How Centuries-Old Flywheels Can Improve the Electric Grid

By Chris Ladd in Popular Mechanics, November 24 2009

The 2000-pound cylinder of fiberglass, resin and carbon fiber, glossy as a vinyl record, hangs from a mechanical winch above its thick steel chamber. For millennia, flywheels have powered everything from potter's wheels to steam engines, storing kinetic energy in their momentum as they spin. Now, the flywheel has found a higher purpose in the electrical grid: Wound around a 500-pound rotor, this 5-foot-tall, 3-foot-diameter flywheel assembly at Beacon Power's plant in Tyngsboro, Mass., appears poised to be the great green hope of that unsung, unsexy, absolutely essential energy niche that is frequency regulation.

Capped and sealed and pumped to a vacuum roughly equivalent to that of near-Earth orbit, the flywheel's motor will use excess power in the grid to accelerate the massive wheel to speeds of up to 16,000 rpm, effectively storing the surplus electricity in its own momentum. Then, when grid power dips, it will switch its motor into generation and draw on the inertia of its flywheel, heavy as a Honda Civic and spinning faster than an F-16 flies, to feed the formerly excess power back into the grid. The goal is to keep supply in perfect concert with demand, maintaining the frequency of alternating current at as close to 60 cycles per second as possible. Too much power in the grid drives its frequency up, too little drives it down, and too much variation in either direction means fines for utilities, wasted energy and, in extreme cases, brownouts.

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