Analyzing Rotor Tilt Angles on the Efficiency of Monopole Tower Floating Offshore Wind Turbines

Abstract

The development of Floating Offshore Wind Turbines (FOWTs) has typically sought to achieve superior power efficiency as well as economic performance by enhancing the application of wind turbine technology. One common difficulty of floating offshore wind turbines is the effect of airflow which contextualises the angles and an unstable floating platform. The condition can cause the wind turbine axis to move out of its vertical alignment, which can reduce the area of the rotor blades exposed to the wind, preventing the blades from maximizing the extraction of energy from the available wind flow. In order to manage this particular challenge, this study aims to examine the effects of the rotor tilt angles upon the efficiency of wind turbine performance; and to compare two different types of wind turbines: the FOWTs, and the fixed tower wind turbines. The study made use of wind turbine modeling in a wind tunnel along with Computational Fluid Dynamics (CFD) simulation models. The experimental wind tunnel measured 4.5 m in length, 3 m in height, and 4 m in width, with a square airflow duct measuring 1 m2 located in the center of the tunnel. The models of both wind turbine types used in the wind tunnel experiments made use of 82 cm diameter R1235 airfoil blades, and the measurements in the tunnel were taken using an anemometer, a tachometer, and an angle meter. The wind speeds varied between 2-5.5 m/s during the testing procedure and comparisons were drawn between the rotational speeds, power coefficients and tip speed ratios for the two different models. The tilt angle values obtained in the wind tunnel experimental model were then applied in the CFD simulation model in order to draw comparisons between the rotational speed, tip speed ratio, and power coefficient for the FOWTs experimental model and the CFD model with the wind speed set in the range of 2-5.5 m/s. The wind tunnel experimental results revealed that the FOWTs offered lower rotational speeds than that of the fixed tower wind turbines, as a consequence of the altered tilt angles, which accounted for average percentage difference of rotational speed of 36.8%. The FOWTs experimental model presented tilt angles in the range of 3.5°- 6.1° while the wind speeds ranged from 2-5.5 m/s. The outcome was comparable with that of the fixed tower wind turbines in the wind tunnel and FOWTs in the CFD, with average percentage difference of 17.7%. However, the CFD simulation results for the FOWTs exceeded the findings from the wind tunnel experiments, with the average percentage difference of 16.4%. These results for the FOWTs, whether from the experiments or the CFD simulation, were not as high as those recorded for the fixed tower wind turbine. In the case of the FOWTs, the values for the tip speed ratio and power coefficient were successfully held at an optimal level, both during the experimental models and the CFD simulations, when the power coefficient was in the range of 0.35-0.36, the tip speed ratios in the range of 7.7-9.6, the wind speed varied from 3-5 m/s at tilt angles from 3.9°- 5.8°. Meanwhile, the results from the fixed tower wind turbine showed that the optimal values were held when the wind speed was in a much narrower range of 2-2.5 m/s, and the power coefficient ranged from 0.33-0.36. The analysis of the two wind turbine types via the wind tunnel experiments and the CFD simulations led to the conclusion that when the wind speeds reach 3-5 m/s and the tilt angles measure 3.9°- 5.8°, it is possible to maintain the wind turbine power efficiency at an optimal value. In addition, the findings offered further understanding of the new theory of maintaining the power coefficient by utilizing an adjustable tilt angle for small to medium fixed pitch FOWTs. Thus, the utilization of these floating offshore wind turbines could decrease the expense of the pitch control system.