Photovoltaic technology is being rapidly adopted to generate electricity from solar energy, both for local use and for supplying power to electrical grids. Photovoltaic systems may be implemented on structures, such as buildings and houses. In addition, light weight photovoltaic modules are now being adopted for transportation applications such as trucks, cars, and boats. Photovoltaic cells are the basic units of such systems. One or more photovoltaic cells are typically arranged into a photovoltaic module, which may be then used to form a photovoltaic array.
In one embodiment, a flexible photovoltaic module with a length and a width may be provided. The flexible photovoltaic module may include a flexible top sheet, a flexible bottom sheet, a plurality of photovoltaic cells electrically connected to each other and positioned in a sealed space between the flexible top sheet and the flexible bottom sheet. The sealed space may include a first region at a first end of the flexible photovoltaic module, a second region at a second end of the flexible photovoltaic module, and a central region interposed between the first region and the second region that encompasses at least 75% of the photovoltaic cells. The flexible photovoltaic module may also include a first external electrical connector that extends into the first region and is electrically connected, in the first region, to the plurality of photovoltaic cells, a second external electrical connector that extends into the first region and a bus bar that includes interlaced copper strands and an external insulation, has a length of at least 1 meter and a thickness of about 0.5 millimeters or less, has a conductivity that can conduct at least 3.0 amps of current, extends substantially the length of the flexible photovoltaic module and is positioned in the first region, the central region, and the second region, and is electrically connected, in the first region, to the second external electrical connector, and, in the second region, to the plurality of photovoltaic cells.
In some embodiments, an outer surface of the flexible top sheet within the central region may vary in planarity by less than 5% of planar.
In some such embodiments, each of the photovoltaic cells may include a photovoltaic layer and may be positioned such that the photovoltaic layer faces the flexible top sheet, and the bus bar may be positioned in-between the plurality of photovoltaic cells and the flexible bottom sheet.
In some embodiments, the interlaced copper strands may be flattened.
In some embodiments, an outer surface of the flexible bottom sheet within the central region may vary in planarity by less than 5% of planar.
In some embodiments, an outer surface of the flexible top sheet within the central region may vary in planarity by less than 5% of planar and an outer surface of the flexible bottom sheet within the central region may vary in planarity by less than 5% of planar.
In some embodiments, the bus bar may be configured to deform in a direction parallel to the flexible top sheet more than in a direction perpendicular to the flexible top sheet.
In some embodiments, repeated heating and cooling of the flexible photovoltaic module does not cause the bus bar to plastically deform one or more of the flexible top sheet and the flexible bottom sheet.
In some embodiments, the length of the flexible photovoltaic module may be at least 1.7 meters long.
In some embodiments, the length of the flexible photovoltaic module may be between about 1.7 meters long and 6 meter long, and the length of the bus bar may be at least 75% of the length of the flexible photovoltaic module.
In some embodiments, the bus bar may have a width of at least 4 millimeters.
In some embodiments, the interlaced copper strands may be plated by titanium nitride.
In some embodiments, the first external electrical connector and the second external electrical connector may have opposite polarities.
In some embodiments, the plurality of photovoltaic cells may be electrically connected to each other in series.
In some such embodiments, the plurality of photovoltaic cells may include N photovoltaic cells, the first photovoltaic cell may be positioned in the first region, and the Nth photovoltaic cell may be positioned in the second region.
In some such embodiments, the bus bar may be physically connected to the Nth photovoltaic cell.
In some embodiments, the bus bar may be an electrical return for the plurality of photovoltaic cells.
In some embodiments, the flexible photovoltaic module may further include a plurality of electrical interconnects, the plurality of photovoltaic cells may be electrically interconnected to each other by the plurality of electrical interconnects such that each electrical interconnect forms an electrical connection between two photovoltaic cells, and the bus bar may not electrically interconnect two of the photovoltaic cells together.
In some such embodiments, each electrical interconnect may be a current collector for each corresponding photovoltaic cell such that the current generated by each of the photovoltaic cells is collected by the plurality of electrical interconnects and the bus bar may not be a current collector for the plurality of photovoltaic cells.
In some embodiments, the first region may include one photovoltaic cell, the second region may include another photovoltaic cell, and the central region may include the remaining photovoltaic cells of the plurality of photovoltaic cells.
