Vessel for containing hydrogen and / or helium

Abstract

A vessel for containing hydrogen and / or helium, comprising an outer skin and a hollow internal space, the hollow internal space being suitable for receiving hydrogen and / or helium, the skin comprising a plurality of skin panels each comprising a layer of metal foil (201,202), sandwiched with a first layer (203) of unidirectional fibres embedded in a cured resin matrix and a second layer (204) of unidirectional fibres embedded in a cured resin matrix, wherein the second layer of unidirectional fibres are oriented at an angle to the first layer of unidirectional fibres.

Description

[0001] Vessel for Containing Hydrogen and / or Helium

[0002] Technical Field

[0003] The invention relates to a vessel for containing hydrogen and / or helium, comprising an outer skin and a hollow internal space, suitable for use as a hydrogen and / or helium storage tank or as an aerostat, and to its method of manufacture.

[0004] Background to the invention

[0005] Low latency access to mobile communications and general information services is becoming vital for economic and social well-being. The Covid Pandemic has accelerated this process. New applications (for example, autonomous driving, remote medical support) and more generally the Metaverse, require reliable, low latency and high bandwidth mobile communications.

[0006] Compared with satellites, aerostat-supported platforms have several advantages, primarily because the distance from a transmitter to a receiver on Earth can be much less, with geostationary satellites typically at 36,000 km altitude and around 1000 km altitude for a “Low Earth Orbit” or LEO satellite.

[0007] This relative nearness of tethered aerostat platforms can result in much stronger signals relayed to Earth and avoid the expense of rocket launches as well as providing shorter development times, and allows power and backhaul connection via the tether.

[0008] Recently-developed, lightweight, very large-capacity phased array antennas have the potential to transform global mobile and fixed line connectivity e.g. by delivering cellular telephone services, including linking to the internet. This is dependent upon them being positioned appropriately, and at typically between 200m and 2500m over most geographies. Suitable tethered aerostats are therefore needed to support such antennas, that are reliable in all weathers at moderate elevations, with power and fibre optic cables as well as lighting and lightning protection being part of the tether. Such aerostats need to be situated below commercial aviation traffic, but sufficiently elevated to provide links of up to 80 km range for low population densities, up to 30 km for moderate rural densities and 5 km in urban areas.

[0009] Similarly, there are a range of earth observation, meteorological data collection, and astronomical data collection systems which could substantially benefit from being supported by a suitable tethered aerostat system that is reliable in all weathers at elevations at typically between 200m and 22000m.

[0010] A tethered aerostat has the potential to be far cheaper than satellite systems with similar or improved functionality: power supply and fibre optic cable links can be supported by or be an integral part of the aerostat tether, avoiding the expensive and limited backhaul systems and power systems required for satellites or aircraft. Furthermore data latency effects are important to many applications (including but not exclusively, augmented reality, autonomous driving, health care, interactive video games, video conferencing, remote control of UAVs etc.); there are significant problems with latencies offered by satellites, even low earth orbit satellites.

[0011] The need for improved mobile connectivity has prompted a resurgence of interest in using alternative delivery technologies rather than ever larger numbers of mobile communication masts e.g. low earth orbit satellites, stratospheric platforms and tethered aerostats. All of these solutions have issues; respectively: data capacity and latency, technology readiness level, and wind stability.

[0012] Current tethered aerostat systems include tethered rigid and non-rigid airships (blimps) and hybrid balloon / kite systems. Airship-type designs are usually inclined to the horizontal to provide both aerodynamic lift as well as buoyancy. The principle of a hybrid balloon / kite system is that the balloon provides lift in low wind speed conditions and the kite provides lift in high wind speed conditions. Hybrid balloon / kite systems are usually less expensive than airships. However, for such balloon / kite systems in high wind speed conditions the horizontal drag on the balloon is high and the kite therefore needs to be large, incurring considerable drag forces to provide sufficient lift to maintain altitude. These large drag forces then require a very strong and hence, heavy, tether, which has its own associated drag that requires a still larger balloon / kite system to support it, thus reducing payload carrying capacity.

