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."

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 [wikipedia.org] has been around for years, and is in effective use around the world in various guises.

The original article reads more like a marketing shot from Vycon's PR department than a news bulletin.

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:Regenerative braking?

[tftp]

The kinetic energy of 100 [short] tons moving at 80 mph would be 58 MJ [wolframalpha.com]. The energy of 4 MW during 30 seconds will be 90 MJ [wolframalpha.com]. So the numbers appear to be correct, plus or minus my guesses on the weight and speed and everything else.

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: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: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: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.