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WIND ENERGY SYSTEMS - Cd3wd

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Chapter 4—Wind Turbine Power 4–21<br />

b =<br />

P eR<br />

u k R − uk c<br />

(22)<br />

As mentioned in Chapter 2, the Rayleigh distribution is a special case of the Weibull<br />

distribution with k = 2 and is often sufficiently accurate for analysis of wind power systems.<br />

This value of k should be used if the wind statistics at a given site are not well known.<br />

AplotofP e versus u is shown in Fig. 16, for k =2. P e varies as u k between the cut-in and<br />

rated wind speeds. It is then assumed to be a constant value between the rated and furling<br />

wind speeds. At the furling wind speed u F the turbine is shut down to protect it from high<br />

winds.<br />

Figure 16: Model wind turbine output versus wind speed.<br />

5 <strong>ENERGY</strong> PRODUCTION AND<br />

CAPACITY FACTOR<br />

We have seen that the electrical power output of a wind turbine is a function of the wind<br />

speed, the turbine angular velocity, and the efficiencies of each component in the drive train.<br />

It is also a function of the type of turbine (propeller, Darrieus, etc.), the inertia of the system,<br />

Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001

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