Device for storing hydrogen in solid form

Abstract

The present invention relates to a hydrogen storage pellet that allows the production of compact, modular, safe and energy-efficient hydrogen storage. The pellet of the present invention comprises a peripheral ring (4) of an outer diameter of expanded natural graphite (ENG) of a predetermined height, surrounding a wafer of metal hydride (5) in the form of a compressed powder.

Description

[Technical field]

[0001] The invention relates to a device for storing hydrogen in solid form, in particular for producing a compact and modular tank for storing hydrogen in the form of a metal hydride at low pressure.

[0002] Hydrogen is used in many industrial sectors, in particular as a fuel or reagent, and in this context, given its volume in gaseous state and its explosiveness in air, it is desirable for hydrogen to be stored in a form that occupies a limited space and ensures safe containment.

[0003] Currently there are three main technologies:

[0004] The first involves storing hydrogen gas at very high pressures (350 bar to 700 bar) by compressing the hydrogen gas in tanks that are designed to withstand high pressures and are therefore expensive. This type of storage also requires a significant amount of energy to compress and cool the hydrogen. Thus, there is a poor energy balance when using hydrogen in this storage method.

[0005] The second technique involves storing hydrogen in liquid form, where the temperature must be kept below -252.87 °C to liquefy the hydrogen in the tanks. This type of storage requires a significant amount of energy to keep the hydrogen in a liquefied state.

[0006] A third technique involves storing gaseous hydrogen in a solid medium in the form of compressed metal hydride powder.

[0007] This technology offers safer storage conditions and limited energy costs. Some metals or alloys can reversibly incorporate hydrogen atoms into their crystal lattices. Hydrogen is absorbed / desorbed by these materials as a function of temperature and pressure conditions. Examples include palladium (Pd), magnesium (Mg), ZrMn2, ZrMn2, Mg2Ni, or alloys such as Mg-Mg2Ni or alanates.

[0008] By convention, the term "metal hydride" as used herein includes metals that are partially or fully loaded with hydrogen, depending on the process step.

[0009] Two types of metal hydrides are generally recognized: deuterides (mainly LaNi5, and alloys such as ferro-titanium or Ti-V-Cr alloys) and light hydrides (mainly magnesium and lithium).

[0010] In the deuterated form, hydrogen is absorbed at ambient temperature and pressure. The exotherm of the reaction is usually moderate (less than 35 kJ / mol H2). Hydrogen is subsequently desorbed at ambient temperature and pressure during use. The energy input required to use hydrogen is reasonable.

[0011] Conversely, for light hydrides, absorption of hydrogen by light metal hydrides requires higher temperatures (about 300 °C for MgH2). The reaction is highly exothermic (75 kJ / mol H2). Thus, the energy input required to initiate the hydrogen absorption reaction is moderate. On the other hand, the absorption reaction will stop spontaneously if the generated heat is not removed. Furthermore, during use, hydrogen desorption requires a significant heat input and the reaction is endothermic.

[0012] The use of hydrides, especially light hydrides, therefore requires very precise thermal management, both during absorption and desorption of hydrogen.

[0013] Furthermore, regardless of the type of hydride used, the absorption / desorption reactions will cause hydride expansion / contraction, ie, the volume expansion / contraction of the hydride during charging / discharging of hydrogen.

[0014] Volumetric and thermal variations must be taken into account when manufacturing the tank and filling it with the metal hydride, since mechanical stresses applied to the tank walls can cause said tank to crack and even burst.

[0015] Finally, it has been noted that after a significant number of cycles have been completed, even though the cycles are consistent with normal use, the compressed metal hydride media tends to "decrepitate," i.e., crumble and return to a powder-like state.

[0016] The present invention is intended to provide devices and tanks for storing hydrogen in a safe solid form (i.e., without the risk of bursting under mechanical stress due to volume fluctuations during absorption / desorption) that are easy to manufacture and provide rapid hydrogen absorption kinetics.

[0017] US6969545 describes a tank for storing hydrogen in solid (hydride) form, comprising a single large volume of hydride with a single inlet and a single outlet, said single large volume being surrounded by a layer of expanded natural graphite (ENG), itself in contact with the rigid (non-deformable) wall of the tank, which allows heat transfer and can be compressed during hydrogen absorption.

