WIND DESCRIPTION/ENERGY CONTENT

Energy of wind

Introduction
A wind turbine obtains its power input by converting the force of the wind into a torque (turning force) acting on the rotor blades. The amount of energy which the wind transfers to the rotor depends on the density of the air, the rotor area and the wind speed.
The cartoon shows how a cylindrical slice of air 1 m thick moves through the 1,500 m2 rotor of a typical 600 kW wind turbine. With a 43 m rotor diameter each cylinder actually weighs 1.9 tonnes, i.e. 1,500 times 1.25 kg.

Density of air
The kinetic energy of a moving body is proportional to its mass (or weight). The kinetic energy in the wind thus depends on the density of the air, i.e. its mass per unit of volume. In other words, the "heavier" the air, the more energy is received by the turbine. At normal atmospheric pressure and at 15° Celsius air weighs some 1.225 kg per cubic metre, but the density decreases slightly with increasing humidity. Also, the air is denser when it is cold than when it is warm. At high altitudes, in mountains, the air pressure is lower and the air is less dense.
In the next table the density of air at standard atmospheric pressure is given. Note that the density of dry air at standard atmospheric pressure at sea level at 15° C is used as a standard in the wind industry.

Temperature (° Celsius) Temperature (° Farenheit) Density of air (kgr/m3)
Max. water content (kgr/m3)
-25
-13
1.423
-20
-4
1.395
-15
5
1.368
-10
14
1.342
-5
23
1.317
0
32
1.292
0.005
5
41
1.269
0.007
10
50
1.247
0.009
15
59
1.225
0.013
20
68
1.204
0.017
25
77
1.184
0.023
30
86
1.165
0.030
35
95
1.146
0.039
40
104
1.127
0.051

Rotor area
A typical 600 kW wind turbine has a rotor diameter of 43-44 m, i.e. a rotor area of some 1,500 m2. The rotor area determines how much energy a wind turbine is able to harvest from the wind. Since the rotor area increases with the square of the rotor diameter, a turbine which is twice as large will receive 22 = 2 x 2 = four times as much energy.

Video frames
- Variation of density: values, equations