Taking the heat out of the situation

Did you know that in 1906 the average temperature on London’s Bakerloo line was approximately 14c?

What is it today?

21c.

Anyone who uses London’s underground transport network can’t help but ask the question ‘Why is it so hot on the Tube?’

They’ll also notice that as a rule of thumb, the only time we feel cool air is when it’s being forced through a tunnel by an oncoming train.

Which is ironic really because, to answer the question, trains are largely the problem; the principal cause behind the rise in temperature down there. So too, to an extent, is the large number of passengers that the underground carries – but more on this in a moment.

It would also be accurate to point out that London is becoming what’s known as a ‘heat island’. With the continual construction, in the London zone, of large, flat surfaces that all absorb solar heat, and the continuing reduction in the amount of foliage and trees in the south east of England, the temperature in London will only continue to rise.

The underground temperature has increased so much in recent decades that the surrounding ground around tunnels and shafts has significantly warmed through, many metres away from the installations.

To compare and contrast the issue, London’s surface air temperatures normally fluctuate between a maximum of around 35c in summer down to -3c in winter.

The Tube’s underground air temperatures range between 30c to 16c.

So we can see that even in the depths of our modern-day winter, the air is 2c warmer than it was in the summer of 1906!

And so to the big questions.

How do you let the hot air:
a. out of the tube carriages (you hadn’t seen that one coming had you!), and
b. out of the underground network?

It isn’t as easy as just digging more holes down from the surface. Expelling the hot air is only part of the problem – if you relied solely on this you would need to build a method to replace the extracted hot air with cold.

Alternatively you could just cool the hot air that’s already underground.

Or deploy a mixture of both methods – but, whatever methods you do deploy, they have to be cost-limited and efficient.

On the first option – that of digging more holes down from the surface – imagine a turbine the size of a light aircraft propeller. You would need several of those in place just to COOL one platform of people – not counting what you would need to pump in cool air.

And we haven’t even looked at the inside of the carriages yet – we really do need to keep that issue on the radar too. But first let’s just consider the stationary components in the underground network – the passenger areas.

Although there are more modern parts, a significant portion of the underground network was designed and built over a hundred years ago. The design intention stated two constraints:
1. ventilation shafts for small passenger groups and
2. non-electric trains.

Small passenger groups?

These days Transport for London (TfL) moves more than the population of a small Scandinavian country.

Every day.

The capacity planners of a hundred years ago would not have been able to comprehend how their Victorian engineering could deliver mass transit functionality on a daily basis.

Unsurprisingly, the tube line with the most ventilation problems is The Bakerloo, whilst The Jubilee Line (the most modern) has fewer issues.

Everyone, from politicians in central government and city hall, to capacity planners in TfL recognises that if something isn’t done the problem will continue to worsen.

Indeed The Mayor of London recently offered a cash prize of £100,000 to anyone who could come up with a solution to the problem. It’s worth noting that the prize remains unclaimed!

It is also recognised that the temperature in the carriages and in the underground stations is a safety-driven issue. The commitment to deal with the problem is beyond doubt.

But what to do about it?

The answer is that there isn’t just one solution.

Some people have said that TfL should just sling air conditioning units on to the carriages, but when the people who make that suggestion are faced with the ancillary question ‘what do we do with massive increase in air temperature that the air conditioning units generate (because they are, after all, just heat exchangers), the scope of the problem starts to become apparent.

A number of cooling methods are being investigated/trialled; some are focussed on the passenger areas of the Tube whilst others are looking at lowering the temperatures in the carriages. Some of the options under investigation include:

• Regenerative Braking – a method of harnessing the heat and energy that is generated when Tube trains brake – can produce around 25% of ‘recyclable energy’ which could be used to cool carriages
• Ground Water Cooling – a kind of ‘central heating’ in reverse – is under trial at a few locations. This method brings its own difficulties; the rapid clogging of water filters is an issue, as is finding readily available supplies of clean(ish) water throughout the underground network.
• The provision of new Mid-Tunnel Ventilation Shafts are also one of the methods under investigation, but each new shaft would need to be capable of moving twice as much air as the shafts that the underground was built with.
• Station Fans – this is an interim measure which, in a recent satisfaction survey, found that 94% of customers felt the enormous fan units had a beneficial effect.

Apart from Regenerative Braking, the other cooling methods above are all focussed on the underground passenger areas, not the carriages. As mentioned, the major difficulty with air-conditioning carriages is what to do with the all of the hot air that the air-conditioning process generates.

But it isn’t an old problem.

In 1935 London Underground (as it was then) held a long air-conditioning trial on the Northern Line. One way or another, AirCon has been on the radar since then.

Hot on the ideas board are:

• A Hybrid Air-Conditioning concept
• Phase Change Media
• Evaporative Cooling

The last method – where an ultra fine mist of cold water is injected in to the underground atmosphere – is currently undergoing experimental trials in Madrid.

Also under consideration are methods to reduce the heat that is generated. For example, high on the list of actions is to switch lighting to LED. This is not only slightly more fuel efficient, but produces less heat.

So there we have it, the problem is widely recognised, various methods to cool the underground are under investigation, and the escalators, platforms and carriages are all under consideration.

The only thing that is for certain is, given the nature and scope of the problem, it’s going to take a while to get something done, but the prospect of a tactical fix being implemented while a long term solution is designed is a real prospect.

B.