GB2491794A

GB2491794A - Orientating and supporting a casing of a geothermal borehole heat exchanger

Info

Publication number: GB2491794A
Application number: GB1218685.4A
Authority: GB
Inventors: James Jonathan Stewart; Michael Lewis Ardis
Original assignee: Greenfield Master IPCO Ltd
Current assignee: Greenfield Master IPCO Ltd
Priority date: 2011-11-11
Filing date: 2012-09-07
Publication date: 2012-12-12
Anticipated expiration: 2032-09-07
Also published as: GB2491794B; WO2013068570A2; EP2776766A2; WO2013068570A3; JP2014533332A; US20140299291A1; GB2491746A; GB2491664A; GB201119470D0; CA2854841A1; CN104040270A; GB201218685D0; KR20140127799A; GB201215986D0; GB2491664B; GB2491746B

Abstract

A casing support of a geothermal borehole heat exchanger having an outer casing is provided. The casing support comprises a base support element 30 incorporating an aperture 36 through it, the base support element being arranged to be supported by a ground surface around a borehole. An annular orientation guide element 58 is provided at an upper surface of the base support element and has a central conduit (60, fig.7) communicating with the aperture. The orientation guide element has an upper surface 59 at a selected angle relative to a lower support surface 57 of the base support element. A casing support ring 54 is fitted around an outer casing 46 of a geothermal borehole heat exchanger, the casing support ring being coupled to the orientation guide element to support the casing in the borehole, where the outer casing extends through the ring, the conduit and the aperture. An associated method includes the step of drilling a borehole through the central conduit and the aperture at an orientation preset by the orientation guide element.

Description

en pnd supporting a casing of a coaxial gcgtSma1borchp1 The present invention relates to a casing support and to a method for orienting and supporting a casing of a coaxial geothermal borehole heat exchanger of a geothermal energy system. The method also relates to a geothermal borehole heat exchanger and to a method of installing a geothermal borehole heat exchanger.

Geothermal energy has been exploited around the globe in various forms for power generation and direct heating for more than a century. Typically these installations have been located in areas of volcanic activity where high enthalpy source rocks are located relatively close to or at the Earth's surface e.g. Western USA, Iceland or Philippines. Less well known, but of increasing importance, has been the development in recent decades of low enthalpy geothermal resources through, for example, the application of low temperature turbo-generators and through the use of ground source heat pumps (GSHP) for heating, cooling and thermal energy storage.

The basic principle involved is the use of the stable thermal conditions existing in the ground formations below approximately 10 meters below surface. This stability derives from the mass of the Earth and the geothermal heat flux that originates in the molten core of the Earth.

This heat flux is for all practical purposes renewable and limitless since the molten core of the Earth is sustained by nuclear decay. Under controlled conditions, the ground formations can supply, absorb or store large quantities of thermal energy by means of tubular heat exchangers inserted into the ground and coupled to a heat pump (single-acting or reversible configuration) at surface utilising a working fluid as the heat transfer medium.

It is known to extract low temperature geothermal energy for heating a building by means of an installation of one or more borehole heat exchangers (BHE), each installed in the ground, combined with a heat pump (H***.P) The system applies a reversible refrigeration cycle that operates between the ground and the building's inner space. A variety of specialist arrangements are known that may employ a working fluid in a closed or open circuit. Such systems and methods are known in the art as comprising "Ground Source Heat Pump (GSHP)" technology.

I

It is known to use a vertical BHE which is known variously as the "Coaxial" or "Concentric" configuration. In its basic form this is a tube-in-tube arrangement comprising an outer cylindrical casing that is used to line and support the borehole wall and within which is installed a matching tubing of smaller diameter that is suspended so as to locate its open end a short distance above the bottom of the borehole.

ideally, although not always the case, the inner tube is centralised in the bore of the outer casing so as to facilitate optimisation of the thermal and hydraulic flows in the BHE, The closed loop is then formed by water circulation either down die inner tube and back up the annulus between the inner tube and outer casing or the reverse depending upon the design considerations. The heat transfer is by conduction to the flow of water in the annulus and the efficiency benefits from the larger effective contact area of the water with the ground formations offered by the outer casing, providing the hydraulic conditions are optimised.

