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How To Build A Solar-Powered Still To Purify Drinking Water

solar still

The author’s solar still, with one pan in it. The still actually has room for two of these. Note that the glass top is at an angle, allowing the water to flow down to the catch tube.

Everyone agrees that water is needed for survival and articles abound for how to find water and purify it for drinking. But all those articles have one thing in common: They are talking about purifying water from biological hazards.

Normally, the biggest hazards we face from drinking water are microorganisms: bacteria, protozoa and other parasites which can enter our system and make us quite sick. These are a legitimate concern and one that every survivalist must take into account. A case of dysentery can kill you, especially if you are already weakened or injured.

While those biological hazards are important, they aren’t the only thing we can find in our drinking water. Chemicals, radiation, salt and minerals can all be there as well. While a fine-enough filter can get rid of biological hazards, particulate minerals and radiation, it can’t do a thing about salt or chemicals. Activated charcoal will help some with some types of chemicals, but the only sure way to get rid of those is through distillation.

Distillation is a simple process, although it can be difficult to accomplish in quantity. It consists of heating the water to the point where it evaporates quickly or steam forms. Then, that water vapor is captured and cooled, allowing it to condense back to water.

The really great thing about distillation is that nothing else evaporates with the water. The only chemicals that could evaporate with it are those that have a lower vapor point than water does. There aren’t too many chemicals normally found in a liquid state at typical ambient temperatures, which also have a lower vapor point than water does. Alcohol is one of the few.

The problem with distillation is that it is a slow process, which means that heat has to be applied to the water for a long time, so that it can evaporate and be captured. In a crisis situation, fuel might be scarce, making the process of distilling water rather expensive, from a resources point of view. This can be solved by harnessing the power of the sun in a solar still.

When most people talk about a solar still, they are talking about a covered hole in the ground, which is used to purify water in an emergency situation, typically in a desert or other arid place. That’s not what I’m talking about here. I’m talking about building a permanent solar still, which can be used to distill water on a constant basis.

A solar still, like many solar collector devices, consists of a glass-covered box, which is painted on the inside. A pan of water is placed inside and the device is placed in the sun. Sunlight enters the box and heats the water, promoting evaporation. When the water vapor hits the glass, it condenses on it, running down the glass to a catch tube.

Directions to Make the Solar Still

To make a solar still of this type, you’ll need to make a box out of plywood. You can’t see it well in the picture, but the white line at the back of the still is a door, which flips down to allow the water pans to be placed inside it. For water pans, I used two large rectangular Pyrex dishes. To accommodate them, the inside dimensions of the box are 19 inches wide by 23 inches long. I was careful about this measurement, as glass commonly comes in pieces that are 24, 36 and 48 inches. Had I made the box just a little bit bigger, I would have needed to buy a 36-inch piece of glass and waste most of it.

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The front of the box (the end with the catch tube) is 6 inches tall, while the back of the box is 9-1/2 inches tall. This allows sufficient slope to cause the condensed water to run down to the catch tube (more on that in a moment). The sides of the box are constructed of two layers of half-inch plywood, while the bottom is of 3/4-inch plywood. That gave me sufficient thickness at the bottom to make it easy to connect it together.

The sides and ends of the box are double thickness, providing a notch at the top for the glass “collector” to mount into. The outer piece of plywood should stick up half an inch above the inner one. I purposely made the outer piece of plywood “short” so that it would provide a ready handle for carrying and moving the solar still.

When inserting the glass, it should be held in place by silicone caulk. Quarter round or base shoe can be placed above it to finish off the box. Be careful when nailing the quarter round in place, so as to not hit the glass with the nail or hammer.

A piece of three-fourths or one-inch thick aluminized insulated sheathing needs to be cut to fit the inside of the box and placed in the bottom, aluminum side up. The aluminum surface of this piece of sheathing and the entire inside of the box should be painted flat black to absorb as much light as possible, converting it to heat. The aluminum coating on the sheathing will readily absorb and transfer this heat to the Pyrex baking dishes. Pyrex is used to reduce the risk of breakage.

diagram solar still

The hardest and most important part of the design is the drip edge and catch tube. I used a rubber door seal (without the metal backer) as the drip edge. You can glue it in place with silicone caulk, allowing the caulk to dry fully before turning the glass over. Be sure to fill the “uphill” side, where the drip edge attaches to the glass, to provide a smooth transition for the water drops to flow down. The drip edge needs to be located directly above the center of the catch tube. So, you will need to place the glass on the box, mark it for the drip edge and then remove it to install the drip edge to the glass, before installing the glass into the solar still.

