Wind-Powered Electricity Generation
Wind turbines have fairly high power densities when the rate measures the flux of wind’s kinetic energy moving through the working surface (the area swept by blades) of this now so popular energy converter. In the windiest, mid-continental regions of America this power density is commonly above 400 W/m2 – but power density expressed as electricity generated per m2 of the area occupied by a large wind farm is a small fraction of that high rate. This is primarily due to necessarily generous spacing of wind turbines (no less than five and up to ten rotor diameters) that is required in order to minimize wake interference. As a result, even a wind farm composed of large 3 MW Vestas turbines with a rotor diameter of 112 m and spacing of six diameters apart will have peak power density of 6.6 W/m2 and even a relatively high average capacity factor of 30% would bring that down to only about 2 W/m2.
Actual power densities vary with average wind speeds and turbine sizes. Altamont, America’s pioneering large wind farm in California, rates only 0.6 W/m2, Puget Sound Energy’s Wild Horse (with a high capacity factor of 32%) has power density of 2 W/m2. The world’s largest offshore wind installation, London Array in the outer Thames estuary – designed to have a capacity of 1 GWp, annual generation of 31 TWh (354 MW) and an area of 245 km2 – will have power density of just 1.44 W/m2. A good approximation of expected power densities for large scale wind generation (year-round average, not the peak power) should not be thus higher than 2 W/m2. If 10% of the US electricity generated in 2009 (395 TWh or 45 GW) were to be produced by large wind farms their area would have to cover at least 22,500 km2, roughly the size of New Hampshire.