New entropy battery pulls energy from difference in salinity between fresh water and seawater
March 25, 2011 by Bob Yirka 
Image credit: ACS
(PhysOrg.com) -- A team of researchers, led by Dr. Yi Cui, of Stanford and Dr. Bruce Logan from Penn State University have succeeded in developing an entropy battery that pulls energy from the imbalance of salinity in fresh water and seawater. Their paper, published in Nano Letters, describes a deceptively simple process whereby an entropy battery is used to capture the energy that is naturally released when river water flows into the sea.
Up to now, this kind of process has been accomplished by passing seawater though a membrane which unfortunately is too costly to merit creating large-scale operations.
The new process works like this:
Step 1 - Two types of nanorod electrodes are placed in river water; one silver anionic electrode contains Cl- ions and one manganese dioxide cationic electrode contains Na+ ions. The battery charges as the river water’s low salinity concentration of salt pulls the chorine and the sodium from the respective electrodes.
Step 2 - The river water is slowly replaced with seawater, causing a potential difference between the two concentrations of ions in the combined water. This is due to the Cl- ions, or anions, traveling to the silver electrode and the Na+ sodium ions, or cations, traveling to the manganese dioxide electrode.
Step 3 - Ions in the electrodes discharge into the seawater when the electrodes receive more ions than they can accommodate.
Step 4 - The salt water is slowly replaced with river water. This lessens the potential difference of the two electrodes which charges the battery. More energy was released in Step3 into the saltwater than is needed to charge the battery, thus the battery collects and stores the energy that has been building up as the ions have been moving in and out of the crystal lattice of the electrodes.
With the entropy battery, costs are much lower than other ways of accomplishing the same thing due to the absence of replaceable membranes.
Cui believes that the entropy battery might eventually contribute up to 13% of total energy needs. He also believes that by moving the two electrodes closer, he might be able to improve his efficiency rate from 74% percent to 85%.
Because the entropy battery operates in both warm and cold conditions it is a completely renewable resource; one that might lead to mass energy production in both developed countries and those in the third world.
More information: Batteries for Efficient Energy Extraction from a Water Salinity Difference, by Fabio La Mantia et al., Nano Lett., Article ASAP, Publication Date (Web): March 17, 2011. http://pubs.acs.or … 21/nl200500s
© 2010 PhysOrg.com
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Mar 25, 2011
Rank: 1.7 / 5 (3)
What kind of cycle time are we talking about here? In other words, what is the charge/discharge time? How much water?
Mar 25, 2011
Rank: 1 / 5 (2)
If so, the salt water is continuously contaminating the fresh water.
Seems like a bad idea to use up fresh water to make electricity.
Mar 25, 2011
Rank: 5 / 5 (1)
Mar 25, 2011
Rank: 5 / 5 (3)
1 - we do it all the time. biofuels require a LOT of freshwater, which is just discharged into rivers, which goes to the ocean.
2 - they would be using river water which is going to mix with the ocean anyway. No fresh water is being wasted that wasn't already being wasted.
no, the output water would be the same as if the river mixed naturally with the ocean.
The stumbling blocks for technologies in this area are cost and reliability. The membrane method produces electricity, but is too costly to be of practical use. These guys will have to find a way to make this cheap (relatively) and robust. Easier said than done.
Mar 25, 2011
Rank: not rated yet
Mar 25, 2011
Rank: 5 / 5 (2)
Based on supporting information, freely available here:
http://pubs.acs.o..._001.pdf
They cite 49 mJ per cm^2 of electrode (Fig S8.) That's about 0.0000136 Wh. So to get 1 Wh, you'd need about 7.36 square meters of electrode surface.Per Fig S7, full charge/discharge cycle takes about 5 hours.
Doubt this can be very cost-effective, particularly considering they use silver in one of the electrodes...
Mar 25, 2011
Rank: 5 / 5 (1)
Mar 26, 2011
Rank: 5 / 5 (1)
So where a river flows into the sea, there is a lot of energy potentially available.
However as PinkElephant points out, this process needs a huge electrode area. At 0.45V they aceived a cycle time of 1.25 hours and 41 mJ/cm2. That's only 410 J/m2 in 4500 seconds, or roughly ten square meters of electrode per Watt.
So they need several orders of magnitude improvement in power density to have any hope of this being practical.
Creative idea, though.
Mar 26, 2011
Rank: 1 / 5 (1)
If the efficiency is already 65%, and they are hoping for 80%, several orders of magnitude improvement is impossible. Remember we are extracting the ENTROPY, which is orders of magnitude smaller than the energy differences (usually).
