FR2963411A1 - THERMAL SOLAR SENSOR ABSORBER, SENSOR COMPRISING SAME, AND PROCESS FOR PREPARING SAME. - Google Patents

Abstract

A solar thermal collector absorber comprising channels each having a first end and a second end, the first ends of the channels opening into a first end chamber and the second ends of the channels opening into a first end second end chamber, wherein said absorber consists of a single extruded part of a solid nanocomposite material comprising a polymer matrix in which nano-objects and / or nanostructures are incorporated. Thermal solar collector comprising said absorber and a cover transparent to solar radiation on said absorber. Process for the preparation of said absorber

Description

THERMAL SOLAR SENSOR ABSORBER, SENSOR COMPRISING SAME, AND METHOD FOR ITS PREPARATION.

TECHNICAL FIELD The invention relates to an absorber for a solar thermal collector. The invention further relates to a solar collector comprising this absorber. The invention also relates to a process for preparing said absorber.

The technical field of the invention can be defined generally as that of solar thermal collectors.

STATE OF THE PRIOR ART A solar thermal collector, as shown in FIG. 1, comprises as an essential element an absorber (1) in which circulates a coolant, such as brine. The absorber (1) is placed in a box or casing (2), generally made of steel, under a transparent cover (3), generally made of glass. The cabinet (2) is usually lined internally by a thermal insulator, usually mineral wool (4).

The absorber (1) comprises, as shown in FIGS. 1 and 2, channels (5, 21), of circular section or the like, generally arranged parallel to one another. All of these channels (5, 21) form the central part or body of the absorber (22). The first ends (23) of the channels (21) are connected to a first hollow end piece (24) forming an inlet volume for the coolant and the second ends (25) of the channels (21) are connected to a second hollow end piece (26) forming an outlet volume for the coolant circulating in the channels (5, 21). The absorber (1) constitutes the active part of the thermal sensor. The absorber (1) must have very particular optical properties: it must indeed absorb (6) at most the solar spectrum (7) while limiting the maximum radiation losses that is to say the re-emission of photons (8) induced by the heating of the absorber. The absorption of the solar spectrum by the absorber is a property which is characterized by the absorption coefficient as, which must therefore be the greatest possible, while the re-emission of photons is a property which is characterized by emissivity And N. The emissivity of a material is the ratio between the energy it radiates and that which a black body radiates at the same temperature. It is therefore a measure of a body's ability to absorb and re-emit radiated energy. The black body absorbs and re-emits all the energy (E = 1). In the case of the absorber, the lowest possible emissivity, often determined at 10 ° C., is desired.

Overall, the energy balance of the sensor takes into account the solar energy (7), the energy 3 absorbed by the absorber (6), the energy re-emitted by irradiation (black body) (8), and the energy reflected by the underside of the cover (9), to finally give the energy lost by the sensor (10).

In order to provide the absorber with these optical properties, the absorber is provided on its upper surface, generally metallic, with a selective spectral coating (11) which limits the energy re-emitted by irradiation by the absorber.

In the most common sensors, and as shown in FIG. 3, the material used for the absorber and in particular for the tubes, channels, of the absorber (31) which carry the coolant, is a good conducting metal. thermal device such as copper or aluminum on which is deposited a selective spectral treatment generally consisting of a stack (32) of two or three layers, or more, to obtain good performance both in terms of the absorption coefficient that the emissivity Etn. These layers are generally cermet layers with different metal contents or with a gradient and an anti-reflective layer. This same structure is used in the case of plastic substrates that do not exhibit intrinsic absorption. Solar thermal collectors combining a plastic base and one or more surface treatments have been the subject of several patents and patent applications over the last 25 years. In most of these patents and patent applications there is disclosed a plastic absorber coated with one or more specific layers which provide spectral selectivity.

US-A-4,112,921 discloses solar thermal collectors in which the use of glass and metal is almost completely eliminated, which in particular reduces the cost and weight of the sensor. The absorber consists of very low thermal conductivity materials such as plastics. US-A-4,556,048 discloses solar thermal collectors in which the surface of the polyolefin resin which constitutes the absorber, receives a two-layer treatment to achieve a selective absorption of solar radiation. The first layer applied to the surface is mainly made of a thermoplastic acrylic resin, or an alkyd resin, a chlorinated polyolefin resin, an epoxy resin, and a metal powder. This first layer has the effect of reducing the infrared radiation. The second layer which has selective absorption properties of solar radiation is mainly composed of a metal oxide and at least one resin selected from acrylic resins, fluorinated resins, urethane resins, and alkyd resins.

CN-A-101158513 describes an absorber and its method of preparation. This absorber comprises circular tubes which are arranged on the same surface and interconnected by fins, a top plate forming a cover and a bottom plate forming a support. A solar energy absorbing layer is disposed on the inner wall of the tubes and a reflective layer is provided on the bottom plate to increase the number of photons transmitted in the absorber. US-A1-2002 / 0002972 discloses a solar collector comprising a plurality of identical reflective elements, arranged in rows, which concentrate the incident solar radiation on a coolant pipe. The reflective surface is coated with a reflective layer such as a vacuum deposited aluminum layer. The reflective elements may be prepared by molding, in particular in a compression mold of a material comprising hydrated aluminum silicates.

US-A-4,060,070 discloses the use of a polymeric material such as polypropylene or polyethylene, charged with carbon black to make a solar collector absorber. This absorber comprises a main body which is prepared by extruding the polymer to thereby define a base plate and a plurality of spaced parallel hollow tubes. The plate is attached at each of its ends to an extruded plastic manifold optionally comprising tubular adapters.

The tubes and collectors are assembled by heating, for example by induction. FR-A-2 498 614 relates to a product for producing a heat exchanger of a solar collector which is constituted by a mixture of ethylene propylene diene monomer (EPDM), carbon black, precipitated silica intended for facilitate the extrusion of the product or other reinforcing mineral filler, an EPDM-compatible plasticizer, a vulcanization system, a coupling agent for silica or mineral filler and EPDM.

