WO2018061036A1

WO2018061036A1 - A backsheet for a photovoltaic module

Info

Publication number: WO2018061036A1
Application number: PCT/IN2017/050441
Authority: WO
Inventors: Roshan Shah
Original assignee: Individual
Current assignee: Individual
Priority date: 2016-10-02
Filing date: 2017-09-29
Publication date: 2018-04-05
Anticipated expiration: 2019-04-02

Abstract

The invention relates to a backsheet for a photovoltaic module, the backsheet having a plurality of coextruded layers, wherein one or more coextruded layer comprises a polyolefin; and a polymer with a glass transition temperature lower than that of the polyolefin in at least one of the coextruded layers.

Description

TITLE OF THE INVENTION

A BACKSHEET FOR A PHOTOVOLTAIC MODULE

FIELD OF THE INVENTION

This disclosure relates generally to the field of photovoltaic modules particularly to the use of polymer films therein, and more specifically to an improved backsheet therein, and a process for making the backsheet.

BACKGROUND OF THE INVENTION

[001] Photovoltaic ("PV") modules are large-area optoelectronic devices that convert solar radiation directly into electrical energy. PV modules are made by interconnecting individually formed and separate solar cells, e.g., multi-crystalline or mono-crystalline silicon solar cells, and then mechanically supporting and protecting the solar cells against environmental degradation by integrating the cells into a laminated PV module. The laminated modules generally comprise a rigid and transparent protective front panel or sheet, and a rear panel or sheet which is typically called a backsheet. Forming a sandwiched arrangement between the front panel and backsheet are the interconnected solar cells and an encapsulant which is transparent to solar radiation. The front panel and backsheet encapsulate the solar cell(s) and provide protection from environmental damage . Alternatively, a thin film material may be deposited on a rigid transparent layer, such as glass, and bonded to a backsheet with a transparent adhesive.

[002] PV modules have been formed using a backsheet consisting of a thermoplastic polymer (e.g., a resin), glass, or some other material. Known backsheets, for example, comprises a laminated structure of polyvinyl fluoride (PVF)/polyester ( PET (/ethylene vinyl acetate (EVA or primer), also PVF/PVD F//PET//P V F/P V DF or PV F/PV DF//PET//E V A/PE or PET//Polyolefin. They are laminated together using adhesives due to which there is a high likelihood of these getting delaminated during the lifespan of the PV module, thereby, exposing the cells to environment and eventually degrading the PV module. Such a laminated structure, however, is not fully impervious to moisture, and as a consequence over time the power output and/or the useful life of PV modules made with this kind of backsheet material is reduced, e.g., due to electrical shorting resulting from absorbed moisture. Thus, the basic design and assembly process of PV modules can exhibit certain drawbacks.

[003] A goal of the PV industry, however, is to have PV modules with an effective working life of decades. Thus, the materials used in constructing PV modules are selected with concern for providing adequate resistance to damage from impact and physical and thermal shock, maximizing the amount of usable solar radiation received by the cells, avoiding short-circuiting and electrical leakage, and minimizing degradation from such environmental factors as moisture, temperature, and ultra-violet sunlight-induced chemical reactions. A further concern of the PV industry is that the useful life goal of PV modules be attained at a commercially acceptable cost.

[004] Both WO2013003541 and WO2013051403 discloses a multi-layer film laminate for an electronic device such as a photovoltaic module wherein the various layers of the film are laminated together to each other and there is a presence of a separate seal layer that seals the various layers to produce a single laminate. However, there is a high likelihood of this laminate getting delaminated over time.

[005] WO/2015/168174 discloses photovoltaic backsheets with multiple layers of co-extruded polymers wherein each layer may include polyolefin. However, machine direction crack propagation is seen after lamination to the EVA encapsulant on glass during the thermal cycling i.e. -45C to +80C.

[006] In addition to improving material properties of films, such as improved ductility, impact resistance, and thermal performance, materials for use in PV backsheets will need to maintain their operating performance in real world conditions, including, for example, during continuous use as backsheets on PV modules operating in a multitude of climate conditions.

SUMMARY OF INVENTION

[007] The present invention relates to a backsheet for a photovoltaic module with plurality of co-extruded layers, wherein one or more layer comprises of polyolefin and a polymer having low glass transition than that of polyolefin. The polyolefin can be polypropylene and/or polyethylene. In another aspect, the layers can also be laminated.

[008] In another aspect, the invention relates to process for preparing a backsheet for a photovoltaic module comprising the steps of co-extruding plurality of polymer layers, wherein one or more layer is prepared by mixing a polyolefin and a polymer with a glass transition lower than the polyolefin. The layers are then co-extruded in one extrusion to obtain a final extruded backsheet.

