Regarding safety issues with flywheels, I notice that some manufacturers install their flywheels below ground to solve this. A simple and elegant solution. Verticle axis spinning in a hole. I guess the hope is that ALL of the parts would fly sideways into the ground. I think I would still want a two or three inch thick manhole cover over top of it though.
I'm wondering if anybody here has experimented with really high mass and more reasonable rotational speeds for storing wind and solar energy. I'm thinking in terms of hundreds of pounds (or tons) of cement rotating at one or two thousand rpm instead of 50 pounds of stuff rotating at 20,000 rpm. I know the energy storage is only linear with mass, and goes up as the square of speed. Perhaps this is one place where it makes sense to limit the advantages of "the power of squaring" in order to make a more practical homebrew solution.
Just thinking out loud here...Maybe fill a 4 foot long section of that concrete form tube 18" or 24" in diameter with fiber reinforced concrete and welded together rebar. Maybe come up with a simple way of post-tensioning it or wrapping it with something that has more strength, like a few hundred feet of braided steel cable.
Then you dig an 8 foot deep hole and put a heavy duty bearing on a concrete pad at the bottom of the hole. Hoist your concrete drum down into the hole and set up a top bearing and alternator/motor on the top. Then cover it up with dirt and park your spare tank M-1 tank on top of it.
Then you do a destructive test to see how much it can take. Then you dig new hole and start over, and limit the speed to something significantly below the empirical limit.
Anybody out there know the math and the unit constants to turn this into a simple "kilowatt-hour" per rpm per ton? I don't know which box my college physics book is in.
BTW, I'm new here.
Hope this sort of blue-skying is well received around here. If not, I refuse to appologise. 
