But the system is designed to do more than capture power from the subway's dynamic braking system, said Audrey Zibelman, the chief executive officer of Viridity Energy Inc., the Conshohocken smart-grid innovator that devised the project for SEPTA.
The power-storage system is potentially so large - each battery array would store one megawatt of power - that SEPTA could further reduce its electric bill by buying cheap power at night to use or resell during expensive peak hours.
And SEPTA also could collect fees from the regional grid operator, PJM Interconnection L.L.C., by providing power on short notice - one or two seconds - to stabilize regional power flows on the grid.
Zibelman said the transit agency could realize energy savings "substantially higher" than the 20 percent target Viridity typically sets for a project. SEPTA currently spends about $20 million a year to buy electricity from Peco Energy Co. to power its trains, subways, and electrified trolley and bus fleet.
The pilot project, involving a single battery array at a SEPTA electric substation in Kensington, would cost about $1.5 million.
The Pennsylvania Energy Development Authority is underwriting the project with a $900,000 grant. Viridity will underwrite the remaining capital cost, Zibelman said.
If the project proves economic - Viridity estimates one battery array will generate $500,000 a year in value - SEPTA envisions installing the technology at all 33 electric substations that serve its subway and trolley lines.
The system will take advantage of regenerative-braking capacity already installed in the Market-Frankford Line and SEPTA's electrified buses and trolleys.
When applied, the brakes now convert the train's kinetic energy into electricity, which is transmitted into the third-rail system for use by other trains.
But when there are no other trains nearby to consume the electricity, the power is lost. Excess electricity from the brakes is converted into heat that is dissipated from vents in the carriage rooftops.
About half the power produced by the regenerative brakes is now lost, Gillespie said.
SEPTA says the batteries, in addition to capturing the regenerated power that is now lost, will help increase the system's electrical efficiency and stabilize the voltage - it operates on 600-volt direct-current power.
Zibelman envisions other transit agencies' adopting Viridity's technology.
"This is an opportunity," she said, "for us to develop a market."
Contact staff writer Andrew Maykuth at 215-854-2947 or firstname.lastname@example.org.