River water, salty water `to generate electricity`

A battery to generate electricity from fresh and sea water developed.

Updated: Apr 02, 2011, 13:32 PM IST

Washington: Researchers have developed a battery that they claim takes advantage of the difference in salinity between fresh and sea water to generate electricity.

Anywhere freshwater enters the sea, such as river mouths or estuaries, could be potential sites for a power plant using such a battery, said lead researcher Yi Cui of Stanford University.

The theoretical limiting factor, he said, is the amount of freshwater available. "We actually have an infinite amount of ocean water; unfortunately we don`t have an infinite
amount of freshwater," he said.

As an indicator of the battery`s potential for producing power, the researchers calculated that if all the world`s rivers were put to use, their batteries could supply about 2 terawatts of electricity annually -- that`s roughly 13 per cent of the world`s current energy consumption.

The battery itself is simple, consisting of two electrodes -- one positive, one negative -- 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. That makes
it possible to reap far more electricity than the amount used to charge the battery, the `Nano Letters` journal reported.

"The voltage really depends on the concentration of the sodium and chlorine ions you have. 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," Cui said.

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 said.