[009] The features and advantages described herein are not all-inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification and claims. Moreover, it should be noted that the language used in the specification has been selected principally for readability and not to limit the scope of the inventive subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

[001] Reference will be made to embodiments of the invention, examples of which may be illustrated in accompanying figures. These figures are intended to be illustrative, not limiting. Although the invention is generally described in context of these embodiments, it should be understood that it is not intended to limit the scope of the invention to these particular embodiments. [0010] Figure 1 illustrates a graph showing the thermal cycling test carried out bon the PV module comprising the backsheet in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0011] The present invention relates to a backsheet for a photovoltaic module. The said backsheet includes plurality of co-extruded layers, one or more layer comprising of a polyolefin and a polymer with a lower glass transition than that of the polyolefin added to improve the impact strength of the backsheet.

[0012] In an aspect of the invention, the polyolefin can be polypropylene and/or polyethylene.

[0013] The polypropylene is a homopolymer, copolymer or random copolymer and the polyethylene may be low density polyethylene (Low Density Polyethylene (LDPE), Linear low- density polyethylene (LLDPE), High Density Polyethylene (HDPE), Ultra High Molecular Weight Polyethylene (UHMWPE), optionally linear.

[0014] In another aspect of the invention, the low glass transition polymer may be selected from Styrenic Thermoplastic Elastomers, Styrenic Block Copolymers and/or Hydrogenated Styrenic Block copolymers like styrene-ethylene-butylene-styrene (SEBS), styrene-ethylene-butadiene- styrene (SEBS), hydrogenated polydiene polymers, Propylene based Ethylene -octene polymer or ethylene-propylene-diene-monomer (EPDM) to improve the impact strength of the backsheet.

[0015] In this regard, in a typical case, the backsheet does not consist of any adhesive or seal layers as the layers are co-extrudeded to each other. Numerous configurations and variations can be adapted to the invention. In another aspect, the layers may also be laminated.

[0016] Each layer may further comprise additives known within the art. The layers preferably comprise at least one additive selected from UV stabilizers, UV absorbers, anti-oxidants, thermal stabilizers, nucleating agents, flame retardants, halogen free flame retardants and/or hydrolysis stabilisers. When such additives, stabilizers are used, the polymer composition contains from 0.01 wt. % to 20 wt.% additive, based on the total weight of the polymer combination.

[0017] In a preferred embodiment, the weight percentage of polypropylene is between 50-80%, lower glass transition temperature polymer is between 10-30% and polyethylene is between 0.01-10 % based of the weight of each layer. The said backsheet can comprise three layers i.e. an air-side layer, a cell-side layer and a mid-layer. In some embodiments, the middle layer includes a pigment which may be any type of organic or inorganic pigment, including carbonaceous pigments, titanium dioxide, silica, calcium carbonate, hydrophobic silica, fly ash and/or carbon black. The weight percentage of the pigment in the middle layer is 1-15% of the weight of the backsheet. Additionally, said pigment in the backsheet may reflect light back to the solar cells, thereby increasing the efficiency of the module.

[0018] In another preferred embodiment, the layers of said backsheet may additionally comprise of additives such as antioxidants (AO), hindered amine light stabilizers (HALS), ultraviolet light absorbers (UV), antiozonants, organophosphorus, boron compounds, phosphates, organohalogens, metal hydroxides such as aliminium trihydroxide and magnesium dihydroxide and thio-di-propionate peroxide decomposers such as distearyl thio-di-propionate (DSTDP). The antioxidants (AO) have a weight percentage of 0.01-5% on the weight percentage of the protective film.

[0019] The UV light absorbers are selected from benzophenones and benzotriazoles such as hydroxyphenylbenzotriazole and hydroxyphenyltriazines. UV light absorbers are generally in the weight percentage range of 0.01-5 weight percentage on the weight percentage of the backsheet. Preferably, the HALS are in the weight percentage of 0.01-5 on the weight percentage of the protective backsheet. Additionally, the HALS include butanedioic acid, dimethyl ester, 4- hydroxy-2,2,6,6-tetramethyl-l-peperidine and its other derivatives. [0020] In another embodiment, the antioxidants consists of hindered phenols, secondary aromatic amines, benzofuranones, l,3,5-tris(4-tert-butyl-3-hydroxy-2,6-dimethylbenzyl, 1,3,5- triazine-2,4,6-(lH,3H,5H)-trione, and/or tris(2,4-di-t-butylphenyl) phosphite.

[0021] With the optional addition of one or more of these additives, the layers obtained by co- extruding fulfills all essential requirements for solar-cell module backsheet, such as weathering stability (UV and hydrolysis resistance), enhanced thermal cycling, heat resistance, mechanical protection, electrical insulation and good adhesion.

[0022] It is to be noted that the addition of polymer with lower glass transition temperature greatly improves the results during the thermal cycling tests at varied ranges of temperatures.

[0023] The thickness of the solar-cell module backing layer is preferably from 0.1 to 1 mm, more preferably from 0.15 to 0.8 mm, even more preferably from 0.2 to 0.75 mm.

