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Picturing the Wind...
Date: 12 April 2011
A pioneering project now underway in Denmark aims to produce real-time 3-D images of wind movements up to 200m above ground level. As Tony Sacks from Drives and Controls reported, this technology, which could revolutionise the design and economics of wind turbines, relies on advanced motion engineering developed by Heason Technology.
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We are surrounded by air that is shifting dynamically in three
dimensions. We can feel these movements as breezes and wind, but it
is almost impossible to "see" them. But a project now underway in
Denmark may change this, by allowing us to view 3D air movements in
real time.
This information could be useful for architects, civil engineers
and, in particular, for designers and builders of wind turbines. At
present, when deciding where to site wind turbines, engineers have
to rely on a combination of guesswork, modelling and limited data
gathered by erecting costly monitoring towers - some of them more
than 100m high - which provide information only from the precise
location of the instruments that they carry. With some turbine
rotors now spanning diameters of 120m, a single point measurement
of windspeed is of limited use.
The Danish project, being run by Risų DTU - Denmark's National
Laboratory for Sustainable Energy, which is part of the Technical
University of Denmark - will do away with the need for towers, by
using transportable ground-based equipment to produce detailed 3D
pictures of air movements. This will allow turbines to be built at
locations that will maximise their outputs, as well as helping to
optimise the designs of the turbines themselves. The technology
could also take wind patterns into account when planning and
designing bridges, buildings and other large structures.
The technology, called WindScanner, is based on three laser beams
that are fired upwards and reflected by particles in the moving
air. The reflected beams' wavelength is shifted slightly by the
Doppler effect and from this the speed and direction of the wind
vectors can be deduced over a large volume. The Lidar (light
detection and ranging) technology is already well established in
fields such as biology, meteorology and the military. The
technology has also been used to measure the distance from the
earth to the moon, and to check vehicle speeds. Lidars were even
used at the 2008 Olympics to gauge wind speeds during the yachting
events.
The Danish researchers are basing their WindScanner system on the
ZephIR laser anemometer. They have already used the ZephIR in
another application in which they mounted the Lidar system in the
nacelle of a wind turbine and aimed its laser beam through a window
in the front of the nacelle to get a conical view of the incoming
wind. This information could be used to adjust the yaw (direction)
of the turbine and the pitch of its blades in real time to optimise
its performance to match the incoming wind.
Such techniques - which are now starting to be commercialised -
will also help to compensate for the effects of gusts, wind shear
and yaw errors, and could reduce stresses on turbines as well as
maximising their output. In future, such wind-sensing systems could
become standard on wind turbines and may even be used to control
the geometry of the blades.
The WindScanner system is far more complex than a single
nacelle-mounted Lidar and is based on three coordinated,
ground-based Lidars, whose beams are moved quickly and precisely to
scan a large volume of air (roughly the shape of a triangular
pyramid with 150m-long sides, and reaching up to 200m above ground
level). The way the Danish team is doing this is to direct the
laser beams via a pair of accurate beam-steering prisms (known as
Risley prisms), which are moved in small, precise steps to cover
the whole volume. The beams will be moved hundreds of times every
second, through a 120-degree x 120-degree field in two axes, to
cover the whole volume in less than a second.
The optics and mechanics for the WindScanner's two-axis steerable
scan
heads and focus axis have been designed by Risų DTU and built by
IPU of Lyngby in Denmark. For motion engineering expertise, the
Danish researchers turned to UK-based Heason Technology.
Heason is designing and manufacturing the WindScanner's two-axis
scanning and one-axis focus positioning systems and their
associated motion controls, as well as the interfacing needed to
synchronise the Lidars' wind speed data and the steering system
servo loop. The positioning system is based on a combination of
motion control technologies including: ceramic servomotors and
drives from Nanomotion; brushless servomotors and drives from
Kollmorgen; and a 32-axis motion control system with a fibre optic
interface and high-speed dual-port RAM, supplied by Delta
Tau.
Two axes are needed to steer the laser beams, while a third, linear
axis is used to focus them. An array of three Lidars will require
nine axes to be controlled in real time. A fourth axis is used to
compensate for vibrations. The scan head's two axes are rotated
using belts driven by two 800kW Kollmorgen servomotors.
The Risų DTU researchers are at present fine-tuning the first Lidar
device before deploying the three-device array - scheduled for the
summer of 2010. So far, the project has received about DKK18m
(£2.2m) in internal funding from Risų DTU and a further DKK25m
(£3m) from the Danish Government. The Danish researchers are also
proposing a pan-European WindScanner project with a budget of
€50-100m. This would bring together research organisations, wind
turbine manufacturers and others in a collaborative venture to
install WindScanners permanently at test sites across Europe, as
well as building several mobile systems to investigate sites with
complex terrains, forested areas and even offshore.
The prototype WindScanner now being tested in Denmark is based on a
Lidar that emits a continuous wave laser beam. But there is another
type of Lidar that uses laser beams broken into short pulses. This
pulsed Lidar technology will extend the sensing range of a
WindScanner to 500m or more, and will form the basis for a
subsequent project at Risų DTU, catering for future generations of
larger wind turbines.
The information from WindScanner systems could be used in many
areas where this type of detailed, real-time 3D wind data has not
been available before, including:
>> Improving the ability to predict wind turbine power
production, loads andstructural dynamics;
>> Characterising wind flows in hilly and forested terrains; and
>> Analysing wake interference in wind
farms.
Having a better understanding of wind flows could also help to
boost wind turbine efficiencies - an improvement of just 1 or 2%
could affect the economic case for erecting turbines at some sites.
The new knowledge could also be used to reduce blade weights and,
with this, the size and cost of turbine towers and their
foundations.
If the Danish researchers succeed in their aims, the WindScanner
technology could revolutionise our understanding of how the wind
works and how turbulence fields affect wind turbines, thus
rewriting the economics of wind power production.