Energy Storage Could Make Electric Trains More EfficientEnergy Storage Could Make Electric Trains More Efficient

Utility companies have long known how to capture the braking energy from electric trains, but they've usually found it difficult to get any large-scale benefits from such projects -- until now.

An energy storage project in Philadelphia promises to capture braking energy and provide a voltage boost for electric trains, while offering so-called "frequency regulation" for utilities. The companies behind the project hope that it will reduce the need for fossil fuel generation, thereby creating economic and environmental benefits. "The central aspect of the Philadelphia system is that it doesn't just do regenerative braking and power-assist," Jim McDowall, business development manager for Saft, told us. "It helps with the demand side of the equation."

The new system could help utilities use regenerative braking power more intelligently. In the past, regenerative braking energy from train motors was typically pushed back into the electric train's third rail, raising the voltage. When the voltage rose too high, however, excess energy was diverted through coils and dissipated as heat.

The Recycled Energy and Optimization project takes a different approach. Using a dc/dc converter from Envitech Energy, it senses when track voltage is too high (about 800V). It then sucks DC power out of the third rail and pushes it into a giant lithium-ion battery pack provided by Saft, until it's needed. "When the third rail's voltage drops too low, they can bring the power out of the battery and then put it back in the third rail," McDowall said.

Saft's batteries are located in a substation that serves five or six stations on the Southeastern Pennsylvania Transportation Authority's (SEPTA's) electric elevated train line. Using lithium-ion batteries with a well-known nickel-cobalt-aluminum (NCA) chemistry, the substation offers about 1.5MW of charge and discharge capability. Saft employed a high-power (rather than high-energy) battery configuration, largely because big electric trains on the line can capture 3MW during a 15-second period.

The company's Max20 lithium-ion energy storage system is roughly equivalent in size to 280 Toyota Prius battery packs. It employs 290 modules, each with 14 cells rated at 30 A-h.

The big potential benefit of the new system, however, goes beyond its ability to capture braking energy. By employing software from Viridity Energy and piggybacking a frequency regulation signal atop the power flow information, the system can modulate the loads at the substation in response to signals from the grid operator. As a result, it can change the amount of power that gets fed into the substation, and therefore eliminate the need to dissipate excess power as heat. "So a megawatt of storage might displace a megawatt of fossil fuel generation," McDowall said. "That means less cost for the market and less cost for society."

Engineers who are part of the pilot project believe that strategy could provide a return on investment that wouldn't otherwise be available by simply capturing braking energy and pushing it back into the third rail. "If we can prove the value proposition, then other cities will look at this for their substations, as well," McDowall said. "This is a very high-visibility project."

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