Abstract
This paper exposes the risk of generalization of wind conditions from a single met-mast measurement to be representative of the actual flow field in a wind farm situated in complex terrain. As a case study, Large-Eddy Simulation (LES) of the neutral Atmospheric Boundary Layer (ABL) flow for a mid-western Sweden wind farm is performed. The site-specific complex topography and the forest properties like the Plant Area Density and the tree heights are extracted from the Airborne Laser Scanning (ALS) 3D data, thus the forest is heterogeneous. To emphasize the impact of the local topography and surface roughness on the wind field, the wind turbines are not included in the numerical simulations. The predicted wind speeds using LES are compared to wind speed from the nacelle-mounted anemometers taken from the wind farm's turbine SCADA data, focusing on the wake-free turbines. A sufficient degree of match is observed, supporting the accuracy of the numerical simulations. The results show that inflow variables i.e., mean wind speed, shear exponent and turbulence intensity vary at each wind turbine location justifying the need for turbine-specific assessment of the wind resource in a wind farm located in forested complex terrain. The inter-turbine (between turbines in the wind farm) differences in wind resource is quantified in terms of the difference in turbine-specific structural and mechanical loads by running wind turbine mechanical simulations using the extracting the wind fields predicted by the LES. The results show that not only inter-turbine loads varying significantly in the wind farm, but the turbine loads also differ significantly if a homogeneous assumption is made for the forest. Most importantly, it was found that the homogeneous forest assumption predicted a higher turbulence intensity compared to a heterogeneous forest resulting.
Highlights
Wind power is known as one of the most environment-friendly sources of renewable and clean energy which has been a pioneering renewable technology in recent decades
The red dots indicate the location of eight wind turbines of the wind farm where all are placed in the highest elevation of the neighbourhood region
In Eq (1b), the Smagorinsky subgrid model [104] is used to model the small scales given by where dij, nsgs, sij, fv, k, d, D and yþ denote Kronecker delta, turbulent viscosity, strain rate tensor, van Driest damping function [105,106], von Karman constant, wall distance and filter-width computed by the local grid size, respectively
Summary
Wind power is known as one of the most environment-friendly sources of renewable and clean energy which has been a pioneering renewable technology in recent decades. The capability of advanced CFD of including topographical phenomena in ABL simulation flow provides a more realistic wind farm flow prediction It requires large computational resources for wind resource assessment over a complex terrain. Contrary to the horizontally homogeneous forest, in heterogeneous forest, LAD/PAD profile varies in both horizontal and vertical directions This inhomogeneity has a significant impact on the variability of wind field [93,94,96] and it is more pronounced for wind farms located in a complex terrain. The focus of the study is to employ a highfidelity CFD method - the so-called Large-Eddy Simulation (LES) to model the airflow inside and over complex terrains and around each wind turbine in a mid-western Swedish wind farm while the difference between the heterogeneous and homogeneous forest assumptions on the dynamic response of the wind turbines in a wind farm is investigated.
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