Mar 26, 2011
Rank: 5 / 5 (1)
What I don't like is the surface area needed to extract serious watts out of it, you probably would need nanostamping on a roll to roll print process to make costeffective sheets and panels and finding a cheaper nanocomposite alloy for the silver would be nice
Mar 26, 2011
Rank: 4.7 / 5 (7)
luckily elektrodes can be made with a huge surface compared to their size, much like a sponge
Mar 26, 2011
Rank: 5 / 5 (1)
Mar 26, 2011
Rank: not rated yet
The stumbling-blocks seem to be basic without need for extensive research.
I hope to see more work on this.
Mar 26, 2011
Rank: 1 / 5 (1)
How about migrating fish etc?
Mar 26, 2011
Rank: not rated yet
It is not the thermodynamic efficiency which needs massive improvement - even 65% is impressive. As stated within the text you quoted, what needs orders of magnitude improvement is the power density - the energy per membrane area per unit time.
Delemming: Yes, one can pack a huge area into a small volume.
However from the photographs in the link PinkElephant sent, it looks like the researches have already exploited this to a considerable exent. And the more one does this, the more silt, bacteria, etc. tend to clog the system in real-world use. So getting the areal power density way up is a key step in making this practical.
Mar 26, 2011
Rank: 5 / 5 (1)
Not necessary. Most shipping will already pass through a system of designated channels. This system could simply be built on the margins. You wouldn't be able to process the whole volume of water, but then you weren't going to be able to do that anyway.
Mar 26, 2011
Rank: 3.3 / 5 (3)
Mar 26, 2011
Rank: 5 / 5 (1)
But space-based energy is not an immediate solution (launch costs too high and non-trivial to bring energy back to earth), so there is plenty of need for other scientists to fill in gaps here at home.
And why look to the moon - everything there is stuck in a gravity well. Asteroids and comets are better sources of materials. We have to stop seeing NEOs are problems and view each as 'a gift from the gods'.
Mar 26, 2011
Rank: 1 / 5 (1)
But best is to put up a space elevator, which should be possible in about twenty years, and use it to move electrical generation and (long-distance) transmission to space.
As for the idea here, it would be used in tidal estuaries. A much better example than the Mississippi is the Chesapeake Bay, a huge area where the salinity changes with the tides. An installation covering a few square miles near the Bridge/Tunnel would be relatively small. Of course, to keep costs down, you would need the electrodes to pack about 1 kW/square meter of Bay surface area. Could be done...
Mar 26, 2011
Rank: 1 / 5 (1)
Mar 26, 2011
Rank: not rated yet
However, until we get the space elevators going (I'm sure there will be at least three.) there is a need for concentrated power generation, which means coal or nuclear. Don't let the news media coverage of the nuclear "crisis" in Japan mislead you. Chernobyl--which this is no where near as bad as killed fewer than a thousand people. The tsunami/earthquake death toll will be around 30,000. Three Mile Island about duplicated the Wash 1400 "maximum credible accident" in the US. The report figured that the top cause of deaths from such a failure would be either due to air pollution from the coal plants that would replace the power--or grade crossing accidents involving coal trains. :-(
Mar 27, 2011
Rank: not rated yet
Mar 27, 2011
Rank: not rated yet
If I remember correctly from working it out a few decades ago, with piano wire a space elevator cable would be something like 10^23 times the cross-sectional area at the geostationary height as at the bottom, with the best glass fibers then available it would take 10^6 times more area at the geostationary height, but with perfect graphite sheet (what would now be called graphene) it would take less than 4x the area at the geostationary height - that's less than twice the diameter!
The biggest hurdle will be using microwaves to bring power to earth. The accuracy scales nicely with size, but the costs are still unproven and there may well be public resistance, even though the energy densities being mooted are lower than sunlight.
Mar 28, 2011
Rank: 5 / 5 (1)
While I agree with you that we should think bigger, it is also important to realize that eventually we will run out of resources. Even if we learn to colonize the stars, we will still run out of stars to colonize eventually. What then?
We won't run away from this problem, its important to realize that our environment will one day reach 'carrying capacity'. Thats when true, systematic progress will begin, perhaps this is when we will truly build upwards. Skyscrapers are erected because we have run out of room. Think of the figurative skyscrapers that will be built when we run out of energy for instance this!! This is a fine piece of the sustainable 'solution', pun intended. ;o)
Mar 30, 2011
Rank: not rated yet
Apr 02, 2011
Rank: not rated yet
That's wrong.
It's 41mJ
This particular part of the paper is poorly written, IMO, and it isn't exactly clear how often this 41mJ is produced.
The article here claims they think this could potentially provide up to 13% of human energy needs, but if the PAPER is saying just 41mJ per cm^2 per 40 minutes, that would require probably coating the entire planet in these electrodes...
On the other hand, if that is just a misunderstanding, and instead it should be 41milliwatts per cm^2, then that would be 410watts per meter square, which would be better than any marketable solar panel I'm aware of...
Apr 15, 2011
Rank: not rated yet