 0.1 wt% or less and an aromatics content of 1 wt% or less, wherein the proportion of hydrocarbons having 10 or fewer carbon atoms is less than 100 ppm by mass. US 2016 / 145523 A cites an experiment in which a polyalphaolefin was mixed with the antifoam dimethylpolysiloxane and prepared according to the disclosed process. The mixture is then used for the compression of hydrogen. However, the decomposition temperature of the dimethylpolysiloxane is already reached at approx. 200 °C, so that only limited pressure conditions would be achievable.Furthermore, the addition of antifoam additives negatively affects the surface tension and thus the lubricating properties in axial piston or radial piston pumps. This fluid would therefore be unsuitable for the application according to the invention. As described at the beginning of WO 2006 / 034748 A1, piston compressors are regularly used for the compression of gaseous media in order to keep the medium to be compressed separate from the medium driving the piston, for example hydraulic oil. When compressing hydrogen, precisely fitting cylinders with pistons and effective dynamic sealing systems are required. These lead to high production and maintenance costs. Therefore, for such applications, More cost-intensive compression technologies, such as diaphragm compressors and lubrication-free piston compressors, are used. Against this background, WO 2006 / 034748 A1 discloses a method and a device for compressing hydrogen, in which compression is carried out using a working fluid in which the gas does not dissolve and / or which can be separated from the gas without leaving residue. Thus, pistons made of a solid material are replaced by a non-compressible liquid column. The working fluid can be an ionic liquid, a high-boiling hydraulic oil, a liquid with a very low vapor pressure (e.g., vacuum pump oils, molten salts, and low-melting-point metals), or a liquid with a gas solubility of less than 10 -4 mol / 1 bar. WO 2006 / 120145 A1 also relates to a working fluid for compressing hydrogen, which transfers the force required for compression directly to the gas and has a vapor pressure of less than 10 -3 mbar. The working fluid can be a molecular liquid, e.g., a mineral oil, a silicone oil, or a synthetic oil. DE 10 2011 101 504 A1 discloses another method for compressing hydrogen using a working fluid. The working fluid can be an ionic liquid, a perfluorinated polyether, or a thermal oil. DE 198 48 234 A1 relates to a method and a compressor for compressing a gas. The gas and a working fluid are contained in a container, and the gas is compressed by raising the level of the working fluid. A liquid with a low vapor pressure and / or that does not react with the gas, such as silicone oils, water, or hydrocarbons, can be used as the working fluid. Finally, DE 10 2015 016327 A1 describes a filling station for filling storage tanks in mobile vehicles with hydrogen. The hydrogen is stored in one or more constant-pressure storage tanks, which make the gas available at a specific, constant pressure. For this purpose, the constant pressure accumulator has a cylinder which is divided into two areas by a movable separating piston. The first area holds the hydrogen. The second area holds a liquid, for example a hydraulic fluid. During the storage phase, the pressure of the hydrogen in the first area is kept constant by moving the separating piston, whereby the first area is enlarged and the second area is reduced. During the discharging phase, the volume of the first area is reduced by pumping more liquid into the second area, thus increasing the volume of the second area so that the pressure in the first area is kept constant. As described in EP 3 514 380 A1, sealing a classic compressor piston made of a solid body is very complicated and time-consuming. This is particularly true for the compression of hydrogen, which places extremely high demands on the seals. Frequent seal replacement is therefore necessary. EP 3 514 380 A1 proposes a compressor in which seal replacement is automated. If the high-pressure seal can no longer seal the high-pressure piston due to wear or damage, a changing device is activated, with which the high-pressure seal is replaced with a replacement high-pressure seal. If all (replacement) high-pressure seals are worn out, the magazine as a whole can be replaced with a corresponding magazine containing unused replacement high-pressure seals, or new replacement high-pressure seals can be inserted into the magazine.Thus, this seal changer makes replacing the seal much easier. However, it would be desirable to further improve the sealing