AU2020290036A1

AU2020290036A1 - Hybrid radiation absorber for solar power plant, and method for preparing such an absorber

Info

Publication number: AU2020290036A1
Application number: AU2020290036A
Authority: AU
Inventors: Sylvain Pare
Original assignee: Individual
Current assignee: Individual
Priority date: 2019-06-13
Filing date: 2020-05-26
Publication date: 2022-01-20
Also published as: CA3142844A1; IL288880A; CN114127484A; KR20220024542A; FR3097304A1; JP2022536366A; WO2020249885A1; EP3983730A1; FR3097304B1

Abstract

The present invention relates to a solar radiation absorber for a concentrated solar thermal power plant, the absorber being characterised in that it is formed from a monolithic piece of silicon carbide, the absorption surface of which is, for example, coated with tungsten dendrites, in particular for the production of a collector or a system for a solar power plant. The invention also relates to a method for preparing such an absorber.

Description

HYBRID RADIATION ABSORBER FOR SOLAR POWER PLANT, AND METHOD FOR PREPARING SUCH AN ABSORBER

Field of the invention

The present invention relates to the field of energy absorbers, the characteristics of which are similar to the behavior of a black body.

The black body is an ideal object that would perfectly absorb all the electromagnetic energy it receives, without reflecting or transmitting it. Under the effect of thermal agitation, the black body emits electromagnetic radiation. In thermal equilibrium, emission and absorption are balanced and the radiation actually emitted depends only on the temperature (thermal radiation).

Typical non-limiting applications: Gaseous fluid: Stirling/Ericsson type external combustion engine, hot air turbine (turbo alternators), industrial processes, cooking, etc., the liquid fluid can be water that one wishes to heat, a liquid to be sterilized, a production of steam to supply a standard turbo alternator, DHW (domestic hot water/heating), various fluids, etc.

The invention relates more particularly to the field of absorbers intended for the production of energy from solar radiation by solar thermal power plants supplemented by a flame that is ideally an HHO or renewable flame Thermal solar methods have better efficiencies than photovoltaic methods, of the order of 30%, but they are bulkier and more suitable for a large production of electricity.

New devices coupling a Stirling engine with a concentrator are currently being developed to produce electric current. However, the thermodynamic efficiency is correlatively linked to the input temperature, which must then be high enough for the best efficiency. Existing devices are limited to 650/800 0C and therefore cannot exceed % efficiency. The invention makes it possible to reach 1200 0C and therefore to reach and exceed 60% net efficiency.

Furthermore, the absorber according to the invention allows the hybridization of different heat sources, for example solar and "solar fuel" = HHO (or biogas, petroleum derivatives, etc.), thus allowing continuous operation of an installation at full power despite variations or absences of solar flux.

Prior Art

There are different techniques for concentrating solar radiation, for transporting and possibly storing heat, and for converting heat into electricity. In any case, one of the essential elements of a concentrated solar thermal power plant is the solar radiation absorber element that forms part of the receiver. In order to maximize the efficiency of the absorber, the latter generally comprises a coating, called a selective coating or selective treatment. The selective coating is intended to allow maximum absorption of incident solar energy while re-emitting as little infrared radiation as possible (black body principle). In particular, such a selective coating is considered perfect if it absorbs all wavelengths less than a cutoff wavelength and reflects all wavelengths greater than this same cutoff wavelength. The optimum cutoff wavelength depends on the operating temperature of the absorber element considered and is generally between 1.5 pm and 2.5 pm. It is, for example, about 1.8 pm for a temperature of the order of 650 K.

Patent application US2015033740 describes a solar receiver comprising: • a low-pressure fluid chamber configured to operate at pressures up to 2 atmospheres, and comprising a fluid inlet, a fluid outlet, and an opening for receiving concentrated solar radiation; • a solar absorber housed within the low-pressure fluid chamber; and • a plurality of transparent objects that define a segmented wall of the low pressure fluid chamber; • wherein the concentrated solar radiation received through the opening passes through the segmented wall and between transparent objects to pass into the low-pressure fluid chamber and impinges upon the solar absorber.

