From Stanford University: A unique idea that has great potential (pun intended). The only catch is that availability and quality of fresh water is one of the biggest environmental concerns now, way ahead of global warming.
Stanford researchers use river water and salty ocean water to generate electricity
Stanford researchers have developed a battery that takes advantage of the difference in salinity between freshwater and seawater to produce electricity.
Anywhere freshwater enters the sea, such as river mouths or estuaries, could be potential sites for a power plant using such a battery, said Yi Cui, associate professor of materials science and engineering, who led the research team.
The theoretical limiting factor, he said, is the amount of freshwater available. “We actually have an infinite amount of ocean water; unfortunately we don’t have an infinite amount of freshwater,” he said.
As an indicator of the battery’s potential for producing power, Cui’s team calculated that if all the world’s rivers were put to use, their batteries could supply about 2 terawatts of electricity annually – that’s roughly 13 percent of the world’s current energy consumption.
The battery itself is simple, consisting of two electrodes – one positive, one negative – immersed in a liquid containing electrically charged particles, or ions. In water, the ions are sodium and chlorine, the components of ordinary table salt.
Initially, the battery is filled with freshwater and a small electric current is applied to charge it up. The freshwater is then drained and replaced with seawater. Because seawater is salty, containing 60 to 100 times more ions than freshwater, it increases the electrical potential, or voltage, between the two electrodes. That makes it possible to reap far more electricity than the amount used to charge the battery.
“The voltage really depends on the concentration of the sodium and chlorine ions you have,” Cui said. “If you charge at low voltage in freshwater, then discharge at high voltage in sea water, that means you gain energy. You get more energy than you put in.”
Once the discharge is complete, the seawater is drained and replaced with freshwater and the cycle can begin again. “The key thing here is that you need to exchange the electrolyte, the liquid in the battery,” Cui said. He is lead author of a study published in the journal Nano Letters earlier this month.
In their lab experiments, Cui’s team used seawater they collected from the Pacific Ocean off the California coast and freshwater from Donner Lake, high in the Sierra Nevada. They achieved 74 percent efficiency in converting the potential energy in the battery to electrical current, but Cui thinks with simple modifications, the battery could be 85 percent efficient.
To enhance efficiency, the positive electrode of the battery is made from nanorods of manganese dioxide. That increases the surface area available for interaction with the sodium ions by roughly 100 times compared with other materials. The nanorods make it possible for the sodium ions to move in and out of the electrode with ease, speeding up the process.
Other researchers have used the salinity contrast between freshwater and seawater to produce electricity, but those processes typically require ions to move through a membrane to generate current. Cui said those membranes tend to be fragile, which is a drawback. Those methods also typically make use of only one type of ion, while his battery uses both the sodium and chlorine ions to generate power.
Cui’s team had the potential environmental impact of their battery in mind when they designed it. They chose manganese dioxide for the positive electrode in part because it is environmentally benign.
The group knows that river mouths and estuaries, while logical sites for their power plants, are environmentally sensitive areas.
“You would want to pick a site some distance away, miles away, from any critical habitat,” Cui said. “We don’t need to disturb the whole system, we just need to route some of the river water through our system before it reaches the ocean. We are just borrowing and returning it,” he said.
The process itself should have little environmental impact. The discharge water would be a mixture of fresh and seawater, released into an area where the two waters are already mixing, at the natural temperature.
One of Cui’s concerns is finding a good material for the negative electrode. He used silver for the experiments, but silver is too expensive to be practical.
His group did an estimate for various regions and countries and determined that South America, with the Amazon River draining a large part of the continent, has the most potential. Africa also has an abundance of rivers, as do Canada, the United States and India.
But river water doesn’t necessarily have to be the source of the freshwater, Cui said.
“The water for this method does not have to be extremely clean,” he said. Storm runoff and gray water could potentially be useable.
