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Atmospheric humidity for water production.

  1. Mar 26, 2015 #1
    *Suggestions for Water Production for California.*

    California is still in the midst of a huge drought:

    Annie Sneed Science 03.23.15 7:00 am
    California’s About to Run Out of Water. We Have to Act Now.
    http://www.wired.com/2015/03/californias-run-water-act-now/

    There appear to be however relatively simple methods of providing extra water. One is by deriving water from humidity in the air, the other by distilling the water from the ocean. Both would appear to have relatively low cost solutions.

    First the humidity solution. The amount of water in the form of water vapor is substantial, especially at high humidity. A key fact is air can store more water vapor at higher temperatures. But the point is the areas in California with the highest drought level are the areas with routine high temperatures, such as Los Angeles. So lets calculate the amount of water in air in Los Angeles. This page gives a graph of average relative humidity levels in LA:

    Average Weather For Los Angeles, California, USA.
    https://weatherspark.com/averages/30699/Los-Angeles-California-United-States

    You see that the relative humidity commonly reaches the 80% range and above, especially during the warmer months. The relative humidity is the percentage of the maximum possible water vapor the air can hold based on that temperature, called the saturated vapor density. This page gives a calculator for the saturated vapor density based on temperature.

    Relative Humidity Calculation.
    http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/relhum.html

    For a temperature of 80 degrees F, the saturated vapor density is given as 25.4 gm/m^3. And for a relative humidity of 80%, this corresponds to an actual amount of water in the air of 20.3 gm/m^3, 0.0203 kg/m^3.

    *Residential Solutions.*

    Some ceiling fans can move quite large amounts of air at a time resulting in quite large amounts of water vapor inflow at a time, if placed for example in residential windows. For instance some fans on this page at their higher speed settings can move 5,000 cubic feet per minute (CFM) of air while using only 30 watts:

    ENERGY STAR Most Efficient 2015 — Ceiling Fans 52 inches and under.
    http://www.energystar.gov/index.cfm?c=most_efficient.me_ceiling_fans_under_52_inches

    Lower speeds give better energy efficiency, but at the cost of lowered air flow amounts. We’ll use the high speed numbers since we want large air flow to get high water vapor inflow.

    At this high amount of hot air inflow though we would probably want the fan in a basement or attic since we would also need a window for the hot air, once the water is removed, to exit.

    How much water would need to be produced? An average person uses about 50 gallons per day of water:

    City Utilities: Water Tips.
    http://www.cityutilities.net/resident/pgms/watertips.htm

    So say a residence for a family of 4 needed 200 gallons of water per day. A gallon is 3.785 liters, so this is 757 liters per day. Water weighs 1 kilo per liter so this is 757 kilos per day. Then how long would it take for a fan blowing in 5,000 cubic feet per minute to bring in this much water(vapor)? At 5,000 cfm this is 5,000/3.28^3 = 141.7 meters per minute. Each of these cubic meters of air would contain 0.0203 kg of water vapor. So this would amount to 2.88 kg per minute of water. This would take 262.8 minutes, about 4.4 hours.
    This amount of power, only 30 watts used for 4.4 hours, is also quite small in energy costs compared to what the county of Los Angeles charges for water.

    An additional question to be resolved however is how can we convert this water vapor to liquid water? Air conditioners are able to do this, by accident, by chilling the air. Air dehumidifiers also commonly work this way. This causes water to condense out like happens for example in cool morning temperatures with morning dew. On our relative humidity calculator page, at 80 degrees F and 80% relative humidity, the dew point is only 73.4 F. However, both air conditioners and air humidifiers use quite high power levels. We want to minimize additional power used.

    Some possibilities:

    1.)If the water produced this way is an adjunct to the water received from the city, then we can use the cool water coming from the city water supply, typically around 50 degree F, to cool this air and get the water to condense out.

    2.)It would be nice though, since the calculation showed this air-produced water alone is sufficient to supply the entire household water needs, to find a way that didn’t use the city water supply.

    a.)Another type of air dehumidifier uses desiccants to absorb water vapor. The desiccant material is then heated to release the water as liquid and the same desiccant is used over again. However, this material typically is a silica compound and you would not want remnants of this to be left in the water. This also uses additional power for the heating step. If a desiccant could be found that is a type of mineral you would normally see for example in spring water then this might work. You would though need to find a way to get the water to be released as a liquid. Heating as with air dehumidifiers would work. However it may be at the high temperature of southern California would be sufficient so this would happen naturally.

    b.) A similar possibility derives from the fact that rain droplets can frequently condense in the air out of water vapor at temperatures higher than they would normally do by having nucleation sites:

    Cloud condensation nuclei.
    http://en.wikipedia.org/wiki/Cloud_condensation_nuclei

    Then we could add nucleators into the air stream to get the air to condense. These nucleators though again would have to be a non-toxic if ingested. Ideal would again be some type of mineral commonly found in mineral water.

