Small Wind electric Systems: efficiency depends a lot on the diameter of the rotor

Today’s small wind turbines are sophisticated machines, involving large aero-dynamic blades and new materials (composite carbon materials and fiberglass) and electronic controllers.

All comprise a rotor, a generator, a tail (attached to a frame, to keep the turbine facing the wind) and a number of electronic controllers to automate the operation of the system, to turn DC electricity into AC power, and to connect the system to the utility grid or to a battery bank.
Modern small wind electric systems are sophisticated devices, largely automated. Some modern hybrid solar-wind systems have GPS-controlled tracking mechanisms, mounted in the wind system tower, to rotate the solar panels to the more advantageous position.

The importance of the diameter of the rotor

The rotor of the turbine and the site's average wind speed (which in most cases is closely related to the height of the turbine tower) are the most determining factors of success.

The power generated by a horizontal axis wind turbine tends to be directly proportional to the swept area of the rotor.

In other words: a turbine with a larger rotor will generate more electricity than one with a smaller rotor, even if it has the same rated capacity.  

The rated capacity is a rather theoretical value. Two turbines with 5 kW generators (rated capacity), one with a 10 foot diameter rotor and the other with a 14 foot diameter rotor, have in fact very different electrical outputs. The second, with twice the swept area, will produce about twice as much power, for the same wind resources...

See: Rated Capacity & Electric Output of Wind Systems.

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