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Comment by memracom | Hacker News Reader

Here is an idea that I sent to the Underground in 2006 when they solicited suggestions from the public.

Add cool to the tunnels, rather than taking heat out.

Build liquid air plants above ground, 2 or 3 floors up in the air so that the heat of the pumps is released above street level and the noise can be kept away from the street. Feed the liquid air into the tube tunnels through insulated pipes which takes up far less volume than air vents. Let gravity bring the liquid air down the pipes. Release the liquid into the tunnels near platforms where the air pump effect of moving trains caused lots of air circulation. Also the car doors open on the platforms.

Since you are liquifying the air, not just the oxygen, it can be safely released anywhere in the tunnels. And if your air intakes are high up you will actually be improving the air quality in the tunnels as well, i.e. cleaner air flows in.

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This is a terrible idea. First lets ignore the thermodynamic efficiency losses compared to a regular chilled water cooling system. Liquid air is a potent oxidizer and the Underground has myriad sources of ignition and flammable things like people. Plus the risk of frostbite from exposure is very real. A control failure could kill people and ignite a raging fire underground.

Liquid air is the same as normal air. During its liquid stage it would be entirely enclosed in pipes. It can be safely expanded using the technology used in mine rescue suits like those from Draegerman.

You are confusing it with liquid oxygen which is just as dangerous in its gas form as it is when liquid. Air has some 70+ percent nitrogen in it, whether gas or liquid, and that prevents it from being any more corrosive or flammable than plain air.

For those who also didn't know about liquid air: "Liquid air is air that has been cooled to very low temperatures (cryogenic temperatures), so that it has condensed into a pale blue mobile liquid. To protect it from room temperature, it must be kept in a vacuum insulated flask." (c) Wiki

Plan sounds great and very expensive.

The flask is for small amounts in a lab. An ore processing plant liquifies gases on an industrial scale and if the temperature is low enough, you can keep it in large vats like any other liquid. Of course I am suggesting something in between where you only make enough to keep the pipes full. Of course it might make sense to make it in big vats overnight and drain them down throughout the day.

This could be tested with a single installation in a single tunnel because it requires no change to the tube system or the cars.

That sounds ridiculously expensive. It's much cheaper just to pump cool air through the tunnels starting in the evening, and through the night.

This is the correct answer! Tied to the spot price of power in London, it could be very inexpensive if run only at night (as well as using power delivered back into the grid by trains using regenerative instead of friction braking).

Air use not very good at transferring heat. To have effective air cooling you need to pump lots of air. The current ventilation shafts certainly have their limits and especially night time the noise may become an issue if you put serious jet engines there.

You'd pipe chilled coolant down into The Tube and perform the heat exchange below ground (cooling the air below). The energy intensive chillers would run on the surface.

But first, regenerative braking; you must stop adding more energy into the system before you consider a method to extract the existing thermal energy.

They already use regenerative braking and are running all of their ventilation shafts at full blast all the time (with upgraded fans).

The groundwater system in the article more or less does what you suggest, without the expense of the chillers.

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