WO2014136031A1

WO2014136031A1 - Optical concentrator

Info

Publication number: WO2014136031A1
Application number: PCT/IB2014/059382
Authority: WO
Inventors: Aldo Righetti; Francesco Morichetti; Andrea Annoni; Maria Chiara Ubaldi; Giorgio Grasso
Original assignee: FONDAZIONE CENTRO INTERNAZIONALE DELLA FOTONICA PER ENERGIA
Current assignee: FONDAZIONE CENTRO INTERNAZIONALE DELLA FOTONICA PER ENERGIA
Priority date: 2013-03-04
Filing date: 2014-03-03
Publication date: 2014-09-12
Anticipated expiration: 2015-09-04
Also published as: ITMI20130317A1; EP2965365A1

Abstract

System (10) and photovoltaic panel (100) comprising a photovoltaic cell and a photovoltaic solar concentrator (1 ) comprising a reflective system (6) and a refractive system (7), the reflective system comprising two mutually facing half-portions and having concavities directed towards each other, the refractive system comprising a lower portion supporting an upper portion, wherein minimum width of the lower portion is greater than the maximum width of the upper portion. The photovoltaic cell (30) lies on the lower side of the lower portion (15) of the refractive system. The sectional profile of the upper portion has an aspect ratio less than or equal to 0.43.

Description

DESCRIPTION

"OPTICAL CONCENTRATOR"

The present invention is situated in the field of optical concentrators, in particular photovoltaic solar concentrators, more particularly in the field of photovoltaic concentration systems, still more particularly with low concentration.

Photovoltaic concentration systems are known in which the solar radiation is concentrated by means of an optical system on the photovoltaic material, in order to substitute part of the high-cost photovoltaic material with an optics that can be made with low-cost technology. The geometric gain (G) of the optical concentration systems, defined as the ratio between the area of the inlet opening of the solar radiation and the area occupied by the photovoltaic material on which the radiation is concentrated, can range from values comprised between approximately 1 and 10, for which one refers to low concentration, up to values of 100 and above (high concentration), passing through intermediate values comprised between approximately 10 and 100 (medium concentration). Generally, the low concentration systems, unlike the medium or high concentration systems, have the advantage of not requiring a sun follower system and allow the collection of a significant fraction of the diffused light. Such low concentration photovoltaic systems have good performances all year round, even when installed in a static manner (i.e. fixed) or quasi-static manner (i.e. with few, preferably only two, adjustments of their orientation during the course of the year).

The patent document AU-A-70929/87 and the article "Prism-coupled compound parabola: a new ideal and optimal solar concentrator", Ian R. Edmonds, OPTICS LETTERS, Vol. 1 1 , No. 8, August 1986 - full reference is made herein to the content of these documents - describe a two- dimensional photovoltaic solar concentrator (2-D) with low concentration comprising a pair of parabolic reflectors mirroring each other, each parabola, in orthogonal section, having axis parallel to the direction of incidence of the radiation incident at the predefined acceptance angle and focus on the apex of a refractive prism positioned between the two reflectors and having base on the optical output of the concentrator (at which the photovoltaic cells are positioned). This structure, according to the author of the aforesaid documents, allows the collection on the photovoltaic cells of all of the radiation incident within the acceptance angle.

The Applicant has found that the known optical concentrators, including the aforesaid concentrator, do not lack drawbacks and can be improved with regard to various aspects.

In general, the Applicant has found that the known optical concentrators are characterized by an unsatisfactory concentration efficiency (see below) and/or by high bulk, in particular the height along the axis of symmetry (see below), and/or by a complex manufacturing and/or installation and/or by a high manufacturing cost.

In particular, the Applicant has found that the known concentrators comprising a reflective system and a refractive system (including that described above by Edmonds) lead to complications in manufacturing a photovoltaic panel in which a plurality of 2-D concentrators are arranged adjacent and parallel to each other, since they require that each refractive system is separately coupled to the respective reflective system, with consequent increase in terms of production time and costs of the relative panel. In particular, the concentrator described by Edmonds requires that each prism is separately situated at the center of the reflective system. The Applicant has realized that in order to evaluate the performances of a concentration optics, an important parameter is the optical concentration efficiency (Pr), defined as the ratio between the optical power that reaches the optical output of the concentrator (and hence the photovoltaic cell) and the total optical power incident on the optical input of the concentrator. The value assumed by the parameter Pr is an indicator of the optical losses of the concentrator (Pr = 1 in the case of ideal concentrators without losses) and constitutes a parameter extremely important for determining the reduction of the quantity of the electrical energy produced by the photovoltaic material and hence for evaluating the actual economical validity thereof. The concentration C (ratio between the outgoing and incoming light intensity) is given by the following relation C=Pr*G.

The Applicant has found that the known concentrators comprising a reflective system and a refractive system (including that described above by Edmonds), for a given solar concentration factor and a given concentration efficiency (Pr), are characterized by a high value of the aspect ratio (defined as the ratio between the overall height of the concentrator and the width of the optical output). On the other hand, the Applicant has established an industrial standard (or in any case a few industrial standards) is being confirmed with regard to the size and/or weight of the photovoltaic panels without solar concentration and for the relative structural work and installation mode. On the matter, the Applicant has realized that it is advantageous, in terms of cost and ease of manufacturing, to use the same structural work and the same installation modes of the standard panels also for those static or quasi-static with low concentration. In order to allow this, it is however necessary that the concentration panels are similar to the standard ones in terms of size (including height) and weight.

