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Presentation of Wind Data

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.

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Presentation of Wind Data

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  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.

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