Patent application US4047517 describes a radiant energy receiver comprising a plurality of elongated vane structures arranged in a converging configuration from an exterior portion thereof to an interior throat portion thereof, the exterior to intermediate surfaces of the vanes being at least in part a reflective surface and the intermediate to interior surfaces of the vanes being at least in part of a selective surface that absorbs radiant energy impinging upon the selective surface at a small angle of incidence, but reflects such energy impinging at a greater angle of incidence, whereby radiant energy impinging upon the exterior portions of the vane is reflected toward the converging throat of the vanes and radiant energy in the interior portion impinging upon the selective surface at a relatively small angle of incidence, as would indicate incipient or actual reversal of direction of travel of the radiant energy relative to the vanes is absorbed while that impinging upon the selective surface at a relatively large angle of incidence is reflected into the throat of the vanes to generate elevated temperature adjacent to the throat of the vanes.

Drawbacks of the Prior Art

The performance of the solutions of the state of the art is limited by the energy conversion capacities of the absorber, which leads to limited efficiencies. Furthermore, the known absorbers are exposed to the open air, which generates a significant heat loss. The known absorbers have a smooth, poorly absorbent and highly emissive collection surface. The materials of the known absorbers do not allow use in high temperatures and cannot withstand excessive pressures or stresses.

Solution provided by the invention

In order to remedy these drawbacks, the invention relates in its most general sense to a solar radiation absorber for a concentrated solar thermal power plant, characterized in that a monolithic piece of silicon carbide is formed, the absorption surface of which is for example coated with tungsten dendrites (or other substrate) The invention also relates to a thermal collector for a concentrated solar thermal power plant, characterized in that it is formed by a cavity, for example a graphite cavity, with a transparent inlet window in which is disposed the absorber according to the invention, formed by a monolithic piece of silicon carbide, the absorption surface of which is ideally coated with tungsten (or other) dendrites.

Advantageously, the collector comprises a burner arranged inside said cavity, directing a flame in the direction of said absorber.

According to one variant, it comprises an optical fiber transporting solar energy to said absorber.

Advantageously, at least part of the interior surface of the cavity has cavities behaving like a light trap (conical honeycomb).

The invention also relates to a system consisting of a thermal collector for a concentrated solar thermal power plant thermally and mechanically coupled to the inlet of a thermal machine, characterized in that said collector is formed by a graphite cavity with a transparent inlet window in which is disposed an absorber formed by a monolithic piece of silicon carbide, the absorption surface of which is coated with tungsten dendrites.

Advantageously, said expansion machine with upper part made of silicon carbide.

The invention also relates to a method for preparing an absorber according to the invention, characterized in that it comprises a step of depositing a thin layer absorbing the radiation that may for example consist of a plasma spray or concentrated solar flux of tungsten dendrites on the surface of a monolithic silicon carbide piece. Said layer can also advantageously be deposited as soon as it comes out of the molding, the paste obtained being relatively tacky and thus allowing easy fixing of the dendrites by simple mechanical spraying or powdering.

According to one variant, the method comprises a step of laser projection of tungsten dendrites onto the surface of a monolithic part made of silicon carbide.

According to other variants, the invention relates to:

A thermal collector for a concentrated solar thermal power plant, characterized in that it is formed by a cavity isolated under vacuum, for example a graphite cavity, with a transparent inlet window in which is disposed an absorber formed by a monolithic piece of silicon carbide of high purity, the absorption surface of which is coated with tungsten dendrites.

Advantageously, this solar radiation absorber for a concentrated solar thermal power plant has: - a honeycomb configuration, the cells of which are conical/flared with a greater height in the center and presenting microcavities. - a sealed spherical upper/lower supporting interface acting as an assembly/sealing flange with a support having a honeycomb and fins and a threaded connection of a pipe on a thermodynamic device. - fins for heat exchange with the fluid in the form of rosettes and having microcavities with a higher height in the center. - a sealing disc and fluid passage ports. - a central helical frustoconical section with a 900 return and a flared/conical shape. - a burner disposed inside said cavity, directing a flame in the direction of said absorber. - exchange surfaces with microcavities. The invention also relates to a system consisting of a thermal collector for the aforementioned concentrated solar thermal power plant thermally and mechanically coupled to a tubing (hot fluid outlet) or to the inlet of a thermal machine, characterized in that said collector is formed by a graphite cavity with a transparent inlet window in which is disposed an absorber formed by a monolithic piece of silicon carbide, the absorption surface of which is coated with tungsten dendrites.

Preferably, said expansion machine with upper part made of silicon carbide.

