Patent Application
20140000682
1. Field of the Disclosure
The present invention relates to back-sheets for photovoltaic cells and modules, and more particularly to back-sheets with integrated electrically conductive circuits, and back-contact photovoltaic modules with electrically conductive circuits integrated into the back of the modules.
2. Description of the Related Art
A photovoltaic cell converts radiant energy, such as sunlight, into electrical energy. In practice, multiple photovoltaic cells are electrically connected together in series or in parallel and are protected within a photovoltaic module or solar module.
As shown in
Photovoltaic cells typically have electrical contacts on both the front and back sides of the photovoltaic cells. However, contacts on the front sunlight receiving side of the photovoltaic cells can cause up to a 10% shading loss.
In back-contact photovoltaic cells, all of the electrical contacts are moved to the back side of the photovoltaic cell. With both the positive and negative polarity electrical contacts on the back side of the photovoltaic cells, electrical circuitry is needed to provide electrical connections to the positive and negative polarity electrical contacts on the back of the photovoltaic cells. U.S. Patent Application No. 2011/0067751 discloses a back-contact photovoltaic module with a back-sheet having patterned electrical circuitry that connects to the back contacts on the photovoltaic cells during lamination of the solar module. The circuitry is formed from a metal foil that is adhesively bonded to a carrier material such as polyester film or Kapton® polyimide film. The carrier material may be adhesively bonded to a protective layer such as a backsheet laminate comprised of polyester and fluoropolymer film layers. The layers are provided to bring different properties to the protective back-sheet such as strength, electrical resistance, moisture resistance, and durability.
PCT Publication No. WO2011/011091 discloses a back-contact solar module with a back-sheet with a patterned adhesive layer with a plurality of patterned conducting ribbons placed thereon to interconnect the solar cells of the module. Placing and connecting multiple conducting ribbons between solar cells is time consuming and difficult to do consistently.
Multilayer laminates have been employed as photovoltaic module back-sheets. One or more of the laminate layers in such back-sheets conventionally comprise a highly durable and long lasting polyvinyl fluoride (PVF) film. PVF films are typically laminated to other polymer films that contribute mechanical and dielectric strength to the back-sheet, such as polyester films, as for example polyethylene terephthalate (PET) films. There is a need for durable and economical back-sheets for a back-contact photovoltaic module with integrated conductive circuitry.
An integrated back-sheet for a solar cell module with a plurality of electrically connected solar cells is provided. The back-sheet comprises a homogeneous polymer substrate having opposite first and second surfaces. The polymer substrate has a thickness of at least 0.25 mm, and is comprised of 20 to 95 weight olefin-based elastomer and 5 to 75 weight percent of inorganic particulates, based on the weight of the polymer substrate. A plurality of electrically conductive metal wires are attached to the homogeneous polymer substrate with the homogeneous polymer substrate adhering to said metal wires. The metal wires are at least partially embedded in the homogeneous polymer substrate. The metal wires may be disposed directly on and partially embedded in the surface of said homogeneous polymer substrate. Alternatively, the metal wires may be buried in the homogeneous polymer substrate with vias connecting the buried metal wires in the homogeneous polymer substrate to the first surface of the polymer substrate.
In one embodiment, the homogeneous polymer substrate has a thickness of from 0.4 to 1.25 mm. In a preferred embodiment, the homogeneous polymer substrate comprises 25 to 90 weight percent olefin-based elastomer, 10 to 70 weight percent of inorganic particulates, and 5 to 50 weight percent of adhesive selected from thermoplastic polymer adhesives and rosin based tackifiers, based on the weight of the polymer substrate.
In another preferred embodiment, the polymer substrate comprises 10 to 65 weight percent of inorganic particulates based on the weight of the polymer substrate, and the inorganic particulates have an average particle diameter between and including any two of the following diameters: 0.1, 0.2, 15, 45, and 100 microns. The inorganic particulates are preferably selected from the group of calcium carbonate, titanium dioxide, kaolin and clays, alumina trihydrate, talc, silica, silicates, antimony oxide, magnesium hydroxide, barium sulfate, mica, vermiculite, alumina, titania, wollastonite, boron nitride, and combinations thereof.
A back-contact solar module is also provided. The module has a front light emitting substrate, a solar cell array of at least four solar cells each having a front light receiving surface, an active layer that generates an electric current when the front light receiving surface is exposed to light, and a rear surface opposite the front light receiving surface, the rear surface having positive and negative polarity electrical contacts thereon. The front light receiving surface of each of the solar cells of the solar cell array is preferably disposed on the front light emitting substrate. The homogeneous polymer substrate with electrically conductive wires, as described above, is adhered to the rear surface of the solar cells. The positive and negative polarity electrical contacts on the rear surface of the solar cells of the solar cell array are physically and electrically connected to said electrically conductive metal wires attached to said homogeneous polymer substrate.
In one embodiment of the solar module, the plurality of metal wires are buried in the homogeneous polymer substrate, a first surface of the homogeneous polymer substrate directly adheres to the rear surface of said solar cells, and vias connect the buried metal wires in the homogeneous polymer substrate to the first surface of said homogeneous polymer substrate. A polymeric conductive adhesive is disposed in the vias and connect to the first surface of said homogeneous polymer substrate, such that the plurality of metal wires are physically and electrically connected to the positive and negative polarity electrical contacts on the rear surface of the solar cells by the polymeric conductive adhesive.
