Mineral insulated cable

Abstract

A mineral insulated cable (5), comprising: an elongate core (10) comprising: a resistive tube (12) extending along a central axis (A) of the mineral insulated cable (5), wherein the resistive tube (12) comprises an outer surface and an inner surface, the inner surface defining an internal bore that also extends along the central axis (A); and a semi-conducting filler (14) packed into the internal bore of the resistive tube (12) and in electrical contact with the resistive tube (12) along a length thereof; an electrically insulating layer (16) concentrically enveloping around the elongate core (10), the electrically insulating layer (16) comprising a mineral material; and an outer sheath (18) concentrically enveloping around the electrically insulating layer (16); wherein an opening (11) is defined in the resistive tube (12), the opening (11) extending between the outer and inner surfaces of the resistive tube (12).

Description

[0001] SP3225

[0002] MINERAL INSULATED CABLE

[0003] Field of the Invention

[0004] This invention relates to a mineral insulated cable and a method of manufacturing a mineral insulated cable. Background of the invention

[0005] The environmental impact of greenhouse gases, primarily carbon dioxide (CO₂) and methane (CH₄), has been the subject of much public debate over the past several decades. Large amounts of greenhouse gases are emitted when fossil fuels are produced and burned for e. g., electricity production, transport, and heating.

[0006] One promising alternative to carbon-containing fossil fuels is the use of hydrogen (H₂) gas. Hydrogen can, for example, be used as an energy carrier for storing energy produced by renewable energy sources such as solar panels and wind turbines. Hydrogen can also be used in fuel cells to produce electricity, or via combustion to generate heat. Advantageously, the combustion of hydrogen gas yields only water as a reaction product.

[0007] Traditionally, hydrogen has primarily been produced using fossil fuels, for example using natural gas conversion in a steam reformer, which does little to reduce the emission of greenhouse gases into the atmosphere. Alternatively, hydrogen gas may be generated by the electrolysis of water into hydrogen gas and oxygen, but this requires a substantial amount of electricity. At least some of the required electricity may be obtained from renewable sources (e. g., wind, solar, and hydroelectric). However, all renewable energy used for hydrogen production is renewable energy that can' t be used for other purposes and hence may need to be compensated for by the use of more fossil fuels.SP3225

[0008] 2

[0009] One alternative production method is to mine natural resources of hydrogen gas. In US Patent Application No. 63 / 566507, hydrogen is generated from mafic and ultramafic rock formations through serpentinization reactions using a mineral insulated heater cable. In particular, hydrogen is generated by providing the mineral insulated heater cable inside a wellbore in a mafic or ultramafic rock formation and heating the rock formation to a temperature of about 200 to 600 degrees Celsius. The mineral insulated heater cable advantageously allows the rock formations to be heated to temperatures that exceed the typical temperatures associated with naturally occurring serpentinization reactions. At these higher temperatures -up to about at least 600 degrees Celsius - the serpentinization ion reaction is accelerated and larger amounts of hydrogen are generated. This process is known as in-situ thermally assisted serpentinization.

[0010] The mineral insulated heater cable described in US Patent Application No. 63 / 566507 is a temperature limited heater, meaning that it reduces heat output above a specified temperature without the use of external controls such as temperature controllers, power regulators, rectifiers, or other devices, To this end, the mineral insulated heater cable makes use of an elongate core with a negative temperature coefficient, which causes the resistance of the cable to decrease as the overall temperature increases. This beneficially mitigates against hotspots and possible thermal runaway events in the cable, but at the same time reduces its power output.

[0011] It is an aim of the current invention to overcome at least some of the disadvantages of the prior art.SP3225

[0012] 3

[0013] Summary of the Invention

[0014] According to an aspect of the invention, this aim is achieved by providing a new mineral insulated cable, comprising:

[0015] -an elongate core comprising:

[0016] - a resistive tube extending along a central axis of the mineral insulated cable, wherein the resistive tube comprises an outer surface and an inner surface, the inner surface defining an internal bore that also extends along the central axis; and

[0017] - a semi-conducting filler packed into the internal bore of the resistive tube and in electrical contact with the resistive tube along a length thereof;

[0018] - an electrically insulating layer concentrically enveloping around the elongate core, the electrically insulating layer comprising a mineral material; and

[0019] - an outer sheath concentrically enveloping around the electrically insulating layer;

[0020] wherein an opening is defined in the resistive tube, the opening extending between the outer and inner surfaces of the resistive tube.

[0021] Current passing through the mineral insulated cable passes through both the semi-conducting filler and the resistive tube along the length of the mineral insulated cable. When the temperature of the elongate core is low, the elongate core has high resistivity and so produces heat as current passes therethrough, which is then conveyed outwardly through the electrically insulating layer and the outer sheath, thereby allowing the mineral insulated cable to heat a substance in contact therewith. However, since the semi-conducting filler has a negative temperature coefficient, it means that when the temperature of the elongate core is high, the resistivity of the semi-conducting filler, and hence the elongateSP3225

[0022] 4

[0023] core, decreases and so the temperature of the elongate core decreases also. In this way, the elongate core is able to self-regulate its temperature when it becomes too high, and it prevents the occurrence of hotspots at locations along the length of the mineral insulated cable and hence thermal runaway events there.

[0024] The opening reduces the material of the resistive tube, thereby increasing the electrical resistance of the resistive tube. As a result, the elongate core produces more heat energy when the same amount of current is provided to the mineral insulated cable, and hence the mineral insulated cable has a higher power output, while still being able to self-regulate its temperature. Even greater power output is possible when a higher voltage is applied to the mineral insulated cable. Such high power is particularly advantageous when the mineral insulated cable is used as a heater, in particular to heat mafic or ultramafic rock formations for hydrogen production, since the higher temperatures improve the efficiency of such serpentinization reactions and ensure a higher yield of hydrogen therefrom.

[0025] The resistive tube is therefore not in the form of a resistive wire helically coiled to form a helix around the semi-conducting material. The resistive tube may be made of a metal material. The outer sheath may be metallic.

[0026] In an embodiment, the opening is completely enclosed by at least a portion of the resistive tube. When a plurality of openings is defined in the resistive tube, each opening may be completely enclosed by a respective portion of the resistive tube. The (or each) opening may define a periphery that extends entirely around the outside thereof, a portion of the resistive tube providing said periphery.Optionally, a plurality of openings are defined in the resistive tube. Each opening may extend between the outer and inner surfaces of the resistive tube. Each opening may be separate and distinct. The greater the number of openings in the resistive tube, the more resistance provided by the resistive tube, and hence the greater power outputted by the mineral insulated cable.

