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."

Comments

Isn't this an old idea?

[atari2600a]

I thought EV's have been using the flywheel braking trick for years now...

Re:Isn't this an old idea?

[atari2600a]

Mother fucker... http://en.wikipedia.org/wiki/Regenerative_braking I'm fine with them proposing to retrofit an entire fleet of trains, but a press release like this really yells SLEASY OR FRAUD

Re:Isn't this an old idea?

[Anonymous Coward]

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.

Re:Isn't this an old idea?

[ScottyLad]

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.

Re:Isn't this an old idea?

[brusk]

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.

Re:Isn't this an old idea?

[nzac]

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:Isn't this an old idea?

[Anonymous Coward]

Yes, it's an old idea in general. But trains, despite being full-electric or diesel-electric-hybrids for decades, have traditionally just thrown out their braking energy as heat, either kinetic->electric->resistor bank or kinetic->friction brakes. So it's not new technology, it's just moving existing technology into a slow-moving, capital-intensive industry.

Re:Isn't this an old idea?

[jijitus]

And it isn't even the coolest train-related article on that site. Check out the one on moving platforms

Re:Isn't this an old idea?

[MichaelSmith]

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.

Re:Isn't this an old idea?

[hedwards]

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.

Re:Isn't this an old idea?

[Anonymous Coward]

I remember reading about a city doing this, maybe Boston?

Re:Isn't this an old idea?

[wagnerrp]

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?

[Technician]

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.

Re:Isn't this an old idea?

[zippthorne]

Ahh yes, the ol, brachistochrone railroad technique.

Re:Isn't this an old idea?

[Anonymous Coward]

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

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.

Re:Isn't this an old idea?

[Anonymous Coward]

The reason they don't do this is that it would require an insanely large inverter and insane amounts of current through the third rails.

Re:Isn't this an old idea?

[Warwick Allison]

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.

Re:Isn't this an old idea?

[errhuman]

They do this in the HK MTR system.

Re:Isn't this an old idea?

[ryanov]

Many stations in New York are already like that.

Re:Isn't this an old idea?

[morethanapapercert]

As others have mentioned, that's been done. I just have a couple of thoughts to throw out there:

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 the below ground infrastructure.

3)In some cities the main track has to deal with enough changes in elevation as it is; enough, I suspect to negate the benefits of elevated track at the stations. The Bloor-Danforth line rises up out of the Don River valley quite a ways before it meets up with the Scarborough RT. IIRC from my local history class; that line was mostly done with the cut and cover method, which is much cheaper, as long as you follow the surface terrain loosely, otherwise some areas the trenches get impractically deep. (which is why some of the line was tunneled)

Re:Isn't this an old idea?

[Z00L00K]

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.

Re:Isn't this an old idea?

[kanweg]

Next step: Put the trains in vacuum tubes and use magnets to levitate the trains above tracks, and the trains can move around while hardly using any energy.

Bert

Re:Isn't this an old idea?

[FooAtWFU]

And they're doing this on the new Second Avenue Subway in New York City, as well.

Re:Isn't this an old idea?

[adolf]

The first subway operated on vacuum, half the time.

Re:Isn't this an old idea?

[icebike]

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.

Re:Isn't this an old idea?

[MichaelSmith]

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 too.

Re:Isn't this an old idea?

[MichaelSmith]

In this case 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.

Re:Isn't this an old idea?

[Mindcontrolled]

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?

Re:Isn't this an old idea?

[ESD]

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.

Re:Isn't this an old idea?

[JWSmythe]

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 be captured at a station with whatever means (batteries, capacitors, flywheel, or whatever), it could then be used when it starts moving also.

Even if the 3rd rail is only for a few blocks at most, there is a common power grid that it can feed back into. The question would then be, can the power grid support it? They are already geared up for the heavy load of a train starting to move from a stop, but would the introduction of excess energy in such short bursts and high current, be useful or harmful to the grid?

Re:Isn't this an old idea?

[JWSmythe]

    That makes me think of the "Gravity train". I know, it isn't what you were referencing, nor quite on topic, but it's a neat (and implausible) idea.

Re:Isn't this an old idea?

[vadim_t]

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?

[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

Re:Isn't this an old idea?

[Patch86]

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.

Re:Isn't this an old idea?

[isorox]

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?

[Smallpond]

Let's see. 3-4 MW x 30 seconds = 100 MJ. A high power car battery can hold about 4MJ so it might seem plausible, but the batteries would have to be able to support very high charge-discharge rates and hundreds of cycles per day, which would quickly wear out existing lead-acid batteries. Flywheel storage seems like a cheaper alternative. Not sure about the conversion of kinetic to electrical energy then back to kinetic in the flywheel. Maybe there is a better way.

Re:Isn't this an old idea?

[Nemyst]

The entire Montreal subway system is designed this way. On top of providing some measure of breaking power, it also helps with water flooding by draining it all towards specific locations that can then be connected to the rest of the city's water system.

Re:Isn't this an old idea?

[kheldan]

YES, it is: It's called "regenerative braking". Install ultracapacitors or similar electric energy storage technology in the cars, and use the motors that otherwise drive the cars as generators during braking, shunting the electricity generated into temporary storage instead of converting that energy to heat, then use the stored energy as part of the start-up current to get the car moving again. Not a new idea by far.

Re:Isn't this an old idea?

[Miamicanes]

The biggest problem isn't engineering, it's regulatory. About 15 or 20 years ago, Deutschebahn came up with a similar idea for its (then new) ICE trains -- for certain stops with asymmetric passenger traffic (lots more passengers getting off than getting on), they could have extra trains that served only arriving passengers, put them all in the last few cars of the train, then uncouple it before arriving in the station and use automatic train control (or a cross-trained conductor) to bring it into the station while the rest of the train continued without stopping. I'm not sure whether the scheme involved coasting an unpowered tail section into the station, or whether it involved EMUs that could power themselves the last kilometer or two.

Either way, the regulators *freaked*. On so many levels, DB just decided it was a completely hopeless lost cause and gave up. Railroad laws going back to the 19th century regulate everything from how braking systems work to the exact procedures that have to be followed for coupling and uncoupling them. Union agreements govern minutiae of their actual operation, in ways that would have basically required a full staff of engineers, conductors, and support personnel for that tail segment from the moment it broke away until the moment it arrived in the station. And if that doesn't kill it, you still have actual passengers who are in the wrong place at the wrong time & end up missing their station or getting off by mistake. And this is a "moving platform" type solution that's relatively STRAIGHTFORWARD, engineering-wise. God help anyone who tries to implement anything that additionally requires precise alignment of parallel tracks, precision speed control, and countless safety mechanisms to ensure that the trains can never, ever break apart and shear a passenger in half as he or she changes trains. I can't even fathom how the parallel-train idea could work with track that isn't dead straight.

