WO2013164538A1

WO2013164538A1 - Solar collector with a wall made from a reflective material

Info

Publication number: WO2013164538A1
Application number: PCT/FR2013/050949
Authority: WO
Inventors: Audrey Durand; Nathalie ELKHIATI; Paul Sargood; Catherine Jacquiod
Original assignee: Saint-Gobain Quartz S.A.S
Priority date: 2012-05-04
Filing date: 2013-04-29
Publication date: 2013-11-07
Also published as: FR2990265A1; FR2990265B1

Abstract

The invention concerns a device for collecting solar radiation comprising a receiver of solar radiation and a wall comprising a surface positioned facing the receiver, the surface being made from a reflective ceramic material comprising particles having a diameter of between 100nm and 100µm or pores having a diameter of between 100nm and 100µm. The reflective ceramic material makes it possible to recover heat not directly received by the receiver or escaping from the receiver, in order to send it back to said receiver.

Description

SOLAR COLLECTOR WITH WALL IN REFLECTIVE MATERIAL

The invention relates to the field of solar concentrators by which the solar light energy is converted into heat and generally into electricity. The invention relates more particularly to the use of a ceramic material to send back to the solar concentrator receivers direct or reflected solar radiation or infrared emitted by the receiver itself. In the context of the present application, this material may be called "reflective ceramic material".

Solar concentrators are based on the principle of collecting light rays over an enlarged surface, generally of the mirror type, to return them concentrated to a point collector or in line. The collection and return of sunlight is usually achieved by flat or curved mirrors according to the principle of the concentrator. Thus, Fresnel mirrors are known, a set of plane mirrors arranged so as to approximate by their juxtaposition a parabolic form favorable to the return to a line or a focal point. Solarite heliostats mirrors reflecting solar light at the top of a solar tower are also known. In all these systems, the collected light is collected by a heat collector, also called receiver, whose interior can be traversed by a heat-transfer material, for example of the heat transfer fluid type. Depending on the case, the heat transfer fluid may be liquid water or steam, a molten salt, a synthetic oil, or a fluidized bed powder or granules in a fluidized bed. In some solar radiation collectors, the receiver is disposed in an enclosure comprising an opening for the passage of incident concentrated solar radiation. The placement of the receiver in an enclosure (that is to say in a cavity) allows to confine the heat received and minimize losses. Receivers of solar radiation must hold at high temperatures and are usually ceramic like silicon carbide.

WO201 1 135501 teaches a device for storing and transporting the thermal energy of a power generation system, said device receiving solar radiation and being based on the use of a fluidized bed of granules or particles acting as heat transfer fluid and an associated heat exchanger.

WO201 1/045301 teaches a solar energy receiving device comprising a cavity closed by a quartz porthole. The porthole serves to keep the heat in the cavity. Such porthole quartz is expensive and limited in size because of manufacturing difficulties.

Until now, little attention has been paid to the material in the immediate environment of the recipient. Thus, the material commonly used around recipient tubes is aluminum. The use of aluminum is limited in temperature and moreover, it is a material that deforms under the effect of heat. According to the invention, a ceramic material, especially silica fiber, particularly reflective and insulating, is placed near the receiver in order to return a maximum of radiation. Generally, the ceramic material used according to the invention mainly provides a diffuse reflectivity. Depending on the case, it can provide a diffuse reflectivity combined with a specular component. This is particularly the case for example if the material is silica and if one carries out a glaze (involving a superficial melting) controlled silica on the surface of the material.

The invention relates to a solar radiation collector device comprising a solar radiation receiver, said collector comprising a wall comprising a surface facing the receiver, said surface being made of a reflective ceramic material. There are indeed walls at least behind the receiver with respect to solar radiation impacting the receiver, said wall acting as a building element contributing to the fixing of the receiver or thermal insulation element. The receiver may be a set of several recipient elements juxtaposed or not. A receiver may in particular comprise a tube of any shape, for example of circular or square or rectangular section and whose interior is traversed by a heat transfer fluid. Thus, the receiver may be a set of a plurality of such tubes. The tubes may for example be arranged substantially parallel to each other within a cavity of a solar radiation collector device. If the receiver equipping the collecting device comprises elements (such as tubes) imperfectly juxtaposed due to construction constraints, part of the incident solar radiation passes by these elements and directly impacts the partition behind them. This is a significant loss of which we have so far not been concerned. According to the invention, it is proposed to recover this incident solar radiation directly impacting the partition located behind the receiver by sending it efficiently to the receiver at least partially by reflection. If the elements of the receiver are juxtaposed (or if the receiver is in a single room), then the solar radiation can not impact the wall behind the elements of the receiver by passing between them. However, as the receiver is very hot, it emits heat, especially in the infrared field, and the wall behind it is then used to send back its own radiation by reflection. The recipient may be a plate. This plate can be hollow, the heat transfer fluid can circulate inside. According to the invention the solar radiation collector device may comprise a receiver comprising at least two elements (such as two tubes) receiving solar radiation not completely juxtaposed, the reflective ceramic material being placed in the path of solar radiation passing between the two elements.

