Many moons ago made one from an alternator, and a large plastic radiator fan. Mounted on a swivel on the roof of the workshop. Did a good job of keeping spare batteries topped up. Not sure it would be much good for anything else. Was still going strong when we sold up years later and left it for the new owners. Have always intended to build another one, just never got round to it.
radiator fan is an interesting idea. I vaguely recall one of the videos showing someone making a huge slow speed long diameter prop and getting 400 watts out of their setup. I think that's a novel thing, but to get 400 max even with a prop designed for slow speed suggests 1kw hr on a good day, and damage in a storm.
A specific use for something making that much power could still be other than pointless, though.
Years ago, a guy put a low head high volume low speed water turbine on YT. He showed the making of the entire thing (which wasn't really a beginner's product). It probably cost him $100 to make the thing and it wasn't theoretically efficient.
However, he had it in a stream on a far end of his property where he wanted to keep a small building and a fridge without running power to it (think really rural). He showed the turbine running, which it had been doing, and the little fixes that he'd made to prevent it from getting plugged by trash or fish and said that it generated about 350 watts. It was genius for what he wanted, big crude and loosely made and robust and ran two fridges or a fridge and a freezer.
the comments were about half amazed (I was) at the guy's ability to design something crude, simple and effective and reliable for his situation. The other half were people deriding him about the design of the turbine fins, which were pretty much a fan on the end of a rod hanging below the device, efficiency or whatever else, and how dumb he was.
350 watts of continuous power for something made from salvage parts that's proven to literally work years mostly unattended is no joke.
He eventually put up a revised video that was really short and didn't show the making, and then removed it entirely.
Point in this case, most people with a 3kw air conditioner would think 350 watts is pointless. To a guy who has no grid tie for half a mile and even running it would have to pay a separate meter fee since it's not part of the residence, having 350 watts of continuous power that only involved sweat equity is just about priceless.
He reminds me of my tinkering uncle - when someone asked him where he read about it, he said he didn't read a whole lot other than the Bible. My tinkering uncle was spatially intelligent, excellent with electronics and electrical stuff because he understood how it worked, but I'm fairly convinced he was functionally illiterate.