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Joseph K.W. Lam, Fluent Europe
A wind farm is a plot of land where a number of wind turbines operate
concurrently. The power delivered by the wind to a turbine is proportional
to the swept area of the rotor blades and the wind speed cubed. Wind turbines
start to generate electricity at wind speeds of about 10 mph, and reach
their maximum or rated power output at about 33 mph. Depending on the
location, a wind farm will produce electricity for about 80-85% of the
time, mostly at low wind speeds. The site of the farm, in particular the
topology of the land at and surrounding the farm, can play a significant
role in the efficiency of the collective energy output of the turbines.
A close-up of a typical wind map, showing the locations of the turbines
At Renewable Energy Systems in the UK, FLUENT has been used to predict
the wind speeds for an existing wind farm at Coal Clough, Lancashire.
There are 24 turbines at Coal Clough providing about 6,000 homes with
their electricity needs. The analysis was done to generate a “wind
map”, or high resolution contour map of wind speeds at a certain
height above the ground. The best wind maps take into account the variations
in the local terrain, including the topography of the land and the presence
of nearby structures. A substantial amount of measured wind speed data
was available, and was used for calibration of the CFD results. A well
calibrated wind map can provide wind speeds at every location of the wind
farm site. Accurate maps for the surface that slices through the turbine
hub centers are essential for planning purposes, especially because of
the strong dependence of wind speed on power.
For the analysis, a rectangular footprint of land was considered that
is oriented in the direction of the prevailing wind, with sufficient upstream
and downstream distance from the existing core turbine region. Over 160,000
points of terrain height data were used for a 20km wide strip of land,
with a resolution of 50m horizontally and 1m vertically. A mesh of one
million hexahedral cells was generated. The grid was progressively coarsened
in the vertical direction, with the first cell layer approximately 0.05m
off the ground and gradually increasing to 25m in height at the top boundary
of the domain.
The prevailing wind was found to have a height-dependent profile taken
from anemometer measurements at the site of the turbines. The measured
velocity profiles were applied at the upstream inlet to the domain through
the use of a user-defined function. Because the terrain is hilly near
the site of the turbines, the resulting CFD predictions for velocity at
the turbine site were greater than the measured values by about 50% in
the initial runs. By calibrating the inlet profiles using the measured
velocities at the turbine, the adjusted predictions at the turbines were
brought to within 10% of the measured values. By repeating this process,
using anemometer data from other nearby turbines and re-calibrating the
inlet profiles, the wind speed map was developed into an accurate tool
for predicting the flow field at all locations at the site. This project
will allow the company to explore further the potential of CFD, to improve
knowledge of wind conditions at existing and prospective sites.
Land topography used as a boundary for the simulations
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