In another embodiment of the back-contact solar module, the electrically conductive metal wires are disposed on the first surface of the homogeneous polymer substrate. A polymeric interlayer dielectric layer having opposite first and second sides is disposed between the electrically conductive metal wires on the back-sheet and the rear surface of the solar cells of the solar cell array. The interlayer dielectric layer has openings arranged in a plurality of columns, and the interlayer dielectric layer is adhered on its first side to the rear surface of the solar cells of the solar cell array and on its second side to the first side of said polymer substrate over said conductive metal wires. The plurality of columns of openings in the interlayer dielectric layer are arranged over the conductive wires adhered to the first side of the polymer substrate such that the openings in each column of openings are aligned with and over one of the plurality of electrically conductive wires. The openings in the interlayer dielectric layer are aligned with the positive and negative polarity electrical contacts on the rear surfaces solar cells of the solar cell array, and the positive and negative polarity electrical contacts on the rear surfaces of the solar cells are electrically connected to the conductive wires through the openings in the interlayer dielectric layer.
The detailed description will refer to the following drawings, wherein like numerals refer to like elements:
a and 2b are schematic plan views of the back side of arrays of back-contact solar cells.
a is a schematic representations of a back-sheet with integrated wires.
b is a schematic representations of another embodiment of a back-sheet with integrated wires.
a-4c are cross-sectional views illustrating one disclosed process for forming a back-contact solar cell module in which a back-sheet has integrated conductive wires connected to the back contacts of solar cells.
d shows another embodiment of a back-contact solar cell module in which a back-sheet has integrated conductive wires placed for connection to the back contacts of solar cells.
a-5d illustrate steps of a process for forming a back-contact solar cell module in which an array of back-contact solar cells are electrically connected in series by conductive wires that are integrated into the back-sheet of the solar cell module.
To the extent permitted by the United States law, all publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety.
The materials, methods, and examples herein are illustrative only and the scope of the present invention should be judged only by the claims.
The following definitions are used herein to further define and describe the disclosure.
The terms “comprises,” “comprising,” “includes,” “including,” “has,” “having” or any other variation thereof, are intended to cover a non-exclusive inclusion. For example, a process, method, article, or apparatus that comprises a list of elements is not necessarily limited to only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Further, unless expressly stated to the contrary, “or” refers to an inclusive or and not to an exclusive or. For example, a condition A or B is satisfied by any one of the following: A is true (or present) and B is false (or not present), A is false (or not present) and B is true (or present), and both A and B are true (or present).
The terms “a” and “an” include the concepts of “at least one” and “one or more than one”.
Unless stated otherwise, all percentages, parts, ratios, etc., are by weight.
When the term “about” is used in describing a value or an end-point of a range, the disclosure should be understood to include the specific value or end-point referred to.
As used herein, the terms “sheet”, “layer” and “film” are used in their broad sense interchangeably. A “frontsheet” is a sheet, layer or film on the side of a photovoltaic module that faces a light source and may also be described as an incident layer. Because of its location, it is generally desirable that the frontsheet has high transparency to the incident light. A “back-sheet” is a sheet, layer or film on the side of a photovoltaic module that faces away from a light source, and is generally opaque. In some instances, it may be desirable to receive light from both sides of a device (e.g., a bifacial device), in which case a module may have transparent layers on both sides of the device.
“Encapsulant” means material used to encase the fragile voltage-generating solar cell layer to protect it from environmental or physical damage and hold it in place in a photovoltaic module. Encapsulant layers are conventionally positioned between the solar cell layer and the incident front sheet layer, and between the solar cell layer and the back-sheet backing layer. Suitable polymer materials for these encapsulant layers typically possess a combination of characteristics such as high transparency, high impact resistance, high penetration resistance, high moisture resistance, good ultraviolet (UV) light resistance, good long term thermal stability, adequate adhesion strength to front-sheets, back-sheets, other rigid polymeric sheets and solar cell surfaces, and long term weatherability.
As used herein, the terms “photoactive” and “photovoltaic” may be used interchangeably and refer to the property of converting radiant energy (e.g., light) into electric energy.
As used herein, the terms “photovoltaic cell” or “photoactive cell” or “solar cell” mean an electronic device that converts radiant energy (e.g., light) into an electrical signal. A photovoltaic cell includes a photoactive material layer that may be an organic or inorganic semiconductor material that is capable of absorbing radiant energy and converting it into electrical energy. The terms “photovoltaic cell” or “photoactive cell” or “solar cell” are used herein to include photovoltaic cells with any types of photoactive layers including, crystalline silicon, polycrystalline silicon, microcrystal silicon, and amorphous silicon-based solar cells, copper indium(gallium)diselenide solar cells, cadmium telluride solar cells, compound semiconductor solar cells, dye sensitized solar cells, and the like.
As used herein, the term “photovoltaic module” or “solar module” (also “module” for short) means an electronic device having at least one photovoltaic cell protected on one side by a light transmitting front sheet and protected on the opposite side by an electrically insulating protective back-sheet.
The term “copolymer” is used herein to refer to polymers containing copolymerized units of two different monomers (a dipolymer), or more than two different monomers.
Disclosed herein are integrated back-sheets for back-contact solar cell modules and processes for forming such integrated back-sheets. Also disclosed are back-contact solar modules with an integrated conductive circuitry and processes for forming such back-contact solar modules with integrated circuitry.
