Technology Makes Cheap Drinking Water From Air : 3 Steps

“Generating Cheap Unlimited Drinking Water
From Air”

H.Vogel 3/2021

INTRODUCTION:

How can we best apply basic technology to assist 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 and death. 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 into water 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 65F. 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 ambient air through the ground-chilled piping system. We can even force our dry, cooled piped air to exhaust into a tent-like structure to provide air conditioning as an added bonus, but this adds the penalty of both power and an increased fan size 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-green” 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 capable of 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 (lbm) of reference dry-air, (2) the specific volume or the ratio of a unit-volume of local air to its unit-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 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 or “dry bulb” 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 (24/7) of water to fill the cistern (fill rates will change at night):

1. Determine the Air Specific Volume Based on Ambient Air Temperature and Relative Humidity

In our example, the “local” air (dry-bulb) temperature is assumed Td=106F, at a relative humidity of RH= 55%. Fig-2 indicates what the resultant Humidity Ratio (HR) (the intersection of Td=106F line and RH=55% line, then horizontal to HR value) is. This is the amount of water contained in a unit weight of dry air or HR= 0.0253 Lbs-water/Lb-Dry-Air. Using HR with RH we interpolate to yield the air specific volume “mv” which is mv=14.7 ft3/Lb-Dry-Air. This value sets the optimum unit airflow for our given ambient conditions, and will create a ballpark pipe length to diameter ratio needed later. It represents the basic unit of ambient 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 air volume will scale up the additional amounts of water that can be collected.

2. Mass of Water Contained in Every Pound of Ambient Air Based on Temperature and Relative Humidity

As the inlet air cools down to a temperature of Td=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= ~89.7F, which is its “dew-point temperature” where the corresponding local RH is 100%. At this Ts 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 underground 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, the mass of available water is mw= 0.0253 – 0.009 = 0.0163 pounds of absolute pure and distilled potable water that precipitates onto the inside pipe wall maintained at temperature Tu=~55F. The resultant airflow is amixture of water droplets and dryer air flowing through to the pipe exit and is collected by dumping onto the cistern walls before the dryer air exhausts from our cistern to the outside world.

3. Finding Water Flow Yield for Every 100 CFM of Ambient Airflow

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. So for every Va=100 ft3 of local volumetric airflow movement per minute (CFM, cubic feet per minute) through the pipe (this translates into an airflow of ma= Va/mv= 100/14.7 = 6.8 lbs. of dry air per minute or 6.8 * 60 = 408 lbs. dry air per hour [PPH]) we will get a water-flow yield of mwf=ma * mw = 408 * 0.0163 = 6.65 PPH water (from above mw=0.0163 Lbs-water/Lb-Dry-Air) or 6.65/8.345 = 0.8 GPH water per every 100 CFM of air moved. An industrial fan rated at 1.27kW DC (1.71 HP) can typically move 1500 CFM at 3450 RPM and a pressure of 9.6-iwc (inches water column), to continuously produce 15 * 0.8 = 12 GPH of pristine potable water. This is based on ambient air values of Td=106F and RH=55% with an underground soil temperature of 55F.

For comparison, if our RH were at 70% with same Td=106 F, we establish a dew point or saturation temperature of Ts=~98.7F (below which the moisture in the air starts to condense at a volume rate proportional to its drop below Ts), an mw= 0.0343 – 0.009 = 0.0253 lbs., and an ma=100/15.1 = 6.6 lbs. of dry air per minute or 6.6 * 60 = 397 lbs. per hour to get a water-flow yield of mwf= ma* mw = 397 * 0.0253 = 10.0 PPH or 10 /8.345 = 1.2 GPH for every 100 CFM of air moved, which at 1500 CFM turns out to be 18.0 GPH. This would be a near max value for condensing water from air at 106F, rated at 70% RH.

4. Actual System Design Details Are Beyond Present Scope

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 this current project.

5. Powering Our Fan with Humans

HUMAN POWERED FAN (HPF): We attach a geared bike-like system to generate electricity and run this fan with human power. Exercise bikes exist that can generate electricity, and the concept of direct bike coupling to create fan motion is best for efficiency! Stairmasters may work better because you basically use all your weight to create energy. This makes it easier, produces more power and requires less human endurance than biking. We have: StepPowerOutput (SPO) = Weight x Step Height x Steps Per Second (SPS) = 175lbs x 0.8 ft x 2.5 SPS = 350 ft-lbs/sec. SPO is synonymous with HPF which translates into 1.0 HorsePower or 745.7 (W) of energy available for maybe a 2 – 3 hour steady clip by a person? This enables a 3450 RPM fan to pump 900 CFM of 106F airflow to yield 7.2 GPH of water at 55% RH, or 10.8 GPH at 70% RH.

6. Determining Fan Power Requirements

FAN POWER REQUIRED (FPR): Now comparing HPF to the actual power required (FPR) to run our fan and yield 12 GPH. FPR varies with required volumetric flowrate, length of piping, and number of fan blades available to yield our 12 GPH target. Assume a 5 bladed fan with n=95% air movement efficiency and: a) For a volumetric airflow of ma = 1500 CFM (or 0.708 cms), which translates into an air velocity of = Va/Area = 22 mps for an 8” ID pipe, producing a Reynolds number of 2.8E+05 and yielding a dynamic head of 272 Pa, b) We are capable of forcing airflow through a total pipe length of L = 186 ft., given c) A fric = pipe friction factor = 0.025, as based on the above Reynolds number; this yields a requirement of: FPR = 0.95 x [0.025 x (186/ (8/12) x 272] x 0.708 = 1267 Watts (1.7 HP). When compared to HPF, it implies a shortfall of 0.7 HP in our human powered fan and therefore can’t provide the necessary power to flow our required airflow of 1500 CFM through an 8” ID pipe of length 186 ft. However HPF is still capable of managing an airflow of 900 CFM through our required 186 ft. of piping to yield a 7.2 GPH water supply!

Required Fan Power vs. Fan Flowrate, Pressure Drop and Efficiency