WO2011064311A2

WO2011064311A2 - A luminescent solar concentrator device

Info

Publication number: WO2011064311A2
Application number: PCT/EP2010/068242
Authority: WO
Inventors: Marco Romagnoli; Paola Galli
Original assignee: Marco Romagnoli; Paola Galli
Priority date: 2009-11-27
Filing date: 2010-11-25
Publication date: 2011-06-03
Also published as: ITMI20092101A1; WO2011064311A3; IT1396635B1

Abstract

The present invention relates to a luminescent solar concentrator device comprising a receiving surface adapted to receive the solar radiation and a structure made of photoluminescent material, said photoluminescent material being adapted to absorb the solar radiation and to emit, with a net optical gain that is positive or zero, a light radiation having a spectral content that is different from the absorbed solar radiation. The luminescent solar concentrator device also comprises a transmission surface adapted to transmit the light radiation emitted by said photoluminescent material, said transmission surface being optically associable with at least one photovoltaic cell. There are also provided filtering means, adapted to select the spectral content of the light radiation emitted by said photoluminescent material and to convey the light radiation emitted by said photoluminescent material along a predefined main propagation direction.

Description

"A LUMINESCENT SOLAR CONCENTRATOR DEVICE "

DESCRIPTION

The present invention relates to the field of apparatus for the photovoltaic conversion of solar radiation.

More in particular, the present invention relates to a luminescent solar concentrator device based on the use of photoluminescent material.

It is widely known how fossil or nuclear fuels represent energy sources with noteworthy problems in terms of environmental sustainability and/or procurement.

For this reason, the use of clean and renewable energy sources is considered by many as the only possible solution for sustainable development in the long term.

Photovoltaic conversion systems, based on the use of photovoltaic cells made of semiconductor materials, capable of converting solar radiation into electrical energy, can contribute towards reducing the use of conventional fuels to produce electrical energy.

A noteworthy problem of solar radiation photovoltaic conversion systems consists in the high costs of the electrical energy thus obtainable.

It has been proven how these high costs are mainly due to the industrial manufacturing costs of photovoltaic cells, which require large quantities of semiconductor material and complex treatment procedures of this semiconductor material.

For this reason, the constant need has been felt over the years to develop technological solutions that allow reductions in the cost of the installed capacity (€/W) of photovoltaic conversions systems.

In particular, attempts have been made to use photovoltaic cells in a more efficient manner, in order to reduce the quantity of active semiconductor material required to generate the same amount of electrical power.

Therefore, new production procedures have been introduced to produce photovoltaic cells capable of making better use of the solar spectrum for photovoltaic conversion.

In this sense, a known example is provided by the photovoltaic cells made of semiconductor material with intermediate energy gap, i.e. greater with respect to silicon (Si) but lesser with respect to the energy level corresponding to the wavelength of the solar radiation to be converted.

Technical solutions aimed at increasing the density of the solar radiation in the absorption area of the photovoltaic cells have also been proposed .

In this sphere, an approach conventionally followed was that of providing concentrator apparatus of optical type, for example parabolic mirrors, adapted to focus solar radiation on the surface of the photovoltaic cells.

However, apparatus of this kind requires to track the movement of the sun and are generally characterised by relatively high installation costs.

An alternative approach is that of using luminescent solar concentrator devices based on the use of photoluminescent materials.

These devices, also known as LSC, are generally constituted by a structure comprising photoluminescent material adapted to absorb solar radiation and to emit a light radiation of different wavelength.

Most of the solar radiation thus emitted remains trapped in the photoluminescent material, by total reflection with the outer surfaces thereof and, by means of subsequent internal reflections, can be guided towards a transmission surface, optically associated with the photovoltaic cells.

The photoluminescent materials used in LSC devices comprise, for example, materials comprising organic dyes dispersed in polymer matrices or appropriately doped semiconductor materials, for example Si doped with compounds such as CdSe or ZnSe.

The LSC devices mentioned above have undoubted advantages.

The light radiation emitted by the photoluminescent material advantageously has a greater wavelength and consequently lower energy levels with respect to that of solar radiation.

These energy levels are closer to the band edge of the semiconductor material used in the photovoltaic cells, improving the photovoltaic conversion efficiency thereof.

In LSC devices, the solar radiation absorption efficiency depends only slightly on the orientation of the incident solar radiation. Therefore, it is possible to avoid the use of complex solar tracking systems.

