The London Underground is, for some reason I don't understand, quite a bit warmer than the streets above it. There is also a lot of it. It's four hundred kilometres long in total and over 3.5 metres in diameter everywhere. Assuming this to be semicircular, that gives it a volume of over 1900000 cubic metres. This volume of air seems to be heated to around five degrees K above the atmosphere above ground. I can't do the maths because I don't know enough about heat capacity and stuff, but it seems to me that the energy required to heat such a huge volume of air must be considerable. Most people travelling on the Tube don't seem to enjoy the warmth, although in the winter I've found it quite nice.

So: why not install Stirling engines, in the tubes to refrigerate them down to the ambient temperature and transfer the heat onto the streets while generating either power or mechanical energy? The ground surface can be warmed in winter to melt snow and ice, which can then run off into the water supply, generating further hydroelectric power as it goes and providing a grey water source for cisterns. Meanwhile the energy can either be added to the underground rail system mechanically, perhaps in the form of wheels between the tracks which accelerate the trains, or used to provide electrical power for the underground itself.

I'm almost certain they already air condition the Underground, so the system is already partly in place to do this.

I'm pretty sure this is by no means a negligible power source.
-- nineteenthly, Apr 27 2017

https://en.wikipedi...Underground_cooling [hippo, Apr 27 2017]

amazed at how small those tubes are. I had to look it up.
-- po, Apr 27 2017


Me too. I think that's the minimum size though, and I think they have smaller trains running through them. Also, there is more space elsewhere on the system such as above platforms and in station concourses.

Assuming the tunnels are three-quarters of a circle in cross-section, that they are 3.56 m in diameter and 402 km long rather than doing all my rounding down, that makes the volume of the system over three million cubic metres, and that's still ignoring all the extra bits. But what's the volume of all the trains added together?
-- nineteenthly, Apr 27 2017


The trains are mostly hollow shells, so the volume they occupy is possibly more ignore-able than the volume of all the passengers (body heat from the passengers would explain some of "mystery" warmth --each person's body produces about the same total warmth as a 100-watt incandescent light bulb, radiating from a larger surface area).
-- Vernon, Apr 27 2017


In addition to body heat, is it not also heat given off by the trains as they move, and by electric motors and the like?
-- nineteenthly, Apr 27 2017


And being closer to the centre of the Earth - that's why deep mines are always hot places.

The linked article says that the heat comes from trains, and that the clay surrounding the tube has heated up from 14C since the tube was built to 19-26C now. It also discusses the use of heat pumps.
-- hippo, Apr 27 2017


Examining the map, the obvious solution to cooling is to drill vertically up from the tunnels at selected locations. This would ensure that the temperature of the system was rapidly lowered to that of the Thames.
-- 8th of 7, Apr 27 2017



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