For what it's worth, other countries looked at DB's idea, because it's sensible and cool. Even Amtrak looked at it. They all came to the same conclusion -- the same regulatory problems that "derailed" it in Germany would be as bad, if not worse, in their own countries. I believe the closest it came to fruition anywhere in the world was Israel (a small country starting out with no real rail network to speak of, and thus no bureaucratic legacy the way you'd have in Germany or the US), but I'm not sure what happened to stop in in Israel (I think it just never got funded).

Re:Isn't this an old idea?

[Miamicanes]

Sounds like the peoplemover at Atlanta International Airport years ago. From what I remember, a red light flashed, a robotic voice said, "Stop boarding ! Stop boarding! Stop boarding!", and on the third "Stop boarding," the doors would slam shut like they really, really *meant* it. I suspect the system has been mellowed out a bit over the past 10-20 years, but I remember being somewhat amused at the time by its cold determination to slam the doors shut (in contrast to the wimpy, uber-cautious creepingly-slow doors found on peoplemovers at most airports at the time).

Re:Isn't this an old idea?

[Miamicanes]

I don't think vacuum-evacuated tubes are really practical for a subway environment, where you have stations every mile or less. They're really more practical for long-distance trains.

Also, you wouldn't necessarily run them as a complete vacuum. I believe most proposals involve a partial vacuum with increased oxygen concentration. The idea is that you're removing most of the air resistance, but putting enough pure oxygen into what remains to enable people to breathe and survive, and keeping enough pressure to keep passengers who evacuate a train from getting the bends. You'd have emergency systems in place to flood the tunnel with atmospheric air if passengers had to evacuate a train (so they wouldn't spend extended amounts of time at low air pressure), and have maintenance crews depressurize before going in for extended time (kind of like deep sea diving in reverse).

I believe the best-researched proposal to date is probably the one for SwissMetro, because its backers actually HAVE done a fair amount of work so far to balance commercial viability against risks & benefits.

Re:Isn't this an old idea?

[Ferzerp]

Battery? Why did you choose the most awful energy storage medium for this application. If we only had some sort of storage medium that is great for relatively short term storage, high charge and discharge rates...

If only someone would create such a device... It would probably be, oh, about 10x the weight of a battery for a given amount of energy storage. I bet it would be pretty easy to shock yourself with one, too, since the potential discharge rate would be so high... Won't someone think of the subways!

Re:Isn't this an old idea?

[Miamicanes]

It's been done for years. The catch is that it's only really practical for bored tunnels (cut & cover tunnels would require too much deep excavation), and the point that's optimal from an energy standpoint would be unacceptable to passengers (think: roller coaster, literally.) Remember, subway cars depend upon having a certain percentage of passengers standing up and walking around while the train is in motion, so G-forces that would be fine in a jet that's taking off (or in a private car with one passenger who's wearing a seat belt) would be completely unacceptable in a subway car.

It's the same reason why passenger trains aren't equipped with the same technology as antilock brakes to enable them to quickly stop on a dime to avoid crossing accidents. In the US, at least, a speeding train will generally slice through any car or truck (besides maybe a gas truck) that's stopped across the tracks, and the passengers on board will barely notice anything besides the noise and subsequent delay. On the other hand, if the engineer actually tried to STOP a speeding passenger train within a few hundred feet, anybody standing in the aisle would end up getting hurled against the car's interior, and everything not literally screwed down (laptops, drinks, purses, small infants) would turn into missiles. You'd save the life of 1-6 people in a trapped car stopped across the tracks, and kill or injure several hundred passengers on the train in the process.

Re:Isn't this an old idea?

[downhole]

I actually thought of this a while ago too, more in terms of replacing air travel with long-distance vacuum maglev subways. It'd be cool in that you could just keep accelerating to the halfway point with no effective top speed - travel times might be an order of magnitude shorter than aircraft. Main downsides I can think of are:

Constructing even one such inter-city tunnel would be insanely expensive, much less a network comparable to the current air travel system.

Nobody knows how much of a pain in the ass it would be to maintain the vacuum. And it could be a big failure point - if you end up needing pumping stations every few miles and one of them fails, do you suddenly pressurize the tubes and effectively vaporize some train running at 50,000mph or so?

Loading and unloading will be a pain in the ass. You'll need long-term reliable very low-leakage doors and couplings on the trains and stations. And then how do you fix broken ones without re-pressurizing the whole system? Maybe we should make them full airlocks on both the trains and stations...

Re:Isn't this an old idea?

[joe_frisch]

Yes, I remember a flywheel regenerative braking test on new york subways done in the late 70's. I don't know why it wasn't adopted back then. Like many of the "new" energy ideas floating around now, it is a re-do of ideas from the previous "energy crisis"

Re:Isn't this an old idea?

[whimmel]

When I was piloting Disney's monorails, a fully loaded train would draw up to 2000A at 600VDC accelerating out of a station. The regen braking would also generate 2000A slowing to about 20mph before the friction brakes stepped in. Unfortunately that power is burned off in resistor banks located between each car.

Re:Isn't this an old idea?

[gumbi west]

in NYC the subways are DC powered by DC generators. They are on their own grid. When the power went out in 2002(?) it was only the lights (used for collision prevention) that used the grid's AC power and prevented the subways from running.

I think the point is that you would have to generate less for the subway than that you could use the power right away in another facility.

Re:Isn't this an old idea?

[JWSmythe]

    Since you obviously know more about their system than me (seriously, not sarcastically), where does the power come from? Oh, never mind, I found the answer.

    The NYC subway system is amazing. I, like most people who have used it, know that we get from Point A to Point B rather efficiently.

Re:Isn't this an old idea?

[atari2600a]

My thoughts exactly, when someone pointed out the third line resistance, if it really is a problem.

Re:Isn't this an old idea?

[gumbi west]

Huh, I hadn't realized the got rid of the DC generators. Thanks for sharing that.

Re:Isn't this an old idea?

[Coren22]

Just passed through this weekend. Yes, they have changed quite a bit. I do recall it saying something about not trying to block the doors, but I didn't notice the doors slamming especially hard.

hm

[Osgeld]

never really thought about how much juice a subway train draws on startup. thanks

Regenerative braking?

[hawguy]

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?

[kevinqtipreedy]

I was thinking the same thing. I'm guess what they meant to say was 3-4 Megawatt Hours over the 30 seconds, which would bring and amperage to 42-56A.

Re:Regenerative braking?

[tftp]

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.

Re:Regenerative braking?

[Nemyst]

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.

Re:Regenerative braking?

[Anonymous Coward]

I don't see how feeding energy into local flywheels is any different than feeding it back into the grid?