of a compressor piston when compressing hydrogen, so that replacing the seal is not necessary at all, or at least occurs less frequently. WO 2013 / 079692 A1 discloses a different type of ball valve with an external sealing arrangement in which a sealing fluid is used. The use of a polyalkylene glycol as the sealing fluid is described. The working fluids mentioned in the prior art have proven to be unsuitable or disadvantageous for the applications described above. Some of the working fluids known in the prior art are highly viscous, for example mineral oils, hydraulic oils, synthetic oils and perfluorinated polyethers. High viscosity is associated with a high tendency to foam, which can increase the cavitation tendency of the working fluid and consequently the wear of system components. The working fluid can also be carried over from the intended process space. This problem is critical, for example, when refueling a vehicle with hydrogen, when the hydrogen is transferred from the hydrogen storage tank into the vehicle's hydrogen tank. Carryover of the working fluid into the vehicle's hydrogen tank must be avoided at all costs.Other working fluids known in the prior art, such as thermal oils, water, and hydrocarbons, have high gas solubility or a high vapor pressure and therefore tend to enter the gas phase. The working fluids known in the prior art can therefore be detrimental to stable process control and the service life of the systems in the applications described above. The object of the present invention is therefore to alleviate or eliminate at least some of the disadvantages of the prior art. In particular, there is a need for working fluids and sealing fluids that ensure high process stability in the applications described above and minimize wear on the system components. This object is achieved by a hydrogen compressor and / or displacer according to claim 1, a method for compressing and / or displacing hydrogen according to claim 7, a housing-piston arrangement according to claim 8, a rotor according to claim 12 and a method for sealing the movement of a solid-state piston relative to a housing according to claim 14. In the first embodiment of the invention as a hydrogen compressor and / or displacer, the working fluid containing the polyalphaolefin can be used to achieve the required compression and / or displacement of the hydrogen. In the second embodiment of the invention, as a housing-piston arrangement, the movement of the solid-state piston can be sealed against the housing with the aid of the sealing fluid containing the polyalphaolefin. In both embodiments, the use of a polyalphaolefin (PAO) is surprising, since particular advantages are achieved without having to accept the disadvantages typical of PAO. Practical tests have shown that a particular advantage of PAO for the applications according to the invention is its low viscosity, i.e. its low viscosity. This can avoid the problem, for example, of gases released into the fluid due to pressure changes causing the fluid to foam when the pressure is released again. Such foaming could, for example, increase the tendency towards cavitation in a pump that supplies the working fluid, preferably an axial or radial piston pump, which would increase wear on the pump. When refueling a vehicle with hydrogen, the foaming of the working fluid could also cause fluid to be carried over into the hydrogen tank, which, however, should be avoided. In the prior art, highly viscous working fluids, for example ionic liquids, have been proposed. These have a high tendency to foam, which should be compensated for by very low gas solubility.In practice, however, the required low gas solubility has not actually been achieved. Other working fluids such as mineral oils, synthetic oils and hydraulic oils are so viscous that foaming would make their use in the applications according to the invention very disadvantageous or even impossible. On the other hand, thermal oils are unsuitable for use in systems that provide hydrogen for fuel cell-powered vehicles due to their tendency to carry over. The use of PAO according to the invention is based on the finding that the low foaming tendency of PAO can reliably prevent foaming without the need for extremely low gas solubility. In the applications according to the invention, the low foaming of PAO can be optimally utilized even though the gas solubility of PAO is quite low in absolute terms, but higher than that of ionic liquids. A "polyalphaolefin" is understood to mean a poly-1-olefin which has been produced by polymerization of alphaolefins. The term "polyalphaolefin" includes polyalphaolefin