Method for manufacturing latent heat storage element

Abstract

A method of manufacturing a latent heat storage element. A method of manufacturing a latent heat storage element (12), comprising the following steps:-providing a graphite sheet (1) made of expanded graphite, the sheet (1) having a sheet plane (1a) and comprising a graphite layer oriented substantially parallel to the sheet plane (1a), such that the graphite sheet (1) has a higher thermal conductivity in a direction parallel to the sheet plane (1a) than perpendicular thereto,-forming a phase change material in a liquid state, the invention relates to a method for producing a phase change material, comprising the steps of:-providing a phase change material,-infiltrating a graphite flake (1) with the phase change material to obtain an infiltrated graphite flake (1),-cooling the infiltrated graphite flake (1) to a temperature below the melting point of the phase change material,-optionally cutting or grinding the infiltrated graphite flake (1) to obtain at least one infiltrated graphite body,-optionally sealing the infiltrated graphite flake (1) by means of an impermeable encapsulation thereof when the phase change material is in its liquid state, the infiltrated graphite flakes (1) or each infiltrated graphite body are completely encapsulated.

Description

[0001] The present invention relates to a method for producing a latent heat storage element. Furthermore, the present invention relates to a latent heat storage element obtained by said method and a transport container for transporting temperature-sensitive goods containing the latent heat storage element.

[0002] Latent heat storage elements are passive temperature control elements commonly used in various transportation applications to maintain temperature-sensitive cargo within a predetermined temperature window. These elements are capable of storing thermal energy in a phase change material (PCM) whose latent heat of fusion, dissolution, or absorption is significantly greater than the PCM's ability to store heat due to its normal specific heat capacity. For example, PCMs are used as latent heat storage materials in passively cooled pallet-sized air freight containers for transporting temperature-sensitive cargo.

[0003] The PCM must be encapsulated to prevent leakage and contamination of the transported cargo. In a typical cold pack, the PCM is encapsulated in a plastic shell with relatively thick walls. This is necessary to prevent leakage due to diffusion of the PCM through the encapsulation. To effectively control the temperature of the cargo, a latent heat storage element is typically placed within or along the walls of the transport container. However, due to the thickness of the encapsulation, the thickness of the transport container's walls cannot be optimized. First, the thickness of the plastic encapsulation wall increases the thickness of the container wall. Second, because cold packs are generally not completely filled with PCM and include a plastic structure to insert the PCM and close the encapsulation, the cold pack cannot be arranged so that the PCM continuously surrounds the interior space of the container. This reduces the amount of PCM in the container wall relative to the wall thickness and leads to localized thermal bridges where heat can bypass the latent heat storage element and enter the container interior space.

[0004] To optimize the walls of the transport container in terms of maximum PCM amount and minimum wall thickness and to avoid thermal bridges between cold packs, the walls of the envelope must be as thin as possible. However, thin walls do not prevent diffusion of PCM through the wall.

[0005] Another significant issue with conventional encapsulation methods is the low thermal conductivity of PCM, which leads to localized melting of the PCM if a non-uniform heat load is applied. Consequently, the full potential of the PCM cannot be utilized. Furthermore, due to the low thermal conductivity, the freezing and refilling processes of the container take longer and are less uniform. To address this issue, it has been proposed to combine PCM with highly thermally conductive materials, such as graphite or expanded graphite.

[0006] WO 2011 / 124624 A1 describes the production of a composite material comprising expanded graphite and PCM. After PCM infiltration, the composite material is comminuted, with the PCM in a solid or liquid state. This comminution disrupts the thermally conductive connections between the graphite flakes in the expanded graphite sheet. This results in a relatively low thermal conductivity for a given portion of the graphite in the resulting composite material.

[0007] EP 2836784 B1 describes the infiltration of microencapsulated PCM into expanded graphite blanks. Further production steps, such as grinding, cutting, and external encapsulation, are not described. Compared to pure PCM, microencapsulated PCM has a lower specific enthalpy. For gravimetric and volumetric efficiency, it is important that the PCM is pure without losing gravimetric enthalpy density through microencapsulation. On the other hand, infiltrating expanded graphite sheets with pure PCM as described in EP 2836784 B1 results in composite graphite PCM components that are very fragile and difficult to handle in further production steps. In addition, due to the lack of external encapsulation, there is a risk of leakage and diffusion of the PCM in the liquid phase.

