TECHNICAL FIELD

The invention relates to an intermediate welding device for welding together two ends of a coaxial plastic pipe structure comprising at least two coaxial pipes, in one step, and to a method of welding two ends of by using the intermediate welding device.

BACKGROUND

The welding of pipe structures, which comprise at least two coaxial pipes, thus two pipes arranged within each other, poses certain difficulties, such as increased cross section of the plastic pipe structure around the welding region and complicated welding operations involving many manual steps.

Pipe structures or pipes comprising at least two coaxial pipes are, among others, used in gasoline and fuel pipe systems and in the geothermal energy field, specifically for coaxial borehole exchangers.

The document WO99/46532 illustrates a prior art technique of welding pipe structure comprising two coaxial pipes together (Figs.i and 2 of

WO99/46532) and a fusion welding socket for welding together two ends of a plastic pipe, the welding socket being configured to receive the pipe parts to be coupled together with a tight fit (Fig. 3 of WO99/46532). The inside of the fusion welding socket comprises an intermediate part, which is shorter than an outer part of the fusion welding socket and which comprises passageways for bridging the leaking detection space. The intermediate part comprises a smaller inner diameter than the outer part of the welding socket. The outer part and the intermediate part each comprise a coil on the inner side, whereby the outer part comprises a coil in each of the two end regions. The intermediate part receives the ends of the inner pipe up to a stop flange. The outer part receives the ends of the outer pipes, which are shortened, leaving a gap between the intermediate part and the ends of the outer pipes. After fitting the inner - and the outer pipes a voltage can be applied to the coils, which generates heat in the coils and the material surrounding the coils, by heat conduction, and thus welds the welding socket to the inner pipe and the pipe, thereby connecting the pipe ends.

Prior to inserting the ends of the outer pipes into the fusion welding socket, both ends of the outer pipes need to be shortened, either by pushing the outer pipe ends in a longitudinal direction of the outer pipe away from the inner pipe ends or by cutting away a part of the outer pipe ends.

SUMMARY

It is an object of the present invention to provide an intermediate welding device that facilitates the welding process and enhances reliability of the welding.

The above object is solved by an intermediate welding device according to claim l.

Disclosed herein is an intermediate welding device for use when welding together two ends of a coaxial plastic pipe structure in one step. The plastic pipe structure has at least an innermost pipe and an intermediate pipe. The intermediate welding device comprises a hollow section. The hollow section has an inner side configured to receive the innermost pipe with a tight fit and an outer side. The intermediate welding device further comprises a

circumferentially closed inner heating element arranged in order to heat the inner side by thermal conduction, wherein the intermediate welding device further comprises a circumferentially closed outer heating element arranged in order to heat the outer side by thermal conduction, which outer side is configured to glide into the intermediate pipe with a tight fit.

In the following circumferentially closed means that the inner heating element and the outer heating element each form a closed loop on the inner side and the outer side.

In order that the circumferentially closed outer heating element and the circumferentially closed inner heating element may heat the outer side and inner side, respectively, the circumferentially closed outer heating element and the circumferentially closed inner heating element may be arranged on or at least in close proximity of the outer side and inner side, respectively.

The intermediate welding device allows welding the coaxial plastic pipe structure in one step without cutting or preparing the intermediate pipe ends prior to the welding.

The term coaxial means, that the coaxial plastic pipe structure comprises at least two pipes arranged within each other, thus coaxially arranged in respect to each other.

The coaxial pipe structure may be a coaxial borehole heat exchanger. The circumferentially closed inner heating element and the circumferentially closed outer heating element may be arranged axially fully offset relative to each other. Thereby high currents can be used for welding to ensure that the intermediate welding device is properly welded to the innermost pipe and the intermediate pipe of the coaxial plastic pipe structure. Without this offset, the intermediate welding structure could melt during welding with high currents and the large temperatures involved.

The circumferentially closed inner- and outer heating elements may each comprise two circumferentially closed heating portions arranged on either side of a middle portion of the hollow section. The hollow section has a continuous and constant cross section over its entire length, as measured along the longitudinal direction.

Additionally the circumferentially closed inner- and outer heating elements may be arranged on the inner- and outer surface, respectively, of the hollow section or embedded on the inner- and outer side, respectively, of the hollow section.

