Wind turbine power performance testing using a Lidar - A field study

Abstract

In recent years, the installed capacity of wind turbines has increased rapidly, thereby raising the takes for wind turbine power performance testing. The current standard for power performance testing, IEC 61400-12-1, prescribes to measure the wind speed at hub height using a meteorological mast. This study tested whether Lidar, a remote measurement technology, could improve power performance testing. Potential benefits include mobility of the measurement device and increased insight in wind speed, with measurements up to approximately 200 meter height.

A measurement campaign was held at a site with flat terrain by positioning a ZephIR 300 Lidar next to an IEC compliant meteorological mast to measure the power performance of two wind turbines. Wind speed measurements were compliant to Norsewind criteria with a linear regression slope of 0.989 indicating that the Lidar measured lower wind speeds than the meteorological mast. Three major conclusions were drawn. (I) The difference in wind speed measurement has a large effect on annual energy production estimation (2.5 % for a Rayleigh function with a mean annual wind speed of 7 m/s), but difference in power performance of two wind turbines was found by either using the meteorological mast or the Lidar. (II) The rotor equivalent wind speed approach reduced scatter in the power curve for the Lidar compared to hub height wind speed, but not compared to hub height wind speed measurement by the meteorological mast. (III) Inner- and inner+outer envelope guaranteed power curve conditions showed a difference in annual energy production of approximately 1 % for a Rayleigh function with a mean annual wind speed of 7 m/s.

The most important implications are the following. (I) It is advized to use a Lidar to compare the power performance of multiple wind turbines or to use it for comparing performance over time, but it is advised not compare power performance measured for one test with a Lidar and for another test using a meteorological mast. (II) For power performance guarantee negotiations Lidar measurements could be included as option for power performance measurement and with REWS calculations the inner envelop conditions could be enlarged, but a different guaranteed power curve than when measured using a meteorological mast might be needed. (III) For flat terrain with grassland and wind turbines of approximately 100 meter hub height, rotor equivalent wind speed and veer calculations are not adding much insight and are not needed for inclusion in the power performance testing, except when used for extending inner envelope conditions.