The receiver may be metal, especially steel, or ceramic, especially silicon carbide. In particular, it may be a metal plate or ceramic tubes. The receiver may comprise a tube or a hollow plate traversed by a heat transfer fluid.

It was found that the receivers were subjected to very high thermal gradients because solar radiation only hit them on one side. The placement according to the invention of a reflective ceramic material opposite the receiver, in particular behind and on the sides thereof, provides homogeneity in the temperature of the receiver by decreasing the thermal gradients and the phenomena derived from them such as thermal stresses. and mechanical that can lead to breakage or deformations.

According to the invention, the reflective ceramic material is placed behind or next to the receiver while minimizing the direct impact of solar radiation on the receiver. The reflective ceramic material is therefore normally not in the path of incident solar rays that can directly impact the receiver. This is for the reflective ceramic material to recover the maximum heat not received directly by the recipient or escaping from the receiver to return it to it. To be effective, if the Reflector ceramic material receives directly solar radiation, it must be as reflective as possible in all wavelengths of solar radiation, that is to say between 0.25 μιτι and 2.5 μιτι. In particular, the ceramic material reflects at least 60% and preferably at least 70% and more preferably at least 85% and even more preferably at least 90% of the incident radiation in the wavelength range between 0.5 and 2.5μηη. This reflectivity can be measured on the basis of a spectrometer, in a manner known to those skilled in the art. In any case, whether the reflective ceramic material receives solar radiation or not, it should send back to the receiver the receiver's own heat, which requires good reflection properties at least in the field of IR, of 2, 5 μιτι to 10 μιτι. In some cases, walls form a true cavity, which is provided with an opening for passing the incident solar radiation, the receiver being placed inside and at the bottom of said cavity. In this case, the reflective ceramic material is advantageously placed not only behind the receiver with respect to the incident solar radiation but also on the sides of the receiver without, of course, being in the path of the incident solar radiation. In other words, the reflective ceramic material is advantageously placed anywhere on the surface of the interior of the cavity containing the recipient. Thus, the receiver is inside a cavity and the surface of the wall of the reflective ceramic material opposite the receiver is the inner face of the cavity. It can be considered that the opening of the cavity generally comprises an axis passing in its center (the center of gravity of the surface of the opening) and perpendicular to the opening itself, said axis being preferably directed towards the sun. In particular, the ceramic material is also advantageously placed at the bottom of the cavity behind the receiver and in the axis of the opening. The expression "opposite" used to say that the material is opposite the receiver means that it comprises a surface capable of sending radiation directly (by reflection or emission) to the receiver without encountering a solid obstacle. This surface of the wall can therefore have very different orientations. It is enough that it is possible to find a point of the surface of the wall and a point of the surface of the receiver between which there is no solid obstacle. Generally, the reflective ceramic material is at a distance of between 1 cm and a few meters from the receiver, for example 5 m from the receiver. This means that the points closest to the receiver and the reflective ceramic material are generally spaced from each other from 1 cm to 5 m without the presence of other solid between them.

In the present application, the incident solar radiation is called solar radiation intended to directly impact the receiver of the solar radiation collector device, it being understood that this solar radiation has generally already been reflected at least once by a mirror.

The reflective ceramic material is a ceramic material comprising particles or pores with a diameter of between 100 nm and 1 μm. The term "pores" denotes any cavity or bubble in the material and especially any open or closed pore. Advantageously, the ceramic material comprises both particles with a diameter of between 100 nm and Ι μm and pores with a diameter of between 100 nm and Ι μm. The existence in the reflective ceramic material of particles having a diameter of between 100 nm and Ι ΟΌμιτι is preferred with respect to the existence of pores with a diameter of between 100 nm and Ι ΟΌμιτι.