[0027] In an embodiment, the size, shape, position, orientation, number and / or distribution of the openings varies along the length of the resistive tube. This allows different levels of resistance to be achieved at different locations along the length of the cable, i.e., along central axis A, and hence different temperature outputs at each of these locations.

[0028] In an embodiment, the resistive tube comprises a cylindrical sheet. The at least one opening may comprise at least one perforation defined in the cylindrical sheet. The or each perforation may have been bored into the cylindrical sheet.

[0029] In an embodiment, the resistive tube comprises a mesh of interlaced strands. The at least one opening may comprise or consist of a plurality of gaps defined between the strands of the mesh. The mesh may be a wire mesh, i.e., where the strands may be wires. To provide circular, triangular, square, rectangular and / or hexagonal gaps, a round, triangular, square, rectangular and / or hexagonal mesh may be used.

[0030] In embodiments, the at least one opening comprises a circular opening, an elliptical opening, a triangular opening, a square opening, a rectangular opening, a pentagonal opening and / or a hexagonal opening. The openings may be round e. g., circular or elliptical ( e. g., oval ), or polygonal e. g., triangular, a square, rectangular, pentagonal hexagonal etc. The openings may beSP3225

[0031] 6

[0032] substantially regularly polygonal. The at least one opening may comprise at least one slot in the resistive tube. A slot is a long and narrow opening in the resistive tube. Openings that are elliptical (e. g., oval) or rectangular can be understood as slots.

[0033] Optionally, the or each opening is defined by a maximum surface dimension. The maximum surface dimension may be between 1mm and 150mm.

[0034] When the openings are slots, i. e., elliptical or rectangular, they may be defined by a ( surface) length and a ( surface ) width, where the length is longer than the width. In these embodiments, the maximum surface dimension may correspond to the length of the slot. For all other shapes, the maximum surface dimension may be defined by the ( surface) diameter of each opening. When such openings are non-circular, this may correspond to the diameter of a circle circumscribing the opening.

[0035] Each opening may also be defined by a radial depth. The depth may be the radial distance that the opening extends through the resistive tube between the outer and inner surface thereof, i. e., corresponding to the wall thickness of the resistive tube. The openings are preferably substantially uniform along their radial depth.

[0036] In an embodiment, the size, thickness and / or material of the resistive tube varies along the length of the resistive tube. This is another way of varying the resistance of the resistive tube, and hence the power output of the mineral insulated cable, along its length.

[0037] Optionally, the elongate core further comprises a barrier layer concentrically enveloping around the resistive tube, preferably wherein the barrier layer is a flexible ceramic fabric such as 3M™ Nextel™. The barrier layer or sleeve advantageously prevents certain semi-SP3225

[0038] 7

[0039] conducting fillers, especially certain powders, from leaking out of the elongate core through the opening ( s).

[0040] In an embodiment, a second resistivity of the filler at 20 °C is higher than a first resistivity of the resistive tube at 20 °C, optionally wherein the second resistivity of the semi-conducting filler is at least 1000 μΩ·m at 20°C and the first resistivity of the resistive tube is between 0.05 μΩ·m and 5 μΩ·m at 20°C. The first resistivity of the resistive tube at 20°C is preferably between 0.1 μΩ·m and 5 μΩ·m, more preferably between 0.3 μΩ·m and 5 μΩ·m, and more preferably between 0.5 μΩ·m and 5 μΩ·m. C.

[0041] In an embodiment, the second resistivity of the filler at a temperature of operation above a predetermined elevated temperature is lower than the first resistivity of the resistive tube at said temperature of operation. The second resistivity of the filler at a temperature of operation above a predetermined elevated temperature may be ten times lower than the first resistivity of the resistive tube at said temperature of operation. The predetermined elevated temperature may be from 1 °C to 100 °C above a design operating temperature. The design operating temperature relates the temperature range within which the mineral insulated cable is designed to work under normal operating conditions.

[0042] The semi-conducting filler may have a first electric bandgap, and the electrically insulating layer may have a second electric bandgap, wherein the second electric bandgap is larger than the first electric bandgap.

[0043] The invention also extends to a method of heating a substance, comprising: providing a mineral insulated cable described above; bringing the mineral insulated cable in heat exchanging contact with a substance to be heated; passing an electrical current through the elongate core inSP3225

[0044] 8

[0045] a direction along the central axis; and transferring heat from the elongate core to the substance.

[0046] The invention also extends to a method of heating a rock formation for generating hydrogen, comprising: providing a mineral insulated cable described above inside a wellbore in a mafic or ultramafic rock formation, passing an electrical current through the elongate core in a direction along the central axis; and transferring heat from the elongate core to the rock formation.

[0047] The invention also extends to a system e. g., for heating a substance, comprising: a mineral insulated cable described above; and a current supply in electrical connection with the elongate core of the mineral insulated cable, arranged to pass an electrical current through the elongate core in a direction along the central axis.

[0048] The invention also extends to a method of manufacturing a mineral insulated cable, comprising: providing a resistive tube, a semi-conducting filler, an electrically insulating layer comprising a mineral material, and an outer sheath, wherein the resistive tube extends along a central axis and comprises an outer surface and an inner surface, the inner surface defining an internal bore of the resistive tube that also extends along the central axis, and wherein an opening defined in the resistive tube, the opening extending between the outer and inner surfaces thereof; manufacturing an intermediate assembly comprising: forming an elongate core by packing the semi-conducting filler into the internal bore of the resistive tube such that the semi-conducting filler is in electrical contact with the resistive tube along a length thereof; concentrically enveloping the electrically insulating layer around the elongate core; and concentrically enveloping the outer sheath around the electrically insulating layer; and subj ecting theSP3225

[0049] 9

[0050] intermediate assembly to diameter reduction, comprising mechanically working and / or heat treating the intermediate assembly.

[0051] Providing a resistive tube may comprise providing a sheet, boring a perforation into the sheet, and forming the sheet into a cylinder ( sheet). Alternatively, providing a resistive tube may comprise providing a sheet, forming the sheet into a cylinder ( sheet), and boring a perforation into the cylinder ( sheet).

[0052] Providing a resistive tube may comprise providing a mesh of interlaced strands, and forming the mesh into a cylinder (mesh).

[0053] Brief Description of the Drawings

[0054] Figure 1 schematically illustrates a transverse cross-sectional view of a mineral insulated heater cable according to a first embodiment.

[0055] Figure 2 schematically illustrates a longitudinal cross-sectional view of the mineral insulated heater cable of Figure 1.

[0056] Figure 3 schematically illustrates a longitudinal cross-sectional view of a mineral insulated heater cable according to a second embodiment.

[0057] Figure 4 schematically illustrates a longitudinal cross-sectional view of a mineral insulated heater cable according to a third embodiment.

[0058] Figure 5 schematically illustrates a longitudinal cross-sectional view of a mineral insulated heater cable according to a fourth embodiment.

