Polyolefin-based rear-face film for photovoltaic modules

Abstract

The invention relates to a photovoltaic module comprising a translucent cover layer, a layer which is located under the translucent cover layer and which contains at least one photovoltaically active cell that is embedded into an embedding material, and a backsheet (1) arranged under said layer. The backsheet (1) is designed as a polyolefinic film with at least two plies (2, 3, 4), wherein at least one ply (2) has a black pigment. The backsheet (1) has a reflection of more than 30%, preferably more than 40%, in particular more than 50%, for electromagnetic radiation in the range of 800 to 1700 nm.

Description

[0001] POLYOLEFIN-BASED BACKSHEET FOR PHOTOVOLTAIC MODULES

[0002] Description

[0003] The invention relates to a photovoltaic module having a translucent cover layer, a layer located thereunder containing at least one photovoltaically active cell embedded in an embedding material, and a backsheet arranged thereunder, wherein the backsheet is formed as a polyolefinic film having at least two layers and wherein at least one layer comprises a black pigment.

[0004] Photovoltaic modules, or solar modules, are widely known and are increasingly being used for sustainable electricity generation. Photovoltaic cells convert sunlight directly into electrical current, which is then conducted via suitable electrical conductors and fed into a grid.

[0005] The cover layer can be made of glass with a thickness of 4-5 mm or a polymer front sheet, for example. The embedding material is usually based on ethylene vinyl acetate, which is applied as a film or strip on and around the electrically active cells and cross-linked using heat.

[0006] The backsheet of these modules forms a rear cover for the layer containing the photovoltaic cells. This backsheet is commonly made from a fluoropolymer-based plastic film, as fluoropolymers are highly temperature-resistant. The backsheet is formed as a multi-layer laminate, with the individual layers bonded together using suitable adhesives.

[0007] However, such backsheets, constructed as multi-layer laminates, have the disadvantage that the bonding areas between the individual layers represent weak points, as the layers can separate from each other over extended use. DE 10 2012 022 450 A1 discloses a photovoltaic module with a plastic-based backsheet that does not require lamination adhesive. The photovoltaic module is frameless, and the backsheet is coated with an adhesive on at least its top and bottom surfaces. At least one fastening element for attaching the photovoltaic module to a substructure is glued to the underside of the backsheet.

[0008] The backsheet typically provides electrical insulation for the solar module and protects the photovoltaic module from environmental influences, primarily moisture. Backsheets typically contain several layers of different materials, each of which can serve a different purpose. Most commonly, at least one of the materials contains a fluoropolymer. In addition to fluoropolymers, commonly used materials include polyethylene terephthalate (PET) polymers and polyethylene naphthalate (PEN) polymers.

[0009] This structure represents an economic and ecological disadvantage due to the materials it contains, which are comparatively expensive and environmentally unfriendly. In many cases, bonding layers or other additional components must be provided to ensure sufficient cohesion between the individual layers and to bond them together and with the encapsulation layers of the solar module.

[0010] Other backsheets comprise layers of polyester and / or polyamides. A disadvantage of many state-of-the-art materials is that the individual layers cannot be combined into a multilayer backsheet in a single process step; instead, they must be bonded together separately. This can lead to damage to the entire photovoltaic module, particularly with regard to the long-term stability of the comparatively unstable interlayer bond.EP 2 390 093 B1 describes a multilayer backsheet for solar modules, comprising an optional cover layer, an insulating layer, and a backsheet. The cover layer, the insulating layer, and the backsheet contain a polyolefin as the main component. The polyolefin is selected from polyethylene homopolymers and copolymers, the polyethylene copolymers containing more than 50 mol% ethylene-derived units and alpha-olefin comonomers. The layers are coextruded, and the backsheet contains no interlayers. Polyolefin-based films can be recycled and, from this point of view, are more environmentally friendly than the known backsheets.

