Advancements in Wind Energy Metrology – UPWIND 1A2.3

An overview of wind related metrology research made at Risø DTU over the period of the UPWIND project is given. A main part of the overview is devoted to development of the Lidar technology with several sub-chapters considering different topics of the research. Technical problems are not rare for this new technology, and testing against a traditional met mast have shown to be efficient for gaining confidence with the ground based Lidar technology and for trust in accuracy of measurements.

In principle, Lidar measurements could be traceable through the fundamental measurement principles, but at this stage of development it is not found feasible. Instead, traceability is secured through comparison with met masts that are traceable through wind tunnel calibrations of cup anemometers. The ground based Lidar measurement principle works almost acceptable in flat terrain.

In complex terrain and close to woods the measurement volume is disturbed because the flow is no longer horizontally homogeneous. These conditions require special attention and correction methods. Due to the large measurement volume, ground based Lidars perform a spatial averaging which has the effect of a low pass filter on turbulence measurements.

Theory and measurements seem to be in good agreement. Lidar measurements from a rotating spinner have been performed. The analysis show good perspectives for scanning the incoming wind, which may lead to better controlled wind turbines. Lidars have also been used to scan the wake of wind turbines. These measurements document the meandering wake pattern.

The second part of the overview considers power performance measurements. A new investigation on the influence of wind shear points to a revision of the definition of a power curve. A new measurement method has been developed which has a good chance of being implemented in the present revision of the IEC performance standard.

Also, a turbulence normalization method has been tested but not found efficient enough for inclusion in the IEC standard. In relation to the coming IEC standard on performance verification with the use of nacelle anemometry, IEC61400- 12-2-CD, nacelle anemometry has been studied, both with experiments and in theory.

An alternative to nacelle anemometry has been developed, the socalled spinner anemometer. This type of sensor measures yaw-error with high absolute accuracy, and avoids the draw-backs of nacelle anemometry because the spinner anemometer is positioned in front of the rotor. Advances in classic mast measurement technologies have also been made.

A mast flow distortion correction method has been developed to improve classical state of the art mast measurements. Finally, an optical method for measurements of turbine vibrations is considered.