Estimating atmospheric boundary layer turbulence in the marine environment using lidar systems with applications for offshore wind energy

Abstract

Estimating turbulence in the marine-atmospheric boundary layer is critical to many industrial, commercial and scientific fields, but of particular importance to the wind energy industry. Contributing to both the efficiency of energy extraction and the life-cycle cost of the turbine itself, turbulence in the atmospheric boundary layer is estimated within the wind energy industry as Turbulence Intensity (TI) and more recently by Turbulent Kinetic Energy (TKE). Traditional in-situ methods to measure turbulence are extremely difficult to deploy in the marine environment, resulting in a recent movement to and dependence on remote sensing methods. One type of remote sensing instrument, Doppler lidars, have shown to reliably estimate the wind speed and atmospheric turbulence while being cost effective and easily deployable, and hence are being increasingly utilized as a standard for wind energy assessments.

In this thesis, the ability of lidars to measure turbulence up to a height of 200 m above mean sea level in the marine-atmospheric boundary layer was tested using a 7-month data set spanning winter to early summer. Lidar-based TI and TKE were estimated by three methods using observations from a highly validated lidar system and compared under both convective and stable atmospheric stability conditions. Convective periods were found to have higher turbulence at all the heights compared to stable conditions, while mean wind speed and shear were higher during stable conditions. The study period was characterized by generally low turbulent conditions with high turbulence events occurring at timescales of a few days. Mean vertical profiles of TKE were non-uniformly distributed in height during low turbulent conditions. During highly turbulent events, TKE increased more strongly with height. The definition of TI--following the industry or meteorology conventions --

had no real effect on the results, and differences between cup or sonic anemometers and lidar TI values were small except at low wind speeds. All the three lidar-based TKE methods tested corresponded closely to independent estimates, and differences between the methods were small relative to the temporal variability of TKE observed at the offshore site.