Full of trains, electronics and people packed in like sardines, subway tunnels are pretty warm places. In theory, all that heat could present an untapped source of energy, if only it could be captured. Now, researchers at EPFL 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.

The air in underground railway tunnels and stations is sapping heat from many different sources. It rises up from the ground. Trains braking and accelerating heat the air around them. Add in heat from electronic devices like lights and signals, and body warmth from all the passengers, and you’ve got plenty of potential energy floating around down there. It’s no wonder these places need such complex ventilation and temperature control systems.

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 important 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 says the system would be relatively cheap and energy-efficient to install, and have an expected lifespan up to a century. The heat pumps are the most vulnerable piece of the puzzle, needing to be replaced every 25 years or so.

As an example, 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 percent 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,” says Margaux Peltier, lead researcher on the study. “Switching from gas-fired heating would cut the city’s CO2 emissions by two million tons per year.”

This isn’t the only attempt to tap into waste energy from the subway. A few years ago Transport for London trialled a regenerative braking system for trains in the London Tube that would feed energy back into the grid.

There’s a lot of excess energy all around us, it’s just a matter of finding ways to harvest it.

Turning train tunnels into giant “fridges” could heat thousands of homes [New Atlas]