The document WO-A1-01 / 34698 describes an elastomer mixture, in particular based on EPM or EPDM to which a thermally conductive component such as graphite, carbon black, calcium oxide, an agent, is added. crosslinking. This elastomeric mixture is used for the extrusion manufacture of pipes for absorbers and heat exchangers. The document FR-A-2 430 578 relates to a solar collector absorber which is made of a generally transparent or translucent material, which allows at least 50% of the solar radiation to pass through. In addition, the heat transfer fluid contains a dispersed pigment and / or a dissolved dye capable of absorbing solar energy. US-A-4,161,942 relates to a solar energy sensor comprising one or more tubes arranged in a helix on which is fixed a heat transfer coating. The heat transfer coating comprises carbon black and plaster of Paris.

US-A-4,376,801 relates to a selective coating for surfaces exposed to solar energy of a solar collector, which comprises an organic compound or a substance having a high molecular weight and a high carbon content, such as an oil, oil, fat, or wax, vegetable or animal that is pyrolyzed to give a pigmented black carbon varnish. DE-A1-3228274 relates to a method for producing a selective absorber layer on the surface of a metal solar collector in which carbon black with a particle size of 10 to 300 nm is dispersed in a binder and this dispersion is applied with a suitable solvent on said surface and then dried. In general, the absorbers are traditionally prepared by a method (FIG. 2) of extruding two pieces (27, 28) each forming half of the channel body (22) of the absorber, that is to say the half -absorbeurs (27, 28) then to assemble them with the two other end pieces (29, 210) in which are defined the end chambers (24, 26) of the absorber, that is to say the hollow chambers in which the channels of the absorber open. The absorber is thus prepared by assembling four parts, namely the two parts (27, 28) which constitute the channel body of the absorber, the part (29) which constitutes the first end part and finally the part ( 210) which constitutes the second end portion. This process is long, complex, expensive, and unreliable. Indeed, it implements complex and expensive steps such as thermoforming and welding steps. The absorbers obtained by this process have particularly sealing problems especially at the level of the connections between the parts constituting the end portions, and the channel body. There is therefore a need for a solar thermal collector absorber which can be prepared by a simple method, having a limited number of steps, reliable, reproducible, and of reduced duration and cost. There is still a need for an absorber that is similarly simple, reliable, low cost and whose properties and in particular the thermal and optical properties can be easily optimized. The object of the present invention is to provide a solar collector absorber and a method of preparing this absorber, which meet the needs mentioned above. The object of the present invention is further to provide a solar collector absorber and a method of preparing this absorber which do not exhibit the defects, disadvantages, limitations and disadvantages, absorbers and methods of preparing absorbers of the prior art and which solve the problems of the absorbers and methods of the prior art. SUMMARY OF THE INVENTION This and other objects are achieved, in accordance with the invention, by a solar thermal collector absorber comprising channels with each having a first end and a second end, the first ends of the channels opening into a first end chamber and the second ends of the channels opening into a second end chamber, wherein said absorber is constituted by a single extruded part of a solid nanocomposite material comprising a polymer matrix in which are incorporated nano-objects and / or nanostructures. Advantageously, the polymer of the matrix is chosen from thermoplastic polymers such as polyolefins such as polyethylenes and polypropylenes; polymers and copolymers of cyclic olefins polystyrenes; polyamides; polyesters such as polycarbonates, poly ((meth) acrylates), poly (ethylene terephthalate) s or PETs, poly (ethylene naphthalate) s; and their mixtures. Advantageously, the content of nano-objects and / or nanostructures may be less than or equal to 5% by weight, preferably less than or equal to 1% by weight, more preferably from 10 ppm to 0.5% by weight of the mass of the nanocomposite material. Advantageously, the nano-objects may be chosen from nanotubes, nanowires, nanoparticles, nanocrystals, and mixtures thereof. Advantageously, the nano-objects or nanostructures may be functionalized, in particular chemically. Advantageously, the nano-objects and / or nanostructures are chosen from nano-objects and / or nanostructures which confer thermal and / or electrical and / or magnetic and / or optical properties on the nanocomposite material; and among the nano-objects and / or nanostructures that improve the thermal and / or electrical and / or magnetic and / or optical properties of the nanocomposite material. Advantageously, the material constituting the nano-objects and / or nanostructures may be chosen from carbon; metals such as gold, copper, manganese or aluminum; metal alloys such as alloys of copper, gold, manganese or aluminum; metal oxides such as rare earth oxides possibly doped; organic polymers; and materials comprising a plurality of materials from the aforesaid materials. Advantageously, the nano-objects may be carbon nanotubes; nanoparticles of metals such as copper, gold, manganese or aluminum; nanoparticles of metal alloys such as alloys of copper, gold, manganese or aluminum; nanoparticles of metal oxides; magnetic nanoparticles such as nanoparticles of AgMn, Fe2O3 or Fe3O4; or a mixture thereof. Advantageously, carbon nanotubes and magnetic nanoparticles are incorporated in the polymer matrix. Advantageously, the nanostructures may be core-filament nanostructures, in particular core filament nanostructures with a core made of alumina and filaments consisting of carbon nanotubes. Nanostructures of this type impart good thermal properties to the material, i.e. the polymer matrix, in which they are dispersed. In other words, nanostructures of this type provide a significant improvement in the thermal properties of the material for a minimal addition of nanostructures. Advantageously, the nano-objects and / or nanostructures can be distributed homogeneously in the polymer matrix. Advantageously, the upper face of the absorber, capable of being exposed to solar radiation, is provided with a coating made of a material transparent to radiation having a wavelength in the wavelength range of the solar spectrum and a reflectivity to radiation having a wavelength greater than the wavelength range of the solar spectrum. Advantageously, said transparent material may be chosen from transparent conducting or OTC oxides such as indium tin oxide or ITO, and zinc oxide doped with aluminum or ZnO: Al. Advantageously, said coating of a transparent material comprises a single layer, preferably a single layer of zinc oxide doped with aluminum or ZnO: Al.

