WO2013121392A1

WO2013121392A1 - Flexible photovoltaic module and method for the manufacture thereof

Info

Publication number: WO2013121392A1
Application number: PCT/IB2013/051244
Authority: WO
Inventors: Michele TORRI
Original assignee: TORRI SOLARE Srl
Current assignee: TORRI SOLARE Srl
Priority date: 2012-02-16
Filing date: 2013-02-15
Publication date: 2013-08-22
Anticipated expiration: 2014-08-16
Also published as: ITBS20120021A1; EP2815435A1

Description

"Flexible photovoltaic module and method for the manufacture thereof"

* * * *

FIELD OF THE INVENTION

The present invention relates to a flexible photovoltaic module, in particular made of crystalline silicon, and a method for its manufacturing.

STATE OF THE ART

In general a photovoltaic module can be defined as device for directly converting light energy to electric power.

Nowadays two typologies of photovoltaic modules are established on the market: the modules made of amorphous silicon and the modules made of crystalline silicon. The present invention relates to the second type of modules.

In modules made of amorphous silicon the silicon atoms are chemically deposed in amorphous form, that is structurally unorganized, on the surface of a flexible support, for example a film or a strip, forming a uniform layer of some microns. Generally a throughput (or efficiency) lower on average than the throughput of modules made of crystalline silicon is ascribed to the photovoltaic modules made of amorphous silicon, where the term throughput refers to the ratio between the electric power produced by the module and the light energy hitting the surface thereof; in addition the throughput of the amorphous silicon modules is prone to decay over time. Usually the modules made of amorphous silicon have a throughput lower than 10%.

A huge advantage of the amorphous silicon modules, in addition to the lower manufacturing cost with respect to the crystalline silicon modules, is given by being flexible and thus able to be fitted to the curved surfaces. In practice the amorphous silicon modules are commercialized in form of strips similar to carpets which can be laid out on flat or curved surfaces, and in case rolled up.

The crystalline silicon modules are arranged as an assembly of electrically interconnected cells; each cell consists of a flat and stiff, that is not flexible, wafer, of monocrystalline or polycrystalline silicon. The connection among cells is obtained by metal strips. In its turn a single photovoltaic module can be included in a supporting frame to form a photovoltaic panel which can be installed outdoor. The throughput of a photovoltaic module (and thus of the panel thereof) is the ratio between the electric power produced by the module and the light energy hitting against the surface of the cells thereof. Usually the crystalline silicon modules have a throughput comprised between 15% and 17%.

The highest manufacturing cost of these modules with respect to the amorphous silicon modules is related to the purification process of the starting silicon oxide; the wafers constituting the cells are in fact realized with silicon whose purity has to be the highest possible.

The crystalline silicon modules have the not negligible disadvantage of being stiff, that is to say they can not undergo bending with respect to the lying plane of the cells thereof, which otherwise would break impairing the operation of the module itself.

Just in order to prevent the crystalline silicon cells from breaking when subjected to bending forces, the structure of modules is absolutely stiff. In practice the modules are rolled according to a sandwich shape. The laminate, or sandwich, comprises in the order:

- a rear supporting member, usually called backsheet, intended to be facing downwards, made of an insulating material having poor thermal expansion, typically a tempered glass plate or a sheet made of a polymeric material such as tedlar;

- a first sheet of ethylene vinyl acetate, often named with the acronym EVA;

- the monocrystalline or poly crystalline silicon cells connected in series and/ or in parallel by metal strips;

- a second sheet of ethylene vinyl acetate;

- a transparent and tempered glass plate, intended to be facing upwards, whose function is to protect the photovoltaic cells from atmospheric agents, such as rain or hail, or from debris brought by the wind, leaves, etc.

The above described assembly undergoes a hot-rolling and under vacuum process in a hot-rolling oven. The ethylene vinyl acetate melts and polymerizes, acting as inert glue which stabilizes the module structure, by isolating from the air the cells and the strips which electrically connect them.