[0008] Other approaches involve mixing graphite powder with PCM. Again, the thermally conductive connections between the graphite flakes, as found in expanded graphite sheets, are missing. To still achieve acceptable thermal conductivity, the graphite fraction must be relatively high. For applications in transport containers, it is important to be as weight-efficient as possible. A high graphite fraction results in a low thermal energy density for latent heat storage devices.

[0009] Therefore, the present invention aims to provide a latent heat storage element that addresses the above-mentioned issues. In particular, the latent heat storage element should prevent leakage (including diffusion) of the liquid PCM through its envelope without requiring thick walls. Furthermore, the latent heat storage element should have high thermal conductivity to avoid localized melting of the PCM while minimizing the amount of highly thermally conductive material.

[0010] A production process for PCM composite latent heat storage elements suitable for use in air freight transport containers for temperature-sensitive goods with improved weight and volume efficiency should be provided. The volume fraction of pure PCM should be maximized. The PCM should remain uniformly distributed within the latent heat storage element, regardless of where it is stored or used. The packaging should be space-efficient: it should be possible to place adjacent latent heat storage elements in close proximity without sacrificing space due to packaging walls, foil welds, or other elements that reduce energy density for a given wall thickness of the transport container. Efficient mass production of latent heat storage elements should be possible, allowing the creation of latent heat storage elements with arbitrary shapes suitable for various application requirements.

[0011] To address these and other objectives, a first aspect of the present invention provides a method of manufacturing a latent heat storage element, comprising the steps of:

[0012] - providing a graphite sheet made of expanded graphite, the sheet having a sheet plane and comprising graphite layers oriented substantially parallel to the sheet plane, such that the graphite sheet has a higher thermal conductivity in a direction parallel to the sheet plane than perpendicular thereto,

[0013] - infiltrating the graphite sheet with the phase change material while the phase change material is in a liquid state to obtain an infiltrated graphite sheet,

[0014] - cooling the infiltrated graphite sheet to a temperature below the melting point of the phase change material,

[0015] - optionally cutting or grinding the infiltrated graphite sheet to obtain at least one infiltrated graphite body,

[0016] - Completely encapsulating the infiltrated graphite sheet or each infiltrated graphite body by virtue of its impermeable encapsulation by the phase change material when it is in its liquid state.

[0017] The present invention is based on the concept of containing a phase change material (PCM) within the interlayer spaces of expanded graphite, where capillary forces are sufficiently strong to maintain the PCM in its liquid state. This means that once the PCM has infiltrated the expanded graphite, outflow of the PCM is minimized. Thus, the graphite helps contain the PCM and prevent leakage. Consequently, the external envelope only partially contributes to the encapsulation function of the latent heat storage element and can be designed accordingly. In particular, the external envelope can be implemented with a significantly reduced thickness compared to prior art embodiments. Consequently, the present invention allows for latent heat storage elements with a particularly high percentage of PCM based on the total volume of the latent heat storage element.

[0018] Furthermore, arranging the PCM within the interlayer spaces of the expanded graphite helps ensure that the PCM is evenly distributed within the packaging and remains evenly distributed over time, regardless of where the packaging is stored or used. Conventional packaging with cold packs suffers from the disadvantage that the PCM, if in liquid form, can flow within the packaging. This can lead to uneven distribution and affect the long-term performance of the shipping container.

[0019] Another advantage of using expanded graphite as the primary encapsulation for PCM in shipping containers is the achievable fill factor. As mentioned above, when PCM is used as latent heat storage in shipping containers, its packaging should occupy as little space as possible. When other encapsulation methods, such as cold packs, are used to encapsulate PCM, the volume fill factor is typically around 80%-95%. When using expanded graphite with thin walls to encapsulate PCM, fill factors exceeding 95% can be achieved.