Advantageously, it comprises a step of plasma spraying tungsten dendrites on the surface of a monolithic piece made of silicon carbide and/or a step of deposition by powdering during the production in a pasty phase of tungsten dendrites on the surface of a monolithic silicon carbide piece.

Detailed description of a non-limiting example of the invention

The present invention will be better understood on reading the detailed description of a non-limiting example of the invention which follows, with reference to the appended drawings, where: - Fig. 1 shows an absorber seen in section, upper part toward the top (sun/flame) comprising the honeycomb, the sealed interface in the middle (3) and the fins of the fluid exchanger below - Fig. 2 shows a bottom view of the exchanger with its helical cone in the center. - Fig. 3 shows a section of the interface and of the lower part alone. - Fig. 4 shows a view of the honeycombs - Fig. 4A shows a view of the dendrites - Fig. 5 is a simplified representation of the honeycomb matrix - Fig. 6 shows a first form of enlarged tungsten dendrite - Fig. 6A shows another form of enlarged tungsten dendrite - Fig. 7 shows a close-up of fused dendrites on the CSi support - Fig. 8 shows an external containment enclosure (unit for a solar concentrator). - Fig. 9 shows a detailed section of the helical cone - Fig. 10 shows a 3D representation of the helical cone for the understanding of the device.

Description of the usage context of an absorber according to the invention

The thermal collector absorbs solar radiation to transform it into heat. This heat is then transmitted to a heat transfer fluid. A collector is made up of an absorber, a heat transfer fluid, an insulation, sometimes a glazing and reflectors.

The absorber is one of the most important parts of a thermal collector; it converts solar radiation into heat.

The absorber is characterized by two parameters: - the solar absorption factor a* (or absorptivity): the ratio of the light radiation absorbed by the incident light radiation; - the infrared emission factor e (or emissivity): the ratio between the energy radiated in the infrared when the absorber is hot and that which a black body would radiate at the same temperature.

In solar heating applications, the aim is to obtain the best solar absorption factor/infrared emission factor ratio. This ratio is called selectivity. The constituent material of the absorber is generally made of copper or aluminum, but also sometimes of plastic. The properties of some materials used as absorbers.

The structure of the absorber is illustrated in Fig. 1, 2, 3 and 8.

It comprises a honeycomb structure (1) exposed to solar radiation through a window (10). It is fixed to the enclosure by a flange (2). A membrane (3) forms a sealed interface. A seal (4) ensures the tightness between the flange (2) and an internal shoulder of the enclosure. A structure (6, 12) has a central helical frustoconical section with lower fins (5). It is fixed by screws (7).

A sealing disc (11) extends under the structure (12). In the area between the window (10) and the honeycomb structure (1), a burner injects hot gases. This area also has discharge openings (14).

The honeycomb structure receives the heat flow, and has a sealed interface in the center, with a flange on the sides with the fins over the entire height (makes it possible to withstand high pressures), below in the center the helical cone (allows the fluid to be returned to 900), at the bottom the sealing disc = allows the fluid circuit to be sealed and allows circulation from the periphery to the center and vice versa (reversible/alternative)

Table 1 Materials absorptivity emissivity selectivity Max temperature a* a*/ E

Black nickel 0.88- 0.98 0.03-0.25 3.7- 32 300 0 C Graphitic films 0.876- 0.92 0.025-0.061 14.4- 36.8 250 0 C Black copper 0.97- 0.98 0.02 48.5- 49 250 0 C Black chrome 0.95- 0.97 0.09-0.30 3.2- 10.8 350-425 0 C

In order to obtain better efficiency, certain systems therefore consist of a specific coating.

The heat transfer fluid makes it possible to discharge the heat stored by the absorber and to transmit it to where it is to be consumed. A good heat transfer fluid must take into account the following conditions: - be chemically stable when it reaches a high temperature, in particular when the collector is stagnant; - possess antifreeze properties correlated with local weather conditions; - possess anticorrosive properties depending on the nature of the materials present in the collector circuit; possess high specific heat and thermal conductivity in order to efficiently transport heat; - be non-toxic and have a low impact on the environment; - have a low viscosity in order to facilitate the task of the circulation pump; - be readily available and inexpensive

The correct compromise with respect to these criteria is a mixture of water and glycol (used in automotive coolant), although it is not uncommon to find systems operating with pure water or simply with air depending on the use.

The glazing protects the inside of the collector against the effects of the environment and improves the efficiency of the system by greenhouse effect.

