Probe For Collecting Thermal Energy From The Ground For A Heat Pump, And A Collection Network Equipped With Such Probes
The probe (10) comprises a circuit for circulating a heat-conveying fluid, between a fluid inlet (28) and a fluid outlet (34) connected to a heat pump. The circuit comprises at least two tubes (12, 14) extending in parallel, comprising a fluid admission tube (14) and a fluid return tube (12), the admission and return tubes being put into communication (24) with each other at their distal ends (18, 22). The tubes are made with a wall in common over their entire length, advantageously a wall that is isothermal. They may be engaged one inside the other (12, 14) or they may be adjoining. The assembly forms a single tubular element suitable for burying having a distal end that is free that can easily be introduced into a tunnel of small diameter that has been dug by a miniature boring machine. The return tube is provided with portions in relief (44) suitable for creating turbulence in the fluid flowing in said tube, whilst the inner wall of the fluid admission tube is smooth to encourage laminar flow of the fluid flowing in said tube.
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The invention relates to a probe for collecting thermal energy from the ground for heat pumps, which pumps may be of the so-called “water/water” type or of the “gas/water” type.
Such equipment serves to collect the thermal energy available in the upper layers of the earth's crust, to concentrate said energy (raise it to a higher temperature), and to deliver it in said concentrated form to feed a heater circuit.
The core of the pump comprises a compressor and two heat exchangers connected respectively to the collection network and to the heat delivery network, together with a refrigerant fluid circuit comprising a condenser, an expander, and an evaporator. The compressor concentrates the energy collected from the ground at the evaporator and delivers the energy for delivering to the heater circuit at the condenser.
In the collection network, there is provided a “collector probe” constituted by a circuit for a heat-conveying fluid, generally a liquid such as water having ethylene glycol added thereto, but which could equally well be a gaseous fluid. This collector fluid or heat-conveying fluid is cooled by the evaporator of the heat pump, and is then sent into the ground in order to be heated by coming into contact with the surrounding medium, thereby extracting heat energy therefrom. Each linear meter of the probe buried in the surrounding medium in question can thus bring a few joules of thermal energy to the heat pump when the circuit is in operation, and the fluid as heated in this way then returns to the heat pump which concentrates and delivers the thermal energy as collected in this way.
The technique described above should be distinguished from the technique described for example in U.S. Pat. No. 5,561,985 which does not make use of a “collector probe” in the meaning of the invention, i.e. a circuit in which a heat-conveying fluid circulates (and not a refrigerant fluid), without changing phase. That document proposes burying an evaporator, i.e. a heat exchanger in which the refrigerant fluid coming from the compressor is taken to the evaporator while in the liquid phase and leaves the evaporator in the vapor phase. DE-A-103 27 602 describes a comparable technique in which a tube receives a CO2 condensate that flows downwards in the form of a film along the walls of the tube to a deep zone at higher temperature where the CO2 vaporizes and is returned under pressure by rising up the tube. In contrast, a “collector probe” is never directly connected to the compressor, the heat-conveying fluid that circulates therein being different from the refrigerant fluid of the circuit including the compressor.
Collector probes are generally implemented in the form of a tube constituting a loop connected at each of its ends to a respective terminal of the heat pump. The nature of the tube, and most particularly the thermal conductivity of its wall, determines its heat exchange characteristics with the surrounding medium. Furthermore, the diameter of the tube and the longer or shorter length of the loop determine the heat exchange area, and thus the mass of the surrounding medium from which heat is collected.
A first technique known as “horizontal” collection consists in burying the tube in the ground to a shallow depth (about 50 centimeters (cm) to 70 cm), causing it to follow a sinuous path so as to occupy a maximum area of ground in order to engage a sufficient mass of the surrounding medium. For the purposes of burying the probe, said technique requires the ground to be stripped over a large extent, or else it requires trenches to be dug, with various constraints that stem therefrom: earth-moving costs; impossibility of placing the collection network under a house; and restrictions on how the ground can be used after the tube has been buried, for example no possibility of planting trees therein.
