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| | Background | | |  | | Author: Thor Thorsen, SINTEF, Norway. The principle of salinity gradient energy is the exploitation of the entropy of mixing freshwater with saltwater. The potential energy is large, corresponding to 2.6 MW m3/sec freshwater when mixed with seawater. This energy source is not easy to understand, as it is not directly sensed in nature in the form of heat, waterfalls, wind, waves, or radiation. 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 semipermeable membranes. The energy can then be extracted as pressurized brackish water by pressure retarded osmosis (PRO) or direct electrical current by reverse electrodialysis (RED). No technical plants using these methods are operating in 2001. | | | | How it works | | | | With the Red method, ion selective membranes are used in alternate chambers with freshwater and seawater, where salt ions migrate by natural diffusion through the membranes and create a low voltage direct current. With the PRO method, another type of membrane, similar to reverse osmosis membranes used for sea water desalination, is used. These PRO method membranes are much more permeable to water than to salt. If fresh and saltwater are separated by such membranes, natural osmosis will force the freshwater through the membrane to the saltwater side where hydrostatic pressure up to 26 bars can be created. The two methods are quite different in their working principles, but it is the same potential energy that is exploited. | | |  | |
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