# Application of Simultaneous Symmetric and Cambered Airfoils in Novel Vertical Axis Wind Turbines
## 1 Introduction  
Increasing the share of renewable energy, and specifically wind energy, to meet increased demand for energy is one important strategy for reducing the burden of greenhouse gas emissions and local air pollution. To enhance the cost-effectiveness of electricity generation from wind energy, increased attention to the technological advancement of wind turbines combined with the installation of large-scale wind farms is emerging as an increasing source of new energy supply [1]. VAWTs are well-known and popular types of wind turbines, particularly for small-scale power extraction. Most of the VAWT types have been developed based on the [PERSON] design, invented by the French engineer in 1925 [2, 3]. This type of turbine has a proper power coefficient, particularly at the small scale used in urban areas [3]. Despite this advantage, self-start capability is one of the critical shortcomings of VAWTs; they cannot start rotating without external help, such as an electrical motor [4]. Due to the importance of vertical axis wind turbines and their positive features compared to horizontal ones, research has focused on improving their performance. The research studies follow two main paths: vertical axis wind turbine self-start capability and powercoefficient. Several studies reveal that the focus on improving self-start capability has a negative impact on power production. Alteration of the main characteristics of VAWTs, such as several blades or airfoils, has been assumed as one way of improving self-starting ability. For instance, three-bladed wind turbines can have better self-starting than two-bladed ones [5], but increasing the number of blades usually reduces the power coefficient [6]. Furthermore, cambered airfoils can help VAWTs rotate at lower wind speeds [7]. However, these airfoils usually reduce the power coefficient [8]. Due to the popularity of NACA airfoils, some studies have tried to find suitable NACA airfoils for self-starting purposes. By way of illustration, NACA2418, a cambered NACA airfoil, has better self-starting with a 1.5\({}^{\circ}\) fixed pitch angle than NACA0018, NACA0015, NACA0022, and NACA0024 airfoils [9]. On the other hand, NACA0015 showed the highest power production among them under the same conditions [10]. Another effective way of improving the self-starting ability is modifying the geometry of airfoils [7]. Apart from changing the main parameters of VAWTs (as mentioned above), some innovations can improve self-starting by reducing power production. For instance, double-blade VAWTs, which have two blades in one strut, are useful for this purpose, suggested by [PERSON] et al. [11]. Usage of helical blades instead of straight ones introduced by [PERSON] et al. is another solution for starting characteristics [12]. The idea of [PERSON] and [PERSON], based on combining a Darrieus rotor with a Savonius one, was beneficial for self-starting ability [13]. They placed the Savonius rotor in the interior area of the Darrieus rotor. Another beneficial arrangement is placing a Savonius rotor above a Darrieus one. This type of VAWT was used in railways by [PERSON] et al. [14]. They proved that coupling a batch-type VAWT with a Darrieus rotor can yield a higher power coefficient as compared to combining one Savonius rotor with one Darrieus one. However, the generated power is still lower than the power coefficient of the H-type VAWTs [15]. An idea that has been proposed recently is using an auxiliary blade close to the main blades. It has been confirmed experimentally and numerically that these VAWTs have a higher self-starting ability [16; 17]. A large number of studies have been conducted to increase power production while neglecting the self-starting problem [18; 19; 20; 21; 22; 23; 24; 25]. An important inherent parameter of any VAWT is airfoil geometry. It has been shown by [PERSON] et al. that the thickness chord ratio is more effective on the power coefficient than the maximum thickness and the diameter of the leading edge [10].