Date: 27 September 2017
Venue: DONG Energy, Gentofte (5 km North of Copenhagen), Denmark
Workshop leader: Rozenn Wagner, DTU Wind Energy
Nacelle lidars have been shown to be very promising alternatives to mast mounted cup anemometers for wind turbine power performance verification. The technology has been tested and adopted for internal use by many stakeholders of the wind energy industry including turbine OEMs, wind farm developers and certification bodies over the last 5 years. Common understanding and standardisation for this technique are needed at this stage. The purpose of this workshop was to gather and discuss experience and to identify the barriers of using nacelle lidars for wind turbine power performance verification.
The workshop was organised around two sub-topics:
1. The calibration of nacelle lidars and related uncertainties
Best practices from four calibration institutes were presented. They all adopted the so-called White-box approach (calibrating the lidar geometry and the LOS velocities along each LOS). They were however applied with different set-ups (e.g. lidars placed on the ground with different inclination angles or placed on a platform several meters above the ground level). It was however pointed out that there is no consensus on the estimation of the measurement uncertainties. Nevertheless all have been using a cup anemometer as a reference wind speed measurement, which represents the largest contribution to the final lidar measurement uncertainty. An alternative approach, using a flywheel as reference was presented for short range cw lidars.
2. The wind turbine power curve measurement using nacelle lidars
Perspectives from different stakeholders were presented, including a turbine OEM, two wind farm developers (including floating wind turbines project), two research and development bodies and three nacelle lidar manufacturers. The main points that came out of the presentations and following discussion were:
- The requirement to measure at 2.5D upstream for the coming wind turbines with very large rotor diameters (especially offshore) is challenging the limitations of the measurement geometry of currently available nacelle lidars:
- the maximum measurement range is typically of 300-400m,
- even if the measurement is possible at long ranges, the question of the decorrelation due to time lag arises,
- the distance between the lidar Line-of-sights increases significantly, invalidating the assumption of horizontal homogeneity.
- An alternative approach has been tested: use measurements at shorter range and somehow correct for the induction effect with some analytical model of induction. The method is promising as good comparisons to mast measurements were obtained.
- Nacelle lidars would also be a good solution for power performance testing in complex terrain however several challenges need to be overcome:
- the flow inhomogeneity (in moderately complex terrain, the issue may be limited, results obtained so far are promising; the method based on short range measurements seems could be good solution for complex terrain as well but needs to be further tested with experimental data)
- the terrain topography around the turbine needs to be taken into account to estimate the wind at the right height.
- The installation of the lidar on the nacelle should be facilitated by some collaboration between the lidar manufacturer (to make smaller and lighter lidars) and the turbine OEMs (to include a dedicated place/bracket for the nacelle lidars in their turbine design).
The outcome of the workshop were presented to the kick-off meeting of PT61400-50-3 in charge of writing the new IEC standard regarding nacelle mounted lidars for wind measurements.
More details can be found the slides of the presentations and the Minutes of the Meeting available on this webpage.