Could subway tunnels act as a geothermal heat-recovery system?

Heat transfer happens in various ways in rail tunnels. For instance, when trains brake or accelerate, they produce heat that warms the surrounding air. That hot air mixes with other air in the tunnel and with heat radiating from the ground.

Until now, engineers have been unable to accurately calculate the amount of heat that tunnel air contains. Researchers at EPFL’s Soil Mechanics Laboratory (LMS) have overcome that problem by precisely estimating the convection heat transfer coefficient. Their findings have been published in Applied Thermal Engineering. 

This breakthrough paves the way for innovative applications involving so-called energy tunnels that can supply energy to built environments. The team also tested its model on Lausanne’s future M3 metro line which, once complete, will carry passengers between the city’s train station and the Blécherette district to the north.

Enough heat for 1,500 apartments

“Our research shows that fitting the heat-recovery system along 50–60% of the planned route – or 60,000 square meters of tunnel surface area – would cover the heating needs of 1,500 standard 80m2 apartments, or as many as 4,000 Minergie-certified energy-efficient units,” explains Margaux Peltier, a scientific assistant at the LMS, whose Master’s research forms the basis of the article. The system also allows heat to be stored so it can be supplied to homes when needed. “Switching from gas-fired heating would cut the city’s CO2 emissions by two million tons per year,” adds Peltier. Her calculation does not include the savings gleaned from metro stations or at the planned rolling-stock depot in the north of the city, which could also benefit from the system.

As the temperature naturally stabilizes in underground tunnels, excess heat or cold is evacuated to the surface. The resulting warm airflow can often be felt at metro tunnel entrances. The idea behind the innovation is to harness this surplus warmth in addition to heat naturally present in the ground.

The system works in a similar way to a refrigerator, with plastic pipes containing heat-transfer fluid, or simply water, placed at regular intervals inside the concrete tunnel walls and connected to a heat pump. In winter, cold water will be pumped into the pipes, emerging hot at the surface. The opposite will happen in summer. According to the researchers, the system would be cheap and energy-efficient to install and would have a lifespan of between 50 and 100 years, with only the heat pumps having to be replaced every 25 years.