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Gravity Waves and Wind-Farm Efficiency in Neutral and Stable Conditions
Dries Allaerts1, Johan Meyers1
1Department of Mechanical Engineering, KU Leuven, Celestijnenlaan 300, Box 2421, 3001 Leuven, Belgium.
Boundary-Layer Meteorology
|January 31, 2020
Summary
Stable stratification significantly impacts wind farm energy extraction by enhancing gravity waves. These waves cause upstream flow deceleration, reducing turbine output and introducing non-local efficiency considerations.
Area of Science:
- Atmospheric Science
- Renewable Energy Engineering
- Fluid Dynamics
Background:
- Stable stratification influences atmospheric boundary layer (ABL) dynamics.
- Wind farms extract energy from the ABL, interacting with atmospheric flows.
Purpose of the Study:
- Investigate the impact of stable stratification on gravity wave excitation and energy extraction in large wind farms.
- Analyze the interplay between wind farm characteristics and gravity wave dynamics.
Main Methods:
- Employed large-eddy simulations (LES) to model ABL transition from neutral to stable conditions over 8 hours.
- Utilized two distinct cooling rates to simulate stratification development.
- Compared LES results with a simplified one-dimensional model.
Main Results:
- Turbulence decay initially affects energy extraction, but inertial oscillations dominate later, enhancing gravity wave excitation.
- Gravity waves induce upstream flow deceleration, reducing turbine power output.
- Introduced a non-local wind farm efficiency metric, comparable to standard wake efficiency.
Conclusions:
- Stable stratification significantly alters wind farm performance through gravity wave generation.
- Gravity waves create non-local effects impacting overall wind farm efficiency.
- Energy flux analysis confirms gravity waves' influence on wind farm flow dynamics.
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