The so formed module is completed with some accessories before becoming a commercial solar panel. The electrical terminations of the metal strips are clamped in a sealed terminal block, also called junction box, generally fastened to the backsheet, in line with one of the cells, and an aluminium frame is glued to the perimeter of the module in order to allow the fastening thereof to structures such as buildings or supporting frames.

The mounting of solar panels built with the photovoltaic modules of crystalline silicon is not easy. Since the modules are not flexible, and thus can not be bent to fit to possible not flat surfaces, such as for example some roofs of industrial buildings, the panels have to be fastened to proper custom-made supporting frames, for example feet or brackets made of metal or plastic previously positioned in the roof of the target building. Obviously prearranging the supporting frames and fastening structures takes time and often results in risks for the operator safety, in addition to an increase of the costs for installation and materials themselves.

In order to partially solve the described drawbacks a solution characterized by a certain flexibility of the module has been proposed.

The document US 8,039,739 describes a photovoltaic module according the pre-characterizing part of the enclosed claim 1. The module comprises a solar cell, made of microcrystalline silicon too, individually encapsulated in a protective layer that can be combined to only one side of the solar cell, to more sides thereof or else enclose it integrally. The protective layer can be combined with the solar cell by means of a rolling process.

In the embodiment shown in figure 1A the device comprises a solar cell, a protective layer combined with the entire outer surface of the solar cell, a first and a second sealing and layers opposingly arranged with respect to the protective layer, an upper barrier layer, combined with the first sealing layer and a lower supporting layer, combined with the second sealing layer, all combined in a sandwich. Sealing junctions extended along the entire thickness of the sandwich are also indicated laterally to the device.

The supporting layer can be realized with several materials, among which combinations of Tedlar with a number of polymers, thermoplastic polyurethanes of aliphatic or aromatic ethers or esters, thermoplastic polyolefins, acrylic sheets or aluminium. The first and second sealing layers can be made of several plastic materials among which thermoplastic polyurethane, thermosetting ethylene vinyl acetate EVA, thermoplastic fluoropolymer. The barrier layer can be made of flexible material such as for example an ethylene- tetrafluoroethylene copolymer (ETFE) or acrylic resins cross linked by ultraviolet radiation.

This solution provides the module flexibility, but not the integrity of the photovoltaic cells in any use condition. In fact an excessive module flexion can cause its breaking.

Also the documents GB 2428331, US 2009/044852 and WO 2006/122376 describe modules according to the known art.

A further drawback is related to the mode of connecting several panels to form a photovoltaic system. Typically, several panels are connected together by electrical wires and by means of the terminal block of each panel, to form the so called strings; the strings are then connected with one or more direct current DC switchboard; generally several DC switchboards are needed, in their turn connected in parallel. The DC switchboards are then connected to one or more inverters which converts the direct current produced by the solar panels into alternating current AC. The inverters are then connected to corresponding AC electrical switchboards, in their turn connected to the home or ordinary mains. Sometimes a module to remotely control the photovoltaic system is provided.

A drawback of all solutions known in the art is given by the electrical switchboards and inverters being remarkably bulky and this often requires providing a proper room or cabinet in the buildings. Furthermore high electric power leaks are usually observed in the connection between the panels and the DC electrical switchboards.

Other drawbacks pertain to the "standard" technology. One of these is given by the overheating of the terminal block of each panel during the operation, and being it arranged next to some cells causes their easily overheating, causing a drop of the throughput of the entire module and a decrease in producibility.

Another drawback is given by the fact that the now available photovoltaic modules are heavy; the upper covering glass plate of the sandwich adversely affects the weight. This complicates the transportation and installation.

Objects and summary of the invention

Therefore a first object of the present invention is to provide a photovoltaic module made of crystalline silicon which is flexible, that is to say it can bend with respect to the lying plane of its cells, and lighter than the known solutions.

A second object of the present invention is to provide a photovoltaic module made of crystalline silicon which can be easily installed and in an embodiment also serves as a covering.

