Philadelphia Metro System to Sell Power Back to Grid

Tough economic times mean lower revenues for America’s public transportation systems. Government funding and ridership drop in tandem with tax revenues and incomes, and many transit systems have recently had to raise fares and cut back on services to stay afloat.

But the old saying, “Necessity is the mother of invention,” is true also in the world of buses and trains.

In Philadelphia, a city of 1.5 million, the Southeastern Pennsylvania Transportation Authority (SEPTA) has teamed up with a local company specializing in smart grid technology to explore how to harness electricity from braking subway trains. The pilot project with Viridity Energy is expected to bring the Philadelphia subway system an extra $500,000 annually — a figure that could swell to millions if and when it expands.

The key to this innovation is a battery or similar energy storage device that will be installed at an electric substation along the subway tracks. The device will capture and store energy created whenever a train hits the brakes and forward it to other trains that must pick up speed, saving the subway system on electricity costs.

But here’s the best part: Whatever power remains in the battery can then be sold back to the local electric grid. Viridity’s smart grid software would signal to SEPTA when there’s a high demand for electricity in the local power grid, for example when air conditioners are turned up on hot summer days, and the price for power is right.

That could create a new revenue source for SEPTA and other cash-strapped metropolitan transit systems. The subway smart grid system would also help the transit system reduce its greenhouse gas emissions. SEPTA has pledged toreduce its carbon footprint 5% annually (PDF, 127KB) between  2010 and 2014, using 2009 as a baseline year. The project with Viridity Energy should help the system reach that goal, officials say.

Today’s substations already transfer brake-generated power back to the subway system through overhead lines or a third rail and use it for trains that need to accelerate. That can occur, however, only if the train that receives the electricity picks up speed at the same time that the other train hits the brakes — or the power will be lost. It’s estimated that at least 80% of the energy generated by braking locomotors is now wasted as heat.

“In Pennsylvania, in particular, where we have mainly coal-generated electric power, every kilowatt-hour of power that is not consumed matters,” said Erik Johanson, a SEPTA strategy and sustainability planner. “We’ll essentially be recycling electricity.”

Another potential benefit, Johanson said, is that by capturing and reusing electricity, subway systems can avoid a drop in voltage during high-traffic hours — a problem that sometimes brings trains to a halt, frustrating travelers.

SEPTA received a state grant in August that will pay for the first battery, and batteries currently cost as much as $1 million or more apiece. The subway system expects to have its technology in place for testing by next summer.

The transit authority has also applied for a $2.7 million grant from the Federal Transit Authority, that could pay for batteries at other substations throughout its system.

A number of other public transit authorities have contacted SEPTA to learn more about the subway power project, intrigued by its potential, said Andrew Busch, a SEPTA spokesman. “They’re keeping an eye on it and want to see how we’re doing,” he said. “Hopefully, we’ll have a success story to share with them.”

(This is a product of the Bureau of International Information Programs, U.S. Department of State.)

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