Cool Reflections for Overheating Metros!

“Why paint trains with high-performance solar reflective paint ……….and fit low emissivity glass windows?”

Author: Calvin R Barrows, BSc (Hons), CEng, MICE

 

(N.B. The focus of this article is on mixed surface and sub-surface Metros like London Underground. Many of these, like London Underground, do not have sub-surface cooling systems throughout.)

There are many heat sources in Metro systems. However, only those that are seasonal are under consideration in this paper. Indeed, it is worth noting that, in the cooler weather the combined effect of the non-seasonal heat sources is insufficient to heat trains to a temperature in which customers would feel comfortable and so in winter these heat sources need to be augmented by the addition of heating from train saloon heaters.

However, there are two heat sources that increase in summer: outside ambient air and solar irradiation. The former is an important and notable seasonal variant, but whilst ambient air temperature varies throughout the daily cycle, it does not dramatically change over a 30 minute period. It cannot, therefore, explain the cause of the considerable increase in train temperatures on the surface often achieved over that same time period, so this paper will focus primarily on the role of solar irradiation.

Even though we all experience the effect of sunshine in summer, there does not appear to be more than a superficial appreciation of just how powerful solar irradiation can be! In this link, under the “Measurement” section it explains that “…direct sunlight at Earth’s surface when the Sun is at the zenith (directly above) is about 1050 W/m2”.

This is generally supported by the monitoring and other evidence presented in my article “Cooling the Tube – On Ice till 2030”, and also by the fact that in the summer of 2018 my pale coloured sandstone patio was as red-hot as the sand on a Mediterranean beach.

However, the sun’s power is also affected by the Angle of Incidence, which reduces its intensity when it is at shallower angles. For example, at a 450 angle of incidence, although solar radiation can cover a 40 percent greater area, it is then 30 percent less intense than when at its maximum angle of incidence of 900. Consequently, we also need to consider how this changing intensity might affect trains.

Prof. Piercarlo Romagnoni and Prof. Fabio Peron of the Università Iuav di Venezia produced a factsheet which examines the temperature impact solar radiation has on thermal insulation materials in roofing applications. Although some train roofs are not insulated, this factsheet gives an indication of the potential external skin temperatures.

One test they undertook was on a roof sheet formed of a curved sandwich panel:

• The external surface was aluminium metal sheet, thickness 7/10mm, painted red;
• The insulation layer was a 40mm thickness of polyurethane foam;
• The internal surface was a galvanized, corrugated metal sheet, thickness 4/10mm.

Whilst this is not exactly how a train roof is constructed, I would submit the external skin of a train roof would absorb similar levels of solar irradiation and reach similar temperatures. The maximum temperatures on curved roofs in more moderate climate zones like Venice can reach 67°C. Summer ambient air temperatures in Venice can reach 40°C, so the surface temperature of the aluminium external skin of this sandwiched panel is potentially around 25°C above ambient, which is akin to the difference between train rail temperatures and ambient air temperatures.

The next consideration is, how much of the train should we coat with solar reflective paint? Some railway staff have focused on the roof. However, focusing on the roof alone will limit the potential benefits.

The average carriage size is 2.9m wide with a “passenger compartment” body height of around 2.5m. Taking that as a basis, when the sun is at its zenith, it produces 1050 W/m2 x 2.9m per metre length of carriage or 3045 W/m (3.045 kW/m) on its roof.

However, with the sun at an “angle of incidence” of 450 and hence with the sun on both the roof and one or other side of the train, there is approximately 2.5m + 2.9m of train body exposed to the sun. At this angle the sun’s intensity is 1050 W/m2 x 70% (30% less than at its zenith) = 735 W/m2 but this is now shining on 5.4m per metre length of train body. In this scenario then the sun produces 735 W/m2 x 5.4m per metre length of carriage or 3969 W/m (3.969 W/m).