[0059] Figure 6 schematically illustrates a longitudinal cross-sectional view of a mineral insulated heater cable according to a fifth embodiment.SP3225

[0060] 10

[0061] Figure 7 schematically illustrates a longitudinal cross-sectional view of a mineral insulated heater cable according to a sixth embodiment.

[0062] Figure 8 schematically illustrates a longitudinal cross-sectional view of a mineral insulated heater cable according to a seventh embodiment.

[0063] These drawings depict one or more implementations in accordance with the present teachings, by way of example only, not by way of limitation.

[0064] Detailed Description of the Drawings

[0065] Figures 1 to 8 each show a mineral insulated cable 5 for heating a substance according to the invention.

[0066] The mineral insulated cable 5 comprises an elongate core 10, an electrically insulating layer 16 comprising a mineral material and an outer sheath 18. The elongate core 10 comprises a resistive tube 12 extending along a central axis A of the mineral insulated cable 5. The resistive tube 12 comprises an outer surface and an inner surface, the inner surface defining an internal bore of the resistive tube 12 that also extends along the central axis A. The elongate core 10 further comprises a semiconducting filler 14 packed into the internal bore of the resistive tube 12 and in electrical contact with the resistive tube 12 along a length thereof. The electrically insulating layer 16 concentrically envelops around the elongate core 10, and the outer sheath 18 concentrically envelops around the electrically insulating layer 16. An opening 11 is defined in the resistive tube 12 that extends between the outer and inner surfaces of the resistive tube 12.

[0067] Current passing through the mineral insulated cable 5 passes through both the semi-conducting filler 14 and the resistive tube 12 along the length of the mineralSP3225

[0068] 11

[0069] insulated cable 5. When the temperature of the elongate core 10 is low, the elongate core 10 has high resistivity and so produces heat as current passes therethrough, which is then conveyed outwardly through the electrically insulating layer 16 and the outer sheath 18, thereby allowing the mineral insulated cable 5 to heat a substance in contact therewith. However, since the semi-conducting filler 14 has a negative temperature coefficient, it means that when the temperature of the elongate core 10 is high, the resistivity of the semi-conducting filler 14, and hence the elongate core 10, decreases and so the temperature of the elongate core 10 decreases also. In this way, the elongate core 10 is able to self-regulate its temperature, and it prevents hotspots at locations along the length of the mineral insulated cable 5 and hence thermal runaway events.

[0070] The opening 11 reduces the material of the resistive tube 12, thereby increasing the electrical resistance of the resistive tube 12. As a result, the elongate core 10 produces more heat energy when the same amount of current is provided to the mineral insulated cable 5, and hence the mineral insulated cable 5 has a higher power output, while still being able to self-regulate its temperature. Even greater power output is possible when a higher voltage ( such as between 1, 000V and 15, 000V) is applied to the mineral insulated cable 5. The mineral insulated cable 5 is therefore capable of delivering more than 7 kW / m of cable length, for example up to 15 kW / m, and at a core temperature in a range of between 600 °C and 850 °C, preferably between 700 °C and 850 °C, and a temperature differential between core 10 and sheath 18 of between 250 °C and 400 °C.

[0071] Such a high power output is particularly advantageous when the mineral insulated cable 5 is used toSP3225

[0072] 12

[0073] heat mafic or ultramafic rock formations for hydrogen production, since the higher temperatures improve the efficiency of the serpentinization reactions and ensure a higher yield of hydrogen therefrom.

[0074] Now more details about the mineral insulated cable 5 will be provided.

[0075] As best seen in Figure 1, the elongate core 10 is arranged at the centre of the mineral insulated cable 5 on the central axis A. The elongate core 10 is surrounded by the electrically insulating layer 16, such that the electrically insulating layer 16 concentrically envelops the elongate core 10. The electrically insulating layer 16 is itself surrounded by the outer sheath 18, such that the outer sheath 18 concentrically envelops around the electrically insulating layer 16. Hence, the elongate core 10, the electrically insulating layer 16 and the outer sheath 18 are all concentrically arranged about the central axis A, with the electrically insulating layer 16 arranged between the elongate core 10 and the outer sheath 18. Hence, the entire mineral insulated cable 5 extends along central axis A (as best seen in Figures 2 to 8 ).

[0076] The outer diameter of the entire mineral insulated cable 5 is approximately between 25 and 60 mm.

[0077] Now each of the elongate core 10, the electrically insulating layer 16 and the outer sheath 18 will be described in turn.

[0078] As stated above, the elongate core 10 provides heating of the mineral insulated cable 5 when a current is passed therethrough. Furthermore, the elongate core 10 is able to self-regulate its temperature to prevent hotspots along its length. To this end, the elongate core 10 comprises a resistive tube 12 and a semi-conducting filler 14 packed into the bore of the resistive tube 12.SP3225

[0079] 13

[0080] In Figures 1 to 8, the resistive tube 12 is made up of a substantially cylindrical wall, i. e., with a substantially circular cross-section. As such, the outer and inner surfaces of the resistive tube 12 are curved, and the internal bore defined by the inner surface (and surrounded by the cylindrical wall of the resistive tube 12 ) is also substantially cylindrical, i. e., with a substantially circular cross-section.

[0081] The wall thickness of the resistive tube 12 may be in a range of from about 0.5 mm to about 2.5 mm. The selected thickness will be influenced by the resistivity of the resistive tube 12 in combination with the desired heat output, length of cable 5, and available drive voltage, and thus larger or smaller thicknesses may be used depending on these parameters.

[0082] The diameter of the internal bore of the resistive tube 12 may be in a range of between about 3. 5 mm and about 38 mm, preferably between about 5 mm and 38 mm, more preferably between about 10 mm and 38 mm. The selected diameter will depend on the desired LRR ratio of the core 10 ( see below), and on the type of semi-conducting material that is packed inside, and in some cases it may even be outside of the range given.

[0083] As stated above, the resistive tube 12 is provided with one opening (or more than one opening) 11 to increase its resistance, thereby increasing the power output of the mineral insulated cable 5. Each of the openings 11 extends all the way between the outer and inner surfaces of the resistive tube 12, thereby connecting the outside of the resistive tube 12 with the internal bore within. As best seen in Figures 2 to 8, each opening 11 is completely enclosed / surrounded by at least a portion of the resistive tube 12 such that each opening 11 is separate and distinct from all other openings 11. In other words,each opening 11 defines a periphery that extends entirely around the outside of said opening 11, wherein a portion of the resistive tube 12 defines said periphery.

[0084] Preferably, a resistance of 0. 001 - 2 Ohm / m is achieved along the length of the resistive tube 12 by way of the openings 11. In general, the more of the resistive tube 12 that is eliminated through opening ( s ) 11, the higher resistance and hence power output of the elongate core 10.