The diameter of a particle is the diameter of the smallest circle through which the particle could pass entirely. The diameter of a pore is the diameter of the largest sphere that can be contained entirely in the pore. The particles are not independent but are bonded to the material either through a material of different composition or by sintering. These particles may be grains or fibers and are visible by optical microscope or scanning electron observation, if appropriate after acid etching revealing the grain boundaries. Thus, the particles can have very diverse shapes with high form factors. In particular, in the case of a particle in the form of circular section fiber, the diameter of the particle corresponds to the diameter of the fiber in its circular section.

Advantageously, at least 80% of the mass of the reflective ceramic material consists of particles whose diameter is between 100 nm and Ι ΟΌμιτι. Preferably, the reflective ceramic material is a ceramic material of which at least 50% of the mass consists of particles whose diameter is between 100 nm and 30 μm.

A ceramic is an essentially inorganic and non-metallic material. It is a crystallized or vitreous material, and formed by a thermal process (melting then cooling in the case of vitreous, sintering in the case dispersed materials (powder, fiber)). It can comprise (or be completely made up of) an oxide, a carbide, a nitride. As oxide, there is an oxide of aluminum, zirconium, titanium, silicon or zinc. The ceramic may comprise (or consist entirely of) a mixture of at least two of these oxides, especially silica and alumina. It can therefore especially be quartz, an aluminosilicate, mullite.

By nature, the reflective ceramic material holds very well the temperatures encountered in solar collectors operating, that is to say generally up to 800 ° C can even reach 1000 ° C. This is a significant advantage of the use according to the invention of a ceramic material with respect to metal reflectors which do not maintain a temperature above 500 ° C. Indeed, metal materials deform, oxidize and tarnish, resulting in a significant drop in reflectivity properties. Thanks to the invention, the temperatures accessible in the recipients are increased.

Preferably, the reflective ceramic material is porous so as to have a thermal insulator character. In particular, the total porosity (open + closed) may be between 10 and 90% by volume and preferably between 40 and 90% by volume. The porosity can be determined by measuring the difference between the theoretical density of the dense material constituting the material and the geometric density of the material. Advantageously, at least 80% by volume of the total porosity has a diameter of between 100 nm and 100 μm.

Advantageously, the reflective ceramic material is intrinsically transparent in the range 400 nm to 2.5 μm. Transparency means that the internal transmittance of the material is greater than 95% per cm and preferably greater than 99% per cm in the wavelength range from 400 nm to 2.5 μιτι. This internal transmittance may in particular be measured using a Perkin Elmer spectrophotometer on polished samples. These are completely dense samples made specifically to characterize an intrinsic property of the material.

Advantageously, the reflective ceramic material comprises silica, which is more preferably in the form of fiber. Pure silica is a material that is very transparent to light. However, it was found that a material comprising several thicknesses of silica particles (fibers or grains) was a excellent reflector of light in a broad spectrum of light and IR, that is to say IR to UV, which is paradoxical a priori given the high transparency of silica. It is probable that it is the multiple reflections inside the particles, moreover, of sufficient thickness and which do not absorb the light, and also thanks to a suitable refractive index, which are at the origin of this phenomenon. Preferably, the particles are arranged in the material in a manner that is not totally compact, which is at the origin of a certain porosity conferring on the material good insulating properties. Preferably, the reflective ceramic material comprises silica fiber. Particles silica in the material, especially in the form of fiber therefore has a dual function: excellent reflectivity despite the transparency of the silica, and good insulation properties. This combination of properties makes it a material of choice for the interior lining of solar radiation collectors.

The silica material at the surface of the wall according to the invention preferably contains more than 90% and preferably more than 99% and more preferably more than 99.9% by weight of SiO 2. Thus, this material may consist essentially of silica. This purity in S1O2 is at the origin of the good thermal behavior of the material and its excellent reflectivity.

The reflective ceramic material at the surface of the wall according to the principle of the invention preferably comprises more than 50% of silica fibers, preferably more than 50% by weight of pure silica fiber (more than 99.9% of SiO 2 ). In addition to the silica fiber, it may comprise non-fiber-like silica such as grains or other forms, in particular capable of forming bridges between fibers. In particular, the reflective ceramic material may comprise sintered silica fiber and silica powder (fiber and powder being sintered together). Thus, the invention also relates to a solar radiation collector device comprising a solar radiation receiver and a wall comprising a surface placed opposite the receiver, said surface being made of a ceramic material comprising at least 50% by weight of silica fiber having a diameter between 5 and 30 μιτι.