 

The 1972 British Rail Research calculation [abstract], had previously raised a concern about the challenge of how solar irradiation can be calculated. TfL have kindly provided figures from the original, full report (which no longer appears to be directly available in the RSSB’s SPARK Rail Knowledge Hub’s archive). The process of arriving at these figures, as set out in the abstract, appears completely unintelligible and this historic calculation of 7.8kW per trailer car or approximately 0.5kW per metre length of carriage, is clearly way off the mark for the following reasons:

• Train saloon heaters for winter operation produce around 15kW of heat per carriage, approximately twice the 1972 claimed power of the summer sun!
• Even in low ambient winter temperatures one’s body can be pleasantly comfortable when in the
  winter sunshine.
• Recent posts on TfL’s Yammer Network about a trial investigation on the Central Line were brought to my attention. This investigation compared the levels of solar heat gain through saloon windows fitted with tinted film to those without, and the extent to which untreated windows contributed towards the elevated temperature in the carriages, in open sections of track, during the hottest portion of the day. A significant temperature difference was recorded, the saloons with the tinted glass windows being up to 4.50C cooler. The limited, low performance measures implemented by the year 2015 would only have delivered these notable improvements, if the effect of solar irradiation were significant, which it clearly is!

Let us now consider the benefits of using high-performance solar reflective paint on train bodies. The trials undertaken by the Australian Navy on their patrol boats have shown that the low solar-absorbing formulation reduced their surface temperature by 15-200C compared to conventional coatings, and consequently lowered the power load for on-board cooling systems. In the context of metro trains, reflecting the solar irradiation would:

1. Reduce the train’s internal temperature in summer;
2. Reduce the required size of the air conditioning, delivering greater payload and reduced weight;
3. Reduce the in-train air conditioning running costs;
4. Improve the passenger experience in terms of comfort; and very importantly,
5. Reduce the safety risk to passengers in a stalled train event.

Similarly, in the context of reducing tunnel heat, reflecting the solar irradiation when travelling on the surface in summer would:

1. Reduce the train’s external skin temperature; which in turn will reduce the absorbed heat load into the train body and thus the heat emitted from the train body into the tunnel;
2. Reduce the train’s internal temperature, and consequently reduce the heat being discharged from the train’s air conditioning (if fitted) on entry into the tunnel; and
3. As a result of 1 and 2 above, subsequently reduce the heat transferred across the network from hotter lines to cooler lines by the “pressure / suction” wave caused by the movement of trains, whilst satisfying the Laws of Thermodynamics – heat will go to cold.

However, it should be noted that the benefits do not stop there ….. For tunnels with adequate and comprehensive cooling systems, with the use of solar reflective paint these systems may become less crucial, be under-run or even become fully or partially redundant. For those where such measures have not already been installed, solar reflective paint may obviate the need to install them at all.

As obvious as all the above may be by now, testing still needs to be carried out. Whilst it is simple to compare the reductions in internal train temperatures, accurately assessing the cumulative reduction in tunnel temperatures as each train is treated is less straightforward.

Testing should involve the continuous temperature measurements, along its route, of all the relevant train temperatures and be related to time, location and external ambient temperature: but there are several things that will need careful consideration in order to quantify accurately the collective benefits in respect of reduced tunnel temperatures. The sooner these relevant train temperature readings can commence the sooner a reliable baseline can be established.

Undoubtedly the optimum test would be to treat all the trains on one line, and I would suggest the most appropriate line would be the Central Line, especially bearing in mind the 40+0C temperatures experienced this last summer (2018). Doing this optimum test would be the only way to measure, rather than theoretically predict, the overall reduction of tunnel temperatures. It would not be deemed sensible to treat all the trains without some evidence of the benefit; however, the limitations of data from a less comprehensive test will need to be fully understood and well thought through.

Such a comprehensive test would also avoid skewing the results with such elements as the carry-in heat of untreated trains being transferred within the tunnel, to the treated trains – we have a possible Catch 22 situation here!

Monitoring the winter temperature would be useful to compare the train’s external skin temperatures with those in the summer. On the face of it, there would seem little to be gained monitoring internal saloon temperatures with their heaters operating. However, it is yet another legitimate cyclical heat source to be considered within the seasonal differential and I believe it would be useful, provided it is interpreted appropriately. Unlike the practice to date, ALL possible heat sources should be robustly accounted for!

Monitoring the train’s external skin might best be achieved with sensors on the internal face of the external skin, suitably insulated from the train’s internal space. This will mitigate the inaccuracies caused by unrepresentative cooling effects – e.g. local eddy currents caused by turbulent air flows, which have certainly proved difficult in the context of modern cars. Monitoring of undercarriages and bogies would need some serious thought, in order to be able to identify the effects of the various heat sources operating on these – see below.

