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Power Transportation Hardware

Tapping Subway Trains For Energy 229

Posted by Soulskill
from the make-them-pedal dept.
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."
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Tapping Subway Trains For Energy

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  • by hawguy (1600213) on Saturday September 03, 2011 @11:14PM (#37300250)

    How is this different from traditional regenerative braking (they even mention regenerative braking in the article) that's already in wide use by electrified transit providers? I don't see how feeding energy into local flywheels is any different than feeding it back into the grid? Surely a grid that's capable of delivering megawatts of power for to start a train is capable of absorbing (fewer) megawatts of power for braking?

    Is the 30 seconds @ 3 - 4MW figure mentioned in the article accurate? That's a 6000 amp draw for a 600V system, sounds like a lot of current over a relatively small conductor -- the conductors that I've seen appear to be around a 4/0 gauge, which is only rated for around 250A. Granted, for only 30 seconds it could exceed this rating, but 6000A?

    • by tftp (111690) on Saturday September 03, 2011 @11:25PM (#37300302) Homepage
      The kinetic energy of 100 [short] tons moving at 80 mph would be 58 MJ [wolframalpha.com]. The energy of 4 MW during 30 seconds will be 90 MJ [wolframalpha.com]. So the numbers appear to be correct, plus or minus my guesses on the weight and speed and everything else.
    • by Nemyst (1383049) on Saturday September 03, 2011 @11:36PM (#37300348) Homepage

      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.

      • by Dunbal (464142) * on Sunday September 04, 2011 @01:02AM (#37300758)
        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.
    • by robbak (775424)

      two points: Overhead systems like you see normally use much higher voltages than 600V, for the reasons that you quote. Third-rail systems can deal with much higher currents.
      Secondly, A system that can deliver that current could only absorb it if it has somewhere else to send it - another accelerating train. Another poster suggested that, most of the time, the energy in the braking currents are, at least partially, lost in the resistances of the third rail, carrying it miles until it finds an accelerating tr

      • by timeOday (582209)
        Has anybody tried just building the stations on hills? (I.e. putting the platforms at less depth than the rest of the tunnel?) Then the trains are slowed coasting uphill into the station, and pick up speed going downhill out of the station.

        I guess you could even have the trains pull onto a teeter-totter tilted up as they arrive and down as they leave, by a piston, so they wouldn't have to go a train length on level ground before being accelerated by the downhill.

        • Has anybody tried just building the stations on hills?

          http://en.wikipedia.org/wiki/Victoria_line [wikipedia.org]

          The line has hump-backed stations which allow trains to store gravitational potential energy as they slow down and release it when they leave a station. This provides an energy saving of 5% and makes the trains run 9% faster.

          I suppose it may be hard to retrofit the hills. Although, wiki calls it a slight hill. If it were only a few feet then I would think it would be worth it.

          Some quick math shows that an object at 40 mph (18 m/s) has about the same energy as it would gain from a 16 meter fall. That's certainly doable, but probably too much for a retrofit, as it would require new tunnels. Then again, any hill would help a little.

    • by Anonymous Coward on Sunday September 04, 2011 @12:21AM (#37300594)

      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.

    • by sam0737 (648914)

      6000A maybe a bit stretch, but it's not out of the ball park.
      It takes 1500A@750V to accelerate a 6-car train in Beijing.

      Scale to 10-car and 600V, you get 3000A.

    • by sjames (1099)

      The difference is a smoother draw from the grid and a lot more sales of flywheels. I doubt there's much net energy savings.

      I'm sure there's a lot more than a single 4/0 gauge wire providing power to the track.

  • Energy != work (Score:3, Insightful)

    by zill (1690130) on Saturday September 03, 2011 @11:19PM (#37300272)

    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."

    For how long?

    • by webdog314 (960286)

      The way I read it, 30 seconds.

    • 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."

      For how long?

      For 30 seconds, more or less, if a home is ~ 2-3 kW.

  • Better idea (Score:5, Funny)

    by SuperKendall (25149) on Saturday September 03, 2011 @11:31PM (#37300330)

    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.

    • by amiga3D (567632)

      Careful there, somebody might not realize you're joking.

    • by formfeed (703859) on Saturday September 03, 2011 @11:46PM (#37300410)

      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. [...] All you need to make it work is some very good bumpers and perhaps strengthening the hand-straps.

      I'd add a pair of gigantic springs.

    • by Ryanrule (1657199)
      Well, you could use the energy of the train stopping on one side to fire off the train on the other side.
    • That only works if you've got perfectly inelastic trains.....

    • by Tablizer (95088)

      Flintstones lives!

    • by jovius (974690)

      Solution: Build a sort of docking tunnel, through which the trains come to the station. The tunnel has hydraulic/pneumatic system which the train drags along and loads while braking. The loaded energy is transferred to another train (or the same train) by a system at the other end of the station to give the train some initial speed. Or have the hand-straps be connected to a dynamo, so everybody can participate. All in all the system should make hisses and poofs and generally create a sense of great wonder.

    • by sjames (1099)

      And just think of all of those opportunities to meet new people when everyone piles up on one end of the car :-)

      That and science teachers can go on field trips to show practical examples of kinetic energy transfer in a collision.

    • by Splab (574204)

      Magnets! Big strong magnets, less damage on the trains. Put the front train in neutral and the one behind will push it smoothly out of the way.

