OSLO (AP) — Norwegian energy company Statkraft on Tuesday unveiled what it called the world's first salt power generator, a system which harnesses the energy produced when fresh water and sea water mix.

The prototype was built for testing and development purposes in an old paper mill. State-owned Statkraft said it hopes to be able to build "a commercial osmotic power plant within a few years."

It estimates that globally, salt power could produce 1,600-1,700 terawatt hours, equivalent to half of the European Union's total annual power production.

Salt power, whose waste product is brackish water, is widely considered an environmentally friendly energy source. However, there are concerns that the brackish water emitted by the plants could affect local marine ecology.

The technology is also referred to as osmotic power because it relies on osmosis — the tendency of water to move from one side of a porous divider to the other to even out the salt concentration on either side. That displacement in turn produces energy.

The prototype, about 40 miles (60 kilometers) south of the Norwegian capital, is located where a river empties into the Oslo fjord on a creek.

Owned solely by the Norwegian government, Statkraft has 3,200 employees in 20 countries.