Borehole cooling (Air-conditioning of London Underground)

Air-conditioning of London Underground - reality or dream?

E&T analyses logistics, priorities and engineering solutions of installing air-conditioners in London Underground.

It was alright in the beginning - the Victorians knew they had to have some form of air-conditioning in their steam-driven trains. In fact, conditions were so pleasant in 1906 that placards proclaimed the underground 'The coolest place in hot weather'. But that was before deep lines, millions of passengers - and time. Now temperatures can rise above 30C. In the summer of 2001, about 600 passengers were treated for heat problems when three trains were brought to a halt for 90 minutes on the Victoria Line. London Underground now issues tips for passengers on how to keep cool: 'Carry a bottle of water with you. If you are feeling unwell, contact a member of staff who will be able to help.'

About 80 per cent of heat generated in London's underground system comes from braking and motors, another 15 per cent from other equipment, and only 5 per cent from passengers. After around 30 years of operation, the ground surrounding tunnels now heats up to about 30C and that heat stays, it does not go away.

London's Underground is the oldest in the world and its deep tunnels have a narrow bore. Pride from being first in development brings initial advantages, but then come the unknowns, with many of the ensuing problems beyond the vision of the pioneers. They did not foresee the effect of heat retention, the huge growth in the number of passengers and the subsequent need for air- conditioning. Newer systems learnt from London's mistakes and built larger tunnels with higher ceilings.

Heat-reducing aims

London Underground introduced a programme 'Cooling the Tube', which is ongoing. David Waboso, director of London Underground Upgrade, points out that there are two types of lines on the Underground's subsurface averaging about 5m (16ft 5in) below the surface and deep level, about 20m (65ft 7in) - they both require different approaches. He believes the subsurface lines can be controlled through air-conditioned trains being deployed.

The deep lines, however, need more complicated solutions, including methods to cool the clay surrounding the tunnels and to use cool underground ground water. Waboso stressed the need to keep trains, and hence air, moving. Stationary trains retain heat, so straightforward measures have to be pursued, such as introducing new trains, relaying track, updating signalling, making efforts to reduce the generation of heat at source by trying to control braking and use coasting where possible; renewing old ventilation shafts and fans, and installing new ones.

London Underground has embarked on a programme of restoring ventilation shafts and last year work had been completed on 83 of them. By 2011, 13 of the Victoria Line shafts will also have been upgraded. The trains make use of the piston effect by driving forward air, which it has sucked down from ventilation shafts and platforms. It only works when the train is moving, though.

To be effective, numerous shafts are required but there are, as always, problems to overcome. Some shafts need to be more than 30m deep, their fans are noisy and have to be baffled. Unfortunately, suitable sites are not readily available in London and, when they are, they are costly.

Air can be kept moving by another method - fans. Industrial, or 'Blue', fans have been installed in about 30 stations of the network to blow air across them. They may not reduce the temperature, but do make the atmosphere feel more pleasant.

Regenerative raking

Waboso explains how the friction, caused by braking, produces heat. One way to reduce this heat is the regenerative braking process. This is a sort of feedback system where connections are switched to turn the train's motor into a generator. Less friction is needed and, hence, less heat generated. London Underground has also started making the approach to stations a gently rising gradient to slow down the train and reduce the required level of braking.

Tackling subsurface and deep lines

Air flows more freely on subsurface lines that run partially in the open, which also allows for air-conditioned carriages. 190 new S-Stock trains started coming into operation in 2009, first on the Metropolitan and later the District, Circle, Hammersmith and City Lines, as well as the Edgware Road to Wimbledon one. These have standard air-conditioning, because the tunnels are large enough to dissipate hot air. They also have interconnecting gangways, so that passengers can pass through the train to help reduce overcrowding. Other Circle Line trains have had their height reduced and vent slots cut into the extra space.

Deep lines are the problem. Not only are they deep, they are also narrow, with no space for high-ceiling cooling, and ventilation shafts have to be deep. The ground surrounding the tunnels retains heat, which becomes saturated and does not cool.

Deep-line trains cannot have air-conditioning installed, because the heat thrown off by this process cannot be dissipated, so it continues to build up and the train gets hotter. This is particularly the case with the Bakerloo and Central Lines.

Train drivers spend much longer underground than passengers. Terry Wilkinson, an executive member of ASLEF, which represents most drivers, confirms that standard air-conditioning is provided in the drivers' cabs. Unfortunately, this provision cannot be extended to the carriages because of the build-up of heat.

