New entropy battery pulls energy from difference in salinity between fresh water and seawater

Mar 25, 2011 by Bob Yirka report
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 are placed in river water; one silver anionic electrode contains Cl- 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 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 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 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 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.

Explore further: Carbyne morphs when stretched: Calculations show carbon-atom chain would go metal to semiconductor

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. pubs.acs.org/doi/abs/10.1021/nl200500s

Related Stories

Renewable Energy Made by Mixing Salt and Fresh Water

Sep 02, 2009

(PhysOrg.com) -- When a river flows into the sea, the location is more than just a haven for water commerce. The mixing of fresh and salt water that occurs at an estuary also dissipates energy, as the different ...

Wastewater produces electricity and desalinates water

Aug 06, 2009

A process that cleans wastewater and generates electricity can also remove 90 percent of salt from brackish water or seawater, according to an international team of researchers from China and the U.S.

Increasing Electric Car Battery Performance

Sep 23, 2009

(PhysOrg.com) -- Researchers have found that by replacing conventional graphite electrodes with silicon nanotube electrodes, lithium-ion batteries can store 10 times more charge.

Recommended for you

An anti-glare, anti-reflective display for mobile devices?

Jul 16, 2014

If you've ever tried to watch a video on a tablet on a sunny day, you know you have to tilt it at just the right angle to get rid of glare or invest in a special filter. But now scientists are reporting in the journal ACS Ap ...

New materials for future green tech devices

Jul 15, 2014

From your hot car to your warm laptop, every machine and device in your life wastes a lot of energy through the loss of heat. But thermoelectric devices, which convert heat to electricity and vice versa, ...

User comments : 27

Adjust slider to filter visible comments by rank

Display comments: newest first

Parsec
1.7 / 5 (3) Mar 25, 2011
So... the power required to pump the water from the high to low salinity sources comes from where exactly? 85% of what, the entropy differences? I wonder what that would be in terms of watt-hours.

What kind of cycle time are we talking about here? In other words, what is the charge/discharge time? How much water?
tpb
1 / 5 (2) Mar 25, 2011
It appears from the text that the fresh river water and salt water are mixed with the ratio of salt to river water slowly changed back and forth.
If so, the salt water is continuously contaminating the fresh water.
Seems like a bad idea to use up fresh water to make electricity.
Sonhouse
5 / 5 (1) Mar 25, 2011
It appears from the text that the fresh river water and salt water are mixed with the ratio of salt to river water slowly changed back and forth.
If so, the salt water is continuously contaminating the fresh water.

It would use water from major rivers like the Mississippi where the fresh water of the river is going into the salt water anyway.

My question would be, does the salinity of the salty body increase as a result of this conversion?
Seems like a bad idea to use up fresh water to make electricity.

fmfbrestel
5 / 5 (3) Mar 25, 2011

Seems like a bad idea to use up fresh water to make electricity.


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.

My question would be, does the salinity of the salty body increase as a result of this conversion?


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.
fmfbrestel
not rated yet Mar 25, 2011
I don't mean to sound like this techs cheer leader, but it is important to be skeptical for the right reasons.
PinkElephant
5 / 5 (2) Mar 25, 2011
@Parsec,

Based on supporting information, freely available here:

http://pubs.acs.o..._001.pdf

I wonder what that would be in terms of watt-hours.
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.
What kind of cycle time are we talking about here?
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...
Caliban
5 / 5 (1) Mar 25, 2011
From my reading of the article, it sounds as if they mean that this technology is to be deployed in intertidal zones at river mouths. Not a big problem in the case of non-navigable rivers, but seems like it would be kind of dicey in the case of a Mississippi or other major river. Although I suppose it could be deployed in some kind of diversion channel.
RealScience
5 / 5 (1) Mar 26, 2011
The energy of mixing between fresh water and average ocean water is the equivalent of a 600-foot water fall.
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.
Parsec
1 / 5 (1) Mar 26, 2011
The energy of mixing between fresh water and average ocean water is the equivalent of a 600-foot water fall.
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.

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).
Husky
5 / 5 (1) Mar 26, 2011
What i like is that it has no need for biofaulable membranes, also i would expect since silverions are used in antimicrobiological sprays, the silver electrode would keep relatively clean as well, i wouldn't know about the manganese.

