Too bad AC power can't be stored directly in some kind of device. That would make wind and solar more compelling.
I was thinking some kind of resonant system. Of course, there is always the option of a rotating mass (momentum). The windmill itself is a rotating mass. Too simple, though.
Storage and conversion losses would have to be very low.
There would be a lot of bucks (probably Obama bucks) if someone one came up with a working prototype.
You could maybe store the energy in one or many giant spinning flywheel/gyro contraptions.
lol
i should have read the rest of your post.