Wake Fast Simulation Method And Its Application In Wind Farm

The power losses of a large wind farm caused by wake effect usually account for 10%to 20%of the annual energy production.Accurate wake model is the foundation for assessing the energy production and economics of a wind farm.During the micro-siting of a wind farm,the position of each wind turbine should be reasonably arranged to minimize the wake losses and maximize the overall economic efficiency of the wind farm.For the wind farms in operation,the power generation of the entire power plant can be maximized through an integrated control strategy to adjust the operating state of the upstream wind turbines to reduce the impact of its wake effect on the downstream ones.During the practical micro-siting and optimal control of a wind farm,it is necessary to calculate the wake up to 10,000 times in a short time.Therefore,a wake simulation method requires not only high accuracy but also high computational efficiency in order to meet the engineering requirements.Focus on the high-precision wake simulation method and its application on the micro-siting and control of a wind farm,the main work in this thesis is as follows:(1)The physical principles of overestimating/underestimating the velocity loss in the wake region are explored by the improved Jensen model and the Frandsen model,and this lays a theoretical foundation for the model improvements.The mass and momentum exchange inside and outside the control volume in the two models are analyzed.The results show that the former model overestimates the wake velocity loss due to the neglect of the exchanging mass;while the latter model underestimates it since the actual momentum in the atmosphere into the control volume is lower than the theoretical momentum.(2)A two-dimensional J-F wake model is developed.The Gaussian distribution function is used to fit the wake velocity.The two-dimensional model solves the maximum velocity loss in the wake center based on the mass conservation and calculates the standard deviation based on the momentum conservation.The performance of the J-F model is validated by wind tunnel experiment data and wind farm measurements.And the results show that the J-F wake model can simulate the flow characteristics of the wind turbine wake with higher accuracy.(3)A fast wake simulation method based on the engineering wake model is proposed.First,considering the fact that higher wake turbulence intensity leads to faster wake recovery,two calculation methods for velocity in the wake overlap region are developed,i.e.k-Based method and MEB superposition model.Second,the coordinate transformation is used to calculate the relative position between wind turbines under different wind directions.Third,the problem of partial wake effect when using the two-dimensional wake model is solved through linear integration of the wake velocity.The proposed method is applied to calculate the power of two offshore wind farms.The results show that the two superimposed wake methods are more accurate than the classical model.(4)A CFD pre-calculation simulation method for single wake is proposed.To solve the efficiency problem of the classical CFD method when applied in the practical micro-siting,the numerical simulation of the wind turbine wake under different inflow speeds is carried out in advance using the actuator disk model.In this way,a database of the flow characteristics under various conditions can be obtained.And then,the piecewise cubic Hermite interpolation is applied to obtain the wake velocity under any incoming conditions.The accuracy of the proposed pre-calculation method is validated by the wind tunnel experiment and the measurement data of the wind farm.The results show that the pre-calculation method consumes the practically acceptable time to achieve similar accuracy compared to the actuator disk method,which solves the contradiction between model accuracy and calculation efficiency.The proposed method is also applied in the practical wind farm micro-siting and energy production assessment to compare its performance to the engineering wake models.The results show that lower cost per kilowatt is obtained under three typical wind conditions,and the optimized layout of wind turbines shows a high degree of symmetry.(5)A method for calculating the power output of a wind turbine at any blade tip speed ratio is proposed.Since the poor performance,the optimal control strategy based on a theoretical power calculation model cannot be applied to the engineering practices.To solve this problem,a method for calculating the wind turbine power output at any blade tip speed ratio is proposed based on the power curve and the thrust coefficient curve.To maximizing the power output of a wind farm,an optimal control model is established.Taking the Lillgrund offshore wind farm as an example,the power output of the whole wind farm is increased by 8.4%when the incoming flow is 8 m/s and the inflow direction is 120°,when the flow direction is 222°,this percentage is slightly decreased to 6%.The results show that the proposed control model can achieve higher power improvements when the wake loss of the wind farm is higher.