The co-axial configuration has not found widespread acceptance to date in the GSHP industry. The reasons for this include higher capital cost and the perception of complexity relative to the U-tube design. Historically, die limited number of co-axial installations has been exclusively carried out by oil and gas and water well drilling contractors with little awareness of the GSHP market, inappropriate price structures and lack of innovation.

Consequently, relatively little research and development on coaxial systems has been carried out i.n support of the GSHP industry in the past. However, this situation is now changing, with a drive towards higher fiRE efficiency to match the requirements of large capacity GSHP installations. As a consequence, the inherent advantages of die coaxial designs are getting increased attention. This is in part also driven by a considerable body of research into large scale geothennal thennal storage applications where the co-axial design is favoured for the same reasons. To date, the application of vertical fiREs, in general, to large scale installations has been in the form of large arrays comprising tens or hundreds of boreholes typically drilled to depths of 50-200m using conventional water well drilling equipment and completed with U-tubes, Because of the need to maintain a minimum separation between the horeholes to avoid thermal interaction, the surface area required can be considerable. The overall efficiency of the drilling and operation of this design approach is low for reasons discussed above.

There have been a number of designs of SUEs over the last 25 years. The majority of close circuit GSHP installations utilise the two main practical designs for a vertical BHE, the first being the so-called U-tube (typically a loop of flexible plastic pipe) and the second being the coaxial (tube in tube) design. The coaxial design is known to have a more thermally efficient geometry, but is less practical for the majority of installers due to requirement for heavy equipment during installation, However, industrial scale projects can support the coaxial design. Both types of these BHE are filled with a working fluid, typically water containing an antifreeze solution, A surface collector system is provided for the SHE installation to gather or distribute thermal energy to or front substantial buildings. Such a surface collector system may consist of up to 100% extra length of pipe in addition to the total vertical pipe kngth provided for the in ground heat transfer process. This additional surface piping causes constant operating losses, such as thermal energy and pressure losses. This in turn requires additional electrical energy for compensation of the operating losses, as well as an increased cost for construction and maintenance of the extensive surface collector systems. This has, for a long time, been a limiting factor for large GSHP installations.

Referring to Figures 1 and 2, there is shown a sectional view of a known installation of a coaxial geothermal borehole heat exchanger (SHE). Figure 1 shows the assembly during installation and Figure 2 shows a BHE and well head after installation. Figure 1 has an inclined a vertical SHE orientation and Figure 2 has a vertical SHE orientation. One or multiple BHFs are typically installed in an inspection chamber 4 pre-set within the ground 2 and below ground level prior to the drilling operation commencing. The chamber 4 includes a concrete base 6 and a sidewall 8 extending upwardly therefront The sidewall 8 may comprise a stack of concrete tubes and may, as shown, include internal access steps 24 within the chamber 4. The borehole heat exchanger is installed, at a selected angle, through the concrete base 6.

During the installation of the coaxial geothermal borehole heat exchanger (BHE), it is standard practice to set, extending through a hole 10 in the base 6, a temporary surface casing 12 that is not cemented in place in the base 6 to enable isolation of the unstable surface rock fonnations to enable drilling of the main borehole to continue. Thi.s temporary casing 12 is designed to be retrieved and reused to reduce costs or it can be left in place depending on time it takes to retrieve and the length used. Typically the length of temporary casing 12 is between 5 and 30 meters, although it may be longer or shorter. The coaxial geothermal borehole heat exchanger (BRE) 14 extends downwardly through the temporary casing 12 to the bottom of the drilled borehole.

As shown in Figure 2, after the main borehole section has been drilled through to the bottom of the borehole, the temporary casing 12 is removed and a permanent casing 12, which surrounds the coaxial BHE 14, is rim through to the bottom of the already drilled borehole and supported by the bottom of the borehole.