[0013] Current tethered airship designs and balloon / kite systems do not survive strong winds of more than 50 knots for smaller systems and for very large systems winds of more than 70 knots or exceptionally 100 knots. For continuous operation at useful elevations of typically over 400m, high wind speeds of over 100 knots will have to be sustained. With high winds, existing systems become unstable, moving uncontrollably and are ultimately blown over.

[0014] Because of these effects it has not been possible hitherto to design and build a reliable tethered aerostat system to deliver a high availability service because very high wind conditions will be encountered almost everywhere from time to time, particularly at suitable altitudes needed for many different applications. That, in turn, has meant that any potential use of a tethered kite / balloon aerostat carrying a system that requires high availability and reliability has not been possible.

[0015] Lighter than air balloon / kite systems are described in US 2011 / 0222077A1 with the following examples: US 2,398,745 and US 2,431,938 to Jalbert, US 4,029,273 to Cristofel Jr., in US 6,016,998 to R. Allsopp, and in U.S.Pat.No.6,499,695 to Talamo. In US 6,555,932 to Mizzi, there is described a combined buoyant aerofoil for use in generating electricity with wind power or for aerial advertising. Such combination balloon / kite systems are also available commercially for surveillance and advertising use, such as the SkyDoc TM Aerostat, supplied by Floatograph Technologies LLC, of SilverSpring Md., or the Helikite, supplied by Allsopp Helikites Ltd of Fordingbridge, Hampshire, UK. Such systems have been used for military and civilian use, and the data generated has been described as being conveyed to and from the ground station by means of wireless, cable or optical fibre. US 2018 / 0050797 Al discloses a conceptual tethered lighter-than-air unmanned aerial vehicle that has a wing and a fuselage. However, no consideration is given to how to maintain such an arrangement so that it is stable in the atmosphere. KR 10-2016-0081328 discloses a buoyant aerostat that is tethered to ground level. However, no consideration for how such an apparatus can be kept stable in the atmosphere is given. US 2016 / 0122014 Al discloses a conventional blimp attached to the ground by two tethers. US 3,620,292 discloses a known type of wing balloon.

[0016] An aerostat system highly stable in extreme weathers for a wide variety of locations with the ability to withstand meteorological challenges including: extreme gusts, lightning strikes; prevention of snow and ice build-up; and safety features to prevent accidents with low flying aircraft, is therefore likely to be valuable in a whole variety of applications including but not limited to those described above.

[0017] However, such aerostats have to be large to provide the buoyancy required to lift suitable payloads. Furthermore, significant buoyancy is required to support the tether which has to contain lighting conductors required for all-weather operation at suitable altitudes (above 200m preferably 1000m and most preferably 1500m). Typical required aerostat volumes are at least 300 m3, more preferably above 1000 m3. An example of a vertical buoyant aerofoil is given in international patent application no. PCT / GB2022 / 053290.

[0018] Conventionally large aerostats are transported when uninflated and folded. However, the foldable materials currently in use suffer from significant helium permeability if they are to be suitably lightweight and strong and such aerostats typically need to be topped up with helium every two to six weeks by winching them down to the ground or some other means, which is highly undesirable since the launch and recovery of large aerostats is a significant undertaking requiring suitable weather conditions (moderate wind gusts) and careful management. The time taken will also lead to significant interruptions to communications service. Aerostat skins are therefore required where the strength to weight ratio is as high as possible and where the skin prevents significant helium or hydrogen permeation.

[0019] Such skin properties are also required for liquid hydrogen and liquid helium cryogenic tanks for use in aircraft or drones where light weight, high strength and low permeability are also required but with the added requirement of being able to withstand being cooled, preferably repeatedly, to the relevant cryogenic temperatures: typically, between a few Kelvins, and a few tens of Kelvins.