A third object of the present invention is to provide a photovoltaic module made of crystalline silicon which is easy to connect to the alternating current mains.

A further object of the present invention is to provide a photovoltaic panel made of crystalline silicon which does not suffer from overheating of cell with respect to the other cells.

A further object of the present invention is to provide a method for realizing a photovoltaic module made of crystalline silicon flexible while resistant to impacts and deformations which could lead the photovoltaic cells to break.

Therefore, in its first aspect, the present invention relates to a photovoltaic module according to claim 1.

In particular, the invention concerns a photovoltaic module comprising: - a first backsheet which is substantially flat and flexible with respect to lying plane thereof;

- one or more photovoltaic cells made of monocrystalline or polycrystalline silicon and arranged on the upper face of the first backsheet either directly contacting the latter or with the interposition of at least one first sheet of ethylene vinyl acetate EVA1;

- at least one transparent plastic film, defined as frontsheet, arranged on the photovoltaic cells, either directly in contact with the latter or with the interposition of at least one second sheet of ethylene vinyl acetate, in order to isolate them from the outside;

- a second backsheet, flexible with respect to the lying plane thereof, positioned on the first backsheet, on the opposite side with respect to the cells, and a third sheet of ethylene vinyl acetate interposed between the first backsheet and the second backsheet, wherein said second backsheet is a metal plate having a thickness comprised between 0.5 mm and 5 mm,

and wherein the first backsheet, the photovoltaic cells, the transparent plastic film, the second backsheet and every ethylene vinyl acetate sheet are hot-rolled together to form a sandwich flexible with respect to the lying plane of any one of its layers, that is able to take a convex or concave configuration and avoid the breaking of the photovoltaic cells at the same time.

Advantageously the module according to the present invention is free from completely stiff plates or completely stiff layers such as, for example, glass plates which do not bend. Substantially the sandwich comprises flexible sheets or members, that is to say able to fit the curvature given to the module. The presence of the second backsheet causes the module not to reach such curvatures leading to cell breaking, while being flexible. In other words the module can take a concave or convex configuration without breaking. This becomes very useful both during the installing step and also during the module transportation to the installing site. In fact the transportation often assumes the handling of modules with cranes or equivalent means and a backsheet excessive softness could easily impair the cell integrity in case of excessive bends or impacts.

Furthermore providing a backsheet as above described facilitates the realization of fastening eyelets and maximizes the heat dissipation.

In an embodiment the backsheet, that is the second metal sheet, is a covering member for buildings. In this circumstance the photovoltaic module is made by directly rolling the above described members along with a covering member. In this way the installation on building roofs is simplified and the step of gluing the photovoltaic modules to other supporting or covering members is avoided.

Experimental tests showed that by embedding the photovoltaic cells - which are stiff, as mentioned - in a sandwich structure as the described one, the cells themselves can fit to curvatures given to the module, without breaking.

For example a rectangular module 1 meter long is able to bend until the ends thereof lie on orthogonal planes. Longer modules, for example of 6 meters length, can be even rolled up to facilitate the transportation thereof while not exceeding the highest allowed curvature.

For example, the sheets of ethylene vinyl acetate are shaped to be rolled up, and thus polymerized, at a temperature higher than or equal to about 150°C for a time shorter than 30 minutes. Ethylene vinyl acetate films adapted for realizing the photovoltaic module object of the invention are sold with Vistasolar trade name.

Preferably the first backsheet is made of a material selected from the polyvinyl fluoride, polyvinylidene fluoride, polyethylene terephthalate, a styrene- or polyolefin- or polyurethane- or polyester- or polyamide- based thermoplastic elastomer. More preferably the first backsheet is made of polyvinyl fluoride. For example a material suitable to be used as first backsheet is sold in films by the DuPont Company with the trade name Tedlar.

Alternatively the first backsheet is a metal sheet, for example a thin aluminium sheet having a thickness lower than 0.5 mm, that is a metal sheet.

