This idea is entirely new to me. Osmosis is being used to generate electricity in a pilot project in Norway:
Freshwater and saltwater are channeled into separate chambers, separated by an artificial membrane. The salt molecules in the seawater draw the freshwater through the membrane, causing the pressure on the seawater side to increase. This pressure is equivalent to a water column of 120 meters or, in other words, quite a significant waterfall. This pressure can be used in a turbine to make electricity.