DESCRIPTION

[TECHNICAL FIELD]

The present invention relates to a reflective panel, in particular for a concentrated thermal solar plant.

The invention further relates to a concentrated thermal solar plant comprising said panel, as well as to a method for manufacturing said panel.

[PRIOR ART]

In CSP (Concentrated Solar Power) and CST (Concentrated Solar Thermal) plants, solar concentration is achieved by means of an optical system consisting of reflective surfaces moved by suitable drives that allow them to follow the apparent motion of the sun and to direct the solar radiation beams, via the optical reflection mechanism, towards a linear receiver, e.g. linear parabolic troughs and Fresnel reflectors, or towards a punctiform receiver, e.g. solar tower or solar dish systems.

Linear parabolic concentrators are made up of long lines of linear parabolic reflectors that concentrate the solar radiation incident on a linear receiver integral with the parabolic reflector and positioned along the latter' s focal axis. Such linear receiver comprises a duct, referred to as receiver or absorber tube, into which a heat transfer fluid flows, which is heated and transfers heat to a thermoelectric plant for power generation or to another plant directly using this thermal energy .

In Fresnel reflector systems, solar concentration is achieved by means of long parallel rows of reflectors equidistant from the ground, moved by suitable drives. Fresnel reflectors have a flat or slightly curved geometry for better focusing capability. The linear receiver, which is similar to the one used in parabolic reflectors, is fixed and not integral with the optical system that concentrates the solar radiation.

In solar tower systems, an optical system, typically consisting of individually driven flat square mirrors capable of turning about two perpendicular axes, directs the reflected beam of solar radiation towards a punctiform receiver located at a height of several tens of metres.

Like solar tower systems, solar dish systems concentrate the solar radiation towards a punctiform receiver. In this case, however, the punctiform receiver is located on the geometric focus of the optical system and is integral therewith. In solar dish systems, the dish is in some cases made up of a number of smaller reflective elements, mechanically integral with one another and driven by a single biaxial drive.

Notwithstanding the clear differences in the configurations of the various optical systems employed in CSP and CST systems, they must have some fundamental characteristics, such as:

(i) high reflectivity of the reflective surfaces,

(ii) high geometric precision of the reflective surface,

(iii) high mechanical rigidity,

(iv) low weight,

(iv) long life, which must be aligned with the lifetime of CSP and CST systems, i.e. approx. 25 years, and

(v) low cost, since this is one of the system components that most affect the total cost.

The reflective surfaces currently in use can be divided into three main types: glass mirrors (thick or thin), aluminium mirrors, and polymeric mirrors consisting of a thin layer of silver or aluminium coated with polymethylmethacrylate (PMMA) .

Except for thick glass mirrors (4-6 mm), all other surfaces are manufactured as thin, flat and flexible foils having a thickness ranging from some tenths of a millimetre, as in the case of aluminium and polymeric mirrors, to one millimetre, as in the case of thin glass. Such foils require a support for assuming the geometry and rigidity necessary for their use.

When it is necessary that the reflector has a curved geometry, as in the case of parabolic or Fresnel or solar dish reflectors, it must be very accurate, with spatial deviations relative to the ideal geometry of approx. one tenth of a millimetre and angular deviations of approx. one thousandth of a radian. In fact, the efficiency of a concentrated thermal solar power plant as a whole mostly depends on the optical performance of the reflector, i.e. on the capability of effectively concentrating the solar radiation incident on the surface of the receiver.

In order to achieve efficient concentration, it is necessary that the reflective panels are slightly convex relative to one or two axes of rotation; therefore, in the manufacturing process the main difficulty is due to the need to create an extremely accurate curvature rapidly and at a very low cost. Moreover, the reflector must be sufficiently rigid from a structural viewpoint, in order not to jeopardise the optical-geometrical precision in strong-wind conditions or during the movements occurring while tracking the sun.

Another fundamental property of solar reflectors is their life, i.e. the capability of keeping their performance essentially unchanged throughout the lifespan of the solar plant. Finally, since this is a component of a technology intended for energy production from renewable sources, and therefore, more in general, for the paradigm of green economy and environmental sustainability, another characteristic of fundamental importance for reflective panels is their low environmental impact, as concerns both their production and their disposal when their life is over.