[0011] Many European countries have signed the European Plastics Pact. According to this pact, all packaging and plastic designs must be reusable or recyclable by 2025. Therefore, the focus will also be on more sustainable product design, such as the construction of more monomaterials, better separation at the source, and further technical developments around automatic sorting and decolorization.

[0012] According to the state of the art, the backsheets used today for photovoltaic modules are black. To produce black backsheets, carbon black is usually added to the masterbatch before extrusion. However, carbon black has the disadvantage that it does not reflect the infrared radiation used to separate or sort plastic streams in recycling, preventing the plastic material from being sorted into the appropriate grade. A polyolefin backsheet therefore cannot be separated and therefore cannot be recycled.

[0013] The object of the present invention is to provide a photovoltaic module with a backsheet that meets the requirements of the Plastics Pact 2025 and is fully recyclable. The backsheet should be detectable and separable in conventional sorting systems. Furthermore, the backsheet should optimally support the performance of the photovoltaic module. The backsheet should protect the photovoltaic module from environmental influences and ensure its use for as long as possible. Furthermore, the backsheet should be harmless to health and ecologically sustainable.

[0014] This object is achieved according to the invention by a photovoltaic module according to the main claim and by a method and use according to the independent main claims. Preferred variants can be found in the subclaims, the description, the exemplary embodiment, and the drawing.

[0015] According to the invention, the backsheet has a reflection of the electromagnetic radiation in the range 800 to 1700 nm of more than 30%, preferably more than 40%, in particular more than 50%.

[0016] Reflection refers to the rebounding of waves at an interface where the wave impedance or refractive index of the propagating medium changes. The angle of reflection corresponds exactly to the angle of incidence, and both angles lie in the same plane as the normal.

[0017] Near infrared (NIR) is the region of the electromagnetic spectrum that extends beyond visible light in the longer wavelength range. This range of infrared light extends from 780 to 3000 nm and thus encompasses the IR-A and IR-B spectral ranges. The region of the infrared spectrum used for the detection of polymer material in recycling sorting systems lies in the range from 800 to 1700 nm.

[0018] Ideally, at least one layer of the backsheet contains a black pigment and is free of the commonly used carbon black. Nevertheless, this layer is formed with a high degree of blackness and can reflect infrared light. This increases the efficiency of the photovoltaic module and enables the polyolefin backsheet to be sorted in modern recycling plants.

[0019] In a multi-layer backsheet, two or three or more layers may contain a black pigment.

[0020] In a particularly advantageous variant of the invention, the black pigment can be designed as an organic pigment.

[0021] Advantageously, the organic pigment can be a pigment or pigment mixture from the perylene group. Perylene is a chemical compound from the class of polycyclic aromatic hydrocarbons. It is a solid that forms shiny yellow platelets. Substituted perylenes are color-intense and resistant to heat and, in general, to chemicals, making them particularly suitable for use in weather-exposed photovoltaic modules.

[0022] Ideally, the black pigment, preferably the black organic pigment, can be in the form of a masterbatch and can be mixed together with the polyolefinic starting material, in particular the polypropylene, melted and extruded into a film.

[0023] In a favorable variant of the invention, the black layer has a proportion of a masterbatch with black pigment of more than 2 wt. %, preferably more than 4 wt. %, in particular more than 5 wt. % and / or less than 16 wt. %, preferably less than 12 wt. %, in particular less than 8 wt. %. Even a small proportion of the masterbatch for one layer is sufficient to achieve the deep black formation of the layer and to enable the reflection of infrared light. In one variant of the invention, the black layer of the backsheet is not reflective, but allows the electromagnetic radiation in the range from 800 to 1700 nm to be fully transmitted through the black layer.

[0024] In a particularly advantageous variant of the invention, at least one layer of the backsheet is white, for example, due to a proportion of titanium dioxide. Ideally, the white layer reflects electromagnetic radiation in the range of 800 to 1700 nm almost completely.