However, with every further treated train, first on the Central Line and then throughout the network, the cumulative reductions on tunnel temperatures, and thus sub-surface network temperatures would become significant. Hence the fullest benefits will only become apparent when all lines, with the possible exceptions of the Victoria and the Waterloo & City Lines, have had all their trains treated.

Whilst the foregoing looks at the general solar irradiation effect on the passenger compartment of the train, it should not be overlooked that the undercarriage (bogies) will also be affected. Two NON- seasonal heat sources acting on these are obvious: traction and braking. However, there are three seasonal ones that need to be considered and the overlooking of these may have skewed the previous perception of the heat load from braking.

1. The considerable mass of steel in the bogies and wheels, which will be affected when the sun is
   striking one or other side of the train;
2. The heat being constantly emitted / transferred from the rails, which in the summer sun can often
   reach 20°C above ambient; and
3. The solar irradiated sleepers and ballast, which will also absorb the sun’s heat, reflected to the
   underside of the train and bogies.

These additional heat loads are less likely to have a significant effect on the passenger compartments when overground but will add to the carry-in heat within the tunnels and so it would make total sense to treat the exposed areas of undercarriage and wheels with the high-performance solar reflective paint.

It should be noted that more recently Railtrack have been painting non-wearing exposed surfaces of rails with white paint to mitigate rail buckling. Additionally, implementing this on London Underground will reduce the heat absorbed by the rails and consequently what they “emit” when the train passes over them.

In considering the issues raised in this paper, clearly painting the train body and undercarriage in this way is still only part of the solution, albeit a significant one. In addition, there is the matter of the windows, which at present is a crucial factor in turning carriages into “greenhouses”.

The effect of solar gain through glass windows into a room (aka a carriage) is well illustrated. The next logical step
to the use of high-performance solar reflective paint would be to fit “low emissivity glass” windows in all trains.

Managing the two-way passage of heat through glass has come a long way with Low-E glass. Given the high percentage of window glass in the body of a carriage, using this highly reflective glass meansthat the external solar heat will not be absorbed whilst the train is on the surface (as with solar reflective paint) and so:

•  the internal carriage temperatures will be reduced; and
•  the external glass surface temperature will be reduced, which in turn will reduce the carry-in heat being emitted when the trains pass into the tunnels.

Treatment with solar reflective measures would probably be best undertaken in stages:

1. Establish the baseline for tunnel temperatures with “all trains being untreated”;
2. Treat one train, including the undercarriage, initially with high-performance solar reflective paint
   and low emissivity windows;
3. Compare the reduction in heat absorbed by this treated train with the baseline data;
4. Extrapolate the above results to get a sense of the cumulative reductions in tunnel temperatures that can be achieved once all trains are treated.

Clearly treating only one train will have a negligible effect on tunnel temperatures but treating all the line’s rolling stock will achieve a significant reduction in tunnel temperatures across the treated line.

Some insight as to the consequences of heat emittance on the treated train from the untreated trains in the tunnel would also be useful, since such a transfer of heat would result in an under-estimation of the ultimate benefits. However, by virtue of an inefficient transfer mechanism, the effect of overheated tunnel air on the treated train skin may not be of great significance.

Of course, when the temperature of one line (the hottest line) is reduced significantly, other lines would now be relatively hotter, as then by the Laws of Thermodynamics there would be a transfer of heat from untreated lines to the now cooler line. Therefore, as a result of the interdependencies of one line with the others, after the completion of the preceding hottest line, a decision on the treatment sequence of the other lines would require identifying the next hottest remaining line!

In conclusion, a metro that has no surface lines would not benefit from having its trains painted with solar reflective paint or being fitted with low emissivity glass windows, provided stabled trains are properly managed. Conversely, metros with a mix of surface and sub-surface lines and indeed overground trains would benefit greatly from having their rolling stock painted with solar reflective paint and windows fitted with highly reflective glass.

Furthermore, in an era of heightened and pressing environmental concern, the application of solar reflective paint and the incorporation of highly reflective glass windows are very cost-effective, particularly when compared to the capital cost of installing mechanical cooling solutions, and longer term will deliver real on-going reductions in maintenance and energy running costs.

