Wind speeds across Kenya, a key indicator of wind energy potential
The successful exploitation of wind energy is highly site specific and largely depends on the wind resources of the area being exploited. Electricity generation from wind energy requires a wind speed greater than 5 meters per second (m/s). For windpumps, lower wind speeds can be sufficient. However, most windpumps will not start below a wind speed of 3 m/s and will furl at about 12 to 15 m/s.
The output of any wind-powered machine is directly proportional to the square of the rotor diameter and to the cube of the wind speed passing through it. • A dense (multi-bladed) rotor extracts torque from the wind at low wind speeds and shaft rotations per minute (r.p.m.). Unfortunately, it has to supply its maximum torque to lift the water and pump rod weight the very first time it rotates. • A two or three bladed rotor as used in wind generators, extracts torque from the wind in proportion to the wind speed. It does this by either very fast r.p.m. as in the case of small DC wind generators, or a very large rotor diameter as in the case of the large AC wind turbines. Although the shaft rotations in large machines may be fairly low due to the very large diameter, the blade tips can be passing through the sound barrier, which has repercussions for the actual design and noise pollution. • The speed of the wind across any surface is influenced by the ‘’friction’’ it encounters, which is obviously much greater where there are trees or tall buildings near the wind pump. This friction is gradually reduced when the rotor is high above the source of that friction.