Solar Still Challenge!

Discussion in 'Hobby Engineering' started by oscarg, Sep 5, 2013.

  1. oscarg

    oscarg Member

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    Hi Everyone.

    I have had this idea going around in my head for a while, and I thought this would be a great place to work it...

    I been trying to design a solar stilll (a device to extract drinkable water from seawater) using only 2 standard plastic softdrink bottles, with probably the use of just a knife.

    Most shorelines around the world have bottles washed up in them, and just running through the various ways of putting them together makes my mind spin..

    I will be putting forward my own designs in this thread.
    I challenge anyone to beat my designs

    Heres the challenge...
    Produce enough drinking water from seawater to sustain 1 person
    Produce a design that can be easily repeated.
    Show proof the design works
    Design is public domain.. the plan is to tell everyone in the world.
    Use only 2 plastic bottles​

    Winner will be the system that produces the most drinkable water.

    It is possible that this task is impossible.... thoughts?

    Oscar
     
  2. HeXa

    HeXa Member

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    why reinvent the wheel?



    EDIT: guess the tube breaks your rules
     
    Last edited: Sep 5, 2013
  3. OP
    OP
    oscarg

    oscarg Member

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    Not Solar, only works once

    Points for fast comment :)

    Would love that tube..

    This is not solar.

    I was thinking of variations using gaffa tape and tubes and black paint would all make it much more efficient..

    Also, this would not extract the salt from seawater, this would boil freshwater... technically if you have several days, you can build a bucket filter then just leave the cleanishwater in the bottle in the sun for a couple of days and it kills all the bacteria/parasites anyway.

    What is bizarre about that video is that it is raining.. but I think he is talking about radioactive fallout..
    In my humble opinion... if you are in the presence of enough fallout that the rain is not drinkable, then getting a drink is the least of your worries :)

    EDIT Actually it is a still :)
     
    Last edited: Sep 5, 2013
  4. HeXa

    HeXa Member

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    Last edited: Sep 5, 2013
  5. OP
    OP
    oscarg

    oscarg Member

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    Good Stuff,

    I should really have googled more, :)
    Although technically the cup is a big advantage -

    I wonder if a small bottle and a big bottle would count..

    I wonder how many of these would be needed to supply drinkable water for a single person..

    Im sure you can see the "castaway" scenario I am aiming this at..

    Im going to test...
     
  6. pantner

    pantner Member

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    a cup is just a small bottle with the top cut off.

    so, get a 2L bottle and then a 600ml/390ml/can, cut the top off so the sides are straight, and put it inside the same way.

    EDIT - and actually, the first one could probably be solar, it would just be much slower. fire=fast :)
     
    Last edited: Sep 6, 2013
  7. Widman

    Widman Member

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    Also the bubbling of water up the tube using the method in the first video will carry crap as well. I guess if you are post-apocalyptic or lost in the desert it won't matter that much, but it would probably be worth treating the water 2 or 3 times if not in imminent danger of dying....
     
  8. OP
    OP
    oscarg

    oscarg Member

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    First Try... Glad I didnt actually need a drink

    Based on a YouTube Vid... Fourth post

    From a scale of 0 (absolutely nothing produced) to 10 (industrial desalination plant) This one scored 0.00001

    Amount of drinkable water produced: Significantly less than 1ml

    After 3 hours of direct, if slightly cloudy sunlight on my sunny morning balcony (which has in the past been compared to the chronicles of Riddick in terms of sunlight power) it got misty in there, but very little water actually appeared at the bottom.

    Im not even sure if the couple of droplets of moisture at the bottom of the bottle werent there before :)

    There are a couple of variables to play with here;

    - less water water in the cup next time?
    - maybe a solid or tinted cup would have heated up more..
    - maybe a tinted/painted bottle would have heated up more.
    - I had an issue with joining the top to the bottom, since I cut them where they were even, I will next time make the upper part slightly smaller so it slides inside the bottom part.

    [​IMG][​IMG]


    There is about 2 drops... you probably cant even see them in this photo, so its a good thing you werent relying on them for anything... like survival.

    Trying again tomorrow morning..
     
    Last edited: Sep 8, 2013
  9. mtma

    mtma Member

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    Yes your thing wouldn't be capable of capturing much of the energy that hit it due to the fact that it is mostly clear. keeping the same design I would probably make the cup inside black.

    The other thing is that there's nothing in particular promoting the condensation of the water after vaporisation. Cooling the outside of the bottle even with a bit of spray and fanning should improve performance considerably.
     
  10. OP
    OP
    oscarg

    oscarg Member

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    Pressure is key as well.

    Pressure also important..

    It would be great to get it airtight, but there would no way to get the cup in.
    Gaffa tape would be perfect, but the challenge is for just 2 bottles... although any variant is acceptable.

