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

Maas, Oliver; Raasch, Siegfried

doi:10.5194/wes-7-715-2022

Cited by 38 publications

(59 citation statements)

References 37 publications

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“…The flow through and around wind farms of this scale can be significantly different than the flow through and around smaller wind farms on the sub-gigawatt scale, as recently published results show (Maas and Raasch, 2022). For example, large wind farms can cause significant wake deflection and inversion layer displacement.…”

Section: Introductionmentioning

confidence: 92%

“…The aim of this study is to provide this direct, systematic comparison by performing LESs of a small wind farm (0.96 GW) and a large wind farm (11.52 GW) in a convective boundary layer, which is the most common boundary layer type in the North Sea (Maas and Raasch, 2022). The comparison focuses on the boundary layer flow inside the wind farm, but also in the far wake and the overlying free atmosphere.…”

Section: Introductionmentioning

confidence: 99%

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From gigawatt to multi-gigawatt wind farms: wake effects, energy budgets and inertial gravity waves investigated by large-eddy simulations

Maas

2022

Preprint

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Abstract. The size of newly installed offshore wind farms increases rapidly. Planned offshore wind farm clusters have a rated capacity of several gigawatt and a length of up to one hundred kilometers. The flow through and around wind farms of this scale can be significantly different than the flow through and around smaller wind farms on the sub-gigawatt scale. A good understanding of the involved flow physics is vital for accurately predicting the wind farm power output as well as predicting the meteorological conditions in the wind farm wake. To date there is no study that directly compares small wind farms (sub-gigawatt) with large wind farms (super-gigawatt) in terms of flow effects or power output. The aim of this study is to fill this gap by providing this direct comparison by performing large-eddy simulations of a small wind farm (0.96 GW, 13 km length) and a large wind farm (11.52 GW, 90 km length) in a convective boundary layer, which is the most common boundary layer type in the North Sea. The results show that there are significant differences in the flow field and the energy budgets of the small and large wind farm. The large wind farm triggers an inertial wave with a wind direction amplitude of approximately 10° and a wind speed amplitude of more than 1 ms-1. In a certain region in the far wake of a large wind farm the wind speed is greater than far upstream of the wind farm, which can be beneficial for a downstream located wind farm. The inertial wave also exists for the small wind farm, but the amplitudes are approximately 4 times weaker and thus may be hardly observable in real wind farm flows, that are more heterogeneous. Regarding turbulence intensity, the wake of the large wind farm has the same length than the wake of the small wind farm and is only a few kilometers long. Both wind farms trigger inertial gravity waves in the free atmosphere, whereas the amplitude is approximately twice as large for the large wind farm. The inertial gravity waves induce streamwise pressure gradients inside the boundary layer, affecting the energy budgets of the wind farms. The most dominant energy source of the small wind farm is the horizontal advection of kinetic energy, but for the large wind farm the vertical turbulent flux of kinetic energy is 5 times greater than the horizontal advection of kinetic energy. The energy input by the gravity wave induced pressure gradient is greater for the small wind farm, because the pressure gradient is greater. For the large wind farm, the energy input by the geostrophic forcing (synoptic-scale pressure gradient) is significantly enhanced by the wind direction change that is related to the inertial oscillation. For both wind farms approximately 75 % of the total available energy is extracted by the wind turbines and 25 % is dissipated.

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Section: Introductionmentioning

confidence: 92%

Section: Introductionmentioning

confidence: 99%

From gigawatt to multi-gigawatt wind farms: wake effects, energy budgets and inertial gravity waves investigated by large-eddy simulations

Maas

2022

Preprint

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“…Wind-farm induced gravity waves absorb energy within the ABL, transporting it at higher altitudes (Smith, 2010(Smith, , 2022Wu and Porté-Agel, 2017;Allaerts and Meyers, 2017Lanzilao and Meyers, 2021;Devesse et al, 2022;Maas and Raasch, 2022). The energy is then released when the waves break down (Nappo, 2002;Sutherland, 2010).…”

