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.
A power curve has four regions: zero output below cut-in (about 3 m/s); a steep, roughly cubic rise up to rated speed (10–13 m/s); a flat section at nameplate power; and shutdown or soft ramp-down at cut-out (about 25 m/s).
Manufacturers publish warranted curves measured to IEC 61400-12-1, which specifies a met mast 2–4 rotor diameters upwind, 10-minute averaging and binning in 0.5 m/s intervals, normalised to a reference air density (usually 1.225 kg/m³). At lower density, as at high altitude or in warm climates, the curve shifts to the right.
Annual energy production (AEP) combines the curve with the site's wind speed frequency distribution, normally a Weibull distribution:
The result is gross AEP. Net AEP subtracts wake losses (5–15%), availability (2–5%), electrical losses (1–3%), blade degradation, icing and curtailment; total losses of 15–20% are common in bankable assessments.
Field performance diverges from the warranted curve because of turbulence intensity, wind shear across the rotor, inflow angle and leading-edge erosion. Turbulence typically raises output near the knee of the curve and lowers it near rated speed.
Related terms
All terms →- Cut-in wind speed
The lowest wind speed, measured at hub height, at which a wind turbine starts producing net electrical power, typically 2.5 to 4 m/s for modern machines.
- 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.
- Cut-out wind speed
The wind speed at which a turbine shuts down or ramps down to protect itself from excessive loads, typically 25 m/s (10-minute mean) for utility-scale machines.
- Weibull distribution
A two-parameter probability distribution used to describe how often each wind speed occurs at a site, defined by a scale factor c and a shape factor k.
- Betz limit
The theoretical maximum fraction of kinetic energy an open rotor can extract from the wind: 16/27, or about 59.3%.
- Capacity factor
The ratio of the energy a power plant actually produces over a period to the energy it would produce running at full nameplate capacity for the whole period.