[0020] Expanded graphite is characterized by its light weight and can have a theoretical thermal conductivity of up to 600 W / mK. Expanded graphite (also known as exfoliated graphite) is produced by inserting foreign components (intercalations) between graphite layers. Such expandable graphite intercalations are typically prepared by dispersing graphite particles in a solution containing an oxidant and a gaseous compound to be intercalated. Commonly used oxidants include nitric acid, potassium chlorate, chromic acid, potassium permanganate, and the like. For example, concentrated sulfuric acid is used as the compound to be stored. When heated to a temperature above the so-called starting temperature, the expandable graphite intercalation compound undergoes a strong volume increase with an expansion coefficient greater than 200. This is due to the fact that the intercalation compound inserted into the graphite layer structure decomposes due to the rapid heating to this temperature, with the formation of gaseous substances, so that the graphite layers are driven apart in an accordion-like manner, i.e., the graphite particles expand or swell perpendicular to the layer plane.

[0021] For example, fully expanded graphite sheets can be produced by compressing it under pressure, with the graphite layers aligned perpendicular to the direction of pressure. This results in the graphite sheets having graphite layers aligned substantially parallel to the sheet plane, leading to anisotropic thermal conductivity. Specifically, the graphite sheets have a higher thermal conductivity parallel to the sheet plane than perpendicular to it, which has the effect of evenly distributing the thermal energy applied to the latent heat storage element disposed in a transport compartment surrounding the transport container. In contrast, the thermal conductivity of the graphite sheets is lower perpendicular to the sheet plane, significantly reducing the amount of heat transferred into the transport compartment across the latent heat storage element.

[0022] Preferably, the thermal conductivity of the graphite sheet parallel to the sheet plane is at least 10 W / mK, while the thermal conductivity of the graphite sheet perpendicular to the sheet plane is <10 W / mK, preferably <5 W / mK, more preferably <1 W / mK, such as between 0.2 W / mK and 10 W / mK.

[0023] Preferably, the expanded graphite sheet has a g / cm 3 density.

[0024] Preferably, the graphite sheet has a thickness of 10-50 mm.

[0025] The size of expanded graphite sheets can be within the following dimensions: length 80-800mm x width 80-800mm x thickness 10-50mm.

[0026] While the phase change material is in a liquid state, the PCM is introduced into the interlayer spaces of the expanded graphite sheet by infiltrating the graphite sheet with the PCM. This process is preferably performed by placing the graphite sheet in an impregnation vessel filled with liquid PCM. The PCM can be infiltrated into the expanded graphite without the application of a vacuum. For PCMs with a melting point of 5°C, the infiltration process can be performed at room temperature. For PCMs with a melting point in the range of 15°C-25°C, the infiltration can be performed at temperatures >30°C, preferably >35°C, to accelerate the process. More generally, the infiltration step is preferably performed at a temperature >5°C, preferably >10°C, above the melting point of the phase change material.

[0027] Preferably, the step of infiltration is carried out until a filling factor of >95 vol.-%, based on the total volume of the latent heat storage element, is achieved.

[0028] Any type of PCM may be used for manufacturing the latent heat storage element of the present invention, in particular PCMs that undergo a phase change from liquid to solid at the temperature that needs to be maintained during transport of temperature-sensitive goods, such as pharmaceutical products.

[0029] Preferably, the phase change material has a melting point < 25°C. Preferably, the phase change material may have a melting point in the range of 2-10°C or 2-25°C or -82 to -72°C or -15 to -30°C.

[0030] Suitable PCMs include paraffins (such as n-hexadecane or n-tetradecane), esters (such as methyl esters), linear alcohols, ethers, organic anhydrides, salt-hydrates, water-salt-mixtures, salt solutions and / or water-based solutions.

[0031] According to a preferred embodiment of the infiltration step, a plurality of graphite sheets can be infiltrated in an impregnation vessel filled with PCM. The graphite sheets can be stacked on a support structure comprising a plurality of separation elements or support trays for separating adjacent graphite sheets from one another. This support structure allows for the simultaneous infiltration of a large number of graphite sheets, thereby saving time. Furthermore, it allows for the separation of the graphite sheets, facilitating extraction of the infiltrated graphite sheets from the PCM bath. It also allows for weighing down the entire stack of graphite sheets while preventing them from breaking.

