Operational-Based Annual Energy Production Uncertainty: Are its Components Actually Uncorrelated?

TL;DR

This study evaluates the assumption that components of annual energy production uncertainty in wind plants are uncorrelated, finding that considering correlations slightly increases uncertainty estimates and can impact financial assessments.

Contribution

The paper provides the first large-scale empirical assessment of correlations between AEP uncertainty components using data from over 470 wind plants.

Findings

Ignoring correlations underestimates AEP uncertainty by up to 0.5%.

Positive and negative correlations between specific uncertainty components were identified.

Considering correlations slightly increases the total uncertainty estimate, affecting financial decisions.

Abstract

Calculations of annual energy production (AEP) from a wind power plant-whether based on preconstruction or operational data-are critical for wind plant financial transactions. A popular industry practice is to assume that different uncertainty components within an AEP calculation are uncorrelated and can therefore be combined as the sum of their squares. We assess the practical validity of this assumption for operational-based uncertainty, by performing operational AEP estimates for more than 470 wind plants in the United States, mostly in simple terrain. We apply a Monte Carlo approach to quantify uncertainty in five categories: revenue meter data, wind speed data, regression relationship, long-term correction, and future interannual variability. We identify correlations between categories by comparing the results across all 470 wind plants. We observe a positive correlation between interannual variability and the linearized long-term correction; a negative correlation between wind resource interannual variability and linear regression; and a positive correlation between reference wind speed uncertainty and linear regression. Then, we contrast total operational AEP uncertainty values calculated by omitting and considering the correlations between the uncertainty components. We quantify that ignoring these correlations leads to an underestimation of total AEP uncertainty of, on average, 0.1%, and as large as 0.5% for specific sites. Although these are not large increases, these would still impact wind plant financing rates; further, we expect these values to increase for wind plants in complex terrain. Based on these results, we conclude that correlations between the identified uncertainty components should be considered when computing the total AEP uncertainty.