FIELD OF THE INVENTION

The invention relates to the engineering of the optical properties of materials and more specifically to the engineering of absorption properties.

BACKGROUND AND PRIOR ART

Solar thermal receivers operate at high temperature and are fuelled by concentrated sunlight radiations. Their efficiency depends on the balance between sunlight radiations absorbed and infrared re-emission at operating temperature (i.e. the temperature required to insure proper heat transfer to the thermodynamic fluid circulated through the receiver).

Therefore, one problem relates to the design of an optical selective surface that can absorb as much as possible of the incoming radiation (visible spectrum - wide band spectrum) while radiating as low as possible in the infra-red range so as to improve the yield of heat transfer to the working fluid.

At present, in particular two types of wide-band selective absorbers are currently used in the thermo-solar domain.

A first type is based on metal-dielectric composites named cermets where absorption is insured by metallic particles dispersed in a ceramic matrix. Cermets are a special class of composite materials made of finegrained mixtures of metallic nanoparticles randomly dispersed in dielectric host materials.

A second type is based on thin-films multilayers.

One may cite the following articles with respect to both:

C.E. Kennedy - Review of Mid to High Temperature Solar Selective

Absorber Materials - NREL - July 2002, G. D. Mahan, Phys. Rev. B 38, 14, 9500 (1988).

M. Gomez, L. Fonseca and G; Rodriguez, Ferroelectrics Letters Section 2, 1, 17-24 (1984).

S. Gresillon, L. Aigouy, A. C. Boccara, J. C. Rivoal, X.Quelin, C. Desmarest, P. Gadenne, V. A. Shubin, A. K. Sarychev, and V. M. Shalaev, Phys. Rev. Lett. 82, 4520 (1999).

V. M. Shalaev and A. K. Sarychev, Phys. Rev. B 57,

13265 (1998). In addition, metal-dielectric metamaterials structured at the subwavelength scale have been found very effective for wide band absorption but are not used due to their high fabrication costs (see ] K. Aydin, V. E. Ferry, R. M. Briggs and H. A. Atwater, Nat. Comms. 2, 517 (2011). See also S. Chen et al, Appl. Phys. Lett., 99, 253104 (2011); P. Zhu et al., Appl. Phys. Lett., 101, 241116 (2012); I. Massiot et al., Apppl. Phys. Lett. 101, 163901 (2012) ; "Design of a perfect black absorber at visible frequenscies using plamonic metamaterials", Hedayati, M.K.; Javaherirahim, M.; Mozooni, B.; Abdelaziz, R.;Tavassolizadeh, A.; Chakravadhanula, V.S.K.; Zaporojtchenko, V.; Strunkus, T.; Faupel, F.; Elbahri, M. Advanced Materials 2011, WILEY).

Furthermore resonant plasmonic structures have been developed to engineer the absorption spectra. These solutions are based on plasmonic resonances of stand-alone particles without any interactions between particles( see for example T.V. Teperik et al., Nat. Photonics 2, 299-301 (2008) ; A. Moreau, C. Ciraci, J.J. Mock, R. T. Hill, Q. Wang, B.J. Wiley, A. Chilkoti and D. R. Smith, Nature 492, 86-89 (2012) ; L. Li, K.-.Q. Peng, B. Hu, X. Wang, Y. Hu, X.-L. Wu, Appl. Phys. Lett., 100, 223902 (2012)).

It has to be noticed that none of the selective coatings already in use on Concentrated Solar Power (CSP) technologies can withstand the harsh operating conditions of solar central receivers (receivers used on solar tower under high irradiation fluxes, at very high temperature and under air).

Consequently, there is a real need for a new approach of solar thermal receivers.

In the state of the art, US2013/0092221 relates to an intermediate band solar cell having solution processed colloidal quantum dots and nano particles.

This document does not relate to a solar thermal receiver. In particular, it has to be beard in mind that the temperature of photovoltaic device shall be regulated and not exceed a certain temperature for not reducing the efficiency of the photovoltaic cell, whereas for a solar thermal receiver, the temperature shall be as high as possible.