    c.)To get the water to condense we could also expand the air flow. Rapidly expanding the air would cause the temperature to drop thereby chilling the water. A problem here though is the air flow is so large it might require an unreasonable size of expansion needed to get the needed temperature drop.

    d.)Another possibility would be by increasing the pressure of the air. Just as increasing the pressure increases the temperature at which water makes the transition from liquid to gas, the boiling point, so also the temperature at which it makes the transitions from gas to liquid, the dew point, also increases. This page gives a calculator for how the pressure changes the dew point:

    Dew Point Conversion Calculator.
    http://www.howelllabs.com/resources/dew-point-conversion-calculator/

    Enter in 73.4 degrees F in the known dew point field for our 80 degree F and 80% relative humidity scenario. Enter in 0 for the “psig” field, which measures how far this is above standard pressure in psi. Then a psig of only 4 gives a dew point of 80.3 F. That is an increase in pressure of less than 30% results in the water condensing out.

    We might be able to generate this amount of extra pressure by circulating the air around in a circle by centrifugal force. A problem though is the size of the air stream coming from a large ceiling fan size diameter might make the circle size needed impractically large. We could constrict the air coming from the fan into a smaller pipe diameter, but by the Bernoulli principle this would reduce the pressure. It still may be possible though that some combination of restricted pipe diameter and circulation diameter size could provide the needed pressure change at a practical size.

    *Water Utility Solution.*

    For the water company using this method to produce extra water it becomes particularly simple. The large ceiling-fan type fans discussed in the residential solution only move air in the range of 3 to 4 miles per hour. But wind speeds commonly are above this speed especially in coastal areas. Then you would not even need to use fans for the water utility solution. You would just collect the air driven by the wind in large tubes for processing. Also, for California communities near the coast, using the ocean to supply the necessary cooling to condense the water vapor becomes especially simple.

    *Individual Water Production.*

    For a scenario where one is stranded out in the wild or in a life raft without water, the amount of power needed for the fan is so small that it could probably be generated by hand for a single individual just for enough water to sustain life. And considering the wind speed needed is only in the range of 3 to 4 mph a fan might not be needed at all.
    Also, at least for an 80 F and 80% relativity humidity scenario, the 30% extra pressure needed to allow water to condense could be easily supplied by hand. By the Ideal Gas Law PV = RT, to get a 30% increase in pressure we would need to make less than a 23% decrease in volume, assuming we did the compression slowly so as not to increase the temperature. For instance, a piston in a foot long cylinder would only need to make a 3 inch compression to get the needed pressure.
    The life boat case would also be very simple because the condensing could instead be done by using the cool temperatures of the ocean water.

    Bob Clark
     
  2. jcsd
  3. Mar 26, 2015 #2

    DaveC426913

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    Gold Member

    Perhaps I am being naive but aren't your numbers for average humidity in the L.A. are based on normal weather cycles? If there is a drought, does it not mean that the amount of vapour in the air is substantially lower than that average?
     
  4. Mar 26, 2015 #3

    phyzguy

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    Science Advisor

    There's a lot of scare-mongering around the situation in California. Here are some things to remember:

    * Household water users use less than 10% of California's water, so even if all of California's residents cut their water use to zero by the techniques you describe, it would only reduce the water usage by less than 10%, and the cost would be quite substantial. Also, even at the current levels of rainfall, there is more than enough water to supply all household water needs.

    * Agriculture uses the majority of California's water. The value of most of the crops grown is simply not high enough to pay for desalination or condensing water out of the air. To give you some rough numbers, water from the huge Carlsbad desalination plant being built near San Diego will cost about $2000 per acre-foot of water ( an acre-foot of water is one acre covered to a depth of one foot, also 43,560 cubic feet, about 320,000 gallons or about 1.2 M liters). A crop like alfalfa, one of California's largest water users, needs something between 1-4 feet of water per year, depending on rainfall, so it would cost $2000-$8000 to water an acre of crop for a year. But an acre of alfalfa only produces on the order of $1000 worth of crop (depending on yield, prices, etc.). I'm being very rough, but the point is that there are very few if any agricultural crops that are worth enough to pay for the very expensive methods you propose. Remember that the agricultural industry in California has grown up with water provided from rivers and snow melt, which is very cheap (on the order of $100 per acre-foot or less) compared to desalination or condensing water out of the air.

    Many people may disagree, but I think the bottom line is that if the rains don't return, much of California's agriculture may have to be grown somewhere else. High value crops like fruits, nuts, and vegetables will probably continue to make sense, but low value crops like alfalfa, hay, and rice, which consume most of California's agricultural water today, may have to be grown elsewhere.
     
  5. Mar 26, 2015 #4

    russ_watters

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    Staff: Mentor

    Welcome to PF, Robert.

    While you stated a power input for bringing the air into the building (way too low, but at least you tried - ceiling fans do not bring air into a house), that's the easy part. Condensing the water vapor is what takes most of the energy. So you can't really know how much this is going to cost unless you do that calculation.
     
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