In the following description, numerous specific details are set forth in order to provide a thorough understanding of the presented embodiments. The disclosed embodiments may be practiced without some or all of these specific details. In other instances, well-known process operations have not been described in detail to not unnecessarily obscure the disclosed embodiments. While the disclosed embodiments will be described in conjunction with the specific embodiments, it will be understood that it is not intended to limit the disclosed embodiments.
Flexible photovoltaic modules are made of flexible materials that allow these modules to bend and conform to various non-planar installation surfaces. Such modules can include two flexible sealing sheets and a set of flexible photovoltaic cells sealed between these sheets. Flexible modules may be easier to handle and install than their rigid glass counterparts. For example, flexible modules are less susceptible to damage when dropped or stepped on. Further, such modules may be positioned directly onto supporting surfaces without any intermediate mounting hardware. Flexible materials used for constructing photovoltaic modules may be easier to cut or otherwise shape to fit these modules into available installation areas. Flexible sealing sheets may be bonded directly to various installation surfaces, such as rooftop polymer membranes, and may be used for additional protection of these surfaces after installation. Further, a sealed space formed between the installation surface and the flexible module may be used to house and protect various components of a photovoltaic array, such as connectors, lines, inverters, converters, and the like.
Flexible photovoltaic modules allow new photovoltaic applications, not available with conventional rigid modules. For example, flexible modules may be used on substantially horizontal rooftops, which are common on commercial buildings. Horizontal rooftops use different roofing materials and are subject to different environmental conditions than the typically sloped rooftops of residential buildings. For example, flat horizontal rooftops tend to accumulate water and snow. Freezing and thawing cycles cause substantial thermal and mechanical stresses to be exerted on rooftop structures. Further, flat rooftops may have greater temperature fluctuations because of their construction material. Flat roofs are often used as walkways to different parts of the roof. Photovoltaic modules used on horizontal rooftops may need to withstand exposure to rain, snow, stresses associated with freeze and thaw cycles, temperature fluctuations, and foot traffic.
An example embodiment of a flexible photovoltaic module that is the subject of the present disclosure will now be discussed.
In
The sealed space 104, identified in dark shading in
The flexible top sheet 112 may be considered a light-facing sheet. The flexible top sheet 112 and flexible bottom sheet 114 may be sealing sheets that include flexible materials, such as polyethylene, polyethylene terephthalate (PET), polypropylene, polybutylene, polybutylene terephthalate (PBT), polyphenylene oxide (PPO), polyphenylene sulfide (PPS) polystyrene, polycarbonate (PC), ethylene-vinyl acetate (EVA), fluoropolymers (e.g., polyvinyl fluoride (PVF), polyvinylidene fluoride (PVDF), ethylene-terafluoethylene (ETFE), fluorinated ethylene-propylene (FEP), perfluoroalkoxy (PFA) and polychlorotrifluoroethane (PCTFE)), acrylics (e.g., poly(methyl methacrylate)), silicones (e.g., silicone polyesters), and polyvinyl chloride PVC, as well as multilayer laminates and co-extrusions of these materials. A typical thickness of a sealing sheet is between about 125 microns and 2,540 microns or, more specifically, between about 254 microns and 1,270 microns, though other thicknesses may be used as well. In certain embodiments, a flexible sealing sheet includes a metallized layer to improve its water permeability characteristics. For example, a metal foil may be positioned in between two insulating layers to form a composite flexible sealing sheet.
In certain embodiments, the flexible top sheet 112 may be made from material that is different than a material of the flexible bottom sheet 114, and the flexible top and bottom sheets 112 and 114 may be of different thicknesses from each other. In some embodiments, these sheets may have a thickness less than 2,032 microns or, more specifically, less than 1,016 microns or even less than 508 microns.
In certain embodiments, the module 100 may have an encapsulant layer positioned in between the flexible top sheet 112 and the photovoltaic cells 102, as well as another encapsulant layer between the flexible bottom sheet 114 and the photovoltaic cells 102. One or more of the encapsulant layers may also be positioned around portions of the bus bar 110. Examples of encapsulant layer materials include non-olefin thermoplastic polymers or TPO, such as polyethylene (e.g., a linear low density polyethylene), polypropylene, polybutylene, PET, PBT, polystyrene, polycarbonates, fluoropolymers, acrylics, ionomers, silicones, and combinations thereof.