WO2009105662 relates to methods and systems for treating cell proliferation disorders using plasmonic enhanced photospectral therapy and exciton plasmon enhanced phototherapy. This document does not relate to a solar thermal receiver.

In addition, structured meta-materials have very high fabrication costs and so are not envisaged for large scale equipment.

For this purpose, the invention proposes a solar thermal receiver for a solar thermal energy plant comprising a light absorber comprising nanoparticles in a host material which is transparent in an absorbing range of the light absorber and where the nanoparticles exhibit plasmonic resonances within the absorbing range of the light absorber, wherein the dispersion of the nanoparticles in the host material is controlled in such a way that the mean distance between said nanoparticles is lower than the wavelengths of light in the absorbing range of the light absorber for generating near field radiation interactions between said nanoparticles, where the size and the distribution of the nanoparticles is chosen to obtain a a cutting wavelength also known as cutoff wavelength, where light of higher wavelength than the cutting (cutoff) wavelength is less absorbed than light at shorter wavelengths with respect to the cutting (cutoff) wavelength reducing losses due to infrared radiation of the absorber.

Thanks to the near field interaction, the absorption coefficient may be enhanced at a high level over a wide band of wavelength.

In addition, losses due to infrared radiation of the absorber may be reduced enhancing thus the total efficiency of the solar thermal receiver.

According to other features taken alone or in combination:

When the absorbing range is in the visible range, the nanoparticles may be are metallic nanoparticles, for example made of silver and/or of gold.

When the absorbing range is in the infrared range, the nanoparticles may be polarized polar nanoparticles.

In one embodiment, the nanoparticles are for example at least of a first material and of a second material different from the first material.

In another embodiment, that might be combined with the previous one, the nanoparticles comprise nanoparticles of a first mean size and nanoparticles of a second mean size, different from the first mean size.

Thus, one disposes of several parameters that allow to shape and adapt the absorption curve of the absorber.

Furthermore the mean radius of said nanoparticles may be less than lOOnm.

The nanoparticles may of spherical shape.

The invention also relates to a solar receiver for a solar thermal energy plant comprising a light absorber as described above.

Within the solar receiver, the light absorber is for example arranged as a layer having a backside in contact with a heat conducting medium.

BRIEF DESCRIPTIONS OF THE DRAWINGS

Other advantages and characteristics will appear with the reading of the description of the following figures, among which:

FIG. 1 is a schematic of a concentrated solar power plant, FIG. 2 is a schematic representation of a solar receiver in particular for a concentrated solar power plant as in figure 1,

FIG. 3 shows a first example a distribution of nanoparticles of a unitary cell of an absorber according to the invention,

FIG. 4 shows a top view of the unitary cell of figure 3,

FIG. 5 shows in a diagram the absorption coefficient in function of the wavelength for an absorber having a unitary cell as shown in figures 3 and 4,

FIG. 6 shows a second example a distribution of nanoparticles of a unitary cell of an absorber according to the invention,

FIG. 7 shows a top view of the unitary cell of figure 6,

FIG. 8 shows in a diagram the absorption coefficient in function of the wavelength for an absorber having a unitary cell as shown in figures 5 and 6,

FIG. 9 shows a third example a distribution of nanoparticles of a unitary cell of an absorber according to the invention,

FIG. 10 shows a top view of the unitary cell of figure 9, FIG. 11 shows in a diagram the absorption coefficient in function of the wavelength for an absorber having a unitary cell as shown in figures 9 and 10,

DETAILED DESCRIPTION

On all the figures the same references refer to the same elements.

The filling factor f of a lattice can be defined by f=V_part/V_tot Where V_tot = lx * ly * lz, is for example the volume of a unitary cell and V_part the volume occupied by the particles in the unitary cell.

FIG.l schematically illustrates an example of a schematic of a concentrated solar power (CSP) plant with a solar power tower 1.

In order to collect the sun rays 3 emitted by the sun 5, heliostats 7 supporting mirrors 9 that may be driven by motors in order to reflect and focus the incoming sun rays on a small area at the top of the solar sun power tower 1 are arranged around the solar power tower 1.