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
delemming
4.7 / 5 (7) Mar 26, 2011
That's only 410 J/m2 in 4500 seconds, or roughly ten square meters of electrode per Watt.


luckily elektrodes can be made with a huge surface compared to their size, much like a sponge
TJ_alberta
5 / 5 (1) Mar 26, 2011
right on delemming. the specific surface area is what is important here.
JamesThomas
not rated yet Mar 26, 2011
This could turn out to be a very inexpensive and practical way of producing energy.
The stumbling-blocks seem to be basic without need for extensive research.
I hope to see more work on this.
kaasinees
1 / 5 (1) Mar 26, 2011
So we put a huge sponge where the river meets the ocean?
How about migrating fish etc?
RealScience
not rated yet Mar 26, 2011
Parsec - The 600-foot waterfall equivalent is the energy of mixing, which is indeed the entropy difference.
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.
jscroft
5 / 5 (1) Mar 26, 2011
From my reading of the article, it sounds as if they mean that this technology is to be deployed in intertidal zones at river mouths. Not a big problem in the case of non-navigable rivers, but seems like it would be kind of dicey in the case of a Mississippi or other major river. Although I suppose it could be deployed in some kind of diversion channel.


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.
hevans1944
3.3 / 5 (3) Mar 26, 2011
It is so depressing to see real scientists grasping at such small energy straws. Are we to believe that at the very best this "technology" could provide only 13% of current total world energy needs? We need science that provides 10,000% and more of current world energy needs. How else will the human race ever fulfil the dreams of the Star Trek generation? Look outward to space. There is virtually unlimited energy out there. Go get it! Bring it back to Earth! The Moon is a good place to start in our search for new energy sources. Or have we stopped dreaming and are now content to just share the leftovers? Two terawatts? Think bigger. Much bigger.
RealScience
5 / 5 (1) Mar 26, 2011
hevans - many real scientists do indeed think much bigger, and space does indeed have almost unlimited energy and it is easier to gather there than on earth (especially if the energy is needed as heat).

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'.
eachus
1 / 5 (1) Mar 26, 2011
Lol! First on space energy. Solar arrays (with concentrating mirrors) in orbit are very practical, as is microwave transmission to Earth. Assuming you put your solar collector in geosynchronous (or higher), you can target different microwave receivers on Earth to redistribute the power, for example between Europe and Africa, or the US East and West coasts.

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...
kaasinees
1 / 5 (1) Mar 26, 2011
eachus, stop with the scientology.
eachus
1 / 5 (1) Mar 26, 2011
Think of 10,000 sheets of back to back electrodes. You would need some way to keep sea water from shorting the around the edges of the planes. That's an engineering/design issue. Since you would want a passive design to feed the sea water and fresh water through, but could be done.

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. :-(
zuggerjack
not rated yet Mar 27, 2011
This promises to be a highly viable alternative energy source that will supplement space solar power (as described in David Kagan's book Sunstroke), wind and ocean current power generation to help wean our planet from fossil fuels and the uranium atom. Great story!
RealScience
not rated yet Mar 27, 2011
eachus - Agreed on space elevators - that's why I put the 'immediate' in my reply to hevans that space energy is not an immediate solution due in part to launch costs.

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.

antonima
5 / 5 (1) Mar 28, 2011
We need science that provides 10,000% and more of current world energy needs. How else will the human race ever fulfil the dreams of the Star Trek generation? ... Think bigger. Much bigger.


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)
saltydog
not rated yet Mar 30, 2011
Would a higher level of salinity change the efficiency of this process? Example would be the Salton Sea in California or the Salt Lake in Utah?
Quantum_Conundrum
not rated yet Apr 02, 2011
I wonder what that would be in terms of watt-hours.
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.

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...
hevans1944
not rated yet Apr 15, 2011
Space-based energy is not an immediate solution, unless you count solar cell arrays on Earth harvesting energy from our only working thermonuclear power plant... the Sun. Exploitation of space is ultimately the ONLY solution that is sustainable. As for the gravity well of the Moon, it is much shallower than that of Earth. Once you have a self-sustaining base colony on the Moon, with families living there and calling it home, you can with relative ease export energy back to Earth and then move further into space to retrieve other resources, including minerals and energy. My point is you have to sustain this effort now, while we still have the energy resources to bootstrap ourselves into space, and plan for the long term. Our "one trick pony" stunt of landing a few people on the Moon in the last century was just that... a stunt. If we aren't "out there" before the beginning of the 22nd Century our human civilization will become extinct, drowned in its own excrement and apathy.