The absorber according to the invention has never been described in the prior art. The absorber according to the invention does not have the drawbacks of the absorbers of the prior art and solves the problems of the absorbers of the prior art.

Indeed, the absorber according to the invention fundamentally differs from the absorbers of the prior art first of all in that it consists of a single extruded part. Such an absorber in which all, the whole of the absorber, namely channels, end chambers or input and output volumes connected to said channels is integrally constituted by a single extruded piece "monoblock" n ' has never been described or suggested in the prior art.

Because the absorber according to the invention consists of a single piece extruded in a single material, and not by several parts assembled for example by welding, the absorber according to the invention has no connection, joined between the parts.

The absorber according to the invention is therefore extremely simple, extremely reliable, and in particular does not exhibit the sealing problems encountered with the absorbers of the prior art constituted by the assembly of several parts.

The absorber according to the invention is also of a lower cost. The absorber according to the invention is also fundamentally different from the absorbers of the prior art in that it consists of a specific material, namely a solid nanocomposite comprising a polymer matrix in which nano-objects are incorporated. Such a nanocomposite material is totally compatible with the extrusion process and it makes it possible to impart to the absorber all the desired properties, for example thermal, mechanical, magnetic or other properties. The choice of the polymer of the matrix and / or nano-objects makes it possible to easily and specifically adjust the properties of the nanocomposite material with a view to its implementation in a solar thermal collector absorber. The nanocomposite material used may have as desired all the desired specific properties, such as improved magnetic and thermal properties, particularly with respect to a plastic charged with carbon black while remaining compatible with the extrusion process. It has been seen above that the absorber according to the invention is generally provided, on its upper face, capable of being exposed to solar radiation with a coating made of a material transparent to radiation having a wavelength in the range. wavelength of the solar spectrum and a reflectivity preferably a significant reflectivity, to the radiation having a wavelength greater than the range of wavelength of the solar spectrum. In general, and thanks to the fact that the absorber consists of a solid nanocomposite material, this coating comprises only one layer of a transparent material instead of a stack of layers for the absorbers of the prior art. made for example of copper. The invention further relates to a solar thermal collector comprising an absorber according to the invention as described above and a cover transparent to solar radiation on said absorber (on the side exposed to solar radiation). The sensor according to the invention inherently has all the advantageous properties, already listed above, of the absorber according to the invention which constitutes the fundamental element of this sensor. Advantageously, the transparent cover of the sensor according to the invention may be coated on its underside (that is to say the face facing the absorber) with a coating made of a material transparent to radiation having a length of wave in the wavelength range of the solar spectrum and reflectivity to radiation having a wavelength greater than the wavelength range of the solar spectrum. Advantageously, said transparent material may be chosen from transparent conducting or OTC oxides, such as indium tin oxide or ITO, and zinc oxide doped with aluminum or ZnO: Al. Advantageously, said The coating of a transparent material comprises a single layer, preferably a single layer of zinc oxide doped with aluminum or ZnO: Al. The solar thermal collector according to the invention inherently has all the advantageous properties due to the absorber. The sensor according to the invention, when it comprises, both on the lower face of its transparent cover and on the upper face of the absorber, a coating made of a material transparent to radiation having a wavelength in the range of wavelength of the solar spectrum and a reflectivity to the radiation having a wavelength higher than the range of wavelength of the solar spectrum, sees its thermal and optical properties, and in particular its capacities of absorption of the solar spectrum, optimized . The invention furthermore relates to a process for the preparation of the absorber as described above, in which the extrusion of a melt of the solid nanocomposite is continuously carried out, and the step is successively formed in one step. first end chamber, the channels, and the second end chamber. The preparation by a technique of continuous extrusion and in a single step of the whole, all of an absorber, namely channels, end chambers or volumes of input and output connected to said channels, said absorber being finally integrally constituted by a single extruded part, has never been described in the prior art. The method according to the invention has only one step and is therefore much simpler, much faster and much less expensive, much more reliable, than the methods of the prior art which generally involve the assembly of several pieces made separately. The method according to the invention eliminates in particular the most expensive steps of the processes of the prior art such as thermoforming and welding.

Advantageously, the first end chamber, the channels and the second end chamber of the absorber can be formed using a single extrusion head or die. Advantageously, several absorbers can be formed in series and continuously. This produces additional gains in terms of cost and time. Advantageously, during the extrusion, a fluid can be injected into the molten nanocomposite material in order to define the cavities, channels, and chambers of the absorber. Advantageously, the injected fluid may be selected from gases such as air, argon, nitrogen, and mixtures thereof; and liquids such as oils, for example silicone, mineral, synthetic, semi-synthetic oils, and mixtures thereof. Advantageously, after the extrusion is deposited, for example by a physical vapor deposition process ("PVD") on the upper face of the absorber or absorbers, may be exposed to radiation solar a coating of a radiation-transparent material having a wavelength in the wavelength range of the solar spectrum and a reflectivity to radiation having a wavelength greater than the wavelength range of the solar spectrum . Advantageously, this coating may be a transparent conductive oxide, such as indium tin oxide (ITO) or zinc oxide doped with aluminum or ZnO: Al.

