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Fred King
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Tonight I visited a friends farm and watched one kid wind the rope swing the other was sitting on until the ropes were twisted almost there full length. When the swing was released the kid on it became a blur. A lot of energy had been stored and released.  I had to wonder about the efficiency of a dc motor/generator winding more and /or longer ropes with solar or wind power and unwinding when power was needed. Should be cheaper and longer lasting batterys. Just a thought from Colorado where weed is legal but maybe worth a try.Now where can I find some cheap ropes?
 
Tim Bermaw
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The energy release you witnessed was not actually due to the ropes being twisted.  It was due to the child on the seat being lifted higher as the combined rope length shortened.  Basically gravitational potential energy.

Ep = mgh

Thus a 30kg child lifted 20cm higher (due to the twisting of the ropes making them shorter) now has potential energy of:

Ep = 30 * 9.8 * 0.2
Ep = 58.8 J
Ep = 58.8/3,600,000 kWh
Ep = 0.000016333 kWh

So assuming you pay $0.30/kWh for electricity, the amount of energy stored (and hence the absolute maximum amount of energy that could be harnessed upon release) would be worth:

Value = amount * unitValue
Value = 0.000016333 * 0.3
Value = $0.0000049
Value = 0.00049¢

The value of the energy produced would never recoup the cost of the rope (before it wears out), so it will never be economically viable.  Sorry.

That's not to say the idea can't actually be used to do something useful.  Just get a falling mass to power a generator and produce electricity (like this https://gravitylight.org/how-it-works) and you can have an easy-to-construct and hard-to-break 'power supply' that can, in certain circumstances, be a quite satisfactory solution.

The thing that breaks your original idea is the failure rate for twisting ropes.  If you replace twisting ropes with a rope over a pulley, then the rope lasts longer and your economic efficiency increases.  If you replace ropes with steel cables and large diameter pulleys, it gets even better.  If you instead have chains pulling a cart with rocks up a rail track on the side of a hill it gets even better.  If you throw all that away and just use solar power (generated during the day) to pump water up a hill and into a reservoir, and then release that water at night through a turbine (when the PV panels aren't generating anything) to get the energy back, then you're pretty much at the point that we're at now (it's called 'pumped hydro').  All of them are essentially the same:  Use surplus energy at one point in time, to lift a mass (give it potential energy), then later release the mass and recapture some of that energy.

PS:  Anyone lucky enough to live in a hilly area with an elevated water tank (or the potential to install one) already has most of what they need to store renewable energy during the day and use it at night with a round-trip efficiency of over 60%...  https://en.wikipedia.org/wiki/Pico_hydro
 
Daniel Schmidt
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Location: Jacksonville, FL
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I have seen some designs that try to use the stored potential energy to convert it in to electricity, but it could be much more efficient to use it to directly drive something. For instance I vaguely remember seeing something similar for a clothes washing machine, but it was human powered. I believe it would be fairly straightforward to use direct wind power or to divert excess renewable energy generation to a motor to lift a weight to store potential energy.

There were ways to store torsional energy in ropes, such as the Roman Onager, but I would imagine spring steel would be cheaper and much more durable. It would probably be easy to repurpose used springs for such a thing.

I have been thinking of using a large compressed air tank to store excess solar energy as compressed air to run tools and other things. There are tons of ways to use excess energy as storage, but each time you change energy forms it tends to lose energy from heat and friction Going from electricity>kinetic energy>potential energy>kinetic energy>electricity and potentially turning the electricity to something else at the point of use has a lot of places to lose efficiency along the way.
 
Travis Johnson
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I have logged a lot in my lifetime, and the way the wind tosses huge trees around due to their height, and wind swept area of their boughs always made me think that if ropes could be tied to the top of a tree, then run through a pulley attached to a generator. As the wind tossed the tree over, the rope would produce a lot of electrical generation, the rope being returned by weight or spring. I don't have a dull design worked out, but the concept is simple; use a tree as a already-made windmill of sorts.
 
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