Go into almost any subway station, and it's probably going to be almost unbearably hot at this time of year. You would think that going underground would chill things down, but tunnels don't work that way. The heat from trains, crowds of people, electronics like lights, and signals all add up to make them really warm. Now, researchers have found a way to use all that hot air.

Researchers at the Swiss Federal Institute of Technology in Lausanne, EPFL, have looked at the heat transfer numbers in the heated air in train tunnels and designed a geothermal heat recovery system that could be used to provide heat and cooling to thousands of nearby houses.

Scientists have been aware of the potential energy in subway tunnels, but they weren't able to calculate it accurately. But now, the researchers from EPFL have solved the puzzle, according to an article in New Atlas. The team found a way to develop a model that allows them to very precisely calculate the convection heat transfer coefficient of individual tunnels.

This formula could be used to develop systems that can harness the energy and pump it back to the surface where it can be used to heat or cool nearby apartment buildings.

The technology is similar to a refrigerator. Plastic pipes can be built into the sides of the tunnel and filled with a heat transfer fluid or even just ordinary water. Cold liquid is then pumped through the pipes where it is warmed by the tunnel heat and comes out as a heated liquid that can be pumped through buildings for heating. In the summer, the system can be reversed and used to cool the buildings with the removed heat going into the ground.

The researchers said that the system is cheap, easy to install, and could last up to 100 years, except for the heat pumps, which would have to be replaced approximately every 25 years.

The team applied their model to the metro line that is under-development in Lausanne, Switzerland, to calculate the benefits.

"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," Margaux Peltier, lead researcher on the study said in the paper that was published in the August 2019 journal Applied Thermal Engineering.

He concluded, "Switching from gas-fired heating would cut the city's CO2 emissions by two million tons per year." That's a very respectable number.

There is a lot of excess energy – not just in subway tunnels – that can be used in geothermal systems. A Swedish company, Climeon, is using heat from under the ground to make electricity. Iceland has used geothermal energy from their numerous volcanoes to heat homes for a long time, and by 2015, 85 percent of households were heated that way.

The world needs to switch to renewable energy as quickly as possible if we are going to meet the 2030 climate goals. Besides solar and wind power, geothermal energy will make a big dent whether it occurs naturally from volcanoes or it is humanmade like in subway tunnels.

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