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Comment by Y_Y | Hacker News Reader

> Historically, the Underground infrastructure offered a respite from warm weather, indicated in Austin Cooper’s ‘It is cooler below’ poster, issued in 1924 by the Underground group to promote a more comfortable experience of travel during warm weather.

A century of burrowing commuter-worms unfortunately managed to bake all the beautiful wet clay that kept the tunnels tolerable when the sun was shining about.

It seems straightforward to me that it would be enough to rehydrate the ground. Just need (approximately),

  400km of track * 25m average depth * 3m tunnel width * 20% moisture content of wet clay
  = 6 billion litres of water

Sounds like a lot but it's only about 1/300th of the yearly flow of the Thames.

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I don’t think the hydration of the clay is the important element here. Rather I suspect it’s simple just the sheer mass of clay, wet or otherwise, that’s involved.

There’s a reason why ground source heat pumps work so well. It’s because the ground is such a fantastically huge heat sink/source that in most scenarios we consider it capable of sinking or sourcing a practically unlimited amount of heat.

Unfortunately one of the scenarios where this breaks down, is when you stick a bunch of tunnels in the ground, then pump a crap ton of energy into those tunnels years round, and expect the ground to sink all the heat away. Turns out, if you do that, the ground itself starts heating up, and given that clay is a reasonable good insulator, it’s like wrapping all those tunnels in wool jumpers.

I would point out as well that all these tunnels are “deep level” tunnels running at an average depth of 24 meters and getting as low as 67 meters. The heat of the sun on the ground surface will have approximately zero impact on the tunnel temperatures. 24 meters of clay is a lot of insulation to work with.

I think you are right. The graph at the bottom of the article shows that temperatures dropped around 2020 (presumably less traffic due to the pandemic).

So the ground can take up more heat, but it can't do it quickly enough.

My original comment was meant to be tounge-in-cheek, but I would like to add the advantage of wet clay is really in its thermal conductivity[0] which is roughly two to four times higher than dry clay. It will also have higher thermal capacity, so that high frequency fluctuations (like day/night) will be smoothed out.

[0] https://www.mdpi.com/2571-8789/8/2/47

(I don't think my comment implied that the sunshine makes it hot in the tunnel, only that when it's hot above ground it's nice to be cool below, like in the poster. And I did include a figure of 25m for average depth, I've spent enough time on their endless stairs to know that the TfL delved to greedily and too deep. This spiral staircase has 320 steps, I shudder.)

<man walks into sauna room> Ooh, it's a bit hot in here! I better throw some water on these rocks to cool them down.

Joking aside, I actually don't know how dry it is in the underground, and therefore whether adding water for evaporative cooling would work. I would have assumed it was quite humid, but maybe not?

It would be if they tried cooling it with water, for sure; evaporative cooling would only work if there's enough airflow, else they'd just get 100% humidity so it would be both hot and humid.

It seems implausible to me that the clay is dehydrated. The Victoria line was only built in the 60s and has a waterproof lining. (It's also built with asbestos cement, unfortunately, which is no doubt a problem when they need to cut it for whatever reason)

The cement can negatively effect the water retenction of the clay, as does of course the temperature. The water needn't seep into the tunnel, just be moved away from it to adjacted areas.

There's a nice article here which goes into more detail: https://www.ianvisits.co.uk/articles/cooling-the-london-unde...

At a couple of stations a river passes by and that is used to cool the station.

I think Oxford Circus is one.

> Austin Cooper’s ‘It is cooler below’ poster

As I see it, it is just a marketing problem with easy solutions.

There is psychology and reverse psychology. You could fool people into embracing the heat by redecorating the stations and trains to make it a 'journey to hell, via the centre of the earth'. The stops south of the river could be marketed as Dante's inferno and hellish.

Or you could go the other way, to make it kitch Hawaiian themed, so people expect the heat and to embrace it as a mini-holiday.

Or, the stations could be redecorated as if it were a trip to the Antarctic, so people see images of penguins braving the ice, with murals from the Shackleton days, to imagine they are cold.

In places like Moscow the underground train stations give the impression that people are in splendid palaces, this works pretty well. As I see it, TfL just need to up their decor game, problem solved.

I hate that I think Theres a lot to this

Have you thought of suggesting this to TfL? There has to be something here.

Unfortunately, hydrating clay is extremely hard to do. Clay is what you use as a water-tight material in dams, artificial lakes, waste dumps and stuff like that, because water doesn't really pass through it.

Yes, people whose day job is to explore all possible options have surely missed this extremely obvious idea.

Alright smart arse.

In fairness, TfL ran a public competition to source cooling ideas some years ago, so the people whose day job is to explore all the possibilities clearly felt like they were out of ideas.

It was just a bit of a shower thought to be honest, but it looks like there are cleverer and more responsible people who've considered similar schemes.

As I posted in a cousin comment, I found this nice blog post examining some of the more serious ideas, including water-cooling: https://www.ianvisits.co.uk/articles/cooling-the-london-unde...

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