Cooling down the London underground

As we shiver our way through winter, the prospect of barbecues and balmy summer evenings is deeply enticing. Yet despite our hankering for heat it won’t be long before front-page tales of winter woe are displaced by stories of sweltering commuter hell aboard London’s maligned underground system

For the capital’s commuters and tourists the Tube in summer can be a seriously unpleasant experience — and many believe that it’s only a matter of time before soaring summer heat, stalled trains and crowded carriages combine to cause disaster.

Indeed, the Tube has come close in the past: in the summer of 2001 around 600 passengers were treated for heat problems when three trains became stuck for 90 minutes on the Victoria line near Highbury & Islington station.

Three years ago the gravity of the situation prompted London’s mayor, Ken Livingstone, to offer a much-publicised reward for an enterprising solution. Today, following a rash of largely ridiculous suggestions — ranging from strapping giant bags of frozen peas to the top of trains to encouraging commuters to travel naked — Livingstone’s £100,000 remains unclaimed. But the baton has now been taken up by the Tube’s bosses, and a serious engineering initiative is underway that should, it is claimed, start turning down the heat as early as this spring.

David Waboso, London Underground’s director of engineering, explained to The Engineer that the Tube is getting hotter for a variety of reasons. ‘We’ve got deep tunnels, and as time has gone on we’ve put more and more trains in, more power and more people. The tunnels have heated up and the clay that used to act as a sponge soaking up the heat has now reached the point where it’s at the same temperature as the tunnel. In heat terms it is saturated.’

LU has therefore set up a steering group to address the problem and over £100m has already been allocated through Transport for London’s (TFL) £10bn investment programme. The short-term aim is by this spring to reduce the temperature in some areas of the tube system by 2–3°C. In summer temperatures can exceed 40°C.

The engineering and logistical challenges are considerable. The Tube is the oldest underground railway in the world and while modern metro systems, such as lines recently opened in Singapore, have larger tunnels that allow air-conditioning systems to be built into the ceilings and under the floors of trains, the situation faced by those attempting to reduce the temperature on London’s deepest tunnels is rather more complex.

Jonathon Faragher, the engineer charged with spearheading LU’s cooling initiative explained: ‘We’re dealing with a system that is very old and very constrained, with small-bore tubes that are deep underground. The engineering we’re contemplating is monumental in some respects.’

Waboso explained that a range of solutions are being considered and each approach is likely to have specific advantages for different bits of the network. For instance, the cut-and-cover tunnels found on the capital’s sub-surface lines are far more spacious than the small-bore tunnels found on the deep lines, so there are plans to equip trains travelling on these lines with some form of air-conditioning. The deep lines, however, will rely on other solutions, such as platform-based fans to cool the stations, technology that cools the clay around the tunnels, or ground-based systems that exchange heat and water with the ground.

The group is also looking at the continued roll-out of regenerative braking systems. Currently 40 per cent of the train fleet uses regenerative braking (which saves 20–25 per cent of energy that would otherwise be lost as heat).

Faragher’s team will be looking at introducing simple housekeeping measures as well, such as asking station supervisors to switch off unnecessary equipment and open windows and doors. ‘This is a systems engineering project, consisting of a combination of engineering solutions and operational changes,’ said Faragher.

But one of the most ambitious and exciting approaches to the problem is being researched at London’s South Bank University, where a group led by Prof Graeme Maidment has developed an underground cooling system that uses naturally occurring ground water.

The system, which has been developed with funding from both LU and the Carbon Trust, will be trialled on a concourse between two running tunnels at Victoria station this summer, and Maidment expects it to reduce rush-hour temperatures significantly.

Maidment explained that London has a plentiful supply of ground water. In an unending effort to prevent structural damage, LU currently pumps 30 million litres of ground water a day from parts of its network. But this water, while troublesome, is at a seasonally constant temperature of around 10°C which, according to Maidment, is perfect for cooling.

The cooling system developed by his team is simple, working on a similar principle to a car radiator. Work begins early this year, and will see a large heat exchanger installed in the ceiling of one of Victoria station’s main concourse areas. During operation, ground water will be pumped from an existing ground water tank, and circulated through the pipes of the heat exchanger. Fans will be used to promote the flow of air over the surface of these pipes, and the cold water passing through them will be warmed as it absorbs heat from the environment causing the temperature in the concourse to fall.

‘We will be putting six litres per second through the system, which will equate to 150kW of cooling. There are many variables, but we predict a significant reduction in temperature in the concourse area,’ explained Maidment.

