Taking the heat off the Tube

Passengers will not be reassured by Transport for London programme director Kevin Payne, who says temperatures are likely to go "from warm to uncomfortable at present to somewhere we really won't want to be". The problem is caused mainly by the energy input by the trains. Since the first deep level tube line was opened with electric traction in 1890, the power consumed by the system has increased steadily, reaching 300 million kWh per year in 2006. The number of passengers also continues to increase – a billion in 2006, expected to climb to 1.4 billion in 2030, requiring the use of yet more trains and even more power. According to Payne, heat sources are: 38% braking losses, 16% drive losses, 22% mechanical losses, 3% passengers on trains, 13% train auxiliaries, 4% tunnel systems and 4% passengers in stations. Some 79% of the generated heat goes into the tunnel walls, 11% is removed by the piston effect as trains move the air ahead of them down the tunnels and 10% is removed by ventilation. The problem is that the clay behind the tunnel walls is now heating up significantly: 5ºC more than the 14-15ºC natural ambient over much of the central area and up to 11ºC more in some places. Left to its own devices, the problem is likely to get worse. Upgrades to the Victoria line, to be completed by 2012, will see trains drawing 4500 amps at 630V when they accelerate, drawing and then dissipating 2.8MW of heat as they do so. And there are going to be a lot more trains. The Challenge Our challenge this month is to find the most cost-effective ways of cooling the Tube to tolerable levels. Air conditioning the whole system with refrigerated air is a non-starter, because of the immense cost. According to Kevin Payne, taking heat out this way costs 10 to 50 times as much as putting heat in and it is almost impossible to find sites to build heat dissipation units above ground. What is required is ingenuity. In 2005, Mayor of London Ken Livingstone offered £100,000 for the best solution, although we can offer no guarantee that anyone will actually receive this sum. Transport for London is piloting a whole raft of ideas, a summary of which may be found below. For those without access to the web, this material will be in our December issue. See if you can come up with your own solution to take the heat out of this problem. Solution Transport for London is piloting a whole raft of innovative solutions to cooling the system. The first is to reduce energy braking losses. Much could be done by computer controlling the trains so that they accelerate fast, coast under inertia, and then use regenerative braking to stop. Although four quadrant inverters are common on surface electric trains, they are not used on London Underground trains, although Kevin Payne said that there will be one inverter controlled train running on a trial basis, "By Christmas 2007". At Victoria Station, water at 16 to 17 deg C from the now underground River Tyburn, which tends to seep into the station and thus has to be pumped out, is being used to cool the exit area to the District and Circle lines between the Victoria line platforms. In January/February next year, work will start on boreholes at Stockwell station, one of which will be used to extract water from beneath the clay, while another, 200m away will be used for re-injection. Ventilation fans in shafts, many of which had fallen into disuse, are being put back into service, and in some cases doubled in capacity. Warren Street is expected to see the trial of a scheme whereby air from the underside of trains, heated by the brakes and traction equipment, will be drawn out under the platforms. There are plans to use water evaporative cooling, both with wet pads and very fine sprays in air ducts and tunnels. A 100kW heat capacity chiller based air conditioning system has been installed above the Oxford Circus ticket hall. And a 6m long mock-up section of a train should be working by Christmas 2007, to trial freezing water in containers on trains that go above the surface, and then using the ice to cool them when they go underground. Ultimately, Kevin Payne told us, "I can personally see the day when we can sell out waste heat to domestic users for preheating water". TFL is presently in preliminary discussions with Swim London for pre heating swimming pool water and helping warm sports halls.