A solar still is a simple way of distilling water, using the heat of the Sun to drive evaporation from humid soil, and ambient air to cool a condenser film. Two basic types of solar stills are box and pit stills.
In a solar still, impure water is contained outside the collector, where it is evaporated by sunlight shining through clear plastic. The pure water vapor condenses on the cool inside plastic surface and drips down from the weighted low point, where it is collected and removed.
The box type is more sophisticated. The basic principles of solar water distillation are simple, yet effective, as distillation replicates the way nature makes rain. The sun’s energy heats water to the point of evaporation.
As the water evaporates, water vapor rises, condensing on the glass surface for collection. This process removes impurities, such as salts and heavy metals, and eliminates microbiological organisms. The end result is water cleaner than the purest rainwater.
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.
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 inches) 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.
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 an inch is optimal with a decreasing trend if more is used.
The “wick” type solar still is a glass-topped box constructed and held at 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 attached to the wicks which separates 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 salt water and more product can be made. A plastic net can also catch salt water 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 application, with glass is that it is not flexible if the solar still is not a standard shape.
The system is inefficient for how much work is put into it versus the water output. In desert environments water needs can exceed 1 US gallon (3.8 L) per day for a person at rest, while still production may average 8 US fluid ounces (240 mL) 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.
Because of this, it is advised to make a solar still to supplement another water source, such as a reverse-osmosis unit or water purification tablets.
In 1952 the United States military developed a portable solar still for pilots stranded on the ocean, which comprises an inflatable 24-inch 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 2.5 US quarts (2.4 l) of fresh water could be produced. On an overcast day, 1.5 US quarts (1.4 l) was produced. Similar sea water stills are included in some life raft survival kits, though manual reverse osmosis desalinators have mostly replaced them.