[0085] The resistive tube openings 11 in each of the mineral insulated cables 5 of Figures 1 to 8 will now be overviewed.

[0086] In relation to the mineral insulated cable 5 of Figures 1 and 2, the resistive tube 12 is in the form of a sheet or plate of e. g., metal that has been formed into a cylinder, i. e., a cylindrical sheet.

[0087] Each opening 11 is in the form of a perforation in the resistive tube 12, wherein each perforation was bored into the sheet either before or after being formed into a cylinder. Each opening 11 has a circular cross-section such that they may be understood as circular openings 11. Each opening 11 has a ( surface) diameter that is between 1mm and 150mm.

[0088] All of the openings 11 are the same size and shape, but the distribution of the openings 11 varies along the length of the resistive tube 12 such that in the middle there is a lower density of openings 11 ( i. e., the openings are more spread out ) while on either side thereof there is a greater density of openings 11 (i. e., the openings are more closely arranged together). This advantageously achieves different resistances along the length of mineral insulated cable 5 and hence different power outputs there.SP3225

[0089] 15

[0090] In this embodiment, the elongate core 10 further comprises a barrier layer or sleeve 15 that concentrically envelops around the resistive tube 12. In other words, the resistive tube 12 is surrounded by the barrier layer 15. The barrier layer 15 is also concentrically arranged about the central axis A and is arranged between the resistive tube 12 and the electrically insulating layer 16. The barrier layer 15 is preferably a flexible ceramic fabric such as 3M™ Nextel™. The barrier layer 15 can be used to prevent certain semi-conducting fillers (e. g., certain powders ) 14 from leaking out of the elongate core 10 through the openings 11.

[0091] The mineral insulated cable 5 of Figure 3 is the same as the mineral insulated cable 5 of Figures 1 and 2, except for the size, shape and number of the openings 11 in the resistive tube 12. In particular, the openings 11 take the form of slots, i. e., long and narrow openings 11. In particular, each opening 11 has an elliptical (or oval) cross-section, and hence may be understood as elliptical (or oval) openings 11. Each opening 11 is defined by a ( surface) length and a ( surface) width, and the length of each opening 11 is between 1mm and 150mm.

[0092] Furthermore, the slots are diagonally-extending, meaning that they extend both along the central axis A and around the circumference of the resistive tube 12.

[0093] Furthermore, each slot extends in the same direction such that the slots may be understood as being arranged in parallel.

[0094] In this embodiment all of the openings 11 are the same size and shape, and the distribution of openings 11 varies along the length of the resistive tube 12 such that in the middle there is a lower density of openings 11 while on either side thereof there is a greater density of openings 11. This again achieves different resistances,SP3225

[0095] 16

[0096] and hence different power outputs, along the length of the mineral insulated cable 5.

[0097] The mineral insulated cable 5 of Figure 4 is the same as the mineral insulated cable 5 of Figure 3, except for the size, position and orientation of the openings 11. In particular, the slots are longitudinally-extending, meaning that they extend along the central axis A only. The slots are again arranged in parallel, but some groups of slots are arranged in rows around the circumference of the resistive tube 12.

[0098] In this embodiment all of the openings 11 are the same size and shape, and the distribution of openings 11 varies along the length of the resistive tube 12 such that in the middle there is a lower density of openings 11 while on either side thereof there is a greater density of openings 11. This again achieves different resistances, and hence different power outputs, along the length of the mineral insulated cable 5.

[0099] The mineral insulated cable 5 of Figure 5 is the same as the mineral insulated cables 5 of Figures 1 to 4, except that the resistive tube 12 is in the form of a mesh of interlaced strands, and the openings 11 are in the form of gaps defined between the strands of the mesh. The mesh takes the form of a wire mesh, i. e., made up of wire strands, formed / bent into a cylinder, i. e., into a cylindrical mesh. The mesh is a hexagonal mesh such that each opening (i. e., gap) 11 has a hexagonal cross-section (i. e., is hexagonal).

[0100] Each hexagon has the same size and orientation, and the distribution of openings 11 is the same across the length of the resistive tube 12, such that the power output along the length of the resistive tube 12 is substantially the same.The mineral insulated cable 5 of Figure 6 is the same as the mineral insulated cable 5 of Figure 5, except that it includes two different kinds of hexagonal mesh, one with larger hexagonal openings 11 at the centre and another with smaller hexagonal openings 11 on either side thereof. Since there is less resistive tube 12 in the middle compared to the sides, there is higher resistance here. This again achieves different resistances, and hence different power outputs, along the length of the mineral insulated cable 5.

[0101] The mineral insulated cable 5 of Figure 7 is the same as the mineral insulated cables 5 of Figures 1 to 4, except that it includes some of the circular openings 11 of Figures 1 and 2, some of the diagonally-extending slots of Figure 3 and some of the longitudinally-extending slots of Figure 4. By varying the size, shape, position, orientation, number and / or distribution of the openings 11 along the length of the resistive tube 12 in this way, it is possible to carefully control the resistance of the resistivity, and hence the power output, at every point along the length of the cable 5.

[0102] The mineral insulated cable 5 of Figure 8 is the same as the mineral insulated cable 5 of Figure 1, except that the elongate core 10 does not include a barrier layer or sleeve 15. The barrier layer 15 is not needed in this embodiment because the semi-conducting filler 14 is maintained within the elongate core 10 without it. For example, the semi-conducting filler 14 may be a non-powder semi-conducting filler or a powder material made into (e. g., un- ) crushable blocks that do not leak out of the openings 11 when crushed.

[0103] In addition to the above, it is noted that a resistive tube 12 may have openings 11 of any shape. For example, the openings 11 may be round (e. g., circular orSP3225

[0104] 18

[0105] elliptical ) or polygonal ( e. g., triangular, square, rectangular, pentagonal hexagonal etc ).

[0106] In terms of size, each opening 11 has a maximum surface dimension which is preferably between 1mm and 150mm. When the openings 11 are slots, i. e., elliptical or rectangular, they can be understood as being defined by a ( surface ) length and a ( surface ) width, where the length is longer than the width. In these embodiment s, the maximum surface dimension corresponds to the length of the slot. For all other shapes, the maximum surface dimension may be the ( surface ) diameter of each opening 11. When such openings 11 are non-circular, the maximum surface dimension may be the diameter of a circle circumscribing the opening 11.

[0107] Each opening 11 is also defined by a radial depth. The depth is the radial distance that the opening 11 extends through the resistive tube 12 between the outer and inner surface thereof, i. e., corresponding to the wall thickne ss of the resi stive tube 12. The openings 11 are preferably substantially uniform along their radial depth.

