How to Build a Solar Still and Save Your Life

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©2019 C. Henry Martens

How long can a human being function without water? In cool weather, perhaps three days, in hot, as little as one day. And death follows similarly. Cool weather, about five days and in hot, about two days.

So when the faucet goes dry, what do you do?

If you have a high-tech purification option in your back pocket, like desalination or reverse osmosis, then fine… but usually these methods are not easily portable and are usually limited by geography.

If you are in dire straits, a place with no potable water, a simple low-tech water purifier can be a life saver.

Natural hydrological cycles purify water and have inspired the solar still, a man-made solution. Low-tech stills can be built and used in emergency situations, deserts, at sea, and in poverty-stricken areas… and they help to reduce energy costs and pollution as solar stills are environmentally friendly.

Using the power of the sun, a solar still purifies water, mimicking natural processes like the hydrological cycle. The sun drives the mechanism to distill water.

Impure water is evaporated by the sun to be cooled, condensed, and collected in potable form. No fossil fuels or artificial heat sources necessary. Unlike industrial forms of water purification, solar power is the only energy needed, and you get pure, drinkable liquid. Water safe for cooking or human consumption.

In the natural hydrological cycle, oceans and other bodies of water are exposed to the sun and heat up, evaporating surface water to be carried into the atmosphere and trapped in clouds. Evaporated water condenses in clouds as it cools, forming liquid rain. The rain falls to return to the earth, forming streams and bodies of water, and the cycle starts all over again. Natural purified water.

To construct a solar still, you need two containers. One for contaminated water or whatever you are extracting water from, and the other to collect purified water. Usually, the container containing the contaminated water, or hydrological water source, is larger in order to expose as much surface area to the sun as possible. A box with a black interior is built with a piece of glass angled above the contaminated water surface to catch the sun, angling downward into a collection trough. The trough leads to a catch basin, collection container, preferably a deep trap with little surface area and entirely shaded.

The scientific principles involved in distilling water are evaporation and condensation.

Evaporation: Evaporation separates impurities from water by turning the liquid water into vapor that rises from the surface. Impurities are heavier and left behind. The solar energy, realized as heat, makes the water molecules more active until they separate from the cooler liquid water and rise as vapor. Energy absorbed creates vapor. Vapor rises and impurities don’t.

Certain impurities with a lower boiling point than water are not suitable for distillation. Oils and fuels can make water difficult to purify adequately.

A solar still can be integrated with other heat sources to make the evaporation process faster. A solar heater can be used to heat the bottom of the contaminated water container.

Condensation: The second step of the distillation process is condensation. The evaporated vapor rises inside the enclosed space of the heating box to make contact with the glass surface. This is where the vapor condenses against the glass. The vapor loses energy as it cools, forming liquid just as it does inside a cloud. As the liquid water accumulates, gravity causes it to flow down the surface of the glass and into the trough at the lowest edge. The box should be built with the lower trough shaded from the sun and the trough should be angled to allow collected water to fill the catch basin container.

The process results in water that is free of minerals, bacteria, and other contaminants that remain unable to evaporate.

There are two basic kinds of solar stills.

A single basin still will hold contaminated water in a single container, and it will have only one outlet.

A multi-basin still has more than one basin to hold contaminated water. The additional basins are made of clear materials to allow solar radiation to pass through into lower containers. This type of still increases surface area of the contaminated liquid without increasing the size of the still.

Solar stills can be built to help in survival situations. You can harvest water in areas that you might never think you could find water.

What you will need:

  • A tin can, cup, small cooking pot, or some type of container to catch water.
  • A sheet of clear plastic, best to be at least four feet square and larger is better.
  • Some rocks.
  • A shovel

Optional equipment to make your life easier would be some tape and a length of flexible plastic tubing, and if there is any vegetation around that will be a bonus.

Dig a pit in the ground. Just wide enough that you can cover it with your piece of plastic sheeting. In a smaller hole centered inside the pit, place your container to catch water. Use the plastic tubing to reach the bottom of the catch basin container, placing it so it will reach outside of the pit and the plastic cover. If there is vegetation available, gather it and place it in the pit. Bruised vegetation works best. Don’t cover the catch basin. Cover the pit with your plastic sheet by using larger rocks to hold the plastic suspended above the bottom of the pit and anything in it. Place a small rock above the catch basin. This is critical. The rock needs to be centered accurately so the condensation drops into the container. The rock should weigh the plastic down enough to create a forty-five-degree angle. Be sure to cover the edges of the plastic cover with dirt so no moisture escapes.

Heavy labor is best done in the coolest time of the day. If you have enough water to get you through to nighttime, build your still in the dark. Once built, it is time to be patient. Do not open the plastic sheet. The water you need will take time to accumulate. If you don’t have the plastic tubing you will have to wait at least a full day before accessing the water in your catch container.

A solar still built in the ground can produce anything from half a cup to a full liter of liquid per day. A lot depends on how much moisture is in the soil, as well as the weather you get and the size of your still. Adding bruised vegetation can increase the efficiency substantially.

Solar stills have some benefits. As solar stills use natural cycles, the PH levels remain balanced. This means the water should taste natural, unlike commercial distilled water which tastes flat because of the boiling process.

The cost/benefit in solar water purification is tremendously efficient. Once built, a solar still will run cost free with little if any maintenance.

Simplicity of design makes construction easy.

The disadvantages in solar stills usually involves the time it takes to produce a usable amount of water. There are places where there are no viable options, and people there have developed huge stills which generate large amounts of liquid. Because of the low maintenance involved and relatively low cost compared to importing water, these stills are cost efficient.

Solar stills are a great way to purify water. Over time they have proven themselves in both survival situations and in commercial applications. You can make a simple still out of scraps, or you can purchase one relatively cheaply. There are a lot more designs than the ones I have described, and researchers continue to find more efficient ways to build units that improve results.

I like to pay attention to new information as it comes to light. Solar power is increasingly found to be usable as economies of scale become part of the equation. I look forward to seeing what the future holds.

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