salinity gradient power
When salty water is exposed to water with less salt water flows from low salinity to high salinity; this process is called osmosis. Separated by a membrane the osmotic pressure of flowing water can be used as fuel to generate electric power.
Osmotic power, salinity gradient power or blue energy is the energy available from the difference in the salt concentration between seawater and river water. Two practical methods for this are reverse electrodialysis (RED) andpressure retarded osmosis (PRO). Both processes rely on osmosis with ion specific membranes. The key waste product is brackish water. This byproduct is the result of natural forces that are being harnessed: the flow of fresh water into seas that are made up of salt water. In 1954 Pattle suggested that there was an untapped source of power when a river mixes with the sea, in terms of the lost osmotic pressure, however it was not until the mid ‘70s where a practical method of exploiting it using selectively permeable membranes by Loeb was outlined. The method of generating power by pressure retarded osmosis was invented by Prof. Sidney Loeb in 1973 at the Ben-Gurion University of the Negev, Beersheba, Israel. The idea came to Prof. Loeb, in part, as he observed the Jordan River flowing into the Dead Sea. He wanted to harvest the energy of mixing of the two aqueous solutions (the Jordan River being one and the Dead Sea being the other) that was going to waste in this natural mixing process. In 1977 Prof. Loeb invented a method of producing power by a reverse electrodialysis heat engine. The technologies have been confirmed in laboratory conditions. They are being developed into commercial use in the Netherlands (RED) and Norway (PRO). The cost of the membrane has been an obstacle. A new, lower cost membrane, based on an electrically modified polyethylene plastic, made it fit for potential commercial use. Other methods have been proposed and are currently under development. Among them, a method based on electric double-layer capacitortechnology. and a method based on vapor pressure difference.
Source: Wikipedia - Osmotic power