FIELD OF THE INVENTION

The present invention relates to a system to extract heat from hot rocks and a geothermal plant that utilizes it.

STATUS OF THE ART

The exploitation of the geothermal Energy is well known since many years whether not much diffused.

First we must distinguish between low enthalpy geothermal energy, which utilizes tight wells (for example 10-20 cm) and not much deep (for example 20-200 m), and high enthalpy geothermal energy, that utilizes wider and deeper wells; the differences between the two systems are not limited to the wells dimensions.

Low enthalpy geothermal energy exploits conceptually the underground, or, better, the superficial underground at low depth, as source of heat at constant temperature (usually low, typically 12-17° C): during winter the heat is transferred to the houses to be heated, while during summer the excessive heat of the building is transferred to the soil; this work is made possible thanks to the presence of a heat pump and one or more geothermal probes (that can be horizontally or vertically deployed). Low enthalpy geothermal power us commonly used in small plants suitable for few housing units.

High enthalpy geothermal energy exploits the underground, or, better, the hot deep rocks as source of heat at high temperature (typically 130-250° C); thanks to the heat an high pressure steam flow is generated to move a turbine to generate mechanical energy; the rotation of the turbine creates power thanks to an alternator. High enthalpy geothermal energy is commonly used for large plants, also said geothermal power plants.

Currently three geothermal power plants typologies are known.

The first generation geothermal plants utilize the energetic properties of natural geothermal vapors and steam sources like geysers and fumaroles.

Second and third generation power plants utilize the steam obtained by injecting water into the natural fractures of the earth's crust, in the second generation plants, or into deep artificial wells, in the third generation power plants. Second generation plants exploit vertical fractures that go down to very deep hot rocky layers; these fractures are linked by other natural horizontal fractures in the deep layers; the water is injected and it is collected as steam thanks to these natural horizontal fractures. In the third generation plants two adjacent wells are drilled to reach very deep hot rock layers where some natural horizontal fractures are available; the water is injected in the descending well and it returns in form of steam from the ascending well thanks to the horizontal natural fractures. The hot rock layers that can be used in the second and third generation power plants are commonly located at a depth ranging from 500 m and 7000 m, apiece the geographic area; the depth depends, among other things, to the depth where the magma is located.

From what has been said is clear that all the geothermal power plants are strictly dependent from environmental conditions, and this is a critical issue; the second and the third generation power plants require the presence of natural fractures in very deep and hot rock layers, and their persistence notwithstanding seismic events.

In all these geothermal power plants is exploited the energy generated by the hot layers of the earth's crust; there are the following major issues:

- dispersion of thermal and mechanical energy due to the ascent of the steam, which is cooled back and is dispersed;

- increase of the local seismicity due to the action of water injection and / or dispersion of the high pressure steam;

- transport of toxic substances present in the deep subsoil (such as hydrogen sulfide, carbon dioxide, ammonia) dragged by the rising steam to the atmosphere on the surface;

- transport of corrosive substances present in the deep subsoil (such as hydrogen sulfide, carbon dioxide, ammonia) entrained by the rising steam and then, on one hand, corrosion of the walls of the wells and the other, of the devices that receive the steam to be turned into mechanical energy (as regards the second aspect is essential to treat the steam before it comes in contact with the turbine blades);

- unpredictability of the steam flow available for the generation of mechanical energy due to the many variables related to uncontrollable environmental conditions that cannot be precisely determined in advance.

These problems involve the need to create very large geothermal plants.

From the German patent application No. 102007025905 is known a "probe head" for geothermal applications comprising a cylindrical bulb; the bulb is adapted to be inserted in the subsoil; a fluid is circulated in the bulb; the motion of the fluid in the bulb is of substantially laminar type and follows a spiral shape path as a result of the action of a fluid flow guiding device placed in the lower zone of the bulb itself. From the American patent application No. 2007/0023163 are known various exchanging heat solutions to be inserted into the ground which comprise a cylindrical shape bulb. Although the title refers to "coaxial flow", all solutions provide a descending flow at the center of the bulb and a rising flow along the walls of the bulb; the descending flow is laminar and rectilinear, the ascending flow is laminar and spiral shaped, so it is inappropriate to speak of "coaxial flow." One (see Fig. 43) of the various solutions is to use a corrugated tube as casing of the bulb and a central helical structure which serves to guide the ascending flow; these corrugations of the pipe create some vortices. Nonetheless the difference in inclination between this spiral shaped channel and these horizontal corrugations is relatively small and these corrugations are small. Therefore these corrugations can cause a mild mixing that affects only the fluid flowing in the outermost part of the channel; in other words, there is not a real turbulent motion in the fluid and the vortices are located in the areas adjacent to the cylindrical walls of the bulb. Therefore, this specification solution improves only slightly the thermal transfer. SUMMARY

The general object of the present invention is to overcome the drawbacks of the prior art.

A first more specific object is to provide a system for extracting heat from hot rocks that does not rise pollution problems at the surface.

A second more specific object is to provide a system for extracting heat from hot rocks which does not rise corrosion problems.

A third more specific object is to provide a system for extracting heat from hot rocks that is highly efficient in terms of energy output.

A fourth more specific object is to provide a system for extracting heat from hot rocks that do not disperse substances in the subsoil.

A fifth more specific object is to provide a system for extracting heat from hot rocks that is not strongly dependent on environmental conditions.

