The Power Curve of a Wind Turbine
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.
Power Curve
The graph shows a power curve for a typical Danish 600 kW wind turbine.
Power curves are found by field measurements, where an anemometer is placed on a mast reasonably close to the wind turbine (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 to the left.
Uncertainty in Measurement of Power Curves
In reality, one will see 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 per cent error in wind speed measurement, then the energy in the wind may be 9 per cent higher or lower (remember that the energy content varies with the third power of the wind speed).
Consequently, there may be errors up to plus or minus 10 per cent 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 you how much power a wind turbine will produce at a certain average wind speed. You would not even be close, if you used that method!
Remember, that the energy content of the wind varies very strongly with the wind speed, as we saw in the section on the energy in the 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, you may remember from the example in the section on the power density function , that most of the wind energy is available at wind speeds which are twice the most common wind speed at the site.
Finally, we need to account for the fact that the turbine may not be running at standard air pressure and temperature, and consequently make corrections for changes in the density of air.
© Copyright 1997-2003 Danish Wind Industry Association
Updated 1 June 2003
http://www.windpower.org/en/tour/wres/pwr.htm
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