Arrays of back-contact solar cells are shown from their rear side in
Each of the solar cells 22 has multiple positive and negative polarity contacts on back side of the solar cell. The contacts on the back side of the solar cells are typically made of a metal to which electric contacts can be readily formed, such as silver or platinum contact pads. The contacts are typically formed from a conductive paste comprising an organic medium, glass frit and silver particles, and optionally inorganic additives, which is fired at high temperature to form metal contact pads. The solar cells shown in
The disclosed integrated back-sheet comprises an electrically insulating polymer substrate to which electrical circuitry is embedded or buried. The disclosed polymer substrate is a homogeneous polymer substrate having opposite first and second surfaces and a thickness of at least 0.25 mm. The polymer substrate comprising 20 to 95 weight percent olefin-based elastomer and 5 to 70 weight percent of inorganic particulates, based on the weight of the polymer substrate. The olefin-based elastomer is a copolymer comprised of at least 50 weight percent of monomer units selected from ethylene and propylene monomer units based on the weight of the olefin-based elastomer.
One preferred polymer substrate, the olefin-based elastomer is comprised of an ethylene propylene diene terpolymer (“EPDM”). EPDM is an ethylene-propylene elastomer with a chemically saturated, stable polymer backbone comprised of ethylene and propylene monomers combined in a random manner. A non-conjugated diene monomer is terpolymerized in a controlled manner on the ethylene-propylene backbone to provide reactive unsaturation in a side chain available for vulcanization. Two of the most widely used diene termonomers are ethylidene norbornene (ENB) and dicyclopentadiene (DCPD). Different dienes incorporate with different tendencies for introducing long chain branching or polymer side chains that influence processing and rates of vulcanization by sulfur or peroxide cures. Specialized catalysts are used to polymerize the monomers including Zeigler-Natta catalysts and metallocene catalysts. Particularly useful EPDM terpolymers are comprised of 40 to 90 mole percent ethylene monomer, 2 to 60 mole percent propylene monomer, and 0.5 to 8 mole percent diene monomer. Specific examples of these EPDM terpolymers include ethylene propylene norbornadiene terpolymer and ethylene propylene dicyclopentadiene terpolymer. EPDM terpolymers are commercially available from DSM Elastomers, Dow Chemical Company, Mitsui Chemicals and Sumitomo Chemical Company among others. The EPDM polymers preferably have Mooney viscosity of 15 to 85 at 125° C. when tested according to ASTM D 1646.
Another preferred substrate is one in which the olefin-based elastomer is a copolymer comprised of at least 50 weight percent of ethylene and/or propylene derived units copolymerized with a different alpha olefin monomer unit selected from C2-20 alpha olefins. Such preferred olefin-based elastomers are of high molecular weight with a melt index of less than 25 g/10 min, and more preferably less than 15 g/10 min, and even more preferably less than 10 g/10 min based on ASTM D1238. Such preferred olefin-based elastomers are polymerized using constrained geometry catalysts such as metallocene catalysts. The preferred olefin-based elastomers provide excellent electrical insulation, good long term chemical stability, as well as high strength, toughness and elasticity. A preferred olefin-based elastomer is comprised of more than 70 wt % propylene derived units copolymerized with comonomer units derived from ethylene or C4-20 alpha olefins, for example, ethylene, 1-butene, 1-hexane, 4-methyl-1-pentene and/or 1-octene. A preferred propylene-based elastomer is a semicrystalline copolymer of propylene units copolymerized with ethylene units using constrained geometry catalysts, having a melt index of less than 10 g/10 min (ASTM D1238), that can be obtained from ExxonMobil Chemical of Houston, Tex., under the product names “Vistamaxx™ 6102” and “Vistamaxx™ 6202”. Such propylene-based elastomers are generally described in U.S. Pat. No. 7,863,206. Another preferred olefin-based elastomer is comprised more than 70 wt % ethylene derived units copolymerized with comonomer units derived from C3-20 alpha olefins, for example, 1-propene, isobutylene, 1-butene, 1-hexane, 4-methyl-1-pentene and/or 1-octene. A preferred ethylene-based elastomer is a flexible and elastic copolymer comprised of ethylene units copolymerized with alpha olefin units using constrained geometry catalysts, having a melt index of 5 g/10 min (ASTM D1238; 190° C./2.16 Kg), that can be obtained from the Dow Chemical Company of Midland, Mich. under the product name Affinity™ EG8200G. Such ethylene-based elastomers are generally described in U.S. Pat. Nos. 5,272,236 and 5,278,236.
The olefin-based elastomer containing substrate further comprises 5% to 75% by weight of inorganic particulates, and more preferably 10% to 70% of inorganic particulates, and even more preferably 25% to 65% of inorganic particulates. The inorganic particulates preferably comprise amorphous silica or silicates such as crystallized mineral silicates. Preferred silicates include clay, kaolin, wollastonite, vermiculite, mica and talc (magnesium silicate hydroxide). Other useful inorganic particulate materials include calcium carbonate, alumina trihydrate, antimony oxide, magnesium hydroxide, barium sulfate, alumina, titania, titanium dioxide, zinc oxide and boron nitride. Preferred inorganic particulate materials have an average particle size less than 100 microns, and preferably less than 45 microns, and more preferably less than 15 microns. If the particle size is too large, defects, voids, pin holes, and surface roughness of the film may be a problem. If the particle size is too small, the particles may be difficult to disperse and the viscosity may be excessively high. Average particle diameters of the inorganic particulates are preferably between and including any two of the following diameters: 0.1, 0.2, 1, 15, 45 and 100 microns. More preferably, the particle diameter of more than 99% of the inorganic particulates is between 0.1 and 45 microns, and more preferably between about 0.2 and 15 microns.
