At Stanford University researchers have developed a “rechargeable battery” that uses freshwater and seawater to create electricity. Aided by 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.

Salt and Fresh Water Lab Model Cycling Generator. Click image for more info.

Yi Cui, Associate Professor of Materials Science and Engineering, who led the research team said, “We actually have an infinite amount of ocean water; unfortunately we don’t have an infinite amount of freshwater.”  Anywhere freshwater enters the sea, such as river mouths or estuaries, could be potential sites for a power plant using such a battery.

Cui’s team calculated that if all the world’s rivers were put to use, the Stanford batteries could supply about 2 terawatts of electricity annually – that’s roughly 13 percent of the world’s current energy consumption.

Except, it’s not a battery – it just uses the chemistry principles common to chemical batteries as a cyclic generator.

The cyclic generator 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.

Saline and Fresh Water Cycling Electric Generation. Click image for more info.

To start the cyclic generator is filled with freshwater and a small electric current is applied to “charge it up” by driving out the sodium and chlorine ions. 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.

Cui explains, “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 seawater that means you gain energy. You get more energy than you put in.”

Once the electrodes are saturated with ions the discharge is complete.  Then 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.

Cui is lead author of a study published in the journal Nano Letters in March; it’s a worthwhile read.

Cui points out that, “It is the opposite process of water desalination, where you put in energy and try to generate freshwater and more concentrated saltwater. Here you start with freshwater and concentrated saltwater, and then you generate energy.”

Cui’s team converted to electricity 74% of the potential energy that exists between saltwater and freshwater, with no decline in performance over 100 cycles. Placing the electrodes closer together, Cui says, could allow the battery to achieve 85% efficiency.

How far does the math take this idea?  Cui says a power plant using this technology would be based near a river delta where freshwater meets the sea. Drawing at 50 cubic meters of river water per second, a power plant could produce up to 100 megawatts of power. He calculates that if all of the freshwater from all of the world’s coastal rivers were harnessed, the salinity-gradient process could generate 2 terawatts, or approximately 13 percent of the energy currently used around the world.

In their lab experiments, Cui’s team used seawater they collected from the Pacific Ocean off the California coast and freshwater from Donner Lake, high in the Sierra Nevada. That sourcing achieved the 74% efficiency in converting the potential energy in the battery to electrical current, but Cui thinks with simple modifications, the battery could be 85 percent efficient.

The Stanford team enhances efficiency of the positive electrode by making them 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 with ease, speeding up the process.

Other researchers, most notably in Norway where a the world’s first osmotic power plant is in operation at Tofte, outside Oslo use the salinity contrast between freshwater and seawater to produce electricity, but those processes typically require ions to move through a membrane to generate current.

Cui said those membranes tend to be fragile, which is a drawback. Those methods also typically make use of only one type of ion, while his battery uses both the sodium and chlorine ions to generate power.

Using river water has already set off the environmentalists.  But, Cui’s team had the potential environmental impact of their battery in mind when they designed it. The process itself should have little environmental impact. The discharge water would be a mixture of fresh and seawater, released into an area where the two waters are already mixing, at the natural temperature.  The group knows that river mouths and estuaries, while logical sites for their power plants, are environmentally sensitive areas. They 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 said. “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,” he said.

The positive electrode seems to be developed, leaving Cui’s last concern to finding a good material for the negative electrode. He used silver for the experiments, but silver is too expensive to be practical.  Perhaps the team will push that 85% higher.

Cui is also noting that storm runoff and other sources not pristine will do saying, “The water for this method does not have to be extremely clean.”

While water is a matter of concern, primarily having enough in arid areas and clean enough everywhere, using water just as it would re-enter the oceans is a grand idea.  What returns from a cycling are ions from sodium and chlorine in a weak solution that will quickly return to ocean salt as it mixes with the seawater.  No harm if engineered and executed carefully.

Press on Stanford – the Norwegians are already making power and there is no reason the Stanford cycling generator can’t make a worthwhile addition in the electrical supply at likely a very low cost.


1 Comment so far

  1. David Trahan on May 30, 2011 9:10 PM

    Interesting post. I know lab to commercial can be a long winding road full of potholes but this one sure seems like it could be out there sooner than later.

    Check out They sell a salt water powered ice chest they say is good for 100 hours before changing the anode.

    Isn’t this the same with anodes on ships?

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