[0025] In a particularly advantageous variant of the invention, the black, transmissive layer and the white, reflective layer are arranged one above the other, with the black layer facing the photovoltaic cells. Incident electromagnetic radiation transmits through the black layer and is reflected by the white layer. The reflected electromagnetic radiation transmits through the black layer of the backsheet and enters the photovoltaic cells, thereby advantageously increasing the performance of the photovoltaic module.

[0026] In addition, a backsheet with a combination of a white and a black layer that reflects electromagnetic radiation in the range of 800 to 1700 nm can be detected by sorting systems during recycling and is therefore also recyclable.

[0027] The special properties of the backsheet are achieved through the use and selection of special polymers.

[0028] Ideally, the backsheet is made of a single-grade polypropylene with a density of more than 0.88 g / cm 3 , preferably more than 0.89 g / cm 3 and / or less than 0.93 g / cm 3 , preferably less than 0.92 g / cm 3 and / or whose melt flow rate according to ASTM 1238 (at 230 °C and 2.16 kg) is more than 0.6 g / 10 min, preferably more than 0.7 g / 10 min and / or less than 1.0 g / 10 min, preferably less than 0.9 g / 10 min.

[0029] A particularly advantageous feature is that the backsheet is not designed as a laminate, meaning that delamination processes cannot be initiated and / or promoted by weather influences. The likelihood of a defective backsheet and, in the medium term, a defective or functionally impaired photovoltaic module is significantly reduced by the coextruded design.

[0030] Under these circumstances, it seems particularly advantageous that the backsheet is free of laminating adhesives. These can promote delamination of a laminated backsheet and thus lead to a defective photovoltaic module.

[0031] In the interest of ecological design, the backsheet is free of fluoropolymers and / or similar elements, which ensures the recyclability of the backsheet after the end of the photovoltaic module's service life.

[0032] The filler content can be determined using well-known measurement methods such as ashing. A sample with a known initial weight is heated to a temperature at which the polymer thermally decomposes but the filler does not. A temperature of 560 °C, for example, has proven effective for this purpose. The sample weight is then measured again. The polymer content per square meter can be calculated from the difference between the initial and final weight.

[0033] As an alternative to ashing, a TGA measurement is possible, in which the weight of a sample is continuously measured during heating. This test method can also clearly differentiate between polymer and filler and allows the polymer content of the film to be determined. In a favorable variant of the invention, at least one additional layer comprises an inorganic filler, with the filler content being more than 5 wt.%, preferably more than 7.5 wt.%, in particular more than 10 wt.%.

[0034] Ideally, the filler is titanium dioxide, which allows for the creation of a white layer with advantageous opacity.

[0035] Opacity is the opposite of transparency. It is a measure of the light impermeability or opacity expressed as a percentage. Specifically, the opacity of a completely opaque film is 100%, and a completely or fully transparent film has an opacity of 0%.

[0036] In a particularly advantageous variant, the additional layer of the backsheet has an opacity according to DIN 53416 of more than 55%, preferably more than 70%, especially more than 80%. This allows the additional layer to reflect infrared light almost completely.

[0037] In a particularly inexpensive variant, two or more layers of the backsheet contain an inorganic filler in the form of titanium dioxide and reflect infrared light much more intensively.

[0038] In the three-layer backsheet variant, the middle layer is thicker than the two surrounding layers by a factor of more than 1.25, preferably more than 1.5, and especially more than 1.75. Ideally, the middle layer contains a proportion of titanium dioxide, which allows for particularly good infrared light reflection.

[0039] The film thickness was measured according to DIN 53370 and reported as an average value. The backsheet has a thickness of less than 250 pm, preferably less than 210 pm, in particular less than 180 pm, and / or more than 100 pm, preferably more than 130 pm, in particular more than 160 pm.

[0040] Ideally, the backsheet is designed to be particularly thin and save material. Conventional and well-known backsheets are usually thicker to achieve the mechanical properties of the backsheet according to the invention. The backsheet's construction thus complies with the design specifications of the Plastics Pact. Nevertheless, due to the special manufacturing process, the film exhibits excellent properties that make it an ideal backsheet, especially for photovoltaic modules.