In addition, the Applicant observes that the aforesaid known two- dimensional concentrator described by Edmonds has a plane of symmetry, a main extension along a longitudinal axis belonging to the plane of symmetry and a continuum of sections orthogonal to the longitudinal axis equal to each other along the entire longitudinal extension axis, the section of the plane of symmetry defining an axis of symmetry of the orthogonal section. In such context, the radiation incident on the concentrator is characterized by two incidence angles with respect to the axis of symmetry of the section of the concentrator, taken along two planes orthogonal to each other, where the first angle (hereinbelow indicated with 6_Ns), is taken along the orthogonal section plane (on which the aforesaid parabolas are defined) and the second angle (indicated with θ_Εο) on the plane of symmetry of the 2-D concentrator. More precisely, the actual angle of incidence with respect to the axis of symmetry can be projected on the two aforesaid orthogonal planes. In order to facilitate the exposition, in the present application reference will be made to such projections with the expression first and second incidence angle.

Given that stated above, the Applicant observes that Edmonds considered, for example for the calculation of the concentration efficiency of the aforesaid concentrator, only the first incidence angles 6_Ns, assuming the second incidence angle to be equal to zero. The Applicant has found that in reality, it is opportune to consider also the second incidence angle ΘΕΟ, since when the radiation forms a second non-zero angle ΘΕΟ, the Applicant discovered that the behavior of the concentrator (for incident radiation with a first non-zero angle 6_Ns) diverges from that with θ_Εο equal to zero, with a deterioration of the concentration efficiency that is greater the higher the aforesaid second incidence angle ΘΕΟ (and the higher the aforesaid first angle 6NS)-

The Applicant has realized that in an actual application of a 2-D concentrator, in which the latter is oriented with the longitudinal extension axis (along which the sectional geometry remains constant) parallel to the East-West geographical direction and with the aforesaid orthogonal section parallel to the North-South geographic direction, the solar direction in the course of the day varies its second incidence angle (θ_Εο) up to a maximum interval that ranges from -90° up to +90° (with free horizon).

Therefore, according to the Applicant, the overall efficiency in the course of an entire day of the 2-D concentrator would not be that expected for ΘΕΟΓ equal to zero, except in the ideal case in which the first incidence angle 6_Ns is zero (concentrator oriented in a manner such that the elevation of the axis of symmetry is equal to the elevation of the sun, a condition that can only be verified for at most a few days of the year) . On the contrary, the overall efficiency would typically be less than that expected.

The Applicant has found that in a photovoltaic solar concentrator with reflective and refractive system for static or quasi-static installation, it is possible to obtain a low aspect ratio and a high concentration efficiency, and the same time simplify the production process of a relative photovoltaic panel, configuring the refractive system in the shape of a sheet with a suitable ribbing.

In one aspect, the invention relates to an optical concentrator, in particular a photovoltaic solar concentrator, having a main extension along a longitudinal axis and a section orthogonal to the longitudinal axis (substantially) equal over a continuum of orthogonal sections taken along at least a portion of the longitudinal extension of the concentrator.

The expression "optical concentrator" (hereinbelow also referred to as only "concentrator") also includes, due to the known principle of reciprocity of optical paths, the optical projectors, i.e. the devices in which the optical radiation is projected from an input opening to an output opening having area greater than the input opening.

The concentrator comprises a primary reflective system (hereinbelow also indicated as only reflective system) and a secondary refractive system (hereinbelow also indicated as only refractive system) both longitudinally extended and having a respective profile on the orthogonal section. The primary reflective system defines, at two opposite ends thereof, respectively an optical input and an optical output and comprises two mutually facing half-portions having concavities directed towards each other. The refractive system comprises a lower portion supporting an upper portion. The minimum width of the lower portion is greater than the maximum width of the upper portion. The optical output lies on a lower side of the lower portion, the minimum width of the lower portion being greater than or equal to the width of the optical output. The sectional profile of the upper portion has an aspect ratio (ratio between the height and the maximum width) less than or equal to 0.43, preferably less than or equal to 0.3. The sectional profile of each half-portion of the reflective system is extended from a respective initial point on an upper side of the lower portion and laterally adjacent to or in proximity to the upper portion up to an end point on the optical input, and comprises preferably a segment substantially shaped according to a parabola.

In the present application, the terms 'height', 'vertical', 'lower', 'upper' and 'surmounted' are defined in relation to a direction lying on the orthogonal section and generically directed according to a low-high sense that extends from the optical output to the optical input.

In addition, in the present application, the surfaces (or in section, the sides) which delimit the portions of the refractive systems can correspond to actual surfaces of separation between different materials (such as the dielectric material / air separation surface) or they can correspond to ideal surfaces being extended inside a continuous dielectric material (as will be clear in the following description, for example from that shown in figure 2 or 7).