The reflective ceramic material is preferably present at a thickness of at least 1 mm and more preferably at least 2 mm and more preferably at least 4 mm from the outer surface of the wall opposite the receiver. In the case where the material is fibrous, it is this thickness which guarantees the sufficient multiplicity of fibers in the path of the light rays and makes it possible to return them sufficiently.

The reflective ceramic material may be obtained by sintering fibers (in particular silica), if appropriate mixed with powder (in particular silica). In the case of the presence of a silica fiber in the reflective ceramic material, the diameter of the fiber is preferably between 5 and 30 μιτι. In the case of the presence of a silica fiber in the reflective ceramic material, its length is preferably between 100 and 1500 μηη.

To produce a ceramic reflective material made of silica and comprising silica fiber and silica powder, a slip of a mixture of silica powder and silica fiber can be produced. The silica powder reduces the friable nature of the final material. The silica used was of purity greater than 99.9% by weight. The internal transmittance of this material is greater than 99% per cm. The slip is shaped in a mold, then, after sedimentation, is demolded and sintered between 1300 and 1400 ° C. A porous silica block, rich in silica fibers, is thus obtained. To achieve this, one can mix silica fiber and silica powder, the silica powder may be present in a proportion of 0 to 50% by weight of the mixture, in particular 10 to 40% by weight of the mixture. The density of this material is preferably between 0.2 and 1.5 g / cm ³ and preferably between 0.3 and 0.8 g / cm ³ . This material is easily machinable, for example by a diamond band saw, so that building elements can be easily cut. This material appears matte (not glossy) to the naked eye and its reflectivity is essentially diffuse. In this material, the fibers generally have a random orientation.

The reflective ceramic material (especially silica) may also be a fiber textile (in particular silica). In this case, it does not usually contain powder. The textile can be prepared by weaving or knitting (2 or 3 dimensions) or braiding of sized threads. The textile structure gives the material an ease of shaping. Once shaped, the size is removed by pyrolysis, for example from the first use in a solar concentrator. Such a material reflects light with a diffuse component and a specular component. The reflective ceramic material (especially silica) may also be in a felt of long cut fibers, generally at a length between 1 and 10 cm, and needled.

Preferably, the reflective ceramic material appears white to the naked eye. FIG. 1 represents the reflectivity of a part of a sintered silica material

(99.9% SiO 2) produced by sintering a mixture of 70% by weight of silica fiber and 30% by weight of silica powder and secondly that of a conventional refractory material of the alumino type -Silicate comprising 57% of S1O2 and 43% of Al2O3. These two materials have high reflectivities, the silica material being much higher up to 5 μιτι, the performance of the two materials being substantially equivalent beyond. The reflectivity of these samples was determined using two spectrometers to cover the entire wavelength range of measurement.

A spectrometer with a built-in sphere enabled measurements from 250 nm to 2.5 μηη (PERKIN ELMER Lambda 900 model). For measurements in the infrared beyond 2.5μηη a Fourier transform spectrometer has been used (THERMO-NICOLET 6700). This device is equipped with an integration sphere that allows diffuse reflectivity measurements up to 25μηη.

FIG. 2 represents a scanning electron microscope photo of a sintered silica material made by sintering a mixture of 70% by weight of silica fiber and 30% by weight of silica powder, said material having given rise to to the establishment of the reflectivity curve of Figure 1. This product has a total porosity of 70% by volume. This porosity is totally open.

FIG. 3 represents a collector device for solar radiation 30 in the form of an enclosure comprising a cavity 31 inside which a receiver is arranged in the form of recipient tubes 32. Inside these receiver tubes circulates the coolant . The collecting device comprises an opening 33 through which incident solar radiation from mirrors can penetrate. The enclosure comprises two materials, one 36 being concrete and the other 37, disposed over the entire inner surface of the enclosure is 100% silica and was made by sintering 70% by weight of silica fiber (fiber diameter: 9 and 14 μιτι), and 30% by weight of silica powder, according to the invention. This ceramic material is present on a thickness of 1 cm from the external surface of the wall opposite the receivers. The enclosure comprises a rear wall 38 located behind the receivers with respect to incident solar rays, and two side walls 39 and 40. All these walls are placed facing the receiver within the meaning of the invention since they can send radiation directly. to the receiver, either reflected solar radiation or emitted IR radiation. It will be noted that certain solar rays 34 directly impact the receivers 32 while the solar rays 35 pass between the receivers and impact the wall 38 located behind the receivers.