A power plant operating with 50 cubic meters of freshwater per second could produce up to 100 megawatts of power, according to the team’s calculations. That would be enough to provide electricity for about 100,000 households.
Cui said it is possible that even treated sewage water might work.
“I think we need to study using sewage water,” he said. “If we can use sewage water, this will sell really well.”
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Jenne
The roposed system is different to the one described in your link. Your version uses osmosis to generate a pressure difference across a membrane that can be used to drive a turbine. The system described in the main article uses the different properties of fresh and saline water to generate power by electrolysis. They are completely different processes.
We do have an inexhaustible supply of fresh water–as long as it rains.
All of these diffuse low-density power schemes/sources have two things (at least) in common, massive hungers: real estate, and capital. That’s on the input side. On the output side, two more: uncontrollable variability, and minute output. “100,000 homes” with 100 MW sounds impressive, but that’s one hotplate per household. Actual averages are at least 5X higher, and peak loads 7-10X higher (for large samples).
And it would take 10 of those for a GW? Yawn. One half-ar*ed Nat Gas plant would outperform them at a fraction of the cost AND be able to match demand fluctuations. Plus make a moderate contribution to ending the planet’s CO2 famine!
But insane megaprojects that sap the resources of a nation or culture are nothing new. The Pyramids were a fine example, and on a smaller scale so is the Vatican. Or maybe you prefer the Aral Sea irrigation diversions?
For the moment, every wind turbine is a monument to Stupidity Rampant. Unfortunately, that’s the only capital crime in Nature.
“John Marshall says:
March 30, 2011 at 1:57 am
We do have an inexhaustible supply of fresh water–as long as it rains.”
As long as the sun shines
Steinar Midtskogen says:
March 29, 2011 at 1:32 pm
> A working prototype for this kind of power plant has already been build in Norway.
That’s for an osmotic power plant – diffusion across a membrane increases pressure/lifts liquid and then you get energy out be a pressure differential.
The Standford thing is an electrochemical battery, the Norwegian thing is a hydro-electric process.
Doug Jones says:
March 29, 2011 at 5:32 pm
Thanks, your’s is the first comment that begins to explain how this critter works. I don’t have a good sense about how the voltage varies with the ion concentration. I think part of the problem is in the drawing. When the battery is charged in freshwater, the negative terminal pushes the ions into solution and positive ions would cluster around that electrode. When the fresh water is replace with salt water, then I guess the greater number of negative ions would force some to the electrode and force the voltage lower.
I think I’m still missing a lot – help?
George E. Smith says:
March 29, 2011 at 2:37 pm
> let me guess, this works better than sticking a copper wire and an iron nail in a lemon or orange; and way better than an apple; right !! ??
That’s electrochemical and the electrodes get consumed. This is ion exchange and no chemistry. I think.
> Come to think of it, why couldn’t those California Central Valley….
How about Utah’s Great Salt Lake vs spring runoff?
Maybe not a dam, but certainly a large diversion channel at the mouth of a stubstantial river.
I do like the commenters idea of using this on the freshwater discharge from seaside power plants… though wisdom of seaside power plants of a certain sort is questionable.
All the rivers run into the sea,
Yet the sea is not full;
To the place from which the rivers come,
There they return again.
I’m fairly positive that even if this form of power generation were to become economically exploitable, someone would discover a 1/4 inch long fish or a snail that would be endangered by it.
What happens when you charge manganese dioxide a nano-rods matrix in “fresh” water? The positive ions (H+) migrate to it and the voids fill with “fresh” water. The surface of the nano-rods is reduced to manganese oxide, manganese, manganese hydride, or some combination. The combination depends on the potential that is being used to charge this battery. What happens when you fill this charged battery with sea water? The negative ions will be attracted to this electrode. While the voids still contain “fresh” water, O– will react with the surface to form manganese dioxide. After the sea water diffuses into the voids, manganese chloride will form. Both “fresh” water and sea water contain other positive and negative ions in different concentrations that can poison the battery. How long do you expect it to have any kind of efficiency? How would it compare to Ni-CD?