Advantageously, the coating made of a transparent material comprises a single layer, preferably a single layer of zinc oxide doped with aluminum or ZnO: Al. Advantageously, the process according to the invention comprises a final stage during which one pierces the heat transfer fluid inlets and outlets in the absorber or the absorbers, then the fluid is optionally drained, and finally connections are welded to said heat transfer fluid inlet and outlet. The invention will be better understood on reading the detailed description which follows in connection with the accompanying drawings in which: BRIEF DESCRIPTION OF THE DRAWINGS - Figure 1 is a schematic vertical sectional view of a solar thermal collector. - Figure 2 is a schematic view of a thermal solar collector absorber of the prior art. 18

- Figure 3 is a schematic vertical sectional view of a tube of a absorber of the prior art provided on its upper face with a stack of layers forming a selective spectral coating. - Figure 4 is a schematic view of a solar thermal collector absorber according to the invention. FIG. 5 is a graph which represents the absorption spectrum of a nanocomposite material comprising a polyamide matrix in which carbon nanotubes are dispersed at a rate of 0.1% by mass. In ordinate is carried the absorption (in%), and in abscissa is carried wavelength (in microns). - Figure 6 is a schematic view of a specific extrusion head or die for carrying out the method according to the invention for preparing an absorber according to the invention. FIG. 7 is a cross-sectional view of the cylindrical part of the second zone of the extrusion head or die of FIG. 6. FIG. 8 schematically represents for each of the positions of the inner cylinder of the cylindrical part the corresponding extruded profiles obtained. FIG. 9 shows the continuous and series extrusion of five absorbers according to the invention. - Figure 10 is a graph that gives the total reflectivity of an aluminum doped zinc oxide coating. The abscissa is the wavelength (in microns) and the ordinate is the total reflectivity. Figure 11 is a graph showing reflectance versus wavelength of light (pine). Reflectance is the ratio of reflected and incident solar energy. Below 1.4 g / cm of wavelength, the solar energy is absorbed by the absorber material, beyond which there is a re-emission caused by the heating of the material. Figure 12 is a graph which shows in continuous line the solar spectrum seen as a distribution of the energy density as a function of the wavelength, and in broken line the distribution of the energy density according to the length of the re-emission of the absorber caused by its heating at different temperatures, namely 100 ° C, 200 ° C and 300 ° C. In ordinate is carried the density of energy (in GW / m2), and in abscissa is carried the wavelength (in pin).

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS The absorber according to the invention as shown in FIG. 4 has a structure analogous to that of a conventional absorber such as has already been described above and as it is described. 2, with the fundamental difference, however, that the absorber according to the invention consists of a single piece, element, extruded block (41) made of a solid nanocomposite material comprising a polymer matrix in which nano -objects.

This material can also be called simply "polymer-nanocomposite". The absorber according to the invention, like the conventional absorbers, comprises channels (42) each having a first end (43) and a second end (44), the first ends (43) of the channels opening to the interior of a first end chamber (45) and the second ends (44) of channels opening into a second end chamber (46). These channels (42) and these first and second end chambers (45, 46) are defined in one and the same extruded part (41). In other words, the absorber according to the invention consists entirely of a single element extruded in a nanocomposite material (41) which integrates the channels (42) and the zones, volumes, chambers, reserve input ( 45) and outlet (46) of heat transfer fluid. The shapes of this element (41) are obtained in a single step, by extrusion, without using other thermoforming and welding steps.

The channels (42) may have a circular cross section with a diameter generally of 1 to 7 mm, or a polygonal cross section, for example square or rectangular. The length of the channels (42) is generally between 0.5 and 4 m. The channels (42) are generally arranged parallel to each other with a channel spacing of 0.1 to 3 mm. The channels (42) are generally 4 to 20, for example 8 as shown in FIG. 4. The thickness of the absorber is generally 0.5 to 1.5 mm and the width of the absorber is usually 0.1 to 1 m. According to the invention, and as indicated above, the absorber according to the invention is made of a nanocomposite material with a polymer matrix. Recall that nanocomposite materials with a polymer matrix are multiphase materials, in particular two-phase materials, which comprise a polymer matrix forming a first phase in which nano-objects or nanostructures such as nanoparticles forming at least a second phase are dispersed. generally denotes reinforcement phase or charge.

The nanocomposites are so called because at least one of the dimensions of the objects such as particles forming the reinforcing phase or charge is at the nanoscale, namely generally less than or equal to 100 nm, for example from one nanometer to one or a few tens of nanometers, for example 100 nm. As a result, these objects and particles are called nano-objects or nanoparticles. The nanocomposite materials make it possible, generally for relatively low charge rates, namely less than 10% by weight, and even less than 1% by weight, to obtain a significant improvement in the properties of the material, be it mechanical, electrical, thermal, magnetic, optical or other properties ....

