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Comment by Aachen | Hacker News Reader

Isn't heat free energy in a place like London? I know very little about metro systems so please correct me if this is insane: wouldn't the people living above the tubes be happy to get a heat exchanger (passive) or heat pump (active, but takes more of the heat) that prewarms their hot water supply? People still take warm showers and boil tea and rice/pasta in summer, and in winter the purpose should be obvious. If the water comes in at 30 instead of 10 degrees C, you need to add only a few degrees for showers and floor heating

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A problem is the clay surrounding the tunnels insulates them - it traps heat because heat flows through it very slowly. So you drill down and put a heat exchanger pipe down there, you pump heat from 3cm of clay around the pipe and now no heat flows through the clay to your pipe even though there’s a lot of heat still down there.

Your pipe becomes a tiny worm of cold pipe in a big lump of hot clay and you’ve done very little to cool the underground or warm your water. That is, if heat moved easily through the stuff then the problem of heat buildup would be easy to solve but in that case heat wouldn't build up so there wouldn't be a problem; and vice-versa.

You could use this kind of technology to extract the heat: https://enerdrape.com/en/solution/

Have you seen how small the clearances involved are? The trains are very close to the walls of the tunnels.

Definitely an unfortunate domain name they've got there.

From their FAQ:

> WILL THIS GEOTHERMAL PANEL SYSTEM COOL MY UNDERGROUND SPACE?

> No, the Enerdrape system primarily draws its energy from the ground. This does not affect the comfort of people using the underground space.

Yes but the only places we care about extracting heat are the tunnels, so you'd just run pipes through the tunnels themselves to extract heat. I think the bigger issue is the amount of energy needed to get the water down there and back up just means it doesn't make much sense on its own.

The problem isn’t so much finding uses for the heat, it’s getting the heat out of the tunnels to begin with.

These are some of the deepest tunnels going under some of the most built up parts of the UK.

I wonder if an extremely tall subterranean windcatcher [0] with its bottom at the top of a tunnel could passively cool the tunnel.

[0] https://en.wikipedia.org/wiki/Windcatcher

These basically already exist around the network. But that simply isn’t even close to moving enough energy to make a difference.

The catch here is the cost of the buildings you'd need to destroy in order to build the surface portion of the windcatcher. The deep underground lines run under heavily populated areas of London.

To build enough windcatchers to move the needle on tunnel temps, you'd need to buy many plots in one of the most expensive cities in the world.

Your are correct in principal, though implimenting your idea, now, is essentialy impossible as installing the plumbing after the fact might cost more than just starting over with a whole new line, and would in fact make things much worse durring the many years it takes to find out if the added systems even work. Given that there is only clay under London, it is by far better to start over and build a whole new line, and/or go all in on a mega high tech ,high pressure refrigeration systems for the human occupancy areas, and hope that there are no break downs in the "hot zones" orrible mess

Heat is to energy as feces to food. It's not quite valueless by mass or by volume, but close enough that for most purposes the major issue is rejection.

Nice analogy, in hindsight only it’s in the name: waste heat. It takes the tiniest bit of impolite thoughts to make that lovely connection, thanks for that morning coffee bit of trivia.

Well, it's the human condition, once a day more or less whenever healthy. That's not only unpleasant, though! Also part of that condition is being a member of a species with the wit to invent soap.

>wouldn't the people living above the tubes be happy to get a heat exchanger (passive) or heat pump (active, but takes more of the heat) that prewarms their hot water supply

Ground source heat pumps are expensive to build, even more so in a dense area like London. So even if everything you said is true, I suspect the juice isn't worth the squeeze.

I think you are correct.

In this case, though, the excess heat is a major burden, so there is room to negotiate with a district thermal provider that pays that provider to absorb and redistribute the heat, as long as it's less than the cost to pump it out to the environment.

I'm not saying it's easy (it will likely be a bespoke solution). Given the organizations, I expect the difficulty to be as much business (setting the prices) and political (defending the prices set) as technical.

There is at least one on this line (north of Kings Cross) and one on the Northern line (north of Moorgate). It's for district heating or electricity generation.

You need a temperature gradient to turn heat into energy, and the smaller the gradient, the less efficient your heat engine will be.

What do you do in the summer when the homes don’t want the heat?

People still take hot showers and use hot water

That doesn't have to be a problem in practice.

The entire issue is that the earth surrounding the tubes is acting as a giant buffer. Enough heat has been dumped into it over the years that it has permanently warmed up. Draw heat from it during the winter to warm up homes, and it'll be able to absorb more heat from the tunnel air during the summer.

And because it's permanently warmed up, the long term consequence is the line becomes a health hazard and has to be closed for increasingly long periods.

When wet bulb > body temp people start getting heat stroke, which leads to fainting and potentially death - a bad look for a public transport system.

The likely remedy is to install gigantic refrigeration units in the ventilation shafts and pump in cold air. This will be hugely expensive to build and run.

But the alternative is a tube line that can't be used. So there may not be much choice.

It won't be zero so spreading it across enough people might already solve it. If that still leads to insufficient demand during the hottest weeks, idk, it's energy, surely there's something useful you can do? Store it for next week, pre-heat water for the nearest steam engine (e.g. gas power plants are steam engines running on methane, so if they have to heat the water by fewer degrees.. The problem will be finding a steam engine close to the heat source), supply it to an industrial process that needs temperatures above ambient (egg breeding for vaccine production? Idk), create electricity from the temperature differential between this system and the Thames water using the Peltier effect

I've surely got a too naïve view of economics but if the goal were to not waste resources then there will be things you can do before dumping it into the hot summer air

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