The fluid in high pressure storage is a liquid, making the storage much cheaper. Liquid N2 (most of air) would require over 40 times more pressure or cold temperatures. Purifying out CO2 or any gas is generally a negligible cost.
I remember following that amazing accident. But as I recall, the turbine jumped up out of its vault, and that's what started the whole failure.
IIRC, the speed of a water-powered generator is regulated by the load on it. Take that load away, it spins faster and faster (think that's what happened to the other generators). So that one guy had to climb up to the top of the dam to close the gate.
Wow, that is pretty amazing article. About 2 orders of magnitude more energy than I think we're talking about here, but still. These things are scary when they get away from you.
You are missing something (but don't worry, it is good to brainstorm like this).
At maximum speed (maximum energy storage) you want every part of the flywheel to be about to break. By introducing a non-structural material (liquid water), you have added mass but not the strength to get to higher speeds.
Imagine filling a bucket with water and spinning it around your body. All of the force to keep the bucket from flying away has to be held by the handle of the bucket. A better bucket flywheel would be all handle... Hopefully someone else can give you a better analogy.
