Desalinaztion on board.

Discussion in 'Wooden Boat Building and Restoration' started by thudpucker, Dec 30, 2012.

  1. thudpucker
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    thudpucker Senior Member

    Professional Desalinators on board.
    Hunderds of Dollars and Noisy operation.

    I got to thinking....yeah I know that's a problem:p....that we might be able to make a Solar Still.
    A cookie sheet of salt water in the Sun would evaporate the water and leave the salt.

    So could we put a tent over the Cookie sheet to capture the water evap and let it run down into a collector?

    I know I'm not the first guy to bring this one up. Has anybody made it work?
     
  2. Steve W
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    Steve W Senior Member

    Of course they have, in fact you can buy the things for emergency survival. The trick is to scale it up to get a meaningfull amount of water.
     
  3. thudpucker
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    thudpucker Senior Member

    Here's a schemetic view of the process.
    [​IMG]
     
  4. Landlubber
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    Landlubber Senior Member

    ...it would of course work, till you go to sea....
     
  5. PAR
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    PAR Yacht Designer/Builder

    I've read about more then one survival situation, where this was the only way they made it, many finding they had to pee into the still to keep up the volume necessary.
     
  6. CDK
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    CDK retired engineer

    At night you can retrieve water from moist air with a plastic tent, even in the desert, but the yield is low.
    A good solar collector produces steam within minutes, so that would be much more effective, although a solution must be found to prevent clogging. One possibility is to interrupt the process at regular intervals to flush the system for a few minutes before the salinity reaches the point where deposits are formed.
     
  7. jonr
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    jonr Senior Member

    Google for "solar still".
     
  8. Steve W
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    Steve W Senior Member

    Question is how big of a system would you need to get a meaningfull amount of water, how much precious real estate would it take up. as i said before you can buy or easily make a system for survival but enough to take a shower is another story. Before the days of RO systems there were large condensation systems that ran off the engine but not much use for a sailboat.

    Steve.
     
  9. PAR
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    PAR Yacht Designer/Builder

    A gallon a day, per person is meaningful, though a wee bit light for most of us, but you'll live.
     
  10. Steve W
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    Steve W Senior Member

    Yeah, but you may not smell pretty. Lots of cruisers these days can make enough RO water for the whole crew to shower every day, though at a price.

    Steve.
     
  11. Autodafe
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    Autodafe Senior Member

    I've always thought this was an interesting idea, because the high power consumption and fragile membranes have been putting me off RO.
    However it would be fair to say that any solar still producing household quantities of water will be large.

    An average temperate-region insolation is around 5kWh/day/m^2 (18MJ/day/m^2) so assuming 100% efficiency that would evaporate just under 8 litres/day/m^2 (~0.2us gal/day/sqft).
    A good still may get 60% efficiency, but given the weight and robustness constraints for a boat still I suspect 30-50% would be more likely.

    So an cruising couple using 20L/day would need a solar still collector of at least five square meters. Not totally impossible, but difficult to retrofit without inconvenience.

    Note also the difficulty in making the collector storm and wave proof without making it grotesquely heavy.

    However if the purpose is just to extend the cruising range given limited tankage then the calculation is different. A comparatively small collector could usefully extend range, and provide an emergency "survival" supply of water, perhaps with as little as 0.5m^2/person.

    The output water of a well designed still should be superior in quality to RO water.
     
  12. michael pierzga
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    michael pierzga Senior Member

    They produced solar stills that looked like plastic inflatable beach lie lows. Ive not seen one in years.

    Now everyone uses hand pump desalinators for emergency backup.

    You best solution on a small simple boat is several water tanks and huge capacity.

    When you leave the dock your waterline should be under the water.
     
  13. CDK
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    CDK retired engineer

    Somehow I get the feeling your calculation is too pessimistic.

    There is a small DeLonghi dehumidifier humming in our guest apartment. It has an easy job now, running maybe at 5-10% duty cycle, but if the air were really moist, the little machine collects 30 liters in 24 hours. During that time it consumes 24 x 320W = 7.68 KW, so approx. 0.25KW for 1 liter of potable water.

    A 1 sq.m. solar collector in bright sunlight receives 1 KW in 1 hour, that is 860 Kcal, enough to heat 10 liters of seawater from 14 C. to boiling temperature. I know it takes another 6 KW to boil dry in free air, but that would mean a total of 7 KW for 10 liters, assuming 100% efficiency.

    The dehumidifier does 3 times better, without boiling! It uses an evaporator, a fan and the heat loss from the compressor to maintain enough temperature differential for water to condense.
    It seems to me a solar still should have a large evaporating area under glass and a seawater cooled condensing surface under the evaporator, perhaps also a small fan for forced circulation.
    The whole construction could be part of a boat deck, so it doesn't take any space at all. What remains is the question how to remove the brine without contaminating the collected water.
     
  14. Autodafe
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    Autodafe Senior Member

    CDK, I'm not sure how the de-humidifier is relevant, as it doesn't include the energy input to evaporate the water in the first place.
    True, if you could guarantee that the ambient humidity will be high you could use a de-humidifier as a water maker with no solar still at all. The downside of that is that the de-humidifier requires electricity which probably comes from a solar panel at 13% efficiency, so the "collector" area is even greater.

    You can check the insolation for your part of the world if you don't like my figures. Where I am 5kWhr/day is a bit high in winter and quite a bit low in summer, so a fair average for a water maker.

    It is worth pointing out that you can make a still with over 100% "efficiency" as I used the term. The enthalpy of vaporisation is captured by the condenser, and can be partly re-used if you're prepared to use a multi-stage still with some vacuum.
    I don't consider this practical on a small boat but it is possible.
     

  15. Autodafe
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    Autodafe Senior Member

    I seem to come across as overly negative in my last.

    I'm not saying solar stills are a bad idea, rather the opposite. However we need to start with a realistic comparison of the performance required and the performance possible.

    I would say:
    If you use a lot of water while cruising, get RO, a solar still is unlikely to satisfy.
    If you have a boat that can take the load, add more tanks, it both cheapest and most reliable.
    If you don't need the volume of water a RO setup can give and you can't afford the extra weight of tanks, then a solar still may be useful.

    Solar stills are common on land in remote and third-world areas, so performance data is not hard to find, however remember that they are usually starting with brackish water rather than seawater, and they don't roll around, both of which will lower the efficiency on a boat.
     
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