    I have been thinking of doing a variant using the spray nozzle, since there is an outside chance one could wash up on a beach. It is feasible you could find one in a medium/small boat and grab it if the boat was sinking... the tube and screw top would probably be essential to make a still that would work in the rolling and rocking environment of a boat.

    Something like this...
    [​IMG]

    For the moment I will concentrate on a simple two bottle method.
     
    Last edited: Sep 6, 2013
  11. RobRoySyd

    RobRoySyd Member

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    Fairly good chance of finding a sheet of plastic washed up.
    Dig hole in moist sand or soil. Adding vegetation in hole can help. Place cup or coconut shell in middle to collect water.
    Cover with plastic leaving it loose enough for smallish rock to create a dimple in the middle. Seal edge of plastic sheet edge with sand or soil.
    This device should yield enough clean water per day to sustain a human.

    This will work much better than a putting water in a bottle, the surface area from which the water can evaporate is much larger.
     
  12. OP
    OP
    oscarg

    oscarg Member

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    2nd Attempt.. better, but still thirsty

    Worked a couple of bugs out.

    Started with 200mls of water in the dare cup

    Produced about 10mls of drinking water

    Used some magic tape to seal it up a bit, just because I knew this one would suck, and was more looking for proof of concept..
    EDIT: technically it would blow since the hot air and moisture would be escaping out.
    It still had holes though, and water definitely leaked to outside the bottle..

    Heres what I learned..

    1) before you cut the top off the outside bottle, mark it, so you can orient the two parts to their original position re each other... to get a solid seal is vital (more pressure means more heat means more condensation)

    2) The outside bottle should be as clear as possible, while the inside one should be as dark and heavy as possible (absorbs more light and bigger thermal mass).

    3) A tool that can puncture would make the work alot easier especially if you are making a complex one.
    The Gerber tool I am using always scares me a little, it is VERY sharp and always wide open when unfolded.
    Its very small and effective for its size though.

    4) Its not just the direct sunlight that does the work (although it does alot) leaving it overnight had (a small effect).

    5) a bottle with flat sides is the easiest to get a good seal (thats my next one)

    [​IMG][​IMG][​IMG]
     
    Last edited: Sep 8, 2013
  13. 1shot1kill

    1shot1kill Member

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    Your first problem is that using even 2 litre bottles, you can't produce enough water to sustain one person.

    http://iwtc.info/wp-content/uploads/2010/09/PERFORMANCE-OF-SOLAR-STILL-WITH-A-WICK.pdf
     
    Last edited: Sep 8, 2013
  14. OP
    OP
    oscarg

    oscarg Member

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    Point is to see what can be done.

    Outstanding Link 1shot.

    The point is more to see what can be done...
    At the moment it looks like a single bottle and a drink or tin can could be capable of producing 150ml per day, or possibly more.

    The second video (4th reply) seems to imply he can get that after 2 hours

    I am assuming 2 litres per day minimum is required.
    This means if you found (for example) 10 plastic bottles and used them separately with other means (Roys for example), you could produce enough to live indefinitely..

    The question (changed slightly) would be...
    "What is the ideal design to get the most water from 1 or 2 plastic bottles"
    becomes a vital question.

    I want to test it for myself.

    Your link does show the maths based on ml/m^2
    at some points gets to 3600 ml/m^2
    Ill do the maths and see what I come up with...
     
    Last edited: Sep 8, 2013
  15. OP
    OP
    oscarg

    oscarg Member

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    Third Attempt... Left it too long

    Got together some more "ideal" parts.

    A big V8 2 litre square bottle and a LOL can.
    [​IMG]
    If you have to ask why I used the LOL can, you wouldnt understand the answer ;)
    I cut the can off at about the height of the eyes.


    I put in 100 mls water... probably should have used 200.


    Vertical sides are important for the bottle to get a good seal,

    [​IMG][​IMG]

    left it for almost three days..

    It was cloudy on the first day, and barely got steamy.
    Second day it got pretty wet inside, but not alot seemed to collect on the bottom.
    The seal was good, but not perfect so it seems most of the water simply evaporated out after 3 days..
    Means I had an air leak.

    At the end there was around 10 mls in the bottle and the can was empty...

    [​IMG]
    Was about half a mounthful.

    There is too much lame and not enough fame here...

    So I am going to make a more advanced one using 2 bottles, some gravel, one of the spray heads and 750 mls of ordinary household bleach and maybe a tiny amount of tape... stay tuned...
     