The reflective panel, which is the largest and heaviest component of a concentrated thermal solar plant along with the metal structure that supports it, is preferably made from recyclable and/or renewable materials that are safe for people's health and for the environment. Rigid structural members having high-precision curved surfaces can be manufactured by using various techniques, most of which imply, however, slow and costly machining processes, such as material removal by milling on large areas, hot-bending of laminated components, calendering of thick foils, or adjustable beam structures, or milling of large surfaces, followed by lapping or grinding, calendering, pressing, etc. These manufacturing technologies, necessary for fulfilling the required optical precision constraints, however imply slower production and higher production costs.

One effective manufacturing technology for producing reflectors meeting the requirements of mechanical rigidity, geometric precision and low weight is represented by panels made of laminated glass, in which one of the two outer foils is a reflective surface or acts as a support for a thin reflective foil.

A laminated glass structure is made up of two or more superimposed glass sheets, wherein such layers are alternated with inserts filled with solid thermoplastic material. For the multi-layer structure of glass sheets alternated with inserts of thermoplastic material to become a laminated glass sheet, it must be subjected to a vacuum hot-lamination process. Following said process, the thermoplastic material will soften and come into perfect contact with the glass surface, thus simultaneously acting as an adhesive and as a separator between the glass plates. Once brought back to room temperature, the multi-layer glass will look like a single glass sheet, but it will provide amplified thermal, mechanical and acoustic performance. Since the thermoplastic material employed in the glass lamination processes is generally transparent, the resulting laminated glass is often used as a substitute for single thick sheets for safety reasons. When it breaks, laminated glass will not explode into many fragments and splinters; on the contrary, the pieces will hold their initial position, remaining glued to the adhesive plastic matrix.

The thermoplastic materials employed for glass lamination are: polyvinyl butyral (PVB) , ethylene vinyl acetate (EVA) and thermoplastic polyurethane (TPU) .

The above-mentioned materials are thermoplastic polymers formed by linear or slightly ramified chains not bound to one another (i.e. not cross-linked) . The absence of cross-linking implies that, as temperature increases, the thermoplastic materials will soften and take the characteristic aspect and behaviour of highly viscous liquids. When a thermoplastic material is subjected to mechanical stress, it will show an elastic behaviour for small deformations only, whereas when it is subjected to larger deformations it will tend to become permanently deformed. In these materials, the polymer molecules are not bound to one another by chemical bonds, unlike cross- linked polymers (e.g. thermosetting polymers or elastomers), and can move relative to one another. The bonds among the different polymer molecules are effected by intermolecular attraction forces that are weaker than chemical bonds (e.g. van der Waals forces or hydrogen bonds) . As temperature increases, the effort required for deforming a thermoplastic polymer will decrease.

As aforesaid, the reflective panels used in solar concentrators are slightly curved and must keep their curvature unchanged throughout the lifespan of the solar plant. The cold- bending of flat glass sheets implies the application of a force that is balanced by the adhesion forces of the glass - thermoplastic material interface and by the elastic deformation of the thermoplastic polymer. However, such panels will have to operate outdoors and will be subject to the continuous temperature variations of the outside environment, and hence to repeated heating and cooling cycles. During the heating phases, thermoplastic materials will reduce their resistance to mechanical stress and will tend to become permanently deformed. As a consequence, the mechanical balance conditions of the laminated structure will change, with the latter tending to progressively losing its original structural characteristics, i.e. the curvature imposed by the lamination process, and to restore the original flat geometry of the starting materials.

In order to limit the tendency to deformation of the thermoplastic layer, some precautions can be taken, such as the use of thin glass sheets, which, compared to thick glass, will show less resistance to the bending imposed by the curvature of the panel. Another measure is the use of films of thermoplastic material as thin as possible, so that surface adhesion forces will prevail over the forces resisting the elastic deformation of the material, thereby limiting the effects of any plastic deformation on the curvature of the reflective panel. In the long term, however, the repeated heating and cooling cycles may nevertheless alter the curvature of the laminated glass panel, and hence the optical characteristics of its reflective surface, thus substantially affecting the efficiency of the solar radiation concentration system.

