Renewable Energy from Mixing Salty Water with Fresh Water

What if we could harness the mixing of salt water and fresh water to create a renewable energy source? Better yet, what if we could use sea water and even saltier water from a desalination plant to create a renewable energy source?

That is exactly what Doriano Brogioli has proposed in a paper to be published in an upcoming issue of Physical Review Letters. Dr. Brogioli has created a process that is basically the reverse of desalination in which an ion gradient between fresh water and salty water creates electricity. Because the universe tends to go toward disorder, energy is released as a system becomes more disorganized. Salt water has more ions, or charged particles, in it than fresh water. When the two are mixed, the ions spread out or diffuse into a larger volume of water and become more disorderly, thus releasing energy. Brogioli has found a method to harness this release of energy to create electricity and brackish water. The prototype salination cell that Brogioli created uses two electrodes of porous carbon to first separate the positive ions and negative ions from the saltwater. The cell then is flushed with fresh water. Diffusion battles the electrostatic forces at the electrodes increasing the energy of the system and creating the potential for work. This work is harnessed as electricity. A great discussion of the science behind the work can be found at Physics Central: Buzz Blog. Previous work on using the salinity gradient focused on using membranes to separate the fresh water from the saltwater and harnessing the diffusion of ions across the membrane to create electricity. Unfortunately, maintaining the membranes proved to be cost prohibitive compared to other sources of renewable energy. According to Physics Central,

A typical cell would require about three dollars worth of activated carbon, and, given a steady flow of water, the cell could produce enough electricity to meet the needs of a small house. It's the equivalent, in hydroelectric power, of running your appliances from a personal 100 meter (338 feet) high waterfall.

This type of power generation is ideal for locations, such as river deltas, that already have fresh water flowing into salty water. In such naturally occurring places, the brackish water produced would not be a pollutant, but a normal consequence of the natural process. According to comments written by Dr. Brogioli in response to a recent article in Science News Online,

Finally, as each alternative energy source, it must be used where it can be used. If you have not enough fresh water to drink, you cannot use fresh water to produce energy! But there are places where you have polluted river water, but also a lot of drinking water. There, you can apply my method.

Also, Brogioli goes on to say that you can use salt water and water with very high salinity with his method. This would allow his salinity gradient cell to be used in desert regions with water from desalination plants. Several people have expressed skepticism of the scalability of this system from kilowatt production to megawatt production. Additionally, salt water is corrosive and may lead to problems in implementing this method of renewable energy. Despite the skepticism, here is another method to think about in the quest for renewable energy.