Kite power technology generates electricity through wings or kites, linked to the ground by tethers (or cables), which reach altitudes of about 800-1000 m above the ground. At these high elevations, cross winds are strong and constant. This strong and consistent wind force provides for increased power generation from the kite, but also causes stress to the tethers which both hold the kites and generate the power through resistance mechanisms on the ground. As a result, it is important for the tethers to be durable and efficient.
Air exerts a drag force on any object moving through it by directly opposing motion. A moving object has a high-pressure area on its front side. Air flows smoothly over the contours of the front side and eventually separates from the object toward the back side. A moving object also leaves behind a turbulent wake region where the air flow is fluctuating or agitated, resulting in lower pressure behind it. The size of the wake affects the amount of drag on the object. Due to the high speeds incurred during operation of kite power technology there is a strumming effect (Aeolian vibration) as the tether whips through the air.
And, the amount of surface over which the air flow must travel also affects the amount of wake, such that a greater amount of smooth surface creates a greater amount of wake, and thus a greater amount of drag. Surface depressions on an object travelling through air create a thin turbulent boundary layer of air that clings to the surface. This allows the smoothly flowing air to follow the surface farther around the back side of the object, thereby decreasing the size of the wake, and thus decreasing the drag.
Cross wind can cause torsion, which increases member shear stress. This stress is added to the existing shear stress due to vertical and lateral applied loads. A fluted exterior profile when subjected to crosswinds has been shown to lower the drag force on cylindrical objects by inducing turbulent flow across the object which reduces drag force on the object. A unidirectional helical profile can present an issue of inducing a torsional “wind up and release” which is not desirable. Example “wind up and release” can produce a cyclic torsional stress loading which can be detrimental to the service life of the components comprising a tether cable.
An aspect of the present disclosure relates to a cable extending along a longitudinal axis between a first end and a second end. The cable includes an outer jacket defined about the longitudinal axis. The jacket includes a plurality of fluting elements between the cable first end and the cable second end. The fluting elements are configured to reduce the drag forces, and the torsion along the cable axis result from the cross-wind.
An example cable (or tether) core can have a carbon fiber construction insulated with an aluminum fluoropolymer, for example the TEFZEL™ product. The outer jacket can be constructed of material capable of having a fluid state and a solid state. Examples of such outer jacket material include thermoplastics, such as PVC, polyurethane, polyethylene, polypropylene, nylon and silicone rubber.
An example cable can have fluting elements extending on the jacket to reduce drag from cross-winds, thus increasing efficiency of the system and therefore higher energy gains by the system. This design can create more turbulent flow at the surface of the tether, resulting in lower resultant drag from the crosswind. This design forces the boundary layer of the airflow around the tether for lower drag. This proposed designs minimize strumming from coordinated vortex shedding.
The illustrated cable 100 can include oscillating helical fluting features 108 on the outer surface 106. This example cable 100 includes a plurality of oscillating helical fluting features 108 extending in parallel to each other from the first end 102 to the second end 104. An example cable can include between about 1 and about 40 parallel fluting features 108, more preferably between about 10 and about 30 parallel fluting features, and most preferably about 20 parallel fluting features. Each one of the oscillating helical fluting features 108 can alternate (from the first end 102 to the second end 104) between counter-clockwise oriented sections 110 and clockwise oriented sections 112, and vice versa. The sections of the oscillating helical fluting features 108 can extend at an acute angle orientation with respect to the axis X1, and can alternate in direction between the counter-clockwise section 110 and the clockwise section 112.
The oscillating helical profile can reduce the potential for torsional “wind up and release,” since the net torsion created by wind traveling down the helical profiles (while the direction reverses periodically) counteracts the torsion, resulting in almost a net zero torsion on the cable along the axis.
An example fluting feature 108 can be a groove in the jacket 106. The grooves can have a depth of between about 0.1 mm and about 1.0 mm, preferably between about 0.3 mm and 0.7 mm, and more preferably about 0.5 mm. The grooves can be semi-circular, with a diameter of between about 0.1 mm and about 1.0 mm, preferably between about 0.3 mm and 0.7 mm, and more preferably about 0.5 mm.
Alternatively, the fluting features 108 can be grooves with alternative geometries, for example square, or triangular.
An example jacket can have a thickness of between about 0.1 mm and about 3.0 mm, preferably between about 1.0 mm and about 2.0 mm, and more preferably about 1.5 mm. The ratio between the depth of the fluting features 108 to the thickness of the jacket can be between 1:4 and 1:2, and more preferably about 1:3.
An example jacket can have a diameter of between about 20 mm and about 40 mm, preferably between about 25 mm and about 35 mm, and more preferably about 31 mm. The ratio between the depth of the fluting features 108 to the diameter of the jacket 106 can be between about 1:20 to about 1:400, preferably between about 1:50 to about 1:83, and more preferably about 1:62. Thus more preferably, the depth of the fluting features 108 can be about 1.6% of the diameter of the jacket 106.
The thickness of the outer jacket 106 can be between about 10% and about 70%, preferably between about 20% and about 60%, and more preferably about 30% of the diameter of the cable core 100.
There are several possible solutions to achieve the fluted outer jackets described above. A first example can be an extrusion line system, such as the example described in
The illustrated extrusion die 200 includes a die profile 202 which can be a passageway or aperture through which the cable core can pass after being coated with a hot liquid material that forms the outer jacket. The die profile 202 can have an outer circumference 210 intersected by a plurality of flanges (or teeth) 204 which extend inwardly from the outer circumference. The number of flanges 204 to be used is determined by the number of fluting elements 208 to be formed on the outer jacket 206 during extrusion. The outer circumference 210 of the die profile 202 is defined by a diameter that is determined by the preferred diameter of the outer jacket 206 to be formed. The distance which the plurality of flanges 204 extend inwardly from the outer circumference 210 is determined by the preferred depth of the fluting elements 208 to be formed in the outer jacket 206.
