You may be familiar with a survival procedure taught to travelers to remote areas, where they spread a small sheet of black 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.
When some people choose a really remote location to be off-the-power-grid, they often had not realized that they may need to have a very deep well bored to get water, but that the large truck that hauls such equipment may not easily be able to get to the site. Also, once it is installed, generally a high-power water pump is required to then raise water forever after, meaning that a rather large supply of electricity is necessary. This system MIGHT be able to eliminate all those expenses and complications. 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.
There are actually three separate components to this very efficient system. Each are simple and inexpensive to create at the location.
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. The other two components if this system are to maximize the relative humidity in the air going into this tube and to heat that air to also increase its ability to contain moisture. A Chart 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! (Much of the operation of the combination of the three devices is so effective that it is off the right hand side of that Chart!)
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 seawater or rainwater and some local dead vegetation are the only necessary materials! The first two components HEAT the air and the water to increase the amount of water vapor in the air that goes into the underground tube. This concept uses the fact that deep underground, the soil is naturally cooler than the daytime air temperature.
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 cooler than the air temperature during hot summer days, and usually during winter days as well. The fact that it might not be cooler than the nighttime temperature is taken care of by the first device involved, the HG 3a unit.
This amazingly simple and inexpensive system can realistically supply 250 gallons of perfectly pure water every day! There is essentially nothing which can break down, so it should reliably provide water for many, many years.
We are showing the three devices from right to left. (1) The (green circle here) HG 3a unit is normally designed as a heating system, but the exhaust coming out of it is excellent for this system. The HG 3a unit actually PRODUCES around SIX POUNDS OF WATER PER HOUR from the very material of the dead vegetable material put into it. This is due to the chemical reaction of the decomposition of cellulose and the other organic materials into carbon dioxide and water vapor, the opposite of what happened when the plant grew due to photosynthesis. The plant material generally also contains some water as well, so the total amount of water coming out of the HG 3a unit is generally greater than the 6 pounds per hour. (That alone is around 18 gallons of perfectly pure distilled water per 24 hours.)
The HG 3a also can send around 90,000 Btu/hr of heat out in those same exhaust gases, all in the range of 130ºF to 150ºF (or 54ºC to 66ºC). (2) This amount of heat is sent into the second component of this system, a relatively simple "heat bag" over a very shallow pond of seawater or rainwater. That rather hot air passing over the seawater or rainwater causes some of it to evaporate, which further increases the moisture contained in the air inside the chamber bag. Since it takes roughly 1000 Btu to evaporate one pound of water, the 90,000 Btus provided to the bag by the HG 3a device has the (maximum) capability of evaporating nearly 90 pounds of water (or about 12 gallons) per hour. This amount is actually less because some of the heat is lost upward through the bag, except in the middle of very sunny days in Equatorial locations. At such times, the evaporation is even greater than this, due to the added heating due to the Sun's heat. At night the performance is less due to greater heat losses through the top of the bag. A 30-foot-square tarp as a water tray, with one-inch deep of seawater or rainwater, contains around 600 gallons of water. A thinner layer of water would heat up better and faster, and therefore more would evaporate to provide humidity in the air for the final device.
(3) After the air has been heated and humidified by the first two devices, it goes into the third device, the underground tube, where the coolness of the deep soil causes much of the moisture contained in that hot humid air to condense as tiny water droplets on the walls of the buried tube. Those water droplets go downhill and into the collection pipe area and then the collection tank. A conventional hot water tank can usually be considered to be clean of previous chemicals, while a 55-gallon drum might not be. A simple hand-crank pump can raise water for each family that brought a container for it.
The rightmost device, the HG 3a unit is fully described, with all the engineering information included, in HeatGreen heating system. That page includes a Big Bag version that is very crude but very simple and inexpensive
The construction instructions for the HG 3a is provided at HeatGreen heating system HG 3a construction
The information and construction guidelines for the underground device is at Underground water condensation device.
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 0.046 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 46 pounds of water, or around six gallons.
The same Psychrometric Chart shows that at that temperature and humidity, one pound of air takes up around 16 cubic feet, so we are talking about 16,000 cubic feet of air. If we hope to produce ten gallons of this Distilled Water per hour, we then only need to send around 450 cubic feet of air through the tube every minute ((16,000 * 10/6)/60), a relatively moderate airflow. (Obviously, additional buried tubes or additional HG 3a devices or larger evaporation pond areas can be arranged for greater water production per day.)
This is roughly 250 gallons of absolutely pure water to drink and for washing and batheing every day! All from a rather simple arrangement of three simple and inexpensive devices!
The initial ocean seawater is not drinkable due to the high salt content in it. Ditto regarding trying to drink rainwater, which might have picked up contaminents from the air as the drops fell, or from roof shingles or other materials that the water might have contacted. Existing systems which try to desalinate saltwater are incredibly expensive, complex and high-tech. Unfortunately, they generally do not work on seawater because the high salt content quickly clogs up microscopic filters and accumulated salt deposits clog up nearly everything else. This simple system, of the underground tube in combination with the two rather simple accessories, can provide as much as ten gallons of perfectly pure Distilled water every hour, 24 hours each day, or around 250 gallons of water that is desalinated to a purity even BETTER than when expensive high-tech equipment is used!
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 the seawater or rainwater which is available, since when that water evaporates from its source, all the contaminants are left and only the pure water evaporates.