The catch tube is a piece of half-inch PVC pipe. The outside diameter of half-inch schedule 40 PVC is slightly less than seven-eighths of an inch, making it easy to put a hole through the side of the still for it with a spade bit or forstner bit. Drill partially through the opposite wall of the still’s box from the inside, so that there is a pocket for the catch tube to sit into. Glue it into this partial hole with silicone caulk to both seal the wood and hold the drip tube in place.

catch tube solar still

To make the catch tube, make two lengthwise cuts in a piece of half-inch PVC pipe, about 120 degrees apart. It is easier to do this if you make the cuts too long, and then cut the tube to length. The cut-out portion of the tube should span the entire still, with the full tube starting where it goes through the side of the box.

The drip tube should be held in place by a pin at the outlet end. This can be a small nail. Just drill a hole down through the side of the still’s box and insert the pin to keep the tube from pulling out. A half-inch slip to half-inch threaded coupler is attached to the end of the catch tube, with a barbed fitting connected to it. A short piece of tubing is connected to this fitting.

Using the Solar Still

The solar still can be used all day long, even if it is slightly overcast. If it is properly sealed with paint, it can be left out even in a rainstorm. To use it, place the still in a place where it will receive good sunlight, without any possibility of it being shaded. Point it toward the south so that it can capture the most sunlight. Place a water jug or food-grade five-gallon bucket so that the drip tube will empty into the bucket or jug.

Fill both Pyrex pans with water and place them inside the still, ensuring that the door closes securely, without any air gaps. While the solar still is not fully airtight, you don’t want the water vapor escaping into the atmosphere. Leave the solar still and come back later to collect your water.

You can use literally any water with a solar still. It does not need to be pre-filtered in any way. Debris left in the water will stay in the Pyrex pans, while the clean water will be distilled out. If the water used is dirty, full of debris and silt, you will want to rinse out the Pyrex pans when it is time to refill them.

© 2008-2014 Off The Grid News

30 comments

  1. Some closeups of the inside would be nice. I don’t quite understand how you have the drip edge and why it would be attached to the glass. I would like to build one of these for our cabin in the woods. We have rain barrels but not on site clean water source. This would be the ticket.

  2. This looks rather dangerous with the high powered lens. Waterwise makes a small stainless steel distiller which is the size of a large pot and which is non electric. A small wood fire (or any heat source) gives you
    distilled water.

    • The Fresnel Lens he is using is extremely powerful as shown. Not sure if the colored glass was required or a weak link. I will experiment with small less efficient lens when they arrive. The experiment is another good indication of a smokeless/flameless distillation technique. Since my colloidal silver generator requires distilled water, might be my go to method for distillation.

  3. silicone can’t be used for aquariums because it is toxic to fish so i’m thinking there might be a safer sealant for humans too. that’s the only improvement I can think of.

    • I’ve used my aquarium for years with silicone seal….fish did not die from it…

      • Though I don’t do fish, except for dinner, it was my understanding that silicone sealant was safely used with aquariums, thought I had also seen it on the TV series. Might require one that does not have dangerous additives? Your fish/pet store should carry SAFE products.

  4. I don’t know how I would make the cuts in the pvc. A router table perhaps ?

  5. Is it possible to use double pane glass for these projects ? How about glass from skylights ?

    • Insulated glass would hider the process as the airspace in double pane glass is the insulator which slows the transfer fo heat. Skylights often have a partially-reflective coating on the glass.

  6. I ordered a Solar book that had this solar still plan in it. I built it to specifications but even in 100 degree weather, it would only produce about a cup of water a day. This isn’t enough to keep even one person alive in a crisis. Anyone else build one of these stills? I thought about adding metal wings to it to reflect more heat. I wonder if this would set the wood on fire?

  7. OMG!! Rich! You furnished the information WITHOUT my having to order a book or to watch a video!!
    OMG!! I love you, Rich. You made my life 100 times easier today. Thank you! Thank you!! Thank you!!!
    Mary

  8. I’m thinking your article should be amended to make it clear that only labeled “food grade” sealant should be used and NOT run of the mill regular silicone or caulks. If the choice is death by waterborne pathogen or potential slow debilitating illness from sealant, I guess use what you have, but if the idea is prepping now while resources are available, why use inferior and harmful materials?

