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Almost passive atmospheric water generator

Darko Savic
Darko Savic Mar 06, 2021
Atmospheric water generators are not new. What this idea proposes is a fairly inexpensive DIY design of a large, almost passive unit with a minimal number of moving parts and minimal upkeep. It is to be mounted on the side of a house and custom-sized to provide a sufficient amount of drinking water for the household.

It effectively makes a "spring" of clean drinking water from the air. I built a 3 meters long prototype. The only thing left to do is mount it on the side of the house and test it in action. I will get it done soon and post the results.

  1. A stainless steel pipe passing through a stainless steel cooling tank.
  2. The tank is kept cold either via a compressor or Peltier elements and powered via renewable power.
  3. In suitable climates, cooling can be completely passive because the cooling tank is mounted on the outside of the house while warm, humid air circulates on the inside.
  4. Warm and humid air passes through the pipe. Either passively or assisted by a fan.
  5. In the sections where the pipe intersects with the tank, water condensation droplets form and stick to the wall of the pipe.
  6. The gravity then pulls the droplets into the water filter where it gets remineralized.
  7. Filtered water is then stored and kept sterile by UV lights inside the drinking water tank.

Imagine mounting this on the outside wall of a house or a greenhouse. The pipe goes through the wall on both ends, so that the air inlet and outlet are inside the house. The cooling tank section remains outside of the house, on the wall facing away from the sun.

The air would be more humid and warm inside because of people or plants. All the water vapor would exit the room via one-way only - condensed and filtered for drinking.

In the colder climates, the outside air would be cooled by the environment for half of the year. No additional energy would be required to cool the cooling tank. In the summertime or in warmer climates, the cooling tank could be insulated and cooled via renewable energy. Either using a compressor system (much like air conditioning units) or Peltier elements.

If the indoor air is always airconditioned, it would make sense to reverse the location and keep the cooling tank indoors while the water harvesting would be done on the outside. In that case an additional filter would be placed over the air intake.

The cooling tank could also be cooled geothermally. Pipes would be dug deep into the ground where the temperature is significanlty lower. The coolant water could then be circulated trough the undergorund pipes to lower its temperature.

The necessary conditions for atmospheric water to condense

Water vapor in the air is composed of H2O molecules that have enough kinetic energy that they bounce apart and do not stick together to form droplets of liquid water. When those H2O molecules strike a surface that is colder than the water vapor, they transfer some of their kinetic energy to the atoms within that surface. When a water molecule touches a surface that is cold enough to borrow enough energy from that water molecule, that H2O molecule can no longer take off from the surface. It sticks - this is called adsorption. If the air is saturated with water molecules (high relative humidity) and the surface of the object is very cold, then the process of adsorption will happen faster than the air can reevaporate the water off of the surface on which it has adsorbed - so plenty of condensation. If the air humidity is low, the adsorbed water will reevaporate before more water molecules can be added to it - so not enough condensation.

The water that can be extracted from the air depends on relative humidity, temperature and pressure:

For example, if the air temperature is 27°C (80°F) and the relative humidity is 75% then any object with a temperature of 22°C (71°F) or lower will become covered by water droplets fairly quickly.

Some things that I have yet to put some serious thought behind:

  • How would this work in very hot regions that have minimal relative humidity in the air?
  • How can the water filtration and remineralization be made to require minimal upkeep and lowest cost?
Creative contributions

Atmospheric water harvesting in hot and dry climates

Darko Savic
Darko Savic Mar 07, 2021
The water that can be extracted from the air depends on relative humidity, temperature, and pressure. If the conditions are unfavorable, what can we do to make them better?

In climates where the atmospheric air is very dry but there is a source of water unfit for drinking (sea, river) a solution would be to use the energy from the sun to make the water evaporate into a containment vessel, thereby saturating the contained air with humidity, then extracting it similarly as originally described.

This could be:
  • A greenhouse, irrigated with dirty water.
  • A plastic dome over a pond of seawater or river water.
  • A half-filled long black pipe that is heated from the sun. Undrinkable water passes through the bottom half while humid air is harvested from the upper half.

Atmospheric water vs. rainwater harvesting

Povilas S
Povilas S Mar 08, 2021
I wonder what are the benefits of atmospheric water collecting that you described over rainwater collecting? In places where air humidity levels are high rain is usually frequent too. The purification process you mention for atmospheric water (filters and UV lamps) is the same for rainwater. The collecting process is easier for rainwater (you just need a flat surface area and tanks to catch it). One obvious benefit is that atmospheric water can be collected at any time whether for rain you have to wait, but rainwater is collected in larger quantities, and therefore its resources last longer. In the case of rainwater, you also don't need additional energy for the cooling process. However, those two methods could be combined to produce more reliable availability of drinking water.

