A new form of hydroelectric power – no dam needed

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

The initial state of the battery developed by Yi Cui, with two electrodes immersed in fresh water. Purple and orange dots represent positively and negatively charged ions, respectively. In Step 1, a small electric current is applied to charge the battery, pulling ions out of the electrodes and into the water. Step 2, the fresh water is purged and replaced with seawater. Note much higher abundance of charged ions in the salt water. Step 3, electricity is drawn from the battery for use, draining the battery of its stored energy. Ions return to electrodes. Step 4, seawater is discharged and replaced with river water, for the cycle to begin anew. Courtesy of Yi Cui, Stanford University

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 mouth of the Amazon River, where the world’s largest drainage basin flows into the Atlantic Ocean. A location such as this, where fresh and sea water mix, is a good spot for generating electricity with Yi Cui’s new battery. Credit: NASA

“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.

The manganese dioxide nanorods used to make the postiive electrode of Yi Cui's new battery that generates power by using the salinity contrast between fresh and salt water. Credit: Courtesy of Yi Cui.


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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This is one of those, “why the hell didn’t I think of that?” moments.
It may not be commercially viable, but it would have been an easy graduate thesis.


I wrote a book in 2003… and it included the discovery of this technology.
Although in my book, the discovery was of the ancient world…


i know how to fix this. We will use Barrie Harrop’s windmill driven reverse osmosis plants to generate the fresh water so that we can generate the electricity. A veritable perpetual motion machine of the first kind.


There is a small little problem… they need to pump the water up from the ocean. This is where there electricity will go into.


“The battery itself is simple, consisting of two electrodes – one positive, one negative – immersed in a liquid containing electrically charged particles, or ions. ” and coated with slime and zebra mussells.
“They chose manganese dioxide for the positive electrode in part because it is environmentally benign”. But we may have to consider lead pipes, copper sulfate coatings or add clorine to the diverted river water.
“But river water doesn’t necessarily have to be the source of the freshwater, Cui said.”
We can use the water from a de-salinization plant just as well!
““I think we need to study using sewage water,” he said. “If we can use sewage water, this will sell really well.”, or potatos, last year my daughter brought home a potato battery, really cool, we could power all of Prince Edward Island with just a couple of tons of nails some wire and the potatos in the ground! Free electicity.

Lonnie E. Schubert

I’ve only skimmed so far, and I’ll be looking for educational comments, but this sounds crank to me–perpetual motion machine style. They admit it has limits, but I’m not sure I see any potential at all.

Gary Pearse

Many years ago I wondered if Great Salt Lake could be made into a giant battery. It’s not as goofy as I thought.

Harry Bergeron

To me the obvious use would be refining aluminum from bauxite.
This take huge amounts of DC if I remember correctly.
There are large losses in conversion to AC, transmission, and
conversion back to DC which could be avoided.


Actually, mods, if you don’t mind indulging me for a moment, here’s the part of my book (yes, 2003) that is relevant:

“It’s so obvious, I can’t believe neither of us thought of it. I have the images on a webpage. I want you to see them, then see if you come to the same conclusion I did.”
He gave me the address and I typed it into my computer. There were the grainy but colorful results of the scan.
“Wow,” I whistled. “You’re right, it’s so obvious. But what about control?”
“Tell me what you see.”
“I see a salt cavern, possibly the remains of an ancient sea, with metallic plates embedded throughout. Water no doubt trickles through, making it a gigantic natural battery.”
“Excellent. Can you estimate the current and voltage we are looking at?”
“No,” I said, smiling. “Assuming the red ones are copper and the blue ones are, what, silver? Gold?”
“Gold, maybe. My own estimate is that this is several tens of thousands of amperes, and by the number of plates it would be about ten thousand volts. Direct current.”
“This is larger than the entire visible pyramid complex,” I said, amazed.
“They probably selected the location for the pyramids based on the availability of this cavern,” he said. “If we had thought of this, we might be running towns and cities on natural battery power.”
“It does seem rather obvious, doesn’t it? Then again, the sheer cost of the metals might be high.”
“Not compared to a nuclear plant,” he pointed out.
“But, what turns it on and off? Surely this has not been consuming power for five thousand years, there would be corrosion and crud on the plates.”
“At the moment, I have no answer, but now we know where the power is coming from. Can you imagine, an almost free power source underground, the only pollution is a bit of ionized saline leeching into surrounding subterranean areas. We could self-sustain our electrical needs for millennia.
“Centuries, anyway,” I said, thinking. “We’d have to maintain the plates from time to time.”


85% efficiency still says they are only getting out 85% of the energy they put in. Thermodynamics dictates this isn’t the panacea they claim. It could make an excellent storage system for surplus power on the grid though and used to even out generation capacity, because 85% seems pretty good for battery efficiencies.


Nearly all riparian estuaries have brackish water resulting from tidal exchanges. What sort of infrastructure will be required to transport of pure fresh water to plant where the mixing and generation occurs? Can it be cost effective?

