Solar still is an emergency survival tool
The solar still functions under the general principle of the “greenhouse effect”. Solar energy heats the ground by passing through a clear plastic barrier. Moisture from the soil then evaporates, rises and condenses on the underside of the plastic barrier above.
Desert solar still
There are only 2 essential components to constructing the solar still — a container to catch the water and a 6 x 6-footsheet of clear plastic. A shovel or trowel, a length of plastic tube and tape are all optional.
The container can be a collapsible cup, an empty plastic bottle, a small cooking pot or just about anything with a large enough opening to catch falling drops of water. In a pinch, even tin foil or a sandwich bag can be fashioned into a workable receptacle.
The sheet of clear plastic can be a ground cloth used under tents when backpacking or a thin painting drop cloth. Both work well as long as there are no tears or holes. This is the one item that should be carried at all times, since there is no natural substitute out in the boonies. I keep a 6 x 12-foot plastic drop cloth taped inside my daypack, large enough to make two stills if necessary. Some desert rats like to keep their plastic sheets folded inside a hip sack or as part of their first-aid kits.
A 6-foot length of flexible plastic tubing, similar to the kind used in fish tanks is a non-essential but desirable addition to the still components. This will allow you to drink accumulated water without needing to break down the solar still, inevitably affecting its efficiency.
The best part of this life-saving device is that for something that collects water from seemingly nothing, the solar still is amazingly simple to build. Here’s how it is done:
- Dig a pit approximately 4 feet wide and 3 feet deep. Use a shovel, hand trowel, a digging stick or even your hands in soft soil or sand. Look for a sandy wash or a depression where rainwater might collect.
- In the center of the pit, dig another small hole deep enough for the water container.
- Place the container inside, then run the tubing from the container to the outside of the pit. If there is tape available, tape the tubing to the inside of the container.
- Blanket the pit with the plastic sheet, evenly on all sides, but not touching the bottom of the pit. Anchor the corners with rocks.
- Find a small rounded rock to place in the center of the sheet, over the water container. This will keep the plastic centered and control any flapping from the wind. Gently push down on the center weight until the sides slope to a 45º angle. If the pit is dug deep enough, this should leave the center weight just a few inches above the water container.
- Next, secure the edges of the plastic sheet with rocks and dirt. Make sure there are no places where moisture can escape.
- Close the tubing end with a knot, or double it and tie it closed.
Within 2 hours, the air inside the still will become saturated with moisture and begin to condense onto the underside of the plastic sheeting. Because of the angle of the plastic, water will run down towards the center. Finally, drops will gather and fall from the apex down into the water container. As the container fills, simply sip fresh, sterile water from the plastic tubing. In especially dry conditions, water output can be increased by placing succulent plant material inside the still.
The solar still only takes about an hour to build. If constructed correctly, it can yield about a quart of water a day. And although the palm trees may be noticeably absent, you will have made your very own oasis in the desert, quicker than Hollywood itself could.
CAUTION: Solar stills are not a primary water source, nor a substitute for carrying adequate amounts of water in the desert. Always carry a minimum of one gallon per day per person.
Home-built solar still
With just a few basic building materials, a sheet of glass and some sunshine, you can purify your own water at no cost and with minimal effort. Distilled water is not just for drinking, and it’s always worth keeping a few gallons of it on hand. Clean water free of chemicals and minerals has a number of valuable uses:
- Always refill the lead-acid batteries used for solar energy systems or automobiles with distilled water
- Water delicate plants like orchids with distilled water; minerals and additives like fluoride or chlorine that are present in most tap water can harm plants
- Distilled water mixed with antifreeze is recommended for car radiators, as it’s less corrosive
- Steam irons become clogged with mineral deposits unless you use distilled water
How to make a solar still box
The box is built from 3/4 ” BC-grade plywood, painted black on the inside to absorb heat. We used a double layer of plywood on the sides to resist warping and to help insulate the box, with an insulated door at the back and a sheet of glass on top.
Finding a good lining or container to hold the water in the inside of the box as it heats and evaporates can be complicated. The combination of high heat and water containing salt or other contaminents can corrode metals faster than usual and cause plastic containers to break down or offgas, imparting an unpleasant taste to the distilled water. The best liners are glass or stainless steel, although you can also coat the inside of the box with two or three coats of black silicone caulk. Spread the caulk around the bottom and sides with a taping knife. After it dries and cures thoroughly, just pour water in—the silicone is impervious to the heat and water.
We chose to paint the inside black and use two large glass baking pans to hold the water. Glass baking pans are a safe, inexpensive container for dirty or salty water, and they can easily be removed for cleaning. We used two 10 x 15″ pans, which hold up to 8 quarts of water when full. To increase the capacity of the still, just increase the size of the wooden box and add more pans.
The operation of the distiller is simple. As the temperature inside the box rises, water in the pans heats up and evaporates, rising up to the angled glass, where it slowly runs down to the collector tube and then out to a container.
The runoff tube is made from 1″ PEX tubing. Stainless steel can also be used. However, use caution with other materials—if in doubt, boil a piece of the material in tap water for 10 minutes, then taste the water after it cools to see if it added any flavor. If it did, don’t use it.
DIY Solar still step by step
- Mark and cut the plywood pieces according to the cutting list. Cut the angled end pieces with a circular saw or table saw set to a 9 degree angle.
- Cut the insulation the same size as the plywood base, then screw both to the 2 x 4 supports with 2 1/2″ screws.
- Screw the first layer of front and side pieces to the base and to each other, then add the back piece. Predrill the screws with a countersink bit.
- Glue and screw the remaining front and side pieces on, using clamps to hold them together as you predrill and screw. Use 1 1/4″ screws to laminate the pieces together and 2″ screws to join the corners.
- Glue and screw the hinged door pieces together, aligning the bottom and side edges, then set the door in position and screw on the hinges. Add a pull or knob at the center.
- Paint the inside of the box with black high-temperature paint. Cover the back and the door with reflective foil glued with contact cement. Let the paint dry for several days so that all the solvents evaporate off.
- Apply weather seal around the edges of the hinged door to make the door airtight.
- Drill a hole for the PEX drain. The top of the PEX is 1/2″ down from the top edge. Clamp a scrap piece to the inside so the drill bit doesn’t splinter the wood when it goes through.
- Mark the first 19″ of PEX, then cut it in half with a utility knife. Score it lightly at first to establish the cut lines.
- Drill three 1/8″ holes in the side of the PEX for screws, then insert the PEX through the hole. Push it tight against the other side, then screw it in place, sloping it about 1/4″.
- Wipe a thick bead of silicone caulk along the top edge of the PEX to seal it against the plywood.
- Shim the box level and tack a temporary stop to the top edge to make it easy to place the glass without smearing the caulk. Spread a generous bead of caulk on all the edges, then lay the glass in place. Tape it down around the edges with painter’s tape, then let it set up overnight.
CAUTION: One fluid never to be used is radiator fluid, as its toxins will vaporize and poison the water.