Hello. I went to a talk last night about cooling the London Underground. It was interesting. I took notes, and have written them up. This isn't in the exact order the talk was in; I moved some stuff around to make more sense (the speaker mentioned that his slides weren't in the optimal order, so his talk wouldn't be either).
The talk was at the Building Centre in Bloomsbury, and the speaker was the director of TfL's Cooling The Tube programme, Kevin Payne.
When an underground train tunnel is first constructed, it's at the native soil
temperature of around 14°C. Temperature problems start to show up
20–30 years later; this timescale has been seen over and over again in
different lines/metro systems throughout the world. Basically the
tunnel is a closed environment with a lot of energy sources. The soil
around the tunnel gradually dries out and becomes a much better
insulator — they've measured this on the Victoria line and found that
the soil is dried out for several metres' distance from the tunnel.
The subsurface lines (e.g. Circle, Hammerchapel) are relatively well
ventilated, so conventional aircon can be used there — in fact, trains
with aircon are being brought into service next year. The deep lines
are the problem. There's very little free air as the tunnel sizes are
so constrained; also, many of them were built without adequate
ventilation — the Jubilee Line is the best of the bunch as it was built
later, with the benefit of hindsight.
Aircon (really, air cooling) basically works by moving heat from one
place to another. The reason conventional aircon is unsuitable for
use on the deep line trains is that while moving heat from the inside
of the train to the outside of the train is all well and good when the
train is moving, it actually makes things worse when the train is at a
standstill; there's nowhere for the heat to go and so the temperature
in the tunnel increases and increases to the point where the aircon
has to cut out. The conditions inside the train then become
intolerable much faster than if there had been no aircon in the first
place (they've tested this; he didn't give details of how).
Temperature isn't actually the only factor in play here; the others
are exposure time, air movement, and humidity. Humans have a fairly
narrow tolerance range — optimal conditions are around 24–26°C,
1.5m/s airflow, and 30–45% relative humidity. Currently, we can only
really control exposure time. Humidity isn't an enormous problem
because the network is actually fairly dry; relative humidity in the
tunnels is already around 35–45%. (This does cause some difficulties
in other technical/operational aspects — but it's not a problem in
terms of comfort.)
Climate change isn't a huge factor driving heat increases in the
deep-level lines; the increase in passenger journeys is more
important. The overall trend is ever upwards; although certain
economic events may stall the increase for a while (not sure what
these are — there was one around 1992–1994ish and another one a bit
later), it never goes backwards. 3 million journeys per day used to
be a "very busy" day — in December 2007 we hit a max of 3.9 million
and a month later a max of 4.1 million.
(As an aside, we were also shown a graph of temperatures on the
Northern Line between Chalk Farm and Hampstead; while the outside
temperature spiked up and down during day and night, the temperature
in the tunnel barely varied at all. Indeed at some points the outside
temperature was higher than the tunnel temperature — a high outside
temperature would have to persist for quite some time before the
tunnel temperature would start to rise.)
London's special climate plays a part too; compared to the rest of the
UK we're what's known as a "heat island" (see this PDF of an essay on the topic). The many dark surfaces along with
all the bricks, mortar, and steel soak up heat to the point where
London's temperature may be up to 6°C higher than that of the
surrounding more rural areas.
Various cooling schemes have been thought up, and a number of them are
in deployment or trialling at the moment.
Although the conventional aircon method of moving heat from the train
to the tunnel is a net loss, there's no harm in using natural
ventilation for this; that is, methods that don't involve adding
energy to the system. This has been tested on one Central Line train
— these trains have rather large windows, so they replaced this with a
smaller window plus a natural ventilation, er, thingy, above the
window. This simple idea was very successful — it's no good in a
stalled train, but it does help a lot when the train is moving.
Groundwater cooling makes use of water in the ground surrounding the
underground part of a station. It's currently being trialled at
Victoria Station; this is a suitable site because the station gets a
fair bit of seepage (much of it from the River Tyburn). The
groundwater's collected in a sump; it comes in at around 16–17°C
and is then circulated through heat exchangers (which work by blowing
warm air across coils of cold water pipes). Once the water is warm,
it's returned to the external environment via the sewers. The
environmental impact is tiny. This trial is on a fairly small scale —
he mentioned a figure of 60 kW — but it's working well so they plan to
increase the cooling power as part of the overall upgrade of Victoria
They'd love to be able to do this on the entire network, but they're
constrained by the lack of reliable and decent quality sump water —
they expect to be able to do it at three or four more sites at most.
They've looked at lots of sites and found that the groundwater is very
polluted at some of them; and although the cooling system is a closed
one, there's always the chance of leaks, and they just don't want to
risk pumping polluted water around so close to people.
Other things currently being deployed in stations are fans and
chillers (i.e. fairly conventional aircon, the
kind which wouldn't work in tunnels). There's a large fan installed
in Oxford Circus ticket hall, and there'll be one at Euston soon and
one at Waterloo "at some point". There are also a number of portable
fans (the ones in cages) which can be used as an interim measure;
they're getting more of these (and also getting "nicer cages").
Another option for cooling the tunnels is mid-tunnel ventilation
shafts, which work by a combination of fans in the shafts and the
"piston" effect of a train in a tunnel pushing a plug of air ahead of
it. There are lots of these already, built 20–50 years ago and hidden
all over London. Some of them are quite interesting, architecturally.