The difference is, if they feed the energy back into the grid, then Vycon doesn't sell as many flywheels.

Re:Regenerative braking?

[robbak]

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 train to use it. Absorbing it in a local storage flywheel and pumping it back a few minutes latter makes good sense.

Re:Regenerative braking?

[Anonymous Coward]

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.

Re:Regenerative braking?

[timeOday]

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.

Re:Regenerative braking?

[sam0737]

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.

Re:Regenerative braking?

[DaleSwanson]

Has anybody tried just building the stations on hills?

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

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.

Re:Regenerative braking?

[Dunbal]

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.

Re:Regenerative braking?

[Dunbal]

Has anybody tried just building the stations on hills?

Yah lots of hills in downtown Manhattan. You could just knock some of those buildings down land is cheap over there I hear. Didn't they like buy the whole island for $24 or something? Or you could dig the tunnels a little deeper because it's just dirt, right? And there's no ground-water from the Hudson river to worry about when you blast your way deep enough....

Re:Regenerative braking?

[ryanov]

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?

Re:Regenerative braking?

[Urkki]

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) when talking about electrical energy, to avoid confusion.

Re:Regenerative braking?

[Anonymous Coward]

They would create the hills by digging, asshole. What, you think those subway tunnels are already part of the natural landscape?

The comment you're replying to even says this ("... building the stations on the hills? (I.e. putting the platforms at less depth than the rest of the tunnel?)") (emphasis mine). He's not talking about building subway lines around existing landscape, he's talking about shaping the landscape to fit the subway line (with inclines around the platforms).

Re:Regenerative braking?

[Anonymous Coward]

Large motors arent operated like this. They will indeed draw those amps the whole 30 seconds. If the train would just be switched to the grid like you described it would be very uncomfortable to ride.

Rule of thumb: The current in an elecric motor equals torque (multiplied by a machine specific value). That holds true even for non-DC motors which nowadays are operated trough variable-frequency drives.

Re:Regenerative braking?

[sjames]

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.

Re:Regenerative braking?

[Anonymous Coward]

i believe this is how the london underground (in the most part) is built

Re:Regenerative braking?

[jo_ham]

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 a fool.

Re:Regenerative braking?

[Anonymous Coward]

And 50A would be more than adequate bring a subway train up to, say, 5 mph.

Moreover, you fail to grasp the basics of unit conversion -- 3 MWh in 30s is 1h/30s = 120x the power of 3 MW, not 1/120th. I'm not sure what sort of half-assery causes that mistake, but if your answer makes no sense (like ~50hp for a train), you double-check your math and then you might have caught & fixed it.

Maybe next time actually think about the numbers you're pulling out of your ass before you claim trains run on the power of a 500cc sport bike, huh?

Re:Regenerative braking?

[ChumpusRex2003]

Yes. The power levels given are correct. A large subway train can draw around 3 MW during acceleration, potentially up to 4 MW if it is a modern 'all wheel driven' type train with high acceleration capability. This is very dependent on the size and speed of train - but taking something like the http://en.wikipedia.org/wiki/London_Underground_S_Stock as an example, the trains can be about 150 meters long, with acceleration of 1.3 m/s2.

The typical electrical energy cost of accelerating such a subway train, up to 'operating' speed is around 20-30 kWh.

Re:Regenerative braking?

[Anonymous Coward]

short tons, mph and then MJ? Shouldn't you be using something like Giga BTUs?

Re:Regenerative braking?

[vlm]

.... 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.

This strikes me as spectacularly unlikely. Where does the 6 MW go? If into motive power, at 700 some watts per HP, that's way over 6 thousand horsepower.

Please compare your little people-trams with a fractional mile long Amtrak line, with far lower HP. Or compare that little people tram with a mountain moving coal train, that might by combining a couple million pound engines. Speaking of million pound engines, you can push as many HP into steel on steel wheels as you want, but you'll never get over about a HP per thousand pounds without wheel spin, unless you aggressively sand. I know the average American's weight has increased over the years with HFCS and all that, but the average dinky people mover cannot weigh 7 million pounds.

Another non-engineering analysis is to compare the little people mover to a "real" 6 MW train, the TGV

http://en.wikipedia.org/wiki/TGV#Rolling_stock

Is a 6 MW class people mover 700 feet long and travels at 186 miles per hour? I think not.

It may be designed to draw 6 MW momentarily while wheels are locked before the fuse blows, kind of like how Sears (are they still in business?) used to advertise "5 horsepower" vacuum cleaners and "5 horsepower" air compressors that miraculously operated off 110 volt lamp cords connected to conventional 15 amp shared circuits at merely a couple amps real world draw. Then again they were also famous for advertising their compressor CFM at 20 psi instead of 90 psi like everyone else.

Re:Regenerative braking?

[Anonymous Coward]

[[I work for the Power department of the New York City Subway system]]
Citation needed.

Re:Regenerative braking?

[Anonymous Coward]

Actually the resistance does not change at all. The moving coil/magnetic field produces a back emf just like a generator. So the required current becomes I = (Vorig - Vgen)/R

Re:Regenerative braking?

[AmiMoJo]

And that explains why a flywheel is a good idea. It is difficult to store large amounts of electrical energy and then release it very quickly, but a flywheel can be mechanically coupled with a simple clutch.

Re:Toyota called...

[artor3]

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.

Energy != work

[zill]

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?

Re:Energy != work

[webdog314]

The way I read it, 30 seconds.

Re:Energy != work

[bromoseltzer]

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.

Re:Energy != work

[Anonymous Coward]

That's not the point. The train, while accelerating, consumes 3-4MW of power, placing 3-4MW worth of strain on the grid's energy budget, which is being equivocated to the load of 1300 average homes. The grid is rated for peak instantaneous power, not work.

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.

3MW 1300 average homes 4MW

so

2.3kW 1 average home 3.1kW

The numbers themselves seem reasonable.

Re:Energy != work

[knuthin]

As humans made conscious about every little energy we waste by TV and internet and other stuff, we are so desperate to find new sources of energy.

After extracting energy unsuccessfully from cow fart, cow dung and human pee, now we do this. :/

Re:Energy != work

[Idarubicin]

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 per train. Not good.

The advantage here is that the energy is stored (in flywheels at each station) and fed back to the same train a little bit later. Nothing goes back to the grid, and a lot less is drawn from it. That's how this (or other, similar systems which have been proposed in the past) would reduce the strain on the grid.

Re:Energy != work

[Dunbal]

IANAElectrician, but if you're storing the power locally couldn't you isolate sections of the track from each other and the grid in general and have an "acceleration" track to avoid what you describe? The connection with the rest of the rail could be broken when a train is about to leave, and the flywheel + grid is fed into a predetermined length of isolated track. The train accelerates with power from the flywheel (plus some grid) and crosses onto the main track via inertia but since it's at speed already the current draw is not that much so there shouldn't be a huge change when the train switches to the main track. Likewise the decelerating train leaves the main track and pumps energy back into the flywheel system only, not the whole network.

