This ultra green, almost chartreuse, body-heat design works especially well in Sweden, a land of soaring fuel costs, legendary hard winters, and ecologically minded citizens. 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…

Part of the appeal of heating buildings with body heat is the delicious simplicity of finding a new way to use old technology (just pipes, pumps and water). Hands down, it’s my favorite form of renewable energy.

What could be cozier than keeping friends and strangers warm? Or knowing that by walking briskly or mousing around the shops, you’re stoking a furnace to heat someone’s chilly kitchen?

How about the reciprocity of a whole society, everyone keeping each other warm?