Optimizing Wind Energy

Investigation of atmospheric turbulence using lidars.

In order to counter the challenges of climate change, it has become critical that renewable energy sources such as wind energy satisfy the energy demands of the society. There is thus an everincreasing need to optimize wind energy production such that it becomes at least as affordable as the traditional energy sources. Wind turbines experience enormous fluctuating forces due to atmospheric turbulence that influence design, operation, maintenance and power production. In order to optimize wind turbine control in real time, novel approaches using remote-sensing instruments such as nacelle-mounted lidars are being contemplated to measure the wind field in front of the wind turbine. 

Currently, lidars are capable of measuring only the mean wind field but not precisely the turbulence. Also, a nacelle-mounted lidar cannot measure exactly at the rotor plane but quite some distance in front of the wind turbine. Using ‘Taylor's hypothesis’ a wind speed time series some distance in front of the wind turbine will be interpreted as that at the plane of the rotor blades. The validity of ‘Taylor's hypothesis’ is dependent on the turbulence level, separation distance between the two points and atmospheric stability. Wind turbines operate under all conditions, and the validity of this hypothesis is not yet known for wind energy. In this individual postdoc project, at first new generic methods for measuring atmospheric turbulence using lidars will be developed such that they can either be used as a ground-based or nacelle-mounted remote-sensing instrument. Subsequently, investigations on the validity of ‘Taylor's hypothesis’will be carried out, and necessary corrections will be applied that address the practical challenges of wind turbine control in real time. 

Project objectives
  • To develop new methods that make turbulence measurements using wind lidars possible
  • To investigate the validity of ‘Taylor's hypothesis’ for practical wind turbine applications using lidars.           

In recent years the interest in using remote-sensing instruments like lidars has increased significantly and there have been several measurement campaigns for wind energy purposes. These lidars perform conical scanning and use the velocity azimuth display (VAD) technique to deduce the components of the velocity field. Reliable measurements are however restricted to only 10-min mean wind speeds [Courtney et al., 2008], which is the current state-of-the-art concerning lidar measurements. Novel ideas are now being contemplated to measure the wind field by placing lidars at the top of a wind turbine tower, i.e. mounted on a nacelle. Such lidars will measure the wind speed in front of the wind turbine, where advance information from the measurements can be fed to the controller and preventive action can be taken accordingly, either to minimize the loads or maximize the power. A prerequisite for this approach is that lidars are able to measure turbulence. Sathe and Mann [2011], Sathe et al. [2011] concluded that with the current measurement configuration these lidars cannot be used to measure turbulence precisely. Thus new methods are required that make turbulence measurements using lidars possible. 

Inherent to nacelle-mounted lidar measurement is that they cannot measure exactly at the plane of the rotor blades but some distance away from the wind turbine. This brings into focus the validity of ‘Taylor's hypothesis’ [Taylor, 1938], which provides a link between the spatial and temporal turbulence field, i.e. instead of measuring the turbulence at different points in space, we can measure at only one point over a certain period of time. Some of the fundamental studies on the validity of ‘Taylor's hypothesis’ and deriving corrections for the same have been carried out by Lin [1953], Lumley [1965], Wyngaard and Clifford [1977]. Hill [1996] extended the analysis of Lumley [1965], Wyngaard and Clifford [1977], and derived state-of-the-art corrections for any turbulent statistic of any order. Investigating certain classes of turbulence flows for the validity of this hypothesis is still ongoing [Burghelea et al., 2005, He et al., 2010].

Until now there have been very limited studies on the validity of ‘Taylor's hypothesis’ for practical wind turbine applications. The state-of-the-art for quantifying turbulence in the wind turbine design standards [IEC, 2005] is the Mann [1994] turbulence model, which assumes that ‘Taylor's hypothesis’ is valid under all atmospheric conditions. Using lidars for turbulence measurements and particularly for wind turbine control will add a completely new dimension to wind turbine operation, and significantly help in optimizing the energy production. 

This individual Postdoc project is funded by Det Frie Forskningsråd - Teknologi og Produktion for a period between April 1, 2012 – March 31, 2015