Follow Slashdot stories on Twitter


Forgot your password?
Power Earth Science

Electricity From Salty Water 301

BuzzSkyline writes "It's possible to produce energy by simply mixing fresh and salty water. Although chemists and physicists have long known about the untapped energy available where fresh water rivers pour into salty oceans — it's equivalent to 'each river in the world ending at its mouth in a waterfall 225 meters [739 feet] high' — the technology for exploiting the effect has been lacking. An Italian physicist seems to have solved the problem with the experimental demonstration of a 'salination cell' that creates power given nothing more than input sources of salty and fresh water. The researcher believes that this renewable, environmentally friendly energy source could be deployed in coastal areas and could provide another addition to the green-tech roster. A paper describing the technology is due to be published in an upcoming issue of the journal Physical Review Letters."
This discussion has been archived. No new comments can be posted.

Electricity From Salty Water

Comments Filter:
  • Double Duty? (Score:4, Interesting)

    by drrck ( 959788 ) on Friday July 24, 2009 @01:47PM (#28809657)
    So can we expect this to work in parallel with existing hydro power generation techniques?
  • by Lordplatypus ( 731338 ) on Friday July 24, 2009 @02:12PM (#28809981)
    It would be interesting if this could make desalinization more energy efficient. After you finish desalinization you end up with clean water and very salty water. If you mixed the less salty sea water with your now very salty water, could they recover part of the huge amounts of energy that desalinization requires?
  • by sliverstorm ( 942764 ) <> on Friday July 24, 2009 @02:13PM (#28810003)
    This is actually really interesting! Think about it. We've been limited to solar cells for a long time for producing electricity, and those have limitations we are constantly struggling against. But... Now, you can make a simple isolated enviroment consisting of water and salt. Design it such that fresh water runs down from a resivoir into a lower resivoir with salt. Expose the lower resivoir to sunlight, and use the greenhouse effect to speed up the evaporation of the water. Direct the vapors up to the upper reservoir, where they precipitate out, and flow back down! Thus, we generate electricity and use the sun to separate the two components to repeat the cycle. (plus if you want, you can capture the heat from the condenser, for even more energy) Not something you could put in your car, but on a large scale I bet this could work. Similar to large steam powered plants.
  • Some ideas (Score:4, Interesting)

    by drinkypoo ( 153816 ) <> on Friday July 24, 2009 @02:19PM (#28810093) Homepage Journal

    One of the best places (potentially) to grow algae for biofuels is in the desert. You could pump seawater inland, and circulate it in pools. If you covered those pools with greenhouses (which could just be big clear balloons... or not-so-big ones, if you use arrays of small pools) and collected water they'd make you some fresh water, which could then be combined with incoming salt water to produce energy to help run the system, whether that would be the pumps, mixing devices which keep the pools circulating, or what ever else have you.

    Another idea for the waste water produced from this process is to pump it inland and use it in the algae pools... so you can have coastal plants whose effluent is used to grow algae for carbon-neutral biofuels, and [optionally] to raise the water table in the desert.

  • Re:Double Duty? (Score:2, Interesting)

    by mcgrew ( 92797 ) on Friday July 24, 2009 @02:20PM (#28810105) Homepage Journal

    At first I thought "probably not". Hydroelectric is actually gravity and solar power; you need a waterfall, or a place where the river is channelled into a smaller space (like a dam). Hoover Dam and Tom Sauk come to mind.

    Then I realized that not every river is as big as the Mississippi; Even though most damns are far inland, perhaps you could dam small streams or rivers flowng into the ocean. They dam the big ones bacause the bigger the river, the more power you can get from it.

    What I'd like more explanation of is how this technique works - TFA doesn't say. I got the impression that the writer didn't understand, either. Can any of you chemists/physicists explain this phenomena in layman's terms for us?

  • Re:Double Duty? (Score:5, Interesting)

    by Wrath0fb0b ( 302444 ) on Friday July 24, 2009 @02:43PM (#28810421)

    I got the impression that the writer didn't understand, either. Can any of you chemists/physicists explain this phenomena in layman's terms for us?

    Intuitively, anything that happens spontaneously (e.g. water falling down in a gravitational field) must be downwards in free energy or else it wouldn't happen (with any significant probability). So you know that when you pour together your rum and coke into a glass, the final state (uniform mix) must be lower in free energy than the initial state (rum on the bottom, coke on top).

    Slightly less intuitively, you can understand it very simply with a lattice model of solution under the assumption that there are no energetic effects (true to first order). Imagine the solvent as a lattice in which each square/cube (2D or 3D, your choice) can be occupied by solute or not -- now count up the configurations that correspond to a mixed solution versus an unmixed solution. That difference is configurational entropy and drives it to seek the macroscopic state with the most microscopic realizations since, in the absence of significant energetic effects, every microscopic state is equally likely.

