How to Spot a Psychopath

Using a roof ventilator as a wind turbine (I think this is the home site of the video creators) is a neat idea. It'd really work, and make a great science fair project - just hook up a little brush motor to the rotor, connect an LED (or several) to it, and see how much wind you need before your LED pops.

This design will not, however, really give the advertised "cheap" power on a dollars per watt basis, since I doubt you could put more than a few watts of generator braking on one of those roof turbines without stalling it in anything short of a gale.

A roof-ventilator turbine could still be useful for charging a battery to run minor stuff occasionally, like a radio and a small light in a shed somewhere where the wind blows but the sun doesn't shine much. Or as a supplemental charger for a system that has a solar panel as well. Plus, you could still use it as a roof ventilator.

There's a reason why you don't see a lot of vertical turbines, though. Yes, they're simple - most importantly, you don't need a mechanism to make them face into the wind, and power take-off is very easy when you've got a simple vertical shaft to work with - but they just don't work very well. This is not just because they don't suck as much energy out of the air as a propeller-type turbine does (see also: Paddle-Wheels, And Why We Don't Use A Lot Of Them Any More), but also because they usually don't turn fast enough.

Slow-turning turbines can be good if you want to direct-drive a pump or something, but generators usually want more RPM, and gearing up a wind turbine is a great way to make it stall.

There are other vertical turbine designs that're very quick to make and have a lot more blade area for the wind to push on. The "Savonius" versions made from oil drums cut in half (technically, roof ventilators are a multi-bladed Savonius turbine) are the price/performance winners for small jobs, I think. But the world is not short of other ideas.

There are also commercial vertical turbine designs, the domestic-sized versions of which have a tendency to be scams.