In an embodiment, the hollow section is a sleeve. The hollow section may have any cross sectional shape, such as rectangular, circular, elliptic, etc., depending on the cross section of the innermost pipes and intermediate pipes of the coaxial plastic pipe structure.

In a further embodiment the hollow section comprises a front end portion and a back end portion, said front- and back end portion being conically shaped with an increasing cross section towards the middle portion of the hollow section.

This facilitates the insertion of the intermediate welding device in the space in between the innermost pipe and the intermediate pipe. The hollow section may comprise at least one longitudinal passage.

Such a longitudinal passage may ensure that the space in between the innermost pipe and the intermediate pipe one side of the intermediate welding device is in fluid communication with the space in between the innermost pipe and the intermediate pipe on the other side of the

intermediate welding device, when the intermediate welding device is in place and welded to the coaxial plastic pipe structure.

In an embodiment the hollow section is made of a weldable plastic.

This may ensure that the plastic of the hollow section, which plastic is heated by heat conduction via the circumferentially closed inner- and outer heating elements, bonds with the plastic of the coaxial plastic pipe structure. The plastic of the coaxial plastic pipe structure is also heated via heat conduction from the circumferentially closed inner- and outer heating elements.

In another embodiment the circumferentially closed outer heating element and the circumferentially closed inner heating element are embodied in the form of outer and inner coils.

By applying a voltage to the terminals of the outer and inner coils, the outer and inner coils are heating up and the intermediate welding device is welded to the coaxial plastic pipe structure. In a preferred embodiment the outer and inner coils are made of a single resistance wire or filament.

Using a single resistance wire or filament has the advantage that only two plus and minus terminals need to be connected to a voltage source in order to energize the circumferentially closed inner- and outer heating elements.

The resistance wire or filament may be arranged on the inner side or on the outer side of the hollow section. In order to facilitate the insertion of the intermediate welding device into the space in between the innermost pipe and the intermediate pipe, the outer and inner coils may be embedded or slightly embedded in the inner- and outer side, respectively.

The outer and inner coils may comprise two terminals, said terminals being arranged in a middle portion of the hollow section on the outer side, configured to extend beyond the intermediate pipe.

This eases the connection of a voltage source to the terminals. The terminals may extend away from the middle portion of the hollow section.

In a further preferred embodiment the outer coil is arranged offset from the inner coil as seen in a direction perpendicular to the longitudinal direction of the intermediate welding device.

This may prevent the melting of the entire cross section of the hollow section when the outer and inner coils are energized.

In another preferred embodiment the circumferentially closed outer heating element and the circumferentially closed inner heating element are embodied in the form of outer and inner induction weldable layers.

Using outer and inner induction weldable layers has the advantage that no terminals need to be connected to a voltage source, thus eliminating one work step. The outer and inner induction weldable layers are energized/heated by applying an electromagnetic field. In a preferred embodiment the outer induction weldable layer is arranged offset from the inner induction weldable layer as seen in a direction perpendicular to the longitudinal direction L of the intermediate welding device. This may prevent the melting of the entire cross section of the hollow section, at least in a region of the induction weldable layers, when said layers are energized.

The outer and inner induction weldable layers may comprise a plurality of susceptors. In another embodiment the outer and inner induction weldable layers may additionally comprises a matrix material.

The susceptors are preferably embedded in the matrix material, thereby forming a paste, whereby this past may be injection moulded. The outer and inner induction weldable layers may be injection moulded at the same time as the hollow section or prior or post to injection moulding the hollow section.

Disclosed herein is further a method for axially welding together two coaxial borehole heat exchanger parts via the intermediate welding device. The coaxial borehole heat exchanger parts each comprising an innermost pipe and an intermediate pipe, the method comprising the steps of:

- removing axial ribs of the intermediate pipes in an end region of each of the coaxial borehole heat exchanger parts;

- inserting the intermediate welding device into a space in between the intermediate pipes and the innermost pipes; and - energizing the circumferentially closed outer- and the

circumferentially closed inner heating element via a power source.

