Amount of Electricity from Wind Energy
Wind speeds are often measured in metres per second. Below is a rough conversion from speeds in kilometers per hour (km/h):
|
m/s |
4 |
6 |
8 |
10 |
12 |
14 |
16 |
|
km/h |
14.4 |
21.6 |
28.8 |
36.0 |
43.2 |
50.4 |
57.6 |
The amount of energy available in the wind is proportional to the cube of the wind speed (e.g. if the wind speed doubles, the amount of energy in the wind goes up by 2 x 2 x 2 = 8 times!). Typically, wind speeds greater than 3 m/s are needed before a wind energy system can begin to generate electricity; this is known as the “cut-in” speed. The “cut-out” speed, usually around 20 m/s, is where the turbine stops to protect itself from damage. The precise amount of energy that can be extracted from the wind is complicated and depends on such factors as the variability and distribution of wind speed, height of the rotor, diameter of the area swept by the rotor, and density of the air.
When energy is extracted from the wind, the wind’s local speed decreases. In theory, if you took all the energy out of the wind, the wind would stop completely! In reality, however, you cannot remove all the energy from the wind. The most energy that an ideal wind energy system can extract is approximately 59% -- known as the Betz limit.
To determine how much electricity can be produced by a turbine from the wind, you need to know the wind speed over time and the amount of electricity a turbine generates at different wind speeds. Wind speed is often expressed as a wind speed distribution curve, which describes the number of hours per year the wind blows at different speeds.
 A mathematical ‘Rayleigh distribution’ (see figure to the right) provides a good approximation of a given site's wind distribution. Each wind turbine model is tested by the manufacturer or a third party facility to measure the electricity output at different wind speeds, known as a turbine ‘power curve’. The combination of a site’s wind distribution curve and a particular turbine’s power curve yields the estimated electricity that the turbine could generate on that site.
Hills, ridges and valleys can block the wind or create undesirable turbulence for a wind energy system, so mounting a wind energy system on a hill and on a tower will increase the amount of wind energy available. Due to the ground’s friction, wind speed increases as you move higher. For most open spaces, wind speed increases 12% each time the height is doubled. A small increase in wind speed leads to a large increase in energy output as the energy available in the wind is proportional to the cube of the wind speed.
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