If you’ve ever walked over a grating in the city that sits above a metro line, you’ve probably felt a warm gust of air. That’s because subway tunnels are full of trains, electronics and people packed in like sardines, which turns them into pretty hot 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 would work a bit like a fridge, using plastic pipes to move liquids that can transfer heat. The 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.