The invention concerns a method for manufacturing thermal insulation units with long life expectancy, particularly vacuum insulation units constituting a part of insulation boards / panels with ultra low thermal conductance properties for use in applications such as cryogenic transport and storage.

Background of the invention:

Vacuum insulation is an advanced thermal insulation technology that significantly outperforms conventional materials. Typically, vacuum insulation is 3 to 7 times better than conventional insulation of polyurethane foam, polystyrene foam, and fiberglass matting. A vacuum insulation panel (VIP) is the most common embodiment of vacuum insulation. These panels are normally flat, but may also have shaped structures that, when holding a vacuum, provides the resistance to thermal conduction. The pressure levels in these VIPs are often in the range less than 25 kPa or 0.25 bar.

In order to sustain the correct insulating value for the whole VIP lifespan, it is of great importance that the panel remains fluid/vaportight, and these absolute requirements have traditionally set a minimum wall thickness of such panels (see e.g. EP 0857 833 Al). On the other hand, in many applications where thermal insulation is required, it is advantageous to have as small insulation thickness as possible, for example to avoid space limitations.

The importance of maintaining low thermal conductance is easily recognizable in the field of liquid gas transport on board of vessels. In this particular application a company may achieve significantly lower costs if the average boil-off rates are reduces. For example, during transport of liquid natural gas (LNG) in conventional storage tanks a boil-off rate (BOR) of 0.15 % BOR/day is not uncommon. Hence, a tank initially storing 150Ό00 m ³ LNG would during a 20 days journey have a loss of about 4'220 m ³ , amounting to an economical loss of about $1.56 million assuming an LNG cost per m ³ of $370. However, if the BOR is reduced to for example 0.06 % the corresponding economical loss during the same journey would be about $630'000, thus reducing the economical loss of over $930'000 per trip. By estimating 160 transport days during one full year the company would thus have the potential of saving around $7.5 million per vessel. Even a reduction of only 0.05%, i.e. from 0.15 % to 0.1 %, would results in a potential saving of around $4.1 million. Improved thermal insulation is therefore of utmost importance for companies storing and transporting liquid gas at cryogenic temperatures. High-frequent welding or ultrasonic welding is an industrial technique whereby high-frequency ultrasonic acoustic vibrations are locally applied to workpieces being held together under pressure to create a solid-state weld. The benefits of ultrasonic welding are that it is much faster than conventional adhesives or solvents, since the pieces do not need to remain in a jig for long periods of time waiting for the joint to dry or cure. The welding can easily be automated, making clean and precise joints; the site of the weld is very clean and rarely requires any touch-up work. Furthermore, the low thermal impact on the materials involved enables a greater number of materials to be welded together. Use of ultrasonic welding of thin films for manufacturing VIPs is known in the field. An example is the Japanese publication JP 20061 18637 which discloses a method for manufacturing a vacuum isolating material that inter alia comprises insertion of a core material inside a pocket formed between three ultrasonically welded sides of two Al-foils, with subsequent evacuation and sealing. However, the manufactured panels are only suited for isolating fluids having temperatures above room temperature. In addition, these VIPs have a rather complex configuration with multiple layers.

Another known method is the use of heat welding for manufacturing VIPs suitable for isolation of refrigerators. An example is given in the European publication EP 1647759 A2 which discloses a manufacturing method for vacuum isolation panels used in refrigerant applications in which three sides are joined by heat welding followed by insertion of a core material into the formed pocket, and finally evacuating and sealing the pocket. The manufacturing disclosed in this publication does not employ ultrasonic welding, and the final VIP has a rather complex configuration with multiple layers. Due to the low thermal impact on the materials compared to for example heat welding, use of ultrasonic welding enables a greater number of materials to be joined together.

It is therefore an object of the present invention to provide a method for manufacturing of thermal insulation units that overcomes the above mentioned disadvantage, i.e. manufacturing simple vacuum insulation units as building blocks for insulation boards having ultralow thermal conductance properties even at cryogenic temperatures.