Moreover, LSC devices can be produced with production processes that are relatively simple to implement at industrial level.

However, in LSC devices currently available there are still aspects that require further improvement.

One problem found frequently relates to non-negligible scattering of the light radiation emitted by the photoluminescent material.

To limit this scattering, the surface of the LSC device, not optically associated with the photovoltaic cells, can be coated with layers reflecting the light radiation emitted by the photoluminescent material.

Alternatively, appropriate optical devices can be arranged outside the concentrator device, to correspond with the free surfaces. However, these solutions imply a significant increase in the structural complexity of the LSC device and in the relative industrial production costs.

A further problem of known LSC devices is represented by the significant presence of reabsorption phenomena of the light radiation emitted by the photoluminescent material. This contributes towards considerably limiting the power density of the light radiation that reaches the photovoltaic cells.

LSC devices of know type also have significant limitations with regard to modulation of the spectral range of the light radiation emitted by the photoluminescent material.

This causes difficulties in adapting this spectral content to the intrinsic characteristics of the semiconductor material of the photovoltaic cells.

On the basis of the above, it can be stated that, at the state of the art, there remains the need to develop luminescent solar concentrator devices that allow an increase in the light power density received by the photovoltaic cells, that allow improvement of photovoltaic conversion efficiency, that are easy to produce at industrial level and that are economically competitive with regard to installation and operating costs.

To meet this requirement, the present invention provides a luminescent solar concentrator device according to claim 1 and to the relative dependent claims, as proposed hereunder. In its most general definition, the concentrator device according to the present invention comprises:

a receiving surface adapted to receive the solar radiation; and

a photoluminescent material adapted to absorb the solar radiation and to emit a light radiation having a spectral content that is different from the absorbed solar radiation, with a net optical gain that is positive or zero:

a transmission surface that transmits the light radiation emitted by the photoluminescent material, said transmission surface being optically associable with at least one photovoltaic cell.

According to the invention, the concentrator device comprises spectral and spatial filtering means that select the spectral content of the light radiation emitted by the photoluminescent material and that induce the light radiation emitted by said photoluminescent material to propagate along a predefined main direction, preferably towards said transmission surface of the concentrator device.

The presence of photoluminescent centres capable of emitting/absorbing light radiation with positive or zero optical gain ensures a high power density for the light radiation emitted by the photoluminescent material, which due to the action of the filtering means has a high degree of spatial and spectral confinement.

The light radiation emitted by the photo luminescent material can thus be easily directed towards the photovoltaic cells, with consequent reduction in scattering through the surfaces of the concentrator device.

Moreover, its spectral content can be selected with high resolution, in order to considerably reduce reabsorption phenomena in the photoluminescent material and, simultaneously, to adapt it as best possible to the intrinsic characteristics of the semiconductor material constituting the photovoltaic cells.

It is thus possible to optimise photon collection by the semiconductor material in the photovoltaic cells, simultaneously reducing the occurrence of heating phenomena.

The concentrator device according to the present invention allows high concentration factors of solar radiation to be obtained, considerably reducing the quantity of active semiconductor material required to produce photovoltaic conversion, to generate the same amount of electrical power.

Natural spheres of application for the concentrator device according to the present invention are constituted by photovoltaic conversion apparatus for electrical power production systems or for lighting systems.

However, the concentrator device according to the present invention could be used in photovoltaic conversion apparatus, appropriately designed to be integrated in constructional structures for buildings that have surfaces exposed to solar radiation, such as windows or cladding panels.

Further characteristics and advantages of the concentrator device according to the present invention can be better understood with reference to the description given below and to the accompanying figures, provided purely for illustrative and non-limiting purposes, wherein:

Fig. 1 shows a schematic diagram of the concentrator device according to the present invention; and

Figs. 2A-2B schematically show some simplified energy models relative to possible light radiation absorption/emission processes in the concentrator device according to the present invention; and

Fig. 3 schematically shows the spectral range of the light radiation absorbed and emitted by the photoluminescent material in the concentrator device according to the present invention; and

Figs. 4-13 schematically show some embodiments of the concentrator device according to the present invention; and Fig. 14 schematically shows an example of embodiment of the concentrator device according to the present invention; and

Fig. 15 schematically shows a simplified energy model for the embodiment of Fig. 14. Description of the invention

With reference to the aforesaid figures, the present invention relates to a concentrator device 1 for photovoltaic conversion apparatus.

