Wind farm. Wind turbines sihouetted against the glow of a sunset. Each turbine has three blades which rotate around a horizontal axis, turning a shaft which drives an electric generator. Wind turbines are tall and are placed on hills or by the sea so that they receive as much wind as possible. This is because the power which they generate increases with longer blades and higher wind speeds. Most machines are designed to operate in windspeeds between 21 and 60 kilometers per hour. The blades have to be turned to face directly into the wind to work efficiently. Photographed in California, USA.