ZephIR® laser anemometer

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THE PRINCIPLES OF LIDAR


“An eye-safe infrared beam illuminates natural aerosols in the atmosphere
(such as dust, pollen and water droplets) and a small fraction of the light is
back scattered into a receiver. Motion of the target particles along the beam direction leads to a
change in the light’s frequency through Doppler shift which is then accurately measured.
A conical scan pattern is used to move the
beam and intercept the wind at different angles,
building up a series of measurements around a
disc of air from which the wind speed vector is
obtained.
ZephIR obtains each measurement in just 20
milliseconds and one second of data can be
used to derive the horizontal and vertical wind speed components and wind direction. This can
then be repeated from 10 metres up to a height of 200 metres, at five or more user-defined
heights by focussing the transmitted beam.”
Michael is invited to present lidar papers and tuition sessions within a wide range of organisations
across Europe and North America. He has conducted fundamental optics research at JILA
(Boulder, Colorado) and the University of Essex.


NATURAL POWER’S METHODOLOGY IN COMPLEX TERRAIN


the state of the art in the applica tion of remote sensing and CFD
Complex wind farm sites present various challenges which
can often limit the applicability of conventional wind resource
and energy yield measurement assessment methods, due to
the complexity of the flow physics at play and the spatial and
temporal variations in the flow conditions.
The effects of this are varied; the typically steep slopes
encountered on complex terrain sites often violate the
assumptions on which linearised flow models are based,
notably the assumption that topography is a perturbation
to the horizontal flow, rather than a true 3-D effect. An
off-horizontal flow can result in measurement errors with
anemometry. Additionally, forestry cannot be accurately
modelled using simple methods for a number of reasons.
Regions with significant veer can complicate wake loss
and wind prediction calculations. Finally, strong wind speed
gradients make the energy yield very sensitive to the precise
location of the measurement systems used.
Measuring and predicting wind flows in complex terrain
requires a methodology that addresses the above challenges.
Natural Power has developed a best-practice methodology
which combines several leading technologies to reduce
and quantify wind resource and energy yield prediction
uncertainties.
The Natural Power methodology uses a combination of highdensity
measurement campaigns alongside an advanced CFD
flow model (VENTOS®). VENTOS® has been specifically and
intensively validated for application on complex terrain and
forested sites.
A map is produced showing the area of highest flow
complexity in terms of turbulence, shear, inflow angle, veer
and deviation from conventional flow modelling results. This
identifies areas where flow modelling uncertainty is highest
and informs where further measurement campaigns are
best performed in order to deliver an optimised reduction in
prediction uncertainty. Measurements are then performed at
as many locations as deemed necessary, using a combination
of conventional anemometry and ZephIR (a lidar wind
profiler) as required to obtain an optimum balance between
measurement density and increased certainty in the results.


MEASURING TURBULENCE

ZephIR calculates the turbulence intensity (TI) that a
conventional cup would have obtained at the same
measurement height by analysing the variation in individual
wind speed values during a 10-minute averaging period.
This value of TI is automatically logged in the output data.
The calculation takes into account the difference between
point measurements obtained from a cup anemometer, and
spatially-averaged lidar data where a volume is interrogated.
ZephIR’s measurements of turbulence have been investigated
in a number of independent studies against calibrated met
masts in flat, offshore and complex terrain, and at different
heights above ground*.