Lightweight conductive tethers, fairings around tethers

[Windy_Skies]

One of the prerequisites for flygen AWE is a lightweight conductive tether I think, like a lightweight motor is a prerequisite for powered flight, maybe a fairing around a tether is too, or an airfoil shaped tether.

This topic is to explore that, or this idea:

@Windy_Skies

@tallakt

I found this, 1200 V DCDC capable of 250 kW peak at 30 kg. Expect a weight for a optimized unit for 1 MW to be around 200-300 kg. [guesstimate]. Much lighter than the cable

The higher the voltage, the more interesting it becomes I think, but the higher the voltage the more insulation you need IIRC. Perhaps you could only insulate the bit at the beginning and end, letting the air around the cable be the insulator for the rest, and only send electricity through the cable while it is fully reeled out. You’d have some load at altitude to absorb electricity for when the cable is not fully reeled out.

This nice calculator could help:

@aokholm

On the topic of cable voltage and insulation. I made this interactive tether designer when we considered the possibilities with flygen systems. Should be applicable to ASWES Hello World!

Do you still remember what formula you used for the conductive wire area and the insulation thickness @aokholm?

This calculator lets the diameter of the dyneema rope determine the number of conductive wires, which I either don’t like, because it multiplies the insulation and jacket weights, or don’t understand.

I would like to be able to use a different design than the calculator uses, though that seems somewhat standard for flygen:

@Rodread

Makani report has
The tether comprises a mechanical load-bearing core of pultruded carbon fiber rods, surrounded by a bedding layer, a layer of spiral-wound insulated electrical conductors, and covered in an aerodynamically fluted jacketing material.

This for example, or something with a separate conductive wire and dyneema rope and a fairing around them:

@Windy_Skies

@tallakt

Some testing equipment could maybe be made by putting a regular wire inside the hollow core of a dyneema rope…

The physics of it is similar to a braking cable, which is governed by the capstan equation. (Cable Mechanism Maths: Designing Against The Capstan Equation | Hackaday ) So as long as you add enough slack to the wire to account for the lengthening of the dyneema rope when it is under tension, the wire can still slide freely when the rope is under tension, you don’t include too many turns, and there aren’t any pinch points, I think this should work, which I think is counterintuitive.

Questions might be:

• What are different transformer options
• Could you design a transformer that is better suitable for AWE
• How many wind turbines should you put on a wing
• How do you calculate cable weights outside of the bounds the calculator allows
• What are possible fairing designs

[Windy_Skies]

@Windy_Skies

I would like to be able to use a different design than the calculator uses, though that seems somewhat standard for flygen

Or I don’t know. With 0.5MW, 10kV, 0.95 voltage drop, 1 insulation safety factor, 1500 meter length tether you get a tether weight of 0.2 kg/m, which seems good enough.

[Rodread]

Length becomes critical because of flutter is an issue raised previously in tether fairing studies and by the TU Delft study on flutter for Makani
was mentioned previously on the forum

Theoretical drag of a Tether

Length of a faired tether is really important. From what I understand… The resulting aspect ratio of a long faired tether will be very high which will significantly increase the likelihood of flutter occurring See the tests on flutter TUDelft did for Makani. Is it fair to say Fairings on short tethers… Should fair better? Doesn’t sound very scientific

[Windy_Skies]

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https://www.reddit.com/r/technology/comments/196g002/magic_balls_installed_by_drones_may_soon_be/

https://www.reddit.com/r/technology/comments/196g002/magic_balls_installed_by_drones_may_soon_be/khtp0id/
It has everything to do with heat and sag. Because the reason why you need a certain gauge to carry a certain amount of current? It’s not “magic” - it’s heat.

A wire has a certain resistance - which means that some of the current that flows through it will be converted to heat, and that will heat up the wire. If you push more current than what the wire can handle? The wire will eventually get too hot, and bad things will happen. What bad things? For the wiring in your house, it’s the insulation melting. For the large power lines that have no plastic insulation? It’s wire sag.