The polymer of the matrix of the nanocomposite material which constitutes the absorber according to the invention is generally chosen from thermoplastic polymers. There is no limitation as to the nature of this thermoplastic polymer. It may be chosen from polyolefins such as polyethylenes, polypropylenes, polymers and copolymers of cyclic olefins; polystyrenes; polyamides; polyesters such as polycarbonates, poly ((meth) acrylates), poly (ethylene terephthalate) s or PETS, poly (ethylene naphthalate) s; and their mixtures. These polymers are readily available commercially and the absorber according to the invention can be manufactured simply using commercial grade polymers. Particularly preferred polymers are polyamides, optionally filled with glass fibers to provide mechanical reinforcement and avoid creep. Glass fibers have dimensions much larger than nano-objects or nanostructures and should not be confused with them. By "nano-objects" is generally meant any object that is alone or linked to a nanostructure of which at least one dimension is less than or equal to 100 nm, for example from one nanometer to one or a few tens of nanometers and up to 100 nm. . These nano-objects can be, for example, nanoparticles, nanowires, nanocrystals, nanotubes, for example single-walled carbon nanotubes (CNTs) ("SWCNT" or "Single Walled Carbon Nanotube" in English) or multiwall ("MWCNT"). or "Multi Walled Carbon Nanotube" in English), or a mixture thereof. By "nanostructure" is generally meant an architecture consisting of an assembly of nanoobjects which are organized with functional logic and which are structured in a space ranging from cubic nanometer to cubic micrometer. As mentioned above, the nano-objects or nanostructures are generally selected from nano-objects and nanostructures which improve the thermal, and / or mechanical and / or magnetic and / or electrical and / or optical properties of the material. solid nanocomposite and / or confer such properties to the solid nanocomposite material. The material constituting these nano-objects or nanostructures is not particularly limited and can be chosen from carbon, metal metals and alloys, metal oxides, organic polymers; and mixtures thereof. Preferred nano-objects are in particular carbon nanotubes ("CNT" or "Carbon Nanotubes" in the English language) whether it is single-walled carbon nanotubes ("SWCNT" or "Single Walled Carbon Nanotubes" in English) or multi-walled nanotubes. 15 24 ("MWCNT" or "Multi Walled Carbon Nanotubes" in English), and nanoparticles of metals and alloys. Nanostructures can be constructions, assemblies whose bricks are nano-objects. The nanostructures may for example be carbon nanotubes "decorated" with platinum nanoparticles, copper, gold, iron; Silicon nanowires "decorated" with gold, nickel, platinum, iron, etc. Among the nanostructures, mention may in particular be made of the ZnO-Ni nanostructure which is a three-dimensional structure of ZnO terminated by nickel nanospheres. Among the nanostructures, it may also be mentioned, as already mentioned above, the core-filament nanostructures, in particular the core-filament nanostructures with a core consisting of alumina, more precisely by a particle of a powder of alumina, and filaments consisting of carbon nanotubes. Nanostructures of this type impart good thermal properties to the material, i.e. the polymer matrix, in which they are dispersed. In other words, nanostructures of this type provide a significant improvement in the thermal properties of the material for a minimal addition of nanostructures.

The nanocomposite material may contain only one type of nano-object or nanostructure or it may contain both several types of nanoobjects and / or nanostructures which may differ in their shape and / or the material constituting them and / or their size. In the case where several types of nano-objects or nanostructures are present, they can each confer a property to the material and / or improve a property of this material. Thus, the material can comprise nanoobjects such as mineral and / or organic fillers such as carbon nanotubes to improve its thermomechanical properties, and other nano-objects that make the material magnetic. For example, the composite material may contain both carbon nanotubes ("NTC") and magnetic nanoparticles, for example particles Fe2O3r Fe3O4, AgMn, in the proportions indicated above, for example less than or equal to 1% in weight. Generally, the content of nano-objects and / or nanostructures may be less than or equal to 10% by weight, preferably less than or equal to 5% by weight, more preferably less than or equal to 1% by weight, more preferably 10% by weight. ppm to 0.1% by weight of the mass of the nanocomposite material.

It is preferable that the nano-objects and / or nanostructures are homogeneously distributed in the nanocomposite material. A nanocomposite material with a polymer matrix that is particularly suitable for manufacturing the absorber according to the invention is described in the documents FR-A1-2 934 600 and WO-A1-2010 / 012813 to the description of which reference may be made. These documents describe a nanocomposite material in which nano-objects and / or nanostructures are distributed homogeneously, especially at low concentrations as mentioned above. . Indeed, to guarantee the "nano" effect, the quality of the dispersion of the nano-objects and / or nanostructures in the matrix is generally essential and it is therefore recommended to use a high-performance dispersion method such as that described. in the documents FR-A1-2 934 600 and WO-A1-2010 / 012813 cited above. In the process described in these documents, agglomerates or capsules, optionally lyophilized, are prepared in which nano-objects and / or nanostructures coated with macromolecules of polysaccharides are homogeneously distributed, said macromolecules forming in at least a part of each of them. agglomerates, a gel by crosslinking with positive ions. A nanocomposite material is then prepared by incorporating said agglomerates into a polymer matrix. For this purpose, it is possible, for example, to mechanically mix the prepared agglomerates with the polymer, then to dry the mixture and then to treat it by a plastics processing process such as extrusion. Recall that extrusion consists in merging n-materials and mixing them along a screw or a twin-screw with a temperature profile and an optimized rotation speed to obtain an optimal mix. At the end of this bi-screw or single-screw, there is a die which shapes the mixture before complete solidification. The shape can be a ring, a film or have any type of profile.

Incorporation of nano-objects or nanostructures in the polymer matrix can of course be done by a method, such as a plastics process, in which the nano-objects or nanostructures are not in the form of agglomerates.

The preferred homogeneous distribution of the nano-objects, nanostructures in the material of the absorber according to the invention, guarantees the improvement of the properties (mechanical, electrical, thermal, magnetic, optical, etc.) of this material because of the nano -objects even at lower concentrations. Thus, a nanocomposite comprising a polyamide matrix in which are homogeneously dispersed (process described in documents FR-A1-2,934,600 and WO-A1-2010 / 012813) carbon nanotubes at a rate of 0.1% by weight. On the one hand, the mass has a much higher thermal conductivity than a commercially available polymer (for example, a conductivity 3 to 5 times greater) and, on the other hand, a maximum absorption of the solar spectrum. This absorption is about 96% for the nanocomposite as shown in FIG. 5, whereas it is only a few percent for the polyamide alone without carbon nanotubes. The absorber according to the invention is prepared in a single step by an extrusion process. According to the invention, an extruder of a type already mentioned above, which is fed with the only nanocomposite material already prepared, or which is fed firstly with the nano-objects or nanostructures for example in the form of agglomerates, and secondly with the polymer. At the outlet of the extruder, a stream of molten nanocomposite material is collected.