Therefore, manufacturing a reflective panel made of laminated glass that possesses at the same time all of the above- mentioned geometric, optical, mechanical, economical and environmental features required by the application is a problem that cannot be easily solved.

Document US 2009/101208 Al describes stiffening members for reflectors used in solar concentration apparatus, and a method for making said stiffening members.

Note that the reflectors described in US 2009/101208 Al require stiffening members whereas, as will become apparent hereinafter, the present invention relates to a reflective panel that, thanks to its very structure, does not need to be associated with any mechanical reinforcement members.

Furthermore, the reflector described in US 2009/101208 Al is equipped with a pre-curved rigid structure, whereon the reflective glass sheet is glued, whereas, as will become apparent hereinafter, the panel of the present invention is made up of flat sheets that are then cold-bent, preferably by lamination on a mould.

In addition to the above, the layers that make up the reflector described in US 2009/101208 Al have a thickness greater than or equal to 2 mm. This prevents the achievement of significant curvatures by cold-bending, unlike the present invention, wherein the layers have a maximum thickness of 1.5 mm.

Note also that US 2009/101208 Al does not describe the execution of a step of cross-linking the layer of ethylene vinyl acetate (EVA) . On the contrary, the present invention provides for using a layer of cross-linked EVA, which will prevent any mutual slipping of the foils.

Document JP 2002 162503 A describes a reflector made up of multiple layers, comprising a layer of cross-linked ethylene vinyl acetate (EVA) .

However, the cross-linking agents associated with EVA in JP

2002 162503 A have a generic function of reinforcing the mechanical properties of the adhesive layer (paragraph [0046]) . On the contrary, as will become apparent below, the cross- linked EVA used in the present invention has a more specific and targeted function. In particular, the cross-linking of the EVA gives the latter elastomeric properties, thereby increasing its elastic properties. As a result, the deformations induced both by the internal stresses generated by the bending of the glass sheets and by the external forces acting upon the reflector (gravity and wind) will be elastic and hence not irreversible (permanent) .

In other words, the cross-linking (and the resulting acquisition of elastomeric properties) of the EVA in the present invention aims at keeping the curvature of the manufactured product unchanged over time, by increasing its elastic properties and preventing any permanent deformations induced by the forces acting thereupon and by the heating cycles undergone by the reflector, since the latter will be installed outdoors.

In summary, therefore, the purpose of EVA cross-linking within the frame of the present invention is not to achieve mechanical strength (note that the panel according to the invention has very good mechanical strength even without EVA cross-linking), but to achieve elasticity of the material, i.e. to transform it from a plastic material into an elastomeric material .

Document US 4,422893 A describes a method of manufacturing mirrors and mirrors so obtained. In this document, it is indicated that the adhesives used have a melting temperature of 150°C, preferably ranging from 60°C to 120°C. On the contrary, within the scope of the present invention, as will become apparent hereinafter, the multi-layer structure is heated to a temperature ranging from 120°C to 160°C in order to achieve cross-linking of the EVA (which by the way is not even mentioned in US 4,422893 A), so as to obtain the above-mentioned characteristics of elasticity.

Document US 2013/265667 Al describes a curved reflective mirror and a manufacturing method thereof. This document mentions the possibility of using ethylene vinyl acetate (EVA) as an adhesive. However, unlike the present invention, this material is not subjected to cross-linking. Therefore, the structure described in US 2013/265667 Al cannot attain the advantages of the present invention in terms of effectiveness of the bond between the various foils.

[OBJECTS AND SUMMARY OF THE INVENTION ]

It is the object of the present invention to provide a reflective panel that features high reflectivity of the reflective surface, high geometric precision of the reflective surface, high mechanical rigidity, low weight, a sufficiently long life, as well as low production costs.

Preferably, the reflective panel of the invention comprises a first reflective foil and two or more additional foils, wherein: said first foil is fixed to the second foil, the second foil is fixed to the third one, and so on, by means of a stratification process in which these foils are alternated with intermediate layers made of thermoplastic material; the foils are preferably made of thin transparent glass; the thermoplastic material is highly cross-linked ethylene vinyl acetate (EVA) .

