(a) Schematic representation of the working principle behind a complete cycle of the mixing entropy battery, showing how energy extraction can be accomplished: step 1,
charge in river water; step 2, exchange to seawater; step 3, discharge in seawater; step 4, exchange to river water. (b) Typical form of a cycle of battery cell voltage (?E) vs
charge (q) in a mixing entropy battery, demonstrating the extractable energy.
Estuary Battery Employs Salinity Difference
In coastal estuaries where fresh water rivers meet saltwater seas, the difference in salinity can represent about a kilowatt of free, renewable energy for every liter of water.
Extracting that energy efficiently to create a useful form of energy has been a challenge, but scientists in Standford University's Department of Materials Science and Engineering
developed a battery that taps into that electrochemical energy. The team says their "mixing entropy battery" could potentially supply 13 percent of the world's energy needs.
The battery itself is based on a simple concept that consists of one positive and one negative electrode.
River water and sea water are alternately flushed through the battery; both kinds of water contain charged particles (ions), but seawater contains 60 to 100 times more ions than
freshwater. Therefore, when freshwater and its ions are flushed out and replaced with seawater, the battery produces a charge. The team of scientists estimates that a power plant built near an estuary could potentially produce up to 100 megawatts. The process for generating electrical energy can also be reversed to remove salt from sea water to produce drinking water. The water for this
method does not have to be extremely clean and could use stormwater runoff, graywater or even sewer water. This particular battery is simple to fabricate, and the team is modifying it
for commercial use. To enhance efficiency, the positive electrode of the battery is made from nanorods of manganese dioxide, which 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 and providing the capability to be applied to a variety of salt waters. The team has demonstrated energy extraction efficiencies of up to 74 percent.
Considering the flow rate of river water into oceans as the limiting factor, the renewable energy production could potentially reach 2 terrawatts, or approximately 13 percent of the current world energy consumption.
Researchers at Stanford University demonstrated a device called a "mixing entropy battery" that uses the salinity difference between seawater and fresh river water to extract