Ric Werme says:
March 30, 2011 at 6:09 am
But Rick, the electrodes DO get consumed in this device by side reactions, and the cathodes by direct result of operation. The clues to the non-success of this device on scale-up are given in the professor’s own words. To paraphrase, “Looking for a better cathode” means, in layman’s terms, that the electrodes, even if silver, become sacrificial to side reactions. I’m sure the MnO2 pellets become sacrificial to side reactions as well. I am sure the first thing he would have done is “look” at all available electrode materials, which knowledge has been available for a hundred years. This is the big bugaboo of all electrode systems.
But these little fellers do not like Ag+ ions, or any heavy metal ion, including Mn, especially Mn(VII). If we come up with another corrosion resistant non-ferrous alloy, like Cr-Mn-Cu alloy, can yo imagine the outcry when Mn(VII) or Cr(VI) ions are detected in the water? By the way, Erin Brockovich became a millionaire over her claims (erroneous) that Cr(VI) caused cancer. These highly oxidized heavy metals also will kill beneficial bacteria and diatoms and, therefore, aquatic animals.
Of course, we could use RO to remove all these from the effluent water afterwards!
As for the osmotic membrane differential processes, the big drawback is also in materials.
For an RO unit to be kept operable, for example a laboratory demineralization unit, it has to be kept relatively sterile. The RO membranes are stored in formaldehyde to prevent bacterial breakdown. Bacteria love to eat polymers! Yum! I am sure the maintenance rate would be a little lower in a cold place like Norway, but those little fellers exploit every environmental niche in no time flat.
“Initially, the battery is filled with freshwater and a small electric current is applied to charge it up. The freshwater is then drained and replaced with seawater.”
OK, so we charge the battery, then drain out the charged electrolyte and replace it with another electrolyte …
Just in time for April.
Oh, right. We’re charging the electrodes, not the electrolyte. That could have been pointed out sooner.
There’s nothing new about this. And it’s just basic physics. It takes energy to free dissolved salt from water, so you get energy out when you salinate it. Just observe any river flowing into the sea and you will see a mist, which is the energy release from the salination. Big deal.
Where the concept entirely falls down is energy density. Rather coyly, the Stanford researchers decline to mention anything with respect to power density, i.e. the amount of extracted energy you get from the size of the infrastructure. If it’s like tidal power, the energy generated will be so low density that it’s uselessly uneconomic except for some tiny niche applications.
Nice try, Codetech, but salt domes form over millions or tens of millions of years. I really don’t want to have to wait about 10,000 years or so to accumulate enough charge to run my refrigerator. Take your salt dome, for example. Work out what the total energy release was and then divide it by the time over which it formed. You will come out with a very low number.
Some things to think about. If the process works like a storage battery, which it seems to do because of the necessity of charging the device, then there must be an electrolyte better than pure water which is a poor conductor. That would require some salt or acid to make it conductive. Then, since the charging puts ions into the electrolyte, either the electrodes begin as compounds of, say, sodium manganese dioxide and silver chloride or have sodium and chlorine absorbed in them. Then the charging with a high enough voltage might place just sodium and chlorine ions into the electrolyte in the latter case. In the former case maybe sodium, chlorine, silver, and manganese dioxide ions would go into the electrolyte. Would replacing the electrolyte with sea water with a possibly higher concentration of ions be able to discharge to a state where the electrodes have a composition different than their beginning state? If they could, then why not just begin with the electrodes and sea water and let the ions in the sea water react with the electrodes? Can one get more energy from a storage battery than than the energy stored in the battery just by changing the electrolyte after charging it? The terminal voltage may be different, but the total volts x amps x time = energy will be the same for charging and discharging with some heat loss.
How many charge/discharge cycles did they demonstrate? How fragile (and expensive) are the manganese dioxide nanorods? What was the power required to charge the device? How much power was produced when the device was discharged?