I seems to me that all they are doing is charging a capacitor in low conductivity water and then rapidly discharging it in the high conductivity sea water. Also, by using this type of material, they may be producing concentration cells.

“… 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.” [emphasis added]
Seems like a pie-in-the-sky projection to me, for a result that is self limiting. I cannot imagine even 10% of the world’s rivers being put to such use; then we have massive constructs providing no more power than wind turbines.

Don K

From my notes on energy sources:
===Osmotic Power===
Osmotic power is a label for various techniques for exploiting the fact that water containing salts is denser than fresh water. This difference can be exploited to generate power when the less dense fresh water is mixed with saltier water. In the case where rivers are the source of the fresh water, the mixing would take place anyway, so the power is essentially free — other than the possibly considerable cost of the generation facility.
Osmotic power is renewable and does not release significant carbon. It may however be disruptive to the environment where many species that currently live in the brackish water transition zone between fresh and salt water might be threatened.
The amount of power available is unknown with some sources claiming a potential for 1.6 to 1.7 Twh (over what time span?) This appears to work out to a “sticker” capacity of about 900GW (Note: These numbers do not appear to make sense), but it seems fairly unlikely that actual generation will reach anything like those levels. Currently there is a single 4kw pilot plant in operation in Norway.
The article isn’t very clear, but I suspect that either the fresh water has to end up salty or the salt water fresh. If not, it sure looks like a violation of conservation laws. A perpetual motion machine as others have said.


“unfortunately we don’t have an infinite amount of freshwater”
Yeah, it keeps running downhill into oceans ….
“Manganese is mined in South Africa, Australia, China, Brazil, Gabon, Ukraine, India and Ghana and Kazakhstan.”


Given that most places where fresh and salt water mix are subject to tidal oscillations anyway, wouldn’t it be simpler and more efficient to build tide-powered generators? And on the same subject, why is nobody using the pressure of tidal flows to drive osmotic desalination systems?


Maybe a good option for Fjordland, but pretty useless everywhere else because of environmental constraints. In researching tidal power options as an electrical engineer, I found out that most coastal areas have at least 7 distinct stakeholders each of which has veto power over projects.
Essentially, the technology is irrelevant because regulations will prevent it from even being tested.

Jeff Carlson

nice experiment that ignores the energy costs of moving the salt and fresh water to the lab …
in the real world I expect alot of losses for water transport (both ways, you’re not going to store it locally after its is used) …
pie in the sky …
I worked on a project in school that used the differences in water temperature in the ocean (surface vs hundreds of feet down) to vaporize ammonia, run it thru large turbines and condense it for reuse … sure it could generate power … but trying to scale it up would be a very different issue …
a solar cell generate free electricity as well … try powering an auto plant with them …

A working prototype for this kind of power plant has already been build in Norway.


“… 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…”
Valves, pumps, lifting mass (water) etc.
Where these items considered in the efficiency calculations? I’m guessing “no.”
This is essentially an echo of Ray’s post above. He sees also sees the problem.

Since this idea relies on the difference between salt and fresh water, would it also generate current with salt water of different concentrations on either side, vs. only salt & fresh?
At first glance, it seems to me that it would work, although somewhat less efficiently.

Ken Harvey

This could work. I’m going to give it some thought and see if I can devise a contrivance that will push sea water up hill, with a zero energy requirement. Of course, once I have got my contrivance working properly we won’t need to mess around with batteries!


I’d think that using the sun to distill some salt water would be a solution to the problem of fresh water – you’d still have to remove the excess salt from the distillation equipment.
TANSTAFL in any case.


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.”
And it cost a gazillion and produces enough energy to warm a cup of coffee, if i remember correctly. Fits perfectly to Dogbert’s fake green breakthrough press releases.


On a casual read – it seems rather illogical to me! As others have commented it doesnt look like it could be very efficient but my first thoughts were actually environmental based. Like, what happens to the seawater and freshwater afterwards – how do the ions replace their charge ‘taken’ by the battery? As I recall my basic conservation of energy law – I am wondering where in the electrochemical chain will the effect of ionic/charge removal cause an eventual problem?


The brainiacs always come up with such fanciful and wonderfully (expensive?) and equally applicable as W.E. Coyote finds the A.C.M.E. inventions.
Just create a sales pitch, like so: Lose weight, keep in shape, power the city lights, and save the planet!
Let the unsuspecting hippie, for statistics in this particularly well researched area has it that mostly the hippies will be attracted to the aforementioned sales pitch so no civilians will be hurt, climb into the caged wheel, then, with Flash-envious speed, just stuff three peeled red thai hot chilies up his bum and slam the door shut.
Sit back and try and figure out who’s whining the most, the hippies saving the planet or the generators.
Just don’t go for the truly green energy with ecological hydroponically grown green chillies, they just don’t have the same energy potency. :p

Bob Diaz

Simple and practical, which is why all the environmentalists will be against it.


why do you need to move water uphill? The freshwater is always flowing down to the ocean, to refill the battery you could pipe it(the freshwater) from upstream and at an angle from the battery, an angle to get the battery out of the mixing zone of where the river enters the ocean,then when you have charged up the battery you could just let ocean currents flush out the fresh water maybe a series of barges anchored in a line

George E. Smith

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 !! ??
So per million gallons of FRESH water :be my guest on the salt water; take plenty; it’s cheap ! How many megaWatts peak, or MegaWatt-hrs of electricity do I get. I’m pretty fussy about what I do with my fresh water.
Come to think of it, why couldn’t those California Central Valley farmers use their old salted up dead land, to make a huge salt water battery; it wouold work great evbery tijme we get a deluge period like the present, and have some fresh water we want to pollute for more juice.

