The Energy of Flowing Air
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The Origin of Wind Energy
A windsock is used at airfields and roads to determine the approximate direction and strength of the wind.
The origin of wind energy can be found in the uneven heating of the Earth’s surface by solar radiation. Lower layers of air are heated by the surface and rise due to their lower density, allowing colder air masses to move in. Differences in atmospheric pressure develop, causing air to move from areas of higher pressure towards areas of lower pressure and creating wind.
Parameters Influencing the Wind Energy Utilisation
From an energy point of view, wind turbines provide an opportunity to capture the kinetic energy of flowing air. The most important factor for harnessing wind energy is wind speed. The kinetic energy of a moving air mass is proportional to the square of its speed, while the power available in the wind is directly proportional to air density, the area through which the air flows and the cube of wind speed. From this we can deduce that when wind speed doubles, the power available in the wind increases by a factor of eight. Wind turbines, however, reach their rated output at a certain wind speed, above which their power is controlled and no longer increases with the cube of wind speed. Suitable locations therefore require favourable and sufficiently consistent wind conditions.
Map of wind patterns. Mean wind speed at 10 m above ground level.
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A wind turbine can extract only part of the kinetic energy of the air flowing through its rotor. According to the Betz limit, the theoretical maximum fraction of wind power that an ideal turbine can extract is 59.3%. At this optimum condition, the wind speed far downstream of the turbine is reduced to one third of its original value. Real wind turbines achieve lower power coefficients due to aerodynamic and mechanical losses and other design limitations.
Another important parameter of wind turbine performance is the capacity factor, which is the ratio of the actual energy generated over a given period to the energy that would have been generated if the turbine had operated continuously at its rated power. The capacity factor depends strongly on wind conditions, turbine design and location.
Cup anemometer.
The power output of a wind engine can be determined using the following formula:
Pm = 0.125 × Cp × ρ × v3 × π × D2
where:
| Pm | useful power (W) |
| Cp | power coefficient (efficiency) |
| ρ | air density (kg/m3) |
| v | wind speed (m/s) |
| D | rotor diameter (m) |
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