• 专利附录
  • 专利说明
  • 权利要求
  • 类似技术
  • 同族专利
  • 引用文献
  • 法律状态
  • 优先权
    • 专利摘要:
    Selective solar structure with self-cleaning resistant to high temperatures. The present invention is directed to a structure formed by an upper section comprising an upper layer comprising doped2 tio having a high transmittance in the visible spectrum and a high reflectance in the ir region and properties of self-cleaning, an absorbent intermediate section and a substrate. Due to the aforementioned properties and resistance to high temperatures, the structure is useful as a selective solar structure for tower receivers in concentrated solar power systems (csp). (Machine-translation by Google Translate, not legally binding)
    公开号:ES2575746A1
    申请号:ES201431972
    申请日:2014-12-31
    公开日:2016-06-30
    发明作者:María Elena GUILLÉN RODRÍGUEZ；Ramón Escobar Galindo；Irene HERAS PÉREZ；José Luis Endrino Armenteros；Azucena BELLO；Noelia Martínez Sanz；Frank LUNGWITZ；Marcel NEUBERT
    申请人:Helmholtz Zentrum Dresden Rossendorf eV；Abengoa Research SL；
    IPC主号:
    专利说明:

    The present invention relates to a high temperature solar selective structure, for example tower receivers in solar energy systems by concentration (CSP). The selective solar structure maintains a high absorptivity in the solar region and a low emittance in the infrared spectrum. In addition, the selective solar structure resists high temperatures and is stable in air. STATE OF THE TECHNIQUE
    In a solar power plant by concentration with a central receiver, a set of mirrors concentrates sunlight into / on a receiver placed in / on the top of a tower located at the focal point of the mirrors. The receiver tubes are coated with a selective solar coating and filled with a heat transfer fluid. The selective solar coating must be highly absorbent in the solar region and must have a low thermal emittance at elevated temperatures.
    The coatings commonly used as selective solar coatings for high temperature applications are based on cermets, which are highly absorbent in the visible region and transparent to the infrared region, deposited on a highly reflective substrate that prevents loss of emittance by the transfer fluid .
    Selective absorbent surface coatings can be classified into six different types: a) intrinsic, b) tandem semiconductor / metal, c) multilayer absorbers, d) multidielder composite coatings, e) textured surfaces and f) selective solar transmission coatings on a black body type absorber.
    In relation to the last type of coatings mentioned there are several works in the literature, such as CE Kennedy, "Review of Mid-to High-Temperature Solar Selective Absorber Materials" July 2002, which describes a material based on a highly absorbent coated film by a transparent conductive oxide (OCT) and deposited on a substrate: SnO2: F or SnO2: Sb or In2SO3: Sn or ZnO: Al / black enamel / substrate.


    Selvakumar, N .; Barshilia, H. C. Review of Physical Vapor Deposited (PVD) Spectrally Selective Coatings for Mid-and High-Temperature Solar Thermal Applications. Sol. Energy Mater. Sol. Cells 2012, 98, 1-23, is an example of films based on a black body type (CN) absorber coated by transparent conductive oxides such as In2O3 / Si; G. E. Carver, S. Karbal, A., Donnadieu, Chaoui, A .; Manifacier, J. C. Tin oxide-black molybdenum photothermal solar energy. Mat. Res. Bull. 1982, 17, 527-532, use SnO2 / black molybdenum to provide a coating with selective wavelength properties.
    All references cited above are based on a highly absorbent material coated by a selective absorber that reflects in the IR region and transmits in the visible region. However, none of the coatings mentioned in the state of the art show self-cleaning properties.
    In addition, the state of the art OCT layers are deposited on a CN substrate (where CN means "black body"). This substrate contributes to the optical properties of the material. Therefore, the coating always consists of a single layer.
    The amount of energy provided by the solar system is reduced by dirt from the coating on the surface of the tube. If the coating gets dirty, the incident light that reaches the absorbent material is drastically reduced. The loss of transmission of solar radiation can reach 50% when the dust concentration is 1 mg / cm2 (Mazumderl, M .; Horenstein, M .; Stark, J .; Girouard, P .; Summerl, R. Characterization of Electrodynamic Screen Performance for Dust Removal from Solar Panels and Solar Hydrogen Generators. 2011, 1-8). In addition, surface roughness can reduce transparency due to dispersion losses. Self-cleaning properties can be advantageous to prevent the accumulation of dirt on the tubes and to increase the durability of the coating in the long term. A self-cleaning coating is of special interest for outdoor applications, such as CSP towers, since the self-cleaning properties favor long-term stability, reduction of maintenance work, cost reduction and optimal system performance over the life of the coating, which translates into greater plant efficiency. The reduction of maintenance work is especially interesting in relation to the tower receiver due to the difficult access to it to clean the surface of the coating.


