P50 and P90: WEEK 2

When evaluating the Net Yield, each step of the applied methodology is subject to some uncertainties. The identification and quantification of uncertainties is crucial to determine the correct exceedance probabilities.

It is possible to distinguish two classes:

1.The variation in wind

2.The modelling errors

The first category refers to the variability of the wind resource over time. It is known that large variations could be observed from one year to another with no forecast possible. In this context, an uncertainty is calculated to account for inter-annual variation over the wind farm’s lifetime. The longer the project lifetime, the more likely it is to reach the P50 in cumulated production.

The second category refers to the model uncertainty; i.e. uncertainties related to each step of the applied methodology due to the use of mathematical and numerical models; site measurement, vertical profile, reference data, long-term correction, wind flow modelling, wake modelling, density, and power curve.

Conclusion 

As no one can predict the wind over the next 20 years and because the models and methods used for energy yield assessments are subject to errors, the consideration of an uncertainty is crucial when creating a business model. This is all the more relevant as the wind resource over the wind farm site area can be affected over time by evolution in the immediate vicinity: urbanisation, densification of neighboured wind turbines, tree growth etc.

Greensolver highly recommends to consider the P90 or P75 figures instead of the P50, depending on the quality of the energy yield assessment. To determine the quality of an energy yield assessment, a review performed by an independent expert is essential.

By Carla Vico, Technical Asset Manager