The inorganic particulate material adds reinforcement and mechanical strength to the sheet and it reduces sheet shrinkage and curl. Platelet shaped particulates such as mica and talc and/or fibrous particles provide especially good reinforcement. The inorganic particulates also improve heat dissipation from the solar cells to which the integrated back-sheet is attached which reduces the occurrence of hot spots in the solar cells. The presence of the inorganic particulates also improves the fire resistance of the back-sheet. The inorganic particulates also contribute to the electrical insulation properties of the back-sheet. The inorganic particulates may also be selected to increase light refractivity of the back-sheet which serves to increase solar module efficiency and increase the UV resistance of the back-sheet. Inorganic particulate pigments such as titanium dioxide make the sheet whiter, more opaque and more reflective which is often desirable in a photovoltaic module back-sheet layer. The presence of the inorganic particulates can also serve to reduce the overall cost of the olefin-based elastomer containing layer.
In one preferred embodiment, the olefin-based elastomer containing substrate layer is comprised of one or more of the above-described olefin-based polymers combined with one or more tackifiers or thermoplastic polymer adhesives. For example, the olefin-based elastomer and tackifiers or thermoplastic polymer adhesives may be mixed by known compounding processes. In one aspect, the olefin-based elastomer containing substrate comprises 20 to 95% by weight of olefin-based elastomer as described above, and 1 to 50% by weight of one or more of tackifiers and thermoplastic polymer adhesives, and more preferably and 5 to 40% by weight of one or more of tackifiers and thermoplastic polymer adhesives, and even more preferably and 10 to 30% by weight of one or more of tackifiers and thermoplastic polymer adhesives, based on the weight of the substrate layer. The tackifiers and/or thermoplastic polymer adhesives serve to improve the adhesion of the olefin-based elastomer containing substrate to the conductive circuit and other layers of the photovoltaic module, such as the back of the solar cells, an optional interlayer dielectric layer, or optional thermoplastic polymer protective layers on a surface of the olefin-based elastomer containing substrate facing away from the solar cells.
Tackifiers useful in the disclosed back-sheet substrate include hydrogenated rosin-based tackifiers, acrylic low molecular weight tackifiers, synthetic rubber tackifiers, hydrogenated polyolefin tackifiers such as polyterpene, and hydrogenated aromatic hydrocarbon tackifiers. Two preferred hydrogenated rosin-based tackifiers include FloraRez 485 glycerol ester hydrogenated rosin tackifier from Florachem Corporation and Stabelite Ester-E hydrogenated rosin-based tackifier from Eastman Chemical.
A preferred thermoplastic adhesive is a polyolefin plastomer such as a non-aromatic ethylene-based copolymer adhesive plastomer of low molecular weight with a melt flow index of greater than 250. Such polyolefin adhesive materials are highly compatible with the olefin-based elastomer, they have low crystallinity, they are non-corrosive, and they provide good adhesion to fluoropolymer films. A preferred polyolefin plastomer is Affinity™ GA 1950 polyolefin plastomer obtained from Dow Chemical Company of Midland, Mich. Other thermoplastic polymer adhesives useful in the disclosed olefin-based elastomer containing back-sheet substrate include ethylene copolymer adhesives such as ethylene acrylic acid copolymers and ethylene acrylate and methacrylate copolymers. Ethylene copolymer adhesives that may be used as the thermoplastic adhesive include copolymers comprised of at least 50 wt % ethylene monomer units, copolymerized in one or more of the following: ethylene-C1-4 alkyl methacrylate copolymers and ethylene-C1-4 alkyl acrylate copolymers; ethylene-methacrylic acid copolymers, ethylene-acrylic acid copolymers, and blends thereof; ethylene-maleic anhydride copolymers; polybasic polymers formed of ethylene monomer units with at least two co-monomers selected from C1-4 alkyl methacrylate, C1-4 alkyl acrylate, ethylene-methacrylic acid, ethylene-acrylic acid and ethylene-maleic anhydride; copolymers formed by ethylene and glycidyl methacrylate with at least one co-monomer selected from C1-4 alkyl methacrylate, C1-4 alkyl acrylate, ethylene-methacrylic acid, ethylene-acrylic acid, and ethylene-maleic anhydride; and blends of two or more of these ethylene copolymers. Another thermoplastic adhesive useful in the olefin-based elastomer containing substrate layer of the disclosed integrated back-sheet is an acrylic hot melt adhesive. Such an acrylic hot melt adhesive may serve as the thermoplastic adhesive on its own or in conjunction with an ethylene copolymer adhesive to improve the adhesion of the olefin-based elastomer layer of the back-sheet to the electric wires and/or to an external fluoropolymer film. One preferred acrylic hot melt adhesive is Euromelt 707 US synthetic hot melt adhesive from Henkel Corporation of Dusseldorf, Germany. Other thermoplastic adhesives that may be utilized in the olefin-based elastomer substrate layer include polyurethanes, synthetic rubber, and other synthetic polymer adhesives.
