SIMULATION OF WIND TURBINES UTILISING SMART BLADES
Wind turbine smart blades change their aerodynamic characteristics in response to changes in operating condition, aiming at enhancing the power capture capability or controlling the power and aerodynamic load. Smart blades span a wide range of technologies. Some of these technologies have been proposed and developed specially for wind turbines, while some others have been borrowed from aircraft applications. Adaptive blades, telescopic blades, morphing blades, blades equipped with active control surfaces are some examples of smart blades. This paper presents a summary of wind turbine smart blades and advances in simulation and design of these blades. INTRODUCTION Wind turbines are designed to produce maximum power at the most probable wind speed. At high wind speeds, the generated power by a wind turbine far exceeds the generator capacity. To protect wind turbine operation at high wind speeds it is needed to limit the generated power otherwise it overloads the generator. While power control is an essential control for all wind turbines, aerodynamic load control is the main challenge for large wind turbines. As the size of wind turbine increases, larger blades and larger aerodynamic loads demand for either new blade materials or fast response load alleviation mechanisms in place. Pitch control and stall regulation are the most popular power control systems both based on controlling the flow angle of attack. Stall regulation is mechanically the simplest controlling strategy. In stall regulated wind turbines the blades have been designed to stall in high winds without any other control. The rotor is built with the blades fixed on the hub therefore it is rather simple in construction and the pitch of the blades are adjusted only once when the wind turbine is erected. In order to achieve stall regulation at appropriate wind speeds, the wind turbine blades operate closer to stall and result in lower aerodynamic efficiency below rated power. Stall regulated wind turbines normally do not have a perfectly flat power curve above the rated wind speed. While stall regulation is the simplest power control mechanism, pitch control is the most common means of controlling the rotor mechanical power. It also can be used for quasi steady aerodynamic load control. Conventional pitch control systems are used to limit the rotor mechanical power at a its rated value and to optimise the energy capture below that value. Individual pitch control systems have been successfully developed and utilised to alleviate low frequency fluctuating loads by pitching the blades individually (Caselitz 1997, Bossanyi 2003, Lovera 2003, Larsen 2005, van Engelen 2006) The concept of individual pitch control was first introduced for helicopter rotor blades (Johnson 1982) Still some disadvantages are evident, especially for the large scale application for wind turbine blades. The response time for individual pitch control systems is not fast enough for high frequency load fluctuation. Moreover, actuation of massive large blades requires significant actuation force and energy. As a result of the significant growth of wind turbines in size, blade load control has become the main challenge for large wind turbines (Nijssen 2006, Johnson 2008 and Barlas 2010) Many advanced techniques have been investigated aiming at developing control devices to ease blade loading. Individual pitch control system, adaptive blades, trailing edge microtabs, morphing aerofoils, ailerons, trailing edge flaps, and telescopic