Looking At the Solar Still
By Allan Douglas | Aug 11, 2015
In the mid-1880s, mine owners in the high country of Chile were faced with the problem of providing drinking water for their workers. The only water available was unfit to drink so a means of purifying that liquid had to be found. The solution was a sun-operated distilling apparatus in which an array of glass-covered wooden frames evaporated the contaminated water and condensed it. This array produced as much as 6,000 gallons of fresh water each day.
Oddly enough, this method of producing fresh water was forgotten for a while and fuel operated stills were used whenever it was necessary to convert salty or polluted water to fresh.
During World War II, to help aviators forced down at sea who needed a source of drinking water until they were rescued, Dr. Maria Telkes developed an inexpensive, lightweight plastic still that could be included in even one-man life rafts and would produce a quart of fresh water a day.
Solar stills operate on the same principles that produce rainfall. The sun heats water to the point that it evaporates. In this process, only pure water rises as vapor, contaminants are left below. The water vapor cools and condenses. In nature, this condensed water vapor forms clouds that then drop their water back to earth as rain. In a solar still, the water vapor condenses on a glass or plastic cover. The cover is sloped and the condensed water runs down to be collected. There are no moving parts in a solar still, and only the sun’s energy is required for operation.
Today, interest in solar water stills is increasing due to the need by hikers/campers, survivalists, homesteaders, and poverty and disaster relief efforts to provide potable water where none is available.
I am not a survivalist or off-grid homesteader (not yet anyway), but I am concerned by the prospect of an extended power outage. Our only pure water source is a well. That well is 400 feet deep. At that depth, no hand pump will draw water up to where we can use it. A generator powerful enough to provide enough 240-volt juice to power the submerged 1 hp pump motor is expensive. Too expensive to justify purchase on the chance that the national grid will be taken down by terrorists – or something like that.
We do keep a supply of emergency water frozen in jugs for use during those instances when the power is taken out for a day or three by a storm or when the water lines freeze. But I often wonder about building a solar still to have on hand just in case something horrific happens. There is a small creek 1/4 mile from our house where we could get water if needed. Not a lot of water, and I would not trust it to be drinkable, but it is a source other than catching rain. So what does it take to build a solar still?
The most basic solar still is a simple pit dug in the ground with a container placed in the bottom and a sheet of thin plastic secured over the top. A small rock forms the plastic into a shallow cone. The sun draws moisture from the soil, it condenses on the underside of the plastic, runs down the cone and drips into the container. Crude, but workable in a survival emergency.
A more practical version is a sealed box with a glass cover. Feed water is delivered into the box – which usually is painted black on the inside to absorb more solar heat – and evaporates, condenses on the sloped glass cover, and runs down to a fresh water collection trough. There are as many variations on this basic theme as there are designers. One I came across even incorporates a means to collect rain water.
This one is for long-term use, being large and built on a concrete slab and using bricks to help retain heat so it works longer into the nights. Solar stills work at night? Yes, as long as there is enough heat inside the still to evaporate water, they will continue to produce fresh water, even in the dark.
For intermediate production, multiples of the smaller still can be put together into an array:
The folks at Global Village Institute for Appropriate Technology say this about their design:
“We designed a still which is easy to replicate, using standard building materials, of which 95 percent are available ‘off the shelf.’ The exterior materials were chosen for their ability to withstand our desert climate with minimal maintenance. The still produces an average of 3 gallons per day in the summer months. Winter production is expected to be 1/2 that amount. The Solar Still can utilize a standard size patio glass replacement, 34 inches by 76 inches. The material costs per still are approximately $150. Approximately 8 square feet (of glass cover) will distill around 1 gallon of water per day, over five hours of full sunlight.”
Mother Earth News offers a design that uses a rectangular box propped up on legs and incorporates a black terrycloth towel hung between the back reflector and the glass. Feed water is dripped onto the towel to keep it moist and they claim the water evaporates more quickly, therefore increasing fresh water production.
There is even a solar powered still for use by cyclists that draws drinking water out of the air – although this is probably not the same concept.
Important considerations for any solar still design.
• The body of the box needs to be made of materials that will stand up to the rigors of high humidity without warping or rotting. Redwood is often recommended for this. White oak also works well if it does not come in contact with the soil (set it on bricks).
• The back of the box serves as heat-sink and spine for the box: it needs to be heavy enough for the former and stable enough for the latter. Half-inch marine grade (or at least exterior grade) plywood does well.
• Single strength window glass works well for the cover. Plastic is not recommended.
• The interior of the box needs to be coated with a black, non-toxic material that will absorb heat and protect the wood from water damage as well as preventing leaks at joints. It also needs to stand up under the heat so it does not crack. Global Village Institute for Appropriate Technology recommends black, high temperature silicone rubber sealant; (spread it on with a Bondo squeegee).
The condenser glass needs to be removable so it can be cleaned, inside and outside. In addition, the interior of the box will scale up rapidly as the minerals and pollutants that were in the feed water are deposited by the evaporating water. The box needs to be over-filled to flush it out once a day.
For maximum efficiency the sun’s rays should strike the still at 90 degrees. That will mean either adjusting the still throughout the day to track the sun or finding the best compromise position and leaving it there.
So, what do you think?
Is the idea of building a small solar still as an emergency source of clean water for drinking and cooking of any interest to you? I am interested. The skills and tools needed are well within my realm. The materials are “off the shelf” and inexpensive. A panel of scrap plywood could be strapped over the glass to protect it and the whole thing stored away (after a suitable test period, of course) until it’s needed. And if (or when) we DID need it, it would be great to have it all built and ready to go. Having to build it after some disaster strikes could lead to difficulty in acquiring the materials.
Yes, I believe I’ll have a go at it. What do you think? Have you built one already? If so, please share any advice you have from your experience.
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