To reduce energy use in colder climates, finding more sustainable sources of heat is imperative. One counterintuitive technology is now being championed by some, including the UK government: heat pumps that extract heat from—for example—chilly Welsh seas or London’s River Thames, and concentrate it to heat buildings close to their shores.

Heat pumps work basically like a refrigerator in reverse. Fridges take heat away from their interior, and pump it out into the surrounding room. A heat pump takes warmth from the air or water, and “concentrates” it via a chemical process, then uses it to heat a building.

In one example from Norway, water from a fjord that feels freezing to the touch is in fact 8 degrees Celsius (46.4 degrees Fahrenheit). It’s used to heat liquid ammonia, turning that into a gas. The gas is then pressurized, heating water in the process. Then the gas is de-pressurized, and turns back to liquid form, and the process can repeat.

The UK Department for Energy and Climate Change released an interactive map this week showing where in the country the most heat is used, and where potential sources of heat in rivers, canals and oceans could be tapped into by locals in need of warmth. A million homes could be heated through the technology, DECC said, while energy secretary Ed Davey noted that the drive was part of an attempt to shore up energy security, using “home-grown resources rather than relying on foreign fossil fuels.”

In the UK, use of heat pumps from water is nascent. But Battersea power station—a huge, iconic former power generator in London currently being converted into a complex of flats, offices and shops—said this week it would look to install a heat pump system using water from the Thames.

And one stately home run by conservation body the National Trust, Plas Newydd, already has replaced its oil-fired burner with a heat pump system, using water from the Menai Straits, a narrow tract of water separating the isle of Anglesea from Wales.