WO2016107883A1

WO2016107883A1 - Self-cleaning high temperature resistant solar selective structure

Info

Publication number: WO2016107883A1
Application number: PCT/EP2015/081376
Authority: WO
Inventors: María Elena GUILLÉN RODRÍGUEZ; Ramón ESCOBAR GALINDO; Irene HERAS PÉREZ; José Luis ENDRINO ARMENTEROS; Azucena BELLO; Noelia MARTÍNEZ SANZ; Sibylle GEMMING; Frank LUNGWITZ; Matthias Krause
Original assignee: Helmholtz Zentrum Dresden Rossendorf eV
Current assignee: Helmholtz Zentrum Dresden Rossendorf eV
Priority date: 2014-12-31
Filing date: 2015-12-29
Publication date: 2016-07-07
Anticipated expiration: 2017-06-30
Also published as: ES2575746B1; ES2575746A1; EP3240978A1

Abstract

The present invention is directed to a structure formed by a top section comprising a top layer (1) comprising doped TiO₂which presents high transmittance in visible spectra and high reflecting in IR and self-cleaning properties, an absorbing intermediate section (2) and a substrate (4). Due to the mentioned properties and to a resistance to high temperature, the structure is useful as a solar selective structure for tower receivers in CSP systems.

Description

SELF-CLEANING HIGH TEMPERATURE RESISTANT SOLAR SELECTIVE

STRUCTURE

DESCRIPTION

The present invention relates to a high-temperature solar selective structure e.g. of tower receivers in concentrating solar power (CSP) systems. The solar selective structure maintains high absorptance in the solar region and low emittance in the infrared spectrum. In addition, the solar selective structure resists to high temperature and is stable in air.

STATE OF ART

In a central receiver concentrating solar power plant, a set of mirrors concentrates the solar light into/onto a receiver placed in/on the top of a tower, which is located in the focal point of the mirrors. The receiver tubes are coated by a solar selective coating, and filled by a heat transfer fluid. The solar selective coating must be highly absorbing in the solar region and must have a low thermal emittance at elevated temperature. Coatings usually used as solar selective coatings for high temperature applications are based on cermets, which are highly absorbing in the visible region and transparent to the infrared region, deposited on a highly reflecting substrate which avoids emittance losses from the transfer fluid. Selective absorber surface coatings can be classified in six different types: a) intrinsic, b) semiconductor-metal tandems, c) multilayer absorbers, d) multi-dielectric composite coatings, e) textured surfaces and f) selectively solar transmitting coatings on a black- body-like absorber. With respect to the last type of mentioned coatings, there are some works in the literature such as C. E. Kennedy, "Review of Mid- to High- Temperature Solar Selective Absorber Materials" no. July, 2002, which describes a material based on a highly absorbing film coated by a transparent conductive oxide (TOO) and deposited on a substrate: Sn0₂:F or Sn0₂:Sb or ln₂S0₃:Sn or ZnO:AI /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 (BB) like absorber coated by transparent conductive oxides as; ln₂0₃/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 Sn0₂/Black Molybdenum for providing a coating with wave length selective properties.

All the references reported above are based on a highly absorbing material, coated by a selective absorber which is reflecting in the IR and transmitting in the visible. However, none of the coatings reported in the state of the art exhibits self-cleaning properties.

In addition, the TCO layers of the state of the art are deposited on a BB substrate (where BB stands for "black body"). This substrate contributes to the optical properties of the material. Therefore, the coating is always a single layer.

The amount of energy provided by the solar system is reduced by dirtiness of the coating on the surface of the tube. If the coating becomes unclean, the incoming light reaching the absorber material will be dramatically reduced. The transmission loss of solar radiation can be as much as 50% when the dust concentration is 1 mg/cm² (Mazumderl, M.; Horenstein, M.; Stark, J.; Girouard, P.; Sumnerl, R. Characterization of Electrodynamic Screen Performance for Dust Removal from Solar Panels and Solar Hydrogen Generators. 201 1 , 1-8). Also, surface roughness may hinder transparency due to scatter losses. Self-cleaning properties can be advantageous to avoid dirtiness accumulation on the tubes and to increase long-term durability of the coating. A self- cleaning coating is especially interesting for open air applications like CSP towers as the self-cleaning properties will favor the long term stability, the reduction of maintenance works, cost reduction and the optimal performance of the system during coating lifetime, which results in higher efficiency of the plant. Reduction of maintenance work is especially interesting regarding the receiver in the tower due to the difficult access to clean the coating surface.

