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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I think several commentators have missed the point. As windfarms demonstrate, the technology does not need to work, to be economic or to provide electricity in useful amounts for it to harvest large profits from subsidies.
Indeed, this technology could be even more profitable than windfarms because it would do even more environmental damage than windfarms (that cover the countryside in concrete for their foundations and roads to access them) so ‘greens’ will campaign for it to get even larger subsidies.
Richard
Being a hopeful optimist I find this very interesting, the skeptic in me says, but wait.
First, if a place in an estuary were found where the salinity of the water is dramatically less when the tide is out than when it is in, then wouldn’t the efficiency of adding and removing water to and from the battery be as high as what the paper claims? You just let the river do all of the work.
But how much difference would there need to be in the water’s salinity to make this effective?
I’m going to build a giant treadmill powered by genetically engineered hamsters. I will fed them algae grown in ponds and fertilised with their own poo. It will be at least 250% efficient!
CRS, Dr.P.H. says: “Why not make a whole bunch of these instead?
http://hilaroad.com/camp/projects/lemon/lemon_battery.html
Just one single lemon the size of Phobos could supply 2/5 of the energy requirements of the United States! And the energy is practically FREE! We must build the Great Lemon Power Plant right away to save the Earth from Global Warming! Let us stand in front of the White House, join arms, and sing kumbaya! All together now…!
Hmmm….. sounds suspiciously like perpetual motion at work here.
Got to pump the water, and have to account for the energy consumed in making the manganese dioxide rods.
Nice idea, though. Somebody has been thinking.
stupidboy says:
March 29, 2011 at 3:57 pm
This might warrant a new thread. If you took away all the papers in the West attributed to AGW studies, then China is probably even or surging ahead of the West.
I hereby propose to change “Save the Planet” to “Save the Planet for the emergence of China”.
The Panama Canal Zone, would be an excellent place to try this. Every time a ship moves through the locks, Millions of gallons of fresh and salt water are exchanged. Very little energy is required in the process and since it is already being done to move cargo, the electricity generated would cost only what is needed for constuction and infrastucture.
All scoffing aside, this system *can* work, and is an entirely reasonable extrapolation of battery and ion exchange systems. 100 MW from 50 m3/s comes to about 2000 J/kg, extracted from the entropy difference between salt and fresh water. That’s the energy equivalent of 200 meters of hydrostatic head, which is much *less* than what is needed to power reverse osmosis of fresh water from sea water, so the energy levels are in the realm of feasibility.
This is NOT a perpetual motion system; it takes a lot of energy to extract fresh water from seawater, the inverse process can inherently release a lot of energy. The tricky part is figuring out a good way to harness the entropy change- a naive method would be to run fresh water into a dry dock 200 m below sea level through water turbines, then allow it to diffuse into the ocean at that depth through osmotic membranes. The naive method would be a pain operationally because of silt and sludge in the fresh water, which would probably need perhaps a quarter of the concentrated fresh water to be pumped back up to sea level.
This electrolytic method is very very clever.
It may be time to drag out a plan I came up back in my early high school days while sitting out a detention, brought on by one of the earlier instances of authority figures not appreciating my irreverent sense of humor. I envisaged a power generating system based on scaled up versions of this device.
http://www.scientificsonline.com/famous-drinking-bird.html?gclid=CJuSgbT_9KcCFYwH2god_VBLcw
My original plan called for lining the coasts with giant models, but I soon realized that any body of water of sufficient size would do, even large tanks. This means placement options are almost unlimited. I never had the opportunity, or desire really, to run the numbers for extractable power or optimum size, but given the tsunami of grant money swirling about novel energy schemes nowadays, it may be time to put in the effort. What do ya think?
jonjermey says:
March 29, 2011 at 1:17 pm
This battery is totally useless but so is tide in most places; the head is far too small. Bay of Fundy and Horizontsal falls are possible exceptions but too far from consumption point.
And on the same subject, why is nobody using the pressure of tidal flows to drive osmotic desalination systems?
You probably mean REVERSE osmosis.
Answer: Because at the pressures achieved by tides you’d probably get several pico-grams of fresh water per century per Billion dollars of investment. Most people would consider this a poor ROI.
I admit that I didn’t read below the fold but some things are so absurd that the details don’t matter. Like, for instance, there is no need to read details of an experiment that demonstrates ESP. We know it can not work; the details are irrelevant.
“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.”
I have my doubts about the mechanism and the efficiency, but assuming this is not an early April Fool’s joke, there’s up to 500 cubic metres per second of pure water going into Doubtful Sound from the Manapouri Power Station in New Zealand.
The pure water is not really wanted in the Sound as it affects sea life, but it’s there now. Nobody will object if it’s used for another purpose, e.g. generating 1000 megawatts of power, or even if it was exported for people to drink! There’s a deep water port there. Let’s use it.
I have so much fresh water near my house — SE Corner of Lake Simcoe Ontario — that I am pretty sure I could stuff rocks ion the underpants of every environmentalist on the face of the earth, stuff them in a nearby lake or river and never have to look at them again…
So I am not so sure about this shortage thing — at least not for my current purposes.
Why wouldn’t a design use all gravity fed fresh water, and truck in salts to disolve in as needed for that half of the reaction? You could use it where ever freshwater head pressure wasn’t sufficent for hydro and could saturate the salty side for potentially better performance. Your super saline rinse would also discourage mussels and slime.