The transparent plastic film is made of a material selected from ethylene tetrafluoroethylene, polyethylene terephthalate, fluoro ethylene propylene, polytetrafluoroethylene, or else it is a film obtained by rolling together one or more films made of said materials and/ or one or more films of ethylene vinyl acetate.

In the preferred embodiment of the module, the transparent plastic film, that is the frontsheet, is made of ethylene tetrafluoroethylene ETFE, or it is a film obtained by rolling an ethylene tetrafluoroethylene ETFE film along with one or more films of ethylene vinyl acetate EVA. In practice the transparent film is a single film or else it is itself a laminate made of different films.

The transparent film made as described has shown a greater transparency with respect to the glass plates usually employed for making photovoltaic modules of crystalline silicon.

Preferably the transparent plastic film has a thickness in the range 10 pm - 200 pm. With this thickness a 2%-4% higher transparency with respect to a glass plate has been observed.

The transparent plastic film constitutes an excellent protective screen against potentially dangerous atmospheric agents, such as for example the hail, the dust, etc.

In an embodiment the transparent plastic film is calendared on the other sandwich layers, at least next to some regions. This allows to create a light low relief, or pattern, on the surface of the film exposed to the atmosphere. The effect obtained is to make part of the film protrude in order to realize a sort of bearing for absorbing the impacts of small objects, for example pebbles or dust thrown by the wind against the module. Therefore, as described, the transparent film preferably is not coated by other layers, but is directly exposed to the air.

Preferably the photovoltaic cells are electrically connected by means of wires or strip conductors, usually named ribbons.

The photovoltaic cells can be variously connected. For example in an embodiment the module comprises more strings than cells; each string contains from six to twenty four photovoltaic cells electrically connected in series and the strings are connected one to another in series, or in parallel, or in series and in parallel according to the desired layout.

In the preferred embodiment no further layer is provided between the backsheet and the outside air.

Providing the second sheet, that is the metal backsheet, with a thickness larger than 0,5 mm further allows to obtain valid results as regard to the heat dissipation. In other words, by allowing the module to dissipate the excess heat through a convective thermal exchange between the second backsheet and the atmosphere, overheating of the photovoltaic cells is avoided and thus undesired decreases of the efficiency thereof are avoided.

Preferably the module, still due to the metal backsheet with thickness greater than 0,5 mm comprises through eyelets extending next to the perimeter of the module. The eyelets permit to speed up the installation, and reduce costs due to the purchase of suitable supporting structures. In fact the presence of eyelets directly on the photovoltaic module allows the use of simple fastening means such as screws and bolts of different type which in their turn allow the rapid installation and without using complicated sub-structures.

Preferably each module comprises reinforcing ribs extending at two or more edges of the module, or else it comprises a reinforcing frame extending next to at least part of the perimeter of the module, in order to prevent the module itself to be twisted and making it stiff and loadbearing but keeping the desired bending radius. The mounting of these backsheets is only possible due to the metal backsheet having a thickness greater than 0,5 mm. Preferably the reinforcing ribs and the reinforcing frame are made with metal profiles, or with ripples or bendings of the perimeter of the backsheet or also with metal sheets.

Preferably the sandwich could be rolled directly on a metal sheet which will then directly become a covering product, therefore the backsheet of the above described product will be the covering directly.

Preferably at least some of said reinforcing ribs and at least part of said reinforcing frame are provided with heat sinks, for example of the fin type.

The present invention further relates to a photovoltaic panel comprising one or more of the above described photovoltaic modules, simply arranged in an assembly or also electrically connected.

Preferably the panel comprises one or more modules glued on planar or even calendared and bent folded roofing sheet, of the type used for the coverings of the buildings, becoming effectively a building product.

In its third aspect the present invention relates to a method for manufacturing a photovoltaic module with cells of crystalline silicon.

