GB2442803A

GB2442803A - Multiple depth subterranean heat exchanger and installation method

Info

Publication number: GB2442803A
Application number: GB0620261A
Authority: GB
Inventors: Chris Ford; Jeff Newton
Original assignee: CANNON PILING Ltd
Current assignee: CANNON PILING Ltd
Priority date: 2006-10-12
Filing date: 2006-10-12
Publication date: 2008-04-16
Also published as: GB0620261D0

Abstract

A subterranean heat exchanger is constructed by forming a hole 14 directed, for example, between 0 and 30 degrees from the vertical, inserting a sleeve 16 into the hole and a fluid conduit 20 into the sleeve. The hole may be drilled and may be of similar dimensions to structural piles 6, have a diameter of 150mm to 400mm and be formed during construction of a building. Multiple holes of varying depths may be used and the deeper holes may be located closer to the centre of a building footprint. Their depths may be 6m to 26m and they may be 1.5m to 3m apart. The gap between the hole and the sleeve 18 may be filled by concrete and the sleeve may be made of metal, cardboard or plastic and form part of a closed circuit for a fluid. The conduit may be inserted after construction of the building or embedded in concrete and may be flexible and have sections of increased surface area 22, such as coiled sections, extending over all or part of the length of the hole and arranged such that heat exchange in each hole occurs over a depth range continuous with but not overlapping that in nearby holes. The conduit may be removable such that it can be easily replaced.

Description

A subterranean heat exchanger and methods of making or reconfiguring a

subterranean heat exchanger The present invention relates to subterranean heat exchangers for use with a ground source heat pump and to methods of making and/or reconfiguring subterranean heat exchangers. In particular the invention relates to subterranean heat exchangers provided in an elongated hole extending into the ground in a direction having a vertical component.

Systems for extracting heat that may be used in heating buildings from the ground using heat pumps are known. In some such systems a working medium is circulated in a closed liquid circuit, parts of which are buried below ground. When the working medium passes through buried parts of the liquid circuit the temperature of the working medium gradually adapts to the temperature of its surroundings. The working medium is generally introduced into the buried part of the liquid circuit at a temperature that is lower than the ground temperature. The working medium thus absorbs energy from the ground and the temperature of the working medium increases when the working medium passes through the buried parts of the liquid circuit. A heat pump, often referred to as a ground source heat pump in this context, is used for circulating the working medium through the liquid circuit. An initial energy input, typically in the form of electrical energy is required for operating this pump.

The ratio between the heat energy extracted from the ground using the ground source heat system and the energy used in operating the pump increases with the difference in temperature between the ground and the working medium prior to its feeding into the buried part of the liquid circuit.

Various ways of burying conduits in the ground for use with a ground source heat pump are known. For example, it is known to bury straight or coiled sections of tubing relatively superficially in the ground (e.g. down to a depth of less than one meter) so that the tubing extends substantially horizontally in the ground. Systems using buried conduits of this type require large area of land to be available to achieve an acceptable energy output. The ground temperature at these shallow depths varies considerable with the seasons. Thus the ground is typically coldest at times at which ground source heat is needed the most, namely during winter. Systems of this type thus typically suffer from low efficiency during the colder months of a year.

It is known that the seasonal variations in ground temperature become less marked with increasing depth and below a certain depth a minimum ground temperature is guaranteed. For example, it is known that, while some seasonal temperature variations occur down to large depths, on average the ground temperature in the United Kingdom at a depth of about 1.7 metres does not fall below degrees centigrade. A known system drills for holes to a depth of twenty meters or greater to take advantage of this effect. These holes typically require a large diameter of about 1.2 meter. After completion of the drilling operation a pre-fabricated steel armament is inserted into the hole. This steel armament carries conduits for conveying a working medium below ground and is permanently installed in the hole by setting it in concrete. This way of installing conduits for the working medium in the ground requires specialist machinery for the drilling the holes and for inserting the arrangement into the holes and is thus costly. Further, if the concrete structures comprising the conduits form part of a structural arrangement, for example, a foundation, then building approval is required for the inclusion of the conduits in the structural arrangement, prior to installation.

Systems of this type further suffer from the disadvantages that parts of the conduits extending above ground may be damaged during subsequent construction operations, for example during the course of constructing a building in the vicinity of the subterranean structure.

Systems comprising large heat absorbing structures can further suffer from the disadvantage that they drastically reduce the ground temperature in areas surrounding the heat absorbing structure. Thus these systems reduce their own efficiency.

The present invention seeks to alleviate one or more of the above problems.

According to an aspect of the present invention, there is provided a method of forming a subterranean heat exchanger comprising forming a first elongated hole in the ground so that a longitudinal axis of the first hole extends in a direction having a vertical component, inserting a first sleeve into the first hole and inserting a first fluid conduit into the sleeve so that a working fluid can be conveyed into and/or out of the first hole through the first fluid conduit, the first fluid conduit extending to a first depth.

The present invention permits taking advantage of the relatively stable temperatures found below the above mentioned depth by drilling holes that extend at least partially vertically. The sleeve can define an empty cylindrical space at the centre of the subterranean heat exchanger and can prevent ingress of any concrete into this interior space. It is thus, not necessary that the fluid conduit is installed at the same time as the hole is formed or the sleeve is inserted into the holes. Instead, the conduit can be installed below ground at a later stage. The danger of damaging the liquid conduit during construction of buildings in the vicinity of the subterranean heat exchanger is thus reduced.

Preferably the fluid conduit is arranged to be removable from the sleeve, so that the fluid conduits can be removed for easy repair or replacement. If a fluid conduit has become damaged or undesirable from an efficiency point of view, for example after part of or at the end of its lifetime, the fluid conduit can simply be removed from this interior space and replaced by a new and/or more efficient fluid conduit.