In use relative to the extrusion line in
When forming the outer jacket 106 having straight fluting elements 188, as illustrated in
When forming the outer jacket 106 having oscillating helical fluting elements 108, as illustrated in
When forming the outer jacket 106 having spiral fluting elements 158, as illustrated in
The reverse of the above is also effective, with the pay-off 10, the cable core 11, the cooling/curing trough 16, the capstan 18, and the take-up 20 remaining in a fixed orientation, while the extruder cross head 14 and profile die 202 rotate about the axis.
Each roller 306 is oriented along an axis X3 extending from the backing plate 304 and is supported with respect to a position on the backing plate with a fastener 320. The backing plate 304 can have a series of apertures, arranged to adjust the position of paired rollers 306 with respect to each other. A first fixed position aperture 308 receives a fastener 320 to fix a first roller 306 in a fixed position on the backing plate 304. A second sliding (or translating) position aperture 310 can have an elongated geometry that provides freedom for a fastener 320 to alter its position with respect to the first fixed position aperture 308. In use, a fastener 320 in the second sliding position aperture 310 fastens to a second roller 306 and allows this second roller to move (or translate) relative to an opposing first roller fastened through the first fixed position aperture 308. Such movement of relative position between a pair of opposing first and second rollers 306 provides an adjustment in gripping force applied by the rollers to a cable core 302.
A pair of opposing rollers 306 on the roller assembly 300 grip the cable core 302 between the opposing receivers 312 of each roller, with the opposing pair of barriers 315a, 315b preventing the cable core from accidentally being removed. The pair of opposing receivers 312 of opposing rollers 306 engages and applies pressure to the outer surface of the cable core 302. When received between a pair of opposing receivers 312, the cable core 302 can still progress along the extrusion line (
The roller 306 can have an aperture 314 between the barriers 315a, 315b. The aperture 314 can receive the fastener 320 that is inserted through one of the apertures 308, 310 in the backing plate 304 in order to support the roller with respect to the backing plate. The fastener 320 can be a bolt that includes a threaded insertion portion 324 and a gripping end 324. The gripping end 324 can include a nut-like element and a washer-like element, which do not pass through the aperture 314 in the roller 306 when the fastener 320 is inserted therein. When inserted, the gripping end 323 contacts a bearing 322 that is rotatably fitted into a recessed receiver in one of the barriers 310b.
The threaded insertion portion 324 of the fastener 320 is inserted through the aperture 314 in the roller 306 and through either the fixed aperture 308 or the sliding position aperture 310 in the backing plate 304. A locknut 326 secures to the threaded insertion portion 324 of the fastener 320 on the opposite side of the backing plate 304. A washer 328 can fit over the fastener 320 to prevent the locking nut 326 from passing through the aperture 308, 310 in the backing plate 304. When supported to the backing plate 304 by the fastener 320, the roller 306 can freely rotate about axis X3 with respect to the backing plate, with each roller in an opposing pair rotating in opposite directions to each other.
When the pair of opposing rollers 306 are to closer each other, the amount of gripping force applied to the cable core 302 is increased. When the pair of opposing rollers 306 are farther apart from each other, the amount of gripping force applied to the cable core 302 is decreased. When gripped with sufficient force by the opposing pair of rollers 306, the cable core 302 can be twisted clockwise or counter-clockwise by rotating the roller assembly 300 in a similar direction. Preferably, rotation of the roller assembly 300 will cause a specific section of the cable core 302 to rotate immediately before and after passing through the extrusion cross head 14 without rotating the entire cable core. Such limited twisting assists in achieving higher line speeds and lowers cost.
A water-soluble compound in the shape of sphere 500 is introduced to the jacketing material during the extrusion process, such as the extrusion line in
The dimples can also be formed by blowing agents, for example chemical blowing agents that decompose due to heat may be used to create the dimples. Such compounds, for example triazoles and nitrogen, may be dispersed in the compound and when the correct temperature and pressure is reached during extrusion they give off gas. The gases form cells or bubbles, which if large enough, will come to the surface and appear as dimples.
Instead of using dimples to create turbulent flow around the extruded surface, the reverse of dimples could be applied. By creating little bumps on the surface of the cable laminar flow of air will be lost at high speeds as the cable whips through the air. These bumps, or reverse dimples, may be introduced during the extrusion process. They may be made of glass bubbles or any other light weight material like recycled fluoropolymers that require elevated melt temperatures.
The above specification, examples and data provide a complete description of the manufacture and use of the composition of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention resides in the claims hereinafter appended.
Embodiments of the disclosure may comprise a cable extending along a longitudinal axis between a first end and a second end, the cable comprising jacket defined about the longitudinal axis, the jacket comprising a plurality of dimples between the cable first end and the cable second end, the dimples being configured to reduce the torsional forces of cross-wind.
Embodiments of the disclosure may comprise a system for creating a cable with a core and an outer jacket that reduces torsion from cross winds, the system comprising a plurality of water-soluble spheres which form dimples in the outer jacket when exposed to water.
Embodiments of the disclosure may comprise a system for creating a cable with a core and an outer jacket that reduces torsion from cross winds, the system comprising a plurality of heated rollers comprising a plurality of nodes, wherein the plurality of rollers are configured to melt the outer jacket and form a plurality of dimples in the outer jacket.
This application claims priority to U.S. Provisional Application No. 62/701,047, titled “Oscillating Fluted Outer Covering for Reduced Wind Drag” filed on Jul. 20, 2018, the disclosure of which is hereby incorporated by reference in its entirety.
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Number | Date | Country | |
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62701047 | Jul 2018 | US |