The annual wind speed at a location is useful as an initial indicator of the value of the wind resource.The relationships between annual mean wind speed and potential value of the wind energy resource are listed below.
Anual mean wind speed @ 10 m Height < 4.5 m/s 4.5 - 5.4 m/s 5.4 - 6.7 m/s > 6.7 m/s | Indicated value of wind resource Poor Marginal Good to Very Good Exceptional |
In locations where data are not available, a qualitative indication of a high annual mean wind speed can be inferred from geographical location, topographical features, wind- induced soil erosion, and deformation of vegetation. However, accurate determination of the mean annual wind speed requires anemometer data for at least 12 months. Earlier studies have revealed that the mean wind speed for a given year varies from year to year but within ±10% of the long-term mean. A ±10% variation in mean wind speed would show a variation in energy content of ±30% or more. Part of the kinetic energy of wind can be captured by a rotor and converted into mechanical or potential energy. The major drawback of a wind power system stems from the nature of the source - wind has a low power density, and wind speed and direction are highly variable. Therefore, wind power systems are best for applications which can tolerate varying power input, such as, pumping of water from wells, the traditional windmill application in many parts of the world. The power that can be achieved by a wind system is proportional to the cube of the wind speed and the square of the windmill diameter. That is,
P = nCd2v3 where, P = theoretical wind power (kW) d = windmill diameter (m) v = wind speed (m/sec) C = constant (5.104 kg/m3) n = windmill efficiency (20-40 per cent) |