Summary of the invention:

The above-identified objects are achieved by a method for manufacturing thermal insulation units with long life expectancy in accordance with claim 1 , an insulation board in accordance with claim 10 and a liquefied gas storage tank such as storage of LNG, LPG or LEG in accordance with claim 15. Further beneficial features are defined in the dependent claims. In particular, the invention discloses a method for manufacturing thermal insulation units to be used at cryogenic temperatures and having long life expectancy. The method comprises the following steps:

- arranging at least one first layer of foil onto at least one second layer of foil in a layer-by- layer relationship,

- joining the at least one first layer of foil and the at least one second layer of foil together by performing ultrasonic welding, thereby forming an open envelope with at least one pocket,

- inserting the at least one spacer into at least one of the at least one pocket, the total exterior volume of the at least one spacer being smaller than the inner volume of the at least one pocket,

- evacuating the at least one spacer inserted pocket,

and

- sealing the at least one spacer inserted pocket by ultrasonic welding. The ultrasonic welding is preferably made along the boundaries of the layer-by- layer assembled foils. Note that a boundary is herein defined as the region of the resulting thermal insulation unit extending up to 20 % of the total unit distance (L,W) from the respective outermost edge to the opposite laying outermost edge, preferably up to 10 % of the respective total unit distance (L,W).

At least part of the ultrasonic welding may advantageously be performed along at least two parallel or near parallel paths, thereby creating multiple weld seems. Such multiple weld seems will both increase the lifetime of the vacuum insulation and ensure higher mechanical stability. The resulting pressure inside at least one of the at least one evacuated pocket is preferably less than 25 kPa or 0.25 bar, more preferably less than 10 kPa or 0.1 bar.

Since the thermal conductance of a material is increasing proporsionally with the area of the conductive material the thickness of the foils should be thin to ensure ultralow thermal conductivity through the edge zones where the warm side foil is joined with the cold side foil, preferably less than 1 millimeter, and even more preferably less than 500 micrometers, for example 100 micrometer. This is particularly important for super effective insulation systems where only a minimum of excessive heat transfer can be tolerated. In addition, thin material is easier to bond by ultrasonic welding. With proper choice of spacer composition material the overall thermal conductivity for each of the units can be less than about 1 W/m-K and strong enough to withstand at least 1 bar pressure. More preferably, the overall thermal conductivity for each of the units should be better than conventional insulation such as polyurethane foam, polystyrene foam or fiberglass matting, i.e. an overall thermal conductivity of less than about 0.02 W/m-K. However, the spacer composition material can in principle be any material fulfilling the above mentioned requirements, i.e. having low thermal conductivity while being strong enough to withstand a certain pressure, e.g. around one bar pressure. Examples of spacer materials are one or more of the materials in the group ceramic fibre, cork, cotton, expanded polystyrene, fiberglass, perlite, low- density polymer, polyurethane, rice hulls, aerogels and wood.

The present invention also includes an insulation board for installation on the walls of a fluid storage container storing fluids with an average fluid temperature which differs from the ambient temperature outside the fluid storage container, the board comprising at least one of the units obtainable by the manufacturing method disclosed above. The board may comprise several thermal insulation units mounted in an at least two-dimensional matrix configuration onto the external and/or internal walls of the fluid storage container. In a preferred embodiment the insulation board consists of two or more two-dimensional thermal insulation unit layers, for example three layers, wherein the lateral position of one layer is shifted with respect to its adjacent layer(s). Further, the average fluid temperature may be below the ambient temperature of the tank, for example between 0 K and 200 K.

