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Novel battery to extract and store energy from the salinity difference between seawater and river water could make significant contribution to renewable energy

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.

(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. Typical form of a cycle of battery cell voltage (ΔE) vs charge (q) in a mixing entropy battery, demonstrating the extractable energy.(b) Credit: ACS, La Mantia et al. Click to enlarge.

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:

  1. 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.

  2. 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.

  3. 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.

  4. 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



This sounds very promising! Maybe this could be developed in California so those poor farmers & other folks living in the central valley aren't forced to deplete their aquifers! We need to help those people get more freshwater!


Do I understand correctly, that this process could be the basis of a continuous solar power-plant?

The boiling/precipitation cycle could be emulated in an artificial environment (a power plant).

I would think that one can convert seawater into freshwater by boiling it with concentrated solar rays and move the steam to a cooler place to make it precipitate quickly (a large underwater water reservoir). Possibly precipitation could be done by forcing steam through the water reservoir (though the energy need for this may be too high so a slower method may be more effective).

By this salt/fresh conversion, the process could be closed loop, only the solar energy would come from the outside.

If one can store huge amount of fresh/salt water (separated from each other) near the power plant, then the power plant could serve power after the night (only the freshwater supply would decrease and the seawater supply increase until boiling can be restarted in the morning by the solar concentrator arrays).

Based on the article, 1 m3 water could store ~0,6Kwh energy (2200 KJoule). One of the new water reservoirs here in Hungary has a capacity of ~94 million m3, so in theory, this could hold 56,4 Gwh of energy.

If we take only 30% efficiency for the process (steam pumping...etc) and we take only half of the size, we are still talking about 8,4 Gwh which is a huge amount of energy.

Naturally, a much-much smaller reservoir would be enough for a smaller powerplant.

The Desertec project may just have found the ideal process for their power plants.


I mean "large underground water reservoir".


Cute info.

Frankly, consumers are more interested actual batteries available from prior years of ANNOUNCED breakthroughs: 10X better

will manufacture batteries up to 1,000 times more powerful, 10 times longer-lasting and cheaper than traditional batteries Lithium Ion Battery breakthrough promises 100-fold boost in performance

EEstor, ect, ect.......


This could be done in the Sacramento river delta in California. Much of the fresh water goes out to sea and there is an interface area where the two mix. I would suppose that you could have canal "locks" that would do this without disturbing the fish.


I seem to remember hearing about this idea before - about 40 years ago!



Funny enough, I was googling furiously just a couple weeks ago to find a "salt-water battery" to be used in a method just as you described. Basically, I was looking for a heat engine of sorts that produced electricity based on salinity differences. Unfortunately, my finding was that such a system would have poor power density.


Haven't dissimilar PH levels always yielded a voltage?


This is not new. This idea was proposed some time ago for installation across the Afsluitdijk in The Netherlands which has the sweet water IJsselmeer on one side and the salty Waddenzee on the other.

To call this a battery is sort of misleading I guess. It simply produces energy in a manner similar to a battery, bu you never have to charge it.



Yes, both the power density and the energy density is quite low, so this is absolutely for stationary applications.

However, this can work extremely-large-scale while a lot of battery/fuel-cell technologies seem to have problem with scaling.


yes, theoretically there is potential for energy capture here but the practicalities of building such a 'device' are substantial.
Estuaries, where fresh and salt water meet are environmentally sensitive areas.


Yeah, this pipedream gets pushed about every five years by some new numbnut looking for a university/government grant. It works and looks all cute in the lab, but once you get in out in the field, it simply doesn't work. The size of the system requires such large scales of land, availability to seawater, and sources of fresh water, it simply can't be done.
Take the land area, throw down a solar array.
Take the sea area, put in the new clamshell technology.
Take the fresh water, and hold it between your knees.
...i love saying that...


"Before major dams were built on rivers in the Delta watershed, the salinity interface migrated as far upstream as Courtland along the Sacramento River"

It sounds like salinity intruding too far upstream is not good, so these kinds of barriers could have multiple benefits.


They already have salinity control gates, so this would just be an additional source of power.


Reminds me of wave power.


This may not be practical at all, but then again who knows? Done on a large scale with a low enough cost per watt, low maintenance and longevity then perhaps.

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