According to the Applicant, the aforesaid geometric form of the refractive system having a lower portion surmounted by an upper portion, with the lower portion wider than the upper portion and also wider than the optical output, and the upper portion with an aspect ratio less than or equal to 0.43, and the fact that the sectional profile of each half-portion of the reflective system is extended from a respective initial point on an upper side of the lower portion and laterally adjacent to or in proximity to the upper portion, allow giving structural continuity to the refractive systems of a plurality of concentrators side-by-side in a photovoltaic panel, while collecting sufficient light radiation at the optical output (and hence on the photovoltaic cell). Without wishing to be tied to any theory, the Applicant deems that a comparative refractive system composed of a prism with triangular section, of known type, above a lower portion such as that of the present invention, due to the excess divergence of the optical beam at the base of the triangle, would determine an unsatisfactory collection of radiation on the optical output, after the propagation through the lower portion.

The Applicant has also discovered that the aforesaid structure of the concentrator allows obtaining a low aspect ratio of the concentrator and at the same time a high concentration efficiency, at least at values of the geometric gain comprised between about 3 and 5, for a static or quasi- static installation.

Preferably, the concentrator has a plane of symmetry comprising the longitudinal axis, and an axis of symmetry defined by said plane of symmetry on the orthogonal section. Preferably the upper and lower portions of the refractive system are centered on the axis of symmetry.

Preferably the two half-portions of the reflective system are arranged mirrored with respect to the plane of symmetry.

Preferably the sectional profile of the lower portion of the refractive system is rectangular-shaped, the optical output lying on the lower longer side of the rectangle and the lower side of the upper portion of the refractive system lying on the upper longer side of the rectangle.

Preferably the sectional profile of the upper portion of the refractive system is convex-shaped with upwardly directed convexity, more preferably it is trapezoid-shaped (typically isosceles trapezoid).

Preferably the longer base of the trapezoid lies on the upper longer side of the rectangle. The sectional profile of each half-portion of the reflective system is extended from a point on, or in proximity to, the respective vertex at the base of the trapezoid.

Preferably the internal angle at the base of the trapezoid (indicated with Qb) is greater than or equal to 30°, preferably greater than or equal to 35°, and/or less than or equal to 50°, preferably less than or equal to 45°.

Preferably the width (w) of the optical output (coinciding with the width of the photovoltaic cell) is greater than or equal to 3mm, preferably greater than or equal to 5mm, and/or less than or equal to 20mm, preferably less than or equal to 15mm, for example 10mm.

Preferably the width of the (upper) shorter base of the trapezoid is greater than or equal to 0.3w, preferably greater than or equal to 0.4w, and/or less than or equal to 0.7w, preferably less than or equal to 0.6w, for example 0.5w.

Preferably the height of the trapezoid is greater than or equal to 0.2w, preferably greater than or equal to 0.25w, and/or less than or equal to 0.4w, preferably less than or equal to 0.35w, for example 0.31 w.

Preferably the height of the rectangle is greater than or equal to 0.2w, preferably greater than or equal to 0.25w, and/or less than or equal to 0.4w, preferably less than or equal to 0.35w, for example 0.3w.

Preferably the height of the rectangle is greater than or equal to about 2mm, preferably greater than or equal to 2.5mm, and/or less than or equal to 4 mm, preferably less than or equal to 3.5mm, for example 3mm. In such a manner, advantageously, sufficient strength is ensured for the base sheet of the photovoltaic panel without giving it excessive weight.

Preferably the width of the longer sides of the rectangle is greater than or equal to 2w, preferably greater than or equal to 3w, and/or less than or equal to 6w, preferably less than or equal to 5w, for example 4w (such width typically being equal to Gw).

The Applicant deems that a technical equivalent to the refractive system of the present invention can be each refractive system having as sectional profile not the aforesaid trapezoid and/or rectangle, but rather any form (for example for the trapezoid a cylindrical lens with upward convexity) in which each point of its edge lies less than 10% of the width of the optical output (w) away from the corresponding point of the trapezoid and/or, rectangle, respectively.

Preferably, the sectional profile of each half-portion of the reflective system is substantially shaped according to a parabola. Preferably, the sectional profile of each half-portion of the reflective system lies in the portion of space defined on the lower part by a lower limit parabola obtained by horizontally translating an optimal parabola having equation h(x) = 0.84(x)² + 1 .15(x) by a value D⁺ = 0.1 (1 +x) and on the upper part by an upper limit parabola obtained by horizontally translating the optimal parabola by a value D^" = -0.2x, x and h being respectively the horizontal and vertical coordinates of a Cartesian system starting at a point of connection between the sectional profiles of the upper portion and lower portion, and normalized to the width (w) of the optical output. Such point of connection lies on an upper side of the sectional profile of the lower portion (typically in the aforesaid base vertex of the trapezoid).

Preferably the aspect ratio of the concentrator is greater than or equal to 3 and/or less than or equal to 5.

Preferably the overall height of the concentrator is less than or equal to 200mm, preferably less than or equal to 100 mm.

Preferably the ratio G (geometric gain) between the area of the optical input (or the sectional width thereof) and the area of the optical output (or the sectional width thereof) is greater than 2, preferably greater than or equal to 3, and/or less than or equal to 6, preferably less than or equal to 5.

Preferably the concentrator comprises a convergent lens arranged above the optical input and having width substantially equal to the width of the optical input. According to the Applicant, the lens advantageously contributes to further reducing the aspect ratio.

Preferably the lens comprises a cylindrical lens with axis along the longitudinal axis and width substantially equal to the width of the optical input and having radius on the orthogonal section greater than or equal to 6w, preferably greater than or equal to 7w, and/or less than or equal to 10w, preferably less than or equal to 9w, for example 8w.

