If a membrane is used which allows water molecules to pass through, but not the molecules of dissolved salts, and it is arranged with fresh water on one side and salt water on the other side, as the water goes through the membrane, the water level on the salt water side will rise.  This pressure difference can be used to generate a flow of water which will turn a turbine.

This is the theory of operation of the first pilot plant for generating electricity from the osmotic pressure difference between river water and sea water.

Pilot Plant

In 2009 Norway’s Crown Princess, Her Royal Highness Mette-Marit, opened the word’s first osmotic power plant at Tofte in Norway.  Initially the plant is only expected to produce 2 Kilowatts of power, but it is hoped to be able to increase this considerably.

The Norwegian utility Statkraft is running this plant.  They hope that Pressure Retarded Osmosis will eventually be able to produce 5 Watts per square metre.  Statkraft estimates the global potential of this form of electricity generation at about 1.6 Petawatt hours a year.  This is close to China’s 2002 electricity consumption.

However, personally I seriously doubt whether the actual power production by this method will ever be even close to this colossal estimate.  If it did, there would certainly be serious adverse ecological repercussions. 

But a more modest production could certainly occur and if done sensibly need not harm the environment.

Sources

.

http://news.cnet.com/8301-11128_3-10404158-54.html

http://www.osmoticpower.com/

http://www.statkraft.com/energy-sources/osmotic-power/

Petawatt

http://en.wikipedia.org/wiki/Petawatt#Multiples

http://www.leonardo-energy.org/23-reverse-electrodialysis