Luther Wu

manganese nodules litter the ocean floor

George E. Smith

So far as I am aware, the only way you can get lower on the stored chemical energy Totem Pole; or Food Chain, if you prefer that metaphor; than H2O from burning the abundant supplies of native Hydrogen on Earth, is to add salts to the H2O. That about gets you down to bedrock for your Totem Pole.
And I see that Infinity iws now smaller than it used to be; we din’t ever have an infinity of anything; or even of everything; and now we have an infinity of salt water, and we want to make more. Simply Wunnerful !!

It's always Marcia, Marcia

Something I have wondered, if the Hoover Dam was broken by an earthquake how much flooding would it do, and for how many miles down river?
I’m not trying to be alarmist. I’m just wondering. Anyone know?

Another Gareth

Why would you bother to collect sea water from the Pacific and fresh water from a lake when you could demonstrate the effect easily enough on a kitchen table?
Draining the ‘charged’ fresh water surely destroys the electrolytic cell. If it worked as the diagram suggests you ought to be able to produce electricity just sticking the electrodes in salty water. Perhaps that is the actual effect they are noticing, something like this: Salt water air battery

Luther Wu

It’s always Marcia, Marcia…
that could never, never happen there in never, never land.

CRS, Dr.P.H.

Why not make a whole bunch of these instead?

Bob Maginnis

I heard about the potential energy between salt and fresh water in about 1982, and it isn’t perpetual motion, because to would require about 900 feet of head pressure to reverse the change from fresh to salt water, to desalt the previous fresh water from the river.


I would be nice if pro AGW and deniers etc could debate like this

jack morrow

The G M volt works about as good as this project will.

A G Foster

If the chemistry has been tested, that’s all we need. You don’t have to move water, you just move the batteries up and down–river water floats on the surface for many miles out into the ocean–dozens of miles in the case of the Amazon. The batteries could be positioned on a slowly turning wheel on a floating platform. But I think the temperature differential between shallow and deep water holds more promise for large scale generation. There’s more ammonia than precious metals.


Rossi’s Energy Catalyzers will produce 15 kilowatts continuously for 6 months on a 200g charge of nickel and hydrogen. Replaced twice a year, it will generate 131.4 Mwhrs/yr. World electricity consumption in 2007 was 17,109,665,000 MWhr. Using a 33% efficiency factor to convert E-Cat heat into electricity using the Carnot cycle would require just 174,000 tons of nickel powder (much of it recyclable since all of it would not be converted into copper). Annual world production of nickel is about 1,400,000 tonnes, so about 15% of the world’s nickel production is all that’s needed to generate practically all the electricity consumed by mankind, even when factoring in a 30% transmission loss.


Use the melting arctic/antarctic ice… :o)

Well, solar energy produces molecular motion which creates friction which produces heat. – And as we all know, heat is pure energy so I cannot see any flaws in this one.


Why not just put paddle wheels on your sewer outlet and recuperate some of the energy the city uses to pump the water up your pipes.
In any case, a galvanic cell can be built between clean and salted water. The only problem is that the pure water gets contaminated via the electrolytic bridge. You can even measure a potential between two saline solutions having different concentrations, and of course once both solutions have the same concentration, the difference in potential is zero.


“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.”
The Fraser River in BC has a minimum flow estimated at 867 m-3/s and a maximum flow of 6970 m-3/s.
How big is this plant going to be if you need to divert so much water from a river.
Essentially you almost need to divert a really big river to get a 1000MW.
The fish won’t be happy.


Following up on this idea of generating electricity when you flush…
Think about it this way, a city’s water reservoir can produce electricity since the energy is accumulated in term of height and mass. Now multiply that by the number of toilet reservoir and you get a very big water reservoir. If you take into account the added mass when you flush, we are talking serious megawatts here.

Michael Penny

50 cubic meters per second of water to power 100,000 households is 792,516 gallons per minute, or 11,412 gallons per household per day. That is about 1.5 MW for each 10 feet of lift plus friction losses for the pumping of the water.


Apologies from straying from the topic but this just caught my attention:
I wonder how CAGW related papers alone compare.

Lady Life Grows

Energy flows wherever there is a gradient of any kind–hot air to cold regions (wind), water flowing downhill, etc.
This won’t solve all the world’s energy problems, but it will be practical to build in some places. And that is good enough.
But only carbon fuels can green up the world’s vast deserts.

Richard S Courtney

I think several commentators have