The step of removing the axial ribs may not be necessary in case the intermediate pipe does not comprise axial ribs. This method enables the operator to weld two coaxial plastic pipes, such as the innermost pipes and the intermediate pipes of two coaxial borehole exchanger parts, in one step without prior preparation of the ends of any of the two pipes. - connecting the two terminals to a power source, prior to

energizing the circumferentially closed outer - and the circumferentially closed inner heating element via the power source.

This step may only be necessary if the circumferentially closed inner- and outer heating elements are embodied in the form of outer and inner coils. In case the circumferentially closed inner- and outer heating elements are embodied in the form of outer and inner induction weldable layers, the above step is not necessary.

The power source may provide a voltage or an electromagnetic field. In a further embodiment, the method may additionally comprise the steps of

- installing an outer welding device on outermost pipes of the

coaxial borehole heat exchanger parts; and

- energizing a circumferentially closed inner heating element of the outer welding device via the power source. Since the coaxial borehole heat exchanger part usually comprises a third pipe, such as an outermost pipe, which may also need to be welded together with the outermost pipe of a further coaxial borehole heat exchanger part, the method may include the above steps.

The outer welding device may be a conventional welding sleeve comprising a circumferentially closed inner welding layer on the inside surface of the sleeve.

Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the element, apparatus, component, means, step, portion, etc." are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, portion etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is now described, by way of example, with reference to the accompanying drawings, in which: Fig l schematically illustrates a perspective view of an intermediate welding device according to the invention with a hollow section shown transparent;

Fig. 2 schematically illustrates a front view of the intermediate welding device of figure ι;

Fig. 3 schematically illustrates a side view of the intermediate welding device of figure 1;

Fig. 4 schematically illustrates a view on a cross section of the intermediate welding device cut along line IV-IV of figure 3;

Fig. 5 schematically illustrates a perspective view of an another embodiment of the intermediate welding device according to the invention with the hollow section shown transparent;

Fig. 6 schematically illustrates a front view of the intermediate welding device according to figure 5;

Fig. 7 schematically illustrates a side view on a coaxial borehole heat exchanger; Fig. 8a schematically illustrates a view on a cross section of the coaxial borehole heat exchanger cut along line VIII-VIII of figure 7; Fig. 8b schematically illustrating a welding zone with a part of the

intermediate welding device according to an embodiment of the invention;

Fig. 9 schematically illustrates a view on a cross section of the coaxial borehole heat exchanger cut along ling IX- IX of figure 8a; Fig. 10 schematically illustrates perspective view of a piece of the coaxial borehole heat exchanger cut at line IX-IX of figure 8a;

Fig. 11 schematically illustrates a perspective view of a piece of the coaxial borehole heat exchanger cut at line IX-IX of figure 8a; and

Fig. 12 illustrates a method of welding two coaxial borehole heat exchanger parts together.

DETAILED DESCRIPTION

The invention will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout the description. Figure 1 prospectively illustrates an intermediate welding device 1 according to the invention comprising a hollow section 9 and a circumferentially closed outer heating element 6a and a circumferentially closed inner heating element 6b. For illustrative purposes the hollow section 9, which is preferably made of a weldable plastic is shown transparent in figure 1. In the following circumferentially closed means that the inner heating element 6b and the outer heating element 6a each form a closed loop on the inner side 4 and the outer side 2. The hollow section comprises an outer side 2 and an inner side 4. The outer side 2 and the inner side 4 may alternatively be called outer surface and inner surface. The intermediate welding device 1 and the hollow section 9, respectively further comprise a front end portion 7a and a back end portion 7b.

Still referring to figure 1, the circumferentially closed outer heating element 6a and the circumferentially closed inner heating element 6b are embodied in the form of outer and inner coils 10a, 10b. Each outer coil 10a comprises two circumferentially closed coil portions and each inner coil 10b comprises two circumferentially closed coil portions. The outer coil 10a and the inner coil 10b and the circumferentially closed coil portions are made of a resistance wire or filament 12 and electrically interconnected with each other via the resistance wire or filament 12. The resistance wire or filament 12 is a continuous wire and has a beginning and an end, which beginning and end from the terminals 13a, 13b. The terminals 13a, 13b are extending away from the hollow section 9 in a middle portion 11 of it and are configured to be connected to power source/voltage source (not shown).

