Fundamentals of wind energy - Wind Power Generation and Wind Turbine Design - page 38

Figure 14: Illustration of a dual-blade set wind turbine .

7.6 Wind turbine with contra-rotating rotors

Coaxial contra-rotating propellers have been widely applied to aircrafts, marines,
ships, and torpedoes for improving the propulsion effi ciency and offsetting system
reactive torques. The effi ciency of contra-rotating propellers was found 6-16%
higher than that of normal propellers [102]. Late, the contra-rotating concept has
been introduced into the wind turbine design. The contra-rotating type of wind
turbines has usually two sets of blades that rotate in opposite directions. These two
sets of blades can be arranged either one behind another at the turbine front, or
separately at the turbine front and rear (see Fig. 14).
Schönball [103] is one of the fi rst to describe a mechanism which composes of
two contra-rotating rotors. In the generator, a rotor is driven by one wind wheel
and a regular stationary stator driven by another wind wheel. Owing to the opposed
rotation of two wind wheels, the relative speed between the two rotors may be
doubled and thus the effi ciency is correspondingly increased, compared with a
conventional generator having one rotor. Similarly, McCombs [104] developed a
wind turbine equipped with two sets of blades that propel the rotor and stator
directly. More recently, Wachinski [105] proposed a drive device for an improved
windmill composed of two counter-rotative sets of blades. In 2005, with two
granted US patents [106], Kowintec Inc. successfully built up a 100 kW and 1 MW
contra-rotating wind turbine prototypes. The test results have shown that with the
dual-blade and dual-rotor, the power production effi ciency of the turbine increases
24% and the system cost decreases 20-30%, by comparing with a conventional
single rotor unit [108].
The benefi ts of using contra-rotating wind turbines are:

￿ Enhancing wind energy capturing capability A wind turbine with a contra-

rotating system can capture more wind energy to convert into electricity, by

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