Technologies: Rail Systems
Most of the research and development involving rail transit is concentrated in energy storage systems. Although a variety of high-speed rail technologies are being studied (such as magnetic levitation), as are alternatives to diesel fuel for rail freight (i.e. gas turbines), these efforts deal primarily with long-haul rail transport, rather than the predominantly electrified light- or heavy-rail systems typical of North American urban areas.
As for weight reduction, it is unlikely that the dramatic reductions achieved with composite materials in bus design will be replicated in rail cars, given the more stringent fire safety regulations to which they are subject. Work on improving rail technology is "improving, but without revolutionary breakthroughs," according the David Phelps, a Senior Project Manager at the American Public Transportation Association.
Given that the majority of light rail transit systems in the United States are electrified, their greenhouse gas emissions profiles will match those of the utilities that power them. Reducing emissions from a typical metro system is therefore an issue of increasing the efficiency of an entire system of trains, rather than the individual vehicles that comprise it.
The principle behind the technology for doing so is not so different from the principle behind regenerative braking in a single hybrid-electric bus: the idea is to capture the kinetic energy lost when a vehicle decelerates, to store it, and to use it to accelerate the same or different cars at a later point in time.
In a hybrid-electric or electric bus, the rate at which energy is drawn from and put into the battery is not beyond the performance range of conventional technology. For a system of rail cars, however, the technical challenge lies in finding a way to quickly absorb a relatively large electric charge, and store it long enough to distribute it to a vehicle elsewhere in the system, something which current battery technology is unable to do.
Flywheels and ultracapacitors are two promising energy storage technologies for overcoming this hurdle. Flywheels are devices that store energy in the momentum of large masses revolving with very little friction. Ultracapacitors are, as the name suggests, very large capacitors, devices able to receive and distribute a large electric charge in a short time. As is often the case, gains in efficiency in one part of a system can lead to further gains elsewhere in the system; one maker of flywheels notes that regeneration of braking power reduces heat in subway tunnels, thereby reducing the need to use electric fans to remove it.
Regenerative braking, according to APTA's David Phelps, is "the most exciting area in rail technology advance currently." The Center for Electromechanics at the University of Texas, Austin, is working on a demonstration gas-turbine flywheel locomotive that it hopes to test in Pueblo, Colorado, in 2004. Looking further ahead, UT Austin expects the "commercialization phase of flywheel technology to be about 8 to 10 years away" for high-speed applications.
More relevant for urban transit is "wayside energy storage" in which a flywheel or ultracapacitor is located, not on the locomotive, by beside the track, as part of a power distribution system. A train decelerating into a station would send the energy recuperated from braking to a nearby storage device, which would then discharge it at the appropriate moment.
One such wayside storage device, employing a flywheel, is in demonstration in the London Underground. With the technology as it currently stands, recuperated energy in an electrified system is useless unless there is a second train accelerating at just the moment the first train is slowing down, allowing the power to be sent through the rails for a short distance from one train to the other.
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