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Maxwell supplying ultracapacitors for light rail braking energy recuperation system; 2.8% energy savings

Clockwise from upper left: ESS enclosure, power control unit and ultracapacitor modules. Source: TIGGER, Tri-Met. Click to enlarge.

Maxwell Technologies, Inc. is supplying ultracapacitors for an energy-saving braking energy recuperation system that American Maglev Technology (AMT), is installing on light rail vehicles operated by the Portland, Oregon area’s Tri-County Metropolitan Transportation District (TriMet).

The ultracapacitor-based Energy Storage System (ESS) is an embedded system that captures, stores and discharges 0.7 kWh of energy for use in commercial transit applications. The ESS consists of the ultracapacitors and the required conditioning choppers and auxiliary devices to recapture and store a transit vehicle’s kinetic energy that would otherwise be lost during braking to be re-used for future departures or for the vehicle’s auxiliary power.

TriMet won a $4.2-million grant from the US Department of Transportation’s Federal Transit Administration (FTA) 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 US Department of Energy’s National Renewable Energy Laboratory (NREL), preliminary data collected by TriMet indicate that the roof-mounted ultracapacitor-based energy storage systems will result in annual energy savings of approximately 2.8%.



$4.2 M for a 2.8% reduction in energy usage is quite expensive, but I suppose if you consider it a learning experience, it looks better.

My question is how much would it cost to do this again, in another city, with similar trains.


The same, you now have to pay off a different set of local politicians and union officials.


mahonj, how can you draw that conclusion if you don't know 2.8% of what? If it is 2.8% of 1 MWh, you are correct. If it is 2.8% of 1 TWh, the ROI looks quite different.


Recent (1960+) e-subways already recover a high percentage of the deceleration energy and feed it back to the power lines? Would the added on-board super-caps do better?

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