Encapsulant of a photovoltaic module

Abstract

An encapsulant of a photovoltaic module, intended for coating a photovoltaic cell (10), including: a copolymer of ethylene-alkyl acrylate, the melt flow index (MFI) of the copolymer being 1 g/10 min to 40 g/10 min; and a silane making up 0.1% to 0.5% of the weight of the composition; wherein the encapsulant also includes a cross-linking agent making up 0.1% to 0.5% of the weight of the composition and wherein the copolymer makes up at least 99% of the weight of the composition. Also, a use of such an encapsulant in a photovoltaic module as well as to a photovoltaic module including such an encapsulant.

Claims

1. An encapsulant composition of a photovoltaic module, configured to encase a photovoltaic cell, the encapsulant composition comprising: an ethylene/alkyl acrylate copolymer that has a melt flow index, MFI, between 1 g/10 min and 40 g/10 min, wherein the alkyl acrylate is either methyl acrylate or butyl acrylate; a silane, representing between 0.1% and 0.5% of the weight of said encapsulant composition; wherein the encapsulant composition additionally comprises a crosslinking agent, wherein the crosslinking agent either is 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane or belongs to the monoperoxycarbonate family and represents between 0.1% and 0.5% of the weight of the encapsulant composition, wherein said copolymer represents at least 99% of the weight of said encapsulant composition, and wherein the encapsulant composition possesses a creep measurement less than 4 mm when laminated onto a glass structure and heated at 110° C. for 500 hours.

2. The encapsulant composition as claimed in claim 1 wherein, for the above said copolymer, the weight content of ethylene is between 50% and 85%, and the weight content of alkyl acrylate is between 15% and 50%.

3. The encapsulant composition as claimed in claim 1, wherein the silane consists of a vinyl silane or (meth)acrylic silanes.

4. The encapsulant composition as claimed in claim 3, wherein the silane consists of 3-(trimethoxysilyl)propyl methacrylate.

5. The encapsulant composition as claimed in claim 1, wherein the crosslinking agent represents between 0.1% and 0.3% of the weight of the encapsulant composition.

6. The encapsulant composition as claimed in claim 1, wherein the copolymer has a melt flow index, MFI, between 2 g/10 min and 10 g/10 min.

7. The encapsulant composition as claimed in claim 1, wherein the encapsulant composition consists of the copolymer, the abovesaid crosslinking agent and the abovesaid silane.

8. The encapsulant composition as claimed in claim 1, wherein the encapsulant composition additionally comprises additives intended to confer additional specific properties.

9. A photovoltaic module comprising the encapsulant composition as claimed in claim 1.

10. A photovoltaic module comprising a structure consisting of a combination of at least one encapsulant and a frontsheet or a backsheet, wherein the at least one encapsulant is an encapsulant composition as claimed in claim 1.

11. The encapsulant composition as claimed in claim 1, wherein the crosslinking agent in the encapsulant composition produces an incomplete crosslinking.

12. The encapsulant composition as claimed in claim 1, wherein no acetic acid formation is detected after exposure of said composition to DHT testing, wherein DHT testing occurs at 85° C. in 85% relative humidity for at least 2000 hours.

13. The encapsulant composition as claimed in claim 1, wherein said encapsulant does not become yellow after exposure of said composition to DHT testing, wherein DHT testing occurs at 85° C. in 85% relative humidity for at least 2000 hours.

14. The encapsulant composition as claimed in claim 1, wherein the crosslinking agent does not cause bubbles to form during a lamination step.

Description

DESCRIPTION OF THE APPENDED FIGURES

(1) The description which follows is given solely by way of illustration and without implied limitation with reference to the appended figures, in which:

(2) FIG. 1, which is already described, represents an example of a photovoltaic cell, the portions (a) and (b) being ¾ views, the portion (a) showing an individual cell before connection and the portion (b) a view after connection of 2 individual cells; the portion (c) is a top view of a complete photovoltaic cell.

(3) FIG. 2, which is already described, represents a cross section of a photovoltaic module, the “conventional” photovoltaic sensor of which is encapsulated by an upper encapsulant film and a lower encapsulant film.

DETAILED DESCRIPTION OF THE INVENTION

(4) As regards the ethylene/alkyl acrylate copolymer, it is a component well known to a person skilled in the art. The distinctive features specific to this copolymer, within the context of the present invention, essentially originate from the weight proportions of ethylene and of alkyl acrylate and from the melt flow index, MFI, of the copolymer, expressed in grams per 10 minutes and measured at 190° C. under a load of 2.16 kg.