[0108] When the resistive tube 12 is in the form of a mesh, the res istive tube 12 may include round, triangular, square, rectangular and / or hexagonal etc. me shes to provide circular, triangular, square, rectangular and / or hexagonal etc. openings ( i. e., gaps ) 11 respectively.

[0109] Another way of varying the re sistance of the resistive tube 12, and hence the power output of the mineral insulated cable 5, along it s length i s to vary the size, thicknes s and / or material of the resistive tube 12 along the length of the resi stive tube 12. This may be used in addition to, or instead of, the opening ( s ) 11 described above.

[0110] In terms of materials, the re sistive tube 12 is preferably made out of a high resistivity material. InSP3225

[0111] - 19 -general, electrical resistivity is a fundamental specific property of a material that measures its electrical resistance or how strongly it resists electric current. A low resistivity indicates a material that readily allows electric current, while high resistivity indicates a material that resists the flow of electron, giving off heat as a result.

[0112] To this end, the resistive tube 12 is preferably made of a metal or alloy, in particular those with high resistivity such as tungsten (W), iron ( Fe), constantan (Cu-Ni alloy), chromium (Cr), nichrome (Ni-Cr alloy) or other alloys found in electrical resistive heating devices. Such metals and alloys are additionally advantageous because they are relatively easy to form tubes 12 or meshes out of them. By selecting materials with slightly higher resistivity, it is possible to further increase the heat output of the mineral insulated cable 5.

[0113] The resistivity of the resistive tube 12 at 20 °C may be between 0.05 μΩ·m and 5 μΩ·m, preferably between 0.1 μΩ·m and 5 μΩ·m, more preferably between 0.3 μΩ·m and 5 μΩ·m, and more preferably between 0.5 μΩ·m and 5 μΩ·m.

[0114] Metal / Alloy Approximate resistivity (μΩ·m) Tungsten (W) 0. 056

[0115] Iron (Fe) 0. 097 Constantan (Cu-Ni alloy) 0. 49

[0116] Chromium (Cr) 0. 129

[0117]

[0118] Nichrome (Ni-Cr alloy) 1. 00

[0119] As stated above, the semi-conducting filler 14 is packed into the bore of the resistive tube 12 and is in electrical contact with the resistive tube 12 preferably along the entire length of the elongate core 10. Moreover, since the internal bore of the resistive tube 12 is substantially cylindrical, i. e., with a substantiallySP3225

[0120] 20

[0121] circular cross-section, the semi-conducting filler 14 packed into the internal bore is also substantially cylindrical, i. e., with a substantially circular crosssection. The diameter of the semi-conducting filler 14 may therefore also be in a range of between about 3.5 mm and 38 mm, preferably between about 5 mm and 38 mm, more preferably between about 10 mm and 38 mm. Furthermore, the semi-conducting filler 14 is also arranged concentrically on the central axis A.

[0122] The semi-conducting filler 14 is a semiconductor. Semiconductors are materials which have a conductivity between conductors (generally metals ) and non-conductors or insulators ( such as most ceramics ). Semiconductors can be pure elements, such as silicon or germanium, or compounds such as silicon carbide, silicon nitride, gallium nitride or iron oxide, or mixtures of two or more pure and / or compound semiconductors. Small amounts of impurities may be added to pure semiconductors, to cause large changes in the conductivity of the material.

[0123] Since semiconductors have an intermediate-sized nonzero band gap, they have a negative temperature coefficient. This means that they behave as an insulator at lower temperatures (e. g., 20 °C), but allow thermal excitation of electrons into their conduction bands at elevated temperatures (but below its melting point), thereby reducing the resistance of the material and making it more conductive at elevated temperatures. As the semiconducting filler 14 is in electrical contact with the wall along the length of the resistive tube 12, a current flowing through the elongate core 10 in the longitudinal direction, i. e. along the central axis A, will continuously partition its flow between the resistive tube 12 and the semi-conducting filler 14 to minimise its overall resistance through the core 10. When theSP3225

[0124] 21

[0125] temperature of the elongate core 10 is low, the elongate core 10 has high resistivity and so produces heat as current passes therethrough. However, due to the negative temperature coefficient of the semi-conducting filler 14, at elevated temperatures the resistivity of the semiconducting filler 14, and hence the elongate core 10, decreases and so the temperature of the elongate core 10 decreases also. In this way, the mineral insulated cable 5 is able to effectively reduce heat production locally at elevated temperatures, which advantageously avoids hotspots developing along the length of the cable 5.

[0126] The result is a self-regulating mineral insulated heating cable 5. The heat that is dissipated per unit length in any section of the cable 5 by the electrical current is proportional to local resistance of the core 10 in that section of the cable 5. Should the temperature in a certain section of the cable 5 exceed a certain predetermined elevated temperature (i. e., as a "hotspot"), then the resistivity of the core 10, in that section, will drop and thus also the dissipation rate of heat will drop in that section. The local resistance drop thus proportionally reduces the power dissipation in that section thereby reducing the local temperature at the hotspot to a temperature closer to the design operating temperature (at which the mineral insulated cable 5 is designed to work under normal operating conditions. ). This phenomenon may be referred to as Local Resistance Reduction (LRR). At the hotspot, which is typically a limited section along the length of the mineral insulated heating cable 5, the local electric resistance decreases significantly and the heat generation rate reduces proportionally, thereby reducing the local temperature at the hotspot. The LRR ratio, at any location along the length of the cable 5, is defined as the power that wouldhave been dissipated in the resistive tube 12 if there were no filler material inside the bore ( i. e. if the total current would have been passed through the resistive tube 12 at that location), over the reduced power actually dissipated (as part of the total current flows through the semi-conducting filler 14 rather than through the resistive tube 12 alone).

[0127] Avoiding of local overheating of the cable 5 (local hot spots ) has many advantages, one of which is to avoid damage to the electrically insulating layer 16 which surrounds the core 10 by ensuring the insulating properties are not compromised by overheating. Another advantage of local resistance reduction in hotspots is that the cable 5 can be continuously operated at or as close as possible to the predetermined temperature of a selected heating application, while not damaging the cable 5 or the substance that is being heated.

[0128] The semi-conducting filler 14 may be selected from a variety of materials and does not need to be ferromagnetic. In an embodiment, the semi-conducting material comprises a ceramic material. The semi-conducting filler material 14 may comprise one or more of: germanium, silicon, gallium arsenide, gallium phosphide, cadmium sulfide, silicon carbide, gallium nitride, silicon nitride, boron nitride, barium titanate, boron carbide, spinel and at least one metal oxide including iron oxide, nickel oxide, magnesium oxide, alumina, and copper oxide. In some embodiments, the semi-conducting filler material 14 is in the form of a crystalline powder. The semiconductor material may be doped, but preferably it is undoped to achieve the best LRR ratio of heat production due to employing the maximum possible conductivity contrast of the filler material 14.The resistivity of the semi-conducting filler 14 at 20 °C may be higher than the resistivity of the resistive tube 12 at 20 °C. The resistivity of the semi-conducting filler 14 is preferably at least 1000 μΩ·m at 20 °C.

