Burning Questions

West Australian
25/08/2006 Page: 16

Q. Why do windmills generating electricity have three narrow blades while windmills that pump water have as many wide blades as possible?

A: It all boils down to the available wind speed and the mechanism it is intended to power. Water pumps are usually sited inland where there's not much wind, so a multivane design works quite well because it has to use relatively slowmoving air. But windmills generating electricity need to rotate much faster to avoid the use of heavy (and expensive) gearboxes. So electrical generators are usually sited in windy coastal regions.

All wind turbines generate energy by a mixture of lift and drag. Lift is the more efficient process and increases rapidly with wind velocity. At high wind velocities multiple blades tend to obstruct air flow and cause turbulence whereas at low speeds this is not such a problem. Lift and drag explain also wily a yacht will sail faster with the wind from the beam (lift) as opposed to wind from behind (mostly drag). Similarly, paddlewheels on ships were superseded by propellers and biplanes were replaced by monoplanes as they were simply a lot more efficient.

For windpowered generators, maximum efficiency is reached when the wind velocity is slowed by about a third after passing the rotors. Therefore, whenever the wind velocity is high, two or three fastrotating thin blades are so much more aerodynamically efficient than a large number of wider ones. At very slow wind speeds, where drag predominates, the reverse usually applies.