The concentrator device 1 comprises a main body 100 provided with a receiving surface 11 of the solar radiation LI and a transmission surface 12 of a light radiation L2, having a spectral content that is different from the solar radiation LI .

The transmission surface L2 is optically associable with one or more photovoltaic cells 2. The concentrator device 1 comprises a photoluminescent material adapted to absorb the solar radiation LI and to emit the light radiation L2, with a net optical gain g that is positive or zero.

For reasons of expository clarity, it should be specified that, within the context of the present invention, the term "photoluminescence" refers to the capacity of a material to absorb a light radiation and to emit an electromagnetic radiation, the term "light radiation" refers to an electromagnetic radiation with wavelength between 100 nm and 2000 mn and the term "photoluminescent material" refers to atoms, molecules, ions, semiconductors, polymers or co-polymers that are capable of photoluminescence.

It should also be specified that the term "net optical gain g_h" is intended as the total optical gain offered by the photoluminescent material 13, net of phenomena of scattering, diffusion or the like, or losses of another nature.

Finally it should be specified that the assertion "the photoluminescent material absorbs solar radiation LI and emits light radiation L2, with a net optical gain g that is positive or zero" means that the photoluminescent material 13 behaves from an optical viewpoint respectively as a transparent or beyond transparent material.

In other words, the photoluminescent material 13 emits, in the time unit, a number of photons respectively equal to or greater than the number of photons absorbed, net of phenomena of scattering, diffusion or the like, or losses of another nature.

Preferably, the photoluminescent material 13 is taken to a condition of population inversion by the solar radiation LI absorbed.

In this condition, the population of atoms in excited state in the photoluminescent material 13 exceeds the population of atoms in unexcited state and the net optical gain will undoubtedly be positive. The light radiation absorption/emission process by the photo luminescent material 13 can be based on three main energy levels.

In this case, considering, for evident reasons of descriptive clarity, the simplified energy model of Fig. 2, the light radiation absorption/emission process takes place substantially as described below.

Three main energy levels ,h and I₃, with populations Ni, N₂ and N3 and with energies Ei <

E₂ < E3, respectively, can be identified in the photo luminescent material 13.

Initially, in conditions of thermal equilibrium, the majority of the atoms of the photoluminescent material 13 are in the fundamental state Ii (N₂ ~ N3 ~ 0).

Absorption of the solar radiation LI, by the photoluminescent material 13, excites the atoms from the fundamental state Ii to the upper excited level I₃ (optical pumping, N3 > 0).

Subsequently, with a first non-radiative transition, the atoms are taken to the excited metastable level I₂ (N₂>0).

The atoms that populate the level I₂ can decay spontaneously to the fundamental state Ii, releasing photons with frequency v₂i=(E₂-Ei)/h, where h is the Planck constant (spontaneous emission of solar radiation L2).

Generally, as the level I₂ is metastable, the mean transition time I₂-Ii is greater than the non- radiative mean transition time I₃-I₂.

Therefore, on average the level I₃ is constantly depleted (N3 ~ 0) while the population of the excited intermediate state I₂ increases (N₂>0), until reaching a condition of population inversion, with respect to the fundamental level Ii (N₂>Ni).

In these conditions, there is a high probability that the atoms in the excited intermediate state I₂ will be perturbed by the passage of photons with frequency v₂₁, corresponding to an energy level analogous to that of the energy gap E2-E1.

Following this perturbation, the excited atoms can collapse to the fundamental state Ii inducing the production of further photons, also with frequency v₂₁ (stimulated emission of light radiation L2).

The photons with frequency v₂₁, which perturb the atoms in the excited state I₂, can be generated by spontaneous emission or, in turn, by stimulated emission.

The light radiation absorption/emission process by the photoluminescent material 13 can also be based on four main energy levels, as a function of the type of substances forming the photoluminescent material 13,

In this case, considering, for evident reasons of descriptive clarity and simplicity, the simplified energy model of Fig. 3, the light radiation absorption/emission process takes place substantially as described below.

Four main energy levels Ii, I₂, 13 and I₄, with populations Ni, N₂, N3 and N₄, respectively, and with energies Ei < E₂ < E₃, < E₄, respectively, can be identified in the photoluminescent material 13,

Initially, in conditions of thermal equilibrium, there is a high probability that the atoms of the photoluminescent material 13 will be in the fundamental state Ii (N₂ ~ N3 ~ N₄ ~ 0).