    Self-cleaning coatings for heliostats have been proposed in high-temperature concentrated solar energy systems [Low-Cost Self-Cleaning Reflector Coatings for CSP Collectors. SunShot CSP Program Review. April 23-25, 2013]. However, the possibility of using a self-cleaning coating for a tower receiver has not yet been studied. None of the existing coatings for solar energy systems by high temperature concentration show self-cleaning properties.
    The most commonly used receptor coating is 2500 Pyromark®. Attempts have been made on several occasions to create a selective high temperature solar coating, but most of the coatings showed significant deterioration when exposed to air and at high temperatures.
    Patent application US3968786A describes a tube formed of a plastic material in which a black body material (CN) is distributed throughout the thickness of the tube and an optional coating layer that provides a selective absorbent surface. This coated tube is not stable in applications where the temperature is as high as in tower receivers nor is it composed of multiple layers with controlled selective optical characteristics.
    In patent application EP2243750A1 a glass or glass ceramic disc is described which comprises an infrared reflective layer and is used as a display panel for high temperature applications. This invention does not describe any black body layer, it only comprises an infrared reflective layer. The coating does not achieve selective optical characteristics of absorptivity and emittance to absorb in the solar region and emit in the infrared spectrum, which is required for receiver applications.
    In patent application WO2013044975 A1 a glass tube with a reflective coating of infrared light on the upper surface and a solar energy absorbing coating inside the tube is described. The reflective IR and absorbent layers are deposited on two different surfaces, so that this patent application describes two coatings. In addition, this coating configuration is only suitable for applications of cylindrical-parabolic concentrators.
    Patent application US20070134501 A1 describes a coating comprising


    nanocrystals of a photoactive material that provides self-cleaning properties to the coated surface. That coating is applied on a transparent substrate without absorbent layer. The coating of that patent would not be suitable for tower receivers.
    Therefore, there is a demand for a structure with both selective solar and self-cleaning and high temperature resistant optical properties that can be used for applications that require such properties, such as solar tower receivers. DESCRIPTION OF THE INVENTION
    The present invention provides a solar selective structure that has self-cleaning properties and that can be used, for example, for high temperature outdoor applications, for tower receivers in solar energy systems by concentration (CSP).
    The selective solar structure is based on a highly absorbent material, resistant to high temperatures, covered by a thin layer that meets the following characteristics: self-cleaning capacity, high transmission in the visible region, high reflectance in the IR region and high temperature resistance . The structure is stable at temperatures above 500 ° C, its absorbency is in the range of 0.80 to 0.99 and has an emittance of less than 0.6.
    Thus, according to a first aspect, the present invention is directed to a structure that
    understands:
    a) an upper section comprising an upper layer comprising TiO2
    doped with an element selected from Sb, In, B, F, Sn, Nb, N, Ru or Ta,
    b) an intermediate section comprising a layer of a material that absorbs
    in the wavelength range of 300 to 2500 nm and
    c) a substrate selected from metallic or ceramic materials, in which the intermediate section is disposed between the substrate and the upper section.
    The upper section may consist of a single layer (i.e., the upper layer) or a multilayer stack comprising the upper layer.
    The upper layer of the upper section is resistant to high temperatures, reflected in the