Self-cleaning coatings have been proposed for heliostats in high temperature concentrated solar power 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 been explored yet. None of the existing coatings for high temperature concentrating solar power systems exhibits self-cleaning properties.

Currently 2500 Pyromark® is the receiver coating more commonly used. Several attempts have been made to create a high temperature solar selective coating but most of the coatings have shown significant deterioration when exposed to air in high temperatures.

The patent application US3968786A describes a tube made of plastic material in which a black body (BB) material is distributed throughout the thickness of the tube, and an optional coating layer providing a selective absorbing surface. This coated tube will not be stable at such high temperature applications as tower receivers, also it is not a multilayer with controlled optical selective characteristics.

In the patent application EP2243750A1 , a glass or glass ceramic disc which comprises an infrared reflective layer and which is used as viewing pane for high temperature applications. This invention does not describe a blackbody layer, it only comprises an infrared reflective layer. The coating does not achieve optical selective characteristics of absorptance and emittance, to absorb in the solar region and emit in the infrared spectrum, required for receiver applications.

In the patent application WO2013044975 A1 a glass tube with infrared light reflective coating on the top surface and a solar energy absorptive coating in the interior of the tube is described. The IR reflective and the absorber layer are deposited on two different surfaces, so this patent application describes two coatings. Moreover this coating configuration is only suitable for parabolic trough applications. The patent application US20070134501 A1 describes a coating which comprises nanocrystals of a photoactive material providing self-cleaning properties to the coated surface. That coating is applied over a transparent substrate without an absorber layer. The coating in that patent would not be suitable for tower receivers. Therefore, there is a need for a structure with both solar selective optical and self- cleaning properties and resistant to high temperatures, so it could be used for applications that require said properties, such as solar tower receivers. DESCRIPTION OF THE INVENTION

The present invention provides a solar selective structure which exhibits self-cleaning properties and can e.g. be used for high temperature applications in air, for tower receivers in concentrating solar power (CSP) systems.

The solar selective structure is based on a highly absorbing material, resistant to high temperatures, coated by a thin layer which fulfills the following characteristics: self- cleaning capabilities, high transmission in the visible, high reflectance in the IR and resistance to high temperature. The structure is stable at temperatures over 500 °C, its absorptance is in the 0.80-0.99 range and it has an emittance below 0.6.

Thus, according to a first aspect, the present invention is directed to a structure, which comprises:

a) a top section comprising a top layer comprising Ti0₂ 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 absorbing in the 300-2500 nm wavelength range and

c) a substrate selected from metallic or ceramic materials,

wherein the intermediate section is arranged between the substrate and the top section.

The top section may consist of a single layer (i.e. the top layer), or of a multilayer stack comprising the top layer. The top layer of the top section is resistant to high temperatures, reflecting in the IR, transmitting in the visible region, and with self-cleaning properties

The top layer, due to the presence of Ti0₂, does not absorb in the visible region and it is photocatalytically active. In addition, when doped with metals, it exhibits IR reflective properties. Wide band gap semiconductor Ti0₂ has a melting temperature of 1855°C and exhibits self-cleaning properties due to two different phenomena: photocatalysis and photo- induced superhydrophilicity. Photoexcited titanium dioxide has a strong oxidation and reduction power which may result in the degradation of surface pollutants. Another self- cleaning mechanism different from the photocatalytic decomposition of organic components is the photo-induced hydrophilic conversion of Ti0₂, which produces very small contact angles on the surface of Ti0₂.