Re:Energy != work

[Anonymous Coward]

That sounds like a good idea but even on the "non-acceleration" track you have to allow for a stopped train to draw enough current to get moving again.

Re:Energy != work

[Anonymous Coward]

3-4 MW is 3k - 4k kW. That's 2.3 - 3 kW per household of the 1,300 households. The average home pulls about 1200 kW from the grid at any given moment.

So actually it would produce about 60 seconds of power for the average home.

Re:Energy != work

[FrankieBaby1986]

Power (MW) != Energy (MWh). Not matter how you try to read it, the summary sucks.

Slashvertisement? Overthinking?

[Anonymous Coward]

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 â" that's enough energy to power 1,300 average U.S. homes.

1. Misleading at best. That's "power 1,300 average US homes for 30 seconds".

2. So why not have subway cars stagger start on as many lines as possible? You know, to limit concurrent start of trains. This should limit any "peak power emergencies".

3. Don't train systems have a separate, dedicated power lines with their own transformers and such, effectively limiting the amount of power they can draw from grid at once? This basically "browns out" the subway train system, unaffectedly the larger grid.

4. Seems like slashvertisement.

U-shaped tunnels

[Anonymous Coward]

I read somewhere that subway tunnels dip down between stations, so that the train gets an automatic boost as it departs and breaking assistance as it arrives at the next station. In physics terms, there is a transfer between kinetic energy and gravitational potential energy. It sounds like an elegant low-tech solution, -- no need for flywheels, and nearly as fun as a rollercoaster.

Re:U-shaped tunnels

[ChrisMaple]

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't exceed about 3%. (Although that's a limit for railroads. Subways are powered at each car, so they can tolerate steeper grades.)

To get back to the article, a flywheel seems like a poor idea, unnecessarily complicated and dangerous. If there's energy storage involved, then store it by lifting a weight 50 feet or so.

Re:U-shaped tunnels

[ryanov]

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.

Better idea

[SuperKendall]

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.

Re:Better idea

[amiga3D]

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

Re:Better idea

[formfeed]

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.

Re:Better idea

[Ryanrule]

Well, you could use the energy of the train stopping on one side to fire off the train on the other side.

Re:Better idea

[Anonymous Coward]

Or better yet, don't stop the trains. Just like at Disney World, but much, much faster!

Re:Better idea

[SuperKendall]

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

[zippthorne]

That only works if you've got perfectly inelastic trains.....

Re:Better idea

[Dunbal]

Or have the passengers push the trains to the next station.

Re:Better idea

[Tablizer]

Flintstones lives!

Re:Better idea

[Anonymous Coward]

This is an amusing idea if it wasn't for the whiplash and lawsuits it would induce.

They probably should have just put flywheels in the lead and tail car to give the subway a boost starting up.

I wonder if any of this applies to the LIM's used in the Bombardier ART technology. They pretty much stop and start on a dime if needed.

Re:Better idea

[jovius]

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.

Re:Better idea

[sjames]

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.

Re:Better idea

[Mindcontrolled]

Also, point-shaped ones. In a frictionless vacuum. Emitting and attracting passengers isotropically...

Re:Better idea

[Splab]

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!

Re:Better idea

[orange47]

I have an even better idea. Why stop train at all? Passengers could be sling-shot with some mechanism in moving train, one by one; through open roof or something.

Re:Better idea

[trout007]

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.

Re:Better idea

[Anonymous Coward]

That reads like something out of a Kurt Vonnegut book.

Is it practical?

[Anonymous Coward]

Is it practical to have the passengers push start the subway trains?

Re:Is it practical?

[foniksonik]

How about pedal power? Get your daily exercise but avoid the long walk through the city and get there faster as a group?

Toyota called AGAIN

[ChrisCampbell47]

... 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.

Re:Toyota called...

[Anonymous Coward]

Hmm... it seems like they want to use flywheels to store the energy on that overly green page. IMHO, it seems far easier to just synchronize the trains so one is accelerating while another is decelerating. That's still a complex problem, but it requires far less hardware. Of course, it's a flywheel company. When all you have is a hammer, everything looks like a nail. Doubly so if you make money by hammering.

Re:Toyota called...

[amiga3D]

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.

Re:Toyota called...

[amiga3D]

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.

Re:Toyota called...

[bromoseltzer]

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.

Sounds Interesting

[Anonymous Coward]

So the basic idea is to have a giant flywheel in the train station that is spun up from the incoming train's electric regenerative braking. It sounds ingenious because it doesn't require any fancy equipment, just voltage regulators and some fancy switches. Plus any light rail line could be retrofitted.

90% efficient sounds pretty optimistic, I'd think 60-70% is more realistic without seeing a real-world proof of concept. Still, 60% of four megawatts is a freaking huge amount of power.

Re:Sounds Interesting

[j-beda]

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?

Re:Toyota called...

[Anonymous Coward]

All technology improvements EVER are just refinements or new uses of old tech.

Fixed.

If you go far back enough, you'll find that every form of technology grew out of imitating/advancing things that already existed.

I salute you Cavemen Ugg and your using rocks to break open nuts. It's thanks to you that we can now split atoms.

better to use ultra-caps at the station

[WindBourne]

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.

Re:better to use ultra-caps at the station

[Anonymous Coward]

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.

And the flywheel doesn't "have power"? It's simply a function of the electrical machine (combination motor/generator) you have hooked to it. Anyway, electrolytic photoflash caps beat ultra-caps all hollow for power density, there's a whole range of power/energy density tradeoff -- just as there is with flywheel/machine systems. One assumes that in either case, the energy system would be specified in accordance with the actual energy and power requirements.

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.

Again, same exact thing for flywheels -- friction is a drag, but for short-term storage like this, not a big issue.

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.

And you finally have a point against their proposed implementation. But you could put flywheels at the stations, too, right?

Re:better to use ultra-caps at the station

[calidoscope]

The application calls for a storage system that can provide up to 6MW for about 20 to 30 seconds. My gut reaction is that ultracaps would be a better choice than flywheels, the discharge rate is close to optimal for ultracaps and the cycle life would probably be much higher for ultracaps (especially since the caps would rarely be fully discharged). In addition, the control scheme would be much simpler, the caps would be floating on the third rail supply, not needing the power conversion electronics and motor/alternator on flywheels.

Try it with airplanes

[crow]

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:Try it with airplanes

[BradleyUffner]

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!

Re:Try it with airplanes

[Anonymous Coward]

I believe Carriers already do something fairly efficient - US Nuclear ones at least use compressed steam generated by their reactors to shoot the slingshot that is used to assist take-off.