homopolymers, polymers composed of two or more different monomer units (e.g. polyalphaolefin copolymers, polyalphaolefin terpolymers), and mixtures thereof. The working fluid preferably contains the polyalphaolefin in an amount of 80 wt% or more, more preferably 90 wt% or more, even more preferably 95 wt% or more, and particularly preferably 99 wt% or more, based on the total weight of the working fluid. This allows the positive properties of the polyalphaolefin, such as low gas solubility and low foaming tendency, to be effectively utilized. Depending on the design, the working fluid can contain exactly one polyalphaolefin. In a preferred embodiment, the working fluid comprises a mixture of two or more polyalphaolefins. The physical properties of the working fluid (e.g., viscosity, boiling point) can then be adapted to the respective conditions. The polyalphaolefin can have a straight or branched chain structure (n-alkane or isoalkane). A mixture of a straight polyalphaolefin and an isomer of the same polyalphaolefin, or of a polyalphaolefin with a different number of carbon atoms, can also be used. This allows the boiling point of the working fluid to be adjusted. The branched molecules, in particular, have a significant influence on the viscosity of the working fluid. In a preferred embodiment, at least 90 wt% of the polyalphaolefin has 19 or fewer carbon atoms per molecule, preferably 14 to 18 carbon atoms per molecule, more preferably 16 to 18 carbon atoms per molecule, based on the total weight of the polyalphaolefin. Preferably, at least 95 wt% of the polyalphaolefin has such a number of carbon atoms, even more preferably at least 99 wt%, based on the total weight of the polyalphaolefin. Polyalphaolefins Polyalphaolefins with such a number of carbon atoms exhibit particularly low gas solubility and a particularly low foaming tendency, making them ideal working fluids. If the working fluid contains two or more polyalphaolefins, preferably all polyalphaolefins have such a number of carbon atoms. Preferably, a maximum of 10 wt% of the polyalphaolefin has 20 or more carbon atoms, in particular 20 to 30 carbon atoms, based on the total weight of the polyalphaolefin. Preferably, a maximum of 5 wt% of the polyalphaolefin has such a number of carbon atoms, even more preferably a maximum of 3 wt%, based on the total weight of the polyalphaolefin. This allows the viscosity to be kept low, which is associated with a low foaming tendency. The working fluid preferably contains a maximum of 1% by weight of an aromatic hydrocarbon, preferably a maximum of 0.5% by weight, based on the total weight of the working fluid. This also helps keep the viscosity and, consequently, the tendency to foam low. The proportion of a sulfur compound in the working fluid is preferably a maximum of 10,000 ppm, more preferably a maximum of 1,000 ppm, based on the total weight of the working fluid. This can prevent a cytotoxic effect, particularly in fuel cell-powered vehicles. The working fluid preferably has a decomposition temperature (i.e., a temperature at which the working fluid is irreversibly decomposed) of at least 250°C, preferably of at least 280°C, under ambient conditions (temperature 293.15 Kelvin; atmospheric pressure 101,325 Pascals). However, it is harmless if the working fluid reversibly boils below this decomposition temperature. Preferably, the operating temperature of the working fluid is at least from 0°C to 210°C, or from -50°C to +250°C, or up to 280°C, in each case under ambient conditions. In a further preferred embodiment, the working fluid is free from an antifoam agent, in particular free from Dimethylpolysiloxane. The addition of an antifoam agent or additive would negatively affect the surface tension and thus the lubricating properties, particularly in axial or radial piston pumps. The composition of the working fluid can be determined using two-dimensional gas chromatography coupled to mass spectrometry (2D GC-MS) (Orbitrap Exploris GC 60K MSMS, Thermo Fisher Scientific). For this purpose, a sample containing 4 wt% of the working fluid in dichloromethane can be prepared. The parameters can be set as follows: For gas chromatography (GC): Heat the cold injector (PVT injector) from 40 °C to 300 °C at 2.5 °C / sec; heat the oven from 40 °C to 150 °C at 5 °C / min and then to 300 °C at 3 °C / min; column flow of 0.5 ml / min, transfer at 250 °C, detector flow of 20 ml / min. For the flame ionization detector (FID): 200 Hz, 300 °C, 350 ml / min air, 10 ml / min make-up, 35 ml / min hydrogen. For mass spectrometry (MS): 200 °C, solvent delay 7 min, m / z from 30 to 650, 15 kJ, cold