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.
Wind power density (WPD) measures the kinetic energy flux through a vertical surface, before any turbine efficiency is applied:
The mean of the cubed speed, ⟨v³⟩, matters, not the cube of the mean speed. With a Rayleigh Weibull distribution (k = 2) the mean cube is about 1.91 times the cube of the mean. A site averaging 7 m/s at sea-level density therefore has WPD of about 0.5 × 1.225 × 343 × 1.91 ≈ 400 W/m², not 210 W/m².
Indicative resource classes at 100 m height:
- Below 200 W/m²: marginal for utility wind
- 300–500 W/m²: good onshore sites
- 500–800 W/m²: excellent, such as US Great Plains, Patagonia and Scottish coasts
- Above 800 W/m²: exceptional offshore and ridge sites, North Sea and Southern Ocean
Air density ρ scales WPD directly: it falls about 10% per 1,000 m of altitude and varies about 3–4% per 10°C, so a cold coastal site outperforms a hot plateau with identical wind speeds. Only up to the Betz limit of 59% can be captured, and real turbines convert roughly 40–47%. The Global Wind Atlas maps WPD worldwide at 10, 50, 100, 150 and 200 m heights; see also the site's wind potential map.
Related terms
All terms →- 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%.
- 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.
- Wind shear
The change in wind speed (and sometimes direction) with height above the surface, driven by surface roughness and atmospheric stability.
- 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.