As noted above, the module 100 may include the edge seal 106 that surrounds and seals the photovoltaic cells 102 together with the flexible top sheet 112 and the flexible bottom sheet 114, as well as with other components. The edge seal 106 may prevent moisture from penetrating towards the photovoltaic cells 102. The edge seal 106 may be made from one or more organic or inorganic materials that have low inherent water vapor transmission rates. In certain embodiments, a portion of the edge seal 106 that contacts electrical components (e.g., bus bars, diodes, return lines) of module 100 is made from a thermally resistant polymeric material. The edge seal 106 may also secure flexible top sheet 112 with respect to the flexible bottom sheet 114. In certain embodiments, the edge seal 106 determines at least some of the boundaries of the sealed space 104.
In some embodiments, the module 100 may be manufactured using one or more lamination procedures in which aspects of the module 100 may be heated and pressed. For example, the pressing may be performed by an inflatable bladder, and such lamination may heat the encapsulant and edge seal such that the sealed space 104 is formed in the module 100.
The module may be considered to have three regions, with the first region 120 located at the first end 122 of the module as shown in
The electrical components and configurations of the module 100 will now be discussed. In
Photovoltaic cells may be interconnected by any appropriate conductor that contacts a front side (i.e., the photovoltaic layer that is exposed to light and generates a voltage) of one cell as well as back side of an adjacent cell to interconnect these two cells in-series. In the example of
The shaped wire shown in
Given the nature of interconnects, current collectors, and bus bars, it is generally not feasible to interchange a current collector or interconnect with a bus bar, and vice versa. For example, some electrical interconnects are thin wires that travel a short distance between two adjacent cells, such as 5 mm, 20 mm, or even up to 500 mm. It is advantageous to have these wires very thin, such as about 36 standard wire gauge (SWG) to about 46 SWG, because this small size can reduce the surface area of a photovoltaic cell that is blocked or shadowed by the interconnect and therefore increase efficiency of the photovoltaic cell. It is also advantageous to have these interconnects travel short distances, such as no greater than two times the width of the cell. However, as the thickness and cross-sectional area of the interconnect decreases, so does the conductivity of that interconnect. An interconnect carries the current generated by the two interconnected photovoltaic cells. For thin wire interconnects, a single section of a thin wire may be too small to carry the current. Multiple thin wires, or multiple sections of a single wire, can be used to provide capacity to carry the current generated by the two photovoltaic cells.
Similarly, it is also advantageous to have current collectors be thin, small wires. For optimized and efficient current collection from a photovoltaic cell, it is generally advantageous to have a current collector positioned on various locations of the photovoltaic cell in order to minimize resistive losses and maximize module efficiency as compared to aperture efficiency. But because the current collector is placed directly on the photovoltaic cell, it is advantageous to minimize the surface area of the photovoltaic cell that is blocked or shadowed by the current collector to thereby increase efficiency of the photovoltaic cell. In some instances it has been found that using about a 36 SWG to about 46 SWG, as compared to wires thicker than these gauges in the same configuration, can lead to an increase in efficiency of about 2% to 5%. Like described above, as the thickness and cross-sectional area of the current collector decreases, so does the conductivity of that current collector. The current collector as a whole must be able to carry the current generated by the photovoltaic cell, but for thin wire current collectors, a single section of a thin wire is typically too small to carry the required current. Accordingly, multiple thin wires, or multiple sections of a single wire, are used for the current collector of a photovoltaic cell.
Conversely, a bus bar that is used as an electrical return for a series of interconnected photovoltaic cells that extends for the substantial length of the module, as discussed herein, generally has a much larger cross-sectional area than current collectors and interconnects for several reasons. For example, the bus bar has a cross-sectional area sufficient to carry a current generated by all the photovoltaic cells to which it is electrically connected. This cross-sectional area may be about 2 to 2.5 square millimeters as compared to a 36 SWG wire that has a cross-sectional area of about 0.0293 square millimeters or a 46 SWG wire that has cross-sectional area of about 0.0029 square millimeters. Additionally, the bus bar must be able to withstand the physical and thermal forces associated with being positioned within the module and extending for substantially the entire length of the module, such as modules greater than 1 meter in length. A thin wire used for an interconnect or current collector would not be able to withstand the current generated by an entire string of cells as well as the thermal cycling and associated physical and thermal forces which would cause such a bus bar to break or fail within the module. Accordingly, it would not be feasible to use a single interconnect wire or a current collector as a return bus bar for a photovoltaic module having a length of at least 1 meter and a plurality of photovoltaic cells.