In a preferred embodiment, the structure of the invention also comprises a layer of an IR reflecting material deposited between layer (b) and substrate (c). This additional layer produces a further reduction in emissivity of the structure of the invention. In a more preferred embodiment, the IR reflecting 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 reflecting material is preferably between 50 nm and 500 nm, and more preferably between 100 nm and 250 nm.

In a preferred embodiment, the top section (a) is a single layer comprising Ti0₂ doped with an element selected from Sb, In, B, F, Sn, Nb, N, Ru or Ta, preferably Ta. Preferably, the dopant concentration is equal to or lower than 10 at.%., wherein "at.%" is atomic percent (percentage of one atom relative to the total number of atoms). And more preferably the dopant concentration is between 1 -3 at.%.

In a preferred embodiment, the top section (a) is a single layer of Ti0₂ doped with Ta.

This layer can be dense or porous, with porosity preferably equal to or lower than 70 % v/v (volume fraction), more preferably between 10-50 % v/v. Optical constants of the layer can be tuned changing the porosity. The transmittance in the visible can be raised by raising the porosity, as the optical properties of a porous layer would be the combination of the optical constants of the material and the air located in the porous layer.

Preferably, the thickness of the single layer (a) is lower than 1 μηη, preferably between 10 nm and 500 nm. In another preferred embodiment, the top section (a) is a multilayer stack comprising at least a layer of Ti0₂ doped 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 (TCO) with different refractive index. These two types of layers are deposited alternately and the top layer must be a Ti0₂-based one to provide the self-cleaning properties. TCOs are conductive materials which exhibit selective transmittance. They have a low absorption coefficient in the near UV-vis region, and they are reflective in the IR. Their optical properties depend on the electron mobility, the band gap and the carrier density. They do not absorb in the visible as they are wide band gap semiconducting oxides. IR reflective properties of a TCO are related to its electrical properties (charge carrier mobility and charge carrier density). TCOs can be binary or ternary compounds or they can also be a multicomponent. Non-limiting examples of semiconducting oxides of the secondary TCO for the present invention are Sn0₂, ln₂Sn0₃, SiO_xN_y, SiO_xC_y, ZnO. Preferably, the semiconducting oxides of the secondary TCO is selected from Sn0_2, ZnO or ln₂0_3, and more preferably Sn0_2. These semiconducting oxides secondary TCOs are doped, and possible dopants include, but are not limited to, Ta, N, Nb, Sb, In, B, F, Ru and Sn. In a more preferred embodiment, secondary TCOs are doped with F or Ta. For this multilayer stack embodiment, doped-Ti0₂ layers can also be dense or porous with porosity equal to or lower than 70% v/v, volume fraction, preferably 10-50 % v/v.

Preferably, the dopant concentration would be equal or lower than 10 at.%, wherein "at.%" is atomic percent (percentage of one atom relative to the total number of atoms). And more preferably the dopant concentration would be between 1 at.% and 3 at.%.

Preferably, the thickness of doped-Ti0₂ layer is between 5 nm and 500 nm, preferably between 10 nm and 100 nm and more preferably 10 nm. The thickness of the secondary TCO layer is between 5 nm and 500 nm, preferably between 10 nm and 100 nm, and more preferably 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 multilayer stack thickness ranges between 10 nm and 2 μηη preferably between 20 nm and 500 nm. In another preferred embodiment, the top section (a) is a multilayer stack as described above, comprising at least a Ta doped Ti0₂ layer and a layer of a Ta doped Sn0₂. The multilayer stack may also comprise at least one Ta doped Ti0₂-layer and/or at least one Ta doped Sn0₂ layer. Ti0₂ exhibits TCO properties when doped with metals like tantalum. Carrier density of Ta doped Ti0₂ is high, but its electron mobility is lower than other TCO materials, which will affect its transmittance and IR reflective properties. Sn0₂ is also a wide band gap semiconductor material with a high melting point (1630°C). Ta doped Sn0₂ exhibits high mobility but this TCO has bad dopant activation. This limitation could be solved by increasing dopant concentration (Ta), but Ta sites will also acts as scattering centers decreasing transmittance in the visible.