According to another aspect of the present invention there is provided a device comprising a first sleeve in a first elongated hole in a subterranean structure, the first elongated hole extending in a direction having a vertical component, a first fluid conduit arranged in the first sleeve so that a working fluid can be conveyed into and/or out of the first hole through the first fluid conduit, the first fluid conduit extending to a first depth.

To increase the overall output of a ground source heat system, a second elongated hole is preferably formed in the ground adjacent the first hole, so that a longitudinal axis of the second hole extends in a direction having a vertical component. A second sleeve is inserted into the second hole and a second fluid conduit is inserted into the second sleeve so that a working fluid can be conveyed into and/or out of the second hole through the second fluid conduit.

Energy extracted from an earth volume is replenished by energy flowing from earth volumes spaced apart from the subterranean heat exchangers. In the earth volumes from which heat/energy is extracted an equilibrium temperature reflecting the balance between the amount of energy extracted by the subterranean heat exchangers and the amount of energy provided from surrounding earth volumes establishes after an initial period of time following the start of energy/heat extraction.

If the earth volume from which a subterranean heat exchanger extracts heat is close to a further earth volume from which another subterranean heat extractor extracts heat, then the ability of the earth volume to replenish energy in the further earth volume is diminished. As a consequence, the equilibrium temperatures achieved in these two earth volumes is lower than an equilibrium temperature that could be established if a single subterranean heat exchanger was used in isolation.

The amount of energy extracted by two adjacent subterranean heat exchangers may thus be lower than an amount of energy extracted if the subterranean heat exchangers were grounded in isolation. It will be appreciated that adjacent subterranean heat exchangers influence each other to an increasing degree as the spacing of the earth volumes from which the subterranean heat exchangers extract heat decreases. Accordingly the number of known heat exchangers that can be installed in a predetermined ground surface area, such as the footprint of a house, can be limited.

A preferred embodiment overcomes this problem by arranging the second fluid conduit at a second depth that is smaller than the first depth. The preferred embodiment thus permits extraction of heat from a first depth using the first subterranean heat exchanger and extraction of heat from a second depth using the second subterranean heat exchanger. This effectively increases the spacing of the earth volumes surrounding the main energy absorbing portions of the two subterranean heat exchangers without requiring the holes in which the subterranean heat exchangers are provided to be spaced further apart in a horizontal direction.

Thus an increased amount of energy can be extracted from within a predetermined ground surface area.

This predetermined surface area may be the footprint of a building or any area designated for the installation of a plurality of subterranean heat exchangers, such as for example an area considered particularly suitable for the installation of the described subterranean heat exchangers. Thus the subterranean heat exchangers may, for example, be installed in areas requiring a minimum amount of modification, such as areas not comprising surface coverings or areas that are already in use for a purpose different than heat extraction from the ground, such as, for example, car parks etc. Preferably, a first/deeper hole is formed closer to a centre of the footprint or of that designated area than the second hole. Energy extracted using the second, less deep subterranean heat exchanger may be replenished by energy flowing from earth volumes located at small depths or even earth volumes that may benefit from seasonal increases in the ground temperature. As the second less deep subterranean heat exchanger may be located closer to the periphery/edge of a building any temperature shielding effects the building may have on the ground are less severe, the second subterranean heat exchanger may be able to make use of seasonal temperature increases more readily than the more centrally (and thus more shielded) first, deeper subterranean heat exchanger. Energy extracted by the first subterranean heat exchanger can nevertheless be replenished by energy flowing from earth volumes located at greater depth. Preferably the second hole is formed at a periphery of the footprint or at a periphery of the designated area to minimise the shielding effect a building or structure within this footprint or area may have.

The first hole may alternatively also be formed at a periphery of the footprint or at the periphery of the designated area. In this embodiment the first deeper hole and the second less deep hole can be spaced more closely than would be the case if both these subterranean heat exchangers extracted heat from the same depth.

The second depth is preferably about two thirds of the first depth. In this case the second fluid conduit may be arranged to be able to predominantly extract heat from an earth volume extending from the second depth upwardly for a distance corresponding to about one third of the first depth. The first fluid conduit may further be arranged to be able to predominantly extract heat from an earth volume extending from the first depth upwardly for a distance corresponding to about one third of the first depth.

Energy may further be extracted from a third depth by forming a third elongated hole in the ground adjacent the second hole with the third hole being further away from the centre of the footprint or from the centre of the designated area than the second hole. The longitudinal axis of this third hole preferably also extends in a direction having a vertical component. A third sleeve is inserted into the third hole and a third fluid conduit is inserted into the third sleeve so that a working fluid can be conveyed into and/or out of the third hole through the third fluid conduit. The third fluid conduit extends to a third depth smaller than the second depth.

In preferred embodiments in which fluid conduits extend to three different depths the second depth is preferably about three quarters of the first depth. The second fluid conduit is preferably arranged to be able to predominantly extract heat from an earth volume extending from the second depth upwardly for a distance corresponding to about one quarter of the first depth. In this case the third depth is preferably half of the first depth. The third fluid conduit is preferably arranged to be able to predominantly extract heat from an earth volume extending from the third depth upwardly for a distance corresponding to about one quarter of the first depth and the first fluid conduit is preferably arranged to be able to predominantly extract heat from an earth volume extending from the first depth upwardly for a distance corresponding to about one quarter of the first depth.