A second technique known as “vertical” collection consists in drilling a vertical borehole to a depth that may be as much as 100 meters (m), and then burying over the entire depth one or more looped tubes. Given the depth that is to be reached, this technique requires a borehole of relatively large diameter, of the order of 200 millimeters (mm), thus requiring specialist equipment that is heavy and bulky in use. It can indeed be deployed from a small area of ground, but it suffers from other drawbacks: cost and duration of drilling; poorly-controlled heat exchange; the medium is involved only in the form of a single cylindrical mass surrounding the borehole.
U.S. Pat. No. 5,339,890 describes a “collector probe” embodied in the form of a flexible tubular element provided at one of its ends with both the fluid inlet and the fluid outlet, and in which the other end is a free end. That configuration enables the tubular element to be inserted by means of its free portion into a gallery that opens out to the surface via a single point. The buriable tubular element is strong enough to be capable of being pushed along said gallery over its entire length from only one of its ends.
However, no presently-proposed solution for burying a collector probe for a heat pump has been found to be genuinely satisfactory, neither economically speaking nor in terms of the effectiveness of heat exchange.
One of the objects of the invention is to propose a collector probe, which probe is capable specifically of being optimized from the point of view of heat exchange effectiveness compared with previously-proposed solutions.
The starting point of the invention lies in noticing the importance of controlling the flow conditions by a particular configuration of the respective inside surfaces of the fluid return tube and of the fluid admission tube, to obtain a noticeable increase in thermal efficiency.
The probe proposed by the invention is a probe of the type known in U.S. Pat. No. 5,339,890, that is to say comprising a circuit for circulating a heat-conveying fluid, the circuit having a fluid inlet and a fluid outlet suitable for being connected to respective terminals of a heat pump. The circuit comprises at least two tubes extending in parallel, with a fluid admission tube connected to the fluid inlet and a fluid return tube connected to the fluid outlet. The fluid admission and return tubes are in communication with each other at their distal ends, and they are made with a wall in common over their entire length, thus forming a single tubular element for burying having a proximal end with the fluid inlet and outlet, and a distal end that is free.
In a manner characteristic of the invention, the inside surface of the wall of the fluid return tube is provided with portions in relief suitable for creating turbulence in the fluid flowing in said tube, and the inside surface of the wall of the fluid admission tube is a smooth surface suitable for encouraging laminar flow of the fluid flowing in said tube.
The portions in relief in the fluid return tube provide a return flow that is slow and turbulent, encouraging heat exchange with the surrounding medium, unlike the smooth surface of the admission tube that, on the contrary, encourages a fast flow minimizing heat losses.
U.S. Pat. No. 5,339,890 does not describe any configuration of a probe of this type: the only portions in relief that are present are to be found on the outside surface of the fluid tube, and therefore have no effect on the flow conditions of the fluid circulating inside that tube. On the contrary, that document is of the opinion (column 5, lines 44 to 50) that “there is no need for spacers or fins to control annular spacing between the internal tube 60 and the external tube 42 or to create turbulence because positioning of the internal tube 60 in relation to the external tube 42 is unimportant and fluid flow patterns are controlled by other factors including diameter, lumen, and flow rate”. In contrast, the present invention proposes controlling the respective flow regimes in the fluid admission and return tubes by giving the inside surfaces of these tubes configurations that are suitable for encouraging the looked-for type of flow.
According to various preferred and advantageous characteristics:
- the common wall is an isothermal wall and/or enclosing the insulating cavities;
- the fluid flow section of the return tube is greater than the fluid flow section of the admission tube;
- the outside section of said single tubular element for burying is uniform, in particular circular, over the entire length of said element;
- the outside diameter of said single tubular element for burying is less than 150 mm, preferably less than 100 mm, most preferably less than 50 mm;
- the tubes are made of a flexible material suitable for conferring flexibility to the tubular element for burying;
- the distal end of said single tubular element for burying is provided externally with a fitted endpiece; and
- along selected fractions of its length, the probe may further include reinforced insulation of the fluid admission tube and/or of the fluid return tube.
