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The Power of
the Wind:
Cube of Wind Speed
The wind speed is extremely important for the
amount of energy a wind turbine can convert to electricity: The energy
content of the wind varies with the cube (the third power) of the
average wind speed, e.g. if the wind speed is twice as high it contains
23 = 2 x 2 x 2 = eight times as much energy.
Now, why does the energy in the wind vary with the
third power of wind speed? Well, from everyday knowledge you may be
aware that if you double the speed of a car, it takes four
times as much energy to brake it down to a standstill. (Essentially this
is Newton's second law of motion).
 In the case of the wind turbine we use the
energy from braking the wind, and if we double the wind speed, we
get twice as many slices of wind moving through the rotor every
second, and each of those slices contains four times as much
energy, as we learned from the example of braking a car.
The graph shows that at a wind speed of 8 metres per
second we get a power
(amount of energy per second) of 314 Watts per square metre exposed to the
wind (the wind is coming from a direction perpendicular to the swept rotor
area).
At 16 m/s we get eight times as much power, i.e. 2509
W/m2. The table in the Reference
Manual section gives you the power per square metre exposed to the
wind for different wind speeds.
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