Osmosis Power Generation
Osmosis: The phenomenon wherein a solvent diffuses through a semipermeable membrane to dilute a solute. The big idea is that special membranes will let water pass through but hold back anything else that is dissolved in the water. If you have two containers, one with fresh water and one with salt water, and they are separated by such a membrane, the fresh water will creep its way into the salt water in attempt to equalize the salinity between the two containers. You may remember something about this from a chemistry class, or perhaps you work in the life sciences. In living cells, osmotic pressure helps to transport molecules across membranes. Artificial membranes have also been developed for many kinds of chemical tests as well as to purify water, as with reverse-osmosis.
While slow, this diffusion process can generate very high pressures. That is to say, it's very hard to stop that fresh water from diffusing through the membrane. If the saltwater side is kept enclosed, it could develop pressure in the range of 200psi (1.4MPa) or more, depending on the initial concentration difference between fresh and salty.
Mechanical Power Generation:
In nature, the pairing of freshwater from a river with salt water from the ocean present a very large osmotic potential. There are designs dating back to the 1950s that can generate electricity from the mechanical pressure generated on one side of membranes by this diffusion. In simple terms, fresh water pushes into the salt water through the membrane, and the mixing water then pushes its way through a turbine generator. Here is a very nicely produced video by the Norwegian power company Statkraft. Their proposed power plant that operates this way. You can also see more about the actual pilot plant can be seen here.
Direct Electrical Generation:
A new approach to osmotic power was first described by researchers at the Swiss Federal Institute of Technology (EPFL) in 2016. Their technology uses a refined nanopore membrane to generate electricity directly. Unlike the traditional semipermiable membranes that allow water but not salt to pass, the nanopores hold back water molecules while admitting dissolved ions. The movement of ions across a membrane constitute an electrical current, which can be tapped with electrodes on either side of the membrane. It is essentially a saline gradient fuel cell. The nanopore approach is also more efficient and potentially much more compact than the mechanical approaches. The researchers extrapolated data from the single nanopore they used in their experiments and predict that a squre meter of such membrane could generate up to one MW of power. That is a million watts. A power plant sitting near the mouth of a modest river and tapping this potential could be scaled to rival the power output of most coal plants. Oh, and by the way, there is no pollution, just brackish water of the same concentration as normally found at the mouth of a river!
I encourage and invite you read an article about the research on phys.org from July 13, 2016.