A solar still distills water with substances dissolved in it by using the heat of the Sun 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.
There are many types of solar still, including large scale concentrated solar stills and condensation traps (better known as moisture traps amongst survivalists). In a solar still, impure water is contained outside the collector, where it is evaporated by sunlight shining through clear plastic or glass. The pure water vapour condenses on the cool inside surface and drips down, where it is collected and removed.
Distillation replicates the way nature makes rain. The sun's energy heats water to the point of evaporation. As the water evaporates, water vapour rises, condensing into water again as it cools and can then be collected. This process leaves behind impurities, such as salts and heavy metals, and eliminates microbiological organisms. The end result is pure distilled drinking water.
Today, a method for gathering water in moisture traps is still taught within the Argentinian Army for use by specialist units expected to conduct extended patrols of more than a week's duration in the arid border areas of the Andes.
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 target areas that can lose power for days, solar distillation can provide an alternative source of clean water.
Several methods of trapping condensation exist:
This method was first used by the peoples of the Andes. A pit is dug into the earth, at the bottom of which is placed the receptacle that will be used to catch the condensed water. Small branches are placed with one of their ends inside the receptacle and their other ends up over the edge of the pit, forming a funnel to direct the condensed water into the receptacle. A lid is then built over this funnel, using more small branches, leaves, grasses, etc. The completed trap is left overnight, and moisture can be collected from the receptacle in the 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 water that is lost. The shade produced by the lid also reduces the temperature within the trap, which further reduces the rate of water loss to evaporation.
Today, with the advent of plastic sheeting, the moisture trap has become more efficient.
The method is very similar to that described above, but a single sheet of plastic is used instead of branches and leaves. The greater efficiency of this type of trap arises from the waterproof nature of the plastic, which doesn't let any water vapour pass through it (some water vapour escapes through the leaves and branches of the first method). This efficiency requires a certain amount of diligence of the part of the user, in that the plastic sheet must be firmly attached to the ground on all sides; this is often accomplished by using stones to weight the sheet down and/or covering the edges of the plastic sheet with earth (such as that dug out to make the hole in which the trap sits). Weighting the centre of the plastic sheet down with a stone forms the funnel via which the condensed water will run into the receptacle.
Water can be obtained by placing clear plastic bags over the leafy branch of a non-poisonous tree and tightly closing the bag's open end around the branch. Any holes in the bag must be sealed to prevent the loss of water vapour.
During photosynthesis plants lose water through a process called transpiration. A clear plastic bag sealed around a branch allows photosynthesis to continue, but traps the evaporating water causing the vapour pressure of water to rise to a point where it begins to condense on the surface of the plastic bag. Gravity then causes the water to run to the lowest part of the bag. Water is collected by tapping the bag and then resealing it. The leaves will continue to produce water as the roots draw it from the ground and photosynthesis occurs.
The vapour pressure of water in the sealed bag can rise so high that the leaves can no longer transpire, consequently when using this method, the water should be drained off every two hours and stored. Tests indicate that if this is not done the leaves stop producing water.
If there are no large trees in the area, clumps of grass or small bushes can be placed inside the bag. If this is done the foliage will have to be replaced at regular intervals when water production is reduced, particularly if the foliage must be uprooted to place it in the bag.
Efficiency is greatest when the bag receives maximum sunshine at all times. Exposed roots are tested for water content. Soft, pulpy roots will yield the greatest amount of liquid for the least amount of effort.
Condensation trap efficiency
Condensation traps are not in themselves a sustainable source of water; they are sources for extending or supplementing existing water sources or supplies, and should not be relied on to provide a person's daily requirement for water, since a trap measuring 40 cm (16 in) in diameter by 30 cm (12 in) deep will only yield around 100 to 150 mL (3.4 to 5.1 US fl oz) per day.
One method to increase the water output is to urinate into the pit before placing the receptacle in. This increases the moisture content of the earth, reducing the amount of water vapour that the earth can subsequently absorb.
A simple basin-type solar still can be constructed with 2–4 stones, plastic film or transparent glass, a central weight to make a point and a container for the condensate. A cubic hole in moist ground is created of about 30 cm (12 in) on each side. Into the centre of this hole, a collection container is placed. Then a sheet of plastic film is stretched over the hole. Stills can also be made from water bottles or plastic bags.
An alternative method of the solar still is called the transpiration bag. The bag is a simple plastic bag and it folds over a stemmed plant with a corner pointing down to allow the condensate to pool. From there a person can remove the water by taking the bag off and pouring the water out or one can make a tiny incision into the corner to drip water into a cup. Its advantage over the basin type solar still mentioned before is that it only requires a bag like one can get at the grocery store. It doesn't need to be completely transparent. A disadvantage of the transpiration bag is the requirement for a plant in direct sunlight or heat to take the condensate.
In a study performed in 2009, variations to the angle of plastic and increasing the internal temperature of the hole versus the outside temperature made for better water production. Other methods used included using a brine to absorb water from and adding dyes to the brine to change the amount of solar radiation absorbed into the system. During the adjusted tilt angle experiment, the different angles used by the different researchers created different results and it was difficult for any of them to get a definite answer. In the graph, a bell curve is observed with the maximum water output being at 30 degrees angle adjustment. Each brine depth created a different amount of water and it is noted on the graph that about 25 millimetres (1 in) (2,5 cm) is optimal with a decreasing trend if more is used.
The “wick” type solar still is a glass-topped box constructed and held at an angle to allow sunlight in. Salt water poured in from the top is heated by sunlight, evaporating the water. It condenses on the underside of the glass and drips to the bottom. A pool of brine in the still is attached to the wicks which separate the water into banks to increase surface area for heating. The distilled water comes out of the bottom and, depending on the quality of construction, most of the salt has been purged from the water. The more wicks, the more heat can be transferred to the saltwater, and more products can be made. A plastic net can also catch saltwater before it falls into the container and give it more time to heat up and separate into brine and water. The wick type solar still is made vapor-tight, as in the vapor does not escape to the atmosphere. To aid in absorbing more heat, some wicks are blackened to take in more heat. Glass's absorption of heat is negligible compared to plastic at higher temperatures. A problem, depending on the application, with glass is that it is not flexible if the solar still is not a standard shape.
The pit still may be inefficient as a survival still, requiring too much construction effort for the water produced. In desert environments water needs can exceed 3.8 litres (1 US gal) per day for a person at rest, while still production may average 240 millilitres (8 US fl oz) per day. Even with tools, digging a hole requires energy and makes a person lose water through perspiration; this means that even several days of water collection may not be equal to the water lost in its construction.
In 1952, the United States military developed a portable solar still for pilots stranded on the ocean, which comprises an inflatable 610-millimetre (24 in) plastic ball that floats on the ocean, with a flexible tube coming out the side. A separate plastic 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 by the pilot using the side tube that leads to bottom of the inflatable ball. It was stated in magazine articles that on a good day 2.4 litres (2.5 US qt) of fresh water could be produced. On an overcast day, 1.4 litres (1.5 US qt) was produced. Similar sea water stills are included in some life raft survival kits, though manual reverse osmosis desalinators have mostly replaced them.
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