Solar Stills - Again

Posted by: Doug_Ritter

Solar Stills - Again - 09/16/04 03:51 AM

<sigh> I've gotten a few emails of late asking about solar stills because apparently there have been some more stupid articles written suggesting them as a desert survival tool. THEY DON'T WORK!

This past weekend I was out on a desert survival skills exercise with some associates and Tony Nester (excellent instructor, BTW - ). Since these associates had never done any desert survival, we did the obligatory solar still demo/object lesson. I'd consider the circumstances just about ideal, just short of being able to dig and hit water. It had rained recently and we found an area in a wash at the bottom of what would be falls if the water was running with still damp sand. Dug the hole (105 degrees in the shade, not recommended!, for demo purposes only), added some new growth tamarisk greenery and about half a dozen chopped up swollen prickly pear pads, covered it with the clear plastic and let it sit for 24 hours. Total collected was about a quarter to a third of a pint (see image below). If a solar still will work and produce enough water to offset the energy and sweat to produce it, likely there's fresh water to be found and it's not necessary.

Posted by: Burncycle

Re: Solar Stills - Again - 09/16/04 04:00 AM


I've never tried it, but have always read about doing it. I'll think twice about it now <img src="/images/graemlins/tongue.gif" alt="" />
Posted by: Raspy

Re: Solar Stills - Again - 09/17/04 07:17 AM

The Much Maligned Solar Still

The solar still has been accused of being myth or misinformation perpetuated first by ignorant survival book writers copying and recopying others work. Then furthered by well-intentioned yet gullible Internet posters passing along this erroneous data. The major misconceptions about the solar still are unrealistic expectations, misuse and misunderstanding of the stills operation.

The earliest and most common teaching of the solar stillís use is in military survival manuals. The error that most books make is they don't tell the whole story. How to use it effectively. By that I mean to get the best production possible. They do not discuss the very real limitations it has. So that people that try it do not have such unrealistic expectations. They give the impression that a still made from a 3 or 4 foot square of plastic will produce all the water that you will need to survive. This is why many have come to the conclusion that it is worthless because it is not a perpetual fountain. All things have their limitations.

The solar still does produces an amount of water. While it may not keep you going forever, it will supply some water and that will keep you going a little longer. This time extension could mean the difference between rescue and death. The still may only make a difference of a couple of hours but wouldnít you like those extra hours of life. As other options may present themselves?

Under the best conditions found in a desert a still made from a 6 X 6 foot square sheet will produce somewhere between a pint and a quart of water a day. The smaller ones even less. To supply all the water a single person needs requires several such stills and those will need recharging or repositioning periodically.

But a solar still is not a desert only device. More on that later.

Conversely many of the current survival instructors that now profess that a solar still is dangerous to teach or attempt to use are just as much in error. To bolster their case they put out just as much erroneous information. Here is a typical reply on what is wrong with a solar still.

A solar Still in the desert during the summer heat will get you KILLED!

Here's Why:

1) You lose more water in sweat digging the hole than you make from the still!

2) If water is contaminated by Alkaloids, etc. a still won't separate the contaminants!

3) A lot of the soil in the Desert Southwest is high in various elements you wouldn't want to drink! Including alkaloids, cyanide, arsenic, etc! You can't tell by looking either! If you dig into highly contaminated soil, the water collected will be highly contaminated! Good news is if water is heavily contaminated, you won't have to worry about finding more water!

4) Some IDIOT keeps passing around the story about recycling your Urine! Wrong again! Won't filter contaminants!

5) If water is contaminated with Guardia or viruses, solar still won't kill them either - it's the HEAT of boiling that kills them, not the steam (water vapor) so a solar still won't boil water and hold it at boiling temperature for 10 minutes (FEMA advice for boiling water)

Let me try to answer these questions.

1 Building a solar still wastes more water in sweat than it produces.
Well Duh.
But only if you make it so. If you are short on water you ration your sweat. Now I will agree that this is the point that comes, as close to being valid as any of them. Again the still will not work if there is no water available for it to work on. If you site the still where no water is available it is indeed a total waste of sweat and effort. First you have to pick a spot that is very likely to have moisture in the soil, able to add large amounts of vegetation that water can be extracted from or you are able to supply a source of undrinkable water. Second you need to dig your hole smart rather than fast. True if you go out into the middle of hard pan and dig through something that is bone dry and hard as concrete. Then do it at the hottest part of the day like 105 in the shade and no shade. Add in the fact that you are digging as hard and as fast as you can. Under those conditions you are more likely to die anyway from stupidity rather than building the still. As mentioned only dig where you have a good chance of getting water. If it ainít there you not gonna get any. Digging at random is a waste. Dig when it is the coolest not the hottest. Quite often at night deserts can become quite chilly. You may even need the exercise to help keep you warm. But even if it is warm at night dig slowly expending as little energy as possible to accomplish the task. The idea is to save sweat not work up a good one. Take frequent rest breaks. If you are at the point of building a still you are not planning on going anywhere because stills take time to work. You have plenty of time to dig the hole so is no need to rush. Since you should be digging at night it will not start working until the sun comes up.

