Subway tunnels generate a lot of heat which is why researchers at the EPFL university in Switzerland have crunched the numbers on heat transfer in the air of train tunnels, and outlined a geothermal heat recovery system that could potentially supply heating and cooling to thousands of nearby homes.

Accurately calculating the amount of heat in the air of train tunnels has been a difficult task, but now researchers at EPFL's Soil Mechanics Laboratory (LMS) claim to have cracked it.

The team has developed a model that allows them to precisely calculate the convection heat transfer coefficient of a given tunnel environment.

That formula could be applied to develop systems that harness the extra energy and pump it back up to the surface, where it can be used as heating (or cooling) for nearby apartments.

The concept of the technology works a bit like a giant fridge. Plastic pipes are built into the walls of the tunnel, and filled with a heat-transfer fluid – or failing that, just plain old water.

Cold liquid is pumped through the pipes, where it's warmed by the air in the tunnel and emerges at the surface as a hot liquid.

In summer, the system can be reversed to act as air-conditioning. Heat can be ferried away from homes and dispersed into the ground, which tends to naturally stabilize its own temperature.

The team said the system would be relatively cheap and energy-efficient to install, and have an expected lifespan up to a century although the heat pumps would need to be replaced every 25 years.

The researchers applied the model to a metro line currently in-development in the city of Lausanne, Switzerland, and calculated the potential benefits to the city.

"Our research shows that fitting the heat-recovery system along 50 to 60 per cent of the planned route – or 60,000 sq m (645,000 sq ft) of tunnel surface area – would cover the heating needs of 1,500 standard 80 m2 (860 sq ft) apartments, or as many as 4,000 Minergie-certified energy-efficient units," according to lead researcher, Margaux Peltier.

"Switching from gas-fired heating would cut the city's CO2 emissions by two million tonnes per year."

- This research was first published in the journal of Applied Thermal Engineering

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