At the outlet of the extruder of a type as described above, is disposed a specific extrusion head or die which allows to form the flow of molten nanocomposite before solidification and to prepare in a single step, and continuously, the various parts of the absorber. This specific extrusion head or die is shown in FIG. 6. It first comprises an inlet connection, generally of circular shape (61), connected to the exit of the extruder, for example to the bi zone. -vis, which receives a flow (62) of molten nanocomposite material from the extruder, and which feeds the extrusion head or die with a generally circular flow of molten nanocomposite material. The extrusion head then successively comprises three zones (63, 64, 65) in the direction of flow of the nanocomposite material flow and outlet means (66) for collecting the extruded absorbers. The first zone (63) may be defined as a flow zone, laminar flow, of the inlet flow of molten nanocomposite material; the second zone (64) can be defined as a zone of separation of the flow and possibly of injection of a fluid; and the third zone (65) can be defined as a flow convergence zone and final shaping and shaping of the absorber or absorbers.

The first zone or laminar flow zone (63) generates a laminar flow of the sensor width, for example about 1 m, and a thickness, for example 15 mm, which is greater than the final thickness of the sensor. the absorber.

This first zone (63) generally has walls (67) which diverge from said inlet fitting (61) to subsequently form a duct (68) of the desired width and thickness indicated above. This duct (68) is generally from 5 cm to 15 cm long, sufficient to establish a laminar flow. The laminar flow thus formed then enters the second zone (64) or separation zone of the flow and possible injection of a fluid.

The second zone (64) comprises a cylindrical piece (69) arranged along the width of the duct, channel (68) of the extrusion head, whose main axis is substantially perpendicular to the direction of the laminar flow in the channel, leads. This cylindrical piece (69) passes through the side walls (610, 611) of said channel, leads (68) and extends beyond the sidewalls (610, 611) of said channel (68), the bases (612, 613 ) of said cylindrical piece (69) are located outside said channel, beyond the side walls (610, 611) thereof.

In other words, this cylindrical piece (69) is placed transversely in the duct, in which duct flows the flow, laminar flow of molten nanocomposite material. This cylindrical piece (69) is arranged to separate the flow, laminar flow into two equal parts which flows respectively above and below the cylindrical piece (69), respectively between the upper wall of the channel, leads and the upper wall of the cylindrical piece, and between the bottom wall of the channel and the cylindrical piece. The cylindrical piece (69) comprises two concentric hollow cylinders (71, 72: see Figure 7). With the aid of these hollow and concentric cylinders, it is possible to inject a fluid into the stream of molten nanocomposite material in order to define the hollow spaces of the absorber and possibly to provide the internal surfaces of the absorber. a coating giving it interesting properties.

The third zone (65) is a convergence zone of the flow of nanocomposite material and of the final dimensions and shapes of the absorber, namely generally a width of 1 m and a thickness of 7 mm. The section of the duct, led in this third zone thus decreases progressively from the second zone (64) to the outlet means (66), 31 this section decrease being essentially due to a decrease in the thickness of the channel, which leads . The cylindrical piece disposed in the second zone of the head, extrusion die is shown in Figure 7, it comprises two concentric cylinders, namely a first fixed outer cylinder (71) and a second inner cylinder (72) rotatable . The outer side wall (73) of the first outer cylinder or cylinder (71) is the inner wall of the cylindrical part. The inner side wall (74) of the outer cylinder (71) is in contact with the outer side wall (75) of the inner cylinder, and the inner side wall (76) of the inner cylinder (72) defines a channel (77) in the center of the cylindrical piece. At one end of the cylindrical part and of said channel (77) in the center of the cylindrical part, the base (613) of the two cylinders can be pierced with a through orifice which can be connected to a supply pipe in a fluid ( 614). The outer cylinder (71) is fixed and its side wall is pierced by a window (78) on the side of the cylindrical piece (69) opposite the first zone (63).

This window (78) is generally in the form of a rectangle whose length is arranged in the direction of the width of the duct, in which flows the flow of molten nanocomposite material. The length of this window (78) generally corresponds to 80 cm and its width or height to 5 mm. The inner cylinder (72) is rotatable within the outer cylinder (71). This rotational movement can be produced by means of a motor (615), which drives in rotation a shaft integral with the base (612) of the inner cylinder (72) which is not pierced by the connected through orifice to the fluid supply line. This shaft is generally located along the main axis of the cylindrical part.

In circumferentially spaced positions, the side wall of the inner cylinder (72) is pierced by an orifice, a window or a series of several orifices which are, at each of these positions, arranged longitudinally in the direction of the axis of the cylindrical piece which merges with the main axis of the inner and outer cylinders. For example, apertures, windows or series of orifices (79, 710, 711) may be provided as described in FIG. 7 respectively in three positions (712, 713, 714) spaced apart around the circumference of the inner cylinder. Position 3 (713) is spaced 90 ° from position 2 (712) on the circumference of the inner cylinder in a clockwise direction, and position 4 (714) is spaced 90 ° from the position 3 (713) on the circumference of the inner cylinder in a clockwise direction. Other spacing angles could be provided. One or more positions 1 may be defined on the circumference of the inner cylinder in which the wall of the cylinder is solid and is not pierced with orifices, windows or series of orifices. In this or these position (s) 1, the passage of a fluid in the channel in the center of the cylindrical piece is prohibited. This position 1 may be a position (715) spaced 90 ° from position 4 (714) on the circumference of the inner cylinder in a clockwise direction. In FIG. 7, the positions 1, 2, 3, and 4 are therefore evenly distributed with 90 ° spacings on the circumference of the inner cylinder (72). The number of positions at which windows, orifices or series of orifices are made in the wall of the inner cylinder (72) is generally 3, for an absorber according to the invention, as shown in FIG. 7, but this number could be different and go for example from 3 to 9 for other configurations of the absorber. During the rotation of the inner cylinder (72), for example in a counter-clockwise direction, it is possible to put the part of the wall of the inner cylinder not provided with orifices (position (s) 1), and the orifices, windows or series of orifices (79, 710, 711) provided at various positions 2, 3, 4 (712, 713, 714) in the side wall of the inner cylinder (72) successively in register with the window (78) provided in the side wall of the outer cylinder (71). When the orifices, windows or series of orifices (79, 710, 711) provided at the various positions 2, 3, 4 (712, 713, 714) in the side wall of the inner cylinder (72) coincide with the window (78) provided in the side wall of the outer cylinder (71), the passage of the fluid in the channel in the center of the cylindrical part through said orifices (79, 710, 711) and then through said window is made possible and the fluid can be injected into the stream, flow, flow of melted nanocomposite material in the direction of this flow. By injecting a fluid into the flow, flow, flow of molten nanocomposite material in the direction of this flow, it is thus possible to form in this stream of molten nanocomposite cavities filled with said fluid, these cavities corresponding to different hollow volumes of the absorber that we want to prepare.