[0018] However, the large volume of hydride means that the hydrogen absorption / desorption times are too long to make this tank usable.

[0019] French patent application FR2939784 also provides a hydrogen storage tank that minimizes volume fluctuations. It proposes a hydrogen storage tank using a light metal hydride, in particular magnesium hydride, mixed and compressed with a thermally conductive matrix (selected from the group comprising ENG, metal felt, non-oxide ceramics, and copper foam coated with non-oxide ceramics) and combined with a reversible absorption-heat storage system.

[0020] The compressed material may comprise 80% to 99% by weight magnesium hydride and 20% to 1% by weight ENG.

[0021] The tank comprises at least one tubular container defined by a thermally conductive wall immersed in a phase change material.

[0022] Each tubular vessel contains a plurality of vertically stacked solid pellets formed of a mixture of compressed material containing a metal hydride and particles forming a thermally conductive matrix made of ENG. Each pellet is provided with a central hole intended to receive a permeable tube in fluid communication with the hydrogen inlet and outlet. Metal plates are disposed between each pellet.

[0023] The pellets are in heat transfer relationship with the external phase change material through the walls of each container, which are made of stainless steel. To address the expansion problem, this document proposes providing a mechanical means to hold the pellets in contact with the walls.

[0024] This device is complex, expensive and difficult to implement due to the presence of phase change materials.

[0025] Furthermore, this apparatus can be dangerous because, due to the use of vertically stacked pellets, hydride powder can fall to the bottom of the tank during decrepitation of the pellets, causing an explosion under certain operating conditions.

[0026] The present invention is therefore intended to avoid this risk resulting from the natural and inevitable collapse of metal hydride compacts in pellet form.

[0027] The present invention therefore relates to a solution for storing hydrogen in the form of low-pressure metal hydrides, which allows the design and manufacture of hydrogen tanks that are compact, modular, safe (i.e. no risk of wall rupture under mechanical stress and no risk of explosion) and offer improved energy efficiency (i.e. have higher filling rates).

[0028] To this end, the present invention proposes a specific arrangement of metal hydrides and ENGs that allows all of these problems to be overcome, in particular limiting the mechanical stresses on the tank walls during hydrogen filling / discharging cycles and limiting the risk of explosions associated with collapse, while accelerating the filling speed and filling capacity as a result of improved heat exchange.

[0029] The invention more particularly relates to a pellet for storing hydrogen in solid form intended to be incorporated into a hydrogen storage tank, comprising a peripheral ring of a given outer diameter made of expanded natural graphite (ENG) of a given height, surrounding a wafer of metal hydride in the form of a compressed powder.

[0030] Therefore, rather than mixing the metal hydride and the ENG, the present invention proposes to surround the compressed metal hydride pellets with a ring made of ENG, preferably having a sheet structure, and to separate these two elements using a plate of thermally conductive material.

[0031] According to a particular embodiment: the peripheral ENG ring may be formed by axial stacking of annular ENG sheets having a height less than that of the peripheral ring; and / or The annular ENG sheet may have a height of the order of a few tenths of a millimetre, preferably between one tenth and five tenths of a millimetre.

[0032] The present invention also relates to a tank for storing hydrogen in solid form, comprising: - a hollow cylindrical container extending along a longitudinal axis, closed at a first end and open at a second end, defined by a thermally conductive outer radial wall, comprising an alternating stack of rigid disks of a given diameter made of a thermally conductive material and the above-mentioned pellets, a hollow cylindrical container in which each pellet is interposed between two rigid disks, each rigid disk being pierced by a hole facing a hole in the disk, such that each metal hydride wafer is pierced by a hole forming an axial passage; - a removable cover for reversibly sealing the second end of the hollow cylindrical container, the removable cover comprising hydrogen inlet / outlet orifices; A tank comprising:

[0033] According to a particular embodiment The tank may further comprise a passive hydrogen diffusion tube extending axially along the hollow cylindrical vessel through a hole in the rigid disk and pellet hydride wafers and in sealed fluid communication with an orifice in the removable reversibly sealing cover. · The passive hydrogen diffusion tube may be made of a material that is permeable to hydrogen. The hydrogen inlet / outlet orifices may be in fluid communication with an on-off valve. The hollow cylindrical container may provide free axial expansion space for the stack of pellets between the last rigid disk of the stack and a removable reversibly sealing cover; and / or The tank may be shaped to be used in an extended position in which the longitudinal axis of the container is horizontal with respect to gravity, the tank further comprising a compression spring between the last rigid disc of the stack and a removable reversibly sealing cover.