Preferably the lens further comprises a portion arranged with continuity above the cylindrical lens and having on the orthogonal section a horizontal rectangle shape with width substantially equal to the width of the optical input. According to the Applicant, the rectangular portion confers structural strength to the lens and advantageously allows, in a multi-concentrator photovoltaic panel, integrating the plurality of lenses in a single continuous sheet.

Preferably the sectional profile of each half-portion of the reflective system at the end point on the optical input is substantially rounded along an arc of a circle having radius d less than or equal to 2mm. According to the Applicant, the synergistic effect of the lens and of the rounding allows an optimization of the concentration efficiency, since it reduces the radiation loss on the end point of the reflective system.

Preferably the distance along the vertical direction between the end point of the reflective system and the lens is greater than zero, more preferably it depends on the aforesaid curvature radius, said distance being such that the tangent to the aforesaid arc of a circle at the point of connection coincides with the tangent to the parabola at the same point of connection. Preferably the secondary refractive system and/or the lens is/are made of glass with silica base, such as silica oxide or borosilicates preferably with low iron content, or made of polymer material such as PolyMethylMethAcrylate (PMMA), Polycarbonate, Cyclo-Polyolefin, etc. Preferably the refractive system and/or the lens is/are made by means of roller molding or injection molding or by casting or via extrusion. Preferably the refractive system and/or the lens is/are externally covered with an anti- reflection layer (in order to reduce the radiation reflected at the dielectric material / air interface to below 1 %).

Preferably the refraction index (n₂) of the refractive system is greater than or equal to 1 .3, more preferably greater than or equal to about 1 .5. Preferably, the refraction index (n₂) of the refractive system is less than or equal to 2.5.

Preferably the primary reflective system defines an internal space within which the upper portion of the secondary refractive system (and not the lower portion) is situated and more preferably the portion of the internal space left free of the secondary refractive system is filled with air.

In an alternative embodiment, the aforesaid portion of the internal space left free of the upper portion of the secondary refractive system is filled with a dielectric material having refraction index (n-i) less than that of the refractive system. In such a manner, advantageously, the actual concentration factor on the section plane (first angle 6_Ns) is increased and/or the concentration efficiency (following the 'recovery' of the second incidence angle ΘΕΟ on the plane of symmetry) is increased. Preferably the refraction index (n-i) of the dielectric material in the aforesaid portion of the internal space is less than or equal to 1 .6.

Preferably the orthogonal section is (substantially) equal over a continuum of sections taken along substantially the entire longitudinal extension (except possibly for the two longitudinal ends) of the concentrator.

Preferably the refractive system comprises a non-linear optical material (or fluorescent) adapted to achieve an optical wavelength conversion (thus increasing the energy conversion efficiency of the solar light). Preferably, the refractive system comprises polymer materials doped with florescent materials such as organic molecules or nano-particles capable of absorbing two low-energy infrared photons, emitting one of these with high energy (or generally greater) absorbable by the photovoltaic cell (up- conversion) and/or absorb a high-energy photon, emitting two of these with intermediate energy absorbable by the photovoltaic cell (down- conversion).

Preferably the primary reflective system is made of plastic (polymer) material covered with a thin (film) layer made of reflective material such as aluminum, silver or alloys thereof. Preferably the reflective system is obtained via molding of plastic material laminates. In an alternative and preferred embodiment, the primary reflective system is a metal plate suitably shaped and covered with a film made of reflective material. The reflectivity of the reflective surfaces is preferably greater than 85%, preferably greater than 90%.

In a second aspect, the invention relates to a photovoltaic system comprising the optical concentrator in accordance with the first aspect (in the various embodiments) of the present invention and a photovoltaic cell optically coupled to the optical output, preferably arranged at the optical output (e.g. the width of the optical output coincides with the width of the photovoltaic cell), more preferably fixed (for example with acrylic glue with ultraviolet cross-linking) to a lower surface of the lower portion of the refractive system (e.g. of the rectangle).

Preferably the photovoltaic cell is made of silicon (e.g. mono- or poly- crystalline), preferably with the ohmic contacts all on the same side ("back contacted"), and/or where the silicon is surface treated in a manner so as to cancel the reflected light and enlarge the absorption spectrum ("black silicon"). Alternatively, the photovoltaic cell is made of gallium and indium nitride, and/or of the type with double or triple junction, and/or with semi- transparent thin films such as CdTe or CIGS, etc.

Preferably, the photovoltaic system can comprise a thermal dissipater associated with the photovoltaic cell on the side opposite the refractive system, in order to limit the operating temperature of the cell itself, and consequently to maintain its conversion efficiency at high levels.

In a third aspect, the invention relates to a photovoltaic panel comprising a plurality of photovoltaic systems according to the second aspect (in the various embodiments) of the present invention, arranged side-by-side and with the longitudinal extension axis parallel to each other, in a manner such that each photovoltaic system is in contact with the adjacent system (s).

