Cooling the tube – Engineering heat out of the Underground (2017)
ianvisits.co.uk"Over the years, the heat from the trains soaked into the clay to the point where it can no longer absorb any more heat. Tunnels that were a mere 14 degrees Celsius in the 1900s can now have air temperatures as high as 30 degrees Celsius on parts of the tube network."
Similar issues can happen with geothermal heating/cooling if it's unbalanced - eventually the ground temperature changes and you have to dig deeper or stop using it.
I wondered about that. I know a few people with GT systems and it has never been an issue, perhaps because we have real summer and winter here, so the heat dumped underground in the summer would be offset by the heat extracted in the winter?
You know people that have a personal geothermal system?
They're quite common in some parts of the world.
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.
hardly primitive, it's by far the most efficient form of regenerative braking
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 drive the ventilation system. Unlikely to be practical (you'd have to have a pre-platform door to prevent all the passengers from getting exposed to the pressure increase, I think).
Why not just use air pressure for the whole thing? https://en.wikipedia.org/wiki/Beach_Pneumatic_Transit
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.
Your second point was stated exactly in the parent of your parent comment.
I don't think that's correct. @pbhjpbhj talks about magnetic braking and some sort of pneumatic braking. The sibling to his comment by @fredley mentions it, but I don't recall seeing that yet when I posted mine.
Yes. It's called "electric generator" and can be performed by almost all electric motors.
Indeed. Electricity and magnetic fields are so closely related we have a single term, electromagnetism, the motor and generator are thus almost the same thing and it's relatively easy to build a single device which can serve both purposes.
Actually regenerative braking is common on electric trains. Stock bought since at least the 1990s for London Underground has regenerative braking of various forms, with the more modern being fitted to S-stock (the sub-surface lines which were all gradually replaced with a single type of train, if you went on a "Tube" train and it was actually walk-through the length of the train and seemed vaguely modern inside, that was S-stock and you weren't in the deep tube where heat is a bigger problem)
In the 1970s when the oldest stock still in use was purchased, regenerative braking was not a thing.
Regenerative braking was a thing for decades before the 1970s.
Some old freight routes used electric haulage because the regenerative braking going downhill almost completely offset the electricity consumption going uphill.
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...
It'd at least solve the dust break-pad dust issue
piston effect already does a lot of ventilation work
the more elegant solution in use around in the world is to put the stations on hills
Super pedantry, but I think it's a plunger effect unless you seal around the piston?
Yeah, they mention it with a "grid balancing issue" but it seems that could be solved by adding under-floor battery packs to the trains or the stations themselves - store braking energy to restart the train afterwards.
Less batteries and more ultra capacitors. You might need to handle power requirements in the range of 5-20MW, albeit for just a minute or so.
Definitely. The bonus is that they wouldn't have to be on the train. And the tube electric system is DC already as well. This sounds like an engineering problem that is largely already solved.
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.
>Commuters on the London Underground get a characteristic grey/black snot from the brake dust they inhale.
Well that's horrifying. Is there no health and safety monitoring being conducted?
It is continuously monitored, but then what? There are no feasible solutions as yet.
I love this complex-engineering-thing-you-thought-it-was-trivial-but-it-doesn't-even-try-to-be kind of articles!
> The difficulty of adding more ventilation is the lack of space above ground to put new ventilation shafts. This is always going to be a problem for the older tube tunnels except on rare occasions when a surface development takes place at just the right location and agreements can be made to include a shaft down to the Underground
Sounds like we need to knock down a few houses. This isn't unreasonable for critical infrastructure.
Also sounds like giving people free ground source heat pumps along the tube line would be best - they would be pumping out the heat thats in the clay. The article talk about a scheme, but this needs to be done at scale
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.
I personally don't find Toronto or Montreal subways unbearable (warm, but not crazy), while my time in the London Underground was absolutely miserable - I'm not sure how locals handle that kind of stifling heat. Actually, there was one time I was in the old Toronto Osgoode (pre-opera house) station with a -30 degree wind blowing through - that was chilly!
Actually, I'm kind of sad that Montreal has so few cars left that play "Fanfare" [0]
[0] https://web.archive.org/web/20090823173621/http://observator...
I think the car temps and the station temps are quite different. And maybe it varies by line or car-type in the same system? I've definitely experienced insanely hot cars in Montreal's metro and it previously led me down the rabbit hole to learn why there was no climate control in the system.
The underground gets quite a lot hotter than the legal temperature limit for transporting livestock in the UK. Supposedly the record is 47°C
>I personally don't find Toronto or Montreal subways unbearable
tbh London is fine too. Problem is if it is the main form of transport then the at-risk members of society use it too. Very old & frail etc.