The olefin-based elastomer containing back-sheet substrate may comprise additional additives including, but are not limited to, plasticizers such as polyethylene glycol, processing aides, flow enhancing additives, lubricants, dyes, flame retardants, impact modifiers, nucleating agents to increase crystallinity, antiblocking agents such as silica, thermal stabilizers, hindered amine light stabilizers (HALS), UV absorbers, UV stabilizers, antioxidants, dispersants, surfactants, primers, and reinforcement additives, such as glass fiber and the like. Compounds that help to catalyze cross-linking reactions in EPDM such as inorganic oxides like magnesium oxide or peroxide may also be used. Such additives typically are added in amounts of less than 3% by weight of an EPDM-containing substrate. The total of the additional additives preferably comprises less than 10% by weight of the EPDM-containing substrate and more preferably less than 5% by weight of the EPDM-containing substrate.
a and 3b show an embodiment of the disclosed olefin-based elastomer containing integrated back-sheet. The back-sheet 30 shown in
The thickness of the olefin-based elastomer containing substrate layer ranges from about 0.2 mm to about 2.5 mm or more, and more preferably about 0.25 mm to about 2 mm, and more preferably about 0.4 mm to about 1.5 mm. Where the integrated electric circuits are fully embedded in the olefin-based elastomer containing substrate as shown in
Conductive wires, such as the substantially parallel pairs of electrically conductive wires 42 and 44 may be adhered directly to the surface of the olefin-based elastomer containing substrate 32 that will face the rear surface of the solar cells of the solar cell array or they may be partially embedded in the surface as shown in
The wires 42 and 44 are preferably conductive metal wires. The metal wires are preferably comprised of metal selected from copper, nickel, tin, silver, aluminum, indium, lead, and combinations thereof. In one embodiment, the metal wires are coated with tin, nickel or a solder and/or flux material. The conductive wires may be coated with an electrically insulating material such as a plastic sheath so as to help prevent short circuits in the solar cells when the wires are adhered on the surface of the substrate 32 and are positioned over the back of an array of solar cells. Where the conductive wires are coated with an insulating material, the insulating material can be formed with breaks where the wires are exposed to facilitate the electrical connection of the wires to the back contacts of the solar cells.
The electrically conductive wires preferably each have a cross sectional area of at least 1.5 mm2 along their length, and more preferably have a cross sectional area of at least 2 mm2 along their length. Preferably, the electrically conductive wires have a thickness (depth) of at least 0.5 mm, and preferably a thickness of about 1 to 2.5 mm. The electrically conductive wires of the integrated back-sheet may have any cross sectional shape, but ribbon shaped wires having a width and thickness where the wire width is at least three times greater than the wire thickness, and more preferably where the wire width is 3 to 15 times the wire thickness, have been found to be especially well suited for use in the integrated back-sheet because wider wires makes it easier to align the wires with the back contacts of the solar cells when the integrated back-sheet is formed and applied to an array of back-contact solar cells. Solar cell tabbing wire such as aluminum or copper tabbing wire may be used. In
In one preferred embodiment, the conductive wires are at least partially embedded in the surface of the olefin-based elastomer containing back-sheet substrate. Preferably, the wires are partially embedded in the substrate in order to securely attach the wires to the back-sheet. In a preferred embodiment, the wires are embedded to at least 20% of their thickness in the surface of the substrate, and more preferably to at least 50% of the wire thickness. A top surface of the wires may remain exposed so that electrical contacts can be formed between the solar cell back contacts and the wire circuits of the back-sheet as shown in
The conductive wires on the integrated back-sheet should be long enough to extend over multiple solar cells, and they are preferably long enough to cover all of the solar cells in a column of solar cells in the solar cell array to which the integrated back-sheet is applied. Where the wires are attached to the surface of the olefin-based elastomer containing substrate, the wires can be attached by a batch hot pressing process or a continuous roll-to-roll process where the electrically conductive wires are continuously heated and fed into a nip where the wires are brought into contact with the olefin-based elastomer containing back-sheet substrate and adhered to the substrate by heating the wires and/or the substrate at the nip so as to make the substrate surface tacky. Alternatively, the olefin-based elastomer containing back-sheet substrate can be extruded with the wires fed into the substrate surface during the extrusion process. Where the wires are fully buried in the olefin-based elastomer containing substrate, the wires can be fed between top and bottom layers of the olefin-based elastomer containing substrate as the substrate is being extruded from a die. In another embodiment, the wires and the olefin-based elastomer containing substrate are heated and pressed in a batch press to partially or fully embed the wires into the surface of the olefin-based elastomer containing substrate, or to fully bury the wires between layers of the olefin-based elastomer containing substrate. Heat and pressure may also be applied to the substrate and wires at a heated nip so as to partially or fully embed or bury the conductive wires in the wire mounting layer.
