Why the London Underground is so hot

[IvanV]

A HuffPost article came into my feed talking about the excessive temperatures in the Tube in recent days. Various stations in central London are currently 33C. The article claims that the main source of the heat is train braking, which turns the kinetic energy of the tube train into heat. A commonly mentioned potential source is all the people in there, radiating heat from their bodies. But apparently some calculation easily shows taht train kinetic energy greatly exceeds warm bodies as a heat source. If braking is the main problem, then implementing regenerative braking - increasingly common on trains - should make a big difference.

When the deep tube lines were new, I think they were about 11C down in the deep lines. That's the typical ground temperature at that depth in the London clay, remote from heat-producing operations. But that London clay is very heat retentive. And so it has heated up over the decades, as a result of emission of heat from the tube operations. And that heat moves away very slowly, and so accumulates over time as operations continue. So it is much hotter down in the tube than when I was a kid. There is no easy way of extracting that heat and shedding it, except in a few fortunately located stations that can easily retrofit a solution, or new stations which have heat extraction designed in.

A few years ago I read an article by a retired engineer called Calvin Burrows. He says the idea that brake it is the main problem is demonstrably wrong. And the proof lies in the fact that the tube is so much hotter in the summer than the winter. The seasonal variation is about 8C. If the heat is so thoroughly locked in down there, and tube braking the main source of it, it would remain much more of an even temperature over the seasons. For that summer heat is not conducting down to the tube from the streets. Or at the least, the presenters of the braking theory need to to explain these seasonal temperature variations, which it seems they have not. And it is the high temperatures in the summer that are the problem. So it is more important to address the source of the seasonal variation than the year to year gain. At least for the moment. Some lines are can be about 24C even in midwinter, and as that increases even the winter temperature will be too much.

Burrows points out that much heat must be entering the tube in the summer from some source, and then leaving it in the winter, to create that 8C seasonal variation. And he says that seasonal source of heat is solar gain from when trains are out on overground sections. The trains get hot when out in the sun. Then they take that heat into the tunnels and radiate it out when in there. And in the winter cold trains enter the tunnels and cool it down.

Whilst something like this seems evident - at least to me - as a source of the seasonal variation, does this explain the long term heating over the years? Barrow does not address what is the net balance of that solar gain/atmospheric cooling from moving hot or cool heavy trains in and out of the tunnels over the course of the year. Maybe this solar gain process just explains the seasonal cycle in temperatures, but not the long term gain. Maybe the braking issue is the large reason for the long term trend over the years.

But as Burrows points out, whatever is the average temperature over the year, the 8C higher temperatures is always going to be a very big problem. So addressing the solar gain problem is necessary, whatever other heat sources also need addressing. Mechanisms to address it include use of reflective coatings on trains, and shading of overground lines - which is apparently often implemented in other country. But leaves on the line is a problem and TfL goes around cutting trees down, rather than encouraging them to shade the lines.

There's a variety of articles setting out Barrow's theory, I think this one from Rail Business Daily is the most detailed.

Any thoughts on this?

[nekomatic]

Any Tube user is familiar with the air currents caused by trains moving along narrow tunnels, which must have the effect of mixing outside air into the air underground over reasonably short timescales - in fact if that mixing didn’t happen then the air underground would become unbreathable. Presumably there’s forced air ventilation in places as well, rather than relying purely on train-driven air exchange. That seems a sufficient cause of the seasonal variation?

Edit: I see the article dismisses that as ‘probably not significant’, and the author is a clever engineer, but I’d still like to see the calculation by which he decided to dismiss it.

[bob sterman]

Burrows points out that much heat must be entering the tube in the summer from some source, and then leaving it in the winter, to create that 8C seasonal variation. And he says that seasonal source of heat is solar gain from when trains are out on overground sections. The trains get hot when out in the sun. Then they take that heat into the tunnels and radiate it out when in there. And in the winter cold trains enter the tunnels and cool it down.

Nice idea but TFL data showed that the hottest deep line in summer 2024 was the Victoria Line. And Victoria Line services are entirely undergound. Trains only go above ground when going to and from the depot.