The reflective panel has, therefore, a so-called laminated or multi-layer structure.

Preferably, the glass foils of the multi-layer structure have the same thickness and the same shape.

Preferably, each intermediate layer comprises a sheet of Ethylene Vinyl Acetate (EVA) , the thickness of which lies within the range of 0.25 - 0.75 mm.

After having been subjected to the hot-lamination process, said sheets of Ethylene Vinyl Acetate (EVA) will have adequate properties for the intended application, i.e. strong and long- lasting adhesion to the glass, sufficient resistance to compression forces, low cost, limited permeability to water and gases, good chemical stability, high resistance to environmental chemical agents and UV radiations.

The Applicant observes that EVA is a random copolymer composed of ethylene and vinyl acetate; within the frame of the present invention, the percentage of vinyl acetate is preferably in the range of 28-33% (weight/weight) . The Applicant also observes that, prior to the lamination process, the EVA has typical thermoplastic characteristics, with a thermal softening range of 60-70°C, is slightly opaque, is soft and easily subject to plastic deformation. Thanks to the cross-linking reactions of the copolymer chains during the lamination process, the initial EVA sheet transforms into a highly transparent elastomeric adhesive sheet. The underlying chemical process is the formation of a loose three-dimensional grid that increases the elastomeric characteristics and thermal stability of the material. The cross-linking of EVA is substantially based on radical reactions, obtained through the addition of an organic peroxide or a peroxycarboxylic acid acting as a radical initiator (cross-linking agent) . This cross-linking agent is initially homolitically split into two radical species, which extract hydrogen atoms from the polymeric chains of EVA, preferably from the terminal methyl groups of the side chains of vinyl acetate. In this process, the active radical site is transferred to the methyl group, which reacts with another active site spatially close thereto, thereby creating a chemical bond between the two polymeric chains and transforming the initially thermoplastic EVA material into an elastomer with a three-dimensional polymeric grid. In glass lamination, this radical reaction is activated mostly by heat; in other words, the homolitic breakage of the peroxidic bond is the result of a thermolysis that occurs during the lamination process starting from a process temperature of approx. 130°C. While the subsequent cross-linking reactions include many possible radical reactions, the latter are much faster than the initial homolitic thermolysis of the cross-linking agent. The speed of the cross-linking reaction is therefore mainly controlled by the speed of the homolitic thermolysis of the cross-linking agent, and hence by its concentration and by the temperature of the lamination process. The cross-linking percentage is thus determined by these latter two parameters, together with the duration of the cross-linking process .

In light of the above, it is clear that a conventional multi ^¬ layer glass lamination process would not be able to generate a curved reflective panel having the characteristics required for its use in optical systems for solar radiation concentration. In accordance with the present invention, the desired characteristics can be obtained by selecting a specific initial thermoplastic material, i.e. EVA, to which suitable cross- linking agents are then added, and through a lamination process conducted at appropriate temperatures.

Said lamination conditions of laminated glass allow achieving such structural, rheological and adhesive properties that ensure structural stability of the reflective panel for approx. 25 years of use.

Assuming that the EVA intermediate layers will perfectly transmit the shearing actions between the adjacent faces of the laminated element, the bending resistance of laminated glass is mainly determined by the mechanical characteristics of the glass sheets and in particular, their area being equal, by the thickness thereof. In order to achieve the rigidity necessary for use in a solar concentration system, a panel having a mirror area not smaller than approx. 0.5 m ² will preferably have a minimum thickness of approx. 4 mm. In principle, such a thickness could be obtained with a laminated glass panel made up of two glass sheets having a thickness of 2 mm each. While the rigidity of the multi-layer glass panel must be sufficiently high to give it the necessary resistance to the forces caused by the wind and by its own weight, each foil should have, on the other hand, the least possible resistance to bending, in order to limit the strains generated by the curvature, which are counterbalanced, as aforesaid, by the adhesion forces at the glass-EVA interface that transfer such strains to the bulk of thermoplastic material. Stability over time of the curvature of the reflective panel is thus obtained not only by cross-linking the EVA, but also by limiting as much as possible the bending resistance of the individual glass sheets, while keeping the overall rigidity of the laminated panel unchanged. In order to obtain said characteristics, the glass sheets preferably have a thickness of approx. one millimetre, particularly not greater than 1.5 millimetres. It follows that the laminated glass preferably comprises at least 3 glass sheets, in particular having a thickness not greater than 1.5 mm. As the radius of curvature of the panel decreases and the curvature thereof increases, it is appropriate that the bending resistance of the individual sheets be reduced through the use of progressively thinner foils. For example, for radii of curvature of approx. 5 metres it is preferable that the thickness of the single glass sheets does not exceed 1.3 mm and the laminated panel is made up of at least 4 glass sheets.

