At deltas of rivers where fresh water mixes with salt water, energy associated with the salinity gradient can be harnessed using pressure-retarded reverse osmosis process and associated conversion technologies.
Salinity power is one of the largest potential sources of renewable energy. The potential power is large, corresponding to 2.6 MW for a flow of 1 m3/sec freshwater when mixed with seawater, according to Ocean Energy Europe.
Two practical methods for electricity generation from salinity gradient are being researched: the reverse electrodialysis (RED) method and pressure retarded osmosis (PRO).
Both technologies are dependent on the semi-permeable membrane. A semi-permeable membrane only allows specific substances to pass through the membrane.
The processes rely on ion-specific membranes. The technology RED as well as PRO is in the research and development phase.
Pressure Retarded Osmosis (PRO)
Pressure Retarded Osmosis (PRO) uses the selective diffusion of water across a membrane in order to pressurize seawater. Freshwater and seawater are placed on either side of a membrane, and the seawater side is pressurized. As the seawater side increases in pressure and decreases in salinity, part of the water is discharged through a turbine while the rest is put in a pressure exchanger to pressurize the incoming seawater.
The pressure difference across the membrane is the main supplier of energy.
Reverse Electrodialysis (RED)
Reverse Electro Dialysis is another membrane-based technology that uses an electrochemical reaction rather than osmotic pressure. The form of the device is a stacked series of membranes, half of which are permeable to sodium and half chloride, with seawater and freshwater flowing alternately between each pair of membranes.
The stack controls the diffusion of the sodium and chloride ions in the water, which then cause oxidation and reduction at the iron anode and cathode.