Blue energy is the energy extracted from osmosis of salt and fresh water through a membrane. The difference in the salt concentration between seawater and fresh water across special modified plastic membranes produces electricity. The process is called reverse electro dialysis (RED) (or osmosis) and the waste product in this process is brackish water (salt and fresh water mixed).
The technology of reversed electrodialysis has been confirmed in laboratory conditions but the cost of the membrane is currently stopping a larger scale commercial proof. A new, cheap membrane, based on an electrically modified polyethylene plastic may change this in the near future.
But how much power can you generate? Even though only small currents are generated, cells of membranes can be stacked to increase the output. For example, in the Netherlands, more than 3,300 square meters fresh water runs into the sea per second on average. The energy potential is therefore 3,300 MW, based on an output of 1 MW/m3 fresh water per second.
How does this work?
These special membranes will allow a process called osmosis. Through osmosis these membranes will let fresh water through, but not the salt in the water. So you can divide a tank in two seperated by a membrane. Fill one side with fresh water and the other side with salt water. The salt concentration will naturally try to balance itself. As salt can not pass through the membrane, that means fresh water will travel through to try and make the two sides have the same salt concentration. Almost nothing will flow back the other way. So the water level in the salty side of the tank gradually rises above that in the fresh water side. One side higher than the other = potential energy which you can harness through turbines etc.
Several companies are now working on commercially viable plants. As mentioned previous, the membranes were previously to expensive to make this worth while but new technologies have brought the cost down. One of the most exciting things to come out of this technology is the small space it takes up. Potentially a 250 kW plant will take up the space of about 1 shipping container.
LinksKema - One of the companies working on these plastic membranes.
Article on Osmotic Energy.