Preferably each (e.g. the first and the second) half-portion of each reflective system (excluding those at the transverse ends of the panel) has the end edge at the optical input proximal and in structural continuity (or coinciding) with the end edge at the optical input of a corresponding (i.e. the second and the first, respectively) adjacent half-portion of an adjacent reflective system. Preferably the panel comprises a plurality of metal plates (for example made of aluminum or an alloy thereof), preferably covered as described above, each suitably bent in order to obtain, in a single body, the two aforesaid continuous half-portions of two adjacent different reflective systems. Preferably the panel comprises, on its lower part, a first sheet made in a single piece and made of dielectric material (for example as described above), suitably shaped in order to obtain in a single body the aforesaid plurality of refractive systems parallel to each other, wherein the plurality of photovoltaic cells is mechanically fixed to a lower surface of said sheet.

Preferably a sheet made of EVA (ethylvinylacetate) is interposed between the photovoltaic cells and the refractive system, in order to obtain the mechanical coupling via heat welding between the respective elements. Preferably the panel comprises on its upper part a second sheet made in a single piece and made of dielectric material (for example as described above), suitably shaped in order to obtain, in a single body, the aforesaid plurality of lenses parallel to each other.

Advantageously one such structure is capable of reducing the manufacturing costs and/or installation costs of the panel.

In a fourth aspect, the invention relates to a method for producing a photovoltaic panel according to the third aspect of the invention, comprising:

- arranging the first sheet;

- fixing, for example with acrylic glue with ultraviolet cross-linking, the plurality of photovoltaic cells (each possibly comprising a printed electrical connection board, Printed Circuit Board or PCB) to the lower surface of the first sheet;

- bending, preferably under cold conditions, the plurality of metal plates in a manner so as to obtain the plurality of reflective systems; - fixing, for example with an acrylic glue with ultraviolet cross-linking, the plurality of metal plates to the upper surface of the first sheet, placing the lower end of the sectional profile of each half-portion of the reflective system at the respective vertex at the base of the trapezoid of the refractive system;

- associating, with the first sheet, a perimeter frame (for example made of metal);

- mechanically fixing the second sheet to the perimeter frame.

In a fifth aspect, the invention relates to a method for installing and/or operating a photovoltaic panel according to the fourth aspect of the present invention, wherein the panel is oriented in a manner such that the longitudinal axis is arranged along the EAST-WEST geographic direction. In one embodiment ('static installation') the aforesaid method of installation and/or operation comprises orienting the panel (preferably only during the installation, without further subsequent orientation interventions), in a manner such that the axis of symmetry has an elevation (angle formed with the horizon) substantially equal to that of the sun during equinox days.

In an alternative embodiment ('quasi-static installation'), the aforesaid method for installing and/or operating comprises orienting the panel two times (preferably, but not exclusively, only two times) during a solar year, wherein the first time the axis of symmetry of the concentrator assumes an elevation (angle formed with the horizon) comprised between (preferably equal to the intermediate value between) the elevation of the sun during equinox days and the elevation of the sun on the day of the summer solstice, and wherein the second time the axis of symmetry assumes an elevation comprised between (preferably equal to the intermediate value between) the elevation of the sun during equinox days and the elevation of the sun on the day of the winter solstice. Preferably the first orientation is executed on a day that falls within a month of the spring equinox and the second orientation is executed on a day that falls within a month of the autumn equinox.

Further characteristics and advantages will be clearer from the detailed description of several exemplifying but not exclusive embodiments of an optical concentrator, a photovoltaic system, a photovoltaic panel, a production method and a method of installation and/or operation of a photovoltaic panel in accordance with the present invention. Such description will be set forth hereinbelow with reference to the set of drawings, provided only as a non-limiting example, in which:

- figure 1 shows a schematic perspective view of a possible embodiment of an optical concentrator in accordance with the present invention, with several parts transparent;

- figure 2 shows a schematic orthogonal section of a photovoltaic system containing the concentrator in accordance with the present invention;

- figures 3a and 3b show a schematic and partial orthogonal section of an optical concentrator in accordance with the present invention, in which the paths of the rays of the light radiation are shown, numerically calculated for a value of the first 6NS respectively equal to 0° and 1 2.5° and of the second incidence angle ΘΕΟ equal to 0°;

- figures 4a and 4b show a schematic and partial orthogonal section of an optical concentrator in accordance with the present invention, in which the paths of the rays of the light radiation are shown, numerically calculated for a value of the first 6NS respectively equal to 0° and 1 2.5° and of the second incidence angle θ_Εο equal to 58°;

- figure 5 schematically shows the sectional profile of the half-portion of the reflective system in accordance with the present invention;

- figure 6 shows the results of a numeric calculation of the concentration efficiency of an optical concentrator in accordance with the present invention for different first and second incidence angles;

- figure 7 shows a schematic and partial orthogonal section of a photovoltaic panel in accordance with the present invention. With reference to the enclosed figures, a concentrator according to the present invention is indicated in its entirety with the reference number 1 , a photovoltaic system according to the present invention is indicated in its entirety with the reference number 10 and a photovoltaic panel according to the present invention is indicated in its entirety with the reference number 100. Generally, the same reference number is used for the same elements (and for their sections), possibly in their modified embodiments. The concentrator 1 typically has a plane of symmetry 2, a main extension along a longitudinal axis 3 belonging to the plane of symmetry and a section taken along a plane 4 (shown in fig. 1 by way of example) orthogonal to the longitudinal axis. The section of the plane of symmetry 2 defines an axis of symmetry 5 of the orthogonal section.

With the term section it is intended the result of the intersection of an element, e.g. the concentrator, with a plane, in terms of two-dimensional profiles or shapes of the components of the element.