If effective glazing is desired, it must have the following properties: reflect light radiation to a minimum regardless of its inclination; - absorb light radiation to a minimum; - have good thermal insulation by keeping infrared radiation to the maximum; - withstand, over time, the effects of the environment (rain, hail, solar radiation, etc.) and large temperature variations.

The main glazings used for thermal collectors are based on non-ferruginous glass or acrylic glass, and often have an anti-reflective coating.

The thermal insulation allows thermal losses to be limited, and its characteristic is the coefficient of conductivity; the lower it is, the better the insulation. The main materials used for thermal collectors are rock and glass wool, polyurethane foams or melamine resin.

Some insulators used for thermal collectors:

Table 2 Materials Thermal conductivity Rockwool 0.032 - 0.040 W/m.K Glass wool 0.030 - 0.040 W/m.K Polyurethane foams (waterproofing) 0.022 - 0.030 W/m.K

In the case of glazed thermal collectors, it is also interesting to replace the insulation between the glass and the absorber with air. Indeed, air has a great insulating power, and is thus used in the double glazing. Still with the aim of obtaining better efficiencies, some manufacturers use other gases such as argon or xenon, and when possible, it is even preferred to use a vacuum. Below are the insulation coefficients of gases used as insulators:

Table 3 Gas Thermal conductivity at 283 K, 1 bar. Air 0.0253 W/m.K Argon 0.01684 W/m.K Xenon 0.00540 W/m.K

Description of the absorber according to the invention

The absorber according to the invention consists of a monolithic piece of CSi on which are deposited during the preparation in the pasty phase, or projected by laser or plasma, tungsten dendrites, a crystalline form absorbing 98% of the infrared radiation and which has a melting point above 3,400C.

For the purposes of the present patent, the term "dendrite" is understood to mean a crystalline form obtained by solidification, and having an arborescent shape. A snowflake, for example, has a dendritic structure. Said dendrites are preferably industrial residues or dust or are produced by the solar route at high temperature.

The agglomeration of the tungsten dendrites on the CSi can be carried out in a thin layer and under high temperature or by any other method.

The absorber thus forms a light trap, in particular using microcavities produced during molding, to have characteristics close to a black body.

Detailed description of the invention

The main qualities of an absorber are:

a) Be able to receive and transfer the maximum amount of energy b) Be a very good thermal conductor c) Do not reflect or radiate IR (infrared) d) Support a very high energy density e) Withstand thermal shocks and remain chemically inert f) Do not deteriorate over time g) Have the lowest possible manufacturing cost h) Be easily industrialized i) Have significant mechanical properties

The first quality of an absorber is its ability to receive radiation (sun/flame) and transfer it into a fluid with the best possible efficiency.

The absorber is in a vacuum chamber allowing perfect thermal insulation, which is ideally made of graphite and covered with a window that is transparent to solar radiation and covered with an anti-reflection coating limiting optical losses. The vacuum cavity is equipped with a burner making it possible to supply the necessary energy when there is no solar flux and an outlet is fitted to discharge the combustion residues.

The majority of known absorbers use materials such as stainless steel, with or without an absorbent/selective coating. This material has a very limited absorption rate and re-diffuses a good part of the infrared radiation. Furthermore, its conductivity is very limited at around 20 W/m.K, which is very little compared to other recognized materials such as copper = 386 W/m.k, which has proven to be 20 times better as a thermal conductor, an important property for the quality of an absorber. Then stainless steel can only be used up to 80 0 °C, which limits the advantageous thermodynamic efficiency in high temperature ranges. One of the suitable materials provided in the invention and which will be cited by way of example is CSi (silicon carbide) in relatively pure form.

Pure CSi is an excellent thermal conductor up to 1,200C with a maximum conductivity of around 350 W/m.k close to that of copper, which gives it exceptional properties; in addition, it conducts IR (infrared) perfectly. It withstands significant thermal shocks and its very high hardness and mechanical strength allow parts to be designed that can withstand very high stresses, thus allowing the production of thin parts in with excellent thermal conductivity. Chemically inert, it withstands very high temperatures and does not degrade over time.

Conversely, pure CSi is marred by several problems because it is almost transparent to solar radiation, resembling glass, and therefore does not absorb the concentrated solar flux. Furthermore, it is very difficult to produce the parts with complex geometries that are necessary to produce almost perfect absorbers. Finally, its implementation requires a very large amount of energy and very high temperatures.