Another thing I was thinking about is that if many people would be using these systems it might lower air humidity on a larger scale which then might create consequent atmospheric/ecological problems.
Shubhankar Kulkarni
Shubhankar Kulkarnia month ago
Povilas S I had the same thought - whether it would be economically better compared to purifying harvested rainwater? I would not worry too much about disturbing the air humidity in the region since the air on Earth is one single mass. It constantly alters the humidity and pressure in the different parts to maintain a minimal entropy point. It is highly dynamic which eases compensation. Loss of humidity will be compensated with higher evaporation from the nearest water body. Winds bring in humid air and maintain the percentage of humidity that is allowed by the temperature in the region.
Darko Savic
Darko Savica month ago
I might be wrong but I think the amount of water that circulates in an area is fairly constant. If people "dried up" the air, the water in the area would just re-evaporate that much faster. I'm not sure what the mechanism responsible for this would be, but I "feel it" in my bones:) Physics is not my strong side. I'm thinking atmospheric pressure, fluid dynamics, and entropy would have the answers to this.

If disrupting the ecosystem is a concern then, large rainwater cisterns would temporarily remove more water from the environment than the proposed smaller family-sized, water-on-demand cisterns. In other words, the idea proposed on this page would be less intrusive.

As you said, for rainwater you have to wait and then have the storage capacity (big cisterns) to store enough water to last you between the rainfall. With the method proposed in this water generator idea, you would be generating water on demand.

Controlling the water quality is easier when the water doesn't stay in the same cisterns for longer periods of time and when the cisterns are smaller. You can wash the smaller cisterns by hand on a regular basis. In contrast, larger cisterns would need to be chemically treated (and then you drink from them) - which defeats the main purpose of the proposed idea.

People who are lucky to live in water-rich areas where there are plenty of natural sources of drinkable water wouldn't find much use for the proposed idea. It might still be useful to those who wish to control the water quality and don't want to drink the chlorinated water provided by the local municipality.

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General comments

Shubhankar Kulkarni
Shubhankar Kulkarnia month ago
Great idea and manufacturing Darko Savic ! I have a few questions:
1. How much did the assembly cost you?
2. Is there a maintenance cost associated with it? For example, how frequently do you need to replace the coolant? How frequently do you need to clean the pipe on the inside? How frequently do you need to replace the filter?
3. What is the average humidity and temperature in the area where the assembly is fitted?
4. In those specific environmental conditions, how much water does the apparatus produce per hour? It may not be constant. It will show diurnal and seasonal variation. But just to have an idea, can you mention the amount of water produced at X temperature and Y humidity?
5. Any cool names for the apparatus? :)
Darko Savic
Darko Savica month ago
Thank you Shubhankar Kulkarni 🙂

Most of the answers have yet to be determined. In the summertime, the coolant is either water or refrigerant gas - I haven't yet decided. I will first test it with water, cooled by Peltier elements. In the wintertime, it should be completely passive and not require any refrigeration. I'm not sure what the total cost was. It's basically all welded together from ready-made stainless steel elements. I think the cost is about 5 EUR per kilogram. I can only guess the weight until I put it on a scale. It feels about 70kg.

This shows the temperature and humidity in my area https://meteo.arso.gov.si/uploads/probase/www/climate/table/sl/by_location/maribor/climate-normals_81-10_Maribor.pdf (top section is the temperature, 2nd section is the relative humidity at 7 am, 14:00 and 21:00 hour) - the conditions here are ideal. There is no shortage of water. The device takes indoor air anyway. That can be controlled by adding more plants to the intake room and watering them. The plants do the first filtration.

I haven't yet started working on the filter. I imagine something DIY like this https://i.pinimg.com/originals/09/1a/c3/091ac33b87bd5348042edbc6cd34a68f.jpg mimicking nature as much as possible. I would like to see if I can get a "permanent filter" (ecosystem) going so that it never needs to be replaced.

The water holding tank would be a large stainless steel pot with a UV light inside. The pot would be easily removable for cleaning. It might also be a good idea to use a ready-made drinking water cooler and pipe the water directly into it.

Let's see it perform before we give it a name. Hopefully, it doesn't include the word "failure" within the name:)
Shubhankar Kulkarni
Shubhankar Kulkarnia month ago
Darko Savic haha! I think it won't. :)
The humidity in Maribor is really potent for such an apparatus. It hardly goes below 70%. (https://en.climate-data.org/europe/slovenia/maribor/maribor-882/). Where I live, the humidity hardly goes above 70% (only during the rainy season (mid-June to September).

Please do measure the amount of water you use as a coolant. That would be great for the cost-benefit analysis we can do later. Also, how do you plan on cooling the coolant?

The filter idea looks good. Yes, it is closer to nature. I remember performing that experiment in school. I think we used cotton as one of the layers. It will probably perform the same function as that of the cloth in the image you linked.

All the best! Keep us posted.
Darko Savic
Darko Savica month ago
Shubhankar Kulkarni many places have a very high relative humidity but a problem with drinking water supply. For example, Thailand has perfect conditions for such water harvesting but people in urban areas have to get the water delivered via mobile cisterns.
Shubhankar Kulkarni
Shubhankar Kulkarnia month ago
Darko Savic Such places are the best targets (users of the apparatus).