When gusts threaten, City Hall's turbine bows out

Well, the brakes work.

Days after a wind turbine started spinning on the roof of Boston City Hall, its rotor is motionless. City officials say wind gusts earlier this week triggered the turbine's safety mechanism, shutting it down.

The 40-foot tall turbine is designed to withstand winds of up to 80 miles per hour, according to its manufacturer, but once wind speed exceeds 20 to 25 miles per hour, the rotor slows as a protective measure.

"It was going like gangbusters when we installed it," said James W. Hunt III, Boston's chief of environmental and energy services. "When it gets too windy, the clutches kick in and then it needs to be reset."

The turbine is more symbolic than practical - it only generates about 2 kilowatts of electricity for City Hall, enough to power about 19 100-watt lightbulbs. Earlier this year, engineers shot down Mayor Thomas M. Menino's plan for a larger turbine at City Hall Plaza, saying winds in the area were too turbulent.

Officials said they haven't finished calculating the price of the rooftop project, but estimate it at $13,000. A $5,000 grant from an international energy company was used to partially fund the installation. The turbine itself was donated to the city by Southwest Windpower, an Arizona company.

Andy Kruse, a Southwest Windpower vice president, said in addition to high winds, it is possible the shutdown was caused by a surge in electrical power, or a technological glitch.

"If it [the turbine] gets confused, it just says 'Wait, I gotta figure this out. I need some help,' " Kruse said, and it will either slow its rotors or stop.

Hunt couldn't say when the turbine would be reset, although he noted the process is easy.

"It's just like hooking a laptop up," he said.

But it might be wise to hold off on the restart - today's forecast calls for northeast winds that could reach or exceed 20 miles per hour.

Erin Ailworth can be reached at eailworth@globe.com.