[0024] In another aspect, the invention relates to process for preparing a backsheet for a photovoltaic module comprising the steps of co-extruding plurality of polymer layers, wherein each layer is prepared by mixing a polyolefin and a polymer with a glass transition lower than the polyolefin. In an aspect of the invention, the polyolefin used in different layers may be same or different. The co-extrusion of the plurality of layers avoids the need for the seal layer or adhesive layer thereby preventing delamination and extending the life of the PV module.

[0020] The present invention further relates to a solar-cell module containing the backsheet of the invention.

[0021] The advantages of the backsheet of the present invention includes avoiding delamination, faster production, production in one go thereby resulting in less defects, cost effectiveness, and no bubbling of the backsheet. Also, the process for production may be solventless, and an environment friendly process. The backsheet of the invention has a very good thermal conductivity resulting in better heat dissipation compared to other backsheets available in the market. The high thermal conductivity helps in keeping panel temperature lower thereby increasing the efficiency. EXAMPLE

The following experimental examples are illustrative of the invention but not limitative of the scope thereof:

A protective sheet according to the invention comprising of 3 coextruded layers: an air-side layer, a mid layer and a cell-side layer was prepared. Each layer had a thickness of approximately 75 microns, each mid layer had a thickness of approximately 55 microns and each cell -side layer had a thickness of approximately 69 microns. The sheets were prepared with the following formulation.

EXPERIMENTAL RESULTS

Thermal cycling test:

A comparative test between the sheet comprising a polymer with low transition temperature than polyolefin and the conventionally available sheets was conducted by subjecting the sheets to Thermal cycling test. The purpose of this test was to determine the ability of the PV module to withstand the effects such as material fatigue, temperature stresses etc. during fast changes of temperatures from 85°C to -40°C.

This test was conducted by putting a strain on PV module so that the subsequent effect of different coefficients of thermal expansion of parts of the PV module shows up the hidden defects like poor soldering, cracked cells, delamination, performance reduction and insulation resistance etc. .As ambient temperature and irradiance fluctuates, materials expand or contract. When adjacent materials have mismatched coefficients of thermal expansion (CTE) the interface experiences stress which causes aging such as solder joint fatigue. It was noted that the sheet comprising a polymer with low transition temperature than polyolefin was the only sheet which did not crack after the first 100 cycles. The thermal cycling test involved the sequence as illustrated in the Figure 1. the figure 1 illustrates a graph between the temperature of the module in °C v/s the time. In the first stage (1), the temperature was brought down to -40°C in 30 minutes. In the second stage (2), the dwell time at temperature of -40°C was held for 10 minutes. In the third stage (3), the temperature was increased to 85°C in 75 minutes. In the fourth stage (4), the dwell time at temperature of 85°C was held for 10 minutes. In the fifth stage (5), the temperature was again brought down to 0°C in 56 minutes.

[002] The foregoing description of the invention has been set merely to illustrate the invention and is not intended to be limiting. Since the modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to the person skilled in the art, the invention should be construed to include everything within the scope of the disclosure.

Claims

1. A backsheet for a photovoltaic module, the backsheet having a plurality of coextruded layers, wherein one or more coextruded layer comprises

a polyolefin; and

a polymer with a glass transition temperature lower than that of the polyolefin in at least one of the coextruded layers.

2. The backsheet as claimed in claim 1, wherein the low glass transition polymer is selected from Styrenic Thermoplastic Elastomers, Styrenic Block Copolymer, and Hydrogenated Styrenic Block Polymers.

3. The backsheet as claimed in claim 2, wherein the low glass transition polymer is selected from is styrene-ethylene-butylene-styrene (SEBS), styrene-ethylene-butadiene-styrene (SEBS), hydrogenated polydiene polymers, Propylene based ethylene-octene polymer or ethylene- proplylene-diene-monomer (EPDM).

4. The backsheet as claimed in claim 1, wherein the polyolefin is polypropylene and polyethylene.

5. The backsheet as claimed in claim 4, wherein the polypropylene is selected from a homopolymer, a coploymer and a random copolymer.

6. The backsheet as claimed in claim 2, wherein the polyethylene is selected from a linear low density polyethylene (LDPE), linear low density polyethylene(LLDPE), High Density polyethylene(HDPE) and ultra high molecular weight polyethylene(UHMWPE).

7. The backsheet as claimed in claim 1 wherein the backsheet has thickness ranging from 0.1 mm to 1 mm.

8. The backsheet as claimed in claim 1 further comprises of additives selected from ultraviolet stabilisers, pigments, antioxidants, thermal stabilizers, nucleating agents. Flame retardants, halogen free flame retardants, hydrolysis stabilisers and ultraviolet light absorbers or any combination thereof, in 0.01% to 20% wt. of the layer.

9. A method of preparing a backsheet for a photovoltaic module comprising the steps of: a) preparing one or more layer by mixing a polyolefin and a polymer having a

glass transition lower than the polyolefin;

b) co-extruding the layers to obtain the backsheet.

10. The method as claimed in claim 9 wherein the loading of polymer with glass transition temperature lower than the polyolefin is in a range from 1% w/w to 70% w/w.