The concentrated heat energy may be used in a conventional power cycle or other heat engine to produce mechanical power that drives an electrical generator. CSP power plants can generate important amounts of power (hundreds of megawatts) for utility-scale applications.

In the top of the solar power tower 1 is arranged a solar thermal receiver 10 comprising a light absorber 11 and a heat conducting medium 12 on the back of the light absorber 11.

The heat conducting medium 12 may be a heat conducting substrate like for example a high temperature nickel based super-alloy or a sintered silicon carbide supporting the high temperatures (for example above 650 C up to lOOO C or more) that may be generated through the absorption of concentrated sunlight and in contact with a heat conducting fluid like for example a molten salt.

The light absorber 11 comprises nanoparticles 13 in a host material 15 which is transparent in an absorbing range of the light absorber 11.

When the absorbing range is for example in the visible range (about

350nm to 700nm), the nanoparticles 13 are for example metallic nanoparticles, made for instance of silver and/or of gold. Aluminum, copper, titanium, chromium or platinum may also be used. In the visible absorption range, the host material 15 may be glass, quartz or transparent ceramic material. In other applications than for CSP plants, where the absorbing range is for example in the infrared range (larger than 800nm up to 30μηι), the nanoparticles 13 may be polarized polar nanoparticles, for instance made of materials supporting surface phonon- polaritons in the operating frequency range of the absorbers. For example SiC particles (resonance around 10 microns) could be used in the mid- infrared.

In this case, the host material 15 may be for example a dielectric like Si02.

The nanoparticles 13 may be only out of one type of material.

In another embodiment, nanoparticles 13 are at least of a first material, for example gold, and a second material different from the first material, for example silver. Examples with three or four types of different materials may also be envisaged.

In a further embodiment the nanoparticles 13 may comprise nanoparticles of a first mean size and nanoparticles of a second mean size, different from the first mean size.

The mean radius of said nanoparticles 13 may be less than lOOnm. Furthermore, the nanoparticles 13 are for example of spherical shape, or nearly spherical shape.

The material, size and shape of the nanoparticles 13 are chosen in order exhibit plasmonic resonances within the absorbing range of the light absorber, meaning that they exhibit plasmonic resonances as isolated nanoparticle.

In general, for an absorber operating in the wavelength range [λ^ ; λ max ], the size (radius) of nanop ^A articles is typically between λ mm /50 and λ mm .

The dispersion of the nanoparticles 13 in the host material 15 is controlled in such a way that the mean distance between said nanoparticles is lower than the wavelengths of light in the absorbing range of the light absorber for generating near field radiation interactions between said nanoparticles 13. In other words, the mean distance between nanoparticles is smaller than the attenuation length of surface modes supported by them.

The absorber may be obtained in a chemical way, with a coating of specialized molecules on the particles to control the distance between the nanoparticles 13. Another one is to put nanoparticles in a suspension and use a sol-gel deposition. It is also possible to deposit particles by CVD (chemical vapor deposition) with the help of chemical precursor or by laser ablation.

Through the interactions between the nanoparticles 13 (near field - strong interaction ; coupling of evanescent wave), these nanoparticles absorb at least an important part of the visible spectrum while not radiating much in the infrared range, which highly increases efficiency of the absorber, in particular for an application in a CSP plant.

Contrary to known multilayer structures already in use for selective absorbers operating at medium temperature (T<500 ^° C), the nanoparticles 13 in the light absorber 11 are isolated from each other and from air by the host material 15, which avoids chemical interactions promoted at high temperature. There is no thin film interface that delaminates under operating conditions. Thus the absorber according to the invention may be used at very high temperatures even up to lOOO C.

Contrary to the absorbers already studied in the literature, the light absorber according to the invention creates and benefits from interaction modes between the nanoparticles 13 ("dressed absorption" concept). This allows the creation of multi resonant modes, promoting the absorption over a wide - yet controlled - range of wavelengths.

