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Sonic sound scatter -- Wind monitoring (1)

After years being considered a technology on the brink of commercial success, sodar systems for use in wind measurement are being regarded by some as essential to wind project development. Sodar is like the more familiar sonar technology used in submarines that shoots out pulses of sonic waves and analyses the waves reflected back to the source. In sodar, the medium is air instead of water and reflection is due to the scattering of sound by atmospheric turbulence. It provides three-dimensional data of wind speed over an area far higher and wider than covered by anemometers fixed to wind monitoring masts, which for years have been the industry's standard resource measurement tool.

"It's still on that acceptance curve, and it's still not accepted by everyone, but it's much more widely embraced now. More people have used it for themselves and realise that it indeed gives you reliable results," says Bruce Bailey, president and CEO of AWS Truewind, a company specialising in wind monitoring for project developers and owners. Bailey says his New York based consultancy has done more than 150 site qualifications for wind speed with sodar -- more than any other company in the US and Europe. The results when compared to data collected by anemometers at the same locations are off by no more than 2-3%. "That's excellent, and most of that difference is not resolvable," he says.

Improved accuracy is one reason for sodar's success, but increasing turbine size plays a big role. The average tilt-up wind measurement mast is 60 metres tall and can cost between $15,000 and $35,000 per unit. But the average hub height of a new turbine is 80 metres, with blades stretching nearly 50 metres above that. Data from 60 metre masts is often estimated or "extrapolated" to greater heights, but with less precision than instrumentation.

Tubular anemometry masts above 60 metres for the wind industry exist, but any taller than that and it is necessary to turn to more expensive lattice-style structures that can cost as much as $80,000 for large units. In the US, anything taller than 60 metres triggers a review from the Federal Aviation Administration and requires aviation lights. A rule of thumb is one mast is needed for site assessments for every 10-20 MW of wind plant, depending on geography.

A handful of companies offer Sodar systems, including Atmospheric Research and Technology, Atmospheric Systems Corporation, Remtech, Scintec, and Second Wind, with prices ranging from $35,000-$75,000 a unit depending on configuration. AWS has used most of the units available today, owns two of its own, but often crunches the data from sodar systems leased by developers.

"We don't have any partnership or vested interest in any sodar manufacturer. We like to stay independent so we are free to go and evaluate," says Bailey. Sodar's few drawbacks are that it does not work in rain or snow and the data streams are more complex than those collected by anemometers.

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