Pure Distilled Water for Emergencies when Wells are Unusable

Here is a very simple, very inexpensive system to provide absolutely pure water for situations when existing wells either become contaminated or they run dry, where people would otherwise not have easy sources for safe drinking water. It is essentially a natural dehumidifier, which causes some of the moisture that is in the air as humidity to be condensed out in a cool underground pipe-tunnel, even in semi-arid regions near deserts. Ten gallons (40 liters) of absolutely pure distilled water per day is very realistic for many climates. With some simple accessories, as much as 250 gallons (1,000 liters) of pure water can be obtained every day!

Below, we will give an example of an AVERAGE summer day near Chicago, IL, USA, where an 8-ounce coffee cup full of absolutely pure distilled water is provided every three minutes! This continues constantly for most of the daytime hours, and all straight from the humidity in the atmosphere!

You are probably familiar with the fact that during the summer, concrete basement walls are often damp or even wet. That occurs because the warm outdoor humid air gets into the basement, and when it passes near the colder concrete wall or floor, it cools and loses some of its ability to hold moisture. If the air drops to a temperature low enough, then some of the moisture has to condense into water droplets. That is essentially the concept used here, but this system uses an enclosed chamber to keep the water purer.

You may be familiar with a survival procedure taught to travelers to remote areas, where they spread a small sheet of plastic suspended above the ground. The relatively cool ground beneath it causes the plastic to (often) be cooler than the hot daytime air, and some humidity (moisture) in the air can condense into droplets on that cooler plastic, and then be collected to drink to survive. That very crude method enables capturing a very small amount of the humidity in the air. The system described here is a far more sophisticated and far more effective way of doing that same process.

All atmospheric air contains some moisture, water, which we call humidity. If that air is COOLED, its "RELATIVE" humidity increases, because cooler air cannot contain as much moisture in it. If it is possible to cool it enough, the air gets to 100% relative humidity, and the saturated air starts having tiny droplets of water condense out on cooler surfaces. That water is PERFECTLY PURE water that is called Distilled water.

OK. You are skeptical! How can there be much water in the atmosphere? And, in SOME climates, such as deserts, that concern is valid. Humidity data for a location near Chicago. But look at this graph of the outdoor relative humidity for a location near Chicago, Illinois, USA. See that the outdoor relative humidity is amazingly high in nearly all months! In the morning, it is nearly always at least 80% and in the afternoon when it is usually lowest, it is still generally over 60%. There is a LOT of water in the atmosphere as humidity!

In the Summer, it works impressively. In the winter, the moisture is still in the air, but the ground is probably not cold enough to cause it to condense there. So, for a climate like Chicago, only about six to eight months of substantial water production is possible with the basic system. However, the (discussed) addition of a $200 accessory, an HG 3a device can produce even larger quantities of water every day of the year, and THAT is true in ANY climate, even a desert!

We have found that for many environments, simply blowing hot daytime air through a COOL underground tube, is able to cool the air enough for the condensation to occur. A Chart and also an automatic Calculator below provides the necessary information to know how many gallons of perfectly pure distilled water can be obtained in this way, directly from the atmosphere! (If any local supply of groundwater happens to be available, no matter how contaminated it might be, the performance of this system can even be greatly increased!)

If you live in a cold climate, and ever wear glasses, you know that if you have been outdoors where the glass has gotten cold, that when you enter a warm house, your glasses immediately fog up! What happens is that the warm humid air of the house cools down when it gets near anything cold, such as the glass, and that cooler air cannot contain as much moisture as when it was warmer. If the room is humid enough and the glass is cool enough, the (local) relative humidity gets up to 100% and tiny droplets of water condense out of the air onto the surface of the glasses. (A minute later, the glasses warm up and this problem ends.) Similarly, if house windows are single-pane, on cold winter days, room humidity condenses on the cold window glass and droplets of water form, and can even freeze into ice!

