Tapping Subway Trains For Energy 229
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."
Regenerative braking? (Score:3)
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?
Re:Regenerative braking? (Score:4, Informative)
Re:Regenerative braking? (Score:4, Interesting)
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.
Re:Regenerative braking? (Score:4, Informative)
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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
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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.
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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.
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Clearly you don't realize that this was done on some lines in Manhattan. Do you even live anywhere near there to make such a snarky post on the subject?
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Funnily enough, that's exactly what they did in Manhattan. Some stations are at a higher level than the track to provide gravitation potential energy storage that assists the trains when slowing down and helps when speeding them up.
It's done on the London Underground too, where it saves something like 5% in energy costs compared to 'flat' stations (ie, 95% power use).
If you're going to try to "pwn" someone, perhaps you should actually check some facts first. It tends to help when attempting to not look like
Re:Regenerative braking? (Score:5, Informative)
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.
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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.
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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.
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Mega Watt here, Mega Watt Hour there... what's the difference between friends?
They should use the traditional ((furlongs^2) * (eV / (c^2)))) / (fortnights^2) (1.36962187 × 10^-50 kilowatt hours [google.com]) when talking about electrical energy, to avoid confusion.
Energy != work (Score:3, Insightful)
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?
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The way I read it, 30 seconds.
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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.
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If subway trains use regenerative braking to feed power back to the grid during periods of high demand they are reducing the strain on the grid, lowering demands on peaker generating stations.
No, no, no. If the trains were feeding the braking power back into the grid, it would make the situation worse, not better. Every time a train stopped there would be fifteen seconds of reduced load (by four megawatts or so) during braking, followed fifteen seconds of passenger boarding, followed by thirty seconds of increased load (by three or four megawatts) during acceleration out of the station. Instead of having a swing of four megawatts of grid load, there would be local swings of eight megawatts p
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2-3kW per home is reasonable? (Score:2)
Our house is pretty energy efficient, and our energy use is typically below the norm. I do work in the power industry, and the average that they tend to use is 1.2kW. Not 2 or 3kW average. This is the problem when journalist
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My thoughts exactly - I don't have air-conditioning, but my last energy survey (required to get subsidies for solar PV) was ~8kWh/day. And I have 4 computers, TV, washing machine, etc.
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22 fucking kw/day? do you drive all-electric cars? are you growing pot man?
Reread his post; dj245 said 22kWh per day. (22kWh)/(24h/day) ~ 917W continuous draw.
Better idea (Score:5, Funny)
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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Careful there, somebody might not realize you're joking.
Re:Better idea (Score:5, Funny)
Well I did hope the bit about handstraps was enough of a clue...
But come to think of it, the brilliance of the plan is how it keeps the trains on an exact schedule. Why yes, the train IS leaving at 10:43 even if you try to hold the door.
Re:Better idea (Score:4, Funny)
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.
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That only works if you've got perfectly inelastic trains.....
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Flintstones lives!
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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.
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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.
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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!
Storing energy in track elevation? (Score:4, Interesting)
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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])
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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.
better to use ultra-caps at the station (Score:3)
Ultra-caps are for LOW voltage/current DC (Score:2)
Try it with airplanes (Score:4)
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.)
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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!
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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
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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]
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Power != Energy (Score:5, Insightful)
The corrected sentence is much less impressive: "— that's enough energy to power 1,300 average U.S. homes for 30 seconds."
"Subsidize" city power systems? (Score:2)
Regenarative braking? (Score:3)
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...
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> 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.
The key is synchronization (Score:2)
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
Regenerative braking and its problems (Score:5, Interesting)
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.
What ultra-caps could possibly power a train? (Score:2)
Bombardier already sells these (Score:2)
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...
Deja vu (Score:2)
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
Re:Isn't this an old idea? (Score:4, Informative)
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.
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That actually makes sense - why have the train carry the regeneration unit when it actually can feed it's excess energy back to the grid. Lighter trains means less losses.
But it's hardly a new idea to feed energy back to the grid.
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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.
Why, because electricity doesn't flow down the third rail? Are these 3rd rails composed of multiple short segments, fed every few blocks?
Just askin9, because I've never seen the electrical connections up close.
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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.
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I don't know enough specifics about any given rail system, but ...
I am pretty sure they are provided in limited length circuits, so they aren't dependent on a single power source along a long route. This whole conversation is really best suited for the engineers who actually design and maintain those systems, not for us casual observers with our limited knowledge on the subject.
But, I'll continue my casual observer comments anyways. :)
If they're feeding energy from braking back to the 3rd rail, and it can
Re:Isn't this an old idea? (Score:4, Informative)
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.
Re:Isn't this an old idea? (Score:4, Informative)
Indeed, Regenerative Braking [wikipedia.org] 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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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.
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Ahh yes, the ol, brachistochrone railroad [google.com] technique.
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In this case [atsb.gov.au] I was amazed that the train could take off with such a small grade. Its pretty funny to imagine that train controllers could actually have redirected the train "back up the hill" into the Melbourne suburbs and it would have gone most of the way to the end of the line.
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Many stations in New York are already like that.
