Date of Award

Spring 1-1-2015

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Atmospheric & Oceanic Sciences

First Advisor

Julie K. Lundquist

Second Advisor

John J. Cassano

Third Advisor

John J. Cassano

Fourth Advisor

Jeffrey Weiss

Fifth Advisor

Branko Kosovic

Abstract

There is growing awareness in the wind power industry that boundary-layer stability influences wind turbine performance in meaningful ways. Stability is inextricably tied to the diurnal ebb and flow of heat, momentum, and moisture that drives weather. Boundary-layer stability is closely linked to low-level wind speeds, wind shear, wind veer, and turbulence. It is these myriad consequences of stability which directly impact turbines, both modifying performance and contributing to structural fatigue.

I describe the influence of near-surface stability on the aggregate power output of a utility-scale wind farm in central North America. During convective conditions, the wind farm produced more power than during neutral conditions, while in stable conditions the farm underperformed. These results are statistically significant, despite the uncertainty involved in using nacelle anemometer measurements of wind speed. Next, I use lidar measurements from Iowa to categorize low-level jets and their impact on rotor-layer winds. Observed jets are similar to those studied in the Great Plains, though regional sloping terrain forcing is absent in Iowa. Rotor-layer wind speeds intensify during jet periods, but detrimental wind shear and veer also increase when jets occur. Simulations using the Weather Research and Forecasting (WRF) model with various input data and boundary-layer physics favorably reproduce jet features. I then utilize the same model to examine the impact of switching from maize to soybeans on rotor-layer winds during the peak of the growing season. The crop change was represented in the simulation by surface roughness. The switch produces a statistically significant increase in both wind speed and power output. Finally, I evaluate the performance of the wind farm parameterization (WFP) in WRF using high-resolution large eddy simulations (LES) from the same model. The wind speed and turbulence impacts estimated by the WFP compare favorably to LES flow for both convective and stable conditions.

The results from all four studies exhibited some dependence on stability. Stability is intrinsic to so many boundary-layer processes that it inevitably plays a large role in wind farm conditions. Future work will continue to elucidate that role, with particular emphasis on the wakes of both wind turbines and wind farms.

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