In the multi-layer structure, the first glass sheet is made as a mirror, i.e. a flat sheet of glass, wherein on one of the two faces a highly reflective thin metal foil, typically made of silver or aluminium, has been deposited. The metal foil is insulated from the outside environment via deposition of an insulating protective paint. In laminated glass, the silver or aluminium foil of the first sheet, and its protective foil, are comprised between the first glass sheet and the first intermediate layer of thermoplastic material. In this kind of structure, therefore, the metal foil is effectively insulated from the external agents by the thermoplastic sheet and then by the second glass sheet.

Direct exposition of the reflective metal foil to the atmosphere can thus be limited to the panel edge, which can then be protected by means of other insulating materials.

Preferably, in the reflective panel of the present invention the edge is sealed via application of a sheath comprising a sealing resin having an appropriate composition, or a plastic film, or thermoformed thermoplastic material. Said materials must be sufficiently inert to atmospheric agents, must adhere firmly to the glass and, possibly but not necessarily, must be charged with powder or fabric of mineral fibre in order to enhance their mechanical characteristics.

As an alternative or in addition to the sealing sheath, a frame may be applied, consisting of a "U"-shaped profile made of rubber or plastic material, e.g. extruded, having a "U"- shaped recess of suitable dimensions for receiving the edge of the panel. Said "U"-shaped profile may be made up of separate sections or sections joined together by vulcanization to simplify the operations necessary for assembling it with the laminated panel.

These and other objects are substantially achieved through a reflective panel as described in the appended claims.

[BRIEF DESCRIPTION OF THE DRAWINGS]

Further features and advantages will become more apparent from the following detailed description of some preferred but non-limiting embodiments of the invention.

This description will refer to the annexed drawings, which are also provided merely as explanatory and non-limiting examples, wherein:

- Figure 1 shows a perspective view of a reflective panel in accordance with the present invention;

Figure 2 shows a sectional view of the panel of Figure 1, highlighting the various components thereof;

Figure 3 shows a sectional view of the panel of Figure 1 coupled to a sheath and a sealing frame;

Figure 4 shows a conformation taken by the reflective panel of Figure 1 during the production process;

Figure 5 shows a simplified block diagram of a plant using panels made in accordance with the invention.

[DE TAILED DESCRIPTION OF THE INVENTION ]

With reference to the annexed drawings, 1 designates a reflective panel in accordance with the present invention.

The reflective panel 1, along with other similar panels, is advantageously used in a concentrated thermal solar plant 100.

The plant 100 (Figure 5) preferably comprises a conversion element E, in which a heat transfer fluid flows in a pipe and absorbs thermal energy irradiated on said pipe.

The plant 100 further comprises one or more panels 1 in accordance with the invention, arranged in such a way as to receive solar radiations and direct them towards the conversion element E.

In one embodiment, the plant 100 further comprises one or more actuators A operatively active on the panels 1 in order to move the same as a function of the apparent motion of the sun.

The reflective panel 1 (Figure 1) comprises a first reflective foil 2 made of glass or another material.

The first reflective foil 2 can be made, for example, of thin glass mirror, having a thickness ranging from 0.8 to 1.5 millimetres. As an alternative, the first reflective foil 2 may consist of a support foil of thin transparent glass having a thickness ranging from 0.8 to 1.5 millimetres, on which a reflective film coated with polymeric material adheres. Said polymeric material may be polymethylmethacrylate (PMMA) or mirror aluminium having a thickness ranging from 0.1 to 0.8 millimetres .

