Wind energy

Wind energy

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Wind energy is the kinetic energy
Kinetic energy
The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...

 of air in motion; see also wind power
Wind power


Total wind energy flowing through an imaginary area A during the time t is:

E = ½ m v2 = ½ (A * v * t * ρ) v 2

where v is the wind speed

; ρ is the air density

; (A * v * t) is the volume of air passing through A (which is considered perpendicular to the direction of the wind); (A * v * t * ρ) is therefore the mass m passing per unit time. Note that ( ½ ρ * v2) is the kinetic energy of the moving air per unit volume.

P = E / t = ½ A * ρ * v3

Wind power in an open air stream is thus proportional to the third power of the wind speed; the available power increases eightfold when the wind speed doubles. Wind turbines for grid electricity therefore need to be especially efficient at greater wind speeds.

History of usage by humans

The wind has been used for thousands of years as a source of energy. Sailors capture it in the sails of their ships, and weather has therefore had important historical consequences, for example, the Spanish Armada
Spanish Armada
This article refers to the Battle of Gravelines, for the modern navy of Spain, see Spanish NavyThe Spanish Armada was the Spanish fleet that sailed against England under the command of the Duke of Medina Sidonia in 1588, with the intention of overthrowing Elizabeth I of England to stop English...

 was defeated with the help of stormy weather around the British Isles
British Isles
The British Isles are a group of islands off the northwest coast of continental Europe that include the islands of Great Britain and Ireland and over six thousand smaller isles. There are two sovereign states located on the islands: the United Kingdom of Great Britain and Northern Ireland and...

. The Netherlands

 are famous for the use of windmill

s with four cloth sails. These were used for pumping water to drain polders forming agricultural land. In the UK
United Kingdom

, Norfolk

 borrowed this Dutch expertise to do the same, leaving distinctive features on the flat lowland landscape.

Theoretical power captured by a wind turbine

Total wind power could be captured only if the wind velocity is reduced to zero. In a realistic wind turbine this is impossible, as the captured air must also leave the turbine. A relation between the input and output wind velocity must be considered. Using the concept of stream tube, the maximal achievable extraction of wind power by a wind turbine is 59% of the total theoretical wind power (see: Betz' law
Betz' law
Betz's law is a theory about the maximum possible energy to be derived from a "hydraulic wind engine", or a wind turbine such as the Éolienne Bollée , the Eclipse Windmill , and the Aermotor...


Practical wind turbine power

Further insufficiencies, such as rotor blade friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and/or material elements sliding against each other. There are several types of friction:...

 and drag
Drag (physics)


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