A fjord south of Oslo is the site for a new experimental power plant, where researchers will attempt to extract energy from the osmotic reaction that takes place when fresh river water combines with the saline water of the sea.

When saline and fresh water are mixed at river mouths, they typically warm up by 0.1 degree Celsius (0.2 Fahrenheit).

Scientists in the Netherlands estimate that the energy at all the world's estuaries is equivalent to 20 percent of global electricity demand.

"We might well be able to find new promising solutions such as generating power naturally from osmotic forces occurring when salt and fresh water are mixing," said Liv Monica Stubholt, Norwegian deputy minister of energy.

The Norwegian state-owned power company Statkraft is spending US$20 million (£10 million) on the Oslo test plant.

"Ours will be the world's first saline power plant based on osmosis," said Stein Erik Skilhagen, a spokesperson for Statkraft.

The Dutch Centre for Sustainable Water Technology (Wetsus) will also begin construction soon on a saline power plant at IJsselmeer, a former estuary of the Rhine that was dammed to create a large fresh water lake.

"At the start, it will be on the scale of 100 watts -- but we aim at this salt factory to obtain one to five kilowatts within one year," said Jan Post, a researcher at Wetsus.

The Norwegian and Dutch plants use different methods but both rely on membranes placed between the salt and fresh water.

These crucial membranes are currently expensive and energy-intensive to produce, which could be a major barrier to commercialisation.

"The Achilles' heel for this process is that there is no commercial membrane," said Menachim Elimelech, professor of chemical and environmental engineering at Yale.

"It's not even close to being economical."

"The membrane is the challenge," Skilhagen conceded.

"In tests we have come over three watts per square metre, but we have to reach five."

"When we do that it will be industrially interesting."

The Dutch are close to generating two watts per square metre of membrane.

"In theory, both techniques use the same energy source and you could in theory get the same amount of energy out," said Sybrand Metz, project leader at Wetsus.