Water can hold a huge amount of thermal energy, and a new system to tap into this resource is being trialed in Scotland. A startup called SeaWarm uses heat stored in bodies of water for buildings, pulling four times more heat out than electricity used.

Simply put, heat pumps are designed to take thermal energy (heat) from one medium and give it to another. Your fridge, for instance, takes heat from the air inside and pumps it outside, cooling the interior. An air conditioner works in a similar way, cooling the inside of your house (or heating it in winter).

That, however, requires a lot of energy, because air isn’t a great medium for storing heat. But water, according to SeaWarm, can hold up to 3,400 times more thermal energy than the same volume of air. That represents a pretty enticing energy source, so SeaWarm, a spin-off company from the University of Edinburgh, is working to tap it.

The company’s Water Source Heat Pumps (WSHPs) can harvest heat from bodies of water like oceans, rivers and lakes, or water from underground or mines. The SeaWarm unit can be placed up to 500 m (1,640 ft) from the source, with the water pumped or gravity-fed to the heat exchanger.