Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Orienting and supporting a casing of a coaxial geothermal borehole
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.
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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 50-200m 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.
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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 Figures 1 and 2, there is shown a sectional view of a known
installation of a
coaxial geothermal borehole heat exchanger (BHE). 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 BHE orientation and Figure 2 has a vertical BHE
orientation. One or
multiple BHE's 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 therefrom. 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
formations to enable drilling of the main borehole to continue. This temporary
casing 12 is
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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 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 run 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 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.
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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:
(a) providing a first casing support portion which is supported by a ground
surface
and defines a predetermined drilling angle for a borehole;
(b) drilling a borehole through the first casing support portion, the first
casing support
portion defining a predetermined angle of an upper end of the borehole; and
(c) suspending, from the first casing support portion, a borehole heat
exchanger within
the borehole extending downwardly from the first casing support portion, the
first casing
support portion defining a predetermined angle of an upper end of the borehole
heat
exchanger within the borehole.
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
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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:
(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
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.
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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:
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;
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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 pre-cast
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.
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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
40 of the base support element 30.
In the illustrated embodiments of Figures 1 to 10 the orientation guide
element 38, 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 be provided.
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.
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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 mutually inclined at an acute angle, in
this embodiment
15 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.
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 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 and 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
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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 Figures 3 and 4, with further 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.
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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 installation. 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 BHE.
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.
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Other modifications to the various embodiments of the present invention will
be apparent to
those skilled in the art.
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