[0020] It is well known that helium or hydrogen diffusion rates through metals are far lower generally than through plastics and many attempts have been made to combine fibre reinforced plastic components with thin skins or foils of metals, particularly aluminium, gold and so forth. Typically, fibre reinforced skins are made from woven resin ‘prepregs’ which are composite materials made from "pre-impregnated" fibres and a partially cured polymer matrix, such as epoxy or phenolic resins. These woven ‘prepregs’ have been placed adjacent to thin films of aluminium foil and the system cured, often in an autoclave. Such systems have been highly disappointing - small defects - even as small as a few microns in diameter - in the aluminium foil or joints in the aluminium foil allow the passage of helium or hydrogen and the resin and woven prepreg is not a sufficient barrier to allow a significantly lower diffusion path to helium or hydrogen as compared to existing aluminised plastic systems.

[0021] Fibre composite materials with suitable metal barrier technology and jointing technologies as aerostat skins would allow aerostats to be operated for far longer without helium top-up. Furthermore, with an external metal skin to block UV weathering, such aerostat skins have the potential to last for many years if they could be realised. In this context if a new means of transport of large aerostats with unfoldable skins can be developed, it would be of great economic benefit.

[0022] For many applications, particularly in aerospace, there is a need for a very strong lightweight skin technology that is highly impermeable to helium or hydrogen that can withstand temperature cycling, most preferably cryogenic temperature cycling and extreme weathering for liquid hydrogen or helium tanks. Such applications have great economic value.

[0023] Summary of the Invention

[0024] In a first aspect, the invention relates to a vessel for containing hydrogen and / or helium, comprising an outer skin and a hollow internal space, the hollow internal space being suitable for receiving hydrogen and / or helium, the skin comprising a plurality of skin panels each comprising a layer of metal foil, sandwiched with a first layer of unidirectional fibres embedded in a cured resin matrix and a second layer of unidirectional fibres embedded in a cured resin matrix, wherein the second layer of unidirectional fibres are oriented at an angle to the first layer of unidirectional fibres.

[0025] Such an arrangement has been found to provide for a rigid outer skin that is highly impermeable to hydrogen and / or helium, whilst also being of relatively low weight. The use of unidirectional fibres allows a greater fibre-to-resin content, which gives a greater str ength-to- weight ratio.

[0026] Preferably the metal foil is an outermost layer of the skin panel.

[0027] Preferably the skin comprises an outermost layer of metal foil and an innermost layer of metal foil, sandwiching between them the two or more layers of unidirectional fibres embedded in a cured resin matrix. Additional metal foil layers may be included, which could provide additional gas impermeability, at the cost of increasing the weight of the skin. Having three metal foil layers can be advantageous.

[0028] Optionally, additional layers may be present, such as additional layers of fibres embedded in a cured resin matrix, or other layers such as coatings or paints or other surface treatments. To manufacture the vessel from the skin panels, a jointing system is needed to suitably assemble flat or curved sheets as described above. It has been discovered that it is possible to make joints that can provide minimal hydrogen and / or helium gas leakage and be extremely strong without significant weight increase of the skin, by having butt joints with skin elements on one or both sides of the butt joint. Thus, preferably at least one pair of adjacent skin panels butt together to form a contiguous region of skin with a joining line therebetween, wherein at least one further skin panel is placed to cover both the innermost and / or outermost facing of the joining line.

[0029] Such an arrangement provides an effective seal against hydrogen and / or helium leakage, as the join is covered by one or two further skin panels and provides suitable strength. The main principle of such joint design is that gas diffusion or flow through the join encounters one or more metal foil surfaces, or must travel longitudinally through the further pair of skin panels (as opposed to through the thickness thereof), which provides a much longer route and therefore much greater sealing.

[0030] Preferably the adjacent skin panels have a joining region, adjacent to the joining line, wherein there is no layer of metal foil. This is so that, in the joining region, a continuous resin seal may be made by the placement of the further skin panels.

[0031] Preferably, the further pair of skin panels do not have an innermost layer of metal foil. This is for similar reasons, to allow a continuous resin seal in the joining region.

[0032] It has been found that each joining region extends for a length from the joining line that is from 5 to 50 times the thickness of the skin panel outside the joining region, provides an effective helium seal.

[0033] Alternatively, joints may be formed as overlap joints, wherein at least one pair of adjacent skin panels overlap each other to form a contiguous region of skin with an overlap region, wherein at least one further skin panel is placed to cover the overlap region. In such overlap joints preferably one or both of the adjacent skin panels has some metal foil removed in the overlap region. It is also preferred that at least one further skin panel does not have an innermost layer of metal foil.

