Want to keep your house warm without your furnace? A 30-person dance party is pretty much the only way to do it. By the end of the evening, you'll be opening windows.
Now imagine that your house is Stockholm Central Station, where 125,000 hot bodies hustle to and from their commutes every day. All that body heat has traditionally been somewhat of a nuissance. But now, the city of Stockholm is actually going to capture the body heat and use it to heat a nearby office building.
Apparently the system is fairly simple. Just a bunch of pipes that will collect the heat, and pump it out of the too-warm train station and into a new office building being built nearby. Body heat from Stockholm Central Station will provide roughly 20% of the heat for the new building. For a system that will only cost about $30,000 to build, it's a no-brainer for the City of Stockholm.