This new system operates in a way that is also somewhat similar to how a solar still works, except that the Sun is not necessary, no sheets of glass are necessary, and not even any source for contaminated water is necessary! Rather than a solar still HEATING water to increase the humidity inside the chamber, so that it will condense on a relatively cooler glass cover panel, this approach uses the fact that deep underground, the soil is naturally cooler than the daytime air temperature note 3.

All of these things occur because warm air can hold more water vapor in it than cooler air can, and that the deep soil is always cooler than the air temperature during hot summer days, and usually during winter days as well.

This amazingly simple and inexpensive system can realistically supply 10 gallons (40 liters) of perfectly pure water every day! There is essentially nothing which can break down, so it should reliably provide water for many, many years.

Basic setup:


We are showing the warm daytime air as the red arrow at the right of this drawing, where it goes into and down through an inexpensive PVC 4" plastic sewer pipe arrangement. As tiny droplets of water form on the inner walls of the pipe, gravity causes them to flow downward to collect in the bottom of the Tee shown. A small pipe goes downward from there to a storage tank or simply a kitchen pot. The air continues upward and out, shown now as light blue to indicate that the air has actually also been cooled by the cooler ground, as well as the desired dehumidification that provided the pure water.

We show that the pipe must be at least one meter deep to be in cool soil that is not heated by summer heat. Deeper is ALWAYS better! The length of the underground pipe should be as long as is possible. If only a short pipe is used (say 16 meters or 50 feet) it will work fine for a while, but that deep soil will gradually get heated up by the hot air constantly going through the tube, and the effect gets reduced. Thirty meters or 100 feet should give very reliable water supply for most climates. Longer is ALWAYS better!

If the summer air is around 120°F (49°C) temperature, and the outdoor relative humidity is just 30%, then every pound of that air contains about 0.022 pound of water in it as water vapor. This is standard thermodynamics information, as shown in the Psychrometric Chart presented and discussed below.

If this air can be de-humidified so that it becomes around 20% relative humidity, it would then only contain 0.014 pound of water in it. We would capture the difference, around 0.008 pound of water, as actual water droplets. This does not sound like much, but if we send just 1000 pounds of air through our underground tube, this is 8 pounds of water, or around one gallon (4 liters).

The same Psychrometric Chart shows that at that temperature and humidity, one pound of air takes up just over 15.0 cubic feet, so we are talking about 15,000 cubic feet of air. If we hope to produce one gallon (4 liters) of this Distilled Water per hour, we then only need to send around 250 cubic feet of air through the tube every minute (15,000/60), a relatively reasonable airflow.

Depending on the local (daytime) temperature and humidity and the deep ground temperature, two gallons (8 liters) of water produced per hour is generally very realistic! Roughly 10 gallons (40 liters) of absolutely pure water to drink and for washing and bathing every day! All from a very simple underground tube!

This system can be installed WITH an underground tank and hand-pump, or WITHOUT that tank and pump, if a pit is provided so that people would climb down a ladder to get a jug of water. (drawings below).

There are actually a variety of ways that this performance can be easily enhanced. All the water that is produced by this system COMES FROM THE AIR. That means that it is absolutely pure water, called Distilled Water. Those accessories simply increase the relative humidity of the air entering the underground tube by evaporating any source of water that might be available, since when that water evaporates from its source, all the contaminants are left and only the pure water evaporates.

Possible Complications

If this system is used without either of the accessory components, where the air which enters the tube is directly from the local atmospheric air, then it is possible that dust or even small sand grains can be carried in that air, and therefore get inside the sloping underground tube. So, in certain climates, it is possible that the resulting water might appear to be slightly cloudy rather than perfectly clear. These are materials which are NOT dissolved in the water. They are generally absolutely safe in water, but they can be removed either by letting the water remain in a container for some time to let such things settle out, or the water could be poured through any of many simple filters to remove such materials.

