Photovoltaic module support with cable clamps

Apparatus and techniques for mounting frameless photovoltaic modules reduce module stress induced by the mounting configuration. Cable clamps and cable spacing configured to relieve module stress by reducing or eliminating module sag are used.

BACKGROUND OF THE INVENTION

Photovoltaic cells are widely used for generation of electricity, with multiple photovoltaic cells interconnected in module assemblies. Such modules may in turn be arranged in arrays and integrated into building structures or otherwise assembled to convert solar energy into electricity by the photovoltaic effect. Arrays of modules are typically mounted on racking systems on the roofs of buildings or on ground-based structures. The modules are required to pass load testing to ensure that they can safely withstand snow loading and other environmental conditions. This can be challenging for frameless photovoltaic modules.

SUMMARY OF THE INVENTION

The invention relates generally to apparatus and techniques for mounting frameless photovoltaic modules in a cable-based mounting system to reduce module stress induced by the mounting configuration. The invention involves cable clamps installed on photovoltaic modules and a cable-based mounting systems with cable spacing configured to relieve module stress by reducing or eliminating module sag.

In one aspect, the invention relates to a photovoltaic module assembly. The photovoltaic module assembly includes a frameless photovoltaic module having a frontside sheet and a backside sheet, and cable clamps configured for attachment of the module to a cable across the backside sheet.

In another aspect, the invention relates to a photovoltaic assembly. The photovoltaic assembly includes a frameless photovoltaic module having a frontside sheet and a backside sheet, a cable set, and cable clamps attached to the frameless photovoltaic module across the backside sheet, wherein the frameless photovoltaic module is secured to the cable set via the cable clamps.

Another aspect of the invention relates to a method of installing a frameless photovoltaic module having a frontside sheet and a backside sheet onto a cable set. The method involves providing the cable set and securing the frameless photovoltaic module onto the cable set with cable clamps attached to the backside sheet of the frameless photovoltaic module.

These and other aspects of the invention are described further below with reference to the figures.

DETAILED DESCRIPTION

Frameless Photovoltaic Modules

Photovoltaic modules are required to meet load ratings specified by IEC 61646 and UL 1703, incorporated herein by reference for this purpose. In this regard, a module must be able to pass a 2400 MPa static load test for wind and 5400 MPa static loading test for snow/ice. This load testing requirement can be particularly challenging for a frameless photovoltaic module (a module without a metallic frame around its perimeter) to meet. Further, the structural stability and module integrity can be difficult to preserve in a racking system for frameless photovoltaic modules.

Embodiments of the present invention relate to mounting of frameless photovoltaic modules (also referred to as solar modules or solar panels or, in this application, simply as modules), and associated racking systems and methods.FIG. 1Ashows a not-to-scale cross-sectional view of certain components of a frameless solar module100in accordance with one embodiment of the present invention. The module100includes interconnected solar cells102and front (light-incident) and back layers104and106, respectively, for environmental protection and mechanical support. A light-transmissive thermoplastic polymer encapsulant110is also provided between the solar cells102and the front layer104to provide electrical insulation and further protection to the underlying solar cells by preventing direct contact between the solar cells and the generally rigid front layer104. The same or a different encapsulant layer111may also be provided between the solar cells102and the back layer106for the same reasons. In certain modules, an additional edge material108surrounds the solar cells102, and in this example, is embedded within encapsulating layers110and111.

The front and back layers may be any suitable material that provides the environmental protection and mechanical support required for reliable module operation. In some typical embodiments, the front and back layers are rigid plates, light transmitting in the case of the front layer, such as glass, although other materials, such as polymers, multi-layer laminates and metals that meet the functional requirements may also be used. In other embodiments the typical rigid back layer (e.g., back glass plate) can be replaced with a much lighter weight flexible material, thereby reducing handling costs associated with the module.

