Researchers use the ocean to recharge their batteries

When people think of harvesting energy from the oceans, it's typically in the form of wave or tidal power, in which the motion of the water drives the production of electricity. A paper in Nano Letters suggests an alternate way to turn the ocean into power: using the fact that it's salty. There have been a few ideas about how to extract energy from the salinity difference between salt and fresh water, but the paper suggests a rather intriguing approach: treat the entire ocean a bit like a battery medium.

The battery charge cycle generally involves the exchange of electrons with ions that shuffle between a storage medium and electrodes; normally, the ions themselves remain encased within the battery. The new device takes a very different approach, allowing the ions to exchange freely with water that flows through it.
The device has electrodes that specifically react with some of the salt ions normally found in sea water. One is made of manganese dioxide, which can react with a sodium ion to form Na₂Mn₅O₁₀. The material also happens to be cheap, environmentally benign, and has a high energy density. The authors weren't so careful when they chose the other electrode, as they used silver, which can react with chlorine. Thus, the two electrodes in the device can sequester the ions that form when sodium chloride—common salt—dissolves in water.
This allows for a simple cycle. When salt water flows over the electrodes, they capture the ions, producing a charged battery. When the salt water is replaced by freshwater, the cycle can be reversed, but in order to do so, electrons have to flow between the two electrodes, creating a usable current. The authors call their device a "mixing entropy battery," and show some examples that perform at about 75 percent of the theoretical efficiency, with no decline over 100 cycles. They also show that it works perfectly well with environmental samples.
There's not a huge amount of energy available per device, but the authors calculate that a freshwater flow of 40 cubic meters a second could generate up to 100MW. For context, Niagara Falls sees over 1,800 m³/s. Globally, about 2TW of energy could be harvested. That's not a lot compared to our energy needs, but the approach could be part of a renewable portfolio, since it could run around the clock provided that the fresh water stream is kept separate from the salt.
But the authors also offer a less grand but possibly more compelling application: a simple system for storing solar power. A closed solar still could evaporate fresh water from a salt stock, allowing a source of material to discharge the battery. The authors also demonstrate one of these devices, and show it can run for over 100 cycles with no loss of efficiency.
For all their devices, the authors say that it should be possible to improve the geometry of the electrodes to boost efficiencies even further. This doesn't seem like the sort of thing that's going to set the world on fire—the energy involved is simply too small—but an improved version may provide a simple, flexible way of storing renewable power in some specific contexts.
Nano Letters, 2011. DOI: 10.1021/nl200500s  (About DOIs).

http://arstechnica.com/science/news/2011/04/researchers-use-the-ocean-to-recharge-their-batteries.ars