The only other real complication is that the basic system described here is dependent on the heat from sunlight to evaporate the water into becoming humidity, so the basic system can (usually) only operate during daytime hours. The solution to a larger water production and also 24-hour water production is to include the HG 3a unit as a heat (and water) source.

Technical Information

The following section is some technical info that shows how to determine how much water might be captured from the air in any specific climate. It is based on a standard Psychrometric Chart.

We will use an example of where the air temperature is 120°F (60°C) and the relative humidity is 30%. (Any other local weather conditions can be similarly analyzed). In the Psychrometric Chart below, this is along the very right edge of this chart, at the bottom right end of the red line. We can see that the air contains about 0.022 pound of water in every pound of air (which the chart also shows takes up a little over 15 cubic feet). THIS is the air that we will have enter the start of the buried tube system. As this air is cooled down by contact with the much cooler (70°F or 21°C) walls of the tube, it first cools in a process that is called reversible adiabatic. This means that the Enthalpy of the dry air, the energy content per pound, stays constant during the process. This is represented by our red line toward the left and upward.

We can see that the Relative Humidity percentage keeps rising as the air gets cooled. This is because cool air cannot hold as much moisture as warm air does. This process can continue until the air becomes saturated, or is at what is called the dew-point. Once our air has cooled to around 88°F (31°C), it has gotten up to 100% Relative Humidity, meaning that it cannot hold any more water in it than that.

At this point, the process necessarily moves along the green line in our example, downward and to the left, as the air continues to be cooled in the underground tube. This process is where moisture can condense out of the air, in our case, on the walls of the cool underground tube. By the time it has gotten to the end of the tube and the air is then at around 70°F or 21°C, the Psychrometric Chart shows us that the air which had initially contained 0.022 pound of water per pound of air at the very start of entering the tube, is now fully saturated air which now contains only 0.016 pound of water in it. The remainder of that initial humidity has necessarily condensed into (absolutely pure, distilled) water droplets on the inside of the underground tube. For every pound of air that entered the tube, (0.022 - 0.016 or) 0.006 pound of water forms inside the tube. If 100 cubic feet of air enters the tube every minute, that is about (100 / 15.3) 6.5 pounds of air every minute or 390 pounds of air every hour. This then means that for this situation, (390 * 0.006) 2.4 pounds of water would condense out every hour, around 0.3 gallon per hour. A realistic two gallons of absolutely pure water every day.

Psychrometric Chart

End of technical information!

I have come to realize that I may have been optimistic regarding whether many people could find usefulness in the Psychrometric Chart above! Therefore, I have created a simplified way of getting the needed data, without having to understand the Thermodynamics or Engineering involved! You can use the following automatic calculator to get the results you need, for any location and any circumstances.

Water in the air as humidity
English Metric
Enter Air Temperature:
Enter Relative Humidity (%):
Water in 1000 cubic feet: pounds Water in 1000 cubic meters: Kgrams
Water in 1000 cubic feet: gallons . Water in 1000 cubic meters: liters . .
If local wind (or a blower) is at 12.5 mph (6 m/s) and a single buried 4" tube therefore has 100 cubic feet (2.8 cubic meters) passing through it every minute, then there is around gallons per hour or liters per hour, of water in that air passing through the tube.

We now know the amount of water IN the air. We can also determine how much water would be LEFT in the air after it has passed through the underground tube. Use the same calculator above, but now put in different data: the temperature will be the UNDERGROUND temperature; and the humidity will be 100%, because, in order for any water to have condensed out, the air inside the tube must have risen to 100% at that temperature.

The DIFFERENCE of these two numbers then gives a very accurate estimate of the amount of water that will be condensed out in ANY location and under any circumstances!

An example:

On a sunny day in an African location, the mid-daytime air temperature might be 130°F, and the relative humidity might be 25%. The calculator shows that the air passing through a tube would then contain 1.14 gallons of water per hour. If the deep soil was at 70F, the calculator shows that 0.83 gallons of water would remain in the air, NOT being condensed out, after it passed through the tube. The DIFFERENCE, 0.31 gallon of water per hour, would be what could be collected. In the several hours of daytime sunlight, this could provide an excellent two gallons of absolutely pure water every day!