[0034] The present invention provides a very effective helium sealing with minimal weight and thickness of skin. Therefore preferably each skin panel has a thickness of from 0.5 to 5mm, preferably from 0.5 to 2mm.

[0035] In addition, it is preferable, but not always essential that load transfer should take place without significant shear force on foil surfaces, and in that context the foil surface should not extend to cover the whole interface between the fibre reinforced resin elements of the skin.

[0036] Preferably the metal foils are any metal that is malleable and stable in air, such as aluminium or gold, but preferably aluminium with its low density and cost. The metal foils are typically 0.5 to 200 microns thickness, preferably 5 to 40 microns thickness, most preferably 8 to 25 microns thickness.

[0037] Such foils are manufactured by rolling and often have small holes offering flow paths through their thickness. It has been found that plugging such holes by resin is desirable but not essential, and this process happens naturally if sufficient curing pressure and resin is used. Too much resin is undesirable since to achieve a high strength to weight ratio, a low volume fraction of resin compared with fibre is to be preferred, consistent with having minimal gas voidage in the resin / fibre system.

[0038] Therefore, preferably the volume of cured resin matrix is from 15 to 100%, preferably from 30 to 70%, more preferably from 45 to 70%, of the volume of unidirectional fibres, to provide good strength to weight ratios and good barrier properties.

[0039] The resin may be any suitable thermally curable thermoplastic resins suitable for use in a fibre composite material, such as an epoxy resin. Preferably, the angle between the first layer of unidirectional fibres and the second layer of unidirectional fibres is at least 10°, preferably at least 25°, more preferably from 45 to 90°, which allows the strength of the skin to be tailored to the magnitude of the maximum stresses requirements in different directions.

[0040] Preferably, the unidirectional fibres are carbon fibre or Kevlar™. Prepreg carbon fibre or Kevlar weights of 30 grams per square meter to 500 grams per square meter can be used.

[0041] Gas voidage, e, is defined as the fraction of the total volume of resin / fibre system which is free space available for the flow of fluids, in this case helium or hydrogen. Preferably the cured layers of unidirectional fibres embedded in a cured resin matrix have a gas voidage of less than 5% by volume, preferably less than 1%, most preferably less than 0.1%.

[0042] The vessel may be an entire aerostat, or simply a vessel for storage of hydrogen and / or helium, e.g. for use in an aerostat or aircraft. However, the present invention is particularly applicable when the vessel is an aerostat, the hollow internal space being suitable for receiving sufficient lighter-than-air gas, such that the aerostat can become buoyant in air. Preferably the aerostat has an elongate hollow body having a length extending between an upper end and a lower end in use or can be an aerostat shape with an axi-symmetrical shape of low drag- the long axis being approximately parallel to the incident wind.

[0043] It has furthermore been found to be particularly advantageous when the aerostat is shaped as an aerofoil. Therefore preferably the elongate body, when the upper end is substantially vertically positioned above the lower end in use, has a horizontal cross-section at each point throughout substantially the entire length that is an aerofoil, providing a leading edge and a trailing edge extending between the upper end and lower end, and defining between them, for each horizontal cross-section, a chord line between the leading edge and the trailing edge of the cross-section, having a chord length. Such an arrangement has the appearance of a “vertical wing”, as discussed in more detail below, and can be described as a vertical buoyant aerofoil or aerostat.

[0044] Optionally, the elongate or tall body may take a swept-back or swept-forwards arrangement to provide appropriate mass distribution and stability. Thus, preferably the aerostat comprises regions that are oriented at an angle to the vertical. A swept-back or swept- forwards arrangement can be provided for when the leading edge and trailing edge, in the regions that are oriented at an angle to the vertical, are located within the single vertical plane of symmetry, parallel to the wind direction in use.

[0045] A low drag aerostat is highly preferred for such an aerostat to operate in high winds without an excessively strong and therefore heavy tether. If the aerostat has a higher drag then the aerostat needs to be larger to carry the weight of the heavy tether and payload in high winds. A larger aerostat is more costly and less economic. It has been discovered that for practical utility in being able to carry substantive payloads in high winds of more than 30 metres per second, a drag coefficient of less than 0.35, preferably less than 0.2, more preferably less than 0.06 is required.

