Reply on RC2

Oliver Maas

doi:10.5194/wes-2021-83-ac2

Abstract

Germany’s expansion target for offshore wind power capacity of 40 GW by the year 2040 can only be reached if large portions of the Exclusive Economic Zone in the German Bight are equipped with wind farms. Because these wind farm clusters will be much larger than existing wind farms, it is unknown how they affect the boundary layer flow and how much power they will produce. The objective of this large-eddy-simulation study is to investigate the wake properties and the power output of very large potential wind farms in the German Bight for different turbine spacings, stabilities and boundary layer heights. The results show that very large wind farms cause flow effects that small wind farms do not. These effects include, but are not limited to, inversion layer displacement, counterclockwise flow deflection inside the boundary layer and clockwise flow deflection above the boundary layer. Wakes of very large wind farms are longer for shallower boundary layers and smaller turbine spacings, reaching values of more than 100 km. The wake in terms of turbulence intensity is approximately 20 km long, where longer wakes occur for convective boundary layers and shorter wakes for stable boundary layers. Very large wind farms in a shallow, stable boundary layer can excite gravity waves in the overlying free atmosphere, resulting in significant flow blockage. The power output of very large wind farms is higher for thicker boundary layers, because thick boundary layers contain more kinetic energy than thin boundary layers. The power density of the energy input by the geostrophic pressure gradient limits the power output of very large wind farms. Because this power density is very low (approximately 2 W m−2), the installed power density of very large wind farms should be small to achieve a good wind farm efficiency.

Highlights

At present, the global installed wind power capacity from offshore wind farms is increasing rapidly
In the second section it is discussed how the power output of very large and small wind farms is affected by the variation of the turbine spacing and the meteorological conditions
The wake length is defined as the distance between the wind farm trailing edge and the point at which the wind speed recovers to 90 % of its initial value, i.e. 9 m s−1

Summary

Introduction

The global installed wind power capacity from offshore wind farms is increasing rapidly. According to the expansion targets of the current leading offshore wind markets (the United Kingdom, Germany and China), the offshore wind power capacity will be subject to significant growth over the decades. The otherwise undisturbed flow at offshore sites will be increasingly modified by wind farms, affecting the wind farm power output and the meteorological conditions in the wake. Because no wind farms of this size exist currently, new insights into the behaviour of the flow through wind farms and the resulting power output can only be provided by simulations. In recent years many large-eddy simulations of wind farm flows have been carried out.

Methods

Results

Conclusion