This could reuse industrial waste as well.
This is a wonderful new battery technology but the technology of reverse electrolysis is not new. There have been experiments for decades and one plant is running in Europe. The water at the mouth of the river is a lost resource there. If you dam and divert up stream and leave no remaining flow then there are very adverse out comes for the ecology and fisheries. None of us would advocate completely blocking river flows.
However dams in general block fish population going up stream. This in turn block phosphorous and other soluble elements moving against the water flow leading to soil depletion and the need for fertiliser. The fish at the head waters becomes food for something, mostly insects and birds, on shore which in turn dies (or simply defecates) on the farm. Thus in a rich ecosystem phosphorous etc can go up hill.
Fish ladders work to solve the problem but their often species selective. More research is needed to make a much more species flexible fish ladder or lock.
A fish ladder around this technology would be essential if it blocked a full flow. In some places the river is more saline than the sea and the device could run on the sea water as the low salt input.
In the wet tropics where water is not limiting and flows to the sea are short, fast and continuous this technology will be very important. The catch is this is also where solar and micro hydro have the best prospects and biomass out does everything even oil; making the wet tropics the one place where energy should be abundant. Generally where it is not, you find old hammer and sickle symbols cast in cold war concrete.
There are more ions in the saltwater than in the fresh water. Fair enough. I didn’t see the bit about how the positive ions get separated from the negative ions. Current isn’t going to flow until the opposite charges are moved apart. The diagrams don’t explain that bit.
rbateman
what do you think of this (if you have the time)?
“Fire from Water, hosted by Scotty from Star Trek”
They need a different river than the Amazon. Last I heard, the surface water more than 100 miles out in the Atlantic was drinkable. Trying to put something in the open ocean permanently? i don’t think that has been solved yet.
What are you guys smoking? For example:
Alternatively, you could build it at sea level, and have the fresh water come to you by gravity.
I don’t see this as solving the worlds energy problems and there is a long way to go between a lab experiment and a practical power plant. However provided the source of fresh water is above sea level it should be possible to use the difference in density between fresh and salt water to avoid the need to pump either.
In principle you could have a cell that was open at the bottom and had a valve at sea level. Normally it would fill with sea water. Adding fresh water from the top with the valve closed would force the sea water out – the fresh water would rise above sea level because it is less dense. Then you open the top valve – the “overburden” of fresh water would run out and salt water would force the rest of the fresh water out of the cell. To avoid mixing you could use a floating piston with a buoyancy between that of the fresh and salt water to help separate the two – it would not have to have a perfect seal. Thus the only energy needed would be that required to open and close a couple of valves.
Assuming that treated sewage is sufficiently like fresh water this process could provide a useful byproduct for a treatment plant that normally discharges into the sea.
The test of this is to use its own generated energy to swap out the salt water and freshwater. If it cannot produce enough energy to do this then it is an energy sink, not an energy generator.
Here’s a field test. Quite simple. Fly a fully assembled and tested system and with sufficient capacity for the task to the moon and let it heat itself to keep from freezing.
It won’t work as you can imagine. You don’t need to go to the moon to test it. It will stop working in your basement, too. It needs outside energy. In the electronics industry we call this a resistor.
FAIL
So far no one has mentioned that aquatic organisms would very quickly clog up this system, rendering it useless.
About the hoover dam flooding: The great flood of 1905 by the Colorado river created the Salton Sea over a 2 year period, which was the impetus for building Boulder (now Hoover) Dam. The surface of the Salton Sea is currently 226 ft below sea level and covers about 376 sq mi or 241,000 acres, with a volume of 7,500,000 acre·ft, making it the largest lake in California. Much of Imperial County, CA is below sea level, with El Centro, CA (population 40K) about 40 feet below sea level. Since Hoover dam has an active capacity of 15,853,000 acre·ft, if the dam burst it would flood all the low lying areas with the water eventually draining into the Salton Sea, tripling the volume of the lake.
As to “no-dam” hydroelectric power, I once spent some time in Eastern Nepal near a couple of swift-flowing rivers, pouring down from the Himalayas (the Arun and the Dudh Khosi), dropping thousands of feet in a few miles. It often occurred to me that you could divert some of those streams in canals along near-horizontal mountain contours, then drop the water down to the lower stream bed in the descending valley. You’d have turbines at the bottom of each run, generating electric power. If you built enough of those, you could power most of northern India. No dams needed.
/Mr Lynn
Ah, the Salton sea – such a lovely place.
http://www.ferdyonfilms.com/?tag=salton-sea
It is actually the site of natural as well as unnatural lakes, the current iteration being sustained by agriculture runoff. When I lived in SoCal it was still a good place to go to catch healthy corbina but that was in the 1960s. Now it is a toxic soup that, like an ill-mannered barista that re-uses old coffee grounds, absorbs and recycles life with the cold black heart of a grave robber and beckons from the flyways new death with the crooked finger of promise.
I think only the Berkeley Pit in Montana is more disgusting.
Andrew30- Once the big red mud is generating potato power, perhaps the peislanders can export the juice all over Maritime Canada, enough electricity to warm the peas served beside the hot beef. Can you turn tobacco into batteries ? How about hake ?
Move one, absolutely nothing new here:
http://en.wikipedia.org/wiki/Osmotic_power
http://www.osmoticpower.com/