Survey of Energy Resources 2007
Wind Energy - Energy Production
Contrary to popular opinion, energy yields do not increase with the cube of the wind speed, mainly because energy is discarded once the rated wind speed is reached. It does not make economic sense to build turbines with very high ratings that will only be reached on rare occasions. To illustrate the key parameters and the concept of rated output, a typical power curve for a 1.65 MW machine, 63 m in diameter, is shown in Fig. 12-4. Most machines start to generate at a similar speed - around 3 to 5 m/s - and shut down in very high winds, generally around 20 to 25 m/s.
Annual energy production from the turbine whose performance is charted in Fig. 12-4 is around 1 500 MWh at a site where the wind speed at 60 m height is 5 m/s, 3 700 MWh at 7 m/s and 4 800 MWh at 8 m/s. Wind speeds around 5 m/s can typically be found away from the coastal zones in all five continents, but developers generally aim to find higher wind speeds. Levels of around 7 m/s are to be found in many coastal regions and over much of Denmark; higher levels are to be found on many of the Greek Islands, in the Californian passes - the scene of many early wind developments - and on upland and coastal sites in the Caribbean, Ireland, Sweden, the United Kingdom, Spain, New Zealand and Antarctica. Offshore wind speeds are generally higher than those onshore - around 8 m/s in European coastal waters, for example.