The front, light-incident layer104should transmit visible and near visible wavelengths of the solar spectrum113and be chemically and physically stable to anticipated environmental conditions, including solar radiation, temperature extremes, rain, snow, hail, dust, dirt and wind to provide protection for the module contents below. A glass plate comprising any suitable glass, including conventional and float glass, tempered or annealed glass, combinations thereof, or other glasses, is preferred in many embodiments. The total thickness of a suitable glass or multi-layer glass layer104may be in the range of about 2 mm to about 15 mm, optionally from about 2.5 mm to about 10 mm, for example about 3 mm or 4 mm. As noted above, it should be understood that in some embodiments, the front layer104may be made of a non-glass material that has the appropriate light transmission, stability and protective functional requirements. The front layer104, whether glass or non-glass, transmits light in a spectral range from about 400 nm to about 1100 nm. The front layer104may not necessarily, and very often will not, transmit all incident light or all incident wavelengths in that spectral range equally. For example, a suitable front layer is a glass plate having greater than 50% transmission, or even greater than 80% or 90% transmission from about 400-1100 nm. In some embodiments, the front layer104may have surface treatments such as but not limited to filters, anti-reflective layers, surface roughness, protective layers, moisture barriers, or the like. Although not so limited, in particular embodiments the front layer104is a tempered glass plate about 3 mm thick.

The back layer106may be the same as or different than the front layer104and is also typically a glass plate as described above. However, since the back layer106does not have the same optical constraints as the front layer104, it may also be composed of materials that are not optimized for light transmission, for example metals and/or polymers. And, while the present invention is applicable in more typical module configurations having both front and back glass plate layers, the invention finds particularly advantageous application in embodiments in which the back layer104is a lighter weight flexible material. Such lighter weight modules have manufacturing and transportation benefits, but can present additional challenges for module stability, including compliance with load testing requirements stresses induced by module mounting configurations. In such embodiments, the back layer106may be a flexible yet weatherable laminate that protects the photovoltaic cells and other module components from moisture, UV exposure, extreme temperatures, etc. The back layer laminate may include a weatherable back sheet exposed to the exterior of the module. The back sheet should be resistant to environmental conditions expected to be experienced by the module (e.g., temperatures of about −40 to 90° C.), so that it is stable throughout the range of temperate climate temperatures and conditions so as to retain its properties to perform its protective function.

The back sheet may be composed of a fluoropolymer, including but not limited to polyvinyl fluoride (PVF) (e.g., Tedlar® film available from DuPont), polyvinylidene fluoride (PVDF), ethylene-tetrafluoroethylene (ETFE), fluorinated ethylene-propylene (FEP), perfluoroalkoxy (PFA) and polychlorotrifluoroethane (PCTFE). Other weatherable materials may be used in addition to or instead of a fluoropolymer, including silicone polyesters, chlorine-containing materials such as polyvinyl chloride (PVC), plastisols, polyethylene terephthalate (PET), polypropylene, polybutylene, polybutylene terephthalate, and acrylics or combinations (laminated stacks) of the above. In certain embodiments, any material that meets UL 1703 requirements (incorporated by reference herein) can be used. In one example, the back layer includes PVF (e.g., Tedlar®). In certain examples, the thickness may range from about 2 to about 12 mils, although other thicknesses may be used as appropriate. A suitable flexible back layer laminate may also include a flexible moisture barrier sandwiched between an insulation sheet, for example a sheet of PET, and the weatherable back sheet. A suitable moisture barrier may be a metallic sheet, such as an aluminum foil. A suitable laminate back sheet in accordance with some embodiments of the invention is composed of a polyvinyl fluoride/Al foil/polyethylene terephthalate laminate (e.g., Tedlar®/Al foil/PET). Further description of suitable flexible back layers for photovoltaic cells that may be used in modules in accordance with the present invention is provided in U.S. Published Patent Application No. 2008/0289682 and U.S. Published Patent Application No. 2010-0071756, each of which is incorporated by reference herein for this purpose.

The edge material108may be an organic or inorganic material that has a low inherent water vapor transmission rate (WVTR) (typically less than 1-2 g/m2/day) and, in certain embodiments may absorb moisture and/or prevent its incursion. In one example, a butyl-rubber containing a moisture getter or desiccant is used.

