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posted by takyon on Sunday April 12 2015, @07:57PM   Printer-friendly
from the salty-savior dept.

Justin Gillis writes in the NYT that as drought strikes California, residents "can't help but notice the substantial reservoir of untapped water lapping at their shores — 187 quintillion gallons of it, more or less, shimmering invitingly in the sun."

Once dismissed as too expensive and harmful to the environment, desalination is getting a second look. [...] A $1 billion desalination plant to supply booming San Diego County is under construction and due to open as early as November, providing a major test of whether California cities will be able to resort to the ocean to solve their water woes. [...] "It was not an easy decision to build this plant," says Mark Weston, chairman of the agency that supplies water to towns in San Diego County. "But it is turning out to be a spectacular choice. What we thought was on the expensive side 10 years ago is now affordable."

Carlsbad's product will sell for around $2,000 per acre-foot (the amount used by two five-person U.S. households per year), which is 80 percent more than what the county pays for treated water from outside the area. Water bills already average about $75 a month and the new plant will drive them up by $5 or so to secure a new supply equal to about 7 or 8 percent of the county's water consumption.

Critics say the plant will use a huge amount of electricity, increasing the carbon dioxide emissions that cause global warming, which further strains water supplies. And local environmental groups, which have fought the plant, fear a substantial impact on sea life. "There is just a lot more that can be done on both the conservation side and the water-recycling side before you get to [desalination]," says Rick Wilson, coastal management coordinator with the environmental group Surfrider Foundation. "We feel, in a lot of cases, that we haven't really explored all of those options."

 
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  • (Score: 0) by Anonymous Coward on Sunday April 12 2015, @11:18PM

    by Anonymous Coward on Sunday April 12 2015, @11:18PM (#169459)

    A desalination plant near Perth, Western Australia has a solar plant next to it, which seems like the perfect match to me.

    If the desalination plant isnt required to operate 100% of the time, it can work when their is excess power available and store the result (clean water).

  • (Score: 3, Interesting) by subs on Sunday April 12 2015, @11:57PM

    by subs (4485) on Sunday April 12 2015, @11:57PM (#169469)

    On paper it seems like a reasonable idea, until you run the numbers on it. Desalination is typically done by one of two methods: evaporation or reverse osmosis. Evap is about 1.5x as energy intensive as reverse osmosis and needs sizable amounts of land, but is relatively cheap to implement. Reverse osmosis is more energy conserving and compact, but much more expensive to implement. Now solar also needs lots of land and given that it's typically ~20% efficient, even if you drove reverse osmosis with it, it'd still be about 2-3x less efficient in terms of space usage than a simple evap plant. Not to speak of expensive! First the expensive solar cells & then a reverse osmosis plant.
    Put simply, if you want to use solar power to desalinate water, just use the sunlight directly and cut out the middle man.

    • (Score: 0) by Anonymous Coward on Monday April 13 2015, @01:40AM

      by Anonymous Coward on Monday April 13 2015, @01:40AM (#169492)

      It would be neat if solar thermal energy and desalination could be combined. Pump salt water into tower, heat with mirrors, separate.

    • (Score: 2) by kaszz on Monday April 13 2015, @01:56AM

      by kaszz (4211) on Monday April 13 2015, @01:56AM (#169497) Journal

      So how would a plant that uses solar concentrators for the thermal energy to evaporate sea water, condensate it and sterilize work out economically?

      Or even more advanced. Use solar concentrators to cook water such that it's pushed through an osmosis filter?

      Both solutions without using electricity as a power carrier. Nor any nuclear dirty stuff.

      • (Score: 2) by c0lo on Monday April 13 2015, @03:17AM

        by c0lo (156) Subscriber Badge on Monday April 13 2015, @03:17AM (#169529) Journal

        Use solar concentrators to cook water

        Sorry, I like my water rare - blue rare if possible.

        --
        https://www.youtube.com/watch?v=aoFiw2jMy-0 https://soylentnews.org/~MichaelDavidCrawford
      • (Score: 2) by subs on Monday April 13 2015, @11:43AM

        by subs (4485) on Monday April 13 2015, @11:43AM (#169648)

        I have simplified this topic quite a bit - the details are a lot more complicated - to just focus on the basic energy requirements. Google is your friend if you want the nitty-gritty details. But in short, it boils down to your electricity vs. land cost. If land is cheap, do an evaporation-type process. If electricity is cheap, do RO. As an added bonus, if fossil fuel is cheap, do distillation-type processes (much of the middle east does this).

        Or even more advanced. Use solar concentrators to cook water such that it's pushed through an osmosis filter?

        I'm not aware of any direct solar-driven process which can produce pressures that would be sufficient for RO *and* be more efficient than electrical pumps (steam engines, while certainly cool, are horribly inefficient).
        As you can see [wikipedia.org], both evap-type processes and RO-type processes are fairly similar in energy requirements. My original point was, there's no point in taking RO and then multiplying its energy requirements by ~4-5x by linking it with a solar power plant - i.e. if solar-powered is your goal, just use the sun directly instead of going through an unnecessary, inefficient and expensive electrical conversion step.

        Nor any nuclear dirty stuff.

        "Dirty" is a very relative word and there are certainly ways to address that (although we have collectively decided not to do anything about it). The beauty about heat-engine power plants is that you can desalinate sorta "for free" just using waste heat from the plant without affecting electrical generation.

        • (Score: 2) by kaszz on Monday April 13 2015, @12:41PM

          by kaszz (4211) on Monday April 13 2015, @12:41PM (#169665) Journal

          I'm not aware of any direct solar-driven process which can produce pressures that would be sufficient for RO *and* be more efficient than electrical pumps (steam engines, while certainly cool, are horribly inefficient).
          As you can see [wikipedia.org], both evap-type processes and RO-type processes are fairly similar in energy requirements. My original point was, there's no point in taking RO and then multiplying its energy requirements by ~4-5x by linking it with a solar power plant - i.e. if solar-powered is your goal, just use the sun directly instead of going through an unnecessary, inefficient and expensive electrical conversion step.

          Put it simple. If you have one container with sea water connected to an osmosis filter which is connected to an empty container. And then heat the container with water. I think that the steam pressure will force its way to the empty container until pressure equilibrium is achieved.
          By repeating this process one should be able to continuously desalinate.

          However I have seen a type of desalination which uses a technique which is even better (less energy, less cost) than both evaporation and osmosis. But then I have to dig deeply.

          • (Score: 3, Insightful) by subs on Monday April 13 2015, @12:52PM

            by subs (4485) on Monday April 13 2015, @12:52PM (#169669)

            I think that the steam pressure will force its way

            If you've already invested enough energy to evaporate water to steam, why not just capture & condense the steam to get distilled (i.e. non-salty) water, instead of then adding an additional lossy step for no reason? The steam *is* your product, salts do not evaporate at 100C.

    • (Score: 0) by Anonymous Coward on Monday April 13 2015, @04:51AM

      by Anonymous Coward on Monday April 13 2015, @04:51AM (#169543)

      Solar cells are fairly cheap AND..... have ever been to West Australia? Space is the least of anyone's issues.

      It's literally one big barren desert wasteland almost a third the size of the lower 48 and one city of any size at all and even Perth is under 2 million - and only 2.5 million in total in WA. It's utterly perfect for solar or anything that uses lots of space.