A sixth more specific object is to provide a system for extracting heat from hot rocks that is simple and relatively inexpensive to construct and implement.

These and other objects are achieved thanks to the system to extract heat from hot rocks having the features as set out in the annexed claims, which are an integral part of this description.

According to a further aspect the present invention also relates to a geothermal plant that uses such a system.

The Applicant is directed toward a solution for high enthalpy geothermal because it allows a much higher energy efficiency since it operates with very high temperatures (eg 250-450° C) and therefore does not require heat pumps; it is good to clear at first that the present invention can be used with advantage also operating at medium temperatures (eg 180-250 ° C) or at relatively low temperatures (eg 130-180 ⁰ C).

To avoid contamination and corrosion, the Applicant has thought to use a closed hydraulic circuit in which to circulate a fluid for heat transport that is not in direct contact with rock or soil.

To reduce the cost of installation, the Applicant has to a system that could be inserted into a single well long, for example, 500-3500 m, and large, for example, 1-2 m, resulting from an artificially made borehole; of course, this does not exclude the possibility of providing a plurality of wells with a corresponding plurality of systems placed inside. Even if also deeper drillings could be carried out: with the current technologies it is possible to arrive at 7000-8000 m.

To carry out an efficient heat exchange, the Applicant has thought to a cylindrical relatively short bulb eg. 10-100 m (in the case of deep drilling and / or not too high temperatures the length of the bulb could increase and reach for example 300 m), preferably located at the deep hot rocks or geothermal aquifers, the so-called geothermal "reservoires", which permeate the deep hot rocks. In addition, following the ideas set forth above, the Applicant then realized that the walls of the well can become a source of heat loss instead of an element useful to the solution; in fact, their temperature slowly decreases along the extension of the well from the bottom to the surface, so a fluid at the well bottom heated to the temperature corresponding to the bottom of the well tends to transmit heat to the rocks and to the terrain surrounding the well as it rises to the surface; therefore, the Applicant has provided the thermal insulating means arranged along the well. Finally, from the point of view of the mechanical arrangement, the cylindrical bulb and the cylindrical isolating means are aligned (speaking in mathematical terms we should say "approximately aligned") so as to realize a single cylindrical shape (speaking in mathematical terms we should say "approximately cylindrical") inserted into a well (cylindrical).

In general, the system for extracting heat from hot rocks according to the present invention is able to be inserted entirely into a well which reaches said hot rocks and includes a closed hydraulic circuit suitable for the circulation of a fluid for heat transport; it further comprises:

- a bulb of a cylindrical shape (that is elongated and of substantially constant radial dimensions) adapted to circulate said fluid in its interior and adapted to be placed in contact with said hot rocks on the outside, to absorb heat from them and transfer the heat to said circulating fluid in its interior,

- a first conduit (inside the well) adapted to conduct said fluid to and within said bulb,

- a second conduit (inside the well) adapted to conduct said fluid out of and away from said bulb,

- cylindrical shape thermal insulating means, aligned to said bulb, arranged along said shaft to said bulb, and adapted to thermally isolate said first and second conduits from each other and from the ground and / or the surrounding rocks;

the bulb comprises:

- means for generating turbulent motions in said fluid circulating inside it, and

- a plurality of fins which extend from the peripheral zone of the bulb toward the inside arranged to come into contact with said fluid in turbulent motion, thereby increasing the heat transfer toward said fluid because the increased effective contact area of heat transfer.

It is recalled here that a turbulent regime is a fluid motion type in which the viscous forces are not sufficient to counteract the forces of inertia: the particles motion of the fluid happens in a chaotic way, without following tidy trajectories as in the case of laminar flow regime. A turbulent flow differs from a laminar flow as the vortical structures that are present in the fluid are of different magnitude and speeds which make the flow unpredictable along the time even if the motion is deterministic. The heat transfer is greatly improved not only because the fins surface is much larger than that of a cylindrical surface (straight or corrugated), but also for the fact that thanks to the turbulent motion all the fluid comes in good contact with the fins; therefore this is a synergistic effect.

In the following are synthetically the main advantageous characteristics of the present invention. The diameter of said thermal insulating means, which then corresponds substantially to the diameter of the well where there is the rock, is typically equal to the diameter of said bulb multiplied by a factor between 1.00 and 1.20; is worth to observe here that in the sections of the well crossing ground instead of rock, it will typically be provided a lining around the well, typically realized in the form of coating the wall of the well, to define its size and to prevent collapses.

Typically, both said conduits terminate in correspondence of the upper wall of said bulb in such a way that it is not necessary to provide additional radial space for these links.

Said bulb typically has a length between 10 m and 100 m.

Said bulb typically has a diameter between 1 and 2 m.

Said bulb typically has a length / diameter ratio between 10 and 100.

Said heat insulating means typically have a length from 500 m to 3500 m; this length corresponds, in fact, to the length of the well except for the length of the bulb and a possible small margin.

Said blades extend advantageously up to a central area of said bulb.

Said blades are advantageously radial.

The surface of said blades is advantageously parallel to the axis of said bulb.

According to a preferred embodiment, said blades produce an external shape of said bulb with elongated lobes, which can be called as a "daisy shape".

Said means for generating turbulent motions may advantageously comprise passive rotating mixers; in this way, the flow motions that may be generated are very strong but do not require additional means to realize the rotation.

Said means for generating turbulent motions may advantageously comprise twisted wires or strips, in particular with course and / or random arrangement; said wires or strips can be made of metallic material.

