It's one of the basic tenets learned in high school physics class. The first law of thermodynamics says that energy cannot be created or destroyed; it can only change form.

SEPTA subway cars use a lot of energy, but then cast it off each time one of them stops at a station. Now, the transit agency is about to put that physics lesson to use.

SEPTA is launching a project to build a battery storage network, which will capture and reuse  energy released by braking subway cars. When complete, the network will be one of the nation's largest customer-sited battery storage systems — the first commercially deployed for a transit operation — and serve as a model for using stored energy to supply power in congested parts of the grid.

"It'll save us about a million kilowatt hours each year," explained Erik Johanson, SEPTA's director of innovation. He shared the details of the project with NewsWorks Tonight's Dave Heller.