27-11-07 As the world beefs up its search for the perfect renewable energy source of the future, Norwegian company Statkraft says the answer may have been on the tip of our tongues all along: salt.
The publicly-held energy company aims to begin building the world's first miniature seawater power station next year along the banks of the Oslo fjord.

"Osmotic" power is a clean energy source that according to Statkraft could theoretically supply 1,600 TWh worldwide, or about half of Europe's current energy consumption. "It is totally CO2-free," Statkraft's new energy business developer Jon Dugstad told.
"The only thing we do is that we mix fresh water and sea water. We don't add anything in that process, which is a completely natural process," he added, pointing out that the power is created anywhere where rivers run into the sea.

Osmotic power takes advantage of the different salt concentrations in liquids: When saltwater and freshwater are separated using a filter called a semi-permeable membrane, the water containing the least salt naturally migrates towards the saltwater, creating pressure that can be transformed into energy.
In a small factory in the southern Norwegian town of Hurum, Statkraft is planning to build a miniature osmotic power station capable of pumping out between 2 and 4 kWh, just enough to keep a few light bulbs burning.
"The prototype will only be aimed at verifying the technology," Dugstad said.

If the experiment works, the company is considering building a larger-scale station capable of producing between 160 and 170 GWh, or enough to cover the electricity needs of about 15,000 households. Osmotic power could become competitive around 2015, according to Statkraft.
"This is not just a scientific whim. In the future, we will in any case be pushed to take advantage of all the clean energy sources," said Gerald Pourcelly, who heads the European Membrane Institute, an affiliate of French scientific agency CNRS.

The greatest technological challenge lies in the filter used in the osmotic process. The semi-permeable membrane, which must be permeable enough to allow freshwater to flow into the saltwater but needs to hinder grains of salt from migrating in the opposite direction, is what determines how much energy is produced.
"The problem is that the exchange surface, or the membrane, must be extremely large to harvest enough energy. This means a large number of membranes will be needed to produce relatively little energy per square metre," Pourcelly said.

Statkraft says it in recent years in the lab has reached a power density (the quantity of energy to surface ratio) of 3 Watts per square metre.
"We believe we need 5 Watts per square metre," Dugstad said. Once an osmotic power station is up and running, this type of energy production should be cheap and also offer a constant flow of power -- a great advantage over the sporadic energy provided by solar and wind stations.

Europe, North America, South Africa and certain parts of South America are expected to be the most fertile markets for osmotic power. The new energy source is however expected to pose geographical problems, since the power plants will require a large footprint in delta areas, which are often already densely populated.
Statkraft meanwhile insists the space needed -- the equivalent of "one or two football fields" for a 160 GWh power station -- is much smaller than the area covered by a wind field capable of producing the same amount of energy.