Wind power systems in the stable nocturnal boundary layer
Walter, Kevin Robert
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Hourly-averaged tall-tower data from a 200m tower located near Lubbock, Texas are used to examine static atmospheric stability as a governor of speed and direction shear in the atmospheric boundary layer. Meteorological forcing mechanisms for such shears include the thermal wind, inertial oscillation, Ekman spiral, and others. The inertial oscillation is highlighted as an atmospheric mechanism capable of generating discernable diurnal variations in average speed shear data in regions void of low-level jet association. Theoretical aerodynamic treatment shows the case of direction shear to differ from the case of turbine operation in yawed flow, and has therefore not been studied in wind power systems. Numerical simulations of power production in steady non-turbulent flow fields containing speed and direction shear show instantaneous power gain as great as 0.5% and depletion as great as 6% relative to a no shear baseline. Coupled with joint-probability distributions of speed and direction shear measured at Lubbock, instantaneous losses as great as 3% and gains as great as 0.5% are expected, while the average power change relative to the zero shear case is estimated to be -0.5%. Over the 20 year lifetime of a 100 MW wind power plant this finding translates to a $2.1 million loss in project revenue. Observational evidence shows the correlation coefficient between the average diurnal variation in static stability and power law shear exponent is 1.00. The correlation coefficient between the average diurnal variation in static stability and direction shear magnitude is found to be 0.93. The influence of static stability on speed and direction shear is hypothesized to be globally applicable. Observations from a second data platform in northwest Indiana support the magnitudes of direction shear found at Lubbock, and further suggest that the presence of direction shear is a more general result. Large magnitudes of wind direction shear are found to occur concurrently with large magnitudes of power law shear exponent at hub-height wind speeds greater than 8 m/s, making them present in a critical operating region where wind turbine control transitions from speed control to power regulation.