Both semiconductor oxides (Ti0₂ and Sn0₂) exhibit similar thermal expansion coefficients which favor the multilayer stack formation. In addition, using Ti0₂ as a seed layer for Sn0₂ growth, an epitaxial growth of the latter is promoted, which improves its electronic properties. Physical vapour deposition methods are required to obtain the desirable properties of the multilayer stack. These high energy methods make the epitaxial growth of Sn0₂ layers using Ti0₂ as a seed possible. In addition, good interlayer adhesion and homogeneity are obtained. This multilayer stack based on two TCO materials combines properties of both materials, where doped Ti0₂ acts as an electron source, and doped Sn0₂ as an electron carrier, having an epitaxial growth of the Sn0₂ over the Ti0₂ that favors the performance of the multilayer stack in several aspects: contributes to reduction of tensions during deposition, so one layer grows over the other with no delamination, as if they were the same material. This good interlayer connection favors a good conductivity between layers, which is critical for a good performance of the resulting multilayer stack.

In a preferred embodiment, the total number of single 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 single layers of Ta doped Ti0₂ is at least 2 and the number of single layers of Ta doped Sn0₂ is at least 1 to obtain the desired epitaxial growth of the Ta doped Sn0₂ over the Ta doped Ti0₂. The multilayer stack exhibits self-cleaning properties, is high temperature resistant, has a high transmittance in the visible region and high reflectivity in the IR. In this selectivity behavior the different index of refraction of TCO materials, the number and thickness of layers and the electrical properties of the resulting TCO play a role. We obtain, therefore, a double optimization of optical properties.

The intermediate section is based on a material which is highly absorbing in the 300 nm - 2500 nm wavelength range and resistant to high temperature. In a preferred embodiment, blackbody materials adequate 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, plasma sprayed layers etc. The present configuration allows taking advantage of the good absorption properties of a blackbody absorber, without the problem associated to its high emissivity.

Preferably, the thickness of this intermediate section is in the range 100 nm-1000 μηη.

Thermal emission of the intermediate section is stopped thanks to the presence of the highly IR reflective top layer. The use of an IR reflective top layer is advantageous in two ways: it will avoid high thermal losses at high temperature (the emittance from absorber materials like cermets rises at high temperatures), and it opens the possibility of using BB absorber materials instead of selective absorbers.

In a preferred embodiment, the intermediate section is deposited on a metallic substrate such as steel, stainless steel, copper, aluminum, Inconel® or other nickel alloy or metallic alloys, ceramic materials or any other material forming the receiver tube.

The resultant structure is thermally stable at more than 400 °C, more preferably the structure is stable at between 400 °C and 1000 °C and more preferably the resultant structure is stable at between 500 °C and 700 °C. The resultant structure stability makes it suitable for high temperature applications.

Another aspect of the present invention is the use of the structure described above for tower receivers in concentrating solar power systems. The structure is especially applied to tower receivers where the working temperatures are equal to or higher than 500 °C. Although this solar selective structure could be also applied to mid- and low temperature solar thermal applications. Thus, according to another aspect of the invention the structure of the invention is used in thermal receivers, e.g. for a solar power system. According to this aspect, the tower receiver itself is the substrate of the structure of the invention.

The top and intermediate sections of the structure described above can be deposited by Physical Vapour Deposition, such as but not limited to Magnetron Sputtering, Cathodic Vacuum Arc, Ion Beam Sputtering, Ion Beam Assisted Deposition and High Power Impulse Magnetron Sputtering.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one ordinary skilled in the art to which this invention belongs. Methods and materials similar or equivalent to those described herein can be used in the practice of the present invention. Throughout the description and claims the word "comprise" and its variations are not intended to exclude other technical features, additives, components, or steps. Additional objects, advantages and features of the invention will become apparent to those skilled in the art upon examination of the description or may be learned by practice of the invention. The following examples and drawings are provided by way of illustration and are not intended to be limiting of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1. Represents a structure according to any embodiment of the invention, formed by a top section comprising a single layer or a multilayer stack, resistant to high temperatures, reflecting in the IR, transmitting the visible region, and with self-cleaning properties (1 ), a highly absorbing section resistant to high temperatures (2), and optional IR reflective layer (3), and a substrate (4).

