Wind farm siting, layout and wake effect

A wind farm is a group of grid-connected wind turbines installed at one site to produce bulk electric power, sharing roads, substation and transmission line.

Site selection factors

1. Wind resource: annual mean speed above about 6 m/s at hub height from at least one year of measurement; wind rose, Weibull distribution and low turbulence.
2. Terrain: open land, ridges, hill tops or coast; avoid trees and buildings that cause turbulence and speed-down.
3. Grid access: nearness to a substation or line of enough capacity.
4. Access and ground: roads for heavy transport, firm soil for foundations.
5. Environment and society: noise (distance from houses), bird and bat routes, visual effect, shadow flicker, land cost and acceptance.
6. Extreme conditions: lightning, icing, high winds, earthquakes.

Layout

Turbines are placed in rows at right angles to the prevailing wind. Typical spacing is 3–5 rotor diameters (D) between turbines crosswise and 5–10 D along the wind.

 wind -->   o     o     o     o   (row 1)
 
              o     o     o     o (row 2, staggered)
         <--- 5-10 D --->

Wake effect

Air behind a rotor has lower speed and higher turbulence. A turbine inside this wake produces less power and suffers higher fatigue loads. Power loss from wake is commonly 5–15 % of the farm output.

Reduction: adequate spacing, staggering of rows, laying rows across the dominant wind direction, use of wake models (e.g. Jensen) to optimise the layout, taller towers and control strategies that yaw turbines slightly off the wind.