[0046] In this application, the drag coefficient of a tethered aerostat is defined as CD = F / (’A u2A) , where F is the horizontal aerodynamic drag, p is the air density , u is the horizontal component of wind velocity and - as is conventional in aerofoil theory - well known to those skilled in the art, A is the plan area when seen looking at the aerostat horizontally perpendicular to the wind. The leading edge at any vertical level, is defined by the point which first meets oncoming air, and the trailing edge at any level, the point which last meets oncoming air.

[0047] In a second aspect, the invention relates to a method of manufacturing the skin, or component thereof, of a vessel according to any one of the preceding claims, the method comprising forming the skin panels by layering a layer of metal foil, a first layer of unidirectional fibres embedded in a uncured resin matrix and a second layer of unidirectional fibres embedded in a uncured resin matrix, wherein the second layer of unidirectional fibres are oriented at an angle to the first layer of unidirectional fibres, are brought together into contact with each other, followed by curing the resin by exposure to elevated temperature and optionally elevated pressure, to produce the skin, or component thereof.

[0048] The skin panels are made by laying-up the metal foils and prepreg comprising uncured resin coated unidirectional fibre layers on a solid former with a release film if necessary and vacuum bag surround before being cured at the appropriate resin curing temperature and time as is well known to those skilled in the art of fabrication of composite components. The former can be flat or curved to form elements of the aerostat surface or tank.

[0049] Each skin panel may be manufactured and cured separately, or a plurality of skin panels may be cured together, and optionally the entire set of skin panels in the vessel may be cured together. However, preferably each skin panel is cured separately. This makes it easier for each skin panel to be cured in an autoclave, which can then reach very high pressures, in order to reduce the voidage to acceptable levels.

[0050] For example, pairs of skin panels with their further skin panel or panels at the join may be cured together. Alternatively the skin panels may be separately cured, followed by butting them together, placing the further skin panels in place over the join, and curing the further skin panels in situ.

[0051] In the “laying-up” or placing process it is important that the new layer is laid down without wrinkles or inclusions or air as is well known to those skilled in the art. If a surface is curved in only one direction, so the minimal principal curvature is zero (see Gauss, Theorema Egregium), and the maximum principal curvature is non-zero, thin surfaces can be laid down easily. However, if the surface has two non-zero principal curvatures, the surface will wrinkle unless small elements are used with modest size compared to the minimum principal curvature. It has been discovered that by utilising unidirectional fibre ‘prepregs’ with one or more aluminium foil sheets laid adjacent to the unidirectional fibre prepreg system, curing under pressure typically greater than 1 Bar, preferably greater than 5 Bar with appropriate vacuum suction arrangements known to those skilled in the art, that the voidage, e, in the cured fibre reinforced resin is minimal, typically less than 5%, preferably less than 1%, most preferably less than 0.1%.

[0052] The invention will now be illustrated, by way of example with reference to the following figures.

[0053] Figure la is a section of, and lb a perspective of a known cured conventional woven prepreg sandwiched between two aluminium foils, and illustrates the difficulties of current approaches. Figure 1c shows the form of the weave.

[0054] Figure 2a shows a side sectional view through a skin panel according to the present invention, with figures 2b and 2c showing plan views of the layers of unidirectional fibres in 90° orientation.

[0055] Figure 3 a shows side sectional view through a further skin panel according to the present invention, with figures 3 b and 3 c showing plan views of the layers of unidirectional fibres in 45° orientation.

[0056] Figure 4 shows a side sectional view through two adjacent skin panels as shown in figure 2, with a further pair of skin panels covering the joining line.

[0057] Figure 5 shows detailed views of the adjacent skin panels and the further pair of skin panels shown in figure 4.

