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Potencijal vetra

wind potentialBased on the measurement of the wind it is possible to estimate wind potential. Using sophisticated software the wind potential report summarizing the observed wind resource is normally produced on a monthly, quarterly, or annual basis, as well as at the end of the monitoring program.

The mean wind speed - The mean wind speed is simply the average of the valid speed values for the period. The mean wind speed can sometimes be a misleading indicator of the wind resource, however. If the data span a period much shorter than a full year, the mean will not reflect the full seasonal cycle of wind variations.

Wind shear - The wind shear (the rate of change in horizontal wind speed with height) is typically expressed as a dimensionless power-law exponent known as alpha (α). Time-averaged shear exponents can range from less than 0.10 to more than 0.40, depending on land cover, topography, time of day, and other factors. Complex terrain usually produces higher shear, except on exposed ridges and mountain tops where topographically driven acceleration can reduce shear.

wind shear

Weibull distribution – The speed frequency distribution is a critical piece of information as it is used directly in estimating the power output of a wind turbine. The frequency distribution represents the number of times in the period of record that the observed speed falls within particular ranges, or bins.

A wind and energy rose indicates the directional distribution of the wind resource, which can strongly influence the turbine layout.

probabillity distribution

Wind power density - defined as the flux of kinetic energy in the wind per unit cross-sectional area. Combining the site's wind speed distribution with air density, it provides an indication of the wind energy production potential of the site.

Air density - The air density depends on temperature and pressure (thus altitude) and can vary by as much as 10% to 15% seasonally.

Turbulence - defined as rapid fluctuations in wind speed and direction, can have a significant impact on turbine performance and loading. The analysis shows how the turbulence in the wind varies with wind speed, wind direction, month, or hour of day.

The 3rd edition of 61400-1 standard of the International Electrotechnical Commission, released in 2005 provides a tabular reference for four turbulence categories based on mean turbulence intensity at a wind speed of 15 m/s:

turbulence categories

Wind rose - The directional distribution of the wind resource is a key factor affecting the design of a wind project. In most projects, the spacing between turbines along the principle wind direction is much greater than the spacing perpendicular to it. This configuration maximizes the density of wind turbines while keeping wake interference between the turbines, and hence energy losses, manageable.

Picture bellow shows typical wind parameters - direction and speed which is usually defined through wind rose. The wind rose might be calculated for every month or whole year.

wind rose

The return period - defined as the reciprocal of the probability of exceedence. This is a useful concept in the field of extreme value analysis.

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