Figure 2. Illustrates the process of decreasing of the transmission rate caused by settled dirtiness on the surface of the structure and subsequent transmittance recovery due to self-cleaning. Figure 3. Shows the reflectance spectra of the structure of the invention comprising a multilayer stack Ti0₂:Ta / Sn0₂:F top layer (1 ), BB absorbing intermediate section (2) and a stainless steel alloy as substrate (4).

Figure 4. Shows the reflectance spectra of a structure of the invention with a porous Ti0₂:Ta as top layer (1 ), BB as absorbing intermediate section (2) and stainless steel alloy as substrate (4). Figure 5. Shows the reflectance spectra of a structure of the invention with a porous Ti0₂:Ta as top layer (1 ), BB as absorbing intermediate section (2) and stainless steel alloy as substrate (4), after exposure to dirtiness layer.

Figure 6. Shows the transmittance values of a clean glass (A) and the transmittance values when dirtiness is deposited onto the glass (B).

Figure 7. Shows a comparison of the absorptance after dirtiness exposure of the structure of the invention (B) and a structure not comprising the top section (A). Figure 8. Optical constants of a typical Ti0₂:Ta film.

Figure 9. (A) shows RBS (Rutherford Back Scattering) results for Ti0₂:Ta layer deposited by Magnetron Sputtering. (B) shows XRD (X-ray diffraction) data for Ti0₂:Ta layer deposited by Magnetron Sputtering after annealing at 425 °C.

Figure 10. Shows the ellipsometric angles psi (A) and delta (B) as function of temperature and wavelength for a Ti0₂:Ta sample.

EXAMPLES

Example 1 : Structures of the invention and obtaining process.

Two structures of the invention with the following characteristics were prepared: Structure (1 ): top section is a multilayer stack based on layers of a Ta doped Ti0₂ layer 10 nm thick and a F doped Sn0₂ layer 30 nm thick disposed alternately. The number of layers is five. Ti0₂ is in anatase phase, and Ta dopant concentration is 1 .4 at.%. The most superficial layer and the layer more proximal to the absorbing material section are Ta doped Ti0₂ layers. The intermediate section is a 100 nm thick layer of a blackbody material comprising a cermet based on a silicon nitride, and is deposited on a stainless steel substrate.

Structure (2): top section is a single layer of Ta doped Ti0₂ 82 nm thick, with a porosity of 50 %v/v, where Ti0₂ is in the anatase phase, and Ta dopant concentration is 1 .4 at.%. The intermediate section is a 100 nm thick layer of a blackbody material comprising a cermet based on a silicon nitride, and is deposited on a stainless steel substrate.

Samples are physically vapour deposited by Magnetron Sputtering from a metallic target with a power of 200 W at an approximate pressure of 1 .5- 10^"2 mbar. The oxygen inlet is controlled by a plasma control unit. This unit detects the optical emission line of metal to continuously react to changes in the sputtered plasma. The optical emission of the plasma changes depending on its oxygen content. Therefore the emission signal can be used to control a feedback loop which regulates the oxygen valve very precisely. This gives the opportunity to deposit ceramic materials from a metallic target with a high sputtering rate and a controlled stoichiometry. After deposition, Ti0₂ samples are annealed to 425 °C during 1 hour 30 minutes to obtain anatase phase.

By this process is obtained an epitaxial growth of Sn0₂ using Ti0₂ as seed, which improves electronic properties of the multilayer stack, and contributes to the reduction of tensions during deposition. So one layer grows over the other with no delamination as if they were made of the same material and this interlayer connection will favor a good conductivity between layers. On the other hand, as optical properties depend on electrical properties and those are related to doping density, getting the proper dopant concentration of the oxides is critical. Moreover, a careful control of layer thickness is also important.