    IR region, transmits in the visible region and has self-cleaning properties.
    Due to the presence of TiO2, the upper layer does not absorb in the visible region and has photocatalytic activity. In addition, when doped with metals, it has IR reflective properties.
    The high bandwidth semiconductor TiO2 prohibited has a melting temperature of 1855 ° C and shows self-cleaning properties due to two different phenomena: photocatalysis and photoinduced super hydrophilicity. Photoexcited titanium dioxide has a strong oxidation and reduction power that can cause degradation of surface contaminants. Another self-cleaning mechanism other than photocatalytic decomposition of organic components consists of the photo-induced hydrophilic conversion of TiO2, which creates very small contact angles on the surface of TiO2.
    In a preferred embodiment, the structure of the invention also comprises a layer of an IR reflective material deposited between the layer (b) and the substrate (c). This additional layer produces a further reduction in the emissivity of the structure of the invention. In a more preferred embodiment, the IR reflective material comprises Al, Cu, Ag or Au. In another more preferred embodiment, this material comprises a nitride such as, but not limited to, TiN, ZrN or CrN. The thickness of this IR reflective material is preferably between 50 nm and 500 nm, more preferably between 100 nm and 250 nm.
    In a preferred embodiment, the upper section (a) is a single layer comprising TiO2 doped with an element selected from Sb, In, B, F, Sn, Nb, N, Ru or Ta, preferably Ta.
    Preferably, the concentration of the dopant is equal to or less than 10% at., Where "% at." is the atomic percentage (percentage of an atom in relation to the total number of atoms). More preferably, the concentration of the dopant is between 1 and 3% at ..
    In a preferred embodiment, the upper section (a) is a single layer of TiO2 doped with Ta.
    This layer can be dense or porous, with a porosity preferably equal to or less than


    70% v / v (volume fraction), more preferably between 10 and 50% v / v. The optical constants of the layer can be adjusted by changing the porosity. The transmittance in the visible region can be increased by increasing the porosity, since the optical properties of a porous layer are the combination of the optical constants of the material and the air found in the porous layer.
    Preferably, the thickness of the single layer (a) is less than 1 µm, preferably 10 nm to 500 nm.
    In another preferred embodiment, the upper section (a) is a multilayer stack comprising at least one layer of doped TiO2 with an element selected from Sb, In, B, F, Sn, Nb, N, Ru or Ta, preferably Ta, and a layer of a secondary transparent conductive oxide (OCT) with a different refractive index. These two types of layers are deposited alternately, and the top layer must be one based on TiO2 to provide self-cleaning properties. OCTs are conductive materials that show selective transmittance. They have a low absorption coefficient in the near-visible UV region and are reflective in the IR region. Its optical properties depend on the mobility of electrons, the prohibited band and the density of charge carriers. They do not absorb in the visible region since they are semiconductor oxides of large bandwidth prohibited. The reflective IR properties of an OCT are related to its electrical properties (mobility of the load carriers and density of the charge carriers). The OCT can be binary or ternary compounds,
    or they can also constitute a multicomponent. Non-limiting examples of semiconductor oxides of the secondary OCT for the present invention are SnO2, In2SnO3, SiOxNy, SiOxCy, ZnO. Preferably, the semiconductor oxides of the secondary OCT are selected from SnO2, ZnO or In2O3, and more preferably SnO2.
    These semiconductor oxides of the secondary OCTs are doped and the possible dopants include, but are not limited to Ta, N, Nb, Sb, In, B, F, Ru and Sn. In a more preferred embodiment, the secondary OCTs are doped with F or Ta.
    For this multi-layer stacking embodiment, the doped TiO2 layers can also be dense or porous, with a porosity equal to or less than 70% v / v (volume fraction), preferably 10 to 50% v / v.


