i'm trying to imagine how long/how much energy could be stored in a resonably sized flywheel. a better solution perhaps would be to focus said solar energy on the stirling engine to generate electricy storing hydrogen and oxygen from electrolized water in pressuraized tanks afaik although non trivial it's possible to fire traditional combustion generators off the proper mixture of these gasses
i agree that you would get more storage out of that, however it would require you to constantly refill the water? or is it able to be recycled? why do you choose making an engine out of the hydrogen and oxygen instead of using it as a fuel cell (requires Pt i think).
how big is reasonably sized? about a car tire? a bucket?
in reality, if you want the energy to be kept for longer than an hour, you need to have it suspended inside a vacuum chamber with magnets. i can see that costing quite a bit.http://www.youtube.com/watch?v=mV_b5oMqc2M
that is just a small weight and it can power a flashlight for a little while.