Feeding energy into the electricity grid could prove more valuable than using it to power trains. Grid operators have to balance supply and demand. They now do this by sending signals to power stations that have agreed to operate at less than full capacity, so that the stations can quickly increase or decrease power output, sometimes for only a few seconds at a time. But it takes time for power plants to ramp power up and down, which can make it difficult for grid operators to keep up with fluctuations in demand. A battery can provide jolts of power "almost instantaneously," says Kevin Morelock, director of IT and projects at Viridity.

Providing short bursts of power in response to signals from grid operators is a service called "frequency regulation." At times when electricity prices are high, such as on hot days when demand is high, it may be more lucrative to stop offering frequency regulation and to simply sell power back to the grid instead.

The pilot project, which is being funded by a $900,000 grant from the Pennsylvania Energy Development Authority, faces two primary technical challenges. The first is selecting a battery that can quickly absorb and discharge electricity, potentially for hundreds of thousands of cycles, Morelock says. Viridity is currently evaluating different battery technologies, such as lead acid (which is cheap, but not long-lasting) and lithium-ion (which is being developed by companies such as AltairNano and A123 Systems).

The other challenge is developing software that can quickly analyze data from SEPTA's system, and from various electricity markets, and decide how best to use the stored energy. Viridity has already developed similar software for managing buildings, drawing power from solar panels or moderating power demand from air-conditioning systems and other loads to help stabilize the grid.

Mark Duvall, the director of electric transportation and energy storage at the Electric Power Research Institute in Palo Alto, CA, says the project has set an "aggressive target" for energy savings. But a lot will depend on how much regenerative braking capacity can be harnessed, he says, and being profitable will depend on the cost of the batteries, although these costs should decrease rapidly in coming years. "It's good that they're looking into this. What they learn can help other transit agencies," he says.