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?
Get any cheap small electric DC motor and attach it somehow (pulleys?). It won’t be as efficient as possible, but it’ll work.
But also: sucking power out of it will create resistance and might stop it from spinning in low-medium winds.