Another example:

On a sunny summer day near Chicago, IL, USA, the mid-daytime air temperature might be 100°F, and the relative humidity might be 40%. The calculator shows that the air passing through a tube would then contain 0.82 gallon of water per hour. If the deep soil was at 52F, the calculator shows that 0.45 gallons of water would remain in the air, NOT being condensed out, after it passed through the tube. The DIFFERENCE, 0.37 gallon of water per hour, would be what could be collected. In the several hours of daytime sunlight, this could provide an excellent two gallons of absolutely pure water every day!

These are not spectacular amounts of water, but the equipment can easily be installed, it is extremely cheap to obtain, and it can operate entirely automatically, allowing natural winds to blow the air through the tube. It is obviously also possible to dig ten trenches and install ten of these simple underground tubes, to obtain ten times as much water.

Note also that, with this BASIC system, there can be situations when NO water condenses out! If you use the calculator for 80°F soil temperature, the air LEAVING that tube would still contain (up to) 1.14 gallon of water per hour, so with the African example just given, that is GREATER than the amount of water entering the tube. Therefore NONE would condense out, and no pure water would then be collected.

For such situations, it can be useful to build an HG 3a device as well. The normal operation of the HG 3a can naturally produce a consistent 6 pounds (or 0.73 gallon) of water every hour in its exiting air, at a temperature that is often around 150°F. The airflow rate is slower than the natural wind airflow in our examples above, around 1/10 as fast a flow of air. If the soil temperature is 70°F, then around 0.08 gallon of water (due to the 1/10 airflow rate) will be left in that air (every hour) after it has passed through the underground tube. This leaves 0.65 gallon of pure water that would be condensed and collected every hour. Better, it operates 24 hours a day instead of just the few sunny hours that the BASIC setup can do, so it will consistently provide around 16 gallons of pure water every day (24 * 0.65). If we consider the situation where the deep soil temperature is 80°F or let's examine an extreme 90°F deep soil, it will still work fine at providing pure water! Deep soil at 90°F will cause 0.15 gallon of water to remain in the air after it passes through the tube, so we would collect (0.73 - 0.15) or 0.58 gallon of pure water per hour. That is still around 13 gallons of pure water EVERY DAY.

A CONSISTENT 13 gallons to 16 gallons of absolutely pure water every day, essentially anywhere on Earth! And all with a system which involves a total cost of around $300 to $400! The system is very simple, very automatic, and virtually nothing in it can break down. And even if something ever did, local villagers should be able to figure out how to repair the simple devices involved!

The automatic calculator can also be used in estimating the performance of the "pond" variants of the system, whether with or without an HG 3a device being involved. The air temperature inside the chamber over the pond needs to be measured. If the pond is large enough and the sunlight intense enough, the relative humidity inside the chamber can be near 100%, depending on how fast the airflow is removing that air. If an HG 3a device is used, the consistency of that 100% humidity is better assured, and it is then true 24 hours each day rather than only when the sun was providing heat energy to evaporate the water.

An example:

Say that we are able to have our 100 cfm airflow, but that our pond and chamber are able to provide a consistent supply of 120°F air which is at 100% relative humidity. If we enter these numbers in the automated calculator above, we see that 3.53 gallons of water will be in the air entering the tube every hour. If the deep soil temperature is at 80°F, we also see that the air LEAVING the tube will still have 1.14 gallon of humidity in it. This means that this configuration would provide (3.53 - 1.14) 2.39 gallons of perfectly pure water every hour, or around 58 gallons in every 24 hour day.

By arranging a larger pond size or better using solar heat or additional HG 3a devices, this water production could be increased even more, to provide PLENTY of perfectly pure water for nearly any sized village!