In particular, the method comprises the steps of:

a) arranging the following assembly of members according to the following order:

- a first backsheet which is substantially flat and flexible with respect to lying plane thereof;

- one or more photovoltaic cells made of crystalline silicon arranged directly resting on the upper face of the first backsheet, or arranged on at least one first sheet of ethylene vinyl acetate interposed between the first backsheet and the photovoltaic cells;

- at least one transparent plastic film arranged directly contacting with the upper surface of the photovoltaic cells, in order to isolate them with respect to the outside, or else arranged on at least one second sheet of ethylene vinyl acetate interposed between the transparent plastic film and the photovoltaic cells; - a second backsheet, made of metal and having a thickness comprised between 0,5 mm and 5 mm, which is flexible with respect to the lying plane thereof, positioned on the first backsheet on the opposite side with respect to the cells, and a third sheet of ethylene vinyl acetate interposed between the first backsheet and the second backsheet;

b) hot-rolling the assembly of members until obtaining a photovoltaic module with sandwich structure, flexible with respect to the lying plane of the module itself but anyway not over the breaking point of the photovoltaic cells.

If necessary the step a) comprises the polymerization of the ethylene vinyl acetate and the transparent film in order to form and/ or prearrange the members for the electrical connection in series and/ or in parallel of the photovoltaic cells before hot-rolling the assembly.

Preferably the first backsheet is made of polyvinyl fluoride or metal.

The step b) can be carried out in batch for example, prearranging the assembly of layers to be rolled in a hot-rolling oven and under vacuum.

Alternatively the step b) can be carried out in continuous, mechanically coupling continuous strips of the various layers of the sandwich and feeding the assembly to the hot-rolling oven, inside which vacuum has been created or else the atmosphere is formed by inert gases; in this circumstance the inside of the oven is isolated from the outside by means of opposite nip rollers which press the entering sandwich, to extract the air out of its layers before entering the oven, and at the exit to keep it sealed. In case, the oven can be provided with more pairs of input/ output nip rollers and each pair can be provided with a restraint hood and an air extraction pump suctioning the air from the hood.

This latter technique is perfect for directly rolling the sandwich on the metal backsheet which will then become a covering product.

Brief description of the drawings

More details on the invention will be evident from the following description made with reference to the attached illustrative and not limitative drawings, wherein: - figure 1 is a schematic and exploded view of a photovoltaic module according to the present invention, in a not deformed configuration;

- figure 2 is a front view of a first embodiment of a photovoltaic module according to the present invention, in a deformed configuration;

- figure 3 is a rear view of the photovoltaic module shown in figure 2;

- figure 4 is a perspective view of the photovoltaic module shown in figure 2, in a not deformed configuration;

- figure 5 is a perspective view of a solar panel provided with more photovoltaic modules according to the first embodiment;

- figure 6 is a perspective view of a second embodiment of the photovoltaic module according to the present invention, in a not deformed configuration;

- figure 7 is a perspective, partial and rear view of the photovoltaic module shown in figure 6;

- figure 8 is a schematic, perspective rear and partially exploded view of the photovoltaic module shown in figure 6.

Detailed description of the invention

In figure 1 is shown a constructive layout of a photovoltaic module 1 according to the present invention, comprising a first backsheet Bl, substantially flat and flexible with respect to the lying plane thereof, one or more photovoltaic cells C made of crystalline silicon, arranged on the upper face of the first backsheet Bl, either directly on by the interposition of at least one first sheet of ethylene vinyl acetate EVA1, and a transparent plastic film 2, or frontsheet, arranged on the photovoltaic cells C in order to isolate them from the outside, either directly or by the interposition of at least one second sheet of ethylene vinyl acetate EVA2.

Between the photovoltaic cells C, preferably made of polycrystalline silicon, and the film 2, or else between the photovoltaic cells and the second sheet of ethylene vinyl acetate EVA2, the so called ribbons are interposed that is to say thin welded metal strips which realize the electrical connection of the cells one to another and/ or with the junction box of the module (shown in figure 9).

