Hugh Pickens writes "Depending on the level of activity, the human body generates about 60 to 100 Watts of energy in the form of heat, about the same amount of heat given off by the average light bulb. Now Diane Ackerman writes in the NY Times that architects and builders are finding ways to capture this excess body heat on a scale large enough to warm homes and office buildings. At Stockholm's busy hub, Central Station, engineers harness the body heat issuing from 250,000 railway travelers to warm the 13-story Kungsbrohuset office building about 100 yards away. First, the station's ventilation system captures the commuters' body heat, which it uses to warm water in underground tanks. From there, the hot water is pumped to Kungsbrohuset's heating pipes, which ends up saving about 25 percent on energy bills. Kungsbrohuset's design has other sustainable elements as well. The windows are angled to let sunlight flood in, but not heat in the summer. Fiber optics relay daylight from the roof to stairwells and other non-window spaces that in conventional buildings would cost money to heat. Constructing the new heating system, including installing the necessary pumps and laying the underground pipes, only cost the firm about $30,000, says Karl Sundholm, a project manager at Jernhusen, a Stockholm real estate company, and one of the creators of the system. 'It pays for itself very quickly,' Sundholm 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.'"