But what if your current isn’t continuous? What if you need to fucking send it, and push five times the current, but only for a few minutes? What if you let your wire heat up, and then drop the current to give it time to cool back down? Can you get away with it? Or what if it’s cold outside, and the wire is cooled by the wind and rain? Can you have it carry more continuous current?

The answer is “maybe”. Usually, you don’t actually know what’s happening to the wires out there. And if you don’t know much, you have to plan for the worst case scenario, add a margin, and that’s your safe current limit.

But if you know things like the current temperature of the wire and the exact weather conditions? You don’t have to plan for the worst case. You can plan for the case you have. A cold wire under a strong wind of cold humid air can take more heat than a hot wire in hot dry air on a windless day. So you know whether you can work the same wire harder - and you know whether you are working the wire too hard.

[Windy_Skies]

– https://www.reddit.com/r/explainlikeimfive/comments/1b1gemw/eli5_if_silver_is_the_best_conductor_of/

One of the comment threads is on aluminium vs copper. Here on work hardening and corrosion of aluminium:

Aluminum distribution lines and salty coastal air do not mix well, you see a lot more pitting and chalky white residue on lines near the coast. Jacketed wire can react especially quickly because the salt water will sit in the plastic covering in constant contact with the conductor and won’t be rinsed off by rain. Regardless, all jobs to run in new conductor are always aluminum, copper conductor is used in very short sections that have high high currents and/or work hardening from bends, like the wires that attach to pole equipment (transformers, disconnects, load breaks, reclosers, etc…). Ground wires are also copper, but they have a habit of disappearing to a height around 6 feet from the sidewalk.

But:

A lot of high voltage lines do aluminum cores with copper around them(and then insulator and then the neutral side). This is both because aluminum is more flexible than copper, and because negative charge repels itself, the current density is higher towards the outside of a cylindrical wire.

– https://www.reddit.com/r/ElectricalEngineering/comments/xs2zdd/why_do_high_voltage_power_cables_have_aluminum/

Aluminium is far more flexible than copper. Therefore, it’s beneficial to use aluminium as the central core, as it makes the whole cable bend easier when installing on curved routes.

Why not use aluminium for all of it? Because aluminium is a far worse electrical conductor than copper. You don’t want to compromise the performance of the cable by that much.

[Windy_Skies]

Who Fails First? Electrician’s Legend: Thin vs Thick Wire in Parallel

You can think about higher melting point wire, insulation, and tether, but for example:

Dyneema® is a UHMPE (Ultra High Modulus Polyethylene, also known as UHMWPE or Ultra High Molecular Weight Polyethylene) and is available in a number of different grades. All grades of Dyneema® have excellent fatigue resistance (cyclic bending) and UV, chemical and abrasion resistance, but all have poor heat resistance due to a relatively low melting point (140○C).

Dyneema® Tech Info - Marlow Ropes

[dougselsam]

I don’t have much experience with dyneema, but a major concern I have is abrasion resistance.

[Windy_Skies]

The context here is using it as a tether around a conductive cable. It only needs to be unwrapped and wrapped around a drum once every time the kites take off, not every few minutes like in ground gen. Dyneema is more abrasion and uv resistant than possible alternatives IIRC.

I imagine abrasion becomes an issue you need to account for again if the tether needs to go at the same speed as the kites, just like abrasion is/was an issue for conventional wind turbine blades: Aluminium rigid wings? + Blade Materials & Erosion I don’t know how much a tensioned dyneema tether with a conductive core going at high speeds absorbs the impacts from or is abraded by sand, rain, and so on. Maybe you’d want to put sheathing around it.

[dougselsam]

@Windy_Skies

I imagine abrasion becomes an issue you need to account for again if the tether needs to go at the same speed as the kites, just like abrasion is/was an issue for conventional wind turbine blades

I was thinking more of internal abrasion around tight bends, attachment points, friction between layers and bending around reeling drums, etc. Wind energy involves endless motion and endless cycles, where any opportunity for wear will eventually cause failure. Occasional fair-weather tests will not reveal such flaws.