Wake properties and power output of very large wind farms for different meteorological conditions and turbine spacings: A large-eddy simulation case study for the German Bight

Abstract

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.

Impact of wind farm wakes on flow structures in and around downstream wind farms

“…Cañadillas et al. (2020) determine the wake length as the distance behind the last turbine row at which the velocity has recovered to of the inflow velocity, while Maas and Raasch (2022) select a value of . The variety of definitions stems from uncertainties in inflow velocities in field measurement observations and the limited area for which the wind farm wake is measured or simulated.…”

“…regions of velocity deficit and increased turbulence intensity, have been observed to persist far downstream of wind farms (Cañadillas et al., 2020; Schneemann, Rott, Dörenkämper, Steinfeld, & Kühn, 2020). The wind farm wake development depends on the prevailing atmospheric conditions, as well as the size and layout of the wind farm (Cañadillas et al., 2022; Maas & Raasch, 2022; Nygaard, 2014; Platis et al., 2020; Schneemann et al., 2020), such that our fundamental understanding of wind farm wakes is still limited and is not well captured in wind farm design and siting tools (Porté-Agel, Bastankhah, & Shamsoddin, 2020; Stieren & Stevens, 2021). This emphasizes the importance of studying the impact of wind farm wakes on downstream wind farms.…”

“…However, due to high computational cost, most LES have focused on individual wind turbines or wind farms, not the interaction between wind farms (Porté-Agel et al., 2020; Stevens & Meneveau, 2017). Recently, Maas and Raasch (2022) performed LES to study wind farm wakes in the German Bight. They observe that wind farm wakes are longer when the ABL is lower or when the interturbine spacing in the farm is smaller.…”

“…For this purpose, we perform LES and systematically increase the distance between two identical wind farms, each consisting of 72 wind turbines. Representative for existing and planned wind farm clusters in the North Sea, the distance between the wind farms is varied between and (Cañadillas et al., 2020; Maas & Raasch, 2022). We note that these distances also correspond to the optimal wind farm spacing of 4 to 14 km suggested by Frandsen et al.…”

Large-eddy simulations of the interaction between wind farms and mesoscale effects

“…The reason for this is the underlying assumption in such simulations that the wind turbines reside in the inner region of the ABL and that outer layer e ects are negligible. In later studies, outer layer e ects like thermal stability [24,122,140,[189][190][191][192][193][194] and Coriolis force [81,195,196] received more attention. These studies highlighted the complexity of the interaction between wind farms and the ABL.…”