URGENT: Atmospheric Water Generator

[deleted-957136]

Hello! I want to create an atmospheric water generator for my science fair project. I have a design in mind however im having some trouble fully wrapping my head around the concept of an AWG, mainly how I would condense the air.

Another thing I was wondering about was the purification aspect of the generator. I would like to use a 4 step process involving a mesh layer, UVC, activated carbon, and an ion exchange resin. Based on my research this will remove the vast majority of the dissolved solids in the water including essential minerals. Without these minerals, the water would lack taste and it would not be as healthy. Any suggestions on how to include this into my generator after the purification process?

Any advice or suggestions would be greatly appreciated!
Thank you in advance!

[deleted-957136]

Also, how does a dehumidifier work exactly? It is the same concept as an AWG except the latter is purified for consumption. I found this picture on of it and I understand the general concept however, I still do not understand the compressor or the necessity of the capillary tube.

[kgrivera]

Hello!

To answer your first question, an AWG works in much the same way as a dehumidifier except it takes the water produced by dehumidifying the air and purifies it. Both take advantage of the dew point of water to condense water. The dew point is the the temperature the air needs to be cooled to (at constant pressure) in order to achieve a relative humidity (RH) of 100% (weather.gov). When air with a high humidity comes into contact with a cool surface, the water vapor in the air condenses on the cold surface. A real life example of this is condensation on a cool drink; on a humid day, if you leave a cold drink in a cup sitting on a table you will quickly notice water droplets forming on the outside of the cup.

Both the dehumidifier and the AWG take advantage of this phenomenon to work. The picture you attached shows how they work. A fan pulls moist air from the surrounding area into the evaporator coils. These contain cooled liquid refrigerant. The water vapor in the moist air condenses on the evaporator coils and drops into a collection bucket. Meanwhile, the refrigerant in the evaporator has absorbed the heat from the moist air and become a vapor inside the evaporator coils. The gaseous refrigerant enters the compressor. This part of the dehumidifier both pushes the refrigerant through the system into the condenser coils and pressurizes the refrigerant (which is now at a higher temperature due to the increase of pressure as stated in Gay-Lussac's Law). Now, the cool air from the environment that has passed across the evaporator coils flows over the condenser coils. The air absorbs the heat from the refrigerant, and the warm air passes out of the dehumidifier back into the room. Meanwhile, the refrigerant condenses back into a liquid since it has lost thermal energy to the air. However, before the liquid refrigerant can make it back to the evaporator coils, it needs to be cooled. The capillary tube depressurizes (and therefore cools the refrigerant) before it flows into the evaporator coils. At this point the dehumidification cycle can begin again.

To answer your question about the necessity of the capillary tube, without the capillary tube, the refrigerant will not be cooled. An alternative like a thermal expansion valve or pressure-controlled valve can also be used in place of the capillary tube. These all serve as metering (or flow control) devices, modulating the flow and pressure of the refrigerant as it enters the evaporator. In the capillary tube, the small diameter of the tube decreases the pressure of the refrigerant due to the Hagen-Poiseuille law. This law describes the pressure drop that occurs when a fluid flows through a long cylindrical pipe with a constant cross section. According to this law, change in pressure is inversely proportional to the radius, so a smaller radius tube will result in a greater drop in pressure. Based on Gay-Lussac's law as mentioned above, this drop in pressure is accompanied by a drop in temperature as well. Consequently, the capillary tube, or another metering device, is necessary to cool the refrigerant.

To add minerals back into the water, I would recommend looking up papers that describe remineralization methods for desalinated water. Despite the purified water being obtained through a different process, both should have the same problem with lack of minerals. Some common methods used include acidification with CO2 and dissolution of limestone.

Let me know if you have any further questions!

[deleted-957136]

Thank you! that was very helpful. These are my further inquiries

So, essentially to build my AWG as far as the condensing section is concerned, I would simply place the copper coils in a manner that resembles the picture and then buy a refrigerant to run through it? And then where would I place a pump to transport the refrigerant through the unit? What gauge copper wire should I use for the unit and what gauge for the capillary tube? (The unit for the "dehumidifier" section will be approx. 16''x12'') Finally, what does the compressor do and how would I make it?

Thank you for your assistance!

[kgrivera]

Hi again!

I can answer some of your questions, but the rest are a bit outside of my area of expertise (I'm technically a molecular biologist/biophysicist as I do structural biology research as a PhD student). I recommend making a second post repeating your questions on the Physical Science board for grades 9-12 since there are more likely to be engineers/physicists that will be able to help you with details about how to actually construct the AWG (where to place the pump, gauge of copper wire, how to make the compressor).

As far as what the compressor does, this page (https://smwac.net/hvac-knowledge/how-an ... sor-works/) has some details on the different types of compressors commonly found in air conditioners (which function in a similar manner to dehumidifiers, but the air doesn't pass through the condenser coils). The compressors take the refrigerant (which is a gas after passing through the evaporator) and increase the pressure and temperature by compressing it either using pistons and valves like a car engine (called a reciprocating compressor), or using different designs like a rotary compressor or a scroll compressor.

Sorry that I can't answer all of your questions!

[deleted-957136]

Thank you so much for your help! And thank you for the suggestion to post my question to the physical science board!

[deleted-957136]

A component of my science fair project this year involves building a water filter. Two components of that filter are activated carbon and an ion exchange polymer. The process I have designed will likely remove most, if not all of the preexisting minerals in the water,; meaning that it will not be healthy to drink immediately following the purification process. What is the most efficient way to add healthy minerals and salts back to the desalinated water?

Thank you!

