> You may be interested to know that it takes energy to remove salt from water.

Yes, the osmotic pressure is similar to a head of about 240 meter. In practice it will be less in practice (110-120m), but still significant.

> There are proposals to mix freshwater from the mouth of streams with seawater and capture the energy released.

Not only proposals, but there is a prototype in production on Norway that opened in 2008.

http://www.statkraft.com/energy-sources/osmotic-power/

I think the problem for such power plants is to make the membranes efficient enough, and given that they have problem to remove all salt in this "battery", I guess that they have the same problem. For reverse osmosis I'm pretty sure it is more efficient with lower gradient, so these techniques could be combined.

Since osmotic power is better with larger salt gradients, a power plant that use the gradient between the ocean and the Dead Sea would be perfect. The Dead Sea sinks one meter per year because of excessive use of water for irrigation. There is probably not the right political climate in the area to achieve such things however.

The dead sea idea is interesting. The political climate is fine actually - Israel and Jordan have a pretty good relationship.

There are already plans to collect energy from the elevation drop - adding the salinity change too could be pretty nice. This seems like a rare environmental win - get free energy, and replenish the water level at the same time.