Re:Try it with airplanes

[Baloroth]

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 is used getting up to speed, very little staying there and most of this can be recouped. This is also why hybrid cars are more fuel efficient than regular vehicles. Their theoretical highway mileage would in fact be the same if non-hybrids didn't need such a large engine for acceleration.

Not to say your idea is terrible, it just wouldn't have much impact. Also, storing energy from a plane would be technically extremely difficult, although the US Navy's new electromagnetic launch system might be able to do it.

Re:Try it with airplanes

[Anonymous Coward]

the navy has no need for such a technology. Ever since the USS Enterprise, carriers have had an abundance of power and not much to do with it.

Complicating the landing hook is a stupid idea.

Re:Try it with airplanes

[Dunbal]

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?

Re:Try it with airplanes

[Anonymous Coward]

Sorry, won't work with aircraft. The amount of energy available in a landing plane is a tiny, teeny fraction of the energy consumed during the trip. Most aircraft bun energy to get to altitude, then set for a "most economical" rate of fuel burn for cruise, and then more or less coast into the airport once they get reasonably close.

It takes me about 10 minutes to climb to 5,000 feet in my Cessna 182. When I descend, I also do so at around 500 feet/minute, and I don't use flaps or anything to induce drag until I'm down near the airport, at least in the pattern if not on final. By the time I get anywhere near the ground, all the energy I spent climbing has been spent.

Re:Try it with airplanes

[WindBourne]

actually, our new ones use maglev systems.

Re:Try it with airplanes

[Anonymous Coward]

Difficult? Just put put half the runway on a hill.

Re:Try it with airplanes

[bazorg]

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

Re:Try it with airplanes

[Anonymous Coward]

No, they don't. They use linear motors, which is a completely different concept.

Re:Try it with airplanes

[u38cg]

Try a Planet of the Dead.

Re:Try it with airplanes

[djlemma]

On aircraft carriers, the pilots already are encouraged to make really hard landings to make sure the tailhook catches.. and they also gun the engines as they land, just in case the hook doesn't catch.

Re:Try it with airplanes

[WindBourne]

oops. thanx. I meant to say linear motor. just sloppy.

Re:Try it with airplanes

[Idarubicin]

The energy in subway trains is dwarfed by the energy used and lost on runways for jetliners.

A jetliner starts and stops once every four hours. A subway train starts and stops once every minute or two.

Non-military aircraft (and for that matter, non-carrier-based military aircraft) aren't designed and strengthened to survive being launched or caught by ground-based restraints. Are you sure that you can offset the energy cost of keeping that extra weight (the additional hardware and heavier framing) aloft? For that matter, can you justify the greater risk of accidents caused by the more-complex landing and takeoff system your idea would require?

Re:Try it with airplanes

[Anonymous Coward]

Nuclear Powered Subway Trains? I LIKE it!

Indian Point does generate about 30% of NYC's electricity.

Re:Try it with airplanes

[Anonymous Coward]

Your info is way out of date. A modern jet aircraft doesn’t “fly” off the ground with "wing-lift". It accelerates down the runway until it can tilt back and comfortably shoot up.

Re:Toyota called...

[bertok]

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!

That's 1,300 houses for 30 seconds.

[robbak]

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.

Re:That's 1,300 houses for 30 seconds.

[Dunbal]

Yes especially in Manhattan, it should be 1300 one bedroom studio apartments.

Sounds like a win-win.

[causality]

Well, there's also the issue of idiots being torn to pieces when they try to get through the door at the last moment.

It's okay. There are plenty more where those came from. PLENTY more.

Re:Sounds like a win-win.

[TheLink]

But they might splatter blood on more people's clothes that way.

2-3kW per home is reasonable?

[dj245]

My energy use last month was about 22kW-hr per day according to the utility. That's about 920 watts continuously. This is for a 3 bedroom house inhabited by 2 people, with a 55" TV, a couple computers, air conditioning set at 77F with an average outside temperature of 75F to 90F.

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 journalists abuse measurements like this.

Re:2-3kW per home is reasonable?

[Anonymous Coward]

22 fucking kw/day? do you drive all-electric cars? are you growing pot man?

Re:2-3kW per home is reasonable?

[dwywit]

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.

Re:2-3kW per home is reasonable?

[Lonewolf666]

If you choose your appliances for moderate energy consumption, then even less is possible.
My electrical power consumption (not running my own washing machine, no electrical air conditioning or heating) is around 1000kWh/year or ~3kWh/day. Yet I don't think I'm missing out on much quality of life.

This is made possible by mostly using compact fluorescent lamps, a computer built from energy efficient parts and a well insulated apartment that does not heat up too much in summer, despite being under the roof.

Re:2-3kW per home is reasonable?

[cffrost]

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.

Re:2-3kW per home is reasonable?

[Anonymous Coward]

I live in Houston and have my thermostats programmed to increase temperature when nobody's here, and according to August's bill, the average daily temperature was 102F, and my average daily usage was 93kWh for a 4 bedroom house with two AC units.

What the fuck do you do, use candles and cook with gas?

Subway != Energy Efficiency vs Automobile!

[Anonymous Coward]

So a 10 car train requires 30 MW for 10 sec to accel to cruise speed. That's 300 M Joules.

A car consumes about 100 kW during accel. For 10 sec of accel time, a car consumes 1 M Joule.

It would seem to me a 10 car train better have 300 people on it before it starts consuming less energy/passenger during a given accel time. I doubt this happens except in the extremes of rush hour. 30 people/car in every car of a 10 car train is believable during a rush hour. But the energy savings are much less than I'd of thought. Include the fact, that the train must travel with or without passengers, at all hours, and I'm really starting to lose faith in the proposition that mass transit is more energy efficient let alone much more fuel efficient than personal automobile transport.

At least a subway can be powered by coal or nuclear power and not oil/petrol.

Humm......

Re:Subway != Energy Efficiency vs Automobile!

[The Dawn Of Time]

30 people per car in the peak of rush hour? I guess you don't ride a lot of subways.

Re:Subway != Energy Efficiency vs Automobile!

[Renegrade]

http://www3.ttc.ca/About_the_TTC/Projects_and_initiatives/New_Subway_Train/index.jsp

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'.

Re:Toyota called...

[wagnerrp]

Because the concept of regenerative braking has been around long before the Prius was a twinkle in Toyota's eye. Bringing them into this, alluding that they invented it, is what makes you sound like a troll.

Power != Energy

[FrankDrebin]

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."

Re:Power != Energy

[Anonymous Coward]

Thank you, that was my observation as well.