[0032] The support structure's separating elements or support trays are preferably designed as a grid or plate with through-holes to allow the PCM to flow through. The structure is preferably made of metal. The graphite sheets can be stacked horizontally or vertically. Horizontal stacking is preferred because it facilitates handling of the infiltrated graphite sheets.

[0033] Preferably, the graphite sheet is placed on a support structure, which is located outside the impregnation vessel. To carry out the infiltration step, the support structure is placed in the impregnation vessel together with the graphite sheet.

[0034] After the graphite sheet is placed on a support structure (either inside or outside the impregnation vessel) and pressed down to prevent floating, the impregnation vessel is filled with liquid PCM. The graphite sheet preferably remains covered with liquid PCM throughout the infiltration duration. Since most infiltration occurs during the first hour, it may be necessary to top up the impregnation vessel. The graphite sheet can preferably remain in the PCM bath for 24 hours. This duration allows for a fill rate >95%.

[0035] After the infiltration step, the infiltrated graphite sheets are removed from the PCM bath. They are preferably drained of excess PCM deposited on the surface. This can be done by placing the infiltrated graphite sheets on a flat surface for about 30 minutes, or by allowing the sheets to drip while still on a support structure for about 10 minutes.

[0036] Placing the graphite sheets on the separation element during at least one of the steps of infiltration, dripping off excess PCM and removing these sheets from the impregnation vessel provides support for the graphite sheets, which are usually very fragile and difficult to handle.

[0037] After the infiltration step, the method of the present invention provides a step of cooling the infiltrated graphite sheets to a temperature below the melting point of the phase change material. Preferably, the cooling step is carried out to a temperature >5°C, preferably >10°C, below the melting point of the phase change material. In this way, the PCM transforms into its solid state, thereby providing stability to the infiltrated graphite sheets, which facilitates further handling of these sheets, prevents breakage, and allows subsequent manufacturing steps.

[0038] An optional manufacturing step includes cutting or grinding the infiltrated graphite sheet to obtain at least one infiltrated graphite body. Cutting and / or grinding the infiltrated graphite sheet may be necessary to obtain a graphite body of a desired shape or size. The cutting and grinding steps ensure that the thermally conductive graphite lamellae connections within the expanded graphite sheet remain intact. Crushing and reshaping the infiltrated graphite elements disrupts the graphite lamellae connections and results in a composite material having lower thermal conductivity.

[0039] After an optional manufacturing step, the infiltrated graphite sheet is enclosed in an envelope that is impermeable to the phase change material when it is in its liquid state, such as an oil-tight foil or plastic casing. The envelope is designed to completely surround and seal the infiltrated graphite sheet.

[0040] To ensure a compact and space-efficient encapsulation and to avoid leakage and diffusion of the PCM through the encapsulation, the encapsulation is preferably performed under a vacuum of <100 mbar.

[0041] According to one embodiment, the envelope comprises or consists of a foil, preferably a multilayer foil, comprising at least one aluminum layer and at least one sealable layer, such as polyethylene, in particular LD-PE or HD-PE. The sealable layer can be configured to be sealable, for example by welding.

[0042] The foil can preferably be folded such that the weld seams are not arranged on the side surfaces, but on the top and / or bottom surface of the latent heat storage element, so as not to prevent the latent heat storage elements from being placed closely relative to one another in a heat-exchanging manner. In this context, the top and bottom surfaces of the latent heat storage element are understood to extend parallel to the sheet plane, while the side surfaces extend to connect the top and bottom surfaces.

[0043] Preferably, the foil, in particular the multilayer foil, has a thickness of 0.01 mm - 0.5 mm.

[0044] Encapsulation with a multilayer composite foil containing at least one layer of aluminum may comprise the following steps:

[0045] - Creation of foil tubes.

[0046] -Insert the infiltrated graphite sheet into the foil tube, where the PCM is in a solid state. The foil tube should fit tightly around the graphite sheet, leaving a minimum gap between the sheet and the tube.

