Why Nacelle Power Curves Can Be Misleading for Energy Improvement Assessments

Aug 26, 2020

In our last video, we laid out the basic requirements for a successful methodology to assess the energy improvement from wind turbine upgrades: cost-effectiveness, accuracy, and reproducibility. Now we’re going to dive into the details of some standard energy improvement validation methods to see how they hold up.

 

One of the most common ways of visualizing turbine power performance is the nacelle power curve, a plot of power production versus wind speed measured at the nacelle anemometer.

 

If you want to assess the energy improvement of a turbine upgrade, this might be the most obvious approach. You plot the average curves in the baseline and optimized states. If the optimized power production is higher for a given wind speed below rated power, you might assume that the turbine upgrade was a success.

 

Since nacelle wind speed and power data are almost always available, this method can be cost-effective and reproducible. But is it accurate?

 

Unfortunately, in this case, there is a false assumption that a nacelle power curve is only affected by turbine power performance. In reality, the nacelle power curve can be distorted by many other factors:

  1. Power production is affected by the characteristics of the incoming wind. We can begin to account for one such characteristic—air density—by correcting the wind speed based on temperature or humidity and using this corrected wind speed as the independent variable. However, performance is affected by other atmospheric conditions like turbulence and atmospheric stability. These conditions will most likely be different between our baseline and optimized periods and are not captured by the simple average of wind speed used for the nacelle power curve.
  2. Since the nacelle anemometer is typically placed behind the rotor, the measured flow is slower than in the free stream due to the momentum extracted by the turbine. An attempt is made to correct for this slowing of the flow by deriving a nacelle transfer function. However, this nacelle transfer function can’t be assumed to be valid after an upgrade that changes the turbine power performance, because the turbine thrust coefficient, which quantifies momentum extraction, can also change.
  3. The nacelle anemometer only measures the wind speed at a single point, whereas the rotor extracts power from the wind over its entire projected area. Any changes in wind speed or direction across the rotor as characterized by wind shear and wind veer cannot be captured at the nacelle anemometer, so if these wind conditions are different in the baseline and optimized periods, the nacelle power curves are not comparable.


These deficiencies make nacelle power curves misleading and inaccurate for assessing power or energy production changes from wind turbine upgrades. Keep an eye out for future videos where we’ll be diving into more energy improvement assessment methods to see how they compare.

 

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