Turbulence effects on wind turbines: A wind-tunnel study

Understanding the effects of turbulence on wind turbine output is important for an optimal design of wind energy projects. Of particular interest is the prediction of how wind velocity fluctuations, associated with turbulent eddies, affect the power curve used to calculate energy output as a function of wind speed. In order to study that effect, a series of wind tunnel experiments has been carried out at the St. Anthony Falls Laboratory atmospheric boundary layer wind tunnel. Using temperature controls for both the tunnel floor and the air temperatures, boundary layers have been created with different thermal stratification levels. In addition, different surface roughnesses have been considered. Miniature wind turbines have been placed at different positions in those boundary layers and in the free stream, where the flow has very small turbulence intensity. At scales larger than the wind turbine diameter, fluctuations in the turbine power output appear to be correlated with velocity fluctuations measured with hot-wire anemometry. Differences in the power output are found between cases with the same mean wind velocity at the turbine location, but different level of turbulence due to different turbine height, surface roughness or thermal stratification. The implications of these results for the estimation of turbine power output are discussed.