If 500 cubic feet of air pass through the tube in a minute, that is around 33 pounds of air (the Chart shows us that the air is around 15.1 pound per cubic foot). This means that around 0.25 pound of water would form inside the tube every minute. This is 15 pounds of water in an hour, or just over two gallons (8 liters) of pure distilled water per hour.

This basic system does not usually work at night, but generally should work well for at least five hours each day, meaning that more than ten gallons (40 liters) of pure safe water would be available from this extremely simple system each day. (One of the possible accessories, the HG 3a unit, enables this system to work 24 hours every day.)

Air needs to be passing through the system. It should NOT be necessary to have to use any blower, because the entrance to the tube could be provided with a wind-vane type of tail to turn the intake into the wind at all times.

However, if the climate is such that a blower is sometimes necessary, the 12-volt blower from a car heater system could be used, powered from a standard 12-volt battery which is charged by a simple windmill, such as a Savonius rotor made of an old 55-gallon drum.

The very simple system shown and described above should be wonderfully useful in many places in the world where water supplies are inadequate. It involves digging a trench to bury the tube, which MUST slope downward, with the entire tube at least three feet (one meter) deep and preferably six feet (two meters) deep. The horizontal run of pipe should be at least 50 feet (16 meters) long, and longer still is better, to ensure that all the air passing through it is sufficiently cooled.

The system as shown only involves maybe $60 of new 4" (10cm) PVC pipe. It could also be created using surplus large diameter pipe that might be locally found. It CANNOT use CORRUGATED pipe, as the water droplets would then be trapped in many puddles inside the tube. It should NOT be made of any pipe or materials that had earlier been used to carry dangerous chemicals. It should also NOT be made of "ceramic drain tiles", because the many joints between the tile sections are likely to leak and lose the precious water being collected, or allow insects or bacteria inside the pipe.

There are countless ways that this system could be created using only locally available materials. For example, if 4" (10 cm) or 6" (15cm) or larger sections of iron pipe are found, they might either be coupled with standard pipe fittings or there are standard rubber couplings and hose clamps that are inexpensive to join two sections of such pipe.

Limited Water Needs

This basic system can be made for small scale water needs, where a kitchen pot can be placed under the water collection pipe. This approach obvious requires a pit and ladder for a family member to climb down to get the water container once it has filled. A shutoff valve could be added to allow some amount of water to accumulate inside the system, to fill the container more quickly. Plenty of water for eating and drinking and also sufficient for cleaning food utensils and washing and bathing.

Larger Water Needs

On a larger scale, a tank (represented here by a discarded hot water tank) can be connected to the water collection pipe so the tank would gradually fill. A different pipe connection on the water tank could be used to get water from out of the tank. If people would climb down a ladder, they could use the standard draincock of the tank to remove water.

Alternately, a hand-operated pump could be purchased, where a few cranks on the pump would get out the water their family needs for that day, into a convenient container.

Notice that for each of these, we show simple natural airflow due to wind, to drive the air through the tube. The intake is able to rotate with the wind with a tail like a weathervane, so that the intake is always able to face into the wind. If that does not provide desirable amounts of water, a funnel might be added to the air intake, to catch more air and force it through the tube. Finally, a blower from an automobile heater might be used to actively blow air through the tube.

These arrangements really only produce significant amounts of water when the air temperature is high, in other words on sunny days near the middle of the day. Therefore, if a blower is used, there is no sense in it running except during those few hours during the daytime.

For Climates Where the Natural Humidity is too Low

There are some climates where the Relative Humidity is normally too low for this system to work. The Psychrometric Chart above is provided so that anyone could quickly and easily determine whether it will work in a specific location and even how much water it should supply.

