Underneath a 26-floor office tower in Stockholm, an underground space once used as an archive for a newspaper will soon become a farm. And because of a unique business model, the urban farmers growing greens in the new farm won’t pay rent–their farm will pay for itself in heat.

Like some other indoor farms, the Plantagon CityFarm, set to begin production in early 2018, will grow greens in vertical towers under LED lights. But by capturing the heat from the lights–heat that would normally have to be vented out of the room and require air conditioning to keep the plants from overheating–the farm operators can send it into a heat storage system for the office building, and the heat can be used to help keep the offices warm through the winter.

The system will save the office building 700,000 kilowatt-hours of energy a year, worth roughly three times as much as the previous tenant of the basement was paying in rent.

“[The building owner] agreed to give us a free lease for three years, so we don’t pay one single Swedish kroner for the room,” says Plantagon cofounder Hans Hassle. “This is the challenge, very often, for urban farmers: If you really want to grow things in the city, you have to find new business models that actually make the food not too expensive in the end.”

The company plans to sell food directly to people working in the offices above, along with two restaurants that are located in the high-rise. Roughly a third of the produce will be sold to nearby grocery stores, all close enough that the greens can be delivered without fossil fuels. Another third of the produce will be sold in an on-site store in the skyscraper.

“In Sweden, we have a higher demand for locally grown food than we do for organic food,” Hassle says. “People tend to want to know where the production comes from.”

If organic kale or lettuce travels hundreds or thousands of miles to a store, Hassle says, the environmental footprint could be higher than the same greens, grown without pesticides or herbicides, inside the closed-loop system of the indoor farm. Like other indoor farming, the Plantagon system also uses a tiny fraction of the water used on outdoor farms. The heat is captured in water that travels in tubes over the LED lights, and then sent into a heat pump system. Carbon dioxide from the offices will also be sent to the farm, and fresh oxygen from the plants will be sent back to office workers.