[0041] To ensure recyclability and thus also sorting in modern waste separation plants, such as the sink-float process, the density of the backsheet is less than 1.00 g / cm 3 , preferably less than 0.99 g / cm 3 , in particular less than 0.98 g / cm 3 and / or more than 0.70 g / cm 3 , preferably more than 0.80 g / cm 3 , in particular more than 0.90 g / cm 3 .

[0042] ISO 4892 specifies a procedure in which test specimens are exposed to xenon arc radiation and moisture in an apparatus to simulate the weathering effects that occur when materials are exposed to global radiation in real, end-use environments or to global radiation behind window glass.

[0043] The test method described in ISO 4892-2 is also known as the xenon test, also known as the weather meter or lightfastness test. The xenon test quickly shows how well plastics are resistant to weathering influences such as sunlight and rain. This is why this test method according to 4892-2 is also called artificial accelerated weathering. In the xenon test according to ISO 4892-2, samples are placed in a test chamber where they are then exposed to a xenon arc lamp. This light is filtered to allow the exact light to pass through which the backsheet will be exposed in practice. The most important filter types are so-called daylight filters and window glass filters. Daylight filters illuminate the product with a spectrum very similar to that of sunlight. A window glass filter creates a light spectrum that corresponds to the sunlight passing through a standard window.This means that the samples are protected from UV light, the cause of most damage to plastics.

[0044] Ideally, the backsheet has a UV resistance according to ISO 4892-2 of more than 15,000 h, preferably more than 17,500 h, in particular more than 20,000 h and is therefore very durable for use as a backing in a photovoltaic module that is installed outdoors and subject to weathering.

[0045] Photovoltaic modules can become damaged if the backsheet allows moisture to penetrate. This can lead to delamination of the backsheet and / or cracking. The front cover can also be damaged by moisture penetration, significantly reducing the performance of the photovoltaic module.

[0046] The determination of water vapor permeability is carried out according to DIN 53116 using a gravimetric measuring method. A test container filled with a desiccant is sealed with a backsheet film and exposed to a defined test climate. The amount of water permeating through the sample is determined by weighing. The water quantity can be in the range of 1 - 200 g / (m 2 ■ d) be detected. The detection limit also depends on the sample properties and the sample thickness. Ideally, the backsheet has a water vapor transmission rate of less than 20 g / (m 2 ■ d), preferably less than 2 g / (m 2 ■ d), in particular of less than 1 g / (m 2 ■ d) according to DIN 53122-1 and is therefore ideally suited as a rear wall for photovoltaic modules.

[0047] Films and sheets up to 1 mm thick are tested using strip or shoulder specimens. According to ISO 527-3 and ASTM D 882, strips are the usual specimen shape. This specimen shape can be easily produced using a cutting press or a film cutter. Shoulder specimens are often used in quality control. In this case, a direct-measuring extensometer is required, which can record the specimen strains directly on the specimen. Typical test results include yield stress, yield strain, maximum stress, and nominal strain at break.

[0048] The tensile properties are determined according to DIN EN ISO 527. In the tensile test, a sample strip of a backsheet film is stretched at a constant speed specified in the test standard and the force F is recorded with the change in length AL of the test section Lo.

[0049] Ideally, the backsheet has a tensile strength in the machine direction according to DIN EN ISO 527-3 of more than 500 MPa, preferably more than 650 MPa, and especially more than 750 MPa. This makes the backsheet film particularly stiff and tough, allowing it to achieve the necessary tension as a backsheet in a photovoltaic module.