A final thought! Of all the mitigation works ever implemented under the Cooling the Tube Project, the limited solar reflective foil and film treatment of Central Line trains, reducing the internal temperature of the saloons by some 40C, has proved by far the most successful. If my information is correct, and I believe it is, the Green Park Station Ground Source Cooling Project was tendered at around £12M and the final costs was more like £25M …… and it would have made little or no difference to train temperatures.

For that sort of money, you can buy and apply an awful lot of high-performance solar reflective paint and install a lot of Low-E glass windows.

On reflection – absolutely pun intended – the solar reflective paint and Low-E glass windows will reduce the amount of heat absorbed by trains travelling on the surface in the summer, because the trains will be able to jettison this heat continually along the overground route BEFORE THEY REACH AND ENTER THE TUNNEL.

“I wish we were on the right track!” Think Sun Cream for trains!

 

Author: Calvin R Barrows, BSc (Hons), CEng, MICE

Cooling the Tube – on Ice till 2030?

Author: Calvin R Barrows, BSc (Hons), CEng, MICE

 

Hang onto your handrail, if it’s not too hot, fellow tube users, I am just about to share my controversial theory about Cooling the Tube, which has resulted in some folk getting rather hot under the collar.

Please stay aboard, because for the Cooling the Tube Project ever to be successful, there are fundamental points at issue that still need to be considered and resolved. I am hoping that summarising my findings and research to date along with my interpretation of these, will initiate some serious and constructive debate among engineers, scientists and anyone with an interest in this matter.

In this article, I shall:

• consider the extent of the problem, and how it affects passengers;
• set out the circumstances that led me down this very different route;
• identify the probable causes of the over-heating, based on my direct experiences,observations and temperature measurements;
•  discover how another underground network, conceptually similar, albeit with some
  subtle differences, fares with overheating; and
•  suggest possible solutions specifically for the configuration of London Underground’s
  infrastructure.

Finally – for the reader – there is an enlightening question to be considered!

A well-defined problem is half solved – paraphrased from Charles Kettering.

 

First, the very title of the project “Cooling the Tube” is a misnomer. Consequently, the search for a solution to the problem has been misled. The challenge is not simply “Cooling the Tube” or even “Cooling the Underground Network”: to be precise I would define it as “Cooling the Whole LU Network in the Summer”, since both overground and underground overheat in the summer!

Secondly, having spent years commuting on the Central Line and monitoring the seasonal temperatures differences at various times of day, I came to the irrefutable conclusion that the trains were gaining more heat overground than underground in the summer.

Let’s dispel the myth circulating that the Tube is hot all the year round, because in the winter passengers wear their thick coats and scarves while standing or sitting comfortably inside an electrically heated carriage. Conversely in the summer, passengers wear light clothing – yet are still frequently getting close to heat exhaustion during heatwaves and the hotter weather generally.

The project’s stated objective is two-fold – comfort and safety. For comfort cool trains are the priority, since passengers may spend 5-10 minutes on a platform but perhaps 45 minutes to an hour on a train.

For safety we need cool trains and cool tunnels, the cool tunnels being key to maintaining cool trains when underground. If passengers were trapped for some considerable time in an overheated, stalled train, within an
overheated tunnel, the outcomes for them could be very grave. So, the decision to expend all that effort and money cooling underground stations was erroneous, and indeed has been a wasteful diversion from addressing the real problem – because passengers are free to leave a station, for a place of safety, if they are overheating there!

Heat Sources

Many alternative reasons for the overheating in the summer have been proffered – including the combined effect of the tunnel, train and station systems; traction; braking; mechanical losses; passengers; the heat accumulating in the London clay; and so on.

All these are red herrings – either because they do not change significantly throughout the year, so they don’t explain the seasonal variations, or in the case of the London clay temperature, it’s not a cause – it’s an effect. The key issue
is that many of the tube trains run both over- and underground.

These are inevitably going to be affected by the overground, ambient temperature and solar heat gain. So rather than “the primary heating of the carriages happens in the tunnels”, my assertion is that “the primary heating of the carriages does not happen in the tunnels”.

My initial, subjective observation struck me as odd in that, travelling overground towards Epping during a late summer afternoon with progressively less passengers on board, the carriage seemed to become markedly hotter. My subsequent temperature monitoring clearly confirmed that up to a 6°C increase in temperature was not uncommon in the summer between Leyton and Epping.

The only logical conclusion was that the train was being heated by the surrounding hot, ambient air and “superheated” by solar irradiation (heat gain from the sun).