The fluid conduits may be arranged so that substantially no energy is absorbed from an earth volume immediately below ground and extending to a depth of 2 m or 3 m. This helps prevent foliage being adversely affected by an undue lowering of the ground temperature and may be achieved by configuring the sections of the fluid conduits that extend through this earth volume to be sections of straight tubing.

At least one of the holes may be formed within a footprint of a building before or during construction of the building, so that once the building is constructed the area below the building is used for ground source heat extraction. The fluid conduit may be inserted into a sleeve in such a hole after at least part of or all of the building has been constructed. This can protect the conduit from damage. The hole and sleeve combination may simply be capped or sealed in the meantime to prevent contamination of the subterranean heat exchanger with building debris.

One or more of the holes are preferably formed by drilling. The one or more holes preferably have an inner diameter of between 150 mm and 400 mm, more preferably of between 200 mm and 300 mm and even more preferably of about 250 mm. Preferably the one or more holes have a depth of between 6 m and 26 m, more preferably about or less than twelve meters.

The present invention of course also extends to configurations in which a plurality of holes, sleeves or subterranean heat exchangers extend to the respective first, second or third depths.

When forming the holes for the subterranean heat exchangers, holes for structural piles that may be required for the construction of building may simultaneously be formed within the footprint of the building. Preferably the holes for the subterranean heat exchangers have an inner diameter and/or a depth that substantially corresponds to the inner diameter and/or depth of a structural pile. The machinery required for providing the structural piles can thus also be used for forming the holes for the subterranean heat exchangers. This can reduce the costs of installing the subterranean heat exchangers considerably.

The gap between an outside of the sleeve and an inside of a hole is preferably filled. The gap may be filed with concrete to increase the structural integrity of the subterranean heat exchanger. It will be appreciated that in this case the structural features of the subterranean heat exchanger are isolated from the heat exchange features of the subterranean heat exchanger, so that interference between the two features can be minimised. The gap between outside of the sleeve and the inside of the hole can be filled with an alternative substance that may promote heat conduction from the earth to the sleeve.

The fluid conduits preferably form part closed circuits for fluid circulation. It is, however, also envisaged that the fluid conduits form such a closed circuit with the sleeve of the subterranean heat exchanger, so that fluid is introduced into and removed from the sleeve by the fluid conduit and so that the sleeve is responsible for exchanging heat with the surrounding earth volume.

Preferably a fluid conduit comprises a section with a surface area that is larger than the surface area of other sections of the same fluid conduit, so that heat is absorbed predominantly in the section of the fluid conduit having the larger surface area. One way of increasing the larger surface area of a section is to provide a larger length of tubing in that section, for example by arranging the tubing in a helical fashion. The section having a smaller surface preferably comprises straight sections of tubing.

The section of the fluid having the larger surface area could alternatively extend over the entire length of a hole so that heat is extracted from the earth over the entire height of the hole, either when the fluid is conveyed into or out of the subterranean heat exchanger. The fluid flow in the other direction can be along a straight set of tubing so that heat transfer along this part of the fluid circuit is minimised.

Alternatively the section of the fluid conduits having the larger surface area can extend over only part of the entire length of a said hole. In this manner absorption of energy can be concentrated on a particular depths range along the length of the hole. The section of the conduit having the larger surface area can, for example, extend over about a quarter, over a third or over half of the entire length of the hole.

The holes are preferably spaced between about one and half meters and about three meters. The holes do not need to extend in the vertical direction and the longitudinal axis of the holes may assume an angle of between 0 degrees and 30 degrees relative to the vertical direction.

The sleeve is preferably made of metal, such as steel, aluminium, copper or tin, of plastic or of cardboard. In situations in which a cardboard sleeve is used, the sleeve may loose its structural integrity over time. Such a cardboard sleeve may be used when the subterranean heat exchanger is constructed. The cardboard sleeve may, for example prevent a fluid from flowing into the centre of a hole in the ground thus fulfilling the function of a former defining the inner diameter of an open hole into which the fluid conduit can be inserted. The cardboard sleeve may, for example be put in place in the hole before a gap between the sleeve and the hole is filled with concrete. Once the concrete has set a deterioration of the sleeve does not negatively influence the subterranean heat exchanger.

One working fluid suitable for use in the preferred embodiment is water. Anti-freeze may be provided as part of the working fluid.

The fluid conduit is preferably flexible to aid insertion and removal into and out of the sleeve. The fluid conduit may be made from polyethylene tubing or from polyvinyichlonde tubing.

It has been recognised that being able to reconfigure a subterranean heat exchanger by removing and/or replacing the fluid conduit is advantageous in its own right and according to another aspect of the present invention there is provided a method of reconfiguring a subterranean heat exchanger comprising removing a fluid conduit from a sleeve in a hole in the ground and inserting a replacement fluid conduit into the sleeve.

It has further been recognised that it is particularly advantageous to use holes having a diameter of between 150 mm and 400 mm in the present invention.

According to another aspect of the present invention there is thus provided a device comprising an elongated hole in a subterranean structure, the hole extending in a direction having a vertical component and having an inner diameter of between 150 mm and 400 mm, the device further comprising a fluid conduit arranged in the hole so that a working fluid can be conveyed into and out of the hole through the fluid conduit.

It will be appreciated that, although in this particular arrangement it is not essential that a sleeve be provided in the hole, a number of holes can be drilled to various different depths in a manner consistent with the foregoing discussion relating to subterranean heat exchangers comprising sleeves. Alternatively a plurality of holes may be drilled to the same depth and fluid conduits may be placed in the holes so that their respective main heat absorbing sections extend over different depths ranges in the respective holes, such as, for example, described above in relation to subterranean heat exchangers comprising sleeves.