[MadelineB]

Hello Dominique,

At the suggestion of the expert who has been helping you, I've cross-posted your topic (and merged your posts) in the Physical Science forum so those experts can also help you. Thank you and best of luck.

Madeline
Moderator

[deleted-909690]

Hi dominiqueolivia,

This sounds like a great science fair project! And an incredibly challenging one! Kgrivera has given you excellent advice to get started.

There are many possible ways to construct an AWG, but we'll focus on the one you have described. This is a thermodynamic process called a "cycle." Think of a cycle as a series of steps performed on the "working fluid," which is a refrigerant in your case. The refrigerant runs through the inside of your system and will be returned to its initial state and the cycle starts all over again. The LearnChemE website has some great resources on thermodynamics concepts, which is a course you would take in college as a sophomore or junior chemical engineering student.

There are several thermodynamic cycles used in all sorts of applications, including internal combustion engines (Otto cycle), diesel engines (Diesel cycle), and jet engines (Brayton cycle). The refrigeration cycle in the image you provided is called a vapor compression cycle.

In the vapor compression cycle, you put in "work" to the system using the compressor (using electrical power to compress the volume of the refrigerant). The compressor increases the pressure and temperature of the refrigerant. The compressed refrigerant enters the condenser (inside the coils) and heat will be transferred to the outside air blown over the coils by a fan/blower. The refrigerant then goes through a pressure reducing valve (an expansion valve, capillary tube, an orifice of any kind) to rapidly expand the volume (and reduce the pressure) of the gas. Because of the unique properties of the refrigerant, this rapid expansion also drastically lowers the temperature of the refrigerant. The cold, low-pressure refrigerant goes through the evaporator coils. The humid outside air blown over the evaporator coils produces "condensate" and reheats the refrigerant. The warming refrigerant is returned to the compressor, and the cycle repeats. The LearnChemE website has a youtube video explaining vapor compression cycles in further detail. They have also developed a simulator to help understand the cycle's concepts.

As for how to physically construct a vapor compression cycle-based AWG, I found this thesis describing the process as well as the underlying concepts. It should be a fabulous resource for your science fair project.

Finally, about the water purification aspects... If the purpose of your AWG is to generate drinking water, using an ion exchange membrane is overkill. Ion exchange membranes are typically used to remove sodium or other salts from water. You could easily get away with using a backpacking water filter or purifier, which would remove any bacteria or viruses, respectively, that might have been introduced from the air.

Hope this helps!
Dr. Payne

[deleted-957136]

Hello Dr. Payne,

Thank you so much! That explanation was amazing and it was really helpful. First, am I able to cite you in my research plan? Your assistance, as well as the help of Kgrivera, has helped me alot with my project. I am only a sophmore in high school and although I am taking college courses, I dont really have any teachers that are able to assist me with my project as far as the engineering aspect is concerned; so your help is greatly appreciated!

For the filter, what would be an appropriate configuration? Through my research I have found that most commercial companies utilize an ion exchange resin, but that may not be best for my project. I will think about using a carbon based filter (with mesh as well) and UVC for purification purposes. What else would I add to this if anything? Or do you think it would be best to simply use a commercial filter?

For the AWG it would require a motor correct? Where would that be placed?

Thank you

[deleted-909690]

Hi dominiqueolivia,

No problem! I'm glad you found my explanation helpful. If you have access to a physic teacher, they may also be able to help you with local resources.

As for your questions...

First, am I able to cite you in my research plan?

You generally want to cite peer-reviewed literature or books to support scientific assertions. Any chemical engineering thermodynamics textbook will have a section on thermodynamic cycles. I recommend the "Fundamentals of Chemical Engineering Thermodynamics" text, as it tries to make thermodynamics more understandable at the undergraduate level. Section 5.3 reviews the vapor compression cycle.

If discussions with other scientists/engineers have shaped the direction of your project, you can include that as an acknowledgment rather than a citation.

For the AWG it would require a motor correct? Where would that be placed?

Depending on your design, your system may need two motors - one to drive the compressor and one to drive the pump. The pump is necessary to move the collected water through the filtration system unless you use gravity-based filtration. These will typically be an integral part of the equipment though and will not need to be separately purchased. In a compressor, the motor turns the crankshaft or powers the fan of a centrifugal compressor. You can read about the different types of compressors at this website. The University of Michigan website also has a library of images for industrial-scale chemical process equipment, including compressors, evaporators, and condensers, so you can imagine what this would look like at large scales.

For the filter, what would be an appropriate configuration? Through my research I have found that most commercial companies utilize an ion exchange resin, but that may not be best for my project. I will think about using a carbon based filter (with mesh as well) and UVC for purification purposes. What else would I add to this if anything? Or do you think it would be best to simply use a commercial filter?

The wikipedia page for AWGs has a nice image of how a water filtration system would be integrated into the process design. You would collect the condensing water in a vessel or tank which would then pass through a filter(s) or a membrane system such as reverse osmosis.

Ion exchange water treatment systems are generally used to remove charged contaminants or undesirable minerals from the water, such as salts, calcium, magnesium, heavy metals, etc. Since an AWG is pulling water from the atmosphere rather than from a wastewater source, your water will have relatively few ionic contaminants and minerals, making ion exchange treatment unnecessary. Plus, you would just have to add back some of those minerals to make it potable drinking water.

You will want to treat the water for contaminants that could be introduced through the air, however. This would primarily be to remove biological contaminants (bacteria, viruses, yeast) that naturally exist in the air. The easiest way to do this is to pass the water through a granular filter (could be activated carbon) and then use a chemical disinfectant (such as chlorine) to kill any remaining pathogens. This is how nearly all municipal wastewater is treated in the U.S. You could use a commercial water filter or you could even make your own.

Dr. Payne