At the mouth 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. Another system is based on using freshwater upwelling through a turbine immersed in seawater, and one involving electrochemical reactions is also in development.
Significant research took place from 1975 to 1985 and gave various results regarding the economy of PRO and RED plants. It is important to note that small-scale investigations into salinity power production take place in other countries like Japan, Israel, and the United States. In Europe the research is concentrated in Norway and the Netherlands, in both places small pilots are tested. Salinity gradient energy is the energy available from the difference in salt concentration between freshwater with saltwater. This energy source is not easy to understand, as it is not directly occurring in nature in the form of heat, waterfalls, wind, waves, or radiation.
Salinity Energy Potential
Salinity power is one of the largest sources of renewable energy that is still not exploited. The potential power is large, corresponding to 2.6 MW for a flow of 1 m3/sec freshwater when mixed with seawater. The energy released from 1 m3 fresh water is comparable to the energy released by the same m3 falling over a height of 260 m. The exploitable potential world-wide is estimated to be 2000 TWh/y. The potential cost of energy from this source is higher than most traditional hydropower, but is comparable to other forms of renewable energy that are already produced in full-scale plants. The availability and predictability of salinity gradient energy is much higher than intermittent renewables like wind and sun and therefore sustainable base load energy.
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. The energy can then be extracted as pressurised brackish water by pressure retarded osmosis (PRO) or direct electrical current by reverse electrodialysis (RED).