2 &3 since they are the same thing. Alkaloids, cyanide, arsenic, etc what you are describing are minerals not liquids. They are dissolved solids in the water. They do not evaporate they stay in the ground. The only type only contamination you have to worry about is volatile liquids mixed with the water. Such as liquid chemical wastes. Solar stills work by evaporating the liquid present. It then condenses on the plastic then runs into your container. Simple physics. Therefore it most definitely separates the contaminates you mentioned. Now if your are sloppy you can recontamnate your drinking water.

4 Urine can be recycled. This is no myth. The still distills it does not filter. The same as stills can desalinate seawater. Again the contaminating products are dissolved solids mostly urithric acid. This is left behind as white crystals. So it is not a myth perpetuated by idiots on the Internet.

5 It is the same thing with the biologicals. Even the smallest virus is a solid object. They do not evaporate. The water that does evaporate rises away from them to re-condense on the plastic. So who cares if you kill them. They are left in the soil or within any remaining contaminated liquid you charged the still with. By the way exposure to solar radiation does kill many bio contaminates. That is if you did not put the contaminated water in the same container you used to catch your drinking water.

If you want to see how this works here is a demonstration in reverse.
First make some ersatz urine or the equivalent of contaminated water. Take some tap water and dissolve as much salt or sugar in it as possible. The salt or sugar represents the contaminant and when finished gives you something easily seen. You could take the pan of water and set it outside in the sun to evaporate or if you want to speed things up you could boil the water on the stove. This is not to show how boiling would kill bio contaminants but to get rid of the water. When all the water is gone you will see a white residue coating the bottom of the pan. This shows how the contaminants are left behind. If you were to collect the steam or water vapor and re-condense it you would find that it is purer than the tap water you started with as most tap water contains some dissolved solids and these would be left in the pan also.

A couple of other points about cross contamination. If you were to dig the hole for a solar still in soil that contains poisonous minerals or used the catch container to transport bio contaminated water to charge a still. Then the container could very easily have enough contamination to re-contaminate the water you just purified. It is also possible that an insignificant amount of cross over may be picked up where the plastic contacts the ground at the edge of the hole.

A solar still can produce some water in the desert if conditions are right. But the use of a still in not limited to desert use. Any place that reaches a reasonable temperature a still can be used. Thee warmer it gets the more effective it will be. It can not only draw water from the soil or vegetation but if you have a source of water that is not safe to drink it can purify it. Boiling can kill biologically contaminated water it does nothing if the water contains dissolved substances that could poison you. But a still can separate the water from the contaminants. The only things it can not remove are contaminants that are also volatile liquids. A prime example of these would be petroleum products. They would evaporate and re-condense exactly the same way that water does.

As it can be seen while the solar still is not the end all be all of water procurement. Yet it is not the myth that some tout it to be. Merely a misunderstood tool. That the naysayers would strip from your arsenal of survival weapons.

My favorite rule is options, always more options. A solar still is just one more skill and option.
Posted by: Doug_Ritter

Re: Solar Stills - Again - 09/17/04 12:40 PM

Everyone is entitled to an opinion. I was addressing use of a solar still as a primary means to procure water in the desert. I've built enough solar stills in the desert to determine to my satisfaction that they don't work and are not practical under real survival circumstances. The only times they have worked, water was otherwise available quicker using less resources. I won't be using or recommending a solar still in the desert as a means to procure water. The proof is in the photo. Other uses of a solar still may be viable, but that's not the issue. Lug that 6x6 square of clear plastic anywhere you want to, just don't look for one in my survival gear.
Posted by: Anonymous

Re: Solar Stills - Again - 09/17/04 12:51 PM


What about large transpiration clear bags put over green leaf limbs and sealed? The FAA touts it can produce 2 gallons of water a day for 3 days off one limb and bag.

I never tried it myself.

Posted by: Anonymous

Re: Solar Stills - Again - 09/17/04 12:54 PM


I am confused <img src="/images/graemlins/confused.gif" alt="" /> normal for me.

How does water vapor collecting on the underside of a plastic sheet transfer giardia and contaminents?


Posted by: Doug_Ritter

Re: Solar Stills - Again - 09/17/04 01:08 PM

Transpiration bags will work, but the typical vegetation you find in the desert are not likely to produce gallons of water. In my experience, we have obtained about a 1 quart maximum per bag under ideal conditions, often much less. Remember that most all desert species are designed to limit transpiration, that's how they survive the desert. In other environments you may well get more; I haven't used them myself except in the desert environment so I cannot speak from experience in that regard. The water you get also tastes pretty bad in my experience, but as long as the vegetation isn't toxic, it is safe to drink, just tastes (and smells) like the vegetation it was taken from. The big advantage transpiration bags have is they take little effort to use, so there is much greater likelihood of a net gain in water and little risk trying.
Posted by: GoatRider

Re: Solar Stills - Again - 09/17/04 01:17 PM


He was saying that's one of the myths some trainers are propogating. It can't, unless you used the same jar to bring water to the still as you are using to collect the condensate.
Posted by: Anonymous

Re: Solar Stills - Again - 09/17/04 02:56 PM

Ah ok, read it out of context, makes sense now.