It is observed that within the refractive systems shown in the figures, several lines are traced for the purpose of identifying the different portions thereof or the single refractive systems (such as for example in fig.7). Nevertheless, such lines of demarcation do not necessarily correspond (and typically do not correspond, since they are refractive systems or sheets made in a single piece) to any actual surface of separation between two distinct refractive elements.

As already described above, the radiation incident on the concentrator has an actual incidence angle with respect to the axis of symmetry that can be conventionally projected on two planes that are orthogonal to each other, in a manner so as to define a first and a second incidence angle with respect to the axis of symmetry, wherein the first incidence angle (6NS) is taken along the section plane 4 and the second incidence angle (θ_Εο) is taken along the plane of symmetry 2.

The orthogonal section of the concentrator is (substantially) equal over a continuum of orthogonal sections taken along at least a portion of the longitudinal extension of the concentrator (preferably a substantial portion, for example a longitudinally central portion that is extended over at least 75% or 90% of the overall longitudinal extension of the concentrator).

The concentrator comprises a primary reflective system 6 and a secondary refractive system 7 both longitudinally extended and having a respective profile on the orthogonal section (for example shown in fig. 2). The primary reflective system 6 defines, at two opposite ends thereof along the axis of symmetry 5, respectively an optical input 8 and an optical output 9 both arranged orthogonal to the axis of symmetry 5. The primary reflective system 6 defines an internal space 14, where for example the portion of the internal space left free of the refractive system 7 is filled with air.

Typically the reflective system 6 comprises two half-portions arranged mirrored with respect to the plane of symmetry 2 and having concavity directed towards the axis of symmetry.

Preferably the refractive system comprises a lower portion 1 1 , typically having sectional profile with rectangle shape and an upper portion 12, typically having sectional profile with trapezoid shape, both centered on the axis of symmetry 5, the longer sides of the rectangle being orthogonal to the axis of symmetry, the longer base 13 of the trapezoid lying on the upper longer side 14 of the rectangle (in other words, the rectangle and trapezoid are continuous) and the optical output 9 lying on the lower longer side 15 of the rectangle. Preferably the longer side 14, 15 of the rectangle is longer than both the longer base 13 of the trapezoid and the optical output 9 and typically equal to the width of the optical input 8.

Preferably the sectional profile of each half-portion of the reflective system is extended from an initial point 16 on, or in proximity (see fig. 5) to the respective vertex at the base of the trapezoid up to an end point 17 on the optical input, and comprises preferably a segment 18 substantially shaped according to a parabola.

Although the embodiments described exemplifyingly show the profile of the half-portion of the reflective system in the form of an ideal geometric parabola, nevertheless the present invention also considers the case in which such second part is constituted by multiple substantially rectilinear segments which approximate an ideal parabola segment (for example three or more substantially rectilinear segments).

Preferably the sectional profile of each half-portion of the reflective system at the end point 17 on the optical input is substantially rounded along an arc of a circle.

Preferably the concentrator comprises a convergent lens 20 arranged above the optical input and having width substantially equal to the width of the optical input. Preferably the convergent lens 20 comprises a cylindrical lens 21 with axis along the longitudinal axis and width substantially equal to the width of the optical input and also a rectangular portion 22 arranged with continuity above the cylindrical lens and with width substantially equal to the width of the optical input.

In the exemplifying embodiment of figure 1 , the orthogonal section is (substantially) equal over a continuum of sections taken along substantially the entire longitudinal extension (except for the two longitudinal ends) of the concentrator 1 . Preferably each of the two longitudinal ends of the concentrator 1 is closed by a respective flat wall (not shown in figure 1 ), preferably having reflective surface directed towards the internal space 14 (or alternatively a transparent wall).

The photovoltaic system 10 comprises the solar concentrator 1 and a photovoltaic cell 30 optically coupled to the optical output 9. The photovoltaic cell 30 is preferably arranged at the optical output 9, more preferably fixed (for example with acrylic glue with ultraviolet cross-linking) to a lower surface of the rectangle, possibly by means of interposition of a PCB which is directly fixed (for example with cyanoacrylic glue) to the lower surface.

Preferably the photovoltaic system 10 comprises a pair of metal plates 40 (for example made of aluminum or an alloy thereof), preferably covered with a film made of reflective material, suitably bent in order to obtain the primary reflective system 6.

The figures 3a, 3b, 4a, 4b and 6 show the results of numeric simulations conducted with a processing program with tracing of the solar rays on a mathematical optical concentrator model in accordance with the present invention and having sectional profile as in figure 2.

With regard to the figures 3a, 3b, 4a and 4b, the parameters of the optical concentrator are the following: internal angle at the base of the trapezoid (indicated in figure 2 with 6_b) equal to 38°, width of the upper shorter base 19 of the trapezoid equal to 0.5w (where w is the width of the optical output 9, coinciding with the width of the photovoltaic cell 30), the height of the trapezoid is equal to 0.31 w (aspect ratio of the trapezoid equal to 0.2w), the height of the rectangle is equal to 0.3w, the width of the longer sides of the rectangle is equal to G^*w, the sectional profile of each half- portion of the reflective system is shaped according to a parabola having equation h(x) = 0.84(x)² + 1 .15(x), h and x being as defined above and where h has a maximum value equal to 3w, the radius of the cylindrical lens 21 is equal to 8w, the height of the rectangular portion 22 is equal to 0.3W, the refraction index (n₂) of the refractive system and of the lens is equal to 1 .49, the curvature radius of the reflective system at the point 17 is equal to 0.05w, the vertical distance between the end point 17 of the reflective system and the lens 20 is equal to 0.2w. The resulting geometric gain G is equal to 4.