[0129] When the resistivity of the semi-conducting filler 14 at 20 °C is higher (e. g., orders of magnitude higher) than the first resistivity of the metal material of the resistive tube 12 at 20 °C, electric current supplied to the elongate core 10 preferentially flows through the resistive tube 12 over the semi-conducting filler 14.

[0130] Accordingly, the effective resistance of the elongate core 10 as a whole will be practically equal to the resistance of the resistive tube 12. However, with increasing temperature, a larger number of electrons in the semiconducting filler material 14 will have enough thermal energy to surmount the band gap between the material' s valance band and conduction band and become conduction electrons. This causes a reduction of resistance of the semi-conducting filler material 14 and accordingly an overall reduction of the resistance of the core 10, as a higher fraction of the total current will partition through the semi-conducting filler 14. When that happens in any section of the cable 5, the core resistance in that section will drop due to the current now being able to pass through the filler 14 instead of the resistive tube 12. This will have a limiting effect on the amount of heat that can be generated.

[0131] Furthermore, the resistivity of the filler 14 at a temperature of operation above a predetermined elevated temperature (e. g., between 1 °C and 100 °C above a design operating temperature ) may be lower than the resistivity of the resistive tube 12 at said temperature of operation. The resistivity of the filler 14 may be ten times lower than the resistivity of the resistive tube 12 at saidtemperature of operation. The predetermined elevated temperature is a design parameter which may be based on requirements of a selected heating application.

[0132] Depending on whether the cable 5 is powered by current control or voltage control, the total current through the heater cable 5 may remain the same or increase slightly (due to a slight reduction of overall series resistance of the cable 5 when the resistance in a local section drops ), and thus heat continues to be dissipated in the remaining sections of the cable 5 which do not exceed the predetermined temperature. The local resistance drop is independent from current frequency and advantageously it works with DC current so that reactive power loss can be avoided and all power can be used to heat up a substance.

[0133] Turning now to the electrically insulating layer 16, which is provided to both electrically insulate the elongate core 10 and facilitate conduction of heat between the elongate core 10 and the outer sheet.

[0134] To this end, the electrically insulating layer 16 comprises a mineral material, and so has a much higher resistivity than the semi-conducting filler 14. As such, the electrically insulating layer 16 functions as an electric insulator even at elevated temperatures. To this end, the electric bandgap of the semi-conducting material may be smaller than the electric bandgap of the electrically insulating layer 16. For example, the electric bandgap of the semi-conducting material may be 2eV and the electric bandgap of the mineral material may be 5eV.

[0135] The electrically insulating layer 16 may comprise at least one mineral material including magnesium oxide, alumina, zirconia, beryllium oxide and spinel. Magnesium oxide is particularly advantageous because it providesgood thermal conductivity and good electrical insulation properties such as low leakage current and high dielectric strength. A low leakage current is advantageous because it decreases the possibility of thermal breakdown and a high dielectric strength is advantageous because it decreases the possibility of arcing across the insulator. Thermal breakdown can otherwise occur if the leakage current causes a progressive rise in the temperature of the insulator leading also to arcing thereacross.

[0136] The thickness of the electrically insulating layer 16 is predominantly determined by the maximum desired break down voltage between the core 10 and the outer sheath 18 and by the desired insulating properties of the layer 16. However, for certain high-voltage applications (for example, potential difference of up to 10 kV), and a 85% compacted MgO as insulating layer 16, the thickness may need to be up to 25 mm. For most applications, the thickness range of the electrically insulating layer 16 is from about 4 mm to 25 mm, preferably from about 9 mm to 25 mm.

[0137] Turning now to the outer sheath 18, which provides protection to the mineral insulated cable 5. To this end, the outer sheath 18 is preferably metallic and preferably made of a chemically resistant and mechanically robust material, including at operating temperatures and in contact with the substances to be heated. Alloys that may be used in a desired operating temperature range of the cable 5 include, but are not limited to, 304 stainless steel, 310 stainless steel, Incoloy® 800, and Inconel® 600 ( Inco Alloys International, Huntington, W. Va., U. S. A. ). The outer sheath 18 may itself be coated with one or more protective coating layers.

[0138] The thickness of the outer sheath 18 may have to be sufficient to last for three to ten years in a hot andcorrosive environment. To this end, the thickness may be in a range of between about 1 mm and about 3. 5 mm. Larger or smaller thicknesses may be used, to meet specific application requirements.

[0139] The mineral insulated cable 5 described above is preferably used to heat substances, i. e., to function as a heater.

[0140] A method of heating a substance may include providing the mineral insulated cable 5 and passing an electrical current through the elongated core 10 in a direction along the central axis A. To this end, a current supply (not shown) may be brought into electrical connection with the elongate core 10 of the mineral insulated cable 5. For any substance brought into contact with the mineral insulated cable 5, heat may be transferred from the elongated core 10 to the substance when the current is passed through the core 10.

[0141] Hence, in use, the substance to be heated will be in heat exchanging contact with the mineral insulated cable 5, while an electrical current passes through the elongate core 10 which resistively heats the cable 5. Heat is then transferred from the cable 5 to the substance. Local overheating of the cable 5 ( in a hotspot) is avoided by the provision of self -regulating local resistance reduction anywhere within the cable 5 as described above. Heat exchanging contact may be achieved through direct physical contact or through indirect contact via one or more other intermediate materials.

[0142] Advantageously, the mineral insulated cable 5 described above can be used to perform in-situ thermally assisted serpentinization, i. e., for heating mafic or ultramafic rock formations to generate hydrogen. In particular, the method of heating a rock formation for generating hydrogen may include providing the mineral27

[0143] insulated cable 5 inside a wellbore in a mafic or ultramafic rock formation, passing an electrical current from a power supply through the elongate core 10 in a direction along the central axis A; and transferring heat from the elongate core 10 to the rock formation.

[0144] The high-power output of the mineral insulated cable 5 described above means that high power serpentinization reactions can be performed by these mineral insulated heater cable 5, and high yields of hydrogen from the rock formations are possible

[0145] Now a method of manufacturing the mineral insulated cable 5 described above will be provided. To provide a high-level summary thereof, first an intermediate assembly is formed, and then the intermediate assembly is subj ected to diameter reduction to form the mineral insulated cable 5.

