Study On The Aerodynamic Characteristics And Coupling Dynamic Performance Of Floating Helical VAWT

Optimizing the energy supply structure,improving energy utilization efficiency and building a clean,low-carbon,safe and efficient modern energy system are important strategic objectives of China.Wind resources are large,renewable and pollution-free.It is an important form of renewable energy.With the advantages of rich resource reserves and broad development space,offshore wind energy development has become a new strategy for wind power technology and industrial development of China.With the gradual development of offshore wind farm construction to large-scale and massive,and the continuous increase of single unit capacity,large vertical axis wind turbine(VAWT)has better development potential.The proposal of helical blade solves the problems of low wind energy utilization and large load fluctuation of traditional vertical axis wind turbine effectively.At present,the research of helical VAWT mainly focuses on small-scale landed wind turbine.The theory and technology of megawatt floating helical VAWT are not mature.It is urgent to establish the system coupled simulation model of megawatt floating helical VAWT,and comprehensively explore the aerodynamic characteristics and system coupled dynamic performance of wind turbine,so as to provide a theoretical basis for the research of floating helical VAWT.Supported by NSFC,this paper systematically studies the aerodynamic characteristics and system coupled dynamic performance of floating helical VAWT.The main research contents and conclusions are as follows:(1)Considering the factors such as helical twist angle,dynamic stall,tip loss and tower shadow effect,a three-dimensional blade element momentum(BEM)theoretical model suitable for the aerodynamic load calculation of helical VAWT is established,a three-dimensional unsteady BEM aerodynamic calculation program is developed,and the aerodynamic characteristics of helical VAWT are explored.Based on the single parameter and multi parameter analysis methods,a 5MW helical VAWT is designed,and the influence law of wind turbine parameters on aerodynamic performance is revealed.The influence of wind turbine parameters on wind energy utilization is as follows: height diameter ratio > blade chord length > pitch angle > helical twist angle,and the influence on power fluctuation is as follows: helical twist angle > pitch angle >height diameter ratio > blade chord length.The aerodynamic performance of the proposed 5MW helical VAWT is significantly better than that of the prototype straight blade wind turbine,the rated power is increased by 7.6%,and the fluctuation amplitude of dynamic torque is reduced by 89.0%.(2)In order to explore the aerodynamic characteristics of floating helical VAWT and the coupled dynamic performance of the system,and provide data support for numerical model verification and correction,a 5MW floating helical VAWT model test with a scale ratio of 1:50 was carried out.OC4 semi submersible platform is selected as the floating foundation,the wind turbine blades are made of carbon fiber materials,and the transmission system is designed to control the rotating speed.The natural characteristics and damping characteristics of the floating helical VAWT system are obtained through the free decay test.The heave damping has nonlinear characteristics,and the nonlinear degree increases with the decrease of the initial heave value.By observing the system response under wind only condition,regular wave condition,wind& wave condition under working state and limit condition,the influence law of environmental parameters on the dynamic characteristics of floating helical VAWT is revealed.In the working state,wave load dominates the response amplitude of surge,heave and pitch motion,while wind load dominates the response equilibrium position of surge and pitch motion.Under extreme conditions,the wave frequency is close to the heave natural frequency,resulting in a significant increase in heave response.(3)Considering the aero-hydro-control-rigid flexible coupling,the numerical model of floating helical VAWT system is established,and the full coupled simulation program of floating helical VAWT is developed.The aerodynamic load of wind turbine system is calculated by three-dimensional unsteady BEM model,and the hydrodynamic load is calculated by frequency domain hydrodynamic parameters obtained by Sesam/Wadam software.The floating foundation is considered as a rigid body,and the blades and tower are considered as flexible bodies.Based on the Kane method of multi-body system,the generalized active forces(aerodynamic loads,hydrodynamic loads and other external excitation loads)and generalized inertial forces acting on each part of the system are assembled to obtain Kane dynamic equations,which are solved in time domain.A variable speed control model considering efficiency and load is established to match the rotating speed with the wind speed,and always maintain the target output power under the influence of turbulent wind and large-scale movement of floating foundation.The control strategy of limiting the target speed change rate and low-pass filtering is adopted to ensure the rapid regulation of the wind turbine under low wind speed conditions and stable regulation under high wind speed conditions.The accuracy of the fully coupled numerical model of floating helical VAWT is verified by comparing the numerical and experimental results under wind only condition,regular wave condition,irregular wave condition,wind & wave condition and limit condition.(4)Based on the fully coupled numerical model of floating helical VAWT system,the aerodynamic characteristics and system coupled dynamic performance of 5MW floating helical VAWT are deeply studied.Under the action of turbulent wind and steady wind,there are significant differences in the response of floating foundation and the bending moment at the bottom of tower.Under the action of turbulent wind,the maximum surge and pitch responses of floating foundation increase by 18.3% and 34.8%respectively.Turbulent wind conditions should be considered in the design.The secondorder differential frequency wave load has a significant impact on the surge response of floating foundation.In mooring design,we must pay attention to the influence of second-order wave force.The mathematical model of aerodynamic damping of floating helical VAWT shows that the aerodynamic damping is directly proportional to the incoming wind speed.The aerodynamic damping in pitch degree mainly has an impact in the lower wind speed area,and the impact is the greatest at the rated wind speed.The floating helical VAWT can maintain good power generation performance under the combined action of wind and wave.Compared with the floating straight blade wind turbine,the power fluctuation of the floating helical VAWT is greatly reduced.When the wind speed exceeds the rated wind speed,the maximum power fluctuation deviation is no more than 5%,which can meet the grid connection requirements.