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FEATURE LIDAR FOR WIND ENERGY


The Flidar floating lidar system provides a much more cost-effective method for assessing offshore wind farm sites than installing met masts out at sea


➤ mean the turbine has to be shut down to reduce the load on its mechanics. The International Electrotechnical Commission (IEC) standard gives guidelines on how wind turbines should be designed according to different turbulence classes. ‘That’s important for loads on the turbine and also for the power curve, which is an extremely important value used in the wind energy industry because it is directly related to the annual energy yield from the turbine,’ explains Ameya Sathe, a postdoc researcher at the Technical University of Denmark. ‘The turbine should produce the energy it is designed for, i.e. it produces power at the wind speeds that it’s designed for. Power performance is dependent on turbulence.’ Turbulence is one of the parameters


Lidar measures turbulence as a weighted average of all the velocities at different points in space


where cup and sonic anemometers provide more accurate readings than a lidar system, largely because of the way lidar operates – anemometers measure at a point, whereas lidar measures a volume. Dr Sathe is trying to improve the lidar readings for turbulence. ‘Turbulence is a random process and encompasses a range of scales, [which makes it difficult to measure],’ he explains. ‘It varies according to the wind speed, height above the ground, and the time of the day – turbulence during the afternoon is very different to turbulence during the early morning, which is related to atmospheric stability. ‘Turbulence also differs depending on


where it is taking place, so turbulence over desert will be different to that over water, which in turn will be different to that in cities.’


Lidar measures turbulence as a weighted average of all the velocities at different points


14 PHOTONICS FOR RENEWABLE ENERGY 2013


in space, which is different from anemometers that record point measurements. ‘This is not what we are accustomed to use in the wind energy sector,’ says Dr Sathe. ‘We use point values of turbulence rather than a volume.’ This problem also depends on the type of lidar used, be it continuous wave or pulsed. The amount of averaging increases with height with a CW lidar, whereas it’s the opposite for a pulsed lidar, explains Dr Sathe. As part of his project, which runs until the end of March 2015, Dr Sathe is looking at different ways of processing the data in order to mitigate the volume averaging problem. One possible way to do this is to use turbulence models whereby a turbulence statistic as a function of weighted average is defined. The models can then be fitted to the data to deduce the true point value of interest. Dr Sathe feels there is a lot of scope to improve the various data analysis methods used in terms of how turbulence is calculated.


Blowing in the wind There are ideas for installing lidar systems on the nacelle of a turbine, the structure housing the turbine gears and generators behind the blades, to measure turbulence of the incoming wind field. Research into this area is ongoing, says Dr Sathe. The general idea is for lidar to look forward and measure the incoming wind field, with the data coupled to the controller of the turbine. This then would send signals to the turbine to either pitch the blades or reduce the speed, etc. The main goal would be to reduce the loads on the turbine. ‘This is another huge application, but it’s still a long way from being commercialised,’ Dr Sathe comments. In order to couple lidar to turbine, Dr Sathe adds: ‘The information from the lidar would have to be reliable to be fed into the controller of the turbine. Secondly, maintenance of the lidar, to ensure the data remains reliable while the turbine is running, remains questionable. You still have to make sense of the data and connect the system to the turbine controller. You have to take into consideration when the blades are passing in front of the lidar system, and how fast the blades turn depends on the wind speed.’ Work on this is still ongoing but could add to the efficiency of wind turbines. As it stands, though, the accuracy and measurement range of lidar technology is such that it can help significantly reduce the cost of the assessment phase of planned wind farms. l


FLiDAR NV


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