trap offset 1.5 V. This measurement can be used to determine the content of volatile polyalphaolefins with a specific number of carbon atoms, as well as the content of volatile aromatic hydrocarbons and the content of volatile sulfur compounds. For quantitative determination, appropriate calibration of the analytes is necessary. The instrument is constructed as follows: 1. Column dimension: BPX 5, 20 m x 0.18 mm x 0.18 pm flow modulator with loop 23 cm x 0.53 mm and bleedline 5 m x 0.1 mm; modulation 3 s, with purge time 100 ms 2nd dimension column: BPX 50, 5 mx 0.25 mm x 0.1 pm Detectors: FID, TCD, mass spectrometer (Exploris: electron impact ionization and quadrupole+Orbitrap), split ratio MS 5:1 TCD / FID Preferably, the hydrogen at 50 °C and a pressure of 50 bar has a solubility in the working fluid selected from a range of 0.02 to 0.15 litres (1) of hydrogen per litre of working fluid, preferably substantially 0.07 litres of hydrogen per litre Working fluid. This results in a low tendency for the working fluid to enter the gas phase, or conversely, a low tendency for the hydrogen to dissolve in the working fluid. This allows for good process stability. It is preferred if the working fluid at 70°C has a viscosity selected from a range of 1 to 10 centistokes (cSt), preferably 1 to 5 cSt, in particular 2.5 to 4 cSt. This low viscosity can at least largely prevent hydrogen dissolved in the working fluid due to pressure changes from causing the working fluid to foam when the pressure is released again, which can subsequently increase the cavitation tendency of the working fluid and lead to wear and tear on the system components. The low viscosity of the working fluid can also prevent the working fluid from being entrained and carried away with the gas. Due to this low viscosity, the working fluid according to the invention can indeed have poorer tribological properties, in particular lower friction, compared with working fluids customary in the prior art, e.g. mineral oils or hydraulic oils.However, this can be easily compensated for by hydrostatic lubrication and / or a suitable material pairing. Gas solubility can be measured in a known manner using a pressure decay method under isothermal conditions. The working fluid preferably has exactly one boiling point (when using exactly one PAO) or an upper end of a boiling range (when using two or more PAOs) of a maximum of 250°C. The working fluid preferably has a boiling point or a boiling range selected from an interval of 90°C to 200°C. Due to the low boiling point or boiling range, the working fluid can evaporate due to a temperature increase during operation. If the working fluid is used in a closed circuit, as is preferred, this is not disadvantageous; instead, the working fluid can subsequently condense again, and evaporation of the working fluid due to pressurization can be avoided. In a hydrogen storage system, the working fluid volume can be located below the hydrogen volume. This is due to the fact that the working fluid has a higher density than the hydrogen. The working fluid and hydrogen are in direct contact with each other in the hydrogen storage system, with the hydrogen volume and the working fluid volume bordering each other at a single layer. This layer can lie in a horizontal plane due to gravity. In the hydrogen storage device, a first opening can be arranged on a first side of the hydrogen storage device to supply working fluid into the hydrogen storage device. A second opening can be provided on a second side of the hydrogen storage device to discharge hydrogen from the hydrogen storage device. The second side can be arranged opposite the first side. This allows the volume of the hydrogen storage device to be optimally utilized, and the working fluid volume can be displaced as completely as possible by the working fluid volume when discharging hydrogen. Preferably, the initial pressure of the hydrogen in the hydrogen storage device, i.e., the nominal pressure of the hydrogen before the hydrogen is withdrawn and without replenishment of the working fluid, so that the hydrogen volume and the working fluid volume each remain constant, is more than 500 bar, preferably more than 600 bar, in particular more than 700 bar, for example, essentially 800 bar, and / or a maximum of 875 bar. This pressurization can reduce or completely prevent evaporation of the working fluid. Thus, there can be no or only a minimal boiling loss, albeit a temporary one in a closed hydraulic system. The invention further relates to a system for compressing and / or displacing hydrogen, comprising: a hydrogen compressor and / or displacer in