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Power Science

Harvesting Power When Freshwater Meets Salty 151

Posted by samzenpus
from the power-up dept.
ckwu writes "As a way to generate renewable electricity, researchers have designed methods that harvest the energy released when fresh and saline water mix, such as when a river meets the sea. One such method is called pressure-retarded osmosis, where two streams of water, one saline and one fresh, meet in a cell divided by a semipermeable membrane. Osmosis drives the freshwater across the membrane to the saltier side, increasing the pressure in the saline solution. The system keeps this salty water pressurized and then releases the pressure to spin a turbine to generate electricity. Now a team at Yale University has created a prototype device that increases the power output of pressure-retarded osmosis by an order of magnitude. At a full-scale facility, the estimated cost of the electricity generated by such a system could be 20 to 30 cents per kWh, approaching the cost of other conventional renewable energy technologies."
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Harvesting Power When Freshwater Meets Salty

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  • How well does it scale?

    • by fisted (2295862)

      1. Make freshwater river
      2. Make saltwater river
      3. Connect at both ends
      4. ???
      5. Infinite Energy

      • by RockDoctor (15477)

        1. Make freshwater river
        2. Make saltwater river

        I think you've misunderstood the article. It's more like:

        1. 1. Let nature make a freshwater river for you.
        2. 2. Let it run down to the sea, where you capture and canalise it's flow.

        And carry on from there.

        Oh, and the energy isn't infinite ; it's going to be limited absolutely by the flow rate of the river and the salinity difference between the two water bodies. Which could be substantial (most people are so inconsiderate of the power of the sun to drive such th

    • by perpenso (1613749)
      Why not just use the river to turn a turbine?
      • by Luckyo (1726890)

        Advantage of this is that you can do both. The energy in question here is not potential energy that is converted by convential hydro, but osmotic pressure (effectively chemical energy) from having large sources of fresh and saline water.

        It's basically a new kind of hydro that uses a different energy source.

      • Why not just use the river to turn a turbine?

        Typically in the coastal plains (where a river would meet with saltwater), a river will widen up and it's current slows to a crawl (due to the lack of a "downhill" presence).

        • by perpenso (1613749)

          Why not just use the river to turn a turbine?

          Typically in the coastal plains (where a river would meet with saltwater), a river will widen up and it's current slows to a crawl (due to the lack of a "downhill" presence).

          Sure, but we can put water wheels, turbines or something other than a dam up river a bit. Or many along the river. We don't need to be in the mixing zone.

          • by RockDoctor (15477)

            Sure, but we can put water wheels, turbines or something other than a dam up river a bit.

            That's fine if you're an island nation - as Britain is (currently) and the US is (largely). But generally it can be more complex.

            I can't think of a realistic example in Britain (the Anglo-Scottish border fairly closely follows a break of slope). But as I struggle to remember the details of my North American geography I think of the routine flooding in North Dakota (?) due to the low slope of the Red River (?). Say that

    • That and how long does the membrane last, and does the system produce more energy in that time than it takes to produce the membrane? Either way, it's pretty cool.
  • by Anonymous Coward

    The energy density of this system is crap, plus it has all the problems of water fouling and so maintenance will be a pain. IMO, we should focus our efforts on developing cheap organic photovoltaics, and then paving the desert with them. We need more government funding injected into fundamental materials research. Disclaimer: although I don't pursue this line of research I am a researcher!

    • by RockDoctor (15477)

      The energy density of this system is crap, plus it has all the problems of water fouling and so maintenance will be a pain.

      Valid points. Are you sure you intended to post as an AC?

      IMO, we should focus our efforts on developing cheap organic photovoltaics, and then paving the desert with them.

      If you've got large fresh-water (or even brackish water ; the system will work, but the energy density will be even crappier) rivers, then there's a good chance that you don't have lots of desert to pave.

