When fresh water rivers flow into the sea the concentration difference leads to a change in entropy. Researchers have developed a battery that generates power from this entropy difference. They were able to extract energy with 74 per cent efficiency using manganese dioxide nanorods and silver electrodes.
While entropy-based power generation has been done before but is most reliably done today by separating fresh and seawater with membranes and as ions travel through the membranes they generate currents. The new method extracts energy from the difference in concentration between two solutions by storing it chemically in batteries.
The battery extracts energy through sodium and chlorine ions' movements into and out of the crystal lattice of the electrodes. The battery discharges in seawater as chlorine is taken up by the silver electrode and sodium is taken up by the manganese dioxide electrode. The ions are released when the battery charges in freshwater. Because of the higher ion concentration in seawater, the electrical energy discharged is greater than that needed for the battery to charge in freshwater.
The researchers estimate that if the technology was used in all of the world's rivers, this renewable energy technology would hypothetically generate 2 TW, or approximately 13 per cent of current global consumption.
Batteries for Efficient Energy Extraction from a Water Salinity Difference
Nano Lett., Article ASAP, Publication Date (Web): March 17, 2011
Gopal R. Rao, Ph.D.
Web Science Editor
Materials Research Society (MRS)