The above mentioned insulation board may be used in any application where thermal insulation is necessary, for example tanks for storing cryogenic liquids such as liquid helium, liquid hydrogen, liquid nitrogen, liquefied natural gas (LNG), liquefied petroleum gas (LPG), liquefied ethylene gas (LEG), or for various domestic appliances such as refrigerators and freezers. Further, the thermal insulation may be obtained both by direct or indirect contact of the inventive insulation board with the stored fluids. An example of liquified hydrogen storage by use of vacuum insulation boards / panels are temporary energy storage from surplus electrical power production for use in coming periods of temporary energy deficits or as reliable source of hydrogen supply to hydrogen dependent apparatus.

In the following description, numerous specific details are introduced to provide a thorough understanding of, and enabling description for, embodiments of the claimed method and apparatus. One skilled in the relevant art, however, will recognize that these embodiments can be practiced without one or more of the specific details, or with other components, systems, etc. In other instances, well- known structures or operations are not shown, or are not described in detail, to avoid obscuring aspects of the disclosed embodiments.

Brief description of the drawings:

Preferred embodiments of the present invention will now be described with reference to the attached drawings, in which: Figure 1 shows perspective views of two layers according to the invention positioned in a layer-by- layer orientation,

Figure 2 shows a top view of the assembled layers in figure 1 wherein the welding paths and welding directions are indicated by lines and arrows, respectively, Figure 3 shows a perspective view of the partly assembled thermal insulation unit according to the invention, wherein three of four sides are welded,

Figure 4 shows a perspective view of the thermal insulation unit according to the invention having a spacer partly inserted into the pocket of the unit,

Figure 5 shows a perspective view of the thermal insulation unit according to the invention having a spacer almost completely inserted into the pocket of the unit,

Figure 6 shows a perspective view of a fully assembled thermal insulation unit according to the invention, wherein the pocket with its spacer has been evacuated and then sealed by ultrasonic welding, and

Figures 7 A and B show schematic views of an insulation board according to the invention, wherein figure 7 A shows top-view and side view arrangements of a layer with four thermal insulation units in accordance with the invention, while figure 7 B shows arrangements of three of the layers shown in figure 7 A stacked on top of each other with a lateral directed offset.

Detailed description of the drawings:

Figures 1-6 shows in sequence the different method steps for producing thermal insulation units 1 in accordance with the invention, having ultralow thermal conductance properties. The expression ultralow thermal conductance shall hereinafter be interpret as thermal conductivity lower than 1 W/m-K and typically lower than 0.1 W/m-K. In particular, figure 1 shows a first and second layers 2,3, for example ultrathin aluminum foils both having a typical length L and width W of 50 cm, arranged in a layer-by- layer orientation with their respective lateral edges 5 parallel to each other. However, the choice of material can in principle be any material which allows the formation of stable joints by high-frequency welding. Examples are steel, stainless steel, copper, composite materials, carbon fiber, carbon-fiber-reinforced polymers (CFRP) and thermoplastics. Further, figure 2 shows the ultrasonic or high-frequency welding process 7, 7a, 7b performed in a counterclockwise direction on the layer-by- layer arranged foils 2,3. To ensure maximum degree of thermal insulation from each thermal insulation unit 1 the welding is performed at or close to the lateral edges 5 of each foil 2,3. In this embodiment the foils 2,3 are welded by a double weld seem 7, i.e. two parallel or non-crossing welding paths, thus increasing the mechanical stability of the joints and reducing the risk of leakage. Figure 3 shows the foils 2,3 having three of their sides sealed with double weld seem 7 and the forth unwelded, thus resulting in a pocket 6 with an opening into a cavity. A spacer 4 of a suitable material is then inserted into the cavity as illustrated in figures 4 and 5. The purpose of the spacer 4 is primarily to ensure minimum amount of direct contact between the two foils 2,3, and secondary to ensure high mechanical stability. The spacer material can be chosen freely as long as the overall thermal conductivity across the spacer 4 is kept at an acceptable low level throughout the intended lifetime of the units 1. Table I below gives a non-exhaustive list of spacer material that may be used, together with their typical bulk thermal conductivity or conductivity range at room temperature and atmospheric pressure.

