Philadelphia’s mass transit system used more than 500,000 megawatt-hours (pdf) of electricity in 2011, equivalent to the annual consumption of around 46,000 average U.S. homes. A lot of the electricity went into the city’s two subway lines, but the Southeastern Pennsylvania Transportation Authority, or SEPTA, is now taking innovative steps to try to conserve some of that energy. It has combined a technology called regenerative braking with electricity storage, and other cities are starting to follow suit.



Transportation accounts for about 20 percent of the world’s total energy use and urban transport makes up about 40 percent of that. All those subways, trolleys, light rail and regional rail systems represent a huge opportunity when it comes to regenerative braking. The technology, which also helps power batteries in hybrid cars like the Toyota Prius, works like this: When a train slows, it brakes using its motor rather than friction on the wheels; the motor essentially acts as a generator, turning the kinetic energy of the train back into electrical energy.



The problem is that, in general, the electricity needs to be reused right away. Subway systems can send it out onto the third rail, and if another train is nearby it can use the energy to help it accelerate. But if no other train is ready to tap into that power, it is essentially lost. Other systems dump the regenerative braking energy into banks of resistors on the trains, letting it dissipate as heat. “That excess energy was being wasted,” says Andrew Gillespie, SEPTA’s chief engineer. To address that waste issue, he helped install a 1.5-megawatt bank of lithium ion batteries (pdf) at the Letterly substation in Philadelphia, which was among the first attempts in the world to marry regenerative braking and grid-ready energy storage.



That marriage is growing stronger for a few reasons. Storage technologies like batteries and flywheels have been rapidly improving and dropping in price—an important step for the ongoing growth in renewable energy supply. In addition, utility companies are getting used to differences in how they provide electricity, especially from distributed sources like rooftop solar panels.



Gillespie says the economics of the project in Philadelphia only worked out when SEPTA could participate in the frequency regulation service market, meaning it could sell the power it stored back to utilities to help keep the electric grid running smoothly. At latest count, the Letterly substation alone is saving SEPTA up to $60,000 per year in electricity bills and generating about $200,000 in revenue via the frequency regulation market. And there are 30 substations in all.



Frequency regulation led SEPTA to choose lithium ion batteries as its storage medium, but there are other options that may make more sense for trains and regenerative braking. On Los Angeles County’s Metro, Vycon Energy just finished installing a flywheel system at the Westlake–MacArthur Park station, which should be fully operational by the end of May. Flywheels, mechanical devices that store rotational energy, are an old technology that is starting to come back into vogue for some applications, and the company’s president, Frank DeLattre, says they carry several advantages over batteries. “Large amounts of regenerated electrical energy from the train can be turned to kinetic energy for storage in the flywheel in a short period of time,” he says. “This is key for electric rail, as each braking train can generate two to six megajoules of energy…over a 15- to 20-second braking period.” That’s approaching two kilowatt-hours, or about half of what your iPhone uses in a full year, every time a train pulls into a station. Battery banks that can charge and discharge that quickly, DeLattre says, would have very short life spans: “The flywheels can provide hundreds of thousands of cycles, without loss of energy-storage performance, making them ideal for this application.”



The Westlake–MacArthur Park project was funded by a federal grant of $4.46 million, and the hope is that the two-megawatt system will save as much as 400 megawatt-hours per year—the electricity consumption of about 40 average homes.



Up the coast in Portland, Ore., a city long known for its green tendencies, yet another storage option is taking hold. The new Portland–Milwaukie Light Rail Transit Project line will feature supercapacitors to make use of the electricity from regenerative braking. Supercapacitors have a much higher energy density than batteries, meaning they can charge and discharge extremely quickly; this is useful to meet quick spikes in electricity demand on the rail system. They also can cycle millions of times without degrading whereas batteries have shelf lives on the order of tens of thousands of cycles.



The specifics of a given transit system will likely determine which idea works best, and sometimes even a combination may be in order. Gillespie says that the next installation in Philadelphia, at Griscom substation toward the end of the Market–Frankford line, will combine the lithium ion batteries with a supercapacitor. This will erase a 20 percent shortfall in potential revenue that SEPTA suffered in its first attempt largely due to the somewhat slow energy response time of its batteries (in seconds, rather than milliseconds for a supercapacitor). The hope is to eventually have 10 substations with energy-storage capacity generating enough savings and revenue to finance an expansion of the transit system.



In a 2013 paper on the various technologies available three Newcastle University engineering professors in England noted that on the whole most transit systems could likely reduce their energy usage by 15 to 30 percent with some version of these ideas. That’s a lot of dollars, a lot of megawatts and a lot of tons of carbon dioxide, so many other cities are taking note. DeLattre says Vycon has been in talks with several transit authorities in the U.S. as well as in China and South Korea, and Gillespie has spoken with counterparts in Chicago, Boston and cities in Spain about SEPTA’s experience. He doesn’t think it will stop there either.



“I think as the industry moves forward, and there is an emphasis on energy conservation, you’ll start to see this implemented more and more,” he says.