Re:Power != Energy

[Anonymous Coward]

if you don't have to qualify the time you can make it sound much more impressive.

enough energy to power 1 300 000 average US homes! (...for 0.03 seconds)
enough energy to power the whole world! (.... for 0.00001 seconds) (yes, a guess - I expect there're a lot of houses worldwide that draw no current at all, ever)

Re:Power != Energy

[Anonymous Coward]

or one home for 39,000 seconds

Re:Power != Energy

[Anonymous Coward]

Actually it remains quite impressive to us, in the third world.

Re:Power != Energy

[Anonymous Coward]

It's still one home for a day

Re:Power != Energy

[Anonymous Coward]

The point being it is a significant draw on the grid, especially because it's only for ~30 seconds every time. I do wonder however, if the subway companies around the world care enough for the power grid to try fix the problem. There has to be some kind of incentive to invest in it, and I just can't imagine there is any.

Re:Power != Energy

[Anonymous Coward]

haha, you are the guy with the comment i was looking for. agreed

Re:Power != Energy

[Anonymous Coward]

Except that there is always at least one or more trains leaving a station at any given moment.

So combined, it will be powering more than 1,300 homes.

Re:Power != Energy

[Anonymous Coward]

Why you got modded insightful for something that is blindingly fucking obvious, I don't know. The article and the summary both said 30 seconds. How do I know?

requires a jolt of three to four megawatts of power for 30 seconds to get up to cruising speed

BLAM! Right there. 30 seconds. 1300 homes draws the same amount of power. So.. they get powered for the same amount of time as the train. This is not difficult. Do you call hotel front desks and ask them to give you a wakeup call at 6.30AM in the morning? Perhaps you like cereal in your cereal so you can be unimpressed with whole grain while you're unimpressed with your Wheaties.

The fact that it takes the same power as 1300 homes to start a subway train is impressive. The fact that you think "oh, its only for 30 seconds.. thats nothing" is weird.

Re:Power != Energy

[Anonymous Coward]

or... "Enough energy to power 1 home for 10 hours..."

Re:Toyota called...

[rthille]

It could be argued that the water wheel started the industrial revolution...

"Subsidize" city power systems?

[Radical Moderate]

Don't they mean supplement? I realize it's Saturday, but come on, editors.

Re:Toyota called...

[The Dawn Of Time]

Lighten up, Francis.

Re:Toyota called...

[zippthorne]

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, and you won't be able to do a constant acceleration on either side. 1G deceleration of the arriving train at the beginning of its decel corresponds to a lot more than 1G of acceleration for the departing train....

Re:Toyota called...

[Anonymous Coward]

If something is an entirely new concept it would be impossible to understand since we would have no basis to understand it WITH. At least some element of the past MUST be used to evolve or discover said concept.

Everything comes from everything else. Since all things are based on their *relation* to other things, NO invention or concept can be considered wholly new or unconnected from the past.

Regenarative braking?

[Hamsterdan]

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...

Re:Regenarative braking?

[Maury Markowitz]

> 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.

Re:Regenarative braking?

[Anonymous Coward]

Regenerative braking is only about 30% - 50% efficient. This is probably about 70-90% for flywheels. Also Flywheels could be made to last 20+ years with little degradation in efficiency which batteries can't.

The key is synchronization

[realyendor]

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.

Tap a train?

[Anonymous Coward]

I'd rather Tap YO MAMA's ass for energy.

She got it going on!

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.

How many Libraries of Congress...

[Anonymous Coward]

...would train startup energy power?

startup next train on braking energy of the former

[Anonymous Coward]

This would be the easiest solution wouldn't it ?
As soon as the first train brakes the energy is fed into the grid at which time the next train uses it up to accelerate. Easy.

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:Storing energy in track elevation?

[xaxa]

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)

Re:Storing energy in track elevation?

[Anonymous Coward]

They do in the UK, at least. There track between some stations on the London underground is a couple of meters lower than the track right at the platform. The only problem is when the train in front of you is late leaving so your train has to stop before reaching the upslope...

Re:Storing energy in track elevation?

[Anonymous Coward]

It's been tried on LU's central line. It helps, but not much.

Downhill solution

[Anonymous Coward]

Build all roads and tracks downhill, to allow coasting to the destination.
There. Problem solved.

DC Metro already does this

[Anonymous Coward]

Train cars on the DC Metro have flywheels that recoups some of the energy lost during breaking. If your standing at the right point you can feel/hear the whizz of it.

Re:Toyota called...

[6Yankee]

What's more revolutionary than a flywheel? :)

Re:Toyota called...

[JWSmythe]

    Robin, guessing by your UID, you haven't been around long. :) Don't worry about how people moderate you. You were kind of asking for it with your smart ass post, and attributing the technology to Toyota, when it's been around for longer than Toyota has been around, much less any concept of them building electric (or hybrid) vehicles.

    With that said, Say your peace as you feel appropriate. I've had a lot of fun watching some of my posts get modded up and down so many times that it's hard to tell where it will land. Some people mod down because they don't like what you said, or just because they're asses. Sometimes we just click the wrong option. It happens. That's why the moderation system is in place, so others can fix the mistakes, and set the score properly based on a consensus, rather than one person's opinion.

    If you're always being modded down, and no one else mods you up, you should consider the content of what you're writing. There are plenty of people who mod up when a post has unfairly been modded down (and vice versa). If you're posting interested, informed, and intellectual posts, on average, you'll end up getting positive mods, and usually decent replies.

    Once you've been around for a few years, if you've been giving to the community (i.e., intelligent conversation, not smart ass answers and complaining), you'll find that you have a virtually indestructible karma rating. But you have to earn that. That's why I'm posting this as myself, and not AC. I most likely will be modded down as off-topic, since this clearly is. Will I complain or cry? Nope. It doesn't really matter. And if I can get through to you and a few other lurkers who are afraid of being moderated, it was well worth it. Well, that is assuming you're bringing intellectual conversation to the table.

  If you want to complain about moderation, take it to your journal. It's a good place to vent, and it won't interrupt the thread for the topic.

Ultra-caps are for LOW voltage/current DC

[csirac]

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 (current) for charge capacity/density.

Re:Ultra-caps are for LOW voltage/current DC

[WindBourne]

Well, Lets see.
LRT system vehicles are powered by 900 volts direct current (Vdc) electricity. The Traction Electrification System provides power via a traction power system and
WOW. So, they are using 900 volts DC. So, that means your first part is WRONG.
However, you are right on high voltage. It is 900 volts. But, how do we go from low voltage to high-voltage? Oh yes. SERIAL CONNECTION.
Now, was that so hard? Nope.

Now, as the voltage vs charge cap/density, yes, you are correct that CURRENT tech does sacrifice. However, if storage at the station, then not a big deal. And as to the rest, it is amazing what they can do with power these days. Just amazing.
And as loses? Well, lets see. You take a charge from a train and then give it right back to it several minutes later. Somehow, I do not think that the minor amount of loss in that length of time is a big deal.