The injected fluid may be selected from gases such as air, argon, nitrogen, and mixtures thereof; and liquids such as oils, for example silicone, mineral, synthetic, semi-synthetic oils, and mixtures thereof.

When it is the part of the wall of the inner cylinder not provided with orifices (position 1), which is in coincidence with the window (78) in the side wall of the outer cylinder (71) then the passage of the fluid lying in the channel in the center of the cylindrical part by said window is not possible and the fluid is not injected into the flow stream, molten nanocomposite material. No cavity is then formed in the molten nanocomposite material. In this position 1, one forms the solid parts, for example end of the absorber. The orifices, windows or series of orifices provided in each of said positions 2, 3, 4 are preferably provided in such a way that each of these orifices, windows or series of orifices has a profile allowing by injection of a fluid in the stream, flow, of molten nanocomposite material the formation of cavities corresponding to the different hollow volumes of the absorber that it is desired to prepare. Thus, for example in the positions 2 and 4 (712, 714) can the inner cylinder (72) be pierced by a window (79, 711) shaped elongated rectangle whose length generally corresponds to the internal width of the end chambers of the absorber and whose width or height corresponds to the internal height of the first and second end chambers. In the position 3 (713), the inner cylinder is pierced by holes (710) arranged in a row, these orifices having a shape, for example the shape of a square, and a dimension corresponding to the shape and the internal dimension channels of the absorber. The extrusion of a complete absorber generally amounts to performing a complete revolution of the inner cylinder in a counter-clockwise direction. In Figure 8, schematically shown for each of the positions of the inner cylinder (positions 1, 2, 3, 4) coincident with the window (78) in the wall of the outer cylinder, the corresponding different extruded profiles obtained. 36 The position 1 allows to extrude the solid part of the absorber (81), the position 2 makes it possible to extrude the first end chamber (82) at the first end of the channels, the position 3 makes it possible to extrude the channel body (83), and position 4 allows extruding the second end chamber (84) at the second end of the channels. Several absorbers can be extruded in series and continuously by successively scrolling the positions 1, 2, 3, 4 of the inner cylinder in front of the window of the outer cylinder according to the following cycle: 1234, 1234, 1234, .... Figure 9 thus shows the continuous and series extrusion of five absorbers.

Once the absorber (s) extruded (s), and before the cutting step, can deposit a selective coating (47: see Figure 4)) on the upper face of the absorber or absorbers which is the face that will be exposed to solar radiation.

This coating is made of a radiation-transparent material having a wavelength in the wavelength range of the solar spectrum and a reflectivity to radiation having a wavelength greater than the wavelength range of the solar spectrum. In general, this coating comprises only a single layer of a transparent material instead of a stack of layers for the absorbers of the prior art made for example of copper (FIG. 3), which again leads to significant gains in cost and time. This coating may be, for example, a coating of a conventionally used ITO substitute, which is an aluminum doped zinc oxide (ZnO: Al). Preferably, this coating comprises only one layer of ZnO: Al. The reflectivity characteristics of this coating are illustrated in the graph of FIG. 10. Such a coating can be deposited by a physical vapor deposition process ( "Physical Vapor Deposition" or "PVD" in English). Then a cutting step separates, isolate, each absorber to complete its manufacture. For example, it is possible to carry out a drilling operation of the inlets and outlets of the absorber, then possibly an emptying of the fluid, for example of the nano-fluid, and finally a welding of the connections. The absorber according to the invention is integrated in a solar thermal collector which is substantially similar to that of FIG. 1. Such a solar thermal collector comprises a transparent cover. This transparent cover consists of a transparent thermoplastic polymer which is generally selected from polyolefins: polymers and copolymers of cyclic olefins ("COP", "COC"); polystyrenes; polyesters such as polycarbonates ("PC"), poly ((meth) acrylates) such as poly (methyl methacrylate) ("PMMA"), poly (ethylene terephthalate) s 38 (" PET "), poly (ethylene naphthalate) s (" PEN "); and their mixtures. The transparent cover is usually prepared by extrusion or injection.

The underside of the transparent plate cover is generally coated with a transparent conductive oxide ("OTC"), which has a good transparency on all the wavelengths of the solar spectrum, namely typically between 300 nm and about 1300-1500 nm, and which also has significant reflectivity for wavelengths greater than 1300-1500 nm in order to avoid the energy losses due to the phenomenon of radiation due to the heating of the sensor. The ideal reflectivity spectrum for coating the cover of a solar thermal collector is shown in FIGS. 11 and 12. This wavelength (cut-off wavelength) must be adjustable as a function of the temperature of the solar thermal collector. sensor balance that will impose the minimum wavelength from which the sensor re-emits by radiation. Experimentally, the cut-off wavelength can be adjusted by adjusting the deposition conditions that will modify the optical characteristics of the OTC layer. The coating currently used is based on indium tin oxide (ITO); however, other less expensive materials may be employed, such as, for example, aluminum-doped zinc oxide (ZnO: Al) whose reflectivity spectrum is shown in Figure 10.

The upper face of the transparent cover may optionally be provided with an anti-reflection and / or anti-UV coating which is generally also deposited by a physical vapor deposition process.