Figure 1 illustrates how the resistance wire or filament 12 runs from the beginning and the terminal 13a, respectively around the outer side 2 to form a first circumferentially closed outer portion of the outer coil 10a, is then guided on the hollow section into back end portion 7b and there through the hollow section 9 to the inner side 4, where it forms a first circumferentially closed inner portion of the inner coil 10b. From the first circumferentially closed inner portion of the inner coil 10b the resistance wire or filament is then guided on the inner side 4 to the front end portion 7a where it forms the second circumferentially closed inner portion of the inner coil 10b. From the second circumferentially closed inner portion of the inner coil 10b, the resistance wire or filament 12 is guided through the hollow section 9 where it forms the second circumferentially closed inner portion of the outer coil 10a on the outer side 2. The first and second circumferentially closed inner portions of the outer coil 10a and the first and second circumferentially closed inner portions of the inner coil 10b and the outer and inner coil 10a, 10b, respectively, are arranged offset from each other as seen in a direction perpendicular to the longitudinal direction L (c.f. figure 4) of the intermediate welding device 1.

Each circumferentially closed portion of the inner coil 10b is axially offset from the corresponding circumferentially closed portions of the outer coil 10a. In particular, they are offset in a non-overlapping manner, sequentially along the axial direction. Thus, in general, the circumferentially closed inner heating element and the circumferentially closed outer heating element are arranged axially fully offset relative to each other.

Figure 2 illustrates a front view of the intermediate welding device 1. The terminals 13a, 13b are well visible and, since the hollow body is shown transparent, the passages of the resistance wire of filament 12 (not shown in figure 2) are visible, which passages are arranged in the hollow section 9 at an angle of at least approximately 40 ⁰ from the terminals 13a, 13b, as shown in figure 2. Figure 2 further illustrates an axial passage 8 which is embedded within the hollow section 9 and which axial passage 8 extends from one side of the intermediate welding device 1 to the other side of the intermediate welding device 1. The purpose of this axial passage 8 may be obtained from the description of figure 8a.

From figures 1 and 2 it can be seen, that the front- and back end portion 7a, 7b can be conically shaped (not shown). Thereby the surface of the cross section of the hollow section at one of the two ends is smaller than the cross section of the hollow section closer to the middle section 11. In addition the inner side 4 and the outer side 2 may be conically shaped towards the two ends of the hollow section and in the front and back end portions 7a, 7b.

Figure 3 illustrates a side view of the intermediate welding device 1, illustrating the outer coil 10a with the first and second circumferentially closed coil portions, arranged on the outer side 2. Referring now to figure 4, which illustrates a view on a cross section cut along line IV-IV of figure 3, the inner coil 10b with the first and second

circumferentially closed coil portions, arranged on the inner side 4, is shown. The axial passage 8 running in the longitudinal direction L of the

intermediate welding device 1 is also illustrated in figure 4. As can be seen from figure 4, the outer coil 10a may be embedded in the hollow section 8, this may be done by placing the outer and inner coils 10a, 10b in a form or mould prior to injection moulding of the hollow section 9. This may facilitate the insertion procedure of the intermediate welding device in between two coaxial pipes.

There may be more than one axial passage 8 embedded in the hollow section 9, depending on the requirements. The axial passages 8 may be pipes or little tubes, for example made of steel or plastic, that are embodied in the intermediate welding device 1 during the production and thus during the injection moulding process.

Figure 5 illustrates an alternative embodiment of the intermediate welding device l'comprising the hollow section 9 with the inner side 4 and the outer side 2 and the circumferentially closed outer heating element 6a' and the circumferentially closed inner heating element 6b'. The circumferentially closed outer- and circumferentially closed inner heating element 6a', 6b' are embodied in the form of an outer induction weldable layer 14a and an inner induction weldable layer 14b. The outer induction weldable layer 14a is arranged offset from the inner induction weldable layer 14b. Each induction weldable layer 14a, 14b may comprise a first and a second circumferentially closed induction weldable portion. The first and second circumferentially closed induction weldable portions of the outer induction weldable layer 14a and the inner induction weldable layer 14b are arranged offset from each other, as seen in a direction perpendicular to the longitudinal direction L (c.f figure 4) of the intermediate welding device 1'. The outer and inner induction weldable layers 14a, 14b may comprise a plurality of susceptors and optionally a matrix material. The matrix material may work as a binder that embeds the susceptors. The susceptors are typically made of a metallised film, ceramic or metal such as aluminium flakes. The susceptors have the ability to absorb electromagnetic energy from a power source and turn it into heat. The matrix material may not be needed, since the susceptors may be arranged in a negative form, which is used to injection mould the hollow section 9, prior to the injection moulding of the hollow section 9, so that the susceptors are directly embedded in the plastic of the hollow section 9 during injection moulding. By using outer and inner induction weldable layers 14a, 14b, the terminals 13a, 13b are not required since an electromagnetic power source is used to energize the outer and inner induction weldable layers 14a, 14b.