Groundwater Cooling

In 2006, with the help of research undertaken by Professor Graeme Maidment of London South Bank University, groundwater cooling trials were carried out at Victoria Station.

This system takes advantage of the Underground's existing pumps, which prevent a rising water table from flooding the area. Water is extracted from boreholes at 14C and pumped to heat exchangers located between platforms. Fans blow hot air from the station across the pipes of the exchangers; the cooler air is then blown onto platforms and driven along by trains. The water in the pipes, now warmed, is pumped into the Thames. At Victoria, 200 litres of water a second is drawn from the underground River Tyburn.

The system is now in full operation at Victoria Station, and London Underground hopes to extend this to three or four others. The process is not suitable for all stations, because groundwater has been found to be contaminated at several sites, and so there is a risk of polluted water leaking out of the system.

Deep Abstraction

The deep abstraction process involves drilling deep abstraction wells into the water table. Cold water is pumped to cool the stations by heat exchangers and then the warmed water is pumped back into the water table.

Measures have to be taken to prevent the ejected warm water from heating up the aquifer. The problem with this is the lack of surface locations to drill into. The company Drilcorp has just completed a successful boring at Green Park Station. Drilling of the 450mm borehole continued through London clay and beyond the chalk level to 130m to ensure the borehole does not dry up. A spokesman for Drilcorp said the chalk level was fissured and an acidisation, or well development process, was applied to reduce fissures and facilitate freer movement of the water.

London Underground says just 10 stations would be suitable for this method of cooling. Abstraction is already in use at the Royal Festival Hall, City Hall and Portcullis House.

Permission to drill into boreholes is controlled by the Environment Agency. Open-loop boreholes involve abstracting water from one or more and subsequently discharging it back into ground via a second borehole after it has been through a heat exchanger. If there are no significant problems, long-term abstractions need to be authorised with an abstraction licence, which is typically limited to 12 years.

To discharge back into the ground, an applicant also needs an environmental permit, which may impose conditions to prevent groundwater pollution.

Icy-cold air supply

Engineers from London Underground are also testing carriages with ice units, installed to provide a supply of cold air when underground.

The ice-block system is similar to that used in the kitchen fridge. It is based on phase change, in which a substance alters form from solid, liquid or gas and absorbs or emits heat. Trains have a condenser and evaporator unit in a carriage with a fluid - usually water - that is cooled into a frozen block. You will often hear that the ice block stores 'coolth', the opposite of warmth.

Underground, the air-conditioning is switched off, the ice block thaws, and cool air is passed out over a heat exchanger. When the train surfaces again, the process is reversed and the thawing block refrozen. A series of tests have been completed in the climate chamber of the Motor Industry Research Association for London Underground.

One disadvantage is that the turnaround of trains can be delayed while waiting for the block to be frozen. It can only be used on lines that have a terminus in the open. This rules out the Bakerloo and Northern Lines, which have only one end in the open, and the Victoria Line, which does not see daylight at all.

There are lots of ideas floating around on how to cool the deep lines. Among the most detailed is a method proposed by Dave Hindle of consultants OTB Engineering, who previously worked for the former Tube franchise holder Metronet.

Hindle's solution involves freezing the chalk aquifer below the clay level to cool surroundings with heat exchangers and an underground chamber with a refrigeration unit. This unit would use brine at -35C to freeze the surrounding ground. Cooled air could be pumped into tunnels, causing the section of a tunnel to become colder. Trains could then push cold air into stations. Metronet funded development and intended to include it in its new prospectus.

Hindle says: 'Transport for London's consultants prepared a report that itemised a number of technical questions, which we worked on until the demise of Metronet.' He then sent his scheme to the new Mayor of London, Boris Johnson, 'but it just got passed down the line to Transport for London and the same dead end'. Waboso says that the ground-cooling proposal is under technical review, with no trials planned in the immediate future.

London Underground has been conducting trials with Moscow Underground on evaporative cooling where fine mists of water are sprayed into tunnels. Moscow had relied on frozen ground to prevent high temperatures, but recently things have been warming up. One drawback to be considered is the build-up of humidity.

The greatest handicap for London Underground is that its deep-line tunnels have a narrow bore. The cost of cutting new, bigger tunnels is prohibitive and would cause great disruption. More feasible areas for research could lie in groundwater, the ice block project, chilling the surrounding ground, using disused tunnels and voids for storing cold, and pumping cool water through tubes along the length of tunnels. Any scheme will have to consider logistics, priorities and costs.

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