The overall electrical arrangement between the photovoltaic cells of the module may be in-series, parallel, or a combination of both. For example, the photovoltaic cells 102 of module 100 may all be electrically connected in-series. This in-series arrangement may result in string of photovoltaic cells having opposite polarities at each end of the string and at opposite ends of the module. For instance, referring to
It can advantageous to have the external electrical connections of the module at one end of the module. To have both negative and positive poles at the same end of the module, an electrical return line is used to provide an electrical pathway for one of the poles to the opposite end of the module. The bus bar 110 depicted in
Referring to
The current generated by the module 100 may be transferred to elements external to the module 100, such as other modules in an array of photovoltaic modules, inverters, or a power grid. To form the connections between the module 100 and these external elements, the module may have one or more electrical connectors that are accessed during installation and connected to the external elements, such as electrical connectors of adjacent modules. A module's electrical connectors include electrically conductive elements, such as a metallic wire that may be electrically insulated. An electrical connector may also include, or may be configured to make electrical connections to, standard MC4 photovoltaic connectors or other types of external photovoltaic connectors. For example, a module may have a cable connected to a photovoltaic connector that is electrically connected to the photovoltaic cells such that electricity generated by the cells can be transported to the cable, the photovoltaic connector and to an external electrical connection, such as another module.
The one or more electrical connectors of a flexible photovoltaic module may be electrically connected to the photovoltaic cells that are sealed inside the module and to return lines provided within the module that typically extend along the module. The one or more electrical connectors may be electrically connected to the photovoltaic cells by electrical leads. An electrical lead may have a portion that extends into the sealed space of the module, which may include extending through an edge seal of the module. Electrical leads may be in the form of thin but sufficiently conductive metal strips that may have flat aspect ratios (i.e., their heights may be substantially smaller (e.g., less than 10%) than their widths). In some of the embodiments disclosed herein, the height of an electrical lead may be 0.1 millimeters or 0.125 millimeters, while the width may be 12 mm. An electrical lead may be positioned within a module during manufacturing such that one portion of an electrical lead is located within a sealed space of the module with another portion extending through and outside the sealed space so that it may electrically connect with an electrical connector.
For example, as can be seen further in
Modules that are arranged like that in
For some thin flexible modules that use a solid bus bar that extends substantially the length of the module, like depicted in
The differences in CTEs between the bus bar and other materials of the module cause plastic deformation of the module in areas surrounding the bus bar that are undesirable for the module, such as an undesirable aesthetic which may affect the marketability of the module.
However, as illustrated in
Similar to
A deformation of the module in the y-axis may be considered a bus bar bump regardless of whether the encapsulant, the flexible top sheet, the flexible bottom sheet, or the photovoltaic cells are caused to be plastically deformed. The deformed shape of the module depicted in
It has been observed that this bus bar bump occurs when the solid metal bus bar is longer than about 1 meter; it generally does not occur at a length less than 1 meter so it may therefore not be a consideration for modules less than about 1 meter because the increase in buckling and deformation caused by the increased length due to thermal expansion does not create a bus bar bump in a module. Accordingly, modules that are the subject of this disclosure that are longer than 1 meter, such as between 1.7 meters and 6 meters experience the bus bar bump.
Presented herein as part of this disclosure are embodiments of flexible photovoltaic modules that include a bus bar configured to minimize or eliminate this bus bar bump. This type of bus bar may be the bus bar of the module discussed herein and shown in
The bus bar of the present disclosure is not a solid metal band or strip as discussed above. Rather, the bus bar of the present disclosure is non-monolithic and includes metallic strands that are interlaced. Interlacing includes weaving, intertwining, braiding, and twisting. Weaving is the interlacing of strands passing in one direction with others at a right angle to them while braiding is the interlacing of strands together but not at right angles. In some embodiments, the interlacing may be at least three individual monolithic strands that are interlaced with each other, or twisted together. Such interlacing may include one central monolithic strand and a plurality of monolithic strands surrounding that central strand, all of which are interlaced with each other. In some embodiments, the individual monolithic strands may not be interlaced with each other, but rather they may be grouped into a plurality of groups, with each group having a particular number of monolithic strands, and it is these groups that are interlaced with each other. Each group may have the same number of monolithic strands as the other groups. In some embodiments, the interlacing of these metallic strands may be considered braiding, like with a braided wire, while in some other embodiments, the interlacing may be considered a plurality of wires bundled or wrapped together, like with a stranded wire. For example, a bus bar may have twelve groups that each have four individual wires, and the twelve groups are braided together.
