Supercapacitor-Enhanced Hybrid Storage to Earn Cash for Subways

Photo: SEPTA

A moving train represents a significant amount of energy, which is often lost as carriages slow to stop at a station. Trains in the Philadelphia subway are not only capturing that energy in banks of batteries but also selling it to the local grid operator. This fall, it’ll be capturing even more energy—maybe earning more money from grid operators—because it plans to upgrade the system with a hybrid of both lithium ion batteries and supercapacitors.

The Southeastern Pennsylvania Transit Authority (SEPTA) stores energy produced by braking railway cars, much the way a hybrid car juices its battery when slowing. The spinning wheels turn a motor-generator to charge a bank of batteries via a third rail system. The battery is located at the Lettery substation, which powers a portion of the Market-Frankford Line in Philadelphia, and the autumn upgrade will be installed on the same line about 5 kilometers away.

Capturing energy from braking railway cars, which can reduce energy use by to 30 percent, is relatively new technology, says Jacques Poulin, director of energy storage for public transportation at ABB, which is the systems integrator on the SEPTA project. The Oslo Metro system, for example, sends energy from braking into its network, but it’s still unusual for rail authorities to capture and store that energy, he says. What makes the Philadelphia project unique in the world is that the battery also provides balancing services to the local electricity grid.

"We connect the energy storage to the frequency regulation market locally in order to use that asset 24 hours a day and make money for the train company over and above recovering the braking energy of the train," Poulin says.

Grid operators need to maintain a balance between power supply and power demand to ensure the grid's frequency remains within a narrow band. Typically, natural gas power plants supply power into the grid to match demand and thus, maintain the right frequency. But batteries have begun to fill that role and they have the advantage of providing burst of power very quickly.

In the case of SEPTA, the existing lithium-ion battery can ramp up to its full 1.5 MW of power within one second, compared to a few minutes for a thermal power plant, says Poulin. Local grid operator PJM has implemented a new Federal Energy Regulatory Commission order that pays more to assets that can provide fast frequency regulation services. That makes the economics of the system compelling: SEPTA projects it will earn between $150 000 and $200 000 annually from its battery.

Using software from Viridity Energy, the battery will take signals from PJM and either supply or absorb power for a few minutes at a time.

The addition of the supercapacitors (or, ultracapacitors), which store charge electrostatically rather than chemically, means the trains will be able to capture more energy during braking.

Supercapacitors can rapidly store or discharge power, but are less energy-dense than batteries. So supercapacitors are well suited for recuperating the energy of a braking train or hybrid bus, says John Meaney, an account executive at Maxwell Technologies, which is supplying supercapacitors to the SEPTA project. Including supercapacitors will also reduce the strain on the battery and extend its life, he says.

The existing energy storage system, which has been in place since 2012, can recover between 1.5 megawatt-hours and two megawatt-hours a day. "That amount of energy is equivalent to a solar system that would cost at least twice as much as the cost of our system," notes Poulin. If several energy storage units are put in place, they could provide backup power as well, he adds.

The project has gotten attention outside Philadelphia because it’s been earning money for a municipality, many of which are starved for revenues. In theory, it could be replicated elsewhere, although it’s worth noting that PJM, which serves 13 states in the mid Atlantic U.S., has been very progressive in adopting new rules for frequency regulation.

That means rail authorities in other regions or countries wouldn’t necessarily have the same earnings potential from an energy storage device—at least not until other grid operators start to pay more for fast frequency response services. But if nothing else, other cities may look at the SEPTA project and decide its time to stop wasting all that train energy.

Photos: SEPTA

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