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Consider a maglev train. It weighs a lot, despite being magnetically levitated off the ground. In Physics, though, we want to think about its "mass" more so than its weight.
Now consider the maglev track upon which the train runs. Suppose it was a loop. A big loop, say 25km in diameter. Let's
also suppose the loop is enclosed by a tunnel-like structure, and that we pump all the air out, so this particular maglev train moves through a vacuum. We might consider putting the whole thing underground, so that any roads passing through the area won't be interrupted.
Now imagine that the maglev train occupies the entire circular length (circumference) of the track. All the train cars are alike; there is no first or last car. Another advantage of having a vacuum is that it acts as a heat-insulator. This means we can use cryonic superconductors, and the amount of energy we need to spend, to maintain the cold temperature for the magnets, will be minimal.
All the train cars are fully loaded with rocks. Cheap mass, in other words. It is going to take a LOT of energy to get this train started moving, in a circle!
However, in a maglev train, every car also has a "linear" motor, for powering it along the track. That means we can apply energy equally to all the cars simultaneously, to get the train moving. And because the train is floating frictionlessly in a vacuum, it will keep moving.
One way to think of this is as being a super-big flywheel. The more mass in motion, the more energy it stores. And the faster it moves, the more-SQUARED energy it stores. One advantage of putting the whole thing underground is that the outer-sidewalls, of the tunnel, are supported by lots of mass, and we can put more maglev tracks on those walls.
At speed, then, our train is trying to jump its "ordinary" tracks and scrape the walls, but the secondary maglev tracks prevent it. This lets us run the train even faster, storing even more energy.
The same linear motors that we use to accelerate the train can also be used to efficiently extract energy from this storage device. So, if you have a big windmill farm occupying lots of square miles, a DynaRing is something that could be built to "even out" the supply of electricity to the national grid.
Smaller Version of this Idea
Each size of device has its own niche of usefulness. [Vernon, Apr 08 2012]
ring flywheel 2
[xaviergisz, Apr 08 2012]
Non-ring energy-storage train
A variation on the theme [Vernon, May 23 2016]
Polarallel Drive Mounting
As mentioned in an annotation. [Vernon, Dec 27 2017]
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||[+], but I think you need to explain it in finer detail.
||hmmm, if you got this thing rotating too quickly would the Earths' rotation cause it to rip itself apart from precession?
||So, it's a perpetual motion train?
||How do the passengers get on and off?
||All other things being equal, both the centrifugal force and the hoop stress of a rotating (thin) ring scale with r². However, the circumference is proportional to r, so the force per unit length exerted by the magnets is proportional to r²/r = r. While that's an oversimplification, it does strongly suggest that this idea is indeed better suited to very large flywheels; smaller ones should rely on tension.
||I was going to try to calculate some actual forces, but that would involve making up a whole bunch of parameters. Suffice it to say that for a given energy density, the maglev pressures can be made arbitrarily small by making the radius arbitrarily large (and/or making the train wider and flatter). Whether that equates to a potential cost saving compared with conventional flywheels is a whole nother question.
||If both sides are very lightly loaded permanent magnets then there's no energy wasted (under the assumption that there's a strength point before the magnets start flipping each other off atom by atom).
||But thassahelluvalotta permanent magnet. The motor need not be in the train cars, it can be in the track. And for "rocks" I read "nuclear waste" for purposes of both mass efficiency and dramatic tension.
||If it works like most mag-lev, the motor is strictly on both sides - the permanent magnet rotor on the train / ring, and the wound stator on the track being the sensible configuration here. (It's an interesting, but probably unimportant, question whether it's a linear or rotary motor. Fortunately the terminology is the same either way.)
||This Idea fails to take into account the effect of the Earth's
rotation on the DynaRing (there will be gyroscopic twisting
forces for any DynaRing not located at the North Pole or
South Pole). A solution for that problem is to
build the track/tunnel at such an angle, relative to the
surface of the ground, that the layout of the tunnel is
horizontal relative to the imagined-vertical axis of the
rotation. For more details, see the Polarallel Drive
Mounting Idea (linked).