The salinity gradient when fresh and saltwater meet has been harvested for a new form of renewable energy, albeit on a tiny scale. This could become a clean electricity source that is far more stable than those dependent on the wind and Sun, potentially offering a reliable source of energy in coastal locations.
Dr Kristian Dubrawski of Stanford University is attempting to harness the electric potential of mixing salt and freshwater and use it to power coastal wastewater treatment plants. He built a prototype bank of batteries with Prussian Blue and polypyrrole electrodes and alternately bathed them in salt and freshwater for an hour at a time. To ensure the test was realistic, Dubrawski used seawater collected from Half Moon Bay, California, and the nearly fresh outflow from the Palo Alto Regional Water Quality Control Plant.
In ACS Omega, Dubrawski describes how sodium and chloride ions passed into the battery's electrodes during the saltwater phase, carrying charge with them. When the wastewater replaced the salt, the ions left the electrodes, carrying charge in the opposite direction.
Aside from gates switching which water would flow over the battery, no input energy was required. Electricity was produced throughout the process, although the power available for external use was highest at the start of the saltwater phase.
The head of Dubrawski's Lab, Professor Craig Criddle, sees the technology as an opportunity to provide water treatment plants with a reliable source of clean power, freeing them from the dangers posed if grid power is cut off, such as happened during California's wildfires.
Theoretically, it could be scaled up to use the vast outflows from major rivers. A previous study estimated global estuarine potential as 2 Terrawatts, not far below world electricity consumption, although it is unlikely we will ever tap more than a tiny proportion of this. The output of the world's waste treatment plants alone could theoretically power Argentina.
With perfect efficiency, it is possible to produce 650 watt hours from a cubic meter (220 gallons) each of fresh and seawater – a half-fresh/half-salty backyard swimming pool could power an average American house for a day. Dubrawski's test model achieved two-thirds efficiency, although he acknowledges this may not hold at larger scales.
Others have noticed the potential available from the meeting of fresh and salty water, and many prototypes have been built to capture it. However, these have used prohibitively expensive technologies like membranes or hydrogels that expand or contract depending on their environment.
"Our battery is a major step toward practically capturing that energy without membranes, moving parts or energy input,” Dubrawski said in a statement. Unlike previous efforts, the components are all made from cheap and widely available materials.