Said means for generating turbulent motions may advantageously comprise a plurality of fins which extend from the peripheral zone of the bulb toward the inside.

Said means for generating turbulent motions are typically placed inside of said bulb, in particular in the extreme lower area and / or, at least, in an intermediate area suited to conduct fluid and / or, at least, in an intermediate region suited to heat transfer.

Said insulating means advantageously comprise at least one layer of sand or gravel and / or at least one layer of sealing material; gravel and sand provide a good thermal insulation materials and are cheap and easily available in a place where a borehole is made; the sealing material very effectively prevents the ascent of toxic substances along the well. It 's very advantageous to provide long layers of gravel or sand alternating with short layers of sealing material along the well.

Advantageously, said fluid for heat transport is a nanofluid; in this way, the heat transport is very effective and efficient thanks to the high specific heat of nanofluids.

Said bulb advantageously comprises a plurality of longitudinal modules joined together; in this way it is much more easy to produce industrially bulbs of variable length, depending on specific installation conditions, in particular the rocks temperature and the hot rock layer thickness.

Said bulb advantageously comprises a central pipeline coupled to said first conduit and suited to conduct said fluid in an internal area of said bulb to a lower end of said bulb; in this way, the descending fluid that is colder, flows away from the walls perimeter of the bulb, while the ascending fluid that is warmer flows close to the perimeter walls; in addition the heating of the fluid due to the perimeter walls effects enhances its upward flow strength.

In general, the geothermal power plant, that is to extract heat from the ground, according to the present invention, comprises at least one system as defined above.

Typically, the system further comprises:

- a closed hydraulic circuit with a working fluid to directly generate work,

- a heat exchanger placed in correspondence of an upper end of said second conduit and suited to directly exchange heat between said fluid for transport of heat, coming from said bulb, and said working fluid.

The system advantageously comprises a turbine driven by the working fluid and an electrical machinery mechanically coupled to the turbine and suited to generate electrical power due to the rotation of the turbine.

LIST OF FIGURES

The technical features of the present invention and its advantages shall become apparent from the below description to be considered considered with the attached drawings, wherein:

Fig.1 schematically shows the vertical section view of an embodiment of a geothermal power plant according to the present invention with a system of extraction of heat in a well,

Fig.2 schematically shows the vertical section view of the bulb of the system of Fig.1 from which some components have been removed, ie the laminar elements (fins),

Fig.3 schematically shows the perspective view of a passive rotating mixer placed in the bulb of Fig.2,

Fig.4 shows a schematic vertical sectional view of the bulb of the system of Fig.1 in which are inserted a series of laminar elements, but from which a component has been removed, that is the passive rotating mixer of Figure 3,

Fig.5 schematically shows the partial view in cross section of a laminar element (first variant),

Fig.6 schematically shows the perspective view of a passive rotating mixer usable in the bulb of Fig.2 and Fig.4, in particular fixed to the thin plates of the lamellar element,

Fig.7 schematically shows the perspective view of a passive rotating mixer usable in the bulb of Fig.2 and Fig.4, in particular fixed to the perimeter walls of the bulb, Fig.8 schematically shows the vertical section view of the lower part of a bulb of a system according to the present invention in which are inserted a series of laminar elements,

Fig.9 schematically shows the cross-sectional view of a laminar element (second variant), and

Fig.10 schematically shows the lower part of a particular well that can be used for the system according to the present invention.

DETAILED DESCRIPTION

Both said description and said drawings are to be merely considered for illustrative purposes and are not exhaustive; therefore, the present invention may be implemented according to other and different embodiments; furthermore, it must be taken into account that said figures provide schematic and simplified views. In Fig.1 we see a well 1 resulting from a drilling (cylindrical and substantially vertical) in which is inserted an embodiment of the system 2 of extraction of heat according to the present invention. On the surface, on the ground, are a set 3 of machines connected to system 2 that are suitable to transform the heat extracted from the system 2 first into mechanical energy and then into electrical energy; as will become clearer hereunder, the set 3 corresponds to the processing section of a geothermal plant. It is noted that, in this figure, the system 2 does not reach exactly the lower end, ie the bottom, of the well 1 and does not reach exactly the upper end of the well 1.

It must be cleared that Fig.1 is not to scale (in particular the width has been increased compared to the height) and is much simplified; this has been done to facilitate the readability of the drawing.

At the bottom of the well 1 , outside it, there is a layer 4 of rocks very hot, for example at a temperature of 250-450 ° C.

At the bottom of the well 1 , in its interior, there is a bulb 5 of the system 2 which is cylindrical and hollow; the bulb 5 is, as far as possible, in contact with the hot rocks of the layer 4; the bulb 5 is connected to a first conduit 6 and to a second conduit 7; both conduits 6 and 7 are within the well 1 , in particular substantially parallel, and are surrounded by thermal insulating means that thermally isolate them from each other and from the ground (in the superficial part of the well 1 ) and from the rocks (in the deep part of the well 1 ).

The thermal insulating means correspond essentially to two long layers 81 of gravel or sand. However, in addition, three short layers 82 of sealing material have been provided, one near the bulb 5, one near the surface of the well 1 and one intermediate; in other words, the sealing layers alternate with insulating layers; of course, the number and the size of the layers depend on the particular implementation of the system according to the present invention even if it is advantageous to provide at least a sealing layer on the surface and / or a sealing layer on the bottom.