Absorption of solar radiation LI by the photoluminescent material 13 excites the atoms from the fundamental state L to the upper excited level I₄ (optical pumping N₄ > 0).

With a first non-radiative transition I4-I₃, the atoms are taken to the intermediate upper level I₃ which is metastable.

From the upper excited state I₃, the atoms can undergo a radiative transition towards the lower excited state I₂ and a further non-radiative transition from the lower excited level I₂ to the fundamental level L.

Given that the subsequent non-radiative transition I₂-Ii takes place typically in an extremely rapid manner, the presence of excited atoms at the energy level I₃ automatically implies a population inversion with respect to the lower energy level I₂. The transition I₃-I₂ of the excited atoms, and the relative emission of photons, can therefore take place either spontaneously or in a stimulated manner (spontaneous and stimulated emission of light radiation L2).

As described below, the light radiation absorption/emission process, based on four main energy levels, could also involve energy transfer mechanisms between atoms or groups of atoms of the substances forming the photoluminescent material 13.

It can be noted how the light radiation absorption/emission processes, described above, have dynamics similar to that of laser emission processes.

However, unlike these latter, the light radiation absorption/emission process in the photoluminescent material 13 can comprise mechanisms of amplified spontaneous emission and/or of stimulated emission of a coherent and narrow-band light radiation L2, without requiring to exceed a population inversion threshold or provide a resonant cavity.

The light radiation absorption/emission process can also be of superradiant type, above all in the presence of strong solar illumination LI .

This light radiation absorption/emission process also has similar characteristics to a laser emission process (coherence, narrow band), except for the absence of threshold or of resonant cavity.

According to the invention, the concentrator device 1 also comprises spectral and spatial filtering means 14.

The filtering means 14 are adapted to select the spectral content of the light radiation L2, performing narrow-band filtering of the light radiation L2, emitted by the photoluminescent material 13, for example by means of absorption of the unwanted spectral components.

For the light radiation L2, they can thus impose a given emission band and a relative peak wavelength vo.

A first advantage deriving from the use of the filter means 14 consist in the fact that the spectral content of the light radiation L2 can be modulated so as to reduce the occurrence of reabsorption phenomena in the photoluminescent material 13, as shown in Fig. 3.

In Fig. 3, the curves CI and C2 schematically represent respectively the absorption and emission spectrum of the photoluminescent material 13, while the curve C3 represents the spectral content of the light radiation L2.

It is evident how the spectral content of the light radiation L2 can be selected so as to reduce or avoid the overlap region S with the absorption spectrum of the photoluminescent material 13.

The peak wavelength vo of the light radiation L2 can be regulated in such a manner that the difference between the corresponding energy level and the band edge level of the semiconductor material of the photovoltaic cells 2 is greater than a given threshold E_th.

The value of this threshold E_th can be selected as value of compromise between the need to optimise photon collection and the need to limit the occurrence of any heating phenomena, caused by non-radiative energy exchanges.

According to the invention, the filtering means 14 cause the light radiation L2 to propagate according to a main direction Z.

The light radiation L2, filtered by the filtering means 14, can in fact provide the seed and the wavelength for stimulated emission of light radiation L2 by the photoluminescent material 13. The light radiation L2, generated by stimulated emission, is undoubtedly coherent with the stimulating light radiation and will replicate the direction and sense of the radiation seed. In turn, this radiation can provide the seed and the wavelength for a new stimulated emission of light radiation L2 by the photoluminescent material 13.

In substance, the filtering means 14 induce the occurrence of a positive reaction, in the light radiation absorption/emission process by the photoluminescent material 13, the final effect of which is to ensure that the light radiation L2, emitted by the photoluminescent material 13, is conveyed along a predefined main propagation direction coinciding substantially with the direction of reflection imposed by the filtering means 14. It is noted how this positive reaction phenomenon rapidly becomes dominant due to the fact that the absorption/emission process in the photoluminescent material 13 takes place with a net optical gain g that is positive or zero.

By positioning the filtering means 14 appropriately, it is possible to convey the light radiation L2 in the direction of the transmission surface 12, thereby reducing losses through scattering. It is therefore possible to considerably increase the light intensity at the photovoltaic cells 2, without the use of optical reflection devices operatively associated with the free surfaces of the concentrator device 1.

