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It's 7:30 a.m. on a wintry morning in downtown Stockholm and a sea of Swedes are flooding Central Station to catch a train to work. The station is toasty thanks to the busy shops and restaurants and the body heat being generated by the 250,000 commuters who crowd Scandinavia's busiest travel hub each day. This heat used to be lost by the end of the morning rush hour. Now, however, engineers have figured out a way to harness it and transfer it to a newly refurbished office building down the block. Unbeknownst to them, these sweaty Swedes have become a green energy source: "They're cheap and renewable," says Karl Sundholm, a project manager at Jernhusen, a Stockholm real estate company, and one of the creators of the system.

Using excess body heat to warm a building is not a new concept — the Mall of America in Minneapolis recycles the heat generated from shoppers' bodies to help regulate the temperature of the massive complex during Minnesota's dreadful winters. But Stockholm has taken the idea a step further by successfully transferring excess body heat from one building to another. "This is old technology, but used in a new way," Sundholm explains. "It's just pipes, water and pumps, but we haven't heard of anyone else using this technology in this way before."

Here's how the system, which began operating this month, works: the heat generated by the commuters is captured by the station's ventilation system and used to warm water in underground tanks. The water is then pumped through pipes to the 13-story Kungbrohuset office building about 100 yards away, where it is incorporated into the main heating system. Not only is the system environmentally friendly, it's also cost-effective, says Jernhusen, which owns both the station and the office building. In the long run, the company expects to lower the energy costs in the office building by as much as 20% per year. And constructing the new heating system, including installing the necessary pumps and laying the underground pipes, only cost the firm about $30,000, Sundholm says. "It pays for itself very quickly," he adds. "And for a large building expected to cost several hundred million kronor to build, that's not that much, especially since it will get 15% to 30% of its heat from the station."

Sundholm and his colleague Klas Johansson stumbled upon the idea while drawing on a crinkled napkin during a coffee break two years ago. "[The excess body heat] was previously just let out into the air. We thought we could do something with it," says Johansson, head of Jernhusen's environmental division. Both men are optimistic about the possible future uses of the technology: if they can figure out how to harness excess body heat on a mass scale, it could offer a significantly cheaper way to heat homes and reduce carbon emissions. Sundholm says the aim is to one day transfer body heat generated in residential areas at night to office buildings in the morning, and back again in the afternoon. "It could even be our next project," he says.

But it may not be as easy as it sounds. One obstacle is that the buildings need to be close together for the engineering to work. "It is very hard to move low-temperature heat very far. The buildings would have to be very close together by 100 to 200 yards and they would have had to really do some magnificent engineering to make sure they were not using more energy to pump the hot air over in the train station into the office building," says Lester Lane, director of Carnegie Mellon University's Green Design Initiative. (Sundholm says his system does not require more energy to move the heat than what is produced.) In addition, Lane says, countries like the U.S., where energy is not as expensive as it is in Sweden, may not see the same financial benefits after investing in the insulation, pipes and pumps. Furthermore, if people don't regularly turn up to the train station or other high-density place where the energy is being derived, there won't be enough body heat to fuel the heating systems.

With its freezing winters, ecologically minded citizenry and high energy costs, Sweden has long taken a creative approach to heating its homes. Last year, the city found a novel way to generate energy by burning the carcasses of rabbits that had been culled from local parks to keep the critters' numbers in check. "Sweden has always been very good at engineering and energy," says Ulla Hamilton, Stockholm's deputy mayor, who is heavily involved in the capital's environmental, waste and recycling plans. She attributes this to the Swedish lifestyle. "Most Stockholmers have families living in the countryside so they have a specific relationship towards nature," Hamilton says. "Because of this, sustainability is a large part of our culture."

To maintain their green credentials, all Swedes have to do is keep the heat on. In a crowded train station, that shouldn't be too difficult.

• Tara Kelly / Stockholm
• In Sweden, engineers are using body heat generated by commuters at a train station to keep a nearby office building warm — a test of a future possible low-cost energy source