Instructables

Technology Makes Cheap Drinking Water from Air

INTRODUCTION:  

How can we best apply basic technology to help the underprivileged and/or disaster-hit countries like Haiti? Daily hygiene and nourishment are among the top needs for disaster ridden regions!  Simply put, no water means no hygiene. The Romans understood that over two millennia ago and created their complexly beautiful aqueduct networks for handling both fresh and wastewater! Other ingenious water systems like “air wells” have been found in the city of Theodosia (cf: discovered in 1900 by Zibold, see Zibold’s Collectors/Dehumidifiers) dating back to Greco-Roman times during the Byzantine Empire. These were strictly passive systems that naturally dehumidified air, collecting its potable water in underground basins.

All air, even in relatively dry desert regions, will precipitate or release its natural water content (initially in the form of vapor) through condensation when it hits its dew-point temperature and below. That means you “chill” it to an appropriate level that is anywhere from 5F to 50F below its current air temperature, depending upon how much water content (relative humidity) it has locally absorbed. The condensation of the water vapor releases its internal latent heat (reheating the cooled air) which must be constantly dissipated (absorbed by something) in order for water formation to steadily continue. So how do we dissipate this resultant vapor-heat and chill our air without any infrastructure or electricity, in an underprivileged or disaster-ridden region? We simply bury a long cast-iron or any metallic drain-pipe sufficiently underground where the temperature of the earth is naturally held to a constant at around 45F to 55F. That’s our “free” chiller gift from nature. One end of the pipe, Figure-1,  sticks out of the ground to suck-in local outside hot air, and the other end dumps cooled dry air and water into an underground cistern where it gets collected and is piped to the surface to both exhaust the cooled dry air and connect to a water pump. We need a hand operated water pump to lift up the water above ground, and we need an electric fan to constantly pump air through the ground-chilled piping system. We can even force the cooled piped air to exhaust into a tent-like structure where it provides air conditioning as an added bonus, but this adds the penalty of both power and the increased fan size necessary to drive our required airflow further into an enclosure!

While this concept is not “passive” (requiring electricity to work) like those clever Byzantine air-wells, it will produce much more potable water and within a smaller volume than those elegantly passive historic devices. The electricity for our fan power requirements can be produced by any one of four ways using either “active” or “passive” techniques: 1) An active playground or bike-pedaling-person or oxen-driven mechanism-generator, 2) A passive windmill generator, 3) A passive solar energy collection system that directly generates electricity, or 4) A passive thermo-electric system that directly generates electricity using the Peltier effect, operating solely on temperature differences between the cell’s top and bottom surface (we jury-rig the cool pipe and hot ambient air to contact separate sides of the cell).

Depending upon how much water is needed, the required air volume plus pipe length and diameter, together with the fan will be sized accordingly. We can also configure groups of parallel fan-driven air pipes that are radially fed into the cistern. The sizing of this underground network depends upon the ambient air’s local average temperature and relative humidity (how much water gets absorbed into the air) plus buried pipe depth and effective underground temperatures achieved.

The basic concept is one where we “wring” water from air at some given humidity content. The higher its relative humidity the more water is recovered from the air. The air-wringing process simply chills the air as it scrubs along the cooled internal pipe surface until it starts to rain inside the pipe from condensation onto its surface. The condensation is like the dew that forms on car windows, grass or any cooled surface in the early morning, before the sun comes out and evaporates the dew back into the heating air. A further bonus is that our dew-formed water is naturally distilled and very clean. It is potable water ready to drink without the need for additional sterilizing agents. Of course, we must make sure that the interior piping and cistern network is biologically cleansed before burying it underground. The hand pump with its 10 to 15 foot extended piping to reach the underground cistern must also be cleansed.

The beauty of this constantly replenishable water supply is its convenient underground installation anywhere! After the in-ground installation, we have a virtual, partially passive, no moving parts, non-breakdown system containing above ground total access to all moving parts that could breakdown, namely the water pump and electric fan. Also, it is easily maintained, with few moving parts (water hand-pump and electric fan) and basically lacking any technical complexity which makes it ideal for technologically backward regions.

The example below uses a relatively small industrial fan moving air at 1500 CFM (Cubic Feet per Minute) with a DC motor rated at 1kW. This fan together with our underground piping system will conservatively generate 12 GPH (Gallons Per Hour) of potable drinking water without need for any purification chemistry. Based on an average electrical cost of 14-cents per kWh (kilo-Watt hour), the typical commercial distillation of one gallon of drinking water costs roughly 35-cents as compared to our cost of only 1.2-cents. Furthermore, if we decide to go green and use solar energy for generating our water, it would effectively cost us nothing beyond the initial installation!