(5) The weight content of ethylene being between 50% and 85%, preferably between 60% and 84%, and the weight content of alkyl acrylate is between 15% and 50%, preferably between 16% and 40%;

(6) The melt flow index (MFI) of the copolymer being between 1 g/10 min and 40 g/10 min, preferably between 2 g/10 min and 10 g/10 min.

(7) As nonlimiting example, the applicant company makes use commercially of a component known as LOTRYL®, which is an ethylene/alkyl acrylate copolymer.

(8) A person skilled in the art fully knows how to produce/manufacture such a copolymer, according to the different amounts of each of the two monomers. Hereinafter, the invention is presented with an ethylene/alkyl acrylate copolymer of specific type but it has been demonstrated by the proprietor that the encapsulant composition according to the invention meets the objectives set when the copolymer varies within the ranges of content of ethylene and of alkyl acrylate which are defined above, possibly in a slightly better way when said copolymer has contents of ethylene and of alkyl acrylate which are chosen within the ranges preferred for these two monomers.

(9) As regards the silane, these are chemical compounds which make possible the adhesion interactions between the encapsulant and the glass. As examples of silane, mention may be made of 3-(trimethoxysilyl)propyl methacrylate, vinyltrimethoxysilane or any other silane bearing a function that is reactive with respect to a peroxide-type crosslinking agent. Preferably, the silane in the composition according to the invention is 3-(trimethoxysilyl)propyl methacrylate. Nevertheless, equivalent or substantially equivalent results would be obtained by choosing another silane from the family of vinylsilanes or (meth)acrylic silanes.

(10) Regarding the crosslinking agent, this element, which decomposes to initiate and propagate chemical reactions (with the silane for the grafting of the latter to the copolymer chains) and crosslinking reactions (of the copolymer), is well known to a person skilled in the art and it does not present any difficulties for its manufacture/preparation.

(11) It should be noted here that a particular family of crosslinking agents corresponds best to the objectives set within the context of the present patent application: these are monoperoxycarbonates, and among these in particular OO-tert-butyl O-(2-ethylhexyl) monoperoxycarbonate which is sold especially by the applicant company under the trademark Luperox® TBEC.

(12) The composition forming the encapsulant according to the invention could optionally comprise a certain number of additives intended to confer additional specific properties.

(13) Plasticizers could be added in order to facilitate the processing and to improve the productivity of the process for the manufacture of the composition and of the structures. Mention will be made, as examples, of paraffinic, aromatic or naphthalenic mineral oils, which also make it possible to improve the adhesiveness of the composition according to the invention. Mention may also be made, as plasticizer, of phthalates, azelates, adipates or tricresyl phosphate.

(14) Adhesion promoters, although not necessary, may advantageously be added in order to improve the adhesiveness of the composition when the adhesiveness has to be particularly high. The adhesion promoter is a nonpolymeric ingredient; it may be organic, crystalline, inorganic and more preferably semi-inorganic semi-organic. Mention may be made, among these, of titanates.

(15) In this specific application of the composition with photovoltaic modules, as UV radiation is capable of resulting in a slight yellowing of the composition used as encapsulant of said modules, UV stabilizers and UV absorbers, such as benzotriazole, benzophenone and other hindered amines, may be added in order to ensure the transparency of the encapsulant during its lifetime. These compounds may, for example, be based on benzophenone or on benzotriazole. They may be added in amounts of less than 10% by weight of the total weight of the composition and preferably from 0.05% to 3%.

(16) It will also be possible to add antioxidants in order to limit the yellowing during the manufacturing of the encapsulant, such as phosphorus-based compounds (phosphonites and/or phosphites) and hindered phenolic compounds. These antioxidants may be added in amounts of less than 10% by weight of the total weight of the composition and preferably from 0.05% to 3%.

(17) Flame retardants may also be added. These retardants may be halogenated or nonhalogenated. Among the halogenated retardants, mention may be made of brominated products. Use may also be made, as nonhalogenated retardant, of phosphorus-based additives, such as ammonium phosphate, polyphosphate, phosphinate or pyrophosphate, melamine cyanurate, pentaerythritol, zeolites and the mixtures of these retardants. The composition may comprise these retardants in proportions ranging from 3% to 40%, with respect to the total weight of the composition.

