대기 안정도 효과를 고려한 대형 풍력발전기의 공력 특성 해석

Abstract

Environment in which the wind turbines are installed has a variety of variables. The wind turbine is particularly influenced by the atmospheric condition due to the nature of the turbine mechanism. The stability of the atmosphere can be categorized into three characteristics - neutral, stable, and unstable(convective). Depending on whether the boundary layer is stable, neutral, or unstable, the mean wind speed, direction, and turbulence properties may vary greatly across the tall turbine swept area. This variance can cause tall turbines to produce different amount of power with the wind speed blown at an identical hub height for some time period. Currently, however, IEC standard for the wind turbine design and performance evaluation does not consider the actual atmospheric environment in many respects. The wind profiles described in the IEC 61400 standard are based on the neutral stability conditions considered when the sky is cloudy and has strong geostrophic flow, and are formulated by a power law applying only to limited conditions that does not have a theoretical basis conditions. So we can expect that the Cyclic loadings and Power calculated according to the IEC standard are very different from actual measured value. This research consists of the cell and boundary conditions to avoid stream-wise gradients in the vertical profiles of mean wind velocity and turbulence quantities. This requirement is referred as horizontally homogeneous ABL(Atmospheric Boundary Layer) flow over uniformly rough terrain. Based on this scheme, we observed the characteristics of aerodynamic and wake on the tall wind turbine generating 5MW output by applying the velocity, potential temperature and turbulence quantity profiles based on the Monin-Obukhov Similarity theory considering the atmospheric stability. A flow in the horizontal direction is observed to be more uniform when the expansion rate is low and the distance from the wall to the center of the wall adjacent cell is larger than the roughness height yielding about twenty times roughness length. When considering the aerodynamic performance according to atmospheric stability, there are a little difference of wind velocity between the lowermost part of the rotor and the uppermost part at the unstable case, so the cyclic loading in the root part of blade is not great compared to the stable case. But we find that the output of the wind turbine in the stable case is higher than that of the unstable case. When considering the characteristics of wind turbine wake according to atmospheric stability, the velocity recovery is much faster in the unstable case. The level of turbulence kinetic energy and turbulence intensity is more significant in the stable case than in the unstable case. Also, we can find the phenomenon such as the daytime cooling and nocturnal warming in the wake due to the mixing turbulence caused by wind turbine rotor.