**3. Principles of Wind Energy**

#### *3.1. Wind Energy as Kinetic Energy*

Wind energy is kinetic energy that is transformed from potential energy. Scottish physicist William Rankine stated in 1881 that, "the object is gaining the potential to move 'by the occurrence of such changes, actual energy disappears and is replaced by Potential or Latent Energy'" [23]. Taking the definition of 'work' as the force multiplied by the distance moved in the direction of the force, the amount of energy harnessed from wind is determined by the speed of the wind and volume of air moved. When air-flow passes a wind turbine at a given speed, a moving turbine constructs a hypothetical cylinder with the swept area as the length of the wind blade and the height as the speed of wind per second. The hypothetical cylinder captures air mass, which is kinetic energy, and is eventually transformed into electricity.