However, even in climates where the natural Relative Humidity is too low for this system to work, there may still be ways to enable it to provide excellent pure water. In the field near the tube's entrance, a large tarp might be spread out on the ground on which saltwater or rainwater other non-potable water might be spread. A second tarp, such as polyethylene, would then be supported several feet above that note 2. The heat of sunlight would heat the upper tarp and cause the contents of the chamber created to get quite hot, probably even hotter than the 120°F (49°C) we first assumed. More importantly, any water inside that chamber would get heated, with a lot of it evaporating. This would greatly increase the Relative Humidity inside that chamber, possibly even getting up near 100%. In that case, the water there would evaporate (leaving any salt or other contaminants there on the bottom tarp) to raise the Relative Humidity of the air entering the intake tube. As that much higher humidity air has a lot of its moisture condense inside the tube, the system would provide even larger quantities of absolutely pure distilled water. The additional water production is difficult to predict as it is dependent on many variables, but it can be quite significant.


In the event that there is minimal sunlight and heat available in a particular climate, such as on a mountain, the intake air could be pre-heated by a system such as the HG 3a heating system (shown here as a green circle at the right) described in some other pages in this web-site Domain (linked below). It can provide a consistent 130°F (54°C) to 150°F (66°C) heat source twenty-four hours a day, and also extremely high relative humidity, using only dead field grasses and leaves as the energy source! note 1


Finally, BOTH of these accessories can be added to the basic system. In this case, this simple device, in combination with the two rather simple accessories discussed above, can provide as much as ten gallons (40 liters) of perfectly pure Distilled water every hour, 24 hours each day, or 250 gallons (1,000 liters) of water that is pure to a purity even BETTER than when expensive high-tech equipment is used!

Pure Desalinated Seawater Distilled Water for Off-Grid Residents.

Pure Water Supply for Third World Villages.
Pure Desalinated Seawater for Third World Villages.
Pure Distilled Water for Emergencies when Wells are Unusable.
Pure Desalinated Seawater Distilled Water for Off-Grid Residents.

NOTE: The water that is produced by this system CAN appear slightly cloudy! This can occur if the AIR going through the tube has dust particles in it. For any location where the air might contain a lot of dust or even sand particles, it is a good idea to add a filter over the intake to the underground air tube, and/or pour the resulting water through a cloth or better filter. Extremely tiny particles such as cigarette smoke are so tiny that they are harder to filter out, so an even better filter, such as charcoal, might be desirable.


Brief Functioning of the HeatGreen 3a Device

The chemical reaction of Photosynthesis is generally this one:

(6) H2O + (6) CO2 + sunlight energy gives C6H12O6 + (6) O2.

In words, this says that water from the ground plus carbon dioxide from the air plus the energy from sunlight can produce glucose and free oxygen.

The chemical process of COMPLETE (aerobic) decomposition is exactly the opposite (there are many partial decomposition processes that result in other compounds):

C6H12O6 + (6) O2 gives (6) H2O + (6) CO2 + released energy equal to that absorbed from the sunlight.

In words, this says that glucose combined with oxygen from the air can decompose into water (vapor) and carbon dioxide and a lot of energy, primarily due to the activities of certain types of bacteria which are in soil.

More complex organic molecules such as cellulose are first broken down into glucose to permit this process, gaining some extra energy in the process.

The numbers in parentheses are the number of those molecules which are involved in the reaction. They are important.

In Chemistry, we know that those numbers can be used to describe the number of moles of each compound, so in this case, we have one mole of glucose combines with six moles of oxygen from the air to decompose into six moles of water (vapor) and six moles of carbon dioxide. This tells us the quantities of each which are involved. We need to know the molecular weights of each of the compounds, which are 180, 32 and 18 and 44, respectively. It is then really easy to calculate the WEIGHT of each material involved. 1 * 180 + 6 * 32 gives 6 * 18 + 6 * 44. This is true for any unit of weight/mass: grams, kilograms, ounces, pounds, etc.

We confirm that there is the same amount of mass on both sides, 372 units, which confirms Conservation of Mass. If we use grams, then we now know that 180 grams of glucose will combine with 192 grams of oxygen from the air to create 108 grams of water and 264 grams of carbon dioxide. This natural decomposition occurs worldwide every day, every second.