As already mentioned, Fig.1 shows a simplified example of embodiment; more realistically, 100 meters of thermal insulating means comprise, for example about ten layers of sealing material with a thickness of 1-3 meters, which are then spaced apart by layers of insulating material with a thickness of 9-7 meters; the last stretch of the well, for example the last 100 meters, is advantageously filled entirely of sealing material; a sufficiently sealing and sufficiently economical material is the concrete.

The sealing material can be advantageously high density concrete, and in certain circumstances, such as for example in presence of drinking groundwater layers, such materials may also represent the totality of the material inserted into the well thanks to its sealing and heat insulating characteristics.

The set of the conduit 6, the bulb 5 and the conduit 7 is a closed hydraulic circuit adapted to the circulation of a fluid for heat transport; such a fluid is injected into the conduit 6 to the surface at low temperature (eg 40-90 ° C ), descends along the conduit 6, enters the bulb 5; circulates in the bulb 5 and is heated to a temperature close to that of the layer 4 (for example 250-450 ° C), exits from the bulb 5, rises through the conduit 7 and exits from the conduit 7 to the surface at high temperature; if the thermal insulating means are well made, the fluid will not lose much when it exits from the bulb temperature of from 5 to when it leaves the conduit 7 to the surface; this loss could be, for example 10 -20 ° C.

In Fig.1 , the bulb 5 and insulating means 81 and 82 are perfectly cylindrical, aligned and of the same diameter, and occupy exactly the cylindrical space of the well 1 ; in other words, the diameter of the well, the diameter of the bulb 5 and the diameter of the insulating means 81 and 82 are equal.

Of course, in reality this ideal situation does not occur. The well 1 is derived from a drilling which typically takes place partly in the ground and in part in rocks and therefore can not be perfectly cylindrical, inter alia, if the well passes through layers of soil will probably be necessary to carry out an excavation significantly greater than the shaft and achieve a lining, made typically in the form of coating the wall of the well, for example in concrete and / or metal plate, to hold the soil and allow the continuation of the drilling and the insertion of a system according to the present invention.

The bulb 5, to be able to descend along the well 1 , will typically have a diameter a little 'lower than that of the well, for example up to 20% lower; the light of this consideration, it is anticipated that between the bulb 5 and the wall inside of the well 1 , in the space that remains, being instead of the material with good thermal conductivity (eg high specific density nano-lubricants, consisting of mineral grease and nanoparticles, substances already known in technical and scientific literature, or advantageously a material such as the one described in Italian patent application n° CO2011A000024, incorporated herein by reference, which can be called "nano-concrete", which is a suspension such as aqueous solution containing metal nanoparticles, or a metal such as tin in the molten state containing nanoparticles of a metal such as iron) to obtain a good thermal efficiency and yield of the system according to the present invention. The insulating means 81 and 82, for as they are typically made, will fill the well 1 , and therefore while not perfectly cylindrical, they will have dimensions, in particular the diameter, almost equal to those of the well.

As already said, the solution object of the present invention has been designed for high enthalpy geothermal energy, so the well has to reach rock layers very hot, for example at a temperature of 250-450 ° C; layers of this type are usually located to a depth variable between 500 ft 3500 m, depending on the geographical area; the length of the thermal insulating means will then be approximately equal to that well minus the length of the bulb which, as will be explained better later on, will typically be of 10-100 m (in the case of deep drilling and / or temperatures are not too high, the length of the bulb could increase and reach for example 300 m). The solution object of the present invention can be used with advantage also operating at medium temperatures, for example 180-250° C, or at relatively low temperatures, for example 130-180° C.

The bulb is made in such a way as to absorb a lot of heat from the surrounding rocks and pass it effectively to the fluid that circulates inside it; details of the bulb 5 of Fig.1 are shown in Fig.2 and Fig.4 and will be described below with reference to these figures.

The bulb 5 is substantially cylindrical, in particular, presents superiorly a cap to facilitate the conveyance of the hot fluid in the conduit 7 of ascent; it is hollow and has an internal cavity 9 adapted to circulate the fluid for the heat transport; the cavity 9 is in communication with the conduit 6 of descent of the fluid and with the conduit 7 of ascent of the fluid; the conduit 6 is connected to a central pipe 10 adapted to conduct the fluid (cold) in a zone 11 inside the bulb 5 at its bottom end; the pipe 10 (together with the perimeter walls of the bulb 5) therefore inside the cavity 9 defines an annular zone 12 between the lower end zone 11 and the upper end zone 13 (in the example of Fig.2 and Fig.4: inside of the cap); in the zone 12 it takes place the slope of the fluid and its heating, as a result of the contact with the perimeter walls of the bulb 5; in the zone 12 various types of motion of the fluid take place, including natural convection, and turbulent motions induced, as will become clearer from the following.

In zone 11 , is mounted a rotating passive mixer 14 which is shown, in greater detail in Fig.3. It is basically divided into two parts; the central part is an impeller adapted to receive the fluid coming from the pipe 10 and to make rotate the whole mixer 14 for effect of the pressure exerted by the fluid on its blades;

the peripheral portion serves to impart a whirling and turbulent motion to the fluid; in particular, the peripheral part comprises a plurality (specifically, four) of perforated cups mounted on a perimeter band of the central part - the shape may seem that of an anemometer, but the effect on the fluid is very different. The fluid coming from the pipe 10 traverses the central portion of the mixer 14 and, thus causes its rotation; crossed the mixer 14, the fluid comes in contact with the inner bottom wall of the bulb 5 and reverses its motion, but due to of a particular conformation of this wall is also deflects radially and then ascends in correspondence of the peripheral part of the mixer 14;

in that ascending motion, the fluid is intercepted by the rotating elements of the perforated peripheral part of the mixer 14 which imparts a whirling component to the fluid motion; in addition, by using perforated elements, some turbulent and convective motions (for "Venturi effect") will be generated. The mixer 14 is called "passive" because it does not require any type of engine to operate, but simply exploits the kinetic energy of the fluid that it must shuffle.

