Full two-dimensional rotor plane inflow measurements by a spinner-integrated wind lidar

Introduction
Wind turbine load reduction and power performance optimization via advanced control strategies is an active area in the wind energy community. In particular, feed-forward control using upwind inflow measurements by lidar (light detection and ranging) remote sensing instruments has attracted an increasing interest during the last couple of years. So far, the reported inflow measurements have been along a few measurement directions or at most on a circle in front of the turbine, which is not optimal in a complex inflow such as in the wakes of other turbines. Here, however, we present novel full two-dimensional radial inflow measurements.
Approach
In order to achieve full two-dimensional radial inflow measurements, a special laser beam scanner has been developed at the DTU Wind Energy Department. It is based on two rotating prisms that each deviate the beam by 15°, resulting in a space filling scan pattern within a full opening angle of 60° on an upwind spherical surface. The scanner is similar to the short-range WindScanner developed at the same department. However, this implementation is only using one motor with a fixed gearing between the two prism axes in order to achieve a reliable implementation for turbine control applications.
Main body of abstract
During the summer of 2012, a proof-of-concept field campaign with the two-dimensional upwind scanning wind lidar mounted in the rotating spinner of an operating Vestas NM80 turbine (59 m hub height and 80 m rotor diameter) located at Tjæreborg Enge in western Denmark was conducted. The new two-dimensional scanning device was integrated on top of a modified ZephIR 300 continuous-wave coherent Doppler lidar (ControlZephIR) operating at a wavelength of about 1.565 µm. The lidar was modified to stream averaged Doppler spectra at a rate selectable up to about 500 measurements per second in order to ensure short enough transversal sampling volumes when the prisms are rotating at maximum speed. The maximum scanning speed corresponds to a one second completion time of a two-dimensional scan pattern covering an upwind spherical surface, in the rotating coordinate frame of the spinner, bounded by the perimeter of a cone with its apex in the spinner-mounted lidar and with a full opening angle of 60°. The actual absolute measurement positions were calculated from the instantaneous positions of the two wedge-shaped optical prisms and the instantaneous azimuth position of the spinner-mounted wind lidar measured by an integrated accelerometer. Turbine parameters as well as wind measurements at a nearby met mast were logged and root-bending moments in the blades were acquired by an optical fiber-based strain measurement system. In addition, a proof-of-concept trial with a blade mounted lidar was performed during the measurement campaign and is reported in a separate EWEA 2013 contribution.
Conclusion
The study presented here is the novel full two-dimensional continuation of the previous inflow measurements on a circle presented in the paper “A spinner-integrated wind lidar for enhanced wind turbine control” by T. Mikkelsen et al. in Wind Energy 2012. The new data set with two-dimensional upwind radial wind speeds opens up for interesting discussions about which properties in the measured inflow to extract and how they can be used in wind turbine control algorithms. In summary, this two-dimensional lidar-based turbine inflow measurement technique provides new data, interesting questions and prospects for advanced feed-forward control of turbines in complex inflows.