The Battery That Keeps on Going?

Saline water mixing with Amazon basin. Source: Stanford news

In their lifetimes many people have imagined inventions that will remove the salt from seawater and make it potable, or ones that feature a self-powered electrical energy device.

Idle far-fetched fancies? Not if you happen to be a Stanford associate professor of materials and engineering named Yi Cui, known worldwide for his innovative work with nanotechnology.

Now Cui and his team of researchers have calculated a way to alternate the flow of river water and salty seawater through a battery to produce electricity that can be used for charging purposes. It might also be possible this same process can be reversed to remove salt from seawater to produce drinking water.

Cui’s research team calculated that if all the world’s rivers were put to use for this purpose, the batteries that would be used – referred to by some as mixing entropy batteries – could supply about 2 terawatts of electricity annually, almost 13 percent of the world’s current energy consumption.

A power plant operating with 50 cubic meters of freshwater per second could produce up to 100 megawatts of power, according to the team’s calculations. That would be enough to provide electricity for about 100,000 households, says Cui in a paper titled, “Batteries for Efficient Energy Extraction from a Water Salinity Difference.”

This example has been presented for a location: the mouth of the Amazon River, where the world’s largest drainage basin flows into the Atlantic Ocean. A location such as this, where fresh and sea water mix, is a good spot for generating electricity with Yi Cui’s new battery.

Writing about this news, Louis Bergeron adds this perspective: “By using nanotechnology, the battery employs the difference in salinity between fresh and saltwater to generate a current. A power station might be built wherever a river flows into the ocean.”

The battery that Cui’s team has developed relies on the difference in salinity between freshwater and seawater in order to produce electricity. “Anywhere freshwater enters the sea, such as river mouths or estuaries, could be potential sites for a power plant using such a battery.” says Cui, speculating on the possibilities.

Bergeron adds: “A power plant operating with 50 cubic meters of freshwater per second could produce up to 100 megawatts of power, according to the team’s calculations. That would be enough to provide electricity for about 100,000 households.”

In the press announcement, Cui said the theoretical limiting factor is the amount of freshwater available. Seawater is abundant, fresh water isn’t.

The battery is simple, made with positive and negative electrodes that are immersed in a liquid containing electrically charged ions. In water, the ions are sodium and chlorine – the basics in ordinary table salt.

When the battery is filled with freshwater, a small electric current is applied to charge it. The freshwater is then drained and replaced with seawater. Because seawater is salty, containing 60 to 100 times more ions than fresh water, it increases the voltage, between the two electrodes, makes it possible to harvest more electricity than the amount that was used to charge the battery.

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

Of note, Cui believes river water isn’t the only fresh water source available. Storm runoff water and grey water can also be used. This sounds like another good fit for the purposes of sustainability.