[0034] Other features of the present invention are described in the detailed description given below with reference to the accompanying drawings, which are provided by way of example and show respectively: [Brief description of the drawings]

[0035] [Figure 1] FIG. 2 is a schematic perspective view of a stack of hydrogen storage pellets according to the invention with aluminum discs inserted between them; [Diagram 2] A schematic cross-sectional view of the stack in FIG. [Diagram 3] 1 is a schematic perspective view of two ENG sheets of a stack of sheets forming a peripheral ENG ring according to the invention; [Figure 4] 1 is a schematic cross-sectional view of a tank according to the invention; [Diagram 5] 5 is a schematic cross-sectional view of the tank of FIG. 4 with a plurality of pellets according to the invention; and [Figure 6] FIG. 1 is a schematic cross-sectional view of a tank according to the invention used in an extended position; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0036] Figures 1 and 2 show a pellet 1 for storing hydrogen in solid form according to the invention, which is intended to be incorporated into a hydrogen storage tank (see Figures 5 and 6).

[0037] The pellet 1 comprises a peripheral ring 4 of a given outer diameter D4 made of expanded natural graphite (ENG) of a given height H4, surrounding a wafer of metal hydride 5 in the form of a compressed powder, also of height H4, the compressed powder being subjected to a uniaxial force of several metric tons, which bonds the powder together and provides a wafer of metal hydride 5 which is solid, i.e. free-standing.

[0038] The diameter D4 is equal to the inner diameter of the tank in which the pellet 1 is intended to be installed, to ensure intimate contact between the ENG ring and the wall of the tank.

[0039] 3 shows a particularly advantageous embodiment in which the peripheral ENG ring 4 is formed by axially (in use along the axis XX) stacking a number of annular ENG sheets 4a (this figure shows only two sheets). These sheets 4a have a height H4a that is lower than the height H4 of the peripheral ring 4. Preferably, the annular ENG sheets have a height of the order of a few tenths of a millimeter, preferably between one and five tenths of a millimeter.

[0040] This stacked sheet structure offers surprising efficiency in both radial stress absorption, heat transfer, axial stress offset, and stability over time.

[0041] Preferably, the metal hydride 5 in the form of a compressed powder is a hydride of the AB2 group of metal hydrides having a gravimetric storage capacity of up to 1.8 wt. % (kg_H2 / kg metal hydride) for moderate operating conditions (moderate pressures, i.e. less than 100 bar, and temperatures less than 100°C, preferably ambient temperature).

[0042] Advantageously, a metal hydride that works particularly well with the structure of the pellets according to the invention is the metal hydride sold under the name Hydralloy C5, an alloy based on Ti / Zr / Mn / V / Fe. The powder is initially in the form of particles smaller than 600 μm. After compaction, the apparent density of the metal hydride (mass of powder / apparent volume of powder) is 2.93 g / cm. 3 Its absolute density is 6.41 g / cm 3 It is.

[0043] This arrangement of the pellets according to the invention ensures that the volumetric compression / decompression cycles caused by the charging / discharging of hydrogen are absorbed particularly well laterally by the ENG ring, while maintaining heat transfer, and that the charging / discharging speeds are much faster than in known systems that do not have a surrounding ENG ring.

[0044] According to the invention, the pellets 1 are stacked alternately with rigid discs 2 made of a thermally conductive material.

[0045] In other words, the disks 2 are spaced apart from one another by a peripheral ring 4 of expanded natural graphite (ENG) and a metal hydride wafer 5 on which the disks rest freely, ie without being fixed to the disks.

[0046] A peripheral ENG ring 4 acts as a spacer forming a space between the disks 2 for receiving the metal hydride wafers 5 .

[0047] Each disk 2 has a diameter D2 slightly smaller than the inner diameter of the tank, as the disks 2 are intended to be stacked within the tank so that the disks 2 can expand during heat transfer and come into contact with the tank wall 11b without adding stress to the tank.

[0048] Each disk 2 is pierced by at least one hole 3 (in this case a single central hole 3).