USING A PSYCHROMETRIC CHART TO SIZE OUR WATER SUPPLY:
The following gets a little technical and is only provided for those die-hards who are truly interested in how the science works. Those non-technically schooled may skip this part and not miss the basic concept.

Figure-2 shows a Psychrometric Chart for air. This chart summarizes some of the basic thermodynamic properties of air throughout its typical range of operating temperature. The chart uses six basic air properties that defines the physical chemistry of water evaporation into air:  (1) the enthalpy or total energy contained within a unit of air which is a combination of its internal and external energy, expressed as the amount of BTU-energy per unit mass of reference dry-air, (2) the specific volume or the ratio of a unit volume of local air to its mass of reference dry-air, (3) the humidity ratio or the amount (mass) of moisture in a local unit of air divided by its reference mass of dry-air, (4) the percent relative humidity per unit of local air, or the mass ratio (expressed in percentage form) of the partial pressure of water vapor in the air-water mixture to the saturated vapor pressure of water at those conditions (the relative humidity depends not only on air temperature but also on the pressure of the system of interest),  (5) the dry-bulb temperature or the locally measured air temperature, and (6) the wet-bulb temperature or saturation temperature which is the local air temperature experienced during constant water evaporation (a wet-bulb thermometer is typically used:   a thermometer that measures resultant temperature while wrapped in a water wet-gauze and spun to generate local air movement and max-evaporation) 

1.0   The Process and A Sample Calculation

Our Psychrometric Chart uses six thermodynamic properties that help to determine the amount of water available for extraction from the local ambient air as a function of its temperature, pressure and relative humidity.  Let’s assume the following local ambient conditions for the region we plan to construct our water system at:  (1) Typical daily air temperature Td = 106F and one atmosphere pressure assumed at sea-level, (2) Relative Humidity, RH = 55%, and (3) Typical underground temperature down at six feet is measured at Tu=55F (at 12ft. it drops to ~45F).

This yields the following calculated results for obtaining a steady-state supply (changes at night) of water to fill the cistern:     

1)      In our example, the “local” air (dry-bulb) temperature is Td=106F, at a relative humidity of RH= 55%.  Fig-2 indicates that the resultant Humidity Ratio is HR= 0.0253 Lbs-water/Lb-Dry-Air (intersection of Td=106F line and RH=55% line, then horizontal to HR value).  We then determine the “gulp” of air volume containing the HR Lbs-water which corresponds to the point of intersection of Td and RH. Interpolating on specific volume “mv” yields mv=14.7 ft3/Lb-Dry-Air (this value sets the optimum unit airflow for our given ambient conditions, and creates a ballpark pipe length to diameter ratio needed later). It represents the basic unit of air volume that will enter our underground pipe per given time, and ultimately defines the size of our fan and piping network. For increased water creation, multiples of this unit volume will scale up the additional amounts of water that can be collected.

2)      As the inlet air cools down to a temperature of Tu=55F, from contact with the relatively cold underground pipe, we follow the constant enthalpy line (red upward left-diagonal) from the intersection of Td and RH to its saturated air temperature condition of Ts= ~88F, which is its dew-point temperature where the corresponding local RH=100%.  At this temperature or under, the air precipitates and releases its moisture content, resulting in water condensation onto the pipe walls.  Since our air will chill to a final pipe temperature of Tu=~55F, we follow the RH=100% saturated curve (green) down to yield an HR=~0.009 Lbs-water/Lb-Dry-Air. This is how much water is left in the air when it gets to 55F.  Therefore for every pound of local outside air that enters the pipe, mw=0.0253 – 0.009 = 0.0163 pounds of absolute pure, distilled potable water precipitates onto the inside pipe wall (per pound of dry air that is cooled and dehydrated) to gravity-flow out the pipe exit and into the cistern.

3)      We now convert pounds of air per unit time into a unitized volumetric airflow that yields gallons of hygienically pure potable water production per unit time. For every Va=100 ft3 of local volumetric air movement per minute (CFM) through the pipe, which translates into ma=Va/mv= 100/14.7 = 6.8 lbs. of dry air per minute or 6.8 * 60 = 408 lbs. per hour (PPH), to yield a water-flow of mwf=ma * mw = 408 * 0.0163 = 6.65 PPH or 6.65/8.345 = 0.8 GPH of water.  An industrial fan rated at 1kW DC will typically move 1500 CFM at a pressure of 8-iwc, to continuously produce 15 * 0.8 = 12 GPH of pristine potable water.

