DynaRing

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.