Wind turbines absorb wind energy and convert it into mechanical energy, and are classified according to the orientation of their axis of rotation into horizontal-axis wind turbines (HAWTs) and vertical-axis wind turbines (VAWTs). The HAWTs, represented by three-bladed propeller turbines, are the most common wind turbines because of the highest performance and easy manufacturing as a result of the great advance of aerodynamics and material engineering [1
]. The VAWTs are further grouped into lift-type and drag-type. The lift-type VAWTs, which employ airfoil section blades to generate lift force as HAWTs, have higher rotation speed and better performance than the drag-type. The lift-type VAWTs, represented by the Φ-type and the straight-bladed type of Darrieus turbine, are also well-known [2
]. On the other hand, most of them are used in small-scale applications, although they have several advantages over HAWTs, such as the ability to accept wind from any direction without yawing and the ability to provide direct rotary drive to a fixed load [3
]. Some of the disadvantages of the VAWTs include an inability to self-start and relatively lower efficiency [4
Darrieus turbine blades use airfoil sections designed as aircraft wing profiles. The NACA0012, NACA0015, and NACA0018 profiles are commonly used as blade sections [6
]. Typically, these are designed to operate at small angles of attack (AoAs), lower than ±10°. At angles higher than this, the airfoil stalls and the flow separates on the upper surface of the blade, causing a loss of lift and an increase in drag [7
]. At the startup of the turbine, a zero-tangential flow speed contributes to a large AoA, which then makes the airfoil stall. In this case, the pressure drag of the blade is so high that the lift cannot overcome the drag to self-start [8
], not to mention any additional load from the electricity generator. Previous studies proposed a number of methods to make a self-starting lift-type VAWT. Kirke [9
] reviewed those studies, and stated that high solidity turbines have the potential to increase the self-starting torque, but a higher solidity lowers the maximum power coefficient and narrows the operating range. Beri et al. [10
] maintained that the Darrieus type VAWTs with asymmetrical blades have better self-starting behavior, but unfortunately, this also causes a reduction in peak efficiency. Zamani et al. [11
] numerically investigated a 3-kW straight-bladed Darrieus type VAWT with a designed J-shaped profile. Their results showed that the J-shaped profile could suppress the vortices and improve the self-starting of the turbine. The hybrid wind turbine, comprising two vertical co-axial rotors, is normally constructed as a Darrieus with drag-type blades in the middle [13
]. The primary purpose of the hybrid design is to promote the lack of torque when self-starting and at low tip speed ratios (TSRs) [4
]. They can self-start normally, but after starting, they are less efficient than a turbine with normal lift due to the negative drag created at the middle [14
]. It has been found that the above approaches hardly provide a way to promote both the self-starting ability and the peak power coefficient of the VAWTs at same time.
A particular method to address those issues is to develop a variable pitch (VP)-straight-bladed VAWT to control the pitch angles of the blades on each azimuthal angle in order to maximize turbine torque and start up easily [15
]. A reduction of stall is the main mechanism whereby VP-technology produces significant improvement of the torque at start up and low TSRs [16
]. Besides offering the greatest potential for achieving significantly increased torque at low and intermediate TSRs, VP-technology is also used to promote peak efficiency [17
]. VP-VAWTs consist of active and passive-type turbines. The active designs are defined as those systems that produce blade pitch change through means other than the direct action of the aerodynamic forces acting on them in passive designs [16
]. Staelens et al. [7
] introduced three modifications for the increased power output of a straight-bladed VAWT by varying its pitch. The third modification proposed a continuous variation in the local AoA correction during the rotation cycle using a sinusoidal function. Although the power output obtained by using such a modification is less than the other two, it has the inherent advantage of being practically feasible. Also, Kiwata et al. [4
] described a micro VP-VAWT that varies the pitch angle according to the azimuthal angle. The performance of the VP-VAWT was measured in an open-circuit wind tunnel, and the results were better than those of the fixed pitch (FP)-VAWT. Erickson et al. [20
] tested the effects of cyclic blade pitch actuation on the efficiency and operability of a high-solidity VAWT with a cam and control rod mechanism to prescribe the pitch dynamics in the wind tunnel over a wide range of design and operational variables. The results revealed that a tuned first-order sinusoidal actuation system could achieve a maximum absolute power coefficient of 0.436, which is an increase of 35% over the optimal fixed-blade configuration, with self-starting capabilities and drastically improved performance at a wide range of suboptimal operating conditions. Hwang et al. [21
] investigated the performance of a VP-turbine controlled by a cycloidal strategy. Compared with the FP-turbine, the performance at the upwind half-cycle of the VAWT with the optimized blade pitch angle was greatly improved, particularly at the azimuth region from 90° to 180°; the effective region was expanded, and was nearly twice as large as that of the FP-case. Chougule et al. [17
] designed a 500-W VAWT to implement the pitch control mechanism using the cyclic VP-technology. Its aerodynamics were predicted using the double multiple stream tube (DMST) model, and their results showed that only a 5° increase of the pitch angle amplitude increased the power coefficient of the VAWT by 12%.
Despite the improvement of the self-starting ability and power coefficient, the above VP-approaches have the same limitation, which is mainly about the performance enhancement of azimuths with high AoAs in the case of the FP. In FP-VAWTs, each blade is constantly subjected to a variation of the AoA. Since the AoA constantly changes, the blade will generate large torque in some azimuthal angles, and low torque in other azimuthal angles. The overall performance is determined by the average torque of the blade in a cycle path line. Evidently, the low torques will produce a lower average value, and then degrade the power efficiency. The azimuthal position with the biggest geometric AoA in FP-VAWTs will achieve the biggest increase in the performance of the blade in VP-VAWTs. Assuming that the biggest geometric AoA is close to and below the stall value at the rated TSR, the magnitude of the maximum increase of the AoA would be much smaller if it still used the above-mentioned VP-approaches; thus, the increase of the AoA in other azimuths and the peak efficiency of the turbine would be much smaller, too.
To change this, we proposed a new VP-approach that was intended to mainly promote the performance of the blade working in the azimuths with smaller AoAs in FP-VAWTs. Ultimately, the aim is to largely enhance the peak efficiency of the VAWT in the rated or larger TSR. In this paper, the effect of the new approach on the performance of the blade or the turbine are investigated using a method implementing the double multiple stream tube (DMST) model.