To remedy this, the absorber according to the invention is covered with a thin layer of tungsten dendrites on the face exposed to the heat source. Tungsten dendrites have the property of perfectly capturing solar radiation or radiation from a flame and transmitting it into a support substrate with an efficiency of 98%. To do this, the dendrites are deposited by means, for example, of a plasma torch or any other suitable method, in particular when the CSi comes out of the molding phase, its tacky paste consistency allowing perfect cohesion.

In order to absorb an incident flux with the greatest possible efficiency, it is necessary to produce a particular geometry that can capture and trap the incident radiation. Known absorbers generally have a smooth surface, which reflects a large part of the radiation. The invention has a geometry that acts as a light trap and is compared to a black body. For this, the surface consists of a honeycomb structure whose section is conical, thin at the top, and wide at the bottom. Thus, it is possible to capture the incoming radiation with the greatest possible efficiency because it cannot escape and is perfectly captured by the dendrites, which therefore transfer the flux into the CSi substrate. Furthermore, the conical shape of the honeycomb allows easy stripping Concerning the production, the state of the art does not currently allow the manufacture of parts with complex geometries, especially given that for a good absorber it is necessary to limit the thickness as much as possible, to the detriment of its strength, which is currently not possible in the state of the art because it requires machining with tools whose diameter and length are limited for mechanical reasons.

The invention makes it possible to remedy these problems owing to two innovative methods, one being high pressure isostatic pressing, the second in additive manufacturing by 3D printer. The high-pressure isostatic pressing developed by the inventor makes it possible to send a CSi paste into a mold formed by two or more parts, almost similar to plastic or metal injection, one for the upper part and the second for the lower part, and possibly a third for the central helical cone which may require a screwing/unscrewing function or even two independent molding half-shells. It is thus possible to obtain parts of complex geometry with a very small thickness, which may be of the order of a millimeter, the geometric architecture of the part allowing this type of production. The sealing disc can be added immediately to obtain a monolithic piece.

The second method successfully tested by the inventor is additive printing or 3D printing. A nozzle or a set of nozzles deposits the CSi paste as it goes on a plate progressively forming a part with a geometry whose complexity is almost infinite or obtaining shapes impossible to achieve otherwise. The honeycomb surface ideally consists of a rough surface having microcavities that advantageously absorb light and allow easier attachment of the dendrites. Likewise, the fins of the lower part may have microcavities generating microturbulence, which contribute on the one hand to increasing the heat exchange coefficients, and on the other hand to reducing the friction on the surfaces increasing the overall efficiency.

The parts, after various appropriate treatments, are then sintered in a high temperature furnace, traditionally supplied with gas or electricity, but can also ideally be sintered by the concentrated solar route to drastically reduce production costs. The absence of the solar source is ideally compensated for by the combustion of a HHO mixture, which produces a very high-quality flame at 2800 0C, the residue of which is only water vapor that can be recycled indefinitely. The HHO mixture, also called "solar fuel," can ideally be produced by the solar route and correspondingly lower the energy cost. The other advantage of this solar method is that a controlled annealing can be considered to release the tensions, which is very inexpensive.

It is thus possible to envisage the industrial production of parts with complex geometries at a very high speed and a very low manufacturing cost while having a carbon index close or equal to zero, and therefore no environmental impact.

Although the absorber is monolithic, it is divided here into three sections for the sake of understanding the description. The first section is the upper part receiving the heat flow, and the second is the interface making it possible to support the two main sections and to carry out the assembly in a structure under pressure while guaranteeing tightness.

The third section is the lower part, which is responsible for transmitting thermal energy within a fluid. The assembly is concave in shape so as to optimize the capture and transfer of energy, but also to ensure the best mechanical strength by making the energy flows and the mechanical forces applied to the surfaces homogeneous.

The upper part, described above, is a honeycomb structure of conical shape flared toward the opposite part (Fig. 4) so as to even out the temperature gradients, and its surface is covered with a thin layer of tungsten dendrites. These cones are taller and wider in the center due to the fact that a solar flux or a flame is always greater in its center, thus requiring a higher density of matter that then transmits by conduction to the surrounding elements.