[0049] The metal hydride wafers 5 also comprise holes 5a arranged in a ring relative to the holes 3 so as to leave a free passage through the stack. This free passage allows hydrogen to circulate to and from the metal hydrides 5 of the pellets 1 and to be discharged via the holes 3 and 5a. As specified below, a tube permeable to hydrogen is advantageously inserted through the holes 3, 5a so as to convey the hydrogen to the tank and to the circulation of the hydrogen storage system. This tube also makes it possible to filter the hydrogen, i.e. to prevent metal hydride particles from the wafers 5 from contaminating the hydrogen leaving the tank. Finally, this tube also has the role of a mechanical guide during the manufacture of the stack in the tank, making it possible to perfectly center the pellets 1 and the rigid disks 2.

[0050] Figure 2 shows an embodiment with dimensions given by way of non-limiting example only, the figures are not to scale and are given merely as an example.

[0051] In this figure, the disk 2 is made of aluminium and has a diameter D2 of 111.8 mm and a thickness E2 of 1 mm. The hole 3 has a diameter D3 of 10.2 mm.

[0052] The peripheral ENG ring 4 has a height H4 of 15 mm and a width L4 of 5.6 mm.

[0053] More generally, the peripheral ENG ring 4 has a height H4 equal to 5% to 15% of the radius (D4 / 2).

[0054] Advantageously, the ratio of the width L4 of the peripheral ENG ring to the diameter D2 of the disk is between 3 and 8.

[0055] The material of the disk 2 is chosen to optimize the heat transfer and to allow the evacuation of heat in contact with the walls of the tank and with the metal hydride wafers 5. It is also chosen to have the lowest possible density. It may be chosen, for example, from stainless steel or copper, but is advantageously made of aluminum, which optimizes the heat conduction / density ratio. For example, the aluminum disk has a thickness E2 of about 1 mm.

[0056] Figures 4 and 5 show a tank 10 for storing hydrogen in solid form according to the invention, used vertically, which is shaped to incorporate a plurality of pellets 1 according to the invention.

[0057] The tank 10 comprises a hollow cylindrical vessel 11 extending along a longitudinal axis XX, closed at a first end 11a and defined by a thermally conductive outer radial wall 11b, the vessel 11 having a second open end 11c allowing access to the interior of said vessel.

[0058] The tank 10 also includes a removable cover 12 for reversibly sealing the second end 11c of the hollow cylindrical vessel 11 to allow access to the interior of the vessel for placement of pellets therein, and for sealing the vessel for use in storing / removing hydrogen. The cover 12 also includes a hydrogen inlet / outlet orifice 12a in fluid communication with an on / off valve 14.

[0059] The wall 11b of the container in contact with the storage pellets 1 is as thin as possible to optimize the evacuation of heat. Of course, this wall must be able to withstand the operating pressure of hydrogen and the mechanical compression from the pellets 1 without deformation. In the pellets according to the invention, the mechanical compression from the pellets 1 is very limited since it is absorbed by the surrounding ENG ring. At the second end 11c, the wall 11b is advantageously thicker to allow the fixing of the cover 12.

[0060] Preferably, the tank 10 also includes a passive hydrogen diffusion tube 13 that extends axially along the hollow cylindrical vessel through the holes 3 in the disk 2 and the holes 5a in the pellets. The tube 13 is also in sealed fluid communication with the orifice 12a in the removable reversibly sealing cover.

[0061] The tube 13 also facilitates the insertion of the pellets 1 and the disk 2 into the container 11 by centering the assembly, i.e. ensuring its optimal positioning, in particular with regard to the contact between the peripheral ENG ring 4, the disk 2 and the wall 11b of the container 11. The tube 13 also allows for the filtration of residual metal hydride powder during desorption.

[0062] The passive hydrogen diffusion tube 13 is a tube made of a material permeable to hydrogen so as to allow absorption / desorption of hydrogen into and out of the metal hydride 5 .

[0063] A passive hydrogen diffusion tube 13 extends axially (parallel to the longitudinal axis XX) throughout the vessel and is connected to an on-off valve 14 outside the tank 10 to prevent / allow the circulation of hydrogen from or to the tank 10. Advantageously, the valve 14 can be controlled manually and / or automatically by a central unit (not shown) of the storage system.

[0064] Preferably, the hollow cylindrical container 11 has a removable seal with the last disk 2a of the stack. Between the cover 12 there is provided an axial expansion space 16 for stacking pellets.