The important point here is that for every 180 units of weight of glucose that decomposes, there are 108 units of water created. So 180 pounds of glucose will give 108 pounds of water, about 15 gallons (60 liters).

The HG 3a device always has rather high humidity inside it, so when additional water is created like this, it gets carried out and away through the exhaust connection, along with the carbon dioxide (gas) that was also created. In this case, once that hot and humid air gets outside, it cools and most of the moisture in that air condenses into water droplets, which we choose to do inside the cooled underground tube. So, without actually using the large amount of heat that the HG 3a unit produces, if just the exhaust gases are sent into the underground tube, 15 gallons (60 liters) of water will be produced for each 180 pounds of organic material that is allowed to decompose. This is in addition to the moisture in the natural air itself, of the underground tube alone.

Where the underground tube alone can only function during the day, due to the energy of sunlight, when the HG 3a is added, the system can then produce water 24 hours a day. Since the HG 3a can reasonably be expected to decompose about 10 pounds per hour, or 240 pounds per day, this source therefore can provide an additional 20 gallons (80 liters) of absolutely pure distilled water every day. This is true even in an extreme desert climate where the atmospheric humidity is very near zero.

Water Evaporation Bag Functioning

Tarps could be used to enclose any source of water, of any level of contamination by any chemicals. If this is done without using an HG 3a unit, then it will be dependent on sunlight to heat up and evaporate the water, which thereby becomes humidity in air that is sent into the underground tube to condense as pure water. A moderate amount of extra water can be provided in this way, but the exact amount is difficult to calculate since there are many variables that can affect performance, especially regarding the sunlight.

But if this sort of evaporation chamber is combined with a HG 3a device, then the 90,000 Btus of heat that the HG 3a system can generate can all be made to come out with the exhaust gases, and therefore into the evaporation chamber. The heat of vaporization of water is around 970 Btu per pound, with another 70 Btu/pound or so used in heating the water up. This means that the 90,000 Btu/hr heat output that the HG 3a unit can continuously create can evaporate roughly 90 pounds of water per hour. However, a simple poly tarp cover can allow considerable heat loss at night, so the water evaporation then will be less. During the daytime, the amount of heat lost outward through the tarp may be greater or less than the amount of sunlight heating which comes in through the poly tarp, so the evaporation rate may be less than or greater than the 90 pounds of water per hour. This is roughly 13 gallons 540 liters) per hour or around 320 gallons (1,300 liters) of water evaporated from the chamber per day. Due to the night heat losses and other reasons, a more practical expected amount is around 250 gallons (1,000 liters) of water per day. Included in this is the 20 gallons (80 liters) of water the HG 3a system naturally creates in a full day of operation, so a total of around 250 gallons (1,000 liters) of perfectly pure distilled water is realistic every day. This quantity is relatively independent of local humidity, since most of the heat used is produced by the HG 3a device, although in extremely cold climates, the water production will be less.

Solar Still Operation

The operation of a solar still has many limitations. The tilted glass cover is usually faced to the south, so early in the morning or late in the afternoon, the sunlight cannot easily get in to heat the water because of the angle of the sunlight. The water is not of a color that is particularly absorbent (technically, high emissivity) to solar energy. A critical part of a solar still is that the high humidity air that is produced by the evaporation of some of the water needs to encounter a COOL or COLD surface, such that the effects described in this presentation can occur, where the (local) relative humidity gets up to 100% and therefore water must condense into droplets. Since the glass cover is the surface in a solar still which must also represent that cooler surface, it would be great if it were as cool as possible. However, as the sunlight passes through the glass on the way in, a little of the heat is absorbed. Also, the location of the glass exposes it to the outdoor air, so the glass can never be cooler than the ambient air temperature. And finally, the glass cover is constantly exposed to the warm or hot air inside the chamber, so it generally becomes quite a bit HOTTER than the current ambient air temperature. Per the Psychrometric Chart, these effects all greatly reduce the amount of water that can be produced in a solar still. A little water is better than no water, true, but our approach of having the cool surface fairly deep UNDERGROUND, that factor alone keeps the condensation surfaces 20°F or 30°F (10°C or 15°C) or COOLER than the glass cover of a solar still. The Chart shows the wonderful advantages of this factor.

