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Philly subway to capture energy from braking trains

The smart grid goes underground in Philadelphia, where a battery will store energy from regenerative braking, cutting power costs and earning revenue on the grid.

Martin LaMonica Former Staff writer, CNET News
Martin LaMonica is a senior writer covering green tech and cutting-edge technologies. He joined CNET in 2002 to cover enterprise IT and Web development and was previously executive editor of IT publication InfoWorld.
Martin LaMonica
2 min read

Here's an unusual way to upgrade a subway system: add a giant battery.

Viridity Energy said Monday that it has been awarded a $900,000 grant by the Pennsylvania Energy Development Authority to build a system that will capture the energy from Philadelphia subway cars as they brake to enter a station.

CC Infowidget/Flickr

The regenerative braking system will collect energy in a large battery installed along the busy Market-Frankford Line. The stored energy will be used to power trains when they leave the station and to earn money from energy sold back to the grid.

"Essentially we're creating a microgrid that is integrated with the transmission grid and operated so that its optimized for efficiency and economics," said Audrey Zibelman, president and CEO of Viridity.

The project, which Viridity hopes to be operating by next spring, will have a battery with between 1 megawatt and 1.5MW of power, intended to replace the current system, which cannot capture all the energy from incoming trains.

With the battery in place, the system can power trains when they leave, cutting down on operating costs for Southeastern Pennsylvania Transportation Authority (SEPTA).

The battery will be able to make money, too, by providing services to the grid. Using its stored energy, it can make money from grid operator PJM by supplying quick bursts of energy to maintain a steady frequency. SEPTA can also draw energy from the grid at off-peak times and supply it at peak times when the utility is looking to lower usage because energy prices are high.

Viridity's hosted software is like a "network operating service" that optimizes how the energy is pulled into and dispatched from the battery, said Zibelman. The company, which makes money by getting a percentage of customers' revenue, is now evaluating what types of batteries it will use, she added.

"Electric vehicles are on everyone's mind right now as where we need to go, but we have an electric vehicle system already sitting here. Let's use those first," Zibelman said. "It's something could be done in almost any transit system.

SEPTA estimates that it can save $500,000 a year on its electricity spending. If the project is successful, SEPTA hopes to replicate the model system-wide, Joseph Casey, SEPTA general manager said in a statement.

Updated at 9:45 a.m. PT with corrected term for the battery's expected power.