[0047] - Fold the corners to avoid weld overhang (see Figure 3 )

[0048] -Weld one side as close to the edge of the graphite sheet as possible

[0049] - Create vacuum (<100mbar)

[0050] -Weld the other side while maintaining the vacuum

[0051] -Release the vacuum

[0052] - Fixing the overhanging foil relative to the graphite sheets to allow the graphite sheets to be positioned relative to each other

[0053] According to another embodiment, the package comprises a bottom shell (preferably obtained by thermoforming) and an upper foil that is sealed (preferably welded) to the bottom shell to close the package. For example, the upper foil can be welded or bonded to the edge of the bottom shell using an adhesive. Preferably, the sealing or welding can be performed under a vacuum of <100 mbar.

[0054] According to a preferred embodiment, the bottom shell has a wall thickness of 0.2 mm - 2 mm and the upper foil has a thickness of 0.01 mm - 0.5 mm.

[0055] Encapsulation with thermoformed plastic may include the following steps:

[0056] -Thermoforming of bottom shell

[0057] -Insert the infiltrated graphite sheet into the bottom shell where the PCM is in solid state

[0058] - Create vacuum (<100mbar)

[0059] - Weld the upper foil to the edge of the bottom shell to close the bottom shell

[0060] -Release the vacuum

[0061] According to a second aspect, the present invention provides a latent heat storage element obtained by the method according to the first aspect of the invention. Thus, the latent heat storage element comprises or consists of a graphite sheet made of expanded graphite, which is infiltrated with a phase change material and completely encapsulated with an encapsulation that is impermeable to the phase change material when it is in its liquid state.

[0062] Preferably, the thermal conductivity of the latent heat storage element in a direction parallel to the sheet plane is at least 10 W / mK, and the thermal conductivity of the latent heat storage element perpendicular to the sheet plane is <10 W / mK, preferably <5 W / mK, more preferably <1 W / mK, such as between 0.2 W / mK and 10 W / mK.

[0063] According to a third aspect, the invention provides a transport container for transporting temperature-sensitive goods, said transport container comprising walls on all sides thereof enclosing a transport compartment, wherein at least one of said walls comprises a latent heat storage element according to the second aspect of the invention.

[0064] Preferably, the latent heat storage element is arranged such that its lamella plane extends parallel to the wall plane of the associated wall.

[0065] Preferably, each of said walls comprises a latent heat storage element, and the latent heat storage elements of adjacent walls are arranged in contact with each other to allow the thermal energy acting thereon to be distributed over the entire periphery of the transport compartment.

[0066] The latent heat storage element preferably surrounds the transport compartment of the transport container on all sides without gaps. Thus, for example, the latent heat storage element forms an enclosure within which the transported goods are located. In the case of a cubic transport container, each of the six container walls preferably contains a latent heat storage element or a plurality of adjacent latent heat storage elements, so that the enclosure consists of six planar latent heat storage elements arranged in a cubic shape. The latent heat storage elements, in particular their edge regions, are preferably in direct contact with one another, so that thermal equilibrium occurs throughout the entire transport compartment, allowing heat to be conducted through the enclosure of the latent heat storage element, for example, from one side of the transport compartment to the opposite side.

[0067] Preferably, the wall comprises a plurality of layers arranged one above the other, a first layer being formed by at least one said latent heat storage element, and a second layer being a thermal insulation layer having a thermal conductivity perpendicular to the wall plane of <0.04 W / mK, preferably <0.01 W / mK.

[0068] Preferably, the thermal insulation layer can be a vacuum panel, polyisocyanurate (PIR), expanded polystyrene (EPS), extruded polystyrene foam (XPS), or ISOPET. ISOPET panels are understood to be insulation panels comprising multiple layers of foils arranged in a honeycomb pattern, with a very low-emissivity metal coating (particularly with an emissivity of <0.2, preferably 0.02-0.09), wherein air or a gas with low thermal conductivity, such as krypton or xenon, is located in the cavities between the foils, or the cavities are evacuated. Such insulation panels are described in WO 2012 / 142639 A1.

[0069] The transport container according to the invention is preferably designed as an air transport container and therefore preferably has a diameter of at least 0.4 x 0.4 x 0.4 m 3 , preferably 0.4x0.4x0.4 m 3 -1.6x1.6x1.6 m 3 , preferably 1.0x1.0x1.0 m 3 -1.6x1.6x1.6 m 3 External dimensions.

[0070] Hereinafter, the present invention will be described in more detail with reference to the exemplary embodiments shown in the accompanying drawings.