(18) It is also possible to add pigments, such as, for example, titanium dioxide, dyeing compounds or brightening compounds in proportions generally ranging from 5% to 15%, with respect to the total weight of the composition.

(19) Fillers, in particular inorganic fillers, may also be added to improve the thermomechanical strength of the composition. Examples which will be given are, without implied limitation, silica, alumina or calcium carbonates or carbon nanotubes or also glass fibers. Use may also be made of modified or nonmodified clays which are mixed at the nanoscale; this makes it possible to obtain a more transparent composition.

(20) Crosslinking/Preparation of the Encapsulant and Production of an Encapsulant Film According to the Invention (Intended to be Incorporated in a Photovoltaic Module):

(21) Conventionally, a crosslinking is necessary in order to adjust the thermomechanical properties of the EVA-based encapsulant, in particular when the temperature becomes very high. In this particular case, within the context of the present invention, the crosslinking is not complete owing to a very low content of crosslinking agent(s), but allows the grafting of the silane to the copolymer chains and a partial crosslinking of this copolymer.

(22) The other elements of the composition, namely the silane and optionally the fillers, are added to the crosslinking agent and to the aforesaid copolymer in a conventional manner, well known to a person skilled in the art.

(23) With regard to the aspects targeted above, the handbook entitled “Handbook of Polymer Foams and Technology”, in particular on pages 198 to 204, provides additional instructions to which a person skilled in the art may refer.

(24) As regards the aspects of the invention relating to the use of the thermoplastic composition in a photovoltaic module, a person skilled in the art may refer, for example, to the “Handbook of Photovoltaic Science and Engineering”, Wiley, 2003. This is because the composition of the invention can be used as encapsulant or encapsulant-backsheet in a photovoltaic module, the structure of which is described in connection with the appended figures.

(25) Materials Employed in Order to Form the Test Formulations:

(26) Lotryl® 17BA07: ethylene/butyl acrylate copolymer, the acrylate content of which is 17% by weight of the copolymer and the MFI of which is 7 g/10 min (190° C., 2.13 kg). It is obtained according to an autoclave process and its melting point is 89° C.

(27) In the tables of results presented below, this Lotryl® is denoted by the initials 17BA07.

(28) Lotryl® 20MA08: ethylene/methyl acrylate copolymer, the acrylate content of which is 20% by weight of the copolymer and the MFI of which is 8 g/10 min (190° C., 2.13 kg). It is obtained according to an autoclave process and its melting point is 75° C.

(29) In the tables of results presented below, this Lotryl® is denoted by the initials 20MA08.

(30) Lotryl® 35BA40: ethylene/butyl acrylate copolymer, the acrylate content of which is 35% by weight of the copolymer and the MFI of which is 40 g/10 min (190° C., 2.13 kg). It is obtained according to an autoclave process and its melting point is 66° C.

(31) In the tables of results presented below, this Lotryl® is denoted by the initials 35BA40.

(32) Lotryl® 35BA320: ethylene/butyl acrylate copolymer, the acrylate content of which is 35% by weight of the copolymer and the MFI of which is 320 g/10 min (190° C., 2.13 kg). It is obtained according to an autoclave process and its melting point is 65° C.

(33) In the tables of results presented below, this Lotryl® is denoted by the initials 35BA320.

(34) Lotryl® 28MA07: ethylene/methyl acrylate copolymer, the acrylate content of which is 28% by weight of the copolymer and the MFI of which is 7 g/10 min (190° C., 2.13 kg). It is obtained according to an autoclave process and its melting point is 68° C.

(35) In the tables of results presented below, this Lotryl® is denoted by the initials 28MA07.

(36) Lotryl® 7BA01: ethylene/butyl acrylate copolymer, the acrylate content of which is 7% by weight of the copolymer and the MFI of which is 1 g/10 min (190° C., 2.13 kg). It is obtained according to an autoclave process and its melting point is 105° C.

(37) In the tables of results presented below, this Lotryl® is denoted by the initials 7BA01.

(38) Evatane® 3345PV: ethylene/vinyl acetate copolymer, the acetate content of which is 33% by weight of the copolymer and the MFI of which is 45 g/10 min (190° C., 2.13 kg). In the tables of results presented below, this Evatane® is denoted by the initials 3345PV.