Deep Soil Temperatures in the USA

Map of the US showing average deep soil/well temperatures
For locations within the United States, this map can give a good idea of the underground tube wall temperature as long as it is buried deep enough to not be too affected by hot summer air temperatures. However, note that the underground tube will gradually warm up due to the warm or hot air that is continuously blown through it. If the flow rate (or equivalently, the water production) is limited, and the tube is sufficiently long, the underground tube will easily lose that heat to the deep soil against the outside of the tube, and remain cool near the temperatures indicated in this map. However, if massive production of water is done with this system, for example, by using the HG 3a unit to be producing 90,000 Btu/hr every hour which is then sent through the underground tube, then either a much longer tube should be used or it should be expected that as the soil warms up, the performance of the system will become reduced. A great feature of this system is that even if that occurs, simply waiting a few hours will allow the underground tube and the soil around it to cool back down to their natural temperature, and full performance can again be had.

An example is here near Chicago, where the deep underground temperature is around 52°F. If we have a common summer day where the air temperature is 85°F and the relative humidity is 50%, the Chart shows us that each pound of air contains around 0.0135 pound of water vapor in it. If that "normal air" is sent into the underground tube, then as soon as the air has cooled to around 71°F (parallel to the red line), water will start to condense. Since the underground tube walls are around 52°F, this would assure that virtually all the air passing through the tube would get near enough to some part of the 52°F walls to have its moisture condense out. As the air is cooled down to near that 52°F, we can see that the MAJORITY of the water in the air condenses out (like the green line in the chart). Finally, after that air has exited the tube and again warmed up to the 85°F ambient air temperature (parallel with the red line but now to the right and down), we can see that it will then have only around 5% relative humidity! Less than 0.002 pound of water will then remain in the pound of air. This means that this simple device will have removed around (0.0135 - 0.002) 0.0115 pound of water from every pound of normal atmospheric air passing through the tube. The chart shows that the air density is generally around one pound per 14 cubic feet. So if we send just 200 cubic feet of air per minute (cfm) through the tube, (which is around 14 pounds of air), we would have (200/14) * 0.0115 or about 1/6 pound of water condense in the tube every minute. An 8-ounce coffee cup full of absolutely pure distilled water every three minutes! This is 10 pounds of water per hour, or about 1.5 gallons per hour. An easy ten gallons of absolutely pure water would be removed from atmospheric humidity on a very normal summer day, without even using the accessories that can so greatly improve this system's performance! (Keep in mind that the basic system relies on the warmth of the air, so it generally only produces water during the daytime. The HG 3a accessory unit would be needed if 24 hour water production was considered important.)

For locations OUTSIDE the USA, there are two easy ways of getting a good estimate of the deep soil temperature. The simpler is to simply obtain local weather records, and locate the average HIGH and LOW temperatures for December 21 (or just December) and the average HIGH and LOW temperatures for June 21 (or just June). Add those four numbers together and divide by four. The result is nearly always within one degree of the deep soil temperature. The reason is very logical. By averaging those four temperatures, you are learning the year average AIR temperature for that location. The deep soil gets and loses nearly all of its heat energy from the air, so its temperature, for ANY location on Earth, is necessarily extremely close to the average air temperature.

The second method is to get a standard thermometer, and place it in the water flow from a local well, for at least one minute of constant flow. That water temperature should become very close to the deep soil temperature, as the rapid water flow does not allow the water to significantly heat up in just the brief time of it coming up to the surface.

This presentation was first placed on the Internet in March 2008.

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C Johnson, Theoretical Physicist, Physics Degree from Univ of Chicago