Where the solar cells of the array will be connected in parallel, the full length wires can be used as shown in
In order to prevent electrical shorting of the solar cells, it may be necessary to apply an electrically insulating encapsulant layer or dielectric layer between the conductive wires on the olefin-based elastomer containing substrate and the back of the solar cells of the back-contact solar cell array. This dielectric layer is provided to maintain a sufficient electrical separation between the conductive wires and the back of the solar cells. The dielectric layer, known as an interlayer dielectric (ILD), may be applied as a sheet over all of the wires and the wire mounting layer, or as strips of dielectric material just over the electrically conductive wires. It is necessary to form openings in the ILD as for example by die cutting or punching sections of the ILD, that will be aligned over the back contacts and through which the back contacts will be electrically connected to the conductive wires. Alternatively, an ILD may be applied by screen printing. The printing can be on the back of the solar cells or over the wires on the back-sheet, and can cover the entire area between the back-sheet and the solar cell array or it may be printed only in the areas where the wires need to be prevented from contacting the back of the solar cells. The ILD can be applied to the wires and the back-sheet or it can be applied to the back of the solar cells before the olefin-based elastomer containing substrate and conductive wires are applied over the back of the solar cell array. Alternatively the ILD may be applied as strips over the wires on the portions of the back side of the solar cells over which the conductive wires will be positioned. The thickness of the ILD will depend in part on the insulating properties of the material comprising the ILD, but preferred polymeric ILDs have a thickness in the range of 5 to 500 microns, and more preferably 10 to 300 microns and most preferably 25 to 200 microns. Where the conductive wires on the surface of the olefin-based elastomer containing substrate have a complete insulating coating or sheath, it may be possible to eliminate the ILD between the electrically conductive wires on the integrated back-sheet and the back side of the back-contact solar cells to which the integrated back-sheet is applied. Likewise, where the wires are buried in the olefin-based elastomer containing substrate as shown in
Where an ILD is used, the ILD is preferably comprised of an insulating material such as a thermoplastic or thermoset polymer. For example, the ILD may be an insulating polymer film such as a polyester, polyethylene or polypropylene film. In one embodiment, the ILD is comprised of a PET polymer film that is coated with or laminated to an adhesive or an encapsulant layer such as an EVA film. Preferably, the ILD is comprised of a material that can be die cut or punched, or that can be formed with openings in it. Polymeric materials useful for forming the ILD may also include ethylene methacrylic acid and ethylene acrylic acid, ionomers derived therefrom, or combinations thereof. The ILD may also comprise films or sheets comprising poly(vinyl butyral) (PVB), ethylene vinyl acetate (EVA), poly(vinyl acetal), polyurethane (PU), linear low density polyethylene, polyolefin block elastomers, ethylene acrylate ester copolymers, such as poly(ethylene-co-methyl acrylate) and poly(ethylene-co-butyl acrylate), silicone polymers and epoxy resins. The ionomers are thermoplastic resins containing both covalent and ionic bonds derived from ethylene/acrylic or methacrylic acid copolymers. In some embodiments, monomers formed by partial neutralization of ethylene-methacrylic acid copolymers or ethylene-acrylic acid copolymers with inorganic bases having cations of elements from Groups I, II, or III of the Periodic table, notably, sodium, zinc, aluminum, lithium, magnesium, and barium may be used. The term ionomer and the resins identified thereby are well known in the art, as evidenced by Richard W. Rees, “Ionic Bonding In Thermoplastic Resins”, DuPont Innovation, 1971, 2(2), pp. 1-4, and Richard W. Rees, “Physical 30 Properties And Structural Features Of Surlyn Ionomer Resins”, Polyelectrolytes, 1976, C, 177-197. Other suitable ionomers are further described in European patent EP1781735, which is herein incorporated by reference.
Preferred ethylene copolymers for use in an ILD layer include the adhesives described above that can be mixed with the olefin-based elastomer containing substrate. Such ethylene copolymers are comprised of ethylene and one or more monomers selected from the group of consisting of C1-4 alkyl acrylates, C1-4 alkyl methacrylates, methacrylic acid, acrylic acid, glycidyl methacrylate, maleic anhydride and copolymerized units of ethylene and a comonomer selected from the group consisting of C4-C8 unsaturated anhydrides, monoesters of C4-C8 unsaturated acids having at least two carboxylic acid groups, diesters of C4-C8 unsaturated acids having at least two carboxylic acid groups and mixtures of such copolymers, wherein the ethylene content in the ethylene copolymer preferably accounts for 60-90% by weight. A preferred ethylene copolymer for the ILD includes a copolymer of ethylene and another α-olefin. Ethylene copolymers are commercially available, and may, for example, be obtained from DuPont under the trade-names Bynel®, Elvax® and Elvaloy®.
The ILD may further contain any additive or filler known within the art. Such exemplary additives include, but are not limited to, plasticizers, processing aides, flow enhancing additives, lubricants, pigments, titanium dioxide, calcium carbonate, dyes, flame retardants, impact modifiers, nucleating agents to increase crystallinity, antiblocking agents such as silica, thermal stabilizers, hindered amine light stabilizers (HALS), UV absorbers, UV stabilizers, anti-hydrolytic agents, dispersants, surfactants, chelating agents, coupling agents, adhesives, primers, reinforcement additives, such as glass fiber, and the like. There are no specific restrictions to the content of the additives and fillers in the wire mounting layer as long as the additives do not produce an adverse impact on the adhesion properties or stability of the layer.