In some embodiments, the bus bar with interlaced metallic strands may also be flattened. The flattening may occur by compressing the interlaced metallic strands, such as by drawing the interlaced strands between two rollers thereby pressing the interlaced metallic strands. The flattening may be a flattening of the individual metallic strands, the flattening of the interlaced structure itself, or both. For example, prior to flattening, the interlaced strands may each have circular cross-sectional areas and may be formed into a cylindrical braided wire, and during flattening, the wire is drawn through two rollers which flatten the cylindrical wire to a strip that does not have a cylindrical cross-sectional area and instead has an oblong or generally rectangular cross-sectional area. In some embodiments, this may flatten the individual strands, while in other embodiments this may only flatten the overall interlaced structure and not the individual strands. The cross-sectional area may not be an actual rectangle given that the interlaced strands are generally circular and there may be some gaps between each strand, but rather it may be considered to be encompassed by a rectangle such that the cross-sectional area of the bus bar is at least 75% of the rectangular cross-sectional area.
The interlacing of the metallic strands of the bus bar enables the bus bar of the present disclosure to expand and contract in more directions and/or in more amounts than a sold bus bar. Such additional movement enables the bus bar of the present disclosure to move within at least the x- and/or z-directions which in turn prevents or reduces the buckling and deformation of the bus bar in the y-axis thereby preventing or reducing the bus bar bump.
The metallic strands are a conductive metal or alloy, such as copper. In some embodiments, the metallic strands are plated by another conductive material, such as titanium nitride. The bus bar 110 may also include an external insulation, such as a polymer.
The thermal expansion of the solid metal bus bar and the bus bar that includes interlaced metallic strands in the x-axis is shown in
Accordingly, in some embodiments, the bus bar having interlaced metallic strands may deform less have an overall CTE in a direction perpendicular to the flexible top sheet (e.g., in the y-axis of
In some embodiments, a bus bar having interlaced metallic strands may fully eliminate the bus bar bump over a central region of the module, e.g., section 128 in
The length of the bus bar of the present disclosure, as discussed above, is greater than at least 1 meter because the bus bar bump does not occur for bus bars that are less than about 1 meter in length. In some embodiments, the bus bar extends substantially the length of the photovoltaic module (substantially here means within 15% of the length); some example lengths of the modules include about 1.6 meters to about 6 meters. The thickness of the bus bar (as measured in the y-axis) may be about 0.5 millimeters or less. As discussed above, for thin modules having a thickness of 1 millimeter or less, the bus bars must be less than this overall module thickness, such as about 0.5 millimeters. The width of the bus bar may (as measured in the x-axis), in some embodiments, be about 4 millimeters or about 5 millimeters.
The bus bar of the present disclosure also is able to conduct the current generated by the string of photovoltaic cells. This may include having a conductivity that enables the bus bar to carry a current of at least 3.0 amps, at least 3.5 amps, and between about 3.0 amps to about 13.6 amps, or about 125% of the fuse rating of a solar panel. The bus bar does not electrically interconnect two of the photovoltaic cells together and is not a current collector of the photovoltaic cells; instead the wire network 108 both collects the current from and interconnects the photovoltaic cells while the bus bar 110 acts as an electrical return line for the entire string of photovoltaic cells.
It is to be understood that the configurations and/or approaches described herein are exemplary in nature, and that these specific embodiments or examples are not to be considered in a limiting sense, because numerous variations are possible. The specific routines or methods described herein may represent one or more of any number of processing strategies. As such, various acts illustrated may be performed in the sequence illustrated, in other sequences, in parallel, or in some cases omitted. Likewise, the order of the above described processes may be changed.
The subject matter of the present disclosure includes all novel and nonobvious combinations and sub-combinations of the various processes, systems and configurations, and other features, functions, acts, and/or properties disclosed herein, as well as any and all equivalents thereof.
Unless the context of this disclosure clearly requires otherwise, throughout the description and the claims, the words “comprise,” “comprising,” and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in a sense of “including, but not limited to.” Words using the singular or plural number also generally include the plural or singular number respectively. When the word “or” is used in reference to a list of two or more items, that word covers all of the following interpretations of the word: any of the items in the list, all of the items in the list, and any combination of the items in the list. The term “implementation” refers to implementations of techniques and methods described herein, as well as to physical objects that embody the structures and/or incorporate the techniques and/or methods described herein.