Swept area
The area of the circle traced by a turbine's rotating blades; wind turbine power output is directly proportional to it, and it grows with the square of rotor diameter.
Swept area is the single most important size parameter of a wind turbine, because the power available in the wind is proportional to it:
Doubling rotor diameter quadruples swept area and therefore energy capture at a given wind speed. This quadratic scaling explains the rapid growth of rotors:
- 1990s turbine, 40 m rotor: about 1,260 m²
- Typical onshore turbine of the mid-2020s, 150–170 m rotor: 17,700–22,700 m²
- Large offshore turbine, 220–260 m rotor: 38,000–53,000 m², more than seven football pitches
The ratio of nameplate power to swept area is called specific power (W/m²). Low-wind onshore designs sit at 200–300 W/m²; high-wind and offshore machines at 300–450 W/m². Lower specific power means a lower rated wind speed and higher capacity factor.
Blade mass grows faster than swept area (roughly with D to the power 2.3–2.6 for modern designs, versus a theoretical cube law), which is why carbon-fibre spar caps, segmented blades and transport logistics become limiting factors above 120 m blade length. The same concept applies to tidal stream turbines, where water's density of about 1,025 kg/m³ makes much smaller rotors viable.
Related terms
All terms →- Betz limit
The theoretical maximum fraction of kinetic energy an open rotor can extract from the wind: 16/27, or about 59.3%.
- Power curve
The relationship between hub-height wind speed and a turbine's electrical output, used with a wind speed distribution to estimate annual energy production.
- Rated wind speed
The lowest wind speed at which a turbine reaches its nameplate (rated) power output, typically 10 to 13 m/s for modern utility-scale turbines.
- Hub height
The height of a wind turbine's rotor centre above ground or sea level; taller hubs reach stronger, steadier winds and are a main driver of rising capacity factors.
- Wind power density
The mean power available in the wind per square metre of area perpendicular to the flow, in W/m², the standard measure of wind resource quality.