As shown in Figure 2, after installation each BHE 14 is connected at its upper end to a well head 16. The well head 16 has fittings to connect to a conduit or fluid flow line 22 connecting to the heat exchanger (not shown) of the geothermal energy system. The chamber 4 is closed with a lid 20 located at its upper end above the sidewall 8. The lid 20 may have a central hole for permitting downward passage there through of lengths of BHE 18 during installation.

Setting outer permanent casing 12 on the borehole bottom is inefficient and can lead to inconsistent BITE lengths due to the borehole filling up with drilled formation cuttings and other solids suspended in the drilling fluid after the drilling operation had ended. In order to accommodate such length variation, a significant additional length of excess borehole is drilled and a number of shorter lengths of casing 12 are used to land the casing at the bottom of the borehole because of the uncertain length of the borehole available.

This known installation procedure adds installation costs due to the time it takes to land the casing and the cost of the shorter outer casing lengths. Furthermore, this known installation procedure does not eliminate the result that variable lengths of BHE can be installed) the length varying between different BITEs within a common geothermal system, which in turn leads to variable flow in each fiRE of the heat exchange fluid caused by the varying pressure loss in each BIlE, The variation in flow in each BIlE can lead to inconsistent SHE.

perfor.nance and can only be eliminated by individually choking flow to each BHE to balance the flow to each BIlE. This adds costs both in.i.nstallation time and equipment.

The inconsistent setting depth of the casing 12 also means that each well head 16 can he at varying heights within the chamber 4. This results in the problem that each connection to the borehole flow line 22 will var, requiring customisation on site.

The present invention aims at least partially to overcome these problems of known installations and casing structures of coaxial geothermal borehole heat exchangers.

The present invention provides a casing support of a geothermal borehole heat exchanger having an outer casing, the casing support comprising a base support element incorporating an aperture therethrough, the base support element being arranged to be supported by a ground surface around a borehole, an annular orientation guide element at an upper surface of the base support element and having a central conduit communicating with the aperture, the orientation guide element having an tipper surface at a selected angle relative to a lower support surface of the base support elenient, and a casing support ring fitted around an outer casing of a geothermal borehole heat exchanger, the casing support ring being coupled to the orientation guide element to support the casing in the borehole, the outer casing extending through the ring, the conduit and the aperture.

Optionally, the base support element and the orientation guide element are integral.

Further optionally, the base support element and the orientation guide element are composed of precast concrete.

The invention also provides a chamber comprising a plurality of casing supports according to the invention, each casing support having a respective borehole heat exchanger extending downwardly therefrom at a respective orientation.

The present invention further provides a method of installing a casing support of a geothermal borehole heat exchanger having an outer casing, the method including the steps of: (a) providing a base support element incorporating an aperture therethrough, the base support element being supported by a ground surface, and an annular orientation guide element at an upper surface of the base support element and having a central conduit communicating with the aperture, the orientation guide element having an upper surface at a selected angle relative to a lower support surface of the base support element; and (b) drilling a borehole through the central conduit and the aperture at an orientation preset by the orientation guide element.

Preferred features of all of these aspects of the present invention are defined in the dependent claims.

The preferred embodiments of the present invention can provide a low cost modu!ar system to enable the outer casing of a coaxial geothermal BHE to be supported from an upper surface, in particular a chamber surface. The coaxial geothermal BHE may in particular being suspended or hung from the bottom wall of the chamber rather than supported by the base of the borehole.

The preferred embodiments of the present invention can also provide that the base of the borehole chamber may be sealed.

The preferred embodiments of the present invention can further provide that the orientation of the borehole and inclination of the borehole is predetermine& which can eliminate the possibility of human error during set up of the drilling process.

The modular system of the preferred embodiments of the present invention also allows ease of manufacturing and installation, since a common set of components can be used for various borehole depths and/or inclinations.

In the preferred embodiments of the present invention, by eliminating the problem of variable height between the upper ends of the plural BREs in a unitary geothermal system, optionally there being plural BHEs in a single chamber, then aH connections between the well heads and the flow lines can be standardized and manufactured off site, reducing installation time and installed costs.