The ILD may be coated with an adhesive on the side of the ILD that will initially be contacted with the back side of the solar cells, depending upon the order of assembly. Suitable adhesive coatings on the ILD include pressure sensitive adhesives, thermoplastic or thermoset adhesives such as the ethylene copolymers discussed above, or acrylic, epoxy, vinyl butryal, polyurethane, or silicone adhesives. The openings formed in the ILD correspond to arrangement of the solar cell back contacts when the ILD is positioned between the conductive wires of the integrated back-sheet and the back of the solar cell array. Preferably, the openings are formed by punching or die cutting the ILD, but alternatively the ILD can be formed or printed with the openings.
a-4d illustrate in cross section steps of two processes for making a back-contact solar module with an integrated back-sheet. As shown in
A small portion of an electrically conductive adhesive or solder is provided on each of the contact pads 60 and 61. The portions of conductive adhesive are shown as balls 62 in
b shows the application of an ILD 50 over the back of the solar cell array. The conductive adhesive may alternatively be provided by placing the conductive adhesive in the openings in the ILD.
Another preferred pre-lamination configuration for forming a photovoltaic module is shown in
In one preferred embodiment, a fluoropolymer film layer is laminated to the side of the olefin-based elastomer containing substrate layer that is opposite the solar cell layer. The fluoropolymer film layer may adhere directly to the olefin-based elastomer without the need for an additional adhesive layer. The fluoropolymer film may be comprised of polyvinyl fluoride, polyvinylidene fluoride, polytetrafluoroethylene, ethylene-tetrafluoroethylene copolymers, poly chloro trifluoroethylene, THV and the like. Preferred fluoropolymer films are PVF film or PVDF film. Suitable PVF films are more fully disclosed in U.S. Pat. No. 6,632,518. The thickness of the fluoropolymer film layer is not critical and may be varied depending on the particular application. Generally, the thickness of the fluoropolymer film will range from about 0.1 to about 10 mils (about 0.003 to about 0.26 mm), and more preferably within the range of about 1 mil (0.025 mm) to about 4 mils (0.1 mm). Alternatively, the fluoropolymer layer may be applied as a coating directly to the olefin-based elastomer layer. Such PVDF and PVF fluoropolymer coatings are more fully disclosed in U.S. Pat. No. 7,553,540.
A process for forming a back contact solar cell module with a solar cells connected in series by the integrated back-sheet is shown in
In
d shows the application of bus connections 94, 96, and 98 on the ends of the back-sheet. The terminal buss 94 connects to the wires 44 that are over and will connect to the positive back-contacts on the solar cell at the bottom left hand side of the solar cell array. Likewise, the terminal buss 98 connects to the wires 44 that are over the negative back-contacts on the solar cell at the bottom right hand side of the solar cell array. Positive terminal buss 94 is connected to a positive lead 93 and the negative terminal buss 98 is connected to a negative lead 97. The intermediate buss connectors 96 connect the positive or negative back contacts at the top or bottom of one column of solar cells to the oppositely charged contacts at the same end of the adjoining column of solar cells. The terminal buss connections may alternately be extended through the “Z” direction out through the back-sheet. This would eliminate the need for extra space at the ends of the module for running the buss wires to the junction box. Such “extra space” would reduce the packing density of the cells and reduce the electric power output per unit area of the module.
The solar cell array shown in
The photovoltaic module of
A process for manufacturing the photovoltaic module with an olefin-based elastomer containing back-sheet substrate will now be disclosed. The photovoltaic module may be produced through a vacuum lamination process. For example, the photovoltaic module constructs described above may be laid up in a vacuum lamination press and laminated together under vacuum with heat and standard atmospheric or elevated pressure. In an exemplary process, a glass sheet, a front-sheet encapsulant layer, a back-contact photovoltaic cell layer, and a wire embedded olefin-based elastomer containing back-sheet substrate, as described above, are laminated together under heat and pressure and a vacuum to remove air. Preferably, the glass sheet has been washed and dried. In the procedure, the laminate assembly of the present invention is placed onto a platen of a vacuum laminator that has been heated to about 120° C. The laminator is closed and sealed and a vacuum is drawn in the chamber containing the laminate assembly. After an evacuation period of about 6 minutes, a silicon bladder is lowered over the laminate assembly to apply a positive pressure of about 1 atmosphere over a period of 1 to 2 minutes. The pressure is held for about 14 minutes, after which the pressure is released, the chamber is opened, and the laminate is removed from the chamber.
If desired, the edges of the photovoltaic module may be sealed to reduce moisture and air intrusion by any means known within the art. Such moisture and air intrusion may degrade the efficiency and lifetime of the photovoltaic module. Edge seal materials include, but are not limited to, butyl rubber, polysulfide, silicone, polyurethane, polypropylene elastomers, polystyrene elastomers, block elastomers, styrene-ethylene-butylene-styrene (SEBS), and the like.
The described process should not be considered limiting. Essentially, any lamination process known within the art may be used to produce the back contact photovoltaic modules with integrated back circuitry as disclosed herein.
While the presently disclosed invention has been illustrated and described with reference to preferred embodiments thereof, it will be appreciated by those skilled in the art that various changes and modifications can be made without departing from the scope of the present invention as defined in the appended claims.
The following Examples are intended to be illustrative of aspects of the present invention, and are not intended in any way to limit the scope of the present invention described in the claims.
Damp heat exposure is followed by a peel strength test. The substrate samples with embedded wires are made with at least one end where at least one end of the wires are not embedded in the substrates (“free ends”) for use in peel strength testing. Each sample strip has a section with the wire embedded that is at least four inches long and has a free end.