Claims (26)

REVENDICATIONS1. Solar thermal collector absorber comprising channels (42) each having a first end (43) and a second end (44), the first ends (43) of the channels (42) opening into a first chamber end (45) and the second ends (44) of channels opening into a second end chamber (46), wherein said absorber consists of a single extruded part (41) of a solid nanocomposite material comprising a polymer matrix in which nano-objects and / or nanostructures are incorporated. 2. Absorber according to claim 1, wherein the polymer of the matrix is selected from thermoplastic polymers such as polyolefins such as polyethylenes and polypropylenes polymers and copolymers of cyclic olefins polystyrenes; polyamides; polyesters such as polycarbonates, poly ((meth) acrylates), poly (ethylene terephthalate) s or PETs, poly (ethylene naphthalate) s; and their mixtures. 3. Absorber according to claim 1, wherein the content of nano-objects nano-objects and / or nanostructures is less than or equal to 5% by weight, preferably less than or equal to 1% by weight, more preferably 10 ppm to 0.5% by weight of the mass of the nanocomposite material. 41 4. Absorber according to any one of the preceding claims, wherein the nano-objects are selected from nanotubes, nanowires, nanoparticles, nanocrystals, and mixtures thereof. 5. Absorber according to any one of the preceding claims, wherein the nano-objects and / or nanostructures are functionalized, in particular chemically. 6. Absorber according to any one of the preceding claims, wherein the nano-objects and / or nanostructures are selected from nano-objects and / or nanostructures that confer thermal and / or electrical and / or magnetic and / or optical properties. nanocomposite material; and among the nanoobjects which improve the thermal and / or electrical and / or magnetic and / or optical properties of the nanocomposite material. 7. Absorber according to any one of the preceding claims, wherein the material constituting the nano-objects and / or nanostructures is selected from carbon; metals such as gold, copper, manganese or aluminum; metal alloys such as alloys of copper, gold, manganese or aluminum; metal oxides such as rare earth oxides possibly doped; organic polymers; and materials comprising a plurality of materials from the aforesaid materials. 42 An absorber according to any one of the preceding claims, wherein the nano-objects are carbon nanotubes; nanoparticles of metals such as copper, gold, manganese or aluminum; nanoparticles of metal alloys such as alloys of copper, gold, manganese or aluminum; nanoparticles of metal oxides; magnetic nanoparticles such as nanoparticles of AgMn, Fe2O3 or Fe3O4; or a mixture thereof. An absorber according to claim 8, wherein carbon nanotubes and magnetic nanoparticles are incorporated in the polymer matrix. 10. An absorber according to any one of claims 1 to 7, wherein the nanostructures are core-filament nanostructures, in particular core-filament nanostructures with a core made of alumina and filaments consisting of carbon nanotubes. An absorber according to any one of the preceding claims, wherein the nano-objects and / or nanostructures are homogeneously distributed in the polymer matrix. An absorber according to any one of the preceding claims, wherein the upper face of the absorber, capable of being exposed to solar radiation, is provided with a coating (47) of a radiation-transparent material having a length in the wavelength range of the solar spectrum and a reflectivity to the radiation having a wavelength greater than the wavelength range of the solar spectrum. An absorber according to claim 12, wherein said transparent material (47) is selected from transparent conductive oxides or OTC such as indium tin oxide or ITO and aluminum doped zinc oxide. or ZnO: Al. An absorber according to claim 12 or 13, wherein said coating (47) of a transparent material comprises a single layer, preferably a single layer of zinc oxide doped with aluminum or ZnO: Al. 15. Solar thermal collector comprising an absorber according to any one of claims 1 to 14, and a cover transparent to solar radiation, on said absorber. The sensor of claim 15, wherein the transparent cover is coated on its underside with a coating of a radiation-transparent material having a wavelength in the wavelength range of the solar spectrum and a reflectivity at radiation having a wavelength greater than the wavelength range of the solar spectrum. 17. The sensor of claim 16, wherein the transparent material is selected from transparent conductive oxides or OTC such as indium tin oxide or ITO and zinc oxide doped with aluminum or ZnO: al. 18. Process for the preparation of an absorber according to any one of claims 1 to 14, in which the extrusion of a melt of the solid nanocomposite is continuously carried out and the first chamber is formed successively and in a single step. end, the channels and the second end chamber of the absorber. The method of claim 18, wherein the first end chamber, the channels, and the second end chamber of the absorber are formed with one and the same extrusion head or die. 20. A method according to any one of claims 18 and 19, wherein is formed successively in series and continuously several absorbers. 21. A method according to any one of claims 18 to 20, wherein during the extrusion 45 is injected a fluid into the molten nanocomposite material. 22. The method of claim 21, wherein the injected fluid is selected from gases such as air, argon, nitrogen, and mixtures thereof; and liquids such as oils, for example silicone, mineral, synthetic, semi-synthetic oils, and mixtures thereof. 23. Method according to any one of claims 18 to 22, wherein after the extrusion is deposited on the upper face of the absorber or absorbers, may be exposed to solar radiation coating in one radiation-transparent material having a wavelength in the wavelength range of the solar spectrum and a reflectivity to radiation having a wavelength greater than the wavelength range of the solar spectrum. 24. The method of claim 23, wherein the transparent material is selected from transparent conductive oxides, such as indium tin oxide (IT0) and zinc oxide doped with aluminum or ZnO: al. 25. The method of claim 23 or 24, wherein the coating of a transparent material comprises a single layer, preferably a single layer of zinc oxide doped with aluminum or ZnO: Al. 26. A method according to any one of claims 18 to 25, which comprises a final step during which is pierced heat transfer fluid inlets and outlets in the absorber or absorbers, then optionally performs a drain of the fluid, and finally, connections are welded to said heat transfer fluid inlet and outlet.