The collective interactions between the nanoparticles 13 allow to maintain a strong level of absorption even in diluted lattices where the filling factor f is below 3%. Indeed, the absorption cross section of the nanoparticles 13 is strongly enhanced by the presence of neighborhood nanoparticles 13 and their interactions. Through the control of the material, the size and the distribution of the nanoparticles, a cutting wavelength also known as cutoff wavelength by the man skilled in the art, may be obtained. The cutting wavelength or cutoff wavelength may be defined as a wavelength where light of higher wavelength than the cutting wavelength is less absorbed than light at shorter wavelengths with respect to the cutting wavelength (cutoff wavelength) and the absorption coefficient decreases from the cutting wavelength (cutoff wavelength) to longer wavelengths.

With a proper selection of materials, the invention can so be used at very high temperature.

Some examples will be presented hereafter on the basis of a unitary cell of an n-ary lattice. Such a unitary cell has for example a 200nm respectively in x and y direction and 450nm in z direction. An example of a light absorber according the invention would be obtained through two- dimensional paving of the unitary cells.

In the examples, it has been chosen to have one gold nanoparticle 13- Au and one silver nanoparticle 13-Ag with a radius between 10 to 80nm and a minimal distance of at least 5nm between both nanoparticles. The nanoparticles 13 are immersed in a transparent material of refractive index n= 1,5.

Example 1 :

Figures 3 and 4 show an example where a cutting wavelength (cutoff wavelength) of 450nm is projected. Figure 3 shows the unitary cell as such and figure 4 shows a top view. One can see one gold nanoparticle 13-Au and one silver nanoparticle 13-Ag. 13-Au-N and 13-Ag-N designate a gold respectively silver nanoparticle having its center in a neighboring unitary cell, but extending in the unitary cell shown. In this example, the position of the gold nanoparticle of 32nm radius is (x=51nm; y=0nm; z=217nm) and the position of the silver nanoparticle of 60nm radius is (x=165nm; y=125nm; z=229nm).

As can be seen in figure 5 showing the absorption coefficient in function of the wavelength of a n-ary lattice having a unitary cell as shown in figures 3 and 4, a cutting wavelength is obtained at 450nm.

Example 2 : Figures 6 and 7 show an example where a cutting wavelength (cutoff wavelength) of 500nm is projected. Figure 6 shows the unitary cell as such and figure 7 shows a a top view One can see one gold nanoparticle 13-Au and one silver nanoparticle 13-Ag. 13-Au-N and 13-Ag-N designate a gold respectively silver nanoparticle having its center in a neighboring unitary cell, but extending in the unitary cell shown.

In this example, the position of the gold nanoparticle of 63nm radius is (x=168nm; y=177nm; z=197nm) and the position of the silver nanoparticle of 47nm radius is (x=80nm; y=69nm; z=241nm).

As can be seen in figure 8 showing the absorption coefficient in function of the wavelength of a n-ary lattice having a unitary cell as shown in figures 6 and 7, a cutting wavelength (cutoff wavelength) is obtained at 500nm. It can be seen that the absorption coefficient is quite high (above 80% for wavelengths less than 500nm) and nearly constant. Example 3 :

Figures 9 and 10 show an example where a cutting wavelength (cutoff wavelength) of 550nm is projected. Figure 9 shows the unitary cell as such and figure 10 shows a top view. One can see one gold nanoparticle 13-Au and one silver nanoparticle 13-Ag. 13-Au-N and 13-Ag-N designate a gold respectively silver nanoparticle having its center in a neighboring unitary cell, but extending in the unitary cell shown.

In this example, the position of the gold nanoparticle of 71nm radius is (x=81nm; y=54nm; z=257nm) and the position of the silver nanoparticle of 22nm radius is (x=0nm; y=0nm; z=205nm).

As can be seen in figure 11 showing the absorption coefficient in function of the wavelength of an n-ary lattice having a unitary cell as shown in figures 9 and 10, a cutting wavelength (cutoff wavelength) is obtained at 550nm. It can be seen that the absorption coefficient is quite high (above 80% for wavelengths less than 550nm) and nearly constant. In addition, the decrease of the absorption coefficient beyond 550nm is quite more pronounced.

On may understand that the light absorber according to the invention allows a quite efficient absorption in the projected absorption range and may be used at very high temperatures. Numerical simulations have shown that even light perturbation from a « perfect » lattice order with the unitary cells do only slightly impact the absorption curves shown in the examples above.