TriMet won a $4.2 million grant from the FTA's Transit Investment for Greenhouse Gas and Energy Reduction (TIGGER) program to improve the efficiency of its fleet of 101 light rail vehicles equipped with regenerative braking after determining that the vehicles were able to use only about 70% of the energy generated by braking.
In regenerative braking systems, the electric motor that propels an electric or hybrid vehicle also does most of the braking. When the brakes are applied, instead of employing a conventional friction-based braking process, the system signals the electric motor to run in reverse mode, creating resistance to slow the vehicle. An electric motor running backwards also acts as an electric energy generator or dynamo that converts the kinetic energy of motion into electrical energy that can be stored for reuse, improving efficiency.
In TriMet's system, energy generated by decelerating vehicles is fed into the traction electrification system for use by other vehicles on the system. TriMet is using the TIGGER funding to retrofit 27 vehicles with the American Maglev-designed, ultracapacitor-based, energy storage system, which captures and stores regenerative braking energy as it is generated, uses the stored energy for acceleration and releases surplus energy to other vehicles on the system as needed. The 27 retrofitted vehicles are being paired with non-capacitor-equipped vehicles so that stored energy can be used by both vehicles.
To measure and validate anticipated energy savings, TriMet will compare in-service energy consumption data collected by on-board vehicle propulsion computers before and after the retrofit. According to a bulletin published by the FTA and prepared by the U.S. Department of Energy's National Renewable Energy Laboratory (NREL), preliminary data collected by TriMet indicate the roof-mounted ultracapacitor-based energy storage systems will result in annual energy savings of approximately 2.8%.
"The use of Maxwell's ultracapacitors in our Energy Storage Systems has resulted in unprecedented energy savings as we strive to make light rail vehicles greener and more energy efficient," said Tony J. Morris , the president/CEO of AMT. "We see this as a new, enabling, sustainable technology that can be applied in many other transportation environments."
"With their ability to charge and discharge in as little as fractions of a second, ultracapacitors are highly efficient in capturing energy generated during braking events that last only a few seconds and then quickly releasing stored energy as needed for acceleration," said David Schramm, Maxwell's president/CEO. "In addition to this and other rail installations, fuel-efficiency and emission-reduction benefits are being realized by more than 10,000 Maxwell ultracapacitor-equipped hybrid-electric transit buses in daily operation around the world."
