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FEATURE LIDAR FOR WIND ENERGY


KEEPING TURBINES


Greg Blackman finds out that, when it comes to assessing offshore wind farm sites, lidar offers a cost-effective and flexible measurement technique


TURNING


Wind farm projects – the large ones, anyway – tend to be located out at sea where the wind is constant and predictable. The London Array, the world’s largest offshore wind farm, built 20km off the coasts of Kent and Essex, reached full capacity in April. The wind farm has 175 turbines generating 630MW electricity, enough to power nearly half a million homes a year, with the potential to increase this capacity to 870MW in the future.


Any proposed wind farm site, onshore or


offshore, will have to undergo a 12-month wind resource assessment, to get a picture of the variations in wind speed and direction over the year. Wind speed measurements have traditionally been made by erecting meteorological masts with anemometers, but installing these offshore is incredibly expensive – anything between €5million and €10 million, according to Bruce Douglas, general manager of Flidar, a company that’s developed a floating lidar wind measure system. ‘They’re [met masts] difficult to get permission to install, and it takes time to install them,’ he comments.


Douglas’ Flidar system is a floating wind


measurement device, ideal for making assessments of offshore site proposals. ‘We’ve worked with onshore lidars for many years and we’ve done offshore wind resource assessments using traditional met masts. We saw an opportunity to use the lidar technology offshore,’ he says. Lidar is a remote sensing technique that gives the range of an object. Lidar systems fire a laser at a certain height and angle and the light scattering back from aerosols carried


12 PHOTONICS FOR RENEWABLE ENERGY 2013 In a wind farm assessment, developers


by the wind is detected. The frequency of the received radiation is different from the emitted radiation, called the Doppler shift, which is used to calculate wind speed. In early June, Flidar was deployed in the Irish Sea as part of an offshore wind measurement campaign conducted by Dong Energy, the Danish utility company, which also has a 50 per cent stake in the London Array wind farm. Flidar will remain in position measuring wind data for 12 months. Flidar consists of a marine buoy equipped with lidar technology from Leosphere. ‘We thought we’d put it [lidar] on a floating platform, but you’ve got the issue of the motion of the sea,’ says Douglas. ‘We’ve combined a passive mechanical stabiliser on the buoy and a correction algorithm to correct for the motion. The results are equivalent to a fixed offshore met mast. We’ve managed to achieve the same level of accuracy [as a met mast], but with a much cheaper system.’ In general, lidar systems operate by scanning light out in a cone, where the device is at the apex. Measurements can then be made at different heights within that cone. Flidar measures continuously at 1Hz at 12 different configurable heights anywhere between 40 and 280m. Several of those heights will be focused around the hub height of the turbine, according to Douglas. ‘That also is a huge advantage [of Flidar] over met masts, which are limited to approximately 90m above sea level,’ he says. ‘Ninety metres is about hub height, but we can measure the entire span of the turbine blades and even above it.’


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