4)      Not shown here are the design details of sizing our pipe, fan and solar collection system for electric power requirements using heat transfer principles coupled with a thermodynamic heat balance, and aerodynamic fan performance assessment. These details help to size the electric power generation requirements plus margin used to properly size a solar collector containing further margins for overcast days. The engineering involved here is straight forward but beyond the scope of the current project.


Picture of Technology Makes Cheap Drinking Water from Air
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DragonDon11 months ago
After reading(err, skimming) info and comments, I think a smaller unit is a better way to go. Multiple cheap units rather than one large one. I understand the scale you are talking but if a simple device would be setup, then it removes a ton of other issues (bugs, rodents...). Using the strips from this kit (http://solarpocketfactory.com/collections/solar-panels/products/solar-pocket-kit) could easily be utilized to power simple devices. (Actually, I did buy the DIY solar kit as suggested from this 'ible http://www.instructables.com/id/Five-minute-solar-phone-charger/ but they don't seem to be available, they were cheap. Someone must have them).

I'm sure there is a simple solution out there. Keep digging!
Cool idea! I have a couple concerns. The first is what a couple people mentioned, purity. Organics and other contaminants could become an issue.

My bigger concern is actually the thermo of it. At 100 cfm, you guesstimate you can get around .8 gph of water. But that correlates to a little over 5,000 btus/hr of cooling capacity. This may be realistic to achieve. However, looking at a 1500 cfm fan and wanting to remove that much heat (76,000 btus/hr) you will need a very large or long pipe. The heat transfer of that pipe has to be the equivalent of around a 6 ton hvac system. That's big enough for most small commercial buildings/small retail building.

Now I am not saying it can't be done, 100 cfm seems a bit more realistic for a much more passive system easily running off a bike or much smaller blower. Also keep in mind that blower has to be on non stop to be producing water. In my experience the fan ad blower consume more energy than the the compressor/rest of the hvac system. Point being, powering a blower like that may be more burdensome than you think. I'd look into some calculations to see the diameter and length of pipe you need to maintain heat transfer by flow rate. Then figure out the length of pipe that seems to be justifiable. My guess is any thing above 200-300 cfm is going to require significant piping.

Also, heat transfer is better in metal pipes, which would seep into your water and cost more. Many geothermal systems use plastic tubing because its better for corrosion and such but have to use significantly longer pipes since they are basically attempting to transfer heat using a conductor.

Sorry if my math is wrong, but that's my best guess. Good luck with this, it's a pretty cool idea!
Jayefuu1 year ago
I agree with the others. The idea (is it yours?) is interesting. But it's far too long to read without breaking it up with pictures. I've subscribed, I hope you turn some of these into Instructables.

Some thinks to consider:
1) Disaster zones and solar panels.... possible with aid. But the things that will have the most impact should be built easily from materials to hand. This could be built from salvaged guttering and electronics if it wasn't for the solar collector. Which leads to other things that might be on hand.... car batteries? Bikes?

2) In your response to chesterjohn you say you think the one he suggests couldn't make more than a few gallons per day and discount it because it needs electricity. Again... where are you going to get solar panels in an emergency? It would be interesting to see calculations on how much air a human powered bike could move.
RT-101 (author)  Jayefuu1 year ago

Hi. Thanks for your feedback.

1) IDEA: This is only partially my idea, discussions with friends led to this blog.

 2) INSTRUCTABLES: This website is a great idea (wish I thought of it) and I’m an avid fan. But I’ve got the problem of trying to make something that only works on large scale. Small models won’t work well based on doing the math. Making 12 GPH of water or better would need at least a 50-ft metal pipe, at least 8” to 10” inner diameter and buried at least 6-ft underground. That alone is backhoe excavation work using heavy equipment! Then a ~100-lb electric fan motor, etc. My point is it’s silly of me to actually create an Instructable when the process is like building a house? What do you think?

 3) SALVAGED MATERIALS AT HAND: Boy you really got me there! A worthy challenge. I’ll have to talk with my friends to come up with some ideas.

 4) HUMAN POWERED FAN: Great idea! Attach a geared bike-like system to run a fan. Exercise bikes exist that generate electricity, but your suggestion of direct coupling to fan motion is best for efficiency! Stairmasters might work better because you use your weight to create energy. It’s easier, produces more power with greater human endurance than biking: StepPowerOutput (SPO) = Weight x Step Height x Steps Per Second = 150lbs x 0.5 ft x 1.5 SPS = 113 ft-lbs/sec = 0.205 HorsePower = 159 Watts energy for maybe 2 – 3 hour steady clips?