one of the above embodiments, a pump, in particular an axial piston or radial piston pump, for feeding the working fluid into the working fluid volume. The method according to the invention for displacing and / or compressing hydrogen comprises at least the following steps: (a) providing the hydrogen compressor or displacer in one of the embodiments described above, and (b) Feeding working fluid into the hydrogen compressor or displacer so that the working fluid volume of the hydrogen compressor or displacer is increased and the hydrogen volume of the hydrogen compressor or displacer is reduced. In step (b), the working fluid serves as a liquid piston, transferring the force required for compressing and / or displacing the hydrogen to the hydrogen. By replenishing the working fluid, i.e., increasing the volume of working fluid inside the hydrogen compressor or displacer, the hydrogen volume can be reduced, with the hydrogen being at least partially compressed and / or at least partially removed from the gas storage unit. The extent to which the hydrogen volume is reduced can depend, among other things, on the pressure at which the working fluid is replenished into the gas storage unit and on the density of the hydrogen in the provided hydrogen storage unit. The invention further relates to a method for refueling a hydrogen tank, in particular a vehicle, with hydrogen, comprising the steps: (a) Providing a hydrogen storage device in one of the embodiments described above with a hydrogen volume filled with hydrogen, (b) providing a working fluid reservoir with a working fluid, (c) connecting the hydrogen storage to the hydrogen tank so that hydrogen is transferred from the hydrogen storage to the hydrogen tank in an outflow phase, and (d) connecting the working fluid reservoir to the hydrogen reservoir so that working fluid is pumped from the working fluid reservoir into the hydrogen reservoir in a post-compression phase, preferably overlapping with the outflow phase, whereby the volume of hydrogen filled with hydrogen in the hydrogen reservoir is reduced. This refueling process can also be carried out with at least one additional hydrogen storage unit, whereby the hydrogen storage units can be connected to the hydrogen tank one after the other. By adding working fluid to the respective hydrogen storage unit, the hydrogen in the respective hydrogen storage unit can be recompressed, in particular brought back to its initial pressure. Furthermore, the present disclosure also relates to a refueling system, in particular a mobile refueling system, for refueling a hydrogen tank, in particular of a vehicle, with hydrogen, comprising: a hydrogen reservoir which contains the hydrogen, a working fluid reservoir which contains a working fluid, a control device, in particular with a valve control, for controlling a discharge connection for discharging hydrogen from the hydrogen reservoir to the hydrogen tank and for controlling an inlet connection between the working fluid reservoir and the hydrogen reservoir, and a pump with which working fluid can be pumped from the working fluid reservoir to the first hydrogen reservoir. Furthermore, the present disclosure also relates to a refueling system comprising a refueling system as described above and the vehicle, wherein the hydrogen tank of the vehicle is connected to the hydrogen storage of the refueling system. In the housing-piston arrangement according to the invention, the sealing fluid comprises a polyalphaolefin. For the advantages and effects as well as the preferred compositions of the sealing fluid, reference is made to the explanations above in connection with the working fluid. In a preferred embodiment, the preferably linearly reciprocating solid-state piston has a circumferential recess on its end face facing the sealing fluid. The distance between a sealing fluid-facing In this embodiment, the gap between the end face of the solid-state piston and a side of the housing facing this end face is thus larger in the area of ​​the recess than in a radially inner area. This allows the working volume in the area of ​​the recess to be increased, thereby achieving particularly good sealing. The recess can be curved when viewed in the longitudinal section of the solid-state piston. This can increase the contact area between the sealing fluid and the solid-state piston, allowing the sealing fluid to be pressed against an inner wall of the solid-state piston. This can further improve the sealing effect. In a preferred embodiment, the housing comprises an outer housing part and a cylindrical insert or liner inside the outer housing part. The insert