However, what, if anything, could that have to do with the over-heating in the tunnels? When travelling overground, the sun not only heats the inside of carriages directly, but the solar irradiation is also absorbed by the external skin of the train.

If a child’s slide can get so hot in the sun as to cause them serious burns; if tarmac roads can melt; if a car can become too hot to touch; and if untreated train tracks can reach 20°C above ambient temperature and buckle – then by the same token train bodies can absorb heat from the sun on their external surfaces and then emit some of that “super-heat”.

These overheated “storage radiators” will constantly emit this absorbed heat until they reach a state of equilibrium. However, being continuously heated by the sun, they cannot reach a state of equilibrium before entering the tunnel, so they continue emitting heat into the tunnel. Clearly then overheating is a network problem – the Laws of Thermodynamics can’t have it any other way!

Although the level of heat emission is unharmful on the surface, when these 200 tonnes of train enter the tunnel, with a thermal capacity estimated at 100-200 MJoules/°C, they continue to emit heat to detrimental effect. If all that were not enough, every time these trains enter the portals in the summer, they are also pushing/dragging in a plug of hot, ambient air, adding to the heat load in the underground network.

The yearly cycle from Autumn 2017 has seen ambient overground temperatures vary from -5°C to +35°C. This 40°C temperature difference of the air introduced into the tunnels by the piston and drag effects undeniably makes a significant difference to the tunnel temperatures between summer and winter.

When comparing the London network with that of Glasgow, similar in concept but ALL underground and with shallower tunnels at a depth of ≈ 9 metres (30 feet), their tunnel ambient temperature hovers around 16°C all year-round, even during their 31°C heatwave of the summer of 2018. Because it is wholly underground, it suffers neither from solar irradiation nor the portal piston/drag effect.

Additionally, unlike most of the London Underground rolling stock, Glasgow’s trains are stabled appropriately during the heat of the day, in roofed train sheds with the doors open – parasol effect!

Mitigating the factors causing this specific overheating problem on London Underground’s network really does not need to be difficult but does require an uncommon solution. Whilst many national overground trains are already being successfully air-conditioned, with trains underground the waste heat would be expelled into the overheated tunnels, leading to probable equipment failure.

Moreover, any cooled air in the trains would soon be overwhelmed by this overheated tunnel air. Therefore, installing or retro-fitting air-conditioning to the entire London Underground train stock, even if feasible, would be wholly unrealistic and ineffective.

However, to deal with solar irradiation there is no sensible reason why trains that run both over- and underground should not be coated with a high-performance, solar-reflective paint and be fitted with high-performance, solar-reflective glass windows. Equally, trains should be appropriately stabled in the non-peak times – some depot buildings may not be very suitable, if they are likely to be affected by solar heat gain themselves. It might be better to provide well ventilated shading, built with a finned roof and walls (Venetian blind form) and with the openings facing north – parasol effect again.

Additionally, limiting the transfer of ambient summer air into the underground network should be addressed – indeed The Rail Safety and Standards Board (RSSB) recommends this be considered! On sub-surface lines the piston/drag effect is less marked because of the configuration of the portal openings, the more frequent “cross-passages” between the two tracks and the less restricted, overline ventilation shafts and openings. For the deep lines it’s a different story.

Their portals will probably need some remodelling; perhaps incorporating some form of cooling, diversion or dispersal of the ambient summer air – or a combination of all three.

Clearly, there is a need for appropriate and detailed train temperature monitoring throughout an entire year, to agree upon the real causes of summer overheat – and dispel the myths. Evidently TfL has not undertaken such formal monitoring to date, since they still mistakenly claim that “the primary heating of the carriages happens in the tunnels”.

In summary, whilst not confusing causes with effects, we need to be very clear about both the problem, and the root causes, the removal of which will resolve or dramatically reduce that problem.
Therefore, with an over- and underground network such as London Underground’s, this demonstrates the key factors causing overheating are trains travelling on the OVERGROUND sections of the network combined with the summer weather.

Finally, a quote from the Vienna Metro in their e-mail to me – “We appreciate your interest. As a
matter of fact, the sun is the main cause for the overheating problem for the U6 [be]cause of the long
overground route”.

 

Author: Calvin R Barrows, BSc (Hons), CEng, MICE