Presentation of Wind Data

1. Presentation of Wind Data • The wind energy that is available at a specific site is usually presented on an annual basis. • There are several methods by which this data can be presented. • Some of these methods carry the identical information, presented from slightly different viewpoints. • The goal is to evaluate the energy that can be produced from the wind at a site.

2. Presentation of Wind Data • Wind data is usually summarized by annual averages and totals. • This can be a bit misleading, since seasonal variations are significant. • In some cases, monthly averages are also presented. • For non-grid-connected turbines, the annual peak winds may not correspond to the peak electrical energy demands. • Typical plots that describe annual wind data are shown on the next several slides.

3. Wind Speed Frequency Distribution Plots • One method of presenting wind speed data is to produce a histogram of the number of hours each year that the wind speed is within a certain band. • An example is shown below:

4. Normalized Wind Speed Frequency Diagrams • Rather than report the total time the wind is within a certain band, sometimes the data is normalized by dividing by the total number of hours. • The result is a probability vs. wind speed histogram.

5. The Weibull Distribution • Most wind data is modeled analytically by assuming it follows a Weibull probability distribution function. • If P(V) is the probability of the wind being at speed V, then the Weibull distribution is given by: • The parameters k and C are called the scale and shape parameters, respectively, and are chosen to fit the data. • C has dimensions of velocity, and k is dimensionless. • It is convenient to work with an equation rather than discrete data.

6. The Weibull Distribution Continued

7. The Cumulative Wiebull Distribution • The cumulative distribution Pc(V) gives the probability of the wind speed exceeding a certain value:

8. Plot of the Cumulative Weibull Distribution

9. Development of a Weibull Estimate from Wind Data

10. The Wind Rose • Wind speed frequency diagrams do not provide information on the direction that the wind is blowing. • This is often done by a graph called a “wind rose.” • A wind rose is a polar plot giving the direction, magnitude, and cube of the magnitude of the wind. • The data is usually averaged over a year. • In some cases, shorter time periods, such as months, may be appropriate.

11. Description of the Wind Rose Plot • The polar plot is usually broken up into 12 30° sectors, and average data is presented over each sector. • The first rose “petal” in a given direction is the fraction of the time, normalized to 100%, that the wind is blowing from that direction. • The second petal is the fraction of time wind blows from a particular direction multiplied by the average wind speed in that direction, and normalized to 100%. • The second petal is the fraction of time wind blows from a particular direction multiplied by the average cube of the wind speed from that direction, and normalized to 100%. • The last two petals give information about how “useful” the wind is from a given direction. • For example, if the wind usually blows from a particular direction, but not very hard.

12. Direction Speed Energy Wind Rose Examples

13. Use of the Wind Rose • The wind rose gives a graphical interpretation of much of the data that is considered when evaluating a wind power site. • The second plot (previous slide) shows the wind and most of the wind energy comes from a prevailing direction. • In this case, the site can be designed with this in mind, and turbine technology with limited ability to rotate into the wind can be considered. • Link to Wind Rose Applet: • www.windpower.org/en/tour/wres/roseplot.htm

14. Wind Speed Power Curves for Specific Turbines • A wind turbine power curve shows the rated power as a function of wind speed, as follows:

15. Wind Speed Power Curves • Wind speed power curves, when considered with wind speed frequency diagrams, allow the calculation of the total power that can be produced at a site. • This allows the designer to choose turbine technology appropriate for a given situation. • In addition, the wind rose illustrates the requirements for swiveling, etc.