    Preferably, the concentration of the dopant is equal to or less than 10% at., Where "% at." is the atomic percentage (percentage of an atom in relation to the total number of atoms). More preferably, the concentration of the dopant is between 1% at. and 3% at ..
    Preferably, the thickness of the doped TiO2 layer is between 5 nm and 500 nm, preferably between 10 nm and 100 nm and more preferably at 10 nm. The thickness of the secondary OCT layer is between 5 nm and 500 nm, preferably between 10 nm and 100 nm and more preferably at 30 nm. The number of separate layers is between 2 and 60, preferably between 2 and 30, more preferably between 3 and 15 and more preferably between 3 and 10. The thickness of the multilayer stack ranges between 10 nm and 2 µm, preferably between 20 nm and 500 nm.
    In another preferred embodiment, the upper section (a) is a multilayer stack as described above, comprising at least one layer of TiO2 doped with Ta and a layer of a SnO2 doped with Ta. The multilayer stack may also comprise at least one layer of TiO2 doped with Ta and / or at least one layer of SnO2 doped with Ta. TiO2 shows properties of OCT when doped with metals such as tantalum. The density of charge carriers of TiO2 doped with Ta is high, but its electron mobility is lower than that of other OCT materials, which will affect its transmittance and its reflective IR properties. SnO2 is also a semiconductor material with a high bandwidth prohibited with a high melting point (1630ºC). SnO2 doped with Ta shows high mobility, but this OCT has a poor dopant activation. This limitation could be resolved by increasing the concentration of the dopant (Ta), but the Ta sites would also act as dispersal centers by reducing the transmittance in the visible region.
    Both semiconductor oxides (TiO2 and SnO2) have similar thermal expansion coefficients, which favors the formation of multilayer stacking. In addition, the use of TiO2 as a seed layer for the growth of SnO2 promotes epitaxial growth of the latter, which improves its electronic properties. Steam phase deposition procedures are required by physical method to obtain the desired properties of multilayer stacking. These high energy procedures enable epitaxial growth of the SnO2 layers using TiO2 as a seed. In addition, good adhesion between the layers and homogeneity are obtained.


    This multilayer stack based on two OCT materials combines properties of both materials, where the doped TiO2 acts as an electron source and the doped SnO2 as an electron carrier, presenting an epitaxial growth of the SnO2 over the TiO2 that favors stacking performance. multi-layered in several aspects: it helps reduce tensions during deposition so that one layer grows over the other without delamination, as if they were the same material. This good connection between the layers favors a good conductivity between the layers, which is critical for a good performance of the resulting multi-layer stacking.
    In a preferred embodiment, the total number of individual layers of both oxides is between 3 and 60, preferably between 3 and 30 and more preferably between 3 and 15 and even more preferably between 3 and 10. The number of individual layers of TiO2 doped with Ta amounts to at least 2 and the number of individual layers of SnO2 doped with Ta amounts to at least 1 to obtain the desired epitaxial growth of SnO2 doped with Ta over TiO2 doped with Ta.
    The multilayer stacking shows self-cleaning properties, is resistant to high temperatures, has a high transmittance in the visible region and a high reflectivity in the IR region. The different refractive index of the OCT materials, the number and thickness of the layers and the electrical properties of the resulting OCT influence this selective behavior. Therefore, the authors obtain a double optimization of the optical properties.
    The intermediate section is based on a material that absorbs strongly in the wavelength range from 300 nm to 2500 nm and is resistant to high temperatures. In a preferred embodiment, the black body materials suitable for this layer are, for example, but not limited to, carbon materials such as nanotubes, silicon, Pyromark® and other commercial paints, black enamel, black molybdenum, cermets, layers projected by plasma, etc. The present configuration allows to take advantage of the good absorption properties of a black body absorber without the problem associated with its high emissivity.
    Preferably, the thickness of this intermediate section is in the range of 100 nm to 1000 µm.