I have many inventions in the realm of physical chemistry, organic chemistry, and electronics. I think the failure of education in the patent area is much at fault here, and in promulgating these untested, futuristic ideas.
The international patent treaties are all unanimous in their demand that inventions satisfy three basic legal criteria: novelty, utility, and non-obviousness.
However, the overarching requirement is that an idea or concept is not patentable. One can only patent a thing. The thing has to satisfy the three criteria, but utility is the hardest nut to crack.
Remembering these facts, in my industry we were always admonished to work it out! “Engineer it out ahead of time” was a similar cry. What that meant was to identify the issues, problems and pitfalls, and solve them before the hypothesis was formulated. Then we would be required to run the violent gauntlet of challenges from our peers as we defended our theses, and we had better know the thing backwards and forwards ahead of time. In other words, we had better reduce our idea to practice before we exposed our vulnerable underbelly. (Reduction to practice is also a legal patent stipulation).
Because the grant process has become so lucrative to professors, they have been encouraged to skip steps. Pulling in money for the establishment is the goal. Ideas that had been previously relegated to Arthur C. Clarke or Frederick Pohl novels, now appear in academic circles. Then silly legislators fund these.
I think when someone floated the idea of the Dr. Seuss aerosol boat to make rain a year ago, an alert should have been sounded, but it wasn’t. I can’t blame them for making a living, but I hope that I may have had scruples, and charged ahead instead with something do-able. Who knows what the filthy lucre would have done to me!
Now, half-baked (or 10% baked) ideas are rampant. Even in the most prestigious schools, such as Cal Tech, is this charade going forward.
I feel sorry and worried for our science.
Almost forgot…How many times were they able to charge one of their batteries using the discharge cycle from a second battery?
John Marshall says:
March 30, 2011 at 1:57 am
We do have an inexhaustible supply of fresh water–as long as it rains.
Is there more water underground that above?
The abstract for the paper at Nano Letters gives a more complex manganese electrode than the manganese dioxide electrode that the press release gives. It is sodium (y-x) manganese (x) oxygen (z) rather than my guess of sodium manganese dioxide in my previous post. I can’t get the paper because it is $35 for 48 hours online. As far as I can see, the efficiency is on drawing energy from the charged battery and not some kind of over energy device.
The process is straightforward enough – geophysicists have been using something similar (Spontaneous Potential) since the Schlumberger’s discovered the effect in 1931. Occurs where a permeable rock and near impermeable rock meet and when cations can migrate into the impermeable rock but the much larger anions cannot.
The level is millivolts though.
More Press Release Science. Interesting but not anything to be excited about now. These types of articles used to fill the magazines, Popular Science and Popular Mechanics. Close to 100% of the touted breakthroughs never came to fruition. This one doesn’t look like it will fly either, but you never know.
When a press release falls back on such statements as ‘if all the world’s rivers were put to use, their batteries could supply about 2 terawatts of electricity annually – that’s roughly 13 percent of the world’s current energy consumption.’ it should set off your ‘pie-in-the-sky’ alarm. A two-year-old could tell you ‘no, no, no’ to that….there is simply no way anyone could claim to have carried out such a calculation from the proto-type battery design in the first place — it is just a made up number.
Shame on Standford for issuing such a statement — it is horribly embarrassing.
It occurs to me that the temperature of river water is usually quite different from that of sea water and therefore it would be much easier to extract energy using a simple Sterling engine. Of course, the problems with all these approaches is you got only a modicum of energy out with a lot of mechanics involved, whereas conventional electricity generation produces huge amounts of energy using a can with a fire lit underneath it.
Lake Simcoe is awesome cottage country. The sky is like you wouldn’t believe in the city, you can actually see the Milky Way and smaller and smaller stars in between each other. Oh, look! There goes the satellite 🙂 Still, there are all usual stores nearby.
Pl let me know the email of this person who invented this i like to try this in sri lanka.