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NYC Subways Testing Flywheels
socolow writes "The New York Times (free registration required) has an article about the NYC subway system's use of flywheels to store the braking energy of trains approaching stations. Not only does this advance the development of flywheel energy storage, but it will help relieve a lot of the heat subways generate (always appreciated during the summer)."
Wired ran an article about the new flywheels a while ago.
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If you read the article, you'd know: they can't put power right back into the third rail because the resistance of the rail is too high. And they can't put it into batteries because there aren't batteries big enough to solve the problem. So they use a flywheel.
Please read the article before posting, next time.
Just in case anyone is too lazy to go read it, here's the content of the article:
The people who ensure that electric power is supplied constantly and consistently to the New York City subway get very little attention.
Track work somehow seems more honest -- the hoisting of heavy rails and traversing of dark tunnels. Water work seems more daring -- inflatable skiffs and scuba gear dispatched to save the system from sure inundation. Even motormen and conductors are figures of stoic romance, captaining their 400-ton trains above and below the city.
Try as you might, it is just not as easy to summon that kind of interest in a guy with a pair of alligator clips and an ohmmeter, poring over a schematic of a circuit breaker.
Then again, if it were not for that guy and the 650 volts of direct current that he knows how to dispatch through the third rail, miles of tracks and yards full of subway trains would amount to nothing more than ornamented chunks of steel.
So when an invitation was extended recently to visit a secluded stretch of land along Jamaica Bay, for a peek at what was described as "this incredible new gadget that the power guys are working on," a visit was dutifully paid.
The subway rarely conjures up images of high technology. But in a cinder-block barn near the jet path of Kennedy International Airport, the subway's chief electricians were presiding over something that looked like some kind of miniature mission control, much more rocket science than railroading.
On a desk sat a flat-panel computer screen, covered with numbers and graph patterns and colored lines. In front of the computer sat 10 whirring metal boxes slightly larger than refrigerators, a ping-pong ball levitating mysteriously in an updraft of air above one of them.
At the desk sat Robert W. Lobenstein -- Loby to his friends -- with a radio in his hand and a look of excitement on his face that only someone with an engineering degree can have.
"Five . . . four . . . three . . . two . . . one," he called into his radio. "Full acceleration southbound!"
Despite the distant roar, Mr. Lobenstein, the general superintendent of power operations for the subway, was not launching a shuttle. He was launching a train -- one of the brand-new models starting to appear now on the L line -- along a 10,000-foot test track just outside the barn.
The train and the big metal boxes inside the barn were conspiring to do something that had never been done regularly in the subway. Since the subway first opened, trains have had a one-way relationship with the third rail: they take power from it. (During peak demand, in fact, subways and commuter trains use 600 million watts, enough energy to supply all the homes in Birmingham, Ala.)
But now, harnessing the mass and momentum of the new train cars, the subway's electricians are trying to strike up a better relationship between train and rail. In theory, it works like this: A moving train consumes power. When it stops, however, it can use its motor as a generator and pump some of that power back into the third rail, to be consumed by other trains around it.
The only problem is that when the power goes back into the rail, it is quickly eaten up by the resistance of the metal. So if other trains are not close by, to scoop up the power, the extra electricity dissipates like so many ripples in a pool.
Last summer, transit electricians and officials at the New York Power Authority, which supplies the third-rail, figured out a way to fix that. It was not a radically new idea, nor did it employ especially new technology. But finding anything to fix a problem in a place as huge as the subway is always a big job.
The solution: a battery.
A very, very big battery. Or, to be more accurate, 10 of them, each weighing as much as a Volkswagen Bug and together able to store up to a million watts of power.
A chemical battery, even the biggest around, could not handle this job. So the electricians harnessed a different kind, called a flywheel, which takes electrical energy and converts it to mechanical energy, using a rotating magnetic mass that spins up to 36,000 revolutions per minute.
For the last several weeks, through countless countdowns and test runs, the flywheels have been working like sponges, successfully absorbing the extra energy put out by a braking train. Or, as Robert Schmitt, another transit electrical official, put it, excitedly: "They're sitting here, saying: `Give it to me! Give it to me! Give it to me!' "
And after taking, they have also been giving -- sensing that a train is accelerating and releasing the extra power to help it speed up.
Now, instead of a ragged green line on the computer screen, showing power dips and spikes, the lines have begun to smoothe out nicely. This makes electricians very happy.
Should the batteries be expanded to the whole subway system, they could also make accountants very happy, saving up to $20 million a year in electricity costs.
Before the visit ended the other day, a final question had to be asked: What is the purpose of the floating ping-pong ball?
"Oh, that?" Mr. Lobenstein smiled like a child. "That's just to amuse us. Sometimes, we got bored."
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regenerative braking, not sure what technologies are used by it.
If you drive a car with a manual transmission the flywheel is connected directly to crankshaft at the engine. The clutch then rubs up against the flywheel to transfer power through the transmission to the wheels.
