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Vertical Axis Wind Turbine
Vertical Axis Wind Turbine (VAWT)
The earliest recorded use of wind power is with vertical axis turbines used to mill grains as long ago as 200B.C. Variations on the design (which typically employed four vertical flags of fabric or woven grasses connected with wooden beams to a central column) became quite prominent in modern day Iran starting in the 7th century.

Contemporary versions of the vertical axis wind turbine (named because of the vertical orientation of the central rotor shaft that gives mechanical power to the generator) can be of a number of different design types: Darrieus, Savonius

One advantage of VAWTs is that they can be located in closer proximity to each other (HAWTs must be spaced far enough apart so as not to block or cause interference to the horizontal wind energy of the adjacent turbines). Some studies have shown that dense configurations can actually increase efficiency of the overall installation with turbines picking up wake energy from the rotations of adjacent turbines.

Typically VAWTs have lower cut-in speeds (the wind speed at which they begin to produce electricity) and can be positioned lower to the ground than HAWTs. However, HAWTs still have the higher single unit capacity potential (there are no designs for VAWTs that approach 10 megawatt capacity like some HAWTs).