The Paris Metro is one of the world's great underground railways and not the sort of place you'd expect to find cutting edge satellite technology at work. But for the last year and a half a cooling system developed by the European Space Agency (ESA) for its satellites has been making Trains on Metro Line One more comfortable. The new cooling system works without moving parts and frees up more space to be enjoyed by passengers while saving costs.

If you've ever been caught in an underground train during a power failure, you may have noticed that the situation soon becomes not only extremely frustrating, but also very hot. That's because an underground train is filled with all sort of heat-generating machinery in addition to the passengers, who each put out as much heat as a 100 watt incandescent bulb. In a cylinder like an underground rail tunnel, there isn't any place for this heat to go, so when the cooling system on a train conks out, things can get very unpleasant very quickly.

The trouble is that the machinery that controls this heat is bulky, noisy, expensive to run, prone to breakdowns, and takes up a lot of space on the train. So, when French transportation company Alstom decided to do away with the large fans that currently cool most of the trains, it turned to ESA and its space technology spin-off program.

Subject to the constant glare of the Sun and without the shielding benefit of the Earth's atmosphere, satellites need to stay cool, too, if their electronics aren't to end up frying. On Earth, large devices like computers are kept cool by using fans to blow air across the circuit boards to carry away the heat while smaller ones, like tablets, use air convection to cool off. But with no air in space, neither of these are an option. Also, fans use moving parts and motors, which don't do very well in a space environment.

ESA's solution was an advanced cooling system without moving parts that uses heat pipes to carry away waste heat. These copper, stainless steel, or plastic tubes are filled with liquid that runs in a closed loop between hot spots and cold spots on the satellite, such as between the sunward and leeward sides.

The liquid circulates; using the temperature differential between the two spots to cool the satellite. That is, it has a very low boiling point and turns into vapor on the warm side of the spacecraft. When it reaches the cool side, the vapor condenses and the cycle starts over again.

According to ESA, the tricky bit is to get the liquid circulating through the system without using pumps. The space agency engineers managed this by relying on capillary action. It's the same force that pulls sap up tree trunks, spilled drinks into paper towels, and coffee into sugar cubes.

"Picture dipping the tip of a sugar cube in coffee," says Michel Ganseman, CEO at Euro Heat Pipes. "The coffee is quickly drawn up into the sugar, through the pores, because of what s known as capillary action."

By using this capillary action combined with an air heat exchanger, the heat-pipe technology replaces the conventional mechanical fans on the Metro trains, though ESA says that the technology can also work with everything from computer chips to pigsties. In the meantime, the hope is to expand the cooling system to other trains in French rail transport.

"Currently, we have one experiment on the Parisian Metro line," says Alstom’s Sebastian Nicolau. "But potentially, we can propose this solution for all different trains, from tramways to metros, suburban trains, and high speed trains like the TGV."

Source: ESA

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