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Tapping Subway Trains For Energy
An anonymous reader writes "Industrial flywheel manufacturer Vycon Energy believes that they can tap the immense amount of kinetic energy carried by moving subway trains to subsidize city power systems. Not only would this reduce emissions, but it would also help to avoid peak power emergencies. This energy could the be used to start the trains up again — a 10-car subway train in New York's system requires a jolt of three to four megawatts of power for 30 seconds to get up to cruising speed — that's enough energy to power 1,300 average U.S. homes."
That's not what this is about. It's about putting flywheels in the stations themselves. The energy put back into the 3rd rail is usually wasted since it would require another train to be close to the train braking. Since most trains are guaranteed to stop in a station, absorbing the electricity put back into the rail could be stored for when the train starts. Batteries are insufficient, so they're using flywheels.
This exact same thing comes up every few years on Slashdot. Look it up if you don't believe me.
I'm gonna go out on a limb here and say there's a lot more energy involved in moving subway trains than your typical Prius. Perhaps the trick here is creating a system able to store so much energy efficiently?
We've had airplanes since the Wright brothers in 1903, and jetliners since the early 50s. That doesn't mean that Boeing's 787 is an old idea and not worth talking about. The real advances in engineering are always in the little fiddly bits that screw you over when you first try to scale up.
Forget this fancy regenerative braking nonsense.
What better way to get one train totally stopped, while startup up another? The solution to this problem is obvious, simply let an incoming train hit a parked one. The kinetic energy will be transferred, the parked train will be in motion while the formerly moving train is almost totally stopped.
All you need to make it work is some very good bumpers and perhaps strengthening the hand-straps.
"There is more worth loving than we have strength to love." - Brian Jay Stanley
6000 amps at 625 volts is EXACTLY what a subway train draws when it starts. I should know, I work for the Power department of the New York City Subway system.
The corrected sentence is much less impressive: "— that's enough energy to power 1,300 average U.S. homes for 30 seconds."
Anybody want a peanut?
Some of the newer NYC subway trains do have regenerative braking. All have dynamic braking, where the motor acts as a generator, but in the older cars, the energy is dumped into huge iron resistors.
In the NYC subway, there's usually a train drawing power somewhere in the section of third rail connected to a single substation. So there's usually some load able to take regenerated power. Subway traction power is distributed at 27KV AC, and rectified to about 600VDC at one of 215 substations. Regeneration can only supply power to a single DC section; the substations can't up-convert DC to AC and feed it back upstream. (Interestingly, back when the subway system used rotary converters instead of rectifiers, some power could in theory be fed from the DC system into the AC system.)
If there's no load able to take regenerated power, it has to be dumped somewhere, either into resistors at the substation or on the train.
The question is whether enough unused regenerated power is produced to justify storing it. It's quite likely that during late-night off-peak hours, there may be only one train running on a substation and power will have to be dumped. But late-night power is cheap, and in NYC, mostly from hydro plants. So flywheel energy storage probably isn't worth it.
On-vehicle flywheels have been tried, but ultracapacitors look more promising today.
Traction elevators (with cables, as opposed to hydraulics) have usually been regenerative for decades, both for the gravity and inertial loads.
Exactly right. The problem is that most 3rd rail/4 rail/short-range overhead systems run on DC power - usually around 700 V DC, but with a wide variation. Regenerative braking is widely used on may railways. However, the problem is that when the train's inverters inject DC power back into the rail, the voltage rises on the rail. Hopefully, there will be a nearby accelerating train which can absorb the energy. However, if there isn't the voltage on the rail will continue to rise until the train's inverters redirect the energy into on-board resistors, to permit continued dynamic braking.
Lowering the resistance of the 3rd rail, and making longer interconnected 3rd rail segments can all improve the efficiency of this system. But installing bigger rails, or upgrading to copper/aluminium is very expensive. Additionally, lower resistances increase the severity of potential short-circuit scenarios. Finally, short separated segments of power infrastructure is preferred for reasons of fault isolation. E.g. originally the whole London underground network used fully interconnected power rails, but in such a scenario, the system was unreliable, as a faulty train would degrade the entire network. After a couple of fault induced fires, the system was sectionalised into 1-2 mile segments.
Flywheels are already used on subway systems (for example New York and London Underground) in order to provide another method of capturing regenerated energy before the trains need to dump it into resistors. At strategic points, flywheels are connected to the rails. If the voltage on the rails rises above the normal grid supply voltage, the flywheel controller will accelerate the flywheel keeping the rail voltage controlled. Similarly, under severe acceleration conditions, where the rail voltage falls under load, the flywheel controller will draw energy from the flywheel and inject it into the rails. This allows subway operators to upgrade to faster accelerating trains, or run more trains, without upgrading their grid supply which may be very expensive, or impractical in power constrained cities