They are using regenerative braking. That's what the whole article is about, a new application of regenerative braking in a place that people don't usually associate with relatively new technologies.
"The only problem is that when the power goes back into the rail, it is quickly eaten up by the resistance of the metal. So if other trains are not close by, to scoop up the power, the extra electricity dissipates like so many ripples in a pool."
If other trains are not close by? How close do they want trains to be?
That quip about the heat in the stations is no joke. If you go to the yellow line (N/R/Q/W) stations, it is like 110 deg F down there minimum, at 2 AM! People stagger around down there panting and sweating like they're Ozzy.
The modifications to the trains are actually significant to support this, but it's about how the braking systems work and how the motor controllers work on the trains. There are a class of motor controllers that are not really compatible with regenerative braking, and they are fairly commonly used since they are cheaper than the others. The conversion to regenerative braking may involve replacing a fair bit of gear on the rolling stock. They were considering this kind of thing in San Diego, which is where I picked up lots of this trivia.
Many rail systems and streetcar systems have regenerative braking, but frequently they don't store the energy. What they do is have one unit braking while another is accellerating, so the excess power is in effect transferred via the wire to the other vehicle. Think of cable car systems where the guy at the top of the hill counterbalances the one at the bottom. This is hard to make work though, the timing issues being what they are.
My $.02
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From the out_of_the_topic dept.
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and a million watts is a lot of power.
Let's hope those flywheels are enclosed in something pretty solid.
Storing that much energy is one thing. Accidentally releasing it is another. When I was a student at MIT there was a permanent display in a glass case in the hallway of the biology department showing a centrifuge rotor that exploded, just to remind everyone of what happens when something spins too fast.
Let's also hope there's something to muffle that 600 Hz whine (which is close to the peak of human hearing sensitivity).
And I thought the wheels on Boston's Green Line screeching when going around sharp turns was bad...
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Why not just use regenerative braking
I'm usually a little kinder than this, but you plainly don't know what regenerative braking actually is. It's a lot more than just a feature of your R/C car. The story is all about regenerative braking. Rather than using friction to convert kinetic energy to heat and getting rid of it, using the motor to convert it back to electrical energy. The flywheels are just the most efficient place to hold on to that energy until it's needed again. It's more efficient to store it near where it's generated, since a stopping train is likely to start again, from the place where it stopped, than to send it all along the system on the rail, where it will mostly be wasted in heating the third rail before it reaches a useful load.
The thing i found surprising about this story was learning that they weren't already doing something like this.
Right here A very good article (with illustrations) that tells how flywheels work and store energy. Pretty neat stuff.
http://web.mit.edu/charliew/www/centrifuge.html
Heinlein's "The Roads Must Roll" (1940) predates Clarke's Against the Fall of Night (1953). You can read the Heinlein tale in The Past Through Tomorrow. It tells the story of what happens when the blue-collar workers who run and maintain the moving-sidewalk "roads" go on strike. (Hint: Mayhem.)
IAAEE, so I'll hazard a guess. They say in the article that the 3rd rail uses 650V DC. For power distribution, this is a relatively low voltage. To minimize resistance losses, power is typically distributed at thousands of volts. To be able to easily convert voltages, you need AC, not DC so you can run it through a transformer.
I'll bet that they have high-voltage AC power distribution throughout the system, and they step it down to 650 V and rectify to DC it at frequent intervals along the tracks. The distance the power needs to run at low voltage along a high-resistance steel rail would never be very long, so losses are minimal. (I assume they use DC becuase it's easier to design train motors for DC, or something like that.)
The AC -> DC rectification is not reversible, however, so there would be no way for power generated by a train to get back into the main distribution grid, and the average distance the 650V DC would have to flow throught the 3rd rail to the next train would be too far to be economical.
(Of course, I could be wrong about all of this, since I don't really know anything about their system.)
That's how the system works, yes. The MTA has 214 substations around the city, which are fed something between 11-27 kV AC, transformed to 400 V AC, and rectified to 600 V DC.
:)
It's only within the last ten years that they finally retired all of the old pre-solid-state rotary converters in the system - running power backwards through them would have actually worked.
The new cars actually have AC motors - the DC third rail powers a battery on board, I'm not sure exactly what the AC conversion tech is. There's still a couple thousand DC-motored cars riding the rails, so I'm not expecting to see the system switch over to AC distribution....
Think of it this way: every motor is really an AC motor. The so-called "DC motor" is really an AC motor that performs an internal DC->AC conversion, usually mechanically. So the change is not from DC to AC, but from mechanical to solid-state DC->AC conversion.
My EV1 electric car uses a 3-phase AC induction motor driven by a variable frequency, variable voltage inverter that uses IGBTs (insulated gate bipolar transistors), a hybrid between bipolar transistors and MOSFETs. On routine driving around San Diego, regeneration gives me back maybe 10-20% of the energy I move from the batteries to the car. It's not a large percentage, but every little bit helps, and it's not hard to do.