The upcoming TriMet Portland-Milwaukie light rail line has been designed so that power captured from braking trains can power accelerating ones.
The concept is quite similar to a hybrid car with regenerative brakes, except the battery isn't on board. In fact, there's no battery at all. The Siemens Sitras SES system that's set to be installed in Oregon actually uses a massive supercapacitor to store up juice.
In addition to recapturing energy that would've otherwise been lost as heat during braking, the supercapacitor also works to meet ever-changing power demand. Trains have frequent voltage peaks when they're accelerating. If multiple trains are accelerating at the same time, voltage demand exceeds the available supply. If you've ever been on a subway or light rail car that suddenly stopped for no reason at rush hour, you know exactly how frustrating this can be.
That's where the supercapacitor comes in. Since it's stored energy from braking trains, it can help maintain a constant voltage level or even provide enough electricity to meet a spike in demand. Such a setup reduces possible service interruptions, and also allows TriMet to avoid the costly installation of a utility substation along the line.
A supercapacitor is ideal for such an application. It has a much greater power density than a battery, meaning that it can deliver power to the railway more quickly than traditional cells could. That means it can instantaneously provide enough power to meet a spike in demand.
A supercapacitor can also take more charges and discharges without degrading, which means it will be better equipped to deal with the constant flow of energy in a railway's power grid. Sure, it has a lower energy density than a battery and therefore has to be larger, but that doesn't matter in a railyard.
When the Cologne Transit Authority installed the Siemens supercapacitor, demand for power fell by 15,000 kWh in a single month. Siemens estimates that a single supercapacitor could reduce electricity demand by 500,000 kWh per year, leading to a 300-ton reduction in CO2 emissions.
It's the first application of the Siemens Sitras SES system in the U.S., but one other, large-scale regenerative braking system has been installed in a public transit project, using batteries instead of a super capacitor. In Philadelphia, trains running on the Market-Frankford line send power regenerated from braking back down the third rail to a giant array of batteries. SEPTA, the local transit agency, uses some of that power, and sells the rest of it back to local utilities.
The supercapacitors are due to be installed in Portland this October, and TriMet's 7.9-mile Portland-Milwaukie line is scheduled to open in 2015.
Photo: Timmy Lucas