There's a "temple"-style one (built in 1968) for the
Victoria Line in a landscaped garden in North London; the design of
the surface part of the shaft was inspired by the garden. There's
also a row of Georgian houses in Pimlico where one of the houses is
actually the surface part of a ventilation shaft.
These shafts are currently being upgraded to roughly double the
previous amount of airflow; the Victoria Line ones are first (there
are fourteen of them plus two others that are actually on the
Piccadilly Line but are used to cool the Victoria Line — don't
understand this bit).
As well as the stuff in deployment/trialling, there's also a fair bit
of new technology in the research or testing stage.
In-train hybrid cooling might be the Great Solution, but it's
still at the experimental stage. Basically, conventional aircon is
used when the train is above the surface, and switched off when the
train goes underground. The clever part is that when above the
surface, the aircon is used not only to cool the train but also to
build up a reservoir of cooled material which is then exploited when
underground. They put a lot of effort into looking for a suitable
medium for this — they tried all kinds of exotic substances, some of
which were too toxic, or too flammable, or had unsuitable melting or
freezing points. In the end, they settled on water — you may have
heard things in the news about "Ken's mad plan to put blocks of ice on
Tube trains" — well, it's true, and this is it. (The ice unit will be
under the floors of the carriages.)
This is a fairly radical idea as it's not (to their knowledge) been
done before; although ice has been used to cool trains for over a
century in various parts of the world, this is the first system that
actually creates the ice on board the train. (As an aside, it turns
out that freezing and thawing ice is quite an interesting problem in
itself — you don't want ice to accumulate around the pipes when making
your reservoir, nor do you want water to accumulate around the pipes
when melting your reservoir; it's vital that as much material as
possible crosses the phase boundary as this increases the amount of
energy stored/released.) Hybrid cooling is currently at the mockup
Another option is regenerative braking. Instead of braking
conventionally, using friction (which creates waste
heat), you brake by switching your motor to a generator/alternator,
thus creating electricity instead of heat. The Central, Northern, and
Jubilee Lines already have this. There's still some tweaking to be
done, adjusting the acceleration and deceleration cycles to optimise
the system, but basically railways are way ahead of other forms of
transport on this.
Adiabatic and wet evaporative cooling are due to be trialled this
year. (Adiabatic cooling works by blowing air through a fine water
mist; it's a very ancient technology.) They're not sure these are
right for London since they're most effective in arid conditions. The
Madrid metro is already using this technology.
As already mentioned, although groundwater cooling is quite fantastic,
it's unsuitable for all but a very few sites. A related idea uses
boreholes to exploit the water in the aquifer below London; the water
enters the system at around 12–13°C, is used for cooling, and is
then reinjected into the aquifer at a suitable distance from the
source point. This has already been successfully deployed at the
Royal Festival Hall. There are around ten sites on the Tube network
that are suitable for this — the main problem is finding suitable
places to site the boreholes. I don't entirely understand why this
is, since he also said that when the borehole is complete the external
footprint is of the order of a manhole cover. Anyway, they should
have one of these working within five years, and hopefully much
Another thing in testing at the moment is aimed at tackling the space
aspect of the problem — cooling technology is all very well but you
need somewhere to put it. These are air handling units (water/air
heat exchangers) shaped in cross-section like a circular
segment, and installed in the ceilings above platforms. This idea
was inspired by the passenger overbridges which you can see on some
platforms (you know when
you're on the platform and if you look up you can see people's feet
walking overhead? Those. You can see them at e.g. Paddington, Waterloo, Holborn, Leicester Square, Piccadilly Circus) There's a mockup install at Aldwych (a
disused station) and there'll be another installed at a disused
platform at Charing Cross later this year. Once they've got the
installation process streamlined, they'll be doing a live trial at one
of the Victoria Line stations — because access time for
maintenance work is very precious, they want to make absolutely sure
they can get the thing installed as fast as possible to avoid too much
disruption to passenger service.
Under-platform exhausts are also in development — simply removing heat
from under the train (where a surprising amount of it accumulates)
while the train is in the platform. These are "almost certain" to be
deployed, but he wouldn't commit to which stations will get it first.
The ideas above are all basically about minimising waste heat, and
transferring it out of the system. The next stage of the challenge is
to find a use for all this energy, rather than just throwing it
all away. One possibility is warm water preheating — although there
will never be enough power to heat water to the point where you can
have a bath in it, preheating with waste heat can at least reduce the
amount of energy input needed for the domestic/business hot water
supply. Heating for building environments is also a possibility.
Something along these lines is currently being trialled in
Stockholm, Sweden, where a new office block alongside Central Station will be heated using (in part) waste heat from the station.
At the moment, in London, energy prices are just slightly too low and
the technological problems slightly too difficult for us to have
tipped over into using our waste heat in this way. He ended the talk
by saying that he is very open to the idea, though, and he would love
it if someone would come to him and say "hey, I have this building,
can I have some of your heat?" — though nobody has, yet.
I have already spent way too much time on this, but I may come back later and add some supplementary links/explanations (or perhaps the other Tube obsessives and/or the engineers on my friends list will do it for me). Also, I took some photos of the Building Centre's scale model of Central London, but I haven't had a chance to sort them out yet. Edit: here they are.