It really doesn't matter much - any amount of heat going into the underground system that is not able to be dispelled in a year-round cycle will heat the ground eventually.
Reducing the heat entering the system slows it down, but at some point you have to cool it again.
Every amount of heat generated will have an equilibrium average temperature.
Reducing input heat will lower this equilibrium temperature.
But that equilibrium could be a lethal temperature.
If they added hydraulic hybrid braking to the trains, they could capture 80% or more of the kinetic energy and reuse it to accelerate again. This would drastically cut down on heat. By way of comparison, electric batteries can capture maybe 20-25% of the kinetic energy.
Uncaptured energy is likely turned into pure heat.
Makes me wonder if we can do something more productive with the heat.
They mention energy generation, but what about heat pumps and water heaters for the above ground buildings? Probably cost prohibitive, but wouldn't it be neat if the waste heat from the brakes ended warming a building or hot water heater?
I suspect a big part of the problem is that while the heat is there in adundance its not particularly concentrated. Just ambient everywhere.
Any kind of capture heat and use it because much more workable if concentrated
Converting heat into useful amounts of energy usually requires a temperature gradient that'd be unbearable for humans.
Heat pump extraction into hot water distribution pipes for district use.
Could using magnets to break and start be a solution to reduce the heat generation ?
Using magnets will release exactly the same amount of heat since energy cannot be destroyed (unless you mean by that using regenerative braking, that does work since you can "recycle" the energy)
That's effectively what motors and regenerative braking are (just the magnets are in a circle!)
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!
TFL does pay for electricity, but it pays for it as part of the Crown Commercial Service which aggregates procurement for many large public services in the UK. [1]
TFL used to have its own generating stations, which helped keep costs manageable and also meant the trains kept running during power cuts.
That ended around 2000. Now it has at least four separate connections to the National Grid, and is supposed to be moving to 100% renewables within the next decade.
[1] https://content.tfl.gov.uk/fc-20171205-part-1-item08-tfl-ene...
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.
Source on 40-60%? As some trains are from the early 70s, and others were introduced 10, 5 years ago, which are you referring to?
I'm referring to the oldest ones.
The new ones are much better, although they still recover very little braking energy.
>>put a big resistor in the circuit and waste 50+% of the energy
That Is Insane!!
Rail needs a complete rethink of design from the ground up. There are certainly vast efficiencies from rolling steel wheels on steel rails. But I've seen little evidence of light-weighting and indeed active resistance to steps like making cars using composites, which would save enormous amounts of energy and cost over the lifetime, as well as reduce maintenance costs. Yet their approach to everything seems to be sticking with an approach of 'brute force and ignorance'. Sure that works in the 19th century when you're just starting to put some carbon in iron and make it stronger than pig iron, but materials are sooo far past that now, yet it's still the same old approach. (&yes, I strongly support the Chesterton's Fence checking but this seems way beyond that)
I don't think it's anywhere near 50% that is wasted, since if I understand correctly the resistors are used only during initial acceleration.
But anyway, the affected trains are the oldest in use in Britain, from 1972 (Bakerloo Line, 36 trains) and 1973 (Piccadilly Line, 87 trains), out of 620 passenger trains across the whole system. New trains have been ordered and should be introduced in 2025.
There's been a 10 year delay, due in part to the 1990s attempt to privatize chunks of the system.
Yeah, it's insane because it's 100% not true.
Surprising then that a company sells resistors just for rail speed control applications:
https://www.cressall.com/applications/rail-traction-dynamic-...
(scroll down to "MOTOR STARTING AND SPEED CONTROL RESISTORS")
Thats not to "go slower" but to slow down:
Whenever possible regenerative braking is used. In this case the drive motors convert the kinetic energy of the train into electricity, which is fed back into the power supply and used elsewhere on the network.
If it cannot be absorbed by the system the energy may have to be dissipated with braking resistors. These braking resistors can be either mounted on the rail vehicle or as a fixed (trackside) installation connected to the third rail or overhead catenary via Automatic Assured Receptivity Unit (AARU) systems.
That section starts with "This was the traditional approach...". I guess the Bakerloo and Picadilly line trains from the 1970s might still use them on London Underground? Wikipedia mentions stuff like GTO thyristers and IGBTs on descriptions of stuff like the 1992 (Central line) and 1995 (Northern) stock trains, so the next oldest trains from the 1990s seem to have modern(ish) power control. And new trains are coming to the Picadilly line, and presumably must come to the Bakerloo at some point before the old ones fall apart.
That doesn't mean that's all that's used. It would also imply that it only has 2 speeds or something. Use your head. Or learn some basic electronics.