WIND TURBINE BLADE OPTIMISATION WITH AXIAL INDUCTION FACTOR AND TIP LOSS CORRECTIONS Y. EL KHCHINE 1 , M. SRITI 2 1. Engineering Sciences Laboratory, Polydisciplinary Faculty of Taza, Sidi Mohamed Ben Abdellah University + younes.elkhchine@usmba.ac.ma 2. Engineering Sciences Laboratory, Polydisciplinary Faculty of Taza, Sidi Mohamed Ben Abdellah University + mohammed.sriti@usmba.ac.ma Abstract Tip loss corrections are a critical factor in blade element momentum theory when determining optimum blade shape for maximum power production. Using numerical and analytical optima, this paper compares the optimal tip shape using the classical tip loss correction of Glauert. A semi- analytical solution was proposed to find the optimum rotor considering Shen’s new tip loss model. The optimal blade geometry is obtained for which the maximum power coefficient is calculated at different design tip speed and glide ratio. Our simulation is conducted four S809 rotor wind turbine blade type, produced by National Renewable Energy Laboratory (NREL). Keywords: BEM method, S809 airfoil, Horizontal axis wind turbine, Aerodynamic performances, Tip speed ratio, power coefficient 1. Introduction (12 gras) Optimisation of wind turbine rotors frequently involves the use blade element momentum (BEM) models both analytically, or with numerical methods. These numerical methods can be used where no analytical optimisation is known, as well as maximising two or more objective functions. State-of-the-art BEM models are based on two- dimensional flow models and hence are limited in their ability to accurately determine three-dimensional flow structures. Whilst Navier-Stokes solvers can be incorporated into rotor optimisation for greater accuracy, BEM computations have significant advantages in computational speed and ease of implementation. An alternative to using a more computationally intensive method is to modify the BEM model and apply corrections to the technique. One of the most important corrections to BEM analysis is a tip loss correction. The concept of a tip loss was introduced by Prandtl [4] to account for the difference between an actuator disc with an infinite number of blades and a real wind turbine or propeller with a finite number of blades. Glauert [1] developed blade element- momentum theory based on one-dimensional momentum theory as a simple method to predict wind turbine or propeller performance. In order to make more realistic predictions, Glauert introduced an approximation to Prandtl’s tip loss correction to be included in BEM computations. In his analysis, Glauert assumed that the tip loss only affected the induced velocities but not the mass flux. Later, de Vries [2] corrected both the induced velocity and the mass flux. Recently, Shen et al. [3] showed existing tip loss corrections to be inconsistent and argued that they fail to predict correctly the physical behavior in the proximity to the tip. Shen et al. introduced a new tip loss correction model that gave better predictions of the loading in the tip region. Numerical methods may also be useful with the inclusion of the so called Glauert empirical correction. If the axial induction factor, a, reaches values greater than 0.5, momentum theory is no longer holds and it is common to correct the local coefficient of thrust, CT, using a simple linear relationship. To avoid confusion between the ‘Glauert empirical correction’ and ‘Glauert’s tip correction’ the phrase ‘high thrust modification’ has been used throughout this manuscript. Shen et al. [3] used a value of ac = 1/3 which was followed here. In theory, for a rotor operating at the Betz limit, ac = 1/3 and hence uncorrected momentum theory should hold. Further, it is arguable whether the high thrust modification should be used in wind turbine power computations. The objective of the current work is to maximise the power extraction efficiency of a wind turbine rotor, with a focus on the the effect of various tip loss models on the optimal rotor shape. 2. Mathematical model As the classical theory of wind turbine rotor aerodynamic, the BEM method combines the momentum and blade element theory. The blade is divided into several elements as shown in Fig. 1, by applying the equations of momentum and angular momentum conservation, for each element dr Y. El Khchine, M. Sriti International Journal of Renewable Energy Sources http://www.iaras.org/iaras/journals/ijres ISSN: 2367-9123 105 Volume 2, 2017