Slashdot Mirror
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.
Philosopher (n) - a wise person who is calm and rational; someone who lives a life of reason with equanimity
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.
.sig withheld by request
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?
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.
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?
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
... to let you know that you're playing fast and loose with differing types of braking systems. Flywheel-based KERS, electric motor+battery regen braking, different things that are the same "in principle" only if your principle is "slow down with some mechanism besides direct generation of heat".
Further, nobody is bothering to read the article, is just taking the summary here at face value. But that's par for the course here. Nevermind.
One simple rule for its versus it's
A better way would have been to build the stations at a shallower depth than the tracks between stations. That way kinetic energy can be stored as potential energy when stopped.
http://michaelsmith.id.au
That is interesting. I think the major problem there is figuring out how to keep trains going on schedule. The other issues should be a lot easier to solve.
Well, there's also the issue of idiots being torn to pieces when they try to get through the door at the last moment.
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.
I prefer the "u" in honour as it seems to be missing these days.
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.)
Surely they could just feed the generated electricity back into the grid without all the local flywheels being necessary? As I recall, the Vancouver trolley buses have been doing this type of thing since at least the 1970s. If the grid can handle the output necessary to accelerate the trains, surely it could handle the input of slowing them down?
It's okay. There are plenty more where those came from. PLENTY more.
It is a miracle that curiosity survives formal education. - Einstein
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.
This may be simply a way to store energy on a line with few cars on it. Most power supplies are rectifies and are unable to put excess power back into the grid for storage. Excess regen power must be consumed by another train or dissapated as heat in braking load resistors. I think what they are trying to do is use the flywheel so voltage rise due to excess regenerative braking is captured in the flywheel in the powerhouse.
Most trains do not have this. They rely on braking resistors for excess regenerative braking. Elevators have this same issue in locations that prohibit regenerative breaking putting power back into the grid.
The truth shall set you free!
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?
It could be argued that the water wheel started the industrial revolution...
Awesome furniture, accessories and cabinetry in Santa Rosa, CA: http://humanity-home.com/
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...
I've got better things to do tonight than die.
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 outage thanks to (a) its very efficient linear induction motor propulsion & braking, (b) operating at a reduced speed (to minimize the impact of wind resistance), (c) supplementary power from backup generators, and (d) synchronized arrivals and departures from stations in conjunction with regenerative braking. The synchronization could be done precisely and programatically because it is a fully-automated system.
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.
How high is the energy required to keep the tubes evacuated, though? I am not attacking you there, but I am seriously interested in the energy budget for running a vacuum maglev train - it might just not save enough compared to a conventional subway to be worthwhile. Especially on short hauls like a subway. Perhaps on long distance runs?
Ubi solitudinem faciunt, pacem appellant.
At least for Antwerp, yes. (Although the premetro is not technically a subway, it's a tramway that's been put underground in the city center; it has overhead power instead of a 3rd rail.) The power sections run from station to station (the connection diagrams are on the emergency separator switches in the stations so you know if it switches the section before or after the current station.)
The things you pay attention to as a geek.
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.
What's more revolutionary than a flywheel? :)
Just take a lesson from the Moscow Metro. When the doors are closing they warn:
"Be careful, the doors are closing".
And that's entirely serious, because they slam with such force you can see them bounce back a bit. If you do get caught it leaves a good bruise. Definitely removes all the temptation to try to get through at the last second. Trains run on schedule with no problem at all.
It also helps that the trains pass every 1 to 3 minutes.
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.