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1) I think it'd be pretty expensive and enormously disruptive to raise existing track at the stations along with the stations themselves. Adding a flywheel system, either under the platform or in the service area at the end of the platform would be a lot easier.
2) In some cities, the track at the stations is actually lower than the main line because the station had to be squeezed in under existing buildings or
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I think it'd be pretty expensive and enormously disruptive to raise existing track at the stations along with the stations themselves.
Yeah thats why I said "would have been". I intended this to be considered for totally new infrastructure. But still thinking of energy recovery I wonder what you could do with the air pushed along the tunnel by each train? Perhaps the leading train could be pushed off the blocks by the train behind it. Additionally I wonder if the cables and (sometimes) rails used for energy distribution could be replaced by batteries in this day and age. Batteries are expensive but maintaining all that cable is expensive t
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The first subway [wikipedia.org] operated on vacuum, half the time.
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I know you probably didn't mean it in that way- but that isn't the first subway in global terms. The Metropoliton Railway (the oldest bit of the London Underground) opened in 1863, 7 years earlier than the one you linked. Plus it was a proper passenger railway, rather than just a 100m long tech demonstration.
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That makes me think of the "Gravity train" [wikipedia.org]. I know, it isn't what you were referencing, nor quite on topic, but it's a neat (and implausible) idea.
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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.
Sounds like a win-win. (Score:4, Insightful)
It's okay. There are plenty more where those came from. PLENTY more.
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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.
Re:Isn't this an old idea? (Score:4, Informative)
Re:Isn't this an old idea? (Score:4, Informative)
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.
Re:Isn't this an old idea? (Score:5, Interesting)
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
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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 regenerat
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It means the person writing the article is an imbecile. All electric trains already brake by shunting power out to the grid, they suggest keeping it local to a station in a flywheel. Very little net gain really, just a bit of peak power smoothing. Better to have utility-scale power storage in the grid, for smoothing everything, not just trains. Flywheels is just one (tried and true) system for doing that.
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The idea might be old but getting it working on a much larger scale is not trivial. Being able to store 100 or so megajoules is pretty cool also.
Re:Toyota called... (Score:5, Insightful)
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.
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Almost all technology improvements today are just refinements or new uses of old tech. Look at what we do with nuclear power. We boiled water for steam with coal. Now we boil water with nuclear for steam. We only look tall because we're standing on the shoulders of giants.
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A lot of engineering is refinement, yes, but scientific advancements are often revolutionary.
Electricity for example had no equivalent before it -- it was an entirely new concept. One could argue that the internal combustion engine is just a variant of a piston steam engine, but the steam engine itself was a new concept: converting thermal energy to work by allowing hot gases to expand against a piston in a reciprocal way. That was so revolutionary that it started the Industrial Revolution!
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It could be argued that the water wheel started the industrial revolution...
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I think it might be less complex than you imagine. Instead of trying to manage the entire system like a big dance that has to be carefully coordinated, you just build in some slack and treat each station like a newton's cradle: just hold the departing train until the arriving train.. arrives.
You do have to build in enough slack to make sure that the doors are all shut, and of course there needs to be a plan to deal with the possibility that a train cannot clear the station in time for the arriving train,
Toyota called AGAIN (Score:3)
... 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.
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I know this principle was used a couple of decades before the prius by some university that was building an experimental 100+ mpg auto. Don't know who patented it or even if it is patented. It might even be a lot older than that.
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It's not a revolutionary invention, but it should be very helpful if they can cut the peak and the average power draw on the power grid by a substantial amount. There's an energy cost saving and also transmission grid saving. You don't need such a heavy connection between the train system to the general power grid.
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I've ridden the NYC subways and seen the tracks. If the stations are high points of any significance, I haven't seen it. It seems like a nice idea, but there is one significant problem. Steel (wheels) on steel (tracks) has a low coefficient of friction, particularly if there's water or oil involved. It's not uncommon for a subway train to wait just outside of a station, waiting for the track to clear. It would then have to start up and climb the grade into the station. To do this, the grade probably shouldn
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Only some of the lines in NYC do this, and in some parts, but they definitely do exist. I can't recall which, but I looked into it the first time I heard of the idea.
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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?
That's 1,300 houses for 30 seconds. (Score:2)
During the startup, the train uses as much power as 1,300 houses. (Does anyone else think that "houses" is a silly unit of power?)
So, the energy required to accelerate a train (it takes 30 seconds) could power 1,300 houses for 30 seconds.
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How about pedal power? Get your daily exercise but avoid the long walk through the city and get there faster as a group?
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http://www3.ttc.ca/About_the_TTC/Projects_and_initiatives/New_Subway_Train/index.jsp [www3.ttc.ca]
This train seats 404 (heh). The total would be 1,598. I bet in actual usage it exceeds 2,000-2,200 well-sardined people.
188 KJ per person, versus 1 MJ per person in a car (seeing as the average car around here carries 1.0000001 people), in rush hour.
That's a six car train. A ten-car would have almost 2700 people in it by 'spec'.
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When they would have electric trains, then they could introduce the braking energy into the electric system of the overhead line which can then be used to power other trains. However, this might be difficult with freight trains, as the freight cars do not have a connection to the electric system.