I have access to this decorative windmill. It’s been in place for decades, spins freely in the wind, and has made millions of revolutions in its lifetime. Its RPM’s are impressive even in relatively low wind.
What are your thoughts on how I should harness this energy?
Simple answer: making it do meaningful work will make it harder to rotate (kind of like increasing the setting on a stationary bike) and something this size won’t be enough to power anything useful.
Maybe one of those solar garden lights? But those already run fine on solar.
I’d say leave it and enjoy the free spins in the wind, those are more valuable than the tiny bit of energy this can give you.