Innovations that reduce the cost of offshore wind are crucial if the technology is to expand successfully. So a new kind of control system that uses Lidar (light detection and ranging) technology to turn rotor blades out of the path of a gust of wind before it hits could well help get more turbines turning out at sea. The device uses a laser to scan the entire rotor area in front of the turbine and then feeds the data into a control system, which moves the blades in such a way as to ensure that, even if wind conditions fluctuate, the rotational speed stays more or less constant. This minimises the harmful fatigue and extreme loads acting on the turbine, thereby keeping it more stable. System stability also improves because the amount of electricity fed into the grid doesn’t vary so much.
The early warning system was designed by the Institute of Aircraft Design (IFB) at the University of Stuttgart as part of the Research at Alpha Ventus (Rave) project, a collaborative undertaking that brings together researchers and turbine manufacturers. The idea is to use the Alpha Ventus wind farm in the North Sea to demonstrate, in a variety of ways, that Germany can generate offshore electricity both reliably and cost effectively. “We want to support large-scale use of the early warning technology over the next few years,” says David Schlipf, a researcher at IFB. Existing control systems don’t react fast enough to changes in wind speeds. They only intervene and change the position of the rotor blades once the gust has already hit the turbine, which means it still has to withstand heavy loads. The new feed-forward control acts faster and can thus reduce the additional strain caused by fluctuating wind speeds.
Lidar can also help make the systems that turn the nacelle into the wind much more accurate. Turbines always have to be optimally positioned to catch the wind and produce as much electricity as possible. Right now, anemometers and weather vanes provide the necessary data on wind speed and direction. But the devices are installed on the nacelle, behind the rotor. That means the wind measurements are distorted by the action of the blades, which cause turbulence and slow the wind down behind the rotor. Because Lidar devices scan the area in front of the turbine, they can collect much more accurate data for positioning the nacelle.
A small addition to the nacelle
The innovation could be very good for the offshore wind industry. As things stand, electricity generated at sea is still relatively expensive, with average costs lying somewhere between EUR 0.12 and EUR 0.16 per kilowatt-hour. Manufacturers would welcome a solution that allows them to build larger turbines, as these reduce costs and deliver higher yields. But larger turbines also need to be built of lighter materials to prevent them from becoming too heavy. Unfortunately, there is only so much weight you can shave off a turbine destined for the high seas, since heavy winds and powerful waves put the components under a great deal of strain. If, however, manufacturers had a way of reducing the damaging effects of maritime conditions, they would be able to install more powerful turbines out at sea.
The early warning solution is made up of a Lidar measurement system, a real-time data-processing system and a link to the control system. At the moment, the device is installed on the nacelle, but the idea is that it will eventually be built into the rotor hub. Schlipf explains that the IFB’s Lidar device uses an infrared laser beam and the Doppler effect to measure the speed of tiny airborne particles. The technology builds on the fact that, when a laser beam is emitted, airborne particles (tiny dust or water droplets, say) reflect some of the light back to where it came from. The frequency of the original wavelength changes on its return journey, with the degree of the shift depending on the speed of the particles doing the reflecting. By measuring the change in frequency, the system can determine how fast the particles are travelling and therefore how fast the wind is blowing.
Lidar technology has actually been helping the wind industry for a while now. Ground-based devices, for example, are useful in finding suitable locations for turbines. They can measure wind conditions at altitudes of up to 200 metres. To make the technology part of turbine control and have it measure the space in front of the nacelle, the IFB researchers developed a special scanner and combined it with a ground-based Lidar device. Inside the scanner is a free-moving mirror that can steer the laser beams into any position in front of the turbine. “That means they can take any path necessary to scan individual measuring points. The result is that we get a pretty accurate picture of an incoming wind field and can improve the way the turbine reacts to it,” says Schlipf.
This is an abridged version of the article – the complete text is available in issue 6/2013 of new energy.