Subway Trains to Generate Power for the Grid

A battery will capture power from braking trains.

The Southeastern Pennsylvania Transportation Authority (SEPTA), which runs the transit system in Philadelphia, is piloting a smart electrical grid technology that could cut its electricity bills by up to 40 percent and generate millions of dollars a year.

Powering down: A new project will harness energy from subway trains like this one on Philadelphia’s Market-Frankford Line.

A massive battery installed at one of the authority’s substations will store electricity generated by the braking systems on trains (as the trains slow down the wheels drive generators). The battery will help trains accelerate, cutting power consumption, and will also provide extra power that can be sold back to the regional power grid. The pilot project, which involves one of 38 substations in the transit system, is expected to bring in $500,000 a year. This figure would multiply if the batteries are installed at other substations.

The project shows how cash-strapped public transit agencies that operate major subways and electric rail systems could find a new source of income by tapping into the smart grid. It also highlights one way that the smart grid could save energy, avoid blackouts, and incorporate more renewable power.

Several energy utilities are already experimenting with using large batteries to help smooth out fluctuations in electricity supply, keeping the grid operating at the correct frequency and preventing blackouts. In the new pilot project, this kind of battery technology will be paired with software from a Philadelphia-based smart grid company called Viridity Energy.

The software will decide how to allocate the power stored in the battery, using it to drive trains and smooth out spikes in SEPTA’s power distribution network. SEPTA could also sell the excess power to various electricity markets, depending on a number of factors, including what’s most profitable.

The pilot project, which Viridity expects to complete by next summer, will make use of the regenerative braking capabilities of Philadelphia’s subway trains. These systems convert kinetic energy into electricity as the train slows down. In the existing system, without the battery, that power can be sent back into SEPTA’s power distribution system and used to help another train accelerate–but only if it happens to be accelerating at the same time the power is generated, says Laurie Actman, director of government relations and business development at Viridity. Otherwise that power is lost.

Feeding energy into the electricity grid could prove more valuable than using it to power trains. Grid operators have to balance supply and demand. They now do this by sending signals to power stations that have agreed to operate at less than full capacity, so that the stations can quickly increase or decrease power output, sometimes for only a few seconds at a time. But it takes time for power plants to ramp power up and down, which can make it difficult for grid operators to keep up with fluctuations in demand. A battery can provide jolts of power “almost instantaneously,” says Kevin Morelock, director of IT and projects at Viridity.

Providing short bursts of power in response to signals from grid operators is a service called “frequency regulation.” At times when electricity prices are high, such as on hot days when demand is high, it may be more lucrative to stop offering frequency regulation and to simply sell power back to the grid instead.

The pilot project, which is being funded by a $900,000 grant from the Pennsylvania Energy Development Authority, faces two primary technical challenges. The first is selecting a battery that can quickly absorb and discharge electricity, potentially for hundreds of thousands of cycles, Morelock says. Viridity is currently evaluating different battery technologies, such as lead acid (which is cheap, but not long-lasting) and lithium-ion (which is being developed by companies such as AltairNano and A123 Systems).

The other challenge is developing software that can quickly analyze data from SEPTA’s system, and from various electricity markets, and decide how best to use the stored energy. Viridity has already developed similar software for managing buildings, drawing power from solar panels or moderating power demand from air-conditioning systems and other loads to help stabilize the grid.

Mark Duvall, the director of electric transportation and energy storage at the Electric Power Research Institute in Palo Alto, CA, says the project has set an “aggressive target” for energy savings. But a lot will depend on how much regenerative braking capacity can be harnessed, he says, and being profitable will depend on the cost of the batteries, although these costs should decrease rapidly in coming years. “It’s good that they’re looking into this. What they learn can help other transit agencies,” he says.

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