Faragher said that the system should provide around 10°C worth of cooling in its immediate vicinity and is expected to create a cooling effect of almost 3°C on the platforms, where the action of trains will help push the cooler air along the line. Indeed, one of the key elements of the project will be monitoring this distribution of cooler air.

Maidment is confident that the system could be applied across the entire Tube network, and it need not stop there. ‘There is significant potential to roll this out. LU currently pumps a massive amount of water each day. Ground water represents a massive resource that could be tapped into to produce cooling for buildings as well as the Underground,’ he said.

Faragher also confirmed that there are a number of other locations on the network where similar installations could make use of seepage water. A larger version of Maidment’s system is also likely to be installed as part of the longer-term plan to upgrade Victoria station. If water is derived from other means, the same technology could in addition be installed at about 20 or 30 other stations, claimed Faragher.

The group is particularly interested in the cooling potential offered by deep aquifer bore holes and is discussing with the Environment Agency the feasibility of boring 90m-deep wells close to stations to take cold water from the chalk aquifer.

Another method under the spotlight is the use of cooling derived from combined heat and power (CHP) plants. Faragher said that there are a number of convenient locations where CHP schemes are being planned, and it should be possible to use the waste heat from these installations to drive some form of absorption cooler. This is a type of chemical processor that uses a heat source to produce a cooling effect from a solution of water and lithium bromide.

Faragher added that while the use of absorption coolers on the Underground is still very much at the feasibility stage, a prototype system is actually under development at South Bank University.

But, while most of the technical developments so far have been inspired and carried out by LU, Waboso is keen to stress that every company involved in the running of the network has an important role to play and, in order to succeed, the initiative has to be highly collaborative.

‘We’re going to provide leadership, but the infracos [the companies involved in running the infrastructure of the underground system] have a very big part to play in this,’ he said. ‘Each infraco is contributing to the problem so they also are going to be part of the solution.’

These sentiments are echoed by Andy Bourne, chairman of Tubelines, the company that runs the Jubilee, Northern and Piccadilly lines. Bourne said that without co-operation it would be easy to cancel out each other’s good efforts. ‘We could easily develop solutions here that completely frustrated each other by just shunting hot air into each other’s bits of the system,’ he said.

An alternative cooling method has also been proposed by Metronet, which runs the majority of London’s Tube lines. As with the South Bank’s Graeme Maidment, Metronet engineer Dave Hindle believes the answer lies with London’s natural landscape. But Hindle’s solution involves freezing the chalk aquifer and then using the cold conditions to cool adjacent areas by a series of heat exchangers.

The proposed Metronet system would consist of a surface heat exchanger and an underground chamber between and below the two running tunnels. This chamber would contain an underground heat exchanger and an underground refrigeration unit and would be connected to the running tunnels by a small adit or horizontal passage.

The underground refrigeration unit would use brine at –35°C to freeze the ground to around –10°C. ‘What we’re proposing is freezing a very large block of the chalk at considerable depth below the clay strata and isolating that ground freeze from the actual system,’ he said.

He explained that during the winter months the frozen ground could be trickle charged with energy by exchanging heat between the underground refrigeration unit and the surface heat exchanger. Then, by disconnecting the surface heat exchanger and the underground refrigeration unit and reversing the circulation of the brine through the frozen ground and back up to the underground heat exchanger, cooled air could be pumped into the running tunnel, causing the section of tunnel to become colder. The action of trains passing along the tunnels would then push this cold air into the station.

‘Very rapidly an equilibrium will build up and the stations will get cooler,’ claimed Hindle, who is effusive about the potential of his scheme. ‘We could achieve coverage of the whole of central London, and it could be done progressively starting with the areas that are most seriously affected (for instance around King’s Cross station). If we start now, by the 2012 Olympics 15 installations could be covering most of central London,’ he claimed.

However, Hindle may have to wait a little longer for the go-ahead. Waboso, while clearly pleased that the infracos are developing their own solutions, said that the ground-cooling proposal is under technical review, with no trials planned in the immediate future.

There’s every possibility that the changes being implemented now will help lower the temperature in some of the Tube’s hotspots this summer. Whether they decrease the temperature enough to make travelling on the Underground in summer anything other than unpleasant remains to be seen. But what is certain is that the Tube’s bosses are serious about doing something effective.

In the words of David Waboso, ‘We’ve got to get rid of the heat — doing nothing is not an option’.