[0065] In use (see FIG. 4), the ENG ring 4 of the pellet 1 is placed between the wall 11b of the tank 10 and the compressed metal hydride 5. In this way, the ENG ring 4 reduces the mechanical stresses exerted on the tank wall 11b by absorbing them and improves the thermal conductivity to evacuate heat.

[0066] The disk 2 not only allows for heat conduction towards the tank wall, but also, due to its weight, guides part of the stress resulting from the expansion of the metal hydride 5 during volumetric compression / decompression cycles caused by hydrogen filling / discharging radially towards the ENG ring 4, the surrounding ENG ring i.e. absorbing most of the increase in volume of the metal hydride 5 without transmitting the stress to the tank wall.

[0067] The remaining increase in the volume of the hydride wafer results in a slight increase in the height of said wafer. In parallel, during the absorption of the radial stresses, the ENG ring 4 is compressed against the wall and increases its own height H4 as well as the height of the hydride wafer. All this is possible because the rigid disk 2 is not rigidly connected to the ENG ring 4 or to the metal hydride wafer 5.

[0068] The assembly of pellets 1, discs 2 and expansion space 16 allows the stack to "breathe", which generates very little radial stress on the tank and no axial mechanical compression, since the space 16 allows the pellets to expand axially. The latter only results in an increase in hydrogen pressure that is adapted to the operating pressure and that the tank can easily withstand without mechanical risk.

[0069] Surprisingly, this lateral guidance of the mechanical stress by the rigid plate 2 to the ENG ring 4 is also accompanied by a very substantial improvement in the hydrogen absorption / desorption times compared to pellets not provided with a surrounding ENG ring.

[0070] This time saving is particularly enhanced by the ENG ring formed by axially stacked sheets 4a, as shown in figure 3. This stacking of sheets provides anisotropic heat conduction properties: the conductivity is significantly greater in the direction perpendicular to the axis XX.

[0071] In a tank dimensioned according to the example below, it is thus possible to store 150 g of gaseous hydrogen in less than 10 minutes.

[0072] In an embodiment with dimensions provided only as a non-limiting example, the wall 11a is made of an aluminum alloy and has a thickness E1a of about 5 mm in the part intended to be in contact with the pellets 1 and a thickness E1b of about 20 mm in the part intended to fasten the cover 12. The cover 12, also made of an aluminum alloy, has a thickness E12 of about 12 mm.

[0073] The container 11 has an inner diameter D11 that is substantially equal to the diameter D4 of the ENG ring 4 of the pellet 1.

[0074] By "substantially equal" it is meant that the diameter is equal to D4 within the manufacturing clearances required to insert the pellet into the tank.

[0075] The disk 2 has a diameter D2 slightly smaller than the diameter D4 of the ENG ring 4 to allow its expansion during heat transfer and to contact the tank wall 11b without applying significant stress to the tank wall 11b.

[0076] For example, the inner diameter D11 is 112.1 mm, the ENG ring diameter D4 is 112 mm, and the disk diameter D2 is 111.8 mm.

[0077] The container 11 has a height H11 which is greater than the height of the stack of pellets 1 in order to leave a free axial expansion space 16 between the last pellet 1a of the stack of pellets and the removable reversibly sealing cover 12. For example, the height H11 is about 320 mm, making it possible to store 17 pellets 1 with a total height of 17 mm, leaving an expansion space 16 of height 31 mm.

[0078] A tank according to the invention can easily be made longer or shorter depending on the chosen storage capacity and therefore the number of pellets to be placed (and height at an equivalent diameter).

[0079] The structure of the stack according to the invention of pellets 1 and discs 2 also makes the tank particularly safe.

[0080] Indeed, as the tanks and pellets age, after many hydrogen fill / discharge cycles the metal hydride wafers 5 tend to disintegrate, ie they tend to pulverize again.

[0081] A peripheral ENG ring holds the metal hydride powder between the disks. This peripheral ENG ring 4 ensures that only a very small amount of powder can fall by gravity to the wall 11a of the first end of the tank. Conversely, in known tanks without a peripheral ENG ring, a large amount of powder falls by gravity to the bottom of the tank, creating a risk of explosion.

[0082] 6 shows a further safe embodiment made possible by the construction of the pellet according to the invention, in which the tank is used in a horizontal position, i.e. the longitudinal axis XX of the tank is substantially horizontal.

