Depending on where you live, your public transportation options either provide a quality way to get around, or are the subject of endless frustration. Often it’s a little a little bit of both.

Even in the best public transportation locales, such as New York, it’s a wonder that any project gets completed, let alone expanded upon. The Second Avenue subway, finally under construction, has been in some sort of planning phase since 1929.

Ride the rails toward Philadelphia, however, and you might find yourself in the midst of a transportation system that is focused on the future, at least when it comes to energy use.

For the past two years, the Southeastern Pennsylvania Transit Authority has been capturing its braking energy and then selling it back into the power grid. SEPTA’s initial project has been successful enough that it is launching into a second phase, with future expansions already being planned. Other electric modes of transportation, such as electric cars and trucks, are also participating in frequency regulation markets in PJM and ERCOT

ABB, Viridity Energy, Saft Battery, and Maxwell are providing SEPTA with storage to gather brake energy and sell it into the PJM frequency regulation market. Frequency regulation has always been needed to balance grid fluctuations, but in areas with high penetration of intermittent renewables, faster response times are becoming more important.

“The SEPTA model is quite unique in the world,” said Jacques Poulin, product manager for energy storage and rail at ABB.

Recovering braking energy is not the novel part. That technology is widely used across the globe. The new element is the scope and use of the energy.

“Now we want to get to 100 percent recycled energy,” added Poulin. “The trend we’re trying to establish goes way beyond just regenerative braking.” 

In 2012, SEPTA used a $900,000 Pennsylvania state grant to install an 800-kilowatt, 400-kilowatt-hour battery from Saft at the Letterly substation serving five stations on the Market-Frankford Line. ABB provided its energy recovery expertise and its Enviline energy recovery product, while Philadelphia-based Viridity Energy helped optimize the energy system and sell power into PJM for ancillary services.

One lesson at a time

At first, SEPTA was interested in playing in both the demand response and frequency regulation markets, but demand response was not the ideal application given how the batteries are used for regenerative braking.

There were also many kinks to work out. At first, the trains weren’t providing the amount of energy from regenerative braking that SEPTA and project engineers expected. It turns out that the programmable controllers on the trains were limiting how much they could put back onto the system. That has since been tweaked, and the amount of energy coming off trains is increasing. Additionally, the rolling stock is fifteen or more years old, and the tracks date back nearly a century, so the system likely will never be as efficient as a newer one.

There were also issues to work out between ABB's and Saft’s software, which ABB described as an engaging challenge. Now that most of the issues have been resolved, ABB is ready for "more advanced applications,” said Poulin.

There were unexpected upsides as well. SEPTA engineers initially thought that the highest amount of absorbed energy the system could handle would be about 40 percent to 50 percent. 

“It turned out it was much more receptive,” said Andy Gillespie, chief engineer in the engineering, maintenance and construction division at SEPTA. In some areas of the tracks, the figure was up to 80 percent. The increase in regenerative voltage meant that SEPTA could optimize the voltage it needed to run the trains and adjust how much voltage it needed from the grid. In the first year, the transit authority saw savings of about $1 million.

Paying for itself

The savings from optimizing voltage were a welcome surprise, but it is the ongoing payments from the frequency regulation market that will pay back the initial investment in the system over the long term. In PJM territory (the grid operator where SEPTA is located), payments for fast-responding resources earn higher payments than do slower-responding frequency regulation services, as required by FERC Order 755.

In the first year, SEPTA earned $23,000 in just five days, nearly one-quarter of what it forecasted to make in the entire year. It earned about $150,000 from frequency regulation for the entire year.

However, the system isn’t earning to its full potential, because it can either be selling power into the grid or doing regenerative braking, but can't do both at the same time. When the system jumps out of a frequency regulation event to do regenerative braking, its score is impacted in the PJM system, which in turn determines the level of payment it receives.

For the second project, ABB and Saft proposed supercapacitors that would allow SEPTA, the sixth-largest transit agency in the U.S., to stay in the frequency regulation market longer. The combination of Maxwell supercapacitors with the existing batteries will allow the transit authority to better manage regenerative braking and optimally time them to market conditions. Supercapacitors, also known as ultracapacitors, can store and release energy almost instantaneously, but they do not have the energy storage density of batteries.

“We will recover more braking energy, produce higher revenues from the frequency regulation, and extend the battery life. This innovation demonstrates the full capability and flexibility of our Enviline platform,” Poulin said in a statement.

The payback for each project is expected to be three to five years. The second part is being supported by funding from a $1.2 million federal grant. Eventually, however, the company "should be able to self-fund the expansion,” said Gillespie. “One goal is to make this modular.”

Gillespie said it would take at least four installations for it to be self-funding. In the meantime, the transit authority is looking at other contracting methods. That could be some form of an energy efficiency performance contract, or potentially a public/private partnership that had a revenue sharing scheme.

SEPTA has preliminary plans for ten of these projects across its 33 substations. At some point, however, returns will diminish as most of the braking energy is captured on the different lines.

“I’m excited about this hybrid approach,” Gillespie said of bringing together regenerative braking, storage technologies and grid applications. Others might be excited too. SEPTA has had inquires from other agencies in the U.S., and European agencies are reaching out as well.

“The interest for this model is spreading across the U.S.,” said Poulin. “Ultimately, this is financially more attractive than either part by itself.”