The hole preferably has a length of between 6 m and 26 m and the fluid conduit may optionally be embedded in a concrete matrix.

It has further been recognised that arranging fluid conduits of subterranean heat exchangers at different depth so that they predominantly absorb heat at different depth is advantageous in its own right and according to another aspect of the present invention there is thus provide a device comprising two or more elongated holes in a subterranean structure, each of the two or more holes extending in a direction having a vertical component; and a fluid conduit arranged in each said hole so that a working fluid can be conveyed into and/or out of the hole through the fluid conduit.

Two of the fluid conduits are arranged so that energy absorbed by the working fluid is predominantly absorbed at one range of depths for one of the holes and at a different range of depths for the other one of the holes.

The range of depth and the different range of depth are preferably not overlapping but under certain circumstances may overlap. More preferably the range of depth and the different range of depths are substantially contiguous depths ranges.

The devices preferably comprising three or more holes, wherein the fluid conduits are arranged so that energy absorbed by the working fluid is predominantly absorbed at three different depths ranges.

In order that the present invention may be more fully understood and so that further features thereof may be appreciated, embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings in which: Figure 1 shows a schematic cross-sectional view of a subterranean heat exchanger in accordance with a preferred embodiment of the present invention; Figure 2A shows a top plan view of the footprint of a building with a plurality of subterranean heat exchangers arranged along the periphery of the footprint of the building in accordance with a preferred embodiment of the present invention; Figure 2B shows a view along line 2B-2B of Figure 2A; Figure 3A shows a top plan view of the footprint of a building with a plurality of subterranean heat exchangers arranged inside of and along the periphery of the footprint of the building in accordance with a second preferred embodiment of the present invention; Figure 3B shows a view along line 3B-3B of Figure 3A; Figure 4A shows a top plan view of the footprint of a building with a plurality of subterranean heat exchangers arranged inside of and along the periphery of the footprint of the building in accordance with a further preferred embodiment of the present invention; Figure 48 shows a view along line 48-48 of Figure 4A; Figure 5 shows a plurality of subterranean heat exchangers connected according to a preferred embodiment of the present invention; and Figure 6 shows another plurality of subterranean heat exchangers connected according to another preferred embodiment of the present invention.

Figure 1 shows a building 2 and a subterranean heat exchanger 4 provided below the footprint of the building 2. Structural piles 6 form part of the foundation of the building 2. In the preferred embodiment the ground level 8 below the footprint of building 2 is arranged to be somewhat lower than the base 10 of the building 2.

There is thus a gap 12 between the base 10 of the building 2 and ground level 8.

The subterranean heat exchanger 4 is located inside a substantially vertically extending hole 14. A steel sleeve 16 is located inside the hole 14. A gap 18 between the inner wall of the hole 14 and the outer surface of the sleeve 16 is filled with concrete.

A liquid filled conduit 20 is arranged inside the sleeve 16. The conduit 20 forms a closed circuit and comprises two sections 22 and 24. Section 22 comprises tubing arranged in a helical formation. Conduit section 24 comprises straight tubing that connects the lower end of conduit section 22 with the surface. Section 22 of conduit 20 thus has a larger surface area than section 20, so that the liquid conduit predominantly absorbs heat through section 22 and is only able to absorb heat through section 24 to a limited extent. The two ends 26 and 28 of the liquid conduit are arranged for connection to a closed liquid circuit (not shown) comprising a ground source heat pump (not shown) for circulating liquid into the subterranean heat exchanger 4 and for extracting the energy the liquid has absorbed from the ground.

The subterranean heat exchanger is created by first drilling the hole 14, for example using standard earth drills, also known as augers, that can also be used for drilling the holes required for structural piles 6. Sleeve 16 is subsequently inserted into the hole, prior to filling the gap 18 between the sleeve 16 and the inside wall of the hole 14 with concrete. The liquid conduit 20 is then inserted into the sleeve 16.

The sleeve 16 may be filled with a material promoting the conduction of energy from the formation surrounding the hole 14 to the liquid conduit 20. The conduit 20 is made from a flexible material such as, for example, from polyethylene tubing or from polyvinyichioride tubing.

The liquid conduit 20 may be simply suspended or otherwise removably arranged inside the sleeve 16. Should it turn out that, say after a number of years of service, the liquid conduit 20 is damaged, or that a more efficiently operating liquid conduit 20 has become available, then the liquid conduit 20 can be disconnected from the associated liquid circuit and removed from the sleeve 16. The gap 12 provides sufficient room for pulling a first section of the liquid conduit 20 out of the sleeve 16. The flexible liquid conduit 20 can then simply be bent about the upper opening 30 of the sleeve 16 and fully removed from the sleeve 16. The liquid conduit may subsequently be reinserted into the sleeve 16, for example after repair, or replaced by a new and/or improved liquid conduit.

In use, liquid medium in the liquid circuit associated with the ground source heat pump is circulated through the liquid conduit 20. It is desirable that this is done so that the liquid medium is circulated from the end of the helical section 22 having a low temperature to the end of the helical section 22 having a higher temperature.

Thus, when ground temperatures are high, for example in summer, it may be preferable to circulate the liquid medium into the end 28 of the liquid conduit 20 and down into the subterranean heat exchanger 4 through the straight section 24 of liquid conduit 20. The liquid medium can then return to the end 26 of the liquid conduit 20 through the helical section 22. During the passage of the helical section 22 the liquid medium can gradually adapt to the temperature of its surroundings without losing already absorbed energy to colder surrounding earth volumes.