The absorptance and emittance of the structure (2) was simulated before and after the exposure to dirtiness. The dirtiness has been simulated as a porous partial light blocking layer of 700 nm. This layer of dirtiness produces a transmittance reduction over a glass of 86% to 50% which can be seen in figure 6. Data are shown in the following table:

Table 1

Figure 1 represents a structure according to any embodiment of the invention, formed by a top section comprising a single layer or a multilayer stack resistant to high temperatures, reflecting in the IR, transmitting the visible region, and with self-cleaning properties (1 ), a highly absorbing section resistant to high temperatures (2), an optional IR reflective layer (3), and a substrate (4).

Figure 2 represents the self-cleaning process of the structure. In normal conditions of operation the structure exhibits an absorptance a and an emittance ε. During the night, the structure becomes unclean. Dirtiness diminishes the absorptance and increases the emittance of the structure during morning operation. The surface of the structure is cleaned under exposure to light. Thanks to the self-cleaning properties of the structure, the surface of the receiver is clean again. Figure 3 shows the reflectance spectrum of the structure (1 ) of example 1 of the invention. An absorptance of 0.92 and an emittance of 0.07 are obtained.

Figure 4 shows the reflectance spectrum of the structure (2) of example 1 of the invention. An absorptance of 0.94 and an emittance of 0.07 are obtained.

Figure 5 shows the reflectance spectra after dirtiness accumulation on a structure of the invention comprising a porous Ti0₂:Ta as top section (1 ), a BB as absorbing intermediate section (2) and stainless steel alloy as substrate (4) . The diminution of the transmission properties of the top layer has negative effects on the optical properties of the structure described in Figure 1 . When structure (2) in example 1 has a 700 nm thick layer of dirtiness absorptance decreases to 0.82, and emittance increases to 0.11 (Table 1 ).

Figure 6 shows how the dirtiness layer in Figure 5 diminishes the transmittance when deposited on a glass substrate. Solar transmittance decreases from 86 % to 50 %.

Figure 7 shows a comparison of the structure with no self-cleaning top layer (A) and the structure proposed in this invention comprising a multilayer stack of Ti0₂:Ta and Sn0₂:F top section (B). Dirtiness accumulation on the structure surface without the self- cleaning top layer provides an absorptance of 0.84. The structure of the invention with the same exposure to dirtiness, presents no dirtiness accumulation, providing an absorptance of 0.92.

Figure 8 shows optical constants of a typical Ti0₂:Ta layer deposited by Magnetron Sputtering. Index of refraction (n) and extinction coefficient (k) of the material were obtained from ellipsometry measurements.

Figure 9A shows RBS (Rutherford Back Scattering) results for a Ti0₂:Ta layer deposited by Magnetron Sputtering. This experiment was performed to obtain the atomic composition of the layer. The doped Ti0₂ layers are composed of a 1.4 at.% of Tantalum, a suitable material/dopant ratio for the conductance of the TCO.

Figure 9B shows XRD (X-ray diffraction) data for Ti0₂:Ta layer deposited by Magnetron Sputtering after annealing at 425 °C. Results confirm the anatase phase of the oxide.

Figure 10. shows the ellipsometry data for a Ti0₂:Ta sample deposited by magnetron sputtering. The data show that the optical constants do not change with temperature.

Claims

1 . A structure comprising:

a) a top section comprising a top layer comprising Ti0₂ doped with an element selected from the group consisting in Sb, In, B, F, Sn, Nb, N, Ru and Ta, b) an intermediate section comprising a layer of a material absorbing in the 300-2500 nm wavelength range and

c) a substrate selected from metallic or ceramic materials,

wherein the intermediate section is arranged between the substrate (c) and the top section (a).

2. The structure according to claim 1 , which also comprises a layer of an IR reflecting material between section (b) and substrate (c).

3. The structure according to claim 2, wherein the layer of the IR reflecting material comprises metals selected from Al, Cu, Ag or Au.

4. The structure according to claim 2, wherein the layer of the IR reflecting material comprises nitrides selected from TiN, ZrN or CrN.

5. The structure according to any of claims 2 to 4 wherein the thickness of the layer of the IR reflecting material is between 50 nm and 500 nm.

6. The structure according to claim 5, wherein the thickness of the layer of the IR reflecting material is between 100 nm and 250 nm.