 5) FAN POWER REQUIRED (FPR): The energy required to run our fan varies with the number of blades available. Assuming a 5 bladed system with n=95% air movement efficiency and:  a) volumetric airflow of ma =  1500 CFM, b) a ID = 10-inch diameter pipe, c) L = 50 foot long pipe, and d) fric = pipe friction factor = 0.015:

we have ...

FPR = [(Mass Airflow Kinetic Energy / time) + (Pipe Pressure Drop x Volume Airflow) ] / n 

Where:  
(Mass Airflow Kinetic Energy / time) = ma x (air-density) x (pipe-air-velocity)^2 / 2

 and 
(Pipe Pressure Drop x Volume Airflow) = ma x (air-density) x [fric  x 12 x L / ID x (pipe-air- velocity)^2 / 2
therefore...

FPR = [(83W) plus (72W)] / 0.95 =  163 Watts.

 

Which says it’s doable with a 150 lb person walking steps!

Jayefuu RT-1011 year ago
I would think a bike would be better over steps.
1) It's easier to translate the motion to the fan
2) If a bike has cups on the pedals you can power on the lifting of the leg as well as the pressing down.
How do you keep cockroaches out of it. One thing you don't want is bugs crawling down there and living. How do you protect it from molds and fungus. these are important as these will affect the quality of water not to mention it sanitary condition.
RT-101 (author)  chesterjohn1 year ago
Bugs and algae growth are definitely issues to contend with. We will need periodic hygiene antibiotics to deal with algae. The only way bugs will get into the sealed underground system is either at the pipe inlet or exit. So mosquito screens will be placed at both points. Thanks.
Go here to learn more on how to make a safe drinking atmospheric water generator in your home.
http://www.instructables.com/id/DIY-Atmospheric-Water-Generator/
RT-101 (author)  chesterjohn1 year ago

Thanks.  A very clever device.

My only concerns are that it needs electricity and probably makes no more than a few gallons of water a day, which is maybe good for one person but not more. Typical Haitian camps housed hundreds to thousands of people, had no electricity, yet each person needed at least one gallon of water per day. Multiply that by hundreds of camps throughout the country and that’s a lot of electricity and water needed?

Kiteman2 years ago
TL:DR
RT-101 (author)  Kiteman2 years ago
Agreed!
Remember, those in real need would want even more detail!
kelseymh RT-1012 years ago
I'm confused -- I think this is well suited to be a step-by-step instructable (i.e., break out the sections into individual blocks, with illustrations for each one, rather than one long manifesto).

However, I just saw the comment from one of I'bles' Staff suggesting that you make it a forum topic. I'm not sure why -- your manifesto doesn't really ask questions or set up a discussion. It's completely instructional, and therefore entirely suitable as a true (i.e., step-by-step) I'ble.
RT-101 (author)  kelseymh2 years ago
Thanks. I'm new here and need all advice. I did put this article in both the forum and instructables areas.
This article borders on the step-by-step but obviously needs more to make it an ideal instructable. I thought it too lengthy and detailed to go that far, and Kiteman felt even this article was too much!

QUESTIONS/DISCUSSIONS:
By all means!
I'm not one who claims to know enough and without need for further discussions. No manifesto intended here. I've put forth an idea that has haunted me for awhile and am anxious for feedback regarding its functionality; its possible need; why it hasn't been done before with available technology to yield water far beyond those ancient "air-wells", is it practical; is it too costly, are we truely using green technology, how much water can we get from desert air, etc. Sorry for being lengthy here!
kelseymh RT-1012 years ago
I'bles don't have to be lengthy (mine are, but I'm pompous and long-winded, so there you go :-). Basically a few sentences (a "newspaper-style" paragraph) with pictures on each step. What you've got here would make a great step-by-step I'ble.
RT-101 (author)  kelseymh2 years ago
OK. But, I haven't made one of these, it's all concept so far. So I can draw pictures but not photos of any hardware. I feel drawings are not what I'bles is all about! I'bles = Tangibles
kelseymh RT-1012 years ago
There are lots of conceptual or design-based I'bles. And if someone actually builds based on your design, they can make their own I'ble, too.
Here's a couple links that you might find interesting about this technology (commonly known as "Atmospheric Water Generators"). There's certainly more information on this topic, and some can be linked to from these pages.

http://en.wikipedia.org/wiki/Atmospheric_water_generator

Turning Air into Water
RT-101 (author)  canucksgirl2 years ago
Thanks, it gave me some ideas!