BTW, this also has another advantage. It becomes possible to store energy from the lines for use in various applications. It can even be used to buffer the lines. Caps are pretty handy about that.

I still say go with ultra-caps and use this to help spin up an industry. Once geared up, then they can sell more elsewhere.

What ultra-caps could possibly power a train?

[csirac]

Most datasheets I've seen are 10V, and certainly the dis/charge current is always in mA...

Re:What ultra-caps could possibly power a train?

[Animats]

These Maxwell ultracapacitors packaged as "transportation modules". 63 farads at 125 volts per unit. Max continuous current for an ordinary braking-type operation, 240A. So each such unit could absorb 30KW. The current generation of NYC subway car has a maximum current drain of 313KW per car. So nine such units per car, at 65Kg each, could accelerate a subway car. Stored energy is only 1.3KWh, but that's just about right for accelerating or decelerating a subway car once.

Re:What ultra-caps could possibly power a train?

[AI0867]

How about these? They're 3 kF, 2.7 V and can supply hundreds of amps depending on how long you're going to draw it. (2200 for one second)

So, that's about 11 kJ that it can store or release in a matter of seconds.

Re:Toyota called...

[Anonymous Coward]

Because there's no "moron" or "twit" moderation.

Re:Toyota called...

[Anonymous Coward]

We've had airplanes since the Wright brothers in 1903

Actually since John Stringfellow in 1848.

Dynamic Braking

[midmopub]

Forget the subway. I work for a Class 1 railroad. We haul 19,000 ton coal trains up and down hills all day long. We have four 4,400 HP motors in full dynamics down every hill. We dissipate the generated electrical power as heat in brake grid resistors. What if we had a way to release that tremendous electrical output into the grid? Thousands of trains a day all over the country do this.

Re:Dynamic Braking

[Gothmolly]

It's your power, you made it, why not keep it? Why worry about "the grid" ?

Re:Dynamic Braking

[prefec2]

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.

Re:Dynamic Braking

[Anonymous Coward]

I believe GP's suggestion was to use flywheel, capacitor, or battery energy storage on the train, so you can use the energy from going down one hill to go up the next -- avoiding exactly the issue you mention.

Of course, the trouble is, you need 10s of GJ for the scenario GGP mentions, which translates into 10s of tons (and corresponding costs, naturally) of energy storage. Much more challenging upgrade than light rail as in TFA, or even passenger service on regular rail.

Re:Toyota called...

[m50d]

Water wheels were around for centuries, used to power mills. What started the industrial revolution was learning to refine iron using coal rather than charcoal - which was an evolutionary improvement, but made iron a lot cheaper.

Bombardier already sells these

[Maury Markowitz]

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...

already done in Philly

[Anonymous Coward]

This is not a new idea. Philly has this already, albiet using batteries instead of flywheels:

http://www.smartplanet.com/blog/intelligent-energy/philadelphia-subway-brakes-for-energy-savings/2556

Deja vu

[prefec2]

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 it. Even more the batteries are expensive, they use seldom materials to produce them, and they are toxic waste afterwards. In total it is cleverer to manage trains effectively or have that many trains running on the grid, that the introduction of electricity and the consumption appear smoother due to the many start and stop incidents.

Over all. This idea is again rubbish. Even though other manufacturer provide "packages" with this technology already in action. Still makes the idea second choice.

Re:Deja vu

[Arrepiadd]

It's incredible how you can, not only miss the entire point (by not reading the article) and then say their idea is a total crap, without understanding at all what it is about!

It's NOT a battery system. It's NOT on board storage. It's NOT the typical back to the grid system. Do yourself a favor, go read the fine article and then come back with something to tell us.

Re:Deja vu

[prefec2]

Yes. I admit. I only read the summary. And the summary sounded like something we have read on /. a couple of times over the past years. So I thought. Not again! And wrote in a rush a response which I cannot correct after sending. Definitely a missing feature ;-).

However, I read the article afterwards and find the idea now quite interesting. It is still nothing new about each component in the system. As they use brake energy and they use flywheels for short term energy preservation. Even though the combination of such a system in a subway rail system is new and therefore it is good that they reported this idea. Maybe the energy loss is less than with capacitors or batteries. It can definitely used more often than batteries with their very limited recharge cycles.

So in short: I am sorry posting such crap in the first place.

Re:Your energy calcs are off

[Anonymous Coward]

3 or 4 megawatts for 30 seconds = ballpark 3000 kilowatt-hours.

Yeah, if 30 seconds = ballpark 1 hour. And somebody modded this up?

Re:Toyota called...

[waives]

umm no, surprisingly enough 1G = 1G.

Make the train non stop

[Anonymous Coward]

And save a lot of energy...

Why even stop?

[acidradio]

You guys are putting too much work into this. Maybe just have the trains "slow down" at stations so that everyone can quickly jump on and off. That would save all that energy that it takes to start from a standstill. It'll work, right?

Like a lot of "green" attempts....

[poofmeisterp]

This is not a troll or flamebait comment. It is a request for explanation by physicists, electricians, metallurgists, and other parties that have knowledge on the subject, about its benefit. When I ask that I ask, "Benefit other than a good show of faith?"

You have to use megawatts of power for acceleration, right?

That's from a stop to determined speed.

Now you want to stop the vehicle. You start generating new power by utilizing the movement of the vehicle transferred through the wheels, to the axles, into a generator unit (multiple), right?

Now let's look at the law of conservation of energy... Energy was lost in the process of acceleration via electromagnetic fields, heat, and friction. In the process of stopping the vehicle, energy is transformed, that's right, with a loss via heat, friction, and electromagnetism.

You lost on the upswing, and lost on the downswing. It's effectively throwing back just a hair of power that can power, what, a dozen homes for a minute, if that?

Just to make a note from the perspective of people from another angle, aren't we concerned with the health risk via electromagnetic fields that are many thousands of times more powerful than mobile phones, Wi-Fi, or other 500mW - 1W transceivers? This isn't a concern of mine, I'm just painting a picture of the road ahead... the backlash from other parties, if you will :)

Shame on anyone who thinks this is flame bait or trolling material. I am encouraging a scientific discussion from smart minds on /.

Re:Like a lot of "green" attempts....

[aXis100]

Yes, there are losses through heat and friction, but on large electrical motors that is a very small percentage. Electric motors are generally well over 70% efficient, and can be in the 90's of percent - both in driving and generation mode.

You are seriously underestimating the amount of power involved. Even if the total round trip efficiency of the regenerative system was 60%, that's 2.4MW for 30 seconds, or 20,000 kWh - enough to power 1000 homes all day. It will pay off the investment very quickly and is well worth doing.