The reflective panel 1 further comprises a first intermediate layer 3 of Ethylene Vinyl Acetate (EVA) , preferably having a thickness ranging from 0.1 to 0.75 millimetres. Suitable cross-linking agents are preferably added to the EVA.

Note that, for the purposes of the present invention, it is important that at the end of the panel production process the EVA of the intermediate layer 3 (and of any other intermediate layers of the same panel) will show a high degree of cross- linking .

According to one preferred but non-limiting feature of the invention, this can be obtained, as aforesaid, by combining the EVA with one or more cross-linking agents prior to the start of the panel production process.

Preferably, the EVA is laminated at a temperature of 120°C to 160°C, e.g. 130°C to 150°C.

The reflective panel 1 further comprises a second foil 4 of thin, flat, transparent glass, preferably having a thickness ranging from 0.8 to 1.5 millimetres.

Preferably, the dimensions and material of the second foil 4 are substantially the same as those of the first foil 2, except for the reflective film or surface.

Advantageously, the reflective panel 1 comprises a second intermediate layer 5, substantially equal to the first intermediate layer 3. Advantageously, the reflective panel 1 comprises a third foil 6 of thin, flat, transparent glass, preferably having a thickness ranging from 0.8 to 1.5 millimetres.

In one embodiment, the reflective panel 1 further comprises a third intermediate layer 7, substantially equal to the first and/or second intermediate layers 3, 5.

In one embodiment, the reflective panel 1 further comprises a fourth foil 8 of thin, flat, transparent glass, preferably having a thickness ranging from 0.8 to 1.5 millimetres.

The arrangement of the foils 2, 4, 6, 8 and of the intermediate layers 3, 5, 7 is shown in Figure 1. In brief, between each pair of adjacent foils there is an intermediate layer having the characteristics mentioned above with reference to the first intermediate layer 3.

The Applicant observes that, in light of the above, the reflective panel 1 has a multi-layer structure 13 of laminated glass .

Preferably, the reflective panel 1 further comprises a sheath 9 (Figure 3) .

The sheath 9 may be made from a sealing resin having an appropriate composition.

The sheath 9 may be made of thermoformed thermoplastic material or as a film of polymeric material.

In a plan view, the sheath 9 is arranged on one or more sides of the perimeter of the reflective panel 1.

Preferably, in a plan view, the sheath 9 is arranged along the whole perimeter of the reflective panel 1.

The sealing sheath 9 is positioned along and adheres to the whole height of the edge and the outer surface of the end foils (in the example of Figure 1, the first foil 2 and the fourth foil 8), up to a distance from the edge that may vary, for example, from 1 mm to 10 mm. The material of the sealing sheath 9 can be supported by fabric of mineral fibre to increase the mechanical strength of the edge, for the purpose of preventing the glass foils from breaking or fraying, since they are particularly exposed to damage caused by accidental shocks during panel transportation and/or installation.

In addition to the above-mentioned mechanical protection functions, the sealing sheath 9 also creates a physical barrier against environmental agents that may interact with the reflective foil 2 over time, thus altering its original characteristics. In particular, the material of the sealing sheath 9 prevents atmospheric oxygen and moisture from coming into contact with the silver or aluminium of the reflective foil 2, thereby oxidizing it.

In addition or as an alternative to the sheath 9, the reflective panel 1 may comprise a frame 10.

The frame may be made as a "U"-shaped rubber profile (e.g. EPDM, PVC, Silicone, etc.) or from thermoplastic material or another material, preferably via an extrusion process.

The internal groove of the frame 10 has an internal width equal to or slightly greater than the thickness of the multi ^¬ layer structure 13, so that it can be inserted into and adhere to the edge of the multi-layer structure. The depth of the "U"- shaped recess of the frame 10 may vary, for example, between 1 mm and 10 mm.

The frame 10 of the panel 1, if present, has a twofold function: protective and mechanical. It acts as an additional element for mechanical and physical protection of the panel edge and forms an additional barrier against external agents. The frame 10 can be made by glueing a profile, preferably "U"-shaped, of rubber or thermoplastic material to the sealing sheath 9 or, in the absence of the sheath 9, directly to the edge of the multi-layer structure 13. The profile may surround the entire edge of the panel, or may be positioned only at specific points or along limited portions thereof.