Table I: Possible spacer materials with normal bulk thermal conductivity at room temperature and atmospheric pressure.

Spacer material: W/m-K

Cork 0.04 - 0.07

Cotton 0.03

Ceramic fibre 0.04

Expanded polystyrene 0.03-0.033

Fiberglass 0.045

Perlite 0.031

Polymer, Low-density 0.04 - 0.16

Polyurethane foam 0.02 - 0.021

Rice hulls (ash) 0.062

Rice hulls (whole) 0.0359

Silica Aerogel 0.003-0.03

Wood, oven- dry 0.04-0.17

However, spacer 4 should be interpreted herein in its widest form that fulfills the above mentioned objectives of minimum direct contact between foils and high mechanical stability, while keeping the thermal conductivity low. For example, other possibilities may be use of an at least partly hollow spacer 4 composed of a framework of wires having the required shape, or simply rely on the stiffness of the foil material to maintain a preformed cavity. In these particular cases the major contribution to the low thermal conductivity is due to the formed vacuum inside the pocket 6. Finally, figure 6 shows the resulting thermal insulation unit 1 after complete insertion of the spacer 4, evacuation and final sealing by ultrasonic welding of the two foils 2,3.

The schematic drawings in figure 7 A and B show examples of insulation boards / panels 8 with specific matrix configuration composed of several thermal insulation units 1 at least partly encapsulated with a high-resilient foam 11 such as polyurethane (PU) foam. Figure 7 A illustrates a top view and side views of an embodiment having four PU covered units 1 arranged in a two dimensional matrix configuration. When applied as insulation boards 8 suitable for thermal insulation of liquefied gas such as LNG, LPG or LEG, a typical board volume may be about 1000x1000x50 mm ³ . However, these dimensions should of course be adapted according to specific needs. Figure 7 B illustrates six of these boards 8a-f arranged in three board layers 12', 12", 12" ' and mounted onto an exterior wall 9 of a liquid storing tank 10. The lateral positions of the two layers 12', 12", 12" ' are laterally shifted relative to each other in order to reduce the thermal conductivity bridge between the tank 10 and an ambient area 15 outside the tank 10 and the boards 8. The units 1 are indicated only for the two topmost layers 12', 12". In the particular embodiment shown in figure 7 B each unit 1 is completely encapsulated by PU 11. However, in another embodiment of the invention some units 1 may be stacked directly onto each other without adding any PU 11 in between. In yet another embodiment of the invention only some units 1 are fully encapsulated while others are only partly encapsulated. Completely removing the encapsulation material 11 around some or all of the units 1 are also a possible embodiment of the invention. In figure 7 B connection regions 13 ',13" are shown representing the regions in which the different boards 8a-f are laterally interconnected, i.e. 8a-8b, 8c-8d and 8e-8f. These regions 13 ',13" can advantageously be filled with low conductive material such as PU foam. Further, fastening means 14 such as bolts / pins are illustrated in the lowermost layer 12" ' for establishing a mechanically stable connection between the insulation board and the tank 10.

The ultrasonic sealing method may seal super thin metal film, for example of thickness in the range 30 μιη to 800 μιη, producing long lasting fluid tight joints. By following the above mentioned method steps super thin insulation units 1 are obtained which have ultralow thermal conductance properties even at ultralow temperatures across the vacuum and spacer part of the unit 1, for example down to temperature ranges typical for liquid hydrogen or helium. In the preceding description, various aspects of the apparatus according to the invention have been described with reference to the illustrative embodiment. For purposes of explanation, specific numbers, systems and configurations are set forth in order to provide a thorough understanding of the apparatus and its workings. However, this description is not intended to be construed in a limiting sense. Various modifications and variations of the illustrative embodiment, as well as other embodiments of the apparatus, which are apparent to persons skilled in the art to which the disclosed subject matter pertains, are deemed to lie within the scope of the present invention.