    The thermal emission of the intermediate section is braked thanks to the presence of the highly reflective IR top layer. The use of an IR reflective top layer is advantageous twice: it avoids large thermal losses at high temperatures (the emittance of absorbent materials such as cermets increases at elevated temperatures) and gives the possibility of using CN absorbent materials instead of absorbers selective.
    In a preferred embodiment, the intermediate section is deposited on a metal substrate such as steel, stainless steel, copper, aluminum, Inconel® or other nickel alloy or metal alloys, ceramic materials or any other material that forms the receiver tube.
    The resulting structure is thermostable at more than 400 ° C, more preferably the structure is stable between 400 ° C and 1000 ° C and more preferably the resulting structure is stable between 500 ° C and 700 ° C. The stability of the resulting structure makes it suitable for high temperature applications.
    Another aspect of the present invention consists in the use of the structure described above for tower receivers in solar energy systems by concentration. The structure is especially applied to tower receivers in which the working temperatures are equal to or greater than 500 ° C. However, this selective solar structure could also be used in medium and low temperature solar applications.
    Thus, according to another aspect of the invention, the structure of the invention is used in thermal receivers, for example for a solar energy system. According to this aspect, the tower receiver itself is the substrate of the structure of the invention.
    The upper and intermediate sections of the structure described above can be deposited by vapor deposition by physical method, such as, without limitation, cathodic magnetron spray, vacuum cathode arc, ion beam cathodic spray, beam assisted assisted deposition ions and high power pulsed sputtering by magnetron.


    Unless otherwise indicated, all technical and scientific terms used herein have the meaning normally understood by the person skilled in the art to which this invention pertains. In the practice of the present invention methods and materials similar or equivalent to those described herein may be used. In the description and the claims, the word "understand" and its variants are not intended to exclude other technical characteristics, additives, components or steps. Additional objects, advantages and features of the invention will be apparent to those skilled in the art after examining the description or can be learned by practicing the invention. The following examples and drawings are provided by way of illustration and are not intended to limit the present invention. BRIEF DESCRIPTION OF THE DRAWINGS
    Figure 1. Represents a structure according to any embodiment of the invention, formed by an upper section (1) comprising a single layer or a high temperature resistant multilayer stack reflecting in the IR region, transmitted in the visible region and it has self-cleaning properties, an absorbent intermediate section (2) resistant to high temperatures and an optional IR reflective layer (3) and a substrate (4).
    Figure 2. Illustrates the process of reducing the transmission rate due to the accumulated dirt on the surface of the structure and the subsequent recovery of the transmittance thanks to self-cleaning.
    Figure 3. Shows the reflectance spectrum of the structure of the invention comprising a multilayer stack of TiO2: Ta / SnO2: F as upper section (1), a CN as an absorbent intermediate section (2) and a steel alloy stainless as substrate (4).
    Figure 4. Shows the reflectance spectrum of a structure of the invention with TiO2: Porous Ta as upper section (1), a CN as an absorbent intermediate section (2) and a stainless steel alloy as a substrate (4).
    Figure 5. Shows the reflectance spectrum of a structure of the invention with TiO2: Porous Ta as upper section (1), a CN as absorbent intermediate section (2) and a