The samples are placed into a dark chamber. The samples are mounted at approximately a 45 degree angle to the horizontal. The chamber is then brought to a temperature of 85° C. and relative humidity of 85%. These conditions are maintained for a specified number of hours. Samples are removed and tested after about 1000 hours of exposure, because 1000 hours at 85° C. and 85% relative humidity is the required exposure in many photovoltaic module qualification standards.
After 1000 hours in the heat and humidity chamber, the sample strips were removed for peel strength testing. Peel strength is a measure of adhesion between wire and substrate. The peel strength was measured on an Instron mechanical tester with a 50 kilo loading cell according to ASTM D3167.
UV exposure was tested in a UV exposure simulation test for 1200 hours using an Atlas weather-ometer Model-Ci 65, a water-cooled zenon arc lamp set at 0.55 watts/m2, a borosilicate outer filter, and a quartz inner filter to provide a constant source of 340 nm light.
The ingredients listed in Table 1 were mixed in a tangential BR Banbury internal mixer made by Farrel Corporation of Ansonia, Conn. The non-polymer additives were charged into the mixing chamber of the Banbury mixer and mixed before the ethylene propylene diene terpolymer (EPDM) and any thermoplastic polymer adhesive or rosin tackifier ingredients were introduced into the mixing chamber, in what is know as an upside down mixing procedure. The ingredient quantities listed in Table 1 are by weight parts relative to the parts EPDM.
The speed of the Banbury mixer's rotor was set to 75 rpm and cooling water at tap water temperature was circulated through a cooling jacket around the mixing chamber and through cooling passages in the rotor. The cooling water was circulated to control the heat generated by the mixing. The temperature of the mass being compounded was monitored during mixing. After all of the ingredients were charged into the mixing chamber and the temperature of the mass reached 82° C., a sweep of the mixing chamber was done to make sure that all ingredients were fully mixed into the compounded mass. When the temperature of the compounded mass reached 120° C., it was dumped from the mixing chamber into a metal mold pan.
The compounded mass in the mold pan was then sheeted by feeding the mixture into a 16 inch two roll rubber mill. Mixing of the compound was finished on the rubber mill by cross-cutting and cigar rolling the compounded mass. During sheeting, the mass cooled.
Sample slabs were prepared by re-sheeting the fully compounded mass on a two roll rubber mill in which the rolls were heated to 80° C. The compound was run between the rolls from five to ten times in order to produce a 25 mil (0.64 mm) thick sheet with smooth surfaces. Six inch by six inch (15.2 cm by 15.2 cm) pre-form squares were die cut from the sheet. A number of the pre-forms were put in a compression mold heated to 100° C., and the mold was put into a mechanical press and subjected to pressure. The mold pressure was initially applied and then quickly released and reapplied two times in what is known as bumping the mold, after which the mold pressure was held for 5 minutes. Cooling water was introduced into the press platens in order to reduce the mold temperature. When the mold cooled to 35° C., the press was opened and the sample substrate slabs were removed.
Back-sheet samples were made using at least two sample substrates for each of the slab nos. 1 to 6 of Table 1 above. A 5 mil (127 μm) thick release sheet made of Teflon® PTFE was provided. Eight inch (20.3 cm) long tin-coated copper solar cell tabbing wires with a thickness of about 160 mils (4.1 mm) were also provided. For each sample substrate slab, five of the 8 inch long solar cell tabbing wires were arranged parallel to each other and spaced about 1 inch (2.54 cm) from each other on the release sheet. The 25 mil (0.64 mm) thick single layer EPDM containing substrate sample slabs were each placed over five of the spaced wires. Each of the EPDM-containing slabs were six inch by six inch (15.2 cm by 15.2 cm) pre-form squares such that all of the wires overhung the opposite ends of each substrate by about an inch (2.54 cm) and the outside most wires were spaced in about an inch (2.54 cm) from the edges of each substrate.
The lamination was accomplished by preparing a layered structure of a PTFE based heat bumper, followed by a 5 mil thick cell support release sheet made of Teflon® PTFE, followed by a 1.5 mil (38.1 microns) thick Tedlar® polyvinyl fluoride film, followed by the 25 mil thick single layer of one of the sample slabs of Table 1, followed by the wire structure described in the paragraph above, and then followed by the 5 mil thick cell support release sheet made of Teflon® PTFE. The assemblies were placed into a lamination press having a platen heated to about 110° C. The assemblies were allowed to rest on the platen for about 6 minutes to preheat the structures under vacuum. The lamination press was activated and the assemblies were pressed using 1 atmosphere of pressure for 14 minutes. When heat and pressure were removed, and the PTFE layers were removed, the wires had been partially embedded in surface of the EPDM containing sample substrates.
The peel strength between one of the wires on each set of substrate samples for each of the slabs 1-6 of Table 1 was measured according to ASTM D3167 as referenced above to obtain an initial peel strength for the wire on the sample. The average initial peel strength for each slab (Examples 1-6) is reported on Table 2. One of the sample substrates for each of the slabs of Table 1 was subjected to the damp heat exposure test described above for 1000 hours and then three or four wires on the sample were tested for peel strength. The average peel strength after damp heat exposure is reported on Table 2 below. One of the sample substrates for each of the slabs of Table 1 was subjected to the UV weatherability test described above and then three or four wires on the sample were tested for peel strength. The average peel strength after 1200 hours UV is reported on Table 2 below.
| Number | Date | Country | |
|---|---|---|---|
| 61664909 | Jun 2012 | US |