Embodiments of the present invention will now be described by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic sectional view of a known installation of a coaxial geothermal borehole heat exchanger (BHE). illustrated during installation; Figure 2 is a schematic sectional view of the installation of Figure 1 after installation of the well head; Figure 3 is a schematic sectional view of an installation of a coaxial geothermal borehole heat exchanger (BHE), illustrated during installation, in accordance with a first embodiment of the present invention; Figure 4 is a schematic sectional view of an installation of a coaxial geothermal borehole heat exchanger (BHE), illustrated during installation, in accordance with a second embodiment of the present invention; Figure 5 is a schematic exploded sectional view perspective view of chamber components of the installations of Figures 1 and 2; Figure 6 is a schematic section through an orientation guide of the installation of Figure 1; Figure 7 is a schematic section through an orientation guide of the installation of Figures 2; Figure 8 is a schematic sectional view of a modified structure of the installation of Figure 3, illustrated after installation of the well head; Figure 9 is a schematic sectional view of a modified structure of the installation of Figure 4.

illustrated after installation of the well head; Figure 10 is a schematic sectional view of the installation of Figure 4, illustrated during installation of the orientation guide; and Figure 11 is a schematic section through an alternative base support element incorporating plural integral orientation guides according to another embodiment of the invention Referring to Figures 3 and 4, there are shown schematically an installation system of a borehole heat exchanger of a geothermal energy system in accordance with first and second embodiments of the present invention. Figures 3 and 4 show the assembly during installation. The embodiment of Figure 3 has a vertical BHE orientation and the embodiment of Figure 4 has an inclined BHE orientation. In each embodiment, a. BHE is installed in an inspection chamber pre-set within the ground (not shown) and below ground level prior to the drilling operation commencing. The chamber includes a base support element 30 in the form of a plate. The base support element 30 is typically composed of precast concrete. The base support element 30 has an aperture 36 extending therethrough. The base support element 30 has standard dimensions for all borehole inclinations. The reinforced concrete or other material can support up to 15 tons of weight suspended through the aperture 36.

A sidewall 32 extends upwardly from the base support element 30. The sidewall 32 may comprise a stack of concrete tubes and may, as shown, include internal access steps within the chamber. A lid 34 having an access opening 35 is located, on the sidewall 32. The lid 34, sidewall 32 and base support element 30 are shown in exploded form in Figure 5.

The borehole heat exchanger is installed, at a selected angle, through the base support element 30, as described hereinafter.

Referring additionally to Figures 6 and 7, which respectively correspond to the embodiments of Figures 3 and 4, an annular orientation guide element 38, 58 is fitted onto an upper surface of the base support element 30.

In the illustrated embodiments of Figures 1 to 10 the orientation guide element 33, 58 is separate from but fitted to the base support element 30. In general, the orientation guide element 38, 58 is at the upper surface 40 of the base support element 30, and in alternative embodiments the base support element 30 and the orientation guide element 38, 58 are integral, for example the base support element 30 and the orientation guide element 38, 58 being composed of a single body of precast concrete. The single body may include plural orientation guide units, at respective positions and inclinations to the vertical. With such an integral arrangement, the disposing or mounting of the base support element 30 at the bottom of the inspection chamber also simultaneously disposes or mounts the integral orientation guide element 38, 58 at the upper surface 40 of the base support element 30. Figure 11 shows such a structure, with a base support element 130 and integral orientation guide elements 138, 158, which are a single body, for example of precast concrete. Although two integral orientation guide elements are provided, in the single body, any number may be present and formed together with the base support element, and any orientations or combinations of orientations may he provided.

The orientation guide element 38, 58 has a cenfral conduit 52, 60 communicating with the aperture 36. The orientation guide element 38, 58 has an upper surface 39, 59 at a selected angle relative to a lower support surface 37, 57 of the base support element 30. The base support element 30 and the orientation guide element 38, 58 are provided with interlocking elements 48, 50 which mutually fit together to locate the orientation guide element 38, 58 at a preset rotational position, with respect to a longitudinal axis of the aperture 36, relative to the base support element 30. Typically, the interlocking elements 48, 50 comprise male and female elements, The interlocking elements 48, 50 ensure a faihsafe alignment between the base support element 30 and the orientation guide element 38, 58.