In a first embodiment, the fluid admission and return tubes are tubes engaged one inside the other, one of the tubes being an inner tube open at its distal end, with its wall constituting said common wall, and the other of the tubes being an outer tube containing the inner tube and closed at its distal end. The inside surface of the inner tube is smooth, and the outside surface of the same inner tube is provided with said portions in relief.
In another embodiment, the fluid admission and return tubes are adjoining tubes. There may be a single admission tube associated with a single return tube, the section of the return tube being greater than the section of the admission tube. However, it is also possible to provide at least three tubes, with the number of fluid admission tubes being smaller that the number of fluid return tubes, the total section of the return tube(s) being greater than the total section of the admission tube(s).
The invention also provides a network for collecting thermal energy from the ground for a heat pump, the network comprising a plurality of probes as defined above buried in tunnels dug in the ground. Such a network presents a three-dimensional configuration bounded by an envelope volume extending over a given area of ground and over a given burying depth. Advantageously, the probes include reinforced insulation of the fluid admission tube and/or of the fluid return tube over their portions extending between ground level and said envelope volume.
Typically, the envelope volume extends to a depth lying in the range 0.5 meters (m) to 10 m below ground level, and the probes are placed with their terminal ends at the lowest point in order to avoid bubbles forming.
This characteristic configuration enables the tubular element to be inserted via its free portion into a gallery of small diameter (a few tens of millimeters) that opens out to the surface at a single point.
This is made possible in particular by the flexibility of the buriable tubular element that enables it to accommodate the complex curves that the track of the gallery is likely to follow, while still being strong enough to be capable of being pushed into the gallery over its entire length from only one of its ends.
Drilling machines of small dimension do indeed exist that enable galleries to be dug very easily and at low cost over several tens of meters in length and having a diameter of several tens of millimeters (e.g. a diameter of 50 mm), e.g. the miniature drilling machines or “moles” used for passing water supply tubes under roads or buildings, without it being necessary to dig an open trench. The galleries dug by said drilling machines are not necessarily vertical or horizontal, but may follow any arbitrary curved track adapted to the configuration of the site. Digging such a gallery does not lead to significant damage to the terrain, which eliminate the major drawback of horizontal collection, also making it possible to dig a gallery that is sufficiently deep to avoid interfering with planting trees and to avoid any need to drill holes in the ground.
Finally, drilling such a gallery requires only a small area, and can even be undertaken from inside a building, with heat energy then being collected either in full or in part from under the building.
The use of the collection probe of the invention thus minimizes nuisance while it is being buried and can be implemented at small cost and without putting restrictions on subsequent utilization of the terrain.
There follows a description of an embodiment of the device of the invention given with reference to the accompanying drawings in which the same numerical references are used from one figure to another to designate elements that are identical or functionally similar.
The outer tube 12 is closed at its distal end 18 by any suitable means, for example by a mechanical plug or by being molded into a closed configuration while hot. This end 18 is also advantageously covered by a protective endpiece 20, e.g. made of metal, to make it easier to engage the tube in a tunnel.
The inner tube 14 is open at its distal end 22 so as to leave a gap 24 between said end 22 and the facing closed end 18 of the outer tube 12.
At the proximal end, the outer and inner tubes 12 and 14 are connected to a header connection element 26 enabling the two tubes to be connected to each other, with the inner tube 14 occupying the central position and emerging at 28. The header connection 26 also has longitudinal passages 30 (also visible in the section of
The inner tube 14 may advantageously be made in the manner shown in detail in
An isothermal core 46 serves to provide thermal insulation between the flows flowing on either side of the tube 14. In variant or in addition, the tube walls can be hollow, with cavities such as 48 that provide reinforced insulation of the flow from one portion to another of the wall.
Fluid circulates in the probe as follows.
