As climate change makes our summers hotter and longer, underground temperatures are becoming more extreme. A recent analysis found on-train measurements as high as 33.6°C on the Victoria line and 31.7°C on the Central line.
Not every train on the Underground has the luxury of air-conditioning, like those on the new Elizabeth line. For a global city such as London, it can make travel something to be endured – but engineers are trying to alleviate the issue.
As the UK baked in its hottest ever heatwave in July 2022, Transport for London announced trials of new cooling systems to try to reduce the heat on the Tube network. The project used ‘cooling panels’, which circulated air over pipes holding cold water. That cooled air could then be used to lower temperatures for passengers on Tube platforms.
“This innovative trial is taking place as we are experiencing record high temperatures,” said Paul Judge, TfL project director. “This new technology could play an important role in ensuring we are doing everything we can to protect TfL's network against future temperature increases, helping to keep staff and customers safe and comfortable.”
The initiative was put in place because of the oppressive heat and the risk that it could get even hotter, partly due to the changing climate but also because of additional heat generated by increasing the number of trains running on the Piccadilly line.
Retroactively cooling things down is tricky, however. “They can't do air-conditioning, apparently, because there would be nowhere to dispose of the heat,” says Christian Wolmar, a rail industry consultant. “The Tube is kept ventilated by the trains pushing air through.”
It is a conundrum that vexes engineers. “These tunnels were built over a century ago with very little ventilation and are now surrounded by clay that has absorbed decades of heat,” says Gary Tidman, mobility specialist and lead designer at Only Lifts. “That makes traditional air-conditioning not just difficult, but close to impossible in most cases. So the focus has shifted to more creative, location-specific solutions.”
Cooling panels are “one of the more promising developments”, he says. “In lab conditions, [they have] managed drops of around 10 to 15 degrees locally. From an engineering standpoint, it’s much more efficient than earlier systems and relatively easy to maintain, which is crucial for scaling across other deep-level stations.”
At Victoria station, a groundwater cooling method takes advantage of water already being pumped out of tunnels to keep them dry. “This system simply reuses that naturally cold water to cool the air via heat exchangers before pushing it on to the platform,” says Tidman. “It’s a low-energy, low-waste setup and it’s already proved effective.”
Less well known but still effective features installed across the Tube network include improved ventilation systems, impulse fans on station platforms and borehole cooling units in places like Oxford Circus and Green Park. “These upgrades often get less attention,” says Tidman, “but they make a noticeable difference in overall airflow and platform comfort.”
Another issue that passengers face comes when the trains go above ground and sunlight streams in through the windows. “The time that the trains are in the open air, on some lines, also adds other issues,” says Wolmar. “They gather a lot of heat. One of the ideas they have is to open the doors less when they're in the open air,” he explains, but that has little impact overall.
None of these engineering options are one-size-fits-all solutions that will immediately reduce the oppressive temperatures commuters have to face. “But together they’re moving the Underground in the right direction,” says Tidman. “The key now is securing the investment to expand what’s already working, without compromising daily operations.”
Each change comes with incremental benefits, says Wolmar. “There does seem to be no easy solution to the problem, and it's something that has dogged the Tube for decades,” he says. “It is obviously getting worse with climate change.”
The problem is that as the mercury rises, the need to change things becomes more important – and the challenge of fixing it becomes harder.
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