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I have spotted this on the rooftops of industries, workshops, etc. I guess this is used for providing ventilation. This is not electrically powered. But then how does this work??

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That is a wind-driven ventilator. – Kyle Kanos Sep 30 '13 at 19:22
@KyleKanos- A friend of mine argues that, its the exhaust of warm air from under the roof that makes the ventilator rotate and further assists in the process..I cant counter his point – karthikeyan Sep 30 '13 at 19:31
I am pretty sure that it is the wind on the outside that is driving it. – Kyle Kanos Sep 30 '13 at 20:06
I have recently seen lots of them on sale in an open-air store and with a light breeze they do spin fast. – Peltio Jun 15 '15 at 9:58

I am familiar with such equipment. There are several kinds and are wind-driven. There is a paper A review on wind driven ventilation techniques about passive ventilation techniques.

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