Resource assessment campaigns historically involved installing a met tower and monitoring conditions for about a year to capture data points, such as wind speed and direction, at 40 meters to 60 meters, and then extrapolating the captured data to estimate the wind resource at hub-height, typically at 80 meters. However, because hub-height conditions were typically not directly measured, the extrapolations often resulted in a developer making, in essence, an educated guess.Â
As technological innovation progressed, developers then began to account for deeper meteorological phenomena, such as wind shear, which reflects the variation of wind speeds at various heights. Information about wind shear makes assessments more precise, but data extrapolation is more difficult to achieve.Â
And the stakes could not be higher: If a developer's calculations are off, the potential wind location's average wind speed and overall energy production estimates will also be incorrect. Because lenders and investors are scrutinizing the validity of long-term energy estimates, it has become more essential to corroborate the hub-height extrapolations.Â
Several industry efforts are underway to drive the uncertainty out of wind resource assessment campaigns. In mid-January, Hinesburg, Vt.-based NRG Systems unveiled an 80-meter tilt-up tower, which the company says will allow for more precise hub height measurement. For example, some developers are now beginning to install a series of met towers at varying heights to obtain more precise wind data.Â
With many of the most obvious locations taken already in North America, explains Jean-SÃ©bastien Naud, operations coordinator for the wind resource assessment group at Hatch (formerly GPCo), the locations with more complex terrains need to be carefully analyzed.Â
‘Any developer serious about getting funding should install an 80-meter tower,’ Naud says. For smaller-scale projects, developers can get by with a 60-meter tower. However, he says, for anyone that is really serious, using an 80-meter tower is the best practice. "Installing an 80-meter tower could cost anywhere from $55,000 to $100,000, as well as be subject to a review by the Federal Aviation Authority (FAA). Any tower exceeding 200 feet in height crosses a threshold where the FAA could require lighting atop the tower.Â
Additionally, the wind industry is increasingly moving to more sophisticated methods, such as remote sensing devices, that can alleviate much of the uncertainty regarding extrapolation. A sonic detection and ranging (SODAR) system, for example, measures wind speed and direction across the full-swept area of utility-scale wind turbine blades by sending a series of sound waves into the atmosphere and computing the Doppler shift in their return signals.Â
As new players get involved in the wind power industry and the market becomes more competitive, it is critical for developers to use up-to-date methods to plan for future projects.