Impact of forested fetch on energy yield and maintenance of wind turbines

In a wind farm whose turbines have either a forested or unforested fetch, measurements of the wind speed, wind direction and turbulence intensity are made with a 3D scanning LiDAR, and measurements of the tower head aeroelastic deflections are simultaneously made with an opto-mechanical measurement system. It is seen that in the forested fetch there is up to 2.5 times larger turbulence intensity than in the unforested fetch. Aeroelastic deflections of the tower during normal operation are up to 2.8 times larger for a turbine in forested fetch compared to a turbine in an unforested fetch. It is observed that the turbine with forested fetch has 17% lower annual energy yield compared to a turbine in an unforested fetch. The deficits in the direction-wise energy yields are shown to be primarily a function of the upstream streamwise extent of the forest. Furthermore, an analysis of the maintenance logs of the turbines shows that there are 2.2 times more fault durations per year and 20% shorter time intervals between unscheduled and scheduled maintenances for a turbine in a forested fetch; thus more frequent scheduled maintenances are recommended for turbines with a forested fetch.