[0071] - Figure 1 Showing the internal structure of expanded graphite sheets,

[0072] - Figure 2 Graphite sheet shown arranged on a support structure before being placed in a PCM bath,

[0073] - Figure 3 Showing a cooled infiltrated graphite sheet enclosed in a compact foil,

[0074] - Figure 4 Showing an alternative embodiment of encapsulation,

[0075] - Figure 5 a flow chart showing a manufacturing method according to the present invention, and

[0076] - Figure 6 A transport container with a latent heat storage element according to the invention is shown.

[0077] Figure 1 The internal structure of a graphite sheet 1 made of expanded graphite used in the present invention is shown. Graphite sheet 1 comprises graphite layers oriented substantially parallel to sheet plane 1a. Specifically, the internal structure comprises thermally conductive graphite flake connections 2 and interlayer spaces 3 disposed therebetween. The graphite content of graphite sheet 1 can range from 92-96 vol.% based on the total volume of graphite sheet 1. Consequently, the void content of graphite sheet 1 ranges from 4-8 vol.%.

[0078] like Figure 2 As shown, a plurality of graphite sheets are arranged in a support structure 4 before being placed in a PCM bath. The support structure 4 comprises a frame and a plurality of parallel separate elements, such as support trays 5, on which the graphite sheets are individually deposited.

[0079] Figure 3 A first embodiment of an encapsulated permeable graphite sheet is shown. The encapsulation comprises a foil 6 that is impermeable to the liquid PCM, wherein the foil 6 is tubular and surrounds the graphite sheet, and wherein the corner regions of the foil 6 that protrude from the graphite sheet are suitably folded. The overlapping foil regions at the open ends of the tubular foil 6 are connected by means of welds, wherein the welds are arranged so that, after the overhanging foil is folded and secured back onto the element, they are located on the top or bottom surface of the latent heat storage element, rather than on the sides.

[0080] Figure 4 An alternative embodiment of the encapsulation is shown, in which the infiltrated graphite sheet 1 is placed in a thermoformed lower shell 7 which is closed by an upper foil 8 which is sealed along the circumference under a vacuum of <100 mbar.

[0081] The manufacturing method of the present invention is as follows Figure 5 As shown, the process includes a process P1 of infiltrating a graphite sheet and a process P2 of encapsulating the infiltrated graphite sheet.

[0082] Process P1 includes the following steps:

[0083] S1: Prepare graphite sheets, such as by cutting to size

[0084] S2: Placing the graphite sheet in a support structure

[0085] S3: Placing the support structure in the impregnation vessel

[0086] S4: Filling the impregnation container with liquid PCM

[0087] S5: Wait 24 hours

[0088] S6: Remove the infiltrated graphite sheet from the PCM bath and drain the excess PCM

[0089] S7: Cooling the infiltrated graphite sheet

[0090] S8: Optionally cutting or grinding the infiltrated graphite sheet, wherein the PCM is in a solid state.

[0091] Process P2 includes the following steps:

[0092] S9a: Cut and weld a tubular foil tightly around the infiltrated graphite sheet

[0093] S10a: Fold the corner and weld the tubular foil at one open end

[0094] S11a: Create vacuum and weld the tubular foil at its other open end

[0095] S12a: Fixing the overhanging foil relative to the top or bottom side of the component.

[0096] or:

[0097] S9b: Thermoforming the encapsulated bottom shell

[0098] S10b: Inserted infiltrated graphene sheet

[0099] S11b: Create vacuum and weld the upper foil to the bottom shell.

[0100] Figure 6 A transport container made of a cubical compartment 9 is shown, which comprises walls 10 surrounding the interior of the container on all sides except for the opening. The opening is closed by an inner door 15 and an outer door 16. The walls 10 all comprise the same layer arrangement: an inner lining 11, a layer 12 comprising a latent heat storage element according to the invention, insulation 13, and an outer lining 14.