    Last edited: Sep 10, 2013
  16. OMARK

    OMARK Member

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    oscarg the spray nozzle is cheating. if your on a sinking boat your lucky to grab a life jacket let alone a spray nozzle lol - if you had time to grab that you'd instead grab a caterers roll of cling wrap and just make a proper solar still right.....

    i love the idea and where you're trying to go here but i dont think its possible to produce any amount able to sustain life. if this is a uni assignment, it'll come down to who produces the most ml.

    my only truly constructive idea is 2 bottles top to top, sealed somehow, only enough water in one that it does not flow through when laid horizontally, heat bottle with water over coals/fire, this creates quicker evap/boil stea/vapor can then condense at cooler end (cover with palm leaves?). life sustainable only with multiple bottles
     
  17. Snowblindnz

    Snowblindnz Member

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    Considering your current design I would try:
    -Using less water on the inside bottle
    -Try and have a large as possible surface area at the top of the inside bottle.
    -Drain the water via a small hole from the bottom

    Reason being:
    - Less water: you are using the sunlight to heat up all the water + air in the big bottle (less water = less energy required = less time). You may have to refill it every few hours though.
    -Larger surface area: evaporation only occurs at the waters surface, where it makes contact with the air in the big bottle, so you want as much surface area as possible on the small bottle.
    -Drain the water: The big problem I see with your current design is that the clean water is getting evaporated again once collected at the bottom (possibly at a faster rate than the small bottle as it has a larger surface area).

    Suggestions:
    - Use less water, refill frequently
    - Cut the small bottle in half length ways, sit the big bottle on its side and sit the small bottle in sideways. this should give you 4-5x the surface area of an upright small bottle (depending on the small bottles dimensions)
    - Lean the big bottle on a slight angle so water drains to a bottom corner, poke a small hole, and place the other half of the small bottle underneath to collect the clean water as it drips out. Keep this collection half covered/cool so water collected doesn't evaporate again (would be useful to have a 3rd bottle for this, but you said 2 bottles).

    Disclaimer: I am not a professional, or a scientist.

    Good luck :)
     
  18. BlueRaven

    BlueRaven Member

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    Thought I'd flex my mad MS Paint skills :)lol:) and give this a crack.

    How about something like this?


    Click to view full size!


    Should be fairly self-explanatory. Those familiar with the design of a traditional "hole in the ground + plastic sheet" solar still will see that this is very similar, but on a smaller scale.

    Reasoning behind this design:

    - Larger surface area for evaporation of the liquid to be distilled by using the large bottle as the "source" vessel.

    - Minimised evaporation of the distilled liquid due to smaller surface area of the "destination" vessel.

    - Any liquid that does evaporate from the "destination" vessel will be likely to condense on the upturned top/cap of the large bottle above it, and should run back down into the "destination" vessel along with the rest of the condensate.

    - Bonus: if you could find/make a narrow tube (ie. a straw) and could make a hole of the same diameter in the cap of the large bottle, you could drink any condensate from this device as soon as it was available without disturbing the still.

    Challenges/drawbacks
    :

    - Getting a good seal between the bottom of the large bottle and its upturned upper half. Sand/soil/pebbles might be used to provide some mass to weigh the top half down and help it seat snugly into its base.

    - Preventing the destination vessel from floating in the liquid to be distilled. Small pebbles/rocks might be used to weigh it down.

    - Stably locating the destination vessel under the upturned cap. See above for a potential solution (two birds with one stone, good stuff in design terms. :)).
    Or twigs/sticks etc. could be used as locating "spokes" to help keep the destination vessel in its correct place.
    These would be threaded crosswise through small holes punched around the upper rim of the destination vessel.
    Wouldn't have to be precision engineered, just good enough to stop the destination vessel from rattling around in there.

    - A dark-tinted large bottle would obviously be best. As far as I know, there are still many drinks sold around the world in brown or dark green PET bottles.
    Wrapping the whole apparatus in a dark material such as cloth or plastic might improve performance a bit.

    Whether this design could produce enough drinkable water per day to sustain an adult human is open to debate/experiment. It would certainly be easy to refill, but that might not be good enough.
    It's entirely possible that the restriction of using commonly available plastic bottles (such as might wash up on a beach) limits the available surface area for evaporation too much, regardless of the design.
    With a traditional "hole in the ground" still, your only limitation on evaporative surface area is the size of the plastic sheet you use. You can also place cut vegetation into the hole, which will add its moisture content to the distilled end result.
    That's not really possible with this design, as I think any plant matter placed into the source liquid would just end up absorbing it rather than expiring its existing moisture content into the sealed atmosphere of the bottle.
    EDIT: Actually, scratch that... if trying to distil seawater, the plant matter would just leach its moisture content into the seawater via osmosis due to the higher concentration of salt there.
    This seems an inefficient way to add moisture, when you could simply just top it up with more seawater. :)
     
    Last edited: Nov 7, 2013

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