We will use an example of where the air temperature is 120ºF (49ºC) 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 (80ºF or 27ºC) 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 88ºF (31ºC), 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 mositure condenses 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 80ºF or 27ºC, the Psychrometric Chart shows us that the air which had contained 0.030 pound of water per pound of air at the start of the green (dehumidification) portion of the process, now contains only 0.0225 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 passes through, (0.030 - 0.0225 or) 0.0075 pound of water forms inside the tube.
Air needs to be passing through the system. It may NOT be necessary to have to use any blower, because the entrance to the HG 3a unit could be provided with a wind-vane type of tail to turn an intake funnel 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.
Many variants of such equipment exist, with most being versions of either Reverse Osmosis (RO) or Electrodialysis (ED). The main reason they clog up so extremely often is because 1,000 gallons of seawater contains about 300 pounds of dissolved salt and other chemical ions. Both RO and ED systems work far better on what is called brackish water, which is far less salty than seawater. ED is not even attempted on seawater any more, after attempts were essentially failures, and RO is not particularly successful for seawater.
Distillation is the other main method used for desalinating water. Traditional distillation was considered to be too slow and inefficient, and most recent installations involve variants, such as what is called flash distillation. Due to the large quantities of salts in the seawater, such equipment tends to get encrusted with salt, because such actions increase with temperature and flash distillation uses rather high temperatures. Many modifications have been added to the distillation process to try to deal with such problems, which has made such installations very large and complex and expensive.
The system presented here does not have such problems. The two tarps (bottom as a tray and top as a heat and humidity cover) are simple to clean of accumulated salt deposits, and there are communities in India that collected such sea salt to be sold for significant profit. The underground tube never has any contamination or deposits form as the only thing that enters it is air with humidity in it. If the atmosphere in the area is heavily polluted, it is possible that some of that air pollution could get into the tube and therefore into the resulting distilled water. That is never really a problem for Third World countries or usually even for any remote location where Americans try to go off-grid.
In general, RO and ED systems are designed to filter out enough salt to lower the seawater's normal concentration of 35,000 ppm (parts per million) of salt down to around 1,000 ppm, which is considered usable. If the water is to be potable, it must be lowered even more down to below 500 ppm. In much of the US, the requirement for potable water is to be less than 250 ppm. This represents a reduction of salt content of seawater by a factor of about 140, which is why desalinization of seawater has not become broadly used, until now.
With any Distillation method, including ours, there is ZERO salt content in the resulting Distilled water! The reason is obvious, that the salt CANNOT be evaporated with the water, and therefore can never even get into the underground tube to get condensed. So Distillation methods result in far purer water than RO or ED can even hope to achieve.
There are many other methods which have been tried, such as freezing seawater. The premise is that fresh water freezes at 32ºF or 0ºC, where seawater freezes at a temperature that is several degrees lower. So if seawater is cooled to around 30ºF, only the fresh water can actually freeze, which should result in pure fresh water. Unfortunately, the reality is that this process results in crystals of salt being trapped within the fresh water ice that results, and so there is still significant salt in the resulting water or ice. It turns out that this process also involves massive usage of electricity for refrigeration, and it has generally been ignored as being too expensive for practical use.
Note that the system we describe in this presentation has a total cost of around $60 for the underground tube part and $200 for the HG 3a device and $40 for the simple tarps or $300 total, and it can provide a consistent 250 gallons of pure distilled water per day. Scaling this up, it indicates that around $1.2 million US dollars would be required to provide the same one million gallons of processed seawater. This is not only far less expensive than the $6 to $8 million of current technologies, but it would provide water at a far greater purity, approaching 0 ppm of dissolved materials rather than the 1000 ppm that is currently accepted.
The actual cost of providing water is higher still, because of the extensive need for (externally provided, fossil fuel) power needed for those various systems, as well as for getting supplies of such fuels to remote locations where desalination operations are needed. The fact that highly trained technicians must always be on-site to maintain and repair the equipment adds to the cost of operation. When these factors are combined with the cost of the initial capital, the interest on those funds, and depreciation of the equipment, the following general guidelines result: (obtained from Mark's Handbook for Mechanical Engineers, 1995)
Cost per thousand gallons of water provided
In contrast, modern American Municipal water supplies averaged around $0.15 (in 1995), with an additional $0.20 to $0.40 for distribution costs.
The new system presented here involves no distribution costs, as the water is produced locally for the users. In addition, maintenance and repair are minimal and very simple, where local villagers should generally be able to correct anything that could go wrong, and also clean any of the items that might require such maintenance.
It also involves NO FOSSIL FUELS at all, as it is entirely self-powered by a combination of sunlight and the decomposition of locally available organic materials such as grasses and leaves. A very small amount of other power might be needed for a blower if that is required due to lack of sufficient winds, but a small and crude windmill should be able to provide those minimal requirements.
This results in essentially no costs for fuel or other power, beyond the hauling and loading of that vegetative matter into the HG 3a device every few days, and essentially no costs for labor or maintenance or repair parts. This results in the cost for the water being primarily in amortizing the cost of the initial materials. As these devices should last for at least ten years without requiring replacement, this suggests that 250 gallons of water per day times 3650 days or around 900,000 gallons of water should be provided by the $300 initial costs. This indicates that the operating costs of this new system would be around $0.33 per thousand gallons of pure distilled water provided. That is not quite as economical as American Municipal water supplies, but decently close.
No other method of desalinating seawater is remotely close!
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