The upper surface 39, 59 of the orientation guide element 38. 58 is parallel to, or inclined at an angle to, a lower surface 41, 61 of the orientation guide element 38, 58. In the embodiment of Figures 3 and 6 the upper and lower surfaces of the orientation guide element 38 are parallel whereas in the embodiment of Figures 4 and 7 the upper and lower surfaces of the orientation guide element 58 are mutuafly inclined at an acute angle, in this embodiment degrees. The upper and lower surfaces of the orientation guide element 58 may typically be mutually inclined at any desired angle of from 5 to 45 degrees The orientation guide element 38, 58 is typically composed of pre-cast concrete and has a standard selected borehole inclination, for example zero, 5, 10 or 15 degrees. The reinforced concrete or other material can support up to 15 tons of weight suspended through the aperture 36. :9

A casing support ring 54 is fitted around the upper end 44 of an outer casing 46 of a geothermal borehole heat exchanger. The casing support ring 54 is coupled to the orientation guide element 38, 58 to support the casing 46 in die borehole. The outer casing 46 extends through the ring 54, the conduit 52, 60 and the aperture 36. The casing support ring 54 has an inner annular surface engaging an outer cylindrical surface of an upper end 44 of the outer casing 46. Typically, the inner annular surface of the casing support ring 54 threadably engages the outer cylindrical surface of the upper end 44 of the outer casing 46.

This assembly orients the outer casing 46 at the desired vertical or offvertical orientation, shown by axes B and C in Figures 3 and 4. The chamber has vertical axis A. In the embodiment of Figures 3 and 4, as shown in detail in Figure 10, the casing support ring 54 comprises an inner element 68 mounted around the upper end 72 of the coaxial borehole heat exchanger 80 and an outer landing guide 62 mounted on the orientation guide element 58. Figure 10 shows the arrangement during installation. After installation, the inner element 68 is fitted in the outer landing guide 62. The inner element 68 and the landing guide 62 have complementary outer arid inner conically tapered fitting surfaces 70, 64 to permit the inner element 68 to be downwardly fitted into a conduit 66 of the landing guide 62.

A borehole surface casing 42 surrounds an upper portion of the outer casing 46 of the borehole heat exchanger within the borehole and is fitted to the casing support ring 54, in particular to the outer landing guide 62 of the casing support ring 54, Typically, the borehole surface casing 42 is threadably fitted to an inner annular surface of a downwardly extending flange 75 of the hnding guide 62 In the method of installing die casing support of the geothermal borehole heat exchanger having the outer casing 46, initially die base support element 30 incorporating the aperture 36 therethrough is provided so as to be supported by a ground surface, preferably in a below ground chamber. The annular orientation guide element 38, 58 is fitted onto the upper surface of the base support element 30, the orientation guide element 38, 58 having a central conduit 52, 60 communicating with the aperture 36. The orientation guide element 38, 58 has an upper surface at a selected angle relative to a lower support surface of the base support element 30. A casing support ring 54 is installed so as to be coupled to the orientation guide element 38, 58. A borehole surface casing 42 is fitted to surround an upper portion of the borehole and fitted to the casing support ring 54. A borehole is drilled through the central conduit 52, 60 and the aperture 36 at an orientation preset by the orientation guide element 38, 58 After drilling, the borehole heat exchanger is fined into the borehole and the casing support ring 54 is fined around the outer casing 46 of the geothermal borehole heat exchanger and supports the outer casing 46 in the borehole, the outer casing extending through the ring 54, the conduit 52, 60 and the aperture 36.

In the embodiments of Figures 3 and 4, with ftirther reference to the detail of Figure 10, the casing support ring 54 is installed at the top of the surface casing 42 if the surface casing 42 is not be retrieved after installation. The surface casing 42 is a drilling conductor length of surface casing 42 which is present during drilling and grouting operations to transfer drilling fluids and drilled cuttings to the surface for processing.

