The aerodynamic performance of offshore floating wind turbines (OFWTs) is more complicated than onshore wind turbines due to 6-degree of freedom (DOF) motion of the floating platform. In the current study, the aerodynamic analysis of a horizontal-axis floating offshore wind turbine is performed with the aim of studying the effects of floating platform movement on the aerodynamic characteristics of the turbine in the presence of a pitch angle control system. The National Renewable Energy Laboratory (NREL) 5-MW offshore wind turbine is selected as the baseline wind turbine. For this sake, the unsteady blade element momentum method with dynamic stall and dynamic inflow models have been employed to obtain the unsteady aerodynamic loads. The baseline pitch angle control system is assumed to be coupled with the aerodynamic model to maintain the rated condition of the wind turbine and also to approach a closer model of wind turbine. In case of pitching motion input, the reduction of mean power coefficient for tip speed ratios (TSRs) less that 7 is expected by an amount of 16% to 20% at pitch amplitude of 2 and frequency of 0.1 Hz. For high TSRs, the trend is reverse with respect to fixed-platform case. The mean thrust coefficient is reduced for almost all range of TSRs with maximum loss of 37%.Moreover, the mean control pitch angle that is an index of control system effort is increased. The results also represent the importance of considering the pitch control system for aerodynamic analysis of disturbed OFWT.
K E Y W O R D Sfloating offshore wind turbine, pitch, pitch control system, platform disturbance, unsteady blade element momentum method 1 | INTRODUCTION Nowadays, the numerous advantages of offshore wind turbines (OWTs) lead us to use them in order to harvest energy from offshore wind.Although the construction and installation of OWTs are more expensive than the onshore wind farms, the availability of space and fewer complaints about noise problems make them more attractive compared with the onshore wind turbines. 1 Moreover, the wind energy potential is considerable at offshore. Statistics 2,3 show that the rated capacity of OWTs has grown 62% over the past decade, particularly the mean rated capacity of OWTs has increased about 15.4% from 2015 to 2016.The foundation type of the OWT depends on the sea depth, which could be either fixed or floated. In deep-sea area, the floating platform is utilized instead of fixed-foundation technology. Due to the waves and currents in sea regions, offshore floating wind turbines (OFWTs)
The aim of the present study is to investigate the accuracy of two different dynamic stall approaches for windturbine airfoils. The first approach is the semi-empirical Leishman-Beddoes model (L-B), and the second is the computational fluid dynamic (CFD) results. National Renewable-Energy Laboratory (NREL) S series airfoils are used, and the simulations are performed in Re=10 6. For both approaches, aerodynamic coefficients are represented and compared to experimental data. Validation data refer to Ohio State University (OSU) experiments, which are for pitch oscillation. Results show that the accuracy of the L-B and CFD methods is dependent on mean angle of attack, reduced frequency and the phase of motion. The semi-empirical model has appropriate accuracy as well as low computational cost while the CFD unsteady simulation could be properly used to predict the drag coefficient.
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