It's rare to run across a novel form of renewable energy that hasn't already been touted as "the next big thing." Work on harnessing salinity-gradient power, one aspect of which is also known as "osmotic power," has proceeded in relative obscurity. However, recent developments at Stanford University hold the promise of extracting electricity directly from the difference in salt concentration between fresh water and seawater. If it can be scaled up, it could offer yet another renewable option for coastal communities.
Most forms of energy production, renewable or otherwise, depend on exploiting some kind of gradient—differences in temperature, pressure, or characteristic between two locations. Sometimes, these gradients are inherent in the primary energy source like the pressure gradients that produce the wind harnessed by wind turbines, or temperature gradients that drive geothermal power plants. In other cases, the gradient is created by the process of energy conversion in some device, such as an internal combustion engine, gas turbine or nuclear reactor.
The "mixing entropy battery" the researchers created uses electrodes chosen for their affinity for sodium and chloride ions, with surfaces optimized through the application of nanotechnology. By charging it from an external power source while immersed in fresh water and then discharging it while filled with salt water, they're able to extract more energy than goes into it. This is not perpetual; the extra energy comes from exploiting the concentration difference between the two fluids. Impressively, researchers extracted 74% of that potential in tests (few energy cycles recover >50% of their source energy.
It seems we should add salinity power to the existing long list of renewable options.