Why is London’s Tube so hot? It may be solar gain
Anyone who has taken the London Underground in summer knows that things can get hot – sometimes, dangerously hot. And temperatures appear to be getting worse. The climate crisis may seem an obvious culprit, but there could be another reason for such excessive heat on the train network: solar gain. RailTech asks veteran Chartered Engineer Calvin Barrows to unravel the enigma.
When did you first notice the issue of solar gain?
The first time the idea struck was while I was on London’s Central Line during my homeward commute to Epping. The trains on London’s overground and underground networks had been getting very, very hot. Transport for London (TfL) and London Underground (LU) were convinced that the increasing temperatures were caused by operational issues: acceleration, deceleration, traction, braking, passenger heat, etc. In their view, this was causing the ambient, surrounding air in the tunnels to heat up. However, as a Tube passenger and a civil, structural and forensic engineer, a little rudimentary temperature monitoring within the trains’ saloons soon led me to realise they were only overheating in the warmer periods of the year.
This seasonality alerted me to consider other factors, particularly surface ones. The hypothesis was that, for the most part, the trains first gained heat when they were travelling or stationary above ground. Thereafter, they carried it into the tunnels. But the ultimate eureka moment was watching David Attenborough’s Perfect Planet and hearing his reminder about the sheer power of the sun: “The solar energy that strikes our planet in just an hour, contains more power than that used by all of humanity in an entire year.” I quickly realised I needed to better understand the scientific explanations regarding heat transfer and its effect on the train network – and my findings were absolutely fascinating.
So what is the issue with solar gain and why does it happen?
Essentially, the sun irradiates the entirety of the train’s body whilst it is on the surface, as well as the track overground, ballast, and rails. Simultaneously, this latter “primary” radiation is re-radiated from the ballasts and rails to the train’s undercarriage – what we call “secondary” radiation. When the overheated trains then enter the underground sections of the network, being hotter than the tunnels, they re-radiate the heat into these confined spaces, heating up the tunnel linings and their geological surroundings. This seasonal heat transfer is significantly greater than any ambient heat being produced from year-round operational heat such as traction or braking. The resulting heat from solar gain in carriages and tunnels is the health and safety elephant in the room. In such conditions, passengers can suffer considerable discomfort or even the potentially serious symptoms of heat exhaustion, which can lead to life-threatening heat stroke.
When and where does the danger arise?
Unsurprisingly, during the warmer seasons of the year, and especially during heatwaves. There is an indisputable, direct relationship between LU’s chronic overheating and seasonal higher temperatures. However, these seasonal consequences also depend on the differences between the types of rail networks. Underground-only networks, like in Glasgow, do not overheat because their rolling stock is not routinely exposed to the sun’s radiation, so no solar gain.
Whereas in the context of the LU network, which travels both above and below ground, solar gain affects the entire skin and structure of any rolling stock when it is on the surface. From the start of the day, once irradiated, the trains begin entering the tunnel portals. This heat is progressively discharged (re-radiated) into the cooler tunnels. Consequently, tunnels in mixed networks receive the sum of all heat sources. That includes relatively insignificant operational sources like traction and brakes, but also the re-radiation of the significant, direct, and indirect solar gain from the rolling stock’s surface exposure. Once this heat is carried into the tunnels, it is exceedingly difficult to mitigate.
What is currently being done by London Underground to tackle overheating? What are the most recent developments?
London Underground would assert that they have undertaken many projects, investing vast amounts of money to mitigate rising temperatures. Nevertheless, their accomplishments have been relatively minimal. Some fruitless projects have included the cooling trials at Holborn Station, where a prototype machine circulated air through a matrix of tubes cooled by water – basically air conditioning (AC), but with a different name.
There’s also TfL’s enhanced tunnel ventilation system. Known as the “wind chill factor,” this idea was based on the currently used “piston” effect of trains entering the tunnels to supposedly help push out hot air. But this is ineffective as it would neither remove heat from infrastructure components, nor from the solar-irradiated rolling stock. Essentially, solar gain would still be a problem. TfL and LU did toy around with solar-reflective film on the Central Line around 2017, but with little understanding of what was needed to derive any benefit. Essentially, train operators routinely try to address overheating by using AC – a bad idea in metro rail networks.
Why is that a problem?