    Stainless steel alloy as a substrate (4) after exposure to a layer of dirt.
    Figure 6. Shows the transmittance values of a clean glass (A) and the transmittance values when dirt is deposited on the glass (B).
    Figure 7. Shows a comparison of the absorptivity after exposing the structure of the invention to dirt (B) and a structure that does not comprise the upper section (A).
    Figure 8. Optical constants of a TiO2 film: Typical Ta.
    Figure 9. (A) shows the results of Rutherford backscattering (RBS) for a layer of TiO2: Ta deposited by magnetron sputtering. (B) shows the X-ray diffraction (DRX) data for a TiO2: Ta layer deposited by magnetron sputtering after annealing at 425 ° C.
    Figure 10. Shows the ellipsometric angles psi (A) and delta (B) as a function of temperature and wavelength for a sample of TiO2: Ta.
    EXAMPLES Example 1: Structures of the invention and process of obtaining
    Two structures of the invention were prepared with the following characteristics:
    Structure (a): The upper section is a multilayer stack based on layers of a TiO2 layer doped with Ta of 10 nm thick and a SnO2 layer doped with F 30 nm thick arranged alternately. The number of layers is five. TiO2 is in the anatase phase and the concentration of the Ta dopant is 1.4% at .. The most superficial layer and the layer closest to the absorbent material section are TiO2 layers doped with Ta. The intermediate section is a 100 nm thick layer of a black body material comprising a cermet based on a silicon nitride and deposited on a stainless steel substrate.
    Structure (b): The upper section is a single layer of TiO2 doped with Ta of 82 nm of


    thickness and with a porosity of 50% v / v, the TiO2 being in anatase phase and the concentration of the dopant being Ta 1.4% at .. The intermediate section is a 100 nm thick layer of a black body material that It comprises a cermet based on a silicon nitride and is deposited on a stainless steel substrate.
    The samples are deposited in the vapor phase by physical method by sputtering by magnetron from a metallic target with a power of 200 W at an approximate pressure of 1.5x10-2 mbar. The oxygen input is controlled by a plasma control unit. This unit detects the optical emission line of the metal to react continuously to changes in the sprayed plasma. The plasma's optical emission changes depending on its oxygen content. Therefore, the emission signal can be used to control a feedback loop that regulates the oxygen valve with great precision. This offers the opportunity to deposit ceramic materials from a metallic target with high spray speed and controlled stoichiometry. After deposition, the TiO2 samples are counted at 425 ° C for 1 hour and 30 minutes to obtain the anatase phase.
    Through this process, an epitaxial growth of SnO2 is obtained using TiO2 as a seed which improves the electronic properties of multilayer stacking and helps reduce stresses during deposition. In this way, one layer grows on the other without delamination, as if they were made of the same material, and this connection between the layers favors a good conductivity between the layers. On the other hand, since the optical properties depend on the electrical properties and these are related to the doped density, it is crucial to achieve the appropriate concentration of dopant in the oxides. It is equally important to carefully control the thickness of the layers.
    The absorptivity and emittance of the structure (b) was simulated before and after exposure to dirt. The dirt was simulated in the form of a 700 nm porous layer that partially blocked the light. This layer of dirt produces a reduction in transmittance from 86% to 50% on glass, as can be seen in Figure 6. The data is shown in the following table.


    Absorption (clean structure) Absorption (dirt)Emittance (clean structure)Emittance (dirt)
    Structure (b) 0.940.820.070.11 Table 1
    Figure 1 represents a structure according to any embodiment of the invention, formed by an upper section (1) comprising a single layer or a stack of
    5 multi-layer high temperature resistant reflecting in the IR region, transmits in the visible region and has self-cleaning properties, a high temperature resistant absorbent intermediate section (2), an optional IR reflecting layer (3) and a substrate ( 4).
    Figure 2 represents the self-cleaning process of the structure. Under normal conditions
    10 the structure shows an absorptivity  and an emittance . During the night, the structure gets dirty. Dirt reduces absorptivity and increases the emittance of the structure during morning operation. The surface of the structure is cleaned when exposed to light. Thanks to the structure's self-cleaning properties, the surface of the receiver is clean again.
    Figure 3 shows the reflectance spectrum of the structure (a) of example 1 of the invention. An absorbency of 0.92 and an emittance of 0.07 are obtained.
    Figure 4 shows the reflectance spectrum of the structure (b) of example 1 of the invention. An absorbency of 0.94 and an emittance of 0.07 are obtained.
    Figure 5 shows the reflectance spectrum after the accumulation of dirt on a structure of the invention comprising TiO2: Porous Ta as upper section (1), a CN as an absorbent intermediate section (2) and a stainless steel alloy as a substrate
    25 (4). The deterioration of the transmission properties of the upper layer has negative effects on the optical properties of the structure described in Figure 1. When the structure (b) of example 1 has a 700 nm thick layer of dirt, the absorptivity decreases to 0.82 and the emittance increases to 0.11 (table 1).
    30 Figure 6 shows how the dirt layer of Figure 5 decreases the transmittance when deposited on a glass substrate. Solar transmittance is reduced from 86% to