The solar cells102may be any type of photovoltaic cell including crystalline and thin film cells such as, but not limited to, semiconductor-based solar cells including microcrystalline or amorphous silicon, cadmium telluride, copper indium gallium selenide or copper indium selenide, dye-sensitized solar cells, and organic polymer solar cells. In particular embodiments, the cells are copper indium gallium selenide (CIGS) cells. In other aspects of the invention, the cells can be deposited as thin films on the front, light-incident (e.g., glass) layer104. Direct deposition of a solar cell on glass is described, for example, in U.S. Published Patent Application No. 2009/0272437, incorporated by reference herein for this purpose. In such an embodiment, element110ofFIG. 1Awould be absent and element102would be in contact with the front, light-incident layer104.

Frameless photovoltaic modules are often rectangular in overall shape, as shown inFIG. 1B. For purposes of discussion, references to frameless photovoltaic modules herein will be made in the context of a rectangular module possessing a longitudinal axis or direction and a transverse axis or direction (as depicted inFIG. 1B, diagram (a)), wherein the longitudinal axis is along the major (larger) dimension of the rectangle and the transverse axis is along the minor (smaller) dimension of the rectangle. Similarly, reference may be made to the length and width of the module. The length of a module refers to the major dimension of the rectangle; the width of a module refers to the minor dimension of the rectangle. Of course, frameless photovoltaic modules may take on a variety of forms departing from a rectangle, and reference to rectangular modules, rectangles, and longitudinal or transverse axes, dimensions, or directions, should not be viewed as limiting the invention only to rectangular modules.

Reference is also made in this application to sagging of a frameless photovoltaic module. In some cases, a module will be described as experiencing sagging along a transverse or longitudinal direction. Sag along a transverse direction refers to sagging behavior which manifests as a non-linear displacement of the module from a line running in a transverse direction, as depicted inFIG. 1A, diagram (b). Sag along a longitudinal direction refers to sagging behavior which manifests as a non-linear displacement of the module from a line running in a longitudinal direction, as depicted inFIG. 1A, diagram (c). A module may sag at multiple points depending on the method of support, as depicted inFIG. 1A, diagram (d). Sag may occur along both transverse and longitudinal directions to different degrees at the same time and result in complex overall displacement, as depicted inFIG. 1A, diagram (e).

Frameless Photovoltaic Module Cable Mounting Systems

Frameless photovoltaic modules may be mounted onto cable-based mounting systems when installed at their installation locations. A plan view of an example cable mounting system is shown inFIGS. 2A and 2C, with and without the modules mounted respectively. Respective side views are shown inFIGS. 2B and 2D. Such cable mounting systems200are frequently attached to freestanding support structures, roofs202, carports, walls, or other structures which receive exposure to sunlight and can support the weight of cables206and installed frameless photovoltaic modules208and maintain sufficient cable tension in cables206. Alternatively, cables206may be deployed on freestanding ground-based structures. All such structures are often oriented, or may be re-oriented, to present the mounted frameless photovoltaic modules208in an orientation that promotes efficient solar power generation.

In one embodiment, cable mounting system200includes two or more cables206which support one or more frameless photovoltaic modules208. Cables206may be mounted to a structure, such as roof202, using end mounts209and intermediate mounts210. Mounting cables206may also be attached to a supplemental support structure; the supplemental support structure may elevate or position the cables206in a more optimum manner (e.g., position the cables206such that attached frameless photovoltaic modules208will be oriented towards the sun to a greater extent).

Cable206may be integral to end mounts209. For example, cable206may be swaged, brazed, soldered, or otherwise permanently attached to end mount209. Alternatively, cable206may be removably mounted to end mount209, such as through the use of clamps, flared stops, or eyelets. A tensioning device or mechanism may be incorporated into end mount209or cable206. For example, a turnbuckle may be incorporated into cable206to allow cable slack to be removed.

Intermediate mount212may be clamped onto cable206or slid over cable206. Intermediate mount212may provide support to cable206to mitigate sagging of cable206. Intermediate mount212may also be configured to clamp cable206to prevent slippage of cable206relative to intermediate mount212.

