The picture of the V-112 in Fig. 3.14 shows that the blades of modern turbines have been designed with special shapes to maximize efficiency at all wind speeds and to minimize turbulence.15 Such shapes are also seen in newer airplanes (Fig. 3.15). Blades may evolve further to incorporate scalloped edges, as these have been found to reduce drag on a humpback whale’s flippers.16 As the wind speed varies, each blade’s pitch is changed with a motor to capture the most energy. In very strong winds, the blades are feathered as in airplanes. The fiberglass blades are much thinner than on windmills and there are only three of them per rotor. This design is driven by cost.17 More blades will not only be too expensive but will also require sturdier towers to support in strong winds. The blades are so long that even at only 5 rpm, the tip of a 200-feet (60-m) blade travels at 170 miles per hour (75 m/s).

The diameter of the rotors is very large because these catch more wind when the speed is low. This is explained in Fig. 3.16,18 which is drawn for a situation when the average wind speed is 7.5 m/s (17 m/h). The smooth, peaked curve at the left shows how often each wind speed occurs. The speeds are on the bottom scale. Notice that most of the time, the speed is between 2 and 12 m/s. The rightmost of

the rising curves shows the turbine’s output power for a 50-m diameter rotor. The power is limited by the size of the generator. The curve labeled 50 m-3.0 MW, therefore, rises as the wind speed increases but stops rising and stays flat when the curve reaches 3 MW (at around 16 m/s). Increasing the generator’s capability to

4 or 5 MW permits capturing the energy of the strongest winds, as shown by the uppermost curves on the right. However, these occur only a small part of the time. If, instead, we keep the generator at 3 MW and increase the rotor diameter, we get the colored curves labeled 70, 90, 120, and 150 m. These rotor diameters utilize the slow wind speeds more efficiently, even though the 3-MW generator cuts the curves off when the available power reaches 3 MW.

Even larger rotors would capture more of the slow winds under the peaked curve, but then the towers would have to be even taller than the monsters that we now have. High hub heights also contribute to a turbine’s efficiency. Winds are stronger away from the ground, where the trees, grass, hills, and structures impart a drag. This was a rather technical discussion, but it shows why it pays to tear down old turbines and replace them with fewer large ones.