      Just because

  • Big problem here... (Score:5, Informative)

    by Andy Dodd (701) <atd7&cornell,edu> on Monday December 02, 2013 @05:44PM (#45579031) Homepage

    It requires saline that is MUCH more concentrated than seawater... So you need to somehow concentrate the saltwater before using it.

    Although this might allow for some rather unconventional solar power projects - feeding brine from salt concentration ponds might be workable here.

    • by Acapulco (1289274) on Monday December 02, 2013 @05:49PM (#45579087)

      I know this will probably cause a host of issues that I'm not thinking here, but the (to me) most obvious solution would be to pair this with a de-salinization plant. What if instead of de-salinizing all the water they stop at X% of water remaining in the solution, and then use that super-concentraded saline water with the power generation plant.

      • by Anonymous Coward

        but the (to me) most obvious solution would be to pair this with a de-salinization plant.

        I think that would be the worst thing, trying to extract energy from a process on the one hand that you are using energy to reverse in the other. I think people are so often overlooking the importance of clean water in the search for energy - for example, corn for ethanol, and this as well...

        • by dominux (731134)

          If you have a river then you probably don't require a desalinisation plant. It is just about plausible that concentrated saline from a desalinisation plant could be transported to a distant river (by boat) where it would be used for power generation like this.

          • If it's concentrated enough, why can't you use sea water as "fresh", since it is powered by the difference in salinity, not the absolute value.

          • by perpenso (1613749)

            If you have a river then you probably don't require a desalinisation plant.

            If you have a river you probably don't require a pressure-retarded osmosis power generating plant, just using the kinetic energy of the river to turn turbines.

      • by bobbied (2522392)

        I know this will probably cause a host of issues that I'm not thinking here, but the (to me) most obvious solution would be to pair this with a de-salinization plant.

        Well if the process requires fresh water, why would you have a desalinization plant? Wouldn't it be easier to just treat the fresh water?

        • As I understand it, it doesn't require fresh water, per se. It requires a large difference in salinity. So, technically, you could use discharge brine from a desalinating plant as the "saline" part, and regular sea water as the "fresh" part, as long as the difference in salinity is enough.

          • by RockDoctor (15477)
            This sounds like the plans for your perpetual motion machine. Instead of publishing them, could you get back to me about my bridge-sale proposition. I want to make my sale before Big Oil has you beaten to death with the only working model of your PM machine.
            • Yeah, or you could use your brain for half a second for something besides sarcasm. Consider:

              It takes X watts of power to desalinate 1000g of fresh water
              In desalinating 1000g of fresh water, you get enough brine suitable for generating Y watts of power, where X > Y.

              So, would you rather have 1000g of water, or 1000g of water + Y watts of power? And is X - Y watts of net power used better than X watts of power used, where Y > 0?

              Is regenerative braking on an electric vehicle also perpetual motion in you

      • by rrohbeck (944847)

        Sounds like a perpetual motion device to me.
        Oblig.: In this house we obey the laws of thermodynamics!

        • by Dahamma (304068)

          No more than regenerative braking. Better to reuse the potential generated by desalinization than just dump the brine back into the ocean like most plants do today.

          It's all about energy efficiency, and desalinization is basically just charging a big chemical battery. Why waste it?

          • by rrohbeck (944847)

            No, the desalination will require more energy than you get out of this method. Conversely, if you have freshwater, why use desalination?

            • by AcidPenguin9873 (911493) on Monday December 02, 2013 @10:54PM (#45580927)

              Yes, desalination obviously requires more energy than you get out of this method. But the point of the desalination is not energy production, it's freshwater production. You get freshwater out of your desalination plant. That requires using some amount of energy X. Instead of dumping the waste product of the desalination plant (highly-concentrated brine) somewhere, you use it with one of these devices to produce some amount of energy Y where Y is less than X.

              The net result is that you end up with freshwater, and instead of spending X energy to get it, you had to spend only (X - Y) energy.

