San Francisco International Airport Commissions Wayside Flywheel Energy Storage System on AirTrain Guideway

Electrical engineers at San Francisco International Airport (SFO) and municipal power specialists with the San Francisco Public Utilities Commission Hetch Hetchy Power division commissioned a 1.5-megawatt wayside kinetic flywheel energy storage bank on Saturday morning inside the Central Garage Traction Power Substation serving SFO's automated AirTrain people mover. Operating 24 hours a day across a six-mile elevated guideway connecting all four passenger terminals, the Grand Hyatt hotel, the Rental Car Center, and the Millbrae/SFO BART transfer station, the fleet of 38 driverless Bombardier Innovia APM 100 cars completes more than 1,400 station stops every day.

Previously, when multiple three-car trains braked simultaneously into Terminal 2 and the International Terminal stations, excess regenerative electricity that could not be immediately absorbed by an accelerating train on the same 600-volt AC power rail was dissipated as waste heat through rooftop resistor banks. The new wayside substation array routes that braking current into six magnetically levitated carbon-fiber rotors spinning at up to 36,000 revolutions per minute inside vacuum-sealed steel containment vessels.

Seconds later, when departing trains accelerate out of the station on 3.5 percent guideway grades, the flywheels discharge up to 1,500 kilowatts of stored kinetic energy back onto the power rail, trimming peak substation electricity demand by 28 percent and saving an estimated 1.9 gigawatt-hours of power annually. Unlike chemical lithium batteries, the solid-state kinetic rotors require zero hazardous cooling fluids and are rated for 10 million full charge-discharge cycles over a 25-year service life.

The $3.8 million traction energy recovery project advances SFO's Net-Zero Energy campus roadmap and was partially funded by a Federal Aviation Administration Zero Emissions Vehicle and Infrastructure grant. Travelers can view SFO sustainability metrics and live AirTrain system maps on the SFO Sustainability and Climate Action Portal.