A quantitative evaluation of the effect of spectral and spatial confinement induced by the filtering means 14 is proposed hereunder.

For reasons of descriptive simplicity, a substantially unidimensional model of the concentrator device 1 is used (Fig. 1), wherein:

propagation of the light radiation L2 in the concentrator device 1 takes place according to the main direction Z of propagation;

the filtering means 14 are positioned at a first surface 15 of the body 100 of the concentrator device 1 that is arranged opposite to the transmission surface 12, taking as a reference the main direction (Z) of propagation of the light radiation (L2);

the filtering means 14 have a bandwidth Δν _h with peak wavelength v ₀.

The emission spectrum of the light radiation L2 can be defined as: (v ) = 2-

The total emission intensity along the propagation axis Z, integrated on the entire spectrum of frequencies, can be expressed by the followin relation:

The spontaneous emission intensity for each polarization can be expressed by the following relation:

=— ^π h r v ^AV h -r

s^p 2 A

where A corresponds to the area of the photovoltaic cell 2 and r is the ratio between the angle subtended by the photovoltaic cell 2 and the solid emission angle.

The light intensity, at the photovoltaic cells 2, can thus be expressed as:

I_tot = I_spe^2Shl (1) where 1 indicates the length of the concentrator device 1. If the filtering means 14 were not present, the light intensity, at the photovoltaic cells 2, could be expressed as:

Ι_ω = Ι_ψ (^β'·> - ΐ) (2) where 1 indicates the length of the concentrator device 1.

It can be noted how the main difference between the expressions (1) and (2) consists substantially in the presence of a factor 2 in the exponent of the expression (1).

This factor takes account of the fact that the maximum light intensity obtainable, in the case of the presence of filtering means 14, derives from amplification of the radiation L2 emitted spontaneously in proximity of the transmission surface 12, which initially travels the entire length of the concentrator device 1 until reaching the filtering means 14 and is then forced by these to return towards the transmission surface 12.

Evidently, this cannot happen without the aid of the filtering means 14, in the absence of which the light radiation L2, emitted spontaneously, would naturally be scattered in all directions.

The bandwidth of the light radiation L2 can be expressed as follows:

Therefore, this depends on the value of I_tot which, as seen above, assumes different values depending on whether the filtering means 14 are present.

It is evident how, for finite values of length of the concentrator device 1 , the bandwidth of the light radiation L2 is influenced by the presence of the filtering means 14, even if these latter are positioned at one of the surfaces of the concentrator device 1.

It is also noted how the filtering means 14 make it possible to obtain a light radiation L2 that produces uniform illumination of the photovoltaic cells 2.

This particularity represents a considerable advantage with respect to a laser emission system, using which it would not be possible to obtain uniform illumination due to the effects of simultaneous oscillation on the transverse modes of the light radiation reflected in the resonant cavity.

Description of some embodiments of the invention

According to an embodiment of the present invention, the photo luminescent material 13 could comprise a silica matrix comprising nanocrystalline Si (nc-Si) grains. It has been seen that the presence of nc-Si grains makes it possible to obtain a more extensive absorption spectrum than that of Si.

It has also been seen that it is possible to obtain a relatively large spontaneous emission cross section, with consequent possibility of positive optical gain, in the case of high optical pumping with solar radiation.

This positive optical gain is realised for a wavelength of the light radiation L2 of around 900 nm.

These wavelength values for the light radiation L2 are particularly suitable for crystalline or amorphous Si photovoltaic cells.

In fact, for these values, efficient photon collection can still be achieved by the photovoltaic cell.

Moreover, the energy levels of the photons emitted at these wavelengths are sufficiently close to the band edge, so that the occurrence of heating phenomena in the photovoltaic cells can be limited.

According to another embodiment of the present invention, the photo luminescent material 13 could comprise a silica matrix, doped with Nd³⁺ ions and comprising nc-Si grains.

The spatial proximity of Nd³⁺ ions and Si nanoparticles facilitates the establishment of mechanisms of non-resonant energy transfer from the Si nanoparticle to the Nd³⁺ ion.

This allows the combination of Nd³⁺ ions and Si nanoparticles to constitute a highly efficient light radiation absorption/emission system, with spectral characteristics typical of the Nd³⁺ ions.