              • by rrohbeck (944847)

                Instead of dumping the waste product of the desalination plant (highly-concentrated brine) somewhere, you use it with one of these devices

                ... for which you need freshwater.

                • Nope. Osmosis operates off a relative concentration difference. Regular ocean saltwater is much more dilute than brine and would work just fine as the "freshwater" side of this power generator.
                  • by rrohbeck (944847)

                    You'll still lose enery on the round trip. Your overall energy consumption would be lower if you ran your desal plant at a lower power level and produced less briny output than producing very briny output, diluting it and producing energy from that.

                    • Pick a "brininess" and energy consumption you want to run at. By definition, the brine produced will be more concentrated than the ocean water flowing into your plant. Run the more-concentrated brine and less-concentrated ocean water through this power system and produce whatever energy you can get from it. It will always be less than the energy that you used to produce the freshwater+brine, but it will always be more than 0 which is what you get if you dump the brine back into the ocean. How you want t

                    • by Dahamma (304068)

                      Obviously you'll lose power on every round trip, it's not a perpetual motion machine any more than an electric car with regenerative brakes. The point is to use the energy you put in more efficiently, not make it self-sustaining.

                      Also, part of the advantage to this is that power plants are *already* commonly paired with desalination plants because thermal desalination can use the waste heat generated during power generation. This would effectively be an additional component to a traditional power/desal pla

                • by Dahamma (304068)

                  No, you just need sea water with less salinity than the brine. The potential energy is in the DIFFERENCE, of course.

                  And it can in fact use less energy, chemicals, etc to desalinate relatively clean seawater over more polluted river water. New York is investigating this now.

              • by RockDoctor (15477)

                Instead of dumping the waste product of the desalination plant (highly-concentrated brine) somewhere,

                Someone, several someones, seems to think that desalination plants produce highly concentrated brine. They don't. It's only a few percent more concentrated. Say that you take 2m^3 seawater (35) into your plant and discharge 1m^3 of fresh water (essentially 0) and 1m^3 of "concentrate", what is the solute concentration in the concentrate?

                Answer : 70 Which is still not terribly concentrated.

                One of the const

      • by BitterOak (537666)
        Interesting. You were modded +4 Insightful for suggesting that we power a desalinization plant by mixing fresh water with salt water. Slashdot really does manage to surprise me every day.
        • by Dahamma (304068)

          Well, if you think about it a bit more, it's the *difference* in salinity that matters. Desalinization is basically creating an osmotic potential, just like in a chemical battery. You could then use that hypersalinated water (aka brine) with regular sea water as described to extract the energy back, rather than just dumping it back into the ocean (which is what normally happens). Or you could use the hypersalinated water with river water to make the technique more efficient (foreshadowing, here...)

          In fac

      • by d'baba (1134261)
        Desal is a big thing here on the left coast. Propose projects up and down the coast because ground water and surface water are being oversubscribed. It won't run the desal process but it should at least be looked at to see if using the concentrated saline could give us a little in return. It's in the same category as regenerative braking in e-cars. Every little bit helps.
      • by ganv (881057)
        No, you already input energy to separate the water and the salt. Remixing them will release part of the energy which could be harnessed, but inevitable losses in conversion will make it better to just use your original energy if you didn't need the fresh water. One nice thing about this article is that they explicitly state the most important point...that it is impractical to use this method in the only context where it would have potential for significant impact which is in the mixing of fresh water ri
    • by Luckyo (1726890)

      As I understand it, this is a project for power plant located at the point where river meets an ocean. You have ocean level salinity meeting fresh water, all available in one place.

      • by Andy Dodd (701)

        You didn't RTFA - this new "breakthrough" depends on the availability of brine that is significantly more concentrated than the ocean.

        Basically this "breakthrough" provides zero benefit compared to existing technology when used for ocean water.

