Applications of scanning Doppler Lidar for the wind energy industry

The development of wind energy has increased rapidly along with the size and capacity of wind turbines. These larger turbines require detailed wind resource measurements at higher and higher altitudes. Accurate wind speed, wind direction, and turbulence statistics are required for wind resource assessment and efficient wind farm operation. Tower measurements are limited in coverage and do not provide the three dimensional sampling of the upstream atmospheric processes required for operational decisions. Remote sensing measurements are the most attractive option for wind energy meteorology. However, the measurement volume must be sufficiently small to observe the hub height winds at large ranges upstream and to resolve the important spatial scales of the atmosphere for the wind energy industry. Forecasts of wind power in the 20-60 minute time frame require high quality wind profiles at distances of 20 to 30 km upstream, especially for accurate predictions of ramp events.

High resolution profiles of mean and turbulent statistics of the wind field upstream of a wind farm can be produced using a commercial scanning Doppler lidar such as the WindTracer from Lockheed Martin Coherent Technologies. Careful corrections for the spatial filtering of the wind field by the lidar pulse produce turbulence estimates equivalent to point sensors but with the added advantage of a larger effective sampling volume to increase the statistical accuracy of the estimates. For a well designed lidar system, this permits accurate estimates of the mean windspeed and the turbulent statistics over various subdomains and with sufficiently short observation times to monitor rapid changes in conditions. In addition, new processing algorithms can produce measurements of wind profiles at a large distance upstream that are attractive for short term forecasts of wind power. Results from the analysis of scanning Doppler lidar data collected at the National Renewable Energy Laboratory (NREL) will be presented to highlight the important atmospheric processes for wind power meteorology. The unresolved issues for future applications of this technology will be discussed.