End mounts209and intermediate mounts212may be mounted to roof202, or other mounting structure, through any fastening system compatible with the surface to be mounted to. For example, end mounts209and intermediate mounts212may include a mounting plate with a hole pattern for accepting threaded fasteners. End mounts209and intermediate mounts212may be attached, for example, to roof202using screws. Additional mounting methods and techniques may also be used. For example, screws may be augmented with a layer of waterproof silicone adhesive.

The cables206are preferably manufactured from steel or other high-strength material. Cables206are also preferably manufactured from a corrosion and UV-resistant material, such as stainless steel. The diameter of cable206may be sized to support a given module installation. For example, cable206may have a diameter of 0.25″.

Modules208may be attached to cables206using one or more cable clamps204. Cable clamps204may be clamped onto cables206such that cable clamps204secure module208in place and prevent module208from sliding along cables206.FIG. 2Cdepicts locations of four cable clamps204in plan view with respect to one module208, which is shown with cutaway transparent regions in the vicinity of cable clamps204. Representative cable clamps are discussed in greater detail below with reference toFIGS. 3A-E.

Stop clamps212may be attached to cables206as an installation aid or as a safety device. Stop clamps212may serve as a backup positive stop along cable206and may be used to prevent excessive sliding of modules208during and after installation. Stop clamps212may be installed with or without a gap between stop clamp212and cable clamp204.

Frameless photovoltaic modules mounted to cable mounting systems may experience sagging in areas not directly supported by a cable due to the modules' weight and geometry. In a two-cable mounting system, a frameless photovoltaic module will typically only be externally supported at the cable locations. At the two cable locations, the frameless photovoltaic module may rest on the cables themselves. In areas where the frameless photovoltaic module does not receive external support, the module must be self-supporting, i.e., the module must rely on the material properties and geometry of the module for support.

Two-cable mounting cable systems may be spaced according to the L/4 rule, in which the midpoints of the cables are typically positioned at a distance of L/4 from the transverse edges of a module, where L refers to the length of the module. For example, for a 1611 mm×665 mm module, the L/4 distance would be 402.75 mm.

In a preferred embodiment, the transverse midpoint of each cable in a two-cable cable mounting system is instead positioned approximately 22% of the length of the module from the transverse edges of the module. Thus, for a 1611 mm×665 mm module, the midpoints of the cables would be positioned about 354.4 mm from either transverse edge along the longitudinal axis.

More particularly, the midpoint of each cable in a two-cable mounting system may be positioned approximately 22.3% of the length of the module from a transverse edge of the module. 55.4% of the module would thus be located between the midpoints of the two cables.

Cable Clamps

Frameless photovoltaic module302, shown inFIG. 3A, may be attached to cables306using cable clamps304. Cable clamps304may be individually attached to module302or may comprise a multi-clamp assembly305, such as shown inFIG. 3B. A suitable multi-clamp assembly305may comprise a strip of material with a length substantially matching the transverse width of module302and with features configured to mate with cable clamps304. In an alternate embodiment, multi-clamp assembly305may comprise strip of material308with a length exceeding the transverse width of module302such that, when mounted transversely to module302, multi-clamp assembly305may extend beyond the longitudinal edges of module302. Multi-clamp assembly305may be mounted to the backside sheet of module302, as shown inFIG. 3B, or incorporated within the structure of module302.

It is to be understood thatFIG. 3AandFIG. 3Beach depict two modules on the same cable but facing opposite directions to conveniently convey to the reader the details of the frontside and backside of the modules in one diagram and is not representative of an actual mounting arrangement. In most installations, modules302would all face the same general direction.

It is to be understood that cable clamp304may include sufficient features to clamp or grip cable306, such as upper saddle312and lower saddle316inFIGS. 3C and 3D. However, when used in conjunction with multi-clamp assembly305, cable clamp304may only include some of the features required to clamp or grip cable306; the remaining features may be included as part of multi-clamp assembly305. For example, with reference toFIG. 3C, upper saddle312may be integrated with strip of material308to form multi-clamp assembly305. Finally, cable clamp304may include sufficient features to clamp or grip cable306and may be mounted to multi-clamp assembly305. For example, multi-clamp assembly305may consist of a bar with mounting holes at either end and cable clamp304may simply be mounted to the bar via the mounting holes.