Figure 6 illustrates a front view of the intermediate welding device (ι') according to figure 5, without terminals 13a, 13b. The operator or welder does not need to connect terminals 13a, 13b to a power source, when the circumferentially closed outer heating element 6a' and the circumferentially closed inner heating element 6b' are embodied in the form of outer and inner induction weldable layers 14a, 14b.

Figure 7 illustrates a coaxial borehole heat exchanger 19 comprising two coaxial borehole heat exchanger parts 20, 20' that are welded together by an outer welding device 30 and an intermediate welding device 1, 1'. The borehole heat exchanger 19 and the borehole heat exchanger parts 20, 20' comprise an innermost pipe 22, 22', an intermediate pipe 24, 24' and an outermost pipe 26, 26', as best illustrated in figures 8a and 9. Figure 8a illustrates a cross sectional view onto the cross section cut along the line VIII-VIII of figure 7. The welding zone of figure 8a and 8b is illustrated with an intermediate welding device 1 according to figures 1 to 4, comprising outer and inner coil 10a, 10b. The welding zone and the welding may however be also achieved and embodied with the welding device 1' according to figures 5 and 6. The coaxial borehole heat exchanger parts 20', 20 may each comprise an end region 28, 28', which end region needs to be interconnected with each other by welding and by using the intermediate welding device 1, 1' according to an embodiment of the invention. The intermediate welding device 1 is inserted into an annular space in between the innermost pipe 22, 22' and the intermediate pipe 24, 24' of the coaxial borehole heat exchanger parts 20, 20'. The two coaxial borehole heat exchanger parts 20, 20' are then pushed towards one another so that the intermediate welding device 1 is gliding into the respective first and second end region 28, 28'. In order to ensure that the intermediate welding device 1 is centrally positioned in between the two coaxial borehole exchanger parts 20, 20', it may comprise a protrusion (indicated in figure 8a) or the like on the inner side 4. The protrusion or the like may result in the gap 46, as illustrated in figure 8 a. The axial passage 8, illustrated in figure 4, may be used to interconnect the annular space in between the innermost pipe 22 and the intermediate pipe 24 of one coaxial borehole heat exchanger part 20 with the annular space in between the innermost pipe 22' and the intermediate pipe 24' of the other coaxial borehole heat exchanger part 20'. Still referring to figure 8a, once the intermediate welding device 1 is in place, the terminals 13a, 13b (not shown in figure 8a) may be connected to the power source to apply a voltage. At this stage the outer welding device 30 is still not in place, so that the operator has easy access to the terminals 13a, 13b. By applying a voltage, the outer- and inner coils 10a, 10b heat up and weld the intermediate welding device 1 to the innermost pipes 22, 22' and the intermediate pipe 24, 24'. The welding zone may be supervised by

temperature measurement and visual control.

After welding of the intermediate welding device 1, the outer welding device 30 is shoved over the outermost pipes 26, 26' and over the first and second end regions 28, 28'. The outer welding device 30 comprises an inner side 32, a circumferentially closed inner heating element 34 arranged on the or at least close to the inner side 32, said inner heating element 34 being embodied in the form of a coil 36 made of a resistance wire or filament 36. The coil 36 may comprise two circumferentially closed coil portions, one on each side of the gap 46, when the outer welding device 30 is placed.

By applying a voltage source to the coil 36 via terminals (not shown in figure 8a) the outer welding device 30 is welded to the outermost pipes 26, 26' of the two coaxial borehole heat exchanger parts 20, 20'.

As best illustrated in figures 7 and 8a, the outer welding device 30 may comprise conically shaped end portions, which end portions are conical towards the outermost pipes 26, 26'.