[0058] Figure 6 is an image of a vessel according to the present invention, in the form of a tank. Turning to the figures, figure la is a section of, and figure lb a perspective of, a known cured conventional woven prepreg fibre sheet sandwiched between two aluminium foils. Figure 1c shows the “plain weave” in perspective. It can be seen that figures la and lb are also of plain weave type. The section shown in figure la is in the plane shown in figure lb, defined by the corners 112, 113, 114, and 115.

[0059] The two aluminium foils 101 and 102 are shown at the top and bottom of the section in figure la, and in figure lb part of the top foil 101 has been removed to expose the woven structure of the carbon fibre bundles 105 and 106 with carbon fibre yarn 104 which is made up of grouped smaller carbon fibres (not shown). The carbon fibre bundles have cured resin 108 that holds the bundles in position and attaches them to the aluminium foils.

[0060] However, unless a large excess resin quantity is used, with preferably at least twice the volume of resin as fibre, preferably three or four times the volume of resin as fibre, the separation of the aluminium foils incurred by the weave pattern of the bundles means that voids 107 are formed where resin does not fully occupy the space between the foil sheets 101 and 102. It might be thought that at normal autoclave pressures of c. 8 Bar these voids would be eliminated, but given the small distances involved between adjacent fibre bundles the aluminium foils at thicknesses of only a few microns are capable of bridging the relevant distances. It has been discovered that at higher autoclave pressures for practical thicknesses of foil, too much strain of the aluminium sheets is required to avoid voids without incurring fracture of the foils and losing their barrier properties. Indeed at 8 bar autoclave pressures, dimples 103 are seen protruding from the aluminium foil surfaces where the fibre bundles are impinging on the foil layers.

[0061] Aluminium foils have defects in them - typically micron sized - often up to 25- or 40- micron holes penetrating from one side of the foil to the other and providing a flow path if not blocked. Such unblocked holes 109, 110 are shown in the bottom and top foils respectively. Ill shows a hole that has been blocked by resin 108 being squeezed through it in the curing process. These unblocked holes 109,110 breach if then linked to internal voids such as 107, which are in turn linked provide flow paths for gases to transfer from one side of the skin to the other and to make such a system highly permeable and unsatisfactory. If very large amounts of resin are used some mitigation can be provided but only at expense of the strength to weight ratio of the skin. Different weave patterns can provide some mitigation. Stranded weaves - figure Id - offer some benefit in reducing the amount of resin required to achieve continuous resin between the surfaces of the skin.

[0062] Figure 2 shows a side sectional view through a cured skin panel 200 according to the present invention. Such a skin panel typically has a millimeter or sub-millimeter thickness. The width will normally be far greater than that shown. The skin consists of two layers of cured unidirectional carbon fibres embedded within an epoxy resin, with the fibres oriented at 90 degrees angle of orientation, sandwiched between two layers of aluminium foil, 201 and 202. Figure 2b is a section through such a layer 203, and figure 2c is a section through the other layer 204. The aluminium layers have defects within them - holes penetrating from one side of the layer to the other. If sufficient resin is available in the prepreg, these holes are plugged with resin as shown 205.

[0063] Figure 3 shows a side sectional view through a further cured skin panel 300 according to the present invention, similar to that shown in figure 2 but with a different angle between the layers of unidirectional fibre. The skin 300 consists of a double layer of cured unidirectional carbon fibres embedded within an epoxy resin, with the fibres oriented at 45 degrees angle of orientation, and sandwiched between two layers of aluminium foil as in figure 2. Figure 3b is a section through such a layer 303, and figure 2c is a section through the other layer 304. As in the previous example the aluminium layers have defects within them - holes penetrating from one side of the layer to the other. If sufficient resin is available in the prepreg, these holes are plugged with resin as shown 305.

[0064] Figure 4 shows a pair of adjacent skin panels forming a “butt joint” using the skin panels shown in figure 2. Figure 4a shows the complete joint, and figure 4b shows the same joint but in more detail. The two adjacent skin panels are butted together to form a contiguous region of skin with a joining line therebetween. A further pair of skin panels are placed to cover both the innermost and outermost facing of the joining line. Such a join provides good tension strength, which is required for aerostat and tank envelopes. The detail of this is described in figure 5.

