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Sunlight Helps Turn Salty Water Fresh

Posted by Soulskill on from the that's-very-kind-of-the-sunlight dept.

MTorrice writes "With energy-efficient desalination techniques, water-starved communities could produce fresh water from salty sources such as seawater and industrial wastewater. But common methods like reverse osmosis require pumping the water, which uses a substantial amount of energy. So some researchers have turned to forward osmosis, because in theory it should use less energy. Now a team has demonstrated a forward osmosis system that desalinates salty water with the help of sunlight. The method uses a pair of hydrogels to absorb and squeeze out freshwater."

13 of 58 comments (clear)

  1. Am I missing something? by Anonymous Coward · · Score: 5, Insightful

    How is this distinctly more efficient than simply using sunlight to warm water, which evaporates, and collecting the fresh water that condenses? Desalination plants work like this, except they tend to use energy from some other source to boil the incoming seawater.

    1. Re:Am I missing something? by OhANameWhatName · · Score: 4, Insightful

      How is this distinctly more efficient than simply using sunlight to warm water, which evaporates, and collecting the fresh water that condenses?

      It's patentable.

    2. Re:Am I missing something? by icebike · · Score: 4, Informative

      How is this distinctly more efficient than simply using sunlight to warm water, which evaporates, and collecting the fresh water that condenses? Desalination plants work like this, except they tend to use energy from some other source to boil the incoming seawater.

      Efficiency isn't one of the claims being made here. In fact TFA indicates that at the lease an order of magnitude improvement is required to get this anywhere near competitive.

      But they do mention that the highest temperature required is 30C, with is well withing what you can collect with nothing more than greenhouse, and a heck of a lot lest than evaporator processes need.

      However, the summary jumped to conclusions about the seawater bit.

      The device also struggles with desalinating seawater, which has a salt concentration about 17 times greater than the team’s test solution, As a result, he says the current method would be most useful for purifying industrial wastewater streams that have a lower salt concentration.

      .

      So, not really practical at this time.

      --
      Sig Battery depleted. Reverting to safe mode.
    3. Re:Am I missing something? by Anonymous Coward · · Score: 5, Insightful

      That is generally what I was wondering. And you don't have to actually boil the water, a proper dome will allow the water to evaporate, collect on the inner surface, and drip down sides for collection. Problem there is the large size you need for it to collect enough to be useful.

      I think you're not thinking big enough.

      Imagine a large sphere, maybe 8000+ miles across, illuminated by natural sunlight. You could put salt water on the outer surface of the sphere -- enough to cover 3/4 or so of the surface -- and it would naturally evaporate and condense above the surface of the sphere in certain regions and fall down in drops. A system of canals could be used to conveniently collect the water. All you would have to do is put devices that use water in those regions, and they'd have a steady supply of water. My calculations show that you could easily get 1000 kg per square meter per year in the right areas, without having to put in energy at all.

      Of course, there would also be parts of the sphere where almost no water would fall, but that problem is pretty easy to solve -- just don't put anything that needs water in those areas.

    4. Re:Am I missing something? by Anonymous Coward · · Score: 2, Informative

      Evaporation takes a great deal of energy: about 2 MJ/kg.

      If the solar flux is 500 W/m^2 and the system is otherwise perfectly efficient, that would permit evaporating at most 0.9 L/m^2-hr.

      The system in the article can produce 10 L/m^2-hr with unconcentrated sunlight of that intensity, or 25 L/m^2-hr if concentrated to 2 kW/m^2. So it's at least 10 times more efficient than evaporation, and in practice quite a lot more than that.

      Concentrating the sunlight is less energy-efficent, but allows a given area of their fancy stuff to produce water faster.

    5. Re:Am I missing something? by Anonymous Coward · · Score: 3, Insightful

      I wonder why people choose to live in such inhospitable places.

      At least in the US, it's because the people who live there don't realize that the area they have chosen to live cannot sustain them. They think of it as paradise, since they have "perfect weather year-round". It's up to Someone Else to worry about pumping fresh water over the Tehachapi Mountains, and about where that water is coming from.

  2. Wait, I've heard this one before! by Somebody+Is+Using+My · · Score: 5, Funny

    Desalinating salty water using sunlight?

    Oh right.

    IT'S CALLED RAIN!
    (patent pending)

    1. Re:Wait, I've heard this one before! by bob_super · · Score: 5, Funny

      Your patent is useless, your technology is just a stream served by my Cloud.

    2. Re:Wait, I've heard this one before! by Em+Adespoton · · Score: 2

      Cloud-based desalination will be the next big thing, I'm sure of it! People will be providing DsaaS solutions globally!

      I use three different modules: roof, gutter and barrel, which together take advantage of the synergistic features enabled by cloud-based desalination.

      The patent applications should start flooding in any day now....

  3. What about this system? by GoodNewsJimDotCom · · Score: 2

    Design a robotic software to keep mirrors aimed at a focal point.

    At the focal point, have sea water pumped into a concrete basin.

    Have a steam engine that takes sea water input, and makes electricity and desalinated water output

    Mirrors or silvery material is relatively inexpensive. Once you developed robotic sun tracking/aiming software, that isn't too expensive either. The electricity generated by the system can go towards pumping sea water into it.

    --
    God spoke to me
  4. Re:system that desalinates salty water by gargleblast · · Score: 2

    ... division of redundancies division, at 16:30 PM after afternoon tea.

  5. Use sunlight to pump water... by viperidaenz · · Score: 3, Interesting

    Already done. Over a billion years ago too.
    It's one of the methods plants use to extract nutrients from the soil.
    Osmosis does some of the work, but it is also assisted by the fact the water travels up sealed tubes in the plant to the leaves, which when the water evaporates, creates a negative pressure in that tube, sucking more up.

    How else do you think a 50m tall tree can push water to the top, overcoming 70psi of pressure?

    1. Re:Use sunlight to pump water... by Smauler · · Score: 2

      Already done. Over a billion years ago too.

      If only the... wait, over a billion years ago? There was basically nothing on the earth a billion years ago. No plants, no nothing. There was stuff in the sea.

      Also... yes, people have thought to look at plants techniques. It is one of the starting points.