It’s no secret that crowds cause temperatures to rise, but all those bodies generating heat could be a large source of untapped energy. Inspired by the hordes of people passing through Stockholm’s Central Station each day, a Swedish real estate company, Jernhusen, has found a way to harness the energy created by the 250,000 moving bodies to heat an office building across the street. As a result, the building’s energy bill has decreased by 25 percent.

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“[The people] generate a bit of heat, but they do a lot of activities,” said Klas Johnasson, one of the creators of the system and head of Jernhusen’s environmental division. “All this energy generates an enormous amount of heat. So why shouldn’t we use this heat? It’s there. If we don’t use it then it will just be ventilated away to no avail.”

Jernhusen first proposed the idea back in 2008, and now it has finally come to fruition. Heat exchangers in the Central Station’s ventilation system convert the excess body heat into hot water, which is then pumped into the heating system of the neighboring building. The system works particularly well in Sweden because of the low temperatures and high gas prices. Doug King, a sustainable development consultant told the BBC, “It means a low-grade waste heat source, like body heat, can be used advantageously. It’s worth them spending a little bit of money on electricity to move heat from building to building, rather than spending a lot on heating with gas.”

Johnasson hopes to implement the system in other buildings, possibly even using it to heat the Central Station itself. In a country as eco-friendly and forward-thinking as Sweden, the sustainable method is sure to catch on quickly.