[0065] Figure 5 shows the same butt joint shown in figure 4, but in greater detail. Figure 5a shows two skin panels 501 and 502 joined at the joint 503, providing a joining line 512.

[0066] Figure 5b shows a little more than half of the joint 503. Figure 5c shows the left-hand transition from a single sheet of cured skin 501, to a multi-layered skin system in which an upper further skin panel 504 consisting of an aluminium sheet and two layers of unidirectional fibre at 90 degrees fibre orientation to each other is situated above the skin layers 501 and 502 and a similar further skin panel 505 is placed below the skin layers 501 and 502. Thus, the further pair of skin panels 504, 505 do not have an innermost layer of metal foil.

[0067] Figure 5d shows a further transition zone to the joining region. When fabricating the skin layer 501, the aluminium foils are cut to leave a length 513 without an aluminium layer on both sides of the skin layer 501 at the edge of the skin that is to be joined. This results in the skin layer 501 being adjacent to fibre reinforced plastic for the length 513, rather than having an aluminium layer 506 or 508 interspersed between the layers of carbon fibre embedded in resin for the whole joint.

[0068] Figure 5e shows the central section of the joint where the two skins 501 and 502 are closest together showing that the joining line 512 is a small gap filled with resin. This gap helps to accommodate manufacturing tolerances in cutting the skin width.

[0069] The purpose of the preferred embodiment of the joint arrangement shown is to create a join very strong in tension, lightweight almost helium- impermeable joint. There is a large area relative to the skin cross sectional area, where the top layer 504 and the bottom layer 505 are in contact with the skins 501 and 502 and shear stresses are relatively low compared to the tensile stresses in the skin. The manufacture may be by simple gluing of cured skins to cured overlap layers or, in a preferred method of manufacture, the further skin panels in an uncured or cured embodiment are placed above and below cured skins and the further skin panels are cured in situ by heating tapes and a vacuum system that ensures a 1 bar compressive force normal pressure to the layers providing a strong stable almost impermeable joint.

[0070] Such a system may tolerate aluminium to CFRP interfaces rather than CFRP to CFRP interfaces - so the length 513 essentially becomes zero. Such a system will have better permeation performance but a lower strength.

[0071] Other joint forms are possible including overlap joints and additional resin sheets can be added to improve permeation resistance and strength.

[0072] Figure 6 shows a tank 600 according to the present invention, of approximately Im in length 601, and 0.25m in diameter 602.

[0073] Example

[0074] Using these principles, two vessels of a cylindrical skin with three layers of aluminium foil and two unidirectional carbon fibre reinforced plastic (CFRP) have been manufactured. From the outside the skins were:

[0075] (i) LuxLiv® Premium Heavy Duty 20 Micron Thick Kitchen Foil Catering Aluminium.

[0076] (ii) 130 grams per square meter unidirectional CFRP ‘prepreg’ supplied by Delta-preg Spa with T800s Carbon Fibre, DT120 resin system, 36% resin weight, and EF tack type.

[0077] (iii) Bacofoil™ Catering Aluminium foil 12 micron.

[0078] (iv) as (ii) but aligned at 90 degrees to (ii).

[0079] (v) As (iii).

[0080] The vessels were 0.25m in diameter with hemispherical ends and approximately Im in length. The vessel is shown in figure 6. When pressurised with helium at 0.4 Barg the pressure loss after 4 months was less than 0.01 Bar. This pressure loss is less than 1 / 100 th of that to be expected of commercial aerostat fabrics. The skin on a weight basis is estimated on the basis approximately ten times stronger than a conventional aerostat skin

Claims

Claims1. A vessel for containing hydrogen and / or helium, comprising an outer skin and a hollow internal space, the hollow internal space being suitable for receiving hydrogen and / or helium, the skin comprising a plurality of skin panels each comprising a layer of metal foil, sandwiched with a first layer of unidirectional fibres embedded in a cured resin matrix and a second layer of unidirectional fibres embedded in a cured resin matrix, wherein the second layer of unidirectional fibres are oriented at an angle to the first layer of unidirectional fibres.