  9. Going by the video as opposed to the article;

    This looks like a potential idea for all those people who saved their old DISH or DirecTV satellite dishes when they changed providers. I’ve read where covering the inner part of the satellite dish with “mirror tape” will focus sunlight onto the end of the dish arm and get things quite hot in short order when it is aimed at the sun properly. Stirling engine plans caused me to save my old DirecTV dish when we got cable out here in my neighborhood, and ditto with a much larger old DirecWay internet dish (DirecWay has since renamed to hughes.net) as well. They sit in the garage waiting for me to mirrorize them for solar power projects, some day. For a hobbyist with a metalworking lathe, milling machine and some determination, a Stirling engine could power a small generator, as has been done and documented online quite a bit. But in my geographical area, solar power is too often blocked by clouds/weather and the need to mechanically track the sun to keep the dish aligned. On the other hand, I’d bet even the small satellite dish could be boiling / distilling water fairly rapidly for emergency use, and much simpler to put together…

    • John Z…this is a great idea since I have 3 satellite dishes. Could you send me directions on how to construct for solar water still? I would greatly appreciate it. I’m a disabled senior, so very simple directions by the #’s would be very very helpful to me. I reside in HOT Arizona desert, so sun is not problemo. Thanks so much, and I will be looking for your reply. To your health & survival off the grid!
      Cynthia R

  10. Be aware that only drinking distilled water for a long time will result in mineral deficiencies if there isn’t an adequate diet. Certainly, in a survival situation, clean water is preferred over contaminated, but thought it worth mentioning. Great article!

    • Dangers of mineral deficiencies in distilled water is a long held myth. Think about it – the mineral content of water is a fraction of the minerals in the food we consume daily. The only way we would be deficient in minerals from drinking distilled water is if we fast for days.

  11. I am curious in a warm moist environment such as inside your still won’t bacteria thrive on the glass thus contaminating the water just saying …

    • The temperature inside the box will probably prevent bacteria from growing…not sure though.

      • First, let the water sit in the open so VOC has a chance to evaporate.
        Then distill the water to remove remaining contaminates/ chemicals.
        Store distilled water in bottles.
        If bacteria/ mold is a concern, pasteurize water with the parabolic dish or sun oven.
        Add salt/ citrus juice to water for electrolyte before consumption.
        That is my setup. Water source is rain barrels from shingle roof

  12. This would be a much better article if I could print it off. After all if we need a solar water distiller, we surely are not going to have power to access the internet.

  13. Can u post the schematic drawings plzzzzzxx

  14. Some additional thoughts:
    – Use coke cans painted black and seasoned outside the distiller to off VOC’s (Volatile Organic Compounds) then install them in rows below the glass from side to side to increase heat.

    – Use only pyrex glass cake “pans” instead of aluminum cake pans to avoid contamination. They are also very easy to clean.

    – Do not use Rubber or Plastic inside the distiller instead use a drip edge of 1/4 x 1/4 inch steel bar from hardware store – fasten it to the underside of the single pane glass using JB Weld which will adhere to glass and metel

    – Do not use PVC for catch tube, instead use 1″ copper M grade. A trough 1/3 the diameter of the pipe and desired length can be cut at a machine shop for cheap. Plastic will give off bad chemicals (VOC’s) with the heat of the inside of the box.

    – Extend the catch tube outside the box using a 1″ to 1/2″ reducer and enough 1/2″ copper tubing to enter your glass container. Use open jar with no lid inside of a foam ice chest to minimize evaporation inside the jar.

    Your design using the removable lid instead of hinges is a very good idea….no rusted hinges.

    – I would suggest at least double the size (enough room for 4 pyrex cake “pans”) for greater production. Much larger than that will make the lid too heavy to easily lift off for servicing.

    – I would use a shaded copper coil between the box and collection jar to cool the water and steam coming from the box or else steam will just exit the box through the drain tube back into the atmosphere.

    • When using the 1″ copper tubing make sure you do not cut off one side (length wise) or it.Lift it and straighten it. that way you can use it as the drip edge. You should be able to cut pass have its width to have a longer drip edge.

  15. Nice thought, would love a bit more detail. I don’t think drawing or picture give a very clear on where in the box you are putting this drip edge thing. Would love to make this, can you please improve the instructions? Thx

  16. Marge,search youtube with key words for what you want a video on..as in”solar still’..or water purification w/o power…etc etc…you can find a video with detailed instructions on just about anything you want to do…and I mean anything…its amazing what you can find there…good luck everybody…we’re gonna need it!!!!

  17. Fun little project, but long term I see a problem with keeping the equipment sterile

  18. Why not use a stainless steel pan inside with a small float valve to maintain the water level on the inside and a large container on the outside to gravity flow and keep your pan full?

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