[0146] In more detail, first an elongate core 10 on a central axis A, an electrically insulating layer 16 and an outer sheath 18 are provided. Then, the electrically insulating layer 16 is arranged to concentrically envelop around the elongate core 10, and the outer sheath 18 is arranged to concentrically envelop around the electrically insulating layer 16, thereby forming the intermediate assembly.

[0147] Thereafter, the intermediate assembly is subj ected to diameter reduction. This preferably involves alternating steps of mechanically working and heat treating the intermediate assembly, which causes a compaction of the ceramic material in the electrically insulating layer 16. The target compaction is defined by the desired break down voltage for the cable 5. As a rule of thumb, the target compaction is typically 85% or higher, where 100% compaction is equal to the density ofSP3225

[0148] 28

[0149] crystal material. Usually, a diameter reduction of between 10% and 30% suffices to achieve the target compaction.

[0150] More details can be found in, for example, US pat.

[0151] 10,119,366, which describes a manufacturing process in detail. Reference is also made to Chapter 16 of the Electric Cables Handbook / BICC Cables (3rdedition edited by G. G. Moore, Blackwell Science Ltd., 1997 ).

[0152] In one example of a process to make (form) the mineral insulated cable 5, the outer sheath 18 of the cable 5 starts as a strip of electrically conducting material ( for example, stainless steel), which is formed (e. g., longitudinally rolled) into a partial cylindrical shape. Thereafter, blocks of electrically insulating material ( for example, magnesium oxide blocks ) are inserted into the partially cylindrical sheath 18 - the blocks may be partial cylinder blocks such as halfcylinder blocks. Following the insertion of the blocks, the elongate core 10 is placed in the partially cylindrical sheath 18 and inside the half -cylinder blocks of electrically insulating material. Then, the portion of the sheath 18 containing the blocks and the core 10 may be formed into a complete cylinder around the blocks and the core 10. The longitudinal edges of the strip may be welded to close the cylinder sheath 18 and form the mineral insulated cable 5 with the core 10 and electrical insulator layer 16 enveloping the core 10 inside the sheath 18. This process may then be repeated along a length of sheath 18, to form the intermediate assembly in a desired length.

[0153] To form the elongate core 10, a resistive tube 12 and a semi-conducting filler 14 is first provided, and the semi-conducting filler 12 is then packed into the internal bore of the resistive tube 12 such that the semi-SP3225

[0154] 29

[0155] conducting filler 12 is in electrical contact with the resistive tube 12 along a length thereof.

[0156] To manufacture the resistive tube 12 in the form of a cylinder sheet with perforation ( s ) 11, the method may comprise providing a sheet, boring perforation ( s ) 11 into the sheet, and forming the sheet into a cylinder.

[0157] Alternatively, providing a resistive tube may comprise providing a sheet, forming the sheet into a cylinder, and boring perforation ( s ) 11 into the cylinder. When the resistive tube 12 is in the form of a mesh, the method may comprise providing a mesh of interlaced strands, and forming the mesh into a cylinder.

[0158] There are multiple options to accomplish the packing of the filler material 14 in the internal bore of the resistive tube 12. Three examples are briefly discussed. The first is to provide a tube 12 of resistive material in preferentially vertical arrangement and fill the internal bore of the tube 12 from the top with a powder of the semi-conducting filler material 14. Vibration and / or ramming may be applied, to more effectively pack the powder within the bore. The elongate core 10 thus provided has a predetermined length.

[0159] The second example of manufacturing the elongate core 10 is similar as above wherein, instead of powder, macroscopic consolidated blocks (e. g. cylindrical blocks ) of the semi-conducting filler material 14 are inserted in the internal bore of the tube 12. In this example, the tube 12 may be oriented horizontally. Preferably, the macroscopic consolidated blocks fit snugly inside the bore. Small gaps are acceptable as these may disappear in the subsequent reduction steps. The elongate core 10 thus provided has a predetermined length.

[0160] The third example is a semi-continuous process wherein the resistive material is provided in the form ofSP3225

[0161] 30

[0162] a strip, and subsequently formed around macroscopic consolidated blocks (e. g. cylindrical blocks ) of the semiconducting filler material 14 much like how the metallic sheath 18 is formed around the mineral insulating material as described above. The resistive tube 12 may optionally be welded by the meeting long edges, but in some embodiments welding is not needed. The resulting elongate core 10 made by this example may be indeterminate in length.

[0163] The macroscopic consolidation of the semi-conducting filler material 14 in the second and third examples may be achieved by sintering.

[0164] After the intermediate assembly is formed, further steps may be taken to reduce gaps and / or porosity in the assembly and increase the breakdown voltage. The intermediate assembly may be moved through a progressive reduction system (cold working system) to reduce gaps in the assembly. One example of a progressive reduction system is a roller system. In the roller system, the intermediate assembly may progress through multiple horizontal and vertical rollers with the assembly alternating between horizontal and vertical rollers. The rollers may progressively reduce the size of the intermediate assembly into the final mineral insulated cable 5. Alternatively, the reduction may be achieved in a drawbench drawing process wherein the intermediate assembly is pulled through a successive series of draw dies.

[0165] The mineral insulated cable assembly is preferably heat treated (annealed) between reduction steps. Without wishing to be bound by theory, heat treatment (annealing) of the assembly is believed to help to regain mechanical properties of the metal ( s ) used in the mineral insulated cable 5. Heat treatment (annealing) of the cable may beSP3225

[0166] 31

[0167] described as heat treatment that relieves stress and returns a material (for example, a metal alloy material ) back to its natural state (for example, a state of the alloy material before any cold working or heat treating of the alloy material). For example, as austenitic stainless steels are cold worked, they may become stronger but more brittle until a state is reached where additional cold work may cause the material to break because of its brittleness. The strength of an annealed material, and the strength that may be achieved through cold working before failure may depend (vary) based on the material being treated.

[0168] In some embodiments, heat treatment allows for further reduction (cold working) of the mineral insulated cable 5. For example, the mineral insulated cable assembly 5 may be heat treated to reduce stresses in metal in the assembly after cold working and improve the cold working (progressive reduction) properties of the metal.

[0169] Metal alloys (for example, stainless steel used as the sheath 18 ) in the mineral insulated cable 5 may need to be quenched quickly after being heat treated. The metal alloys may be quenched quickly to solidify the alloy while the components are still in solution rather than allowing the components to form crystals, which may not contribute as needed to the mechanical properties of the metal alloy. During quenching, the metal sheath 18 may be cooled down first, and then heat is more gradually transferred from the inside of the cable 5 through the sheath 18. Thus, the metal sheath 18 contracts and squeezes the electrically insulating layer 16 ( for example, the MgO), which further compacts the electrically insulating layer 16.

