For quite some years now, I have been exploring this seemingly challenging problem. Since 2005 London Underground also has sought to address this in their Cooling the Tube Project but they have not succeeded in making any meaningful progress in reducing the overheating. I firmly believe this is because they are trying to solve the wrong problem.
I do not subscribe to London Underground’s interpretation of the primary cause(s) of overheating because it does not explain why this is uniquely a summer problem. Unfortunately, this interpretation has been accepted largely without critique. The US’s CNBC TV article stated that “But for all the technological advances in the world, nothing will stop the rising heat surrounding the London Underground train network.” (See https://www.cnbc.com/2018/04/20/londons-world-famous-transport-system-could-soon-be-too-hot-to-ride.html) This statement is not true. There is a very simple, technological and cost effective solution that could make significant difference, addressing the root cause, not just the symptoms.
The source of London Underground’s misunderstanding clearly dates back to receiving the report and mathematical model they commissioned from the London South Bank University (LSBU) – published in 2004, which I contend is seriously flawed. In this LSBU report “Underground railway environment in the UK Part 2: Investigation of heat load” the authors’ starting point is the earlier observation (I believe from 1968) by The New York City Transit Authority (NYCTA), who remarked that “the operation of underground railway systems can generate enough heat to raise tunnel and station temperatures as much as 8–11 K”. In the same breath, LSBU refers to the high temperatures in summer within London’s tube network. Interestingly, NYCTA is still grappling with this problem, however, nowhere did they actually identify the root cause of this temperature increase.
On the back of the above, LSBU’s report then unjustifiably claims the following: “It has been shown that the major contributors of heat load in the tunnel are the braking mechanism and the train carriage and its content. In typical summer conditions, the model predicts that over 80% of the heat load in the tunnel of an underground railway network comes from the braking mechanism of the trains”. However, nowhere does the report indicate what the model predicts in winter conditions nor what actually constitutes 100% of the heat load. Furthermore, it claims this, whilst making ill-considered assumptions like “the thermal storage of the carriage could be neglected” – absurd, because omitting any heat source or potential transmitter of heat undermines the overall accurateness of the model; and “in summer, a cold-air stream (ambient air) with a mass flow rate is continuously flowing into the tunnel through the ventilation shafts, and a warmer-air stream (tunnel air) of the same mass flow rate is continuously flowing out of it” – equally absurd, because it cannot possibly be claimed that ambient summer air could be described as cold. Furthermore, the RSSB states in “Internal Flows, Metros and Underground Systems” that “…. pressure waves move along the tunnel at the speed of sound, reflecting at any interface with the open air or large volumes of air, such as at portals or at the top of airshafts and station voids. On reflection, the pressure waves are modified with waves above ambient pressure returning back into the tunnel as waves below ambient pressure and vice versa”. The latter seems to be a much more rational explanation of the air stream behaviour caused by a moving train!
The associated (open access) report by the University of Cambridge, Engineering Department (UCED) “Thermal Modelling and Parametric Analysis of Underground Rail Systems” is considerably more balanced. Despite this, I do have some concerns about their interpretation and I am unclear as to how they too arrived at the conclusion that the effect of the braking heat load is SO significant. What was really interesting in the Cambridge report were Figures 3a and 3b, particularly 3b. There is consensus that caves or non-operational railway tunnels in clay have a background temperature of about 14°C, so their model has predicted that well. However, the figures 3a and 3b show a linear relationship between the rise in ambient temperature and the rise in tunnel and station temperatures. Taking the more complete list of the heat sources from https://en.wikipedia.org/wiki/London_Underground_cooling, some of which UCED may have included within the braking heat load, I submit none of these particular heat outputs would change significantly between winter and summer. There are, however, other seasonal heat sources acting on the trains’ undercarriage, which may have skewed any temperature readings!
The alternative proposal to reduce the heat of the clay across the London network using Ground Source Heat Pumps is anything but viable within the city environment and such discharges would considerably increase above ground city temperatures, as there would be very little or no demand in the summer for this excess heat to be recycled into space heating.
My hypothesis, which is generating serious interest with the railway press and the rail industry advisory and regulatory bodies, is accessible in the link below and is the only one (so far) that comes close to explaining the reason overheating is a summer problem.
See what you think and test it for yourself – when summer arrives again !
(https://drive.google.com/open?id=1w18eRNzbUKd29X9ajTCg2XFTJbDbCr_-)