Claims

1. A method for producing a latent heat storage element (12), comprising the following steps: - providing a graphite sheet (1) made of expanded graphite, the sheet (1) having a sheet plane (1a) and comprising graphite layers oriented substantially parallel to the sheet plane (1a), such that the graphite sheet (1) has a higher thermal conductivity in a direction parallel to the sheet plane than perpendicular thereto, - infiltrating the graphite sheet (1) with the phase change material while the phase change material is in a liquid state to obtain an infiltrated graphite sheet (1), - cooling the infiltrated graphite sheet (1) to a temperature below the melting point of the phase change material, - optionally cutting or grinding the infiltrated graphite sheet (1) to obtain at least one infiltrated graphite body, - Completely encapsulating the infiltrated graphite sheet (1) or each infiltrated graphite body by virtue of the impermeable encapsulation of said phase change material when it is in its liquid state.

2. The method according to claim 1, wherein the expanded graphite sheet (1) has a thermal conductivity of >10 W / mK in a direction parallel to the sheet plane (1a).

3. The method according to claim 1 or 2, wherein the expanded graphite sheet (1) has a thermal conductivity of <10 W / mK, preferably <5 W / mK, more preferably <1 W / mK in a direction perpendicular to the sheet plane (1a).

4. The method according to claim 1, 2 or 3, wherein the expanded graphite sheet (1) has a particle size of <0.15 g / cm 3 density.

5. The method according to any one of claims 1 to 4, wherein the phase change material has a melting point < 25°C.

6. The method according to any one of claims 1 to 5, wherein the infiltration step is performed at a temperature >5°C, preferably >10°C, above the melting point of the phase change material.

7. The method according to any one of claims 1 to 6, wherein the infiltration step is carried out until a filling factor of >95 vol.-%, based on the total volume of the latent heat storage element (12), is obtained.

8. Method according to any one of claims 1 to 7, wherein the graphite sheet (1) is placed on a support structure (4), in particular a grid or a support tray (5), which is introduced into the impregnation vessel for the infiltration step.

9. The method according to any one of claims 1 to 8, wherein the cooling step is performed to a temperature >5°C, preferably >10°C below the melting point of the phase change material.

10. The method according to claim 1 , wherein the phase change material comprises or consists of a paraffin wax, such as n-hexadecane or n-tetradecane, an ester, such as a methyl ester, a linear alcohol, an ether, an organic anhydride, a salt-hydrate, a water-salt mixture, a salt solution and / or a water-based solution.

11. Method according to any one of claims 1 to 10, wherein the envelope comprises or consists of a foil (6), preferably a multilayer foil comprising at least one aluminum layer and at least one sealable layer, such as polyethylene, in particular LD-PE or HD-PE.

12. Method according to claim 11, wherein the foil (6), in particular the multilayer foil, has a thickness of 0.01 mm - 0.5 mm.

13. Method according to any one of claims 1 to 10, wherein the envelope comprises a bottom shell (7) and an upper foil (8) closing the envelope by foil welding or sealing, the bottom shell (7) preferably being obtained by thermoforming.

14. Method according to claim 13, wherein the bottom shell (7) has a wall thickness of 0.2 mm - 2 mm and the upper foil (8) has a thickness of 0.01 mm - 0.5 mm.

15. Method according to any one of claims 1 to 14, wherein the infiltrated graphite sheet (1) or the infiltrated graphite body is subjected to a vacuum, preferably to a pressure of <100 mbar, during the encapsulation step.

16. Latent heat storage element (12) obtainable by the method according to any one of claims 1 to 15.

17. A transport container for transporting temperature-sensitive goods, comprising walls (10) surrounding a transport compartment (9) on all sides thereof, wherein at least one of the walls (10) comprises a latent heat storage element (12) according to claim 16.

18. The transport container according to claim 17, wherein the latent heat storage element (12) is arranged such that its lamella plane (1a) extends parallel to the wall plane of the wall (10).

19. A transport container according to claim 18, wherein each of the walls (10) contains a latent heat storage element (12), and the latent heat storage elements (12) of adjacent walls (10) are arranged in contact with each other to allow the thermal energy acting thereon to be distributed over the entire periphery of the transport compartment (9).

20. Transport container according to claim 17, 18 or 19, wherein the wall (10) comprises a plurality of layers arranged one above the other, a first layer being formed by the latent heat storage element (12), and a second layer being a thermal insulation layer (13) having a thermal conductivity perpendicular to the wall plane of <0.04 W / mK, preferably <0.01 W / mK.

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