Thanks to space, some Parisian Metro riders now enjoy a very high-tech commute. A satellite spin-off is paving the way for more comfortable journeys. Tourists and locals alike are familiar with Metro Line One: traversing the length of the French capital, this underground line is the city's busiest. More than 213 million journeys are made yearly on its 16.6 km track, which serves 25 stations.

What riders might not know is that, for the last year and a half, trains travelling along this route have been equipped with an advanced new cooling system. The technology used comes straight from space - thanks to ESA.

Typically, metro trains use large mechanical fans to keep the their electric power suppliers from overheating.

But, looking to create more room for passengers by reducing the space taken up by machinery, the French transportation company Alstom decided to turn to space, where they found a high-tech alternative.

Learning from cooling systems in space
Because there is no air in space, fans cannot be used to cool electric and other types of equipment on spacecraft. Instead, when it comes to cooling, 'heat pipes' are standard.

These rely on liquid. Circulating through a pipe formed in a closed loop, the liquid carries heat away from a hot spot.

The basic idea is simple. One part of the loop is placed next to the heat source, while another part is next to a cold spot, perhaps the satellite's outer wall.

The challenge is to keep the liquid circulating without the help of mechanical pumps, which are vulnerable to breaking down.

"In space, technology has to last for years or decades, without maintenance or repair," said Michel Ganseman, CEO at Euro Heat Pipes, or EHP, a company that provides heat-pipe cooling for some three quarters of all European scientific and Earth observation satellites.

Like sugar cubes in coffee
Heat-pipes solve this problem by placing a special porous material in one part of the loop. Using something called the capillary system, this material's microstructures automatically suck up the liquid in the pipe.

"Picture dipping the tip of a sugar cube in coffee," explained Michel. "The coffee is quickly drawn up into the sugar, through the pores, because of what s known as 'capillary' action."

Of course, a heat pipe's capillary material - made of copper, stainless steel or plastic - is far more efficient than sugar, and produces an almost inexhaustible capillary force.

Once the cool liquid has been sucked up through this capillary material, it nears the heat source. There, the liquid turns to vapour. Further along the loop, the vapour encounters the cold source, and turns to liquid. The capillary material then sucks up this liquid, bringing it to the heat source, and the cycle begins again.

"It's a way to quickly transport liquid from one place to another without pumps or applying extra energy.

"No mobile part is used, eliminating any risk of wearing parts and potential breakdowns of the system."

The idea to use heat-pipe technology to replace mechanical fans on metro trains came about thanks to feasibility studies funded by ESA's Technology Transfer Programme and working with Belspo, the Belgian Science Policy Office.

These feasibility studies looked at cooling everything from computer chips to data centres, aircraft equipment and even pigsties, here on Earth.

"Anything that has semiconductor material - chips, power modules and so on - produces waste heat, that needs to be removed," said Maxime Vuckovic, marketing and sales manager at Calyos, a spin-off of EHP created in 2011. "This is the key application."

"ESA has always recommended heat pipes as the space solution which guarantees a constant, average temperature for cooling purposes with no external supply of energy," said Jean-Paul Henry, CEO of Creaction, part of ESA's technology transfer broker network.

"Calyos was created to give low-cost solutions to new terrestrial application markets using the EHP's proprietary technologies and knowhow."

Paris Metro into the space age
Soon after Calyos was founded, CEO Olivier de Laet began working with Alstom, one of the world's largest manufacturers of trains and metros, on the train application. Initially, the idea was to target high-speed trains.

Then Alstom suggested a larger market: Paris's subway system, whose ageing fleet is being renewed.

"If the technology was priced competitively for the metro, than it would be priced competitively for any type of train," said Maxime.

One challenge that had to be overcome in the technology transfer was the fact that in space, the capillary system does not have to work against gravity. "There's gravity on Earth," said Michel. "But we managed it."

After being adapted for Earth, the heat pipes were added to new trains built for the One metro line.

"Currently, we have one experiment on the Parisian Metro line," said 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."

Others imagine the technology having an impact even further away: "If you've ever been in the New York City subway in the summer, you know it can get up to 40 degrees in the station," said Michel.

"The reason is, the amount of heat is so big, that the fans must keep dissipating it in stations.

"A more efficient cooling system could make a big difference. Heat can be dissipated between stations, to bring down temperatures. And it will be less noisy for passengers."