Multi-clamp assembly305and/or cable clamp304may include features for mounting multi-clamp assembly305and/or cable clamp304to cable306. For discussion purposes, examples of such features are provided below in the context of cable clamp304, although it is to be understood that such features may also be implemented on multi-clamp assembly305in combination with cable clamp304, as outlined previously.

In one embodiment, cable clamp304may include upper saddle312and lower saddle316, both of which are configured to be clamped around cable306, as shown inFIGS. 3C and 3D. Cable clamp304may also include fasteners314, such as machine screws or bolts, which may be tightened to compress cable306between upper saddle312and lower saddle316. Upper saddle312may be mounted to the backside sheet of module302using adhesive310.

In another embodiment, cable clamp304may include upper jaw318, lower jaw317, and swivel wingnut319, as shown inFIGS. 3E and 3F. Upper jaw may be mounted to the backside sheet of module302using adhesive310. Lower jaw317may be pivoted about pivot320to clamp around cable306. Swivel wingnut319may then be pivoted into a slot in the end of lower jaw317and tightened to draw lower jaw317against upper jaw318and securely clamp cable306. In yet another embodiment, shown inFIGS. 3G and 3H, swivel wingnut319is replaced with draw latch321, which allows for rapid clamp-down during installation.

Multi-clamp assembly305and cable clamp304may each be a single material or an assembly of different materials. For example, multi-clamp assembly305may comprise an extruded aluminum channel. Alternatively, multi-clamp assembly305may comprise a layered composite or a plastic. In yet a further embodiment, multi-clamp assembly305may comprise a metal substrate overlaid with a layered composite or a fiber-reinforced plastic.

In one embodiment, cable clamp304and/or multi-clamp assembly305may include an elastomeric or other compliant material to enhance the clamping grip on cable306. The elastomeric material may also protect cable306from crimping due to direct contact with harder cable clamp materials, such as steel or aluminum. An example saddle clamp featuring elastomeric cushion322on upper saddle312and lower saddle316is shown inFIGS. 3I and 3J. Of course, elastomeric cushion322may not be required to completely encircle cable306when cable306is clamped.

Multi-clamp assembly305may be constant in cross-section along its length or possess a variable cross-section, as shown inFIG. 3B. Multi-clamp assembly305may also incorporate any of a variety of different cross-sections, including solid-core, hollow-core, and open-channel cross-sections. For example, multi-clamp assembly305may include rectangular cross-section. Alternatively, multi-clamp assembly305may consist of a hollow, thin-wall, rectangular cross-section. Multi-clamp assembly305may, in another embodiment, feature a flanged channel cross-section.

Cable clamp304or multi-clamp assembly305may be attached to module302through the use of adhesives, adhesive tape, diffusion bonding, or may even be sandwiched between layers of module302during module assembly. For example, if the backsheet of module302comprises 4 layers of woven composite, multi-clamp assembly305may be installed between the layup of the inner two layers and the outer two layers.

Cable clamps304may be attached to module302such that two cable clamps304are placed approximately 22% of module302's length from the transverse edges of module302.

Installation of Cable Clamps

Cable clamps may be attached to modules at any of several points in time. During manufacture of the module, a cable clamp may be woven into a composite forming the backsheet, as discussed above with respect to multi-clamp assemblies. Such installation would need to be done at the module manufacturing site due to the integrated nature of the cable clamp installation.

An alternative is to glue the cable clamps to the module backsheet. For example, cable clamps may be attached to the module backsheet using a silicone adhesive, such as Dow-Corning PV804™ silicone, which is marketed for use with solar power systems. A UV-stable adhesive may be used to prevent UV degradation.

Alternatively, an adhesive tape, such as 3M acrylic VHB™ may be used to attach the cable clamps to the module. Adhesive tape may be preferable to liquid adhesive due to the relatively instantaneous bond that forms. Such post-module-manufacture installation may be performed at the module manufacturing facility or at a secondary facility. The cable clamps bonding may be performed in controlled conditions to maximize bond strength and quality.

