To remove the salt from water requires energy.  In the little reverse osmosis unit in our shop this energy comes from the pressure of the mains water.  In the much larger desalination plant built by the South Australian state government, the energy required to produce the necessary pressure difference is input in the form of electricity to drive the pumps.

Similarly, it is possible to get energy from the salinity difference between fresh and salt water.  South Australia must be one of the worst places in the world to use this type of power, but there are a number of places in the world where rivers do reach the sea, and flow all the year round.  Two of these places are Norway and Holland.

These two countries have put effort into making this idea work.  They are working on different methods of getting this power.

Pressure Retarded Osmosis

Reverse Electrodialysis

General Problems

Both methods of using the potential power of the salinity difference depend on there being a lot of fresh water reliably flowing out to sea.  This only occurs in certain parts of the world.

There is probably nowhere in southern Australia that would be suitable, and only a few places in northern Australia.

Most of Asia does not have unused reliable flows to the sea.

Parts of northern and western Europe are possibilities.  Some parts of northern North America have the water, and some parts of South America and Africa have the right conditions.

Ecological Disruption

Although the end result of the process is that the fresh water ends up in the sea as it would have done, disruption to the interface of fresh and salt water could be a serious problem for some organisms.  It would be necessary to prevent Humans overdoing a good thing as we often do.  Only a modest percentage of the available water should ever be used for power generation in this way.

Advantages

No Pollution?

Once this is set up, power will be produced without adding any greenhouse gases or heat to the environment.

Continuous

Unlike wind, solar and tidal power, this is offers a continuous source of power.  With proper design, maintenance can be carried out without any interruption of the power supply.

Sources

http://www.osmoticpower.com/