Nowadays, it has been widely recognized that wind is one of the promising green energy sources for its non-regional availability. This has driven significant resource efforts devoted to the study of wind energy (e.g., [1
]). In principal, there are two major classifications for wind turbines, namely vertical axis wind turbine (VAWT) and horizontal axis wind turbine (HAWT). Typically, a VAWT can generate more power at a relatively low wind speed in urban areas compared to the HAWT. Nevertheless, when they both operate at the same wind speed, a HAWT is expected to generate more power since the aerodynamic drag is less and the entire rotation of all blades receives more wind power. Currently, small-scale wind turbines are gaining more attention due to the fact that rising energy costs have driven the consumer to seek alternative solutions of independent power supply. Despite the high efficiency of the HAWT, its main drawback is the potential risk of circuit burnout or structural failure when the wind speed exceeds its critical value. The present research is to provide a solution to this problem of the small-scale HAWT by an innovative passive pitch-control mechanism.
Over the years, there are various solutions proposed, including the short-circuit braking system, Choudhry et al.
] has outlined the control requirements for dynamic stall [5
] for wind turbines, where three passive control methodologies have been investigated. By applying this approach of dynamic stall control, Yen and Ahmed [6
] showed synthetic jet actuation to be effective for a vertical axis wind turbine (VAWT). By the stall-controlled approach, the system may sustain its stable operation generating power under a critical wind speed. However, when operating beyond the critical wind speed, the system will be at the risk of over-running due to a rapid rise of receiving excessive wind power. Another approach proposed by Xie et al.
] was to design a folding blade of turbine rotor, controlled by a servo motor [7
] or a passive mechanism regulation [8
]. The main drawback of this design is its difficulty of retaining dynamic equilibrium at variable speeds of rotation. Another innovative design of a flywheel rotor system was proposed by Jauch and Hippel [9
] to control the rotation of HAWT via inertia effect. Still, this flywheel mechanism has very limited capability of braking when the HAWT is subjected to very high wind speeds. Having borrowed the design of trailing edge flap of airplanes, Barlas et al.
] used active flaps on a small-scale HAWT. An alternative design to protect the HAWT system is the so called “yaw control” mechanism, which applies the principal of misalignment to achieve the goal of reducing rotational speeds. Under operations at high wind speeds, the system shall keep yawing to seek a balance with the incoming wind. Among many innovations in this category, Kragh [11
] proposed a rotor speed-dependent yaw control of wind turbines. Although applicable, this design has a potential problem of structural fatigue occurring at its support due to the dynamic loads arising from the frequent oscillations of the system. For this, Ekelund [12
] presented mathematical models obtained from the equations of motion and proposed to use a yaw servo for attenuation of structural dynamic load oscillations. Additionally, Shariatpanah [13
] developed another model for PMSG-based wind turbine with yaw control. Despite its effectiveness, the active control system requires a complicated design of an electro-mechanical sub-system and thus, the system reliability is still an issue when operated under long lasting sever conditions. Some other active pitch-angle control can be found in [14
]. Still, these systems require relatively delicate electro-mechanical systems for fulfilling the function of active control and its reliability is always an issue of concerns. Moreover, the issue of consuming extra electric power is another drawback of the consumer’s concern for using the active control. To the authors’ personal opinions, designs of passive pitch-angle control on these aspects are comparably more ideal in consideration of system reliability and saving of electricity. Li [18
] presented a self-powered passive adaptive control to adjust pitch angle of blade. The principal of the self-powered passive adaptive control is to use aerodynamic force acting upon on the blade as the control power under balance with the spring force. Hertel et al.
] developed a simple strip theory model for the analysis of a conceptual passive pitch control, providing the aerodynamics as a function of the relative wind vector’s magnitude and local angle of attack.
Penghu, the largest island of the Pescadores Islands in the Taiwan Strait, has a wind turbine test unit. Having complete facilities, this platform has acquired the international certification of small wind certification council (SWCC). For collecting data from the original HAWT system without pitch-control, we set up HAWT systems of 200 W, 400 W, and 1 kW output at the Penghu test unit (Figure 1
). Figure 2
a displays all collected data of the 400 W system, as an example, for the monitoring period of one month (March 2012). From the collected data, it can be clearly seen that the data meets the designed power under normal operation as shown by the green region (before the rated wind speed). However, the blue region, the wind turbine system is operated over the rated wind speeds and should be braked for protection of over power outputs. Under the overloaded operation of up to 600 W, the long-term operation shall lead to generator corruption (Figure 2
b). As the objective of this research, this has inspired us to make the best use of the good wind-farm to stably generate electric power even at over rated wind speeds.