Body Heat: Sweden's New Green Energy Source

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...