In the quest for something better to run our cars on than gasoline, one of the proposals is flywheels. In fact, for a while there were flywheel-powered buses running in Switzerland and Belgium. On one level, it makes a lot of sense: you're storing energy as mechanical motion, and we're pretty good at transmitting mechanical motion from place to place. On another level it scares the living daylights out of me: a car in motion uses tens of kilowatts, so the car must be able to store hundreds of kilowatt-hours. If you let all those loose at once bad things will happen: 100 g of TNT going off releases about a hundred kilowatt-hours. Fortunately it's hard to get gasoline to do this, but a flywheel is just itching to dump all its energy. Batteries are a little scary too, to be honest. But anyway, that's all a digression: I want to talk about some really staggering examples of flywheel energy storage: pulsars and black holes.
My pet pulsar, PSR J1023+0038, is putting out about thirteen times as much power as our Sun. But there's no nuclear burning to power it; all that power is coming from the energy stored up in the pulsar's rotation. And even at the prodigious rate it's being liberated, the pulsar can go on at pretty much its current power levels for billions of years - considerably longer than our Sun. Of course, the power is not coming out in a very convenient form for harvesting: mostly it's a relativistic electron-positron-magnetic-field wind - we would call that beta radiation if there were only a few particles instead of a massive stream. A small fraction, maybe a few percent, comes out as gamma rays, and even less of the power comes out as X-rays. So not the most accessible form, or the most hospitable environment.
Where does all this energy come from? Still not nuclear fusion, as it turns out. This pulsar was once a young pulsar that radiated away almost all its stored rotational energy (only took a few million years!), but then it was "recycled": its companion dumped material onto the pulsar, and this spun the pulsar back up to its current extremely fast rotation. So ultimately the energy comes from gravity. The Big Bang was so inconsiderate as to leave matter strewn all over the universe rather than all concentrated in one spot, a fact that neutron stars and black holes are eager to rectify. And it turns out that with nuclear fission you can get out a tenth of a percent of the fuel's mass as energy, with fusion you get a third of a percent, but with accretion - using gravity alone - you can get fully half the fuel's mass turned into energy.
The fact that gravity can extract a great deal of energy from matter - demonstrated on a small scale by meteors - is relatively easy to work out given how far you can fall towards a black hole before disappearing. Most of it comes out as the matter is on its way in, but it turns out that some of it can be stored in the black hole's rotation.
In fact, early on, a scheme was worked out to actually extract (some of) the energy that had been deposited into a spinning black hole. The basic idea was, you build a ring of space stations around the hole. They drop massive objects down towards the black hole, aiming them so that they just skim the horizon. Now, a spinning black hole obviously doesn't have any surface features to show you it's spinning, but it does produce a weird phenomenon called "frame dragging", in which objects near the black hole tend to get swept up into its rotation (usually just before their total obliteration). But aim an object right, and it can get pulled along a bit before it comes flying back out from near the horizon. Catch it and you can extract that extra bit of energy it got by being pulled along. Plus if you need to refill the black hole with usable energy, you can toss your garbage in along a trajectory that spins the hole up.
This sounds like something from Physics Experiment Land, grist for some of the weirder science fiction. And it is. But it turns out that in fact, lots of black holes are rotating very rapidly, and that rotation actually does put all kinds of energy into nearby material, in some cases ejecting it in massive intergalactic jets. The mechanism involves magnetic fields, not threading the black hole (it can't sustain them) but threading the plasma around it, being stretched, pulled, and strengthened by the motion near the hole. The energy balance is a little delicate - under some conditions, the energy comes directly from the infalling material, and it spins the black hole a little faster, but under other conditions, the black hole drives the material.
And at least you don't need to worry about black holes coming apart if you spin them too fast.