The light radiation L2 thus emitted has wavelengths variable between 920 and 1060 nm, whose corresponding energy levels are suitable to ensure optimal photovoltaic conversion by the crystalline or amorphous Si photovoltaic cells.

It has been found that, by modulating the dimension of the nc-Si grains, it is possible to considerably reduce reabsorption phenomena of the light radiation in the photo luminescent material 13, particularly in conditions of population inversion.

The photo luminescent material 13 could comprise other inorganic materials such as a silica matrix, doped with rare earths different from Nd and optionally comprising nc-Si particles. Alternatively, the photo luminescent material 13 could comprise organic compounds, such as a polystyrene matrix comprising an organic dye known as DCJTB, doped with Alq3.

The Alq3:DCJTB:PS mixture behaves in the same way as the light radiation absorption/emission system constituted by a glass matrix, doped with Nd3+ and comprising nanocrystalline Si. The concentrator device 1 can be structured according to the installation requirements of the photovoltaic conversion apparatus in which it is to be arranged.

According to some embodiments (Figs. 1 and 14), the body 100 of the concentrator device 1 can have a substantially sheet-like structure, for example comprising a thin film of photo luminescent material 13 comprised between passive layers 131 of optically transparent material, such as Si0₂.

The body 100 of the concentrator device 1 could be structured substantially as a stack structure 132, for example comprising staggered and mutually overlapped layers of glass matrix, doped with Nd³⁺ ions and comprising nc-Si, and of transparent material, such as Si0₂, (Figs. 10-11).

In a further alternative, the body 100 could simply be constituted by one or more layers of photo luminescent material 13 (Fig. 9).

In the embodiments of Figs. 1, 11 and 14, the reflection surface 12 is found in lateral position with respect to the receiving surface 11 of the solar radiation.

This configuration is advantageous as it allows the dimensions of the surface of the photovoltaic cells 2 to be limited even in the presence of a large receiving surface 11.

Instead, in the embodiments of Figs. 9-10, the reflection surface 12 is located in the opposite position to the receiving surface 11, taking as a reference the main direction of propagation of the light radiation L2.

This configuration has the advantage of ensuring a longer path of the light radiation L2, through the photo luminescent material 13 and therefore of improving the spatial and spectral concentration, induced by the filtering means 14.

As shown in Fig. 4, the filtering means can comprise a Bragg grating 141, distributed along the main direction of propagation of the light radiation L2.

Alternatively, the filtering means could be positioned at or in proximity of the first surface 15, opposite to the transmission surface 12.

In this case, the filtering means can comprise a Bragg grating 142, of reduced length (Fig. 5), a narrow-band selective reflecting element 143, preferably external to the surface 15, (Fig. 6), a narrow-band selective diffusing element 144 (Fig. 7) or a reflecting element 145, preferably external to the surface 15, operatively associated with a narrow-band filtering element 146 (Fig. 8).

The term "reflecting element" or "diffusing element" indicates, in this context, a discrete element or a layer of material that respectively reflects or diffuses in the space the incident light radiation. The term "selective reflecting element" or "selective diffusing element" indicates, in this context, a discrete element or a layer of material that respectively reflects or diffuses in the space only given wavelengths of the incident light radiation.

In the embodiments described in Figs. 9-10, the filtering means are advantageously positioned at or in proximity of the solar radiation receiving surface 11 and advantageously can comprise a narrow-band reflecting element 147.

Instead, in the embodiment shown in Fig. 11, the filtering means are positioned at several surfaces of the concentrator device 1, for example at the receiving surface 1 1 and at a second surface 16 opposite thereto, and advantageously comprise the narrow-band reflecting elements 148-149, positioned at the surfaces 11 and 16.

The light radiation L2 can thus be reflected several times between the surfaces 11 and 16 before reaching the transmission surface 12.

To increase the density of the solar radiation LI, incident on the receiving surface 11, the concentrator device 1 can be operatively associated with optical devices to reflect the solar radiation, such as a lens 51 (Fig.13) or a focusing mirror 52 (Fig. 12).

