STOCKHOLM (Reuters) - A Swedish state-owned firm has found a cheap, eco-friendly source of energy to warm one of its offices: body heat from a quarter million commuters steaming through Stockholm’s central train station.

Body heat already warms the station itself but the surplus, currently let out in thin air, will be redirected to provide as much as 15 percent of the heating in a planned 4,000 square meter office building, real estate firm Jernhusen said.

“We had a look at it and thought ‘We might actually be able to use this’,” said Karl Sundholm, project leader at Jernhusen, which also owns the station. “This feels good. Instead of just airing the leftover heat out we try to make use of it.”

Jernhusen markets the building as “environment smart” and aims for its energy consumption to be half of what a corresponding building usually is.

The bodily warmth from the central station will be redirected to heat up water. The investment will be around 200,000 Swedish crowns ($31,200), Sundholm said.

“The ventilator aggregates are already there, and even some of the pipes. All we need to do is complement with a few pumps and pipes.”