[0170] As the electrically insulating layer 16 and the elongate core 10 cool, they contract and may leave small voids and may relieve pressure from, for example, seamsbetween electrical insulator blocks inside the mineral insulated cable assembly 5. The small voids or seams may contribute to increased pore volume and / or porosity in the electrically insulating layer 16, and may have an adverse effect on the dielectric breakdown voltage. For example, heat treatment may reduce the breakdown voltage by about 50% or more for typical heat treatments of metals used in the mineral insulated cable 5 described herein. Such reductions in the breakdown voltage may produce shorts or other electrical breakdowns when the mineral insulated cable 5 is used at medium to high voltages (for example, voltages of about 5 kV or higher). A final reduction (cold working) of the mineral insulated cable 5, after heat treatment, may be applied to restore breakdown voltages to acceptable values for long length heaters. The final reduction, however, should preferably not be as large a reduction as previous reductions, to avoid straining or over-straining the metal in the cable assembly 5 beyond acceptable limits. Too much reduction in the final reduction may result in an additional heat treatment being needed to restore mechanical properties to the metals in the mineral insulated cable 5. Thus, the final reduction (cold working) step may reduce a cross-sectional area of the mineral insulated cable 5 enough to compress the electrical insulator and reduce or essentially eliminate voids in the electrical insulator ( for example, decrease pore volume and / or porosity) to restore breakdown voltage properties of the electrical insulator to desirable levels.

[0171] While many possible variations of methods for generating hydrogen have been described above, it will be clear to the skilled person that additional variations and modifications can be made without departing from the scope of the invention as claimed in the appended claims.

Claims

SP3153C L A I M S1. A mineral insulated cable ( 5 ), compris ing:- an elongate core ( 10 ) comprising:- a resistive tube ( 12 ) extending along a central axis (A) of the mineral insulated cable ( 5 ), wherein the resi stive tube ( 12 ) comprise s an outer surface and an inner surface, the inner surface defining an internal bore that also extends along the central axis (A); and - a semi-conducting filler ( 14 ) packed into the internal bore of the resistive tube ( 12 ) and in electrical contact with the resistive tube ( 12 ) along a length thereof;- an electrically insulating layer ( 16 ) concentrically enveloping around the elongate core ( 10 ), the electrically insulating layer ( 16 ) compri sing a mineral material; and - an outer sheath ( 18 ) concentrically enveloping around the electrically insulating layer ( 16 );wherein an opening ( 11 ) is defined in the re si stive tube ( 12 ), the opening ( 11 ) extending between the outer and inner surfaces of the resistive tube ( 12 ).

2. The mineral insulated cable ( 5 ) of claim 1, wherein the opening ( 11 ) is completely enclosed by at lea st a portion of the res istive tube ( 12 ).

3. The mineral insulated cable ( 5 ) of claim 1 or claim 2, wherein a plurality of openings ( 11 ) are defined in the resistive tube ( 12 ), each opening ( 11 ) extending between the outer and inner surfaces of the resistive tube ( 12 ), and wherein each opening ( 11 ) is separate and distinct.

4. The mineral insulated cable ( 5 ) of claim 3, wherein the size, shape, position, orientation, number and / or distribution of the openings ( 11 ) varies along the length of the res istive tube ( 12 ).

5. The mineral insulated cable ( 5 ) of any preceding claim, wherein the res istive tube ( 12 ) comprises a cylindrical sheet, and wherein the at least one opening ( 11 ) compri ses at least one perforation def ined in the cylindrical sheet.

6. The mineral insulated cable ( 5 ) of any preceding claim, wherein the res istive tube ( 12 ) comprises a mesh of interlaced strands, and wherein the at least one opening ( 11 ) comprises a plurality of gaps defined between the strands of the mesh.

7. The mineral insulated cable ( 5 ) of any preceding claim, wherein the at least one opening ( 11 ) comprises a circular opening, an elliptical opening, a triangular opening, a square opening, a rectangular opening, a pentagonal opening and / or a hexagonal opening.

8. The mineral insulated cable ( 5 ) of any preceding claim, wherein the or each opening ( 11 ) is defined by a maximum surface dimension, and wherein the maximum surface dimension is between 1mm and 150mm.

9. The mineral insulated cable ( 5 ) of any preceding claim, wherein the size, thicknes s and / or material of the resistive tube ( 12 ) varies along the length of the resistive tube ( 12 ).

10. The mineral insulated cable ( 5 ) of any preceding claim, wherein the elongate core ( 10 ) further compri ses abarrier layer ( 15 ) concentrically enveloping around the resistive tube ( 12 ), preferably wherein the barrier layer ( 15 ) is a flexible ceramic fabric such as 3M™ Nextel™.

11. The mineral insulated cable ( 5 ) of any preceding claim, wherein a second resi stivity of the filler ( 14 ) at 20 ° C is higher than a first resistivity of the resistive tube ( 12 ) at 20 ° C, optionally wherein the second resistivity of the semi-conducting filler ( 14 ) is at least 1000 μΩ·m at 20 ° C and the first res istivity of the resistive tube ( 12 ) i s between 0.05 μΩ·m and 5 μΩ·m at 20 ° C.

12. The mineral insulated cable ( 5 ) of claim 11, wherein the second resi stivity of the filler ( 14 ) at a temperature of operation above a predetermined elevated temperature is lower than the first resistivity of the resistive tube ( 12 ) at said temperature of operation.

13. A method of heating a substance, comprising:- providing a mineral insulated cable ( 5 ) according to any preceding claim;- bringing the mineral insulated cable ( 5 ) in heat exchanging contact with a substance to be heated;- pa s sing an electrical current through the elongate core ( 10 ) in a direction along the central axi s (A); and- transferring heat from the elongate core ( 10 ) to the substance.

14. A method of manufacturing a mineral insulated cable ( 5 ), comprising:- providing a res istive tube ( 12 ), a semi-conducting filler ( 14 ), an electrically insulating layer ( 16 ) compris ing a mineral material, and an outer sheath,SP322536wherein the resistive tube ( 12 ) extends along a central axis (A) and comprises an outer surface and an inner surface, the inner surface defining an internal bore of the resistive tube ( 12 ) that also extends along the central axis (A), and wherein an opening ( 11 ) is defined in the resistive tube ( 12 ), the opening ( 11 ) extending between the outer and inner surfaces thereof;- manufacturing an intermediate assembly comprising:- forming an elongate core by packing the semiconducting filler into the internal bore of the resistive tube such that the semi-conducting filler is in electrical contact with the resistive tube ( 12 ) along a length thereof;- concentrically enveloping the electrically insulating layer around the elongate core; and- concentrically enveloping the outer sheath around the electrically insulating layer; and- subj ecting the intermediate assembly to diameter reduction, comprising mechanically working and / or heat treating the intermediate assembly.

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