A solar still distills water with substances dissolved in it by using the heat of the

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared rad ...

to evaporate water so that it may be cooled and collected, thereby purifying it. They are used in areas where drinking water is unavailable, so that clean water is obtained from dirty water or from plants by exposing them to sunlight. Still types include large scale

concentrated solar still A concentrated solar still is a system that uses the same quantity of solar heat input (same solar collection area) as a simple solar still but can produce a volume of freshwater that is many times greater. While a simple solar still is a way of ...

s and condensation traps. In a solar still, impure water is contained outside the collector, where it is evaporated by sunlight shining through a transparent collector. The pure

water vapour (99.9839 °C) , - , Boiling point , , - , specific gas constant , 461.5 J/( kg·K) , - , Heat of vaporization , 2.27 MJ/kg , - , Heat capacity , 1.864 kJ/(kg·K) Water vapor, water vapour or aqueous vapor is the gaseous ph ...

condenses on the cool inside surface and drips into a tank. Distillation replicates the way nature makes rain. The sun's energy heats water to the point of evaporation. As the water evaporates, its vapour rises, condensing into water again as it cools. This process leaves behind impurities, such as salts and heavy metals, and eliminates microbiological organisms. The end result is pure (potable) water.

History

Condensation traps have been in use since the pre-

Incan The Inca Empire (also known as the Incan Empire and the Inka Empire), called ''Tawantinsuyu'' by its subjects, (Quechua for the "Realm of the Four Parts", "four parts together" ) was the largest empire in pre-Columbian America. The admin ...

peoples inhabited the

Andes The Andes, Andes Mountains or Andean Mountains (; ) are the longest continental mountain range in the world, forming a continuous highland along the western edge of South America. The range is long, wide (widest between 18°S – 20°S ...

. In 1952, the United States military developed a portable solar still for pilots stranded in the ocean. It featured an inflatable floating plastic ball, with a flexible tube in the side. An inner bag hangs from attachment points on the outer bag. Seawater is poured into the inner bag from an opening in the ball's neck. Fresh water is taken out using the side tube. Output ranged from to of fresh water per day."Sea Water Still"
''Popular Mechanics'', February 1952, p. 113. Similar stills are included in some

life raft A lifeboat or liferaft is a small, rigid or inflatable boat carried for emergency evacuation in the event of a disaster aboard a ship. Lifeboat drills are required by law on larger commercial ships. Rafts (liferafts) are also used. In the mil ...

survival kit A survival kit is a package of basic tools and supplies prepared as an aid to survival in an emergency. Civil and military aircraft, lifeboats, and spacecraft are equipped with survival kits. Survival kits, in a variety of sizes, contain su ...

s, though manual

reverse osmosis Reverse osmosis (RO) is a water purification process that uses a partially permeable membrane to separate ions, unwanted molecules and larger particles from drinking water. In reverse osmosis, an applied pressure is used to overcome osmotic ...

desalinators have mostly replaced them. Today, a method for gathering water in moisture traps is taught within the Argentinian Army for use by specialist units expected to conduct extended patrols of more than a week's duration in the Andes' arid border areas.

Methods

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 Dew is water in the form of droplets that appears on thin, exposed objects in the morning or evening due to condensation. As the exposed surface cools by radiating its heat, atmospheric moisture condenses at a rate greater than that at w ...

frost Frost is a thin layer of ice on a solid surface, which forms from water vapor in an above- freezing atmosphere coming in contact with a solid surface whose temperature is below freezing, and resulting in a phase change from water vapor (a g ...

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 Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase. High concentration of the evaporating substance in the surrounding gas significantly slows down evaporation, such as when humid ...

with the morning sun and thus vanish, but the lid traps the evaporating water and raises the

humidity Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present. Humidity dep ...

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 2–4 stones, plastic film or transparent

glass Glass is a non-Crystallinity, crystalline, often transparency and translucency, transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most ...

, a central weight to make the funnel and a container for the condensate. 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 .

Transpiration

During photosynthesis plants release water through

transpiration Transpiration is the process of water movement through a plant and its evaporation from aerial parts, such as leaves, stems and flowers. Water is necessary for plants but only a small amount of water taken up by the roots is used for growth ...

. Water can be obtained by enclosing a leafy tree branch in clear plastic, capturing water vapour released by the tree. 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

The wick type solar still is a vapour-tight glass-topped box with an angled roof. 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. It reflects sunlight to reduce surface heating. Residual heat that is not reflected is reemitted in a specific (

infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from aroun ...

) 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

superhydrophobic Ultrahydrophobic (or superhydrophobic) surfaces are highly hydrophobic, i.e., extremely difficult to wet. The contact angles of a water droplet on an ultrahydrophobic material exceed 150°. This is also referred to as the lotus effect, after the ...

material, so that the condensate forms into droplets and falls into a collector. A test system yielded of water per day, using a surface or approximately 1.3 L/m² (0.28 gal/ft²⁾ per day.

Efficiency

Condensation traps are sources for extending or supplementing existing water sources or supplies. A trap measuring in diameter by deep yields around per day.

Urinating Urination, also known as micturition, is the release of urine from the urinary bladder through the urethra to the outside of the body. It is the urinary system's form of excretion. It is also known medically as micturition, voiding, uresis, ...

into the pit before adding the receptacle allows some of the urine's water content to be recovered. A pit still may be too inefficient as a survival still, because of the energy/water required for construction. In desert environments water needs can exceed per day for a person at rest, while still production may average only . Several days of water collection may be required to equal the water lost during construction.

Applications

Remote sites

Solar stills are used in cases where rain, piped, or well water is impractical, such as in remote homes or during power outages. In subtropical

hurricane A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. Depe ...

target areas that can lose power for days, solar distillation can provide an alternative source of clean water.

Survival

Solar stills have been used by ocean-stranded pilots and included in life raft emergency kits. Using a condensation trap to

distill Distillation, or classical distillation, is the process of separating the components or substances from a liquid mixture by using selective boiling and condensation, usually inside an apparatus known as a still. Dry distillation is the heat ...

urine will remove the urea and salt, recycling the body's water.

Wastewater Treatment

Solar stills have also been used for the treatment of municipal wastewater, the dewatering of sewage sludge as well as for olive mill wastewater management. Mastoras P., Vakalis S., Fountoulakis M.S., Gatidou G., Katsianou P., Koulis G., Thomaidis N.S., Haralambopoulos D., Stasinakis A.S. (2022) Evaluation of the performance of a pilot-scale solar still for olive mill wastewater treatment. Journal of Cleaner Production 364C, 132695, https://doi.org/10.1016/j.jclepro.2022.132695

References

* *

Patents

* * *

External links

Making a solar still

Solar distillation

Plans for solar distillation
{{DEFAULTSORT:Solar Still Water technology Water treatment Solar power Survival skills