Cutting Edge Energy from Estuaries?

Katy Yan
The Amazon River meets the sea - potential site for future non-dam salinity-based hydro?
The Amazon River meets the sea - potential site for future non-dam salinity-based hydro?
Norman Kuring/NASA

Battery-operated electric vehicles are cutting CO2 emissions and raising awareness of our transportation carbon footprint. Recent cutting edge research now shows that batteries can also be harnessed in rivers and estuaries as a non-dam and hopefully low-impact form of electricity generation.

According to Stanford University, a team lead by Yi Cui (Associate Professor of Materials Science and Engineering), has developed a battery that takes advantage of the difference in salinity between freshwater and seawater to produce electricity.

The technology

Battery in freshwater/saltwater cycle
Battery in freshwater/saltwater cycle
Yi Cui et al, 2011

The battery is simple, consisting of a positive and negative electrode immersed in water containing electrically charged particles, or ions – in this case, sodium and chlorine (i.e. table salt).

  • Step 1: A small electric current charges the battery, pulling ions out of the electrodes and into the water.
  • Step 2: The fresh water is replaced with seawater, thereby increasing the amount of charged ions. Salty seawater contains 60 to 100 times more ions than freshwater.
  • Step 3: The salt water increases the electrical potential, or voltage, between the two electrodes, allowing the battery to generate far more electricity than the amount used to charge it. Electricity is then drawn from the battery for use, draining the battery of its stored energy.
  • Step 4: Seawater is discharged and replaced with river water, and the cycle starts again. 

The impacts and limitations

According to the Stanford article, the potential environmental impact of the battery should be low. They recognize that river mouths and estuaries are environmentally sensitive areas. They chose manganese dioxide for the positive electrode, in part because it is environmentally benign. 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.

That still leaves the questions of how these sites would be chosen, how much water would need to be diverted to make this system cost-effective, and what impacts there might be if these power plants were scaled up. Another concern is what to use for the negative electrode. Yi Cui's team used silver for their experiments with water from the Pacific Ocean and Donner Lake, but silver is expensive. That leads one to wonder of how battery leaks could be prevented, or how they could be safely disposed.

The potential

Despite the risks, these batteries could prove to be highly efficient and versatile. In the lab, they achieved 74 percent efficiency in converting the potential energy in the battery to electrical current, but Cui thinks the battery could achieve 85 percent efficiency.

His group also did an estimate for various regions and countries and determined that the Amazon River has the most potential, followed by Africa, Canada, the US, and India. On the global level, Cui's team calculated that if all the world's rivers were put to use, their batteries could supply about 13 percent of the world's current energy consumption. This would only be true if rivers continue to flow to the sea (and in many places, thanks to large dams, this may be increasingly unlikely).

San Francisco at night
San Francisco at night

One exciting application is urban use. According to Yi Cui, these batteries could be used to generate electricity from storm-water runoff and grey water, and perhaps even treated sewage water. A power plant operating with 50 cubic meters of freshwater per second in a city could provide enough electricity for about 100,000 households. That's enough for a third of the households in San Francisco, which is quite a feat.

While not enough to power entire cities, cutting edge research such as this could prove beneficial for diversifying a city or town's energy mix. If designed to be cheap, easily operable, low-impact, and transportable, it could also prove a boon for coastal communities far from an energy grid.