      Also, it will clean up any lost metal on the tracks, win win!

    • by Adam J. Richter (17693) on Sunday September 04, 2011 @04:27AM (#37301320)
      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.
      • by xaxa (988988)

        The maximum speed of a R142A train (found randomly on Wikipedia) is 55mph (25 m/s). Subway trains don't go very fast. I can't find any information for New York, but in London they barely hit 30-40mph (13-18 m/s) in the centre of the city. (Though that's fast compared to road traffic.)

        That gives an elevation of 8-16m.

        (More technical facts about the London Underground than you could possibly want: Key Facts [tfl.gov.uk])

    • by trout007 (975317)

      You could use regenerative braking for you scheme. The train at rest sits in the station and as the next train starts to brake and dumps the power on the rails the train t rest uses it to start accelerating. That way no flywheel needed.

  • by WindBourne (631190) on Sunday September 04, 2011 @12:03AM (#37300504) Journal
    By using ultra-caps at the station, they get to drop the price of these. In doing so, they make it available for other technology. The advantage of ultra-caps is that it has power. In addition, while some of the ultra-caps do not retain energy for days without loss, this is simply shot back into the system in under 5 minutes. The loss is nominal. Finally, most rail systems have more cars, trains, then stations. It is actually cheaper to put these at the stations, using the electric system, then to retrofit all of the cars. Also, not making the cars carry the charge system around is more efficient.
    • Whereas trains use HIGH voltage and current AC. When building a capacitor, you have to fight against conflicting requirements: high density, high current, high voltage, stable against environmental changes (humidity/temperature), stable against aging characteristics, stable against voltage/charge (and voltage/charge rate), AC/RF response characteristics, dissipation ("leakage"), among other things. Double-layer capacitors ("ultra-caps") sacrifice maximum operating voltage and maximum discharge rate (curre
  • by crow (16139) on Sunday September 04, 2011 @12:05AM (#37300518) Homepage Journal

    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.)

    • Re: (Score:3, Funny)

      by BradleyUffner (103496)

      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.)

      Nuclear Powered Subway Trains? I LIKE it!

    • by Baloroth (2370816)

      No, that wouldn't really work. Most of the energy to propel a plane is used up by wind resistance (deliberately). The plane needs to actually force the air downwards with the wings to fly, so no matter how low a drag the rest of the system has most of the energy isn't going to be retained. Most jumbojets run at pretty close to max power for the whole flight, because they need to due to energy loss. A subway, on the other hand, runs through mostly inertia (similar to how most cars work.) Most of the energy

    • by Dunbal (464142) *
      So would pilots be encouraged to make really really really hard landings in order to create more energy for the departing aircraft? And who would clean up the mess?
    • by bazorg (911295)

      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.

      Prior art [wikipedia.org]

  • Power != Energy (Score:5, Insightful)

    by FrankDrebin (238464) on Sunday September 04, 2011 @12:33AM (#37300650) Homepage

    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.

    The corrected sentence is much less impressive: "— that's enough energy to power 1,300 average U.S. homes for 30 seconds."

  • Don't they mean supplement? I realize it's Saturday, but come on, editors.
  • by Hamsterdan (815291) on Sunday September 04, 2011 @01:49AM (#37300914)

    Like the Prius, the Lexus Hybrids, the Ford Escape, and many of the hybrid cars on the market?

    Montreal's Societe de Transport de Montreal is testing hybrid buses (perfect use for a hybrid vehicule)...

    I can see Delivery vehicules (Purolator, UPS, DHL, FEDEX, restaurant delivery) using that, they are always stop and go, so regenerative braking makes lots of sense.

    If you're only doing highways, a hybrid won't do much, except use more gas for the added battery weight...

    • > If you're only doing highways, a hybrid won't do much, except use more gas for the added battery weight...

      Unless you reduce the size of the engine, which you can because you don't need low end torque.

      My 2006 Honda Civic Hybrid gets 4.3 l/100 km (54.2 m/gal US, 65.2 Imperial) - photos available if need be.

      The non-hybrid version gets about 20 mph less on the hiway. So, you're wrong.

  • If you can synchronize arrivals with departures at the same (or a nearby) station, energy regenerated through braking can be immediately used to power the acceleration of another train. If it is not synchronized, the power is wasted (unless they have batteries or some other power cache, which would surely introduce its own inefficiencies).

    I once heard a story (though unfortunately I have no references--it may very well be an urban legend) that the Vancouver SkyTrain continued operating through a power outag

  • by Animats (122034) on Sunday September 04, 2011 @02:12AM (#37300982) Homepage

    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.

  • Bomber's got an optional package for most of their light rail stuff that uses Maxwell super caps for regen. 25% improvement in efficiency. This is particularly useful on light rail because it means they have enough energy onboard to pull themselves through an intersection if there's a power failure.

    No one buys them. Up-front cost. So next time you complain that people don't buy hybrid cars...

  • The idea is to generate energy from decelerating a train. This is done in modern electric trains all over the place not only subways or trams. SO this is and old idea. The "new" thing is the storage on board instead of introducing the energy in the electric system. However, why should a train carry around heavy batteries, which consume extra energy to be accelerated when the train can provide energy for other trains which apparently accelerate at the time when another decelerate? There is no real logic in i

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