Posted by: brian

Re: Solar Stills - Again - 09/17/04 03:09 PM

I have tried transpiration bags one time on pretty lush vegetation it the heat of a North Texas summer (85-105 degrees in a 24hr period) and had very little luck. I collected about 1oz of water/container in 24hours using tranlucent kitchen-size trash bags. This is something that I have been meaning to work on some more but just haven't had the time lately when I have been out sleeping overnite in the woods (too many other "projects" <img src="/images/graemlins/smile.gif" alt="" />). My point here is that this is obviously something I need to research and practice more so I am also interested in any tips going beyond the information in the Joint Forces Survival Guide and the SAS survial guide. Maybe I should use truely transparent bags instead of translucent? Maybe I need a location that gets direct sunlight all day long instead of half the day? Maybe I need bigger bags? I would love to here ideas and opinions on this.

Posted by: Wellspring

Re: Solar Stills - Again - 09/17/04 05:37 PM

Just to make sure I understand the principle, transpiration bags get the water that is a byproduct of sugar production during photosynthesis.

So that's why the bag has to transmit light. The bag also has to be airtight and weighted down so that water will condense out (rather than venting as vapor) and collect at the bottom of the bag. The branch you pick should also have plenty of leaves and get plenty of sunlight.

Is all this correct? What do you recommend as a good bag type? Grocery store vegetable bags? They're very thin and transparent, but appear to be water-tight.

Is this deciduous only, or will evergreens work in winter?
Posted by: GoatRider

Re: Solar Stills - Again - 09/17/04 05:41 PM

I haven't tried it yet, but I know a bit about plants. All plants transpire somewhat, as part of the pumping mechanism. Deciduous plants will do it the most, followed by coniferous, and various desert plants such as cactii will do it the least. Which is too bad, because the desert is where you need it the most!
Posted by: Chris Kavanaugh

Re: Solar Stills - Again - 09/17/04 08:11 PM

Watching people ride bicycles and riding one the first time are two different things. I would no sooner hand a child a book by Lance Armstrong than trust ANYTHING in a survival book without trying it myself. Options and redundancy are valid only if the methods are. I tried the solar still. I tried drinking seawater. I did these, and other options under guidance in survival schools. My military instructor said the manual page on solar stills was excellent- as tinder. I persisted with 3 more personal attempts in civilian life. I finally found a fantastic option in lieu of that plastic sheeting. It's called a canteen. <img src="/images/graemlins/grin.gif" alt="" />
Posted by: Anonymous

Re: Solar Stills - Again - 09/17/04 08:35 PM

Would using seawater as the condensing fluid work better in a coastl situation? Maybe if you found yourself tropical island stranded a la Tom Cruise <img src="/images/graemlins/wink.gif" alt="" />

Posted by: Burncycle

Re: Solar Stills - Again - 09/17/04 09:04 PM

Tom Hanks <img src="/images/graemlins/wink.gif" alt="" />

I always thought it sucked that he lost the survival thing when he hit the water. That would have made the movie more interesting for me, seeing him use all the gadgets <img src="/images/graemlins/grin.gif" alt="" />
Posted by: Anonymous

Re: Solar Stills - Again - 09/17/04 10:21 PM

Please believe me that I laughed heartilly after I realised when I read Alkaloids and thought Altoids that it confused me why people would think minty water would be bad...


Lysdexics Untie!
Posted by: Raspy

Re: Solar Stills - Again - 09/18/04 05:49 AM

Doug I actually do not recommend the solar sill either. What the intent of the article is to set the record straight because there are errors on both sides. No it is not a great thing to precure water in the desert but it is not completely useless either.

Where I think its best use is as a still to decontaminate bad water. While boiling and chemical means can deal with bio hazards. They do nothing for other water contamination sources such as alkaloids often found in desert water sources or salt from sea water. And as I said it is just another option.

I do not know if it is still available but years ago I ran in to a product called if I remember right was tedlar. This was 1 mil thick and textured on one side. It was a very strong plastic that looked a lot like saran wrap. The texturing helped the water to collect better and run into the container. Being so thin Carrying a one or more large pieces was easy.
Posted by: Anonymous

Re: Solar Stills - Again - 09/18/04 01:12 PM

Actually I've read that most SAR teams prefer you lug an 8x8 piece of plastic. That size is more convenient to wrap your dehydrated corpse in.