The cold fluid leaving the heat pump is introduced (arrow 50) into the proximal free end 28 of the inner tube 14 (the fluid admission tube) from which it flows to the distal end 22, the smooth surface 38 encouraging laminar flow of the fluid in the tube.
The fluid then opens out into the zone 24 situated at the distal end of the outer tube 12 (the fluid return tube), from which it is sent back towards the opposite end of the same tube (arrows 52, 54) over its entire length, in order to be collected (arrow 56) via the passages 30 leading to the outlet 34 of the header connection element 26 (arrow 58). The presence of the portions in relief 44 contributes to create turbulence in the fluid, thereby slowing it down and increasing heat exchange with the outside.
The fluid introduced from the proximal end of the inner tube 14 is taken directly to the distal opening 22 of that tube without negotiating any obstacles other than the curves followed by the tube. On reaching the distal opening, the fluid at the distal end of the outer tube returns towards the proximal end thereof.
While traveling along the outer tube 12, the collector fluid receives heat energy from the surrounding medium, and then returns to the heat pump, which concentrates and extracts said heat energy prior to returning the cooled fluid to the probe in order to renew a collection cycle.
It should be observed that with the configuration shown, heat collection begins in the distal region of the probe, i.e. the region that is likely to be the hottest and the region in which temperature is renewed the most quickly, after which the liquid rises back towards the heat pump which is thus fed with heated fluid.
Under certain circumstances, the flow direction of the fluid can be reversed, i.e. the fluid can be admitted via the orifice 34 into the outer tube 12 (which then becomes a fluid admission tube) in order to travel therealong over its entire length and be collected at the distal end by the tube 14 (which thus becomes the fluid return tube), and be extracted therefrom via the opening 28. Under such circumstances, and unlike the preceding configuration, it is then the portion of the surrounding medium that is closest to the header connection element 26 that is involved the most in heat exchange. It will readily be understood that one or other of those two configurations can be selected merely by reversing the direction in which fluid circulates through the collector probe, thus making it very simple to optimize heat exchange as a function of requirements, or possibly to test both configurations and compare the results obtained.
The outer tube 12 is made of a material that presents strength and semi-stiffness that are sufficient under most circumstances to enable the tube to be pushed along the tunnel 16 after it has been bored. Where necessary, the tube may be pressurized so as to increase its stiffness and its strength. The material should also be selected so as to present sufficient ability to withstanding stretching so as to make it possible, where necessary, to pull the tube along the tunnel from an open remote end thereof. The tube should also be sufficiently strong to withstand being flattened and it should be inert relative to the fluid that is to circulate therealong.
In practice drinking water supply tubes made of polypropylene (diameter 32 mm, thickness 3.6 mm) are entirely suitable for use in most circumstances for heat pumps using a mixture of water and ethylene glycol as the heat-conveying fluid in the heat collector network.
If the heat-conveying fluid of the network is a gas, it is possible to use a metal tube of small diameter, so as to limit the amount of gas used and so as to reduce head losses. In particular, it is possible to use a tube made of stainless steel tubing closed at its end by welding, with tube segments being connected end to end by welding (e.g. orbital TIG welding) either beforehand or else as necessary while the tube is being buried: under such circumstances, it is possible to build up quickly a continuous tube of strength that is entirely uniform along its entire length, using tube segments of arbitrary length.
The inner tube 14 can be a tube of plastics material that is sufficiently flexible, and that is provided with portions in relief 44, e.g. fluting, bulges, etc., integrally molded therewith. Its length is matched to the length of the outer tube so as to ensure that its distal opening 22 is situated a few centimeters away from the end closure 18 of the outer tube, and said distal opening may be chamfered to maximize fluid delivery. Lateral slots (not shown) may be provided to enable fluid to circulate even in the event of the distal portion of the probe being flattened or blocked in some other way.
Like the outer tube 12, the inner tube 14 must be inert relative to the heat-collection fluid. Along its length its minimum radius of curvature must be smaller than or equal to that of the outer tube, and its outside diameter must be smaller than the inside diameter of the outer tube so as to enable it to be engaged inside the outer tube.