[0050] Shrinkage of plastics refers to a change in the dimensional stability of test specimens at temperatures T > TG (amorphous) or T > Ts (semi-crystalline), which is caused by the recovery of molecular orientations and the relaxation of residual stresses. These orientations arise as a result of the processing process (extrusion, injection molding, or deep drawing) and are therefore dependent on processing parameters. These parameters include the temperature of the mold and the melt, the injection and holding pressure, the flow path length, and the cooling gradient of the backsheet film.

[0051] In a favorable variant of the invention, the backsheet exhibits a shrinkage according to DIN 55543-4 of less than 10%, preferably less than 2%, in particular less than 1%. This makes the backsheet ideally suited to withstand the enormous temperature fluctuations between sunlight during the middle of the day and cool nighttime temperatures without exhibiting any expansion or shrinkage.

[0052] In an alternative variant of the invention, the backsheet can have an adhesion promoter layer. This can, for example, create a permanent bond with the embedding material that will remain stable even after years of outdoor use of a photovoltaic module.

[0053] The dielectric breakdown voltage is determined according to ASTM D149. The tests are conducted at 60 Hz. There are three basic methods for determining the dielectric strength of an insulator. These methods are the short-term method, the slow-rise method, and the step method. Each of these three methods has the same basic setup, which consists of placing the test specimen between two electrodes in air or oil.

[0054] For the most common test, the short-time method, a voltage is applied across the two electrodes and increased at a uniform rate from zero until dielectric breakdown occurs. Breakdown occurs when electrical arcing pierces the sample or decomposition occurs within the sample. The rate of voltage increase is determined by the time it takes for the sample to reach dielectric breakdown. The slow-ramp method starts at 50% of the breakdown voltage determined by the short-time method and increases uniformly.

[0055] The step-by-step procedure begins at 50% of the short-term test, then the voltage is increased in equal steps for a specified period of time until breakdown occurs. The test is sometimes performed in oil to prevent arcing from the electrode to the ground.

[0056] Advantageously, the backsheet has a dielectric breakdown voltage of at least 20 kV and can thus form a secure back wall for a photovoltaic module.

[0057] In an alternative variant of the invention, the backsheet may have a coating that provides additional protection against the penetration of moisture or other weather influences.

[0058] In a further variant of the invention, the photovoltaic module is frameless. This can be achieved particularly well by using an adhesive backsheet. This significantly reduces the mass of the photovoltaic module and also allows for flexible mounting on a wide variety of substrates.

[0059] According to the invention, the method for producing a photovoltaic module comprises the following steps: The polypropylene is mixed with a masterbatch or with titanium dioxide and coextruded to form a backsheet with at least two layers. A proportion of a masterbatch with black pigment of more than 2 wt.%, preferably more than 4 wt.%, in particular more than 6 wt.% and / or less than 16 wt.%, preferably less than 12 wt.%, in particular less than 8 wt.% is incorporated into at least one layer of the backsheet. With the aid of embedding material, for example ethylene vinyl acetate, the electrically active cell is connected between a translucent tray and the backsheet to form a photovoltaic module.

[0060] According to the invention, a recyclable backsheet is used as a backsheet in a photovoltaic module.

[0061] Further advantages and features of the invention will become apparent from the description of an embodiment with reference to a drawing and from the drawing itself.

[0062] This shows

[0063] Fig. 1 is a schematic representation of the backsheet according to the invention.

[0064] Fig. 1 shows a schematic representation of the backsheet 1 according to the invention. In this embodiment, the backsheet 1 has three layers 2, 3, 4. Layers 2 and 4 are approximately the same thickness, while layer 3 is twice as thick. Layers 3 and 4 are white and contain 10 wt.% TiO2. Layer 2 is black, faces the electrically active cell, and contains 6 wt.% masterbatch from Avient with the number PE 94000817. The backsheet 1 is made of Repsol Inspire 137 polypropylene, is free of carbon black, and reflects 60% of electromagnetic radiation with a wavelength in the range of 800 to 1700 nm. In addition, the backsheet 1 has a coating 5 on layer 4, which faces the weathered environment, which protects the backsheet from the effects of weathering.

