It is a sign of the amazing advances in technology, driven by commercial considerations, that Don Quixote's "giants" keep getting ever more humongous by the day.
One of the biggest constraining factors with wind power was the need for massive tracts of land to develop reasonably large windfarms. However, in recent years, as windpower emerged as a favored source of renewable energy, manufacturers have been working to make wind mills bigger, so that they can reach faster and steadier winds and the blades can cover larger areas. Wind farm developers too have been trying to get lesser numbers of larger turbines to generate the same power. The current leader in this race is the German wind turbine manufacturer Enercon GmbH.
The Enercon E-126 turbine - with a hub height of 135 m (443 ft), rotor diameter of 126 m (413 ft), total height of 198 m, total weight of 6000 mt - can generate 7.5 MW power per turbine today. Its first turbine was installed at Emden in Germany 2007, and a total of 24 are operational today, with the biggest farm using the turbine being at Estinnes, Belgium. The world's largest wind farm, the Markbygden Wind Farm, with 1,101 turbines covering just 500 sqkm to generate 4000 MW is under construction in northern Sweden and will contain approximately 150 Enercon E-126 7.5 MW wind turbines.
A Times report points to a study commissioned by the European Commission, 'Upwind: Design Limits and Solutions for Very Large Wind Turbines', which has found that a 20 MW turbine — with each blade probably more than 120 meters long — was "feasible".
Research and development work on wind turbines has proliferated around matters like how to pitch, or angle, the blades and how to monitor wind speed and direction at a turbine more accurately, using lasers. Further, since many of the best wind sites have already been claimed, developers are forced to build in place not so windy, which in turn makes innovation all the more crucial for cost-effectiveness.
The most active area of innovation has been in the design of gearbox, which speeds up the wind-powered rotations of the blades. A big breakthrough in recent years has been the use of a technology called direct drive that replaces the less reliable and high maintenance gearbox with a lower-speed generator. One of the key differentiators of the Enercon-126 is the use of a gearless, direct drive mechanism.
See also this graphic of global wind power capacity in 2010.