It should be observed that there is no need for the inner and outer tubes to be strictly coaxial; the inner tube 14 may come into contact with the inside wall of the outer tube 12, e.g. in regions where the probe is curved, without that impeding fluid circulation: flow section remains the same providing the tubes are not flattened, and indeed from the thermal point of view, such a singularity can have the advantage of creating additional turbulence where it occurs.
The material of the inner tube 14 is preferably a material of low thermal conductivity, or else it is constituted by a structure that includes an isothermal core 46 and/or the insulating cavities 48 as shown in
The respective sections of the outer and inner tubes are advantageously selected so as to define an optimum ratio between the admission flow section (in the inner tube 14), and the return flow section (between the outer tube 12 and the inner tube 14). When the admission section is less than the return section, the speed of the admission flow is higher than that of the return flow. An admission flow that is fast minimizes losses in the inner tube 14, while a return flow that is slow and turbulent encourages heat exchange between the outer tube 12 and the surrounding medium.
Other embodiments of the invention can be envisaged, with a tube configuration different from that described above, in which an inner tube 14 is engaged inside an outer tube 12 in the manner shown in
Thus, as shown in
In the variant shown in
In yet another variant, as shown in
In the above, all that has been said relating to the presence of portions in relief 44 suitable for forming turbulence in the return flow, and a smooth surface for the admission flow, and also concerning thermal insulation between the admission and return flows continues to be applicable, mutatis mutandis to the various embodiments shown in
The inlet 28 of the first probe 10 is connected to the fluid outlet 86 of the heat pump 88, the outlet 34 of said first probe is connected to the inlet 28′ of the second probe 10′, and so on, the outlet 34″ of the third probe 10″ being connected to the fluid inlet 90 of the heat pump 88.
It is also possible to connect a plurality of collector probes in parallel, when the heat pump fed by the probes requires a flow rate that cannot validly be satisfied by the internal section of one of the tubes of a single probe.
As will readily be understood, the collector probe of the invention or a plurality of collector probes of the invention can be buried in a tunnel following a path that it defines as a function of topographical constraints and of the nature of the subsoil. The tunnel can equally well be sloping, vertical, initially sloping followed by a horizontal level, curved, etc. It is possible to provide an installation with tunnels that descend to a variety of depths in the ground and that are disposed one above another with sufficient spacing. Such a configuration serves in particular to engage a mass of heat-collection medium that is much greater than is possible with configurations that are linear or in two dimensions, as is the case with conventional collection systems that are vertical or horizontal.
In the example shown, this network has five probes 10 as described above, which are introduced into tunnels dug substantially from the same location, each opening to the atmosphere via a single orifice only.
After being inserted in the tunnels, the probes 10 are connected in series and/or in parallel and they are connected to the heat pump 88.
In the advantageous configuration of
The mass of ground engaged in collecting heat is thus defined by a three-dimensional volume 92 situated at shallow depth and occupying the ground around the heat pump.
This collection volume 92 extends at least 50 cm below ground level, and it is possible to install a network of probes even in the presence of trees 94, and it is also possible to pass under the dwelling 96, as can be seen in
Furthermore, the tubes and probes 10 are advantageously provided with thermal insulation 98, e.g. in the form of insulating sleeves, covering those portions that extend between ground level (manifold or connection to the heat pump 88) and the upper level of the collection volume 92. This serves to avoid ineffective heat exchange in this shallow region of the ground which can drop to a temperature that is too low to provide satisfactory thermal efficiency.
By using a plurality of probes each having a length of 25 m, for example, it is thus possible to engage a large mass of surrounding medium even in a parcel of land that is small, for example having maximum dimensions typically of the order of 35 m to 50 m. For parcels of land that are of even smaller dimensions, for example in areas of low-rise housing, it is possible to provide a plurality of superposed sheets of probes, for example a first sheet occupying a collection volume situated 0.5 m to 1.5 m below ground level, a second sheet in a collection volume situated in the range 3 m to 4 m below ground level, etc., so as to increase the mass of external medium involved in spite of the small ground area.
