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."

Re:Regenerative braking?

[Nemyst]

I've done some digging and it'd appear that the figure is actually correct. This thread [nyctransitforums.com] about the NYC subway system seems to say that the trains actually draw at maximum 10,000 amps, or 6 MW at 600 V. The 3-4MW figure would then be a good estimate.

I'm going to guess that feeding the energy in flywheels causes less power loss than going back and forth the lines, though it may very well be that they just want to keep the city dependent on their flywheels to use the regenerative breaking system they'd implement.

Regenerative braking and its problems

[Animats]

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.

Storing energy in track elevation?

[Adam J. Richter]

More seriously, I wonder if subways currently store some of that kinetic energy by putting the passenger platforms at a slightly higher elevation (not as deep in the ground) in comparison to the other portions of the track. If I have my math right, the kinetic energy of moving at 30 meters per second ( ~67 miles/hour) is approximately the potential energy of an elevation of 45 meters in 1 Earth gravity (0.5mv^2 = mgh --> 0.5v^2 = gh --> h=0.5v^2/g --> h = 0.5(30m/sec)^2/(10m/sec^2) = 45 m/sec). I imagine that that would be much too rollercoastery for a local train, and you wouldn't want to have the train fly off the track so easily for arriving a little too fast, but it wouldn't surprise me if a dip of a meter or two is engineered into subway lines for a bit of energy savings.

Re:Isn't this an old idea?

[ChumpusRex2003]

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