[0083] As in the previous embodiment, the pellets 1 are arranged alternately with the disks 2 around the tube 13. In this embodiment, the tank further comprises a compression spring 17 between the last rigid disk 2a of the stack and the removable reversibly sealing cover 12 in the expansion space 16.

[0084] This spring 17 holds the stack of alternating pellets 1 and discs 2 against the wall of the first end 11a of the tank whilst allowing axial expansion during hydrogen fill / discharge cycles.

[0085] This embodiment is particularly safe: in fact, the peripheral ENG ring holds this metal hydride powder between the disks, nevertheless, when the powder passes between the ENG ring and the disks, said powder falls by gravity against the bottom wall 11d of the tank in the use position.

[0086] This wall 11d is much larger than wall 11a so that powder cannot accumulate and the risk of explosion is even lower than in the vertical position.

[0087] This device according to the invention is simple yet particularly efficient in terms of hydrogen filling time and safety, being able to absorb the mechanical stresses resulting from the expansion of the metal hydride during hydrogen filling. This efficient loading time is surprisingly related to the specific design of the pellet according to the invention, which allows differential absorption of mechanical stresses inside the pellet 1, limiting the axial expansion by the rigid plate 2 and favoring the lateral (or radial) expansion absorbed by the surrounding ENG ring 4.

[0088] The present invention therefore enables the design and manufacture of hydrogen tanks that are compact, lightweight (as most of their walls are thin), modular, safe (i.e. there is no risk of the walls bursting under mechanical stress and there is no risk of explosion) and offer improved energy efficiency (i.e. having a higher filling rate).

Claims

1. A pellet (1) for storing hydrogen in solid form, intended to be incorporated into a hydrogen storage tank (10), comprising: A pellet (1) characterized in that it comprises a peripheral ring (4) of a given outer diameter (D4) made of expanded natural graphite (ENG) of a given height (H4) surrounding a wafer of metal hydride (5) in the form of a compressed powder.

2. 2. The pellet of claim 1, wherein the peripheral ENG ring is formed by an axial stack of annular ENG sheets having a height (H4a) less than the height (H4) of the peripheral ring.

3. 3. A pellet according to claim 2, wherein the annular ENG sheet has a height of the order of a few tenths of a millimeter, preferably between one tenth and five tenths of a millimeter.

4. A tank (10) for storing hydrogen in solid form, comprising: a hollow cylindrical container (11) extending along a longitudinal axis (X-X), closed at a first end (11a) and open at a second end (11c), defined by a thermally conductive outer radial wall (11b), and comprising a stack (1a) of alternating rigid discs (2, 2a) made of a thermally conductive material and of a given diameter (D2) and pellets (1) according to any one of claims 1 to 3, a hollow cylindrical container (11) in which each pellet is sandwiched between two rigid disks (2, 2a), each rigid disk (2, 2a) being pierced by a hole (5a) facing the hole (3) in said disk, so as to form an axial passage; a removable cover (12) for reversibly sealing said second end (11c) of said hollow cylindrical vessel (11), said removable cover (12) comprising hydrogen inlet / outlet orifices; A tank equipped with:

5. 5. The tank (10) of claim 4, further comprising a passive hydrogen diffusion tube (13) extending axially along the hollow cylindrical vessel through the holes (3, 5 a) in the rigid disk (2) and the hydride wafer (5) of the pellet (1) and in sealed fluid communication with the orifice in the removable reversibly sealing cover.

6. 6. The tank (10) of claim 5, wherein the passive hydrogen diffusion tube (13) is made of a material that is permeable to hydrogen.

7. 5. The tank (10) of claim 4, wherein the hydrogen inlet / outlet orifice is in fluid communication with an on-off valve (14).

8. 5. The tank (10) according to claim 4, wherein the hollow cylindrical container (11) comprises a free axial expansion space (16) for the stack of pellets (1, 1a) between the last rigid disc (2a) of the stack and the removable reversibly sealing cover (12).

9. 9. The tank (10) of claim 8, shaped to be used in an extended position in which the longitudinal axis (X-X) of the container is horizontal with respect to gravity, the tank (10) further comprising a compression spring between the last rigid disc (2a) of the stack and the removable reversibly sealing cover (12).

Images