Power Curve of a Wind Turbine

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The power output of a wind turbine varies with wind speed and every wind turbine has a characteristic power performance curve. With such a curve it is possible to predict the energy production of a wind turbine without considering the technical details of its various components. Thus, the powercurve of a wind turbine is a graph that indicates how large the electrical power output will be for the turbine at different wind speeds. It illustrates three important characteristic velocities: 

- The cut-in speed: the wind speed at which the turbine starts to generate power. 

- The rated speed: the wind speed at which the wind turbine reaches rated turbine power. This is often, but not always, the maximum power. 

- The cut-out speed: the wind speed at which the wind turbine shuts down to keep loads and generator power from reaching damaging levels. Power curves are found by field measurements, where an anemometer is placed on a mast reasonably close to the wind turbine, but not on the turbine itself or too close to it, since the turbine rotor may create turbulence and make wind speed measurement unreliable. If the wind speed is not fluctuating too rapidly, then one may use the wind speed measurements from the anemometer and read the electrical power output from the wind turbine and plot the two values together in a graph like the one here. 

Power Curve

Cut-in speed

At very low wind speeds, there is insufficient torque exerted by the wind on the turbine blades to make them rotate. 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.

Rated output power and rate output wind speed
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 to keep the power at the constant level.

Cut-out speed
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 metres per second.

Uncertainty in measurement of power curves 

In reality, there will be a swarm of points spread around the blue line and not the neat curve in the graph. The reason is that in practice the wind speed always fluctuates and one cannot measure exactly the column of wind that passes through the rotor of the turbine.

It is not a workable solution just to place an anemometer in front of the turbine, since the turbine will also cast a "wind shadow" and brake the wind in front of itself. In practice, therefore, one has to take an average of the different measurements for each wind speed and plot the graph through these averages. Furthermore, it is difficult to make exact measurements of the wind speed itself. If one has a 3% error in wind speed measurement, then the energy in the wind may be 9% higher or lower, since the energy content varies with the third power of the wind speed. Consequently, there may be errors up to plus or minus 10% even in certified power curves.

Verifying power curves

Power curves are based on measurements in areas with low turbulence intensity and with the wind coming directly towards the front of the turbine. Local turbulence and complex terrain, e.g. turbines placed on a rugged slope, may mean that wind gusts hit the rotor from varying directions. It may therefore be difficult to reproduce the power curve exactly in any given location.

Pitfalls in using power curves

A power curve does not tell how much power a wind turbine will produce at a certain average wind speed. Remember, that the energy content of the wind varies very strongly with the wind speed. So, it matters a lot how that average came about, i.e. if winds vary a lot or if the wind blows at a relatively constant speed. Also, remember that most of the wind energy is available at wind speeds which are twice the most common wind speed at the site. Finally, it is necessary to account for the fact that the turbine may not be running at standard air pressure and temperature, and consequently it is necessary to make corrections for changes in the density of air.