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Cooling the tube – Engineering heat out of the Underground (2017)

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Re: Cooling the tube – Engineering heat out of the Underground (2017)

#11

Post by zorlack »

jaywalk wrote: post #8
pbhjpbhj wrote: post #6

Earlier quoted context omitted.

I wonder if you could fit a magnetic braking system mainly at platforms, when engaged trains would be slowed. That could reduce weight on the trains themselves? Thinking wildly, tube trains are like pistons (they're actual plungers, I guess) if you had a smooth walled section of tunnel then you could have a seal at the head of the train that fans out and provides pneumatic braking, you could maybe use the pressure to…

Magnetic (eddy current) brakes work by converting kinetic energy into heat, which doesn't really solve the problem here. I'm not sure if anyone has devised a system that actually captures electrical energy from the braking action, or if such a thing is even plausible.

You could put the braking resistors above ground, no?

Or if the braking resistors have to be close the the coils, then you could use a heat-exchanger to move the heat somewhere else...

Re: Cooling the tube – Engineering heat out of the Underground (2017)

#13

Post by konschubert »

aaaaaaaaaaab wrote: post #4

Regenerative braking would improve a lot with not just the heat issue, but also air quality. Commuters on the London Underground get a characteristic grey/black snot from the brake dust they inhale. If you stare into the tunnel after a train left you can literally see a haze of brake dust in the air.

It seems they have been testing it out? https://arstechnica.com/cars/2015/09/london-tube-gains-regen...

Re: Cooling the tube – Engineering heat out of the Underground (2017)

#14

Post by londons_explore »

The main cause of heat is the inefficiency of the trains. All the brake energy is converted to heat. The motors and controllers are pretty inefficient too - modern electric motors+controllers are 95-98% efficient, whereas most tube trains are more like 40-60% efficient. For example, to go slowly, they simply put a big resistor in the circuit and waste 50+% of the energy!!

Perhaps not coincidentally, the inefficiency also has a massive carbon and budget impact... Or it would, if the train system had to pay for the electricity it used - which it doesn't! Trains in the UK are given free unmetered electricity, so there is no reason to conserve energy. Hot tunnels are just an extreme symptom of that.

If they had to pay for usage, at todays wholesale electricity rates, it would cost nearly a billion pounds a year. Which is ~20% of their revenue! I can see why they don't want to pay that bill!

Re: Cooling the tube – Engineering heat out of the Underground (2017)

#15

Post by jamincan »

jaywalk wrote: post #8
pbhjpbhj wrote: post #6

Earlier quoted context omitted.

I wonder if you could fit a magnetic braking system mainly at platforms, when engaged trains would be slowed. That could reduce weight on the trains themselves? Thinking wildly, tube trains are like pistons (they're actual plungers, I guess) if you had a smooth walled section of tunnel then you could have a seal at the head of the train that fans out and provides pneumatic braking, you could maybe use the pressure to…

Magnetic (eddy current) brakes work by converting kinetic energy into heat, which doesn't really solve the problem here. I'm not sure if anyone has devised a system that actually captures electrical energy from the braking action, or if such a thing is even plausible.

Regenerative braking generates electricity and there's really no reason modern trains wouldn't use them.

Another approach that can be used when initially building the system is to have a slope leading into/out of stations so that energy can be stored as gravitional potential. I know the Montreal Metro was built this way.

Re: Cooling the tube – Engineering heat out of the Underground (2017)

#16

Post by namibj »

jaywalk wrote: post #8
pbhjpbhj wrote: post #6

Earlier quoted context omitted.

I wonder if you could fit a magnetic braking system mainly at platforms, when engaged trains would be slowed. That could reduce weight on the trains themselves? Thinking wildly, tube trains are like pistons (they're actual plungers, I guess) if you had a smooth walled section of tunnel then you could have a seal at the head of the train that fans out and provides pneumatic braking, you could maybe use the pressure to…

Magnetic (eddy current) brakes work by converting kinetic energy into heat, which doesn't really solve the problem here. I'm not sure if anyone has devised a system that actually captures electrical energy from the braking action, or if such a thing is even plausible.

Yes. It's called "electric generator" and can be performed by almost all electric motors.

Re: Cooling the tube – Engineering heat out of the Underground (2017)

#17

Post by fredley »

aaaaaaaaaaab wrote: post #4

Regenerative braking would improve a lot with not just the heat issue, but also air quality. Commuters on the London Underground get a characteristic grey/black snot from the brake dust they inhale. If you stare into the tunnel after a train left you can literally see a haze of brake dust in the air.

It's already got a primitive version of regenerative braking! The deep-line tubes—those that have the heating problem—are designed so that the stations are at peaks in the line. As the train is approaching the platform the uphill slope helps it decelerate, and the downhill helps it accellerate again.

Re: Cooling the tube – Engineering heat out of the Underground (2017)

#18

Post by Symbiote »

konschubert wrote: post #13
aaaaaaaaaaab wrote: post #4

Regenerative braking would improve a lot with not just the heat issue, but also air quality. Commuters on the London Underground get a characteristic grey/black snot from the brake dust they inhale. If you stare into the tunnel after a train left you can literally see a haze of brake dust in the air.

It seems they have been testing it out? https://arstechnica.com/cars/2015/09/london-tube-gains-regen...

The new trains on the deep lines should also bring a big improvement, from 2025.

https://en.wikipedia.org/wiki/New_Tube_for_London

Re: Cooling the tube – Engineering heat out of the Underground (2017)

#19

Post by interestica »

I think the Montreal metro faces a lot of these same issues. It works out in the winter where you don't need to actively heat the space. In the summer, it can be unbearable. I'm not sure if the type of rolling stock (rubber tires + tunnel track) has a major difference from other typical subway cars.

Re: Cooling the tube – Engineering heat out of the Underground (2017)

#20

Post by acta_non_verba »

londons_explore wrote: post #14

The main cause of heat is the inefficiency of the trains. All the brake energy is converted to heat. The motors and controllers are pretty inefficient too - modern electric motors+controllers are 95-98% efficient, whereas most tube trains are more like 40-60% efficient. For example, to go slowly, they simply put a big resistor in the circuit and waste 50+% of the energy!! Perhaps not coincidentally, the inefficiency…

This is categorically not true.

In fact, the level of effort required to estimate electricity uses for multiple franchises on one physical section of railway has resulted in nearly all British rolling stock having built-in electricity metering.

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