Improvements in Wind Power Estimates from Numerical Weather Models using Rotor Equivalent Speed and the Full Power Equation

Wind energy is growing around the world and is set to continue into the foreseeable future. Reliable estimations of wind speeds and power are required to sustain the growth. In the present paper, a high quality wind speed resource assessment is created over the USA for the years of 2006–2008. The spatial resolution is 13km and the temporal resolution is one hour. The resource assessment is created from weather assimilation data. The wind speeds are computed for both the hub height, set at 90m above ground level, and the rotor equivalent speed (RES) that takes into account shear across the rotor diameter. The differences between these two speeds are discussed and it is shown that the RES produces a general decrease in the wind power production on wind turbines due to the wind shear in the atmosphere. It is shown that the RES also changes the time series characteristics of the wind energy content throughout the period studied. In addition, the present paper calculates the wind power created by the wind speeds over the contiguous USA in an accurate way by incorporating the wind capacity (Cp) curve, air density, and icing constraints. A new suitability metric is established via the coefficient of variation. The suitability metric should be utilized in assessing good quality wind sites because it incorporates both variability and energy content. It is shown that the variability in wind speeds extends over multi-year timescales even over large geographic regions suggesting higher spatial and temporal resolution datasets and resource assessments are needed over very long time periods to accurately capture the wind energy resource accurately. Finally, the present paper discusses the usefulness of the full power equation rather than the approximation that is predominantly used currently.