Solar still

Solar still built into a pit in the ground

Overview

A solar still is a water collection method used typically for emergencies. The principle of the solar still is to use the sun’s heat energy magnified by a clear sheet of glass or plastic into a hole with a focus point. The water is evaporated out of the ground to land onto the underside of the plastic wrap that acts a dish to prevent water from going into the atmosphere. The condensate from the evaporation pools into the center point created by a weight and drips into a container, typically a bowl or a cup. To maximize water output, a sunny day and damp soil is preferred.

Puits Solaire

Typical materials for a Basin-type Solar Still

2-4 stones, plastic wrap or transparent glass, a weight for the center to make a point and a container for the condensate.

How to build the "standard" basin solar still

Step 1: Dig a hole about 12 by 12 by 12 inches. Step 2: Place a container, like a cup, into the center of the hole. Step 3: Stretch a sheet of plastic wrap over the hole and hold the ends down with rocks or stakes. Make sure that there is a little tension on the plastic wrap Step 4: Place another, slightly smaller, rock onto the center of the plastic wrap creating a dip pointing towards the cup on the inside of the still. Step 5: Let it set for several hours in the sun and at the end of the day, you may retrieve the condensate from the cup. Results may vary on how much sun there was and whether or not the soil was moist.

Variations of the Solar Still

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 is the requirement of the transpiration bag is the requirement of 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.

Another method for the solar still is the “wick” type solar still. A box is constructed and held at angle. Salt water is poured in from the top and then the heat from glass panel magnifying sunlight evaporates the water and has the condensate drip to the bottom leaving puddle of brine in the still attacked to the wicks that 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 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.

This file shows how a wick basin solar still works. Notice how much more complex this is compared to a basin solar still pictured above.

Practical considerations

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 a survival situation where you are stationary, this wouldn’t be too big of an issue if you are being saved from that location.^{[citation needed]} But, if you are constantly on the move, this wouldn’t be a great idea due to the stop time needed to make one.^{[citation needed]}

Seawater still

In 1952 the United States military developed a portable solar still for pilots stranded on the ocean, which comprises a large 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.

In fiction

There are many films and books talking about this very useful technique. There are some:

In Yann Martel's novel Life of Pi, the main character, Piscine Molitor Patel (Pi), survives on a life boat in the Pacific with a Bengal tiger by using twelve solar stills to extract fresh water from saline ocean water.

In the 2002 horror film 28 Days Later about a "zombie apocalypse" in Britain, Frank tries to use this technique after seeing it on television, but he is unable to use it properly.

See also

• Desalination
• Freshwater
• Solar cooker
• Solar water disinfection
• Watermaker
• Solar Pyramid

References

• US 3337418, "Pneumatic solar still" 
• US 4235679, "High performance solar still" 
• US 4966655, "Plastic covered solar still" 
• Jackson RD, Van Bavel CH (1965 Sep 17). "Solar distillation of water from soil and plant materials: a simple desert survival technique". Science 149 (3690): 1377–9. doi:10.1126/science.149.3690.1377. PMID 5826532. 
• Badran AA, Al-Hallaq AA, Salman IAE, Odat MZ (February 2005). "A solar still augmented with a flat-plate collector". Desalination 172 (3): 227–34. doi:10.1016/j.desal.2004.06.203. 

External links

• Making a solar still