Understanding some basic scientific principles will help you build an optimal solar still.
When placing a transpiration bag on a tree limb, ensure you don’t puncture any holes and tie the end tightly.
Visual reference of the typical amount taken out of solar still in a 24-hour period on the left, vs. a transpiration bag on the right.
I no longer get surprised by the vast amount of survival training that has become dogma. What I mean is there is a tremendous amount of information out there that has been repeated so often that it is now widely accepted as valuable and useful. Take for example the idea of the solar still. Pick up nearly any survival manual. Particularly the old-school military ones. You will see some form or variation of a solar still, usually requiring a survivalist to dig a hole in the ground and go about setting up a system by which to collect evaporated water from vegetation that they put in the hole.
There is a serious problem with this process that is very rarely considered: the simple fact that on earthen ground consisting of soil and clay, the building of the still causes a person to burn more water than the still yields. This puts a person into a negative deficit when it comes to their hydration, meaning the person would’ve been better off simply doing nothing at all.
I did some research on the solar still many years and ago. I have been preaching that message for a long time now. However, I had yet to put those findings down on paper until now. What I’ve done for this study is consider the inability of the typical solar still to be valuable. Perhaps more importantly, I want to offer some other solutions to help myself and you, the reader, understand when solar stills can be a viable option, as well as provide other similar solutions for obtaining water.
We must first understand the science of what’s happening. Then we can harness it and work with it to our advantage. The process is most simply referred to as the water cycle. Trees and other leafy plants collect water from the soil, and that water is pushed up through the stem of the plant into the leaves. The bottom side of the leaves have pores called stomata. These stomata allow water to come out of the leaves and escape into the atmosphere. This part of the cycle is called transpiration, and it’s important to understand. This water that leaves the leaves (pun intended) eventually gathers into rain clouds. When enough is collected, it rains again, and the same leaves then collect the rain and utilize it, as well. Excess water seeps into the soil or runs off into creeks, rivers, ponds, lakes and oceans.
The other cool 8th grade science class topic that’s important to understand here is photosynthesis. Plants are like you and me. They get hungry and they need food. You and I usually cook our food, and many plants do, too—by utilizing the sun. The sun’s rays work with the plants to synthesize compounds within the leaves themselves to create a food source. One of the things the sun’s rays work with is carbon dioxide within the plant. A by-product of this process is oxygen for us to breathe and water for you, the survivalist, to drink. Now that we understand that, let’s consider how to best utilize this for survival.
Solar Still Basics
The typical solar still requires a survivalist to dig a hole, place a cup in the middle of the hole, surround it with something that contains water (e.g. vegetation, urine, dirty water), cover it with plastic and then wait. The sun will force water in the hole to evaporate. The water would then normally return to the atmosphere, but because of the plastic barrier, it’s trapped. By placing a rock on the plastic above, gravity and the angle of the plastic force the water to then drip into the cup.
Simple enough, right? Not so much. There are a lot of variables involved in this process:
- The amount of water available. That water can come from placing your still close to an unreliable water source, such as a river, where most of the surrounding soil will be saturated. You can also introduce leaves and stems of plants into the hole. What water is stored in those plants will evaporate.
- The amount of sunlight. The more sunlight there is, the more water that will evaporate, and that equals more water collected.
- The humidity. If the air has a high humidity level, then that air is essentially already saturated. This makes it more difficult for the leaves to expel water because the water concentration in the air is already high.
- The temperature. Much like the sun, a hot day will force the surrounding area to evaporate more water.
Take any one of these parts away and you’ll decrease the efficiency of this process. Think about all those elements needed. Then imagine yourself digging that hole in those same conditions. In most of these situations, you’ll be fortunate to obtain a few simple ounces of water. If you burn off more water than you gather, you’ve made a vital survival mistake: You’ve failed to consider how much energy and water and how many calories you’ve burned off in the process of obtaining more.
The average person loses about 12 ounces of water per day just breathing and another 12 ounces doing nothing at all besides sitting around. That’s already 24 ounces of water that we need to make up for in a survival situation if we’re doing nothing at all. If you put the effort into digging a hole, gathering leaves and the other work that a solar still requires, you’re compounding your water loss. In the early days of my survival school, I was much like many others in that I’d accepted some things as true without verifying them myself. After having students make several solar stills and only producing minimal amounts of water, I knew there must be better ways to handle this for survival situations.
A Better Option
With a bit of deductive reasoning, you can readily determine that it would be best to gather water in some way that doesn’t require so much work. In an arid climate, using a solar still is one of the only options you have. You can urinate into the hole or dig the hole near any vegetation you can find, taking advantage of its roots to get water out of the soil. The reason I suggest this is because digging sand out, even by hand, is an easy process and one that takes very little effort to complete.
But what if you could use the same plastic without putting in all that effort? This is where the same clear plastic you would use for a solar still could be used as a transpiration bag, which exploits the same science I’ve already outlined. However, what it doesn’t do is require you to use as much energy or burn as much water to assemble it.
To create a transpiration bag, simply take a clear garbage bag, place it around green vegetation (like a tree branch) and tie it off at the opening of the bag. The tree collects water in its roots and passes it through its trunk and the out through the leaves. Due to the bag surrounding some of its leaves, the water cannot escape, and it will collect on the inside of the bag. Gravity will eventually pull the water down to the bottom, where it will accumulate.
We now practice this in nearly every class at my school when the conditions are optimal to do so, and we’ve collected as much as 2 cups in one bag within a 12-hour period. That’s not the norm, but it is an indicator of what’s possible.
For the Best Results:
- Choose a clear plastic bag. For the sun to get to the bag and cause photosynthesis to occur, a clear bag works better than a colored one.
- The optimal time of year for water to be moving through a tree is the springtime.
- Make sure the area where you’re setting it up is going to receive full sunlight.
- Gather as many branches into bundles as you can and put them all into the bag together, but do not collect loose green leaves and toss them in. They’ll mix with the water and make it unpalatable.
- Choose tree species that are known for moving large amounts of water from the soil, such as sycamore, willow or water maple.
- Choose low-lying areas where water is more likely to congregate in the soil.
- Look for other small stemmed plants that are water indicators, such as cattails and jewelweed. These always grow near water or in soil that’s saturated with water.
- Set up as many transpiration bags as you can. It’s no different than survival trapping: The more bags you put out, the more you’ll collect.
Solar Stills Disclaimers
If you follow these recommendations, you’re likely to get several ounces of water, and that will be enough to keep you alive. But there are some problems with this method that you should be aware of as well. For one, you’re going to find little to no green vegetation in cold climate conditions. It’ll all be gone or unyielding during those colder seasons of the year. But even in the warmer seasons, you can have the best vegetation, best bag and a great location, but if it’s a cloudy day, our experience shows that you will cut your water gathered by nearly 75 percent. Still, you haven’t put much work into gathering it, and it’s not forcing you into a water deficit by building it. So, even when it’s cloudy, put it out and be patient.
I want to make it clear that my purpose in presenting this argument is twofold. The first is to make sure we all know that a solar still usually requires more water to construct than we’ll get from it. The second is to encourage you to stay hydrated and to carry water and other essentials with you always. It’s far better to be proactive than reactive in survival and disaster readiness.
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