Geothermal energy is putting the world’s most underused resource to work.

What does it take to harness the energy of the human body in order to heat a building? The idea may conjure images of people running on hamster wheels or a bicycle pedal-powered generator, but all it requires – according to the latest Swedish innovation – is the reliable hustle and bustle of people moving through a train station.

The station is Stockholm Central Station, whose 250,000 daily travelers and shoppers make it the busiest train depot in Scandinavia. Engineers for real estate company Jernhusen figured out how to harness their heat energy and direct it to an office building across the street.  According to an article in The 9 Billion (a website that promotes sustainability by 2050, when the world is expected to have a population of nine billion), heat exchanges in the station’s ventilation system convert only the excess body heat into hot water, which is then pumped into the heating system of the other building.

It’s not the first example of harvesting human heat; last year, Paris Habitat, the largest owner of social housing in Paris, announced it would harness body heat from the Paris Metro to help heat a public housing project on rue Beaubourg, close to the Pompidou museum. The plan, which is going into effect this year, will combine the calories emitted by passengers with the heat from moving trains (which keep the corridor temperatures hovering around 60 degrees Fahrenheit year round) and move it to heat exchangers before supplying heating pipes. Francois Wachnick from Paris Habitat told Reuters that he expected the project should slash carbon dioxide emissions by a third compared with using a boiler room connected to district heating. “Luckily, the building is connected to the metro through a staircase,” Wachnick said.

The difference between the Paris project and Sweden’s heat harvesting plan is that Sweden’s is the first instance of heating buildings that aren’t directly connected, according to Jernhusen. Klas Johnasson, one of the creators of the system and head of Jernhusen’s environmental division, told BBC News, “This is old technology being used in a new way. The only difference here is that we’ve shifted energy between two different buildings.” 

The new system is lowering the heat bill of Kunbrohuset (the office block) by about 25 percent. Of course, it helps that Jernhusen owns both the train station and the office block, but Johnasson noted that heat exchange among buildings presents different owners the opportunity for collaborating with each other.

Considering the cost and energy savings, it’s a great idea. What do you think it will take for building owners to work together on harvesting human heat – and what public areas in the U.S. would be the best candidates for a harvest? Cast your vote here.