In times when the temperature at the upper end of the helical section 22 is lower than the temperature at the lower end of the helical section 22, as may be, for example, the case in winter in arrangements in which the helical section 22 extends substantially up to ground level, the system of the preferred embodiment could be operated in reverse, namely by feeding the liquid medium into the end 26 of the liquid conduit 20 so that the liquid medium passes through the helical section 22 before being returned to the end 28 of the liquid conduit 20 through the straightsection of the liquid conduit 20. In one preferred embodiment a switch for reversing the direction of flow of the liquid medium in liquid conduit 20 is provided.

Referring now to Figures 2 to 4, further preferred embodiments of the present invention will now be described. It will be understood that the subterranean heat exchangers in these preferred embodiments comprise a sleeve 16 in a hole 14 with a liquid conduit 20 having a helical section 22 being placed in this sleeve. In the illustrated preferred embodiments the helical section 22 extends only over part of the depth of the hole 14 it is provided in. An upper end of the helical section is connected to end 26 by a straight section of tubing.

Referring now to Figure 2A, the footprint 32 of building 2 is shown in a top plan view. Twelve subterranean heat exchangers 4-1 to 4-12 are arranged around the periphery of the footprint 32 in accordance with a second preferred embodiment of the present invention. In the preferred embodiment subterranean heat exchangers 4-1 to 4-4 and 4-7 to 4-10 are spaced about 1.8 m from each other and subterranean heat exchangers 4-5 and 4-6 and 4-11 and 4-12 are spaced about 2.5 meters from each other. The subterranean heat exchangers 4-1 to 4-12 are further spaced from structural piles 6.

Referring now to Figure 2B, which shows a view along line 2B-2B in Figure 2A and illustrates a cross-section of four subterranean heat exchangers 4-7 to 4-10, it can be seen that holes 14-8 and 14-10 extend to a first depth, while holes 14-7 and 14-9 extend to a second, smaller depth. The first depth is preferably twelve meters and the second depth is preferably eight meters. Each of the subterranean heat exchangers 4-7 to 4-10 comprises a liquid conduit 20-7 to 20-10 with helical sections 22-7 to 22-10. Each of the helical sections 20-7 to 20-10 extends from the lower end of the respective holes 14-7 to 14-10 over a vertical height of about four meters.

Thus, the subterranean heat exchangers 14-7 and 14-9 in use extract heat predominantly from a depth from between about four meters and about eight meters, while the subterranean heat exchangers 14-8 and 14-10 in use extract heat extract predominantly from a depth from about between eight meters and about twelve meters. Thus, adjacent ones of the subterranean heat exchanger 4-7 to 4-10 extract heat from earth volumes at different depths. By varying the extraction depths the spacing between the earth volumes from which energy is extracted is increased. This permits spacing the subterranean heat exchangers 4-7 to 4-10 more closely in a horizontal direction than subterranean heat exchangers that extract heat from earth volumes at the same depth.

While Figure 2B only shows subterranean heat exchangers 4-7 to 4-10, it will be appreciated that subterranean heat exchangers 4-1 to 4-6, 4-11 and 4-12 are also arranged in the illustrated fashion, namely so that adjacent heat exchangers extract heat from different depths.

In a non-illustrated third embodiment all of the holes 14 of the subterranean heat exchangers 4 extend to the same depth and the helical sections 22 of the respective liquid conduits 20 are arranged to extend over alternating depth ranges.

Thus all of the holes of the subterranean heat exchangers 4 can be drilled to a depth of, for example, twelve meters. The liquid conduits 20 in every other hole 14 can then be arranged in these holes so that their helical sections extend from a depth of, for example, between about four meters and about eight meters and between a depth of about eight meters and a depth of about twelve meters. The third preferred embodiment thus achieves the same effect as the preferred embodiment illustrated in Figures 2A and 28. The third preferred embodiment is advantageous in that it does not limit a user with regard to the depth from which heat is extracted. A user can thus, if desired, re-configure the system to extract heat from any depth up to the depth of the holes 14.

A fourth preferred embodiment of the present invention will now be described with reference to Figures 3A and 3B. As can be seen from Figure 3A, twelve subterranean heat exchangers 4-1 to 4-12 are arranged about the periphery of the footprint 32 of a building in the same manner as in the second preferred embodiment described above. This part of the fourth preferred embodiment differs from the first preferred embodiment in that all of the holes 14-1 to 14-12 extend to the same depth.

In the fourth preferred embodiment holes 14-1 to 14-12 are eight meters deep.

Further subterranean heat exchangers 4-13 to 4-16 are provided towards the centre of the footprint 32. These further subterranean heat exchangers 4-13 to 4-16 extend into the ground more deeply than subterranean heat exchangers 4-1 to 4-12, namely to a depth of about twelve meters.

Each of the subterranean heat exchangers 4-1 to 4-16 comprises a liquid conduit 20-1 to 20-16 with respective helical sections 22-1 to 22-16 which extend over a vertical height of four meters from the lower end of the respective holes 4-1 to 4-16. Thus, the subterranean heat exchangers 4-1 to 4-12 arranged about the periphery of the footprint 32 extract heat from respective earth volumes at a depth from about four meters to about eight meters and the subterranean heat exchangers 4-13 to 4-16 located further towards the centre of the footprint 32 extract heat from respective earth volumes at a depth from about eight meters to about twelve meters.

Advantageously therefore, energy replenishing the energy extracted by the subterranean heat exchangers 4-1 to 4-12 can flow towards the subterranean heat exchangers 4-1 to 4-12 from all areas outside of the footprint 32. Energy replenishing the energy extracted by the subterranean heat exchangers 4-13 to 4-16 can flow towards the subterranean heat exchangers 4-13 to 4-16 from deeper lying earth volumes surrounding the subterranean heat exchangers 4-13 to 4-16, without such energy flow being impeded by subterranean heat exchangers 4-1 to 4-12.