    Of course, it's on wikipedia: []

  • by ShadowRangerRIT ( 1301549 ) on Friday July 24, 2009 @02:48PM (#28810483)

    Somewhat inaccurate. They have offered a reward to the first person to make in-vitro meat, where the meat is grown independent of the animal, economically viable. They oppose "unethical treatment," which is defined broadly enough to mean killing or confining an animal for virtually any reason. Bruce Friedrich, a spokesman for PETA, has said that if in-vitro meat were available, he'd eat it in a heartbeat. After all, no animal would have to suffer to provide it. It's a consistent position, which I respect.

    Before anyone starts, I'm aware of hypocrisy in other areas (PETA pet shelters), but I'm addressing only their views on vegetarianism.

  • Re:Double Duty? (Score:3, Interesting)

    by amRadioHed ( 463061 ) on Friday July 24, 2009 @03:50PM (#28811431)

    Desalination is extremely expensive, that would be a waste to use a large amount of energy to make fresh water, and then turn around and spoil it to get back a fraction of your original energy.

    On the other hand, desalination plants do create plenty of waste salt, so I wonder if you could get the same effect from the difference in salinity of sea water and the plants extremely salty waste water. You wouldn't make surplus energy, but it could reduce the costs of making fresh water.

  • by Weedhopper ( 168515 ) on Friday July 24, 2009 @04:14PM (#28811829)

    Here's the problem. Gravity is only interested in moving stuff in one direction, down. At some water will have to move against the gradient. Does the process produce enough energy to do that? If it does, how much water do you need in how much space, but just as importantly, what is the rate of production?

    Oh, we'll put it on the coast, people say. Do the mixing reservoirs have access to the ocean? Good luck with the tides.

  • Re:Double Duty? (Score:5, Interesting)

    by James McP ( 3700 ) on Friday July 24, 2009 @04:26PM (#28812065)

    The article actually has an interesting addendum at the end that explains it, albeit in an interesting vernacular.

    In short, salt water is ionic. A small initial electric charge is given to the two pieces of carbon (one positive, the other negative). The sodium and chlorine ions migrate to the respective carbon and thanks to the very high surface area of activated carbon, you get a very high quantity of ions. The water source then switches to fresh water. Elecrostatic force tries to keep the sodium & chlorine ions near the carbon but diffusion pulls them away. The work done to pull the ions away is what generates the power.

    The inventor that it can generate as much as 1.6KJ / Liter of fresh water. If we diverted 10% of the Missisippi River's outflow into one of these facilities you get ~2.6GW of more or less continuous power. (Mississippi = 572,000 ft^3/s * 28.32 L/ft^3 x 10% x 1.6KJ = 2.6GJ/s = 2.6GW)

  • Re:Double Duty? (Score:2, Interesting)

    by conspirator57 ( 1123519 ) on Friday July 24, 2009 @04:55PM (#28812619)

    i wonder if estuary ecosystems depend on this energy TFA proposes we harvest.

    what effect will exploiting the "green" energy have on the environment?

    will it actually be green or is this going to lead to 1000s of mini Aral seas?

  • by Weedhopper ( 168515 ) on Friday July 24, 2009 @05:24PM (#28813075)

    I think you need to consider this more deeply.

    You need two water sources, fresh and salty. The chambers are flushed alternately from each source. The only way this happens naturally is if you use the tides. Which gives you a total of 1 cycle a day.

    Nature is not going to do all of the work. At some point, water needs to be moved against a gradient.

  • by James McP ( 3700 ) on Friday July 24, 2009 @05:25PM (#28813089)

    He needs a pump in the lab. If you used a river delta, where there's a natural water flow, you only need a series of diversion gates; one to let fresh water in, one to let seawater in.

    He states that in theory you could capture 1.6KJ/Litre of water. 1 Liter water =~ 1 kg. Assuming you want 1m of elevation difference (it simplifies the math and that much head is a pretty solid flush), that's 9.81 J/liter at 100% pump efficiency. Assuming ~30% total pump efficiency, you're at ~30 J/liter. Now say that he has to pump both fresh and salt water, so it takes a total of 60 J of pumping per liter of fresh water reaction. If his system can reach 50% theoretical efficiency (0.8 KJ/liter) then you'd generate ~0.74 KJ/L of fresh water.

    Using my example above of diverting ~10% of the mississippi river, Mississippi = 572,000 ft^3/s * 28 L/ft^3 x 10% x 0.74 KJ =~ 1.2GJ/s =~ 1.2GW.

    I'd say a 1.2GW power plant is a pretty nifty goal.

    Of course at the moment he's generating 0.00005J/liter in his proof-of-concept unit, so that 50% theoretical may be lofty, but with ~60 J/L of overhead, he starts positive power production at 4% theoretical. Not sure what percentage he needs to produce more power than manufacturing the the carbon and other plant facilities requires.

  • Re:Double Duty? (Score:4, Interesting)

    by The_mad_linguist ( 1019680 ) on Friday July 24, 2009 @10:39PM (#28815665)

    Desalination plants produce brine, though, which has more salt than ocean water, so conceivably you could increase the efficiency by combining seawater and brine.

Someday your prints will come. -- Kodak