Experimental investigation of flow over a double ridge with several Doppler lidar systems

For the first time two triple-lidar instruments, each consisting of synchronized and carefully positioned scanning Doppler lidars, were used to map the wind flow over complex terrain. The purpose of the experiment was to create unprecedented dataset of both turbulence and mean wind flow measurements for validation of wind resource estimation, inflow modeling and wind turbine wakes models in complex terrain. The experiment took place in May and June of 2015 over two steep and parallel ridges in Portugal on a site called Perdigao. On one of the ridges a solitary 82 m rotor diameter and 78 m hub height wind turbine was located. The parallel ridges are oriented in the NW - SE direction, which is roughly orthogonal to the prevailing wind direction. The ridges are 1500 m apart and reach a height of almost 500 m, while the lowest part of the valley in between is 250 m. The slopes are covered with low bushes or eucalyptus stands. In order to describe the terrain accurately, a helicopter laser-scanned a 20 square km area surrounding the ridges with a point density of more than 40 points per square m. From these scans the terrain elevation as well as the canopy density profiles could be derived. Three of the six Doppler lidar scanners were pulsed systems having a range of typically 4-5 km. Two were positioned one and a half kilometer apart on the ridge without the turbine and both at an altitude of 470 m, while the third was placed on the turbine ridge at roughly the same height. The other three scanning Doppler lidars systems were based continuous wave lasers and had a range of 150 m and a spatial resolution of approximately 10 m. They were all placed close to the wind turbine. These systems will in the following be called the long- and short-range windscanner systems, respectively. The synchronization meant for the short-range system that three beams were investigating the same position at the same instant allowing for a derivation of the three-dimensional wind vector from the three line-of-site wind speed component measurements. The long-range system typically combined only two beams to derive the horizontal wind vector, while the third was used to scan the line of sight velocity in vertical planes. The landscape is shown below with colors indicating heights in meters, the blue dots the position of the long-range windscanners, the red dot and circle showing the position and size of the wind turbine, and the blue lines illustrating the beams in a "virtual tower" configuration measuring in the center of the valley. Preliminary analysis of the data shows that the speed up over the ridge, which is important for wind resource estimation, depends on atmospheric stability. Both the long range, pulsed systems and the shorter range systems were able to resolve the wind turbine wake. The vertical movement of the wake deficit also seems to depend on stability. The experiment is a precursor for a larger and longer lasting experiment to be performed in 2017, which will include more than 30 meteorological masts, other meteorological equipment and more Doppler lidars. A strong American participation is anticipated. The experiment is a part of the New European Wind Atlas EU project, and was also funded by FarmOpt and UniTTe, Danish projects concerning optimization of wind farms in complex terrain and modeling of wind flow to turbines.