2. A vessel according to claiml, wherein the metal foil is an outermost layer of the skin panel.

3. A vessel according to claim 2, wherein the skin panels comprise an outermost layer of metal foil and an innermost layer of metal foil, the metal foils sandwiching between them the cured resin and fibre layers.

4. A vessel according to any one of the preceding claims, wherein an additional metal foil is sandwiched between the first layer of unidirectional fibres embedded in a cured resin matrix and the second layer of unidirectional fibres embedded in a cured resin matrix.

5. A vessel according to any one of the preceding claims, wherein at least one pair of adjacent skin panels butt together to form a contiguous region of skin with a joining line therebetween, wherein at least one further skin panels is placed to cover the innermost and / or the outermost facing of the joining line.

6. A vessel according to claim 5, wherein the adjacent skin panels have a joining region, adjacent to the joining line, wherein there is no layer of metal foil.

7. A vessel according to claim 5 or claim 6, wherein the at least one further skin panels do not have an innermost layer of metal foil.

8. A vessel according to claim 6 or claim 7, wherein each joining region extends for a length from the joining line that is from 5 to 50 times the thickness of the skin panel outside the joining region.

9. A vessel according to any one of the preceding claims, wherein at least one pair of adjacent skin panels overlap each other to form a contiguous region of skin with an overlap region, wherein at least one further skin panel is placed to cover the overlap region.

10. A vessel according to claim 9, wherein one or both of the adjacent skin panels has some metal foil removed in the overlap region.

11. A vessel according to claim 9 or claim 10, wherein the at least one further skin panels does not have an innermost layer of metal foil.

12. A vessel according to any one of the preceding claims, wherein each skin panel has a thickness of from 0.5 to 5mm, preferably from 0.5 to 2mm.

13. A vessel according to any one of the preceding claims, wherein the metal foil is aluminium foil.

14. A vessel according to any one of the preceding claims, wherein the metal foil has a thickness of from 0.5 to 200 microns, preferably from 5 to 40 microns.

15. A vessel according to any one of the preceding claims, wherein the volume of cured resin matrix is from 15 to 100%, preferably from 30 to 70% of the volume of unidirectional fibres.

16. A vessel according to any one of the preceding claims, wherein the angle between the first layer of unidirectional fibres and the second layer of unidirectional fibres is at least 10°, preferably at least 25°, more preferably from 45 to 90°.

17. A vessel according to any one of the preceding claims, wherein the unidirectional fibres are carbon fibre or Kevlar™.

18. A vessel according to any one of the preceding claims, wherein the first layer of unidirectional fibres and the second layer of unidirectional fibres each have a fibre weight of from 30 to 500 grams per square metre.

19. A vessel according to any one of the preceding claims, wherein the cured layers of unidirectional fibres embedded in a cured resin matrix have a gas voidage, e, of less than 5% by volume, preferably less than 1% by volume.

20. A vessel according to any one of the preceding claims, wherein the vessel is an aerostat, the hollow internal space being suitable for receiving sufficient lighter- than-air gas, such that the aerostat can become buoyant in air.

21. A vessel according to claim 20, wherein the body is an elongate hollow body having a length extending between an upper end and a lower end in use.

22. A vessel according to claim 21, wherein the elongate body, when the upper end is substantially vertically positioned above the lower end in use, has a horizontal cross-section at each point throughout substantially the entire length that is an aerofoil, providing a leading edge and a trailing edge extending between the upper end and lower end, and defining between them, for each horizontal cross-section, a chord line, between the leading edge and the trailing edge of the cross-section, having a chord length23. A method of manufacturing the skin, or component thereof, of a vessel according to any one of the preceding claims, the method comprising forming the skin panels by layering a layer of metal foil, a first layer of unidirectional fibres embedded in a uncured resin matrix and a second layer of unidirectional fibres embedded in a uncured resin matrix, wherein the second layer of unidirectional fibres are oriented at an angle to the first layer of unidirectional fibres, are brought together into contact with each other, followed by curing the resin by exposure to elevated temperature and optionally elevated pressure, to produce the skin, or component thereof.

24. A method according to claim 23 wherein skin panels are cured individually, and are subsequently joined together in cured form.

25. A method according to claim 24, wherein the curing takes place in an autoclave.

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