It is clear that there would be heat transfer between the perimeter walls of the bulb 5 and the fluid which ascends in the intermediate annular zone 12 also without the usage of a mixer in the extreme lower zone 11 ; however, thanks to this device, the efficiency of the heat transfer is greatly increased since the mixer facilitates a uniform heating of all the fluid that comes out of pipe 10.

The conduits 6 and 7 both terminate in correspondence of the upper wall of the bulb 5 that, in the case of the example of Fig.2 has the form of a cap; in this way, the connection of such conduits with the bulb can be realized without the usage of components that protrude radially from to the bulb; as consequence, in the specific embodiment of Fig.2 and Fig.4, the diameter of the well depends only on the diameter of the bulb (and of course the type of soil and rocks in which the perforation is carried out) and not on the size of other elements.

Preferably, the axis of the conduit 6 coincides with the axis of the bulb 5 so as to obtain the most possible power of fall.

The bulb to be used for the present invention typically has a diameter between 1 and 2 m. The length of the bulb can be greatly variable, typically between 10 m and 100 m (but also longer lengths are not to be excluded, for example 300); this fact depends on the temperature of the rocks of the rock layer which is in contact with, on the thickness of such a rock layer, on the amount of heat to be extracted from that rock layer. In a relative sense, the bulb typically has a length / diameter ratio between 10 and 100. The diameter of the inner pipe is typically between 25 cm and 60 cm. The bulb is advantageously made of metallic material, particularly copper, copper alloys or steel; the bulb can also be produced by several superimposed layers of material.

In order to protect the bulb from the corrosive agents present in depth it is advantageous that it is covered on the outside by a protective layer; the material of this protective layer will typically be compact and resistant ceramic material, such as the "ceramic stoneware"; the thickness of this protective layer can range from 0.1 to 1.0 cm and depends on the conditions found on the bottom of the well. This protective layer can also be reinforced with metal mesh, preferably in steel, single or double, to increase resistance, for example in the case of bulbs of large size and / or subjected to particularly high temperatures and / or pressures.

The two conduits 6 and 7 may have different diameters to compensate the friction loss introduced by the bulb and by the expansion of the fluid caused by its heating. According to the present invention to regulate the circulation of the fluid in the circuit of the system a pumping device is typically required, possibly electronically controlled, in order to have at the outlet of the conduit 7 a pressure and a speed that fall into predetermined intervals.

The velocity of the fluid in the bulb, and more generally in the circuit, will typically be in the range from 0.5 m / s to 10 m / s, therefore relatively high. The pressure in the circuit, and particularly in the bulb, will typically be very high; taking into account that every 100 m of well depth create a hydrostatic pressure of about 10 atm, in the case of a well of 3000 m there will be a pressure of at least 300 atm in the bulb.

The present invention teaches other measures for the efficiency of heat transfer, which can be advantageously used as an alternative or in combination with a lower mixer.

A first measure which can be successfully used to increase the heat transfer from the outer walls of the bulb to the fluid that circulates inside it, and which is used in the example of Fig.2 and Fig.4, are elements, in particular the fins or strips, protruding towards the inside of the bulb and designed to increase the heat transfer surface with the fluid; these elements, typically made of good thermal conductor material, can be integrated into the perimeter walls of the bulb or be simply laid to these walls to rapidly transmit heat by conduction to the interior cavity of the bulb where the fluid flows.

According to the advantageous example of Fig.4 and Fig.5, these protrusions are made using a section of cylindrical tube inserted inside the bulb 5 so as to be in contact with the perimeter walls of the bulb 5; to the inner side of the tube are joined a plurality of fins arranged radially and which extend up to a predetermined distance from the axis of the tube; this distance is a function of the diameter of the internal pipe 10 (there is no contact between the fins and pipe); to increase the density of the fins (and therefore the heat exchange surface), these are of different lengths, in particular, in the example of Fig.4 and Fig.5, short fin is alternated to a long fin; in this way a laminar element 15 is realized; pipe and fins are typically made of a good thermal conductor material, preferably copper.

The lamellae or fins of the laminar elements may have surfaces of different types: smooth, grooved, dimpled (ie containing a plurality of protrusions placed side by side in particular in the form of hemisphere or hemielipsoid), with "microcraters" (a plurality of micrometric dimensions recesses placed side by side in particular in form of hemispheric or hemielipsoid cavities) and mixed type; the different surfaces have different heat exchange yields and different manufacturing costs; mixed type surfaces (therefore the mostly varied) are the most efficient because they expose a greater surface area but they are also the most expensive requiring the presence of all the machining operations in a single product. If used, the surface with microcraters obtains an increase of 2/3 of the exposed surface with the same footprint A surface with microcraters with a diameter, for example of 0.1 to 0.01 mm, can be advantageously made via projection of high-speed nebulised flows of acid solutions.

