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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.
Indeed, Regenerative Braking has been around for years, and is in effective use around the world in various guises.
The original article reads more like a marketing shot from Vycon's PR department than a news bulletin.
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It seems a little different. TFA is quite vague about how they are actually putting the energy into the flywheel. It says the wheel would be "housed in the station," but what that means is unclear. Does the train somehow mechanically transfer its kinetic energy to the flywheel? Or use hybrid/EV-style regenerative braking to generate electricity which spins the flywheel which releases energy to start the train again when it leaves? The former is hard to imagine, the latter seems like it involves many inefficiencies but it might still be worth it.
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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.
The kinetic energy of 100 [short] tons moving at 80 mph would be 58 MJ. The energy of 4 MW during 30 seconds will be 90 MJ. So the numbers appear to be correct, plus or minus my guesses on the weight and speed and everything else.
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.
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I've done some digging and it'd appear that the figure is actually correct. This thread 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.
The energy in subway trains is dwarfed by the energy used and lost on runways for jetliners. Imagine a system where, when a plane touches down, the energy is absorbed by a ground-based system that is then used to assist in takeoff for the next plane.
I suppose the natural first use of this would be on aircraft carriers. They already use systems to assist the takeoff, and they use hooks and cables in landing. They just need to efficiently store all that energy for reuse. (Then, again, when you have your own private nuclear reactor, energy for the catapult system may not be such a big deal.)
It's okay. There are plenty more where those came from. PLENTY more.
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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.
They're talking about the latter. Subway systems run an electrified third rail, charged with somewhere between 500-1500VDC. Trains draw power off this rail as needed, and power substations are located periodically throughout the system to supply it with power. They're talking about using the traction motors to stop, instead of brakes, and pumping that power back into the DC rail. Then setting up flywheels attached to the power substations that intelligently buffer the power supplied to the rail.
When the train brakes and dumps power onto the rail, the flywheel sucks it up. When the train wants to take off again, it is powered by the stored energy in the flywheel. Due to the low rolling resistance of metal wheels, trains require surprisingly little power to operate. Between the energy capture efficiency, and low operating needs, such a subway would run on only a small fraction of its current draw.
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?
Electric motors draw maximum amps at the very beginning just before they start turning. After that the draw is significantly less because resistance increases. You know, V=IR rewritten as I = V/R. The voltage is fixed, the resistance is close to zero when the motor is not turning so the amps go sky high. But only for an instant. As soon as the motor starts turning resistance picks up due to electromagnetic effects and the current draw falls. This is why you'll burn out a motor switching it on and off too quickly. You're shooting tremendous amounts of current through a non-turning motor. All that huge amount of current heats up the coils until something melts. However the 10,000 amp figure is peak, not continuous for 30 seconds. Therefore it's not fair to use that figure for calculations over a 30 second acceleration period. The amps drawn would form a curve, and for that you'd need something a little more complex than y = mx+c to figure it out, ie knowing the exact curve for those engine types/trains and some calculus.
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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.
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.
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
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.
The London underground has been doing this for over a century, many stations are higher than the normal track, so trains slow down when they go uphill before stopping, and get a boost when the leave and go downhill.