The first incidence angle is equal to 0° for the figures 3a and 4a, and is equal to 12.5° for the figures 3b and 4b, whereas the second incidence angle is equal to 0° for the figures 3a and 3b and is equal to 58° for the figures 4a and 4b. The figures intuitively show how the concentrator in accordance with the present invention has a good performance in terms of capacity of concentration of the radiation on the optical output also at the extreme values of the incidence angles for quasi-static installations. Figure 5 shows the portion of space defined by two parabolas obtained by horizontally translating the optimal parabola 51 , having equation h(x) = 0.84(x)² + 1 .15(x) (x and h being the respectively horizontal and vertical coordinates of a Cartesian system having origin at the aforesaid base vertex of the trapezoid and normalized to the width (w) of the optical output) respectively by a value D⁺ = 0.1 (1 +x) for defining the lower limit parabola 52 and by a value D^" = -0.2x for defining the upper limit parabola 53.

Following simulations, the Applicant established that the concentrators having reflective system lying in said portion of space allow obtaining the desired performances in terms of aspect ratio (from approximately 3 to approximately 4), geometric gain (from approximately 3 to approximately 5) and concentration efficiency.

Figure 6 shows the simulated concentration efficiency Pr (vertical axis) of the exemplifying concentrator described above for different (degree) values of the second incidence angle (horizontal axis) and for different values of the first incidence angle (according to the figure legend).

The Applicant has also conducted a comparison simulation between the concentrator of the present invention described above and a mathematical model of a comparative optical concentrator, of the type described in the abovementioned patent document AU-A-70929/87.

In the following table, the results are reported of the calculation after optimization of the relative geometries, where the second column refers to the comparative model obtained according to the rules of the abovementioned document and having geometric gain and aspect ratio equal to those of the concentrator that is the subject of the present patent.

comparative invention

G 4 4

Aspect ratio 3.9 3.9

Pr_eq 72% 80% Where Pr_eq is the concentration efficiency Pr integrated on second angles ΘΕΟ that vary in the interval ± 58° and on first angles 6NS which vary in the interval ±1 2.5°, wherein in the integration the actual (astronomic) variation is accounted for of the first and second angle over an entire solar year. Such parameter, termed equivalent concentration efficiency, Preq, is deemed to be a good indicator of the actual performance of the concentrator (and of the relative panel) in the course of a full year, when installed in a quasi-static mode (i.e. with only two adjustments per year) with the longitudinal axis parallel to the EAST-WEST direction. Indeed, in such case, in the course of a day, its second incidence angle ranges from -90° to +90° (although the aforesaid narrower interval is considered meaningful for the purposes of production of electric current) and in the course of a half year it varies its first incidence angle from -1 2.5° to +1 2.5°, in the case in which the axis of symmetry has intermediate elevation between that of the sun at the equinoxes and at the solstices. As can be observed, the concentrator of the present model, given the same G and aspect ratio, allows obtaining, in quasi-static mode, a higher concentration efficiency with respect to the aforesaid known concentrator. Figure 7 shows the schematic and partial orthogonal section of an exemplifying photovoltaic panel 1 00 comprising a plurality (in the figure, only three are shown) of photovoltaic systems 1 0 as exemplifyingly described above, arranged side-by-side and with the longitudinal extension axes parallel to each other, in a manner such that each photovoltaic system is in contact with the adjacent system(s).

Preferably the panel comprises a plurality of metal plates 40 (for example made of aluminum or an alloy thereof), each suitably bent in order to obtain, in a single body, the two aforesaid continuous half-portions of two adjacent different reflective systems.

Preferably the panel comprises on its lower part a first sheet 41 made in a single piece and made of dielectric material, suitably shaped in order to obtain, in a single body, the aforesaid plurality of refractive systems parallel to each other, wherein the plurality of photovoltaic cells is mechanically fixed to a lower surface of the sheet.

Preferably the panel comprises, on its upper part, a second sheet 42 made in a single piece and made of dielectric material, suitably shaped in order to obtain, in a single body, the aforesaid plurality of lenses parallel to each other.

The panel 100 described above with reference to figure 7 can be produced according to the following operations:

- arranging the first sheet 41 ;

- bending, preferably under cold conditions, the plurality of metal plates 40 in a manner so as to obtain the plurality of reflective systems;

- fixing, for example with an acrylic glue with ultraviolet cross-linking, the plurality of metal plates to the upper surface of the first sheet, placing the lower end of the sectional profile of each half-portion of the reflective system at the respective vertex at the base of the trapezoid of the refractive system;

- associating, to the first sheet, a perimeter frame (for example made of metal, not shown);

- mechanically fixing the second sheet to the perimeter frame.

