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Solar still
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==Methods== [[File:Puits Solaire.jpg|thumb|upright=1.2|Solar Well]] === Pit still === A collector is placed at the bottom of a pit. Branches are placed vertically in the pit. The branches are long enough to extend over the edge of the pit and form a funnel to direct the water into the collector. A lid is then built over this funnel, using more branches, leaves, grasses, etc. Water is collected each morning. This method relies on the formation of [[dew]] or [[frost]] on the receptacle, funnel, and lid. Forming dew collects on and runs down the outside of the funnel and into the receptacle. This water would typically [[evaporate]] with the morning sun and thus vanish, but the lid traps the evaporating water and raises the [[humidity]] within the trap, reducing the amount of lost water. The shade produced by the lid also reduces the temperature within the trap, which further reduces the rate of water loss to evaporation. A solar still can be constructed with two–four stones, plastic film or transparent [[glass]], a central weight to make the funnel and a container for the condensate.<ref>{{Cite web|title=Uncle John's Portable Solar Water Distiller ( for Survival )|url=https://www.instructables.com/Uncle-Johns-portable-solar-water-distiller-Fo/|website=Instructables}}</ref> Better materials improve efficiency. A single sheet of plastic can replace the branches and leaves. Greater efficiency arises because the plastic is waterproof, preventing water vapour from escaping. The sheet is attached to the ground on all sides with stones or earth. Weighting the centre of the sheet forms the funnel. Condensate runs down it into the receptacle. One study of pit distillation found that angling the lid at 30 degrees angle captured the most water. The optimal water depth was about {{convert|1|in|order=flip}}.<ref>{{cite journal|last1=Khalifa|first1=Abdul Jabbar N.|last2=Hamood|first2=Ahmad M.|date=November 2009|title=Performance correlations for basin type solar stills|journal=Desalination|volume=249|issue=1|pages=24–28|doi=10.1016/j.desal.2009.06.011|bibcode=2009Desal.249...24K }}</ref> === Transpiration === During photosynthesis plants release water through [[transpiration]]. Water can be obtained by enclosing a leafy tree branch in clear plastic,<ref>{{Cite web|title=Solar Still | website=Practical Survivor|url=http://www.practicalsurvivor.com/solarstill|access-date=2023-01-12}}</ref> capturing water vapour released by the tree.<ref>{{cite book|last1=O'Meagher|first1=Bert|url=http://www.vks737.on.net/pdfs/survival.pdf|title=Aids to survival: a handbook on outback survival|last2=Reid|first2=Dennis|last3=Harvey|first3=Ross|date=2007|publisher=Western Australia Police Academy|isbn=978-0-646-36303-5|edition=25th|location=Maylands, W.A.|page=24|access-date=7 February 2017}}</ref> The plastic allows photosynthesis to continue. In a 2009 study, variations to the angle of plastic and increasing the internal temperature versus the outside temperature improved output volumes. Unless relieved the vapour pressure around the branch can rise so high that the leaves can no longer transpire, requiring the water to be removed frequently. Alternatively, clumps of grass or small bushes can be placed inside the bag. The foliage must be replaced at regular intervals, particularly if the foliage is uprooted. Efficiency is greatest when the bag receives maximum sunshine. Soft, pulpy roots yield the greatest amount of liquid for the least amount of effort. === Wick === [[File:Wick solar still.png|thumb|Wick basin solar still.|upright=1.5]] The wick type solar still is a vapour-tight glass-topped box with an angled roof.<ref>{{cite journal|last1=Manikandan|first1=V.|last2=Shanmugasundaram|first2=K.|last3=Shanmugan|first3=S.|last4=Janarthanan|first4=B.|last5=Chandrasekaran|first5=J.|date=April 2013|title=Wick type solar stills: A review|journal=Renewable and Sustainable Energy Reviews|volume=20|pages=322–335|doi=10.1016/j.rser.2012.11.046|bibcode=2013RSERv..20..322M }}</ref> Water is poured in from the top. It is heated by sunlight and evaporates. It condenses on the underside of the glass and runs into the connecting pipe at the bottom. Wicks separate the water into banks to increase surface area. The more wicks, the more heat reaches the water. To aid in absorbing more heat, wicks can be blackened. Glass absorbs less heat than plastic at higher temperatures, although glass is not as flexible. A plastic net can catch the water before it falls into the container and give it more time to heat. === Additives === When distilling brine or other polluted water, adding a dye can increase the amount of solar radiation absorbed. === Reverse still === A reverse still uses the temperature difference between solar-heated ambient air and the device to condense ambient water vapour. One such device produces water without external power. It features an inverted cone on top to deflect ambient heat in the air, and to keep sunlight off the upper surface of the box. This surface is a sheet of glass coated with multiple layers of a polymer and silver.<ref name=":1">{{Cite web|last=Irving|first=Michael|date=June 24, 2021|title="Reverse solar still" keeps its cool to wring drinking water from air|url=https://newatlas.com/science/passive-drinking-water-harvester/|access-date=2021-06-27|website=New Atlas|language=en-US}}</ref> It reflects sunlight to reduce surface heating. Residual heat that is not reflected is reemitted in a specific ([[infrared]]) wavelength so that it passes through the atmosphere into space. The box can be as much as 15 °C (27 °F) cooler than the ambient temperature. That stimulates condensation, which gathers on the ceiling. This ceiling is coated in a [[Ultrahydrophobicity|superhydrophobic]] material, so that the condensate forms into droplets and falls into a collector. A test system yielded {{Convert|4.6|ml|usoz|abbr=on}} of water per day, using a {{Convert|10|cm|abbr=on}} surface or approximately 1.3 L/m<sup>2</sup> (0.28 gal/ft<sup>2)</sup> per day.<ref name=":1" /> === Inclined Solar Still === [[File:Sketch of an inclined basin-type solar still.jpg|thumb|Sketch of an inclined basin-type solar still]] An inclined solar still operates by allowing short-wave solar radiation to pass through a transparent glass plate while trapping the long-wave radiation emitted by the heated sand and water inside the still.<ref name=":2">{{cite journal|last1=Anjaneyulu|first1=L.|last2=Kumar|first2=E. Arun|last3=Sankannavar|first3=Ravi|last4=Rao|first4=K. Kesava|date=13 June 2012|title=Defluoridation of drinking water and rainwater harvesting using a solar still|journal=Industrial & Engineering Chemistry Research|volume=51|issue=23|pages=8040–8048|doi=10.1021/ie201692q}}</ref> This trapped heat raises the water temperature, increasing the evaporation rate. The resulting water vapor condenses on the inner surface of the glass plate and is collected using a channel. This type of still is utilized to produce potable water from brackish sources and to examine its effectiveness for defluoridation. A variation of this method, known as earth–water distillation, involves using wet sand or soil to extract water in arid regions. Sand is used within the inclined still to retain a stable water layer, preventing overflow. Without sand, feed water would spill over if its free surface height exceeded that of the collection channel.<ref name=":2" />
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