FIG. 4 represents a device 51 that collects solar radiation

53 in the form of an enclosure comprising a cavity 41 inside which is disposed a hollow metal plate 42 acting as a receiver. A heat transfer fluid circulates inside the plate 52. The plate constitutes a recipient tube. The collection device comprises an opening 43 through which incident solar radiation 53 from mirrors can penetrate. The enclosure comprises two materials, one 46 being a concrete and the other 47, disposed over the entire inner surface of the enclosure is 100% silica and was made by sintering 70% by weight of fiber. silica (fiber diameter: 9 and 14 μιτι), and 30% by weight of silica powder, according to the invention. This silica fiber ceramic material is present in a thickness of 1 cm from the outer surface of the wall facing the plate tray. The enclosure comprises a rear wall 48 located behind the plate relative to incident solar rays, and two side walls 49 and 50. All these walls are placed facing the receiver within the meaning of the invention since they can directly send radiation to the receivers, either reflected solar radiation or IR radiation. The wall 48 essentially returns infrared radiation to the receiver plate since the latter shields the solar rays vis-à-vis this wall. The axis of the opening coincides with the direction of the solar rays 53 and passes through the surface of the rear wall 48.

Claims

1. Solar radiation collecting device comprising a solar radiation receiver and a wall comprising a surface placed opposite the receiver, characterized in that the surface is made of a ceramic material comprising particles with a diameter of between 100 nm and ΙΙμιτι or pores of diameter between 100nm and Ι ΟΌμιτι.

2. Device according to the preceding claim characterized in that at least 80% of the mass of the ceramic material consists of particles whose diameter is between 100nm and Ι ΟΟμιτι.

3. Device according to one of the preceding claims, characterized in that the material is heat insulating and comprises a porosity of between 10 and 90% by volume and preferably between 40 and 90% by volume.

4. Device according to one of the preceding claims, characterized in that the material comprises silica.

5. Device according to the preceding claim, characterized in that the material comprises more than 90% and preferably more than 99% by weight and more preferably more than 99.9% by weight of S1O2.

6. Device according to one of the preceding claims, characterized in that the material comprises silica fiber.

7. Device according to the preceding claim, characterized in that the material comprises at least 50% by weight of silica fiber.

8. Device according to the preceding claim, characterized in that the material comprises silica fiber and sintered silica powder.

9. Device according to one of claims 6 to 8, characterized in that the silica fiber has a diameter between 5 and 30 μιτι.

10. Device according to one of claims 6 to 9, characterized in that the silica fiber has a length between 100 and 1500 μιτι.

1 1. Device according to one of the preceding claims, characterized in that at least 50% of the mass of the ceramic material consists of particles whose diameter is between 100 nm and 30 μm.

12. Device according to one of the preceding claims, characterized in that the ceramic material reflects at least 60% and preferably at least 70% and more preferably at least 85% and so still more preferred is at least 90% of the radiation in the wavelength range between 0.5 and 2.5μηη.

13. Device according to one of the preceding claims, characterized in that the ceramic material has an internal transmittance greater than 95% per cm and preferably greater than 99% per cm in the wavelength range from 400 nm to 2.5 μιτι.

14. Device according to one of the preceding claims, characterized in that the recipient is inside a cavity and in that the surface of the wall facing the receiver is the inner face of the cavity.

15. Device according to one of the preceding claims, characterized in that the recipient is inside a cavity, the surface of the wall facing the recipient being in the internal face of the cavity, the cavity comprising an opening of which the axis passes through said surface.

16. Device according to one of the preceding claims, characterized in that the ceramic material is present at a thickness of at least 1 mm and preferably at least 2 mm and more preferably at least 4 mm at count the outer surface of the wall facing the receiver.

17. Device according to one of the preceding claims, characterized in that the receiver comprises a tube or a hollow plate traversed by a heat transfer fluid.

18. Device according to one of the preceding claims, characterized in that the receiver comprises at least two elements not completely juxtaposed, the ceramic material being placed in the path of solar radiation passing between the two elements.

19. A solar radiation collecting device comprising a solar radiation receiver and a wall comprising a surface facing the receiver, characterized in that the surface is made of a ceramic material comprising at least 50% by weight of silica fiber having a diameter. between 5 and 30 μιτι.