The second sheet of ethylene vinyl acetate EVA2 can be absent in case the film 2 already contains an own ethylene vinyl acetate sheet; this is the case, for example, wherein the film 2 is obtained by rolling more layers, one of which - the lower one - is a sheet of ethylene vinyl acetate.

Preferably the transparent film 2 is made of ethylene tetrafluoroethylene ETFE, or it is a film obtained by rolling a film of ethylene tetrafluoroethylene ETFE along with one or more films of ethylene vinyl acetate.

In the example shown in figure 1 the first backsheet Bl is a sheet made of polyvinyl fluoride, for example of Tedlar, material sold by the DuPont Company. Alternatively the first backsheet Bl is made of metal, and preferably is a soft aluminium sheet having thickness lower than 0.5 mm.

The module 1 thus realized is flexible with respect to the lying plane thereof, as shown in figures 2 and 3, that is to say it can take a concave or convex shape and, if enough long, even partly concave and partly convex.

Preferably the backsheet Bl is made of polyvinyl fluoride.

The module 1 is provided with a further backsheet B2, shown in the exploded of figure 1, made of a metal material, more preferably of aluminium or stainless steel. At least one sheet EVA3 of ethylene vinyl acetate is interposed between the backsheets Bl and B2 in order to allow the rolling of the sheet B2 along with the remaining of the module layers in the manufacturing step.

The second backsheet B2 has a thickness comprised between 0,5 mm and 5 mm, that is to say it is thicker than a simple sheet, so to provide structural integrity to the module itself, but still enough thin to assure the assembly flexibility.

Once the layers 2, EVA2, R, C, EVA1, Bl, and the layers EVA3 and B2 have been overlapped, the assembly is inserted in a high temperature hot- rolling oven, for example about 200°C for a time enough to obtain the polymerization of the ethylene vinyl acetate, the compaction of the layers and the extraction of the air out of the gaps thereof. In the hot-rolling oven the transparent film 2 can also undergo a calendaring with proper rollers which impress the proper pattern on the film 2.

When the assembly has been compacted and is aggregated, it is extracted out of the hot-rolling oven, obtaining the module 1.

In the example shown in figures 2 and 3 the module 1 comprises fifteen cells CI, C2,..., CN, arranged on two rows. The cells C of each row are connected in series by the respective ribbons R and the row is connected to the electrical connector 3 or 4.

The not deformed configuration of the module 1 is shown in figure 4. In other words this figure shows the module 1 not bent and lying in the lying plane thereof.

Figure 5 shows a photovoltaic panel P comprising six photovoltaic modules 1 arranged in parallel rows of two modules. In particular the modules are applied to a supporting base 5, for example are glued to a planar or calendared and curved, folded roofing metal sheet of the type usually employed as coverings for building roofs, becoming effectively a building product. The supporting base 5 comprises reinforcing rib 6.

Figure 6 shows in perspective and from top an alternative embodiment of the photovoltaic module 1' according to the present invention. The module V comprises sixty cells C made of polycrystalline silicon arranged according to a 6x10 matrix. The module V comprises the backsheet Bl made of tedlar and the backsheet B2 made of aluminium. The module 1' comprises two reinforcing ribs 7, in particular C-shaped metal profiles, longitudinally applied on the lower backsheet B2, next to two sides, and having the function of limiting the bending of the module 1' within the desired thresholds and making it loadbearing against the snow loads but curved.

Figure 7 shows a part of the module V from the bottom and in perspective. The module V comprises an own inverter 8 fastened to the lower backsheet B2 of the module V by means of a proper bracket 12 which keeps the inverter 8 apart from the backsheet B2 and in an offset relationship with respect to the cells C. In particular the inverter 8 is next to a portion of the module 1' the cells C do not intercept. In this way it allows the correct circulation of air and the localized overheating of the portion of the module 1' at the inverter 8 is avoided. Note that a traditional photovoltaic module is subjected to an efficiency loss corresponding to about 1 W for every degree centigrade of temperature increase over 25°C. Therefore providing the inverter 8 far away from the cells C and well aerated is an advantage.