        • by Luckyo (1726890)

          Actually, you didn't understand it. They are pushing to solve the solution where pressure building up in the membrane breaks it, and the higher the pressure, the more efficient the system becomes.

          So while the plant is designed, according to TFA
          "They have designed methods that harvest the energy released when fresh and saline water mix, such as when a river meets the sea."

          They tested it out with salinity up to five times that of sea water to stress test the membrane and see if the pressure will break it (the

    • by RockDoctor (15477)

      It requires saline that is MUCH more concentrated than seawater...

      No it doesn't ; it requires two fluids of differing concentration of a solute.

      I think you need to go back to your sophomore year chemistry notes and revise osmosis.

  • by Idarubicin (579475) <allsquiet@hotm a i l .com> on Monday December 02, 2013 @05:56PM (#45579155) Journal

    At a full-scale facility...

    So, we're guessing about imagined economies of scale that may or may not, hypothetically speaking, materialize, in the best-case scenario of a fully-developed, mature technology, probably some decades hence.

    ...the estimated cost of the electricity generated by such a system could be 20 to 30 cents per kWh...

    Our wild-assed guess ranges over a factor of 1.5 anyway.

    ...approaching the cost of other conventional renewable energy technologies.

    "Approaching", in this instance, meaning "costing twice as much as" pholtovoltaic systems, which already sit at the expensive end of the renewable spectrum.

  • by Anonymous Coward

    Where salt meets fresh is often an estuary. These are unique and productive habitats. Even traditional political opponents have come together to save these environments in certain cases--the green coalition from the Democrats and duck-hunting and fishing Republicans don't want these places ruined.

    Tread lightly on this. The loss may be greater than what's gained.

  • Continuous Flow (Score:4, Insightful)

    by yanom (2512780) on Monday December 02, 2013 @06:01PM (#45579201)
    It's worth noting that this would have something most other renewables (solar, wind, ... ) lack - a power output that is more or less constant day and night.
    • by Anonymous Coward

      I'm pretty sure hydro works day and night, and is the most used renewable on earth.

    • by olau (314197)

      Sure, but that's not really what you want. Far from it. You want output that follows the consumption. Many existing hydro plants can do this by virtue of the storage in the dam.

      On a related note, cost/kWh figures can be deceptive. For instance, say the cost is 0,20 USD/kWh 24/7. That's great - except at night consumption is low so you may not be able to sell the energy, or will have to sell at a much reduced price; you can still do that if the marginal costs of keeping the plan running are lower than compet

  • If not: problem! Lose lose.

  • by tp1024 (2409684)

    The only thing they will do is to remove zones of brackish water from the environment, that are usually highly prized by greenies as having high biodiversity and such stuff. Of course this is all swept to the wayside once you can make "green energy" out of all this green stuff. You'll even find conspiracy theories thrown out by eco-nuts blaming "big oil" for preventing such "innovative alternative technology" from coming to market. If that should happen, very soon they will have an epiphany, realize that in

    • Wow, this potential "Greenie" evil you are talking about sure is diabolical. People who want clean air, water and land sure are mean and evil.

      Let's stick to REAL examples on what bad people do -- it's not like the world has run out of bad decisions, good intentions going wrong, or inefficiency. You are just being too lazy to prove your point.

      Now unleash the hypothetical hounds to defeat the mythical tigers!

    • How far fetched is this scenario ... look no farther than bio-ethanol.

      Ethanol, made primarily from corn is pushed by ADM, Monsanto and some other corn producers. They make more money when corn is used -- go figure! There are various "greenie" organizations who have called it wasteful and not particularly green. Perhaps the MEDIA driven by the big pockets of corporations failed to mention the controversy around ethanol.

      Of course, I could spend 2 seconds to verify this claim; http://green.autoblog.com/2013/02 [autoblog.com]

  • If fossil fuels paid the true cost, according to the methods described by Adam Smith, the Father of Capitalism, which include the removal of mercantilist tax subsidies such as cheap extraction leases and no cost for pollution, then it might be competitive.

