Wind energy, as a clean and renewable energy, has attracted more and more attention from all over the world. Wind turbines that can transfer wind energy into electricity production have been developing rapidly in the world in recent years. The blades, as one of the key components of wind turbines, cost about 20% of the whole machine. Their good design, reliable quality, and superior performance are the decisive factors to improve the utilization rate of wind energy and ensure stable operation of wind turbines. The key factors for the design of wind turbine blades are the aerodynamic performance and the structural properties of wind turbine airfoils.
At present, the design of airfoil profiles with medium thickness, in which the maximum relative thickness is between 25% and 35%, has been the focus of aerodynamic performance of wind turbine blades. Tangler and Somers [1
] designed 35 kinds of airfoils named the NREL-S (National Renewable Energy Laboratory Special) airfoil series for various wind turbines using Eppler theory and inverse design method. The new airfoil families exhibit high maximum lift coefficients. DU airfoil families with a relative thickness of 15% to 40% were designed using mixed inverse design method [3
]. Airfoils with good aerodynamic performance can be designed by changing the upper-surface thickness and the S-shape lower surface. The result showed that the lift coefficient was improved and relatively insensitive to roughness. Compared with other airfoils, the DU (Delft University) airfoil families exhibited better aerodynamic performance. From the middle of 1990’s, RISØ airfoils were designed by RISØ National Laboratory in Denmark [5
]. In their work a direct design method for wind turbine airfoils based on mathematic optimization and the XFOIL software was presented. Sobieczky [6
] presented a method named PARSEC (Pseudopotential Algorithm for Real-Space Electronic Calculations) to design airfoils; a modern airfoil was designed by controlling the airfoil geometry parameters. Hajek [7
] presented an improved PARSEC method to design airfoils, and a brand-new airfoil was optimized. Ava and Alireza [8
] presented a new method for airfoil shape parameterization to optimize the airfoil at high Reynolds number turbulent flow conditions using a genetic algorithm. It was concluded that this method is capable of finding efficient and optimum airfoils in fewer generations. Chen and Wang [9
] presented a general integral expression of airfoils based on a generalized functional and Trajkovski conformal transformation. CQU-DTU (Chongqing University) and WT (Wind Turbine) series airfoils have been designed successively using a multi-disciplinary optimal method. The aerodynamic performance of the airfoil series has been verified by wind tunnel experiments. Seong-Ho Seo and Cheol-Hyun Hong [11
] studied performance improvement of airfoils for wind turbine blades with groove. It was confirmed that the shape of the groove contributed to recovering velocity around the airfoil wall, and the lift to drag the ratio improvement by the groove was maintained at the given range of Reynolds number. Cheng and Zhu [12
] used this method combined with airfoil self-noise theory, as such CQU-DTU-LN1 airfoils were designed which exhibited low noise emissions. The noise characteristics of the airfoils were verified by wind tunnel experiments. Zhu and Shen [13
] presented an integrated optimal method of airfoil and blade for large wind turbines. The results indicated that the airfoils achieved a high power coefficient and were insensitive to surface roughness. Seyed Mehdi Mortazavi [14
] presented a multi-objective genetic algorithm with objective functions of minimum energy waste and maximum efficiency. The results show that using the second law approach along with the Pareto optimality concept leads to a set of precise solutions. Xingxing Li [15
] put forward a mathematical model of the overall optimization employing airfoil performance evaluation indicators. Based on this model, an integrated optimization platform for thick airfoils was established. The results confirmed that the proposed method effectively balanced airfoil’s complicated requirements and successfully improved the new airfoil’s overall performance. Pierluigi [17
] presented a novel optimal procedure for airfoil profile design based on PARSEC parameterization and genetic algorithm optimization. As a matter of fact, the optimization under Nash equilibria solutions would be more attractive to use when a well posed distinction between player variables exists. Pereira and Timmer [18
] described a design method of airfoils which were suitable to employ actuation in a wind energy environment. The novel airfoil sections are baptized wind energy actuated profiles. The results show that using WAP (wind energy actuated profiles) airfoils provided much higher control efficiency than adding actuation on reference wind energy airfoils, without detrimental effects in non-actuated operation. Miller [19
] used a Bézier curve and XFOIL software to design flatback airfoil families with a high degree of attention to the appropriate selection of design constraints and objectives. The new CU-W1-XX series were shown to have equal or superior performance compared to other airfoils. Ram [20
] designed the airfoil sections named USP07-45XX for a 20 kw wind turbine using the multi-objective genetic algorithm. The USP07-45XX airfoils showed only a slight change during clean and soiled conditions both in experiments and in numerical studies. Besides, there are many other researchers [21
] who have also made other significant contributions to this field. In summary, some new design methods were put forward to optimize wind turbine airfoils from different points of view. Those new wind turbine airfoils not only have improved the aerodynamic performance, but also increased the efficiency of wind energy.
However, for the above studies, whether they were based on an original airfoil profiles, or a mathematical parametrical expression, the objective functions usually were the aerodynamic parameters (such as lift-drag ratio) of the free transition and the fixed transition. The influence between blade stiffness and aerodynamic geometry was not considered. Medium thickness wind turbine airfoils, not only require high aerodynamic performance to obtain high wind energy utilization, but also large structural stiffness characteristics to resist excessive elastic deformation of the blade. Therefore, an optimal design method for the medium thickness wind turbine airfoils in which the aerodynamic performance and blade cross-sectional stiffness is both considered are proposed in this paper. It does not improve the airfoil’s aerodynamic performance unilaterally, but seeks to increase both aerodynamic performance and cross-sectional stiffness of the medium thickness airfoils. An optimal mathematic model which combines the functional integrated expression of airfoils and the stiffness matrix of the composite wind turbine blade cross-section is established. Coupled with RFOIL software [27
] and the blade cross-sectional stiffness calculated program, the aerodynamic performance of the medium thickness airfoil and the stiffness performance of its cross-section are optimized using a genetic algorithm. Lastly, the optimized airfoil named WQ-B300 is compared with the classic DU97-W-300 airfoil and the WQ-A300 airfoil, which were designed without considering the change of the blade layer parameters, to verify that the airfoil has better aerodynamic performance and structural stiffness characteristics.