See gallery: “Cool technologies for hot subways”
IN A quiet backstreet in central London, not far from Buckingham Palace, a small, nondescript building hides the entrance to an unusual tunnel. There are no signs indicating its presence. “We take security very seriously,” says Kevin Payne, the engineer showing me around. It’s easy to understand why: this tunnel is a crucial ventilation shaft for the Victoria line, one of London’s deep tube-train tunnels.
Before we can enter, Payne has to switch off a giant 2-tonne fan that draws hot air out of the tunnels, lest we be blown away like sweet wrappers. Then we descend into a dark, dust-blackened tunnel that connects the north and southbound rail tunnels. The air is warm. As we reach the bottom and the trains rattle past, just centimetres away, I feel a pang of guilt. I’d travelled this section of line earlier, with sweat dripping down my back. With the fan switched off for my visit, am I condemning these travellers to an extra degree of discomfort? Payne reassures me any temperature increase will be minimal. As the person in charge of power and cooling at London Underground (LU), he should know.
Advertisement
The new fan at the top of the shaft is his baby. It is part of a major programme to tackle rising temperatures across the tube network. In these tunnels on the Victoria line, one of the hottest lines in London, platform temperatures can average 32 °C in the summer. In one notorious incident in 2006, temperatures on the Central line were reported to have reached 47 °C.
London is not alone in battling heat underground. Some 170 metro systems around the world carry millions of commuters every day, keeping the world’s major cities running (see map, below) and on many of these engineers are seeking to prevent uncomfortably hot conditions.
In the face of growing concern, LU, the organisation that runs the tube, has begun testing and beefing up conventional ones. As well as sucking hot air out of the tunnels, LU is pioneering ways of pumping cool air in. It has even investigated the idea of trains carrying lumps of ice. It is no small challenge. In 2003, the mayor of London offered £100,000 to anybody who could solve the problem of cooling the underground. The prize was never awarded (and has seemingly been quietly forgotten by the current mayor). These engineers are not just battling heat – they are up against nothing less than the laws of thermodynamics.
Feeling the heat
Here’s the problem. The trains fit snugly in their tunnels and so act like pistons, pushing cool fresh air ahead of them through the tunnels as they move. What’s more, underground stations themselves act like ventilation shafts, allowing cool air into the system: at Highgate station on the Northern line, the icy blasts of wind rushing down the escalators can take your breath away. And yet, air temperatures in these tunnels have doubled in the 100 years since they were built.
That’s partly because the tunnels are so old. The deep lines are carved out of solid clay. At one time, the clay acted as a heat sink. In the 1900s, LU advertised its tunnels as “the coolest place to be in hot weather”. Not any more. After decades of continuous heating, the clay is now hot too.
The sources of heat are easy to identify. The electric motors that drive underground trains pump heat into the tunnels every time they accelerate. And when the trains decelerate using friction brakes, the kinetic energy dissipates as more heat. Even when stationary, electric trains cope with electricity surges by burning off excess power through resistors designed to heat up. LU engineers affectionately call these circuits “toasters”. Together this accounts for 80 per cent of the heat introduced to the network. The rest comes from other sources such as the body heat of the commuters, who are sometimes packed into carriages at sardine densities of up to five people per square metre.
One obvious solution is to use more energy efficient trains. LU has been introducing new trains with regenerative brakes that turn kinetic energy back into electricity rather than into heat. By 2012, all trains running on the Victoria line will have them. This and other energy-saving measures should allow the same service to run using 10 per cent less power, says Ian Flynn, LU’s head of engineering strategy. But plans are also afoot to increase the number of trains operating on the line by 10 per cent. It’s like running to keep still.
Back in the tunnel, we’re clambering through huge baffles that absorb the sound of the rushing air as it is drawn out of the ventilation shaft. Without insulation, noise would be a significant problem. The vent extends up beyond the ground-level entrance, sandwiched between an office building and a residential flat. As the trains race past beneath us, Payne tells me about the scale of the cooling challenge he faces.
“In London the problem is the sheer age of the deep tunnels and the fact they have so little ventilation”
The problems in London are particularly acute. Other cities can suffer because of the extreme temperature and humidity of the atmosphere outside, which is exacerbated in cramped tunnels where airflow can be poor. But in London the problem is the sheer age of the deep tunnels and the fact they have so little ventilation, having been designed for just a few trains per hour.
Payne and his team have scoured the world for inspiration. In Madrid and St Petersburg, for example, engineers pump a fine mist of atomised water into the air which then evaporates, absorbing heat. The human body uses exactly this kind of evaporative cooling and it is now used in many outdoor spaces.
But Payne says this would not work in London because the atmosphere is already humid. Human comfort depends not only on temperature but also on humidity and airflow: increasing the humidity can make people more uncomfortable even though the temperature is lower. Then there is the problem of condensation. “Water and electricity are not a good mix,” he says.
One thing Payne did for starters was to soup up the existing cooling shafts with powerful new fans – the shaft we are in is one of 13 that have been revamped along the Victoria line. It’s no panacea, but it helps. The fans extract hot air at 75 cubic metres per second. “That’s about the volume of a double-decker bus each second,” he says proudly.
But his team’s most innovative project so far consists of an inconspicuous set of heat exchangers – like the radiators in your car – attached to the ceiling in a busy pedestrian tunnel at Victoria station, just a couple of hundred metres from where we’re standing. Stand beneath them and you can feel a steady stream of cool air wafting over you. “The heat exchangers create a plug of cold air in the platform area that each incoming train then pushes down the tunnel,” says Payne.
