In recent years a renewed interest has arisen on vertical axis concept in wind turbines both small and large because of the several advantages. For example, simplicity of design and maintenance with no yaw or pitch mechanisms, ability to withstand rapid changes of wind direction and power density of wind farm potentially greater than horizontal axis wind turbine (HAWT), etc. For this reason, relevant studies have been performed by many researchers. However, previous studies mainly focused on overall performance of wind turbines and loading on the blades. Also, some researchers have conducted a study on the flow measurements, but very limited studies are found in far wake model.
In this study, in order to provide a blockage correction factor and wake profile model, wind tunnel experiments were performed and investigated the blockage effect and the characteristics of wake using the vertical axis wind turbines of Darrieus types which have 3 and 2-straight blade respectively. Preliminary test was performed to determine the measurement point of reference inflow velocity according to up-stream distance from the wind turbine. In the test of blockage effect, using three different types of wind tunnel and AC motor, we measured wind speeds and estimated power coefficients under three different Blockage Ratio (BR) and various Tip Speed Ratios (TSRs). In addition, the blockage correction was conducted with some typical methods and the blockage factor was determined based on the test result. Hot-wire anemometry was used to characterize the distribution of mean velocity deficits and turbulence intensities behind 2 ~ 12D (diameter) from the wind turbine, then the wake profiles obtained by experiment were compared with Ainslie model.
From the wind tunnel test, reference measurement point was selected more than 3.5D in front of the wind turbine to avoid distortion of inflow velocity. The maximum power coefficients and the optimum TSR have been strongly influenced by the blockage ratio, generally increased as the blockage ratio increases. The overestimated power coefficients are reduced from all correction methods considered in this study, however, the correction results show that the proposed method more effectively correct the blockage effects compared to the other correction methods. The parameters of TSR, rotor diameter and solidity affect on the shape of wake profile developed in right front of the wind turbine. However, the shape become insensitive to these parameters as far away from the wind turbine and much depends on the distance from the wind turbine. Overall, the wake profile evaluated with Ainslie model fits well with the measurement results, but doesn’t reflect the peak location of velocity deficit distribution which is shifted from the distribution center.