Subterranean heat exchangers 4-1 to 4-12 can of course also be arranged in the manner shown in Figures 2A and 26, namely so that adjacent heat exchangers extract heat from different depths. Preferably all of the subterranean heat exchangers 4-1 to 4-12 are arranged so as to extract heat from depths that are more shallow than the depths from which subterranean heat exchangers 4-13 to 4-16 extract heat.

A non-illustrated fifth embodiment comprises sixteen heat exchangers located in the positions shown in Figure 3A. The fifth embodiment, however, differs from the fourth embodiment in that all of the holes 14 of the sixteen subterranean heat exchangers extend to the same depth, for example, to a depth of twelve meters. The helical sections 22 of the liquid conduits 20 provided in the four subterranean heat exchangers located towards the centre of the footprint 32 extend upwardly for about four meters from the lower end of the respective holes in the same manner as in the fourth embodiment. In the fifth embodiment, however, the helical sections 22 of the liquid conduits 20 provided in the twelve subterranean heat exchangers located around the periphery of the footprint 32 do not extend upwardly from the lower ends of the holes but rather extend upwardly for a given distance, for example for about four meters, from a point above the lower ends of the holes, for example from depths of eight meters. Thus, in the fifth preferred embodiment the helical sections 22 of the liquid conduits 20 can extend over exactly the same depths ranges as in the fourth embodiment despite the fact that the holes 14 all extend to the same depth. As already discussed with regard to the third embodiment, this is advantageous because a user wishing to reconfigure the system of the fifth embodiment has the freedom to choose alternative and deeper locations for placing the helical sections 22 of the liquid conduits 20 than is possible in the subterranean heat exchangers of the fourth embodiment.

Referring now to Figures 4A and 4B a sixth preferred embodiment of the present invention will be described. As can be seen from Figure 4A, sixteen subterranean heat exchangers 4-1 to 4-16 are provided in the same locations as the sixteen subterranean heat exchangers provided in the fourth embodiment. Two additional subterranean heat exchangers 4-17 and 4-18 are furthermore provided at the centre of the footprint 32.

The holes 14-1 to 14-12 associated with subterranean heat exchangers 4-1 to 4-12 arranged around the periphery of the footprint extend to one depth, for example to about six meters below ground level. The holes 14-13 to 14-16 associated with subterranean heat exchangers 4- 13 to 4-16 extend to another depth, for example to about nine meters below ground level. The holes 14-17 and 14-18 associated with subterranean heat exchangers 4-17 and 4-18 extend to a further depth, for example to about twelve meters below ground level. All of the subterranean heat exchangers 4-1 to 4-18 comprise liquid conduits 20 with helical sections 22 that extend upwardly for three meters from the lower ends of the respective holes 14-1 to 14-18.

In the sixth preferred embodiment therefore the subterranean heat extractors 4-1 to 4-12 located along the periphery of the footprint 32 extract heat from earth volumes at depths from about three meters to about six meters below ground, the subterranean heat exchangers 4-13 to 4-16 located further towards the centre of the footprint 32 extract heat from earth volumes at depths from about six meters to about nine meters below ground and the subterranean heat exchangers 4-17 and 4-18 located at the centre of the footprint 32 extract heat from earth volumes at depths from about nine meters to about twelve meters below ground.

In a non-illustrated seventh embodiment the holes 14-1 to 14-18 all extend to the same depth, for example to a depth of about twelve meters, and the helical sections of the liquid conduits 10-1 to 20-18 are arranged to extend over the same depth ranges as the helical sections of the liquid conduits 20-1 to 20-18 of the sixth preferred embodiment.

Figure 5 shows one of various preferred arrangements for connecting a number N of subterranean heat exchangers 34-1 to 34-N to each other. In the embodiment shown in Figure 5, all of the subterranean heat exchangers 34-1 to 34-N are connected in parallel and all of the input conduits leading to the subterranean heat exchangers 34-1 to 34-N receive liquid medium from a single feeding conduit.

All of the output conduits from the subterranean heat exchangers 34-1 to 34-N feed the liquid medium that has flown through the subterranean heat exchangers 34-Ito 34-N into a single output conduit. Thus the liquid medium received in the single output conduit has a temperature that is an average of all of the temperatures of the liquids supplied from subterranean heat exchangers 34-1 to 34-N.

A further preferred embodiment is now discussed with reference to Figure 6.

In this preferred embodiment a number N of subterranean heat exchangers is again provided in parallel, so that liquid medium is fed into the number N of subterranean heat exchangers 34-Ito 34-N using a common input conduit and received from the subterranean heat exchangers 34-1 to 34-N as a mixture of all of the liquid mediums flows through the* subterranean heat exchangers 34-1 to 34-N in a single output conduit. In Figure 6, however, subterranean heat exchangers 34-1, 34-3 or 34-(N-1) are connected in series with respective subterranean heat exchangers 34-2, 34-4 and 34-N. Thus liquid medium that is fed into a subterranean heat exchanger 34-1, 34-3 or 34-(N-1) also flows through a respective subterranean heat exchanger 34-2, 34-4 and 34-N before arriving at the common output conduit.

This embodiment is advantageous in situations where heat is extracted from two earth volumes that heat different temperatures. In such a situation the liquid medium can be first conveyed through the subterranean heat exchanger 34-1, 34-3 or 34-(N-1) located in the cooler earth volume, so that the liquid medium can initially adapt to the lower temperature. The thus pre-warmed liquid medium is then fed into the respective subterranean heat exchanger 34-2, 34-4 or 34-N located in the warmer earth volume, so that the liquid medium can then adapt to the higher temperature. This arrangement may be particularly advantageous, for example, in arrangements as shown in Figures 2 to 4, where the subterranean heat exchangers extract heat from different depths.