Train drain
Heat exchangers aren’t new but the way these ones handle heat is. It all started in 2001, when Graeme Maidment, an engineer at London’s South Bank University, noticed that the city’s tunnels feature an escape route for excess heat that nobody had spotted – storm drains. Under some of the stations, pumps carry seepage water into the sewers to prevent it pooling and flooding. There’s a sump under platforms at Victoria station in which water collects. So in 2006, LU began pumping cold water from this tank, through the heat exchangers and then sending the slightly warmer water out into the storm drains. Maidment says the system achieves 2 or 3 °C of cooling and has won him and LU a number of awards for innovative, green engineering.
“London’s tunnels feature an escape route for excess heat that nobody had spotted”
It has been so successful that LU is now building a bigger system on the Victoria line, north of us at Green Park station. The plan here is to pump cool water out of a chalk aquifer 80 metres beneath a nearby park, through heat exchangers in the station and then back into the aquifer at a slightly higher temperature, downstream of the source. Payne says that the design has had to pass all kinds of environmental checks to get the project approved. “The water we pump back in will be chemically identical to the water we take out, just a few degrees warmer,” he says. The boreholes and pipework are now finished and LU is waiting until after the London Olympics in 2012 to finish the project (Proceedings of the Institute of Civil Engineering, ).
These ideas are just the beginning. In future, trains could be drastically redesigned, says Flynn. One promising approach is surprisingly simple: make trains lighter and more efficient by reducing the number of wheel chassis, called bogies, on every train. With each weighing 5 tonnes, two per carriage and seven carriages per train, that’s a combined weight of 70 tonnes.
A better option is for carriages to share bogies, so that a single bogie supports the ends of two carriages. Although that would require more carriages, the result would be a train with 10 bogies rather than 14, a saving of 20 tonnes or about 15 per cent of the total mass of the train. This directly reduces the power required for acceleration.
A further 5 per cent drop in the power required to accelerate the train could come if rolling resistance can be reduced. Most comes from the interface between the wheel and the rail but not in the way you might expect. Push a set of wheels along a straight track and the movement is essentially frictionless, says Flynn. But resistance rises dramatically when you come to a curve and the wheel flanges begin to rub against the rail, generating friction – and the infamous screeching noise.
Above ground, this friction can be minimised by keeping tracks as straight as possible and, when a curve cannot be avoided, by raising one rail so that the train banks into the curve. This means less force pushing the wheel flange against the track. But the underground lines in London tend to follow the pattern of roads at the surface, and so twist and turn in unreasonable ways. And raising one rail is not possible because of these trains’ rapid changes in acceleration. For example, a train approaching a curve before a station would enter the curve at high speed but then slow down as it pulls into the platform. The result is that the front and rear of the train take the curve at different speeds, which means friction losses are still high.
“You end up pumping heat out of the train and into the tunnel where you still have the problem of removing it”
Still, some reductions should be possible via track replacement and fewer bogies. If these efficiency measures are achieved, that might allow LU to invest this saved energy in some more exotic cooling technologies.
One of these could be a clever twist on air conditioning. Conventional air conditioning on the deep lines has never made sense. “You end up pumping heat out of the train and into the tunnel where you still have the problem of removing it,” says Maidment. He and his colleagues have been working on an alternative that means more cool air in carriages – they call it “coolth” – by using the trains themselves to store and then release warmth on sections of the line where it will dissipate. Many lines pop out into open air where they could do just that.
Such a system is essentially a giant freezer. Before the train enters the tunnels, it freezes water in an on-board container. Once inside, the freezer is switched off and the ice begins to melt. This process “releases” coolth, or in other words absorbs heat from the carriage and stores it in the increasingly liquid water. When the train returns to open air, the water is converted back into ice, releasing this stored heat ().
LU was impressed with the idea and funded research into the feasibility of retrofitting such a system to trains on the Piccadilly line, which has a significant length of overground line where the heat could be released – unlike the Victoria line which is almost entirely underground. “Although this was a very early prototype, the tests were very successful,” says Payne. However, passengers will have to wait. Flynn admits that introducing trains with major redesigns is not currently on the cards. They’d be unlikely to run before 2020.
The ultimate solution that Payne and Flynn are looking for is to use the excess heat from the underground system elsewhere: to heat water for local businesses and housing, for example, or to de-ice pavements and car parks in winter. That’s not as far-fetched as it sounds. At Stockholm Central Station in Sweden, excess body heat from commuters will help meet the heating needs of a 13-storey office block being built above. The project is due to be completed next year. The real estate company behind the scheme, , hopes this heat will account for 15 per cent of the new building’s heating.
“The thick, black oily dust – mainly human skin – is a reminder of the sheer volume of travellers”
Climbing the staircase from the deep tunnels to the surface, going with the flow of the hot air from the bowels of the city, I try in vain to avoid the walls and handrails because they are caked in a thick, black, oily dust. Payne says it is mostly human skin. It’s a final reminder of the sheer volume of people who use the tube every day – millions of travellers crammed into carriages, fanning their faces with newspapers. Then we emerge, blinking, into the sunlight and go our separate ways.
But the shafts have a strange allure. Twenty minutes later, I’m on a Victoria line train, rattling past the place where I imagine the cooling shaft meets the tunnel. I peer hard through the window but the entrance to the shaft is impossible to see in the darkness. All I can do is imagine the cool air it is drawing into the tunnel and wish, slightly sweatily, that it could draw a little more.
See gallery: “Cool technologies for hot subways“
