Guest blog: Cooling the Tube – Engineering heat out of the Underground

31 Jul 2019

By Ian Mansfield

This is a shortened version, the full article can be read here.

On a hot summers day a trip on the tube to get home can be a dreaded experience, with already hot trains overflowing with sweaty people. But how did we get to a situation where tube trains are stiflingly hot, and what’s being done about it?

The “tube” can be split into two types of service — the tube proper which runs through tube tunnels, and the older sub-surface lines which are just below street level. The older sub-surface tunnels were built for steam trains, so had loads of big holes in the ground included in the design to deal with removing smoke, and they are also much larger than tube tunnels. This has allowed the London Underground to fit air-conditioning units to its new fleet of sub-surface trains (S Stock), and as anyone who uses them appreciates, it’s a boon on hot days to get onto a cool train. The heat from the air conditioning units is easily vented away when the trains are underground thanks to the pre-existing steam train ventilation.

© TfL from the London Transport Museum collection

London Transport Museum

The deep tube tunnels

However, it’s the deep level tube tunnels that cause the biggest problems for both passengers suffering the heat, and London Underground in getting rid of it. One of the biggest problems is a side-effect of what made it possible to dig the deep level tunnels in the first place — namely the very solid and nice to tunnel through London Clay which sits under the city. In fact, when the early tube tunnels were dug, they were so cool down there that the cool tube was seen as a respite from the summer heat on the surface. ‘Why suffer on a bus in the heat when there’s a cool tube to take instead?’ said the marketing men.

So why is the Bakerloo line, once the coolest place to be, now a mobile sauna? While that heavy thick clay is lovely to tunnel through, it is also a heat insulator. 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.

Where does the heat come from?

Well, the passengers aren’t the problem. All those hot sweaty bodies represent roughly 2 percent of the heat in the tunnels. Climate change is also not much of a problem. It’s an impact, but the tunnel temperatures are not much affected by what’s happening up on the surface. During a heat wave in 2006, as the surface temperatures jumped around, the tunnels were pretty much constant.

About 21% of the heat in the tunnels comes from the movement of the trains themselves, from aerodynamic drag and other frictional losses. The motor engines account for 15%, the electrical and auxiliary systems are the remaining 12%. About half the heat in the tunnels though comes from just one source –from the trains slowing down — the conversion of movement into heat by applying the brakes. So it can be seen that cutting the heat from applying the brakes is where the biggest win would be, and indeed, the use of regenerative braking now converts about half the heat loss back into electricity. However, that can only work where trains are accelerating and braking at the same time, on the same electricity sub-station loop. Experiments have been underway to improve that by use of an inverting substation, supplied by Alstom, which can send unused power from braking trains back into the national grid.

Removing the heat

Anyone who has stood on the platforms will know that as the trains approach, there’s a blast of wind. This is intentional, as the trains act as air pistons in the small tunnels, and the effect of pushing air ahead and sucking air from behind soaks away about 11 percent of the heat in the tunnels. So when you curse as your paper flaps in the wind, think about the cooling benefits as well. Mechanical ventilation removes about 10 percent of the heat — that’s the big ventilation shafts that line the tunnels. The older tunnels weren’t built with a lot of ventilation, as it wasn’t thought to be necessary — after all it’s difficult to argue that tunnels will get hot when standing in a tunnel that’s cool enough that you need to wear a jumper.

By the time the Victoria line came along, the engineers were very well aware of the problem and it was built with considerably more ventilation shafts than older tunnels would have been supplied with. Over the past few years, 14 of the Victoria line shafts have been upgraded, and 50 fans across the network have had their airflow doubled, with 10 out-of-action fans brought back into use. Despite that, fully 79 percent of the heat in the tunnels is left to soak into the surrounding clay, which is already at or near its limit thanks to decades of absorbing heat.

The difficulty of adding more ventilation is the lack of space above ground to put new ventilation shafts. People also tend to be wary of having a new ventilation shaft in their neighbourhood, even though the aim is to keep the volume level to that equivalent to background city noise. Even if new shafts are installed, at the moment they represent just 10% of the heat removed from the tunnels, so you can imagine how many extra shafts would be needed to remove a meaningful amount.

An experiment in Islington is using heat from the tube tunnels to warm up a municipal heating service provided to a housing estate. The advantage of this scheme is that it can remove heat in winter when it’s needed above ground. If the clay surrounding the tunnel can be cooled in winter, it has more capacity to absorb heat in the summer. At this particular trial, the fans can also be reversed so that during the summer months, they can suck cool night time air down into the tunnels as well.

The future

As can be seen, even with all the planned upgrades, it’s never going to be viable to remove all of the heat from the tunnels, so the aim going forward is to stop the heat getting in there in the first place. The specifications for the New Tube for London not only include squeezing some sort of air-conditioning units into the carriages, but much more importantly, they need to consume a lot less energy in the first place. Lighter trains, with the ability to coast unpowered for longer with less energy consumed means less heat dumped into the tunnels. The future of the cooling the tube project will be judged not so much by how they cool the hot tunnels, but by how they stop tunnels becoming hot in the first place.

About the author

Ian Mansfield is the founder of the successful blog 'Ian Visits'. He writes about London heritage, tours and free (or cheap) lectures on offer in the capital.

This article is based, in part, on a lecture given at The Institution of Engineering and Technology by Sharon Duffy, Head of Station Systems Engineering, LU Crossrail and Stations. It was organised by the IET’s railway group.