Electricity from Seawater?

July 31, 2009

The Physics Central Web site of the American Physical Society has an article reporting on some recent research by Italian physicist Doriano Brogioli, demonstrating the possibility of generating electricity from the energy released when salt water is mixed with fresh water:

A device that gleans usable energy from the mixing of salty and fresh waters has been developed by University of Milan-Bicocca physicist Doriano Brogioli. If scaled up, the technology could potentially power coastal homes, though some scientists caution that such an idea might not be realistic.

The process, in a certain sense, reverses that of a de-salinization plant, which consumes energy to turn salt water into fresh water.  When fresh water is mixed with salt water, the thermodynamic entropy of the system is increased. leading to a release of energy.  What Brogioli has done is to develop a new kind of salinization cell, in which he uses two chunks of activated carbon (which has a very large surface area relative to its mass) as the “plates” of an electrolytic capacitor.  The cell is initially filled with salt water, and the plates are charged by a power supply.  This cause ions  (e.g., Na+ and Cl) in the salt water to migrate to the oppositely-charged plate.  The cell is then flushed with fresh water.  Diffusion of the ions into the fresh water, working against the electrostatic attraction, produces a positive voltage across the capacitor.  (This is analogous to charging the capacitor, then removing the dielectric.  The capacitance drops but the voltage increases.)   The increased voltage is drained off, and then the cycle is repeasted.  (You might think of it as “Salt, Rinse, Repeat”.)

So far, the idea has only been tested in the lab, but there are hopes that it can be scaled up to practical applications;

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.

“I don’t see any reason why it should not work,” said Yury Gogotsi, director of the A.J. Drexel Nanotechnology Institute at Drexel University in Philadelphia. “Capacitor desalination has been demonstrated and commercialized, and this can be called reverse capacitance desalination. It appears to be a logical approach. Of course the challenge is the practical implementation.

If the process can be scaled up to industrial size, it might be possible to install it in places where fresh and salt water naturally mix, such as river estuaries.  It’s been esyimated that the energy released at a river estuary is the equivalent of a 225-meter waterfall.

Brogioli has demonstrated that it is possible to obtain energy in this way.  I hope a way to scale it up practically is found; at this point, we can use all the alternative approaches to generating energy that we can find.

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