“Why paint trains with high-performance solar reflective
paint ..... .....and fit low emissivity glass windows?”
(N.B. The focus of this document 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 previous paper “Cooling the Tube – On Ice till 2030”, particularly
the links on Page 2, 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 kW/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.
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 means that 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.
Acknowledgements
I must recognise my editor Sylvia Telatycka, a lover of
words: she drew out my intended message, helping me prepare my papers and then
polishing, refining and enhancing them. I also thank fellow engineers Brian
Farley, Philip Hargrave and John Newman, who by their input and questioning
approach added new perspectives to my understanding of the many facets at play.
Finally, I wish to express my appreciation of the many LinkedIn connections,
who by their reactive and proactive engagement with constructive input helped
me keep on track and share the “Metro” message worldwide.
Author: Calvin R Barrows, BSc (Hons), CEng, MICE January
2019
If you have any questions for the author please email Calvin.Barrows (at)hotmail.co.uk
