Small Wind Site Assessment: Lessons Learned
June 17, 2013
Growing concerns about energy costs and environmental impacts have increased the demand for small wind energy systems for homeowners, schools, businesses, and local governments. Over the past decade, the knowledge, skills, and technologies utilized in the proper siting of wind turbines have evolved into a science that can aid in the optimization of a system, allowing it to produce at a peak performance level over a long lifetime. Author Frank Oteri compiled the following lessons learned from recent interviews he conducted with small wind turbine installers and developers regarding site assessments.
Although wind resource maps have improved, site visits are still necessary. The resolution and accuracy of today's wind maps provide an initial estimate for the wind resource of a selected site, but they are not enough to ensure a project's success. According to Pieter Huebner, service manager at Bergey Windpower, inexperienced developers often rely too much on wind maps. When a wind map shows a good resource, they assume that "everywhere is that shade of color." Huebner said that although he thinks the models get better every year, one should not assume that every site will have the resource depicted on the map. Site visits are still recommended.
Relying on Google Earth to discover surrounding structures or obstacles can be a mistake. Technology has become more accessible and hands-on in recent years, but according to Lisa DiFrancisco of North Coast Energy Systems, visiting the site is critical to ensure a successful project.
"Walking on the site, you find that what you see when you get there is often not what you saw on the overhead because it's not quite up to date," DiFrancisco said. "Someone might have built a new building, grown some trees, or even cut some trees down. Maybe the hills are steeper than you imagined, or the site is closer to a set of train tracks than you thought, or there could be a utility right-of-way that wasn't on the map. In my view, the physical site assessment is the single most important thing you can do prior to an installation."
Turbulence can shorten a turbine's life. Wind turbulence is the rapid disturbances or irregularities in the wind speed, direction, and vertical component. It is an important site characteristic because high turbulence levels may decrease power output and cause extreme loading on wind turbine components, resulting in a shorter life, or increase the long-term maintenance costs for a project.
"I've seen energetic sites where installers have chosen shorter towers than they should have, and the turbine sees a lot more mechanical use and turbulence," Huebner said. "Even though the production seems to be as promised or better, the reliability of the turbine and the amount of maintenance required is more than expected because of the turbulent location."
Wind resource measurement is an investment and should be considered under the correct circumstances. When trying to decide whether to install an anemometer and conduct a year-long wind measurement study for your site prior to installing your small wind energy system, you must consider multiple factors.
"It comes down to uncertainty and risk," said Charles Newcomb, director of technical strategy for Endurance Wind Power. "Where my margins are really thin, I should be more careful."
Although Newcomb believes that turbine sizes do not dictate when wind resource measurements should be conducted, Huebner feels that the larger small wind turbines may require further due diligence.
"When you start looking at the 50-kilowatt turbines and above and there's doubt about the wind resource, because of the size of the investment it's probably justifiable to do an actual wind study at the site," Huebner said. He added that the investment is hard to justify for the smaller wind turbines.
Obstacles in the primary wind energy direction have an increased impact on the production of a project by altering the resource or increasing turbulence. There are multiple ways to help avoid this potential conflict, including siting the turbine in a more open area or increasing the tower height. Experts recommend that turbines be sited upwind of buildings and trees and installed 25 to 35 feet above anything within a 300-foot horizontal radius.