7. The structure according to any of claims 1 to 6, wherein the dopant concentration of Ti0₂ of the top layer of the top section (a) is equal to or lower than 10 at.%.

8. The structure according to claim 7, wherein the dopant concentration of Ti0₂ of the top layer of the top section (a) is between 1 and 3 at.%.

9. The structure according to any of claims 1 to 8, wherein the top layer of the top section (a) presents porosity equal to or lower than 70% v/v.

10. The structure according to claim 9 wherein the top layer of the top section (a) presents porosity between 10 and 50% v/v.

1 1 . The structure according to any of claims 1 to 10, wherein the top section (a) is a 5 single layer of Ti0₂ doped with Ta.

12. The structure according to claims 1 to 1 1 , wherein the top section (a) is a single layer with a thickness lower than 1 μηη.

10 13. The structure according to claim 12 wherein the thickness of the top section (a) is between 10 nm and 500nm.

14. The structure according to any of claims 1 to 10, wherein the top section (a) is a multilayer stack comprising at least one layer of Ti0₂ doped with an element

15 selected from Sb, In, B, F, Sn, Nb, N, Ru or Ta and one layer of a secondary transparent conductive oxide.

15. The structure according to claim 14, wherein the Ti0₂ is doped with Ta_.

20 16. The structure according to claim 14 or 15, wherein the layer of the secondary transparent conductive oxide is a doped semiconductor oxide selected from Sn0₂, ZnO or ln₂0₃.

17. The structure according to claim 16, wherein the doped semiconductor oxide is 25 Sn0₂.

18. The structure according to any of claims 14 to 17, wherein the secondary transparent conductive oxide is a semiconductor oxide doped with an element selected from Sb, In, B, F, Sn, Nb, N, Ru or Ta.

30

19. The structure according to claim 18, wherein the semiconductor oxide is doped with F or Ta.

20. The structure according to any of claims 14 to 19, wherein the thickness of the Ti0₂ 35 layer or layers is between 5 nm and 500nm.

21 . The structure according to claim 20, wherein the thickness of the Ti0₂ layer or layers is between 10 nm and 100nm.

22. The structure according to claim 21 , wherein the thickness of the Ti0₂ layer or 5 layers is 10nm.

23. The 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.

10 24. The structure according to claim 23, wherein the thickness of the secondary transparent conductive oxide layer is between 10 nm and 100 nm.

25. The structure according to claim 24, wherein the thickness of the secondary transparent conductive oxide layer is 30 nm.

15

26. The structure according to any of claims 14 to 25, wherein the number of layers of multilayer stack of the top section (a) is between 2 and 60.

27. The structure according to claim 26, wherein the number of layers of multilayer stack 20 of the top section (a) is between 2 and 30.

28. The structure according to claim 27, wherein the number of layers of multilayer stack of the top section (a) is between 3 and 15.

25 29. The structure according to any of claims 14 to 28 wherein the doped Ti0₂-based layer is the top layer, and the other Ti0₂-based layers and the secondary transparent conductive oxide-based layers are deposited alternately.

30. The structure according to any of claims 14 to 29, wherein the thickness of the 30 multilayer stack of the top section (a) is between 10 nm and 2 μηη.

31 . The structure according to claim 30, wherein the thickness of the multilayer stack of the top section (a) is between 20 nm and 500 nm.

32. The structure according to any of claims 1 to 31 , wherein the absorbing material of the intermediate section (b) is selected from nanotubes, silicon, Pyromark®, black enamel, black-molybdenum, cermets or plasma sprayed layers.

33. The structure according to any of claims 1 to 32, wherein the thickness of the intermediate section (b) ranges between 100nm -Ι ΟΟΟμηη.

34. The structure according to any of claims 1 to 33, wherein the substrate (c) is selected from steel, stainless steel, copper, aluminum or nickel-chrome alloys.

35. The structure of any of claims 1 to 34, wherein the substrate is a tower receiver.

36. Use of the structure according to any of claims 1 to 35 for tower receivers in concentrating solar power systems.

37. The use of the structure according to claim 36 where the tower receiver is the substrate of the structure.