    But we have artificial trade barriers in the use of subsidies and exemptions for fossil fuels that drive down the cost of fossil fuels. Things like free naval shipping lane protection by the US navy and air force given to China without cost.

    Fix the source

  • As per the subject, What makes it better than hydro electricity? Hydro is great, is clean, is renewal; really the only downside the ecological destruction associated with damming up the rivers.

    I speculate that this new solution is going to have all the same issues as hydro does, at scale. If not, why not? I see a 'membrane' across the mouth of the river, i see turbines, I see "environmentalists protesting that the fish hatchery is being disrupted..."

    • by gewalker (57809)

      You missed the biggest downside of hydro power. Most of the viable hydro power is already being used. There is a good reason for that -- hydro power is the low-hanging fruit of power generation, so naturally we used it when it was available. Yes, there is some hydro not being used - small basins. The total is quite small compared the the amount we use. Lots of hydro power is not used in base load conditions, it is more valuable for peak production due to it fast ramp-up and the fact that the total water ava

    • Hydro is great, if you happen to have a gigantic dam handy holding back a huge lake before the water falls hundreds of feet. In North America, that means Hoover Dam, Niagra Falls and a couple others. For the other 99.9% of the population, you need another solution. Texas, for example, is the second largest state and I don't think there are any hydro falls anywhere in Texas. I live 120 miles from the coast and my elevation is about 60 feet. You're not going to get hydro power from the river here.

      • I live 120 miles from the coast and my elevation is about 60 feet. You're not going to get hydro power from the river here.

        Gee, that's too bad, since Bonneville Dam [wikipedia.org] is at 70 feet of elevation and 146 river miles from the Pacific, and generates 1200 MW.

        Granted, the Columbia isn't the average river, but elevation and distance from the ocean are only a couple of values that matter.

        • The area around Bonneville Dam has an elevation of around 600-1,600 feet. Bonneville Dam itself is 197 feet high.
          Since the land around it is about 1,000 feet elevation, the water is easily contained.

          > but elevation and distance from the ocean are only a couple of values that matter

          Elevation controls. If the elevation isn't high enough, you can't build a high dam, period.
          Filling a 197 foot reservoir in central Texas would require flooding the gulf coast up through the entire eastern seaboard, north to N

  • "I use salt from the sea to charge special batteries that I've made."

  • by ISayWeOnlyToBePolite (721679) on Tuesday December 03, 2013 @02:48AM (#45581707)
    So how do they arrive at the 20-30 cents/kWh? Infinite durability? This has been tried in Norway http://en.wikipedia.org/wiki/Statkraft_osmotic_power_prototype_in_Hurum [wikipedia.org] with rather underwelming results, outputting 4kW (not a typo) under ideal conditions. Granted this new plant is rated as 60 times more efficient it seems like a long way from a sound investment as the upfront cost is just to high.
    • by Teun (17872)
      So?

      with a water usage of 10 litres of fresh water and 20 litres of salt water per second

      That's the flow from a good fire hose.

      B.t.w, this plant uses PRO or Pressure Retarded Osmosis, not Reverse Osmosis.

      In the Netherlands alone, more than 3,300 m fresh water runs into the sea per second on average, that would deliver in excess of 1MW.
      In 2005 we had an experimental plant near Harlingen with an output of 5kW.
      Dutch research is especially looking at Reverse Electrolysis.

  • He's a straight-laced by-the-book detective straight out of the academy.

    He's a grizzled fisherman from the wrong side of the docks.

    Freshwater and Salty - Wednesday at 7, 8 central, on CBS.

  • Another method to diminish the amount of fresh water - we have far too much of that useless fluid.
  • Estuaries tend to have interesting ecosystems associated with them--Everglades, mangroves, etc that are also important in hurricane protection, among other things. What's the likely environmental impact of building these sorts of power plants?

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