    fifty%.
    Figure 7A shows a comparison of the structure without self-cleaning top layer and Figure 7B the structure proposed in this invention comprising a stack of
    5 multiple layers of TiO2: Ta and SnO2: F as upper section. The accumulation of dirt on the surface of the structure without the self-cleaning top layer provides an absorbency of 0.84. The structure of the invention exposed to the same dirt does not show accumulation of dirt, providing an absorbency of 0.92.
    10 Figure 8 shows the optical constants of a typical TiO2: Ta layer deposited by magnetron sputtering. The refractive index (n) and extinction coefficient (k) of the material were obtained from ellipsometric measurements.
    Figure 9A shows the results of Rutherford backscattering (RBS) for a layer of
    TiO2: Ta deposited by magnetron sputtering. This experiment was carried out to obtain the atomic composition of the layer. The doped TiO2 layers are made up of 1.4% at. of tantalum, a material / dopant relationship suitable for OCT conductance.
    Figure 9B shows the X-ray diffraction (DRX) data for a layer of TiO2: Ta deposited by sputtering by magnetron after annealing at 425 ° C. The results confirm the anatase phase of the oxide.
    Figure 10 shows the ellipsometric data for a sample of TiO2: Ta deposited
    25 by magnetron sputtering. The data shows that the optical constants do not change with temperature.

    权利要求:
    Claims (36)
    [1]
    1. Solar selective structure comprising:
    a) an upper section comprising an upper layer comprising TiO2doped with an element selected from the group consisting of Sb, In, B, F, Sn,Nb, N, Ru and Ta,b) an intermediate section comprising a layer of a material that absorbsin the wavelength range of 300 to 2500 nm andc) a substrate selected from metallic or ceramic materials,
    wherein the intermediate section is disposed between the substrate (c) and the upper section (a).
    [2]
    2. Structure according to claim 1, which also comprises a layer of an IR reflective material between section (b) and the substrate (c).
    [3]
    3. Structure according to claim 2, wherein the layer of the IR reflective material comprises metals selected from Al, Cu, Ag or Au.
    [4]
    Four. Structure according to claim 2, wherein the layer of the IR reflective material comprises nitrides selected from TiN, ZrN or CrN.
    [5]
    5. Structure according to any of claims 2 to 4, wherein the thickness of the layer of the IR reflective material is between 50 nm and 500 nm.
    [6]
    6. Structure according to claim 5, wherein the thickness of the layer of the IR reflective material is between 100 nm and 250 nm.
    [7]
    7. Structure according to any one of claims 1 to 6, wherein the concentration of TiO2 dopant of the upper layer of the upper section (a) is equal to or less than 10% at.
    [8]
    8. Structure according to claim 7, wherein the TiO2 dopant concentration of the upper layer of the upper section (a) is between 1 and 3% at.