It will be appreciated that the above description of the preferred embodiments are made by way of example only and that modifications to the preferred arrangements are possible within the scope of the appended claims. For example, the hole containing the sleeve does not have to extend in a vertical direction as is shown in Figure 1 but may instead be inclined relative to the vertical direction. The lower end of the hole may, for example be arranged to be situated below the footprint of a building and the upper end of the hole may be arranged to be situated outside of the footprint of the building. This can improve access to the liquid conduit in the hole/sleeve for easy reconfiguration, removal or replacement. The holes may further be arranged to fan out below ground from a predetermined area designated for the installation of subterranean heat exchangers to maximise the volume of earth from which heat can be extracted.

It may further not be necessary to fill the gap between the inside of the hole and the outside of the sleeve and the gap may be left empty relying on the sleeve for structural integrity.

The sleeve may further be arranged to form part of the closed liquid circulation system. The sleeve may, for example, be sealed at its lower end in a liquid tight manner, so that the liquid medium can be conveyed into the inside of the sleeve through the liquid conduit so as to fill the sleeve. Liquid that has warmed up inside the sleeve can then be conveyed from the interior of the sleeve to a ground source heat pump using a further open ended liquid conduit extending into the sleeve.

Claims

1. A method of forming a subterranean heat exchanger comprising: forming a first elongated hole in the ground so that a longitudinal axis of the first hole extends in a direction having a vertical component; inserting a first sleeve into the first hole; and inserting a first fluid conduit into the sleeve so that a working fluid can be conveyed into and/or out of the first hole through the first fluid conduit, the first fluid conduit extending to a first depth.

2. A method as claimed in claim 1, further comprising: forming a second elongated hole in the ground adjacent the first hole, so that a longitudinal axis of the second hole extends in a direction having a vertical component; inserting a second sleeve into the second hole; and inserting a second fluid conduit into the second sleeve so that a working fluid can be conveyed into and/or out of the second hole through the second fluid conduit, the second fluid conduit extending to a second depth that is smaller than the first depth.

3. A method as claimed in claim 2, wherein the first hole is formed closer to a centre of a footprint of a building or of an area designated for the installation of a plurality of subterranean heat exchangers than the second hole.

4. A method as claimed in claim 2 or 3, wherein the second hole is formed at a periphery of the footprint of the building or at a periphery of an area designated for the installation of a plurality of subterranean heat exchangers.

5. A method as claimed in claim 4 as dependent from claim 2, wherein the first hole is formed at a periphery of the footprint of the building or at the periphery of the designated area.

6. A method as claimed in claim 2, 3 or 4, further comprising: forming a third elongated hole in the ground adjacent the second hole further away from the centre of the or a footprint of the building or the centre of the or a designated area than the second hole, and so that a longitudinal axis of the third hole extends in a direction having a vertical component; inserting a third sleeve into the third hole; and inserting a third fluid conduit into the third sleeve so that a working fluid can be conveyed into and/or out of the third hole through the third fluid conduit, the third fluid conduit extending to a third depth smaller than the second depth.

7. A method as claimed in any preceding claim, further comprising forming at least one of a said hole within a footprint of a building before or during construction of the building.

8. A method as claimed in claims 7, wherein a said fluid conduit is inserted into a said sleeve after at least part of or all of the building has been constructed.

9. A method as claimed in any preceding claim, wherein the step of forming a said hole further comprises forming structural piles within the footprint of a or the building.

10. A method as claimed in any preceding claim, wherein a said hole is formed by drilling.

11. A method as claimed in any preceding claim, further comprising filling a gap between an outside of a said sleeve and an inside of a said hole.

12. A method as claimed in claim 11, comprising filling the gap with concrete.

13. A device comprising: a first sleeve in a first elongated hole in a subterranean structure, the first elongated hole extending in a direction having a vertical component; a first fluid conduit arranged in the first sleeve so that a working fluid can be conveyed into and/or out of the first hole through the first fluid conduit, the first fluid conduit extending to a first depth.

14. A device as claimed in claim 13, further comprising: a second sleeve in a second elongated hole in the subterranean structure, the second hole extending in a direction having a vertical component; a second fluid conduit arranged in the second sleeve so that a working fluid can be conveyed into and/or out of the second hole through the fluid conduit, the second fluid conduit extending to a second depth smaller than the first depth.

15. A device as claimed in claim 14, wherein the second depth is about two thirds of the first depth.

16. A device as claimed in claim 15, wherein the second fluid conduit is arranged to be able to predominantly extract heat from an earth volume extending from the second depth upwardly for a distance corresponding to about one third of the first depth.

17. A device as claimed in claim 15 or 16, wherein the first fluid conduit is arranged to be able to predominantly extract heat from an earth volume extending from the first depth upwardly for a distance corresponding to about one third of the first depth.

18. A device as claimed in any of claims 13 to 17, wherein the first hole is located further towards the centre of a designated area than the second hole.

19. A device as claimed in any of claims 13 to 18, further comprising: a third sleeve in a third elongated hole in the ground, the third hole extending in a direction having a vertical component; a third fluid conduit arranged in the third sleeve so that a working fluid can be conveyed into and/or out of the third hole through the fluid conduit, the third fluid conduit extending to a third depth smaller than the first depth and than the second depth.

20. A device as claimed in claim 14 or 19, wherein the second depth is about three quarters of the first depth.

21. A device as claimed in claim 20, wherein the second fluid conduit is arranged to be able to predominantly extract heat from an earth volume extending from the second depth upwardly for a distance corresponding to about one quarter of the first depth.

