Tham khảo tài liệu 'wind power 2011 part 7', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Small Wind Turbine Power Controllers 167 wind turbine we have to optimize the value of CP. The coefficient of performance is a strong non- linear function of the Tip Speed Ratio as seen in Figure 2. It depends on such factors as the number of blades and the pitch and shape of the blades. The maximum value CP can attain is theoretically around 57 however the highest performing turbines generally only attain values in the 40 range while values in the 30 range are more common. The tip-speed-ratio is the linear speed of the outer extremity of the turbine blade divided by the wind speed. For a turbine of radius R and rotor speed fflrotor in radians sec the tipspeed-ratio TSR is thus TSR arotorR V . wind 2 Where Vwind is the speed of the wind at the turbine hub height. In order to maximize Cp we can vary the load on the turbine to achieve a rotor speed fflopt of TSR V . iSRoptV wind R 3 The optimum tip-speed ratio TSRopt is a constant dependent on turbine blade design for fixed blade turbines. Thus for every wind speed there will be a different rotor speed for maximum power production. The optimum TSR value is around 5 to 7 for most small turbines meaning that the tip of the turbine blade will have a linear velocity around 6 times the wind speed. Most small wind turbines have 2 or 3 fixed blades rotating about a horizontal axis directly driving a permanent magnet generator. The control system generally consists of a mechanical system for furling the turbine in high winds and with perhaps a power controller and electrical breaking system. Bergey Windpower s 10kW Excell turbine is a notable exception having a mechanical blade pitch control system which adjusts the pitch of the blades as a function of rotor speed Manwell et al. 2002 . Larger turbines commonly have 2 or 3 adjustable pitch blades and 1 or 2 gear stages between the rotor and the generator. A larger number of blades gives a lower operating speed and greater starting torque facilitating turbine starting in low .
