BACK PAGE BASICS: Understanding Wind Speed


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Two completely different wind speed measurements are commonly used when talking about wind energy systems: instantaneous wind speed and annual average wind speed. If we mix these up, we will misunderstand wind site assessment, wind turbine ratings, and projections of wind energy available on your site.

Instantaneous wind speed is an immediate, real-time measurement of what the wind is doing—the “right now” wind speed. Today’s measuring equipment may record it every second or even more often, and we can watch the results on a digital or analog monitor. What you’ll probably notice first while observing an instantaneous wind speed readout is that it varies a lot and quickly. The wind speed can bounce around from a few miles per hour to tens of miles per hour in a matter of seconds.

Instantaneous measurements can help determine the maximum (peak) wind speed at your site over a certain time period—an indication of how severe a site is, and therefore how durable the turbine needs to be. Wind turbine designers use instantaneous wind speeds when looking at governing (over-speed protection) and other controls. And power curves correlate with instantaneous wind speeds, predicting the instantaneous power output at given instantaneous wind speeds (see the power curve article in HP127.) For the end user, instantaneous wind speed measurements are not very useful, and can be distracting.

Instantaneous wind speed measurements range from zero to more than 200 miles per hour at extremely severe sites on mountaintops and at sea. Typical residential wind sites might see 80+ miles per hour, once or several times a year.

The annual average wind speed (AAWS) is the primary wind measurement needed, as an average of all the wind experienced at a site, for all seasons of the year. This measurement normally encompasses many years of data.

We are primarily interested in energy (watt-hours), not power (watts; instantaneous rate of generation). Energy is a result of the average wind speed on a site; power correlates with instantaneous wind speeds. Because wind speed varies significantly over any time period, you can expect the power and, therefore, energy, to vary as well.

But it’s even more complex than this, because wind power is a cubic resource. If we double the wind speed, the available power increases eight times. An annual average wind speed takes this into account (using a standardized “wind distribution”), and gives us a measure of wind energy that is reasonably accurate and reflects the total energy available from a variable resource. Typical residential wind energy sites may have annual average wind speeds of 7 to 14 mph, with the high end being quite a windy site. Utility-scale wind sites start in the 12 to 14 mph range; the best sites may approach or exceed 20 mph annual average.

Keeping these two wind measurements—instantaneous and annual average—clear and distinct will help you make sensible analysis of wind sites and wind turbines. Focus on the annual average wind speed (and kWh generated in that wind regime), and don’t worry too much about instantaneous wind speeds, except to be aware of the peaks.

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