Claims

Patent claims Photovoltaic module a. with a light-permeable cover layer, b. a layer located underneath which contains at least one photovoltaically active cell which is embedded in an embedding material, c. and a backsheet (1) arranged underneath, i. wherein the backsheet (1) is designed as a polyolefin film with at least two layers (2, 3, 4), ii. wherein at least one layer (2) has a black pigment, characterized in that the backsheet (1) has a reflection of the electromagnetic radiation in the range 800 to 1700 nm of more than 30%, preferably more than 40%, in particular more than 50%. Photovoltaic module according to claim 1, characterized in that the layer (2) has a proportion of a masterbatch with black pigment of more than 2% by weight, preferably more than 4% by weight, in particular more than 5% by weight and / or less than 16% by weight, preferably less than 12% by weight.-%, in particular less than 8 wt.%.

3. Photovoltaic module according to claim 1 or 2, characterized in that the backsheet (1) is made of a single-grade polypropylene whose density is more than 0.88 g / cm 3 , preferably more than 0.89 g / cm 3 and / or less than 0.93 g / cm 3 , preferably less than 0.92 g / cm 3 and / or whose melt flow rate according to ASTM 1238 (at 230 °C and 2.16 kg) is more than 0.6 g / 10 min, preferably more than 0.7 g / 10 min and / or less than 1.0 g / 10 min, preferably less than 0.9 g / 10 min.

4. Photovoltaic module according to one of claims 1 to 3, characterized in that at least one further layer (3, 4) comprises an inorganic filler, wherein the proportion of filler is more than 5 wt.%, preferably more than 7.5 wt.%, in particular more than 10 wt.%.

5. Photovoltaic module according to one of claims 1 to 4, characterized in that the backsheet (1) has a thickness of less than 250 pm, preferably less than 210 pm, in particular less than 180 pm and / or more than 100 pm, preferably more than 130 pm, especially more than 160 pm. Photovoltaic module according to one of claims 1 to 5, characterized in that the backsheet (1) has a UV resistance according to ISO 4892-2 of more than 15,000 h, preferably of more than 17,500 h, in particular of more than 20,000 h. Photovoltaic module according to one of claims 1 to 6, characterized in that the backsheet (1) has a water vapor transmission rate of less than 20 g / (m 2 ■ d), preferably less than 2 g / (m 2 ■ d), in particular of less than 1 g / (m 2■ d) according to DIN 53122-1. Photovoltaic module according to one of claims 1 to 7, characterized in that the backsheet (1) has a coating (5). Photovoltaic module according to one of claims 1 to 8, characterized in that the backsheet (1) has a tensile strength in the machine direction according to DIN EN ISO 527-3 of more than 500 MPa, preferably more than 650 MPa, in particular more than 750 MPa. Photovoltaic module according to one of claims 1 to 9, characterized in that the backsheet (1) has an adhesion promoter layer.

11. Photovoltaic module according to one of claims 1 to 10, characterized in that the backsheet (1) has a dielectric breakdown voltage of at least 20 kV.

12. Photovoltaic module according to one of claims 1 to 11, characterized in that the layer (3) is a factor thicker than the Layers (2, 4), wherein the factor is more than 1.25, preferably more than 1.5, in particular more than 1.

75.

13. A method for producing a photovoltaic module comprising the following steps: Coextrusion of a backsheet (1), Connecting a light-permeable tray with an electrically active cell and a backsheet (1) with an embedding material to form a photovoltaic module, characterized in that at least one layer (2) of the backsheet has a proportion of a masterbatch with black pigment of more than 2 wt.%, preferably of more than 4 wt.%, in particular of more than 5 wt.% incorporated and / or of less than 16 wt.%, preferably of less than 12 wt.%, in particular of less than 8 wt.% incorporated.

14. Use of a recyclable backsheet (1) according to one of the Claims 1 to 12 as a backsheet in a photovoltaic module.

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