Re:Like a lot of "green" attempts....

[ResidentSourcerer]

You are correct. Typical motor to track efficiency is on the order of 85%. So you put in 60 MJ, the train now has a kinetic energy of 0.85 * 60 = 51 MJ. It takes a fairly small amount of power to keep it running at that speed. When you stop it, your 85% comes into play again, so you get 0.85 * 51 = 43 MJ back.

I may be wrong about that 85%. Large motors tend to be more efficient, so perhaps it's 90.

Subways and transit buses are appropriate for this tech, as they start and stop frequently. If it's many minutes between stops, then the start/stop energy is small compared to the running energy.

Another place where this is appropriate is on railways that go over mountain passes. Done right, the train running downhill provides most of the power for the train coming up.

Re:Toyota called...

[nedlohs]

Because Toyota didn't invent regenerative braking and claiming they did is clearly just trolling. Because it is impossible to be so stupid as to actually believe they did.

not new

[Anonymous Coward]

This is not new.

The oldest reference to regenerative braking of a production system I could find were German ICE 2 fast trains that were being rolled out about 15 years ago, today its commonplace in municipal transit everywhere in Germany.

Deutsche Bahn claims to feed back about 10% of power used in fast trains that way, numbers should be much better for local transit because of shorter travel times.

Let's do some math

[zippthorne]

You've neglected the fact that work = force applied over a distance, the force is applied over a greater distance for the arriving train at the beginning of the arrival than the departing train at the beginning of the departure over the same time interval, so for energy to balance, the force on the departing train must initially be greater.

It's easiest to compare by using the instantaneous power balance: the power taken from the arriving train must be applied to the departing train (or stored, but we're assuming no storage for this example)

T = m*v^2
P = dT/dt = 2mvdv/dt = 2mva
P_a = P_d
2m_a*v_a*a_a = 2m_d*v_d*a_d

Assuming m_a approx.= m_d, (e.g. both trains are the same mass)

a_d = (v_a)/(v_d)*a_a

The Key is Kinetic Energy Storage

[Anonymous Coward]

The 3-4 megawatts is simply energy. The 30 seconds is time the energy is available, or exists. The problem is to save the kinetic energy generation of a train's deceleration to apply to accelerating the train at a later time. This means to store the train's moving-mass kinetic energy, expressable, among other ways, as equivalent to 3 to 4 megawatts for 30 seconds in electrical energy, so that most of it, (some will be lost in conversions) may be translated, when needed later at start-up, to an equivalent to 3 to 4 megawatts for 30 seconds of electrical energy, as an accelerating force to apply to the train's resting mass to return it to a moving mass.

Current tradition translates the moving-mass kinetic energy of trains to heat, through friction braking or regenerative braking (electrical generation routed to resistors to convert it to heat). Regenerative brakiing "back to the grid", done in 30 or 60 second events, is only academically reclamation since electrical grids can't store such pulses. A 30 second regernation is a spike. It which must be absorbed or dissipated (to prevent it blowing up sensitive electronics).

Flywheels store energy as moving-mass kinetic energy. A flywheel at one side geared to rolling wheels on a moving train, to be spun up by the rolling wheels to decelerate the train athrough that energy transfer, through spinning while the train is stopped would store the kinetic energy traditionally converted to heat and dissipated. The flywheel would hold the energy imparted to it until needed to accelerate the train again.

Connected through torque-converters the flywheel's stored moving mass kinetic energy could be mechanically transferred back to the rolling wheels it was taken from. Connected to a generator the flywheel's stored moving mass energy could be translated to electrical energy, which could be translated to magnetic energy in the train's electric motors and with the magnetism torque-converting, from magnetic energy to moving mass energy as the translation would accelerate the train to a moving mass again.

Both ways part of the kinetic energy of the ten-car train would be 'harvested', then returned to the train it was harvested from.

The electric nature of "3 to 4 megawatts for 30 seconds" is an expression of measure only. It is not, itself, part of the save-and-reuse process.

What are you doing at Slashdot? Get back to work!

[Anonymous Coward]

What are you doing at Slashdot? Get back to work! /John

Background info

[Anonymous Coward]

Background: In underground railways the tunnels are never level, not even in case of a city built on perfectly flat land. The stations are the deepest and the tunnel between them is higher, with the mid-point as the highest. The rationale is that a complete loff of electric supply shall not trap a train within the tunnel, rather gravity shall allow it to descend to the nearest station. Most modern underground railways don't even feature emergency brake levers in the passanger cabin, because the risk of an in-tunnel stoppage for any reason is considered unmanageable, e.g. in case of a fire on-board. Owing to the tight size of tunnels, full derailment in the traditional sense is not possible, so it's best if the train arrives at the station in whatever shape, where large fans will provide fresh air to people, remove smoke and firemen have easy access.

Because of this safety setup, underground railways are highly unefficient, because they must accelerate away from a station, while negotiating a climb and when they approach a station, they must brake away not just their own energy of motion, but also the potential energy they gained due to their slope descent towards the station.

Therefore regenerative braking is a good idea, but subway trains tend to be uncomplicated, because they must be extremely reliable. There are few places (like every 5-7th station) where a stricken train can be shunted away. The time needed to do that usually means a replacement services with buses becomes a necessity for 45-90 minutes, which is a major pain in the arse and also cost.

a better answer?

[Anonymous Coward]

stop stopping so much

Cost analysis

[Anonymous Coward]

What about the cost effectiveness of the solution. I calculate 2.5 MW for 30 seconds is ~ 20.83 KWh each start up. Paying $0.10 a KWh would result in a cost of $2.83 per start up. The cost to the rail operator would probably be less than $0.10 but I am not sure what it would be.

If the train starts 15 times per hour for 14 hrs a day it would be 210 starts a day for $594.3 each day for 300 days a year is $178,290 for a year. Simple pay back 5 years means the replacement would need to cost less than $900,000.

I know this isn't a thorough analysis but perhaps it shows a ballpark number for a cost guesstimate.

Sam

Capturing the power

[bjb]

I remember talking with someone at an "art museum" (read: an old worn down to the point of being dangerously condemned ex-paper plant) noting how the town's power problems could be solved by somehow tapping the energy of the freight trains that would rumble by.

For some reason, I guess this guy figured there was no degenerative effect on the freight train by capturing its power or like it just comes for free. It has to come from somewhere.

Not entirely related to TFA, but felt like making a rant.

Awesome concept

[hesaigo999ca]

I just feel bad that we didnt think of this method 20 years ago, I think of all the lost power we could have been collecting from all subways around the world...
atleast now, we will be making energy from using energy , and with the amount of vibrations a subway causes, I am sure we will make tons!