A wind turbine power curve is a graph representing how much power a turbine can produce at different wind speeds.
This is useful when identifying possible sites for wind farms, or home wind power installations.
If you know the typical range of wind speeds at your site, and you have a wind power curve for the turbine you are considering, you should be able to get a reasonable estimate of its annual output and know whether this is likely to be sufficient for your needs.
To plot a power curve, you would normally use an anemometer sited close to the turbine in order to measure the wind speed at various times, and at the same time, measure the corresponding electrical power output of the wind turbine.
In reality, due to natural variance, this would give you a range of data points that would not exactly sit on the smooth curve shown on this graph.
Instead, the graph would be a line of best fit, taking an average of all the data points plotted in order to provide a reasonable estimation of the output at any given wind speed.
The figure below shows a sketch of how the power output from a wind turbine varies with steady wind speed
Example of a typical wind turbine power curve (for illustration only)
As I explained in the wind energy statistics roundup, the power output of a wind turbine depends on where it is located, as well as the physical characteristics of the turbine itself.
It is highly unlikely that the wind speed will be steady in any location, and therefore the output will vary in line with the speed at any one time. This is where a wind turbine power curve can help to estimate current and near-future output.
At very low wind speeds, there is insufficient torque exerted by the wind on the turbine blades to make them rotate (see how fast does a wind turbine spin).
However, as the speed increases, the wind turbine will begin to rotate and generate electrical power.
The speed at which the turbine first starts to rotate and generate power is called the cut-in speed and is typically between 3 and 4 meters per second.
As the wind speed rises above the cut-in speed, the level of electrical output power rises rapidly as shown. However, typically somewhere between 12 and 17 meters per second, the power output reaches the limit that the electrical generator is capable of.
This limit to the generator output is called the rated power output and the wind speed at which it is reached is called the rated output wind speed.
At higher wind speeds, the design of the turbine is arranged to limit the power to this maximum level and there is no further rise in the output power.
How this is done varies from design to design but typically with large turbines, it is done by adjusting the blade angles so as to keep the power at a constant level.
As the speed increases above the rate output wind speed, the forces on the turbine structure continue to rise and, at some point, there is a risk of damage to the rotor.
As a result, a braking system is employed to bring the rotor to a standstill. This is called the cut-out speed and is usually around 25 meters per second.
The available power in a stream of wind of the same cross-sectional area as the wind turbine can easily be shown to be:
If the wind speed U is in meters per second, the density ρ is in kilograms per cubic meter and the rotor diameter d is in meters then the available power is in watts.
The efficiency, μ, or, as it is more commonly called, the power coefficient, cp, of the wind turbine is simply defined as the actual power delivered divided by the available power.
There is a theoretical limit on the amount of power that can be extracted by a wind turbine from an airstream. It is called the Betz limit. The limit is:
Find out more about the Betz Limit and its implications here.