Section: Introductionmentioning

confidence: 99%

“…To overcome this issue, Klemp and Lilly (1977) introduced a Rayleigh damping layer (RDL) at the top of the domain to damp out gravity waves before they would have reached the upper boundary. Since then, this technique has been used extensively in mountain-wave simulations (Klemp and Lilly, 1977;Durran and Klemp, 1983;Teixeira, 2014), mesoscale models (Klemp et al, 2018;Powers et al, 2017) and LES of large wind farms (Allaerts and Meyers, 2017;Wu and Porté-Agel, 2017;Allaerts and Meyers, 2018;Gadde and Stevens, 2021;Maas and Raasch, 2022). The efficiency of this sponge layer increases with its vertical dimension.…”

Section: Introductionmentioning

confidence: 99%

An improved fringe-region technique for the representation of gravity waves in large-eddy simulation with application to wind farms

Lanzilao¹,

Meyers²

2022

Preprint

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Large-eddy simulations of the atmospheric boundary layer are often performed using pseudo-spectral methods, which adopt a fringe-region approach to introduce inflow boundary conditions. However, we notice that a standard fringe-region technique excites spurious gravity waves when stratified atmospheres are considered, therefore enhancing the amount of energy reflected from the top of the domain and perturbing the velocity and pressure fields downstream. In this work, we develop a new fringe-region method that imposes the inflow conditions without altering the flow downstream. This is achieved by locally damping the convective term in the vertical momentum equation. We first apply the standard and wave-free fringe-region techniques to two-dimensional inviscid-flow simulations subjected to 169 different atmospheric states. A similar study is performed on a three-dimensional domain using a couple of atmospheric states. In all cases, the new fringe-region technique imposes the inflow conditions without altering the surrounding flow. Moreover, we also investigate the performance of two non-reflective upper boundary conditions, that is a Rayleigh damping layer (RDL) and a radiation condition (RC). Results highlight the importance of carefully tuning the RDL to limit the distortion of the numerical solution. Also, we find that the tuned RDL outperforms the RC in all cases. Finally, the tuned RDL together with the wave-free fringe-region method are applied to an LES of a wind farm operating in a conventionally neutral boundary layer, for which we measure a reflectivity of only 0.7%.

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“…Large eddy simulations [7] and mesoscale simulations with e.g. the Weather Research and Forecasting model [8], while covering large areas offshore, are computationally expensive.…”

Section: Introductionmentioning

confidence: 99%

Comparison of near wind farm wake measurements from scanning lidar with engineering models

Anantharaman

Centurelli

Schneemann

et al. 2022

J. Phys.: Conf. Ser.

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The presence of offshore wind farms causes downstream regions of reduced wind velocity, i.e. wind farm (cluster) wakes, which can affect the power of wind farms downstream. Engineering models are now being used to simulate the effects of these wakes, and an important requirement for model validation is a comparison with full-field measurements. Our objective in this paper is to parametrize and validate two engineering wake models with long-range lidar measurements. We use a long-range scanning Doppler lidar to scan the near wake region of a 400 MW offshore wind farm and compare the wind velocities in the wake to the outputs of two engineering models: FarmFlow and flappy. We adapt FarmFlow to solve the flow in highly unstable atmospheres by modifying the boundary conditions, which enables the comparison of velocity profiles behind the farm. The models perform qualitatively well in predicting the wake deficit and shape close to the farm and at lower heights. They predict higher wake losses within the farm when compared to production power data in a strongly unstable atmospheric case. However, the current analysis is limited due to the lack of inflow measurements for model initialization, compounded by limited data availability. We discuss the possibilities and limitations of long-range scanning lidar data for cluster wake model validation and the need for inflow measurements for model initialization. We conclude that with detailed inflow measurements, scanning long-range lidars could serve as a good tool for the validation of wind farm wake models.

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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

Cited by 38 publications

References 37 publications

From gigawatt to multi-gigawatt wind farms: wake effects, energy budgets and inertial gravity waves investigated by large-eddy simulations

From gigawatt to multi-gigawatt wind farms: wake effects, energy budgets and inertial gravity waves investigated by large-eddy simulations

An improved fringe-region technique for the representation of gravity waves in large-eddy simulation with application to wind farms

Comparison of near wind farm wake measurements from scanning lidar with engineering models

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