Beacon Power Corporation has just completed the first installation of its technology at a wind farm in Tehachapi, California. The system will provide frequency regulation for the energy that is produced by the wind so that it is modulated for reliable grid use. When the wind is blowing the flywheels will take on excess capacity, acting as a load on the system. That energy is used to spin the flywheels to as fast as 16,000rpm or twice the speed of sound. As the wind energy slows, this energy is then captured back out of the flywheel via its own internal generator. Each flywheel starting from full rotation has the capacity to power a typical house for a day. They are arranged in series, each one a little over a meter in diameter and 3 about meters tall.
Beacon Power's news release on the installation is here.