Wind turbines operating in cold climates: Reynolds number and turbulence effects on performances

The present study is motivated by several observations of unexpected, recurring, high levels of power for stall-regulated wind turbines operating under very low temperatures. As power levels recorded largely exceed design levels of the rotor, operation in such conditions can cause dramatic damage to turbine. This study aims to understand the origin of such phenomenon by analyzing experimental data gathered from a stall-controlled wind turbine, having a nominal power of more than 500 kW, and comparing the experimental behaviour with numerical simulations. To provide a quantitative estimate of density and atmospheric turbulence effects on power output, a procedure based on the IEC 61400-12 international standard for elaboration of a wind turbine power curve is used. The numerical simulations is based on the solution of the time-averaged, steady-state, incompressible Navier-Stokes equations with an appropriate turbulence closure model. The actuator disk model, together with blade element theory, are used to model the turbines. The stall-regulated turbine analyzed has shown to produce measured power increases significantly higher than increases of density. Regarding the influence of turbulence intensity, it has been observed that for constant hub height incoming wind velocity and density, power output increases with turbulence intensity at low winds, the opposite being true at higher winds. The numerical simulations show a good agrement with the measurements.