liner can be made, for example, from a ceramic material. Alternatively, the insert liner can have a coating of DLG ("diamond-like carbon," i.e., amorphous carbon layers) on its inside. In a preferred embodiment, the piston has at least one circumferential sealing groove, preferably a plurality of sealing grooves spaced apart in the axial direction of the piston. The at least one sealing groove causes pressure reduction, which can prevent the working fluid from leaving the working volume along the outside of the piston. Furthermore, an additional mechanical seal can be accommodated in the sealing groove. The invention further relates to an axial or radial piston pump with a cylinder-piston arrangement in one of the embodiments described above. In the rotor according to the invention, the rotation drive is designed to rotate the housing-piston arrangement about the rotation axis. In a preferred embodiment, the axis of rotation is substantially perpendicular to the longitudinal axis of the solid piston. In an alternative embodiment, the axis of rotation may extend along the longitudinal axis of the piston. The method according to the invention for sealing the movement of a solid-state piston relative to a housing comprises at least the following steps: (a) providing the housing-piston assembly in any of the embodiments described above, wherein the solid-state piston is arranged in a first position relative to the housing, and (b) moving the solid piston relative to the housing from the first to a second position, thereby changing the working volume enclosed by the housing, the movement of the solid piston relative to the housing being sealed with the sealing fluid. In the first position, the end face of the solid-state piston facing the sealing fluid can be spaced further apart from the housing side facing this end face than in the second position. Moving the solid-state piston to the second position then reduces the working volume. The housing-piston assembly is preferably rotated about the rotation axis. This allows a centrifugal force to act on the sealing fluid due to inertia, forcing the sealing fluid outward, thus achieving further improved sealing. Hydrogen can be provided in the working volume, which can be at least partially compressed by the movement of the solid-state piston in step (b) and / or at least partially discharged through an opening arranged in the housing. The opening can be arranged on a side of the housing in the direction of which the piston moves in order to reduce the working volume. The invention is further described below with reference to preferred embodiments which are illustrated in the figures. For the purposes of this disclosure, "hydrogen" always refers to molecular hydrogen (H2). Fig. 1 and 2 show a longitudinal section of a hydrogen storage device in two different states, where a polyalphaolefin is used as a liquid piston for the displacement and / or compression of hydrogen. Fig. 3 and 4 show a longitudinal section of a housing-piston arrangement in two different states, wherein a polyalphaolefin is sealed as a circumferential seal to seal the reciprocating movement of a solid-state piston along the housing designed as a cylinder. Fig. 1 and Fig. 2 show a hydrogen compressor or hydrogen displacer, which here is designed as a hydrogen storage device 1 for storing, discharging and recompressing the hydrogen remaining in the hydrogen storage device 1. According to Fig. 1, the hydrogen storage device is in a first state. The hydrogen storage device 1 has a hydrogen volume filled with hydrogen 2 and a working fluid volume filled with a working fluid 3, which together form a filling space of the hydrogen storage device. The working fluid 3 comprises a mixture of polyalphaolefins which have 16 to 18 hydrocarbon atoms per molecule. The working fluid 3 is designed as a liquid piston which is in direct contact with the hydrogen (i.e. without a solid piston arranged in between), so that the interface to the hydrogen can be shifted with the working fluid 3.The hydrogen storage device 1 has a first opening 4, through which the working fluid 3 can be supplied from a working fluid reservoir (not shown). Furthermore, the hydrogen storage device 1 has a second opening 5, through which the hydrogen 2 can be discharged from the hydrogen storage device 1. In the embodiment shown, the first opening 4 is located at a bottom of the device. Hydrogen storage 1 and the second opening 5 is formed on an upper side of the hydrogen storage 1. For the purposes of this disclosure, the location and direction specifications, such as "top" and "bottom", refer to the intended use