Due to the excellent properties of CSi and the implementation method according to the invention, it is possible to produce convection fins (honeycomb) with a thickness of the order of a millimeter at their upper part. The "leading edge" (terminology?), which receives direct solar radiation or the flame (therefore the top), is rounded to avoid sharp corners that are too fragile and to allow stripping

The other advantage of the honeycomb structure is that it distributes both thermal and mechanical stresses perfectly. Since the height of the honeycomb is greater at the center than at the periphery, both thermal and mechanical stresses are thus uniformly distributed over the entire surface and the structure can therefore undergo much greater pressure at its center, which makes it possible to withstand the most extreme energy and mechanical densities, unlike known absorbers, for example made of stainless steel, the surface of which is smooth, spherical and of constant thickness.

In the continuity of the honeycomb structure is an "interface" (sealed concave disc separating the lower and upper parts), which receives the two exchange parts, upper and lower. This interface ensures the continuity of the seal between the two opposite parts and the proper energy transmission uniformly distributed over its entire surface. Its shape is preferably spherical and its concavity oriented toward the upper part (receiving the flow), which allows the absorber to withstand very high pressures with the thinnest thickness possible, thus contributing to the thermal efficiency. This small thickness also makes it possible to limit the mechanical stress or the known molecular defects in a large thickness as well as the quality of the sintering, which is essential to ensure the durability and reliability of the absorber.

To ensure that the absorber is mounted tightly between the various constituents of a thermal device, the outer periphery consists of a peripheral bearing surface similar to a flange, which is assembled with the external devices. This is of a thickness adapted to the stresses to which it will be subjected and is designed to fit into a cylinder of slightly larger section on which the absorber is positioned and ensure a tight assembly.

To do this, it is possible to envisage a ring identical to a seal, which is produced on the central periphery so as to apply pressure on a limited surface defined as within a flange, ideal in the context of very high applied pressures. This raised ring can also be replaced by a groove receiving a standard seal or be a flat surface for certain flat seals, in particular of the metallic type. An insulating seal, for example made of graphite, can also be ideally considered to withstand high temperatures; the other advantage of this type of gasket is that it constitutes a thermal bridge, thereby avoiding the transmission of heat to the external support.

In this case, the absorber is mounted directly on the outer receiving cylinder as in Fig. 8, and sufficient gas pressure is applied allowing rapid and easy mounting, like tubeless tires, especially considering that the window of the cavity receiving the absorber is ideally under vacuum. This avoids creating a mechanical stress during the expansion differences of the various constituents of the device by allowing self adjustment and displacement of the absorber on the sealed surface.

The lower part of the "flange" allows the mounting of additional components and devices and in this sense has elements allowing their mechanical connection such as threads or any suitable assembly system, in particular a quarter-turn type assembly to allow a quick and economical assembly. Said threads or assembly devices may be on both sides to allow a durable and safe mechanical assembly while allowing perfect maintenance of the pressure to be exerted on the sealing device/seal.

The lower part in Fig. 2 and 3 is made up of fine fins forming a rosette (terminology to be checked) allowing thermal energy to be transmitted to the heat transfer fluid or working fluid/fluid to be heated. The fins are nested in the "flange" part over their entire height so as to obtain a monolithic piece that is particularly resistant to high pressures and to distribute the forces uniformly.

The concentrated solar flux is similar to a Gaussian curve, i.e. with a maximum intensity at its center. As a result, the fluid ideally comes from the periphery toward the center to avoid heat losses at the sealing flange. To this end, a "port" (passage) is made around the entire periphery of the inlet of the fins to allow the passage of the fluid. This port is imposed by a sealing disc affixed to the fins and locked by a method such as mounting lugs to prevent its displacement or any vibrations. Its assembly can also advantageously be in monolithic form depending on the manufacturing method, the latter avoiding the addition of mechanical fixing/holding devices.

This arrangement in portions of circles (rosettes?) nested one inside the other makes it possible to create a certain number of turbulences and to guide the fluid in a very precise direction. In addition, the centripetal force makes it possible to increase the interaction of the fluid on the surface of the fins, thus improving the heat exchange coefficient. This unique arrangement also makes it possible to increase the exchange surfaces and to increase the thermal efficiency of the absorber. The space between the fins is greater at the periphery than at the center for a perfect correlation with the energy density implemented on the surfaces.

Furthermore, the fins are higher in their center than toward the periphery, which on the one hand allows the heat exchanges to be optimized, the greatest energy density being in the center, and on the other hand contributes to the mechanical strength of the assembly when it is subjected to very high pressures made necessary in particular in thermodynamic devices, for example of the Stirling type. It is thus possible to have a very thin interface while ensuring extreme mechanical resistance to very high pressures.

