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Fusion spin-off for Renewable Energy
Whistle down the wind
A new anemometer based on a CO2 laser allows wind turbines to make optimal use of all types of weather conditions. This innovation is an unexpected spin-off from advanced plasma physics research under the nuclear-fusion programme.
The European Union is the world leader in wind energy with more installed capacity than any other region. Over 90% of medium and large turbines installed world-wide are now produced in Europe. And more than half of all turbines come from Denmark.
Wind-turbine technology is moving rapidly. Typical wind turbines weigh less than half as much as they did five years ago, average output has quadrupled, and noise emissions have been markedly reduced. The latest wind turbines use 1-MW to 1.5-MW generators and 50-m to 66-m diameter rotors.
However, there is continuous pressure from users for more flexible units to boost efficiency even further. Today's wind turbines demand a specific wind speed to operate at optimal efficiency. The more the wind deviates from that ideal speed, the lower the output. One way to overcome this is to vary the angle of the turbine blades and the speed of the rotor to increase efficiency in both light and strong winds.
Measuring wind speed at the right place
To achieve this, however, it is necessary to know continuously - and preferably in advance - the current wind speed and direction. Tests carried out at the Wind Energy and Atmospheric Physics Department of the Risø National Laboratory in Denmark showed that the best place to measure wind speed is at a point two to three times the diameter of the blades away from the turbine - 150m to 200m for modern units - and at the same height as the rotor. This avoids distortion feedback from the motion of the blades and provides sufficient time delay to alter the angle of the turbine blades.
Researchers at Risø showed that such a continuous measurement can be achieved using a laser anemometer based on the Doppler effect. The Department for Optics and Fluid Dynamics had already designed such a tool based on `light beating' - mixing a light beam with a reference beam to make it possible to measure very small Doppler changes. This advanced measurement technology had been developed as part of Risø's participation with the German Max-Planck-Institute for Plasma Physics in the thermonuclear fusion research programme. It was used to map turbulence in plasma and to determine the level of gaseous stability needed to produce fusion energy.
A cheap device for every weather
Adapting the CO2-laser anemometer to wind turbines was the brainchild of two of Risø's researchers: Lars Lading and Sten Frandsen. "Many of the properties of CO2 lasers are relevant to equipping windmills," explains Mr Lading. First and foremost, the long wavelength means that, compared with visible light anemometers, CO2 lasers are less affected by fog, rain and snow. In addition, as their wavelength matches the spatial distribution of the reflecting aerosols occurring abundantly in the atmosphere, the optical elements of the laser anemometer do not need to be particularly sophisticated.
"This is important because if you want to see all new windmills fitted with the instrument, it must not make the windmill significantly more expensive," underlines Mr Lading.
The original development of this project is a joint effort involving Risø with wind-turbine manufacturer NEG Micom and consultancy VEA Engineering. Partners produced a prototype of the CO2 laser anemometer that could be housed in a casing 10cm to 15cm in diameter, and 50 cm long. With these dimensions, the instrument does not seriously affect the wind turbine's aerodynamic properties or structural requirements yet is robust enough to withstand harsh conditions.
The two companies are also included in a patent application for the laser system. The Department of Optics and Fluid Dynamics helped fund the preliminary project; steps are now being taken to continue the project under the auspices of the EU.