|Estimated extractable power (GW) from the difference of salinity in different countries, based on flow of river water emptying into the ocean. Credit: ACS, La Mantia et al. Click to enlarge.|
A team from Stanford led by Dr. Yi Cui, and Dr. Bruce Logan from Penn State University have developed a device they call a “mixing entropy battery” which can extract energy from the salinity difference between seawater and river water and store it as useful electrochemical energy.
In a paper in the ACS journal Nano Letters they report demonstrating energy extraction efficiencies of up to 74%. Considering the flow rate of river water into oceans as the limiting factor, renewable energy production using such mixing energy batteries could potentially reach 2 TW, or ~13% of the current world energy consumption, they conclude. The mixing entropy battery is simple to fabricate and could contribute significantly to renewable energy in the future, they suggest.
The large scale chemical energy stored as the salinity difference between seawater and freshwater is another renewable source which can be harvested. The major components of the global water cycle involve distillation of water from oceans by evaporation, precipitation, and collection of this freshwater in rivers, lakes, and aquifers, with mixing of freshwater and salt water in estuaries. Solar energy drives this cycle, creating a significant salinity difference between seawater and freshwater. The entropic energy created by the difference in water salinities is normally dissipated when river water flows into the sea. This reduction in free energy due to the mixing is estimated at 2.2 kJ of free energy per liter of freshwater.
To date this significant and completely renewable energy source has not been fully harnessed, although since Pattle’s pioneering studies in 1954 several types of technologies have been proposed in order to take advantage of this renewable energy source...In this work, we demonstrate a novel electrochemical cell named a “mixing entropy battery”, which extracts energy from the difference in concentration of two solutions and stores it as chemical energy inside the electrode material’s bulk crystal structure.—La Mantia et al.
The mixing entropy battery is a reversible electrochemical system where the salts in the electrolyte are the reactants and the electrode stores ions. The team used two different electrodes: an anionic electrode, which interacts with Cl- ions selectively; and a cationic electrode, which interacts with Na+ ions selectively. Elements of the working principle include:
Charge in river water. The electrodes are initially submerged in river water (a low ionic strength solution) in their discharged states, when the electrode materials contain the respective ions incorporated in their structures. In this dilute solution, the battery is charged by removing the Na+ and Cl- ions from the respective electrodes.
Exchange to seawater. The dilute electrolyte is then exchanged for a concentrated solution (seawater), which is accompanied by an increase in the potential difference between the electrodes.
Discharge in seawater. At the higher potential difference from step 2, the battery is discharged as the anions and cations are reincorporated into their respective electrodes.
Exchange to river water. The concentrated solution is then removed and replaced by the dilute electrolyte (river water), resulting in a decrease in potential difference between the electrodes.
The researchers note that the exchange of solution could also be carried out by a flow process, which “could be attractive for large scale energy production.”
No energy is produced or consumed during steps 2 and 4. During step 1, the battery requires energy to drive the ions out of the crystal structure, while during step 3 the battery produces energy by incorporation of the ions; the energy gain is due to the fact that the same amount of charge is released in step 3 at a higher voltage than consumed in step 1.
They also note that the energy can be easily harvested at low temperatures, and is completely renewable, since the ultimate source is the solar energy which powers the water cycle.
The mixing entropy battery and other processes described here represent not only a novel type of electrochemical system compared to existing technologies but also a very promising technology whose practical application can make a significant contribution to the field of renewable energy production. This process for generating electrical energy could also be reversed, and exploited as a method for water desalination. Future work will focus on cell geometry optimization and the development of materials which can be used to replace silver as the anionic electrode.—La Mantia et al.
Fabio La Mantia, Mauro Pasta, Heather D. Deshazer, Bruce E. Logan, Yi Cui (2011) Batteries for Efficient Energy Extraction from a Water Salinity Difference. Nano Letters doi: 10.1021/nl200500s