The upper end 72 of the coaxial borehole heat exchanger 80 may comprise an additional short joint of casing, typically 50cm in length, for running and installing the inner element 68 in the outer landing guide 62. The well head 60 is then attached to the short joint of casing The inner element 68 transfers the weight of the outer casing 46 string to the outer landing guide 62, and thus to the orientation guide element 38. 58 and then to the base support element 30, the ultimate load bearing support for the outer casing 46 of the BHE.

Referring to the alternative embodiments of Figures 8 and 9,which are modified as compared to Figures 3 and 4 respectively, the borehole surface casing 42 may be only temporary and in position only during installation, and removed after instaIlation In this case, casing support ring 54 comprises the landing guide 62 which is directly fitted, for example by a threaded coupling, to the upper portion 44 of the outer casing 46 of the borehole heat exchanger. in these embodiments, the surface casing 42 is to be retrieved. The casing support ring 54 is installed directly onto the outer casing 46. The casing support ring 54 has a first annular thread for temporarily supporting the borehole surface casing 42 and a second annular thread fitting to the outer casing 46. The casing support ring 54 sits on the orientation guide element 38, 58 and transfers the weight of the outer casing 46 to the orientation guide element 38, 58 and then to the base support element 30, the ultimate load bearing support for the outer casing 46 of the EHE.

The borehole heat exchanger 46 extends downwardly to a depth of greater than 100 metres, optionally from 100 to 200 metres. After installation, a wellhead 60 is fitted to the upper end of the borehole heat exchanger 46 and coupled to flow lines 56 of the geothermal system.

Each casing support has a respective borehole heat exchanger 46 extending downwardly therefrom at a respective orientation, In the geothermal system of plural borehole heat exchangers 46, the orientations of at least some of the borehole heat exchangers 46 are different, each orientation being provided by a corresponding selected orientation of the respective orientation guide element 38, 58. When multiple BHE*'s are installed in a common inspection chamber preset within the ground, the BITEs may have different orientations. The use of multiple horeholes in a single chamber reduces the surface area of the ground required for the boreholes.

When installing the orientation guide element 38, 58 and the casing support ring 54 of any embodiment, the area of contact between the orientation guide element 38, 58 and base support elemer.t 30, and between the casing support ring 54 and the orientation guide element 38, 58 are sealed using a sealing compound. This ensures that surface water coming up from the borehole cannot enter the chamber, thereby protecting the well head and associated connections and flow lines against corrosion.

Other modifications to the various embodiments of the present invention will be apparent to those skilled in the art.

