The London Underground is notorious for its sweltering temperatures during the summer months, a problem seemingly getting worse with each passing year. While climate change might seem the obvious culprit, veteran Chartered Engineer Calvin Barrows points to a different factor: solar gain.
Barrows first noticed the issue during his daily commute on the Central Line, observing trains overheating during the warmer periods of the year. He realised that the trains were absorbing heat while travelling or stationary above ground and carrying it into the tunnels. The "eureka moment," he explains, came after watching David Attenborough, who highlighted the immense power of the sun, leading Barrows to delve deeper into the science of heat transfer.
He discovered that the sun's radiation directly affects the entire body of the train, as well as the track, ballast, and rails above ground. This "primary" radiation is then re-radiated from the ballast and rails to the train's undercarriage, known as "secondary" radiation. As the overheated trains enter the tunnels, they re-radiate the heat into these confined spaces, leading to a significant rise in temperature within the tunnels and their surrounding geological formations.
This seasonal heat transfer, according to Barrows, far surpasses any heat generated by operational factors like traction or braking. The resulting heat from solar gain presents a serious health and safety hazard for passengers, potentially leading to discomfort, heat exhaustion, and even life-threatening heatstroke.
The danger is particularly acute during the warmer months, especially during heatwaves. While underground-only networks like Glasgow's do not suffer from this issue, the London Underground network, with its above and below ground sections, is highly susceptible to solar gain.
Despite the severity of the problem, Transport for London (TfL) and London Underground (LU) have struggled to effectively address it. While they claim to have invested heavily in mitigation efforts, their successes have been limited. Their attempts, including cooling trials at Holborn station and the "wind chill factor" ventilation system, have failed to address the root cause of the problem: solar gain.
Barrows highlights the folly of relying on air conditioning (AC) as a solution, arguing that the hot exhaust air from AC units further exacerbates the issue within the tunnels. He points to New York's subway network, where AC has been in place for decades yet overheating persists.
The current approach by TfL/LU, focusing on mitigating operational factors instead of tackling solar gain, is likened to "copying the errors and replicating the failures made in New York." Furthermore, their recent proposal to install AC on the Piccadilly Line's new rolling stock, despite previously acknowledging its ineffectiveness, is seen as a step in the wrong direction.
So, what are the recommended solutions? Barrows advocates for preventative measures like using solar reflective paint on the train's exterior, specialist coatings for undercarriages and bogies, and solar-controlled glass for windows. He also recommends radiative insulation materials, solar barriers, and green track vegetation, which have proven successful in other parts of Europe.
Barrows believes that these solutions would be significantly less expensive than TfL/LU's current approach, both in terms of capital investment and maintenance costs, while also offering additional operational and environmental benefits.
He warns that without a change in approach and a more accurate understanding of the heat sources and transfer mechanisms, the situation is likely to worsen with increasing summer temperatures. This inaction, he argues, is unacceptable and could ultimately lead to a catastrophic event.
As the autumn season approaches, TfL might have avoided a major incident this year, but the underlying problem persists. The time to act is now before a tragedy occurs.