Kake (nou) wrote,

Cooling the Tube.

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 Station.

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 testing stage.

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 sooner.

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.

Tags: london, tube

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