The module V further comprises a junction box 11 powered by the ribbons of the module 1' and connected to the inverter 8 by standard connectors.

Through and orthogonal eyelets 9 and 10 are provided at the corners of the module 1'. The eyelets allow the insertion of a fastening pin of the module V to an external supporting structure or directly on the roof of an industrial building avoiding the use of many other fastening materials. Furthermore the eyelets 9 and 10 allow the installer personnel to take advantage from a certain adjusting clearance in positioning the module 1'.

Figure 8 schematically shows the configuration of the assembly comprising the junction box 11, in which the connections of the ribbons of the cells C converge, electrically connected to the inverter 8 by means of cables 14+14' and 15+15'. The inverter 8 supplies alternating current AC to an outer user or an AC switchboard through the cable 13. The bracket 12 keeps the inverter 8 apart from the surface of the backsheet B2 and from the junction box 11.

The ribs 7 can be shaped also like a frame extending along the perimeter of the module 1'. Finned surfaces (not shown) can be connected to the ribs 7 in order to maximize the heat dissipation.

The module 1 or 1' is manufactured by providing at first the string fastening of the cells C through ribbons R, that is to say the welding of the cells one to another in order to form strings of cells C. The so connected cells C are arranged on the backsheet Bl, as above explained, according to the desired layout. The sandwich is hot-rolled, and preferably under vacuum, and then afterwards trimmed and finished. The so formed module is provided with the reinforcing ribs 7, the inverter 8 and other accessories to form the panel P.

In case, in the lamination step the transparent film 2 is calendared in order to impress a pattern.

Claims

1. Photovoltaic module (1) comprising:

- a first backsheet (Bl) which is substantially flat and flexible with respect to lying plane thereof;

- one or more photovoltaic cells (C) made of crystalline silicon and arranged on the upper face of the first backsheet (Bl) either directly or with the interposition of at least one first sheet of ethylene vinyl acetate (EVA1);

- at least one transparent plastic film (2) arranged on the photovoltaic cells (C) either directly or with the interposition of at least one second sheet of ethylene vinyl acetate (EVA2) in order to isolate them from the outside;

wherein the first backsheet (Bl), the photovoltaic cells (C), the transparent plastic film (2) and, if present, the sheets of ethylene vinyl acetate (EVA1, EVA2) are hot-rolled together to form a sandwich which is flexible with respect to the lying plane thereof and able to take a convex or concave configuration,

characterized by comprising a second backsheet (B2), flexible with respect to the lying plane thereof, positioned on the first backsheet (Bl), on the opposite side with respect to the cells (C), and a third sheet of ethylene vinyl acetate (EVA3) interposed between the first backsheet (Bl) and the second backsheet (B2), and

wherein also the second backsheet (B2) and the third sheet of ethylene vinyl acetate (EVA3) are hot-rolled together with the other members composing the sandwich structure of the module (1), and

wherein said second backsheet (B2) is a metal plate having thickness comprised between 0,5 mm and 5 mm so to be flexible but at the same time to prevent the photovoltaic cells from being deformed until breaking.

2. Photovoltaic module (1) according to claim 1, wherein the first backsheet (Bl) is made of a material selected from polyvinyl fluoride, polyethylene terephthalate, polyvinylidene fluoride, a thermoplastic elastomer or combinations thereof, or a metal.

3. Photovoltaic module (1) according to claim 2, wherein the first backsheet (Bl) is made of Tedlar or aluminium.

4. Photovoltaic module (1) according to any one of claims 1-3, wherein said transparent plastic film is made of a material selected from ethylene tetrafluoroethylene (ETFE), polyethylene terephthalate, fluoro ethylene propylene, polytetrafluoroethylene, or else it is a film obtained by rolling together one or more films made of said materials and/ or one or more films of ethylene vinyl acetate.

