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    • CommentAuthorJoiner
    • CommentTimeOct 16th 2016
     
    One for the richer charities, or would the principle be available low-tech as a third-world village technology?

    http://www.offgridquest.com/green/waterseer-water-capture
    •  
      CommentAuthorfostertom
    • CommentTimeOct 16th 2016
     
    Hm, this can only work where there's wind, significant water in the air and subsoil is cooler than the air.
    Many places where it would be valuable, the air is bone-dry and subsoil is hot in depth, when solar irradiation is remorseless and there's no source of 'coolth'.
    Or that situation may vary day vs night.
    My guess it's fine in the US SW deserts, where under a clear sky it's cold at night, blisteringly hot by day, and plenty of wind.
    Less useful in the Gobi or Kalahari?
    • CommentAuthorEd Davies
    • CommentTimeOct 16th 2016
     
    Posted By: fostertomHm, this can only work where there's wind,
    Yep, but that's not uncommon.

    significant water in the air
    Pretty common. By my rough calculations there's often about as much water in the air in the Sahara as there is in the air on the west coast of Scotland in the winter (in absolute or specific humidity terms, not relative humidity, of course).

    and subsoil is cooler than the air.
    That's the difficult one to get 24 hours a day. One possibility would be to use a heat pipe to cool the soil at night but act as a “diode” to not contribute to warming it in the day time. This technique is used to maintain the permafrost under things like railways in Siberia.
    •  
      CommentAuthorfostertom
    • CommentTimeOct 16th 2016
     
    Is that hot day/cold night thing common in waterless areas, other than US SW deserts near to entrepreneur-idea land?
    • CommentAuthorEd Davies
    • CommentTimeOct 16th 2016
     
    Yes, it is common though not as extreme as sometimes said. See average highs and lows for Timbuktu, for example:

    https://en.wikipedia.org/wiki/Timbuktu#Climate

    You ought to be able to keep a Passivhaus comfortably cool in Timbuktu without air conditioning by doing your cooling at night. Might need MWRV (mechanical water recovery ventilation) though.

    That thing about the Romans making ice (or was it ice cream?) in the desert is, I think, probably true but depends on carefully arranging heat radiation to the sky while at the same time providing decent insulation from the ground which you could do with a heat pipe cooling arrangement, I imagine.
    • CommentAuthorskyewright
    • CommentTimeOct 17th 2016
     
    Posted By: Ed DaviesThat thing about the Romans making ice (or was it ice cream?) in the desert is, I think, probably true but depends on carefully arranging heat radiation to the sky while at the same time providing decent insulation from the ground which you could do with a heat pipe cooling arrangement, I imagine.

    Vaguely related, in the sense of an indication of the temp differences that can be achieved quite easily, albeit on a small scale...

    You can build a very effective night time cloud cover indicator based on black body radiation. I used to run one built in about 150mm of 110mm drainage pipe. Inside that were 2 tight fitting discs of 50mm EPS. The top disk had a recess ~90mm diameter & 10mm deep. In the base ,of the recess was a metal (heavy alu foil) disk painted black, and in the centre of the disk was a low mass temp sensor. A piece of acrylic (part of a CD case!) was stuck on top of the pipe to prevent draughts. To further isolate the sensor I covered the outside of the pipe with Alu foil tape.

    You then monitor the difference in temp between the sensor in the black body and the normal outdoor temp sensor. If you calibrate using the temp difference on a totally clear night you can then get an estimate of the cloud cover; A zero difference => 100% cover, a difference that is half way to the calibrated max is 50% cover.

    It worked remarkably well. With the above arrangement my clear night temp difference was around -5C IIRC.
    •  
      CommentAuthorfostertom
    • CommentTimeOct 17th 2016
     
    V gd
    •  
      CommentAuthordjh
    • CommentTimeOct 17th 2016
     
    There are other less hi-tech ways to collect water in various circumstances, of course:

    http://www.climatetechwiki.org/content/fog-harvesting

    http://www.desertusa.com/desert-people/water-solar-still.html
    • CommentAuthorEd Davies
    • CommentTimeOct 17th 2016
     
    Yes, different mechanisms would work in different circumstances. That fog thing needs fog. The solar still needs either water to be purified or damp ground so isn't a long-term source for which you need something that extracts the water from the air, one way or another (e.g., from previous rainfall). Any mechanism which extracts water from far-less-than-saturated air is likely to be reasonably high tech, I think.
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