TURBINE OPERATION/ENERGY OUTPUT
The power curve of a wind turbine
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 power curve 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.
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, as sawn in page Energy of wind. 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.
- Mechanical power curves: pitch control-stall control
- Electrical power curves: differences electr.-mech. curves