Due to the particular geometry of the fins, a fast vortex is formed in the center of the lower structure, which is redirected outside the absorber by a helical frustoconical section taking the direction of the initial flow in a perpendicular direction or toward a piping, or toward a piston for certain thermodynamic devices. Said frustoconical section comprises helical fins, which make it possible to direct the flow in the new perpendicular axis on the one hand, and to avoid overheating on the central area most exposed to the incident thermal radiation on the other hand due to an increased flow speed by the Venturi effect. The center of the cone is relatively thick, while its end is thinner. The lower base of the frustoconical section is advantageously curved in order to avoid excessive turbulence and pressure drops that are detrimental to the overall efficiency. This helical shape approaches the sealing disc to avoid losses linked to leaks or guiding faults, preferably in one direction of the fluid.

To ensure the tightness of the lower part, a sealing disc makes it possible to close and thus perfectly direct the fluid intended to be heated. This disc comprises an opening in its center allowing the possible connection by a cylinder section on a piping or sending on a piston, as well as an outer diameter slightly smaller than the diameter of the fins, thus allowing the passage of the fluid from the periphery.

An attachment device with the body of the absorber is produced on the sealing disc, the latter possibly being done in several ways such as lugs, notches or any other assembly method, or constituting a monolithic assembly in the case of additive printing. Another advantageous method being the assembly of the disc as soon as the body of the absorber comes out of molding, adhesion then taking place easily, or even during production by additive printing.

In general, all sharp angles interfering with the movement of the fluid are rounded off to avoid generating turbulence and other detrimental pressure drops.

The lower part is also designed to allow a rapid reciprocating passage without head loss of fluids in the outward and return directions, as for example in the case of an FPSE process (free piston Stirling engine, this with frequencies that can be of the order of several tens of cycles per second.

Claims

Claims

1 - Thermal collector for a concentrated solar thermal power plant, characterized in that it is formed by a cavity isolated under vacuum, for example a graphite cavity, with a transparent inlet window in which is disposed an absorber formed by a monolithic piece of silicon carbide, the absorption surface of which is coated with tungsten dendrites.

2 - Solar radiation absorber for a concentrated solar thermal power plant according to claim 1, characterized in that it has a honeycomb configuration, the cells of which are conical/flared with a greater height in the center and presenting microcavities.

3 - Solar radiation absorber for a concentrated solar thermal power plant according to claim 1, characterized in that it has a sealed spherical upper and lower supporting interface acting as an assembly and sealing flange with a support having a honeycomb and fins and a threaded connection of a pipe on a thermodynamic device.

4 - Solar radiation absorber for a concentrated solar thermal power plant according to claim 1, characterized in that it has fins for heat exchange with the fluid in the form of rosettes and having microcavities with a higher height in the center.

5 - Solar radiation absorber for a concentrated solar thermal power plant according to claim 1, characterized in that it has a sealing disc and fluid passage ports.

6 - Thermal collector for a concentrated solar thermal power plant according to claim 1, characterized in that it has a central helical frustoconical section with a 900 return and a flared and conical shape.

7 - Thermal collector for a concentrated solar thermal power plant according to claim 1, characterized in that it comprises a burner disposed inside said cavity, directing a flame in the direction of said absorber.

8 - Thermal collector for a concentrated solar thermal power plant according to claim 1, characterized in that it comprises exchange surfaces with microcavities.

9 - System consisting of a thermal collector for the concentrated solar thermal power plant according to claim 1, thermally and mechanically coupled to a tubing (hot fluid outlet) or to the inlet of a thermal machine, characterized in that said collector is formed by a graphite cavity with a transparent inlet window in which is disposed an absorber formed by a monolithic piece of silicon carbide, the absorption surface of which is coated with tungsten dendrites.

10 - System for a concentrated solar thermal power plant according to the preceding claim, characterized in that said expansion machine with upper part made of silicon carbide.

11 - Method for preparing an absorber according to claim 1, characterized in that it comprises a step of plasma spraying tungsten dendrites onto the surface of a monolithic part made of silicon carbide.

12 - Method for preparing an absorber according to the preceding claim, characterized in that it comprises a step of deposition by powdering during the production in a pasty phase of tungsten dendrites on the surface of a monolithic silicon carbide piece.

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