Discussion of an embodiment of the invention

The example of embodiment of the concentrator device 1A, shown in Fig. 14, is described. The concentrator device 1A has a main body 100A comprising a thin film of photoluminescent material 13 A, comprised between supporting layers 131 A, made of silica. The main body 100A has a length of 20 cm (axis Z), width of 10 cm and depth of ΙΟΟμιη. The layer of photoluminescent material 13A has a depth of ΙΟΟμιη, and is constituted by a glass matrix, doped with Nd³⁺ ions, with concentration between 0.12 and 1.9 at.% (atomic percent), and comprising nc-Si grains with a diameter of 1.2 nm.

The concentrator device 1 comprises a receiving surface 11A of the solar radiation LI and a transmission surface 12A of the light radiation L2, emitted by the photoluminescent material 13 A.

The transmission surface 12 is optically associable with at least one photovoltaic cell 2.

The filtering means of the concentrator device 1 A are constituted by a narrow-band reflecting element 143 A, with peak wavelength centred in the band ranging from 900-1100 nm.

The reflector 143 is constituted by a dielectric multilayer, positioned at the surface 15 A, opposite the transmission surface 12A of the light radiation L2.

Fig. 15 shows a simplified energy model of the layer of photoluminescent material 13 A.

The level A indicates the fundamental energy level of the nc-Si, the energy level B indicates the excitation level of the nc-Si while the conduction band edge of the nc-Si is indicated as energy level C.

The intraband decay time (i.e. between the levels B and C) was evaluated as negligible as it is extremely short, while the radiative decay time between levels C and A was evaluated in the order of TR = Ιμβεΰ.

This radiative decay time is, at ambient temperature, comparable with the non-radiative decay time between C and A (TNR).

By irradiating the receiving surface 11 with sunlight having a wavelength of 476 nm at T = 300°C, excitation of the nc-Si is determined.

A very high percentage of the excitation (almost 100%) is transferred to the Nd³⁺ ions by means of non-resonant energy transfer between the band BC and the level ⁴Fc>_/2 with a transfer time of TET -150 ns.

The fraction of energy transferred from nc-Si to the Nd³⁺ ion depends on the ion concentration and is maximum for an Nd³⁺ ion concentration equal to 0.43 10¹⁸ cm^"3.

Subsequent decays through the levels of the Nd³⁺ ion have no further influence on the conversion efficiency between nc-Si and Nd³⁺ ion.

Decay between the level ⁴Fc>_/2 and the metastable level ⁴F₃/₂ is very fast, while total decay from ⁴F₃/2 to the fundamental level ⁴Ig/2 was evaluated between τ_1ι2 = 5μβ and 30 μβ, as a function of the concentration of Nd³⁺ ions.

The photoluminescence peak of the Si-nc/Nd³⁺ system was evaluated for a wavelength of 920 nm.

From this it is deduced that, in the example considered, it is possible to achieve efficient conversion of the solar radiation LI into infrared radiation in the wavelength range between 900 and l lOO nm.

Use of the reflector 143 A, centred in this wavelength interval or in a sub-interval, facilitates the spectral and spatial concentration of the light radiation L2 to transmit to the photovoltaic cell 2.

In the example considered, the loss of propagation of the light radiation L2 due to scattering phenomena, was evaluated between 0.5 dB/cm and 2 dB/cm.

It was calculated that the optical gain g assumes values greater than 50 cm^"1 for an emission wavelength of 900 nm.

The pumping saturation intensity (parameter necessary to determine the inversion level) was estimated in the interval from 50 ÷ 300 W/cm².

On the basis of the above, it was evaluated that, to obtain a population inversion for a standard solar illumination of 100 mW/cm², it is advisable to use a single layer of photo luminescent material 13A with maximum thickness of around ΙΟμιη.

Vice versa, in the case of sunlight focusing, for example using optical devices with magnification equal to 10⁴ in area, it is also possible to reach population inversion using several active layers 13 A.

The concentrator device according to the invention has considerable advantages with respect to prior art.

Due to the described filtering means, it allows transmission to the photovoltaic cells of a light radiation with selectable spectral content in such a manner as to optimise the photovoltaic conversion efficiency, simultaneously reducing the occurrence of heating phenomena in the photovoltaic cells and the occurrence of reabsorption phenomena in the photo luminescent material.

This allows a reduction in the photovoltaic conversion surface required to generate the same amount of electrical power, with consequent reduction of installation and operating costs of the photovoltaic apparatus.

The same filtering means, by stimulating an anisotropic propagation of the light radiation, advantageously in the direction of the photovoltaic cells, contribute towards considerably reducing losses through scattering.

