Electric-powered trains have been capturing the energy from regenerative braking for years now. But besides reducing power bills, they haven’t done much with the energy they’ve saved.

Philadelphia-based startup Viridity Energy is seeking to put a grid value on that energy with a new battery-backed, grid-connected system installed with the Southeastern Pennsylvania Transit Authority (SEPTA) train system. After a year and a half of work, Viridity is now turning on the 800-kilowatt battery backed system, and will soon start bidding its energy reductions into demand response and frequency regulation markets.

It’s the latest move by Viridity into the world of so-called microgrids, virtual power plants, or other systems that allow buildings (or trains, in this case) to share their power with the grid. SEPTA’s project taps regenerative braking power at five downtown train stations along the Market-Frankford line, the city's most-used, using power equipment from Envitech and Viridity's "VPower" optimization software, and feeds it to an 800-kilowatt, 400-kilowatt-hour battery from Saft at the substation serving the five stations.

That’s a pretty small battery, compared to the multi-megawatt wind power backup systems being installed around the world. But Kevin Morelock, Viridity’s managing director of technical sales and partnerships, said in a Wednesday interview that Viridity’s ability to manage and optimize the interplay of battery, train power system and grid interconnections should yield returns that make it worth the effort.

How does it work? Well, when trains use their electric motors to slow down (usually approaching a station), that sends voltage down the “third rail” or overhead power line that supplies the trains their power. If there’s a train ahead, it can capture that voltage itself -- but if there isn’t a train, that over-voltage is generally dissipated as heat, Morelock said.

Viridity’s system, on the other hand, taps the third rail and captures that voltage to charge the battery, he said. From there, the battery can be used either to power trains when power prices are high, to cushion substation loads to manage peak power moments, or to bid back into grid power markets, he said.

Those markets include so-called “economic” demand response -- turning down power use when the grid is at peak demand, usually at day-ahead or hour-ahead increments -- as well as frequency regulation markets, which require assets (usually gas-fired power plants) that can react in minutes, if not seconds, to balance fluctuations of grid frequency.

Viridity is already bidding battery-backed power into frequency regulation markets with partner and battery supplier Axion Power, which has installed Viridity’s system at its New Castle, Pa. manufacturing plant.

Saft and Envitech are financing the SEPTA project, helped along with a $900,000 Pennsylvania state grant that Viridity landed last year, Morelock said. With that grant, Viridity is expecting a return on investment of about 2 to 3 years on the project, he said.

Building a similar system without a grant would likely double that payback period to four to six years, he noted. SEPTA is working on a second, similar project, backed by $1.2 million in federal grants, that will use a different set of batteries from an as-yet unnamed supplier, he said. Other companies looking to capture train braking power and apply it to grid needs include flywheel maker Vycon Power and ultracapacitor maker Maxwell Technologies.  

Viridity raised $14 million in January 2011 and is balancing and storing power at several university campus microgrids, a wind power storage management project in Pennsylvania, and other locations. While batteries play a part in some of its projects, others are concentrating on building energy management systems that pre-cool libraries, turn down lights in unused corridors, or adjust thermostats during peak power times to shave energy that can be bid back into power markets.

In February of this year, Viridity launched a partnership with big energy services company ConEd Solutions that could see its technology deployed in offices and other buildings. It’s also working with the developers of the Tres Amigas project, a multi-billion dollar effort to link the United States’ three main power grids via a high-voltage direct current transmission hub in New Mexico.

Tags: batteries, beacon power, demand response, energy efficiency, energy storage, general electric, maxwell, saft, septa, smart grid, subway, train, utilities, viridity energy, vycon