As an alternative, the frame 10 can be realised by adhesion of a film of polymeric material, preferably silicone-based, positioned like the above-described "U"-shaped profile, to the sealing sheath 9 or directly to the edge of the multi-layer structure .

Note that Figure 3 shows, merely by way of example, the sealing sheath 9 and the frame 10 applied on two opposite sides of the panel 1; however, the sealing sheath 9 and/or the frame 10 may be applied on one side only of the panel, or on a number of sides greater than two.

In a plan view, the reflective panel 1 preferably has a rectangular shape, with a first side having a length ranging from 200 mm to 2,000 mm and a second side having a length ranging from 500 mm to 3,000 mm.

By way of example, the first side may have a length substantially equal to 600 mm, and the second side may have a length substantially equal to 1,000 mm.

The reflective panel 1 is curved, preferably with a cylindrical or parabolic geometry. An appropriate convex curvature gives the reflective surface the capability of focusing the reflected solar radiation towards the receiver.

The specific curvature of the reflectors used in the different CSP or CST plants depends on the distance between the reflective panel and the receiver. For Fresnel reflectors with a cylindrical curvature, the preferred radius of curvature of the reflective surface is in the range of 5-15 metres.

The total thickness of the panel 1, not including the sealing sheath 9 and/or the frame 10, is preferably in the range of 3 mm to 6 mm. The panel 1 is preferably formed by three to five thin glass foils, alternated with two to four intermediate EVA layers. In particular, the number of glass foils may be four, and the number of intermediate EVA layers may be three.

The process of fabrication of the panel 1 starts with the making of the multi-layer structure 13, which is obtained by superimposing the foils 2, 4, 6, 8 and the intermediate layers 3, 5, 7. In this processing phase, the intermediate EVA layers 3, 5, 7 are still solid and in contact with the glass surfaces lying underneath and above. Prior to the softening caused by the heat treatment, the EVA sheets do not adhere to the glass, and it is still possible for the different layers making up the multi-layer structure to slip over each other. As a consequence, in this phase the semifinished product is still poorly resistant to bending.

It is thus possible to bend the multi-layer structure 13 by laying the reflective face thereof on the concave surface of a mould 11 and then exerting a pressure, i.e. a force perpendicular to the surfaces of the structure and of the mould. The application of pressure forces the multi-layer structure to take the curvature of the mould. Pressure can be applied by using several tools and methods.

The preferred, though non-limiting, method involves the insertion of the multi-layer panel-mould assembly into a bag 12, inside of which a certain level of vacuum is created by sucking out the air contained therein (the so-called vacuum bag technique) . The preferred vacuum level is a few millibar units. The value of the pressure used will be, therefore, approximately the atmospheric one, i.e. 10,000 Kgf/m ² (the weight force of 10,000 kg applied onto an area of one square metre) . The multi ^¬ layer panel-mould assembly will then be kept under a vacuum and brought to a temperature ranging between a minimum value of 130 °C and a maximum value of 150 °C. For this thermal process, the temperature variation as a function of time follows a trend that depends on the thickness of the glass sheets, the thickness of the EVA sheets, and the number of layers making up the multi- layer structure. Merely by way of example, a typical trend of temperature as a function of time shows a period of pre-heating and thermalization at approx. 90 °C for 40-60 minutes and a subsequent period of approx. 60 minutes at the maximum process temperature, during which the cross-linking radical reactions occur. Once the heat treatment is complete, the temperature of the panel is brought back to room temperature, while maintaining the vacuum condition inside the bag.

As soon as full cooling is achieved, the panel can be removed from the bag and separated from the mould.

The panel 1 thus manufactured will be rigid and will have the desired curvature with maximum geometric deviations within 2 milliradians (deviations are measured as the angular difference between the actual perpendicular to the surface at a specific point of the reflective surface and the theoretical perpendicular at the same point) .

Preparation of the panel 1 is then completed by sealing the edge. Along the latter, a sealing and adhesive resin 9 is applied and, in addition or as an alternative, the frame 10 is glued.