How a 'dinky' energy storage project gives financial oomph to Philadelphia's subway system

PHILADELPHIA -- As the subway train headed north on the Market-Frankford line and pulled into an aboveground station, Erik Johanson shot out a finger toward the opposite track.

"That train right there is accelerating on power this train is generating," he said, with palpable excitement in his voice.

Johanson is the strategy and sustainability planner for the Southeastern Pennsylvania Transportation Authority, or SEPTA, which operates Philadelphia's multimodal public transportation system. Nothing about the subway ride with him last month seemed exceptional; the train started and stopped like normal. But a few stops away, Johanson explained, SEPTA had configured an innovative way to turn an everyday train ride into a method to conserve energy, save money and reduce greenhouse gas emissions.

The year the Letterly Substation in Kensington, Philadelphia, turned 100 years old, SEPTA started up what's believed to be the world's first wayside energy storage project to combine the energy-saving benefits of regenerative braking, or "regen," with the revenue benefits of selling some of that energy back to the grid.

The wayside, or "beside-the-train-track," energy storage project consists of a large stationary battery in a large white container, next to a smaller gray box that controls the system. The project is housed in what looks like an old gymnasium but was initially built to contain a giant rotary converter. The converter now long gone, SEPTA had more than enough space to store a battery.

Since the project came online in April last year, SEPTA has seen a nearly 20 percent reduction in power consumption at the substation. By tapping into the wholesale energy market, the transportation authority -- which is currently facing a $5 billion backlog in maintaining a state of good repair on its assets -- is also starting to turn train braking into a revenue generator.

Getting a battery charge from braking

Every time a train brakes on the 18 miles of track powered by the Letterly Substation, electricity is sent to the third rail system. It used to be that the regen energy went to waste unless another train was accelerating on the same track within a few miles away. This system, therefore, captured only a small portion of available braking energy. But now, with the energy storage device in place, SEPTA engineers calculate that the storage system will capture close to 40 percent of available braking energy.


With the controller and 800-kilowatt lithium-ion battery hooked up to the system, the substation sees when a train is braking and the voltage is rising. The system then opens up a switch and allows energy to flow into the battery. When the train stops braking, the battery will turn the switch off and store that energy until it detects another train speeding up and sends power back out to the rail.

Every time a train accelerates, the big gray controller box starts to emit a high-pitched buzz. If the train is running on battery power, that's electricity SEPTA doesn't have to buy from the utility. But the really innovative part of the system is where the Philadelphia-based utility Viridity Energy stepped in and proposed selling a portion of the battery power to the wholesale energy market.

Andrew Gillespie, chief engineer of SEPTA's engineering, maintenance and construction division, said he didn't believe he could make any money owning a battery. "I thought, 'Who cares about my little dinky wayside storage project?'" he said.

"Then Viridity comes along and says, 'Hey, you can take that same asset and you can leverage it, you can multiply its benefit,'" he said. "Of course, the pessimistic public employee goes, 'Yeah, prove it.' They did prove it."

The initiative was enabled by PJM Interconnection, the Eastern regional grid operator, which was looking for ways to balance electric supply and demand and willing to support storage projects to decentralize its energy production. PJM's interest in SEPTA's wayside energy storage project follows a growing trend toward building smaller, self-sustaining grid systems, or microgrids (ClimateWire, May 1).

According to PJM spokesman Ray Dotter, working with SEPTA and Viridity is about "making sure we provide fair and equal market opportunities to new innovative technologies that will help balance the grid and maintain stability."

Selling juice to the grid

When PJM needs a greater supply of energy, it sends a signal to Viridity to discharge some of SEPTA's battery power. As opposed to a coal- or gas-powered facility, which generally takes about 30 minutes to ramp up, the battery can respond within 4 seconds. PJM pays SEPTA and Viridity for every hour the battery is in the wholesale frequency regulation market.

The combination of SEPTA's localized energy savings through regenerative braking and PJM's demand for frequency regulation services has made the wayside energy project an income-generating asset.

The Letterly Substation project cost $1.8 million. Viridity brought $900,000 in grant money from the Pennsylvania Energy Development Authority and contributed some of its own capital. SEPTA supported the rest.

Viridity also had to make a significant investment in its software programs to manage the frequency regulation application with PJM. This software will now allow the company to invest in other regenerative braking projects or tap into managing the emerging renewable energy market.

"What we've demonstrated with SEPTA is how to harness a different flavor of difficult-to-predict, but not unpredictable, intermittent supply of energy. To us, we can look at it generically and solve the same problem by storing the energy from solar and wind and perhaps other sources as well," said Chad Von Eck, vice president of partner and delivery services at Viridity who oversaw the SEPTA wayside project.

Working with a transportation system is unique, however, because it's an energy source without ever intending to be. Since nearly all of SEPTA's subways, trains and trolleys came standard with regen capacity as they were upgraded over the last couple of decades, it's almost as though the energy they produce is free.

There's also a lot of untapped potential. Connecting the battery to help run trains reduced energy usage at the Letterly Substation by 10 percent. When SEPTA engineers found a way to increase the amount of energy they could capture in a given braking event from 700 volts to 735 volts, the energy savings jumped to nearly 20 percent.

"Basically, every one of these trains is a little generator. So when you increase the output of these little generators, it means there's more power to be used by the other trains," Gillespie said.

SEPTA is now running tests to see whether it's possible to capture 765 and even 790 volts of braking energy, which is expected to further increase the amount of energy available to send to other trains or store in the battery.

Electrifying financial possibilities

Through its energy savings, SEPTA is able to pay back the battery provider Saft Groupe SA as well as ABB Group, which created the control system that links the trains, the battery and PJM all together. Viridity and SEPTA then share the profits from the wholesale energy market. Under this model, SEPTA expects to pay off the project in six years. The life expectancy of the battery is 10 years.

"All of this remains budget neutral," Gillespie said, thanks to the energy savings.

Comparing the end of 2009 with the end of 2012, during which time the wayside energy storage project came online and SEPTA boosted the energy capture to 735 volts, the transit authority saved nearly $500,000 in electricity costs.

"I'm a policy person," Johanson said. "I get excited about this because the numbers are real."

Electricity usage on the Market-Frankford line was increasing until regen was clicked on in April of last year, which means the $500,000 in savings doesn't even reflect a full year of usage. SEPTA also expects savings to grow as it increases the voltage further to 765 by the end of this year.

The next step is to expand. Using money from its energy savings, SEPTA plans to put 10 wayside energy storage devices in place at its substations. By enhancing the potential of regenerative braking by increasing the voltage and building out the 10 storage facilities, SEPTA will cut its emissions by more than 35 million pounds of carbon dioxide equivalent. That's about equal to taking 3,000 cars off the road for one year.

SEPTA has already received a $1.44 million grant from the Federal Transit Administration to build its second project, priced at $1.8 million. The profits from the two projects combined should bring in close to $1 million per year, which can be reinvested into other projects. SEPTA also expects private firms will finance these projects once they see how cost-effective they are.

This model offers a new funding mechanism for the cash-strapped transit authority's sustainability program, Johanson said. The projects do require an upfront investment, but once the system is brought up to scale, the revenues generated can be invested directly into upgrading the transportation system.

"We are a transportation company; we can't justify doing this and then not do a transportation project," he said. "But if you can finance it on the savings, everyone wins."

Like what you see?

We thought you might.

Request a trial now.

Get access to our comprehensive, daily coverage of energy and environmental politics and policy.



Latest Selected Headlines

More headlinesMore headlines

More headlinesMore headlines

More headlinesMore headlines

More headlinesMore headlines