Finally, cable clamps may be attached to the module backsheet at a remote location, such as a solar panel installation jobsite. For example, cable clamps may be attached to modules using silicone, as discussed above, but in the field instead of in the factory. However, installation in a controlled environment is preferred for quality control purposes. For example, field installation runs an increased risk of dirt and other contaminants being trapped between the cable clamps and the module. Such foreign substances may cause a substandard adhesive bond, generate stress concentrations, or become a source for abrasion of the module. Installation in a controlled environment may also allow for any curing process which may be required to be accelerated or kept within required environmental conditions.

While care must be taken to ensure that cable clamps are attached to the module in the correct locations, tolerances for cable clamp installation may be less stringent than for other mounting systems, such as parallel rail systems. One of the advantages of cable mounting systems is that minor tolerance variations in cable clamping locations may be absorbed through the inherent flex of the cable.

Example Modeling

Modeling was conducted in order to demonstrate the advantages provided by various aspects of this invention with regard to the positioning of the mounting locations. The data presented here are intended to better illustrate the invention as described herein and are non-limiting. The analyses shown reflect a rigid rail mounting configuration, although the analysis results may be generally extrapolated to cable mounting systems as well.

FIG. 4Adepicts a plot of the maximum principal stress experienced by a typical module depending on the distance of mounting clamps from the transverse edge of the module. For the analyzed module, positioning mounting locations at approximately 22% of the longitudinal length of the module from either transverse edge reduced the resulting maximum principal stress by approximately 37 MPa relative to the stress induced by a L/4 rail spacing.

FIG. 4Bis a stress contour plot of an example frameless photovoltaic module supported by two mounting rails, each rail attached to the module via two edge clamps. The rail spacing in this plot is approximately 22% of the module longitudinal length from either transverse edge. The combination of sag loading and localized stress concentrations in the regions of the edge clamps results in a peak principal stress of 366 MPa.

Example Installation Process

An example installation process utilizing cable mounting systems in conjunction with cable clamp-equipped modules is diagrammed inFIG. 5. It should be noted that not all of the operations depicted and described are necessarily part of a process in accordance with the present invention; an installation process in accordance with the invention may include all or just some of the operations described. A number of the operations are provided for context to facilitate description and understanding of the invention, but are optional in some embodiments.

Installation process500begins with the installation of end mounts and, if needed, intermediate mounts, onto a support structure, as shown in step505. This may include attaching two or more end mounts to a roof, carport, or other support structure.

In step510, the cable sets may be installed onto the installed end mounts and intermediate mounts, if present. If the cables are permanently attached to the end mounts, this step may be redundant in view of step505.

In step515, the cables may be tensioned appropriately. Tensioning may be repeated throughout the installation process if warranted. For example, the cables may sag after module installation due to the increased distributed loads from the modules. This sagging may be mitigated through re-tensioning. Truing of the cables is largely unnecessary, as cables are self-truing in the horizontal direction.

In step520, cable stops may be installed onto the cables. Cable stops may be installed between each module mounting location, between groups of modules, or not at all, depending on the characteristics of the installation. For example, cables mounted on a steep slope may require more cable stops than cables which are substantially horizontal.

In step525, a module is installed onto the mounted cables. Installing a module may involve placing cable clamps attached to the module onto the mounted cables.

In step530, the cable clamps are clamped onto the cable using associated hardware, such as threaded fasteners or draw latches.

In step535, the installation process returns to step520if any modules remain which will be installed on the installed cable set.

In step540, the installation process returns to step505if there are any cable sets remaining to be installed.

In step545, electrical and control connections are made to the mounted modules, and any support electronics are installed and configured. In step550, the mechanical installation is complete.

Of course, the above steps are merely examples of an installation process using the described technology. The ordering of the steps may be changed significantly—for example, it is not necessary to install the modules for one cable set before installing a second cable set. The order set forth inFIG. 5should not be construed as limiting in any way.

CONCLUSION

Although the foregoing invention has been described in some detail for purposes of clarity of understanding, it will be apparent that certain changes and modifications may be practiced within the scope of the invention. It should be noted that there are many alternative ways of implementing both the processes and apparatuses of the present invention. Accordingly, the present embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein.