Select your localized edition:

Close ×

More Ways to Connect

Discover one of our 28 local entrepreneurial communities »

Be the first to know as we launch in new countries and markets around the globe.

Interested in bringing MIT Technology Review to your local market?

MIT Technology ReviewMIT Technology Review - logo


Unsupported browser: Your browser does not meet modern web standards. See how it scores »

{ action.text }

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.

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.

9 comments. Share your thoughts »

Credit: SEPTA

Tagged: Energy, energy, battery, smart grid

Reprints and Permissions | Send feedback to the editor

From the Archives


Introducing MIT Technology Review Insider.

Already a Magazine subscriber?

You're automatically an Insider. It's easy to activate or upgrade your account.

Activate Your Account

Become an Insider

It's the new way to subscribe. Get even more of the tech news, research, and discoveries you crave.

Sign Up

Learn More

Find out why MIT Technology Review Insider is for you and explore your options.

Show Me