In Fig.4, the bulb 5 contains four laminar elements 15 one above the other spaced by three intermediate zones 16; each of the elements occupies an intermediate zone 17; note that one of the laminar elements is not drawn in the figure to show clearly one of the intermediate zones.

For the realization purposes, it is advantageous to make the bulb and / or the laminar elements according with a modular design concept. For example, the shell of the bulb may realized by combining a glass bottom cylinder, a plurality of equal cylindrical tube sections (for example having a length equal to 1-3 times the diameter of the tube), a glass cylindrical upper (in the inverted position). With regard to the laminar elements may be a plurality (as in Fig.4), for example in a number equal to the cylindrical tube sections with a length 10-20% less than that of tube sections. In this way, to produce bulbs of different length will be sufficient to combine a different number of tube sections and to insert a corresponding number of laminar elements.

Thanks to the laminar elements as that of Fig.5, it is possible to increase the heat transfer surface area of 8-12 times and equally the efficiency of the heat exchange process.

A second measure that can be successfully adopted to increase the heat transfer from the outer walls of the bulb to the fluid that circulates within it, are the means for generating turbulent motions in the fluid that circulates within the bulb which allow to break the laminar flows; such means are advantageously made by means of rotating passive mixers such as, for example, those shown in Fig.5 and Fig.6. Means for generating turbulent motions can be placed within the bulb in various positions; for example they can be placed in intermediate zones aimed primarily to stream the fluid, such as for example the area 16 in Fig.4, and / or in the intermediate zones designed primarily for the thermal transfer, as for example the area 17 in Fig.4.

The rotating passive mixer 18 in Fig.6 has been designed to be mounted on the surface of the fins of the laminar elements. It comprises a shaft on which are mounted a plurality of cups by means of a corresponding plurality of transverse arms to the shaft; the cups have axis perpendicular to both the axis of the respective arms and the shaft axis; the cups can be closed (as in figure) or open; the shaft is inserted in a corresponding hole made perpendicularly in the surface of the fin.

The rotating passive mixer 19 in Fig.7 is designed to be mounted on the inner surface of the bulb where there are no fins. It includes, in a lower part 191 , a shaft on which are mounted a plurality of cups by a corresponding plurality of arms transversal to the shaft; the cups have their axis perpendicular to the axis of the respective arms, but parallel to the axis of the shaft ; the cups have tangential discharges; the ascending fluid enters into the cups and exits by the discharges realizing at the same time the rotation of the mixer and the generation of turbulent flow motions. Moreover, it advantageously comprises, in an upper part 192, a shaft (integral with the shaft of the lower part 191 ) on which are mounted a plurality of vanes; the vanes are slightly inclined with respect to the axis of the shaft and have a sawtoothing on the margin in order to increase the turbulence introducing a vortex in correspondence of each tooth.

As already said, in the annular zone 12 some convective motions are created; in particular there will be an ascending warm flow in areas adjacent to the perimeter walls of the bulb 5, and a cold descending flow in areas adjacent to the pipe 10. In order to withdraw from the bulb 5 only the warmer part of the ascending fluid, one element (for example a truncated toroidal) can be placed in the upper end 13 of the bulb 5 capable to act as "invitation" to the colder part of the flow to re- descend and to act as a barrier to it for entering conduit 7; said in other words, they increase the convective motions and increases the temperature of the fluid entering the conduit 7.

The fluid for the heat transport, that is preferable to use in the circuit of the system according to the present invention, is a nanofluid that is a suspension of nanoparticles in a liquid.

The nanoparticles (eg from 1 to 100 nanometers) are typically made of materials such as carbon, aluminum oxide, copper, micaceous compounds, in a percentage varying between 5% and 45% by volume.

The liquid is typically a material such as water, ethylene glycol and its derivatives; however, since the circuit of the system according to the present invention is closed, there are no major constraints on this liquid although, of course, is advantageous to use non-toxic and / or non-pollutants liquid, to take account of possible failures and fluid leakage into the environment; some elements which determine the choice are: the cost, the specific heat, the thermal conductivity and the viscosity of the resulting fluid, the temperatures of operation.

The adoption of a suspension to 30% of nanoparticles increases the heat exchange properties of the fluid up to 70% of the value of the original fluid.

Is to be noted that many nanofluids can work as lubricants, and this is a considerable advantage for the rotating mixers placed inside the bulb; for example, the nanofluids with nanoparticles of micaceous materials (eg K2AI4-6Si8O20 (OH, F) 4 ) are excellent lubricants.

The use of a mixed nanofluid, that is containing particles of different sizes and / or different materials, allows to obtain simultaneously two technical effects very useful for the practical realization of the present invention: a higher heat exchange and a higher lubrication.

The system to extract heat from hot rocks according to the present invention is designed to be typically used in a geothermal power plant; such geothermal power plant is therefore made of a section of extraction of heat from the ground, which corresponds precisely to the extraction system, and a section of converting heat into electrical energy.

Is to be noted that the extraction system according to the present invention could also used for the transformation of existing geothermal plants; in other words, in the case of an existing geothermal power plant, one could drill a new well, enter a system of extraction according to the present invention, and then connect the system to the plant.

In general, the geothermal power plant comprises, in addition to the extraction system, a closed hydraulic circuit with a working fluid to directly generate work, a heat exchanger placed at the surface capable of directly exchanging heat between the fluid for heat transport (from the bulb of the system that is placed in depth) and the working fluid.