Claims

1 . Optical concentrator (1 ) having a main extension along a longitudinal axis (3) and a section (4) orthogonal to the longitudinal axis substantially equal over a continuum of orthogonal sections taken along at least a portion of the longitudinal extension of the concentrator, wherein the concentrator comprises a reflective system (6) and a refractive system (7) both longitudinally extended and having a respective profile on the orthogonal section, wherein the reflective system respectively defines, at two opposite ends thereof, an optical input (8) and an optical output (9) and comprises two mutually facing half-portions having concavities directed towards each other, wherein the refractive system comprises a lower portion (1 1 ) supporting an upper portion (12), wherein the minimum width of the lower portion is greater than the maximum width of the upper portion, wherein the optical output lies on a lower side of the lower portion, the minimum width of the lower portion being greater than or equal to the width of the optical output, wherein the sectional profile of the upper portion has an aspect ratio less than or equal to 0.43 and wherein the sectional profile of each half-portion of the reflective system is extended from a respective initial point (16) on an upper side of the lower portion and laterally adjacent to or in proximity to the upper portion up to an end point (17) on the optical input.

2. Concentrator (1 ) according to the preceding claim, wherein the sectional profile of the lower portion (1 1 ) of the refractive system is rectangular- shaped, the optical output (9) lying on the lower longer side (15) of the rectangle, and wherein the sectional profile of the upper portion (12) of the refractive system is convex-shaped with upwardly directed convexity.

3. Concentrator (1 ) according to claim 1 or 2, wherein the sectional profile of the upper portion is trapezoid-shaped and wherein said initial point (16) of the sectional profile of each half-portion of the reflective system lies on, or in proximity to, the respective vertex at the base of the trapezoid.

4. Concentrator (1 ) according to claims 2 and 3, wherein the trapezoid is an isosceles trapezoid, the longer side of the rectangle (14, 15) is longer than both the longer base of the trapezoid and the optical output, and the internal angle at the base of the trapezoid (6_b) is greater than or equal to 30°, preferably greater than or equal to 35°, and/or less than or equal to 50°, preferably less than or equal to 45°.

5. Concentrator (1 ) according to claims 2 and 3, or according to claim 4, wherein the width (w) of the optical output is greater than or equal to 3 mm and/or less than or equal to 20 mm, the width of the shorter base of the trapezoid is greater than or equal to 0.3w and/or less than or equal to 0.7w, the height of the trapezoid is greater than or equal to 0.2w and/or less than or equal to 0.4w, the height of the rectangle is greater than or equal to 0.2w and/or less than or equal to 0.4w, the height of the rectangle is greater than or equal to about 2 mm and/or less than or equal to 4 mm, the width of the longer sides of the rectangle is greater than or equal to 2w and/or less than or equal to 6w.

6. Concentrator (1 ) according to any one of the preceding claims, wherein the sectional profile of each half-portion of the reflective system comprises a segment (18) substantially shaped according to a parabola, and lies in the portion of space defined on the lower part by a lower limit parabola (52) obtained by horizontally translating an optimal parabola (51 ) having equation h(x) = 0.84(x)² + 1 .15(x) by a value D⁺ = 0.1 (1 +x) and on the upper part by an upper limit parabola (53) obtained by horizontally translating the optimal parabola (51 ) by a value D^" = -0.2x, x and h being the respectively horizontal and vertical coordinates of a Cartesian system starting at a point of connection between the sectional profiles of the upper portion and the lower portion, and normalized to the width (w) of the optical output.

7. Concentrator (1 ) according to any one of the preceding claims, also comprising a convergent lens (20) arranged above the optical input and having width substantially equal to the width of the optical input, the convergent lens (20) comprising a cylindrical lens (21 ) with axis along the longitudinal axis and width substantially equal to the width of the optical input and having radius on the orthogonal section greater than or equal to 6w and/or less than or equal to 10w, and a portion (22) arranged with continuity above the cylindrical lens and having on the orthogonal section a horizontal rectangle shape with width substantially equal to the width of the optical input.

8. Concentrator (1 ) according to any one of the preceding claims, wherein the sectional profile of each half-portion of the reflective system at the end point (17) on the optical input is substantially rounded along an arc of a circle having radius d less than or equal to 2 mm.

9. Photovoltaic panel (100) comprising a plurality of photovoltaic systems (10) each comprising the concentrator (1 ) according to any one of the preceding claims and a photovoltaic cell (30) arranged at the respective optical output (9), preferably fixed to a lower surface of the refractive system, the photovoltaic systems being arranged side-by-side and with the longitudinal extension axes parallel to each other, in a manner such that each photovoltaic system is in contact with the adjacent system(s), wherein each half-portion of each reflective system, excluding those at the transverse ends of the panel, has the end edge (17) proximal and in structural continuity with the end edge (17) of the optical input of a corresponding adjacent half-portion of an adjacent reflective system.

10. Photovoltaic panel (100) according to claim 9, comprising a plurality of metal plates (40), each suitably bent in order to attain, in a single body, the two said continuous half-portions of two adjacent different reflective systems, wherein the panel comprises on its lower part a first sheet (41 ) made in a single piece and made of dielectric material, suitably shaped for attaining in a single body the aforesaid plurality of refractive systems that are parallel to each other, wherein the plurality of photovoltaic cells (30) is mechanically fixed to a lower surface of said sheet, and wherein the panel comprises on its upper part a second sheet (42) made in a single piece and made of dielectric material, suitably shaped for attaining in a single body the aforesaid plurality of lenses that are parallel to each other.