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.
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.
I remember getting a wide band oxygen sensor and tuning the gain in the new ECU (and by tuning I mean eventually realizing turning it all the way up minimizes steady state error). When I worked with the setup, it had no supercharger and was a lab demo for the newly reintroduced engines class. (getting to be almost a decade ago, I should go visit) Did you guys do research with it?
"Mayvenn will send you a check for your earned commissions every month. You can do anything you want with it – pay rent, pay for your car or buy those shoes you wanted!"
From Mayvenn's site. Does this seem insulting to anyone else?
Perhaps you know the math, but I think it is important to be clear to readers that a penny earned is LESS than a penny saved.
Put another way, are making $10,000 more dollars per year and spending $10,000 less per year the same? No. You will retire exponentially (literally, if not colloquially) sooner by spending less.