Referring to Fig.1 , the ascending conduit 7 conveys the "hot" fluid to the surface where the section 3 converts the thermal energy into electrical energy. Here the hot fluid releases its heat directly to the working fluid, which is typically water, via a heat exchanger; then the working fluid, which has become steam and is very hot, enters directly into a turbine; the shaft of the turbine is mechanically coupled to an electric machine, typically an alternator to generate electricity; the cooled fluid at the exit of the exchanger is fed directly into the descending conduit 6; the working fluid at the exit of the turbine, which is typically still in the steam state, is conveyed again to the heat exchanger to be re-heated.

Is to be noted that the heat transport fluid never comes in direct contact with toxic materials and / or corrosive present in depth, and then the plant is very safe for the environment.

Furthermore, it is also to be noted that by operating at high temperatures, expensive and complicated heat pumps are not required, an the efficiency of the plant is very high

The deep insertion of the bulb requires the excavation of the well to reach the deep hot rock layers. This excavation will typically be carried out in two phases: the first corresponds to a normal drilling and the second corresponds to the intervention of a device for excavation of tunnels.

The drilling of an initial small diameter well, serves, among other things, to carry out a detailed study of the geological layers where the system according to the present invention has to be installed; during this drilling can be carried out analysis on the materials crossed, on the presence of gas or water at high temperature, on the temperature of the rock layers; in addition, the drilling of the initial well guides the execution of the next excavation and therefore facilitates the realization of a perfectly vertical excavation.

The excavation of the actual well may advantageously use a machine known and used in the construction of tunnels and pipelines to distribute water from reservoirs: the so-called "boring machine"; in this case, however, the boring machine will operate vertically rather than horizontally.

This machine comprises a frontal rotating shield which crushes the rock; the machine moves forward into the rock and the collected material is conveyed and expelled to the rear.

The larger boring machines have an internal plant for the production of concrete used to create the coating material of the tunnel, during the advancement of the excavation.

In the this case, the boring machine will be small (4 to 6 m in diameter) and will work vertically. The resulting material will be conveyed to the surface by means of lifting equipment such as storage bins of balanced cables or hoist. The technology of excavation and lifting materials are known and marketed by specialized companies.

During the excavation phase, in case of passage through geological unstable friable layers, the walls of the well are armed with concrete walls, put in place, for example, in segments and preferably from the excavating machine itself.

Another technique of well coating, already known and used is the projected concrete (also called "sprayed concrete" or "spritzbeton") which can be used during the crossing of relatively stable rock layers.

Once finished digging, the bulb is inserted. During the lowering phase the segments of the descending and ascending flow conduits (for example isolated and integrated into a single pipeline) are connected, for example, with appropriate connecting flanges. The pipeline can be maintained vertical with supports connected to the suspension cables where the bulb is anchored to; these cables regulate the lowering process of the bulb into the well, and can be left in place even after the finalization of the installation work.

Once the bulb and the pipeline have reached the bottom of the well, the excavation can be filled with sand layers alternating with layers of lightweight concrete which serve as permanent sealing of the well itself and, in this way, the system according to the present invention is fixed firmly to the geological strata which it traverses.

At this point in the system put in place you can to circulate the fluid for the heat transport and then make the connections and thus complete the geothermal system.

As already mentioned, the present invention may be implemented according to embodiments more or less different from those of the figures here appended.

The Italian Patent Application No. CO2011A000023 filed July 8, 2011 entitled "METODO Dl PREPARAZIONE Dl CAMPI GEOTERMALI" that is "METHOD OF PREPARATION OF GEOTHERMAL FIELDS" and the Italian patent application No. CO2011A000024 filed July 8, 2011 entitled "NANOFLUIDO per INFILTRAZIONI TERMICAMENTE CONDUTTIVE" that is "NANOFLUID FOR THERMALLY CONDUCTIVE INFILTRATIONS" describe some very interesting and innovative technical solutions that can be used with great advantage in combination as expressly described herein, so these two patent applications are incorporated herein by reference.

According to a first variant, the bulb of the system to extract heat (for example the bulb shown in Fig.2 and Fig.4) can be associated to a bar of absorbing heat. Such a bar may be attached to the bulb in its lower part in particular fixed to the lower face. Advantageously, the bar can be made of a material good thermal conductor in particular copper or an alloy thereof. The bar may be constituted by a solid cylinder element, in particular, the element can be advantageously cylinder coaxial to the bulb. The dimensions of the bar may correspond for example to a diameter of 0.5-1.0 m and a length of 20-200 m (more typically 50-150 m).

The function of this bar is to absorb heat from the hot rocks surrounding and transfer by conduction to the bulb; in this way, it is possible to further increase the yield of the system according to the present invention because it increases the heat exchange surface and because the bar is in contact with rocks even hotter, being deeper than those where the bulb is.

From what has been described in relation to the realization of the excavation for the implementation of the system to extract heat according to the present invention, it is understood that the implementation of a possible absorbing heat bar may not require additional operations (besides the fact to lower the bar itself); the excavation is derived from the drilling of an initial guide well of small diameter so it will be sufficient that the initial drilling is a bit deeper (as mentioned before, for example of 20-200 m) to accommodate the bar.

Other very advantageous variants are described in Fig.8, Fig.9 and Fig.10; in addition these variants can be advantageously combined with each other, for example in pairs.

Fig.8 schematically shows the vertical section view of the lower part of a bulb of a system according to the present invention.