The circumferentially closed inner heating element 34 of the outer welding device 30 may alternatively be embodied in the form of an induction weldable layer (not shown), of the above described type. In a further embodiment, which is not illustrated in the figures, the outer and inner coils 10a, 10b of the intermediate welding device 1 may be electrically interconnected with the coil 36 of the outer welding device 30 so that the innermost pipes 22, 22', the intermediate pipes 24, 24' and the outermost pipes 26, 26' can be welded in one single step. In case the outer welding device and the intermediate welding device 1' comprise induction weldable layers both solutions are possible, namely firstly welding the intermediate welding device 1' to the innermost pipes 22, 22' and the intermediate pipes 24, 24' and secondly welding the outer welding device to the outer pipes 26, 26' or welding the intermediate welding device 1' and the outer welding device simultaneously.

A simultaneous welding may however not always be wanted for quality reasons. Figure 8b illustrates an enhanced view of the welding zone illustrating the outer welding device 30, the intermediate welding device 1 and the innermost pipe 22, the intermediate pipe 24 and the outermost pipe 26.

Figure 9 illustrates a view on a cross section of the coaxial borehole heat exchanger 19 and the coaxial borehole heat exchanger part 20, respectively, cut along line IX-IX of figure 7.

The innermost pipe 22, the intermediate pipe 24 and the outermost pipe 26 are arranged coaxially with respect to one another. The intermediate pipe 24 comprises a plurality of axial ribs 24a and a plurality of axial grooves 24b. The axial ribs 24a and axial grooves 24b may be formed for insulation purposes.

Similar to the intermediate pipe 24, the outermost pipe 26 may comprise a plurality of axial ribs 26a and a plurality of axial grooves 26b.

In order to fit the intermediate welding device 1, 1' into the annular space, the axial ribs 24a of the intermediate pipes 24, 24' may be removed at least in a first and second end region 28, 28' of the coaxial heat exchanger parts 20, 20'. The removal may be done for example mechanically by grinding.

Figures 10 illustrates a perspective view of a piece of the coaxial borehole heat exchanger part 20 away from the welding zone, cut along line IX-IX of figure 8a, and thus without the intermediate welding device 1.

Figure 11 illustrates a perspective view of a piece of the coaxial borehole heat exchanger part 20 in the welding zone, cut along line XI -XI of figure 8a, and thus with the intermediate welding device 1 visible in embedded in the annular space in between the innermost pipe 22 and the intermediate pipe 24.

Figure 12 illustrates method steps of a method for welding two coaxial borehole heat exchanger parts 20, 20' together. The method comprises the steps of: Removing S01 axial ribs 24a of the intermediate pipes 24, 24' in an end region 28, 28' of each of the coaxial borehole heat exchanger parts 20, 20';

Inserting SO2 the intermediate welding device 1 into the annular space in between the intermediate pipes 24, 24' and the innermost pipes 22,

22';

Connecting S03 the two terminals 13a, 13b to a power source, prior to energizing S04 the circumferentially closed outer - and the

circumferentially closed inner heating element via the power source; and

Energizing S04 the circumferentially closed outer- and the

circumferentially closed inner heating element 6a, 6b via a power source.

It is clear that removing the axial ribs 24a may not be necessary in case the intermediate pipes 24, 24' do not comprise axial ribs 24a. Further the terminals 13a, 13b only need to be connected in case the circumferentially outer- and circumferentially inner heating elements 6a, 6b are embodied in the form of outer and inner coils 10a, 10b, thus when using fusion welding. In case the circumferentially outer- and circumferentially inner heating elements 6a', 6b' are embodied in the form of induction weldable layers 14a, 14b, this step is not necessary, which is indicated in figure 12.

Finally when the above steps are performed, the method may additionally comprise a checking and controlling step (not illustrated in figure 12) and then the further steps of: - Installing S05 an outer welding device 30 on outermost pipes 26, 26' of the coaxial borehole heat exchanger parts 20, 20'; and

Energizing S06 a circumferentially closed inner heating element 34 of the outer welding device 30 via the power source.

The welding of the outermost welding device 30 may additionally also comprise a controlling and checking step after the welding. l8

The invention has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention, as defined by the appended patent claims