state of the hydrogen storage device 1. In a preferred embodiment, the hydrogen storage device 1 is a component of a refueling system which can be connected to a hydrogen vehicle in order to fill a hydrogen tank of the hydrogen vehicle with hydrogen (not shown). According to Fig. 1, the working fluid occupies a first working fluid volume inside the hydrogen storage device 1. Thus, in the first state, the working fluid has a first fill level. By adding working fluid 3 to the hydrogen storage unit 1, the working fluid volume can be increased and the hydrogen volume correspondingly reduced. A portion of the hydrogen 2 can be compressed in the hydrogen volume, while another portion can be removed from the hydrogen storage unit 1 and, in a preferred application, fed into the tank of the hydrogen vehicle. Fig. 2 shows the hydrogen storage device 1 of Fig. 1 in a second state, with working fluid 3 being replenished. The working fluid occupies a second working fluid volume, which is greater than the first working fluid volume. Thus, in the second state, the working fluid has a second fill level, which is higher than the first fill level. Figures 3 and 4 show a second embodiment of the invention in which the polyalphaolefin is used as a sealing fluid. Fig. 3 shows a housing-piston assembly 6 in a first state. The housing-piston assembly 6 comprises a housing 7, which in the embodiment shown comprises an outer housing part and a cylindrical insert or liner. which fills a cylindrical recess in the outer housing part. The insert bushing can be made from a ceramic material, for example. Alternatively, the insert bushing can have a DLC coating on the inside. The housing 7 encloses a working volume in which hydrogen 8 is accommodated. Furthermore, the housing-piston arrangement 6 has a solid-state piston 9 which, in order to change the working volume, can be axially displaced essentially precisely along the inner wall of the housing 7. Unlike in the previous embodiment, according to Fig. 3 and Fig. 4, no liquid piston is provided, but rather the solid-state piston 9 which is designed as a machine component that is essentially unable to change its shape.To seal the movement of the solid-state piston 9 relative to the housing 7, a sealing fluid 10 is provided, which prevents the hydrogen 8 from reaching the side of the piston 9 facing away from the working volume. In addition, at least one circumferential sealing groove, preferably a plurality of sealing grooves spaced apart in the axial direction of the piston 9, can be provided on the circumference of the piston 9. The at least one sealing groove ensures pressure reduction, thereby preventing the working fluid from leaving the working chamber along the piston 9. In addition, an additional mechanical seal can be accommodated in the sealing groove. In the embodiment shown, the sealing fluid 10 consists of a mixture of polyalphaolefins containing 16 to 18 hydrocarbon atoms per molecule. The solid-state piston 9 has, on its end face facing the sealing fluid 10, a circumferential recess 11 that is curved in the longitudinal section of the piston 9. An opening 12 for discharging the hydrogen 8 is arranged in one side of the housing 7, in the direction of which the piston 9 is moved to reduce the working volume. In Fig. 3, the piston 9 is arranged in a first position relative to the housing 7. In this first position, the end face of the solid-state piston 9 facing the sealing fluid 10 is at a comparatively large distance from a side of the housing 7 facing this end face. Accordingly, the working volume is comparatively large. By axially moving the By moving the solid-state piston 9 toward a second position, the working volume enclosed by the housing 7 can be reduced. A portion of the hydrogen 8 can thereby be compressed, while another portion can be discharged from the housing 7 through the opening 12. Fig. 4 shows the housing-piston arrangement 6 of Fig. 3 in a second state. The solid-state piston 9 is arranged in a second position in which the working volume is smaller than in the first state of the housing-piston arrangement 6. The sealing fluid 10 fills the sealing points adjacent to the inside of the housing 7, as a result of which the movement of the solid-state piston 9 is sealed off from the housing 7. By rotating the housing-piston arrangement 6 about an axis of rotation 14 which is perpendicular to a longitudinal axis of the solid-state piston 9, a centrifugal force can act on the sealing fluid 10, as a result of which the sealing fluid 10 can be pressed even better against the inside of the housing 7, so that the seal is further improved.

