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 (HP). 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.
It is known to use a vertical BHE which is known variously as the “Co-axial” 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 the 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, the 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 co-axial systems has been carried out in support of the GSHP industry in the past. However, this situation is now changing, with a drive towards higher BHE efficiency to match the requirements of large capacity GSHP installations. As a consequence, the inherent advantages of the co-axial designs are getting increased attention. This is in part also driven by a considerable body of research into large scale geothermal thermal storage applications where the co-axial design is favoured for the same reasons. To date, the application of vertical BHEs, 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 50-200 m using conventional water well drilling equipment and completed with U-tubes.
Because of the need to maintain a minimum separation between the boreholes 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 BHEs 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 BHE installation to gather or distribute thermal energy to or from substantial buildings. Such a surface collector system may consist of up to 100% extra length of pipe in addition to the total vertical pipe length 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
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 formations to enable drilling of the main borehole to continue. This 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 (BHE) 14 extends downwardly through the temporary casing 12 to the bottom of the drilled borehole.
As shown in
As shown in
Setting outer permanent casing 12 on the borehole bottom is inefficient and can lead to inconsistent BHE 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 BHEs within a common geothermal system, which in turn leads to variable flow in each BHE of the heat exchange fluid caused by the varying pressure loss in each BHE. The variation in flow in each BHE can lead to inconsistent BHE performance and can only be eliminated by individually choking flow to each BHE to balance the flow to each BHE. This adds costs both in installation time and equipment.
The inconsistent setting depth of the casing 12 also means that each well head 16 can be at varying heights within the chamber 4. This results in the problem that each connection to the borehole flow line 22 will vary, 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 geothermal borehole heat exchanger supported in a borehole by a casing support, the casing support being fitted around an outer casing of the geothermal borehole heat exchanger and suspending the borehole heat exchanger within a borehole extending downwardly from the casing support, the casing support defining a predetermined angle of an upper end of the borehole heat exchanger within the borehole.
The present invention further provides a chamber comprising a plurality of geothermal borehole heat exchangers 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 geothermal borehole heat exchanger, the method including the steps of:
The present invention further 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 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.
The present invention further 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:
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 modular 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 predetermined, 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 BHEs in a unitary geothermal system, optionally there being plural BHEs in a single chamber, then all 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:
Referring to
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
The borehole heat exchanger is installed, at a selected angle, through the base support element 30, as described hereinafter.
Referring additionally to
In the illustrated embodiments of
The orientation guide element 38, 58 has a central 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 fail-safe 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
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.
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 the 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 off-vertical orientation, shown by axes B and C in
In the embodiment of
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 landing guide 62.
In the method of installing the casing support of the geothermal borehole heat exchanger having the outer casing 46, initially the 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 fitted into the borehole and the casing support ring 54 is fitted 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
The upper end 72 of the coaxial borehole heat exchanger 80 may comprise an additional short joint of casing, typically 50 cm 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
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 pre-set within the ground, the BHEs may have different orientations. The use of multiple boreholes 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 element 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.
Number | Date | Country | Kind |
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1119470.1 | Nov 2011 | GB | national |
1215986.9 | Sep 2012 | GB | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP2012/072332 | 11/9/2012 | WO | 00 | 5/6/2014 |