The problem is that the hot exhaust air from the AC units is discharged directly into the Tube network. Cooling stations like Oxford Circus, Green Park, and Bond Street by installing massive AC units may bring down temperatures on the platforms, but it would not even begin to have any influence on the overheating in the tunnels. The only way this could work on mixed networks is fully air-conditioning both the networks and the trains, but this is so environmentally unfriendly and costly in terms of installation, running, and maintenance.
Ultimately, limiting AC to train saloons not only fails to resolve the problem, but actually exacerbates it. New York’s network has had AC for some 30 years: the trains and tunnels still overheat, and the AC is overwhelmed and regularly breaks down. The reality is that on a mixed overground and underground network, AC will only compound the problem within tunnels that are not air-conditioned.
So are TfL/LU currently trying anything different to their previous efforts? How does this compare to other metro systems?
For the most part, it is not different. This is because TfL/LU continue to focus on mitigating the wrong issues like traction, braking, and passenger-generated heat, copying the errors and replicating the failures made in New York. In fact, I would say they’ve backtracked. Despite historically being adamant that AC would not work because they did not know how they were going to disperse the waste heat, these days they are surprisingly big fans. TfL are now proposing to install AC on the Piccadilly Line’s new rolling stock. In the Jubilee Line extension and the Elizabeth Line, some may assume the installation of platform doors is intended to stop the tunnels overheating. This is not the case. They are intended purely for operational health and safety reasons, to separate the passengers from the trains.
In truth, we know little about how most metros are addressing overheating, or whether like TfL, they are hoping to get by on a wing and a prayer. Underground-only systems in Europe like Glasgow, Warsaw, and Prague remain relatively cool, even in summer. In Moscow, the metros have cool air ventilation shafts that run deep down into their network, and the trains incorporate an effective method of capturing and distributing this air.
Over in the US, most subways appear to rely on train saloon AC in one form or another. These tend to break down frequently as a result of extremely hot air being drawn into the AC’s inlet ducts. The problems they have faced are similar to those in London. Interestingly, in India, they have recognised the need to address the problem of solar gain in many diverse applications and are way ahead of us in delivering it.
What solutions would you recommend to TfL/LU regarding solar gain?
Back when I first recognised the serious impact of surface irradiation, I advised many of TfL’s senior engineers, managerial staff, and board members that they should consider preventative options. That could include using solar reflective paint on the rolling stock’s external surfaces exposed to the sun. This could deliver a 15 to 20°C drop in temperatures. I would also suggest similar specialist coatings for undercarriages and bogies. Solar-controlled glass, which can reflect some 75 per cent of solar irradiation, would help.
Using radiative insulation materials – essentially solar barriers – and green track vegetation would also really improve the situation. The latter has been used successfully in Europe, where there are currently more than 800 kilometres of green “light-rail” tracks. As a direct consequence of the consultative work undertaken with my co-author for the UK’s Rail Safety and Standards Board (RSSB), they published a report recommending the use of solar reflective materials. It states that such techniques could bring “many safety and comfort advantages to the rail industry (and the travelling public).” Unfortunately, TfL has closed their minds to third-party inputs and consequently paid little attention to the advantages of solar reflective measures.
What would the estimated cost of implementing these solutions be?
It depends on many factors unique to each network operator, but I believe it will inevitably be much, much less expensive in both capital investment, maintenance, and running costs than TfL’s current approach. Moreover, many of the ideas have other operational and environmental benefits as well. For example, green tracks are attractive as well as environmentally and cost-beneficial.
How do you see the issue evolving on the LU network if nothing changes?
Without intelligently targeted monitoring to establish and confirm the predominant heat sources, as well as the progressive increase in heat and the mechanisms of heat transfer, TfL and their advisers will continue to misinterpret the evidence. Their modelling will be flawed as they massively underestimate the amount of heat carried into the tunnels and its potential impact on passengers. Increasing summer temperatures makes this a more pressing issue, as it is likely to continue to make surface, solar irradiation of rolling stock even worse.
When trains’ internal saloon temperatures have already exceeded 40°C in the UK, not acting to deal with the issue – and then playing politics around the identification of the real cause – is unacceptable and negligent. In fact, not accepting the problem – when it’s already well established and accepted by rail regulators – is the height of stupidity. Now that autumn is coming, TfL might have gotten away with it for another year. But let’s be clear: a catastrophe is inevitable if they continue down this blind alley.
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