A.J. Neste

A giant battery bank installed by the side of the Southeast Pennsylvania Transit Authority’s subway tracks a little over a month ago is saving about nine megawatt-hours of power a week, its manufacturer says, which is more electricity than the typical apartment-dweller uses in a year.

The battery system, which I wrote about last year, is allowing the trains to run a bit like Prius hybrids. When they slow down at a station, their motors turn into generators, converting torque into current. Before the battery bank was installed, some of that current was returned to the third rail; but if the voltage got too high, it was shunted instead into a giant electric heater under the train, which simply dissipated the energy as heat.

Now the battery captures excess current, about 3.5 to 4 kilowatt-hours per train that stops, and puts it back on the line when a train is accelerating. Sometimes it does this for several trains at once.

The battery bank is also receiving signals every four seconds from the regional grid operator and either absorbing energy or giving it back to the grid to help balance supply and demand.



With the house current known as AC, for alternating current, electrons change direction at 60 cycles per second; if there is more supply than demand, they run a bit faster, and if there is more load than supply, they run a little more slowly. Essentially the battery owners get paid for helping keep the system as close to 60 cycles as possible.

Jim McDowall, the business development manager at Saft, which built the battery, said the combination of energy storage plus grid balancing would make the project profitable. The nine megawatt-hours cost about $90 each, he said, which would put the weekly savings in the range of $800.

As a storage device, the battery is more like a peanut butter jar than a wine jug; electrically speaking, it has a wide mouth and can empty and fill quickly, although its volume is small. It can accept or send out power equal to 1.5 megawatts, which is enough to light up a good-size shopping center, but it holds only 420 kilowatt-hours. That’s plenty for this application, however.

Officials held a ribbon-cutting ceremony for the battery bank on Tuesday. Saft and the developer of the system, Viridity Energy, are hunting for other applications.