1. A collector probe for collecting heat energy from the ground for a heat pump, the probe (10) comprising a circuit for circulating a heat-conveying fluid, the circuit having a fluid inlet (28) and a fluid outlet (34) suitable for being connected to respective terminals (86, 90) of a heat pump (88), the circuit comprising at least two tubes (12, 14; 64, 66; 74, 76; 80, 82, 84) extending in parallel, with a fluid admission tube (14; 66; 76; 84) connected to the fluid inlet and a fluid return tube (12; 64; 74; 80, 82) connected to the fluid outlet, in which probe the fluid admission and return tubes are in communication with each other at their distal ends, are made with a wall in common over their entire length, and form a single tubular element for burying having a proximal end with the fluid inlet and outlet, and a distal end that is free,
- the probe being characterized in that the inside surface of the wall of the fluid return tube is provided with portions in relief (44) suitable for creating turbulence in the fluid flowing in said tube, and the inside surface of the wall of the fluid admission tube is a smooth surface suitable for encouraging laminar flow of the fluid flowing in said tube.
2. The collector probe of claim 1, in which said common wall (36, 40, 46) is a wall with a thermally insulating core (46) and/or enclosing insulating cavities (48).
3. The collector probe of claim 1, in which the fluid flow section of the return tube is greater than the fluid flow section of the admission tube.
4. The collector probe of claim 1, in which the outside section of said single tubular element for burying is uniform over the entire length of said element.
5. The collector probe of claim 4, in which the outside section of said single tubular element for burying is circular section.
6. The collector probe of claim 1, in which the outside diameter (d) of said single tubular element for burying is less than 150 mm, preferably less than 100 mm, most preferably less than 50 mm.
7. The collector probe of claim 1, in which the tubes are made of a flexible material suitable for conferring flexibility to said single tubular element for burying.
8. The collector probe of claim 1, in which the distal end of said single tubular element for burying is provided externally with a fitted endpiece (20).
9. The collector probe of claim 1, in which said fluid admission and return tubes are tubes (12, 14) engaged one inside the other, one of the tubes being an inner tube (14) open at its distal end (22), with its wall constituting said common wall, and the other of the tubes being an outer tube (12) containing the inner tube and closed at its distal end (18), the inside surface (38) of the inner tube (14) being smooth and the outside surface (42) of the same inner tube (14) being provided with said portions in relief (44).
10. The collector probe of claim 1, in which said fluid admission and return tubes are adjoining tubes (64, 66; 74, 76; 80, 82, 84).
11. The collector probe of claim 10, comprising a single fluid admission tube (66; 76) and a single fluid return tube (64; 74), and in which the section of the return tube is greater than the section of the admission tube.
12. The collector probe of claim 11, comprising at least three tubes (80, 82, 84), with the number of fluid admission tubes (84) being less than the number of fluid return tubes (80, 82), and in which the total section of the return tube(s) is greater than the total section of the admission tube(s).
13. The collector probe of claim 1, further comprising, over selected fractions of its length, reinforced insulation (98) of the fluid admission and/or fluid return tube.
14. A collector network for collecting thermal energy from the ground for a heat pump, the network being characterized in that:
- it comprises a plurality of probes (10) according to any one of claims 1 to 13, the probes being buried in tunnels dug in the ground; and
- it presents a three-dimensional configuration defined by an envelope volume (92) extending over a given area of ground and over a given burying depth.
15. The collector network of claim 14, in which the probes (10) have reinforced insulation (98) of the fluid admission and/or return tubes over the portions thereof that extend between ground level and said envelope volume.
16. The collector network of claim 14, in which said envelope volume extends to a depth lying in the range 0.5 m to 10 m below ground level.
17. The collector network of claim 14, in which said probes are disposed with their terminal ends at their lowest points.