    [9]
    9. Structure according to any one of claims 1 to 8, wherein the upper layer of the upper section (a) has a porosity equal to or less than 70% v / v.
    [10]
    10. Structure according to claim 9, wherein the upper layer of the upper section (a) has a porosity of 10 to 50% v / v.
    [11]
    eleven. Structure according to any one of claims 1 to 10, wherein the upper section (a) is a single layer of TiO2 doped with Ta.
    [12]
    12. Structure according to claims 1 to 11, wherein the upper section (a) is a single layer with a thickness of less than 1 µm.
    [13]
    13. Structure according to claim 12, wherein the thickness of the upper section
    (a) is between 10 nm and 500 nm.
    [14]
    14. Structure according to any one of claims 1 to 10, wherein the upper section (a) is a multilayer stack comprising at least one layer of doped TiO2 with an element selected from Sb, In, B, F, Sn, Nb , N, Ru or Ta and a layer of a secondary transparent conductive oxide.
    [15]
    fifteen. Structure according to claim 14, wherein the TiO2 is doped with Ta.
    [16]
    16. Structure according to claim 14 or 15, wherein the secondary transparent conductor oxide layer is a doped semiconductor oxide selected from SnO2, ZnO or In2O3.
    [17]
    17. Structure according to claim 16, wherein the doped semiconductor oxide is SnO2.
    [18]
    18. Structure according to any of claims 14 to 17, wherein the secondary transparent conductive oxide is a doped semiconductor oxide with an element selected from Sb, In, B, F, Sn, Nb, N, Ru or Ta.

    [19]
    19. Structure according to claim 18, wherein the semiconductor oxide is doped with F or Ta.
    [20]
    twenty. Structure according to any of claims 14 to 19, wherein the thickness of the TiO2 layer or layers is between 5 nm and 500 nm.
    [21]
    twenty-one. Structure according to claim 20, wherein the thickness of the TiO2 layer or layers is between 10 nm and 100 nm.
    [22]
    22 Structure according to claim 21, wherein the thickness of the TiO2 layer or layers is 10 nm.
    [23]
    2. 3. Structure according to any of claims 14 to 22, wherein the thickness of the secondary transparent conductive oxide layer is between 5 nm and 500 nm.
    [24]
    24. Structure according to claim 23, wherein the thickness of the secondary transparent conductive oxide layer is between 10 nm and 100 nm.
    [25]
    25. Structure according to claim 24, wherein the thickness of the secondary transparent conductive oxide layer is 30 nm.
    [26]
    26. Structure according to any of claims 14 to 25, wherein the number of layers of the multilayer stack of the upper section (a) is between 2 and 60.
    [27]
    27. Structure according to claim 26, wherein the number of layers of the multilayer stack of the upper section (a) is between 2 and 30.
    [28]
    28. Structure according to claim 27, wherein the number of layers of the multilayer stack of the upper section (a) is between 3 and 15.
    [29]
    29. Structure according to any of claims 14 to 28, wherein the doped TiO2-based layer is the upper layer and the other TiO2-based layers and the layers based on the secondary transparent conductive oxide are alternately deposited.

    [30]
    30. Structure according to any of claims 14 to 29, wherein the thickness of the multi-layer stacking of the upper section (a) is between 10 nm and 2 µm.
    [31]
    31. Structure according to claim 30, wherein the thickness of the multilayer stack of the upper section (a) is between 20 nm and 500 nm.
    [32]
    32 Structure according to any one of claims 1 to 31, wherein the material
    The absorbent intermediate section (b) is selected from nanotubes, silicon, Pyromark®, black enamel, black molybdenum, cermets or plasma-projected layers.
    [33]
    33. Structure according to any one of claims 1 to 32, wherein the thickness
    of the intermediate section (b) ranges from 100 nm to 1000 µm. fifteen
    [34]
    34. Structure according to any of claims 1 to 33, wherein the substrate
    (c) is selected from steel, stainless steel, copper, aluminum or nickel alloys.
    A structure according to any one of claims 1 to 34, wherein the substrate is a tower receiver.
    [36]
    36. Use of the structure according to any of claims 1 to 35 for
    tower receivers in solar energy systems by concentration. 25
    [37]
    37. Use of the structure according to claim 36, wherein the tower receiver is the substrate of the structure.

     DRAWINGS 

    FIG. 6A

    FIG. 6B
    FIG. 7A

    FIG. 7B

    FIG. 9A
    FIG. 9B

    FIG. 10A
     FIG. 10B
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