5. Photovoltaic module (1) according to any one of claims 1-4, wherein said transparent plastic film has a thickness in the range from 15 μιη to 100 μιη.

6. Photovoltaic module (1) according to any one of claims 1-5, wherein the photovoltaic cells are electrically connected by means of wires or strip conductors interposed either between the photovoltaic cells and the transparent film, or between the photovoltaic cells and the second sheet of ethylene vinyl acetate (EVA2).

7. Photovoltaic module (1) according to any one of claims 1-8, comprising either reinforcing ribs (7) extending at two or more edges of the module, or a reinforcing frame extending next to at least part of the perimeter of the module, in order to prevent the module itself to be twisted and bent beyond a desired threshold.

8. Photovoltaic module (1) according to claim 7, wherein said reinforcing ribs (7) and said reinforcing frame are made of metal profiles.

9. Photovoltaic module (1) according to claim 7 or claim 8, wherein at least some of said reinforcing ribs (7) and at least part of said reinforcing frame are provided with heat sinks.

10. Photovoltaic module according to any one of the preceding claims 1- 9, wherein said second metal sheet is itself a covering member for buildings so that the photovoltaic module is integrated in such a member.

11. Photovoltaic panel (P) comprising one or more photovoltaic modules (1) according to any one of claims 1-11.

12. Photovoltaic panel (P) according to claim 11, wherein the photovoltaic module or modules (1) are glued to a folded roofing sheet of the type used for covering buildings so as to form an assembly which can be directly installed on roofs.

13. Panel according to claim 12, wherein said folded roofing sheet is provided with one or more reinforcing ribs.

14. Method for manufacturing a photovoltaic module (1) with cells (C) made of crystalline silicon, comprising the steps of:

- a transparent plastic film (2) arranged either directly on the photovoltaic cells or with the interposition of at least one second sheet of ethylene vinyl acetate (EVA2) in order to isolate them from the outside;

b) hot-rolling the assembly of members to form a photovoltaic module (1) with a sandwich structure which is flexible with respect to the lying plane of the module itself,

characterized in that the assembly of members referred to in step a) further comprises a second backsheet (B2), made of metal and having a thickness comprised between 0,5 mm and 5 mm, which is flexible with respect to the lying plane thereof, positioned on the first backsheet (Bl) on the opposite side with respect to the cells (C), and a third sheet of ethylene vinyl acetate (EVA3) interposed between the first backsheet (Bl) and the second backsheet (B2).

15. Method according to claim 14, wherein the step b) is carried out until the transparent film (2) and the ethylene vinyl acetate (EVA1, EVA2, EVA3) are polymerized.

16. Method according to claim 14 or claim 15, wherein the step a) further comprises, before hot-rolling the assembly, arranging members (R) for electrically connecting the photovoltaic cells (C) in series and/ or in parallel.

17. Method according to any one of claims 14-16, wherein the first backsheet (Bl) is made of a material selected from polyvinyl fluoride, polyethylene terephthalate, polyvinylidene fluoride, a thermoplastic elastomer or combinations thereof, or it is made of a metal.

18. Method according to any one of claims 14-17, wherein said transparent plastic film is made of a material selected from ethylene tetrafluoroethylene (ETFE), polyethylene terephthalate (PET), fluoro ethylene propylene, polytetrafluoroethylene, or else it is a film obtained by rolling together one or more films made of said materials and/ or a film of ethylene vinyl acetate.

19. Method according to any one of the preceding claims 14-18, wherein the step b) is carried out in a hot-rolling oven under vacuum or under a controlled atmosphere using one or more pairs of opposite nip rollers to press the sandwich entering and leaving the oven so as to extract the controlled atmosphere out of its several layers before it is hot-rolled and to keep the oven isolated from the outside.

20. Method according to any one of the preceding claims 14-19, wherein said second backsheet is itself a covering member for roofs of buildings, so that the thus rolled module becomes itself a covering member for buildings which can be directly positioned in situ.