This allows an increase in the power density of the light radiation that reaches the photovoltaic cells, avoiding or reducing the use of optical devices, operatively associated with the surface of the concentrator device to confine the light radiation inside the photo luminescent material of the concentrator device, consequently simplifying the structure of the photovoltaic apparatus.

As is evident from the description above, the concentrator device according to the invention has an extremely simple structure, easily adaptable to the installation requirements of the photovoltaic apparatus.

It can be easily produced at industrial level using known processes, with considerable advantages in terms of limiting industrial production costs.

On the basis of the description provided, other characteristics, modifications or improvements for the invention thus conceived are possible and evident to those with average skill in the art. These characteristics, modifications and improvements are therefore to be considered part of the present invention.

Claims

1. A luminescent solar concentrator device (1, 1A) characterised in that it comprises:

- a receiving surface (11, 11A) adapted to receive the solar radiation (LI); and

- a photo luminescent material (13, 13 A), adapted to absorb the solar radiation and to emit a light radiation (L2) having a spectral content that is different from the absorbed solar radiation, with a net optical gain (g ) that is positive or zero; and

- a transmission surface (12, 12A) of the light radiation (L2) emitted by said photoluminescent material, said transmission surface being optically associable with at least one photovoltaic cell (2); and

- spectral and spatial filtering means (14, 141, 142,143, 143 A, 144, 15, 146, 147,

148, 149) adapted to select the spectral content of the light radiation (L2) emitted by said photoluminescent material, said filtering means inducing the light radiation (L2) emitted by said photoluminescent material to propagate along a predefined main direction (Z).

2. A luminescent solar concentrator device according to claim 1, characterised in that said filtering means (14) are arranged so as to induce the light radiation (L2) emitted by said photoluminescent material, to propagate towards said transmission surface (12).

3. A luminescent solar concentrator device according to one or more of the preceding claims, characterised in that absorption of the solar radiation (LI) takes said photoluminescent material to a condition of population inversion.

4. A luminescent solar concentrator device according to one or more of the preceding claims, characterised in that said receiving surface (11) is arranged laterally to said transmission surface (12), taking as a reference the main direction (Z) of propagation of the light radiation (L2) emitted by said photoluminescent material.

5. A luminescent solar concentrator device according to one or more of the claims from 1 to 3, characterised in that said receiving surface (11) is arranged opposite to said transmission surface (12), taking as a reference the main direction (Z) of propagation of the light radiation (L2) emitted by said photoluminescent material.

6. A luminescent solar concentrator device according to claim 4, characterised in that said filtering means comprise a Bragg grating (141), distributed along the main direction (Z) of propagation of the light radiation (L2) emitted by said photoluminescent material.

7. A luminescent solar concentrator device according to claim 4, characterised in that said filtering means comprise:

- a Bragg grating (142); or

- a narrow-band reflecting element (143, 143 A); or

- a narrow-band diffusing element (144); or

- a reflecting element (145) operatively associated with a narrow-band filtering element (146);

at a first surface (15) that is arranged opposite to said transmission surface (12), taking as a reference the main direction (Z) of propagation of the light radiation (L2) emitted by said photo luminescent material.

8. A luminescent solar concentrator device according to claim 5, characterised in that said filtering means comprise a narrow-band reflecting element (147), at said receiving surface (11).

9. A luminescent solar concentrator device according to claim 4, characterised in that said filtering means comprise a first narrow-band reflecting element (148), at said receiving surface (11) and a second narrow-band reflecting element (149) at a second surface (16) that is arranged opposite to said receiving surface (11), taking as a reference the main direction (Z) of propagation of the light radiation (L2) emitted by said photoluminescent material.

10. A concentrator device according to one or more of the preceding claims, characterised in that said photoluminescent material comprises:

- silica doped with Nd³⁺ ions; or

- silica comprising nc-Si; or

- silica doped with Nd³⁺ ions and comprising nc-Si; or

- polystyrene comprising a DCJTB dye doped with Alq3.

11. A photovoltaic conversion apparatus characterised in that it comprises at least one photovoltaic cell (2) and a luminescent solar concentrator device (1) according to one or more of the preceding claims, operatively associated with said at least one photovoltaic cell.

12. An electrical power production system or a lighting system or a window or a building cladding panel characterised in that it comprises a photovoltaic conversion apparatus according to claim 11.