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?
I believe you’re thinking of momentum in a narrow sense: the energy an already-moving object has that allows it to move for a while before stopping. And it was meant in a more general sense ie the amount of energy being generated and doing work.