(39) Dynasylan MEMO: 3-MethacryloyloxypropylTrimethoxySilane sold by Evonik.

(40) In the tables of results presented below, this silane is denoted by the initials MTS.

(41) Luperox® TBEC: OO-tert-butyl O-(2-ethylhexyl) monoperoxycarbonate sold by the applicant company Arkema, denoted hereinafter by TBEC.

(42) Luperox® 101: 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, sold by the applicant company Arkema, denoted hereinafter by 101.

(43) Production of the Test Films and Formulations:

(44) Preparation of the Films:

(45) The encapsulant films are obtained by extrusion of granules of impregnated polymers:

(46) The silanes, and where appropriate, the peroxide are added by impregnation of Lotryl or Evatane granules. Granules and liquid are placed in a flask and the flask is positioned on a roll mixer for 3 hours at a speed of 60 rotations per minute.

(47) After impregnation, these granules, and also optionally additional granules, are placed in the feed hopper of an extruder with a slot die having a width of 10 cm.

(48) The extrusion is carried out at a temperature appropriate to the composition; thus, for the compositions based on Luperox TBEC, this temperature is limited to 90° C., as, above this temperature, the peroxide would decompose. For compositions based on Luperox 101, this temperature may reach 100° C. to 110° C.

(49) This extrusion makes it possible to obtain a reel of film, the drawing of which at the extruder outlet is adjusted so as to obtain a film with a thickness of between 350 and 550 μm (micrometers).

(50) Preparation of the Test Modules:

(51) In order to characterize the formulations, test modules are obtained by hot lamination.

(52) The structure of a test module can be varied according to the characterizations to be carried out: Measurement of creep and of optical properties by transmission: Glass (4 mm)/Encapsulant film/Glass (4 mm) Measurement of adhesion: Glass (4 mm)/Encapsulant film/Apolhya backsheet

(53) The laminator used is provided by P energy. The lamination conditions are dependent on the composition of the laminated films.

(54) Thus, in the case of a formulation based on TBEC, the cycle observed is the following:

(55) TABLE-US-00001 Duration (s) T (° C.) V.sub.up (mbar) V.sub.down (mbar) Prestart 10 85 0 1000 1 10 85 0 0 2 180 85 0 0 3 10 85 900 0 4 10 85 1000 0 5 600 150 1000 0 6 360 150 1000 0 7 10 150 0 0 8 10 150 0 0 9 — 50 0 1000

(56) Furthermore, in the case of a formulation based on Luperox 101, the temperature is adjusted owing to the higher decomposition temperature of this peroxide. The cycle observed is therefore the following:

(57) TABLE-US-00002 Duration (s) T (° C.) V.sub.up (mbar) V.sub.down (mbar) Prestart 10 110 0 1000 1 10 110 0 0 2 180 110 0 0 3 10 110 900 0 4 10 110 1000 0 5 600 170 1000 0 6 360 170 1000 0 7 10 170 0 0 8 10 170 0 0 9 — 50 0 1000
Tests Carried Out on the Test Specimens (Compositions E1 to E4 and CE1 to CE5):

(58) The present invention is illustrated in more detail by the following nonlimiting examples.

(59) The compositions denoted E1, E2, E3 and E4 in the table below are compositions in accordance with the invention while the compositions CE1, CE2, CE3, CE4 and CE5 are compositions according to the prior art and/or not in accordance with the present invention.

(60) TABLE-US-00003 Content (% Content (% Content (% by weight of by weight of by weight of Constituent 1 the composition) Constituent 2 the composition) Constituent 3 the composition) E1 28MA07 99.5 MTS 0.3 TBEC 0.2 E2 17BA07 99.6 MTS 0.3 101 0.1 E3 20MA08 99.6 MTS 0.3 101 0.1 E4 35BA40 99.3 MTS 0.3 TBEC 0.4 CE1 EVA3345PV 98.2 MTS 0.3 TBEC 1.5 CE2 20MA08 99.7 MTS 0.3 — — CE3 7BA01 99.6 MTS 0.3 101 0.1 CE4 35BA320 99.2 MTS 0.3 TBEC 0.5 CE5 35BA40 98.7 MTS 0.3 TBEC 1.0

(61) It will be noted that the test specimens targeted above exhibit identical amounts of silane, fixed at 0.3% of the weight of the composition. Nevertheless, additional tests have made it possible to identify that the amount of silane in the composition could be between 0.1% and 0.5% by weight of said composition.

