The design tip speed ratio depends closely on the number of rotor blades. The maximum power coefficient of a three-bladed rotor is reached at a tip speed ratio of between 7 and 8, whereas for two-bladed rotors the figure is 10 and for a one-bladed rotor about 15. However, these values can vary depending on the system design. The optimum tip speed ratio Aopt also defines the design wind speed along with the rotor radius r and the rotational speed of the rotor n in min-1.
For example, the optimum tip speed ratio of a three-blade wind turbine with a rotor radius of r = 22 m and a rotational speed of n = 28 min-1 = 0.467 s-1 is Aopt = 7.5, and therefore the design wind speed is vD = 8.6 m/s. The wind turbine has its maximum efficiency at this wind speed. The design wind speed is very important for wind turbines with constant speed. Systems with variable speed can obtain the optimum efficiency at other wind speeds also. In this case, the design wind speed range should replace the design wind speed.
At very low wind speeds the operation of the wind turbine makes little sense. No power or only a very little power can be taken from the wind and the wind generator can even become a power consumer. Therefore, the rotor brake should stop the wind turbine below a predefined starting wind speed, or cut-in wind speed vcut-in.
The design wind speed vD was explained above; the rated or nominal wind speed vN of a wind turbine is usually different. At the nominal wind speed, the wind turbine generates the rated power. The nominal wind speed is usually higher than the design wind speed. Above the nominal wind speed, the power of the wind turbine must be limited. If the wind speed becomes too high, the wind power plant can be overloaded and damaged. Therefore, wind turbines cut out at high wind speeds, vcut-out: the rotor brakes stop the wind turbine o.b
Wind speed at hub height in m/s
Wind speed at hub height in m/s
Source: data from Enercon, 1997
Figure 5.13 Generator Active Power and Power Coefficient against Wind Speed for the 500-kW Enercon E-40 Wind Generator and the rotor is turned out of the wind if possible. The different wind speeds have the following typical ranges:
It must be considered that some companies give their generator curves for wind speeds at a height of 10 m, others at hub height (40-100 m). Figure 5.13 shows the generator active power and the power coefficient for a variablespeed 500-kW wind turbine as a function of the wind speed.
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