Comment by srean
4 days ago
Just yesterday I was mentioning about the shared fascination with everything knitting, weaving, knitting, tatting, crocheting and braiding.
https://news.ycombinator.com/item?id=46039952
Wonder if I should braid my wired earphones for storage to prevent tangling. I can keep the cable inside a pouch with the earpieces out but that's not very satisfactory.
My current fascination knitted ropes/cables/cords. These are not the typical ropes that are spun and coiled and held together by friction. These ones made of synthetic fibres look like woven tubes, but the insides aren't hollow. The insides seem packed with more woven tubes.
What I really want to see though, are 3d knitted heavy duty carbon fibre flywheels of optimal shape such that it's under equal radial stress everywhere. The shape is interesting to compute for a solid one.
out of curiosity, what would you want a carbon fiber flywheel for? usually the value of a flywheel is for storing kinetic energy which a low density material like carbon fiber would not be suitable for.
What you lose in mass could be made up in velocity and radial distance. Depends on the breaking strength of the fibres. I haven't done any calculations myself to see if they are more promising than a steel one. A carbon fibre flywheel exploding might be somewhat less dangerous than steel if the bits flying away can crumple dissipatively.
Regarding where can they be used... they are just batteries with a different form factor. You can put them on the grid to have some inertia and be a place to dump or extract energy spikes. They may be commercially viable if rare earth based batteries become very expensive. At a smaller scale one could use them as a mechanical UPS for a building/datacenter. Maybe even to power golf carts, not sure how well they will steer because of the angular momentum.
That was somewhat relevant for data centers years ago when inverters were much more expensive, and even then only for the 20 seconds it took to start the diesel by connecting a simple soft starter to the diesel's induction generator (bringing it up to 50/60 rps (1500/1800 rpm) in about 5 seconds, then waiting for the turbo to spool up before it can deliver full power).
Even on the grid, batteries for sub-hour duration storage are cheap, as long as you place them at an already existing AC/DC converter site like a solar plant (or a modern internally-DC datacenter's centralized grid rectifier (AC/DC converter)).
Or even a HVDC transmission line.
Or a sufficiently modern aluminum/zinc smelter. Pretty much anything large enough to bother that has at least a boost-PFC on the input. Because with those you could just put them there, beef up the capacitor a bit or better yet, use a native 3-phase PFC that doesn't have strong 100/120 Hz ripple on that capacitor, and then literally just control the already there input transistors to do your grid jobs. If it was the very cheap low efficiency rectifier approach, it also needs the rectifier upgraded to be controlled, so just use it on the higher efficiency ones before upgrading the others. (It's like 1% efficiency difference on 240V, and 2% on 120V power supplies.)
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