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Transit agency TriMet has received a $4.2 million grant from the Federal Transit Administration to improve regenerative braking on light rail, TriMet announced today.

Regenerative braking is a mechanism that captures energy released when a vehicle brakes and converts it into energy that can power that vehicle or another one. But currently, when TriMet trains brake, some of that energy is wasted if it cannot be used immediately. The grant will fund the installation of 20 storage units that can store the energy until it is needed.

Compared to the 70 percent of energy used by the current system, the new energy capacitors will capture and use nearly 100 percent of the power generated by braking trains, said TriMet spokeswoman Bekki Witt.

“The storage units give more flexibility to capture that power and use it in a more robust way,” Witt said.

Energy saved by TriMet’s current regenerative braking can only be used if a braking train and an accelerating train are in the same area. Some energy is lost if another train is not in the same line segment to utilize the energy of the braking train. The energy generated from the friction of the braking is immediately sent up into the overhead lines that power the train. The new storage units will trap that energy instead and keep it for immediate or future use.

TriMet’s current regenerative braking system saves 12 million kilowatt hours of the overall energy needed to operate the light-rail system each year. With the new storage units, 70,000 kilowatt hours will be saved annually for each equipped train; 20 light-rail vehicles will save 1.4 million kilowatt hours annually and $168,000 a year in energy costs.

TriMet, one of 27 recipients of the Transit Investments for Greenhouse Gas and Energy Reduction grant program out of a total of 173 applicants, will begin installing the units on MAX trains in 2011.

“We’re trying it out on these 20 trains to see exactly how effective it is,” Witt said. “Hopefully we’ll be able to begin retrofitting the rest of our trains with them in the future.”