In the extreme lower zone 11 the turbulent flow motions are generated by means of a plurality of fins 110 which project inward from the inner surface of the bottom of the bulb 5; this inner surface is preferably shaped and is also correspondingly shaped the exit of the pipe 10 (reaching up to the proximity of this inner surface) in such a way as to guide the fluid in the change of direction from descending to ascending.

The fluid is driven right in the direction of the fins 110 and collides against them, generating considerable vortices 111 , so the flow regime changes from substantially laminar to essentially turbulent.

The fins are preferably arranged transversely to the direction of fluid flow in such a way as to maximize the generation of turbulent flow motions (is to be taken into account that the bottom of the bulb 5 has a circular contour section).

The fins 110 may be constituted for example by small plates of a few centimeters, for example 5 cm x 5 cm, placed in random positions over the inner surface of the bottom of the bulb 5; these plates may be welded to the bottom.

Fig.9 schematically shows the cross-sectional view of a laminar element.

The element of Fig.9 is to be considered an evolutive variant of the element shown in Fig.5 and has been studied to obtain a higher exchange surface in contact with the external heat coming from the hot rocks; it typically has from 5 to 9 times the external exchange surface of the Fig.5; consequently also the collected energy will typically be from 5 to 9 times higher than that collected by the element of Fig 5.

The outer profile is no longer circular, but is constituted by a plurality of elongated lobes similarly to a daisy.

Each lobe has a separate duct 121 ; each lobe is associated with two walls, arranged radially which correspond to the fins of Fig.5;

in the central area these walls are then combined two by two (detail 123 in Fig.9) to form a single fin;

these unique fins are joined to a cylindrical element 120;

each lobe is separated from the two adjacent respectively by two spaces 124 in which the natural and / or artificial material (such as the nanoconcrete mentioned before) can penetrate and transmit very efficiently the heat coming from the surrounding hot rocks to the bulb, and then to fluid rising inside the bulb.

A solution like the one just described is also remarkably robust.

Note that, in this case the pipe 10 is internal to the cylindrical element 120 and is made of thermally insulating material, for example, of carbon foam or cellular glass.

The solution of Fig.9 or an equivalent one, is particularly suitable to be used in situations where the temperature surrounding the bulb is relatively low, for example 130-180 ° C. In the laminar element of Fig.9 means 125 are provided for generating turbulent flow motions that include twisted strips or wires. In this figure, the wires or strips are twisted with random pattern and have a substantially regular disposition inside the ducts 121 (but may also present a more random arrangement); in particular these means 125 are placed in correspondence of the spaces 124. In this example the wires or strips are made of metallic material.

In the case of wires, the agglomeration of metallic wire may have a density typically ranging from 20 and 100 g/dm3.

In the case of strips the agglomeration of metal plate may have a width typically ranging from 5 to 15 mm and a density typically ranging from 10 to 100 g/dm3. These or equivalent agglomerates impose a tortuous path to the ascending fluid generating many micro-vortices (which, among other things, increase the convection) and increasing the efficiency of heat exchange with only small flow velocity loss and small friction loss increments.

Fig.10 schematically shows the lower part of a particular well that can be used for the system according to the present invention; this figure shows also the lower part of the bulb 5.

Between the lower end of the bulb and the lower end of the well there is a space 83 (the figure is not to be intended in a limiting sense, and therefore this space can be larger or smaller). One or more sub-wells 84 start from the space 83; in the figure, the central sub-well is substantially vertical, while the lateral sub-wells have diverging path designed according to the local isotherms.

The space 83 and / or the sub-wells 84 are intended to increase the amount of heat that reaches the bulb 5, heat which is transferred to the fluid that circulates in the bulb 5.

To achieve this effect, the space 83 and / or the sub-wells 84 are appropriately filled of material so that the filling material transports the heat towards the bulb 5. These elements can be filled, for example, of a metal (which is a good thermal conductor), in particular a metal having high thermal conductivity and low melting temperature; in this way, the filling material remains liquid due to the temperature of the surrounding rocks.

Alternatively, one could use an high density nanofluid (eg 80-90%). The nanofluid may be aqueous based, in this case, the water of the aqueous base remains liquid because of the high pressure environment.

The nanofluid may be metal-based; for example it can provide a first metal, such as tin, suitable to be in the liquid state at the temperature of the surrounding hot rocks and a second metal (which constitutes the nanoparticulate) such as the iron, able to be solid state at temperatures of the surrounding rocks.

With a liquid filling material the heat transfer into the sub-wells can be effected not only by conduction, but also, very effectively, by convection.

The sub-wells 84, if present, may be in a number that ranges from a minimum of 1 to a maximum of 100; most typical numbers range from 10 to 30.

If the sub-wells 84 are few, their diameter can be relatively large, for example from

0.5 to 1.0 m, and if there are many, the diameter can be reduced for example, to

8-15 cm.

The length of the sub-wells 84 may range from 100 to 5000 m, but more typically from 100 to 1000 m; given the length of the sub-wells, these typically reach rocks even deeper and warmer than those close to the bulb, and the amount of heat transported towards the bulb increases very significantly.

Advantageously, the path of these sub-wells is perpendicular to the isotherms that are created in the rock around the bulb. These isotherms form because the volume of rock around the bulb tends to cool because of the energy absorption. To follow the trend of the isotherms the sub-wells paths may be curved.

To realize curved sub-wells will be exploited some drilling techniques already used by the oil industry which involve the usage of directional drilling heads.