Salinity Gradient Technology
Osmotic power or salinity gradient power is the energy retrieved from the difference in the salt concentration between seawater and river water.
Salinity Gradient Power is the highest energy concentration (i.e., energy density) of all marine renewable energy sources, an ultra-dense energy resource. It was recognized in the 1970s, that salinity power or “the energy represented by the salinity concentration gradient between fresh water and seawater” could be an attractive, large and untapped resource to explore.
Resources: The osmotic pressure difference between fresh water and seawater is equivalent to 240 m of hydraulic head. In theory, a stream flowing at 1 m3/s could produce 1 MW of electricity. The worldwide fresh to seawater salinity resource is estimated at 2.6 TW comparable to the ocean thermal gradient (estimated at 2.7 TW). Inland highly saline lakes (hypersaline) have an even greater potential. For example, the Dead Sea osmotic pressure differential corresponds to a head of 5,000 m (almost twenty times greater than seawater). Underground salt solutions or deposits have also been recognized.
The Potential Resource for the Osmotic Pairing of Fresh Water and Seawater
Source: European Ocean Energy Association
Technology: Several methods have been proposed to extract this power. Among them are the difference in vapor pressure above freshwater and saline water and the difference in swelling between fresh and saline waters by organic polymers. However, the most promising method is the use of semi-permeable membranes.
In the osmotic process two solutions with different salt-concentrations are involved (often freshwater and salt-water). A semi permeable membrane, which is an organic filter, separates the solutions. The membrane only lets small molecules like water molecules pass.
The water aspires to decrease the salt-concentration on the side of the membrane that contains most salt. The water therefore streams through the membrane and creates a pressure on the other side. This pressure can be utilized in order to gain energy, for example by using a turbine and a generator.