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Anywhere freshwater enters the sea, such as river mouths or estuaries, could be potential sites for a power plant using such a battery, but the amount of available freshwater could be a limiting factor.

“We actually have an infinite amount of ocean water; unfortunately we don’t have an infinite amount of freshwater,” says Yi Cui, associate professor of materials science and engineering at Stanford University.

The battery itself is simple, consisting of two electrodes—one positive and one negative—that are immersed in a liquid containing electrically charged particles, or ions. In water, the ions are sodium and chlorine, the components of ordinary table salt.

Initially, the battery is filled with freshwater and a small electric current is applied to charge it up. The freshwater is then drained and replaced with seawater. Because seawater is salty, containing 60 to 100 times more ions than freshwater, it increases the electrical potential, or voltage, between the two electrodes making it possible to reap far more electricity than the amount used to charge the battery.

“The voltage really depends on the concentration of the sodium and chlorine ions you have,” Cui says. “If you charge at low voltage in freshwater, then discharge at high voltage in sea water, that means you gain energy. You get more energy than you put in.”

Once the discharge is complete, the seawater is drained and replaced with freshwater and the cycle can begin again. “The key thing here is that you need to exchange the electrolyte, the liquid in the battery,” Cui says.

Details of the study are published in the journal Nano Letters.

Using sea water collected from the Pacific Ocean off the California coast and freshwater from Donner Lake, high in the Sierra Nevada, Cui achieved 74 percent efficiency in converting the potential energy in the battery to electrical current, but he thinks with simple modifications, the battery could be 85 percent efficient.

To enhance efficiency, the positive electrode of the battery is made from nanorods of manganese dioxide, that increases the surface area available for interaction with the sodium ions by roughly 100 times compared with other materials. The nanorods make it possible for the sodium ions to move in and out of the electrode easily, speeding up the process.

Previous studies using the salinity contrast between freshwater and seawater to produce electricity, have  typically required ions to move through a membrane to generate current. Those membranes tend to have the drawback of being too fragile and also typically make use of only one type of ion. Cui’s battery uses both the sodium and chlorine ions to generate power.

Knowing that river mouths and estuaries, while logical sites for power plants, are environmentally sensitive areas, Cui chose manganese dioxide for the positive electrode in part because it is environmentally benign.

“You would want to pick a site some distance away, miles away, from any critical habitat,” Cui says. “We don’t need to disturb the whole system, we just need to route some of the river water through our system before it reaches the ocean. We are just borrowing and returning it.”

The process itself should have little environmental impact. The discharge water is a mixture of fresh and seawater, released into an area where the two waters are already mixing, at a natural temperature.

An estimate for various regions and countries for a prime water source indicates that South America, with the Amazon River draining a large part of the continent, has the most potential. Africa also has an abundance of rivers, as do Canada, the United States, and India.

But river water isn’t necessarily the only source of freshwater, Cui says. “The water for this method does not have to be extremely clean.” Storm runoff and gray water could potentially be useable.

A power plant operating with 50 cubic meters of freshwater per second could produce up to 100 megawatts of power. That would be enough to provide electricity for about 100,000 households.

“I think we need to study using sewage water,” he says. “If we can use sewage water, this will sell really well.”

King Abdullah University of Science and Technology (KAUST) and the U.S. Department of Energy provided funding for the research.

More news from Stanford University: http://news.stanford.edu/