London Underground cooling

In summer, temperatures on parts of the London Underground can become very uncomfortable due to its deep and poorly ventilated tube tunnels: temperatures as high as 47°C (116 °F) were reported in the 2006 European heat wave. Posters may be observed on the Underground network advising that passengers carry a bottle of water to help keep cool.

Source
The heat in the tunnels is generated by the trains (motors and braking systems etc.), station equipment and body heat from the passengers. About 80% of the heat comes from the operation of the trains, 15% from other equipment, and 5% from people—it is calculated that tube passengers account for about 56 gigawatt-hours of heat energy emitted in an average year.

Temperatures underground slowly increase as the ground around the tube tunnels warms up. When a new line is built, the temperature of the surrounding ground, and of the air in the tunnels, is about 14°C; however, unless the line has very high capacity ventilation, the air warms up as soon as trains begin to operate, and gradually transfers heat to the ground. Over about thirty years, the background temperature rises by ten or fifteen degrees.

Conventional brakes on trains rely on friction to slow the train down. This friction transforms the train's kinetic energy into heat. More modern trains feature regenerative braking systems that can feed the energy from braking back into the power supply. This means that the energy derived from braking is used somewhere else to power a train. This way energy is "re-used" and the amount of heat generated by braking is minimized. Another advantage is minimizing the amount of brake dust that is produced by the trains. This dust collects inside the tunnel systems; although it has been shown not to be hazardous to health in the quantities involved, it certainly detracts from the appearance of the tube.

Piston effect
Trains moving through the tight deep-level tunnels push air forward through the tunnels, and draw air down into the tunnels through stations and draught-relief shafts. This is called the piston effect. This effect can produce a strong wind blowing though the tunnels and stations. The usefulness of the piston effect as a way of drawing air through the system varies from place to place, its key limitation being that it ceases to operate if trains stop moving.

Stations
Heat pumps were trialled in 1938 and have been proposed again recently to overcome this problem. Following a successful demonstration in 2001 funds were given to the School of Engineering at London's London South Bank University to develop a prototype; work began in April 2002. A prize of £100,000 was offered by the Mayor of London during the hot summer of 2003 for a solution to the problem, but the competition ended in 2005 without a winner.

A year-long trial of a groundwater cooling system began in June 2006 at Victoria station. If successful the trial will be extended to 30 other deep-level stations. For this trial Metronet installed London South Bank University's system comprising three fan coil units which use water that has seeped into the tunnels and is pumped from the tunnels to absorb the heat after which it is discharged in the sewer system. The scheme was one of the winners in the Carbon Trust's 2007 Innovation Awards.

Tube trains
Newspapers such as Metro are often discarded onto the existing air vents behind seats, which increases the problem.

Conventional air conditioning has been ruled out on the deep lines because of the lack of space for equipment on trains and the problems of dispersing the waste heat these would generate. Different systems have been proposed to cool Underground trains, most notably the use of massive blocks of ice inside the train. The blocks would be kept in refrigeration units, preventing them from melting completely.

Subsurface trains
In 2010, new S-stock trains were delivered to replace the A, C and D stock trains on the subsurface Lines (Metropolitan, Circle, Hammersmith & City, and District). These have standard air-conditioning, as the subsurface tunnels are large enough to displace the exhausted hot-air.