Claims

CLAIMS1. A casing support of a geothermal borehole heat exchanger having an outer casing, the casing support comprising a base support element incorporating an aperture therethrough, the base support element being arranged to be supported by a ground surface around a borehole, an annular orientation guide element at an upper surface of the base support element and having a central conduit communicating with the aperture, the orientation guide element having an upper surface at a selected angle relative to a lower support surface of the base support element, and a casing support ring fitted around an outer casing of a geothermal borehole heat exchanger, the casing support ring being coupled to the orientation guide element to support the casing in the borehole, the outer casing extending through the ring, the conduit and the aperture.
2 A casing support according to claim I wherein the base support element and the orientation guide element are integral.
3. A casing support according to claim 2 wherein the base support element and the orientation guide element are composed of precast concrete.
4. A. casing support according to any one of claims I to 3 wherein the orientation guide element is located at a preset rotational position, with respect to a longitudinal axis of the aperture, relative to the base support element.
5. A casing support according to any one of claims 1 to 4 wherein the upper surface of the orientation guide element is inclined at an angle to a lower surface of the base support elementS & A. casing support according to claim 5 wherein the angle is from 5 to 45 degrees 7. A casing support according to any one of claims I to 6 wherein the casing support ring has an inner annular surface engaging an outer cylindrical, surface of an upper end of the outer casing.8. A casing support according to claim 7 wherein the inner annular surface of the casing support ring threadably engages the outer cylindrical surface of an upper end of the outer casing 9. A casing support according to claim 7 or claim 8 wherein the casing support ring comprises an inner element mounted around the upper end of the coaxial borehole heat exchanger and an outer landing guide mounted on the orientation guide element, the inner element being fitted in the outer landing guide.10. A casing support according to claim. 9 wherein the inner element and the bnding guide have complementary outer and inner tapered fitting surfaces to permit the inner element to be downwardly fitted into the landing guide.11. A casing support according to any one of claims 1 to 10 further comprising a borehole surface casing surrounding an upper portion of the borehole heat exchanger within the borehole and fitted to the casing support ring.12. A casing support according to claim 1 when appendant on claim 9 wherein the borehole surface casing is threadably fitted to an inner annular surface of the landing guide.13 A casing support according to any one of claims I to 12 wherein the borehole heat exchanger extends downwardly to a depth of greater than 100 metres.14. A chamber comprising a plurality of casing supports according to any one of claims 1 to 13 each casing support having a respective borehole heat exchanger extending downwardly therefrom at a respective orientation 15. A chamber according to claim 14 wherein the orientations of at least some of the borehole heat exchangers are different, each orientation being provided by a corresponding selected orientation of the respective orientation guide elementS 16. A method of installing a casing support of a geothermal borehole heat exchanger having an outer casing, the method including the steps of (a) providing a base support element incorporating an aperture therethrough, the base support element being supported by a ground surface, and an annular orientation guide element at an upper surface of the base support element and having a central conduit communicating with the aperture, the orientation guide element having an upper surface at a selected angle relative to a lower support surface of the base support element; and (b) drilling a borehole through the central conduit and the aperture at an orientation preset by the orientation guide element.17. A method according to claim 16 wherein the base support element and the orientation guide element are integral.18. A method according to claim 17 wherein the base support element and the orientation guide element are composed of precast concreteS 19. A method according to any one of claims 16 to 18 wherein the orientation guide element is located at a preset rotational position, with respect to a longitudinal axis of the aperture, relative to the base support element.20. A method according to any one of claims 16 to 19 wherein the upper surface of the orientation guide element is inclined at an angle to a lower surface of the base support element.21. A method according to claim 20 wherein the angle is from 5 to 45 degrees 22. A method according to any one of claims 16 to 21 further comprising installing a casing support ring fitted around an outer casing of a geothermal borehole heat exchanger and coupling the casing support ring to the orientation guide element to support the easing in the borehole, the outer casing extending through the ring, the conduit and the aperture, the easing support ring having an inner annular surface engaging an outer cylindrical surface of an upper end of the outer casing.23. A method according to claim 22 wherein the inner annular surface of the casing support ring threadably engages the outer cylindrical surface of an upper end of the outer casing.24. A method according to claim 22 or claim 23 wherein the casing support ring comprises an inner element mounted around the upper end of the coaxial borehole heat exchanger and an outer landing guide mounted on the orientation guide element, the inner element being fitted in the outer landing guide.25. A method according to claim 24 wherein the inner element and the landing guide have complementary outer and inner tapered fitting surfaces and the inner element is downwardly fitted into the landing guide.26. A method according to any one of claims 22 to 25 further comprising fitting a borehole surface casing to surround an upper portion of the borehole heat exchanger within the borehole and fitted to the casing support ring.27. A method according to claim 26 when appendant on claim 24 wherein the borehole surface casing is threadably fitted to an inner annular surface of the landing guide.28. A method according to any one of claims 16 to 27 wherein the borehole heat exchanger extends downwardly to a depth of greater than 100 metres.29. A. method according to any one of claims 16 to 28 of installing a plurality of casing supports, each casing support having a respective borehole heat exchanger extending downwardly therefrom at a respective orientation.30. A method according to claim 29 wherein the orientations of at least some of the borehole heat exchangers are different, each orientation being provided by a corresponding selected orientation of the respective orientation gui.de element.31. A. method according to claim 29 or claim 30 wherein the upper end of the borehole heat exchanger is instafled at a preset height above the base support element, and further comprising fitting a well head to the upper end of the borehole heat exchanger.