(62) The examples of the composition according to the invention all have the same thicknesses but it is clearly understood that a person skilled in the art could vary them as a function of the application of the photovoltaic module and of the performance of the latter.

(63) Measurements of Optical Properties by Transmittance:

(64) The optical properties by transmittance are measured on glass/encapsulant/glass structures using a spectrocolorimeter of the Minolta brand. The measurement conditions are as follows:

(65) Wavelength: 360 nm-740 nm (nanometers) Illuminant: C Angle: 2° Measurement opening: LAV 25 mm (millimeters) Background: “Cera” white plate+light well
Two pieces of numerical data are taken from this measurement: Haze: the haze corresponds to the degree of haze of the structure studied. It is calculated according to the standard ASTM D-1003-007) Transparency: the degree of transparency is calculated by taking the mean transmittance value between 400 and 740 nm, corrected by respective contributions of the glass layers and of the glass/air and glass/encapsulant interfaces, then standardized to a thickness of 200 μm. The transparency was also evaluated during DHT (damp heat test—85° C./85% relative humidity/2000 h) aging.
In order to meet the requirements of the invention, the transparency test should display a result above 96% and the haze test a result below 20.
Creep Test:
The creep test is carried out on glass/encapsulant/glass structures (with glass sheets having a length L=70 mm). After lamination, the test modules are placed on a metal structure forming an angle of 70° with the horizontal. Each module is held back by an edge covering a portion of the thickness of the first glass layer.
This structure is placed at 100° C. in an oven. Under the weight of the second glass layer, creep may be observed. The creep value measured is thus the distance traveled by the second glass sheet after 500 hours under these conditions. This distance is between 0 mm (no creep) and 70 mm (complete creep, separation of the structure).
In order to meet the requirements of the invention, the creep measurement should display a result of less than 4 mm.
Adhesion Test on a Glass Layer:
The degree of adhesion between the encapsulant and the glass is measured on glass/encapsulant/backsheet structures using a 90° peel test carried out at 50 mm/min (millimeters per minute) on a Zwick 1445 universal testing machine. The backsheet used for this measurement is a monolayer consisting of Apolhya® manufactured and sold by the applicant company. The measurement conditions are as follows: Rate of displacement of the crosspiece: 50 mm/min Test specimen cut-out width: 10 mm Peel angle: 90°
The adhesion result is expressed in N/mm.
In order to meet the requirements of the invention, the adhesion measurement should display a result of greater than 3.5 N/mm.

(66) Tests on the encapsulant were also carried out in order to confirm that this novel structure retains excellent properties, that is to say identical properties, relative to the properties of an encapsulant in accordance with that described in the document WO 09138679, namely in particular relating to its mechanical, thermomechanical and fire retardant properties and its electrical insulation properties. These tests proved to be positive.

(67) The yellowing properties of the encapsulant, formulated as described in the present invention, were evaluated during DHT (damp heat test—85° C./85% relative humidity/2000 h) aging. The results obtained proved to be better than those obtained for a formulation according to the prior art.

(68) The compositions according to the invention thus meet the criteria to be able to be very advantageously used as binder or encapsulant in solar modules.

(69) Results of the Tests Carried Out on the Test Specimens of the Different Formulations:

(70) TABLE-US-00004 Opt. Transmittance Prop. Creep 70°/ to DHT 2000 h 110° C. Compo- Adhesion % T.sub.400 μm % T.sub.400 μm (mm/70 sitions (N/mm) (%) Haze (%) mm - 500 h) E1 6.7 99.5 1.5 98 2.5 E2 3.8 94 15   93.8 1.5 E3 6.2 98 3   97.6 2   E4 4.6 99.7 0.9 99.2 3.5 CE1* 6.2* 99.6* 1*  93.5 0*  CE2 0.9 97.6 3.5 97 70   CE3 3.5 91 25   89.8 1   CE4 4.2 99.6 0.9 99.3 12   CE5 No measurement performed, presence of a very large quantity of bubbles in the encapsulant layer *The results obtained on the CE1 test specimen (formulation according to the prior art) are good, but the formation of acetic acid during the DHT aging is detected and gives rise to an intense yellowing of this encapsulant. This phenomenon is not observed on any of the samples produced according to the present invention.