Claims

Claims:

1. Hydrogen compressor and / or displacer for hydrogen, in particular hydrogen storage (1) for storing hydrogen (2) or housing-piston unit, comprising: a hydrogen volume filled with hydrogen (2), and a working fluid volume filled with a working fluid (3), wherein the working fluid (3) is designed as a liquid piston for changing the hydrogen volume, characterized in that the working fluid (3) comprises a polyalphaolefin.

2. Hydrogen compressor and / or displacer according to claim 1, characterized in that the working fluid (3) comprises the polyalphaolefin in an amount of at least 80% by weight, based on the total weight of the working fluid (3).

3. Hydrogen compressor and / or displacer according to claim 1 or 2, characterized in that at least 90% by weight of the polyalphaolefin has 14 to 18 carbon atoms per molecule, based on the total weight of the polyalphaolefin.

4. Hydrogen compressor and / or displacer according to one of claims 1 to 3, characterized in that a maximum of 10 wt% of the polyalphaolefin has 20 or more carbon atoms per molecule, based on the total weight of the polyalphaolefin.

5. Hydrogen compressor and / or displacer according to one of claims 1 to 4, characterized in that the working fluid (3) has a proportion of an aromatic hydrocarbon of 1% by weight or less, based on the total weight of the working fluid (3).

6. Hydrogen compressor and / or displacer according to one of claims 1 to 5, characterized in that the working fluid at 70°C has a viscosity selected from a range of 1 to 5 cSt, in particular 2.5 to 4 cSt.

7. A method for displacing and / or compressing hydrogen (2), comprising the steps: (a) providing the hydrogen compressor and / or displacer according to any one of claims 1 to 6, and (b) Feeding working fluid (3) into the hydrogen compressor and / or displacer, preferably by means of an axial piston or radial piston pump, so that the working fluid volume of the hydrogen compressor and / or displacer is increased and the hydrogen volume of the hydrogen compressor and / or displacer is reduced.

8. Housing-piston arrangement (6), comprising: a housing (7), preferably a cylinder, wherein the housing (7) encloses a working volume for accommodating hydrogen (8), a solid-state piston (9) which is movable relative to the housing (7) to change the working volume, and a seal for sealing the movement of the solid-state piston (9) relative to the housing (7), wherein a sealing fluid (10) is provided as the seal, characterized in that the sealing fluid (10) comprises a polyalphaolefin.

9. Housing-piston arrangement (6) according to claim 8, characterized in that at least 90 wt% of the polyalphaolefin has 14 to 18 carbon atoms per molecule, based on the total weight of the polyalphaolefin.

10. Housing-piston arrangement (6) according to claim 8 or 9, characterized in that the solid-body piston (9) has a circumferential recess on its end face facing the sealing fluid (10).

11. Housing-piston arrangement (6) according to one of claims 8 to 10, characterized in that the recess is curved when viewed in the longitudinal section of the solid-state piston (9).

12. Rotor, comprising: a rotation axis (13, 14), a rotation drive, and a housing-piston arrangement (6) according to one of claims 8 to 11, wherein the rotation drive is adapted to Housing-piston arrangement (6) to rotate about the rotation axis (13, 14).

13. Rotor according to claim 12, characterized in that the axis of rotation (14) is perpendicular to the longitudinal axis of the solid-state piston (9).

14. Method for sealing the movement of a piston (9) relative to a housing (7), comprising the steps: (a) providing a housing-piston arrangement (6) according to one of claims 8 to 11, wherein the solid-state piston (9) is arranged in a first position relative to the housing (7), and (b) moving the solid-state piston (9) relative to the housing (7) from the first to a second position, thereby changing the working volume enclosed by the housing (7), wherein the movement of the solid-state piston (9) relative to the housing (7) is sealed with the sealing fluid (10).

15. Method according to claim 14, characterized in that the housing-piston arrangement (6) is rotated about a rotation axis (13, 14).

16. Axial or radial piston pump with a cylinder-piston arrangement according to one of claims 8 to 11.

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