22. A device as claimed in claim 19, 20 as dependent from claim 19 or 21 as dependent from claim 19, wherein the third depth is about half of the first depth.

23. A device as claimed in claim 22, wherein the third fluid conduit is arranged to be able to predominantly extract heat from an earth volume extending from the third depth upwardly for a distance corresponding to about one quarter of the first depth.

24. A device as claimed in any of claims 13, 14 and 19 to 23, wherein the first fluid conduit is arranged to be able to predominantly extract heat from an earth volume extending from the first depth upwardly for a distance corresponding to about one quarter of the first depth.

25. A device as claimed in any of claims 19 and 20 to 24 as dependent from claim 19, wherein the third hole is located further away from the centre of the or a designated area than the second hole.

26. A device as claimed in any of claims 13 to 25, wherein at least one of the fluid conduits is arranged so that substantially no energy is absorbed from an earth volume immediately below ground and extending to a depth of about 2 m or 3 m.

27. A device as claimed in claim 26, wherein sections of the at least one of the fluid conduits that extend through the said earth volume are sections of straight tubing.

28. A device as claimed in any of claims 13 to 27, wherein at least one of the first fluid conduit, the second fluid conduit and the third fluid conduit is arranged to be removable from the respective first sleeve, second sleeve and third sleeve.

29. A device as claimed in any of claims 13 to 28, arranged so that the first, second and/or third fluid conduit forms part of one or more closed circuits for fluid circu'ation.

30. A device as claimed in any of claims 13 to 29, wherein a said hole has an inner diameter of between about 150 mm and 400 mm.

31. A device as claimed in any of claims 13 to 29, wherein a said hole has an inner diameter of between about 200 mm and 250 mm.

32. A device as claimed in any of claims 13 to 29, wherein a said hole has an inner diameter that substantially corresponds to the diameter of a structural pile adjacent the hole.

33. A device as claimed in any of claims 13 to 32, wherein the hole has a depth of between 6 m and 26 m.

34. A device as claimed in any of claims 13 to 32, wherein the hole has a depth of about twelve meters.

35. A device as claimed in any of claims 13 to 32, wherein the hole has a depth of less than twelve meters.

36. A device as claimed in any of claims 13 to 35, wherein a said fluid conduit is arranged so that the fluid remains in the fluid conduit when being conveyed into and out of a respective hole.

37. A device as claimed in any of claims 13 to 36, wherein at least one of the first fluid conduit, the second fluid conduit and the third fluid conduit comprises a section having a larger surface area than other sections of the same fluid conduit, so that heat can be absorbed by working fluid in the fluid conduit predominantly in the section having the larger surface area.

38. A device as claimed in any of claims 37, wherein the section having a larger surface area comprises a section of helical tubing.

39. A device as claimed in claim 37 or 38, wherein the section having a larger surface area extends over an entire length of a said hole.

40. A device as claimed in claim 37 or 38, wherein the section having a larger surface area extends over part of an entire length of a said hole.

41. A device as claimed in claim 40, wherein the section having a larger surface area extends over about a quarter, a third or half of the entire length of the hole.

42. A device as claimed in any of claims 13 to 41, wherein a said fluid conduit is flexible.

43. A device as claimed in any of claims 13 to 42, wherein a said fluid conduit is made from polyethylene tubing or from polyvinylchloride tubing.

44. A device as claimed in any of claims 13 to 43, wherein at least one of the holes is substantially cylindrical.

45. A device as claimed in any of claims 13 to 44, wherein the holes are spaced between one and a half meters and three meters.

46. A device as claimed in any of claims 13 to 45, wherein a longitudinal axis of a said hole is arranged at an angle of between 0 degrees and 30 degrees to the vertical direction.

47. A device as claimed in claim 13, 14 or 19, wherein the sleeve is made of a metal, such as steel, aluminium, copper or tin, of plastic or of cardboard.

48. A device as claimed in any of claims 13 to 47, wherein a said fluid conduit and a said sleeve associated with the said fluid conduit from part of a closed circuit for conveying working fluid.

49. A device comprising an elongated hole in a subterranean structure and a fluid conduit, the hole extending in a direction having a vertical component and having an inner diameter of between about 150 mm and about 400 mm; and a fluid conduit arranged in the hole so that fluid can be conveyed into and out of the hole through the fluid conduit.

50. A device as claimed in claim 49, wherein the hole has a length of between about 6 m and about 26 m.

51. A device as claimed in claim 49 or 50, wherein the fluid conduit is embedded in a concrete matrix.

52. A device comprising: two or more elongated holes in a subterranean structure, each of the two or more holes extending in a direction having a vertical component; and a fluid conduit arranged in each said hole so that a working fluid can be conveyed into and/or out of the hole through the fluid conduit; wherein two of the fluid conduits are arranged so that energy absorbed by the working fluid is predominantly absorbed at one range of depths for one of the holes and at a different range of depths for the other one of the holes.

53. A device as claimed in claim 52, wherein the range of depth and the different range of depth are not overlapping.

54. A device as claimed in claim 52 or 53, wherein the range of depth and the different range of depths are substantially contiguous depths ranges.

55. A devices as claimed in claim 52, 53 or 54, comprising three or more holes, wherein the fluid conduits are arranged so that energy absorbed by the working fluid is predominantly absorbed at three different depths ranges.

56. A method of reconfiguring a subterranean heat exchanger comprising: removing a fluid conduit from a sleeve in a hole in the ground; and inserting a replacement fluid conduit into the sleeve.