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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.
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 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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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
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....